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

Benzylation of 4,4'-azopyridine, followed by counterion exchange, yields th e 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-dioxy naphtho-38-crown-10 (1/5DN38C10) with K-a values of 90 and 880 M-1, respect ively, in acetonitrile. When a 4,4'-azopyridinium unit is introduced along with a bipyridinium unit into a tetracationic cyclophane - either in its fr ee 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'-hydrazopyr idinium unit is located alongside the cavity of the macrocyclic polyether w hile the other dicationic unit of the tetracationic cyclophane namely the 4 ,4'-bipyridinium unit - is located inside. Variable temperature H-1 NMR spe ctroscopy demonstrated that the 4,4'-hydrazopyridinium unit rotates in solu tion around the [N . . .N] axis defined by its two pyridinium nitrogen atom s. The energy barrier for this dynamic process is ca. 14 kcal mol(-1) in bo th the free tetracationic cyclophane and in the [2]catenane incorporating B PP34C10. However, the energy barrier for this dynamic process is only 11.7 kcal mol-l 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 (DeltaG(c)double dagger 14.0 kcal mol(-1)) in solution about their [N . . .N] and [O . . .O] axes, respectively. In the former [2]catenane, incorporating BPP34C10, the macro cyclic polyether circumrotates through the cavity of the tetracationic cycl ophane against an energy barrier of 11.7 kcal mol(-1).