Arene Ruthenium Complexes as Versatile Catalysts in Water in both Transfer Hydrogenation of Ketones and Oxidation of Alcohols. Selective Deuterium Labeling of <i>rac</i>-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-<i>N</i>,<i>N</i>-dmobpy)]­X (arene = benzene, X = Cl^– (<b>[1]­Cl</b>), BF₄^– (<b>[1]­[BF</b>_<b>4</b><b>]</b>), TsO^– (<b>[1]­TsO</b>), PF₆^– (<b>[1]­[PF</b>_<b>6</b>]); arene = <i>p</i>-cymene (<i>p</i>-cym), X = Cl^– (<b>[2]­Cl</b>), BF₄^– (<b>[2]­[BF</b>_<b>4</b><b>]</b>), TsO^– (<b>[2]­TsO</b>), PF₆^– (<b>[2]­[PF</b>_<b>6</b>]); arene = 2-phenoxy-1-ethanol (phoxet), X = Cl^– (<b>[3]­Cl</b>), BF₄^– ([<b>3]­[BF</b>_<b>4</b><b>]</b>), TsO^– (<b>[3]­TsO</b>), PF₆^– (<b>[3]­[PF</b>_<b>6</b>])). The structures of <b>[1]­Cl</b>, <b>[1]­TsO</b>, <b>[2]­TsO</b>, <b>[2]­[BF</b>_<b>4</b><b>]</b>, and <b>[2]­[PF</b>_<b>6</b><b>]</b> 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 <i>rac</i>-1-phenylethanol to acetophenone with ^tBuOOH at pH 7, both in aqueous solution. For the transfer hydrogenation with <i>p</i>-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-<i>p</i>-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