Hydrogen for Fluorine Exchange in C6F6and C6F5H by Monomeric [1,3,4-(Me3C)3C5H2]2CeH: Experimental and Computational Studies

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Abstract
The net reaction of monomeric Cp‘2CeH [Cp‘ = 1,3,4-(Me3C)3(C5H2)] in C6D6 with C6F6 is Cp‘2CeF, H2, and tetrafluorobenzyne. The pentafluorophenylmetallocene, Cp‘2Ce(C6F5), is formed as an intermediate that decomposes slowly to Cp‘2CeF and C6F4 (tetrafluorobenzyne), and the latter is trapped by the solvent C6D6 as a [2+4] cycloadduct. In C6F5H, the final products are also Cp‘2CeF and H2, which are formed from the intermediates Cp‘2Ce(C6F5) and Cp‘2Ce(2,3,5,6-C6F4H) and from an unidentified metallocene of cerium and the [2+4] cycloadducts of tetra- and trifluorobenzyne with C6D6. The hydride, fluoride, and pentafluorophenylmetallocenes are isolated and characterized by X-ray crystallography. DFT(B3PW91) calculations have been used to explore the pathways leading to the observed products of the exergonic reactions. A key step is a H/F exchange reaction which transforms C6F6 and the cerium hydride into C6F5H and Cp‘2CeF. This reaction starts by an η1-F−C6F5 interaction, which serves as a hook. The reaction proceeds via a σ bond metathesis where the fluorine ortho to the hook migrates toward H with a relatively low activation energy. All products observed experimentally are accommodated by pathways that involve C−F and C−H bond cleavages.