Molecular meccano Part 50 Diazapyrenium-containing catenanes and rotaxanes

Diazapyrenium units have been incorporated into the pi-electron deficient c omponents of four [2]pseudorotaxanes, four [2]catenanes, and two [2]rotaxan es, each having a dioxyarene-based macrocyclic polyether as the pi-electron rich component. A dramatic increase in the association constants that char acterize the formation of diazapyrenium-containing [2]pseudorotaxanes, rela tive to those of their bipyridinium-based analogs, was observed in solution studies. These results indicate that the intercomponent non-covalent bondi ng interactions are reinforced significantly when diazapyrenium, instead of bipyridinium, recognition sites are employed in the pi-electron deficient components of this kind of [2]pseudorotaxane. Not surprisingly, therefore, in two asymmetric [2]catenanes which incorporate one diazapyrenium and one bipyridinium recognition site within their tetracationic cyclophane compone nts, the diazapyrenium unit is located inside the cavity of the pi-electron rich macrocyclic component as revealed in the solid state by X-ray crystal lographic analyses and by H-1 NMR spectroscopic studies in solution. Variab le temperature H-1 NMR spectroscopic studies showed that, in the [2]catenan es, the free energy barriers associated with the circumrotation of one macr ocyclic component through the cavity of the other and vice versa increase w hen diazapyrenium, instead of bipyridinium, recognition sites are employed, reflecting the stronger intercomponent non-covalent bonding interactions i nvolving the former.