Ligand and solvent effects on the kinetics of the electrochemical reduction of Ni(II) complexes: Experiment and quantum chemical modeling

The two-electron reduction of some Ni(II) complexes with α-diimine ligands ([Ni(bpy)₃]²⁺ and [Ni(pybox)₂]²⁺) at a platinum electrode from acetonitrile (AN) and dimethylformamide (DMF) solutions is investigated by cyclic voltammetry (CV). Digisim simulations are employed to determine kinetic parameters. For [Ni(bpy)₃]²⁺ in AN the first electron transfer (ET) step was found to be rate-controlling. The rate constants of the first and second steps (k₁ and k₂) are practically the same in the DMF solution. In contrast, the second ET step is rate controlling for the [Ni(pybox)₂]²⁺ complex in AN, while the difference between k₁ and k₂ for this complex reactant in DMF becomes less noticeable. Molecular modeling based on the quantum mechanical theory of electron transfer and on density functional theory (DFT) calculations is performed for the first time to predict the activation barriers of the two-step heterogeneous redox processes and to elucidate the role of the ligands and solvents in the kinetics of these processes. Three-dimensional reaction free energy surfaces are constructed along the solvent and intramolecular coordinates. The electronic transmission coefficients are estimated as well. The results of model calculations are in qualitative agreement with the experimental data.