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.