Large versus 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) andiPr(2)Im (= 1,3-diisopropyl-imidazoline-2-ylidene), with olefins, ketones and aldehydes are reported. The reaction of [Ni(Mes(2)Im)(2)]1with ethylene or methyl acrylate afforded the complexes [Ni(Mes(2)Im)(2)(eta(2)-C2H4)]3and [Ni(Mes(2)Im)(2)(eta(2)-(C,C)-H2C=CHCOOMe)]4, as it was previously reported for [Ni-2(iPr(2)Im)(4)(mu-(eta(2):eta(2))-COD)]2as a source for [Ni(iPr(2)Im)(2)]. In contrast to2, complex1does not react with sterically more demanding olefins such as tetramethylethylene, 1,1-diphenylethylene and cyclohexene. The reaction of [Ni(NHC)(2)] with more pi-acidic ketones or aldehydes led to formation of complexes withside-on eta(2)-(C,O)-coordinating ligands: [Ni(iPr(2)Im)(2)(eta(2)-O=(CHBu)-Bu-t)]5, [Ni(iPr(2)Im)(2)(eta(2)-O=CHPh)]6, [Ni(iPr(2)Im)(2)(eta(2)-O=CMePh)]7, [Ni(iPr(2)Im)(2)(eta(2)-O=CPh2)]8, [Ni(iPr(2)Im)(2)(eta(2)-O=C(4-F-C6H4)(2))]9, [Ni(iPr(2)Im)(2)(eta(2)-O=C(OMe)(CF3))]10and [Ni(Mes(2)Im)(2)(eta(2)-O=CHPh)]11, [Ni(Mes(2)Im)(2)(eta(2)-O=CH(CH(CH3)(2)))]12, [Ni(Mes(2)Im)(2)(eta(2)-O=CH(4-NMe2-C6H4))]13, [Ni(Mes(2)Im)(2)(eta(2)-O=CH(4-OMe-C6H4))]14, [Ni(Mes(2)Im)(2)(eta(2)-O=CPh2)]15and [Ni(Mes(2)Im)(2)(eta(2)-O=C(4-F-C6H4)(2))]16. The reaction of1and2with these simple aldehydes and ketones does not lead to a significantly different outcome, but NHC ligand rotation is hindered for the Mes(2)Im complexes3,4and11-16according to NMR spectroscopy. The solid-state structures of3,4,11and12reveal significantly larger C-NHC-Ni-C(NHC)angles in the Mes(2)Im complexes compared to theiPr(2)Im complexes. As electron transfer in d(8)- (or d(10)-) ML(2)complexes to pi-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 pi-acidic ligands. Furthermore, we identified two different side products from the reaction of1with benzaldehyde,trans-[Ni(Mes(2)Im)(2)H(OOCPh)]17and [Ni-2(Mes(2)Im)(2)(mu(2)-CO)(mu(2)-eta(2)-C,O-PhCOCOPh)]18, which indicate that radical intermediates and electron transfer processes might be of importance in the reaction of1with aldehydes and ketones.