Tetranuclear Co<sup>II</sup><sub>4</sub>O<sub>4</sub> Cubane Complex: Effective Catalyst Toward Electrochemical Water Oxidation
Ezhava Manu Manohar, Hariharan N. Dhandapani, Soumalya Roy, Robert Pełka, Michał Rams, Piotr Konieczny, Srinu Tothadi, Subrata Kundu, Atanu Dey, Sourav Das
Abstract
The reaction of Co(OAc) 2 ·6H 2 O with 2,2′-[{(1 E,1′ E )-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH 2 ) a multisite coordination ligand and Et 3 N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co 4 (L) 2 (μ-η 1:η 1 -OAc) 2 (η 2 -OAc) 2 ]· 1.5 CH 3 OH· 1.5 CHCl 3 ( 1 ). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co 4 O 4 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 ( D 1 = 8.8, D 2 = 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 Co 3 O 4 (5.242 Ω), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.