Preparation of polyoxometalate-based metal-organic cuboctahedron and its selective dye adsorption property

Metal-organic polyhedra (MOPs) have attracted a great deal of chemists attention, attributing to their fascinating structural topologies and potential applications in the fields of sorption/separation, drug delivery, catalysis, and as molecular building blocks to concatenate to extended structures. In the past several decades, many MOPs with various shapes and sizes have been synthesized assembled from different building blocks and functionalized organic ligands. The building blocks formed by metal atoms or metal clusters act as the vertices of the polyhedra, while linear/bent or facial ligands act as the edges or the faces of polyhedra. For the construction of MOPs, the selection of a suitable organic ligand with certain characteristics is crucial, such as advisable angles, flexibility and widespread binding modes. Polyoxovanadates (POVs) as secondary building blocks exhibit a variety of geometries and structures owing to the redox activity of vanadium ions and coordination diversity. VMOP-20 was obtained by the reaction of H₃TATAB and VOSO₄ in N,N-dime- thylformamide (DMF)/CH₃OH (2:1, v:v) at 130°C for 2 d. VMOP-20 crystallized in cubic P-43n space group and exhibited the similar cuboctahedral geometry when the polyoxovanadate [V₆O₆(OCH₃)₉(SO₄)] building units and organic ligands are considered as triangular faces of polyhedra. Each cuboctahedron was surrounded by eight neighboured cuboctahedra via strong C–H···π interactions, leading to a three-dimensional (3D) open supramolecular structure. In addition, Infrared (IR) spectrum, thermogravimetric analysis (TGA), X-ray powder difraction (PXRD) and UV-Vis spectroscopy of VMOP-20 were also investigated in detail. PXRD patterns confirm the phase purity of the sample. UV-Vis spectroscopy displays the absorption ability of VMOP-20 toward ionic dyes. Only cationic dyes can be absorbed into the cavity of VMOP-20 except neutral and anionic dyes, which indicating the cationic dyes absorption process belongs to the ion- exchange. The discovery of VMOP-20 not only adds a representative architecture to the limited family of POV-based metal-organic polyhedra, but also may inspire researchers to synthesize unprecedented MOPs and improve their application.