Isolation of a germanium(II) cation and a germylene iron carbonyl complex utilizing an imidazolin-2-iminato ligand [original]

Dalton

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Cite this: Dalton Trans., 2015, 44,

10952

Received 24th April 2015,

Accepted 13th May 2015

DOI: 10.1039/c5dt01554e

www.rsc.org/dalton

Isolation of a germanium(II) cation and a

germylene iron carbonyl complex utilizing an

imidazolin-2-iminato ligand†

Tatsumi Ochiai, Daniel Franz, Xiao-Nan Wu and Shigeyoshi Inoue*

The novel amino(imino)germylene 1 was prepared by the con-

version of HNIPr (NIPr = bis(2,6-diisopropylphenyl)imidazolin-

2-imino) with 1 equiv. of Ge[N(SiMe

)

]

. Germylene 1 reacts with B

)

to aﬀord the borate salt 2

[MeB(C

)

]

−

in a methyl-

abstraction and ring-closing reaction. The conversion of 2 with

(CO)

furnishes the germylene iron carbonyl complex 3 with a

trigonal planar-coordinate germanium atom.

For decades, divalent compounds of germanium, mostly

referred to as germylenes, have been attractive targets for fun-

damental, as well as applied research. Succeeding the seminal

report of Harris and Lappert on the bis(amino)germylene Ge[N

(SiMe

)

]

in 1974,

a huge variety of germylenes are known to

date.

In sharp contrast to the neutral germylenes, the field of

germyliumylidenes, that is, germanium(II) monocations of the

type [RGe:]

(R = anionic ligand), is still in its infancy. This

may be due to the poor thermodynamic stability of this system

which demands for additional electron pair donor ligands at

the metal center. Despite the synthetic challenges, this class of

compounds has received great attention because of its pro-

nounced Lewis ambiphilic nature. Since the characterisation

of the half-sandwich germanocene cation [(C

)Ge:]

Jutzi and co-workers in 1980,

several types of donor-stabilised

germyliumylidenes have been described. For example, Dias

and co-workers implemented an aminotroponiminate ligand

to stabilise the highly reactive Ge(II) cation species (I, Fig. 1),

whereas a β-diketiminate ligand was introduced by Power (II,

Fig. 1).

Müller and co-workers synthesised the germyliumyli-

dene III (Fig. 1) by protonation of the respective uncharged

germylene at the ligand backbone.

Following a diﬀerent syn-

thetic approach, a number of germyliumylidenes have been

described that were formed via auto-ionisation of germanium

dichloride imposed by neutral chelate systems

7–10

or a carbo-

diphosphorane

ligand. Recently, Jones, Krossing and co-

workers reported a germanium(II) monocation stabilised by a

weak arene interaction.

Remarkably, germanium(II) dications

have also been reported.

We figured that a strongly electron-donating ligand would

grant thermodynamic stability to a highly electrophilic germy-

liumylidene species. The imidazolin-2-iminato ligand may act

as a 2σ- and either a 2π-ora4π-electron donor and, hence,

suits this criterion. Also, variation of its steric bulk may be

used to fine-tune the kinetic protection of a metal center. Over

the last two decades the properties of transition-metal com-

plexes of the imidazolin-2-iminato ligand have been investi-

gated thoroughly by Tamm and co-workers.

In contrast,

research on main group element compounds comprising this

system has been neglected in recent years. Rivard and co-

workers reported on the bis(imino)germylene IV (Fig. 1) and a

related silicon compound.

Our group isolated the Cp*-substi-

tuted iminosilylene V

(Fig. 1) and discovered a number of

applications for boron-,

as well as aluminium

complexes of

this ligand. Prior to this work we described the tin(II) mono-

Fig. 1 Selected low-valent group 14 element compounds: the germa-

nium(II) compounds I–IV (top), as well as imidazoline-2-iminato com-

plexes of silicon(II) and tin(II)(V,VI, bottom).

†Electronic supplementary information (ESI) available: Experimental details,

crystallographic data and details of the DFT calculations. CCDC 1054431 and

1054432. For ESI and crystallographic data in CIF or other electronic format see

DOI: 10.1039/c5dt01554e

Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135,

10623 Berlin, Germany. E-mail: shigeyoshi.inoue@tu-berlin.de

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cation VI (Fig. 1).

The straightforward synthesis of VI by

conversion of its amino(imino)stannylene precursor with tris-

(pentafluorophenyl)borane prompted us to examine the

germanium chemistry of this ligand system. Herein we

describe the synthesis and structural characterisation of the

germanium analogue of VI and an amino(imino)germylene

iron carbonyl complex.

The reaction of HNIPr with Ge[N(SiMe

)

]

in toluene at

50 °C furnishes the novel amino(imino)germylene 1in high

yield (87%, Scheme 1) which was confirmed by

H-,

C-, and

Si NMR analysis, as well as high resolution mass spectro-

metry. As already reported, a very similar synthetic approach

grants access to the tin congener of 1.

Following our established concept for the synthesis of VI

(Fig. 1) equimolar amounts of amino(imino)germylene 1and

B(C

)

were brought into contact in hexane solution

(Scheme 2). After work-up 2

[MeB(C

)

]

−

was obtained as a

crystalline solid in moderate yield (48%). The multinuclear

NMR study, as well as a single crystal X-ray diﬀraction analysis

verified our formulation of this borate salt and high resolution

mass spectrometry complemented its characterisation.

Colourless crystals of 2

[MeB(C

)

]

−

were grown in

toluene at −30 °C and the solid-state structure shows discrete

cations and anions (Fig. 2). We find the shortest distance

between the germanium center and a fluorine atom at 3.96 Å.

In comparison, the sum of the van der Waals radii of these

atoms amounts to 3.38 Å (1.98 Å for Ge and 1.40 Å for F).

The four-membered N

SiGe ring in the cation of 2

is planar

(the sum of the internal angles of the N

SiGe ring amounts to

360°). Interestingly, the Ge1–N4 bond length of 1.8437(15) Å in

resembles the Ge–N distances in uncharged Ge[N(SiMe

)

]

(1.878(5) Å and 1.873(5) Å).

However, it is clearly reduced as

compared to Iand II in which the Ge–N bond lengths range

from 1.894(2) Å to 1.917(5) Å.

4,5

In contrast, the Ge1–N1 dis-

tance of 1.9694(14) Å is significantly increased with respect to

that in Iand II. Note that the germanium(II) cation III also

exhibits a longer Ge–N distance to the imino nitrogen-atom

(1.9884(18) Å) and a shorter Ge–N contact to the amino nitro-

gen-atom (1.8286(21) Å).

In accordance with the conclusions

drawn by Müller for III, the X-ray study of 2

suggests a high

dative character of the Ge1–N1 bond and a significantly stron-

ger interaction between Ge1 and N4 in comparison. As a

result, one approximation to describe the bonding situation in

the germyliumylidene 2

is represented by resonance structure

A1 with the imino ligand functioning as an intramolecular

electron-pair donor to the germanium center (Scheme 3).

Scheme 3 Selected resonance structures of 2

(Dip = 2,6-diiso-

propylphenyl).

Scheme 1 Synthesis of amino(imino)germylene 1(Dip = 2,6-diiso-

propylphenyl).

Scheme 2 Synthesis of the four-membered germyliumylidene salt

[MeB(C

)

]

−

and the germylene iron carbonyl complex 3(Dip = 2,6-

diisopropylphenyl).

Fig. 2 ORTEP representation of the molecular structure of the cation

of 2

[MeB(C

)

]

−

in the solid state. Thermal ellipsoids are at the 40%

probability level. Dip groups are depicted as stick models. Hydrogen

atoms are omitted for clarity. Selected bond lengths (Å) and bond angles

(°): Ge1–N1, 1.9694(14); Ge1–N4, 1.8437(15); Si1–N1, 1.7814(14); Si1–N4,

1.7332(16); Si1–C28, 1.8426(19); Si1–C29, 1.849(2); Si2–N4, 1.7499(15);

C1–N1, 1.335(2); C1–N2, 1.358(2); C1–N3, 1.357(2); N1–Ge1–N4, 80.54

(6); N1–Si1–N4, 89.13(7); Si1–N1–Ge1, 92.09(6); Si1–N4–Ge1, 98.14(7).

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The C1–N1 distance of 1.335(2) Å in 2

is very similar to

the one observed for the imino group in its tin congener VI

(1.331(3) Å) and lies between that of a typical C–N single and a

CvN double bond.

As it had been demonstrated for VI

this

indicates the delocalisation of the positive charge of 2

into

the imidazoline moiety which is illustrated by the structural

formula A2 (Scheme 3).

In order to gain further insight into the electronic pro-

perties of the germanium(II) cation 2

, quantum-mechanical

calculations were carried out (see the ESI†). The Natural Popu-

lation Analysis (NPA) of 2

determines the charges at the ger-

manium center, the C

imine

atom and the two nitrogen atoms of

the imidazoline ring to +1.240, +0.609 and −0.348/−0.352

(Fig. S15†). The relation to the less positive/more negative NPA

charges in 1(+1.135, +0.580 and 0.399/−0.410) points out the

germanium(II) cation character of 2

and supports our charge

assignment in the structural formulations A1 and A2

(Scheme 3). The calculated Wiberg Bond Index (WBI) for the

Ge–N

imine

bond in 2

amounts to 0.476 which is considerably

lower than that of the Ge–N

amine

interaction (0.729). For com-

parison, we calculated a WBI of 0.774 for the Ge–N

imine

bond

in 1. The WBI of the C

NHC

–N

imine

bond in 2

(1.291) is lower

than that of the C

NHC

–N

imine

bond in 1(1.590). These results

underline the conclusion of our X-ray study on 2

(vide supra).

The LUMO+1 of 2

majorly exhibits orbital lobes at the five-

and six-membered rings of the iminato ligand and a p-type

orbital at the Ge(II) atom (Fig. 3). In the LUMO a p-orbital at

the germanium(II) center engages in non-bonding interaction

with π-symmetric components at the adjacent N

amine

atom, as

well as the imino group. In the HOMO of 2

an orbital lobe

observed at the Ge(II) center hints towards an expected lone

pair in an s-type orbital. However, the major contributions to

the HOMO derive from orbital components with high p-char-

acter at the N atoms and the imidazoline moiety.

With the intention to assess the properties of amino(imino)

germylene 1as a ligand to transition metals we treated it with

1.1 equiv. of Fe

(CO)

in toluene (Scheme 2). After work-up,

yellow crystals of the iron carbonyl compound 3were obtained

in 62% yield from hexane (Fig. 4, two independent molecules

were found in the asymmetric unit). The germylene iron

complex 3features a trigonal-bipyramidal coordinated Fe atom

and a trigonal planar Ge moiety (the sum of the bond angles

around the Ge center amounts to 359°). Notably, most related

iron carbonyls exhibit a germanium center that is either tetra-

coordinate (germylene–iron complexes)

or found at the apex

of a trigonal pyramid (ferrogermylenes).

From our literature

we retrieved the germylene complex (2,6-tBu

4-Me-C

GeFe(CO)

as the only respective example with a

trigonal planar-coordinate germanium center.

The Fe1–Ge1

bond length in 3was determined to be 2.3026(5) Å [2.3041(5)

Å] which is longer than that reported for (2,6-tBu

-4-Me-

GeFe(CO)

(2.240(2) Å)

but does not exceed the

range found for the majority of related iron carbonyls (germy-

lene–iron complexes: 2.298(2)–2.348(1) Å; ferrogermylenes:

2.4415(11) Å, 2.496(2) Å).

22,23

Interestingly, the Ge–N

imine

distance in 3(Ge1–N1 = 1.755

(2) Å [1.752(2) Å]) is considerably decreased in comparison

with those in Rivard’s bis(imino)germylene IV (1.8194(15) Å)

and approaches the typical GevN bond lengths in germai-

mines (1.691(3)–1.704(5) Å).

Moreover, we determined the

WBI of the Ge–N

imine

bond in 3to 0.857 which is larger than

that calculated for 1(0.774, see the ESI, Fig. S16†). We con-

ceived that this might hint towards a significant 1-germa-

2-azaallene character of 3resembling the 1-sila-2-azaallene

nature reported for the tris(pentafluorophenyl)borane adduct

of the iminosilylene V

(Fig. 1, adduct not shown). As another

indicator of a possible 1-germa-2-azaallene character the

NHC

–N

imine

bond length in 3(1.296(3) Å [1.300(3) Å]) is

longer than the one in Ge(NIPr)

(IV, 1.273(2) Å).

However,

the Ge1–N1–C1 bond angle in 3amounts to 136.12(19)°

Fig. 3 (a) LUMO+1, (b) LUMO and (c) HOMO of 2

Fig. 4 ORTEP representation of the molecular structure of one of the

two independent molecules of 3in the solid state. Thermal ellipsoids are

at the 40% probability level. Hydrogen atoms are omitted for clarity. Dip

groups are depicted as stick models. Selected bond lengths (Å) and

bond angles (°); values for the not depicted molecules are given in

square brackets: Ge1–N1, 1.755(2), [1.752(2)]; Ge1–N4, 1.839(2), [1.835

(2)]; Ge1–Fe1, 2.3026(5), [2.3041(5)]; N1–C1, 1.296(3), [1.300(3)]; N2–C1,

1.381(3), [1.377(3)]; N3–C1, 1.378(3), [1.373(3)]; N1–Ge1–N4, 104.87(10),

[105.54(10)]; Fe1–Ge1–N1, 131.97(7), [132.89(7)]; Fe1–Ge1–N4, 121.69(7),

[120.87(7)]; Ge1–N1–C1, 136.12(19), [133.75(18)].

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[133.75(18)°] which is more similar to that in bent Ge(NIPr)

(IV, 131.21(13)°) than it resembles the wider C

NHC

–N

imine

–Si

bond angle (158.7(3)°) in the silylene–borane adduct of V.

Consequently, the relevant 1-germa-2-azaallene character of

3is negligible.

Conclusions

In summary, we synthesised the amino(imino)germylene 1

which serves as a precursor to the novel Ge(II) cation salt

[MeB(C

)

]

−

. Compound 2

features a two-coordinate ger-

manium atom incorporated into a four-membered metalla-

cycle and is formed via a rare type of methyl-abstraction and

ring-closing reaction using B(C

)

as a Lewis acid. This syn-

thetic approach to monocationic complexes of germanium(II)

is without precedence in the literature. Moreover, conversion

of 1with Fe

(CO)

aﬀords the germylene iron carbonyl

complex 3. In the latter, the germanium atom is found in a tri-

gonal planar coordination environment which is a scarce

structure motive for this type of compound. In future research

we will examine the use of 2

as a ligand in transition metal

complexes.

Acknowledgements

We are exceptionally grateful to the Alexander von Humboldt

Foundation (Sofja Kovalevskaja Program) for financial

support. We thank Dr. Elisabeth Irran for reviewing the X-ray

diﬀraction data.

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