"Buckyball" redirects here. For other uses, see Buckyball (disambiguation)

Buckminsterfullerene is a type of fullerene with the formula C 60 . 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 [ edit ]

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 C 60 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 [ edit ]

The discoverers of the allotrope named the newfound molecule after Buckminster Fuller, who designed many geodesic dome structures that look similar to C 60 . 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 [ edit ]

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

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.

C 60 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 C 60 and C 70 . 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 C 60 cluster that could be enhanced further by allowing the plasma react longer. They also discovered that the C 60 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 C 60 .[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 C 60 molecules, approximately 1.0 nm in van der Waals diameter[20] as well as the expected molecular mass of 720 u for C 60 (and 840 u for C 70 ) 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 C 60 led to the exploration of a new field of chemistry involving the study of fullerenes.

Synthesis [ edit ]

60 -fullerene derivative. Slow diffusion into the anode (right side) yields the characteristic purple color of pure C 60 . High-vacuum electrolysis of a C-fullerene derivative. Slow diffusion into the anode (right side) yields the characteristic purple color of pure C

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 C 60 , 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 [ edit ]

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]

60 under "ideal" spherical (left) and "real" icosahedral symmetry (right). Electronic structure of Cunder "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 [ edit ]

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 C 60 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 C 60 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 C 60 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". C 60 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

59 N or C 59 B 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 [ edit ]

60 solution solution

C

60 solution, showing reduced absorption for the blue (~450 nm) and red (~700 nm) light that results in the purple color. Optical absorption spectrum ofsolution, 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 C 60 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 C 60 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 C 60 in benzene solution yields triclinic crystals with the formula C 60 ·4C 6 H 6 . Like other solvates, this one readily releases benzene to give the usual fcc C 60 . Millimeter-sized crystals of C 60 and C

70 can be grown from solution both for solvates and for pure fullerenes.[34][35]

Solid [ edit ]

60 solid solid

C

60 crystal structure crystal structure

In solid buckminsterfullerene, the C 60 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 C 60 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, C 60 converts from a semiconductor into a conductor or even superconductor.[36][39]

Chemical reactions and properties [ edit ]

Hydrogenation [ edit ]

C 60 exhibits a small degree of aromatic character, but it still reflects localized double and single C–C bond characters. Therefore, C 60 can undergo addition with hydrogen to give polyhydrofullerenes. C 60 also undergoes Birch reduction. For example, C 60 reacts with lithium in liquid ammonia, followed by tert-butanol to give a mixture of polyhydrofullerenes such as C 60 H 18 , C 60 H 32 , C 60 H 36 , with C 60 H 32 being the dominating product. This mixture of polyhydrofullerenes can be re-oxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone to give C 60 again.

Selective hydrogenation method exists. Reaction of C 60 with 9,9′,10,10′-dihydroanthracene under the same conditions, depending on the time of reaction, gives C 60 H 32 and C 60 H 18 respectively and selectively.[40]

C 60 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 C 60 or C 48 B 12 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 H 2 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 [ edit ]

Addition of fluorine, chlorine, and bromine occurs for C 60 .

Fluorine atoms are small enough for a 1,2-addition, while Cl 2 and Br 2 add to remote C atoms due to steric factors. For example, in C 60 Br 8 and C 60 Br 24 , 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 C 60 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 C 60 framework into a drum-shaped molecule.[40]

Addition of oxygen atoms [ edit ]

Solutions of C 60 can be oxygenated to the epoxide C 60 O. Ozonation of C 60 in 1,2-xylene at 257K gives an intermediate ozonide C 60 O 3 , which can be decomposed into 2 forms of C 60 O. Decomposition of C 60 O 3 at 296 K gives the epoxide, but photolysis gives a product in which the O atom bridges a 5,6-edige.[40]

Cycloadditions [ edit ]

The Diels–Alder reaction is commonly employed to functionalize C 60 . Reaction of C 60 with appropriate substituted diene gives the corresponding adduct.

The Diels–Alder reaction between C 60 and 3,6-diaryl-1,2,4,5-tetrazines affords C 62 . The C 62 has the structure in which a four-membered ring is surrounded by four six-membered rings.

62 derivative [C 62 (C 6 H 4 -4-Me) 2 ] synthesized from C 60 and 3,6-bis(4-methylphenyl)-3,6-dihydro-1,2,4,5-tetrazine A Cderivative [C(C-4-Me)] synthesized from Cand 3,6-bis(4-methylphenyl)-3,6-dihydro-1,2,4,5-tetrazine

The C 60 molecules can also be coupled through a [2+2] cycloaddition, giving the dumbbell-shaped compound C 120 . The coupling is achieved by high-speed vibrating milling of C 60 with a catalytic amount of KCN. The reaction is reversible as C 120 dissociates back to two C 60 molecules when heated at 450 K (177 °C; 350 °F). Under high pressure and temperature, repeated [2+2] cycloaddition between C 60 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 [ edit ]

Reactions of C 60 with free radicals readily occur. When C 60 is mixed with a disulfide RSSR, the radical C 60 SR• forms spontaneously upon irradiation of the mixture.

Stability of the radical species C 60 Y• depends largely on steric factors of Y. When tert-butyl halide is photolyzed and allowed to react with C 60 , a reversible inter-cage C–C bond is formed:[40]

Cyclopropanation (Bingel reaction) [ edit ]

Cyclopropanation (the Bingel reaction) is another common method for functionalizing C 60 . Cyclopropanation of C 60 mostly occurs at the junction of 2 hexagons due to steric factors.

The first cyclopropanation was carried out by treating the β-bromomalonate with C 60 in the presence of a base. Cyclopropanation also occur readily with diazomethanes. For example, diphenyldiazomethane reacts readily with C 60 to give the compound C 61 Ph 2 .[40] Phenyl-C 61 -butyric acid methyl ester derivative prepared through cyclopropanation has been studied for use in organic solar cells.

Redox reactions – C 60 anions and cations [ edit ]

C 60 anions [ edit ]

The LUMO in C 60 is triply degenerate, with the HOMO–LUMO separation relatively small. This small gap suggests that reduction of C 60 should occur at mild potentials leading to fulleride anions, [C 60 ]n− (n = 1–6). The midpoint potentials of 1-electron reduction of buckminsterfullerene and its anions is given in the table below:

Reduction potential of C 60 at 213 K Half-reaction E° (V) C 60 + e− ⇌ C −

60 −0.169 C −

60 + e− ⇌ C 2−

60 −0.599 C 2−

60 + e− ⇌ C 3−

60 −1.129 C 3−

60 + e− ⇌ C 4−

60 −1.579 C 4−

60 + e− ⇌ C 5−

60 −2.069 C 5−

60 + e− ⇌ C 6−

60 −2.479

C 60 forms a variety of charge-transfer complexes, for example with tetrakis(dimethylamino)ethylene:

C 60 + C 2 (NMe 2 ) 4 → [C 2 (NMe 2 ) 4 ]+[C 60 ]−

This salt exhibits ferromagnetism at 16 K.

C 60 cations [ edit ]

C 60 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 [nBu 4 N] [AsF 6 ].[40]

Reduction potentials of C 60 oxidation at low temperatures Half-reaction E° (V) C 60 ⇌ C +

60 +1.27 C +

60 ⇌ C 2+

60 +1.71 C 2+

60 ⇌ C 3+

60 +2.14

Which the [C 60 ]2+ ion is very unstable, and the third process can be studied only at low temperatures.

The redox potentials of C 60 can be modified supramolecularly. A dibenzo-18-crown-6 derivative of C 60 has been made, featuring a voltage sensor device, with the reversible binding of K+ ion causing an anodic shift of 90mV of the first C 60 reduction.

Metal complexes [ edit ]

C 60 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 + C 60 → M(η2-C 60 )(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-C 2 H 4 )(PPh 3 ) 2 + C 60 → Pt(η2-C 60 )(PPh 3 ) 2 + C 2 H 4

Titanocene complexes have also been reported:

(η5-Cp) 2 Ti(η2-(CH 3 ) 3 SiC≡CSi(CH 3 ) 3 ) + C 60 → (η5-Cp) 2 Ti(η2-C 60 ) + (CH 3 ) 3 SiC≡CSi(CH 3 ) 3

Coordinatively unsaturated precursors, such as Vaska's complex, for adducts with C 60 :

trans-Ir(CO)Cl(PPh 3 ) 2 + C 60 → Ir(CO)Cl(η2-C 60 )(PPh 3 ) 2

One such iridium complex, [Ir(η2-C 60 )(CO)Cl(Ph 2 CH 2 C 6 H 4 OCH 2 Ph) 2 ] has been prepared where the metal center projects two electron-rich 'arms' that embrace the C 60 guest.[42]

Endohedral fullerenes [ edit ]

Metal atoms or certain small molecules such as H 2 and noble gas can be encapsulated inside the C 60 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 C 60 cage, and its motion has been followed by using NMR spectroscopy.[42]

Applications [ edit ]

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 C 60 were discovered to exert an inhibition on the three isoforms of nitric oxide synthase, with slightly different potencies.[44]

The optical absorption properties of C 60 match solar spectrum in a way that suggests that C 60 -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 C 60 –polymer cells.[46]

Safety [ edit ]

Solutions of C 60 dissolved in olive oil are nontoxic to rodents.[47]

References [ edit ]

Bibliography [ edit ]