New bis-ferrocenyl dibenzo-18-crown-6 ligands that can electrochemically sense group 1 and 2 metal cations

New bis-ferrocenyl dibenzo-18-crown-6 ligands have been prepared in which the ferrocene redox centre is appended to the benzo crown ether moiety by unsaturated imine (Schiff bases) and saturated methylene amine bond linkages (secondary amine derivatives). H-1 and C-13 NMR titration studies reveal that both types of ligand form 1:1 complexes with sodium and potassium cations. Magnesium cations form 2Mg(2+):Schiff base complexes in which H-1 NMR and electronic spectral evidence suggest that the divalent metal cation primarily interacts with the imine bond. Electrochemical investigations have demonstrated that, whereas the respective ferrocene redox couples of all ligands are insensitive to K+, the electrochemical sensing of Na+ guest cations is only observed with ligands (Schiff bases) that contain the conjugated metallocene cyclopentadienyl-imine-phenyl bond linkage in combination with a crown ether binding site. Similar redox-responsive behaviour is observed with Mg2+, except electrochemical observations with 1,2-dimethoxy-4-(imino-ferrocenyl)benzene indicate that the crown ether recognition site is not required for the detection of this metal dication.