θ-[Mo8O26]4−-based multifunctional hybrid material for high catalytic performance on the cycloaddition of CO2, esterification and knoevenagel condensation

Abstract

Developing materials with excellent properties has become the norm in the field of basic research, prompting us to explore highly robust hybrid materials based on electron-rich POMs and electron-deficient MOFs. Herein, a θ-[Mo₈O₂₆]⁴⁻-based hybrid material of [Cu₂(BPPP)₂]{θ-[Mo₈O₂₆]} (NUC-62) with excellent physicochemical stability was self-assembled under acidic solvothermal conditions from Na₂MoO₄ and CuCl₂ in the presence of a designed chelated ligand of 1,3-bis(3-(2-pyridyl)pyrazol-1-yl)propane (BPPP), which has sufficient coordination sites, spatial self-regulation and great deformation ability. In NUC-62, each of two tetra-coordinated Cu^II ions and two BPPP are unified into one dinuclear unit serving as the cation, which is interactively linked to θ-[Mo₈O₂₆]⁴⁻ anions via rich hydrogen bonds of C–H⋯O. Because of the unsaturated Lewis acidic Cu^II sites, NUC-62 exhibits high catalytic performance on the cycloaddition reactions of CO₂ with epoxides under mild conditions with a high turnover number and turnover frequency. Furthermore, NUC-62, as a recyclable heterogeneous catalyst, shows high catalytic activity for the esterification of aromatic acid under refluxing, which is much better than the inorganic acid catalyst of H₂SO₄ in terms of turnover number and turnover frequency. Moreover, because of open metal sites and rich terminal oxygen atoms, NUC-62 shows high catalytic activity for Knoevenagel condensation reactions of aldehydes and malononitrile. Hence, this study lays the groundwork for constructing heterometallic cluster-based microporous MOFs with excellent Lewis acidic catalysis and chemical stability. Therefore, this study lays a foundation for the construction of functional polyoxometalate complexes.