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( t Bu-MesPDIMe) (3- t Bu) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide; MesPDIMe = 2,6-((Mes)­N=CMe)2C5H3N; t Bu-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( t Bu-MesPDIMe)­(THF) (1- t Bu). These uranyl complexes contain singly reduced pyridine­(diimine) ligands suggesting formation occurs via cooperative ligand/metal oxidation. Treating 3 or 3- t Bu with stoichiometric equivalents of Me3SiI results in stepwise oxo silylation to form (Me3SiO)2UI2(MesPDIMe) (5) or (Me3SiO)­UI2( t Bu-MesPDIMe) (5- t Bu), 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.