The Nature of the UC Double Bond: Pushing the Stability of High-Oxidation-State Uranium Carbenes to the Limit

Treatment of [K(BIPMMesH)] (BIPMMes={C(PPh2NMes)2}2; Mes=C6H2-2,4,6-Me3) with [UCl4(thf)3] (1equiv) afforded [U(BIPMMesH)(Cl)3(thf)] (1), which generated [U(BIPMMes)(Cl)2(thf)2] (2), following treatment with benzyl potassium. Attempts to oxidise 2 resulted in intractable mixtures, ligand scrambling to give [U(BIPMMes)2] or the formation of [U(BIPMMesH)(O)2(Cl)(thf)] (3). The complex [U(BIPMDipp)(-Cl)4(Li)2(OEt2)(tmeda)] (4) (BIPMDipp={C(PPh2NDipp)2}2; Dipp=C6H3-2,6-iPr2; tmeda=N,N,N,N-tetramethylethylenediamine) was prepared from [Li2(BIPMDipp)(tmeda)] and [UCl4(thf)3] and, following reflux in toluene, could be isolated as [U(BIPMDipp)(Cl)2(thf)2] (5). Treatment of 4 with iodine (0.5equiv) afforded [U(BIPMDipp)(Cl)2(-Cl)2(Li)(thf)2] (6). Complex 6 resists oxidation, and treating 4 or 5 with N-oxides gives [{U(BIPMDippH)(O)2- (-Cl)2Li(tmeda)] (7) and [{U(BIPMDippH)(O)2(-Cl)}2] (8). Treatment of 4 with tBuOLi (3equiv) and I2 (1equiv) gives [U(BIPMDipp)(OtBu)3(I)] (9), which represents an exceptionally rare example of a crystallographically authenticated uranium(VI)carbon sigma bond. Although 9 appears sterically saturated, it decomposes over time to give [U(BIPMDipp)(OtBu)3]. Complex 4 reacts with PhCOtBu and Ph2CO to form [U(BIPMDipp)(-Cl)4(Li)2(tmeda)(OCPhtBu)] (10) and [U(BIPMDipp)(Cl)(-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)2CC(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 UC double bonds.