Catenanes and rotaxanes are molecular compounds"] that hold considerable promise for use in molecular devices as a consequence of the relative movements of their ring and dumbbell-shaped components. Initially researchers learned how to make simple rotaxanes,[21 followed by degenerate rotaxanes with molecular shuttling characteristic^^^] and eventually nondegenerate rotaxanes whose dynamic properties could be addressed selectively with external stimuli of a chemical, electrochemical, or photochemical nature to induce them to behave as molecular switches. 141 However, to our knowledge, no constitutionally asymmetric, two-site [2] rotaxane that exhibits 100% selectivity for one of the binding sites between ring and dumbbell-shaped components has yet been reported. Here, we describe the self-assembly of a [2] rotaxane that possesses two very dissimilar cationic binding sites within its dumbbell-shaped component-specifically, a secondary dialkylammonium center and a bipyridinium unit-ach of which shows different affinitiesr5] toward dibenzo [24] crown-8 (DB24C8), the ring component of the [2] rotaxane. We also report the pH-controlled shuttling of the DB24C8 ring between the two very different recognition sites, or" stations", in this simple molecular shuttle. The [2] rotaxane 2-H. 3 PF, was self-assembled (Scheme 1) by using a template-directed threading approach.[61 A pseudorotaxaneL7.* I [DB24C8* lH][PF,], was formed initially in solution between the DB24C8 and 1-H. 2PF, J9] which already contains the ammonium center and a stopper (3, 5-di-tert-butylbenzyl) to prevent unthreading of the ring. Subsequently a single stoppering reaction with 3, 5-di-tert-butylbenzyl bromide led to the formation of the bipyridinium dicationic unit and the dumbbellshaped component."']

The'HNMR spectrum of 2-H. 3PF6 recorded in CD,-COCD, at 20" C shows signals corresponding to the formation of a [2] rotaxane. The spectrum (Figure lb) demonstrates that the DB24C8 ring binds selectively with the secondary dialkylammonium center, since the characteristic resonance at 6= 4.87-4.85 for the four CH, protons adjacent to the NH; group is observed. These protons experience a downfield shift of about A6= 0.2 upon association with DB24C8: compare the