New heteroleptic bis-(diphenylphosphino)ethane appended dialkyldithiophosphate cobalt(iii) cations: apt electrocatalysts for the heterogeneous oxygen evolution reaction and homogeneous hydrogen evolution reaction

Abstract

Two new heteroleptic cobalt(III)dialkyldithiophosphate complex cations with a 1,2-bis(diphenylphosphino)ethane (dppe) ancillary ligand having formula [Co{S₂P(OR)₂}₂(dppe)](B(C₆H₅)₄) (R = –C₂H₅ (Co–Et); –CH(CH₃)₂ (Co–Pr)) have been synthesized and characterized spectroscopically as well as via single crystal X-ray diffraction analysis. The single crystal X-ray diffraction analyses reveal that the immediate geometries around Co(III) in both complexes are distorted octahedral, which are satisfied by four sulfur atoms of two dialkyldithiophosphate ligands and two phosphorus centers of the dppe ligand. These complex cations are neutralized by tetraphenyl borate B(C₆H₅)₄ anions. The solid-state framework of both complexes are stabilized by C–H⋯π and C–H⋯C interactions. The nature of these interactions have been addressed with the help of Hirshfeld surface analysis, and percent contributions of pertinent interactions in the crystal structure of these complexes have been addressed using fingerprint plots. Both complexes have been used as heterogeneous electrocatalysts for the oxygen evolution reaction (OER). Results suggest that Co–Et is a better electrocatalyst for the OER, displaying an onset potential of 1.68 V and Tafel slope of 114 mV dec⁻¹. Furthermore, in the homogeneous electrocatalysis of the hydrogen evolution reaction (HER) using trifluoroacetic acid, the overpotential for Co–Et and Co–Pr have been 1.05 V and 0.92 V, respectively, with turnover frequencies of 1518 s⁻¹ and 287.5 s⁻¹, respectively. Overall, heterogeneous OER and homogenous HER results suggest that Co–Et can be used as an efficient molecular electrocatalyst for oxygen/hydrogen evolution reactions.