Influence of Ligand Sterics on Electrocatalytic Oxygen Evolution Reaction in Molecular Half-Sandwich Nickel(II) N-Heterocyclic Carbene Complexes: Experimental and DFT Insights

Abstract In this study, two half-sandwich Ni(II) N-heterocyclic carbene (NHC) complexes, NiCpL1Br and NiCpL2Br, were synthesized from 2,6-diethylphenyl-1,2,4-triazolium bromide salts bearing varying benzocoumarin substituents, designated as HL1Br and HL2Br, respectively and thoroughly characterized. The resulting complexes were found to adopt a two-legged piano-stool configuration, with the Ni center exhibiting a trigonal planar coordination geometry. Further, the Ni(II) complexes were immobilized onto carbon cloth (CC) via noncovalent interactions to fabricate molecularly modified electrodes to evaluate their performance in the oxygen evolution reaction (OER) in an alkaline medium. NiCpL1Br and NiCpL2Brmodified CC electrodes were compared in terms of overpotentials at a current density of 10 mA/cm2, Tafel slopes, double-layer capacitance, electrochemical active surface area, and Nyquist plots. The long-term chronoamperometric measurement was conducted and analyzed using polarization curves and post-electrolysis morphological characterization. Density functional theory revealed that the tilted benzocoumarin ring in NiCpL1Br due to its reduced dihedral angle boosts electronic interaction with the Ni center, thereby increasing the orbital overlapping, and further strengthens π-conjugation, leading to increased OER activity.