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

The sterically demanding 1,2,4-tri-t-butylcyclopentadienylligand (1,2,4-(Me3C)3C5H2, hereafter Cp') has been used to preparemonomeric cerium metallocenes, Cp 2CeX (X = Cl, I, OSO2CF3), which areused to synthesize the benzyl, Cp'2CeCH2C6H5. The benzyl is a usefulstarting 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 slowlydecomposes in solution to toluene and a metallacycle,[Cp'][(Me3C)2C5H2(CMe2CH2)]Ce. The ring CMe3 groups of both themetallacycle and the hydride, Cp'2CeH, can be fully deuterated byprolonged exposure to C6D6, providing a useful labeling tool inmechanistic studies.The hydride activates C-F and/or C-H bonds influorobenzenes, C6HxF6-x , x = 0-5. The reactions are selective, with theselectivity depending on the presence of two fluorines ortho to thereaction site more than on the type of bond activated. Complexes of thetype Cp'2CeC6HxF5-x , x = 0-4, are formed as intermediates, which slowlydecompose in solution to Cp'2CeF and fluorobenzynes, C6HxF4-x, x = 0-4,which are trapped. The rate at which Cp'2CeC6HxF5-x complexes decomposeincreases as the number of fluorines decreases. Complexes with oneortho-fluorine decompose much faster than those with two ortho-fluorines.The metallacycle activates only C-H bonds in fluorobenzenes, permittingthe synthesis of specific Cp'2CeC6HxF5-x complexes. The crystal structureof Cp'2CeC6F5 is presented. The hydride and the metallacycle react withfluoromethanes, CH4-xFx, x = 1-3, through postulated Cp'2CeCH3-xFxintermediates to generate Cp'2CeF and other products. The other products,CH4, tri-t-butylbenzenes, tri-t-butylfluorobenzenes, and a presumedmetallocene cerium fluoride with one Cp' and one (Me2EtC)(Me3C)2C5H2ligand, suggest a decomposition pathway for Cp'2CeCH3-xFx , x = 1-3, thatinvolves carbenes or carbenoids, which are trapped. The hydridepolymerizes ethylene, but hydrogenates other olefins. The metallacycleactivates C-H bonds in olefins and aromatics to generate new complexeswith Ce-C bonds. The hydride reacts with one equivalent of CO in pentaneto generate (Cp'2Ce)2CH2O, whose crystal structure shows the presence ofa bridging dianionic formaldehyde ligand. (Cp'2Ce)2CH2O reacts H2 to givethe hydride and Cp'2CeOMe, or with a mixture of H2 and CO to generateCp'2CeOMe exclusively. (Cp'2Ce)2CH2O or the hydride can react with anadditional equivalent of CO to generate dimeric enediolate,(Cp'2CeCHO)2