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The Nature of the UC Double Bond: Pushing the Stability of High‐Oxidation‐State Uranium Carbenes to the Limit
Author(s) -
Cooper Oliver J.,
Mills David P.,
McMaster Jonathan,
Tuna Floriana,
McInnes Eric. J. L.,
Lewis William,
Blake Alexander J.,
Liddle Stephen T.
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201300071
Subject(s) - chemistry , medicinal chemistry , oxidation state , catalysis , organic chemistry
Treatment of [K(BIPM Mes H)] (BIPM Mes ={C(PPh 2 NMes) 2 } 2− ; Mes=C 6 H 2 ‐2,4,6‐Me 3 ) with [UCl 4 (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 (OEt 2 )(tmeda)] ( 4 ) (BIPM Dipp ={C(PPh 2 NDipp) 2 } 2− ; Dipp=C 6 H 3 ‐2,6‐ i Pr 2 ; tmeda= N , N , N ′, N ′‐tetramethylethylenediamine) was prepared from [Li 2 (BIPM Dipp )(tmeda)] and [UCl 4 (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) 2 Li(tmeda)] ( 7 ) and [{U(BIPM Dipp H)(O) 2 (μ‐Cl)} 2 ] ( 8 ). Treatment of 4 with t BuOLi (3 equiv) and I 2 (1 equiv) gives [U(BIPM Dipp )(O t Bu) 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 )(O t Bu) 3 ]. Complex 4 reacts with PhCO t Bu and Ph 2 CO to form [U(BIPM Dipp )(μ‐Cl) 4 (Li) 2 (tmeda)(OCPh t Bu)] ( 10 ) and [U(BIPM Dipp )(Cl)(μ‐Cl) 2 (Li)(tmeda)(OCPh 2 )] ( 11 ). In contrast, complex 5 does not react with PhCO t Bu and Ph 2 CO, which we attribute to steric blocking. However, complexes 5 and 6 react with PhCHO to afford (DippNPPh 2 ) 2 CC(H)Ph ( 12 ). Complex 9 does not react with PhCO t Bu, Ph 2 CO 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.