Highly efficient conversion of carbon dioxide catalyzed by half-sandwich complexes with pyridinol ligand: The electronic effect of oxyanion

Half-sandwich complexes with 4,4′-dihydroxy-2,2′-bipyridine (DHBP) or 4,7-dihydroxy-1,10-phenanthroline (DHPT) are highly efficient catalysts for the hydrogenation of bicarbonate in an alkaline aqueous solution without an amine additive. The generation of an oxyanion by the deprotonation of the two hydroxy substituents on the catalyst ligand caused a dramatic enhancement of catalytic activity due to the strong electron-donating ability of the oxyanion. Turnover frequencies (TOF) up to 42,000 h−1 and turnover numbers (TON) up to 222,000 have been obtained by using iridium catalysts under 6 MPa at 120 °C. The production of formate (TOF = 3.5 h−1) was observed even under ambient conditions (0.1 MPa, 30 °C).