Inorganic Chemistry of Fullerenes

Like in most other cases too, it is hard to distinguish inorganic from organic chemistry of fullerenes. This section primarily deals with aspects of structural chemistry, whereas reactions with metahorganic agents to organic derivatives of

Crosslinked Fullerene Film
Figure 2.42 Examples of organometallic fullerene complexes with one metal center (a), two metal centers arranged at opposite sites (b) and neighbouring sites (c) and a complex comprising three metal centers in close proximity (d).

fullerenes will be discussed in the Section 2.5.5 on organic chemistry of fullerenes (Section 2.5.5).

Atoms of transition metals can coordinate the olefinic (6,6)-bonds present in C60 as these are already positioned suitably to complexation due to the curvature of the fullerene surface. Usually, the n2 - complexes typical also of other olefins are formed. The coordination to the metal center causes an extension of the respective (6,6)-bond and consequently reduces the fullerene's symmetry. At the same time, the affected carbon atoms' degree of hybridization is shifted further toward sp3 . Figure 2.42 collects some examples that exhibit in some cases totally different stabilities. The complex (n2-C60)Mo(CO)3(dppe), for instance (Figure 2.42a), is fairly inert and disintegrates upon strong heating only, while other compounds decompose already at room temperature.

n2-coordinated complexes are formed in particular with metals known for their affinity toward electron-deficient olefins, that is, with elements from subgroups 6-8 like Pt, Pd, Fe, Co, Ni, Ir, Rh, Ru, Os, Mn, and Re as well as Ta, Mo, and W. Titanium may also form olefin complexes with fullerenes. Especially the low-valent complexes of the aforementioned metals readily incorporate olefins in their coordination sphere.

With certain complexes it is also possible to obtain multiple coordination of the fullerene. One way to achieve this is the application of a large excess of the metal complex to be coordinated, for example, in the preparation of (n2-C60)[Ir(CO) Cl(PPh3)2]2 - Due to the size of the ligands the two iridium centers on the C60 are situated in trans- 1-position toward each other in this complex (Figure 2.42b). Furthermore succeeds the generation of bridged complexes with C60. These may contain metal-metal bonds or bridging ligands. The structure of such complexes causes a close proximity of the transition metal atoms. An example is depicted in Figure 2.42c. The red complex Ru3(CO)9(|3-n2, n2, n2-C60) represents an extreme, as all double bonds of one single six-membered ring are engaged in the coordination of the three metal centers (Figure 2.42d). This species should be distinguished from the n6-complexes, which C60 does not form for its structure containing localized double bonds and for the associated low aromaticity of its six- membered rings. This may as well be deduced from considering the orbitals: their overlap does not suffice for n6-coordination due to their radial orientation on the surface, which causes an ever growing distance of orbitals toward the outside. In a metal-lacyclopropane at n2-complexation of C60- on the other hand, the overlap is even stronger than in the reference molecule benzene.

The complex Tl[C60Ph5- represents a particularity as the thallium cation is n5-bound in this species. A five-membered ring entirely surrounded by phenyl groups serves as a coordinating unit (Figure 2.43).

These kinds of metal complexes with singly or multiply coordinating fullerenes are known as well for the higher homologs of C60. The structural variation, however, is much wider and more intricate due to their lower symmetry. C70 for instance possesses four kinds of olefinic (6,6)-bonds. The shortest of them, that is, the ones


Figure 2.43 n5-Tl[Q0Ph5]-a complex with fivefold coordination of the thallium atom which is n5-complexed by the cyclopentadienyl structure.

Ph with the most distinct double bond character, are situated at the polar caps of the carbon cage. In even higher fullerenes, a large number of possible isomers complicate the situation even more in addition to the existence of different olefinic bonds. Complexation on these fullerenes inevitably yields a multitude of structural isomers.

Inorganic compounds may also enter into o-additions with fullerenes. Osmium tetroxide in the presence of pyridine, for example, adds to a (6,6)- bond of C6 0 forming two carbon-oxygen bonds (Figure 2.31). The stoichiometry of this reaction can be controlled by the amount of osmium tetroxide added. With an excess of OsO4, the major product is the bisadduct that is obtained as a mixture of five out of the eight possible regio-isomers. These might be separated by HPLC.

In the presence of 4-tert-butylpyridine, an analogous adduct bearing additional ligands on the metallic centers is obtained. This substance has been the first derivative of C60 to be examined by X-ray analysis, thus confirming the cage-like structure of the fullerene molecule (Figure 2.31). The X-ray structure also reveals that the carbon atoms carrying the addend are slightly extricated from the fullerene cage, indicating a higher degree of pyramidalization.

C60 also participates in addition reactions with other inorganic compounds. The reaction with di-rhenium decacarbonyl, for instance, yields the respective 1,4-bisad-duct of Re(CO)5 (refer to Section, and with lithium alkyls the products of a 1,2-addition are obtained. With larger residues, however, the reaction no longer occurs in 1,2-position, but at more distant sites on the carbon cage. For example, C60(Si'Bu(Ph)2)2 is a 1,6-adduct (Figure 2.44a).

Fullerenes possess a distinct ability to form intercalation compounds with other substances, especially with solvents. From benzene for instance, C-0 crystallizes as a compound C60-4 (C6H6). Similar crystallizations are known for further solvents like cyclohexane or CCl4.

Figure 2.44 (a) Product of the 1,6-addition of a silicon compound to C60; (b) co-crystallizate of C70 and ferrocene. An attack by the latter does not occur as its reduction power is too low (© Elsevier 1999).

Figure 2.44 (a) Product of the 1,6-addition of a silicon compound to C60; (b) co-crystallizate of C70 and ferrocene. An attack by the latter does not occur as its reduction power is too low (© Elsevier 1999).

Figure 2.45 (a) Inclusion compounds of C70 with sulfur. The enclosure of the fullerene cage by the sulfur crowns is evident (© Neue Schweiz. Chem. Ges. 1993); (b) ternary cocrystal-lizate (© ACS 1996).

Figure 2.45 (a) Inclusion compounds of C70 with sulfur. The enclosure of the fullerene cage by the sulfur crowns is evident (© Neue Schweiz. Chem. Ges. 1993); (b) ternary cocrystal-lizate (© ACS 1996).

Contrasting many other substances however, fullerenes may also cocrystallize with molecules that cannot be numbered among the solvents, like hydroquinone, ferrocene or elemental phosphorus, to name some well-known examples. Although ferrocene is a reducing agent, its reducing power does not suffice to generate C-0 or C-0, respectively. Upon crystallization, they form substances of the type C60-2 [n5-C5H5)2Fe] (Figure 2.44b). On cocrystallization with elemental phosphorus, compounds with a composition of C60-2 P4 are obtained. Here the phosphorous tetrahedrons are situated in the gaps of the C60-lattice with a tetrahedral face oriented in parallel with a six-membered ring of an adjacent fullerene molecule. Slowly evaporating solutions of C60 in CS2 yields a cocrystallizate of the fullerene with elemental sulfur S8. The same is true for C70 and C76. The respective complexes, for example, C-o-6 S8. feature a fullerene entirely surrounded by sulfur molecules (Figure 2.45 a).

Even inorganic complexes like (PhCN)2PdCl2 react with C60 forming intercalation compounds. In this case a black, crystalline substance with the composition C60-2 (Pd6Cl12) -2.5 C6H6 is obtained. The three components C60, benzene, and Pd6 Cl12 (a cubic structure with the Pd- atoms on the faces and the Cl- atoms on the centers of the edges) join in a common crystal lattice (Figure 2.45b). Furthermore, a reaction of C-o with cobalt porphyrines has been described. In the products the fullerene is quite close to the center of the porphyrine system, but still too far away to call it a real chemical bond. In solid state the planar porphyrines and the fullerene cages stack over each other to form long, columnar aggregates.

Fagan studied the reaction of C60 with the complex [Cp*Ru(CH3CN)3] and found that it did not lose all three molecules of acetonitrile to form a n6-complex as observed for aromatic systems, but rather gave a n2-olefin complex by releasing just one ligand of acetonitrile. With some sufficiently nucleophilic hydrido complexes, C60 does not yield n2-complexes, but a hydrometallation occurs instead. The zirconocene complex Cp2Zr(H)Cl reacts with C60 to give a red solution containing Cp2ZrCl(C60H). From the acidic hydrolysis of this complex, isomerically pure 1,2-dihydro-C60 can be obtained (refer to Section

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