Catalytic Properties of [PSiP] Pincer Cobalt(II) Chlorides Supported by Trimethylphosphine for Alkene Hydrosilylation Reactions.

In this paper, six silyl [PSiP] pincer cobalt(II) chlorides [(2-PhPCH)MeSiCo(Cl)(PMe)] (), [(2-PhPCH)HSiCo(Cl)(PMe)] (), [(2-PhPCH)PhSiCo(Cl)(PMe)] (), [(2-PrPCH)HSiCo(Cl)(PMe)] (), [(2-PrPCH)MeSiCo(Cl)(PMe)] (), and [(2-PrPCH)PhSiCo(Cl)(PMe)] ()) were prepared from the corresponding [PSiP] pincer preligands (), CoCl and PMe by Si-H bond activation. The catalytic activity of complexes for alkene hyrdosilylation was studied. It was confirmed that complex is the best catalyst with excellent regioselectivity among the six complexes. Using as the catalyst, the catalytic reaction was completed within 1 h at 50 °C, predominantly affording Markovnikov products for aryl alkenes and -Markovnikov products for aliphatic alkene substrates. During the investigation of the catalytic mechanism, the Co(II) hydrides [(2-PhPCH)MeSiCo(H)(PMe)] () and [(2-PrPCH)MeSiCo(H)(PMe)] () were obtained from the stoichiometric reactions of complex and with NaBHEt, respectively. Complexes and could also be obtained by the reactions of preligands and with Co(PMe) via Si-H bond cleavage. More experiments corroborated that complex is the real catalyst for this catalytic system. Under the same catalytic conditions as complex , using complex as a catalyst, complete conversion of styrene was also achieved in 1 h, and the selectivity remained unchanged. Based on the experimental results, we propose a plausible mechanism for this catalytic reaction. The addition of B(CF) to catalyst can reverse the selectivity of styrene hydrosilylation from the Markovnikov product as the main product (/ = 99:1) to the -Markovnikov product as the main product (/ = 40:60). Further study indicated that using the (CoCl + ) system instead of complex , the selectivity was changed from Markovnikov to -Markovnikov product (/ = 1:99.7). Therefore, the selectivity for the substrate styrene is influenced by the presence of a PMe ligand. The different selectivities may be caused by different active species. For the system of complex , a cobalt(II) hydride is the real catalyst, but for the (CoCl + ) system, a cobalt(I) complex is proposed as active species. The molecular structures of Co(II) compounds and were resolved by single-crystal X-ray diffraction.