La complexation des lanthanides par les macrocycles

Abstract

Complexes of the complete series of lanthanide nitrates and Perchlorates with various macrocycles have been synthesized under anhydrous conditions. Spectroscopic studies indicate that the complexation by cyclic ligands depends on the dimension of the internal cavity, on the rigidity of the macrocycle, on the nature of its donor atoms and on the complexing properties of the counter ion. These factors lead to some selectivity in the complexation of the rare earths. The structure of the complexes was deduced by nuclear magnetic resonance spectroscopy of paramagnetic compounds. It was established that a non-substitued diaaza tetraoxa cycle adopts a non-planar conformation of reduced cavity size. The influence of the lanthanide contraction was demonstrated in the study of sandwich-type complexes with a small tetraoxa macrocycle. The electrochemical behaviour of the systems Yb(III)-Yb(II) and Sm(III)-Sm(II) in the presence of macrocycles was investigated by polarography. A hexaoxa macrocycle was found to have the unique property of stabilizing the II oxydation state by complexation. Stability constants were deduced from an analysis of partially irreversible Polarographie waves. A tetraaza tetraacetic macrocycle was used to complex the lanthanides in aqueous solution. This ligand forms the most stable rare earth complexes known so far and can act as a masking agent in the separation of rare earths from transition metal ions and alkalis.