Functional polyoxometalate assemblies: from host-guest complexes to porous frameworks

The host-guest chemistry of two sets of isopolyoxometalate clusters is investigated. In particular the binding modes of cationic alkali and alkali earth metals through coordinative interactions with the cluster anions {Mo36} and {W36} were compared and contrasted. It was shown that the ionic radii of the cations are crucial in the isolation of discrete molecules or infinite 3D frameworks. Crystal engineering allowed the introduction of organic amine guest molecules by the formation of a set of intermolecular interactions between the cluster anions and the amine cations. In addition it was shown that by engaging the ammonium guest molecules in additional supramolecular interactions, the framework assembly in the solid state could be directed. Further investigations focused on the assembly of supramolecular polyoxometalate-based framework materials where organic ammonium cations were used as hydrogen-bond donors. The structural effects of three amines were compared and contrasted and it was established that the use of rigid planar molecules resulted in the formation of 2D networks whereas the use of flexible amines gave supramolecular 1D chains. Based on these results the synthesis of a functional framework was achieved; a three-component approach allowed the formation of a chiral, porous framework which shows structural stability and reversible solvent sorption properties. In a different approach, the cross-linking of polyoxometalates using transition metal linkers in organic solvents was studied. It was shown that silver(I) cations are highly versatile linking units and allow the linking of {V10} cluster anions into 1D zigzag chains and 2D planar networks. The silver units assemble into supramolecular, trinuclear complexes which are supported by bridging organic ligands. Careful choice of the reaction conditions allowed the formation of a 3D framework based on {W12} units. The tungstate clusters are cross-linked by dinuclear {Ag2} linkers which are held together by argentophilic silver-silver interactions and result in the formation of a porous framework. The material features reversible sorption capabilities and can be used to sequester small molecules as well as transition metal cations from organic solvents

Polyoxometalate‐Single Atom Catalysts (POM‐SACs) in Energy Research and Catalysis

Single atom catalysts—SACs—have received intense interest in sustainable energy research due to their enormous application potential and broad catalytic scope. In particular, SACs anchored on molecular metal oxides—polyoxometalates (POMs)—offer unrivalled possibilities as models to understand the function of these complex systems on the atomic level. Research questions which are difficult to address for classical heterogeneous SACs can be addressed by experiment and theory using POM-SACs as prototype catalysts. This review reviews the emerging field of POM-SAC research with a focus on fundamental properties and their application in energy conversion and storage technologies including water splitting, CO/CO2- as well as N2-activation. Current limitations of the field are identified and possible routes to overcome these are described. Future areas of development, including polyoxometalate-single-site catalysts, are highlighted, and critically assessed to enable the community to develop this fast-moving field further

Instantaneous formation of polyoxometalate-based cerium vanadium oxide gels

et al.The instantaneous formation of mechanically stable cerium vanadium oxide gels starting from soluble polyoxovanadates is reported together with initial application studies. Upon addition of phosphoric acid to solutions containing a vanadium oxide source (e.g. (nBuN)[VO]) and Ce, instantaneous formation (reaction time <1 s) of a vanadium oxide gel is observed. The gel shows unique mechanical and thermal stabilities (up to ∼180 °C). High permeability of the gel is observed, allowing its use for long-term acid delivery into aqueous media or for the adsorption of organic aromatic dye pollutants from solution. A range of spectroscopy and electron microscopy techniques provide insight into the gel formation and the gel composition: an intertwined 3D matrix of nanowires (d ∼ 10 nm) containing cerium, vanadium oxide and phosphate is identified as an inorganic matrix which enables the formation of the mechanically stable gel.Financial support by the Deutscher Akademischer Austauschdienst (DAAD) EU COST Action CM1203, Ulm University, Friedrich-Alexander-Universität Erlangen-Nürnberg and Fundación General CSIC (Programa ComFuturo) is gratefully acknowledged.Peer Reviewe

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

The Reactivity and Stability of Polyoxometalate Water Oxidation Electrocatalysts

This review describes major advances in the use of functionalized molecular metal oxides (polyoxometalates, POMs) as water oxidation catalysts under electrochemical conditions. The fundamentals of POM-based water oxidation are described, together with a brief overview of general approaches to designing POM water oxidation catalysts. Next, the use of POMs for homogeneous, solution-phasewater oxidation is described togetherwith a summary of theoretical studies shedding light on the POM-WOC mechanism. This is followed by a discussion of heterogenization of POMs on electrically conductive substrates for technologically more relevant application studies. The stability of POM water oxidation catalysts is discussed, using select examples where detailed data is already available. The review finishes with an outlook on future perspectives and emerging themes in electrocatalytic polyoxometalate-based water oxidation research

POM@ZIF Derived Mixed Metal Oxide Catalysts for Sustained Electrocatalytic Oxygen Evolution

The design of efficient and stable oxygen evolution reaction (OER) catalysts based on noble-metal-free materials is crucial for energy conversion and storage. In this work, it was demonstrated how polyoxometalate (POM)-doped ZIF-67 can be converted into a stable oxygen evolution electrocatalyst by chemical etching, cation exchange, and thermal annealing steps. Characterization by X-ray photoelectron spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy and Raman spectroscopy indicate that POM-doped ZIF-67 derived carbon-supported metal oxides were synthesized. The resulting composite shows structural and compositional advantages which lead to low overpotential (306 mV at j=10 mA ⋅ cm−2) and long-term stability under harsh OER conditions (1.0 M aqueous KOH)

Polyoxometalates on Functional Substrates: Concepts, Synergies, and Future Perspectives

Polyoxometalates (POMs) are molecular metal oxide clusters that feature a broad range of structures and functionalities, making them one of the most versatile classes of inorganic molecular materials. They have attracted widespread attention in homogeneous catalysis. Due to the challenges associated with their aggregation, precipitation, and degradation under operational conditions and to extend their scope of applications, various strategies of depositing POMs on heterogeneous substrates have been developed. Recent ground-breaking developments in the materials chemistry of supported POM composites are summarized and links between molecular-level understanding of POM-support interactions and macroscopic effects including new or optimized reactivities, improved stability, and novel function are established. Current limitations and future challenges in studying these complex composite materials are highlighted, and cutting-edge experimental and theoretical methods that will lead to an improved understanding of synergisms between POM and support material from the molecular through to the nano- and micrometer level are discussed. Future development in this fast-moving field is explored and emerging fields of research in POM heterogenization are identified

Covalent Linkage of BODIPY-Photosensitizers to Anderson-Type Polyoxometalates Using CLICK Chemistry

The covalent attachment of molecular photosensitizers (PS) to polyoxometalates (POMs) opens new pathways to PS-POM dyads for light-driven charge-transfer and charge-storage. Here, we report a synthetic route for the covalent linkage of BODIPY-dyes to Anderson-type polyoxomolybdates by using CLICK chemistry (i. e. copper-catalyzed azide-alkyne cycloaddition, CuAAC). Photophysical properties of the dyad were investigated by combined experimental and theoretical methods and highlight the role of both sub-components for the charge-separation properties. The study demonstrates how CLICK chemistry can be used for the versatile linkage of organic functional units to molecular metal oxide clusters. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH Gmb