Role of Solvent Coordination and Ligand Structure on the Electrochemical Redox Cycle of Nickel (II) bis(dithiocarbamate) Complexes for Energy Storage Applications

Abstract

Nickel (II) diethyldithiocarbamate, NiII(Et2dtc)2, undergoes a 2e- ligand-coupled electron transfer (LCET) oxidation at a single potential to form [NiIV(Et2dtc)3]+ in acetonitrile (MeCN) solvent. However, in different nonaqueous solvents NiII(dtc)2 can exhibit divergent redox behavior based on the solvent’s coordination properties, producing significant changes on the electrochemistry of the complex during oxidation and reduction. Pyridine (Py), dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), MeCN, methanol (MeOH), acetone (Ac), and dichloromethane (DCM) were studied here due to their dissimilar coordination abilities towards the nickel metal center. Cyclic voltammetry data in the above-mentioned solvents show distinct behavior which is expected as solvent coordination ability varies. For low coordinating solvents like MeCN and Ac, 2e- oxidation of NiII(Et2dtc)2 to [NiIV(Et2dtc)3]+ occurred at a single potential. Stronger coordinating solvents like MeOH, DMF, DMSO, and Py disrupted the 2e- oxidation by coordinating to the Ni(III) intermediate to form [NiIII(Et2dtc)2(sol)x]+ complexes. The decay of these complexes through ligand exchange with NiII(Et2dtc)2 and disproportionation to yield [NiIV(Et2dtc)3]+ was monitored as a function of scan rate and temperature to extract rate constants and activation parameters. A thorough analysis of activation parameters revealed that ΔHapp‡ generally increased with solvent donor number, suggesting solvent dissociation was a key factor in the rate limiting step. However, ΔSapp‡ was found to be negative for all solvents, suggesting an associative mechanism in line with dimer formation with NiII(dtc)2 to facilitate ligand exchange. In a mixed solvent composition of acetonitrile, pyridine, and dichloromethane in a ratio of 90:7.5:2.5, the apparent activation entropy (∆Sapp‡) decreases as the steric bulk of the substituent R groups replacing an ethyl (Et) group increase. This trend suggests that greater steric bulk promotes a more associative mechanism. Primary amine-based dtc ligands were introduced to improve solubility of Ni(II) complexes where solubility of NiII(Etdtc)2 was four-fold than NiII(Et2dtc)2 in MeCN.