Nickel N-heterocyclic Carbene Research Articles

Intramolecular Nitrile C–H Bond Activation in Nickel NHC Complexes: A Route to New Nickelacycles

Cyclopentadienyl N-heterocyclic carbene (NHC) nickel complexes of general formula [Ni(NHC)XCp] (NHC = 1-(propylnitrile)-3-methylimidazol-2-ylidene, 1-(2,4,6-trimethylphenyl)-3-(butylnitrile)imidazol-2-ylidene, 1-(2,4,6-trimethylphenyl)-3-(pentylnitrile)imidazol-2-ylidene, 1-(2,4,6-trimethylphenyl)-3-(hexylnitrile)imidazol-2-ylidene; X = Cl, Br, I; Cp = η5-C5H5), which bear an alkylnitrile side chain attached to one of the nitrogen atoms of the NHC ring, were prepared by the direct reaction of nickelocene with the corresponding imidazolium salts (NHC·HX). The new complexes [Ni{Me-NHC-(CH2)2CN}ICp] (1a), [Ni{Mes-NHC-(CH2)3CN}ClCp] (1b), [Ni{Mes-NHC-(CH2)4CN}ICp] (1c), and [Ni{Mes-NHC-(CH2)5CN}BrCp] (1d) were obtained in good yields and were fully characterized by standard spectroscopic techniques and elemental analyses and, in the cases of 1a,b, by single-crystal X-ray crystallography. Structural studies established their two-legged piano-stool geometry. The cationic derivatives [Ni{Mes-NHC-(CH2)nCN}(NCMe)Cp]+ (2b–d; n = 3–5) were prepared from the reaction of their neutral homologues 1b−d with KPF6 in acetonitrile at room temperature. Upon treatment of the neutral complexes 1 or of the cationic compounds 2 with KO-t-Bu, a C–H bond α to the nitrile group in each molecule underwent a base-promoted C–H activation and the new nickelacyclic complexes [Ni{Me-NHC-CH2CH(CN)}Cp] (3a) and [Ni{Mes-NHC-(CH2)nCH(CN)}Cp] (3b–d; n = 2–4) were obtained. All of these metallacycles contain Ni–C σ-bonds, and their synthesis generated a new asymmetric carbon center. The cyclic complexes 3 were fully characterized by 1H and 13C NMR spectroscopy, IR spectroscopy, and elemental analyses. The structures of complexes 3b and 3d, which contain six- and eight-membered metallacyclic rings, respectively, were determined by single-crystal X-ray diffraction studies. DFT studies, carried out to probe the mechanism of these cyclonickelation reactions, indicated that the mechanism of formation of these nickelacycles was similar to that observed for the formation of cyanomethyl ligands from coordinated acetonitrile. Nevertheless, the base deprotonation of an α-C–H ligand in a side arm, while thermodynamically comparable to that of a simultaneously coordinated acetonitrile ligand, is kinetically favored, and this leads to the formation of nickelacycles rather than cyanomethyl complexes in the case of the base-assisted activation of the cationic complexes 2.

Structure and bonding of three-coordinate N-heterocyclic carbene nickel nitrosyl complexes: Theoretical study

The structure and bonding of the for C3N3H2X2Ni(Cp)NO (X = H, F, Cl, Br) and their linkage isomers C3N3H2X2Ni(Cp)ON has been studied by carrying out density functional theory. The bonding nature of NiC bonds has been further explored by means of AIM method and natural bond orbital (NBO) analysis. Nucleus-independent chemical shift (NICS) values calculated at several points above ring center indicate aromaticity of heterocyclic cycle. Also, the effect of substitution (X = F, Cl, Br, CN) in N-heterocyclic carbene on the properties of complex has been shown.

Nickel N-heterocyclic carbene–pyridine complexes that exhibit selectivity for electrocatalytic reduction of carbon dioxide over water

We report a homologous series of nickel(II) complexes supported by N-heterocyclic carbene-pyridine ((R)bimpy, R = Me, Et, Pr) ligands that exhibit high selectivity for reducing carbon dioxide over water under electrocatalytic conditions.

Recyclable NHC-Ni Complex Immobilized on Magnetite/Silica Nanoparticles for C-S Cross-Coupling of Aryl Halides with Thiols

A new type of magnetite/silica nanoparticle-supported N-heterocyclic carbene nickel catalyst (Mag-NHC-Ni) was developed from imidazolium with N-picolyl moieties and used as an efficient catalyst in the C-S coupling of various aryl halides with thiols. Moreover, the catalyst was easily recovered from the reaction mixture by simple filtration and recycled with almost consistent activity.

Nickel N-Heterocyclic Carbene Catalyst for Cross-Coupling of Neopentyl Arenesulfonates with Methyl and Primary Alkyl Grignard Reagents

Nickel N-heterocyclic carbene (NHC) catalytic system prepared in situ by the reaction of Ni(acac)(2) with NHC precursor efficiently catalyzed the cross-coupling reaction of alkoxysulfonylarenes with methyl, neopentyl, and benzyl Grignard reagents at ambient temperature.

The Role of Free N‐Heterocyclic Carbene (NHC) in the Catalytic Dehydrogenation of Ammonia–Borane in the Nickel NHC System

Enders' N-heterocyclic carbene (NHC) dehydrogenates ammonia-borane with a relatively low barrier, producing NH(2)BH(2) and NHC-(H)(2). The nickel NHC catalyst present in the reaction media can activate the NHC-(H)(2) produced to regenerate the free NHC and release H(2). The release of free NHC enables further dehydrogenation of ammonia-borane.

The Role of Free N‐Heterocyclic Carbene (NHC) in the Catalytic Dehydrogenation of Ammonia–Borane in the Nickel NHC System

AbstractEnders' N‐heterocyclisches Carben (NHC) dehydriert den Ammoniak‐Boran‐Komplex mit einer vergleichsweise niedrigen Aktivierungsbarriere zu NH2BH2 und NHC‐(H)2. Ein Nickel‐NHC‐Katalysator im Reaktionsmedium zersetzt das gebildete NHC‐(H)2 in H2 und freies NHC, das wiederum Ammoniak‐Boran‐Komplex dehydrieren kann.magnified image

N-donor functionalized N-heterocyclic carbene nickel(II) complexes in the Kumada coupling

The synthesis and characterization of novel nickel(II) complexes bearing two bidentate N-heterocyclic carbene ligands functionalized with anionic N-donor moieties are described. Two different N-donor groups are employed, namely amido and benzimidazolato moieties. The solid-state structures of three of these complexes have been determined by X-ray crystallography. The amido-functionalized low-spin, square-planar Ni(II) complexes exhibit a cis geometry around the metal centre, while the benzimidazolato-functionalized complex crystallizes as the trans isomer. The activity of these novel complexes in the Kumada cross-coupling of phenylmagnesium chloride with 4-chloroanisole and 4-fluoroanisole was investigated. One of the benzimidazolato-functionalized complexes shows the highest activity in this reaction reported to date, yielding the desired product in quantitative yields within 30 min (4-chloroanisole), or 150 min (4-fluoroanisole) with only 1 mol% catalyst.

Preparation of a N-Heterocyclic Carbene Nickel(II) Complex. Synthetic Experiments in Current Organic and Organometallic Chemistry

A four-part experiment that leads to the synthesis of a cyclopentadienyl chloro–nickel(II) complex bearing a N-heterocyclic carbene (NHC) ligand is presented. In the first part, the preparation of 1,3-bis-(2,4,6-trimethylphenyl)imidazolium chloride (IMes·HCl) in a one-pot procedure by reaction of 2,4,6-trimethylaniline with paraformaldehyde, glyoxal, and hydrochloric acid is described. The reaction of NiCl2·6H2O with aqueous ammonia to give [Ni(NH3)6]Cl2, and the reaction of sodium in neat dicyclopentadiene at 160 °C to produce sodium cyclopentadienide in a one-pot synthesis are both delineated in the second part of this experiment. The reaction of [Ni(NH3)6]Cl2, with NaCp to yield [Ni(η5-C5H5)2] forms the third part. Finally, the procedure ends with the reaction of nickelocene with IMes·HCl to afford the nickel(II) half-sandwich complex [Ni(η5-C5H5)(IMes)Cl]. This sequence of experiments provides an excellent example of NHC–ligand preparation and its subsequent complexation to a transition metal and is a...

Synthetic, Structural, and Thermochemical Studies of N-Heterocyclic Carbene (NHC) and Tertiary Phosphine Ligands in the [(L)2Ni(CO)2] (L = PR3, NHC) System

Two new dicarbonyl N-heterocyclic carbene nickel(0) complexes of the type (NHC)2Ni(CO)2 (NHC = ICy, [N,N′-bis(cyclohexylimidazol)-2-ylidene (2), IMes [N,N′-bis(2,4,6-trimethylphenyl)imidazol)-2-ylidene] (3)) have been prepared by a substitution reaction of (NHC)Ni(CO)2 (NHC = ItBu [N,N′-bis(tert-butylimidazol)-2-ylidene], IAd [N,N′-bis(1-adamantylimidazol)-2-ylidene]) and 2 equivalents of ICy or IMes. Single-crystal X-ray analyses confirmed the monomeric 18-electron compositions of [(ICy)2Ni(CO)2] (2) and [(IMes)2Ni(CO)2] (3). The greater electron-donating properties of the NHC ligands compared to tertiary phosphines are also demonstrated through calorimetric studies and enabled the determination of average bond dissociation enthalpies for Ni−L (L = NHC and tertiary phosphine).

Nickel(II) Complexes of Bidentate N‐Heterocyclic Carbene/Phosphine Ligands: Efficient Catalysts for Suzuki Coupling of Aryl Chlorides.

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Nickel(II) Complexes of Bidentate N‐Heterocyclic Carbene/Phosphine Ligands: Efficient Catalysts for Suzuki Coupling of Aryl Chlorides

Nickel(II) complexes of bidentate N-heterocyclic carbene (NHC)/phosphane ligand L were prepared and structurally characterized. Unlike palladium, which forms [PdCl(2)(L)], the stable nickel product isolated is the ionic [Ni(L)(2)]Cl(2). These Ni(II) complexes are highly robust in air. Among different N-substituents on the ligand framework, the nickel complex of ligand L bearing N-1-naphthylmethyl groups (2 a) is a highly effective catalyst for Suzuki cross-coupling between phenylboronic acid and a range of aryl halides, including unreactive aryl chlorides. The activities of 2 a are largely superior to those of other reported nickel NHC complexes and their palladium counterparts. Unlike the previously reported [NiCl(2)(dppe)] (dppe=1,2-bis(diphenylphosphino)ethane), 2 a can effectively catalyze the cross-coupling reaction without the need for a catalytic amount of PPh(3), and this suggests that the PPh(2) functionality of hybrid NHC ligand L can partially take on the role of free PPh(3). However, for unreactive aryl chlorides at low catalyst loading, the presence of PPh(3) accelerates the reaction.

Highly Diastereoselective Preparation of anti-1,2-Diols by Catalytic Addition of Alkynylsilanes to α-Silyloxyaldehydes

The catalytic, diastereoselective coupling of alpha-silyloxy aldehydes and alkynylsilanes catalyzed by a nickel(0) N-heterocyclic carbene complex provides an effective entry to anti-1,2-diols. The scope of couplings and extent of diastereoselection are excellent across a range of substrates.

Synthesis, structure, and olefin polymerization with nickel(ii) N-heterocyclic carbene enolates

Two novel N-heterocylic carbene enolate nickel complexes have been prepared and shown to be active for ethylene and propylene polymerization to yield linear polymers.

Preparation, Structure, and Olefin Polymerization Behavior of Functionalized Nickel(II) N-Heterocyclic Carbene Complexes

Picolyl-functionalized N-heterocyclic carbene complexes have been synthesized by a route involving carbene transfer from Ag(I) carbene precursors. The Ag complexes undergo facile reaction with Ni(PPh3)2Cl2 to yield the carbene complexes Ni(C∧N)2Cl2 (C∧N = 3-methyl-1-picolylimidazolin-2-ylidene (2a), 3-benzyl-1-picolylimidazolin-2-ylidene (2b)). Complexes 2a,b have been characterized by IR and 1H and 13C NMR spectra and elemental analyses. The molecular structures of complexes 2a,b have been confirmed by X-ray single-crystal analyses. The carbene complex 2a shows high catalytic activities of up to 2.6 × 107 g of PNB (mol of Ni)-1 h-1 for the addition polymerization of norbornene in the presence of methylaluminoxane (MAO) as cocatalyst and exhibits moderate catalytic activity (3.3 × 105 g of PE (mol of Ni)-1 h-1) for ethylene polymerization. Catalytic activities, polymer yield, molecular weights, and molecular weight distributions of polynorbornene have been investigated under the various reaction conditions.