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(OtBu)6 with 1 or 2 equiv of C6F5OH generates U(OtBu)5(OC6F5) (1) and U(OtBu)4(OC6F5)2 (2) in 70% and 65% yields, respectively. Complexes 1 and 2 have been fully characterized, and their solution redox properties have been determined by cyclic voltammetry. Complex 1 exhibits a reversible reduction feature at E1/2 = −0.60 V (vs [Cp2Fe]0/+), while 2 exhibits a reversible reduction feature at −0.24 V (vs [Cp2Fe]0/+). Attempts to isolate the other tert-butoxide/pentafluorophenoxide complexes, U(OtBu)6-n(OC6F5)n (n = 3−6), did not generate the intended products. For instance, reaction of U(OtBu)6 with 6 equiv of C6F5OH in CH2Cl2 results in the formation [Li(HOtBu)2][U(OC6F5)6] (3). The source of the lithium cation in 3 is likely LiI, which is present from the initial synthesis of the U(OtBu)6. However, reaction of LiI-free U(OtBu)6 with 6 equiv of C6F5OH results in the formation of a uranyl complex, UO2(OC6F5)2(HOtBu)2 (4), along with isobutylene and tBuOC6F5. To probe the mechanism of this transformation, U(OtBu)6 was reacted with C6F518OH·0.5DME. This produces UO2(18OC6F5)2(DME) (5-18O) along with tBu18OC6F5 as determined by GC/MS, which suggests that oxo formation only occurs by tert-butyl cation elimination and not aromatic nucleophilic substitution. Several other synthetic pathways to UVI(OC6F5)6 were also investigated. Thus, addition of 10 equiv of C6F5OH to [Li(THF)]2[U(OtBu)6] in Et2O followed by addition of DME results in the formation of [Li(DME)3]2[U(OC6F5)6] (7). Oxidation of 7 with 2 equiv of AgOTf in CH2Cl2 or toluene generates [Li(DME)3][U(OC6F5)6] (8) or [Ag(η2-C7H8)2(DME)][U(OC6F5)6] (9), respectively. However, no evidence for the formation of UVI(OC6F5)6 was observed during these reactions.