Influence of Pyridine on the Multielectron Redox Cycle of Nickel Diethyldithiocarbamate

Abstract

Two-electron (2e^-)-transfer reactions for monometallic complexes of first-row transition metals are uncommon because of the tendency of these metals to proceed through sequential one-electron (1e^-)-transfer pathways. For this chemistry to be observed, structural changes upon electron transfer are often needed to shift the 1e^- redox potentials to a condition of potential inversion where 2e^- transfer becomes favorable. Nickel(II) dithiocarbamate complexes take advantage of these conditions to drive 2e^- oxidation from Ni^II to Ni^IV. Here, we have studied the electrochemistry of Ni^II(dtc)₂, where dtc^- is <i>N</i>,<i>N</i>-diethyldithiocarbamate in an acetonitrile solvent as a function of the scan rate and added pyridine to gain further insight into the mechanism for its 2e^- oxidation to [Ni^IV(dtc)₃]⁺. The scan rate dependence revealed evidence for an ECE mechanism in which the chemical step constituted ligand exchange between [Ni^III(dtc)₂]⁺ and Ni^II(dtc)₂. A pseudo-first-order rate constant for this reaction of 34 s^-1 was obtained at 1 mM Ni^II(dtc)₂. The addition of pyridine to the electrolyte solution showed pronounced changes to the cyclic voltammetry (CV) that were consistent with the formation of a pyridine-bound Ni^III complex, [Ni^III(dtc)₂(py)₂]⁺, which was stable at high scan rates but decomposed to [Ni^IV(dtc)₃]⁺ at low scan rates. The observed decomposition rate constant was well modeled with two parallel decay pathways, one through the dipyridine [Ni^III(dtc)₂(py)₂]⁺ and another through a monopyridine [Ni^III(dtc)₂py]⁺. Overall, these data point to a mechanism for oxidation from Ni^II(dtc)₂ to [Ni^IV(dtc)₃]⁺ that proceeds through an undercoordinated [Ni^III(dtc)₂]⁺ complex, which can be trapped on the time scale of CV experiments using pyridine ligands. These studies provide insight into how we may be able to control 1e^- versus 2e^- redox chemistry using the coordination environment and nickel oxidation state.