Salophen chromium(iii) complexes functionalized with pyridinium salts as catalysts for carbon dioxide cycloaddition to epoxides

Abstract

The catalytic properties of a series of novel chromium(III) salophen complexes having different pyridinium chloride units (pyridinium, 2,6-dimethylpyridinium or 4-(dimethylamino)pyridinium ones) have been studied in the reaction of carbon dioxide cycloaddition to phenyl glycidyl ether. The examined complexes were found to be capable of catalyzing cycloaddition under relatively mild reaction conditions without any additional nucleophilic co-catalyst. However, their catalytic activity depended strongly on the structure and number of pyridinium salt units in the ligand molecule. The complex with a single unit of 4-(dimethylamino)pyridinium chloride turned out to be the most active among the examined ones. A TOF of up to 1480 h⁻¹ was obtained in the presence of this catalyst under the following conditions: 120 °C, 2 h, 6 bar, 0.05 mol% (74% epoxide conversion, and >99% selectivity). The most active complex has also been examined as a catalyst in the reactions of CO₂ with a series of ten other terminal epoxides. High catalytic activity (TOF = 220–5045 h⁻¹) was observed in most cases, except for the reaction of CO₂ with allyl glycidyl ether.