Case Study of N- i Pr versus N-Mes Substituted NHC Ligands in Nickel Chemistry: The Coordination and Cyclotrimerization of Alkynes at [Ni(NHC) 2 ]

• Published in: Chemistry : a European journal 2021-12, Vol.27 (71), p.17849-17861
• Main Authors: Tendera, Lukas, Helm, Moritz, Krahfuss, Mirjam J, Kuntze-Fechner, Maximilian W, Radius, Udo
• Format: Article
• Language: English
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202103093

A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC) ] (NHC= Pr Im 1 , Mes Im 2) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni ( Pr Im ) (μ-(η  : η )-COD)] B/ [Ni( Pr Im ) (η -COD)] B' or [Ni(Mes Im) ] 2, respectively, with alkynes afforded complexes [Ni(NHC) (η -alkyne)] (NHC= Pr Im : alkyne=MeC≡CMe 3, H C C≡CC H 4, PhC≡CPh 5, MeOOCC≡CCOOMe 6, Me SiC≡CSiMe 7, PhC≡CMe 8, HC≡CC H 9, HC≡CPh 10, HC≡C(p-Tol) 11, HC≡C(4- Bu-C H ) 12, HC≡CCOOMe 13; NHC=Mes Im: alkyne=MeC≡CMe 14, MeOOCC≡CCOOMe 15, PhC≡CMe 16, HC≡C(4- Bu-C H ) 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- Bu-C H ), 11 and 12, which were formed by addition of a C-H bond of one of the NHC N- Pr 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 1 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) ]. DFT calculations reveal that the differences between 1 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 C -Ni-C bite angle, and to less steric repulsion of the smaller NHC Pr Im .