Macrocyclic hosts for fullerenes: Extreme changes in binding abilities with small structural variations.

Article abstract of DOI:10.1021/ja111072j

Exploiting the shape and electronic complementarity of C₆₀ and C₇₀ with π-extended derivatives of tetrathiafulvalene (exTTF), we have very recently reported a macrocyclic receptor featuring two exTTF recognizing units which forms 1:1 complexes with C₆₀ with log K _a = 6.5 ± 0.5 in chlorobenzene at 298 K. This represents one of the highest binding constants toward C₆₀ reported to date and a world-record for all-organic receptors. Here, we describe our efforts to fine-tune our macrocyclic bis-exTTF hosts to bind C₆₀ and/or C ₇₀, through structural variations. On the basis of preliminary molecular modeling, we have explored p-xylene, m-xylene, and 2,6-dimethylnaphthalene as aromatic spacers between the two exTTF fragments and three alkene-terminated chains of different length to achieve macrocycles of different size through ring closing metathesis. Owing to the structural simplicity of our design, all nine receptors could be accessed in a synthetically straightforward manner. A thorough investigation of the binding abilities of these nine receptors toward C₆₀ and C₇₀ has been carried out by means of UV-vis titrations. We have found that relatively small variations in the structure of the host lead to very significant changes in affinity toward the fullerene, and in some cases even in the stoichiometry of the associates. Our results highlight the peculiarities of fullerenes as guests in molecular recognition. The extreme stability of these associates in solution and the unique combination of electronic and geometrical reciprocity of exTTF and fullerenes are the main features of this new family of macrocyclic hosts for fullerenes.

Full text of DOI:10.1021/ja111072j

ARTICLE
pubs.acs.org/JACS
Macrocyclic Hosts for Fullerenes: Extreme Changes in Binding
Abilities with Small Structural Variations.
David Canevet,† María Gallego,‡ Helena Isla,‡ Alberto de Juan,‡ Emilio M. Pꢀerez,*,†,‡ and Nazario Martín*,†,‡
†IMDEA-Nanociencia, Facultad de Ciencias, Ciudad Universitaria de Cantoblanco, E-28049 Madrid, Spain
‡Departamento de Química Orgꢀanica, Facultad de Química, Universidad Complutense de Madrid, E-28040 Madrid
S Supporting Information
b
ABSTRACT: Exploiting the shape and electronic complementarity
of C60 and C70 with π-extended derivatives of tetrathiafulvalene
(exTTF), we have very recently reported a macrocyclic receptor
featuring two exTTF recognizing units which forms 1:1 complexes
with C60 with log Ka = 6.5 ( 0.5 in chlorobenzene at 298 K. This
represents one of the highest binding constants toward C60 reported to
date and a world-record for all-organic receptors. Here, we describe our
efforts to fine-tune our macrocyclic bis-exTTF hosts to bind C60 and/
or C70, through structural variations. On the basis of preliminary
molecular modeling, we have explored p-xylene, m-xylene, and 2,6-dimethylnaphthalene as aromatic spacers between the two exTTF frag-
ments and three alkene-terminated chains of different length to achieve macrocycles of different size through ring closing metathesis. Owing to
the structural simplicity of our design, all nine receptors could be accessed in a synthetically straightforward manner. A thorough investigation of
the binding abilities of these nine receptors toward C60 and C70 has been carried out by means of UV-vis titrations. We have found that
relatively small variations in the structure of the host lead to very significant changes in affinity toward the fullerene, and in some cases even in
the stoichiometry of the associates. Our results highlight the peculiarities of fullerenes as guests in molecular recognition. The extreme stability
of these associates in solution and the unique combination of electronic and geometrical reciprocity of exTTF and fullerenes are the main
features of this new family of macrocyclic hosts for fullerenes.
to date.3c,3s,3v,9a,9b,9f For example, the Zn(II) metalloporphyrin
1. INTRODUCTION
congener shows a log Ka = 5.8 toward C60, while the free base shows
The search for molecular receptors for fullerenes was initiated
log Ka = 5.9, both in benzene at room temperature.9f The world-
soon after their discovery1 with the pioneering work of the
groups of Diederich, Ringsdorf, and Wennerstr€om.2 The main
record in complex stability is the Ir(III) metalloporphyrin derivative
motivations that have kept it as a very active area of research3,4 are
which shows a log Ka = 8.1 for C60 in 1,2-dichlorobenzene (o-DCB)
at room temperature.3s It is noteworthy that in the latter case, the
the selective association of specific fullerenes to achieve their
purification from complex mixtures3g,3q,3r and the construction of
organized electroactive nanostructures by self-assembly.4 In both
cases the formation of associates with fullerenes of high stability in
solution—that is, the construction of hosts that show large binding
constants toward the fullerenes—is one of the main goals. To
achieve this objective, a broad variety of molecular fragments capable
of establishing positive noncovalent interactions with the outer surface
of fullerenes have been explored. These include derivatives of calix-
[n]arenes,3b,3j,3n,5 cyclotriveratrylenes,3q,3r,6 corannulenes,3p,7 cyclic
paraphenyleneacetylenes,8 π-extended tetrathiafulvalenes,3h,3l,3o,3t
and, most frequently, porphyrins.3c,3s,3v,9
authors indicate that the supramolecular nature of the host-guest
system is arguable, since iridium was found to bind a 6,6 junction of
the fullerene in a η2 fashion. Recently, a macrocycle featuring three
porphyrins10 and a “nanobarrel” formed by four porphyrin units11
have been shown to bind C60 with log Ka = 6.2 and 5.7, respectively,
both in toluene at room temperature.
2. DESIGN AND SYNTHESIS
We have very recently reported a macrocyclic receptor that
associates C60 with log Ka = 6.5 ( 0.5 in PhCl at room tempera-
ture (p-xylmac12 in Chart 1).12 The design of the macrocyclic
host was based on our initial tweezers-like receptors, in which
two exTTF units were connected through an isophthalic diester
spacer. This simple receptor served as proof-of-principle for the
utilization of exTTF as recognizing motif in the construction of
hosts for fullerenes, and showed respectable affinities toward C60
Although the experimental results from different groups are
sometimes difficult to compare due to the use of different methods
to assess the binding constants and to the different solvents
utilized, taking C60 as a reference fullerene guest, it is fair to say
that obtaining hosts that show binding constants of log Ka = 5 or
larger remains a major challenge. Aida’s bisporphyrin macrocycles—
in which porphyrin units are connected via flexible alkyl spacers—
constitute perhaps the most successful family of hosts for fullerenes
Received: December 9, 2010
Published: February 14, 2011
r
2011 American Chemical Society
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Chart 1. Chemical structures of the nine receptors
Scheme 1. General Scheme for the Synthesis of the Macro-
cyclic Hostsa
we conserved all the basic traits of the exTTF tweezers, but included
an alkyl linker with terminal alkenes to achieve macrocyclization
through RCM. In the present study, we describe a collection of
macrocyclic hosts with systematic structural modification of both the
alkyl and aromatic spacers. Considering the precedents and pre-
liminary molecular modeling, we decided to investigate p-xylene, m-
xylene and 2,6-dimethylnaphthalene as aromatic spacers, and hexene,
heptene and octene as alkyl spacers and precursors for RCM. The
structures of the receptors synthesized are shown in Chart 1. The
nomenclature utilized identifies the aromatic spacer and the length of
the alkenyl spacers.
The p-xylmac and m-xylmac families (n = 1-3) were synthe-
sized following a procedure analogous to that reported for p-
xylmac12 (Scheme 1).12 The synthesis starts with the Williamson
etherification of anthraflavic acid with the corresponding bromoalk-
ene under standard conditions, followed by dialkylation of either
para or meta-R,R0-dibromoxylene with the remaining phenolic OH,
which yielded linear tetraketone precursors. These were subjected to
Horner-Wadsworth-Emmons olefination with dimethyl 1,3-
dithiol-2-ylphosphonate, to afford the bis-exTTF linear precur-
sors, which were finally treated with Grubb’s first generation
catalyst to obtain the desired macrocycles. The synthesis of the
naphmac family follows the same procedure, but utilizing 2,6-
bis(hydroxymethyl)naphthalene and a Mitsunobu protocol to
obtain linear bis-anthraquinone precursors (Scheme 1). This was
followed by olefination to form the exTTF derivatives and RCM
to yield the macrocycles as a chromatographically inseparable
mixture of E/Z isomers, which was used as such. The identity
and purity of the nine macrocyclic hosts and all synthetic inter-
mediates were unambiguously established by standard spectro-
scopic and analytical techniques (see the Supporting Information
for full experimental details and characterization).
a Conditions: (a) K2CO3, NaI (cat.), DMF, reflux; (b) for p-xylmac
and m-xylmac families, R,R0-p-dibromoxylene, or R,R0-m-dibro-
moxylene, K2CO3, NaI (cat.), DMF, 60 °C; (c) for naphmac family,
2,6-bis(hydroxymethyl)naphthalene, Ph3P, diethyl azodicarboxy-
late, THF, reflux; (d) dimethyl 1,3-dithiol-2-ylphosphonate, BuLi,
THF, -78 °C to room temp; (e) Grubb’s first generation catalyst,
CH2Cl2, room temp.
3. ASSOCIATION OF C60 AND C70
Binding constants were estimated through UV-vis titrations.
In a typical experiment, we prepared stock solutions of the macro-
cyclic receptor in PhCl at a concentration approximately equal to the
reciprocal of the binding constant. Those solutions were utilized as
solvents in the solution of fullerene, to ensure working at cons-
tant concentration of host. Aliquots of the fullerene solution were
considering its lack of preorganization (log Ka = 3.5 in PhCl at
room temperature).3t To increase the degree of preorganization,
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Figure 1. Spectral changes in UV-vis titration experiments of (a) p-xylmac12, (b) m-xylmac12, and (c) naphmac12 against C60 in PhCl at 298 K.
Table 1. Logarithm of the Calculated Binding Constants for the
Macrocyclic Hosts towards C60 and C70, asEstimatedfromThree
Separate UV-vis Titration Experiments in PhCl at 298 K
C60
C70
p-xylmac10
p-xylmac12
p-xylmac14
m-xylmac10
m-xylmac12
m-xylmac14
naphmac10
naphmac12
naphmac14
4.3 ( 0.5
6.5 ( 0.5
3.5 ( 0.6
4.0 ( 0.5
4.8 ( 0.4
4.1 ( 0.4
4.0 ( 0.2
5.6 ( 0.6
3.4 ( 0.1
6.0 ( 0.4; 12.3 ( 0.6a
b
5.9 ( 0.3
4.4 ( 0.4; 8.3 ( 0.5a
b
5.4 ( 0.3
4.7 ( 0.3
5.6 ( 0.4
6.1 ( 0.2
a First binding constant corresponds to log K1:1 and second to log K2:1
.
b Titration data did not provide satisfactory fits (see main text).
added to the host solution until reaching 3-4 mol equiv, and
UV-vis spectra recorded at room temperature on a Cary UV-50
or on a Shimadzu UV-3600 spectrophotometer equipped with a
temperature control unit. Analysis of the UV-vis data to deter-
mine binding constants was carried out utilizing Specfit multi-
variable analysis software. Experiments were repeated three times.
The values of the binding constants are reported in Table 1 and
were calculated as the mean value of the three experiments, with
the error estimated as the standard deviation.
The changes in the absorption spectra during the titrations
against C60 are very similar for all nine receptors. As representa-
tive examples, Figure 1 shows the evolution of the spectra of the
best host of each series, p-xylmac12, m-xylmac12, and naph-
mac12 upon increasing C60 concentration. In all cases, we
observe the decrease in absorption of the band at λmax ≈ 425-
Figure 2. Side and top views of the energy-minimized (AMBER)
models of the associates: (a) p-xylmac10 C60; (b) p-xylmac12 C60;
3
3
(c) p-xylmac14 C60. The macrocycles are depicted in stick representa-
3
tion with carbon atoms in green, sulfurs in yellow, and oxygens in red.
Hydrogen atoms are not shown for clarity. [60]Fullerenes are depicted
as space-filling models and colored in dark red.
best host for C60, and naphthalene in an intermediate position.
With regards to the alkyl spacer, the C12 derivative consistently
shows the highest binding constant, with a very significant increase
compared to C10 (too small) and C14 (too large). These results
are qualitatively well reproduced by molecular mechanics calcu-
lations in the gas phase.14 Figure 2 shows the geometry optimized
models for the p-xylmac series to illustrate this point.
430 nm, and the appearance of a charge-transfer band at λmax
≈
470-475 nm. An isosbestic point at around 450 nm is also clearly
observable. These spectral changes distinctively indicate associa-
tion of C60 inside the cavity of the macrocycle.12 The relative
intensity of the changes correlates very well with the calculated
binding constants (see Figure 1). For p-xylmac12 (log Ka = 6.5
( 0.5) and naphmac12 (log Ka = 5.6 ( 0.6) both the decrease in
the absorption of the exTTF and the increase in absorption of the
charge-transfer band are significantly more marked than for
m-xylmac12 (log Ka = 4.8 ( 0.4).
In p-xylmac10 C60 (Figure 2a) the macrocyclic cavity is
3
too small to associate C60, and the host adopts a bowl-type
conformation, accommodating the fullerene above the plane of
the macrocycle. In contrast, in p-xylmac12 C60 (Figure 2b) C60
3
fits perfectly inside the cavity of the macrocycle, with both exTTF
units, the aromatic linker, and the alkyl spacer closely wrapping
the fullerene unit. Finally, C60 fits loosely in p-xylmac14, so only
part of the alkyl chain can approach the [60]fullerene and provide
stabilizing van der Waals interactions.
The titration data versus C60 for all nine macrocycles were
fitted successfully to a 1:1 binding mode,13 yielding the binding
constants summarized in Table 1. From the values of the binding
constants, it is clearly apparent that the m-xylylene spacer is the
least adequate of the aromatics, with p-xylylene producing the
The evolution of the spectra during the titrations against C70 is
much less uniform across the different families of macrocycles
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attending to the aromatic spacer. The spectral changes in the
naphmac series mirror what we have previously observed for
exTTF-cyclotriveratrylene hosts,3h namely, during the first additions
the absorbance at λmax = 430 nm decreases, with the concomitant
increase of a charge-transfer band centered at λ = 471 nm. As the
concentration of C70 increases the spectral changes become
difficult to trace due to spectral overlap. Figure 3 depicts the
experimental results obtained in the titration of naphmac14 vs
C70 (PhCl, 298 K). All three titrations for naphmac10, naph-
mac12, and naphmac14 fitted successfully to a 1:1 binding model
(see Figure 3 inset), yielding larger binding constants as the size of
the macrocyclic cavity increases (seeTable1), ascould be expected.
Thus, the most efficient binder for C70 was found to be naph-
mac14, with a remarkable binding constant of log Ka = 6.1 ( 0.2.
On the other hand, the p-xylmac and m-xylmac families showed
parallel trends, depending on the length of the alkyl spacer. The
larger members of each series, p-xylmac14 and m-xylmac14
behaved similarly to the naphmac hosts, and analysis of their
titration data was performed satisfactorily utilizing a 1:1 binding
model, yielding binding constants of log Ka = 5.9 ( 0.3 and 5.4 (
0.3, for p-xylmac14 and m-xylmac14, respectively.
offers major and informative differences. During the first three
additions of the titration, where the concentration of the macro-
cycle is significantly larger than that of the [70]fullerene, two very
well-defined isosbestic points at λ = 402 and 432 nm can be
noted (Figure 4b, inset). As the concentration of C70 increases
these isosbestic points are lost, indicating the coexistence of more
than one type of associates in solution. All these features point to
the formation of complexes of both 2:1 and 1:1 host/guest stoi-
chiometry, with the former prevailing in the early stages of the
titration, and complexes of both stoichiometries being present in
solution when the concentration of C70 increases sufficiently.
Indeed, the titration data did not provide satisfactory fits when
assuming either a 1:1 or a 2:1 stoichiometry exclusively. On the
contrary, the multivariable analysis software converged nicely
when we utilized a model where both types of associates are pre-
sumed to exist in solution (Figure 4a, inset). The calculated bind-
ing constants for p-xylmac10 were log K1:1 = 6.0 ( 0.4 and log
K2:1 = 12.3 ( 0.6, in agreement with both types of associates
showing similar stability. Analogously, we estimated log K1:1
=
4.4 ( 0.4 and log K2:1 = 8.3 ( 0.5 for m-xylmac10, which also
shows smaller binding constants toward C70, as was the case with
C60. Conclusive evidence for this model was provided by Job’s
plot analysis, which showed a broad maximum centered at a
molar fraction of 0.3-0.45 (Figure 4c), in an intermediate situa-
tion between 2:1 and 1:1, where the maxima would be located at
0.33 and 0.5, respectively.
Unexpectedly, the smaller macrocycles p-xylmac10 and m-
xylmac10 showed singular spectroscopic changes during the
titration experiments. Figure 4a shows the variations in the
electronic absorption spectrum of p-xylmac10 upon additions
of 0.1 equiv aliquots of C70 in PhCl at 298 K. Although at first
sight the general trends are similar to those we observed for
naphmac hosts (see Figure 3), a closer look at the titration data
Finally, p-xylmac12 and m-xylmac12 are apparently left in an
intermediate situation between the C10 and C14 members, and,
although association of C70 seems to take place considering the
spectral changes, we could not achieve correct fitting of their titra-
tion data with any binding model. For instance, for m-xylmac12
the best fit was obtained with a 1:1 model, and yielded a clearly
overestimated binding constant of log Ka = 9.9 ( 2.6, with
excessive error and less than modest fitting for the binding isothe-
rms (Figure 5). Similarly, the best fit for the data corresponding
to p-xylmac12 vs C70 affords log Ka = 8.4 ( 2.5.12
As with C60, the experimental results can be rationalized in
terms of a simple lock and key model through molecular mecha-
nics calculations. The energy-minimized models of p-xylmac10 C70
3
and naphmac10 C70 are shown in Figure 6 structures a and c. The
3
cavity of p-xylmac10 is too small to accommodate C70, and leaves a
significant part of its ellipsoidal surface exposed, so that it can be
bound by another molecule of the host (Figure 6b) without steric
congestion. On the other hand, the slight increase in size provided
by the naphthalene spacer in naphmac10 is sufficient to allow
C70 to penetrate deeper into its cavity, stabilizing the 1:1 associate
Figure 3. UV-vis spectra as recorded during the titration of naph-
mac14 vs C70 (PhCl, 298 K). Inset shows the binding isotherms at
430 nm (blue squares) and 471 nm (red triangles); solid lines represent
the fits. For this particular experiment a log Ka = 6.0 was calculated.
Figure 4. (a) UV-vis spectra as recorded during the titration of p-xylmac10 vs C70 (PhCl, 298 K). Inset shows the binding isotherms at 430 nm (blue
squares) and 472 nm (red triangles), solid lines represent the fits. (b) First 10 additions, up to 1 equiv of C70. Inset shows the first three additions.
(c) Job’s plot ([p-xylmac10] þ [C70] = 1.0 ꢀ 10-4 M).
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Figure 7. Plot of the natural logarithm of Ka vs 1/T for the association
of C60 with p-xylmac12 in PhCl.
Figure 5. UV-vis spectra as recorded during the titration of p-
xylmac12 vs C70 (PhCl, 298 K). Inset shows the binding isotherms at
430 nm (blue squares) and 471 nm (red triangles); solid lines represent
the fits.
chose the best binder of the family: p-xylmac12 and C60 as a host.
A plot of the natural logarithm of Ka vs 1/T is shown in Figure 7.
The data show a clear linear tendency and was fitted to a straight
line (r2 = 0.995) with a slope of 19557 ( 1416 and an intercept
of -51 ( 5. From these data, the standard enthalpy of associa-
tion was calculated to be ΔH° = -163 ( 12 kJ mol-1 (-38 ( 3
kcal mol-1) and ΔS° = -424 ( 42 J mol-1 (-101 ( 10 cal
mol-1). Thus, the association of fullerenes by our exTTF-
based macrocyclic hosts is driven by strong enthalpic interac-
tions. For comparison, the enthalpy value is approximately
two to three times higher than that calculated for the associa-
tion of C60 by calix[4]naphthalenes in toluene, which show a
log Ka = 2.8.15
The appearance of a charge-transfer band during the titration
suggests that electronic interactions between the fullerenes and
our bis-exTTF macrocycles take place. These interactions in the
ground state are also noticeable in cyclic voltammetry (CV)
experiments. Though adsorption phenomena prevent a reliable
measure of the oxidation potential of exTTF units, the cathodic
part of the CV allowed us to draw interesting conclusions.
Figure 8a shows the CVs of p-xylmac12 (black), C60 (blue),
and a mixture of both compounds (red). In the presence of a
macrocycle, the half-wave potentials associated with the reduc-
tion signals of [60]fullerene are clearly shifted, which definitely
confirms an interaction in the ground state between the host and
the guest, even though both positive and negative shifts were
detected. Similar measurements (Figure 8b), performed with
naphmac14 and C70, indicate a strong electronic communication
in the complex with a half-wave potential shift as high as -150
mV for the fourth reduction of C70.
To further investigate the influence of the electronic commu-
nication on the stability of the complexes, we carried out
titrations in solvents of different polarities. In the more polar
benzonitrile (εr = 25.2), where charge-transfer interactions
should be favored, we calculated a binding constant of log Ka =
7.5 ( 0.9. The increase in binding constant cannot be ascribed
solely to stabilization of the complex through enhanced charge-
transfer interactions, since the solubility of C60 in benzonitrile is
0.4 mg/mL. This is conclusively confirmed in the nonpolar
toluene (εr = 2.4), in which a decrease in binding constant to
log Ka = 5.4 ( 0.9 was found. Considering the decrease in
solubility of C60 when moving from chlorobenzene (6.4 mg/mL)
to toluene (2.4 mg/mL)16 this is evidence of the contribution of
Figure 6. Energy-minimized (AMBER) models of the associates: (a) p-
xylmac10 C70; (b) p-xylmac10 C70 p-xylmac10; (c) naphmac10
3
3
3
3
C70, side view and (d) top view; (e) naphmac14 C70 with the guest
3
perpendicular to the plane of the macrocycle; (f) naphmac14 C70 with
3
the guest parallel to the plane of the macrocycle. The macrocycles are
depicted in stick representation with carbon atoms in green, sulfurs in
yellow, and oxygens in red. Hydrogen atoms are not shown for clarity.
[70]Fullerenes are depicted as space-filling models and colored in dark red.
(Figure 6c,d). Meanwhile, naphmac14 is the only macrocycle
which is large enough to be capable of hosting C70 with its longer
axis parallel to the plane of the macrocycle (Figure 6f). This provides
an alternative to the perpendicular binding we observe for all other
macrocycles, and can be responsible for the increase in stability.
To complete our understanding of the recognition process of
these macrocyclic systems and gain an insight into its thermo-
dynamic nature, we carried out titrations at different tempera-
tures (288, 298, 308, and 318 K) in PhCl. As model system, we
the electronic interactions to the stabilization of the p-xylmac12
3
C
60 associate, since a decrease in binding constant with increas-
ing solubility is typically found.3u Such effect can be attributed to
the solvation penalty associated with the formation of the
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a simple lock and key model through undemanding molecular
mechanics calculations.
To shed light on the driving force for binding, we have
determined the thermodynamic parameters for the formation
of p-xylmac12 C60. To that end, we have carried out titrations
3
at different temperatures (318, 308, 298, and 288 K). The
positive slope of the ln Ka vs 1/T indicates that association is
driven by enthalpy. Indeed, we calculated a remarkably high
ΔH° = -163 ( 12 kJ mol-1 (-38 ( 3 kcal mol-1). The
spectral features observed during the UV-vis titrations, cyclic
voltammetry data, and titrations carried out in solvents of
different polarity suggest that electronic interactions between
the exTTF and C60 contribute significantly to the stabilization of
the complex.
In the near future, we plan to exploit the extreme stability of
the bis-exTTF macrocycle-fullerene complexes, the lessons
extracted from the structure-affinity relationships, and the
relatively simple synthetic access to the macrocycles to construct
chiral hosts for the enantioselective molecular recognition of the
inherently chiral higher fullerenes.3g,3v
’ ASSOCIATED CONTENT
Figure 8. Cyclovoltammograms of (a) p-xylmac12 (black), C60 (blue),
S
Supporting Information. Supplementary figures, char-
b
and p-xylmac12 C60 (red); (b) naphmac14 (black), C70 (blue), and
3
naphmac14 C70 (red), in PhCl, Bu4NClO4 (0.1 M), v = 0.1 V s-1
,
acterization and full experimental procedures. This material is
available free of charge via the Internet at http://pubs.acs.org.
3
[fullerene] = [macrocycle] = 5 ꢀ 10-4 M. Potentials are indicated vs the
Ag/AgNO3 redox couple.
’ AUTHOR INFORMATION
supramolecular assembly. Since a significant portion of the mole-
cular surfaces of both the receptor and the guest becomes unavai-
lable for solvation upon complexation, the solvation penalty is
more severe in solvents which solvate the separated entities more
efficiently.
Corresponding Author
emilio.perez@imdea.org; nazmar@quim.ucm.es
’ ACKNOWLEDGMENT
We thank Prof. Enrique Ortí (Instituto de Ciencia Molecular,
Universidad de Valencia) for fruitful discussions on the theore-
tical calculations. Financial support by the MICINN of Spain
(CTQ2008-00795/BQU, PIB2010JP-00196, and CSD2007-
00010), the ESF (SOHYD MAT2006-28170-E), and the CAM
(MADRISOLAR-2 S2009/PPQ-1533), is acknowledged. H.I.
and M.G. thank the MICINN for an FPU Studentship and EMP
for a Ramꢀon y Cajal Fellowship, cofinanced by the European
Social Fund. D.C. thanks IMDEA-Nanoscience for a postdoctoral
grant.
4. CONCLUSIONS
We have synthesized a collection of nine macrocycles
bearing two units of exTTF as recognition elements for the
fullerenes, connected through three different aromatic and
alkyl spacers, and investigated their binding abilities toward
C
60 and C70. It was found that p-xylmac12 is the best binder
for C60 with log Ka = 6.5 ( 0.5 for C60 in PhCl and log Ka =
7.5 ( 0.9 in benzonitrile. These values make p-xylmac12 one
of the most efficient receptors for C60 reported to date. With
regards to C70, naphmac14 and p-xylmac14 show log Ka = 6.1 (
0.2 and 5.9 ( 0.3, respectively, in PhCl at 298 K. These are
two of the very few examples of hosts capable of associating
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70 with micromolar affinity.3h,17 From the general point of view
of the design of hosts for fullerenes, our results show that
relatively small structural variations in the receptor lead to large
changes in binding affinities, and in, some cases, even stoichi-
ometry. For example, while m-xylmac14 forms 1:1 com-
plexes with C70 with a respectable binding constant of log Ka =
5.4 ( 0.3, the smaller member m-xylmac10 forms associates of
both 1:1 and 2:1 stoichiometries, with log K1:1 = 4.4 ( 0.4 and
log K2:1 8.3 ( 0.5. With regards to C60, an impressive change in
binding constant of nearly 3 orders of magnitude is found when
moving from p-xylmac12 (log Ka = 6.5 ( 0.5) to p-xylmac14
(log Ka = 3.5 ( 0.6). These differences underline the singularities
of fullerenes as hosts in molecular recognition and highlight the
importance of fine-tuning the structure of the receptors. Remark-
ably, all our experimental results can be qualitatively explained with
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(13) Continous variation plots to confirm the stoichiometries of the
associates were also carried out. For the sake of brevity, only those
indicating 2:1 stoichiometry are shown. The Job’s plot of p-xylmac12 vs
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