Controlling the Kinetic and Electrochemical Properties of Eu^II-Containing Complexes Using Peripheral Charges

The development of Eu-based contrast agents for magnetic resonance imaging requires careful balance between electrochemical potential and inertness with respect to metal dissociation. Using coordination chemistry, this study aimed to understand how the distance between peripheral anionic groups and Eu^II in macrocyclic complexes affects these properties. Four Eu^II-containing complexes were synthesized with different maximum-possible distances between the Eu^II ion and peripheral anionic groups of the ligand. Electrochemical potentials were measured using cyclic voltammetry, and dissociation rates were measured at pH 7 using an electrochemical method and at pH 1 using an acid-catalyzed dissociation method. Electrochemical studies show that increasing the maximum-possible distance between Eu^II and peripheral charges shifts electrochemical potentials to more positive values. Additionally, a decrease in the dissociation rates was observed at pH 7 with increasing maximum-possible distance. The Eu^II-containing complex with the largest maximum-possible distance exhibited the slowest dissociation rate. Statistical analysis confirmed that trends are significant. The findings are expected to provide a framework for the rational design of Eu^II-based contrast agents for magnetic resonance imaging in which redox accessibility and kinetic inertness can be fine-tuned independently by controlling the distance of peripheral charges from Eu^II.