Urea-based covalent organic crown polymers and KI electrostatic synergy in CO2 fixation reaction: A combined experimental and theoretical study

Multifunctional organic polymer materials are supposed to be the most promising catalysts in the utilization of carbon dioxide (CO2) to five-membered cyclic carbonates (5CCs). Herein, functional one dimensional (1D) organic polymer materials (1D-UCP and 1D-UP) were successfully synthesized and their structural features were thoroughly characterized by FT-IR, C-13 CP MAS NMR, SEM, TGA and XPS spectroscopy. These materials showed excellent reaction performance in the CO2 cycloaddition reaction. Notably, the 1D-UCP decorated by crown ether group and urea unit simultaneously showed excellent yields of 5CCs under solvent-free conditions and low pressure of CO2. The outstanding performance was attributable to the synergistic effect of activated KI by coordinating with crown ether fragment and urea unit as hydrogen bonding donor facilitating implementation of speed-determined step of this reaction. In combination with density functional theory (DFT) calculations including intermediates structure optimization and transition states free energy profile, a possible catalytic mechanism was proposed that the urea group accelerated the reaction by vicinal dual hydrogen bonding and increasing nucleophilicity of I anion.