Tris(polypyridine)nickel(II) complexes: Synthesis, DFT and electrochemistry

A series of 13 tris(polypyridine)nickel(II) complexes were synthesized and subjected to electrochemical (using cyclic voltammetry) and theoretical (using density functional theory, DFT) studies. Experimentally, one oxidation and a couple successive reduction processes are observed for the tris(polypyridine)nickel(II) complexes. The experimental Ni(II/III) oxidation potential follows the same trend as the experimental metal(II/III) oxidation potential of related tris(polypyridine)metal(II) complexes, metal = Co, Os, Fe, Ru and Mn, with the tris(polypyridine)nickel(II) being oxidized at the most positive potential. DFT calculations show that the tris(polypyridine)nickel(II) complexes have an octahedral coordination sphere, while the oxidized tris(polypyridine)nickel(III) complexes exhibit a Jahn-Teller distorted geometry. The change in geometry from octahedral to Jahn-Teller distorted upon oxidation of tris(polypyridine)nickel(II), may contribute to the large peak current potential separations with small peak current ratios experimentally measured for the Ni(II/III) redox process. The DFT calculations further show that the reduced tris(polypyridine)nickel(II) complexes, [Ni(polypyridine)3]n+, contain a Ni(II) central ion ferromagnetically coupled to one (n = 1), two (n = 0) or three (n = -1) polypyridine radicals. The tris(polypyridine)nickel(II) complexes thus exhibit polypyridine ligand-based reduction.