4-Vinylbenzyl-substituted silver(I) N-heterocyclic carbene complexes and ruthenium(II) N-heterocyclic carbene complexes: synthesis and transfer hydrogenation of ketones

4-Vinylbenzyl-substituted Ag(I) N-heterocyclic carbene (NHC) complexes and Ru(II) NHC complexes have been synthesized. The Ag(I) complexes were synthesized from the imidazolium salts and Ag₂O in dichloromethane at room temperature. The Ru(II) complexes were prepared from Ag(I) NHC complexes by transmetallation. The six 4-Vinylbenzyl-substituted Ag(I) NHC complexes and six 4-Vinylbenzyl-substituted Ru(II) NHC complexes have been characterized by spectroscopic techniques and elemental analyses. The Ru(II) NHC complexes show catalytic activity for the transfer hydrogenation of ketones.     

N-Heterocyclic carbene functionalized iridium phosphinidene complex [Cp*(NHC)Ir=PMes*]: comparison of phosphinidene,imido, and carbene complexes

The novel phosphinidene complex [Cp*(NHC)Ir=PMes*] (3; NHC=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) was prepared in high yield from [Cp*(NHC)IrCl(2)] (2) and [LiPHMes*].3 THF. It represents the first example of an NHC ligated transition metal phosphinidene complex. The X-ray crystal structure for 3 is also reported. DFT calculations on the N-heterocyclic carbene containing parent complexes [Cp(NHC)Ir=E] (E=PH, NH, CH(2)) show that the NHC ligand acts as good sigma-donor/weak pi-acceptor ligand and forms strong Ir-C(NHC) single bonds. The Ir=E double bonds result from strong triplet-triplet interactions between [Cp(NHC)Ir] and E.     

[(NHC)CrCl(μ-Cl)(THF)]_2 and(NHC)_2CrCl_2(NHC=1,3-Diisopropyl-4,5-dimethylimidazole-2-ylidene):Syntheses,Structures,and Polymerization Activities

The preparation of divalent chromium N-heterocyclic carbene(NHC,1,3-diisopropyl4,5-dimethylimidazole-2-ylidene) compounds is reported.The reaction of 1:1 molar ratio of NHC with CrCl2 led to an isolation of [(NHC)CrCl(μ-Cl)(THF)]2(1),while that of 2:1 ratio resulted in the formation of(NHC)2CrCl2(2).1 can be considered as an intermediate in the formation of 2 and further converted into 2 by the addition of another equiv.of NHC.The reaction of 2 with CpNa afforded an ion pair compound [(NHC)2CrCp]+[Cp]-(3),indicating a strong coordination ability of NHC supplanting one of the ionic Cr-Cp bonding.In combination of methylalumoxane(MAO) as cocatalyst 1 and 2 both are active for catalyzing ethylene polymerization.     

Stabilisation of iridium(III) fluoride complexes with NHCs

The first neutral, [IrClF(2)(NHC)(COD)] and [IrClF(2)(CO)(2)(NHC)] (NHC = IMes, IPr), and cationic, [IrF(2)py(IMes)(COD)][BF(4)] and [IrF(2)L(CO)(2)(NHC)][BF(4)] (NHC = IMes, L = PPh(2)Et, PPh(2)CCPh, py; NHC = IPr, L = py), NHC iridium(III) fluoride complexes, have been synthesised by the xenon difluoride oxidation of iridium(I) substrates. The stereochemistries of these iridium(III) complexes have been confirmed by multinuclear NMR spectroscopy in solution and no examples of fluoride-trans-NHC arrangements were observed. Throughout, CO was found to be a better co-ligand for the stabilisation of the iridium(III) fluoride complexes than COD. Attempts to generate neutral trifluoroiridium(III) complexes, [IrF(3)(CO)(NHC)], via the ligand substitution reaction of [IrF(3)(CO)(3)] with the free NHCs were unsuccessful.     

The Quest for Stable Silaaldehydes: Synthesis and Reactivity of a Masked Silacarbonyl

The first donor–acceptor complex of a silaaldehyde, with the general formula (NHC)(Ar)Si(H)OGaCl₃ (NHC=N-heterocyclic carbene), was synthesized using the reaction of silyliumylidene–NHC complex [(NHC)₂(Ar)Si]Cl with water in the presence of GaCl₃. Conversion of this complex to the corresponding silacarboxylate dimer [(NHC)(Ar)SiO₂GaCl₂]₂, free silaacetal ArSi(H)(OR)₂, silaacyl chloride (NHC)(Ar)Si(Cl)OGaCl₃, and phosphasilene–NHC adduct (NHC)(Ar)Si(H)PTMS unveil its true potential as a synthon in silacarbonyl chemistry.     

Structures and stabilities of group 13 adducts [(NHC)(EX3)] and [(NHC)2(E2X(n))] (E=B to In; X=H, Cl; n=4, 2, 0; NHC=N-heterocyclic carbene) and the search for hydrogen storage systems: a theoretical study

Quantum chemical calculations using density functional theory at the BP86/TZVPP level and ab initio calculations at the SCS-MP2/TZVPP level have been carried out for the group 13 complexes [(NHC)(EX(3))] and [(NHC)(2)(E(2)X(n))] (E=B to In; X=H, Cl; n=4, 2, 0; NHC=N-heterocyclic carbene). The monodentate Lewis acids EX(3) and the bidentate Lewis acids E(2) X(n) bind N-heterocyclic carbenes rather strongly in donor-acceptor complexes [(NHC)(EX(3))] and [(NHC)(2)(E(2)X(n))]. The equilibrium structures of the bidentate complexes depend on the electronic reference state of E(2)X(n), which may vary for different atoms E and X. All complexes [(NHC)(2)(E(2)X(4))] possess C(s) symmetry in which the NHC ligands bind in a trans conformation to the group 13 atoms E. The complexes [(NHC)(2)(E(2)H(2))] with E=B, Al, Ga have also C(s) symmetry with a trans arrangement of the NHC ligands and a planar CE(H)E(H)C moiety that has a E=E π bond. In contrast, the indium complex [(NHC)(2)(In(2) H(2))] has C(i) symmetry with pyramidal-coordinated In atoms in which the hydrogen atoms are twisted above and below the CInInC plane. The latter C(i) form is calculated for all chloride systems [(NHC)(2)(E(2)Cl(2))], but the boron complex [(NHC)(2)(B(2)Cl(2))] deviates only slightly from C(s) symmetry. The B(2) fragment in the linear coordinated complex [(NHC)(2)(B(2))] has a highly excited (3)(1)Σ(g)(-) reference state, which gives an effective B≡B triple bond with a very short interatomic distance. The heavier homologues [(NHC)(2)(E(2))] (E=Al to In) exhibit a anti-periplanar arrangement of the NHC ligands in which the E(2) fragments have a (1)(1) Δ(g) reference state and an E=E double bond. The calculated energies suggest that the dihydrogen release from the complexes [(NHC)(EH(3))] and [(NHC)(2)(E(2)H(n))] becomes energetically more favourable when atom E becomes heavier. The indium complexes should therefore be the best candidates of the investigated series for hydrogen-storage systems that could potentially deliver dihydrogen at close to ambient temperature. The hydrogenation reaction of the dimeric magnesium(I) compound [LMgMgL] (L=β-diketiminate) with [(NHC)(EH(3))] becomes increasingly exothermic with the trend B相似文献   

Palladium(II) N-heterocyclic carbene complexes: synthesis,structures and cytotoxicity potential studies against breast cancer cell line

Abstract

Mononuclear trans-Pd(II)–NHC complexes (where NHC?=?N-heterocyclic carbene) bearing asymmetrically substituted NHC-ligand have been synthesized via transmetalation reaction between Ag(I)–NHC complexes and [Pd(NCCH₃)₂Cl₂]. The NHC precursors are accessible in two steps by N-n-alkyl reactions of benzimidazole. The resultant benzimidazolium salts were deprotonated with Ag₂O by in situ deprotonation to facilitate the formation of mononuclear Ag(I)–NHC complexes. Single-crystal structural study for Pd(II)–NHC shows that the palladium(II) ion exhibits a square-planar geometry of two NHC ligands and two chloride ions. The cytotoxicity study was investigated against breast cancer cell line (MCF-7). The Ag(I)–NHC complexes exhibit better activities than their corresponding Pd(II)–NHC complexes, whereas all benzimidazolium salts are inactive toward MCF-7 cancer cell line.     

Structure and bonding in three-coordinate N-heterocyclic carbene adducts of iron(II) bis(trimethylsilyl)amide

The molecular structures, chemical bonding and magnetochemistry of the three-coordinate iron(II) NHC complexes [(NHC)Fe{N(SiMe(3))(2)}(2)] (NHC = IPr, 2; NHC = IMes, 3) are reported.     

The Fate of NHC‐Stabilized Dicarbon

The attempted synthesis of NHC‐stabilized dicarbon (NHC?C?C?NHC) through deprotonation of a doubly protonated precursor ([NHC?CH?CH?NHC]²⁺) is reported. Rather than deprotonation, a clean reduction to NHC?CH?CH?NHC is observed with a variety of bases. The apparent resistance towards deprotonation to the target compound led to a reinvestigation of the electronic structure of NHC→C?C←NHC, which showed that the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) gap is likely too small to allow for isolation of this species. This is in contrast to the recent isolation of the cyclic alkylaminocarbene analogue (cAAC?C?C?cAAC), which has a large HOMO–LUMO gap. A detailed theoretical study illuminates the differences in electronic structures between these molecules, highlighting another case of the potential advantages of using cAAC rather than NHC as a ligand. The bonding analysis suggests that the dicarbon compounds are well represented in terms of donor–acceptor interactions L→C₂←L (L=NHC, cAAC).     

Imidazolium carboxylates as versatile and selective N-heterocyclic carbene transfer agents: synthesis, mechanism, and applications

N,N'-Disubstituted imidazolium carboxylates, readily synthetically available, isolable, air- and water-stable reagents, efficiently transfer N-heterocyclic carbene (NHC) groups to Rh, Ir, Ru, Pt, and Pd, to give novel NHC complexes, e.g., [Pd(NHC)3OAc]OAc and [Pt(NHC)3Cl]Cl (NHC = 1,3-dimethyl imidazol-2-ylidene). The NHC esters are also effective. Tuning the reaction conditions for NHC transfer can give either mono- or bis-NHCs, or bis- and tris-NHCs. A net N to C rearrangement of the N-alkyl imidazole complex to the corresponding NHC complex was seen with (MeO)2CO (DMC). DFT calculations identify the steps needed to form the carboxylate from imidazole and DMC: SN2 methyl transfer from DMC to imidazole, followed by proton transfer from the imidazolium CH to the carboxylate counterion, produces the free NHC H-bonded to MeOH with a weakly associated CO2. The nucleophilic NHC attacks CO2 to form NHC-CO2. NHC transfer to the metal with loss of CO2 has been calculated for Rh(cod)Cl. A proposed two-cis-site reactivity model rationalizes the experimental data: two such vacant sites at the metal are needed to allow coordination of the NHC-CO2 carboxylate and subsequent CC cleavage with NHC transfer. Partial cod decoordination or chloride loss is thus required for Rh(cod)Cl. Chloride dissociation, calculated to be easier in polar solvent, is confirmed experimentally from the retarding effect of excess chloride.     

Stepwise synthesis of a hydrido, N-heterocyclic dicarbene iridium(III) pincer complex featuring mixed NHC/abnormal NHC ligands

We describe a stepwise synthesis of the hydrido, N-heterocyclic dicarbene iridium(III) pincer complex [Ir(H)I(C(NHC)CC(aNHC))(NCMe)] (3) which features a combination of normal and abnormal NHC ligands. The reaction of the bis(imidazolium) diiodide [(CH(imid)CHCH(imid))]I(2) (1) with [Ir(μ-Cl)(cod)](2) afforded first the mono-NHC Ir(I) complex [IrI(cod)(CH(imid)CHC(NHC))]I (2), which was then reacted with 2 equiv. of Cs(2)CO(3) in acetonitrile at 60 °C for 40 h to yield 3. These observations support our previously proposed mechanism for the formation of hydrido, N-heterocyclic dicarbene iridium(III) pincer complexes from the reaction of bis(imidazolium) salts with weak bases involving a mono-NHC Ir(I) intermediate. We describe the reactivity of the mono-NHC Ir(I) complex 2 under various conditions. By changing the reaction solvent from MeCN to toluene, we observed the cleavage of the imidazol-2-ylidene ring and the formation of an iminoformamide-containing mono-NHC Ir(I) complex [IrI(cod){[NHCH=CHN(Ad)CHO]CHC(NHC)}] (4). Complex 4 was also prepared in high yield from the reaction of 2 with strong bases (potassium tert-butoxide or potassium hexamethyldisilazane), via the initial formation of the complex [IrI(cod)(CH(NHC)CHC(NHC))] (5), which contains a coordinated NHC moiety and a free carbene arm, followed by subsequent hydrolysis of the latter. The bis(imidazolium) salt 1 can be deprotonated by strong bases to form the bis(carbene) ligand C(NHC)CHC(NHC) (6), which readily reacts with [Ir(μ-Cl)(cod)](2) to give the dinuclear complex [{IrI(cod)}(2)(μ-C(NHC)CHC(NHC))] (7), in which the N-heterocyclic bis(carbene) ligand bridges the two metals through the carbene carbon atoms.     

Direct Synthesis of N-Heterocyclic Carbene-Stabilized Copper Nanoparticles from an N-Heterocyclic Carbene–Borane

N-Heterocyclic carbene (NHC)-stabilized copper nanoparticles (NPs) were synthesized from an NHC–borane adduct and mesitylcopper(I) under thermal conditions (refluxing toluene for 2.5 h). NPs with a size distribution of 11.6±1.8 nm were obtained. The interaction between Cu NPs and NHC ligands was probed by X-ray photoelectron spectroscopy, which showed covalent binding of the NHC to the surface of the NPs. Mechanistic studies suggested that NHC–borane plays two roles: contributing to the reduction of [CuMes]₂ to release Cu⁰ species and providing NHC ligands to stabilize the copper NPs.     

Syntheses, crystal structures, reactivity, and photochemistry of gold(III) bromides bearing N-heterocyclic carbenes

Gold(I) complexes bearing N-heterocyclic carbenes (NHC) of the type (NHC)AuBr (3a/3b) [NHC = 1-methyl-3-benzylimidazol-2-ylidene (= MeBnIm), and 1,3-dibenzylimidazol-2-ylidene (= Bn(2)Im)] are prepared by transmetallation reactions of (tht)AuBr (tht = tetrahydrothiophene) and (NHC)AgBr (2a/2b). The homoleptic, ionic complexes [(NHC)(2)Au]Br (6a/6b) are synthesized by the reaction with free carbene. Successive oxidation of 3a/3b and 6a/6b with bromine gave the respective (NHC)AuBr(3) (4a/4b) and [(NHC)(2)AuBr(2)]Br (7a/7b) in good overall yields as yellow powders. All complexes were characterized by NMR spectroscopy, mass spectrometry, elemental analysis and single crystal X-ray diffraction. Reactions of the Au(III) complexes towards anionic ligands like carboxylates, phenolates and thiophenolates were investigated and result in a complete or partial reduction to a Au(I) complex. Irradiation of the Au(III) complexes with UV light yield the Au(I) congeners in a clean photo-reaction.     

Dynamic behaviour attributed to chiral carbohydrate substituents of N-heterocyclic carbene ligands in square planar nickel complexes

Nickel complexes having acetylated glucopyranosyl group incorporated N-heterocyclic carbene (NHC) ligands with methyl or benzyl groups as an N-substituent exhibit two kinds of dynamic behaviours in solution (1)H NMR spectroscopy. One of the dynamic behaviours is attributed to the anti- and syn-rotamers, which occur by the rotation of the unsymmetrical NHC ligands around the axes of the Ni-C bonds. The other is attributed to the diastereomers of the syn-rotamers, which occur by opposite rotation of the imidazolylidene rings and the chiral carbohydrate group incorporated into the NHC ligands. Crystallographic analysis of the nickel complex having the NHC ligand with acetylated glucopyranosyl and benzyl groups as N-substituents showed CH-π interaction between the glucopyranosyl unit of each NHC ligand and the phenyl ring of the other NHC ligand in the complex in the solid state.     

Palladium, rhodium and platinum complexes of ortho-xylyl-linked bis-N-heterocyclic carbenes: Synthesis, structure and catalytic activity

New Pd(II), Pt(II) and Rh(I) N-heterocyclic carbene (NHC) complexes containing two NHC units linked by an ortho-xylyl group are described and structurally and spectroscopically characterised. The Pt(II) complexes represent the first examples of Pt-bis(NHC) complexes where the NHC units are linked by an ortho-xylyl group. Functionalisation of the bis(NHC) ligands with heptyl groups has been used as a means of enhancing the solubility of the complexes, in order to facilitate spectroscopic characterisation and catalytic studies. The catalytic activity of the palladium(II) complexes in Heck and Suzuki cross-coupling reactions has been examined to investigate any effects of the diverse structural changes, though these appear to be insignificant.     

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.     

Ionic Pd/NHC Catalytic System Enables Recoverable Homogeneous Catalysis: Mechanistic Study and Application in the Mizoroki–Heck Reaction

N-Heterocyclic carbene (NHC) ligands are ubiquitously utilized in catalysis. A common catalyst design model assumes strong M–NHC binding in this metal–ligand framework. In contrast to this common assumption, we demonstrate here that lability and controlled cleavage of the M−NHC bond (rather than its stabilization) could be more important for high-performance catalysis at low catalyst concentrations. The present study reveals a dynamic stabilization mechanism with labile metal–NHC binding and [PdX₃]⁻[NHC-R]⁺ ion pair formation. Access to reactive anionic palladium intermediates formed by dissociation of the NHC ligands and plausible stabilization of the molecular catalyst in solution by interaction with the [NHC-R]⁺ azolium ion is of particular importance for an efficient and recyclable catalyst. These ionic Pd/NHC complexes allowed for the first time the recycling of the complex in a well-defined form with isolation at each cycle. Computational investigation of the reaction mechanism confirms a facile formation of NHC-free anionic Pd in polar media through either Ph–NHC coupling or reversible H–NHC coupling. The present study formulates novel ideas for M/NHC catalyst design.     

Imidazol(in)ium hydrogen carbonates as a genuine source of N-heterocyclic carbenes (NHCs): applications to the facile preparation of NHC metal complexes and to NHC-organocatalyzed molecular and macromolecular syntheses

Anion metathesis of imidazol(in)ium chlorides with KHCO(3) afforded an easy one step access to air stable imidazol(in)ium hydrogen carbonates, denoted as [NHC(H)][HCO(3)]. In solution, these compounds were found to be in equilibrium with their corresponding imidazol(in)ium carboxylates, referred to as N-heterocyclic carbene (NHC)-CO(2) adducts. The [NHC(H)][HCO(3)] salts were next shown to behave as masked NHCs, allowing for the NHC moiety to be readily transferred to both organic and organometallic substrates, without the need for dry and oxygen-free conditions. In addition, such [NHC(H)][HCO(3)] precursors were successfully investigated as precatalysts in two selected organocatalyzed reactions of molecular chemistry and polymer synthesis, namely, the benzoin condensation reaction and the ring-opening polymerization of d,l-lactide, respectively. The generation of NHCs from [NHC(H)][HCO(3)] precursors occurred via the formal loss of H(2)CO(3)via a concerted low energy pathway, as substantiated by Density Functional Theory (DFT) calculations.     

基于非血红素铁模型配合物[FeⅣ(O)(N4Py)]2+的新型N杂环卡宾配合物的理论设计

基于四价非血红素铁模型配合物[FeⅣ(O)(N4Py)]2+, 通过理论计算设计出一种新型N杂环卡宾配合物[FeⅣ(O)(N4Py)]2+. 采用密度泛函理论B3LYP方法, 计算了[FeⅣ(O)(N4Py)]2+的几何结构和电子结构, 并研究了[FeⅣ(O)(N4Py)]2+使环己烷C-H键羟基化的反应机理. 结果表明, [FeⅣ(O)(N4Py)]2+的五重态能量比基态三重态能量高约5.7 kJ/mol, 故五重态几乎不能参与反应. 赤道方向的配位基N杂环卡宾(NHC)对FeO单元的σ-贡献要大于N4Py的贡献, 而它的空间位阻效应也大于N4Py, 因此2+的稳定性强于[FeⅣ(O)(N4Py)]2+. [FeⅣ(O)(N4Py)]2+的三重态的反应能垒比[FeⅣ(O)(N4Py)]2+的三重态反应能垒高2.0 kJ/mol, 且为单态反应, 所以[FeⅣ(O)(N4Py)]2+的反应活性要高于[FeⅣ(O)(N4Py)]2+.     

First crystallographic elucidation of a high-valent molybdenum oxo N-heterocyclic carbene complex [CpMo(VI)O2(IBz)]2[Mo6O19

A high-valent molybdenum NHC dioxo complex [CpMoO(2)(IBz)](2)[Mo(6)O(19)] (IBz = 1,3-dibenzylimidazol-2-ylidene) has been isolated in its hexamolybdate form and crystallographically elucidated to show a Mo-C(NHC) bond of 2.173(4) ?. (NHC = N-heterocyclic carbene).