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

Abstract

DFT calculations show that, due to Jahn-Teller distortion, the d7 [Co(N,N)3]2+ complexes, with S = ½ (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 = ½, 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 CoIII/II, CoII/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.