Mechanical Bond-Induced Radical Stabilization

A homologous series of [2]rotaxanes, in which cyclobis(paraquat-p-phenylene) (CBPQT(4+)) serves as the ring component, while the dumbbell components all contain single 4,4'-bipyridinium (BIPY2+) units centrally located in the midst of oligomethylene chains of varying lengths, have been synthesized by taking advantage of radical templation and copper-free azide-alkyne 1,3-dipolar cycloadditions in the formation of their stoppers. Cyclic voltammetry, UV/vis spectroscopy, and mass spectrometry reveal that the BIPY center dot+ radical cations in this series of [2]rotaxanes are stabilized against oxidation, both electrochemically and by atmospheric oxygen. The enforced proximity between the BIPY2+ units in the ring and dumbbell components gives rise to enhanced Coulombic repulsion, destabilizing the ground-state co-conformations of the fully oxidized forms of these [2]rotaxanes. The smallest [2]rotaxane, with only three methylene groups on each side of its dumbbell component, is found to exist under ambient conditions in a monoradical state, a situation which does not persist in acetonitrile solution, at least in the case of its longer analogues. H-1 NMR spectroscopy reveals that the activation energy barriers to the shuttling of the CBPQT(4+) rings over the BIPY2+ units in the dumbbells increase linearly with increasing oligomethylene chain lengths across the series of [2] rotaxanes. These findings provide a new way of producing highly stabilized BIPY center dot+ radical cations and open up more opportunities to use stable organic radicals as building blocks for the construction of paramagnetic materials and conductive molecular electronic devices.