A cationic zinc hydride cluster stabilized by an N-heterocyclic carbene: Synthesis, reactivity, and hydrosilylation catalysis

Article abstract of DOI:10.1002/anie.201408346

The trinuclear cationic zinc hydride cluster [(IMes)₃Zn₃H₄ (THF)](BPh₄)₂ (1) was obtained either by protonation of the neutral zinc dihydride [(IMes)ZnH₂]₂ with a Bronsted acid o

Full text of DOI:10.1002/anie.201408346

Angewandte
Chemie
DOI: 10.1002/anie.201408346
Zinc Catalysis
A Cationic Zinc Hydride Cluster Stabilized by an N-Heterocyclic
Carbene: Synthesis, Reactivity, and Hydrosilylation Catalysis**
Arnab Rit, Alessandro Zanardi, Thomas P. Spaniol, Laurent Maron,* and Jun Okuda*
Abstract: The trinuclear cationic zinc hydride cluster
[(IMes)3Zn3H4(THF)](BPh4)2 (1) was obtained either by
protonation of the neutral zinc dihydride [(IMes)ZnH2]2 with
a Brønsted acid or by addition of the putative zinc dication
[(IMes)Zn(THF)]2+. A triply bridged thiophenolato complex
À
2 was formed upon oxidation of 1 with PhS SPh. Protonolysis
of 1 by methanol or water gave the corresponding trinuclear
dicationic derivatives. At ambient temperature, 1 catalyzed the
hydrosilylation of aldehydes, ketones, and nitriles. Carbon
dioxide was also hydrosilylated under forcing conditions when
using (EtO)3SiH, giving silylformate as the main product.
Scheme 1. Preparation of a cationic zinc hydride cluster 1.
S
ince its discovery by Schlesinger et al. in 1947, zinc
dihydride (ZnH2)[1] has been used as hydride transfer reagent
in organic synthesis.[2] Owing to its pronounced thermal
sensitivity, the precise structure of zinc dihydride in the
bulk[1f,g] has remained unknown until this day. More recently,
zinc dihydride was stabilized as thermally robust dimeric N-
heterocyclic carbene (NHC) adducts,[3] following several
reports on zinc monohydrides of the general formula
(LX)ZnH (LX = monoanionic ligand).[4] While anionic zinc
hydrides[5] such as structurally characterized Na2[Zn2Et4(m-
H)2] and Na3[Zn2iPr6(m-H)][5e] had been known, a cationic
zinc hydride [(IPr)ZnH(THF)](OTf) (IPr= 1,3-bis(2,6-diiso-
propylphenyl)imidazol-2-ylidene) was reported only recently
by Rivard et al.[5f] We describe here the synthesis, reactivity,
and enhanced hydrosilylation activity of a trinuclear cationic
zinc hydride cluster [(IMes)3Zn3H4(THF)](BPh4)2 (1)
(IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene)
stabilized by an N-heterocyclic carbene.[6]
86% yield (Scheme 1). Alternatively, 1 was prepared in 82%
yield by treatment of [(IMes)ZnH2]2 with the putative zinc
dication [(IMes)Zn(THF)]2+, generated in situ by salt meta-
thesis of [(IMes)ZnCl2][7] with two equiv of NaBPh4
(Scheme 1). The zinc hydride cluster 1 is moderately soluble
in THF and stable for over a week in THF solution. Isolated
crystals of 1 are stable for months at À358C and decompose
only at temperatures above 1658C.
Formation of a new hydrido complex was revealed by
a hydride signal at d = 1.79 ppm (showing no correlation in
the 13C–1H HSQC NMR spectrum) in the 1H NMR spectrum
in [D8]THF. One set of signals was observed for both the IMes
and (BPh4)À anions. Variable temperature (À80 to 508C)
1H NMR spectra in [D8]THF did not lead to decoalescence of
the hydride signals, which is most likely due to fast exchange
among the hydrido ligands. The coordinated THF in 1 was
detected as broad signals at d = 1.61 and 3.16 ppm in CD2Cl2.
In the 13C{1H} NMR spectrum, the zinc-coordinated NHC
signal was detected at d = 169.25 ppm in [D8]THF, in the
range of other zinc(II)–NHC complexes.[3,7]
The cationic zinc hydride [(IMes)3Zn3H4(THF)](BPh4)2
[3]
(1) was obtained by protonation of [(IMes)ZnH2]2 with
[PhNMe2H]BPh4, a Brønsted acid containing a weakly coor-
dinating anion, under dihydrogen generation, and isolated in
The structure analysis of crystals grown from THF/hexane
solution of 1 shows a trinuclear zinc dicationic core connected
by four hydrides (Figure 1). Hydride 1 can be regarded as the
[(IMes)Zn(THF)]2+ dication bonded to the neutral
[(IMes)ZnH2]2 through two terminal hydrides. To the best
of our knowledge, 1 is the first example of a hydride-bridged
cationic molecular zinc cluster.
[*] Dr. A. Rit, Dr. A. Zanardi, Dr. T. P. Spaniol, Prof. Dr. J. Okuda
Institute of Inorganic Chemistry, RWTH Aachen University
Landoltweg 1, 52056 Aachen (Germany)
E-mail: jun.okuda@ac.rwth-aachen.de
Prof. Dr. L. Maron
Universitꢀ de Toulouse et CNRS, INSA, UPS, CNRS; UMR 5215
LPCNO
135 avenue de Rangueli, F-31077 Toulouse (France)
E-mail: laurent.maron@irsamc.ups-tlse.fr
All three zinc centers in 1 are tetrahedrally coordinated.
À
As expected for a cationic compound, the Zn m-H (1.65(3)–
[**] We thank the Deutsche Forschungsgemeinschaft for financial
support through the International Research Training Group
“Selectivity in Chemo- and Biocatalysis” (GRK 1628). L.M. is a fellow
of the Alexander von Humboldt Foundation. We thank Toni Gossen
for assistance with the NMR spectroscopy.
À
1.75(4) ꢀ) and Zn CNHC bonds (1.974(4)–1.995(4) ꢀ) are
shorter than those in the neutral parent complex
[3]
À
À
[(IMes)ZnH2]2 (Zn m-H 1.71(4)–1.81(3) ꢀ and Zn CNHC
2.052(3)–2.054(3) ꢀ), reflecting the higher Lewis acidity of
the zinc atoms in 1. Three zinc atoms form a triangle with two
hydrides bridging the short Zn2···Zn3 distance (2.4137(7) ꢀ,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408346.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
Figure 1. Molecular structure of the dication [(IMes)3Zn3H4(THF)]2+ in
1·2THF (the hydrogen atoms except those connected to zinc, co-
crystallized THF molecules, BPh4 anions, and the methyl groups of the
mesityl moieties are omitted for clarity).
Scheme 2. Reactivity studies of 1.
as found in RZn(m-H)ZnR)[8] and one hydride bridging each
of the two long Zn···Zn separations (Zn1···Zn2 2.8116(7) ꢀ,
Zn1···Zn3 2.8008(7) ꢀ). The (BPh4)À anions do not show
significant interactions with the cationic part.
C6D6) and biphenyl (detected by GC-MS analysis). Similar
oxidation of (BPh4)À has also been observed with other metal
complexes.[10] Complex 2 is readily soluble in dichlorome-
1
thane, but sparingly soluble in THF. The H NMR spectrum
The hydrido ligands in 1 did not exchange with D2 (1 bar)
in [D8]THF over a week at 258C; [(IMes)ZnH2]2 was also
found to be inert.[3] The concentration dependence of the
1H NMR chemical shifts of the IMes together with the
hydrido ligands and the observation of free THF in [D8]THF
solution indicate that the coordinated THF is labile. When
1 was reacted with 1 equiv of PMe3 in CD2Cl2, the coordinated
of 2 in CD2Cl2 shows one set of signals for each of the IMes
and thiophenolate ligands and (BPh4)À anions in 2:3:1 ratio, in
agreement with the formulation of 2 as [(IMes)2Zn2(SPh)3]-
(BPh4).
Single crystals of 2 suitable for X-ray diffraction were
grown from a saturated THF solution. The molecular
structure contains
a non-crystallographic C3-symmetric
1
THF was completely replaced, as revealed by the H NMR
Zn2S3 core with three sulfur atoms forming an almost
equilateral triangle connected to a zinc atom on each side
of the plane (Figure 2). The Zn1···Zn2 distance of 2.791(6) ꢀ
is close to the Zn···Zn separations in 1 that are bridged by one
hydrido ligand.
Zinc hydride cation 1 reacted readily with four equiv-
alents of methanol or water at room temperature under
immediate evolution of dihydrogen to give the methoxy
spectrum, and the coordination of PMe3 to the zinc center is
indicated by a signal at d = À46.96 ppm in the 31P{1H} NMR
spectrum.[9] No nZn–H stretching frequencies for 1 could be
assigned, as they overlap with the aromatic vibrations of the
IMes ligand.[3,8]
Bonding analysis of 1 at the DFT level (see the Supporting
Information) gave geometric parameters close to the exper-
À
imental values. The computed Zn H bond lengths (1.69–
1.76 ꢀ) agree with the experimental values (1.65(3)–
1.75(4) ꢀ). Similarly, the Zn···Zn distances are reproduced:
the short Zn2···Zn3 distance (2.43 ꢀ vs 2.4139(7) ꢀ) as well as
the two long Zn1···Zn2/Zn3 distances (2.86–2.88 ꢀ vs.
2.8008(7)–2.8116(6) ꢀ). The NBO analysis indicates that the
charge at Zn1 is almost twice of that at Zn2 and Zn3 (+ 0.92
vs. + 0.50), in line with a formal + 2 charge located at Zn1.
While the Wiberg index of 0.33 for the Zn2···Zn3 distance
indicates a partial bonding interaction, the Wiberg bond
indexes of 0.14 calculated for Zn1···Zn2 and Zn1···Zn3 point
to weaker interactions.
À
With 2.25 equiv of PhS SPh, the hydrido ligands in 1 were
readily oxidized to dihydrogen with concomitant reduction of
À
À
PhS SPh to (PhS) to give the dinuclear thiophenolato
complex [(IMes)2Zn2(SPh)3](BPh4) (2) as colorless crystals in
69% yield (Scheme 2). Dihydrogen formation was observed
by 1H NMR spectroscopy. Complex 2 is formed by one-
electron oxidation of one of the (BPh4)À anions under release
of BPh3 (detected by 11B NMR spectroscopy; d = 67 ppm in
Figure 2. Molecular structure of the monocation [(IMes)2Zn2(SPh)3]+
in 2·THF (hydrogen atoms, co-crystallized THF molecules, BPh4
anions, and the methyl groups of the mesityl moieties are omitted for
clarity).
2
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
Table 1: Hydrosilylation of aldehydes and ketones catalyzed by 1.
[(IMes)3Zn3(OMe)4(THF)](BPh4)2 (3a) and hydroxy
[(IMes)3Zn3(OH)4(THF)](BPh4)2 (3b) complexes in 83 and
80% yield, respectively (Scheme 2). All of the hydrido
ligands in 1 reacted within 15 min; no protonation of the
IMes ligands was observed under these conditions. Com-
plexes 3a,b are readily soluble in THF and dichloromethane
and are stable at À358C as solids.
Entry
R1, R2
Silane
Time
[min]
Catalyst
loading
[mol%]
Conv.
[%][a]
The methoxy unit in 3a is characterized by one singlet at
1
2
3
4
5
6
7
8
9
10
Ph, H
Ph, H
Ph, H
4-F-C6H4, H
4-Br-C6H4, H
4-OMe-C6H4, H
4-OMe-C6H4, H
nBu, H
PhSiH3
10
15
60
45
50
120
30
180
180
60
0.5
2.2
2.2
2.2
2.2
2.2
2.2
2.7
3.3
3.3
>98
>98
>98
>98
>98
>98
>98
97
1
d = 2.20 ppm in the H NMR spectrum and by one signal at
PhMeSiH2
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
PhMeSiH2
(EtO)3SiH
(EtO)3SiH
PhMeSiH2
d = 55.62 ppm in the 13C{1H} NMR spectrum ([D8]THF). The
hydroxy unit in 3b was detected as singlet at d = À1.36 ppm in
the 1H NMR spectrum ([D8]THF) and at 3648 cmÀ1 in the IR
spectrum, which are in the same regions of those of other
hydroxide bridged zinc complexes.[11] 1H NMR patterns
similar to those of 1 were observed for both the IMes and
BPh4 counterions in 3a,b and the integration ratios of
methoxy/hydroxy units: IMes ligands: (BPh4)À anion of
4:3:2 confirmed the retention of the trinuclear structures as
for 1. The molecular structure of 3b, confirmed by single
crystal X-ray diffraction, showed a trinuclear structure similar
to that of 1, with zinc centers bridged by m2-hydroxide ligands
(see the Supporting Information). As observed for 1, the THF
ligands in 3a,b are also labile in [D8]THF. IMes coordination
to zinc was revealed by signals at d = 170.14 (3a) and 171.83
Ph, Ph
Ph, Me
95
87
1
[a] determined by H NMR spectroscopy.
Table 2: Hydrosilylation of nitriles catalyzed by 1.[a]
1
(3b) ppm in the 13C{1H} NMR spectra in [D8]THF. H NMR
Entry
R1
Silane
Time
Conv.
[%][b]
spectroscopic monitoring showed that 3a react with silanes
PhSiH3, PhMeSiH2, or (EtO)3SiH at room temperature to
generate 1 quantitatively.
[h]
1
2
3
4
5
6
7
Ph
Ph
(EtO)3SiH
PhMeSiH2
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
9
7
4
8
6
>98
>98
90
Preliminary studies showed fast insertion of PhCHO into
3-Br-C6H4
4-OMe-C6H4
2-naphthyl
tBu
À
the Zn H bonds in 1, even though a single product could not
94
>98
92
be isolated from the reaction with 4 equiv of benzaldehyde.
This observation along with the conversion of the alkoxy
complex 3a into 1 when reacted with silanes prompted us to
probe the suitability of 1 as catalyst for hydrosilylation
reaction. Recently, zinc hydride species have proved to be an
alternative to precious-metal-based catalysts for various
organic transformations.[12] Hydrosilylation of carbonyl and
nitrile substrates results in reduction to form silyl ethers and
silyl imines, respectively.[13,14]
48
24
4-pyridyl
41[c]
[a] 3.3 mol% catalyst loading. [b] Determined by 1H NMR spectroscopy.
[c] Catalyst deactivation.
only with PhMeSiH2, no catalysis was observed with tertiary
silanes.[5f] Complex 1 shows comparatively better catalytic
activity than some transition-metal systems, which requires
either higher temperature or longer reaction time.[16]
Employing 0.5 mol% of 1, quantitative hydrosilylation of
PhCHO with primary silane PhSiH3 (Table 1) to give 95% of
PhSiH(OCH2Ph)2 was observed within 10 min at room
Nitriles can also be catalytically hydrosilylated by 1 to give
monoreduced N-silyl imines at room temperature (Table 2).
This is a rare example where such a process is catalyzed by
a non-transition-metal catalyst.[12d] Benzonitrile was hydro-
silylated with both PhMeSiH2 and (EtO)3SiH (entries 1–2).
Electron-withdrawing/releasing substituents on the phenyl
ring are tolerated and facile hydrosilylation was observed
(entry 3–4). 2-Naphthonitrile was hydrosilylated faster than
benzonitrile (entry 5). Alkyl nitrile tBuCN was reduced
slowly (entry 6). For 4-pyridinecarbonitrile, only 41% con-
version was observed within 1 d before the catalyst was
deactivated (entry 7). Complex 1 shows catalytic activity
comparable with that reported for [(Dipp-nacnac)ZnH]
(Dipp-nacnac = CH{(CMe) (2,6-iPr2C6H3N)}2).[12d]
À
temperature. PhCHO can also be inserted into all three Si
H bonds of PhSiH3 to give PhSi(OCH2Ph)3, an alternative
synthetic approach towards PhSi(OR)3 derivatives.[12a] Sec-
ondary silane PhMeSiH2 works also efficiently (entries 2 and
7). While the tertiary silane PhMe2SiH was less reactive,
comparatively inexpensive (EtO)3SiH shows high activity.[15]
PhCHO is hydrosilylated with (EtO)3SiH to the correspond-
ing silyl ether within 1 h (entry 3). Notably, this catalytic
system tolerates different substituents on aldehydes, such as
nitro, cyano, halide, and alkoxy groups (see the Supporting
Information). While an electron-withdrawing group margin-
ally increases the reaction rates (entries 4–5), an electron-
donating group (OMe) slows down the rate (entry 6). Beside
aromatic aldehydes, alkyl aldehyde nBuCHO (entry 8) as well
as ketones (entries 9 and 10) were also efficiently hydro-
silylated at room temperature. Complex [(IPr)ZnH(THF)]-
(OTf) was found to catalyze the hydrosilylation of Ph2CO
Competitive hydrosilylation experiment with 1:1 mixture
of benzaldehyde and benzonitrile when using PhMeSiH2
(2.2 mol% 1, 258C) showed that only benzaldehyde was
quantitatively reduced to PhSiHMe(OCH2Ph) within 15 min
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
[3] A. Rit, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. Int. Ed.
2013, 52, 4664 – 4667; Angew. Chem. 2013, 125, 4762 – 4765.
[4] For selected references, see: a) A. Looney, R. Han, I. B. Gorrell,
M. Cornebise, K. Yoon, G. Parkin, A. L. Rheingold, Organo-
metallics 1995, 14, 274 – 278; b) H. Hao, C. Cui, H. W. Roesky, G.
Bai, H.-G. Schmidt, M. Noltemeyer, Chem. Commun. 2001,
1118 – 1119; c) M. P. Coles, S. M. El-Hamruni, J. D. Smith, P. B.
Hitchcock, Angew. Chem. Int. Ed. 2008, 47, 10147 – 10150;
Angew. Chem. 2008, 120, 10301 – 10304; d) J. Spielmann, D.
Piesik, B. Wittkamp, G. Jansen, S. Harder, Chem. Commun.
2009, 3455 – 3456; e) B. Gutschank, S. Schulz, D. Blꢁser, R.
Boese, C. Wçlper, Organometallics 2010, 29, 6133 – 6136; f) D.
Mukherjee, A. Ellern, A. D. Sadow, J. Am. Chem. Soc. 2010, 132,
7582 – 7583; g) W. Sattler, G. Parkin, J. Am. Chem. Soc. 2011,
133, 9708 – 9711; h) N. J. Brown, J. E. Harris, X. Yin, I. Silver-
wood, A. J. P. White, S. G. Kazarian, K. Hellgardt, M. S. P.
Shaffer, C. K. Williams, Organometallics 2014, 33, 1112 – 1119;
i) P. Jochmann, D. W. Stephan, Chem. Commun. 2014, 50, 8395 –
8397.
at room temperature, leaving benzonitrile unchanged (see the
Supporting Information).
Based on the observations that 1 readily inserts benzal-
dehyde and the resulting alkoxy compound (not isolated) as
well as 3a regenerates 1 when reacted with silanes (see the
Supporting Information), the following catalytic cycle is
À
proposed. Insertion of carbonyl functions into the Zn H
bonds is followed by the reaction of the resulting alkoxy
intermediates with silanes to regenerate 1 along with the
release of silyl ethers. This proposal is supported by NMR
experiments. When the deuterium isotopomer 1-D4 was
reacted with 4 equiv of PhCHO followed by addition of the
silane PhSiH3, PhMeSiH2, or (EtO)3SiH, quantitative forma-
tion of 1-H4 was observed. On the other hand, when 1-H4 was
treated with 4 equiv of PhCHO followed by PhSiD3, the
deuterium isotopomer 1-D4 was obtained quantitatively (see
the Supporting Information). A similar mechanism has been
[5] a) G. J. Kubas, D. F. Shriver, J. Am. Chem. Soc. 1970, 92, 1949 –
1954; b) D. F. Shriver, G. J. Kubas, J. A. Marshall, J. Am. Chem.
Soc. 1971, 93, 5076 – 5079; c) E. C. Ashby, R. G. Beach, Inorg.
Chem. 1971, 10, 2486 – 2488; d) E. C. Ashby, J. J. Watkins, Inorg.
Chem. 1973, 12, 2493 – 2503; e) A. Lennartson, M. Hꢂkansson, S.
Jagner, Angew. Chem. Int. Ed. 2007, 46, 6678 – 6680; Angew.
Chem. 2007, 119, 6798 – 6800; f) P. A. Lummis, M. R. Momeni,
M. W. Lui, R. McDonald, M. J. Ferguson, M. Miskolzic, A.
Brown, E. Rivard, Angew. Chem. Int. Ed. 2014, 53, 9347 – 9351;
Angew. Chem. 2014, 126, 9501 – 9505.
[6] For reviews on N-heterocyclic carbenes, see: a) F. E. Hahn,
M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122 – 3172;
Angew. Chem. 2008, 120, 3166 – 3216; b) M. Melaimi, M.
Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed. 2010, 49,
8810 – 8849; Angew. Chem. 2010, 122, 8992 – 9032.
postulated
for
the
hydrosilylation
catalyzed
by
a [(NHC)CuIH][17] and a [{2,6-(iPr2PO)2C6H3}NiH] cata-
lyst.[16b] Alternatively, coordination of Si H s-bond or
À
oxidative addition of silanes to a zinc center,[18] proposed
for other systems, is quite unlikely to operate here, as 1 did not
react with PhMeSiH2 or (EtO)3SiH over one day at room
temperature in [D8]THF.
Compound 1 was also found to catalyze the hydrosilyla-
tion of more challenging substrate CO2 (40 bar, THF, 608C).
Complete conversion was achieved after 64 h when
(EtO)3SiH and 1 mol% of 1 were used. Triethoxysilylformate
(1H NMR: 8.01 ppm (CHO); 13C NMR: 157.9 ppm (CHO);
29Si NMR: À86.4 ppm) was identified as the major product
[7] D. Wang, K. Wurst, M. R. Buchmeiser, J. Organomet. Chem.
2004, 689, 2123 – 2130.
(> 80%)
along
with
bis(triethoxysiloxy)
methane
(< 20%).[12a]
[8] a) Z. Zhu, R. J. Wright, M. M. Olmstead, E. Rivard, M. Brynda,
P. P. Power, Angew. Chem. Int. Ed. 2006, 45, 5807 – 5810; Angew.
Chem. 2006, 118, 5939 – 5942; b) Z. Zhu, M. Brynda, R. J.
Wright, R. C. Fischer, W. A. Merrill, E. Rivard, R. Wolf, J. C.
Fettinger, M. A. Olmstead, P. P. Power, J. Am. Chem. Soc. 2007,
129, 10847 – 10857.
[9] a) F. A. Cotton, G. Schmid, Polyhedron 1996, 15, 4053 – 4059;
b) D. J. Darensbourg, M. S. Zimmer, P. Rainey, D. L. Larkins,
Inorg. Chem. 2000, 39, 1578 – 1585.
In conclusion, we have isolated a cationic zinc hydride
cluster[19] that retains hydridic character and shows versatile
hydrosilylation activity toward electrophiles such as carbonyl
compounds, nitriles, and importantly more challenging sub-
strate carbon dioxide. Detailed mechanistic studies including
the nature of the active species are underway.
Received: August 18, 2014
Published online: && &&, &&&&
[10] W. J. Evans, S. A. Kozimer, J. W. Zeller, Chem. Commun. 2005,
4681 – 4683.
[11] a) D. L. Reger, A. E. Pascui, P. J. Pellechia, M. D. Smith, Inorg.
Chem. 2013, 52, 11638 – 11649; b) G. Anantharaman, K. Elango,
Organometallics 2007, 26, 1089 – 1092.
[12] a) W. Sattler, G. Parkin, J. Am. Chem. Soc. 2012, 134, 17462 –
17465; b) D. Mukherjee, R. R. Thompson, A. Ellern, A. D.
Sadow, ACS Catal. 2011, 1, 698 – 702; c) P. Jochmann, D. W.
Stephan, Angew. Chem. Int. Ed. 2013, 52, 9831 – 9835; Angew.
Chem. 2013, 125, 10014 – 10018; d) C. Boone, I. Korobkov, G. I.
Nikonov, ACS Catal. 2013, 3, 2336 – 2340; e) A. Rit, T. P. Spaniol,
J. Okuda, Chem. Asian J. 2014, 9, 612 – 619.
Keywords: carbon dioxide · homogeneous catalysis · hydrides ·
N-heterocyclic carbenes · zinc
.
[1] a) A. E. Finholt, A. C. Bond, Jr., H. I. Schlesinger, J. Am. Chem.
Soc. 1947, 69, 1199 – 1203; b) J. J. Watkins, E. C. Ashby, Inorg.
Chem. 1974, 13, 2350 – 2354; c) E. C. Ashby, J. J. Watkins, D.
Greig, D. F. Shriver, Inorg. Synth. 1977, 17, 6 – 8; d) A. J.
De Koning, J. Boersma, G. J. M. Van Der Kerk, J. Organomet.
Chem. 1980, 186, 159 – 172; e) E. C. Ashby, A. B. Goel, J.
Organomet. Chem. 1981, 204, 139 – 145; f) For gas phase
structure, see: A. Shayesteh, D. R. T. Appadoo, I. E. Gordon,
P. F. Bernath, J. Am. Chem. Soc. 2004, 126, 14356 – 14357; g) For
noble gas matrix isolation study, see: T. M. Greene, W. Brown, L.
Andrews, A. J. Downs, G. V. Chertihin, N. Rueneberg, P.
Pyykkç, J. Phys. Chem. 1995, 99, 7925 – 7934.
[13] The Chemistry of Organic Silicon Compounds, Vol. 3 (Eds.: Z.
Rappoport, Y. Apeloig), Wiley, New York, 2001.
[14] For reviews on hydrosilylation, see: a) Hydrosilylation:
A
Comprehensive Review on Recent Advances (Ed.: B. Marciniec),
Springer, Heidelberg, 2009; b) O. Riant, N. Mostefaꢃ, J. Cour-
marcel, Synthesis 2004, 2943 – 2958.
[15] Hydrosilylation of aldehydes with (EtO)3SiH gave a mixture of
silyl ethers (R1CH2O)xSi(OEt)4Àx (x = 0–3) by redistribution of
alkoxide ligands, which has also been observed previously;[12d,16a]
this is presumably catalyzed by a metal hydride species.
[2] a) Y. Gao, H. Urabe, F. Sato, J. Org. Chem. 1994, 59, 5521 – 5523;
b) Y. Gao, K. Harada, T. Hata, H. Urabe, F. Sato, J. Org. Chem.
1995, 60, 290 – 291.
4
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
[16] Selected references: a) E. Peterson, A. Y. Khalimon, R. Sim-
ionescu, L. G. Kuzmina, J. A. K. Howard, G. I. Nikonov, J. Am.
Chem. Soc. 2009, 131, 908 – 909; b) S. Chakraborty, J. A. Krause,
H. Guan, Organometallics 2009, 28, 582 – 586.
[17] a) H. Kaur, F. K. Zinn, E. D. Stevens, S. P. Nolan, Organo-
metallics 2004, 23, 1157 – 1160; b) S. Dꢄez-Gonzꢅlez, N. M. Scott,
S. P. Nolan, Organometallics 2006, 25, 2355 – 2358; c) S. Dꢄez-
Gonzꢅlez, E. D. Stevens, N. M. Scott, J. L. Petersen, S. P. Nolan,
Chem. Eur. J. 2008, 14, 158 – 168.
[18] a) I. Ojima, T. Kogure, M. Kumagai, S. Horiuchi, T. Sato, J.
Organomet. Chem. 1976, 122, 83 – 97; b) G. Du, P. E. Fanwick,
M. M. Abu-Omar, J. Am. Chem. Soc. 2004, 126, 8072 – 8073;
c) T. Steinke, C. Gemel, M. Cokoja, M. Winter, R. A. Fischer,
Angew. Chem. Int. Ed. 2004, 43, 2299 – 2302; Angew. Chem.
2004, 116, 2349 – 2352.
[19] CCDC 1014080 (1), 1014081 (2), and 1014082 (3b) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Zinc Catalysis
Competition for transition metals: A
cationic trinuclear zinc hydride cluster
with Zn3H4 core efficiently catalyzes the
hydrosilylation of aldehydes, ketones, and
nitriles, and notably also carbon dioxide.
A. Rit, A. Zanardi, T. P. Spaniol,
L. Maron,* J. Okuda*
&&&& — &&&&
A Cationic Zinc Hydride Cluster
Stabilized by an N-Heterocyclic Carbene:
Synthesis, Reactivity, and Hydrosilylation
Catalysis
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!