Since their first detection and bulk production, the fullerenes have
gained a primary role on the scientific scene, reaching their climax when
the 1996 Nobel Prize for Chemistry was awarded to Kroto, Curl and Smalley
for their seminal discovery. The unique physical and chemical properties
of these new forms of carbon led many scientists to predict several
technological applications. This created a heavy disappointment when it
was clear that fullerene-based materials would not soon be ready for the
market. However, the fullerenes have so far delighted several dozens of
researchers who found that C
 60
 and its relatives undergo a
variety of chemical reactions. In most cases, the new derivatives retain
the main properties of the original fullerene, and it is now not unlikely
that some functionalized fullerenes may find useful applications in the
field of materials science and technology. In this Article we summarize
the basic principles of the organic chemistry of fullerenes, together with
a description of the physicochemical properties that have made these
carbon cages popular in materials science, and review the most recent
achievements in the functionalization of fullerenes aimed at the
production of new molecular materials.

[60]Fullerene chemistry for materials science applications

Initially envisaged as rather unreactive, aromatic-like molecules, the fullerenes instead undergo a wide variety of reactions characteristic of alkenes. The many derivatives of C60, and the few of C70, that have now been reported offer new directions for organic chemistry.

The Chemistry of Fullerenes

M@C(60) and related endohedral metallofullerenes comprise a significant portion of the metallofullerene yield in the traditional arc synthesis, but their chemistry and potential applications have been largely overlooked because of their sparse solubility. In this work, procedures are described to solublize Gd@C(60) species for the first time by forming the derivative, Gd@C(60)[C(COOCH(2)CH(3))(2)](10), and its hydrolyzed water-soluble form, Gd@C(60)[C(COOH)(2)](10). Imparting water solubility to Gd@C(60) permits its evaluation as a magnetic resonance imaging (MRI) contrast agent. Relaxometry measurements for Gd@C(60)[C(COOH)(2)](10) reveal it to possess a relaxivity (4.6 mM(-1) s(-1) at 20 MHz and 40 degrees C) comparable to that of commercially available Gd(III) chelate-based MRI agents. An in vivo MRI biodistribution study in a rodent model reveals Gd@C(60)[C(COOH)(2)](10) to possess the first non-reticuloendothelial system (RES) localizing behavior for a water-soluble endohedral metallofullerene species, consistent with its lack of intermolecular aggregation in solution as determined by light-scattering measurements. This first derivatization and use of a M@C(60) species suggests new potential for metallofullerene technologies by reducing reliance on the chromatographic purification procedures normally employed for the far less abundant M@C(82) and related endohedrals. The recognition that water-soluble fullerene derivatives can be designed to avoid high levels of RES uptake is an important step toward fullerene-based pharmaceutical development.

First soluble M@C60 derivatives provide enhanced access to metallofullerenes and permit in vivo evaluation of Gd@C60[C(COOH)2]10 as a MRI contrast agent.

Radical reactions of fullerenes: from synthetic organic chemistry to materials science and biology.

A rapid and inexpensive method for the large-scale purification of C60 is the simple filtration of the toluenesoluble extract of commercial fullerene soot through a short plug of charcoal/silica gel with toluene as the eluent. Reactions of C60 with ethyl and tert-butyl diazoacetates in refluxing toluene lead to the formation of the (alkoxycarbonyl)methylene-bridged isomers 1a–3a and 1b–3b, respectively, which can be equilibrated, upon further heating, into the single compounds 1a and 1b, respectively. Isomers 1a/b possess the methano bridge at the 6–6 ring junction, whereas structures 2a/b and 3a/b are bridged at the 6–5 junction. A dramatic influence of local and π-ring current anisotropic effects of the fullerene sphere on the NMR chemical shifts of the methine protons in the bridge is observed: the chemical shifts of the protons located over a pentagon ring in 2a/b and over a hexagon ring in 3a/b differ by Δδ = 3.47 and 3.45 ppm, respectively. The analysis of the 13C-NMR chemical shifts of the bridgehead C-atoms and the 1J(C,H) coupling constants for the methano-bridge atoms reveals conclusively that the 6-5-ring-bridged structures 2a/2b and 3a/3b are π-homoaromatic (‘open’ transannular bond) and the 6-6-ring-bridged structures 1a/b are π-homoaromatic (‘closed’ transannular bond). The electronic absorption spectra show that π-homoconjugation in 2a/b and 3a/b represents a much smaller electronic perturbation of the original C60 chromophore than σ-homoconjugation in 1a/b. The results of this study demonstrate an impressive linkage between the chemistry of methano-bridged annulenes and methano-bridged fullerenes.

Improved purification of C60 and formation of σ- and π-homoaromatic methano-bridged fullerenes by reaction with alkyl diazoacetates

Benzyne reacts with C 60 to form a series of compounds [C 60 +(C 6 H 4 ) n ], n=1,2,3,4, identified by mass spectrometry; the monoaddition product was isolated by chromatography and structurally characterized by 1 H- and 13 C-NMR spectroscopies

Reaction of fullerenes and benzyne

The fullerences C60 and C70 form adducts with toluene, p-xylene and other aromatic compounds when refluxed in these solvents in the presences of FeCl3. Benzene derivatives are produced only when bromine is also present. Mass spectra of the reaction mixtures indicate the presence of derivatives including 1:1 adducts, as well as products of multiple additions of the aromatic molecules and their dehydrogenation products. Tandem mass spectrometry provides evidence for CC bonds formation between the adducts and fullerences.

Aromatic hydrocarbon derivatives of fullerences

Fullerene derivatives of naphthalene have previously been synthesized by adding 1,8-naphthyne to the fullerene. The authors synthesized these compounds by pyrolytically decomposing 1,8-diiodonaphthalene in the presence of C{sup 60}.

Jay Molstad

Papers

Reaction of fullerenes and benzyne

Aromatic hydrocarbon derivatives of fullerences

Pyrolytic Syntheses of a C60 Derivative of Naphthalene and Some Congeners

Pyrolytic and gas phase fullerene derivatization reactions

Reaction of Fullerenes and Benzyne.