Catalytic and Stoichiometric Reactivity of β-Silylamido Agostic Complex of Mo: Intermediacy of a Silanimine Complex and Applications to Multicomponent Coupling

The reaction of complex (ArN=)(2)Mo(PMe3)(3) (Ar = 2,6-diisopropylphenyl) with PhSiH3 gives the beta-agostic NSi-H center dot center dot center dot M silyamido complex (ArN=)Mo(SiH2Ph)(PMe3)-(eta(3)-ArN-SiHPh-H) (3) as the first product. 3 decomposes in the mother liquor to a mixture of hydride compounds, including complex {eta(3)-SiH (Ph)-N(Ar)-SiHPh-H center dot center dot center dot}MoH3(PMe3)(3) characterized by NMR Compound 3 was obtained on preparative scale by reacting (ArN=)(2)Mo(PMe3)(3) with 2 equiv of PhSiH3 under N-2 purging and characterized by multinuclear NMR, IR, and X-ray diffraction. Analogous reaction of (Ar'N)(2)Mo(PMe3)(3) (Ar' = 2,6-dimethylphenyl) with PhSiH3 affords the nonagostic silylamido derivative (Ar'N=)Mo(SiH2Ph)(PMe3)(2)(NAr'{SiH2Ph}) (5) as the first product. 5 decomposes in the mother liquor to a mixture of {eta(3)-PhHSi-N(Ar')-SiHPh-H center dot center dot center dot}MoH3(PMe3)(3), (Ar'N=)Mo(H)(2)(PMe3)(2)(eta(2)-Ar'N=SiHPh), and other hydride species. Catalytic and stoichiometric reactivity of 3 was studied. Complex 3 undergoes exchange with its minor diastereomer 3' by an agostic bond-opening/closing mechanism. It also exchanges the classical silyl group with free silane by an associative mechanism which most likely includes dissociation of the Si H agostic bond followed by the rate-determining silane sigma-bond metathesis. However, labeling experiments suggest the possibility of an alternative (minor) pathway in this exchange including a silanimine intermediate. 3 was found to catalyze dehydrogenative coupling of silane, hydrosilylation of carbonyls and nitriles, and dehydrogenative silylation of alcohols and amines. Stoichiometric reactions of 3 with nitriles proceed via intermediate formation of eta(2)-adducts (ArN=)Mo(PMe3)(eta(2)-ArN=SiHPh)(eta(2)-Nm CR), followed by an unusual Si N coupling to give (ArN=)Mo(PMe3)(kappa(2)-NAr-SiHPh-C(R)=N-). Reactions of 3 with carbonyls lead to eta(2)-carbonyl adducts (ArN=)(2)Mo(O=CRR') (PMe3) which were independently prepared by reactions of (ArN=)(2)Mo(PMe3)(3) with the corresponding carbonyl In the case of reaction with benzaldehyde, the silanimine adduct (ArN=)Mo(PMe3)(eta(2)-ArN=SiHPh)(eta(2)-O=CHPh) was observed by NMR. Reactions of complex 3 with olefins lead to products of Si-ag-C coupling, (ArN=)Mo (Et) (PMe3) (eta(3)-NAr-SiHPh-CH=CH2) (17) and (ArN=)Mo(H)(PMe3)(eta(3)-NAr-SiHPh-CH=CHPh), for ethylene and styrene, respectively. The hydride complex (ArN=)Mo(H)(PMe3)(eta(3)-NAr-SiHPh-CH=CH2) was obtained from 17 by hydrogenation and reaction with PhSiH3. Mechanistic studies of the latter process revealed an unusual dependence of the rate constant on phosphine concentration, which was explained by competition of two reaction pathways. Reaction of 17 with PhSiH3 in the presence of BPh3 leads to agostic complex (ArN=)Mo(SiH2Ph)(eta(3)-NAr-Si(Et)Ph-H)(eta(2)-CH2=CH2) (24) having the Et substituent at the agostic silicon. Mechanistic studies show thatthe Et group stems from hydrogenation of the vinyl substituent by silane. Reaction of 24 with PMe3 gives the agostic complex (ArN=)Mo(SiH2Ph)(PMe3)(eta(3)-NAr-Si(Et)Ph-H), which slowly reacts with PhSiH3 to furnish silylamide 3 and the hydrosilylation product PhEtSiH2. A mechanism involving silane attack on the imido ligand was proposed to explain this transformation.