Encapsulation of Fluoride/Chloride in the C3v‐Symmetric Cleft of a Pentafluorophenyl‐Functionalized Cyanuric Acid Platform Based Tripodal Amide: Solid and Solution‐State Anion‐Binding Studies

Abstract

AbstractA simple electron‐deficient cyanuric acid based tripodal amide, 1,3,5‐tris[2‐(2,3,4,5,6‐pentafluorobenzamido)ethyl]‐1,3,5‐triazinane‐2,4,6‐trione (L), was synthesized and characterized by NMR spectroscopy, ESI mass spectrometry, and single‐crystal X‐ray crystallographic studies. The binding of various anions towards L was thoroughly examined bysingle‐crystal X‐ray crystallography as well as solution‐state isothermal titration calorimetry (ITC). The crystallographic results show that L has an unsymmetrical cleft, where the third arm is perpendicularly disposed to the other two arms. Interestingly, L upon complexation with tetrabutylammonium fluoride/chloride shows encapsulation of monotopic fluoride/chloride in the C3v‐symmetric cleft by means of N–H···X (X = F–, Cl–) hydrogen‐bonding interactions in complexes 1 and 2. In complex 2, the encapsulated chloride ion shows evidence of anion–π interactions with the pentafluorophenyl moiety. A detailed solution‐state ITC study of L with tetrabutylammonium salts of different halides in acetonitrile showed an exothermic binding profile with 1:1 (host/guest) stoichiometry for fluoride (log Ka = 4.86 M–1), chloride (log Ka = 3.83 M–1), and bromide (log Ka = 2.97 M–1). In the case of iodide, no such binding was observed. Oxyanions like acetate and benzoate also show an exothermic binding profile with a 1:1 (host/guest) binding pattern, whereas other oxyanions like phosphate, sulfate, and nitrate failed to exhibit a 1:1 binding model. The presence of [L(X)]– (X = F, Cl) species as the base peak in the ESI (negative) mass spectra further confirmed the strong binding of the halide ions in the gaseous phase.