Carbon atom encapsulated

Figure 24.9 Cluster carbonyls of rhenium containing an encapsulated carbon atom, (a) Octahedral [HjReeC-(C0)i8] . (b) Monocapped octahedral [Re7C(CO)2i] . (c) tran -bicapped octahedral [RegC-(CO)24] . (d) and (e) isomers of [Re7HC(CO)2i] differing in the position of their /r-H atom.

A (Figure 4.9). The diameter of such a neck, 2.3 A, is sufficiently large for a linear C-C chain to pass, but too small to also be an equilibrium adsorption position. The largest compound allowed inside the pores is a linear molecule limited in length to four carbon atoms due to the distance between two subsequent necks [103]. Another example of shape-selective behavior is found in a Zn-based MOF able to encapsulate linear hexane while branched hexanes are blocked [104]. 

It is constituted by a hexaosmium octahedron (encapsulating a carbon atom), four faces of which apically bear a Os(CO)3 group. 

It can be considered to be constituted by two Co6 prisms, each encapsulating a carbon atom, having a common apex (Col) but rotated with respect to each other. The whole assembly is bound, on opposite sides, by a pentacoordinate cobalt atom (Col2 and Col3).ld Clearly, the structural complexity results in inequivalent bond distances. 

Our choice was the two series of dendritic polymers 5 and 6, depicted in Figure 4, which have all their open-shell centers (or trivalent carbon atoms) sterically shielded by an encapsulation with six bulky chlorine atoms in order to increase their life expectancies and thermal and chemical stabilities. Indeed, it is very well known that the monoradical counterpart of both series of polyradicals, the perchlo-rotriphenyl methyl radical, shows an astonishing thermal and chemical stability for which the term of inert free radical was coined. The series of dendrimer polymers 5 and 6 differ in the nature and multiplicity (or branching) of their central core unit, N, as well as in their branch-juncture multiplicities, N Thus, series 5 has a hyperbranched topology with = 3 and = 4, while dendrimer series 6 has a lower level of branching with = 3 and = 2, and the topology of a three-coordinated Cayley tree. 

The structure of 4 (Fig. 5) comprises an open butterfly arrangement of four iron atoms, each bearing three terminal carbonyls (8). The hitherto encapsulated carbon atom is equidistant (1.99 0.03 A) from the metal atoms and is now bonded to the carbomethoxy group. The reaction may be visualized as in Scheme 1. The exposed carbon atom undergoes carbon -carbon bond formation with carbon monoxide to give a ketenylidene group,... 

Fig. 35. Co,3C2(CO)fc, 28, as in its (PhCH2N(CH3)3)+ salt (68). The two encapsulated carbon atoms occupy trigonal prismatic cavities, which share a common vertex [Co(l)]. The two prisms, which are outlined with solid lines for clarity, are rotated with respect to one another (see text), and two further cobalt atoms [Co(4) and Co(5)] assume capping positions. Average Co-Co distance = 2.57 A average Co-C distance = 1.98 A.

The vibrational frequencies of the encapsulated carbon atom in [Osl0C(CO)24]2-, 21 (Fig. 26), and its protonated derivative H2OS 0C(CO)24 have been identified by Oxton et al. (57) and the assignments confirmed by isotopic enrichment. The carbon atom in the tetrahedral Os,0 dianion resides in an octahedral cavity, and at room temperature a band at 753 cm-1 is observed and assigned to Vos.c- On protonation three absorptions of approximately equal intensity are observed in this region and 3C enrichment of the central carbon atom identified these absorptions, at 772.8,... 

In the case of rhodium, however, it was demonstrated early that in the synthesis of [Rh6C(CO)l5]2 the encapsulated carbon atom originated as chloroform, which had reacted with the rhodium carbonyl anion [Rh7(CO)l6]3- (59). In the cobalt analog, [Co6C(CO)l5]2-, the carbon atom is derived indirectly from carbon tetrachloride [via Co3(CO)9CCl] (60) Both these syntheses are performed under mild conditions, and there are apparently no examples of carbidocarbonyl clusters of cobalt or rhodium prepared directly from the metal carbonyls under pyrolysis conditions. 

In clusters with octahedrally encapsulated carbon atoms, the observed values for the radius of the interstitial atom are much smaller. An illuminating comparison is provided by contrasting the optimum carbon radius in [Co8C(CO)l8]2- (0.74 A) with that for Co6C(CO)fr (0.56 A) 63), which has an octahedral core. Indeed all the octahedral carbido clusters have similarly compressed interstitial cavities (Table IV) and as has been pointed out previously, this fact militates against invoking minimum steric requirements of the carbon atom to explain distortions in octahedral carbido clusters. 

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]. 

Fig. 15 Three-dimensional view of Nitschke s tetrahedral assembly for the protective encapsulation of P4. Iron atoms are drawn in purple, carbon atoms gray, nitrogen atoms blue, and phosphorous atoms are orange. The sulfonate groups, which help solubilize the assembly in water, are yellow and red [24], Reprinted with permission from AAAS...

The skeleton of W4(/Lt4-C)(0)(0H2Bu )i2 is given in 7-XII. The carbon atom is frequently found as a discrete C4- unit encapsulated in scores of clusters of metals such as iron, ruthenium, osmium, cobalt, rhodium, nickel, and rhenium but a few contain encapsulated Q units (see later). 

Rhodium also forms an unusually large number of clusters containing encapsulated hetero atoms as, for example, the [Rhn(S)2(CO)32]3 species whose skeletal structure is shown in Fig. 16-15, and a number that contain carbon atoms. It also forms both smaller, for example, Rh CO), and larger, for example, Rh H CO) clusters that have no encapsulated atoms. The Rh12 species has a very novel structure in which two Rl octahedra are linked together to form an Rh6 octahedron between them. Finally, rhodium also forms many mixed metal clusters, especially with plati-... 

Using DFT to optimise fully these pre-optimised cluster structures is computationally expensive, even using the rather low C2 symmetry. For this particular cluster, in addition to its relatively large size and complexity, another reason for the high computational expense in the DFT optimisations came from an unexpected large structural relaxation of the ruthenium-encapsulated carbon atoms, which escape to the surface of the cluster upon optimisation. This structural change could not be... 

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