An unexpected mechanism of hydrosilylation by a silyl hydride complex of molybdenum

Article abstract of DOI:10.1021/ic201550a

Carbonyl hydrosilylation catalyzed by (ArN)Mo(H)(SiH₂Ph) (PMe₃)₃ (3) is unusual in that it does not involve the expected Si-O elimination from intermediate (ArN)Mo(SiH₂Ph)(O ⁱPr)(PMe₃)

Full text of DOI:10.1021/ic201550a

Communication
pubs.acs.org/IC
An Unexpected Mechanism of Hydrosilylation by a Silyl Hydride
Complex of Molybdenum
Andrey Y. Khalimon,† Stanislav K. Ignatov,‡ Razvan Simionescu,† Lyudmila G. Kuzmina,§
Judith A. K. Howard,# and Georgii I Nikonov*,†
†Chemistry Department, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario L2S 3A1, Canada
‡Chemistry Department, N. I. Lobachevsky State University of Nizhnii Novgorod, Gagarin Avenue 23, 603950 Nizhny Novgorod,
Russia
§N. S. Kurnakov Institute of General and Inorganic Chemistry, 31 Leninskii prospect, Moscow 119991, Russia
#Chemistry Department, University of Durham, South Road, Durham DH1 3LE, U.K.
S
* Supporting Information
Scheme 1. Preparation of Complex 3
ABSTRACT: Carbonyl hydrosilylation catalyzed by
(ArN)Mo(H)(SiH2Ph)(PMe3)3 (3) is unusual in that it
does not involve the expected Si−O elimination from
intermediate (ArN)Mo(SiH2Ph)(OiPr)(PMe3)2 (7). In-
stead, 7 reversibly transfers β-CH hydrogen from the
alkoxide ligand to metal.
he need for inexpensive and less toxic catalysts has
Trecently fueled significant interest in nonprecious metal
catalysis.1 In the field of carbonyl hydrosilylation,2 titanium,3
zirconium,4 molybdenum,5,6 tungsten,7 rhenium,8 iron,9 and
analysis. Rewardingly, 3 turned out to be a much better catalyst
than compound 1 (Table 1).14 However, to our surprise, an
X-ray study of 3 revealed a geometry very different from what
was expected for compound 2. First of all, the hydride ligand in
3 unexpectedly occupies the site trans to the imido group.15
Second, the Mo−P distance to the PMe3 trans to the silyl is
very close to the Mo−P bond lengths to two mutually trans
phosphines [2.4699(5) Å vs 2.4671(5) and 2.4861 (5) Å]. This
decreased silyl trans influence is obviously a result of a
decreased Si−Mo−Ptrans bond angle of 131.78(2)°. Although in
3 the bulky PMe3 and ArN groups are placed cis to each other,
the electronic factor (trans influence) is clearly not at play, and
thus the overall geometry should be dictated by sterics.16
To understand better the increased catalytic activity of 3,
stoichiometric reactions were carried out. 1H EXSY NMR
revealed fast exchange between the silicon-bound protons and
free PhSiH3, but no exchange with the molybdenum-bound
hydride in the range 30−50 °C, ruling out Si−H elimination as
the first step in silyl/silane exchange.17
nickel10 catalysts have been developed. Mechanistic studies
revealed several reaction pathways based on Si−H oxidative
addition:6a,11 Si−H addition to MO bonds,8a ionic hydro-
silylation,7,8b and Si−H heterolytic splitting on M−O
bonds.6b,10 In particular, our group found that hydrosilylation
of PhC(O)H by (ArN)Mo(H)(Cl)(PMe3)3 (1) proceeds via
dissociation of PMe3 trans to hydride and carbonyl
coordination to give trans-(ArN)Mo(Cl)(H)(η2-C(O)HPh)-
(PMe3)2 followed by the rate-determining rearrangement into
(ArN)Mo(Cl)(OBn)(PMe3)3. In order to eliminate this rate-
determining step, we sought to prepare an analogue of 1 having
either the hydride or silyl (the required components of the
Ojima mechanism11) in the cis position to the incoming
carbonyl. We reckoned that the hypothetical complex 2
(Chart 1) would be an ideal target because it would place
Chart 1. Isolobal Relationship between 1 and 2
We then looked at the possibility of PMe3 dissociation from
3 and a σ-bond metathesis or oxidative addition/reductive
elimination type of sequence for the silyl/silane exchange. To
our surprise, a variable-temperature 31P−31P EXSY NMR study
revealed a much more facile intramolecular phosphine exchange
the PMe3 ligands trans to each of the strongest trans-influence
ligands (imido,12 hydride, and silyl13). Attempts to prepare 2
resulted in its isomer (ArN)Mo(H)(SiH2Ph)(PMe3)3 (3),
which catalyzes hydrosilylation by an unusual mechanism.
Complex 3 was prepared according to Scheme 1 and
characterized by spectroscopic methods and X-ray diffraction
0.1) × 10−2 s−1]18 than the intermolecular
intra
[k
= (9.1
295.1
inter
295.1
exchange with the free PMe3 [k
= (1.80 0.08) × 10−3 s−1].19
Received: July 20, 2011
Published: December 21, 2011
© 2011 American Chemical Society
754
dx.doi.org/10.1021/ic201550a | Inorg. Chem. 2012, 51, 754−756

Inorganic Chemistry
Communication
a
Table 1. Hydrosilylation of Organic Substrates at Room Temperature with 5 mol % Load of 3
c
entry
substrate
silane
time
products
PhH2Si(OBn)/PhHSi(OBn)2
Ph(Me)HSi(OBn)
yield, %
1
2
3
4
PhHC(O)
PhHC(O)
PhHC(O)
Me2C(O)
PhMeC(O)
EtOH
PhSiH3
15 min
18 h
47/53
PhMeSiH2
(EtO)3SiH
PhSiH3
100
18 h
(EtO)3Si(OBn)/(EtO)2Si(OBn)2/(EtO)Si(OBn)3
PhH2Si(OiPr)/PhHSi(OiPr)2
41/29/30
73/17
3.4 h
b
5
PhSiH3
24 h
PhH2Si[OCH(Me)Ph]/PhHSi[OCH(Me)Ph]2/PhCH2CH3
PhH2Si(OEt)/PhHSi(OEt)2
45/33/22
67/33
6
7
PhSiH3
20 min
19.7 h
PhCN
PhSiH3
(PhHCN)SiH2Ph
20
c
a
b
Conditions: 5.0 mol % 3; solvent C6D6; the ratio of substrate/silane is 1:1; RT unless stated otherwise. 1 mol % 3 was used; 50 °C. Yields were
1
determined according to H NMR, using tetramethylsilane as the standard.
The positive entropy of activation (ΔS⧧
= 14.6
5.2
to the imido, which is not involved in the exchange with silane,
and another cis to the imido and two silyl groups. The latter
hydride is formed upon Si−H addition and is involved in the
exchange. We found experimental evidence for the feasibility of
such an unusual molybdenum(VI) bis(silyl) intermediate.26
The NMR reaction of complex trans-(ArN)Mo(SiH2Ph)-
(OiPr)(PMe3)2 (7) vide infra with excess PhSiH3 at −45 °C
showed the fast formation of (ArN)Mo(H)2(SiH2Ph)2(PMe3)2
(8) in a mixture with the hydrosilylation products PhH2SiOiPr
and PhHSi(OiPr)2.
In a further attempt to understand the mechanism of
carbonyl hydrosilylation, the reaction of 3 with acetone was
studied. With 1 equiv of acetone, carbonyl insertion across the
Mo−H bond takes place. The product 7 was observed in a
mixture with the starting complex 3 and acetone, suggesting an
equilibrium (Scheme 3).27 The addition of excess PMe3 to the
intra
cal K−1 mol−1) suggests a dissociatively activated mechanism,
but its details remain unknown.20 Fluxional octahedral
complexes are rare and are typically observed for polyhydride
compounds.21 The fluxionality of 3 is particularly remarkable in
comparison with the rigid structure of the closely related
compound 1. The large positive ΔS⧧ for the intermolecular
PMe3 exchange (ΔS⧧inter = 30.9 10.6 cal K−1 mol−1) suggests
a dissociative mechanism that should lead to a coordinatively
unsaturated species (ArN)Mo(H)(SiH2Ph)(PMe3)2, which can
be prone to interact with the free silane. To our further
surprise, the rate of silyl/silane exchange [k295.1 = (2.7 0.1) ×
10−3 s−1] is slightly faster than that for the bound/free
phosphine exchange, although the activation parameters (ΔS⧧ =
34.6 5.6 cal K−1 mol−1 and ΔH⧧ = 31.0 1.7 kcal mol−1)
suggested that the overall process is still dissociative.22 This
puzzle (the dissociative mechanism of silane exchange but
slower PMe3 dissociation and much slower PhSiH3 dissocia-
tion) was solved by the remarkable observation that the bound/
Scheme 3. Preparation and Reactivity of 7
free PMe3 exchange is facilitated by the addition of excess
PhSiH3 [k
= (4.00 0.05) × 10−3 s−1].23 To the best of our
inter
295.1
knowledge, this is the first observation of a dissociatively
activated X/Y interchange24 in which the incoming nucleophile
Y (in this case, silane) lacks a lone pair of electrons.
Consistent with our experimental observations, the density
functional theory (DFT) study25 of gas-phase reactions of the
model complex (PhN)Mo(H)(SiH2Me)(PMe3)3 (4) suggested
easier PMe3 than silane dissociation (Scheme 2). The addition
mixture does not result in the expected Si−O elimination but
merely shifts equilibrium toward 3, suggesting β-CH activation
in the OiPr group. Reversible carbonyl insertion into an early
transition metal−hydride bond is very rare28 because alkoxide
ligands to electropositive early metals usually defy CH
activation. β-CH activation in the alkoxide ligand was further
confirmed by the treatment of 7 with 1 equiv of benzonitrile,
which resulted in fast transfer hydrogenation of the nitrile and
quantitative formation of the benzylidenamide 9 formed as a
mixture of two isomers.29 Even more surprisingly, the addition
of a different silane, m-TolSiH3, to 7 results in quantitative
regeneration of 3 with complete retention of the SiH2Ph group.
Therefore, the silyl ligand plays an unusual role of a spectator
ligand, which is at odds with the classical Ojima mechanism of
hydrosilylation.11
Scheme 2. DFT-Calculated Mechanism of Hydrosilylation by
4 (Free Energies in kcal mol−1)
The DFT study of a possible mechanism of hydrosilylation
suggested that acetone coordination to 5 gives the high-energy
adduct 10. Carbonyl insertion into the Mo−H bond gives
complex 11, a model for 7 (Scheme 2). Complex 11 undergoes
heterolytic splitting of silane on the Mo−O bond6 via the rate-
determining TS11/12 to give the silyl ether adduct 12, which
easily dissociates (barrier of 2.2 kcal mol−1) the product
MeH2Si(OiPr) and regenerates the catalyst 5.
of MeSiH3 to the 16-electron intermediate (PhN)Mo(H)-
(SiH2Me)(PMe3)2 (5) goes via a barrier of 12.8 kcal mol−1 to
give the MoVI d0 product (PhN)Mo(H)2(SiH2Me)2(PMe3)2
(6). This compound exhibits two types of hydrides: one trans
In conclusion, we discovered unexpected features of the
mechanism of hydrosilylation catalyzed by the silyl hydride
complex 3, such as the spectator role of the silyl ligand and a
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Inorganic Chemistry
Communication
Darcel, C. Adv. Synth. Catal. 2011, 353, 239. (l) Bhattacharya, P.;
Krause, J. A.; Guan, H. Organometallics 2011, 30, 4720.
(10) (a) Fontaine, F.-G.; Nguyen, R.-V.; Zargarian, D. Can. J. Chem.
2003, 81, 1299. (b) Chakraborty, S.; Krause, J. A.; Guan, H.
Organometallics 2009, 28, 582. (c) Tran, B. L.; Pink, M.; Mindiola,
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(11) In the classical Ojima mechanism, carbonyl adds to a silyl
hydride complex, followed by migration of either silyl or hydride
ligands and elimination of Si−O (or C−H) bonds: Ojima, I.; Kogure,
T.; Kumagai, M.; Horiuchi, S.; Sato, T. J. Organomet. Chem. 1976, 122,
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ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data in CIF format and experimental and
computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
(12) (a) Nugent, W. A.; Mayer, J. M. Metal−ligand multiple bonds;
Wiley: New York, 1988. (b) Wigley, D. E. Prog. Inorg. Chem. 1994, 42,
239.
Corresponding Author
*E-mail: gnikonov@brocku.ca.
(13) (a) Dioumaev, V. K.; Procopio, L. J.; Carroll, P. J.; Berry, D. H.
J. Am. Chem. Soc. 2003, 125, 8043. (b) Sola, E.; Garcıa-
́
Camprubí, A.;
ACKNOWLEDGMENTS
■
Andres, J. L.; Martín, M.; Plou, P. J. Am. Chem. Soc. 2010, 132, 9111.
́
This work was supported by the Petroleum Research Fund,
administered by the American Chemical Society (grant to
G.I.N.), and by an OGS fellowship to A.Y.K. S.K.I. thanks the
RFBR (Russia), and L.G.K. and J.A.K.H. thank the Royal
Society of London.
(14) For example, hydrosilylation of PhC(O)H catalyzed by 1
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refined isotropically. Final R1 = 0.0320.
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N/Si separation (3.515 Å) rule out any Si−N interaction as a driving
force for this unusual geometry. Similarly, the large silicon···hydride
separation of 2.328 Å is not consistent with any Si−H interaction.
(17) A labeling experiment with D3SiPh, however, proves the
occurrence of a slow Mo−H/PhSiD2-D exchange.
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