Doubly docked pseudorotaxanes

The complexation phenomena associated with the ditopic crown ether bis-p-phenylene[34] crown-10 (BPP34C10) and four bisammonium salts, each endowed with two bulky 3,5-di-tert-butylbenzyI termini and a pair of NH₂⁺ centers that are linked through a suitable spacer unit, have been studied. These studies have led to a route to the supramolecular syntheses of singly stranded, doubly docked [2]pseudorotaxanes, in which each NH₂⁺ center interacts simultaneously with the crown ether's independent polyether arcs, so that one dicationic unit can interpenetrate the cavity of one BPP34C10 macroring by means of hydrogen-bonding interactions. NMR spectroscopy, mass spectrometry, and X-ray crystallography demonstrate (in solution, in the gas phase, and in the solid state, respectively) that the doubly docked [2]pseudorotaxanes are generated through the self-assembly of BPP34C10 with bisammonium dications bearing p-xylylene, 2,6-naphthalenebis(methylene), or hexa-methylene spacer units. In contrast, X-ray crystallography shows that a supermolecule, possessing a hot-dog-like co-conformation, is synthesized noncovalently when BPP34C10 self-assembles with a bisammonium salt in which the NH₂⁺ centers are separated by a shorter pentamethylene spacer unit. The double docking of one of the bisammonium dications within BPP34C10's cavity has been utilized in a prototypical chromophoric supramolecular device that operates in response to changes in its surrounding pH. A 1:1:1 solution of the hexafluorophosphate salt of this bisammonium dication with BPP34C10 and a 4,4'-bipyridinium salt is colorless, since the crown ether complexes preferentially with the bisammonium dication. Conversely, it is red in the presence of iPr₂NEt because the NH₂/⁺ centers are deprotonated, forcing the crown ether to interact with the 4,4'-bipyridinium salt by means of, inter alia, charge-transfer interactions. This process is reversible, since the solution is decolorized upon treatment with CF₃CO₂H.