Two different types of folding of isomeric macrocycles induced by metal ion co-ordination

The crystal structure of the dicopper(I) complex of a novel 30-membered macrocyclic Schiff-base type ligand with two dithiadiimine co-ordination sites, linked by two meta-substituted benzene rings, has revealed an achiral folding of the macrocycle (‘squeezed ring’), with two tetrahedral co-ordination sites and π-stacked benzene rings. The dicopper(I) compound of the isomeric 32-membered macrocycle with π-stacked para-substituted benzene rings has a chiral double helical (‘twisted ring’) conformation. The co-ordination geometries of the two copper(I) chromophores of both structures are very similar, and this emerges also from the solid-state UV/VIS spectra [metal-to-ligand charge-transfer transitions at 375 and 355 nm for the dicopper(I) species with the 30- and 32-membered macrocycle, respectively]. Proton NMR experiments and UV/VIS spectroscopy indicate that the folded structures are preserved in solution and that the metal-free compounds both have an open cyclic structure.