Redox Chemistry of tris(β-diketonate)cobalt(III) Complexes: A Molecular View

Abstract

Reduction of low spin S = 0 tris(β-diketonate)cobalt(III) complexes leads to an intermediate spin S = 3/2 anion. This electron rearrangement upon reduction could be the reason why a large peak potential difference between the reduction and re-oxidation peaks was observed during electrochemical analysis, using cyclic voltammetry. It is shown that experimental redox potentials obtained from the following methods, namely (i) polarography, (ii) cyclic voltammetry on a hanging Hg electrode, as well as (iii) cyclic voltammetry on a carbon fibre surface, related linearly to various empirical (Gordy scale group electronegativities, Hammett meta-substituent constants, pK a and the Lever electronic parameters) and density functional theory (DFT) calculated (LUMO energy, adiabatic electron affinity, chemical potential, global electrophilicity index and molecular electrostatic potential on Co) reactivity parameters. Deviations from these linear trends were due to a resonance effect of aromatic substituent groups. The linear relationships obtained for complexes containing aliphatic substituent groups can be used to predict redox potential and the electrophilic reactivity of related tris(β-diketonate)cobalt(III) complexes.