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

Abstract

In this paper, six silyl [PSiP] pincer cobalt(II) chlorides 1-6 [(2-Ph₂PC₆H₄)₂MeSiCo(Cl)(PMe₃)] (1), [(2-Ph₂PC₆H₄)₂HSiCo(Cl)(PMe₃)] (2), [(2-Ph₂PC₆H₄)₂PhSiCo(Cl)(PMe₃)] (3), [(2-ⁱPr₂PC₆H₄)₂HSiCo(Cl)(PMe₃)] (4), [(2-ⁱPr₂PC₆H₄)₂MeSiCo(Cl)(PMe₃)] (5), and [(2-ⁱPr₂PC₆H₄)₂PhSiCo(Cl)(PMe₃)] (6)) were prepared from the corresponding [PSiP] pincer preligands (L1-L6), CoCl₂ and PMe₃ by Si-H bond activation. The catalytic activity of complexes 1-6 for alkene hyrdosilylation was studied. It was confirmed that complex 1 is the best catalyst with excellent regioselectivity among the six complexes. Using 1 as the catalyst, the catalytic reaction was completed within 1 h at 50 °C, predominantly affording Markovnikov products for aryl alkenes and anti-Markovnikov products for aliphatic alkene substrates. During the investigation of the catalytic mechanism, the Co(II) hydrides [(2-Ph₂PC₆H₄)₂MeSiCo(H)(PMe₃)] (8) and [(2-ⁱPr₂PC₆H₄)₂MeSiCo(H)(PMe₃)] (9) were obtained from the stoichiometric reactions of complex 1 and 5 with NaBHEt₃, respectively. Complexes 8 and 9 could also be obtained by the reactions of preligands L1 and L5 with Co(PMe₃)₄ via Si-H bond cleavage. More experiments corroborated that complex 8 is the real catalyst for this catalytic system. Under the same catalytic conditions as complex 1, using complex 8 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(C₆F₅)₃ to catalyst 1 can reverse the selectivity of styrene hydrosilylation from the Markovnikov product as the main product (b/l = 99:1) to the anti-Markovnikov product as the main product (b/l = 40:60). Further study indicated that using the (CoCl₂ + L1) system instead of complex 1, the selectivity was changed from Markovnikov to anti-Markovnikov product (b/l = 1:99.7). Therefore, the selectivity for the substrate styrene is influenced by the presence of a PMe₃ ligand. The different selectivities may be caused by different active species. For the system of complex 1, a cobalt(II) hydride is the real catalyst, but for the (CoCl₂ + L1) system, a cobalt(I) complex is proposed as active species. The molecular structures of Co(II) compounds 5 and 9 were resolved by single-crystal X-ray diffraction.