Publications

The two-electron oxidation of water (2e-WOR) has been studied in the past as a possible method for the alternative preparation of hydrogen peroxide. Often, fluorinated tin oxide (FTO) is used as an anode and FTO itself was found also to be active for 2e-WOR. Because one use of H2O2 is as an oxygen donor for BaeyerVilliger oxidation of ketones catalyzed by tin compounds and materials, presently we were interested in studying the use of in situ formed H2O2 for these reactions. First, the formation of H2O2 was verified in an acetonitrile/water solvent in a 2e-WOR reaction, which is more efficient than a comparable reaction in water in terms of the H2O2 concentration attained and faradaic efficiency at comparable potentials, that is, ∼3 V vs SHE. Second, initial studies on oxygenation of reactive substrates such as sulfides showed normalized reaction rates (NRRs) for two-electron oxidation reactions that were about 3 times higher than the NRR for H2O2 formation, indicating the formation of an active oxygen-donating or oxidizing species on the electrode surface prior to the formation and release of H2O2 into solution. Third, the BaeyerVilliger oxygenation of 2-adamantanone at 2.1 V versus SHE in acetonitrile/water showed both the formation of the expected lactone product and hydroxylation at both tertiary and secondary CH bonds. Hydroxylation is most easily explained by the presence of hydroxyl radical species as supported by the formation of a spin adduct and its identification by electron paramagnetic resonance. However, the potential used, 2.1 V versus SHE, is an underpotential for the formation of a solvated hydroxyl radical in solution, thereby leading to the conclusion that surface-bound hydroxyl species, OH*, are those that are reactive for the apparent one-electron water oxygenation reaction. Fourth, it was shown that although H2O2 can be thermally activated on FTO as a catalyst to a minor degree, electrochemical activation is by far more significant, leading to the use of FTO as an electrochemical catalyst for activation of H2O2 for the BaeyerVilliger oxygenation and also alkene epoxidation.

Carbon monoxide dehydrogenase (CODH) enzymes are active for the reversible CO oxidationCO2 reduction reaction and are of interest in the context of CO2 abatement and carbonneutral solar fuels. Bioinspired by the activesite composition of the CODHs, polyoxometalates triply substituted with firstrow transition metals were modularly synthesized. The polyanions, in short, {SiM3W9} and {SiM2MW9}, M, M, M=CuII, NiII, FeIII are shown to be electrocatalysts for reversible CO oxidationCO2 reduction. A catalytic Tafel plot showed that {SiCu3W9} was the most reactive for CO2 reduction, and electrolysis reactions yielded significant amounts of CO with 98% faradaic efficiency. In contrast, FeNi compounds such as {SiFeNi2W9} 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.A modular synthesis of trimetallosubstituted polyanions enables the preparation of designer catalysts. Here this approach was used to prepare electrocatalysts for the selective reduction of CO2 to CO using a tricoppersubstituted polyanion. In contrast, FeNisubstituted polyanions preferably catalyzed the oxidation of CO to CO2, showing a reactivity profile reminiscent of the reactivity of FeNibased carbon monoxide dehydrogenase enzymes.

The article presents and discusses the results of Residue Analysis performed on 27 pottery vessels, placed as offerings in burials dating to the Iron I period (ca. 1050900 BCE) at the site Ḥorvat Tevet (Israel). The results show that heated beeswax was used during the burial ceremonies and placed in variety of vessels. These results shed new light on burial practices of South Levantine rural communities. They also contribute to the growing body of evidence regarding bee-product economy in the Southern Levant during the beginning of the Iron IIA.

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 PO₄^3-, 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

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 aerobic, selective oxygenation of alkanes via C-H bond activation is an important research challenge. Photocatalysis offers the potential for the introduction of additional concepts for such reactions. Visible light photoactive semiconductors such as bismuth oxyhalides (BiOX, X = Cl and Br) used in this research typically oxidize organic compounds through photocatalyzed formation of strongly oxidizing holes. The reactive oxygen species formed react with organic compounds in one-electron processes, leading to radical intermediates and nonselective oxidation. Such oxidation reactions generally lead to total oxidation. Here, impregnation of BiOX with a polyoxometalate, H-5 PV2Mo10O40, as a strong electron acceptor changed the reactivity of BiOX, leading to Mars-van Krevelen-type reactivity, that is, photoactivated oxygen donation from BiOX to the organic substrate followed by reoxidation by O-2 and catalysis. This conclusion was supported by mechanistic studies involving isotope labeling studies. In this way, ethane was selectively oxidized to acetaldehyde in a flow reactor with a turnover number (24 h) of 415.

Commonly, iron(ii) and copper(i) complexes bind dioxygen (O-2) and then activate O-2 through a reductive reaction pathway. There is, however, significant interest in low temperature oxygenation with O-2 without the use of a sacrificial reductant. Here, earth-abundant metal complexes (Fe-II, Co-II, Ni-II and Cu-II) coordinated by two different tetra-dentate mono-carbon bridged bis-phenanthroline ligands, (1,10-Phen)(2)-2,2-CR1R2, where R-1 = n-butyl and R-2 = 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 R-1 = n-butyl; R-2 = 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-R-1 bond cleavage (Ni and Cu). The different product assemblages are explained by different reaction pathways that are metal specific. For quaternary carbon bridged ligands, R-1 = R-2 = 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 O-18(2), 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 O-2 activation.

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 [(CoW12O40)-W-III](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+H2OArOH+H-2. 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 H-2 is a side-reaction.

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 O-2, the two-electron one more so. The reactions are first-order in the polyoxometalate and O-2. Solvents also have a considerable effect, protic solvents being preferred over aprotic ones. H5PV2Mo10O40 was reduced either by an electron transfer reaction (H-2) 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 O-2 to form superoxo and then peroxo eta(2)-O-2 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, O-2 coordination occurs mostly at the Mo CUSs, preferably those where the vanadium centers are distal to each other.

The coordination of CO2 to Lewis bases is well-known; however, its binding to Lewis acids is much less common. The ligation of CO2 to a zinc-substituted Wells-Dawson type polyoxometalate yielded [(n-C8H17)(4)N](8)[(2)-P2W17O61Zn(CO2)], which was studied by variable temperature H-1 NMR, C-13 NMR, and P-31 NMR spectroscopy from room temperature to 201 K. The spectra were analyzed in consideration of predicted end-on coordination, POM-Zn-O-C-O, and side-on coordination where in addition an adjacent basic oxygen atom in the polyoxometalate binds the carbon atom of CO2. After adding 2,4,6-trimethypyridine into the solution, a plausible adduct, POM-CO2-2,4,6-trimethypyridine, was observed.

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.

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, (H5PW2W10O40)-W-V, 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.

Based on the knowledge that o-aminomethylphenylboronic acids reversibly bind to carbohydrates, relevant water-soluble derivatives of the former were prepared by appending hydrophilic tethers. In this way the phenylboronic acid derivatives were used to hydrolytically dissolve, i.e. depolymerize cellulose in water at nearly neutral pH values. Some of these hydrophilic tethers consisted of moieties that were surmised to be able to promote hydrolysis of the glycosidic bonds such as a carboxylic acid, phosphonic acid as weak Bronsted acids, or an imidazole functionality as a nucelophilic catalyst; water-soluble polyethylene glycol and polyethyenelimine appendages were also used. The results show that at around 120 °C efficient hydrolysis of cellulose to form water-soluble oligosaccharides could be attained in a period of 24 h. Importantly preimpregnation of a morpholine substituted o-aminomethylphenylboronic acid led to the very selective formation of glucose.

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 ππ stacking and CHπ 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 Mo₁₃₂capsule.

Various ruthenium(II) complexes with proximal oxophilic phenylselenium groups of the general formula [Ru^IIL^AL^B]X₂{L^A= L^B= 6,6-bis[(4-methoxyphenyl)selanyl]-2,2-bipyridine; 6,6-bis[(nitrophenyl)selanyl]-2,2-bipyridine; 3,6-bis(phenylselanyl)dipyrido[3,2-a:2,3-c]phenazine; L^A= 6,6-bis(phenylselanyl)-2,2-bipyridine, L^B= terpyridine} were prepared. The substitution patterns of these compounds were designed to have different electron-withdrawing/-donating properties or different binding motifs in comparison to the previously reported compound with L^A= L^B= 6,6-bis(phenylselanyl)-2,2-bipyridine. The research objective was to evaluate the potential of these compounds to activate ground-state molecular oxygen to form higher-valent RuOSe bonds by cleavage of the OO bond of O₂. All of the compounds prepared indeed activated O₂to form RuOSe moieties, as observable by UV/Vis spectroscopy, mass spectrometry, or X-ray crystallography.

Manganese(IV,V)-hydroxo and oxo complexes are often implicated in both catalytic oxygenation and water oxidation reactions. Much of the research in this area is designed to structurally and/or functionally mimic enzymes. On the other hand, the tendency of such mimics to decompose under strong oxidizing conditions makes the use of molecular inorganic oxide clusters an enticing alternative for practical applications. In this context it is important to understand the reactivity of conceivable reactive intermediates in such an oxide-based chemical environment. Herein, a polyfluoroxometalate (PFOM) monosubstituted with manganese, [NaH₂(Mn-L)W₁₇F₆O₅₅]^q-, has allowed the isolation of a series of compounds, Mn(II, III, IV and V), within the PFOM framework. Magnetic susceptibility measurements show that all the compounds are high spin. XPS and XANES measurements confirmed the assigned oxidation states. EXAFS measurements indicate that Mn(II)PFOM and Mn(III)PFOM have terminal aqua ligands and Mn(V)PFOM has a terminal hydroxo ligand. The data are more ambiguous for Mn(IV)PFOM where both terminal aqua and hydroxo ligands can be rationalized, but the reactivity observed more likely supports a formulation of Mn(IV)PFOM as having a terminal hydroxo ligand. Reactivity studies in water showed unexpectedly that both Mn(IV)-OH-PFOM and Mn(V)-OH-PFOM are very poor oxygen-atom donors; however, both are highly reactive in electron transfer oxidations such as the oxidation of 3-mercaptopropionic acid to the corresponding disulfide. The Mn(IV)-OH-PFOM compound reacted in water to form O₂, while Mn(V)-OH-PFOM was surprisingly indefinitely stable. It was observed that addition of alkali cations (K⁺, Rb⁺, and Cs⁺) led to the aggregation of Mn(IV)-OH-PFOM as analyzed by electron microscopy and DOSY NMR, while addition of Li⁺ and Na⁺ did not lead to aggregates. Aggregation leads to a lowering of the entropic barrier of the reaction without changing the free energy barrier. The observation that O₂ formation is fastest in the presence of Cs⁺ and ∼fourth order in Mn(IV)-OH-PFOM supports a notion of a tetramolecular Mn(IV)-hydroxo intermediate that is viable for O₂ formation in an oxide-based chemical environment. A bimolecular reaction mechanism involving a Mn(IV)-hydroxo based intermediate appears to be slower for O₂ formation.

The synthesis of benzaldehyde derivatives by oxygenation of methylarenes is of significant conceptual and practical interest because these compounds are important chemical intermediates whose synthesis is still carried out by nonsustainable methods with very low atom economy and formation of copious amounts of waste. Now an oxygenation reaction with a 100% theoretical atom economy using a polyoxometalate oxygen donor has been found. The product yield is typically above 95% with no "overoxidation" to benzoic acids; H₂ is released by electrolysis, enabling additional reaction cycles. An electrocatalytic cycle is also feasible. This reaction is possible through the use of an aqueous sulfuric acid solvent, in an aqueous biphasic reaction mode that also allows simple catalyst recycling and recovery. The solvent plays a key role in the reaction mechanism by protonating the polyoxometalate thereby enabling the activation of the methylarenes by an electron transfer process. After additional proton transfer and oxygen transfer steps, benzylic alcohols are formed that further react by an electron transfer-proton transfer sequence forming benzaldehyde derivatives. (Chemical Equation Presented).

Terrestrial plants contain ∼70% hemicellulose and cellulose that are a significant renewable bioresource with potential as an alternative to petroleum feedstock for carbon-based fuels. The efficient and selective deconstruction of carbohydrates to their basic components, carbon monoxide and hydrogen, so called synthesis gas, is an important key step towards the realization of this potential, because the formation of liquid hydrocarbon fuels from synthesis gas are known technologies. Here we show that by using a polyoxometalate as an electron transfer-oxygen transfer catalyst, carbon monoxide is formed by cleavage of all the carbon-carbon bonds through dehydration of initially formed formic acid. In this oxidation-reduction reaction, the hydrogen atoms are stored on the polyoxometalate as protons and electrons, and can be electrochemically released from the polyoxometalate as hydrogen. Together, synthesis gas is formed. In a hydrogen economy scenario, this method can also be used to convert carbon monoxide to hydrogen.

High-valent oxo compounds of transition metals are often implicated as active species in oxygenation of hydrocarbons through carbonhydrogen bond activation or oxygen transfer and also in water oxidation. Recently, several examples of cobalt-catalyzed water oxidation have been reported, and cobalt(IV) species have been suggested as active intermediates. A reactive species, formally a dicobalt(IV)-mu-oxo polyoxometalate compound [(alpha(2)-P2W17O61Co)(2)O](14-), [(POMCo)(2)O], has now been isolated and characterized by the oxidation of a monomeric [alpha(2)-P2W17O61CoII(H2O)](8), [(POMCoH2O)-H-II], with ozone in water. The crystal structure shows a nearly linear Co-O-Co moiety with a CoO bond length of similar to 1.77 A. In aqueous solution [(POMCo)(2)O] was identified by P-31 NMR, Raman, and UV-vis spectroscopy. Reactivity studies showed that [(POMCo)(2)O](2)O] is an active compound for the oxidation of H2O to O-2, direct oxygen transfer to water-soluble sulfoxides and phosphines, indirect epoxidation of alkenes via a Mn porphyrin, and the selective oxidation of alcohols by carbon-hydrogen bond activation. The latter appears to occur via a hydrogen atom transfer mechanism. Density functional and CASSCF calculations strongly indicate that the electronic structure of [(POMCo)(2)O](2)O] is best defined as a compound having two cobalt(III) atoms with two oxidized oxygen atoms.

Alkylated polyethylenimine or homochiral polyethylenimine derivatives, or both, were used to stabilize palladium nanoparticles dispersed in water. Such constructs have hydrophobic regions that enabled the aqueous biphasic hydrogenation of imines formed in situ by the reaction of ketones and chiral primary amines. In the presence of MgCl₂, good yields of the direct amination product were obtained with high diastereoselectivity. In the presence of a homochiral polyethylenimine, enantiospecific direct amination was observed via kinetic resolution, ostensibly as a result of preferred access of one enantiomer to the achiral reaction center.

Trifluoroethanol was oxidized with O₂ to trifluoroethyl trifluoracetate (>98% selectivity) using Pt^II(dppz)Cl₂ as a catalyst in the presence of H₂SO₄; Pt^II(phen)Cl₂ was inactive. Kinetic isotope effects suggest the CH bond activation as the rate determining step and DFT calculations showed different frontier orbitals for Pt^II(dppz) and Pt^II(phen)-based catalysts.

Metal oxides as a rule oxidize and oxygenate substrates via the Mars-van Krevelen mechanism. A well-defined α-Keggin polyoxometalate, H ₅PV₂Mo₁₀O₄₀, can be viewed as an analogue of discrete structure that reacts via the Mars-van Krevelen mechanism both in solution and in the gas phase. Guided by previous experimental observations, we have studied the key intermediates on the reaction pathways of its reduction by various compounds using high-level DFT calculations. These redox reactions of polyoxometalates require protons, and thus such complexes were explicitly considered. First, the energetics of outer-sphere proton and electron transfer as well as coupled proton and electron transfer were calculated for seven substrates. This was followed by identification of possible key intermediates on the subsequent reaction pathways that feature displacement of the metal atom from the Keggin structure and coordinatively unsaturated sites on the H₅PV₂Mo₁₀O₄₀ surface. Such metal defects are favored at vanadium sites. For strong reducing agents the initial outer-sphere electron transfer, alone or possibly coupled with proton transfer, facilitates formation of metal defects. Subsequent coordination allows for formation of reactive ensembles on the catalyst surface, for which the selective oxygen-transfer step becomes feasible. Weak reducing agents do not facilitate defect formation by outer-sphere electron and/or proton transfers, and thus formation of metal defect structures prior to the substrate activation is suggested as an initial step. Calculated geometries and energies of metal defect structures support experimentally observed intermediates and demonstrate the complex nature of the Mars-van Krevelen mechanism.

Focus The chemical industry is intimately linked with the realities of transformation of fossil fuels to useful compounds and materials, energy consumption, and environmental sustainability. The questions that concern us are the following: how can one minimize energy consumption in the chemical industry and reduce waste formation in chemical reactions, and can we transform this industry from one based primarily on petroleum to one that utilizes renewable feedstocks? In this chapter we will see how this \u201cgreening\u201d of this major industrial and energy sector will require the use of catalysis and development of new catalysts. Synopsis Thomas Jefferson stated that \u201cthe earth belongs in usufruct to the living.\u201d This premise is the basis of present recognition that, if the natural capacity of planet Earth to deal with pollution and waste is exceeded, then our lifestyle will become unsustainable. In this context, it is difficult to imagine life in the twenty-first century without accounting for the role the chemical industry plays in our modern society. For example, in the transportation sector, the most obvious aspect is the production of efficient fuels from petroleum, but one also should consider catalytic converters that reduce toxic emissions, and engineering polymers and plastics that reduce vehicle weight and therefore reduce fuel consumption. In daily consumer life we use chemicals in products such as paints, DVDs, synthetic carpets, refrigerants, packaging, inks and toners, liquid-crystal displays, and synthetic fibers. Pesticides and fertilizers are needed in order to increase agricultural productivity and pharmaceuticals to keep us healthy. In this chapter we will try to understand the aspects involved in the sustainability of the chemical industry. We will describe how to measure and control environmental performance and then define what we mean by green processing in the chemical industry, specifying green process metrics, introducing key concepts such as atom economy, E-factors, and effective mass yields. With these concepts together with the evaluation raw material costs, waste treatment, and unit processes needed for production of a chemical one can appreciate the \u201cgreenness\u201d of a chemical process. The second part of this chapter will describe the role catalysis has in chemical transformations and how catalysis is used to reduce energy consumption and waste formation, together leading to increased sustainability. Examples will be given for a spectrum of applications ranging over oil refining, ammonia production, manufacture of important chemical intermediates and materials, and use of catalysis for the production of commodity chemicals. Finally, we will discuss the role catalysis may play in the replacement of fossil-fuel feedstocks with renewable ones, and how catalysis may contribute to our search for solar fuels.

Protonated phosphovanadomolybdates of the Keggin structure, H _3+xPV_xMo_12-xO₄₀ where x = 0, 1, 2, and derivatives with surface defects formed by loss of constitutional water were studied using high-level DFT calculations toward determination of the most stable species and possible active forms in oxidation catalysis in both the gas phase and in polar solutions. The calculations demonstrate that protonation at bridging positions is energetically much more favorable than protonation of terminal oxygen atoms. The preferential protonation site is determined by the stability of the metal-oxygen bond rather than the negative charge on the oxygen atom. In H₃PMo₁₂O₄₀, maximum distances between protons at bridging oxygen atoms are energetically favored. In contrast, for H₄PVMo₁₁O₄₀ and H₅PV ₂Mo₁₀O₄₀ protons prefer nucleophilic sites adjacent to vanadium atoms. Up to three protons are bound to the nucleophilic sites around the same vanadium atom in the stable isomeric forms of H ₅PV₂Mo₁₀O₄₀ that result in strong destabilization of oxo-vanadium(V) bonding to the Keggin unit. Such behavior arises from the different nature of the Mo-O and V-O bonds that can be traced to the different sizes of the valence d orbitals of the metals. Coordination of two protons at the same site yields water and an oxygen defect as a result of its dissociation. The energetic cost for the formation of surface defects decreases in the order: O_t ≫ O_c ≲ O_e and is lower for the sites adjacent to vanadium atoms. Vanadium atoms near defects also have a significant contribution to the LUMO. Thus, vanadium-substituted polyoxometalates with defects near and, especially, between vanadium atoms present a plausible active form of polyoxometalates in oxidation reactions.

Gone in 7200 Seconds: Heteroaromatic thiophene derivatives, refractory to hydrodesulfurization, are removed from hydrocarbons by oxidative polymerization over H₅PV₂Mo₁₀O₄₀/SiO₂ as a recyclable, heterogeneous catalyst. The levels of sulfur-containing compounds reach sub-ppm to nondetectable levels, as analyzed by gas chromatography with a flame photometric detector.

A porous, homochiral titanium-phosphonate material based on a tripodal peptide scaffold was used as a heterogeneous reaction medium for the enantioselective hydration (>99%) of styrene oxide. This titanium-phosphonate material, which was shown to contain confined chiral spaces, was prepared by polymerization of L-leucine onto a tris(2-aminoethyl)amine initiator, followed by capping with phosphonate groups and completed by non-aqueous condensation with titanium isopropoxide. Circular dichroism confirmed that the peptide tethers yielded a secondary structure. X-ray powder diffraction and transmission electron microscopy supported by a semi-empirical model showed the likely formation of a porous, lamellar material that was quantified by nitrogen adsorption.

A ditopic 1,2-bis(2,2-bipyridyl-6-yl)ethyne ligand, L, has been synthesized for the first time by consecutive Suzuki and Sonogashira coupling reactions either in a one- or two-step synthesis. Coordination of L with some first-row transition metals, Fe, Mn and Co showed a very rich structural diversity that can be obtained with this ligand. Reaction of L with Mn ^II(OAc)₂ yielded a dimanganese(ii) complex, [Mn ₂L(μ-OAc)₃]PF₆, (1) where the two somewhat inequivalent trigonal-bipyramidal Mn atoms separated by 3.381 Å are bridged by L and three acetate moieties. A similar reaction of L with Mn ^III(OAc)₃ yielded a very different dimanganese complex [Mn₂L(OH)(OAc)₂(DMF)₂]PF ₆·DMF (2) where L is a E-1,2-bis(2,2-bipyridyl- 6-yl)ethene fragment that was formed in situ. The L ligand bridges between the two Mn centers, despite its trans configuration, which leads to a very strained ethene bridging moiety. The Mn atoms are also bridged by two acetate ligands and a hydroxy group that bridges between the Mn atoms and the ethene fragment; DMF completes the octahedral coordination around each Mn atom which are separated by 3.351 Å. A comproportionation reaction of L with Mn^II(OAc)₂ and n-Bu₄NMnO₄ yielded a tetramanganese compound, [Mn₄(μ₃-O)₂(OAc) ₄(H₂O)₂L₂](PF₆) ₂·2CH₃CN (3). Compound 3 has a dimer of dimers structure of the tetranuclear Mn core that consists of binuclear [Mn ₂O(OAc)₂L]⁺ fragment and a PF₆ anion. BVS calculations indicate that 3 is a mixed-valent 2Mn^II plus 2Mn^III compound where two [Mn^II₂O(OAc) ₂L]⁺ fragments are held together by Mn^III-O inter-fragment linkers which have a distorted octahedral geometry. The Mn atoms in the [Mn₂O(OAc)₂L]⁺ fragments have a capped square-pyramid configuration where an aqua ligand is capped on one of the faces. Although the aqua ligand is well within a bonding distance to a carbon atom of the proximal ethyne bridge, there does not appear to be an oxygen-carbon bond formation, rather the ligand is constrained in this position, as deduced by the observation that the bond lengths and angles of the ligand are essentially the same as those for the free ligand, L. Reaction of L with perchlorate or triflate salts of Fe(ii), Mn(ii) and Co(ii) in dry acetonitrile yielded binuclear triple helicate structures (2:3 metal to L ratios) [Fe₂L ₃](CF₃SO₃)₄·CH₃CN (4), [Mn₂L₃](ClO₄)₄·1. 7CH₃CN·1.65EtOEt (5) and [Co₂L₃] (ClO₄)₄·2CH₃CN·2EtOEt (6) where each M(ii) center with a slightly distorted octahedral geometry is bridged by three of the ditopic ligands. The M-M distances varied; 5.961 Å (Mn), 6.233 Å (Co) 6.331 Å (Fe). Reaction of L with Co(ClO ₄)₂·6H₂O in wet acetonitrile yielded a dicobalto(iii) compound, [Co₂L₃(O) ₂](ClO₄)₂·H₂O (7), with two types of L fragments; one bridging between the two Co centers and two non-bridging ligands, each bonded to a Co atom via one bipyridyl group where the other is non-bonding. The octahedral coordination sphere around each Co atom is completed by the formation of a cobalt-carbon bond from the two carbon atoms of the ethene moiety of the bridging ligand and by a hydroxy moiety that is also bonded to the ethene group of the non-bridging ligand. Reaction of L with Co(ClO₄)₂·6H₂O in dry acetonitrile in the presence of Et₃N yielded the tetracobalto(ii) complex {[Co ₂L₄(OH)₄](ClO₄)₄} ₂ (8) with a unique twisted square configuration of cobalt ions with Co-Co distances of 3.938 to 4.131 Å. In addition to the L bridging ligand the Co atoms are linked by hydroxy moieties. Some preliminary catalytic studies showed that the Mn compounds 1 and 2 were active (high yield within 3 min) for alkene epoxidation with peracetic acid and hydrogen peroxide dismutation (catalase activity).

In this Forum Article, we discuss the use of dioxygen (O₂) in oxidations catalyzed by polyoxometalates. One- and two-electron-transfer oxidation of organic substrates is catalyzed by H₅PV ₂Mo₁₀O₄₀ and often occurs via an outer-sphere mechanism. The reduced polyoxometalate is reoxidized in a separate step by O₂ with the formation of water. H₅PV₂Mo ₁₀O₄₀ also catalyzes electron transfer-oxygen transfer reactions. Here, in contrast to the paradigm that high-valent oxo species are often stronger oxygen-transfer species than lower-valent species, the opposite occurs. Thus, oxygen transfer from the catalyst is preceded by electron transfer from the organic substrate. The monooxygenase-type reduction of O₂ with polyoxometalates is also discussed based on the formation of a stable iron(III) hydroperoxide compound that may have implications for the oxidation of other lower-valent polyoxometalates such as vanadium(IV)- and ruthenium(II)-substituted polyoxometalates. Finally, the formation of hybrid compounds through the attachment of electron-accepting polyoxometalates to coordination compounds can modify the reactivity of the latter by making higher-valent oxidation states more accessible.

A polyoxometalate of the Keggin structure substituted with Ru(III), (6)Q(5)[Ru(III)(H(2)O)SiW(11)O(39)] in which (6)Q=(C(6)H(13))(4)N(+), catalyzed the photoreduction of CO(2) to CO with tertiary amines, preferentially Et(3)N, as reducing agents. A study of the coordination of CO(2) to (6)Q(5)[Ru(III)(H(2)O)SiW(11)O(39)] showed that 1) upon addition of CO(2) the UV/Vis spectrum changed, 2) a rhombic signal was obtained in the EPR spectrum (g(x)=2.146, g(y)=2.100, and g(z)=1.935), and 3) the (13)C NMR spectrum had a broadened peak of bound CO(2) at 105.78 ppm (Delta(1/2)=122 Hz). It was concluded that CO(2) coordinates to the Ru(III) active site in both the presence and absence of Et(3)N to yield (6)Q(5)[Ru(III)(CO(2))SiW(11)O(39)]. Electrochemical measurements showed the reduction of Ru(III) to Ru(II) in (6)Q(5)[Ru(III)-(CO(2))SiW(11)O(39)] at -0.31 V versus SCE, but no such reduction was observed for (6)Q(5)[Ru(III)(H(2)O)SiW(11)O(39)]. DFT-calculated geometries optimized at the M06/PC1//PBE/AUG-PC1//PBE/PC1-DF level of theory showed that CO(2) is preferably coordinated in a side-on manner to Ru(III) in the polyoxometalate through formation of a Ru-O bond, further stabilized by the interaction of the electrophilic carbon atom of CO(2) to an oxygen atom of the polyoxometalate. The end-on CO(2) bonding to Ru(III) is energetically less favorable but CO(2) is considerably bent, thus favoring nucleophilic attack at the carbon atom and thereby stabilizing the carbon sp(2) hybridization state. Formation of a O(2)C-NMe(3) zwitterion, in turn, causes bending of CO(2) and enhances the carbon sp(2) hybridization. The synergetic effect of these two interactions stabilizes both Ru-O and C-N interactions and probably determines the promotional effect of an amine on the activation of CO(2) by [Ru(III)(H(2)O)SiW(11)O(39)](5-). Electronic structure analysis showed that the polyoxometalate takes part in the activation of both CO(2) and Et(3)N. A mechanistic pathway for photoreduc

Reaction of nickel (II) perchlorate with the ligand N,N-bis-(3,5-dipiperidin-1-yl-[2,4,6]triazin-1-yl)-pyridin-2-ylmethyl-ethane-1,2-diamine yields an octahedral Ni(II) high-spin complex 1 ([C₄₀H₅₆N₁₄Ni(H₂O)(CH₃OH)](ClO₄)₂(CH₃OH)₂) with moderate zero-field splitting (ZFS) axial distortion parameter D/k_B = 5.37 K. The ligand contributes a N4 donor set; the remaining two coordinating positions are occupied by coordinating solvents molecules. Exchange of the coordinating solvents molecules in complex 1 to thiocyanate moieties leads to formation of complex 2 ([C₄₀H₅₆N₁₄Ni(NCS)₂](CHCl)₃) with an extended parameter D/k_B = 8.80 K. The analysis of the structural and magnetic properties of complexes 1 and 2 led to the design of dinuclear complex 3 ([C₄₀H₅₆N₁₄NiN₃]₂(ClO₄)₂(CH₃OH)₂), where two azido groups were utilized as bridging ligands. The double azido bridges in complex 3 cross each other to form a rarely observed non-coplanar (N₃)₂ structure. The magnetic behavior of complex 3 reveals ferromagnetic coupling interactions characterized by J/k_B = 23.25 K, D₁/k_B = 7.90 K, D₂/k_B = 0.54 K.

Cornets are cone-shaped ceramic vessels, characteristic of the Chalcolithic period (ca. 4700-3700 BC) in Israel and Jordan. Their contents and use are unknown. Gas chromatography with flame ionization and mass-selective detection, showed that extracts of cornets from five different sites with different related activities (domestic, habitation cave and a cultic complex) all contain the same assemblage of mainly n-alkanes adsorbed within their walls. This assemblage differs from those found in other types of ceramic vessels from the same sites, as well as from the residues found within the associated sediments. The assemblage of odd and even-numbered n-alkanes found in the cornets is almost identical to that found in the residues of beeswax heated on modern ceramic fragments, as well as in a beehive from the Iron Age IIA strata at Tel Rehov, Israel. Thus the cornets are most likely to have contained beeswax. The presence of beeswax in the cornets contributes to our understanding of the Chalcolithic period; a time when secondary products such as milk, olive oil and wine are thought to have come into use.

Both closed and open framework structures were designed for copper complexes with N,N-bis-pyridin-2-ylmethyl-ethane-1,2-diamine (2-bpen)-based ligands. The design included substitution at the bridging aliphatic nitrogen atoms by reaction with cyanuric chloride to yield the 3,5-dichloro-2,4,6-triazine derivative. The chloride atoms on the triziane rings were further substituted by either electron donating amines, or an electron withdrawing thiomethyl moiety. The substitution of bridging nitrogen atoms by more electron donating aminated 2,4,6-triazines led to the formation of copper(II) complexes with closed square pyramidal architecture. On the other hand, substitution of bridging nitrogen atoms by the more electron withdrawing 2,4,6-triazines with thioether groups led to the formation of square planar or tetrahedral copper complexes with an open framework architecture whose specific structure and oxidation state depended on the anion.

Beehives were discovered in a densley built area in the Iron Age city of Rehov (tenth-ninth century BC). They consisted of hollow clay cylinders, each with a little hole at one end (for the bee) and a removable lid at the other (for the bee keeper). These beehives, the earliestfound in the Near East, were identified by analogy with examples pictured on Egyptian tombs and in use by traditional peoples. The suggested identification was confirmed by chemical analysis.

A polyoxometalate with two alkyl thiol appendages, Q₄[SiW ₁₁O₄₀(SiCH₂CH₂CH₂SH) ₂] (Q = ammonium salt) stabilized the formation of palladium nanoparticles. This tethered polyoxometalate-palladium binary catalyst was effective for the aerobic oxydehydrogenation of vinylcyclohexene and vinylcyclohexane to styrene as the major product via activation of the allylic, tertiary carbon-hydrogen bond.

The geometries and electronic structures of the five isomers of [PV ₂Mo₁₀O₄₀]^5- and its mono- and direduced species were theoretically investigated by means of B3LYP calculations, with an attempt to understand their role as oxidative catalysts in electron-transfer-initiated processes. The calculations reveal the following features: (a) Either in the gas phase or in a solvent, the reduction of the [PV₂Mo₁₀O₄₀]^5- produces [PV ₂Mo₁₀O₄₀]^6,7- ions in which the excess electrons are localized on the vanadium atoms in the corresponding δ orbitals. By contrast, the direduced [PV₂Mo ₁₀O₄₀]^7- isomers where the one electron is localized on molybdenum are high in energy. Consequently, whereas the five isomers of [PV₂Mo₁₀O₄₀]^5- have roughly a statistical distribution, in the direduced species, the 1,4-[PV ₂Mo₁₀O₄₀]^7- isomer becomes more stable than others. (b) The gas-phase reduction of the parent [PMo ¹²O₄₀]^3- anion is exceedingly more favorable (by ca. 250 kcal/mol) than the reduction of [PV₂Mo₁₀O ₄₀]^5-. By contrast, contribution of the solvent exerts a strong leveling effect and makes the reduction potentials of the two species very similar, (c) Since the reduction of [PV₂Mo₁₀O ₄₀O₄₀]^5- concentrates high negative charge near the O=VO₄ moiety, this negative charge accumulation will act as an attractor that binds the organic radical cation (cation), which causes a subsequent proton transfer to an oxo ligand of the vanadium. As such, in terms of an electron-transfer catalytic effect, the presence of vanadium in [PV ₂Mo₁₀O₄₀]^5- creates function for the catalyst, (d) The computations reveal that [PV₂Mo₁₀O ₄₀]^7- is a diradicaloid that possesses two virtually degenerate states, with singlet and triplet spins. Thus, we may expect two-state reactivity (TSR) in the catalytic reactions of [PV₂Mo ₁₀O₄₀]^5-. Some predictions are made based on these features.

A cross-linked polyethyleneimine polymer containing the [ZnWZn ₂(H₂O)₂(ZnW₉O₃₄) ₂]^12- polyoxometalate was prepared from branched polyethyleneimine (M_w = 600), the polyoxometalate, and a n-octylamine-epichlorohydrin cross-linking reagent. This catalytic assembly was active for the selective oxidation of 2-alkanols to 2-alkanones with aqueous H₂O₂ with reactions presumably occurring at a hydrophobic domain. Most importantly, the catalyst showed distinctive lipophiloselectivity, that is selectivity as a function of the lipophilic nature of a reaction substrate. The lipophiloselectivity was proportional to the relative partition coefficient (1-octanol/water) of the substrates.

This work uses density functional calculations to design a new high-valent Fe(V)=O catalyst [Mo₅O₁₈Fe=O]^3-, which is based on the Lindqvist polyoxometalate (Mo₆O₁₉^2-). Because the parent species is stable to oxidative conditions, one may assume that the newly proposed iron-oxo species will be stable, too. The calculated Mössbauer spectroscopic data may be helpful toward an eventual identification of the species. The calculations of C-H hydroxylation and C=C epoxidation of propene show that, if made, [Mo₅O₁₈Fe=O] ^3- should be a potent oxidant that will be subject to strong solvent effect. Moreover, the Lindqvist catalyst leads to an intriguing result; the reaction that starts along an epoxidation pathway with C=C activation ends with a C-H hydroxylation product (⁴6) due to rearrangement on the catalyst. The origins of this result are analyzed in terms of the structure of the catalyst and the electronic requirements for conversion of an epoxidation intermediate to a hydroxylation product. Thus, if made, the [Mo ₅O₁₈Fe=O]³ will be a selective C-H hydroxylation reagent.

An EPR spectrum of as synthesized [G.A. Tsigdinos, C.J. Hallada, Inorg. Chem. 7 (1968) 437-441], orange colored, H₅PV₂Mo₁₀O₄₀ polyoxometalate showed the presence of a reduced vanadium(IV) addenda atom. Surprisingly, further ³¹P ENDOR (electron-nuclear double resonance) measurements indicated the absence of a phosphorous heteroatom leading to the suggestion that H₅V^VV^IVMo₁₁O₄₀ exists as a previously unrecognized impurity in the typically synthesized H₅PV₂Mo₁₀O₄₀ compound. H_5/4PV^VO₄V^IV/VMo₁₁O₃₆ was then synthesized in low yield (0.8 mol%) by omitting the addition of phosphate in a typical H₅PV₂Mo₁₀O₄₀ preparation. The molecular formulation and structure was supported by X-ray crystallography, infrared and mass spectrometry. Further use of EPR/ENDOR/ESEEM (electron-spin echo envelope modulation) allowed the formulation of [V^VV^IVMo₁₁O₄₀]^5- as [V^VO₄V^IVMo₁₁O₃₆]^5-. Accordingly, the polyoxometalate has a multiscripts(VO, 4, mml:none(), mml:none(), 3 -) heteroatom core with 11 molybdenum addenda and one VO²⁺ moiety at the polyoxometalate surface. The redox potential and the catalytic activity of the new vanadomolybdate polyoxometalate compound were essentially identical to the often-studied H₅PV₂Mo₁₀O₄₀ polyoxometalate isomeric mixture.

The synthesis of a new vanadium phosphonate material (TPPhA-V), prepared by the non-hydrolytic condensation of a vanadium(V) alkoxide with an arylphosphonic acid, is presented. Transmission electron microscopy (TEM) of TPPhA-V showed the specimen to be granular with small highly contrasting dots situated within the a more poorly contrasting substance, while the scanning electron microscopy (SEM) examination revealed formation of spongelike amorphous material as the only type of the specimen morphology. Room-temperature electron spin resonance measurement of the TPPhA-V isotropic broad singlet resonance line indicated strong interaction between neighboring vanadium (IV) atoms. The test of the catalytic properties of TPPhA-V showed that the oxidation generally proceeded smoothly with the formation of corresponding benzylic aldehydes at high conversion and selectivity.

Micelle directed polyoxometalate nanoparticles were synthesized by depositing H_3+xPV_xMo_12-xO₄₀ (x = 0, 2) by precipitation on micelles prepared from cesium dodecyl sulfate. The cryo-TEM image showed particles of about ∼10 nm roughly consistent with the particle size computed from an idealized model. HRTEM coupled with EELS imaging to map the distribution of the elements also supported the formation of micelle directed polyoxometalate nanoparticles. In the aerobic oxidation of various sulfides to sulfoxides and sulfones, the clustered polyoxometalate assemblies supported on hydrophilic silica showed significantly higher catalytic activity versus that of nonclustered assemblies.

Selective aerobic oxidation of benzylic alcohols and of activated aromatic hydrocarbons occurs in supercritical CO₂ as reaction medium using H₅PV₂Mo₁₀O₄₀ as a quasi-heterogeneous catalyst without further additives or co-solvents; efficient recycling is possible and no metal leaching is detectable in the product stream.

A new method for the synthesis of vicinal diols from alkenes has been developed. Reaction of molecular iodine in the presence of a polyoxometalate as oxidation catalyst under aerobic conditions in acetic acid solvent leads to the oxidative iodoacetoxylation of an alkene, i.e. formation of a 1,2-iodoacetate. Further in situ substitution of the iodide by water yields the 1,2-diol monoacetate with a predominantly (ca. 4.5:1) cis-configuration. Further esterification under the reaction's acidic conditions leads also to the cis-diacetate. The method may be valuable for the synthesis of cis-vicinal diols without use of toxic osmium catalysts.

Ag and Ru nanoparticles stabilized by H₅PV₂MO ₁₀O₄₀, prepared by a sequence of redox reactions and supported on α-alumina, were effective catalysts for the direct aerobic epoxidation of alkenes in the liquid phase.

A phenanthroline ligand has been covalently modified at the 2 and 9 positions by an aminophenylhexamolybdate substituent. The 1H NMR spectrum indicated a strong electron-withdrawing effect of the hexamolybdate (Mo ₆O₁₉^2-) moiety on the phenanthroline ligand. UV-vis and cyclic voltammetry showed extended conjugation of the hybrid phenanthroline-polyoxometalate compound and the possibility of easy oxidation of the extended phenanathroline ligand. Further EPR experiments provided strong evidence for an intramolecular charge-transfer process with the formation of a phenanthroline cation radical and a reduced hexamolybdate.

Two new paradigms for separating the homogeneous catalyst from the substrate and products of oxidation were reviewed. The first example demonstrated the functionalization in water of hydrophobic substrates, i.e., hydrocarbons, with methane monooxygenase biomimetic complexes embedded in a derivatized surface silica system using tert-butyl hydroperoxide/O₂ as the oxidants. For example, cyclohexane was oxidized to a mixture of cyclohexanone, cyclohexanol, and cyclohexyl-tert-butyl peroxide, in a ratio of ∼ 3:1:2. The balance between polyethylene oxide and polypropylene oxide, tethered on the silica surface, was crucial for maximizing the catalytic activity. The mechanism for the silica-based catalytic assembly occurred via the Haber-Weiss process. The second precatalyst separation paradigm, the use of the fluorous solvents, which was predicted on solubilizing the precatalyst in a fluorocarbon solution, allowed the functionalization of alkanes and alkenes, while selective oxidation of alcohols to aldehydes was also possible; both precatalyst and product were in separate solvent phases.

Nine- and 27-armed dendrimers with a peroxophosphotungstate core were synthesized by an ionic-bonding approach and used as air-stable, recoverable catalysts for oxidation reactions using hydrogen peroxide.

We have demonstrated that a bipyrimidinylplatinum-polyoxometalate, [Pt(Mebipym)Cl₂]⁺[H₄PV₂Mo₁₀O₄₀]^-, supported on silica is an active catalyst for the aerobic oxidation of methane to methanol in water under mild reaction conditions. Further oxidation of methanol yields acetaldehyde. The presence of the polyoxometalate is presumed to allow the facile oxidation of a Pt(II) intermediate to a Pt(IV) intermediate and to aid in the addition of methane to the Pt catalytic center.

Regenerable and reusable dendritic catalysts have been synthesized by the ionic assembly of polyammonium dendrimers and polyoxometalate (POM) trianionic units. These dendrimers (see example depicted, in which the POM is [PO ₄{WO(O₂)₂}₄]^3-) are found to catalyze, at ambient temperature, the quantitative epoxidation of olefins by H₂O₂ in water/ CDCl₃ and the selective and quantitative oxidation of thioanisole to its sulfone.

Eleven W-based catalyst systems for alkene epoxidation with aqueous H ₂O₂ were compared under identical conditions and at equal level of 0.1 mol % W-atoms. Of these, those based on a combination of H ₂WO₄ and a methyltrioctylammonium phase transfer catalyst turned out to be most active in particular systems that contain a source of phosphate. Evidence is presented that under our conditions the actual epoxidizing species in H₂WO₄-based catalyst systems without phosphate source is mononuclear [WO(OH)(O₂)₂] ^- rather than binuclear [{WO(O²)₂} ₂O]^2- that is usually thought to be active. For large-scale applications, however, the polyoxometalate Na₁₂[WZn ₃-(ZnW₉O₃₄)₂] (NaZnPOM) in combination with a suitable phase transfer catalyst such as methyltrioctylammonium chloride is preferred over H₂WO ₄-based catalysts. This preference results from the fact that use of H₂WO₄ requires a catalyst activation step that is troublesome on a large scale, whereas epoxidations catalyzed by NaZnPOM start without induction period on addition of H₂O₂. Optimizations of epoxidations catalyzed by QCl/NaZnPOM or QCl/H ₂WO₄ have shown that the optimum Q/W ratio depends on the alkene that is epoxidized and differs from that expected from catalyst stoichiometry. An attractive feature of NaZnPOM from the viewpoint of industrial applicability is that epoxidations and other reactions with H₂O ₂ are efficiently catalyzed by a readily available aqueous solution of NaZnPOM prepared through self-assembly. A 1 mol scale example is provided of an epoxidation catalyzed by a combination of self-assembled NaZnPOM and Luviquat mono CP as a multifunctional cocatalyst with emulsifying, buffering, and phase-transferring properties.

A metal-organic-polyoxometalate hybrid compound with two functional centers consisting of a rhodium(I)bis(diphenylphospine) unit connected through two alkylene bridging groups to a lacunary Keggin type polyoxometalate was synthesized and used as an effective, recyclable hydrogenation catalyst in monophasic and aqueous biphasic reaction modes.

The epoxidation of allylic alcohols is shown to be efficiently and selectively catalyzed by the oxidatively resistant sandwich-type polyoxometalates, POMs, namely [WZnM₂(ZnW₉O ₃₄)₂]^q- [M = OV(IV), Mn(II), Ru(III), Fe(III), Pd(II), Pt(II), Zn(II); q = 10-12], with organic hydroperoxides as oxygen source. Conspicuous is the fact that the nature of the transition metal M in the central ring of polyoxometalate affects significantly the reactivity, chemoselectivity, regioselectivity, and stereoselectivity of the allylic alcohol epoxidation. For the first time, it is demonstrated that the oxovanadium(IV)-substituted POM, namely [ZnW(VO)₂(ZnW ₉O₃₄)₂]^12-, is a highly chemoselective, regioselective, and also stereoselective catalyst for the clean epoxidation of allylic alcohols. A high enantioselectivity (er values up to 95:5) has been achieved with [ZnW(VO)₂(ZnW₉O ₃₄)₂]^12- and the sterically demanding TADOOL-derived hydroperoxide TADOOH as regenerative chiral oxygen source. Thus, a POM-catalyzed asymmetric epoxidation of excellent catalytic efficiency (up to 42 000 TON) has been made available for the development of sustainable oxidation processes. The high reactivity and selectivity of this unprecedented oxygen-transfer process are mechanistically rationalized in terms of a peroxy-type vanadium(V) template.

Co-crystallization of a tri-ammonium cation with short and somewhat flexible 'arms', [N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide]³⁺, with a polyoxometalate trianion, PW₁₂O₄₀^3-, yielded an insoluble channeled or microporous structure. The polyoxometalate clusters are arranged in a layered and zig-zag fashion along the xy plane. Looking along the x-axis, channels of a dimension of ∼3.5 × ∼6.5 Å are observed. It was found that C-H⋯O bonds aided in determining the crystal packing by providing directionality to the anion-cation interaction. On the other hand the co-crystallization of a tetraammonium cation with an extended and rigid tetrahedral configuration, 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane tetraiodide, with a polyoxometalate tetracation, SiW₁₂O ₄₀^4-, yielded a lamellar structure with alternating layers with spacing of 16.6 Å of the inorganic-organic hybrid material.

We have demonstrated that a simply prepared water-soluble polyoxometalate, Na₁₂[WZnZn₂(H₂O)₂(ZnW₉O₃₄)₂], synthesized from readily available zinc and tungsten salts in the presence of nitric acid, is an effective catalyst for selective alcohol oxidation with hydrogen peroxide in biphasic (water-alcohol) reaction media. Experiments have shown that the "self-assembled" catalyst in its mother liquor was as active as the isolated catalyst. The aqueous catalyst solution is easily separated from the water-insoluble products and can be recycled without loss in activity or selectivity.

Experimental observations show that direct olefin epoxidation by H₂O₂, which is extremely sluggish otherwise, occurs in fluorinated alcohol (R_fOH) solutions under mild conditions requiring no additional catalysts. Theoretical calculations of ethene and propene epoxidation by H₂O₂ in the gas phase and in the presence of methanol and of two fluorinated alcohols, presented in this paper, show that the fluoro alcohol itself acts as a catalyst for the reaction by providing a template that stabilizes specifically the transition state (TS) of the reaction. Thus, much like an enzyme, the fluoro alcohol provides a complementary charge template that leads to the reduction of the barrier by 5-8 kcal mol^-1. Additionally, the fluoro alcohol template keeps the departing OH and hydroxyalkenyl moieties in close proximity and, by polarizing them, facilitates the hydrogen migration from the latter to form water and the epoxide product. The reduced activation energy and structural confinement of the TS over the fluoro alcohol template render the epoxidation reaction observable under mild synthetic conditions.

Sandwich-type polyoxometalates (POMs), namely [VZnM₂(ZnW₉O₃₄)₂]^q- [M = Mn(II), Ru(III), Fe(III), Pd(II), Pt(II), Zn(II); q = 10-12], are shown to catalyze selectively the epoxidation of chiral allylic alcohols with 30% hydrogen peroxide under mild conditions (ca. 20 °C) in an aqueous/organic biphasic system. The transition metals M in the central ring of polyoxometalate do not affect the reactivity, chemoselectivity, or stereoselectivity of the allylic alcohol epoxidation by hydrogen peroxide. Similar selectivities, albeit in significantly lower product yields, are observed for the lacunary Keggin POM [PW₁₁O₃₉]^7-, in which a peroxotungstate complex has been shown to be the active oxidizing species. All these features support a tungsten peroxo complex rather than a high-valent transition-metal oxo species operates as the key intermediate in the sandwich-type POM-catalyzed epoxidations. On capping of the hydroxy functionality through acetylation or methylation, no reactivity of these hydroxy-protected substrates [1a(Ac) and 1a(Me)] is observed by these POMs. A template is proposed to account for the marked enhancement of reactivity and selectivity, in which the allylic alcohol is ligated through metal - alcoholate bonding, and the H₂O₂ oxygen source is activated in the form of a peroxotungsten complex. 1,3-Allylic strain promotes a high preference for the threo diastereomer and 1,2-allylic strain a high preference for the erythro diastereomer, whereas tungsten - alcoholate bonding furnishes high regioselectivity for the epoxidation of the allylic double bond. The estimated dihedral angle α of 50 - 70° for the metal - alcoholate-bonded template of the POM/H₂O₂ system provides the best compromise between ^1,2A and ^1,3A strain during the oxygen transfer. In contrast to acyclic allylic alcohols 1, the M-POM-catalyzed oxidation of the cyclic allylic alcohols 4 by H₂O₂ gives significant amounts of enone.

Benzylic, allylic, and aliphatic alcohols are oxidized to aldehydes and ketones in a reaction catalyzed by Keggin-type polyoxomolybdates, PV_xMo_(12-x)O₄₀^-(3+x) (x = 0, 2), with DMSO as a solvent. The oxidation of benzylic alcohols is quantitative within hours and selective, whereas that of allylic alcohols is less selective. Oxidation of aliphatic alcohols is slower but selective. Further mechanistic studies revealed that, for H₃PMo₁₂O₄₀ as a catalyst and benzylic alcohols as substrates, the sulfoxide is in fact an oxygen donor in the reaction. Postulated reaction steps as determined from isotopelabeling experiments, kinetic isotope effects, and Hammett plots include (a) sulfoxide activation by complexation to the polyoxometalate and (b) oxygen transfer from the activated sulfoxide and elimination of water from the alcohol. The mechanism is supported by the reaction kinetics.

A manganese(III)-substituted polyoxometalate of the "sandwich" structure, [Mn^III₂ZnW(ZnW₉O₃₄)₂]^10-, catalyzed the highly selective (>99.9%) epoxidation of alkenes, such as 1-octene, 2-octene, and cyclohexene with nitrous oxide. Reactions occurred in homogeneous media at 150 °C under 1 atm N₂O. The epoxidation had a linear reaction profile; turnover frequencies of 0.5-1.4 h^-1 were measured. The reactions were also stereoselective; for example, cis-stilbene gave cis-stilbene oxide. From ESR spectroscopy, it was shown that a Mn(II) octahedral species is reversibly formed by reaction between the original Mn(III) polyoxometalate and N₂O. Therefore, it would appear that a Mn(V)-oxo active species is not formed; it is possible that the activation of nitrous oxide was by its oxidation by the Mn(III) polyoxometalate.

Oxidatively and solvolytically persistent sandwich-type polyoxometalates, namely [WZnM2(ZnW9O34)(2)](q-) [M = Mn(II), Ru(III), Fe(Ill), Zn(II)], catalyze chemoselectively, diastereoselectively and regioselectively the epoxidation of chiral allylic alcohols with 30% hydrogen peroxide through a tungsten peroxo species, in which the allylic alcohol is coordinated as alcoholate (template effect).

The film preparation properties and amplified spontaneous emission (ASE) features of a conjugated polymer blended with a zirconium-organosilicon xerogel glassy matrix were studied. Dip and spin coating methods were used to perform polarization and washing steps for the conjugated polymer. Silicon wafer covered with a 7μm thick silica layer was used as a waveguide substrate. The characterization of the absorption and fluorescence spectra of the films signified the evidence of an enhanced gap between the two bands.

New metallosilicate catalysts were prepared by reacting a silanol capped dendrimer, Si[CH₂CH₂Si(CH₃)₂OH]₄ with MCp₂Cl₂ (M = Ti^IV, Mo^VI, W^VI and V^V). The resulting Si[CH₂CH₂Si(CH₃)₂OMCp ₂Cl]₄ compounds were incorporated in a silica matrix by the sol-gel method. The catalytic activity of the metallosilicates after calcination revealed excellent activity and selectivity towards epoxidation of alkenes with tert-butylhydroperoxide. Maximum activity was observed with molybdenum-containing materials. Analysis of the catalytic activity revealed that the catalysts were truly heterogeneous.

Composites of PPV in PVA, PVK and other polymeric matrixes were prepared by an in-situ polymerization of 1,4 phenylene dimethylene-bis-(tetramethylene sulfonium chloride) or □□□-dibromo-p-xylene. The monomer was converted directly to PPV in the polymeric matrix. This polymerization technique was successfully carried out by the use of a base, at room temperature, without requiring removal of oligomers and was completed in several minutes. EL and PL of the composites were measured and compared with PPV. A blue shift was found in the PL spectrum of the PPV-PVA composite compared to PPV. Other composites showed similar PL to that of PPV.

A sterol biosynthetic intermediate from sea cucumbers is parkeol (1). By mutation of residues at two positions, the enzyme lanosterol synthase was changed enough to make parkeol as its major product, along with lanosterol (2) and lanost-24-ene-3β,9α-diol (3) as by-products.

The catalytic electrophilic activation of hydrogen peroxide with transition metal compounds toward reaction with nucleophiles is a matter of very significant research and practical interest. We have now found that use of perfluorinated alcoholic solvents such as 1,1,1,3,3,3-hexafluoro-2-propanol in the absence of catalysts allowed electrophilic activation of hydrogen peroxide toward epoxidation of alkenes and the Baeyer−Villiger oxidation of ketones.

For the first time, mixed-addenda vanadium-substituted polyfluorooxometalates, PFOMs, have been synthesized. Depending on the workup procedure used, two types of compounds were prepared. The first PFOM was a quasi Wells-Dawson type compound, [H₂F₆NaV(V)W₁₇O₅₆]^8-, and the second a mixture of vanadium-substituted polyfluorooxometalates of the Keggin structure, XV(IV)W₁₁F(n)O(40-n) (X = H₂, V, W; n = 1-4). From the X-ray diffraction analysis, [H₂F₆NaV(V)W₁₇O₅₆]^8- has an elliptic (egg) shape with a central sodium atom surrounded by six fluorine atoms in a trigonal prism coordination. One may differentiate between two types of addenda atoms to be found in belt and capped positions. According to ¹H, ¹⁹F, and ⁵¹V NMR analysis, it is concluded that vanadium is isomorphically substituted in both the belt and capped position of [H₂F₆NaV(V)W₁₇O₅₆]^8-. The mixture of vanadium-substituted PFOMs of the Keggin structure was shown, by HPLC and ESR, to contain at least two species of different charge and of a different vanadium environment. The [H₂F₆NaV(V)W₁₇O₅₆]^8- PFOM was active for the catalytic aerobic oxidation of alkyl aromatic compounds in biphasic (water-catalyst and substrate) media. The reaction selectivity (autoxidation versus oxydehydrogenation) depended on the substrate and reaction conditions such as temperature and oxygen pressure. The selectivity to oxydehydrogenation was significantly higher compared to the prototypical cobalt acetate catalytic system.

The purpose of this study was to characterize the intraocular pressure (IOP) lowering activity and possible mechanism of action of the synthetic, non-psychotropic cannabinoid dexanabinol (HU-211) [(+)(3S,4S), 7-hydroxy-Delta-6-tetrahydrocannabinol 1, 1 dimethylheptyl], following intravenous (i.v.) administration in the rabbit. IOP (pneumatonometry), aqueous humor inflow rate (fluorophotometry), blood pressure, and heart rate (computerized physiograph system connected to central ear artery cannula) were measured in unanesthetized albino rabbits. Intravenous administration of HU-211 resulted in a dose-related reduction in IOP; a maximal IOP reduction of 5.0 +/- 0.2 mmHg was observed 4 hr after a 0.5 mg/kg dose. No significant changes in blood pressure or heart rate were observed during the first hr following this dose of HU-211. Pupil diameter did not change significantly during the 5 hr following the 0.5 mg/kg i.v. dose. No significant change in the rate of aqueous humor inflow occurred during the 6 hr after a 0.5 mg/kg dose of HU-211, thereby implicating outflow changes as the major source of IOP reduction. IOP reduction by HU-211 following pre-treatment with the alpha(2) adrenergic antagonist, yohimbine (1 mg/kg, i.v.), was only 30% of that of HU-211 alone. IOP reduction following pretreatment with the alpha(2) agonist, clonidine (0.5 mg/kg i.v.), was twice as large as that of HU-211 alone. Pretreatment with the beta-adrenergic antagonist, propranolol (0.5 mg/kg i.v.), resulted in a 50% reduction in the IOP-lowering effect of HU-211. In summary, HU-211, administered i.v., is an effective IOP-lowering agent, devoid of any significant side effects (blood pressure, heart rate or pupil diameter, all of which have been reported previously for cannabinoids). Involvement of the adrenergic system is indicated in mediating the IOP-lowering effects of HU-211 that appear to reflect a change in fluid outflow from the eye.

Excellent deoxygenation of ketones and aldehydes is achieved with Keggin-type polyoxometalates in the presence of hydrogen (see Equation (1) for an example). The mixed addenda phosphovanadomolybdate [PV₂Mo₁₀O₄]_5- was found to be the best catalyst. X-ray diffraction and IR studies suggest that the polyoxometalates are structurally stable under the strongly reducing conditions.

Poly(phenylenevinylene) (PPV) grafted with triethylenetetramine (TETA), PPV:TETA, unlike PPV, can interact with anionic sulfonated polymer dyes and can also coordinate metal ions, eg Eu, by chelation with the grafted TETA moiety. We have used these interactions and chelation to construct two types of self-assembled films using the layer-by-layer self-assembly technique. Structural characterization by means of specular X-ray reflectivity of heterostructures containing PPV:TETA:Eu showed that in self-assembled films containing metal ions there is a clear Bragg reflection (up to the third harmonic) indicating the formation of well modulated heterostructures. Photoluminescence (PL) measurements of heterostructures containing PPV:TETA and anionic polymer dyes showed evidence for energy transfer between the PPV:TETA and the polymer dyes.

New transition metal substituted silicate xerogels, ML(n) -SiO₂, have been prepared by the sol-gel technique and have been used as heterogeneous catalysts for the side-chain oxidation of alkyl arches with anhydrous t- butylhydroperoxide. The cobalt substituted silicates were the most active and selective, for example, ethylbenzene was cleanly oxidized to acetophenone at 65% conversion and > 99% selectivity. The metal salt precursor used was a key factor in determining the activity of the metallosilicate. Diffuse reflectance UV-vis and IR measurements indicated that when using Co(OSV)₂ as metal precursor, the acetate group was detached from the metal and Co(II) was tetrahedral and site substituted in the silicate network. Addition of the oxidant led to stabilization of a Co(III) oxidation state and/or retention of the tetrahedral configuration.

The biomimetic, methane monooxygenase enzyme (MMO) precatalyst, [Fe ₂O(ν1-H₂O)(ν¹-OAc)(TPA)₂] ³⁺ (TPA) tris[(2-pyridyl)methyl]amine), 1, formed in situ at pH 4.2 from [Fe₂O(μ-OAc)(TPA)₂]³⁺, 2, was embedded in an amorphous silicate surface modified by a combination of hydrophilic poly(ethylene oxide) and hydrophobic poly(propylene oxide). The resulting catalytic assembly was found to be a biomimetic model for the MMO active site within a hydrophobic macroenvironment, allowing alkane functionalization with tert-butyl hydroperoxide (TBHP)/O₂ in an aqueous reaction medium (pH 4.2). For example, cyclohexane was oxidized to a mixture of cyclohexanone, cyclohexanol, and cyclohexyl-tert-butyl peroxide, in a ratio of ∼3:1:2. The balance between poly- (ethylene oxide) and poly(propylene oxide), tethered on the silica surface, was crucial for maximizing the catalytic activity. The silica-based catalytic assembly showed reactivity somewhat higher in comparison to an aqueous micelle system utilizing the surfactant, cetyltrimethylammonium hydrogen sulfate at its critical micelle concentration, in which functionalization of cyclohexane with TBHP/O2 in the presence of 1 was also studied at pH 4.2 and was found to provide similar products: cyclohexanol, cyclohexanone, and cyclohexyl-tert-butyl peroxide, in a ratio of ∼2:3:1. Moreover, the mechanism for both the silica-based catalytic assembly and the aqueous micelle system was found to occur via the Haber-Weiss process, in which redox chemistry between 1 and TBHP provides both the t-BuO and t-BuOO radicals. The t-BuO radical initiates the C-H functionalization reaction to form the carbon radical, followed by O₂ trapping, to provide cyclohexyl hydroperoxide, which produces the cyclohexanol and cyclohexanone in the presence of 1, whereas the coupling product emanates from t-BuOO and cyclohexyl radicals. A discussion concerning both approaches for alkane functionalization in water will be presented.

A review of recent research in Jerusalem concerning electroluminescence phenomena from conjugated polymers with emphasis on poly(arylenevinylene) polymers and copolymers as the active emitting polymers is presented. Electroluminescence (EL) is described in both simple spin-cast thin films and multilayers produced by a layer-by-layer deposition technique.

The gas-phase oxidative dehydrogenation of 4-vinylcyclohexene (VCH) to styrene in high selectivities was successfully carried out at moderate temperatures, 200-260°C, using a vanadium substituted polyoxometalate, PV₂Mo₁₀O^5-₄₀, supported on carbon as catalyst. The major co-product was ethylbenzene and only a small amount of over-oxidation to CO_x was observed. Maximum conversions and selectivity were obtained at a O₂/VCH ratio of ∼1.9. The identity of the counter cation also affected the results with activity and selectivity decreasing in the following order H₅∼(NH₄)₄K>Cs₃H ₂≫(NH₄)₅. Ethylbenzene and styrene are not formed by the same reaction pathway. For ethylbenzene formation, oxydehydrogenation is preceded by isomerization of the exocylic double bond to an endocyclic position, whereas for styrene formation there is no such isomerization. A mechanism is proposed whereby the active catalyst is a polyoxometalate - carbon support complex, which yields in the presence of oxygen quinone/hydroquinone or aroxy/phenol redox couples responsible for the oxydehydrogenation.

We report on holographic storage with relatively high diffraction efficiencies in films ~2 mm to 20 mm thick! of conjugated polymer/glass and conjugated polymer/polymer composites with emphasis on the former. Anindex of refraction change Dn as high as 8.631023 was obtained. The grating formation was attributed primarily to the photochromic effect. However, the anomalous large two-beam coupling that was observed inthese materials cannot be explained by a photochromic mechanism. This, together with the small nonlinear electro-optic coefficient r eff50.42 pm/V may suggest that a weak photorefractive process also contributes tothe hologram formation in these materials. Holograms written on these conjugated polymer composite films are stable in the dark for ~at least! one year. @S0163-1829~98!50420-1#

We have studied the current versus voltage (I-V) and the luminescence versus voltage (EL-V) characteristics in PPV-based LED devices for different film thickness, d. For voltages below the EL threshold voltage, the I-V curves exhibit a linear regime (I∼V) at low voltages and quadratic (I∼V²) regime for higher voltages. The later is attributed to the space charge limited current mechanism assuming a single shallow trap level. The fit of the model to the experimental results is very good with a single fitting parameter, namely, the effective mobility. The extracted mobilities are compared to those measured independently by time-of-flight (TOF) photoconductivity. The SCLC model gives a reasonable explanation for the voltage threshold in EL.

The oxidation of organic substrates catalyzed by 'sandwich' type transition metal substituted polyoxometalates of the general formula, Na(x)M₂Zn₃W₁₉O₆₈, (M = Ru, Mn, Zn, Pd, Pt, Co, Fe, Rh) was examined in three different reaction media. The manganese analog was dissolved in a 1,2-dichloroethane phase using a lipophilic quaternary ammonium counter cation. Various organic substrates were oxidized with 30% aqueous H₂O₂. Alkenes reactivity increased as a function of the nucleophilicity of the double bond, but decreased as a function of steric crowding in the cyclohexene series. Alkenols with primary hydroxyl groups reacted chemo- and stereoselectively to form the corresponding epoxy alcohols. On the other hand, alkenols with secondary hydroxyl units did not react chemoselectively; both ketones and epoxy alcohols were formed. Diols were oxidized in most cases to ketols, except for 1,4-butanediol which yielded γ-butyrolactone. Secondary amines yielded hydroxyl amines except for piperidine which reacted with the solvent. A manganese containing catalyst supported on a functionalized silica particle was as active and selective as the organic solvent containing biphasic system for the oxidation of alkenes and alkenols. Reactions were also carried out by dissolving Na(x)M₂Zn₃W₁₉O₆₈ in aqueous solutions of 30% H₂O₂, 70% t-butylhydroperoxide or 0.02 M potassium persulfate in the absence of solvent. Hydrogen peroxide degraded all the TMSP compounds. One degradation product was an effective and chemo- and stereoselective catalyst for the epoxidation of primary alkenols. In alcohol oxidation only the ruthenium precursor was active. For oxidations with 70% t-butylhydroperoxide all compounds were stable but only the Na₁₁Ru₂Zn₃W₁₉O₆₈ compound was active. Alcohols were oxidized selectively, however, alkenols yielded a mixture of products. With persulfate, some catalytic effects were observed in double bond oxidation.

We describe a method to microfabricate a light emitting diode (LED) pixel array based on conjugated electroluminescent polymers sandwiched between ITO and aluminum. The method, based on direct photoablation using a 193 nm excimer laser, maintains intact the properties of the polymers. The technique as described here has already achieved array of 20 μm × 20 μm pixels with enhanced electroluminescence (EL) from pixels. The method can be extended to achieve nanometer sizes using near-field nanolithography. The microfabrication of the LED array requires also the patterning of the ITO and the aluminum electrode. For better performance of the device it is important to map the conductivity of the patterned electrodes. For that purpose we have used a novel mm-wave conductivity microscope which is capable to measure the local conductivity of the patterned film with a spatial resolution of ∼10-30 μm.

The fabrication and characterization by means of photoluminescence (PL), UV-vis absorption, electro-luminescence (EL) and X-ray reflectivity of multilayer heterostructures consisting of alternate layers of conjugated and non-conjugated polymers are studied. The heterostructures are prepared by the layer-by-layer self-assembly technique, using two-types of polyelectrolytes. The thickness of bilayer or multilayer was controlled by changing the non-conjugated polymer layer. Fabrication of light emitting diodes (LEDs) which emit in the blue region is accomplished by using the assembly technique.

This chapter contains sections titled:IntroductionThe Polyoxometalate Reduction-Substrate Oxidation Reaction Mode: Oxygen as Primary OxidantThe Polyoxometalate Oxidation-Substrate Oxidation Reaction Mode: Peroxygen Compounds as Primary OxidantsTransition Metal Substituted PolyoxometalatesOther Uses of Polyoxometalates in CatalysisA Final Word and a View to the FutureCiting Literature

A new immobilization technique for homogeneous catalysts is represented by the title process. Polyethers that are covalently attached to silica surfaces (shown schematically on the right) act as solvent and/or ligands for the catalysts, in this case polyoxometalates (POM). Such systems allow, for example, the quantitative oxidation of cyclooctene to cyclooctene oxide. The catalyst can be recycled without loss of activity.

We describe a method to microfabricate a light emitting diode array with pixels based on conjugated electroluminescent polymers sandwiched between appropriate electrodes. This method, based on direct photoablation with the 193 nm emission of an excimer laser, maintains the properties of these unique polymers. The technique as described here has already achieved an array of 20 μm×20 μm pixels with enhanced electroluminescence (EL) from these pixels and possible spectral tuning of the EL by the application of varying external field. This method can be extended to achieve nanometer dimensionalities using near-field nanolithography.

We present photoluminescence (PL), UV absorption, electroluminescence and x-ray reflectivity studies of self-assembled multilayer films containing alternate layers of conjugated copolymers, and nonconjugated insulating polymers. We show that the PL emission properties of these organic quantum wells can be "tuned" by a proper choice of the conjugated copolymer and the thickness of the insulating layers. Particularly, some of the self-assembled ultrathin films containing thin (∼7 Å) insulating polymeric layers exhibit a blue shift upon decreasing the thickness of the assembly. The PL shift is roughly proportional to 1ld where d is the thickness of the assembly, as expected for confined photogenerated electron-hole pair in an infinite square potential well. In contrast, the PL emission of similar assemblies but containing thick (∼40 Å) insulating layers is independent of the assembly thickness and exhibit emission in the blue. This may suggest a strong spatial confinement. Light emitting diodes based on self-assembled multilayer films with improved efficiency and stability and with threshold voltage as low as 2.6 V could be fabricated.

Composites of a sol-gel glass and a conjugated polymer have the potential to combine their unique properties: the high stability and ease of preparation of the sol-gel host and the optical fluorescence of the polymers. Two methods of preparation of such new composites are described, one of which successfully leads to composites with photoluminescence similar to that of homogeneous films of conjugated polymers. This result opens up new vistas in the fabrication of devices based on thin films of conjugated polymers using composite materials.

The polyoxometalates substituted with noble metals, Pd(II), Pt(II) and Ru(III), K₁₂{[WZnPd^II₂(H₂O)₂](ZnW₉O₃₄)₂}· 38H₂O, K₁₂{[WZnPt^II₂(H₂O)₂](ZnW₉O₃₄)₂}·36H₂O, and Na₁₁{[WZnRu^III₂(OH)(H₂O)](ZnW₉O₃₄)₂}·42H₂O, were prepared by exchange of labile zinc atoms with noble metal atoms from the isostructural starting material, Na_12-{[WZn₃(H₂O)₂](ZnW₉O₃₄)₂]}·46H₂O. The X-ray crystal structure of the ruthenium compound shows a structure compatible with a sandwich-type structure type with a WRuZnRu (Ru and W, Zn at opposing sides) ring between two B-XW₉O₃₄ units. Magnetic susceptibility studies as a function of temperature provide convincing evidence of two ruthenium (III) centers with no magnetic interaction between them. The EPR spectrum is supportive of this formulation showing an anisotropic spectrum of a ruthenium (III) atom (S = ¹/₂) in an octahedral field. The IR and UV-vis spectra of the ruthenium compound as well as of the diamagnetic palladium and platinum compounds are consistent with an isostructural series of compounds. The water soluble polyoxometalates may be extracted into an organic phase e.g. 1,2-dichloroethane by the addition of methyltricaprylammonium chloride to form their quaternary ammonium salts. The catalytic activity of these compounds was tested for the oxidation of alkenes and alkanes using aqueous 30% hydrogen peroxide and 70% tert-butyl hydroperoxide as oxidants. The alkene oxidation proceeded in high reactivity and moderate selectivity to the epoxide product using 30% H₂O₂. Kinetic profiles as well as UV-vis and IR spectra before, during and after the reaction indicate that the catalysts are stable throughout the reaction. Formation of epoxides rather than ketonization in the reaction of terminal alkenes as well as low reactivity with iodosobenzene indicates that the reaction is tungsten centered and not noble metal centered. Oxidation of alkenes with tert-butyl hydroperoxide gave mostly allylic oxidation and/ or addition of tert-butyl alcohol to the double bond. Oxidation of cyclic alkanes such as cyclohexane and adamantane was successful with tert-butyl hydroperoxide with catalytic activity 10 times higher than previously found for transition metal substituted Keggin compounds. Ratios of hydroxylation of adamantane at tertiary vs secondary positions indicates different active species in the palladium-, platinum-, and ruthenium substituted-polyoxometalates.

Color tuning of the electro and photoluminescence from naphthalenevinylenePPVbased lightemitting diodes between yellowgreen and red is reported. Band structure considerations are employed to explain that chemical selectivity and selective bandgap tuning are observed in some of the copolymers studied but not in others. It is also found that light emission can be tuned by changing the thickness of the copolymer film in the LED.

The synthesis of (2-bromoethyl)benzene by the anti-Markovnikov addition of gaseous hydrogen bromide to styrene has been found to be promoted by αpha;-bromo carbonyl compounds such as 2-bromo-2-methylpropanal. These compounds were found to catalyze the \u201cabnormal\u201d addition in a variety of solvents such as ethyl acetate, heptane, toluene and dioxane. High concentrations of 2-bromo-2-methylpropanal and hydrogen bromide and low concentrations of styrene favor formation of (2-bromoethyl)benzene. Using the free-radical catalyzed cyclization of 6-bromo-l-hexene as a probe we have found that the 2-bromo-2-methylpropanal does not in itself initiate a free-radical chain reaction by thermal formation of radicals. Instead, radicals may react with 2-bromo-2-methylpropanal to form relatively stable 2-methylpropanal radicals. The presence of such radicals increases the effective length or inhibits termination of the free-radical chain reaction (propagation) and in the case of hydrobromination of styrene raises the yield of (2-bromoethyl)benzene.

In a one-pot reaction, transition metal substituted polyoxomolybdates are shown to be bifunctional catalysts for epoxidation of alkenes by via oxygen transfer from intermediate alkylhydroperoxides; the latter being formed by catalytic autoxidation with molecular oxygen.

NA

Molecular compatibility of a guest porphyrin in a thermotropic nematic liquid crystalline host phase was shown to control the orientation of the porphyrin chromophore. In this manner, the normal alignment of a porphyrin plane parallel to the director in a nematic phase was inverted to a perpendicular orientation by attachment of mesogenic type appendages orthogonal to a porphyrin ring on one side of the ring plane. This molecular design also leads to a head-to-tail ordering of the porphyrin even in isotropic phases. The conclusions reached are based on detection and analysis of time-resolved EPR spectra of photoexcited triplet states.

The orientation of the porphyrin ring in nematic liquid-crystalline phases is shown to be controlled by modification of guest-host dynamics through molecular design. Thus, attachment of rod-like mesogenic residues or "arms" to α,α,α,α,-meso-tetrakis(o -aminophenyl)porphyrin yields the porphyrin, MesogenP, having molecular compatibility with standard liquid-crystalline compounds. Both the free-base and zinc porphyrin were photoexcited to the triplet state and detected by time-resolved EPR spectroscopy. The spectra show that the orientation, porphyrin ring parallel to the director, found for the simple tetraphenyl porphyrin, is inverted in the case of the MesogenP so that the porphyrin ring is perpendicular to the director in both frozen and fluid nematic phases.

The mixed addenda heteropoly acid H₆PMo₁₀V₂O₄₀ dissolved in 1,2-dichloroethane with tetraglyme, forming the (tetraglyme)₃-H₆PMo₁₀V₂O₄₀complex, catalyzes the aromatization of cyclic dienesat moderate temperatures in the presence of molecular oxygen. Dehydrogenations of exocyclic dienes such as limonene show that dehydrogenation is preceded by isomerization to their endocyclic isomers. Aromatization takes place by successive one-electron transfers and proton abstractions from the organic substrate to the heteropoly acid, the latter being reoxidized by dioxygen coupled with the formation of water.

A ruthenium heteropolyanion, SiRu(H₂O)W₁₁O ₃₉^5-, has been synthesized which catalyses the liquid phase oxidation of alkanes and alkenes with various primary oxidants including potassium persulphate, sodium periodate, t-butyl hydroperoxide, and iodosylbenzene; the activity and selectivity vary with the oxidant used.

Toluene and methylbenzene ring substituted with electron-withdrawing substituents were oxidized to the corresponding carboxylic acids. The oxidation system is biphasic consisting of an organic substrate phase and an aqueous sodium hypochlorite phase with both ruthenium and quaternary ammonium salts acting as catalysts. Yields are essentially quantitative after 2 h at room temperature with a stoichiometric ratio hypochlorite/toluene of 4.5:1 provided the pH of the aqueous phase is between 8.0 and 10.5. Kinetic studies show the reaction to be of first order in the substrate, zero order in sodium hypochlorite, and combined first order in the catalysts. The reaction mechanism consists of a Ru0₄-catalyzed hydride abstraction and a phase-transfer-catalyzed proton-dependent step.

The known Hofmann degradation of quaternary ammonium salts under basic phase-transfer catalytic conditions has been studied. The base-catalysed isomerization of p-allylanisole to p-methoxy-β-methylstyrene was used as a kinetic probe to find experimentally the rate constant and activation energy of the Hofmann decomposition without isolating the quaternary ammonium basic salt R₄N⁺B^-(B^-=base anion). Reactions performed at various temperatures showed that the higher the temperature the greater was the initial rate but the lower the final conversion in the isomerization reaction. The quaternary ammonium hydroxide was found to catalyse the isomerization and the Hofmann degradation more effectively than the corresponding alkoxide. This indicates that the former is a stronger base in the nonpolar aprotic solvents common in phase-transfer catalysis.

The use of quaternary ammonium salts (Quats) as phase-transfer catalysts is analyzed from a technological and economic point of view. The limiting price for Quats Is determined by establishing a qualitative relationship between technical parameters and economic factors. Specific examples show that the present price of Quats limits their commercial use to the area of high-priced commodities and specialty chemicals. Quats may be used in commodity manufacture provided they are produced on a larger scale. A simulation of such a large-scale production of a representative Quat (tetrabutylammonium bromide) has been made and its consequences are discussed.

Aromatic aldehydes were oxidized to carboxylic acids in high yields and selectivity under mild conditions. The reactions were performed using aqueous sodium hypochlorite as oxidant in a liquid-liquid phase transfer catalytic system using quaternary ammonium salts as catalysts. The reaction was strongly pH-dependent, with maximum reaction rates at pH 911. Similarly, extraction of the hypochlorite ion was maximal at these pHs. The maxima are attributed to coextraction of hypochlorous acid together with the hypochlorite anion into the organic phase, the former significantly increasing the reaction rate.

The complexation of alkali metal salts by polyethylene glycol is strongly dependent on the counter anion of the salt. In general, potassium salts are more easily complexed. In fact, sodium salts at low free energy of formation are barely complexed at all. By conductometric and index of refraction measurements it was shown that anions capable of hydrogen bonding such as OH⁻, F⁻, HSO⁻₄ and HCO⁻₃ were ion paired to the complexed potassium cation and existed as relatively \u201cnaked\u201d anions. Other anions such as Cl⁻, Br⁻, I⁻, SCN⁻, NO⁻₃, NO⁻₂ AND HF⁻₂ existed as disassociated anions with significant solvation shells. The use of polyethylene glycol as a phase transfer catalyst considering the results found is discussed.

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