Highly selective recognition of diols by a self-regulating fine-tunable methylazacalix[4]pyridine cavity: Guest-dependent formation of molecular-sandwich and molecular-capsule complexes in solution and the solid state

The molecular recognition of methylazacalix[4]pyridine (MACP-4; 1) towards various diols was investigated by using H-1 NMR spectroscopic and Xray diffraction analysis. As a unique macrocyclic host molecule that undergoes conformational inversions very rapidly in solution, MACP-4 has been shown to self-regulate its conformation, through the formation of different conjugations of the four bridging nitrogen atoms with their adjacent pyridine rings, to form a cavity that best fits the guest species through intermolecular hydrogen-bond, C-H center dot center dot center dot pi, and pi-pi interactions between the host and guest. As a consequence, depending upon the diol structure and geometry, MACP-4 forms a 1:1 molecular sandwich, 2:1 molecular capsule, and 1:2 butterfly-layered complex with the guests. As a result of favorable enthalpy and entropy effects, MACP-4 exhibits excellent selectivity in the recognition of resorcinol, thus resulting in a very stable 1:1 sandwich complex with resorcinol with a binding constant of 6000M(-1). The dynamic H-1 NMR spectroscopic study demonstrated that the 1,3-alternate conformation of the macrocyclic ring of the MACP-4.resorcinol complex (1.3) is stable at low temperature (T< 243 K), and its conformational inversion requires a larger activation energy (Delta G double dagger = (45.5 +/- 2.2) kJ mol(-1)). In the presence of an excess amount of resorcinol, however, the conformational inversion of the MACP-4.resorcinol complex proceeds more readily with a decreased activation energy (Delta G double dagger = (33.5 +/- 1.5) kJ mol(-1)) owing most probably to the favorable enthalpy effect of the [3...1...3]double dagger transition state.