Binding discrepancy of fluoride in quaternary ammonium and alkali salts by a tris(amide) receptor in solid and solution states

Abstract

A dinitrophenyl functionalized tris(amide) receptor, L, showed distinct complexation behaviour towards the F⁻ anion when tetrabutylammonium fluoride (TBAF) and potassium fluoride (KF) salts were individually employed for the recognition of F⁻ with L. X-ray crystallography analyses revealed the formation of a F⁻–encapsulated complex (1 : 1 host–guest) stabilized by three N–H⋯F⁻ and three C–H⋯F⁻ hydrogen bonds when TBAF was employed as the F⁻ source, whereas in the KF complex of L (1 : 1 host–guest), the receptor is involved in side-cleft binding of a hydrated KF contact ion-pair governed by amide N–H⋯F⁻, aryl C–H⋯F⁻ and lp(F⁻)⋯π interactions. The binding of hydrated KF is identical to the side-cleft binding of solvents such as DMSO and DMF via N–H⋯O, aryl C–H⋯O and lp(O) ⋯π interactions, which has been exemplified by X-ray crystallography and a detailed Hirshfeld surface analyses of the crystals. The binding discrepancy of F⁻ in the TBAF and KF complexes of L has also been manifested in the solution state by ¹H NMR and 2D NOESY NMR experiments. In the ¹H NMR analyses, a huge downfield shift of the coordinating –NH and ortho–CH protons was observed in the TBAF complex in comparison to the KF complex whereas, in the 2D NOESY NMR experiments, a disappearance of the signals corresponding to the through-space NOE coupling between the –NH and ortho–CH protons was observed in the former when compared to the latter and free receptor, L.