Al(III) phthalocyanine catalysts for CO2 addition to epoxides: Fine-tunable selectivity for cyclic carbonates versus polycarbonates

Abstract

A sustainable synthetic methodology to prepare bifunctional cationic imidazolyl and neutral tert-butylphenoxy Al(III)-phthalocyanine complexes is described, using ultrasound irradiation to synthesize the phthalonitrile precursors, followed by microwave-assisted cyclotetramerization. The metallophthalocyanine catalysts were applied in the coupling reaction of CO2 with epoxides, where the structure of the phthalocyanine was shown to determine the reaction selectivity towards the formation of cyclic carbonates versus polycarbonates. The cationic imidazolyl phthalocyanines operate as efficient bifunctional catalysts to promote the CO2 cycloaddition to terminal epoxides (styrene oxide, epichlorohydrin, propylene oxide and allyl glycidyl ether), leading to TONs up to 1414 and 100% selectivity for cyclic carbonates, without the need of using any co-catalyst. On the other hand, the neutral 4-tert-butylphenoxy Al(III) phthalocyanine catalyzed the copolymerization reaction between CO2 and cyclohexene oxide, in the presence of PPNCl as co-catalyst, with TON = 814, providing the first example of a metallophthalocyanine catalyst with full selectivity for formation of polycarbonates.