2]Rotaxane with multiple functional groups.

Abstract

High-yield syntheses of Cu(II)- and Ni(II)-templated [2]pseudorotaxane precursors (CuPRT and NiPRT, respectively) were achieved by threading bis(azide)bis(amide)-2,2'-bipyridine axle into a bis(amide)tris(amine) macrocycle. Single-crystal X-ray structural analysis of CuPRT revealed complete threading of the axle fragment into the wheel cavity, where strong aromatic π-π stacking interactions between two parallel arene moieties of the wheel and the pyridyl unit of axle are operative in addition to metal ion templation. Attachment of a newly developed bulky stopper molecule with a terminal alkyne to CuPRT via a Cu(I)-catalyzed azide-alkyne cycloaddition reaction failed as a result of dethreading of the azide-terminated axle under the reaction conditions. However, the synthesis of a metal-free [2]rotaxane containing triazole with other functionalities in the axle was achieved in ∼45% yield upon coupling between azide-terminated NiPRT and the alkyne-terminated stopper. The [2]rotaxane was characterized by mass spectrometry, 1D and 2D NMR (COSY, DOSY, and ROESY) experiments. Comparative solution-state NMR studies of the [2]rotaxane in its unprotonated and protonated states were carried out to locate the position of the wheel on the axle of the metal-free [2]rotaxane. Furthermore, a variable-temperature (1)H NMR study in DMSO-d6 of [2]rotaxane supported the kinetic inertness of the interlocked structure, where the newly developed stopper prevents dethreading of the 30-membered wheel from the axle.