A ring-in-ring assembly strategy is used to prepare this hetero[3]catenane and while it has previously been shown that [12]CPP and 18-crown-6 have a moderate binding affinity, 18-crown-6 is too small for axle molecules containing aryl groups. Therefore, 24-crown-8 is chosen as the inner ring connecting [12]CPP and a dibenzylammonium thread. The dibenzylammonium thread is designed with terminal alkyne and azide groups, allowing ring-closure via copper(I)-catalysed azide–alkyne cycloaddition to obtain the [3]catenane product. Analysis of a single crystal of [12]CPP and 24-crown-8 shows that 24-crown-8 is completely encapsulated by [12]CPP, with an average distance of 3.3 Å between crown ether hydrogen atoms and the centre of the CPP aryl units. This distance decreases to around 3.1 Å when a shortened thread molecule is present, however, it should be noted that single crystals of the [3]catenane product could not be grown, likely owing to the flexibility of two of the macrocycles in this structure.
1H nuclear magnetic resonance spectroscopic analysis of these macrocycles reveals that the proton signals of 24-crown-8 undergo a 0.22 ppm upfield shift, compared to free 24-crown-8, upon interlocking with the dibenzylammonium macrocycle and a 1.89 ppm upfield shift when part of the [3]catenane structure. The synthesis of this hetero[3]catenane product allows for the study of its dynamic motion, using molecular dynamics and well-tempered metadynamics, showing that the larger CPP ring can slip across the smaller crown ether ring. This ring-in-ring assembly strategy therefore provides a valuable way to make interesting mechanically interlocked molecules which may be applied to the synthesis of other such structures, as well as hetero[3]rotaxanes and polyrotaxanes, in the future.
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