Polyoxometalate-encapsulated metal-organic frameworks with diverse cages for the C–H bond oxidation of alkylbenzenes

Exploring new heterogeneous catalysts to achieve efficient C–H bond oxidation is momentous in industrial chemical production. Herein, three Fe-incorporated polyoxometalate-encapsulated metal-organic frameworks ([email protected]), [Fe(H2O)3(dtb)][Fe(dtb)2][HBW12O40]·12H2O (1), [Fe(H2O)2(dtb)]2[Fe(dtb)2(Hdtb)][SiWVI9WV3O40]·16H2O (2), [Fe(H2O)2]4(dtb)5[PMoVI11MoVO40]2·18H2O (3), (dtb ​= ​1,4-di[4H-1,2,4-triazol-4-yl]-benzene), were hydrothermally synthesized for catalytic C–H bond oxidation. They exhibited three-dimensional (3-D) POM-based metal-organic host-guest frameworks with diverse cages. In compound 1, cationic metal-organic frameworks constructed by binuclear [Fe2(dtb)6(H2O)2]4+ clusters and dtb ligands present a pcu alpha-Po primitive cubic topological net and abundant twisted quadrangular prism-shaped cages, in which the monoprotonated [HBW12O40]4– polyoxoanions are encapsulated. In compound 2, two-dimensional (2-D) cationic metal-organic layers constructed by trinuclear [Fe3(dtb)6(H2O)4]6+ clusters and dtb ligands display a sql/Shubnikov tetragonal plane net topology, in which the rhomboid-shaped metal-organic windows in adjacent parallel-arranged 2-D layers are enclosed to form an open parallelepiped cage. The [SiWVI9WV3O40]7– polyoxoanions locate in the parallelepiped cages between 2-D bilayers. In compound 3, the 3-D cationic metal-organic frameworks constructed by binuclear [Fe2(dtb)3(H2O)4]4+ clusters and dtb ligands possess a bnn hexagonal BN topology, in which the hexagonal prismatic metal-organic cages accommodate four [PMoVI11MoVO40]4– polyoxoanions. Employing diphenylmethane (DPM) oxidation as C–H bond oxidation model reaction, compounds 1–3 displayed distinct catalytic activities owing to the synergistic effect of polynuclear Fe nodes and polyoxoanions. Among them, [PMoVI11MoVO40]4–-contained compound 3 exhibited higher catalytic activity than polyoxotungstate-based compounds 1–2 with 99% DPM conversion and 99% benzophenone (BP) selectivity within 6 ​h as well as good recyclability and structural stability.