Influence of Pyridine on the Multielectron Redox Cycle of Nickel Diethyldithiocarbamate

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 N,N-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)(2)(py](+). 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.