Stability palladium

Figure 3.96 Macrocyclic ligands used to stabilize palladium(III) and platinum(III).

Table 9.4 Phenanthroline-stabilized palladium nanoparticles in PEG for the hydrogenation of alkenes.a) (Adapted from [68])...

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... 

Schmid, G. et al., Synthesis and catalytic properties of large ligand stabilized palladium clusters, J. Mol. Catal. A, 107, 95, 1996. 

Meier MAR, FUali M, Gohy J-F, Schubert US (2006) Star-shaped block copolymer stabilized palladium nanoparticles for effident catalytic Heck cross-coupling reactions. J Mater Chem 16 3001-3006... 

In early studies, it was observed that when the NHG was already attached to the metal center, reaction times were shortened since the time for the deprotonation of the salt and coordination to the metal center was no longer required. The use of well-defined systems also allows for a better understanding of the actual amount of stabilized palladium available in the system. Herrmann reported on two similar Pd(0) complexes bearing two carbenes, 37 and 38. The latter was used in 2002 as the first example of coupling of aryl chlorides (activated and unactivated) with arylboronic acids at room temperature, in high yields, and reaction times between 2 and 24 h in the presence of GsF as base. 

Good yields of carbonyl compounds have also been obtained from the vapor-phase oxidation of alkenes by steam and air over palladium catalysts supported on charcoal.413 In this case, no copper cocatalyst is needed, presumably because palladium(II) is not reduced to palladium(O), but remains in the form of a stabilized palladium(Il) hydride which can react with 02 to give the hydroperoxidic species. 

It should be noted that heterogeneous palladium acetoxylation catalysts do not contain copper cooxidants, presumably because the support stabilizes the resulting palladium(II) hydride such as (136) and prevents the formation of metallic palladium. The stabilized palladium hydride (136) may react with 02 to give the hydroperoxide (137), which is probably an important intermediate for the regeneration of the initial Pd11 catalyst. 

Hou Z, Theyssen N, Brinkmann A et al (2005) Biphasic aerobic oxidation of alcohols catalyzed by polyethylene glycol)-stabilized palladium nanoparticles in supercritical carbon dioxide. Angew Chem Int Ed 44(9) 1346—1349... 

Wang X, Yang H, Feng B et al (2009) Functionalized polyethylene glycol)-stabilized palladium nanoparticles as an efficient catalyst for aerobic oxidation of alcohols in supercritical carbon dioxide/poly(ethylene glycol) biphasic solvent system. Catal Lett 132 34-40... 

Some other recent examples are the use of palladium nanoparticles entrapped in aluminum hydroxide [181], resin-dispersed Pd nanoparticles [182], and poly (ethylene glycol)-stabilized palladium nanoparticles in scC02 [183]. Although in some cases the activities for activated alcohols obtained with these Pd-nanoparti-cles are impressive, the conversion of aliphatic alcohols is still rather slow. 

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... 

Brij-35 [polyoxyethylene (23) lauryl ether] stabilized palladium nanoparticles, obtained on attempted sonochemical reduction of PdCl2, by sodium sulfite in water... 

Albert, K., Gisdakis, P., Roesch, N. On C-C Coupling by Carbene-Stabilized Palladium Catalysts A Density Functional Study of the Heck... 

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]. 

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. 

Scheme 1 Mechanistic proposal for the selective hydrogenation of linoleic acid with Pd SSCs (L,Pd—H = stabilized palladium hydrido species).

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]. 

These data confirm that the perovskite stabilizes palladium and suggest that the activity in catalytic combustion can be more efficiently recovered, through a regeneration oxidizing process, for these systems than for the usual Pd/y-Al203 supported catalyst, due to the palladium moiety re-incorporated in the perovskite structure at a high oxidation state (Pd" ). 

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. ... 

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