Photochemical reduction of carbon dioxide to carbon monoxide in water using a nickel(II) tetra-azamacrocycle complex as catalyst

Visible-light illumination of a CO2-saturated aqueous solution containing tris(2,2′-bipyridyl)ruthenium(II) chloride as photosensitiser, ascorbic acid as electron donor, and 1,4,8,11-tetra-azacyclotetradecanenickel(II) chloride as catalyst results in the production of both carbon monoxide and hydrogen. The yields of both CO and H2 are pH dependent. The production of CO is proposed to proceed via CO2 insertion into the Ni–H bond that is formed upon reaction of the reduced nickel species with a proton. This is followed by dissociation to form CO and H2O. The production of hydrogen is also proposed to proceed via the reaction of H+ with the same Ni–H species. Some H2 is also produced in the absence of catalyst, presumably by a photoreaction of the sensitiser. By a mechanism that is not yet understood, both CO2 and CO enhance the photoproduction of H2.