Aluminium porphyrins catalyse the hydrogenation of CO2 with H2

Abstract

Boron-based frustrated Lewis pairs (FLPs) have become well-established catalysts for the hydrogenation of a wide range of functional groups. Conversely, aluminium-based FLP hydrogenation catalysts are less common, especially for CO₂ reduction. They are mostly confined to the hydrogenation of imines, alkenes, and alkynes even though aluminium is much more abundant than boron and forms structurally related compounds. Moreover, aluminium forms penta- and hexa-coordinated complexes, which remain untested in FLP hydrogenation catalysis. Herein, we demonstrate that cationic, hexa-coordinated diaqua-meso-tetraphenylporphyrin aluminium complexes [Al(TPP)(OH₂)₂]X and [Al(^tBuTPP)(OH₂)₂]X (X = Cl⁻, OTf⁻, ClO₄⁻) form FLPs with nitrogen bases, activate H₂, and reductively couple CO₂ to amines, yielding N-formylamines and water. Our experimental results and DFT analysis indicate that H₂ activation involves the formation of an FLP, base-promoted CO₂ reduction and formate salt elimination from the FLP, as proposed for transition metal-catalyzed N-formylations. These similarities in the reaction mechanism and structure of aluminium complexes brings Al-based FLPs closer to transition metal catalysis and may enable us to apply this knowledge to ligand design to enhance main group metal-promoted hydrogenations.