Cyclic Carbonate Synthesis from Epoxides and CO₂ Catalyzed by Aluminum–Salen Complexes Bearing a nido-C₂B₉ Carborane Ligand

The active and well-designed Schiff base ligands are considered “privileged ligands”. The so-called salen ligands, i.e., the tetradentate [O, N, N, O] bis-Schiff base ligands, have also found broad applications in many homogeneous catalytic reactions. Modification of the salen ligands has concentrated on altering the substituents in the phenolate rings and variations in the diamine backbones. Herein, o-carborane-supported salen ligands (2) were designed and prepared. A series of aluminum–salen complexes (3·(sol)₂), which were supported by the nido-C₂B₉ carborane anions, were synthesized. These Al­(III) complexes showed high activities (TOF up to 1500 h^–1) in catalyzing the cycloaddition of epoxides and CO₂ at atmospheric pressure and near room temperature. Complexes 3·(sol)₂ are one of the rare examples of Al-based catalysts capable of promoting cycloaddition at 1 bar pressure of CO₂. Density functional theory (DFT) studies combined with the catalytic results reveal that the catalytic cycles occur on two axial sites of the Al­(III) center.