Synthesis,
Characterization, and Stoichiometric U–O
Bond Scission in Uranyl Species Supported by Pyridine(diimine) Ligand
Radicals
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Abstract
Two
uranium(VI) uranyl compounds, Cp*UO<sub>2</sub>(<sup>Mes</sup>PDI<sup>Me</sup>) (<b>3</b>) and Cp*UO<sub>2</sub>(<sup><i>t</i></sup>Bu-<sup>Mes</sup>PDI<sup>Me</sup>)
(<b>3-</b><sup><i><b>t</b></i></sup><b>Bu</b>) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide; <sup>Mes</sup>PDI<sup>Me</sup> = 2,6-((Mes)N=CMe)<sub>2</sub>C<sub>5</sub>H<sub>3</sub>N; <sup><i>t</i></sup>Bu-<sup>Mes</sup>PDI<sup>Me</sup> = 2,6-((Mes)N=CMe)<sub>2</sub>-<i>p</i>-C(CH<sub>3</sub>)<sub>3</sub>C<sub>5</sub>H<sub>2</sub>N; 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(<sup>Mes</sup>PDI<sup>Me</sup>)(THF) (<b>1</b>), Cp*U(<sup>Mes</sup>PDI<sup>Me</sup>)(HMPA) (<b>1-HMPA</b>), and Cp*U(<sup><i>t</i></sup>Bu-<sup>Mes</sup>PDI<sup>Me</sup>)(THF) (<b>1-</b><sup><i><b>t</b></i></sup><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><sup><i><b>t</b></i></sup><b>Bu</b> with stoichiometric equivalents of
Me<sub>3</sub>SiI results in stepwise oxo silylation to form (Me<sub>3</sub>SiO)<sub>2</sub>UI<sub>2</sub>(<sup>Mes</sup>PDI<sup>Me</sup>) (<b>5</b>) or (Me<sub>3</sub>SiO)UI<sub>2</sub>(<sup><i>t</i></sup>Bu-<sup>Mes</sup>PDI<sup>Me</sup>) (<b>5-</b><sup><i><b>t</b></i></sup><b>Bu</b>), respectively.
Additional equivalents result in full uranium–oxo bond scission
and formation of UI<sub>4</sub>(1,4-dioxane)<sub>2</sub> 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
