Tuning Uranium–Nitrogen Multiple Bond Formation with Ancillary Siloxide Ligands

The
new homoleptic ate U­(III) siloxide [K­(18c6)]­[U­(OSi­(OtBu)3)4] 2 was prepared in 69% yield by reduction of [U­(OSi­(OtBu)3)4] 3 with KC8. The reaction of the neutral U­(III) siloxide complex [U­(OSi­(OtBu)3)2(μ-OSi­(OtBu)3)]2 1 with adamantyl azide leads to the isolation of the dinuclear
U­(VI) imido complex [U2(NAd)4(OSi­(OtBu)3)4] 4. The
X-ray crystal structure shows the presence of a “cation–cation
interaction” between the two [U­(NAd)2]2+ groups. In contrast the reactions of 2 with the trimethylsilyl
and adamantyl azides afford the U­(V) imido complexes [K­(18c6)]­[U­(NSiMe3)­(OSi­(OtBu)3)4] 5-TMS and [K­(18c6)]­[U­(NAd)­(OSi­(OtBu)3)4] 5-Ad pure in 48%
and 66% yield, respectively. The reaction of 2 with CsN3 in THF at −40
°C yields a mixture of products from which the azido
U­(IV) complex [K­(18c6)]­[U­(N3)­(OSi­(OtBu)3)4] 7 and the μ-nitrido
diuranium­(V) complex [KU­(μ-N)­(OSi­(OtBu)3)]2 8 were isolated. The crystal
structure
of 8 shows the presence of a rare U2N2 core with two nitrido atoms bridging two uranium centers
in a diamond-shaped geometry. In contrast, the reaction of 1 with CsN3 affords the diuranium­(IV) complex Cs­{(μ-N)­[U­(OSi­(OtBu)3)3]2} 9 presenting a nitrido ligand bridging two uranium
and one cesium cations. These results show the importance of the coordination
environment in the outcome of the reaction of U­(III) with azides.