Copper and Nickel Complexes of Oxamate−Phenol Containing Ligands: A Structural Dichotomy in Oxidized Species

The copper and nickel complexes of two tetradentate ligands derived from bis(aminophenol) and bis(phenol) architectures connected by an oxamate linker were isolated. Depending on the metal and ligand, the complex is isolated with either an intact (deprotonated) ligand (12−), one‐electron oxidized ligand (2−) or quinone form (3). Surprisingly, the Mannich base is easier to oxidize than the amidophenol derivatives. The complexes were characterized by X‐ray diffraction, cyclic voltammetry, UV‐Vis‐NIR and EPR spectroscopies. Complex 1 shows two reversible oxidation waves assigned to the successive iminosemiquinone/aminophenolate redox systems. Complex 2− shows an intense NIR feature, as well as an EPR signal at giso=2.043, consistent with a metallic contribution to the main ligand radical SOMO. Complex 3 shows the typical feature of an isolated Cu(II) complex. Spectro‐electrochemistry coupled to DFT calculations demonstrate a ligand‐centered oxidative redox chemistry for all the complexes. The oxamate−phenol association induces a duality in the oxidation locus in the complexes: While the oxidation site would lean toward the metal due to the amide, the ligand would provide a plausible alternative. We establish that the oxidative events are ligand‐centered and occur at low potential. Remarkably, phenolates are easier to oxidize than amidophenolates featuring an identical substitution pattern.