Synthesis,Characterization, and Reactivity of a Uranium(VI) Carbene Imido Oxo Complex

We report the uranium(VI) carbene imido oxo complex [U(BIPM^TMS)(NMes)(O)(DMAP)₂] ( 5 , BIPM^TMS=C(PPh₂NSiMe₃)₂; Mes=2,4,6‐Me₃C₆H₂; DMAP=4‐(dimethylamino)pyridine) which exhibits the unprecedented arrangement of three formal multiply bonded ligands to one metal center where the coordinated heteroatoms derive from different element groups. This complex was prepared by incorporation of carbene, imido, and then oxo groups at the uranium center by salt elimination, protonolysis, and two‐electron oxidation, respectively. The oxo and imido groups adopt axial positions in a T‐shaped motif with respect to the carbene, which is consistent with an inverse trans‐influence. Complex 5 reacts with tert‐butylisocyanate at the imido rather than carbene group to afford the uranyl(VI) carbene complex [U(BIPM^TMS)(O)₂(DMAP)₂] ( 6 ).     

Two‐Electron Reductive Carbonylation of Terminal Uranium(V) and Uranium(VI) Nitrides to Cyanate by Carbon Monoxide

Two‐electron reductive carbonylation of the uranium(VI) nitride [U(Tren^TIPS)(N)] ( 2 , Tren^TIPS=N(CH₂CH₂NSiiPr₃)₃) with CO gave the uranium(IV) cyanate [U(Tren^TIPS)(NCO)] ( 3 ). KC₈ reduction of 3 resulted in cyanate dissociation to give [U(Tren^TIPS)] ( 4 ) and KNCO, or cyanate retention in [U(Tren^TIPS)(NCO)][K(B15C5)₂] ( 5 , B15C5=benzo‐15‐crown‐5 ether) with B15C5. Complexes 5 and 4 and KNCO were also prepared from CO and the uranium(V) nitride [{U(Tren^TIPS)(N)K}₂] ( 6 ), with or without B15C5, respectively. Complex 5 can be prepared directly from CO and [U(Tren^TIPS)(N)][K(B15C5)₂] ( 7 ). Notably, 7 reacts with CO much faster than 2 . This unprecedented f‐block reactivity was modeled theoretically, revealing nucleophilic attack of the π* orbital of CO by the nitride with activation energy barriers of 24.7 and 11.3 kcal mol^?1 for uranium(VI) and uranium(V), respectively. A remarkably simple two‐step, two‐electron cycle for the conversion of azide to nitride to cyanate using 4 , NaN₃ and CO is presented.     

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 U(IV)═C, U(V)═C, and U(VI)═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)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.     

An Inverted‐Sandwich Diuranium μ‐η5:η5‐Cyclo‐P5 Complex Supported by U‐P5 δ‐Bonding

Reaction of [U(Tren^TIPS)] [ 1 , Tren^TIPS=N(CH₂CH₂NSiiPr₃)₃] with 0.25 equivalents of P₄ reproducibly affords the unprecedented actinide inverted sandwich cyclo‐P₅ complex [{U(Tren^TIPS)}₂(μ‐η⁵:η⁵‐cyclo‐P₅)] ( 2 ). All prior examples of cyclo‐P₅ are stabilized by d‐block metals, so 2 shows that cyclo‐P₅ does not require d‐block ions to be prepared. Although cyclo‐P₅ is isolobal to cyclopentadienyl, which usually bonds to metals via σ‐ and π‐interactions with minimal δ‐bonding, theoretical calculations suggest the principal bonding in the U(P₅)U unit is polarized δ‐bonding. Surprisingly, the characterization data are overall consistent with charge transfer from uranium to the cyclo‐P₅ unit to give a cyclo‐P₅ charge state that approximates to a dianionic formulation. This is ascribed to the larger size and superior acceptor character of cyclo‐P₅ compared to cyclopentadienyl, the strongly reducing nature of uranium(III), and the availability of uranium δ‐symmetry 5f orbitals.     

Alkali-metal mediated reactivity of a diaminobromoborane: mono- and bis-borylation of naphthalene versus boryl lithium or hydroborane formation

Reaction of lithium with PDABBr [PDA = C(6)H(4)-1,2-(NTripp)(2), Tripp = 2,4,6-Pr(i)(3)C(6)H(2)] and naphthalene afforded 2- and 2,6-borylated naphthalenes; conversely, use of high-sodium lithium (0.5% Na) afforded the lithium boryl [(PDAB)Li(THF)(2)]; this work establishes that main group reagents can achieve selective borylations of fused polycyclic aromatics under mild conditions in good yields.     

A Very Short Uranium(IV)–Rhodium(I) Bond with Net Double‐Dative Bonding Character

Reaction of [U{C(SiMe₃)(PPh₂)}(BIPM)(μ‐Cl)Li(TMEDA)(μ‐TMEDA)_0.5]₂ (BIPM=C(PPh₂NSiMe₃)₂; TMEDA=Me₂NCH₂CH₂NMe₂) with [Rh(μ‐Cl)(COD)]₂ (COD=cyclooctadiene) affords the heterotrimetallic U^IV?Rh^I₂ complex [U(Cl)₂{C(PPh₂NSiMe₃)(PPh[C₆H₄]NSiMe₃)}{Rh(COD)}{Rh(CH(SiMe₃)(PPh₂)}]. This complex has a very short uranium–rhodium distance, the shortest uranium–rhodium bond on record and the shortest actinide–transition metal bond in terms of formal shortness ratio. Quantum‐chemical calculations reveal a remarkable Rh U^IV net double dative bond interaction, involving Rh^I 4d ‐ and 4d_xy/xz‐type donation into vacant U^IV 5f orbitals, resulting in a Wiberg/Nalewajski–Mrozek U?Rh bond order of 1.30/1.44, respectively. Despite being, formally, purely dative, the uranium–rhodium bonding interaction is the most substantial actinide–metal multiple bond yet prepared under conventional experimental conditions, as confirmed by structural, magnetic, and computational analyses.     

F-block N-heterocyclic carbene complexes

N-Heterocyclic carbenes (NHCs) can bind as two-electron sigma-donor ligands to lanthanide and actinide metal cations. In this review we summarise how the incorporation of an anionic group (alkoxide or amido), to form heterobidentate NHC ligands, allows the synthesis of a range of f-block NHC adducts. The tethering group also allows the lability of the NHC group, and its subsequent reactivity, to be studied. We include a brief survey of the known, structurally characterised f-element-NHC bond distances, and a range of substrates that react to displace the metal-bound NHC group.