Chromogenic behaviour of a family of hydrazine and hydrazone metal complexes

A family of five tridentate hydrazone-based ligands with an N,N,N-, N,N,O- or N,N,S-donor environment is reported. The tridentate ligands are derived from either 2-(1-methylhydrazinyl)pyridine or 2-hydrazinyl-pyridine and have been obtained by condensation with a series of aldehydes and ketones carrying one potential metal-binding site. To study their metal complexation behaviour, the five ligands and one of the bidentate precursor compounds have been reacted with ten different transition and main group metal ions. An interesting chromogenic behaviour has been found: ligands 1, 3 and H6 (with an N,N-, N,N,N- and N,N,O-donor environment) give very specific colour changes in the presence of Fe2+, whereas ligand H7 specifically forms a dark brown coloured complex with Co2+ only. All ligands form yellow to orange complexes with Pd2+. Three of these metal complexes were characterized by single-crystal X-ray diffraction analysis: the molecular structures of [Pd(1)Cl-2], [Cu(1)Cl-2](2), and [Cu(6)Cl] give an insight into the metal-binding behaviour of the ligands and nicely confirm the bidentate binding of ligand 1 and the tridentate binding of the deprotonated ligand 6(-). It is proposed that the chromogenic behaviour of the ligands can be useful for the development of selective metal ion sensors. (C) 2015 Elsevier B.V. All rights reserved.