Arene platform based hexa-amide receptors for anion recognition: Single crystal X-ray structural and thermodynamic studies

Five newly synthesized hexa-amide receptors (L1-L5) and previously reported three such receptors (L6-L8) have been explored to investigate binding propensity of anions of various shapes and sizes. Single crystal X-ray structures of five new anion complexes of receptors (L1-L5), complexes (1-5) and previously reported seven such complexes of (L6-L8), complexes (6-12) fall under four categories of conformations such as aaabbb (A), ababab (B), aabaab (C) and aaaaaa (D) depending upon basicity of the anions as well as substituents on the receptor backbones. Moderately basic guest; chloride exhibits two different conformers with A and B patterns with L1 (R = -oNO₂C₆H₄, complex 1) and L4 (R = -pFC₆H₄, complex 2) respectively in 1:2 (host:guest) stoichiometry. On the other hand, strongly basic acetate complexes of L2 (R = -pNO₂C₆H₄), L3 (R = -mCF₃C₆H₄), L5 (R = 4-pyridyl), L6 (R = -C₆F₅), L7 (R = -mNO₂C₆H₄) and L8 (R -oCF₃C₆H₄) i.e. complexes 3-8 respectively, exhibit a chair like conformer (A) with 1:2 (host:guest) stoichiometry. Our previous studies showed recognition of nitrate-water cluster (9) by L6 and nitrate (10) by L8 with (B) and (C) conformations respectively and an unusual conformation (D) was isolated in cases of L7 and L8 with hydrated fluoride clusters [(F)₄(H₂O)₁₀]^4- (11) and [(F)₄(H₂O)₆]^4- (12) respectively. Solution state ITC and NMR studies have also shown 1:2 stoichiometry of host:guest binding of acetate and fluoride with L1-L8.