Acid/base controlled size modulation of capsular phosphates, hydroxide encapsulation, quantitative and clean extraction of sulfate with carbonate capsules of a tripodal urea receptor

Abstract

A simple tris-(2-aminoethyl) amine based pentafluorophenyl substituted tripodal urea receptor L has been extensively studied as a versatile receptor for various anions. Combined ¹H-NMR, Isothermal Titration Calorimetry (ITC) and single crystal X-ray diffraction studies reveal that mononegative anions like F⁻, OH⁻ and H₂PO₄⁻ are encapsulated into the pseudocapsular dimeric assemblies of L with 1 : 1 stoichiometry whereas dinegative anions like CO₃²⁻, SO₄²⁻ and HPO₄²⁻ form tight capsular dimeric assemblies of L with 1 : 2 stoichiometries. Single crystal X-ray diffraction study clearly depicts that the size of the dimer of H₂PO₄⁻ encapsulated pseudocapsule is 13.8 Å whereas the size of the tight HPO₄²⁻ encapsulated capsular assembly is only 9.9 Å. The charge dependent anion encapsulated capsular size modulation of phosphates has been demonstrated by simple acid/base treatment via solution state ³¹P-NMR and single crystal X-ray diffraction studies. L is also capable of encapsulating hydroxide in its C_3v-symmetric cavity that is achieved upon treating a DMSO solution of L with tetrabutylammonium (TBA) cyanide and characterized by single crystal X-ray diffraction study. To the best of our knowledge this is the first report on the encapsulation of hydroxide in a neutral synthetic receptor. The excellent property of L to quantitatively capture aerial CO₂ in the form of CO₃²⁻ capsules [L₂(CO₃)][N(n-Bu)₄]₂ in basic DMSO solution has been utilized to study the liquid–liquid extraction of SO₄²⁻ from water via anion exchange. Almost quantitative and clean extraction of SO₄²⁻ from water (99% from extracted pure mass and >95% shown gravimetrically) has been unambiguously demonstrated by NMR, FT-IR, EDX, XRD and PXRD studies. Selective SO₄²⁻ extraction is also demonstrated even in the presence of H₂PO₄⁻ and NO₂⁻. On the other hand the mixtures of L and TBACl (to solubilize L in CHCl₃) results impure sulfate extraction even when 1 : 1 L/TBACl is used. Similar impure SO₄²⁻ extraction is also observed when organic layers containing [L(Cl)][N(n-Bu)₄] are used as the extractant, obtained upon precipitating SO₄²⁻ from the extracted mass, [L₂(SO₄)][N(n-Bu)₄]₂ in the carbonate capsules method using aqueous BaCl₂ solution.