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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
Author(s) -
Gong HanYuan,
Wang DeXian,
Xiang JunFeng,
Zheng QiYu,
Wang MeiXiang
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700498
Subject(s) - pyridine , molecular recognition , solid state , chemistry , capsule , materials science , polymer chemistry , combinatorial chemistry , molecule , organic chemistry , botany , biology
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, CH⋅⋅⋅π, and π–π 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‐4⋅resorcinol complex ( 1 ⋅ 3 ) is stable at low temperature ( T <243 K), and its conformational inversion requires a larger activation energy (Δ G ≠ =(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 (Δ G ≠ =(33.5±1.5) kJ mol −1 ) owing most probably to the favorable enthalpy effect of the [ 3 ⋅⋅⋅ 1 ⋅⋅⋅ 3 ] ≠ transition state.