Electron-deficient State Research Articles

Reductive routes to stabilized nanogold and relation to catalysis by supported gold

Evidence is presented and discussed for the involvement of ionic gold, probably Au(1+) in catalysis by supported gold. Various catalytic systems and various experimental techniques have been used for reactions involving carbon monoxide. Mössbauer effect spectroscopy on gold-titania and gold-titania-zirconia reveals the co-existence of Au(0), Au(I) and Au(III), but only the Au(I) content correlates directly with the activity of these solids for CO oxidation. XPS data suggest that a small (ca. 6%) fraction of the gold present in gold-titania-zirconia catalysts is present in an electron deficient state relative to Au(0). For gold–iron oxide catalysts, high water-gas shift activity is obtained for samples in which a partial reduction of the gold, initially present as Au(III), has occurred. For gold-HY zeolites, in which the gold is initially introduced as Au(III) by ion-exchange from [Au(en 2)] 3+, samples become catalytically active only after a considerable induction period has been exceeded. The induction period is shortened and the activity of the catalysts increased by pre-treatment with sodium borohydride, and some degree of reduction of the gold is assessed to have occurred. Further evidence that non-equivalent states of gold exist (i.e. metallic and ionic gold) in active CO oxidation catalysts based on gold-titania is provided by the fact that partial removal of gold occurs when the solids are treated with solutions containing cyanide ions under oxidative conditions. The specific activity per unit of gold increases with decreasing gold content when the gold content is lowered by such treatment. The results are consistent with the requirement for Au(I) involvement in the catalytic reaction, though a role for Au(0) is not ruled out.

High oxidation state alkylidene and alkylidyne complexes

Electron deficient high oxidation state early transition metal complexes that contain metal-carbon double or triple bonds and bulky supporting ligands have been found to be highly reactive catalysts for the alkene and alkyne metathesis reactions, respectively.

Development of TTF-based self-assembled monolayer systems and their electronic properties

Self-assembled monolayer (SAM) systems of alkanethiol and TTF-substituted alkanethiol molecules on Au substrate and metal nanoparticles are investigated. TTF-substituted alkanethiol molecules form charge transfer complex SAM with TCNQ molecules, where charge transfer (CT) rate becomes similar to that in bulk TTF.TCNQ crystal. TTF-substituted alkanethiol SAMs with long alkyl chains shows Coulomb-blockade-type electron transport owing to the resistance of the long alkyl chain bridge. In Pt nanoparticles with alkanethiol SAMs on their surface, CT takes place from core Pt nanoparticle to surface SAMs, producing an electron deficient state in Pt core nanoparticle. Pd nanoparticles with SAMs of mixtures of alkanethiol and TTF-substituted alkanethiol molecules take a large reduction of the Pauli paramagnetic susceptibility, which is brought about by CT.

Characteristics of Al-MCM-41 supported Pt catalysts: effect of Al distribution in Al-MCM-41 on its catalytic activity in naphthalene hydrogenation

Naphthalene hydrogenation was carried out in a high-pressure batch reactor over platinum catalysts supported on Al-MCM-41 where aluminum was incorporated through two different methods: a direct sol–gel method (Pre) and post-synthetic grafting method (Post). The catalytic reaction was also performed in the presence of dibenzothiophene to investigate the sulfur tolerance. The hydrogenation activity, selectivity and the sulfur tolerance strongly depended on the acidic nature of Al-MCM-41 support. It was suggested that the acid sites of Al-MCM-41-Post be more accessible than those of Al-MCM-Pre due to different aluminum distribution within the pore wall. The naphthalene and tetralin conversion increased with the acid amount of the supports in Pt/Al-MCM-41 catalysts. The acid sites in bifunctional catalysts seemed to contribute to alternative pathway by the spillover hydrogen in the acid–metal interfacial region for naphthalene hydrogenation, since the metal dispersions were kept constant for Pt/Al-MCM-41 catalysts. The trans-decalin selectivity generally increased with temperature or acid amount. The acid sites seemed to enhance the sulfur tolerance of supported platinum catalysts due to the electron-deficient state of metal.

Reduction of permanganate by thioanisole: lewis acid catalysis

The oxidation of sulfides and sulfoxides by permanganate in anhydrous acetone solutions is catalyzed by Lewis acids such as iron(III) chloride, zinc chloride, and mercury(II) chloride. The reaction kinetics unequivocally confirm that the function of these catalysts is to activate the oxidant by forming permanganate/Lewis acid complexes analogous to the protonation of Mn by Bronsted acids. A Hammett analysis of the rate constants for the oxidation of a series of substituted thioanisoles gives a negative rho value (-1. 11) indicative of an electron deficient transition state. No secondary kinetic isotope effect is observed when the hydrogens of the methyl group are replaced by deuterium. Despite previous observations that sulfoxides are preferentially oxidized in competitive experiments, sulfides are oxidized more rapidly when individual rates are measured. All of these observations are most consistent with a mechanism in which the reductant reacts with the oxidant via initial ligation.

Catalytic and electrocatalytic oxidation of aniline to nitrobenzene over vanadium silicate molecular sieves: VS-1 usingt-butyl hydroperoxide (TBHP) as oxidant

A comparison of the results of catalytic and electrocatalytic oxidation of aniline using VS-1 in the presence of H2O2 and TBHP indicates remarkable differences in conversion and selectivity. VS-1 catalyzes the oxidation of aniline selectively to nitrobenzene (73%) in the presence oft-butyl hydroperoxide (TBHP), while azoxybenzene (95.2%) is formed selectively when H2O2 is used. Cyclic voltammetric studies show a three-step oxidation of aniline to nitrobenzene in H2O2 but in the presence of TBHP only one step is observed. Electrocatalytic oxidation of aniline to nitrobenzene occurs at a potential 700 mV less than that corresponding to H2O2 as oxidant along with a selectivity of 91.8%. The enhancement of electrocatalytic rate is attributed to the stabilization of electron deficient transition state.

A mechanism for hydroxylation by tyrosine hydroxylase based on partitioning of substituted phenylalanines.

The iron-containing enzyme tyrosine hydroxylase catalyzes the hydroxylation of tyrosine to dihydroxyphenylalanine. A series of 4-X-substituted (X = H, F, Br, Cl, CH3, or CH3O) phenylalanines have been characterized as substrates to gain insight into the mechanism of hydroxylation. Multiple hydroxylated products were formed in most cases. As the size of the substituent at the 4-position increased, the site of hydroxylation switched from the 4- to the 3-position of the aromatic ring. The total amount of product formed with each amino acid showed a very good correlation with the sigma parameter of the substituent, with rho values of -4.3 +/- 0.7 or -5.6 +/- 0.8 when tetrahydrobiopterin or 6-methyltetrahydropterin, respectively, was used as cosubstrate. These values are consistent with a highly electron deficient transition state for hydroxylation. Oxygen addition at the 4-position resulted in either elimination of the substituent to form tyrosine or an NIH shift to form the respective 3-X-tyrosine. The relative amount of the product due to an NIH shift decreased in the order Br > CH3 > Cl >> F approximately CH3O approximately 0. A chemical mechanism for hydroxylation by tyrosine hydroxylase is presented to account for product formation from the various 4-substituted phenylalanines.

Osmium (VIII) catalyzed kinetics and mechanism of indoles oxidation with Aryl-N-haloamines in alkaline medium

The kinetics of oxidation of the title substrates by sodium N-haloarylsulfonamides (or ary-N-haloamines), chloramine-T (CAT), bromamine-T (BAT), chloramine-B (CAB), and bromamine-B (BAB), catalyzed by osmium(VIII) in alkaline medium has been studied at 30°C. The corresponding oxindoles and arylsulfonamides have been characterized as reaction products. The reaction rate shows a first-order dependence each on |indole|0 and |oxidant|0, a fractional-order on |Os(VIII)|, and an inverse first-order on |OH−|. Addition of arylsulfon-amide, chloride and bromide, and variation of ionic strength of the medium have no effect on the reaction rate. There is a negative effect of dielectric constant of the solvent. Activation parameters have been calculated from the Arrhenius and Eyring plots. Hammett correlation of substituent effects indicates an LFE relationship with ρ = −1.0, showing the formation of an electron deficient transition state. From enthalpy-entropy relationships and Exner correlations, the isokinetic temperatures (333 K and 326 K) have been determined for the reactions of CAT and BAT, respectively. Proton inventory studies in H2O-D2O mixtures have shown the involvement of a single exchangeable proton of OH− ion in the transition state. A mechanism consistent with the observed kinetics has been proposed. © 1996 John Wiley & Sons, Inc.

General and Efficient Insertions of Carbons Carrying Aryl and Heteroaryl Groups: Synthesis of alpha-Aryl- and alpha-Heteroaryl-Substituted Ketones.

Anions formed from the lithiation of a variety of 1-(arylmethyl)- and 1-(heteroarylmethyl)benzotriazoles 1 with n-BuLi underwent addition to aliphatic and aromatic aldehydes and cyclic and acyclic ketones. Subsequent in situ thermal rearrangements of the intermediates in the presence of zinc bromide provided one-carbon chain-extended or ring-expanded alpha-aryl- and alpha-heteroaryl-substituted ketones 2 in moderate to excellent yields in simple one-pot operations with excellent regioselectivity in most cases. Substituent effects on the relative migration rates were investigated in the insertion reactions of 1-(4-methoxybenzyl)benzotriazole (1e) with XC(6)H(4)COPh. The small and negative Hammett rho(+) value (-0.92) suggested that the rearrangements proceed via early, reagent-like, electron deficient transition states.

Kinetics of oxidation of some ortho, meta, and para substituted S‐phenylmercaptoacetic Acids by N‐Chloro‐3‐Methyl‐2,6‐Diphenyl piperidin‐4‐one in buffered ethanol‐water

AbstractThe kinetics of oxidation of a number of ortho, meta, and para substituted S‐phenylmercaptoacetic acids by N‐Chloro‐3‐Metyl‐2,6‐Diphenylpiperidin‐4‐one (NCP) has been studied in buffered ethanol‐water (75:25 v/v) of pH 5.46. The reaction is of first order each in [oxidant] and [substrate]. The rate constant decreases with increase in pH from 5.22 to 5.70. The reaction is accelerated by electron releasing and retarded by electron withdrawing substitutents. The ρ value obtained for this reaction is −1.88 at 10°C with a good correlation coefficient of 0.99 showing an electron deficient transition state. The effect of ortho substitutents also is discussed in detail.

Structure and function of real catalysts

This paper addresses issues such as the relation between the physical and chemical nature of active centers, activation of reactant molecules, mechanisms of catalytic reactions and distribution of promoters in real catalysts which are invariably multicomponent multiphase systems. Interactions among the catalyst components and phases often result in the formation of compounds and defects that do not exist in the separate components. Although such interactions give rise to a great variety of morphologic, chemical, and electronic properties of the real catalysts, they can be controlled, mostly by careful preparation and doping procedures, to the degree that the catalyst displays reproducible activity and selectivity. Most of the examples given in this paper are based on the results from the author's laboratory concerning the activity and selectivity of copper-based catalysts for the hydrogenation of carbon monoxide and carbon dioxide, the water gas shift reaction, and some reactions of amines. Evidence is presented for the interactions between the copper and zinc oxide then analyzed in terms of defect equilibria using models and constants partially known from the literature and adapted for the present system. It is shown on the basis of boundary layer theory for small particles that charge transfer between copper metal particles, also present in the system, and the zinc oxide phase is insignificant. On the other hand, electron equilibria between the solute copper species and the zinc oxide matrix are dictated primarily by intrinsic ionization Cu 0 → Cu + + e - and oxygen vacancy formation. Optical absorption spectroscopy strongly corroborates the theoretical contention that a significant portion of the copper is in electron deficient state, and there is some evidence based on Auger spectroscopy for the presence of Cu + species on the catalyst surface. It is reiterated, as has been proposed before in our earlier work, that these copper species activate substrates such as carbon monoxide or unsaturated hydrocarbons through back-bonding of the copper d-orbitals into the π ∗ orbitals of the substrates. In a paper by D.L. Roberts and G.L. Griffin at this Symposium, additional evidence is presented that the same finely dispersed Cu species are the chemisorption and activation sites for hydrogen. Some significant mechanistic features of carbon monoxide hydrogenation are demonstrated by the enhancement of methanol synthesis rates and carbon-carbon bond formation in the presence of alkali promoters. The nature and concentration of the alkali ions on the catalyst surface determine the outcome of the carbon monoxide hydrogenations in the following way: (i) of all the alkali and alkaline earth promoters, cesium displays the most pronounced effects; (ii) at high temperatures and low hydrogen-to-carbon monoxide ratios, maximum amount of n-propanol and 2-methyl-propanol is observed in the product over the Cs/Cu/ZnO catalysts, consistent with the function of the alkali as base catalysts in aldol condensation of aldehydic or enolic surface intermediates; (iii) at low temperatures and high hydrogen-to-carbon monoxide ratios, cesium enhances methanol synthesis as well as water gas shift rates in water- and CO 2-free synthesis gas, retards the methanol synthesis rate in synthesis gas containing intermediate amounts of water, primarily due to loss of surface area upon cesium doping, and again accelerates the synthesis in water-rich synthesis gas. These latter effects point to a mechanism in which the rate of formation of surface formate is enhanced by cesium in water-free synthesis gas and a rapid removal of surface hydroxyls free sites that activate hydrogen in water-rich synthesis gas. The role of Group VIII metals as promoters of the Cu/ZnO catalysts for low alcohol and hydrocarbon synthesis is represented by the effects of small additions of iron. Product composition is intermediate between that in methanol and Fischer-Tropsch syntheses, with significant amount of low alcohols formed. Characterization of the 1%Fe/Cu/ZnO catalyst by analytical electron microscopy reveals two forms of iron, a fine dispersion in the Cu/ZnO catalyst and metallic particles, suggesting that the alcohol and hydrocarbon products are formed over two different parts of the catalyst. In the concluding remarks it is contended that in many systems the catalytic activity and selectivity is primarily controlled by chemical entities, defects with respect to the pure components, that are induced and stabilized by solid state reactions involving a significant part of the bulk of the catalyst particles. As the various spectroscopic techniques as well as the tools of high resolution and analytical electron microscopy are used in an increasingly refined way for the analysis of real catalysts, it seems inevitable that the traditional art of catalyst preparation will take advantage of the new understanding of catalyst structure and will become a branch of materials science firmly footed in solid state chemistry and physics.

Structure and function of real catalysts

This paper addresses issues such as the relation between the physical and chemical nature of active centers, activation of reactant molecules, mechanisms of catalytic reactions and distribution of promoters in real catalysts which are invariably multicomponent multiphase systems. Interactions among the catalyst components and phases often result in the formation of compounds and defects that do not exist in the separate components. Although such interactions give rise to a great variety of morphologic, chemical, and electronic properties of the real catalysts, they can be controlled, mostly by careful preparation and doping procedures, to the degree that the catalyst displays reproducible activity and selectivity. Most of the examples given in this paper are based on the results from the author's laboratory concerning the activity and selectivity of copper-based catalysts for the hydrogenation of carbon monoxide and carbon dioxide, the water gas shift reaction, and some reactions of amines. Evidence is presented for the interactions between the copper and zinc oxide then analyzed in terms of defect equilibria using models and constants partially known from the literature and adapted for the present system. It is shown on the basis of boundary layer theory for small particles that charge transfer between copper metal particles, also present in the system, and the zinc oxide phase is insignificant. On the other hand, electron equilibria between the solute copper species and the zinc oxide matrix are dictated primarily by intrinsic ionization Cu0 → Cu+ + e- and oxygen vacancy formation. Optical absorption spectroscopy strongly corroborates the theoretical contention that a significant portion of the copper is in electron deficient state, and there is some evidence based on Auger spectroscopy for the presence of Cu+ species on the catalyst surface. It is reiterated, as has been proposed before in our earlier work, that these copper species activate substrates such as carbon monoxide or unsaturated hydrocarbons through back-bonding of the copper d-orbitals into the π∗ orbitals of the substrates. In a paper by D.L. Roberts and G.L. Griffin at this Symposium, additional evidence is presented that the same finely dispersed Cu species are the chemisorption and activation sites for hydrogen. Some significant mechanistic features of carbon monoxide hydrogenation are demonstrated by the enhancement of methanol synthesis rates and carbon-carbon bond formation in the presence of alkali promoters. The nature and concentration of the alkali ions on the catalyst surface determine the outcome of the carbon monoxide hydrogenations in the following way: (i) of all the alkali and alkaline earth promoters, cesium displays the most pronounced effects; (ii) at high temperatures and low hydrogen-to-carbon monoxide ratios, maximum amount of n-propanol and 2-methyl-propanol is observed in the product over the Cs/Cu/ZnO catalysts, consistent with the function of the alkali as base catalysts in aldol condensation of aldehydic or enolic surface intermediates; (iii) at low temperatures and high hydrogen-to-carbon monoxide ratios, cesium enhances methanol synthesis as well as water gas shift rates in water- and CO2-free synthesis gas, retards the methanol synthesis rate in synthesis gas containing intermediate amounts of water, primarily due to loss of surface area upon cesium doping, and again accelerates the synthesis in water-rich synthesis gas. These latter effects point to a mechanism in which the rate of formation of surface formate is enhanced by cesium in water-free synthesis gas and a rapid removal of surface hydroxyls free sites that activate hydrogen in water-rich synthesis gas. The role of Group VIII metals as promoters of the Cu/ZnO catalysts for low alcohol and hydrocarbon synthesis is represented by the effects of small additions of iron. Product composition is intermediate between that in methanol and Fischer-Tropsch syntheses, with significant amount of low alcohols formed. Characterization of the 1%Fe/Cu/ZnO catalyst by analytical electron microscopy reveals two forms of iron, a fine dispersion in the Cu/ZnO catalyst and metallic particles, suggesting that the alcohol and hydrocarbon products are formed over two different parts of the catalyst. In the concluding remarks it is contended that in many systems the catalytic activity and selectivity is primarily controlled by chemical entities, defects with respect to the pure components, that are induced and stabilized by solid state reactions involving a significant part of the bulk of the catalyst particles. As the various spectroscopic techniques as well as the tools of high resolution and analytical electron microscopy are used in an increasingly refined way for the analysis of real catalysts, it seems inevitable that the traditional art of catalyst preparation will take advantage of the new understanding of catalyst structure and will become a branch of materials science firmly footed in solid state chemistry and physics.

Oxidation of benzaldehyde semicarbazone by Tl(III) acetate—a structure reactivity correlation

The kinetics of Tl(III) acetate oxidation of semicarbazones of benzaldehyde and substituted benzaldehydes have been investigated in aqueous (90%) acetic acid. Electron-releasing substituents in the phenyl ring enhance the rate of the reaction, whereas electron-withdrawing substituents retard the rate. The rho value of −1 favours electron-deficient transition state. To account for the major product of the reaction,viz respective benzaldehyde, a suitable scheme involving N-thallated intermediate has been proposed

TEM and XPS studies of Ni/WS2 catalysts for thiophene hydrogenolysis

Differences in the structure of WS2 samples with SBET=7.6 and 75 m2/g are manifested in decreasing lengths of basal faces of some small crystallites in the more dispersed WS2. This effect increases the relative fraction of lateral faces on the catalyst surface. Ni ions in the Ni/WS2 catalysts are in an electrondeficient state compared to NiS.

Composition and catalytic activity of supported sulphided metal catalysts prepared via anchored complexes I. Tungsten catalysts prepared via anchoring of W(C4H7)4 ON SiO2

Anchored organometallic complexes of tungsten were prepared by supporting tetramethallyltungsten on SiO 2 . The composition of the surface compounds obtained by treatment of the anchored complexes with H 2 S was studied. The activities of sulphided catalysts containing different surface species of tungsten in the hydrogenolysis of thiophen were measured. An increase in turn-over number (per surface atom of tungsten) was observed for catalysts containing small sulphide aggregates. The electron-deficient state of W and S in these aggregates by comparison with WS 2 was shown by the x.p.s. technique.

Catalytic activity of nickel-containing zeolites reduced by sodium borohydride

1. By the use of NaBH4 it is possible to reduce individual nickel cations, in zeolites and on silica gel, at temperatures below room value. 2. The fact that the boron-containing metal resulting from such reduction is highly active in olefin hydrogenation is due, in part, to the high degree of disperson of the metal and, in part, to the participation of the reactive B-H bonds in the hydrogenation process. 3. The fact that the boron-containing metal shows low catalytic activity in the hydrogenation of benzene may be due to retention of an electron-deficient state by the metal particles in the reaction mixture.

An electron spectroscopy study of platinum/Y - zeolite catalysts

The electronic properties of small platinum metal particles in two platinum/Y - zeolites have been studied using A1 K α radiation principally through examination of the platinum 4f photoelectron lines. It has been established that there are more electron deficient platinum states in a 3 wt% Pt/La - Y zeolite than in a 3 wt% Pt/Na - Y zeolite. The origin of electron deficient states is not well established. Variation in the strength of electrostatic fields with different exchangeable cations in the two zeolites investigated may be the principal factor. Other factors related to particle size effects and to variability in site location cannot however be completely eliminated in the present study.

Valence state and interaction of platinum and germanium on γ-Al 2O 3 investigated by X-ray photoelectron spectroscopy

An X-ray photoelectron spectroscopic study of the reducibility of Pt γ- Al 2O 3 , Ge γ- Al 2O 3 , and Pt, Ge γ- Al 2O 3 systems has shown that: (i) treatment with H 2 at temperatures up to 650 °C leads to interaction between Pt and Ge, giving rise to the formation of PtGe alloys; (ii) in the absence of platinum, germanium is difficult to reduce and at 650 °C seems to dissolve in the alumina matrix whereas in the presence of platinum, it remains in a exposed state. The results strongly suggest that in both Pt γ- Al 2O 3 and Pt, Ge γ- Al 2O 3 “platinum atoms” are present in an electron-deficient state.

The 347.2 nm Laser Photolysis of Tetranitromethane in Polar, Nonpolar, and Micellar Solutions

AbstractTetranitromethane in various solvents (water, ethanol, t‐butanol, acetone, n‐hexane, neo‐pentane, carbon tetrachloride, mixtures of n‐hexane and ethanol, sodium lauryl sulfate and dodecyltrimethylammonium chloride micelles) was photolyzed by a pulsed, frequency‐doubled ruby laser (3.57 eV quantum of energy). The products of the photolysis were monitored in the 10−8 to 10−3 s range by recording the optical absorption and electrical conductivity of the solutions. In polar solvents, the anion of nitroform, C(NO2)3−, is formed during the 20 ns laser pulse. The yields of C(NO2)3− are highest in ethanol and lowest in aqueous solutions. In water, ionic dissociation of tetranitromethane is the main photolytic decomposition mode. In the alcohols and acetone a two step mechanism is proposed involving CN bond rupture to form the trinitromethyl radical which subsequently picks up an electron from the solvent to form C(NO2)3− and an electron deficient solvent state. The latter could optically be detected in the case of acetone. In hydrocarbon solutions, aci‐nitroform is produced via H‐atom abstraction by the trinitromethyl radical. In mixtures of n‐hexane and ethanol (0.1 to 5 vol.%), this radical is only able to abstract an electron from clusters of ethanol molecules and not from free, unassociated ethanol. When tetranitromethane is incorporated in the lipoidic core of ionic micelles, the extent of photolytic decomposition is substantially enhanced relative to that in water. In aqueous solutions containing 0.1 to 3 M sodium chloride, the trinitromethyl radical forms a complex with the chloride anion.Energy considerations involving the formation of electron deficient solvent states are used to explain the observed effects.

Substituenteneinfluss der phosphino- und der phosphinylgruppe auf die reaktivität aromatischer verbindungen—II : Störungstheoretische diskussion des substituenteneinflusses

To discuss the reactivities of some aromatic compounds substituted by the phosphino (Y 2P) or the phosphinyl group (Y 2PO) the MO's of the unsubstituted compounds were calculated by an SCF-π-MO-method. This was done for the ground state as well as for some transition-state like structure. Perturbation theory was used to analyze the effect of the substituents in question on both states. There are the following results. (1) The phosphino and the phosphinyl group stabilize electron rich transition states relative to neutral ground states by interactions of the non-bonding unoccupied d-orbitals on phosphorus with the occupied orbitals of the rest, the phosphinyl group being more effective by an extra stabilizing charge interaction. (2) The phosphinyl group destabilizes electron deficient transition states relative to neutral ground states by charge interactions, while the phosphino group slightly stabilizes those states by weak interactions of the hybrid orbital on phosphorus with unoccupied orbitals of the rest.