A Novel Type of Isomerism in [3]Catenanes

The self-assembly of three [3]-catenanes based on a single tetracationic cyclophane—cyclobis(bipyridinium-1,4-di-ethoxybenzene)—with pairs of identical interlocked crown ethers—bis-p-phenyl-ene[34]crown-10, bis-1,5-dioxynaphthal-ene[38]crown-10, or tetrafluoro-p-phenyl-ene-p-phenylene[34]crown-10—has been achieved in yields of 34, 31, and 33%, respectively. The solid-state structures of these [3]catenanes, determined by X-ray crystallography, are consistent with molecules having approximately C2h symmetry. In the solution state, the π-electron rich aromatic ring systems of the crown ether components, residing within the cavity of the tetracationic cyclophane, can adopt two different relative orientations, thus giving rise to two distinct isomeric forms possessing either C2h or D2 symmetries. Two dynamic processes have been characterized in the [3]catenane incorporating bis-1,5-dioxynaphthalene[38]-crown-10 macrocycles, by means of variable-temperature 1H NMR spectroscopy and subsequent lineshape analyses. The slower process is the exchange between C2h and D2 isomers, and the faster process is the rotation of the bipyridinium units of the cyclophane around their NN axes. The evaluation of the free energies of activation at 298 K, which are, for the slower process, ca. 16 kcal mol−1, and for the faster one, ca. 14.5 kcal mol−1, has enabled us to propose a scenario in which the two included 1,5-dioxynaphthalene ring systems move continuously about the center of the cavity of the tetracationic cyclophane. A partial dissociation of one 1,5-dioxynaphthalene ring system allows the adjacent bipyridinium units to rotate about their long axes, whereas a complete dissociation is presumably necessary to allow a 1,5-dioxynaphthalene ring system to rotate about its OO axis. The isomerism between the C2h and the D2 isomers of the [3]catenane incorporating the two bis-1,5-dioxynaphthalene[38]crown-10 rings is dependent upon 1) translational motions between the two crown ether rings and the central tetracationic cyclophane and 2) conformational changes within at least one of the two crown ether rings.