Rhodium-platinum cluster

Harada, M., Asakura K., and Toshima, N., Structural analysis of polymer-protected platinum/rhodium bimetallic clusters using extended x-ray absorption fine structure spectroscopy. Importance of microclusters for the formation of bimetallic clusters, J. Phys. Chem., 98, 2653, 1994. 

There are few reports of reactions between alkynes and trinuclear clusters of metals other than iron, ruthenium, or osmium. Some rhodium, platinum, and mixed-metal clusters undergo metal-metal bond rupture in reactions with alkynes (54-56), while in other cases the alkyne coordinates to the trinuclear unit without causing any major changes in framework geometry (56-59), as illustrated in Eq. (3). 

The selective production of methanol and of ethanol by carbon monoxide hydrogenation involving pyrolysed rhodium carbonyl clusters supported on basic or amphoteric oxides, respectively, has been discussed. The nature of the support clearly plays the major role in influencing the ratio of oxygenated products to hydrocarbon products, whereas the nuclearity and charge of the starting rhodium cluster compound are of minor importance. Ichikawa has now extended this work to a study of (CO 4- Hj) reactions in the presence of alkenes and to reactions over catalysts derived from platinum and iridium clusters. Rhodium, bimetallic Rh-Co, and cobalt carbonyl clusters supported on zinc oxide and other basic oxides are active catalysts for the hydro-formylation of ethene and propene at one atm and 90-180°C. Various rhodium carbonyl cluster precursors have been used catalytic activities at about 160vary in the order Rh4(CO)i2 > Rh6(CO)ig > [Rh7(CO)i6] >... 

Chemistry similar to that described above for iridium clusters has also been observed for rhodium clusters. Several authors [16-18] have prepared [Rh6(CO)i6] in NaY zeolite [R1i4(CO)i2] has also been formed [18], and each of these has been decarbonylated with minimal changes in the metal frame, as shown by EXAFS spectroscopy [18]. Thus there appears to be some generality to the method of forming small clusters in zeolite cages by synthesis of stable metal carbonyl precursors followed by decarbonylation. However, the method is limited. Attempts to use it to prepare zeolite-supported platinum clusters that are structurally simple and uniform have apparently not been successfiil. The literature of platinum carbonyl clusters in zeolites is not considered here because it is still contradictory. 

In a similar vein, Sakai, Takenaka and Torikai produced Nafion/metal microcomposites by reducing palladium, rhodium, platinum and silver ion-exchanged membranes with H, at high temperatures (100-300°C) (26). The affected metal clusters, distributed homogeneously, were about 50 A in size. As in Risen s work, a polymer matrix template effect appears to be operative. These systems were considered and evaluated within the context of gas separations technology. 

Some rhodium and platinum clusters exhibit dynamic properties, i.e., skeletal fluxionality. In the RhNMR of [Rh9P(CO)2i] , which has a capped antiprism structure, three signals possessing the intensity ratio 4 4 1 are observed at 183 K, while at 298 K there is only one signal. This experiment shows that, due to fluxionality of the skeleton, all rhodium atoms becomes equivalent. The same is indicated also by the presence of a decet in the PNMR spectrum of this compound. Similarly, [RhioS(CO)22] and [RhioE(CO)22] (E = P, As) are rigid at low temperatures but fluxional at high temperatures. 

The synthesis and characterisation of the platininn-containii species (PPh3)HPt(/i-PPh2XM"CO)M(CO)4 (M = Cr, Mo, W) has been reported by Shao et al The range of rhenium-platinum clusters has been extended by the preparationof the complex [Re3Pt(/L(-H)3(CO)i4]. Osmium-rhodium complexes are also mentioned ... 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

Figure 4.28. STM image of a PtRh(lOO) surface. Although the bulk contains equal amounts of each element, the surface consists of 69% of platinum (dark) and 31 % of rhodium (bright), in agreement with the expected surface segregation of platinum on clean Pt-Rh alloys in ultrahigh vacuum. The black spots are due to carbon impurities. It is seen that platinum and rhodium have a tendency to cluster in small groups of the same elements.

Because of- the similarity in the backscattering properties of platinum and iridium, we were not able to distinguish between neighboring platinum and iridium atoms in the analysis of the EXAFS associated with either component of platinum-iridium alloys or clusters. In this respect, the situation is very different from that for systems like ruthenium-copper, osmium-copper, or rhodium-copper. Therefore, we concentrated on the determination of interatomic distances. To obtain accurate values of interatomic distances, it is necessary to have precise information on phase shifts. For the platinum-iridium system, there is no problem in this regard, since the phase shifts of platinum and iridium are not very different. Hence the uncertainty in the phase shift of a platinum-iridium atom pair is very small. 

Recently we reported EXAFS results on bimetallic clusters of iridium and rhodium, supported on silica and on alumina (15). The components of this system both possess the fee structure in Efie metallic state, as do the components of the platinum-iridium system. The nearest neighbor interatomic distances in metallic iridium and rhodium are not very different (2.714A vs. 2.690A). From the results of the EXAFS measurements, we concluded that the interatomic distances corresponding to the various atomic pairs (i.e., iridium-iridium, rhodium-rhodium, and iridium-rhodium) in the clusters supported on either silica or alumina were equal within experimental error. Since the Interatomic distances of the pure metals differ by only 0.024A, the conclusion is not surprising. 

Iron-platinum carbonyl clusters, characteristics, 8, 415 Iron reagents, in Kumada-Tamao-Corriu reaction, 11, 21 Iron-rhodium clusters, as heterogeneous catalyst precursors,... 

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