Electrocatalytic water oxidation with bioinspired cubane-type CoII complexes

Abstract

Synthesis, characterization, and electrocatalytic water oxidation studies of the cubane-type complexes [(µ³-L¹O)CoCl(MeOH)]₄ (1) and [(µ³-L²O)CoCl(MeOH)]₄ (2) are herein reported. Cubanes 1 and 2 were obtained in high yields under mild conditions by self-assembly of the ligands L¹OH = 1-H-2-benzimidazolylmethanol and L²OH = 1-methyl-2-benzimidazolylmethanol with CoCl₂·6H₂O in basic methanolic solution. Both compounds feature a cubane-type structure in which the central {Co₄O₄} units are built by four Co^II centers coordinated by alkoxide-bridged oxygen and nitrogen atoms from the deprotonated LⁿOH ligands and stabilized by MeOH molecules and chloride ions. Magnetic studies allowed determination of g = 2.42 and g = 2.57 values for 1 and 2, respectively. At low temperature, 1 shows antiferromagnetic behavior, while 2 shows ferromagnetic coupling. DFT analyses support the antiferromagnetic behavior of 1. Unfortunately, the same method was not effective in explaining the ferromagnetic character of 2. Such inconsistency was explained through an exhaustive ac-susceptibility study by considering the single molecular magnet (SMM) behavior of 2. Cyclic Voltammetry of 1 and 2 in phosphate buffer solution (pH = 7.4) displayed catalytic currents at 1.65 and 1.68 V vs Standard Hydrogen Electrode (SHE), corresponding to water oxidation, with TOF values of 1.04 and 1.99, overpotential values of 710 and 680 mV, faradaic efficiencies of 88% and 90%, and TON values of 2.8 and 3.5 for 1 and 2, respectively. Electrochemical Quartz Microbalance (EQCM) analysis showed the robustness of 1 and 2 since only around 0.05% of their mass deposited on the electrode surface. Subsequent SEM-EDX microanalysis demonstrated that although 1 converts to CoO_x during electrolysis, it also changes into an undetermined form of the original catalyst, containing C, O, and Co. In the case of 2, analysis of the electrode after WOC did not allow the detection of Co, C, or O, establishing 2 as a more stable catalyst than 1.