Synthesis, Structure, and Solution Dynamics of Neutral Allylnickel Complexes of N-Heterocyclic Carbenes

Neutral allylnickel complexes of the general formula [Ni(η3-C3H5)(Im)Br] (where Im = 1,3-di-tert-butylimidazol-2-ylidene (tBu2Im), 1,3,4,5-tetramethylimidazol-2-ylidene (Me4Im), 1,3-dimethylimidazol-2-ylidine (Me2Im)) were prepared from the reaction of the dimer [Ni(η3-C3H5)Br]2 with 2 equiv of the
corresponding free N-heterocyclic carbene (NHC). The halide exchange of Br- by I- can be easily
performed, giving rise to compounds of the type [Ni(η3-C3H5)(Im)I]. The new complexes [Ni(η3-C3H5)
(tBu2Im)Br] (1), [Ni(η3-C3H5)(Me4Im)Br] (2), [Ni(η3-C3H5)(Me2Im)Br] (3), [Ni(η3-C3H5)(Me2Im)I] (4),
and [Ni(η3-C3H5)(Me4Im)I] (5) were obtained in good yields and were fully characterized by elemental
analysis and NMR spectroscopy. The X-ray crystal structures of 1, 2, 4, and 5 reveal a square-planar
geometry at the nickel atom and a tilt angle of the NHC ring (in relation to the Ni square plane) dependent
on the bulkiness of both the N substituents and the halogen bound to Ni. Variable-temperature NMR
experiments in solution show that compounds 1−5 are stereochemically nonrigid. Three simultaneous
dynamic processes are observed with increasing temperature:  (a) NHC rotation about the nickel−carbon
bond, starting at lower temperatures (ΔG⧧ = 14−18 kcal mol-1 for compounds 2−5), (b) allyl rotation
about the Ni−η3-allyl axis, which is responsible for the cis−trans isomerization observed at intermediate
temperatures (ΔG⧧ = 16.4 kcal mol-1 for compound 4), and (c) π−σ−π allyl isomerization, occurring
at higher temperatures. DFT calculations were performed in order to elucidate the possible mechanisms
involved and suggest (1) NHC rotation is mainly controlled by steric factors imposed by the N-substituent
groups and to a lesser extent by the halogen and (2) there is a “spin-forbidden” mechanism for η3-allyl
rotation, involving spin singlet and triplet species. Thermodynamic activation parameters obtained by
DFT agree well with the experimental values.