Selectivity Control in Synergistic Liquid–Liquid Anion Exchange of

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Selectivity Control in Synergistic Liquid−Liquid Anion Exchange of Univalent Anions via Structure-Specific Cooperativity between Quaternary Ammonium Cations and Anion Receptors Christopher J. Borman, Peter V. Bonnesen, and Bruce A. Moyer* Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6119, United States S Supporting Information *

ABSTRACT: Two anion receptors enhance liquid−liquid anion exchange when added to quaternary alkylammonium chloride anion exchangers, but with a striking dependence on the structure of the alkylammonium cation that suggests a supramolecular cooperative effect. Two anion receptors were investigated, mesooctamethylcalix[4]pyrrole (C4P) and the bisthiourea tweezer 1,1′-(propane-1,3diyl)bis(3-(4-sec-butylphenyl)thiourea (BTU). Whereas synergism is comparatively weak when either methyltri(C8,10)alkylammonium chloride (Aliquat 336) or tetraheptylammonium chloride is used with the BTU receptor, synergism between C4P and Aliquat 336 is so pronounced that anion exchange prefers chloride over more extractable nitrate and trifluoroacetate, effectively overcoming the ubiquitous Hofmeister bias. A thermochemical analysis of synergistic anion exchange has been provided for the first time, resulting in the estimation of binding constants for C4P with the ion pairs of A336+ with Cl−, Br−, OAcF3−, NO3−, and I−.

S

Scheme 1. Model Representation of Simple Synergistic Anion Exchange for Univalent Anions with Chloride as a Reference

eparation techniques based on liquid−liquid ion exchange (LLIX) have been applied in analytical chemistry,1−3 as well as in the hydrometallurgical3,4 and nuclear3,5 industries for six decades. LLIX principles have also been applied for many years in the liquid membranes used in ion-selective electrodes.6,7 Typically, the observed selectivity favors charge-diffuse anions, as high hydration energies cause smaller, more chargedense anions to be less easily removed from aqueous solution.8−12 In practice, this type of selectivity, which has been termed Hofmeister bias,13 has proven extremely useful, but there is an obvious incentive to find ways to overcome its limitations to develop anion separation systems with predetermined non-Hofmeister selectivity patterns. In efforts to gain greater control of anion selectivity, researchers have turned to the use of anion receptors.14−16 Recent work has shown that the structure of the anion receptor plays a critical role in enhancing the selectivity for sulfate extraction in so-called synergistic anion exchange using a quaternary ammonium extractant.17,18 The question has naturally arisen as to whether the structure of the quaternary ammonium cation also has an effect on anion selectivity for a given receptor. For sulfate exchange with nitrate, this has been shown to be the case,19 leading us to seek a more general understanding of the underlying cooperative effects. Toward this end, in this paper we present the results of a survey type of study of selectivity among univalent anions in synergistic anion exchange. Scheme 1 provides a simple equilibrium model for understanding how selectivity responds to addition of a receptor, R, to an anion exchanger, Q+X−, where Q+ is a lipophilic quaternary ammonium cation and X− is a univalent anion. In the scheme, as well as in later equations, overbars (and alternatively the subscript “org”) indicate species © 2012 American Chemical Society

in the organic phase. Since liquid−liquid anion exchange normally conforms to the Hofmeister bias,8,10 the position of the anion-exchange equilibrium in the absence of R effectively lies in the direction favoring the anion with lower charge density. If the ion pairs in Scheme 1 were completely dissociated in the organic phase, as could be assured by a suitable choice of conditions (diluent of high relative permittivity and sufficiently low ionic strength), then the magnitude of the shift of the equilibrium to the left or right upon addition of R simply measures the relative affinity of R for X− or Cl−. However, since our choice of conditions favors ion pairing (vide infra), the shift of the equilibrium measures the cooperative effect of R with Q+ on selectivity. An experiment was designed to test such cooperativity effects based on Scheme 1. Chloride was chosen as the reference anion Received: May 30, 2012 Accepted: August 30, 2012 Published: August 30, 2012 8214

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ion-pair receptor by simultaneously H-bonding to the anion and accommodating the partial insertion of cations of appropriate size into the electron-rich C4P cavity of the C4P cone conformation. Bound cationic species characterized by Xray structures so far include Cs+,33 distal C−H groups of bis-Nmethylimidazolium cations,33,34 aromatic C−H groups of pyridinium,34 an N-methyl group of tetramethylammonium,31 methyltributylammonium (see Figure S-1, Supporting Information),19,30 or octyltrimethylammonium,30 the N-methylene (NCH2−) group or terminal methyl group of tetraalkylammonium,32 and N-CH2− groups of benzyltriethylammonium.31 Other mounting evidence that C4P plays the role of an ion-pair receptor in solution stems from extraction of cations33,35 and anions,18,19 NMR studies,30−34,36 determination of binding constants by isothermal titration calorimetry (ITC),30,31 and various applications of theory and modeling.19,30,36 In the present study, we subject the anion-exchange survey data to a thermochemical analysis that allows calculation of the binding constants of C4P with A336 ion pairs in the DCE phase and a comparison with results determined independently by a homogeneous-phase ITC method.30

herein due to both the convenient radiometric analysis afforded by the 36Cl tracer and the intermediate position of chloride among typical univalent anions spanning a range of high to low relative extractability.20 If the easy-to-obtain chloride form of a lipophilic quaternary ammonium salt is employed as the anionexchange extractant, then a selectivity survey can be carried out conveniently with a series of aqueous sodium salts. As before,20 1,2-dichloroethane (DCE) was chosen as the diluent, which allows access to previously determined Gibbs free energies of anion partitioning.9,10,20−22 Furthermore, DCE is a mildly polar, water-immiscible diluent that tends to suppress the unwanted tendency of quaternary ammonium salts to aggregate.23 This allows exchange reactions under our conditions to be treated in terms of monomeric ion pairs,20,24 facilitating interpretation of behavior according to the model shown in Scheme 1. For purposes of the survey, two quaternary ammonium extractants and two anion receptors were chosen for comparison (shown in Figure 1). The two quaternary

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EXPERIMENTAL SECTION Materials. All commercial reagents, with the exception of Aliquat 336, were used as received without further purification (see the Supporting Information). Aliquat 336 was purified by washing a chloroform solution (ca. 50 g/100 mL) successively with 1 M aqueous NaOH (3×), 0.1 M aqueous NaCl (at least 5×), and deionized water (until the pH was neutral). Chloroform was removed via rotary evaporation, and the resulting viscous oil was dried in a vacuum oven (99.9% confidence, having a slope differing from unity by 6.3 standard deviations. A slope of slightly less than unity is indeed theoretically expected on the basis of the somewhat tighter chloride ion pairing with A336+ as compared with the symmetrical Hp4N+ cation.10

A remarkable effect of the cation for the case of C4P stands out in Figure 2b. Deviations are significant in favor of Cl− exchange by A336 for larger anions, and scatter is large about the dotted trend line added for visualization. By contrast, A336 and Hp4N behave essentially identically for BTU (Figure 2c). The difference between the dashed trend line and solid reference line is insignificant, statistically equivalent at 0.54 standard deviation. The special enhancement of competitive chloride extraction upon addition of C4P to A336 is also evident by replotting DCl with R vs DCl without R (DCl,Q+R vs DQ, Figure S-2, Supporting Information). In this paper we will focus henceforth on the remarkable C4P−A336 combination. 8216

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The pronounced cooperative effect between C4P and A336 indicates formation of a mixed complex with chloride as indicated in Scheme 1. First, if the system were dissociated into free organic-phase Q+ and RX− species, A336 and Hp4N would behave identically. Second, control experiments with C4P used alone (see the Supporting Information) show negligible DCl near the limit of quantitation. Resultant calculated synergistic factors [DCl,Q+R/(DCl,Q + DCl,R)] (Table S-2, Supporting Information) prove that the extraction of chloride vs larger anions resulting from combining C4P with A336, and to a much lesser extent with Hp4N, is greater than the sum of the receptor and anion exchanger acting independently. This behavior fits the standard definition of synergism, normally taken to indicate that the two reagents combine together to form a mixed complex with the extracted species.3,37 In view of the structural, spectroscopic, thermodynamic, and theoretical background described in the introduction, the extraction behavior here is consistent with the expected role of C4P as an ion pair receptor. Anion size is a rough but useful predictor of anion exchange based on electrostatic ion solvation principles.9,10 Figure 3 shows plots of log DCl for each anion in the three A336 systems (Table S-1, Supporting Information) against the respective reciprocal thermochemical ionic radius (1/rionic; see Table S-3, Supporting Information). The distribution ratios observed in this survey span over 4 orders of magnitude and reveal two trends over the series of anions as indicated by two best fit line segments in each plot. A rising trend with the expected scatter (given the known inadequacies both of the purely electrostatic model using the simple 1/rionic parameter and of treating multiatomic ions as spheres)9,10 may be observed along with a level region for small anions. The rising trend line in each plot indicates decreasing competitiveness with Cl− as one progresses to smaller, more hydrated anions (Hofmeister bias8,10,20). Both receptors induce an attenuation of the Hofmeister bias relative to the anion exchangers used alone,13 as reflected by decreased line slopes. Increased scatter follows from perturbations ascribable to the receptors’ binding selectivities.13,38−40 These perturbations are minor relative to the steepness of the trend lines except for the case of C4P combined with A336. In this remarkable case, actual selectivity for Br−, NO3−, OAcF3−, and Cl− is almost level. In fact, chloride is slightly preferred over the larger NO3− and OAcF3− anions. Analogous plots for the Hp4N systems are given in the Supporting Information (Figure S-3), showing only mild attenuation of Hofmeister bias. The second line segment indicates a “leveling off” representing an apparent maximum distribution ratio, 2.04 ± 0.26 overall. All three of the anions smaller than Cl− fall on the level (plateau) trend lines in a narrow range (i.e., average 2.04 ± 0.26) with no statistically significant dependence on the anion, quaternary ammonium cation, or receptor. A satisfactory explanation for this level behavior is the slight partitioning of the quaternary ammonium chloride salts to the aqueous phase (Table S-4, Supporting Information), which would be limiting for the anions smaller and more hydrophilic than Cl− but not for the less hydrophilic anions. Thermochemical Model of Synergized Anion Exchange. The availability of values of the standard molar Gibbs energies of partitioning from water to DCE (ΔG°p,W→DCE; see Table S-3, Supporting Information) for the anions studied herein9,10,20−22 affords an opportunity to put the equilibrium model of Scheme 1 to a more rigorous test. For this purpose,

Figure 3. Chloride distribution ratios from A336 experiments as a function of reciprocal anionic radii. Except where shown in the plots, error bars lie within data markers. Solid lines correspond to linear regressions for subsets of the anions (omitting outlier OTf−). The inset tables report the slopes, standard deviations (in parentheses), and regression correlation coefficients.

the separation factor αCl/X, defined as the ratio DCl/DX (eq 1), is employed (Table S-1, Supporting Information). The hypothetical model for synergized anion exchange of univalent anions as presented in Figure 4 defines the net exchange process in terms of anion partitioning, ion pairing, and binding. Going in the forward direction, Cl− is taken as the reference anion (the displacing anion) originating in the aqueous (upper, less dense) phase. Each forward reaction is opposed by a corresponding reverse reaction for the competing anion X−, which returns to the aqueous phase. The net standard molar Gibbs energy changes of anion partitioning, ion pairing, and binding are given as corresponding difference terms, as shown in the lower part of the figure. The standard molar Gibbs energy change of the overall exchange then ends up being written as a series of the three difference terms shown 8217

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Figure 4. Model for synergized anion exchange for univalent anions Cl− (reference anion) and X−, univalent lipophilic cation Q+, and receptor R.

at the bottom of Figure 4. Without an anion receptor, the observed process is taken to be the formation of Q+Cl− via partitioning and ion pairing. Binding is defined as the 1:1 reaction of R with Q+Cl− or Q+X− ion pairs. Possible binding interactions of R with individual ions will not be considered, as all ions are predominantly ion paired under the conditions employed (see below). In general, the difference terms shown in Figure 4 can all be evaluated, and it will be shown that binding constants can also be estimated for certain ion pairs by making reasonable assumptions. The evaluation is specifically carried out only for exchange of Cl− with the larger anions Br−, OAcF3−, NO3−, I−, OTf−, and ClO4− using C4P in combination with A336, as these are the only observed exchange processes in which a cooperative effect between R and Q+ is significant. Analysis for Case with No Anion Receptor. Anion exchange in the absence of a receptor was evaluated decades ago,11,12 though without the benefit of experimental values of standard molar Gibbs energies of anion partitioning. A thorough updated treatment has been given.10 As shown in Figure 4, the exchange process consists of anion-partitioning and ion-pairing processes, amounting to the horizontal process indicated in Scheme 1. At infinite dilution, where ion pairs are completely dissociated, the exchange process is Cl− + X− ⇌ Cl− + X−

Figure 5. Chloride selectivity expressed as the separation factor (αCl/X) as a function of the difference in standard molar Gibbs energy of partitioning (water to DCE, ΔG°p,Cl− − ΔG°p,X−) for the chloride reference vs each anion: (a) A336Cl systems, (b) Hp4NCl systems. Bold solid reference lines represent the theoretical reference case for infinite dilution given by eq 2b. Gray solid horizontal lines represent the location of “neutral” selectivity for chloride vs chloride (i.e., unity). Dotted and dashed lines correspond to the apparent asymptotic limits on selectivity with BTU and C4P as the receptor, respectively (see the text). Data were taken from Table S-1 (Supporting Information).

When plotted vs ΔGp,Cl ° − − ΔGp,X ° − in Figure 5, the experimentally determined separation factor logs αCl/X,Q indicate a minor ion-pairing effect. Points plotted for systems without an anion receptor tend to track the two solid reference lines but do not lie exactly on the lines, implying a nonzero value for the term ΔG°ip,QCl − ΔG°ip,QX in Figure 4. However, deviations from the line indeed tend to be small as compared with the approximately 5 orders of magnitude total range of αCl/X,Q across the 10-anion series. It may be concluded that the Hofmeister bias selectivity in the absence of an anion receptor reflects the predominance of ion partitioning vs ion pairing, in agreement with previous observations for large cations.10−12,20 Exchange in the case of ion-paired species in the absence of anion receptor is given as

(2a)

where the quaternary ammonium cation (not shown) acts as a “spectator” ion. The corresponding standard molar Gibbs energy change is simply the difference between the standard molar Gibbs energies of anion partitioning (Table S-3, Supporting Information). The corresponding separation factor is identical with the equilibrium concentration quotient, αCl/X,ref = ([Cl−]org[X−]aq)/([Cl−]aq[X−]org), the subscript “ref” indicating the hypothetical reference case of infinite dilution. Since activity coefficients are unity, the separation factor equates to the thermodynamic equilibrium constant for eq 2a, so that log αCl/X,ref

⎛ ⎞ 1 ⎟(ΔG ° − − ΔG ° −) = ⎜− p,Cl p,X ⎝ 2.303RT ⎠

Q+X− + Cl− ⇌ Q+Cl− + X−

(3a)

This reaction is taken to be the observed reaction under the experimental conditions chosen, which is to say that DCl ≈ [QCl]org/[Cl−]aq and DX ≈ [QX]org/[X−]aq. By implication, the experimental separation factor (αCl/X,Q = DCl/DX) may be equated to the equilibrium constant for the exchange in eq 3a, assuming canceled activity coefficients. The corresponding logarithmic form of the separation factor may then be expressed as

(2b)

Taking the difference ΔG°p,Cl− − ΔG°p,X− as the independent variable, eq 2b plots as a straight line with slope −0.175 by definition as shown by the solid reference line in Figure 5. Deviations from the reference line must correspond to differences in the strength of ion pairing (and binding when a receptor is added). 8218

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⎛ ⎞ 1 ⎟[(ΔG ° − − ΔG ° −) log αCl/X,Q = ⎜ − p,Cl p,X ⎝ 2.303RT ⎠ ° ° )] + (ΔGip,QCl − ΔGip,QX

pattern is repeated, though less conspicuously at approximately 1 order of magnitude above the solid reference lines, for the other combinations, C4P + Hp4N, BTU + Hp4N, and BTU + A336. Asymptotic limits are indicated by dashed and dotted lines in Figure 5 on the basis of the assumption that perchlorate and triflate are negligibly bound by the receptor. If the charge density of the competing anion diminishes sufficiently across the series until binding is effectively nil, then the resulting enhancement in separation factor will become constant, reflecting the binding affinity for chloride alone. This case matches the observed behavior. It follows that the binding affinities for the competing anions can be obtained by difference. Following the mathematical procedure elaborated in the Supporting Information, the binding constant for QCl by R may be found from the expression αCl/X,Q + R αCl/X,Q + R ≈ Kbind,QRCl = αCl/X,Q [R]̅ αCl/X,Q [R]̅ init (5)

(3b)

where the subscript “Q” indicates the case with an anion exchanger, no receptor added. Subtraction of eq 2b from eq 3b gives log KΔip = log αCl/X,Q − log αCl/X,ref ⎛ ⎞ 1 ⎟(ΔG ° ° ) = ⎜− ip,QCl − ΔGip,QX ⎝ 2.303RT ⎠

(4)

which is the difference in the ion pair association constants in logarithmic form, log Kip,QCl − log Kip,QX. Graphically, eq 4 gives the vertical distance between the points without an anion receptor and the solid reference lines in Figure 5. Values obtained for log KΔip (Table S-4, Supporting Information) are evidently small, as the separation factors determined in the absence of a receptor fall close to the reference line in Figure 5. Further discussion is provided in the Supporting Information. Analysis for Case with an Anion Receptor. On addition of either of the anion receptors to the anion-exchange systems, the resulting separation factors shown in Figure 5 reflect the unique enhancement in chloride selectivity that occurs when C4P is combined with A336. Otherwise, the other reagent combinations (C4P + Hp4N, BTU + Hp4N, and BTU + A336) produce much smaller and comparable enhancements for a given anion. The enhancement for trifluoroacetate and nitrate (and nearly so for bromide) when C4P is combined with A336 is strong enough to overcome the Hofmeister bias, in which case the net exchange selectivity is actually reversed; that is, the more hydrophilic anion is thermodynamically selected in the exchange. For the hydrophilic anions fluoride, acetate, and hydrogen carbonate, both anion receptors exhibit weak or altogether undetectable effects on selectivity. These small anions are known to be highly hydrated in the organic phase,10 presenting a competitive barrier to binding apparently not overcome by either receptor. Thus, little or no thermodynamic evidence for antagonism (shift to the left) in Scheme 1 is observed, though it was found that the synergistic factor for fluoride falls slightly below 1 (Table S-2, Supporting Information). The primary effect observed is a synergistic shift to the right in favor of chloride exchange vs less hydrated anions. For the four systems shown in Figure 5 with either C4P or BTU, the behavior of αCl/X as the anion size increases from Cl− to ClO4− suggests that the binding strength of X− decreases and then becomes negligible for OTf− and ClO4−. In general, the positive enhancement of chloride exchange for anions bromide to perchlorate is interpreted as being a measure of the difference in the greater binding affinity of the receptor for chloride vs the competing anion. If binding of X− decreases but remains significant from left to right in Figure 5, then the separation factors should continue to diverge from the black reference line as the relative binding of chloride grows vs increasingly charge-diffuse competing anions. Instead, the plotted points to the right of Cl− at first diverge from the reference line and then appear to follow a downward trend parallel to it. This behavior is more easily seen for the special combination of C4P and A336, in which the corresponding parallel trend line (dashed line in Figure 5a) is approximately 3 orders of magnitude above the solid reference line. The same

where the equilibrium concentration of free R, [R]org, may be approximated as its initial concentration [R]init, since there is negligible binding of OTf− and ClO4− anions and DCl is small (Table S-1, Supporting Information). From the experimental separation factors, the binding constant log Kbind,QRCl was found to be 5.01 ± 0.09 as an average for the OTf− and ClO4− systems (Table S-5, Supporting Information). The thermochemical analysis is validated by the reasonable agreement of this estimate with the slightly larger value 5.4 obtained by isothermal titration calorimetry (ITC) on the reaction of A336Cl with C4P in DCE at 25 °C.30 Indeed, the ITC value would be expected to be somewhat larger, as it refers to a dry system, whereas the exchange results obtained in this work refer to a water-saturated system in which choride anion will be hydrated to a degree10 and thus slightly more weakly bound by anion receptors. Values for the binding constants corresponding to QRX, Kbind,QRX, were found for the series Br− to I− by mass-action analysis (Table S-5, Supporting Information). Since it is assumed that QX and QRX are both significant species, and QCl is negligible, the separation factor becomes αCl/X,Q + R ≈

([QRCl])[X −] [Cl ]([QRX] + [QX]) −

(6)

The ratio αCl/X,Q+R/αCl/X,Q then gives the simple expression by use of the equilibrium quotients for KQRCl and KQRX: αCl/X,Q + R αCl/X,Q

=

=

=

[QRCl][QX] [QCl]([QRX] + [QX])

( (

[QRCl] [QCl]

[QRX] [QX]

) )

+1

Kbind,QRCl[R]̅ Kbind,QRX[R]̅ + 1

(7)

Equation 7 has two unknown quantities, [R]org and Kbind,QRX. Using the mass-balance equations [QRX]org = [R]org,init − [R]org − [QRCl]org and [QX]org = [Q]org,init − [QRCl]org − [QRX]org with the approximation [QRCl] ≈ [Cl]org,tot allows an analytical solution for [R]org and Kbind,QRX. Alternatively, the procedure used was to iteratively refine trial values of [R]org until Kbind,QRX 8219

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study with dry solvent. The binding of the large A336Cl ion pair is remarkably strong, coming at the expense of extending the internuclear charge separation of the initial ion pair. Since the A336Cl ion pair association constant log Kip,QCl is on the order of 4.4 (see the Supporting Information), a significant fraction of the initial Coulombic energy of the ion pair must be lost on binding. Since binding also requires a specific orientation of the cation, entropy is also lost. It would be desirable to compare the binding constant for C4P binding of the free Cl− anion vs that for the A336Cl ion pair, but the former value is not available. However, in the extraction of CsCl by C4P into nitrobenzene, the binding of the CsCl ion pair by C4P (estimated log Kbind,CsRCl ≈ 6.7) may be seen to be much stronger than the binding of free Cl− by C4P (log Kbind,RCl− = 3.7). It seems likely that the analogous observation would be true in the present system, implying that the cooperativity of the cation and the C4P receptor molecule in the anion-binding process is also strongly positive. Much more typically, binding of an ion by a neutral receptor tends to weaken the ion pairing of the ion with its counterion, resulting in a negative cooperativity of the receptor and counterion. The remarkable positive cooperativity between A336 and C4P in chloride binding is not fully understood and should be studied further.

values calculated separately from eq 7 and from the massbalance expressions became equal. An approximate graphical interpretation of the binding constants by reference to Figure 5a is discussed in the Supporting Information. Figure 6 shows

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Figure 6. Anion binding constants as logarithms vs standard molar Gibbs energy of partitioning (water to DCE, ΔG°p,X−) for the C4P− A336 system at 25 °C. Data points are given by circle markers. The linear regression line is based only on halides Cl−, Br−, and I−; “+” markers show the regressed values corresponding to evaluated anions Cl− to I−, including extrapolation for OTf− and ClO4−.

ASSOCIATED CONTENT

S Supporting Information *

Previously reported X-ray structure of C4P[Bu3MeN]Cl, relations for anion concentrations from distribution measurements, preparations of 1,3-diisothiocyanatopropane and BTU, tabulated chloride distribution ratios and separation factors, enhancement of DCl upon addition of receptors to exchangers, control experiments for extraction of chloride by C4P used alone, synergistic coefficients and enhancement factors for C4P, table of ionic radii and standard molar Gibbs energies of anionpartitioning, results for partitioning of A336Cl from DCE to water, plot of log DCl,Q+R vs log DCl,Q for Hp4N systems, detailed thermochemical analysis of synergized anion exchange, and supplemental conclusions. This material is available free of charge via the Internet at http://pubs.acs.org.

that the estimated binding constants for C4P binding of A336 ion pairs of anions in the series Cl−, Br−, OAcF3−, NO3−, and I− follow a bias-type selectivity, correlating linearly with the standard molar Gibbs energies of partitioning of the anions from water to DCE.

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CONCLUSIONS Our results demonstrate the clear benefit of a supramolecular approach when employing anion receptors in liquid−liquid anion exchange, so-called synergistic anion exchange. The unique cooperativity of C4P and A336 in chloride exchange in competition with less hydrated anions was sufficiently strong that the separation factor for Cl− vs the more lipophilic anions Br− and OAcF3− exceeds unity, the perturbed Hofmeister extraction order being F− ≈ OAc− ≈ HCO3− < OAcF3− < NO3− < Cl− < Br− < I− < OTf− < ClO4−. In light of literature precedent, it is inferred that the C4P receptor plays the role of receptor for both the cation and anion in that the electron-rich aromatic bowl of C4P in the cone conformation complements the methyl group of the A336 quaternary ammonium cation, while C4P converges its four H-bonds toward the halide center. In good agreement with the value 5.4 determined by calorimetry,30 a log Kbind,QRCl value of 5.01 for the binding of the A336Cl ion pair by C4P was determined from the exchange data using a thermochemical model presented here for the first time. The good agreement between a value determined in a strictly one-phase system and our two-phase study supports the validity of the incorporation of discrete exchange and binding steps in the thermochemical model. The slightly lower value obtained in our two-phase study is even in line with the expected weakly competing effect of partial hydration of the A336Cl ion pair, an effect that is absent in the single-phase

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AUTHOR INFORMATION

Corresponding Author

*Phone: (865) 574-6718. Fax: (865) 576-8559. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Radu Custelcean for redrawing the X-ray structure from Gross et al.31 and Nathan L. Bill and Prof. Jonathan L. Sessler of the Department of Chemistry and Biochemistry, The University of Texas, Austin, for the sample of mesooctamethylcalix[4]pyrrole used in this study. This research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.

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REFERENCES

(1) Morrison, G. H.; Freiser, H. Solvent Extraction in Analytical Chemistry; John Wiley & Sons: New York, 1957. (2) Coleman, C. F.; Blake, C. A., Jr.; Brown, K. B. Talanta 1962, 9, 297−323. 8220

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(3) Rydberg, J., Cox, M., Musikas, C., Choppin, G. R., Eds. Principles and Practices of Solvent Extraction; Marcel Dekker: New York, 2004. (4) Coleman, C. F.; Brown, K. B.; Moore, J. G.; Crouse, D. J. J. Ind. Eng. Chem. 1958, 50, 1756−1762. (5) Coleman, C. F. Process Chemistry; Pergamon Press: Oxford, U.K., 1969; Vol. 4, pp 233−285. (6) Bakker, E.; Pretsch, E. Angew. Chem., Int. Ed. 2007, 46, 5660− 5668. (7) Johnson, R. D.; Bachas, L. G. Anal. Bioanal. Chem. 2003, 376, 328−341. (8) Custelcean, R.; Moyer, B. A. Eur. J. Inorg. Chem. 2007, 1321− 1340. (9) Marcus, Y. Ion Solvation; Wiley: Chichester, U.K., 1985. (10) Moyer, B. A.; Bonnesen, P. V. In Supramolecular Chemistry of Anions; Bianchi, A., Bowman-James, K., Garcia-Espana, E., Eds.; WileyVCH: New York, 1997; pp 1−44. (11) Scibona, G.; Byrum, J. F.; Kimura, K.; Irvine, J. W., Jr. In Solvent Extraction Chemistry; Dyrssen, D., Liljenzin, L. O., Rydberg, J., Eds.; North-Holland Publishing Co.: Amsterdam, 1967; pp 399−407. (12) Scibona, G.; Nathan, R. A.; Kertes, A. S.; Irvine, J. W., Jr. J. Phys. Chem. 1966, 70, 375−379. (13) Kavallieratos, K.; Moyer, B. A. Chem. Commun. 2001, 1620− 1621. (14) Bianchi, A., Bowman-James, K., Eds. The Supramolecular Chemistry of Anions; VCH: Weinheim, Germany, 1997. (15) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, Germany, 1995. (16) Sessler, J. L.; Gale, P. A.; Cho, W.-S. Anion Receptor Chemistry; The Royal Society of Chemistry: Cambridge, U.K., 2006. (17) Fowler, C. J.; Haverlock, T. J.; Moyer, B. A.; Shriver, J. A.; Gross, D. E.; Marquez, M.; Sessler, J. L.; Hossain, M. A.; BowmanJames, K. J. Am. Chem. Soc. 2008, 130, 14386−14387. (18) Moyer, B. A.; Frederick, V.; Sloop, J.; Fowler, C. J.; Haverlock, T. J.; Kang, H.-A.; Delmau, L. H.; Bau, D. M.; Hossain, M. A.; Bowman-James, K.; Shriver, J. A.; Bill, N. L.; Gross, D. E.; Marquez, M.; Lynch, V. M.; Sessler, J. L. Supramol. Chem. 2010, 22, 653−671. (19) Borman, C. J.; Custelcean, R.; Hay, B. P.; Bill, N. L.; Sessler, J. L.; Moyer, B. A. Chem. Commun. 2011, 47, 7611−7613. (20) Levitskaia, T. G.; Maya, L.; Van Berkel, G. J.; Moyer, B. A. Inorg. Chem. 2007, 46, 261−272. (21) Czapkiewicz, J.; Czapkiewicz-Tutaj, B. J. Chem. Soc., Faraday Trans. 1980, 76, 1663−1668. (22) Marcus, Y. Ion Properties; Marcel Dekker: New York, 1997. (23) Marcus, Y.; Kertes, A. S. Ion Exchange and Solvent Extraction of Metal Complexes; Wiley Interscience: New York, 1969; pp 737−814. (24) Inoue, Y.; Tochiyama, O. Bull. Chem. Soc. Jpn. 1980, 53, 1618− 1624. (25) Gale, P. A.; Sessler, J. L.; Kral, V.; Lynch, V. J. Am. Chem. Soc. 1996, 118, 5140−5141. (26) Rothemund, P.; Gage, C. L. J. Am. Chem. Soc. 1955, 77, 3340− 3342. (27) Bryantsev, V. S.; Hay, B. P. J. Am. Chem. Soc. 2006, 128, 2035− 2042. (28) Nishizawa, S.; Kamaishi, T.; Yokobori, T.; Kato, R.; Cui, Y.-Y.; Shioya, K.; Teramae, N. Anal. Sci. 2004, 20, 1559−1565. (29) Pichierri, F. J. Mol. Struct. 2002, 581, 117−127. (30) Gross, D. E.; Schmidtchen, F. P.; Antonius, W.; Gale, P. A.; Lynch, V. M.; Sessler, J. L. Chem.Eur. J. 2008, 14, 7822−7827. (31) Sessler, J. L.; Gross, D.; Cho, W.-S.; Lynch, V. M.; Schmidtchen, F. P.; Bates, G. W.; Light, M. E.; Gale, P. A. J. Am. Chem. Soc. 2006, 128, 12281−12288. (32) Wu, Y.-D.; Wang, D.-F.; Sessler, J. L. J. Org. Chem. 2001, 66, 3739−3746. (33) Custelcean, R.; Delmau, L. H.; Moyer, B. A.; Sessler, J. L.; Cho, W.-S.; Gross, D.; Bates, G. W.; Brooks, S. J.; Light, M. E.; Gale, P. A. Angew. Chem., Int. Ed. 2005, 44, 2537−2542. (34) Caltagirone, C.; Bill, N. L.; Gross, D. E.; Light, M. E.; Sessler, J. L.; Gale, P. A. Org. Biomol. Chem. 2010, 96−99.

(35) Wintergerst, M. P.; Levitskaia, T. G.; Moyer, B. A.; Sessler, J. L.; Delmau, L. H. J. Am. Chem. Soc. 2008, 130, 4129−4139. (36) Kríz, J.; Dybal, J.; Makrlík, E.; Vanura, P.; Moyer, B. A. J. Phys. Chem. B 2011, 115, 7578−7587. (37) Irving, H. M. N. H. In Solvent Extraction Chemistry International Conference; Dyrssen, D., Liljenzin, L. O., Rydberg, J., Eds.; NorthHolland Publishing Co.: Gothenburg, Sweden, 1966; pp 91−110. (38) Sisson, A. L.; Clare, J. P.; Taylor, L. H.; Charmant, J. P. H.; Davis, A. P. Chem. Commun. 2003, 2246−2247. (39) Levitskaia, T. G.; Marquez, M.; Sessler, J. L.; Shriver, J. A.; Vercouter, T.; Moyer, B. A. Chem. Commun. 2003, 2248−2249. (40) Sisson, A. L.; Clare, J. P.; Davis, A. P. Chem. Commun. 2005, 5263−5265.

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dx.doi.org/10.1021/ac301315c | Anal. Chem. 2012, 84, 8214−8221