Tetranuclear Co^II₄O₄ Cubane Complex: Effective Catalyst Toward Electrochemical Water Oxidation

The reaction of Co(OAc)₂·6H₂O with 2,2'-[{(1E,1'E)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH₂) a multisite coordination ligand and Et₃N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co₄(L)₂(μ-η¹:η¹-OAc)₂(η²-OAc)₂]· 1.5 CH₃OH· 1.5 CHCl₃ (1). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co₄O₄ cubane core consisting of two completely deprotonated ligands [L]^2- and four acetate ligands. Two distinct types of Co^II centers exist in the complex, where the Co(2) center has a distorted octahedral geometry; alternatively, Co(1) has a distorted pentagonal-bipyramidal geometry. Analysis of magnetic data in 1 shows predominant antiferromagnetic coupling (J = -2.1 cm^-1), while the magnetic anisotropy is the easy-plane type (D₁ = 8.8, D₂ = 0.76 cm^-1). Furthermore, complex 1 demonstrates an electrochemical oxygen evolution reaction (OER) with an overpotential of 325 mV and Tafel slope of 85 mV dec^-1, required to attain a current density of 10 mA cm^-2 and moderate stability under alkaline conditions (pH = 14). Electrochemical impedance spectroscopy studies reveal that compound 1 has a charge transfer resistance (R_ct) of 2.927 Ω, which is comparatively lower than standard Co₃O₄ (5.242 Ω), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.