Xiu-Li Hao

Assembly of new organic–inorganic hybrids based on copper-bis(triazole) complexes and Keggin-type polyoxometalates with different negative charges

Three new organic–inorganic hybrid compounds based on copper-bis(triazole) complexes and Keggin-type polyoxometalates (POMs) with different negative charges, [Cu(I)BBTZ]3[PMo12O40] (1), [Cu(I)BBTZ][Cu(I)(HBBTZ)2][SiMo12O40]·2.5H2O (2) and [Cu(I)BBTZ]6[SiWV2WVI10O40]·2H2O (3) (BBTZ = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene), have been hydrothermally prepared and characterized by elemental analyses, IR spectra, TG analyses, XPS spectra, powder X-ray diffraction and single-crystal X-ray diffraction analyses. Compounds 1 and 2 exhibit the POM-templated 3D host–guest supramolecular frameworks. Compound 3 displays a POM-supported 3D open framework. In these three hybrid compounds, the structural transformation from host–guest framework to open framework were modulated by the introduction of Keggin-type polyoxoanions with different negative charges. Moreover, the electrochemical properties of compounds 1–3 were investigated and all these compounds show the electrocatalytic activities towards the reduction of nitrite.

New entangled coordination networks based on charge-tunable keggin-type polyoxometalates.

Investigation into a hydrothermal reaction system with transition-metal (TM) ions, 1,4-bis(1,2,4-triazol-1-lmethyl)benzene (BBTZ) and various charge-tunable Keggin-type polyoxometalates (POMs) led to the preparation of four new entangled coordination networks, [Co(II) (HBBTZ)(BBTZ)2.5 ][PMo12 O40 ] (1), [Cu(I) (BBTZ)]5 [BW12 O40 ]⋅H2 O (2), [Cu(II) (BBTZ)]3 [AsW(V) 3 W(VI) 9 O40 ]⋅10 H2 O (3), and [Cu(II) 5 (BBTZ)7 (H2 O)6 ][P2 W22 Cu2 O77 (OH)2 ]⋅6 H2 O (4). All compounds were characterized by using elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. The mixed valence of W centers in compound 3 was further confirmed by using XPS spectroscopy and bond-valence sum calculations. In the structural analysis, the entangled networks of 1-4 demonstrate zipper-closing packing, 3D polythreading, 3D polycatenation, and 3D self-penetration, respectively. Moreover, with the enhancement of POM negative charges and the use of different TM types, the number of nodes in the coordination networks of 1-4 increased and the basic metal-organic building motifs changed from a 1D zipper-type chain (in 1) to a 2D pseudorotaxane layer (in 2) to a 3D diamond-like framework (in 3) and finally to a 3D self-penetrating framework (in 4). The photocatalytic properties of compounds 1-4 for the degradation of methylene blue under UV light were also investigated; all compounds showed good catalytic activity and the photocatalytic activity order of Keggin-type species was initially found to be {XMo12 O40 }>{XW12 O40 }>{XW12-n TMn O40 }.

Polyoxometalate-based metal–organic coordination networks for heterogeneous catalytic desulfurization

Three new polyoxometalate (POM)-based metal–organic coordination networks (MOCNs) with chemical formulae of [Co(BBTZ)1.5(HBBTZ)(H2O)2][PW12O40]·H2O (1), [Co2.5(BBTZ)4(H2O)2] [BW12O40]·4H2O (2) and [Cu(BBTZ)2]5[BW12O40]2·4H2O (3) (BBTZ = 1,4-bis-(1,2,4-triazol-1-ylmethyl)benzene) were hydrothermally synthesized in a reaction system containing Keggin-type POMs, transition metal salts (cobalt salts and copper salts) and BBTZ ligands. All compounds were characterized by elemental analyses, IR, powder X-ray diffraction, TG analyses and single-crystal X-ray diffraction analyses. Compound 1 exhibits a POM-encapsulated 3-D supramolecular network, while compounds 2 and 3 display POM-supported 3-D coordination networks. Using the oxidative desulfurization of dibenzothiophene (DBT) as the model, the catalytic activities of compounds 1–3 are investigated. All three compounds show efficient catalytic activity for the oxidation of DBT with the order of 2 > 3 > 1. It is found that the POM species of compounds 1–3 play the main role in the catalytic oxidative desulfurization process, while the TM ions, the loading amounts of POMs, and the structural features of these POM-based MOCNs are also necessary factors that affect the catalytic activities. Furthermore, a surfactant-assisted hydrothermal synthesis method has been developed to prepare nanocrystal 2. SEM reveals that the as-synthesized nanocrystalline 2 is about 245 nm in diameter. The catalytic oxidative desulfurization experiments show that nanocrystal 2 possesses much higher catalytic activities than those of the large single-crystal products of 2.

Isopolymolybdate-induced organic–inorganic hybrid assemblies with copper ions and bichelate-bridging ligands

The heptamolybdate [NH4]6[Mo7O24], as a common polyoxometalate (POM) precursor with dynamic configuration transformation in aqueous solution by changing the pH, has been introduced into the hydrothermal reaction system containing copper ions and bichelate-bridging ligands, leading to the isolation of seven new organic–inorganic hybrid compounds [CuI2Lo2][Mo6O19] (1), [CuII2Lo2][β-Mo8O26]·2H2O (2), [CuII4Lo2(pypz)4][α-Mo8O26]·3H2O (3), [CuII4Lp4(H2O)4][α-Mo8O26][β-Mo8O26]·4H2O (4), [CuI4Lp3][β-Mo8O26]·H2O (5), [CuI4Lp4][α-Mo8O26] (6), and [CuI2Lp2(Emim)2][β-Mo8O26] (7) (bis(3-(2-pyridyl)pyrazole-1-ylmethyl)benzene = bppmb, Lo = 1,2-bppmb, Lp = 1,4-bppmb, pypz = 3-(2-pyridyl)pyrazole, Emim = 1-ethyl-3-methylimidazolium). All compounds were characterized by elemental analysis, IR spectroscopy, TG analysis, powder X-ray diffraction and single-crystal X-ray diffraction analysis. The structural analyses show that the isopolymolybdate clusters act as a type of “molecular adaptor”, which modulate the coordination mode of metal–organic subunits, leading to a series of new metal–organic secondary building units, such as the “Mobius-Ring”-type binuclear unit, “S”-type binuclear unit, butterfly-type unit, helical chain and mesomeric helical chain. As a result, the POM anions and the metal–organic subunits have suitable matching modes to form new hybrid self-assemblies. The photocatalytic properties of compounds 1–7 for the degradation of Rhodamine-B (RhB) under UV irradiation were investigated. All compounds showed good catalytic activity. Furthermore, the luminescent properties of compounds 1–7 have also been explored, indicating an organic ligand-based emission mode for all compounds.

Polyoxometalate-Based Metal–Organic Frameworks for Selective Oxidation of Aryl Alkenes to Aldehydes

Polyoxometalates (POMs) show considerable catalytic performance toward the selective oxidation of alkenes to aldehydes, which is commercially valuable for the production of pharmaceuticals, dyes, perfumes, and fine chemicals. However, the low specific surface area of POMs as heterogeneous catalysts and poor recyclability as homogeneous catalysts have hindered their wide application. Dispersing POMs into metal-organic frameworks (MOFs) for the construction of POM-based MOFs (POMOFs) suggests a promising strategy to realize the homogeneity of heterogeneous catalysis. Herein, we report two new POMOFs with chemical formulas of [Co(BBTZ)2][H3BW12O40]·10H2O (1) and [Co3(H2O)6(BBTZ)4][BW12O40]·NO3·4H2O (2) (BBTZ = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene) for the selective oxidation of alkenes to aldehydes. Compound 1 possesses a non-interpenetrated three-dimensional (3D) cds-type open framework with a 3D channel system. Compound 2 displays a 3D polyrotaxane framework with one-dimensional channels along the [100] direction. In the selective oxidation of styrene into benzaldehyde, compound 1 can achieve a 100% conversion in 4 h with 96% selectivity toward benzaldehyde, which is superior to that of compound 2. A series of control experiments reveal that the co-role of [BW12O40]5- and Co2+ active center as well as a more open framework feature co-promote the catalytic property of the POMOFs in this case. This work may suggest a new option for the development of POMOF catalysts in the selective oxidation of alkenes.