Endohedral fullerenes, also called endofullerenes, are
fullerenes that have additional
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
s,
ions, or
clusters enclosed within their inner spheres. The first
lanthanum C
60 complex called La@C
60 was
synthesized in 1985.
The @ (
at sign
The at sign () is an accounting and invoice abbreviation meaning "at a rate of" (e.g. 7 Widget (economics), widgets @ £2 per widget = £14), now seen more widely in email addresses and social media platform User (computing), handles. It is norm ...
) in the name reflects the notion of a small molecule trapped inside a shell. Two types of endohedral complexes exist: endohedral metallofullerenes and non-metal doped fullerenes.
Notation
In a traditional
chemical formula
A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...
notation, a
buckminsterfullerene (C
60) with an atom (M) was simply represented as MC
60 regardless of whether M was inside or outside the fullerene. In order to allow for more detailed discussions with minimal loss of information, a more explicit notation was proposed in 1991,
where the atoms listed to the left of the @ sign are situated inside the network composed of the atoms listed to the right. The example above would then be denoted M@C
60 if M were inside the carbon network. A more complex example is K
2(K@C
59B), which denotes "a 60-atom fullerene cage with one
boron
Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...
atom substituted for a
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
in the geodesic network, a single
potassium
Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
trapped inside, and two potassium atoms adhering to the outside."
[
The choice of the symbol has been explained by the authors as being concise, readily printed and transmitted electronically (the at sign is included in ]ASCII
ASCII ( ), an acronym for American Standard Code for Information Interchange, is a character encoding standard for representing a particular set of 95 (English language focused) printable character, printable and 33 control character, control c ...
, which most modern character encoding schemes are based on), and the visual aspects suggesting the structure of an endohedral fullerene.
Endohedral metallofullerenes
Doping fullerenes with electropositive metals takes place in a 1 kV direct current
Direct current (DC) is one-directional electric current, flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor (material), conductor such as a wire, but can also flow throug ...
arc reactor or via laser evaporation. Synthesis is unspecific, and (for unclear reasons) only relatively noncoordinating metals can be inserted this way. Thus endofullerenes containing rare earth metal
The rare-earth elements (REE), also called the rare-earth metals or rare earths, and sometimes the lanthanides or lanthanoids (although scandium and yttrium, which do not belong to this series, are usually included as rare earths), are a set o ...
s like scandium
Scandium is a chemical element; it has Symbol (chemistry), symbol Sc and atomic number 21. It is a silvery-white metallic d-block, d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the lantha ...
, yttrium
Yttrium is a chemical element; it has Symbol (chemistry), symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost a ...
, and the lanthanides
The lanthanide () or lanthanoid () series of chemical elements comprises at least the 14 Metal, metallic chemical elements with atomic numbers 57–70, from lanthanum through ytterbium. In the periodic table, they fill the 4f orbitals. Lutetium ...
; alkaline earth metal
The alkaline earth metals are six chemical elements in group (periodic table), group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar p ...
s like barium
Barium is a chemical element; it has symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element.
Th ...
and strontium
Strontium is a chemical element; it has symbol Sr and atomic number 38. An alkaline earth metal, it is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is exposed to ...
; alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
s like potassium
Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
; and certain tetravalent metals like uranium
Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
, zirconium
Zirconium is a chemical element; it has Symbol (chemistry), symbol Zr and atomic number 40. First identified in 1789, isolated in impure form in 1824, and manufactured at scale by 1925, pure zirconium is a lustrous transition metal with a greyis ...
and hafnium
Hafnium is a chemical element; it has symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dm ...
, have all been demonstrated, but not the other transition metals. The other transition metals, do, however, catalyze metal insertion.
Besides unfilled fullerenes, endohedral metallofullerenes develop with different cage sizes like La@C60 or La@C82 and as different isomer cages. Aside from the dominant presence of mono-metal cages, numerous di-metal endohedral complexes and the tri-metal carbide fullerenes like Sc3C2@C80 were also isolated.
In 1999 a discovery drew large attention. With the synthesis of the Sc3N@C80 by Harry Dorn and coworkers, the inclusion of a molecule fragment in a fullerene cage had succeeded for the first time. This compound can be prepared by arc-vaporization at temperatures up to 1100 °C of graphite rods packed with scandium(III) oxide iron nitride and graphite powder in a K-H generator in a nitrogen atmosphere at 300 Torr
The torr (symbol: Torr) is a Pressure#Units, unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (unit), atmosphere (101325 Pa). Thus one torr is exactly (≈ ).
Historically, one torr was intended to be ...
.
Endohedral metallofullerenes are characterised by the fact that electrons will transfer from the metal atom to the fullerene cage and that the metal atom takes a position off-center in the cage. The size of the charge transfer is not always simple to determine. In most cases it is between 2 and 3 charge units, in the case of the La2@C80 however it can be even about 6 electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s such as in Sc3N@C80 which is better described as 3N">c3Nsup>+6@ 80">80sup>−6. These anionic fullerene cages are very stable molecules and do not have the reactivity associated with ordinary empty fullerenes. They are stable in air up to very high temperatures (600 to 850 °C).
The lack of reactivity in Diels-Alder reactions is utilised in a method to purify 80">80sup>−6 compounds from a complex mixture of empty and partly filled fullerenes of different cage size.[ In this method Merrifield resin is modified as a ]cyclopentadienyl Cyclopentadienyl can refer to
* Cyclopentadienyl anion, or cyclopentadienide,
** Cyclopentadienyl ligand
* Cyclopentadienyl radical, •
* Cyclopentadienyl cation,
See also
* Pentadienyl
{{Chemistry index ...
resin and used as a solid phase against a mobile phase containing the complex mixture in a column chromatography
Column chromatography in chemistry is a chromatography method used to isolate a single chemical compounds, chemical compound from a mixture. Chromatography is able to separate substances based on differential absorption of compounds to the adsorbe ...
operation. Only very stable fullerenes such as 3N">c3Nsup>+6@ 80">80sup>−6 pass through the column unreacted.
In Ce2@C80 the two metal atoms exhibit a non-bonded interaction. Since all the six-membered rings in C80-Ih are equal[ the two encapsulated Ce atoms exhibit a three-dimensional random motion. This is evidenced by the presence of only two signals in the 13C-]NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which atomic nucleus, nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing ...
spectrum. It is possible to force the metal atoms to a standstill at the equator as shown by x-ray crystallography
X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
when the fullerene is exahedrally functionalized by an electron donation silyl group in a reaction of Ce2@C80 with 1,1,2,2-tetrakis(2,4,6-trimethylphenyl)-1,2-disilirane.
Gd@C82(OH)22, an endohedral metallofluorenol, can competitively inhibit the WW domain
The WW domain (also known as the rsp5-domain or WWP repeating structural motif, motif) is a modular protein domain that mediates specific interactions with protein ligands. This domain is found in a number of unrelated signaling and structural pro ...
in the oncogene YAP1 from activating. It was originally developed as an MRI contrast agent
MRI contrast agents are contrast agents used to improve the visibility of internal body structures in magnetic resonance imaging (MRI).
The most commonly used compounds for contrast enhancement are gadolinium-based contrast agents (GBCAs). Suc ...
.
Non-metal doped fullerenes
Endohedral complexes He@C60 and Ne@C60 are prepared by pressurizing C60 to ca. 3 bar in a noble-gas atmosphere. Under these conditions about one out of every 650,000 C60 cages was doped with a helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
atom.
The formation of endohedral complexes with helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
, neon
Neon is a chemical element; it has symbol Ne and atomic number 10. It is the second noble gas in the periodic table. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with approximately two-thirds the density of ...
, argon
Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
, krypton
Krypton (from 'the hidden one') is a chemical element; it has symbol (chemistry), symbol Kr and atomic number 36. It is a colorless, odorless noble gas that occurs in trace element, trace amounts in the Earth's atmosphere, atmosphere and is of ...
and xenon
Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
as well as numerous adducts of the He@C60 compound was also demonstrated with pressures of 3 kbars and incorporation of up to 0.1% of the noble gases.
While noble gas
The noble gases (historically the inert gases, sometimes referred to as aerogens) are the members of Group (periodic table), group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some ...
es are chemically very inert and commonly exist as individual atoms, this is not the case for nitrogen
Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
and phosphorus
Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
and so the formation of the endohedral complexes N@C60, N@C70 and P@C60 is more surprising.
The nitrogen atom is in its electronic initial state (4S3/2) and is highly reactive. Nevertheless, N@C60 is sufficiently stable that exohedral derivatization from the mono- to the hexa adduct of the malonic acid ethyl ester
In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
is possible.
In these compounds no charge transfer of the nitrogen atom in the center to the carbon atoms of the cage takes place. Therefore, 13C-couplings, which are observed very easily with the endohedral metallofullerenes, could only be observed in the case of the N@C60 in a high resolution spectrum as shoulders of the central line.
The central atom in these endohedral complexes is located in the center of the cage. While other atomic traps require complex equipment, e.g. laser cooling
Laser cooling includes several techniques where atoms, molecules, and small mechanical systems are cooled with laser light. The directed energy of lasers is often associated with heating materials, e.g. laser cutting, so it can be counterintuit ...
or magnetic traps, endohedral fullerenes represent an atomic trap that is stable at room temperature and for an arbitrarily long time. Atomic or ion traps are of great interest since particles are present free from (significant) interaction with their environment, allowing unique quantum mechanical phenomena to be explored. For example, the compression of the atomic wave function
In quantum physics, a wave function (or wavefunction) is a mathematical description of the quantum state of an isolated quantum system. The most common symbols for a wave function are the Greek letters and (lower-case and capital psi (letter) ...
as a consequence of the packing in the cage could be observed with ENDOR spectroscopy. The nitrogen atom can be used as a probe, in order to detect the smallest changes of the electronic structure of its environment.
Contrary to the metallo endohedral compounds, these complexes cannot be produced in an arc. Atoms are implanted in the fullerene starting material using gas discharge (nitrogen and phosphorus complexes) or by direct ion implantation
Ion implantation is a low-temperature process by which ions of one element are accelerated into a solid target, thereby changing the target's physical, chemical, or electrical properties. Ion implantation is used in semiconductor device fabrica ...
. Alternatively, endohedral hydrogen fullerenes can be produced by opening and closing a fullerene by organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
methods.
A recent example of endohedral fullerenes includes single molecules of water encapsulated in C60.
Noble gas endofullerenes are predicted to exhibit unusual polarizability. Thus, calculated values of mean polarizability of Ng@C60 do not equal to the sum of polarizabilities of a fullerene cage and the trapped atom, i.e. exaltation of polarizability occurs. The sign of the Δ''α'' polarizability exaltation depends on the number of atoms in a fullerene molecule: for small fullerenes (), it is positive; for the larger ones (), it is negative (depression of polarizability). The following formula, describing the dependence of Δα on n, has been proposed: Δ''α'' = ''α''Ng(2''e''−0.06(''n'' – 20)−1). It describes the DFT-calculated mean polarizabilities of Ng@C60 endofullerenes with sufficient accuracy. The calculated data allows using C60 fullerene as a Faraday cage, which isolates the encapsulated atom from the external electric field. The mentioned relations should be typical for the more complicated endohedral structures (e.g., C60@C240 and giant fullerene-containing "onions" ).
Molecular endofullerenes
Closed fullerenes encapsulating small molecules have been synthesized. Representative are the synthesis of the dihydrogen endofullerene H2@C60, the water endofullerene H2O@C60, the hydrogen fluoride endofullerene HF@C60, and the methane endofullerene CH4@C60. The encapsulated molecules display unusual physical properties which have been studied by a variety of physical methods. As shown theoretically, compression of molecular endofullerenes (e.g., H2@C60) may lead to dissociation of the encapsulated molecules and reaction of their fragments with interiors of the fullerene cage. Such reactions should result in endohedral fullerene adducts, which are currently unknown.
See also
* Fullerene ligands
* Inclusion compounds
References
{{reflist, 30em
External links
Movie "Helium atom trapped in fullerene (C60) and dodecahedrane (C20H20)" (Youtube)
Fullerenes
Supramolecular chemistry