Cation-reinforced donor-acceptor pseudorotaxanes

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 [M-2.1.2](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 [Li-2.1](2+) and [Li-2.1.2](2+).