Showing papers on "Disproportionation published in 1999"

Mono- and Dinuclear Silica-Supported Titanium(IV) Complexes and the Effect of TiOTi Connectivity on Reactivity

Abstract: The room-temperature reactions of excess Ti(OiPr)4 with the hydroxyl groups of a nonporous silica yield dinuclear surface complexes regardless of the degree of partial dehydroxylation of the silica. The surface reactions were studied by in situ IR transmission spectroscopy, 13C CP/MAS NMR, GC/MS, and elemental analysis. The spontaneous stoichiometric formation of both 2-propanol and propene during grafting indicates that interaction of Ti(OiPr)4 with the silica surface induces disproportionation of alkoxide ligands with concomitant formation of Ti−O−Ti bridges. A synthetic route to mononuclear silica-supported Ti alkoxide complexes was developed by reaction of grafted amide complexes (⋮SiO)nTi(NEt2)4-n (n = 1 or 2) with alcohols. Subsequent reactions of the mononuclear surface alkoxide complexes with Ti(OiPr)4 yield dinuclear species identical to those prepared by the direct reaction of Ti(OiPr)4 with silica. Both mono- and dinuclear supported alkoxide complexes undergo ligand-exchange reactions with tert...

Thermal Decomposition Processes in Aromatic Polycarbonates Investigated by Mass Spectrometry

Abstract: The thermal decomposition pathways leading to the formation of volatile compounds and to char residue in poly(bisphenol A carbonate) (PC), poly(resorcinol carbonate) (PRC), and poly(hydroquinone carbonate) (PHC) have been investigated by mass spectrometry. The structure of the volatile compounds obtained in the temperature range 300−700 °C, by direct pyrolysis mass spectrometry (DPMS), suggests that these polycarbonates undergo thermal decomposition by a number of different pyrolysis processes. In the initial stage of the thermal degradation are generated cyclic oligomers by an intramolecular exchange reaction, whereas the evolution of CO2 and H2O is spread over all the pyrolysis temperature range, being responsible for the formation of ether bridges (decarboxylation) and phenolic end groups (hydrolysis). A disproportionation reaction of the BPA isopropylidene bridges of PC itself takes place at higher temperature yielding phenyl and isopropylidene end groups, whereas pyrolysis products containing dibenzo...

General aspects of the chemistry of radicals

Abstract: EPR Spectroscopy: The Basics (G. Buettner). Measurements of Rate Constants for Radical Reactions in the Gas Phase (T. Wallington & O. Nielsen). Measurements of Rate Constants for Radical Reactions in the Liquid Phase (G. Buxton). Models for Abstraction and Addition Reactions of Free Radicals (E. Denisov). Combination and Disproportionation of Free Radicals (Z. Alfassi). Radical Ions: Generation, Characterization and Reactions (J. Gebicki & A. Marcinek). Free Radical Polymerization and Chain Reactions (O. Ito). Reactions of Free Radicals in Supercritical Fluids (J. Chateauneuf). Numerical Simulations of Free Radical Dynamics (E. Bittner). Thermochemistry of Organic Free Radicals (N. Cohen). Measurement of Reduction Potentials of Inorganic Radicals in Aqueous Solution (D. Stanbury). Measurement and Estimation of Redox Potentials of Organic Radicals (K. Daasbjerg, et al.). Correlations Between Rate Parameters and Molecular Properties (G. Marston, et al.). Empirical Correlational Solvent Effect on Free Radical Chemistry (Z. Alfassi). Organic Synthesis by Radical Reactions (C. Chatgilialoglu & P. Renaud). Index.

NO Disproportionation Reactivity of Fe Tropocoronand Complexes

Abstract: The synthesis and characterization of divalent [Fe(TC-5,5)] (1) and trivalent [Fe(OTf)(TC-5,5)] (2) tropocoronand complexes are described. Compound 1 reacts with 1 equiv of NO to form the {FeNO}7 complex 3. A single-crystal X-ray structure determination of 3 reveals a trigonal bipyramidal geometry with a linearly coordinated nitrosyl (Fe−N−O = 174.3(4)°) having a short Fe−N distance of 1.670(4) A. EPR and Mossbauer spectroscopy, SQUID susceptometry, and normal coordinate analysis indicate 3 to be a low-spin {FeIII(NO-)}2+ species. In the presence of excess NO, 3 converts to a metastable nitrosyl−nitrito complex that decomposes by losing NO2, which subsequently nitrates the aromatic tropolone rings of the ligand. The final products of the NO disproportionation reaction are N2O and [Fe(NO)(TC-5,5-NO2)] (4). The νNO stretching band of 4 is increased to 1716 cm-1 from its value of 1692 cm-1 in 3, owing to the electron-withdrawing nitro groups on the ligand, and the compound no longer promotes the disproportio...

Acidity of Beta zeolite determined by TPD of ammonia and ethylbenzene disproportionation

Abstract: The acidity of Beta zeolites with Si/Al ratios ranging from 18 to 33 was determined using appropriate temperature programmed desorption of ammonia conditions and their catalytic behavior was evaluated by disproportionation of ethylbenzene. Samples with greater Si/Al ratios presented higher acid strength by retaining ammonia at higher temperatures and showed a higher ethylbenzene disproportionation activity per aluminum atom. These results indicate that as the aluminum content in zeolite Beta is reduced in that range, the strength of the associated acid sites increases.

[Al7{N(SiMe3)2}6]−: A First Step towards Aluminum Metal Formation by Disproportionation

Abstract: A “naked” aluminum atom links two aluminum tetrahedra in the [Al7{N(SiMe3)2}6]− ion (see picture), which results from the reaction of a metastable AlCl solution with LiN(SiMe3)2 and crystallizes with [Li(OEt2)3]+ as cation. This unique structure among molecular metal atom clusters represents a small but characteristic section of cubic close-packed aluminum.

Kinetics and mechanisms of the reverse Boudouard reaction over metal carbonates in connection with the reactions of solid carbon with the metal carbonates

Abstract: The decomposition processes of alkali or alkaline earth carbonates with a large excess of carbon, and the reverse Boudouard reaction given by CO2/C→2CO over metal carbonates, were compared. The carbonates of Li+, Na+, K+, Cs+, Sr2+ and Ba2+ generated CO exclusively by an intermolecular redox reaction given by CO32-+C→2CO+O2-. The reverse Boudouard reaction over these metal carbonates at 700°C proceeded at a steady rate until just before the carbon was completely consumed, and in the cases of Li+, Sr2+ and Ba2+, the rates agreed with the initial rates of the intermolecular redox reaction. On the other hand, the rates over the carbonates of Na+, K+ and Cs+, the oxides of which undergo a disproportionation reaction to produce gas-phase metal and liquid-phase metal peroxide, were much higher than the initial rates of the intermolecular redox reaction. This discrepancy can be explained by the presence of a catalytic process on the metal-covered surface of the silica wool that was used for preventing the highly basic gas-phase metals from escaping.

Kinetics and Mechanism of Nitric Oxide Disproportionation upon Reaction with Ruthenium(II) Porphyrin Carbonyls: Evidence for Dinitrosyl Intermediates1

Abstract: The kinetics of the reaction of nitric oxide with the ruthenium porphyrin carbonyl complexes Ru(P)(CO) (P = octaethylporphyrin (OEP) or tetra-m-tolylporphine (TmTP)) to give the respective nitrosyl...

A Critical Evaluation of the Redox Properties of Uranium, Neptunium and Plutonium Ions in Acidic Aqueous Solutions

Abstract: Standard redox potentials, E8s and redox processes of U, Np and Pu ions in acidic aqueous solutions are reviewed and evaluated critically. The E8s of reversible redox processes, MO2 /MO2 ‡ and M/M (M: U, Np or Pu) adopted are those proposed mainly by Riglet et al. on the basis of the precise correction of formal potentials, E8 s, according to the improved theoretical approach to estimate the activity coef®cient. Electrode processes of the U, Np and Pu ions are discussed in terms of current±potential curves, measured so far by polarography, voltammetry or ow coulometry. Special attention is paid to the irreversible MO2 / M reactions. Disproportionation reactions of MO2 ‡ are also discussed. New substances are introduced as intermediates during reductions of MO2 ‡ to M or disproportionations of MO2 .

A new process for the synthesis of diphenyl carbonate from dimethyl carbonate and phenol over heterogeneous catalysts

Abstract: The two-step synthesis of diphenyl carbonate (DPC) from dimethyl carbonate (DMC) and phenol has been compared in liquid phase and gas phase, both over heterogeneous catalysts In the first step, equilibrium yields of methyl phenyl carbonate (MPC) in the transesterification of DMC and phenol were very low at low temperatures in the liquid phase although reaction rates were fast This endothermic reaction was more favorable at high temperatures in the gas-phase reaction Titanium oxide catalysts supported on SiO2 or activated carbon were found to be effective in a continuous gas flow reactor In case of the second step, the disproportionation of MPC, selective formation of DPC was not feasible in the gas-phase reaction due to extensive side reactions However, there was no by-product in the liquid-phase reaction over the TiO2/SiO2 catalyst Therefore, our proposed two-step synthesis process consists of the gas-phase transesterification of DMC and phenol followed by the liquid-phase disproportionation of MPC to DPC, both over the TiO2/SiO2 catalyst

Catalytic Isomerization and Disproportionation of Olefins Promoted by Group 4/d0 Benzamidinate Complexes

Abstract: The group 4/d0 benzamidinate complexes cis-[p-CH3-C6H4C(NSiMe3)2]2Zr(CH3)2 (1), C3-tris([N-trimethylsilyl][N‘-myrtanyl]benzamidinate)zirconium chloride (2) and cis-[p-CH3C6H4C(NSiMe3)2]2Ti(CH3)2 (3) were found to be active catalytic precursors, in the presence of MAO (methylalumoxane), for the isomerization of aliphatic olefins (1-octene, 2-E-octene, 3-methyl-1-butene, 2-methyl-1-butene, allylbenzene) and the disproportionation of 1,4- and 1,3-cyclohexadienes. The active species was found to be the cationic hydride complex obtained for aliphatic olefins by the insertion of the methyl ligand with concomitant β-hydrogen elimination, and for the alicyclic dienes by an activation of the allylic hydrogen and accompanying elimination of methane. Kinetic studies and Eyring analysis for the isomerization of allylbenzene show that the reaction is first order in both complex and olefin with ΔH† = 17.8(6) kcal·mol-1 and ΔS† = −25.1(2) eu. These findings support the epimerization mechanism proposed for these types of...

Noncatalytic Cannizzaro-type Reaction of Formaldehyde in Hot Water

Abstract: In hot water heated up to 250 °C and 4 MPa, methanol and formic acid are produced from the Cannizzaro-type reaction of formaldehyde without a catalyst, although this disproportionation reaction is well-known to occur in the presence of a large amount of base catalysts in ambient conditions. Formic acid further undergoes the hydride transfer reaction with formaldehyde, and the final yield of methanol exceeds 50%.

Theoretical Study of Bimolecular Reactions between Carbenium Ions and Paraffins: The Proposal of a Common Intermediate for Hydride Transfer, Disproportionation, Dehydrogenation, and Alkylation

Abstract: The mechanism of the bimolecular reactions of ethyl cation with ethane and propane and of s-propyl cation with ethane, propane, and isopentane is theoretically investigated by means of the B3PW91 density functional method. The study includes complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved, calculation of the reaction enthalpies and activation energies for the different elemental steps, and obtainment of the equilibrium constants and relative reaction rate constants by means of transition state theory. It is found that the interaction of a carbenium ion with a saturated alkane always results in formation of a stable carbonium ion intermediate and that different intramolecular rearrangements of this common intermediate can explain the mechanism of acid-catalyzed hydrocarbon reactions, such as hydride transfer, disproportionation, dehydrogenation, and alkylation.

Adsorption studies of gases on Pt-Rh bimetallic catalysts by reversed-flow gas chromatography

Abstract: In the present work, the relatively new technique of reversed-flow gas chromatography was applied for the study of adsorption of carbon monoxide, oxygen, and carbon dioxide on Pt-Rh bimetallic catalysts. Using suitable mathematical analysis, equations were derived by means of which rate constants for adsorption, desorption, and disproportionation reaction were calculated. From the variation of these rate constants with temperature and the nature of the catalyst (Pt content), as well as from the finding that the CO adsorption is a dissociative process, useful conclusions concerning the mechanism for the CO oxidation reaction over Pt-Rh bimetallic catalysts were extracted. The catalytic fractional conversions for the CO disproportionation reaction were found to be higher for the Pt-Rh bimetallic catalysts than those for the pure Pt catalyst, indicating the presence of beneficial Pt-Rh synergism.

Oxidative Addition of Aryl and Benzyl Trifluoroacetates to Zerovalent Palladium Complexes with Two Modes of C–O Bond Cleavage Processes

Abstract: The aryl trifluoroacetates oxidatively added to a zerovalent palladium complex 2a with acyl-O bond cleavage under mild conditions to give the corresponding trans-(aryloxo)(trifluoroacetyl)palladium complexes 3a—3c. But 4-nitrophenyl trifluoroacetate reacted with 2a to yield cis-[Pd(OC6H4-4-NO2)2(PMe3)2] 3d, which was produced with C–O bond activation followed by disproportionation reaction. In contrast, benzyl trifluoroacetates reacted with the Pd(0) complex with benzyl–O bond fission to form benzyl(trifluoroacetato)palladium complexes 4a—4d. Complexes 4a—4d are in equilibrium in solutions between trans and cis isomers, with the proportion of the cis isomer increasing in polar solvents. Palladium-catalyzed carbonylation of benzyl trifluoroacetate has been achieved in the presence of benzyl alcohol and triethylamine to yield benzyl phenylacetate.

Multielectron Oxidation of Anthracenes with a One-Electron Oxidant via Water-Accelerated Electron-Transfer Disproportionation of the Radical Cations as the Rate-Determining Step

Abstract: The six-electron oxidation of anthracene and the four-electron oxidation of 9-alkylanthracene occur with [Ru(bpy)3]3+ (bpy = 2,2-bipyridine) in acetonitrile (MeCN) containing H2O to yield anthraquinone and 10-alkyl-10-hydroxy-9(10H)anthracenone, respectively. The direct detection of radical cations of anthracene and its derivatives formed in the multielectron oxidation with [Ru(bpy)3]3+ and the extensive kinetic analysis are performed with the use of a stopped-flow technique. Both the rates of decay of anthracene radical cations and the formation of [Ru(bpy)3]2+ obey the second-order kinetics. The kinetic deuterium isotope effects and the dependence of the rates on the concentrations of [Ru(bpy)3]3+, anthracenes, and H2O have revealed that the six-electron oxidation of anthracene and the four-electron oxidation of alkylanthracene proceed via the rate-determining electron-transfer disproportionation of radical cations of anthracene and alkylanthracene, which is accelerated by H2O due to the complex formati...

Solvent and counterion effects in the asymmetric cyclopropanation catalysed by bis(oxazoline)–copper complexes

Abstract: The reaction of styrene with ethyl diazoacetate, promoted by the complexes of two bis(oxazolines) with Cu(I) and Cu(II) salts (chloride and triflate), was studied in CH 2 Cl 2 and nitroethane. The use of nitroethane reduces the selectivity with regard to the diazoacetate, and also slightly reduces the trans / cis selectivity. In the reactions carried out with triflates the use of nitroethane gives rise to a slight decrease in the enantioselectivity, whereas the reverse is true for copper chlorides. The influence of the solvent and the counterion on the enantioselectivity is explained on the basis of their influence in the disproportionation of Cu(I) to Cu(II) and Cu(0), the latter acting as a non-chiral catalyst. The rate of this side reaction also depends on the structure of the bis(oxazoline) ligand.

Incorporation of a binary alloy in an oxide matrix via single source precursor CVD process

Abstract: Using a Ni−Sn heterometal alkoxide, [Ni2Sn2(OtBu)8], in a chemical vapor deposition (CVD) process, thin films of biphasic composite, Ni3Sn4/SnO2, have been obtained. Due to the presence of two metal atoms in a single molecule, the decomposition occurs at a molecular level resulting in homogeneous incorporation of intermetallic Ni3Sn4 in an SnO2 matrix. The CVD experiments performed at different temperatures (450−550 °C) show that the obtained composite results from two chemical processes: (i) disproportionation of Sn(II) species and (ii) the redox reactions ocurring between Sn(II) and Ni(II) species. Fragmentation of the precursor and disproportionation of the tin(II) component dominate up to 500 °C, resulting in the formation of NiO, Sn(0), and SnO2. Redox reactions are favored at higher temperature (550 °C) which lead to the formation of the Ni3Sn4 alloy. This alloy−metal oxide composite has been deposited on different substrates (steel, copper, silicon wafer), and no heterogeneity was observed on a mi...

Formation of carbon nanotubes from the carbon monoxide disproportionation reaction over Co/Al2O3 and Co/SiO2 catalysts

Abstract: The ammonia method has been successfully used for preparing thermostable and well dispersed alumina‐supported catalysts with a surface average size of cobalt particle Ds= 5.7 nm. The disproportionation reaction of CO over this Co/Al2O3 catalyst and a similar Co/SiO2 catalyst leads to the formation of carbon nanotubes demonstrating the same morphology. The amount of nanotubes over Co/Al2O3, however, is much larger than that obtained over Co/SiO2, because of a faster ageing in the latter solid. Similar support effects have already been reported for other catalytic reactions involving carbon oxides.