title: 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.
abstract: The molecular recognition of methylazacalix[4]pyridine (MACP-4; 1) towards various diols was investigated by using (1)H NMR spectroscopic and X-ray 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...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 6000 (M-1). The dynamic (1)H NMR spectroscopic study demonstrated that the 1,3-alternate conformation of the macrocyclic ring of the MACP-4resorcinol complex (13) is stable at low temperature (T<243 K), and its conformational inversion requires a larger activation energy (DeltaG++=(45.5+/-2.2) kJ mol(-1)). In the presence of an excess amount of resorcinol, however, the conformational inversion of the MACP-4resorcinol complex proceeds more readily with a decreased activation energy (DeltaG++=(33.5+/-1.5) kJ mol(-1)) owing most probably to the favorable enthalpy effect of the [3...1...3]++transition state.
authors.affiliation: Beijing National Laboratory for Molecular Sciences, Laboratory of Chemical Biology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China.
authors.forename: Han-Yuan
authors.initials: HY
authors.lastname: Gong
authors.forename: De-Xian
authors.initials: DX
authors.lastname: Wang
authors.forename: Jun-Feng
authors.initials: JF
authors.lastname: Xiang
authors.forename: Qi-Yu
authors.initials: QY
authors.lastname: Zheng
authors.forename: Mei-Xiang
authors.initials: MX
authors.lastname: Wang
mesh_terms: D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D011725:Pyridines / Q000737:chemistry*; D012996:Solutions
chemical_list: D011725:Pyridines; D012996:Solutions