Towards Multistation Rotaxanes Using Metalloporphyrin Coordination Templating

This paper describes some approaches toward the templated synthesis of rotaxanes incorporating straped metalloporphrin moieties as the shuttle unit, with the thread component containing both a neutral diimide "station" and a functionalised pyridine moiety, the latter acting not onle as a template but also as a second binding motif. In the first instance, the use of appropriately 3,5-difunctionalised pyridine esters and naphthoquinol-strapped rhodium(III) chloride porphyrins ina stoppering approach to rotaxanes produced only unlinked components: the flexibility of the strap allowed sufficient room for the potential thread unit to bind on the same face of the porphyrin as the strap, while not being interlocked through it. An alternative strategy involving a 1,3-dipolar cycloaddition reaction (a "click" reaction) between azides and alkynes, producing triazole linkers in the thread component of rotaxanes, was more successful. Both porphyrinic (zinc, free base, and rhodium(III) derivatives) and crown ether rotaxanes were successfully produced, with multifunctional (triazole and naphthodiimide) thread units. The potential for molecular motion through the use of stimuli such as acid, solvent, and competing ligands was investigated, with limited success. The same cycloaddition methodology was extended to pyridine-templated analogues of the thread components in the Rh(III)-strapped porphyrins, but again, only unlinked thread and porphyrin shuttle units were produced.