Ruthenium(0) complexes with triazolylidene spectator ligands: Oxidative activation for (de)hydrogenation catalysis

Transmetallation of silver triazolylidene intermediates with the ruthenium(0) precursor [Ru(Cp]O) (CO)2]2 afforded low-valent ruthenium(0) complexes containing a triazole-derived NHC ligand (Cp] O ¼ 3,4-di(4-methoxyphenyl)-2,5-diphenyl-cyclopentadienone). Protonation of the carbonyl group of the Cp]O ligand significantly reduces the p character of the RueCO bond as deduced from nCO analysis. The new triazolylidene ruthenium(0) complexes were evaluated as catalyst precursors in transfer hydrogenation of 4-fluoro-acetophenone and in the acceptorless dehydrogenation of benzyl alcohol. Low activities were noted, though in both reactions, catalytic performance is markedly increased when cerium( IV) was added. Electrochemical analysis indicates that activation of the catalyst precursor proceeds via cerium-mediated oxidation of the ruthenium center, which facilitates dissociation of a CO ligand to enter the catalytic cycle. Such oxidative activation of catalyst precursors may be of more general scope.