The
3+ and 2+ oxidation states of europium have drastically different
magnetic and spectroscopic properties. Electrochemical measurements
are often used to probe Eu<sup>III/II</sup> oxidation state changes,
but a full suite of spectroscopic characterization is necessary to
demonstrate conversion between these two oxidation states in solution.
Here, we report the facile conversion of an europium(III) tetraglycinate
complex into its Eu<sup>II</sup> analogue. We present electrochemical,
luminescence, electron paramagnetic resonance, UV–visible,
and NMR spectroscopic data demonstrating complete reversibility from
the reduction and oxidation of the 3+ and 2+ oxidation states, respectively.
The Eu<sup>II</sup>-containing analogue has kinetic stability within
the range of clinically approved Gd<sup>III</sup>-containing complexes
using an acid-catalyzed dissociation experiment. Additionally, we
demonstrate that the 3+ and 2+ oxidation states provide redox-responsive
behavior through chemical-exchange saturation transfer or proton relaxation,
respectively. These results will be applicable to a wide range of
redox-responsive contrast agents and Eu-containing complexes
