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Acknowledgments: We are grateful to the NSF
(CHE-0924410), U.S. Department of Energy
(DE-FG02-09ER16069), and DFG for financial support
of this work and to the Japan Society for the Promotion
of Science for a postdoctoral fellowship (R.K.). We thank
E. Stoyanov for helpful advice in infrared spectroscopy.
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27 April 2011; accepted 3 June 2011
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insertion of a molecule through it—wide var-
ieties of endohedral fullerenes could be synthe-
sized by organic synthesis because restoration of
the small opening should be easier than that of
the larger one. We report the macroscopic syn-
thesis of H2O@C60 using such a dynamic control
of the opening size with a series of organic re-
actions, as well as the structure and the properties
A Single Molecule of Water
Encapsulated in Fullerene C60
Kei Kurotobi and Yasujiro Murata*
Water normally exists in hydrogen-bonded environments, but a single molecule of H2O without
any hydrogen bonds can be completely isolated within the confined subnano space inside fullerene of the molecule in which a molecule of H2O is
C60. We isolated bulk quantities of such a molecule by first synthesizing an open-cage C60
derivative whose opening can be enlarged in situ at 120°C that quantitatively encapsulated one
water molecule under the high-pressure conditions. The relatively simple method was developed
to close the cage and encapsulate water. The structure of H2O@C60 was determined by
single-crystal x-ray analysis, along with its physical and spectroscopic properties.
completely isolated in a confined space.
The synthetic route for open-cage C60 deriva-
tives is outlined in Fig. 1A. Applying our previ-
ous synthetic route (13, 14), diketones 3a and
3b were synthesized in 29 and 43% isolated
yields, respectively, from the reaction of C60 with
he unusual properties of bulk water, such (ii) ion implantation to empty C60 and C70, and pyridazine derivatives 1a and 1b followed by
as its high boiling and melting points, high (iii) high-pressure and high-temperature treatments photochemical cleavage of one of the C=C double
dielectric constant, and ability to act as of empty fullerenes with rare gases (5). However, bonds on the rim of the opening on intermediates
T
both acid and base, arise from hydrogen bonding these methods are not suitable to obtain endohe- 2a and 2b, respectively (15) (figs. S1 to S12). The
in bulk. Clusters of H2O molecules, both in the dral fullerenes encapsulating small molecules. lowest unoccupied molecular orbital (LUMO) of
gas phase (1) and confined in nanoscale spaces The fourth method for synthesizing endohe- open-cage C60 with a similar opening motif is
defined by the hydrophobic interiors of carbon dral fullerenes is the molecular surgical approach mainly located on the conjugated butadiene
nanotubes (2) or a self-assembled coordination (6), which includes construction of an opening moiety (13), and sulfur (13, 14) and hydrazine
cage (3), also undergo hydrogen bonding. How- on an empty C60 or C70 cage, insertion of small derivatives (11) were reported to react with that
ever, a single molecule of H2O without any hy- molecules through it, and then restoration of the moiety. N-Methylmorpholine N-oxide (NMMO) is
drogen bonding to other organic molecule or original framework, retaining the encapsulated known as a nucleophilic oxidant (16), and we
coordination to metal is rare so far (4).
species (7). By using such an approach, hydro- found that the reaction of 3 with 2.3 equivalents
The inner space of the fullerene C60, which gen molecules have been entrapped in C60 and of NMMO in wet tetrahydrofuran (THF) at room
is spherical with a diameter of 3.7 Å, is suitable C70 in macroscopic quantities (8, 9). To apply temperature led to the synthesis of 5a and 5b,
to entrap a water molecule. When atoms or mol- this method to a larger molecule than H2, devel- respectively, in good yields after purification with
ecules are encapsulated in fullerenes, it is often opments of synthetic methods are needed to create silica gel chromatography (figs. S13 to S18).
possible to control the properties of the outer open-cage fullerenes with an opening large enough
The structure of 5a determined by single-
carbon cage as well as to study the isolated spec- for the small molecule to pass through (10). crystal x-ray analysis (Fig. 1B) shows that the
ies. Endohedral fullerenes encapsulating a wide Fullerenes with large openings to encapsulate an opening is constructed by the 13-membered ring
variety of species, including metal ions, rare gases, H2O molecule have been reported (11, 12), but containing two hemiacetal carbons, in addition to
and nitrogen atoms, have been synthesized with the encapsulated H2O is in an equilibrium, with two carbonyl carbons (table S1). This molecule
physical methods under harsh conditions such as outer H2O molecules existing in the system in can be considered as the hydrate of tetraketone
(i) arc discharge of carbon rods containing metal, large amounts. Furthermore, attempts to close 4a having the 16-membered-ring opening with
such large openings have not been reported.
four carbonyl carbons. Because wet THF was used
If the size of an opening on fullerenes can be in this reaction, a water molecule attacked one of
controlled dynamically—that is, a small opening the carbonyl carbons in 4a to give 5a. When the
changes into a larger one in situ under specific hydrate 5a was heated at reflux temperature in
conditions and regenerates itself again after toluene for 30 min, complete transformation of
Institute for Chemical Research, Kyoto University, Uji, Kyoto
611-0011, Japan.
*To whom correspondence should be addressed. E-mail:
613