Giant Pd clusters

Complex 5 is unstable in air. It readily loses all hydride atoms imder the action of O2 to form H2O and a polynuclear compound of empirical formula Pd9phen(OAc)3 (6) this compound is highly soluble in water and acetic acid, less so in other polar solvents e.g., MeCN, DMF, DMSO) and their mixtures with water.  

The molecular mass of 6 was estimated to be (1.0 + 0.5) x 10 from the data on the rates of sedimentation of its aqueous solution on ultracentrifugation, by use of the Stokes Einstein law. - This method is analogous to the technique used for determination of the molecular mass of the cluster Au55(PPh3)i2Cl6 in the pioneering work by G. Schmid. 

The HREM study showed some additional features of the packing of the Pd atoms. Although some cluster metal particles have a five-fold axis typical of an icosahedron (Fig. 2a), the more abundant metal particles seemed from the micrographs to be arranged according to face-centered cubic (f.c.c.) packing (Fig. 2b), and this was confirmed by the electron diffractograms for the same cluster samples. 

One possible reason for this structure might be the non-uniformity of the material under study. The HREM study of several samples of 6 obtained after the same 

Another reason for the observed discrepancy between EXAFS and HREM-ED data might be the different experimental conditions of the two techniques. The EXAFS experiments were performed at normal temperatures and in air, and no noticeable damage to a sample exposed to a filtered, low-intensity beam of syn- 

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Vargaftik, M.N. et al., Giant Pd clusters as catalysts of oxidative reactions of olefins and alcohols, J. 

Volkov, V. V. et al., Long- and short-distance ordering of the metal cores of giant Pd clusters, J. Cryst. 

It seems that the material obtained experimentally consists of the giant Pd cluster species the metal core of which is far from the perfect f.c.c. packing. The less stable icosahedral and/or multiply-twinned polyhedra form initially. The more stable f.c.c.-packed cluster metal cores are, however, formed either on heating e.g. under conditions of the electron microscopic experiments) or in the course of chemical transformations such as substitution of OAc ligands under ambient conditions, alfording cluster 7. 

Examination of the idealized models for giant Pd cluster 6 showed that because of steric hindrance only 60-62 phen molecules can be coordinated in a bidentate manner at the outer layer of the 561-atom metal cluster. The OAc ligands constituting the cluster can be arranged mainly as outer-sphere anions. Another... 

Moiseev and coworkers [68, 86] had previously shown that giant Pd clusters (nowadays known as Pd nanoclusters) are good catalysts for the oxidation of alcohol moieties and selectively oxidize allylic C—H bonds in alkenes. More recently, Pd nanoparticles supported on hydroxyapatite [87] were shown to be an excellent catalyst for aerobic alcohol oxidations. 

Kaneda et al. immobilised giant Pd clusters with five shells, Pd56iphen6o(OAc)i8o on the surface of metal oxides, i.e. Ti02 and studied their catalytic application. The authors reported that the immobilised giant Pd clusters (2-3 nm mean particle size) efficiently catalysed the oxidation of primary allylic alcohols in the presence of molecular oxygen with conversion levels higher than 90%. The active sites are Pd-Pd paired sites with an oxidation state smaller than - -2, where the oxidations took place via multiple interactions of these sites with the allylic alcohols. 

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]. 

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]. 

As with the allylic oxidation of olefins (see above) the giant Pd-561 cluster was also found to catalyze benzylic acetoxylation under mild conditions in acetic acid [10]. 

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). 

If the metal atoms are packed like spheres from a central metal, each atom is surrounded by 12 atoms that form the first layer then the second layer contains 42 atoms, etc. In this way, giant clusters are accessible. The n layer contains (10 n -E 2) atoms. For instance, a 5-layer cluster contains 561 atoms. Indeed, Moiseev has characterized [Pd56i(phen)6o(OAc)i8o] whose diameter is about 25 A. Continuing this strategy, Schmid has synthesized Pd clusters up to the 8 layer with the formula Pd2os7(phen)840i6oo- X-ray absorption spectroseopy showed that the Pd-Pd distances are very close to those of Pd metal these giant clusters are nanoparticles. - ... 

Moiseev, I.I., Tsirkov, G.A., Gekhman, A.E., and Vargaftik, M.N., Facile hydrogen- transfer reduction of multiple bonds by formic acid catalysed with a Pd-561 giant cluster, Mendeleev Commun., 7,1-3,1997. 

Kovtun G, Kameneva T, Hladyi S et al (2002) Oxidation, redox disproportionation and chain termination reactions catalysed by the Pd-561 giant cluster. Adv Synth Catal 344(9) 957-964... 

Figure 5. Giant clusters of different magic nuclearities, (Pd the circles corresponding to the diameters of the clusters calculated on the basis of the effective volume of an individual nanocrystal.

The giant clusters could be reproducibly formed starting from Pd561 nanocrystals in water, ethanol and ethanol-water mixtures and from sols with very different concentrations of the nanocrystals. It is possible that the formation of the giant clusters is facilitated by the polymer shell that encases them. Unlike Pd nanocrystals coated with alkanethiols, which self-assemble to form ordered arrays, the polymer shell effectively magnifies the facets of the metallic core, thereby aiding a giant assembly of the nanocrystals. The surface properties of the polymer-coated nanocrystals are clearly more favorable in that the interparticle interaction becomes sufficiently attractive. 

The outer-sphere OAc anions can be replaced by other anions. For instance, the and PF anions readily substitute for OAc anions in an aqueous solution containing KPFft, affording the giant cluster with the idealized formula [Pdsei LeoOeoKPFeleo [Ik 16, 17]. The Pd-561 clusters exhibit a high catalytic activity in alkene acetoxylation in an AcOH solution under mild conditions (20-60 °C at 0.1 MPa). Besides reaction (1), the clusters provide the oxidative acetoxylation of propylene to allyl acetate (eq. (6)) or of toluene to benzyl acetate (eq. (7)). 

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