Binding Modes of Cucurbit[6]uril and Cucurbit[7]uril with a Tetracationic Bis(viologen) Guest

Abstract

Binding behaviors of two cucurbit[n]urils (CB[n]) hosts with the [CH₃bpy(CH₂)₆bpyCH₃]⁴⁺ (bpy = 4,4‘-bipyridinium) guest were investigated by ¹H NMR and MALDI-TOF-MS experiments. While the CB[6] and CB[7] form [2]pseudorotaxanes with the host located over the hexamethylene chain of the guest, only the CB[7] forms a [3]pseudorotaxane with both host molecules residing over the bipyridinium groups. The initial CB[7] host vacates the inclusion of the hexamethylene chain as a result of the electrostatic and steric repulsions that would arise in simultaneous binding of adjacent aliphatic and aromatic portions of the guest.