Synthesis and Redox Chemistry of High-Valent Uranium Aryloxides

2009-04-06T00:00:00Z (GMT) by Skye Fortier Guang Wu Trevor W. Hayton
Alcoholysis of U(O<sup>t</sup>Bu)<sub>6</sub> with 1 or 2 equiv of C<sub>6</sub>F<sub>5</sub>OH generates U(O<sup>t</sup>Bu)<sub>5</sub>(OC<sub>6</sub>F<sub>5</sub>) (<b>1</b>) and U(O<sup>t</sup>Bu)<sub>4</sub>(OC<sub>6</sub>F<sub>5</sub>)<sub>2</sub> (<b>2</b>) in 70% and 65% yields, respectively. Complexes <b>1</b> and <b>2</b> have been fully characterized, and their solution redox properties have been determined by cyclic voltammetry. Complex <b>1</b> exhibits a reversible reduction feature at <i>E</i><sub>1/2</sub> = −0.60 V (vs [Cp<sub>2</sub>Fe]<sup>0/+</sup>), while <b>2</b> exhibits a reversible reduction feature at −0.24 V (vs [Cp<sub>2</sub>Fe]<sup>0/+</sup>). Attempts to isolate the other <i>tert-</i>butoxide/pentafluorophenoxide complexes, U(O<sup>t</sup>Bu)<sub>6-<i>n</i></sub>(OC<sub>6</sub>F<sub>5</sub>)<sub><i>n</i></sub> (<i>n</i> = 3−6), did not generate the intended products. For instance, reaction of U(O<sup>t</sup>Bu)<sub>6</sub> with 6 equiv of C<sub>6</sub>F<sub>5</sub>OH in CH<sub>2</sub>Cl<sub>2</sub> results in the formation [Li(HO<sup>t</sup>Bu)<sub>2</sub>][U(OC<sub>6</sub>F<sub>5</sub>)<sub>6</sub>] (<b>3</b>). The source of the lithium cation in <b>3</b> is likely LiI, which is present from the initial synthesis of the U(O<sup>t</sup>Bu)<sub>6</sub>. However, reaction of LiI-free U(O<sup>t</sup>Bu)<sub>6</sub> with 6 equiv of C<sub>6</sub>F<sub>5</sub>OH results in the formation of a uranyl complex, UO<sub>2</sub>(OC<sub>6</sub>F<sub>5</sub>)<sub>2</sub>(HO<sup>t</sup>Bu)<sub>2</sub> (<b>4</b>), along with isobutylene and <sup>t</sup>BuOC<sub>6</sub>F<sub>5</sub>. To probe the mechanism of this transformation, U(O<sup>t</sup>Bu)<sub>6</sub> was reacted with C<sub>6</sub>F<sub>5</sub><sup>18</sup>OH·0.5DME. This produces UO<sub>2</sub>(<sup>18</sup>OC<sub>6</sub>F<sub>5</sub>)<sub>2</sub>(DME) (<b>5-</b><sup><b>18</b></sup><b>O</b>) along with <sup>t</sup>Bu<sup>18</sup>OC<sub>6</sub>F<sub>5</sub> as determined by GC/MS, which suggests that oxo formation only occurs by <i>tert</i>-butyl cation elimination and not aromatic nucleophilic substitution. Several other synthetic pathways to U<sup>VI</sup>(OC<sub>6</sub>F<sub>5</sub>)<sub>6</sub> were also investigated. Thus, addition of 10 equiv of C<sub>6</sub>F<sub>5</sub>OH to [Li(THF)]<sub>2</sub>[U(O<sup>t</sup>Bu)<sub>6</sub>] in Et<sub>2</sub>O followed by addition of DME results in the formation of [Li(DME)<sub>3</sub>]<sub>2</sub>[U(OC<sub>6</sub>F<sub>5</sub>)<sub>6</sub>] (<b>7</b>). Oxidation of <b>7</b> with 2 equiv of AgOTf in CH<sub>2</sub>Cl<sub>2</sub> or toluene generates [Li(DME)<sub>3</sub>][U(OC<sub>6</sub>F<sub>5</sub>)<sub>6</sub>] (<b>8</b>) or [Ag(η<sup>2</sup>-C<sub>7</sub>H<sub>8</sub>)<sub>2</sub>(DME)][U(OC<sub>6</sub>F<sub>5</sub>)<sub>6</sub>] (<b>9</b>), respectively. However, no evidence for the formation of U<sup>VI</sup>(OC<sub>6</sub>F<sub>5</sub>)<sub>6</sub> was observed during these reactions.