Binding of Ru(NH3) - American Chemical Society

Sep 1, 2009 - 41012 SeVille, Spain. ReceiVed: April 22, 2009; ReVised Manuscript ReceiVed: August 3, 2009. The reactions [Ru(NH3)5pz]2+ + S2O8...
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J. Phys. Chem. B 2009, 113, 12721–12726

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Binding of Ru(NH3)5pz2+ to 4-Sulfocalix[4]arene Sodium Salt. Effects of the Host-Guest Interaction on Electron Transfer Processes P. Lopez-Cornejo,* B. Bote, R. Felix, I. Infantes, P. Lopez, A. Martin, E. Mateos, M. Perez, A. Rojas, and R. Suarez Department of Physical Chemistry, Faculty of Chemistry, UniVersity of SeVille, c/ Prof. Garcia Gonzalez s/n, 41012 SeVille, Spain ReceiVed: April 22, 2009; ReVised Manuscript ReceiVed: August 3, 2009

The reactions [Ru(NH3)5pz]2+ + S2O82- and [Ru(NH3)5pz]2+ + [Co(C2O4)3]3- have been studied in solutions of 4-sulfocalix[4]arene sodium salt. Results show a binding of the ruthenium complex to the calixarene with a 2:1 stoichiometry; that is, a ruthenium molecule binds to two calixarene molecules. This stoichiometry changes when NaCl is added to the medium. Thus, a mixture of 1:1 and 2:1 adducts is found in the presence of 0.1 mol dm-3 NaCl and only 1:1 adducts when the salt concentration is increased up to 0.3 mol dm-3. Results show that the binding of the ruthenium complex to the calixarene is due to electrostatic and nonelectrostatic interactions. Kinetic data are interpreted by using the pseudophase model and taking into account the stoichiometry of the ruthenium binding to calixarene. The presence of a supporting electrolyte in the medium produces ion pair formation which exerts an influence on the kinetic rate constants. Introduction A calixarene is a cyclic oligomer based on a hydroxialquilation product of an aldehyde and a phenol, resulting in cavities made of several phenolic units linked via methylene groups. The word calixarene, given by Gutsche et al.,1 derives from the structure or shape of this macrocyclic which resembles a vase (calix in Greek) and from the incorporation of aromatic rings in its assembly (arene). The size of the macrocycle is specified in the name by a bracketed number inserted between calix and arene. Calixarenes have hydrophobic cavities that can hold smaller molecules,2,3 that is, because they belong to the receptor family known in host-guest chemistry.4 However, they also show a hydrophilic character when the benzenes that form the ring are functionalized, either the upper rim or lower rim of the calixarene framework, with ionic groups.5,6 Therefore, they contain both hydrophobic and hydrophilic compartments together in the same molecule. This fact increases the interest for these macrocyclic systems because, given their characteristics, they exhibit a great ability to be used as drug delivery systems.7 Besides, they also show potential applications in different fields such as analysis and separation,8 enzyme mimetic systems,9 self-assembly membranes,10 etc. Compared to other host systems, calixarenes have a peculiar characteristic with respect to its structure: they can adopt different conformations. The number of conformations increases with the number of benzene units in the system,11 although this also depends on the solvent and the nature of the guest.12 There are different types of interactions that happen between a guest and a host: Coulombic, hydrophobic, hydrogen-bond, Π-stacking, van der Waals, etc.13 Some authors have demonstrated that electrostatic interactions are generally excluded as a crucial role in the host-guest complexation when calixarenes act as a receptor.14 In the same way, we also obtained previously that electrostatic interactions were less important than nonelec* To whom correspondence should be addressed. E-mail: [email protected].

Figure 1. Structure of 4-sulfocalix[4]arene sodium salt.

trostatic ones in the noncovalent binding of a ruthenium complex to calixarenes.15 Here, we have studied the effects that the presence of 4-sulfocalix[4]arene sodium salt exerts on the following electron transfer processes: the reaction [Ru(NH3)5pz]2+ + S2O82- and the reaction [Ru(NH3)5pz]2+ + [Co(C2O4)3]3-. According to results, a binding of the ruthenium complex to two 4-sulfocalix[4]arene salt sodium molecules happens. This 2:1 type binding becomes 1:1 type when high salt concentrations are added to the medium. It is interesting to note the importance that nonelectrostatic interactions have in a host-guest binding. However, it is demonstrated that electrostatic interactions must also be taken into account in such complexation. Experimental Section Materials. The complexes [Ru(NH3)5pz](ClO4)2 and Na3Co(C2O4)3 were prepared and purified according to the procedure described in the literature.16,17 Na2S2O8 from Fluka and NaCl from Merck were used as received. 4-Sulfocalix[4]arene sodium salt (see Figure 1) from Aldrich was used as purchased. Spectra. The spectra of all of the reactants were recorded at different calixarene concentrations with a Cary 500 scan UV-vis-NIR spectrophotometer. The concentrations of the complexes used were the same as those used in kinetic measurements. Kinetic Measurements. Kinetic runs were studied in a stopped-flow spectrophotometer from Applied Photophysics.

10.1021/jp903715t CCC: $40.75  2009 American Chemical Society Published on Web 09/01/2009

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Lopez-Cornejo et al. TABLE 2: Observed Rate Constants, kobs/s-1, for the Process [Ru(NH3)5pz]2+ + [Co(C2O4)3]3- in Aqueous Solutions of 4-Sulfocalix[4]arene Sodium Salt with Different NaCl Concentrations kobs/s-1

Figure 2. Absorption spectra of [Ru(NH3)5pz]2+ in aqueous solutions of 4-sulfocalix[4]arene sodium salt: (A) [calixarene] ) 0 mol dm-3; (B) [calixarene] ) 2.1 × 10-3 mol dm-3.

TABLE 1: Observed Rate Constants, kobs/s-1, for the Process [Ru(NH3)5pz]2+ + S2O82- in Aqueous Solutions of 4-Sulfocalix[4]arene Sodium Salt [calixarene]/ mol dm-3

kobs/s-1

[calixarene]/ mol dm-3

kobs/s-1

0 1.0 × 10-4 1.5 × 10-4 2.0 × 10-4 3.0 × 10-4 7.0 × 10-4 9.0 × 10-4

70.4 6.44 5.50 3.08 1.89 1.16 1.08

1.0 × 10-3 2.0 × 10-3 3.6 × 10-3 5.0 × 10-3 6.3 × 10-3 7.1 × 10-3 9.0 × 10-3

0.86 0.83 0.81 0.90 0.92 0.91 0.89

The kinetics measurements of the processes [Ru(NH3)5pz]2+ + S2O82- and [Ru(NH3)5pz]2+ + [Co(C2O4)3]3- were made following the changes in absorbance of the ruthenium complex. The maximum wavelength of this complex depended on the calixarene concentration. The reactions were studied in pseudo-first-order conditions by using an excess of the oxidant species: peroxodisulphate ions and cobalt complex. The rate constants were obtained from the plots of ln(At - A∞) vs t, with At and A∞ being the absorbances corresponding to the ruthenium complex at time t and when the reaction finished, respectively. The reactant concentrations used in the kinetic measurements were [Ru(NH3)5pz2+] ) 4 × 10-5 mol dm-3, [Co(C2O4)33-] ) 2 × 10-3 mol dm-3, and [S2O82-] ) 3.5 × 10-3 mol dm-3. The concentration of 4-sulfocalix[4]arene sodium salt was in the range (0-9) × 10-3 mol dm-3. All of the measurements (kinetics and spectra) were done at 298.2 ( 0.1 K. The water used in preparation of the solutions had a conductivity of ∼10-6 S m-1. Results Spectra. Spectra of the [Ru(NH3)5pz]2+ complex show a change of the maximum wavelength (∼20 nm) as well as a decrease of the intensity band when the calixarene concentration was changed (see Figure 2). Thus, a blue shift of the band is observed by increasing the host concentration. No change of the spectrum of the cobalt complex is observed when the host concentration is changed. Kinetic Measurements. Tables 1 and 2 show the observed rate constants, kobs, corresponding to the processes [Ru(NH3)5pz]2+ + S2O82- and [Ru(NH3)5pz]2+ + [Co(C2O4)3]3- studied, respectively. A decrease of kobs by increasing the calixarene concentration is always obtained. Discussion Taking into account kinetic results, a decrease of the rate constant by increasing the host concentration is due to an

[calixarene]/ mol dm-3

[NaCl] ) 0 mol dm-3

[NaCl] ) 0.1 mol dm-3

[NaCl] ) 0.3 mol dm-3

0 1 × 10-4 1.5 × 10-4 2 × 10-4 2.5 × 10-4 3 × 10-4 3.5 × 10-4 4 × 10-4 5 × 10-4 7 × 10-4 9 × 10-4 2 × 10-3 3 × 10-3 4 × 10-3 5 × 10-3 7 × 10-3

23.0 1.73 0.97 0.64 0.43 0.33 0.27 0.23 0.19 0.15 0.13 0.12 0.12 0.11 0.11 0.11

5.88 3.83 3.15 2.67 2.31 2.03 1.81 1.64 1.28 1.07 0.95 0.52 0.48 0.44 0.42 0.39

2.62 2.14 2.02 1.91 1.83 1.76 1.70 1.64 1.54 1.41 1.28 1.04 0.90 0.87 0.84 0.76

SCHEME 1

interaction of at least one of the reactants with the host, that is, the calixarene. Bearing in mind the characteristics of the reactants and the calixarene used, one must think that the [Ru(NH3)5pz]2+ complex will interact with this host. Therefore, the ruthenium complex will be distributed between the aqueous solution and the host (see Scheme 1, where Sw represents the S2O82- ions and the [Co(C2O4)3]3- complex), while [Co(C2O4)3]3- and S2O82- will be mainly located in the aqueous bulk of the system due to repulsive interactions with the host. This behavior is confirmed by the spectroscopy results obtained: the band of the ruthenium complex shows a blue shift when the calixarene concentration is increasing as well as a slight decrease of the intensity of these bands, while no change is observed in the case of the cobalt complex. We could not obtain any information on the band of the peroxodisulphate molecules because this reactant absorbs in a region of the electromagnetic spectrum where calixarene and other ions of the medium also absorb. In any case, the negative charge of these ions seems to indicate that, as happens with the cobalt complex, they will also mainly locate in water. The observed behavior in the rate constant can be explained by using the known pseudophase model proposed by Menger and Portnoy.18 Therefore, considering the distribution of the ruthenium complexes between the aqueous bulk and the calixarene (see Scheme 1), one can write the following equation:

kobs )

kw + kcalixK[calixarene] 1 + K[calixarene]

(1)

where kw and kcalix are the rate constants of the processes taking place in water and in the presence of calixarene, respectively. K is the equilibrium constant for the binding of the ruthenium complex to the calixarene, and it is expressed as

Ruthenium Binding to Calixarene

K)

[Ru/calixarene] [Ruw][calixarene]

J. Phys. Chem. B, Vol. 113, No. 38, 2009 12723

(2)

The equation corresponding to the pseudophase model in its simplest version, that is, eq 1, cannot fit the set of data of the two reactions studied. To find an explanation for this behavior, a study on the type of interaction that happens between the [Ru(NH3)5pz]2+ complex and the calixarene has been done. More concretely, the stoichiometry of the guest/host complex formation has been studied by using the continuous variation method.19 This method, also called Job’s method, is commonly used to determine the composition of complexes in solution. For this, a series of solutions are prepared containing a fixed amount of ruthenium complex, with a varying concentration of calixarene. The absorbance of all of these solutions is measured at 472 nm, the maximum wavelength of the ruthenium complex in water. Absorbance increments, ∆A, of the solutions are plotted versus the molar fraction of the complex (considering for the calculation of this molar fraction the sum of the complex and calixarene concentrations). The absorbance increment is obtained as the absorbance of the solution without calixarene minus the absorbance of a solution containing a determined amount of calixarene. This type of measurement was done in the absence and presence of different amounts of NaCl. The plots show a maximum which gives information about the molar relationship between calixarene and ruthenium. Results show the formation of an inclusion complex of the type 2:1; that is, a ruthenium complex binds to two calixarene molecules. This stoichiometry in the binding changes when a salt, NaCl, is added to the medium. Thus, when a NaCl concentration equal to 0.1 mol dm-3 is added, there is a mixture of 1:1 and 2:1 adducts in the medium. However, when the salt concentration is increased up to 0.3 mol dm-3, only 1:1 adducts are formed. Figure 3 shows the Job curves obtained in the absence and presence of NaCl. The addition of salt to a charged host produces a condensation of the counterions of the salt, Na+ ions in this case, in the surface of the host. This will cause a decrease of the surface potential of the host and, therefore, a decrease of the attractive electrostatic interaction between the ruthenium complex and the calixarene. Although some authors have shown in previous works14 that the electrostatic interactions do not play a crucial role in the host-guest binding, we think that this fact depends on the characteristics of the two species, host and guest, that are acting in such complexation. According to this, we also obtained in a previous paper for the binding of [Ru(NH3)5pz]2+ to psulfonatocalix[n]arene (n ) 4, 6, and 8) that this was principally due to nonelectrostatic interactions.15 Experimental results obtained in the present work demonstrate that the 4-sulfocalix[4]arene sodium salt supports a higher negative charge than the acid form of the same calixarene used in the previous paper. Therefore, electrostatic interactions play or do not play an important role depending on the characteristics of the system. Of course, interactions such as Π-stacking, hydrogen bonds, hydrophobic, etc. (nonelectrostatic interactions in general), are also acting in the binding of the ruthenium complex to the 4-sulfocalix[4]arene sodium salt. Thus, for example, the pyrazine ligand of the [Ru(NH3)5pz]2+ complex can bind by Π-stacking interactions with the benzene groups of the calixarene, and the terminal nitrogen of this ligand can also form hydrogen bonds with the hydrogen atoms from the calixarene ring. The binding studied here can be described by the stepwise mechanism shown in Scheme 2, involving the initial formation

Figure 3. Plots of the absorbance increments, ∆A, of the complex solutions with calixarene versus the molar fraction of the ruthenium complex in the presence of different quantities of NaCl (black circles, [NaCl] ) 0 mol dm-3; blue circles, [NaCl] ) 0.1 mol dm-3; pink circles [NaCl] ) 0.3 mol dm-3). Points correspond to experimental data and lines to a fit to find the maximum of each plot.

SCHEME 2

of a 1:1 adduct followed by the addition of a second calixarene molecule to give a 2:1 adduct. Therefore, an oxidant, S2O82- or Co(C2O8)33-, could react with the free ruthenium complex located in the water as well as with the 1:1 and 2:1 adducts formed. Rate constants corresponding to these processes can be defined as kw, k1, and k2, respectively. According to a kinetic scheme of this type, one can write the following equation for the observed rate constant:

kobs )

kw + k1K1[calixarene] + k2K′2[calixarene]2 1 + K1[calixarene] + K′2[calixarene]2

(3) with K2′ being the product K1K2. This rate constant equation can be used when 1:1 and 2:1 adducts coexist in the reaction medium. However, other cases can happen. Therefore, when 1:1 adduct is only formed, or K1 . K2, eq 3 must be rewritten as

kobs )

kw + k1K1[calixarene] 1 + K1[calixarene]

(4)

Note that eq 4 is similar to that corresponding to the pseudophase model, in its simplest version, previously described (see eq 1). On the other hand, when only 2:1 adducts are formed, or K2 . K1, eq 3 must be rewritten as

kobs )

kw + k2K2′ [calixarene]2 1 + K′2[calixarene]2

(5)

Equations 3, 4, and 5 are used to explain the behavior obtained in the kinetic measurements. Thus, according to results obtained by using the Job method, eq 5 is used to fit the kinetic

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Lopez-Cornejo et al.

TABLE 3: Values of the Best Fit Parameters Obtained from eq 3, 4, or 5 According to the NaCl Concentration Used [NaCl]/mol dm-3

kw/s-1

k1/s-1 2+

0 0 0.1 0.3

[NaCl]/mol dm-3

[Ru(NH3)5pz] 70.4 ( 0.2

+ S2O8

0.96 ( 0.06

K2/mol-1 dm3

[Ru(NH3)5pz]2+ + S2O82[Ru(NH3)5pz] 0 0.1 0.3

0.11 ( 0.01 0.33 ( 0.02

K2′/mol-2 dm6 1.1((0.1) × 109

0 2+

k2/s-1

2-

[Ru(NH3)5pz]2+ + [Co(C2O4)3]323.0 ( 0.3 5.91 ( 0.02 1.73 ( 0.04 2.43 ( 0.03 0.61 ( 0.01

K1/mol-1 dm3

+ [Co(C2O4)3]

3-

6.5((0.2) × 10 2.1((0.3) × 103 3

SCHEME 3

3.4((0.1) × 103

1.1((0.2) × 109 2.2((0.1) × 107

rate constant of the two processes studied in the absence of NaCl. Table 3 collects the adjustable parameter values obtained, and Figures 4 and 5 show the quality of the fit. As is shown, the k2 values, that is, the rate constant value corresponding to the electron transfer reaction between the 2:1

Figure 4. Plot of the rate constants of the reaction [Ru(NH3)5pz]2+ + [Co(C2O4)3]3- in aqueous solutions of 4-sulfocalix[4]arene sodium salt in the absence of salt. Points represent experimental data and the line the best fit by using eq 5.

Figure 5. Plot of the rate constants of the reaction [Ru(NH3)5pz]2+ + S2O82- in aqueous solutions of 4-sulfocalix[4]arene sodium salt in the absence of salt. Points represent experimental data and the line the best fit by using eq 5.

adduct and the oxidant, are higher for the calix/[Ru(NH3)5pz]2+/ calix + S2O82- process than for the calix/[Ru(NH3)5pz]2+/calix + [Co(C2O4)3]3- one. This must be related to the charge of the oxidant. The cobalt complex supports a higher negative charge than that for the S2O82- ions. Taking into account the anionic character of the calixarene, the approach of the [Co(C2O4)3]3complex to the host will be less favorable than that of the S2O82ions. Equations 3 and 4 were used to fit experimental data corresponding to the [Ru(NH3)5pz]2+ + [Co(C2O4)3]3- process in the presence of 0.1 and 0.3 mol dm-3 NaCl, respectively. Adjustable parameters are also collected in Table 3. As can be seen, k1 values, that is the rate constant corresponding to the Ru/calix + [Co(C2O4)3]3- process, decrease by increasing the NaCl concentration. The formation of ion pairs between the salt ions and the reactants decreases the electron transfer rate constant. The same effect is observed in the case of the kw rate constant. This behavior can also be explained taking into account the formation of ion pairs. Comparing the rate constants corresponding to the [Ru(NH3)5pz]2+ + [Co(C2O4)3]3-, Ru/calix + [Co(C2O4)3]3-, and calix/[Ru(NH3)5pz]2+/calix + [Co(C2O4)3]3- processes, it is observed that kw > k1 > k2. Electron transfer processes occur in, at least, the three steps20 shown in Scheme 3. The first step represents the formation of the precursor complex from the reactants; the second one, the activation of the precursor complex, electron transfer, and the formation of the successor complex; and the third one, the formation of separate products from the successor complex. That is, the rate constant for the process reactants f products contains contributions from the above steps. The factorization into the different contributions can be done by selecting suitable systems: Thus, if the reaction is accompanied by a major decrease in the free energy, as happens with the processes studied here,21 the contributions from the second and third steps and the reverse process of the second one can be ignored. Nonetheless, the observed rate constant still has contributions from the forward and backward processes of the first step and, of course, from the forward process in the second step. The rate constant corresponding to the latter process is ket, the pure electron transfer rate constant. Under this circumstance, the rate constants kw, k1, and k2 are given by w w kw ) KIP ket

(6)

1 1 k1 ) KIP ket

(7)

2 2 k2 ) KIP ket

(8)

w w 1 1 2 2 ) kAw/k-A , KIP ) kA1/k-A where KIP , and KIP ) kA2/k-A , that is, the equilibrium constants corresponding to the formation of the

Ruthenium Binding to Calixarene

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precursor complexes Ru/Co, Ru/calix/Co, and calix/Ru/calix/ Co, respectively, from the separate reactants. kiet (i ) w, 1, and 2) are the rate constants corresponding to the true electron transfer processes. As is mentioned above, the kw values are higher than the k1 and k2 ones for the processes Ru + Co f products, Ru/calix + Co f products, and calix/Ru/calix + Co f products. This behavior is due to the KIP equilibrium constant. The ruthenium complex has a positive charge higher than that for the Ru/calix, and therefore, the formation of the successor complex Ru/Co will be more favorable than that for the Ru/calix/Co complex. This explains the higher values for kw as compared to k1. With respect to the K2′ values, the equilibrium constant has the same value independent of the oxidant used, as was expected. Besides, K2′ values decrease by increasing the NaCl concentration for the [Ru(NH3)5pz]2+ + [Co(C2O4)3]3- process. This is due to the formation of ion pairs between the ions of the salt and the reactants. K1 values also decrease by increasing the salt concentration for the same reason. A final comment seems pertinent in relation to eq 3. This equation is similar to that obtained from the transition state theory. Thus, according to the transition state theory, the rate constant for a given reaction A + B f products (A ) [Ru(NH3)5pz]2+, B ) S2O82- or Co(C2O4)33-) is given by the equation

kobs ) k0

γAγB γq

(9)

In this equation, γA, γB, and γq are the activity coefficients of the reactants and of the activated complex, respectively. k0 is the rate constant of the process in the reference state. In microheterogeneous systems, it is convenient to take as such the solute in the aqueous solution in constant with the host instead of the habitual reference state, the solute at infinitive dilution. In this way, it is easily shown22 that

γi )

1+

Ki1[calixarene]

1 + Ki1Ki2[calixarene]2 (i ) A, B, q) (10)

K1i

is the equilibrium constant corresponding to the where binding of a reactant i with one calixarene molecule and K2i is the equilibrium constant corresponding to the binding of a species (i/calixarene) with a second calixarene molecule (both equilibrium constants are the same as those represented in Scheme 2 for the ruthenium complex). In the present work, one of the reactants of each process, the oxidant, bears a high negative charge. Thus, it can be assumed that, on average, this reactant will remain far from the calixarene, that is, in the aqueous phase. In other words, it will stay at the reference state and, thus, by definition its activity coefficient will be unity. Therefore, the use of eqs 9 and 10 gives

kobs ) k0

1 + Kq1[calixarene] + Kq1Kq2[calixarene]2 1 + KA1 [calixarene] + KA1 KA2 [calixarene]2

(11)

This expression is similar to eq 3 obtained from the pseudophase model and considering k0 ) kw, k0K1q ) k1K1, k0K1qK2q ) k2K2′, K1A ) K1, and K2′ ) K1AK2A (see ref 22). It is important to note that the transition state also bears a negative charge. At first, one could consider γq ) 1. This consideration would give the following equation for the rate constant:

kobs ) k0

1+

KA1 [calixarene]

1 + KA1 KA2 [calixarene]2

(12) However, an equation of this type does not fit experimental data. Therefore, one must think that the transition state is close enough to the calixarene and we cannot consider its activity coefficient equal to unity. Therefore, as was demonstrated in a previous paper,22 the two models (the Bro¨nsted equation, based on the transition state theory, and the pseudophase model) are equivalent. In conclusion, nonelectrostatic and electrostatic interactions are the cause of the host-guest binding studied here. The role that one or the other plays in the binding process depends on the characteristics of the system. Thus, for the processes studied, the binding of the [Ru(NH3)5pz]2+ complex to the 4-sulfocalix[4]arene sodium salt is due to electrostatic and nonelectrostatic interactions. The former decreases by increasing the NaCl concentration added to the medium. This explains the fact that the [Ru(NH3)5pz]2+ complex only forms 2:1 adducts with the calixarene in the absence of salt, a mixture of 1:1 and 2:1 adducts in the presence of 0.1 mol dm-3 NaCl, and only 1:1 adducts in the presence of 0.3 mol dm-3 NaCl. The rate constants corresponding to the electron transfer processes studied are explained on the basis of the pseudophase model taking into account the stoichiometry of the ruthenium complex binding to the host, the 4-sulfocalix[4]arene sodium salt. The presence of a supporting electrolyte in the medium produces the formation of ion pairs between the ions of the salt and the reactants. This fact exerts an influence on the rate constants kw and k1 due to an influence on the equilibrium constant KIP corresponding to the formation of the precursor complex. Acknowledgment. This work was financed by the D.G.I.C.Y.T. (CTQ2008-00008/BQU) and the Consejerı´a de Educacio´n y Ciencia de la Junta de Andalucı´a. References and Notes (1) Gutsche, C. D.; Muthukrishnan, R. J. Org. Chem. 1978, 43, 4905. (2) (a) Danil de Namor, A. F.; Abbas, I.; Hammud, H. H. J. Phys. Chem. B 2006, 110, 3428. (b) Sessler, J. L.; Gross, D. E.; Cho, W.-S.; Lynch, V. M.; Schmidtchen, F. P.; Bates, G. W.; Light, M. E.; Gale, P. A. J. Am. Chem. Soc. 2006, 128, 12281. (c) Notestein, J. M.; Katz, A.; Iglesia, E. Langmuir 2006, 22, 4004. (d) Bakirci, H.; Koner, A. L.; Dickman, M. H.; Kortz, U.; Nau, W. M. Angew. Chem., Int. Ed. 2006, 45, 7400. (3) (a) Izzet, G.; Rager, M.-N.; Reinaud, O.Dalton Trans. 2007, 771. (b) Valeur, B.; Leray, I.Inorg. Chim. Acta 2007, 360, 765. (c) Arimura, T.; Ide, S.; Suga, Y.; Tachiya, M.J. Oleo Sci. 2007, 56, 149. (d) Baldini, L.; Casnati, A.; Sansone, F.; Ungaro, R.Chem. Soc. ReV. 2007, 36, 254. (4) (a) Mchedlov-Petrossyan, N. O.; Vilkova, L. N.; Vodolazkaya, N. A.; Yakubovskaya, A. G.; Rodik, R. V.; Boyko, V. I.; Kalchenko, V. I. Sensors 2006, 6, 962. (b) Gutsche, C. D.; Nam, K. C. J. Am. Chem. Soc. 1988, 110, 6153. (c) Mohammed-Ziegler, I.; Hamdi, A.; Abidi, R.; Vicens, J. Supramol. Chem. 2006, 18, 219. (d) Gutsche, C. D. In Host Guest Complex Chemistry Macrocycles: Synthesis, Structures, Aplications; Weber, E., Ed.; Springer: Berlin, 1985. (e) Miyagawa, T.; Yamamoto, M.; Muraki, R.; Onouchi, H.; Yashima, E.J. Am. Chem. Soc. 2007, 129, 3676. (f)

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