A NOVEL TYPE OF ISOMERISM IN [3]CATENANES

The self-assembly of three [3]-catenanes based on a single tetracation ic lophane-cyclobis(bipyridinium-1,4-diethoxybenzene) with pairs of id entical interlocked crown ethers-bis-p-phenylene[34]crown-10, bis-1,5- dioxynaphthalene[34]crown-10, or etrafluoro-p-phenylene-p-phenylene[34 ]crown-10-has been achieved in yields of 34, 31, and 33 %, respectivel y, The solid-slate structures of these [3]catenanes, determined by X-r ay crystallography, are consistent with molecules having approximately C-2h symmetry. In the solution state, the pi-electron rich aromatic r ing systems of the crown ether components, residing within the cavity of the tetracationic cyclophane, can adopt two different relative orie ntations, thus giving rise to two distinct isomeric forms possessing e ither C-2h or D-2 symmetries, Two dynamic processes have been characte rized in the [3]catenane incorporating bis-1,5-dioxynaphthalene[38]-cr own-10 macrocycles, by means of variable-temperature H-1 NMR spectrosc opy and subsequent lineshape analyses. The slower process is the excha nge between C-2h and D-2 isomers, and the faster process is the rotati on of the bipyridinium units of the cyclophane around their N ... N ax es. 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 o ne, ca, 14.5 kcal mol(-1), has enabled us to propose a scenario in whi ch the two included 1,5-dioxynaphthalene ring systems move continuousl y about the center of the cavity of the tetracationic cyclophane, A pa rtial 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-dioxynap hthalene ring system to rotate about its O ... O axis. The isomerism b etween the C-2h and the D-2 isomers of the [3]catenane incorporating t he two bis-1,5-dioxynaphthalene[38]crown-10 rings is dependent upon 1) translational motions between the two crown ether rings and the centr al tetracationic cyclophane and 2) conformational changes within at le ast one of the two crown ether rings.