Azopyridinium-containing [2]pseudorotaxanes and hydrazopyridinium-containing [2]catenanes

Abstract

Benzylation of 4,4′-azopyridine, followed by counterion exchange, yields the bis(hexafluorophosphate) salt of the dibenzyl-4,4′-azopyridinium dication, which is bound by bis-p-phenylene-34-crown-10 (BPP34C10) and by 1,5-dioxynaphtho-38-crown-10 (1/5DN38C10) with Ka values of 90 and 880 M-1, respectively, in acetonitrile. When a 4,4′-azopyridinium unit is introduced along with a bipyridinium unit into a tetracationic cyclophane - either in its free or catenated forms - spontaneous reduction to the 4,4′-hydrazopyridinium unit occurs. The X-ray structural analysis of a [2]catenane, incorporating this tetracationic cyclophane and BPP34C10, shows that the 4,4′-hydrazopyridinium unit is located alongside the cavity of the macrocyclic polyether while the other dicationic unit of the tetracationic cyclophane - namely the 4,4′-bipyridinium unit - is located inside. Variable temperature 1H NMR spectroscopy demonstrated that the 4,4′-hydrazopyridinium unit rotates in solution around the [N···N] axis defined by its two pyridinium nitrogen atoms. The energy barrier for this dynamic process is ca. 14 kcal mol-1 in both the free tetracationic cyclophane and in the [2]catenane incorporating BPP34C10. However, the energy barrier for this dynamic process is only 11.7 kcal mol-1 in a [2]catenane incorporating the same tetracationic cyclophane and 1/5DN38C10. In this latter [2]catenane, the 4,4′-bipyridinium unit and the inside 1,5-dioxynaphthalene ring system rotate (ΔGc‡ 14.0 kcal mol-1) in solution about their [N···N] and [O···O] axes, respectively. In the former [2]catenane, incorporating BPP34C10, the macrocyclic polyether circumrotates through the cavity of the tetracationic cyclophane against an energy barrier of 11.7 kcal mol-1.