Water-Soluble Arene Ruthenium Complexes Containing a <i>trans</i>-1,2-Diaminocyclohexane Ligand as Enantioselective Transfer Hydrogenation Catalysts in Aqueous Solution

The cationic chloro complexes [(arene)Ru(H₂N∩NH₂)Cl]⁺ (<b>1</b>: arene = C₆H₆; <b>2</b>: arene = <i>p</i>-MeC₆H₄<i>i</i>Pr; <b>3</b>: arene = C₆Me₆) have been synthesised from the corresponding arene ruthenium dichloride dimers and enantiopure (<i>R,R</i> or <i>S,S</i>) <i>trans</i>-1,2-diaminocyclohexane (H₂N∩NH₂) and isolated as the chloride salts. The compounds are all water-soluble and, in the case of the hexamethylbenzene derivative <b>3</b>, the aqua complex formed upon hydrolysis [(C₆Me₆)Ru(H₂N∩NH₂)OH₂]²⁺ (<b>4</b>) could be isolated as the tetrafluoroborate salt. The molecular structures of <b>3</b> and <b>4</b> have been determined by single-crystal X-ray diffraction analyses of [(C₆Me₆)Ru(H₂N∩NH₂)Cl]Cl and [(C₆Me₆)Ru(H₂N∩NH₂)OH₂][BF₄]₂. Treatment of [Ru₂ (arene) ₂Cl₄] with the monotosylated <i>trans</i>-1,2-diaminocyclohexane derivative (TsHN∩NH₂) does not yield the expected cationic complexes, analogous to <b>1<b>-</b>3</b> but the neutral deprotonated complexes [(arene)Ru(TsN∩NH₂)Cl] (<b>5</b>: arene = C₆H₆; <b>6</b>: arene = <i>p</i>-MeC₆H₄<i>i</i>Pr; <b>7</b>: arene = C₆Me₆; <b>8</b>: arene = C₆H₅COOMe). Hydrolysis of the chloro complex <b>7</b> in aqueous solution gave, upon precipitation of silver chloride, the corresponding monocationic aqua complex [(C₆Me₆)Ru(TsHN∩NH₂)(OH₂)]⁺ (<b>9</b>) which was isolated and characterised as its tetrafluoroborate salt. The enantiopure complexes <b>1</b>-<b>9</b> 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 °C) with <b>7</b>, giving a catalytic turnover frequency of 43 h^-1 and an enantiomeric excess of 93 %.