Switching S to O: Enhancing OER and HER molecular heterogeneous electrocatalysis with trithio-/dithio-carbonate Co(III)-dppe complexes

The development of new molecular electrocatalysts especially the transition metal dithiolates through rational design is imperative for advancing efficient water-splitting reactions. In this context, the present study reports syntheses and characterization of two new 1,2-Bis(diphenylphosphino)ethane (dppe) appended heteroleptic cobalt(III) complex salts with compositions NEt₄[Co(S₂C[dbnd]S)₂dppe)] (Co-1) and NEt₄[Co(S₂C[dbnd]O)₂(dppe)] (Co-2). Single crystal X-ray diffraction study for Co-2 suggests that complex anion adopts distorted octahedral coordination geometry around Co(III) satisfied by the four sulfur of the two S₂C[dbnd]O²⁻ ligands and two P-centers of dppe. The supramolecular architecture of Co-2 is held together by C–O⋯H, C–S⋯H and C–H···π contacts, the characteristics of which are explored through Hirshfeld surface analyses. These complexes have been used as heterogeneous molecular electrocatalysts for oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) and the results suggest that Co-2 offers better electrocatalytic performance in OER with η¹⁰ of 427 mV and an exceptionally low Tafel slope (b) of 49.2 mVꞏdec⁻¹. In HER, Co-2 exhibits moderate activity with η⁵ of 992 mV and b of 184.2 mVꞏdec⁻¹. The EDAX results for Co-2 suggests an in situ oxidative transformation of cobalt at the surface during OER electrocatalysis that lead to the formation of high-valence cobalt oxide or hydroxide species, indicating that Co-2 undergoes structural changes during electrolysis and functions as a pre-catalyst. The post-catalysis stability of Co-1 and Co-2 have been assessed using FESEM that indicates agglomeration of the particles in acidic medium that results in decline in their HER activity. Also, X-ray photoelectron spectroscopy (XPS) has been employed to assess the valence states in these complexes during electrocatalysis that also reverberated the formation of active catalysts during electrocatalysis experiments. This study presents how switching from thiocarbonyl (C[dbnd]S) to carbonyl (C[dbnd]O) entity in the ligand plays a pivotal role in tuning the electrocatalytic performances in these complexes towards electrochemical water splitting reactions.