# Controlling the Kinetic and Electrochemical Properties of EuII–Containing Complexes Using Peripheral Charges

## TL;DR

This study shows how adjusting the distance of charges around a europium ion can control its chemical and electrochemical behavior, which is important for designing better MRI contrast agents.

## Contribution

The paper introduces a method to independently tune redox potential and kinetic inertness in EuII complexes by manipulating peripheral charge distance.

## Key findings

- Increasing the distance between EuII and peripheral charges shifts electrochemical potentials to more positive values.
- Larger distances correlate with slower dissociation rates at pH 7.
- The complex with the largest distance showed the slowest dissociation rate.

## Abstract

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 EuII in macrocyclic complexes
affects these properties. Four EuII-containing complexes
were synthesized with different maximum-possible distances between
the EuII 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
EuII 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 EuII-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 EuII-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 EuII.