Agostic NSi-H...Mo complexes: From curiosity to catalysis

Article abstract of DOI:10.1002/anie.200802147

An uncommon catalyst: The β-agostic NSi-H...complex 1 (Ar=2,6-diisopropylphenyl) catalyzes a variety of hydrosilylation reactions. Stoichiometric reactions of 1 with unsaturated compounds proceed via the silanimine intermediate A and, in the case of olefins or nitriles, give products of Si-C coupling, such as 2.

Full text of DOI:10.1002/anie.200802147

Angewandte
Chemie
DOI: 10.1002/anie.200802147
Hydrosilylation
À
Agostic NSi H···Mo Complexes: From Curiosity to Catalysis**
Andrey Y. Khalimon, Razvan Simionescu, Lyudmila G. Kuzmina, Judith A. K. Howard, and
Georgii I. Nikonov*
Dedicated to Professor Robert West on the occasion of his 80th birthday
Although agostic silylamido complexes A have been known
since 1992 (Scheme 1),[1,2] their reactivity has been virtually
unstudied, so they largely remain a laboratory curiosity.[3] We
recently discovered a silane–imide coupling reaction,[4] allow-
ing for the preparation of agostic silylamido complexes 1 and
The reaction of bis(imido) compound (ArN)2Mo(PMe3)3
(Ar= 2,6-diisopropylphenyl) with two equivalents of PhSiH3
leads to a product of double silane addition, the b-agostic
À
NSi H···Mo complex 3 [Eq. (1)]. The structure of 3 is
1
fluxional at room temperature, but at 223 K the H NMR
spectrum shows an up-field signal characteristic of the proton
of an agostic Si Ha moiety[3] at d = 4.35 ppm (brm), which is
À
À
coupled to a signal assigned to the terminal Si H proton at
2
d = 6.03 ppm (d, JH,H = 5.4 Hz). The diastereotopic protons
of the PhH2Si group give rise to signals at d = 5.68 and
À
5.97 ppm. The agostic Si Ha bond is also clearly seen from the
Scheme 1. Agostic silylamido complexes (Ar=2,6-diisopropylphenyl,
Ar’=2,6-dimethylphenyl).
red-shifted band at 1694 cmÀ1 in the IR spectrum of 3,
À
whereas three classical Si H stretches are detected at 2014,
2a,b with a d2 configuration (Scheme 1).[2] Bonding in these
species can be represented by two canonical forms (B and C;
Scheme 1), one of which has a silanimine character (C). This
fact suggests that 1 and 2a,b could serve as intermediates for
silanimine complexes, which, although very scarce,[5] are
known to exhibit a wealth of reactivity.[5a,b,6] Herein, we
describe the preparation, structure, and reactivity of a new
agostic silylamido complex, 3. For the first time, we report the
catalytic and stoichiometric reactions of such a complex and
provide evidence for the intermediacy of a silanimine
complex.
2041, and 2165 cmÀ1. The 29Si NMR spectrum of 3 contains
two signals. The classical SiH2Ph group gives rise to a triplet at
d = 1.2 ppm (1JSi,H = 153.5 Hz), whereas the silyl group par-
ticipating in the b-agostic interaction gives rise to an up-field-
1
shifted signal at d = À72.9 ppm (dd, JSi,H = 113.0, 245.3 Hz).
a
1
À
The JSi,H coupling constant forthe agostic Si Ha bond
a
(113.0 Hz) is slightly increased in comparison with the values
observed for 1 (96 Hz) and 2a (97 Hz), which may indicate a
À
lowerextent of Si H activation, owing to a greater electron
deficiency of the 16-electron MoIV centerin 3.[7]
The molecular structure of 3 can be described as a
distorted molybdenum-centered trigonal bipyramid having
the PMe3 group and the agostic hydride ligand in the apical
positions (Figure 1). The Mo1 Si1 bond involving the agostic
silyl group (2.634(1) Š) is significantly longer than the Mo1
[*] Dipl.-Chem. A. Y. Khalimon, Dipl.-Ing. R. Simionescu,
Dr. G. I. Nikonov
À
Chemistry Department, Brock University
500 Glenridge Ave., St. Catharines, ON L2S3A1 (Canada)
Fax: (+1)905-682-9020
À
Si2 bond involving the terminal silyl group (2.495(1) Š), but is
À
comparable with the Mo Si bonds involving the agostic silyl
E-mail: gnikonov@brocku.ca
groups in the related complexes 1 (2.646(1) Š) and 2a
Prof. L. G. Kuzmina
Institute of General and Inorganic Chemistry RAS, Moscow (Russia)
À
(2.668(1) Š). The agostic hydride ligand is at a long Mo1
H1b distance of 1.92(5) Š, whereas the Si1 H1b bond of
À
Prof. J. A. K. Howard
Chemistry Department, University of Durham (UK)
1.49(5) Š is normal.
Complex 3 was found to catalyze a surprising variety of
hydrosilylation processes.[8] Thus, the reaction of benzalde-
hyde with PhSiH3 in the presence of 0.5 mol% of 3 gives a
100% conversion of benzaldehyde after 16 h (Table 1,
entry 1). Hydrosilylation of ketones (acetophenone and
[**] This work was supported by the Petroleum Research Fund,
administered by the American Chemical Society. L.G.K. thanks the
Russian Foundation of Basic Research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802147.
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(TON = 20, TOF = 245; Table 1, entries 5 and 6).[14] Hydro-
=
silylation of benzonitrile by PhSiH3 selectively gives PhHC
NSiH2Ph with a 20% conversion after 6 days (Table 1,
entry 7). To our knowledge, such a selective monoaddition
is unprecedented in the catalytic hydrosilylation of nitriles.[15]
To shed more light on these catalytic processes, stoichio-
metric reactions were attempted. Addition of excess PhSiH3
to 3 results in a slow silane coupling and redistribution to yield
PhH2SiSiH2Ph, SiH4, and Ph2SiH2.[9,16] An 1H-1H EXSY NMR
experiment indicated an exchange between free PhSiH3 and
the classical PhH2Si ligand, but not the PhH2Si group of the
agostic ligand. However, an NMR experiment with the
labeled silane PhSiD3 showed, after5 min at or om tempe-r
ature, a nearly statistical distribution of deuterium at all the
silicon centers in [D4]-3, suggesting a fast exchange between
both silyl groups and the free silane (Scheme 2). When 3 is
treated with (m-Tol)SiH3, after 10 min at room temperature,
the tolylsilyl group replaces only the classical PhH2Si ligand
(43% conversion), whereas the PhH2Si group of the agostic
ligand remains unchanged in 3tol (as indicated by 1H-13C
HMBC-GP NMR spectroscopy (HMBC-GP: heteronuclear
multiple bond correlation gradient pulse); Scheme 2). In all
stoichiometric reactions of 3 with unsaturated organic sub-
strates, one equivalent of free PhSiH3 is released. These
observations suggest the intermediate formation of a silani-
mine complex D (Scheme 2).[5]
Figure 1. Molecular structure of 3. Selected distances [Š] and
À
À
À
angles [8]: Mo1 Si1 2.634(1), Mo1 Si2 2.495(1), Mo1 N1 2.062(4),
À
À
À
À
Si1 N1 1.694(4), Mo1 N2 1.754(4), Mo1 H1b 1.92(5), Si1 H1a
À
À
À
1.43(4), Si1 H1b 1.49(5), Si2 H2a 1.44(6), Si2 H2b 1.54(6); Mo1-N1-
Si1 88.50(16), Mo1-Si1-N1 51.49(12), Mo1-N1-C1 140.0(3), Mo1-N2-
C13 169.5(3). Hydrogen atoms not bonded to silicon are omitted for
clarity.
Table 1: Hydrosilylation of organic substrates by PhSiH3 (1:1) at room
temperature.
À
Treatment of 3 with olefins leads to products of Si C
À
coupling, containing only one classical Si H group. Thus,
Entry Substrate c(3)
[mol%]
t
Products
Yield
[%][a]
reaction of 3 with ethylene cleanly gives the ethyl vinylsilyl
derivative 4 (Scheme 3), which was characterized by spectro-
scopic methods and by X-ray diffraction (Figure 2). Similarly,
reaction of 3 with styrene gives the hydrido vinylsilyl complex
5 (Scheme 3), which probably emerges from styrene coupling
with the silanimine intermediate D (Scheme 2), followed by
=
1
2
PhHC O 0.5
16h
PhSiH 2(OCH2Ph)
PhSiH(OCH2Ph)2
43
57
44
29
21
6
34
42
100
33
67
18
82
20
=
PhMeC
1.0
20 days PhSiH2OCHMePh
PhSiH(OCHMePh)2
PhSiH(OCHMePh)3
PhCH2CH3
O
À
C H activation in the CH2CHPh fragment. Similar alkene
=
3
Me2C O 1.0
5 days PhH2Si(OiPr)
PhHSi(OiPr)2
À
insertion into a Zr Si bond has been observed previously by
ꢀ
4
5
PhC CH 2.7
17 h
5 min PhH2Si(OEt)
PhHSi(OEt)2
5 min PhH2Si(OiPr)
PhHSi(OiPr)2
polyphenylacetylene
EtOH
iPrOH
PhCN
5.0
5.0
5.0
6
=
7
6days PhHC NSiH2Ph
[a] Yields were determined with 1H NMR spectroscopy, using tetra-
methylsilane as a standard.
acetone) by PhSiH3 at room temperature is more sluggish
(Table 1, entries 2 and 3). An increase of reaction time and
temperature leads to silane redistribution[9] and to partial
carbonyl reduction to alkanes.[10]
In contrast, 3 turned out to be inactive in the hydro-
silylation of alkenes and alkynes. Thus, reactions of PhSiH3
with 1-hexene, cyclohexene, and styrene result only in partial
reduction to the corresponding alkanes.[11] However, 3
catalyzes the isomerization of 1-hexene into 2-hexene[12] and
polymerizes phenylacetylene (turnover number (TON) = 34,
turnover frequency (TOF) = 2; Table 1, entry 4).[13]
Figure 2. Molecular structure of 4 (one of two independent molecules
À
is shown). Selected distances [Š] and angles [8]: Mo1 Si1 2.9036(8),
À
À
À
À
Mo1 N1 1.769(2), Si1 N2 1.740(2), Mo1 C31 2.283(3), Mo1 C32
À
À
2.238(3), Mo1 C33 2.216(3), Mo1 P1 2.5757(8); Mo1-N2-Si1
99.02(10), N2-Si1-C31 95.92(11), N1-Mo1-N2 139.72(10), N1-Mo1-C33
108.77(11), N2-Mo1-C33 111.01(10), N1-Mo1-P1 84.79(8), N2-Mo1-P1
94.23(6), C33-Mo1-P1 82.52(8). Hydrogen atoms are omitted for
clarity.
Catalytic alcoholysis of PhSiH3 by ethanol orisoporpyl
alcohol is fast and gives a 100% conversion in only 5 min
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attempts to trace the fate of the
extruded silylene fragment
DSiHPh were unsuccessful. The
same ketone adducts can be
easily prepared by the reaction
of (ArN)2Mo(PMe3)3 with the
corresponding ketone. The treat-
ment of 6 or 7 with two equiv-
alents of PhSiH3 results in elimi-
nation of the ketone and selective
regeneration of 3. Catalytic
hydrosilylation of acetophenone
with PhSiH3 mediated by complex
6 (1.0 mol%) is much slowerthan
that catalyzed by 3, affording after
20 days only 35% conversion.
The analogous reaction of 3
with benzaldehyde gives a highly
fluxional product, unstable even
at low temperature, which in its
1H NMR spectrum exhibits a dou-
Scheme 2. Silane/silyl exchange in 3 via the silanimine intermediate D.
blet at d = 13.28 ppm (JP, H
=
=
7.2 Hz) assigned to the O CH
proton, which couples to the phos-
phorous nucleus. This fact, and
the observation of a down-field
À
Si H signal at d = 6.83 ppm (1H),
suggest the formation of a com-
plex with a h1-coordinated ben-
zaldehyde, that is, (ArN)-
1
2
=
[18]
(PMe3)Mo(h -O CHPh)(h -
=
NAr SiHPh).
As forketones,
the reaction of (ArN)2Mo(PMe3)3
with benzaldehyde gives an
h2 complex (8), characterized by
À
an up-field C H signal at d =
5.69 ppm in its 1H NMR spec-
trum.[17] These observations sug-
gest that in the hydrosylation of
carbonyl compounds the active
catalyst is probably formed from
3 aftersilane elimination, but
before silylene extrusion from D.
As in the reaction with ace-
tone, treatment of 3 with nitriles
(CH3CN, PhCN, iPrCN, and
tBuCN) gives initially the h2-
adducts (9–12; Scheme 3), evi-
Scheme 3. Stoichiometric reactions of 3 with organic molecules.
denced
by
the
diagnostic
13C NMR signal at d = 192–
204 ppm forthe cabron nucleus
of the nitrile group.[19] The rate of reaction depends on the
steric demands of the R group and varies from 10 min for R =
Me to a couple of hours for R = tBu. The initial adducts 9–12
are unstable in solution and undergo a slow rearrangement
2
=
Berry et al. for the silanimine complex Cp2Zr(h -NtBu
SiMe2)(PMe3).[5a]
Surprisingly, NMR experiments indicated that reactions
of 3 with acetone and acetophenone lead to the selective
formation of the ketone complexes 6[17] and 7,[18] respectively,
accompanied by the evolution of one equivalent of PhSiH3
and the formation of a difficult-to-characterize mixture of
volatile silicon-containing compounds (Scheme 3). All
À
through the insertion of the C ꢀ N moiety into the Mo Si
bond of the silanimine fragment to give 13–16 (Scheme 3).
The rate of rearrangement is also highly dependent on the
bulkiness of the nitrile substituent (as expected, the slowest
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7703

Communications
Comprehensive Handbook on Hydrosilylation, Pergamon,
Oxford, 1992.
reaction was observed for the R = tBu complex 10). Surpris-
À
=
ingly, NMR studies suggest the formation of a [PhHSi C(R)
N] fragment, rather than N-addition to silicon.[15g,h,20] In
particular, a long-range 1H-13C HMBC NMR study of 13
shows the coupling of SiH, CH, and CH3 protons to the imine
carbon nucleus, thus, indicating the presence of direct
bonding between the silicon atom and the imine carbon atom.
In summary, we discovered stoichiometric and catalytic
reactivity of an agostic silylamido complex of molybdenum,
including the unprecedented selective hydrosilylation of
benzonitrile. Labeling experiments indicated the formation
of a silanimine intermediate.
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Received: May 8, 2008
Published online: September2, 2008
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Keywords: agostic interactions · homogeneous catalysis ·
hydrosilylation · imido ligands · silicon
.
0
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2
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À
[7] The energy barrier to activation of the Si H bond depends on
À
owing to the additional activation of the C H bond of the
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À
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