Palladium giant cluster

Contrary to expectation, the reaction product 8, a black amorphous powder soluble in water and acetic acid, was found to have composition and properties quite different from those expected for the platinum analog of the Pdsgi giant clus-ter.157] According to elemental analysis, substance 8 has the reproducible elemental composition Pt8phen3(0Ac)4(0H)4(H20)6, which is quite different from the expected minimum formula Pt9(phen)o.96(OAc)2.89 for the Pt analog of palladium giant cluster 6. On the basis of the elemental composition, the formal oxidation state of platinum in substance 8 is close to (+1). This value was confirmed by the EXAFS data. ... 

Moiseev, I.I. et al., Palladium-561 giant clusters chemical aspects of self organization on a nano level,... 

One of the challenging aims of cluster chemistry is to elucidate the factors controlling the formation of cluster molecules and small metal crystallites. Despite remarkable achievements in the synthesis and structural characterization of metal clusters, the pathways to the assembly of large numbers of metal atoms in the course of the synthesis of high nuclearity metal clusters remains rather mysterious. Some insight into this problem has been gained by recent studies of so-called giant clusters of palladium and platinum. 

Only for the first members of this series, the 13-atom clusters, has the molecular structure been determined by single-crystal X-ray analysis. The other clusters have been characterized by the indirect methods mentioned above. In this review the results of such combined studies are presented, using as examples palladium and platinum giant clusters with phen, dipy, and phospine/phospide ligands. 

The chemistry of giant metal clusters is still at the start of its development. The studies outlined in this survey show that assembling of palladium and platinum atoms to form giant clusters proceeds via polynuclear complexes, which are precursors difficult to isolate for palladium but are more stable for platinum. 

The catalytic applications of Moiseev s giant cationic palladium clusters have extensively been reviewed by Finke et al. [167], In a recent review chapter we have outlined the potential of surfactant-stabilized nanocolloids in the different fields of catalysis [53]. Our three-step precursor concept for the manufacture of heterogeneous egg-shell - nanocatalysts catalysts based on surfactant-stabilized organosols or hydrosols was developed in the 1990s [173-177] and has been fully elaborated in recent time as a standard procedure for the manufacture of egg-shell - nanometal catalysts, namely for the preparation of high-performance fuel cell catalysts. For details consult the following Refs. [53,181,387]. 

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

Oleshko, V. et al., High resolution electron microscopy and electron energy-loss spectroscopy of giant palladium clusters, Z. Phys. D., 34, 283, 1995. 

Figure 15.5. Giant palladium cluster in Wacker reactions...

In contrast to the usual Wacker-conditions, optimum rates and catalyst stability in the Pd/batophenanthroHne-catalyzed olefin oxidations was observed in the presence of NaOAc (pH s 11.5). Under such conditions, the catalyst-containing aqueous phase could be recycled with about 2-3 % loss of activity in each cycle. In the absence of NaOAc precipitation ofPd-black was observed after the second and third cycles. Nevertheless, kinetic data refer to the role of a hidroxo-bridged dimer (Scheme 8.1) rather than the so-called giant palladium clusters which could easily aggregate to metallic palladium. 

Major trends can be discerned for Pd-catalysts, aimed at increasing the stability and activity. First is the use of palladium-carbene complexes [178]. Although activities are still modest, much can be expected in this area. Second is the synthesis and use of palladium nanoparticles. For example, the giant palladium cluster, Pd561phen6o(OAc)i8o [179], was shown to catalyze the aerobic oxidation of primary allylic alcohols to the corresponding a,/funsaturated aldehydes (Fig. 4.66) [180]. 

N.M. Vargaftik, LI. Moiseev, D.I. Kochubey, K.I. Zamaraev, Giant palladium clusters Synthesis and characterization. Faraday Discuss. Chem. Soc. 1991, 92, 13-29. 

Moiseev and coworkers showed [10,13] that giant palladium clusters with an idealized formula Pd56iL5o(OAc)igo (L = phenanthroline or bipyridine) are highly active catalysts for allylic oxidation of olefins. The catalytically active solution was prepared by reduction of Pd(OAc)2, e. g. with H2, in the presence of the ligand, L, followed by oxidation with O2. The giant palladium cluster catalyzed the oxidation of propylene to allyl acetate under mild conditions. Even in 10% aqueous acetic acid, allyl acetate selectivity was 95-98 % [10]. Oxidation catalyzed by Pd-561 in water afforded a mixture of allylic alcohol (14%), acrolein (2%), and acrylic acid (60%), and only 5% acetone [10]. 

In his pioneering contributions Moiseev has shown that giant cationic palladium clusters , e.g. Pd56iL6o(OAc)i8o (L = phenanthroline, bipyridine), characterized by use of high-resolution TEM, SAXS, EXAFS, IR and magnetic susceptibility data, catalyze, under mild conditions (293 363 K, 1 bar), the oxidative acetoxylation of ethylene into vinyl acetate, propylene into allyl acetate, and toluene into benzyl acetate. The oxidation of primary aliphatic alcohols to esters, and the conversion of aldehydes into acetals were also studied. ... 

Schmid, G., Harms, M., Malm, J.O. et al. (1993) Ligand-stabilized giant palladium clusters -promising candidates in heterogeneous catalysis. J. Am. Chem. Soc., 115, 2046-8. 

FIGURE 6.3 (a) Idealized model of Moiseev s giant palladium cluster Pd 5g[L 6o(OAc) igo (phen = phenanthroline) (adapted from Reference 25) (b) idealized model of a Finke-type Ir nanocluster P2W[jNb30g2 and BU4N+ stabilized Ir joQ (adapted with permission from Reference 14). 

The primary surface reaction steps which include the activation of ethylene to vinyl and the subsequent coupling of vinyl with acetate surface intermediates have been cited as potential rate-determining steps in the Nakamura and Yasui route. Moiseev and Vargaftik carried out experiments over giant palladium clusters comprised of 561 atoms and arrived at a similar set of pathways They suggested, however, that the rate-controlling step for this process involves the shift of ethylene from the 7r-bound mode to a di-cr-bound mode. 

Zamaraev, Faraday Discuss., 1992, 92, 13-29. Giant Palladium Clusters Synthesis and Characterization. 

Schmid G, Harms M, Malm J-O, Bovin J-O, van Ruifirrbeck J, Zanbergen HW, Fu WT (1993) Ligand-stabilized giant palladium clusters ptortrising candidates in heterogeneous catalysis. J Am Chem Soc 115 2046-2048... 

Ebitani, K., Fujie, Y. and Kaneda, K. (1999). Immobilization of a Ligand-Preserved Giant Palladium Cluster on a Metal Oxide Surface and its Nobel Heterogeneous Catalysis for Oxidation of AUylic Alcohols in the Presence of Molecular Oxygen, Langmuir, 15, pp. 3557—3562. 

FIGURE 13.10 Electron micrograph of Moiseev s giant palladium clusters on a carbon support. (Reproduced from Ref. 40a with permission of the Royal Society of Chemistry 1985.)... 

Reetz, M. T., M. Winter, and B. Tesche. 1997. Self-assembly of tetraaUcylammonium salt-stabilized giant palladium clusters on surfaces. Chem. Commun. (2) 147-148. 

PaUadium(ll) is also capable of mediating the oxidation of alcohols via the hydrido-metal pathway shown in Scheme 4.5. Blackburn and Schwarz first reported [73] the PdCl2-NaOAc-catalyzed aerobic oxidation of alcohols in 1977. However, activities were very low, with turnover frequencies of the order of 1 h . Subsequently, much effort has been devoted to finding synthetically useful methods for the palladium-catalyzed aerobic oxidation of alcohols. For example, the giant palladium cluster, Pd56ipheii6o(OAc)i8o [74], was shown to catalyze the aerobic oxidation of primary aUyhc alcohols to the corresponding a,(3-unsaturated aldehydes [Eq. (13)] [75]. 

---