Hydrogen for fluorine exchange in C6F6 and C6F5H by monomeric [1,3,4-(Me3C)3C5H2]2CeH:: Experimental and computational studies

The net reaction of monomeric CP'2CeH [Cp' = 1,3,4-(Me3C)(3)(C5H2)] in C6D6 with C6F6 is CP'2CeF, H-2, and tetrafluorobenzyne. The pentafluoropheny/metallocene, CP'Ce-2(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] cycloadcluct. In C6F5H, the final products are also CP'2CeF and H-2, which are formed from the intermediates CP'Ce-2(C6F5) and CP'Ce-2(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 pentafluoropheny/metallocenes 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 eta(1)-F-C6F5 interaction, which serves as a hook. The reaction proceeds via a sigma 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.