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

Abstract

Bifunctional AlIIIporphyrins with quaternary ammonium halides,2-Cland2-Br, worked as excellent catalysts for the copolymerization of cyclohexene oxide (CHO) and CO2at 120 °C. Turnover frequency (TOF) and turnover number (TON) reached 10 000 h−1and 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 MgIIand ZnIIcounterparts,3-Cland4-Cl, as well as a binary catalyst system,1-Clwith 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−1K−1, and ΔG‡= 21.6 kcal mol−1at 80 °C. Comparative DFT calculations on two model catalysts, AlIIIcomplex2′and MgIIcomplex3′, allowed us to extract key factors in the catalytic behavior of the bifunctional AlIIIcatalyst. 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 asTS3b(ΔH‡= 13.3 kcal mol−1, ΔS‡= −3.1 cal mol−1K−1, and ΔG‡= 14.4 kcal mol−1at 80 °C).