Syntheses and EELS characterization of water-soluble multi-hydroxyl Gd@C 82 fullerenols

Article abstract of DOI:10.1016/S0009-2614(00)00599-6

Various water-soluble multi-hydroxyl fullerenes (fullerenols), C₆₀(OH)_n, C₇₀(OH)_n and C₈₄(OH)_n, and a Gd metallo-fullerenols, Gd@C₈₂(OH)_n have been synthesized by the Kitazawa method. Elementary chemical analyses indicate that all of these fullerenols have 30-40 hydroxyl groups and 11-15 coordinated water molecules via hydrogen bonds. Electron energy loss spectroscopy (EELS) on Gd@C₈₂ fullerenols shows that the π^* peak area in the C K-edge spectra decreases on going from intact Gd@C₈₂ metallofullerene to Gd@C₈₂ fullerenols, indicating an increase in sp³ character of the fullerenols. Furthermore, Gd M_4,5-edge EELS spectra show that the encapsulated Gd atom has a trivalent Gd³⁺ state.

Full text of DOI:10.1016/S0009-2614(00)00599-6

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7 July 2000
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Chemical Physics Letters 324 2000 255–259
www.elsevier.nlrlocatercplett
Syntheses and EELS characterization of water-soluble
multi-hydroxyl Gd@C82 fullerenols
a
b
c
c
Haruhito Kato , Kazutomo Suenaga , Masahito Mikawa , Masafumi Okumura ,
c
d
d
d
Naoto Miwa , Arihiro Yashiro , Hidefumi Fujimura , Akiko Mizuno ,
d
d
a,)
Yoshihiro Nishida , Kazukiyo Kobayashi , Hisanori Shinohara
a
Department of chemistry, Nagoya UniÕersity, Nagoya 464-8602, Japan
b
Japan Science and Technology Corporation, cro Department of Physics, Meijo UniÕersity, Nagoya 458-8502, Japan
c
Nihon Schering K.K., Osaka 532-0004, Japan
d
Department of Molecular Design Engineering, Graduate School of Engineering, Nagoya UniÕersity, Nagoya 464-8603, Japan
Received 6 April 2000; in final form 12 May 2000
Abstract
Various water-soluble multi-hydroxyl fullerenes fullerenols , C60 OH , C70 OH and C84 OH , and a Gd metallo-ful-
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.
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.
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.
n
n
n
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.
lerenols, Gd@C82 OH have been synthesized by the Kitazawa method. Elementary chemical analyses indicate that all of
n
these fullerenols have 30–40 hydroxyl groups and 11–15 coordinated water molecules via hydrogen bonds. Electron energy
loss spectroscopy EELS on Gd@C82 fullerenols shows that the p) peak area in the C K-edge spectra decreases on going
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.
from intact Gd@C82 metallofullerene to Gd@C82 fullerenols, indicating an increase in sp3 character of the fullerenols.
q
Furthermore, Gd M4,5-edge EELS spectra show that the encapsulated Gd atom has a trivalent Gd3 state. q 2000 Elsevier
Science B.V. All rights reserved.
1. Introduction
been synthesized and have shown excellent water
solubility. Chiang et al. synthesized the C60
fullerenols by a reaction of C60 with an aqueous acid
Water-soluble fullerene derivatives have been in-
vestigated to date mostly within the framework of
biological and medical applications. For example,
C60 derivatives have been applied as HIV inhibitors
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10,11 followed by the hydrolysis of ester moieties
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of a polyorganocarboxylated fullerene derivative 12 .
Kitazawa and co-workers also reported a more effi-
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1,2 , radical scavenging capabilities 3,4 and anti-
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cient reaction to obtain C60 OH , using NaOH aq
n
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cancer 5 . Water-soluble endohedral metallo-
and tetrabutylammonium hydroxide as a phase trans-
fullerene derivatives are particularly interesting be-
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fer catalyst 13 . Wilson and co-workers synthesized
. .
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cause of their possible applications to radiotracers 6
and magnetic resonance imaging MRI contrast
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metallo-fullerenols Hox@C82 OH
and examined
y
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.
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their biodistribution 14,15 . Yang et al. reported an
XPS study of water-soluble metallo-fullerenols,
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agents 7–9 . The C60 fullerenols, C60 OH , have
n
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.
n
Pr@C82 OH , in which they observed the C1s, O1s,
)
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Pr 3d3r2 and Pr 3d5r2 transitions 16 . However, the
electronic states of these fullerenols the charge state
Corresponding author. Fax: q81-52-789-2962; e-mail:
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nori@nano.chem.nagoya-u.ac.jp
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 00 00599-6

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256
H. Kato et al.rChemical Physics Letters 324 2000 255–259
.
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lar to Li et al. 8,13 . A toluene solution of isolated
fullerenes C60 , C70, C84, Gd@C82 was vigorously
stirred with 50 wt% of aqueous NaOH containing
of encaged metal atoms, in particular have never
been elucidated.
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Endohedral metallofullerenes have unique struc-
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tures that encapsulate metal atoms and metal clusters
few drops of tetrabutylammonium hydroxide 40%
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17,18 . The electronic properties of endohedral met-
allofullerenes in solid states have been investiga-
in water, TBAH or 15-crown-5 as catalyst at room
temperature under ambient conditions. C84 fullerenes
used for the reaction was a mixture of the two major
isomers with D2 and D2 d symmetry. Within a few
minutes, toluene solution turned colorless and a
brown sludge was precipitated. After removal of
toluene by decantation, the sludge and the remaining
aqueous solution were stirred with additional water
for overnight. The precipitation was rinsed with
MeOH to ensure the removal of the catalyst and
NaOH. The rinsed precipitates was dissolved in wa-
ter again, and the resultant brown solution was put
ted by such experimental techniques as ultraviolet
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photoemission spectroscopy UPS and X-ray pho-
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. w
x
toemission spectroscopy XPS 19–21 . Mono-
metallofullerenes, M@C82 , have been shown to have
electronic structures such as M3q@C
and
3y
82
2y
M2q@C
by an electron transfer from the encaged
82
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metal atom to the carbon cage 17,18,22 . Here, we
report the synthesis of multi-hydroxyl Gd@C82
fullerenols and their characterization based on ele-
mental chemical analysis, IR and electron energy
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loss spectroscopy EELS . The M4,5-edges of Gd
atoms show prominent peaks dominated by dipole
allowed transition of 3d electrons to unoccupied 4f
into a cellulose dialyzer tube cut-off MWs500 in
water for a week for complete removal of residual
catalyst and NaOH. Finally, a freeze-drying fine
aqueous solution gave brown solid product of
fullerenols.
10
n
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states 3d 4f
3d94f nq1 and reflect the 4f unoc-
.
cupied density of states; therefore they can be very
useful to determine the valency. Gd M4,5-edge EELS
spectra unambiguously show that the encapsulated
Gd atom of Gd@C82 fullerenols is a trivalent Gd3q
state.
EELS analyses were done on a Gatan spectrome-
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ter GIF, Gatan Imaging Filter in an electron micro-
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.
scope JEOL 2010F operated at 120 or 200 kV. A
few droplets of ethanol suspension of Gd@C82
fullerenols were put on holey carbon micro-grid. The
incident electron beam was focused on the specimen
within a few tens of nanometer area. The conver-
gence and collective angles for the transmission
EELS analyses were set to 7.5 and 10 mrad, respec-
tively 23,24 . The FT-IR with a KBr disc measure-
ment of fullerenols was performed on an infrared
spectrophotometer Japan Spectroscopic Co.
2. Experimental
Soot containing fullerenes and Gd endohedral
metallofullerenes such as Gd@C82 was synthesized
by the direct current arc method 500 A, 20–25 V
of Gdrgraphite composite rods 0.8 wt% Gd atom,
Toyo Tanso Co. under a 17 lrmin flow of helium at
60 Torr. The soot was soxhlet extracted with carbon
disulfide for 60 h and the residue of the soxhlet
extraction was further refluxed with pyridine. The
purification and isolation of Gd@C82 metallo-
fullerene was achieved by two-stage high-perfor-
mance liquid chromatography HPLC with a Buck-
yprep column 20 mm dia=250 mm, nacalai tesque
and a Buckyclutcher column 21 mm dia=500 mm,
Regis Chemical . The purity of Gd@C82 metallo-
fullerene )99% was quantified by both HPLC and
matrix-assisted laser desorption ionization MALDI
TOF mass spectrometry Shimadzu, UV3100 .
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JASCO , A-3 Grating Infrared Spectrophotometer .
Elemental analyses were made on CHN corders
..
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YANACO, CHN corder MT-5 and MT-3 . The
content of water molecules was determined on a
moisture meter Mitsubishi Chemical Corporation,
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Moisture Meter A-07 .
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3. Results and discussion
.
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3.1. Chemical compositions and IR spectra of the
synthesized fullerenols
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Various fullerenols were synthesized from the
corresponding intact fullerenes by a procedure simi-
The IR spectrum of Gd@C82 fullerenols Fig. 1a
showed broad hydroxyl absorption centered at 3420

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H. Kato et al.rChemical Physics Letters 324 2000 255–259
257
fullerenols possess hydrophilic character. The ob-
tained number of coordinated water molecules and
hydroxyl groups of Gd@C82 , therefore, is similar to
those of C60 , C70, and C84 fullerenols. The present
C84 used is a mixture of D2 and D2 d major isomers
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25–28 . The number of hydroxyl groups and coordi-
nated water molecules may vary depending on the
isomers.
3.2. Electron energy loss spectroscopy on the
Gd@C82 fullerenols
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Fig. 1. IR spectra of a Gd@C82 fullerenols and b acid precipi-
tates of Gd@C82 fullerenols. Gd@C82 fullerenols show broad
hydroxyl absorption centered at 3420 cmy1, and acid precipitates
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Electron energy loss spectroscopy EELS is a
powerful tool for characterizing fullerenes in general
show C-O absorption at 1720 cmy1
.
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29 and endohedral metallofullerenes in particular
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30 , because metallofullerenes have characteristic
cmy1 C–O stretching absorption centered at 1100
cmy1, and C-C absorption around 1600 cmy1
Fullerenols solution was precipitated below pH 3 by
metal atoms inside the fullerene cages which are
suitable for core-level edge measurements. Recently,
Suenaga et al. have reported both C K-edge and Gd
M4,5-edge EELS on the intact Gd@C82 metallo-
.
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adding a HCl solution. The IR spectrum Fig. 1b of
the acid precipitates showed C-O absorption cen-
tered at 1720 cmy1 and broad hydroxyl absorption at
about 3430 cmy1. This acidic condition led to con-
version of vicinal hydroxyl groups to corresponding
ketone structure with ring opening via the pinacol-
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fullerene 31 , which clearly indicated that the
encaged Gd atom had a trivalent charge state by an
electron transfer to the carbon cage. The peak posi-
tions of the Gd M-edge for intact Gd@C82 were
shown to be identical to a reference spectrum for the
Gd3q ion.
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pinacolone rearrangement 11,12 .
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The average compositions of fullerenols were cal-
culated from elemental analyses and coulometric
titrations in the Karl Fisher method for water deter-
mination. The results are shown in Table 1. The
average compositions of fullerenols so determined
Fig. 2 shows the oxygen O K-edge EELS spec-
trum of Gd@C82 fullerenols. The p) peak appears
as a shoulder of a much more intense s) peak. The
p) and s) peak appear around 530 and 538 eV,
respectively. The observed O K-edge is a clear indi-
cation of the presence of hydroxyl groups on Gd@C82
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are C60 OH P 12H 2 O, C 70 OH P 11H 2 O,
29
32
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C84 OH 34 P12H2O and Gd@C82 OH 36 P15H2O.
Approximately one half of carbon atoms of cage has
been connected to hydroxyl groups. Moreover, each
fullerenols possesses 10–17 coordinated water
molecules via hydrogen bonds. All of these
fullerenols. Fig. 3a shows the carbon C K-edge
EELS spectrum of Gd@C82 fullerenols. The C K-
edge spectrum is similar in shape to the O K-edge
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spectrum Fig. 2 . The spectral similarity of the O
and C K-edges suggests that oxygen atoms are cova-
Table 1
Observed and calculated average compositions of fullerenols
b
Ca
Ha
Water w.t.%
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Foundrcalc.
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C60 OH 29 P12H2O
50.39r50.38
52.90r53.11
56.17r55.95
48.53r 48.65
3.40r3.74
2.94r3.43
3.05r3.24
2.88r3.29
15.4r15.1
13.0r12.5
12.2r12.0
12.9r 13.3
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C70 OH 32 P11H2O
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C84 OH 34 P12H2O
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Gd@C82 OH 36 P15H2O
a
From elemental analyses.
From coulometry titrations.
b

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258
H. Kato et al.rChemical Physics Letters 324 2000 255–259
Fig. 2. Oxygen K-edge electron energy loss spectrum of Gd@C82
fullerenols.
lently bonded to carbon atoms, so that the electronic
states of oxygen and carbon atoms are closely cou-
pled with each other.
The p) peak area in the C K-edge spectra de-
creases on going from intact Gd@C82 metallo-
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a Gd@C82 fullerenols
Fig. 4. Gd M4,5-edge EELS spectra of
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and b intact Gd@C82 metallofullerene. The Gd M4,5 M5: 3d5r2
.
4f7r2 , M4: 3d3r2
4f5r2 near-edge structures recorded
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.
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fullerene Fig. 3b to Gd@C82 fullerenols Fig. 3a ,
from the Gd@C82 fullerenols and Gd@C82 metallofullerene. The
experimental spectrum is simulated as a superposition of a power-
law background, a set of Lorentzian profiles to account for the
white-lines, and a continuum term corresponding to the transitions
to empty continuum states modeled by a Hartree–Slater function
provided by the Gatan ELP 3.0 software.
indicating an increased sp3 character for Gd@C82
fullerenols through hydroxylation reactions. The ob-
served broad p) peak for Gd@C82 fullerenols can
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be attributed to a lower degree of degeneracy i.e.,
)
.
density-of-states of unoccupied p MO states due
to the lowering of symmetry of the fullerenols. Pos-
sible presence of the structural isomers of Gd@C82
fullerenols can also contribute to the observed broad
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spectral feature 29 .
To obtain information on the charge state of the
Gd atom in Gd@C82 fullerenols, we performed a Gd
M4,5-edge energy loss experiment. Fig. 4a shows the
Gd M 4,5-edge EELS spectrum of Gd@C82
fullerenols. The observed prominent peaks are the
so-called ‘white lines’ that result from the dipole
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transition of 3d electrons to the empty 4f states M5:
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a Gd@C82 fullerenols
Fig. 3. Carbon K-edge EELS spectra of
.
3d5r2
4f7r2 , M4: 3d3r2
4f5r2 . The peak
energies are equal to those of intact Gd@C82 met-
allofullerene as shown in Fig. 4b. The spectral simi-
and b intact Gd@C82 metallofullerene. The p) peak of Gd@C82
fullerenols decreases relative to the s) peak, being consistent
with the decrease in the conjugated area.
Ž .