Acid-Induced, Oxygen-Atom Defect Formation in Reduced Polyoxovanadate-Alkoxide Clusters

Surface ligand length influences kinetics of H-atom uptake in polyoxovanadate-alkoxide clusters

The uptake of hydrogen atoms (H-atoms) at reducible metal oxide nanocrystal surfaces has implications in catalysis and energy storage. However, it is often difficult to gain insight into the physicochemical factors that dictate the thermodynamics and kinetics of H-atom transfer to the surface of these assemblies. Recently, our research group has demonstrated the formation of oxygen-atom (O-atom) defects in polyoxovanadate-alkoxide (POV-alkoxide) clusters via conversion of surface oxido moieties to aquo ligands, which can be accomplished via addition of two H-atom equivalents. Here, we present the dependence of O-atom defect formation via H-atom transfer at the surface of vanadium oxide clusters on the length of surface alkoxide ligands. Analysis of H-atom transfer reactions to low-valent POV-alkoxide clusters [V6O7(OR)12]1- (R = Me, Et, nPr, nBu) reveals that the length of primary alkoxide surface ligands does not significantly influence the thermodynamics of these processes. However, surface ligand length has a significant impact on the kinetics of these PCET reactions. Indeed, the methoxide-bridged cluster, [V6O7(OMe)12]1- reacts ∼20 times faster than the other derivatives evaluated. Interestingly, as the aliphatic linkages are increased in size from -C2H5 to -C4H9, reaction rates remain consistent, suggesting restricted access to available ligand conformers as a result of the incompatibility of the aliphatic ligands and acetonitrile may buffer further changes to the rate of reaction.

Manipulating Ligand Density at the Surface of Polyoxovanadate-Alkoxide Clusters

We present the post-synthetic modification of a polyoxovanadate-alkoxide (POV-alkoxide) cluster via the reactivity of its cationic form, [V6O7(OCH3)12]1+, with water. This result indicates that cluster oxidation increases the lability of bridging methoxide ligands, affording a ligand exchange reaction that serves to compensate for the increased charge of the cluster core. This synthetic advance affords the isolation of a series of POV-alkoxide clusters with varying degrees of μ2-O2- ligands incorporated at the surface, namely, [V6O8(OCH3)11], [V6O9(OCH3)10], and [V6O10(OCH3)9]. Characterization of the POV-alkoxide clusters is described; changes in the infrared and electronic absorption spectra are consistent with the oxidation of the cluster core. We also examine the consequences of ligand substitution on the redox properties of the series of POV-alkoxide clusters via cyclic voltammetry; decreased alkoxide ligand density translates to a cathodic shift of analogous redox events. Ligand substitution also increases comproportionation constants of the Lindqvist core, indicating electron exchange between vanadium centers is promoted in structures with greater numbers of μ2-O2- ligands.

Reversible Electrodeposition of Potassium-bridged Molecular Vanadium Oxides: A New Approach Towards Multi-Electron Storage

Molecular metal oxides, so-called polyoxometalates (POMs), have shown outstanding performance as catalysts and lately attracted interest as materials in energy conversion and storage systems due to their capability of storing and exchanging multiple electrons. Here, we report the first example of redox-driven reversible electrodeposition of molecular vanadium oxide clusters, leading to the formation of thin films. The detailed investigation of the deposition mechanism reveals that the reversibility is dependent on the reduction potential. Correlating electrochemical quartz microbalance studies with X-ray photoelectron spectroscopy (XPS) data gave insight into the redox chemistry and oxidation states of vanadium in the deposited films in dependence on the potential window. A multi-electron reduction of the polyoxovanadate cluster, which facilitates the potassium (K+ ) cation-assisted reversible formation of potassium vanadium oxide thin films was confirmed. At anodic potentials, re-oxidation of the polyoxovanadate and complete stripping of the thin film is observed for films deposited at potentials more positive than -500 mV vs. Ag/Ag+ , while electrodeposition at more negative cathodic potential reduces the electrochemical reversibility of the process and increases the stripping overpotential. As proof of principle, we demonstrate the electrochemical performance of the deposited films for potential use in potassium-ion batteries.

Making Heterometallic Metal-Metal Bonds in Keggin-Type Polyoxometalates by a Six-Electron Reduction Process

Polyoxometalates (POMs) represent a promising class of molecular electron reservoirs. However, their multielectron reduction gives rise to intricate physical-chemical phenomena that must be fully understood for their future use in energy-storage devices. Herein, we show that bulk electrolysis of the archetypal Keggin-type POM [Si(W^VI₂Mo^VIO₁₀)(W^VI₃O₁₀)₃]^4- in aqueous solution leads to the six-electron-reduced derivative [Si(W^IV₂Mo^IVO₇(H₂O)₃)(W^VI₃O₁₀)₃]^4- (notated SiW₁₁Mo-VI') in which the mixed-metal triad acts as a storage unit for six electrons and six protons. X-ray diffraction analysis and multinuclear NMR (¹⁸³W and ⁹⁵Mo) studies reveal that this electron-rich species represents the first example of POMs containing heterometallic metal-metal bonds between addenda centers. This electron-rich POM can be further reduced through multielectronic events, while its full oxidation restores the structure of the oxidized parent ion. Remarkably, the formation of SiW₁₁Mo-VI' results from a fast clustering process compared to that observed for the entirely W-based analogue, revealing that the formation of metal-metal bonds in the mixed-metal Mo/W POM is facilitated because the reaction rate is not limited by a slow disproportionation step. Last, we evaluate the supramolecular properties of SiW₁₁Mo-VI' using a method based on the cloud-point measurement of a nonionic surfactant. This investigation demonstrates that the clustering process has dramatic consequences on the solution behavior of the POM, canceling its superchaotropic character due to a local structuring effect of the hydration shell. These fundamental results pave the way for applications using the massive electron-storage properties of mixed-metal POMs.

Regioselectivity of concerted proton-electron transfer at the surface of a polyoxovanadate cluster

Proton-coupled electron transfer (PCET) is an important process in the activation and reactivity of metal oxide surfaces. In this work, we study the electronic structure of a reduced polyoxovanadate-alkoxide cluster bearing a single bridging oxide moiety. The structural and electronic implications of the incorporation of bridging oxide sites are revealed, most notably resulting in the quenching of cluster-wide electron delocalization in the most reduced state of the molecule. We correlate this attribute to a change in regioselectivity of PCET to the cluster surface (e.g. reactivity at terminal vs. bridging oxide groups). Reactivity localized at the bridging oxide site enables reversible storage of a single H-atom equivalent, changing the stoichiometry of PCET from a 2e-/2H+ process. Kinetic investigations indicate that the change in site of reactivity translates to an accelerated rate of e-/H+ transfer to the cluster surface. Our work summarizes the role which electronic occupancy and ligand density play in the uptake of e-/H+ pairs at metal oxide surfaces, providing design criteria for functional materials for energy storage and conversion processes.

H2 Evolution at a Reduced Hybrid Polyoxometalate and Its Vanadium-Oxo Derivative Used as Molecular Models for Reducible Metal Oxides

We herein report our investigations on the use of a tris-silanol-decorated polyoxotungstate, [SbW₉O₃₃(tBuSiOH)₃]^3-, as a molecular support model to describe the coordination of an isolated vanadium atom at metal oxides, focusing on the reactivity of the reduced derivatives in the presence of protons. Accumulation of electrons and protons at an active site is a main feature associated with heterogeneous catalysts able to conduct the (oxy)dehydrogenation of alkanes or alcohols. Our results indicate that two-electron reduced derivatives release H₂ upon protonation, a reaction that probably takes place at the polyoxotungstic framework rather than at the vanadium center.

Coordination-induced bond weakening of water at the surface of an oxygen-deficient polyoxovanadate cluster

Hydrogen-atom (H-atom) transfer at the surface of heterogeneous metal oxides has received significant attention owing to its relevance in energy conversion and storage processes. Here, we present the synthesis and characterization of an organofunctionalized polyoxovanadate cluster, (calix)V6O5(OH2)(OMe)8 (calix = 4-tert-butylcalix[4]arene). Through a series of equilibrium studies, we establish the BDFE(O-H)avg of the aquo ligand as 62.4 ± 0.2 kcal mol-1, indicating substantial bond weaking of water upon coordination to the cluster surface. Subsequent kinetic isotope effect studies and Eyring analysis indicate the mechanism by which the hydrogenation of organic substrates occurs proceeds through a concerted proton-electron transfer from the aquo ligand. Atomistic resolution of surface reactivity presents a novel route of hydrogenation reactivity from metal oxide surfaces through H-atom transfer from surface-bound water molecules.

Connecting Thermodynamics and Kinetics of Proton Coupled Electron Transfer at Polyoxovanadate Surfaces Using the Marcus Cross Relation

Here, we evaluate the efficacy of multiple methods for elucidating the average bond dissociation free energy (BDFE) of two surface hydroxide moieties in a reduced polyoxovanadate cluster, [V₆O₁₁(OH)₂(TRIOL^NO2)₂]^-2. Through cyclic voltammetry, individual thermochemical parameters describing proton coupled electron transfer (PCET) are obtained, without the need for synthetic isolation of intermediates. Further, we demonstrate that a method involving a series of open circuit potential measurements with varying ratios of reduced to oxidized clusters is most attractive for the direct measurement of BDFE(O-H) for polyoxovanadate clusters as this approach also determines the stoichiometry of PCET. We subsequently connect the driving force of PCET to the rate constant for the transfer of hydrogen atoms to a series of organic substrates through the Marcus cross relation. We show that this method is applicable for the prediction of reaction rates for multielectron/multiproton transfer reactions, extending the findings from previous work focused on single electron/proton reactions.

Mechanistic insight into rapid oxygen-atom transfer from a calix-functionalized polyoxovanadate

We report accelerated rates of oxygen-atom transfer from a polyoxovanadate-alkoxide cluster following functionalization with a 4-tertbutylcalix[4]arene ligand. Incorporation of this electron withdrawing ligand modifies the electronics of the metal oxide core, favoring a mechanism in which the rate of oxygen-atom transfer is limited by outer-sphere electron transfer.

Oxidation Catalysis over Solid-State Keggin-Type Phosphomolybdic Acid with Oxygen Defects

Keggin-type phosphomolybdic acid (PMo₁₂O₄₀), treated with pyridine (Py), forms a crystalline material (PyPMo-HT) following heat treatment under an inert gas flow at ∼420 °C. Although this material is known to have attractive catalytic properties for gas-phase oxidation, the origin of this catalytic activity requires clarification. In this study, we investigated the crystal structure of PyPMo-HT. PyPMo-HT comprises a one-dimensional array of Keggin units and pyridinium cations (HPy), with an HPy/Keggin unit ratio of ∼1.0. Two oxygen atoms were removed from the Keggin unit during crystal structure transformation, which resulted in an electron being localized on the Mo atom in close contact with the adjacent Keggin unit. Upon the introduction of molecular oxygen, electron transfer from this Mo atom resulted in the formation of an electrophilic oxygen species that bridged two Keggin units. The electrophilic oxygen species acted as a catalytically active oxygen species, as confirmed by the selective oxidation of propylene. PyPMo-HT showed excellent catalytic activity for the selective oxidation of methacrolein, with the methacrylic acid yield being superior to that obtained with PMo₁₂O₄₀ and comparable to that obtained with an industrial Keggin-type polyoxometalate (POM) catalyst. The oxidation catalysis observed over PyPMo-HT provides a deeper understanding of POM-based industrial catalytic processes.

Charge-State Dependence of Proton Uptake in Polyoxovanadate-alkoxide Clusters

Here, we present an investigation of the thermochemistry of proton uptake in acetonitrile across three charge states of a polyoxovanadate-alkoxide (POV-alkoxide) cluster, [V₆O₇(OMe)₁₂]ⁿ (n = 2–, 1–, and 0). The vanadium oxide assembly studied features bridging sites saturated by methoxide ligands, isolating protonation to terminal vanadyl moieties. Exposure of [V₆O₇(OMe)₁₂]ⁿ to organic acids of appropriate strength results in the protonation of a terminal V=O bond, generating the transient hydroxide-substituted POV-alkoxide cluster [V₆O₆(OH)(OMe)₁₂]ⁿ⁺¹. Evidence for this intermediate proved elusive in our initial report, but here we present the isolation of a divalent anionic cluster that features hydrogen bonding to dimethylammonium at the terminal oxo site. Degradation of the protonated species results in the formation of equimolar quantities of one-electron-oxidized and oxygen-atom-efficient complexes, [V₆O₇(OMe)₁₂]ⁿ⁺¹ and [V₆O₆(OMe)₁₂]ⁿ⁺¹. While analogous reactivity was observed across the three charge states of the cluster, a dependence on the acid strength was observed, suggesting that the oxidation state of the vanadium oxide assembly influences the basicity of the cluster surface. Spectroscopic investigations reveal sigmoidal relationships between the acid strength and cluster conversion across the redox series, allowing for determination of the proton affinity of the surface of the cluster in all three charge states. The fully reduced cluster is found to be the most basic, with higher oxidation states of the assembly possessing substantially reduced proton affinities (∼7 pK_a units per electron). These results further our understanding of the site-specific reactivity of terminal M=O bonds with protons in an organic solvent, revealing design criteria for engineering functional surfaces of metal oxide materials of relevance to energy storage and conversion.

Experimental determination of the surface basicity of polyoxovanadate-alkoxide clusters across three oxidation states reveals a charge-state dependence of proton uptake in molecular, organofunctionalized vanadium oxide assemblies.

Oxygen-Atom Defect Formation in Polyoxovanadate Clusters via Proton-Coupled Electron Transfer

The uptake of hydrogen atoms (H-atoms) into reducible metal oxides has implications in catalysis and energy storage. However, outside of computational modeling, it is difficult to obtain insight into the physicochemical factors that govern H-atom uptake at the atomic level. Here, we describe oxygen-atom vacancy formation in a series of hexavanadate assemblies via proton-coupled electron transfer, presenting a novel pathway for the formation of defect sites at the surface of redox-active metal oxides. Kinetic investigations reveal that H-atom transfer to the metal oxide surface occurs through concerted proton–electron transfer, resulting in the formation of a transient V^III–OH₂ moiety that, upon displacement of the water ligand with an acetonitrile molecule, forms the oxygen-deficient polyoxovanadate-alkoxide cluster. Oxidation state distribution of the cluster core dictates the affinity of surface oxido ligands for H-atoms, mirroring the behavior of reducible metal oxide nanocrystals. Ultimately, atomistic insights from this work provide new design criteria for predictive proton-coupled electron-transfer reactivity of terminal M=O moieties at the surface of nanoscopic metal oxides.

A series of organic hybrid polyoxovanadate clusters incorporating tris(hydroxymethyl)methane derivatives

A series of new organic hybrid polyoxovanadate clusters [V₄O₄(μ-OH)₂(acac)₂(Htri)₂] (1, H₃tri = tris(hydroxymethyl) aminomethane, acac = acetylacetone), [V₄O₄(acac)₂(Htri)₂(L)₂] {HL = methanol (2), ethanol (3a and 3b), ethylene glycol (4) and benzyl alcohol (5)}, {V₄O₄(H₂O)₂(tri-acetamide)₂(CH₃COO)₂} (6, H₃tri-acetamide = N-(2-hydroxy-1,1-bis-hydroxymethyl-ethyl)-acetamide), [V₆O₈(μ-OH)₂(Htri)₃]·6H₂O (7) and [V₁₄O₁₈(tri)₂(Htri)₆(HCOO)(CH₃COO)]·2H₂O (8) were prepared by hydro(solvo)thermal methods and characterized structurally. 1 contains [VO(OH)(acac)] and [VO₂(Htri)] units, which are further interconnected via common edges to build a tetravanadyl cluster [V₄O₄(OH)₂(acac)₂(Htri)₂] with the double-deficient cube [V₄O₆]. The tetravanadyl cluster frameworks of 2-5 can be derived from the tetravanadyl cluster of 1 by replacing two -OH groups with two deprotonated organic alcohol ligands, namely, CH₃O^- (2), CH₃CH₂O^- (3a and 3b), HO(CH₂)₂O^- (4) and C₆H₅CH₂O^- (5). Interestingly, both 3a and 3b have the same chemical structure, but they exhibit different conformational polymorphisms [denoted as α-type (3a) and β-type (3b)]. Such conformational polymorphisms within the polyoxovanadate clusters incorporating tris(hydroxymethyl)methane derivatives emerged for the first time. 6 displays another tetravanadyl cluster {V₄O₄(H₂O)₂(tri-acetamide)₂(CH₃COO)₂} with a [V₄O₁₆] fragment, where the tri-acetamide unit comes from the amidation reaction of H₃tri and acetic acid and caps the tetrahedral void of the tetravanadyl cluster. The polyoxovanadate cluster of 7 can originate from the Lindqvist-type hexavanadyl cluster [V₆O₁₉] by replacing nine μ-oxides with nine alkoxides of three tri-acetamide^3- ligands. 8 exhibits a fully reduced tetradecavanadyl cluster based on the linkage of two heptavanadyl clusters via two O bridges. The magnetic properties of 1-8 show typical antiferromagnetic interactions.

Modelling local structural and electronic consequences of proton and hydrogen-atom uptake in VO2 with polyoxovanadate clusters

We report the synthesis and characterisation of a series of siloxide-functionalised polyoxovanadate-alkoxide (POV-alkoxide) clusters, [V6O6(OSiMe3)(OMe)12] n (n = 1-, 2-), that serve as molecular models for proton and hydrogen-atom uptake in vanadium dioxide, respectively. Installation of a siloxide moiety on the surface of the Lindqvist core was accomplished via addition of trimethylsilyl trifluoromethylsulfonate to the fully-oxygenated cluster [V6O7(OMe)12]2-. Characterisation of [V6O6(OSiMe3)(OMe)12]1- by X-ray photoelectron spectroscopy reveals that the incorporation of the siloxide group does not result in charge separation within the hexavanadate assembly, an observation that contrasts directly with the behavior of clusters bearing substitutional dopants. The reduced assembly, [V6O6(OSiMe3)(OMe)12]2-, provides an isoelectronic model for H-doped VO2, with a vanadium(iii) ion embedded within the cluster core. Notably, structural analysis of [V6O6(OSiMe3)(OMe)12]2- reveals bond perturbations at the siloxide-functionalised vanadium centre that resemble those invoked upon H-atom uptake in VO2 through ab initio calculations. Our results offer atomically precise insight into the local structural and electronic consequences of the installation of hydrogen-atom-like dopants in VO2, and challenge current perspectives of the operative mechanism of electron-proton co-doping in these materials.

Polyoxometalates-Based Semi-flexible Metal-Semiconductor Triboelectric Nanogenerators for Low Frequency and Small Amplitude Mechanical Energy Harvesting

The development of high-performance and low-cost durable triboelectric nanogenerators (TENGs) is essential for converting mechanical energy into electrical energy. Many organic polymer friction materials used widely have thermal stability problems, which makes TENGs with semiconductors as friction materials stand out. Here, we report a semi-flexible TENG based on metal and TiO₂ modified by polyoxometalates (POMs) as pure inorganic friction materials. Six different POMs are firstly selected to modify the friction materials of TENGs, and the output performance of TENGs with different POMs-modified semiconductors and different metals as friction materials are tested. Compared with the unmodified TENGs, the open-circuit voltage (V_OC ) of the optimal Ag-K₆ P₂ Mo₁₈ O₆₂ (P₂ Mo₁₈ )/TiO₂ TENG device is increased by more than 4 times, which is mainly attributed to the strong electron-accepting and storage capabilities of POMs. This study has demonstrated that TENGs modified by POMs have potential application prospects and provided a new method for increasing the electrical output of TENGs.

Physicochemical implications of surface alkylation of high-valent, Lindqvist-type polyoxovanadate-alkoxide clusters

We report a rare example of the direct alkylation of the surface of a plenary polyoxometalate cluster by leveraging the increased nucleophilicity of vanadium oxide assemblies. Addition of methyl trifluoromethylsulfonate (MeOTf) to the parent polyoxovanadate cluster, [V₆O₁₃(TRIOL^R)₂]^2- (TRIOL = tris(hydroxymethyl)methane; R = Me, NO₂) results in functionalisation of one or two bridging oxide ligands of the cluster core to generate [V₆O₁₂(OMe)(TRIOL^R)₂]^1- and [V₆O₁₁(OMe)₂(TRIOL^R)₂]^2-, respectively. Comparison of the electronic absorption spectra of the functionalised and unfunctionalised derivatives indicates the decreased overall charge of the complex results in a decrease in the energy required for ligand to metal charge transfer events to occur, while simultaneously mitigating the inductive effects imposed by the capping TRIOL ligand. Electrochemical analysis of the family of organofunctionalised polyoxovanadate clusters reveals the relationship of ligand environment and the redox properties of the cluster core: increased organofunctionalisation of the surface of the vanadium oxide assembly translates to anodic shifts in the reduction events of the Lindqvist ion. Overall, this work provides insight into the electronic effects induced upon atomically precise modifications to the surface structure of nanoscopic, redox-active metal oxide assemblies.

Atomically precise vanadium-oxide clusters

Polyoxovanadate (POV) clusters are an important subclass of polyoxometalates with a broad range of molecular compositions and physicochemical properties. One relatively underdeveloped application of these polynuclear assemblies involves their use as atomically precise, homogenous molecular models for bulk metal oxides. Given the structural and electronic similarities of POVs and extended vanadium oxide materials, as well as the relative ease of modifying the homogenous congeners, investigation of the chemical and physical properties of pristine and modified cluster complexes presents a method toward understanding the influence of structural modifications (e.g. crystal structure/phase, chemical makeup of surface ligands, elemental dopants) on the properties of extended solids. This review summarises recent advances in the use of POV clusters as atomically precise models for bulk metal oxides, with particular focus on the assembly of vanadium oxide clusters and the consequences of altering the molecular composition of the assembly via organofunctionalization and the incorporation of elemental "dopants".

Supramolecular assembly of a hierarchically structured 3D potassium vanadate framework

The hierarchical assembly of 3D-polyoxovanadate frameworks using host–guest interactions is reported.

Reductive silylation of polyoxovanadate surfaces using Mashima's reagent

Mechanistic insights into the reductive silylation of metal oxide surfaces.

Mo[O8Mo4]3 Lewis acid-base cluster pairs: highly efficient and stable Lewis catalysis fields frustrated in crystalline nanoclusters

By using metal compounds or oxide/organic acid and enhanced reaction temperatures in the controlled solvothermal oxidation of [Mo3O2(MeCO2)6(H2O)3]2+, more interstitial metal atoms were introduced to produce the largest nanoscale MoIV-polyoxomolybdates, [M2@(MoIV3py3)4Mo18Ox]q- (M = Al, V, Mo). Each [H4V2@(MoIV3py3)4Mo18O84]12- (2a) nanocluster is surrounded by 12 [V3Mo12O42] to build a Lewis catalysis field (LCF) composed of MoIV3[O8Mo4]3 Lewis acid-base cluster pairs in the crystalline 2, accounting for the excellent and stable catalysis performance in the hydrazine reduction of nitroarenes to arylamines in varied solvents. The proposed new concept LCF provides a new way of thinking for designed synthesis and real applications of highly efficient LCF catalysts.

Oxygen-atom vacancy formation and reactivity in polyoxovanadate clusters

Overview of recent work detailing oxygen-deficient polyoxovanadate clusters as models for reducible metal oxides: toward gaining a fundamental understanding the consequences of vacancy formation on metal oxide surfaces during catalysis.

Mechanistic insights into polyoxometalate self-assembly in organic solvent: conversion of a cyclic polyoxovanadate-ethoxide to its Lindqvist congener

We report the synthesis of a cyclic hexavanadate polyoxovanadate-alkoxide cluster, [VO(OC2H5)2]6, and its conversion, under solvothermal conditions, to an oxygen-deficient Lindqvist assembly, [V₆O₆(OC₂H₅)₁₂]ⁿ (n = 1−, 0).