Fluoride Abstraction and Reversible Photochemical Reduction of Cationic Uranyl(VI) Phosphine Oxide Complexes

The syntheses, structural and spectroscopic characterization, fluoride abstraction reactions, and photochemical reactivity of cationic uranyl(VI) phosphine oxide complexes are described. [UO₂(OPPh₃)₄][X]₂ (1a, X = OTf; 1b, X = BF₄) and [UO₂(dppmo)₂(OPPh₃)][X]₂ (2a, X = OTf; 2b, X = BF₄) are prepared from the corresponding uranyl(VI) chloride precursor and 2 equiv each of AgX and phosphine oxide. The BF₄^- compounds 1b and 2b are prone to fluoride abstraction reactions in methanol, leading to dinuclear fluoride-bridged uranyl(VI) complexes. Fluoride abstraction of 2b in methanol generates two structural isomers of the fluoride-bridged uranyl(VI) dimer [(UO₂(dppmo)₂)₂(μ-F)][BF₄]₃ (4), both of which have been structurally characterized. In the major isomer 4^C, the four dppmo ligands are all chelating, while in the minor isomer 4^B, two of the dppmo ligands bridge adjacent uranyl(VI) centers. Photolysis of 2b in methanol proceeds through 4 to form the uranium(IV) fluoride complex [UO₂F₂(dppmo)₃][BF₄]₂ (5), involving another fluoride abstraction step. X-ray crystallography shows 5 to be a rare example of a structurally characterized uranium(IV) complex possessing terminal U−F bonds. Complex 5 reverts to 4 in solution upon exposure to air.