Colloidal rhodium

A similar polymer-stabilized colloidal system is described by James and coworkers [66]. Rhodium colloids are obtained by reducing RhCls, 3H2O with ethanol in the presence of PVP. The monophasic hydrogenation of various substrates such as benzyl acetone and 4-propylphenol and benzene derivatives was performed under mild conditions (25 °C and 1 bar H2). The nanoparticles are poorly characterized and benzyl acetone is reduced with 50 TTO in 43 h. 

The liquid-phase hydrogenation of various terminal and internal alkynes under mild conditions was largely described with metal nanoparticles deposited/in-corporated in inorganic materials [83, 84], although several examples of selective reduction achieved by stabilized palladium, platinum or rhodium colloids have been reported in the literature. 

Finally, these particles generated in ionic liquids are efficient nanocatalysts for the hydrogenation of arenes, although the best performances were not obtained in biphasic liquid-liquid conditions. The main importance of this system should be seen in terms of product separation and catalyst recycling. An interesting alternative is proposed by Kou and coworkers [107], who described the synthesis of a rhodium colloidal suspension in BMI BF4 in the presence of the ionic copolymer poly[(N-vinyl-2-pyrrolidone)-co-(l-vinyl-3-butylimidazolium chloride)] as protective agent. The authors reported nanoparticles with a mean diameter of ca. 2.9 nm and a TOF of 250 h-1 in the hydrogenation of benzene at 75 °C and under 40 bar H2. An impressive TTO of 20 000 is claimed after five total recycles. 

Biologically important substrates can be hydrogenated in aqueous solution by using sulfonated triarylphosphine ligands that confer water solubility upon the catalyst. However, it was later reported that rhodium metal was formed from these complexes in the presence of hydrogen. It was claimed that the role of the ligands is merely to prevent aggregation of the rhodium colloid produced. 

The system was very sensitive to the transition metal ion added to the platinum. When it was nickel, hydrocin-namaldehyde was obtained in 97% selectivity.75 The intermediates in these reductions are probably metal hydride clusters. When the colloidal platinum is supported on magnesium oxide, without another transition metal, the reduction produces the unsaturated alcohol with 97% selectivity.76 A rhodium colloid stabilized by the same polymer was used with a water-soluble phosphine in the hydro-formylation of propylene to produce 1 1 mixture of /r-bu-tyraldehyde and isobutyraldehyde in 99% yield.77 It could be used at least seven times, as long as it was not exposed to air. 

In 1999, the group of Roucoux studied a new series of easily synthesized ionic surfactants that efficiently stabilize active suspensions of rhodium colloids in the hydrogenation of arenes in a biphasic Uquid-Uquid medium [34-36]. The synthesis of N,N-dimethyl-N-alkyl-N-(2-HydroxyEthyl)Ammonium salts which provide an electrosterical stabilization has been obtained by one step quaternarization of N,N-dimethylethanolamine with the appropriate functionaUzed alkanes or by ion exchange. These salts HEA-CnX bear an alkyl chain containing n= 12-18 carbon atoms and can be prepared with various counter-anions X such as Br, Cl, 1, CH3SO3, BF4 (Scheme 11.2). 

Hanaoka et al. (1999) reported the effect of additive and of rhodium colloid catalyst (Rh4(CO)i2) for selective photocatalytic transfer hydrogenation of 1,5-COD to COE. The solution of rhodium colloid particles was prepared with an irradiation method that consists of stirring a solution with Rh4(CO)n in 2-propanol and acetone for 32 min... 

Hanaoka, T., Matsuzaki, T., Sugi, Y. (1999). Selective photocatalytic transfer hydrogenation to 1,5-cyclooctadiene with fight transition metal modified rhodium colloid catalyst. Journal... 

The mechanism for this ostensibly homogeneous process, the Chalk-Harrod mechanism, [264] was based on classical organometallic synthetic and mechanistic research. Its foundation lies in the oxidative addition of the silane Si-H bond to the low oxidation state metal complex catalyst, a reaction which is well established in the organometallic literature. Lewis reported in 1986 that the catalyti-cally active solutions contained small (2.0 nm) platinum particles, and demonstrated that the most active catalyst in the system was in fact the colloidal metal. [60, 265] Subsequent studies established the relative order of catalytic activity for several precious metals to be platinum > rhodium > ruthenium = iridium > osmium. [266] In addition, a dependence of the rate on colloid particle morphology for a rhodium colloid was observed. [267]... 

The high stability of the block copolymer-colloid approach was also illustrated by the use of poly(A-vinyl-2-pyrrohdone) protected rhodium colloid (Rh-PVP) that was used as a catalyst for methanol carbonylation under elevated temperature (140 °C) and high pressure (5.4 MPa). During the reaction, the catalyst was still in a colloidal state as verified by TEM observations, even after repeated uses and a total TON reaching 19 700 cycles per atom of rhodium. Toshima and Shiraishi also demonstrated the possibility to enhance the catalytic activity of silver colloids (Ag-PVP) in the oxidation of ethylene by the addition of alkali metal ions such as cesium. Bimetallic catalysts in colloidal dispersions composed of two distinct metals also appeared in the literature with often better activity... 

Schulz, E., S. Levigne, A. Roucoux, and H. Patin. 2002. Aqueous rhodium colloidal suspension in reduction of arene derivatives in hiphasic system A significant physico-chemical role of surfactant concentration on catalytic activity. Adv. Synth. Catal. 344 (3 ) 266-269. 

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