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

A General Synthetic Route to NHC‐Phosphinidenes: NHC‐mediated Dehydrogenation of Primary Phosphines

The dehydrocoupling of primary phosphines with N-heterocyclic carbenes (NHCs) to yield NHC-phosphinidenes is reported. The reaction of two equivalents of the NHCs Me$_2$Im (1,3-dimethylimidazolin-2-ylidene), Me$_4$Im (1,3,4,5-tetramethylimidazolin-2-ylidene), iPr$_2$Im (1,3-di-iso-propylimidazolin-2-ylidene) and Mes$_2$Im (2,4,6-trimethylphenylimidazolin-2-ylidene) with PhPH$_2$ and MesPH$_2$ led to the NHC stabilized phosphinidenes (NHC)PAr: (iPr$_2$Im)PPh (1), (Mes$_2$Im)PPh (2), (Me$_4$Im)PPh (3), (Mes$_2$Im)PMes (4), (Me$_2$Im)PMes (5), (Me$_4$Im)PMes (6) and (iPr$_2$Im)PMes (7). The reaction of tBuPH$_2$ with two equivalents of the NHCs afforded the corresponding NHC stabilized parent phosphinidenes (NHC)PH: (iPr$_2$Im)PH (8), (Mes$_2$Im)PH (9) and (Me$_4$Im)PH (10). Reaction of 1 with oxygen and sulfur led to isolation of iPr$_2$Im-P(O)$_2$Ph (11) and iPr$_2$Im-P(S)$_2$Ph (12), whereas the reaction with elemental selenium and tellurium gave (NHC)PPh cleavage with formation of (iPr$_2$Im)Se (13), iPr$_2$ImTe (14) and different cyclo-oligophosphines. Furthermore, the complexes [{(iPr$_2$Im)PPh}W(CO)$_5$] (15), [Co(CO)$_2$(NO){(iPr$_2$Im)PPh}] (16) and [(η$^5$-C$_5$Me$_2$)Co(η$^2$-C$_2$H$_4$){(iPr$_2$Im)PPh}] (17) have been prepared starting from 1 and a suitable transition metal complex precursor. The complexes 16 and 17 decompose in solution upon heating to ca. 80 °C to yield the NHC complexes [Co(iPr$_2$Im)(CO)$_2$(NO)] and [(η$^5$-C$_5$Me$_5$)Co(iPr$_2$Im)(η$^2$-C$_2$H$_4$)] with formation of cyclo-oligophosphines. The reaction of 1 with [Ni(COD)$_2$] afforded the diphosphene complex [Ni(iPr$_2$Im)$_2$(trans-PhP=PPh)] 18

A General Synthetic Route to NHC‐Phosphinidenes: NHC‐mediated Dehydrogenation of Primary Phosphines

The dehydrocoupling of primary phosphines with N-heterocyclic carbenes (NHCs) to yield NHC-phosphinidenes is reported. The reaction of two equivalents of the NHCs Me$_2$Im (1,3-dimethylimidazolin-2-ylidene), Me$_4$Im (1,3,4,5-tetramethylimidazolin-2-ylidene), iPr$_2$Im (1,3-di-iso-propylimidazolin-2-ylidene) and Mes$_2$Im (2,4,6-trimethylphenylimidazolin-2-ylidene) with PhPH$_2$ and MesPH$_2$ led to the NHC stabilized phosphinidenes (NHC)PAr: (iPr$_2$Im)PPh (1), (Mes$_2$Im)PPh (2), (Me$_4$Im)PPh (3), (Mes$_2$Im)PMes (4), (Me$_2$Im)PMes (5), (Me$_4$Im)PMes (6) and (iPr$_2$Im)PMes (7). The reaction of tBuPH$_2$ with two equivalents of the NHCs afforded the corresponding NHC stabilized parent phosphinidenes (NHC)PH: (iPr$_2$Im)PH (8), (Mes$_2$Im)PH (9) and (Me$_4$Im)PH (10). Reaction of 1 with oxygen and sulfur led to isolation of iPr$_2$Im-P(O)$_2$Ph (11) and iPr$_2$Im-P(S)$_2$Ph (12), whereas the reaction with elemental selenium and tellurium gave (NHC)PPh cleavage with formation of (iPr$_2$Im)Se (13), iPr$_2$ImTe (14) and different cyclo-oligophosphines. Furthermore, the complexes [{(iPr$_2$Im)PPh}W(CO)$_5$] (15), [Co(CO)$_2$(NO){(iPr$_2$Im)PPh}] (16) and [(η$^5$-C$_5$Me$_2$)Co(η$^2$-C$_2$H$_4$){(iPr$_2$Im)PPh}] (17) have been prepared starting from 1 and a suitable transition metal complex precursor. The complexes 16 and 17 decompose in solution upon heating to ca. 80 °C to yield the NHC complexes [Co(iPr$_2$Im)(CO)$_2$(NO)] and [(η$^5$-C$_5$Me$_5$)Co(iPr$_2$Im)(η$^2$-C$_2$H$_4$)] with formation of cyclo-oligophosphines. The reaction of 1 with [Ni(COD)$_2$] afforded the diphosphene complex [Ni(iPr$_2$Im)$_2$(trans-PhP=PPh)] 18

NHC Nickel-Catalyzed Suzuki–Miyaura Cross-Coupling Reactions of Aryl Boronate Esters with Perfluorobenzenes

An efficient Suzuki–Miyaura cross-coupling reaction of perfluorinated arenes with aryl boronate esters using NHC nickel complexes as catalysts is described. The efficiencies of different boronate esters (<i>p</i>-tolyl-Beg, <i>p</i>-tolyl-Bneop, <i>p</i>-tolyl-Bpin, <i>p</i>-tolyl-Bcat) and the corresponding boronic acid (<i>p</i>-tolyl-B­(OH)₂) in this type of cross-coupling reaction were evaluated (eg, ethyleneglycolato; neop, neopentylglycolato; pin, pinacolato; cat, catecholato). Aryl-Beg was shown to be the most reactive boronate ester among those studied. The use of CsF as an additive is essential for an efficient reaction of hexafluorobenzene with aryl neopentylglycolboronates

Preparing (Multi)Fluoroarenes as Building Blocks for Synthesis: Nickel-Catalyzed Borylation of Polyfluoroarenes via C–F Bond Cleavage

The [Ni­(IMes)₂]-catalyzed transformation of fluoroarenes into arylboronic acid pinacol esters via C–F bond activation and transmetalation with bis­(pinacolato)­diboron (B₂pin₂) is reported. Various partially fluorinated arenes with different degrees of fluorination were converted into their corresponding boronate esters

CCDC 2009924: Experimental Crystal Structure Determination

Related Article: Sara Sabater, David Schmidt, Heidi (née Schneider) Schmidt, Maximilian W. Kuntze-Fechner, Thomas Zell, Connie J. Isaac, Nasir A. Rajabi, Harry Grieve, William J. M. Blackaby, John P. Lowe, Stuart A. Macgregor, Mary F. Mahon, Udo Radius, Michael K. Whittlesey|2021|Chem.-Eur.J.|27|13221|doi:10.1002/chem.20210148