Binding studies on an arene-capped bicyclic cyclophane with π-rich neutral guests and anions

Abstract

Structural aspects of binding of π-rich neutral guests with L and anions with [H₆L]⁶⁺ are examined thoroughly. L forms inclusion complexes with π-rich solvents, 2DMSO⊂L (1), 3DMF⊂L₂ (2), (DMF·benzene·DMF)⊂L₂ (3), MeCN⊂L (4), and MeCOMe⊂L (5) in dimethylsulfoxide (DMSO), dimethyl formamide (DMF), benzene/DMF, acetonitrile (MeCN), and acetone (MeCOMe) respectively. The single crystal X-ray structural analysis of complexes illustrates cavity and cleft binding of these guests via N–H···O interactions in 1, 2, 3, 5 and N–H···N interactions in 4 with the secondary nitrogen center of L and the hydrogen bonding acceptor atoms of the solvent guests. Inclusion of benzene in the side pocket is also observed in 3. Our efforts to isolate single crystals with solvents such as MeOH, EtOH, CHCl₃, and CH₂Cl₂ are unsuccessful. Single crystal X-ray diffraction study has also shown the encapsulation of nitrate in the cleft of [H₆L]⁶⁺ via N–H···O hydrogen bonding interactions in [H₆L][NO₃]₆·HNO₃·6H₂O (6), whereas in [H₆L]₂[ClO₄]₁₂·CH₃OH·17H₂O (7) perchlorates are recognized in the cavity and side pockets of [H₆L]⁶⁺. This receptor has previously shown encapsulation of iodide (8), and Cl–···H₂O (9). A potentiometric study of L exhibits the maximum concentration of [H₆L]⁶⁺ species at pH 2–3 in MeOH/H₂O 1:1 (v/v) binary solvent. Anion binding studies with L at pH 2.0 in MeOH/H₂O 1:1 (v/v) solvent system are examined by isothermal titration calorimetric (ITC) experiments.