Capture and Stabilization of the Hydroxyl Radical in a Uranyl Peroxide Cluster**

Abstract Here we describe the synthesis and characterization of a new uranyl peroxide cluster (UPC), U 60 Ox 30 * , which captures and stabilizes oxygen‐based free radicals for more than one week. These radical species were first detected with a nitroblue tetrazolium colorimetric assay and U 60 Ox 30 * was characterized by single crystal X‐ray diffraction as well as infrared (IR), Raman, UV‐Vis‐NIR, and electron paramagnetic resonance (EPR) spectroscopies. Identification of the free radicals present in U 60 Ox 30 * was done via room temperature solid and solution state X‐band EPR studies using spin trapping methods. The spin trapping agent 5,5‐dimethyl‐1‐pyrroline N‐oxide (DMPO) was definitive for identifying the free radicals in U 60 Ox 30 * , which are hydroxyl radicals (⋅OH) that are stable for up to ten days that also persist upon addition of the metalloenzymes catalase and superoxide dismutase. Addition of the spin trapping agent α‐(4‐pyridyl N‐oxide)‐N‐tert‐butylnitrone (POBN) further confirmed the radicals were oxygen based, and deuteration experiments showed that the origin of the free radicals was from the decomposition of H 2 O 2 in water. These results demonstrate that highly oxidizing species such as the ⋅OH radical can be stabilized in UPCs, which alters our understanding of the role of free radicals present in spent nuclear fuel.