Effect of Silylene Ligand on Catalytic Pathway of Alkene Hydrosilylation Catalyzed by Phosphine Cobalt(III) Hydrides

In this paper, two cobalt(III) hydrides, [(PMe$_3$)$_3$Co(H)(Cl)(SiH$_2$Ph)] (3) and [(LSi:)$_2$(PMe$_3$)Co(H)(Cl)(SiH$_2$Ph)] (LSi: = {PhC (N$^t$Bu)$_2$}SiCl) (4), were used as catalysts to compare their catalytic performance on the hydrosilylation of alkenes. The research results indicate that both cobalt(III) complexes 3 and 4 are good catalysts for the hydrosilylation of alkenes, affording Markovnikov products for aryl alkenes and anti-Markovnikov products for alkyl alkenes. It was found that the catalytic activity of phosphine cobalt(III) hydride 3 is higher than that of silylene cobalt(III) hydride 4 for aryl alkenes. On the contrary, the catalytic activity of silylene cobalt hydride 4 is better than that of phosphine cobalt hydride 3 for alkyl alkenes. Different experiments were designed to study the catalytic mechanism for hydrosilylation of alkenes catalyzed by complexes 3 and 4. We found that complex 3 first reacted with Ph$_2$SiH$_2$ when it was used as a catalyst. From the stoichiometric reaction between complex 3 with Ph$_2$SiH$_2$ we obtained the known complex 5 [(Ph$_2$ClSi)Co(H)$_2$(PMe$_3$)$_3$]. Further experiment indicates that hydrido cobalt(I) intermediate 5b, [HCo (PMe$_3$)$_3$], is an active intermediate in the hydrosilylation of alkenes catalyzed by complex 3. ... mehrWhen silylene cobalt(III) hydride 4 was used as a catalyst, complex 4 reacted at first with styrene, and four-coordinated bis (silylene) cobalt(I) chloride Co (LSi:)$_2$(PMe$_3$)Cl was proposed as the active intermediate in the hydrosilylation of alkenes. Based on the experimental results, two different possible catalytic mechanisms catalyzed by complexes 3 and 4 were proposed, respectively.