Uranium-Carbene-Imido Metalla-Allenes: Ancillary-Ligand-Controlled cis-/trans-Isomerisation and Assessment of trans Influence in the R2 C=U(IV) =NR' Unit (R=Ph2 PNSiMe3 ; R'=CPh3 ).

Uranium(IV)-carbene-imido complexes [U(BIPM(TMS) )(NCPh3 )(κ(2) -N,N'-BIPY)] (2; BIPM(TMS) =C(PPh2 NSiMe3 )2 ; BIPY=2,2-bipyridine) and [U(BIPM(TMS) )(NCPh3 )(DMAP)2 ] (3; DMAP=4-dimethylamino-pyridine) that contain unprecedented, discrete R2 C=U=NR' units are reported. These complexes complete the family of E=U=E (E=CR2 , NR, O) metalla-allenes with feasible first-row hetero-element combinations. Intriguingly, 2 and 3 contain cis- and trans-C=U=N units, respectively, representing rare examples of controllable cis/trans isomerisation in f-block chemistry. This work reveals a clear-cut example of the trans influence in a mid-valent uranium system, and thus a strong preference for the cis isomer, which is computed in a co-ligand-free truncated model-to isolate the electronic trans influence from steric contributions-to be more stable than the trans isomer by approximately 12 kJ mol(-1) with an isomerisation barrier of approximately 14 kJ mol(-1) .

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(PPh2 NSiMe3)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).

The nature of the U=C double bond: pushing the stability of high-oxidation-state uranium carbenes to the limit.

Treatment of [K(BIPM(Mes)H)] (BIPM(Mes)={C(PPh2NMes)2}(2−); Mes=C6H2-2,4,6-Me3) with [UCl4(thf)3] (1 equiv) afforded [U(BIPM(Mes)H)(Cl)3(thf)] (1), which generated [U(BIPM(Mes))(Cl)2(thf)2] (2), following treatment with benzyl potassium. Attempts to oxidise 2 resulted in intractable mixtures, ligand scrambling to give [U(BIPM(Mes))2] or the formation of [U(BIPM(Mes)H)(O)2(Cl)(thf)] (3). The complex [U(BIPM(Dipp))(µ-Cl)4(Li)2(OEt2)(tmeda)] (4) (BIPM(Dipp)={C(PPh2NDipp)2}(2−); Dipp=C6H3-2,6-iPr2; tmeda=N,N,N',N'-tetramethylethylenediamine) was prepared from [Li2(BIPM(Dipp))(tmeda)] and [UCl4(thf)3] and, following reflux in toluene, could be isolated as [U(BIPM(Dipp))(Cl)2(thf)2] (5). Treatment of 4 with iodine (0.5 equiv) afforded [U(BIPM(Dipp))(Cl)2(µ-Cl)2(Li)(thf)2] (6). Complex 6 resists oxidation, and treating 4 or 5 with N-oxides gives [{U(BIPM(Dipp)H)(O)2- (µ-Cl)2Li(tmeda)] (7) and [{U(BIPM(Dipp)H)(O)2(µ-Cl)}2] (8). Treatment of 4 with tBuOLi (3 equiv) and I2 (1 equiv) gives [U(BIPM(Dipp))(OtBu)3(I)] (9), which represents an exceptionally rare example of a crystallographically authenticated uranium(VI)­carbon σ bond. Although 9 appears sterically saturated, it decomposes over time to give [U(BIPM(Dipp))(OtBu)3]. Complex 4 reacts with PhCOtBu and Ph2CO to form [U(BIPM(Dipp))(µ-Cl)4(Li)2(tmeda)(OCPhtBu)] (10) and [U(BIPM(Dipp))(Cl)(µ-Cl)2(Li)(tmeda)(OCPh2)] (11). In contrast, complex 5 does not react with PhCOtBu and Ph2CO, which we attribute to steric blocking. However, complexes 5 and 6 react with PhCHO to afford (DippNPPh2)2C=C(H)Ph (12). Complex 9 does not react with PhCOtBu, Ph2CO or PhCHO; this is attributed to steric blocking. Theoretical calculations have enabled a qualitative bracketing of the extent of covalency in early-metal carbenes as a function of metal, oxidation state and the number of phosphanyl substituents, revealing modest covalent contributions to U=C double bonds.

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.

Reactivity of the uranium(IV) carbene complex [U(BIPM(TMS))(Cl)(µ-Cl)2Li(THF)2] (BIPM(TMS) = {C(PPh2NSiMe3)2}) towards carbonyl and heteroallene substrates: metallo-Wittig, adduct formation, C-F bond activation, and [2 + 2]-cycloaddition reactions.

The reactivity of the uranium(IV) carbene complex [U(BIPM(TMS))(Cl)(µ-Cl)2Li(THF)2] (1, BIPM(TMS) = {C(PPh2NSiMe3)2}) towards carbonyl and heteroallene substrates is reported. Reaction of 1 with benzophenone proceeds to give the metallo-Wittig terminal alkene product Ph2C=C(PPh2NSiMe3)2 (2); the likely "UOCl2" byproduct could not be isolated. Addition of the bulky ketone PhCOBu(t) to 1 resulted in loss of LiCl, coordination of the ketone, and dimerisation to give [U(BIPM(TMS))(Cl)(µ-Cl){OC(Ph)(Bu(t))}]2 (3). The reaction of 1 with coumarin resulted in ring opening of the cyclic ester and a metallo-Wittig-type reaction to afford [U{BIPM(TMS)[C(O)(CHCHC6H4O-2)]-κ(3)-N,O,O'}(Cl)2(THF)] (4) where the enolate product remains coordinated to uranium. The reaction of PhCOF with 1 resulted in C-F bond activation and oxidation resulting in isolation of [U(O)2(Cl)2(µ-Cl)2{(µ-LiDME)OC(Ph)=C(PPh2NSiMe3)(PPh2NHSiMe3)}2] (5) along with [U(Cl)2(F)2(py)4] (6). The reactions of 1 with tert-butylisocyanate or dicyclohexylcarbodiimide resulted in the isolation of the [2 + 2]-cycloaddition products [U{BIPM(TMS)[C(NBu(t)){OLi(THF)2(µ-Cl)Li(THF)3}]-κ(4)-C,N,N',N''}(Cl)3] (7) and [U{BIPM(TMS)[C(NCy)2]-κ(4)-C,N,N',N''}(Cl)(µ-Cl)2Li(THF)2] (8). Complexes 2-8 have been variously characterised by single crystal X-ray diffraction, multi-nuclear NMR and FTIR spectroscopies, Evans method solution magnetic moments, variable temperature SQUID magnetometry, and elemental analyses.

Synthesis and structure of [U{C(PPh2NMes)2}2] (Mes = 2,4,6-Me3C6H2): A homoleptic uranium bis(carbene) complex with two formal U=C double bonds.

Treatment of H2C(PPh2NMes)2 (1, Mes = 2,4,6-Me3C6H2) with two equivalents of ButLi afforded the methandiide complex [Li2{C(PPh2NMes)2}2]2 (2); reaction of 2 with [UI3(THF)4] gave [U{C(PPh2NMes)2}2] (3), which is the first homoleptic uranium bis(carbene) complex with two formal U=C double bonds.

Synthesis and reactivity of the yttrium-alkyl-carbene complex [Y(BIPM)(CH(2)C(6)H(5))(THF)] (BIPM = {C(PPh(2)NSiMe(3))(2)}).

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.