Buckminsterfullerene

Buckminsterfullerene (or bucky-ball) is a spherical fullerene molecule with the formula C₆₀. It has a cage-like fused-ring structure (truncated icosahedron) which resembles a soccer ball, made of twenty hexagons and twelve pentagons, with a carbon atom at each vertex of each polygon and a bond along each polygon edge.

It was first generated in 1985 by Harold Kroto, James R. Heath, Sean O'Brien, Robert Curl, and Richard Smalley at Rice University. Kroto, Curl and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of buckminsterfullerene and the related class of molecules, the fullerenes. The name is a reference to Buckminster Fuller, as C₆₀ resembles his trademark geodesic domes. Buckminsterfullerene is the most common naturally occurring fullerene molecule, as it can be found in small quantities in soot. Solid and gaseous forms of the molecule have been detected in deep space.

Buckminsterfullerene is one of the largest objects to have been shown to exhibit waveâ€"particle duality; as stated in the theory every object exhibits this behavior. Its discovery led to the exploration of a new field of chemistry, involving the study of fullerenes.

Etymology

Buckminsterfullerene derives from the name of the noted futurist and inventor Buckminster Fuller. One of his designs of a geodesic dome structure bears great resemblance to C₆₀; as a result, the discoverers of the allotrope named the newfound molecule after him. The general public, however, sometimes refers to buckminsterfullerene, and even Mr. Fuller's dome structure, as buckyballs.

History

The structure associated with fullerenes was described by Leonardo da Vinci. Albrecht Dürer also reproduced a similar icosahedron containing 12 pentagonal and 20 hexagonal faces but there are no clear documentations of this.

Discovery

Theoretical predictions of buckyball molecules appeared in the late 1960s â€" early 1970s, but they went largely unnoticed. In the early 1970s, the chemistry of unsaturated carbon configurations was studied by a group at the University of Sussex, led by Harry Kroto and David Walton. In the 1980s a technique was developed by Richard Smalley and Bob Curl at Rice University, Texas to isolate these substances. They used laser vaporization of a suitable target to produce clusters of atoms. Kroto realized that by using a graphite target, any carbon chains formed could be studied. Another interesting fact is that, at the same time, astrophysicists were working along with spectroscopists to study infrared emissions from giant red carbon stars. Smalley and team were able to use a laser vaporization technique to create carbon clusters which could potentially emit infrared at the same wavelength as had been emitted by the red carbon star. Hence, the inspiration came to Smalley and team to use the laser technique on graphite to create the first fullerene molecule.

C₆₀ was discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley. Using laser evaporation of graphite they found C_n clusters (where n>20 and even) of which the most common were C₆₀ and C₇₀. A solid rotating graphite disk was used as the surface from which carbon was vaporized using a laser beam creating hot plasma that was then passed through a stream of high-density helium gas. The carbon species were subsequently cooled and ionized resulting in the formation of clusters. Clusters ranged in molecular masses but Kroto and Smalley found predominance in a C₆₀ cluster that could be enhanced further by letting the plasma react longer.[3, 6] They also discovered that the C₆₀ molecule formed a cage-like structure, a regular truncated icosahedron.

For this discovery they were awarded the 1996 Nobel Prize in Chemistry. The discovery of buckyballs was surprising, as the scientists aimed the experiment at producing carbon plasmas to replicate and characterize unidentified interstellar matter. Mass spectrometry analysis of the product indicated the formation of spheroidal carbon molecules.

The experimental evidence, a strong peak at 720 atomic mass units, indicated that a carbon molecule with 60 carbon atoms was forming, but provided no structural information. The research group concluded after reactivity experiments, that the most likely structure was a spheroidal molecule. The idea was quickly rationalized as the basis of an icosahedral symmetry closed cage structure. Kroto mentioned geodesic dome structures of the noted futurist and inventor Buckminster Fuller as influences in the naming of this particular substance as buckminsterfullerene.

Further developments

The versatility of fullerene molecules has led to a large amount of research exploring their properties. One interesting property is fullerene's large-capacity internal spaces. Atoms of different elements may be placed inside the molecular cage formed by the carbon atoms, producing a shrink-wrapped version of these elements.

Beam-experiments conducted between 1985 and 1990 provided more evidence for the stability of C₆₀ while supporting the closed-cage structural theory and predicting some of the bulk properties such a molecule would have. Around this time, intense theoretical group theory activity also predicted that C₆₀ should have only four IR-active vibrational bands, on account of its icosahedral symmetry.

In 1989 physicists Wolfgang Krätschmer and Donald R. Huffman observed unusual optical absorptions in thin carbon films produced by arc-processed graphite rods. Among other features, the IR spectra showed four discrete bands in close agreement to those proposed for C₆₀. A paper published by the group in 1990 followed on from their thin film experiments, and detailed the extraction of a benzene soluble material from the arc-processed graphite. This extract had crystal and X-ray analysis consistent with arrays of spherical C₆₀ molecules, approximately 0.7 nm in diameter.

Synthesis

In 1990, W. Krätschmer and D. R. Huffman developed a simple and efficient method of producing fullerenes in gram and even kilogram amounts, which has boosted the fullerene research. In this technique, carbon soot is produced from two high-purity graphite electrodes by igniting an arc discharge between them in an inert atmosphere (helium gas). Alternatively, soot is produced by laser ablation of graphite or pyrolysis of aromatic hydrocarbons. Fullerenes are extracted from the soot using a multistep procedure. First, the soot is dissolved in appropriate organic solvents. This step yields a solution containing up to 75% of C₆₀, as well as other fullerenes. These fractions are separated using chromatography. Generally, the fullerenes are dissolved in hydrocarbon or halogenated hydrocarbon and separated using alumina columns.

Properties

Molecule

The structure of a buckminsterfullerene is a truncated icosahedron with 60 vertices and 32 faces (20 hexagons and 12 pentagons where no pentagons share a vertex) with a carbon atom at the vertices of each polygon and a bond along each polygon edge. The van der Waals diameter of a C
60 molecule is about 1.01 nanometers (nm). The nucleus to nucleus diameter of a C
60 molecule is about 0.71 nm. The C
60 molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered "double bonds" and are shorter than the 6:5 bonds (between a hexagon and a pentagon). Its average bond length is 0.14 nm. Each carbon atom in the structure is bonded covalently with 3 others.

The C
60 molecule is extremely stable, withstanding high temperatures and high pressures. The exposed surface of the structure can selectively react with other species while maintaining the spherical geometry. Atoms and small molecules can be trapped within the molecule without reacting.

C
60 undergoes six reversible, one-electron reductions to C6âˆ'
60, but oxidation is irreversible. The first reduction needs ~1.0 V (Fc/Fc+
), showing that C₆₀ is a moderately effective electron acceptor. C
60 tends to avoid having double bonds in the pentagonal rings, which makes electron delocalization poor, and results in C
60 not being not "superaromatic". C₆₀ behaves very much like an electron deficient alkene and readily reacts with electron rich species.

A carbon atom in the C
60 molecule can be substituted by a nitrogen or boron atom yielding a C
59N or C₅₉B respectively.

Solution

Fullerenes are sparingly soluble in aromatic solvents such as toluene and carbon disulfide, but insoluble in water. Solutions of pure C₆₀ have a deep purple color which leaves a brown residue upon evaporation. The reason for this color change is the relatively narrow energy width of the band of molecular levels responsible for green light absorption by individual C₆₀ molecules. Thus individual molecules transmit some blue and red light resulting in a purple color. Upon drying, intermolecular interaction results in the overlap and broadening of the energy bands, thereby eliminating the blue light transmittance and causing the purple to brown color change.

C
60 crystallises with some solvents in the lattice ("solvates"). For example, crystallization of C₆₀ in benzene solution yields triclinic crystals with the formula C₆₀·4C₆H₆. Like other solvates, this one readily releases benzene to give the usual fcc C₆₀. Millimeter-sized crystals of C₆₀ and C
70 can be grown from solution both for solvates and for pure fullerenes.

Solid

In solid buckminsterfullerene, the molecules C₆₀ stick together via the van der Waals forces in the fcc motif. At low temperatures the individual molecules are locked against rotation. Upon heating, they start rotating at about âˆ'20 °C. This results in a first-order phase transition to a face-centered cubic (fcc) structure and a small, yet abrupt increase in the lattice constant from 1.411 to 1.4154 nm.

C
60 solid is as soft as graphite, but when compressed to less than 70% of its volume it transforms into a superhard form of diamond (see aggregated diamond nanorod). C
60 films and solution have strong non-linear optical properties; in particular, their optical absorption increases with light intensity (saturable absorption).

C
60 forms a brownish solid with an optical absorption threshold at ~1.6 eV. It is an n-type semiconductor with a low activation energy of 0.1â€"0.3 eV; this conductivity is attributed to intrinsic or oxygen-related defects. Fcc C₆₀ contains voids at its octahedral and tetrahedral sites which are sufficient large (0.6 and 0.2 nm respectively) to accommodate impurity atoms. When alkali metals are doped into these voids, C₆₀ converts from a semiconductor into a conductor or even superconductor.

Band structure and superconductivity

In 1991, Haddon et al. found that intercalation of alkali-metal atoms in solid C₆₀ leads to metallic behavior. In 1991, it was revealed that potassium-doped C₆₀ becomes superconducting at 18 K. This was the highest transition temperature for a molecular superconductor. Since then, superconductivity has been reported in fullerene doped with various other alkali metals. It has been shown that the superconducting transition temperature in alkaline-metal-doped fullerene increases with the unit-cell volume V. As caesium forms the largest alkali ion, caesium-doped fullerene is an important material in this family. Recently, superconductivity at 38 K has been reported in bulk Cs₃C₆₀, but only under applied pressure. The highest superconducting transition temperature of 33 K at ambient pressure is reported for Cs₂RbC₆₀.

The increase of transition temperature with the unit-cell volume had been believed to be evidence for the BCS mechanism of C₆₀ solid superconductivity, because inter C₆₀ separation can be related to an increase in the density of states on the Fermi level, N(ε_F). Therefore, there have been many efforts to increase the interfullerene separation, in particular, intercalating neutral molecules into the A₃C₆₀ lattice to increase the interfullerene spacing while the valence of C₆₀ is kept unchanged. However, this ammoniation technique has revealed a new aspect of fullerene intercalation compounds: the Mott transition and the correlation between the orientation/orbital order of C₆₀ molecules and the magnetic structure.

The C₆₀ molecules compose a solid of weakly bound molecules. The fullerites are therefore molecular solids, in which the molecular properties still survive. The discrete levels of a free C₆₀ molecule are only weakly broadened in the solid, which leads to a set of essentially nonoverlapping bands with a narrow width of about 0.5 eV. For an undoped C₆₀ solid, the 5-fold h_u band is the HOMO level, and the 3-fold t_1u band is the empty LUMO level, and this system is a band insulator. But when the C₆₀ solid is doped with metal atoms, the metal atoms give electrons to the t_1u band or the upper 3-fold t_1g band. This partial electron occupation of the band may lead to metallic behavior. However, A₄C₆₀ is an insulator, although the t_1u band is only partially filled and it should be a metal according to band theory. This unpredicted behavior may be explained by the Jahn-Teller effect, where spontaneous deformations of high-symmetry molecules induce the splitting of degenerate levels to gain the electronic energy. The Jahn-Teller type electron-phonon interaction is strong enough in C₆₀ solids to destroy the band picture for particular valence states.

A narrow band or strongly correlated electronic system and degenerated ground states are important points to understand in explaining superconductivity in fullerene solids. When the inter-electron repulsion U is greater than the bandwidth, an insulating localized electron ground state is produced in the simple Mott-Hubbard model. This explains the absence of superconductivity at ambient pressure in caesium-doped C₆₀ solids. Electron-correlation-driven localization of the t_1u electrons exceeds the critical value, leading to the Mott insulator. The application of high pressure decreases the interfullerene spacing, therefore caesium-doped C₆₀ solids turn to metallic and superconducting.

A fully developed theory of C₆₀ solids superconductivity is still lacking, but it has been widely accepted that strong electronic correlations and the Jahn-Teller electron-phonon coupling produce local electron-pairings that show a high transition temperature close to the insulator-metal transition.

Chemical reactions and properties

Hydrated fullerene (HyFn)

Hydrated fullerene C₆₀HyFn is a stable, highly hydrophilic, supra-molecular complex consisting of С₆₀ fullerene molecule enclosed into the first hydrated shell that contains 24 water molecules: C₆₀@(H₂O)₂₄. This hydrated shell is formed as a result of donor-acceptor interaction between lone-electron pairs of oxygen, water molecules and electron-acceptor centers on the fullerene surface. Meanwhile, the water molecules which are oriented close to the fullerene surface are interconnected by a three-dimensional network of hydrogen bonds. The size of C₆₀HyFn is 1.6â€"1.8 nm. The maximal concentration of С₆₀ in the form of C₆₀HyFn achieved by 2010 is 4 mg/mL.

Hydrogenation

C₆₀ exhibits a small degree of aromatic character, but it still reflects localized double and single C-C bond characters. Therefore C₆₀ can undergo addition with hydrogen to give polyhydrofullerenes. C₆₀ also undergoes Birch reduction. For example, C₆₀ reacts with lithium in liquid ammonia, followed by tert-butanol to give a mixture of polyhydrofullerenes such as C₆₀H₁₈, C₆₀H₃₂, C₆₀H₃₆, with C₆₀H₃₂ being the dominating product. This mixture of polyhydrofullerenes can be re-oxidized by 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone to give C₆₀ again.

Selective hydrogenation method exists. Reaction of C₆₀ with 9,9',10,10'-dihydroanthracene under the same conditions, depending on the time of reaction, gives C₆₀H₃₂ and C₆₀H₁₈ respectively and selectively.

C₆₀ can be hydrogenated, suggesting that a modified buckminsterfullerene called organometallic buckyballs (OBBs) could become a vehicle for "high density, room temperature, ambient pressure storage of hydrogen". These OBBs are created by binding atoms of a transition metal (TM) to C₆₀ or C₄₈B₁₂ and then binding many hydrogen atoms to this TM atom, dispersing them evenly throughout the inside of the organometallic buckyball. The study found that the theoretical amount of H₂ that can be retrieved from the OBB at ambient pressure approaches 9 wt %, a mass fraction that has been designated as optimal for hydrogen fuel by the U.S. Department of Energy.

Halogenation

Addition of fluorine, chlorine, and bromine occurs for C₆₀.

Fluorine atoms are small enough for a 1,2-addition, while Cl₂ and Br₂ add to remote C atoms due to steric factors. For example, in C₆₀Br₈ and C₆₀Br₂₄, the Br atoms are in 1,3- or 1,4-positions with respect to each other.

Under various conditions a vast number of halogenated derivatives of C₆₀ can be produced, some with extraordinary selectivity on one or two isomers over the other possible ones.

Addition of fluorine and chlorine usually results in a flattening of the C₆₀ framework into a drum-shaped molecule.

Addition of oxygen atoms

Solutions of C₆₀ can be oxygenated to the epoxide C₆₀O. Ozonation of C₆₀ in 1,2-xylene at 257K gives an intermediate ozonide C₆₀O₃, which can be decomposed into 2 forms of C₆₀O. Decomposition of C₆₀O₃ at 296K gives the epoxide, but photolysis gives a product in which the O atom bridges a 5,6-edige.

Cycloadditions

The Diels-Alder reaction is commonly employed to functionalize C₆₀. Reaction of C₆₀ with appropriate substituted diene gives the corresponding adduct.

The Diels-Alder reaction between C₆₀ and 3,6-diaryl-1,2,4,5-tetrazines affords C₆₂. The C₆₂ has the structure in which a four-membered ring is surrounded by four six-membered rings.

The C₆₀ molecules can also be coupled through a [2+2] cycloaddition, giving the dumbbell-shaped compound C₁₂₀. The coupling is achieved by high-speed vibrating milling of C₆₀ with a catalytic amount of KCN. The reaction is reversible as C₁₂₀ dissociates back to two C₆₀ molecules when heated at 450 K (177 °C; 350 °F). Under high pressure and temperature, repeated [2+2] cycloaddtion between C₆₀ results in a polymerized fullerene chains and networks. These polymers remain stable at ambient pressure and temperature once formed, and have remarkably interesting electronic and magnetic properties, such as being ferromagnetic above room temperature.

Free radical reactions

Reactions of C₆₀ with free radicals readily occur. When C₆₀ is mixed with a disulfide RSSR, the radical C₆₀SR• forms spontaneously upon irradiation of the mixture.

Stability of the radical species C₆₀Y• depends largely on steric factors of Y. When tert-butyl halide is photolyzed and allowed to react with C₆₀, a reversible inter-cage C-C bond is formed:

Cyclopropanation (Bingel reaction)

Cyclopropanation (Bingel reaction) is another common method for functionalizing C₆₀. Cyclopropanation of C₆₀ mostly occurs at the junction of 2 hexagons due to steric factors.

The first cyclopropanation was carried out by treating the β-bromomalonate with C₆₀ in the presence of a base. Cyclopropanation also occur readily with diazomethanes. For example, diphenyldiazomethane reacts readily with C₆₀ to give the compound C⁶¹Ph₂. Phenyl-C61-butyric acid methyl ester derivative prepared through cyclopropanation has been studied for use in organic solar cells.

Redox reactions â€" C₆₀ anions and cations

The LUMO in C₆₀ is triply degenerate, with the HOMO-LUMO separation relatively small. This small gap suggests that reduction of C₆₀ should occur at mild potentials leading to fulleride anions, [C₆₀]^nâˆ' (n = 1â€"6). The midpoint potentials of 1-electron reduction of buckminsterfullerene and its anions is given in the table below:

C₆₀ forms a variety of charge-transfer complexes, for example with tetrakis(dimethylamino)ethylene:

C₆₀ + C₂(NMe₂)₄ â†' [C₂(NMe₂)₄]⁺[C₆₀]^âˆ'

This salt exhibits ferromagnetism at 16K.

The paramagnetic fulleride ion [C₆₀]^2âˆ' has been isolated as the [K(crypt-222)]⁺ salt. It is synthesized by treating C₆₀ with metallic potassium in the presence of 2.2.2-Cryptand. The most common fulleride ion, however, is [C₆₀]^3âˆ'. Alkali metal salts of this trianion are superconducting. In M₃C₆₀ (M = Na, K, Rb), the M⁺ ions occupy the interstitial holes in a lattice composed of ccp lattice composed of nearly spherical C₆₀ anions. In Cs₃C₆₀, the cages are arranged in a bcc lattice.

C₆₀ oxidizes with difficulty. Three reversible oxidation processes have been observed by using cyclic voltammetry with ultra-dry methylene chloride and a supporting electrolyte with extremely high oxidation resistance and low nucleophilicity, such as [ⁿBu₄N] [AsF₆].

Which the [C₆₀]²⁺ ion is very unstable, and the third process can be studied only at low temperatures.

The redox potentials of C₆₀ can be modified supramolecularly. A dibenzo-18-crown-6 derivative of C₆₀ has been made, featuring a voltage sensor device, with the reversible binding of K⁺ ion causing an anodic shift of 90mV of the first C₆₀ reduction.

Metal complexes

C₆₀ forms complexes akin to the more common alkenes. Complexes have been reported molybdenum, tungsten, platinum, palladium, iridium, and titanium. The pentacarbonyl species are produced by photochemical reactions.

M(CO)₆ + C₆₀ â†' M(η²-C₆₀)(CO)₅ + CO (M = Mo, W)

In the case of platinum complex, the labile ethylene ligand is the leaving group in a thermal reaction:

Pt(η²-C₂H₄)(PPh₃)₂ + C₆₀ â†' Pt(η²-C₆₀)(PPh₃)₂ + C₂H₄

Titanocene complexes have also been reported:

(η⁵-Cp)₂Ti(η²-(CH₃)₃SiC≡CSi(CH₃)₃) + C₆₀ â†' (η⁵-Cp)₂Ti(η²-C₆₀) + (CH₃)₃SiC≡CSi(CH₃)₃

Coordinatively unsaturated precursors, such as Vaska's complex, for adducts with C₆₀:

trans-Ir(CO)Cl(PPh₃)₂ + C₆₀ â†' Ir(CO)Cl(η²-C₆₀)(PPh₃)₂

One such iridium complex, [Ir(η²-C₆₀)(CO)Cl(Ph₂CH₂C₆H₄OCH₂Ph)₂] has been prepared where the metal center projects two electron-rich 'arms' that embrace the C₆₀ guest.

Endohedral fullerenes

Metal atoms or certain small molecules such as H₂ and noble gas can be encapsulated inside the C₆₀ cage. These endohedral fullerenes are usually synthesized by doping in the metal atoms in an arc reactor or by laser evaporation. These methods gives low yields of endohedral fullerenes, and a better method involves the opening of the cage, packing in the atoms or molecules, and closing the opening using certain organic reactions. This method, however, is still immature and only a few species have been synthesized this way.

Endohedral fullerenes show distinct and intriguing chemical properties that can be completely different from the encapsulated atom or molecule, as well as the fullerene itself. The encapsulated atoms have been shown to perform circular motions inside the C₆₀ cage, and its motion has been followed by using NMR spectroscopy.

Applications

No application of C₆₀ has been commercialized. In the medical field, elements such as helium (that can be detected in minute quantities) can be used as chemical tracers in impregnated buckyballs. Buckminsterfullerene also inhibits the HIV virus. In particular, C₆₀ inhibits a key enzyme in the human immunodeficiency virus known as HIV-1 protease; this could inhibit reproduction of the HIV virus in immune cells.

Water-soluble derivatives of C₆₀ were discovered to exert an inhibition on the three isoforms of nitric oxide synthase, with slightly different potencies.

The optical absorption properties of C₆₀ match solar spectrum in a way that suggests that C₆₀-based films could be useful for photovoltaic applications. Because of its high electronic affinity it is one of the most common electron acceptor used in donor/acceptor based solar cells. Conversion efficiencies up to 5.7% have been reported in C₆₀-polymer cells.

References

Bibliography

• Katz, E. A. (2006). "Fullerene Thin Films as Photovoltaic Material". In Sōga, Tetsuo. Nanostructured materials for solar energy conversion. Elsevier. pp. 361â€"443. ISBN 978-0-444-52844-5. 

Further reading

• Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smalley, R. E. (November 1985). "C₆₀: Buckminsterfullerene". Nature 318 (14): 162. Bibcode:1985Natur.318..162K. doi:10.1038/318162a0.  â€" describing the original discovery of C₆₀
• Hebgen, Peter; Goel, Anish; Howard, Jack B.; Rainey, Lenore C.; Vander Sande, John B. (2000). "Fullerenes and Nanostructures in Diffusion Flames". Proceedings of the Combustion Institute 28: 1397â€"1404. doi:10.1016/S0082-0784(00)80355-0.  â€" report describing the synthesis of C₆₀ with combustion research published in 2000 at the 28th International Symposium on Combustion

External links

• History of C₆₀'s discovery carried out by the Chemistry Department at Bristol University
• A brief overview of buckminsterfullerene described by the University of Wisconsin-Madison
• A report by Ming Kai College detailing the properties of buckminsterfullerene
• Donald R. Huffman and Wolfgang Krätschmer's paper pertaining to the synthesis of C₆₀ in Nature published in 1990
• A thorough description of C₆₀ by the Oak Ridge National Laboratory
• An article about buckminsterfullerene on Connexions Science Encyclopaedia
• Extensive statistical data compiled by the University of Sussex on the numerical quantitative properties of buckminsterfullerene
• A web portal dedicated to buckminsterfullerene, authored and supported by the University of Bristol
• Another web portal dedicated to buckminsterfullerene, authored and supported by the Chemistry Department at the University of Bristol
• A brief article entirely devoted to C₆₀ and its discovery, structure, production, properties, and applications
• American Chemical Society's complete article on buckminsterfullerene
• Buckminsterfullerene at The Periodic Table of Videos (University of Nottingham)

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