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.