Hydrogen for Fluorine Exchange in CH₄_-<i>_x</i>F<i>_x</i> by Monomeric [1,2,4-(Me₃C)₃C₅H₂]₂CeH: Experimental and Computational Studies

Abstract

The monomeric metallocenecerium hydride, Cp‘2CeH (Cp‘ = 1,2,4-tri-tert-butylcyclopentadienyl), reacts instantaneously with CH3F, but slower with CH2F2, to give Cp‘2CeF and CH4 in each case, a net H for F exchange reaction. The hydride reacts very slowly with CHF3, and not at all with CF4, to give Cp‘2CeF, H2, and 1,2,4- and 1,3,5-tri-tert-butylbenzene. The substituted benzenes are postulated to result from trapping of a fluorocarbene fragment derived by α-fluoride abstraction from Cp‘2CeCF3. The fluoroalkyl, Cp‘2CeCF3, is generated by reaction of Cp‘2CeH and Me3SiCF3 or by reaction of the metallacycle, [(Cp‘)(Me3C)2C5H2C(Me2)CH2]Ce, with CHF3, and its existence is inferred from the products of decomposition, which are Cp‘2CeF, the isomeric tri-tert-butylbenzenes and in the case of Me3SiCF3, Me3SiH. The fluoroalkyls, Cp‘2CeCH2F and Cp‘2CeCHF2, generated from the metallacycle and CH3F and CH2F2, respectively, are also inferred by their decomposition products, which are Cp‘2CeF, CH2, and CHF, respectively, which are trapped. DFT(B3PW91) calculations have been carried out to examine several reaction paths that involve CH and CF bond activation. The calculations show that the CH activation by Cp2CeH proceeds with a low barrier. The carbene ejection and trapping by H2 is the rate-determining step, and the barrier parallels that found for reaction of H2 with CH2, CHF, and CF2. The barrier of the rate-determining step is raised as the number of fluorines increases, while that of the CH activation path is lowered as the number of fluorines increases, which parallels the acidity.