- Endohedral hydrogen fullerene
Endohedral hydrogen fullerene or H2@C60 is an endohedral fullerene containing molecular
hydrogen . Thischemical compound has a potential application inmolecular electronics and was synthesized in 2005 atKyoto University by the group of Koichi Komatsu ["Synthesis, Structure, and Properties of Novel Open-Cage Fullerenes Having Heteroatom(s) on the Rim of the Orifice" Yasujiro Murata, Michihisa Murata, and Koichi Komatsu Chem. Eur. J. 2003, 9, No. 1600] ["Encapsulation of Molecular Hydrogen in Fullerene C60 by Organic Synthesis" Koichi Komatsu, Michihisa Murata, Yasujiro Murata Science 2005 VOL 238 307] . 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 byorganic synthesis 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 groups and apyridine group for reasons ofsolubility and reacted in 1,2-dichlorobenzene with pristine C60 fullerene 2 in aDiels-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 withsinglet oxygen incarbon tetrachloride which causes one of the ringalkene groups to oxidize to aketone . The 12-ring is extended to a 13-ring by reaction with elementalsulfur in presence oftetrakis(dimethylamino)ethylene .The proposed
reaction mechanism is depicted in a plat surface rendition in "scheme 2". In the first step the triazine reacts with the fullerene in aDiels-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+4]cycloaddition to an intermediate 4 with twocyclopropane rings. This intermediate quickly rearranges in a retro [2+2+2] cycloaddition to the 8 membered ring product 5.In silico calculations show that the electrons in theHOMO reside primarily in the double bonds of the butadiene part of the ring and indeedsinglet oxygen reacts at these positions through the dioxetane intermediate 6 with alkene cleavage to diketone 7 (only one isomer shown). Elementalsulfur 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 byone-electron reduction or by complexation.From
X-ray crystallography 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 oxidizes the sulfur group to asulfoxide group which can then be extracted from the ring by aphotochemical reaction under visible light in toluene. The two ketone groups are re-coupled in aMcMurry reaction withtitanium tetrachloride and elementalzinc . 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 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 (sodium thiolate , Se8) exploiting larger C-Sebond length . Filling cracked-open fullerene now takes 8 hours at 190 °C at 760 atmospheres (77 MPa) of hydrogen and release between 150 °C and 180 °C is three times as fast compared to the sulfur analogue. Theactivation energy for release is lowered by 0.7 kcal/mol to 28.2 kcal/mol (2.9 to 118 kJ/mol). ["Fine tuning of the orifice size of an open-cage fullerene by placing selenium in the rim: insertion/release of molecular hydrogen" Shih-Ching Chuang, Yasujiro Murata, Michihisa Murata, Sadayuki Mori, Shuhei Maeda, Fumiyuki Tanabe and Koichi KomatsuChem. Commun. , 2007, 1278 - 1280, DOI|10.1039/b616478a ]There is evidence that hydrogen incarcered in the fullerene cage is not completely shielded from the oudside world as one study found that H2@C60 is more efficient at quenching
singlet oxygen than empty C60 ["Can H2 Inside C60 Communicate with the Outside World?" Juan Lopez-Gejo, Angel A. Martı, Marco Ruzzi, Steffen Jockusch, Koichi Komatsu, Fumiyuki Tanabe, Yasujiro Murata and Nicholas J. TurroJ. AM. CHEM. SOC. 2007, 129, 14554-14555 DOI|10.1021/ja076104s ] .References
Wikimedia Foundation. 2010.