[2]Rotaxane with Multiple Functional Groups

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 1H 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.