Katharina Kastner

A Molecular Placeholder Strategy To Access a Family of Transition‐Metal‐Functionalized Vanadium Oxide Clusters

Systematic access to metal-functionalized polyoxometalates has thus far been limited to lacunary tungsten oxide and molybdenum oxide clusters. The first controlled, stepwise bottom-up assembly route to metal-functionalized molecular vanadium oxides is now presented. A di-vacant vanadate cluster with two metal binding sites, (DMA)2[V12O32Cl](3-) (DMA = dimethylammonium) is formed spontaneously in solution and characterized by single-crystal X-ray diffraction, ESI mass spectrometry, (51)V NMR spectroscopy, and elemental analyses. In the cluster, the metal binding sites are selectively blocked by hydrogen-bonded DMA placeholder cations. Reaction of the cluster with transition metals TM (Fe(3+), Co(2+), Cu(2+), Zn(2+)) gives access to mono-functionalized vanadate clusters (DMA)[{TM(L)}V12O32Cl](n-) (L = ligand). Metal binding is accomplished by significant distortions of the vanadium oxide framework reminiscent of a pincer movement. Cluster stability under technologically relevant conditions in the solid-state and solution is demonstrated.

Oxidation-driven self-assembly gives access to high-nuclearity molecular copper vanadium oxide clusters

We report a general fragmentation-and-re-assembly route which gives access to high-nuclearity, mixed-metal polyoxometalate clusters. Reduced vanadium(IV) precursors are oxidatively dis-assembled into reactive fragments which subsequently re-aggregate under template control in a one-pot reaction. It is shown that the oxidative dis-assembly is required, as the use of vanadium(V)-based precursors results in the formation of smaller clusters. The principle is exemplified by the synthesis of a ca. 1.8 × 1.7 × 1.0 nm3, 36-nuclear copper vanadium oxide cluster, (nBu4N)4[Cu6V30O82(NO3)2(CH3CN)6]. The cluster is characterized in the solid-state and in solution by single-crystal XRD, ESI-MS and other spectroscopic and electrochemical measurements. Several lines of evidence show that the compound is indeed formed exclusively by fully oxidized vanadium(V) centres. In addition, primary fragmentation products of the type [VO(dmso)5]2+ were isolated. The cuprovanadate cluster features pentagonal secondary building units of the type {(V)M5} (M = Cu, V) which show similar structural function as the well-known {(Mo)Mo5} pentagons observed in giant molybdate clusters. The observation suggests that more complex vanadate clusters might be accessible based on these pentagonal units.

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.

Solvent-shielding allows the self-assembly of supramolecular 1D barium vanadate chains

A proof of principle study is reported which introduces a new synthetic route for the assembly of 1-dimensional barium-linked polyoxovanadate chains. Bulky coordinating solvents featuring a binding site and a blocking site are employed and allow the controlled linkage of decavanadate clusters, [V10O28]6−, by barium(II) centres. Using the bulky solvent N-methyl-2-pyrrolidone (NMP), it is shown that a complex supramolecular architecture, {[Ba(nmp)4(H2O)]2[H4V10O28]}{[Ba(nmp)3(H2O)2][H3V10O28]}2·2H2O·10 NMP}∞ (1) can be accessed where mono- and dinuclear barium units link decavanadate clusters into a linear chain. Using N,N-dimethyl formamide (DMF), a less complex architecture, [(Ba(dmf)4]2[H2V10O28] (2), is formed where only dinuclear barium(II) linkages are observed. Theoretical Hirshfeld-analysis of the crystal lattices is used to examine the ‘shielding’ effect of the bulky ligands. ESI-mass-spectrometric studies give insight into potential fragments formed in solutions of 1 and 2.

Redox-active organic–inorganic hybrid polyoxometalate micelles

A redox-active hybrid organic–inorganic polyoxometalate surfactant showed solvent-dependent self-assembly to form nano-scale architectures. The supramolecular assemblies exhibited contrasting electronic structure and redox activity to their molecular building units, and were found to be stable under electrochemical reduction and re-oxidation.

Orbital Engineering: Photoactivation of an Organofunctionalized Polyoxotungstate

Tungsten-based polyoxometalates (POMs) have been employed as UV-driven photo-catalysts for a range of organic transformations. Their photoactivity is dependent on electronic transitions between frontier orbitals and thus manipulation of orbital energy levels provides a promising means of extending their utility into the visible regime. Herein, an organic-inorganic hybrid polyoxometalate, K6 [P2 W17 O57 (PO5 H5 C7 )2 ]⋅6 C4 H9 NO, was found to exhibit enhanced redox behaviour and photochemistry compared to its purely inorganic counterparts. Hybridization with electron-withdrawing moieties was shown to tune the frontier orbital energy levels and reduce the HOMO-LUMO gap, leading to direct visible-light photoactivation of the hybrid and establishing a simple, cheap and effective approach to the generation of visible-light-activated hybrid nanomaterials.

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.

Stabilization of Low-Valent Iron(I) in a High-Valent Vanadium(V) Oxide Cluster

Low-valent iron centers are critical intermediates in chemical and bio-chemical processes. Herein, we show the first example of a low-valent FeI center stabilized in a high-valent polyoxometalate framework. Electrochemical studies show that the FeIII -functionalized molecular vanadium(V) oxide (DMA)[FeIII ClVV12 O32 Cl]3- (DMA=dimethylammonium) features two well-defined, reversible, iron-based electrochemical reductions which cleanly yield the FeI species (DMA)[FeI ClVV12 O32 Cl]5- . Experimental and theoretical studies including electron paramagnetic resonance spectroscopy and density functional theory computations verify the formation of the FeI species. The study presents the first example for the seemingly paradoxical embedding of low-valent metal species in high-valent metal oxide anions and opens new avenues for reductive electron transfer catalysis by polyoxometalates.

3D printable photochromic molecular materials for reversible information storage

The formulation of advanced molecular materials with bespoke polymeric ionic-liquid matrices that stabilize and solubilize hybrid organic-inorganic polyoxometalates and allow their processing by additive manufacturing, is effectively demonstrated. The unique photo and redox properties of nanostructured polyoxometalates are translated across the scales (from molecular design to functional materials) to yield macroscopic functional devices with reversible photochromism. These properties open a range of potential applications including reversible information storage based on controlled topological and temporal reduction/oxidation of pre-formed printed devices. This approach pushes the boundaries of 3D printing to the molecular limits, allowing the freedom of design enabled by 3D printing to be coupled with the molecular tuneability of polymerizable ionic liquids and the photoactivity and orbital engineering possible with hybrid polyoxometalates.

Aerobic Oxidation Catalysis by a Molecular Barium Vanadium Oxide

Aerobic catalytic oxidations are promising routes to replace environmentally harmful oxidants with O2 in organic syntheses. Here, we report a molecular barium vanadium oxide, [Ba4 (dmso)14 V14 O38 (NO3 )] (={Ba4 V14 }) as viable homogeneous catalyst for a series of oxidation reactions in N,N-dimethyl formamide solution under oxygen (8 bar). Starting from the model compound 9,10-dihydroanthracene, we report initial dehydrogenation/ aromatization leading to anthracene formation; this intermediate is subsequently oxidized by stepwise oxygen transfer, first giving the mono-oxygenated anthrone and then the di-oxygenated target product, anthraquinone. Comparative reaction analyses using the Neumann catalyst [PV2 Mo10 O40 ]5- as reference show that oxygen diffusion into the reaction mixture is the rate-limiting step, resulting in accumulation of the reduced catalyst species. This allows us to propose improved reactor designs to overcome this fundamental challenge for aerobic oxidation catalysis.