Reactions of Si2Br6 with N-Heterocyclic Carbenes

Article abstract of DOI:10.1002/zaac.201800163

A combined experimental and theoretical study on the reaction of Si₂Br₆ with N-heterocyclic carbenes is reported. Employment of an imidazole-2-ylidene with methyl groups in C⁴- and C⁵-position results in the disproportionation of Si₂Br₆ into the adducts NHC→SiBr₂ and NHC→SiBr₄. According to expectation, the hydrogenated derivative 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene forms analogous disproportionation products of Si₂Br₆ at low temperatures, whereas reaction at higher temperatures furnishes the 4-SiBr₃-substituted NHC. The underlying formation mechanism explored by means of density functional theory calculations features an abnormal carbene intermediate.

Full text of DOI:10.1002/zaac.201800163

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201800163
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Reactions of Si2Br6 with N-Heterocyclic Carbenes
Julia I. Schweizer,[a][‡] Alexander G. Sturm,[a][‡] Timo Porsch,[a] Matthias Berger,[a] Michael Bolte,[a]
Norbert Auner,*[a] and Max C. Holthausen*[a]
Dedicated to Prof. Alexander C. Filippou on the Occasion of his 60th Birthday
Abstract. A combined experimental and theoretical study on the reac-
forms analogous disproportionation products of Si2Br6 at low tempera-
tion of Si2Br6 with N-heterocyclic carbenes is reported. Employment tures, whereas reaction at higher temperatures furnishes the 4-SiBr3-
of an imidazole-2-ylidene with methyl groups in C4- and C5-position substituted NHC. The underlying formation mechanism explored by
results in the disproportionation of Si2Br6 into the adducts
means of density functional theory calculations features an abnormal
NHCǞSiBr2 and NHCǞSiBr4. According to expectation, the hydro- carbene intermediate.
genated derivative 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene
action of HSiCl3 with NHC or, alternatively, by reduction of
Introduction
NHCǞSiCl4 with KC8.[11a] Both, NHCǞSiCl2 and
NHCǞSiCl4 can also be obtained through base-induced dis-
proportionation of Si2Cl6.[15] The same groups recently re-
ported the isolation of a stable, biradicaloid SiF2 base ad-
duct[16] employing cyclic alkyl(amino) carbenes, which were
first synthesized by Bertrand and co-workers.[17] In 2013,
Filippou et al. reported formation of NHCǞSiI2 en route to
an NHC stabilized Si2+ ion.[11c]
The isolation of low-valent silicon compounds has long in-
trigued chemists,[1] particularly in view of the distinct differ-
ences to the corresponding carbon analogs. In defiance of Mul-
liken’s[2] and Pitzer’s[3] (putatively universal) double-bond
rule,[4] Brook and West heralded a new era of intense research
activities in main group chemistry in 1981 by characterization
of the first stable Si=C and Si=Si double bond, respectively.[5]
After exploiting possibilities for the intramolecular stabiliza-
tion of such bonding motifs,[6,7] the use of N-heterocyclic carb-
enes (NHC) for trapping of otherwise fleeting reactive main
group species has proven exceptionally well suited over the
last decade[8] and spurred a “renaissance in main group chem-
istry”.[9] Utilization of these strong σ-donors has successfully
led to the isolation and characterization of a variety of com-
pounds with silicon in unusual oxidation states and coordina-
tion environments,[10] including NHC adducts of transient di-
halosilylenes.[11]
Bare SiX2 (X = H, F, Cl, Br, I) is a highly reactive species
with pronounced ambiphilic character.[12] Coordination of a
Lewis base (LB) effectively saturates the electrophilicity of
the subvalent silicon atom and the resulting adduct exhibits an
enhanced Lewis basic character,[11a–11c,12d,13] enabling subse-
quent transformations with Lewis acids (LA) to push-pull
compounds of the type LBǞSiX2ǞLA.[14] In 2009, Roesky,
Stalke, and co-workers isolated an NHCǞSiCl2 adduct by re-
Also in 2009, the first stable adduct of SiBr2 was prepared
by Filippou et al.[11b] Reaction of SiBr4 with one equivalent
of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (NHCH)
in hexane led to formation of ion-pair BH,ion, which was sub-
sequently reduced with two equivalents of KC8 in THF to give
A
H in 48% isolated yield (Figure 1). AH is a yellow, air-sensi-
tive solid soluble in THF and benzene. The reasonably short
Si–C bond in AH was characterized as a strong CǞSi donor-
acceptor single bond by quantum-chemical means. Formation
of the neutral pentacoordinate NHCǞSiBr4 adduct BH in tolu-
ene was described by Ghadwal et al. in 2009 (Figure 1).[18]
Starting from NHCǞSiBr2, Filippou’s group was recently able
to synthesize an NHC-stabilized disilavinylidene,[10k] and only
few other reports on NHC adducts of silicon bromides exi-
st.[10i,19] However, the reactivity of higher bromosilanes
towards NHCs remains elusive to date.
* Prof. Dr. N. Auner
E-Mail: auner@chemie.uni-frankfurt.de
* Prof. Dr. M. C. Holthausen
Fax: +49-69-798-29417
E-Mail: Max.Holthausen@chemie.uni-frankfurt.de
[a] Institut für Anorganische Chemie
Goethe-Universität
Max-von-Laue-Strasse 7
60438 Frankfurt/Main, Germany
[‡] Authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201800163 or from the au-
thor.
Figure 1. Reported products of the reaction of 1,3-bis(2,6-diisoprop-
ylphenyl)imidazol-2-ylidene with SiBr4.
Z. Anorg. Allg. Chem. 0000, ᭿,0–0
1
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Herein, we report a combined experimental and theoretical
study on the reactions of Si2Br6 with NHCH and 1,3-bis(2,6-
diisopropyl)-4,5-methyl-imidazole-2-ylidene (NHCMe
,
Scheme 1). According to expectation, the disproportionation
products NHCǞSiBr2 (A) and NHCǞSiBr4 (B) form at mod-
erate temperatures, whereas reaction at higher temperatures
furnishes a 4-SiBr3-functionalized NHC (CH). The formation
mechanism for all experimental products detailed in the fol-
lowing involves an abnormal carbene intermediate occurring
along the path to CH.
Figure 2. Molecular structure of [HCH]Br with thermal ellipsoids
shown at the 50% probability level. Hydrogen atoms on the substitu-
ents are omitted for clarity. Selected experimental bond lengths /Å and
angles /°: Si1–C1 1.863(10), Si1–Br1 2.160(4), Si1–Br2 2.182(4), Si1–
Br3 2.174(3), C1–C3 1.350(12), C1–N1 1.379(13), C3–N2 1.352(12),
C2–N1 1.324(12), C2–N2 1.324(12), C1–Si1–Br1 107.0(4), C1–Si1–
Br2 111.4(4), C1–Si1–Br3 112.2(4), N1–C1–Si1 131.3(7).
pared to protons attached to the C2 position (calculated pKa
difference of 9 units).[21] Notably, Gates and co-workers syn-
thesized a 4-phosphino-substituted NHC in 2009 and pos-
tulated involvement of an abnormal carbene intermediate in its
formation mechanism.[22] These so-called “abnormal carb-
enes” (aNHCs), which bear the carbene center in 4- or 5-posi-
tion, have long been fleeting curiosities in main-group chemis-
Scheme 1. Products formed in the reaction of Si2Br6 with 1,3-bis(2,6-
diisopropyl)-4,5-methyl-imidazole-2-ylidene (NHCMe: AMe, BMe) and
1,3-bis(2,6-diisopropyl)imidazole-2-ylidene (NHCH: AH, BH, CH,
[HCH]Br), respectively.
Results and Discussion
The reaction of two equivalents of NHCH with Si2Br6 at try[23] until Bertrand and co-workers isolated the first crystal-
room tempeature in toluene selectively gives AH (C6D6: δ29Si line aNHC in 2009.[24] aNHCs are stronger σ-donors than nor-
= 10.9 ppm[11b]) and BH (C6D6: δ29Si = –90.2 ppm[18]). In mal NHCs;[25] reactions with silanes lead to formation of 4-
agreement with Filippou, NMR spectroscopic analyses of BH silyl-functionalized imidazolium salts.[26] To date, a variety of
in CD2Cl2 show a singlet resonance at δ29Si = –63.9 ppm,[11b] 4-silyl-substituted NHCs and their corresponding imidazolium
corresponding to the BH,ion salt (Scheme 1, top). Warming the salts have been reported (Figure 3),[26b,27] most of which result
reaction mixture to 60 °C or above leads to carbene CH, which from reactions of an NHC with the corresponding silane upon
is SiBr3-functionalized in 4-position (D6D6: δ29Si
=
addition of a strong base. Robinson and co-workers have estab-
–49.9 ppm; Scheme 1, bottom), and [HNHCH]Br, which we lished more convenient access to this compound class through
suggest as HBr source for the transformation of CH to C4-lithiated NHCs.[27f,28]
[HCH]Br. CH is detectable as a dominant species in solution,
Dimethylation of the NHC backbone inhibits the formation
whereas single crystals of [HCH]Br and [HNHCH]Br suitable of C4-functionalized products. In fact, reaction of NHCMe with
for X-ray diffraction analysis were obtained from crystalli- Si2Br6 in n-hexane at –78 °C yields a mixture of AMe and
zation experiments. The molecular solid-state structure of BMe,ion as pale yellow precipitate, from which AMe was iso-
[HCH]Br is shown in Figure 2. While these results add to the lated by washing with benzene. The NMR spectrum of the
overall mechanistic picture (see below), we did not attempt to concentrated filtrate shows a single signal at δ29Si = 14.0 ppm,
obtain [HCH]Br in quantitative yields.
similar to the NMR signal of AH (δ29Si = 10.9 ppm[11b]). This
[HCH]Br crystallizes as ion pair in the orthorhombic space assignment is supported by the 13C NMR signal for the C2
group P212121. The imidazolium ring unit is planar and the atom at δ = 162.5 ppm (AH: δ13C = 164.5 ppm[11b]). Colorless
bond lengths C2–N1 and C2–N2 are identical (1.324 Å). Com- crystals suitable for X-ray diffraction analysis were obtained
pared to Filippou’s ionic complex BH,ion the Si–C bond in from a solution of AMe in benzene at 5 °C (Figure 4).
[HCH]Br (1.863 Å) is slightly shortened by 0.017 Å and gen-
AMe crystallizes in the monoclinic space group P21/c with
erally in the range of Si–C(sp2) single bonds.[11b] The Si1 atom one benzene molecule in the unit cell. The molecular structure
in [HCH]Br is almost tetrahedral with angles between 107– is very similar to that of the stable carbene adduct AH.[11b] The
112° and the average Si–Br bond length of 2.172 Å is compar- SiBr2 group shows a significant torsion to the essentially
able to the averaged values of BH,ion (2.175 Å[11b]) and SiBr4 planar imidazolium ring, as seen by the quite different torsion
(2.183 Å[20]).
angles Br1–Si1–C1–N1 [83.6(7)°] and Br2–Si1–C1–N2
The activation of the C4–H bond (C1 in Figure 2) in CH is [–17.1(9)°]. The silicon center is distinctly trigonal pyramidal
intriguing as the acidity of C4 protons in NHCs is minute com- (sum of angles at Si = 297.2°) and the Si–C bond length of
Z. Anorg. Allg. Chem. 0000, 0–0
www.zaac.wiley-vch.de
2
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Figure 3. Examples of 4-silyl-functionalized NHCs.
Figure 4. Molecular structure of AMe with thermal ellipsoids shown
at the 50% probability level. Hydrogen atoms and benzene are omitted
for clarity. Selected experimental bond lengths /Å and angles /°: Si1–
C1 2.008(8), Si1–Br1 2.326(3), Si1–Br2 2.336(3), C2–C3 1.349(11),
C1–N1 1.336(9), C1–N2 1.351(10), C3–N1 1.387(9), C2–N2 1.406(9),
C1–Si1–Br1 97.2(3), C1–Si1–Br2 102.8(2), Br1–Si1–Br2 97.23(10),
N1–C1–N2 104.9(6).
Figure 5. Molecular structure of BMe,ion with thermal ellipsoids shown
at the 50% probability level. Hydrogen atoms are omitted for clarity.
Selected experimental bond lengths /Å and angles /°: Si1–C1
1.900(18), Si1–Br1 2.178(6), Si1–Br2 2.161(7), Si1–Br3 2.171(6), C2–
C3 1.35(2), C1–N1 1.34(2), C1–N2 1.42(2), C3–N1 1.459(19), C2–
N2 1.31(2), C1–Si1–Br1 107.6(6), C1–Si1–Br2 111.7(6), C1–Si1–Br3
112.2(6), N1–C1–Si1 122.6(12), N1–C1–N2 105.5(15).
2.008 Å is slightly elongated compared to AH (1.989 Å[11b]).
The formation mechanisms of A, B, and C were explored
Both Si–Br bond lengths (2.326 Å and 2.336 Å) are in good by means of density functional theory calculations, which were
agreement with the values determined for AH (2.338 Å and benchmarked against high-level coupled-cluster theory.[30] Ini-
2.361 Å[11b]).
tial interaction between Si2Br6 and NHCH leads to exergonic
The colorless residue that remained after washing with benz- adduct formation with a moderate activation barrier (Δ‡GH
=
ene is readily dissolved in dichloromethane. One main 29Si 11.3 kcal·mol–1, Scheme 2). In the presence of excess NHC,
NMR signal is detected at δ = –63.4 ppm, compatible with the adduct 1H thus is the resting state of Si2Br6. Subsequent [1,2]-
chemical shift of the ionic salt BH,ion. The same holds for the bromide migration furnishes 2H that carries the pentacoordi-
characteristic 13C NMR chemical shift for the C2 atom at δ13C nate silicon site in terminal position; in other words, 2H repre-
[11b,29]
= 138.2 ppm vs. 136.3 ppm in BH,ion
.
The identical sents an adduct of the NHC-stabilized silylene with SiBr4. Lib-
NMR signature was observed in an additional experiment em- eration of the former via TS2H is rate-limiting with an effec-
ploying NHCMe and SiBr4. BMe,ion was obtained from CD2Cl2 tive activation barrier of 16 kcal·mol–1. With respect to 1H,
at –20 °C as a white powder with single-crystals of low quality formation of the experimentally observed products
(Figure 5).
Me,ion crystallizes as ion pair in the monoclinic space group The corresponding pathway for the reaction of NHCMe with
NHCHǞSiBr2 (3H) and NHCHǞSiBr4 (4H) is thermoneutral.
B
Cc with four imidazolium cations, four bromine anions, and Si2Br6 is closely related, with the NHCMe·SiBr4 adduct 4Me
two benzene molecules in the asymmetric unit. The N-hetero- slightly disfavored.
cycle is planar in all four molecules. The silicon atom in
Overall, the reaction sequences leading to silylene extrusion
BMe,ion is almost tetrahedral with angles between 107–112° from Si2Br6 bear close resemblance to the mechanism of the
[11b]
analogously to [HCH]Br and BH,ion
and the Si–C bond is amine-induced disproportionation reaction of Si2Cl6 reported
in the same range (1.880 Å vs. 1.900 Å in BH,ion). The average recently.[12d] At variance with the reactivity of Si2Cl6, primary
Si–Br bond length of 2.170 Å compares well with that in adduct formation is exergonic here, presumably resulting from
CH, BH,ion (2.175 Å[11b]) and SiBr4 (2.183 Å[20]).
the strongly σ-donating character of the NHC base.[17b,31] Con-
Z. Anorg. Allg. Chem. 0000, 0–0
www.zaac.wiley-vch.de
3
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Scheme 2. NHC-induced formation of NHCǞSiBr2 and NHCǞSiBr4 from Si2Br6 (ΔG298 in kcal·mol–1). * 1Me: different isomer (see Supporting
Information).
Scheme 3. NHC-induced formation of [NHC–SiBr3]Br from SiBr4; NPA charges of NHCH, 4H, and 5H in red (ΔG298 in kcal·mol–1).
sequently, adduct 1 and NHC-stabilized 3 and 4 are predicted ide dissociation and association reactions shifts the pentacoor-
to exist in equilibrium, although NMR signatures of 1 were dinate silicon center from C4- to C2-position. The resulting
not detected in our experiments.
isomer 10H can undergo SiBr4 elimination via TS8H to furnish
In line with earlier experimental work of Ghadwal et al.,[18] carbene 11H, which bears a SiBr3-group in 4-position, in an
in which the NHCH adduct of SiBr4 was characterized crystal- overall exergonic reaction. This scenario is supported by the
lographically, 4H forms exergonically from the reactants with NMR spectroscopic detection of CH in solution. Formation of
a low activation barrier (Scheme 3). Formation of 4Me is kinet- the [H11H]Br salt, corresponding to crystallized product
ically slightly more feasible, but thermodynamically disfa- [HCH]Br, can either proceed via HBr transfer from 6H or by
vored. Subsequent dissociation of a bromide ion yielding 5 deprotonation of 5H in 4-position (see Supporting Information
is endergonic for both NHCs. We attribute the experimental for details). A related mechanistic picture has been put forth
observation of Bion corresponding to 5 to solvent and crystalli- by Bertrand and co-workers, who postulate formation of an
zation effects not accounted for in our calculations.
abnormal carbene as fleeting intermediate in the reaction be-
According to natural population analysis (NPA), the acidity tween NHCH and benzoyl chloride.[27b]
of the hydrogen atoms in the NHC backbone is enhanced by
partial delocalization of the positive charge within the imid-
azolium ring-system in ion-pair 5H. Consequently, the most
Conclusions
favorable route to the 4-substituted NHC CH is initiated by
In this work, we investigated the reaction of Si2Br6 with N-
deprotonation of 5H in C4-position by another equivalent of heterocyclic carbenes. Employment of an imidazole-2-ylidene
NHCH via TS4H (Scheme 4).[32] The effective activation bar- with methyl groups in C4- and C5-position results in the dis-
rier of 28 kcal·mol–1 (computed with respect to 4H) for the proportionation of Si2Br6 into adducts NHCMeǞSiBr2 and
formation of imidazolium salt 6H and the abnormal carbene 7H NHCMeǞSiBr4. Reaction of the disilane with the correspond-
is in line with a reaction taking place only at elevated tempera- ing NHCH at room temperature leads to formation of the analo-
tures. Alternative deprotonation of 3H is kinetically feasible gous products NHCHǞSiBr2 and NHCHǞSiBr4, which were
(Δ‡GH = 26 kcal·mol–1), but thermodynamically strongly disfa- isolated in earlier work by Filippou and Roesky from the reac-
vored (ΔRG = 20 kcal·mol–1, see Supporting Information). Ad- tion of NHCH with SiBr4 and further treatment with KC8.[11b,
18]
dition of SiBr4 to aNHC 7H leading to 8H is kinetically and
However, the reaction at elevated temperatures yields a 4-
thermodynamically favored. A sequence of low-barrier brom- SiBr3-substituted carbene, which we characterized by single-
Z. Anorg. Allg. Chem. 0000, 0–0
www.zaac.wiley-vch.de
4
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Scheme 4. H-abstraction from 5H by NHCH; RЈ = Dipp (ΔG298 in kcal·mol–1).
Supporting Information (see footnote on the first page of this article):
ORTEP and crystallographic data of [HCH]Br (CCDC 1025332), AMe
(CCDC 1025333), and BMe (CCDC 1025334). Details to the chemi-
cals, syntheses, and general procedures. 29Si NMR spectra of the prod-
uct mixtures. Computational details and additional information on the
reaction mechanism. XYZ coordinates of all computed structures.
crystal X-ray diffraction analysis. We elucidated the underly-
ing reaction mechanism by DFT calculations. The quantum-
chemical results are in line with the experimental observations
and suggest formation of an abnormal carbene intermediate
en route to the experimentally characterized C4-functionalized
NHC.
Acknowledgements
Experimental Section
All reactions were performed employing standard Schlenk techniques
using N2 as inert gas. Solvents were dried with appropriate drying
agents, distilled and stored in PTFE-valved flasks. NMR spectra were
recorded with a Bruker AV400 equipped with a TXI 5N zgrad ATMA
probe and a Bruker AV-500 spectrometer equipped with a Prodigy
We thank Dr. Jan Tillmann for help with the X-ray crystallography.
Quantum-chemical calculations were performed at the Center for Sci-
entific Computing (CSC) Frankfurt on the FUCHS and the LOEWE-
CSC high-performance compute clusters.
1
BBO 500 S1 probe. H NMR spectra were calibrated to the residual
solvent proton resonance ([D6]benzene δ1H = 7.16 ppm, CD2Cl2 δ1H
=
5.32 ppm). 13C{1H} NMR spectra were calibrated to the solvent reso- Keywords: N-heterocyclic carbenes; Bromosilanes; Silyl-
nance ([D6]benzene δ13C = 128.06 ppm, CD2Cl2 δ13C = 53.84 ppm).
29Si NMR chemical shifts are referenced to tetramethylsilane.
enes; Reaction mechanism; Density functional theory
Z. Anorg. Allg. Chem. 0000, 0–0
www.zaac.wiley-vch.de
5
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Huch, D. Scheschkewitz, Chem. Commun. 2016, 52, 2799–2802;
References
p) I. Alvarado-Beltran, A. Rosas-Sánchez, A. Baceiredo, N. Saf-
fon-Merceron, V. Branchadell, T. Kato, Angew. Chem. Int. Ed.
2017, 56, 10481–10485; q) A. Burchert, S. Yao, R. Müller, C.
Schattenberg, Y. Xiong, M. Kaupp, M. Driess, Angew. Chem. Int.
Ed. 2017, 56, 1894–1897; r) D. Wendel, D. Reiter, A. Porzelt,
P. J. Altmann, S. Inoue, B. Rieger, J. Am. Chem. Soc. 2017, 139,
17193–17198.
[1] a) F. S. Kipping, J. E. Sands, J. Chem. Soc. Trans. 1921, 119,
830–847; b) F. S. Kipping, J. Chem. Soc. Trans. 1923, 123, 2590–
2597.
[2] a) R. S. Mulliken, J. Am. Chem. Soc. 1950, 72, 4493–4503; b)
R. S. Mulliken, J. Am. Chem. Soc. 1955, 77, 884–887.
[3] K. S. Pitzer, J. Am. Chem. Soc. 1948, 70, 2140–2145.
[4] P. Jutzi, Angew. Chem. Int. Ed. Engl. 1975, 14, 232–245.
[5] a) R. West, M. J. Fink, J. Michl, Science 1981, 214, 1343–1344;
b) A. G. Brook, F. Abdesaken, B. Gutekunst, G. Gutekunst, R. K.
Kallury, J. Chem. Soc., Chem. Commun. 1981, 191–192; c) A. G.
Brook, S. C. Nyburg, F. Abdesaken, B. Gutekunst, G. Gutekunst,
R. Krishna, M. R. Kallury, Y. C. Poon, Y. M. Chang, W. N. Win-
nie, J. Am. Chem. Soc. 1982, 104, 5667–5672; The first P=P
[11] a) R. S. Ghadwal, H. W. Roesky, S. Merkel, J. Henn, D. Stalke,
Angew. Chem. Int. Ed. 2009, 48, 5683–5686; b) A. C. Filippou,
O. Chernov, G. Schnakenburg, Angew. Chem. Int. Ed. 2009, 48,
5687–5690; c) A. C. Filippou, Y. N. Lebedev, O. Chernov, M.
Strassmann, G. Schnakenburg, Angew. Chem. Int. Ed. 2013, 52,
6974–6978; d) J. Tillmann, M. Moxter, M. Bolte, H.-W. Lerner,
M. Wagner, Inorg. Chem. 2015, 54, 9611–9618.
double bond was also characterized in 1981: d) M. Yoshifuji, I. [12] a) J. C. Thompson, J. L. Margrave, Science 1967, 155, 669–671;
Shima, N. Inamoto, K. Hirotsu, T. Higuchi, J. Am. Chem. Soc.
1981, 103, 4587–4589.
b) R. S. Ghadwal, R. Azhakar, H. W. Roesky, Acc. Chem. Res.
2013, 46, 444–456; c) E. Rivard, in Functional Molecular Silicon
Compounds II (Ed.: D. Scheschkewitz), Springer, 2013, pp. 203–
227; d) J. I. Schweizer, L. Meyer, A. Nadj, M. Diefenbach, M. C.
Holthausen, Chem. Eur. J. 2016, 22, 14328–14335; e) F.
Neumeyer, J. I. Schweizer, L. Meyer, A. G. Sturm, A. Nadj, M. C.
Holthausen, N. Auner, Chem. Eur. J. 2017, 23, 12399–12405; f)
J. Teichmann, M. Wagner, Chem. Commun. 2018, 54, 1397–1412.
[6] Intramolecular stabilization is possible either (i) thermodynami-
cally by π-donor substituents or (ii) kinetically through sterically
demanding ligands shielding the subvalent silicon site. For the
first stable examples, see: a) 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; b) M. Kira,
S. Ishida, T. Iwamoto, C. Kabuto, J. Am. Chem. Soc. 1999, 121, [13] a) F. Meyer-Wegner, A. Nadj, M. Bolte, N. Auner, M. Wagner,
9722–9723.
M. C. Holthausen, H.-W. Lerner, Chem. Eur. J. 2011, 17, 4715–
4719; b) J. Tillmann, L. Meyer, J. I. Schweizer, M. Bolte, H.-W.
Lerner, M. Wagner, M. C. Holthausen, Chem. Eur. J. 2014, 20,
9234–9239.
[7] a) C. Boehme, G. Frenking, J. Am. Chem. Soc. 1996, 118, 2039–
2046; b) M. Asay, C. Jones, M. Driess, Chem. Rev. 2011, 111,
354–396; c) B. D. Rekken, T. M. Brown, J. C. Fettinger, H. M.
Tuononen, P. P. Power, J. Am. Chem. Soc. 2012, 134, 6504–6507; [14] a) J. Li, S. Merkel, J. Henn, K. Meindl, A. Döring, H. W. Roesky,
d) 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; e) M. Kira, Proc. Jpn.
Acad. Ser. B 2012, 88, 167–191.
R. S. Ghadwal, D. Stalke, Inorg. Chem. 2010, 49, 775–777; b) G.
Tavcˇar, S. S. Sen, R. Azhakar, A. Thorn, H. W. Roesky, Inorg.
Chem. 2010, 49, 10199–10202; c) R. S. Ghadwal, H. W. Roesky,
S. Merkel, D. Stalke, Chem. Eur. J. 2010, 16, 85–88; d) R.
Azhakar, G. Tavcˇar, H. W. Roesky, J. Hey, D. Stalke, Eur. J. In-
org. Chem. 2011, 2011, 475–477; e) R. S. Ghadwal, R. Azhakar,
K. Pröpper, J. J. Holstein, B. Dittrich, H. W. Roesky, Inorg. Chem.
2011, 50, 8502–8508; f) S. M. I. Al-Rafia, A. C. Malcolm, R. Mc-
Donald, M. J. Ferguson, E. Rivard, Chem. Commun. 2012, 48,
1308–1310; g) E. Rivard, Dalton Trans. 2014, 43, 8577–8586; h)
J. I. Schweizer, M. G. Scheibel, M. Diefenbach, F. Neumeyer, C.
Würtele, N. Kulminskaya, R. Linser, N. Auner, S. Schneider,
M. C. Holthausen, Angew. Chem. Int. Ed. 2016, 55, 1782–1786.
[8] a) D. Bourissou, O. Guerret, F. P. Gabbaï, G. Bertrand, Chem.
Rev. 2000, 100, 39–92; b) M. Albrecht, Chem. Commun. 2008,
3601–3610; c) M. Melaimi, M. Soleilhavoup, G. Bertrand, An-
gew. Chem. Int. Ed. 2010, 49, 8810–8849; d) Y. Wang, G. H.
Robinson, Inorg. Chem. 2011, 50, 12326–12337; e) C. D. Martin,
M. Soleilhavoup, G. Bertrand, Chem. Sci. 2013, 4, 3020–3030; f)
H. W. Roesky, J. Organomet. Chem. 2013, 730, 57–62; g) Y.
Wang, G. H. Robinson, Inorg. Chem. 2014, 53, 11815–11832.
[9] a) J. Arnold, Dalton Trans. 2008, 4334–4335; b) T. Chivers, J.
Konu, Comments Inorg. Chem. 2009, 30, 131–176; c) C. Jones, [15] R. S. Ghadwal, K. Pröpper, B. Dittrich, P. G. Jones, H. W. Roesky,
G. A. Koutsantonis, Aust. J. Chem. 2013, 66, 1115–1117.
[10] a) Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. Schaefer III, P.
v. R. Schleyer, G. H. Robinson, Science 2008, 321, 1069–1071;
b) Y. Xiong, S. Yao, R. Müller, M. Kaupp, M. Driess, Nat. Chem.
2010, 2, 577–580; c) T. Yamaguchi, A. Sekiguchi, M. Driess, J.
Am. Chem. Soc. 2010, 132, 14061–14063; d) K. Leszczyn´ska, K.
Abersfelder, A. Mix, B. Neumann, H.-G. Stammler, M. J. Cowley,
P. Jutzi, D. Scheschkewitz, Angew. Chem. Int. Ed. 2012, 51,
6785–6788; e) Y. Xiong, S. Yao, S. Inoue, E. Irran, M. Driess,
Angew. Chem. Int. Ed. 2012, 51, 10074–10077; f) M. J. Cowley,
V. Huch, H. S. Rzepa, D. Scheschkewitz, Nat. Chem. 2013, 5,
Inorg. Chem. 2011, 50, 358–364.
[16] S. Sinhababu, S. Kundu, A. N. Paesch, R. Herbst-Irmer, D. Stalke,
I. Fernández, G. Frenking, A. C. Stückl, B. Schwederski, W.
Kaim, H. W. Roesky, Chem. Eur. J. 2018, 24, 1264–1268.
[17] a) V. Lavallo, Y. Canac, C. Präsang, B. Donnadieu, G. Bertrand,
Angew. Chem. Int. Ed. 2005, 44, 5705–5709; b) M. Soleilhavoup,
G. Bertrand, Acc. Chem. Res. 2014, 47, 256–266; c) M. Melaimi,
R. Jazzar, M. Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed.
2017, 56, 10046–10068.
[18] R. S. Ghadwal, S. S. Sen, H. W. Roesky, G. Tavcar, S. Merkel,
D. Stalke, Organometallics 2009, 28, 6374–6377.
876–879; g) S. Inoue, C. Eisenhut, J. Am. Chem. Soc. 2013, 135, [19] a) A. C. Filippou, O. Chernov, G. Schnakenburg, Chem. Eur. J.
18315–18318; h) K. C. Mondal, H. W. Roesky, M. C. Schwarzer,
G. Frenking, I. Tkach, H. Wolf, D. Kratzert, R. Herbst-Irmer, B.
Niepötter, D. Stalke, Angew. Chem. Int. Ed. 2013, 52, 1801–1805;
i) M. I. Arz, D. Geiß, M. Straßmann, G. Schnakenburg, A. C.
Filippou, Chem. Sci. 2015; j) S. U. Ahmad, T. Szilvási, E. Irran,
S. Inoue, J. Am. Chem. Soc. 2015, 137, 5828–5836; k) P. Ghana,
M. I. Arz, U. Das, G. Schnakenburg, A. C. Filippou, Angew.
Chem. Int. Ed. 2015, 54, 9980–9985; l) Y. Wang, M. Chen, Y.
Xie, P. Wei, H. F. Schaefer III, P. v. R. Schleyer, G. H. Robinson,
Nat. Chem. 2015, 7, 509–513; m) M. I. Arz, G. Schnakenburg,
A. Meyer, O. Schiemann, A. C. Filippou, Chem. Sci. 2016, 7,
4973–4979; n) K. C. Mondal, S. Roy, B. Dittrich, D. M. Andrada,
G. Frenking, H. W. Roesky, Angew. Chem. Int. Ed. 2016, 55,
3158–3161; o) D. Nieder, C. B. Yildiz, A. Jana, M. Zimmer, V.
2011, 17, 13574–13583; b) T. Agou, N. Hayakawa, T. Sasamori,
T. Matsuo, D. Hashizume, N. Tokitoh, Chem. Eur. J. 2014, 20,
9246–9249; c) A. C. Filippou, B. Baars, O. Chernov, Y. N. Le-
bedev, G. Schnakenburg, Angew. Chem. Int. Ed. 2014, 53, 565–
570.
[20] T. Iijima, H. Jimbo, M. Taguchi, J. Mol. Struct. 1986, 144, 191–
192.
[21] a) R. W. Alder, P. R. Allen, S. J. Williams, J. Chem. Soc., Chem.
Commun. 1995, 1267–1268; b) A. M. Magill, K. J. Cavell, B. F.
Yates, J. Am. Chem. Soc. 2004, 126, 8717–8724; c) O. Schuster,
L. Yang, H. G. Raubenheimer, M. Albrecht, Chem. Rev. 2009,
109, 3445–3478.
[22] J. I. Bates, P. Kennepohl, D. P. Gates, Angew. Chem. Int. Ed.
2009, 48, 9844–9847.
Z. Anorg. Allg. Chem. 0000, 0–0
www.zaac.wiley-vch.de
6
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
[23] aNHCs are well-established in transition-metal and f-block ele-
trand, J. Am. Chem. Soc. 2010, 132, 7264–7265; c) R. S. Ghad-
wal, H. W. Roesky, M. Granitzka, D. Stalke, J. Am. Chem. Soc.
2010, 132, 10018–10020; d) G. R. Whittell, E. I. Balmond,
A. P. M. Robertson, S. K. Patra, M. F. Haddow, I. Manners, Eur.
J. Inorg. Chem. 2010, 2010, 3967–3975; e) H. Cui, C. Cui, Dalton
Trans. 2011, 40, 11937–11940; f) R. S. Ghadwal, S. O.
Reichmann, E. Carl, R. Herbst-Irmer, Dalton Trans. 2014, 43,
13704–13710; g) H. Schneider, D. Schmidt, U. Radius, Chem.
Eur. J. 2015, 21, 2793–2797.
ment chemistry, for reviews see: P. L. Arnold, S. Pearson, Coord.
Chem. Rev. 2007, 251, 596–609; M. Albrecht, Chem. Commun.
2008, 3601–3610; O. Schuster, L. Yang, H. G. Raubenheimer, M.
Albrecht, Chem. Rev. 2009, 109, 3445–3478. Yet, only few exam-
ples of NHC functionalization in C4- and C5-position by main-
group elements have been reported prior to 2009: A. J. Arduengo,
F. Davidson, H. V. R. Dias, J. R. Goerlich, D. Khasnis, W. J. Mar-
shall, T. K. Prakasha, J. Am. Chem. Soc. 1997, 119, 12742–12749;
M. K. Denk, J. M. Rodezno, J. Organomet. Chem. 2001, 617–
618, 737–740; P. L. Arnold, S. T. Liddle, Chem. Commun. 2005,
5638–5640; T. W. Graham, K. A. Udachin, A. J. Carty, Chem.
Commun. 2006, 2699–2701. For the first mention of an
aNHC, see: S. Grundemann, A. Kovacevic, M. Albrecht, J. W.
Faller Robert, H. Crabtree, Chem. Commun. 2001, 2274–2275.
[24] E. Aldeco-Perez, A. J. Rosenthal, B. Donnadieu, P. Parameswa-
ran, G. Frenking, G. Bertrand, Science 2009, 326, 556–559.
[25] a) A. P. Singh, P. P. Samuel, K. C. Mondal, H. W. Roesky, N. S.
Sidhu, B. Dittrich, Organometallics 2013, 32, 354–357; b) A. R.
Chianese, A. Kovacevic, B. M. Zeglis, J. W. Faller, R. H.
Crabtree, Organometallics 2004, 23, 2461–2468; c) D. J. Nelson,
S. P. Nolan, Chem. Soc. Rev. 2013, 42, 6723–6753; d) T. Dröge,
F. Glorius, Angew. Chem. Int. Ed. 2010, 49, 6940–6952.
[28] Y. Wang, Y. Xie, M. Y. Abraham, P. Wei, H. F. Schaefer, P.
v. R. Schleyer, G. H. Robinson, J. Am. Chem. Soc. 2010, 132,
14370–14372.
[29] Two minor NMR signals were detected at δ29Si = –47.3 and
–34.0 ppm.
[30] Geometry optimizations were performed at the SMD-RI-PBE-D3/
BS1 level of theory (BS1: 6-31+G(d,p) for H, C, N, Si and
LANL2DZdp and ECP for Br). Improved energies and wave-
functions were obtained from subsequent SMD-RI-B2GP-PLYP-
D3/def2-TZVPP single-point calculations. See the Supporting In-
formation for full computational details.
[31] M. N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature
2014, 510, 485.
[32] Related deprotonation reactions of positively charged imid-
azolium complexes have also been observed in the dimerization
of carbenes: R. W. Alder, M. E. Blake, L. Chaker, J. N. Harvey,
F. Paolini, J. Schütz, Angew. Chem. Int. Ed. 2004, 43, 5896–5911;
J. Vignolle, X. Cattoën, D. Bourissou, Chem. Rev. 2009, 109,
3333–3384.
[26] a) D. Rottschäfer, F. Ebeler, T. Strothmann, B. Neumann, H. G.
Stammler, A. Mix, R. S. Ghadwal, Chem. Eur. J. 2018, 24, 3716–
3720; b) Y. Wang, Y. Xie, P. Wei, H. F. Schaefer, G. H. Robinson,
Dalton Trans. 2016, 45, 5941–5944.
[27] a) H. Cui, Y. Shao, X. Li, L. Kong, C. Cui, Organometallics 2009,
28, 5191–5195; b) D. Mendoza-Espinosa, B. Donnadieu, G. Ber-
Received: April 23, 2018
Z. Anorg. Allg. Chem. 0000, 0–0
www.zaac.wiley-vch.de
7
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
J. I. Schweizer, A. G. Sturm, T. Porsch, M. Berger, M. Bolte,
N. Auner,* M. C. Holthausen* .......................................... 1–8
Reactions of Si2Br6 with N-Heterocyclic Carbenes
Z. Anorg. Allg. Chem. 0000, 0–0
www.zaac.wiley-vch.de
8
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim