Organic and organometallic derivatives of dihydrogen-encapsulated [60]fullerene

Article abstract of DOI:10.1021/ja056077a

Regioselective multi-addition reaction of organocopper and amine compounds onto dihydrogen-encapsulated [60]fullerene, H₂@C₆₀, produced a variety of organic and organometallic derivatives of H₂@C₆₀. The X-ray crystallographic analysis of dihydrogen-encapsulated bucky ferrocene, Fe(H₂@C₆₀Ph₅)C₅H₅, showed the presence of the dihydrogen molecule located almost in the center but slightly away from the ferrocene moiety. The ¹H NMR chemical shift values for the encapsulated molecular hydrogen indicated that these values are susceptible to the magnetic environment of the inside as well as the outside of the fullerene cage. Copyright

Full text of DOI:10.1021/ja056077a

Published on Web 11/18/2005
Organic and Organometallic Derivatives of Dihydrogen-Encapsulated
[60]Fullerene
Yutaka Matsuo,‡ Hiroyuki Isobe, Takatsugu Tanaka, Yasujiro Murata,§ Michihisa Murata,§
Koichi Komatsu,§ and Eiichi Nakamura*.‡,
Nakamura Functional Carbon Cluster Project, ERATO, Japan Science and Technology Agency and
Department of Chemistry, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, and
Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan
Received September 2, 2005; E-mail: nakamura@chem.s.u-tokyo.ac.jp
The dihydrogen-encapsulated [60]fullerene, H2@C601 is unique,
among other atom- or molecule-encapsulated fullerenes, in that it
has been synthesized rationally from C60 through “molecular
Scheme 1. Synthesis of Organometallic and Water-Soluble
Derivatives of H2@C60 by Regioselective Multifunctionalization
Reactions
surgery” method2 on a gram scale with maximum 100% H2
incorporation. This makes the molecule an attractive target of further
studies on its properties and applications. Herein we report the first
syntheses and X-ray structures of organic and organometallic
derivatives of H2@C60 and the use of the encapsulated molecular
hydrogen as a magnetic shielding probe.
Regioselective penta-addition of organocopper compounds3 to
[60]fullerene has created a variety of R5C60H molecules and has
led to the development of new classes of compounds and materials,
including fullerene-metallocene hybrid molecules,4,5 liquid-crystal-
forming nano shuttlecocks,6 and bilayer vesicles.7 The reaction can
be performed easily on a 10-g scale. We therefore became intrigued
with how the encapsulated H2 of the penta-adducts is useful as the
magnetic shielding probe. Thus, H2@C60 (H2@C60/C60 ) 4/1) was
treated with 15 equiv of a phenylcopper prepared in situ from equi-
argon atmosphere, and those of 3 by slow diffusion of ethanol into
a CS2 solution of 3. Diffraction data for both compounds were
collected at 143 K under nitrogen gas flow.
molar amounts of PhMgBr and CuBr‚SMe2 under the same condi-
tions as those applied for empty C60 (Scheme 1). The reaction af-
forded the desired product 1 as an orange powder in 92% isolated
yield. Compound 1 was characterized by 1H and 13C NMR measure-
ments and further converted to its potassium and iron complexes.
Deprotonation of 1 by 1.1 equiv of KH in THF at room
temperature for 30 min afforded a dark-red solution, from
which [K(thf)6][H2@C60Ph5] (2) was obtained. We also synthe-
sized the dihydrogen-encapsulated pentaphenyl bucky ferrocene
Fe(H2@C60Ph5)C5H5 (3) in a manner similar to that applied to the
synthesis of empty buckyferrocene.4 Heating 1 and [Fe(C5H5)-
(CO)2]2 in benzonitrile at 180 °C for 20 h afforded 3, which was
obtained as air- and moisture-stable red crystals in 71% isolated
yield after purification by silica gel column chromatography and
recrystallization from a mixture of CS2 and ethanol.
We also synthesized compounds that are soluble in a variety of
solvents including water. Thus, H2@C60 and piperidine were dis-
solved in a mixture of chlorobenzene and dimethyl sulfoxide
(DMSO) in the presence of molecular oxygen to obtain tetrapip-
eridinofullerene epoxide 4a in 80% yield.8 Similarly, the reaction
with 4-(2-hydroxyethyl)piperidine gave an amphiphilic aminofuller-
ene 4b in 78% yield. The reaction rates in this and the penta-addi-
tion reactions were found to be qualitatively the same as those of
the empty fullerene, reflecting that the encapsulation of dihydrogen
does not affect the reactivity of the fullerene cage as expected.
The structures of the above products were determined by X-ray
crystallographic analysis for 2 and 3. Single crystals of 2 were
obtained by slow diffusion of hexane into a THF solution of 2 under
The crystal structure of 2 consists of two crystallographically
independent ion-pairs, K(H2@C60Ph5)(thf)3 (2a; Figure 1a) and
[K(thf)6][H2@C60Ph5] (2B; Figure 1b). Comparison of these
structures with those reported for [K(thf)n][C60Ph5]9 indicates that
the encapsulated dihydrogen does not change either the molecular
structures or supramolecular and crystal packing structures.
Differential Fourier (Fo - Fc) analysis for the crystals of 2
indicates the presence of hydrogen atoms: There was an electron
density peak with a height of 0.31 electrons Å-3 in the center of
the cage of 2a and 2b, whereas there was no such electron density
in the empty counterpart [K(thf)n][C60Ph5].9 Full structure solution
was performed by placing one hydrogen atom10 with 1.6 occu-
pancy11 at the differential Fourier peak in the center of C60 and
isotropic refinement of the H2 position. After convergence of least-
squares refinement, the electron density of dihydrogen in the
fullerene cage was found in PLATON12 contour Fo maps (see
Supporting Information). The center of the dihydrogen molecule
is located 0.10 Å downward from the centroid of the 60 carbon
atoms of the fullerene skeleton.
The crystal structure of 3 is shown in Figure 1c. A differential
Fourier map, obtained after placement of iron and carbon atoms
during structure solution, exhibited the strongest electron density
peak (0.53 electrons Å-3) at the center of the fullerene cage. Contour
Fo maps obtained after fully solving the structure11 also showed
high electron density at the center of the cage. Note that no electron
density appears in the cage of the empty bucky ferrocene. The center
of the dihydrogen molecule is located 0.20 Å downward from the
centroid of the 60 carbon atoms of the fullerene skeleton.
‡ ERATO, Japan Science and Technology Agency.
The University of Tokyo.
§ Kyoto University.
9
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10.1021/ja056077a CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
was observed as a singlet at -10.74, -10.76, -10.80, and -10.85
ppm in THF-d8, DMSO-d6/toluene-d8, DMSO-d6, and D2O/DMSO-
d6, respectively. The upfield shifting of the encapsulated H2 follows
qualitatively the magnetic susceptibility of the solvents,18 suggesting
that a nonspecific solvent effect contributes to the change in
chemical shifts.19
In summary, we have synthesized the organic and organometallic
derivatives of dihydrogen-encapsulated [60]fullerene 2-4 in good
yield and showed that the uniquely upfield-shifted singlet signal
of the encapsulated dihydrogen can act as a sensitive probe for
inside and outside environment of the fullerene cage. With the
success of the multi-addition reactions in hand, we expect that a
number of other reactions known for fullerenes can be performed
readily on H2@C60 and will produce an array of new compounds
for further studies.
Figure 1. X-ray crystal structures of 2 and 3. (a) K[H2@C60Ph5](thf)3 (2a).
(b) [K(thf)6][H2@C60Ph5] (2b). (c) Fe(H2@C60Ph5)(C5H5) (3). Red line-
works in the fullerene cages represent differential Fourier peaks.
Table 1. Chemical Shift of Encapsulated H2 for 1-4
compound
solvent
δa
H2@C60
1
2
3
4a
4b
4b
4b
4b
1,2-Cl2C6D4
CDCl3/CS2
THF-d8
CDCl3/CS2
CDCl3
THF-d8
DMSO-d6/toluene-d8
DMSO-d6
D2O/DMSO-d6
-1.44b
-10.39
-9.79
-10.44
-10.77
-10.74
-10.76
-10.80
-10.85
Acknowledgment. We thank Monbukagakusho (the 21st Cen-
tury COE Program for Frontiers in Fundamental Chemistry and
grant-in-aid for Scientific Research, Young Scientists (A)) for partial
financial support.
Supporting Information Available: Synthetic procedures and
spectral data (PDF) of the new compounds as well as CIF files for
2 and 3. This material is available free of charge via the Internet at
http://pubs.acs.org.
c
c
a The spectra were referenced internally to tetramethylsilane as a standard.
b Reference 1. c Two solvents are mixed at 1:1 ratio (v/v).
References
As in the He@C60,13 the 1H NMR chemical shift of the
encapsulated H2 molecule serves as a sensing probe for investigation
of the magnetic and electronic properties of the fullerene π-con-
jugated system (cf. Table 1). The 1H NMR signals due to the
cyclopentadiene of 1 (δ ) 5.29) and the phenyl protons remain
the same as those of C60Ph5H (cyclopentadiene; δ ) 5.30). The
singlet signal of the encapsulated H2 of 1 appears at δ ) -10.39
(Table 1) as opposed to δ ) -1.44 for H2@C60.1 This extraordinary
upfield shift (∆δ 8.95 ppm)14 indicates that the penta-addition causes
a major change in the shielding/deshielding currents, either increased
shielding or reduced deshielding effect; the phenylation reaction
destroys six deshielding [5]radialene moieties15 out of the total of
12 in [60]fullerene, but only five shielding cyclohexatriene
moieties15 out of the total of 20. Therefore, the overall outcome of
these structural changes must result in strong shielding of the center
of the cage. A similar upfield shift was observed with amino-
fullerenes 4a and b that have the same number of [5]radialene and
cyclohexatriene moieties.16
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nitrogen temperature, which would make it difficult to determine precisely
its location under the conditions we employed.
The 1H NMR measurement of the potassium cyclopentadienide
2 in THF-d8 indicates a time-averaged C5V symmetric structure. The
signal due to the encapsulated H2 appears as a singlet at δ )
-9.79, which is 0.6 ppm lower than 1, despite the formation of a
6π-electron shielding cyclopentadienide. Signals due to the phenyl
groups of 2 appeared at essentially the same magnetic field as in
the empty [K(thf)6][C60Ph5].3 The 13C NMR spectra of 1 and 2 were
essentially the same as those of the empty counterparts.
(11) Since starting materials contain H2@C60 and empty C60 in 4:1 ratio,
refinement of hydrogen atom is performed with 1.6 occupancy.
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(16) We ascribe the difference of chemical shift of the dihydrogen between
1
It is interesting to note that the H NMR chemical shift of the
C
60Ph5H and aminofullerenes to the effect of the carbon and the nitrogen
diydrogen in the potassium cyclopentadienide 2 (δ ) -9.79) is
the most deshielded among all related compounds in Table 1. While
it is difficult to rationalize the origin of this deshielding, we may
tentatively ascribe this to the location of the dihydrogen relative to
the anionic 6π-cyclopentadienide moiety in 2; that is, one of the
hydrogen atoms may be located in a deshielding region of the
cage.17 Further experimentation is necessary to discuss this issue
in detail.
groups attached to the fullerene cage.
(17) Theoretical calculations (GIAO-SCF/6-31G*//HF/6-31G*) suggest that the
shielding effect increases from the center toward the bottom. See
Supporting Information.
(18) Molar diamagnetic susceptibilities of the solvents (øM): H2O ) -12.96
× 106, DMSO ) -43.6 × 106, THF ) -50.4 × 106, toluene ) -66.52
× 106 cm3‚mol-1, respectively. See Gupta, R. R. Landolt-Bo¨rnstein
Numerical Data and Functional Relationship in Science and Technology,
New Series, II/16, Diamagnetic Susceptibility; Springer: Berlin, 1986.
Takahashi, F.; Sakai, Y.; Nakazawa, Y.; Mizutani, Y. Bull. Chem. Soc.
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The amphiphilic aminofullerene 4b can be dissolved in a variety
(19) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry, 3rd ed.;
Wiley: Weinheim, 2003.
1
of solvents, which allowed us to study solvent effects on the H
NMR chemical shift of the encapsulated H2 (Table 1). Its signal
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