Stabilized Palladium Colloid

Palladium nanoparticles, stabilized in micelles formed by polystyrene-co-poly(ethylene oxide) copolymer (PS-PEO) and acetylpyridinium chloride (CPC) as a surfactant, have been used to catalyze heterocyclization of N-methylsulfonyl-o-iodoaniline with phenylacetylene leading to formation of a substituted indole. The activity of the colloidal palladium catalytic system is comparable to that of the low-molecular-we ht palladium complexes, whereas the stabUity of the colloidal palladium system is much h her. The reuse of the catalyst PS-PEO-CPC was demonstrated in experiments with fresh starts as well as by thermomorphous separation of the catalyst from products (20060M154). 

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Chauhan BPS, Rathore JS, Chauhan M, Krawicz A (2003) Synthesis of polysiloxane stabilized palladium colloids and evidence of their participation in silaesterification reactions. 1 Am Chem Soc 125 2876-2877... 

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. 

The propene carbonate-stabilized palladium colloid is an excellent catalyst for the hydrogenation of a great number of different fatty acids, fatty esters, and triglycerides. Table 2 gives a survey of results with sunflower, palm-kernel and rapeseed oils, acids, and esters. The yield of C18 1 products after hydrogenation is in the range of 86-93%. In all examples the reaction time is very short. 

The immobilized, colloidal palladium catalyst, Si02-(C3H6SH)nPd is reported to induce the Heck reaction [14a] between ethyl iodide and ethyl acrylate. XPS data showed the presence of Pd(Il) on the surface of the colloidal Pd particles, owing to air oxidation this explains the different behavior of this and the Pd/C catalyst. Addition of BujN.HI and iodine greatly reduced the induction period. The catalytic activity of propylene carbonate-stabilized palladium colloids in the Heck reaction has been investigated [14b]. 

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. 

Bradley et al. employed P VP-stabilized palladium colloids, which were pretreated with the base and the olefin at the reaction temperature prior to addition of the aryl halide (Table 1) [19]. The colloids were prepared by stirring [Pd(dba)2] in the presence of PVP at variable hydrogen pressures, affording particles of different average particle size (diameter 1.7 0.5 nm to 3.7 0.3 nm). The initial activities of Heck reactions catalyzed by these colloids of different average particle size were correlated with the number of low-coordinated atoms, i.e., comer and edge atoms in the palladium particles rather than all surface atoms. A general difficulty in such considerations is that the particles are not monodisperse, and also are not ideal cuboctahedra. 

Employing the same dendrimer-stabilized palladium colloids for Heck reactions in supercritical carbon dioxide, after 24 h reaction time and 55% conversion no further reaction was observed for reasons not clarified. Remarkably, methyl 2-phenylacrylate was formed exclusively instead of the expected methyl cinnamate (Scheme 1) [37]. 

Scheme 7.6 Thermal preparation of ammonium salt-stabilized palladium colloids.

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. 

A systematic study of the influence of different ionic liquids on the activity of the methoxycarbonylation of iodobenzene promoted by Pd colloids reveals the structure dependence on the catalytic activity tetraalkyl ammonium and pyridinium salts are much more effective additives than imidazolium-based ILs to stabilize palladium colloids. The yield of methoxycarbonylation reaction catalyzed by Pd colloid decreases in the following order NBu4-Br> NBu4-I > NBu4-Cl> BMpy-PFe >... 

BOnnemann, H., Brijoux, W., Siepen, K., Hormes,)., Franke, R., Pollmann, J. and Rothe, J. (1997) Surfactant stabilized palladium colloids as precursors for ds-selective alkyne-hydrogenation catalysts. Applied Organometallic Chemistry, 11, 783-96. 

By shaking a solution of ammonia and ammonium pyruvate with colloidal palladium in the presence of hydrogen, employing starch paste to stabilize the colloid. Aubel and Bourguel, Compt. rend. 186, 1844 (1928). 

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. 

Greater durability of the colloidal Pd/C catalysts was also observed in this case. The catalytic activity was found to have declined much less than a conventionally manufactured Pd/C catalyst after recycling both catalysts 25 times under similar conditions. Obviously, the lipophilic (Oct)4NCl surfactant layer prevents the colloid particles from coagulating and being poisoned in the alkaline aqueous reaction medium. Shape-selective hydrocarbon oxidation catalysts have been described, where active Pt colloid particles are present exclusively in the pores of ultramicroscopic tungsten heteropoly compounds [162], Phosphine-free Suzuki and Heck reactions involving iodo-, bromo-or activated chloroatoms were performed catalytically with ammonium salt- or poly(vinylpyrroli-done)-stabilized palladium or palladium nickel colloids (Equation 3.9) [162, 163],... 

A new class of heterogeneous catalyst has emerged from the incorporation of mono- and bimetallic nanocolloids in the mesopores of MCM-41 or via the entrapment of pro-prepared colloidal metal in sol-gel materials [170-172], Noble metal nanoparticles containing Mex-MCM-41 were synthesized using surfactant stabilized palladium, iridium, and rhodium nanoparticles in the synthesis gel. The materials were characterized by a number of physical methods, showed that the nanoparticles were present inside the pores of MCM-41. They were found to be active catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene, and... 

Michaelis, M. and Henglein, A., Reduction of palladium (II) in aqueous solution stabilization and reactions of an intermediate cluster and palladium colloid formation, J. Phys. Chem., 96, 4719, 1992. 

In addition, other forms of Pd(0) stabilized in less conventional ways should be cited. Catalytically active palladium colloids are obtained by reduction of palladium acetate in DMSO, or in the presence of a number of polymers. " Some of the latter are easy to recycle and avoid the leaching of Pd during the catalytic runs. 

At the opposite of the molecular chemistry described until now, nanoparticles are reminiscent of heterogeneous catalysts. However, these colloid-derived materials have been shown to catalyze efficiently in water coupling reactions which have been previously described in pure homogeneous systems. For instance, poly(N-vi-nyl-2-pyrrolidine)-stabilized palladium nanopartides promote the Suzuki crosscoupling in aqueous media with high yields (see also Section 6.6) [87]. 

In a recent perspective article G. Schmid et al. summarized some general phenomena and the great catalytic potential of ligand-stabilized transition-metal clusters and colloids. The catalytic properties of large ligand-stabilized palladium clusters has been described. ... 

Beller et al. have shown for the first time that palladium colloids are effective catalysts for the olefination of aryl bromides (Heck reaction). Reetz et al. have studied Suzuki and Heck reactions catalyzed by preformed palladium clusters and palladium/nickel bimetallic clusters and further progress was achieved by Reetz and Lohmert using propylene carbonate stabilized nanostructured palladium clusters as catalysts in Heck reactions. In addition, the use of nanostructured titanium clusters in McMurry-type coupling reactions has been demonstrated by Reetz et... 

The interface between two immiscible electrolyte solutions offers the means to combine two-phase catalysis, colloid catalysts, and electrocatalysis. In the study of Lahtinen et al. [158] citrate-stabilized palladium and gold colloids were prepared by a traditional chemical reduction method. The voltammetric response of a system with an aqueous colloid and an electron donor in the organic phase revealed an irreversible voltammetric wave as the potential was swept positive. The response was detected only in the presence of both the colloid and the electron-donor DCMFc. The response was concluded to result from heterogeneous charging of the colloid with electrons from DCMFc. 

Propylene carbonate-stabilized palladium nanopartides were also shown to be active catalysts for the Heck reaction [116, 117]. The formation of olefins from aldehydes and ketones via McMurry-type coupHng reactions was reported using Bu4NBr-stabihzedTi colloids (3nm) [22]. TheTHF-protectedTi,3-nanocluster (Fig. 

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]. 

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