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 absen ce of amine or other additives, under mild conditions. Reaction of:[RuCl2(P TA)(4)] and H-2 (60 bar) gives the hydrides [RuH2(PTA)(4)] (at pH = 12.0) a nd [RuH(PTA)(4)X] (X = Cl- or H2O) (at pH = 2.0). In presence of excess PTA , formation of the unparalleled cationic pentakis-phosphino species, [HRu(P TA)(5)](+), was unambiguously established by H-1 and P-31 NMR measurements. The same hydrides were observed when [Ru(H2O)(6)][tos](2) (tos = toluene-4 -sulfonate) reacted with,PTA under Hz,pressure. The rate of CO2 hydrogenati on 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 soluti ons, respectively). H-1 and P-31 NMR studies together-with the icinetic mea surements suggested that HCO3- was the real substrate and [RuH(PTA)(4)X] th e catalytically active hydride species in this reaction. Hydrogenation of H CO3- showed an induction period which:could be ascribed to the slow formati on of the catalytically active hydride species.