This document summarizes the synthesis, isolation, characterization, and electrochemical behavior of some calcium-containing metallofullerenes. Specifically, it describes the first isolation of Ca@C76, Ca@C88, and two isomers of Ca@C90 using an improved arc discharge method and multi-stage HPLC separation. UV-Vis-NIR spectroscopy was used to analyze the electronic structures and possible symmetries. Cyclic voltammetry and differential pulse voltammetry provided information on the electrochemical properties and reversible reduction behaviors compared to other metallofullerenes. The features of the calcium metallofullerenes' electronic structures are also discussed.

1. Synthesis, isolation, spectroscopic and electrochemical
characterization of some calcium-containing metallofullerenes
Ya Zhang a,b
, Jianxun Xu a
, Ce Hao b
, Zujin Shi a,*, Zhennan Gu a
a
State Key Laboratory of Rare Earth Materials Chemistry and Applications, Department of Chemistry, Peking University, Beijing 100871, PR China
b
State Key Laboratory of Fine Chemicals, Department of Chemistry, Dalian University of Technology, Dalian 116024, PR China
Received 10 March 2005; accepted 17 August 2005
Available online 3 October 2005
Abstract
A few Ca-containing mono-metallofullerenes, i.e. Ca@C76, Ca@C88, Ca@C90 (I, II), were synthesized by an improved DC arc dis-
charge method and isolated by a multi-stage HPLC method for the first time. These isomer-separated metallofullerenes were character-
ized by LD-TOF MS and UV–vis–NIR spectrometry. Their HOMO-LUMO band gaps and possible molecular geometries are discussed
according to the absorption spectra in this report. In addition, the cyclic and differential pulse voltammetry of Ca@C76 was conducted in
MeCN/C6H5CH3 (1:4 v/v). The voltammograms of Ca@C82 (II, III) and Ca@C84 (II) were also recorded. Their electrochemical behav-
iors are discussed compared with those of corresponding ytterbium metallofullerenes. The features of the Ca metallofullerenesÕ electronic
structures are also discussed.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Metallofullerenes; Arc discharge; Absorption spectrometry; Electrochemistry
1. Introduction
Endohedral metallofullerenes have attracted much
attention because of their unique structures and novel
properties [1]. This kind of materials are expected to show
promising applications in diverse areas, such as material
and biology science. The so-called divalent metallofulle-
renes are of special attraction because of their abundant
isomers based on different cages (usually ranging from 72
to 84), for example, Ca@C2n, Sm@C2n, Yb@C2n and
Eu@C2n [2–7]. In contrast, the trivalent metallofullerenes
are mostly based on C82 and C84 cages. Up to now, quite
a few Ca-containing metallofullerenes have been isolated
and characterized. In 1995, kubozono et al. succeeded in
isolating the first Ca-containing metallofullerene, i.e.
Ca@C60, using aniline as the extractant [8]. In 1997, Cao
et al. observed a series of Ca-containing metallofullerenes
from Ca@C58 to Ca@C100 in CS2 extract [9]. In addition,
Shinohara and co-workers obtained some isomer-separated
Ca-containing metallofullerenes, namely, Ca@C2n
(2n = 72, 74, 80, 82, 84) by a multi-step HPLC method
[2–4]. Furthermore, Ca@C82 (III) was determined to be
C2 symmetry by 13
C NMR [10]. However, the synthesis
and isolation of some other cage-based Ca-containing
metallofullerenes were neglected in previous work.
Here we present the first isolation and characterization
of some Ca-containing metallofullerenes, i.e. Ca@C76,
Ca@C88 and Ca@C90 (I, II). They were synthesized by
an improved arc discharge method and isolated in iso-
mer-free form by multi-step HPLC. The purities (>98%)
were determined by LD-TOF MS. In addition, the UV–
vis–NIR spectroscopy, cyclic voltammetry (CV) and differ-
ential pulse voltammetry (DPV) were conducted for these
‘‘new’’ species. The valence state of the endohedral calcium
ion, the possible molecular symmetries and the electronic
structures were discussed according to the absorption spec-
tra and electrochemical voltammograms.
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2005.08.018
*
Corresponding author. Tel.: +861062751495; fax: +861062751708.
E-mail address: zjshi@pku.edu.cn (Z. Shi).
www.elsevier.com/locate/carbon
Carbon 44 (2006) 475–479

2. 2. Experimental Section
Soot containing calcium metallofullerenes was produced
by the DC arc discharge method. The anode is a
B6 · 150 mm graphite rod with a drilled B4 · 120 mm
hole, filled with a powder mixture of graphite, CaC2 and
Ni (10:1:1, atomic ratio). The cathode was a B10 mm
graphite rod with a tapered end to prevent the formation
of deposit. The arc was generated at 65 A in 720 Torr he-
lium static atmosphere, while the gap between the anode
and cathode was kept as large as possible by adjusting
the anode. Usually, a 12-cm long anode rod yields 4.5 g
soot. The raw soot was collected under the protection of
nitrogen to avoid air degradation. The soot was extracted
with o-dimethylbenzene for several times under sonication
until the extract was colorless. The LD-TOF mass spec-
trum of the o-dimethylbenzene extract showed that, under
our experimental conditions, the relative abundance of
each isomer was much different compared with previous re-
port. Especially, Ca@C76 was much more abundant, which
was not isolated in isomer-free form in prior work.
Isolation was conducted by multi-stage HPLC (LC-908-
C60, Japan Analytical Industry) equipped with three differ-
ent columns: 5PYE, Buckyprep, and Buckyprep-M
(20 · 250 mm, Cosmosil Nacalai Tesque). Toluene was
used as mobile phase at a flow rate of 15 mL/min detected
at 335 nm. The detail isolating process can be seen in
supporting materials.
The purities of the isolated Ca-containing metallofulle-
renes were determined to be above 98% by MALDI-TOF
mass spectrometry (BIFLEX III, Bruker Inc.) with radia-
tion pulses (10À8
s) from a N2 laser operating at 337 nm.
UV–vis–NIR spectra of the isolated Ca metallofulle-
renes were recorded between 400–2000 nm in toluene solu-
tion with Shimadzu UV-3100 spectrophotometer.
A BAS-100B Electrochemical Analyzer was used to
record cyclic and differential pulse voltammograms of the
Ca metallofullerenes, i.e. Ca@C76, Ca@C82 (II, III),
Ca@C84 (II) and Ca@C88. All the measurements were con-
ducted in MeCN/C6H5CH3 (1:4 v/v) containing 0.1 M
(n-Bu)4NClO4 with a three-electrode configuration, a glass
carbon disk (4 mm in diameter) as the working electrode, a
platinum wire as the auxiliary electrode and a Ag wire
coated with AgCl as the reference electrode. All the exper-
iments were performed in an ice-water bath. Cyclic voltam-
mograms were recorded at 50 mV/s and DPV were
obtained at 10 mV/s using a pulse amplitude of 50 mV, a
pulse width of 50 ms, and a pulse period of 200 ms. The
electrochemical cell was purged with high purity nitrogen
for about ten min. before each measurement. All the poten-
tials in this report are with respect to the ferrocene/ferro-
cenium redox couple.
3. Results and discussion
3.1. UV–vis–NIR absorption spectroscopy
The absorption spectrum of Ca@C76 and Ca@C88 are
shown in Fig. 1a. There are several characteristic peaks
around 465, 608 and 710 nm and a broad absorption band
between 800 and 1300 nm. The onset arises from 1700 nm.
The onset of the absorption spectrum of a fullerene is a
good measure of its band gap energy. The band gap of
Ca@C76 is estimated to be about 0.7 eV according to
the onset of its absorption spectrum. As can be seen in
the absorption curve, Ca@C88 shows characteristic peaks
at around 705, 786, 1097 nm, relative weak peaks at
495, 530, 579, 933 nm and a board absorption band at
1400 nm. The absorption onset of Ca@C88 is around
1700 nm, indicating a small HOMO-LUMO gap. The
absorption spectrum of Ca@C88 is quite similar to that
of Eu@C88 (I), which was the first report of the spectro-
scopic characterization of the C88-based metallofullerenes
[7]. But there are still some small differences between the
two spectra. For example, there is a distinct peak at
1097 in the absorption spectrum of Ca@C88, but this
region is featureless in the absorption spectrum of
Eu@C88 (I).
400 600 800 1000 1200 1400 1600 1800 2000
Absorbance/a.u.
Ca@C88
Wavelength / nm
Ca@C76
(a)
400 600 800 1000 1200 1400 1600 1800 2000
Ca@C90(II)
Wavelength / nm
Ca@C90(I)
Absorbance/a.u.
(b)
Fig. 1. The absorption spectra of the isomer-separated (a) Ca@C76, Ca@C88 and (b) Ca@C90 (I, II).
476 Y. Zhang et al. / Carbon 44 (2006) 475–479

3. The absorption spectra of Ca@C90 (I, II) are given in
Fig. 1b. Compared with the absorption spectra of
Dy2@C90 (I, II) and Er2@C90 (I, II), the spectra of
Ca@C90 (I, II) are much different [11,12]. However, the
absorption spectra of Ca@C90 (I) and Eu@C90 are very
similar, even though the absorption peaks of Eu@C90 are
not as visible as those of Ca@C90 (I) [7]. The similarity be-
tween the two absorption spectra indicates the same sym-
metry of these two species. In addition, in our work there
was a second isomer, i.e. Ca@C90 (II), isolated and charac-
terized spectroscopically. The absorption spectrum of the
second isomer differs remarkably from that of the first
one. As can be seen in the absorption curve of Ca@C90
(I), there are two broad bands around 800, 110 nm and
two distinct peaks at 569 and 670 nm. For the second iso-
mer, several absorption peaks are observed at 1128, 972,
874, 834, 606 and 554 nm. The different absorption spectra
of Ca@C90 (I, II) indicate their different molecular
symmetries.
3.2. Cyclic and differential pulse voltammograms
The cyclic and differential pulse voltammograms of
Ca@C76, Ca@C82 (II, III) and Ca@C84 (II) were recorded
in the potential range from À3.0 to 1.3 V in our experi-
ments. None of oxidation peak was been observed within
such potential range, which was attributed to the closed
shell electronic structures of these isomers. Meanwhile, five
reduction peaks for each isomer were observed. The fifth
one (E = À2.23 V) was assigned to an impurity original
from the toluene used as HPLC solvent. The correlative
discussion was demonstrated elsewhere [13]. In addition,
there is a tiny peak to be observed between the second
and third reduction in each of the differential pulse voltam-
mograms of Ca@C82 (II, III) and Ca@C84 (II), which can
be attributed to a coexisted ‘‘isomer impurity’’.
The first four reduction potentials of the Ca metallo-
fullerenes are listed in Table 1. Both DPV and CV peak
potentials are given. All the reductions are reversible,
which are estimated from the cyclic voltammograms.
The cyclic and differential pulse voltammograms of
Ca@C76 are present in Fig. 2. As can be seen, there are four
reduction peaks observed within the potential window. The
four reversible couples appear as sets of two (E1 and E2, E3
and E4). This is consistent with the prediction that Ca@C76
has a closed-shell electronic structure. Our early report
showed the electrochemical behaviors of Yb@C76 (I, II)
[13]. The voltammograms of Ca@C76 resemble those of
Yb@C76 (II) to some extent, which indicates their similar
electronic structures.
Fig. 3 shows the cyclic and differential pulse voltam-
mograms of Ca@C82 (II, III). Four reduction peaks can
be seen in both voltammograms of these two isomers. To
be notable, the first reduction of Ca@C82 (III) is followed
immediately by the second one. The similar case was also
observed for Yb@C82 (II) [13]. In fact, these two metallo-
fullerenesÕ voltammograms are almost the same. This
means that the two isomers have the same electronic and
cage structures, i.e. C2. Dennis and Shinohara determined
the symmetry of Ca@C82 (III) to be C2 from the NMR
spectrum [10]. Our above discussion is in agreement with
the NMR result. The voltammograms of Ca@C82 (II) is
different from those of M@C82 (I, II, III) (Ms = Yb, Sm,
Tm), indicating its new cage structure [13–15]. The UV–
vis–NIR of Ca@C82 (I, II, III, IV) were given by Xu
et al. [4]. By comparing the absorption spectra of Ca@C82s
and M@C82s, one can see that Ca@C82 (I, III, IV) corre-
spond to M@C82 (I, II, II) respectively. That is to say,
Ca@C82 (II) is the ‘‘fourth’’ isomer. This new cageÕs struc-
ture is still unknown. But from its voltammograms, it can
be deduced that the electron affinity of Ca@C82 (II) is com-
parative with that of Ca@C82 (III). And all the four reduc-
tion peaks of Ca@C82 (II) are negatively shifted compared
with those of Ca@C82 (III), indicating the higher levels of
the front molecular orbitals of Ca@C82 (II).
The cyclic and differential pulse voltammograms of
Ca@C84 (II) are given in Fig. 4. The voltammograms of
Table 1
Half-cell potentialsa
and DPV peak potentialsb
of the Ca metallofulle-
renes, versus Fc/Fc+
(in volts)
red
E1
red
E2
red
E3
red
E4
Ca@C76 DPV À0.58 À0.94 À1.51 À1.93
CV À0.61 À0.99 À1.57 À1.97
Ca@C82 (II) DPV À0.63 À0.94 À1.52 À1.87
CV À0.65 À0.96 À1.55 À1.90
Ca@C82 (III) DPV À0.55 À0.70 À1.25 À1.66
CV À0.59 À0.74 À1.30 À1.70
Ca@C84 (II) DPV À0.59 À0.84 À1.22 À1.60
CV À0.64 À0.90 À1.27 À1.65
a
The voltammetry experiments were conducted in MeCN/C6H5CH3
(1:4 v/v) containing 0.1 M (n-Bu)4NClO4; scan rate: 50 mV/s.
b
Pulse amplitude: 50 mV; pulse width: 50 ms; pulse period: 200 ms; scan
rate: 10 mV/s.
0 -500 -1000 -1500 -2000 -2500 -3000
Current
E (mv) vs Ferrocene
Ca@C76
1 µA
Fig. 2. The cyclic and differential pulse voltammograms of Ca@C76
recorded in MeCN/C6H5CH3 (1:4 v/v) solution containing 0.1 M
(n-Bu)4NClO4. The peak (Es = 2.23 V) is assigned to an impurity.
Y. Zhang et al. / Carbon 44 (2006) 475–479 477

4. Ca@C84 (II) are quite similar to those of Yb@C84 (II),
which indicates their similar cage structures and electronic
structures [13]. On the other hand, these two metallofulle-
renesÕ absorption spectra are also almost identical. This is
consistent with the above conclusion. The ‘‘impurity’’ peak
is very high due to that the concentration of Ca@C84 (II)
toluene solution is extra low. In addition, there is a reduc-
tion peak around À2.65 V. This peak is poorly definitive
due to that it is of a low intensity and the reduction poten-
tial is near the potential window. This reduction process
was also observed in the voltammograms of Yb@C84 (II).
4. Conclusion
In summary, a few Ca-containing metallofullerens,
namely Ca@C76, Ca@C88, Ca@C90 (I, II), were synthe-
sized and successfully isolated under our improved experi-
mental conditions for the first time. These isomer-separated
metallofullerenes were characterized by LD-TOF MS and
UV–vis–NIR spectrometry. Their HOMO-LUMO band
gaps and possible molecular geometries are discussed
according to the absorption spectra in this report. In addi-
tion, the cyclic and differential pulse voltammetry of
Ca@C76 was conducted in MeCN/C6H5CH3 (1:4 v/v).
The voltammograms of Ca@C82 (II, III) and Ca@C84
(II) were also recorded. Their electrochemical behaviors
are discussed compared with those of corresponding ytter-
bium metallofullerenes. The features of the Ca metallo-
fullerenesÕ electronic structures are also discussed.
Acknowledgements
This work was supported by National Natural Science
Foundation of China (Nos. 20151002, 50272004 and
50472023), and Scientific Research Foundation for the Re-
turned Overseas Chinese Scholars, State Education Minis-
try of China.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.carbon.
2005.08.018.
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