Synthesis and X-ray crystallographic studies of novel proton-ionizable nitro- and halogen-substituted acridono-18-crown-6 chromo- and fluorogenic ionophores

Starting from acridono-18-crown-6 ligand () seven new proton-ionizable chromogenic and fluorogenic ionophores () containing NO2 group(s) and/or Br or Cl atom(s) in the aromatic rings were prepared by electrophilic substitution. The precursor macrocycle was obtained by a modification of the reported procedure which made chromatography unnecessary in purification and gave higher yield. X-ray crystallographic studies of the complexes of acridono-18-crown-6 type ligands , , , and show that the proton-ionizable units are in the acridone tautomeric form and that the ligands invariably bind a water molecule in their cavities by multipodal hydrogen bonding. In two cases ( and ) an additional DMF solvent molecule is also bound at the crown perimeter in the solid state.