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Counterion Binding in Solutions of p-Sulfonatocalix[4]arene Nuno Basilio,† Luis Garcı´a-Rı´o,*,† and Manuel Martı´n-Pastor‡ Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, UniVersidad de Santiago, 15782 Santiago, Spain, and Unidad de Resonancia Magne´tica, RIAIDT, Edif. CACTUS, UniVersidad de Santiago, 15782 Santiago, Spain ReceiVed: February 17, 2010; ReVised Manuscript ReceiVed: April 23, 2010
23 Na relaxation NMR measurements and self-diffusion coefficients for sodium cations and p-sulfonatocalix[n]arenes (SCn) were obtained to confirm that monovalent inorganic cations are complexed by SC4. In the absence of added salts and at neutral pH, the cavity of p-sulfonatocalix[4]arene (SC4) fully binds an Na+ counterion. Our results provide evidence that when investigating the complexation of inorganic cations by SC4 a competitive binding scheme must be considered if more than one cation is present in solution. Moreover, it has been shown for the first time that SC6 and SC8 can also complex monovalent inorganic (sodium) cations.
Introduction Calixarenes are phenol-based macrocyclic molecules that can form host-guest complexes with cations, anions, and neutral molecules.1 Due to their relatively facile chemical modification2 a number of properties;including solubility, conformational flexibility, and binding ability;can be tuned through the introduction of functional groups in the basic skeleton. Of all the calixarene derivatives, p-sulfonatocalixarenes are an important class of synthetic receptors due to their high water-solubility, relative innocuity,3 and diverse biomedical applications, which include antiviral, antithrombotic activities, enzyme blocking, and protein complexation.4 p-Sulfonatocalixarenes can form complexes with various guests, including neutral or cationic organic molecules and inorganic cations.5 Due to their π-rich cavities and additional anchoring points provided by the sulfonate groups, p-sulfonatocalixarenes display particularly strong binding abilities for organic cations as a result of cooperative interactions, which include electrostatic, hydrophobic, π-stacking, and cation-π and CH-π interactions. On the other hand, the complexes formed between p-Sulfonatocalix[4]arene (SC4) and metal cations were shown to be strongly charge-dependent. As revealed by microcalorimetry6 and molecular dynamics,7 di- and trivalent metal cations remain outside the SC4 cavity, where they form strong outer-sphere complexes with sulfonate groups located on the upper rim of the calixarene. The process was totally entropycontrolled, with the thermodynamic parameters reflecting the desolvation of the charged groups upon binding (∆rH > 0 and T∆rS . 0). The interactions between SC4 and monovalent metal cations have been qualitatively detected in 1H NMR coalescence studies. However, in early works on water-soluble calixarenes by Shinkai and co-workers8 the complexation of these cations by SC4 remained uninvestigated. Two research groups recently confirmed the formation of such complexes almost simultaneously. First, the displacement of a fluorescent azoalkane guest from the SC4 cavity by inorganic metal cations was explored by Nau and co-workers9 as a proposed method for the detection and * To whom correspondence should be addressed. E-mail: luis.garcia@ usc.es. † Departamento de Quı´mica Fı´sica, Facultad de Quı´mica. ‡ Unidad de Resonancia Magne´tica, RIAIDT, Edif. CACTUS.
quantification of the binding of these cations to SC4. It was found that monovalent metal cations form complexes with SC4 at pH 2 and 7 with binding constants that vary between 70 M-1 for Li+ at pH 2 and 280 M-1 for Cs+ at pH 7. The binding of several monovalent alkali cations was also studied by microcalorimetry at pH 210 but weaker binding constants were obtained in this work. Interestingly, the thermodynamic parameters were in sharp contrast with those obtained for multivalent cations (∆rH , 0 and T∆rS < 0 or ∼0) and led to the conclusion that the observed negative enthalpy is directly associated with the interactions between the cation and the π-electrons of the phenyl groups within the calixarene cavity. Moreover, according to the authors, significant heat effects were not detected for Na+ and Ag+. It was suggested, in contrast with the results published by Nau et al.,9 that these cations are not complexed. Recently, further evidence for monovalent cation complexation appeared in the bibliography. It was shown by 133Cs diffusion NMR spectroscopy11 that Cs+ is complexed by SC4 and a paramagnetic induced 13C relaxation study12 demonstrated that Cs+ and Tl+ are located well within the SC4 cavity. The latter study provides further evidence that the interaction involved in the complexation of monovalent metal cations by SC4 is cation-π rather than electrostatic. Since inorganic cations can be complexed by SC4, these ions will compete with other guests in binding to SC4 (as shown by Nau et al.9) and the quantification of monovalent alkali cation binding constants is of particular importance. This is especially the case for sodium because SC4 is itself a source of Na+; this cation is always present in solution at neutral pH and Na+ is also the most common counterion used in the preparation of buffer solutions. In this work we report the results of a 23Na NMR relaxation study (longitudinal T1 and transversal T2 relaxation times) along with results from a 1H and 23Na diffusion ordered spectroscopy (DOSY) study on the complexation behavior of monovalent metal cations (Na+ and Li+) with SC4. Experimental Section Materials. All commercial reagents were of the highest purity available and were used as received. p-Sulfonatocalix[4]arene (SC4) was prepared by ipso-sulfonation of p-tert-butylcalixarene in H2SO4 at 80 °C. The pentasodium salt (SC4Na) of SC4 was
10.1021/jp101474a 2010 American Chemical Society Published on Web 05/06/2010
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Figure 1. 23Na signal decay for the determination of T1 (left) and T2 (right) in a 20 mM SC4Na sample in D2O at 25 °C. The solid line shows the nonlinear fit to the monoexponential equation.
TABLE 1: Longitudinal (T1) and Transversal (T2) 23Na Relaxation Times for Different NaCl and SC4Na Samples in D2O at 25 °C sample
T1 (10-2 s)
T2 (10-2 s)
NaCl, 0.002 M NaCl, 0.1 M NaCl, 1 M SC4, 0.0004 M SC4, 0.02 M SC4, 0.05 M SC4, 0.10 M SC4, 0.20 M
4.99 ( 0.04 5.21 ( 0.01 4.91 ( 0.01 4.85 ( 0.12 4.22 ( 0.01 3.78 ( 0.02 3.58 ( 0.01 3.12 ( 0.01
4.0 ( 0.2 4.4 ( 0.2 3.9 ( 0.2 4.04 ( 0.13 3.4 ( 0.2 3.40 ( 0.05 3.08 ( 0.10 2.63 ( 0.06
obtained by neutralization of the acid form of SC4 with Na2CO3 in H2O. The product was then decolorized with activated charcoal and filtered through Celite. Finally, the sodium salt was purified by recrystallization three times from water/methanol mixtures. Similarly, the lithium salt of SC4 (SC4Li) was obtained by neutralization of the parent acid with LiOH to pH 6-7. Unlike the sodium salt, SC4Li is soluble in methanol. The purification of SC4Li was achieved by dissolution in methanol and precipitation with ethyl acetate. This procedure was repeated at least three times. Both forms of SC4 were dried at 80 °C under high vacuum. p-Sulfonatocalix[6]arene (SC6) and p-sulfonatocalix[8]arene (SC8) were obtained by using a similar procedure as described for SC4. Relaxation Experiments. Longitudinal (T1) and transversal (T2) relaxation times of 23Na were measured in an 11.7 T Bruker DRX-500 spectrometer (500 MHz proton resonance) equipped with a 1H/Broad-Band inverse NMR probe with PFG shielded Z-gradient capability. The broad-band channel of the probe was tuned to 23Na (132.29 MHz) and the spectra were acquired at a temperature of 300 K without sample spinning. The resulting pseudo-2D spectra were processed with MestreC v.3.9 software (Mestrelab Inc.). The processing involved Fourier-Transformation, phase-correction, and baseline correction along the frequency dimension. The 23Na chemical shifts were referenced to a sample of NaCl in D2O (0 ppm). The nonlinear fit of the experimental integrals to determine relaxation times was performed with Origin V.6.0 software (Originlab Inc.). Diffusion NMR. NMR spectra were recorded at 25 °C on a Varian Inova 500 spectrometer equipped with a 5 mm 1H/X indirect probe with Z-shielded gradients. The NMR experiments were processed with MestreC v.3.9 software (Mestrelab Inc.). 1 H and 23Na diffusion spectra were acquired independently with the Hahn spin-echo based PGSE pulse sequence.13 In both cases, rectangular-shaped pulsed gradients (G) were applied with a power level linearly incremented from 4 to 65 G cm-1 in 32
steps. The duration of the pulse field gradients (δ) applied to encode and decode the diffusion was set to 1 ms for 1H and 3 ms for 23Na. The diffusion delay period ∆ of the experiment was optimized to 100 ms for 1H and 40 ms for 23Na. Such an optimized ∆ value provided a convenient sampling of the exponential decay of the signal intensity during the diffusion experiment and this was essential to achieve accurate results for the determined diffusion coefficients.14 Calibration of the absolute gradient strength was provided by the spectrometer and the particular probe was calibrated with the actual diffusion pulse sequence by using a compound of known diffusion as a reference.15 The reference sample for the 1H diffusion experiments was 99% D2O at 25 °C (D ) 1.87 × 10-5 cm2 s-1) and for the 23Na diffusion experiments the reference was 2 M NaCl solution in 10% D2O in H2O at 25 °C (D ) 1.14 × 10-5 cm2 s-1). As expected, both reference samples provided the same gradient strength with an error of less than 1%. Results and Discussion Relaxation Experiments. 23Na longitudinal relaxation times, T1, were measured with the inversion-recovery sequence (d1-180°-τ-90°-acq) without proton decoupling. The experiment was acquired as a pseudo-2D spectrum by linearly varying the recovery delay τ of the experiment in the range 0.5 to 165 ms along 32 1D subspectra. Each 1D subspectrum was acquired with 4 scans and the interscan delay d1 was 3 s. The FID was detected with 16k complex points during an acquisition time of 3.2 s. The 23Na signal in each subspectrum was integrated and the 32 integrals obtained were fitted to the following equation to determine T1-23Na of the only signal in the spectrum.
I(τ)/I0 ) 1 - 2 exp(-τ/T1)
(1)
where I(τ) is the integral in a spectrum obtained at a given value of the τ period and I0 is the integral of the reference spectrum obtained with the lowest value of τ (see Figure 1,left for a representative plot). 23 Na transversal relaxation times, T2, were measured with the CPMG experiment16 without proton decoupling. The train of 180° pulses of the CPMG block in the sequence was separated by 1 ms. The experiment was acquired as a pseudo-2D spectrum by linearly varying the τ period and controlling the duration of the CPMG block in the range 2 to 94 ms along 64 1D subspectra. Each 1D subspectrum was acquired with 16 scans and the interscan delay d1 was 3 s. The FID was detected with 8k complex points during an acquisition time of 1.61 s. The 23 Na signal of each subspectrum was integrated and the 64
Counterion Binding in p-Sulfonatocalix[4]arene
Figure 2. 1H (left) and Stejskal-Tanner eq 3.
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Na (right) signal decay for a 10 mM SC4Na sample in D2O at 25 °C. The solid line shows the nonlinear fit to the
integrals obtained were fitted (see Figure 1,right) to the following equation to determine T2-23Na of the only signal in the spectrum.
I(τ) ) I0 exp(-τ/T2)
[NaCl] (M)
NaCl DNa (10-5 cm2 s-1)
0.005 0.020 0.050 0.250
1.10 ( 0.03 1.11 ( 0.01 1.08 ( 0.01 1.09 ( 0.01
(2)
The values of T1 and T2 in different samples of SC4Na and in aqueous solutions of NaCl were determined by using 23Na NMR relaxation techniques. The results are summarized in Table 1. It can be seen that the T1 and T2 values are approximately constant for the NaCl solutions, [Na+] ) (0.002-1.000) M. In dilute SC4Na solutions the 23Na T1 and T2 values are similar to that of NaCl (free Na+) but as the concentration of SC4Na increases these values decrease. Complexation of Na+ with macromolecules leads to a decrease in the 23Na relaxation times17 and our results are therefore indicative of Na+ complexation by SC4. Comparison of 23Na T1 and T2 values for the same Na+ concentration in the presence and absence of calixarene clearly shows the complexation of Na+ counterions by the SC4Na. Reported values of T1 for 0.1 M NaCl (T1 ) 5.21 × 10-2 s) and 0.02 M SC4Na (T1 ) 4.22 × 10-2 s) show the decrease in the value of 23Na T1 in the presence of SC4Na due to binding of Na+ to the calixarene. The same trend is observed in the presence of a higher Na+ concentration: T1 values of T1 ) 4.91 × 10-2 s for 1 M NaCl and T1 ) 3.12 × 10-2 s for 0.2 M SC4Na. As can be seen from the results in Table 1, the same behavior is observed for the transversal relaxation times. Diffusion NMR. The complexation of Na+ with SC4 was investigated in more detail by performing 1H and 23Na DOSY experiments on several solutions of SC4Na and NaCl with equivalent concentrations of Na+ ions for comparison, bearing in mind that at neutral pH the pentanionic form of SC4 predominates.18 Quantitative analysis of the intensity of a relevant echo peak in the diffusion spectrum provided the respective translational diffusion coefficient of the corresponding molecule or ion. This was achieved by nonlinear fitting of the signal intensity to the Stejskal-Tanner equation (eq 3) [ref 15 and references therein]:
I ) I0 exp[-Dγ2G2δ2(∆ - δ/3)]
TABLE 2: Observed Self-Diffusion Coefficients for Na+ NaCl Ions (DNa ) in Different Aqueous NaCl Solutionsa
(3)
where I is the measured signal intensity, I0 is the signal intensity at the lowest gradient pulse power, γ is the magnetogyric ratio of the observed nucleus, and the rest of the parameters are defined above. In all experiments, the intensity decay of the signals gave good fits to the monoexponential eq 3, which shows that they represent a single self-diffusion coefficient. As an example, the experimental intensities and the respective
a
All measurements were carried out in D2O at 25 °C.
TABLE 3: Observed Self-Diffusion Coefficients for Na+ SC4 Ions (DNa ) in Different SC4Na Samples and the Corresponding Self-Diffusion Coefficients for the SC4 Host a (DSC4 H ) [SC4] (M)
SC4 DNa (10-5 cm2 s-1)
DHSC4(10-5 cm2 s-1)
Bound χNa
0.001 0.004 0.010 0.050
1.01 ( 0.02 0.989 ( 0.010 0.968 ( 0.008 0.939 ( 0.005
0.308 ( 0.001 0.303 ( 0.001 0.310 ( 0.001 0.299 ( 0.001
0.11 0.15 0.15 0.19
a Bound The fraction of complexed Na+ (χNa ) was calculated by using eq 4. All measurements were carried out in D2O at 25 °C.
nonlinear fit to eq 1 of a 1H and a 23Na signal are shown in Figure 2. The observed diffusion coefficients, DNaCl Na , for the investigated samples at different NaCl concentrations are shown in Table 2. For the NaCl concentrations under investigation the diffusion coefficient seems to be independent of the Na+ concentration. The diffusion coefficients for the calixarene, DHSC4, and Na+, SC4 , in the absence of added NaCl for different SC4Na DNa concentrations are given in Table 3. The diffusion coefficient obtained for SC4Na measured by 1H DOSY was found to vary between 0.299 × 10-5 and 0.310 × 10-5 cm2 s-1. These values are comparable with some of the values reported in the bibliography: 0.327 × 10-5 cm2 s-1,19 0.321 × 10-5 cm2 s-1,20 and 0.302 × 10-5 cm2 s-1.21 Since the self-diffusion coefficient of SC4 (DHSC4) does not vary significantly in the range of concentrations studied, self-aggregation processes can be ruled out. From the results in Table 3 it can be observed that the SC4 ) in self-diffusion coefficients obtained for Na+ cations (DNa SC4Na solutions are lower than those obtained in NaCl NaCl solutions (DNaCl Na ) (Table 2). It is also interesting that the DNa values for the latter solutions are approximately constant in SC4 decreases on the range of concentrations measured, but DNa increasing the SC4Na concentration. These results, as well as those from T1 and T2 relaxation times, support the complexation of Na+ by the calixarene. In this way the
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NaCl SCn TABLE 4: Observed Self-Diffusion Coefficients for Na+ Ions in NaCl (DNa ) and SCn (DNa ) Solutions and the Corresponding a Self-Diffusion Coefficients for the SCn host (DSCn ) H
a
sample
NaCl SCn DNa or DNa (10-5 cm2 s-1)
DHSCn (10-5 cm2 s-1)
Bound χNa
NaCl, 0.050 M SC4, 0.010 M SC6, 0.006 M SC8, 0.005 M
1.08 ( 0.01 0.968 ( 0.008 0.929 ( 0.008 0.907 ( 0.011
0.310 ( 0.001 0.229 ( 0.001 0.197 ( 0.001
0.15 0.18 0.20
Bound The fraction of complexed Na+ (χNa ) was calculated by using eq 4. All measurements were carried out in D2O at 25 °C.
calixarene should not be fully dissociated, but rather a fraction of the Na+ counterions should be bound by the macrocycle and the negative charge should be partially neutralized. One of the main advantages of DOSY in the study of host-guest systems is that it is possible to calculate the fraction of complexed species from single-point experiments. Assuming a fast exchange equilibrium that leads to the transient formation of a complex between SC4 and Na+ with a 1:1 stoichiometry,9 the observed diffusion, Dobs, can be expressed as a molar average of free and bound species
Dobs ) (1 - χBound)Dfree + χBoundDBound
(4)
If we consider that the diffusion coefficient for complexed Na+ is the same as that for SC4, which is true when the hydrodynamic radius of the host is not changed by guest complexation, we can calculate the fraction of bound Na+ (χBound ). Application of eq 4 to our data yields χBound for each Na Na concentration reported in Table 3. The errors associated with such single-point calculations are expected to be high. Bound , However, these results reveal a consistent trend in χNa which increases with the SC4Na concentration. It is worth noting that in Table 3 the same host/guest molar ratio of 1:5 is considered in all the measurements and an increase in the fraction of complex is expected at higher total concentrations. To ascertain whether other p-sulfonatocalix[n]arenes (SCn) can also complex sodium ions, we measured the sodium and proton diffusion coefficients in p-sulfonatocalix[6]arene (SC6) and p-sulfonatocalix[8]arene (SC8) samples. The results are summarized in Table 4. As can be observed, Na+ is complexed not only by SC4 but also by SC6 and SC8. At neutral pH SC6 exists in an octaionic form and the decaionic form of SC8 Bound indicate that in predominates.22 The calculated values of χNa SC6 solution 1.42 sodium cations are complexed and in SC8 solution this value rises to 1.76. These results indicate that SC6 and SC8 can form 1:2 host-guest complexes with Na+. Even though the previous results provide good evidence for the presence of complexed Na+ in SC4Na solutions, further proof to confirm the validity of our results was obtained by means of a competitive binding experiment. The influence of the concentration of LiCl on the observed diffusion coefficient of Na+ for a 10 mM solution of SC4Na is represented in Figure 3. As expected, Dobs increases with the concentration of LiCl due to the complexation of Li+ and SC4, which leads to the subsequent release of Na+ to the bulk solution. This finding confirms that Na+ and Li+ are both complexed with SC4, a finding in agreement with the results reported by Nau et al.9 An alternative explanation for the rather insignificant heat effects on the complexation of Na+ and Ag+ with SC4 found in ref 10 is that, according to our results, a high fraction of SC4 is already occupied with Na+ (95% at 0.05 M SC4) and therefore only small heat variations should be observed under these conditions. On the other hand, if the heat of complexation of Na+ is similar to that of Ag+, these changes could be compensated
Figure 3. Influence of LiCl concentration on the observed diffusion coefficient of Na+ in a 0.010 M solution of SC4Na in D2O at 25 °C. The black line represents the theoretical value of Dobs calculated considering both equilibria 3 and 4 with KLi ) 75 M-1 and KNa ) 100 M-1 (see text below).
Figure 4. Influence of SC4Li concentration on the observed diffusion coefficient of Na+ in a 0.040 M solution of NaCl in D2O at 25 °C. The red line represents the theoretical value for Dobs considering a simple host-guest complexation process (KNa ) 100 M-1) and the black line represents the theoretical value of Dobs calculated considering both equilibria 5 and 6 with KLi ) 80 M-1 and KNa ) 100 M-1.
in the interchange and heat variations would not be observed at all. The reaction could, of course, be purely entropically driven, but according to the results obtained for other monovalent cations10 this does not seem to be the case. In an effort to gain further insights into the complexation behavior of Na+ with SC4 and to obtain more reliable data, we decided to synthesize SC4 with Li+ counterions (SC4Li) and determine the Na+ diffusion coefficient for various concentrations of SC4Li at a constant Na+ concentration. The results are summarized in Figure 4. It can be seen that the Na+ diffusion coefficient decreases on increasing the concentration of SC4Li as a consequence of Na+ complexation with SC4. However, from the results in Figure 4 it is clear that the Dobs values do not approximate the DHSC4 value at an infinite concentration of SC4Li, as would be expected on the basis of eq 2.
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Figure 5. Influence of KCl (left) and CsCl (right) concentration on the observed diffusion coefficient of Na+ in a 0.010 M solution of SC4Na in D2O at 25 °C. The black line represent the theoretical value of Dobs calculated considering both equilibria 3 and 4 with KK ) 110 M-1 and KCs ) 280 M-1.
To account for these results a competitive binding model can be proposed, where both the complexation of Na+ and Li+ are considered. KLi
SC4 + Li+ y\z SC4Li KNa
SC4 + Na+ y\z SC4Na
(5)
(6)
By using the equilibrium constants KLi ) [SC4Li]/([SC4][Li+ ]) and KNa ) [SC4Na]/([SC4][Na+ ]) and the mass balances [SC4]0 ) [SC4] + [SC4Na] + [SC4Li], [Li+ ]0 ) [Li+ ] + [SC4Li], and [Na+ ]0 ) [Na+ ] + [SC4Na] we can obtain the following equation for the uncomplexed concentration of SC4.
a[SC4]3 + b[SC4]2 + c[SC4] - [SC4]0 ) 0
(7)
a ) KNaKLi
(8)
where
b ) [KNaKLi([Na+]0 + [Li+]0 - [SC4]0) + KNa + KLi] (9) c ) [KNa([Na+]0 - [SC4]0) + KLi([Li+]0 - [SC4]0) + 1] (10) Equation 7 was solved numerically with use of MATLAB software (version R2009b) for each estimate of the binding constants KNa and KLi. The concentration of free, uncomplexed SC4 obtained was used in conjunction with eqs 11 and 4 to obtain the theoretical values of Dobs, which are plotted against the experimental data.
[SC4Na] )
KNa[Na+]0[SC4] 1 + KNa[SC4]
(11)
The solid line in Figure 4 shows the best fit obtained with KLi ) 80 M-1 and KNa ) 100 M-1. As can be observed the theoretical model is in good agreement with the experimental data. This model can also be applied to fit the data shown in Figure 3 and the results (KLi ) 75 M-1 and KNa ) 100 M-1) are compatible with those obtained in the last experiment. Additional data were obtained for other alkali cations. In Figure 5 we show the results obtained in an experiment similar to that presented in Figure 3. As can be observed the influence of the concentration of K+ and Cs+ on the observed diffusion
coefficient of Na+ for a 10 mM solution of SC4Na shows the same behavior as that obtained for Li+. By fitting these data to the competitive binding model we obtained the equilibrium binding constants for K+ and Cs+ (KK ) 110 M-1 and KCs ) 280 M-1). All the values obtained here are in good agreement with those obtained in a previous work.9 Conclusions It has been demonstrated by 23Na diffusion NMR measurements that both Li+ and Na+ are complexed by SC4 and the respective binding constants were determined. Combined 23Na and 1H diffusion NMR spectroscopy proved to be a simple and powerful tool to describe complex host-guest systems where various complexable species might be present. For practical purposes this work has several implications, as pointed out previously in the work published by the Nau research group.9 First, great care should be taken when analytical data (equilibrium constants) are to be determined and/or compared since the effects of competing cations in solution must be taken into account. On the other hand, a competitive binding model should be applied when physical data are determined for SC4 complexes, since if these values are obtained by simple titration models they can be quite different from the results obtained when competitive models are applied. For example, the selfdiffusion coefficient for the complexed Na+ obtained without considering the competitive model is 0.90 × 10-5 cm2 s-1, even though it is known that this value should be close to that of free SC4. Acknowledgment. This work was supported by the Ministerio de Ciencia y Tecnologı´a (Project CTQ2008-04420/BQU) and Xunta de Galicia (PGIDIT07-PXIB209041PR). N.B. acknowledges FCT for a Ph.D. Grant (SFRH/BD/29218/2006). References and Notes (1) Asfari, Z.; Bohmer, V.; Harrowfield, J.; Vicens, J., Eds. Calixarenes 2001; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2001. (2) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713–745. (3) Coleman, A. W.; Jebors, S.; Cecillon, S.; Perret, P.; Garin, D.; MartiBattle, D.; Moulin, M. New J. Chem. 2008, 32, 780–782. (4) Perret, F.; Lazar, A. N.; Coleman, A. W. Chem. Commun. 2006, 2425–2438. (5) Guo, D.; Wang, K.; Liu, Y. J. Inclusion Phenom. Macrocyclic Chem. 2008, 62, 1–21. (6) Bonal, C.; Israe¨li, Y.; Morel, J. P.; Morel-Desrosiers, N. J. Chem. Soc., Perkin Trans. 2 2001, 1075–1078. (7) Mendes, A.; Bonal, C.; Morel-Desrosiers, N.; Morel, J. P.; Malfreyt, P. J. Phys. Chem. B 2002, 106, 4516–4524.
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(8) Shinkai, S.; Araki, K.; Kubota, M.; Arimura, T.; Matsuda, T. J. Org. Chem. 1991, 56, 295–300. (9) Bakirci, H.; Koner, A. L.; Nau, W. M. Chem. Commun. 2005, 5411– 5413. (10) Morel, J. P.; Morel-Desrosiers, N. Org. Biomol. Chem. 2006, 4, 462–465. (11) Cuc, D.; Canet, D.; Morel, J. P.; Morel-Desrosiers, N.; Mutzenhardt, P. Chem. Phys. Chem. 2007, 8, 643–645. (12) Cuc, D.; Bouguet-Bonnet, S.; Morel-Desrosiers, N.; Morel, J. P.; Mutzenhardt, P.; Canet, D. J. Phys. Chem. B 2009, 113, 3499–3503. (13) Price, W. S. Concepts Magn. Reson. 1997, 9, 299–336. (14) Antalek, B. Concepts Magn. Reson. 2002, 14, 225–258. (15) Price, W. S. Concepts Nucl. Magn. Reson. 1998, 10, 197–237. (16) Meiboom, S.; Gill, D. ReV. Sci. Instrum. 1958, 29, 688–691.
Basilio et al. (17) Torres, A. M.; Philp, D. J.; Kemp-Harper, R.; Garvey, C.; Kuchel, P. W. Magn. Reson. Chem. 2005, 43, 217–224. (18) Yoshida, I.; Yamamoto, N.; Sagara, F.; Ishii, D.; Ueno, K.; Shinkai, S. Bull. Chem. Soc. Jpn. 1992, 65, 1012–1015. (19) Dalgarno, S. J.; Fisher, J.; Raston, C. L. Chem.sEur. J. 2006, 12, 2772–2777. (20) Krause-Heuer, A. M.; Wheate, N. J.; Tilby, M. J.; Pearson, D. G.; Ottley, C. J.; Aldrich-Wright, J. R. Inorg. Chem. 2008, 47, 6880–6888. (21) Specht, A.; Ziarelli, F.; Bernard, P.; Goeldner, M.; Peng, L. HelV. Chim. Acta 2005, 88, 2641–2653. (22) Suga, K.; Ohzono, T.; Negishi, M.; Deuchi, K.; Morita, Y. Supramol. Sci. 1998, 5, 9–14.
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