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.

Case Study of N‐iPr versus N‐Mes Substituted NHC Ligands in Nickel Chemistry: The Coordination and Cyclotrimerization of Alkynes at [Ni(NHC)2]

“…In general, the stability of complexes [Ni(Mes 2 Im) 2 ( η 2 ‐R 1 C≡CR 2 )] depend on the steric demand of the alkyne used, but also on the electronic properties of the alkyne ligand. As observed previously for olefin complexes, [11b] the steric bulk of the NHC ligand Mes 2 Im of complex 2 limits the coordination of a third ligand to the nickel atom, which is in stark contrast to the behavior of complexes 1/1 Me . Alkynes with electron withdrawing substituents increase π ‐backbonding from the nickel atom to the alkyne and increase the stability of the alkyne complex in solution at room temperature.…”

“…Unlike the complexes 3 – 13 , the NMR spectra of the compounds 14 – 18 reveal remarkably broadened resonances for the bulkier NHC ligand Mes 2 Im due to hindered rotation, as it was previously reported by us for similar π ‐complexes with ketone or aldehyde ligands. [11b] Even the low temperature NMR spectra of 14 , 16 and 17 revealed some signal broadening. Nevertheless, all characteristic resonances were assigned and the integration of the resonances is consistent with the presence of one alkyne ligand per two NHC ligands in complexes of the type [Ni(Mes 2 Im) 2 ( η 2 ‐alkyne)].…”

“…Strong backbonding from the metal atom to the ligand is also reflected in the 13 C{ 1 H} NMR spectra of these complexes as a significant low‐field coordination shift of 41.7–61.9 ppm occurs upon complexation. [ 10 , 11b ] The observed IR stretching vibrations of the alkyne triple bonds (1659–1785 cm −1 ) in the complexes 3 – 13 are also significantly shifted to lower wavenumbers compared to the uncoordinated alkynes, which show typical stretching vibrations between 2100 cm −1 and 2310 cm −1 , and thus reflect a lower bond order upon coordination to nickel. [19] The ν C≡C coordination shift (▵ ν C≡C ) of complex 5 (1754 cm −1 ), for example, is −469 cm −1 compared to uncoordinated diphenylacetylene (2223 cm −1 ) and much larger compared to ▵ ν C≡C reported for the corresponding phosphine complex [(PPh 3 ) 2 Ni( η 2 ‐PhC≡CPh)] (−419 cm −1 ).…”

“…The C1−C2 separation of 1.439(2) Å is consistent with C=C bond lengths observed for other [Ni(NHC) 2 ( η 2 ‐olefin)] complexes. [11b] The nickel atom, the olefin carbon atoms C1, C2 and the NHC carbon atom C7 are perfectly aligned in a plane and the intact NHC ligand is nearly perfectly perpendicular oriented to this plane (88.58(9)°). The NHC ligand of the metallacycle (i.e., plane N3−C6−N4) is twisted towards the plane C1−Ni1−C2 with an angle of 32.51(11)°.…”

Noble Metal Complexes of a Bis-Caffeine Containing NHC Ligand

“…This is due to their electronic properties, most notably the strong σ-donating character, which makes them useful ligands for the coordination of transitionmetal ions [5,6]. Since then, complexes with NHC-type ligands have been intensively studied in the context of photophysical applications (e.g., with iron [7,8] or ruthenium [9]) as well as catalysis (e.g., with nickel [10], iron [11] or palladium [12]). Thereby, the ligand design has a strong influence on the properties of the resulting complexes, with steric and electronic properties having the most significant contributions [3,13,14].…”