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

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.

Improved solubility of titanium-doped polyoxovanadate charge carriers for symmetric non-aqueous redox flow batteries

Non-aqueous redox flow batteries constitute a promising solution for grid-scale energy storage due to the ability to achieve larger cell voltages than can be readily accessed in water. However, their widespread application is limited by low solubility of the electroactive species in organic solvents. In this work, we demonstrate that organic functionalization of titanium-substituted polyoxovanadate-alkoxide clusters increases the solubility of these assemblies over that of their homoleptic congeners by a factor of >10 in acetonitrile. Cyclic voltammetry, chronoamperometry, and charge-discharge cycling experiments are reported, assessing the electrochemical properties of these clusters relevant to their ability to serve as multielectron charge carriers for energy storage. The kinetic implications of ligand variation are assessed, demonstrating the role of ligand structure on the diffusivity and heterogeneous rates of electron transfer in mixed-metal charge carriers. Our results offer new insights into the impact of structural modifications on the physicochemical properties of these assemblies.

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.

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.

Systematic Approach to the Synthesis of Cobalt-Containing Polyoxometalates for Their Application as Energy Storage Materials

New energy storage materials are an object of study within the framework of the global energy transition. The development of renewable sources is being boosted thanks to stationary energy storage systems such as redox flow batteries (RFBs). This work reports the synthesis of the cobalt-containing Keggin-type polyoxometalates [CoW₁₂O₄₀]^6- (CoW₁₂) and [Co(H₂O)SiW₁₁O₃₉]^6- (CoSiW₁₁), which have previously been shown to have applicability in RFBs. These procedures were reassessed to meet the strict requirements associated with the further implementation of RFBs, including fast and affordable synthetic procedures with high reaction yields. In contrast to the lengthy and complicated synthetic approaches published to date, the optimized synthesis reported in this work enables the isolation of the pure crystalline salt of the CoW₁₂ anion with a 75% reduction of the time of the whole reaction procedure, eliminating tedious steps such as the recrystallization and including a 20% increased yield. The control of the stoichiometry, fine-tuning of reaction conditions, and the identification of intermediate species, as well as the acidic equilibria taking place during the process, were monitored via thermal, spectroscopic, and structural analyses. In the case of the CoSiW₁₁ anion, its preparation was based on a simple and highly efficient procedure. Moreover, promising electrochemical properties were observed with the use of the one-pot synthetic approach, in which the stoichiometric amounts of the starting reagents are dissolved in the supporting electrolyte to be directly implemented as the electrolyte for a RFB.

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.

PEG-Infiltrated Polyoxometalate Frameworks with Flexible Form-Factors

We present the synthesis of metal oxide frameworks composed of the Preyssler anion, [NaP₅W₃₀O₁₁₀]^14-, bridged with transition-metal cations and infiltrated with polyethylene glycol. The frameworks can be dissolved in water to form freestanding rigid or flexible films or gels. Powder X-ray diffraction shows that all form-factors maintain the short-range order of the original crystals. Raman spectroscopy reveals that, similar to hydrogels, the macroscopic mechanical properties of these composites are dependent on the water content and the extent of hydrogen-bonding within the water network. The understanding gained from these studies facilitates solution-phase processing of polyoxometalate frameworks into flexible form factors.

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.

Computational Insights into the Nucleation of Mixed-Valent Polyoxovanadate Alkoxide Clusters

The synthesis of novel tunable electroactive species remains a key challenge for a wide range of chemical applications such as redox catalysis, energy storage, and optoelectronics. In recent years, polyoxovanadate (POV) alkoxide clusters have emerged as a new class of compounds with highly promising electrochemical applications. However, our knowledge of the formation pathways of POV alkoxides is rather limited. Understanding the speciation of POV alkoxides is fundamental for controlling and manipulating the evolution of transient species during their nucleation and therefore tuning the properties of the final product. Here, we present a computational study of the nucleation pathways of a mixed-valent [(V^V_6-nV^IV_nO₆)(O)(O-CH₃)₁₂]^(4-n)+ POV alkoxide cluster in the absence of reducing agents other than methanol.

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.

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".

Density functional theory and thermodynamics analysis of MAl12 Keggin substitution reactions: Insights into ion incorporation and experimental confirmation

Polyaluminum cations, such as the MAl₁₂ Keggin, undergo atomic substitutions at the heteroatom site (M), where nanoclusters with M = Al³⁺, Ga³⁺, and Ge⁴⁺ have been experimentally studied. The identity of the heteroatom M has been shown to influence the structural and electronic properties of the nanocluster and the kinetics of ligand exchange reactions. To date, only three ε-analogs have been identified, and there is a need for a predictive model to guide experiment to the discovery of new MAl₁₂ species. Here, we present a density functional theory (DFT) and thermodynamics approach to predicting favorable heteroatom substitution reactions, alongside structural analyses on hypothetical ε-MAl₁₂ nanocluster models. We delineate trends in energetics and geometry based on heteroatom cation properties, finding that Al³⁺-O bond lengths are related to heteroatom cation size, charge, and speciation. Our analyses also enable us to identify potentially isolable new ε-MAl₁₂ species, such as FeAl₁₂ ⁷⁺. Based upon these results, we evaluated the Al³⁺/Zn²⁺/Cr³⁺ system and determined that substitution of Cr³⁺ is unfavorable in the heteroatom site but is preferred for Zn²⁺, in agreement with the experimental structures. Complimentary experimental studies resulted in the isolation of Cr³⁺-substituted δ-Keggin species where Cr³⁺ substitution occurs only in the octahedral positions. The isolated structures Na[AlO₄Al_9.6Cr_2.4(OH)₂₄(H₂O)₁₂](2,6-NDS)₄(H₂O)₂₂ (δ-Cr_nAl_13-n-1) and Na[AlO₄Al_9.5Cr_2.5(OH)₂₄(H₂O)₁₂](2,7-NDS)₄(H₂O)_18.5 (δ-Cr_nAl_13-n-2) are the first pieces of evidence of mixed Al³⁺/Cr³⁺ Keggin-type nanoclusters that prefer substitution at the octahedral sites. The δ-Cr_nAl_13-n-2 structure also exhibits a unique placement of the bound Na⁺ cation, which may indicate that Cr³⁺ substitution can alter the surface reactivity of Keggin-type species.

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.

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.

Redox Tuning via Ligand-Induced Geometric Distortions at a YMn3O4 Cubane Model of the Biological Oxygen Evolving Complex

The function of proteins involved in electron transfer is dependent on cofactors attaining the necessary reduction potentials. We establish a mode of cluster redox tuning through steric pressure on a synthetic model related to Photosystem II. Resembling the cuboidal [CaMn₃O₄] subsite of the biological oxygen evolving complex (OEC), [Mn₄O₄] and [YMn₃O₄] complexes featuring ligands of different basicity and chelating properties were characterized by cyclic voltammetry. In the absence of ligand-induced distortions, increasing the basicity of the ligands results in a decrease of cluster reduction potential. Contraction of Y-oxo/Y–Mn distances by 0.1/0.15 Å enforced by a chelating ligand results in an increase of cluster reduction potential even in the presence of strongly basic donors. Related protein-induced changes in Ca-oxo/Ca–Mn distances may have similar effects in tuning the redox potential of the OEC through entatic states and may explain the cation size dependence on the progression of the S-state cycle.

The redox properties of [YMn₃O₄] and [Mn₄O₄] complexes featuring bridging ligands of different basicity and chelating properties are reported. In the absence of ligand-induced geometric distortions, increasing the basicity of the ligands results in a decrease of cluster reduction potential. A chelating ligand results in contractions of Y-oxo distances by ∼0.1 Å, which correlates with an increase of cluster reduction potential even in the presence of strongly basic donors.

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.

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.