Selective extraction of higher fullerenes using cyclic dimers of zinc porphyrins

Article abstract of DOI:10.1021/ja0489947

Higher fullerenes (≥C₇₆) were selectively extracted from a fullerene mixture obtained from a combustion-based industrial production source by cyclic dimers of β-unsubstituted porphyrin zinc complexes 2_C5-2_C7 with C5-C7 alk

Full text of DOI:10.1021/ja0489947

Published on Web 05/11/2004
Selective Extraction of Higher Fullerenes Using Cyclic Dimers of Zinc
Porphyrins
Yoshiaki Shoji, Kentaro Tashiro, and Takuzo Aida*
Department of Chemistry and Biotechnology, School of Engineering, The UniVersity of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received February 23, 2004; E-mail: aida@macro.t.u-tokyo.ac.jp
Since the discovery of C60 in 1985,1 spherical carbon nanoclusters
of different sizes have caught special attention, as they are expected
to possess high potentials in broad scientific areas and industrial
applications. In particular, higher fullerenes2 are interesting, which
allow studies on structure-property relationships of discrete
π-conjugated materials. However, a practical difficulty in isolating
such low-yield products has prevented exploration of their physical
properties as well as their applications in materials sciences.
Examples of obtaining fullerenes enriched in higher homologues
(gC76) include (1) utilization of a HfC-doped graphite electrode
for the arc-discharge production,3 (2) multistep chromatographic
separation from carbon soot,4 and (3) electrochemical reduction to
enhance their solubilities for separation, followed by reoxidation.5
However, unlike the case of C60,6 host molecules that can selectively
extract higher fullerenes have never been reported to date.7 Recently,
we have reported that cyclic dimers of metalloporphyrins 1C6 (zinc
complex; Chart 1) serve as excellent host molecules for C60 and
C70.8,9 On the basis of this finding, we expected that a synthetic
Figure 1. HPLC profiles on 5-PBB of (a) an as-received fullerene mixture
flexibility of 1C6 could allow us to design metalloporphyrin hosts
and extracts with (b) 2C5, (c) 2C6, and (d) 2C7, monitored at 356 nm with
that are capable of selectively binding higher fullerenes. Here we
report the first host-guest approach using a series of cyclic dimers
of zinc porphyrins 2C5-2C7 (Chart 1) as designer host molecules
for selective extraction of higher fullerenes from a combustion-
based industrial production source.
chlorobenzene as eluent (1 mL min-1). The peak assignment was made on
the basis of MALDI-TOF-MS measurements.
graphed on alumina, where a fraction containing the zinc porphyrin
host was isolated and treated with 4,4′-bipyridine (1 mg) to allow
dissociation of included fullerenes from the host cavity.8a The
reaction mixture was subjected to size exclusion chromatography
(SEC) to isolate a fraction containing fullerenes, which was washed
with aqueous AcOH (0.1 M) to remove 4,4′-bipyridine and was
then subjected to analytical HPLC on 5-PBB with chlorobenzene
as eluent. As shown in Figure 1a, C60 and C70 are the most abundant
fullerenes in the as-received fullerene mixture, while the total
content of higher fullerenes (gC76) is only 10 abs %.11 On the other
hand, single extraction of the fullerene mixture with 2C5-2C7
resulted in considerable enrichment in higher fullerenes (Figure 1,
HPLC profiles b-d). For example, when 2C5 was used as the host,
the total content of higher fullerenes (gC76) was increased to 74
abs %. Use of 2C6 and 2C7 bearing longer spacers for the extraction
resulted in further enrichment in higher fullerenes up to 93 and 97
abs %, respectively, while only negligible contents of C60 and C70
were detected. The HPLC profiles (Figure 1) also showed that the
extracts with 2C5-2C7 are highly enriched in C96 (0.4 abs % in the
as-received fullerene mixture), where 2C6 achieved a much greater
enrichment in C96 (36 abs % in total fullerenes) than the other two
host molecules (2C5, 10 abs %; 2C7, 25 abs %). It is interesting to
note that such a subtle difference in spacer length of the host
significantly affects the affinity toward fullerenes. We also found
that the structure of the porphyrin moieties affects the selectivity.
Chart 1
Cyclic hosts 2C5-2C7 with C5-C7 alkylene spacers were
synthesized by alkaline-mediated coupling of the corresponding
bromoalkylated zinc porphyrins with a zinc porphyrin bearing
phenol functionalities, and unambiguously characterized by MALDI-
TOF-MS and 1H NMR analyses.10 A typical example of the
extraction of fullerenes with 2C5-2C7 is described below. A host
molecule (0.2 mg) was added to a toluene solution (60 mL) of a
fullerene mixture obtained from carbon soot (as-received fullerene
mixture; 20 mg), and the resulting solution, after being stirred for
30 min at room temperature, was concentrated to a volume of 20
mL. To this solution was added THF (20 mL), and the resulting
suspension was allowed to stand for 5 min and then filtered. The
filtrate was evaporated to dryness, and the residue was chromato-
For example, cyclic host 1C6, a â-substituted analogue of 2C6
,
showed a lower efficiency than 2C6 for the extraction of higher
fullerenes (gC76, 68 abs %; C96, 6 abs % in total fullerenes
extracted).10
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C O M M U N I C A T I O N S
extraction with 1 mg of 2C6. In the third-stage extraction, 0.1 mg
of 2C6 was submitted to a fullerene mixture extracted in the second
stage (Figure 3c). Interestingly, the extract finally obtained (Figure
3d) was considerably enriched in C102-C110 (C102, 15 abs %; C104
,
16 abs %; C106, 9 abs %; C108, 16 abs %; C110, 26 abs % in total
fullerenes), while C96 was less abundant (5 abs %). Furthermore,
C60 and C70 were hardly detected even in the second-stage extract
(Figure 3c).
In conclusion, we have demonstrated the first example of
selective extraction of higher fullerenes (gC76) from a fullerene
mixture, obtained from a combustion-based industrial production
source, by using cyclodimeric zinc porphyrins 2C5-2C7 with C5-
C7 alkylene spacers as the hosts. The selectivities toward higher
fullerenes are much dependent on the size of the host cavity and
the structure of the porphyrin moieties. Three cycles of the
Figure 2. Association constants (Kassoc) of 1C6 (blue bars) and 2C6 (red
bars) with C60, C70, and C96 in toluene/THF (1/1) at 25 °C.
extraction with 2C6 allow enrichment in very rare fullerenes C102
-
C110 (<0.1 abs %) up to 82 abs % of total fullerenes. Use of such
cyclic host molecules immobilized on solid supports for chromato-
graphic separation13 is one of the interesting subjects worthy of
further investigation.
Acknowledgment. We thank Frontier Carbon Corporation for
a generous supply of a fullerene mixture obtained from a combus-
tion-based industrial production source.
Supporting Information Available: Synthesis of 2C5-2C7, spectral
data of inclusion complexes of 1C6 and 2C5-2C7 with fullerenes, and
isolation and characterization of C96 for titration. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 3. Abundances (abs %) of C60-C114 in (a) an as-received fullerene
mixture and extracts with 2C6 after (b) the first-, (c) second-, and (d) third-
stage extractions, as estimated by HPLC monitored at 356 nm.
References
(1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E.
Nature 1985, 318, 162.
(2) Diederich, F.; Whetten, R. L. Acc. Chem. Res. 1992, 25, 119.
(3) Murphy, T. A.; Carl, S.; Wolf, C.; Mertesacker, B.; Weidinger, A.;
Lehmann, A. Synth. Met. 1996, 77, 213.
(4) (a) Ettl, R.; Chao, I.; Diederich, F.; Whetten, R. L. Nature 1991, 353,
149. (b) Diederich, F.; Whetten, R. L.; Thilgen, C.; Ettl, R.; Chao, I.;
Alvarez, M. M. Science 1991, 254, 1768. (c) Kikuchi, K.; Nakahara, N.;
Honda, M.; Suzuki, S.; Saito, K.; Shiromatsu, H.; Yamauchi, K.; Ikemoto,
I.; Kuramochi, T.; Hino, S.; Achiba, Y. Chem. Lett. 1991, 1607.
(5) Diener, M. D.; Alford, J. M. Nature 1998, 393, 668.
(6) (a) Andersson, T.; Nilsson, K.; Sundahl, M.; Westman, G.; Wennerstro¨m,
O. J. Chem. Soc., Chem. Commun. 1992, 604. (b) Diederich, F.; Effing,
J.; Jonas, U.; Jullien, L.; Plesnivy, T.; Ringsdorf, H.; Thilgen, C.;
Weinstein, D. Angew. Chem., Int. Ed. Engl. 1992, 31, 1599. (c) Atwood,
J. L.; Koutsantonis, G. A.; Raston, C. L. Nature 1994, 368, 229. (d) Suzuki,
T.; Nakashima, K.; Shinkai, S. Chem. Lett. 1994, 699.
(7) A mass spectrometric investigation on competitive interaction of C60 and
C84 with an acyclic Cu porphyrin dimer has been briefly described in:
Sun, D.; Tham, F. S.; Reed, C. A.; Chaker, L.; Boyd, P. D. W. J. Am.
Chem. Soc. 2002, 124, 6604.
(8) (a) Tashiro, K.; Aida, T.; Zheng, J.-Y.; Kinbara, K.; Saigo, K.; Sakamoto,
S.; Yamaguchi, K. J. Am. Chem. Soc. 1999, 121, 9477. (b) Zheng, J.-Y.;
Tashiro, K.; Hirabayashi, Y.; Kinbara, K.; Saigo, K.; Aida, T.; Sakamoto,
S.; Yamaguchi, K. Angew. Chem., Int. Ed. 2001, 40, 1857. (c) Tashiro,
K.; Hirabayashi, Y.; Aida, T.; Saigo, K.; Fujiwara, K.; Komatsu, K.;
Sakamoto, S.; Yamaguchi, K. J. Am. Chem. Soc. 2002, 124, 12086. (d)
Yamaguchi, T.; Ishii, N.; Tashiro, K.; Aida, T. J. Am. Chem. Soc. 2003,
125, 13934.
Analogous to C60 and C70, higher fullerenes form inclusion
complexes with the cyclic hosts. For example, upon mixing with
12
C96 in toluene/THF (1/1), 2C6 displayed a bathochromic shift in
the Soret absorption band from 413 to 420 nm. 1H NMR
spectroscopy of a mixture of 2C6 and C96 (1:2) in chlorobenzene-
d5 at 60 °C showed a singlet signal due to meso-H at δ 10.3 ppm.
In contrast, 2C6 without guest C96 showed a rather complicated
spectral profile with two singlet meso-H signals at δ 10.1 and 10.4
ppm, due to the existence of conformational isomers.8 The above
spectral changes upon mixing of 2C6 with C96 are characteristic of
the inclusion of C60 with cyclodimeric zinc porphyrins.8a
A
spectroscopic titration of 2C6 with C96 in toluene/THF (1/1) at 25
°C gave an association constant (Kassoc) of 1.3 × 107 M-1, which
is 118 and 16 times larger than those with C60 (1.1 × 105 M-1
)
and C70 (7.9 × 105 M-1), respectively (red bars, Figure 2). In
contrast, when â-substituted 1C6 with a lower preference toward
C96 in the extraction was titrated with these three fullerenes (blue
bars, Figure 2), the Kassoc value with C96 (3.0 × 107 M-1), compared
to that with C60 (5.1 × 105 M-1), was still rather high, whereas it
was only 2.3 times as large as that with C70 (1.3 × 107 M-1).
Since populations of individual fullerenes in the single-step
extracts are greatly affected by those in the as-received fullerene
mixture, we conducted a sequential three-stage extraction with the
best-behaved 2C6 to estimate its inherent preference. Thus, in the
first stage, 500 mg of the as-received fullerene mixture (Figure 3a)
was extracted with 10 mg of 2C6, and a mixture of the inclusion
complexes was isolated and then treated with 4,4′-bipyridine. A
fullerene mixture thus released from the cavity of 2C6 (Figure 3b;
3.2 mg) was collected and then subjected to the second-stage
(9) An example of host-guest chemistry of fullerenes with metalloporphyrin
dimers: Sun, D.; Tham, F. S.; Reed, C. A.; Chaker, L.; Burgess, M.;
Boyd, P. D. W. J. Am. Chem. Soc. 2000, 122, 10704.
(10) See Supporting Information.
(11) Peak integral ratio (%) in HPLC monitored at 356 nm.
(12) C96 isolated by multistage preparative HPLC was used. Analytical HPLC
on Wakosil II5C18AR with toluene/MeCN (3/2) displayed three major
peaks, indicating that C96 is a mixture of isomers (see Supporting
Information).
(13) Utilization of monomeric free-base and zinc porphyrins as stationary phases
for chromatography: Xiao, J.; Savina, M. R.; Martin, G. B.; Francis, A.
H.; Meyerhoff, M. E. J. Am. Chem. Soc. 1994, 116, 9341.
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