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: [LMeFe(μ-F)]2 (12) and LtBuFeF (2) (L = bulky β-diketiminate). These complexes react with donor molecules (L‘), yielding trigonal-pyramidal complexes LRFeF(L‘). The fluoride ligand is activated by the Lewis acid Et2O·BF3, forming LtBuFe(OEt2)(η1-BF4) (3), and is also silaphilic, reacting with silyl compounds such as Me3SiSSiMe3, Me3SiCCSiMe3, and Et3SiH to give new thiolate LtBuFeSSiMe3 (4), acetylide LtBuFeCCSiMe3 (5), and hydride [LMeFe(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.