Anion Binding in the <i>C</i>_3<i>v</i>-Symmetric Cavity of a Protonated Tripodal Amine Receptor: Potentiometric and Single Crystal X-ray Studies

Tris(2-aminoethyl)amine (tren) based pentafluorophenyl-substituted tripodal <b>L</b>, 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 <b>L</b> in its triprotonated form are examined thoroughly. Crystallographic results show binding of fluoride in the <i>C</i>_3<i>v</i>-symmetric cavity of [H₃<b>L</b>]³⁺ where spherical anion fluoride is in tricoordinated geometry via (N–H)⁺<b>···</b>F interaction in the complex [H₃<b>L</b>(F)]·[F]₂·2H₂O, (<b>3</b>). In the case of complexes [H₃<b>L</b>(OTs)]·[OTs]₂, (<b>4</b>) and [H₃<b>L</b>(OTs)]·[NO₃]·[OTs], (<b>5</b>), tetrahedral <i>p</i>-toluenesulphonate ion is engulfed in the cavity of [H₃<b>L</b>]³⁺ via (N–H)⁺<b>···</b>O interactions. Interestingly, complex [(H₃<b>L</b>)₂(SiF₆)]·[BF₄]₄·CH₃OH·H₂O, (<b>6</b>) shows encapsulation of octahedral hexafluorosilicate in the dimeric capsular assembly of two [H₃<b>L</b>]³⁺ units, via a number of (N–H)⁺<b>···</b>F interactions. The kinetic parameters of <b>L</b> 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 <i>p</i>-toluenesulphonate)