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

Abstract

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.