Structure, stability and relaxivity of trinuclear triangular complexes

Abstract

The self-assembly of a carbonylpyridine-based heptadentate ligand with Ln^III results in the formation of triangular trinuclear europium complexes, which exhibit interesting luminescent properties in the solid state and in solution. With a view to developing multimodal responsive systems, we report here the preparation and characterisation of analogous complexes with Gd^III. The X-ray crystal structure of Gd₃L2₃ indeed reveals the isostructurality with the Eu^III complexes. A combination of ¹H NMRD and variable temperature studies yields the parameters elucidating the exchange of coordinated water and relaxivity properties. Conveniently, the competitive spectrophotometric titrations with EDTA and NTA are used to determine the thermodynamic stability constants of the europium complexes in aqueous media. In addition, the exchange reaction with EDTA is monitored with NMR and fluorimetry. The interactions of the Eu^III trinuclear complex with some potentially interfering ligands are qualitatively investigated by means of luminescence titrations.