Two 2D Layered P4Mo6 Clusters with Potential Bifunctional Properties: Proton Conduction and CO2 Photoreduction

Efficient reduction of CO2 and inhibition of hydrogen precipitation by polyoxometalate photocatalyst modified with the metal Mn

Photocatalytic reduction of CO2 to chemical fuels is attractive for solving both the greenhouse effect and the energy crisis, but the key challenge is to design and synthesize photocatalysts with remarkable performance under visible light irradiation. Efficient catalytic carbon dioxide reduction (CO2RR) with light is considered a promising sustainable and clean approach to solve environmental problems. Herein, we found a new photocatalyst ([Mn(en)2]6[V12B18O54(OH)6]) (abbreviated as Mn6V12) based on the modifiability of polyoxometalates, in which Mn acts as a modifying unit to efficiently reduce CO2 to CO and effectively inhibit the hydrogen precipitation reaction. This Mn modified polyoxometalate catalyst has a maximum CO generation rate of 4625.0 μmol g-1 h-1 and a maximum H2 generation rate of 499.6 μmol g-1 h-1, with a selectivity of 90.3% for CO generation and 9.7% for H2 generation. This polyoxometalate photocatalyst can effectively reduce CO and inhibit the hydrogen precipitation reaction. It provides a new idea for the efficient photocatalytic carbon dioxide reduction (CO2RR) with polyoxometalate catalysts.

Two {CuI[P4Mo6]2}-Based Coordination Polymers Incorporating In Situ Converted Tetrapyridyl Ligands for Trace Analysis of Nitrofuran Antibiotics

Nitrofurans are important synthetic broad-spectrum antibacterial drugs with the basic structure of 5-nitrofuran. Due to their toxicity, it is essential to develop a sensitive sensor with strong anti-interference capabilities for their detection. In this work, two {P4Mo6O31}12--based compounds, [H4(HPTTP)]2{CuI[Mo12O24(OH)6(PO4)3(HPO4)(H2PO4)4]}·xH2O (x = 13 for (1), 7 for (2); HPTTP = 4,4',4″,4‴-(1H-pyrrole-2,3,4,5-tetrayl)tetrapyridine), exhibiting similar coordination but distinct stacking modes. Both compounds were synthesized and used for the electrochemical detection of nitrofuran antibiotics. The tetrapyridine-based ligand was generated in situ during assembly, and its potential mechanism was discussed. Composite electrode materials, formed by mixing graphite powder with compounds 1-2 and physically grinding them, proved to be highly effective in the electrochemical trace detection of furazolidone (FZD) and furaltadone hydrochloride (FTD·HCl) under optimal conditions. Besides, the possible electrochemical detection mechanisms of two nitro-antibiotics were studied.

Bridging Component Strategy in Phosphomolybdate-Based Sensors for Electrochemical Determination of Trace Cr(VI)

Rapid and sensitive electrochemical determination of trace carcinogenic Cr(VI) pollutants remains an urgent and important task, which requires the development of active sensing materials. Herein, four cases of reduced phosphomolybdates with formulas of the (H2bib)3[Zn(H2PO4)]2{Mn[P4Mo6O31H7]2}·6H2O (1), (H2bib)2[Na(H2O)]2[Mn(H2O)]2{Mn[P4Mo6O31H6]2}·5H2O (2), (H2bib)3[Mo2(μ2-O)2(H2O)4]2{Ni[P4Mo6O31H2]2}·4H2O (3), and (H2bib)2{Ni[P4Mo6O31H9]2}·9H2O (4) (bib = 4,4'-bis(1-imidazolyl)-biphenyl) were hydrothermally synthesized under the guidance of a bridging component strategy, which function as effective electrochemical sensors to detect trace Cr(VI). The difference of hybrids 1-4 is in the inorganic moiety, in which the reduced phosphomolybdates {M[P4MoV6O31]2} (M{P4Mo6}2) exhibited different arrangements bridged by different cationic components ({Zn(H2PO4)} subunit for 1, [Mn2(H2O)2]4+ dimer for 2, and [MoV2(μ2-O)2(H2O)4]6+ for 3). As a result, hybrids 1 and 3 display noticeable Cr(VI) detection activity with low detection limits of 14.3 nM (1.48 ppb) for 1 and 6.61 nM (0.69 ppb) for 3 and high sensitivities of 97.3 and 95.3 μA·mM-1, respectively, which are much beyond the World Health Organization's detection threshold (0.05 ppm) and superior to those of the contrast samples (inorganic Mn{P4Mo6}2 salt and hybrid 4), even the most reported noble-metal catalysts. This work supplies a prospective pathway to build effective electrochemical sensors based on phosphomolybdates for environmental pollutant treatment.

"Tree"-like Multidentate Ligand-Assisted Synthesis of Polymolybdate-Based Architectures with Multinuclear Metal Clusters: Supercapacitor and Electrochemical Sensing Performances

In this work, aiming for constructing multinuclear metal cluster-modified polymolybdate-based architectures with novel conformation, the "tree"-like multidentate ligand 5-(3-pyridyl)-1H-tetrazole) (3-ptzH) is introduced into the polymolybdate reaction system. Three new polymolybdate-based architectures with various multinuclear metal clusters, H₄[Cu₆(μ₃-OH)₂(3-ptz)₆(γ-Mo₈O₂₈) (H₂O)₂]·2H₂O (BOHU-1), H₂[Ag₄(3-ptz)₂(Mo₈O₂₆)] (BOHU-2), and H₄[Cu₅(3-ptzH)₂(3-ptz)₂(MnMo₉O₃₂)₂(H₂O)₄] (BOHU-3) (BOHU = Bohai University), have been prepared via the hydrothermal method and structurally characterized. In BOHU-1, a kind of pentanuclear copper cluster unit: [Cu₅(μ₃-OH)₂(3-ptz)₆]²⁺ is formed, which connects to construct a one-dimensional (1D) cluster-based chain. The 1D chains are extended to a two-dimensional (2D) layer via the Cu ions, which are further linked by the 4-connected [γ-Mo₈O₂₈]^8- anions to build a three-dimensional (3D) framework. In BOHU-2, when a Ag^I ion was used as the central metal, the 3-ptz adopts different coordination modes to link the Ag ions, forming hexanuclear [Ag₆(3-ptz)₄]²⁺ cluster and finally 1D chains. These 1D cluster-based chains are connected by the 6-connected [γ-Mo₈O₂₆]^4- anions to establish a 2D layer, which is further extended by [Mo₈O₂₆]_n^4n- 1D chains to a 3D framework. For BOHU-3, the chiral [MnMo₉O₃₂]^6- anions are introduced and coordinated with the Cu ions to build left- and right-handed 1D chains, which are connected via the [Cu₃(3-ptz)₄]²⁺ cluster to form a 1D ladder-like chain. The effects of 3-ptz on the formation of multinuclear clusters, as well as the metals and polymolybdates on the multinuclear clusters and final structures of BOHU-1∼3, are discussed. The electrochemical performances of BOHU-1∼3 as electrode materials for supercapacitors and electrochemical sensors are investigated.

Construction of Water-Stable Rare-Earth Organic Frameworks with Ambient High Proton Conductivity Based on Zirconium Sandwiched Heteropolytungstate

Water-stable proton-conducting materials owning excellent performances at ambient temperatures are currently one of the crucial challenges. Herein, four water-stable three-dimensional polyoxometalate-based rare-earth organic frameworks have been successfully synthesized and formulated as H{Ln₄(L)₂(H₂O)₂₁[Zr₃(OH)₃(PW₉O₃₄)₂]}·15H₂O (1-3) (Ln = La (1), Ce (2), Pr (3); L = 3,5-pyridine dicarboxylic acid), which are the first examples of MOFs constructed by a zirconium sandwiched polyoxoanion. There are abundant coordinated water molecules functionalizing the Pr^III centers, and simultaneously, plenty of lattice water molecules are fitted into the channel of the framework. A continuous H-bonding network is found between the architectures and plays an important role in stabilizing the structure. Benefiting from the consecutive H-bonding networks, compounds 1-3 showed high proton conductivities at ambient temperature (up to 1.05 × 10^-3 S·cm^-1 under 98% RH) by a synergistic effect of the combined components.

Two 1D Anderson-Type Polyoxometalate-Based Metal-Organic Complexes as Bifunctional Heterogeneous Catalysts for CO2 Photoreduction and Sulfur Oxidation

Sulfur oxides from the combustion of petrol and excessive emissions of carbon dioxide (CO₂) are currently the main causes of environmental pollution. Considerable interest has been paid to solving the challenge, and catalytic reactions seem to be the desired choice. Due to the high density of Lewis acid active sites, polyoxometalates are considered to be the ideal choice for these catalytic reactions. Herein, two captivating polyoxometalate-based metal-organic complexes, formulated as [Co(H₂O)₂DABT]₂[CrMo₆(OH)₅O₁₉] ({Co-CrMo₆}) and [Zn(H₂O)₂DABT]₂[CrMo₆(OH)₅O₁₉] ({Zn-CrMo₆}) (DABT = 3,3'-diamino-5,5'-bis(1H-1,2,4-triazole)) were successfully obtained under hydrothermal conditions. The structural analysis demonstrates that {Co-CrMo₆} and {Zn-CrMo₆} are isostructural with two different transition metal (Co/Zn) ions based on quadridentate Anderson-type [CrMo₆(OH)₅O₁₉]^4- polyanions. A fan-shaped unit of {Co-CrMo₆}/{Zn-CrMo₆} is linked to generate a one-dimensional (1D) ladder-like structure. Intriguingly, benefitting from rich Co centers with a suitable energy band structure, {Co-CrMo₆} displays better photocatalytic activity than {Zn-CrMo₆} for converting CO₂ into CO, endowing the CO formation of 1935.3 μmol g^-1 h^-1 with high selectivity. Meanwhile, {Co-CrMo₆} also exhibits a satisfactory removal rate of 99% for oxidizing dibenzothiophene at 50 °C, which suggests that {Co-CrMo₆} may be utilized as a potential dual functional material with immense prospects in photocatalytic CO₂ reduction and sulfur oxidation for the first time.

An isolated doughnut-like molybdenum(V) cobalto-phosphate cluster exhibiting excellent photocatalytic performance for carbon dioxide conversion

An isolated doughnut-like molybdenum(V) cobalto-phosphate cluster with the formula (C₁₁NH₁₀)₂{[Co(H₂O)₆]@[H₂₉Co₁₆Mo₁₆(H₂O)₁₆(PO₄)₂₄O₃₆]}(H₂PO₄)·25H₂O has been successfully synthesized by a hydrothermal method. Single crystal X ray diffraction analysis shows that four {Co₄O₆₀} tetramers and eight {Mo₂O₁₀} dimers are linked by oxygen atoms and phosphate groups to construct a doughnut-type structure for [Co@{Co₁₆Mo₁₆}], in which one [Co^II(H₂O)₆]²⁺ octahedron is enclosed. More importantly, [Co@{Co₁₆Mo₁₆}] exhibits promising photocatalytic performance for CO₂ reduction with the CO formation rate of 6764.3 μmol g^-1 h^-1 and the selectivity of 96.89%. In addition, the cycling test indicated that [Co@{Co₁₆Mo₁₆}] can be reused for at least four cycles without significant loss of catalytic activity. The result of this work may provide new insight for the synthesis of highly efficient POM-based photocatalysts for CO₂ reduction.

Ligand Induced Double-Chair Conformation Ln12 Nanoclusters Showing Multifunctional Magnetic and Proton Conductive Properties

Many methods have been utilized to adjust the size of superatomic metal nanoclusters, while tuning the geometric conformations of specific nanoclusters is rare. Here, we demonstrate that conformation variation can be realized by slightly modifying the ligand under maintaining the nuclei number of metal atoms. A series of novel "double-chair" conformation Ln₁₂ (Ln = Sm (1), Eu (2), Gd (3), Tb (4), and Dy (5)) clusters were generated by replacing 3-formylsalicylic acid with 2,3-dihydroxybenzoic acid in the Ln₁₂ nanocluster. Intriguingly, Dy₁₂ displays slow magnetic relaxation at low temperatures, while Gd₁₂ shows a large magnetocaloric effect (MCE) of 35.63 J kg^-1 K^-1 at 2 K for ΔH = 7 T. Additionally, the introduction of numerous coordination water molecules in these clusters enables Dy₁₂ and Gd₁₂ with high proton conductivity, namely, 2.13 × 10^-4 and 3.62 × 10^-4 S cm^-1 under 358 K and 95% RH humidity conditions.

Two polymolybdate-directed Zn(ii) complexes tuned by a new bis-pyridine-bis-amide ligand with a diphenylketone spacer for efficient ampere sensing and dye adsorption

Two polymolybdate-based Zn(ii) complexes were constructed from a new bis-pyridine-bis-amide ligand, which can be used as electrocatalysts and electrochemical sensors for Cr(vi), KBrO3, H2O2 and AA, and exhibit selective adsorption of CV and MB.

Efficient visible-light-driven reduction of hexavalent chromium catalyzed by conjugated organic species modified hourglass-type phosphomolybdate hybrids

Four conjugated-organic-species modified hourglass-type phosphomolybdate hybrids with a 0-D + 1-D → 3-D supramolecular structure exhibited favorable photocatalytic activity and stability towards Cr(vi) reduction.

A Purely Inorganic Quasi-Keggin Polyoxometalate for Photocatalytic Conversion of Carbon Dioxide to Carbon Monoxide

A pure inorganic cluster, H₄₇ Na₂ Co₄ Mo₂₄ (PO₄ )₁₁ O₇₂  ⋅ 15H₂ O (denoted as {Co₄ Mo₂₄ }), has been successfully synthesized by hydrothermal method. Notably, the assembly of a central {Co₂ PO₄ } tetrahedron and four peripheral {Co[P₄ Mo₆ ]} fragments gives rise to a rare "quasi-Keggin" structure of {Co₄ Mo₂₄ }, in which Co linkers continue to bridge adjacent substructures, resulting in the generation of 3D framework with large cavities. Benefitting from the combination of strong reductive {P₄ Mo₆ } units and Co active centers, the photocatalytic system with {Co₄ Mo₂₄ } as heterogeneous catalyst exhibits excellent activity for CO₂ conversion to CO, offering the CO formation rate of 1848.3 μmol g^-1 h^-1 with high selectivity of 97.0 %. Besides, thermogravimetric and X-ray diffraction analysis confirm that {Co₄ Mo₂₄ } can maintain stable during the photocatalytic reaction process.

Two three-dimensional polyanionic clusters [M(P4Mo6)2] (M = Co, Zn) exhibiting excellent photocatalytic CO2 reduction performance

Two captivating {P4Mo6}-based compounds, formulated as (H2bbi)2{[Co2(bbi)][Co2.33(H2O)4][H9.33CoP8Mo12O62]}·4H2O (1) and (H2bbi){[Zn(Hbbi)]2[Zn0.75(bbi)][K2Zn(H2O)4][H8.5ZnP8Mo12O62]} (2) [bbi = 1,1'-(1,4-butanediyl)bis(imidazole)], were successfully synthesized under hydrothermal conditions. Structural analysis demonstrates that compounds 1 and 2 are constructed from hourglass-shaped structures [M(P4Mo6O31)2]n- (M = Co, Zn), which are all made up of molybdophosphates and one transition metal ion as the central connecting node. Compounds 1 and 2 feature three-dimensional (3D) frameworks, which are all connected to form a 3D structure by metal ions and bbi ligands. More interestingly, compound 1 exhibits higher catalytic activity than 2 in CO2 photoreduction due to the suitable energy band structure of Co species in {P4Mo6} clusters. The CO yield was 3261 μmol g-1 with high selectivity in 8 h for compound 1 in photocatalytic CO2 reduction, which is highly promising in the photocatalytic field. Additionally, the photoluminescence properties of 2 were investigated.

Proton transfer in polyamine-P2Mo5 model adducts: exploring the effect of polyamine cations on their proton conductivity

Constructing acid-base pairs is one of the efficient strategies for the design of proton conductors with high conductivity, due to the ultrafast proton-hopping with a low energy barrier between a proton donor (acid group) and an acceptor (base group). In this study, an acid-base adduct polyamine-P₂Mo₅ model system was established, including adducts [C₆N₄H₂₂][H₂P₂Mo₅O₂₃]·H₂O (P₂Mo₅-TETA), [C₄N₃H₁₆]₂[P₂Mo₅O₂₃]·H₂O (P₂Mo₅-DETA), and [C₂N₂H₁₀]₂[H₂P₂Mo₅O₂₃] (P₂Mo₅-EN), (TETA = triethylenetetramine, DETA = diethylenetriamine, EN = ethanediamine). Proton conductivity analyses showed that adduct P₂Mo₅-EN exhibited the highest proton conductivity 1.13 × 10^-2 S cm^-1 at 65 °C and 95% RH, which was one and three orders of magnitude greater than those of P₂Mo₅-DETA and P₂Mo₅-TETA under the same conditions. E_a values of all three adducts are lower than 0.4 eV, which indicates that their proton transfer is attributed to the Grotthuss mechanism. Combined with visual structure analysis, the proton transport pathways of three adducts are highlighted. Moreover, we use this model system to discuss in detail the effect of pK_a, proton density and size of polyamine molecules on the proton conductivity of organic amine-POM adducts.