Cation–π interaction: a case for macrocycle–cation π-interaction by its ureidoarene counteranion

Abstract

We report the solid state structures of homocomplex [1·K]⁺[I]⁻ and of heterocomplexes [1·K]⁺[3·I]⁻ and [1·K]⁺[4·I]. In the [1·K]⁺[I]⁻ complex the apical position of the K⁺ in the macrocycle is occupied by a –CH₂– moiety of a neighboring 18-crown-6, consistent with a close contact and corresponding to ‘agostic’ interactions between K⁺ and –CH₂– moieties. In the [1·K]⁺[3·I]⁻ complex the second apical site of the cation is coordinated by a bridging water molecule which is simultaneously H-bonded to both the phenol hydroxyl and the iodide anion. In the [1·K]⁺[4·I]⁻ complex the second apical site of the cation is occupied by the indole moiety. Moreover, the new heterocomplex system [1·K]⁺[4·I]⁻ presented here, despite the multiple possibilities of K⁺–π contacts with the sterically available phenyl, phenyl-indole and pyrrole-indole rings of 4, shows that the pyrrolo C2C3 double bond is a versatile π-donor. ¹H NMR results led us to conclude that the complexes adopt similar conformations in solution to those observed in the solid state.