Unravelling the role of –OCH3 positional isomerism and dihedral angle in Ni(ii)-dppe dithiolates for enhanced heterogeneous electrocatalytic oxygen evolution reaction (OER)

Abstract

Three new positional isomeric 1,2-bis(diphenylphosphino)ethane (dppe) appended nickel(II) dithiolates, namely, [Ni(dppe)2-cyano-2-(o-methoxyphenyl)ethene-1,1-dithiolate] (Ni-L1), [Ni(dppe)2-cyano-2-(m-methoxyphenyl)ethene-1,1-dithiolate] (Ni-L2) and [Ni(dppe)2-cyano-2-(p-methoxyphenyl)ethene-1,1-dithiolate] (Ni-L3), were synthesized and characterized using spectroscopic and single crystal X-ray analyses. Single crystal X-ray diffraction studies suggested that in all the three complexes, the immediate geometry around the nickel(II) was distorted square planar, which was defined by two sulfur and two phosphorus centres of dithiolate and dppe ligands, respectively, in a bidentate chelating manner. The supramolecular architectures of these complexes were sustained by intermolecular π⋯π, C–H⋯π, N⋯H, O⋯H and S⋯H interactions, and the nature of these interactions was studied with the aid of Hirshfeld surface analysis. The positional isomeric methoxy group engendered trimeric and dimeric motifs for Ni-L1 and Ni-L2, respectively, while it led to the chain formation in Ni-L3, thereby highlighting the influence of the position of the methoxy group on supramolecular frameworks. The heterogeneous electrocatalytic properties of these complexes in the oxygen evolution reaction (OER) were studied using modified glassy carbon electrodes. Results indicated that the Ni-L3 is a superior electrocatalyst for the OER amongst all three complexes, which exhibited an onset potential of 1.56 V and a TOF of 0.05 s⁻¹. Thus, this study unveiled the influence of factors such as the position of the isomeric –OMe group, thedihedral angle between the NiS₂C₂ and –(C₆H₄-OMe) planes and natural charges on nickel center on the electrocatalytic properties of these complexes towards OER.