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) @(CB[8]) and (G2) @(CB[8]) . 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 4.36×10 and 6.43×10  m for 1:1 complexes G1@CB[8] and G2@CB[8], respectively, and H NMR spectroscopy experiments in D O 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.