Tetrahedral nickel(ii) and cobalt(ii) bis-o-iminobenzosemiquinonates

The new bis-o-iminobenzosemiquinonate nickel and cobalt complexes (imSQt-Bu)2M (M = Ni (1), Co (2)), where imSQ is a radical anion of 4,6-di-tert-butyl-N-(tert-butyl)-o-iminobenzoquinone, were synthesized and characterized in detail. The molecular structures of 1 and 2 have been established by single-crystal X-ray analysis. The metal atoms in 1 and 2 have a distorted tetrahedral environment, and the dihedral angles between the planes of two radical imSQ ligands are approximately 80° in both complexes. According to the structural and spectroscopy data along with magnetic susceptibility measurements the electronic structure of the complexes should be interpreted definitely as a high spin metal center NiII (d8, S = 1) in 1 and CoII (d7, S = 3/2) in 2 bonded with two o-iminobenzosemiquinonate radicals (Srad = 1/2). The strong antiferromagnetic metal-ligand spin interactions in both complexes lead to the observed St = 0 and St = 1/2 ground states in 1 and 2, respectively. The computational DFT UB3LYP/6-311++G(d,p) studies performed on 1 and 2 are in good agreement with experimental data. Complexes 1 and 2 have similar electrochemical properties. The electrochemical reduction of the complexes includes two quasi-reversible one-electron-transfer waves in the cathodic region corresponding to the formation of the anions [M(AP)2]2- and [(imSQ)M(AP)]1- (AP - dianion of 4,6-di-tert-butyl-N-(tert-butyl)-o-iminobenzoquinone), while in the anodic region only one quasi-reversible redox process was registered. All redox processes are shown to be ligand-based.