Synthesis, Characterization, and Stoichiometric U–O Bond Scission in Uranyl Species Supported by Pyridine(diimine) Ligand Radicals

Two uranium­(VI) uranyl compounds, Cp*UO₂­(^MesPDI^Me) (<b>3</b>) and Cp*UO₂(^<i>t</i>Bu-^MesPDI^Me) (<b>3-</b>^{<i><b>t</b></i>}<b>Bu</b>) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide; ^MesPDI^Me = 2,6-((Mes)­N=CMe)₂C₅H₃N; ^<i>t</i>Bu-^MesPDI^Me = 2,6-((Mes)­N=CMe)₂-<i>p</i>-C­(CH₃)₃C₅H₂N; Mes = 2,4,6-trimethylphenyl), have been synthesized by addition of <i>N</i>-methylmorpholine <i>N</i>-oxide to trianionic pyridine­(diimine) uranium­(IV) precursors, Cp*U­(^MesPDI^Me)­(THF) (<b>1</b>), Cp*U­(^MesPDI^Me)­(HMPA) (<b>1-HMPA</b>), and Cp*U­(^<i>t</i>Bu-^MesPDI^Me)­(THF) (<b>1-</b>^{<i><b>t</b></i>}<b>Bu</b>). These uranyl complexes contain singly reduced pyridine­(diimine) ligands suggesting formation occurs via cooperative ligand/metal oxidation. Treating <b>3</b> or <b>3-</b>^{<i><b>t</b></i>}<b>Bu</b> with stoichiometric equivalents of Me₃SiI results in stepwise oxo silylation to form (Me₃SiO)₂UI₂(^MesPDI^Me) (<b>5</b>) or (Me₃SiO)­UI₂(^<i>t</i>Bu-^MesPDI^Me) (<b>5-</b>^{<i><b>t</b></i>}<b>Bu</b>), respectively. Additional equivalents result in full uranium–oxo bond scission and formation of UI₄(1,4-dioxane)₂ with extrusion of hexamethyldisiloxane. The uranium complexes have been characterized via multinuclear NMR, vibrational, and electronic absorption spectroscopies and, in some cases, X-ray crystallography