Synthesis, Structure, and Reactivity of bis(1,2,4-tri-t-butylcyclopentadienyl) Complexes of Cerium
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Synthesis, Structure, and Reactivity of bis(1,2,4-tri-t-butylcyclopentadienyl) Complexes of Cerium

  • Author(s): Werkema, Evan L.
  • et al.
Abstract

The sterically demanding 1,2,4-tri-t-butylcyclopentadienyl ligand (1,2,4-(Me3C)3C5H2, hereafter Cp') has been used to prepare monomeric cerium metallocenes, Cp 2CeX (X = Cl, I, OSO2CF3), which are used to synthesize the benzyl, Cp'2CeCH2C6H5. The benzyl is a useful starting material for preparing other complexes in the Cp'2CeZ system (Z = BF4, F, NH2, C6H5, H). X-ray crystal structures of Cp'2CeOSO2CF3, Cp'2CeF, Cp'2CeCH2C6H5, and Cp'2CeH are presented. The benzyl slowly decomposes in solution to toluene and a metallacycle, [Cp'][(Me3C)2C5H2(CMe2CH2)]Ce. The ring CMe3 groups of both the metallacycle and the hydride, Cp'2CeH, can be fully deuterated by prolonged exposure to C6D6, providing a useful labeling tool in mechanistic studies.The hydride activates C-F and/or C-H bonds in fluorobenzenes, C6HxF6-x , x = 0-5. The reactions are selective, with the selectivity depending on the presence of two fluorines ortho to the reaction site more than on the type of bond activated. Complexes of the type Cp'2CeC6HxF5-x , x = 0-4, are formed as intermediates, which slowly decompose in solution to Cp'2CeF and fluorobenzynes, C6HxF4-x, x = 0-4, which are trapped. The rate at which Cp'2CeC6HxF5-x complexes decompose increases as the number of fluorines decreases. Complexes with one ortho-fluorine decompose much faster than those with two ortho-fluorines. The metallacycle activates only C-H bonds in fluorobenzenes, permitting the synthesis of specific Cp'2CeC6HxF5-x complexes. The crystal structure of Cp'2CeC6F5 is presented. The hydride and the metallacycle react with fluoromethanes, CH4-xFx, x = 1-3, through postulated Cp'2CeCH3-xFx intermediates to generate Cp'2CeF and other products. The other products, CH4, tri-t-butylbenzenes, tri-t-butylfluorobenzenes, and a presumed metallocene cerium fluoride with one Cp' and one (Me2EtC)(Me3C)2C5H2 ligand, suggest a decomposition pathway for Cp'2CeCH3-xFx , x = 1-3, that involves carbenes or carbenoids, which are trapped. The hydride polymerizes ethylene, but hydrogenates other olefins. The metallacycle activates C-H bonds in olefins and aromatics to generate new complexes with Ce-C bonds. The hydride reacts with one equivalent of CO in pentane to generate (Cp'2Ce)2CH2O, whose crystal structure shows the presence of a bridging dianionic formaldehyde ligand. (Cp'2Ce)2CH2O reacts H2 to give the hydride and Cp'2CeOMe, or with a mixture of H2 and CO to generate Cp'2CeOMe exclusively. (Cp'2Ce)2CH2O or the hydride can react with an additional equivalent of CO to generate dimeric enediolate, (Cp'2CeCHO)2.

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