Nickel boryl complexes and the nickel-catalyzed alkyne borylation

The first nickel bis-boryl complexes cis [Ni(iPr2ImMe)2(Bcat)2], cis [Ni(iPr2ImMe)2(Bpin)2] and cis [Ni(iPr2ImMe)2(Beg)2] are reported, which were prepared via the reaction of a source of [Ni(iPr2ImMe)2] with the diboron(4) compounds B2cat2, B2pin2 and B2eg2 (iPr2ImMe = 1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene; B2cat2 = bis(catecholato)diboron; B2pin2 = bis(pinacolato)diboron; B2eg2 = bis(ethylene glycolato)diboron). X-ray diffraction and DFT calculations strongly suggest that a delocalized, multicenter bonding scheme dictates the bonding situation of the NiB2 moiety in these square planar complexes, reminiscent to the bonding situation of “non-classical” H2 complexes. [Ni(iPr2ImMe)2] also efficiently catalyzes the diboration of alkynes using B2cat2 as the boron source under mild conditions. In contrast to the known platinum-catalyzed diboration, the nickel system follows a different mechanistic pathway, which not only provides the 1,2 borylation product in excellent yields, but also provides, additionally, an efficient approach to other products such as C–C coupled borylation products or rare tetra-borylated compounds. The mechanism of the nickel-catalyzed alkyne borylation was examined by means of stoichiometric reactions and DFT calculations. Oxidative addition of the diboron reagent to nickel is not dominant; the first steps of the catalytic cycle are coordination of the alkyne to [Ni(iPr2ImMe)2] and subsequent borylation at the coordinated and, thus, activated alkyne to yield complexes of the type [Ni(NHC)2(η2-cis-(Bcat)(R)C=C(R)(Bcat))], exemplified by the isolation and structural characterization of [Ni(iPr2ImMe)2(η2-cis-(Bcat)(Me)C=C(Me)(Bcat))] and [Ni(iPr2ImMe)2(η2-cis-(Bcat)(H7C3)C=C(C3H7)(Bcat))]

NHC-substituierte Nickel(0)-Komplexe: Bindungsaktivierung, Redoxeigenschaften und Katalyse

This thesis describes the synthesis and reactivity of bis-NHC ligated nickel(0)-complexes and their application in catalytic cyclization and borylation reactions of alkynes. The focus of the presented work lies on the investigation of the electronic and steric impact of different NHC ligands on the reactivity and catalytic activity of [Ni(NHC)2] complexes. Since d10 ML2 complexes play a decisive role for numerous catalytic reactions, such as the Suzuki-Miyaura cross-coupling, the first chapter provides an overview about the general properties of NHCs and the chemistry of NHC-ligated nickel complexes, their synthesis, characterization, reactivity, and application in catalysis.Die vorliegende Arbeit befasst sich mit der Synthese und Reaktivität von zweifach NHC-stabilisierten Nickel(0)-Komplexen sowie deren Anwendung als Katalysatoren in Zyklisierungs- und Borylierungsreaktionen von Alkinen. Der Fokus liegt auf der Untersuchung von elektronischen und sterischen Einflüssen verschiedener NHC-Liganden auf die Reaktivität und katalytische Aktivität von [Ni(NHC)2]-Komplexen. Da solche d10-ML2 Komplexe heute für eine Vielzahl von katalytischen Reaktionen von immenser Bedeutung sind, wie z. B. der Suzuki-Miyaura-Kreuzkupplung, wird im ersten Kapitel ein Überblick über die grundlegenden Eigenschaften von NHCs und die Chemie NHC-substituierter Nickel-Komplexe, deren Synthese, Charakterisierung, Reaktivität und Anwendung in der Katalyse, gegeben

Cationic Nickel d9^{9}‐Metalloradicals [Ni(NHC)2_{2}]+^{+}

A series of five new homoleptic, linear nickel d$^{9}$‐complexes of the type [Ni$^{I}$(NHC)$_{2}$]$^{+}$ is reported. Starting from the literature known Ni(0) complexes [Ni(Mes$_{2}$Im)$_{2}$] 1, [Ni(Mes$_{2}$Im$^{H2}$)2] 2, [Ni(Dipp$_{2}$Im)$_{2}$] 3, [Ni(Dipp$_{2}$Im$^{H2}$)$_{2}$] 4 and [Ni(cAAC$^{Me}$)$_{2}$] 5 (Mes$_{2}$Im=1,3‐bis(2,4,6‐trimethylphenyl)‐imidazolin‐2‐ylidene, Mes$_{2}$Im$^{H2}$=1,3‐bis(2,4,6‐trimethylphenyl)‐imidazolidin‐2‐ylidene, Dipp$_{2}$Im=1,3‐bis(2,6‐diisopropylphenyl)‐imidazolin‐2‐ylidene, Dipp$_{2}$Im$^{H2}$=1,3‐bis(2,6‐diisopropylphenyl)‐imidazolidin‐2‐ylidene, cAAC$^{Me}$=1‐(2,6‐diisopropylphenyl)‐3,3,5,5‐tetramethylpyrrolidin‐2‐yliden), their oxidized Ni(I) analogues [Ni$^{I}$(Mes$_{2}$Im)$_{2}$][BPh$_{4}$] 1$^{+}$, [Ni$^{I}$(Mes$_{2}$Im$^{H2}$)$_{2}$][BPh$_{4}$] 2$^{+}$, [Ni$^{I}$(Dipp$_{2}$Im)$_{2}$][BPh$_{4}$] 3$^{+}$, [Ni$^{I}$(Dipp$_{2}$Im$^{H2}$)$_{2}$][BPh$_{4}$] 4$^{+}$ and [Ni$^{I}$(cAAC$^{Me}$)$_{2}$][BPh$_{4}$] 5$^{+}$ were synthesized by one‐electron oxidation with ferrocenium tetraphenyl‐borate. The complexes 1$^{+}$–5$^{+}$ were fully characterized including X‐ray structure analysis. The complex cations reveal linear geometries in the solid state and NMR spectra with extremely broad, paramagnetically shifted resonances. DFT calculations predicted an orbitally degenerate ground state leading to large magnetic anisotropy, which was verified by EPR measurements in solution and on solid samples. The magnetic anisotropy of the complexes is highly dependent from the steric protection of the metal atom, which results in a noticeable decrease of the g‐tensor anisotropy for the N‐Mes substituted complexes 1$^{+}$ and 2$^{+}$ in solution due to the formation of T‐shaped THF adducts

Case Study of N-i^{i}Pr versus N-Mes Substituted NHC Ligands in Nickel Chemistry: The Coordination and Cyclotrimerization of Alkynes at [Ni(NHC)2_{2}]

A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC)$_{2}$] (NHC=$^{i}$Pr$_{2}$Im$^{Me}$ 1$^{Me}$, Mes$_{2}$Im 2) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni$_{2}$($^{i}$Pr$_{2}$Im$^{Me}$)$_{4}$(μ-(η$^{2}$ : η$^{2}$)-COD)] B/ [Ni($^{i}$Pr$_{2}$Im$^{Me}$)$_{2}$(η$^{4}$-COD)] B’ or [Ni(Mes$_{2}$Im)$_{2}$] 2, respectively, with alkynes afforded complexes [Ni(NHC)$_{2}$(η$^{2}$-alkyne)] (NHC=$^{i}$Pr$_{2}$Im$^{Me}$: alkyne=MeC≡CMe 3, H$_{7}$C$_{3}$C≡CC$_{3}$H$_{7}$ 4, PhC≡CPh 5, MeOOCC≡CCOOMe 6, Me$_{3}$SiC≡CSiMe$_{3}$ 7, PhC≡CMe 8, HC≡CC$_{3}$H$_{7}$ 9, HC≡CPh 10, HC≡C(p-Tol) 11, HC≡C(4-$^{t}$Bu-C$_{6}$H$_{4}$) 12, HC≡CCOOMe 13; NHC=Mes$_{2}$Im: alkyne=MeC≡CMe 14, MeOOCC≡CCOOMe 15, PhC≡CMe 16, HC≡C(4-$^{t}$Bu-C$_{6}$H$_{4}$) 17, HC≡CCOOMe 18). Unusual rearrangement products 11 a and 12 a were identified for the complexes of the terminal alkynes HC≡C(p-Tol) and HC≡C(4-$^{t}$Bu-C$_{6}$H$_{4}$), 11 and 12, which were formed by addition of a C−H bond of one of the NHC N-$^{i}$Pr methyl groups to the C≡C triple bond of the coordinated alkyne. Complex 2 catalyzes the cyclotrimerization of 2-butyne, 4-octyne, diphenylacetylene, dimethyl acetylendicarboxylate, 1-pentyne, phenylacetylene and methyl propiolate at ambient conditions, whereas 1$^{Me}$ is not a good catalyst. The reaction of 2 with 2-butyne was monitored in some detail, which led to a mechanistic proposal for the cyclotrimerization at [Ni(NHC)$_{2}$]. DFT calculations reveal that the differences between 1$^{Me}$ and 2 for alkyne cyclotrimerization lie in the energy profile of the initiation steps, which is very shallow for 2, and each step is associated with only a moderate energy change. The higher stability of 3 compared to 14 is attributed to a better electron transfer from the NHC to the metal to the alkyne ligand for the N-alkyl substituted NHC, to enhanced Ni-alkyne backbonding due to a smaller C$_{NHC}$−Ni−C$_{NHC}$ bite angle, and to less steric repulsion of the smaller NHC $^{i}$Pr$_{2}$Im$^{Me}$

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