Anion binding in the C_3v-symmetric cavity of a protonated tripodal amine receptor: potentiometric and single crystal x-ray studies

Abstract

Tris(2-aminoethyl)amine (tren) based pentafluorophenyl-substituted tripodal L, tris[[(2,3,4,5,6-pentafluorobenzyl)amino]ethyl]amine receptor is synthesized in good yield and characterized by single crystal X-ray diffraction analysis. Detailed structural aspects of binding of different anionic guests toward L in its triprotonated form are examined thoroughly. Crystallographic results show binding of fluoride in the C_3v-symmetric cavity of [H₃L]³⁺ where spherical anion fluoride is in tricoordinated geometry via (N–H)⁺···F interaction in the complex [H₃L(F)]·[F]₂·2H₂O, (3). In the case of complexes [H₃L(OTs)]·[OTs]₂, (4) and [H₃L(OTs)]·[NO₃]·[OTs], (5), tetrahedral p-toluenesulphonate ion is engulfed in the cavity of [H₃L]³⁺ via (N–H)⁺···O interactions. Interestingly, complex [(H₃L)₂(SiF₆)]·[BF₄]₄·CH₃OH·H₂O, (6) shows encapsulation of octahedral hexafluorosilicate in the dimeric capsular assembly of two [H₃L]³⁺ units, via a number of (N–H)⁺···F interactions. The kinetic parameters of L upon binding with different anions are evaluated using a potentiometric study in solution state. The potentiometric titration experiments in a polar protic methanol/water (1:1 v/v) binary solvent system show high affinity of the receptor toward more basic fluoride and acetate anions, with a lesser affinity for other inorganic anions (e.g., chloride, bromide, nitrate, sulfate, dihydrogenphosphate, and p-toluenesulphonate).