Template-directed approaches – i.e., methods that exploit the enthalpic contribution of a template to compensate for the entropy loss of interlocking – represent the method of choice for the synthesis of mechanically interlocked molecules (MIMs). Among the various classes of MIMs, polyrotaxanes – which are comprised of a long chain terminated by two stoppers and encircled by numerous rings – have attracted considerable attention because of the unprecedented mechanical and physicochemical properties imparted by the presence of the mechanical bonds. In general, however, the threading of rings is driven by stabilizing noncovalent bonding interactions, which persist in the final, thermodynamically stable, MIMs, constraining the relative movements of the rings with respect to the dumbbells. Following an introduction on the nature of the mechanical bond, I will discuss how the non-equilibrium operation of artificial molecular pumps can be used to guide the production of a new class of MIMs where the interlocking does not rely on stabilizing interactions – namely, enthalpically and entropically demanding polyrotaxanes (Fig. A). We have designed and synthesized a prototype bearing two artificial molecular pumps at the termini of an oligomeric ionene, which acts as the collecting chain for multiple rings. Controlled-potential coulometry (Fig. B and C) can be used to promote the repetitive pump operation in situ – i.e., the controlled threading of multiple rings. The corresponding rotaxanes have been isolated with good conversions after one and two cycles, respectively, and characterized using 1H, 2D and VT NMR spectroscopies. The high-energy nature of the [5]rotaxane – i.e., the sliding around of rings on account of their experiencing reciprocal repulsions – is reflected in the broadening of the proton resonances associated with the central part of the collecting chain.

Rise and promise of the mechanical bond in chemistry and beyond

254th National Meeting (2017)

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