6s-3d {Ba3Zn4}–Organic Framework as an Effective Heterogeneous Catalyst for Chemical Fixation of CO2 and Knoevenagel Condensation Reaction

The exquisite combination of Ba²⁺ and Zn²⁺ with the aid of 2,4,6-tri(2,4-dicarboxyphenyl)pyridine (H₆TDP) under the condition of solvothermal self-assembly generates one highly robust [Ba₃Zn₄(CO₂)₁₂(HCO₂)₂(OH₂)₂]-organic framework of {[Ba₃Zn₄(TDP)₂(HCO₂)₂(OH₂)₂]·7DMF·4H₂O}_n (NUC-27), in which adjacent 2D layers are interlaced via hydrogen-bonding interactions to form a 3D skeleton with peapod-like channels and nano-caged voids. It is worth emphasizing that both Ba²⁺ and Zn²⁺ ions in NUC-27 display the extremely low coordination modes: hexa-coordinated [Ba(1)] and tetra-coordinated [Ba(2), Zn(1), and Zn(2)]. Furthermore, to the best our knowledge, NUC-27 is one scarcely reported 2D-based nanomaterial with an unprecedented Z-shaped hepta-nuclear heterometallic cluster of [Ba₃Zn₄(CO₂)₁₂(HCO₂)₂(OH₂)₂] as SBUs, which not only has plentiful low-coordinated open metal sites but also has the excellent physicochemical properties including omni-directional opening pores, ultrahigh porosity, larger specific surface area, and the coexistence of Lewis acid-base sites. Just as expected, thanks to its rich active metal sites and pyridine groups as strong Lewis acid-base roles, completely activated NUC-27 displays high catalytic efficiency on the chemical transformation of epoxides with CO₂ into cyclic carbonates under mild conditions and effectively accelerates the reaction process of Knoevenagel condensation.