Chemical fixation of CO2 with epoxides catalyzed by a Cr(iii)-metalated guanidinium-based covalent organic polymer†

Abstract

Cr(III) metal centers have been successfully incorporated in the terephthalaldehyde-triaminoguanidium chloride COP, Te-TG_Cl. Free COP, Te-TG_Cl and metalated COP, Te-TG-Cr(III) were characterized with the help of various techniques, viz., FT-IR, ¹³C CP-MAS NMR, PXRD, XPS, FESEM, SEM-EDX, HR-TEM, TGA and BET. Due to the presence of the Lewis acidic Cr(III) center, Te-TG-Cr(III) showed superior activity (ECH, 99% conv.) compared to non-metalated COP, Te-TG_Cl (ECH, 75% conv.) in the cycloaddition reaction of CO₂ to epoxides, which resulted in chemically valuable cyclic carbonate products. Moreover, compared to Cr(NO₃)₃·9H₂O (ECH, 45% conv.), COP-immobilized Cr(III), Te-TG-Cr(III), shows much higher activity (ECH, 99% conv.), signifying the importance of COP in the enhancement of catalytic activity. Moreover, Te-TG-Cr(III) discriminates the substrates on the basis of the length of the epoxide side chains, showing much higher conversions for smaller substrates ECH and EBH (99% and 96% conv., respectively) compared to larger substrates BGE, AGE, PGE, EH and SO (68%, 60%, 69%, 57% and 35% conv., respectively).