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)]

A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC)(2)] (NHC=( i )Pr(2)Im(Me) 1(Me) , Mes(2)Im 2) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni(2)(( i )Pr(2)Im(Me))(4)(μ‐(η ( 2 ) : η(2))‐COD)] B/ [Ni(( i )Pr(2)Im(Me))(2)(η(4)‐COD)] B’ or [Ni(Mes(2)Im)(2)] 2, respectively, with alkynes afforded complexes [Ni(NHC)(2)(η(2)‐alkyne)] (NHC=( i )Pr(2)Im(Me): alkyne=MeC≡CMe 3, H(7)C(3)C≡CC(3)H(7) 4, PhC≡CPh 5, MeOOCC≡CCOOMe 6, Me(3)SiC≡CSiMe(3) 7, PhC≡CMe 8, HC≡CC(3)H(7) 9, HC≡CPh 10, HC≡C(p‐Tol) 11, HC≡C(4‐( t )Bu‐C(6)H(4)) 12, HC≡CCOOMe 13; NHC=Mes(2)Im: alkyne=MeC≡CMe 14, MeOOCC≡CCOOMe 15, PhC≡CMe 16, HC≡C(4‐( t )Bu‐C(6)H(4)) 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‐( t )Bu‐C(6)H(4)), 11 and 12, which were formed by addition of a C−H bond of one of the NHC N‐( i )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(Me) 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 1(M) (e) 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(NHC)−Ni−C(NHC) bite angle, and to less steric repulsion of the smaller NHC ( i )Pr(2)Im(Me).