A cyclic dimer of metalloporphyrin forms a highly stable inclusion complex with C60 [16]

Full text of DOI:10.1021/ja992416m

J. Am. Chem. Soc. 1999, 121, 9477-9478
9477
A Cyclic Dimer of Metalloporphyrin Forms a Highly
Stable Inclusion Complex with C60
Kentaro Tashiro,† Takuzo Aida,*,† Jiang-Yu Zheng,‡
Kazushi Kinbara,‡ Kazuhiko Saigo,*,‡ Shigeru Sakamoto,§ and
Kentaro Yamaguchi§,|
Department of Chemistry and Biotechnology, Graduate School
of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan, Department of Integrated
Biosciences, Graduate School of Frontier Sciences,
The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan, Chemical Analysis Center,
Chiba UniVersity, Yayoi-cho, Inage-ku, Chiba 2638522, Japan
ReceiVed July 12, 1999
Host molecules for inclusion of fullerenes are of great
importance because of their application to extraction and chemical
modification of fullerenes.1 In particular, inclusion via π-electronic
donor-acceptor interactions is highly interesting in view of
possible supramolecular modulation of electronic properties of
fullerenes. Herein we report that a face-to-face cyclic dimer of
zinc porphyrin (1) forms a highly stable 1:1 inclusion complex
with C60 via donor-acceptor interactions.
Figure 1. Spectroscopic titration of receptor 1 with C60 in benzene at
25 °C: [1] ) 1.96 × 10-6 M; [C60]/[1] ) 0, 0.74, 1.48, 2.96, 5.91, 10.35,
14.77.
Zinc porphyrin cyclic dimer 1 was synthesized by hydrogena-
tion of 2 having rigid diacetylenic spacers (Scheme 1). A benzene
solution of 1 upon mixing with C60 showed a marked color change
from bright reddish purple to dark red. In the electronic absorption
spectrum of a mixture of 1 and C60, the Soret absorption band of
1 was observed to shift bathochromically from 410.5 to 417.5
nm, suggesting an electronic interaction of 1 with C60 (Figure
1).2 Interestingly, when an equimolar mixture of 1 and C60 was
Figure 2. ESI-MS spectrum of a THF solution of a mixture of 1 and
C60.3
subjected to TLC on alumina with benzene as eluent, only a single
spot was observed at Rf ) 0.13 without any other spots at 0.72
and 0.85 due to 1 and C60, respectively. On the other hand, when
either 1 or C60 was present in excess with respect to the
counterpart, the TLC trace showed an additional spot due to free
1 or C60. These observations indicate that 1 and C60 form a highly
stable complex. Accordingly, the complex between 1 and C60
could be easily isolated by column chromatography on alumina.
When benzene solutions of 1 and C60 (1.9 × 10-6 M) were
mixed at varying volume ratios (Job’s plots) at 25 °C, the change
in absorbance at 410.5 nm displayed a maximum at a mole ratio
1:C60 of unity. Furthermore, electrospray ionization mass spec-
trometry (ESI-MS) of a THF solution of a mixture of 1 and C60
clearly showed two sets of isotopic distributions centered at m/z
2785.89 and 1393.12 (Figure 2),3 which correspond to the mono
and dication, respectively, of a 1:1 complex between 1 and C60.
To determine the binding constant of the complexation of 1 with
C60, we titrated a benzene solution of 1 with C60 at 25 °C, where
the absorption spectral change showed a clear isosbestic point at
418.0 nm (Figure 1). From the change in absorbance at 410.5
* Authors of correspondence.
† Department of Chemistry and Biotechnology, The University of Tokyo.
‡ Department of Integrated Biosciences, The University of Tokyo.
§ Chiba University.
| Responsible for ESI-MS.
(1) Selected examples of C60 receptors: (a) Andersson, T.; Nilsson, K.;
Sundahl, M.; Westman, G.; Wennerstro¨m. 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.
(e) Yoshida, Z.; Takekuma, H.; Takekuma, S.; Matsubara, Y. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 1597. (f) Haino, T.; Yanase, M.; Fukazawa, Y. Angew.
Chem., Int. Ed. Engl. 1998, 37, 997.
(2) Several covalently linked porphyrin/C60 systems have been reported to
show similar red shifts in the Soret absorption bands. For selected examples,
(a) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.; Okada,
T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118, 11771. (b)
Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust, D.;
Drovetskaya, T.; Reed, C. A.; Boyd, P. D. W. J. Phys. Chem. 1996, 100,
15926. (c) Baran, P. S.; Monaco, R. R.; Khan, A. U.; Schuster, D. I.; Wilson,
S. R. J. Am. Chem. Soc. 1997, 119, 8363. (d) Bourgeois, J.-P.; Diederich, F.;
Echegoyen, L.; Nierengarten, J.-F. HelV. Chim. Acta 1998, 81, 1835. (e) Dietel,
E.; Hirsch, A.; Eichhorn, E.; Rieker, A.; Hackbarth, S.; Ro¨der, B. Chem.
Commun. 1998, 1981. (f) Cheng, P.; Wilson, S. R.; Schuster, D. I. Chem.
Commun. 1999, 89.
(3) C60 was added to a THF solution of 1 (2.2 × 10-4 M), and the mixture
was sonicated for 20 min and subjected to ESI-MS (JEOL Type JMS-700T)
with a four-sector (BEBE) tandem mass spectrometer. Conditions: needle
volt, 2.0 kV; current, 300-700 nA; acceleration volt, 5.0 kV; resolution, 5000;
chamber temperature, 150 °C; flow rate, 10 µL min-1
.
10.1021/ja992416m CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/28/1999

9478 J. Am. Chem. Soc., Vol. 121, No. 40, 1999
Communications to the Editor
Scheme 1
nm, the binding constant of the 1:1 complexation was evaluated
to be 6.7 × 105 M-1.4 To the best of our knowledge, this is the
highest binding constant reported to date for host-guest interac-
tions with C60 in organic solvents.1f In sharp contrast, cyclic dimer
2, having rigid diacetylenic spacers between the two zinc
porphyrin units, showed no spectral change at the visible region
upon mixing with C60 under similar conditions.
Host molecule 1 in solution exists as a mixture of conforma-
tional isomers due to its flexible spacers:5 1H NMR spectrum of
1 in C6D6 at 25 °C showed two sets of three singlet signals at δ
10.60-9.57 and 3.07-2.67 ppm due to meso and â-methyl
resonances, respectively. 1H NMR saturation transfer profile6 of
1 indicated that such conformational isomers are transformed with
one another at a rate slower than the NMR time scale. On the
other hand, when an equimolar amount of 4,4′-bipyridine (bpy)7
with respect to 1 was added to the solution, the 1H NMR spectrum
became much simplified to show single meso (δ 10.53) and
â-methyl (δ 3.06) signals together with two upfield-shifted doublet
signals due to bpy (δ 8.79, 7.05 f 3.42, 2.09). Thus, bpy was
incorporated between the two zinc porphyrin units in an induced-
fit fashion.8 A similar NMR spectral change was observed upon
addition of C60 to a C6D6 solution of 1, where the meso and
â-methyl resonances showed singlet signals at δ 10.32 and 2.87
ppm, respectively. These results indicate the formation of an
inclusion complex between 1 and C60 (Figure 3). When the
inclusion complex was mixed with an equimolar amount of bpy,7
C60 was released quantitatively from the cavity, to form a complex
of 1 coordinated with bpy.
Figure 3. A computer-generated molecular model of the inclusion
complex between 1 and C60, optimized with a CAChe molecular
mechanics calculation. For ease of calculation, the peripheral hexyl groups
of 1 were replaced by methyl groups.
Cl2 showed that the first reduction of C60 occurs at -1.11 V vs
Fc/Fc+.9 Comparison of the observed redox potential with that
of free C60 (E1/2 ) -1.05 V) indicates that C60 becomes less
subject to reduction upon complexation with 1. Therefore, it is
likely that a π-electronic interaction is operative between 1 and
C60 in the inclusion complex.10
In conclusion, we have developed a novel macrocyclic receptor
for the inclusion of C60 via π-electronic donor-acceptor interac-
tions, where the very high binding constant (6.7 × 105 M-1
)
enables chromatographic separation of C60. The change in redox
potentials of the included C60 also suggests a potential utility of
the supramolecular approach for the modulation of the electronic
properties of fullerenes.
13C NMR spectrum of the inclusion complex of 1 and C60 in
C6D6 at 30 °C showed a single signal due to the included C60 at
δ 140.10 ppm, which is upfield-shifted from that of free C60 (δ
143.21). Such an upfield shift is considered to reflect the shielding
effect and/or the electronic effect of the zinc porphyrin π-cloud.
Cyclic voltammetry of an equimolar mixture of 1 and C60 in CH2-
Acknowledgment. K.S. and J.Y.Z. thank JSPS for financial support.
K.T. thanks JSPS Research Fellowship for Young Scientists. We are
grateful to Dr. F. Hasegawa of Science University of Tokyo for HRMS
analysis of 1 and 2.
Supporting Information Available: Details for synthesis and
characterization of 1 and 2, together with 1H NMR spectral profiles of 1,
an equimolar mixture of 1 and 4,4′-bipyridine (bpy), and that of 1 and
C60 in C6D6 at 25 °C. This material is available free of charge via the
Internet at http://pubs.acs.org.
(4) Spectral change was analyzed by a nonlinear curve fitting method (R2
) 0.999417).
(5) For 1H NMR spectra, see Supporting Information. Presence of confor-
mational isomers in analogous dimeric porphyrin systems has been reported
in Golubchikov, O. A.; Mamardashvili, N. Zh.; Semeikin, A. S. Zh. Org. Khim.
1993, 29, 2445.
JA992416M
(6) Sanders, J. K. M.; Hunter, B. K. Modern NMR Spectroscopy; Oxford
University: Oxford, U.K., 1987.
(9) Cyclic voltammograms of C60 in the absence and presence of 1 were
recorded in CH2Cl2 (10-4 M) on a BAS Type CV-27 voltammetry controller
with Bu4NPF6 (0.1 M) at Pt using a Ag wire pseudo reference electrode and
recalculated against internal Fc/Fc+.
(10) A comparable cathodic shift of the reduction potential has been reported
in ref 2d.
(7) The binding constant of bpy in benzene at 25 °C was 3.2 × 108 M-1
(8) Bampos, N.; Marvaud, V.; Sanders, J. K. M. Chem. Eur. J. 1998, 4,
,
as determined spectroscopically.
335.