Self-Assembled, Iron-Functionalized Polyoxovanadate Alkoxide Clusters

Heterometal Dopant Changes the Mechanism of Proton-Coupled Electron Transfer at the Polyoxovanadate-Alkoxide Surface

The transfer of two H-atom equivalents to the titanium-doped polyoxovanadate-alkoxide, [TiV5O6(OCH3)13], results in the formation of a V(III)-OH2 site at the surface of the assembly. Incorporation of the group (IV) metal ion results in a weakening of the O-H bonds of [TiV5O5(OH2)(OCH3)13] in comparison to its homometallic congener, [V6O6(OH2)(OCH3)12], resembling more closely the thermodynamics reported for the one-electron reduced derivative, [V6O6(OH2)(OCH3)12]1-. An analysis of early time points of the reaction of [TiV5O6(OCH3)13] and 5,10-dihydrophenazine reveals the formation of an oxidized substrate, suggesting that proton-coupled electron transfer proceeds via initial electron transfer from substrate to cluster prior to proton transfer. These results demonstrate the profound influence of heterometal dopants on the mechanism of PCET with respect to the surface of the assembly.

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.

Coupled reaction equilibria enable the light-driven formation of metal-functionalized molecular vanadium oxides

The introduction of metal sites into molecular metal oxides, so-called polyoxometalates, is key for tuning their structure and reactivity. The complex mechanisms which govern metal-functionalization of polyoxometalates are still poorly understood. Here, we report a coupled set of light-dependent and light-independent reaction equilibria controlling the mono- and di-metal-functionalization of a prototype molecular vanadium oxide cluster. Comprehensive mechanistic analyses show that coordination of a Mg2+ ion to the species {(NMe2H2)2[VV12O32Cl]}3- results in formation of the mono-functionalized {(NMe2H2)[(MgCl)VV12O32Cl]}3- with simultaneous release of a NMe2H2+ placeholder cation. Irradiation of this species with visible light results in one-electron reduction of the vanadate, exchange of the second NMe2H2+ with Mg2+, and formation/crystallization of the di-metal-functionalized [(MgCl)2VIVVV11O32Cl]4-. Mechanistic studies show how stimuli such as light or competing cations affect the coupled equilibria. Transfer of this synthetic concept to other metal cations is also demonstrated, highlighting the versatility of the approach.

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.

H8 [PV5 Mo7 O40 ] - A Unique Polyoxometalate for Acid and RedOx Catalysis: Synthesis, Characterization, and Modern Applications in Green Chemical Processes

Polyoxometalates (POMs) are a fascinating group of anionic metal-oxide clusters with a broad variety of structural properties and several catalytic applications, especially in the conversion of bio-derived platform chemicals. H8 [PV5 Mo7 O40 ] (HPA-5) is a unique POM catalyst that ideally links numerous fascinating research fields for the following reasons: a) HPA-5 can be synthesized by rational design approaches; b) HPA-5 can be well characterized using multiple analytical tools explaining its catalytic properties; and c) HPA-5 is suitable for multiple important catalytic transformations of bio-based feedstock. This Review combines the fields of synthesis, spectroscopic, electrochemical, and crystallographic characterization of HPA-5 with those of sustainable catalysis and green chemistry. Selected catalytic applications include esterification, dehydration, and delignification of biomass as well as selective oxidation and fractionation of bio-based feedstock. The unique HPA-5 is a fascinating POM that has a broad application scope for biomass valorization.

Vanadyl as a Spectroscopic Probe of Tunable Ligand Donor Strength in Bimetallic Complexes

Incorporation of secondary metal ions into heterobimetallic complexes has emerged as an attractive strategy for rational tuning of compounds' properties and reactivity, but direct solution-phase spectroscopic interrogation of tuning effects has received less attention than it deserves. Here, we report the assembly and study of a series of heterobimetallic complexes containing the vanadyl ion, [VO]²⁺, paired with monovalent cations (Cs⁺, Rb⁺, K⁺, Na⁺, and Li⁺) and a divalent cation (Ca²⁺). These complexes, which can be isolated in pure form or generated in situ from a common monometallic vanadyl-containing precursor, enable experimental spectroscopic and electrochemical quantification of the influence of the incorporated cations on the properties of the vanadyl moiety. The data reveal systematic shifts in the V-O stretching frequency, isotropic hyperfine coupling constant for the vanadium center, and V(V)/V(IV) reduction potential in the complexes. These shifts can be interpreted as charge density effects parametrized through the Lewis acidities of the cations, suggesting broad potential for the vanadyl ion to serve as a spectroscopic probe in multimetallic species.

Computational Insights into Iron Heterometal Installation in Polyoxovanadate-Alkoxide Clusters

Polyoxovanadate-alkoxide clusters are a new class of electroactive species with applications in a wide variety of fields from redox catalysis to energy storage. Heterometallic installation in these species can be used to modulate the redox properties of polyoxovanadate-alkoxide clusters and thus their applications. However, the formation mechanism of heterometallic polyoxovanadate alkoxides during the solvothermal process is unknown, limiting our understanding regarding what thermodynamic driving forces and/or kinetic barriers are present in the heterometal insertion. Here, we present a computational study on the nucleation pathways of the iron-functionalized mixed-valent hexameric [V^V₂V^IV₃O₅(μ₆-O)(μ₂-OCH₃)₁₂(Fe^IIICl)] polyoxovanadate-alkoxide cluster.

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.

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.

Effect of Heterometal-Functionalization and Template Exchange on the Redox Chemistry of Molecular Vanadium Oxides

Polyoxometalates (POMs) have emerged as material of interest in many applications such as energy storage and conversion due to their redox activity and molecularly defined structure. However, especially for polyoxovanadates a lack of understanding between structural modifications and physicochemical properties remains. The present study leverages a lacunary dodecavanadate to systematically investigate the electronic effect of heterometal functionalization. While structural distortion affects the stability of the cluster, the redox potentials correlate with the overall cluster charge. Furthermore, we report the first bromide-templated analogue of this cluster family. While the halide anion is crucial for the formation of the cluster, no major effect on the electrochemical properties is observed. By improving the understanding of structure-property relationship in this work, we hope to enable a more predictable tuning of redox-properties of polyoxovandates.

Molecular Vanadium Oxides for Energy Conversion and Energy Storage: Current Trends and Emerging Opportunities

Molecular vanadium oxides, or polyoxovanadates (POVs), have recently emerged as a new class of molecular energy conversion/storage materials, which combine diverse, chemically tunable redox behavior and reversible multielectron storage capabilities. This Review explores current challenges, major breakthroughs, and future opportunities in the use of POVs for energy conversion and storage. The reactivity, advantages, and limitations of POVs are explored, with a focus on their use in lithium and post-lithium-ion batteries, redox-flow batteries, and light-driven energy conversion. Finally, emerging themes and new research directions are critically assessed to provide inspiration for how this promising materials class can advance research in sustainable energy technologies.

Crystal structure of catena-poly[(5H-pyrrolo[3,2-b:4,5-b′]dipyridine-κ2 N,N′)-(μ4-hexaoxidodivanadato)dizinc(II)],C10H9N3O6V2Zn

C10H9N3O6V2Zn, monoclinic, C2/c (no. 15), a = 8.0106(17) Å, b = 29.957(6) Å, c = 12.050(3) Å, β = 95.025(4) Å, V = 2880.6(10) Å3, Z = 8, R gt (F) = 0.0512, wR ref (F 2) = 0.1975, T = 296(2) K.

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".

Characterizing Polyoxovanadate-Alkoxide Clusters Using Vanadium K-Edge X-Ray Absorption Spectroscopy

A number of technologies would benefit from developing inorganic compounds and materials with specific electronic and magnetic exchange properties. Unfortunately, designing compounds with these properties is difficult because metal⋅⋅⋅metal coupling schemes are hard to predict and control. Fully characterizing communication between metals in existing compounds that exhibit interesting properties could provide valuable insight and advance those predictive capabilities. One such class of molecules are the series of Lindqvist iron-functionalized and hexavanadium polyoxovanadate-alkoxide clusters, which we characterized here using V K-edge X-ray absorption spectroscopy. Substantial changes in the pre-edge peak intensities were observed that tracked with the V 3d-electron count. The data also suggested substantial delocalization between the vanadium cations. Meanwhile, the Fe^III cations were electronically isolated from the polyoxovanadate core.

Isolated Mixed-Valence Iron Vanadium Malate and Its Metal Hydrates (M = Fe2+, Cu2+, Zn2+) with Reversible and Irreversible Adsorptions for Oxygen

Isolated octanuclear iron-vanadium malate (NH₄)₃(CH₃NH₃)₃[Fe^III₂V^IV₂V^V₄O₁₁(mal)₆]·7.5H₂O (1; H₃mal = malic acid) and its family of metal hydrates M'_3n[M^II(H₂O)₂]_1.5n[Fe^III₂V^IV₂V^V₄O₁₁(mal)₆]_n·xnH₂O (2 or 2-Fe, M' = NH₄⁺, M = Fe, x = 7.5; 3 or 3-Cu, M' = K⁺, M = Cu, x = 10; 4 or 4-Zn, M' = K⁺, M = Zn, x = 6.5) have been obtained by self-assembly in water. The cluster anion [Fe₂V₆O₁₁(mal)₆]^6- (1a) shows an interesting iron bicapped-triangular-prismatic structure, which is bridged by M²⁺ hydrates (M = Fe, Cu, Zn) to construct isostructural metal organic frameworks (MOFs) 2-4. The mixed-valence vanadium systems in 1-4 were determined by theoretical bond valence calculations (BVS) and charge balance. The magnetic susceptibilities are further elucidated as high spin for Fe³⁺ in 1a and bridging Fe²⁺ in 2-Fe, respectively. A strong ferromagnetic interaction was also observed for 2-Fe at 3 K. 2-Fe, 3-Cu, and 4-Zn have similar hydrophilic channels with diameters of 6.8, 6.5, and 6.6 Å, respectively, which show obvious affinity for O₂ in comparison with no adsorption of N₂, H₂, CO₂, and CH₄ at room temperature under different pressures. Moreover, 2-Fe and 4-Zn exhibit irreversible O₂ absorptions, which may be attributed to charge transfer between O₂ and open metal sites (OMSs) formed during vacuum heating pretreatment. UV-vis and EPR spectra show a change in electronic structure of 2-Fe after O₂ adsorption. The reversible adsorption observed in 3-Cu suggests a weak interaction between O₂ and Cu²⁺ due to the Jahn-Teller effect. The properties of gas adsorption provide an insight into the performances of small molecules in the channels constructed by synthetic octanuclear model compounds, which are related to the interactions between the gas substrate and the heterometal cluster in biology.

Electronic Consequences of Ligand Substitution at Heterometal Centers in Polyoxovanadium Clusters: Controlling the Redox Properties through Heterometal Coordination Number

The rational control of the electrochemical properties of polyoxovanadate-alkoxide clusters is dependent on understanding the influence of various synthetic modifications on the overall redox processes of these systems. In this work, the electronic consequences of ligand substitution at the heteroion in a heterometal-functionalized cluster was examined. The redox properties of [V5 O6 (OCH3 )12 FeCl] (1-[V5 FeCl]) and [V5 O6 (OCH3 )12 Fe]X (2-[V5 Fe]X; X=ClO4 , OTf) were compared in order to assess the effects of changing the coordination environment around the iron center on the electrochemical properties of the cluster. Coordination of a chloride anion to iron leads to an anodic shift in redox events. Theoretical modelling of the electronic structure of these heterometal-functionalized clusters reveals that differences in the redox profiles of 1-[V5 FeCl] and 2-[V5 Fe]X arise from changes in the number of ligands surrounding the iron center (e.g., 6-coordinate vs. 5-coordinate). Specifically, binding of the chloride to the sixth coordination site appears to change the orbital interaction between the iron and the delocalized electronic structure of the mixed-valent polyoxovanadate core. Tuning the heterometal coordination environment can therefore be used to modulate the redox properties of the whole cluster.

Solid-state-stabilization of molecular vanadium oxides for reversible electrochemical charge storage

The solid-state stabilization of a molecular vanadium oxide cluster using molecular crystal engineering is reported together with its performance in electrochemical energy storage.

Site-selective halogenation of mixed-valent vanadium oxide clusters

Here, we expand on the synthesis and characterization of chloride-functionalized polyoxovanadate-alkoxide (POV-alkoxide) clusters, to include the halogenation of mixed-valent vanadium oxide assemblies.

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.

Atomically Defined Nanopropeller Fe3Co6Se8(Ph2PNTol)6: Functional Model for the Electronic Metal-Support Interaction Effect and High Catalytic Activity for Carbodiimide Formation

Atomically defined interfaces that maximize the density of active sites and harness the electronic metal-support interaction are desirable to facilitate challenging multielectron transformations, but their synthesis remains a considerable challenge. We report the rational synthesis of the atomically defined metal chalcogenide nanopropeller Fe₃Co₆Se₈L₆ (L = Ph₂PNTol) featuring three Fe edge sites, and its ensuing catalytic activity for carbodiimide formation. The complex interaction between the Fe edges and Co₆Se₈ support, including the interplay between oxidation state, substrate coordination, and metal-support interaction, is probed in detail using chemical and electrochemical methods, extensive single crystal X-ray diffraction, and electronic absorption and Mössbauer spectroscopy.

A simple one-step synthetic route to access a range of metal-doped polyoxovanadate clusters

A simple one-step synthetic route to access a range of metal-doped polyoxovanadate clusters.

Nitric oxide activation facilitated by cooperative multimetallic electron transfer within an iron-functionalized polyoxovanadate-alkoxide cluster

Cooperative multimetallic electron transfer to accommodate substrate binding.

A series of NO-bound, iron-functionalized polyoxovanadate–alkoxide (FePOV–alkoxide) clusters have been synthesized, providing insight into the role of multimetallic constructs in the coordination and activation of a substrate. Upon exposure of the heterometallic cluster to NO, the vanadium-oxide metalloligand is oxidized by a single electron, shuttling the reducing equivalent to the {FeNO} subunit to form a {FeNO}⁷ species. Four NO-bound clusters with electronic distributions ranging from [VV3VIV2]{FeNO}⁷ to [VIV5]{FeNO}⁷ have been synthesized, and characterized via¹H NMR, infrared, and electronic absorption spectroscopies. The ability of the FePOV–alkoxide cluster to store reducing equivalents in the metalloligand for substrate coordination and activation highlights the ultility of the metal-oxide scaffold as a redox reservoir.

Tuning the redox profiles of polyoxovanadate-alkoxide clusters via heterometal installation: toward designer redox Reagents

Herein, we describe a rational synthetic approach for tuning the electrochemical profiles of a series of Lindqvist polyoxovanadate-alkoxide clusters through heterometal functionalization. Synthetic procedures for group(IV) functionalization of the mixed-valent POV-alkoxide cluster, [V₆O₇(OCH₃)₁₂], are established, resulting in the heterometallic species, [NBu₄][V₅O₆(OCH₃)₁₂MOCH₃] (M = Ti, Zr, Hf). We demonstrate that these d⁰, heterometallic dopants anodically shift the potential of the electrochemical processes associated with the cluster, making the molecule more resistant to oxidation. Conversely, incorporation of electron rich heterometals yields a more readily oxidized molecule, with redox processes shifted cathodically. The predictable tuning and remarkable electrochemical profiles of this family of heterometal-functionalized polyoxometalates highlights their potential use as designer redox agents.

Site-selectivity in the halogenation of titanium-functionalized polyoxovanadate–alkoxide clusters

Syntheses and site differentiated reactivity for titanium-functionalized polyoxovanadate–alkoxide clusters are presented. We present insights into the mechanism and electronic consequences metal oxide halogenation.