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

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 degrees 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: Delta H double dagger = 12.4 kcal mol(-1), Delta S double dagger = -26.1 cal mol(-1) K-1, and Delta G double dagger = 21.6 kcal mol(-1) at 80 degrees 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 (Delta H double dagger = 13.3 kcal mol(-1), Delta S double dagger = -3.1 cal mol(-1) K-1, and Delta G double dagger = 14.4 kcal mol(-1) at 80 degrees C).