Electroanalytical, kinetic, and mechanistic investigations of coordination-inspired electron transfer between Fe(II)/Cu(II)

Abstract

Herein, we report that the thermodynamic barrier for solution-phase electron transfer (ET) between Cu(II) and Fe(II) in aqueous acidic media can be overcome through the addition of 2,9-dimethyl-1,10-phenanthroline (Neocuproine [NC]) to the reaction mixture. A detailed discussion of the kinetic and mechanistic aspects of this coordination-inspired ET is presented. We attribute the observed change in the thermodynamic feasibility to the change in the reduction potential of Cu(II)–Cu(I) couple on its ligation with NC. The reaction was found to be slow, following first-order kinetics with respect to each Cu(II) and Fe(II). In the presence of excess NC, the reaction was observed to proceed with a pseudo-second-order rate constant of 3.37 ± 0.05 dm3 mol−1 s−1 at 298 K, with an activation barrier of ca. 26.22 kJ mol−1. The slow reaction is attributed to the significant reorganization energy associated with large-scale changes in the coordination sphere of the oxidant. A two-step mechanism that explains the experimental observations is proposed for the investigated reaction.