Inorganic–organic hybrid compounds based on octamolybdates and metal–organic fragments with flexible multidentate ligand: syntheses, structures and characterization

Abstract

Six polyoxometalate-based (POM) hybrid materials based on octamolybdate building blocks and metal–organic fragments with flexible multidentate ligand, namely [Cu^II₂(ttb)₂(β-Mo₈O₂₆)(H₂O)₂]·2H₂O (1), Cu^I₄(ttb)₂(β-Mo₈O₂₆)(H₂O) (2), [Cu^I₄(ttb)₃(β-Mo₈O₂₆)] (3), [Ni₂(ttb)₂(β-Mo₈O₂₆)(H₂O)₆]·2H₂O (4), [Zn₂(ttb)₂(α-Mo₈O₂₆)(H₂O)₂] (5), and [Ag₄(ttb)₂(β-Mo₈O₂₆)] (6), where ttb = 1,3,5-tris(1,2,4-triazol-1-ylmethyl)-2,4,6-trimethyl benzene, have been synthesized under hydrothermal conditions. Their structures have been determined by single-crystal X-ray diffraction analyses and further characterized by elemental analyses, TGA, IR spectra, and electrochemistry. Compounds 1 and 2 exhibit three-dimensional (3D) 2-fold interpenetrating frameworks with (3,6)-connected (4¹·6²)(4²·6¹⁰·10³) and (3,4)-connected (6²·8¹)₂(6²·8⁴) topologies, respectively. We are not aware of any other examples of interpenetrating (3,6)- and (3,4)-connected frameworks which involves the octamolybdates. Compound 3 shows a rare two-dimensional (2D) 2-fold interpenetrating network structure. In compound 4, a 3D supramolecular framework with the channels is constructed by the hydrogen-bonding interactions between (β-Mo₈O₂₆)⁴⁻polyanions and the Ni-ttb double layers, in which the guest (β-Mo₈O₂₆)⁴⁻ anions are included. If Zn⋯O interactions are considered, the structure of 5 is a 3D (3,4)-connected framework with (4·8²)(4·8²·10³) topology. In 6, the ttb ligand as a tetradentate ligand links the Ag atoms to yield a 2D POM-based network. By careful inspection of the structures of 1–6, it can be seen that the ttb ligand, the metal ion and the coordination mode of the octamolybdate anion play important roles in the formation of the POM-based MOFs.