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⁺.