Electric conductivity of nanocluster polyoxomolybdates in the solid state and solutions

The electric conductivity of nanocluster polyoxomolybdates (POMs) Mo₁₃₂ and Mo₇₂Fe₃₀ with a keplerate structure in aqueous solutions, of these POMs in the solid state, and of Mo₁₃₈ with a toroidal structure was studied. Data on the concentration dependence of the equivalent electric conductivity in solutions and on the specific conductivity of the solid samples were obtained. Solid Mo₁₃₂ showed predominantly electron type of conductivity at nearly room temperatures due to the presence of molybdenum at different oxidation levels; Mo₇₂Fe₃₀, which has metal components at the highest oxidation levels, was found to be poor electric conductor; Mo₁₃₈, which has a lower content of low-valence molybdenum than Mo₁₃₂, showed intermediate conductivity and a reversible transition between the proton and electron types of conductivity.     

Effect of polyoxomolybdate nanocluster doping on the dielectric characteristics of polyvinyl alcohol nanocomposite films

Electrophysical parameters of polyvinyl alcohol (PVA) films comprising nanocluster molybdenum-based polyoxometalates (POMs), namely electrical conductivity, dielectric loss tangent, and dielectric permittivity, have been studied. Frequency and concentration curves have been plotted for these properties, which characterize the relaxation parameters of polymeric macromolecules and interactions of the polymer with spheroidal Keplerate POMs (Mo₁₃₂ and Mo₇₂Fe₃₀) and toroidal POMs (Mo₁₃₈).     

Features of thermophotoinitiated degradation of nanocluster polyoxomolybdate Mo<Subscript>132</Subscript> and its polymer-containing composites

An EPR spectroscopy study was made into the simultaneous effect of intense degradation factors (UV radiation, elevated temperature) on nanocluster porous polyoxomolybdate Mo₁₃₂ of the Keplerate type in the solid state and also into the behavior of film composites including water-soluble nonionogenic polymers (polyvinylpyrrolidone, polyethylene glycol) under the action of these factors. The effect of mutual stabilization of polyoxometalate and polymer components in the studied composites was detected.     

Viscosity and Electrophysical Characteristics of Solutions Containing Nanocluster Polyoxometalates and Polyvinylpyrrolidone

The physicochemical characteristics of solutions intended for preparing composites that contain a water-soluble polymer, polyvinylpyrrolidone (PVP), and nanocluster polyoxometalates (POMs) based on molybdenum with Keplerate (Mo₁₃₂ and Mo₇₂Fe₃₀) and toroidal structure (Mo₁₃₈) are studied. Concentration dependences of the kinematic viscosity of solutions upon changes in the absolute and relative contents of the polymer component are obtained along with the frequency dependences of the dielectric loss tangents of such solutions that correlate with them. The data allow us to confirm the composition of POM—polymers associates that form, and to estimate the relaxation parameters of ions in solutions.     

Encapsulation of Arenes within a Porous Molybdenum Oxide {Mo132} Nanocapsule

The use of confined space to modulate chemical reactivity and to sequester organic compounds spans significant disciplines in chemistry and biology. Here, the inclusion and assembly of arenes into a water‐soluble porous metal oxide nanocapsule [{(Mo^VI)Mo^V₅O₂₁(H₂O)₆}₁₂{Mo^V₂O₄(CH₃COO)}₃₀]^42? (Mo₁₃₂) is reported. The uptake of benzene, halobenzenes, alkylbenzenes, phenols, and other derivatives was studied by NMR, where it was possible to follow the encapsulation process from the outside of the capsule through its pores and then into the interior. The importance of size or shape of the arenes, and various intermolecular bond interactions contributed by the benzene substituent on the encapsulation process was studied, showing the importance of π–π stacking and CH–π interactions. Furthermore, by using NOESY, ROESY, and HOESY NMR techniques it was possible to understand the interaction of the encapsulated arenes and the acetate linkers or ligands that line the interior of the Mo₁₃₂ capsule.     

Fundamental Aspects of Xanthene Dye Aggregation on the Surfaces of Nanocluster Polyoxometalates: H- to J-Aggregate Switching

The induced aggregation of the xanthene dye rhodamine B (RhB) on metal oxide centers belonging to the highly symmetric surfaces of precise nanoscale templates with Keplerate (Mo₁₃₂) or toroidal (Mo₁₃₈) structures has been studied. With the joint use of the Langmuir isotherm and full Stern–Volmer models, the thermodynamic reasons for dye adsorption on the nanocluster surface, such as a mixture of monomer, H-aggregate (H-dimer), and J-aggregate forms (which can coexist or switch one into another under the exact conditions), were established: this was shown through UV/Vis and fluorescence spectroscopies. By using the framework of the exciton model, it is shown that the angle (α) between the transition dipole moments of RhB is very sensitive to surface strain inside the dye sub-monolayer. As a result, it is possible to switch from H- to J-aggregates by the post-functionalization of polyoxometalate (POM)-RhB associates by the surfactant bilayer shell, which allows the surface strain to grow. Recommendations are provided for managing the appearance of H- or J-aggregates on metal oxide (or polyelectrolyte) surfaces during photovoltaic or bioimaging applications.     

Pore “Softening” and Emergence of Breathability Effects of New Keplerate Nano-Containers

The self-assembly of porous molecular nanocapsules offer unique opportunities to investigate a range of interesting phenomena and applications. However, to design nanocapsules with pre-defined properties, thorough understanding of their structure-property relation is required. Here, we report the self-assembly of two elusive members of the Keplerate family, [Mo₁₃₂Se₆₀O₃₁₂(H₂O)₇₂(AcO)₃₀]⁴²⁻ {Mo₁₃₂Se₆₀} 1 and [W₇₂Mo₆₀Se₆₀O₃₁₂(H₂O)₇₂(AcO)₃₀]⁴²⁻ {W₇₂Mo₆₀Se₆₀} 2 , that have been synthesised using pentagonal and dimeric ([Mo₂O₂Se₂]²⁺) building blocks and their structures have been confirmed via single crystal X-ray diffractions. Our comparative study involving the uptake of organic ions and the related ligand exchange of various ligand sizes by the {Mo₁₃₂Se₆₀} and previously reported Keplerates {Mo₁₃₂O₆₀}, {Mo₁₃₂S₆₀} based on the ligand exchange rates, revealed the emergence of increased “breathability” that dominates over the pore size as we transition from the {Mo₁₃₂S₆₀} to the “softer” {Mo₁₃₂Se₆₀} molecular nano-container.     

Coordination Chemistry of the Nanocluster [HxPMo12O40?H4Mo72Fe30(O2CMe)15O254(H2O)98]

The ligand exchange chemistry for the iron-molybdenum nanocluster [H₂PMo₁₂O₄₀⊂H₄Mo₇₂Fe₃₀(O₂CMe)₁₅O₂₅₄(H₂O)_98-x(EtOH)_x], {Mo₇₂Fe₃₀(Mo₁₂P)}-EtOH, with 3,5-lutidene, 3-butylpyridine, octanol, octanoic acid, 1-hexadecanethiol, tetraethylene glycol, and dodecylbenzenesulfonic acid is reported. The structure of {Mo₇₂Fe₃₀(Mo₁₂P)} is preserved throughout the reaction and TGA analysis indicates between 5 and 15 ligands could be attached per {Mo₇₂Fe₃₀(Mo₁₂P)}. AFM height measurements increase with increased ligand length; however, the apparent particle diameter appears to be smaller than expected as the ligands increase in size consistent with the adopting a non-extended conformation in a similar manner to that observed for self-assembled monolayers.     

Aerobic {Mo72V30} nanocluster‐catalysed heterogeneous one‐pot tandem synthesis of benzimidazoles

A novel heterogeneous one‐pot protocol is developed for tandem aerobic synthesis of benzimidazoles through dehydrogenative coupling of primary benzylic alcohols and aromatic diamines co‐catalysed by Keplerate‐type {Mo₇₂V₃₀} polyoxometalate and N‐hydroxyphthalimide (NHPI). The catalytic system also works well for the synthesis of benzimidazoles using benzaldehydes, as commonly used starting materials, in the absence of NHPI. The high activity of the solid nanocluster provides standard conditions avoiding current limitations of oxidation methods including high catalyst loadings. The spectral results and leaching experiments revealed that the nanocapsule preserved its structural integrity after being reused in consecutive runs.     

Structural Evolution of Giant Polyoxometalate: From “Keplerate” to “Lantern” Type Mo132 for Improved Oxidation Catalysis

Structural variants of high-nuclearity clusters are extremely important for their modular assembly study and functional expansion, yet the synthesis of such giant structural variants remains a great challenge. Herein, we prepared a lantern-type giant polymolybdate cluster ( L -Mo₁₃₂ ) containing equal metal nuclearity with the famous Keplerate type Mo₁₃₂ ( K -Mo₁₃₂ ). The skeleton of L -Mo₁₃₂ features a rare truncated rhombic triacontrahedron, which is totally different with the truncated icosahedral K -Mo₁₃₂ . To the best of our knowledge, this is the first time to observe such structural variants in high-nuclearity cluster built up of more than 100 metal atoms. Scanning transmission electron microscopy reveals that L -Mo₁₃₂ has good stability. More importantly, because the pentagonal [Mo₆O₂₇]ⁿ⁻ building blocks in L -Mo₁₃₂ are concave instead of convex in the outer face, it contains multiple terminal coordinated water molecules on its outer surface, which make it expose more active metal sites to display superior phenol oxidation performance, which is more higher than that of K -Mo₁₃₂ coordinated in M=O bonds on the outer surface.     

Layered ionic liquid-crystalline organisations built from nano-capsules [Mo132O312S60(SO4)x(H2O)132−2x](12 + 2x)− and DODA+ cations

Two dimethyldioctadecylammonium (DODA⁺) salts of a new keplerate with the general formula [Mo₁₃₂O₃₁₂S₆₀(SO₄)_x(H₂O)_132–2x]^{(12 + 2x)?} and abbreviated DODA_n?Mo₁₃₂S₆₀ (n = 44, 56) were synthesised and characterised. Both clusters were fully characterised by the combination of Polarised Optical Microscopy, Differential Scanning Calorimetry and Small-angle X-Ray Diffraction showing self-organisation in lamellar (L) liquid crystalline phases. We demonstrated that the lamellar periodicity h of the mesophases can be controlled with the number of DODA⁺ associated to the clusters. Finally, these new results were compared to those gained from a previously published analogue, the fully oxo keplerate noted DODA₃₆?Mo₁₃₂ that also self-organise with temperature, but in a slightly more structured lamellar liquid crystalline phase.     

Thermal behavior of polyoxometalate Mo132

Thermal destruction of (NH₄)₄₂[Mo ₇₂ ^VI Mo ₆₀ ^V O₃₇₂(H₃CCOO)₃₀(H₂O)₇₂]30H₃CCOONH₄ · 250H₂O, polyoxometalate Mo132, was analyzed. Differential scanning calorimetry (DSC) and mass spectrometry were investigative tools. Thermal destruction occurs in air and in nitrogen in three main stages. First, water is eliminated. The thermolysis products at higher temperatures include acetamide and acetonitrile. Molybdenum oxides are formed in solid products through an amorphous stage. The utility of NMR spectroscopy for analyzing poly(oxometalates) is demonstrated.     

New data for molybdenum polyoxometallate with the buckyball structure containing acetate groups and compositions based thereon

Synthesis and characterization of the polyoxometallate (NH₄)₄₂[Mo₇₂^VIMo₆₀^VO₃₇₂(H₃CCOO)₃₀(H₂O)₇₂]30H₃CCOONH₄ · 250H₂O (Mo132) with the buckyball structure were carried out. New crystallographic data were obtained. Stability of Mo132 was studied as a function of external factors, acidity, concentration of the solution, and the presence of a polymeric component. The composition of the most stable complexes of buckyballs with polymers was confirmed. The aggregation of buckyballs in the presence of polyvinylpyrrolidone were studied. Thermochemical studies of Mo132 interaction with polyvinyl alcohol in films were carried out. Electromigration of polymer-salt complexes in solutions was established, the possibility of change of the sign of associates upon interaction of Mo132 anions with lanthanum cations was confirmed.     

Keplerate巨型纳米多孔聚氧钼酸盐作为可重复使用催化剂用于合成1,2,4,5-四取代咪唑(英文)

The Keplerate‐type giant nanoporous isopolyoxomolybdate (NH4)42[MoVI72MoV60O372‐(CH3COO)30(H2O)72], denoted {Mo132}, has been used as a catalyst for the synthesis of1,2,4,5‐tetrasubstituted imidazoles by the one‐pot, four‐component thermal reaction of benzil with aromatic aldehydes, primary amines, and ammonium acetate under solvent‐free conditions. The catalyst was prepared according to a previously published literature procedure using inexpensive and readily available starting materials, and subsequently characterized by FT‐IR, UV and X‐ray diffraction spectroscopy, as well as microanalysis. The results showed that {Mo132} exhibited high catalytic activity towards the synthesis of 1,2,4,5‐tetrasubstituted imidazoles, with the desired products being formed in good to high yields. Furthermore, the catalyst was recyclable and could be reused at least three times without any discernible loss in its catalytic activity. Overall, this new catalytic method for the synthesis of 1,2,4,5‐tetrasubstituted imidazoles provides rapid access to the desired compounds following a simple work‐up procedure, and avoids the use of harmful organic solvents. This method therefore represents a significant improvement over the methods currently available for the synthesis of tetrasubstituted imidazoles.     

Synthesis and Characterizations of Keplerate Nanocapsules Incorporating L- and D-Tartrate Ligands

Five new tartrate-containing Keplerate compounds have been synthesized and characterized in the solid state and in solution. These characterizations evidenced the total replacement of inner sulfate ligands by L- or D-tartrate ligands in aqueous medium under heating during several days. To our knowledge these compounds correspond to the first Keplerate molecules incorporating chiral ligands. The ¹H NMR studies supported by X-ray crystallographic analysis are consistent with the coordination of 24–30 tartrates within the Mo₁₃₂ capsule which are located in close vicinity. The NMR signals of the encapsulated ligands appear particularly broad which precludes the use of advanced NMR methodologies but the solid state NMR provided further characterization of ligand substitution within the capsule by carboxylates. To our knowledge it is the first time that a solid state NMR study of a Keplerate is reported in the literature.     

Supramolecular stabilization of the phosphite-based polyoxomolybdate [Mo6(PO3)(HPO3)3O18]

A novel phosphite-based hetero-polyoxomolybdate, [Mo₆(PO₃)(HPO₃)₃O₁₈]⁹⁻, has been isolated and structurally characterized. The most striking feature of this polyanion is the presence of peripheral phosphite groups linked to the MoO₆ octahedra. In the solid state, this cluster shows strong hydrogen bonding interactions that apparently play a key role in its stabilization and isolation from solution.     

Study of the stability of solid polyoxometalate Mo72Fe30 with a buckyball structure

Samples of polyoxometalate Mo₇₂Fe₃₀: [Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₁₂{Mo₂O₇(H₂O)}₂ {H₂Mo₂O₈(H₂O)}(H₂O)₉₁] · ??150H₂O with a buckyball structure, which can be both crystalline and amorphous, were synthesized. It was shown that such samples can be studied by neutron diffraction. The stability of Mo₇₂Fe₃₀ to heating and UV light exposure (in poly(vinyl alcohol) and polyvinylpyrrolidone films) was studied by IR, EPR, and electronic absorption spectroscopy; thermal analysis; and mass spectrometry. Mo₇₂Fe₃₀ was found to be less stable to heating and irradiation in a poly(vinyl alcohol) film as compared with the related polyoxometallate Mo₁₃₂ free of iron. The sorption properties of Mo₇₂Fe₃₀ to organic vapors and its stability under sorption conditions were studied. It was demonstrated that, in addition to sorption, organic substances cause the destruction of buckyballs.     

Two novel windmill-like tetrasupporting heteropolyoxometalates: [MoVI7MoVVIV8O40(PO4)][M(phen)2(OH)]2[M(phen)2(OEt)]2 (M=Co,Ni)

Two interesting neutral tetrasupporting heteropolyoxometalates: [Mo^VI₇Mo^VV^IV₈O₄₀(PO₄)][M(phen)₂(OH)]₂[M(phen)₂(OEt)]₂·xH₂O (phen=1,10-phenanthroline, EtOH=ethanol, M=Co, x=7, 1; M=Ni, x=6, 2) were hydrothermally prepared and structurally characterized. The mixed molybdenum–vanadium polyoxoanion [Mo^VI₇Mo^VV^IV₈O₄₀(PO₄)]⁴⁻ exist in both two complexes, which acts as a bridge to covalently link two pairs of transition metal complex fragments, generating neutral windmill-like trimetallic nanocluster polyoxometalates. Variable-temperature magnetic susceptibility measurements of complexes 1 and 2 reveal that antiferromagnetic exchange interaction exists in this type of trimetallic tetrasupporting heteropolyoxometalates.     

Densely Packed Hydrophobic Clustering: Encapsulated Valerates Form a High‐Temperature‐Stable {Mo132} Capsule System

Porous molecular nanocontainers of {Mo₁₃₂}‐type Keplerates offer unique opportunities to study a wide variety of relevant phenomena. An impressive example is provided by the highly reactive {Mo₁₃₂‐CO₃} capsule, the reaction of which with valeric acid results in the very easy release of carbon dioxide and the uptake of 24 valerate ions/ligands that are integrated as a densely packed aggregate, thus indicating the unique possibility of hydrophobic clustering inside the cavity. Two‐dimensional NMR techniques were used to demonstrate the presence of the 24 valerates and the stability of the capsule up to ca. 100 °C. Increasing the number of hydrophobic parts enhances the stability of the whole system. This situation also occurs in biological systems, such as globular proteins or protein pockets.