The monoanionic π-radical redox state of α-iminoketones in bis(ligand)metal complexes of nickel and cobalt

The electronic structures of nickel and cobalt centers coordinated by two α-iminoketone ligands have been elucidated using density functional theory calculations and a host of physical methods such as X-ray crystallography, cyclic voltammetry, UV-vis spectroscopy, electron paramagnetic resonance spectroscopy, and magnetic susceptibility measurements. In principle, α-iminoketone ligands can exist in three oxidation levels: the closed-shell neutral form (L)⁰, the closed-shell dianion (L ^red)^2-, and the open-shell monoanion (L ^•)^-. Herein, the monoanionic π-radical form (L^•)^- of α-iminoketones is characterized in the compounds [(L^•)₂Ni] (1) and [(L^•) ₂Co] (3), where (L^•)^- is the one-electron-reduced form of the neutral ligand (t-Bu)N=CH-C(Ph)=O. The metal centers in 1 and 3 are divalent, high-spin, and coupled antiferromagnetically to two ligand π radicals. These bis(ligand)metal complexes can be chemically oxidized by two electrons to give the dications [trans-(L)₂Ni(CH ₃CN)₂](PF₆)₂ (2) and [trans-(L) ₂Co(CH₃-CN)₂](PF₆)₂ (4), wherein the ligands are in the neutral form.