Metal-induced regulation of fullerene complexation with double-calix[5]arene

Article abstract of DOI:10.1039/b108121g

The metal-induced regulation of fullerene complexation with double-calix[5]arene is described. The receptor shows strong binding to C₇₀ only in the presence of Cu⁺.

Full text of DOI:10.1039/b108121g

Metal-induced regulation of fullerene complexation with
double-calix[5]arene†
Takeharu Haino, Yuko Yamanaka, Hiromi Araki and Yoshimasa Fukazawa*
1-3-1 Kagamiyama, Higashi-Hiroshima, Japan. E-mail: fukazawa@sci.hiroshima-u.ac.jp;
Fax: +81-824-24-0724; Tel: +81-824-24-7427
Received (in Columbia, MO, USA) 30th October 2001, Accepted 7th January 2002
First published as an Advance Article on the web 5th February 2002
The metal-induced regulation of fullerene complexation
with double-calix[5]arene is described. The receptor shows
strong binding to C70 only in the presence of Cu+.
the two calix[5]arenes come closer to produce a deep cavity to
take up fullerenes. This structural regulation by the transition
metal complexation should lead to the regulation of the
fullerene binding.
The allosteric effect is one of the fascinating features in
biological systems. In allosteric regulation, binding of an
effector to a remote site leads to a conformational change at the
active site. This conformational change facilitates or deactivates
the binding affinity to its substrate and, hence, it can regulate the
activity of the enzyme. It is of interest to imitate this feature in
simple artificial organic molecules.1,2
There has been intense activity on developing fullerene
receptors.3 We have reported that the calix[5]arene and its
derivatives gave effective binding to fullerene.4 In this paper, a
bridged double calix[5]arene having two bipyridine units as a
ligation site is described. The receptor has two conformationally
coupled binding units: one is a calix[5]arene part for fullerene,
the other is a bipyridine part toward a transition metal. When the
transition metal binds to the bipyridine in a tetrahedral fashion,
The synthesis of bipyridine unit 3 was carried out through
palladium chemistry. Introduction of the trimethyltin group
onto bromomethylnicotinate 2 under palladium catalysis,
following Stille’s coupling with 2,5-dibromopyridine afforded
5A-bromo-5-methoxycarbonyl 2,2A-bipyridine. Treatment of the
bipyridine with hexamethylditin with Pd(0) furnished bipyr-
idine derivative 3. The linker unit bridging the two calix[5]ar-
enes was synthesized. Ditosylate 4,5 which is readly available
from resorcinol, was treated with sodium azide in DMF at 80 °C
to give the corresponding diazide. Hydrogenolysis of the azide
groups then gave diamine 5 (Scheme 1). The synthesis of
double-calix[5]arene 1 (Fig. 1) started from calix[5]arene 66)
according to Scheme 2. Treatment of aluminium trichloride,
followed by iodination with BTMAICl2 gave the iodocalix-
[5]arene in good yield. Protection of the five hydroxy groups on
the lower-rim produced pentaacetyl calix[5]arene 7. Coupling
reaction of 7 with 3 was performed under Still’s conditions to
give the coupled product. Hydrolysis of the ester group and
deprotection of the acetyl groups then afforded carboxylic acid
Fig. 1 Structure of 1 with transition metal.
Scheme 1 a) Me3SnSnMe3, (Ph3P)2PdCl2–dioxane; b) 2,5-dibromopyr-
idine, (Ph3P)4Pd–DMF 22% 2 steps; c) Me3SnSnMe3, (Ph3P)2PdCl2–
dioxane 69%; d) NaN3–DMF 70%; e) H2, Pd/C–MeOH 86%.
† Electronic supplementary information (ESI) available: spectroscopic and
titration data. See http://www.rsc.org/suppdata/cc/b1/b108121g/
Scheme 2 f) AlCl3, phenol–toluene 71%; g) BTMAICl2, CaCO3–MeOH–CH2Cl2 74%; h) Ac2O, pyridine–CH2Cl2 90%; i) (Ph3P)4Pd, 3, Bu4NCl–DMF 43%;
j) NaOH–MeOH–THF–H2O 86%; k) WSCI·HCl, TEA, HOBt–CH2Cl2 43%.
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This journal is © The Royal Society of Chemistry 2002

8. Connecting the two units of 8 with 5 was carried out in the
usual way with EDCI and HOBt to give 1.
We have demonstrated above the regulation of fullerene
complexation with receptor 1 through the structural constraints
The formation of the metal complex with Cu+ was achieved
by a simple mixing of 1 with one equivalent of [Cu+
induced by copper( ) complexation.
I
This work was supported by a Grant-in-Aid (12740348) for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
2
(MeCN)4]PF6 in dichloromethane. The resulting complex
[1·Cu+]PF62 is a brown solid. Complex [1·Cu+]PF6 showed
2
an absorption band at 370 nm in CHCl2CHCl2 while that of 1
appeared at 331nm. This characteristic red-shift indicates the
Notes and references
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formation of the Cu( ) tetrahedral complex with the two
I
bipyridines. Further evidence was obtained by MALDI-TOF
mass spectrometry. The mass measurement of the complex gave
a peak attributable to the loss of counterion, [M-PF62]+{avg.
m/z = 1794 for [1·Cu+]}.
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To study the binding properties of 1 and [1·Cu+]PF6 for
60 or C70 (Fig. 2), titration experiments were carried out by
2
C
UV-vis spectrometry in CHCl2CHCl2. The stoichiometry of
receptor 1 to C60 and C70 was established by Job’s plot. The
binding constants of the receptor with and without Cu+ for C60
or C70 were determined by the Benesi-Hildebrand method
(Table 1). Metal-induced regulations were observed. Receptor 1
showed 1+2 binding to C60 while Cu+ drove the complexation in
a 1+1 fashion. In contrast, C70 bound to 1, resulting in a 1+1
complex in the presence and absence of Cu+. Allosteric
regulation was seen in C70 binding, which was enhanced by Cu+
complexation. These characteristic changes of the binding
ability through the metal complexation are rationalized by the
internal flexibility.
The linker moiety of 1 is highly flexible. The entropic cost on
the internal flexibility is too high to form the 1+1 complex with
C60, and leads the low binding ability toward C70. The metal
complexation to the bipyridine units fixes the flexible chain to
overcome the high entropic cost. Hence, the fixation brings
about the regulation of the guest binding; the change of the
binding mode and the enhancement of the binding ability.
Complex [1·Cu+]PF6 preferentially binds (4-fold excess)
60 to C70. The selectivity of the guest binding can be explained
2
C
by the difference of the guest volumes: C60; 510 Å3, C70; 600
Å3. While the cavity volume8 of the receptor (591 Å3, estimated
by MacroModel9 using AMBER* force field) fits well to that of
C60, the volume of the larger guest exceeds that of the host
cavity. Of course the cavity can be expanded to some extent to
accommodate the larger guest, the resulting deformation of the
receptor causing extra strain because of the rigid nature of the
metal complexed receptor.10
Table 1 Binding constants (M21) of 1 and [1·Cu+]PF62 for C60 and C70 at
rt in CHCl2CHCl2. a) Binding constant of calix[5]arene 94a,7
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2
1
[1·Cu+]PF6
C60
C70
98 2a
250 20
3800 300
950 50
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9 was listed in Table 1.
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Fig. 2 The calculated structure of the complex 1·Cu+ with C60 (purple).
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