Cluster compounds Encapsulated atoms

N NMR spectroscopy has been used extensively in the study of dinitrogen complexes, and a large body of data now exists on these compounds (143). It has also been used to study reactive intermediates Attempts, so far unsuccessful, have been made to identify the unknown ion [HN2]+ in the diazotization of NH3 with H N02 (144). The cyclic anion [CNy] and the isomers of [HCN7] formed in its protonation have been examined (145). Cluster compounds containing encapsulated nitrogen atoms (146) and mononuclear nitro complexes have also been studied (147) by both N and N NMR spectroscopy. The latter show a range of shifts from 66 to 174 ppm. 

It is clear that in the past few years a broad and diverse chemistry of interstitial cluster compounds has developed. Synthetic routes have been established and some clues to the functions of the encapsulated atoms have emerged. The stereochemical... 

For many clusters, the number of valence electrons may be increased as a result of formation of encapsulated compounds containing H, C, N, P, Sb, and S in the interstices between metal atoms (Table 3.12). Recently electron poor clusters containing encapsulated metal atoms such as K and Be and atoms of other elements of the second... 

Metal carbonyl carbide cluster compounds may be formed either by the carbon-oxygen bond breaking of the CO molecule or by the decomposition of an organic compound present in the reaction mixture. Chloroform is a convenient source of the encapsulated carbon atom because in this case the synthesis may be carried out under mild conditions ... 

Cobalt and rhodium also form a series of high nuclearity carbido-cluster compounds. Contrasting with carbido compounds of the iron group which have a clear tendency to put the carbide atom in octahedral cavities, carbides of the group 9 often place it in trigonal prismatic cavities. As shown in scheme in Fig. 3.12, the parent compound [Co6(CO)i5C] of a series of encapsulated car-bido-cobalt species may be prepared by the reaction of Co3(CO)9CCl with [Co(CO)4]. The same scheme also describes some carbido-cobalt cluster interconversions. Rhodium carbide clusters are in general similar to the cobalt ones. 

A number of fullerene compounds have been developed that have unusual chemical, physical, and biological characteristics, and are being used or have the potential to be used in a host of new applications. Some of these compounds involve atoms or groups of atoms that are encapsulated within the cage of carbon atoms (and are termed endo-hedral fullerenes). For other compounds, atoms, ions, or clusters of atoms are attached to the outside of the fullerene shell (exohedral fullerenes). 

In the field of carbonyl clusters it is not rare to find compounds in which atoms such as C, N and P become trapped in interstitial or semi-interstitial positions inside the metal cage. We will briefly consider this type of compound in order of increasing encapsulation of the interstitial atom. 

X-ray structure of the mesitylene derivative was reported shortly afterward.11 This represented the second structurally characterized cluster containing an interstitial atom [the structure of FesC(CO)i5 having already been established]12 and the first example of a cluster with a completely encapsulated carbide atom. At the time that the synthesis of 2 was first reported, another paper described the synthesis of a cluster also obtained from 3 when heated to 150°C in either benzene or cyclohexane. Based on an estimation of the mass of this compound from a differential vapor pressure measurement, the authors suggested that this compound corresponded to Ru6(CO)18.13 It was subsequently noted from a comparison of vco IR data and a structural determination that this compound was in fact 2. 

The carbide atom in 1 is located in the center of the square face such that it is partially exposed whereas the carbide atom in 2 is completely encapsulated by the six ruthenium atoms. From a spectroscopic viewpoint, carbide atoms are very distinctive and the earlier reviews have dealt with these aspects in detail.7 8 The IR spectrum of 1 contains peaks at 701 (s) and 670(m) cm 1, and 2 contains peaks at 717(sh), 703(s), 680(m), and 669(m) cm-1.22 I3C-NMR spectra of 1 and 2 do not appear to have been reported. This is probably due to the low yields in which these compounds were initially obtained at a time when, 3C-NMR was still not in widespread use in cluster chemistry. In general, the 13C-NMR resonance of carbide atoms ranges from 8 250 to 500. The high frequency resonances exhibited in 13C-NMR spectra reflect the different diamagnetic and paramagnetic effects experienced by a nucleus in such an unusual chemical environment.23... 

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]. 

Many compounds have been synthesized, often fortuitously, in which one or more atoms have been partially or completely encapsulated within metal clusters. The most common of these cases have been the carbide clusters, with carbon exhibiting coordination numbers and geometries not found in classic organic structures. Examples of these unusual coordination geometries are shown in Figure 15-22. 

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