Catalytic and stoichiometric reactivity of β-silylamido agostic complex of Mo: Intermediacy of a silanimine complex and applications to multicomponent coupling

Andrey Y. Khalimon, Razvan Simionescu, Georgii I. Nikonov

Research output: Contribution to journalArticlepeer-review

32 Citations (Scopus)

Abstract

The reaction of complex (ArN=)2Mo(PMe3)3 (Ar = 2,6-diisopropylphenyl) with PhSiH3 gives the β-agostic NSi-H ⋯Msilyamido complex (ArNd)Mo(SiH2Ph) (PMe3)- (η3-ArN-SiHPh-H) (3) as the first product. 3 decomposes in the mother liquor to a mixture of hydride compounds, including complex {η3-SiH(Ph)-N(Ar)-SiHPh-H ⋯ }MoH 3(PMe3)3 characterized by NMR. Compound 3 was obtained on preparative scale by reacting (ArN=)2Mo(PMe 3)3 with 2 equiv of PhSiH3 under N2 purging and characterized by multinuclear NMR, IR, and X-ray diffraction. Analogous reaction of (Ar′N=)2Mo(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 {η3-PhHSi- N(Ar′)-SiHPh-H ⋯ }MoH3(PMe3)3, (Ar′N=)Mo(H)2(PMe3)22- 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 σ-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 η2- adducts (ArN=)Mo(PMe3)(η2-ArN=SiHPh) (η2-NtCR), followed by an unusual Si-N coupling to give (ArN=)Mo(PMe3)(κ2-NAr-SiHPh-C(R)=N-). Reactions of 3 with carbonyls lead to η2-carbonyl adducts (ArN=) 2Mo(OdCRR0)(PMe3) which were independently prepared by reactions of (ArN=)2Mo(PMe3)3 with the corresponding carbonyl OdCRR′. In the case of reaction with benzaldehyde, the silanimine adduct (ArN=)Mo(PMe3)(η2-ArN=SiHPh)- (η2-O=CHPh) was observed by NMR. Reactions of complex 3 with olefins lead to products of Siag-C coupling, (ArN=)Mo(Et)(PMe 3)(η3-NAr-SiHPh-CH=CH2) (17) and (ArN=)Mo(H)(PMe3)(η3-NAr-SiHPh-CH=CHPh), for ethylene and styrene, respectively. The hydride complex (ArN=)Mo(H)(PMe 3)(η3-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)(η3-NAr-Si(Et)Ph-H) (η2-CH2=CH2) (24) having the Et substituent at the agostic silicon. Mechanistic studies show that the Et group stems from hydrogenation of the vinyl substituent by silane. Reaction of 24 with PMe 3 gives the agostic complex (ArN=)Mo(SiH2Ph)(PMe 3)(η3-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.

Original languageEnglish
Pages (from-to)7033-7053
Number of pages21
JournalJournal of the American Chemical Society
Volume133
Issue number18
DOIs
Publication statusPublished - May 11 2011

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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