Putting ammonia into a chemically opened fullerene

Article abstract of DOI:10.1021/ja805579m

We put ammonia into an open-cage fullerene with a 20-membered ring (1) as the orifice and examined the properties of the complex using NMR and MALDI-TOF mass spectroscopy. The proton NMR shows a broad resonance corresponding to endohedral NH₃ at δ_H = -12.3 ppm relative to TMS. This resonance was seen to narrow when a ¹⁴N decoupling frequency was applied. MALDI spectroscopy confirmed the presence of both 1 (m/z = 1172) and 1 + NH₃ (m/z = 1189), and integrated intensities of MALDI peak trains and NMR resonances indicate an incorporation fraction of 35-50% under our experimental conditions. NMR observations showed a diminished incorporation fraction after 6 months of storage at -10°C, which indicates that ammonia slowly escapes from the open-cage fullerene.

Full text of DOI:10.1021/ja805579m

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Putting Ammonia into a Chemically Opened Fullerene
Keith E. Whitener, Jr.,† Michael Frunzi,† Sho-ichi Iwamatsu,*,‡ Shizuaki Murata,*,‡
R. James Cross,*,† and Martin Saunders*,†
Department of Chemistry, Yale UniVersity, P.O. Box 208107, New HaVen, Connecticut
06520-8107, and Graduate School of EnVironmental Studies, Nagoya UniVersity, Chikusa-ku,
Nagoya 464-8601, Japan
Received July 17, 2008; E-mail: james.cross@yale.edu
Abstract: We put ammonia into an open-cage fullerene with a 20-membered ring (1) as the orifice and
examined the properties of the complex using NMR and MALDI-TOF mass spectroscopy. The proton NMR
shows a broad resonance corresponding to endohedral NH3 at δH ) -12.3 ppm relative to TMS. This
resonance was seen to narrow when a 14N decoupling frequency was applied. MALDI spectroscopy
confirmed the presence of both 1 (m/z ) 1172) and 1 + NH3 (m/z ) 1189), and integrated intensities of
MALDI peak trains and NMR resonances indicate an incorporation fraction of 35-50% under our
experimental conditions. NMR observations showed a diminished incorporation fraction after 6 months of
storage at -10 °C, which indicates that ammonia slowly escapes from the open-cage fullerene.
Introduction
activation barrier for insertion of the dopant molecule. A high
incorporation fraction of small molecules into the fullerene can
Ever since the discovery of fullerenes in 1985,1 it was realized
that the space inside could hold atoms and even small molecules.
Several methods have been developed for producing endohe-
drally doped fullerenes, including ion bombardment and high-
pressure/high-temperature techniques.2-9 The main drawbacks
of these methods are low yield (<1%) and the need for extreme
reaction conditions. A relatively new approach to producing
endohedral fullerenes has been the “molecular surgery” method,
whereby a hole is chemically opened on the surface of the
fullerene cage, and a gas molecule is inserted reversibly through
the orifice.10-24 This method avoids the drawbacks of the others
because the orifice in the fullerene considerably lowers the
thus be achieved under relatively mild reaction conditions.
Komatsu et al. added groups to C60 to create a 13-membered
ring in the carbon cage.14 This is large enough to permit H2
and He to enter.15,16 They then devised a sequence of reactions
that closed the hole, trapping H2 inside to produce H2@C60 in
high yield.17
Several synthetic groups have been working to produce novel
open-cagefullerenederivativeswithvariousorificesizes.12,13,16,19-22
In particular, Iwamatsu et al.20,21 have succeeded in synthesizing
a fullerene with a 20-membered ring as the orifice (1), the largest
opening synthesized on a fullerene to date; see Figure 1.
The size of this orifice is such that a water molecule
spontaneously incorporates into the fullerene cage at room
temperature.21 Furthermore, it has been shown that, under
suitable reaction conditions, CO can be incorporated into 1.23
We report here the synthesis of a new endohedral fullerene,
† Yale University.
‡ Nagoya University.
(1) Kroto, H. W.; Heath, J. R.; Obrien, S. C.; Curl, R. F.; Smalley, R. E.
Nature 1985, 318, 162–163.
(2) Weiske, T.; Bo¨hme, D. K.; Hrusa´k, J.; Kra¨tschmer, W.; Schwarz, H.
Angew. Chem., Int. Ed. Engl. 1991, 30, 884–886.
(3) Weiske, T.; Hrusa´k, J.; Bo¨hme, D. K.; Schwarz, H. Chem. Phys. Lett.
1991, 186, 459–462.
(4) Saunders, M.; Jime´nez-Va´zquez, H. A.; Cross, R. J.; Poreda, R. J.
Science 1993, 259, 1428–1430.
(13) Murata, Y.; Murata, M.; Komatsu, K. J. Am. Chem. Soc. 2003, 125,
7152–7153.
(5) Saunders, M.; Jime´nez-Va´zquez, H. A.; Cross, R. J.; Mroczkowski,
S.; Gross, M. L.; Giblin, D. E.; Poreda, R. J. J. Am. Chem. Soc. 1994,
116, 2193.
(14) Murata, Y.; Murata, M.; Komatsu, K. Chem.-Eur. J. 2003, 9, 1600–
1609.
(15) Stanisky, C. M.; Cross, J.; Cross, R. J.; Saunders, M.; Murata, M.;
Murata, Y.; Komatsu, K. J. Am. Chem. Soc. 2005, 127, 299–302.
(16) Komatsu, K.; Murata, M.; Murata, Y. Science 2005, 307, 238–240.
(17) Murata, M.; Murata, Y.; Komatsu, K. J. Am. Chem. Soc. 2006, 128,
8024–8033.
(18) Murata, Y.; Maeda, S.; Murata, M.; Komatsu, K. J. Am. Chem. Soc.
2008, 130, 6702.
(19) Iwamatsu, S.-i.; Murata, S.; Andoh, Y.; Minoura, M.; Kobayashi, K.;
Mizorogi, N.; Nagase, S. J. Org. Chem. 2005, 70, 4820–4825.
(20) Iwamatsu, S.; Uozaki, T.; Kobayashi, K.; Re, S.; Nagase, S.; Murata,
S. J. Am. Chem. Soc. 2004, 126, 2668–2669.
(21) Iwamatsu, S.-i.; Murata, S. Tetrahedron Lett. 2004, 45, 6391–6394.
(22) Iwamatsu, S.-i.; Murata, S. Synlett 2005, 2117–2129.
(23) Iwamatsu, S.; Stanisky, C. M.; Cross, R. J.; Saunders, M.; Mizorogi,
N.; Nagase, S.; Murata, S. Angew. Chem., Int. Ed. 2006, 45, 5337–
5340.
(6) Saunders, M.; Cross, R. J.; Jime´nez-Va´zquez, H. A.; Shimshi, R.;
Khong, A. Science 1996, 271, 1693.
(7) Shimshi, R.; Cross, R. J.; Saunders, M. J. Am. Chem. Soc. 1997, 119,
1163–1164.
(8) Saunders, M.; Cross, R. J. Putting Nonmetals into Fullerenes. In
Endofullerenes: A New Family of Carbon Clusters; Akasaka, T.,
Nagase, S., Eds.; Kluwer: Dordrecht, 2002; Vol. 3, pp 1-11.
(9) Cross, R. J.; Khong, A.; Saunders, M. J. Org. Chem. 2003, 68, 8281–
8283.
(10) Hummelen, J. C.; Prato, M.; Wudl, F. J. Am. Chem. Soc. 1995, 117,
7003.
(11) Arce, M. J.; Viado, A. L.; An, Y. Z.; Khan, S. I.; Rubin, Y. J. Am.
Chem. Soc. 1996, 118, 3775.
(12) Rubin, Y.; Jarrosson, T.; Wang, G. W.; Bartberger, M. D.; Houk, K. N.;
Schick, G.; Saunders, M.; Cross, R. J. Angew. Chem., Int. Ed. 2001,
40, 1543.
(24) Xiao, Z. J. Am. Chem. Soc. 2007, 129, 16149–16162.
9
13996 J. AM. CHEM. SOC. 2008, 130, 13996–13999
10.1021/ja805579m CCC: $40.75
2008 American Chemical Society

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Putting Ammonia into a Chemically Opened Fullerene
A R T I C L E S
Figure 1. Iwamatsu’s open-cage fullerene, 1.
NH3@1, and confirm its existence via NMR and MALDI mass
spectroscopy.
Experimental Section
Sample Preparation. The open-cage fullerene 1 was prepared
as described in the literature.19,22 Liquid ammonia was prepared
from an ammonium salt. Briefly, an aqueous solution of ammonium
sulfate and sodium hydroxide was refluxed. A vessel attached to
the top of the condenser was immersed in a dry ice/acetone bath to
condense ammonia from the reflux vapors. About 2 mL of liquid
ammonia was obtained in this way. The ammonia was then
transferred to a cooled sample tube containing 1 (10 mg) dissolved
in 3 mL of 1,1,2,2-tetrachloroethane. The tube was placed into a
high pressure vessel (Autoclave Engineering), and the vessel was
sealed. The vessel was left at room temperature and allowed to
remain under pressure for 20 h. The pressure of ammonia gas inside
the vessel was about 7 atm, the vapor pressure at room temperature.
After 20 h, the sample was removed and purified on a short (10
cm) silica gel column eluting with 50% toluene in ethyl acetate.
The crude material was brown and opaque with some solid material
suspended in the solution. The purified material was a clear red
solution. The solvent was removed without heating, yielding a red
powder, which was stored under nitrogen at -10 °C. The yield of
1 is about 50%. About 35-50% contains an ammonia molecule
(see below). Increasing the temperature causes increased decom-
position of 1 but gives a higher incorporation of ammonia. NMR
spectroscopy of the purified material revealed no trace of free
ammonia dissolved in solution, indicating that the purification
process removes any ammonia that is not incorporated into 1.
NMR Spectroscopy. All spectra were taken in a solution of
CDCl3 except for the low-temperature spectra, which were taken
in a solution of CD2Cl2.
Figure 2. Downfield part of the proton NMR spectrum of NH3@1.
an extraction delay time of 200 ns. Scans with and without use of
1,8,9-anthracenetriol matrix were performed.
Results and Discussion
The sample of NH3@1 was prepared as described above.
After purification, the incorporation of ammonia into 1 at room
temperature is found to be between 35% and 50% by NMR
and MALDI-TOF mass spectroscopy. Higher incorporation
fractions can be achieved with higher temperature, but the
accelerated reaction of ammonia with the external structure of
1 at elevated temperatures destroys a significant fraction of the
fullerene. Also, ammonia is observed to incorporate in small
quantities (<5%) simply by adding liquid ammonia to an open
vessel containing a solution of 1 in 1,1,2,2-tetrachloroethane
and letting the mixture warm to room temperature.
The MALDI spectrum of NH3@1 in a 1,8,9-anthracenetriol
matrix reveals the presence of both 1 (m/z ) 1172) and NH3@1
(m/z ) 1189) in a roughly 2:1 ratio. When the matrix is removed
or the laser power is increased, the intensity of the NH3@1 peak
is dramatically reduced, indicating that the laser is causing the
escape of the gas from the cage. Because this process can be
assumed to occur to some extent even in the presence of a
matrix, it was concluded that the incorporation fractions obtained
from the MALDI spectra were likely unreliable and that NMR
studies would give more accurate values.
The downfield part of the proton NMR spectrum of NH3@1
is shown in Figure 2. The spectrum is almost identical to that
of empty 1. There are two doublets, at 3.0 and 3.5, due to two
methylene protons at the throat of 1 (see Figure 1). These peaks
have been previously shown to shift by small amounts when
an atom or molecule is put inside 1.23 In the present case, the
peaks show a small splitting because some of the open-cage
fullerene is filled and some is empty (see the inset in Figure 2).
However, the splitting is not large enough to get a good ratio
of peak areas to determine the incorporation fraction.
The upfield region of the proton NMR is shown in the top
panel of Figure 3. There is a very broad peak at δH ) -12.3
ppm, fwhm ≈ 45 Hz (peak B). This is contrasted with the peak
at δH ) -11.4 ppm due to H2O@1, which has been previously
seen and which has a fwhm of less than 1 Hz (peak A). The
integrated intensity of the peak at δH ) -12.3 ppm in the
spectrum indicated that the incorporation fraction of ammonia
was between 35% and 50%. The dramatic upfield shift from
1H Spectroscopy. The 1H NMR spectrum was taken on a Bruker
Avance 500 MHz spectrometer running at a 1H frequency of 499.95
MHz. Typically, 32 pulses were used with a width of 9.45 µs and
a recycling delay of 0.5 s. All chemical shifts were relative to TMS.
Line broadening of up to 0.3 Hz was used to improve the signal-
to-noise ratio.
14N Spectroscopy. The 14N NMR spectrum was taken on the
same spectrometer running at a 14N frequency of 36.11 MHz.
Typically, 32 pulses were used with a width of 25 µs and a recycling
delay of 5 s. All chemical shifts were reported relative to
tetraethylammonium chloride in D2O. Line broadening of up to 3
Hz was used to improve the signal-to-noise ratio.
1
14N Decoupling. We modified a standard H-13C CW decou-
pling sequence by changing the 13C frequency. This was used to
1
decouple 14N from H spins. A 14N pulse width of 10.9 µs was
used. The decoupling frequency was set at -87.75 ppm relative to
the 14N shift of tetraethylammonium chloride in D2O. The power
was attenuated typically by 40 dB.
MALDI-TOF Mass Spectroscopy. MALDI-TOF mass spectra
were acquired on an Applied Biosystems Voyager DE Pro MALDI-
TOF fitted with a nitrogen laser (337 nm, 3 ns pulses) in negative
ion reflector mode and using an accelerating voltage of 25 kV and
9
J. AM. CHEM. SOC. VOL. 130, NO. 42, 2008 13997

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A R T I C L E S
Whitener et al.
Table 1. Chemical Shifts
NH3
NH3@1
∆δ
1H
0.465
-66.7
-12.3
-87.8
-12.8
-21.1
14N
The first hypothesis was tested by decoupling the 14N nucleus
from the surrounding protons. An rf pulse at the 14N resonant
frequency causes rapid transitions among the three m states in
14N. The pulse sequence was tested on tetraethylammonium
chloride salt dissolved in D2O. Here, the tetrahedral symmetry
around the 14N means that the electric field gradient at the
nucleus is zero, and the quadrupolar relaxation is not present.
The characteristic 1 Hz N-H j-coupling in the methyl protons
of the salt disappeared when the rf was set to the salt’s 14N
resonant frequency. Gaussian 0334 GIAO calculations at the
B3LYP/6-311G++(2p,2d) level indicated that the 14N frequen-
cies of the ammonium salt and the endohedral ammonia were
separated by ∼100 ppm. A 14N decoupling frequency of δN )
-87.75 ppm (relative to the ammonium salt) collapsed the large
ammonia proton line at δH ) -12.3 ppm from 45 to 2 Hz fwhm,
as seen in the bottom panel of Figure 3. This observation
indicates that the extreme broadening of the line is caused
mainly by rapid quadrupolar relaxation of the 14N nucleus,
which, in turn, relaxes the protons bound to it. It should be
noted that the measured intensities of the water peak (peak A)
in the two panels are the same; the ordinate has been rescaled
on the bottom panel.
Figure 3. Upfield region of the proton NMR of NH3@1. Top panel:
Undecoupled spectrum. Bottom panel: With 14N decoupling applied. Peak
A is due to H2O@1, and peak B is due to NH3@1. Note: Spectra have
been rescaled (see text).
We measured the upfield shifts for both protons and 14N
caused by putting ammonia into 1 (Table 1). The proton NMR
of free ammonia showed a single narrow peak at 0.465 ppm.
The peak is narrow and not split by 14N because the protons
rapidly exchange with trace amounts of water in the solvent.
This difference is yet another proof that the ammonia that we
are studying is inside 1 and protected from exchange. The
upfield shift due to the incorporation is then 12.8 ppm. As stated
above, we find the 14N resonance in NH3@1 is at -87.75 ppm
relative to tetraethylammonium ion. Using 14N NMR, we found
the resonance for NH3 is at -66.67 ppm relative to tetraethyl-
ammonium ion. This gives an upfield shift for the nitrogen of
21.1 ppm. The difference between the effects on protons and
14N is due to the different positions in the open-cage fullerene
cage and to the different effects of the solvent for the dissolved
ammonia.
To test the second hypothesis, the sample was cooled to
-80 °C without the decoupling field. There are two orienta-
tions of the ammonia molecule in the cage: with the
hydrogens pointed toward or away from the mouth. If the
interconversion is sufficiently slow, the proton peak will
broaden. If the temperature is decreased, the interconversion
slows still further, and the NMR spectrum would show two
peaks. The ammonia proton fwhm decreased from 45 to 13
Hz, probably due to the decreased quadrupole relaxation of
the 14N, but did not split into two lines. Also, several other
lines too broad to be distinguished from the noise appeared
to be sharpened as well, although not as much as when
decoupled. When the spectrum was decoupled at -80 °C,
no additional effects beyond those reported in the room-
temperature decoupling experiment relating to peak height
or area were seen. This suggests that the small extra
resonances observed at low temperatures are not additional
ammonia dissolved in CDCl3 (δH ) 0.465 ppm) is characteristic
of endohedral molecules inside fullerenes, as the ring currents
induced by the external static magnetic field produce an
opposing field inside the fullerene, which shields the nuclei
inside. This effect has been predicted theoretically and observed
experimentallyinanumberofotherendohedralfullerenes.16,20,25-33
Two potential causes were hypothesized for the significant
broadening of the observed proton resonance. First, the broaden-
ing could be due to the coupling with the nuclear spin of the
nitrogen. Because 14N has a nuclear spin of I ) 1, the proton
peak should be split into a 1:1:1 triplet. However, 14N has a
large electric quadrupole moment, and, in an asymmetric
environment, the tumbling of the molecule causes rapid enough
relaxation of the 14N spin to broaden the three proton peaks to
give a single broad line. This effect is common in protons
bonded to nitrogen. Second, the umbrella inversion of ammonia
inside the open-cage fullerene might be slow on the NMR time
scale, leading to a broadening of the line as the protons spend
more time in unequal chemical environments.
¨
(25) Buhl, M.; Thiel, W.; Jiao, H.; Schleyer, P. v. R.; Saunders, M.; Anet,
F. A. L. J. Am. Chem. Soc. 1994, 116, 6005.
(26) Cioslowski, J. Chem. Phys. Lett. 1994, 227, 361–364.
(27) Saunders, M.; Jime´nez-Va´zquez, H. A.; Cross, R. J.; Mroczkowski,
S.; Freedberg, D. I.; Anet, F. A. L. Nature 1994, 367, 256.
(28) Bu¨hl, M.; Thiel, W. Chem. Phys. Lett. 1995, 233, 585.
(29) Khong, A.; Jimenez-Vazquez, H. A.; Saunders, M.; Cross, R. J.;
Laskin, J.; Peres, T.; Lifshitz, C.; Strongin, R.; Smith, A. B. J. Am.
Chem. Soc. 1998, 120, 6380–6383.
(30) Shabtai, E. J. Am. Chem. Soc. 1998, 120, 6389–6393.
(31) Syamala, M. S.; Cross, R. J.; Saunders, M. J. Am. Chem. Soc. 2002,
124, 6216–6219.
(32) Sears, D. N.; Jameson, C. J. J. Chem. Phys. 2003, 118, 9987–9989.
(33) Sternfeld, T.; Saunders, M.; Cross, R. J.; Rabinovitz, M. Angew. Chem.,
Int. Ed. 2003, 42, 3136–3139.
(34) Frisch, M. J.; Gaussian 03; Gaussian, Inc.: Pittsburgh, PA, 2003.
9
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Putting Ammonia into a Chemically Opened Fullerene
A R T I C L E S
Summary
conformations of NH3 inside 1, but rather NH3 inside
chemically modified derivatives of 1, likely byproducts of
the reaction of ammonia with the external scaffolding around
the orifice of 1.
We have demonstrated the incorporation of NH3 into the
open-cage fullerene 1. NMR and MALDI spectroscopy indicate
the formation of NH3@1 where the ammonia is highly shielded
by the fullerene cage and the protons undergo rapid relaxation
in the presence of the 14N electric quadrupole moment. The
gas is observed to slowly escape over time, reverting to the
original empty open-cage fullerene.
After more than 6 months of storage under N2 at -10 °C,
the resonance at δH ) -12.3 ppm was still observed in the
1H NMR spectrum of our sample, but at greatly reduced
intensity. The small resonance due to H2O@1, however,
remained at roughly the original intensity. These results
indicate that the incorporation of ammonia into the open-
cage fullerene is reversible and that the incorporated gas leaks
out of the cage over a long period of time. Water apparently
has a low barrier for going into or out of 1. Therefore,
H2O@1 rapidly equilibrates with the trace water in the NMR
solvent. Ammonia that escapes from 1 diffuses out of the
NMR tube and is lost.
Acknowledgment. We are grateful to Dr. Eric Paulson for his
help in doing the NMR spectroscopy.
Supporting Information Available: Complete ref 34. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JA805579M
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