Tuning Uranium–Nitrogen Multiple Bond Formation with Ancillary Siloxide Ligands

The new homoleptic ate U(III) siloxide [K(18c6)][U(OSi(O(t)Bu)3)4] 2 was prepared in 69% yield by reduction of [U(OSi(O(t)Bu)3)4] 3 with KC8. The reaction of the neutral U(III) siloxide complex [U(OSi(O(t)Bu)3)2(μ-OSi(O(t)Bu)3)]2 1 with adamantyl azide leads to the isolation of the dinuclear U(VI) imido complex [U2(NAd)4(OSi(O(t)Bu)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(O(t)Bu)3)4] 5-TMS and [K(18c6)][U(NAd)(OSi(O(t)Bu)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(O(t)Bu)3)4] 7 and the μ-nitrido diuranium(V) complex [KU(μ-N)(OSi(O(t)Bu)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(O(t)Bu)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.