θ -[Mo 8 O 26 ] 4- -based multifunctional hybrid material for high catalytic performance on the cycloaddition of CO 2 , esterification and knoevenagel condensation.

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 -[Mo 8 O 26 ] 4- -based hybrid material of [Cu 2 (BPPP) 2 ]{ θ -[Mo 8 O 26 ]} (NUC-62) with excellent physicochemical stability was self-assembled under acidic solvothermal conditions from Na 2 MoO 4 and CuCl 2 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 -[Mo 8 O 26 ] 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 CO 2 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 H 2 SO 4 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.