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Bistable or Oscillating State Depending on Station and Temperature in Three‐Station Glycorotaxane Molecular Machines
Author(s) -
Busseron Eric,
Romuald Camille,
Coutrot Frédéric
Publication year - 2010
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.201000777
Subject(s) - pyridinium , chemistry , amide , rotaxane , deprotonation , activation energy , kinetic energy , gibbs free energy , hydrogen bond , arrhenius plot , thermodynamics , molecule , crystallography , supramolecular chemistry , medicinal chemistry , organic chemistry , physics , ion , quantum mechanics
Abstract High‐yield, straightforward synthesis of two‐ and three‐station [2]rotaxane molecular machines based on an anilinium, a triazolium, and a mono‐ or disubstituted pyridinium amide station is reported. In the case of the pH‐sensitive two‐station molecular machines, large‐amplitude movement of the macrocycle occurred. However, the presence of an intermediate third station led, after deprotonation of the anilinium station, and depending on the substitution of the pyridinium amide, either to exclusive localization of the macrocycle around the triazolium station or to oscillatory shuttling of the macrocycle between the triazolium and monosubstituted pyridinium amide station. Variable‐temperature 1 H NMR investigation of the oscillating system was performed in CD 2 Cl 2 . The exchange between the two stations proved to be fast on the NMR timescale for all considered temperatures (298–193 K). Interestingly, decreasing the temperature displaced the equilibrium between the two translational isomers until a unique location of the macrocycle around the monosubstituted pyridinium amide station was reached. Thermodynamic constants K were evaluated at each temperature: the thermodynamic parameters Δ H and Δ S were extracted from a Van′t Hoff plot, and provided the Gibbs energy Δ G . Arrhenius and Eyring plots afforded kinetic parameters, namely, energies of activation E a , enthalpies of activation Δ H ≠ , and entropies of activation Δ S ≠ . The Δ G values deduced from kinetic parameters match very well with the Δ G values determined from thermodynamic parameters. In addition, whereas signal coalescence of pyridinium hydrogen atoms located next to the amide bond was observed at 205 K in the oscillating rotaxane and at 203 K in the two‐station rotaxane with a unique location of the macrocycle around the pyridinium amide, no separation of 1 H NMR signals of the considered hydrogen atoms was seen in the corresponding nonencapsulated thread. It is suggested that the macrocycle acts as a molecular brake for the rotation of the pyridinium–amide bond when it interacts by hydrogen bonding with both the amide NH and the pyridinium hydrogen atoms at the same time.