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Relevant academic research and scientific papers

A rotaxane-like complex with controlled-release characteristics.

A rotaxane-like complex, based on a dumbbell-shaped component containing an NH(2)(+) recognition site for a [25]crown-8 ring component and a slippage stopper in the form of a p-(tert-butyl)phenyl group, has been synthesized by a "threading-followed-by-sto

Do dibenzo[22-30]crown ethers bind secondary ammonium ions to form pseudorotaxanes?

In this study, we synthesized dibenzo[22-30]crown ethers and dumbbell-like secondary ammonium salts having stoppers of varying bulk. These crown ethers formed pseudorotaxanes with the ammonium ions in solution and in the gas phase, as evidenced using NMR spectroscopy and MS/MS spectrometry, respectively. The association constants in solution were obtained through regulation of the association and dissociation rates by varying the nature of the stopper groups of the dumbbell-like ammonium ions. The [25]- and [26]crown ether/ isopropylphenyl group, the [27]- and [30]crown ether/tert-butylphenyl group, and the [22]- and [23]crown ether/furyl group were matched pairs. 1 HNMR spectra of mixtures (CDCI3/CD3CN) of the crown ethers and their matched ammonium salts indicated the presence of three sets of signals in solution: those of the crown ether, the ammonium salt, and their pseudorotaxane. Integration of pertinent signals allowed the association constants of pseudorotaxanes to be determined readily. Among the [22-30]crown ethers, the highest value of the association constant was that for the [24]crown ether/dibenzylammoniumion system; the [25-30]crown ether/ammonium ion systems exhibited moderate values of their association constants [Kexp - 35-114 M-1 when using (ArCH2)2NH 2+PF6- as the ammonium salt at 27 °C; Kexp = 21 M-1 when using (ArCH2) 2NH2+TsO- at 27 °C]. The [22]- and [23]crown ethers interacted weakly [Kexp = 6-14 M-1 when using (ArCH2)2NH2+TsO - at27°C].

Supramolecular daisy chains

Two series of self-complementary daisy chain monomers, in which a secondary ammonium ion-containing arm is grafted onto a macrocycle with either a [24]- or [25]crown-8 constitution, have been synthesized. In the solid- and 'gas'-phases, the parent [24]crown-8-based monomer forms dimeric superstructures, as revealed by X-ray crystallography and mass spectrometry, respectively. Elucidation of the complicated solution-phase behavior of this compound was facilitated by the synthesis and study of both deuterated, and fluorinated, analogues. These investigations revealed that the cyclic dimeric superstructure also dominates in solution, except when extremes of either concentration (low), temperature (high), or solvent polarity (highly polar, e.g., dimethyl sulfoxide) are employed. Whereas, upon aggregation, the [24]crown-8-based daisy chain monomers have the capacity to form stereoisomeric superstructures further complicating the study of this series of compounds. The assembly of [25]crown-8-based monomers gives only achiral superstructures. The weaker association exhibited between secondary dialkylammonium ions and crown ethers with a [25]crown-8 constitution, however, resulted in limited oligomerization-only dimeric and trimeric superstructures were formed at experimentally attainable concentrations-of [25]crown-8-based daisy chain monomers.