New heteroleptic tertiary phosphane-based Ni(ii) 3,4-dimethoxyphenylacetonitriledithiolates: bifunctional electrocatalysts for homogeneous HER and heterogeneous OER

Construction of highly efficient non-platinum complexes as catalysts for electrochemical water splitting is the need of time. Herein, three new Ni(II)-based dithiolates, with formulas of [(dpdt)Ni(dppe)] (Ni-dpe), [(dpdt)Ni(dppf)] (Ni-dpf) and [(dpdt)Ni(PPh3)2] (Ni-pph) (dpdt = 3,4-dimethoxyphenylacetonitriledithiolate, dppe = 1,2-bis(diphenylphosphino)ethane, dppf = 1,1′-bis(diphenylphosphino)ferrocene and PPh3 = triphenylphosphane), were synthesized and characterized. Single-crystal X-ray diffraction analysis of Ni-dpe revealed a distorted square planar coordination environment around nickel(II) fulfilled by two S and two P of the dpdt2− and dppe ligands, respectively. Various non-covalent interactions, such as C–H⋯C, C–H⋯S and C–H⋯O, stabilized the lattice of Ni-dpe, and their nature was investigated using Hirshfeld surface analysis. Furthermore, the activities of these complexes as homogeneous electrocatalysts for HERs using trifluoroacetic acid (TFA) as a proton source and heterogeneous electrocatalysts for OERs in an aqueous KOH solution were evaluated. For homogeneous electrocatalysis, turnover frequencies (TOFs) of 402, 655 and 1761 s−1 for Ni-dpe, Ni-dpf and Ni-pph were observed, respectively, at 100 mM TFA. The better electrocatalytic activity of Ni-pph was ascribed to the spatial flexibility of the PPh3 ligands, which facilitated attainment of a more stable transition state than those of other complexes. For heterogeneous OERs, the results suggested that Ni-dpf displayed the lowest Tafel slope of 64 mV dec−1 and the highest current density of 53.47 mA cm−2, thereby exhibiting the best electrocatalytic activity.