Encaged clusters

Chen, N., Zhang, E.Y., Tan, K. et al. (2007) Size effect of encaged clusters on the exohedral chemistry of endohedral fullerenes a case study on the pyrrolidino reaction of ScjGd3 xN Cgo (x = 0-3). Organic Letters, 9, 2011-2013. 

In the pages that follow, we summarize methods for the synthesis of clusters in cages, their structural characterization, reactivity, and catalytic and other properties. The literature of encaged clusters is limited to clusters in zeolites, and thus little is included here about other molecular sieves or potential cluster hosts. The literature dtations are not comprehensive rather, examples are dted to illustrate prindples and to emphasize the unique properties of encaged clusters and the... 

The encaged clusters considered in this chapter are almost exclusively metal carbonyls and metals (including bimetallics). llte encaged metal carbonyls that have been most investigated include [Rh (CO)i6], [Ir4(CO)i2], and the isomers of [Ir (CO),d the crystal structures of the iridium clusters are shown in Figure 4-6. Some of the most thoroughly characterized encaged metal dusters have been made from these metal carbonyls. Brief mention is made of metal oxide and also nonmetal clusters ionic dusters are scarcely considered. Synthesis, characterization, reactivity, and catalytic and other properties are considered for these materials. 

Ikble 4-3. Techniques that have been used to characterize encaged clusters. 

The literature includes only a few examples of zeolite encaged clusters which have been characterized with EXAFS spectroscopy with a thorough analysis of the data. Examples of relatively thorough data analysis are those for clusters inferred to be triosmium carbonyls [53] in NaNj-treated NaY zeolite, [Ir4(CO)i2] [67] and [Irg(CO)i6] [5] in NaY zeolite, and [Ir6(CO)i5] [60] in NaX zeolite. 

These results also illustrate the advantages of using zeolite encaged dusters as precursors for well defined metal catalysts. Some encaged clusters appear to be stable to cycling through oxidation and reduction without forming crystallites on the external zeolite surface. 

There has been a heightened interest in catalysis by encaged clusters in recent... 

Metal NMR spectroscopy is also beginning to gain favor as a technique for the characterization of encaged clusters. [156] For example, Zhang et al. [157] used Co spin echo NMR spectroscopy to characterize the size and location of the Co dusters in the cages of NaY zeolite. 

Intrazeolitic MOCVD chemistry is relatively new. A synthetic chemistry for the preparation of encaged clusters from metd alkyl precursors (alkyl is often methyl) has been suggested by a few researchers. [220, 221] In the first step, a vapor phase precursor is introduced into the zeolite supercages. One of the methyl groups can then react with a Brpnsted add site, thereby produdng CH4 and a concurrent anchoring of the organometallic precursor to the zeolite internal surface (Eq. 4.16, Z represents the zeolite framework). 

Gas hydrates are an ice-like material which is constituted of methane molecules encaged in a cluster of water molecules and held together by hydrogen bonds. This material occurs in large underground deposits found beneath the ocean floor on continental margins and in places north of the arctic circle such as Siberia. It is estimated that gas hydrate deposits contain twice as much carbon as all other fossil fuels on earth. This source, if proven feasible for recovery, could be a future energy as well as chemical source for petrochemicals. 

Figure 29. Water clusters with H30+ or metal ions (depicted by a shaded circle) encaged inside the clathrate (bfeOho. Taken with permission from Int. J. Mass Spectrom. Ion Proc. 1994, 131, 233-264.

X. L. Bai and W. M. H. Sachtler, Methylcyclopentane conversion catalysis by zeolite encaged palladium clusters and palladium-proton adducts, J. Catal. 129, 121-129 (1991). 

Co2(CO)g has been used to obtain encapsulated cobalt clusters in Y-faujasite, which have been used as model catalysts for methane homologation [152]. The gas phase adsorption of Co2(CO)8 under N2 rendered predominately encaged Co4(CO)i2 species whereas Co,s(CO)iis was obtained when the impregnation of Co2(CO)8 was carried out under a CO/H2 atmosphere [152, 155], Samples were oxidized at 80°C, subsequently reduced at 400 °C and then structurally characterized by EXAFS. Clusters of two and three cobalt atoms were formed from encaged Co4(CO)i2 and COis(CO)iis, respectively. Higher methane conversion and selectivity to C2+ products in the CH4 homologation reaction have been obtained for the two atoms-size cluster sample the results were discussed using a DFT model [152]. 

Up to 1999, only metal atoms [1-5], metal clusters [6,7], metal nitrides [55-57], and noble gas atoms [58-60] were observed to be encaged inside C60, C70, or various sizes of higher fullerenes. The experimental evidence for carbon atoms or metal-carbon compounds (carbides) being encapsulated inside fullerenes had not yet been observed. In 2000, Shinohara et al. succeeded in the first production, isolation, and spectroscopic characterization of a scandium carbide endohedral fullerene (Sc2C2) C84. Following this, the first experimental evidence based on synchrotron X-ray diffraction was presented and revealed that the Sc carbide is encapsulated in the form of a lozenge-shaped Sc2C2 cluster inside the D2d-C84 fullerene [8]. 

As clarified later, such a classification is reasonable and useful because electron transfers exist from the encaged metallic species to the fullerene cages so that the structures and properties depend strongly on the encapsulated atom(s). Particularly, cluster metallofullerenes show different properties from those containing only metals (mono-metallofullerenes and di-metallofullerenes), which, in return, strongly affects the synthesis and extraction processes, structures, chemical reactivities, and their applications. Consequently, we must, to a certain degree, address cluster metallofullerenes separately in the following text. 

With the confinement of CNTs, direct observations of the dynamic motions of EMFs and the encaged metallic species were also achieved using high-resolution transmission electronic microscopy (HRTEM). Figure 7.18 shows that the motions of the fullerene cage and the encaged ErsN cluster are clearly resolved [185],... 

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