Keplerate Type Research Articles

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

AbstractStructural 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.

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 [Mo6 O27 ]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.

Prerequisites and prospects for the development of novel systems based on the Keplerate type polyoxomolybdates for the controlled release of drugs and fluorescent molecules

Synthetic approaches were proposed to the development of hybrid organic—inorganic systems based on giant Keplerate polyoxometalates (POM Mo132), biocompatible polymers, and fluorescent molecules. A concept was formulated for the production of new systems for the prolonged release of drugs bearing a constant or temporary (protonated group) positive charge and fluorescent labels for tissue staining during electrophoretic introduction. In particular, a possibility of covalent functionalization of the POM Mo132 surface by the organosilicon molecules (aminopropyltrimethoxysilane) was shown. A promising procedure of the synthesis of NHS-esters of rhodamine B for the subsequent inclusion into the hybrid system was developed. The kinetic studies of POM destruction in an aqueous medium were carried out. The influence of association of rhodamine B with Mo132 on the character of fluorescence was studied.

Novel Keplerate type polyoxometalate-surfactant-graphene hybrids as advanced electrode materials for supercapacitors

The development of novel materials for enhanced electrochemical energy storage applications, in particular for the fabrication of supercapacitors (SCs) displaying increased properties, is a milestone of both fundamental and technological relevance. Among nanostructured materials, polyoxometalates (POMs) combined with various carbon-based nanostructures represent a very promising class of hybrid systems for energy storage, yet, guidelines for their rational design and synthesis leading to high-performance SCs is still lacking. Here, we have produced a novel hybrid architecture based on Keplerate type POM (Mo132) functionalized with dodecyltrimethylammonium bromide (DTAB), which upon mixing with electrochemically exfoliated graphene (EEG) nanosheets results in the formation of porous 3D superstructures. Mo132-DTAB-EEG combines the redox activity of POMs and high electrical conductivity of graphene, all synergically mediated by the surfactant-assisted porosity enhancement, to form new electrode materials for SCs. Cyclic voltammetry and galvanostatic charge/discharge electrochemical studies on Mo132-DTAB-EEG performed in aqueous H2SO4 electrolyte revealed that the unique combination of these three components yields highly efficient energy storage materials. In particular, our highly porous hybrids system exhibits high specific capacitance of 65 F g−1 (93 F cm−3, 93mFcm−2) combined with excellent stability (99% of specific capacitance retained) after 5000 charge/discharge cycles at different current densities, overall displaying significantly improved performance compared to pristine electrochemically exfoliated graphene material. Strong non-covalent interactions between Keplerate type polyoxometalate Mo132-DTAB and graphene surface offer higher stability compared to other hybrid POM/carbon-based systems, and unique electrical properties of the multicomponent structure, thereby paving the way towards the development of novel, and potentially multifunctional, POM-based architectures to be exploited as SC electrode materials.

Iontophoretic transport of associates based on porous Keplerate-type cluster polyoxometalate Mo72Fe30 and containing biologically active substances

The possibility of iontophoretic transport through the native membranes of biologically active substances (vitamin B1 and insulin) associated with porous clusters Mo72Fe30 polyoxometalate of the Keplerate type is demonstrated for the first time in an experimental setup. The diffusion coefficient is estimated. The possibility of transferring Keplerate ions with a protective coating of biocompatible polymer polyvinylpyrrolidone is also shown.

Features of thermophotoinitiated degradation of nanocluster polyoxomolybdate Mo132 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.

Nano isopolyoxomolybdate catalyzed Biginelli reaction for one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones and 3,4-dihydropyrimidin-2(1H)-thiones under solvent-free conditions

The Biginelli reaction of β-ketoesters with aryl aldehyde and urea or thiourea in the presence of a Keplerate type giant nanoporous isopolyoxomolybdate, (NH4)42[Mo 72 VI Mo 60 V O372(CH3COO)30(H2O)72], presented as {Mo132}, as an efficient catalyst under solvent free conditions is reported. The catalyst was synthesized according to the earlier published procedure using inexpensive and readily available starting materials. The catalyst could be recovered and reused efficiently in six cycles. Other beneficial features of this new synthetic approach include short reaction time, high yields, clean reaction profiles, and a simple work-up procedure.

Fast and Solvent-Free Synthesis of Polyhydroquinolines Catalyzed by a Keplerate Type Giant Nanoporous Isopolyoxomolybdate as a Reusable Catalyst

A novel application of (NH4)42[MoVI72MoV60O372(CH3COO)30(H2O)72] catalyst, a Keplerate type giant nanoporous isopolyoxomolybdate, denoted as ({Mo132}), in the synthesis of polyhydroquinolines is reported. The catalyst was prepared using inexpensive and readily available materials and characterized using FT-IR and UV spectra. The results showed that this catalyst has high catalytic activity and the desired products were obtained within a few seconds or minutes (for some cases) in high yields under solvent-free conditions at 120°C. Furthermore, the catalyst could be recycled after a simple work-up, and reused at least four times without significant reduction in its catalytic activity.

Destruction of porous spherical Mo132 nanocluster polyoxometallate of keplerate type in aqueous solutions

The mechanism and kinetics of the decomposition of the Mo132 nanocluster polyoxomolybdate (POM) with a keplerate structure in aqueous solutions, including those containing calcium ions, were studied. The destruction mechanism of POM and the lifetime of Mo132 depend on the initial concentration of its solutions, and this, in turn, is related to the thermodynamic factors. The destruction of keplerate can be described as an autocatalytic process. The calcium ions stabilize POM to different degrees depending on their content in solution.

A unique fluoride nanocontainer: porous molecular capsules can accommodate an unusually high number of "rather labile" fluoride anions.

The present work refers to the challenging issue of fluoride anion recognition/binding in water by taking advantage of the unique possibilities offered by the porous molecular nanocontainers of the {Mo132} Keplerate type allowing the study of a variety of new phenomena. Reaction of the highly reactive carbonate-type capsule with aqueous HF results in the release of carbon dioxide and integration of an unprecedentedly large number of fluoride anions--partly as coordinated ligands at both the pentagonal units and the linkers, partly as a disordered water/fluoride assembly inside the cavity. The internal assembly and some of the fluoride ligands are easily released, which provides interesting options for future studies regarding coordination chemistry and catalysis under confined conditions.

A Unique Fluoride Nanocontainer: Porous Molecular Capsules Can Accommodate an Unusually High Number of “Rather Labile” Fluoride Anions

AbstractThe present work refers to the challenging issue of fluoride anion recognition/binding in water by taking advantage of the unique possibilities offered by the porous molecular nanocontainers of the {Mo132} Keplerate type allowing the study of a variety of new phenomena. Reaction of the highly reactive carbonate‐type capsule with aqueous HF results in the release of carbon dioxide and integration of an unprecedentedly large number of fluoride anions—partly as coordinated ligands at both the pentagonal units and the linkers, partly as a disordered water/fluoride assembly inside the cavity. The internal assembly and some of the fluoride ligands are easily released, which provides interesting options for future studies regarding coordination chemistry and catalysis under confined conditions.

Correction: Towards applications in catalysis: investigation on photocatalytic activities of a derivative family of the Keplerate type molybdenum–oxide based polyoxometalates

Correction for ‘Towards applications in catalysis: investigation on photocatalytic activities of a derivative family of the Keplerate type molybdenum–oxide based polyoxometalates’ by Yunshan Zhou et al., RSC Adv., 2014, 4, 54928–54935.

Application of a Keplerate type giant nanoporous isopolyoxomolybdate as a reusable catalyst for the synthesis of 1,2,4,5-tetrasubstituted imidazoles

The Keplerate-type giant nanoporous isopolyoxomolybdate (NH4)42[MoVI72MoV60O372-(CH3COO)30(H2O)72], denoted {Mo132}, has been used as a catalyst for the synthesis of 1,2,4,5-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.

Third-order optical nonlinearities of a series of compounds derived from a Keplerate type molybdenum-oxide-based polyoxometalate

By reacting the unique Keplerate type molybdenum-oxide based polyoxometalate (NH4)42·[Mo132O372·(CH3COO)30(H2O)72]·ca.300H2O·ca.10CH3COONH4(1) with tetramethylammonium bromide, a new derivative (NH4)26[TMA]16{Mo132O372(H2O)72(CH3COO)30}·ca.7NH4CH3COO·ca.189H2O(2, TMA=tetramethylammonium) was prepared. Compound 2 was characterized by Fourier transform infrared spectroscopy(FTIR), UV-Vis, elemental and thermogravimetric analyses. By the well-established Z-scan technique, investigations on the nonlinear optical(NLO) properties of the series of compounds derived from the Keplerate type molybdenum-oxide-based poly-oxometalate, namely, the newly prepared compound 2, the three previously reported compounds, included compound 1, (NH4)18(TBA)24{Mo132O372(H2O)72(CH3COO)30}·ca.7NH4CH3COO·ca.173H2O(3, TBA=tetrabutylammonium) and (DODA)40(NH4)2[(H2O) n Mo132O372(CH3COO)30(H2O)72](4, DODA=dimethyldioctadecylammonium), reveal that the third-order nonlinearity[χ (3)] values of compounds 1, 2 and 3 in the DMF/H2O solution and compound 4 in chloroform are almost the same, which indicates that the counter cations with different length of alkyl chains show ignorable impacts on the NLO susceptibility. In other words, the remarkable third-order nonlinearities[χ (3)≈10−19 m2/V2] mainly come from the [Mo132O372(CH3COO)30(H2O)72]42− anions. This fact reveals that the applications of the NLO active polyoxometalates in various environments(such as hydrophilic, hydrophobic, polar, apolar, etc.) can be achieved by simply varying cations to meet the demands in the design of diverse devices.

ChemInform Abstract: Capsules with Highly Active Pores and Interiors: Versatile Platforms at the Nanoscale

AbstractReview: [emphasis on metal oxide based capsules of the {M132} Keplerate type; 87 refs.

Capsules with Highly Active Pores and Interiors: Versatile Platforms at the Nanoscale

Spherical porous capsules offer new exciting approaches in chemistry, materials sciences, and in context of physical and biological phenomena. The underlying concepts are reported with particular emphasis on metal oxide based capsules of the {M132 } Keplerate type which display-due to their exceptional structural features and easy variation/derivatization as well as exchange of building units-an unmatched range of properties and offer unique opportunities for investigating a variety of basic aspects of nanoscience, including the discovery of some new phenomena, especially those related to hydrophobicity issues that are of significance for everyday life. This relies in particular on the existence of a large number of flexible crown ether type pores/channels and the possibility of changing the interior from completely hydrophilic to completely hydrophobic due to the presence of numerous easily exchangeable internal ligands/functionalities; the capsules can even be constructed so that they enclose a large number of highly active Lewis and Brønsted acid sites. The manifold of possible applications/uses are outlined as subtitles with reference to results as well as possible future studies. There are, among many others, options to control passing cations under different internal frames allowing also their separations, to conduct studies about hydrophobic recognitions and clustering of biological interest in water, controlled internal ion transport, nanoscale dewetting, and to carry out basic as well as new types of reactions under confined conditions.

Towards applications in catalysis: investigation on photocatalytic activities of a derivative family of the Keplerate type molybdenum–oxide based polyoxometalates

The photocatalytic potential of a derivative family of the unique Keplerate type nanoporous Mo–O based polyoxometalates has been evaluated.

Lipophilic polyoxomolybdate nanocapsules in constitutional dynamic hybrid materials

Dynamic hybrid materials based on Müller’s porous Keplerate type molybdenum-oxide based nanocapsules are described. The present efforts involve the preparation and properties of hybrid materials formed between lipophilic MCM41-mesoporous or octadecyldimethylsilica with Keplerate type molybdenum-oxide based Mo132 nanocapsules – designed by encapsulation into DODA – dimethyldioctadecylammonium cationic surfactants (DODA) 40 Mo132. In particular, the use of a “ dynamic reversible hydrophobic interface” between (DODA) 40 Mo132 and lipophilic silica can render the emerging hybrid mesophases self-adaptive. The reversible hydrophobic interactions allow to both capsule and inorganic silica components to mutually (synergistically) adapt their spatial constitution during simultaneous (collective) formation of self-organized hybrid domains. This might provide new insights into the features that control the design of novel complex materials.

Cellular cation transport studied by 6, 7Li and 23Na NMR in a porous Mo132 Keplerate type nano-capsule as model system

Li+ ions can interplay with other cations intrinsically present in the intra- and extra-cellular space (i.e. Na+, K+, Mg2+ and Ca2+) have therapeutic effects (e.g. in the treatment of bipolar disorder) or toxic effects (at higher doses), likely because Li+ interferes with the intra-/extra-cellular concentration gradients of the mentioned physiologically relevant cations. The cellular transmembrane transport can be modelled by molybdenum-oxide-based Keplerates, i.e. nano-sized porous capsules containing 132 Mo centres, monitored through 6/7Li as well as 23Na NMR spectroscopy. The effects on the transport of Li+ cations through the 'ion channels' of these model cells, caused by variations in water amount, temperature, and by the addition of organic cationic 'plugs' and the shift reagent [Dy(PPP)2](7-) are reported. In the investigated solvent systems, water acts as a transport mediator for Li+. Likewise, the counter-transport (Li+/Na+, Li+/K+, Li+/Cs+ and Li+/Ca2+) has been investigated by 7Li NMR and, in the case of Li+/Na+ exchange, by 23Na NMR, and it has been shown that most (in the case of Na+ and K+, all (Ca2+) or almost none (Cs+) of the Li cations is extruded from the internal sites of the artificial cell to the extra-cellular medium, while Na+, K+ and Ca2+ are partially incorporated.

Synthetic Ion Channels via Self-Assembly: A Route for Embedding Porous Polyoxometalate Nanocapsules in Lipid Bilayer Membranes

Porous polyoxometalate nanocapsules of Keplerate type are known to exhibit the functionality of biological ion channels; however, their use as an artificial ion channel is tempered by the high negative charge of the capsules, which renders their spontaneous incorporation into a lipid bilayer membrane unlikely. In this Letter we report coarse-grained molecular dynamics simulations that demonstrate a route for embedding negatively charged nanocapsules into lipid bilayer membranes via self-assembly. A homogeneous mixture of water, cationic detergent, and phospholipid was observed to spontaneously self-assemble around the nanocapsule into a layered, liposome-like structure, where the nanocapsule was enveloped by a layer of cationic detergent followed by a layer of phospholipid. Fusion of such a layered liposome with a lipid bilayer membrane was observed to embed the nanocapsule into the lipid bilayer. The resulting assembly was found to remain stable even after the surface of the capsule was exposed to electrolyte. In the latter conformation, water was observed to flow into and out of the capsule as Na(+) cations entered, suggesting that a polyoxometalate nanocapsule can form a functional synthetic ion channel in a lipid bilayer membrane.