Three hybrid coordination networks that were constructed from ɛ-Keggin polyoxometalate building units and imidazole-based bridging ligands were prepared under hydrothermal conditions, that is, H[(Hbimb)₂(bimb){Zn₄PMo^V8Mo^VI₄O₄₀}]⋅6 H₂O (1), [Zn(Hbimbp)(bimbp)₃{Zn₄PMo^V8Mo^VI₄O₄₀}]⋅DMF⋅3.5 H₂O (2), and H[Zn₂(timb)₂(bimba)₂Cl₂{Zn₄PMo^V8Mo^VI₄O₄₀}]⋅7 H₂O (3) (bimb=1,4-bis(1-imidazolyl)benzene, bimbp=4,4′-bis(imidazolyl)biphenyl, timb=1,3,5-tris(1-imidazolyl)benzene, bimba=3,5-bis(1-imidazolyl)benzenamine). All three compounds were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, and single-crystal X-ray diffraction. The mixed valence of the Mo centers was analyzed by XPS spectroscopy and bond-valence sum calculations. In all three compounds, the ɛ-Keggin polyoxometalate (POM) units acted as nodes that were connected by rigid imidazole-based bridging ligands to form hybrid coordination networks. In compound 1, 1D zigzag chains extended to form a 3D supramolecular architecture through intermolecular hydrogen-bonding interactions. Compound 2 consisted of 2D curved sheets, whilst compound 3 contained chiral 2D networks. Because of the intrinsic reducing properties of ɛ-Keggin POM species, noble-metal nanoparticles were loaded onto these POM-based coordination networks. Thus, compounds 1–3 were successfully loaded with Ag nanoparticles, and the corresponding composite materials exhibited high catalytic activities for the reduction of 4-nitrophenol.

A new cationic triazole-based metal–organic framework encapsulating Keggin-type polyoxometalates, with the molecular formula [Co(BBPTZ)₃][HPMo₁₂O₄₀]⋅24 H₂O [compound 1; BBPTZ=4,4′-bis(1,2,4-triazol-1-ylmethyl)biphenyl] is hydrothermally synthesized and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. The structure of compound 1 contains a non-interpenetrated 3D CdSO₄ (cds)-type framework with two types of channels that are interconnected with each other; straight channels that are occupied by the Keggin-type POM anions, and wavelike channels that contain lattice water molecules. The catalytic activity of compound 1 in the oxidative desulfurization reaction indicates that it is not only an effective and size-selective heterogeneous catalyst, but it also exhibits distinct structural stability in the catalytic reaction system.

The reaction of Keggin-type polyoxometalate (POM) units, transition-metal (TM) ions, and a rigid bis(imidazole) ligand (1,4-bis(1-imidazolyl)benzene (bimb)) in a hydrothermal environment led to the isolation of four new POM-based metal–organic networks, [H₂L][CuL][SiW₁₂O₄₀]⋅2 H₂O (1), [H₂L]₂[Co(H₂O)₃L][SiW₁₁CoO₃₉]⋅6 H₂O (2), KH[CuL]₂[SiW₁₁CoO₃₉(H₂O)]⋅2 H₂O (3), and [CuL]₄[GeW₁₂O₄₀]⋅H₂O (4; L=bimb). All four compounds were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. Compounds 1 and 3 are new 3D networks with 1D channels. Compounds 2 and 4 contain 2D networks, which further stack into 3D supramolecular networks. The contributions of pH value, the negative charge of the POM, and the TM coordination modes to the construction of 3D networks were elucidated by comparing the synthetic conditions and structures of compounds 1–4. The photocatalytic properties of compounds 1–4 were investigated using methylene blue (MB) degradation under UV light. All compounds showed good catalytic activity and structural stability. The possible catalytic mechanism was discussed on the basis of active-species trapping experiments. The different photocatalytic activities of compounds 1–4 were explained by comparison of the band gaps of different POM species and different packing modes of POM units in these hybrid compounds.

The introduction of an extended bridging bis(triazole) ligand, that is, 4,4′-bis(1,2,4-triazol-1- ylmethyl)biphenyl (BBPTZ), into the hydrothermal reaction system containing transition metal ions and Keggin-type polyoxometalates (POMs) led to the isolation of three new organic–inorganic hybrid entangled coordination networks, [Cu^I₂Cu^II(BBPTZ)₆][SiW₁₂O₄₀]⋅12 H₂O (1), [Ni(BBPTZ)₂(H₂O)][H₂SiW₁₂O₄₀]⋅11 H₂O (2), and [Ni₂(BBPTZ)₄(H₂O)₂][SiW₁₂O₄₀]⋅3 H₂O (3). All three compounds were characterized by elemental analysis, IR spectroscopy, TG analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. Compound 1 contains a 2-D POM-based metal–organic layer entangled with 1-D ladder-like metal–organic chains. The adjacent 2-D networks are parallel to each other, further stacking into a 3-D supramolecular framework with 1-D channels. Compound 2 exhibits a 1-D cantilever-type loop-containing chain. The Keggin-type POMs act as the cantilever groups, leading to the adjacent catilever-type chains interwaving together to form a 3-D supramolecular open framework with two types of channels. Compound 3 possesses a 3-D open framework based on 2-D metal–organic undulated layer and Keggin-type POM clusters. Three sets of such frameworks further interpenetrate with each other to form an interesting three-fold interpenetrating framework. The photocatalytic activities of compounds 1–3 for the decomposition of methylene blue (MB) under UV light have been investigated.