Cation-reinforced donor-acceptor pseudorotaxanes

Abstract

Formation of a series of pseudorotaxanes from an electron-rich crown ether (host 1), pyromellitic diimide (guest 2) and alkali salt templates MX, where M = Li+ and Na+, and X− = Br−, I−, CF3SO3−, [B(C6F5)4]− and [B{3,5-(CF3)2(C6H3)}4]−, is reported. Mixing of 1 and 2 in CH2Cl2 or a mixture of CHCl3 and MeOH (98∶2) gave a pale yellow solution indicative of a very weak charge-transfer interaction. Upon addition of MX, brightly coloured solutions were obtained, resulting from a red shift and an increase in the intensity of the charge-transfer band. Structural and kinetic studies of the pseudorotaxanes were performed by NMR. The solution-phase structures of [M2·1·2]2+ are in good agreement with the solid-phase structure determined by X-ray crystallography. The remarkable templating properties of Li+ for the 1·2 donor-acceptor complex are due to the almost perfect coincidence of coordinative geometries in [Li2·1]2+ and [Li2·1·2]2+.