Ferrocene-amino acid macrocycles as hydrazone-based receptors for anions

We report the synthesis of a family of new macrocyclic hydrazone-based anion receptors. Formed from the reaction between isophthalaldehyde and a helical amino acid-disubstituted ferrocene dihydrazide, these macrocycles contain from one to eight ferrocene moieties. The isolation of the four smallest of the macrocycles and their characterisation by UV-Vis, CD and NMR spectroscopy is described. The conformation of the macrocycles is explored, particularly with reference to the formation of a helical intramolecularly hydrogen-bonded structure. An investigation of the use of these macrocycles as anion-receptors shows that they are all effective hosts; the larger macrocycles show the highest affinities for anions. Studies using NMR spectroscopy suggest that the anion-recognition results primarily from the formation of multiple hydrogen bonds between the anions and the electropositive N-H protons of the acylhydrazones.