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

The electronic structures of nickel and cobalt centers coordinated by two alpha-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, alpha-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 pi-radical form (L-.)(-) of alpha-iminoketones is characterized in the compounds [(L-.)(2)Ni] (1) and [(L-.)(2)Co] (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 pi radicals. These bis(ligand) metal complexes can be chemically oxidized by two electrons to give the dications [trans-(L)(2)Ni(CH3CN)(2)](PF6)(2) (2) and [trans-(L)(2)CO(CH3- CN)(2)](PF6)(2) (4), wherein the ligands are in the neutral form.