1. School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, UK
2. Rigaku Europe, Chaucer Business Park, Watery Lane, Sevenoaks, Kent TN15 6QY, UK

Correspondence to: Polly L. Arnold¹Jason B. Love¹ Correspondence and requests for materials should be addressed to P.L.A. (Email: Polly.Arnold@ed.ac.uk) or J.B.L. (Email: Jason.Love@ed.ac.uk).

Uranium occurs in the environment predominantly as the uranyl dication [UO₂]²⁺. Its solubility renders this species a problematic contaminant^{1, 2, 3} which is, moreover, chemically extraordinarily robust owing to strongly covalent U–O bonds⁴. This feature manifests itself in the uranyl dication showing little propensity to partake in the many oxo group functionalizations and redox reactions typically seen with [CrO₂]²⁺, [MoO₂]²⁺ and other transition metal analogues^{5, 6, 7, 8, 9}. As a result, only a few examples of [UO₂]²⁺ with functionalized oxo groups are known. Similarly, it is only very recently that the isolation and characterization of the singly reduced, pentavalent uranyl cation [UO₂]⁺ has been reported^{10, 11, 12}. Here we show that placing the uranyl dication within a rigid and well-defined molecular framework while keeping the environment anaerobic allows simultaneous single-electron reduction and selective covalent bond formation at one of the two uranyl oxo groups. The product of this reaction is a pentavalent and monofunctionalized [O = U^...OR]⁺ cation that can be isolated in the presence of transition metal cations. This finding demonstrates that under appropriate reaction conditions, the uranyl oxo group will readily undergo radical reactions commonly associated only with transition metal oxo groups. We expect that this work might also prove useful in probing the chemistry of the related but highly radioactive plutonyl and neptunyl analogues found in nuclear waste.

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