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 ABSTRACT: Carbonyl hydrosilylation catalyzed by (ArN)Mo(H)(SiH 2 Ph)(PMe 3 ) 3 (3) is unusual in that it does not involve the expected Si-O elimination from intermediate (ArN)Mo(SiH 2 Ph)(O i Pr)(PMe 3 ) 2 (7). In- stead, 7 reversibly transfers β-CH hydrogen from the alkoxide ligand to metal. T he need for inexpensive and less toxic catalysts has recently 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 nickel 10 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)(PMe 3 ) 3 (1) proceeds via dissociation of PMe 3 trans to hydride and carbonyl coordination to give trans-(ArN)Mo(Cl)(H)(η 2 -C(O)HPh)- (PMe 3 ) 2 followed by the rate-determining rearrangement into (ArN)Mo(Cl)(OBn)(PMe 3 ) 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 mechanism 11 ) 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 the PMe 3 ligands trans to each of the strongest trans-influence ligands (imido, 12 hydride, and silyl 13 ). Attempts to prepare 2 resulted in its isomer (ArN)Mo(H)(SiH 2 Ph)(PMe 3 ) 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 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 PMe 3 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-P trans bond angle of 131.78(2)°. Although in 3 the bulky PMe 3 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. 1 H EXSY NMR revealed fast exchange between the silicon-bound protons and free PhSiH 3 , 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 We then looked at the possibility of PMe 3 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 31 P- 31 P EXSY NMR study revealed a much more facile intramolecular phosphine exchange [k 295.1 intra = (9.1 ± 0.1) × 10 -2 s -1 ] 18 than the intermolecular exchange with the free PMe 3 [k 295.1 inter = (1.80 ± 0.08) × 10 -3 s -1 ]. 19 Received: July 20, 2011 Published: December 21, 2011 Chart 1. Isolobal Relationship between 1 and 2 Scheme 1. Preparation of Complex 3 Communication pubs.acs.org/IC © 2011 American Chemical Society 754 dx.doi.org/10.1021/ic201550a | Inorg. Chem. 2012, 51, 754-756