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

Two
uranium­(VI) uranyl compounds, Cp*UO2­(MesPDIMe) (3) and Cp*UO2(tBu-MesPDIMe)
(3-tBu) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide; MesPDIMe = 2,6-((Mes)­N=CMe)2C5H3N; tBu-MesPDIMe = 2,6-((Mes)­N=CMe)2-p-C­(CH3)3C5H2N; Mes = 2,4,6-trimethylphenyl),
have been synthesized by addition of N-methylmorpholine N-oxide to trianionic pyridine­(diimine) uranium­(IV) precursors,
Cp*U­(MesPDIMe)­(THF) (1), Cp*U­(MesPDIMe)­(HMPA) (1-HMPA), and Cp*U­(tBu-MesPDIMe)­(THF) (1-tBu). These uranyl complexes contain singly reduced pyridine­(diimine)
ligands suggesting formation occurs via cooperative ligand/metal oxidation.
Treating 3 or 3-tBu with stoichiometric equivalents of
Me3SiI results in stepwise oxo silylation to form (Me3SiO)2UI2(MesPDIMe) (5) or (Me3SiO)­UI2(tBu-MesPDIMe) (5-tBu), 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.