Large vs. Small NHC Ligands in Nickel(0) Complexes: The Coordination of Olefins, Ketones and Aldehydes at [Ni(NHC)2]

Investigations concerning the reactivity of Ni(0) complexes [Ni(NHC)\\(_{2}\\)] of NHCs (N‐heterocyclic carbene) of different steric demand, Mes\\(_{2}\\)Im (= 1,3‐dimesitylimidazoline‐2‐ylidene) and iPr\\(_{2}\\)Im (= 1,3‐diisopropyl‐imidazoline‐2‐ylidene), with olefins, ketones and aldehydes are reported. The reaction of [Ni(Mes\\(_{2}\\)Im)\\(_{2}\\)] 1 with ethylene or methyl acrylate afforded the complexes [Ni(Mes\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐C\\(_{2}\\)H\\(_{4}\\))] 3 and [Ni(Mes\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐(C,C)‐H\\(_{2}\\)C=CHCOOMe)] 4, as it was previously reported for [Ni\\(_{2}\\)(iPr\\(_{2}\\)Im)\\(_{4}\\)(µ‐(η\\(^{2}\\):η\\(^{2}\\))‐COD)] 2 as a source for [Ni(iPr\\(_{2}\\)Im)\\(_{2}\\)]. In contrast to 2, complex 1 does not react with sterically more demanding olefins such as tetramethylethylene, 1,1‐diphenylethylene and cyclohexene. The reaction of [Ni(NHC)\\(_{2}\\)] with more π‐acidic ketones or aldehydes led to formation of complexes with side‐on η\\(^{2}\\)‐(C,O)‐coordinating ligands: [Ni(iPr\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=CH\\(^{t}\\)Bu)] 5, [Ni(iPr\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=CHPh)] 6, [Ni(iPr\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=CMePh)] 7, [Ni(iPr\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=CPh\\(_{2}\\))] 8, [Ni(iPr\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=C(4‐F‐C\\(_{6}\\)H\\(_{4}\\))\\(_{2}\\))] 9, [Ni(iPr\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=C(OMe)(CF\\(_{3}\\)))] 10 and [Ni(Mes\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=CHPh)] 11, [Ni(Mes\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=CH(CH(CH\\(_{3}\\))\\(_{2}\\)))] 12, [Ni(Mes\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=CH(4‐NMe\\(_{2}\\)‐C\\(_{6}\\)H\\(_{4}\\)))] 13, [Ni(Mes\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=CH(4‐OMe‐C\\(_{6}\\)H\\(_{4}\\)))] 14, [Ni(Mes\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=CPh\\(_{2}\\))] 15 and [Ni(Mes\\(_{2}\\)Im)\\(_{2}\\)(η\\(^{2}\\)‐O=C(4‐F‐C\\(_{6}\\)H\\(_{4}\\))\\(_{2}\\))] 16. The reaction of 1 and 2 with these simple aldehydes and ketones does not lead to a significantly different outcome, but NHC ligand rotation is hindered for the Mes\\(_{2}\\)Im complexes 3, 4 and 11–16 according to NMR spectroscopy. The solid‐state structures of 3, 4, 11 and 12 reveal significantly larger C\\(_{NHC}\\)‐Ni‐C\\(_{NHC}\\) angles in the Mes\\(_{2}\\)Im complexes compared to the iPr\\(_{2}\\)Im complexes. As electron transfer in d\\(^{8}\\)‐ (or d\\(^{10}\\)‐) ML\\(_{2}\\) complexes to π‐acidic ligands depends on the L–M–L bite angle, the different NHCs lead thus to a different degree of electron transfer and activation of the olefin, aldehyde or ketone ligand, i.e., [Ni(iPr\\(_{2}\\)Im)\\(_{2}\\)] is the better donor to these π‐acidic ligands. Furthermore, we identified two different side products from the reaction of 1 with benzaldehyde, trans‐[Ni(Mes\\(_{2}\\)Im)\\(_{2}\\)H(OOCPh)] 17 and [Ni\\(_{2}\\)(Mes\\(_{2}\\)Im)\\(_{2}\\)(µ\\(_{2}\\)‐CO)(µ\\(_{2}\\)‐η\\(^{2}\\)‐C,O‐PhCOCOPh)] 18, which indicate that radical intermediates and electron transfer processes might be of importance in the reaction of 1 with aldehydes and ketones.