Formation and Characterization of Water-Soluble Hydrido-Ruthenium(II) Complexes of 1,3,5-Triaza-7-phosphaadamantane and Their Catalytic Activity in Hydrogenation of CO2 and HCO3- in Aqueous Solution

The water-soluble tertiary phosphine complex of ruthenium(II), RuCl2(PTA)4, (PTA = 1,3,5-triaza-7-phosphaadamantane) was used as catalyst precursor for hydrogenation of CO2 and bicarbonate in aqueous solution, in the absence of amine or other additives, under mild conditions. Reaction of RuCl2(PTA)4 and H2 (60 bar) gives the hydrides RuH2(PTA)4 (at pH = 12.0) and RuH(PTA)4X (X = Cl- or H2O) (at pH = 2.0). In presence of excess PTA, formation of the unparalleled cationic pentakis-phosphino species, HRu(PTA)5+, was unambiguously established by 1H and 31P NMR measurements. The same hydrides were observed when Ru(H2O)6tos2 (tos = toluene-4-sulfonate) reacted with PTA under H2 pressure. The rate of CO2 hydrogenation strongly depends on the pH. The highest initial reaction rate (TOF = 807.3 h(-1)) was determined for a 10% HCO3-/90% CO2 mixture (pH = 5.86), whereas the reduction was very slow both at low and high pH (CO2 and Na2CO3 solutions, respectively). 1H and 31P NMR studies together with the kinetic measurements suggested that HCO3- was the real substrate and RuH(PTA)4X the catalytically active hydride species in this reaction. Hydrogenation of HCO3- showed an induction period which could be ascribed to the slow formation of the catalytically active hydride species.