Nickel N-heterocyclic Carbene Research Articles

Synthesis of Imidazolium Cations Linked to Para-t-Butylcalix[4]arene Frameworks and Their Use as Synthons for Nickel-NHC Complexes Tethered to Calix[4]arenes.

A series of cationic p-tert-butylcalix[4]arenes, with side-arms that are functionalized with imidazolium groups, have been synthesized in good yields. The parent tetrahydroxy para-t-butyl-calix[4]arene was dialkylated at the phenolic hydrogen atoms using α,ω-dibromo-alkanes to yield bis(mono-brominated) alkoxy-chains of variable length. The brominated side-arms in these compounds were then further alkylated with substituted imidazoles (N-methylimidazole, N-(2,4,6-trimethyl-phenyl)imidazole, or N-(2,6-di-isopropylphenyl)imidazole) to yield a series of dicationic calixarenes with two imidazolium groups tethered, via different numbers of methylene spacers (n = 2-4), to the calixarene moiety. Related tetracationic compounds, which contain four imidazolium units linked to the calix[4]arene backbone, were also prepared. In all of these compounds, the NMR data show that the calixarenes adopted a cone configuration. All molecules were characterized by NMR spectroscopy and by MS studies. Single crystal X-ray diffraction studies were attempted on many mono-crystals of these cations, but significant disorder problems, partly caused by occluded solvent in the lattice, and lack of crystallinity resulting from partial solvent loss, precluded the good resolution of most X-ray structures. Eventually, good structural data were obtained from an unusually disordered single crystal of 5a, (1,3)-Cone-5,11,17,23-tetra-t-butyl-25,27-di-hydroxy-26,28-di-[2-(N-2,6-diisopropylphenyl-imidazolium)ethoxy]calix[4]arene dibromide and its presumed structure was confirmed. The structure revealed the presence of H-bonded interactions and some evidence of π-stacking. Some of these imidazolium salts were reacted with nickelocene to form the nickel N-heterocyclic carbene (NHC) complexes 7a-7d. A bis-carbene nickel complex 8 was also isolated and its structure was established by single crystal X-ray diffraction studies. The structure was disordered and not of high quality, but the structural data corroborated the spectroscopic data.

Activation Mechanism of Nickel(0) N-Heterocyclic Carbene Catalysts Stabilized by Fumarate Ligands.

Nickel(0) catalysts of N-heterocyclic carbenes (NHCs) that are stabilized by electron-deficient alkenes possess desirable properties of air tolerance and ease of handling while also retaining high catalytic activities. Since catalyst stability often comes at the expense of catalytic activity, we have undertaken a detailed study of the activation mechanism of an IMes-nickel(0) catalyst stabilized by di(o-tolyl) fumarate that converts the stable precatalyst form into a catalytically active species. Computational evaluation provided evidence against a simple ligand exchange as the activation mechanism for this catalyst, and a stoichiometric activation process that covalently modifies the stabilizing ligand was identified. A detailed computational picture for the activation process was developed, with predictive insights that elucidate an unexpected catalyst activation pathway that operates when ligand exchange is thermodynamically unfavorable.

Stepwise synthesis and catalysis in C-S cross-coupling of pyridine-functionalized N-heterocyclic carbene nickel(II) complexes by mechanochemistry.

The synthesis of three N-heterocyclic carbene complexes by stepwise grinding is described in this paper. The benzimidazolium salts ([H2L]Br2 and [H2L](PF6)2 ([H2L] = 1,1'-di(2-picolyl)-3,3'-methylenedibenzoimidazolium)) were initially prepared. Their reactions with Ni(OAc)2·4H2O by grinding afforded three nickel complexes, [NiL]Br2·CH3OH (1), [NiL]Br2·2H2O (1') and [NiL](PF6)2·0.5CH3CN (2), respectively. A five-coordinated complex [NiLBr]PF6 (3) was further obtained by grinding NH4PF6 with complex 1 or 1', or grinding KBr with complex 2. Complex 3 can also be obtained by direct grinding of complexes 1/1' and 2. Complex 3 was subsequently used as a catalyst in the C-S bond cross-coupling of 2-bromoacetophenone and 2-mercaptobenzothiazole. The entire procedure from the synthesis to the catalytic reaction was performed by mechanochemistry. The green metrics E-factor and EcoScale close to ideal values showed the eco-friendly nature of the entire procedure.

Ni-Catalyzed Borylation of Aryl Sulfoxides.

A nickel/N‐heterocyclic carbene (NHC) catalytic system has been developed for the borylation of aryl sulfoxides with B2(neop)2 (neop=neopentyl glycolato). A wide range of aryl sulfoxides with different electronic and steric properties were converted into the corresponding arylboronic esters in good yields. The regioselective borylation of unsymmetric diaryl sulfoxides was also feasible leading to borylation of the sterically less encumbered aryl substituent. Competition experiments demonstrated that an electron‐deficient aryl moiety reacts preferentially. The origin of the selectivity in the Ni‐catalyzed borylation of electronically biased unsymmetrical diaryl sulfoxide lies in the oxidative addition step of the catalytic cycle, as oxidative addition of methoxyphenyl 4‐(trifluoromethyl)phenyl sulfoxide to the Ni(0) complex occurs selectively to give the structurally characterized complex trans‐[Ni(ICy)2(4‐CF3‐C6H4){(SO)‐4‐MeO‐C6H4}] 4. For complex 5, the isomer trans‐[Ni(ICy)2(C6H5)(OSC6H5)] 5‐I was structurally characterized in which the phenyl sulfinyl ligand is bound via the oxygen atom to nickel. In solution, the complex trans‐[Ni(ICy)2(C6H5)(OSC6H5)] 5‐I is in equilibrium with the S‐bonded isomer trans‐[Ni(ICy)2(C6H5)(SOC6H5)] 5, as shown by NMR spectroscopy. DFT calculations reveal that these isomers are separated by a mere 0.3 kJ/mol (M06/def2‐TZVP‐level of theory) and connected via a transition state trans‐[Ni(ICy)2(C6H5)(η2‐{SO}‐C6H5)], which lies only 10.8 kcal/mol above 5.

Design and Synthesis of Nickel N-Heterocyclic Carbene Catalyst Systems and Their Application in the Cross-Coupling of Silyloxyarenes

Design and Synthesis of Nickel N-Heterocyclic Carbene Catalyst Systems and Their Application in the Cross-Coupling of Silyloxyarenes

Hydroboration of Alkenes Catalysed by a Nickel N-Heterocyclic Carbene Complex: Reaction and Mechanistic Aspects.

The pentamethylcyclopentadienyl N-heterocyclic carbene nickel complex [Ni(η5 -C5 Me5 )Cl(IMes)] (IMes=1,3-dimesitylimidazol-2-ylidene) efficiently catalyses the anti-Markovnikov hydroboration of alkenes with catecholborane in the presence of a catalytic amount of potassium tert-butoxide, and joins the very exclusive club of nickel catalysts for this important transformation. Interestingly, the regioselectivity can be reversed in some cases by using pinacolborane instead of catecholborane. Mechanistic investigations involving control experiments, 1 H and 11 B NMR spectroscopy, cyclic voltammetry, piezometric measurements and DFT calculations suggest an initial reduction of the NiII precursor to a NiI active species with the concomitant release of H2 . The crucial role of the alkoxo-catecholato-borohydride species resulting from the reaction of potassium tert-butoxide with catecholborane in the formation of an intermediate nickel-hydride species that would then be reduced to the NiI active species, is highlighted.

Aggregation and Degradation of White Phosphorus Mediated by N-Heterocyclic Carbene Nickel(0) Complexes.

The reaction of zerovalent nickel compounds with white phosphorus (P4) is a barely explored route to binary nickel phosphide clusters. Here, we show that coordinatively and electronically unsaturated N‐heterocyclic carbene (NHC) nickel(0) complexes afford unusual cluster compounds with P1, P3, P5 and P8 units. Using [Ni(IMes)2] [IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene], electron‐deficient Ni3P4 and Ni3P6 clusters have been isolated, which can be described as superhypercloso and hypercloso clusters according to the Wade–Mingos rules. Use of the bulkier NHC complexes [Ni(IPr)2] or [(IPr)Ni(η6‐toluene)] [IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene] affords a closo‐Ni3P8 cluster. Inverse‐sandwich complexes [(NHC)2Ni2P5] (NHC=IMes, IPr) with an aromatic cyclo‐P5 − ligand were identified as additional products.

Synthesis and catalytic application of cyclopentadienyl nickel(II) N-heterocyclic carbene complexes

A series of ester-functionalized benzimidazolium salts 2a–c were prepared by quaternization of 1-{(ethoxycarbonyl)methyl}benzimidazole (1) with 3,5-dimethylbenzyl bromide, 2,5-dimethylbenzyl chloride and 3-methoxybenzyl chloride, respectively. Refluxing 2a–c with nickelocene in THF yielded the neutral cyclopentadienyl NHC nickel(II) complexes 3a–c. Their structures were defined by NMR, IR and elemental analysis techniques. Molecular weights of 3a–c were affirmed by MALDI-TOF mass spectrometry. Catalytic tests of 3a–c were performed in Kumada coupling of some aryl chlorides with phenylmagnesium bromide at 25 °C.

Nickel(ii) N-heterocyclic carbene complexes as efficient catalysts for the Suzuki—Miyaura reaction

Catalytic activity of nickel(ii) and palladium(ii) N-heterocyclic carbene (NHC) complexes derived from imidazole, benzimidazole, and 1,2,4-triazole was comparatively evaluated in the cross-coupling reactions of aryl halides with arylboronic acids. Readily available nickel bis-NHC complexes (NHC)2NiX2 (X = Cl, Br, or I) exhibited the activity comparable to that of the structurally related palladium complexes and, consequently, can be applied as efficient catalysts for the Suzuki—Miyaura reaction.

Reactions of a Bis(vinyltrimethylsilane)nickel(0) N-Heterocyclic carbene complex with organic azides

The reactions of three-coordinate iron(0) and cobalt(0) alkene complexes (NHC)M(η2-CH2CHSiMe3)2 (NHC = N-heterocyclic carbene, M = Fe, Co) with organic azides proved an effective way for the preparation of iron and cobalt terminal imido complexes. The relevant reactions of nickel(0) alkene complex (NHC)Ni(η2-CH2CHSiMe3)2 with organic azides, however, have remained unexplored. Herein, we report the study on the reactions of [(IPr)Ni(η2-CH2CHSiMe3)2] (IPr = 1,3-bis(2′,6′-diisopropylphenyl)imidazole-2-ylidene) with organic azides. The interaction of [(IPr)Ni(η2-CH2CHSiMe3)2] with 2,6-dimesitylphenyl azide (DmpN3) produced the two-coordinate imido complex [(IPr)Ni(NDmp)] (1) in a trace amount with the starting materials being largely remained. The reaction of [(IPr)Ni(η2-CH2CHSiMe3)2] with 2,6-diisopropylphenyl azide (DippN3) gave IPrNNNDipp (2) as the only isolable product. The interaction of [(IPr)Ni(η2-CH2CHSiMe3)2] with PhC(CF3)2N3 produced an azanickelacyclobutane compound [(IPr)Ni(κ-C,N–N(C(CF3)2Ph)CH(SiMe3)CH2)] (3) in high yield. The different products formed in these reactions reflect the influence of the steric and electronic nature of organic azides on reaction outcomes. Further study on the azanickelacyclobutane complex 3 indicated that it can convert into a new azanickelacyclobutane [(IPr)Ni(κ-C,N–N(C(CF3)2Ph)CH2CHSiMe3)] (4). A nickel(II) amido alkyl complex [(IPr)Ni(NHC(CF3)2Ph)(CH2CHNC(CF3)2Ph)] (5) was also isolated from the decomposition reaction of 3.

Synthesis and characterization of nickel-N-heterocyclic carbenes linked to a calix[6]arene platform and their applications in Suzuki-Miyaura cross-coupling catalysis

The synthesis and catalytic activity of a series of tris-half sandwich cyclopentadienyl nickel (II) N-heterocyclic carbene (NHC) complexes, that have approximate C3 symmetry, is described. The molecules consist of [Ni(NHC)BrCp] groups (Cp = η-C5H5) in which the NHC ligand is grafted on to the lower rim of a 1,3,5 trimethoxy p-tert butylcalix [6] arene via methylene spacers. These species are all synthesized from 1,3,5-trimethoxy-2,4,6 tris-substituted imidazolium tribromide salts via reactions with nickelocene. All complexes were fully characterized by 1H and 13C NMR spectroscopy and by mass spectrometry. Their catalytic activity in the Suzuki-Miyaura cross coupling reaction was probed.

Synthesis, characterization, in vitro antimicrobial and anticancer studies of new platinum N-heterocyclic carbene (NHC) complexes and unexpected nickel complexes

A series of bidendate pyridine-functionalized metal N-heterocyclic carbene (NHC) complexes with various wingtip substituents (R = methyl, phenyl and tert-butyl) had been prepared from silver oxide-mediated transmetalation method. The basic backbone of the NHC ligands were prepared in two stepwise reactions: reduction of the commercially available 2-benzoylpyridine, followed by halogenation to yield 2-(chloro(phenyl)methyl)pyridine. Subsequent reactions with different imidazoles gave the desired imidazolium salts (or NHC precursors) with different R groups at the N wingtip (R = Me, Ph, t-Bu). Transmetalation of the imidazolium salts into platinum NHC complexes were achieved by using Ag2O mediated method with Pt(cod)Cl2 as the transmetalating agent. On the other hand, nickel NHC complexes were obtained via one pot synthesis method with nickel chloride as the transmetalating agent and reflux in the presence of a base. Following recrystallisation, the molecular structure of Pt NHC complex ( ± )-5a had been successfully elucidated via single crystal X ray diffraction (XRD) analysis. The six-membered ring of the platinum NHC complexes adopted a boat conformation with the phenyl ring arranged at the axial position. Meanwhile, two unexpected nickel complexes, i.e. a simple nickel coordination complex and a nickelate complex, were obtained and studied using XRD. These Pt NHC complexes and unexpected Ni complexes were evaluated for their in vitro antimicrobial and anticancer activities against a panel of pathogenic microorganisms and three selected human cancer cell lines, including breast (MCF7), colon (HCT116) and oral (H103) cancer cells. Generally, the Pt NHC complexes displayed enhanced antimicrobial activities upon coordination to metals, as compared to the corresponding imidazolium salts i.e. NHC precursors, with minimum inhibitory concentration (MIC) values in micromolar (μM) range. A couple of platinum NHC complexes synthesized exhibited antimicrobial activities with MIC as low as 2 μM, which are comparable to silver NHC complexes with renowned antimicrobial profiles. Similarly, upon coordination to the metals, the cytotoxic effects of the platinum NHC complexes increased significantly as compared to the imidazolium salts. Among all, platinum NHC complex (±)-5c displayed significant cytotoxicities towards the cancer cells with IC50 values that are two to three times lower than that of the anticancer drug cisplatin. In summary, evidences for influence of both wingtip substituents and metals on the biological activities of the complexes have been found.

Benzothiazole Nickelation: An Obstacle to the Catalytic Arylation of Azoles by Cyclopentadienyl Nickel N-Heterocyclic Carbene Complexes

NiCp†L(NHC)](+) complexes (Cp† = Cp (η5-C5H5), Cp* (η5-C5Me5); NHC = N-heterocyclic carbene; L = Cl− or NCMe) have been tested as pre-catalysts for the direct arylation of benzothiazole in the presence of an alkoxide. Only the pentamethylcyclopentadienyl derivative, [NiCp*Cl(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenylimidazol-2-ylidene), enabled low conversion to the desired coupling product with phenyl iodide as the electrophilic coupling partner. In contrast, all cyclopentadienyl complexes proved to be inactive. 1H NMR studies of the “catalytic” reaction mixtures demonstrate that they cleanly convert to an unreactive C(2)-benzothiazolyl derivative, whose identity has been confirmed by an independent synthesis and characterization. The latter constitutes a potential energy well that quenches all further reactivity, and provides a rare example of C(2)-metallated azolyl complex.

Magnetic Nanoparticle Decorated N-Heterocyclic Carbene–Nickel Complex with Pendant Ferrocenyl Group for C–H Arylation of Benzoxazole

Magnetic nanoparticle decorated N-heterocyclic carbene–nickel complex with pendant ferrocenyl group has been prepared by multi-step procedure. The formation of complex was confirmed on the basis of analytical techniques such as Fourier transform infrared (FT-IR), Fourier transform Raman (FT-Raman) and X-ray photoelectron spectroscopy (XPS) as well as by X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) analysis. The complex proved to be an efficient heterogeneous catalyst for C–H arylation of benzoxazole with aryl boronic acids. The recycling studies revealed that complex could be reused for six times without significant decrease in catalytic activity.

Bench-Stable N-Heterocyclic Carbene Nickel Precatalysts for C-C and C-N Bond-Forming Reactions.

Herein, we introduce a new class of bench‐stable N‐heterocyclic carbene (NHC) nickel‐precatalysts for homogeneous nickel‐catalysis. The nickel(II) complexes are readily activated to Ni0 in situ under mild conditions, via a proposed Heck‐type mechanism. The precatalysts are shown to facilitate carbonyl‐ene, hydroalkenylation, and amination reactions.

Site-Selective Linear Alkylation of Anilides by Cooperative Nickel/Aluminum Catalysis.

We report meta- and para-selective linear alkylation reactions of anilides with alkenes by nickel/N-heterocyclic carbene (NHC) and aluminum catalysis. With a less bulky NHC, the alkylation reaction of N-methyl-N-phenylcyclohexanecarboxamides proceeded mainly at the meta position. In contrast, a bulky NHC ligand led to the para-selective alkylation of N-sec-alkyl anilides.

Site‐Selective Linear Alkylation of Anilides by Cooperative Nickel/Aluminum Catalysis

AbstractWe report meta‐ and para‐selective linear alkylation reactions of anilides with alkenes by nickel/N‐heterocyclic carbene (NHC) and aluminum catalysis. With a less bulky NHC, the alkylation reaction of N‐methyl‐N‐phenylcyclohexanecarboxamides proceeded mainly at the meta position. In contrast, a bulky NHC ligand led to the para‐selective alkylation of N‐sec‐alkyl anilides.

Computational Insight Into the Hydroamination of an Activated Olefin, As Catalyzed by a 1,2,4-Triazole-Derived Nickel(II) N-Heterocyclic Carbene Complex.

A density functional theory (DFT) investigation performed at the B3LYP/TZVP//B3LYP/6-31G(d)-LANL2DZ level of theory on the hydroamination of dimethylamine (Me2NH) on an activated olefin (namely, acrylonitrile (CH2═CHCN)), as catalyzed by a 1,2,4-triazol based nickel(II) N-heterocyclic carbene complex (namely, [1,4-dimethyl-1,2,4-triazole-5-ylidene]2 nickel dichloride) revealed that the olefin coordination pathway is favorable over the amine coordination pathway, although the initial olefin coordination step is higher in energy than the initial amine coordination step. Significantly enough, the reaction involved a crucial 1,3-proton transfer step between the resonance intermediates, i.e., the C-bound [(NHC)2Ni(CH(CN)CH2NHMe2)]+ (D) species or N-bound [(NHC)2Ni(NCCHCH2NHMe2)]+ (E) species and the intermediate [(NHC)2Ni(NCCH2CH2NMe2)]+ (F), depicting the cleavage of a N-H bond and the formation of a C-H bond facilitated by a water-assisted/amine-assisted proton shuttle. Overall, among the various pathways explored, the lowest energy pathway involved alkene coordination, followed by an amine-assisted 1,3-proton transfer step.

A hybrid imidazolylidene/imidazolium nickel NHC complex: an isolated intermediate

Macrocyclic ligand systems with a variety of (different) donor sites oftentimes give rise to very exciting and unexpected multinuclear metal complexes. We report herein the structure of a trinuclear mixed imidazolylidene/imidazolium nickel N-heterocyclic carbene (NHC) complex, namely di-μ-chlorido-bis{μ-calix[2]imidazolium[2]imidazolylidene[2]pyrazolate}trinickel(II) tetrakis(hexafluoridophosphate) acetonitrile tetrasolvate, [Ni3(C24H24N12)2Cl2](PF6)4·4CH3CN or [Ni3(LMe)2Cl2](PF6)4·4CH3CN, that can be understood as a trapped reaction intermediate during the synthesis of the respective [Ni2LMe](PF6)2 product. The structure not only contains protonated next to deprotonated imidazole heterocycles, but also Ni2+ ions with fundamentally different coordination modes within one molecule. Two of the three metal atoms are coordinated in a square-pyramidal fashion by half a ligand molecule and one chloride ligand, whereas the third Ni2+ ion is bound octahedrally by four pyrazolate moieties and two chloride anions.

Selective Reduction of Nitroarenes with Silanes Catalyzed by Nickel N‐Heterocyclic Carbene Complexes

AbstractAn efficient catalytic system for the reduction of nitroarenes to amines was developed using a well‐defined nickel–NHC (N‐heterocyclic carbene) complex as catalyst and phenylsilane as reducing agent. The excellent activity of the catalyst provides access to anilines containing a wide array of reactive functionalities at 20 °C, and without using any base or additive. Notably, the catalytic system allows the reduction of 5,10,15,20‐tetra‐(nitrophenyl)porphyrin (TNPP) and CuII β‐nitroporphyrin to the corresponding aminoporphyrins.