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-diethoxybenzene) with pairs of identical interlocked crown ethers bis-p-phenylene[34]crown-10, bis- 1,5-dioxynaphthalene[38]crown-10, or tetrafluoro-p-phenylene-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 C(2h) 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 C(2h) 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 C(2h) and D₂ isomers, and the faster process is the rotation of the bipyridinium units of the cyclophane around their N · · · N axes. The evaluation of the free energies of activation at 298 K, which are, for the slower process, ca, 16 kcalmol^-1 and for the faster one, ca, 14.5 kcalmol^-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 O · · · O axis. The isomerism between the C(2h) 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.