New positional isomeric homoleptic Ni(II) dithiolate anions as molecular homogeneous electrocatalysts for hydrogen evolution reaction

Abstract

Three new Ni(II) based positional isomeric homoleptic dithiolate complex anions with general formula [Et₄N]₂[NiL₂] (L = 2-methoxyphenylacetonitriledithiolate (Ni-o-OMe); 3-methoxyphenylacetonitriledithiolate (Ni-m-OMe) and 4-methoxyphenylacetonitriledithiolate (Ni-p-OMe) have been synthesized and characterized by microanalysis and spectroscopic techniques such as FT-IR, UV–Vis., ¹H, and ¹³C NMR. The geometry optimization of these complex anions suggest that, the immediate geometry around Ni(II) is distorted square planar which is satisfied by four sulfur centers of two dinegative dithiolate anions. These complex anions have been used as homogeneous electrocatalysts for hydrogen evolution reactions using trifluoroacetic acid (TFA) as a proton source. The electrocatalysis investigation reveals that Ni-o-OMe, Ni-m-OMe and Ni-p-OMe exhibited a turnover frequency (TOF) of 199, 242 and 450 s⁻¹, with overpotentials (η) of 0.799, 0.763 and 0.710 V, respectively, on adding 115 mM TFA. Thus, Ni-p-OMe with the highest TOF and least η, shows the best electrocatalytic activity. The observed variation in electrocatalytic properties of these isomeric complex anions have been explained by employing theoretical studies which reveals that, Ni-p-OMe possess a larger HOMO-LUMO energy gap compared to other isomeric complexes. This suggests that it is relatively more stable and hence offers better electrocatalytic properties than other two isomers. Furthermore, the planar architecture of Ni-p-OMe and existence of p-OMe group could have easily facilitated better electron communication, thereby elevating its electrocatalytic performance.