Insights into the supramolecular and molecular electrocatalytic properties of 1,2-bis(diphenylphosphine)ethane appended nickel(ii) 1,2-dithiosquarate for oxygen and hydrogen evolution reactions

A new heteroleptic Ni(ii) dithiolate [Nidppe(dtsq)] (Nidtsq) (dppe = 1,2-bis-(diphenylphosphino)ethane, dtsq(2-) = 3,4-dioxocyclobut-1-ene-1,2-dithiolate) has been synthesised and characterized spectroscopically and by single crystal X-ray diffraction. The single crystal X-ray diffraction study reveals a square planar geometry around the Ni(ii) center wherein dtsq(2-) and dppe ligands exhibit bidentate chelating coordination modes by their respective sulfur and phosphorus centers. The solid-state framework of Nidtsq is stabilized by C-H & ctdot;O, C & ctdot;H, C & ctdot;O and pi & ctdot;pi intermolecular interactions and the complex displays an interesting pair of intramolecular C-H & ctdot;Ni anagostic interactions. The nature of these interactions has been addressed with Hirshfeld surface analysis, density functional theory, quantum theory of atoms and molecules (QTAIM) and non-covalent interaction-reduced density gradient (NCI-RDG) analyses. Furthermore, the electrocatalytic properties of Nidtsq have been assessed for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The electrocatalytic properties of Nidtsq have been found to be better than those of the previously reported Ni-mtsq complex.(17) In the OER, the complex displayed an overpotential of 0.37 V with eta(10) of 414 mV and a Tafel slope of 237 mV dec(-1) and in the HER it exhibited an overpotential of -0.35 V with eta(10) of -670 mV and a Tafel slope of 368 mV dec(-1). Chronoamperometry and chronopotentiometry experiments for four hours in 0.1 M KOH and 0.5 M H2SO4 indicate the robustness of the complex in alkaline and acidic environments and the post-electrocatalytic stability of Nidtsq has been assessed with the aid of FESEM and X-ray photoelectron spectroscopy.