Oxidation of Alcohols to Carboxylates with N2O Catalyzed by Ruthenium(II)-CNC Complexes.

Air-stable ruthenium-(II) complexes based on a picoline-derived CNC pincer ligand, [RuH-(CNC)-(CO)L]X (L = PPh3, X = Br; L = CO, X = BF4), were found to catalyze under basic conditions the oxidation with N2O of a series of alcohols to carboxylates. Both [RuH-(CNC)-(CO)L]X complexes react readily with strong bases (tBuOK or KHMDS), giving rise to a Ru-(II) complex containing a deprotonated CNC* ligand (when L = PPh3) or a Ru(0)-CNC derivative (for L = CO). Furthermore, the mechanism of the catalytic reaction has been elucidated through density functional theory (DFT) calculations. The catalytic cycle has been shown to proceed through an outer-sphere mechanism comprising four key transformations, which involve Ru-(II) intermediates based on the deprotonated CNC* ligand: (i) alkoxide dehydrogenation to yield a Ru-(II) hydride complex and an aldehyde molecule, (ii) N2O insertion into the ruthenium-hydride bond to yield a hydroxy ruthenium species and N2, (iii) nucleophilic attack of the hydroxo ligand in the Ru-OH complex to the intermediate aldehyde, and (iv) dehydrogenation of the formed alcoholate to regenerate the catalytically active Ru-(II) hydride and produce the carboxylate product.