Highly Efficient, Tripodal Ion-Pair Receptors for Switching Selectivity between Acetates and Sulfates Using Solid–Liquid and Liquid–Liquid Extractions

A tripodal, squaramide-based ion-pair receptor 1 was synthesized in a modular fashion, and 1H NMR and UV-vis studies revealed its ability to interact more efficiently with anions with the assistance of cations. The reference tripodal anion receptor 2, lacking a crown ether unit, was found to lose the enhancement in anion binding induced by presence of cations. Besides the ability to bind anions in enhanced manner by the “single armed” ion-pair receptor 3, the lack of multiple and prearranged binding sites resulted in its much lower affinity towards anions than in the case of tripodal receptors. Unlike with receptors 2 or 3, the high affinity of 1 towards salts opens up the possibility of extracting extremely hydrophilic sulfate anions from aqueous to organic phase. The disparity in receptor 1 binding modes towards monovalent anions and divalent sulfates assures its selectivity towards sulfates over other lipophilic salts upon liquid–liquid extraction (LLE) and enables the Hofmeister bias to be overcome. By changing the extraction conditions from LLE to SLE (solid–liquid extraction), a switch of selectivity from sulfates to acetates was achieved. X-ray measurements support the ability of anion binding by cooperation of the arms of receptor 1 together with simultaneous binding of cations.


NMR titration experiments
The 1 H NMR titration was performed on a Bruker 300 spectrometer, at 298K in DMSO-d6. In each case, a 500 μL of freshly prepared 1.7 mM solution of receptor 1 (1.4 mM of receptor 2; 2.0 mM of receptor 3) was added to a 5 mm NMR tube. In the case of ion pair titration receptor was firstly pretreated with one or three equivalent of NaClO4 or KPF6 . Then small aliquots of solution of TBAX, containing receptor at constant concentration, were added and a spectrum was acquired after each addition. The resulting titration data were analyzed using BindFit (v0.5) package, available online at http://supramolecular.org.

UV-vis titration experiments
UV-vis titration experiments were performed on a Thermo Spectronic Unicam UV 500 spectrophotometer in CH3CN solution at 298K. To 10 mm cuvette was added 2.5 mL of freshly prepared (receptor 1 -c = 1.2x10 -5 M , receptor 2 -c = 1.4x10 -5 M, receptor 3 -c = 2.1x10 -5 M ) solution of studied receptor and in case of ion pair binding studies 1 mol equivalent of cation (KPF6 or NaClO4) was added prior titrations. Small aliquots of ca. 1.1×10 -3 M TBAX solution containing receptor 1, receptor 2 or receptor 3 at the same concentration as in cuvette, were added and a spectrum was acquired after each addition. The resulting titration data were analyzed using BindFit (v0.5) package, available online at http://supramolecular.org.                      (000), 2414 e -. The all appropriate crystal data and refinement parameters are collected in Table   S2.

S41
The measured sample was oligocrystalline containing a few slightly rotated domains. Due to partial overlap of the diffraction spots integration of the reflections and scaling were based on two main domains followed by the merging data to HKLF4 format with the refined twin fractions yielding 0.7012:0.2988.
The structure is severely disordered and contains in its asymmetric part one tripodal ligand and nonstoichiometric amount of sodium trifluoroacetate with some solvent species (ethyl acetate, acetonitrile, methanol, water). Numbering scheme of atoms in the ligand is presented in Figure S72. To preserve reasonable geometry of disordered moieties number of distance and angles restraints (SIMU instructions) was used together with restrains for atomic displacement parameters (SADI instructions). In the checkCIF report two Alert type B are present, corresponding to too short distance between H atoms. One of this alert results from slightly crowded ordered central part of the ligand, whereas the second denotes possible O-H…O hydrogen bond between methanol and ethyl acetate.
All major component, non-hydrogen atoms with occupancies larger than 50% were refined anisotropically. Most of hydrogen atoms were placed in calculated positions and refined within the riding model. Positions of six hydrogen atoms of amide fragments engaged in hydrogen bonds were refined. The temperature factors of all hydrogen atoms were not refined and were set to be equal to either 1.2 or 1.5 times larger than Ueq of the corresponding heavy atom. Hydrogen atoms of hydroxy group in methanol solvent molecules as well as all H atoms in water molecules were not assigned. The atomic scattering factors were taken from the International Tables [S6]. Molecular graphics was prepared using Mercury CSD 2020 program [S7]. Thermal ellipsoids parameters, with numbering corresponding to Figure 1X, are presented at 50% probability level in Figure S73. Packing diagrams of 1 + Na_triFlac crystal structure are shown in Figure S74.