Electrocatalytic Reduction of Dinitrogen to Ammonia with Water as Proton and Electron Donor Catalyzed by a Combination of a Tri-ironoxotungstate and an Alkali Metal Cation.

The electrification of ammonia synthesis is a key target for its decentralization and lowering impact on atmospheric CO2 concentrations. The lithium metal electrochemical reduction of nitrogen to ammonia using alcohols as proton/electron donors is an important advance, but requires rather negative potentials, and anhydrous conditions. Organometallic electrocatalysts using redox mediators have also been reported. Water as a proton and electron donor has not been demonstrated in these reactions. Here a N2 to NH3 electrocatalytic reduction using an inorganic molecular catalyst, a tri-iron substituted polyoxotungstate, {SiFe3W9}, is presented. The catalyst requires the presence of Li+ or Na+ cations as promoters through their binding to {SiFe3W9}. Experimental NMR, CV and UV-vis measurements, and MD simulations and DFT calculations show that the alkali metal cation enables the decrease of the redox potential of {SiFe3W9} allowing the activation of N2. Controlled potential electrolysis with highly purified 14N2 and 15N2 ruled out formation of NH3 from contaminants. Importantly, using Na+ cations and polyethylene glycol as solvent, the anodic oxidation of water can be used as a proton and electron donor for the formation of NH3. In an undivided cell electrolyzer under 1 bar N2, rates of NH3 formation of 1.15 nmol sec-1 cm-2, faradaic efficiencies of ∼25%, 5.1 equiv of NH3 per equivalent of {SiFe3W9} in 10 h, and a TOF of 64 s-1 were obtained. The future development of suitable high surface area cathodes and well solubilized N2 and the use of H2O as the reducing agent are important keys to the future deployment of an electrocatalytic ammonia synthesis.

Residue analysis evidence for wine enriched with vanilla consumed in Jerusalem on the eve of the Babylonian destruction in 586 BCE.

The article presents results of residue analysis, based on Gas Chromatograph Mass Spectrometer (GC-MS) measurements, conducted on 13 ceramic storage jars unearthed in the Babylonian destruction layer (586 BCE) in Jerusalem. Five of the jars bear rosette stamp impressions on their handles, indicating that their content was related to the kingdom of Judah's royal economy. The identification of the original contents remains is significant for the understanding of many aspects related to the nutrition, economy and international trade in the ancient Levant. The study shed light on the contents of the jars and the destruction process of the buildings in which they were found. The jars were used alternatively for storing wine and olive oil. The wine was flavored with vanilla. These results attest to the wine consumption habits of the Judahite elite and echo Jerusalem's involvement in the trans-regional South Arabian trade of spices and other lucrative commodities on the eve of its destruction by Nebuchadnezzar.

Molecular Transition Metal Oxide Electrocatalysts for the Reversible Carbon Dioxide-Carbon Monoxide Transformation.

Carbon monoxide dehydrogenase (CODH) enzymes are active for the reversible CO oxidation-CO2 reduction reaction and are of interest in the context of CO2 abatement and carbon-neutral solar fuels. Bioinspired by the active-site composition of the CODHs, polyoxometalates triply substituted with first-row transition metals were modularly synthesized. The polyanions, in short, {SiM3 W9 } and {SiM'2 M''W9 }, M, M', M''=CuII , NiII , FeIII are shown to be electrocatalysts for reversible CO oxidation-CO2 reduction. A catalytic Tafel plot showed that {SiCu3 W9 } was the most reactive for CO2 reduction, and electrolysis reactions yielded significant amounts of CO with 98 % faradaic efficiency. In contrast, Fe-Ni compounds such as {SiFeNi2 W9 } preferably catalyzed the oxidation of CO to CO2 similar to what is observed for the [NiFe]-CODH enzyme. Compositional control of the heterometal complexes, now and in the future, leads to control of reactivity and selectivity for CO2 electrocatalytic reduction.

Photoelectrochemical Reduction of Carbon Dioxide with a Copper Graphitic Carbon Nitride Photocathode.

Research on the photoreduction of CO2 often has been dominated by the use of sacrificial reducing agents. A pathway that avoids this problem would be the development of photocathodes for CO2 reduction that could then be coupled to a photoanodic oxygen evolution reaction. Here, we present the use of copper-substituted graphitic carbon nitride (Cu-CN) on a fluorinated tin oxide (FTO) electrode for the photoelectrochemical two-electron reduction of CO2 to CO as a major product (>95 %) and formic acid (<5 %). The results show that at a potential of -2.5 V versus Fc\Fc+ the CO2 reduction activity of Cu-CN on FTO electrode improves by 25 % upon illumination by visible light with a faradaic efficiency of nearly 100 %. Independently, X-ray photoelectron spectroscopy conclusively shows a pronounced increase in the electrical conductivity of the Cu-CN upon white light illumination under vacuum and a contactless measuring configuration. This photo-assisted charge mobility is shown to play a key role in the increased reactivity and faradaic efficiency for the reduction of CO2 .

Guest Transition Metals in Host Inorganic Nanocapsules: Single Sites, Discrete Electron Transfer, and Atomic Scale Structure.

Host-guest solution chemistry with a wide range of organic hosts is an important and established research area, while the use of inorganic hosts is a more nascent area of research. In the recent past in a few cases, Keplerate-type molybdenum oxide-based porous, spherical clusters, shorthand notation {Mo132}, have been used as hosts for organic guests. Here, we demonstrate the synthetically controlled encapsulation of first-row transition metals (M = Mn, Fe, and Co) within a Keplerate cluster that was lined on the inner core with phosphate anions, {Mo132PO4}. The resulting M2+x⊂{Mo132PO4} host-guest complexes were characterized by 31P NMR and ENDOR spectroscopy that substantiated the encapsulation of the first-row transition metal guest. Magnetic susceptibility measurements showed that the encapsulation of up to 10 equiv showed little magnetic interaction between the encapsulated metals, which indicated that each guest atom occupied a single site. Visualization of the capsules and differentiation of the Mo atoms of the capsule framework and the encapsulated transition metal were possible using spherical and chromatic double aberration-corrected electron microscopy combined with energy-filtered TEM (EFTEM) elemental maps. In addition, use of visible light-induced XPS for chemically resolved electrical measurements (CREM) confirmed the successful encapsulation of M within {Mo132PO4} and furthermore showed photoinduced electron transfer from M to Mo. In the future, such targeted electron transfer between host {Mo132} and a transition metal guest could be used as photoinitiated switches using inorganic compounds and for single site photocatalytic reactions in confined space.

Selective Oxidation by H5[PV2Mo10O40] in a Highly Acidic Medium.

Dissolution of the polyoxometalate (POM) cluster anion H5[PV2Mo10O40] (1; a mixture of positional isomers) in 50% aq H2SO4 dramatically enhances its ability to oxidize methylarenes, while fully retaining the high selectivities typical of this versatile oxidant. To better understand this impressive reactivity, we now provide new information regarding the nature of 1 (115 mM) in 50% (9.4 M) H2SO4. Data from 51V NMR spectroscopy and cyclic voltammetry reveal that as the volume of H2SO4 in water is incrementally increased to 50%, V(V) ions are stoichiometrically released from 1, generating two reactive pervanadyl, VO2+, ions, each with a one-electron reduction potential of ca. 0.95 V (versus Ag/AgCl), compared to 0.46 V for 1 in 1.0 M aq H2SO4. Phosphorus-31 NMR spectra obtained in parallel reveal the presence of PO43-, which at 50% H2SO4 accounts for all the P(V) initially present in 1. Addition of (NH4)2SO4 leads to the formation of crystalline [NH4]6[Mo2O5(SO4)4] (34% yield based on Mo), whose structure (from single-crystal X-ray diffraction) features a corner-shared, permolybdenyl [Mo2O5]2+ core, conceptually derived by acid condensation of two MoO3 moieties. While 1 in 50% aq H2SO4 oxidizes p-xylene to p-methylbenzaldehyde with conversion and selectivity both greater than 90%, reaction with VO2+ alone gives the same high conversion, but at a significantly lower selectivity. Importantly, selectivity is fully restored by adding [NH4]6[Mo2O5(SO4)4], suggesting a central role for Mo(VI) in attenuating the (generally) poor selectivity achievable using VO2+ alone. Finally, 31P and 51V NMR spectra show that intact 1 is fully restored upon dilution to 1 M H2SO4.

Visible-Light Photochemical Reduction of CO2 to CO Coupled to Hydrocarbon Dehydrogenation.

Research on the photochemical reduction of CO2 , initiated already 40 years ago, has with few exceptions been performed by using amines as sacrificial reductants. Hydrocarbons are high-volume chemicals whose dehydrogenation is of interest, so the coupling of a CO2 photoreduction to a hydrocarbon-photodehydrogenation reaction seems a worthwhile concept to explore. A three-component construct was prepared including graphitic carbon nitride (g-CN) as a visible-light photoactive semiconductor, a polyoxometalate (POM) that functions as an electron acceptor to improve hole-electron charge separation, and an electron donor to a rhenium-based CO2 reduction catalyst. Upon photoactivation of g-CN, a cascade is initiated by dehydrogenation of hydrocarbons coupled to the reduction of the polyoxometalate. Visible-light photoexcitation of the reduced polyoxometalate enables electron transfer to the rhenium-based catalyst active for the selective reduction of CO2 to CO. The construct was characterized by zeta potential, IR spectroscopy, thermogravimetry, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). An experimental Z-scheme diagram is presented based on electrochemical measurements and UV/Vis spectroscopy. The conceptual advance should promote study into more active systems.

The formyloxyl radical: electrophilicity, C-H bond activation and anti-Markovnikov selectivity in the oxidation of aliphatic alkenes.

In the past the formyloxyl radical, HC(O)O˙, had only been rarely experimentally observed, and those studies were theoretical-spectroscopic in the context of electronic structure. The absence of a convenient method for the preparation of the formyloxyl radical has precluded investigations into its reactivity towards organic substrates. Very recently, we discovered that HC(O)O˙ is formed in the anodic electrochemical oxidation of formic acid/lithium formate. Using a [CoIIIW12O40]5- polyanion catalyst, this led to the formation of phenyl formate from benzene. Here, we present our studies into the reactivity of electrochemically in situ generated HC(O)O˙ with organic substrates. Reactions with benzene and a selection of substituted derivatives showed that HC(O)O˙ is mildly electrophilic according to both experimentally and computationally derived Hammett linear free energy relationships. The reactions of HC(O)O˙ with terminal alkenes significantly favor anti-Markovnikov oxidations yielding the corresponding aldehyde as the major product as well as further oxidation products. Analysis of plausible reaction pathways using 1-hexene as a representative substrate favored the likelihood of hydrogen abstraction from the allylic C-H bond forming a hexallyl radical followed by strongly preferred further attack of a second HC(O)O˙ radical at the C1 position. Further oxidation products are surmised to be mostly a result of two consecutive addition reactions of HC(O)O˙ to the C[double bond, length as m-dash]C double bond. An outer-sphere electron transfer between the formyloxyl radical donor and the [CoIIIW12O40]5- polyanion acceptor forming a donor-acceptor [D+-A-] complex is proposed to induce the observed anti-Markovnikov selectivity. Finally, the overall reactivity of HC(O)O˙ towards hydrogen abstraction was evaluated using additional substrates. Alkanes were only slightly reactive, while the reactions of alkylarenes showed that aromatic substitution on the ring competes with C-H bond activation at the benzylic position. C-H bonds with bond dissociation energies (BDE) ≤ 85 kcal mol-1 are easily attacked by HC(O)O˙ and reactivity appears to be significant for C-H bonds with a BDE of up to 90 kcal mol-1. In summary, this research identifies the reactivity of HC(O)O˙ towards radical electrophilic substitution of arenes, anti-Markovnikov type oxidation of terminal alkenes, and indirectly defines the activity of HC(O)O˙ towards C-H bond activation.

Aerobic oxygenation catalyzed by first row transition metal complexes coordinated by tetradentate mono-carbon bridged bis-phenanthroline ligands: intra- versus intermolecular carbon-hydrogen bond activation.

Commonly, iron(ii) and copper(i) complexes bind dioxygen (O2) and then activate O2 through a reductive reaction pathway. There is, however, significant interest in low temperature oxygenation with O2 without the use of a sacrificial reductant. Here, earth-abundant metal complexes (FeII, CoII, NiII and CuII) coordinated by two different tetra-dentate mono-carbon bridged bis-phenanthroline ligands, (1,10-Phen)2-2,2'-CR1R2, where R1 = n-butyl and R2 = n-butyl or H were synthesized. The structures all showed the expected metal complexation in the equatorial plane by the bridged bis-phenanthroline ligands. For R1 = n-butyl; R2 = H, where the ligand has a tertiary carbon bridging group, fast intramolecular oxygenation occurred at the pseudobenzylic position. Depending on the transition metal the main products formed were oxygen bridged dimers of the metal complexes (Co and Fe) or metal complexes with a carbonyl moiety at the bridging pseudobenzylic position as a result of C-R1 bond cleavage (Ni and Cu). The different product assemblages are explained by different reaction pathways that are metal specific. For quaternary carbon bridged ligands, R1 = R2 = n-butyl, the complexes catalytically activated C-H bonds of cyclohexene under catalytic conditions, showing higher effective turnover numbers at low catalyst loading. The reactivity observed is commensurate with a room temperature autooxidation reaction although the initiation of the free radical reaction is metal specific. In general labelling studies with 18O2, UV-vis and EPR spectroscopy as well as cyclic voltammetry measurements led to a conclusion that the reaction pathways involved both C-H bond activation and O2 activation.

A Thiourea Tether in the Second Coordination Sphere as a Binding Site for CO2 and a Proton Donor Promotes the Electrochemical Reduction of CO2 to CO Catalyzed by a Rhenium Bipyridine-Type Complex.

The electrochemical reduction of CO2 has been extensively investigated in recent years, with the expectation that a detailed mechanistic understanding could achieve the goal of finding a stable catalyst with high turnover frequencies and low reduction potentials. In the catalytic cycle of the carbon dioxide hydrogenase enzyme, it has been suggested that the reduced metal center reacts with CO2 to form a carboxylate intermediate that is stabilized by hydrogen bonding using a histidine moiety in the second coordination sphere. Using the well-known fac-Re(I)bipyridine(CO)3Cl complex as a starting point, the bipyridine ligand was modified in the second coordination sphere with a thiourea tether that is known to form hydrogen bonds with carbonyl moieties. The resulting Re(I) catalyst was an excellent electrocatalyst for the selective reduction of CO2 to CO, with a turnover frequency of 3040 s-1. The binding of CO2 to the thiourea tether was observable by 1H NMR, and NOE experiments showed that the hydrogen atoms of the thiourea group were labile. Further experiments indicated that the thiourea moiety is also a local proton source and addition of an external proton source actually inhibits catalysis. The absence of a kinetic isotope effect was explained through DFT calculations that showed that the proton invariably jumps to the nearest CO2 oxygen atom to form a metal-carboxylic acid without going through any minimum or transition state. EPR and NMR spectroscopies were used to identify the various reduced intermediates. Thus, the thiourea tether in the second coordination sphere can bind CO2, stabilize carboxylic acid reaction intermediates, and directly act as a local proton source, leading to a significantly more active catalyst.

The kinetics and mechanism of oxidation of reduced phosphovanadomolybdates by molecular oxygen: theory and experiment in concert.

The reactivity of the H5PV2Mo10O40 polyoxometalate and its analogues as an electron transfer and electron transfer-oxygen transfer oxidant has been extensively studied in the past and has been shown to be useful in many transformations. One of the hallmarks of this oxidant is the possibility of its re-oxidation with molecular oxygen, thus enabling aerobic catalytic cycles. Although the re-oxidation reaction was known, the kinetics and mechanism of this reaction have not been studied in any detail. Experimentally, we show that both the one- and two-electron reduced polyoxometalate are reactive with O2, the two-electron one more so. The reactions are first-order in the polyoxometalate and O2. Solvents also have a considerable effect, protic solvents being preferred over aprotic ones. H5PV2Mo10O40 was reduced either by an electron transfer reaction (H2) or an electron transfer-oxygen transfer reaction (Ph3P). Similar rate constants and activation parameters were observed for both. DFT calculations carried out on the re-oxidation reactions strongly suggest an inner-sphere process. The process involves first the formation of a coordinatively unsaturated site (CUS) and subsequently the binding of O2 to form superoxo and then peroxo η2-O2 adducts. Most interestingly, although vanadium is the reactive redox centre as well as a necessary component for the oxidative activity of H5PV2Mo10O40, and a CUS can be formed at both Mo and V sites, O2 coordination occurs mostly at the Mo CUSs, preferably those where the vanadium centers are distal to each other.

Electrochemical Hydroxylation of Arenes Catalyzed by a Keggin Polyoxometalate with a Cobalt(IV) Heteroatom.

The sustainable, selective direct hydroxylation of arenes, such as benzene to phenol, is an important research challenge. An electrocatalytic transformation using formic acid to oxidize benzene and its halogenated derivatives to selectively yield aryl formates, which are easily hydrolyzed by water to yield the corresponding phenols, is presented. The formylation reaction occurs on a Pt anode in the presence of [CoIII W12 O40 ]5- as a catalyst and lithium formate as an electrolyte via formation of a formyloxyl radical as the reactive species, which was trapped by a BMPO spin trap and identified by EPR. Hydrogen was formed at the Pt cathode. The sum transformation is ArH+H2 O→ArOH+H2 . Non-optimized reaction conditions showed a Faradaic efficiency of 75 % and selective formation of the mono-oxidized product in a 35 % yield. Decomposition of formic acid into CO2 and H2 is a side-reaction.

Self-Assembly through Noncovalent Preorganization of Reactants: Explaining the Formation of a Polyfluoroxometalate.

High-order elementary reactions in homogeneous solutions involving more than two molecules are statistically improbable and very slow to proceed. They are not generally considered in classical transition-state or collision theories. Yet, rather selective, high-yield product formation is common in self-assembly processes that require many reaction steps. On the basis of recent observations of crystallization as well as reactions in dense phases, it is shown that self-assembly can occur by preorganization of reactants in a noncovalent supramolecular assembly, whereby directing forces can lead to an apparent one-step transformation of multiple reactants. A simple and general kinetic model for multiple reactant transformation in a dense phase that can account for many-bodied transformations was developed. Furthermore, the self-assembly of polyfluoroxometalate anion [H2 F6 NaW18 O56 ]7- from simple tungstate Na2 WO2 F4 was demonstrated by using 2D 19 F-19 F NOESY, 2D 19 F-19 F COSY NMR spectroscopy, a new 2D 19 F{183 W} NMR technique, as well as ESI-MS and diffusion NMR spectroscopy, and the crucial involvement of a supramolecular assembly was found. The deterministic kinetic reaction model explains the reaction in a dense phase and supports the suggested self-assembly mechanism. Reactions in dense phases may be of general importance in understanding other self-assembly reactions.

Dioxygen in Polyoxometalate Mediated Reactions.

In this review article, we consider the use of molecular oxygen in reactions mediated by polyoxometalates. Polyoxometalates are anionic metal oxide clusters of a variety of structures that are soluble in liquid phases and therefore amenable to homogeneous catalytic transformations. Often, they are active for electron transfer oxidations of a myriad of substrates and upon reduction can be reoxidized by molecular oxygen. For example, the phosphovanadomolybdate, H5PV2Mo10O40, can oxidize Pd(0) thereby enabling aerobic reactions catalyzed by Pd and H5PV2Mo10O40. In a similar vein, polyoxometalates can stabilize metal nanoparticles, leading to additional transformations. Furthermore, electron transfer oxidation of other substrates such as halides and sulfur-containing compounds is possible. More uniquely, H5PV2Mo10O40 and its analogues can mediate electron transfer-oxygen transfer reactions where oxygen atoms are transferred from the polyoxometalate to the substrate. This unique property has enabled correspondingly unique transformations involving carbon-carbon, carbon-hydrogen, and carbon-metal bond activation. The pathway for the reoxidation of vanadomolybdates with O2 appears to be an inner-sphere reaction, but the oxidation of one-electron reduced polyoxotungstates has been shown through intensive research to be an outer-sphere reaction. Beyond electron transfer and electron transfer-oxygen transfer aerobic transformations, there a few examples of apparent dioxygenase activity where both oxygen atoms are donated to a substrate.

Real-time molecular scale observation of crystal formation.

How molecules in solution form crystal nuclei, which then grow into large crystals, is a poorly understood phenomenon. The classical mechanism of homogeneous crystal nucleation proceeds via the spontaneous random aggregation of species from liquid or solution. However, a non-classical mechanism suggests the formation of an amorphous dense phase that reorders to form stable crystal nuclei. So far it has remained an experimental challenge to observe the formation of crystal nuclei from five to thirty molecules. Here, using polyoxometallates, we show that the formation of small crystal nuclei is observable by cryogenic transmission electron microscopy. We observe both classical and non-classical nucleation processes, depending on the identity of the cation present. The experiments verify theoretical studies that suggest non-classical nucleation is the lower of the two energy pathways. The arrangement in just a seven-molecule proto-crystal matches the order found by X-ray diffraction of a single bulk crystal, which demonstrates that the same structure was formed in each case.

Hydrogen-Atom Transfer Oxidation with H2O2 Catalyzed by [FeII(1,2-bis(2,2'-bipyridyl-6-yl)ethane(H2O)2]2+: Likely Involvement of a (µ-Hydroxo)(µ-1,2-peroxo)diiron(III) Intermediate.

The iron(II) triflate complex (1) of 1,2-bis(2,2'-bipyridyl-6-yl)ethane, with two bipyridine moieties connected by an ethane bridge, was prepared. Addition of aqueous 30% H2O2 to an acetonitrile solution of 1 yielded 2, a green compound with λmax=710 nm. Moessbauer measurements on 2 showed a doublet with an isomer shift (δ) of 0.35 mm/s and a quadrupole splitting (ΔEQ) of 0.86 mm/s, indicative of an antiferromagnetically coupled diferric complex. Resonance Raman spectra showed peaks at 883, 556 and 451 cm-1 that downshifted to 832, 540 and 441 cm-1 when 1 was treated with H218O2. All the spectroscopic data support the initial formation of a (µ-hydroxo)(µ-1,2-peroxo)diiron(III) complex that oxidizes carbon-hydrogen bonds. At 0°C 2 reacted with cyclohexene to yield allylic oxidation products but not epoxide. Weak benzylic C-H bonds of alkylarenes were also oxidized. A plot of the logarithms of the second order rate constants versus the bond dissociation energies of the cleaved C-H bond showed an excellent linear correlation. Along with the observation that oxidation of the probe substrate 2,2-dimethyl-1-phenylpropan-1-ol yielded the corresponding ketone but no benzaldehyde, and the kinetic isotope effect, kH/kD , of 2.8 found for the oxidation of xanthene, the results support the hypothesis for a metal-based H-atom abstraction mechanism. Complex 2 is a rare example of a (µ-hydroxo)(µ-1,2-peroxo)diiron(III) complex that can elicit the oxidation of carbon-hydrogen bonds.

Photochemical Reduction of CO2 with Visible Light Using a Polyoxometalate as Photoreductant.

The photochemical reduction of CO2 to CO requires two electrons and two protons that, in the past, have been derived from sacrificial amine donors that are also non-innocent in the catalytic cycle. Towards the realization of a water-splitting reaction as the source of electrons and protons for CO2 reduction, we have found that a reduced acidic polyoxometalate, H5 PWV2 W10 O40 , is a photoactive electron and proton donor with visible light through excitation of the intervalence charge-transfer band. Upon linking the polyoxometalate to a dirhenium molecular catalyst, a cascade of transformations occurs where the polyoxometalate is electrochemically reduced at a relatively low negative potential of 1.3 V versus Ag/AgNO3 and visible light, a 60 W tungsten lamp, or a red LED is used to transfer electrons from the polyoxometalate to the dirhenium catalyst active for the selective reduction of CO2 to CO.

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 [{(MoVI )MoV5 O21 (H2 O)6 }12 {MoV2 O4 (CH3 COO)}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.

Aerobic epoxidation catalysed by transition metal substituted polyfluorooxometalates.

First row transition metal substituted polyfluorooxmetalates with quasi Wells-Dawson structures and a nitro terminal ligand, [NaH2M(NO2)W17F6O55](q-), were used as catalysts for the aerobic epoxidation of cyclic alkenes. The Cu(NO2) analog combined the best traits of conversion and selectivity. Some C-C bond cleavage was also observed and cis isomers reacted preferentially without stereochemical inversion indicating an oxygen atom to double bond concerted reaction.

On the effect of ion pairing of Keggin type polyanions with quaternary ammonium cations on redox potentials in organic solvents.

The electrochemical properties of Keggin type polyoxometalates Qn[XW12O40] (X = P, Si, B; Q = n-tetraoctylammonium and n-trioctylmethylammonium) in organic solvents were investigated in order to understand the interrelation between the redox potentials, solvents and ion pairing. A logarithmic correlation between the dielectric constant of the solvent (ε ranged from 4.8 to 46.6) and the reduction potential of the [PW12O4](3-)/[PW12O4](4-) couple was found. This reduction potential increased significantly when the surface charge of the polyoxometalate went from 3- to 5-. The investigation of the ion pairing properties by diffusion NMR revealed the presence of intimate ion pairs in less polar solvents (e.g. dichloromethane) and less compact ion pairs in more polar solvents (e.g. DMSO). Using a V atom within the polyoxometalate an NMR experiment showed that a n-trioctylmethyl ammonium cation bonded to the polyoxometalate anion more intimately than a n-tetraoctyl ammonium cation.