Synthesis, structure, and solution dynamics of neutral allylnickel complexes of N-heterocyclic carbenes

L C Silva, P T Gomes, L F Veiros, Sofia I Pascu, M T Duarte, S Namorado, J R Ascenso, A R Dias

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Abstract

Neutral allylnickel complexes of the general formula [Ni(eta(3)-C3H5)(Im)Br] (where Im = 1,3-di-tert-butylimidazol-2- ylidene ((t)Bu(2)Im), 1,3,4,5-tetramethylimidazol-2-ylidene (Me(4)Im), 1,3-dimethylimidazol-2-ylidine (Me(2)Im)) were prepared from the reaction of the dimer [Ni(eta(3)-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(eta(3)-C3H5)(Im)I]. The new complexes [Ni(eta(3)-C3H5)((t)Bu(2)Im)Br] (1), [Ni(eta(3)-C3H5)(Me(4)Im)Br] (2), [Ni(eta(3)-C3H5)(Me(2)Im)Br] (3), [Ni(eta(3)-C3H5)(Me(2)Im)I] (4), and [Ni(eta(3)-C3H5)(Me(4)Im)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 (Delta G(#) = 14-18 kcal mol(-1) for compounds 2-5), (b) allyl rotation about the Ni-eta(3)-allyl axis, which is responsible for the cis-trans isomerization observed at intermediate temperatures (Delta G(#) = 16.4 kcal mol(-1) for compound 4), and (c) pi-sigma-pi 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 eta(3)-allyl rotation, involving spin singlet and triplet species. Thermodynamic activation parameters obtained by DFT agree well with the experimental values.
Original languageEnglish
Pages (from-to)4391-4403
Number of pages13
JournalOrganometallics
Volume25
Issue number18
DOIs
Publication statusPublished - 2006

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carbenes
synthesis
Halogens
halogens
isomerization
Isomerization
Nickel
Discrete Fourier transforms
nickel
Temperature
nuclear magnetic resonance
halides
temperature
Dimers
dimers
Nuclear magnetic resonance spectroscopy
activation
Carbon
thermodynamics
Crystal structure

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Silva, L. C., Gomes, P. T., Veiros, L. F., Pascu, S. I., Duarte, M. T., Namorado, S., ... Dias, A. R. (2006). Synthesis, structure, and solution dynamics of neutral allylnickel complexes of N-heterocyclic carbenes. Organometallics, 25(18), 4391-4403. https://doi.org/10.1021/om060444p

Synthesis, structure, and solution dynamics of neutral allylnickel complexes of N-heterocyclic carbenes. / Silva, L C; Gomes, P T; Veiros, L F; Pascu, Sofia I; Duarte, M T; Namorado, S; Ascenso, J R; Dias, A R.

In: Organometallics, Vol. 25, No. 18, 2006, p. 4391-4403.

Research output: Contribution to journalArticle

Silva, LC, Gomes, PT, Veiros, LF, Pascu, SI, Duarte, MT, Namorado, S, Ascenso, JR & Dias, AR 2006, 'Synthesis, structure, and solution dynamics of neutral allylnickel complexes of N-heterocyclic carbenes', Organometallics, vol. 25, no. 18, pp. 4391-4403. https://doi.org/10.1021/om060444p
Silva, L C ; Gomes, P T ; Veiros, L F ; Pascu, Sofia I ; Duarte, M T ; Namorado, S ; Ascenso, J R ; Dias, A R. / Synthesis, structure, and solution dynamics of neutral allylnickel complexes of N-heterocyclic carbenes. In: Organometallics. 2006 ; Vol. 25, No. 18. pp. 4391-4403.
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abstract = "Neutral allylnickel complexes of the general formula [Ni(eta(3)-C3H5)(Im)Br] (where Im = 1,3-di-tert-butylimidazol-2- ylidene ((t)Bu(2)Im), 1,3,4,5-tetramethylimidazol-2-ylidene (Me(4)Im), 1,3-dimethylimidazol-2-ylidine (Me(2)Im)) were prepared from the reaction of the dimer [Ni(eta(3)-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(eta(3)-C3H5)(Im)I]. The new complexes [Ni(eta(3)-C3H5)((t)Bu(2)Im)Br] (1), [Ni(eta(3)-C3H5)(Me(4)Im)Br] (2), [Ni(eta(3)-C3H5)(Me(2)Im)Br] (3), [Ni(eta(3)-C3H5)(Me(2)Im)I] (4), and [Ni(eta(3)-C3H5)(Me(4)Im)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 (Delta G(#) = 14-18 kcal mol(-1) for compounds 2-5), (b) allyl rotation about the Ni-eta(3)-allyl axis, which is responsible for the cis-trans isomerization observed at intermediate temperatures (Delta G(#) = 16.4 kcal mol(-1) for compound 4), and (c) pi-sigma-pi 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 eta(3)-allyl rotation, involving spin singlet and triplet species. Thermodynamic activation parameters obtained by DFT agree well with the experimental values.",
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T1 - Synthesis, structure, and solution dynamics of neutral allylnickel complexes of N-heterocyclic carbenes

AU - Silva, L C

AU - Gomes, P T

AU - Veiros, L F

AU - Pascu, Sofia I

AU - Duarte, M T

AU - Namorado, S

AU - Ascenso, J R

AU - Dias, A R

PY - 2006

Y1 - 2006

N2 - Neutral allylnickel complexes of the general formula [Ni(eta(3)-C3H5)(Im)Br] (where Im = 1,3-di-tert-butylimidazol-2- ylidene ((t)Bu(2)Im), 1,3,4,5-tetramethylimidazol-2-ylidene (Me(4)Im), 1,3-dimethylimidazol-2-ylidine (Me(2)Im)) were prepared from the reaction of the dimer [Ni(eta(3)-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(eta(3)-C3H5)(Im)I]. The new complexes [Ni(eta(3)-C3H5)((t)Bu(2)Im)Br] (1), [Ni(eta(3)-C3H5)(Me(4)Im)Br] (2), [Ni(eta(3)-C3H5)(Me(2)Im)Br] (3), [Ni(eta(3)-C3H5)(Me(2)Im)I] (4), and [Ni(eta(3)-C3H5)(Me(4)Im)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 (Delta G(#) = 14-18 kcal mol(-1) for compounds 2-5), (b) allyl rotation about the Ni-eta(3)-allyl axis, which is responsible for the cis-trans isomerization observed at intermediate temperatures (Delta G(#) = 16.4 kcal mol(-1) for compound 4), and (c) pi-sigma-pi 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 eta(3)-allyl rotation, involving spin singlet and triplet species. Thermodynamic activation parameters obtained by DFT agree well with the experimental values.

AB - Neutral allylnickel complexes of the general formula [Ni(eta(3)-C3H5)(Im)Br] (where Im = 1,3-di-tert-butylimidazol-2- ylidene ((t)Bu(2)Im), 1,3,4,5-tetramethylimidazol-2-ylidene (Me(4)Im), 1,3-dimethylimidazol-2-ylidine (Me(2)Im)) were prepared from the reaction of the dimer [Ni(eta(3)-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(eta(3)-C3H5)(Im)I]. The new complexes [Ni(eta(3)-C3H5)((t)Bu(2)Im)Br] (1), [Ni(eta(3)-C3H5)(Me(4)Im)Br] (2), [Ni(eta(3)-C3H5)(Me(2)Im)Br] (3), [Ni(eta(3)-C3H5)(Me(2)Im)I] (4), and [Ni(eta(3)-C3H5)(Me(4)Im)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 (Delta G(#) = 14-18 kcal mol(-1) for compounds 2-5), (b) allyl rotation about the Ni-eta(3)-allyl axis, which is responsible for the cis-trans isomerization observed at intermediate temperatures (Delta G(#) = 16.4 kcal mol(-1) for compound 4), and (c) pi-sigma-pi 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 eta(3)-allyl rotation, involving spin singlet and triplet species. Thermodynamic activation parameters obtained by DFT agree well with the experimental values.

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