Catalytic Alkene Hydrosilylation and Dinitrogen Silylation by Bis(silylene) and Bis(diphenylphosphino) Cobalt Chlorides

Abstract

Three [SiC(sp3)Si]-pincer ligands (L1–L3) and three [PC(sp3)P]-pincer ligands (L4–L6) were reacted with CoCl(PMe3)3 to evaluate how silylene and phosphine coordination groups influence both stoichiometric C(sp3)–H activation behavior and catalytic performance in olefin hydrosilylation and dinitrogen silylation. Notably, the [SiCH2Si]-pincer ligand L1 underwent oxidative addition into a bridging methylene C(sp3)–H bond upon reaction with CoCl(PMe3)3, yielding a cobalt(III) hydride complex (1); in contrast, the [PC(sp3)P]-pincer ligand L4─bearing the identical dipyrromethene scaffold─underwent only ligand exchange to afford a cobalt(I) chloride complex (4). In catalysis, complex 1 proved most effective for alkene hydrosilylation, operating under mild conditions and delivering excellent reactivity and regioselectivity. Mechanistic studies confirmed that initial C(sp3)–H activation at the bridging methylene group is crucial for generating the active cobalt species and thereby enabling high catalytic efficiency. Conversely, complex 4 exhibited superior activity in dinitrogen silylation relative to bis(silylene)-supported cobalt complexes. Critically, this catalytic reaction achieves N2 activation with a turnover number (TON) of 1228 equiv using lithium metal as a stoichiometric reductant, which is the highest value reported to date for cobalt-catalyzed dinitrogen silylation. This result indicates that the [PC(sp3)P] pincer ligand framework enables more effective dinitrogen silylation compared to the [SiC(sp3)Si] analogue.