Arene Ruthenium Complexes as Versatile Catalysts in Water in both Transfer Hydrogenation of Ketones and Oxidation of Alcohols. Selective Deuterium Labeling of rac-1-Phenylethanol

The preparation of three series of arene Ru­(II) half-sandwich compounds with the functional ligand 4,4′-dimethoxy-2,2′-bipyridine (dmobpy) is described. The new cationic derivatives have the general formula [(η6-arene)­RuCl­(κ2-N,N-dmobpy)]­X (arene = benzene, X = Cl ([1]­Cl), BF4 ([1]­[BF4]), TsO ([1]­TsO), PF6 ([1]­[PF6]); arene = p-cymene (p-cym), X = Cl ([2]­Cl), BF4 ([2]­[BF4]), TsO ([2]­TsO), PF6 ([2]­[PF6]); arene = 2-phenoxy-1-ethanol (phoxet), X = Cl ([3]­Cl), BF4 ([3]­[BF4]), TsO ([3]­TsO), PF6 ([3]­[PF6])). The structures of [1]­Cl, [1]­TsO, [2]­TsO, [2]­[BF4], and [2]­[PF6] were determined by X-ray crystallography. All of the complexes except the PF6 salts were water-soluble, and they behaved as active catalysts in two different processes: the transfer hydrogenation of water-soluble and -insoluble ketones to the corresponding alcohols, using HCOONa as the hydrogen source at pH 4, and the oxidation of rac-1-phenylethanol to acetophenone with tBuOOH at pH 7, both in aqueous solution. For the transfer hydrogenation with p-cymene complexes the aqua, formato, and hydride species were detected by means of 1H NMR experiments in D2O. It was found that the cationic hydrido complex was [(η6-p-cymene)­RuD­(dmobpy)]+. The reversible and pH-dependent formation of the hydroxo derivative was also observed. When the catalytic transfer hydrogenation was performed in D2O, the 1-phenylethanol obtained was selectively deuterated at the benzylic carbon. Mechanistic proposals are also included.