Effect of Ligand Chemistry on the Electronic Properties and Reactivity of Cobalt Acetate Autoxidation Catalysts

Autoxidation chemistry catalyzedby cobalt(II) acetate is an industrialpathway for converting hydrocarbons into oxygenated compounds andhas promising potential applications in plastic waste deconstruction.However, the chemical properties of the Co acetate-based catalystsand their roles in the reaction mechanism are poorly understood, andas a result, attempts to redesign the autoxidation chemical processfor novel applications lack clear direction. In this work, we investigatethe structure and electronic properties of a series of multinuclearCo(III) compounds that have been proposed as active participants incatalytic autoxidation. Through a combination of X-ray spectroscopicmeasurements [Co K-edge X-ray absorption spectroscopy (XAS), extendedX-ray absorption fine structure (EXAFS), K & beta; X-ray emission spectroscopy(XES), and high-energy-resolution fluorescence detection (HERFD) XAS]and theoretical methods, we characterize the interactions of Co withacetate and hydroxyl ligands in these compounds. We show that thesubstitution of acetate ligands with hydroxyls in these compoundscauses an increase in the electron density on Co, driven by the lossof a & pi; back-bonding interaction between Co and acetate and anincrease in electron donation to Co from the hydroxyl ligands. Wefurther classify the bonding between Co and acetate based on the orbitaloverlap and show that the experimental absorption and emission spectraare well-described by the resulting molecular orbitals. Finally, wepredict the impacts of acetate/hydroxyl ligand exchange on autoxidationcatalytic properties, showing that the reaction free energies forkey Co oxidation and reduction steps are extremely sensitive to metal-ligandbonding interactions and thus have meaningful impacts on overall reactivity.The results of this study constitute an important set of design criteriafor developing novel homogeneous autoxidation catalysts.