Distorted octahedral cobalt(ii)–acylpyrazolone complex with a tunable lattice-strain structure – an efficient electrocatalyst for overall water splitting

Abstract

Transition metal Schiff base complexes are of recent research interest, especially in the field of catalysis. In this study, mononuclear cobalt(II)–acylpyrazolone complexes, [Co(OH₂)₂(PMBP)₂] (1) and [Co(OH₂)₂(PMTP)₂] (2), have been prepared and explored as electrocatalysts for catalyzing water splitting reactions for the first time. EPR and XRD data confirm that the Co(II) ion is coordinated with the acylpyrazolone ligand to form metal complexes and the complexes reveal dense crystallite morphologies as shown by SEM images. Based on the spectroscopic and molecular orbital data, a six-coordinate geometry around the cobalt center is proposed. The cobalt–acylpyrazolone complexes (1) and (2) towards the OER in 1.0 M KOH electrolyte, respectively, require a lower potential of 1.37 and 1.55 V vs. RHE@10 mA cm⁻² and show a Tafel slope of 50 and 90 mV dec⁻¹, which reveal the superior electrocatalytic activity of complex (1). Similarly, the complexes (1) and (2) towards the HER in 1.0 M KOH demanded a potential of −0.91 and −0.77 V vs. RHE@10 mA cm⁻² with a Tafel slope of 148 and 129 mV dec⁻¹, respectively. This exceptional bifunctional activity and stability of the cobalt–acylpyrazolone complexes towards water oxidation are due to the high synergy between the cobalt ion and the chelating acylpyrazolone ligand. The optimized structure and global reactivity descriptors obtained using density functional theory (DFT) correlate the structure–property and postulate that complexes (1) and (2) are strong electrophiles.