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

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.