Interaction between tetramethylcucurbit[6]uril and some pyridine derivates

Interaction between tetramethylcucurbit[6]uril (TMeQ[6], host) with hydrochloride salts of 2-phenylpridine (G1), 2-benzylpyridine (G2), and 4-benzylpyridine (G3) (guests) have been investigated by using ¹H NMR spectroscopy and electronic absorption spectroscopy and theoretical calculations. The ¹H NMR spectra analysis established an interaction model in which the host selectively included the phenyl moiety of the HCl salt of the above three guests, and formed inclusion complexes with a host - guest ratio of 1:1. Absorption spectrophotometric analysis allowed quantitative measurement of the stability of these host - guest inclusion complexes. Particularly, we have established a competitive interaction in which one host - guest inclusion complex pair is much more stable than another host-guest inclusion complex pair. The stability constants for the three host - guest inclusion complexes of TMeQ[6]-G1, TMeQ[6]-G2, and TMeQ[6]-G3 are ∼2 × 10⁶, 60.7, and 19.9 mol^-1·L, respectively. To understand how subtle differences in the structure of the title guests lead to a significant difference in the stability of the corresponding host - guest inclusion complexes with the TMeQ[6], ab initio theoretical calculations have been performed, not only for the gas phase but also the solution phase (water as solvent) in all cases. The calculation results revealed that when the phenyl moiety of the three pyridine derivate guests was included, the host - guest complexation reached the minimum, and the corresponding energy differences for the formation of the title host - guest inclusion complexes are qualitatively consistent with the experimental results.