The Nature of the UC Double Bond: Pushing the Stability of High‐Oxidation‐State Uranium Carbenes to the Limit |
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| Authors: | Oliver J Cooper Dr David P Mills Dr Jonathan McMaster Dr Floriana Tuna Prof Eric J L McInnes Dr William Lewis Prof Alexander J Blake Dr Stephen T Liddle |
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| Institution: | 1. School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD (UK);2. EPSRC National UK EPR Facility, School of Chemistry and Photon Science Institute, The University of Manchester, Oxford Road, Manchester M13 9PL (UK) |
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| Abstract: | Treatment of K(BIPMMesH)] (BIPMMes={C(PPh2NMes)2}2?; Mes=C6H2‐2,4,6‐Me3) with UCl4(thf)3] (1 equiv) 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)(μ‐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 (0.5 equiv) afforded U(BIPMDipp)(Cl)2(μ‐Cl)2(Li)(thf)2] ( 6 ). Complex 6 resists oxidation, and treating 4 or 5 with N‐oxides gives {U(BIPMDippH)(O)2‐ (μ‐Cl)2Li(tmeda)] ( 7 ) and {U(BIPMDippH)(O)2(μ‐Cl)}2] ( 8 ). Treatment of 4 with tBuOLi (3 equiv) and I2 (1 equiv) gives U(BIPMDipp)(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(BIPMDipp)(OtBu)3]. Complex 4 reacts with PhCOtBu and Ph2CO to form U(BIPMDipp)(μ‐Cl)4(Li)2(tmeda)(OCPhtBu)] ( 10 ) and U(BIPMDipp)(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. |
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| Keywords: | bond theory carbenes oxidation transition metals uranium |
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