Catalytic Investigation of CO₂ Chemical Fixation and the Knoevenagel Condensation Reaction for a Tm^III–Organic Framework

The delicate combination of wave-like [Tm₂(CO₂)₆(OH₂)₂]_n chains and [Tm­(CO₂)₄(NO₃)­(OH₂)] units with the aid of 2,6-bis­(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)­pyridine (H₅BDCP) generates one highly robust dual-channel material of {[(CH₃)₂NH₂]₂­[Tm₃(BDCP)₂­(NO₃)­(OH₂)₃]­·4DMF·2H₂O}_n (NUC-28) with excellent physicochemical properties including solvent-free dual channels, a bigger specific surface area, high porosity, water tolerance, and thermal stability. As far as we know, NUC-28 is one scarcely reported nitrate-functionalized microporous metal–organic framework (MOF) with NO groups as Lewis base sites protruding on the inner surface of the channels. Thanks to the coexistence of Lewis acid–base sites including rich hexa-/hepta-coordinated Tm³⁺ ions, NO and CO groups from μ₁-η¹:η¹ NO^3– and CO^2–, and N_pyridine atoms, NUC-28 displays high catalytic activity in the cycloaddition reaction of epoxides and CO₂ into related cyclic carbonates under mild, solvent-free reaction conditions. In addition, Knoevenagel condensation reactions with aldehydes and malononitrile as substrates could be greatly accelerated in the presence of NUC-28. Hence, these catalytic results confirm that the introduction of Lewis base sites of NO^3– anions on open metal sites in MOFs could effectively enhance its catalytic efficiency, which should be attributed to the recognized synergistic effect of Lewis base–acid active sites.