Oxidative stabilization of aqueous divalent europium for potential use as a contrast agent for ultra-high field magnetic resonance imaging

The magnetic and optical properties of divalent europium are of interest for catalysis, materials, magnetic, diagnostic-medical, and luminescence applications. Use of the aqueous Eu⊃II⊃ ion in these applications is limited due to the instability of Eu⊃II⊃ in the presence of air. To increase the use of Eu⊃II⊃ in these applications, it is desirable to increase the oxidative stability of the Eu⊃II⊃ ion in aqueous solution. Many attempts have been made to increase the oxidative stability of aqueous Eu⊃II⊃ with little success. The most stable aqueous Eu⊃II⊃ complex, before my research, was reported with the ligand [2.2.2]cryptand. My specific research strategy involved modifying this previously most stable ligand for aqueous Eu⊃II⊃ using principles of coordination chemistry. Four strategies were used based on principles of coordination chemistry and they were (i) increasing steric bulk to minimize metal–environment interactions; (ii) reducing Lewis basicity of the donor atoms to favor coordination to Eu⊃II⊃, which is electron rich with respect to Eu⊃III⊃; (iii) changing cavity size to favor coordination of the larger Eu⊃II⊃ ion over the Eu⊃III⊃ ion; and (iv) modifying hard–soft acid–base properties to favor coordination of the softer Eu⊃II⊃ ion with respect to the harder Eu⊃III⊃ ion. Cryptands 1–6 were reacted with Eu(NO⊂3⊂)⊂3⊂ to obtain aqueous Eu⊃II⊃ cryptate complexes in situ after bulk electrolysis. The oxidation potentials of these aqueous Eu⊃II⊃ cryptate complexes were determined using cyclic voltammetry. All cryptands produced using the four strategies were able to oxidatively stabilize the aqueous Eu⊃II⊃ ion, but the modification of hard–soft acid–base properties was the most effective strategy. The most oxidatively stable aqueous Eu⊃II⊃ complex in the study, 5,6-(Benzo)-4,7-dioxa-13,16,21,24-tetrathia-1,10-diazabicyclo[8.8.8]hexacos-5-ene europium(II) complex (6–Eu⊃II⊃), is the most stable aqueous Eu⊃II⊃ complex reported to date. It is more oxidatively stable than Fe⊃II⊃ in human hemoglobin under the same conditions, indicating the potential for future in vivo application