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, ⁵¹V 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 (Fe³⁺, Co²⁺, Cu²⁺, Zn²⁺) 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.     

Supramolecular Recognition Influences Magnetism in [X@HVIV8VV14O54]6− Self‐Assemblies with Symmetry‐Breaking Guest Anions

Mixed‐valence polyoxovanadates(IV/V) have emerged as one of the most intricate class of supramolecular all‐inorganic host species, able to encapsulate a wide variety of smaller guest templates during their self‐assembly formation process. As showcased herein, the incorporation of guests, though governed solely by ultra‐weak electrostatic and van der Waals interactions, can cause drastic effects on the electronic and magnetic characteristics of the shell complex of the polyoxovanadate. We address the question of methodology for the magnetochemical analysis of virtually isostructural {V^IV/V₂₂O₅₄}‐type polyoxoanions of D_2d symmetry enclosing diamagnetic VO₂F₂^? (C_2v), SCN^? (C_∞v), or ClO₄^? (T_d) template anions. These induce different polarization effects related to differences in their geometric structures, symmetry, ion radii, and valence shells, eventually resulting in a supramolecular modulation of magnetic exchange between the V(3d) electrons that are partly delocalized over the {V₂₂O₅₄} shells. We also include the synthesis and characterization of the novel [V^VO₂F₂@HV^IV₈V^V₁₄O₅₄]^6? system that comprises the rarely encountered discrete difluorovanadate anion as a quasi‐isolated guest species.     

Configurational Isomerism in Polyoxovanadates

A water‐soluble derivative of the polyoxovanadate {V₁₅E₆O₄₂} (E=semimetal) archetype enables the study of cluster shell rearrangements driven by supramolecular interactions. A reaction unique to E=Sb, induced exclusively by ligand metathesis in peripheral [Ni(ethylenediamine)₃]²⁺ counterions, results in the formation of the metastable α₁* configurational isomer of the {V₁₄Sb₈O₄₂} cluster type. Contrary to all other polyoxovanadate shell architectures, this isomer comprises an inward‐oriented vanadyl group and is ca. 50 and 12 kJ mol^?1 higher in energy than the previously isolated α and β isomers, respectively. We discuss this unexpected reaction in light of supramolecular Sb?O???V and Sb?O???Sb contacts manifested in {V₁₄Sb₈O₄₂}₂ dimers detected in the solid state. ESI MS experiments confirm the stability of these dimers also in solution and in the gas phase. DFT calculations indicate that other, as of yet elusive isomers of {V₁₄Sb₈}, might be accessible as well.     

Soluble Hetero-Polyoxovanadates and Their Solution Chemistry Analyzed by Electrospray Ionization Mass Spectrometry

Polyoxometalates (POMs) are an intriguing class of compounds due to their tremendous structural variety and the wide spectrum of resulting properties, which make them interesting for applications in fields such as catalysis, material science or nanotechnology. Their ability to form large supramolecular architectures by self-assembly offers an entry to complex, functional systems. After an introduction into the structure and synthesis of POMs of the early transition metals, recently discovered water-soluble antimonato polyoxovanadates (Sb-POVs) and the investigation of their chemical reactivity are discussed. Electrospray ionization mass spectrometry (ESI-MS) is presented as an analytical technique suitable to investigate the structure of complex POM assemblies in solution and to probe the underlying reactivity and formation mechanisms. This Minireview highlights the first studies on the soluble Sb-POVs and how the knowledge of their reactivity obtained by ESI-MS has fostered the syntheses of numerous novel Sb-POV compounds.     

Visible‐Light‐Driven Water Oxidation by a Molecular Manganese Vanadium Oxide Cluster

Photosynthetic water oxidation in plants occurs at an inorganic calcium manganese oxo cluster, which is known as the oxygen evolving complex (OEC), in photosystem II. Herein, we report a synthetic OEC model based on a molecular manganese vanadium oxide cluster, [Mn₄V₄O₁₇(OAc)₃]^3?. The compound is based on a [Mn₄O₄]⁶⁺ cubane core, which catalyzes the homogeneous, visible‐light‐driven oxidation of water to molecular oxygen and is stabilized by a tripodal [V₄O₁₃]^6? polyoxovanadate and three acetate ligands. When combined with the photosensitizer [Ru(bpy)₃]²⁺ and the oxidant persulfate, visible‐light‐driven water oxidation with turnover numbers of approximately 1150 and turnover frequencies of about 1.75 s^?1 is observed. Electrochemical, mass‐spectrometric, and spectroscopic studies provide insight into the cluster stability and reactivity. This compound could serve as a model for the molecular structure and reactivity of the OEC and for heterogeneous metal oxide water‐oxidation catalysts.     

High Nuclearity Antimonato‐Polyoxovanadate Cluster {V15Sb6O42} as a Synthon for the Solvothermal in situ Generation of α‐ and β‐{V14Sb8O42} Isomers

New heteroatom polyoxovanadates (POVs) were synthesized by applying a water‐soluble high‐nuclearity cluster as new synthon. The [V₁₅Sb₆O₄₂]^6? cluster shell exhibiting D₃ symmetry was in situ transformed into completely different cluster shells, namely, the α‐[V₁₄Sb₈O₄₂]^4? isomer with D_2d and the β‐[V₁₄Sb₈O₄₂]^4? isomer with D_2h symmetry. The solvothermal reaction of {Ni(en)₃}₃[V₁₅Sb₆O₄₂(H₂O)_x] ? 15 H₂O (x=0 or 1; en=ethylenediamine) in water led to the crystallization of [{Ni(en)₂}₂V₁₄Sb₈O₄₂] ? 5.5 H₂O containing the β‐isomer. The addition of [Ni(phen)₃](ClO₄)₂ ? 0.5 H₂O (phen=1,10‐phenanthroline) to the reaction slurry gave the new compound {Ni(phen)₃}₂[V₁₄Sb₈O₄₂] ? phen ? 12 H₂O with the α‐isomer. Both transformation reactions are complex due the change of symmetry, the chemical composition, and rearrangement of the VO₅ square pyramids and Sb₂O₅ handle‐like moieties.     

Controlling Nucleation and Crystal Growth of a Distinct Polyoxovanadate Cluster: An In Situ Energy Dispersive X‐ray Diffraction Study under Solvothermal Conditions

The formation of the antimonato polyoxovanadates [V₁₄Sb₈(C₆H₁₅N₃)₄O₄₂(H₂O)] ? 4H₂O ( 1 ), (C₆H₁₇N₃)₂[V₁₅Sb₆(C₆H₁₅N₃)₂O₄₂(H₂O)] ? 2.5H₂O ( 2 ), {C₆H₁₅N₃}₄[V₁₆Sb₄O₄₂] 2H₂O ( 3 ) (C₆H₁₅N₃=1‐(2‐aminoethyl)piperazine, AEP) has been studied under solvothermal conditions by using in situ energy dispersive X‐ray diffraction (EDRXD). The syntheses were performed with an identical ratio for Sb₂O₃ and NH₄VO₃. If the reactions slurries are not stirred during the solvothermal reaction and by applying 70–75 % amine concentration, the products contain all three compounds, whereas 3 is observed at 80 %. Under stirring conditions, variation of the concentration of AEP led to crystallization of the three different compounds at distinct concentrations, that is, 1 is formed at 75 %, 1 and 2 between 75 and 80 % and 3 at 80 %. At an amine concentration of 77.5 %, first reflections of 2 occurred and at later stages, compound 1 started to crystallize. The sample with the lowest number of V^IV species was formed at the lowest amine concentration, whereas crystallization of 3 required the highest concentration. The formation of the compounds occurred without crystalline intermediates and/or precursors. With increasing reaction temperature, the incubation time was significantly reduced.     

Layered polyoxovanadate V15: from structure to highly anisotropic magnetism§

Abstract

In this article, we give a short overview of the theoretical background and properties of the unique polyoxovanadate K₆[V^IV₁₅As₆O₄₂(H₂O)]·8H₂O (abbreviated V₁₅) containing a cluster of the fifteen V^IV ions and exhibiting layers of different magnetizations. Analysis of the magnetic and spectroscopic properties of this fascinating system constitutes a distinct trend in the study of complex nanoscopic polyoxometalates. V₁₅ cluster exhibits strong magnetic anisotropy closely related to the unquenched orbital angular momentum in the degenerate ground state. The degeneracy is interrelated with the spin frustration in V₁₅ which gives rise to a variety of interesting phenomena observed in this polyoxovanadate. We summarize the studies of the isotropic and antisymmetric exchange interaction and analyze its manifestations in magnetic properties, electron paramagnetic resonance, and inelastic neutron scattering spectra as well as in the spin-phonon relaxation in V₁₅. We briefly discuss also a long coherence time (Rabi oscillations) first discovered in molecular magnets due to which the V₁₅ cluster proved to be a prerequisite for the use of molecular magnets in quantum computing.     

A Lacunary Polyoxovanadate Precursor and Transition-Metal-Sandwiched Derivatives for Catalytic Oxidation of Sulfides

A SeO₃-centered lacunary Keggin-type heteropolyoxovanadate (hetero-POV) K₆H₂[SeV₁₀O₂₈(SeO₃)₃] ⋅ 14 H₂O ( 1 ) was isolated by one-pot reaction of KVO₃ and SeO₂ under acidic conditions. X-ray studies revealed that it comprised a single {VO₅}-capped trivacant B-α-type Keggin ion [SeV₉O₃₃(VO)]¹⁴⁻ with its lacunary sites decorated by three {SeO₃} pyramids. Interestingly, this new basic hetero-POV building block was further used as a precursor to assemble with different transition-metal (TM) ions, yielding a series of TM-sandwiched POVs K₆H₈[(SeV₁₀O₂₈(SeO₃)₃)₂(M(H₂O)₄)] ⋅ 24 H₂O (M²⁺=Mn²⁺ ( 2 ), Co²⁺ ( 3 ), Zn²⁺ ( 4 )). All four compounds were characterized by single-crystal X-ray structure analysis, IR, X-ray photoelectron spectroscopy (XPS), EPR, and ⁵¹V NMR spectroscopy. Importantly, three TM-sandwiched derivatives exhibited effective catalytic activity for the heterogeneous oxidative desulfurization of sulfides at room temperature.