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

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 C3v 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