Molecular Metal Oxide Research Articles

A Simple Approach to the Visible-Light Photoactivation of Molecular Metal Oxides.

This study explores a new method to maximize the visible-light-driven photocatalytic performance of organic-inorganic hybrid polyoxometalates (POMs). Experimental and theoretical investigations of a family of phosphonate-substituted POMs show that modification of grafted organic moieties can be used to tune the electronic structure and photoactivity of the metal oxide component. Unlike fully inorganic polyoxotungstates, these organic-inorganic hybrid species are responsive to visible light and function as photocatalysts (λ > 420 nm) in the decomposition of a model environmental pollutant. The degree of photoactivation is shown to be dependent on the nature of the inductive effect exerted by the covalently grafted substituent groups. This study emphasizes the untapped potential that lies in an orbital engineering approach to hybrid-POM design and helps to underpin the next generation of bespoke, robust, and cost-effective molecular metal oxide photoactive materials and catalysts.

Towards establishing the structure of liquid molecular transition metal oxides through a joint experimental and computational investigation

Towards establishing the structure of liquid molecular transition metal oxides through a joint experimental and computational investigation

Enhanced Capacitive Energy Storage in Polyoxometalate‐Doped Polypyrrole

High‐performance batteries and supercapacitors require the molecular‐level linkage of charge transport components and charge storage components. This study shows how redox‐tunable Lindqvist‐type molecular metal oxide anions [VnM6–nO19](2+n)− (M = W(VI) or Mo(VI); n = 0, 1, 2) can be incorporated in cationic polypyrrole (PPy) conductive polymer films by means of electrochemical polymerization. Electron microscopy and (spectro‐)electrochemistry show that the electroactivity and morphology of the composites can be tuned by Lindqvist anion incorporation. Reductive electrochemical “activation” of the Lindqvist–PPy composites leads to significantly increased electrical capacitance (range: ≈25–38 F g−1, increase up to ≈25×), highlighting that this general synthetic route gives access to promising capacitive materials with suitable long‐term stability. Electrochemical, electron microscopic, and Raman spectroscopic analyses together with density functional theory (DFT) calculations provide molecular‐level insight into the effects of Lindqvist anion incorporation in PPy films and their role during reductive activation. The study therefore provides fundamental understanding of the principles governing the bottom‐up integration of molecular components into nanostructured composites for electrochemical energy storage.

Cobalt based water oxidation catalysis with photogenerated Ru(bpy)33+: Different kinetics and competent species starting from a molecular polyoxometalate and metal oxide nanoparticles capped with a bisphosphonate alendronate pendant

Polyoxometalates (POMs) are a class of inorganic compounds that have been extensively considered in the development of robust water oxidation catalysts (WOCs). In particular, cobalt based POMs have shown remarkable activity as WOCs under both dark and light activated cycles, although current studies focus on their possible evolution to different forms of cobalt oxides under operating conditions. In this paper, we investigate the activity in water oxidation catalysis of a heptanuclear cobalt based POM [{(B-α-PW9O34)Co3(OH)(H2O)2(O3PC(O)(C3H6NH3)PO3)}2Co]14−, hereafter Co7(Ale)14− and of 5nm cobalt oxide nanoparticles, Co3O4(Ale), both carrying the same bisphosphonate alendronate ligand. Under the application of an anodic bias in borate buffer pH 8, both species lead to the formation of an active cobalt oxide layer at the working electrode, although in the case of Co7(Ale)14− participation of the POM scaffold in the electrodeposition is evidenced by the presence of W(VI) in the active film. In particular, Co7(Ale)14− and Co3O4(Ale) were investigated in a photocatalytic cycle with Ru(bpy)32+ (bpy=2,2′-bipyridine) as the photosensitizer and persulfate as the electron acceptor, where the photogenerated oxidant is Ru(bpy)33+. Key findings are: (i) ion pairing and aggregate formation of anionic Co7(Ale)14− and Co3O4(Ale) with cationic Ru(bpy)32+ are limited under photocatalytic regime (20mM aqueous buffer, pH 8), as confirmed by the solubility product constant KSP=2.1±0.4×10−15M4 for a Co7(Ale)14−:Ru(bpy)32+ 1:3 adduct, emission quenching experiments, and bimolecular electron transfer (ET) kinetics; (ii) fast ET to Ru(bpy)33+, approaching diffusional rates, occurs from Co7(Ale)14− (kET=9.4±0.3×108 M−1s−1) overarching by four orders of magnitude the Co3O4(Ale) nanoparticles; (iii) Co7(Ale)14− enables up to 18 photoinduced ET events in 90ms, while in the same timeframe, a non-complete bleaching recovery (ET events<1) results from the Co3O4(Ale) nanoparticle analogs; (iv) photocatalytic oxygen evolution is observed with similar rates for both cobalt based species (TOF per cobalt=1.2–1.7×10−3s−1); (v) dark O–O bond formation is the likely rate determining step of water oxidation for both systems, as the O2 evolution kinetics do not depend on photoinduced ET rates and oxygenic activity, and light irradiation intensity (λ=450nm, 0.4–1.6×10−7einsteins−1); (vi) leaching of Co(II) ions from Co7(Ale)14− in catalytic conditions is estimated below 2% by 31P NMR line broadening experiments and supported by persistent flash photolysis kinetics; (vii) the ultimate fate of Co7(Ale)14− under oxygenic turnover involves a polyoxometalate structural rearrangement, while no evidence of cobalt oxide nanoparticles formation is provided by TEM images; (viii) conversely, Co3O4(Ale) nanoparticles preserve their size along the light driven cycle. Therefore, this study supports the involvement of POM based intermediates in water oxidation catalysis, while future perspectives should be directed toward the identification of such main characters, and of the mechanism of O2 construction.

A dielectric anomaly observed for doubly reduced mixed-valence polyoxometalate.

Benzimidazolium is revealed to form quasi-isostructural crystals of mixed-valence molecular metal oxides that consist of [PMoMoO40]5- and fully oxidized [BWO40]5-. The structure and dielectric behaviour were compared and a dielectric anomaly was evident due to the electric dipole relaxation in the mixed-valence system.

Covalent Photosensitizer-Polyoxometalate-Catalyst Dyads for Visible-Light-Driven Hydrogen Evolution.

A general concept for the covalent linkage of coordination compounds to bipyridine-functionalized polyoxometalates is presented. The new route is used to link an iridium photosensitizer to an Anderson-type hydrogen-evolution catalyst. This covalent dyad catalyzes the visible-light-driven hydrogen evolution reaction (HER) and shows superior HER activity compared with the non-covalent reference. Hydrogen evolution is observed over periods >1 week. Spectroscopic, photophysical, and electrochemical analyses give initial insight into the stability, electronic structure, and reactivity of the dyad. The results demonstrate that the proposed linkage concept allows synergistic covalent interactions between functional coordination compounds and reactive molecular metal oxides.

(Invited) In-Situ X-Ray Structural Characterization of Water-Splitting Catalysts in Artificial Photosynthesis

Resolution of structures and structural dynamics for catalysts at active electrode interfaces is necessary to resolve mechanisms for function and assembly in solar fuels applications. We have been developing porous, high-surface area, low X-ray background, 3-D electrodes for interfacial structural analysis of molecular and amorphous metal oxide water-splitting catalysts using X-ray scattering and atomic pair distribution function (PDF) analysis. We demonstrate the opportunity to use ALD-ITO coated microchannel plates and other micro- to nano-porous substrates for characterization of domain structures for amorphous metal oxides water-oxidation catalysts used in artificial leaf assemblies. This presentation will compare domain structures resolved for amorphous cobalt oxide films formed electrolytically as a function of ligand and electrolyte anion variation. For cobalt oxide catalysts, the results show that phosphate is unique in restricting domain size to small, single-layer domains where defects and edge distortions can be resolved. The results show that catalytic currents for do not scale with the domain edge content in the cobaltate films, which are the presumed sites for water oxidation catalysis, but instead track the extend of domain layering and conductivity properties of the films. These results support the view that overall catalytic activity in amorphous cobaltate thin films is determined by the charge accumulation and transport properties. We have extended these techniques demonstrate the opportunity to measure PDF for homogeneous water oxidation catalysts. PDF measurements with 0.2 Å spatial resolution are found to be sufficient to resolve redox and catalytic state dependent changes in ligand structure. This work establishes a foundation for extending the X-ray PDF method for the analysis of atomic structure changes in molecular catalysts driven by excited-state, sequential single electron photo-chemistry.

Polyoxometalate-based ionic liquids-promoted CO2 conversion

Polyoxometalates (POMs) are a class of molecular metal oxides, showing numerous applications in various chemical processes due to their unique acid/base and redox features. By adjusting the tunable molecular structures of the anions and counter cations, plenty of POM-based ionic liquids (POM-based ILs) have been fabricated to be used in various fields, such as catalysis, structural chemistry and material science. As a class of excellent catalysts, POM-based ILs have shown advantages in the emerging field of CO2 utilization such as CO2 capture, cycloaddition of CO2 to epoxides, and reduction of CO2, owing to the efficient activation of CO2 by POM anions. This review summarizes recent advances in the catalysis of POM-based ILs, and particularly highlights the areas that are related to CO2 conversion.

"CLICKable" azide-functionalized phosphonates for the surface-modification of molecular and solid-state metal oxides.

The covalent functionalization of metal oxide surfaces with organic ligands gives unique organic-inorganic hybrids. Here we report a bifunctional organic tether combining a phosphonate for metal oxide anchoring with an organic azide for attachment of organic groups. Stable binding of the tether to molecular and solid-state metal oxides is demonstrated and the subsequent "CLICKability" of the TiO2-anchored tether is shown. The phosphonate-azide reported could in future allow the general linkage of functional organic groups to metal oxides.

Challenges in polyoxometalate-mediated aerobic oxidation catalysis: catalyst development meets reactor design.

Selective catalytic oxidation is one of the most widely used chemical processes. Ideally, highly active and selective catalysts are used in combination with molecular oxygen as oxidant, leading to clean, environmentally friendly process conditions. For homogeneous oxidation catalysis, molecular metal oxide anions, so-called polyoxometalates (POMs) are ideal prototypes which combine high reactivity and stability with chemical tunability on the molecular level. Typically, POM-mediated aerobic oxidations are biphasic, using gaseous O2 and liquid reaction mixtures. Therefore, the overall efficiency of the reaction is not only dependent on the chemical components, but requires chemical engineering insight to design reactors with optimized productivity. This Perspective shows that POM-mediated aerobic liquid-phase oxidations are ideal reactions to be carried out in microstructured flow reactors as they enable facile mass and energy transfer, provide large gas-liquid interfaces and can be easily upscaled. Recent advances in POM-mediated aerobic catalytic oxidations are therefore summarized with a focus on technological importance and mechanistic insight. The principles of reactor design are discussed from a chemical engineering point of view with a focus on homogeneous oxidation catalysis using O2 in microfluidic systems. Further, current limitations to catalytic activity are identified and future directions based on combined chemistry and chemical engineering approaches are discussed to show that this approach could lead to sustainable production methods in industrial chemistry based on alternative energy sources and chemical feedstocks.

ChemInform Abstract: Polyoxometalate‐Based Phase Transfer Catalysis for Liquid‐Solid Organic Reactions: A Review

AbstractReview: 79 refs.

Assembly of Tungsten-Oxide-Based Pentagonal Motifs in Solution Leads to Nanoscale {W48}, {W56}, and {W92} Polyoxometalate Clusters.

We report an approach to synthesize molecular tungsten-oxide-based pentagonal building blocks, in a new {W21 O72} unit, and show how this leads to a family of gigantic molecular architectures including [H12W48O164](28-) {W48}, [H20W56O190](24-) {W56}, and [H12W92O311](58-) {W92}. The {W48} and {W56} clusters are both dimeric species incorporating two {W21} units and the {W56} species is the first example of a molecular metal oxide cluster containing a chiral "double-stranded" motif which is stable in solution as confirmed by mass spectrometry. The {W92} anion having four {W21} units is one of the largest transition metal substituted isopolyoxotungstates known.

Assembly of Tungsten‐Oxide‐Based Pentagonal Motifs in Solution Leads to Nanoscale {W48}, {W56}, and {W92} Polyoxometalate Clusters

AbstractWe report an approach to synthesize molecular tungsten‐oxide‐based pentagonal building blocks, in a new {W21O72} unit, and show how this leads to a family of gigantic molecular architectures including [H12W48O164]28− {W48}, [H20W56O190]24− {W56}, and [H12W92O311]58− {W92}. The {W48} and {W56} clusters are both dimeric species incorporating two {W21} units and the {W56} species is the first example of a molecular metal oxide cluster containing a chiral “double‐stranded” motif which is stable in solution as confirmed by mass spectrometry. The {W92} anion having four {W21} units is one of the largest transition metal substituted isopolyoxotungstates known.

Efficient photocatalytic H2 evolution catalyzed by an unprecedented robust molecular semiconductor {Fe11} nanocluster without cocatalysts at neutral conditions

An unprecedented molecular polyanionic metal oxide cluster of eleven-FeIII substituted antimoniotungstate Na27[Fe11(H2O)14(OH)2(W3O10)2(α-SbW9O33)6]·103H2O(1) based on earth abundant elements has been successfully synthesized. A careful physical investigation of this molecule reveals that VB=−6.34eV/CB=−4.39eV, this unique physical properties ensures that 1 can be defined as a new type of nanoscale molecular semiconductor. This compound showed remarkable catalytic property for light driven H2 evolution activity. A H2 evolution rate of 820μmolh−1g−1 was achieved in the presence of 1 without adding any co-catalyst at neutral condition, which is a very high rate among all polyoxotungstate photocatalysts reported thus far. Six evidences proved that 1 is a true stable molecular catalyst during the photocatalytic hydrogen evolution.

Investigating the formation of "molybdenum blues" with gel electrophoresis and mass spectrometry.

The reduction of solutions of acidified molybdate leads to the formation of a family of nanostructured molybdenum blue (MB) wheels which are linked together in a series of complex reaction networks. These networks are complex because the species which define the nodes are extremely labile, unstable, and common to many different networks. Herein, we combine gel electrophoresis and electrospray ionization mass spectrometry (ESI-MS) to investigate the effect of the pH and the ratio of reactants and reducing agents, R (R = [S2O4(2-)]/[MoO4(2-)]), on the complex underlying set of equilibria that make up MBs. By mapping the reaction parameter space given by experimental variables such as pH, R, solvent medium, and type of counterion, we show that the species present range from nanostructured MB wheels (comprising ca. 154 Mo atoms) to smaller molecular capsules, [(SO3)2Mo(V)2Mo(VI)16O54](6-) ({S2Mo18}), and templated hexameric [(μ6-SO3)Mo(V)6O15(μ2-SO3)3](8-)({S4Mo6}) anions. The parallel effects of templation and reduction on the self-assembly process are discussed, taking into consideration the Lewis basicity of the template, the oxidation state of the Mo centers, and the polarity of the reaction medium. Finally, we report a new type of molecular cage (TBA)5[Na(SO3)2(PhPO3)4Mo(V)4Mo(VI)14O49]·nMeCN (1), templated by SO3(2-) anions and decorated by organic ligands. This discovery results from the exploration of the cooperative effect of two anions possessing comparable Lewis basicity, and we believe this constitutes a new synthetic approach for the design of new nanostructured molecular metal oxides and will lead to a greater understanding of the complex reaction networks underpinning the assembly of this family of nanoclusters.

Controlled reactivity tuning of metal-functionalized vanadium oxide clusters.

Controlling the assembly and functionalization of molecular metal oxides [Mx Oy ](n-) (M=Mo, W, V) allows the targeted design of functional molecular materials. While general methods exist that enable the predetermined functionalization of tungstates and molybdates, no such routes are available for molecular vanadium oxides. Controlled design of polyoxovanadates, however, would provide highly active materials for energy conversion, (photo-) catalysis, molecular magnetism, and materials science. To this end, a new approach has been developed that allows the reactivity tuning of vanadium oxide clusters by selective metal functionalization. Organic, hydrogen-bonding cations, for example, dimethylammonium are used as molecular placeholders to block metal binding sites within vanadate cluster shells. Stepwise replacement of the placeholder cations with reactive metal cations gives mono- and difunctionalized clusters. Initial reactivity studies illustrate the tunability of the magnetic, redox, and catalytic activity.

Polyoxometalate-conductive polymer composites for energy conversion, energy storage and nanostructured sensors.

The exchange of electric charges between a chemical reaction centre and an external electrical circuit is critical for many real-life technologies. This perspective explores the "wiring" of highly redox-active molecular metal oxide anions, so-called polyoxometalates (POMs) to conductive organic polymers (CPs). The major synthetic approaches to these organic-inorganic hybrid materials are reviewed. Typical applications are highlighted, emphasizing the current bottlenecks in materials development. Utilization of the composites in the fields of energy conversion, electrochemical energy storage, sensors and nanoparticle "wiring" into conductive materials are discussed. The outlook section presents the authors' views on emerging fields of research where the combination of POMs and CPs can be expected to provide novel materials for groundbreaking new technologies. These include light-weight energy storage, high-sensitivity toxin sensors, artificial muscles, photoelectrochemical devices and components for fuel cells.

Annealing-free highly crystalline solution-processed molecular metal oxides for efficient single-junction and tandem polymer solar cells

The compatibility of polyoxometalates as hole extraction layers in single-junction organic photovoltaics and as recombination layers in polymer tandem cells is demonstrated.

Polyoxometalates in Visible-Light Photocatalysis and Solar Energy Conversion

Polyoxometalates (POMs) are an important class of anionic molecular metal oxides, boasting an enormous range of structures and properties. They are commonly based on the high oxidation states of V, Mo and W and show strong absorptions in the ultraviolet (UV) due to their O-to-metal charge transfer transitions. This feature, and the ability of many species to act as stable, multi-electron acceptors means that POMs have a well-established UV photochemistry, primarily in the oxidation of organic substrates. The last decade has witnessed increasing efforts to achieve photochemistry with POMs under visible light, through sensitization with molecular or semiconductor chromophores, or by extending the visible absorption of the POMs themselves. Notable achievements have included light driven oxidation of water, and light driven reduction of protons to H2. In this review, we summarize these achievements and provide a perspective on a growing body of work exploiting POMs in light energy conversion.

ChemInform Abstract: Recent Advances in Polyoxometalate‐Based Heterogeneous Catalytic Materials for Liquid‐Phase Organic Transformations

AbstractReview: 220 refs.