Supramolecular and homogeneous electrocatalytic HER properties of new heteroleptic cyanoacetamide dithiolate-based Ni(ii) tertiary phosphanes: effect of co-ligand flexibility on electrocatalytic performance

Abstract

Three new tertiary phosphane-appended heteroleptic Ni(II)-dithiolates with the formula [Ni(CAM)(dppe)] (Ni-1), [Ni(CAM)(dppf)] (Ni-2) and [Ni(CAM)(PPh₃)₂] (Ni-3) (CAM = cyanoacetamide dithiolate; dppe = 1,2-bis-(diphenylphosphinoethane); dppf = 1,1′-bis-(diphenylphosphino)ferrocene and PPh₃ = triphenylphosphane) have been synthesized and characterized spectroscopically and using single crystal X-ray diffraction technique. The single crystal X-ray analysis of Ni-1, Ni-2 and Ni-3 reveal distorted square planar geometry around Ni(II), wherein Ni(II) is coordinated to the two sulfur centres of a CAM ligand in the bidentate chelating mode and two phosphorus centers of dppe, dppf and PPh₃ ligands. Solid-state frameworks in these complexes are stabilized by C–O⋯H, C–N⋯H, C–S⋯H and C–H⋯C non-covalent intermolecular interactions. Ni-2 exhibits intramolecular (Ar)C–H⋯Ni anagostic interaction, which is absent in Ni-1 and Ni-3. Ni-3 exhibits intramolecular π⋯π stacking between the phenyl rings. The nature of these interactions have been assessed using Hirshfeld surface analyses, density functional theory (DFT) and quantum theory of atoms-in-molecules (QTAIM) calculations. Furthermore, the complexes have been employed as a homogeneous electrocatalyst for electrochemical hydrogen evolution reactions (HERs) using trifluoroacetic acid (TFA) as the hydrogen source. Electrochemical studies reveal turnover frequency (TOF) values of 319, 341 and 543 s⁻¹ for Ni-1, Ni-2 and Ni-3, respectively, at 50 mM concentration of TFA. The plausible reason for the relatively better electrocatalytic activity of Ni-3 is ascribed to the coordination flexibility of the PPh₃ ligand and the relatively larger positive natural charge over the nickel center and smaller HOMO–LUMO energy gap.