Redox-inactive ions control the redox-activity of molecular vanadium oxides

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.

Hybrid Molecular Magnets with Lanthanide- and Countercation-Mediated Interfacial Electron Transfer between Phthalocyanine and Polyoxovanadate

A series of {V₁₂}-nuclearity polyoxovanadate cages covalently functionalized with one or sandwiched by two phthalocyaninato (Pc) lanthanide (Ln) moieties via V-O-Ln bonds were prepared and fully characterized for paramagnetic Ln = Sm^III-Er^III and diamagnetic Ln = Lu^III, including Y^III. The LnPc-functionalized {V₁₂O₃₂} cages with fully oxidized vanadium centers in the ground state were isolated as (nBu₄N)₃[HV₁₂O₃₂Cl(LnPc)] and (nBu₄N)₂[HV₁₂O₃₂Cl(LnPc)₂] compounds. As corroborated by a combined experimental (EPR, DC and AC SQUID, laser photolysis transient absorption spectroscopy, and electrochemistry) and computational (DFT, MD, and model Hamiltonian approach) methods, the compounds feature intra- and intermolecular electron transfer that is responsible for a partial reduction at V(3d) centers from V^V to V^IV in the solid state and at high sample concentrations. The effects are generally Ln dependent and are clearly demonstrated for the (nBu₄N)₃[HV₁₂O₃₂Cl(LnPc)] representative with Ln = Lu^III or Dy^III. Intramolecular charge transfer takes place for Ln = Lu^III and occurs from a Pc ligand via the Ln center to the {V₁₂O₃₂} core of the same molecule, whereas for Ln = Dy^III, only intermolecular charge transfer is allowed, which is realized from Pc in one molecule to the {V₁₂O₃₂} core of another molecule usually via the nBu₄N⁺ countercation. For all Ln but Dy^III, two of these phenomena may be present in different proportions. Besides, it is demonstrated that (nBu₄N)₃[HV₁₂O₃₂Cl(DyPc)] is a field-induced single molecule magnet with a maximal relaxation time of the order 10^-3 s. The obtained results open up the way to further exploration and fine-tuning of these three modular molecular nanocomposites regarding tailoring and control of their Ln-dependent charge-separated states (induced by intramolecular transfer) and relaxation dynamics as well as of electron hopping between molecules. This should enable us to realize ultra-sensitive polyoxometalate powered quasi-superconductors, sensors, and data storage/processing materials for quantum technologies and neuromorphic computing.

Solvent-controlled ion-coupled charge transport in microporous metal chalcogenides

Interactions between ions and itinerant charges govern electronic processes ranging from the redox chemistry of molecules to the conductivity of organic semiconductors, but remain an open frontier in the study of microporous materials. These interactions may strongly influence the electronic behavior of microporous materials that confine ions and charges to length scales comparable to proton-coupled electron transfer. Yet despite mounting evidence that both solvent and electrolyte influence charge transport through ion-charge interactions in metal-organic frameworks, fundamental microscopic insights are only just beginning to emerge. Here, through electrochemical analysis of two open-framework chalcogenides TMA₂FeGe₄S₁₀ and TMA₂ZnGe₄S₁₀, we outline the key signatures of ion-coupled charge transport in band-type and hopping-type microporous conductors. Pressed-pellet direct-current and impedance techniques reveal that solvent enhances the conductivity of both materials, but for distinct mechanistic reasons. This analysis required the development of a fitting method that provides a novel quantitative metric of concerted ion-charge motion. Taken together, these results provide chemical parameters for a general understanding of electrochemistry in nanoconfined spaces and for designing microporous conductors and electrochemical methods used to evaluate them.

A General Access Route to High-Nuclearity, Metal-Functionalized Molecular Vanadium Oxides

Molecular metal oxides are key materials in diverse fields like energy storage and conversion, molecular magnetism and as model systems for solid-state metal oxides. To improve their performance and increase the variety of accessible motifs, new synthetic approaches are necessary. Herein, we report a universal, new precursor to access different metal-functionalized polyoxovanadate (POV) clusters. The precursor is synthesized by a novel solid-state thermal treatment procedure. Solution-phase test reactions at room temperature and pressure show that reaction of the precursor with various metal nitrate salts gives access to a range of metal-functionalized POVs. The first nitrate-templated molecular calcium vanadate cluster is reported. We show that this precursor could open new access routes to POV components for molecular magnetism, energy technologies or catalysis.

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.

Tunable Band Gaps in MUV-10(M): A Family of Photoredox-Active MOFs with Earth-Abundant Open Metal Sites

Titanium-based metal-organic frameworks (Ti-MOFs) have attracted intense research attention because they can store charges in the form of Ti³⁺ and they serve as photosensitizers to cocatalysts through heterogeneous photoredox reactions at the MOF-liquid interface. Both the charge storage and charge transfer depend on the redox potentials of the MOF and the molecular substrate, but the factors controlling these energetic aspects are not well understood. Additionally, photocatalysis involving Ti-MOFs relies on cocatalysts rather than the intrinsic Ti reactivity, in part because Ti-MOFs with open metal sites are rare. Here, we report that the class of Ti-MOFs known as MUV-10 can be synthetically modified to include a range of redox-inactive ions with flexible coordination environments that control the energies of the photoactive orbitals. Lewis acidic cations installed in the MOF cluster (Cd²⁺, Sr²⁺, and Ba²⁺) or introduced to the pores (H⁺, Li⁺, Na⁺, K⁺) tune the electronic structure and band gaps of the MOFs. Through the use of optical redox indicators, we report the first direct measurement of the Fermi levels (redox potentials) of photoexcited MOFs in situ. Taken together, these results explain the ability of Ti-MOFs to store charges and provide design principles for achieving heterogeneous photoredox chemistry with electrostatic control.

Double-Shelled Hollow SiO2 @N-C Nanofiber Boosts the Lithium Storage Performance of [PMo12 O40 ]3

Polyoxometalates (POMs)-based materials, with high theoretical capacities and abundant reversible multi-electron redox properties, are considered as promising candidates in lithium-ion storage. However, the poor electronic conductivity, low specific surface area and high solubility in the electrolyte limited their practical applications. Herein, a double-shelled hollow PMo₁₂ -SiO₂ @N-C nanofiber (PMo₁₂ -SiO₂ @N-C, where PMo₁₂ is [PMo₁₂ O₄₀ ]^3- , N-C is nitrogen-doped carbon) was fabricated for the first time by combining coaxial electrospinning technique, thermal treatment and electrostatic adsorption. As an anode material for LIBs, the PMo₁₂ -SiO₂ @N-C delivered an excellent specific capacity of 1641 mA h g^-1 after 1000 cycles under 2 A g^-1 . The excellent electrochemical performance benefited from the unique double-shelled hollow structure of the material, in which the outermost N-C shell cannot only hinder the agglomeration of PMo₁₂ , but also improve its electronic conductivity. The SiO₂ inner shell can efficiently avoid the loss of active components. The hollow structure can buffer the volume expansion and accelerate Li⁺ diffusion during lithiation/delithiation process. Moreover, PMo₁₂ can greatly reduce charge-resistance and facilitate electron transfer of the entire composites, as evidenced by the EIS kinetics study and lithium-ion diffusion analysis. This work paves the way for the fabrication of novel POM-based LIBs anode materials with excellent lithium storage performance.

High Proton-Conductivity in Covalently Linked Polyoxometalate-Organoboronic Acid-Polymers

The controlled bottom-up design of polymers with metal oxide backbones is a grand challenge in materials design, as it could give unique control over the resulting chemical properties. Herein, we report a 1D-organo-functionalized polyoxometalate polymer featuring a purely inorganic backbone. The polymer is self-assembled from two types of monomers, inorganic Wells-Dawson-type polyoxometalates, and aromatic organo-boronates. Their covalent linkage results in 1D polymer strands, which combine an inorganic oxide backbone (based on B-O and Nb-O linkages) with functional organic side-chains. The polymer shows high bulk proton conductivity of up to 1.59×10-1  S cm-1 at 90 °C and 98 % relative humidity. This synthetic approach could lead to a new class of organic-inorganic polymers where function can be designed by controlled tuning of the monomer units.

Unlocking Phase Diagrams for Molybdenum and Tungsten Nanoclusters and Prediction of their Formation Constants

Understanding and controlling aqueous speciation of metal oxides are key for the discovery and development of novel materials, and challenge both experimental and computational approaches. Here we present a computational method, called POMSimulator, which is able to predict speciation phase diagrams (Conc. vs pH) for multispecies chemical equilibria in solution, and which we apply to molybdenum and tungsten isopolyoxoanions (IPAs). Starting from the MO₄ monomers, and considering dimers, trimers, and larger species, the chemical reaction networks involved in the formation of [H₃₂Mo₃₆O₁₂₈]^8- and [W₁₂O₄₂]^12- are sampled in an automatic manner. This information is used for setting up ∼10⁵ speciation models, and from there, we generate the speciation phase diagrams, which show an insightful picture of the behavior of IPAs in aqueous solution. Furthermore, we predict the values of 107 formation constants for a diversity of molybdenum and tungsten molecular oxides. Among these species, we could include several pentagonal-shaped species and very reactive tungsten intermediates as well. Last but not least, the calibration employed for correcting the density functional theory (DFT) Gibbs energies is remarkably similar for both metals, which suggests that a general rule might exist for correcting computed free energies for other metals.

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".

Supramolecular assembly of a hierarchically structured 3D potassium vanadate framework

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