Document [original]

Spatially Well-Defined Silylene Transition-Metal

Complexes in Selective Catalysis

vorgelegt von

M. Sc.

Shweta Kalra

ORCID: 0000-0002-8282-3806

an der Fakultät II – Mathematik und Naturwissenschaften

der Technischen Universität Berlin

zur Erlangung des akademischen Grades

Doktor der Naturwissenschaften

- Dr. rer. nat. -

genehmigte Dissertation

Promotionsausschuss:

Vorsitzende: Prof. Dr. Maria Andrea Mroginski

Gutachter: Prof. Dr. Matthias Drieß

Gutachter: Prof. Dr. Christian Müller

Tag der wissenschaftlichen Aussprache: 09.03.2023

Berlin 2023

Acknowledgements

First and foremost, I would like to thank my PhD supervisor Prof. Dr. Matthias Driess,

for providing me the opportunity to conduct research on the exciting and challenging chemistry

in his research group. I am immensely grateful for his constant guidance, encouragement and

endless support during my doctoral studies. This research work would not have been possible

without his fruitful scientific suggestions.

I am extremely thankful to Prof. Dr. Christian Müller for being the external referee for

my PhD dissertation. I would also like to thank Prof. Dr. Maria Andrea Mroginski for chairing

the doctoral committee.

My sincere thanks go to Prof. Dr. Karsten Meyer, Daniel Pividori, and Dominik Fehn

(FAU, Germany) for their collaboration work, including magnetic measurements of the Mn(0)

compounds. In addition, I am thankful to Prof. Jun Zhu (Xiamen University, China) for his

contribution to theoretical calculations on the Mn(0) complexes.

I would like to acknowledge the analytical centers colleagues of the Institut für Chemie

at the Technische Universität Berlin for their wonderful assistance and help. I am grateful to

Dr. Sebastian Kemper and Dr. Jan Dirk Epping for the NMR measurements. I would like to

convey my huge gratitude to Paula Nixdorf and Dr. Elisabeth Irran from single-crystal X-ray

diffraction facility for their outstanding assistance. I thank Dr. Maria Schlangen and Marc Griffel

for their dedicated service in mass spectrometry and Juana Krone for the elemental analysis

measurements. My sincere thank goes to Dr. Stefan Kohl and Dr. Julia Kohl for their kind

assistance during these years.

I am highly indebted to the members of Cluster of Excellence UniSysCat and Berlin

Graduate School of Natural Sciences and Engineering (BIG-NSE) for a doctoral fellowship,

particularly to Dr. Jean-Philippe Lonjaret for his immense support in realizing the

administrative task and organizing wonderful social activities for BIG-NSE candidates. I would

also like to especially thank my BIG-NSE friends, Dr. Avijit Roy and Benyapa Kaewmee, for

being so supportive and cheerful since the beginning of my doctoral studies.

I am grateful to my co-workers from AK Driess, Sarah Kobosil and Artemis Saddington,

for their constructive suggestions and corrections in polishing my dissertation. I am immensely

thankful to Dr. Shenglai Yao for proofreading this dissertation.

A huge thank goes to the secretary assistant of the Driess group, Ms. Andrea Rahmel,

for her continuous help in achieving all the administrative tasks promptly. I am immensely

thankful to Stefan Schutte for his kind help and outstanding assistance in the lab activities. I

would like to thank Jian Xu, Basundhara and Dr. Marcel-Philip Lücke for being great lab

partners and providing the supportive working environment. I am extremely thankful to all the

members of the Driess group for their invaluable support and mutual respect. This includes

the group members Dr. Carsten Walter, Dr. Vijay Gonela, Dr. Indranil Mondal, Dr. Yun Xiong,

Dr. Shenglai Yao, Dr. Prashanth Menezes, Dr. Lukas Reith, Dr. Ziliang Chen, Changkai Shan,

Anna Wolfrom, Artemis Saddington, Basundhara Dasgupta, Sarah Kobosil, Jian Xu, Suptish

Mondal, Dr. Niklas J. Hausmann, Hongyuan Yang and the former group members Dr.

Terrence Hadlington, Dr. Rodrigo Beltrn Suito, Dr. Biswarup Chakraborty, Dr. André

Hermansdorfer, Dr. Ernesto Ballestero, Dr. Marcel-Philip Lücke, Dr. Yuwen Wang, Min Ha

Kim, Christopher Eberle, Frank Czerny and Viktoria Forstner.

I would like to convey my deep gratitude to Dr. Selvarajan Nagendran for the inspiration

and guidance during my Master’s study at Indian Institute of Technology Delhi. I express my

profound gratitude to all the teachers from the Department of Chemistry at Gargi College,

University of Delhi, for thorough training and constant supervision during my graduate studies.

Last but not least, I am infinitely thankful to my parents and siblings Akash, Chetna

and Shivam for their unconditional love and support throughout these years. I also want to

thank my friends in India for their endless encouragement through the distance. I am eternally

thankful to Avijit for his utmost support and inspiration in all possible ways during the whole

process.

The work described in this dissertation was conducted at the Laboratory of Metalorganics and

Inorganic Materials of the Institut für Chemie, Technische Universität Berlin, under the

supervision of Prof. Dr. Matthias Driess, from January 2019 to July 2022. Some results of my

dissertation are published partly or completely in the following papers:

Publications during doctoral study

1. “A bis(silylene)pyridine pincer ligand can stabilize mononuclear manganese(0)

complexes: facile access to isolable analogues of the elusive d7-Mn(CO)5 radical”.

Shweta Kalra, Daniel Pividori, Dominik Fehn, Chenshu Dai, Shicheng Dong,

Shenglai Yao, Jun Zhu, Karsten Meyer, Matthias Driess*,

Chem. Sci., 2022, 13, 8634–8641.

Publications during doctoral study from the collaboration

2. “Manganese sulfide enables the formation of a highly active β-MnOOH

electrocatalyst for effective alkaline water oxidation”.

Carsten Walter,† Shweta Kalra,† Rodrigo Beltrán-Suito, Michael Schwarze,

Prashanth Wilfried Menezes*, Matthias Driess*,

Mater. Today Chem., 2022, 24, 100905. (†equal contribution)

3. “A Low-Temperature Molecular Precursor Approach to Copper-Based Nano-Sized

Digenite Mineral for Efficient Electrocatalytic Oxygen Evolution Reaction”.

Biswarup Chakraborty,† Shweta Kalra,† Rodrigo Beltrán-Suito, Chittaranjan Das,

Tim Hellmann, P. W. Menezes, Matthias Driess*,

Chem. Asian J., 2020, 15, 852–859. (†equal contribution)

Conference attended during the doctoral study

1. Poster presentation: “Bis(silylene)pyridine Manganese (0) Complex: An Active Pre-

catalyst for Hydroboration of N-heteroarenes”, 19th International Symposium on

Silicon Chemistry (ISOS-2020), Virtual Conference (July, 2021).

2. Oral presentation: “A Pincer-Type Bis(silylene) Manganese Complex With

Remarkable Reactivity Mimicking The [Mn(CO)5] Radical”, EC2-BIG-NSE Retreat,

Virtual (July, 2021).

3. Oral presentation: “A Pincer-Type Bis(silylene) Manganese Complex With

Remarkable Reactivity Mimicking The [Mn(CO)5] Radical”, 29th International

Conference on Organometallic Chemistry (ICOMC-2022), Prague, Czech

Republic (July, 2022).

4. Summer School on “X-ray crystallography” at Freie Universität Berlin, Germany

(May, 2019).

Zusammenfassung

Zweiwertigen N-heterozyklische Silylene (NHSis) wurden aufgrund ihrer einzigartigen

sterischen und elektronischen Eigenschaften bereits erfolgreich für die Aktivierung kleiner

Moleküle und die Stabilisierung von niederwertigen Hauptgruppenelementen eingesetzt. In

meiner Dissertation werden Bis(NHSis)pyridine-Liganden zur Stabilisierung reaktiver

niedervalenter Übergangsmetallkomplexe und deren katalytische Anwendung in der

homogenen Katalyse eingesetzt.

Der Ergebnis- und Diskussionsteil meiner Dissertation ist in vier Abschnitte unterteilt. Der erste

Abschnitt beschreibt die Synthese und strukturelle Charakterisierung des ersten NHSi

stabilisierten Mn(0)-Komplexes, 3-[SiNSi]Mn(dmpe). Die Synthese und Isolierung dieses

Komplexes erfolgte durch die Reduktion von Bis(silylen)pyridin-stabilisierten Dihalido-Mn(II)-

Komplexen in Gegenwart vom chelatbildenden 1,2-Bis(dimethylphosphino)ethan (dmpe)

unter Verwendung von KC8 als Reduktionsmittel. Die spektroskopische Charakterisierung und

theoretische Berechnungen bestätigten die pseudo-quadratische Pyramide eines

fünffachkoordinierten Mn(0)-Monoradikals, das durch einen Bis(silylen)pyridin-Liganden in

einer Low-Spin-Umgebung stabilisiert wird, wie das schwer fassbare Mn(CO)5-Radikal

vorhergesagt wird. Die Reaktivität des isolierten Mn(0)-Komplexes mit -akzeptor-Liganden

wie CO und Isocyanid und ihre katalytischen Aktivitäten werden in Abschnitt 3.2. diskutiert.

Der isolierte Tricarbonyl-Mn(0)-Komplex, der durch einen Bis(silylen)pyridin-Liganden

stabilisiert wird, ist ein beispielloses Bis(NHSi)-Analogon des schwer fassbaren Mn(CO)5-

Radikals. Diese erfolgreichen Liganden-Substitutionsreaktionen schon einem Einblick in die

Eignung von 3-[SiNSi]Mn(dmpe) als Vorläufer für die Isolierung analoger Mn(0)-Komplexe.

Bemerkenswerterweise wirkt der Bis(silylen)-koordinierte Mn(0)-Komplex 3-[SiNSi]Mn(dmpe)

auch als effizienter Katalysator bei der regioselektiven Hydroborierung von N-Heteroaren mit

Pinacolboran, um selektiv die 1,2-hydroborierten Produkte zu liefern.

In Abschnitt 3.3. wird die vergleichende Studie beschrieben, die das unterschiedliche

Koordinationsverhalten von Bis(silylen)pyridin [SiNSi] und Bis(phosphin)pyridin [PNP]

gegenüber Alkylmangankomplexen zeigt. Der starke Donor-Charakter des Bis(NHSi)-

Liganden führte zum zweikernigen Alkyl-Mn(II)-Komplex, während die Verwendung des

Bis(Phosphin)-Liganden den Dialkyl-Mn(II)-Komplex bei der Salz-Metathese-Reaktion ihrer

entsprechenden Dichlorid-Mn(II)-Komplexe mit (Trimethylsilyl)methyllithium ergab.

Schließlich wird in Abschnitt 3.4 die Synthese und strukturelle Charakterisierung von

Bis(silylen)pyridin [SiNSi]-stabilisierten Ni(II)-Komplexen und deren katalytische Anwendung

in der Sonogashira-Kreuzkupplungsreaktion diskutiert. Die CN-Streckschwingungsmoden

der entsprechenden Ni(0)-Komplexe deuten darauf hin, dass der auf Si(II)-Zentren basierende

[SiNSi]-Ligand ein stärkerer -Donor-Ligand ist als der [PNP]-Ligand. Der Bis(silylen)pyridin-

basierte Dibromido-Ni(II)-Komplex wirkt als hervorragender Katalysator für die Sonogashira-

Kreuzkupplungsreaktion mit einer Vielzahl von Acetylenen und Vinyliodiden.

ABSTRACT

Divalent N-heterocyclic silylenes (NHSis) have been successfully used for the

activation of small molecules and stabilization of low-valent main-group elements due to their

unique steric and electronic properties. In this dissertation, bis(NHSis) are utilized as a ligand

to stabilize reactive low-valent transition-metals and to perform catalytic application in

homogeneous catalysis.

The results and discussion part of this dissertation is divided into four sections. The

first section describes the synthesis and structural characterization of the first NHSi-stabilized

Mn(0) complex, 3-[SiNSi]Mn(dmpe). The isolation of this complex was achieved through the

reduction of bis(silylene)pyridine dihalido Mn(II) complexes in the presence of chelating

bis(phosphine) 1,2-bis(dimethylphosphino)ethane (dmpe) using KC8 as reducing agent. The

careful spectroscopic characterization and theoretical calculations confirmed the pseudo-

square pyramidal of a five-coordinate Mn(0) monoradical stabilized by bis(silylene)pyridine

ligand in a low-spin environment, as predicted for the elusive Mn(CO)5.radical. The reactivity

of isolated Mn(0) complex with -acceptor ligands such as CO and isocyanide, and their

catalytic activities are discussed in section 3.2.. The isolated tricarbonyl Mn(0) complex

stabilized by bis(silylene)pyridine ligand is an unprecedented bis(NHSi) analogue of the

elusive Mn(CO)5.radical. These successful ligand-substitution reactions shed light on the

suitability of 3-[SiNSi]Mn(dmpe) as a precursor to isolate analogous Mn(0) complexes.

Remarkably, the bis(silylene) coordinated Mn(0) complex, 3-[SiNSi]Mn(dmpe) also acts as

an efficient catalyst in the regioselective hydroboration of N-heteroarene with pinacolborane

to selectively furnish the 1,2-hydroborated products.

In section 3.3., the comparative study exhibiting the distinct coordination behaviour of

bis(silylene)pyridine [SiNSi] and bis(phosphine)pyridine [PNP] towards alkyl manganese

complexes is described. Owing to the strong donor character of bis(NHSi) ligand, it led to the

dinuclear alkyl Mn(II) complex, whereas the utilization of [PNP] ligand afforded the dialkyl

Mn(II) complex on the salt-metathesis reaction of their corresponding dichloride Mn(II)

complexes with (trimethylsilyl)methyllithium.

Finally, the synthesis and structural characterization of the [SiNSi] Ni(II) complexes

and their catalytic application in Sonogashira cross-coupling reaction is discussed in section

3.4. The CN stretching vibration modes of the corresponding Ni(0) complexes suggested that

the [SiNSi] ligand based on Si(II) centers is a stronger -donor ligand than the [PNP] ligand.

The [SiNSi] dibromido Ni(II) complex acts as an excellent catalyst for the Sonogashira cross-

coupling reaction of a wide range of different acetylenes and vinyl iodides.

CONTENT

1. INTRODUCTION

1.1. Pincer ligands

1.1.1. Nomenclature

1.1.2. Steric and electronic control over pincer ligands

1.1.3. Major breakthroughs in catalysis and cooperative bond

activation employing pincer ligands

1.2. Isolable divalent Si compounds

1.2.1. Synthetic methods for NHSis

1.2.2. Coordination towards transition-metals

1.3. Bis(silylenes)

1.3.1. Classification of chelating silylene ligands

1.3.2. Synthetic methods for bis(silylenes)

1.4. Application of bis(silylenes) as chelating ligands

1.4.1. bis(silylenes) as chelating ligands in homogeneous catalysis

1.4.2. bis(silylene) stabilized low-valent main-group elements

2. MOTIVATION AND OBJECTIVE

3. RESULTS AND DISCUSSION

3.1. Synthesis and characterization of bis(silylene) Mn(0) complex

3.1.1. Synthesis and characterization of bis(silylene) Mn(II) complexes

3.1.2. Strategies utilized for the isolation of bis(silylene) Mn(0) complex

3.2. Reactivity and catalytic application of the bis(silylene) Mn(0)

complex 7

3.2.1. Reaction with CO

3.2.2. Reaction with 2,6-dimethylphenyl isocyanide

3.2.3. Regioselective hydroboration of N-heteroarenes

3.2.3.1. Catalytic screening

3.2.3.2. Substrate scope

3.2.3.3. Mechanistic investigation

3.3. The comparative study on the reactivity of bis(silylene) and

bis(phosphine) in accessing alkyl Mn(II) complexes

3.3.1. background

3.3.2. Synthesis and characterization of bis(phosphine)pyridine

stabilized Mn(II) complexes

3.3.3. Synthesis and characterization of NHSi stabilized alkyl Mn(II)

complexes

3.3.4. Reactivity of bis(silylene)pyridine with LiCH2SiMe3

3.4. Synthesis of bis(silylene)pyridine Ni complexes and their

application in Sonogashira cross-coupling reaction

3.4.1. Background

3.4.2. Synthesis and characterization of bis(silylene) Ni(II) complexes

3.4.3. Synthesis and characterization of bis(silylene) Ni(0) complexes

3.4.4. Synthesis and characterization of bis(phosphine) Ni complexes

3.4.5. Optimization of reaction condition for Sonogashira cross-

coupling reaction

3.4.6. Substrate scope

3.4.7. Mechanistic investigation for catalytic Sonogashira cross-

coupling reaction

101

4. SUMMARY

109

5. EXPERIMENTAL SECTION

117

5.1. General considerations

117

5.2. Analytical methods

118

5.3. Synthesis of starting materials

121

5.4. Synthesis and chracterization of the new compounds

122

5.4.1. Synthesis of Mn(II) complex 2

122

5.4.2. Synthesis of Mn(II) complex 3

123

5.4.3. Synthesis of Mn(II) complex 4

123

5.4.4. Synthesis of Mn(II) complex 5

124

5.4.5. Synthesis of Mn(II) complex 6

125

5.4.6. Synthesis of Mn(0) complex 7

126

5.4.7. Synthesis of Mn(0) complex 8

127

5.4.8. Synthesis of Mn(0) complex 9

128

5.4.9. Synthesis of bis(phosphine) 13

128

5.4.10. Synthesis of complex 14

129

5.4.11. Synthesis of complex 15

130

5.4.12. Synthesis of complex 16

131

5.4.13. Synthesis of complex 18

131

5.4.14. Synthesis of complex 19

132

5.4.15. Synthesis of complex 21

133

5.4.16. Synthesis of complex 22

134

5.4.17. Synthesis of complex 23

135

5.4.18. Synthesis of complex 24

136

5.4.19. Synthesis of complex 25

137

5.4.20. Synthesis of complex 26

138

5.4.21. Synthesis of complex 30

139

5.5. Additional experiments

5.6. Catalysis

140

141

6. REFERENCES

149

7. APPENDIX

161

7.1. Crystal data and structure refinement

161

7.2. Computational details

181

ABBREVIATIONS

Angstrom

APCI

Atmospheric Pressure Chemical Ionization

ATR

Attenuated Total Reflection

B.M.

Bohr Magneton

Calcd

calculated

CFT

Crystal Field Theory

cod

coordinated 1,5-cyclooctadiene

COD

1,5-cyclooctadiene

Cp*

pentamethylcyclopentadienyl

Continuous-wave

chemical shift

DFT

Density Functional Theory

Dipp

2,6-diisopropylphenyl

dme

1,2-dimethoxyethane

dmpe

1,2-bis(dimethylphosphino)ethane

DOSY

Diffusion-ordered spectroscopy

dvtms

1,3-divinyltetramethyldisiloxane

Et2O

diethyl ether

EPR

Electro Paramagnetic Resonance

equiv

equivalent

ESI

Electrospray ionization

exptl

experimental

Fourier-transformation

gram(s)

general procedure

High-resolution

HOMO

Highest Occupied Molecular Orbital

HMQC

Heteronuclear Multiple Quantum Coherence

HSOMO

Highest Singly Occupied Molecular Orbital

Hertz

IPr

1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidin

iPr

iso-propyl

Infrared

coupling constant

LiHMDS

Li[N(TMS)2]

LUMO

Lowest Unoccupied Molecular Orbital

molecular mass or metal

µeff

effective magnetic moment

Mes

mesityl

mol%

mole percent

M.p.

Melting point

Mass spectrometry

nBuLi

n-butyllithium

NHC

N-heterocyclic carbene

NMR

Nuclear Magnetic Resonance

NHSi

N-heterocyclic silylene

phenyl

ppm

parts per million

PSQP

Pseudo-Square Pyramidal

pyridine

room temperature

SQUID

Super-conducting Quantum Interference Device

tBu

tert-butyl

THF

tetrahydrofuran

THF-d8

deuterated tetrahydrofuran

TMS

trimethylsilyl

TOF

Turnover-frequency

valence electron

variable temperature

ν

wavenumber

XRD

X-ray diffraction

INTRODUCTION

1. INTRODUCTION

The development of transition-metal (TM) catalyzed activation of inert and nonpolar

chemical bonds has opened a new avenue in catalysis research. At least one catalytic step is

involved in the manufacturing of 90% of all commercial compounds.[1] The development of

catalytic processes on a colossal scale made it possible to meet the demands of a rapidly

growing world population. Some of these important processes are the Haber-Bosch process,

the Fischer-Tropsch synthesis, the Cativa process, Hydroformylation, and the metathesis of

olefins.[2] However, the rising cost and demand for noble-metal precursors triggered a growing

interest in finding alternative and more abundant homogeneous catalysts. Homogenous

catalysis has benefitted from the successful development of organometallic chemistry.[3] The

vital parameters such as robustness expressed in terms of the turnover number (TON) and

the activity expressed in terms of the turnover frequency (TOF) have always been an important

concern for the development of catalysis processes.[4] The growth in the domain of

organometallic chemistry is largely dependent on the ligand framework design and their

distinctive effects on the geometry, electronic structure, and reactivity of resultant transition-

metal complexes.[4] In the recent years, the choice of transition-metals for the homogeneous

catalysts development has gradually refocused on 3d metals. This shift is not only due to their

ubiquity, availability and often lower toxicity but also due to changes in the coordination

geometry of first-row 3d metal complexes and the diversity of available spin-states, which are

more common compared to their heavier homologues.[5]

One of the ways to discover new active TM complexes is through the development of

ligands with different donor atoms such as N, O and divalent silicon. Wilkinson et al. developed

the first TM complex coordinated to a silicon-based ligand in 1956.[6] Silicon being the second

most abundant element in the Earth’s crust, holds a crucial role for a variety of industrial

applications.[7] The successful isolation of previously elusive divalent and zero-valent group

14 compounds has promoted spectacular advancement in the field of low-valent group 14

element chemistry.[8] In the past three decades, low-valent Si(II) compounds especially, N-

heterocyclic silylenes (NHSis), have been studied enormously as excellent ligands in TM

mediated catalysis due to their strong -donor and -acceptor properties.[9] In this regard, the

investigation of silylene-incorporated pincer-type 3d TM complexes is a promising area of

research.

The introduction of my dissertation is divided into four parts. The first part aims on the

development of pincer-type ligands in accessing the transition-metal complexes in their

unusual oxidation states and their role as ligands in homogeneous catalysis. The second part

is about the development of the isolable silylenes particularly, N-heterocyclic silylenes;

covering their synthetic strategies, reactivities and bonding behaviour as ligands towards

INTRODUCTION

transition-metals. Since the main goal of this dissertation is related to the application of the

pincer-type bis(silylenes) as ligands for the isolation of metals in low-oxidation states, the third

part covers the existing chelating bis(silylenes) and their synthetic routes. In the last part, the

catalytic activity of bis(silylene)-based transition-metal complexes in the field of homogeneous

catalysis is emphasized presenting their superiority as ligands as compared to N-heterocyclic

carbene (NHC) and phosphine supported transition-metal complexes (Figure 1.).

Figure 1. Overview on the introduction of this dissertation.

1.1. Pincer ligands

The concept of employing multidentate ligands for synthesizing organometallic

complexes was first introduced by Shaw and co-workers in 1976.[10] The early reports on

pincer complexes by van Koten and Shaw demonstrated the exceptional kinetic and thermal

stability provided by the pincer-type ligands, which essentially promoted the pincer ligand

concept.[10-14] The ease of modification to functionalize the ligand backbone at different sites

uncovered numerous facile routes to isolate versatile pincer-type novel ligand systems with

high tuneability.

1.1.1. Nomenclature

The term pincer was first introduced in a review article entitled “Tuning the reactivity of

metals held in rigid ligand environment” by van Koten in 1989.[11] Since then a surge in the

reports on pincer complexes has been witnessed. As a simple statistic , a Web of ScienceTM

search with the term “pincer complexes” finds 6668 hits.[15] The term pincers in fact derives

from the the coordination mode of the ligand, with two “pincing” arms. Pincer ligands are

tridentate and generally coordinate to a metal center with a clear preference for a meridional

INTRODUCTION

geometry.[10] Figure 1.1.1. depicts the general classification of pincer-type complexes on the

basis of symmetry and total charge of the ligand motifs.[16]

Figure 1.1.1. Common types of pincer ligands.[16]

For simplicity, a short representation [EXE] is used for pincers, depicting the donor

atoms E and X being coordinated to metal centres in their coordination sphere. Classical

pincer-type ligands comprise of donor arms (E) usually be made up of a two-electron donor

Lewis base groups (e.g., E = PR2, NR2, or the carbon atom from an NHC), which are bridged

via a linker group (Y) (where Y = CH2, O, NH or NR) to the central mono-anionic or neutral

donor site (X; such as, a pyridine, phenyl/alkyl or silyl group). Pincer-ligands can be classified

as palindromic or non-palindromic on the basis of their symmetry and ionic or neutral

according to their binding motifs. Most pincers are palindromic such as PCP, PNP etc.

However, non-palindromic pincers, such as NNP, PCN are little explored. Neutral palindromic

(e.g., PNP) ligands as six electron donors and anionic palindromic (such as PCP) pincers with

C as the ipso-C of an aryl group, are the most commonly employed ligands and were among

the first reports. Owing to the high charge of trianionic palindromic pincers, they are considered

as suitable candidates for stabilizing high-valent metal species.

Hereafter, the referenced pincer ligand systems in my dissertation will be depicted

according to their donor atoms such as [EXE]. Subsequently, upon coordination to a metal, it

will be referred in the form of [EXE]MLn, where Ln represents the co-ligands present at the

metal center, X the central donor atom, and E the donor atom from the flanking arms.

1.1.2. Steric and electronic control over pincer ligands

The ease of modification of pincer ligands enables the entry of various functionalities

in the ligand backbone. The introduction of certain groups in the ligand backbone not only

tunes the reactivity at the metal centre, but can also improves the physical properties such as

solubility of the system.[17] Figure 1.1.2. describes the general structure of an aromatic pincer-

ligand and the potential sites for modification to tune the electronic and steric effects on the

INTRODUCTION

metal center. The various alterable sites on pincer ligands provide a toolbox for synthetic

chemists to subtly manipulate the structure and resultant reactivity of a pincer-metal complex

without significant modification of the coordination geometry at the metal. For example, (i) the

bulky R group has a direct influence on the steric control on the metal, (ii) the substituent on

the pendent linker arm (Y) exerts a profound control on the chirality, bulkiness, ring size and

bite angle effects (iii) the stereo-electronic nature of a substituent on the central ring, Z

modulates the electronic properties and solubility, (iv) the nature of central donor atom X has

a direct and important impact on the electronic properties through the trans-effect.[17]

Figure 1.1.2. General structure of pincer-type metal complexes representing the potential

steric and electronic control over pincer ligands.

The EXE 3-meridional coordination of pincer ligand to transition metals gives rise to

the formation of coplanar five-membered metallacycles, which often render an exceptional

balance of thermodynamic stability and reactivity, which is of great value in homogeneous

catalysis.[18] The virtue of having a strong binding ligand that also provides thermal stability

permits metal-pincer complexes to react with external ligands and reactive entities, such as

CO, CO2, isocyanides, H2, oxidizing and reducing agents, or even Na or K metal, while keeping

the pincer coordination of the ligand intact, even at elevated temperatures. [16-18] Through these

reactions with ligands and reagents, the robustness of pincers is an important factor in the

synthesis of metal complexes in unusual formal oxidation states. A recent interesting example

was reported by Fout and co-workers, where a Ni(IV) pincer complex was successfully isolated

by the oxidation of an anionic palindromic pincer bis(NHC)-Ni(II) complex with PhICl2.[19]

Whilst modification of the pincer, by varying one of the amine linker arms of a symmetric pincer

ligand with a hemilabile arm was proven to enhance the catalytic efficiency and turnover

number (TON) in the Sonogashira coupling between alkynes and alkyl halides. [20] In this work,

the authors ingeniously extended the scope of robust amido nickel complexes to Sonogashira

cross-coupling reactions (Scheme 1.1.1.).[20a] Additionally, the modification of 1 to access 2

INTRODUCTION

led to a new hemilabile pincer complex [Ni2], which in turn led to milder reaction conditions

compared to catalyst [Ni1].[20b] The kinetic study of Sonogashira cross-coupling further

showed that the dissociation of a labile amine donor is the turnover-determining step of the

catalysis.

Scheme 1.1.1. Catalytic activity of pincer type [NNN]NiCl complex in Sonogashira cross-

coupling reaction reported by Hu et al.[20]

1.1.3. Major breakthroughs in catalysis and cooperative bond activation employing

pincer ligands

After the first PCP type pincer complexes, a myriad of transition-metal complexes with

diverse pincer arms have been studied by various research groups, consisting of a vast family

of carbene pincers[21], PNP, [22] PNN, [23] POP,[24] NSiN,[25] SiCSi,[26] SiNSi, [27] as well as boron

functionalized[28] pincer complexes.[29] Some important milestones that have been achieved in

the past decades include: dehydrogenation of alkanes using [PCP]Ir and [PCP]Rh

complexes,[30] pincer-type [NNN]Ni catlayzed cross-coupling reactions,[20] catalytic CO2

reduction reactions catalyzed by [PNP]NiH and [PCP]NiH complexes,[31,32] catalyic ammonia

synthesis using pincer-based Mo, Re, Fe complexes,[33] transfer-hydrogenation reaction using

[PNP]Mn complexes.[34] These developments imply the great potential of pincer complexes in

the synthesis of value-added products from small molecules as attractive feedstock. Herein,

a few selected and important examples are described on the basis of their significance and

the similarities of the systems studied in this dissertation.

INTRODUCTION

A methane complex: Brookhart and co-workers were able to characterize a -type methane

Rh(I) complex [Rh(PNP(CH4))]+ (where PNP is 2,6-bis(di-tert-butylphosphinito)pyridine)

supported by a pincer ligand in solutions (Scheme 1.1.2).[35] In this complex, a metal-C–

H(methane) interaction was observed by multinuclear NMR spectroscopy, with a half-life of 83

min at –87 °C. This work highlighted the steric protection provided by the bulky [PNP] pincer

ligands in stabilization of labile methane-bound complex.

Scheme 1.1.2. Formation of a methane-bound cationic rhodium(I) complex in solution-phase

reported by Brookhart et al.[35]

Dinitrogen fixation : Using a PNP pincer-based trichlorido Mo(III) complex, Nishibayashi and

co-workers isolated a nitrogen-bonded dinuclear molybdenum complex and introduced it as a

N2-fixation pre-catalyst (Scheme 1.1.2).[36] Later, the same group reported the profound

influence of various substituents on the phosphine donors, leading to an enhanced catalytic

efficiency of [PNP]Mo pincer complex.[36d] Furthermore, the addition of an electron-withdrawing

group at the para-position of the pyridine ring improved the yield of ammonia.This work

highlighted the subtle effects of ligand design and showed that bulky substituents at the P

atoms better protect the reactive Mo core.

Scheme 1.1.3. Formation of a nitrogen-bonded molybdenum complex and its catalytic

application in N2-fixation and reduction to NH3 reported by Nishibayashi et al.[36]

Catalytic reduction of CO2: In last few years, pincer complexes of first-row transition-

metals were extensively explored due to their high natural abudance and lower toxicity.

Kirscher and co-workers introduced a pincer based manganese complex for the catalytic

INTRODUCTION

production of formate by the reaction CO2 as C1 chemical feedstock and H2 (Scheme

1.1.4.).[37] In this report, along with a conventional metal-centered mechanism, a new reaction

pathway was proposed with the participation of the N–H bond of the PNP ligand backbone.

This included the loss of aromaticity of pyridine ring via deprotonation of ligand as a key step.

Nevertheless, this kind of metal-ligand cooperativity (MLC) does not the affect the formal +I

oxidation state of the metal center throughout the catalytic cycle. Remarkably, the

experimental data suggested that the activity of a different Mn complex whose PNP structure

does not allow for metal–ligand cooperation due to the presence of NMe groups instead of NH

groups on the backbone, is much lower than that of Mn1. At the same time, Beller and co-

workers have been instrumental in developing various air-stable manganese(I) based pincer

complexes for the hydrogenation of aldehydes and ketones (Scheme 1.1.4.).[38] The chemical

non-innocent behaviour of such ligand-assisted mechanism, and the effects of ligand

structural modifications on catalytic transformations have been a topic of several subsequent

mechanistic studies.[39] The bifunctional role of such PNP type ligands have opened up a new

field of pincer complex design, where the choice of ligand can influence the activation of inert

bonds through metal-ligand coopertivity.

Scheme 1.1.4. Hydrogenation of carbon dioxide catalyzed by [PNP]Mn(H)(CO)2 pincer

complex reported by Kirchner et al.[37]

Scheme 1.1.5. Hydrogenation of carbonyl compounds catalyzed by [PNP]Mn(Br)(CO)2

pincer complex reported by Beller et al.[38]

INTRODUCTION

The abovementioned examples underline the important applications of pincer ligands

in accessing highly reactive transition-metal complexes for homogeneous catalysis. Generally,

these reactions involve the formation of highly reactive electron rich intermediate, supported

by rigid ligand framework that can facilitate the activation of inert chemical bonds. Moreover,

modifications to the pincer ligand that can increase the electron density at the metal centre

are shown to be crucial for developing superior catalytic performance. Thus, investigating

stronger electron donors, such as NHSis, in pincer ligands is a worthwhile task to access

highly electron-rich, first-row transition-metal complexes, which can potentially be isolated and

characterized, that could unveil new paths of unexplored reactivity and improved catalytic

activity. The past decades have witnessed the emergence of NHSis as powerful -donor

ligands in the low-valent group 14 chemistry.[40] Furthermore, theoretical calculations and

experimental results have demonstrated that NHSis possess similar or even in some cases

even stronger -donor strength with respect to carbene and phosphine counterparts.[40-42]

Importantly, the conceptualization of utilizing chelating NHSis as functional ligands has

stimulated the growth of transition-metal mediated homogeneous catalysis. Therefore, the

proposition of employing pincer-type silylene ligands for coordination chemistry and catalysis

is described in this dissertation. In the next section, this class of compounds are described in

a brief account relating their synthetic aspects, reactivity, and coordination behaviour toward

transition-metals.

1.2. Isolable divalent Si compounds

Silylenes are divalent species with +II formal oxidation state and are the heavy

analogue of carbenes. The divalent species derived from group 14 elements are also termed

as tetrylenes. Generally, low-valent tetrylenes are difficult to isolate and becomes even more

challenging on moving up in the group 14 from Pb to C. Divalent silicon compounds, also

known as silylenes were limited to laboratory curiosities for a very long time. Although,

silylenes are the heavy analogues of carbenes, their chemical properties vary widely.[42] Unlike

the simplest parent carbene (:CH2, A), the analogous silylene (:SiH2, B) favours a singlet

ground state (Figure 1.2.1).[8c] The larger size difference of 3s and 3p orbitals make silicon

atom reluctant to undergo hybridization as compared to carbon. Hence, the lone pair of

electrons essentially locates on an orbital with high s-character (HOMO) manifesting a high

s,p-energy gap. As a result, silylene (:SiH2) prefers a singlet ground state while methylene

(:CH2) with smaller energy gap between the HOMO and LUMO, due to an effective sp2

hybridization between 2s and 2p valence orbitals, prefers a triplet ground state. In addition, a

large singlet-triplet energy gap (EST= E(triplet)−E(singlet)]) for the parent silylene (16.7 kcal

INTRODUCTION

mol–1 for :SiH2) as compared to carbene (–14 kcal mol–1 for :CH2) was reported using

theoretical calculations.[8c, 43] This trend of singlet-state energy is closely related to the

relativistic effect of caused by the inert pair of electrons (inert-pair effect), as the non-bonding

orbital (HOMO) has a higher s-character on moving down the group 14. [42]

Figure 1.2.1. Different electronic ground states of parent carbene and silylene.

Owing to the presence of vacant 3p-orbital and a lone pair with s-character, silylenes

exhibit high reactivity towards both electrophiles and nucleophiles, respectively. This intrinsic

amphiphilic nature thus prevented their isolation as single molecules under normal laboratory

conditions for a very long time. The pioneering work of Goldstein on the detection of the parent

silylene (:SiH2) and their derivatives with relatively small substituents as reactive intermediates

laid the foundation for the low-valent silicon chemistry.[44] Transient silylenes were investigated

spectroscopically via cryogenic-temperature matrix-isolation studies, gas-phase kinetic

experiment and laser photolysis studies of silanes. [45] In this regard, a significant insight was

the dimerization of the transient dimesityl-substituted silylene (:SiMes2, Mes = 2,4,6-Me3C6H2)

to a disilene Mes2Si=SiMes2 at 77 K. [46] At ambient temperature, the reactive silylenes would

immediately undergo self-dimerization or further polymerization. Hence, thermodynamic and

electronic protection of their vacant 3p-orbitals is a prerequisite for the strategic stabilization

of silylenes (Figure 1.2.2.). [8c]

Generally, heteroatom substituents with lone-pair of electrons on silicon partially

donate electron density in the vacant 3p-orbitals of silicon via -bonding interaction. The

kinetic stabilization strategy generally utilizes the steric inhibition effect by employing the

sterically demanding substituents around the silicon atom to protect silylenes from self-

oligomerization.

INTRODUCTION

Figure 1.2.2. Thermodynamic and kinetic stabilization of silylenes using either Lewis base or

bulky alkyl/aryl substituents in the ligand backbone, respectively.

Prior to the 1990s, theoretical calculations and experiments suggested that carbenes

and silylenes were highly reactive and non-isolable. Jutzi and co-workers established the first

example of an isolable Si(II) compound via the reduction of corresponding dichlorosilane

precursor. The thermodynamic stabilization effect by -interactions of the Cp* (Cp* = C5Me5)

ligand allowed them to isolate decamethylsilicocene (I-1).[47] The major milestone in the area

of stable N-heterocyclic silylene chemistry by West and Denk was highly influenced by the

discovery of analogous N-heterocyclic carbene (NHC) (I-2) by Arduengo and co-workers

(Figure 1.2.3).[48,49] The first stable NHC was isolated by taking advantage of a N-heterocycle

for thermodynamic stabilization and adamantyl substituents for kinetic stabilization. Similarly,

West and Denk isolated the first two-coordinate NHSi (I-3, I-4) at room temperature through

the reductive dechlorination of corresponding silicon(IV) dichloride using elemental

potassium.[49] Due to the presence of delocalized 6-electrons with an unsaturated backbone,

I-3 is more stabilized and relatively less reactive compared to I-4. The reduced electrophilicity

of silicon by the stabilization of 3p orbital in I-3 is essentially reflected by upfield shifted 29Si{1H}

NMR of I-3 (δ = 78.3 ppm) compared to I-4 (δ = 119 ppm).[49]

Figure 1.2.3. Early examples of isolable carbene and silylenes.

INTRODUCTION

Since then, remarkable advancements are made in the area of stable silylene

chemistry, which is reflected in precedent reports on the successful isolation of various

silylenes. Figure 1.2.4 highlights some of the examples of two-coordinate cyclic and acyclic

silylenes, such as the dialkyl cyclic silylenes and acyclic diaminosilylene.

Figure 1.2.4. Selected examples of two-coordinate cyclic and acyclic silylenes.

In 1999, the successful isolation of the cyclic dialkyl silylene I-5 by Kira and co-workers

embarked a new path to access silylenes even in the absence of strong -donor ligands.[50]

Although, I-5 is kinetically stabilized by four bulky trimethylsilyl groups, it slowly undergoes

isomerization to a cyclic silene via 1,2-silyl migration at ambient temperature in solution-phase

In 2011, our group isolated the carbocyclic silylene I-6 with two bulky phosphorus ylide

substituents employed for thermodynamic stabilization.[51] Based on the pioneering work of

Jutzi, West and Kira, a large number of stable cyclic silylenes such as cyclic

(alkyl)(amino)silylene I-7,[52] bora-ylide stabilized I-8,[53] and several isolable NHSis have been

successfully synthesized (Figure 1.2.4).

Among stable silylenes, the isolation of two-coordinate acyclic silylenes always

remained as one of the challenging synthetic targets. In 2003, the first example of acyclic

INTRODUCTION

silylene [(Me3Si)2N]2Si: (I-9) was achieved through the reduction of [(Me3Si)2N]2SiBr2 with

potassium graphite at −78 °C by West et al.[54] However, due to rapid decomposition above

0°C, its existence was only confirmed by low-temperature, solution-phase spectroscopic

studies. Power and co-workers isolated the first stable acyclic silylene I-10, stabilized by a

bulky terphenyl thiolate ligand.[55] The higher electronegativity of thiolate ligands is proven to

be an important factor for both its stability and inert behaviour towards dihydrogen, consistent

with its large singlet-triplet band gap of 4.26 eV. In 2012 Jones and Aldridge, reported the first

example of a room temperature-stable two-coordinate acyclic silylene, Si{B(NDippCH)2}{N-

(SiMe3)Dipp}, (I-11) substituted with a strong σ-donor and sterically crowded B(NDippCH)2

moiety.[56] Owing to a small singlet-triplet energy gap, I-11 demonstrates facile oxidative

addition reaction with dihydrogen and with alkyl C–H bonds, like transition-metal based

systems. Later, Jones and co-workers reported a single-step synthesis of a thermally stable

acyclic (silyl)silylene I-12,[57] starting from a Si(IV) precursor. Furthermore, I-12 also shows a

room-temperature reactivity towards oxidative addition of dihydrogen and C–H activation at

elevated temperatures.

Although, various cyclic and acyclic silylenes have been isolated in the recent years,

however, the extreme reactivity and challenging synthetic routes have limited their application

as coordinating ligands. On the other hand, N-heterocyclic silylenes owing to their compromise

between stability and reactivity and ease of synthesis, have been most widely studied.

Therefore, the rest of the introduction will focus on the synthesis and applications of N-

heterocyclic silylenes (NHSis).

1.2.1. Synthetic methods for NHSis

The N-heterocyclic silylenes are conventionally classified into Lewis-donor free and

Lewis-donor stabilized silylenes. In general, Lewis-donor stabilized silylenes are three-

coordinate and Lewis base free are two-coordinate in nature. The common synthetic

methodologies for accessing the two-coordinate silylenes (Lewis-base free NHSi) such as I-3

and I-5, are generally based on the dehalogenative reduction of their Si(IV) dihalides with

suitable reducing agents.[50] Another synthetic route is based on thermal or photochemical

reduction of suitable Si(IV) precursors.[58] For example, the photochemical reductive

elimination of a disilane from a bis-silyl precursor of type R2Si(SiR’3)2 produces the

corresponding silylene R2Si:.[58] Our group isolated a zwitterionic cyclic NHSi I-17, with an

unsaturated, conjugated six-membered β-diketiminato ligand backbone through the reduction

of corresponding dibromosilane I-16 with potassium graphite (KC8) as a reducing agent

(Scheme 1.2.1.).[59] I-17 shows a chemical shift of δ = 88.4 ppm in the 29Si NMR spectrum,

comparable to that of NHSi I-3 (29Si NMR, δ = 78.3 ppm). Due to the presence of exocyclic

INTRODUCTION

unsaturated methylene unit in ligand backbone, silylene I-17 shows, a strong ambivalent

reactivity along with a high nucleophilicity of the Si(II) center.

Scheme 1.2.1. Synthetic route of NHSi I-17.[59]

Scheme 1.2.2. Synthetic route of NHSi I-20.[61,62]

Three-coordinate donor-stabilized silylenes are thermodynamically stabilized via inter-

or intramolecular coordination by Lewis bases. The donor-stabilized chlorosilylene such as

I-20 and NHC-stabilized dichlorosilylenes are by far the most widely studied Si(II) compounds

due to their facile functionalization properties (Scheme 1.2.2.).[60] In 2006, Roesky and co-

workers reported the isolation of a first four-membered chlorosilylene I-20 with an N,N'-di(tert-

butyl)amidinato backbone on silicon by the reduction of pentacoordinated trichlorosilane

precursor I-18 with elemental potassium in THF (Scheme 1.2.2.).[61] The 29Si{1H} NMR

spectrum of I-20 show a upfield shift at 14.9 ppm compared to classical NHSis (29Si NMR, δ =

79-199 ppm), due to the stabilization of 3p orbital on Si atom by an additional intramolecular

INTRODUCTION

base. The obtained 10 % yield was extremely low. Therefore, an alternative procedure was

employed for the multi-gram isolation of I-20, where dichlorosilane I-19 undergoes

dehydrochlorination on reaction with a strong base such as LiHMDS.[62] The notion of

functionalization of silylenes through the silicon–heteroatom bond uncovered a new domain

of silylene chemistry leading to even more interesting electronic and geometrical features.

Functionalized silylenes based on N,N'-di(tert-butyl)amidinato backbone with halogens

as external donor are the most widely explored class of three coordinate systems. NHSi I-20

serves as a crucial building block in synthetic low-valent group 14 chemistry. [63,64] The

presence of a labile Si–Cl bond in I-20 has opened new avenues to access several new

heteroatom functionalities through simple salt-metathesis reaction (I-20→I-21).[61] The

introduction of such functionalities not only strongly influences the electronic structure, but

also enhances the reactivity of Si(II)-center in these systems. Additionally, the reactivity

pattern of silylene I-20 towards organic molecules further emphasizes that their coordination

to TMs cannot be merely deemed as isoelectronic to phosphines.[62,65] Therefore, on

coordination to metals, careful synthetic methods should be employed to prevent possible side

reactions with solvents or with the ancillary ligands on the metal complex precursor.

Scheme 1.2.3. Synthetic route of NHSi I-23.[66]

In 2021, Nakata and co-workers realized the new NHSi I-23, supported by an

iminophosphonamide ligand.[66] It was synthesized from the dehydrochlorination of

corresponding dichlorosilane [Ph2P(tBuN)2]SiHCl2 (I-22), using LiHMDS as a strong base

(Scheme 1.2.3.). This particular silylene I-23 is of great interest, as it shows to have stronger

-donor strength compared to common N-heterocyclic carbenes and other NHSis, even

though consisting a negatively charged chloride substituent. Owing to the presence of

extremely high-energy HOMO level, NHSi I-23 exhibits unique coordination behaviour towards

Rh(I) complexes.[67]

INTRODUCTION

1.2.2. Coordination of NHSi towards transition-metals

The presence of a s-like lone pair of electrons on Si(II) centre enables silylenes to

potentially act as coordinating ligands for transition-metals. To date, numerous silylene based

transition-metal complexes are reported.[68,69] Several reviews have highlighted the

coordination behaviour of NHSis ligands towards diverse range of TM centres. [68,70] Herein

selected examples based on amidinato chlorosilylene ligand I-20 are introduced. The

widespread application of amidinato silylene I-20 in coordination chemistry can be credited to

several factors which include (i) the intramolecularly-stabilization makes them more stable and

easier to handle compared to classical NHSi such as I-3 and I-4; (ii) they possess multiple

modification sites, which is an important aspect for the isolation of numerous multidentate

ligands; (iii) their relatively easier synthetic route. In addition to this, in some instances, they

have proven to be stronger donor ligands than NHCs.

Scheme 1.2.4. Coordination behaviour of NHSi I-20 towards various TM complexes (where

dmpe = 1,2-dimethylphosphinoethane, tmeda = tetramethylethylenediamine).[68,70]

INTRODUCTION

Generally, the existing metal complexes bearing silylene I-20 consist of metal centers

in low-oxidation state. Since carbonyls are predominantly used as co-ligands, it makes their

further application a difficult task. However, the examples of silylene-based TM complexes

without supporting ligand or only with labile ligands are limited. The traditional ways of

accessing the silylene TM complexes involves the substitution reaction, where silylenes

displaces one of the coligands coordinated to metal precursors.

1.3. Bis(silylenes)

Bis(silylenes) are the bidentate analogues of NHSis containing two independent

reactive divalent silicon(II) centers in +II oxidation state bridged through a linker organic

framework. Multidentate ligands with two or more donor site are regarded as chelating ligand.

Their strong binding affinity and thermodynamic stability have strongly influenced the rapid

growth of chelating ligands. Since the discovery of stable silylenes in early 1990s, the

chemistry of monodentate NHSis has witnessed rapid progress, while the chemistry of

chelating bis(silylenes) is comparatively less explored.[68] Only in the recent decades, the era

of multidentate silylene ligands have seen rapid growth due to their enhanced -donor and -

acceptor strength and cooperativity.[9,71,72]

1.3.1. Classification of chelating silylene ligands

Unlike monodentate silylene ligands, bidentate silylene ligand can further be divided

into two categories: (i) heteroleptic or mixed bidentate silylene ligands I-24 to I-27 containing

one silylene donor site in addition to one non-silylene donor unit such as carbene, phosphine

or other heteroatom functionalities (Figure 1.3.1).[73-76] (ii) homoleptic bidentate silylene often

referred as bis(silylene) ligands where both the donor sites are silylene units (Figure

1.3.2).[77-85] However, in this section, we will focus more on homoleptic bidentate silylene such

as. bis(silylene) ligands.

Figure 1.3.1. Selected examples of heteroleptic (mixed) bidentate silylene ligands.

INTRODUCTION

Based on the type of linkage between both divalent silicon centres, all the bis(silylenes)

can additionally be classified into two types: (i) interconnected bis(silylenes) in which the

silylene units are directly connected by a silicon-silicon chemical bond and formally possess

+I oxidation state and (ii) spacer separated bis(silylenes) in which two silylene units are

separated by a linker/spacer moiety which normally does not interact with the silylene centres.

Further spacer separated bis(silylene) can be divided into two types based on the number of

potential coordination sites in the molecule: (i) bidentate bis(silylene), which are connected via

a non-coordinating spacer group. such as I-28 to I-33 and (ii) tridentate or pincer bis(silylene)

comprises an additional potential donor site along with two flanking silylene arms in the system

such as, bis(silylene)pyridine [SiNSi] I-35 pincer ligand (Figure 1.3.2).[40]

Figure 1.3.2. Selected examples of bis(silylene) compounds based on NHSi I-20.

By taking advantage of the ease of functionalization, N,N-di(tert-butyl)amidinato

silylene ligands make up a large series of electronically modified silylene ligands with diverse

INTRODUCTION

-donor strength shown in Figure 1.3.2. Depending on the nature of the spacer used, a wide

range of bis(silylenes) could be synthesized, possessing distinct -donor strengths. In this

view, the 29Si{1H} NMR chemical shift of theses ligands essentially reflect the extent of their

donor strength.[40] This series of bis(silylenes) ligands, based on chlorosilylene I-20, can be

categorized as following: i) strong -donating ligands bearing nitrogen and oxygen

substitutions with 29Si{1H} NMR chemical shifts from –24.0 to –14.9 ppm;[78,80,85] ii) moderately

strong -donating ligands bearing carbon substitutions with 29Si{1H} NMR chemical shifts from

16.8 to 18.9 ppm;[81,82,84] iii) the weakest -donating ligand bearing the ferrocene backbone

with a 29Si{1H} NMR chemical shift of 43.3 ppm[79] (Figure 1.3.3).

Figure 1.3.3. -donor strength of bis(silylene) ligands based on N,N-di(tert-butyl)amidinate

backbone with 29Si{1H} NMR chemical shifts as a decisive parameter.

1.3.2. Synthetic methods for bis(silylenes)

Owing to the intrinsic ambiphilic reactivity of silylenes, the isolation of a predesigned

bis(silylene) is experimentally challenging. Nevertheless, in 2005, Gehrhus and Lappert

successfully isolated the bis(silylene) I-37 through the reductive dehalogenation of a spacer

separated bis(dichlorosilane) precursor I-36 (Scheme 1.3.1. top).[86] However, due to the

rigidity of the backbone in I-37, its coordination chemistry is similar to that of monodentate

silylenes. Slightly similar to this approach, in 2010, Driess and co-workers isolated an oxygen

bridged chelating bis(NHSi) I-39, via dehydrochlorination of disiloxane I-37 with a strong base

LiHMDS (Scheme 1.3.1. bottom).[77]

INTRODUCTION

Scheme 1.3.1. Synthetic routes of I-37 and I-39 by the reduction of bis(chlorosilanes).

Another method involves the reduction of trichlorosilane precursors to synthesize

bis(silylenes). Interestingly, Roesky and co-workers isolated a disilylene I-41 (29Si NMR: δ =

75.71 ppm), with a Si(I)–Si(I) bond, via the reduction of corresponding trichlorosilane precursor

I-40 using three molar equivalents of potassium graphite (Scheme 1.3.2.).[87] The single-crystal

X-ray diffraction (XRD) analysis of the latter showed that the lone pair of electrons on each Si

atom occupies one of the coordination site in a tetrahedral geometry.

Scheme 1.3.2. Synthetic routes to the disilylenes I-41 and I-43 by the reduction of

trichlorosilanes.

INTRODUCTION

In 2011, Kato and co-workers demonstrated the formation of a phosphine stabilized

disilylene I-43 via dehalogenative reduction of trichlorosilane precursor I-42 with three molar

equivalents of lithium, similar to the previous work reported by Roesky et al.[88] Both the silicon

centres in I-42 and I-43 are in +I formal oxidation state. However, in comparison to I-42, I-43

possess higher multiple bond character due to the intramolecular -donation from phosphine

ligands. Therefore, compound I-43 can also be regarded as a stable disilyne-bis(phosphine)

adduct.

Intrigued by the versatile nature of N,N’-phenylaminidinato chlorosilylene I-20, our

group have established a diverse family of strong -donor chelating bis(silylenes) by using

various linkers (Figure 1.3.3.). The presence of a labile Si–Cl moiety in chlorosilylene I-20

further makes it a suitable candidate to introduce them into well-defined organic frameworks

via simple salt-metathesis reaction. Inspired by the fascinating chemistry of pincer ligands in

homogeneous catalysis,[29] in 2012, our group ingeniously utilized a 4,6-di-tert-butylresorcinol

(I-44) spacer with a central aryl ring, to isolate the first chelating bis(NHSi) I-34 of its kind.[78]

A new pincer ligand [SiCSi]-type bis(silylene) I-34 was synthesized by the deprotonation of I-

44 with nBuLi to afford the corresponding resorcinolate dianion I-45. The reaction of I-45 with

NHSi I-20 in the molar ratio of 1:2 leads to the first [SiCSi] pincer bis(silylene) ligand I-34,

which could be isolated as yellow crystals in 79% yields (Scheme 1.3.3. top). The 29Si{1H}

NMR spectrum of bis(silylene) I-34, featured a singlet signal at a chemical shift of δ = –24.0

ppm, which is similar to that observed for alkoxy substituted phenylamidinatosilylenes LSiOR

(R = tBu, iPr, Me) reported by Roesky et al.[62]

Scheme 1.3.3. Synthetic routes to the bis(silylene) I-34 and I-28.

INTRODUCTION

Scheme 1.3.4. Synthetic routes of bis(silylene) I-35.

Remarkably, the first neutral pincer-type bis(silylene)pyridine ligand I-35 was realized

via a one-pot synthetic method using 2,6-diamine-N,N′-diethylpyridine (I-48) as a precursor.[80]

Herein, dilithiated 2,6-bis(ethylamino)pyridine was synthesized in-situ and subsequently

reacted with two molar equivalents of chlorosilylene I-20 to afford bis(silylene) I-35 (Scheme

1.3.4). Interestingly, single-crystal XRD structure analysis displayed the presence of only one

of the conformers (symmetric) in the crystal lattice with both silylene subunits pointing out of

the pyridine moiety. However, the fluxional behaviour with regards to the symmetric and

unsymmetric conformers of I-35 was confirmed by solution-phase DOSY NMR

measurements.

Scheme 1.3.5. Synthetic routes to bis(silylenes) I-29 and I-30.

By utilizing the similar strategy, later in 2016, our group reported a chelating

bis(silylene) with an ortho-carborane (CB) backbone.[81] The bis(silylene) [Si(CB)Si] (I-29) was

INTRODUCTION

synthesized via a salt-metathesis reaction of in-situ generated dimetalated ortho-carborane

(LiC)2B10H10 with N,N′-di-tert-butyl(phenylamidinato)chlorosilylene I-20 (Scheme 1.3.5. top).

An XRD analysis of I-29 revealed an intermolecular Si⋅⋅⋅Si distance of 3.267 Å with Si(II) atoms

pointed towards each other crafting a pre-organized chelating potential “pocket” for TM

coordination.

Intrigued by the flourishing coordination chemistry of Xantphos ligand, the Driess group

developed the analogous bis(silylene) ligand I-30 based on xanthene backbone (Scheme

1.3.5. bottom).[82] I-30 was isolated via dilithiation of 4,5-dibromo-9,9-dimethylxanthene (I-50)

with two molar equivalents of secBuLi, subsequent salt-metathesis reaction with the

chlorosilylene I-20, yielding I-30. Single-crystal XRD analysis demonstrated an intermolecular

Si⋅⋅⋅Si distance of 4.316 Å, a suitable space for the potential coordination of transition-metals

and activation of small molecules.

Scheme 1.3.6. Synthetic routes to I-31 and its ambivalent reactivity.

In case of acenaphthene scaffold, a hypercoordinated disilene I-31 with the longest

Si=Si bond distance (2.623(1) Å) to date was obtained due to a spatially forced interaction

between two Si(II) units (Scheme 1.3.6).[83] Bis(silylene) I-31 was synthesized using 5,6-

dibromoacenaphthene I-51 as a linker, via salt-metathesis reaction of in-situ generated 5,6-

dilithioacenaphthene with chlorosilylene I-20 as a black powder in 75% yields (Scheme 1.3.6.).

The 29Si{1H} NMR analysis featured a considerably upfield shifted chemical shift of δ = −36.5

ppm from the related amidinatosilylenes, indicating an excess of electron density at the Si(II)

INTRODUCTION

atom. Interestingly, I-31 showed an ambivalent reactivity with both disilene as well as

bis(silylene) like reactivity as it reacted with ethylene to form [2+2] cycloaddition product I-52

and with Ni(cod)2 to furnish the bis(silylene) coordinated Ni(0)-(cod) complex I-53 (Scheme

1.3.6.).

Scheme 1.3.7. Synthetic route to pincer-type bis(silylene) I-55.

In 2020, Li’s group reported the new pincer-type bis(silylene) [SiCSi] ligand I-55 with

bis(pyrrolyl)methane (I-54) as the spacer framework featuring a C(sp3) atom as a potential

anchor.[89] I-55 was synthesized via a salt-metathesis reaction in which bis(pyrrol-2-yl)methane

with two molar equivalents of the chlorosilylene in toluene was reacted with LiHMDS at −55 °C

(Scheme 1.3.7.). Notably, the 29Si{1H} NMR spectrum displayed a chemical shift at δ = −16.4

ppm, which is similar to that observed for the [SiNSi] I-35 ligand.

Scheme 1.3.8. Synthetic routes to bis(silylenes) I-32 and I-59.

Most recently, our group isolated a new type of chelating bis(silylene) I-32 with a

flexible terphenyl based linker by utilizing 1,4-bis(2-bromophenyl)benzene (I-56) as a

precursor (Scheme 1.3.8).[84] The 29Si{1H} NMR spectrum featured a singlet at δ = 16.8 ppm

INTRODUCTION

comparable to other carbon-functionalized bis(silylenes) with carborane (δ = 18.9 ppm) and

xanthene (δ = 17.3 ppm) backbones (Figure 1.3.3. top). In spite of the considerably long

intermolecular Si∙∙∙Si distance of 7.505 Å as evident from its solid-state structure, bis(silylene)

I-32 exhibits a cooperative behaviour upon coordination to Ni(0) metal complex.[84] In the same

year, Kretschmer and co-workers also synthesized the bis(chlorosilylene) ligand I-59 from the

precursor I-57, instead the silylene units were bridged through the backbone of the amidinate

ligand (Scheme 1.3.8. bottom).[90] I-59 was isolated by the double deprotonation of I-57,

followed by the reaction with trichlorosilane afforded the bis(dichlorosilane) I-58, which on

dehydrochlorination with LiHMDS yielded bis(chlorosilylene) I-59 (Scheme 1.3.8).

Interestingly, the experimental results based on the reactivity of I-59 with benzil pointed out

individual activation tendency rather than a cooperative activation.

Recently, the new chelating bis(silylene) I-33 employing aniline as linker was

developed by our group. [85] N,N-bis(silylenyl)aniline I-33 was obtained by the salt-metathesis

reaction of dilithiated aniline I-61 with chlorosilylene I-20 in a molar ratio of 1 : 2 (Scheme

1.3.9). Due to the relatively shorter intramolecular Si(II)⋅⋅⋅Si(II) distance of 2.895(1) Å, in

comparison to [Si(CB)Si] I-29 (3.2666(6) Å),[81] and [Si(Xant)Si] I-30 (4.3155(9) Å),[82] a unique

metal-free, cooperative bis(silylene)-assisted P4 activation was realized.

Scheme 1.3.9. Synthetic routes to bis(silylenes) I-33.

1.4. Application of bis(silylenes) as chelating ligands

Transition-metal catalyzed activation of small molecule such as H2, CO, CO2, NH3, P4

has widely been studied before.[91] Divalent silicon compounds with highly reactive Si(II)

centres could potentially exhibit distinctive reactivity towards inert small organic molecules.[92]

Figure 1.4. summarizes the diverse applications of bis(NHSi) ligands for the stabilization of

highly reactive low-valent transition-metal and main-group elements in homogeneous TM

mediated catalysis. They have also been employed for cooperative small molecule

activation.[8,12,13,71,93]

INTRODUCTION

Figure 1.4. Applications of bis(silylene) ligands.

In the following sections, the role of bis(silylene) as chelating ligands in homogeneous

catalysis is discussed. Thereafter, Section 1.4.2. provides a brief account of the compounds

containing bis(silylene) supported low-valent main group elements.

1.4.1. Bis(silylenes) as chelating ligands in homogeneous catalysis

The introduction of novel -donor ligands in the emerging area of homogeneous

catalysis is crucial for the development of efficient catalytic processes. The excellent -donor

and -acceptor properties of NHSi ligands have fuelled a huge interest for their role in TM-

mediated homogeneous catalysis. Contrary to its NHC counterpart, the area of transition-

metal NHSi complex is still an emerging field, with many catalytic reaction pathways still left

to be discovered.[8,13] The potential of silylene ligands to provide exceptional chemo-, regio-,

and stereoselectivity in TM-mediated catalytic processes is large.[13,94,95] Latest studies have

also suggested that catalytic efficiencies of numerous organic reactions can be improved by

silylene ligands because of their stronger -donating nature along with cooperative effects of

the Si(II) atoms in the catalytic cycle.[82,84] Recent theoretical studies have also indicated that

silylenes can match or even surpass classic carbene and phosphines owing to their

advantageous combination of σ-donor and -acceptor strength, ligand-to-metal charge

transfer, and steric properties.[14,96,97]

INTRODUCTION

Figure 1.4.1. Catalytic applications of bis(silylene) TM complexes.

An outline of the role of silylene as powerful ligands in boosting catalytic organic

transformations is shown in Figure 1.4.1. In this section, the application of silylenes as steering

ligands in selected transition-metal mediated homogeneous catalysis will be discussed.

1.4.1.1. Cross-coupling reactions

The idea of employing silylene (NHSi) coordinated transition-metal compounds as

catalyst was first introduced to the Suzuki cross-coupling reaction in 2001 by Fürstner et al.[98]

The dinuclear Pd(0) complex I-62, where NHSi ligand acts as bridging ligand, showed good

catalytic performance for the cross-coupling of aryl boronic acids with bromoarenes (Scheme

1.4.1.). After seven years, the second report by Roesky and co-workers presented that a

mononuclear NHSi-Pd(0) complex based on I-3 ligand can catalyze the Heck coupling of

bromoacetophenone with styrene, with complete conversion after 24 h at 140 °C.[99]

Scheme 1.4.1. First example of silylene ligands in the Suzuki cross-coupling reaction as

demonstrated by Fürstner et al.[98]

INTRODUCTION

Later in 2013, Inoue and Enthaler described the first catalytic reaction by implicating

the oxygen-bridged bis(silylene) I-39 as a ligand in the nickel-catalyzed C-C cross-coupling of

aryl halides with organometallic zinc- (Negishi coupling) and Grignard (Kumada coupling)

reagents (Scheme 1.4.2.).[100] The precatalyst I-63, was synthesized by reacting I-39 with a

Ni(0) precursor Ni(cod)2.[77] In addition to the chelating bis(silylene) I-39, a η4-coordination of

COD at the reactive nickel center was observed, resulting in the stable 18 VE Ni(0) complex

I-63.

Scheme 1.4.2. First example of bridged bis(silylene) ligands in the Negishi (top) and Kumada

(bottom) cross-coupling reaction by Inoue et al.[100]

In 2013, our group reported the catalytic Sonogashira cross-coupling reaction of

phenylacetylene with (E)-1-iodo-1-octene by applying the Ni(II) pincer complex I-64 as

precatalyst (Scheme 1.4.3.).[101] The pincer-type bis(silylene) nickel(II) complex I-64 was

prepared via the reaction of silylene ligand I-34 with NiBr2(dme) in the presence of excess

base triethylamine. Furthermore, the mechanistic investigations suggested a cooperative

mechanism involving the in-situ formation of a silylene coordinated heterobimetallic nickel-

copper complex I-65 as a key intermediate in the catalytic cycle. Notably, an single-crystal

XRD analysis of intermediate I-65 revealed the presence of a three-center two-electron Si(II)-

Ni(II)-Cu(I) bonding unit in the molecular structure.

INTRODUCTION

Scheme 1.4.3. Sonogashira cross-coupling reaction catalyzed by pincer-type [SiNSi]Ni(II)

complex I-64.[101]

Scheme 1.4.4. Buchwald-Hartwig amination reaction of catalyzed by various Ni(II) complexes

I-66.[81]

Our group also reported the synthesis of various Ni(II) complexes stabilized by

chelating bis(silylene) and bis(phosphine) ligands with ortho-carborane as linker.[81] They were

found to catalyze the Buchwald-Hartwig amination of aryl halides with secondary amines in

moderate to good yields (Scheme 1.4.4.). The catalyst screening of amination reaction

suggested Ni(II) complex I-66a (93% yield) to be an efficient catalyst compared to I-66b (66%

yield) and I-66c (63% yield). This outcome further supported the fact that stronger sigma donor

INTRODUCTION

strength of bis(silylene) ligands compared to related phosphine ligands is crucial for catalytic

systems.

1.4.1.2. C-H borylation reaction

Scheme 1.4.5. C-H borylation of benzene catalyzed by pincer type [SiCSi] based Ni(II)

complex I-67.[78] and C-H borylation of substituted pyridines and furans catalyzed by pincer

type bis(silylene) Co(II) complex I-68.[102]

In the context of establishing a reactivity pattern based on the strikingly stronger σ-

donating properties of silylenes in selective catalysis compared to germylenes and

phosphines, our group developed a series of [Ir(H)(Cl)(coe)] complexes (I-67a to I-67d)

coordinated to different donor ligands (Scheme 1.4.5).[78] As per the Dewar-Chatt-Duncanson

model, a highly electron-rich metal centre promotes stronger back-bonding to the olefin, which

further leads to a weakening of the C=C bond. This synergistic effect also reflects well in the

upfield NMR chemical shifts of the olefinic groups and therefore can serve as a crucial

parameter for examining the electronic nature of the metals. Comparison of the NMR chemical

shift of olefinic protons and 13C nuclei and C=C bond length in complexes I-67a, I-67d,

INTRODUCTION

revealed the following order of increasing donor strength of ligands: [tBuPCPtBu] < [iPrNPCPNiPr]

< [GeCGe] < [SiCSi]. The application of these complexes towards C–H borylation of arene

showed that silylene coordinated Ir(III) complex I-67a (90%) was the most effective catalyst in

comparison to complexes I-67b (80%), I-67c (64%) and I-67d (21%) respectively.

Later, our group demonstrated that tridentate bis(silylene) [SiNSi] coordinated Co(II)

complex I-68 with NaBHEt3 as a substrate could also catalyze the C–H borylation of numerous

substituted arenes, pyridines and furans with B2pin2 at 100 °C in THF solvent (Scheme

1.4.5.).[102]

1.4.1.3. Reduction reactions of unsaturated bonds

In 2014, our group isolated a novel pincer-type Fe(0) complex I-69 stabilized by

tridentate bis(silylene) I-35 in combination with PMe3 as co-ligands.[80] The complex I-69 was

obtained via two ways: (i) by the reaction of bis(silylene) I-35 with Fe(0) precursor Fe(PMe3)4.

(ii) by the dehalogenative reduction of corresponding Fe(II) complex using KC8 in the presence

of excess PMe3. The Fe(0) complex I-69 demonstrated good catalytic performance in the

hydrosilylation reaction of carbonyl compounds (Scheme 1.4.6.a). Interestingly, stoichiometric

experiments and theoretical analysis suggested a unique unprecedented peripheral

mechanism for the hydrosilylation process.[103] Several cross-over, deuterium labelling and

competitive experiments indicated that the iron centre is not directly involved in the

hydrosilylation. However, the whole catalytic process takes place at the silyl ligand bound at

the periphery of the metal centre.

Scheme 1.4.6. (a). Hydrosilylation of carbonyls catalyzed by pincer-type Fe(0) complex I-

69.[80] (b). Hydrogenation of carbonyls catalyzed by Fe(0) complex I-70.[104]

INTRODUCTION

The reduction of carbonyl compounds was achieved by utilizing η6–benzene

coordinated bis(silylene) Fe(0) complex I-70 as a precatalyst at 50 bar H2 (Scheme

1.4.6.b).[104]

Scheme 1.4.7. (a). Hydrogenation of olefins catalyzed by bis(silylene) Ni(0) complex I-71.[82]

(b). Hydrogenation of olefins catalyzed by bis(silylene) Ni(0) complex I-72.[84]

In 2017, our group established the first example of bis(silylene)-stabilized Ni(0)

complex I-71 with η2-coordination from COD, catalyzed hydrogenation of olefins with very

good functional group tolerance (Scheme 1.4.7.a).[82] Notably, complex I-71, promotes the

chemoselective hydrogenation of several electronically modified alkenes under 1 bar H2

atmosphere at room temperature. The DFT calculations suggested a unique cooperative

activation mode for H2 cleavage which involves low-valent Si(II) centres of the chelating

bis(silylene) ligand. Very recently, a new type of 16 VE Ni(0) complex I-72 based on terphenyl

bis(silylene) was obtained, where an intramolecular -donor stabilization from the central

arene ring of the terphenyl spacer was observed.[84] The complex I-72 also shows exceptional

catalytic activity toward the chemoselective hydrogenation of olefins with high TONs under

1 bar H2 pressure at room temperature (Scheme 1.4.7.b). It is worth mentioning that, the

bis(silylene) Ni(0) complex even outperforms its analogous bis(phosphine) Ni(0) complex in

the hydrogenation of norbornene under identical conditions. Moreover, the experimental

evidences suggested the participation of divalent Si(II) centres in the Ni(0) mediated activation

of dihydrogen in the catalytic system.

INTRODUCTION

1.4.2. Silylene stabilized low-valent main-group elements

As described in the previous section, silylenes act as exceptional donor ligands to

stabilize transition metals in low oxidation states, resulting in highly reactive electron-rich

transition-metal complexes, which are important in boosting homogeneous catalysis. Recent

example of silylenes have even shown to stabilize low-valent main group elements such as

B(I), C(0), C(II), Al(lI), Si(0), Si(II), P(I), Ga(I), Ge(0) in monoatomic states.

In 2003, Kira’s group successfully isolated the first heavier analogue of an allene such

as trisilaallene I-73 (E=Si) with a formally sp-hybridized silicon atom in a bent geometry

(Figure 1.4.2.).[105]. Generally, compounds with a main group element in the zero-oxidation

state are termed as ylidones and possess two electron pairs that are stabilized by a donor-

acceptor interaction with the σ-donor silylene ligands.[106] In last few years, our group realized

the isolation of a series of zero-valent group 14 elements such as Si(0) silylone [107] Ge(0)

germylone,[108] Sn(0) stannylone[109] and Pb(0) plumbylone[110] stabilized by a xanthene-

bridged bis(silylene) ligand I-33 (Figure 1.4.2.). Furthermore, the reactive silylone I-77 was

also able to heterolytically cleave H2 in the presence of BPh3 as Lewis acid.[107] In case of

carborane bridged bis(silylene), the stabilization of a neutral borylene compound I-75, [111]

silylone I-81[112] and germylone I-82[113] was reported by Xie’s and our group respectively.

However, when tridentate bis(silylene) I-35 was employed for synthesis of germylone, only a

lewis-acid such iron tetracarbonyl stabilized Ge(0) compound I-80 could be isolated.[114a]

Figure 1.4.2. Some reported examples of low-valent main-group elements stabilized by

silylene ligands.[105-114]

INTRODUCTION

Scheme 1.4.8. Stabilization of low-valent P2 unit and its functionalization using bis(silylene)

I-30.[93a]

A very recent example was reported by our group on a unique metal-free phosphorus

functionalization by only employing bis(silylene) I-30 that has fuelled up an enormous interest

towards examining the cooperative activation using low-valent group 14 compounds.[93a,

114b, 114c] I-30 shows a remarkable reactivity in direct functionalization of white P4 and towards

the stabilization of the novel zero-valent di-phosphorous compound I-83 (Scheme 1.4.8.).

Notably, the bis(silylene)-stabilized P2 complex I-83 also served as single phosphorus anion

transfer reagent, as on reaction with Cr(CO)6 and W(CO)6, it formed I-84 and I-85 where TMs

are coordinated to the phosphaketenide (PCO-) ligand via the transfer of P– anion and

formation of the bis(silylene)-stabilized phosphorus counter cation.

MOTIVATION AND OBJECTIVE

2. MOTIVATION AND OBJECTIVE

Given the unique steric and electronic properties of bis(NHSis), they have been

explored for isolation of a range of different main-group and TM elements in unusual oxidation

states and TM-mediated catalytic applications. Therefore, this dissertation is devoted to the

design and synthesis of reactive 3d TM complexes by utilizing the potentially tridentate

bis(silylene)pyridine and to investigate their catalytic activity towards organic transformation,

with a specific focus on the largely uninvestigated Mn(0) complexes as well as new types of

Ni(II) complexes to build on previous work in the group.

In contrast to the numerous reports on NHSi based zero-valent TM (late 3d row such

as Fe and Ni) compounds, NHSi-supported manganese(0) chemistry is hitherto an unexplored

area of research. Therefore, the first objective is to synthesize a bis(silylene)-ligated

manganese(0) compounds in addition to their structural and spectroscopic characterization.

The choice of suitable supporting ligands is expected to have tremendous influence on the

isolation of reactive manganese(0) sites (Section 3.1.). Another follow-up objective is to also

study the reactivity of such bis(silylene) manganese(0) compounds toward strong -acceptor

ligands, using CO and xylyl isocyanide (XylylNC) and investigating their catalytic abilities

(Section 3.2.).

Scheme 2.1. The coordination behaviour of bis(silylene)pyridine ligand towards manganese

compounds in different oxidation states and their catalytic application.

The thermodynamic stabilization introduced by the bis(silylene) ligands in the

stabilization of several TM compounds and reactive intermediates is of paramount

significance. The alkyl manganese complexes containing Mn–C -bonds are considered as

reactive intermediates in numerous catalytic transformations. I therefore, decided to examine

MOTIVATION AND OBJECTIVE

the difference in the reactivities of bis(silylene) and bis(phosphine)-ligated compounds in

accessing the reactive alkyl manganese(II) complexes (Section 3.3.). The synthesis of

structurally analogous bis(phosphine)pyridine is envisioned as well.

Scheme 2.2. The application of pincer bis(silylene) as coordinating ligand in accessing alkyl

Mn(II) and Ni complexes and its comparison with the analogous bis(phosphine) ligand.

Although, the ability of anionic bis(silylene), [SiCSi] Ni(II) complex to catalyze the

Sonogashira cross-coupling reaction was highly encouraging and revealed a fascinating Cu/Ni

intermediate, however, it led only to moderate catalytic performance and only one example

was investigated.[101] Therefore, my next objective is to synthesize and test the neutral

bis(silylene)pyridine coordinated Ni(II) and Ni(0) complexes, in the Sonogashira cross-

coupling reaction, to see if there would be an improved performance. The catalytic activity is

also compared with structurally similar analogous chelating bis(phosphine)pyridine nickel

complexes. Furthermore, I aim to investigate the role of the widely used cocatalyst CuI for the

better understanding of the mechanism of the Sonogashira cross-coupling reaction through

stoichiometric experiments.

RESULTS AND DISCUSSION

3. RESULTS AND DISCUSSION

3.1. Synthesis and characterization of bis(silylene) Mn(0) complex

Manganese with an electronic configuration [Ar] 3d54s2; is a TM that belongs to the

group 7 of the periodic table.[115] Most of the known stable compounds of manganese are found

in oxidation states +II, +IV and +VII. The only known Mn(0) text book example, binuclear

decacarbonyl dimanganese [Mn2(CO)10], is regarded as one of the simplest manganese(0)

carbonyl complex, that has been widely used in chemical transformations.[116] Owing to

synergistic bonding, which is the combined effect of -donation to a metal center and -back-

donation from the metal to the * orbitals of the CO ligands are excellent at facilitating the

isolation of stable zero-valent TM compounds (Figure 3.1.1.).[116] Due to the extreme reactivity

and instability of monomeric open shell manganese pentacarbonyl monoradical, it has

remained transient with a strong propensity to dimerize at room temperature.[117] Thus, it only

existed as a dinuclear manganese(0) compound Mn2(CO)10, with the aid of kinetic stabilization

through a covalent Mn–Mn bond. Over the past few decades, despite the high reactivity of

mononuclear Mn(0), considerable progress has been made. In particular, the generation of

the [Mn(CO)5] radical via photolytic cleavage of the Mn–Mn bond in the Mn2(CO)10 both

experimentally and theoretically have gained immense attention by the scientific

community.[118] The research on isolable analogues of [Mn(CO)5] metalloradical has made

rapid progress due to its structural significance and its key role as an intermediate in the

radical-type olefin hydrogenation.[119]

Figure 3.1.1. a) Chemical structure of decacarbonyl manganese(0) complex; b) Molecular

orbital picture diagram depicting the synergistic bonding interactions in low-valent TM carbonyl

compounds.[116]

In the early 1970’s, Turner and co-workers established the low-temperature matrix-

isolation strategy to isolate the five-coordinated 17 VE Mn(CO)5 monoradical in the gaseous

phase.[120] The UV photolysis of HMn(CO)5 in a CO matrix generated the Mn(CO)5 radical,

which was confirmed by the IR and UV/vis spectroscopy. Surprisingly, the extreme sensitivity

RESULTS AND DISCUSSION

of Mn(CO)5 was observed when formation of a weak chemical bond to the surrounding Kr

matrix was observed for [Mn(CO)5], generated from -irradiation of HMn(CO)5 by EPR

spectroscopy.[121] Later, Brown and co-workers were also able to characterize the

Mn(CO)3(PR3)2 complexes generated via photo-substitution of dinuclear manganese

complexes [Mn2(CO)8L2] (L = CO, phosphine), in solution phase through EPR spectroscopy.

[122] However, their existence was limited to only in solution phase at low-temperatures. The

high reactivity of Mn(0) species had prevented their isolation at room temperature until the

beginning of 21st century.

Figure 3.1.2. Some previously reported examples of Mn(0) complexes.[123-126]

Almost two decades later, Roesky and coworker isolated the novel two-coordinated

manganese compound A by taking advantage of the strong -acceptor and radical-anion

character of cyclic alkyl(amino) carbenes (cAAC).[123] However, the spectroscopic

investigation revealed the significant electron density distribution over the cAAC ligands.

Recently, Jones and co-workers ingeniously utilized the bulky amido and Nacnac-

magnesium(I) substituents (Nacnac = N-acetyl-N-acetonates) to kinetically stabilize Mn(0)

complex B (Figure 3.1.2.).[124] Alternatively, compound B could also be referred as Mn(I)-Mg(I)

compound, or as Mn(0)-Mg(II) compound based on their electronegativities. One year later,

Figueroa and co-workers achieved the isolation of a five-coordinate Mn(0) complex through

the comproportionation reaction of corresponding Mn(–I) and Mn(+I) complexes.[125] It is worth

to mention that the presence of sterically crowded isocyanide ligands have significantly

contributed to the kinetic stabilization of Mn(0) compound C (Figure 3.1.2.).[126] Recently, N-

heterocyclic carbenes (NHCs) in addition to dvtms (1,3-divinyltetramethyldisiloxane) ligands

were employed to achieve the isolation of mononuclear Mn(0) complexes by Deng and co-

workers (Figure 3.1.2.).[126] However, the obtained Mn(0) complex D were extremely

RESULTS AND DISCUSSION

temperature sensitive. Nevertheless, the isolation of a genuine, carbonyl free, five-coordinate

17 VE manganese(0) compound is still a long-standing curiosity.

As discussed in chapter 1.3., silylenes offer exceptional steric and electronic protection

to realize the isolation of highly reactive species through kinetic stabilization. This includes

their ability to stabilize low-valent reactive main-group elements; such as zero-valent Group

14 elements also referred as metallylones as discussed in chapter 1.4. As mentioned in

section 1.2.2, the presence of lone-pair of electrons at the Si(II) center enables silylenes to act

as stronger -donor coordinating ligand towards TMs. The bonding behaviour of the silylenes

based ligands resembles other -acceptor ligands due to the presence of formally vacant 3p

orbitals at the Si(II) (base-free silylene; Figure 3.1.3.a) and the antibonding * orbital of the

silicon-base bond (base-stabilized silylene; Figure 3.1.3.b).[116] Additionally, the bidentate

counterpart such as chelating bis(silylenes) have also been shown to have favourable

influence on the stabilization of first-row transition metal centers in less common low oxidation

states such as iron(0), cobalt(I), nickel(0) complexes. Regardless of the plentiful reports of

NHSi-transition-metal(0) complexes, the analogous NHSi-Mn(0) complexes are currently

unknown. The only instance was where a silylene I-20 was reacted with the manganese(0)

precursor Mn2(CO)10 leading to a disproportionation reaction and thereby resulted in a

silylene-Mn(I) complex as reported by Roesky and co-workers (see Scheme 1.2.4.). [127]

Therefore, I became interested in utilizing the strongly donating chelating bis(silylene)

ligands to achieve the isolation of a 17 VE novel bis(silylene) manganese(0) complexes. With

this aim in mind, I planned to apply the dehalogenative reduction of corresponding Mn(II)

precursor to synthesize the desired open-shell Mn(0) complexes.

Figure 3.1.3. a) Molecular orbital interactions in base-free silylene TM complexes; b)

Molecular orbital interactions in base-stabilized silylene TM complexes [116]

3.1.1. Synthesis and characterization of bis(silylene) manganese(II) complexes

The reaction of potentially tridentate bis(silylene) I-35 with MnX2 (X= Cl, Br) in an

equimolar ratio in THF, resulted in pale yellow solutions at room temperature (Scheme

RESULTS AND DISCUSSION

3.1.1.).[128] Due to polar nature of [SiNSi]MnX2 complexes (1 and 2), they are partially soluble

in diethyl ether and toluene. Purification by washing with diethyl ether subsequent filtration

and drying under vacuum afforded 1 as off-white powder in 92% yields and 2 as light-yellow

powder in 90% yields, respectively. The 1H NMR spectra of both 1 and 2 featured broad and

shifted signals indicative of their paramagnetic nature.

Scheme 3.1.1. Synthesis of the bis(silylene)pyridine Mn(II) complexes 1 and 2.

Figure 3.1.4. Molecular structure of compound 2. Thermal ellipsoids are drawn at 50%

probability. The hydrogen atoms and solvent molecules are omitted for clarity; selected bond

lengths (Å) and angles (°): Mn1−Si1 2.560(7), Mn1−Si2 2.567(6), Si1−Mn1−Si2 114.28(2),

Br1−Mn1−Br2 108.837(15).

The obtained characterization data for 1 was in accordance with the literature report.[94]

Electrospray ionization mass spectrometry (ESI-MS) confirmed the presence of dibromido

Mn(II) coordinated by I-35, that is, complex 2 (M•+ calcd. 896.2092, experimental = 896.2083).

Single-crystals of 2, suitable for the XRD structure analysis were obtained as colorless

rectangular crystals by cooling concentrated THF solutions at –20 °C overnight. Complex 2

crystallizes in a orthorhombic crystal system with the Pbca space group. The solid-state

RESULTS AND DISCUSSION

structure shows the presence of a four-coordinate manganese (II) center in a distorted

tetrahedral geometry, with a Si1−Mn1−Si2 bond angle of 114.28(2)° (Figure 3.1.4.). Therefore,

1 and 2 could be better ascribed as 2-[SiNSi]MnX2 complexes (X = Cl, Br). The reluctance

of the pyridine ring in [SiNSi] ligand towards coordination to manganese could be attributed to

the enhanced -donor strength of the [SiNSi] ligand (I-35). Because of this property, an

increase in the electron density on Mn(II) center occurs, which as a result, reinforces the

manganese centre to rather adopt a tetrahedral coordination over five-fold coordination.

However, this outcome is in contrast to the reported Mn(II) complexes based on [PNP] and

[NNN] type of pincer ligands, in which a pentacoordinate manganese center is present (Figure

3.1.5.).[129-132] The molecular structure of 2 displays Mn−Si distances of 2.560(7) and 2.567(6)

Å which are consistent with those observed in the previously reported bis(silylene) Mn(II)

complex 1.[94]

Figure 3.1.5. Some examples of Mn(II) complexes bearing pincer ligands as well as a carbene

ligands for comparison.

Figure 3.1.6. Temperature-dependent SQUID magnetization data (2–300 K at 1 T) for two

independently synthesized samples of 1(left) and 2(right), sample 1 (green squares) and

sample 2 (blue squares), plotted as a function of the effective magnetic moment (µeff) vs.

temperature (T).

RESULTS AND DISCUSSION

Figure 3.1.7. a) CW X-band EPR spectra of 1 recorded as a 1mM solution in THF at 95 K

(black trace), in benzene at 95 K (blue trace), and in benzene at 293 K (green trace).

Experimental conditions: microwave frequency  = 8.959 GHz, modulation amplitude = 1.0

mT (benzene); 0.5 mT (THF), microwave power = 1.0 mW, modulation frequency = 100 kHz,

time constant = 0.1 s. b) CW X-band EPR spectrum of 2 recorded as a 1 mM solution in THF

(black trace) at 95 K. Experimental conditions: microwave frequency  = 8.959 GHz,

modulation amplitude = 1.0 mT, microwave power = 1.0 mW, modulation frequency = 100

kHz, time constant = 0.1 s.

To further gain insights into the electronic environment at the manganese center in 1

and 2, electron paramagnetic resonance (EPR) spectroscopy and SQUID magnetometry

(SQUID = superconducting quantum interference device) measurements were carried out. As

anticipated for a high-spin Mn(II) center, d5 electronic configuration with S = 5/2, in tetrahedral

geometry (e2, t23), an effective magnetic moment (µeff) of 6.10 B.M. and 5.99 B.M. was

determined for 1 and 2, respectively, through variable temperature (VT) SQUID magnetometry

(Figure 3.1.6.). Herein, all the solid-state SQUID measurements were conducted twice on two

independently synthesized samples of 1 and 2 to minimize the errors related to reproducibility.

Furthermore, the measured solution-phase µeff for 1 and 2 by Evans method compare well

with that determined in solid-state via VT-SQUID magnetometry.[133,134] Notably, the

experimental effective magnetic moment values agree well with calculated spin-only value for

a high-spin d5 system with S = 5/2 (µeffs.o. = 5.93 B.M.). The VT-SQUID measurements of 2

demonstrates a temperature independent behaviour of effective magnetic moment in the 150-

300 K range. However, µeff decreases to 5.0 B.M. below 150 K, which could be possibly due

to the zero-field splitting (ZFS) in a high-spin manganese(II) complex (Figure 3.1.6.). For

complex 2 a highly rhombic S = 5/2 spectrum with resonances between 50 and 600 mT,

corresponding to geff = 10 – 1 were detected by EPR spectroscopy. Additional spectral features

RESULTS AND DISCUSSION

such as a six-line-pattern, in the low-field region, may derive from the partially resolved

hyperfine coupling to one 55Mn (I = 5/2, 100% natural abundance) nucleus (Figure 3.1.7.b). In

case of compound 1, the magneto-chemical properties are similar to 2, as shown in Figures

3.1.6. and 3.1.7..

Scheme 3.1.2. Synthesis of bis(silylene) I-30 stabilized Mn(II) complex 3.

Figure 3.1.8. Molecular structure of 3 with thermal ellipsoids at 50% probability. Hydrogen

atoms are omitted for clarity; selected bond lengths (Å) and angles (°): Mn1−Si1 2.5965(8),

Mn1−Si2 2.6029(8), Mn1−Cl1 2.3391(8), Mn1−Cl2 2.3484(8), Cl1−Mn1−Cl2 108.837(15),

Si1−Mn1−Si2 108.35(3).

In order to further expand the scope of bis(silylene) Mn(II) precursors, the bis(silylene)

I-30 was employed as a coordinating ligand. The treatment of bis(silylene) I-35 with MnCl2 in

THF, resulted in pale yellow solutions at room temperature (Scheme 3.1.2.). Extraction with

Et2O and subsequent crystallization from concentrated Et2O solutions afforded complex 3 as

colorless crystals in 84% yield. A single-crystal XRD structural analysis revealed the formation

of a four-coordinate Mn(II) complex with a Si1−Mn1−Si2 bond angle of 108.35(3) (Figure

RESULTS AND DISCUSSION

3.1.8). The distorted tetrahedral coordination geometry is similar to 1 and bis(carbene)-MnCl2

complex F (Figure 3.1.4.). [129] The crystals adopt a monoclinic crystal system with the P21/c

space group. The Mn1−Cl1 and Mn1−Cl2 bond distances of 2.3391(8) and 2.3484(8) Å are

comparable to those of 2. The Mn1−Si1 and Mn1−Si2 bond distances 2.5965(8) and 2.6029(8)

Å, respectively, are similar to those observed in compound 1 and 2.

Scheme 3.1.3. Synthesis of the bis(silylene) I-29 Mn(II) complex 4.

Figure 3.1.9. Molecular structure of 4 with thermal ellipsoids at 50% probability. Hydrogen and

solvent atoms are omitted for clarity; selected bond lengths (Å) and angles (°): Mn1−Si1

2.6132(7), Mn1−Si2 2.7676(7), Mn1−Cl1 2.3167(7), Mn1−Cl2 2.4087(6), Mn1−Cl2’ 2.7115(6),

Cl1−Mn1−Cl2 121.70(3), Si1-Mn1-Si2 79.585(19), Cl1-Mn1-Si1 114.30(2), Cl2−Mn1−Cl2’

84.86(2).

RESULTS AND DISCUSSION

Treatment of two molar equivalents of carborane-based bis(silylene) ligands with two

molar equivalents of MnCl2 afforded the Mn(II) compound 4 as dark-yellow crystalline solid in

89% yields. Due to the highly polar nature of 4, it precipitated out as yellow crystals from the

reaction mixture at room temperature. The single-crystals suitable for an XRD structural

analysis were obtained from the saturated solutions in THF at room temperature. Compound

4 crystallizes in a monoclinic crystal system in the P-1 space group. X-ray crystallographic

analysis of the latter revealed the formation of a five-coordinate symmetric dinuclear Mn(II)

moiety with two chloride ligands acting as bridging ligands (Figure 3.1.9.).To the best our

knowledge, complex 4 is first dinuclear Mn complex with a Mn2Si4 core motif. Notably, 4

displays different sets of Mn−Cl bond distances with the shorter Si1−Mn1−Si2 bond angles of

79.585(19) compared to compounds 1, 2 and 3. The Mn1−Cl1 bond distance 2.3167(7) Å is

the shortest among another Mn1−Cl bond in 4, which essentially reflects the role of Cl1 as a

terminal ligand. As a result of Cl2 taking part in forming the bridge bond with the other Mn1’

center, Mn1−Cl2 (2.4087(6) Å) bond becomes slightly weaker and hence bond elongation is

observed. The Mn1−Si1 bond distance 2.6132(7) Å compares well with the previously reported

bis(silylene) Mn(II) compounds 1 and 2. However, Mn1−Si2 bond distance 2.7676(7) Å is

significantly longer than those Mn(II)−Si(II) bond distances in compounds 1, 2 and 3.[94]

The solution magnetic moment (Evans method, THF-d8, 298 K) of 4 was determined

to be 11.82 B.M. with 5.91 B.M. per manganese atom. [133,134] The observed effective magnetic

moment is consistent with the calculated spin only magnetic moment for a d5 high-spin

environment. These observations further indicated that the dinuclear nature of Mn(II) complex

4 remains intact even in the solution phase.

3.1.2. Strategies utilized for the isolation of bis(silylene) Mn(0) complex

With the aim to isolate bis(silylene) manganese(0) complexes, firstly the reduction of

the isolated Mn(II) precursor complexes 1, 2, 3 and 4 was envisioned. We commenced our

investigation by subjecting bis(silylene)pyridine I-35 stabilized Mn(II) complexes for reduction

reactions. The treatment of complexes 1 and 2 with 2.4 molar equivalents of KC8 under N2 in

the absence of any supporting ligands resulted in oily red residues which could not be

characterized. These observations prompted us to utilize additional donor ligands during

reductive dehalogenation of Mn(II) precursors , which could stabilize reactive Mn(0) complex.

The reduction of 1 and 2 was carried out in the presence of bis(olefin) 1,3-

divinyltetramethyldisiloxane (dvtms) using KC8 at −40 °C, which resulted in the rapid

development of deep purple solutions (Scheme 3.1.4.). Extraction with diethyl ether and

subsequent crystallization afforded purple crystals of complex 5 in 40% (1→5) and 47% (2→5)

RESULTS AND DISCUSSION

yields. X-ray quality crystals of complex 5 were obtained from concentrated n-hexane solutions

after cooling for three days at −20 °C.

Scheme 3.1.4. Attempts to synthesize bis(silylene)pyridine Mn(0) complexes from 1 and 2

using dvtms as supporting ligand leading to 5.

Figure 3.1.10. Molecular structure of 5 with thermal ellipsoids drawn at 50% probability.

Hydrogen atoms and solvent molecules are omitted for clarity; selected bond lengths (Å) and

angles (°): Mn1−Si2 2.505(2), Mn1−N2 2.220(8), Mn1−C45 2.143(8), Mn1−C41 2.129(9),

N2−Mn1−Si2 76.0(2), N2−Mn1−C41 121.4(3), C41-Mn1-C45 98.8(3), Si2-Mn1-C45 122.0(2).

A single-crystal XRD structural analysis of 5 revealed the formation of a four-coordinate

Mn(II) complex occupying a distorted tetrahedral geometry (Figure 3.1.10.). Complex 5

crystallizes in a triclinic crystal system with the P-1 space group. Surprisingly, the molecular

structure of 5 consists a silylene-stabilized dialkyl manganese(II) complex with a N-pyridine

coordination from the pincer ligand backbone. The formation of 5 could be explained by the

in-situ generation of reactive Mn(0) species, which undergoes the cycloaddition of bis(olefin)

dvtms ligand around the Si(II)−Mn(0) bond and results in the oxidation of Si(II)→Si(IV) moiety

RESULTS AND DISCUSSION

along with Mn(0)→Mn(II) complex redox change. The Mn1−Si2 distance of 2.505(2) Å is

slightly shorter than those of the related bis(silylene) coordinated Mn(II) compounds 1 and 2.

The FT-IR spectrum of 5 features an intense vibrational band at 1232 cm−1, suggesting the

presence of Si−O bonding motifs in the molecular structure.[135] Furthermore, ESI-MS and

elemental analysis confirmed the composition of 5.

I then decided to utilize a sufficiently chemically innocent ligand such as the chelating

1,2-bis(dimethylphosphino)ethane (dmpe), to isolate the extremely reactive Mn(0) complex.

Treatment of the dihalido Mn(II) precursors 1 and 2 with KC8 in THF, followed by the addition

of dmpe resulted in deep-red solutions. Filtrations, extraction in n-hexane and subsequent

crystallization from concentrated n-hexane solutions afforded dark-red crystals of 6. The

complex 6 crystallizes in a monoclinic crystal system with P21/c space group. An single-crystal

XRD analysis of the 6 showed the presence of a six-coordinate Mn(II) complex exhibiting a

distorted octahedral geometry with a Si1−Mn1−Si2 angle of 147.50(3)° (Figure 3.1.11.). In

contrast to the expected Mn(0) complex, the molecular structure indicated the formation of a

silylene-stabilized manganese(II) (silyl)hydride complex with a N-pyridine coordination from

the pincer backbone, which is described as 3-[SiIINSiIV]Mn(H)(dmpe) (6). The existence of a

Mn−H bond in 6 was further supported by a weak stretching vibrational band located at ν =

1745 cm–1 in the FT-IR spectrum.[136]

Scheme 3.1.5. Attempts to synthesize bis(silylene)pyridine Mn(0) complexes from 1 and 2

using dmpe as supporting ligand, resulting in the formation of 6.

The mechanism for the formation of 6 is unknown. However, I propose that it possibly

takes place through the in-situ generation of putative Mn(0) species. The extreme reactivity of

the latter promotes the silylene-mediated facile intramolecular phenyl C−H activation of the

amidinate moiety in the ligand. As a consequence, silylation of the phenyl ring takes places

followed by the Si−H insertion to Mn(0) species to realize complex 6. Owing to the difference

in the oxidation states of silicon atoms, complex 6 features two significantly different Mn–Si

bond distances. The slightly elongated Mn1–Si1 distance of 2.320(8) Å is ascribed for silyl-

Mn(II) bonding interaction, which is comparable to that observed for the

(hydrido)(silyl)manganese(II) complex [CpMn(dmpe)(H)(SiPh2H)] (2.319(4) Å).[137] The

RESULTS AND DISCUSSION

Mn1– Si1 distance 2.213(8) Å corresponds to the Si(II)−Mn(II) moiety which is slightly longer

compared to the bis(phosphine)-stabilized silylene manganese(I) hydrido complex cis-

[(dmpe)2MnH(=SiPh2)] (2.148(2) Å) reported by Emslie et al..[138,139] This difference could

presumably arise due to the presence of a two-coordinate donor-free silylene ligand and Mn(I)

center in cis-[(dmpe)2MnH(=SiPh2)].

Figure 3.1.11. Molecular structure of 6 with thermal ellipsoids drawn at 50% probability.

Hydrogen atoms (except hydrogen atom bonded to metal atom) are omitted for clarity;

selected bond lengths (Å) and angles (°): Mn1−Si1 2.320(8), Mn1−Si2 2.213(8),

Mn1−N12.076(2), Mn1−P1 2.221(7), N1−Mn1−Si2 82.04(6), N1−Mn1−P2 95.55(6),

Si1−Mn1−Si2 147.50(3), N1−Mn1−P1 175.07(7), P1−Mn1−P2 84.59(3), P1−Mn1−Si1

93.75(3).

The isolation of 6 inspired me to reorganize the synthetic protocol to render the

successful isolation of desired Mn(0) compound feasible. Therefore, I have carried out the

addition of bis(phosphine) ligand dmpe to THF solution of Mn(II) precursor complexes 1 and

2 prior to their reaction with reducing agent KC8 (Scheme 3.1.6). The last step proceeded with

a rapid transformation of the reaction mixture to a deep purple solution. After stirring overnight,

filtration and subsequent extraction with n-hexane followed, by crystallization at −20 °C, led to

the isolation of 7 as a dark purple crystalline solid in high yields. Compound 7 is stable under

inert gas conditions for several days at room temperature. However, the latter is extremely

sensitive towards air, and the presence of moisture affords rapid decomposition under an non-

inert atmosphere. The 1H NMR spectrum features shifted broad NMR signals in the chemical

shift range of −20 to +30 ppm, characteristic of a paramagnetic compound.

RESULTS AND DISCUSSION

Scheme 3.1.6. Synthesis of the novel bis(silylene)pyridine Mn(0) complex 7 using dmpe as

supporting ligand from 1 and 2.

Figure 3.1.13. Molecular structure of 7 with thermal ellipsoids at 50% probability. Hydrogen

atoms are omitted for clarity; selected bond lengths (Å) and angles (°): Mn1−Si1 2.2423(13),

Mn1−Si2 2.2141(13), Mn1−N1 2.111(3), Mn1−P2 2.1765(11), Mn1−P5 2.1901(13),

N1−Mn1−Si1 80.27(10), N1−Mn1−Si2 79.44(10), P2−Mn1−Si2 96.04(5), P2−Mn1−Si1

104.26(5), N1−Mn1−P2 175.36(11), P2−Mn1−P5 86.11(5), Si1−Mn1−Si2 147.43(5).

The diamond-shaped crystals suitable for a single-crystal XRD structure analysis were

reproducibly obtained from the saturated hexane solutions of 7 at room temperature. The latter

crystallizes in a monoclinic P21 space group. As anticipated, the molecular structure of 7

indeed confirmed the formation of a neutral zero-valent bis(silylene)-stabilized manganese

complex with a pseudo-square pyramidal (PSQP) coordination geometry (Addison-Reedijk 5

geometry index = 0.41) (Figure 3.1.13.).[140,141] The similar PSQP geometry was also observed

in case of analogous [SiNSi] I-35 stabilized Fe(0) compound I-69.[80] The square pyramidal

geometry is also consistent with the previously reported five-coordinate Mn(0) complex

[Mn(CO)3(CNAr*)2] (Ar* = 2,6- (2,6-diisopropylphenyl)-phenyl) (Figure 3.1.2.).[125] Moreover,

the structural geometry of 7 comprises an average axial/basal-plane angle of 96.4° which is

in excellent agreement with that determined by IR spectroscopy for the transient [Mn(CO)5]

RESULTS AND DISCUSSION

radical (96(±3)°) isolated in a low-temperature CO matrix.[115,120] The molecular structure of 7

possesses a N-pyridine coordination to Mn with two Si(II) donor arms and one of the P(III)

atoms of dmpe in the equatorial plane, and one P atom in axial position. The Mn1−Si1 and

Mn1−Si2 distances of 2.214(13) and 2.242(13) Å respectively, are substantially shorter than

those in corresponding Mn(II) precursor complexes 1 and 2 (Figure 3.1.13.). The shortening

of the Mn−Si distances could be illustrated based on the stronger Mn−Si bonding interaction

owing to an increase in the electron density of the Mn(0) center which, in turn, induces more

-backdonation to the empty 3p-orbitals at Si(II) atoms. The high resolution ESI-MS spectrum

of 7 confirmed the molecular composition of 7 ([M+2H]+: calcd. 888.4629; exptl. 888.4618)

(Figure 3.1.14.).

Figure 3.1.14. ESI-MS spectrum of complex 7 (Top: observed spectrum, bottom: calculated

spectrum).

To get more in-depth information on the electronic structure, microcrystalline samples

of 7 were examined by VT SQUID magnetometry. Evans measurement (C6D6 298 K) revealed

a µeff of 2.65 B.M. for 7 in solution phase.[133,134] Figure 3.1.15.a. illustrates the effective

magnetic moment (µeff) of 7, plotted as a function of temperature. Complex 7 possesses a

magnetic moment of 1.95 B.M. at 300 K, somewhat higher than the spin-only-value for a one

unpaired electron (µeffs.o. = 1.73 B.M.), Nonetheless, this observation is consistent the

presence of a single-unpaired electron in a d7 electronic configuration of Mn(0) in a low-spin

environment with S = 1/2 doublet ground state. In light of that, the EPR spectrum of 7 in THF

contains a broad, isotropic signal with an effective g-value of giso = 2.07, in accordance with

SQUID measurement (Figure 3.1.15.b).

RESULTS AND DISCUSSION

Figure 3.1.15. a) Temperature-dependent SQUID magnetization data (2–300 K at 1 T) for two

independently synthesized batches of 7, sample 1 (green squares) and sample 2 (blue

squares), plotted as a function of the effective magnetic moment (µeff) vs. temperature (T) and

corrected for temperature independent paramagnetism, TIP = 420·10–6 emu. A simulation for

S = 1/2 and gavg = 2.07 is represented as a solid red trace as a comparison. b) CW X-band

EPR spectrum of 7 recorded as a 1 mM solution in THF at 95 K (black trace) and its simulation

(red trace). Experimental conditions: microwave frequency ν = 8.959 GHz, modulation

amplitude = 1.0 mT, microwave power = 1.0 mW, modulation frequency = 100 kHz, time

constant = 0.1 s. Simulation parameters: effective g-value giso = 2.07, linewidths Wiso = 7.06

10–4 cm–1 / GHz, pseudo-Voigt lines used with ratios (Lorentz = 0, Gauss = 1) Viso = 0.00.

Figure 3.1.16. The highest singly occupied molecular orbital (HSOMO) of the compound 7.

The isosurfaces with 0.040 au isovalue are plotted for the orbital. Two views are given for a

better understanding of HSOMO.

RESULTS AND DISCUSSION

Figure 3.1.17. Crystal field theory (CFT) diagram of Mn(0) with d7 electronic configuration and

S =1/2 ground state in square pyramidal geometry.

To gain further insights into the electronic structure around the Mn(0) center in complex

7, density functional theory (DFT) calculations were carried out at PBE0-D3BJ/Def2-SVP∼ma-

TZVP level of theory.[142,143] The principal interacting orbital (PIO) analysis determines the

strength of orbital interactions through PIO-based bond index (PBI).[143,144] PIO analysis

developed by Lin is an intuitive bonding analysis tool by dominant interacting semi-localized

orbitals between fragments. Subsequently, principal interacting spin orbitals (PISO) analysis

is an extension of the PIO analysis to open-shell systems.[144]

Table 3.1.1. The relative electronic energy (kcal mol-1) of 7 in the three spin states.

State

relative electronic energy (kcal mol-1)

doublet

quartet

3.2

sextet

4.5

As per the DFT calculation, the bis(silylene) Mn(0) complex 7 exhibits a doublet ground

state with an unpaired electron (Table 3.1.1.). The DFT-derived highest singly occupied orbital

(HSOMO) of 7 shows that the unpaired spin density is predominantly located on a d-orbital of

the Mn atom but also distributed over one of the Mn-P bonds (Figure 3.1.16.).

RESULTS AND DISCUSSION

Figure 3.1.18. PISO analysis on the bonding modes of Mn-Si1 in the compound 7. Hydrogen

atoms in 3D structures are omitted for clarity. The total PBI value of Mn-Si1 is 0.56, with the

contribution of 0.29 from α system and 0.27 form β system. The isosurfaces with 0.080 au

isovalue are plotted for the PISO pairs.

The PISO analysis was performed on the bonding fragments, such as Mn and Si (P or

N) atoms, rather than an entire molecule to conclusively estimate the contribution of an

individual bonding mode. Each PISO pair (α and β) results in a bonding PISMO (principal

interacting spin molecular orbital). The PBI quantifies the strength of the selected bonding

interaction. The total PBI values for Mn-Si1, Mn-P2, and Mn-N1 are 0.56, 0.66, and 0.32,

respectively. The α and β interactions of Mn-Si, Mn-N1 and Mn-P2 bonds display similar

patterns. In particular, the first α- and β-PISO pairs with alike PBI values (0.19 and 0.22) are

indicative of a dative bond between Mn and Si1, with contributions of 0.92 e from Mn and 1.45

e from Si1 in total (Figure 3.1.18). Aside from the first α- and β-PISO pairs, the second α- and

β-PISO pairs demonstrate -backdonation from the d orbitals of the Mn atom to the vacant 3p

orbitals of the Si1 atom (Figure 3.1.18.).

RESULTS AND DISCUSSION

Scheme 3.1.7. Attempts to synthesize bis(silylene) Mn(0) complexes starting from 3 and dmpe

as supporting ligand.

To further extend the generality of abovementioned reductive dehalogenation

approach used for the isolation of [SiNSi] stabilized Mn(0) complex 7, I examined the reduction

of isolated Mn(II) complexes 3 and 4 stabilized by different bis(silylene) ligands. The treatment

of xanthene bis(silylene) stabilized dichlorido manganese(II) complex 3 with dmpe and

subsequent reaction with KC8 afforded dark red solutions (Scheme 3.1.7.). However, even

after several efforts of purification, the product could not be identified and characterized.

Attempts to detect the desired Mn(0) species through ESI-MS were also unsuccessful.

Scheme 3.1.8. Attempts to synthesize a bis(silylene) Mn(0) complexes, starting from 4 and

dmpe as supporting ligand.

Likewise, when the complex 4 was employed for dehalogenative reduction using KC8

to access analogous Mn(0) complex, a rapid formation of dark-purple solutions was observed.

However, the poor solubility of resulted product residue in general organic solvent such as

diethyl ether, hexane and toluene, prevented the isolation of a pure compound. Several

crystallization attempts with a variety of solvent mixtures only resulted in the precipitation of

unidentified products The presence of the desired Mn(0) species could only be detected in the

ESI-MS spectrum of reaction mixture in THF (Scheme 3.1.8.).

Therefore, based on these experimental results, we comprehended that the presence

of intramolecular additional donor ligand such as pyridine in pincer bis(silylene) I-35, plays a

RESULTS AND DISCUSSION

decisive role in facilitating the stabilization of highly reactive electron-rich Mn(0) complex 7

and favours low-spin environment (Figure 3.1.17.). The unique geometrical parameters such

as bite-angle, denticity, pincer-type arrangement, offered by different bis(silylene) ligands

towards TM coordination, are one of the most important factors for enabling the isolation of

extremely reactive TM centers in unusual oxidation states.

RESULTS AND DISCUSSION

3.2. Reactivity and catalytic application of bis(silylene) Mn(0)

complex 7

3.2.1. Reaction with CO

The isolated five-coordinate 17 VE genuine Mn(0) complex 7 stabilized with sterically

encumbering and strong -donating bis(silylene)pyridine ligand along with labile dmpe

coligand is unprecedented. With the unique Mn(0) complex 7 in hand, I became interested in

investigating its ligand substitution behaviour with stronger -acceptor ligands, such as carbon

monoxide (CO) to access analogous Mn(0) complexes, despite the fact that, previous reports

by Deng and co-workers demonstrated that the NHC stabilized Mn(0) complex undergoes

Mn−Mn dimerization to afford dinuclear manganese(0) carbonyl complex on exposure to CO

atmosphere (Scheme 3.2.1.).[126]

Scheme 3.2.1. Reaction of NHC-based Mn(0) compound with CO to form dinuclear Mn

carbonyl compound reported by Deng et al.[126]

Scheme 3.2.2. Ligand exchange reaction of 3-[SiNSi]Mn(dmpe) (7) with CO to form Mn(0)

carbonyl complex, 2-[SiNSi]Mn(CO)3 (8) .

The reaction of 7 with CO gas under 1 bar atmosphere in toluene resulted in a rapid

color change from dark-purple to wine-red solutions to form the new bis(silylene)pyridine

Mn(0) carbonyl complex (8) (Scheme 3.2.2). The latter was isolated in good yields by

extracting the residue with hexane, followed by crystallization at ambient temperatures. The

1H NMR spectrum of 8 includes broad, shifted NMR signals, characteristic of a paramagnetic

compound.

RESULTS AND DISCUSSION

Figure 3.2.1: FT-IR spectrum of compound 8 showing three distinctive CO bands at 1844,

1811 and 1716 cm–1.

The FT-IR spectrum of 8 exhibits three characteristic CO stretching bands at 1844,

1811 and 1716 cm–1, suggesting the incorporation of three CO ligands to the Mn(0) precursor

(Figure 3.2.1). However, the IR absorption bands are relatively bathochromic-shifted

compared to previously reported isolobal [Mn(CO)3(CNArDipp)2], [125]. This observation could be

presumably attributed to the prominent σ-donor strength of Si(II) in contrast to CO and thus

leads to a substantial increase in the Mn→CO  back-donation, The ESI-MS spectrum of 8

also authenticated the molecular composition formulation of [SiNSi]Mn(CO)3 complex (Figure

3.2.2).

Figure 3.2.2. ESI-MS spectrum of compound 8 (Top: observed spectrum, bottom: calculated

spectrum) (a) for [M+2H]+ (b) for [M–CO]+.

RESULTS AND DISCUSSION

Figure 3.2.3. Molecular structures of 8 with thermal ellipsoids drawn at 50% probability.

Hydrogen and solvent atoms are omitted for clarity. Selected bond lengths (Å) and angles (°):

Mn1−Si1 2.224(18), Mn1−Si2 2.362(18), Mn1−C41 1.801(7), Mn1−C40 1.774(7), Mn1−C42

1.778(7), Si1−Mn1−Si2 93.85(6), C42−Mn1−Si2 174.4(2) C41−Mn1−Si1 144.1(2),

C41−Mn1−Si2 90.5(2), C40−Mn1−C42 90.8(3), N2−Si1−Mn1 126.94(18), N1−Si2−Mn1

96.69(15).

Suitable crystals for an single-crystal XRD analysis were grown from a slow diffusion

of diethyl ether into the benzene solutions of 8. Complex 8 crystallizes in a monoclinic crystal

system with the P21 space group. The molecular structure of the latter exhibited a pseudo-

square pyramidal (PSQP) coordination geometry (Addison-Reedijk 5 geometry index = 0.13)

around Mn center with a Si1–Mn1–Si2 bond angle close to 90° (Figure 3.2.3).[140,141] Notably,

this geometrical situation is reminiscent of geometry predicted for the elusive Mn(CO)5

radical.[115] Interestingly, the PSQP geometry of five-coordinate Mn(0) complex essentially

favours the silylene donor arms to occupy the cis position to each other in the equatorial plane

along with two other carbonyl groups, leaving one of the carbonyl groups at the axial position.

Therefore, complex 8 can be better described as 2-[SiNSi]Mn(CO)3. It is important to note

that herein upon CO coordination, Mn center loses the N-pyridine coordination, contrary to the

precursor 3-[SiNSi]Mn(dmpe) (7), instead a N-pyridine coordination to one of the silylene

donor arms was observed, leading to a pentacoordinate Si(II) center. Such kind of coordination

lability of N-pyridine was also previously observed in case of reported Ge(0)→Fe(0) complex

(I-80) bearing the same pincer-type bis(silylene) ligand (Figure 1.4.2.).[114] The structural

orientation and flexibility of the pincer-type bis(silylene) I-35 has shown to play a decisive role

in impeding the dimerization of the Mn(0) radical species and retaining the oxidation state on

exposure to the CO atmosphere.

RESULTS AND DISCUSSION

Owing to the strong σ-donation from the pincer-type bis(silylene) [SiNSi] ligand,

complex 8 possess slightly shorter Mn–C distances of 1.801(7) Å, 1.774(7) Å and 1.778(7) Å

compared to its bis(isocyanide) analogue [Mn(CO)3(CNArDipp2)2] (1.882(6) and 1.883(5) Å)

reported by Figueora and co-workers (Figure 3.1.2).[125] The Mn1−Si1 and Mn1−Si2 distances

2.224(18) and 2.362(18) Å respectively, are significantly different, possibly because one of

Si(II) simply possess one additional coordination from pyridine. This leads to a relatively

weaker Mn1−Si2 bonding interaction and therefore elongation of bond distance is realized.

Though, Mn1−Si1 distance is comparable to its precursor Mn(0) complex 7.

The electronic structure of 2-[SiNSi]Mn(CO)3 (8) was further studied by PISO analysis.

The calculated relative electronic energies of the possible spin-states, implied that the

bis(silylene)pyridine ligand favours doublet ground state for the d7 Mn(0) compound 8 (Table

3.2.1.). The DFT derived highest singly occupied molecular orbital (HSOMO) of the latter

indicated the predominant localization of unpaired electron density on the d-orbital of the Mn

along the Mn-C bond (Figure 3.2.4).

Figure 3.2.4. The highest singly occupied molecular orbital (HSOMO) of the compound 8. The

isosurfaces with 0.040 au isovalue are plotted for the orbital.

Table 3.2.1. The relative electronic energy (kcal mol-1) of compound 8 in the three states.

State

Relative electronic energy (kcal mol-1)

doublet

quartet

33.8

sextet

100.2

RESULTS AND DISCUSSION

Figure 3.2.5. a) Temperature-dependent SQUID magnetization data (2–300 K at 1 T) for two

independently synthesized batches of 8, sample 1 (green squares) and sample 2 (blue

squares), plotted as a function of the effective magnetic moment (µeff) vs. temperature (T). b)

CW X-band EPR spectra of 8 recorded as a 5 mM solution in benzene (black trace) at 293 K.

Experimental conditions: microwave frequency



= 8.959 GHz, modulation amplitude = 1.0

mT, microwave power = 1.0 mW, modulation frequency = 100 kHz, time constant = 0.1 s.

The solution magnetic moment of 8, determined by the Evans method (C6D6, 298 K),

was found to be 1.80 B.M., consistent with the low-spin d7 Mn(0) ground state formulation.

[133,134] Contrary to the solution-phase µeff, the solid-state SQUID magnetization data is highly

temperature-dependent and reproducibly low, with values ranging from 0.5 B.M. at 2 K to 1.18

B.M. at 300 K (Figure 3.2.5.a). Nonetheless, the EPR spectrum of a benzene solution of 8 at

293 K, consistent with the S = 1/2 spin-state, contains one broad resonance centered at geff ≈

2.01 with a partly-resolved, six-line spectrum (Figure 3.2.5.b). The latter splitting pattern

perhaps could arise due to hyperfine coupling to the nuclear spin of a single Mn nucleus (55Mn,

I = 5/2, 100% natural abundance).

Moreover, the synthetic approaches to synthesize 8 through the dehalogenative

reduction of corresponding Mn(II) complexes 1 and 2 in the presence of CO gas were

unsuccessful. These results further explain the extreme reactivity of in-situ formed Mn(0)

species. However, the successful isolation of 2-[SiNSi]Mn(CO)3 (8) via ligand substitution by

dmpe highlights the crucial benefits of 7 as a suitable precursor for accessing more Mn(0)

complexes.

RESULTS AND DISCUSSION

3.2.2. Reaction with 2,6-dimethylphenylisocyanide

Scheme 3.2.3. Reaction of 3-[SiNSi]Mn(dmpe) (7) with 2,6-dimethylphenylisocyanide to

afford 9.

As already observed in the previous section on complex 7, a 17 VE zero-valent Mn(0)

complex in a d7 ground state served as a suitable precursor to access the bis(silylene)

analogue of highly unstable [Mn(CO)5] metalloradical (Scheme 3.2.2.). The reaction of 7 with

two molar equivalents of 2,6-dimethylphenyl isocyanide (xylyl isocyanide) in toluene results in

a dark-green solution affording к3-[SiNSi]Mn(CNXylyl)2(dmpe) (9) in 80% yields. (Scheme

3.2.3.). The FT-IR spectrum of 9 features two stretching vibration bands at 1900 and 1866 cm-1

which could be attributed to the xylyl isocyanide ligands at Mn center. The presence of two

xylyl isocyanide ligands in 9 was further confirmed by the high-resolution atmospheric

pressure chemical ionization mass spectrum (APCI-MS) ([M]+: calcd. 1148.5943; exptl.

1148.5951) (Figure 3.2.6.).

Figure 3.2.6. APCI-MS spectrum of complex 9 (Top: observed spectrum, bottom: calculated

spectrum)

RESULTS AND DISCUSSION

To gain more insights into the magnetic properties of 9 in the solid-state, VT SQUID

measurements were performed, which showed highly reproducible, temperature dependent

magnetic moment. The effective magnetic moment was found to be 1.00 B.M. at 2 K and 1.84

B.M. at 300 K (Figure 3.2.7.a). This observation is consistent with the presence of a low-spin

d7-Mn(0) ion in the doublet (S=1/2) ground state. The solution-phase magnetic moment

determined by the Evans method (1.86 B.M., C6D6, 298 K) was in complete accordance with

the calculated spin-only magnetic moment.[115] The EPR spectra of 9, measured in frozen and

liquid solutions at 9, 95 and 293 K, in various solvents (such as benzene, toluene, THF),

revealed complicated signals centered around g ≈ 2. All spectra reproducibly display

additional, partly resolved spectral features, presumably because of the hyperfine coupling to

the 55Mn and, possibly, 31P, nuclei (55Mn: I = 5/2, natural abundance 100%; 31P: I = 1/2, natural

abundance 100%).

Figure 3.2.7. a) Temperature-dependent SQUID magnetization data (2–300 K at 1 T) for two

independently synthesized batches of 9, sample 1 (green squares) and sample 2 (blue

squares), plotted as a function of the effective magnetic moment (µeff) vs. temperature (T). b)

CW X-band EPR spectrum of 9 recorded as a 5 mM solution in benzene at 293 K (black trace).

Experimental conditions: microwave frequency



= 8.959 GHz, modulation amplitude = 1.0

mT, microwave power = 1.0 mW, modulation frequency = 100 kHz, time constant = 0.1 s.

RESULTS AND DISCUSSION

3.2.3. Regioselective hydroboration of N-heteroarenes

TM catalyzed regioselective reduction of N-heteroarenes is an active area of research

owing to the diverse application of partially reduced products in agrochemical and

pharmaceutical industries.[145] However, the range of homogeneous catalysts used to carry

out this reaction was mostly dominated by the precious transition-metals such as Rh, [146] La[147]

and Th[148] based complexes. In recent years, several novel approaches have been developed

utilizing more earth-abundant, less toxic, first-row TMs such as Fe,[149] Ni,[150] and Zn[151]

complexes (Figure 3.2.8.). Manganese being the fifth most abundant metal in the earth’s crust,

its application as a homogeneous catalyst in chemical transformations is currently an

emerging field of catalysis. Though, the reports on manganese catalyzed selective

hydroboration of N-heteroarenes are unknown. Therefore, I became interested in examining

the catalytic activity of isolated Mn(0) complexes towards hydroboration of N-heteroarenes.

Figure 3.2.8. Selected examples of regioselective hydroboration of quinolines utilizing 3d

transition-metal complexes.

3.2.3.1. Catalytic screening

For optimizing the reaction parameters for catalytic hydroboration of N-heteroarenes,

quinoline was chosen as the model substrate. The initial optimization studies indicated the

catalytic efficacy of Mn(0) complex 7 towards the hydroboration of quinoline (10a) with

pinacolborane (HBpin) along with high regioselectivity to yield 1,2-dihydroborated product 11a

(11a:11a’ = 85:15) at 50 °C (Table 3.2.2., entry 3). The rather superior catalytic performance

of 7 compared to 8 and 9, perhaps could be presumably attributed to the substantially labile

nature of dmpe ligand as observed in complex 7 (Table 3.2.2., entry 3-5). Interestingly, their

corresponding Mn(II) precursors 1 and 2 have not demonstrated any catalytic activity for the

same reaction (Entry 1 and 2).

RESULTS AND DISCUSSION

Table 3.2.2. Optimization study for catalytic 1,2-hydroboration of quinoline.

Entry

Catalyst (mol %)

Borane

(H–BR2)

Conversiona

(1:2::1,4)

HBpin

97 (85:15)

HBpin

17(85:15)

HBpin

30(90:10)

HBcat

40(20:80)

9-BBN

55(20:80)

7+Hgb

HBpin

97 (85:15)

aConversion was monitored from 1H NMR spectroscopy using mesitylene as internal standard.

Regioisomeric ratio (1,2- vs 1.4- addition products) ratio has been shown in parentheses. bHg to catalyst

ratio 100:1.

RESULTS AND DISCUSSION

While screening the hydroborane, maximum conversions were obtained with HBpin in

comparison to catecholborane (HBcat) and 9-borabicyclo[3.3.1]nonane (9-BBN) dimer to

achieve the selective 1,2-hydroboration of quinolines (Table 3.2.2., entry 6-7). Decreasing the

reaction temperature to room temperatures negatively impacted the conversion. Additionally,

to address the issue of homogeneity of catalyst, the reaction performed in the presence of

mercury. The conversion of 10a remained unaffected and thus ensured the catalytic

hydroboration of quinoline to be homogeneous process. (Table 3.2.2., entry 8).

3.2.3.2. Substrate scope

After the preliminary catalytic screening and optimization of reaction conditions, the

substrate scope was extended to various electronically modified quinolines. Complex 7

demonstrated excellent functional group tolerance towards substrates comprising electron-

withdrawing substituents such as Cl-, Br-, and F- and electron donating groups such as methyl

group at various positions (Scheme 3.2.3.). Remarkably, under the identical optimized

conditions, catalyst 7 displayed improved selectivity for the hydroboration reactions of 10b and

10c when compared with traditional Mn(0) precursor Mn2(CO)10 at the same catalytic amounts.

Likewise, 3-methylquinoline (10f), under standard conditions, resulted exclusively in 1,2-

hydroboration product when performed in the presence of 7 (11b:11b’ = 100:0) than with

Mn2(CO)10 (11b:11b’ = 90:10). The hydroboration of 3-bromoquinoline (10d) also followed the

similar regioselective pattern (11d:11d’ = 100:0) with complex 7 than 85:15 with

Mn2(CO)10.These observations further validated the superior catalytic performance of 7

towards regioselective hydroboration of N-heteroarene in comparison to Mn2(CO)10.

When the reaction was performed with isoquinoline (10j), an exclusive 1,2-

hydroborated product (11j) was obtained in >99% yields at ambient temperature by utilizing 7

as a precatalyst. Apart from a variety of quinolines, 7 is also shown to catalyze the

hydroboration of other N-heterocycles such as pyrimidine (10k-l), phenanthridine (10n), and

acridine (10m) under standard condition. However, complex 7 did not show any catalytic

reactivity towards quinoline derivatives comprising Cl-, Br-, and phenyl-groups present at 2-

and 8-positions. A rationale for this observation can be ascribed to the steric restraint caused

by the sterically demanding groups in the proximal position of the putative active catalyst.

Moreover, the hydroboration of 6-nitroquinoline also resulted in complicated mixtures which

could not be identified.

RESULTS AND DISCUSSION

aAll reactions were performed in 0.1 mmol scale in a J. Young NMR tube with 0.45 mL C6D6. Conversion

was determined from 1H NMR spectroscopy using mesitylene as internal standard. Regioisomeric ratio

(1,2- vs 1.4- addition products) ratio has been shown in parentheses. bFor 36 h. cAt room temperature

(2h). d4.0 equiv of HBPin was used. eFor 48 h.

Scheme 3.2.3. Catalytic hydroboration of N-Heteroarenes with HBpin using к3-

[SiNSi]Mn(dmpe) (7) as a precatalyst.

RESULTS AND DISCUSSION

3.2.3.3. Mechanistic investigation

To get further insights into the reaction mechanism, deuterium labelling experiments

were carried out using deuterated pinacolborane (DBpin) under standard conditions. DBpin

was synthesized according to literature procedure (Scheme 3.2.4.).[152] The treatment of 10a

with DBpin indeed led to the selective formation of 1,2-deuteroborated quinoline (12a) as

major product with 73% deuterium incorporation (Scheme 3.2.5.). To understand the relative

rates of involved individual reaction steps in a catalytic reaction, kinetic isotope effect (KIE)

experiments were conducted. The KIE analysis showed a KH/KD ratio of 1.84, consistent with

the dissociation of H−Bpin bond being a rate determining step (Figure 3.2.9.). The values

obtained are also in good agreement with the KIE study for reaction of HBcat with ruthenium

complexes previously reported by Hartwig and co-workers.[153]

Scheme 3.2.4. Synthesis of DBpin using literature procedure. [152]

Scheme 3.2.5. Catalytic hydroboration of quinoline with DBpin using к3-[SiNSi]Mn(dmpe) (7)

as a precatalyst. The deuteration grades were monitored from the 1H NMR spectrum of crude

reaction mixture.

Notably, the reaction of complex 7 with HBpin performed at 50 °C, indicated the

inclusion of dmpe ligand in the active catalyst based on the 31P NMR spectrum. The control

experiment including the reaction of 7 with HBpin monitored by 1H NMR spectroscopy showed

RESULTS AND DISCUSSION

the emergence of a hydridic signal at δ =−9.6 ppm indicative of the formation of a diamagnetic

Mn−H species 7a. 1H-31P HMQC 2-D NMR experiments further provided support for the

proposed 7a complex as a correlation signal was appeared for the hydridic proton at δ = −9.6

ppm with one of the phosphorus nuclei. Subsequent addition of 3-methylquinoline (10f) into

the latter reaction mixture resulted into a selective 1,2-hydroboration product (11f) in good

yields. However, several attempts to isolate the proposed active catalyst were futile. Moreover,

to exclude the formation of any catalytically active cationic boron species, a blank catalytic

experiment was conducted using dmpe (5 mol%) and HBpin under standard conditions in the

absence of 7, but no product formation was detected by NMR spectroscopy.

Figure 3.2.9. KIE experiment for regioselective hydroboration of quinoline with HBpin and

DBpin respectively using 7 as a precatalyst.

Based on these experimental evidences, a probable mechanism as depicted in Figure

3.2.10. for the catalytic reaction pathway was proposed. The hydroboration reaction

presumably starts with the in situ reaction of 7 with two molar equivalents of HBpin to form a

diamagnetic Mn(I) complex 7a with the loss of one phosphine coordinated to the Mn center

(Figure 3.2.10.). The formed active catalyst 7a proceeds to afford 7b through a 1,2-hydride

migration of quinoline via a four-membered transition state (TS1). Once formed intermediate

7b, on reaction with HBpin results into 1,2-hydroborated quinoline and regenerates back the

active catalyst 7a. The proposed reaction mechanism is in parallel with the previous reports

on the regioselective hydroboration of N-heteroarenes.[150]

RESULTS AND DISCUSSION

Figure 3.2.10. Proposed reaction mechanism for the 1,2-hydroboration of quinoline with

HBpin using к3-[SiNSi]Mn(dmpe) (7) as a precatalyst.

RESULTS AND DISCUSSION

3.3. The comparative study on the coordination reactivity of

bis(silylene) versus bis(phosphine) in accessing alkyl Mn(II)

complexes

3.3.1. Background

As discussed in section 1.4, bis(silylene) ligands, owing to their unique steric and

electronic properties have been implemented in stabilizing a variety of reactive TM centers.[13]

Despite a plethora of reports on the isolable silylene-TM complexes, silylene-stabilized alkyl

TM complexes are yet unexplored. Alkyl TM complexes are an important class of

organometallic complexes since they include TM-carbon bonds and therefore they are highly

valued in the field of homogeneous catalysis.[154] A variety of industrially important catalytic

transformations such as alkene hydroformylation, alkene isomerization and olefin

hydrogenation are suspected to include M–C -bonds in the catalytically active

intermediates.[2] However, owing to the propensity of β-hydrogen containing alkyl TM species

to undergo β-hydrogen elimination, the isolable alkyl TM complexes remained laboratory

curiosities for a very long time (Figure 3.3.1.).[155]

Figure 3.3.1. Organometallic β-elimination reaction in β-hydrogen containing TM alkyl

complexes (top); some examples of the alkyl groups without the presence of β-hydrogen

(bottom).

Nevertheless, the seminal reports on the isolation of alkyl lithium and alkylmagneisum

compounds comprising alkyl groups with missing β-hydrogens such as trimethylsilyl methyl

ligand ((Me3Si)CH2) unblocked routes to access various kinetically stabilized dialkyl

manganese(II) complexes which would be otherwise difficult to attain (Figure 3.3.1.).[155-158]

RESULTS AND DISCUSSION

The pioneering work of Lappert and Wilkinson on such magnesium dialkyls and alkyl lithium

compounds led to the isolation of monomeric dialkyl Mn(II) complexes via salt-metathesis

reaction.[159] Subsequently, a wide range of donor ligands such as tetramethylethylenediamine

(TMEDA), 1,2-bis(dimethylphosphino)ethane (dmpe), pyridine, 1,4-dioxane were utilized to

synthesize the donor-stabilized monomeric dialkyl manganese complexes.[160] In recent years,

alkyl manganese complexes have attracted rapid attention due to their potential applications

in the catalytic polymerization of olefins.[161]

Figure 3.3.2. Some reported examples of alkyl manganese complexes.

Recently, Emslie and co-workers succeeded in the structural characterization of the

four-coordinate, dialkyl manganese(II) complex (dmpe)Mn(CH2SiMe3)2 (A, Figure 3.3.2.),

reported earlier by Wilkinson and co-workers.[159,162] On the contrary, an α,α’-diiminato

pyridine-stabilized MnCl2 complex on alkylation with (Me3Si)CH2Li underwent reduction and

subsequently produced the unique monoalkylated anionic manganese complex B.[129]

However, when comparatively stronger -donor ligands such as NHC; bis(2,6-

diisopropylphenyl)-imidazol-2-ylidene (IPr) and terpyridine are applied, successful kinetic

stabilization of dialkyl Mn(II) complexes (C and D) was realized.[163,164]

The aforementioned fascinating examples of alkyl manganese complexes (Figure

3.3.2., A-D) inspired us to explore the coordination abilities of chelating silylene and phosphine

ligands towards alkyl manganese(II) complexes. In this section, the synthesis, distinct

coordination reactivity and molecular structure of a pincer-type ligand with two-phosphine

RESULTS AND DISCUSSION

donor arms and the analogous bis(silylene)pyridine ligand which enable the isolation of

different alkyl manganese complexes will be discussed.

3.3.2. Synthesis and characterization of bis(phosphine)pyridine Mn(II)

complexes

The deprotonation reaction of I-48 with two molar equivalents of LiHMDS as base and

subsequent salt-metathesis with 2-chloro-1,3-diisopropyl-1,3,2-diazaphospholidine[165] in

toluene at ambient temperature resulted in a yellow suspension. Further filtration over celite

to remove LiCl salt and concentration under vacuum afforded the potentially tridentate ligand

2,6-bis((1,3-diisopropyl-1,3,2-diaza-phospholidin)-N-ethylamino) pyridine [PNP] 13, as yellow

oil in 78% yields (Scheme 3.3.1.). The 1H NMR spectrum of 13 featured the characteristic

signal for the pyridine backbone at δ = 6.75 and 7.19 ppm as doublet and triplet for the 3,5-

CH and 4-CH protons on the pyridine backbone, respectively. The 31P{1H} NMR spectrum of

the latter exhibits a resonance signal at δ = 99.8 ppm indicative of the symmetrical

structure.[166]

Scheme 3.3.1. Synthesis of bis(phosphine)pyridine ligand (13) .

The reaction of bis(phosphine)pyridine ligand 13 with MnCl2 in THF at 60 °C afforded

the desired manganese(II) complex 14 as yellow powder in 89% yields (Scheme 3.3.2.).

Complex 14 is NMR silent, characteristic of the paramagnetic nature of Mn(II) center. Evans

measurement (THF-d8, 298 K) of the latter showed an effective magnetic moment of 5.88

B.M., indicative of the presence of a high-spin d5 Mn(II) center with a S = 5/2 spin

environment.[133,134]

The colorless rectangular crystals suitable for an XRD structure analysis were grown

by keeping concentrated THF solutions of 14 overnight at –20 °C. 14 crystallizes in the

orthorhombic crystal system with the P212121 space group. The molecular structure of 14

revealed a five-coordinate manganese complex in a square-pyramidal geometry (Figure

RESULTS AND DISCUSSION

3.3.3.). Therefore, complex 14 could be depicted as 3-[PNP]MnCl2. Interestingly, [PNP] 13

serves as tridentate pincer in contrast to the analogous bis(silylene) MnCl2 complex (1), where

[SiNSi] I-35 acts as a bidentate ligand. The geometric features of 14 are fully consistent with

those of the [{NNN}MnCl2] complex earlier reported by Gambarotta et al.[129] However, the

Mn1–N1(pyridine) distance of 2.418(19) Å in complex 14 is substantially longer compared to

2.210(5) Å in related [{NNN}MnCl2] complex.

Scheme 3.3.2. Synthesis of bis(phosphine)pyridine stabilized Mn(II) complex (14).

Figure 3.3.3. Molecular structures of 14 with thermal ellipsoids drawn at 50% probability.

Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Mn1–P1

2.5415(7), Mn1–P2 2.5439(7), Mn1–N1 2.4180(19), Mn1–Cl1 2.3555(7), Mn1–Cl2 2.3297(7);

N1-Mn1-P1 71.15(5), Cl2-Mn1-P1 106.26(3), P1-Mn1-P2 133.76(2), N1-Mn1-Cl1 144.86(5),

P2-Mn1-Cl2 108.30(2).

RESULTS AND DISCUSSION

Scheme 3.3.3. Synthesis of bis(phosphine)pyridine stabilized dialkyl Mn(II) complex (15)

through salt-metathesis reaction with LiCH2SiMe3.

To access the corresponding [PNP] stabilized dialkyl manganese(II) complex, the salt-

metathesis reaction of 14 with two molar equivalents of (trimethylsilyl)methyllithium

(Me3SiCH2Li) was performed in diethyl ether at –78 °C, resulting in deep-orange solutions

(Scheme 3.3.3.). Filtrations, and subsequent crystallization of 15 in concentrated diethyl ether

at –20 °C afforded a large crop of orange crystals in 34% yield. The 1H NMR spectrum of 15

exhibited broad and shifted NMR signals characteristic of the paramagnetic nature of the

complex. The solution-phase magnetic moment was determined to be 5.79 B.M. (Evans

method, C6D6, 298K), fully consistent for a high-spin Mn(II) complex five unpaired electrons

present in the S = 5/2 spin environment.[133,134] Furthermore, the composition of 15 was

corroborated by ESI-MS and elemental analysis.

Figure 3.3.4. Molecular structures of 15 with thermal ellipsoids drawn at 50% probability.

Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Mn1–P1

2.6609(5), Mn1–P2 2.6167(5), Mn1–C1 2.1803(17), Mn1–C2 2.1552(17), C2-Mn1-C1

119.25(7), C2-Mn1-P2 107.36(5), C1-Mn1-P2 98.78(5), P1-Mn1-P2 117.904(15).

The single-crystal XRD structural analysis of 15 revealed the four-coordinate Mn(II)

center in distorted tetrahedral geometry (Figure 3.3.4.). Contrary to complex 14, in complex

RESULTS AND DISCUSSION

15, Mn center loses the pyridine coordination from the [PNP] ligand, and therefore could be

described as 2-[PNP]Mn(CH2SiMe3)2 (15). The observed coordination lability of the pyridine

in the latter complex can be attributed to the increased electron density on the manganese

center through the strong -donor (trimethylsilyl)methyl ligand. On the other hand, the stronger

-acceptor terpyridine ligand reinforces coordination mode to remain intact even on alkylation

with Me3SiCH2Li forming complex D (Figure 3.3.2.).[163] The Mn1–P1 2.6609(5) and Mn1–P2

2.6167(5) Å bond distances in 15 are considerably longer than those with 2.5415(7) and

2.5439(7) Å, in 14, respectively. Such bond elongation could most likely arise from the strong

-donation from (trimethylsilyl)methyl ligand combined with the weaker [N(pyridine)]–Mn(II)

synergistic bonding interaction in 15.[80] Nonetheless, the Mn–P bond distances are fully

consistent with those reported for [(dmpe)Mn(CH2SiMe3)2] (A) (2.6541(8) Å) by Emslie et al

(Figure 3.3.2.).[162] Furthermore, the Mn–C bond distances of 2.1552(17) and, 2.1803(17) Å

are also in accordance with those of complex D [2.172(3) and 2.177(3) Å], reported by Zheng

et al (Table 3.3.1.).[163]

Table 3.3.1. Comparison of Mn–C distances for selected alkyl manganese complexes.

Compounds

Mn–C (Å)

Reference

[PNP]Mn(CH2SiMe3)2 (15)

2.1803(17),

2.1552(17)

this work

{[SiNN]Mn(CH2SiMe3)}2 (16)

2.145(3)

this work

[Si]Mn(amd)(CH2SiMe3)a (19)

2.139(3)

this work

(dmpe)Mn(CH2SiMe3)2 b (A)

2.1320(14)

162

[TMEDA]Mn(CH2SiMe3)2 c

2.1379(15)

160b

{Li(OEt2}{[PDI]Mn(CH2SiMe3)}d (B)

2.147(6)

129

[terpy]Mn(CH2SiMe3)2 e (D)

2.172(3),

2.177(3)

163

[IPr]Mn(CH2SiMe3)2 f (C)

2.129(1)

164

aamd = PhC(NtBu)2, bdmpe = 1,2-bis(dimethylphosphino)ethane, cTMEDA =

tetramethylethylenediamine, dPDI = 2,6-pyridinediiminato, eterpy = terpyridine, fIPr = bis(2,6-

diisopropylphenyl)-imidazol-2-ylidene.

RESULTS AND DISCUSSION

3.3.3. Synthesis and characterization of bis(silylene)pyridine manganese(II)

complexes

The successful isolation of [PNP]-based dialkyl manganese(II) complex 15 enthused

me to investigate the coordination reactivity of analogous bis(silylene)pyridine I-35 stabilized

MnCl2 complex (1) towards LiCH2SiMe3. In this regard, the reaction of corresponding

dichloride Mn(II) precursor 2-[SiNSi]MnCl2 complex 1 with two molar equivalents of

Me3SiCH2Li in diethyl ether at –78 °C afforded dark-orange solutions (Scheme 3.3.4.). Further

filtration and concentration of its diethyl ether solution kept in freezer at –20 °C for two days

afforded rod-shaped dark orange crystals of {3-[SiNN]Mn(CH2SiMe3)}2 (I-16) in 59% yields.

Scheme 3.3.4. Synthesis of dinuclear silylene alkyl Mn(II) complex (16) through salt-

metathesis reaction of 1 with LiCH2SiMe3 (* indicates that 17 was not isolated).

A single-crystal XRD structural analysis revealed the formation of a centrosymmetric

dinuclear mono(trimethylsilyl)methyl mono(ethyl)amido Mn(II) complex I-16 (Figure 3.3.5.). In

fact, we hypothesize that the formation of 16 is accompanied by nucleophilic Si(II)–N scission

and the subsequent elimination of the (trimethylsilyl)methyl-silylene 17 as side-product

(Scheme 3.3.4.). The proposed side-product, silylene 17 was earlier isolated by Cabeza and

co-workers.[167] Although, the mechanism that proceeds the reaction 1→16, is not known but

we suggest that the kinetic instability of desired bis(silylene)-stabilized

bis[(trimethylsilyl)methyl] Mn(II) complex 16a, may possibly have triggered its further reaction

with another molecule of [SiNSi]Mn(CH2SiMe3)2 to form dinuclear manganese complex 16 as

RESULTS AND DISCUSSION

an end product (Scheme 3.3.4.). The high reactivity of putative intermediate 16a could derive

from the stronger -donor strength of the [SiNSi] ligand compared to that of the related [PNP]

ligand 13 or other bis(phosphines) and bis(NHC) ligands.[78] Such Si(II)–N bond cleavage in

[SiNSi] ligand has also been witnessed earlier by Roesky and co-workers while examining the

thermal stability of a [SiNSi]Ca{N(SiMe3)2}2 complex.[168]

Figure 3.3.5. Molecular structures of 16 with thermal ellipsoids drawn at 50% probability.

Hydrogen and solvent atoms are omitted for clarity. Symmetry transformations applied to

generate equivalent atoms: –x+1, y, –z+1/2. Selected bond lengths (Å) and angles (°): Mn1–

Si1 2.6746(7), Mn1–N1 2.1679(18), Mn1–N2’ 2.1368(19); C1–Mn1–N1 117.12(9), N2–Mn1–

C1 112.92(9), N2–Mn1–N1 127.32(7), C1–Mn1–Si1 111.70(9), N1–Mn1–Si1 72.12(5), N2–

Mn1–Si1 103.87(5).

Complex 16 adopts a monoclinic crystal system with C2/c space group. The molecular

structure of the latter complex represents a dimeric arrangement of manganese centers linked

through two silylene functionalized pyridylamide ligands (Figure 3.3.5.). Complex 16 features

an Mn∙∙∙Mn’ interatomic distance of 3.1141(8) Å, that is somewhat longer than the sum of the

van der Waals radii (2.79 Å) and considerably higher than the reported covalent Mn–Mn bonds

(2.2-2.8 Å).[18] Each four-coordinate Mn center is coordinated to one N(pyridine), pyridyl-

amido, (trimethylsilyl)methyl, and a silylene ligand. As shown in Table 3.3.1., the Mn1–C1

distance of 2.149(3) Å is consistent with the other reported dialkyl Mn(II) complexes.[162-164]

The Mn1–Si1 distance of 2.6784(12) Å is comparable to that in the precursor complex 1.[94] To

the extent of our knowledge, the obtained dinuclear manganese(II) complex 16 that features

RESULTS AND DISCUSSION

a silylene-pyridyl-amido ligand is unprecedented. As expected, complex 16 is paramagnetic

as evidenced by the observed magnetic moment 11.90 B.M. per molecule and 5.95 B.M. per

Mn center, implying the presence of five unpaired electrons on each Mn center.[133,134]

To circumvent the Si(II)–N bond cleavage and the elimination of one silylene motif from

the [SiNSi] ligand I-35, as observed in the formation of 16, I planned to employ a potentially

chelating NHSi with only one silylene donor site. Therefore, with the aim to isolate the

anticipated dialkyl Mn(II) compound, we decided to utilize the chelating monosilylene I-26,

which is a bidentate analogue of [SiNSi] ligand I-35.[75] The reaction of I-26 with MnBr2 in 1:1

molar ratio in THF at 50 °C afforded clear yellow solution (Scheme 3.3.5.). The desired

complex 18 was isolated as light-yellow solid in 87% yields.

Scheme 3.3.5. Synthesis of dinuclear dibromido Mn(II) complex 18 stabilized by chelating

silylene I-26.

The crystals suitable for an single-crystal XRD analysis were obtained on cooling the

concentrated THF solution of 18 at –20 °C overnight. The structural analysis of the latter

revealed the formation of a dinuclear manganese(II) complex bridged through two bromide

ligands. Each five-coordinate Mn center occupies the square pyramidal geometry (Figure

3.3.6.). The slight elongation of Mn1–Br1 (2.5297(7) Å) bond distance compared to Mn1–Br2

(2.4629(7) Å) is clearly attributed to the role of Br1 as bridging ligand. The Mn1–Si1 distance

of 2.5298(10) Å in complex 18 is slightly shorter than those observed for related 2-

[SiNSi]MnBr2 complex 2 (Mn1−Si1 2.560(7) and Mn1−Si2 2.567(6) Å).

RESULTS AND DISCUSSION

Figure 3.3.6. Molecular structures of 18 with thermal ellipsoids drawn at 50% probability.

Hydrogen atoms are omitted for clarity. Symmetry transformations used to generate

equivalent atoms in 5: –x+1, –y+1, –z+1. Selected bond lengths (Å) and angles (°): Mn1–Si1

2.5298(10), Mn1–N1 2.340(4), Mn1–Br1 2.5297(7), Mn1–Br2 2.4629(7), Mn1’–Br1 2.8533(8);

N1–Mn1–Si1 72.33(9), N1–Mn1–Br1 94.04(9), N1–Mn1–Br2 98.11(8), Mn1–Br1–Mn1’

90.14(2), Br2–Mn1–Br1 113.43(3).

Scheme 3.3.5. Synthesis of alkyl Mn(II) complex 19 stabilized by in-situ formed silylene 17.

Treatment of the isolated dinuclear dibromido Mn(II) complex 18 with four molar

equivalents of Me3SiCH2Li in Et2O at –78 °C, resulted in rapid formation of dark-yellow solution

(Scheme 3.3.5.). Subsequent crystallization from the concentrated Et2O solution of the

resultant reaction mixture, produced rectangular-shaped yellow crystals in 24% yields. An

single-crystal XRD analysis demonstrated the presence of a four-coordinate amidinato

[(trimethylsilyl)methyl] manganese(II) complex in 19 stabilized by silylene 17 in a distorted

RESULTS AND DISCUSSION

tetrahedral geometry (Figure 3.3.7.). The latter crystallizes in an orthorhombic crystal system

with Pca21 space group. Complex 19 exhibits a Mn1–Si1 bond distance of 2.6270(8) Å,

comparable to the reported silylene stabilized Mn(II) complexes.[94,128] As shown in Table

3.3.1., the Mn1–C1 distance of 2.139(3) Å compares well with reported monoalkyl Mn

complexes.[129]

Figure 3.3.7. Molecular structures of 19 with thermal ellipsoids drawn at 50% probability.

Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Mn1–Si1

2.6270(8), Mn1–N3 2.143(2), N4–Mn1–C1 118.81(14), N4–Mn1–N3 62.29(9), C1–Mn1–N3

128.32(12), C1–Mn1–Si1 114.28(11).

3.3.3. Reactivity of bis(silylene)pyridine (I-35) with LiCH2SiMe3

The structural analysis of complex 19 implied that its formation might have been

accompanied by additional side-products, but unfortunately, none of the by-products could be

detected or isolated. Similar to Scheme 3.3.4., a Si(II)–N cleavage of silylene I-26 presumably

have taken place to realize the formation of (trimethylsilyl)methyl-silylene 17 motif, which

further undergoes in-situ complexation to alkyl Mn(II) center. Nevertheless, this observation

further verifies my speculation, that 17 could be a plausible side-product during the formation

of 16 via a rather facile Si(II)–N bond-scission of [SiNSi] I-35 ligand (Scheme 3.3.4.).

RESULTS AND DISCUSSION

Scheme 3.3.6. Reaction of bis(silylene)pyridine I-35 with LiCH2SiMe3.

Figure 3.3.8. 1H NMR (500 MHz, THF-d8) spectrum of reaction mixture (17+20) at ambient

temperature.

The unexpected isolation of alkyl manganese(II) complexes 16 and 19 explained that,

upon in-situ formation of reactive dialkyl Mn(II) species, a rather facile Si(II)–N cleavage of

amidosilylene occurs. To further investigate that whether this phenomenon is assisted by the

presence of alkyl manganese species, I became interested in examining the reactivity of

metal-free bis(silylene)pyridine I-35 ligand with Me3SiCH2Li.[80] I-35 was allowed to react with

RESULTS AND DISCUSSION

one molar equivalent of Me3SiCH2Li at –78 °C in Et2O, resulting in the rapid development of a

light-yellow solution. The 1H NMR analysis of the reaction mixture in THF-d8 indicated the

quantitative conversion of bis(silylene)pyridine ligand into 17 and 20 (Figure 3.3.8).

Furthermore, the 1H and 29Si{1H} NMR spectra of the reaction mixture verified the silylene 17

to be one of product, since the obtained NMR signals are fully consistent with the reported

NMR characterization data.[167] The 1H NMR spectrum in d8-THF displays characteristic

signals for the non-equivalent NCH2CH3 at δ = 3.00 and 3.13 ppm, as quartets in compound

20 (Figure 3.3.8.). The proton resonances appeared at δ = 5.32 and 5.41 ppm could

unambiguously be attributed to the aromatic CH protons at the 3- and 5-position in the

unsymmetrical pyridine backbone of 20. The 29Si{1H} NMR spectrum features a singlet signal

for 20 at δ = –19.1 ppm, somewhat upfield shifted when compared to free

(amidinate)amido(silylene)pyridine ligand I-26, suggesting the weaker bonding interaction

between silylene and lithium ion.[19] The 7Li NMR spectrum shows a broad singlet signal at δ

= 1.4 ppm, characteristic for the lithium amide species.[169]

Figure 3.3.9. 1H-29Si HMQC NMR spectrum (coupling constant corrected for 3JSiH = 7 Hz) in

THF-d8 spectrum of reaction mixture (17+20)

RESULTS AND DISCUSSION

Figure 3.3.10. Molecular structures of 20 with thermal ellipsoids drawn at 50% probability.

Hydrogen atoms, tert-butyl, phenyl, and ethyl groups are omitted for clarity. Selected bond

lengths (Å) and angles (°): Si1–Li1 2.791(4), Li1–N1 2.012(4), N6–Li1 2.115(4), N9–Li1

2.124(4), N1–Li1–N6 121.4(2), N1–Li1–N9 115.25(18), N9–Li1–Si1 115.65(17).

Besides, a few colorless crystals of 20 were obtained from cooling the concentrated

Et2O solution of reaction mixture residue, at –78 °C, the similar solubilities of both products 17

and 20 in common non-polar solvents such as n-hexane and toluene prevented their isolation

by fractional recrystallization in larger quantities. Compound 20 crystallizes in a monoclinic

crystal system with the P21c space group. An XRD analysis of 20 disclosed the formation of

trimeric lithium amide species stabilized by a trifunctional silylene ligand [SiNN], which resulted

from cleavage of one of the Si(II)–N bond of the [SiNSi] ligand backbone (Figure 3.3.10.). The

observed Si1–Li1 bond distance of 2.791(4) Å for compound 20 is slightly longer than the

monomeric bis(silylene)stabilized lithium amide compound reported by Mo and co-workers.[170]

RESULTS AND DISCUSSION

3.4. Catalytic Sonogashira cross-coupling reaction utilizing

bis(silylene)pyridine nickel complexes as precatalyst

3.4.1. Background

Owing to their striking steric and electronic properties, bis(silylene) ligands enable the

isolation of elusive TM complexes in unusual oxidation states. In recent years, theoretical and

experimental studies on bis(silylene) assisted TM-mediated catalytic reactions have further

emphasized the paramount importance of TM–Si(II) cooperative effects in facilitating the

activation of rather inert substrates such as dihydrogen.[82,84] In particular, the tridentate

bis(silylene)pyridine ligand I-35 has shown promising results as a powerful chelating ligand

towards the stabilization of electron-rich TM centers as well as its vital role in selective

catalysis, as discussed in section 3.1. and 3.2. Taking this into account, I wished to study its

coordination chemistry towards nickel complexes and to evaluate their potential application in

catalyzing Sonogashira cross-coupling reactions. Sonogashira coupling has proven to be a

versatile reaction to access numerous biologically active molecules, essential drugs,

especially in late-stage modifications.[171-174] Traditional palladium-based catalysts with

cuprous iodide (CuI) cocatalyst, have long been utilized to serve this purpose.[173] However,

the scarcity of palladium sources on the Earth's crust highlights the urgency of developing

catalysts based on earth-abundant metals such as nickel and iron. Despite this, the nickel-

catalyzed Sonogashira cross-coupling reaction have been less widely explored thus far.[175]

Scheme 3.4.1. General scheme for palladium catalyzed Sonogashira cross-coupling reaction

first reported by Sonogashira in 1975.[171]

With regards to the mechanistic understanding of this particular catalysis, the involved

reaction pathways have mostly remained unclear, with only a few catalytically relevant

intermediates that have been structurally characterized and isolated. A Pd(0)-catalyzed cross-

coupling reaction generally occurs firstly by the oxidative addition of R1–X, followed by the

transmetallation with copper acetylides and finally a reductive elimination furnishes the desired

cross-coupled product (Figure 3.4.1).[173]

RESULTS AND DISCUSSION

Figure 3.4.1. General mechanism for the Sonogashira cross-coupling reaction catalyzed by

Pd/Cu catalytic system.[173]

Figure 3.4.2. General mechanism for the Sonogashira cross-coupling reaction catalyzed by

bis(silylene) nickel(II) complex I-64 with CuI as cocatalyst.[101]

On the contrary, previously our group demonstrated a different pathway for a nickel(II)

in combination with CuI catalyzed cross-coupling of 1-octyne and (E)-1-iodo-1-octene (Figure

3.4.2.).[101] In this mechanism, the formation of a bimetallic Ni/Cu complex I-65 stabilized by

bis(silylene) was observed upon the transmetallation reaction of bis(silylene)-Ni(II) complex

RESULTS AND DISCUSSION

I-64 with copper(I)-phenylacetylide. Subsequently, I-65 further undergoes oxidative addition

of (E)-1-iodo-1octene, followed by reductive elimination to furnish the final cross-coupling

product. These findings shed light on the crucial role of CuI in the catalytic Sonogashira cross-

coupling reaction. However, the catalytic efficiency of this particular anionic bis(silylene) based

Ni/Cu system was moderate. This prompted us to employ the neutral tridentate bis(silylene)

ligand I-35 to synthesize suitable Ni complexes that would potentially catalyze the

Sonogashira cross-coupling reaction more efficiently. In this section the synthesis,

characterization and catalytic activity of bis(silylene)pyridine stabilized Ni complexes is

discussed. Furthermore, to get more insights into the role of cocatalyst CuI in the reaction

mechanism, stoichiometric transmetallation experiments with copper(I)-phenylacetylide are

also examined.

3.4.2. Synthesis and characterization of bis(silylene) nickel(II) complexes

The reaction of tridentate bis(silylene) I-35 with [NiBr2(dme)] (dme = 1,2-

dimethoxyethane) in an equimolar ratio in THF at ambient temperatures resulted in an

immediately formation of a brick-red suspension (Scheme 3.4.2.). Due to suspected ionic

nature of 21, it is partially soluble in THF and poorly soluble in diethyl ether and toluene. The

desired complex 21 was isolated as bright orange powder in 86% yields. The 1H NMR

spectrum of 21 in CD2Cl2 showed the resonance for tert-butyl protons as singlet at δ = 1.24

ppm indicative of C2v symmetry in the molecular structure (Figure 3.4.3.). The 29Si{1H} NMR

signal for 21 observed at δ = 13.6 ppm is downfield shifted compared to the metal-free

bis(silylene) ligand (δ = –14.9 ppm for the symmetric conformer and –13.8 and –17.1 ppm for

the asymmetric conformer).[27]

Scheme 3.4.2. Synthesis of the bis(silylene)pyridine Ni(II) complex {3-[SiNSi]NiBr}Br (21),

starting from I-35.

RESULTS AND DISCUSSION

Figure 3.4.3. 1H NMR spectrum of the bis(silylene)pyridine dibromido Ni(II) complex 21 in

CD2Cl2 at 500 MHz.

Figure 3.4.4. Molecular structure of 21 with thermal ellipsoids drawn at 50% probability.

Hydrogen atoms and bromide counterion are omitted for clarity. Selected bond lengths (Å)

and angles (°): Ni1−Si1 2.1957(6), Ni1−Si2 2.1964(6), Ni1−Br1 2.2974(4), Ni1−N1 1.9345(17);

Si1−Ni1−Si2 170.33(3), N1−Ni1−Br1 179.54(5), N1−Ni1−Si1 85.08(5), N1−Ni1−Si2 85.29(5),

Si1−Ni1−Br2 94.749(19), Si2−Ni1−Br1 94.87(2).

The orange-colored, needle-like crystals suitable for single-crystal XRD structural

analysis were obtained by cooling concentrated THF solutions of 21 overnight at –20 °C. The

RESULTS AND DISCUSSION

molecular structure of the latter revealed the presence of Ni(II) center in a square-planar

geometry where both the Si(II) centers are trans to each other (Si1−Ni1−Si2 = 170.33(3)°)

along with a non-coordinating bromide counterion (Figure 3.4.4.). This geometric situation is

typical for the known pincer-type Ni(II) complexes stabilized by strong -donor ligands. The

crystal structure of 21 features a Ni1−Br1 distance of 2.2974(4) Å, which is significantly shorter

than the related bis(silylene) Ni(II) pincer complex I-64 (2.3410(5) Å) (Figure 3.4.4.).[101]

However, the Ni−Si bond distances of 2.1957(6) Å and 2.1964(6) Å in 21 compare well with

the related bis(silylene) stabilized dibromido nickel(II) complex I-64 (2.1737(7) and 2.1716(7)

Å) but are relatively longer than those observed for carborane bis(silylene) stabilized Ni(II)

complex I-66a (2.1378(5) Å and 2.1447(5) Å).[81,101]

Scheme 3.4.3. Reactivity of {3-[SiNSi]NiBr}Br (21) towards donor ligands (L) such as PMe3

and 2,6-dimethylphenyl isocyanide (XylylNC) respectively.

In order to further examine the influence of additional donor ligands on the catalytic

activity of Ni(II) complex 21, I was prompted to study its coordination behaviour towards

ligands (L) such as trimethylphosphine (PMe3) and 2,6-dimethylphenylisocyanide (XylylNC).

The equimolar reaction of 21 with PMe3 proceeded rapidly to afford bright-orange suspension

(Scheme 3.4.3.). The desired complex 22 could be isolated as bright orange powder in 89%

yields. The 1H NMR spectrum of the latter in CD2Cl2 shows the resonances for tert-butyl and

methyl protons (PMe3) at δ = 1.29 and 1.94 ppm as singlet and doublet (2JP-H = 10.1 Hz),

respectively. The 29Si{1H} NMR signal for 22 appeared as doublet at δ = 40.9 ppm (2JSi-P =

74.3 Hz) is shifted strongly downfield compared to its precursor 21 (δ = 13.6 ppm).

Similarly, the reaction of 21 with 2,6-dimethylphenyl isocyanide in an equimolar ratio

in THF immediately resulted in a green-yellow suspension at ambient temperatures (Scheme

3.4.3.). The desired complex 23 was isolated as yellow powder in 86% yields. The NMR signal

for 2,6-CH3 protons of coordinated xylyl isocyanide ligand appeared at δ = 2.52 ppm as singlet

in the 1H NMR spectrum. The 1H NMR resonance signal for tert-butyl protons appeared as

RESULTS AND DISCUSSION

two singlets at δ = 1.22 and 1.33 ppm. Similar to 22, the 29Si{1H} NMR signal for 23 was

observed as singlet at δ = 39.9 ppm, which is also strongly downfield shifted compared to its

precursor 21 (δ = 13.6 ppm). Furthermore, the obtained FT-IR spectrum of the latter exhibited

a strong C≡N stretching vibration band at a lower wave number 2082 cm−1, which is consistent

with the coordinated isocyanide ligand.

Figure 3.4.5. Molecular structures of 22 (top) and 23 (bottom) with thermal ellipsoids drawn

at 50% probability. Hydrogen atoms, counter ion and solvent molecules are omitted for clarity.

Selected bond lengths (Å) and angles (°) for 22 a): Ni1−Si1 2.2181(7), Ni1−Si2 2.2179(7),

Ni1−Br1 2.5871(5), Ni1−N1 2.0082(19), Ni1−P1 2.1465(7); Si1−Ni1−Si2 159.40(3),

N1−Ni1−P1 165.75(6), N1−Ni1−Br1 97.80(6), N1−Ni1−Si1 82.95(6), N1−Ni1−Si2 83.40(6),

P1−Ni1−Si2 95.82(3), P1−Ni1−Si1 93.65(3); Selected bond lengths (Å) and angles (°) for 23

b): Ni1−Si1 2.172(3), Ni1−Si2 2.172(3), Ni1−Br1 2.544(2), Ni1−N1 1.982(8), Ni1−C1

1.775(12), N2−C1 1.172(16), Si1−Ni1−Si2 160.11(16), C1−Ni1−N1 159.3(5), C1−Ni1−Si1

91.9(4), C1−Ni1−Si2 93.9(4), N1−Ni1−Si1 83.6(3), N1−Ni1−Si2 84.3(3), C1−Ni1−Br1

107.6(4), N1−Ni1−Br1 93.1(3).

RESULTS AND DISCUSSION

Table 3.4.1. Selected bond distances for bis(silylene)-stabilized nickel(II) complexes 21, 22

and 23.

Bond distances/ Å

Br(1)−Ni(1)

2.2974(4)

2.5806(15)

2.544(2)

Ni(1)−N(1)

1.9345(17)

1.996(6)

1.982(8)

Ni(1)−Si(1)

2.1957(6)

2.223(2)

2.172(3)

Ni(1)−Si(2)

2.1964(6)

2.227(2)

2.189(3)

The crystals of 22 and 23 suitable for single-crystal XRD structural analysis were grown

from cooling their respective concentrated THF solutions at –20 °C overnight. The molecular

structure of both 22 and 23 revealed a five-coordinate Ni(II) center in an almost square-

pyramidal geometry along with a non-coordinating bromide counterion (Figure 3.4.6.).

Therefore, complex 22 and 23 can be best described as {3-[SiNSi]Ni(L)Br}Br; with, L = PMe3

and XylylNC. The Addison-Reedijk parameter (5) for 22 was found to be 0.1, suggesting about

10% deviation from the perfect square pyramidal geometry.[140,141] As depicted in Table 3.4.1.,

the structural features of 22 and 23 resemble each other except the slightly longer Ni−Si bond

distances in 22 in comparison to 23. Due to a stronger -donor and relatively stronger -

acceptor nature of the isocyanide ligand in the case of 23 compared to 22 with PMe3, a

somewhat strengthening of the Ni−Si and Ni−Br bonds is observed in 23. This essentially gets

reflected by the shortened Ni−Si and Ni−Br distances in 23 (Table 3.4.1.).The observed

Ni1−C1 bond distance of 1.172(16) Å for the isocyanide moiety in 23 compares well with the

related Ni(II) complex [NiI2(PPh3)(XylylNC)] (1.806(13) Å) reported by Yamamoto et al..[176]

3.4.3. Synthesis and characterization of bis(silylene) nickel(0) complexes

With the aim in mind to isolate the corresponding bis(silylene)pyridine Ni(0) complexes

from abovementioned bis(silylene) stabilized dibromide Ni(II) complexes 21, 22 and 23,

reductive debromination reactions using the reducing agent KC8 were performed. The

treatment of 21 and 22 with KC8 in the absence of any supporting ligand, resulted only in

unidentified product mixtures, from which no species could be isolated. However, the reaction

of 23 with KC8 resulted in the formation of clear deep-red solutions, from which dark-red

crystals of 24 could be isolated in 78% yields (Scheme 3.4.4.). Similar results were obtained

when the reduction was performed using 21 as a precursor with KC8 in the presence of xylyl

RESULTS AND DISCUSSION

isocyanide ligand. Furthermore, the FT-IR spectrum of the isolated Ni(0) complex 24 features

a strongly red-shifted C≡N stretching vibration at 1897 and 1863 cm−1 compared to 23. The

splitting of IR stretching band further indicates a decrease in the symmetry on going from

23→24. The 1H NMR spectrum of 24 in C6D6 displays resonance for 2,6-methyl protons at δ

= 2.18 ppm. The 29Si{1H} NMR of the latter exhibited a downfield signal at δ = 46.4 ppm

compared to the corresponding Ni(II) complex 23 (δ = 39.9 ppm).

Scheme 3.4.4. Synthesis of bis(silylene)pyridine stabilized Ni(0) complex 24.

Figure 3.4.6. Molecular structures of 24 (a) front view, (b) side view with thermal ellipsoids

drawn at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)

and angles (°): Ni1−Si1 2.1803(7), Ni1−Si2 2.1730(7), Ni1−N1 2.0383(18), Ni1−C1 1.754(2),

N2−C1 1.206(3); Si1−Ni1−Si2 138.04(3), C1−Ni1−N1 124.56(9), C1−Ni1−Si1 111.06(8),

C1−Ni1−Si2 109.48(8), N1−Ni1−Si1 82.93(5), N1−Ni1−Si2 83.04(5).

The structural analysis by single-crystal XRD revealed the presence of four-coordinate

nickel(0) center in a distorted tetrahedral geometry (Figure 3.4.6.). The molecular structure of

24 features a Si1−Ni1−Si2 bond angle 138.04(3)°, which is significantly shifted from the

RESULTS AND DISCUSSION

precursor 23 (160.11(16)°). These observations further corroborate the importance of the

hemilability of this particular pincer bis(silylene) ligand I-35 in realizing a tetrahedral

coordinated Ni(0) complex. As a consequence of change in the oxidation state from

Ni(II)→Ni(0), an increase in the electron density on Ni centers occurs, causing more -

backdonation. Because of this a pronounced variation in the Ni−Si and Ni1−C1 distances is

observed in comparison to its Ni(II) precursor complex 23. A similar impact is also indicated

by the slight elongation of N2−C1 1.206(3) compared to its precursor Ni(II) complex 23.

3.4.4. Synthesis and characterization of bis(phosphine) nickel(0) complexes

In order to make a comparison with the widely employed phosphine-based pincer

complexes, the synthesis of sterically and electronically similar bis(phosphine)pyridine

stabilized nickel complexes was envisioned. The reaction of analogous bis(phosphine) 13 with

NiBr2 at elevated temperatures indeed furnished the bis(phosphine) dibromido Ni(II) complex

25 which is structurally related to 21 (Scheme 3.4.5.). Complex 25 was isolated as dark-yellow

powder in 78% yields.

Scheme 3.4.5. Synthesis of the bis(phosphine)pyridine Ni(II) (25) and Ni(0) (26) complexes,

starting from 13.

The 1H NMR spectrum of the latter in CD2Cl2 exhibits a multiplet resonance signal for

the iso-propyl groups (CH(CH3)2) at δ = 1.42 ppm and a triplet signal for the N-ethyl (NCH2CH3)

RESULTS AND DISCUSSION

at δ = 1.32 ppm. The distinct resonance signal for 3,5-aromatic protons on the pyridine

backbone appeared as a doublet at δ = 6.40 ppm in the 1H NMR spectrum. The coordination

of [PNP] ligand 13 to the Ni(II) center in 25 in solution was confirmed by the 31P{1H} NMR

spectrum featuring a singlet a δ = 104.5 ppm, slightly shifted than that of the free [PNP] ligand

13 (δ = 99.8 ppm).

A single-crystal XRD analysis of yellow crystals of 25 grown from the concentrated

THF solutions at room temperature further proved the structure as {3-[PNP]NiBr}Br (Figure

3.4.7.a). Similar to the bis(silylene)-Ni(II) complex 21, the molecular structure of 25 displays

the four-coordinate Ni(II) center in a square planar geometry, with the PNP ligand coordinating

in a meridional configuration and thereby establishing P1−Ni1−P2 = 176.60(4)° and

N1−Ni1−Br1 = 178.44(8)°. The Ni−P bond distances of 2.1464(8) Å and 2.1475(9) Å in

complex 25 are significantly shorter than related [PNP]-based Ni(II) complex A (2.2394(6) and

2.2413(6) Å) and B (2.1880(6) and 2.1924(6) Å) (see Figure 3.4.8.).[177] However, these Ni−P

bond distances are in good agreement with complex C (2.136(2) and 2.151(2) Å) reported by

Driess et al..[101]

Figure 3.4.8. Selected known examples of bis(phosphine) stabilized Ni(II) pincer complexes.

Complex 25 was then subjected to reduction using KC8 in the presence of 2,6-

dimethylphenyl isocyanide to realize the bis(phosphine) stabilized Ni(0) compound 26,

analogous to 24 (Scheme 3.4.5.). The desired complex could be isolated as brown-red powder

in 69% yields. The 1H NMR spectrum of 25 in THF-d8 exhibits the doublet resonance signal

for the iPr groups (CH(CH3)2) at δ = 1.19, 1.12, 1.09 and 1.02 ppm and as triplet (3JHH= 6.7

Hz) signal for the N-ethyl (NCH2CH3) protons at δ = 0.77 ppm. The resonance signal for 2,6-

CH3 protons of coordinated isocyanide ligand appeared at δ = 2.20 ppm as singlet in the 1H

NMR spectrum. The 31P{1H} NMR spectrum of Ni(0) complex 26 featured a singlet a δ = 152.9

ppm, significantly shifted in comparison to the precursor 25 (δ = 104.5 ppm) . The FT-IR

spectrum of the isolated Ni(0) complex 26 features a C≡N stretching vibration shifted to lower

wavenumber at 1944 cm−1 characteristic for a coordinated isocyanide group. The enormous

blue-shift in stretching vibration compared to 24 can be unambiguously attributed to a higher

degree of d-p backdonation to isocyanide ligand in the analogous bis(silylene)Ni(0) complex

RESULTS AND DISCUSSION

24 (1897 and 1863 cm−1). This is in accordance with the fact that silylenes act as an

exceptional strong -donor and therefore promote more -backdonation.

Figure 3.4.7. Molecular structures of 25 (top) and 26 (bottom) (a) front view (b) side view with

thermal ellipsoids drawn at 50% probability. Hydrogen atoms, counter ion and solvent

molecules are omitted for clarity. Selected bond lengths (Å) and angles (°) for a) 25: Ni1−P1

2.1464(8), Ni1−P2 2.1475(9), Ni1−Br1 2.2859(6), Ni1−N1 1.880(2); P1−Ni1−P2 173.60(4),

N1−Ni1−Br1 178.44(8), N1−Ni1−P1 87.07(7), N1−Ni1−P2 86.82(7), P1−Ni1−Br1 92.44(3),

P2−Ni1−Br1 93.63(3); for b) 26: Ni1−P1 2.1391(4), Ni1−P2 2.1535(4), Ni1−C1 1.7697(16),

Ni1−N1 1.9880(12), N2−C1 1.191(2); P1−Ni1−P2 136.88(19), C1−Ni1−N1 125.44(6),

C1−Ni1−P1 108.28(5), C1−Ni1−P2 111.34(5), N1−Ni1−P1 85.11(4), N1−Ni1−P2 85.61(4).

The dark-red crystals suitable for an single-crystal XRD analysis were obtained from

cooling the concentrated n-hexane solutions of 26 at –20 °C overnight. Complex 26

crystallizes in a monoclinic crystal system with the P21/c space group. The structural analysis

RESULTS AND DISCUSSION

of complex 26 revealed the presence of a four-coordinate Ni(0) center in a distorted tetrahedral

geometry with a P1−Ni1−P2 = 136.88(19)° (Figure 3.4.7.). The observed Ni1−C1 bond

distance 1.7697(16) Å of metal-isocyanide bonding motif in 26 is comparable to the analogous

bis(silylene) Ni(0) complex 24 (1.754(2) Å).

3.4.5. Optimization of reaction conditions for Sonogashira cross-coupling

reaction

After the successful isolation of bis(silylene) nickel(II) complexes 21, 22 and 23,

catalytic evaluation studies were envisioned to test whether they can serve as an alternative

to the Pd catalyzed Sonogashira cross-coupling reaction. For optimization of reaction

condition the Sonogashira cross-coupling reaction of phenylacetylene (25a) and (E)-1-iodo-1-

octene (26a) was chosen as the model reaction (Table 3.4.2.). Firstly, the effect of various

bases such as LiOtBu, Li2CO3, NaOtBu, Na2CO3, K2CO3 and Cs2CO3 was examined for the

efficient catalytic cross-coupling reaction. Though, the highest conversion was obtained when

two molar equivalents of K2CO3 were utilized as a base for deprotonation of terminal acetylene

(Table 3.4.2., Entry 5). On varying the solvent from THF to slightly non-polar solvents such as

1,4-dioxane and toluene deteriorated the conversions (Table 3.4.2., Entry 7-8). Lowering the

temperature to room temperature negatively impacted the conversions (Table 3.4.2., Entry 9).

Therefore, K2CO3 was chosen as optimal base and THF as suitable solvent for the

Sonogashira cross-coupling reaction.

Table 3.4.2. Effect of various bases and solvents on the catalytic performance of 21 towards

Sonogashira cross-coupling reaction.

Entry

Base

Solvent

Conversion

LiOtBu,

THF

Li2CO3

THF

trace

NaOtBu

THF

RESULTS AND DISCUSSION

Na2CO3

THF

K2CO3

THF

Cs2CO3

THF

K2CO3

1,4-dioxane

K2CO3

toluene

K2CO3

THF

aAll reactions were performed in 0.1 mmol scale. Conversion was determined from 1H NMR

spectroscopy using mesitylene as internal standard. cThe reaction was performed at room temperature.

After the optimizing the reaction parameters such as choice of base, solvent,

temperature and catalyst loading for the efficient cross-coupling catalysis, I became interested

in comparing the role of different ligands such as bis(silylene) vs bis(phosphine) on the

catalytic activity of Ni(II) catalyst. The determined standard reaction conditions include 5 mol%

of catalyst, K2CO3 as base, and 5 mol% CuI in THF at 50 °C. Thereafter, the catalytic screening

was performed under standard conditions using different catalysts as shown in Table 3.4.3.

When bis(silylene)pyridine Ni(II) complex 21 (5 mol%) along with CuI (5 mol%) as cocatalyst

was employed for catalytic Sonogashira cross-coupling reaction, relatively higher conversion

to cross-coupled product (29a) was obtained compared to the bis(phosphine) counterpart 25

(Table 3.4.3., Entry 1 and 5). Interestingly, when the reaction was performed with catalytic

amounts of 22 and 23, the conversions were negatively impacted (Table 3.4.3., Entry 3 and

4). Furthermore, to rule out the sole activity of CuI, when performed with only Cu source such

as CuI or copper(I)-phenylacetylide (CuCCPh), only traces of product was observed (Table

3.4.3., Entry 6 and 7). Finally, the homogeneity of the employed catalytic system was further

confirmed by the observed unaffected catalytic conversion even in the presence of catalytic

amounts of mercury (as mercury is known to stabilize metal nanoparticles) (Table 3.4.3., Entry

8).

Table 3.4.3. Evaluation of various Ni based catalysts for the catalytic Sonogashira cross-

coupling reaction.

RESULTS AND DISCUSSION

Entry

Catalyst

Conversion (%)

{к3-[SiNSi]NiBr}Br (21)

1:1 [SiNSi]+NiBr2(dme)

{к3-[SiNSi]Ni(PMe3)Br}Br (22)

{к3-[SiNSi]Ni(XylylNC)Br}Br (23)

{к3-[PNP]NiBr}Br (25)

CuI

1:1 [SiNSi]+CuCCPh

{к3-[SiNSi]NiBr}Br (21)+Hg

aAll reactions were performed in 0.1 mmol scale. Conversion was determined from 1H NMR

spectroscopy using mesitylene as internal standard.

3.4.4. Substrate scope

With optimized reaction conditions in hand, I then expanded the substrate scope with

regards to: (i) variation of vinyl halide substrates and (ii) variation of terminal alkyne substrates

for the catalytic Sonogashira cross-coupling reaction. Scheme 3.4.6. represents the

compatibility of various electronically modified vinyl halide substrate with phenylacetylene

towards bis(silylene)stabilized Ni(II) 21 catalyzed cross-coupling reaction. The reaction of (Z)-

ethyl-3-iodoacrylate (26b) with phenylacetylene occurred smoothly to afford the cross-coupled

product 27b in 92% yield with the retention of stereochemistry. Moreover, the cross-coupling

reaction of trans-isomer E-ethyl-3-iodoacrylate, also resulted in 85% yield of cross-coupled

product 27c. Remarkably, cross-coupling of electron-rich vinyl halides such as (E)-1-(2-

Iodovinyl)benzene and (E)-1-(2-Bromovinyl)benzene were also realized in excellent yields

(Entry 3 and 4).

RESULTS AND DISCUSSION

aAll reactions were performed in 0.5 mmol scale. All the reaction were performed for 24h.

Scheme 3.4.6. Cross-coupling of various vinyl halides with phenylacetylene using {к3-

[SiNSi]NiBr}Br (21) as a catalyst.

We then investigated the cross-coupling of (Z)-ethyl-3-iodoacrylate with a variety of

alkynes with different steric and electronic properties, using Ni(II) complex 21 as catalyst

(Scheme 3.4.7). Interestingly, under standard conditions, the catalyst 21 demonstrated

excellent functional group tolerance towards a wide range of aryl acetylenes with a retention

in stereochemistry. Various electron-withdrawing substituents such as halogens (X = Br, Cl

and F) and electron-donating groups such as methyl, tert-butyl, at various positions of aryl

acetylenes were well tolerated. Also, when a para-amino substituted aryl acetylene was used

as a substrate, a decrease in the yield of cross-coupled product (28f) was observed.

RESULTS AND DISCUSSION

100

aAll reactions were performed in 0.5 mmol scale. All the reaction were performed for 24h.

Scheme 3.4.7. Cross-coupling of various acetylenes with (Z)-ethyl-3-iodoacrylate using {к3-

[SiNSi]NiBr}Br (21) as a catalyst.

However, the reaction of acetylene with highly electron-withdrawing group such as 4-

nitrophenylaceylene, resulted in somewhat mediocre yield. Overall, in some instances, the

yields of cross-coupled products were considerably decreased when aryl acetylenes with

electron-withdrawing substituents were utilized. Additionally, when alkyl acetylene such as 1-

octyne (25k) was employed for Sonogashira cross-coupling reaction, yield was slightly

lowered compared to phenylacetylene. Remarkably, the substrate with two terminal alkyne

25m, also demonstrated successful double cross-coupling reaction with two molar equivalents

of (Z)-ethyl-3-iodoacrylate to furnish 28m in 70% yield. Furthermore, in order to assess the

tolerance of catalyst 21 towards acetylene with heteroaromatic substitution, the cross-coupling

RESULTS AND DISCUSSION

101

reaction of 2-ethynylpyridine (25l) was performed, resulting in moderate yield of cross-coupled

product (28l).

3.4.4. Mechanistic investigation for the catalytic Sonogashira cross-coupling

reaction

To understand the crucial role of CuI cocatalyst in the efficient Sonogashira cross-

coupling reaction, the investigation of elementary transmetallation step is of paramount

importance. Keeping this in mind, Cu(I)-phenylacetylide CuCCPh, was synthesized as bright

yellow compound using literature protocol.[178,179] Due to polymeric structure of copper

acetylides, their solubility in general arene solvents such as benzene and toluene is very

poor.[180]

Scheme 3.4.8. Synthesis of Cu(I)-phenylacetylide [CuCCPh]4 29 via literature

procedure.[178,179]

Consequently, the reaction of {к3-[SiNSi]NiBr}Br (21) in a J. Young NMR tube with four

molar equivalents of Cu(I)-phenylacetylide 29 in THF-d8 was monitored using multinuclear

NMR spectroscopy. The use of lower molar equivalents of 29 resulted in incomplete

conversions. The 1H NMR analysis of transmetallation reaction (21→31) showed the

quantitative conversion to a new species 31 with CuBr coming off as precipitate (Scheme

3.4.9.). Moreover, the 29Si{1H} NMR spectrum of 31 displayed two distinct resonance signals

at δ = 52.5 and  25.0 ppm, suggesting the unsymmetrical nature of formed molecular entity.

The considerably downfield signal in the 29Si{1H} NMR spectrum at δ = 52.5 ppm compared to

its precursor 21 (δ = 13.6 ppm) could be assigned to the coordinated silylene donor arm.

However, the upfield signal at  25.0 ppm somewhat indicates the Si(II)→Si(IV) oxidation state

change in the formed product. The 1H NMR spectrum also featured two sets of signals for tert-

butyl groups, N-methyl group and 3,5-protons at pyridine ring, consistent with the 29Si{1H} NMR

spectrum. The scaling up of the reaction also resulted in the similar outcome as confirmed by

the NMR spectroscopy. However, keeping the resultant residue for more than 24h, resulted in

the decomposition of formed product as indicated by the NMR measurements and the

subsequent deposition of metallic lustre on the walls of reaction flask was observed. This

instability of the formed reactive species hindered its isolation and structural characterization

by other spectroscopic methods such as single-crystal XRD analysis and HRMS.

RESULTS AND DISCUSSION

102

Scheme 3.4.9. Synthesis of 30 by salt-metathesis of 21 with lithium phenylacetylide and the

reactivity study of 21 and 30 with Cu(I)-phenylacetylide and CuBr respectively.

Figure 3.4.10. Molecular structures of 30 with thermal ellipsoids drawn at 50% probability.

Hydrogen and solvent atoms are omitted for clarity. Selected bond lengths (Å) and angles (°):

Ni1−C1 1.9598(15), Ni1−C2 1.8515(16), Ni1−Si2 2.1223(5), Ni1−N1 1.9640(13), C2−C1

1.289(2), C10−C9 1.210(2); C2−Ni1−C1 39.42(7), C2−Ni1−Si2 117.14(5), C1−Ni1−N1

116.54(6), C1−Ni1−Si2 153.87(5), N1−Ni1−Si2 86.75(4), C2−C1−Si1 161.20(14),

Si1−C9−C10 172.44(14), C1−C2−C3 141.44(15).

RESULTS AND DISCUSSION

103

Figure 3.4.11. 1H NMR(500 MHz, THF-d8) spectrum of reactivity study of 21 and 30 with Cu(I)-

phenylacetylide (top) and CuBr(bottom) respectively.

These outcomes prompted me to consider a more systematic approach in a stepwise

reaction manner wherein, firstly, the reaction of bis(silylene)Ni(II) complex 21 with lithium

acetylides then consecutive reaction with CuI was envisioned. Thus, with the anticipation of

realizing desired Ni(II) acetylide, the reaction of 21 with two molar equivalents of lithium

phenylacetylide (LiCCPh) was conducted at –78 °C in Et2O, affording a red colored solution

with rapid precipitation of lithium bromide (Scheme 3.4.9.; 21→30). Further, filtrations and

keeping the concentrated Et2O solutions at –20 °C afforded a large crop of dark-red crystals

of 30 in 72% yields. The 1H NMR analysis of complex 30 showed the resonances for tert-butyl

group as two singlets at δ = 0.97 and 1.50 ppm indicative of the removal of C2v symmetry as

present in the precursor 21. Additionally, 29Si{1H} NMR spectrum of the latter featured two

singlet signals at δ = 44.5 and  60.8 ppm respectively, further substantiating the speculation

of unsymmetric molecular structure of 30. Thereafter, the 30 was allowed to react with an

excess of CuBr in a J. Young NMR tube in THF-d8 affording the identical species as 31, as

confirmed by the multinuclear NMR spectroscopy (Scheme 3.4.9.; Figure 3.4.11.). This

particular conversion 30→31, provided key insights into the speculated molecular structure of

bimetallic Ni(0)Cu(I) complex 31.

RESULTS AND DISCUSSION

104

Figure 3.4.12. Known examples of Ni(0) complexes with η2-coordinated phenylacetylene

ligand.

A single-crystal XRD structural analysis of 30 rather revealed the presence of a Ni(0)

complex, η2-coordinated by an in-situ generated silyl-substituted phenylacetylene ligand,

instead of a Ni(II)-phenylacetylide complex (Figure 3.4.10.). The molecular structure of 30

demonstrates that one of the divalent silicon atoms has indeed oxidized to a tetrahedral Si(IV)

center with the addition of two phenylacetylene substituents, consistent with the 29Si{1H} NMR

spectrum. Owing to the -coordination, one of the acetylene units shows considerable

deviation from alkyne linearity (180°) and thus exhibits a bent geometry with C2−C1−Si1 =

161.20(14)° and C1−C2−C3 = 141.44(15)°. However, the non-coordinated one shows small

deviation at the alkynyl carbons Si1−C9−C10 = 172.44(14)°. Likewise, the CC bond

distance (1.289(2) Å) for C1−C2 also shown to be longer compared to the free

silylphenylacetylene unit (C10−C9 = 1.210(2) Å) in complex 30. Nonetheless, Ni−C bond

distances are slightly longer compared to that reported for D (1.890(3) and 1.850(3) Å) and E

(1.877(2) and 1881(2) Å) (Figure 3.4.12.).[181,182] The Ni1−Si2 bond distance 2.1223(5) is

shortened compared to the previously isolated bis(silylene) stabilized Ni(0) complex 24.

The formation of 30 could, however, be explained on the basis of mechanism displayed

in Scheme 3.4.13. I speculate that bis(silylene) dibromido Ni(II) complex 21, on salt-elimination

reaction with two molar equivalents of lithium phenylacetylide forms the intermediate

(silyl)Ni(acetylide) 30’. The isolated Ni(0) species probably results from the thermodynamic

instability of proposed intermediate (silyl)Ni(acetylide) complex 30’. The presumably facile

reductive elimination step at nickel(II) center in the latter affords the newly formed

(silyl)phenylacetylene, which remains connected to the pincer backbone. The presence of

acetylene bonding motif in the pincer backbone further drives the realization of -stabilized

Ni(0) complex 30 feasible. Zargarian’s group also observed a similar rearrangement for pincer-

type bis(phosphine)-Ni(acetylide) complex (F), wherein the phenylacetylide transforms into a

phosphinoalkyne ligand to form -stabilized Ni(0) complex F1 (and the cis-isomer F2) (Figure

3.4.14.).[183] However, unlike my system, the formed phosphinoalkyne ligand gets detached

from the pincer-backbone.

RESULTS AND DISCUSSION

105

Figure 3.4.13. Plausible reaction pathway for the salt-metathesis reaction of 21 with LiCCPh

to form complex 30.

Figure 3.4.14. Decomposition pathway for the nickel acetylide complex F to form -stabilized

Ni(0) complexes F1 and F2.

After the careful structural characterization of 30 and 31, I was curious to also test their

suitability for the Sonogashira cross-coupling reaction of phenylacetylene and (Z)-ethyl-3-

iodoacrylate under standard conditions. However, the insufficient stability of 31 hampered its

isolation beyond NMR scale, and therefore could not be applied as a catalyst for the cross-

coupling reaction. Remarkably, when 30 was employed as catalyst, the obtained yield of the

cross-coupled product (86%) was comparable to that of the precursor Ni(II) complex 21 (92%)

(Scheme 3.4.10.). This control experiment further suggested that Ni(0) complex 30 to be a

catalytically active species, generated during the catalytic Sonogashira cross-coupling

reaction using catalyst 21.

RESULTS AND DISCUSSION

106

Scheme 3.4.10. Sonogashira cross-coupling reaction catalyzed by the -stabilized Ni(0)

complex 30.

Figure 3.4.15. Proposed reaction pathway for the catalytic Sonogashira cross-coupling

reaction of phenylacetylene with vinyl iodides catalyzed by Ni(II) complex 21 and CuI using

K2CO3 as a base.

Based on the abovementioned stoichiometric reactions and control experiments, we

proposed a plausible reaction mechanism for the Sonogashira cross-coupling reaction of vinyl

RESULTS AND DISCUSSION

107

iodides with terminal acetylenes catalyzed by the bis(silylene)pyridine Ni(II) complex 21

(Figure 3.4.15.). Firstly, the precatalyst 30 reacts with two molar equivalents of Cu(I)-

phenylacetylide (CuCCPh) generated from the reaction of phenylacetylene with CuI in the

presence of K2CO3 (base) to furnish the reactive Ni(0) complex. The Ni(0) complex on

coordination with CuI generates a bimetallic Ni-Cu intermediate which, due to its unstable

nature, remains in equilibrium with the relatively stable Ni(0) species 30. The latter species,

undergoes oxidative addition in the presence of vinyl iodide to produce the Ni(II) intermediate

I. Thereafter, the latter intermediate undergoes transmetallation with Cu(I)-phenylacetylide

generated from the Cu-cycle, to produce the desired cross-coupled product. Moreover, the

latter step also includes the regeneration of Ni(0) complex 30 which is still in equilibrium with

the bimetallic Ni-Cu intermediate and proceeds to undergoes next catalytic cycles to realize

the cross-coupled product. The particular instability of the intermediate species 31, was also

observed previously by our group for the pincer-type [SiCSi] based bimetallic Ni/Cu

complex.[101] However, in this case the Ni/Cu complex was stable enough for a structural

characterization by single-crystal XRD analysis, whereas, the present [SiNSi] system only

partially stabilizes the bimetallic motif and therefore, only be characterized through

multinuclear NMR spectroscopy.

SUMMARY

108

SUMMARY

109

4. SUMMARY

This dissertation focused on the coordination chemistry of multidentate bis(silylene)

pyridine ligands towards first-row transition-metals such as manganese and nickel. The

bis(silylene) I-35 has proven to be a suitable ligand for realizing the first N-heterocyclic

silylene-stabilized, highly reactive 17 VE manganese(0) compounds. The bis(silylene) I-35

transition-metal complexes were able to efficiently catalyze organic transformations such as

regioselective hydroboration of N-heteroarenes and Sonogashira cross-coupling reaction of

terminal acetylenes and vinyl iodides.

Scheme 4.1. Synthetic routes to isolate bis(silylene)pyridine I-35 stabilized Mn(II) and Mn(0)

complexes.

The chapter 3.1. describes the synthesis and characterization of various bis(silylene)

Mn(II) complexes 1 and 2, which were employed as a precursor for the isolation of reactive

manganese(0) complexes (Scheme 4.1.). However, the successful isolation of desired

genuine, carbonyl-free, bis(silylene) Mn(0) compound was achieved only when reductive

dehalogenation of 1 and 2 was conducted in the presence of 1,2-

bis(dimethylphosphino)ethane (dmpe) using KC8 (Scheme 4.1.). The sequence of adding the

reagents had huge impact on the isolation of desired Mn(0) complex. An single-crystal XRD

structural analysis of 7 revealed the pseudo-square pyramidal coordination geometry at the

Mn(0) center, which resembles with that predicted for the reactive [Mn(CO)5] metalloradical

(Figure 4.1. left). The spectroscopic investigation of 7 by EPR spectroscopy and SQUID

magnetometry verified that the bis(silylene)pyridine I-35 favours a five-coordinate d7 Mn(0)

species in S =1/2 spin environment. The DFT calculations also demonstrated that the unpaired

electron density is largely distributed over the Mn(0) center, consistent with the magnetic

measurements.

Chapter 3.2. focused on the reactivity and catalytic activity of isolated Mn(0)

compounds. On exposure to a CO atmosphere, bis(silylene) Mn(0) complex 7 transforms into

an unprecedented bis(silylene) tricarbonyl Mn(0) complex 8 (Scheme 4.2.). The structural

characterization by single-crystal XRD analysis proved the structure as 2-[SiNSi]Mn(CO)3

SUMMARY

110

(Figure 4.1.right). The ESI-MS and FT-IR spectra confirmed the incorporation of three carbonyl

ligands as indicated by the solid-state structure of 8. The electronic structure characterization

by the EPR spectroscopy, SQUID and DFT calculations suggested that spin electron density

mostly resides on the Mn(0) atom. The reaction of 7 with different -acceptor ligand, xylyl

isocyanide, led to the isolation of 9 as a green powder (Scheme 4.2.). The composition of 9

was determined by means of FT-IR, APCI-MS and elemental analysis. The EPR and SQUID

measurements confirmed the presence of a 17 VE d7 Mn(0) species with S =1/2 spin state.

Scheme 4.2. Reactivity of the bis(silylene) Mn(0) compound 3-[SiNSi]Mn(dmpe) 7 towards

CO and 2,6-dimethylphenylisocyanide.

Figure 4.1. Molecular structures of the first bis(silylene) manganese(0) compounds 7 (left)

and 8 (right), respectively.

The isolated Mn(0) compounds 7, 8, and 9 were applied as catalysts for the

regioselective hydroboration of quinoline using HBpin. The Mn(0) complex,

SUMMARY

111

3-[SiNSi]Mn(dmpe) (7), shows excellent catalytic performance at 50 °C with high

regioselectivity for 1,2-hydroborated products (Scheme 4.3.). Moreover, Mn(0) complex 7 also

exhibits high functional group tolerance as it successfully catalyzes different N-heteroarenes

bearing, with moderate to high product conversions. The deuterium labelling experiments and

KIE measurements suggested the reaction of HBpin with 7, as rate-determining step in the

catalytic cycle.

Scheme 4.3. Catalytic activity of the Mn(0) complex 3-[SiNSi]Mn(dmpe) 7 towards

regioselective hydroboration of N-heteroarenes using HBpin.

The next part of this work was aimed at examining the difference between the

potentially tridentate bis(silylene)pyridine [SiNSi] I-35 and bis(phosphine)pyridine [PNP] 13

with respect to their coordination ability towards alkyl manganese(II) complexes. The

bis(phosphine)pyridine 13 was synthesized, characterized, and utilized for the synthesis of

Mn(II) complex 3-[PNP]MnCl2 14 on reaction with MnCl2, analogous to its bis(silylene)

analogue species 2-[SiNSi]MnCl2 (1). Interestingly, the [PNP] ligand shows tridentate

coordination in 14, while the [SiNSi] exhibited two-fold coordination in 1. Complex 14 was then

subjected to the alkylation with LiCH2SiMe3, affording the anticipated dialkyl Mn(II) complex

15 (Figure 4.2.). Whereas, the same alkylation of the bis(silylene) Mn(II) complex 1 furnished

a unique dinuclear silylene(alkyl)(pyridylamido) Mn(II) complex 16, by the elimination of one

silylene unit from the pincer backbone (Figure 4.2.). This particular distinction of reactivity

pattern between both ligands could be explained by the strong -donor properties of N-

heterocyclic silylenes in comparison to the related phosphines and N-heterocyclic carbenes.

SUMMARY

112

Figure 4.2. Synthesis and molecular structures of alkyl manganese(II) compounds 15 and 16

through salt-metathesis reaction of their corresponding Mn(II) compounds (14 and 1) with

LiCH2SiMe3 respectively. (a) Molecular structure of 15, (b) Molecular structure of 16.

Scheme 4.4. The reaction of bis(silylene)pyridine I-35 with LiCH2SiMe3.

Inspired by the unusual result, I studied the reaction of the metal-free bis(silylene)

ligand [SiNSi] I-35, with alkyl lithium LiCH2SiMe3, giving trimeric lithium amide 20 along with a

new product (trimethylsilyl)methyl(silylene) 17 (Scheme 4.5.). The lithium amide 20 was

characterized by 1H, 29Si{1H}, 7Li NMR spectroscopy and single-crystal XRD analysis.

SUMMARY

113

Additionally, the detection of 17 by multinuclear NMR analysis further validated the speculation

regarding the removal of this particular silylene from the pincer-backbone during the alkylation

of 2-[SiNSi]MnCl2 complex 1 (Scheme 4.4.).

Chapter 3.4. describes the catalytic application of bis(silylene)pyridine-based Ni(II)

complexes in catalyzing the Sonogashira cross-coupling reaction of terminal acetylenes with

vinyl idides in the presence of catalytic amounts of CuI (5 mol%). The Ni(II) complexes 21 and

25 were prepared by the complexation reactions of the corresponding bis(silylene) and

bis(phosphine) ligands with NiBr2(dme) and NiBr2, respectively (Scheme 4.5.). They were

characterized by multinuclear NMR, ESI-MS, IR and single-crystal XRD analyses. In both

complexes, the Ni(II) center adopts square planar coordination geometry where only one of

the bromide ions remains in the coordination sphere of Ni.

The subsequent reduction of these Ni(II) complexes in the presence of xylyl isocyanide

afforded corresponding Ni(0) complexes 24 and 26 respectively. The exceptional -donor

strength of I-35, reflected well in the IR stretching vibration modes of coordinated isocyanide

ligands in 24 in comparison to 26 (Scheme 4.5.).

Scheme 4.5. Synthetic routes to isolate the bis(silylene) and bis(phosphine) Ni(II) complexes

21 and 25, and Ni(0) complexes 24 and 26, respectively.

SUMMARY

114

Figure 4.3. Molecular structures of the nickel(II) pincer complex cations 21 (left) and 25 (right)

respectively.

Subsequently, catalytic screening experiments for Sonogashira cross-coupling

reaction indicated a superior catalytic performance of 21, in comparison to the bis(phosphine)

counterpart 25. The substrate scope was successfully explored for a variety of electronically

modified acetylenes and vinyl halides with high yields. The Ni(II) catalyst 21 demonstrated

high functional tolerance and efficient catalytic conversions (Scheme 4.6.).

Scheme 4.6. Catalytic activity of Ni(II) compound {3-[SiNSi]NiBr}Br 21 towards Sonogashira

cross-coupling reaction of vinyl iodides with acetylenes using CuI as cocatalyst.

Some mechanistic details were obtained by stoichiometric transmetallation

experiments involving the reaction of 21 with Cu(I)-phenylacetylide (CuCCPh) to afford 31,

which was structurally characterized by multinuclear NMR spectroscopy. Remarkably, the

stepwise reaction of 21 with lithium phenylacetylide (CuCCPh) yielded the -Ni(0) complex

30, which on treatment with CuI formed complex 31. Complex 30 was characterized by

multinuclear NMR spectroscopy, IR and single-crystal XRD analyses. Thus, based on these

findings, we were able to propose a plausible mechanism for the catalytic Sonogashira cross-

coupling reaction using 21 as a catalyst (Figure 4.4.).

SUMMARY

115

Figure 4.4. Proposed reaction pathway for the Sonogashira cross-coupling reaction catalyzed

by {3-[SiNSi]NiBr}Br 21 and CuI.

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5. EXPERIMENTAL SECTION

5.1. General considerations

All experiments were performed under dry oxygen-free nitrogen using standard

Schlenk technique or in a MBraun glovebox with an atmosphere of purified nitrogen in flame-

dried glassware. All the solvents were purified using conventional procedures and freshly

distilled under N2 atmosphere prior to use. Solvents (hexane, tetrahydrofuran, diethyl ether,

and toluene) were dried and deoxygenated in a solvent purification system (MBraun), with a

subsequent treatment with the sodium-benzophenone mixture under reflux conditions, and

stored under molecular sieves (3-4 Å) prior use. THF-d8 and C6D6 were dried with the sodium-

benzophenone mixture and stored under molecular sieves (3-4 Å) prior use. Dichloromethane,

CD2Cl2, chloroform and chloroform-d were dried by stirring over CaH2 at ambient temperature.

The glassware used in all manipulations was dried at 150 °C prior to use, cooled to ambient

temperature under high vacuum, and flushed with N2. The handling of solid samples and the

preparation of samples for spectroscopic measurements were carried out inside a glove-box,

where the O2 and H2O levels were normally kept below 1 ppm.

The solvents or solutions were transferred via stainless steel cannulas, which were stored in

the oven at 120 °C. The transfer was enabled by the vessel of origin being left under a positive

pressure of inert gas, and the receptacle vessel of closed with a pressure release value.

Filtrations were carried out using stainless steel cannula containing a filter head at one end.

Whatman (GF/B 25) filters were affixed to the filter end of the cannula with Teflon tape. After

use, cannulas were cleaned immediately by thorough rinsing with acetone, followed by dilute

HCl, water, and acetone.

For low-temperature reactions, Dewar vessels were filled with acetone/dry-ice mixture until

reach the desired temperature.

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5.2 Analytical methods

Nuclear Magnetic Resonance (NMR) Spectroscopy: NMR spectra were recorded on a

Bruker AV 400 or 500 Spectrometer. The 1H and 13C{1H} NMR spectra were referenced to the

residual solvent signals as internal standards. The spectra were referenced using the

standards H and 13C chemical shifts are reported in parts per million (ppm) referenced using

residual solvent as the internal standard (1H NMR: C6D6 δ = 7.16 ppm and THF-d8 δ = 1.72

and 3.58 ppm; CDCl3: δ = 7.26 ppm. 13C NMR: C6D6 δ = 128.0 ppm, THF-d8 δ = 25.31 and

67.21 ppm and CDCl3: δ = 77.16 ppm. The signals of the 1H NMR spectra were integrated

relative to the number of protons, and their multiplicity is described using the following

abbreviations are used: s = singlet; d = doublet; t = triplet; q = quartet; sept = septet; m =

multiplet.

Infra-red (IR) Spectroscopy: IR spectra (4000~400 cm-1) were measured from powder

samples (ATR-Diamond) inside a nitrogen filled glovebox using a Thermo fisher Nicolet iS5

IR Spectrometer. The vibrational bands are given in wavenumbers ν (cm-1). The obtained was

processed using the OMNIC program.

Melting points: Melting points were determined by using the Stuart SMP30 melting point

apparatus. The samples were prepared inside sealed glass capillaries under vacuum.

Mass Spectrometry (MS): All the mass spectrum were performed at the Institute fr Chemie

in Technische Universität Berlin. Mass spectrums were recorded using APCI or ESI as

ionization source and a LTQ Orbitrap XL as an analyzer and the raw data evaluated using the

X-caliburTM computer program. The solutions of samples were prepared inside the glove box.

The samples were submitted inside a Schlenk tube filled with N2 atmosphere.

Elemental analyses: The C, H, and N analysis of all the compounds were performed on a

Thermo Finnigan Flash EA 1112 Series instrument. Air and/or moisture sensitive samples

were prepared inside the glove box.

Magnetic Measurements (Evans method): Solution magnetic susceptibility measurements

were performed on a Bruker AV 200 spectrometer using the Evans NMR method.[133,134] The

material (8-10 mg) was dissolved in THF-d8 or C6D6. The solution was thoroughly mixed, and

approximately 0.5 mL was placed in a NMR tube containing a THF-d8 or C6D6 capillary with

standard. The calculations required to determine the number of unpaired electrons based on

the data collected have been described elsewhere. The values are considered to be accurate

to ±0.2 μB.

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Magnetism data (SQUID): Magnetic measurements were performed at the Department of

Chemistry & Pharmacy, Friedrich-Alexander-University, Erlangen–Nürnberg (FAU).

Magnetism data of microcrystalline and powdered samples (15.8–22.1 mg), loaded within

polycarbonate gel capsules, were collected on a Quantum Design MPMS-3 SQUID

magnetometer. To test for reproducibility, two independently synthesized samples were

measured for each compound. DC susceptibility was recorded in the temperature range of 2–

300 K with an applied DC field of 1 T, if not stated otherwise. Values of the magnetic

susceptibility were corrected for core diamagnetism of the sample using tabulated Pascal’s

constants.[184] For simulation and analysis of the data, the program “JulX2”, written by Dr.

Eckhard Bill (MPI CEC, Mlheim/Ruhr) was used.[185]

Electron Paramagnetic Resonance (EPR) Spectroscopy: EPR spectroscopic

measurements were performed at the Department of Chemistry & Pharmacy, Friedrich-

Alexander-University, Erlangen - Nürnberg (FAU). EPR spectra were recorded on a JEOL

continuous wave spectrometer JES-FA200, equipped with an X-band Gunn diode oscillator

bridge, a cylindric mode cavity, and a helium cryostat. The samples were measured in solution

under a nitrogen atmosphere in quartz glass EPR tubes at 293, 95, and 7 K. The spectra

shown were measured using the following parameters: microwave frequency = 8.959 GHz,

modulation amplitude 1.0, 0.5, and 0.1 mT, microwave power 1.0 mW, modulation frequency

100 kHz, time constant of 0.1 s. Data analysis and simulation of the data was performed using

the software “eview” and “esim”, written by Dr. Eckhard Bill (MPI CEC, Mlheim/Ruhr),[186,187]

on the basis of a spin Hamiltonian description of the electronic ground state:

𝐻

=𝐷(𝑆󰆹𝑧

2−1

3𝑆(𝑆+1)+𝐸

𝐷(𝑆󰆹𝑥

2−𝑆󰆹𝑦

2))+𝜇𝐵𝑔𝑆

Here, 𝑆 represents the total spin quantum number of the coupled system, 𝐷 and 𝐸/𝐷 are the

axial and rhombic zero-field parameters, respectively, and 𝑔 is the g-matrix. Calculations are

based on the S = 5/2 routines developed by Gaffney and Silverstone.[188] EPR line widths, W,

are given in units of mT and 10–4 cm–1 / GHz at full-width-half-maximum (FWHM).

Cyclic Voltammetry Measurement: Cyclic Voltammetry (CV) measurements were carried

out at 295 K by using a Biologic SP-150 potentiostat and a three-electrode setup inside a

glove-box. Pt-wire was used as an auxiliary electrode. A freshly polished glassy carbon disc

(3 mm diameter) as a working electrode and a pseudo reference electrode Ag/Ag+ was used.

All cyclic voltammograms were referenced against the Cp2Fe/Cp2Fe+ redox couple which was

used as an internal standard. As an electrolyte, 0.3 M solutions of TBAPF6 in THF was used.

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120

The iR-drop was determined and compensated by using the impedance measurement

technique implemented in the EC-Lab Software V10.

Single-crystal XRD structure analysis: Crystals were mounted on a glass capillary in

perfluorinated oil and measured in a cold N2 flow. The data for all compounds were collected

on an Agilent Technologies SuperNova (single source) at 150 K (Cu-Kα radiation, λ = 1.5418

Å). All structures were solved by direct methods and refined on F2 with the SHELX-97

software[189]. The positions of the hydrogen atoms were calculated and considered isotopically

corresponding to a riding model. Compound 2 crystallizes with a “free” THF molecule in the

asymmetric unit. Compound 3 crystallizes with a free hexane molecule in the asymmetric unit.

Compound 5 crystallizes with two overlapped benzene molecules in the asymmetric unit.

CCDC: 2175816 (2), 2175817 (3), 2175818 (4), and 2175819 (5) contain the supplementary

crystallographic data for this dissertation. These data can be obtained free of charge from The

Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures/

Quantum chemical calculations:

Manganese(0) complexes: The structures for complex 7 and 8 were calculated by the Zhu

group at the Xiamen University, China. The DFT calculations were performed with Gaussian

16 (Revision A.03) program.[190] Geometry optimizations and frequency calculations were

conducted at the PBE0[191]-D3BJ[192]/Def2-SVP[193]∼ma-TZVP[194,195] level of theory in the gas

phase. The ma-TZVP is the abbreviation of def2-TZVP with minimal augmentation, proposed

by Truhlar and co-workers. All the principal interacting orbital (PIO)[142,143] and principal

interacting spin orbitals (PISO)[144] analyses were performed by NBO 7.0 program[196] at the

same level based on the optimized structure. All the orbitals were plotted with the help of

Multiwfn[197] and VMD programs.[198]

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5.3 Synthesis of precursor compounds

Commercially available reagents were purchased from Sigma-Aldrich, Acros, Alfa-Aesar or

abcr and were used as received. All the liquids obtained commercially were distilled, degassed

and stored under nitrogen. 1,2-Bis(dimethylphosphino)ethane was bought from abcr

specifically, due to its availability in air and moisture free sealed tube.

The following precursors were synthesized according to literature procedures. The

corresponding references are shown in Table 5.3.1

Table 5.3.1: Starting materials and references

Compound

References

I-20

CBSi2

XantSi2

I-26

CuCCPh

178,179

26a

199

26c

200

5.3.1. Modified synthesis of 2-[SiNSi]MnCl2 (1)

A Schlenk tube equipped with a stir bar was charged with bis(silylene) [SiNSi] (I-35)

(682 mg, 1.0 mmol, 1.0 equiv.) and MnCl2 (126 mg, 1.0 mmol, 1.0 equiv.). To this 30 ml THF

were added with the cannula. The resultant dark yellow suspension was stirred for 16 h at

room temperature and a clear yellow solution was obtained. All volatiles were removed under

vacuum for two hours. The residue was washed with diethyl ether (10 mL) and filtered with

cannula filter, and drying under vacuum resulted in 743 mg of an off-white solid in 92% yield.

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122

The obtained magnetic moment by Evans method, IR spectroscopy, and elemental

analysis data was matched the reported data.[94] It was further characterized by EPR

spectroscopy and SQUID measurement.

Melting Point T/°C: 220 (decomp.)

SQUID: µeff, plateau = 6.10 B.M. at 300 K (average value obtained from two

independent batches).

Evans (THF-d8, tetramethylsilylsilane capillary, 200 MHz, 298 K): 6.15 B.M.

Elemental analysis Calcd. for C39H59N7Si2MnCl2: C, 57.98; H, 7.36; N, 12.14. Found: C,

57.84; H, 7.29; N, 12.06.

5.4. Synthesis and Characterization of New Compounds

5.4.1. Synthesis of 2-[SiNSi]MnBr2 (2)

A mixture of bis(silylene) [SiNSi] (I-35) (682 mg, 1.0 mmol, 1.0 equiv.) and MnBr2 215

mg, 1.0 mmol, 1.0 equiv.) was taken in a 100 ml Schlenk tube. To this 30 ml THF was added

with the cannula. After stirring 16 h at room temperature a clear yellow solution was obtained.

All volatiles were removed under vacuum. The residue was washed with diethyl ether (10 mL)

and filtered with cannula and dried under vacuum resulted in a light-yellow solid in 90% (807

mg) yield. Colorless rectangular shaped crystals for an XRD analysis were obtained by

keeping concentrated THF solution of 2 at –20 °C overnight.

Melting Point (T/°C): 240 (decomp.).

Evans (THF-d8, tetramethylsilylsilane capillary, 200 MHz, 298 K): 5.51 B.M.

SQUID µeff,plateau = 5.99 B.M. at 300 K (average value obtained from two

independent batches).

ESI-MS m/z (%): calculated for [M].+ [C39H59N7Si2MnBr2]+ = 896.2092, found =

896.2083.

Elemental analysis Calcd. for C39H59N7Si2MnBr2: C, 52.11; H, 6.84; N, 10.91. Found: C,

52.04; H, 6.79; N, 10.86.

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123

5.4.2. Synthesis of [Si(Xant)Si]MnCl2 (3)

A mixture of bis(silylene) I-30 (0.363 mg, 0.5 mmol, 1.0 equiv.) and MnCl2 (0.63 mg,

0.5 mmol, 1.0 equiv.) was taken in a 100 ml Schlenk tube. To this 20 ml THF was added with

the cannula. After stirring 12 h at room temperature a light-yellow solution was obtained. All

volatiles were removed under vacuum. The residue was washed with diethyl ether (10 mL)

and filtered with cannula and dried under vacuum resulted in an off-white solid in 84% yield

(0.389 mg). Colorless crystals for an single-crystal XRD analysis were obtained by keeping

concentrated THF solution of 3 at –20 °C overnight.

Melting Point (T/°C): 225 (decomp.).

Evans (THF-d8, tetramethylsilylsilane capillary, 200 MHz, 298 K): 5.51 B.M.

Elemental analysis Calcd. for C45H58Cl2MnN4OSi2C4H8O: C, 63.62; H, 7.19; N, 6.06.

Found: C, 63.58; H, 7.12; N, 6.02

IR (ATR) ν/cm–1 = 633(m), 707(s), 763(s), 982 (m), 1205(s),1228(s) 1364(m),

1390(vs), 1471(vw), 2865(w), 2968(w)

5.4.3. Synthesis of {[Si(CB)Si]MnCl2}2(4)

A mixture of bis(silylene) I-29 (661 mg, 1.0 mmol, 2.0 equiv.) and MnBr2 (215 mg, 1.0

mmol, 2.0 equiv.) was taken in a 100 ml Schlenk tube. To this 30 ml THF was added with the

cannula. After stirring 12 h at room temperature a dark yellow solution was obtained. All

volatiles were removed under vacuum. The residue was washed with diethyl ether (10 mL)

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124

and filtered with cannula and dried under vacuum afforded 700 mg of a yellow solid in 89%

yield (1 equiv). Dark-yellow rectangular shaped crystals for an XRD analysis were obtained

by keeping concentrated THF solution of 4 at room temperature overnight.

Melting Point (T/°C): 250 (decomp.).

Evans (THF-d8, tetramethylsilylsilane capillary, 200 MHz, 298 K): 11.82 B.M.

Elemental analysis Calcd. for C64H112B20Cl4Mn2N8Si4: C, 48.84; H, 7.17; N, 7.12. Found:

C, 48.78; H, 7.12; N, 7.09.

IR (ATR) ν/cm–1 = 631(w), 710(m), 731(w), 760(s), 766(w), 827(w), 1060(m),

1193(m), 1258(m), 1364(s), 1393(vs), 1445(m), 2595(m, B–H),

2872(w), 2972(w).

5.4.4. Synthesis of complex 5

A 100 ml Schlenk flask equipped with stir bar was charged with 500 mg (1.0 equiv.) of

Mn(II)-dihalide complex 1 (0.619 mmol) or 2 (0.557 mmol) and 2.4 equivalents of KC8 (201 mg

or 181 mg) in a glovebox. To this 15 mL of cold THF was added with stirring at –40 °C. After

stirring the mixture at this temperature for 10 min, a solution containing 1 molar equiv. of

1,1,3,3-tetramethyl-1,3-divinyldisiloxane (dvtms) (142 µL or 128 µL) in 5 mL THF solution was

added. After stirring for 24 h, the reaction mixture was filtered and the volatiles were removed

under vacuum. The purple residue was then extracted in 40 mL of hexane. The solution was

concentrated to 5 mL and cooled at –20 °C overnight. The compound was isolated as dark-

purple crystals in 40% (250 mg starting from 1) and 47% (235 mg starting from 2) yield.

However, subsequent filtration and drying of the crystals for 2 h under vacuum, resulted in a

sticky solid. Single crystals suitable for an XRD analysis were obtained by keeping a

concentrated hexane solution at –20 °C for three days.

Melting Point (T/°C): 135 (decomp.)

Evans (THF-d8, tetramethylsilylsilane capillary, 200 MHz, 298 K): 5.92 B.M.

EXPERIMENTAL SECTION

125

Elemental analysis Calcd. for C45H75N7Si2MnP2: C, 63.05; H, 6.14; N, 10.95. Found: C,

62.01; H, 6.18; N, 10.98.

IR (ATR) ν/cm–1 = 580(w), 636(w), 692(m), 743(w), 760(w), 870(w), 982(m),

1152(m), 1232(s), 1387(w), 1433(w), 1560(w), 1581(w), 2965(w).

5.4.5. Synthesis of [SiIINSiIV]Mn(H)(dmpe) (6)

A 100 ml Schlenk tube was charged with 500 mg (1.0 equiv.) of Mn(II)-dihalide complex

1 (0.619 mmol) or 2 (0.557 mmol) and 2.4 equiv. KC8 (201 mg or 181 mg) in a glovebox. To

this 15 mL of cold THF was added with stirring at –40 °C. After stirring the mixture at this

temperature for 10 min, a solution of 1 equiv.1,2-Bis(dimethylphosphino)ethane (103 µL or 93

µL) in 5 mL THF solution was added. After stirring for 24 h, the reaction mixture was filtered

and the volatiles were removed under vacuum. The red-brown residue was then extracted in

40 mL of hexane. The solution was concentrated and kept at –20 °C overnight. The compound

was isolated as dark red crystals in 19% (170 mg starting from 1) and 25% (223 mg starting

from 2) yield. However, subsequent filtration and drying of the crystals for 2 h under vacuum,

resulted in a sticky solid. Single crystals suitable for an XRD analysis were obtained by

keeping a concentrated hexane solution at –20 °C for two days.

Melting Point (T/°C): 145 (decomp.)

Evans (THF-d8, tetramethylsilylsilane capillary, 200 MHz, 298 K): 3.70 B.M.

ESI-MS m/z (%): calculated for [M]+ [C45H75N7Si2MnP2]+ = 886.4473, found =

886.4512.

Elemental analysis Calcd. for C45H75N7Si2MnP2: C, 60.85; H, 8.63; N, 11.04. Found: C,

60.81; H, 8.58; N, 10.98.

IR (ATR) ν/cm–1 = 590(w), 604(w), 623(m), 659(w), 685(w), 706(w), 717(m),

748(m), 790(w), 842(w), 920(m), 935(m), 1021(m), 1034(s), 1116,

1163(m), 1176(m), 1261(w), 1312(w), 1368(m), 1388(w), 1412(s),

1445(m), 1469(w), 1478(w), 1526(w), 1579(w), 1745(w, MnH),

2864(w), 2896(w), 2966(w).

EXPERIMENTAL SECTION

126

5.4.6. Synthesis of 3-[SiNSi]Mn(dmpe) (7)

A 100 ml Schlenk tube equipped with a stir bar was charged with one equiv. of

compound 1 (800 mg, 0.990 mmol) or 2 (800 mg, 0.892 mmol). To this 25 mL of THF was

added by cannula, followed by an addition of one equivalent 1,2-

bis(dimethylphosphino)ethane (166 µL or 149 µL) in a 5 mL THF solution. This mixture, was

stirred for 2 hours. The solution was cooled to 0 °C, which was then transferred to a well-

stirred cold suspension containing 2.4 equiv. KC8 (321 mg or 289 mg) in THF at –40 °C. After

stirring the mixture for 16 h, it was filtered using cannula filter. The filtrate was dried under

vacuum for 2 hours. The residue was then extracted in hexane (2×50mL) as dark blue-black

solution. The filtrate was concentrated to 10-15 mL and kept in freezer overnight giving a crop

of diamond shape black crystals. Subsequent filtration and evaporation under vacuum

afforded black shiny crystalline solid in 39% (340 mg, starting from 1) and 50% (396 mg,

starting from 2) yield. Single crystals suitable for XRD analysis were obtained from a

concentrated hexane solution at room temperature overnight at room temperature.

Melting Point (T/°C): 135 (decomp.)

Evans (C6D6, tetramethylsilylsilane capillary, 200 MHz, 298 K): 2.65 B.M.

SQUID: µeff, plateau = 1.95 B.M. at 300 K; 1.62 B.M. at 2 K. (values are obtained

from the average of two independent batches)

ESI-MS m/z (%): calculated for [M+2H]+ [C45H77N7Si2MnP2]+ = 888.4629, found

= 888.4631.

Elemental analysis Calcd. for C45H75N7Si2MnP2: C, 60.78; H, 8.73; N, 11.03. Found: C,

60.72; H, 8.69; N, 10.96.

IR (ATR) ν/cm–1 = 601(w), 705(vs), 785(w), 920(w),1015(w), 1154(s), 1201(s),

1232(s), 1309(w), 1354(m), 1406(m), 1440(w), 1472(vw), 1573(w),

1609(vw), 2985(w), 2959(w).

EXPERIMENTAL SECTION

127

5.4.7. Synthesis of [SiNSi]Mn(CO)3 (8)

A toluene solution of compound 7 (70 mg, 0.079 mmol) was exposed to a CO

atmosphere after three freeze−pump−thaw cycles. The resulting reaction mixture was stirred

for 6 hours at room temperature. The dark black-violet color of the reaction faded to wine red

color over the course of the reaction. The volatiles were removed under vacuum. The residue

was extracted in 30 mL hexane and was concentrated to 5 mL. Cooling it at –20 °C overnight

afforded the first crop of desired product (30 mg). Further concentration, crystallization, from

the remaining hexane solution afforded the second crop of product (25 mg) for a total yield of

55 mg of the product (85%). Single crystals suitable for an XRD analysis were obtained from

a slow diffusion of concentrated diethyl ether/benzene solution at room temperature after two

days.

Melting Point (T/°C): 160 (decomp.)

Evans (THF-d8, tetramethylsilylsilane capillary, 200 MHz, 298 K): 1.80 B.M.

SQUID: µeff = 1.18 B.M. at 300 K

ESI-MS m/z (%): calculated for [M+2H]+ [C42H59MnN7O3Si2]+ = 822.3749, found

= 822.3753.

Elemental analysis Calcd. for C42H57MnN7O3Si2: C, 61.29; H, 7.47; N, 11.91. Found: C,

61.22; H, 7.49; N, 11.88.

IR (ATR) ν/cm–1 = 631(w), 674(m), 706(w), 755(w), 927(w), 1072(w), 1110 (w),

1156(m), 1199(m), 1259(s), 1312(m), 1364(w), 1407(m), 1441(s),

1471(m), 1573(s), 1716(s), 1811(s), 1844(s), 2963(w).

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128

5.4.8. Synthesis of [SiNSi]Mn(XylylNC)2(dmpe) (9)

Compound 7 (50 mg, 0.056 mmol) and 2.0 molar equiv. 2,6-dimethylphenyl isocyanide

(14.7 mg, 0.112 mmol) were weighed inside the glove-box in a 50 mL Schlenk tube. To this

10 mL of toluene was added at room temperature. The reaction mixture was stirred overnight,

affording a dark-green solution. All the volatiles were removed under vacuum. The residue

was washed with (2×3 mL) cold hexane and dried to give a green powder in 80% yield (52.0

mg).

Melting Point (T/°C): 150 (decomp.)

Evans (THF-d8, tetramethylsilylsilane capillary, 200 MHz, 298 K): 1.86 B.M.

SQUID: µeff, plateau = 1.83 B.M. at 300 K

APCI-MS m/z (%): calculated for [M]+ [C63H93MnN9P2Si2]+ = 1148.5943, found =

1148.5951.

Elemental analysis Calcd. for C63H93MnN9P2Si2: C, 65.71; H, 8.32; N, 10.95. Found: C,

65.68; H, 8.29; N, 10.91.

IR (ATR) ν/cm–1 = 619(w), 668(s), 705(s), 754(s), 925(w), 1080(w), 1156(m),

1198(m), 1256(s), 1309(w), 1441(s), 1585(m), 1866(s), 1900(s),

2963(w).

5.4.9. Synthesis of bis(phosphine)pyridine (13)

A mixture of 2,6 N,N’-diethylamino pyridine (1.00 g, 0.65 mmol) and LiHMDS (2.07 g,

12.4 mmol) was dissolved in toluene (20 mL). A toluene (50 mL) solution of 2-chloro-1,3-

diisopropyl-1,3,2-diazaphospholidine (2.52 g, 12.1 mmol, 2.0 equiv.) was subsequently added

via syringe, the reaction mixture allowed to warm to ambient temperature and stirred for 16

EXPERIMENTAL SECTION

129

hours. The reaction mixture was filtered over celite and filtrate was collected. All volatiles were

subsequently removed from the light-yellow solution, affording a light-yellow oil in 78% yields

(2.4 g). Bis(phosphine) 13 is air-stable and is well soluble in general organic solvents (Et2O,

hexane, THF, toluene).

1H NMR (CD2Cl2, 500 MHz, 298 K): δ(ppm) = 7.19 (t, 3JHH = 8.0 Hz, 1H, 4-

Caroma.H py), 6.75 (d, J = 8.0 Hz, 2H, 3,5-Caroma.H py), 3.54 (q, 3JHH =

6.9 Hz, 4H, 2 x CH2CH3), 3.42 – 3.38 (m, 4H, 4 x CH(CH3)2, 3.38 –

3.30 (m, 4H, N(CH2)2N), 3.08 – 3.03 (m, 4H, N(CH2)2N), 1.16 (d, 3JHH

= 3.0 Hz, 12H, 2 x CH(CH3)2, 1.15 (d, 3JHH = 3.1 Hz, 12H, 2 x

CH(CH3)2), 1.09 (t, 3JHH = 6.9 Hz, 6H, 2 x CH2CH3);

13C{1H} NMR (C6D6, 125 MHz, 298 K): δ(ppm) = δ 158.5 (d, 2JC,P = 23.4 Hz, 2,6-

Caroma.H py), 137.1 (s, 4-Caroma.H py), 102.2 (d, 3JC,P = 29.8 Hz, 3,5-

Caroma.H py), 48.4 (d, 2JC,P = 23.1 Hz, N(CH2)2N), 46.3 (d, 2JC,P = 8.9

Hz, CH(CH3)2), 38.0 (d, 3JC,P = 4.8 Hz, CH2CH3), 22.5 (d, 2JC,P = 8.3

Hz, CH(CH3)2), 22.3 (d, 2JC,P = 8.5 Hz, CH(CH3)2), 15.4 (s, CH2CH3);

31P{1H} NMR (C6D6, 202 MHz, 298 K): δ(ppm) = 99.8.

IR(ATR) ν/cm–1 = 674(s), 700(s), 783(m), 926(s), 966(s), 992(m), 1129(w),

1206(s), 1256(m), 1327(w), 1358(w), 1434(s), 1571 (s, C=N), 2964.

5.4.10. Synthesis of 3-[PNP]MnCl2 (14)

A Schlenk tube equipped with a stir bar was charged with 13 (500 mg, 0.99 mmol) and

THF (25 mL). The resultant solution was transferred to a Schlenk tube containing MnCl2 (124

mg, 0.99 mmol) via cannula. The reaction mixture was stirred at 60 °C for 16 hours. After

cooling down to room temperature, the volatiles were subsequently removed under vacuum.

The obtained oily residue was washed with diethyl ether (2 x 10 mL), followed by filtration and

drying under vacuum for one hour afforded yellow solid in 89% yield (560 mg).

Melting Point 165 °C (dec./melt)

EXPERIMENTAL SECTION

130

Evans (THF-d8, tetramethylsilylsilane capillary, 200 MHz, 298K): 5.88 B.M.

Elemental analysis anal. calcd. for C25H49Cl2MnN7P2: C, 47.25%; H, 7.77 %; N, 15.43 %;

found: C, 46.96 %; H, 7.57 %; N, 15.34 %

IR(ATR) ν/cm–1 = 646(w), 680(w), 722(s), 739(w), 777(s), 870(w), 910(w),

936(w), 1048(s), 1120(w), 1165(m), 1239(w), 1310(w), 1387(w),

1433(s), 1582(s, C=N), 2855(w), 2959(w).

5.4.11. Synthesis of {2-[PNP]Mn(CH2SiMe3)2} (15)

A Schlenk flask equipped with a stir bar was charged with 14 (0.50 g, 0.08 mmol) and

20 mL of Et2O was added to it via canula. The resulting suspension was cooled to –78 °C and

a 1.0 mM solution of LiCH2SiMe3 (0.16 mmol, 2.02 equiv.) was added to it in a dropwise

fashion using a syringe. The reaction was stirred at this temperature for 15 min and then stirred

at room temperature for one hour. Over this time the solution turned from yellow to dark-

orange. All the volatiles were subsequently removed in vacuo, and the solid residue was

extracted in Et2O (20 mL), followed by filtration. Concentration of the filtrate to 5 mL, and

storage at –20 °C for five days resulted in the formation of a large crop of dark-orange crystals

in 73% yield (365 mg).

Melting Point 110 °C (dec./melt)

Evans (C6D6, tetramethylsilylsilane capillary, 200 MHz, 298K): 5.79 B.M.

ESI-MS m/z (%): calculated for [M+H]+ [C33H72MnN7P2Si2]+ 739.4238, observed

739.4554;

Elemental analysis anal. calcd. for C33H71MnN7P2Si2: C, 53.63 %; H, 9.68 %; N, 13.27 %;

found: C, 52.28 %; H, 9.53 %; N, 13.15 %

IR(ATR) ν/cm–1 = 636(w), 661(w), 715(w), 781(m), 852(s), 962(w), 1047(m),

1171(m), 1242(m), 1305(w), 1387(w), 1437(s), 1574 (s, C=N) 2838(w),

2931(w), 2949(w).

EXPERIMENTAL SECTION

131

5.4.12. Synthesis of {[SiNN]Mn(CH2SiMe3)}2 (16)

A Schlenk flask containing a suspension of 1 (300 mg, 0.37 mmol, 2.00 equiv) in Et2O

(25 mL) was cooled to –78 °C and a 1.0 mM solution of LiCH2SiMe3 (0.7 mL, 0.74 mmol, 4.04

equiv) was added to it dropwise using a syringe. The reaction was stirred at this temperature

for 15 min and then stirred at room temperature for 1 hour. Over this time the solution turned

from light yellow to deep orange. All volatiles were subsequently removed in vacuo, and the

solid residue extracted in Et2O (20 mL), followed by filtration. Concentration of the filtrate to 5

mL, and storage at –20 °C for two days resulted in the formation of a large crop of dark-orange

crystals in 59% yield (120 mg).

Melting Point 115 °C (dec./melt);

Evans (C6D6, tetramethylsilylsilane capillary, 200 MHz, 298K): 11.90 B.M.

ESI-MS m/z (%) calculated for [M-H]+ [C56H94Mn2N10Si4]+ 1126.5339, observed

1126.5490;

Elemental analysis anal. calcd. for C56H94Mn2N10Si4: C, 59.54 %; H, 8.39 %; N, 12.40 %;

found: C, 57.87 %; H, 8.31 %; N, 12.29 %.

IR(ATR) ν/cm–1 = 615(w), 678(w), 706(m), 766(m), 865(m), 919(w), 1053(w),

1141(w), 1156(w), 1174(w), 1205(m), 1305(w), 1403(m), 1436(s),

1555(m), 1577 (m, C=N), 2865(w), 2967(w).

5.4.13. Synthesis of {2-[SiN]MnBr2}2 (18)

EXPERIMENTAL SECTION

132

A Schlenk flask was charged with I-26 (600 mg, 1.63 mmol, 2.00 equiv) and MnBr2

(350 mg, 1.63 mmol, 2.00 equiv). To this THF (25 mL) was added via cannula. The reaction

mixture was stirred at 50 °C for 16 hours. After cooling down to room temperature, the volatiles

were subsequently removed under vacuum. The obtained oily residue was washed with diethyl

ether (2 x 10 mL), followed by filtration and drying under vacuum for one hour to afford a pale-

yellow solid in 87% yield (830 mg). Single-crystals suitable for XRD analysis were obtained by

cooling the concentrated THF solutions of 18 overnight at –20 °C.

Melting Point 160 °C (dec./melt)

Evans (THF-d8, tetramethylsilylsilane capillary, 200 MHz, 298K): 12.37 B.M.,

Elemental analysis anal. calcd. for C42H62Br4Mn2N8Si2: C, 43.31 %; H, 5.37 %; N, 9.62 %;

found: C, 43.29 %; H, 5.34 %; N, 9.57 %.

IR(ATR) ν/cm–1 = 599(m), 693(w), 710(w), 732(w), 772(m), 870(m), 1200(w),

1308(m), 1365(w), 1389(s), 1440(w), 1468(m), 1597 (m, C=N),

2972(w).

5.4.14. Synthesis of {[SiR]Mn(CH2SiMe3)} (19)

A suspension of 5 (0.60 g, 0.52 mmol) in Et2O (25 mL) was cooled to –78 °C and a 1.0

mM solution of LiCH2SiMe3 (2.10 mL, 2.10 mmol, 4.04 equiv) was added to it in a dropwise

fashion using a syringe. The reaction was stirred at this temperature for 15 min and then stirred

at room temperature for 1 hour. Over this time the solution turned from pale-yellow to dark

yellow-orange. All the volatiles were subsequently removed in vacuo, and the solid residue

extracted in Et2O (20 mL), followed by filtration. Concentration of the filtrate to 5 mL, and

storage at –20 °C for 7 days resulted in the formation of deep yellow crystals in 24 % yield (90

mg, 0.12 mmol).

Melting Point 110-113 °C (dec./melt);

Evans (C6D6, tetramethylsilylsilane capillary, 200 MHz, 298K): 5.71 B.M.,

EA anal. calcd. for C38H68MnN4Si3: C, 63.38 %; H, 9.52 %; N, 7.78 %;

found: C, 62.23 %; H, 9.47 %; N, 8.84 %.

EXPERIMENTAL SECTION

133

IR(ATR) ν/cm–1 = 597(w), 705(s), 762(m), 830(s), 1014(w), 1052(w), 1194(m),

1243(w), 1301(w), 1353(w), 1405(s), 1474(w),1590 (w, C=N),

2958(w).

5.4.15. Synthesis of {3-[SiNSi]NiBr}Br (21)

A Schlenk tube equipped with a stir bar was charged with of bis(silylene)pyridine ligand

I-35 (300 mg, 0.439 mmol) and 136 mg of NiBr2(dme) (0.439 mmol). To this mixture, 20 ml of

THF was added via cannula. After stirring the reaction mixture for 12 h at room temperature,

volatiles were removed under vacuum. The crude mixture was washed with 20 mL Et2O

additionally. Drying the residual solvents for one hour under vacuum afforded brick-red powder

of the complex in 86% yield.(340 mg). Complex is partially soluble in THF, Et2O and insoluble

in n-hexane. X-ray quality single crystals were obtained by keeping a saturated THF solution

of 21 in freezer at –20 °C overnight.

Melting Point 260 °C (dec./melt)

1H NMR (500 MHz, CD2Cl2, 298K): δ(ppm) = 7.69 – 7.53 (m, 11H, Caroma.H Ph,

4-Caroma.H py), 6.14 (d, J = 7.8 Hz, 2H, 3,5-Caroma.H py), 3.48 – 3.38

(m, 4H, NCH2CH3), 1.41 (t, 3JHH = 6.8 Hz, 6H, 2 x NCH2CH3), 1.32 (s,

36H, 4 x NC(CH3)3).

13C{1H} NMR (126 MHz, CD2Cl2, 298K): δ(ppm) = 181.0 (NCN), 167.7 (2,6-Caroma.H

py), 145.5 (Caroma.H Ph), 135.1 (Caroma.H Ph), 132.9 (Caroma.H Ph),

132.7 (Caroma.H Ph), 132.2 (Caroma.H Ph), 132.1 (Caroma.H Ph), 132.0

(Caroma.H Ph), 103.2 (3,5-Caroma.H py), 58.4 NC(CH3)3, 57.0

(NC(CH3)3), 43.6 (NCH2CH3), 34.1 (NC(CH3)3), 17.6 (NCH2CH3).

29Si{1H} NMR (99 MHz, CD2Cl2, 298K): δ(ppm) = 13.6.

ESI-MS (positive-ion mode, THF) m/z (%): calculated for [M]+

[C39H59N7Si2NiBr]+ = 820.2881 found = 820.2871.

Elemental analysis: Calcd. for C39H59NiN7Br2Si2: C, 52.01; H, 6.60; N, 10.89. Found: C,

52.03; H, 6.58; N, 10.05.

EXPERIMENTAL SECTION

134

General procedure (GP) for the synthesis of complexes 22 and 23 of type {3-

[SiNSi]Ni(L)Br}Br, (L = PMe3, XylylNC)

In a Schlenk tube, a 10 ml THF solution of 21 (200 mg, 0.222 mmol), a 5 mL THF

solution of one molar equivalent of ligand ‘L’ where L = PMe3 (22.8 µL, 0.222 mmol), or 2,6-

dimethylphenyl isocyanide (29.1 mg, 0.222 mmol); was added dropwise using syringe. After

stirring the reaction mixture for two hour, precipitate was allowed to settled down, followed by

decanting the solvent and drying the residue under vacuum for one hour afforded desired

complexes in good to moderate yields. Single-crystals for X-ray measurement were obtained

from saturated THF solution of the respective complexes.

5.4.16. Synthesis of {3-[SiNSi]Ni(PMe3)Br}Br (22)

Following GP mentioned above, the brick-red colored complex 22 was isolated in 89% yield

(192 mg).

1H NMR (500 MHz, CD2Cl2, 298K): δ(ppm) = 7.73 – 7.63 (m, 2H, Caroma.H Ph,

4-Caroma.H py), 7.60 – 7.52 (m, 4H, Caroma.H Ph), 7.38 (d, 3JHH = 8.5 Hz,

3H, Caroma.H Ph), 6.17 (d, 3JHH = 8.0 Hz, 2H, 3,5-Caroma.H py), 3.46 (q,

3JHH = 7.0 Hz, 4H, NCH2CH3), 1.94 (d, 2JPH = 10.1 Hz, 9H, P(CH3)3),

1.44 (t, 3JHH = 7.0 Hz, 6H, NCH2CH3), 1.29 (s, 36H, NC(CH3)3).

13C{1H} NMR (101 MHz, CD2Cl2, 298K): δ = 177.7 (NCN) , 162.0 (2,6-Caroma.H py),

140.1 (Caroma.H Ph), 132.2 (Caroma.H Ph), 129.4 (Caroma.H Ph), 129.3

(Caroma.H Ph), 129.2, 129.2, 126.8 (Caroma.H Ph), 99.8 (3,5-Caroma.H py),

55.8 (NC(CH3)3), 39.4 (NCH2CH3), 31.9 (NC(CH3)3), 23.9 (d, 1JCP =

33.8 Hz, P(CH3)3), 14.3 (NCH2CH3).

31P{1H} NMR (202 MHz, CD2Cl2, 298K): δ = -8.62 (br, s).

29Si{1H} NMR (99 MHz, CD2Cl2, 298K): δ(ppm) = 40.9 (d, 2JSi-P = 74.3 Hz).

Melting Point T/°C: 235 (decomp.)

Elemental analysis: Calcd. for C42H68NiN7Br2PSi2: C, 51.65; H, 7.02; N, 10.04. Found: C,

51.62; H, 6.99; N, 10.01.

EXPERIMENTAL SECTION

135

5.4.17 Synthesis of {3-[SiNSi]Ni(CNXylyl)Br}Br (23)

Following GP mentioned above, a yellow-colored compound 23 in 86% yields (197

mg).

1H NMR (500 MHz, CD2Cl2, 298K): δ(ppm) = 7.65 – 7.58 (m, 6H, Caroma.H Ph),

7.55 (d, J = 6.8 Hz, 3H, Caroma.H Ph), 7.45 – 7.42 (m, 2H, Caroma.H Ph),

7.28 (d, J = 6.7 Hz, 1H, 2,6-CH3C6H3), 7.24 (d, J = 7.3 Hz, 2H, 2,6-

CH3C6H3), 7.02 (d, J = 7.5 Hz, 2H, Caroma.H Ph), 6.23 (d, 3JHH = 8.1 Hz,

2H, 3,5-Caroma.H py), 3.50 (br, s, 4H, NCH2CH3), 2.52 (s, 6H, 2,6-

CH3C6H3), 1.44 (t, 3JHH = 6.9 Hz, 6H, NCH2CH3), 1.33 (s, 18H,

NC(CH3)3), 1.22 (s, 18H, NC(CH3)3).

13C{1H} NMR (101 MHz, CD2Cl2, 298K): δ = 176.7 (NCN), 161.1 (2,6-Caroma.H py),

140.3 (4-Caroma.H py), 133.4(Caroma.H Ph), 131.8(Caroma.H Ph),

129.1(Caroma.H Ph), 128.96(Caroma.H Ph), 128.8(Caroma.H Ph),

128.7(Caroma.H Ph), 128.6 (Caroma.H Ph), 126.6 (Caroma.H Ph), 99.7 (3,5-

Caroma.H py), 39.5 (NCH2CH3), 31.2 (NC(CH3)3), 19.4 (2,6-CH3C6H3),

13.9 (NCH2CH3).

29Si{1H} NMR (99 MHz, CD2Cl2, 298K): δ(ppm) = 39.9.

Melting Point T/°C: 240 (decomp.)

Elemental analysis Calcd. for C48H68NiN8Br2Si2: C, 55.88; H, 6.60; N, 10.89. Found: C,

55.82; H, 6.56; N, 10.83.

IR(ATR) ν/cm–1 = 685(w), 738(s), 779(vs), 885(m), 937(w), 961(w), 1052(s),

1093(m), 1143(m), 1161(vs), 1250(m), 1315(s), 1392(s), 1463(s),

1569(w), 1594(m), 2083(w, CN), 2152(m, CN), 2870(w), 2937(w),

2966(w).

EXPERIMENTAL SECTION

136

5.4.18. Synthesis of 3-[SiNSi]Ni(CNXylyl) (24)

A 50 mL Schlenk flask equipped with stir bar was weighed with 23 (200 mg, 0.194

mmol) and 2.1 equiv. of KC8. (55 mg, 0.407 mmol). To this 20 mL of 1:1 THF:toluene cooled

at –40 °C was added slowly under stirring. After stirring for 2 h, the precipitate was separated

via cannula filtration. All volatiles were removed under vacuum. The resultant dark red oily

residue was then extracted in 40 mL n-hexane. The dark-red crystals of Ni(0) complex 24 were

isolated in 78% yield (132 mg) by cooling its concentrated n-hexane solution at –20 °C

overnight.

1H NMR (400 MHz, THF-d8, 298K): δ(ppm) = 7.59 – 7.44 (m, 11H, Caroma.H Ph,

4-Caroma.H py), 6.73 (d, 3JHH = 7.4 Hz, 2H, 2,6-CH3C6H3), 6.67 – 6.59

(m, 1H, 2,6-CH3C6H3), 5.84 (d, 3JHH = 8.0 Hz, 2H, 3,5-Caroma.H py), 3.40

(dq, 2JHH = 14.0, 3JHH = 7.0 Hz, 2H, NCH2CH3), 3.26 – 3.18 (m, 2H,

NCH2CH3), 2.18 (s, 6H, 2,6-CH3C6H3), 1.35 (t, 3JHH = 6.9 Hz, 6H,

NCH2CH3), 1.29 (s, 18H, NC(CH3)3), 1.03 (s, 18H, NC(CH3)3).

13C{1H} NMR (101 MHz, THF-d8, 298K): δ(ppm) = 191.7(NC(Xylyl)), 163.4 (NCN),

160.0 (2,6-Caroma.H py), 133.5(Caroma.H Ph), 132.4(Caroma.H Ph),

130.4(Caroma.H Ph), 130.0(Caroma.H Ph), 128.5(Caroma.H Ph),

127.6(Caroma.H Ph), 126.8(Caroma.H Ph), 126.0(Caroma.H Ph),

125.9(Caroma.H Ph), 125.8(Caroma.H Ph), 124.8(Caroma.H Ph),

123.2(Caroma.H Ph), 119.5(Caroma.H Ph), 92.2 (3,5-Caroma.H py), 51.8

(NC(CH3)3), 51.2 (NC(CH3)3), 35.5 (NCH2CH3), 29.2 (NC(CH3)3), 28.7

(NC(CH3)3), 17.3 (2,6-CH3C6H3), 12.9 (NCH2CH3).

29Si{1H} NMR (99 MHz, THF-d8, 298K): δ(ppm) = 46.4.

Melting Point T/°C :140 (decomp.)

IR(ATR) ν/cm–1 = 621(s), 1153(s), 1261(s), 1389(s), 1440(s), 1575(m),

1589(m), 1862(s, CN), 1897(s, CN), 2865(w), 2972(m).

EXPERIMENTAL SECTION

137

Elemental analysis Calcd. for C48H68NiN8Si2: C, 66.12; H, 7.86; N, 12.85. Found: C, 66.10;

H, 7.82; N, 12.83.

5.4.19. Synthesis of {3-[PNP]NiBr}Br (25)

A Schlenk tube equipped with stir bar was charged with of bis(phosphine)pyridine

ligand 13 (800 mg, 1.57 mmol) and 343 mg of NiBr2 (1.57 mmol). To this mixture, 20 ml of

THF was added via cannula. After stirring the reaction mixture for 12 h at 60 °C, all the volatiles

were removed under vacuum. The resultant yellowish residue was washed with 20 mL of Et2O

via cannula filtration. Drying the residual solvents under vacuum for one hour afforded dark-

yellow powder of the complex 25 in 78% yield. (892 mg). Complex is partially soluble in THF,

Et2O and insoluble in n-hexane. X-ray quality single crystals of 25 were obtained by cooling

its concentrated THF solution in freezer at –20 °C overnight.

1H NMR (500 MHz, CD2Cl2, 298K) δ(ppm) = 7.97 (t, 3JHH = 7.0 Hz, 1H, 4-

Caroma.H py), 6.40 (d, 3JHH = 7.8 Hz, 2H, 3,5-Caroma.H py), 3.77 – 3.71

(m, 4H, CH(CH3)2), 3.51 – 3.43 (m, 4H, N(CH2)2N), 3.41 – 3.38 (m, 4H,

NCH2CH3), 3.34 – 3.28 (m, 4H, N(CH2)2N), 1.42 (t, 3JHH = 7.0 Hz, 24H,

CH(CH3)2), 1.32 (t, 3JHH = 7.0 Hz, 6H, NCH2CH3).

13C{1H} NMR (126 MHz, CD2Cl2, 298K) δ(ppm) = 158.24 (t, 3JCP = 13.8 Hz, 2,6-

Caroma.H py), 144.40 (4-Caroma.H py), 101.19 (3,5-Caroma.H py), 48.06 (t,

3JCP = 6.6 Hz, (CH(CH3)2), 44.25 (NCH2N), 38.9 (NCH2CH3), 22.74

(CH(CH3)2), 21.77 (CH(CH3)2), 13.73 (NCH2CH3).

31P{1H} NMR (202 MHz, CD2Cl2, 298K) δ(ppm) = 104.6.

Melting Point T/°C : 230 (decomp.)

Elemental analysis Calcd. for C25H49Br2N7NiP2: C, 41.24; H, 6.78; N, 13.47. Found: C,

41.20; H, 6.73; N, 13.41.

EXPERIMENTAL SECTION

138

5.4.20. Synthesis of {3-[PNP]Ni(CNXylyl)} (26)

A 50 mL Schlenk flask equipped with stir bar was weighed with 25 (500 mg, 0.686

mmol), 2,6-dimethylphenyl isocyanide (90 mg, 0.686 mmol). To this 20 mL of THF was added,

and the reaction mixture was then stirred for 1 h. Then, 2.1 equiv. of KC8. (195 mg, 1.44 mmol)

was added to it at room temperature. After stirring for 2 h, the precipitate was separated via

cannula filtration. All volatiles were removed under vacuum. The resultant dark-red oily residue

was then extracted in 50 mL n-hexane. The dark-red crystals of Ni(0) complex 24 were isolated

in 69% yield (331 mg) by cooling its concentrated n-hexane solution at –20 °C overnight.

1H NMR (500 MHz, THF-d8, 298K): δ(ppm) = 7.17 (t, 3JHH = 8.0 Hz, 1H, 4-

Caroma.H py), 6.91 – 6.71 (m, 3H, 2,6-CH3C6H3), 5.87 (d, 3JHH = 8.0 Hz,

2H, 3,5-Caroma.H py), 4.38 (qd, 2JHH = 13.0, 3JHH = 6.5 Hz, 2H), 3.48

(ddd, J = 10.2, 6.8, 3.3 Hz, 2H), 3.39 (ddd, J = 9.7, 8.6, 4.7 Hz, 2H),

3.26 – 3.16 (m, 4H), 3.11 – 3.02 (m, 2H), 2.20 (s, 6H), 1.19 (t, J = 7.0

Hz, 6H), 1.12 (d, J = 6.8 Hz, 6H), 1.09 (d, J = 6.5 Hz, 6H), 1.02 (d, J =

6.7 Hz, 6H), 0.77 (d, J = 6.7 Hz, 6H).

13C{1H} NMR (126 MHz MHz, THF-d8, 298K): δ = 156.64 (2,6-Caroma.H py), 134.74,

134.12, 133.47, 128.06, 124.18(Caroma.H), 96.49(3,5-Caroma.H py),

45.29, 45.21, 45.13, 44.96, 44.84, 44.73, 41.87, 40.52, 38.58, 26.43,

25.69, 25.53, 21.82, 21.77, 20.69, 20.50(CH(CH3)2), 19.79(2,6-

CH3C6H3), 14.94 (NCH2CH3).

31P{1H} NMR (203 MHz, THF-d8, 298K): δ(ppm) = 152.9 (s).

Melting Point T/°C: 155 (decomp.)

Elemental analysis Calcd. for C34H58NiN8P2: C, 58.38; H, 8.36; N, 16.02. Found: C, 58.34;

H, 8.31; N, 16.00.

IR(ATR) ν/cm–1 = 573(w), 677(m), 702(s), 753(m), 847(w), 962(m), 1041(s),

1066(m), 1175(s), 1225(w), 1264(w), 1314(m), 1381(m), 1443(s),

1564(w), 1589(w), 1879(w), 1944(s, CN), 2891(w), 2961(w).

EXPERIMENTAL SECTION

139

5.4.21. Synthesis of compound 30

A Schlenk flask containing a suspension of 21 (355 mg, 0.395 mmol) in Et2O (25 mL)

was cooled to –78 °C and a 1.0 mM solution of LiCCPh (0.8 mL, 0.809 mmol, 2.05 equiv) was

added to it dropwise using a syringe. The reaction was stirred at this temperature for 15 min

and then stirred at room temperature for 1 hour. Over this time the solution turned from light

yellow to bright red. All volatiles were subsequently removed in vacuo, and the solid residue

extracted in Et2O (20 mL), followed by cannula filtration. Concentration of the filtrate to 5 mL,

and storage at –20 °C for three days resulted in the formation of a large crop of dark-red

crystals in 72% yield (268 mg).

1H NMR (500 MHz, C6D6, 298K): δ(ppm) = 7.87 (s, 2H, Caroma.H), 7.55 (d, 3JHH

= 7.0 Hz, 1H, Caroma.H), 7.46 (t, 3JHH = 8.1 Hz, 1H,), 7.08 (t, 3JHH = 4.7

Hz, 4H, Caroma.H), 6.95 (m, 6H, Caroma.H), 6.61 (d, 3JHH = 8.1 Hz, 1H,

3,5-Caroma.H py), 6.16 (d, 3JHH = 8.0 Hz, 1H, 3,5-Caroma.H py), 3.30 (q,

3JHH = 6.9 Hz, 2H, NCH2CH3), 1.50 (s, 18H, NC(CH3)3), 1.35 (dt, JHH =

13.8, 7.0 Hz, 6H), 0.97 (s, 18H, NC(CH3)3).

13C{1H} NMR (126 MHz, C6D6, 298K): δ = 169.9 (NCN), 164.0 (NCN), 162.7 (2,6-

Caroma.H py), 159.9 (2,6-Caroma.H py), 142.4 (4-Caroma.H py),

136.3(Caroma.H), 133.1(Caroma.H), 132.2(Caroma.H), 130.9(Caroma.H),

130.1(Caroma.H), 129.7(Caroma.H), 127.2(Caroma.H), 126.6 (Caroma.H),

103.2 (3,5-Caroma.H py), 97.7 (3,5-Caroma.H py), 54.7 (NC(CH3)3),, 53.8

(NC(CH3)3),, 42.6 (NCH2CH3), 38.1 (NCH2CH3),, 32.8 (NC(CH3)3),

31.2 (NC(CH3)3), 15.4 (NCH2CH3), 14.8 (NCH2CH3).

29Si{1H} NMR (99 MHz, C6D6, 298K): δ(ppm) = δ 44.5, -60.8.

Melting Point T/°C: 155 (decomp.)

Elemental analysis Calcd. for C55H69N7Si2Ni: C, 70.05; H, 7.38; N, 10.40. Found: C, 70.01;

H, 7.32; N, 10.37.

IR(ATR) ν/cm–1 = 614(w), 634(w), 690(s), 706(s), 869(w), 921(w), 1007(w),

1157(w), 1194(w), 1252(s), 1392(w), 1440(s), 1582(m), 1771(w,

CC,coordinated), 1938(vw), 2025(vw), 2160(vw), 2865(w), 2924(w),

2965(w), 3052(w).

EXPERIMENTAL SECTION

140

5.5. Additional experiments

5.5.1. Reaction of LiCH2SiMe3 with [SiNSi] (I-35)

A Schlenk flask containing a solution of I-35 (300 mg, 0.44 mmol) in Et2O (25 mL) was

cooled to –78 °C and a 1.0 mM solution of LiCH2SiMe3 (0.9 mL, 0.89 mmol, 2.02 equiv) was

added to it in a dropwise fashion using a syringe. The reaction was stirred at this temperature

for 15 min and then stirred at room temperature for 1 hour. Over this time the solution turned

from pale-yellow to deep-yellow. At this stage, 1H NMR analysis of the reaction mixture

indicated effectively 1:3 quantitative formation of the compound 20 and17 respectively.[6] All

solvents were subsequently removed in vacuo, and the solid residue was analyzed by multi-

nuclear NMR spectroscopy in C6D6 and THF-d8. Concentration of the filtrate to 10 mL, and

storage at –20 °C for two days resulted in the formation of a few colorless crystals of 20.

For compound 17:

1H NMR (C6D6, 500 MHz, 298 K): δ(ppm) = 7.14‒6.92 (m, 5H, HPh), 1.13 (s,

18H, 2 x C(CH3)3), 0.58 (s, 2H, CH2), 0.43 (s, 9H, SiMe3).

29Si{1H} NMR (C6D6, 99 MHz, 298 K): δ(ppm) = 61.1 (s), 0.03 (s) ppm.

1H NMR (THF-d8, 500 MHz, 298 K): δ(ppm) = 7.51‒7.20 (m, 5H, HPh), 1.15 (s,

18H, 2 x C(CH3)3), 0.30 (s, 2H, CH2), 0.18 (s, 9H, SiMe3).

29Si{1H} NMR (THF-d8, 99 MHz, 298 K): δ 58.9, -1.8 ppm.

For compound 20 (for one Li unit of trimer):

1H NMR (C6D6, 500 MHz, 298 K): δ(ppm) = 7.42‒7.39 (m, 1H, 4-Caroma.H py),

7.14‒6.92 (m, 5H, HPh), 6.12 (d, 3JHH = 8.5 Hz, 1H, Caroma.H py), 5.64

(d, 3JHH = 7.5 Hz, 1H, Caroma.H py), 3.54 (q, 3JHH = 6.9 Hz, 2H,

NCH2CH3), 3.20 (q, 3JHH = 6.9 Hz, 2H, NCH2CH3), 1.77 (s, 9H,

EXPERIMENTAL SECTION

141

C(CH3)3), 1.63 (t, 3JHH = 7.2 Hz, 3H, NCH2CH3), 1.37 (t, 3JHH = 6.9 Hz,

3H, NCH2CH3), 1.21 (s, 9H, C(CH3)3).

1H NMR (THF-d8, 500 MHz, 298 K): δ(ppm) = 7.51‒7.20 (m, 6H, HPh), 6.87 (t,

3JHH = 7.8 Hz, 1H), 5.41 (d, 3JHH = 7.0 Hz, 1H), 5.32 (d, 3JHH = 8.2 Hz,

1H), 3.13 (q, 3JHH = 6.6 Hz, 2H, NCH2CH3), 3.04 – 2.96 (m, 2H,

NCH2CH3), 1.21 (t, 3JHH = 7.1 Hz, 6H, 2 x NCH2CH3), 1.17 (s, 18H, 2 x

C(CH3)3).

29Si{1H} NMR (THF-d8, 99 MHz, 298 K): δ(ppm) = -19.1 ppm.

3Li NMR (THF-d8, 194 MHz, 298 K): δ 1.4 ppm.

5.6. Catalysis

5.6.1 General procedure for the regioselective hydroboration of N-heteroarenes

In a J. Young type NMR tube, 5 mol% of 7 (4.4 mg, 5.0 µmol) was added to a solution

containing N-heteroarene (0.1 mmol) in 0.5 mL C6D6 inside a glovebox. Then, HBpin (0.2-0.4

mmol) was added to the resulting mixture. The closed NMR tubes were taken out of the

glovebox and heated at 50 °C for specified time. The reactions were monitored by NMR

spectroscopy. The NMR yields were determined using mesitylene as an internal standard.

5.6.2 Mercury Test Experiment

A 25 mL Schlenk tube was charged with quinoline (26.4 µL, 0.223 mmol), mesitylene

(26.4 µL, 0.091 mmol, internal standard) and complex 7 (9.9 mg, 11.1 µmol) and 1 mL C6D6

was added into the Schlenk tube. Then mercury (250 mg, 1.15 mmol) was added into the

mixture. After stirring the reaction mixture at 50 °C for 12 h, conversion was determined by 1H

NMR spectroscopy. The hydroboration of quinoline was unaffected with >97% conversion in

the presence of Hg, indicative of a homogeneous process.

5.6.3 Kinetic isotope effect

In a glovebox, two NMR-samples, each containing 5 mol% of 7 (4.4 mg, 5.0 µmol) in

0.5 mL C6D6 containing mesitylene as internal standard (0.05 mmol, 6.9 µL), quinoline (0.1

mmol, 12.8 µL) and 2.0 equivalents of HBpin or DBpin were prepared. 1H NMR spectra were

obtained at regular intervals approx. 1 h and the NMR tubes were shaken after each

measurement. The concentration of quinoline was plotted against time and the data points

were fitted with a linear function (R2 = 0.99/0.98).

EXPERIMENTAL SECTION

142

quinoline

HBpin

mesitylene

millimoles

0.005

0.10

0.20

0.05

Concentration (mol/L)

0.010

0.22

0.40

0.10

The KIE was calculated using the following equation:

𝐾𝐼𝐸=𝑘𝐻

𝑘𝐷=−0.0237

−0.0129=1.84

5.6.4 General procedure for the Sonogashira cross-coupling reaction

In a glovebox, to a 10 mL Schlenk tube, containing alkyne (0.5 mmol), K2CO3 (138 mg,

1.0 mmol, 2.0 equiv), 5 mol% CuI (4.8 mg, 25 µmol) , 4 mL of THF was added. Then, 5 mol%

of 21 (22.5 mg, 25 µmol) and 0.5 mmol of vinyl halide were added. The closed tubes were

taken out of the glove box and heated for 24 h at 50 °C. After cooling the reaction mixtures to

room temperature, all the volatiles were evaporated and residues were then redissolved in n-

hexane. Subsequently purification by column chromatography by eluting with n-hexane/ethyl

acetate afforded the corresponding products. All the volatiles were then removed under

vacuum and isolated products were then analyzed by NMR spectroscopy. All the

characterization data was in accordance with the already reported data for corresponding 1,3-

enyne compounds.

5.6.5. Characterization of products of Sonogashira cross-coupling reaction

1H NMR (500 MHz, CDCl3, 298K) δ(ppm) = 7.50 – 7.38 (m, 2H), 7.35 – 7.18 (m, 3H), 6.27 (dt,

J = 15.8, 7.1 Hz, 1H), 5.77 – 5.63 (m, 1H), 2.18 (ddd, J = 14.7, 7.3, 1.6 Hz, 2H), 1.51 – 1.39

(m, 2H), 1.39 – 1.21 (m, 8H), 0.92 (s, 3H). 13C{1H} NMR (126 MHz, CDCl3, 298K) δ(ppm) =

145.40, 131.56, 128.39, 127.98, 123.82, 109.64, 88.54, 87.99, 33.38, 31.82, 28.95, 28.89,

22.74, 14.22.

EXPERIMENTAL SECTION

143

1H NMR (400 MHz, CDCl3, 298K) δ(ppm) = 7.53 (ddd, J = 3.6, 3.1, 1.4 Hz, 2H), 7.35 (dd, J =

4.7, 1.4 Hz, 3H), 6.36 (dd, J = 11.5, 0.8 Hz, 1H), 6.14 (dd, J = 11.4, 0.9 Hz, 1H), 4.34 – 4.21

(m, 2H), 1.34 (dd, J = 7.6, 6.7 Hz, 3H). 13C NMR (101 MHz, CDCl3, 298K) δ(ppm) = 164.97,

132.19, 129.31, 128.52, 128.41, 122.94, 101.30, 86.50, 60.58, 14.44.

1H NMR (500 MHz, CDCl3, 298K) δ(ppm) = 7.50 – 7.46 (m, 2H), 7.38 – 7.33 (m, 3H), 6.98 (d,

J = 15.8 Hz, 1H), 6.31 (d, J = 15.8 Hz, 1H), 4.24 (q, J = 7.1 Hz, 2H), 1.32 (t, J = 7.1 Hz, 3H).

13C NMR (126 MHz, CDCl3, 298K) δ(ppm) = 166.05, 132.10, 130.23, 129.43, 128.61, 125.20,

122.39, 98.37, 86.51, 60.90, 14.37.

1H NMR (500 MHz, CDCl3, 298K) δ(ppm) = 7.50 – 7.46 (m, 2H), 7.45 – 7.41 (m, 2H), 7.38 –

7.28 (m, 6H), 7.05 (d, J = 16.2 Hz, 1H), 6.39 (d, J = 16.2 Hz, 1H). 13C NMR (126 MHz, CDCl3,

298K) δ(ppm) = 141.42, 136.51, 131.67, 128.89, 128.77, 128.49, 128.33, 126.46, 123.60,

108.32, 91.90, 89.06.

1H NMR (400 MHz, CDCl3, 298K) δ(ppm) = 7.53 – 7.33 (m, 2H), 7.37 – 7.22 (m, 2H), 6.33 (d,

J = 11.4 Hz, 1H), 6.15 (d, J = 11.4 Hz, 1H), 4.28 – 4.23 (m, 2H), 1.32 (t, J = 7.1 Hz, 3H). 13C

NMR (101 MHz, CDCl3, 298K) δ(ppm) = 164.84, 135.45, 133.37, 128.91, 128.80, 122.63,

121.30, 99.91, 87.34, 60.60, 14.41.

EXPERIMENTAL SECTION

144

1H NMR (500 MHz, CDCl3, 298K) δ(ppm) = 7.50 – 7.42 (m, 2H), 7.40 – 7.30 (m, 2H), 6.31 (d,

J = 11.4 Hz, 1H), 6.14 (d, J = 11.4 Hz, 1H), 4.24 (q, J = 7.1 Hz, 2H), 1.31 (t, J = 7.2 Hz, 3H).

13C NMR (126 MHz, CDCl3, 298K) δ(ppm) = 164.76, 133.50, 131.80, 128.82, 123.72, 122.57,

121.73, 99.90, 87.47, 60.56, 14.38.

1H NMR (500 MHz, CDCl3, 298K) δ(ppm) = 7.52 (dd, J = 8.9, 5.4 Hz, 2H), 7.04 (t, J = 8.8 Hz,

2H), 6.33 (d, J = 11.4 Hz, 1H), 6.14 (d, J = 11.4 Hz, 1H), 4.26 (q, J = 7.1 Hz, 2H), 1.33 (t, J =

7.1 Hz, 3H). 13C NMR (126 MHz, CDCl3, 298K) δ(ppm) = 164.93 (s), 163.23 (d, J = 251.3 Hz),

134.23 (d, J = 8.7 Hz), 128.47 (s), 122.82 (s), 118.97 (d, J = 3.5 Hz), 115.91 (d, J = 22.2 Hz),

100.20 (s), 86.32 (s), 60.58 (s), 14.43 (s). 19F NMR (471 MHz, CDCl3, 298K) δ(ppm) = -109.30.

1H NMR (500 MHz, CDCl3) δ(ppm) = 8.21 (d, J = 9.0 Hz, 2H), 7.67 (d, J = 8.9 Hz, 2H), 6.36

(d, J = 11.5 Hz, 1H), 6.24 (d, J = 11.5 Hz, 1H), 4.27 (q, J = 7.1 Hz, 2H), 1.33 (t, J = 7.1 Hz,

3H). 13C NMR (126 MHz, CDCl3) δ(ppm) = 164.59, 147.72, 132.85, 130.52, 129.61, 123.77,

121.84, 98.19, 90.78, 60.83, 14.41.

EXPERIMENTAL SECTION

145

1H NMR (500 MHz, CDCl3) δ(ppm) = 7.35 (d, J = 8.6 Hz, 2H), 6.61 (d, J = 8.7 Hz, 2H), 6.35

(d, J = 11.4 Hz, 1H), 6.03 (d, J = 11.4 Hz, 1H), 4.26 (q, J = 7.1 Hz, 2H), 1.33 (t, J = 7.1 Hz,

3H). 13C NMR (126 MHz, CDCl3) δ(ppm) = 165.32, 147.81, 133.99, 126.32, 123.66, 114.77,

112.04, 103.29, 85.69, 60.40, 14.50.

1H NMR (500 MHz, CDCl3) δ(ppm) = 7.48 (d, J = 8.9 Hz, 2H), 6.87 (d, J = 8.9 Hz, 2H), 6.35

(d, J = 11.4 Hz, 1H), 6.08 (d, J = 11.4 Hz, 1H), 4.26 (q, J = 7.1 Hz, 2H), 3.82 (s, 3H), 1.34 (t,

J = 7.1 Hz, 4H).13C NMR (126 MHz, CDCl3) δ(ppm) = 165.16, 160.60, 133.97, 133.90, 127.34,

127.24, 123.39, 123.29, 114.93, 114.23, 101.98, 85.91, 60.49, 55.51, 14.46.

1H NMR (500 MHz, CDCl3) δ(ppm) = 7.44 (d, J = 8.1 Hz, 2H), 7.20 – 7.10 (m, 2H), 6.37 (d, J

= 11.4 Hz, 1H), 6.12 (d, J = 11.4 Hz, 1H), 4.28 (q, J = 7.1 Hz, 2H), 2.38 (s, 3H), 1.35 (t, J =

7.1 Hz, 3H). 13C NMR (126 MHz, CDCl3) δ(ppm) = 165.08, 139.70, 132.18, 129.33, 127.91,

123.16, 119.80, 101.81, 86.15, 60.55, 21.74, 14.47.

EXPERIMENTAL SECTION

146

1H NMR (500 MHz, CDCl3) δ(ppm) = 7.51 – 7.44 (m, 2H), 7.39 – 7.34 (m, 2H), 6.36 (d, J =

11.4 Hz, 1H), 6.10 (d, J = 11.4 Hz, 1H), 4.27 (q, J = 7.1 Hz, 2H), 1.34 (t, J = 7.1 Hz, 3H), 1.32

(s, 9H). 13C NMR (126 MHz, CDCl3) δ(ppm) = 165.01, 152.73, 131.97, 127.91, 125.51, 123.10,

119.80, 101.73, 86.10, 60.48, 34.99, 31.24, 14.45.

1H NMR (500 MHz, CDCl3) δ(ppm) = 7.38 – 7.32 (m, 2H), 7.23 (t, J = 7.6 Hz, 1H), 7.19 – 7.13

(m, 1H), 6.35 (d, J = 11.4 Hz, 1H), 6.12 (d, J = 11.4 Hz, 1H), 4.27 (q, J = 7.1 Hz, 2H), 2.34(s,

3H), 1.34 (t, J = 7.1 Hz, 3H). 13C NMR (126 MHz, CDCl3) δ(ppm) = 164.96, 138.20, 132.71,

130.23, 129.29, 128.39, 128.18, 123.04, 122.61, 101.62, 86.19, 60.51, 21.27, 14.42.

1H NMR (500 MHz, CDCl3) δ(ppm) = 6.13 (dt, J = 11.4, 2.5 Hz, 1H), 6.01 (d, J = 11.4 Hz, 1H),

4.21 (q, J = 7.1 Hz, 2H), 2.43 (td, J = 7.2, 2.4 Hz, 2H), 1.64 – 1.52 (m, 2H), 1.46 – 1.37 (m,

2H), 1.29 (t, J = 7.1 Hz, 7H), 0.88 (t, J = 7.0 Hz, 3H). 13C NMR (126 MHz, CDCl3) δ(ppm) =

165.06, 127.54, 124.04, 104.34, 77.84, 60.34, 31.47, 28.76, 28.53, 22.65, 20.24, 14.38, 14.15.

1H NMR (500 MHz, CDCl3) δ(ppm) = 8.62 (ddd, J = 4.9, 1.7, 0.9 Hz, 1H), 7.67 (td, J = 7.7, 1.8

Hz, 1H), 7.55 (dt, J = 7.8, 1.1 Hz, 1H), 7.29 – 7.20 (m, 1H), 6.37 (d, J = 11.5 Hz, 1H), 6.22 (d,

J = 11.5 Hz, 1H), 4.26 (q, J = 7.1 Hz, 2H), 1.33 (t, J = 7.1 Hz, 4H). 13C NMR (126 MHz, CDCl3)

δ(ppm) = 164.63, 150.42, 143.09, 136.24, 130.31, 127.87, 123.48, 122.18, 99.48, 85.52,

60.74, 14.38.

EXPERIMENTAL SECTION

147

1H NMR (500 MHz, CDCl3) δ(ppm) = 7.49 (s, 2H), 6.35 (d, J = 11.4 Hz, 1H), 6.15 (d, J = 11.4

Hz, 1H), 4.25 (q, J = 7.1 Hz, 2H), 1.32 (t, J = 7.1 Hz, 3H).13C NMR (126 MHz, CDCl3) δ(ppm)

= 164.83, 132.10, 128.94, 123.52, 122.57, 100.50, 88.55, 60.63, 14.41.

REFERENCES

148

REFERENCES

149

6. REFERENCES

[1] (a) N. V. Tzouras, I. K. Stamatopoulos, A. T. Papastavrou, A. A. Liori, G. C.

Vougioukalakis, Coord. Chem. Rev. 2017, 343, 25–138. (b). Q.-L. Zhou, Angew.

Chem. Int. Ed. 2016, 55, 5352–5353.

[2] (a) D. Steinborn, Fundamentals of Organometallic Catalysis; Wiley-VCH: Weinheim,

2012; DOI: 10.1002/anie.201201768. (b) R. H. Crabtree, in The Organometallic

Chemistry of the Transition Metals, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2005,

pp. 235–273.

[3] W. A. Herrmann, B. Cornils, Angew. Chem. Int. Ed. Engl. 1997, 36, 1048–1067.

[4] D. Astruc, Organometallic chemistry and catalysis (Vol. 291). Springer, Berlin, 2007.

[5] L. Alig, M. Fritz, S. Schneider, Chem. Rev. 2019, 119, 2681–2751.

[6] T. S. Piper, D. Lemal, G. Wilkinson, Naturwissenschaften 1956, 43, 129–129.

[7] N. R. Thomas, Silicon 2010, 2, 187–193.

[8] For the reviews of the divalent group 14 element chemistry, see: (a) M. Haaf, T. A.

Schmedake, R. West, Acc. Chem. Res. 2000, 33, 704–714. (b) M. Kira, J. Organomet.

Chem. 2004, 689, 4475–4488. (c) Y. Mizuhata, T. Sasamori, N. Tokitoh, Chem. Rev.

2009, 109, 3479–3511. (d) M. Asay, C. Jones, M. Driess, Chem. Rev. 2011, 111, 354–

396.

[9] (a) B. Blom, M. Stoelzel, M. Driess, Chem. - Eur. J. 2013, 19, 40–62. (b) B. Blom, D.

Gallego, M. Driess, Inorg. Chem. Front. 2014, 1, 134–148. (c) S. Raoufmoghaddam,

Y.-P. Zhou, Y. Wang, M. Driess, J. Organomet. Chem. 2017, 829, 2–10.

[10] (a) C. J. Moulton, B. L. Shaw, J. Chem. Soc., Dalton Trans. 1976, 1020–1024. (b) H.

D. Empsall, E. M. Hyde, R. Markham, W. S. McDonald, M. C. Norton, B. L. Shaw, B.

J. Weeks, Chem. Soc., Chem. Commun. 1977, 589–590. (c) C. Crocker, H. D. Empsall,

R. J. Errington, E. M. Hyde, W. S. McDonald, R. Markham, M. C. Norton, B. L. Shaw,

B. Weeks, J. Chem. Soc., Dalton Trans. 1982, 1217–1224.

[11] G. van Koten, Pure Appl. Chem. 1989, 61, 1681–1694.

[12] S. Yao, Y. Xiong, A. Saddington, M. Driess, Chem. Commun., 2021, 57, 10139–10153.

[13] Y. Zhou, M. Driess, Angew. Chem. Int. Ed. 2019, 58, 3715–3728.

[14] Z. Benedek, T. Szilvási, RSC Advances 2015, 5, 5077–5086.

[15] The search result includes developments in English only (as of November 2022).

[16] E. Peris, R. H. Crabtree, Chem. Soc. Rev. 2018, 47, 1959–1968.

[17] (a) M. Albrecht, G. van Koten, Angew. Chem. Int. Ed. 2001, 40, 3750–3781. (b) A.

Vigalok, D. Milstein, Acc. Chem. Res. 2001, 34, 798–807.

[18] M. Vogt, R. Langer, Eur. J. Inorg. Chem. 2020, 2020, 3885–3898.

REFERENCES

150

[19] G. E. Martinez, C. Ocampo, Y. J. Park, A. R. Fout, J. Am. Chem. Soc. 2016, 138,

4290–4293.

[20] a) O. Vechorkin, D. Barmaz, V. Proust, X. Hu, J. Am. Chem. Soc. 2009, 131, 12078–

12079. b) P. M. Perez Garcia, T. Di Franco, A. Epenoy, R. Scopelliti, X. Hu, ACS Catal.

2016, 6, 258–261.

[21] J. A. Loch, M. Albrecht, E. Peris, J. Mata, J. W. Faller, R. H. Crabtree, Organometallics

2002, 21, 700–706.

[22] D. Benito-Garagorri, K. Kirchner, Acc. Chem. Res. 2008, 41, 201–213.

[23] Y. Kim, J. Lee, Y.-H. Son, S.-U. Choi, M. Alam, S. Park, J. Inor. Biochem. 2020, 205,

111015.

[24] G. M. Adams, A. S. Weller, Coord. Chem. Rev. 2018, 355, 150–172.

[25] F. J. Fernández-Alvarez, R. Lalrempuia, L. A. Oro, Coord. Chem. Rev. 2017, 350, 49–

60.

[26] D. Gallego, A. Brück, E. Irran, F. Meier, M. Kaupp, M. Driess, J. F. Hartwig, J. Am.

Chem. Soc. 2013, 135, 15617–15626.

[27] D. Gallego, S. Inoue, B. Blom, M. Driess, Organometallics 2014, 33, 6885–6897.

[28] H. Kameo, H. Nakazawa, Chem. Asian J. 2013, 8, 1720–1734.

[29] L. Piccirilli, D. Lobo Justo Pinheiro, M. Nielsen, Catalysts 2020, 10, 773.

[30] C. M. Jensen, Chem. Commun. 1999, 2443–2449.

[31] H. Tanaka, K. Arashiba, S. Kuriyama, A. Sasada, K. Nakajima, K. Yoshizawa, Y.

Nishibayashi, Nat Commun 2014, 5, 3737.

[32] (a) S. Chakraborty, J. Zhang, Y. J. Patel, J. A. Krause, H. Guan, Inorg. Chem. 2013,

52, 37–47. (b) N. A. Eberhardt, H. Guan, Chem. Rev. 2016, 116, 8373–8426.

[33] M. Hölscher, M. H. G. Prechtl, W. Leitner, Chem. Eur. J. 2007, 13, 6636–6643.

[34] (a) M. Garbe, K. Junge, M. Beller, Eur. J. Org. Chem. 2017, 2017, 4344–4362. (b) N.

Gorgas, K. Kirchner, Acc. Chem. Res. 2018, 51, 1558–1569.

[35] W. H. Bernskoetter, C. K. Schauer, K. I. Goldberg, M. Brookhart, Science 2009, 326,

553–556.

[36] a) K. Arashiba, Y. Miyake, Y. Nishibayashi, Nature Chem 2011, 3, 120–125. b) M. J.

Chalkley, M. W. Drover, J. C. Peters, Chem. Rev. 2020, 120, 5582–5636. c) E.

Kinoshita, K. Arashiba, S. Kuriyama, Y. Miyake, R. Shimazaki, H. Nakanishi, Y.

Nishibayashi, Organometallics 2012, 31, 8437–8443. d) S. Kuriyama, K. Arashiba, K.

Nakajima, H. Tanaka, N. Kamaru, K. Yoshizawa, Y. Nishibayashi, J. Am. Chem. Soc.

2014, 136, 9719–9731. e) Y. Tanabe, Y. Nishibayashi, Chem. Soc. Rev. 2021, 50,

5201–5242.

[37] F. Bertini, M. Glatz, N. Gorgas, B. Stöger, M. Peruzzini, L. F. Veiros, K. Kirchner, L.

Gonsalvi, Chem. Sci. 2017, 8, 5024–5029.

REFERENCES

151

[38] S. Elangovan, C. Topf, S. Fischer, H. Jiao, A. Spannenberg, W. Baumann, R. Ludwig,

K. Junge, M. Beller, J. Am. Chem. Soc. 2016, 138, 8809–8814.

[39] S. Kostera, S. Weber, M. Peruzzini, L. F. Veiros, K. Kirchner, L. Gonsalvi,

Organometallics 2021, 40, 1213–1220.

[40] Y. Zhou, M. Driess, Angew. Chem. Int. Ed. 2019, 58, 3715–3728.

[41] Z. Benedek, T. Szilvási, RSC Advances 2015, 5, 5077–5086.

[41] (a) D. Bourissou, O. Guerret, F. P. Gabbaï, G. Bertrand, Chem. Rev. 2000, 100, 39–

92. (b) M. Kira, S. Ishida, T. Iwamoto, Chem. Record 2004, 4, 243–253.

[42] M. Asay, C. Jones, M. Driess, Chem. Rev. 2011, 111, 354–396.

[43] (a) G. Trinquier, J. Am. Chem. Soc. 1990, 112, 2130–2137. (b) K. Krogh-Jespersen,

J. Am. Chem. Soc. 1985, 107, 537–543.

[44] P. S. Skell, E. J. Goldstein, J. Am. Chem. Soc. 1964, 86, 1442–1442.

[45] (a) J. M. Jasinski, R. Becerra, R. Walsh, Chem. Rev. 1995, 95, 1203–1228. (b) A. G.

Moiseev, W. J. Leigh, Organometallics 2007, 26, 6277–6289. (c) Wataru. Ando, Akira.

Sekiguchi, Kazuyoshi. Hagiwara, Akihito. Sakakibara, Hitoaki. Yoshida,

Organometallics 1988, 7, 558–559.

[46] R. West, M. J. Fink, J. Michl, Science 1981, 214, 1343–1344.

[47] (a) P. Jutzi, D. Kanne, C. Krüger, Angew. Chem. Int. Ed. Engl. 1986, 25, 164–164. (b)

P. Jutzi, U. Holtmann, D. Kanne, C. Krüger, R. Blom, R. Gleiter, I. Hyla‐Kryspin, Chem.

Ber. 1989, 122, 1629–1639.

[48] A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1991, 113, 361–363.

[49] M. Denk, R. Lennon, R. Hayashi, R. West, A. V. Belyakov, H. P. Verne, A. Haaland,

M. Wagner, N. Metzler, J. Am. Chem. Soc. 1994, 116, 2691–2692.

[50] M. Kira, S. Ishida, T. Iwamoto, C. Kabuto, J. Am. Chem. Soc. 1999, 121, 9722–9723.

[51] M. Asay, S. Inoue, M. Driess, Angew. Chem. Int. Ed. 2011, 50, 9589–9592.

[52] T. Kosai, S. Ishida, T. Iwamoto, Angew. Chem. Int. Ed. 2016, 55, 15554–15558.

[53] A. Rosas-Sánchez, I. Alvarado-Beltran, A. Baceiredo, D. Hashizume, N. Saffon-

Merceron, V. Branchadell, T. Kato, Angew. Chem. Int. Ed. 2017, 56, 4814–4818.

[54] G.-H. Lee, R. West, T. Müller, J. Am. Chem. Soc. 2003, 125, 8114–8115.

[55] B. D. Rekken, T. M. Brown, J. C. Fettinger, H. M. Tuononen, P. P. Power, J. Am. Chem.

Soc. 2012, 134, 6504–6507.

[56] A. V. Protchenko, K. H. Birjkumar, D. Dange, A. D. Schwarz, D. Vidovic, C. Jones, N.

Kaltsoyannis, P. Mountford, S. Aldridge, J. Am. Chem. Soc. 2012, 134, 6500–6503.

[57] A. V. Protchenko, A. D. Schwarz, M. P. Blake, C. Jones, N. Kaltsoyannis, P. Mountford,

S. Aldridge, Angew. Chem. Int. Ed. 2013, 52, 568–571.

[58] G. R. Gillette, G. Noren, R. West, Organometallics 1990, 9, 2925–2933.

REFERENCES

152

[59] (a) M. Driess, S. Yao, M. Brym, C. van Wüllen, D. Lentz, J. Am. Chem. Soc. 2006, 128,

9628–9629. (b) M. Driess, S. Yao, M. Brym, C. van Wüllen, Angew. Chem. Int. Ed.

2006, 45, 6730–6733.

[60] (a) S. S. Sen, S. Khan, P. P. Samuel, H. W. Roesky, Chem. Sci. 2012, 3, 659–682. (b)

A. C. Filippou, O. Chernov, G. Schnakenburg, Angew. Chem., Int. Ed. 2009, 48, 5687–

5690.

[61] (a) C.-W. So, H. W. Roesky, J. Magull and R. B. Oswald, Angew. Chem., Int. Ed.,

2006, 45, 3948–3950. (b) C.-W. So, H. W. Roesky, J. Magull, R. B. Oswald, Angew.

Chem. 2006, 118, 4052–4054.

[62] S. S. Sen, H. W. Roesky, D. Stern, J. Henn, D. Stalke, J. Am. Chem. Soc. 2010, 132,

1123–1126.

[63] (a) M. Tretiakov, H. W. Roesky, in Efficient Methods for Preparing Silicon Compounds,

Elsevier, 2016, pp. 243–255. (b) R. Azhakar, R. S. Ghadwal, H. W. Roesky, H. Wolf,

D. Stalke, Organometallics 2012, 31, 4588–4592.

[64] R. S. Ghadwal, R. Azhakar, H. W. Roesky, Acc. Chem. Res. 2013, 46, 444–456.

[65] R. S. Ghadwal, S. S. Sen, H. W. Roesky, M. Granitzka, D. Kratzert, S. Merkel, D.

Stalke, Angew. Chem., Int. Ed. 2010, 49, 3952–3955.

[66] S. Takahashi, E. Bellan, A. Baceiredo, N. Saffon‐Merceron, S. Massou, N. Nakata, D.

Hashizume, V. Branchadell, T. Kato, Angew. Chem. 2019, 131, 10416–10420.

[67] S. Takahashi, J. Sekiguchi, A. Ishii, N. Nakata, Angew. Chem. Int. Ed. 2021, 60, 4055–

4059.

[68] T. Iwamoto, S. Ishida, in Organosilicon Compounds (Ed.: V.Ya. Lee), Academic Press,

2017, pp. 361–532.

[69] W. Petz, Chem. Rev. 1986, 86, 1019–1047

[70] (a) B. Blom, S. Enthaler, S. Inoue, E. Irran, M. Driess, J. Am. Chem. Soc. 2013, 135,

6703–6713. (b) G. Tan, B. Blom, D. Gallego, M. Driess, Organometallics 2014, 33,

363–369.

[71] C. Shan, S. Yao, M. Driess, Chem. Soc. Rev. 2020, 49, 6733–6754.

[72] S. Inoue in Discovering the Future of Molecular Sciences (Hrsg.: B. Pignataro), Wiley-

VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2014, S. 243.

[73] G. Tan, S. Enthaler, S. Inoue, B. Blom, M. Driess, Angew. Chem. Int. Ed. 2015, 54,

2214–2218.

[74] Y. Bai, J. Zhang, C. Cui, Chem. Commun. 2018, 54, 8124–8127.

[75] X. Qi, T. Zheng, J. Zhou, Y. Dong, X. Zuo, X. Li, H. Sun, O. Fuhr, D. Fenske,

Organometallics 2019, 38, 268–277.

[76] Y. Xiong, D. Chen, S. Yao, J. Zhu, A. Ruzicka, M. Driess, J. Am. Chem. Soc. 2021,

143, 6229–6237.

REFERENCES

153

[77] W. Wang, S. Inoue, S. Yao, M. Driess, J. Am. Chem. Soc. 2010, 132, 15890–15892.

[78] A. Brück, D. Gallego, W. Wang, E. Irran, M. Driess, J. F. Hartwig, Angew. Chem. Int.

Ed. 2012, 51, 11478–11482

[79] (a) W. Wang, S. Inoue, S. Enthaler, M. Driess, Angew. Chem. Int. Ed. 2012, 51, 6167–

6171. (b) W. Wang, S. Inoue, E. Irran, M. Driess, Angew. Chem. 2012, 124, 3751–

3754.

[80] D. Gallego, S. Inoue, B. Blom, M. Driess, Organometallics 2014, 33, 6885–6897.

[81] Y.-P. Zhou, S. Raoufmoghaddam, T. Szilvási, M. Driess, Angew. Chem. Int. Ed. 2016,

55, 12868–12872.

[82] Y. Wang, A. Kostenko, S. Yao, M. Driess, J. Am. Chem. Soc. 2017, 139, 13499–13506.

[83] A. Kostenko, M. Driess, J. Am. Chem. Soc. 2018, 140, 16962–16966.

[84] M.-P. Lücke, S. Yao, M. Driess, Chem. Sci. 2021, 12, 2909–2915.

[85] Y. Xiong, S. Dong, S. Yao, C. Dai, J. Zhu, S. Kemper, M. Driess, Angew. Chem. Int.

Ed. 2022, 61, DOI 10.1002/anie.202209250.

[86] B. Gehrhus, P. B. Hitchcock, M. F. Lappert, Z. anorg. allg. Chem. 2005, 631, 1383–

1386.

[87] S. S. Sen, A. Jana, H. W. Roesky, C. Schulzke, Angew. Chem. Int. Ed. 2009, 48, 8536–

8538.

[88] R. Rodriguez, D. Gau, Y. Contie, T. Kato, N. Saffon-Merceron, A. Baceiredo, Angew.

Chem. Int. Ed. 2011, 50, 11492–11495.

[89] S. Li, Y. Wang, W. Yang, K. Li, H. Sun, X. Li, O. Fuhr, D. Fenske, Organometallics

2020, 39, 757–766.

[90] M. Dehmel, M. A. Wünsche, H. Görls, R. Kretschmer, Eur. J. Inorg. Chem. 2021, 2021,

4806–4811

[91] J. I. van der Vlugt, Eur. J. Inorg. Chem. 2012, 2012, 363–375.

[92] P. P. Power, Nature 2010, 463, 171–177.

[93] (a) Y. Wang, T. Szilvási, S. Yao, M. Driess, Nat. Chem. 2020, 12, 801–807. (b) M.

Luecke, L. Giarrana, A. Kostenko, T. Gensch, S. Yao, M. Driess, Angew. Chem. 2022,

134, DOI 10.1002/ange.202110398. (c) S. Yao, T. Szilvási, Y. Xiong, C. Lorent, A.

Ruzicka, M. Driess, Angew. Chem. Int. Ed. 2020, 59, 22043–22047.

[94] Y.-P. Zhou, Z. Mo, M.-P. Luecke, M. Driess, Chem. Eur. J. 2018, 24, 4780–4784.

[95] M. Schmidt, B. Blom, T. Szilvási, R. Schomäcker, M. Driess, Eur. J. Inorg. Chem. 2017,

2017, 1284–1291.

[96] M. J. Krahfuß, J. Nitsch, F. M. Bickelhaupt, T. B. Marder, U. Radius, Chem. Eur. J.

2020, 26, 11276–11292.

[97] M. J. Krahfuss, U. Radius, Dalton Trans. 2021, 50, 6752–6765.

[98] A. Fürstner, H. Krause, C. W. Lehmann, Chem. Commun. 2001, 2372.

REFERENCES

154

[99] M. Zhang, X. Liu, C. Shi, C. Ren, Y. Ding, H. W. Roesky, Z. anorg. allg. Chem. 2008,

634, 1755–1758.

[100] C. I. Someya, M. Haberberger, W. Wang, S. Enthaler, S. Inoue, Chem. Lett. 2013, 42,

286–288.

[101] D. Gallego, A. Brück, E. Irran, F. Meier, M. Kaupp, M. Driess, J. F. Hartwig, J. Am.

Chem. Soc. 2013, 135, 15617–15626.

[102] H. Ren, Y.-P. Zhou, Y. Bai, C. Cui, M. Driess, Chem. Eur. J. 2017, 23, 5663–5667.

[103] T. T. Metsänen, D. Gallego, T. Szilvási, M. Driess, M. Oestreich, Chem. Sci. 2015, 6,

7143–7149.

[104] M.-P. Luecke, D. Porwal, A. Kostenko, Y.-P. Zhou, S. Yao, M. Keck, C. Limberg, M.

Oestreich, M. Driess, Dalton Trans. 2017, 46, 16412–16418.

[105] (a) S. Ishida, T. Iwamoto, C. Kabuto, M. Kira, Nature 2003, 421, 725–727. (b) T.

Iwamoto, T. Abe, C. Kabuto, M. Kira, Chem. Commun. 2005, 5190–5192.

[106] (a) Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. Schaefer, P. von R. Schleyer, G. H.

Robinson, Science 2008, 321, 1069–1071. (b) N. Takagi, T. Shimizu, G. Frenking,

Chem. Eur. J. 2009, 15, 8593–8604. (b)

[107] Y. Wang, M. Karni, S. Yao, A. Kaushansky, Y. Apeloig, M. Driess, J. Am. Chem. Soc.

2019, 141, 12916–12927.

[108] Y. Wang, M. Karni, S. Yao, Y. Apeloig, M. Driess, J. Am. Chem. Soc. 2019, 141, 1655–

1664.

[109] J. Xu, C. Dai, S. Yao, J. Zhu, M. Driess, Angew. Chem. Int. Ed. 2022, 61, e202114073

[110] J. Xu, S. Pan, S. Yao, G. Frenking, M. Driess, Angew Chem Int Ed 2022, 61, DOI

10.1002/anie.202209442.

[111] H. Wang, L. Wu, Z. Lin, Z. Xie, J. Am. Chem. Soc. 2017, 139, 13680–13683.

[112] S. Yao, A. Kostenko, Y. Xiong, A. Ruzicka, M. Driess, J. Am. Chem. Soc. 2020, 142,

12608–12612.

[113] S. Yao, A. Kostenko, Y. Xiong, C. Lorent, A. Ruzicka, M. Driess, Angew. Chem. Int.

Ed. 2021, 60, 14864–14868.

[114] (a) Y.-P. Zhou, M. Karni, S. Yao, Y. Apeloig, M. Driess, Angew. Chem. Int. Ed. 2016,

55, 15096–15099. (b) Y. Xiong, S. Dong, S. Yao, C. Dai, J. Zhu, S. Kemper, M. Driess,

Angew. Chem. Int. Ed. 2022, 61, e202209250. (c) Y. Xiong, S. Dong, S. Yao, J. Zhu,

M. Driess, Angew. Chem. Int. Ed. 2022, 61, e202205358.

[115] M. Elian, R. Hoffmann, Inorg. Chem. 1975, 14, 1058–1076.

[116] H. Nakazawa, J. Koe, Eds., Organometallic Chemistry:, Royal Society Of Chemistry,

Cambridge, 2021.

[117] M. C. Baird, J. Organomet. Chem. 2014, 751, 50–54.

[118] M. Wrighton, Chem. Rev. 1974, 74, 401–430.

REFERENCES

155

[119] R. L. Sweany, J. Halpern, J. Am. Chem. Soc. 1977, 99, 8335–8337.

[120] S. P. Church, M. Poliakoff, J. A. Timney, J. J. Turner, J. Am. Chem. Soc. 1981, 103,

7515–7520.

[121] S. A. Fairhurst, J. R. Morton, R. N. Perutz, K. F. Preston, Organometallics 1984, 3,

1389–1391.

[122] D. R. Kidd, C. P. Cheng, T. L. Brown, J. Am. Chem. Soc. 1978, 100, 4103–4107.

[123] P. P. Samuel, K. C. Mondal, H. W. Roesky, M. Hermann, G. Frenking, S. Demeshko,

F. Meyer, A. C. Stückl, J. H. Christian, N. S. Dalal, L. Ungur, L. F. Chibotaru, K.

Pröpper, A. Meents, B. Dittrich, Angew. Chem. Int. Ed. 2013, 52, 11817–11821.

[124] J. Hicks, C. E. Hoyer, B. Moubaraki, G. L. Manni, E. Carter, D. M. Murphy, K. S. Murray,

L. Gagliardi, C. Jones, J. Am. Chem. Soc. 2014, 136, 5283–5286.

[125] D. W. Agnew, C. E. Moore, A. L. Rheingold, J. S. Figueroa, Angew. Chem. Int. Ed.

2015, 54, 12673–12677.

[126] J. Cheng, Q. Chen, X. Leng, Z. Ouyang, Z. Wang, S. Ye, L. Deng, Chem 2018, 4,

2844–2860.

[127] R. Azhakar, S. P. Sarish, H. W. Roesky, J. Hey, D. Stalke, Inorg. Chem. 2011, 50,

5039–5043.

[128] S. Kalra, D. Pividori, D. Fehn, C. Dai, S. Dong, S. Yao, J. Zhu, K. Meyer, M. Driess,

Chem. Sci. 2022, 13, 8634–8641.

[129] D. Reardon, G. Aharonian, S. Gambarotta, G. P. A. Yap, Organometallics 2002, 21,

786–788.

[130] M. H. Al-Afyouni, V. M. Krishnan, H. D. Arman, Z. J. Tonzetich, Organometallics 2015,

34, 5088–5094.

[131] S. Fu, Z. Shao, Y. Wang, Q. Liu, J. Am. Chem. Soc. 2017, 139, 11941–11948.

[132] F. Kallmeier, T. Irrgang, T. Dietel, R. Kempe, Angew Chem Int Ed 2016, 55, 11806–

11809.

[133] D. F. Evans, J. Chem. Soc. 1959, 81, 2003.

[134] T. Ayers, R. Turk, C. Lane, J. Goins, D. Jameson, S. J. Slattery, Inorganica Chim. Acta

2004, 357, 202–206.

[135] A. L. Smith, Spectrochimica Acta 1960, 16, 87–105.

[136] W. F. Edgell, J. W. Fisher, G. Asato, W. M. Risen, Inorg. Chem. 1969, 8, 1103–1108.

[137] J. Sun, R. S. Lu, R. Bau, G. K. Yang, Organometallics 1994, 13, 1317–1325.

[138] J. S. Price, D. J. H. Emslie, J. F. Britten, Angew. Chem. Int. Ed. 2017, 56, 6223–6227.

[139] J. S. Price, D. J. H. Emslie, Chem. Sci. 2019, 10, 10853–10869.

[140] A. W. Addison, T. N. Rao, J. Reedijk, J. Van Rijn, G. C. Verschoor, J. Chem. Soc. Dalt.

Trans. 1984, 1349–1356.

REFERENCES

156

[141] The Addison-Reedijk parameter  is a relationship between angles in a

pentacoordinated metal complex to define its geometry.  = (β – α)/60, where β is the

largest of the basal angles. For a perfect trigonal bipyramidal geometry β = 180°, α =

60°, then  = 1. For a perfect square-pyramidal β = 90°, α = 90°, then  = 0.

[142] J.-X. Zhang, F. K. Sheong, Z. Lin, Chem. Eur. J. 2018, 24, 9639–9650.

[143] J. X. Zhang, F. K. Sheong, Z. Lin, Wiley Interdiscip. Rev. Comput. Mol. Sci. 2020, 10,

e1469.

[144] F. K. Sheong, J. X. Zhang, Z. Lin, Phys. Chem. Chem. Phys. 2020, 22, 10076–10086.

[145] J. A. Bull, J. J. Mousseau, G. Pelletier, A. B. Charette, Chem. Rev. 2012, 112, 2642–

713.

[146] K. Oshima, T. Ohmura, M. Suginome, J. Am. Chem. Soc. 2012, 134, 3699–3702.

[147] A. S. Dudnik, V. L. Weidner, A. Motta, M. Delferro, T. J. Marks, Nature Chem 2014, 6,

1100–1107.

[148] H. Liu, M. Khononov, M. S. Eisen, ACS Catal. 2018, 8, 3673–3677.

[149] F. Zhang, H. Song, X. Zhuang, C.-H. Tung, W. Wang, J. Am. Chem. Soc. 2017, 139,

17775–17778.

[150] J. Liu, J.-Y. Chen, M. Jia, B. Ming, J. Jia, R.-Z. Liao, C.-H. Tung, W. Wang, ACS Catal.

2019, 9, 3849–3857.

[151] J. L. Lortie, T. Dudding, B. M. Gabidullin, G. I. Nikonov, ACS Catal. 2017, 7, 8454–

8459.

[152] M. Espinal-Viguri, S. E. Neale, N. T. Coles, S. A. Macgregor, R. L. Webster, J. Am.

Chem. Soc. 2019, 141, 572–582.

[153] J. F. Hartwig, S. Bhandari, P. R. Rablen, J. Am. Chem. Soc. 1994, 116, 1839–1844.

[154] (a) E. Frankland, Q. J. Chem. Soc. 1850, 2, 263–290. (b) F. A. Cotton, Chem. Rev.

1955, 55, 551–594. c) E. Hager, A. Sivaramakrishna, H. Clayton, M. Mogorosi, J.

Moss, Coord. Chem. Rev. 2008, 252, 1668–1688. d) G. Cahiez, C. Duplais, J.

Buendia, Chem. Rev. 2009, 109, 1434–1476.

[155] a) P. J. Davidson, M. F. Lappert, R. Pearce, Chem. Rev. 1976, 76, 219–242; b) G.

Wilkinson, Science 1974, 185, 109–112.

[156] R. A. Anderson, G. Wilkinson, J. Chem. Soc., Dalton Trans., 1977, 809–811.

[157] P. B. Hitchcock, M. F. Lappert, W. P. Leung, N. H. Buttrus, J. Organomet. Chem. 1990,

394, 57–67.

[158] P. B. Hitchcock, J. A. K. Howard, M. F. Lappert, W.-P. Leung, S. A. Mason, J. Chem.

Soc., Chem. Commun.1990, 847–849.

[159] C. G. Howard, G. Wilkinson, M. Thornton-Pett, M. B. Hursthouse, J. Chem. Soc. Dalt.

Trans. 1983, 2025–2030.

REFERENCES

157

[160] (a) C. G. Howard, G. S. Girolami, G. Wilkinson, M. Thornton-Pett, M. B. Hursthouse,

J. Chem. Soc. Dalt. Trans. 1983, 263, 2631–2637. (b) A. Alberola, V. L. Blair, L. M.

Carrella, W. Clegg, A. R. Kennedy, J. Klett, R. E. Mulvey, S. Newton, E. Rentschler, L.

Russo, Organometallics 2009, 28, 2112–2118. (c) R. A. Layfield, Chem. Soc. Rev.

2008, 37, 1098–1107.

[161] K. Yliheikkilä, K. Axenov, M. T. Räisänen, M. Klinga, M. P. Lankinen, M. Kettunen, M.

Leskelä, T. Repo, Organometallics 2007, 26, 980–987.

[162] J. S. Price, P. Chadha, D. J. H. Emslie, Organometallics 2016, 35, 168–180.

[163] G. Zhang, H. Zeng, J. Wu, Z. Yin, S. Zheng, J. C. Fettinger, Angew. Chem. Int. Ed.

2016, 55, 14369–14372.

[164] A. R. Kennedy, J. Klett, R. E. Mulvey, S. D. Robertson, Eur. J. Inorg. Chem. 2011,

2011, 4675–4679.

[165] M. S. Messina, J. M. Stauber, M. A. Waddington, A. L. Rheingold, H. D. Maynard, A.

M. Spokoyny, J. Am. Chem. Soc. 2018, 140, 7065–7069.

[166] H. H. Cramer, B. Chatterjee, T. Weyhermüller, C. Werlé, W. Leitner, Angew. Chem.

Int. Ed. 2020, 59, 15674–15681.

[167] J. A. Cabeza, P. García-Álvarez, M. Gómez-Gallego, L. González-Álvarez, A. D.

Merinero, M. A. Sierra, Chem. Eur. J. 2019, 25, 2222–2225.

[168] X. Sun, T. Simler, K. Reiter, F. Weigend, P. W. Roesky, Chem. Eur. J. 2020, 26,

14888–14895.

[169] a) J. Cámpora, P. Palma, C. M. Pérez, A. Rodríg-uez-Dalgado, E. Álvarez, E.

Gutiérrez-Puebla, Organometallics 2010,29, 2960–2970. b) P. Crewdson, S.

Gambarotta, G. P. A. Yap, L. K. Thompson, Inorg. Chem. 2003, 42, 8579–8584. c) R.

H. Duncan Lyngdoh, H. F. Schaefer, R. B. King, Chem. Rev. 2018, 118, 11626–11706.

[170] X. Chen, H. Wang, S. Du, M. Driess, Z. Mo, Angew. Chem. Int. Ed. 2022, 61,

e202114598

[171] K. Sonogashira, Y. Tohda, N. Hagihara, Tetrahedron Lett., 1975, 16, 4467–4470.

[172] V. Brun, M. Legraverend, D. S. Grierson, Tetrahedron Lett., 2001, 42, 8169–8171

[173] R. Chinchilla, C. Nájera, Chem. Rev. 2007, 107, 874–922.

[174] R. Chinchilla, C. Nájera, Chem. Soc. Rev. 2011, 40, 5084.

[175] P. P. Nair, R. M. Philip, G. Anilkumar, Org. Biomol. Chem. 2021, 19, 4228–4242.

[176] Y. Yamamoto, K. Ehara, K. Takahashi, H. Yamazaki, BCSJ 1991, 64, 3376–3383.

[177] S. Lapointe, E. Khaskin, R. R. Fayzullin, J. R. Khusnutdinova, Organometallics 2019,

38, 1581–1594.

[178] K. Fukatsu, M Harada, S. Hinuma, Y. Ito, S. Sasaki, N. Suzuki, T. Yasuma, Patent EP

1559422 A1,2005;Takeda Pharmaceutical Company Limited.

[179] K. Osakada, R. Sakata, T. Yamamoto, Organometallics 1997, 16, 5354–5364.

REFERENCES

158

[180] S. Díez-González, in Advances in Organometallic Chemistry, Elsevier, 2016, pp. 93–

141.

[181] U. Rosenthal, C. Nauck, P. Arndt, S. Pulst, W. Baumann, V. V. Burlakov, H. Görls, J.

Organomet. Chem. 1994, 484, 81–87.

[182] B. L. Edelbach, R. J. Lachicotte, W. D. Jones, Organometallics 1999, 18, 4040–4049.

[183] J. Hao, B. Mougang-Soumé, B. Vabre, D. Zargarian, Angew. Chem. 2014, 126, 3282–

3286.

[184] G. A. Bain, J. F. Berry, J. Chem. Educ. 2008, 85, 532–536.

[185] E. Bill, SQUID Program julX2, 2019.

[186] E. Bill, EPR Program eview, 2019.

[187] E. Bill, EPR Program esim, 2019.

[188] B. J. Gaffney, J. J. Silverstone, Simulation of the EMR Spectra of High-Spin Iron in

Proteins (Plenum Press, New York, 1993).

[189] G. M. Sheldrick, SHELX-97 Program for Crystal Structure Determination, Universität

Göttingen, Germany (1997).

[190] Gaussian 09, Revision A.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,

M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji,

X. Li, M. Caricato, A. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci,

H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F.

Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D.

Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M.

Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,

H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M.

Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R.

Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J.

Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L.

Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox, Gaussian, Inc.,

Wallingford CT, 2016.

[191] C. Adamo, V. Barone, J. Chem. Phys. 1999, 110, 6158–6170.

[192] S. Grimme, S. Ehrlich, L. Goerigk, J. Comput. Chem. 2011, 32, 1456–1465.

[193] F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 2005, 7, 3297–3305.

[194] J. Zheng; X. Xu, D. G. Truhlar, Theor. Chem. Acc. 2010, 128, 295–305.

[195] E. Papajak, J. Zheng, X. Xu, H. R. Leverentz, D. G. Truhlar, J. Chem. Theory. Comput.

2011, 7, 3027–3034.

[196] E. D. Glendening, J.K. Badenhoop, A. E. Reed, J. E. Carpenter, J. A. Bohmann, C. M.

Morales, C. R. Landis, F. Weinhold, NBO 7.0; Theoretical Chemistry Institute,

University of Wisconsin: Madison, WI, 2013. http://nbo7.chem.wisc.edu/.

REFERENCES

159

[197] T. Lu, F. Chen, J. Comput. Chem. 2012, 33, 580–592.

[198] W. Humphrey, A. Dalke, K. Schulten, 1996, 14, 33–38.

[199] Y. Zhao, V. Snieckus, Org. Lett. 2014, 16, 390–393.

[200] C. Morrill, R. H. Grubbs, J. Org. Chem. 2003, 68, 6031–6034.

APPENDIX

160

APPENDIX

161

7. APPENDIX

7.1. Crystallographic data

For complex 2:

Empirical formula C43H67Br2MnN7OSi2 (2)

Formula weight 968.97

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Orthorhombic

Space group Pbca

Unit cell dimensions a = 16.52850(10) Å α= 90°.

b = 17.37800(10) Å = 90°.

c = 34.2234(2) Å = 90°.

Volume 9830.06(10) Å3

Z 8

Density (calculated) 1.309 Mg/m3

Absorption coefficient 4.848 mm-1

F(000) 4040

Crystal size 0.270 x 0.120 x 0.080 mm3

Theta range for data collection 2.582 to 73.917°.

Index ranges -20<=h<=20, -21<=k<=21, -42<=l<=36

Reflections collected 68256

Independent reflections 9930 [R(int) = 0.0296]

Completeness to theta = 67.684° 100.0 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 9930 / 0 / 519

Goodness-of-fit on F2 1.028

Final R indices [I>2sigma(I)] R1 = 0.0341, wR2 = 0.0838

R indices (all data) R1 = 0.0387, wR2 = 0.0878

Largest diff. peak and hole 1.346 and -0.952 e.Å-3

APPENDIX

162

For complex 3:

Empirical formula C49H68Cl2MnN4O2Si2

Formula weight 927.09

Temperature 149.99(10) K

Wavelength 1.54184 Å

Crystal system Monoclinic

Space group P21/c

Unit cell dimensions a = 19.3488(3) Å α = 90°.

b = 15.5819(2) Å = 112.097(2)°.

c = 18.4655(2) Å  = 90°.

Volume 5158.26(14) Å3

Z 4

Density (calculated) 1.194 Mg/m3

Absorption coefficient 3.782 mm-1

F(000) 1972

Crystal size 0.27 x 0.18 x 0.07 mm3

Theta range for data collection 3.759 to 73.912°.

Index ranges -23<=h<=24, -19<=k<=19, -16<=l<=22

Reflections collected 21904

Independent reflections 10180 [R(int) = 0.0262]

Completeness to theta = 67.684° 99.8 %

Absorption correction Semi-empirical from equivalents

Max. and min. transmission 1.00000 and 0.37579

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 10180 / 0 / 557

Goodness-of-fit on F2 1.049

Final R indices [I>2sigma(I)] R1 = 0.0549, wR2 = 0.1501

R indices (all data) R1 = 0.0646, wR2 = 0.1605

Extinction coefficient n/a

Largest diff. peak and hole 2.090 and -0.906 e.Å-3

APPENDIX

163

For complex 4:

Empirical formula C36H64B10Cl2MnN4OSi2

Formula weight 859.03

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Triclinic

Space group P-1

Unit cell dimensions a = 13.9098(6) Å α = 70.407(4)°.

b = 14.3209(5) Å = 69.447(4)°.

c = 14.3227(7) Å  = 61.168(4)°.

Volume 2291.9(2) Å3

Z 2

Density (calculated) 1.245 Mg/m3

Absorption coefficient 4.163 mm-1

F(000) 906

Crystal size 0.230 x 0.150 x 0.070 mm3

Theta range for data collection 3.365 to 67.487°.

Index ranges -15<=h<=16, -12<=k<=17, -17<=l<=17

Reflections collected 15818

Independent reflections 8258 [R(int) = 0.0374]

Completeness to theta = 67.487° 99.9 %

Absorption correction Semi-empirical from equivalents

Max. and min. transmission 1.00000 and 0.46513

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 8258 / 0 / 517

Goodness-of-fit on F2 1.017

Final R indices [I>2sigma(I)] R1 = 0.0384, wR2 = 0.0929

R indices (all data) R1 = 0.0505, wR2 = 0.1007

Extinction coefficient n/a

Largest diff. peak and hole 0.589 and -0.310 e.Å-3

APPENDIX

164

For complex 5:

Empirical formula C47H77MnN7OSi4

Formula weight 923.45

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Triclinic

Space group P-1

Unit cell dimensions a = 9.2529(5) Å α = 83.863(4)°.

b = 12.7440(6) Å = 85.515(4)°.

c = 24.0197(13) Å = 68.724(4)°.

Volume 2621.8(2) Å3

Z 2

Density (calculated) 1.170 Mg/m3

Absorption coefficient 3.225 mm-1

F(000) 994

Crystal size 0.220x0.150x0.070 mm3

Theta range for data collection 3.705 to 69.993°.

Index ranges -11<=h<=11, -15<=k<=14, -29<=l<=28

Reflections collected 18092

Independent reflections 9910 [R(int) = 0.0700]

Completeness to theta = 67.684° 99.9 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 9910 / 0 / 554

Goodness-of-fit on F2 1.106

Final R indices [I>2sigma(I)] R1 = 0.1232, wR2 = 0.3331

R indices (all data) R1 = 0.1406, wR2 = 0.3424

Extinction coefficient n/a

Largest diff. peak and hole 2.141 and -0.606 e.Å-3

APPENDIX

165

For complex 6:

Empirical formula C48H82MnN7P2Si2

Formula weight 930.26

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Monoclinic

Space group P21/c

Unit cell dimensions a = 13.5113(3) Å a= 90°.

b = 13.8328(3) Å = 92.402(2)°.

c = 28.2254(5) Å = 90°.

Volume 5270.67(19) Å3

Z 4

Density (calculated) 1.172 Mg/m3

Absorption coefficient 3.330 mm-1

F(000) 2008

Crystal size 0.252 x 0.086 x 0.074 mm3

Theta range for data collection 3.134 to 67.496°.

Index ranges -16<=h<=16, -16<=k<=16, -33<=l<=27

Reflections collected 35838

Independent reflections 9507 [R(int) = 0.0515]

Completeness to theta = 67.496° 100.0 %

Absorption correction Semi-empirical from equivalents

Max. and min. transmission 1.00000 and 0.31548

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 9507 / 0 / 564

Goodness-of-fit on F2 1.024

Final R indices [I>2sigma(I)] R1 = 0.0445, wR2 = 0.1058

R indices (all data) R1 = 0.0654, wR2 = 0.1198

Extinction coefficient n/a

Largest diff. peak and hole 0.531 and -0.354 e.Å-3

APPENDIX

166

For complex 7:

Empirical formula C45H75MnN7P2Si2

Formula weight 887.18

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Monoclinic

Space group P21

Unit cell dimensions a = 9.52200(10) Å a= 90°.

b = 18.7589(2) Å = 97.0220(10)°.

c = 13.6576(2) Å  = 90°.

Volume 2421.25(5) Å3

Z 2

Density (calculated) 1.217 Mg/m3

Absorption coefficient 3.602 mm-1

F(000) 954

Crystal size 0.280 x 0.170 x 0.090 mm3

Theta range for data collection 3.260 to 67.496°.

Index ranges -11<=h<=11, -22<=k<=22, -16<=l<=16

Reflections collected 16262

Independent reflections 8126 [R(int) = 0.0485]

Completeness to theta = 67.496° 99.9 %

Absorption correction Semi-empirical from equivalents

Max. and min. transmission 1.00000 and 0.11869

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 8126 / 1 / 533

Goodness-of-fit on F2 1.054

Final R indices [I>2sigma(I)] R1 = 0.0407, wR2 = 0.1048

R indices (all data) R1 = 0.0447, wR2 = 0.1093

Absolute structure parameter -0.014(5)

Extinction coefficient n/a

Largest diff. peak and hole 1.158 and -0.392 e.Å-3

APPENDIX

167

For complex 8:

Empirical formula C48H65MnN7O3Si2

Formula weight 899.19

Temperature 293(2) K

Wavelength 1.54184 Å

Crystal system Monoclinic

Space group P21

Unit cell dimensions a = 10.1252(3) Å a= 90°.

b = 18.0882(5) Å b= 103.606(3)°.

c = 13.3966(5) Å g = 90°.

Volume 2384.69(13) Å3

Z 2

Density (calculated) 1.252 Mg/m3

Absorption coefficient 3.105 mm-1

F(000) 958

Crystal size 0.260 x 0.190 x 0.130 mm3

Theta range for data collection 3.394 to 73.826°.

Index ranges -10<=h<=12, -22<=k<=16, -16<=l<=12

Reflections collected 9625

Independent reflections 6463 [R(int) = 0.0446]

Completeness to theta = 67.684° 99.9 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 6463 / 1 / 564

Goodness-of-fit on F2 1.034

Final R indices [I>2sigma(I)] R1 = 0.0582, wR2 = 0.1444

R indices (all data) R1 = 0.0686, wR2 = 0.1577

Absolute structure parameter -0.017(8)

Extinction coefficient n/a

Largest diff. peak and hole 0.736 and -0.627 e.Å-3

APPENDIX

168

For complex 14:

Empirical formula C25H49Cl2MnN7P2

Formula weight 635.49

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Orthorhombic

Space group P212121

Unit cell dimensions a = 10.95510(10) Å α = 90°.

b = 13.2628(2) Å  = 90°.

c = 22.1018(2) Å  = 90°.

Volume 3211.29(6) Å3

Z 4

Density (calculated) 1.314 Mg/m3

Absorption coefficient 6.020 mm-1

F(000) 1348

Crystal size 0.250 x 0.160 x 0.120 mm3

Theta range for data collection 3.887 to 72.740°.

Index ranges -13<=h<=13, -16<=k<=16, -21<=l<=27

Reflections collected 23863

Independent reflections 6305 [R(int) = 0.0296]

Completeness to theta = 67.684° 100.0 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 6305 / 0 / 344

Goodness-of-fit on F2 1.024

Final R indices [I>2sigma(I)] R1 = 0.0229, wR2 = 0.0561

R indices (all data) R1 = 0.0253, wR2 = 0.0575

Absolute structure parameter -0.0071(19)

Largest diff. peak and hole 0.187 and -0.228 e.Å-3

APPENDIX

169

For complex 15:

Empirical formula C33H71MnN7P2Si2

Formula weight 739.02

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Monoclinic

Space group P21/c

Unit cell dimensions a = 12.91850(10) Å α = 90°.

b = 12.5124(2) Å  = 90.6250(10)°.

c = 26.3472(3) Å  = 90°.

Volume 4258.55(9) Å3

Z 4

Density (calculated) 1.153 Mg/m3

Absorption coefficient 3.994 mm-1

F(000) 1604

Crystal size 0.270 x 0.080 x 0.060 mm3

Theta range for data collection 3.355 to 72.598°.

Index ranges -15<=h<=14, -15<=k<=14, -22<=l<=32

Reflections collected 18221

Independent reflections 8228 [R(int) = 0.0270]

Completeness to theta = 67.684° 99.9 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 8228 / 0 / 422

Goodness-of-fit on F2 1.025

Final R indices [I>2sigma(I)] R1 = 0.0332, wR2 = 0.0798

R indices (all data) R1 = 0.0409, wR2 = 0.0849

Largest diff. peak and hole 0.332 and -0.310 e.Å-3

APPENDIX

170

For complex 16:

Empirical formula C56H94Mn2N10Si4

Formula weight 1129.65

Temperature 150(2) K

Wavelength 0.71073 Å

Crystal system Monoclinic

Space group C2/c

Unit cell dimensions a = 14.2242(4) Å α = 90°.

b = 24.0815(5) Å  = 108.041(3)°.

c = 21.4539(5) Å  = 90°.

Volume 6987.5(3) Å3

Z 4

Density (calculated) 1.074 Mg/m3

Absorption coefficient 0.468 mm-1

F(000) 2424

Crystal size 0.380 x 0.080 x 0.060 mm3

Theta range for data collection 1.691 to 26.286°.

Index ranges -17<=h<=17, -26<=k<=29, -25<=l<=26

Reflections collected 14230

Independent reflections 6862 [R(int) = 0.0268]

Completeness to theta = 25.242° 99.3 %

Absorption correction Semi-empirical from equivalents

Max. and min. transmission 1.00000 and 0.37106

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 6862 / 1 / 336

Goodness-of-fit on F2 1.037

Final R indices [I>2sigma(I)] R1 = 0.0449, wR2 = 0.1175

R indices (all data) R1 = 0.0585, wR2 = 0.1266

Largest diff. peak and hole 0.325 and -0.542 e.Å-3

APPENDIX

171

For complex 18:

Empirical formula C21H30Br2MnN4Si

Formula weight 581.34

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Trigonal

Space group R-3

Unit cell dimensions a = 35.8312(5) Å α = 90°.

b = 35.8312(5) Å  = 90°.

c = 11.94920(10) Å  = 120°.

Volume 13285.9(4) Å3

Z 18

Density (calculated) 1.308 Mg/m3

Absorption coefficient 7.285 mm-1

F(000) 5274

Crystal size 0.210 x 0.080 x 0.060 mm3

Theta range for data collection 2.466 to 67.494°.

Index ranges -42<=h<=42, -42<=k<=42, -14<=l<=12

Reflections collected 31626

Independent reflections 5330 [R(int) = 0.0318]

Completeness to theta = 67.494° 100.0 %

Absorption correction Semi-empirical from equivalents

Max. and min. transmission 1.00000 and 0.42381

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 5330 / 0 / 269

Goodness-of-fit on F2 1.058

Final R indices [I>2sigma(I)] R1 = 0.0414, wR2 = 0.1124

R indices (all data) R1 = 0.0473, wR2 = 0.1169

Largest diff. peak and hole 0.893 and -0.540 e.Å-3

APPENDIX

172

For complex 19:

Empirical formula C38H68MnN4Si3

Formula weight 720.17

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Orthorhombic

Space group Pca21

Unit cell dimensions a = 11.74210(10) Å α = 90°.

b = 17.8160(2) Å  = 90°.

c = 20.8514(2) Å  = 90°.

Volume 4362.06(7) Å3

Z 4

Density (calculated) 1.097 Mg/m3

Absorption coefficient 3.454 mm-1

F(000) 1564

Crystal size 0.250 x 0.080 x 0.050 mm3

Theta range for data collection 2.480 to 72.718°.

Index ranges -14<=h<=14, -20<=k<=22, -24<=l<=25

Reflections collected 17442

Independent reflections 7551 [R(int) = 0.0240]

Completeness to theta = 67.684° 100.0 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 7551 / 1 / 433

Goodness-of-fit on F2 1.018

Final R indices [I>2sigma(I)] R1 = 0.0353, wR2 = 0.0905

R indices (all data) R1 = 0.0375, wR2 = 0.0926

Absolute structure parameter 0.003(4)

Largest diff. peak and hole 0.228 and -0.432 e.Å-3

APPENDIX

173

For complex 20:

Empirical formula C76H118Li3N15OSi3

Formula weight 1362.94

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Monoclinic

Space group P21/c

Unit cell dimensions a = 18.1030(3) Å α = 90°.

b = 18.3528(3) Å  = 94.3550(10)°.

c = 24.5390(4) Å  = 90°.

Volume 8129.3(2) Å3

Z 4

Density (calculated) 1.114 Mg/m3

Absorption coefficient 0.922 mm-1

F(000) 2952

Crystal size 0.320 x 0.080 x 0.070 mm3

Theta range for data collection 2.448 to 67.499°.

Index ranges -21<=h<=20, -21<=k<=21, -29<=l<=29

Reflections collected 55983

Independent reflections 14638 [R(int) = 0.0466]

Completeness to theta = 67.499° 100.0 %

Absorption correction Semi-empirical from equivalents

Max. and min. transmission 1.00000 and 0.93185

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 14638 / 15 / 960

Goodness-of-fit on F2 1.018

Final R indices [I>2sigma(I)] R1 = 0.0521, wR2 = 0.1315

R indices (all data) R1 = 0.0733, wR2 = 0.1492

Largest diff. peak and hole 0.757 and -0.447 e.Å-3

APPENDIX

174

For complex 21:

Empirical formula C39H59Br2N7NiSi2

Formula weight 900.64

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Monoclinic

Space group P2/n

Unit cell dimensions a = 15.9519(2) Å α = 90°.

b = 15.9643(2) Å = 91.9420(10)°.

c = 19.6316(2) Å  = 90°.

Volume 4996.53(10) Å3

Z 4

Density (calculated) 1.197 Mg/m3

Absorption coefficient 3.127 mm-1

F(000) 1872

Crystal size 0.250 x 0.190 x 0.110 mm3

Theta range for data collection 2.768 to 73.948°.

Index ranges -19<=h<=19, -19<=k<=19, -24<=l<=21

Reflections collected 36360

Independent reflections 9963 [R(int) = 0.0322]

Completeness to theta = 67.684° 100.0 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 9963 / 0 / 475

Goodness-of-fit on F2 1.040

Final R indices [I>2sigma(I)] R1 = 0.0376, wR2 = 0.1012

R indices (all data) R1 = 0.0428, wR2 = 0.1050

Extinction coefficient n/a

Largest diff. peak and hole 1.710 and -0.564 e.Å-3

APPENDIX

175

For complex 22:

Empirical formula C42H70Br2N7NiOPSi2

Formula weight 994.73

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Monoclinic

Space group P21/c

Unit cell dimensions a = 18.9613(4) Å α = 90°.

b = 17.2285(3) Å = 116.879(3)°.

c = 17.6427(4) Å  = 90°.

Volume 5140.8(2) Å3

Z 5

Density (calculated) 1.607 Mg/m3

Absorption coefficient 4.233 mm-1

F(000) 2600

Crystal size 0.280 x 0.160 x 0.070 mm3

Theta range for data collection 3.662 to 72.587°.

Index ranges -23<=h<=22, -19<=k<=20, -21<=l<=21

Reflections collected 21245

Independent reflections 9955 [R(int) = 0.0288]

Completeness to theta = 67.684° 99.8 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 9955 / 0 / 558

Goodness-of-fit on F2 1.042

Final R indices [I>2sigma(I)] R1 = 0.0356, wR2 = 0.0902

R indices (all data) R1 = 0.0456, wR2 = 0.0980

Extinction coefficient n/a

Largest diff. peak and hole 0.730 and -0.568 e.Å-3

APPENDIX

176

For complex 23:

Empirical formula C48H68Br2N8NiSi2

Formula weight 1031.81

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Monoclinic

Space group Pn

Unit cell dimensions a = 10.1944(11) Å α = 90°.

b = 14.7922(12) Å = 93.397(9)°.

c = 16.7832(13) Å  = 90°.

Volume 2526.4(4) Å3

Z 2

Density (calculated) 1.356 Mg/m3

Absorption coefficient 3.170 mm-1

F(000) 1076

Crystal size 0.250 x 0.130 x 0.080 mm3

Theta range for data collection 2.987 to 73.013°.

Index ranges -12<=h<=11, -14<=k<=18, -20<=l<=20

Reflections collected 18083

Independent reflections 8563 [R(int) = 0.0652]

Completeness to theta = 67.684° 100.0 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 8563 / 2 / 566

Goodness-of-fit on F2 1.024

Final R indices [I>2sigma(I)] R1 = 0.0754, wR2 = 0.1801

R indices (all data) R1 = 0.0944, wR2 = 0.1993

Absolute structure parameter -0.08(4)

Extinction coefficient n/a

Largest diff. peak and hole 1.706 and -0.803 e.Å-3

APPENDIX

177

For complex 24:

Empirical formula C48H70N8NiSi2

Formula weight 874.01

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Monoclinic

Space group P21/n

Unit cell dimensions a = 11.3011(3) Å α = 90°.

b = 24.2185(4) Å = 92.816(2)°.

c = 17.6822(3) Å = 90°.

Volume 4833.70(17) Å3

Z 5

Density (calculated) 1.501 Mg/m3

Absorption coefficient 1.701 mm-1

F(000) 2350

Crystal size 0.260 x 0.130 x 0.070 mm3

Theta range for data collection 3.097 to 72.731°.

Index ranges -13<=h<=12, -26<=k<=29, -21<=l<=16

Reflections collected 38007

Independent reflections 9488 [R(int) = 0.0578]

Completeness to theta = 67.684° 99.9 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 9488 / 0 / 548

Goodness-of-fit on F2 1.009

Final R indices [I>2sigma(I)] R1 = 0.0414, wR2 = 0.0923

R indices (all data) R1 = 0.0678, wR2 = 0.1052

Extinction coefficient n/a

Largest diff. peak and hole 0.311 and -0.260 e.Å-3

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178

For complex 25:

Empirical formula C25H49Br2N7NiP2

Formula weight 728.18

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Orthorhombic

Space group Pbca

Unit cell dimensions a = 11.9338(2) Å α = 90°.

b = 17.1234(3) Å = 90°.

c = 32.3823(5) Å  = 90°.

Volume 6617.23(19) Å3

Z 8

Density (calculated) 1.462 Mg/m3

Absorption coefficient 4.789 mm-1

F(000) 3008

Crystal size 0.240 x 0.120 x 0.070 mm3

Theta range for data collection 2.729 to 72.563°.

Index ranges -14<=h<=14, -21<=k<=20, -40<=l<=31

Reflections collected 25891

Independent reflections 6501 [R(int) = 0.0452]

Completeness to theta = 67.684° 100.0 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 6501 / 0 / 344

Goodness-of-fit on F2 1.029

Final R indices [I>2sigma(I)] R1 = 0.0366, wR2 = 0.0904

R indices (all data) R1 = 0.0514, wR2 = 0.1004

Extinction coefficient n/a

Largest diff. peak and hole 0.588 and -0.683 e.Å-3

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179

For complex 26:

Empirical formula C34H58N8NiP2

Formula weight 699.53

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Monoclinic

Space group P21/c

Unit cell dimensions a = 11.4055(2) Å α = 90°.

b = 26.5182(4) Å = 109.974(2)°.

c = 14.1423(2) Å  = 90°.

Volume 4020.09(12) Å3

Z 4

Density (calculated) 1.156 Mg/m3

Absorption coefficient 1.691 mm-1

F(000) 1504

Crystal size 0.220 x 0.0120 x 0.080 mm3

Theta range for data collection 3.333 to 72.680°.

Index ranges -14<=h<=13, -32<=k<=32, -15<=l<=17

Reflections collected 16278

Independent reflections 7810 [R(int) = 0.0221]

Completeness to theta = 67.684° 100.0 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 7810 / 0 / 446

Goodness-of-fit on F2 1.026

Final R indices [I>2sigma(I)] R1 = 0.0317, wR2 = 0.0808

R indices (all data) R1 = 0.0381, wR2 = 0.0853

Extinction coefficient n/a

Largest diff. peak and hole 0.464 and -0.282 e.Å-3

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180

For complex 30:

Empirical formula C46H65N7NiSi2

Formula weight 830.94

Temperature 150(2) K

Wavelength 1.54184 Å

Crystal system Triclinic

Space group P-1

Unit cell dimensions a = 17.3930(4) Å α = 106.612(2)°.

b = 17.4684(4) Å = 92.898(2)°.

c = 26.7481(4) Å  = 115.278(2)°.

Volume 6900.8(3) Å3

Z 7

Density (calculated) 1.400 Mg/m3

Absorption coefficient 1.632 mm-1

F(000) 3122

Crystal size 0.350 x 0.210 x 0.120 mm3

Theta range for data collection 2.867 to 72.668°.

Index ranges -21<=h<=21, -21<=k<=21, -22<=l<=33

Reflections collected 52673

Independent reflections 26669 [R(int) = 0.0182]

Completeness to theta = 67.684° 99.8 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 26669 / 0 / 1199

Goodness-of-fit on F2 1.045

Final R indices [I>2sigma(I)] R1 = 0.0404, wR2 = 0.1089

R indices (all data) R1 = 0.0455, wR2 = 0.1122

Extinction coefficient n/a

Largest diff. peak and hole 0.980 and -0.323 e.Å-3

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181

7.2. Computational details

Figure 7.1. The highest singly occupied molecular orbital (HSOMO) of the 7 (top). PISO

analysis on the bonding modes of Mn-Si1, Mn-P1 and Mn-N1 in the compound 7 (bottom).

Hydrogen atoms in 3D structures are omitted for clarity. .Total PBI value of Mn-Si1, Mn-P1

and Mn-N1 is 0.56, 0.66 and 0.32, respectively. The isosurfaces with 0.080 au isovalue are

plotted.

Figure 7.2. PISO analysis was performed on the bonding modes of Mn−Si1, Mn−C1 and

Si2−N1 in the compound 8. The total PBI value of Mn−Si1, Mn−C1 and Si2−N1 is 0.47, 0.86

and 0.26, respectively. The isosurfaces with 0.080 au isovalue are plotted

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182