Synthesis, Characterization, and Catalytic Carbon Dioxide Reduction Reactivity of a Macrocyclic CNC Pincer Ligand Bound to Nickel

Toward the development of earth-abundant catalysts for carbon dioxide (CO2) reduction, we prepared nine new nickel CNC pincer complexes incorporating N-heterocyclic carbenes (NHCs) and a central pyridine ring. The wingtips were varied between long-chain olefin wingtips (1R), a macrocyclic monounsaturated ring (2R), and a macrocyclic saturated ring (3R). The para-position on the pyridine was varied with R = H, Me, or OMe, giving nine combinations. Two crystal structures for 1Me and 2OMe are reported, thus confirming the geometry of the Ni(II) pincer. The catalytic activity of all nine nickel complexes was evaluated for CO2 reduction in the presence of a photosensitizer, (Ir(ppy)3) (ppy = 2-phenylpyridine), and formate was identified as the major product with CO as a minor product. The catalytic activity increased in the order R = OMe > Me > H, which reflects an electron donor group facilitating CO2 reduction, and the saturated macrocyclic wingtip was optimal (maximum TONHCO2– = 207 for 3OMe at 24 h). Excited-state lifetimes for two compounds are reported using transient absorption spectroscopy (TAS), and these are related to the catalytic behavior. Overall, these catalysts are moderately active in the presence of a photosensitizer, but they are unable to perform self-sensitized catalysis.