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 (CO 2 ) 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 ( 1 R ), a macrocyclic monounsaturated ring ( 2 R ), and a macrocyclic saturated ring ( 3 R ). The para-position on the pyridine was varied with R = H, Me, or OMe, giving nine combinations. Two crystal structures for 1 Me and 2 OMe are reported, thus confirming the geometry of the Ni(II) pincer. The catalytic activity of all nine nickel complexes was evaluated for CO 2 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 CO 2 reduction, and the saturated macrocyclic wingtip was optimal (maximum TON HCO2– = 207 for 3 OMe 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.