Synthesis and Redox Chemistry of High-Valent Uranium Aryloxides

Alcoholysis of U((OBU)-B-t)(6) with 1 or 2 equiv of C6F5OH generates U((OBu)-Bu-t)(5)(OC6F5) (1) and U((OBU)-B-t)(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 E-1/2 = -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((OBu)-Bu-t)(6-n)(OC6F5)(n) (n = 3-6), did not generate the intended products. For instance, reaction of U((OBU)-B-t)(6) with 6 equiv of C6F5OH in CH2Cl2 results in the formation [Li((HOBu)-Bu-t)(2)][U(OC6F5)(6)] (3). The source of the lithium cation in 3 is likely Lil, which is present from the initial synthesis of the U(OtBu)(6). However, reaction of Lil-free U((OBu)-Bu-t)(6) with 6 equiv Of C6F5OH results in the formation of a uranyl complex, UO2(OC6F5)(2)((HOBu)-Bu-t)(2) (4), along with isobutylene and (BuOC6F5)-Bu-t. To probe the mechanism of this transformation, U((OBu)-Bu-t)(6) was reacted with (C6F5OH)-O-18 center dot 0.5DME. This produces UO2((OC6F5)-O-18)(2)(DME) (5-O-18) along with (BuOC6F5)-Bu-t-O-18 as determined by GUMS, which suggests that oxo formation only occurs by tert-butyl cation elimination and not aromatic nucleophilic substitution. Several other synthetic pathways to U-VI(OC6F5)(6) were also investigated. Thus, addition of 10 equiv of C6F5OH to [Li(THF)](2)[U((OBu)-Bu-t)(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(eta(2)-C7H8)(2)(DME)][U(OC6F5)(6)] (9), respectively. However, no evidence for the formation of U-VI(OC6F5)(6) was observed during these reactions.