Metal frameworks

Figure 25.11 Metal frameworks of some high-nuclearity binary carbonyl and carbonylate clusters of osmium (a) Os5(CO)i6 (trigonal bipyramid) (b) Os6(CO)ig (bicapped tetrahedron, or capped trigonal bipyramid) (c) [Os6(CO)ig] (octahedron) (d) Os7(CO)2i (capped octahedron) (e) [Osg(CO)22] (bicapped octahedron) (f) [Osi7(CO)36] (3 shaded atoms cap an Osu trigonal bipyramid).

Compounds isotypic with the k phases arc found among intcrmetallics, borides, carbides and oxides and also with silicides, germanides, arsenides, sulfides and sclcnides no nitrides, however, are found. The mode of filling the various voids in the metal host lattice of the k phases follows the schemein Ref. 4 and is presented in Table 1 for all those compounds for which the atom distribution is well known from x-ray or neutron diffraction. Accordingly, B atoms in tc-borides, Zr, Mo, W, Re)4B and Hfy(Mo, W, Re, Os)4B , occupy the trigonal prismatic interstices within the parent metal framework of a Mn, Aln,-type structure (see Table 1 see also ref. 48). Extended solid solutions are found for (Hf, Al)[Pg.140]

Considering the mode of filling the voids in the metal framework of rj phases with the Ti2Ni type (see Ref. 1) (Table 1), Re3Al2B is the only boride member of this group, with B atoms entering the large icosahedral center in 16d, occupied by metal... 

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

A novel series of Au—Ag supraclusters whose metal frameworks are based on vertex-sharing polyicosahedron have been reported. In the structure of these compounds the basic building block... 

Another interesting idea that has been explored without much success so far is the use of clusters with a chiral metal framework as catalysts for asymmetric hydrogenation, since only the intact cluster would induce enantioselectivity. 

Reaction with sulfur ligands leads to rupture of the metal framework, and the structure of one of the compounds isolated is shown in Fig. 51. The reaction of the parent carbonyl with the ion [HOs3(CO)a] ... 

In many studies in which carbonyl compounds have been used as precursors in the preparation of catalysts there is no straightforward characterization in terms of the number of metallic atoms in the supported metaUic entities, there being uncertainties about true structural considerations. Analysis of the catalytic behavior is interpreted mainly in the Ught of electron microscopy analysis, and indirect characterization methods, such as infrared (IR) spectroscopic analysis of (de)carbonylation of the metal framework, and so on. 

The accessibility of new techniques such as EXAFS brings researchers a powerful tool for unambiguous determination of the true core metallic framework of such systems. Thus, the relationship between the parent carbonyl precursor, the support and the final metal-supported particles has been studied at the structural atomic level in some cases. This can allow differentiation of the catalytic behavior of supported metal particles with bulk-like properties from that of supported metal clusters, opening the way to understanding the mechanism of metal-catalyzed reactions and extending the concept of sensitive or insensitive structure reactions from metal aggregates to clusters. 

Other works related to the use of platinum carbonyl species in the preparation of catalysts, which lie beyond the scope of this chapter, refer to the synthesis in situ of supported platinum carbonylate species. For this purpose, the impregnation or exchanging of Pt precursors as [Pt(NH3)4], [Ptcy and [PtCLt] " on an appropriate support is carried out. Then, the carbonylation renders a carbonylate species that could be naked to metal frameworks by appropriate decarbonylation. 

Ir4(CO)i2 reacts with the surface of MgO to generate surface species in which the tetrahedral metal framework is preserved. The structures obtained after decar-bonylation under H2 at 573 K depend on the degree of hydroxylation of the support The iridium cluster nuclearity of 4 was maintained for a low degree of MgO hydroxylation (MgO pretreated at 973 K), but it increased to 6 when the MgO was highly hydroxylated (MgO pretreated at 573 K) [206, 207]. The activity in propane hydrogenolysis of the tailored catalyst is two orders of magnitude less than that of the conventional catalyst at atmospheric pressure and 200 °C. 

The most outstanding examples of heteronuclear gold clusters are the series of Au-Ag supraclusters whose metal frameworks are based on vertex-sharing. In the structure of these compounds, the basic building block is the 13-metal atom (Au7Ag6) icosahedra. These high nuclearity clusters have been termed clusters of clusters and they follow a well-defined growth sequence by successive additions of... 

---