Synthesis of Mixed Silylene–Carbene Chelate Ligands from N‐Heterocyclic Silylcarbenes Mediated by Nickel

The Ni^II‐mediated tautomerization of the N‐heterocyclic hydrosilylcarbene L²Si(H)(CH₂)NHC 1 , where L²=CH(C?CH₂)(CMe)(NAr)₂, Ar=2,6‐iPr₂C₆H₃; NHC=3,4,5‐trimethylimidazol‐2‐yliden‐6‐yl, leads to the first N‐heterocyclic silylene (NHSi)–carbene (NHC) chelate ligand in the dibromo nickel(II) complex [L¹Si:(CH₂)(NHC)NiBr₂] 2 (L¹=CH(MeC?NAr)₂). Reduction of 2 with KC₈ in the presence of PMe₃ as an auxiliary ligand afforded, depending on the reaction time, the N‐heterocyclic silyl–NHC bromo Ni^II complex [L²Si(CH₂)NHCNiBr(PMe₃)] 3 and the unique Ni⁰ complex [η²(Si‐H){L²Si(H)(CH₂)NHC}Ni(PMe₃)₂] 4 featuring an agostic Si? H→Ni bonding interaction. When 1,2‐bis(dimethylphosphino)ethane (DMPE) was employed as an exogenous ligand, the first NHSi–NHC chelate‐ligand‐stabilized Ni⁰ complex [L¹Si:(CH₂)NHCNi(dmpe)] 5 could be isolated. Moreover, the dicarbonyl Ni⁰ complex 6 , [L¹Si:(CH₂)NHCNi(CO)₂], is easily accessible by the reduction of 2 with K(BHEt₃) under a CO atmosphere. The complexes were spectroscopically and structurally characterized. Furthermore, complex 2 can serve as an efficient precatalyst for Kumada–Corriu‐type cross‐coupling reactions.     

(NHC)NiH‐Catalyzed Intermolecular Regio‐ and Diastereoselective Cross‐Hydroalkenylation of Endocyclic Dienes with α‐Olefins

Highly selective cross‐hydroalkenylations of endocyclic 1,3‐dienes at the least substituted site with α‐olefins were achieved with a set of neutral (NHC)Ni^IIH(OTf) catalysts and cationic Ni^II catalysts with a novel NHC ligand. Under heteroatom assistance, skipped dienes were obtained in good yields, often from equal amounts of the two substrates and at a catalyst loading of 2–5 mol %. Rare 4,3‐product selectivity (i.e., with the H atom at C4 and the alkenyl group at C3 of the diene) was observed, which is different from the selectivity of known dimerizations of α‐olefins with both acyclic Co and Fe systems. The influence of the various substituents on the NHC, 1,3‐diene, and α‐olefin on the chemo‐, regio‐, and diastereoselectivity was studied. High levels of chirality transfer were observed with chiral cyclohexadiene derivatives.     

NHC/Nickel(II)-Catalyzed [3+2] Cross-Dimerization of Unactivated Olefins and Methylenecyclopropanes

Cross-dimerization of a methylenecyclopropane ( 1 ) and an unactivated alkene ( 2 ) with typical hydroalkenylation reactivity was observed for the first time by using a [NHC-Ni(allyl)]BAr^F catalyst (NHC=N-heterocyclic carbene). Results show that the C−C cleavage of 1 did not involve a Ni⁰ oxidative addition, which was crucial in former systems. Thus the method reported here emerges as a complementary method for attaining highly chemo- and regioselective synthesis of methylenecyclopentanes ( 3 ) with broad scope. An efficient NHC/Ni^II-catalyzed rearrangement of 1 leads to the convergent synthesis of 3 in the presence of 2 .     

[(NHC)NiIIH]‐Catalyzed Cross‐Hydroalkenylation of Cyclopropenes with Alkynes: Cyclopentadiene Synthesis by [(NHC)NiII]‐Assisted C−C Rearrangement

A cross‐hydroalkenylation/rearrangement cascade (HARC), using a cyclopropene and alkyne as substrate pairs, was achieved for the first time by using new [(NHC)Ni(allyl)]BAr^F catalysts (NHC=N‐heterocyclic carbenes). By controlling the (NHC)Ni^IIH relative insertion reactivity with cyclopropene and alkyne, a broad scope of cyclopentadienes was obtained with highly selectively. The structural features of the new (NHC)Ni^II catalyst were important for the success of the reaction. The mild reaction conditions employed may serve as an entry for exploring (NHC)Ni^II‐assisted vinylcyclopropane rearrangement reactivity.     

Synthesis,crystal structures,spectral FT‐IR,NMR and UV–Vis investigations and Hirshfeld surface analysis of two new 2‐hydroxyethyl‐substituted N‐heterocyclic carbene precursors

This study discusses the synthesis of two new 2‐hydroxyethyl substituted N‐heterocyclic carbene (NHC) precursors. The NHC precursors were prepared from 1‐(alkyl/aryl)benzimidazole and alkyl halides. They were characterized using ¹H NMR, ¹³C NMR, FT‐IR, UV–Vis spectroscopy, and elemental analysis techniques. Molecular and crystal structures of 1 and 2 were determined using the single‐crystal X‐ray diffraction method. Crystal structure of the compounds features NHC precursors and chloride anions. Additionally in 2 , the asymmetric unit has a water molecule, which forms a tetrameric chloride‐hydrate assembly with the chloride anion. The chloride anions play an important role in the stabilization of crystal structures to form a two‐dimensional supramolecular architecture. The 3D Hirshfeld surface and the associated 2D fingerprint plots were also drawn to gain insights into the behavior of the interactions in the compounds.     

Synthesis of a Metallo‐Iminosilane via a Silanone–Metal π‐Complex

Facile oxygenation of the acyclic amido‐chlorosilylene bis(N‐heterocyclic carbene) Ni⁰ complex [{N(Dipp)(SiMe₃)ClSi:→Ni(NHC)₂] ( 1 ; Dipp=2,6‐ⁱPr₂C₆H₄; N‐heterocyclic carbene=C[(ⁱPr)NC(Me)]₂) with N₂O furnishes the first Si‐metalated iminosilane, [DippN=Si(OSiMe₃)Ni(Cl)(NHC)₂] ( 3 ), in a rearrangement cascade. Markedly, the formation of 3 proceeds via the silanone (Si=O)–Ni π‐complex 2 as the initial product, which was predicted by DFT calculations and observed spectroscopically. The Si=O and Si=N moieties in 2 and 3 , respectively, show remarkable hydroboration reactivity towards H−B bonds of boranes, in the former case corroborating the proposed formation of a (Si=O)–Ni π‐complex at low temperature.     

NHC‐Pd complex based on 1,3‐bis (4‐ethoxycarbonylphenyl) imidazolium chloride: synthesis,structure and catalytic activity in the synthesis of axially chiral benzophenone hydrazone

A novel NHC–Pd complex of 1,3‐bis (4‐ethoxycarbonylphenyl) imidazolium chloride has been synthesized and characterized by ¹H NMR, ¹³C NMR, IR and X‐ray single‐crystal diffraction studies. TG analysis shows that the NHC‐Pd complex is stable under 208 °C. The catalytic activities have been explored for the synthesis of axially chiral N‐(2′‐methoxy‐1,1′‐binaphthalen‐2‐yl) benzophenone hydrazone. The result indicates that the novel NHC‐Pd complex can achieve better catalytic activity than the Pd‐phosphine catalysts in the synthesis of axially chiral N‐(2′‐methoxy‐1,1′‐binaphthalen‐2‐yl) benzophenone hydrazone.     

General and Mild Ni0‐Catalyzed α‐Arylation of Ketones Using Aryl Chlorides

A general methodology for the α‐arylation of ketones using a nickel catalyst has been developed. The new well‐defined [Ni(IPr*)(cin)Cl] ( 1 c ) pre‐catalyst showed great efficiency for this transformation, allowing the coupling of a wide range of ketones, including acetophenone derivatives, with various functionalised aryl chlorides. This cinnamyl‐based Ni–N‐heterocyclic carbene (NHC) complex has demonstrated a different behaviour to previously reported NHC‐Ni catalysts. Preliminary mechanistic studies suggest a Ni⁰/Ni^II catalytic cycle to be at play.     

Protic N‐Heterocyclic Carbene Versus Pyrazole: Rigorous Comparison of Proton‐ and Electron‐Donating Abilities in a Pincer‐Type Framework

Evaluation of the acidity of proton‐responsive ligands such as protic N‐heterocyclic carbenes (NHCs) bearing an NH‐wingtip provides a key to understanding the metal–ligand cooperation in enzymatic and artificial catalysis. Here, we design a CNN pincer‐type ruthenium complex 2 bearing protic NHC and isoelectronic pyrazole units in a symmetrical skeleton, to compare their acidities and electron‐donating abilities. The synthesis is achieved by direct C?H metalation of 2‐(imidazol‐1‐yl)‐6‐(pyrazol‐3‐yl)pyridine with [RuCl₂(PPh₃)₃]. ¹⁵N‐Labeling experiments confirm that deprotonation of 2 occurs first at the pyrazole side, indicating clearly that the protic pyrazole is more acidic than the NHC group. The electrochemical measurements as well as derivatization to carbonyl complexes demonstrate that the protic NHC is more electron‐donating than pyrazole in both protonated and deprotonated forms.     

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.     

Nickel(I) Monomers and Dimers with Cyclopentadienyl and Indenyl Ligands

The reaction of (μ‐Cl)₂Ni₂(NHC)₂ (NHC=1,3‐bis(2,6‐diisopropylphenyl)‐1,3‐dihydro‐2H‐imidazol‐2‐ylidene (IPr) or 1,3‐bis(2,6‐diisopropylphenyl)imidazolidin‐2‐ylidene (SIPr)) with either one equivalent of sodium cyclopentadienyl (NaCp) or lithium indenyl (LiInd) results in the formation of diamagnetic NHC supported Ni^I dimers of the form (μ‐Cp)(μ‐Cl)Ni₂(NHC)₂ (NHC=IPr ( 1 a ) or SIPr ( 1 b ); Cp=C₅H₅) or (μ‐Ind)(μ‐Cl)Ni₂(NHC)₂ (NHC=IPr ( 2 a ) or SIPr ( 2 b ); Ind=C₇H₉), which contain bridging Cp and indenyl ligands. The corresponding reaction between two equivalents of NaCp or LiInd and (μ‐Cl)₂Ni₂(NHC)₂ (NHC=IPr or SIPr) generates unusual 17 valence electron Ni^I monomers of the form (η⁵‐Cp)Ni(NHC) (NHC=IPr ( 3 a ) or SIPr ( 3 b )) or (η⁵‐Ind)Ni(NHC) (NHC=IPr ( 4 a ) or SIPr ( 4 b )), which have nonlinear geometries. A combination of DFT calculations and NBO analysis suggests that the Ni^I monomers are more strongly stabilized by the Cp ligand than by the indenyl ligand, which is consistent with experimental results. These calculations also show that the monomers have a lone unpaired‐single‐electron in their valence shell, which is the reason for the nonlinear structures. At room temperature the Cp bridged dimer (μ‐Cp)(μ‐Cl)Ni₂(NHC)₂ undergoes homolytic cleavage of the Ni?Ni bond and is in equilibrium with (η⁵‐Cp)Ni(NHC) and (μ‐Cl)₂Ni₂(NHC)₂. There is no evidence that this equilibrium occurs for (μ‐Ind)(μ‐Cl)Ni₂(NHC)₂. DFT calculations suggest that a thermally accessible triplet state facilitates the homolytic dissociation of the Cp bridged dimers, whereas for bridging indenyl species this excited triplet state is significantly higher in energy. In stoichiometric reactions, the Ni^I monomers (η⁵‐Cp)Ni(NHC) or (η⁵‐Ind)Ni(NHC) undergo both oxidative and reductive processes with mild reagents. Furthermore, they are rare examples of active Ni^I precatalysts for the Suzuki–Miyaura reaction. Complexes 1 a , 2 b , 3 a , 4 a and 4 b have been characterized by X‐ray crystallography.     

N‐Heterocyclic Carbene Stabilized Dicarbondiphosphides: Strong Neutral Four‐Membered Heterocyclic 6π‐Electron Donors

In contrast to cyclic π‐conjugated hydrocarbons, the coordination chemistry of inorganic heterocycles is less developed. Dicarbondiphosphides stabilized by N‐heterocyclic carbenes (NHCs) NHC→C₂P₂←NHC ( 1 a , b ) (NHC=IPr or SIPr) contain a four‐membered C₂P₂ ring with an aromatic 6π‐electron configuration. These heterocycles coordinate to a variety of complex fragments with metals from groups 6, 9, and 10, namely [M⁰(CO)₃] (M=Cr, Mo), [Co^I(CO)₂]⁺, or [Ni^IIBr₂], through an η⁴‐coordination mode, leading to complexes 2 a , b , 3 a , b , 5 a , b , and 6 a , b , respectively. These complexes were characterized by X‐ray diffraction methods using single crystals, IR spectroscopy, and DFT calculations. In combination these methods indicate that 1 a , b behave as exceptionally strong 6π‐electron donors.     

Ammonia Activation by a Nickel NCN‐Pincer Complex featuring a Non‐Innocent N‐Heterocyclic Carbene: Ammine and Amido Complexes in Equilibrium

A Ni⁰‐NCN pincer complex featuring a six‐membered N‐heterocyclic carbene (NHC) central platform and amidine pendant arms was synthesized by deprotonation of its Ni^II precursor. It retained chloride in the square‐planar coordination sphere of nickel and was expected to be highly susceptible to oxidative addition reactions. The Ni⁰ complex rapidly activated ammonia at room temperature, in a ligand‐assisted process where the carbene carbon atom played the unprecedented role of proton acceptor. For the first time, the coordinated (ammine) and activated (amido) species were observed together in solution, in a solvent‐dependent equilibrium. A structural analysis of the Ni complexes provided insight into the highly unusual, non‐innocent behavior of the NHC ligand.     

Rhodium(I) and Silver(I) Complexes of 4, 5‐Dicyano‐1, 3‐dimesityl‐ and 4, 5‐Dicyano‐1, 3‐dineopentylimidazol‐2‐ylidene

The new N‐heterocyclic carbene (NHC) precursors 4, ‐dicyano‐1, ‐dimesityl‐ ( 9 ) and 4, 5‐dicyano‐1, 3‐dineopentyl‐2‐(pentafluorophenyl)imidazoline ( 14 ) were synthesized. The structure of 9 could be determined by X‐ray crystallography. With the 2‐pentafluorophenyl‐substituted imidazolines 9 and 14 , the [AgCl(NHC)], [RhCl(COD)(NHC)], and [RhCl(CO)₂(NHC)] complexes [NHC = 4, 5‐dicyano‐1, 3‐dimesitylimidazol‐2‐ylidene ( 3 ) and 4, 5‐dicyano‐1, 3‐dineopentylimidazol‐2‐ylidene ( 4 )] were obtained. Crystal structures of [AgCl( 3 )] ( 15 ), [RhCl(COD)( 3 )] ( 17 ), [RhCl(COD)( 4 )] ( 18 ), and [RhCl(CO)₂( 3 )] ( 19 ) were solved and with the crystal data of 19 , the percent buried volume ( %V_bur) of 31.8(±0.1) % was determined for NHC 3 . Infrared spectra of the imidazolines 9 and 14 and of the complexes 15 – 20 were recorded and the CO stretching frequencies of complexes 19 and 20 were used to determine the Tolman electronic parameters of the newly obtained NHCs 3 (TEP: 2060 cm^–1) and 4 (TEP: 2061 cm^–1), thus proving that 1, 3‐substitution of maleonitrile‐NHCs does not have a significant effect for the high π‐acceptor strength of these carbenes.     

N‐Phenyl‐substituted carbene precursors and their silver complexes: synthesis,characterization and antimicrobial activities

A series of unsymmetrically substituted N‐heterocyclic carbene (NHC) precursors ( 1a , 1b , 1c , 1d , 1e ) were synthesized from the reaction of N‐phenylbenzimidazole with various alkyl halides. These compounds were used to synthesize NHC–silver(I) complexes ( 2a , 2b , 2c , 2d , 2e ). The five new 1‐phenyl‐3‐alkylbenzimidazolium salts ( 1a , 1b , 1c , 1d , 1e ) and their NHC–silver complexes ( 2a , 2b , 2c , 2d , 2e ) were characterized by the ¹H NMR, ¹³C NMR and FT‐IR spectroscopic methods and elemental analysis techniques. Also, the two NHC–silver complexes 2b and 2c were characterized by single‐crystal X‐ray crystallography, which confirmed the linear C―Ag―Cl arrangements. The antibacterial activities of the NHC precursor and NHC–silver complexes were tested against three Gram‐positive bacterial strains (Bacillus subtilis, Listeria monocytogenes and Staphylococcus aureus) and three Gram‐negative bacterial strains (Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa) using the microdilution broth method. The NHC–silver complexes showed higher antibacterial activity than the NHC precursors. In addition, silver complexes 2a , 2b , 2c , 2d showed high antibacterial activity against the Gram‐positive bacteria L. monocytogenes and S. aureus compared to the standard, tetracycline. Copyright © 2014 John Wiley & Sons, Ltd.     

Phosphine‐NHC Manganese Hydrogenation Catalyst Exhibiting a Non‐Classical Metal‐Ligand Cooperative H2 Activation Mode

Deprotonation of the Mn^I NHC‐phosphine complex fac‐[MnBr(CO)₃(κ²P,C‐Ph₂PCH₂NHC)] ( 2 ) under a H₂ atmosphere readily gives the hydride fac‐[MnH(CO)₃(κ²P,C‐Ph₂PCH₂NHC)] ( 3 ) via the intermediacy of the highly reactive 18‐e NHC‐phosphinomethanide complex fac‐[Mn(CO)₃(κ³P,C,C‐Ph₂PCHNHC)] ( 6 a ). DFT calculations revealed that the preferred reaction mechanism involves the unsaturated 16‐e mangana‐substituted phosphonium ylide complex fac‐[Mn(CO)₃(κ²P,C‐Ph₂P=CHNHC)] ( 6 b ) as key intermediate able to activate H₂ via a non‐classical mode of metal‐ligand cooperation implying a formal λ⁵‐P–λ³‐P phosphorus valence change. Complex 2 is shown to be one of the most efficient pre‐catalysts for ketone hydrogenation in the Mn^I series reported to date (TON up to 6200).     

Chelation‐Assisted Nickel‐Catalyzed Oxidative Annulation via Double C−H Activation/Alkyne Insertion Reaction

A nickel/NHC system for regioselective oxidative annulation by double C?H bond activation and concomitant alkyne insertion is described. The catalytic reaction requires a bidentate directing group, such as an 8‐aminoquinoline, embedded in the substrate. Various 5,6,7,8‐tetrasubstituted‐N‐(quinolin‐8‐yl)‐1‐naphthamides can be prepared as well as phenanthrene and benzo[h]quinoline amide derivatives. Diarylalkynes, dialkylalkynes, and arylalkylalkynes can be used in the system. A Ni⁰/Ni^II catalytic cycle is proposed as the main catalytic cycle. The alkyne plays a double role as a two‐component coupling partner and as a hydrogen acceptor.     

Phenylimidorhenium(V) Complexes with 1,3‐Diethyl‐4,5‐dimethylimidazole‐2‐ ylidene Ligands

The phenylimidorhenium(V) complexes [Re(NPh)X₃(PPh₃)₂] (X = Cl, Br) react with the N‐heterocyclic carbene (NHC) 1,3‐diethyl‐4,5‐dimethylimidazole‐2‐ylidene (L^Et) under formation of the stable rhenium(V) complex cations [Re(NPh)X(L^Et)₄]²⁺ (X = Cl, Br), which can be isolated as their chloride or [PF₆]^? salts. The compounds are remarkably stable against air, moisture and ligand exchange. The hydroxo species [Re(NPh)(OH)(L^Et)₄]²⁺ is formed when moist solvents are used during the synthesis. The rhenium atoms in all three complexes are coordinated in a distorted octahedral fashion with the four NHC ligands in equatorial planes of the molecules. The Re–C(carbene) bond lengths between 2.171(8) and 2.221(3) Å indicate mainly σ‐bonding between the NHC ligand and the electron deficient d² metal atoms. Attempts to prepare analogous phenylimido complexes from [Re(NPh)Cl₃(PPh₃)₂] and 1,3‐diisopropyl‐4,5‐dimethylimidazole‐2‐ylidene (L^i?Pr) led to a cleavage of the rhenium‐nitrogen multiple bond and the formation of the dioxo complex [ReO₂(L^i?Pr)₄]⁺.     

Pyridine‐Enhanced Head‐to‐Tail Dimerization of Terminal Alkynes by a Rhodium–N‐Heterocyclic‐Carbene Catalyst

A general regioselective rhodium‐catalyzed head‐to‐tail dimerization of terminal alkynes is presented. The presence of a pyridine ligand (py) in a Rh–N‐heterocyclic‐carbene (NHC) catalytic system not only dramatically switches the chemoselectivity from alkyne cyclotrimerization to dimerization but also enhances the catalytic activity. Several intermediates have been detected in the catalytic process, including the π‐alkyne‐coordinated Rh^I species [RhCl(NHC)(η²‐HC?CCH₂Ph)(py)] ( 3 ) and [RhCl(NHC){η²‐C(tBu)?C(E)CH?CHtBu}(py)] ( 4 ) and the Rh^III–hydride–alkynyl species [RhClH{? C?CSi(Me)₃}(IPr)(py)₂] ( 5 ). Computational DFT studies reveal an operational mechanism consisting of sequential alkyne C? H oxidative addition, alkyne insertion, and reductive elimination. A 2,1‐hydrometalation of the alkyne is the more favorable pathway in accordance with a head‐to‐tail selectivity.     

Ringkontraktion durch NHC‐induzierte Pnictogen‐Abstraktion

Die Reaktion von [Cp′′′Co(η⁴‐P₄)] ( 1 ) (Cp′′′=1,2,4‐tBu₃C₅H₂) mit ^MeNHC (^MeNHC=1,3,4,5‐tetramethylimidazol‐2‐ylidene) führt über eine NHC‐induzierte Phosphorkationen‐Abstraktion zum Ringkontraktionsprodukt [(^MeNHC)₂P][Cp′′′Co(η³‐P₃)] ( 2 ), welches das erste Beispiel eines anionischen CoP₃‐Komplexes repräsentiert. Solche von NHCs induzierten Ringkontraktionsreaktionen lassen sich ebenfalls auf Tripeldecker‐Sandwich‐Komplexe anwenden. So werden die Komplexe [(Cp*Mo)₂(μ,η^6:6‐E₆)] ( 3 a , 3 b ) (Cp*=C₅Me₅; E=P, As) zu den Komplexen [(^MeNHC)₂E][(Cp*M)₂(μ,η^3:3‐E₃)(μ,η^2:2‐E₂)] ( 4 a , 4 b ) transformiert, wobei 4 b das erste strukturell charakterisierte Beispiel eines NHC‐substituierten As^I‐Kations darstellt. Darüber hinaus führt die Reaktion des Vanadium‐Komplexes [(Cp*V)₂(μ,η^6:6‐P₆)] ( 5 ) mit ^MeNHC zur Bildung der neuartigen Komplexe [(^MeNHC)₂P][(Cp*V)₂(μ,η^6:6‐P₆)] ( 6 ), [(^MeNHC)₂P][(Cp*V)₂(μ,η^5:5‐P₅)] ( 7 ) bzw. [(Cp*V)₂(μ,η^3:3‐P₃)(μ,η^1:1‐P{^MeNHC})] ( 8 ).