POM-based Compounds Research Articles

Constructing a P2W18O626--Containing Hybrid Photocatalyst via Noncovalent Interactions for Enhanced H2 Production.

Polyoxometalates (POMs) have gained significant research attention because of their excellent properties in photocatalytic (PC) hydrogen production. Exploring POM-based compounds for heterogeneous photocatalysis is an ongoing task. Here, we obtain a water-insoluble inorganic-organic hybrid compound, (P2W18O62)3(C12N3H10)6(C12N3H11)6·9.5H2O (P-PW), formed by Dawson-type POM P2W18O626- (P2W18) anions and protonated 2-(pyridin-4-yl)-1H-benzo[d]imidazole (PHB) cations via noncovalent interactions. In the presence of the sacrificial agent triethanolamine, P-PW exhibits a PC H2 generation rate of 0.418 mmol/g/h, surpassing that of P2W18 and PHB by 15 and 17 times, respectively. This enhancement in PC performance of P-PW can be attributed to its band structure change from the precursor compounds, leading to increased light absorption and therefore more efficient PC hydrogen production.

Three Polyoxometalate-Based Ag-Organic Compounds as Heterogeneous Catalysts for the Synthesis of Benzimidazoles.

Three new POM-based compounds, with formulae [Na0.63Ag3(Htba)2.37(tba)0.63(H2O)2(PMo12O40)]·4H2O (Ag3PMo), [Ag4(Htba)4(H2O)2(PMo12O40)](NO3)·H2O (Ag4PMo), and [Ag3(Htba)2(tba)(PW12O40)0.5](NO3)0.5·13H2O (Ag3PW), were prepared with a 3-(4H-1,2,4-triazol-4-yl)benzoic acid (Htba) ligand, Keggin-type anions ([PMo12O40]3-/[PW12O40]3-), and a silver ion (Ag+). The structural features of these compounds are particularly different from the multinuclear subunits, which are [Ag3(tba)3] clusters in Ag3PMo, [Ag4(tba)3] chains in Ag4PMo, and [Ag3(tba)3]2 clusters in Ag3PW, connected by multidonor atom tba ligands and Ag+ ions. Meanwhile, in these compounds, polyanions act as polydentate ligands to link adjacent Ag-tba metal-organic units and expand their spatial dimensions. These compounds, as heterogeneous catalysts, exhibit high stability and excellent catalytic activity to construct benzimidazoles. Ag3PMo could efficiently catalyze the condensation of benzene-1,2-diamines and benzaldehydes and produce benzimidazoles in good yields. In addition, Ag3PMo could be reused up to 7 times and was suitable for gram-scale reactions.

N-H Bond Activation Catalyzed by an Anderson-Type Polyoxometalate-Based Compound: Key Role of Transition-Metal Heteroatom.

Polyoxometalates (POMs) have a broad array of applied platforms with well-characterized catalysis to achieve N-H bond activation. Herein, the mechanism of the Anderson-type POM-based catalyst [FeIIIMoVI6O18{(OCH2)3CNH2}2]3- ([TrisFeIIIMoVI6O18]3-, Tris = {(OCH2)3CNH2}2) for the N-H bond activation of hydrazine (PhHNNHPh) was investigated by density functional theory calculations. The results reveal that [TrisFeIIIMoVI6O18]3- as the active species is responsible for the continuous abstraction of two electrons and two protons of PhHNNHPh via a proton-coupled electron transfer pathway, resulting in the activation of two N-H bonds in PhHNNHPh and thus the product PhNNPh. H2O2 acts as an oxidant to regulate catalyst regeneration. Based on the proposed catalytic mechanism, the key role of the heteroatom FeIII in [TrisFeIIIMoVI6O18]3- was disclosed. The d-orbital of FeIII in [TrisFeIIIMoVI6O18]3- acts as an electron receptor to promote the electron transfer (ET) in the rate-determining step (RDS) of the catalytic cycle. The substitution of the heteroatom FeIII of [TrisFeIIIMoVI6O18]3- with CoIII, RuIII, or MnIII is expected to improve the catalytic activity for several reasons: (i) the unoccupied molecular orbitals of POM-based compounds containing CoIII or RuIII are low, which is beneficial for the ET of RDS; (ii) For N-H bond activation catalyzed by the MnIII-containing POM-based compound, the transition state of RDS is stable because the d-orbital of its active site is half-filled, which results in a low free-energy barrier.

Four octamolybdate compounds with properties of organic dye adsorption and photocatalytic reduction of Cr(VI).

Hydrothermal synthesis was used to create four different POM-based compounds, namely {[Co(Hptpm)2(β-Mo8O26)0.5(ξ-Mo8O26)0.5]}·H2O (1), [Co(Hptpm)2(δ-Mo8O26)]·H2O (2), [Co(Hptpm)2(β-Mo8O26)] (3) and [Zn(Hptpm)2(β-Mo8O26)] (4) (ptpm = 4-[3-(3-pyridine-2-yl-[1,2,4]triazol-4-yl)-propyl]-morpholine). 1-4 containing different octamolybdate isomers were characterized. Compounds 1-4 showed good electrochemical performance and can be utilized as bifunctional sensors for NO2-, H2O2, Cr(VI) and Fe(III). Taking compound 1 as an example, the detection limits are 0.081 μM for NO2-, 0.072 μM for H2O2, 0.054 μM for Cr(VI) and 0.063 μM for Fe(III), respectively. Compounds 1-4 have good capacitance. Moreover, compounds 1-4 also show good adsorption properties for organic cationic dyes. The cationic dyes include methylene blue (MB), crystal violet (CV) and neutral red (NR). In addition, 1-4 have excellent characteristics that can reduce Cr(VI) to Cr(III) by photocatalytic technology. Within 30 min, the reduction rates were 95.85% for 1, 93.99% for 2, 90.29% for 3 and 88.18% for 4.

POM-based compounds modified by mono- and bis-triazole derivatives: photocatalytic, electrochemical, and supercapacitor properties

Using triazole derivatives 5 POM-based compounds with electrocatalytic and capacitive properties were obtained under hydrothermal conditions. The compounds have good photocatalytic activity for the degradation of organic dyes and the reduction of Cr(vi).

Two POM-based compounds containing Zn-capped Keggin anions as decent heterogeneous catalysts for sulfur oxidation and CO2 cycloaddition reactions.

Carbon dioxide (CO2) and the combustion of sulfide in gasoline are the main causes of air pollution. A great deal of attention has been paid to solving the problem and the catalytic reaction seems to be a decent choice. Due to the high-density of Lewis acidic active sites, polyoxometalates are undoubtedly an ideal choice for the sulfur oxidation reaction. With the reasons foregoing, two novel Zn-capped polyoxometalate-based organic-inorganic hybrids, {[α-PMoV2MoVI10O39(OH)Zn2][bbbm]3}·0.5C2H5OH (1) and TBA2{[ε-PMoV8MoVI4O37(OH)3Zn4][phim]3} (2) ((where bbbm = 1-(4-imidazol-1-ylbutyl) imidazole) and phim = 2-phenylimidazole) were successfully obtained by hydrothermal synthesis. In the two compounds, the N-donor ligands in a monodentate or bidentate coordination mode are directly connected to the Keggin anions by Zn-capped atoms, forming an extended one-dimensional chain. It is noteworthy that compound 2 ends up with an interesting spiral infinite chain possibly thanks to the TBA+ cations residing in gaps as structure-directing agents. Simultaneously, the catalytic properties indicate that compounds 1 and2 as efficient heterogeneous catalysts display a decent catalytic activity in the sulfur removal process. Especially, 2 enabled satisfying catalytic oxidation of dibenzothiophene (DBT) to produce more valuable dibenzothiophene sulfone (DBTO2) at 55 °C, and the conversion almost reached 99%. Besides, compound 2 also shows satisfactory catalytic effectiveness in the oxidation of various epoxides in the CO2 cycloaddition reaction, which suggests that compound 2 has the potential to function as a dual functional material with tremendous prospects in sulfur oxidation and carbon dioxide cycloaddition for the first time.

A series of POM-based compounds by tuning coordination groups and spacers of ligands: electrocatalytic, capacitive and photoelectrocatalytic properties

A series of POM-based compounds can be used as amperometric detectors for NO2− and Cr(vi). They also have capacitive properties and can effect the high-efficiency photoelectrocatalytic degradation of organic dyes.

A Series of Polyoxometalate-Viologen Photochromic Materials for UV Probing, Amine Detecting and Inkless and Erasable Printing.

In this work, by using two kinds of viologen ligands three POM-based Compounds were obtained under hydrothermal conditions, namely [AgI (bmypd)0.5 (β-Mo8 O26 )0.5 ] (1) (bmypd ⋅ 2Cl=1,1'-[Biphenyl-4,4'-bis(methylene)]bis(4,4'-bipyridyinium)dichloride), [AgI 2 (bypy)4 (HSiW12 O40 )2 ] ⋅ 14H2 O (2) and [AgI (bypy)(γ-Mo8 O26 )0.5 ] (3) (bypy⋅Cl=1-Benzyl-4,4'-bipyridyinium chloride). The structures were characterized by Fourier transform infrared spectroscopy (FT-IR), Powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS) and single crystal X-ray diffraction. Compounds 1-3 show excellent photochromic ability with fast photoresponse under the irradiation of ultraviolet light with different degrees of color changes. So compounds 1-3 can be used as visible ultraviolet detectors. Compounds 1-3 also possess photoluminescence properties with fast and excellent fluorescence quenching effect. Compounds 1-3 also can be used as inkless and erasable printing materials with suspensions of 1-3 applied to filter paper. Compounds 1-3 can also produce color changes in amine vapor environment, especially in an NH3 atmosphere. Compounds 1-3 can be used as organic amine detectors.

POM-based compounds as capacitor materials and their photoelectric-sensing properties toward inorganic ions

By using different transition metal ions three isostructural Anderson based compounds were obtained, [M2 II(H3bdpm)2TeMo6O24(H2O)6] (M = Co (1); Cd (2); Zn (3)). Compounds 1-3 can act as photoelectric sensors for detecting inorganic ions. As electrochemical sensors, 1 and 2 can detect nitrite with excellent sensitivity and wide concentration range; 2 exhibits better electrochemical properties than 1, with response time of 2.16 s, sensitivity of 43.10375 μA mM−1 and detection limit of 5.11 × 10−5 M. As fluorescent sensors, 1-3 can detect Cr2O7 2– with low concentration. Furthermore, the capacitor performances of 1-3 were explored, in which 3 shows the highest specific capacitance (409 F g−1 at 1 A g−1 with 80% of cycle efficiency after 1000 cycles).

Five compounds based on [TeMo6O24]6− and [β-Mo8O26]4− anions synthesized by using different symmetrical and asymmetric N-donor ligands

A series of POM-based compounds can be used as amperometric detectors and the compounds based on [β-Mo8O26]4− can not only detect the cation Cr(vi), but also can photocatalytically reduce Cr(vi).

A series of POM-based compounds constructed by piperazine and morpholine derivatives: Characterization, selective photocatalytic and electrochemical/fluorescence sensing properties

Through tunning the N and O coordintion donors in piperazine and morpholine derivatives, six POM-based compounds were obtained, namely [CuII(tep)][HCr(OH)6Mo6O18]·H2O (1), [CoII(tep)2][H4(γ-Mo8O26)] (2), [AgI2(tem)4][H2PW12O40]2·2H2O (3), [M(tem)2][H2(γ-Mo8O26)] (M ​= ​CuII 4, CoII 5, ZnII 6) (tep ​= ​1-(2-(4H-1,2,4-Triazol-4-yl)ethyl)piperazine, tem ​= ​4-(2-4H-1,2,4-Triazol-4-yl)ethyl)morpholine). Compounds 1–6 show 0D, 1D, 2D and 3D structures, respectively. In compound 1, two Anderson type anions hang on either side of the copper dimer [Cu2(tep)2]4+, forming a discrete structure. In compound 2, the N donor in piperazine coordinates with a Co atom, inducing a 3D framework. Compond 3 exhibits a 0D structure containing discrete Keggin anions and binuclear [Ag2(tem)4]2+ clusters. Compounds 4, 5 and 6 are isostructural, showing 3D grid structures with 2D POM-based layers linked by metal-organic lines. Compounds 2, 4, 5 and 6 contain Mo–N bonds to enhance structural stability. Tunning N and O coordintion donors in tep and tem ligands is reasonable to construct different POM-based compounds. Furthermore, the electrochemical and selective photocatalytic properties of the title compounds were also studied. Compound 3 can act as a fluorescence sensor for sensing Hg2+.

Polyoxometalate-Based Compounds for Photo- and Electrocatalytic Applications.

Photo/electrocatalysis of water (H2 O) splitting and CO2 reduction reactions is a promising strategy to alleviate the energy crisis and excessive CO2 emissions. For the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and CO2 reduction reaction (CO2 RR) involved, the development of effective photo/electrocatalysts is critical to reduce the activation energy and accelerate the sluggish dynamics. Polyoxometalate (POM)-based compounds with tunable compositions and diverse structures are emerging as unique photo/electrocatalysts for these reactions as they offer unparalleled advantages such as outstanding solution and redox stability, quasi-semiconductor behaviour, etc. This Minireview provides a basic introduction related to photo/electrocatalytic HER, OER and CO2 RR, followed by the classification of pristine POM-based compounds toward different catalytic reactions. Recent breakthroughs in engineering POM-based compounds as efficient photo/electrocatalysts are highlighted. Finally, the advantages, challenges, strategies and outlooks of POM-based compounds on improving photo/electrocatalytic performance are discussed.

CNTs synthesized with polyoxometalate-based metal–organic compounds as catalyst precursors via the CVD method and their adsorption performance towards organic dyes

CNTs were synthesized with POM-based compounds as catalyst precursors via the CVD method, and they possess a high adsorption capacity towards organic dyes.

Five Keggin-based compounds modified by bis- and mono-[1,3,4]triazole ligands: characterization, and selective photocatalytic and electrochemical properties

By using three kinds of triazole-containing ligands under hydrothermal conditions, five kinds of 0D to 3D POM-based compounds were successfully synthesized. These compounds have good photocatalytic selectivity and electrocatalytic activity.

A series of pH and time dependent POM-based compounds by using a double chelate ligand: Syntheses, structures and electrochemical properties

Three new Polyoxometalate-based compounds, namely [Ag2(dpq)][(Mo3O10)] (1), [(dpq)2(H4SiMo12O40)]·8H2O (2) and [Ag2(dpq)2(H2SiMo12O40)] (3) (dpq = 2,3-Di-pyridin-2-yl-quinoxaline), have been hydrothermally synthesized by using a double chelate ligand and characterized by single-crystal X-ray diffraction analysis, IR spectra and elemental analyses. Compounds 1 and 2 were obtained under the same conditions except for different pH. In compound 1, there are infinite Mo-O chains {Mo3O10}x2x− linked by double chelate dpq ligands to construct a 2D layer. Compound 2 shows a supramolecular layer through hydrogen bonding interactions. When the reactive time was changed from 3d for discrete 2 to 4d, a new compound 3 was obtained. In 3, adjacent Keggin anions are connected by a pair of [Ag(dpq)]+ subunits to form a 1D chain. In addition, we also investigated the electrochemical and photocatalytic properties of compounds 1–3.

Synthesis and Characterization of a Novel POM-Based Compound Contained Bi-Capped Bi Keggin Anion and Organic Ligand for Multifunctional Catalytic Property

A new organic–inorganic hybrid compound contained bi-capped Bi Keggin anion, [biim][PMo 1 V Mo 11 VI Bi1.24O40]·3H2O (1) (biim = biimidazole) has been firstly synthesized and characterized by IR, UV–Vis, XRD, XPS, elemental analyses, fluorescent spectrum and TG analysis. Compound 1 exhibits a novel 2-D supramolecular network structure constructed from {PMo 1 V Mo 11 VI Bi1.24O40} anions by O···O interactions, and further extends a 3-D supramolecular framework by bridges of biimidazole with N–H···O hydrogen bonding interactions. The compound 1 performs well catalytic property for epoxidation of styrene to styrene oxide. Moreover, compound 1 displays excellent photodegradation activity for methyl orange dye.

A series of 0D to 3D Anderson-type polyoxometalate-based compounds obtained under ambient and hydrothermal conditions

Through using flexible ligands, a series of Anderson POM-based compounds have been obtained under ambient and hydrothermal conditions, respectively.

Two new POM-based compounds modified by lanthanide-Schiff base complexes with interesting NLO properties.

Two POM-based lanthanide derivatives, namely {triaqua[2,6-diacetylpyridine bis(semicarbazone)-κ5O,N,N',N'',O']terbium(III)}-μ-oxido-[tricosa-μ2-oxido-dodecaoxido-μ12-phosphato-dodecamolybdenum(VI)] pentahydrate (1), [Tb(C11H15N7O2)(H2O)3][PMo12O40]·5H2O, and the dysprosium(III) analogue (2), have been isolated successfully by the reaction of Keggin-POM [PMo12O40]3- (abbreviated as PMo12), the Ln3+ ion and the Schiff base 2,6-diacetylpyridine bis(semicarbazone) (DAPSC) ligand under hydrothermal conditions. [Ln(DAPSC)(H2O)3][PMo12O40]·5H2O is a PMo12-supported cluster featuring a lanthanide-Schiff base complex [denoted Ln-L(Schiff base)]. Single-crystal X-ray diffraction analysis reveals that the LnIII ion is in a distorted tricapped trigonal prismatic arrangement, coordinated by six O atoms and three N atoms. Two O atoms and three N atoms are provided by one DAPSC ligand, while the additional O atoms originate from a PMo12 cluster and three water molecules. Hydrogen-bonding interactions between adjacent clusters form an interesting three-dimensional supramolecular structure. The identities of 1 and 2 were characterized by IR spectroscopy, thermogravimetric analysis and powder X-ray diffraction. Interestingly, both compounds possess excellent two-photon absorption (TPA) responses to the third-order nonlinear optics (NLO) (2264 GM for 1 and 941 GM for 2), suggesting that they have potential applications in the field of nonlinear optics (NLO). To our knowledge, 1 and 2 are the first POM-based Ln-L(Schiff base) complexes showing excellent two-photon responses. Meanwhile, the electrochemical properties of both compounds were studied in detail.

A series of POM-based compounds constructed by mono- and mixed-ligands synergetically: structures, electrochemical and selective photocatalytic properties

By using pyrazine (pz) and its derivatives 2,3-naphthalenylphenazine (npz) and 2,3-diphenylpyrazine (dpz), one Lindqvist- and six Keggin-based compounds, [AgI2(npz)4(W6O19)]·npz ( 1 ), [AgI2(npz)3(HPW12O40)] ( 2 ), [AgI3(npz)4-(PMo12O40)] ( 3 ), [AgI4(pz)5(SiMo12O40)] ( 4 ), [AgI4(npz)2(pz)(PMoVMo11VIO40)] ( 5 ), [AgI4(npz)4(dpz)2(H2O)2(SiW12O40)] ( 6 ) and {AgI5KI2(npz)4(dpz)2[H(PWVW11VIO40)2]} ( 7 ), have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction analyses. In compound 1 , there is a distorted “I”-type [Ag(npz)2]+ subunit with two npz arranging vertically. Two [Ag(npz)2]+ subunits support the [W6O19]2− anion symmetrically. In compound 2 , there are [Ag2-(npz)3]2+ clusters with one linking and two terminate npz, which are further linked by [PW12O40]3− anions to form a “sunrise”-like chain. In compound 3 , there exist a discrete [Ag2(npz)3]2+ cluster and a mono-supporting anion [AgI(npz)- (HPW12O40)]−. In compound 4 , there are two kinds of cycles in honeycomb-like Ag-pz layer: six-membered cycle I and eight-membered cycle II. An anion dimmer linked by a AgI ion embeds in bigger cycle II. In compound 5 , one pz and two npz are connected by four AgI ions to build a linear [Ag4(npz)2(pz)]4+ subunit. The PMo12 anions link the linear subunits alternately and a 1D chain is formed. In compound 6 , there exists discrete [Ag2(npz)2(dpz)(H2O)]2+ subunit and Keggin anion. In compound 7 , the Keggin-K inorganic chains are connected by AgI-npz-dpz subunits to form a 2D layer. The adjacent layers are further linked by anions to build a 3D framework. Moreover, we also have investigated the electrochemical and photocatalytic properties of 1 -- 7 .

A series of pH-dependent POM-based compounds as photocatalysts and electrochemical sensors

By tuning pH, a Zn complex and five POM-based compounds decorated by a pyrazole derivative 1,1′-bis(3,5-dimethyl-1H-pyrazol-4-yl)methane (H2bdpm), [Ag4(H2bdpm)4(SiW12O40)]·4H2O (1), [Ag3(H2bdpm)3(PMo12O40)] (2), [Cu(H2bdpm)2(H2O)2(HPMo12O40)] (3), [Zn(H2bdpm)2(β-Mo8O26)0.5]·3H2O (4), [(ZnO)2(H2O)2(H2bdpm)] (5) and [(γ-Mo8O26)(H2bdpm)2]·2H4bdpm·4H2O (6), have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction analysis, IR spectra and elemental analyses. In compound 1, the infinite wave-like metal–organic chains are linked by POM anions to form a 3D framework. In the compound 2, the H2bdpm ligands are linked by Ag+ ions to construct a chain with [PMo12O40]3− anions suspending up and down this chain by hydrogen bonding interactions. In compound 3, there is a mono-supporting [PMo12O40]3− anion with one [Cu(H2bdpm)2(H2O)2]2+ subunit. By tuning pH of [Mo7O24]6−/Zn2+/H2bdpm system with three values, three distinct structures 4–6 were constructed. In compound 4, two Zn ions and two H2bdpm form a bi-nuclear [Zn2(H2bdpm)2]4+ cycle. Adjacent cycles connect each other vertically to build a cycle-connecting-cycle chain. The discrete [β-Mo8O26]4− anions exist hydrogen bonding interactions with metal–organic chains. In compound 5, the cycle-like [(ZnO)4(H2O)4] subunits are linked by a pair of H2bdpm alternately to form a 1D chain. Compound 6 contains only a [γ-Mo8O26]4− anion functionalized by two H2bdpm through Mo-N bonds. The pH factor is essential for construction of this series. The 1-, 3- and 4-CPEs not only possess excellent multifunctional electrocatalytic activities but also can be used as electrochemical sensors for the detection of hydrogen peroxide, bromate and nitrite. Photocatalytic investigation indicates that compounds 1–4 and 6 are active for the degradation of methylene blue under UV irradiation.