1 : 1 vs. 2 : 1 coordination of pentadentate hydrazone-type ligands to lanthanide(III) ions. Formation of cationic as well as dicationic complexes.

Bis-acylhydrazones of 2,6-pyridinedicarbaldehyde are introduced as planar pentadentate ligands for the coordination of lanthanide(III) ions. With large cations (e.g. lanthanum(III)) 1 : 1 as well as 2 : 1 ligand-to-metal complexes are formed depending on the stoichiometry of the reaction. The different complexes are distinguished by NMR spectroscopy and ESI MS in solution and they can be characterized by X-ray diffraction. With the small lutetium(III) ion only the 1 : 1 complex is formed. In the 1 : 1 compounds, anions such as nitrate fill up the coordination site at the metal ion. However, in some cases anion substitution by solvent molecules is observed in the solid state leading to cationic and even dicationic coordination compounds.