Abstract
Catenanes with desymmetrized ring components can undergo co-conformational rearrangements upon external stimulation and can form the basis for the development of molecular rotary motors. We describe the design, synthesis and properties of a [2]catenane consisting of a macrocycle—the ‘track’ ring—endowed with two distinct recognition sites (a bipyridinium and an ammonium) for a calix[6]arene—the ‘shuttle’ ring. By exploiting the ability of the calixarene to thread appropriate non-symmetric axles with directional selectivity, we assembled an oriented pseudorotaxane and converted it into the corresponding oriented catenane by intramolecular ring closing metathesis. Cyclic voltammetric experiments indicate that the calixarene wheel initially surrounds the bipyridinium site, moves away from it when it is reduced, and returns in the original position upon reoxidation. A comparison with appropriate model compounds shows that the presence of the ammonium station is necessary for the calixarene to leave the reduced bipyridinium site.
Highlights
The chemistry of catenanes—i.e., mechanically interlocked molecules (MIMs) consisting of two or more entwined macrocycles [1,2,3]—has lately experienced a remarkable development, in particular thanks to improved synthetic protocols [4] that disclosed the realization of molecular architectures of increasing complexity
In view of our interest in the development of novel stimuli-responsive molecular machines [26,28], we report on the synthesis and electrochemical properties of the calix[6]arene-based [2]catenane 3 (Figure 2)
The synthetic strategy involves a ring closing metathesis reaction performed on an oriented pseudorotaxane, self-assembled by taking advantage of the directional threading of the calixarene by non-symmetric bipyridinium axles
Summary
The chemistry of catenanes—i.e., mechanically interlocked molecules (MIMs) consisting of two or more entwined macrocycles [1,2,3]—has lately experienced a remarkable development, in particular thanks to improved synthetic protocols [4] that disclosed the realization of molecular architectures of increasing complexity. The first synthetic attempts to obtain these interlocked molecules relied on a statistical approach [5] that, yielded negligible amounts of the target catenanes. An interesting behavior of properly designed catenanes is the ability of the rings to rotate with respect to one another, that is, to behave as molecular machines [2,13,14]. To accomplish this task, specific functional units have to be incorporated into the macrocycles so that controlled co-conformational rearrangements can be triggered by appropriate stimuli. The most common design involves the presence on one ring (the ‘track’) of two different recognition sites for the other ring (the ‘shuttle’); upon switching off and on the affinity of the primary site, the ‘shuttle’ ring moves onto and away from the secondary one, and reversible pirouetting can be achieved (Figure 1a) [15,16,17,18,19]
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