Aluminum porphyrins with quaternary ammonium halides as catalysts for copolymerization of cyclohexene oxide and CO2: metal-ligand cooperative catalysis

Abstract

Bifunctional Al^III porphyrins with quaternary ammonium halides, 2-Cl and 2-Br, worked as excellent catalysts for the copolymerization of cyclohexene oxide (CHO) and CO₂ at 120 °C. Turnover frequency (TOF) and turnover number (TON) reached 10 000 h^-1 and 55 000, respectively, and poly(cyclohexene carbonate) (PCHC) with molecular weight of up to 281 000 was obtained with a catalyst loading of 0.001 mol%. In contrast, bifunctional Mg^II and Zn^II counterparts, 3-Cl and 4-Cl, as well as a binary catalyst system, 1-Cl with bis(triphenylphosphine)iminium chloride (PPNCl), showed poor catalytic performances. Kinetic studies revealed that the reaction rate was first-order in [CHO] and [2-Br] and zero-order in [CO₂], and the activation parameters were determined: ΔH ^‡ = 12.4 kcal mol^-1, ΔS ^‡ = -26.1 cal mol^-1 K^-1, and ΔG ^‡ = 21.6 kcal mol^-1 at 80 °C. Comparative DFT calculations on two model catalysts, Al^III complex 2' and Mg^II complex 3', allowed us to extract key factors in the catalytic behavior of the bifunctional Al^III catalyst. The high polymerization activity and carbonate-linkage selectivity originate from the cooperative actions of the metal center and the quaternary ammonium cation, both of which facilitate the epoxide-ring opening by the carbonate anion to form the carbonate linkage in the key transition state such as TS3b (ΔH ^‡ = 13.3 kcal mol^-1, ΔS ^‡ = -3.1 cal mol^-1 K^-1, and ΔG ^‡ = 14.4 kcal mol^-1 at 80 °C).