DOUBLY DOCKED PSEUDOROTAXANES

The complexation phenomena associated with the ditopic crown ether bis -p-phenylene[34]crown-10 (BPP34C10) and four bisammonium salts, each e ndowed with two bulky 3,5-di-tert-butylbenzyl termini and a pair of NH 2+ centers that are linked through a suitable spacer unit, have been s tudied. These studies have led to a route to the supramolecular synthe ses of singly stranded, doubly docked [2]pseudorotaxanes, in which eac h NH2+ center interacts simultaneously with the crown ether's independ ent polyether arcs, so that one dicationic unit can interpenetrate the cavity of one BPP34C10 macroring by means of hydrogen-bonding interac tions. NMR spectroscopy, mass spectrometry, and X-ray crystallography demonstrate (in solution, in the gas phase, and in the solid state, re spectively) that the doubly docked [2]pseudorotaxanes are generated th rough the self-assembly of BPP34C10 with bisammonium dications bearing p-xylylene, 2,6-naphthalenebis(methylene), or hexamethylene spacer un its. In contrast, X-ray crystallography shows that a supermolecule, po ssessing 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 do uble docking of one of the bisammonium dications within BPP34C10's cav ity has been utilized in a prototypical chromophoric supramolecular de vice that operates in response to changes in its surrounding pH. A 1:1 :1 solution of the hexafluorophosphate salt of this bisammonium dicati on with BPP34C10 and a 4,4'-bipyridinium salt is colorless, since the crown ether complexes preferentially with the bisammonium dication, Co nversely, it is red in the presence of iPr(2)NEt because the NH2+ cent ers are deprotonated, forcing the crown ether to interact with the 4,4 '-bipyridinium salt by means of, inter alia, charge-transfer interacti ons. This process is reversible, since the solution is decolorized upo n treatment with CF3CO2H.