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 NiII to NiIV. Here, we have studied the electrochemistry of NiII(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 [NiIV(dtc)3]+. The scan rate dependence revealed evidence for an ECE mechanism in which the chemical step constituted ligand exchange between [NiIII(dtc)2]+ and NiII(dtc)2. A pseudo-first-order rate constant for this reaction of 34 s-1 was obtained at 1 mM NiII(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 NiIII complex, [NiIII(dtc)2(py)2]+, which was stable at high scan rates but decomposed to [NiIV(dtc)3]+ at low scan rates. The observed decomposition rate constant was well modeled with two parallel decay pathways, one through the dipyridine [NiIII(dtc)2(py)2]+ and another through a monopyridine [NiIII(dtc)2py]+. Overall, these data point to a mechanism for oxidation from NiII(dtc)2 to [NiIV(dtc)3]+ that proceeds through an undercoordinated [NiIII(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.