Sonochemical Synthesis of Cobalt-Oxo-Cubanes as Redox-Active Platform for Proton-Coupled Electron Transfer

Cobalt-oxo-cubanes (COCs) are efficient, earth-abundant analogs of the tetrameric manganese clusters of the natural photosystems. Yet, the synthesis of COCs remains a challenging affair. In this study, we report a simple sonochemical approach for the gram-scale synthesis of a series of pyridine-substituted COCs. The representative electronic variation via para-and meta-substituted pyridines provided a handle to evaluate the effect of the electronic nature of COCs on the overall redox activity. Furthermore, the electrochemical behavior of COC-XPy and its kinetics were investigated in a homogeneous system to provide insight into their solution-phase electrochemical behavior under neutral aqueous conditions. Cyclic voltammetry (CV) studies revealed a one-electron oxidation pathway at the [Co₄O₄]⁴⁺ core, while pH-dependent studies revealed a proton-coupled electron transfer (PCET) mechanism. Comparison of crude and purified samples by CV revealed that trace Co²⁺ contaminants disrupt the intrinsic cubane redox chemistry and likely contributed to the oxidative currents reported in earlier studies. Overall, our approach offers a convenient route for the synthesis of COC clusters and may facilitate further studies aimed at understanding their redox behavior and relevance to artificial photosynthetic models.