Synthesis and reactions of diphosphinidenecyclobutene ruthenium complexes relevant to catalytic hydrosilylation of terminal alkynes

The complex [RuCl(mu-Cl)(CO)(DPCB-OMe)](2) (1a), bearing a low-coordinated phosphorus ligand (DPCB-OMe = 1,2-bis(4-methoxyphenyl)-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene), is readily reduced to [RuH(mu-Cl)(CO)(DPCB-OMe)](2) (2a) by the reaction with water and HSiMe2Ph. The reaction proceeds via a [RuCl2(CO)(H2O)(DPCB-OMe)] intermediate, which is characterized by X-ray diffraction analysis. Complexes la and 2a serve as highly efficient catalysts for Z-selective hydrosilylation of phenylacetylene. The reason for the high catalyst efficiency of DPCB-OMe complexes has been investigated by reaction and structure analysis of the presumed intermediate [Ru(CH=CHPh)Cl(CO)(DPCB-OMe)] (3a). It has been found that 3a has ample space to associate with hydrosilane and, therefore, readily undergoes metathesis between Ru-C and H-Si bonds. This structural feature in conjunction with the strong pi-accepting ability of the DPCB-OMe ligand leads to highly efficient catalysis for Z-selective hydrosilylation of terminal alkynes.