Palladium solvent-stabilized colloids

A new development is biphasic hydrogenation using solvent-stabilized colloid (SSCs) catalysts [39-41]. Palladium colloid systems, especially, were proven to give high reactivity and selectivity. Best solvents are dimethylformamide and particularly the two cyclic carbonic acid esters, ethylene carbonate and 1,2-propene carbonate. In these solvents sodium tetrachloropalladate - stabilized by a sodium carbonate buffer - is reduced with hydrogen to yield the solvent-stabilized palladium colloid. Transmission electron microscopy of the palladium colloid demonstrates that the colloid particles are spherical with an average diameter of 4 nm. 

Reetz et al. reported on catalytically active solvent-stabilized colloids in propylene carbonate, which were prepared electrochemically or by thermal decomposition of [Pd(OAc)2 assisted by ultrasound. The colloidal particles had sizes of 8 to 10 nm, as determined by TEM. After addition of aryl bromide, styrene, and base to the colloid solution, satisfactory conversions were obtained within reaction times of 5-20 h. Isolation of the particles stabilized by propylene carbonate was not possible, however [16]. The same authors also reported Suzuki and Heck reactions with electrochemically prepared Pd or Pd/Ni colloids stabilized by tetraalkylammonium, as well as polyvinylpyrrolidone (PVP)-stabilized palladium colloids prepared by hydrogen reduction (Table 1) [17]. It was assumed that the reaction occurs on the nanopartide surfaces. 

In one example, colloidal Pd particles are obtained by the electrolysis of aqueous solutions of palladium chloride at pH 1 in a two-layer bath in the presence of a hydrocarbon solvent and epoxy dianic resin or PVA. Electrolysis results in flie formation of colloidal palladium organosols stabilized by the chemisorption of tiie polymer. Metal-lopolymers containing up to 90-95% of Pd remain after the removal of solvent and residual electrolyte. They are formed under high cathode polarization where concomitant elimination of hydrogen adsorbed on the nanoparticles (5.5-7.S nm in diameter) occurs. 

PVP, a water soluble amine-based pol5mer, was found to be an optimum protective agent because the reduction of noble metal salts by polyols in the presence of other surfactants often resulted in non-homogenous colloidal dispersions. PVP was the first material to be used for generating silver and silver-palladium stabilized particles by the polyol method [231-233]. By reducing the precur-sor/PVP ratio, it is even possible to reduce the size of the metal particles to few nanometers. These colloidal particles are isolable but surface contaminations are easily recognized because samples washed with the solvent and dried in the air are subsquently not any more pyrophoric [231,234 236]. 

Trivino GC, Klabunde KJ, Dale EB (1987) Living colloidal palladium in nonaqueous solvents. Formation, stability, and film-forming properties. Clustering of metal atoms in organic media. 14. Langmuir 3 986-992... 

Stabilized Pd nanoparticles of compounds featuring perfluorinated chains 6-10 were described by Moreno-Mahas et al. [18,19]. The Pd nanoparticles were obtained by the reduction of PdCl2 with methanol in the presence of 6-10, respectively. The presence of such nanoparticles was confirmed by transmission electron microscopy. Due to the stabilization by the perfluorinated ligand, the palladium colloids are soluble in perfluorinated solvents. Pd nanoparticles stabilized by l,5-bis(4,4 -bis(perfluorooctyl)phenyl)-l,4-pentadien-3-one (6) were active in Heck and Suzuki couplings [18]. 

Polypropylene imine dendrimers with covalently attached perfluorinated poly-(propylene oxide) end-groups have been employed for the stabilization of palladium colloids in Heck reactions in fluorous solvents by Crooks et al. [36] (Table 2). Relatively low activities were obtained, which were further reduced upon re-use of the fluorous phase in a second cycle. From the results of repeated Heck reactions without an added base, it can be asstuned that the reduction in activity upon recycling is due to protonation of the dendrimer scaffold, serving as a base. No leaching of palladium from the fluorous phase was detected (< 0.01 ppm) however, this value was not related to the overall palladium loading (cf. also Section 4.2). 

The syntheses of palladium and platinum organosols [82-85] by the thermolyds of such precursors as palladium acetate, palladium acetylacetonate, and platinum acetylacetonate in hi boiling organic solvents like methyl- o-butylketone have been reported. Likewise, bimetallic colloids of copper and palladium have been prepared from the thermolysis of mixtures of their acetates in similar solvents. [86] These preparations were performed in the absence of stabilizing polymers, and as a residt, relatively broad size distributions and large partides were observed. 

Uniform bimetallic copper-palladium colloids (the metals are completely miscible) have been prepared by thermolysis of mixtures of the acetates in high boiling solvents such as bromobenzene, xylenes, and methyl- o-butyl ketone [86] in the absence of polymer stabilizers. The resulting agglomerated bimetallic particles often contained CuO in addition to the metals. Smaller particle sizes and narrower size distributions without oxide formation were reported to result from the analogous reaction in 2-ethoxyethanol in the presence of PVR [38] In both cases the homogeneous composition of the particles was established during the electron microprobe analysis used to determine the Cu Pd ratio in the individual particles. 

A further report concerning the use of microgels as catal5dic centres involves microgel-stabilized palladium colloids (65). Microgels of this t5q>e can be conveniently loaded with Pd + ions, which become subsequently reduced. The resulting metal colloid can be readily precipitated out of solution and redispersed in suitable solvents for subsequent use. PreHminaiy experiments with these systems show them to be active catalysts for the vinyliation of aiyl iodides and bromides. 

Water-soluble calix[n]arenes are powerful receptors for non-polar substrates in aqueous solution. These compounds are promising candidates as carrier molecules for the transport of non-polar substrates through bulk water as well as inverse phase-transfer catalysts, as proven for the Suzuki coupling of iodobenzene with phenyl boronic acid [91]. 1.5-bis(4,4 -bis(perfluorooctyl)penta-l,4-dien-3-one (39) stabilizes palladium 0) nanoparticles (transmission electron microscopy) formed in the reduction of palladium dichloride with methanol. These palladium colloids are soluble in perfluorinated solvents, and they are efficient recoverable catalysts for Suzuki crosscoupling under fluorous biphasic conditions (Equation 69) [92]. 

In addition to the well-known organic-phase transformations, the Suzuki reaction also offers methods for the crosscoupling of aryl halides with hydrophilic functional groups. These reactions therefore, require polar solvents such as water. El-Sayed s group prepared 3.6 0.73 nm PVP-stabilized palladium nanoparticles and showed them to act as efficient catalysts for the Suzuki crosscouplings as colloids... 

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