Palladium characterization

Ferrandon, M., Carno, J., Jaras, S., and Bjornbom, E. Total oxidation catalysts based on manganese or copper oxides and platinum or palladium characterization. Catal A Gen. 1999, 180, 141-151. 

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

Our general survey of palladium in organic synthesis must now come to an end. At the very least, we hope that our brief foray into this fascinating area conveys some of the vitality that characterizes research in this area. The remainder of this chapter will address the first total synthesis of rapamycin by the Nicolaou group. This work is predicated on a novel variant of the Stille reaction. 

An extensive range of mono- and binuclear halide complexes of platinum and palladium exist. Of the tetrahalometallate(II) ions, some like PtF4 and Pdl4 are elusive, the latter only having been characterized in solution. 

A few isocyanides of palladium and platinum are known in the zerovalent oxidation state. The best characterized compounds involve triangular M3 clusters with M-M bonds. 

Mononuclear complexes of palladium and platinum in the +3 oxidation state have only recently been unequivocally characterized [157]. The major advance has come in complexes with macrocyclic ligands such as 1,4,7-trithiacyclononane (ttcn) and 1,4,7-triazacyclononane (tacn) (Figure 3.96). 

In contrast, the same substrate (76) gave only the monomeric palladium complex (78), characterized by X-ray analysis and spectra [MePdBr(OPPhMe2)2, THF, 20°C >95%]. ... 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

Metallation and oxymetallation reactions have been observed with the salts of only a few metals, namely mercury(II) (66, 67), thallium(III) (66,67), lead(IV) (66, 67), palladium(II) (100), gold(III) (63), and platinum-(II) (29). These facts correlate well with what Chatt (1) has termed class b, and Pearson (130) has called "soft acid character. Soft acids are characterized by low charge, large size, and, often, d electrons in their outer shell. No class b metal is known, in fact, which contains fewer than five d... 

Helquist et al. [129] have reported molecular mechanics calculations to predict the suitability of a number of chiral-substituted phenanthrolines and their corresponding palladium-complexes for use in asymmetric nucleophilic substitutions of allylic acetates. Good correlation was obtained with experimental results, the highest levels of asymmetric induction being predicted and obtained with a readily available 2-(2-bornyl)-phenanthroline ligand (90 in Scheme 50). Kocovsky et al. [130] prepared a series of chiral bipyridines, also derived from monoterpene (namely pinocarvone or myrtenal). They synthesized and characterized corresponding Mo complexes, which were found to be moderately enantioselective in allylic substitution (up to 22%). 

Supported platinum-palladium. This aspect of the study focused on the characterization of platinum and palladium on alumina. The analytical capability of STEM Is fully demonstrated In a problem of this type, because of the rapid manner In which crystallite composition can be analyzed. This study Is especially Interesting because of Che use of platinum and palladium combinations In automotive catalysis. 

This technique is the most widely used and the most useful for the characterization of molecular species in solution. Nowadays, it is also one of the most powerful techniques for solids characterizations. Solid state NMR techniques have been used for the characterization of platinum particles and CO coordination to palladium. Bradley extended it to solution C NMR studies on nanoparticles covered with C-enriched carbon monoxide [47]. In the case of ruthenium (a metal giving rise to a very small Knight shift) and for very small particles, the presence of terminal and bridging CO could be ascertained [47]. In the case of platinum and palladium colloids, indirect evidence for CO coordination was obtained by spin saturation transfer experiments [47]. 

Palladium hydride is a unique model system for fundamental studies of electrochemical intercalation. It is precisely in work on cold fusion that a balanced materials science approach based on the concepts of crystal chemistry, crystallography, and solid-state chemistry was developed in order to characterize the intercalation products. Very striking examples were obtained in attempts to understand the nature of the sporadic manifestations of nuclear reactions, true or imaginary. In the case of palladium, the elfects of intercalation on the state of grain boundaries, the orientation of the crystals, reversible and irreversible deformations of the lattice, and the like have been demonstrated. 

This argument is confirmed by the study of CO pulse chemisorption by Biffis at al., mentioned above. In this piece of investigation, the authors prepared a 2% (w/w) palladium catalyst supported by Lewatit UCP 118, a macroreticular resin (nominal cld = 18 %) from Bayer. Its TEM characterization showed a remarkably heterogeneous distribution of the metal nanoclusters, which are apparently located close to the surface of the polymer nodules [62] (Figure 9). 

Vargafdk MN, Moiseev II, Kochubey DI, Zamaraev KI. 1991. Giant palladium clusters— S3mthesis and characterization. Earaday Discuss 92 13-29. 

We have studied the hydrogenolysis of 2-(perfluorohexyl)ethane thiocyanate to 2-(perfluorohexyl)ethane thiol. It was discovered that perfluoroalkyl thiocyanates can be reduced to thiols and co-product hydrogen cyanide with molecular hydrogen in the presence of a carbon-supported palladium-tin catalyst. This result is surprising since it is known that palladium and other gronps 8 to 10 metal catalysts are poisoned by the product thiol, traces of hydrogen snlfide byprodnct, and the hydrogen cyanide co-product. For that reason, we characterized the catalyst to understand why it was so robust under conditions that would normally poison snch a catalyst. 

The effects of tin/palladium ratio, temperatnre, pressnre, and recycling were studied and correlated with catalyst characterization. The catalysts were characterized by chemisorption titrations, in situ X-Ray Diffraction (XRD), and Electron Spectroscopy for Chemical Analysis (ESCA). Chemisorption studies with hydrogen sulfide show lack of adsorption at higher Sn/Pd ratios. Carbon monoxide chemisorption indicates an increase in adsorption with increasing palladium concentration. One form of palladium is transformed to a new phase at 140°C by measurement of in situ variable temperature XRD. ESCA studies of the catalysts show that the presence of tin concentration increases the surface palladium concentration. ESCA data also indicates that recycled catalysts show no palladium sulfide formation at the surface but palladium cyanide is present. 

This study involved the preparation and characterization of poly(N-phenyl 3,4-dimethylenepyrrolidine) and the subsequent oxidation and reduction of this polymer. The parent polymer was not very soluble, so it was difficult to characterize. However, after oxidizing in the presence of palladium on carbon in nitrobenzene, the resultant poly(N-phenyl 3,4-dimethylenepyrrole) was soluble in several organic solvents. Attempts to reduce the original polymer to the pyrrolidone were unsuccessful. 

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