The Eu^II ion rivals Gd^III in its ability to enhance contrast in magnetic resonance imaging. However, all reported Eu^II-based complexes have been studied in vitro largely because the tendency of Eu^II to oxidize to Eu^III has been viewed as a major obstacle to in vivo imaging. Herein, we present solid- and solution-phase characterization of a Eu^II-containing cryptate and the first in vivo use of Eu^II to provide contrast enhancement. The results indicate that between one and two water molecules are coordinated to the Eu^II core upon dissolution. We also demonstrate that Eu^II-based contrast enhancement can be observed for hours in a mouse.

The Eu^II ion rivals Gd^III in its ability to enhance contrast in magnetic resonance imaging. However, all reported Eu^II-based complexes have been studied in vitro largely because the tendency of Eu^II to oxidize to Eu^III has been viewed as a major obstacle to in vivo imaging. Herein, we present solid- and solution-phase characterization of a Eu^II-containing cryptate and the first in vivo use of Eu^II to provide contrast enhancement. The results indicate that between one and two water molecules are coordinated to the Eu^II core upon dissolution. We also demonstrate that Eu^II-based contrast enhancement can be observed for hours in a mouse.

The kinetic stabilities and relaxivities of a series of Eu²⁺-containing cryptates have been investigated. Transmetallation studies, which monitored the change in the longitudinal relaxation rate of water protons in the presence of Ca²⁺, Mg²⁺, and Zn²⁺, demonstrated that the cryptate structure influenced the stability, and two of the cryptates studied were inert to transmetallation in the presence of these endogenous ions. The efficacy of these cryptates was determined at different magnetic field strengths, temperatures, and pH values. Cryptate relaxivity was found to be higher at ultrahigh field strengths (7 and 9.4 T) relative to clinically relevant field strengths (1.4 and 3 T), but the efficiency of these cryptates decreased as the temperature increased. In addition, a variation in pH did not yield significant changes in the efficacy of the cryptates. These studies establish a foundation of important properties that are necessary to develop effective positive contrast agents for magnetic resonance imaging from Eu²⁺-containing cryptates.

Recent advances in the coordination chemistry of Eu²⁺ are reviewed. Common synthetic routes for generating discrete Eu²⁺-containing complexes reported since 2000 are summarized, followed by a description of the reactivity of these complexes and their applications in reduction chemistry, polymerization, luminescence, and as contrast agents for magnetic resonance imaging. Rapid development of the coordination chemistry of Eu²⁺ has led to an upsurge in the utilization of Eu²⁺-containing complexes in synthetic chemistry, materials science, and medicine.