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 i Pr 2 Im (= 1,3‐di iso propyl‐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 ( i Pr 2 Im) 4 ( µ‐( η 2 :η 2 ) ‐COD)] 2 as a source for [Ni( i Pr 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( i Pr 2 Im) 2 (η 2 ‐O=CH t Bu)] 5 , [Ni( i Pr 2 Im) 2 (η 2 ‐O=CHPh)] 6 , [Ni( i Pr 2 Im) 2 (η 2 ‐O=CMePh)] 7 , [Ni( i Pr 2 Im) 2 (η 2 ‐O=CPh 2 )] 8 , [Ni( i Pr 2 Im) 2 (η 2 ‐O=C(4‐F‐C 6 H 4 ) 2 )] 9 , [Ni( i Pr 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 i Pr 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( i Pr 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.