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- Title
Case Study of N‐<sup>i</sup>Pr versus N‐Mes Substituted NHC Ligands in Nickel Chemistry: The Coordination and Cyclotrimerization of Alkynes at [Ni(NHC)<sub>2</sub>].
- Authors
Tendera, Lukas; Helm, Moritz; Krahfuss, Mirjam J.; Kuntze‐Fechner, Maximilian W.; Radius, Udo
- Abstract
A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC)2] (NHC=iPr2ImMe1Me, Mes2Im 2) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni2(iPr2ImMe)4(μ‐(η2 : η2)‐COD)] B/ [Ni(iPr2ImMe)2(η4‐COD)] B' or [Ni(Mes2Im)2] 2, respectively, with alkynes afforded complexes [Ni(NHC)2(η2‐alkyne)] (NHC=iPr2ImMe: alkyne=MeC≡CMe 3, H7C3C≡CC3H74, PhC≡CPh 5, MeOOCC≡CCOOMe 6, Me3SiC≡CSiMe37, PhC≡CMe 8, HC≡CC3H79, HC≡CPh 10, HC≡C(p‐Tol) 11, HC≡C(4‐tBu‐C6H4) 12, HC≡CCOOMe 13; NHC=Mes2Im: alkyne=MeC≡CMe 14, MeOOCC≡CCOOMe 15, PhC≡CMe 16, HC≡C(4‐tBu‐C6H4) 17, HC≡CCOOMe 18). Unusual rearrangement products 11 a and 12 a were identified for the complexes of the terminal alkynes HC≡C(p‐Tol) and HC≡C(4‐tBu‐C6H4), 11 and 12, which were formed by addition of a C−H bond of one of the NHC N‐iPr methyl groups to the C≡C triple bond of the coordinated alkyne. Complex 2 catalyzes the cyclotrimerization of 2‐butyne, 4‐octyne, diphenylacetylene, dimethyl acetylendicarboxylate, 1‐pentyne, phenylacetylene and methyl propiolate at ambient conditions, whereas 1Me is not a good catalyst. The reaction of 2 with 2‐butyne was monitored in some detail, which led to a mechanistic proposal for the cyclotrimerization at [Ni(NHC)2]. DFT calculations reveal that the differences between 1Me and 2 for alkyne cyclotrimerization lie in the energy profile of the initiation steps, which is very shallow for 2, and each step is associated with only a moderate energy change. The higher stability of 3 compared to 14 is attributed to a better electron transfer from the NHC to the metal to the alkyne ligand for the N‐alkyl substituted NHC, to enhanced Ni‐alkyne backbonding due to a smaller CNHC−Ni−CNHC bite angle, and to less steric repulsion of the smaller NHC iPr2ImMe.
- Subjects
COORDINATE covalent bond; ALKYNES; ETHYNYL benzene; METHYL groups; LIGANDS (Chemistry); CATALYSTS; SONOGASHIRA reaction
- Publication
Chemistry - A European Journal, 2021, Vol 27, Issue 71, p17849
- ISSN
0947-6539
- Publication type
Article
- DOI
10.1002/chem.202103093