Carbon–Fluorine Bond Activation of Tetrafluoroethylene on Palladium(0) and Nickel(0): Heat or Lewis Acidic Additive Promoted Oxidative Addition

The C–F bond cleavage reaction of tetrafluoroethylene (TFE; CF2CF2) with an M(0) complex (M = Pd, Ni) was investigated. The treatment of an M(0) precursor with TFE in the presence of the appropriate monodentate phosphine ligand led to a clean formation of the corresponding η2-TFE adduct (η2-TFE)­M­(PR3)2. In the case of the Ni(0) species, in particular, the choice of phosphine ligands is crucial for the preparation of the desired η2-TFE complex: the use of either PCy3 or PiPr3 resulted in the target adduct, while less sterically hindered phosphines such as PPh3 and PnBu3 gave the known octafluoronickelacyclopentane as a result of the oxidative cyclization of two TFE molecules. Thermolysis of both palladium and nickel η2-TFE adducts bearing PCy3 as the ligand resulted in a C–F bond activation reaction and gave the corresponding (trifluorovinyl)metal fluorides, trans-(PCy3)2M­(F)­(CFCF2). The reaction of (η2-TFE)­Pd­(PPh3)2 with LiI as an additive allowed cleavage of the C–F bond in THF, even at room temperature, and gave trans-(PPh3)2Pd­(I)­(CFCF2) with a concomitant formation of lithium fluoride. Other metal halides, such as MgBr2 and AlCl3, also promoted the C–F bond cleavage of TFE. In addition, the use of either BF3·Et2O or B­(C6F5)3 exerted a similar accelerative effect on the C–F bond activation of TFE on either nickel or palladium. The molecular structures of a series of η2-TFE and trifluorovinyl complexes were unambiguously determined by means of X-ray crystallography. The resultant (trifluorovinyl)palladium or -nickel species have shown the potential to utilize a key intermediate in cross-coupling reactions with organometallic reagents to prepare a variety of trifluorovinyl compounds.