Binding Studies on an Arene-Capped Bicyclic Cyclophane with π‑Rich Neutral Guests and Anions

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.