Oxidation Chemistry of Uranium(III) Complexes of Tpa: Synthesis and Structural Studies of Oxo, Hydroxo, and Alkoxo Complexes of Uranium(IV)

The crystal structure of the complex [U(tpa)2]I3, 1 (tpa = tris[(2-pyridyl)methyl]amine), has been elucidated. The
complex exists as only one enantiomer in the crystal leading to the chiral space group P212121. The coordination
geometry of the metal can be described as a distorted cube. Accidental oxidation of [U(tpa)2]I3 led to the isolation
of the unusual mononuclear bishydroxo complex of uranium(IV) [U(tpa)2(OH)2]I2·3CH3CN, 2, which was structurally
characterized. The controlled reaction of [U(tpa)2]I3 with water resulted in the oxidation of the metal center and led
to the formation of protonated tpa and of the trinuclear U(IV) oxo complex {[U(tpa)(μ-O)I]3(μ3-I)}I2, 3. The solid
state and solution structures of this trimer are reported. The pathway suggested for the formation of this complex
is the oxidation of the [U(tpa)2]I3 complex by H2O to form a U(IV) hydroxo complex which then decomposes,
eliminating mono-protonated tpa. The comparison with the reported reaction with water of cyclopentadienyl derivatives
points to a higher reactivity toward water reduction of the bis(tpa) complex with respect to the cyclopentadienyl
derivatives. The reaction of U(III) with methanol in the presence of the supporting ligand tpa leads to formation of
alkoxo complexes similarly to what is found for amide or cyclopentadienyl derivatives. The monomethoxide complex
[U(tpa)I3(OMe)], 4, has been prepared in good yield by alcoholysis of the U(III) mono(tpa) complex. The crystal
structure of this complex has been determined. The reaction of [U(tpa)2]I3 with 2 equiv of methanol in acetonitrile
allows the isolation of the bismethoxo complex of U(IV) [U(tpa)I2(OMe)2], 5, in 35−47% yield, which has been fully
characterized. To account for the oxidation of U(III) to U(IV) the suggested mechanism assumes that hydrogen is
evolved in both reactions.