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 = (N,N'-bis-(R)-imidazolium: 1a, IMe, R = Me; 1b, IMes, R = 2,4,6-trimethylphenyl; 1c, IPr, R = 2,6-diisopropylphenyl)] were prepared and characterized by spectroscopic methods. In addition, the salts 1b [(IMesH)⁺ F^-] and 1c [(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 1a [(IMeH)⁺ F^-] and 1b [(IMesH)⁺ F^-] with nickelocene led to the salt 2 [Ni(η⁵-C₅H₅)(IMe)₂]⁺ F^- and to the square planar complex 3atrans-[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 2, in particular, exhibited a wide range of interesting H-bonded interactions in the solid state. Complexes 2 and 3a were tested as catalysts for Suzuki-Miyaura coupling but were not promising: complex 2 was inactive, and while 3a did indeed catalyze the reaction, it gave widely diverging results owing to its instability in solution.