Hong-Ying Zang

Construction and property investigation of transition-metal complexes modified octamolybdate hybrid materials based on V-shaped organic ligands

Six new transition-metal complexes modified octamolybdate compounds have been successfully synthesized with two kinds of V-shaped flexible organic ligands under hydrothermal conditions, namely, [CuII(HL1)2(H2O)2(Mo8O26)] (1), [CuII(L1)2(Mo8O26)0.5] (2), [CuII(HL2)2(Mo8O26)]·2H2O (3), [CuII2(L2)4(Mo8O26)]·3H2O (4), [CuI3(L2)2(Mo8O26)0.5Cl] (5), [CuI4(L2)4(Mo8O26)] (6) where L1 = 3-((1H-imidazol-1-yl)methyl)pyridine, L2 = 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine. Their crystal structures have been determined by X-ray single-crystal diffraction, elemental analyses, IR spectra, and thermogravimetric analyses (TGA). With L1 ligand, two compounds were obtained: compound 1 is a 1D β-octamolybdate based chain and compound 2 is a 2D 4-connected network most strikingly, whose octamolybdate anion shows a θ-isomer. With L2 ligand, compounds 3, 4, 5 and 6 were obtained, respectively. In compound 3, 1D β-octamolybdate-based metal–organic chains are further extended by hydrogen bonds to build a 3D supramolecular structure, while in compound 4, every two adjacent double-helical metal–organic chains are bridged by β-octamolybdate anions to generate a 2D 3-connected network. When CuSO4·5H2O was substituted by CuCl, two CuI compounds were attained. Interestingly, in compound 5 the octamolybdate anion threads through the metal–organic loop presenting a fascinating 2D polythreading topology, and in compound 6 the octamolybdate anions pillar the railroad-like metal–organic strands to generate a 2D 3-connected network. The structural differences among compounds 1–6 indicate the importance of the different coordination mode of organic ligands and various polyoxoanions, and of the pH values of the systems for the framework formation. Furthermore, the semiconducting properties of compounds 1–5 were investigated, and photoluminescence as well as photoconductivity for compound 5 were also studied. Compound 5 has a comparatively lower band gap and is photoconductive in the UV-vis range, so it can be potentially applied as a semiconductor of photoelectronic detectors.

N-Doped graphene-coated molybdenum carbide nanoparticles as highly efficient electrocatalysts for the hydrogen evolution reaction

In our efforts to explore promising substitutes for Pt-based electrocatalysts for the hydrogen evolution reaction (HER), a new type of molybdenum carbide nanoparticle coated with graphene shells with nitrogen dopants (abbr. MoCx@C-1) is prepared from an entangled polyoxometalate-encapsulated coordination polymer (PECP), [Zn(bimbp)2]3[PMo12O40]2·2H2O (PECP-1) (bimbp = 4,4′-bis(imidazolyl)biphenyl) via the annealing and etching processes. The synergistic effects between highly dispersive MoCx particles, graphene coatings and N dopants in MoCx@C-1 lead to remarkable HER performance in acidic media with a very positive onset potential close to that of commercial 20% Pt/C catalysts, a low Tafel slope of 56 mV dec−1, a high exchange current density of 0.27 mA cm−2, and superior long-term cycle stability. In particular, MoCx@C-1 exhibiting an overpotential of 79 mV at a current density of 10 mA cm−2 represents one of the currently best reported MoCx-based HER electrocatalysts in acidic media. Such performance is also better than that of uncoated MoCx-2 nanoparticles prepared by carburizing another PECP [Bu4N][Zn3(bimb)4Cl(MoO4)][PMoVMoVI11O40]·4H2O (PECP-2) (bimb = 1,4-bis(1-imidazolyl)benzene). This work provides a new feasible route to prepare nanostructured hybrids composed of transition metal carbides, graphene and N dopants with higher HER activity and stability.

Two eight-connected self-penetrating porous metal–organic frameworks: configurational isomers caused by different linking modes between terephthalate and binuclear nickel building units

Two novel nanoporous metal–organic frameworks (MOFs) based on the same binuclear nickel secondary building units, {[Ni2(H2O)(BDC)2(BIMB)2]·6DMF}n (1) and {[Ni2(H2O)(BDC)2(BIMB)2]·5DMAc}n (2) (H2BDC = terephthalic acid, BIMB = 1,4-bis(1-imidazolyl)benzene and DMF = N,N′-dimethylformamide, DMAc = N,N′-dimethylacetamide) have been synthesized by self-assembly of Ni(NO3)2·6H2O, BIMB and H2BDC in different solvents (DMF and DMAc), respectively. Different linking modes between terephthalate and binuclear nickel building units in 1 and 2 lead to an interesting phenomenon that their host frameworks are isomeric to each other and have the same three periodic eight-connected self-penetrating nets with novel 420.68 topology. Powder X-ray diffraction (PXRD) results indicate that 1 and 2 both have reversible framework transformations in an acetone/guest solvent molecule system.

N-Carbon coated P-W2C composite as efficient electrocatalyst for hydrogen evolution reactions over the whole pH range

Development of a highly efficient and stable hydrogen evolution reaction (HER) electrocatalyst based on non-precious-metal remains a great challenge. Herein, we report P-modified tungsten carbide encapsulated in N-doped carbon (P-W2C@NC) as an efficient and stable HER electrocatalyst over the whole pH range (0–14), prepared by annealing polyoxometalate (H3PW12O40·xH2O) and dicyandiamide at a high temperature. To achieve a current density of 10 mA cm−2, P-W2C@NC required an overpotential of 89 mV in acid, 63 mV in alkaline solution and 185 mV in neutral solution. This is the first case of a tungsten-carbide-based HER electrocatalyst operating at all pH values with high activity and stability, and might provide some new routes for the exploration of tungsten-based Pt-like electrocatalysts for the HER.

Assembly of organic–inorganic hybrid materials constructed from polyoxometalate and metal–1,2,4-triazole units: synthesis, structures, magnetic, electrochemical and photocatalytic properties

A series of organic–inorganic hybrid compounds built from Keggin and Wells–Dawson polyoxometalates and TM-trz complexes (Htrz = 1,2,4-triazole) were obtained at different pH values under hydrothermal conditions, namely, [Co2(H2O)2(Htrz)5][SiW12O40]·2.5H2O (1), [Zn5(Htrz)8(trz)2(H2O)8][SiW12O40]2·10H2O (2), [CuI2CuII4(Htrz)4(trz)4Cl2][GeW12O40]·15H2O (3), [CuI2CuII4(Htrz)4(trz)4Cl2][SiW12O40]·15H2O (4), K2Na2(H2O)2[Co11(trz)14(H2O)14][P2W18O62]2·29H2O (5) and [Cu12(trz)10(Htrz)2(OH)4(SO4)2(H2O)6][P2W18O62]·28H2O (6). All of the compounds were characterized using single-crystal X-ray diffraction, TG analysis and powder X-ray diffraction. Compound 1 contains a kind of bimetallic–organic segment [Co2(Htrz)5]4+, which could be regarded as a quasi-sinusoid-like chain. 2 is a dimer constructed from two [SiW12O40]4− units linked by a [Zn5(trz)2(Htrz)8]8+ motif. In 3 and 4, the 2D wall-like layers constructed by the Cu(II) ions and Htrz ligands are extended into 3D frameworks by polyanions. 5 shows a complicated three-dimensional (3D) framework with a {412·612·84}{46}2 topology. Compound 6 is also a complicated 3D structure based on a triangular trinuclear Cu-trz unit and [P2W18O62] polyanions. To the best of our knowledge, 6 represents the largest copper cluster linked by amines in a POM system. Structural diversities reveal that the pH value of the reaction system plays a crucial role in the assembly of POM-based hybrids. Moreover, the optical band gaps, electrochemical properties, magnetic properties and the photocatalytic properties of the compounds have been investigated.

MoP/Mo2C@C: A New Combination of Electrocatalysts for Highly Efficient Hydrogen Evolution over the Entire pH Range

During the exploration of highly efficient noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER), a promising and challenging strategy is to fabricate composite nanocatalysts by finely tuning metal and/or nonmetal element components. Herein, we report a new HER electrocatalyst, which is composed of molybdenum phosphide and molybdenum carbide composite nanoparticles (NPs) coated by few-layer N-doped graphitic carbon shells (denoted as MoP/Mo2C@C). Such a new combination mode of electrocatalysts is realized by a one-step annealing route with the mixture of a Mo/P-based polyoxometalate (POM) and dicyandiamide. On the basis of this method, the simultaneous phosphorization and carbonization in a nanoscale confined space can be easily achieved by the use of POM as the molecular-element-regulating platform. MoP/Mo2C@C exhibits more remarkable HER performance over the whole pH range than those of MoP, Mo2C, and the physical mixture of MoP and Mo2C. The low overpotentials of 89, 136, and 75 mV were obtained at a current density of 10 mA cm-2 in the media of pH = 0, 7, and 14, respectively. Furthermore, MoP/Mo2C@C shows a long-term durability for 14 h over the entire pH range (0-14). Because of the protection of carbon shells, such composite electrocatalyst also possesses better transition-metal tolerance exemplified by Fe2+, Co2+, and Ni2+ than that of 20% commercial Pt/C. This work demonstrates the advantage of POM precursors in adjusting the component and properties of nanoscale composite electrocatalysts for HER, which may suggest new options for the fabrication of highly efficient composite electrocatalysts.

Synthesis and characterization of two {Mo6}-based/templated metal–organic frameworks

Here, we report two novel POM-based MOFs: a nano-caged hybrid compound [Ni2(BIMB)2(MoVI4MoV2O19)] (1), whose cages are based on [Mo6O19]4− units and a [Mo6O19]2−-templated framework [Zn4(BIMB)4(PO4)2(Mo6O19)]·2H2O (2) (BIMB = 1,4-bis(1-imidazolyl)benzene). According to topological analysis, the first is a 3D (4,6)-connected self-penetrating framework with polycatenanes motif. In contrast, the second metal–organic framework has no interpenetration for the templated [Mo6O19]2− occupying the cavity. The UV-vis absorption spectrum of 1 was investigated to prove the existence of MoV atoms and the typical peak at 577 nm can be attributed to MoV → MoVI charge transfer. Furthermore, the diffuse reflectivity spectrum shows that its band gap can be assessed as 3.06 eV, which can be regarded as a wide gap semiconductor. Additionally, cyclic voltammogram of compound 1 shows two redox peaks in the range of 0–800 mV in H2SO4 aqueous solution.

Inorganic-organic hybrid compounds based on the co-existence of different isomers or forms of polymolybdate†

Three novel compounds co-existing isomers or forms of polymolybdate, namely, {[Ni(γ-Mo8O26)(H2L1)2(H2O)2](γ-Mo8O26)(H2L1)}·2H2O (1), {[Co(γ-Mo8O26)(H2L1)2(H2O)2](β-Mo8O26)[Co(H2O)6]}·6H2O (2) and [CuI6L24(β-Mo8O26)(Mo6O19)] (3), where L1 = 1,1′-bis(pyridin-3-ylmethyl)-2,2′-biimidazole, L2 = 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine, have been successfully synthesized. Crystal structure analysis reveals that 1 is a 1D + 1D supramolecular structure containing both γ-[Mo8O26N2] and γ-[Mo8O28] units; 2 has γ-[Mo8O26]4− as well as β-[Mo8O26]4− anions and 3 is a (3,4,6)-connected 2D network containing both β-[Mo8O26]4− and [Mo6O19]2− anions. According to quantum chemical calculations, the transformation from β-[Mo8O26]4− to [Mo6O19]2− is spontaneous in aqueous solution. Furthermore, we have investigated the optical band gap of compounds 1 and 3, with 3 showing a lower band gap (1.69 eV). Additionally, the photoluminescent property of 3 is also studied, which can be assigned to metal-to-ligand charge transfer (MLCT).

Anion-directed genuine meso-helical supramolecular isomers of two 1D Ag(I) complexes based on arene-linked bis(pyrazolyl)methane ligands

Two genuine supramolecular isomers (Lp = p-[CH(pz)2]2C6H4), have been successfully obtained under similar solvothermal conditions by the reaction of arene-linked p-bis[bis(1-pyrazolyl)methyl]benzene ligand with AgNO3. Both of the compounds are made of unusual meso-helical chains and present a three-dimensional (3D) supramolecular structure. Compounds 1 and 2 were characterized by elemental analyses, IR spectra, single-crystal X-ray diffraction and thermogravimetric (TG) analyses. The fluorescence property of them has also been studied.

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.