Nickel(II) Complexes Bearing Electron Donating OR/OH Groups on a Tetradentate Ligand Scaffold for Photocatalytic Carbon Dioxide Reduction

Nickel(II) complexes bearing tetradentate NCCN ligands composed of optionally protic pyridine and N-heterocyclic carbene (NHC) donors have been synthesized and used as catalysts for carbon dioxide reduction. These complexes were synthesized bearing OMe, OBn, or OH substituents on the pyridine rings and were characterized by ¹H NMR, ¹³C NMR, UV–vis, IR, HR-MS, and single crystal X-ray diffraction. The OH substituent was partially deprotonated, as shown by the crystal structure. Electrochemical studies show that these nickel complexes undergo two electron reduction events prior to CO₂ reduction. Catalytic current enhancement under CO₂ relative to N₂ is not observed under dry conditions, but the addition of proton sources leads to modest current enhancement (i_cat/i_p < 2). Visible light driven photochemical CO₂ reduction with a photosensitizer (Ir(ppy)₃, where ppy = 2-phenylpyridine) and sacrificial electron and proton donors was studied, and formate is the major product with ∼10:1 formate to CO production. Electron donor groups (OMe, OBn, OH) do not enhance formate production (relative to the unsubstituted analogue), and CO production is only slightly enhanced. Overall with Ni(II), the tetradentate ligands are comparable to recently published pincer ligands for sensitized CO₂ reduction, but pincer ligands offer a clear advantage in self-sensitized catalysis.