Synthesis of Dinuclear Cobalt Silylene Complexes and Their Catalytic Activity for Alkene Hydrosilylation Reactions

A novel dinuclear silylene cobalt complex [((Me₃P)₂Co)(PMe₂)(CoCl(PMe₃))(Si(NCH₂PPh₂)₂C₆H₄)] (1) supported by the [PSi(silylene)P] ligand was prepared through the reaction of N-heterocyclic [PSiP] pincer ligand L1 (HSiCl(NCH₂PPh₂)₂C₆H₄) with Co(PMe₃)₄. Complex [((Me₃P)₂Co)₂(Si(NCH₂PPh₂)₂C₆H₄)] (2) was formed through the reaction of complex 1 with MeLi. To the best of our knowledge, complexes 1 and 2 are the first examples of dinuclear silylene cobalt complexes supported by the [PSi(silylene)P] ligand. A new preligand L2 (SiCl₂(NCH₂PPh₂)₂C₆H₄) was synthesized, and the reaction of preligand L2 with Co(PMe₃)₄ afforded silyl cobalt complex [((Me₃P)₂Co)(SiCl(NCH₂PPh₂)₂C₆H₄)] (3). The reaction of 3 with CO delivered cobalt carbonyl complex [((Me₃P)(CO)Co)(Si(NCH₂PPh₂)₂C₆H₄)]₂O (4). The catalytic activity of cobalt complexes 1-4 on the hydrosilylation of alkenes was explored. Among the four complexes, complex 1 has the best catalytic activity. The catalytic process could be promoted with NaBHEt₃ as an additive, and a complete conversion with an excellent selectivity of 98:2 (b/l) could be reached at 120 °C within 8 min for aryl alkenes. A possible catalytic cycle was proposed on the basis of the experimental results and literature reports, with a cobalt hydride complex as an active intermediate. The molecular structure of complexes 1-4 was determined by single-crystal X-ray diffraction analysis.