Bistable or Oscillating State Depending on Station and Temperature in Three-Station Glycorotaxane Molecular Machines

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 H-1 NMR investigation of the oscillating system was performed in CD2Cl2. The exchange between the two stations proved to be fast on the NMR time-scale 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 Delta H and Delta S were extracted from a Van't Hoff plot, and provided the Gibbs energy Delta G. Arrhenius and Eyring plots afforded kinetic parameters, namely, energies of activation E-a, enthalpies of activation Delta H-+/-, and entropies of activation Delta S-+/-. The Delta G values deduced from kinetic parameters match very well with the Delta 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 H-1 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.