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

• Published in: Chemical science (Cambridge) Vol. 11; no. 22; pp. 5669 - 5675
• Main Authors: Deng, Jingyuan, Ratanasak, Manussada, Sako, Yuma, Tokuda, Hideki, Maeda, Chihiro, Hasegawa, Jun-ya, Nozaki, Kyoko, Ema, Tadashi
• Format: Journal Article
• Language: English
• Published: 10.06.2020
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d0sc01609h

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 CO 2 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 [CO 2 ], and the activation parameters were determined: Δ H ‡ = 12.4 kcal mol −1 , Δ S ‡ = −26.1 cal mol −1 K −1 , and Δ G ‡ = 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 (Δ H ‡ = 13.3 kcal mol −1 , Δ S ‡ = −3.1 cal mol −1 K −1 , and Δ G ‡ = 14.4 kcal mol −1 at 80 °C). Bifunctional Al porphyrins worked as excellent catalysts for the copolymerization of cyclohexene oxide (CHO) and CO 2 .