Role of Solvent Coordination in the Multi-electron Redox Cycle of Nickel Diethyldithiocarbamate

Nickel(II) diethyldithiocarbamate, Ni^II(dtc)₂, is known to undergo a 2e^- ligand-coupled electron transfer (LCET) oxidation to form [Ni^IV(dtc)₃]⁺. However, the thermodynamics and kinetics of this 2e^- process can be greatly affected by solvent coordination. For low coordinating solvents like acetonitrile and acetone, 2e^- oxidation is observed via cyclic voltammetry (CV) at a single potential while stronger coordinating solvents like methanol, <i>N</i>,<i>N</i>-dimethylformamide, dimethyl sulfoxide, and pyridine exhibit a 1e^- oxidation wave by formation of [Ni^III(dtc)₂(sol)_<i>x</i>]⁺ intermediates. The decay of these complexes to eventually yield [Ni^IV(dtc)₃]⁺ was monitored as a function of CV scan rate and temperature to extract rate constants and activation parameters. A thorough analysis of activation parameters revealed that Δ<i>H</i>_app^⧧ generally increased with solvent coordination ability, suggesting solvent dissociation was a key factor in the rate limiting step. However, Δ<i>S</i>_app^⧧ was found to be negative for all solvents, suggesting an associative mechanism in line with dimer formation with Ni^II(dtc)₂ to facilitate ligand exchange. Density function theory calculations supported the competitive nature of dissociative and associative steps. Using these calculations, we propose two paths for decay of [Ni^III(dtc)₂(sol)_<i>x</i>]⁺ species based on the coordination strength of the solvent. These studies point to the ability of solvents to either aid or hinder multielectron LCET reactions.