Five-State Molecular Shuttling of a Pair of [2]Rotaxanes: Distinct Outputs in Response to Acid and Base Stimuli

Monosulfonated dibenzo-24-crown-8 and its properties

In this paper, we describe a method for preparing a monosulfonated dibenzo-24-crown-8 ether, SDB24C8, by direct sulfonation of the parent crown (DB24C8). Since neutral DB24C8 readily interacts with cationic guests, permanently charged SDB24C8 is an advantageous candidate for future supramolecular applications. SDB24C8 can be isolated as a sulfonic acid to be used as it is or converted to a salt of choice. The crystallographic analysis provides the first known host-guest assembly with a DB24C8-based scaffold complexing hydronium and potassium cations. Supramolecular investigations of the interactions of this anionic macrocycle with alkali cations were also performed. According to the expectations, the introduction of the sulfonic group into the DB24C8 scaffold increases the affinities of the receptor. An unusual selectivity of SDB24C8 towards a sodium cation was also observed and further investigated with DFT calculations.

Utilization of a Crown Ether/Amine-Type Rotaxane as a Probe for the Versatile Detection of Anions and Acids by Thin-Layer Chromatography

A crown ether/amine-type [2]rotaxane was synthesized and utilized as a probe for the detection of acids and anions. The addition of acids to the amine-type [2]rotaxane solution generated corresponding crown ether/ammonium-type [2]rotaxanes, which were purified by silica gel column chromatography as ammonium salts. The isolated yields of the [2]rotaxanes, possessing a variety of anions, depended on the acidity and polarity of the counter anions. The behaviours of the ammonium-type [2]rotaxanes on thin-layer chromatography (TLC) silica gel reflected the properties of the counter anions. The treatment of the amine-type [2]rotaxane with acids afforded the corresponding ammonium-type [2]rotaxanes bearing several different anions. The ammonium-type [2]rotaxanes behaved similarly to the purified [2]rotaxanes on the TLC silica gel. Furthermore, we succeeded in the analysis of anions using mixtures of the amine-type [2]rotaxane and salts in an appropriate solvent. We demonstrated the detection of anions by the combination of TLC and the utilization of the [2]rotaxane probe.

Distinguishing Two Ammonium and Triazolium Sites of Interaction in a Three-Station [2]Rotaxane Molecular Shuttle

This paper reports on the synthesis of a tri-stable [2]rotaxane molecular shuttle, in which the motion of the macrocycle is triggered by either selective protonation/deprotonation or specific carbamoylation/decarbamoylation of an alkylbenzylamine. The threaded axle is surrounded by a dibenzo[24]crown[8] (DB24C8) macrocycle and contains three sites of different binding affinities towards the macrocycle. An N-methyltriazolium moiety acts as a molecular station that has weak affinity for the DB24C8 macrocycle and is located in the centre of the molecular axle. Two other molecular stations, arylammonium and alkylbenzylammonium moieties, sit on either side of the triazolium moiety along the molecular axle and have stronger affinities for the DB24C8 macrocycle. These two ammonium moieties are covalently linked to two different stopper groups at each extremity of the thread: a tert-butylphenyl group and a substituted DB24C8 unit. Owing to steric hindrance, the former does not allow any π-π stacking interactions with the encircling DB24C8 macrocycle, whereas the latter residue does; therefore, this allows the discrimination of the two ammonium stations by the surrounding DB24C8 macrocycle in the fully protonated state. In the deprotonated state, the contrasting reactivity of the amine functional groups, as either a base or a nucleophile, allows for selective reactions that trigger the controlled shuttling of the macrocycle around the three molecular stations.