Influence of structural diversity in Co(ii)–oxamide complexes towards single molecule magnets and electrochemical oxygen evolution reaction

Abstract

Cobalt complexes continue to attract extensive attention owing to their wide range of potential applications in catalysis, electrochemistry, and magnetism. Three cobalt complexes of different nuclearity namely [Co(L)(H₂O)₂Py₂]·0.5Py (1), [Co₂(μ₂-L)(N₃)₂(H₂O)₂Py₄] (2) and [Co₄(L)₄(μ₄-O)(μ₂-H₂O)₂]·2TEAH·DMF·2H₂O (3) were synthesized by reaction of a benzothiazole based oxamide ligand (H₂L) with CoCl₂·2H₂O (1 : 2 ratio) varying the solvent, reaction conditions, and base. All the complexes were characterized by various spectroscopic and analytical techniques, as well as SCXRD study. In the crystal structure of the tetranuclear complex, the metal centers are in a distorted trigonal bipyramidal (TBP) geometry supported by a μ₄-oxo bridge and two μ₂-aqua bridges. Variable temperature magnetic moment measurement reveals intra-metallic ferromagnetic exchange interactions between metal centers (J = 4.25 cm⁻¹) for the dinuclear complex (2). The tetra-nuclear complex (3) shows temperature and frequency-dependent AC magnetic susceptibility without applying any external magnetic field, a distinctive feature of zero-field single-molecule magnets (SMMs). The Cole–Cole plot of the molecule indicates that slow magnetic relaxation occurs via a single relaxation pathway. Furthermore, the electrocatalytic activities of all complexes were examined for oxygen evolution reaction (OER). Among the complexes, 3 showed a significantly lower Tafel slope of 73 mV dec⁻¹ compared to the commercial Co₃O₄ catalyst (163 mV dec⁻¹), indicating more favorable kinetics for the oxygen evolution reaction (OER).