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

A large no. of complexes of the 1st-row transition metals with noninnocent ligands was characterized in the last few years. The localization of the oxidn. 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 oxidn. site in square-planar Ni complexes of redox-active ligands is explored herein. The tetradentate ligands employed herein incorporate two di-tert-butylphenolate (pro-phenoxyl) moieties and one o-phenylenediamine spacer. The links between the spacer and both phenolates are either two imines ([Ni(L1)] H2L1 = 1,2-bis(3,5-tert-butyl-2-hydroxybenzimino)benzene), two amidates ([Ni(L3)]2- H4L3 = 1,2-bis(3,5-tert-butyl-2-hydroxybenzamido)benzene), or one amidate and one imine ([Ni(L2)]- H3L2 = 2-(3,5-tert-butyl-2-hydroxybenzamido)-1-(3,5-tert-butyl-2-hydroxybenzimino)benzene). The structure of each Ni(II) complex is presented. In the noncoordinating solvent CH2Cl2, the 1-electron-oxidized forms are ligand-radical species with a contribution from a singly occupied d orbital of the Ni. In the presence of an exogenous ligand, such as pyridine, a NiIII closed-shell ligand form is favored: axial ligation, which stabilizes the trivalent Ni in its octahedral geometry, induces an electron transfer from the metal(II) center to the radical ligand. The affinity of pyridine for the phenoxyl-Ni(II) species is correlated to the N-donor ability of the linkers. [on SciFinder(R)]


Soumis le 12 avril 2018