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).