Reduction with palladium colloidal

Palladium catalysts have been prepared by fusion of palladium chloride in sodium nitrate to give palladium oxide by reduction of palladium salts by alkaline formaldehyde or sodium formate, by hydrazine and by the reduction of palladium salts with hydrogen.The metal has been prepared in the form of palladium black, and in colloidal form in water containing a protective material, as well as upon supports. The supports commonly used are asbestos, barium carbonate, ... 

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%... 

One of the most important methods for the identification of unsaturated linkages, and a method which has had important consequences in the study of ring systems in general and alkaloids in particular, is the Paal-Skita method of reduction with colloidal metals, in which finely divided palladium or platinum acts as... 

An acetylene may be reduced to an olefin by sodium in liquid ammonia, ° by electrolytic reduction at a spongy nickel cathode, or by partial hydrogenation over metal catalysts. Catalysts for the hydrogenation include nickel, ° iron, colloidal palladium, and palladium on barium sulfate or calcium carbonate. Pure trans olefins are obtained from dialkylacetylenes by reduction with sodium in liquid ammonia. The yields ate better than 90%. Catalytic hydrogenation leads to mixtures of cis and trans olefins in which the cis isomers predominate. ° Mono- and di-arylacetylenes have also been reduced. ... 

Recently, novel nanomaterials have become a new frontier for SERS experiments, where different metals are collected together to form, for example, bimetallic particles. Thus, the same nanoparticle could be responsible for both SERS effect and catalytic activity. This is the case of the Ag/Pd colloids synthesized by chemical reduction with sodium borohydride (NaBH4) of silver nitrate (AgNOs) and palladium nitrate (Pd(N03)2), with a 96 4 Ag/Pd molar ratio [11]. The silver nanoparticles provide the SERS enhancement for the ligand molecules, while palladium may induce catalytic reactions. Also, in this case, TEM microscopy provides an important help to characterize these composite materials. In Fig. 20.6 TEM images at different magnifications are reported for bimetallic Ag/Pd particles, in comparison with those constituted by pure silver. While these latter present spheroidal shapes, bimetallic particles show more irregularities, due to palladium clusters in contact with the silver core surface. 

When nitrobenzene (23) is reduced with colloidal palladium, the rate is found to be independent of the pH of the solution (22,5). When this reduction is conducted with colloidal rhodium, the rate is found to be maximal in the alkaline range, median in the acid range, and minimal in neutral solution. Further, when substituted nitrobenzene derivatives are reduced with these catalysts, it is found that the rate of reduction with rhodium is dependent both on the nature and position of the substituent group. With palladium, however, the rate of reduction is virtually independent of these factors. In all cases, the reduction was found to be first order with respect to the catalyst and zero order with respect to the substrate. 

Reduction of /3-chlorocodide in dilute acetic acid with a colloidal palladium catalyst gives mainly dihydrodesoxycodeine-D [ix] but also a considerable amount of tetrahydrodesoxycodeine [vm] [30, 34]. The former is presumably formed by initial 1 4-reduction of the system... 

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]. 

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. 

Biffis et al. employed soluble microgels to stabilize colloidal palladium particles [27, 28]. These cross-linked polyacrylates, characterized by gel permeation chromatography, contained amino groups, which coordinate palladium ions. Reduction results in colloids with particle sizes of 3 to 9 nm. Particle sizes can be influenced by the degree of crosslinking of the microgel. Heck reactions of activated aryl bromides proceed with high activities (Table 1). 

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). 

Based on the above results, we have proposed a classical Chalk-Harrod-like mechanism for the Pd nanocluster catalyzed reaction (Scheme 3.10). " The first step involves the reduction of Pd(OAc)2 to zero-valent palladium nanoparticles followed by stabilization with a polysiloxane network. These particles followed the conventional pathway of oxidative-addition and reductive-elimination cycles to generate sUyl esters. After the transformation was over, these palladium colloids precipitated out of the solution as a black solid supported on a polysiloxane network. The presence of the soluble polysiloxane matrix, which was not fully condensed in the form of silica, allows particle redispersion— thus allowing particles to be reused as catalyst... 

The curve follows the fit of the model describing the size dependence. The three experimental points match the curve quite well and we can now understand why even 15 nm palladium colloids show magnetic behavior still far from that of the bulk. Even for a 1 micron sample, one can expect a reduction in x of about 1 % Values for A and X obtained from different experiments for similarly sized particles agree quite well with the results discussed here. [61-64] Finally, we see that the magnetic properties of ligand stabilized particles, even if they are very huge particles like colloids, differ considerably from those found in the bulk. The... 

Similarly irradiation of Pd(NH3)4Q2 in aqueous uo-propanol results in the reduction of the palladium complex to colloidal palladium by both solvated electrons and 1-hydroxy-1-methylethyl radicals (CH3)2COH (formed by reaction of iyo-propanol with the initial radiolysis products H and OH). [102] Since mixtures of acetone and iso-propanol also yield (CH3)2COH under both radiolytic and photolytic conditions, they have been used in many studies on the radical chemistry of metal colloids. In many of these studies, the techniques of flash photolysis and pulse radiolysis were used to detect short lived intermediates, measure the kinetics, and determine the mechanisms of metal cation reduction, nucleation, and colloidal cluster formation. 

Some Pd-Pt bimetallic colloids protected by polymers [162-165] have been prepared by the coreduction of the metal salts with aqueous alcohols. Similar bimetallic sols in nonpolar organic solvents were obtained by N2H4 or NaBH4 reduction of palladium and platinum salt mixtures after their extraction into the organic phase (cyclohexane or chloroform) with such surfactants as trioctylphos-phine oxide or distearyldimethylammonium chloride. [166]... 

Colloids of alloys have been made by the chemical reduction of the appropriate salt mixture in the solution phase. Thus, Ag-Pd and Cu-Pd colloids of varying composition have been prepared by alcohol reduction of mixtures of silver nitrate or copper oxide with palladium oxide (Vasan and Rao 1995). Fe-Pt alloy nanocrystals have been made by thermal decomposition of the Fe and Pt acetylac-etonates in high-boiling organic solvents (Sun et al. 2000). Au-Ag alloy nanocrystals have been made by co-reduction of silver nitrate and chloroauric acid with sodium borohydride (Sandhyarani et al. 2000 He et al. 2002). 

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]. 

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]. 

Ru(bipy)3 formed in this reaction is reduced by the sacrificial electron donor sodium ethylenediaminetetra-acetic acid, EDTA. Cat is the colloidal catalyst. With platinum, the quantum yield of hydrogenation was 9.9 x 10 . The yield for C H hydrogenation was much lower. However, it could substantially be improv l by using a Pt colloid which was covered by palladium This example demonstrates that complex colloidal metal catalysts may have specific actions. Bimetalic alloys of high specific area often can prepared by radiolytic reduction of metal ions 3.44) Reactions of oxidizing radicals with colloidal metals have been investigated less thoroughly. OH radicals react with colloidal platinum to form a thin oxide layer which increases the optical absorbance in the UV and protects the colloid from further radical attack. Complexed halide atoms, such as Cl , Br, and I, also react... 

By reduction of the corresponding acetylene with hydrogen and colloidal palladium. Bourguel, Bull. soc. chim. 41, 1475 (1927). 

---