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Fluorine is an essential component in many highly effective pharmaceutical drugs, however the selective fluorination of organic molecules poses a challenge. A common route to installing fluorine involves C-F bond cleavage, which is often accomplished using second- or third-row transition metals. Base metal catalysts such as nickel may provide a facile, sustainable, and cheaper alternative for C-F activation. Monodentate N-heterocyclic carbene (NHC) nickel complexes have been reported to undergo C-F activation, however bis-bidentate NHC (RNHC2R1; R, R1 = alkyl or aryl) analogs remain underexplored. This work reports a series of RNHC2R1 nickel(0) complexes with various R1 linkers to determine the effect of the linker on the C-F activation of hexafluorobenzene. Comparisons include a reference nickel(0) complex with two monodentate NHC ligands, and results show that low-valent nickel NHC complexes readily break the C-F bond in C6F6 via oxidative addition. Crystallographic and NMR characterization demonstrate that ligand design and denticity affect the cis versus trans orientation of the final product, with the possibility for additional ligand C-H activation. 

This work investigates possible mechanisms and intermediates in the reactivity of p-toluenesulfonyl fluoride with a new bidentate N-heterocyclic carbene (NHC) nickel complex, (MesNHC2oXy)Ni(COD) (Mes = 2,4,6-trimethylphenyl, oXy = ortho-xylyl, COD = cyclooctadiene). (MesNHC2oXy)Ni(COD) was synthesized from the corresponding bis(imidazolium) salt precursor, [MesNHC2oXy][Br]2, and both were structurally characterized. (MesNHC2oXy)Ni(COD) reacts with one equivalent of p-toluenesulfonyl fluoride to furnish (MesNHC2oXy)Ni(2-SO2), HF, and ½ equivalent of 4,4’-dimethylbiphenyl. (MesNHC2oXy)Ni(2-SO2) was structurally characterized and has a unique side-on SO2 coordination mode with a Ni-S-O angle of 60.49(5)°. DFT calculations of (MesNHC2oXy)Ni(2-SO2) are consistent with a Ni(II) center and an activated SO2 fragment. DFT calculations also support an initial oxidative addition at either the S-F or S-C positions, which have similar energetics.