Fine tuning of the oxidation locus, and electron transfer, in nickel complexes of pro-radical ligands

A large number of complexes of the first-row transition metals with non-innocent ligands has been characterized in the last few years. The localization of the oxidation site in such complexes can lead to discrepancies when electrons can be removed either from the metal center (leading to an M(n+1)+ closed-shell ligand) or from the ligand (leading to an Mn+ open-shell ligand). The influence of the ligand field on the oxidation site in square-planar nickel complexes of redox-active ligands is explored herein. The tetradentate ligands employed herein incorporate two di-tert-butylphe-nolate (pro-phenoxyl) moieties and one orthophenylenediamine spacer. The links between the spacer and both phenolates are either two imines ([Ni(L-1)]), two amidates ([Ni(L-3)](2-)), or one amidate and one imine ([Ni(L-2)](-)). The structure of each nickel((II)) complex is presented. In the non-coordinating solvent CH2Cl2, the one- electron-oxidized forms are ligand-radical species with a contribution from a singly occupied d orbital of the nickel. In the presence of an exogenous ligand, such as pyridine, a Ni-III closed-shell ligand form is favored: axial ligation, which stabilizes the trivalent nickel in its octahedral geometry, induces an electron transfer from the metal((II)) center to the radical ligand. The affinity of pyridine for the phenoxylnickel((II)) species is correlated to the N-donor ability of the linkers.