Oxidatively stable, aqueous europium(II) complexes through steric and electronic manipulation of cryptand coordination chemistry.

A series of cryptands have been prepared and they demonstrate the relationship between oxidative stability of aqueous Eu and ligand properties (see figure). One of these Eu complexes is more stable than Fe in hemoglobin and appears to be the most oxidatively-stable aqueous Eu species known. The high stability of Eu is expected to enable the use of the unique magnetic and optical properties of this ion in vivo.