Aryl-aryl linked bi-5,5′-p-tert-butylcalix[4]arene tweezer for fullerene complexation

Article abstract of DOI:10.1021/ol0607591

We report the synthesis and complexing properties of the new cavity-extended receptor 1, a C,C-linked bi-calix[4]arene that selectively binds [70]fullerene in a 2:1 tennis-ball fashion.

Full text of DOI:10.1021/ol0607591

ORGANIC
LETTERS
2006
Vol. 8, No. 12
2571-2574
Aryl
Bi-5,5
−
Aryl Linked
′
-p-tert-butylcalix[4]arene Tweezer
for Fullerene Complexation
Jose´ Carlos Iglesias-Sa´nchez,† Alex Fragoso,† Javier de Mendoza,‡ and
Pilar Prados*,†
UniVersidad Auto´noma de Madrid, Facultad de Ciencias, Departamento de Qu´ımica
Orga´nica, 28049 Madrid, Spain, and The Institute of Chemical Research of Catalonia
(ICIQ), 43007 Tarragona, Spain
pilar.prados@uam.es
Received March 29, 2006
ABSTRACT
We report the synthesis and complexing properties of the new cavity-extended receptor 1, a C,C-linked bi-calix[4]arene that selectively binds
[70]fullerene in a 2:1 tennis-ball fashion.
Recognition of quasi spherically shaped fullerenes by several
molecular receptors based in hollow structures has been
reported.1-4 Particularly interesting is the separation of
fullerene mixtures on the basis of p-tert-butylcalix[8]arene
for the selective precipitation of fullerene C60.5 On the other
hand, receptors containing two or more calixarene subunits
have attracted special attention as a result of their enhanced
complexation properties.6 In most cases, a flexible spacer
acts as a linker between the two calixarene cavities, resulting
in a poorly preorganized overall structure.7 The search for
new fullerene receptors prompted us to examine the forma-
tion of wide-rim “head-to-head” double p-tert-butylcalix[4]-
arenes directly linked through their wider rims. We expected
that the tert-butyl groups at the rings not directly involved
in the connection would result in deeper and more rigid
cavities, with an improved complementarity to the fullerene
surface and likely enhanced selectivity. To date, the only
examples of “head-to-head” bicalix[n]arenes (n ) 4,5,6,8)
directly connected through their wide rims, have been
reported by Neri et al.8 and Gutsche et al.9 and arise from
the oxidative coupling of the corresponding p-unsubstituted
moieties. In the solid state, both cavities of bicalix[4]arene
† Universidad Auto´noma de Madrid.
‡ The Institute of Chemical Research of Catalonia.
(1) Calixarenes: (a) Wang, J.; Gutsche, C. D. J. Org. Chem. 2002, 67,
4423-4429. (b) Mizyed, S.; Georghiou, P. E.; Ashram, M. J. Chem. Soc.,
Perkin Trans. 2 2000, 277-280. (c) Haino, T.; Yanase, M. C.; Fukunaga,
L.; Fukazawa, Y. Tetrahedron 2006, 62, 2025-2035 and references therein.
(2) Thiacalixarenes: Kunsagi-Mate, S.; Szabo, K.; Bitter, I.; Nagy, G.;
Kollar, L. Tetrahedron Lett. 2004, 45, 1387-1390.
(3) Cyclotriveratrylenes: (a) Steed, J. W.; Junk, P. C.; Atwood, J. L.;
Barnes, M. J.; Raston, C. L.; Burkhalter, R. S. J. Am. Chem. Soc. 1994,
116, 10346-10347. (b) Zhang, S.; Palkar, A.; Fragoso, A.; Prados, P.; de
Mendoza, J.; Echegoyen, L. Chem. Mater. 2005, 17, 2063-2068.
(4) Cyclodextrins: (a) Andersom, T.; Wastman, G.; Stenhagen, G.;
Sundahl, M.; Wennerstro¨m, O. Tetrahedron Lett. 1995, 36, 597-600. (b)
Kim, H.-S.; Jeon, J.-S. Chem. Commun. 1996, 817-818.
(5) (a) Atwood, J. L.; Koutsantonis, G. A.; Raston, C. L. Nature 1994,
368, 229-231. (b) Suzuki, T.; Nakashima, K.; Shinkai, S. Chem. Lett. 1994,
699-702.
(6) (a) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713-745.
(b) Gutsche, C. D. Calixarenes ReVisited; The Royal Society of Chemis-
try: Cambridge, U.K., 1998.
(7) Ikeda, A.; Shinkai, S. Chem. ReV. 1997, 97, 1713-1734.
10.1021/ol0607591 CCC: $33.50
© 2006 American Chemical Society
Published on Web 05/19/2006

display an anti orientation.8a Among these receptors, only
the de-tert-butylated bicalix[5]arene described by Gutsche
et al.9 has been found to complex fullerenes with association
constants of 43 M-1 for C60 and 233 M-1 for C70 in CS2, as
determined by UV-vis titrations. As expected for the dimeric
nature of this receptor, these values were about 1 order of
magnitude higher than those observed for the parent calix-
[5]arene. However, the selectivity toward C70, of the bicalix-
[5]arene, measured as the ratio KC70/KC60 was 30 times
lower.9 To the best of our knowledge, there is no example
of a fullerene receptor based on “head-to-head” linked calix-
[4]arenes, despite the inherently simpler synthetic chemistry
of calix[4]arene as compared with that of its higher homo-
logues. We report herein the synthesis, molecular structure,
and fullerene encapsulation properties of 5,5′-bi-(11,11′,17,-
17′,23,23′-hexa-p-tert-butyl)calix[4]arene 1.
state. Crystals suitable for X-ray analysis were obtained by
the slow evaporation of a dichloromethane solution of 1.
Unlike its p-unsubstituted analogue,8a both calixarene
subunits are syn oriented in 1 (Figure 1a), with the biphenyl
Figure 1. (a) X-ray structure of 1. Hydrogen atoms have been
omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level. (b) Wireframe representation of the pseudocage
formed by two molecules of compound 1 in the solid state.
The synthesis of 1 was achieved in two steps from tris-
O-3,5-dinitrobenzoyl calix[4]arene 2 (see Supporting Infor-
mation). Initially, we tried to perform the oxidative coupling
with FeCl3 in acetonitrile, as previously reported,8,9 but this
reaction resulted in the formation of the corresponding
quinone as the major product, along with traces of 1.
Therefore, we choose the classical biphenyl formation via
halogen derivatives. Because attempts to directly brominate
the p-position of 2 with Br2/CHCl3 prior to aryl-aryl
coupling failed, we tried N-bromosuccinimide. Surprisingly,
bicalixarene 3 was formed directly (Scheme 1).10 Removal
linkage tilted by 43°. As a result, the tert-butyl groups are
lined up around a wall that deepens the cavity and increases
its volume. Moreover, several CH-π and CH-O interactions
stabilize the structure. Most interestingly, the crystal packing
shows that two molecules of 1 are “head-to-head” oriented,
forming a pseudocage in which up to four solvent molecules
are included (Figure 1b).
Complexation of C60 and C70 was investigated by UV-
vis titrations in toluene. Both fullerenes gave stable com-
plexes whose guest/host stoichiometries ranged from 1:1 to
1:2, depending on the concentration (Job plot analysis, Figure
2). At submillimolar concentrations, both stoichiometries
Scheme 1. Synthesis of Receptor 1
of the aroyl protecting groups under basic conditions afforded
1 in a 69% yield overall from 2.
In solution, both linked calixarenes display a flattened cone
conformation, as indicated by the presence of two broad high-
field AX systems in the 1H NMR spectrum (CDCl3) and two
signals at ∼32 ppm for the methylene bridges in the 13C
NMR spectrum.11 This structure was confirmed in the solid
Figure 2. Job plots for the complexation of 1 with C60 (a) and C70
(b) at 0.14 mM (9) and 1.4 mM (b).
coexist, especially in the case of C70. At higher concentra-
tions, however, the formation of a 1:2 complex is more
evident, as indicated by the maximum close to ø ) 0.67.
The titration data was analyzed using global multivariate
factor analysis12 for a guest/host stoichiometry of 1:2 and
considering the parent fullerene and its complex as a colored
species (see Supporting Information). Thus, the stability
constants for the formation of a 1:2 complex were evaluated
(8) “Head-to-head” bicalix[4]arenes: (a) Neri, P.; Bottino, A.; Cunsolo,
F.; Piattelli, M.; Gavuzzo, E. Angew. Chem., Int. Ed. 1998, 37, 166-169.
(b) Bottino, A.; Cunsolo, M. F.; Piattelli, M.; Gavuzzo, E.; Neri, P.
Tetrahedron Lett. 2000, 41, 10065-10069. Bicalix[6] and [8]arenes: (c)
Bottino, A.; Cunsolo, F.; Piattelli, M.; Garozzo, D.; Neri, P. J. Org. Chem.
1999, 64, 8018-8020.
(9) “Head-to-head” bicalix[5]arenes: Wang, J.; Borige, S. G.; Watson,
W. H.; Gutsche, C. D. J. Org. Chem. 2000, 65, 8260-8263.
(10) A radical pathway for the aryl-aryl coupling is likely, as the reaction
does not occur in the dark. TLC and MS analysis revealed that the p-bromo
derivative is formed in the first stages of the reaction but is rapidly
transformed into 3. Interestingly, when the reaction was attempted on 11,-
17,23,29-tetra-p-tert-butyl-32,33,34,35-tetra-(3,5-dinitrobenzoyloxy)calix-
[5]arene, no coupling was observed.
(11) Jaime, C.; de Mendoza, J.; Prados, P.; Nieto, P. M.; Sa´nchez, C. J.
Org. Chem. 1991, 56, 3372-3376.
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Org. Lett., Vol. 8, No. 12, 2006

as logK12 ) 4.2 ( 0.2 and 5.1 ( 0.4 for C60 and C70,
respectively (Figure 3).13
Figure 4. Side views of optimized structures of the 1:2 complexes
of 1 with (a) C60 and (b) C70 (Insight II/Discovery in vacuo, cvff
force field calculation).
mixture showed the three characteristic cathodic waves
corresponding to its first, second, and third reductions,
respectively (Figure 5a).14 The peak-to-peak separation is
Figure 3. Variations on the UV-vis spectra of 0.5 mM C60 (top)
and 0.15 mM C70 (bottom) upon addition of 1 in toluene at 295 K.
The titration curves at selected wavelengths are shown in the inset.
Thus, 1 binds C70 about eight times more strongly than
C60. This behavior differs markedly from that reported by
Gutsche et al. for a bicalix[5]arene, which only forms a
relatively weak 1:1 complex with both fullerenes.9 Selectivity
toward C70 could arise from a better fitting of this fullerene
into a dimeric cage made from two bicalix[4]arenes orthogo-
nally oriented in a “tennis-ball” fashion, with intertwining
tert-butyl groups. Indeed, an examination of optimized
molecular models suggests that the “egg-shaped” form of
C70 fits better than the “soccer-ball” shaped C60 into the
dimeric cavity (Figure 4).
The only previous examples of fullerene complexation by
related receptors correspond to calix[4]naphthalenes1b or
thiacalix[4]arenes.2 Therefore, to the best of our knowledge,
this is the first example of the complexation of fullerenes
by p-tert-butyl calix[4]arenes.
The complexation of C60 by 1 was also studied by
electrochemical techniques. The cyclic voltammogram (CV)
of a 1 mM solution of C60 in a 5:1 toluene/acetonitrile
Figure 5. CV (a) and DPV (b) responses of 1 mM C60 (s) and in
the presence of 1 equivalent of 1 (‚‚‚) in toluene/acetonitrile, 5:1.
Supporting electrolyte: 0.1 M NBu4PF6. Working electrode: 0.03
cm2 Pt disk. Scan rate: 100 mV/s.
(12) (a) SPECFIT, version 3.0; Spectra Software Associates. (b) Gampp,
H.; Maeder, M.; Meyer, C. J.; Zuberbu¨hler, A. D. Talanta 1985, 32, 95-
101. (c) Gampp, H.; Maeder, M.; Meyer, C. J.; Zuberbu¨hler, A. D. Talanta
1986, 33, 943-951.
(13) For a 1:1 binding model, the association constants would translate
into log K11 ) 1.9 ( 0.2 and 2.3 ( 0.3 for C60 and C70, respectively.
relatively large (200-220 mV) most likely a result of
uncompensated resistance effects provoked by the low
(14) Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593-
601.
Org. Lett., Vol. 8, No. 12, 2006
2573

polarity of the solvent. Upon addition of 1 equiv of 1, the
signal intensity decreases by about 40%, indicating the
formation of a C60-1 complex that reduces the diffusion rate
of C60 to the electrode. Interestingly, the reduction waves
are split into two signals, as is more evident by examining
the differential pulse voltammograms (DPV; Figure 5b). As
can be seen, the first two reduction signals show two peaks,
one at the same potential of free C60 and a second one that
is shifted to more negative potentials (indicated with an
asterisk), which might correspond to the C60-1 complex.
This means that electron incorporation to the complex is less
favored in the presence of 1, as expected as a result of its
electron-rich character. The ip free/ip complex ratio for the first
and second reduction steps were 0.95 and 1.16, respectively.
These values correlate reasonably well with the concentra-
tions of free and complexed C60 because, according to the
distribution diagram for this system, at 1 mM C60 and 1 mM
1, the concentrations of C60 and C60-1 were 0.56 mM and
0.44 mM, respectively, with [C60]/[ C60-1] ) 1.27. It should
also be noted that the second reduction is more difficult than
the first one, as evidenced by the increased cathodic shift of
this peak (-100 mV) when compared with the gain of the
first electron (-30 mV). A similar signal spitting has been
previously observed and was also explained in terms of
host-guest complexation.15,16 In these cases, however, the
CV changes were observed only after several days of
interaction, in contrast to the present results where splitting
was immediately observed. This accounts for a relatively fast
complexation of C60 by 1. The almost complete absence of
anodic currents in the CV indicates that the electrochemical
process is now chemically and electrochemically irreversible.
After 1 h, the voltametric response of free C60 is partially
recovered due to decomposition of the complex.
In summary, we have reported the synthesis of the new
cavity-extended receptor 1, which represents the first example
of fullerene complexation by a p-tert-butylcalix[4]arene. The
complexing ability of this receptor toward other guests as
well as the preparation of higher homologues is currently
under study.
Acknowledgment. This work has been supported by
CYCIT (project BQU2002-03536) and ICIQ Foundation.
J.C.I.-S. and A. F. acknowledge the Ministerio de Educacio´n
y Ciencia of Spain for fellowships.
Supporting Information Available: Synthetic procedure
and crystallographic data for 1 and complexation studies.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL0607591
(15) Chen, Z.; Fox, J. M.; Gale, P. A.; Pilgrim, A. J.; Beer, P. D.;
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