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Silylenes |Hot Paper|

An Isolable Bis(Silanone–Borane) Adduct

Marcel-Philip Luecke,Elron Pens, ShenglaiYao,and Matthias Driess*[a]

Abstract: The reaction of bis(silylenyl)-substitutedferro-

cene 1with two molar equivalents of BPh3yields the cor-

respondingbis(silylene–borane) Lewis adduct 2.The latter

is capable to activate CO2to furnish the borane-stabilized

bis(silanone) 3through mono-oxygenation of the dative

SiII!Bsilicon centers under release of CO. Removal of

BPh3from 3with PMe3affords the corresponding 1,3,2,4-

cyclodisiloxane and the Me3P@BPh3adduct. All isolated

new compounds were characterized and their molecular

structures were determinedbysingle-crystal X-ray diffrac-

tion analyses.

The activation of small molecules using non- and semi-metal-

based compounds is an attractive field in main-group chemis-

try which led to the discovery of new activation modes and

types of reactions.[1] In this context, the concept of frustrated

Lewis pairs (FLPs) for cooperative activation of inert bonds em-

ploying Lewis acids and bases, firstly reported by Stephan,

Erker and co-workers, is alandmark discovery.[2] Since then, the

rapid expansion of FLP chemistry haspaved the way to differ-

ent inter- and intramolecular systems in which the majority is

based on sterically encumbered phosphorus- and nitrogen-

centeredLewis bases and organoboranes as Lewis acids.[3] Al-

thoughdivalent carbon species such as N-heterocyclic car-

benes (NHCs) have also been successfully probedinFLP

chemistry for the activation of CO2,H

2and N2O, the use of

analogousLewis pairs-containingsilylenes is less known.[4,5]

The silicon(II) atom in silylenes exhibits an ambiphilic character

due to its vacant 3p orbital (LUMO) and the 3s-centered lone

pair (HOMO). Owing to their interesting property and reactivity,

stable N-heterocyclic silylenes (NHSis), the heavieranalogues

of NHCs, have been utilized successfully for the metal-free acti-

vation of small molecules[6] and as powerful steering ligandsin

homogeneous catalysis.[7] After the first isolation of an N-heter-

ocyclic silylene in 1994 by Denk and West, the formation of a

silylene–borane adduct was reported two years later,which,

however,slowly rearranges to asilylborane through SiII inser-

tion into the B@CbondofB(C6F5)3.[8] Since then, an increasing

number of compounds containing adative SiII!BIII bond with

four- and five- coordinate SiII centers have been isolated and

structurally characterized.[9]

Due to alarge polarization of the Si=Obond and the re-

markably weak Si@Opbond (58.5 kJmol@1)compared to the

Si@Os-bond strength (119.7 kJmol@1), compounds with aSi=O

bond are intrinsically susceptible to auto-oligomerization to

the corresponding polysiloxanes.[10] Thus, introductionofan

electron donor at the Si atom or/and an acceptor at the O

atom are needed to disfavor head-to-tail oligomerization of

the polar Si=Obond.[11] This led to the first Lewis acid-base

supported silanonecomplex, the silaformamide–borane A

(Scheme 1), which wasreportedbyusin2007, starting from a

silylene and H2O·B(C6F5)3.[12] Roesky et al. described in 2011the

isolation of the acid anhydride Bgenerated from the reaction

of achlorosilylenewith H2O·B(C6F5)3in the presence of NHC.[13]

Similarly,Roesky et al. reported also the silaformyl chloride

complex C,resulting from an NHC-stabilized silylene and

H2O·B(C6F5)3.[14] In 2019, the isolation of the first donor–accept-

or-supported silaaldehyde Dwas accomplished by the Inoue

group.[15] Remarkably,Kippings dream of isolable genuine sila-

nones was realized in 2014 with the isolation of the first metal-

losilanonebyFilippou[16] and 2017 by the groupsofInoue and

Rieger.[17] Very recently,asilicon analogue of aketone with an

unperturbed Si=Obond was synthesized by Iwamoto and co-

workers.[18]

Starting from an in situ generated silylene–borane adduct,

Teng et al. reportedin2016 on the activation of THF leading

to the isolation of acorresponding ring-opening product.[20]

Recently,Braun and co-workers used asilylene–borane Lewis

adduct as atool for trapping asingle water molecule, affording

azwitterionic silanol stabilized by intramolecular hydrogen

bonds.[23] In 2017, our group reported the first intramolecular

silylene–borane FLP which activatesH

2,O

2,CO

2and even dehy-

drogenates water yielding aborane-stabilized silanone Ewith

adative Si=O!Bbond.[19] Herein, we present the synthesis of

the bis(silylene–borane) adduct 2with the ferrocenespacer

and its mild oxidation with CO2yielding the first borane-stabi-

lized bis(silanone) adduct 3.Removal of BPh3from 3by addi-

tion of PMe3leads to the corresponding 1,3,2,4-cyclodisiloxane

through intramolecular Si=Ohead-to-tail dimerization. More-

over,the reactionof2with elemental sulfur yields abis(sila-

thione) with two ‘borane-free’ Si=Smoieties.

The reaction of the ferrocene-derived bis(silylene)[24] 1with

two molar equivalents of triphenylborane in toluene at room

[a] M.-P.Luecke, E. Pens, Dr.S.Yao, Prof. Dr.M.Driess

Department of Chemistry:Metalorganics and Inorganic Materials

Technische Universit-tBerlin

Strasse des 17. Juni 115, Sekr.C2, 10623 Berlin (Germany)

E-mail:matthias.driess@tu-berlin.de

Supporting information and the ORCID identification number(s) for the

author(s) of this articlecan be found under:

https://doi.org/10.1002/chem.202000235.

T2020 The Authors. Published by Wiley-VCH Verlag GmbH&Co. KGaA.

This is an open access article under the terms of Creative Commons Attri-

bution NonCommercial-NoDerivs License, which permits use and distribu-

tion in any medium, provided the originalwork is properly cited, the use is

non-commercial and no modifications or adaptations are made.

Chem. Eur.J.2020,26,4500 –4504 T2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim4500

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doi.org/10.1002/chem.202000235

temperature leads to the formation of the bis(silylene-borane)

adduct 2whichwas isolatedin74% yields as ared crystalline

solid (Scheme 2). The identity of 2was provenbyelemental

analysis, single-crystal X-ray diffraction analysis and multinu-

clear NMR spectroscopy in the solid state and in solution. Crys-

tals suitable for an X-ray diffraction analysis were obtained in a

concentrated toluene solution of 2at @308C, the crystals are a

mixture of the two rotational conformers (Figure 1; see also

the Supporting Information).

Compound 2crystallizes in the monoclinicspace group

P121/c1inwhich both silicon centersadopt adistorted tetrahe-

dral geometry (8Si1=356.728,8B1=319.808)with Si@Bdistan-

ces of 2.089(2) and 2.077(2) a,similar to those of relatedsilico-

n(II)–boranes adducts (1.9624(5)–2.108(2) a).[9] Given the low

solubility of 2in deuterated benzene and THF,only abroad

29Si NMR signal of low intensity was observed at d=54.0 ppm

which is low-field shifted compared to 1(d=43.3 ppm). The

solid-state 29Si NMR (VACP/MAS) spectrum of 2shows

asinglet at d=48.6 ppm (1:d=41.6 ppm). The iso-

tropic 11Bchemical shift was observed in [D8]THF sol-

utions at d=@7.8 ppm (Dn1/2 =356 Hz) whichis, as

expected, low-field shifteddue to its coordination to

the SiII center (BPh3:d(11B)=55.2 ppm, C6D6).[9]

Compound 2is inert towards H2and CO but reacts

with CO2in C6D6under ambient conditions (1 bar,

298 K), resulting in the simultaneousformation of a

pale-yellow solid and CO as confirmedbyanaddi-

tional 13C-labeling experiment (See the Supporting In-

formation,S11). Resolving the solid in [D8]THF and re-

cording its multinuclear NMR spectra revealed the

formation of anew specieswith astrongly high-field

shifted 29SNMR singlet resonanceatd=@44.7 ppm

(2:d=++54.1 ppm). An X-ray diffraction analysis of

single crystals revealed the formation of the borane-

stabilized bis(silanone) 3,was isolated in 94%yields

(Figure 2).

Scheme1.SelectedLewis acid/base-supportedSi=Ocompounds.

Scheme2.Synthesis of the bis(silylene–borane) adduct 2from 1and its reactivity to-

wards CO2to give 4and 3,respectively.

Figure 1. Molecular structure of 2(only one of the two rotational conform-

ers) with thermal ellipsoids drawn at the 50%probability level. Hydrogen

and solvent atoms are omittedfor clarity.Selected bond lengths [a]: Si1@B1

2.089(2), Si2@B2 2.077(2). Selectedbondangles [8]: C2-Si1-B1 130.74(9), C9-

Si2-B2 130.63(9).

Chem. Eur.J.2020,26,4500 –4504 www.chemeurj.org T2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim4501

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The silicon centerin3adopts adistorted tetrahedral geome-

try with ashort Si@Odistance of 1.557(4) and 1.537(4) ain ac-

cordancewith related four-coordinated Lewis acid stabilized si-

lanones(1.531–1.579 a)containing aSi=Odouble

bond.[12–15,19,21–22] The Si@Odistance is only slightly elongated

when compared with recently reported genuine silanones

(1.518–1.537 a).[15–18] Bis(silanone) 3is remarkable stable in so-

lution ([D8]THF) and no changes in the 1HNMR spectra were

observed upon heatingto608C. Compound 3represents a

rare example of borane-stabilized silanones. Aldridge and co-

workers achievedthe isolation of astabilized silaaldehyde

throughchloride–hydride substitution using K[HBEt3].[21] Addi-

tion of B(C6F5)3to acyclic amino(bora-ylide(silanone)) reported

by Kato et al.,increased the stability of the pre-formedfree si-

lanone.[22] In the presence of B(C6F5)3,Roesky et al. accom-

plished the isolation of adonor–acceptor stabilized silaformyl

chloride.[14] However,i

solation of ab

orane-stabilized silanone

startingfrom asilylene–borane system is not reported so far.

To remove the boranes from the bis(silanone–borane) com-

plex 3,trimethylphosphane (PMe3,5equiv) wasadded. This re-

sulted in the clean formation of the correspondingLewis pair

Me3P!BPh3(31PNMR: @15.3 ppm) and the 1,3,2,4-cyclodisilox-

ane 4(head-to-tail dimer of Si=Omoieties). The latter is identi-

cal with the isolated product from the reaction of 1with CO2

in 76%yields (Scheme 2). Single crystals of 4suitablefor X-ray

diffractionanalysiswere obtained from aconcentrated solution

in a1:1 benzene/hexane mixture at room tempera-

ture (Figure 3). The formation of Me3P!BPh3was ad-

ditionally confirmed by asingle-crystal X-ray analysis

obtainedinthe reaction mixture of 3and PMe3in

THF solutions (see the Supporting Information).

As expected, the five-coordinatesilicon centersin

4show adrastically high-field shifted 29Si NMR chemi-

cal shift at d=@92.1 ppm (3:d=@44.7 ppm). The

Si@Odistance of 1.709(4) and 1.681(4) aare elongat-

ed compared to those observed for 3(1.557(4),1.537(4) a)in

accordance with the presence of Si@Osingle bonds.[25] Reac-

tion of 4with an excess amount of BPh3in tolueneatroom

temperature does not regenerate 3.

Interestingly,reaction of the bis(silylene–borane) 2with

10 equivalents of PMe3led to the formation of anew species

2’ in the course of borane-deprotection of oneSi

II moiety in 2

(Scheme 3, see the Supporting Information). This process is re-

versible because removal of the solvent and PMe3in vacuum

and re-dissolving of the residue in C6D6furnishes compound 2

as shown by NMR spectroscopy.

In contrasttothe oxygenation of 2with CO2,treatment of 2

with elemental sulfur in toluene at room temperature leads to

the selective formation of the ‘borane-free’ bis(silathione) 5.

Compound 5is identical with the product from the reaction of

bis(silylene) 1with elementalsulfur in toluene at room temper-

ature, which was isolated in 54%yield (Scheme 4). Similar to

the product of an intramolecular silylene–borane FLP with ele-

mental sulfur reported by our group,[19] no Si=S!Binteraction

was observed. The structure of 5(Figure 4) features two Si=S

bonds with alow-field shiftedsinglet 29Si NMR signal at d=

12.1 ppm. The Si=Sdistances of 1.9867(13) and 1.9858(13) a

are consistent with relatedsilathiones with four-coordinatesili-

con atoms [{PhC(NtBu)2}Si(S)Cl](2.079(6) a)and as reported for

aSi=Sproduct from sulfuration of an intramolecular silylene–

borane FLP with elementalsulfur (1.9795(10) a).[19,26] Bis(sila-

thione) 5is stable in C6D6solutions over aperiod of several

weekswhich can be explained by aless polarized Si=Sbond

(DEN=0.7) compared to the Si=Obond (DEN=1.7) based on

their electronegativities(EN).

In summary,the synthesis of bis(silylene-borane) Lewis

adduct 2containing two SiII–BPh3moieties in asingle molecule

was presented. Exposure of 2to CO2yields the corresponding

Figure 2. Molecular structure of 3with thermal ellipsoids drawn at the 50%

probability level. Hydrogen atoms and solvent molecules are omitted for

clarity.Selected distances [a]: Si1@O1 1.557(4), Si2@O2 1.537(4), O1@B1

1.545(7), O2@B2 1.541(7);selected bond angles [8]: B1-O1-Si1 157.59,B2-O2-

Si2 145.96,C9-Si2-B2 130.63(9).

Figure 3. Molecular structure of 4with thermal ellipsoids drawn at the 50%

probability level. Hydrogen atoms and solvent molecules are omitted for

clarity.Selected bond lengths [a]: Si1@O1 1.709(4), Si2@O2 1.681(4). Selected

bond angles [8]: Si1-O1-Si2 93.5(2), Si1-O2-Si293.9(2), O2-Si2-O1 84.4(2).

Scheme3.Reversible reaction of 2with PMe3formingthe monoborane adduct 2’.

Chem.Eur.J.2020,26,4500 –4504 www.chemeurj.org T2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim4502

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doi.org/10.1002/chem.202000235

borane-supported bis(silanone) complex 3featuring two Si=

O!Bunits.Removal of the borane with PMe3yields 1,3,2,4-cy-

clodisiloxane 4through intramolecular Si=Ohead-to-tail dime-

rization.Incontrast, the reaction of 2with elemental sulfur

yields exclusively the borane-free bis(silathione) 5which shows

no tendency to undergo dimerization.

Acknowledgements

This work was fundedbythe Deutsche Forschungsgemein-

schaft (DFG, German Research Foundation) under Germany’s

Excellence Strategy–EXC 2008/1-390540038 (Gefçrdert durch

die Deutsche Forschungsgemeinschaft (DFG) im Rahmender

Exzellenzstrategie des Bundesund der L-nder–EXC 2008/1-

390540038) and with aPhD fellowship by the Einstein Founda-

tion Berlin (M.-P.L.). We thank Paula Nixdorf for the assistance

in the XRD measurements.

Conflict of interest

The authors declare no conflict of interest.

Keywords: FLP-chemistry ·silanones ·silylene ·small-molecule

activation

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Scheme4.Reactionof1or 2with elemental sulfur affording 5.

Figure 4. Molecular structure of 5with thermal ellipsoids drawn at the 50%

probability level. Hydrogen and solvent atoms are omitted for clarity.Select-

ed bond lengths [a]: Si1@S1 1.9867(13), Si1@S2 1.9858(13). Selectedbond

angles [8]: C1-Si1-S1 120.70(12).

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Manuscript received:January 15, 2020

Accepted manuscript online:February 4, 2020

Version of record online:March 11,2020

Chem. Eur.J.2020,26,4500 –4504 www.chemeurj.org T2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim4504

Chemistry—A European Journal

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doi.org/10.1002/chem.202000235