Ammonium redox-responsive receptors containing multiple ferrocene and quinone redox-active centres attached to di- and tri-aza crown ether macrocycles

Abstract

Eight new redox-active di- and tri-aza crown ether macrocyclic ligands containing multiple ferrocene and quinone moieties have been prepared, including the first reported mixed ferrocene–quinone macrocyclic ligand. Solution ¹H and ¹³C NMR complexation studies suggest these ligands form 1:1 stoichiometric complexes with ammonium as guest cation. Single-crystal structures of the hydrogen hexafluorophosphate salt of 7,16-bis(ferrocenylmethyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane and the ammonium complex of 4,10,16-tris(ferrocenylmethyl)-1,7,13-trioxa-4,10,16-triazacyclooctadecane have also been determined. Electrochemical investigations reveal the respective ferrocene–ferrocenium redox couples of most of the ligands are perturbed to more positive potentials on co-ordination of NH₄⁺ and K⁺ guest cations. The above tris(ferrocenylmethyl) ligand selectively binds and electrochemically recognises NH₄⁺ in the presence of equimolar concentrations of K⁺. Secondary amide-linked ferrocene aza crown ether ligands electrochemically sense the NH₄⁺ guest cation primarily via host ferrocenecarboxide carbonyl oxygen–H–NH₃⁺ hydrogen-bonding electrostatic interactions.