Tuning Binding Strength of Multiple Intermediates towards Efficient pH-universal Electrocatalytic Hydrogen Evolution by Mo8 O26 -NbNx Oy Heterocatalysts

Developing efficient and robust hydrogen evolution reaction (HER) catalysts for scalable and sustainable hydrogen production through electrochemical water splitting is strategic and challenging. Herein, heterogeneous Mo8 O26 -NbNx Oy supported on N-doped graphene (defined as Mo8 O26 -NbNx Oy /NG) is synthesized by controllable hydrothermal reaction and nitridation process. The O-exposed Mo8 O26 clusters covalently confined on NbNx Oy nanodomains provide a distinctive interface configuration and appropriate electronic structure, where fully exposed multiple active sites give excellent HER performance beyond commercial Pt/C catalyst in pH-universal electrolytes. Theoretical studies reveal that the Mo8 O26 -NbNx Oy interface with electronic reconstruction affords near-optimal hydrogen adsorption energy and enhanced initial H2 O adsorption. Furthermore, the terminal O atoms in Mo8 O26 clusters cooperate with Nb atoms to promote the initial H2 O adsorption, and subsequently reduce the H2 O dissociation energy, accelerating the entire HER kinetics.

A Novel Anderson-Type POMs-Based Hybrids Flame Retardant for Reducing Smoke Release and Toxicity of Epoxy Resins

Smoke emission and smoke toxicity have drawn more attention to improving the fire safety of polymers. In this work, a polyoxometalates (POMs)-based hybrids flame retardant (P-AlMo6 ) epoxy resin (EP) is prepared with toxicity-reduction and smoke-suppression properties via a peptide coupling reaction between POMs and organic molecules with double DOPO (bisDOPA). It combines the good compatibility of the organic molecule and the superior catalytic performance of POMs. Compared to pure EP, the glass transition temperature and flexural modulus of EP composite with 5 wt.% P-AlMo6 (EP/P-AlMo6 -5) are raised by 12.3 °C and 57.75%, respectively. Notably, at low flame-retardant addition, the average CO to CO2 ratio (Av-COY/Av-CO2 Y) is reduced by 33.75%. Total heat release (THR) and total smoke production (TSP) are lowered by 44.4% and 53.7%, respectively. The Limited Oxygen Index (LOI) value achieved 31.7% and obtained UL-94 V-0 rating. SEM, Raman, X-ray photoelectron spectroscopy, and TG-FTIR are applied to analyze the flame-retardant mechanism in condensed and gas phase. Outstanding flame retardant, low smoke toxicity properties are attained due to the catalytic carbonization ability of metal oxides Al2 O3 and MoO3 produced from the breakdown of POMs. This work advances the development of POMs-based hybrids flame retardants with low smoke toxicity properties.

Cations Modulated Assembly of Triol-Ligand Modified Cu-Centered Anderson-Evans Polyanions

Counter-cations are essential components of polyoxometalates (POMs), which have a distinct influence on the solubility, stabilization, self-assembly, and functionality of POMs. To investigate the roles of cations in the packing of POMs, as a systematic investigation, herein, a series of triol-ligand covalently modified Cu-centered Anderson-Evans POMs with different counter ions were prepared in an aqueous solution and characterized by various techniques including single-crystal X-ray diffraction. Using the strategy of controlling Mo sources, in the presence of triol ligand, NH₄⁺, Cu²⁺ and Na⁺ were introduced successfully into POMs. When (NH₄)₆Mo₇O₂₄ was selected, the counter cations of the produced POMs were ammonium ions, which resulted in the existence of clusters in the discrete state. Additionally, with the modulation of the pH of the solutions, the modified sites of triol ligands on the cluster can be controlled to form δ- or χ-isomers. By applying MoO₃ in the same reaction, Cu²⁺ ions served as linkers to connect triol-ligand modified polyanions into chains. When Na₄Mo₈O₂₆ was employed as the Mo source to react with triol ligands in the presence of CuCl₂, two 2-D networks were obtained with {Na₄(H₂O)₁₄} or {{Na₂(H₂O)₄} sub-clusters as linkers, where the building blocks were δ/δ- and χ/χ-isomers, respectively. The present investigation reveals that the charges, sizes and coordination manners of the counter cations have an obvious influence on the assembled structure of polyanions.

Versatile post-functionalisation strategy for the formation of modular organic-inorganic polyoxometalate hybrids

Hybrid structures incorporating different organic and inorganic constituents are emerging as a very promising class of materials since they synergistically combine the complementary and diverse properties of the individual components. Hybrid materials based on polyoxometalate clusters (POMs) are particularly interesting due to their versatile catalytic, redox, electronic, and magnetic properties, yet the controlled incorporation of different clusters into a hybrid structure is challenging and has been scarcely reported. Herein we propose a novel and general strategy for combining multiple types of metal-oxo clusters in a single hybrid molecule. Two novel hybrid POM structures (HPOMs) bis-functionalised with dipentaerythritol (R-POM1-R; R = (OCH2)3CCH2OCH2C(CH2OH)) were synthesised as building-blocks for the formation of heterometallic hybrid triads (POM2-R-POM1-R-POM2). Such a modular approach resulted in the formation of four novel heterometallic hybrids combing the Lindqvist {V6}, Anderson-Evans {XMo6} (X = Cr or Al) and trisubstituted Wells-Dawson {P2V3W15} POM structures. Their formation was confirmed by multinuclear Nuclear Magnetic Resonance (NMR), infrared (IR) and UV-Vis spectroscopy, as well as Mass Spectrometry, Diffusion Ordered Spectroscopy (DOSY) and elemental analysis. The thermal stability of the hybrids was also examined by Thermogravimetric Analysis (TGA), which showed that the HPOM triads exhibit higher thermal stability than comparable hybrid structures containing only one type of POM. The one-pot synthesis of these novel compounds was achieved in high yields in aqueous and organic media under simple reflux conditions, without the need of any additives, and could be translated to create other hybrid materials based on a variety of metal-oxo cluster building-blocks.

Microwave-Assisted Synthesis of Tris-Anderson Polyoxometalates for Facile CO2 Cycloaddition

Four new tris-Anderson polyoxometalates (POMs), (NH₄)₄[ZnMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (1), (NH₄)₄[CuMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (2), (TBA)₃(NH₄)[ZnMo₆O₁₇(C₅H₉O₃)₂(OH)]·10H₂O (3) (TBA = n-C₁₆H₃₆N), and (NH₄)₄[CuMo₆O₁₈(C₅H₉O₃)₂]·16H₂O (4), were synthesized by a microwave-assisted method. Single-crystal X-ray diffraction revealed that 1 and 2 contained a tris (trihydroxyl organic compounds) ligand grafted on one side, while two tris ligands were grafted on two sides to form χ/δ and δ/δ isomers in 3 and 4, respectively. ¹H and ¹³C NMR spectra of the χ/δ isomer 3 were obtained for the first time, with six methylenes showing six peaks in the ¹H NMR spectrum and only four peaks in the ¹³C NMR spectrum. Mass spectrometry monitoring revealed that during the microwave-assistant process the tris ligand can graft onto POMs to form 1, while tris directly coordinates with metallic heteroatoms to form isopolymolybdates during the conventional reflux synthesis process. In addition, 1-4 can catalyze CO₂ with epoxides into cyclic carbonates with high selectivity and yields at an atmospheric pressure of CO₂, which is lower than the pressure of CO₂ in other catalysis using POMs as catalysts. Furthermore, 1-4 showed good catalytic stability and cycling properties. Mechanism studies substantiated POMs cocatalyzed with Br^- to improve the catalytic yields.

Chromium-catalysed efficient N-formylation of amines with a recyclable polyoxometalate-supported green catalyst

A simple and efficient protocol for the formylation of amines with formic acid, catalyzed by a polyoxometalate-based chromium catalyst, is described. Notably, this method shows excellent activity and chemoselectivity for the formylation of primary amines; diamines have also been successfully employed. Importantly, the chromium catalyst is potentially non-toxic, environmentally benign and safer than the widely used high valence chromium catalysts such as CrO₃ and K₂Cr₂O₇. The catalyst can be recycled several times with a negligible impact on activity. Finally, a plausible mechanism is provided based on the observation of intermediate and control experiments.

Removal of nitrate nitrogen from water by phosphotungstate-supported TiO2 photocatalytic method

Nitrate nitrogen in water, especially in groundwater, is a major problem in the current drinking water environment. In this study, copper- and nickel-modified phosphotungstate catalysts supported on TiO₂ were prepared by the sol-gel solvothermal method, and photocatalytic reduction by phosphotungstate was used to remove nitrate nitrogen in water under ultraviolet irradiation. The maximum removal rate was 59.60% with 0.8 g/L Cu-H₃PW₁₂O₄₀/TiO₂, 90 mg/L nitrate nitrogen, and 60 min reaction time. For Ni-H₃PW₁₂O₄₀/TiO₂, the maximum removal rate of nitrate nitrogen was 54.58%, achieved with a catalyst concentration of 0.8 g/L, nitrate nitrogen concentration of 120 mg/L, and reaction time of 30 min. Both catalysts could remove nitrate nitrogen from water under the condition of photocatalysis.

Unprecedented coupling reaction between two anionic species of a closo-decahydrodecaborate cluster and an Anderson-type polyoxometalate

A novel decahydrodecaborate-functionalized Anderson type polyoxometalate has been synthesized and characterized in solution by ESI-MS, various NMR techniques and electrochemical methods. DFT studies provide strong support to understand the properties of this hybrid system.

Binding of a Fatty Acid-Functionalized Anderson-Type Polyoxometalate to Human Serum Albumin

The Anderson-type hexamolybdoaluminate functionalized with lauric acid (LA), (TBA)₃[Al(OH)₃Mo₆O₁₈{(OCH₂)₃CNHCOC₁₁H₂₃}]·9H₂O (TBA-AlMo₆-LA, where TBA = tetrabutylammonium), was prepared via two synthetic routes and characterized by thermogravimetric and elemental analyses, mass spectrometry, IR and ¹H NMR spectroscopy, and powder and single-crystal X-ray diffraction. The interaction of TBA-AlMo₆-LA with human serum albumin (HSA) was investigated via fluorescence and circular dichroism spectroscopy. The results revealed that TBA-AlMo₆-LA binds strongly to HSA (63% quenching at an HSA/TBA-AlMo₆-LA ratio of 1:1), exhibiting static quenching. In contrast to TBA-AlMo₆-LA, the nonfunctionalized polyoxometalate, Na₃(H₂O)₆[Al(OH)₆Mo₆O₁₈]·2H₂O (AlMo₆), showed weak binding toward HSA (22% quenching at a HSA/AlMo₆ ratio of 1:25). HSA binding was confirmed by X-ray structure analysis of the HSA-Myr-AlMo₆-LA complex (Myr = myristate). These results provide a promising lead for the design of novel polyoxometalate-based hybrids that are able to exploit HSA as a delivery vehicle to improve their pharmacokinetics and bioactivity.

A fatty acid-functionalized Anderson-type polyoxometalate, (TBA)₃[Al(OH)₃Mo₆O₁₈{(OCH₂)₃CNHCOC₁₁H₂₃}]·9H₂O (TBA-AlMo₆-LA), was synthesized and characterized in detail. The final organic−inorganic hybrid shows an increased affinity toward the transport protein human serum albumin (HSA) in comparison to its unmodified counterpart, Na₃(H₂O)₆[Al(OH)₆Mo₆O₁₈)]·2H₂O (AlMo₆). This is of medical importance because HSA is a well-known drug carrier and can therefore serve as a delivery system for AlMo₆. This study provides a rational design for the synthesis of bioactive polyoxometalates with enhanced pharmacokinetic properties.

Unprecedented Halide-Ion Binding and Catalytic Activity of Nanoscale Anionic Metal Oxide Clusters

One halide ion (X^- ) can bind on the surface of nanoscale Anderson-type polyoxometalate (POMs) clusters [(n-C₄ H₉ )₄ N]₃ {AlMo₆ O₁₈ (OH)₃ [(OCH₂ )₃ CCH₃ ]}, and form stable complexes in solution with binding constant K=1.53×10³ . Single-crystal structural analysis showed that this binding behavior occurs through multiple hydrogen bonding between X^- and three hydroxy groups on the uncapped side of the cluster. This supramolecular interaction in the cluster systems means that their catalytic activities, evaluated from the oxidation of alcohols to aldehydes, can be switched upon the introduction of halide ions and water molecules. The halide ions work as inhibitors by blocking the active sites of the clusters while they can be re-activated by the addition of water.

{VMo9O31[RC(CH2O)3]}6−: the first class of triol ligand covalently-decorated Keggin-type polyoxomolybdates

The first class of triol-ligand covalently-decorated Keggin-type polyoxomolybdates through C–O–Mo bonds were obtained and characterized.

Hybrid polyoxometalates as post-functionalization platforms: from fundamentals to emerging applications

Polyoxometalates (POMs) represent an important group of metal-oxo nanoclusters, typically comprised of early transition metals in high oxidation states (mainly V, Mo and W). Many plenary POMs exhibit good pH, solvent, thermal and redox stability, which makes them attractive components for the design of covalently integrated hybrid organic-inorganic molecules, herein referred to as hybrid-POMs. Until now, thousands of organic hybrid-POMs have been reported; however, only a small fraction can be further functionalized using other organic molecules or metal cations. This emerging class of 'post-functionalizable' hybrid-POMs constitute a valuable modular platform that permits coupling of POM properties with different organic and metal cation functionalities, thereby expanding the key physicochemical properties that are relevant for application in (photo)catalysis, bioinorganic chemistry and materials science. The post-functionalizable hybrid-POM platforms offer an opportunity to covalently link multi-electron redox responsive POM cores with virtually any (bio)organic molecule or metal cation, generating a wide range of materials with tailored properties. Over the past few years, these materials have been showcased in the preparation of framework materials, functional surfaces, surfactants, homogeneous and heterogeneous catalysts and light harvesting materials, among others. This review article provides an overview on the state of the art in POM post-functionalization and highlights the key design and structural features that permit the discovery of new hybrid-POM platforms. In doing so, we aim to make the subject more comprehensible, both for chemists and for scientists with different materials science backgrounds interested in the applications of hybrid (POM) materials. The review article goes beyond the realms of polyoxometalate chemistry and encompasses emerging research domains such as reticular materials, surfactants, surface functionalization, light harvesting materials, non-linear optics, charge storing materials, and homogeneous acid-base catalysis among others.

Highly selective and efficient olefin epoxidation with pure inorganic-ligand supported iron catalysts

Over the past two decades, there have been major developments in the transition iron-catalyzed selective oxidation of alkenes to epoxides; a common structure found in drug, isolated natural products, and fine chemicals. Many of these approaches have enabled highly efficient and selective epoxidation of alkenes via the design of specialized ligands, which facilitates to control the activity and selectivity of the reactions catalyzed by iron atom. Herein, we report the development of the olefin epoxidation with inorganic-ligand supported iron-catalysts using 30% H₂O₂ as an oxidant, and the mechanism is similar to iron-porphyrin type. With the catalyst 1, (NH₄)₃[FeMo₆O₁₈(OH)₆], various aromatic and aliphatic alkenes were successfully transformed into the corresponding epoxides with excellent yields as well as chemo- and stereo-selectivity. This catalytic system possesses the advantages of being able to avoid the use of expensive, toxic, air/moisture sensitive and commercially unavailable organic ligands. The generality of this methodology is simple to operate and exhibits high catalytic activity as well as excellent stability, which gives it the potential to be used on an industrial scale, and maybe opens a way for the catalytic oxidation reaction via inorganic-ligand coordinated iron catalysis.

Photoluminescence Properties of Two Closely Related Isostructural Series Based on Anderson-Evans Cluster Coordinated With Lanthanides [Ln(H2O)7{X(OH)6Mo6O18}]•yH2O, X = Al, Cr

The paper describes synthesis and structural characterization of the whole series of two closely related lanthanide coordinated chromium or aluminum hexamolybdates (Anderson-Evans cluster) including twelve new members hitherto unreported: [Ln(H₂O)₇{X(OH)₆Mo₆O₁₈}]·4H₂O and [Ln(H₂O)₇{X(OH)₆Mo₆O₁₈}Ln(H₂O)₇]{X(OH)₆Mo₆O₁₈}·16H₂O where X = Al or Cr and Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y. Crystal structures of all the solids were established by powder and single crystal X-ray diffraction techniques. The two series are dictated by a different aggregation of the same set of molecular species: Lighter lanthanides favor coordination interaction between lanthanide ions and molybdate cluster forming 1D chains (Series I) while the heavier lanthanides result in the stacking of a cation, a pair of lanthanide hydrates coordinating to the cluster, and an anion, the discrete cluster is further stabilized through a large number of water molecules (Series II). Crystallization with Er³⁺ and Tm³⁺ ions results in a concomitant mixture of Series I and II. Photoluminescence of single crystals of all the chromium molybdates was dominated by a ruby-like emission including those which contain optically active ions Pr, Sm, Eu, Tb, Dy, and Tm. In contrast, aluminum analogs showed photoluminescence corresponding to characteristic lanthanide emissions. Our results strongly suggest a possible energy transfer from f levels of lanthanide ions to d levels of chromium (III) causing the quenching of lanthanide emission when coordinated with chromium molybdates. Intensity measurements showed that the emission from chromium molybdates are almost two orders of magnitude lower than naturally occurring ruby with broader line widths at room temperature.

Experimental and Theoretical Investigation of the Light-Driven Hydrogen Evolution by Polyoxometalate-Photosensitizer Dyads

The visible-light-driven hydrogen evolution reaction (HER) by covalent photosensitizer-catalyst dyads is one of the most elegant concepts in supramolecular homogeneous solar energy conversion. The intricacies of catalyst reactivity and photosensitizer-catalyst interactions require a detailed fundamental understanding of the system to rationalize the observed reactivities. Here, we report three dyads based on the covalent imine-bond linkage of an iridium photosensitizer and an organo-functionalized Anderson polyoxometalate anion [MMo₆ O₁₈ {(OCH₂ )₃ CNH₂ }₂ ]^3- (M=Mn³⁺ , Fe³⁺ , Co³⁺ ). Modification of the central metal ion M is used to modulate the HER activity. Detailed theoretical and experimental studies examine the role of the central metal ion M and provide critical understanding of the redox activity and light-driven HER activity of the novel dyads. Thus, the study enables a knowledge-based optimization of HER dyads by chemical modification of the reactive metal oxide components.

Synthesis of novel bis(Triol)-functionalized Anderson clusters serving as potential synthons for forming organic–inorganic hybrid chains

Three novel bis(Triol)-functionalized Anderson cluster derivatives (POM–L–POM, POM–L, L–POM–L) were efficiently synthesized, which manifested interesting self-assembly and synergistic effect.

[Ni(OH)3W6O18(OCH2)3CCH2OH](4-): the first tris-functionalized Anderson-type heteropolytungstate

Na₂[TMA]₂[Ni(OH)₃W₆O₁₈(OCH₂)₃CCH₂OH]·9H₂O represents the first covalent tris-functionalized Anderson-type heteropolytungstate and was characterized by single-crystal X-ray diffraction, electrospray ionization mass spectrometry, TGA and IR spectroscopy.

Na₂[TMA]₂[Ni(OH)₃W₆O₁₈(OCH₂)₃CCH₂OH]·9H₂O represents the first covalent tris-functionalized Anderson-type heteropolytungstate and was characterized by single-crystal X-ray diffraction, electrospray ionization mass spectrometry, TGA and IR spectroscopy. Zeta potential measurements in solutions containing human serum albumin were performed to investigate electrostatic interactions with [Ni(OH)₃W₆O₁₈(OCH₂)₃CCH₂OH]^4–.

Unprecedented χ isomers of single-side triol-functionalized Anderson polyoxometalates and their proton-controlled isomer transformation

The μ2-O atom in Anderson polyoxometalates was regioselectively activated by the introduction of protons, which, upon functionalization with triol ligands, could afford a series of unique χ isomers of the organically-derived Anderson cluster {[RCC(CH2O)3]MMo6O18(OH)3}(3-). Herein proton-controlled isomer transformation between the δ and χ isomer was observed by using the fingerprint region in the IR spectra and (13)C NMR spectra.