Mononuclear Ni(II) Complexes with a S3O CoordinationSphere Based on a Tripodal Cysteine-Rich Ligand: pH Tuning of theSuperoxide Dismutase Activity

The superoxide dismutase (SOD) activity of mononuclear NiII complexes, whose structures are inspired by the NiSOD, has been investigated. They have been designed with a sulfur-rich pseudopeptide ligand, derived from nitrilotriacetic acid (NTA), where the three acid functions are grafted with cysteines (L3S). Two mononuclear complexes, which exist in pH-dependent proportions, have been fully characterized by a combination of spectroscopic techniques including 1H NMR, UV–vis, circular dichroism, and X-ray absorption spectroscopy, together with theoretical calculations. They display similar square-planar S3O coordination, with the three thiolates of the three cysteine moieties from L3S coordinated to the NiII ion, together with either a water molecule at physiological pH, as [NiL3S(OH2)]−, or a hydroxo ion in more basic conditions, as [NiL3S(OH)]2–. The 1H NMR study has revealed that contrary to the hydroxo ligand, the bound water molecule is labile. The cyclic voltammogram of both complexes displays an irre...