Anion selective disruption of strong intramolecular -NH...O=C Hydrogen bonds in a nonchromogenic tripodal benzoylthiourea receptor to display colorimetric response

Abstract

Tris(2-aminoethyl)amine based tripodal benzoylthiourea receptor (L) showed colorimetric response for fluoride and acetate in aprotic solvents within a range of competitive anions studied. The strong intramolecular -NH...O=C hydrogen bonding in the C sub(3) symmetric receptor was not disrupted upon addition of any other anions except for fluoride and acetate as confirmed by sup(1) H-NMR experiments in DMSO-d sub(6). The cavity of the tripodal receptor is locked due to the inward orientation of the intramolecular hydrogen bonded donor-acceptor groups as confirmed by single crystal X-ray crystallography. Fluoride and acetate could selectively disrupt the intramolecular -NH...O=C hydrogen bonds to establish intermolecular hydrogen bonds with both the -NH protons from each receptor side arm and showed visual colour change from colourless to yellow. However, hydrogen bonding observed in the solution state does not provide sufficient stabilization to the receptor fluoride/acetate complexes to be crystallized in the solid state and the receptor crystallised as solvates both from DMSO and THF in the presence of excess fluoride/acetate. From UV-vis and sup(1) H-NMR experiments, it was confirmed that the colorimetric sensing by the nonchromogenic benzoylthiourea receptor is due to strong hydrogen bonding between thiourea -NH groups and fluoride/acetate.