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

Abstract

Investigations concerning the reactivity of Ni(0) complexes [Ni(NHC)2] of NHCs (N-heterocyclic carbene) of different steric demand, Mes2Im (= 1,3-dimesitylimidazoline-2-ylidene) and iPr2Im (= 1,3-diisopropyl-imidazoline-2-ylidene), with olefins, ketones and aldehydes are reported. The reaction of [Ni(Mes2Im)2] 1 with ethylene or methyl acrylate afforded the complexes [Ni(Mes2Im)2(η2-C2H4)] 3 and [Ni(Mes2Im)2(η2-(C,C)-H2C=CHCOOMe)] 4, as it was previously reported for [Ni2(iPr2Im)4(µ-(η2:η2)-COD)] 2 as a source for [Ni(iPr2Im)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 η2-(C,O)-coordinating ligands: [Ni(iPr2Im)2(η2-O=CHtBu)] 5, [Ni(iPr2Im)2(η2-O=CHPh)] 6, [Ni(iPr2Im)2(η2-O=CMePh)] 7, [Ni(iPr2Im)2(η2-O=CPh2)] 8, [Ni(iPr2Im)2(η2-O=C(4-F-C6H4)2)] 9, [Ni(iPr2Im)2(η2-O=C(OMe)(CF3))] 10 and [Ni(Mes2Im)2(η2-O=CHPh)] 11, [Ni(Mes2Im)2(η2-O=CH(CH(CH3)2))] 12, [Ni(Mes2Im)2(η2-O=CH(4-NMe2-C6H4))] 13, [Ni(Mes2Im)2(η2-O=CH(4-OMe-C6H4))] 14, [Ni(Mes2Im)2(η2-O=CPh2)] 15 and [Ni(Mes2Im)2(η2-O=C(4-F-C6H4)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 Mes2Im complexes 3, 4 and 11–16 according to NMR spectroscopy. The solid-state structures of 3, 4, 11 and 12 reveal significantly larger CNHC-Ni-CNHC angles in the Mes2Im complexes compared to the iPr2Im complexes. As electron transfer in d8- (or d10-) ML2 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(iPr2Im)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(Mes2Im)2H(OOCPh)] 17 and [Ni2(Mes2Im)2(µ2-CO)(µ2-η2-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.