2:2 Complexes from Diphenylpyridiniums and Cucurbit[8]uril: Encapsulation‐Promoted Dimerization of Electrostatically Repulsing Pyridiniums

Rigid linear compounds G1 and G2 , which contained two 4‐phenylpyridinium (PhPy⁺) units, have been prepared to investigate their binding with cucurbit[8]uril (CB[8]). X‐ray crystallographic structures revealed that in the solid state both compounds were included by CB[8], through antiparallel stacking, to form 2:2 quaternary complexes ( G1 )₂@(CB[8])₂ and ( G2 )₂@(CB[8])₂. For the former complex, CB[8] entrapped G1 by holding two heterodimers of its Py⁺ and benzyl units, which were at opposite ends of the backbone. In contrast, for the first time, the second complex disclosed parallel stacking of two cationic Py⁺ units of G2 in the cavity of CB[8] in the solid state, despite the generation of important electrostatic repulsion. Isothermal titrations in water afforded high apparent association constants of 4.36×10⁶ and 6.43×10⁶ m ^?1 for 1:1 complexes G1 @CB[8] and G2 @CB[8], respectively, and ¹H NMR spectroscopy experiments in D₂O confirmed a similar stacking pattern to that observed in the solid state. A previous study and crystal structures of the 2:1 complexes formed between three new controls, G3–5 , and CB[8] did not display such unusual stacking of the cationic Py⁺ unit; this may be attributed to the multivalency of the two CB[8] encapsulation interactions.