Fullerene receptor based on calix[5]arene through metal-assisted self-assembly

Article abstract of DOI:10.1016/S0040-4039(01)00420-8

A self-assembled fullerene receptor based on calix[5]arene has been developed. Silver cation complexation held together the two calix[5]arenes with bipyridine units providing a large enough cavity to take up C₆₀ or C₇₀. The formation of the supramolecular complex with C₆₀ or C₇₀ was established by using the electrospray mass spectrometry.

Full text of DOI:10.1016/S0040-4039(01)00420-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3203–3206
Fullerene receptor based on calix[5]arene through metal-assisted
self-assembly
Takeharu Haino, Hiromi Araki, Yuko Yamanaka and Yoshimasa Fukazawa*
Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8526, Japan
Received 19 January 2001; revised 2 March 2001; accepted 9 March 2001
Abstract—A self-assembled fullerene receptor based on calix[5]arene has been developed. Silver cation complexation held together
the two calix[5]arenes with bipyridine units providing a large enough cavity to take up C60 or C70. The formation of the
supramolecular complex with C60 or C70 was established by using the electrospray mass spectrometry. © 2001 Elsevier Science
Ltd. All rights reserved.
There have been many reports on synthetic receptors1
having multiple binding sites implemented within their
covalent frameworks. It takes multi-step synthesis to
achieve a proper orientation of the binding groups. An
alternative approach can be envisioned in which the
easy construction of a large and elaborate molecular
architecture can be achieved by self-assembly. In this
approach, modular components are held together by
self-assembly with non-covalent interactions, bringing
together the key groups in the active site.
We have studied a number of calixarene-based recep-
tors capable of binding neutral guests through non-
covalent forces in organic media,5,6 and have reported
that the covalent linking of the two calix[5]arenes
brings strong binding to C60 or C70 within the cavity
composed of the two calix[5]arenes.6b In this paper
we describe an alternative way to construct the large
cavity with the two calix[5]arenes by self-assembly
through metal complexation. The complexation of the
two calix[5]arene subunits 1 with Ag+ will give a tetra-
hedral Ag+-bis-bipyridine complex, bringing the two
binding sites to close proximity. This approach has
the advantage of not only bringing together the host
units by metal-chelation, but it can also detect the
formation of the supramolecular complex by mass
spectrometry due to the presence of the metal ion.
Such self-assembly is commonly seen in biological
systems.2 Viruses and enzymes are well-known exam-
ples. Synthetic self-assembly is a new field in organic
chemistry, and has become a current focus of molecu-
lar recognition studies.3 One of the convenient ways
to construct the elaborate supramolecular architecture
is a coordination-driven self-assembly methodology,4
which offers an alternative and extended function.
The key subunit 1 was prepared from calix[5]arene 27
by straightforward steps according to Scheme 1.
* Corresponding author. Tel.: +81 824 24 7427; fax: +81 824 24 0724; e-mail: fukazawa@sci.hiroshima-u.ac.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00420-8

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T. Haino et al. / Tetrahedron Letters 42 (2001) 3203–3206
Scheme 1.
Deprotection of the t-butyl group under retro
Friedel–Crafts conditions proceeded smoothly. Fol-
lowing iodination with BTMAICl2, protection of the
hydroxyl groups gave iodide 3 in good yield. Intro-
duction of the metal-binding unit to 3 was carried
out by palladium catalyzed coupling reaction with 4.8
Deprotection of the acetyl groups gave the desired
compound 1.
appeared at 143 ppm in CDCl2CDCl2 (Fig. 1a). Dur-
ing the addition of the complex 1·Ag+·1, the C60 sig-
nal shifted upfield by the formation of the complex
C60@1·Ag+·1. Finally, the upfield shift value reached
1.8 ppm (Fig. 1b), which is consistent with that of
the covalently linked double-calix[5]arene reported
previously.6a The large upfield shift suggests that C60
resides within the shielded interior of the cavity com-
posed of the calix[5]arenes, and gives promising sup-
port on the formation of the C60@1·Ag+·1.
The formation of the metal complex with Ag+ was
achieved by simply mixing 2 equivalents of 1 with 1
equivalent of AgOTf in nitromethane. The resulting
complex 1·Ag+·1 is a pale yellow and air stable solid.
The NMR signals of the complex 1·Ag+·1 in
CDCl2CDCl2 were rather broadened. The downfield
shifted signals of the bipyridine unit in the subunit 1
clearly indicated that the silver cation bound to the
bipyridine nitrogens. The spectra were concentration
dependent, which comes from the dynamic equi-
librium between the subunit 1 and the ternary com-
plex 1·Ag+·1. The plotting analysis of the chemical
shift changes around the bipyridine protons gave the
dimerization constant9 to be 5700 300 M−1 at 27°C.
The study of non-covalently bound complexes by
electrospray ionization mass spectrometry10 has
rapidly gained momentum. A large number of reports
have dealt with the mass spectrometric characteriza-
tion of noncovalent complexes, such as supramolecu-
lar metal complexes,11 knots and catenanes,12 cavitand
and guest13 and capsule and guest.14 ESI-MS spec-
trometry has proven to be an indispensable tool in
supramolecular chemistry. In our system, the silver
cation labeling of the assembly was an advantage for
ESI-MS measurements in nonpolar aprotic solvent.
CHCl2CHCl2 was used for the ESI measurements
because both fullerenes and the complex are easily
soluble in it.
The binding study for C60 was carried out with 13C
NMR spectroscopy. The 13C resonance of C60
The ESI-MS measurement of the complex 1·Ag+·1
gave a peak attributable to the loss of the triflate
counterion, [M−OTf]+ {avg. m/z=1590 for [1·Ag+·1]}.
The measured isotope pattern for [1·Ag+·1] nicely
matched the calculated abundance, (Fig. 2a) and
confirmed the elemental composition and the monoca-
tionic nature of the complex. The addition of 2
equivalents of C60 into the solution of complex 1·Ag+
·1 brought a peak [C60@1·Ag+·1]{avg. m/z=2310},
indicating that C60 comes into the cavity provided by
the assembly. A new peak [C70@1·Ag+·1] {avg. m/z=
2430} was observed in the case of C70. Both com-
plexes contain only one guest within the cavity. In
comparison with the relative intensities for the com-
plexes [C60@1·Ag+·1] and [C70@1·Ag+·1] the difference
should arise from the binding ability of complex
1·Ag+·1 toward C60 and C70. Complex 1·Ag+·1 seems
to bind C60 more strongly than C70 in the opposite
sense
from
the
covalently
linked
double
Figure 1. 13C NMR spectra at 125 MHz of (a) C60 (3.3
mmol/L) and (b) C60 with 4.1 equiv. of complex 1·Ag+·1 in
CDCl2CDCl2.
calix[5]arenes,6b which prefer to bind C70 to C60. The
size and shape of the cavity provided by the assembly
should fit nicely to C60 rather than C70.

T. Haino et al. / Tetrahedron Letters 42 (2001) 3203–3206
3205
Figure 2. ESI mass spectra of CHCl2CHCl2 solution of (a) 1·Ag+·1 (5.0 mmol/L), (b) 1·Ag+·1 and C60 (10.0 mmol/L) and (c)
1·Ag+·1 and C70 (10.0 mmol/L). The inset shows the calculated (straight lines) and experimental isotope pattern (curve).
The binding property of the complex for C60 or C70 was
established by ion labeling ESI mass spectrometry.
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