Highly effective binding of methyl viologen dication and its radical cation by p-sulfonatocalix[4,5]arenes

Article abstract of DOI:10.1021/jo701304g

(Figure Presented) The binding behaviors and thermodynamic origins of p-sulfonatocalix[4]arene (C4AS) and p-sulfonatocalix[5]-arene (C5AS) with methyl viologen (MV²⁺) have been investigated by the methods of isothermal titration calorimetry, NMR, and cyclic voltammetry, showing that the binding abilities of C4AS and C5AS and their host selectivity are dramatically pH-controlled, which is closely discussed from the viewpoint of thermodynamics. Moreover, the radical form of MV^+? can also be effectively included by C4AS and C5AS.

Full text of DOI:10.1021/jo701304g

Highly Effective Binding of Methyl Viologen
Dication and Its Radical Cation by
p-Sulfonatocalix[4,5]arenes
dications and radical cations and thus largely prevent the
dimerization/oligomerization of viologen radical cations.8
On the other hand, calix[n]arenes are pronounced with regard
to the third generation of supramolecular hosts due to their
9
extensive properties. Their water-soluble derivatives, p-sul-
Dong-Sheng Guo, Li-Hua Wang, and Yu Liu*
fonatocalix[n]arenes (CnAS), not only greatly improve the
solubility of calixarenes but also can include various guests to
form stable complexes, particularly with cation guests.10 Fur-
thermore, they have been used as artificial signaling acetylcho-
Department of Chemistry, State Key Laboratory of
Elemento-Organic Chemistry, Nankai UniVersity,
Tianjin, 300071, People’s Republic of China
1
1
12
line receptors, inhibitor of quinine-imine dye deamination,
1
3
yuliu@nankai.edu.cn
and metalloenzyme models, etc. However, to the best of our
knowledge, early studies have only been focused on the
complexation of viologens by C6AS, and the inclusion behaviors
of CnAS with viologen radical cations have not yet been
ReceiVed June 20, 2007
1
4
documented. Especially, investigations concerning the inclu-
sion behaviors of CnAS with viologen radical cations have never
been reported up to now.
In the present work, we report our investigation on the
inclusion complexation behaviors of C4AS and C5AS with
2
+
methyl viologen dication (MV ) using isothermal titration
calorimetry (ITC) and NMR spectroscopy. In addition, the
binding abilities of C4AS and C5AS with methyl viologen
radical cation (MV ) have also been investigated using cyclic
voltammetry. The smaller analogues, C4AS and C5AS, are
employed for their stable preorganized cone shapes and
anticipated size/shape fits with methyl viologen.
The binding behaviors and thermodynamic origins of
+
•
p-sulfonatocalix[4]arene (C4AS) and p-sulfonatocalix[5]-
2+
arene (C5AS) with methyl viologen (MV ) have been
investigated by the methods of isothermal titration calorim-
etry, NMR, and cyclic voltammetry, showing that the binding
abilities of C4AS and C5AS and their host selectivity are
dramatically pH-controlled, which is closely discussed from
the viewpoint of thermodynamics. Moreover, the radical form
The formation of the inclusion complexes between p-
2+
1
sulfonatocalixarenes and MV is evident in H NMR spectro-
scopic experiments in D2O (Figure 1). In the presence of about
+
1
equiv of p-sulfonatocalixarenes, all the protons of MV2
+
•
of MV can also be effectively included by C4AS and
C5AS.
exhibit a visible upfield shift (∆δ) owing to the ring current
2+
effect of the aromatic nuclei, which suggests that the MV
guests are encapsulated into the cavities of p-sulfonatocalix-
arenes. However, the ∆δ value for each proton is different,
which can be used as a powerful evidence to deduce the host-
guest binding manner. Upon addition of C4AS in both acidic
(pD 2.0) and basic (pD 12.0) D2O solution, the ∆δ values of
1
Viologens are one class of important redox couples, widely
2
utilized as herbicides, as subunits in constructing functional
molecular assemblies/machines, as probes to study DNA and
zeolites, and as components of electrochromic display devices.
Thereinto, viologen radical cations, as odd-electron species, are
promised to be suitable electrical conductors and electrochromic
displays as solid, crystalline materials. However, a reversible
π-dimerization constrains their applications to a certain extent.
Therefore, endeavors have been made to stabilize viologen
radical cations, such as chemical modification and dispersion
in appropriate in aprotic matrixes. With development of su-
pramolecular chemistry, the host-guest inclusion complexation
emerges to be another effective way for the mission. As an
excellent example, Kaifer and co-workers reported that cucurbit-
3
4
5
2+
MV protons are in the order of CH3 > a-H > b-H. It indicates
2
+
that MV is immersed into the cavity of C4AS in its axial
orientation with the methyl group being included first. The
6
2
+
5,7
deduced binding manner of C4AS with MV is shown in
Figure 2a.
(
8) Ong, W.; G o´ mez-Kaifer, M.; Kaifer, A. E. Org. Lett. 2002, 4, 1791-
1
794.
(
9) (a) Gutsche, C. D. In Calixarenes ReVisited; Stoddart, J. F., Ed.;
Monographs in Supramolecular Chemistry; Royal Society of Chemistry:
Cambridge, U.K., 1998. (b) B o¨ hmer, V. Angew. Chem., Int. Ed. Engl. 1995,
3
4, 713-745. (c) Ikeda, A.; Shinkai, S. Chem. ReV. 1997, 97, 1713-1734.
[
7]uril can form highly stable complexes with both viologen
(
10) (a) Arena, G.; Casnati, A.; Contino, A.; Lombardo, G. G.; Sciotto,
D.; Ungaro, R. Chem.sEur. J. 1999, 5, 738-744. (b) Arena, G.; Gentile,
S.; Gulino, F. G.; Sciotto, D.; Sgarlata, C. Tetrahedron Lett. 2004, 45, 7091-
7094. (c) Bakirci, H.; Koner, A. L.; Schwarzlose, T.; Nau, W. M. Chem.s
Eur. J. 2006, 12, 4799-4807. (d) Perret, F.; Lazar, A. N.; Coleman, A. W.
Chem. Commun. 2006, 2425-2438.
(
1) Bird, C. L.; Kuhn, A. T. Chem. Soc. ReV. 1981, 10, 49-82.
(2) Summers, L. A. The Bipyridinium Herbicides; Academic Press: New
York, 1980.
(3) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem.,
Int. Ed. 2000, 39, 3348-3391.
(11) Koh, K. N.; Araki, K.; Ikeda, A.; Otsuka, H.; Shinkai, S. J. Am.
Chem. Soc. 1996, 118, 755-758.
(
4) (a) Brun, A. M.; Harriman, A. J. Am. Chem. Soc. 1991, 113, 8153-
8
159. (b) Clennan, E. L. Coord. Chem. ReV. 2004, 248, 477-492.
(12) Tao, W.; Barra, M. J. Org. Chem. 2001, 66, 2158-2160.
(13) Bakirci, H.; Koner, A. L.; Dickman, M. H.; Kortz, U.; Nau, W. M.
Angew. Chem., Int. Ed. 2006, 45, 7400-7404.
(14) (a) Bernardo, A. R.; Lu, T.; C o´ rdova, E.; Zhang, L.; Gokel, G. W.;
Kaifer, A. E. J. Chem. Soc., Chem. Commun. 1994, 529-530. (b) Castro,
R.; God ´ı nez, L. A.; Criss, C. M.; Kaifer, A. E. J. Org. Chem. 1997, 62,
4928-4935.
(5) Monk, P. M. S. The Viologens. Physicochemical Properties, Synthesis
and Applications of the Salts of 4,4′-Bipyridine; John Wiley & Sons:
Chichester, England, 1998.
(
6) Porter, W. W., III; Vaid, T. P. J. Org. Chem. 2005, 70, 5028-5035.
(7) L u¨ , J.-M.; Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2003,
1
25, 12161-12171.
1
0.1021/jo701304g CCC: $37.00 © 2007 American Chemical Society
Published on Web 09/07/2007
J. Org. Chem. 2007, 72, 7775-7778
7775

-
1
TABLE 1. Complex Stability Constants (KS/M ), Enthalpy
-
1
-1
[
∆H°/(kJ‚mol )], and Entropy Changes [T∆S°/(kJ‚mol )] for 1:1
2+
Intermolecular Complexation of MV with p-Sulfonatocalixarenes
in Phosphate Buffer Solution (pH 2.0, 7.2, 12.0) at 298.15 K
conditions
complexes
log KS
∆H°
T∆S°
pH ) 2.0 C4AS + MV2 4.49 ( 0.01 -28.18 ( 0.01 -2.53 ( 0.03
+
2
+
C5AS + MV
3.74 ( 0.01 -20.58 ( 0.06
0.79 ( 0.08
pH ) 7.2 C4AS + MV2 4.97 ( 0.00 -31.98 ( 0.03 -3.62 ( 0.01
+
2
+
C5AS + MV
5.40 ( 0.01 -31.52 ( 0.06 -0.67 ( 0.11
pH ) 12.0 C4AS + MV2 4.97 ( 0.01 -32.83 ( 0.29 -4.46 ( 0.25
C5AS + MV2 5.53 ( 0.02 -33.11 ( 0.05 -1.53 ( 0.15
+
+
2
+
geometry of C5AS with MV . The ROESY spectrum (Figure
2
+
S1) of the complex of C5AS with MV exhibits three clear
cross-peaks representing the correlations of a-H, b-H, and CH3
with aromatic protons of C5AS, respectively. The correlations
between methylene protons of C5AS and protons of MV2 are
not observed, which excludes the possibility of the former
+
2
+
manner (if MV penetrates through the cavity of C5AS, the
distances from methylene protons to a-H or b-H should be in
the region of Overhauser effect). Therefore, we determine the
2
+
binding manner of C5AS with MV , as shown in Figure 2b.
This result is consistent with our previous study that 4,4′-
dipyridinium was accumbently included into the cavity of C5AS
1
5
from the upper rim.
To further quantitatively determine the inclusion complexation
abilities of p-sulfonatocalixarenes with MV , the isothermal
FIGURE 1. The 1H NMR spectra of MV2+ in the absence (and
presence) of C4AS or C5AS: (a) pD ) 2.0; (b) pD ) 12.0.
2+
titration calorimetry (ITC) experiments were performed at acidic,
neutral, and basic conditions. ITC is a powerful tool for
measuring the host-guest complex interactions because it not
only gives the complex stability constants (KS) but can also yield
their thermodynamic parameters (enthalpy and entropy changes
∆
H° and ∆S°). The data obtained are listed in Table 1. In all
cases, the titration data can be well fitted by computer simulation
using the “one set of binding sites” model and repeated as 1:1
complex formation, thereby the higher-order complexes did not
need to be postulated.
As can be seen from Table 1, C4AS and C5AS can all form
2
+
3
5
-1
stable complexes with the MV guest (KS ) 10 -10 M ),
which is different from the larger C6AS that just shows weak
inclusion complexation (KS ) 220 M ) for MV with the
alternative conformation. Moreover, the pH conditions also
play a crucial role in manipulating the complex stability
-
1
2+
1
4b
2+
constants (KS) upon inclusion of the MV guest. The KS values
are in the order of C5AS < C4AS at pH 2.0 and in the order of
C4AS < C5AS at pH 7.2 and 12.0. That is to say, the host
selectivity for C4AS/C5AS pairs is reversed when the pH
increases from acidic to neutral (basic) conditions. According
to the pKa values of lower-rim phenolic hydroxyls (pKa values
of C4AS ) 3.08, 12.02; C5AS ) 4.31, 7.63, 10.96),16 all the
phenolic hydroxyls of C4AS and C5AS are in the protonated
form at pH 2.0. In this case, C4AS possesses the most compact
framework and highest π-electron density of the cavity, leading
FIGURE 2. The deduced binding manners of C4AS (a) and C5AS
2
+
(b) with MV according to NMR spectra.
C5AS possesses similar cone shape to C4AS, but with a wider
size. However, C5AS provides distinct binding geometry for
MV2 from C4AS. According to complex-induced shifts of
MV2 protons by C5AS, the ∆δ sequence by C5AS (b-H >
a-H > CH3) is reversed from that by C4AS in both acidic and
2+
to more effective π-stacking interactions with MV , and then
+
2+
forms a stable complex with MV almost 1 order of magnitude
+
higher than C5AS. This fact can be reflected from the much
more favorable enthalpy change (∆H°C4AS+MV2+,pH2.0
-
1
basic conditions. Judged from the present H NMR results,
-1
∆
H°C5AS+MV2+,pH2.0 ) -7.60 kJ‚mol ).
MV2 may undergo two possible modes when included into
+
2+
the cavity of C5AS. One is that MV penetrates through the
(15) Liu, Y.; Guo, D.-S.; Zhang, H.-Y.; Ma, Y.-H.; Yang, E.-C. J. Phys.
cavity of C5AS in the longitudinal orientation; the other is that
Chem. B 2006, 110, 3428-3434.
MV2+ lies at the upper-rim midsection of C5AS in the latitudinal
(16) (a) Matsumiya, H.; Terazono, Y.; Iki, N.; Miyano, S. J. Chem. Soc.,
Perkin Trans. 2 2002, 1166-1172. (b) Steed, J. W.; Johnson, C. P.; Barnes,
C. L.; Juneja, R. K.; Atwood, J. L.; Reilly, S.; Hollis, R. L.; Smith, P. H.;
Clark, D. L. J. Am. Chem. Soc. 1995, 117, 11426-11433.
orientation. To clarify, 2D ROESY NMR experiments were
carried out to obtain complementary information on the inclusion
7
776 J. Org. Chem., Vol. 72, No. 20, 2007

FIGURE 3. The conformational pinch of C5AS induced by complex-
2
+
ation with MV accompanied with pH increase.
With the solution adjusted from acidic to neutral (basic), some
of the phenolic hydroxyls of calixarenes begin to be deproto-
nated,16 and therefore, the cavities of calixarenes become more
electron-rich and are capable of providing stronger π-stacking
interactions. The enhanced binding constants KS going with pH
value are absolutely driven by the enthalpy term. Dramatically,
C5AS presents much stronger binding ability to MV than
C4AS once the solution is adjusted to neutral or basic. For
example, the inverted host selectivity for C5AS/C4AS pairs is
FIGURE 4. Cyclic voltammetric curves of MV2+ (1.02 mM in pH
7.2 phosphate buffer solution) in the absence and presence of 1 equiv
of C4AS and C5AS. Scan rate is 100 mV/s.
2
+
pH 7.2 and 12.0. The stability of complex C5AS+MV2 at pH
+
1
2
2.0 is enhanced from that at pH 7.2 by 1.3 times. From pH
.0 to 7.2, there should be one hydroxyl deprotonated in C5AS,
2.7 times at pH 7.2 and up to 3.6 times at pH 12.0, which differs
from the host selectivity for C4AS/C5AS pairs of 5.6 times at
pH 2.0. Although the augmentation of π-electron density of
calixarenes arising from the deprotonation of phenolic hydroxyls
plays an important role in the enhancement of binding ability
for p-sulfonatocalixarenes (KS,C4AS+MV2+,pH7.2/KS,C4AS+MV2+,pH2.0
and from pH 7.2 to 12.0, there should be another two hydroxyls
deprotonated in C5AS. It can be seemingly deduced that the
enhancement of KS value from pH 7.2 to 12.0 should be as
prominent as, or even possibly larger than, that from pH 2.0 to
7
.2 if only the augmentation of π-electron density originated
)
3.0), it is not effective enough to enhance the binding ability
from the deprotonation of hydroxyls plays the crucial role during
2
+
of C5AS with MV to such a large extent (KS,C5AS+MV2+,pH7.2/
KS,C5AS+MV2+,pH2.0 ) 45.7) because the augmentations of
π-electron density are considered to be almost identical between
C4AS and C5AS when the condition changes from pH 2.0 to
2
+
the course of complexation of C5AS with MV at different
pH values. However, the factual results do not support the
hypothesis. Therefore, according to the close comparison of KS-
2
+
(
C5AS+MV ) values among the three pH conditions, we can
7
.2. One reasonable explanation for the profound pH-subtle
deduce that the outstanding enhancement of complex stability
from pH 2.0 to 7.2 is mainly attributed to the induced-fit
interaction between the conformation-flexible C5AS and MV2
by template effect of guest, while the accepted enhancement of
complex stability from pH 7.2 to 12.0 is rationally attributed to
the augmentation of π-electron density of C5AS as a result of
the deprotonation of phenolic hydroxyls.
2+
selectivity of C5AS with MV can be inferred from the
viewpoint of binding manner. As proved by the aforementioned
NMR experiments, the wider cavity of C5AS can accommodate
the MV2 guest with the accumbent manner. At acidic condition
of pH 2.0, the restriction of the lower-rim hydrogen bonds makes
the cavity of C5AS appear more like a shallow dish with a
+
+
1
7
certain rigidity, and the size/shape fit between C5AS and
Furthermore, the electrochemical behaviors of MV2 in the
absence and presence of p-sulfonatocalixarenes were also
investigated to determine the binding abilities of C4AS and
+
MV2 is not very good. Upon increasing pH value to neutral
+
(
and basic) conditions, some hydrogen bonds are destroyed,
leading to more conformational flexibility of C5AS, which
makes the cavity of C5AS more adaptable to better accom-
+
•
C5AS with the methyl viologen radical cation (MV ). The
2
+
selected cyclic voltammetric (CV) curves for MV before and
after complexation by p-sulfonatocalixarenes are shown in
Figure 4. In the presence of p-sulfonatocalixarenes, the half-
2
+
modate MV . As illustrated in Figure 3, the conformational
2
+
micro-adjustment of C5AS upon complexation with MV
allows the methyl groups of the MV2+ guest to be more adjacent
to the sulfonate groups of C5AS, which can form strong
hydrogen-bonding interactions, and further strengthens the host-
guest complexation to much extent. This can not only be
reflected from the enthalpy term (∆H°C5AS+MV2+,pH7.2 -
1
2+
wave potential (E1/2 ) of the first one-electron reduction of MV
2
+
+•
(
MV f MV ) is obviously shifted to more negative values.
2
+
It indicates that MV becomes more difficult to be reduced,
reflecting the stabilization offered by the complexation of
p-sulfonatocalixarenes. In line with this interpretation, the shifts
-
1
∆
H°C5AS+MV2+,pH2.0 ) -10.94 kJ‚mol ) but also be validated
1
of E1/2 in the presence of p-sulfonatocalixarenes are more and
by the entropy term. Compared with that in acidic condition,
the framework of calixarenes is more flexible in neutral and
basic conditions owing to the deprotonation of phenolic hy-
droxyls. However, the entropy change is more unfavorable
more pronounced from C4AS to C5AS, which is in the same
1
order of KS values. Moreover, the shifts of E1/2 also imply that
the stability of complexes of p-sulfonatocalixarenes with MV2
decreases upon one-electron reduction. It is reasonable that the
first one-electron reduction decreases the π-electron acceptor
+
2
+
during the course of complexation of C5AS with MV at pH
7
)
.2thanthatofpH2.0(T∆S°C5AS+MV2+,pH7.2 -T∆S°C5AS+MV2+,pH2.0
2
+
and hydrogen-bonding donor abilities of MV and thereby
destabilizes the structure of complexes. Despite the disadvanta-
geous influence arising from the one-electron reduction, p-
sulfonatocalixarenes can also effectively include MV , which
is reflected from the shifts of E1/2 . Besides the first reduction
-
1
-1.46 kJ‚mol ). This is mainly originated from the large
loss of conformational degree of freedom for C5AS and structure
freezing upon strong complexation with MV2 at pH 7.2.
On the other hand, the interpretation can also be validated
by the comparison of the binding abilities of C5AS between
+
+
•
2
+
•
progress, the second one-electron reduction progress (MV
f
MV) is also affected to some extent in the presence of
p-sulfonatocalixarenes. It implies that the MV likewise
(
17) Coruzzi, M.; Andreetti, G. D.; Bocchi, V.; Pochini, A.; Ungaro, R.
+
•
J. Chem. Soc., Perkin Trans. 2 1982, 1133-1138.
J. Org. Chem, Vol. 72, No. 20, 2007 7777

becomes more difficult to be reduced upon complexation by
The strong complexation of p-sulfonatocalixarenes with both
MV and MV is potentially valuable because of the versatile
applications of viologens.
2+
+•
p-sulfonatocalixarenes.
1
Along with the continuous addition of C5AS, E1/2 is further
shifted to negative values more and more and becomes almost
balanceable upon addition of 1.44 equiv of C5AS. Moreover,
primary calculation from the KS value of C5AS with MV
Experimental Section
2
+
2
+
2+
implies that MV exists mostly in C5AS+MV complex form.
Therefore, the E1/21 in the presence of 1.44 equiv of C5AS
should represent the corresponding half-wave potential of the
C5AS+MV2 complex. Thus, the complex stability constant
Materials. The two p-sulfonatocalixarenes {i.e., p-sulfonatocalix-
19
[
4]arene tetrasodium (C4AS) and p-sulfonatocalix[5]arene pen-
16b
tasodium (C5AS)} were synthesized and purified according to
+
the literature reports. Guest molecule, methyl viologen diiodide
+
•
2+
(
KS*) of C5AS with MV can be determined by eq 1.
(MV ), was prepared by the direct reaction of 4,4′-dipyridine with
iodomethane in DMF. The phosphate buffer solution of pH 2.0
was prepared by dissolving sodium dihydrogen phosphate in
distilled, deionized water to make a 0.1 mol‚dm solution, which
was then adjusted to pH 2.0 by phosphoric acid. The phosphate
buffer solution of pH 7.2 was prepared by dissolving disodium
1
2+
1
2+
RT
nF KS*
KS
E1/2 (MV ) ) E (C5AS + MV ) +
ln
(1)
1/2
-3
+
•
The obtained KS* value of C5AS with MV at pH 7.2 is
4
-1
hydrogen phosphate (Na
dihydrogen phosphate (NaH
2
HPO
PO
4
‚12H
2
O, 25.79 g) and sodium
1
.52 × 10 M , which decreases obviously by comparing with
5 -1 2+
2
4
‚2H O, 4.37 g) in distilled, deion-
2
the KS value (2.51 × 10 M ) of C5AS with MV . Neverthe-
-
3
+•
ized water (1000 mL) to make a 0.1 mol‚dm solution. The
phosphate buffer solution of pH 12.0 was prepared by dissolving
disodium hydrogen phosphate in distilled, deionized water to make
less, the stable complexation of C5AS with MV still represents
a typical example of calixarene-stabilizing radicals. The binding
behaviors of p-sulfonatocalixarenes with some other radicals
have been studied before, showing that p-sulfonatocalixarenes
-
3
a 0.1 mol‚dm solution, which was then adjusted to pH 12.0 with
sodium hydroxide.
18
cannot act as ideal hosts for radicals. In the same way, the
KS* value of C4AS with MV at pH 7.2 is also gained as 1.13
Measurements. 1H NMR and 2D ROESY (rotating frame
+
•
Overhauser effect spectroscopy) spectra were recorded in D
solution (pH adjusted by DCl or NaOD) at 25 °C. Chemical shifts
δ, ppm) in water were externally referenced to 2,2-dimethyl-2-
2
O
4
-1
×
10 M . The effective inclusion of C4AS and C5AS with
+
•
MV is promised to prevent the unsatisfactory π-dimerization
of viologen radical cations, which endows the more effective
use of viologens as components of electrochromic display
devices and other applications.2 In addition, taking into
account the binding manners, C5AS may be a more suitable
receptor than C4AS.
(
silapentane-5-sulfonate (DSS) as an external reference in order to
avoid any possible interaction with hosts as well as with the guest
molecule. A thermostated and fully computer-operated isothermal
calorimetry (VP-ITC) instrument was used for all microcalorimetric
experiments. The cyclic voltammetry (CV) measurements were
carried out on an electrochemical analyzer with a C3 cell stand in
phosphate buffer (pH 7.2) at 25 °C.
-5
In summary, both C4AS and C5AS can form stable com-
2+
plexes with MV while their binding manners are distinct from
each other. MV2 penetrates into the cavity of C4AS in the
axial orientation, while the wider cavity of C5AS accommodates
MV2 at its upper-rim midsection with the accumbent manner.
The particular binding geometry of C5AS with MV allows
the right induced fit between host and guest accompanied with
the augmentation of pH value. As a result, C5AS presents much
stronger binding ability to MV2 than C4AS at neutral (basic)
conditions and displays the reversed host selectivity from that
at acidic conditions, as well. Moreover, C4AS and C5AS can
+
Acknowledgment. This work was supported by the 973
Program (2006CB932900), NNSFC (20673061), and the Sci-
ence and Technology Innovation Foundation of Nankai Uni-
versity, which are gratefully acknowledged.
+
2
+
+
Supporting Information Available: Two-dimensional NMR,
2
+
ITC, and CV experiments of C5AS (C4AS) with MV . This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
+•
4
-1
also effectively include MV with the strength of over 10 M .
JO701304G
(
18) (a) Franchi, P.; Lucarini, M.; Pedulli, G. F.; Sciotto, D. Angew.
Chem., Int. Ed. 2000, 39, 263-266. (b) Sueishi, Y.; Negi, M.; Kotake, Y.
Chem. Lett. 2006, 35, 772-773.
(19) Arena, G.; Contino, A.; Lombardo, G. G.; Sciotto, D. Thermochim.
Acta 1995, 264, 1-11.
7
778 J. Org. Chem., Vol. 72, No. 20, 2007