Synthesis and Characterisation of a Paramagnetic [2]Rotaxane Based on a Crown Ether-Like Wheel Incorporating a Nitroxide Motif.

The synthesis of the new nitroxide crown ether 8 and its use as the wheel in a bistable [2]rotaxane, containing dialkylammonium and 4,4'-bipyridinium recognition sites, is reported. The synthesis of 8 was achieved by the sequential addition of substituted phenyl groups to a nitrone derivatives leading to the preferential formation of the cis stereoisomer. Due to charge-dipole interactions between the nitroxide unit and the bipyridinium moiety, it was possible to probe the movement of the macrocycle between the two molecular stations of the [2]rotaxane after addition of a base by measuring the nitrogen hyperfine splitting in the corresponding EPR spectra. The equilibrium constant for the complexation of dibenzyl viologen by the macrocycle 8 was also determined by EPR titration.