Synthesis, Structure, and Solution Dynamics of Neutral Allylnickel Complexes of N-Heterocyclic Carbenes
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- 作者：Leonel C. Silva ; Pedro T. Gomes ; Luis F. Veiros ; Sofia I. Pascu ; M. Teresa Duarte ; Só ; nia Namorado ; José ; R. Ascenso ; Alberto R. Dias
- 刊名：Organometallics
- 出版年：2006
- 出版时间：August 28, 2006
- 年：2006
- 卷：25
- 期：18
- 页码：4391 - 4403
- 全文大小：306K
- 年卷期：v.25,no.18(August 28, 2006)
- ISSN：1520-6041
文摘
Neutral allylnickel complexes of the general formula [Ni(
3-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(3-C3H5)Br]2 with 2 equiv of thecorresponding free N-heterocyclic carbene (NHC). The halide exchange of Br- by I- can be easilyperformed, giving rise to compounds of the type [Ni(3-C3H5)(Im)I]. The new complexes [Ni(3-C3H5)(tBu2Im)Br] (1), [Ni(3-C3H5)(Me4Im)Br] (2), [Ni(3-C3H5)(Me2Im)Br] (3), [Ni(3-C3H5)(Me2Im)I] (4),and [Ni(3-C3H5)(Me4Im)I] (5) were obtained in good yields and were fully characterized by elementalanalysis and NMR spectroscopy. The X-ray crystal structures of 1, 2, 4, and 5 reveal a square-planargeometry at the nickel atom and a tilt angle of the NHC ring (in relation to the Ni square plane) dependenton the bulkiness of both the N substituents and the halogen bound to Ni. Variable-temperature NMRexperiments in solution show that compounds 1-5 are stereochemically nonrigid. Three simultaneousdynamic processes are observed with increasing temperature: (a) NHC rotation about the nickel-carbonbond, starting at lower temperatures (
G
= 14-18 kcal mol-1 for compounds 2-5), (b) allyl rotationabout the Ni-3-allyl axis, which is responsible for the cis-trans isomerization observed at intermediatetemperatures (G = 16.4 kcal mol-1 for compound 4), and (c) 
-
- allyl isomerization, occurringat higher temperatures. DFT calculations were performed in order to elucidate the possible mechanismsinvolved and suggest (1) NHC rotation is mainly controlled by steric factors imposed by the N-substituentgroups and to a lesser extent by the halogen and (2) there is a "spin-forbidden" mechanism for 3-allylrotation, involving spin singlet and triplet species. Thermodynamic activation parameters obtained byDFT agree well with the experimental values.
3-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(3-C3H5)Br]2 with 2 equiv of thecorresponding free N-heterocyclic carbene (NHC). The halide exchange of Br- by I- can be easilyperformed, giving rise to compounds of the type [Ni(3-C3H5)(Im)I]. The new complexes [Ni(3-C3H5)(tBu2Im)Br] (1), [Ni(3-C3H5)(Me4Im)Br] (2), [Ni(3-C3H5)(Me2Im)Br] (3), [Ni(3-C3H5)(Me2Im)I] (4),and [Ni(3-C3H5)(Me4Im)I] (5) were obtained in good yields and were fully characterized by elementalanalysis and NMR spectroscopy. The X-ray crystal structures of 1, 2, 4, and 5 reveal a square-planargeometry at the nickel atom and a tilt angle of the NHC ring (in relation to the Ni square plane) dependenton the bulkiness of both the N substituents and the halogen bound to Ni. Variable-temperature NMRexperiments in solution show that compounds 1-5 are stereochemically nonrigid. Three simultaneousdynamic processes are observed with increasing temperature: (a) NHC rotation about the nickel-carbonbond, starting at lower temperatures (G = 14-18 kcal mol-1 for compounds 2-5), (b) allyl rotationabout the Ni-3-allyl axis, which is responsible for the cis-trans isomerization observed at intermediatetemperatures (G = 16.4 kcal mol-1 for compound 4), and (c) -- allyl isomerization, occurringat higher temperatures. DFT calculations were performed in order to elucidate the possible mechanismsinvolved and suggest (1) NHC rotation is mainly controlled by steric factors imposed by the N-substituentgroups and to a lesser extent by the halogen and (2) there is a "spin-forbidden" mechanism for 3-allylrotation, involving spin singlet and triplet species. Thermodynamic activation parameters obtained byDFT agree well with the experimental values.