Synthesis of a Uranium(VI)-Carbene: Reductive Formation of Uranyl(V)-Methanides, Oxidative Preparation of a [R2CUO]2+ Analogue of the [OUO]2+ Uranyl Ion (R = Ph2PNSiMe3), and Comparison of the Nature of UIVC, UVC, and UVIC Double Bonds

We report attempts to prepare uranyl­(VI)- and uranium­(VI) carbenes utilizing deprotonation and oxidation strategies. Treatment of the uranyl­(VI)-methanide complex [(BIPMH)­UO2Cl­(THF)] [1, BIPMH = HC­(PPh2NSiMe3)2] with benzyl-sodium did not afford a uranyl­(VI)-carbene via deprotonation. Instead, one-electron reduction and isolation of di- and trinuclear [UO2(BIPMH)­(μ-Cl)­UO­(μ-O)­{BIPMH}] (2) and [UO­(μ-O)­(BIPMH)­(μ3-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)­UCl3Li­(THF)2] (4) by 4-morpholine N-oxide afforded the first uranium­(VI)-carbene [(BIPM)­UOCl2] (6). Complex 6 exhibits a trans-CUO linkage that represents a [R2CUO]2+ analogue of the uranyl ion. Notably, treatment of 4 with other oxidants such as Me3NO, C5H5NO, and TEMPO afforded 1 as the only isolable product. Computational studies of 4, the uranium­(V)-carbene [(BIPM)­UCl2I] (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.