A meso-Diamidoindolyl Calix[4]pyrrole as an Efficient Anion Receptor

Abstract

A calix[4]pyrrole derivative (2) bearing two amidoinodole substituents at the diagonal meso positions was synthesized and its anion-binding properties were investigated by 1H NMR spectroscopic analysis in CD₂Cl₂. In comparison to the parent calix[4]pyrrole (1), receptor 2, incorporating additional hydrogen-bond donor and acceptor sites, exhibited markedly enhanced binding affinities toward a series of anions (F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻, and H₂PO₄⁻). The introduction of amidoinodole groups significantly strengthened the anion–receptor interactions, as evidenced by substantial downfield shifts of the pyrrolic and indolic NH resonances upon anion addition. Notably, receptor 2 was found to bind H₂PO₄⁻ via a distinct binding mode compared to those observed for the parent calix[4]pyrrole (1) and for the other anions examined, suggesting a unique cooperative hydrogen-bonding network involving both the core pyrrolic NH units and the appended indole–amide moieties.