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) .