Electrochemical and electronic properties of a series of substituted polypyridine ligands and their Co(II) complexes

DFT calculations show that, due to Jahn-Teller distortion, the d(7) [Co(N,N)(3)](2+) complexes, with S = 1/2 (N,N = bipyridine or substituted bipyridine ligand) have two longer axial and four shorter equatorial Co-N bonds (elongation Jahn-Teller), while [Co(terpyridine)(2)](2+) with S = 1/2, instead has two shorter central (axial) Co-N bonds and four longer distal Co-N bonds (compression Jahn-Teller), since in the latter, the distal Co-N bonds are more flexible than the Co-N axial bonds in the rigid structure of the tridentate terpyridine ligand. The same trend is observed for the related high spin S = 3/2 Co(II) complexes, though less pronounced. The cyclic voltammograms of [Co(terpyridine)(2)](2+) and a series of the [Co(N,N)(3)](2+) complexes show at least three chemically as well as electrochemically reversible redox couples, namely Co-III/III, Co-II/I and a ligand based reduction of the polypyridine-Co(I) complex. The reduction of the uncoordinated free polypyridine ligand is more than 0.5 V more negative than the reduction of the coordinated ligand in the polypyridine-Co(I) complex.