A heteropolytungstate based 2D layered porous framework with high proton conductivity

A heteropolytungstate cluster [{Ru2O(bpy)2}2{Bi2W32O110}]10- (bpy = C10H8N2) was incorporated into a 2 : 1 type layered porous framework by interweaving the Na+ bridged cluster chains through the hydrogen bonding ability of the bpy ligands. It features multiple pore channels rich in hydrogen-bond network, contributing high conductivities > 10-2 S cm-1 at 298-358 K and 85% RH.

A Macrocyclic Polyoxomolybdate with Phosphate and Phosphonate Linkers: Synthesis, Structure, and Proton Conductivity

A coordination macrocycle composed of eight identical [PMo8O27]- ({PMo8}) clusters connected by both organic tetraphosphonates and inorganic phosphates, (C3N2H5)29(NH4)6H12[(PMo8O27)8(C10P4O12N2H20)4(PO4)4Cs(Mo4O10(H2O)4)] (C3N2H5+ = imidazolium), is presented here. The primary building block, {PMo8}, is a tetravacant Keggin-type phosphomolybdate that has never been observed before. The compound shows a high proton conductivity of 9.70 × 10-3 S cm-1 at 373 K and 98% relative humidity. Control experiments on an imidazolium-free sample demonstrate the critical role of the imidazolium counterions as mobile proton carriers. The contribution of imidazolium necessitates a high activation energy (Ea = 0.502 eV) for proton conduction via the vehicle mechanism.

Dual-Functional Eu-Metal-Organic Framework with Ratiometric Fluorescent Broad-Spectrum Sensing of Benzophenone-like Ultraviolet Filters and High Proton Conduction

The construction of attractive dual-functional lanthanide-based metal-organic frameworks (Ln-MOFs) with ratiometric fluorescent detection and proton conductivity is significant and challenging. Herein, a three-dimensional (3D) Eu-MOF, namely, [Eu4(HL)2(SBA)4(H2O)6]·9H2O, has been hydrothermally synthesized with a dual-ligand strategy, using (4-carboxypiperidyl)-N-methylenephosphonic acid (H3L = H2O3PCH2-NC5H9-COOH) and 4-sulfobenzoic acid monopotassium salt (KHSBA = KO3SC6H4COOH) as organic linkers. Eu-MOF showed ratiometric fluorescent broad-spectrum sensing of benzophenone-like ultraviolet filters (BP-like UVFs) with satisfactory sensitivity, selectivity, and low limits of detection in water/ethanol (1:1, v/v) solutions and real urine systems. A portable test paper was prepared for the convenience of actual detection. The potential sensing mechanisms were thoroughly analyzed by diversified experiments. The synergistic effect of the forbidden energy transfer from the ligand to Eu3+, the internal filtration effect (IFE), the formation of a complex, and weak interactions between the KHSBA ligand and BP-like UVFs is responsible for the ratiometric sensing effect. Meanwhile, Eu-MOF displayed relatively high proton conductivity of 2.60 × 10-4 S cm-1 at 368 K and 95% relative humidity (RH), making it a potential material for proton conduction. This work provides valuable guidance for the facile and effective design and construction of multifunctional Ln-MOFs with promising performance.

Two Innovative Fumaric Acid Bridging Lanthanide-Encapsulated Hexameric Selenotungstates Containing Mixed Building Units and Electrochemical Performance for Detecting Mycotoxin

Two particular fumaric acid bridging lanthanide-encapsulated selenotungstates [H₂N(CH₃)₂]₁₆Na₈[Ln₃(H₂O)₇]₂ [W₄O₈(C₄H₂O₄) (C₄H₃O₄)]₂[SeW₆O₂₅]₂[B-α-SeW₉O₃₃]₄·46H₂O [Ln = Ce³⁺ (1), La³⁺ (2)] were acquired by the deliberately designed step-by-step synthetic strategy, which are composed of four trilacunary Keggin [B-α-SeW₉O₃₃]^8- and two original [SeW₆O₂₅]^10- building units together with one fumaric acid bridging heterometallic [Ln₃(H₂O)₇]₂[W₄O₈(C₄H₂O₄) (C₄H₃O₄)]₂²⁸⁺ entity. Particularly, this heterometallic cluster contains four fumaric acid ligands, which play two different roles: one works as the pendant decorating the cluster and the other acts as the linker connecting the whole structure. In addition, the 1@DDA hybrid material was produced through the cation exchange of 1 and dimethyl distearylammonium chloride (DDA·Cl) and its beehive-shaped film of 1@DDA was prepared by the breath figure method, which can be further used to establish an electrochemical biosensor for detecting a kind of mycotoxin-ochratoxin A (OX-A). The 1@DDA beehive-shaped film-based electrochemical biosensor exhibits good reproducibility and specific sensing toward OX-A with a low detection limit of 29.26 pM. These results highlight the huge feasibility of long-chain flexible ligands in building lanthanide-encapsulated selenotungstates with structural complexity and further demonstrate great electrochemical application potentiality of polyoxometalate-involved materials in bioanalysis, tumor diagnosis, and iatrology.

Polyoxometalate-based metal–organic complexes and their derivatives as electrocatalysts for energy conversion in aqueous systems

The research progress on polyoxometalate-based metal–organic complexes and their derivatives as electrocatalysts in sustainable and clean energy conversion applications in aqueous systems is summarized.

High Proton Conduction in Two Highly Water-Stable Lanthanide Coordination Polymers from a Triazole Multicarboxylate Ligand

Two lanthanide coordination polymers (CPs) {[Er(Hmtbd)(H₂mtbd)(H₂O)₃]·2H₂O}_n (1) and [Yb(Hmtbd)(H₂mtbd)(H₂O)₃]_n (2) carrying an N-heterocyclic carboxylate ligand 5-(3-methylformate-1H-1,2,4-triazole-1-methyl)benzen-1,3-dicarboxylate (H₃mtbd) were prepared under solvothermal conditions. The single-crystal X-ray diffraction data demonstrate that 1 and 2 are isostructural and display 1D chain structure. Alternating current (AC) impedance measurements illustrate that the highest proton conductivities of 1 and 2 can attain 5.09 × 10^-3 and 3.09 × 10^-3 S·cm^-1 at 100 °C and 98% relative humidity (RH), respectively. The value of 1 exceeds those of most reported lanthanide-based crystalline materials and ranks second among the described Er-CPs under similar conditions, whereas the value for 2 is the highest proton conductivity among the previous Yb-CPs. Coupled with the structural analyses of the two CPs and H₂O vapor adsorption, the calculated E_a values help to deduce their proton conductive mechanisms. Notably, the N-heterocyclic units (triazole), carboxyl, and hydrogen-bonding network all play key roles in the proton-transfer process. The prominent proton conductive abilities of both CPs show great promise as efficient proton conductors.

Highly efficient oxidation of various thioethers catalyzed by organic ligand-modified polyoxomolybdates

Herein, four hybrid dimers based on the carboxylic acid ligand-modified polyoxomolybdates were prepared, which could rapidly and selectively oxidize various phenyl sulfides and the sulfur mustard simulant 2-chloroethyl ethyl sulfide (CEES).

A CsCl-type inorganic cluster-based high-symmetry crystal built from {Mo4.55V7.45PO40}10.45- with a high ratio of vanadium to molybdenum and {(H2O)0.3@K6(H2O)12}6+ clusters exhibiting proton conduction below the freezing point of water

As a class of anionic oxoclusters of early transition metals in their highest oxidation states, polyoxometalates (POMs) show considerable structural versatility and unique chemical and physical properties, making them promising multifunctional materials. In this study, a Keggin-type POM has been achieved, with a formula of [(H2O)0.3@K6(H2O)12]H4.45[PV7.45Mo4.55O40]·11H2O (1), and its microcrystals and nanocrystals have been obtained, respectively. This POM was characterized by elemental analysis for C, H and N, ICP-MS, TG, PXRD, SEM, X-band EPR and XPS techniques. Single crystal X-ray diffraction analysis demonstrated that 1 shows a rare extended structure with a high-connected three-dimensional (3D) all inorganic network of a Keggin-type POM, built from {Mo4.55V7.45PO40}10.45- polyoxoanions and {(H2O)0.3@K6(H2O)12}6+ clusters with CsCl-type crystal structure. In addition, to the best of our knowledge, 1 shows the highest ratio of vanadium to molybdenum among Keggin-type POMs reported thus far. Most interestingly, 1 exhibits intrinsic proton conduction below the freezing point of water, with a proton conductivity of 6.90 × 10-7 S cm-1 at 249 K and further reaching 3.36 × 10-6 S cm-1 at 272 K and Ea = 0.44 eV at 249-272 K.

High Proton Conductivity Achieved by Encapsulation of Imidazole Molecules into Proton-Conducting MOF-808

Metal-organic frameworks (MOFs), as newly emerging materials, show compelling intrinsic structural features, e.g., the highly crystalline nature and designable and tunable porosity, as well as tailorable functionality, rendering them suitable for proton-conducting materials. The proton conduction of MOF is significantly improved using the postsynthesis or encapsulation strategy. In this work, the MOF-based proton-conducting material Im@MOF-808 has been prepared by incorporating the imidazole molecules into the pores of proton-conducting MOF-808. Compared with MOF-808, Im@MOF-808 not only possesses higher proton conductivity of 3.45 × 10^-2 S cm^-1 at 338 K and 99% RH, superior to that of any imidazole-encapsulated proton-conducting materials reported to date, but also good durable and stable proton conduction. Moreover, the thermal stability of H-bond networks is much improved owing to the water molecules partially replaced by higher boiling point imidazole molecules. Additionally, it is further discussed for the possible mechanism of imidazole encapsulation into the pores of MOF-808 to enhance proton conduction.

Different Protonic Species Affecting Proton Conductivity in Hollow Spherelike Polyoxometalates

Polyoxometalates (POMs), which possess strong acidity and chemical stability, are promising solid proton conductors and potential candidates for proton exchange membrane fuel cell applications. To investigate how factors such as proton concentration and carrier affect the overall proton conduction, we have synthesized new compounds HImMo₁₃₂ (Im, imidazole), HMeImMo₁₃₂, ILMo₁₃₂, and TBAMo₁₃₂ with hollow structures and HImPMo₁₂ with a solid spherelike structure. These crystal models were prepared by encapsulating POM with organic molecules with different proton contents. Among them, the single-crystal sample of the hollow structure HImMo₁₃₂ containing more proton sources shows a high proton conductivity of 4.98 × 10^-2 S cm^-1, which was approximately 1 order of magnitude greater than that of the solid cluster HImPMo₁₂ with the same proton sources and 3 orders of magnitude greater than that of the proton-free organic cation-encapsulated giant ball TBAMo₁₃₂. This study provides a theoretical guidance toward designing and developing new-generation proton conductors and studying their performances at the molecular level.

A fully reduced {VIV18O42} host and VO43−, Cl− as guest anions: synthesis, characterization and proton conductivity

Two polyoxovanadate compounds, [Na₇(H₂O)₁₄][H₈VIV18O₄₂(V^VO₄)]·N₂H₄·7H₂O and [Na₅(H₂O)₁₆][H₈VIV18O₄₂(Cl)]·4N₂H₄·6H₂O, each having fully reduced host cage and accommodating eight acidic protons per formula unit, exhibit moderate proton conductivity.