Endohedral hydrogen fullerene
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Endohedral hydrogen fullerene ( H2@ C60) is an
endohedral fullerene Endohedral fullerenes, also called endofullerenes, are fullerenes that have additional atoms, ions, or clusters enclosed within their inner spheres. The first lanthanum C60 complex called La@C60 was synthesized in 1985. The @ (at sign) in the ...
containing molecular
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
. This
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
has a potential application in
molecular electronics Molecular electronics is the study and application of molecular building blocks for the fabrication of electronic components. It is an interdisciplinary area that spans physics, chemistry, and materials science. The unifying feature is use of mo ...
and was synthesized in 2005 at
Kyoto University , mottoeng = Freedom of academic culture , established = , type = National university, Public (National) , endowment = ¥ 316 billion (2.4 1000000000 (number), billion USD) , faculty = 3,480 (Teaching Staff) , administrative_staff ...
by the group of Koichi Komatsu. Ordinarily the payload of endohedral fullerenes are inserted at the time of the synthesis of the fullerene itself or is introduced to the fullerene at very low yields at high temperatures and high pressure. This particular fullerene was synthesised in an unusual way in three steps starting from pristine C60 fullerene: cracking open the carbon framework, insert hydrogen gas and zipping up by
organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
methods.


Organic synthesis

''Scheme 1'' presents an overview of the first step, the creation of a 13 membered ring orifice on the fullerene surface. A 1,2,4-triazine 2 is fitted with two
phenyl In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen ...
groups and a
pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a d ...
group for reasons of
solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubil ...
and reacted in
1,2-dichlorobenzene 1,2-Dichlorobenzene, or orthodichlorobenzene (ODCB), is an organic compound with the formula CHCl. This colourless liquid is poorly soluble in water but miscible with most organic solvents. It is a derivative of benzene, consisting of two adjacent ...
with pristine C60 fullerene 2 in a Diels-Alder reaction at high temperature and for an extended reaction time. In this reaction nitrogen is expulsed and an 8-membered ring is formed (3). This orifice is further extended by reaction with
singlet oxygen Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O=O (also written as or ), which is in a quantum state where all electrons are spin paired. It is kinetically unstable at ambie ...
in
carbon tetrachloride Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also IUPAC nomenclature of inorganic chemistry, recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVAC ...
which causes one of the ring
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
groups to oxidize to a
ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...
. The 12-ring is extended to a 13-ring by reaction with elemental
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
in presence of
tetrakis(dimethylamino)ethylene Tetrakis(dimethylamino)ethylene (TDAE) is an organic compound with the formula (NMe2)2sub>2 (where Me = CH3). A colorless liquid, this compound is classified as an enamine. Primary and secondary enamines tend to isomerize, but tertiary enamines a ...
. The proposed
reaction mechanism In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage of ...
is depicted in a plat surface rendition in ''scheme 2''. In the first step the triazine reacts with the fullerene in a Diels-Alder reaction. In the second step nitrogen is expulsed from the DA adduct 2 resulting in the formation of a fused aza-cyclohexadiene ring followed by a +4 ycloaddition to an intermediate 4 with two
cyclopropane Cyclopropane is the cycloalkane with the molecular formula (CH2)3, consisting of three methylene groups (CH2) linked to each other to form a ring. The small size of the ring creates substantial ring strain in the structure. Cyclopropane itself ...
rings. This intermediate quickly rearranges in a retro +2+2ycloaddition to the 8 membered ring product 5.
In silico In biology and other experimental sciences, an ''in silico'' experiment is one performed on computer or via computer simulation. The phrase is pseudo-Latin for 'in silicon' (correct la, in silicio), referring to silicon in computer chips. It ...
calculations show that the electrons in the
HOMO ''Homo'' () is the genus that emerged in the (otherwise extinct) genus ''Australopithecus'' that encompasses the extant species ''Homo sapiens'' ( modern humans), plus several extinct species classified as either ancestral to or closely relate ...
reside primarily in the double bonds of the
butadiene 1,3-Butadiene () is the organic compound with the formula (CH2=CH)2. It is a colorless gas that is easily condensed to a liquid. It is important industrially as a precursor to synthetic rubber. The molecule can be viewed as the union of two viny ...
part of the ring and indeed
singlet oxygen Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O=O (also written as or ), which is in a quantum state where all electrons are spin paired. It is kinetically unstable at ambie ...
reacts at these positions through the
dioxetane A dioxetane or dioxacyclobutane is an organic compound with formula C2O2H4, whose backbone is a four-membered ring of two oxygen atoms and two carbon atoms. There are two isomers: * 1,2-dioxetane where the oxygen atoms are adjacent. * 1,3-dioxeta ...
intermediate 6 with alkene cleavage to diketone 7 (only one isomer shown). Elemental
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
S8 is inserted into the single bond of the diene group leading to the extension of the ring to 13 atoms (structures 8 and 9 are identical). Tetrakis(dimethylamino)ethylene activates this bond for electrophilic sulfur addition either by
one-electron reduction Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
or by complexation. From
X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
it is determined that the shape of the orifice in the sulfur compound is roughly a circle. Inserting hydrogen in this compound is an easy step taking place with 100% efficiency. Zipping up the orifice is a reversal of the steps required to open the cage. Care must be taken to keep the reaction conditions below 160 °C on order to prevent hydrogen from escaping.
m-CPBA ''meta''-Chloroperoxybenzoic acid (mCPBA or ''m''CPBA) is a peroxycarboxylic acid. A white solid, it is used widely as an oxidant in organic synthesis. mCPBA is often preferred to other peroxy acids because of its relative ease of handling. mC ...
oxidizes the sulfur group to a
sulfoxide In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...
group which can then be extracted from the ring by a
photochemical Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400  nm), visible light (400–7 ...
reaction under visible light in toluene. The two ketone groups are re-coupled in a
McMurry reaction The McMurry reaction is an organic reaction in which two ketone or aldehyde groups are coupled to form an alkene using a titanium chloride compound such as titanium(III) chloride and a reducing agent. The reaction is named after its co-discoverer, ...
with
titanium tetrachloride Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds o ...
and elemental
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
. The reverse cycloadditions take place at 340 °C in a vacuum splitting of 2-cyanopyridine and diphenylacetylene resulting in the formation of H2@C60 at a 40%
chemical yield In chemistry, yield, also referred to as reaction yield, is a measure of the quantity of moles of a product formed in relation to the reactant consumed, obtained in a chemical reaction, usually expressed as a percentage. Yield is one of the pri ...
starting from pristine fullerene.


Properties

H2@C60 is found to be a stable molecule. it survives 10 minutes at 500 °C and shows the same chemical reactivity as empty C60. The electronic properties are also largely unaffected. The process of hydrogen introduction and release can be facilitated by increasing the size of the orifice. This can be done by replacing sulfur by
selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
(
sodium thiolate Sodium hydrosulfide is the chemical compound with the formula NaHS. This compound is the product of the half-neutralization of hydrogen sulfide () with sodium hydroxide (NaOH). NaSH and sodium sulfide are used industrially, often for similar purp ...
, Se8) exploiting larger C-Se
bond length In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...
. Filling cracked-open fullerene now takes 8 hours at 190 °C at 760
atmospheres The standard atmosphere (symbol: atm) is a unit of pressure defined as Pa. It is sometimes used as a ''reference pressure'' or ''standard pressure''. It is approximately equal to Earth's average atmospheric pressure at sea level. History The s ...
(77
MPa MPA or mPa may refer to: Academia Academic degrees * Master of Performing Arts * Master of Professional Accountancy * Master of Public Administration * Master of Public Affairs Schools * Mesa Preparatory Academy * Morgan Park Academy * Mound ...
) of hydrogen and release between 150 °C and 180 °C is three times as fast compared to the sulfur analogue. The
activation energy In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules pe ...
for release is lowered by 0.7 kcal/mol to 28.2
kcal The calorie is a unit of energy. For historical reasons, two main definitions of "calorie" are in wide use. The large calorie, food calorie, or kilogram calorie was originally defined as the amount of heat needed to raise the temperature of on ...
/ mol (2.9 to 118 kJ/mol). There is evidence that hydrogen in the fullerene cage is not completely shielded from the outside world as one study found that H2@C60 is more efficient at
quenching In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as pha ...
singlet oxygen Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O=O (also written as or ), which is in a quantum state where all electrons are spin paired. It is kinetically unstable at ambie ...
than empty C60.


References

{{reflist Fullerenes Molecular electronics Geodesic polyarenes