Ruthenium Trichloride Catalyst in Water: Ru Colloids versus Ru Dimer Characterization Investigations.

An easy-to-prepare ruthenium catalyst obtained from ruthenium(III) trichloride in water demonstrates efficient performances in the oxidation of several cycloalkanes with high selectivity toward the ketone. In this work, several physicochemical techniques were used to demonstrate the real nature of the ruthenium salt still unknown in water and to define the active species for this Csp3-H bond functionalization. From transmission electron microscopy analyses corroborated by SAXS analyses, spherical nanoobjects were observed with an average diameter of 1.75 nm, thus being in favor of the formation of reduced species. However, further investigations, based on X-ray scattering and absorption analyses, showed no evidence of the presence of a metallic Ru-Ru bond, proof of zerovalent nanoparticles, but the existence of Ru-O and Ru-Cl bonds, and thus the formation of a water-soluble complex. The EXAFS (extended X-ray absorption fine structure) spectra revealed the presence of an oxygen-bridged diruthenium complex [Ru(OH) xCl3- x]2(µ-O) with a high oxidation state in agreement with catalytic results. This study constitutes a significant advance to determine the true nature of the RuCl3·3H2O salt in water and proves once again the invasive nature of the electron beam in microscopy experiments, routinely used in nanochemistry.

Odyssey in Polyphasic Catalysis by Metal Nanoparticles.

Nanometer-sized metal particles constitute an unavoidable family of catalysts, combining the advantages of molecular complexes in regards to their catalytic performances and the ones of heterogeneous systems in terms of easy recycling. As part of this research, our group aims at designing well-defined metal nanoparticles based-catalysts, in non-conventional media (ionic liquids or water), for various catalytic applications (hydrogenation, dehalogenation, carbon-carbon coupling, asymmetric catalysis) in mild reaction conditions. In the drive towards a more eco-responsible chemistry, the main focuses rely on the search of highly active and selective nanocatalysts, in association with an efficient recycling mainly under pure biphasic liquid-liquid conditions. In this Personal Account, we proposed our almost fifteen-years odyssey in the world of metal nanoparticles for a sustainable catalysis.

Chiral ammonium-capped rhodium(0) nanocatalysts: synthesis, characterization, and advances in asymmetric hydrogenation in neat water.

Optically active amphiphilic compounds derived from N-methylephedrine, N-methylprolinol, or cinchona derivatives possessing bromide or chiral lactate counterions were efficiently used as protective agents for rhodium(0) nanoparticles. The full characterization of these surfactants and the obtained nanocatalysts was performed by means of different techniques. These spherical nanoparticles, with sizes between 0.8-2.5 nm depending on the stabilizer, were evaluated in the hydrogenation of model substrates in neat water as a green solvent. The rhodium catalysts showed relevant kinetic properties, but modest enantiomeric excess values of up to 13 % in the hydrogenation of ethyl pyruvate. They were also investigated in the hydrogenation of disubstituted arenes, such as m-methylanisole, providing interesting catalytic activities and a preferential cis selectivity of around 80 %; however, no asymmetric induction was observed.

New ammonium surfactant-stabilized rhodium(0) colloidal suspensions: influence of novel counter-anions on physico-chemical and catalytic properties.

Novel anionic species, such as hydrogen carbonate (HCO(3)(−)), fluoride (F(−)), triflate (CF(3)SO(3)(−)), tetrafluoroborate (BF(4)(−)) and chloride (Cl(−)) were investigated as new partners of water soluble N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium salts, used as a protective agent of rhodium colloids. The effect of the surfactant polar head on the micellar behavior, size and morphology of the nanospecies was studied by adapted physico-chemical experiments (surface tension measurements, dynamic light scattering, thermogravimetric and TEM analyses) and discussed in terms of strong or weak stabilization of the growing nanoparticles surface. Finally, the influence of the nanoenvironment generated by the surfactant with various counter-anions was evaluated via the hydrogenation of aromatics.

N-donor ligands based on bipyridine and ionic liquids: an efficient partnership to stabilize rhodium colloids. Focus on oxygen-containing compounds hydrogenation.

Polynitrogen ligands and/in ionic liquids (ILs) are described as a pertinent tandem to efficiently stabilize rhodium nanoparticles (NPs) in the size range of 2.0 nm for catalytic applications. Several N-donor ligands based on bipyridine skeleton were used as extra protective agents in [BMI][PF(6)] and compared in the hydrogenation of functionalized aromatic compounds at 80 °C and under 40 bar H(2). The nature of the bipyridine derivative and its influence on the coordination mode on the particle surface were proposed to explain the observed different kinetic properties. The hydrogenation of various oxygen-containing arenes was investigated and original results were described in the reduction of anisole and cresols as model lignin compounds, providing a significant ratio of ketone derivatives. A first explanation based on possible reaction routes is proposed to justify the formed products.

Polyhydroxylated ammonium chloride salt: a new efficient surfactant for nanoparticles stabilisation in aqueous media. Characterization and application in catalysis.

A trihydroxyammonium chloride has proved to be an efficient protective agent for Rh(0) nanoparticles and the hydrogenation of arene compounds has been investigated. Significant formation of cyclohexanone in the reduction of anisole has been demonstrated.

Carbohydrate-derived 1,3-diphosphite ligands as chiral nanoparticle stabilizers: promising catalytic systems for asymmetric hydrogenation.

Metallic Ru, Rh, and Ir nanoparticles were prepared by the decomposition of organometallic precursors under H(2) pressure in the presence of 1,3-diphosphite ligands, derived from carbohydrates, as stabilizing agents. Structural modifications to the diphosphite backbone were found to influence the nanoparticles' size, dispersion, and catalytic activity. In the hydrogenation of o- and m-methylanisole, the Rh nanoparticles showed higher catalytic activity than the corresponding Ru nanoparticles. The Ir nanoparticles presented the lowest catalytic activity of the series. In all cases, the hydrogenation of o-methylanisole gave total selectivity for the cis-product, however, the ee of the product was always less than 6 %. A maximum of 81 % cis-selectivity was obtained for the hydrogenation of m-methylanisole, however, no asymmetric induction was observed. These results show that the catalytic activity is affected by a combination of influences from the substrate, the diphosphite ligands, and the metallic nanoparticles.

Catalytically active nanoparticles stabilized by host-guest inclusion complexes in water.

Hydrogenation of arene derivatives can be successfully performed in water by using ruthenium(0) nanoparticles stabilized by 1 : 1 inclusion complexes formed between methylated cyclodextrins and an ammonium salt bearing a long alkyl chain.

Rhodium nanocatalysts stabilized by various bipyridine ligands in nonaqueous ionic liquids: influence of the bipyridine coordination modes in arene catalytic hydrogenation.

Rhodium nanoparticles stabilized by 2,2'-, 3,3'-, 4,4'-bipyridine ligands were prepared in various ionic liquids according to a chemical reduction approach. Zerovalent nanospecies in the size range of 2.0-2.5 nm were characterized. The nature of the bipyridine and its influence on the coordination environment of rhodium nanoparticles were investigated in various nonaqueous ionic liquids according to the cation and anion. The hydrogenation of various aromatic compounds by these colloidal suspensions was carried out at 80 degrees C and under 40 bar of H 2. A first structural explanation based on bipyridine coordination modes is proposed to justify the observed different activities.

Diphosphite ligands derived from carbohydrates as stabilizers for ruthenium nanoparticles: promising catalytic systems in arene hydrogenation.

Ruthenium nanoparticles (RuNPs) were prepared through the hydrogenation of [Ru(COD)(COT)] (COD = 1,5-cyclooctadiene, COT = 1,3,5-cyclooctatriene) in the presence of diphosphites derived from carbohydrates as stabilizing agents, and interestingly, structural modifications of the diphosphite backbone were found to influence nanoparticle size and dispersity, as well as their catalytic activity in arene hydrogenation.

A surfactant-assisted preparation of well dispersed rhodium nanoparticles within the mesopores of AlSBA-15: characterization and use in catalysis.

Well dispersed and efficient Rh(0) hydrogenation catalysts were obtained by the reduction of Rh(III)-exchanged mesoporous aluminosilicates by sodium borohydride in the presence of N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium chloride.

Methylated cyclodextrins: an efficient protective agent in water for zerovalent ruthenium nanoparticles and a supramolecular shuttle in alkene and arene hydrogenation reactions.

Zerovalent ruthenium(0) nanoparticles in the size range of 2.5 nm were easily prepared by chemical reduction of ruthenium salt with an excess amount of sodium borohydride and were efficiently stabilized by methylated cyclodextrins. The optimization of the catalytic system has been carried out in terms of stability and catalytic activity, considering the hydrogenation of olefinic compounds under biphasic liquid-liquid conditions. Efficient and controlled chemoselectivities were obtained in the hydrogenation of arene derivatives by the relevant choice of cavity and methylation degree of the cyclodextrins. Finally, the hydrogenation of alpha- and beta-pinenes leads to the major formation of cis-pinanes, interesting synthons for fine chemistry, with high diastereoisomeric excesses.