Studies on [Ru3(CO)12]-catalysed homogeneous transfer hydrogenation reactions; X-ray structure of [Ru4(CO)10Cl12(OPh)2]

Abstract

Using [Ru₃(CO)₁₂](1) as the homogeneous precatalyst, transfer hydrogenations of cyclohex-2-en-1-one, benzylideneaniline, and carbon tetrahalides by donor alcohols, in particular propan-2-ol, have been studied. Conversion of cyclohex-2-en-1-one into cyclohexanol has been found to proceed via the intermediate formation of cyclohexanone. From the temperature dependence of the overall reaction rates, the precatalysts [Ru₃(CO)₁₂] and [Ru₄H₄(CO)₁₂] are involved in processes with comparable activation energies. Two catalytically active cluster complexes, [Ru₄(CO)₁₂(C₆H₆O)] and [Ru₃(CO)₁₀(C₆H₈O)], were isolated from the reaction of (1) with cyclohex-2-en-1-one. Hydrogenation of the tetranuclear cluster led to the formation of [Ru₄H₄(CO)₁₂] and [Ru₃H₂(CO)₉(C₆H₈O)]. The reaction of complex (1) with benzylideneaniline gave a catalytically active cluster [Ru₃H(CO)₉(PhNCPh)]. With (1) as the precatalyst, analogues of benzylideneaniline of general formula RC₆H₄CHNPh can all be transfer hydrogenated, with the exception of the o-methoxy derivative. The complex [Ru₃H(CO)₉(PhNCPh)] was found to undergo reversible carbonylation. Rational syntheses for [Ru₃(CO)₇X₂(OR)₂] and [Ru₄(CO)₁₀X₂(OR)₂] were designed by treating (1) with the appropriate alcohol and carbon tetrahalide. The X-ray structure of [Ru₄(CO)₁₀Cl₂(OPh)₂] has been determined. The halogenoalkoxo clusters are considered to be active intermediates in the overall catalytic cycle for the transfer hydrogenations of carbon tetrahalides.