Enhanced liquid–liquid anion exchange using macrocyclic anion receptors: effect of receptor structure on sulphate–nitrate exchange selectivity

Abstract

When certain macrocyclic anion receptors are added to a chloroform solution of the nitrate form of a lipophilic quaternary ammonium salt (methyltri-C 8,10 -ammonium nitrate, Aliquat 336N), the extraction of sulphate from an aqueous sodium nitrate solution via exchange with the organic-phase nitrate is significantly enhanced. Eight macrocycles were surveyed, including two derivatives of a tetraamide macrocycle, five derivatives of calix[4]pyrrole and b-decafluorocalix [5]pyrrole. Under the hypothesis that the enhancement originates from sulphate binding by the anion receptors in the chloroform phase, it was possible to obtain reasonable fits to the sulphate distribution survey data based on the formation of 1:1 and 2:1 receptor:sulphate complexes in the chloroform phase. Apparent 1:1 sulphate-binding constants obtained from the model in this system fell in the range log K bind ¼ 2:1 -4:8. Comparison of the results for the various anion receptors included in this study reveals that sulphate binding is sensitive to the nature of the substituents on the parent macrocycle scaffolds in a way that does not follow straightforwardly from simple chemical expectations, such as electron-withdrawing effects on hydrogen-bond donor strength.

Supramolecular organization of calix[4]pyrrole with a methyl-trialkylammonium anion exchanger leads to remarkable reversal of selectivity for sulfate extraction vs. nitrate

“…Previously, we observed that the addition of 1 to methyltrialkyl(C 8-10 )ammonium nitrate (2aÁNO 3 , Aliquat s 336N) in chloroform resulted in enhanced exchange for sulfate (SO 4 2À ) from a solution of sodium nitrate as measured by an increase in the sulfate distribution ratio (D SO 4 = [SO 4 2À ] org /[SO 4 2À ] aq ). 7 Equilibrium modeling showed that the enhanced exchange corresponds to a shift to the right in the Scheme, nitrate binding being relatively weak. 7 Unexpectedly, enhancement with 1 was stronger than with its betaoctafluorinated analog, which normally binds anions much more strongly in aprotic organic solvents, presumably owing to stronger hydrogen-bond (H-bond) donation than 1.…”

“…The enhancement of liquid-liquid anion-exchange selectivity by an anion receptor (R) can be usefully viewed as a standard anion-exchange equilibrium modified by competitive coordination of the exchanging anions by R. 7 Enhanced exchange of a univalent anion (X À ) by a divalent anion (Y 2À ) in the presence of a lipophilic cation (Q + ) and R may be illustrated in the following Scheme (for the case of predominant organicphase ion-pairing):…”

“…[9][10][11] In accord with this principle, a prototypical exchange of charge-dense sulfate from nitrate without added receptor is found to be very energetically unfavorable (exchange lies to the left). [6][7][8] Given the desirability of attaining controlled selectivity patterns, such as extracting sulfate from nitrate, 8 a major challenge for the design of anion receptors for liquid-liquid extraction and other separations has been to effect non-Hofmeister selectivity patterns. 5,11,12 To achieve this aim by design for enhanced anion exchange, it is apparent from the Scheme that the role of the cation must be explicitly considered.…”

“…4,5 In earlier work with liquid-liquid extraction, we proposed an approach to enhancing the selectivity of lipophilic anion exchangers, such as fatty quaternary ammonium salts, by addition of neutral lipophilic anion receptors, such as meso-octamethylcalix [4]pyrrole (C4P, 1). [6][7][8] In exploring the potential for controlling the selectivity of these systems through specific synthetic modification of the anion receptors, we began to question the influence, if any, of the organic-phase cation. 7 Here, we present results of a simple extraction experiment, supported by structural and theoretical results, showing that the organic-phase cation can indeed play a pivotal role to the extent that the apparent selectivity of the anion receptor can be reversed.…”

“…[6][7][8] In exploring the potential for controlling the selectivity of these systems through specific synthetic modification of the anion receptors, we began to question the influence, if any, of the organic-phase cation. 7 Here, we present results of a simple extraction experiment, supported by structural and theoretical results, showing that the organic-phase cation can indeed play a pivotal role to the extent that the apparent selectivity of the anion receptor can be reversed. As detailed below, this effect is ascribed to specific supramolecular interactions that involve both anion and cation recognition by receptor 1.…”