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

Abstract

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)₆][tos]₂ (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.