Poly palladium colloids

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). 

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],... 

An alcohol reduction method has been applied to the synthesis of polymer-stabilized bimetallic nanoparticles. They have been prepared by simultaneous reduction of the two corresponding metal ions with refluxing alcohol. For example, colloidal dispersions of Pd/Pt bimetallic nanoparticles can be prepared by refluxing the alcohol-water (1 1 v/v) mixed solution of palladium(II) chloride and hexachloro-platinic(IV) acid in the presence of poly(/V-vinyl-2-pyrrolidone) (PVP) at about 90-95°C for 1 h (Scheme 9.1.5) (25). The resulting brownish colloidal dispersions are stable and neither precipitate nor flocculate over a period of several years. Pd/ Pt bimetallic nanoparticles thus obtained have a so-called core/shell structure, which is proved by an EXAFS technique (described in Section 9.1.3.3). 

Although the selectivity of palladium catalysts in the hydrogenation of 1,5-COD is thus very high, the results also indicate that the hydrogenation of COE to cyclooctane (COA) does not cease after the maximum yield of COE has been attained. Hirai et al. studied the hydrogenation of 1,5-COD over a colloidal palladium catalyst, prepared by reduction of palladium(II) chloride in the presence of poly(iV-vinyl-2-pyrrolidone) in refluxing methanol with addition of sodium hydroxide, in methanol at 30°C and 1 atm H2, and obtained a mixture consisting of 0.4% 1,5-COD, 0.3% 1,4-COD, 97.8%... 

A colloidal palladium obtained by reducing palladium chloride with methanol in the presence of poly (N-vinyl-2-pyrrolidone) and sodium hydroxide. Cyclooctadienes (25 mmol-dm ) were hydrogenated over the colloidal palladium (0.01 mmol dm 3 for 1,3- 0.02 mmol-dm-3 for 1,4- 0.1 mmol-dm-3 for 1,5-cyclooctadiene) in 20 ml of methanol at 30°C and atmospheric pressure. The yields were those obtained when an equimolar amount of hydrogen had been consumed. 

Colloidal palladium can also be stabilized with poly(7V-vinyl-2-pyrrolidone).78 Other polymeric stabilizers have also been used with palladium, platinum, and other metals. These include poly(2-ethyl-2-oxazoline), poly(methacrylic acid),79 block copolymers containing COOH groups,80 poly(Wisopropyl acrylamide),81 and poly(styrene-b-4-vinylpyridine).82 Colloidal iron has been stabilized not only by poly(Wvinyl-2-pyrrolidone) but... 

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]. 

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). 

N. (1981) Selective hydrogenation of cyclooctadienes catalyzed by colloidal palladium in poly(N-Vinyl-2-pyrrolidone). Makromol. Chem., Rapid Commun., 2, 99-103. 

Poly(V-vinyl-2-pyrrolidone) has been studied as a carrier for palladium in the hydrogenation of oleflns f and nitroaromatics, hydrodehalogenation of organic halides,f t and carbonylation of aUyl halides. - In these cases PVP mostly likely played the role of supporting in situ formed colloidal Pd, rather than that of a well-defined ligand. 

Fluoride-free Hiyama cross-coupling reactions of phenyltrimethoxysilane with aryl halides was performed in water using sodium hydroxide as activator at 110°C under microwave heating [32]. The reaction was catalyzed by poly (N-viityl-2-pyrrolidone) (PVP)-stabilized colloidal palladium NPs. The reaction proceeds quickly under microwave heating (6 min). 

A pH-responsive palladium catalyst 46, prepared by immobilizing palladium nanoparticles on colloidal core-shell micro-spheres that contain a pH-responsive shell of poly(methylacryUc acid) (PMAA) segments and a coordinative core of poly[2-(acetoxy)ethyl methacrylate] (PAEMA), exhibits good activity in the Suzuki and Heck reactions in a mixture of DMF/water (1 1, v/v) or in pure water [131]. The catalyst is highly dispersed under neutral or basic conditions, behaving like a homogeneous catalyst, but heterogeneous under acidic conditions. As such, it could... 

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