Who is triaza-macrocycle Cinderella? A study to find the perfect match for the Fe(III) complexation

In recent years, the biomedical research has started to investigate a new generation of contrast agent for MRI based on high spin Fe(III) complexes.[1] The change from the actual Gd(III) based agents concerns not only the properties that determine their efficiency as contrast agents, but also the ligand requirements to ensure the thermodynamic, kinetic and redox stabilities, essential for their approval for clinical use. The stability of a metal complex is determined by the ligand structure and its donor atoms; in particular the donors and the cavity size should be compatible with the chemical properties and the dimension of the Fe3+ ion. The most common chemical structures reported in the literature for Fe(III) complexation are based on the functionalized nine-membered macrocyclic triamine 1,4,7-triazacyclononane. Starting from the 9- membered macrocycle, in this work we systematically evaluated four triazamacrocycles, expanding the size from a 9-membered ring to 10, 11 and 12- membered rings with the aim to determine the most suitable size for Fe(III) complexation. The best basic structure was also evaluated by comparing the functionalization of the three secondary amines with acetic arms or 2-hydroxybenzilic arms. Two of these macrocycles (9 and 12-terms) are commercially available, while the other two (10 and 11-terms) were synthesized using the modified Richman-Atkins procedure, that exploits tosylamides and glycoltosylates.[2] The protonation constants of the ligands and the stability of the Fe(III) complexes were determined by potentiometry and UV-vis spectrophotometry, evaluating the changes in absorbance as a function of pH at precise wavelength values. In conclusion, the higher is the sum of individual protonation constants, the more basic is the ligand and therefore potentially capable of forming highly stable Fe(III) complexes.