Mechanistic Aspects of Hydrosilylation Catalyzed by (ArN=)Mo(H)(Cl)(PMe₃)₃

The reaction of (ArN=)­MoCl₂(PMe₃)₃ (Ar = 2,6-diisopropylphenyl) with L-Selectride gives the hydrido-chloride complex (ArN=)­Mo­(H)­(Cl)­(PMe₃)₃ (<b>2</b>). Complex <b>2</b> was found to catalyze the hydrosilylation of carbonyls and nitriles as well as the dehydrogenative silylation of alcohols and water. Compound <b>2</b> does not show any productive reaction with PhSiH₃; however, a slow H/D exchange and formation of (ArN=)­Mo­(D)­(Cl)­(PMe₃)₃ (<b>2</b>_<b>D</b>) was observed upon addition of PhSiD₃. Reactivity of <b>2</b> toward organic substrates was studied. Stoichiometric reactions of <b>2</b> with benzaldehyde and cyclohexanone start with dissociation of the <i>trans</i>-to-hydride PMe₃ ligand followed by coordination and insertion of carbonyls into the Mo–H bond to form alkoxy derivatives (ArN=)­Mo­(Cl)­(OR)­(PMe₂)­L₂ (<b>3</b>: R = OCH₂Ph, L₂ = 2 PMe₃; <b>5</b>: R = OCH₂Ph, L₂ = η²-PhC­(O)­H; <b>6</b>: R = OCy, L₂ = 2 PMe₃). The latter species reacts with PhSiH₃ to furnish the corresponding silyl ethers and to recover the hydride <b>2</b>. An analogous mechanism was suggested for the dehydrogenative ethanolysis with PhSiH₃, with the key intermediate being the ethoxy complex (ArN=)­Mo­(Cl)­(OEt)­(PMe₃)₃ (<b>7</b>). In the case of hydrosilylation of acetophenone, a D-labeling experiment, i.e., a reaction of <b>2</b> with acetophenone and PhSiD₃ in the 1:1:1 ratio, suggests an alternative mechanism that does not involve the intermediacy of an alkoxy complex. In this particular case, the reaction presumably proceeds via Lewis acid catalysis. Similar to the case of benzaldehyde, treatment of <b>2</b> with styrene gives <i>trans</i>-(ArN=)­Mo­(H)­(η²-CH₂CHPh)­(PMe₃)₂ (<b>8</b>). Complex <b>8</b> slowly decomposes via the release of ethylbenzene, indicating only a slow insertion of styrene ligand into the Mo–H bond of <b>8