Computational study of the reaction of C₆F₆ with [IrMe(PEt₃)₃]: Identification of a phosphine-assisted C-F activation pathway via a metallophosphorane intermediate

Density functional theory calculations have been used to model the reaction of C₆F₆ with [IrMe(PEt₃)₃], which proceeds with both C-F and P-C bond activation to yield trans-[Ir(C ₆F₅)(PEt₃)₂(PEt₂F)], C₂H₄, and CH₄ (Blum, O.; Frolow, F.; Milstein, D. J. Chem. Soc., Chem. Commun. 1991, 258). Using a model species, trans-[IrMe(PH₃)₂(PH₂Et)], a low-energy mechanism involving nucleophilic attack of the electron-rich Ir metal center at C₆F₆ with displacement of fluoride has been identified. A novel feature of this process is the capture of fluoride by a phosphine ligand to generate a metallophosphorane intermediate [Ir(C₆F ₅)(Me)(PH₃)₂(PH₂EtF)]. These events occur in a single step via a 4-centered transition state, in a process that we have termed "phosphine-assisted C-F activation". Alternative mechanisms based on C-F activation via concerted oxidative addition or electron-transfer processes proved less favorable. From the metallophosphorane intermediate the formation of the final products can be accounted for by facile ethyl group transfer from phosphorus to iridium followed by β-H elimination of ethene and reductive elimination of methane. The interpretation of phosphine-assisted C-F activation in terms of nucleophilic attack is supported by the reduced activation barriers computed with the more electron-rich model reactant trans-[IrMe(PMe₃)₂(PMe₂Et)] and the higher barriers found with lesser fluorinated arenes. Reactivity patterns for a range of fluoroarenes indicate the dominance of the presence of ortho-F substituents in promoting phosphine-assisted C-F activation, and an analysis of the charge distribution and transition state geometries indicates that this process is controlled by the strength of the Ir-aryl bond that is being formed.