Towards Nickel–NHC Fluoro Complexes—Synthesis of Imidazolium Fluorides and Their Reactions with Nickelocene

While hundreds of complexes of the general formula [Ni(η⁵-C₅H₅)(NHC)(X)] exist (NHC = a N-heterocyclic carbene, X = Cl, Br, I), none is yet known with X = F. We attempted to prepare such a species by reacting nickelocene with imidazolium fluorides. Three imidazolium fluorides (ImH)⁺ F^- [Im = (<i>N</i>,<i>N</i>'-<i>bis-</i>(R)-imidazolium: <b>1a</b>, IMe, R = Me; <b>1b</b>, IMes, R = 2,4,6-trimethylphenyl; <b>1c</b>, IPr, R = 2,6-diisopropylphenyl)] were prepared and characterized by spectroscopic methods. In addition, the salts <b>1b</b> [(IMesH)⁺ F^-] and <b>1c</b> [(IPrH)⁺ F^-] were subjected single-crystal X-ray diffraction experiments. The reactions of these imidazolium fluorides with nickelocene did not lead to [Ni(η⁵-C₅H₅)(NHC)(F)] species. Instead, the reaction of <b>1a</b> [(IMeH)⁺ F^-] and <b>1b</b> [(IMesH)⁺ F^-] with nickelocene led to the salt <b>2</b> [Ni(η⁵-C₅H₅)(IMe)₂]⁺ F^- and to the square planar complex <b>3a</b><i>trans-</i>[NiF₂(IMes)₂] respectively. Both complexes were characterized spectroscopically and by single crystal X-ray diffraction. All four X-ray diffraction studies reveal hydrogen bonding and hydrogen interactions with the F atom or anion, and in some cases with solvent molecules of crystallization, and these phenomena are all discussed. Complex <b>2</b>, in particular, exhibited a wide range of interesting H-bonded interactions in the solid state. Complexes <b>2</b> and <b>3a</b> were tested as catalysts for Suzuki-Miyaura coupling but were not promising: complex <b>2</b> was inactive, and while <b>3a</b> did indeed catalyze the reaction, it gave widely diverging results owing to its instability in solution.