Anion complexation of a pentafluorophenyl-substituted tripodal urea receptor in solution and the solid state: selectivity toward phosphate

Abstract

The binding and selectivity of halides (spherical) and oxyanions (tetrahedral) toward a recently reported pentafluorophenyl-substituted tripodal urea-based receptor L¹ are examined thoroughly in the solid state by single-crystal X-ray crystallography as well as in solution by multinuclear NMR techniques. Crystallographic results show proof of a fluoride encapsulation in the cavity of L¹ in complex [L¹(F)][Bu₄N], 1. Fluoride encapsulation inside the C_3v symmetric cleft is observed via six hydrogen bonds to all six urea protons of the receptor. In case of complex 2 crystallographic results show encapsulation of sulfate ion inside a supramolecular cage formed upon 1 : 2 (guest–host) complex formation between sulfate and L¹. Sulfate encapsulation is observed via fourteen hydrogen bonding interactions from all six urea moieties of two L¹ units. Our effort to isolate single crystal of halides/oxyanions complexes of L² always yield single crystals of free L² though literature shows anion binding with this receptor in solution. Solution state binding studies of L¹ are carried out by ¹H-NMR titration to calculate binding constants, which show the following anion binding sequence H₂PO₄⁻ > SO₄²⁻> CH₃COO⁻ > F⁻ > Cl⁻ >> Br⁻ whereas there is no binding with I⁻, NO₃⁻ and ClO₄⁻ guests. Comparison of phosphate and sulfate binding in L¹ and L², show higher binding with the pentafluorophenyl substituted receptor, L¹. Further ¹⁹F and ³¹P-NMR experiments in solution are also carried out to probe the binding of F⁻ and H₂PO₄⁻ with L¹, respectively. Extensive ¹H-NMR experiments in solution and crystallization in the presence of multiple anions are also undertaken to evaluate the selectivity of H₂PO₄⁻ over other anions.