An Anionic Ring Locked into an Anionic Axle: A Metastable Rotaxane with Chemically Activated Electrostatic Stoppers

Correlated translational motions in pseudo-rotaxane complexes controlled by a single chemical stimulus

Coordinated motions are essential in the operation of molecular machines. This feature can be achieved by landscaping the energy surface along the movement coordinates. Herein, we present an approach of using a single stimulus to modify the free energy curve describing the threading and shuttling of a ring along a linear molecule. This approach has been realized by locating two identical ring-binding sites near the axle termini.

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.

An Operative Electrostatic Slipping Mechanism along Macrocycle Flexibility Accelerates Guest Sliding during pseudo-Rotaxane Formation

A pseudo-rotaxane is a host-guest complex composed of a linear molecule encircled by a macrocyclic ring. These complexes can be assembled by sliding the host over the guest terminal groups. If there is a close match between the molecular volume of the flanking groups on the guest and the cavity size of the macrocycle, the slipping might occur slowly or even become completely hindered. We have previously shown that it is possible to overcome the restraints imposed by steric effects on the sliding process by integrating electrostatic attractive interactions during the slipping step. In this work, we extend our electrostatically assisted slipping approach (EASA) to a new host-guest system featuring a flexible macrocyclic ring and a series of asymmetric guests containing a cyclic tertiary ammonium group. Compelling evidence for pseudo-rotaxane formation is presented, along with thermodynamic and kinetic data. Experimental results suggests that the higher conformational flexibility of 24-crown-8 significantly increases the sliding rate, compared with the more rigid dibenzo-24-crown-8, without affecting complex stability. Furthermore, by combining the EASA and macrocycle flexibility, we were capable to slip a large eight-membered cyclic group across the 24-crown-8 annulus, setting a new limit on the ring molecular size that can pass through a 24-membered crown ether.