Synthesis and Reactivity of Low-Coordinate Iron(II) Fluoride Complexes and Their Use in the Catalytic Hydrodefluorination of Fluorocarbons

Transition metal fluoride complexes are of interest because they are potentially useful in a multitude of catalytic applications, including C−F bond activation and fluorocarbon functionalization. We report the first crystallographically characterized examples of molecular iron(II) fluorides:  [L^MeFe(μ-F)]₂ (1₂) and L^tBuFeF (2) (L = bulky β-diketiminate). These complexes react with donor molecules (L‘), yielding trigonal-pyramidal complexes L^RFeF(L‘). The fluoride ligand is activated by the Lewis acid Et₂O·BF₃, forming L^tBuFe(OEt₂)(η¹-BF₄) (3), and is also silaphilic, reacting with silyl compounds such as Me₃SiSSiMe₃, Me₃SiCCSiMe₃, and Et₃SiH to give new thiolate L^tBuFeSSiMe₃ (4), acetylide L^tBuFeCCSiMe₃ (5), and hydride [L^MeFe(μ-H)]₂ (6₂) complexes. The hydrodefluorination (HDF) of perfluorinated aromatic compounds (hexafluorobenzene, pentafluoropyridine, and octafluorotoluene) with a silane R₃SiH (R₃ = (EtO)₃, Et₃, Ph₃, (3,5-(CF₃)₂C₆H₃)Me₂) is catalyzed by addition of an iron(II) fluoride complex, giving mainly the singly hydrodefluorinated products (pentafluorobenzene, 2,3,5,6-tetrafluoropyridine, and α,α,α,2,3,5,6-heptafluorotoluene, respectively) in up to five turnovers. These catalytic perfluoroarene HDF reactions proceed with activation of the C−F bond para to the most electron-withdrawing group and are dependent on the degree of fluorination and solvent polarity. Kinetic studies suggest that hydride generation is the rate-limiting step in the HDF of octafluorotoluene, but the active intermediate is unknown. Mechanistic considerations argue against oxidative addition and outer-sphere electron transfer pathways for perfluoroarene HDF. Fluorinated olefins are also hydrodefluorinated (up to 10 turnovers for hexafluoropropene), most likely through a hydride insertion/β-fluoride elimination mechanism. Complexes 1₂ and 2 thus provide a rare example of a homogeneous system that activates C−F bonds without competitive C−H activation and use an inexpensive 3d transition metal.