Classically, late transition-metal organometallic compounds promote multielectron processes solely through the change in oxidation state of the metal centre. In contrast, uranium typically undergoes single-electron chemistry. However, using redox-active ligands can engage multielectron reactivity at this metal in analogy to transition metals. Here we show that a redox-flexible pyridine(diimine) ligand can stabilize a series of highly reduced uranium coordination complexes by storing one, two or three electrons in the ligand. These species reduce organoazides easily to form uranium-nitrogen multiple bonds with the release of dinitrogen. The extent of ligand reduction dictates the formation of uranium mono-, bis- and tris(imido) products. Spectroscopic and structural characterization of these compounds supports the idea that electrons are stored in the ligand framework and used in subsequent reactivity. Computational analyses of the uranium imido products probed their molecular and electronic structures, which facilitated a comparison between the bonding in the tris(imido) structure and its tris(oxo) analogue.

Manipulation of steric crowding in organoactinide complexes has been explored by examining the insertion chemistry of carbodiimides, RNCNR, and organic azides, RN3, with actinide alkyl, alkynyl, and aryl complexes. iPrNCNiPr reacts with (C5Me5)2AnMe2 to produce the isomorphous methyl amidinates (C5Me5)2AnMe[(iPr)NC(Me)N(iPr)-κ2N,N′], An = Th, 1; U, 2, in high yield. The reaction of iPrNCNiPr with (C5Me5)2U(CCPh)2 forms a similar insertion product, (C5Me5)2U(CCPh)[(iPr)NC(CCPh)N(iPr)-κ2N,N′], 3. (C5Me5)2U(C6H5)2 does not generate an analogous product with iPrNCNiPr, but forms instead a complex formally derived from carbodiimide insertion into a “(C5Me5)2U(C6H4)” intermediate, (C5Me5)2U[(iPr)NCN(iPr)(C6H4)-κN,κC], 4. Adamantyl azide, AdN3, inserts into the An−Me bonds in the (C5Me5)2AnMe2 complexes to make monomethyl actinide triazenido complexes that differ in the mode of triazenido coordination: (C5Me5)2ThMe[(Me)NNN(Ad)-κ2N1,2], 5, and (C5Me5)2UMe[(Me)NNN(Ad)-κ 2N1,3], 6. A κ2N1,3-triazenido complex of thorium was also isolated in a crystal comprised of a mixture of (C5Me5)2ThMe[(Me)NNN(Ad)-κ2N1,3] and (C5Me5)2Th(OH)[(Me)NNN(Ad)-κ2N1,3], 7.

Oxidation of [Li(DME)(3)][U(CH(2)SiMe(3))(5)] with 0.5 equiv of I(2), followed by immediate addition of LiCH(2)SiMe(3), affords the high-valent homoleptic U(V) alkyl complex [Li(THF)(4)][U(CH(2)SiMe(3))(6)] (1) in 82% yield. In the solid-state, 1 adopts an octahedral geometry as shown by X-ray crystallographic analysis. Addition of 2 equiv of tert-butanol to [Li(DME)(3)][U(CH(2)SiMe(3))(5)] generates the heteroleptic U(IV) complex [Li(DME)(3)][U(O(t)Bu)(2)(CH(2)SiMe(3))(3)] (2) in high yield. Treatment of 2 with AgOTf fails to produce a U(V) derivative, but instead affords the U(IV) complex (Me(3)SiCH(2))Ag(μ-CH(2)SiMe(3))U(CH(2)SiMe(3))(O(t)Bu)(2)(DME) (3) in 64% yield. Complex 3 has been characterized by X-ray crystallography and is marked by a uranium-silver bond. In contrast, oxidation of 2 can be achieved via reaction with 0.5 equiv of Me(3)NO, producing the heteroleptic U(V) complex [Li(DME)(3)][U(O(t)Bu)(2)(CH(2)SiMe(3))(4)] (4) in moderate yield. We have also attempted the one-electron oxidation of complex 1. Thus, oxidation of 1 with U(O(t)Bu)(6) results in formation of a rare U(VI) alkyl complex, U(CH(2)SiMe(3))(6) (6), which is only stable below -25 °C. Additionally, the electronic properties of 1-4 have been assessed by SQUID magnetometry, while a DFT analysis of complexes 1 and 6 is also provided.

Addition of the Wittig reagent Ph(3)P═CH(2) to the U(III) tris(amide) U(NR(2))(3) (R = SiMe(3)) generates a mixture of products from which the U(IV) complex U═CHPPh(3)(NR(2))(3) (2) can be obtained. Complex 2 features a short U═C bond and represents a rare example of a uranium carbene. In solution, 2 exists in equilibrium with the U(IV) metallacycle U(CH(2)SiMe(2)NR)(NR(2))(2) and free Ph(3)P═CH(2). Measurement of this equilibrium as a function of temperature provides ΔH(rxn) = 11 kcal/mol and ΔS(rxn) = 31 eu. Additionally, the electronic structure of the U═C bond was investigated using DFT analysis.