Encapsulation of fluoride/chloride in the C_3v-symmetric cleft of a pentafluorophenyl-functionalized cyanuric acid platform based tripodal amide: solid and solution-state anion-binding studies

Abstract

A 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 C_3v-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 K_a = 4.86 M^–1), chloride (log K_a = 3.83 M^–1), and bromide (log K_a = 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.