We report attempts to prepare uranyl(VI)- and uranium(VI) carbenes utilizing deprotonation and oxidation strategies. Treatment of the uranyl(VI)-methanide complex [(BIPMH)UO(2)Cl(THF)] [1, BIPMH = HC(PPh(2)NSiMe(3))(2)] with benzyl-sodium did not afford a uranyl(VI)-carbene via deprotonation. Instead, one-electron reduction and isolation of di- and trinuclear [UO(2)(BIPMH)(μ-Cl)UO(μ-O){BIPMH}] (2) and [UO(μ-O)(BIPMH)(μ(3)-Cl){UO(μ-O)(BIPMH)}(2)] (3), respectively, with concomitant elimination of dibenzyl, was observed. Complexes 2 and 3 represent the first examples of organometallic uranyl(V), and 3 is notable for exhibiting rare cation-cation interactions between uranyl(VI) and uranyl(V) groups. In contrast, two-electron oxidation of the uranium(IV)-carbene [(BIPM)UCl(3)Li(THF)(2)] (4) by 4-morpholine N-oxide afforded the first uranium(VI)-carbene [(BIPM)UOCl(2)] (6). Complex 6 exhibits a trans-CUO linkage that represents a [R(2)C═U═O](2+) analogue of the uranyl ion. Notably, treatment of 4 with other oxidants such as Me(3)NO, C(5)H(5)NO, and TEMPO afforded 1 as the only isolable product. Computational studies of 4, the uranium(V)-carbene [(BIPM)UCl(2)I] (5), and 6 reveal polarized covalent U═C double bonds in each case whose nature is significantly affected by the oxidation state of uranium. Natural Bond Order analyses indicate that upon oxidation from uranium(IV) to (V) to (VI) the uranium contribution to the U═C σ-bond can increase from ca. 18 to 32% and within this component the orbital composition is dominated by 5f character. For the corresponding U═C π-components, the uranium contribution increases from ca. 18 to 26% but then decreases to ca. 24% and is again dominated by 5f contributions. The calculations suggest that as a function of increasing oxidation state of uranium the radial contraction of the valence 5f and 6d orbitals of uranium may outweigh the increased polarizing power of uranium in 6 compared to 5.

Reaction of [YI(3)(THF)(3.5)] with three equivalents of [KBz] (Bz = CH(2)C(6)H(5)) affords the tri-benzyl complex [Y(Bz)(3)(THF)(3)] () in excellent yield. Complex reacts with H(2)C(PPh(2)NSiMe(3))(2) (H(2)BIPM) to afford the yttrium-alkyl-carbene complex [Y(BIPM)(Bz)(THF)] (, BIPM = {C(PPh(2)NSiMe(3))(2)}). Compound reacts with one equivalent of benzophenone to give the alkoxy 1,2-migratory insertion product [Y(BIPM)(OCPh(2)Bz)(THF)] () rather than the alkene Wittig-product Ph(2)C[double bond, length as m-dash]C(PPh(2)NSiMe(3))(2). Reaction of with one or more equivalents of benzophenone does not afford any detectable alkene products, rather it apparently catalyses rearrangement of monomeric to afford dimeric [{Y(micro-BIPM)(OCPh(2)Bz)}(2)] (). Investigations reveal that formation of is proportional to the amount of benzophenone added, but the benzophenone is recovered at the end of the reaction. Reaction of with diphenyldiazene affords the 1,2-migratory insertion product [Y(BIPM){N(Ph)N(Ph)(Bz)}(THF)] () Compounds , , , , and have been variously characterised by X-ray crystallography, multi-nuclear NMR spectroscopy, FTIR spectroscopy, and CHN micro-analyses. Density functional theory calculations on reveal the HOMO to be localised at the Y-C(alkyl) bond which is commensurate with the observed reactivity.

The metal-mediated redox transformation of CO2 in mild conditions is an area of great current interest. The role of cooperativity between a reduced metal center and a Lewis acid center in small-molecule activation is increasingly recognized, but has not so far been investigated for f-elements. Here we show that the presence of potassium at a U, K site supported by sterically demanding tris(tert-butoxy)siloxide ligands induces a large cooperative effect in the reduction of CO2. Specifically, the ion pair complex [K(18c6)][U(OSi(O(t)Bu)3)4], 1, promotes the selective reductive disproportionation of CO2 to yield CO and the mononuclear uranium(IV) carbonate complex [U(OSi(O(t)Bu)3)4(μ-κ(2):κ(1)-CO3)K2(18c6)], 4. In contrast, the heterobimetallic complex [U(OSi(O(t)Bu)3)4K], 2, promotes the potassium-assisted two-electron reductive cleavage of CO2, yielding CO and the U(V) terminal oxo complex [UO(OSi(O(t)Bu)3)4K], 3, thus providing a remarkable example of two-electron transfer in U(III) chemistry. DFT studies support the presence of a cooperative effect of the two metal centers in the transformation of CO2.