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

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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.

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