Synthesis and characterization of dinuclear heterometallic lanthanide complexes exhibiting MRI and luminescence response

Abstract

A molecule bearing a macrocyclic DOTA-type chelator and an acyclic chelator based on the 5-aminoisophthalamide diethylenediaminetetraacid (5A-PADDTA) was synthesized by linking these two moieties via an amide bond. The ligand has the possibility to complex two identical or different lanthanide ions, depending on the desire for its potential application. Luminescence studies involving titrations of the Eu³⁺ or Gd³⁺ complex with Tb³⁺ confirm the formation of heterometallic complexes, as well as the presence of different species in the solution. Comparative ¹H NMR spectra of the ligand, its Eu³⁺ complex, and that containing both Eu³⁺ and Tb³⁺ proves the existence of respective monometallic or bimetallic species. NMR diffusion measurements on 5A-PADDTA as a model compound indicate the formation of aggregates upon the addition of Y³⁺ (chosen as a diamagnetic analogue of lanthanide ions). Hydration values were calculated from the respective luminescence lifetime values. They show the dominance of a q = 1 species for both ions in monometallic complexes, or q = 1 and q = 2 species of ions in aggregated complexes, for DOTA and 5A-PADDTA chelators, respectively.