Synthesis of amino-phenolate manganese complexes and their catalytic activity in carbon dioxide activation and oxidation reactions

Abstract

Two manganese(III) compounds were studied as catalysts for the reaction of carbon dioxide with propylene oxide, styrene oxide, and cyclohexene oxide, and formed cyclic carbonate products selectively under solvent free conditions in the presence of an ionic co-catalyst such as TBAB or PPNCl. Variable temperature kinetic studies allowed the activation energy for propylene carbonate formation to be determined (64 kJ mol−1). The catalysts showed good stability in these reactions and overall turnover numbers (TON) of up to 6000 were observed. The complexes showed low activity for the aerobic oxidation of 4-methyoxybenzylalcohol to the corresponding aldehyde, achieving up to 40% conversion in 72 h. However, near quantitative conversion of 1,2-diphenyl-2-methoxyethanol to provide up to 90% yield of benzaldehyde could be achieved over the course of 5 days. Both complexes showed similar reactivity in the two catalytic processes, and this is likely due to the weakly coordinating nature of the pendant donor within the tetradentate ligand.