Characterization of Intermediates in the C−F Activation of Tetrafluorobenzenes using a Reactive Ni(PEt3)2 Synthon: Combined Computational and Experimental Investigation

The reaction of the anthracene adduct (PEt3)2Ni(η2-C14H10) with 1,2,3,4-tetrafluorobenzene revealed NMR spectroscopic evidence for the immediate equilibrium production of the dinuclear adduct [(PEt3)2Ni]2(μ-η2:η2-C6F4H2) and mononuclear adducts (PEt3)2Ni(η2-C6F4H2). Similarly, the reaction of (PEt3)2Ni(η2-C14H10) with 1,2,4,5- and 1,2,3,5-tetrafluorobenzene revealed NMR spectroscopic evidence for the equilibrium production of the dinuclear adducts and mononuclear adducts and also the C−H activation products (PEt3)2NiH(C6F4H). The conversions to both the adducts and C−H activation products were too minimal to allow for isolation, and the low concentrations hampered complete NMR spectroscopic characterization. The reaction of the readily prepared (PEt3)2NiMe2 with isobutene allowed for the in situ generation of (PEt3)2Ni(η2-H2CCMe2) as a more reactive Ni(PEt3)2 synthon. The reaction of (PEt3)2Ni(η2-H2CCMe2) with tetrafluorobenzenes also generates small equilibrium concentrations of adducts and C−H activation products, but the equilibrium can be driven by the removal of isobutene. The dinuclear adducts [(PEt3)2Ni]2(μ-η2:η2-1,2,3,4-C6F4H2) and [(PEt3)2Ni]2(μ-η2:η2-1,2,4,5-C6F4H2) were isolated and fully characterized by X-ray crystallography. The isolation of the dinuclear adduct of 1,2,3,5-tetrafluorobenzene was hampered by its rapid C−F activation and the generation of the less soluble aryne adduct [(PEt3)2Ni]2(μ-η2:η2-3,4,6-C6F3H), which was fully characterized. The spectroscopic characterization of all the tetrafluorobenzene mononuclear and dinuclear adducts as well as C−H activation products was accomplished by variable-temperature NMR spectroscopy and aided by DFT calculations on the proposed intermediates. The observation of equilibrium concentrations of the intermediates in solution allowed for the determination of the thermodynamic parameters for their interconversion.