Water-soluble arene ruthenium complexes containing a trans-1,2-diaminocyclohexane ligand as enantioselective transfer hydrogenation catalysts in aqueous solution

The cationic chloro complexes [(arene)Ru(H2N boolean AND NH2)Cl](+) (1: arene = C6H6; 2: arene = p-MeC(6)H(4)iPr; 3: arene = C6Me6) have been synthesised from the corresponding arene ruthenium dichloride dimers and enantiopure (R,R or S,S) trans1,2-diaminocyclohexane (H2N boolean AND NH2) and isolated as the chloride salts. The compounds are all water-soluble and, in the case of the hexamethylbenzene derivative 3, the aqua complex formed upon hydrolysis [(C6Me6)Ru(H2N boolean AND NH2)-OH2](2+), (4) could be isolated as the tetrafluoroborate salt. The molecular structures of 3 and 4 have been determined by single-crystal X-ray diffraction analyses of [(C6Me6)Ru(H2N boolean AND NH2)Cl]Cl and [(C6Me6)Ru(H2N boolean AND NH2)OH2][BF4](2).Treatment of [RU2(arene)(2)Cl-4] with the monotosylated trans1,2-diaminocyclohexane derivative (TsHN boolean AND NH2) does not yield the expected cationic complexes, analogous to 1-3 but the neutral deprotonated complexes [(arene)Ru(TsN boolean AND NH2)Cl] (5: arene = C6H6; 6: arene = p-MeC(6)H(4)iPr; 7: arene = C6Me6; 8: arene = C6H5COOMe). Hydrolysis of the chloro complex 7 in aqueous solution gave, upon precipitation of silver chloride, the corresponding monocationic aqua complex {(C6Me6)Ru(TsHN boolean AND NH2)(OH2)](+) (9) which was isolated and characterised as its tetrafluoroborate salt. The enantiopure complexes 1-9 have been employed as catalysts for the transfer hydrogenation of acetophenone in aqueous solution using sodium formate and water as a hydrogen source. The best results were obtained (60 degrees C) with 7, giving a catalytic turnover frequency of 43 h(-1) and an enantiomeric excess of 93%. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)