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

The new bis-o-iminobenzosemiquinonate nickel and cobalt complexes (imSQ(t-Bu))(2)M (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 degrees 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 Ni-II (d(8), S = 1) in 1 and Co-II (d(7), S = 3/2) in 2 bonded with two o-iminobenzosemiquinonate radicals (S-rad = 1/2). The strong antiferromagnetic metal-ligand spin interactions in both complexes lead to the observed S-t = 0 and S-t = 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.