Assembly of Six Polyoxometalate-Based Hybrid Compounds from a Simple Supramolecule to a Complicated Pseudorotaxane Framework via Tuning the pH of the Reaction Systems

Three pairs of inorganic–organic hybrid compounds respectively based on [Mo6O19]2–/[Mo8O26]4–, [PW12]3– and [P2W18]6– anions were obtained at different pH values under hydrothermal conditions, namely, [H2bib]­[Mo6O19] (1), CuII(bib)1.5(H2O)­(β-Mo8O26)0.5 (2), [Hbib]2[HPW12O40] (3), [bib0.5]­[CuII(bib)3­CuI(bib)0.5­(PW12O40)] (4), [H2bib]­[CuII(bib)2.5­(H2O)­(H2P2W18O62)]­·2H2O (5), and [CuIbib]­[(CuIbib)2­(H2KP2W18O62)]­·4H2O (6), (bib = 1,4-bis­(1-imidazol-yl)-2,5-dimethylbenzene). Their structures have been determined by single-crystal X-ray diffraction analyses and characterized by infrared spectra (IR), elemental analyses, and powder X-ray diffraction (PXRD) patterns. Compounds 1 and 3 are zero-dimensional (0D) monomers, in which the protonated bib ligands as countercations combine with [Mo6O19]2–/[PW12O40]3– anions by the intermolecular hydrogen bonding interactions forming a supramolecular layer. In 2, the (4, 4) two-dimensional (2D) layer is formed by β-[Mo8O26]4– anions as double-dentate inorganic ligands to link “rail-like” Cu-bib chains. Compound 4 shows a complicated three-dimensional (3D) framework with a (61·102)­(44·62)­(62·81) topology. Compound 5 is a one-dimensional (1D) polypendant chain, and the adjacent chains form a 1D + 1D → 2D interdigitated architecture by hydrogen bonding interactions. Compound 6 is an interesting 3D architecture with a novel pseudorotaxane framework. After careful investigations of these compounds, we noticed that with the increasing pH of the reaction systems, the compounds with more complicated structures were obtained without reference to the kinds of polyoxometalates (POMs). The isolation of these six compounds is very informative for systematically understanding of the effect of the pH on assembly of POM-based hybrids. Additionally, the electrochemical properties of the compounds have been investigated in detail.