Unusual Structure, Fluxionality, and Reaction Mechanism of Carbonyl Hydrosilylation by Silyl Hydride Complex [(ArN)Mo(H)(SiH₂Ph)(PMe₃)₃]

Abstract

The reactions of bis(borohydride) complexes [(RN=)Mo(BH4)2(PMe3)2] (4: R = 2,6-Me2C6H3; 5: R = 2,6-iPr2C6H3) with hydrosilanes afford new silyl hydride derivatives [(RN=)Mo(H)(SiR'3)(PMe3)3] (3: R = Ar, R'3 = H2Ph; 8: R = Ar', R'3 = H2Ph; 9: R = Ar, R'3 = (OEt)3; 10: R = Ar, R'3 = HMePh). These compounds can also be conveniently prepared by reacting [(RN=)Mo(H)(Cl)(PMe3)3] with one equivalent of LiBH4 in the presence of a silane. Complex 3 undergoes intramolecular and intermolecular phosphine exchange, as well as exchange between the silyl ligand and the free silane. Kinetic and DFT studies show that the intermolecular phosphine exchange occurs through the predissociation of a PMe3 group, which, surprisingly, is facilitated by the silane. The intramolecular exchange proceeds through a new non-Bailar-twist pathway. The silyl/silane exchange proceeds through an unusual Mo(VI) intermediate, [(ArN=)Mo(H)2(SiH2Ph)2(PMe3)2] (19). Complex 3 was found to be the catalyst of a variety of hydrosilylation reactions of carbonyl compounds (aldehydes and ketones) and nitriles, as well as of silane alcoholysis. Stoichiometric mechanistic studies of the hydrosilylation of acetone, supported by DFT calculations, suggest the operation of an unexpected mechanism, in that the silyl ligand of compound 3 plays an unusual role as a spectator ligand. The addition of acetone to compound 3 leads to the formation of [trans-(ArN)Mo(OiPr)(SiH2Ph)(PMe3)2] (18). This latter species does not undergo the elimination of a Si-O group (which corresponds to the conventional Ojima's mechanism of hydrosilylation). Rather, complex 18 undergoes unusual reversible β-CH activation of the isopropoxy ligand. In the hydrosilylation of benzaldehyde, the reaction proceeds through the formation of a new intermediate bis(benzaldehyde) adduct, [(ArN=)Mo(η(2)-PhC(O)H)2(PMe3)], which reacts further with hydrosilane through a η(1)-silane complex, as studied by DFT calculations.

Silyl–Hydrosilane Exchange at a Magnesium Triphenylsilyl Complex Supported by a Cyclen-Derived NNNN-Type Macrocyclic Ligand

“…Thus, the silyl–hydrosilane exchange can also be seen as Brønsted acid–base reaction. A radical pathway cannot be excluded, because phenyl-substituted silanes (Si–H bond dissociation energies 351–368 kJ/mol) are known to form silyl radicals. , A silyl–hydrosilane exchange has so far been reported for a molybdenum phenylsilyl hydride complex [({2,6- i Pr-C 6 H 4 }­N)­Mo­(H)­(SiH 2 Ph)­(PMe 3 ) 3 ] and for [CpCp*M­{Si­(SiMe 3 ) 3 }­Cl] (M = Zr, Hf). − …”

“…A radical pathway cannot be excluded, because phenyl-substituted silanes (Si−H bond dissociation energies 351−368 kJ/mol) are known to form silyl radicals. 44,45 A silyl− hydrosilane exchange has so far been reported for a molybdenum phenylsilyl hydride complex [({2,6-i Pr-C 6 H 4 }N)-Mo(H)(SiH 2 Ph)(PMe 3 ) 3 ] 46 and for [CpCp*M{Si(SiMe 3 ) 3 }-Cl] (M = Zr, Hf). 47−49 As expected from the pKa value of 34.8 in dimethyl sulfoxide for HSiPh 3 , 2 reacted with the relatively stronger Brønsted acids H 3 SiPh (pK a : 31.6) 50 and H 2 SiPh 2 (pK a : 33.9) 50 13 Equilibrium Reaction of 2 with H 2 SiMePh and HSiMePh 2 .…”

Imido–hydrido complexes of Mo(iv): catalysis and mechanistic aspects of hydroboration reactions

“…We have recently reported a related reversible insertion of carbonyls into the M-H bond. 20 But to the best of our knowledge, the reversible insertion of nitriles into an early metal-hydride bond has been previously observed only for Cp 2 *ScðNCHRÞ. 19c A similar reactivity pattern was also observed upon the treatment of benzylideneamide 3 with benzaldehyde, which in the presence of PMe 3 leads to the exclusive formation of the benzoxy complex (ArN)Mo(Cl)(OBn)(PMe 3 ) 3 (12; Scheme 4) 12 along with the release of PhCN.…”

“…At −15 °C, 15 reacts with 2 equiv. of CH 3 to give (ArN)Mo(-N(Bcat)vCHMe)(η 2 -NuCCH 3 )(PMe 3 ) 2 (20) as a mixture of two isomers (Scheme 9; ≈3 : 1 ratio by NMR). Formation of 20 is accompanied with the release of a molecule of PMe 3 and an equivalent of H 2 .…”

“…We have recently reported a related reversible insertion of carbonyls into the M-H bond. 20 But to the best of our knowledge, the reversible insertion of nitriles into an early metal-hydride bond has been previously observed only for Cp 2 *ScðNCHRÞ. along with the release of PhCN.…”