Solvent Effects on the Electrochemistry of Nickel (II) Diethyldithiocarbamate

Abstract

Multielectron molecular redox couples have been used both as anolytes and catholytes in redox flow batteries (RFBs). The energy stored in an RFB is proportional to the number of electrons transferred per molecule. Thus, increasing the number of electrons transferred per molecule can enhance the amount of stored energy. Previous works in this field mostly looked for molecules containing multiple 1e- redox couples. The advantage of a single 2e- redox couple over multiple 1e- redox couples is that both electrons are stored and released at the same potential. Nickel (II) diethyldithiocarbamate, NiII(Et2dtc)2 undergoes 2e- oxidation at a single potential to form [NiIV(Et2dtc)3]+.1 In different nonaqueous solvents NiII(Et2dtc)2 can offer divergent redox behavior based on their coordination properties.2 This divergent redox behavior means significant changes on the electrochemistry of the complex during oxidation and reduction. Pyridine, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), acetonitrile (MeCN), methanol (MeOH), acetone, and dichloromethane (DCM) have been studied in this work as they have dissimilar donor numbers with different coordination abilities towards the Ni center. Both cyclic voltammetry and spectroscopic data in the above-mentioned solvents show distinct behavior which is expected as solvent coordination ability varies. Due to its highest coordination ability among the solvents used in this study, pyridine hinders the Ni (II) to Ni (IV) oxidation by coordinating strongly to the Ni (III) center to form [NiIII(Et2dtc)2(Py)2]+. On the contrary, noncoordinating DCM allows oxidation to Ni (IV) albeit through two distinguishable 1e- oxidation processes. MeCN and acetone allow Ni (II)→(IV) oxidation at a single potential. DMSO, DMF, and MeOH show intermediate redox behavior between 1e- and 2e- transfer reactions. These studies point to the ability of solvent to aid or hinder multielectron redox strategies which use Ligand Coupled Electron Transfer where changes in coordination environment are necessary.3 Mazumder, M. M.; Burton, A.; Richburg, C.; Saha, S.; Cronin, B.; Duin, E.; Farnum, B., Controlling One-Electron vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate. Inorganic Chemistry 2021.Islam, R.; Mazumder, M. M. R.; Farnum, B. H., Solvent Dependent Spectroscopic and Electrochemical Studies of Nickel (II) Diethyldithiocarbamate for Energy Storage, ECS Meeting s, IOP Publishing: 2021; p 54. Saha, S.; Sahil, S. T.; Mazumder, M. M. R.; Stephens, A. M.; Cronin, B.; Duin, E. C.; Jurss, J. W.; Farnum, B. H., Synthesis, characterization, and electrocatalytic activity of bis (pyridylimino) isoindoline Cu (II) and Ni (II) complexes. J Dalton Transactions 2021, 50 (3), 926-935. Figure 1