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Colloidal Gold-Catalyzed Reduction of Ferrocyanate (III) by Borohydride Ions: A Model System for Redox Catalysis

We report results on the large catalytic effect of spherical gold nanoparticles on the rate of reduction of hexacyanoferrate (III) by sodium borohydride in aqueous solution. Because the gold nanoparticles remain stable and no aggregation takes place during the reaction, it can be monitored until completion. The presence of colloidal gold leads to a considerable increase in the observed reaction rate and to a change in the order of reaction. The reaction is first-order with respect to the hexacyanoferrate (III) concentration and gold particle concentration, but the reaction order with respect to borohydride ion is more complex. The activation energy is found to be 15 kJ/mol for 15 nm gold particles. The redox reaction is activation-controlled under most conditions, but the rate of reaction approaches the diffusion limit for higher borohydride concentrations and is over 10(4) times faster than in the absence of the gold catalyst.

Reversible assembly of metal nanoparticles induced by penicillamine. Dynamic formation of SERS hot spots

We report a systematic study of the surface modification of gold and silver nanoparticles with DL-penicillamine (PEN) and N-acetyl-DL-penicillamine (NAP), motivated by the possibility of inducing pH-controlled reversible nanoparticle assembly. The interaction of PEN and NAP with the metal nanoparticle surface was studied by isothermal titration calorimetry (ITC). The results indicate that equilibrium is reached with the formation of a submonolayer corresponding to ca. 40% and 64% of total surface coverage for PEN and NAP, respectively. Both PEN and NAP modified nanoparticles could be reversibly aggregated at acidic pH due to the protonation of the carboxylic groups, leading to a decrease in their stability by electrostatic interactions and the advent of hydrogen bonding interactions which promote interparticle linkage. The process was monitored by UV-Vis spectroscopy, transmission electron microscopy (TEM) and surface enhanced Raman scattering (SERS) spectroscopy. Interestingly, the SERS characterization demonstrated the pH-controlled formation of hot-spots.

Supported Pd Nanoparticles for Carbon–Carbon Coupling Reactions

We intent to present an overview of the available catalysts for the carbon–carbon cross-coupling reactions based on supported palladium (Pd) nanoparticles (NPs). We begin this perspective with a brief introduction about the cross-coupling reactions and the mechanistic implications of using Pd NPs as catalyst, i.e. heterogeneous versus homogeneous catalysis, then we summarize some of the most versatile Pd supported catalysts as a function of its nature. The supported catalysts have been classified in inorganic, organic and hybrid supports. Finally we outline the perspectives for the development of new Pd supported catalysts.

Controllable nitric oxide release in the presence of gold nanoparticles.

A major problem associated with nitric oxide (NO) donors is the release of the desired amount of NO at a specific site. A number of platforms have been developed for the regulation of NO dosage. We present the use of citrate-stabilized gold nanoparticles as a platform to regulate NO release. Because of the affinity between gold and thiols, the characteristic -S-NO bond of S-nitrosothiols (RSNOs) breaks in the presence of gold nanoparticles, thereby releasing NO and modifying the gold nanoparticle surface with the corresponding thiol. This system allows for surface-controlled NO release, where the amount of NO released is proportional to the number of thiols bound to the gold nanoparticle surface. Moreover, by employing an amperometric technique to detect the maximum NO release, we were able to estimate the stoichiometry of the reaction, that is, the number of adsorbed RSNO molecules per gold nanoparticle. A kinetic model for NO release and its subsequent decomposition is proposed and used to fit the experimental results. The reaction was found to be zeroth- and first-order with respect to RSNO and gold nanoparticles, respectively.

Evidence for complexes of different stoichiometries between organic solvents and cyclodextrins

The influence of the organic solvent on the acid and basic hydrolysis of N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) in the presence of alpha- and beta-cyclodextrins has been studied. The observed rate constant was found to decrease through the formation of an unreactive complex between MNTS and the cyclodextrins. In the presence of dioxane, acetonitrile or DMSO, the inhibitory effect of beta-CD decreased on increasing the proportion of organic cosolvent as a result of a competitive reaction involving the formation of an inclusion complex between beta-CD and the cosolvent. The disparate size of the organic solvent molecules resulted in stoichiometric differences between the complexes; the beta-CD-dioxane and beta-CD-DMSO complexes were 1 : 1 whereas the beta-CD-acetonitrile complex was 1 : 2. The basic and acid hydrolysis of MNTS in the presence of alpha-CD showed a different behavior; thus, the reaction gave both 1 : 1 and 2 : 1 alpha-CD-MNTS complexes, of which only the former was reactive. This result was due to the smaller cavity size of alpha-CD and the consequent decreased penetration of MNTS into the cavity in comparison to beta-CD. The acid hydrolysis of MNTS in the presence of alpha-CD also revealed decreased penetration of MNTS into the cyclodextrin cavity, as evidenced by the bound substrate undergoing acid hydrolysis. In addition, the acid hydrolysis of MNTS in the presence of acetonitrile containing alpha-CD gave 1 : 1 alpha-CD-acetonitrile inclusion complexes, which is consistent with a both a reduced cavity size and previously reported data.

Comparative study of nitroso group transfer in colloidal aggregates: micelles, vesicles and microemulsions

A kinetic study was carried out on nitroso group transfer from N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) to different secondary amines: morpholine (MOR), piperazine (PIP), dimethylamine (DMA) and piperidine (PIPER) in micelles of dodecyltrimethylammonium bromide (LTABr) and in vesicles of dioxadecyltrimethylammonium chloride (DODAC). Amine nucleophiles were chosen on the basis of their hydrophobicity and basicity. The observed rate constant, kobs, shows opposite behavior in micelles and vesicular systems. kobs for micelle systems decreases as the surfactant concentration increases. This behavior can be interpreted according to the distribution of the reagents among the different pseudophases of the system and the physicochemical properties of the latter. It has been observed that the product of the rate constant in the micellar pseudophase and the distribution constant of the amine, k2mKmR2NH, retains a sequence similar to the reactivity observed in water. The differences observed can be explained on the basis of the different hydrophobicity of the amines and consequently different values of KmR2NH. In all cases a catalytic effect on the addition of vesicles was observed reaching a limiting value of kobs. The kinetic behavior can be explained quantitatively on the basis of a single pseudophase model. k2vKvR2NH in the vesicular systems of DODAC displays a variation which is analogous with the micellar systems but approximately 35 times greater, which in turn indicates the greater hydrophobic character of the vesicles of DODAC by comparison with the micelles of LTABr. In AOT/isooctane/water microemulsions we have found similar behavior where the product is approximately 22 times lower than in vesicles, indicating that the polarity of the interface of the microemulsions is greater than that of the micelles of LTABr and smaller than that of DODAC vesicles. The comparative analysis of the reactivities in the interface of the microemulsion and in an aqueous medium shows that the reactive position in the interface changes as the hydrophobic character of the amine varies.

SERS study of the controllable release of nitric oxide from aromatic nitrosothiols on bimetallic, bifunctional nanoparticles supported on carbon nanotubes.

A hybrid system comprising bimetallic nanoparticles supported on carbon nanotubes (CNTs) was engineered to maximize the surface-enhanced Raman scattering signal from solution by generating a high density of hot spots with reproducible enhancing activity and long-term colloidal and optical stability. CNT@AgAu was employed as a bifunctional material to catalyze and monitor the controlled release of nitric oxide from aromatic nitrosothiols, as a function of the gold content.

Kinetic study of the nitrosation of 3-substituted indoles

Kinetic studies of the nitrosation reaction of three 3-substituted indoles (3-methylindole, indol-3-yl acetate and indole-3-acetic acid) show that the final state is an equilibrium between the reactants (nitrous acid and indole derivative) and the 1-nitroso derivative. Values of the rate constants for the nitrosation of the three indoles and denitrosation of the corresponding nitrosoindoles as well as values of the equilibrium constants have been obtained. The almost complete insensitivity of the reaction rates to medium acidity, the absence of catalysis by the usual catalysts of nitrosation (halides) and the high reactivity at low acidities, are in contrast to the kinetic characteristics of other N-nitrosation reactions. This atypical behaviour is discussed in terms of possible reaction mechanisms.

Solvent-induced mechanistic changes in nitrosation reactions. Part 2. Effect of acetonitrile–water mixtures in the nitrosation of ureas

The nitrosation of 1,3-dimethylurea in acetonitrile–water mixtures has been studied kinetically. The results obtained show that the addition of acetonitrile until the medium holds 70% acetonitrile by weight inhibits the reaction. The reaction is not catalysed by chloride ions in these circumstances, and the reaction mechanism is probably the same as in pure water. Addition of acetonitrile to a solution already containing more than 70% acetonitrile increases the reaction rate, and catalysis by halides becomes possible. The change in reaction mechanism this suggests was studied in detail in a medium containing 90% acetonitrile. The reaction rate increases non-linearly with increasing halide concentration and acidity, but seems to tend to the same limiting value in all cases, depending only on the nitrous acid and urea concentrations. Nitrosyl halides are therefore good nitrosating agents of ureas, though the catalytic efficiency of the different halides is the reverse of that in water, probably because of solvation-induced changes in their nucleophilicity. The tendency of the reaction rate towards a limiting value is evidence that the mechanism changes with the catalyst or acid concentration. In the limit, the reaction rate will only depend on nitrous acid and urea concentrations; this is consistent with a limiting step consisting of the rearrangement of the ‘nitrosourea’, the nitroso group transferring from the more nucleophilic O atom to the N atom. Thus, there is direct kinetic evidence that the nitrosation of amides occurs initially on oxygen.

Palladium complexes with 3-phenylpropylamine ligands: synthesis, structures, theoretical studies and application in the aerobic oxidation of alcohols as heterogeneous catalysts

The reaction of 3-phenylpropylamine with Pd(OAc)2 by heating in toluene resulted in the nearly square-planar complex trans-[Pd(C6H5(CH2)3NH2)2(OAc)2] (1). Complex 1 reacted with NaCl in methanol to obtain the corresponding product trans-[Pd(C6H5(CH2)3NH2)2Cl2] (2). Treatment of 2 with triphenylphosphine in dichloromethane afforded trans-[Pd(C6H5(CH2)3NH2)2(PPh3)2]2Cl− (3). All the palladium(II) complexes (1–3) were fully characterized by IR and NMR spectroscopy. In addition, the crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analysis. In these structures, the acetate and chloride ligands are in trans geometry. Density functional theory (DFT) calculations gave bond lengths and angles that were noted as experimental values. Palladium nanoparticles that were derived from complexes (1–3) were supported on cucurbit[6]uril (CB[6]) and identified by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), inductively coupled plasma analysis (ICP) and high-resolution X-ray powder spectroscopy (HR-XPS). CB[6]-supported palladium nanoparticles (NPs) were used as heterogeneous catalysts for the aerobic oxidation of alcohols to the corresponding aldehydes or ketones without over-oxidation. CB[6]-Pd NPs (3) (prepared from complex 3) show better catalytic activity than CB[6]-Pd NPs (1), (2), as a higher yield was observed with them in a relatively short time. Factors such as the amount of catalyst, solvent, temperature and reaction time were all systematically investigated to determine their effects on the yield of catalytic alcohol oxidation reactions. This catalytic system displayed high activity and selectivity toward alcohols in mild conditions. The catalyst was reused five times without any significant loss of catalytic activity.