Case Study of N-Pr-i 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)(mu-(eta(2) : eta(2))-COD)] B/ [Ni((i)Pr(2)Im(Me))(2)(eta(4)-COD)] B' or [Ni(Mes(2)Im)(2)] 2, respectively, with alkynes afforded complexes [Ni(NHC)(2)(eta(2)-alkyne)] (NHC=(i)Pr(2)Im(Me): alkyne=MeC equivalent to CMe 3, H7C3C equivalent to CC3H7 4, PhC equivalent to CPh 5, MeOOCC equivalent to CCOOMe 6, Me3SiC equivalent to CSiMe3 7, PhC equivalent to CMe 8, HC equivalent to CC3H7 9, HC equivalent to CPh 10, HC equivalent to C(p-Tol) 11, HC equivalent to C(4-Bu-t-C6H4) 12, HC equivalent to CCOOMe 13; NHC=Mes(2)Im: alkyne=MeC equivalent to CMe 14, MeOOCC equivalent to CCOOMe 15, PhC equivalent to CMe 16, HC equivalent to C(4-Bu-t-C6H4) 17, HC equivalent to CCOOMe 18). Unusual rearrangement products 11 a and 12 a were identified for the complexes of the terminal alkynes HC equivalent to C(p-Tol) and HC equivalent to C(4-Bu-t-C6H4), 11 and 12, which were formed by addition of a C-H bond of one of the NHC N-Pr-i methyl groups to the C equivalent to 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).