Hydrogenation of the water-soluble [{RuCl(2)(mtppms)(2)}(2)] (mtppms = monosulfonated triphenylphosphine) was studied in aqueous solutions in the presence of excess mtppms both with H(2) and with aqueous HCOONa. Depending on the reductant, the pH and H(2) pressure altogether nine hydride species were identified. In acidic solutions at 1 bar H(2) pressure the known [RuHCl(mtppms)(3)] (1) and [{RuHCl(mtppms)(2)}(2)] (3) were formed, however, elevated pressure led to the formation of trans-[RuH(2)(mtppms)(4)] (11). In basic solutions at atmospheric pressure cis-fac-[RuH(2)(H(2)O)(mtppms)(3)] (12) was observed which was readily replaced by [RuH(2)(η(2)-H(2))(mtppms)(3)] (13) at higher H(2) pressures. 13 is the first water-soluble and stable η(2)-H(2) Ru(II)-complex stabilized only by monodentate phosphine ligands. [RuHBr(mtppms)(3)] (9) and [RuHI(mtppms)(3)] (10) were obtained analogously to 1. In concentrated aqueous HCOONa solutions (often used in H-transfer hydrogenations) the major species was trans-[RuH(2)(HCOO)(mtppms)](-) (14) while in dilute solutions trans-[RuH(2)(H(2)O)(mtppms)(3)] (15) could be observed. Formation of these various hydride species offers an explanation for the earlier observed pH and pressure dependence of the rates and selectivities in hydrogenation of unsaturated aldehydes catalyzed by [{RuCl(2)(mtppms)(2)}(2)] + mtppms.

Unsaturated aldehydes were efficiently reduced by transfer hydrogenation from sodium formate in water-2-propanol mixtures using a water-soluble Ru(II)-tertiary phosphine catalyst. The reaction yielded unsaturated alcohols with complete selectivity and without hydrogenation or isomerization of C C bonds of the substrates. Very high reaction rate was observed in the transfer hydrogenation of cinnamaldehyde already at 30 • C with turnover frequency of 160 h −1 and this increased to 3800 h −1 at 70 • C. Consequently, the method is applicable to the synthesis of unsaturated alcohols in case of heat sensitive or highly volatile aldehydes, too. Based on multinuclear NMR investigations, trans-[RuH 2 (H 2 O)(mtppms) 3 ] is suggested as the key catalytic species.

In water/2-propanol mixtures [RhCl(mtppms)(3)] (mtppms = monosulfonated triphenylphosphine) was an efficient catalyst for the selective C=C reduction of trans-3-phenyl-2-propenal (trans-cinnamaldehyde) by hydrogen transfer from formate at temperatures as low as 30 °C. An outstandingly high catalyst turnover frequency of 1214 h(-1) was determined at 70 °C. A possible mechanism of the reaction is suggested on the basis of kinetic studies and (1)H- and (31)P-NMR spectroscopic identification of the major Rh(I) species in the reaction mixtures as cis-mer-[H(2)RhX(mtppms)(3)] (X = HCOO(-) or H(2)O). It was established that a large part but not all of the rate increase observed in water/2-propanol mixtures in comparison with systems with neat water as solvent was the consequence of complete dissolution of trans-cinnamaldehyde on the effect of the co-solvent. Nevertheless, the rate showed a significant further increase with increasing 2-propanol concentration even in homogeneous solution and this was ascribed to changes in the solvent structure. The high catalyst activity in this solvent mixture allowed the transfer hydrogenation of citral. Although good to excellent conversions were observed at 30-70 °C, a useful degree of selectivity in hydrogenation of C=C vs. C=O bonds could not be achieved.

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