Electrochemical and theoretical study of nickel(II) containing different 2,2′:6′,2″-terpyridines

Abstract

A series of bis(terpyridine)nickel(II) complexes were synthesized and subjected to electrochemical (cyclic voltammetry) and theoretical (using density function theory, DFT) studies. DFT calculations have confirmed, consistent with existing literature, that the oxidation and reduction reactions of bis(terpyridine)nickel(II) complexes are centered on nickel and the terpyridine ligand, respectively. Upon oxidation of the pseudo-octahedral Ni(II) complex, large changes in Ni-N bonds occur since a Jahn-Teller distorted Ni(III) complex is formed, leading to a quasi-reversible Ni(II/III) oxidation process. Conversely, small geometric changes observed during the reduction of bis(terpyridine)nickel(II) suggest a reversible process, as experimentally observed. The second reduction is quasi-reversible and is attributed to the instability of the neutral bis(terpyridine)nickel complex containing two tpy ligand radicals. The nature of the substituents on the terpyridine ligand, characterized by Hammett constants, influences the redox potentials. These shifts result in linear relationships between the experimental redox potentials and Hammett constants, as well as DFT-calculated potentials, energies and charges associated with the specific redox process.