Constructing Synergistically Catalytic Lewis Acidic-Basic Sites for Boosting Reactivity of a Flexible Coordination Polymer

Targeted construction of Lewis acidic-basic sites in the skeleton of coordination polymers (CPs) can greatly enhance their catalytic efficiency due to the synergistic effect of acidic and basic sites. However, research on validating the coexistence of Lewis acidic-basic sites for boosting the catalytic activity of CPs toward the Knoevenagel condensation (KC) reaction, widely applied in the synthesis of high-added-value intermediates and products under mild conditions, is missing so far. Based on the above consideration, we have artificially constructed Lewis acidic-basic sites and introduced vacancy in the framework of a new flexible cerium CP {Ce-CP: [Ce3+Ce4+(obb2–)3(OH)­(H2O)­(DMF)]∞} (DMF: N,N-dimethylformamide) via applying the functional ligand 4,4′-oxidibenzoate (obb2–) with the bridging O atom as the Lewis basic site and removing the coordinating solvent molecules and counterions to form cerium coordination unsaturated sites (Ce-CUSs) as Lewis acidic sites. Interestingly, Ce-CP exhibits reversible structural transformation associated with a desolvation and resolvation process. The Lewis acidic and basic sites in the resulting Ce-CP (LAB-Ce-CP) have been confirmed by CO2 temperature-programmed desorption (TPD) and NH3 combined with pyrrole-TPD (NH3–Py-TPD) for the first time. Benefiting from the coexistence of Lewis acidic and basic sites as well as the flexibility of the framework, LAB-Ce-CP shows high activity and excellent recyclability toward KC reactions. Moreover, we have found that (1) the activation temperature of Ce-CP plays a critical role in its porosity, exposure of Lewis acidic-basic sites, and thus reactivity; (2) the stronger electron-withdrawing ability of the substituent groups in benzaldehyde derivatives and the smaller size of the reactants lead to the higher yield of product and turnover number (TON) value when the disparity of electron-withdrawing and electron-donating abilities between the substituent groups in benzaldehyde derivatives is not significant. Hence, this work has exploited a new strategy for designing excellent heterogeneous catalysts with constructed active sites of synergistic catalysis capability toward KC reactions.