The Role of Coordination Environment and pH in Tuning the Oxidation Rate of Europium(II)

The Eu II/III redox couple offers metal‐based oxidation‐sensing with magnetic resonance imaging making the study of Eu II oxidation chemistry important in the design of new probes. Accordingly, we explored oxidation reactions with a set of Eu II ‐containing complexes. Superoxide formation from the reaction between Eu II and dioxygen was observed using electron paramagnetic resonance spectroscopy. Additionally, oxidation kinetics of three Eu II ‐containing complexes with bromate and glutathione disulfide at pH values, including 5 and 7, is reported. In the reaction with bromate, the oxidation rate of two of the complexes increased by 7.3 and 6.7 times upon decreasing pH from 7 to 5, but the rate increased by 17 times for a complex containing amide functional groups over the same pH range. The oxidation rate of a fluorobenzo‐functionalized cryptate was relatively slow, indicating that the ligand used to impart thermodynamic oxidative stability might also be useful for controlling oxidation kinetics.