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Buckminsterfullerene

Names
IUPAC name (C60-Ih)[5,6]fullerene
Other names Buckyball; Fullerene-C60; [60]fullerene
Identifiers
CAS Number	99685-96-8 Y
3D model (JSmol)	Interactive image
Beilstein Reference	5901022
ChEBI	CHEBI:33128 Y
ChemSpider	110185 Y
ECHA InfoCard	100.156.884
PubChem CID	123591
InChI InChI=1S/C60/c1-2-5-6-3(1)8-12-10-4(1)9-11-7(2)17-21-13(5)23-24-14(6)22-18(8)28-20(12)30-26-16(10)15(9)25-29-19(11)27(17)37-41-31(21)33(23)43-44-34(24)32(22)42-38(28)48-40(30)46-36(26)35(25)45-39(29)47(37)55-49(41)51(43)57-52(44)50(42)56(48)59-54(46)53(45)58(55)60(57)59 Y Key: XMWRBQBLMFGWIX-UHFFFAOYSA-N Y InChI=1/C60/c1-2-5-6-3(1)8-12-10-4(1)9-11-7(2)17-21-13(5)23-24-14(6)22-18(8)28-20(12)30-26-16(10)15(9)25-29-19(11)27(17)37-41-31(21)33(23)43-44-34(24)32(22)42-38(28)48-40(30)46-36(26)35(25)45-39(29)47(37)55-49(41)51(43)57-52(44)50(42)56(48)59-54(46)53(45)58(55)60(57)59 Key: XMWRBQBLMFGWIX-UHFFFAOYAU InChI=1S/C60/c1-2-5-6-3(1)8-12-10-4(1)9-11-7(2)17-21-13(5)23-24-14(6)22-18(8)28-20(12)30-26-16(10)15(9)25-29-19(11)27(17)37-41-31(21)33(23)43-44-34(24)32(22)42-38(28)48-40(30)46-36(26)35(25)45-39(29)47(37)55-49(41)51(43)57-52(44)50(42)56(48)59-54(46)53(45)58(55)60(57)59 Key: XMWRBQBLMFGWIX-UHFFFAOYSA-N
SMILES c12c3c4c5c1c6c7c8c2c9c1c3c2c3c4c4c%10c5c5c6c6c7c7c%11c8c9c8c9c1c2c1c2c3c4c3c4c%10c5c5c6c6c7c7c%11c8c8c9c1c1c2c3c2c4c5c6c3c7c8c1c23
Properties
Chemical formula	C60
Molar mass	7002720660000000000♠720.660 g·mol−1
Appearance	Dark needle-like crystals
Density	1.65 g/cm3
Melting point	≈600 ºC (subl.)
Solubility in water	insoluble in water
Vapor pressure	0.4-0.5 Pa (T ≈ 800 K); 14 Pa (T ≈ 900 K) [1]
Structure
Crystal structure	Face-centered cubic, cF1924
Space group	Fm3m, No. 225
Lattice constant	a = 1.4154 nm
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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

Preparation and occurrence

It was first generated in 1984 by Eric Rohlfing, Donald Cox and Andrew Kaldor^[2][3] using a laser to vaporize carbon in a supersonic helium beam. In 1985 their work was repeated by Harold Kroto, James R. Heath, Sean O'Brien, Robert Curl, and Richard Smalley at Rice University, who recognized the structure of C₆₀ as buckminsterfullerine.^[4] 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.

Buckminsterfullerene is the most common naturally occurring fullerene. It can be found in small quantities in soot.^[5][6] The molecule has been detected in deep space.^[7]

Etymology

The discoverers of the allotrope named the newfound molecule after Buckminster Fuller, who designed many geodesic dome structures that look similar to C₆₀. This is slightly misleading, however, as Fuller's geodesic domes are constructed from triangles and not hexagons or pentagons. A common, shortened name for buckminsterfullerene is "buckyballs".^[8]

History

Harold Kroto

Richard Smalley

Many soccer balls have the same arrangement of polygons as buckminsterfullerene, C₆₀.

Theoretical predictions of buckyball molecules appeared in the late 1960s  and early 1970s,^[9][10][11] but these reports 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, Smalley and Curl at Rice University developed experimental technique to generate these substances. They used laser vaporization of a suitable target to produce clusters of atoms. Kroto realized that by using a graphite target,^[12] a range of carbon clusters could be studied.

Concurrent but unconnected to the Kroto-Smalley work, astrophysicists were working with spectroscopists to study infrared emissions from giant red carbon stars.^[13][14][15] 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.^[13][16] Hence, the inspiration came to Smalley and team to use the laser technique on graphite to generate fullerenes.

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.^[17] 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 allowing the plasma react longer. They also discovered that the C₆₀ molecule formed a cage-like structure, a regular truncated icosahedron.^[13][17]

For this discovery Curl, Kroto, and Smalley were awarded the 1996 Nobel Prize in Chemistry.^[9]

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.^[9]

In 1989 physicists Wolfgang Krätschmer, Konstantinos Fostiropoulos, and Donald R. Huffman observed unusual optical absorptions in thin films of carbon dust (soot). The soot had been generated by an arc-process between two graphite electrodes in a helium atmosphere where the electrode material evaporates and condenses forming soot in the quenching atmosphere. Among other features, the IR spectra of the soot showed four discrete bands in close agreement to those proposed for C₆₀.^[18][19]

Another paper on the characterization and verification of the molecular structure followed on in the same year (1990) from their thin film experiments, and detailed also the extraction of an evaporable as well as benzene soluble material from the arc-generated soot. This extract had TEM and X-ray crystal analysis consistent with arrays of spherical C₆₀ molecules, approximately 1.0 nm in van der Waals diameter^[20] as well as the expected molecular mass of 720 u for C₆₀ (and 840 u for C₇₀) in their mass spectra.^[21] The method was simple and efficient to prepare the material in gram amounts per day (1990) which has boosted the fullerene research and is even today applied for the commercial production of fullerenes.

The discovery of practical routes to C₆₀ led to the exploration of a new field of chemistry involving the study of fullerenes.

Synthesis

High-vacuum electrolysis of a C₆₀-fullerene derivative. Slow diffusion into the anode (right side) yields the characteristic purple color of pure C₆₀.

Soot is produced by laser ablation of graphite or pyrolysis of aromatic hydrocarbons. Fullerenes are extracted from the soot with organic solvents using a Soxhlet extractor.^[22] This step yields a solution containing up to 75% of C₆₀, as well as other fullerenes. These fractions are separated using chromatography.^[23] Generally, the fullerenes are dissolved in a hydrocarbon or halogenated hydrocarbon and separated using alumina columns.^[24]

Structure

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.^[25]

Electronic structure of C₆₀ under "ideal" spherical (left) and "real" icosahedral symmetry (right).

The model is built of magnetic balls (5mm diam.); 12 pentagons compose a spherical shell with 60 nodes, demonstrating the Carbon atoms. The model suggests 4 nearest-neighbors to each atom, this is possible because the hexagons are squeezed.

Properties

Buckminsterfullerene is the largest object observed to exhibit wave–particle duality; theoretically every object exhibits this behavior.^[26]

The compound is stable,^[27] withstanding high temperatures and high pressures. The exposed surface of the structure can selectively react with other species while maintaining the spherical geometry.^[28] 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.^[20]

C
₆₀ undergoes six reversible, one-electron reductions to C⁶⁻
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 "superaromatic". C₆₀ behaves very much like an electron deficient alkene and readily reacts with electron rich species.^[20]

A carbon atom in the C
₆₀ molecule can be substituted by a nitrogen or boron atom yielding a C
₅₉N or C₅₉B respectively.^[29]

Orthogonal projections

Centered by	Vertex	Edge 5–6	Edge 6–6	Face Hexagon	Face Pentagon
Image
Projective symmetry	[2]	[2]	[2]	[6]	[10]

Solution

C₆₀ solution

Optical absorption spectrum of C
₆₀ solution, showing reduced absorption for the blue (~450 nm) and red (~700 nm) light that results in the purple color.

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.^[33]

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.^[34][35]

Solid

C₆₀ solid

C
₆₀ crystal structure

In solid buckminsterfullerene, the C₆₀ molecules adopt the fcc (face-centered cubic) motif. They start rotating at about −20 °C. This change is associated with a first-order phase transition to a fcc structure and a small, yet abrupt increase in the lattice constant from 1.411 to 1.4154 nm.^[36]

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.^[37] It is an n-type semiconductor with a low activation energy of 0.1–0.3 eV; this conductivity is attributed to intrinsic or oxygen-related defects.^[38] Fcc C₆₀ contains voids at its octahedral and tetrahedral sites which are sufficiently 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.^[36][39]

Chemical reactions and properties

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.^[40]

C₆₀ can be hydrogenated,^[41] 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.^[40]

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 296 K gives the epoxide, but photolysis gives a product in which the O atom bridges a 5,6-edige.^[40]

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.

A C₆₂ derivative [C₆₂(C₆H₄-4-Me)₂] synthesized from C₆₀ and 3,6-bis(4-methylphenyl)-3,6-dihydro-1,2,4,5-tetrazine

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] cycloaddition 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.^[40]

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:^[40]

Cyclopropanation (Bingel reaction)

Cyclopropanation (the 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.^[40]Phenyl-C₆₁-butyric acid methyl ester derivative prepared through cyclopropanation has been studied for use in organic solar cells.

Redox reactions – C₆₀ anions and cations

C₆₀ anions

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]ⁿ⁻ (n = 1–6). 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 → [C₂(NMe₂)₄]⁺[C₆₀]⁻

This salt exhibits ferromagnetism at 16 K.

C₆₀ cations

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].^[40]

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 → 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 → Pt(η²-C₆₀)(PPh₃)₂ + C₂H₄

Titanocene complexes have also been reported:

(η⁵-Cp)₂Ti(η²-(CH₃)₃SiC≡CSi(CH₃)₃) + C₆₀ → (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 → 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.^[42]

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.^[43]

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.^[42]

Applications

In the medical field, elements such as helium (that can be detected in minute quantities) can be used as chemical tracers in impregnated buckyballs.

Water-soluble derivatives of C₆₀ were discovered to exert an inhibition on the three isoforms of nitric oxide synthase, with slightly different potencies.^[44]

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 ^[45] it is one of the most common electron acceptors used in donor/acceptor based solar cells. Conversion efficiencies up to 5.7% have been reported in C₆₀–polymer cells.^[46]

Safety

Solutions of C₆₀ dissolved in olive oil are nontoxic to rodents.^[47]

References

Bibliography

• 
  Katz, E. A. (2006). "Fullerene Thin Films as Photovoltaic Material". In Sōga, Tetsuo. Nanostructured materials for solar energy conversion. Elsevier. pp. 361–443. 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" (PDF). Nature. 318 (14): 162–163. Bibcode:1985Natur.318..162K. doi:10.1038/318162a0. – 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" (PDF). Proceedings of the Combustion Institute. 28: 1397–1404. CiteSeerX 10.1.1.574.8368. doi:10.1016/S0082-0784(00)80355-0. – 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)