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

Please always quote using this URN: urn:nbn:de:bvb:20-opus-257137
• 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$$^{Me}$$: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$$^{Me}$$ 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$$^{Me}$$.