Multicomponent Self-Assembled Metal–Organic [3]Rotaxanes

A set of environmentally responsive
metal–organic [3]­rotaxanes
is described. These mechanically interlocked macromolecules may be
prepared in quantitative yield via a one-pot procedure involving treatment
of a flexible tetracationic macrocycle, known as the Texas-sized molecular
box, with tri-1,3,5-benzenetricarboxylate anion and silver cations
(Ag+). The use of this three-component mixture gives rise
to a metal–organic [3]­rotaxane via a self-assembly process
that occurs under ambient conditions in DMSO-d6 solution. The complex is stable in the presence of excess
TFA. However, disassembly of the [3]­rotaxane to produce anion-box
associated entities may be triggered by adding a competitive counteranionic
species (e.g., I–). Adding excess Ag+ serves to reverse this decomplexation process. The nature of the
[3]­rotaxane complex could be fine-tuned via application of an external
stimulus. Increasing the temperature or adding small molecules (e.g.,
D2O, methanol-d4, acetonitrile-d3, DMF-d7, acetone-d6, or THF-d8) to
the initial DMSO-d6 solution induces conformational
flipping of the macrocycle within the overall complex (e.g., from
limiting chair to chairlike forms). Support for the molecular stimuli
responsive nature of the various structures came from solution-phase
one- and two-dimensional (1H, 1D and 2D NOESY) NMR spectroscopic
studies carried out in DMSO-d6. The core
metal-linked rotaxane unit was characterized via single-crystal X-ray
diffraction analysis. Initial evidence that the present self-assembly
process is not limited to the use of the Ag+ cation came
from studies involving Cd2+; this replacement results in
formation of 2D metal–organic rotaxane-containing frameworks
(MORFs).