Showing papers on "Bromide published in 2010"

Gold-Catalyzed Three-Component Coupling: Oxidative Oxyarylation of Alkenes

Abstract: The three-component coupling of terminal alkenes with arylboronic acids and oxygen nucleophiles is described. The reaction employs a binuclear gold(I) bromide as a catalyst and Selectfluor reagent as the stoichiometric oxidant. Alcohols, carboxylic acids, and water can be employed as oxygen nucleophiles, thus providing an efficient entry into β-aryl ethers, esters, and alcohols from alkenes.

Shape-controlled synthesis of Mn3O4 nanocrystals and their catalysis of the degradation of methylene blue

Abstract: Various sizes and shapes of Mn3O4 nanocrystals have been prepared in a one-pot synthesis in extremely dilute solution by soft template self-assembly. To better control size and shape, the effects of varying the growth time, reaction temperature, surfactant, and manganese source were examined. The average size of octahedral Mn3O4 crystallites was found to be related to the reaction time, while higher reaction temperature (150 °C) and the use of a cetyltrimethylammonium bromide/poly(vinylpyrrolidone) (CTAB/PVP) mixture allowed construction of a better-defined octahedral morphologies. When PVP or poly(ethylene oxide)-poly(propylene oxide) (P123) was used as template, large-scale agglomeration resulting in loss of the octahedral morphology occurred and crystallites with a quasi-spherical shape were obtained. The nano-octahedral crystallites were shown to be an efficient catalyst for the oxidation of methylene blue.

Oxoboryl Complexes: Boron−Oxygen Triple Bonds Stabilized in the Coordination Sphere of Platinum

Abstract: Monomeric oxoboranes have hitherto been detected only as short-lived species in gas-phase or low-temperature matrix experiments. Here, we report formation of the oxoboryl complex trans-[(Cy3P)2BrPt(B≡O)] (Cy being cyclohexyl) by means of reversible liberation of trimethylsilylbromide from the boron–bromine oxidative addition product of dibromo(trimethylsiloxy)borane and [Pt(PCy3)2] in room-temperature toluene solution. The platinum complex is inert toward oligomerization, even under photolytic conditions and at elevated temperatures. The bromide was substituted by thiophenolate, and spectral parameters of both products as well as results of computational and x-ray diffraction studies are in agreement with the formulation of a triple bond between boron and oxygen. The boron–oxygen distance of 120.5(7) picometers shows a bond shortening of 7.2% as compared with a double bond, which is similar to the shortening observed in carbon–carbon analogs.

Semiclathrate Hydrate Phase Equilibria for CO2 in the Presence of Tetra-n-butyl Ammonium Halide (Bromide, Chloride, or Fluoride)

Abstract: Phase equilibrium data for the semiclathrate hydrates formed in three three-component systems, the CO2 + tetra-n-butyl ammonium bromide (TBAB) + water system, the CO2 + tetra-n-butyl ammonium chloride (TBAC) + water system, and the CO2 + tetra-n-butyl ammonium fluoride (TBAF) + water system, were measured in the pressure range of (0.40 to 3.77 MPa) and temperature range of (280.2 to 293.5 K) at (2.93·10−3 and 6.17·10−3) mole fraction of tetra-n-butyl ammonium halide. The experimental data were generated using an isochoric pressure-search method. The equilibrium data for the CO2 + TBAB + water system were compared with some experimental data from the literature. The effects of tetra-n-butyl ammonium halide concentration on the stability zone of the semiclathrate hydrates were studied. It was shown that TBAB, TBAC, and TBAF all can enlarge the hydrate stability zone, and as the tetra-n-butyl ammonium halide concentration increases, so does the hydrate stability zone. The three-phase equilibrium pressure of ...

Kinetics and Mechanisms of N-Nitrosodimethylamine Formation upon Ozonation of N,N-Dimethylsulfamide-Containing Waters: Bromide Catalysis

Abstract: N,N-Dimethylsulfamide (DMS), a newly identified, ubiquitous degradation product of the fungicide tolylfluanide, has been shown to be a N-nitrosodimethylamine (NDMA) precursor during ozonation. In this study, batch ozonation experiments in ultrapure buffered water, surface water, and tap water were performed to determine the kinetics and elucidate the mechanism of NDMA formation from DMS. It was found that at circumneutral pH, DMS reacts slowly with ozone (k ≈ 20 M−1 s−1) and moderately with hydroxyl radicals (k = 1.5 × 109 M−1s−1). The reaction of DMS with these oxidants does not lead to NDMA. NDMA was only formed if bromide was present during ozonation of DMS-containing waters. Bromide is oxidized to hypobromous acid (HOBr) by ozone which then reacts with the primary amine of DMS to form a Br−DMS species. The rate limiting step of the formation of Br−DMS is the formation of HOBr. The reaction to form Br−DMS has an apparent second order rate constant at pH 8 of >3 × 104 M−1s−1. The Br−DMS is transformed b...

Catalyst-free one-pot four component synthesis of polysubstituted imidazoles in neutral ionic liquid 1-butyl-3-methylimidazolium bromide.

Abstract: A catalyst-free one-pot four component methodology for the synthesis of 1,2,4,5-substituted imidazoles under conventional heating and microwave irradiation using 1-butyl-3-methylimidazolium bromide, [Bmim]Br, as a neutral reaction media is described. A broad range of structurally diverse aldehydes (aromatic aldehydes bearing electron withdrawing and/or electron releasing groups as well as heteroaromatic aldehydes) and primary amines (aromatic and aliphatic) were applied successfully, and corresponding products were obtained in good to excellent yields without any byproduct.

The role of bromide ions in seeding growth of Au nanorods.

Abstract: We report our findings on the important role of bromide ions in the seeding growth process of Au nanorods. The seed-mediated process constitutes a well-developed method for synthesizing gold nanorods in high yield, which is facilitated by a micelle-forming surfactant, cetyltrimethylammonium bromide (CTA-Br). Despite the tremendous work in recent years, the growth mechanism of Au nanorods has not been fully understood. Contrary to the widely accepted mechanism of CTA(+) micelle-templated growth of Au nanorods, we have identified the critical role of bromide ions in the seeding growth of Au nanorods. We found that even when the micelle-forming agent (CTA(+)) concentration is below its critical micelle concentration (cmc), bromide ions added in the form of NaBr can successfully effect the growth of Au nanorods in good yield. By controlling the concentration of externally added bromide ions, the rod shape and dimensions of the resulting Au nanoparticles can be readily controlled in the presence of only a minimum amount of CTABr (as a steric stabilizer for nanorods). High-resolution TEM studies show that the as-formed nanorods are perfectly single crystalline, instead of penta-twinned ones, and are bound by {111} and {100} facets with a [110] direction as the elongation direction. A mechanism is proposed to account for the seeding growth of single crystalline Au nanorods. Overall, this work explicitly demonstrates that Br(-) indeed serves as an important shape-directing agent for gold nanorod formation in the seed-mediated process.

Synthesis, Cytotoxicity and Antibacterial Studies of p-Methoxybenzyl-Substituted and Benzyl-Substituted N-Heterocyclic Carbene-Silver Complexes

Abstract: p-Methoxybenzyl-substituted and benzyl-substituted N-heterocyclic carbene (NHC) [(3a-c) and (6a-c)] precursors were synthesised from the reaction of 1H-imidazole (1a), 4,5-dichloro-1H-imidazole (1b), and 1H-benzimidazole (1c) with p-methoxybenzyl bromide (2) and benzyl bromide (5). These NHC precursors were then treated with silver(I) acetate to yield the NHC-silver complexes [1,3-bis(4-methoxybenzyl)-imidazol-2-ylidene]silver(I) acetate (4a), [4,5-dichloro-1,3-bis(4-methoxybenzyl)imidazol-2-ylidene]silver(I) acetate (4b), [1,3-bis(4-methoxybenzyl)benzimidazol-2-ylidene]-silver(I) acetate (4c), (1,3-dibenzylimidazol-2-ylidene)silver(I) acetate (7a), (1,3-dibenzyl-4,5-dichloroimidazol-2-ylidene)-silver(I) acetate (7b), and (1,3-dibenzylbenzimidazol-2-ylidene)silver(I) acetate (7c), respectively. The NHC precursor 3c, four NHC-silver complexes 4c and 7a-c were characterised by single-crystal X-ray diffraction method. The preliminary antibacterial activity of all the compounds was studied against Gram-negative bacteria Escherichia coli, and Gram-positive bacteria Staphylococcus aureus using the Kirby-Bauer disk-diffusion method. Almost all the NHC-silver complexes have shown high antibacterial activity compared to the NHC precursors. In addition, the NHC-silver complexes had their cytotoxicity investigated through MTT-based preliminary in vitro testing on the Caki-1 cell lines in order to determine their IC 50 values. NHC-silver complexes 4a-c and 7a-c were found to have IC 50 values of 7.3 (+/-6), 12.7 (+/-3), 25.2 (+/-5), 2.5 (+/-3), 10.8 (+/-4) and 12.5 (+/-4) μM respectively on the Caki-1 cell line.

A Versatile Palladium/Triphosphane System for Direct Arylation of Heteroarenes with Chloroarenes at Low Catalyst Loading

Abstract: Put a ring on it: The use of an air-stable, robust palladium/tridentate phosphane catalyst in direct C[BOND]H and C[BOND]Cl activation reactions is reported (see scheme; DMAc=N,N-dimethylacetamide, TBAB=tetra-n-butylammonium bromide). Electron-rich, electron-poor, and polysubstituted furans (X=O), thiophenes (X=S), pyrroles (X=NR5), and thiazoles were arylated with chloroarenes in the presence of the catalyst.

Calorimetric and Structural Studies of Tetrabutylammonium Bromide Ionic Clathrate Hydrates

Abstract: In the present work, characteristic properties of tetrabutylammonium bromide (TBAB) ionic clathrate hydrates structures were studied by single-crystal X-ray structure analysis. The structures of three different tetragonal TBAB ionic clathrate hydrates that were formed in our experiments were based on the same water lattice of tetragonal structure I (TS-I) differing in the ways of including bromide anions and arranging tetrabutylammonium cations. We demonstrated that (1) Br– can be included into the water lattice, replacing two water molecules, (2) the butyl group of the cation can be inserted not only in large T and P cavities but also in small D cavities of the water lattice TS-I, and (3) one of the reasons for polytypism of ionic clathrate hydrates on the basis of TS-I is the occurrence of alternative modes of arrangements of four-compartment cavities in adjacent layers of the water framework. The compositions of three TBAB ionic clathrate hydrates TBAB·38.1H2O, TBAB·32.5H2O, and TBAB·26.4H2O were deter...

Ion specificity at the peptide bond: molecular dynamics simulations of N-methylacetamide in aqueous salt solutions.

Abstract: Affinities of alkali cations and halide anions for the peptide group were quantified using molecular dynamics simulations of aqueous solutions of N-methylacetamide using both nonpolarizable and polarizable force fields. Potassium and, more strongly, sodium exhibit an affinity for the carbonyl oxygen of the amide group, while none of the halide anions shows any appreciable attraction for the amide hydrogen. Heavier halides, however, interact with the hydrophobic methyl groups of N-methylacetamide. Using the present results for a model of the peptide bond we predict that the destabilizing effect of weakly hydrated Hofmeister ions, such as bromide or iodide, is not due to direct interactions with the backbone but rather due to attraction to hydrophobic regions of the protein.

Ionic liquid-coated Fe3O4 magnetic nanoparticles as an adsorbent of mixed hemimicelles solid-phase extraction for preconcentration of polycyclic aromatic hydrocarbons in environmental samples

Abstract: In this paper, ionic liquid (IL)-coated magnetic Fe3O4 nanoparticles (NPs) as an adsorbent of mixed hemimicelles solid-phase extraction (SPE) was investigated for the preconcentration of polycyclic aromatic hydrocarbons (PAHs) from environmental samples. Due to the high surface area and excellent adsorption capacity of the Fe3O4 NPs after modification with ILs, satisfactory extraction recoveries can be achieved with only 80 mg Fe3O4 NPs, 50 mg IL, 300 mL solution at pH = 10 and 10 min for equilibration. A comprehensive study of the adsorption conditions such as the amount of Fe3O4 NPs and ILs, the solution pH, ionic strength, standing time, breakthrough volume, and desorption solvents was presented. The extraction ability of different coating agents, such as 1-hexadecyl-3-methylimidazolium bromide (C16mimBr), 1-decyl-3-methylimidazolium bromide (C10mimBr) and cationic surfactant cetyltrimethylammonium bromide (CTAB) was also compared. Under the optimized conditions, the recoveries for the water samples analysis were between 76 and 105% with relative standard deviations (RSDs) ranging from 3.9 to 6.9%, and the recoveries for soil samples were between 73 and 104% with RSDs ranging from 1.0 to 6.3%. In this method, only a small amount of C16mimBr (50 mg) and Fe3O4 NPs (80 mg) was needed to obtain satisfactory recoveries.

Synthesis of monodisperse [Oct4N(+)][Au25(SR)18(-)] nanoparticles, with some mechanistic observations.

Abstract: A single phase (THF) synthesis of monodisperse [Oct4N+][Au25(SR)18−] nanoparticles is described that yields insights into pathways by which it is formed from initially produced larger nanoparticles. Including the Oct4N+Br− salt in a reported single phase synthetic procedure enables production of reduced nanoparticles having a fully occupied HOMO molecular energy level (Au25(SR)18−, as opposed to a partially oxidized state, Au25(SR)180). The revised synthesis accommodates several (but not all) different thiolate ligands. The importance of acidity, bromide, and dioxygen on Au25 formation was also assessed. The presence of excess acid in the reaction mixture steers the reaction toward making Au25(SR)18; while bromide does not seem to affect Au25 formation, but it may play a role in maintaining the −1 oxidation state. Conducting the nanoparticle synthesis and “aging” period in the absence of dioxygen (under Ar) does not produce small nanoparticles, providing insights into the pathway of reaction product “agin...

Hydrogen Storage in Semiclathrate Hydrates of Tetrabutyl Ammonium Chloride and Tetrabutyl Phosphonium Bromide

Abstract: Original calorimetric and phase equilibrium properties for hydrogen + tetrabutylammonium bromide (TBAB), tetrabutylammonium chloride (TBACl), or tetrabutylphosphonium bromide (TBPB) semiclathrate hydrates were measured using differential scanning calorimetry under pressure. The dissociation temperatures of H2 + TBACl and H2 + TBPB semiclathrate hydrates are very close to the ambient temperature at low pressures around 15.0 MPa. H2 + TBACl and H2 + TBPB systems therefore exhibit better and comparable stability than the H2 + TBAB system at equivalent pressure, respectively. The amount of hydrogen stored in H2 + TBACl and H2 + TBPB semiclathrate hydrates was estimated in terms of the H2-to-water mole ratio (nH2/nH2O) calculated from disssociation enthalpies and (p, T) equilibrium data. In terms of mass fraction deduced from the ratio (nH2/nH2O), H2 + TBACl and H2 + TBPB semiclathrate hydrates can store 0.12 % and 0.14 % of hydrogen, respectively. Moreover, the quantity of hydrogen stored in these two semicla...

Cation Exchange on the Surface of Gold Nanorods with a Polymerizable Surfactant: Polymerization, Stability, and Toxicity Evaluation

Abstract: Gold nanorods were synthesized using a seed-mediated wet chemical approach with a quaternary ammonium surfactant, cetyltrimethylammonium bromide (CTAB), that forms a bilayer on the surface of the nanorods The CTAB molecules in the bilayer were exchanged with a similar polymerizable analog, 11-(acryloyloxy) undecyltrimethyl ammonium bromide (p-CTAB) Mass spectrometric analysis of the degree of exchange of CTAB for p-CTAB, after gold digestion, gave 77 ± 3 and 23 ± 1% for p-CTAB and CTAB, respectively On-rod polymerization with a cationic free-radical initiator was confirmed by FTIR analysis and did not induce aggregation as judged by ultraviolet−visible spectroscopy, transmission electron microscopy, and dynamic light scattering measurements after polymerization In contrast to the nanorods before polymerization, the nanorods with a polymerized bilayer showed improved stability against dialysis as well as enhanced biocompatibility as measured using a viability assay on cultured human cells Our results