Metal Carbene Complexes Research Articles

ChemInform Abstract: Bond Activation by Metal‐Carbene Complexes in the Gas Phase

AbstractReview: 67 refs.

Synthesis and investigation of new cyclic haloamidinium salts

AbstractThe presented work describes the synthesis of new six- and seven-membered haloamidinium salts and their reaction with different metals. The isolated metal complexes were tested in a catalytic reaction. Two different synthetic routes were applied to prepare five different salts. Chloroamidinium salts were very water-sensitive in comparison to their corresponding bromoamidinium salts. Hence, the preparation of the less sensitive bromoamidinium salts was higher prioritized. The formed salts were converted with metal sources to N-heterocyclic carbene (NHC) metal complexes through an oxidative insertion into the C–X bond. This type of formation is less examined for the synthesis of extended NHC metal complexes. Pd(PPh3)4and cobalt powder were applied as metal sources, whereby two palladium complexes were isolated, characterized, and their crystal and molecular structures determined. The palladium complexes were investigated in the Suzuki-Miyaura reaction and showed promising catalytic activity.

Active phase, catalytic activity, and induction period of Fe/zeolite material in nonoxidative aromatization of methane

Catalytic dehydroaromatization of methane has been investigated over 2–6wt.% Fe-modified HZSM-5 or HMCM-22 zeolite in the absence of an added oxidant. After pretreatment in a CH4/Ar/He mixture, Fe/HMCM-22, and more significantly, Fe/HZSM-5 exhibited high catalytic activity in nonoxidative conversion of methane to aromatic hydrocarbons. This conversion actually takes part in three consecutive steps, oxidation of methane by iron oxides, decomposition, and aromatization of methane. During this conversion, XPS and XRD results showed the gradual reduction of Fe2O3 to metallic Fe, followed by the carburization of the metallic species. The fact that no aromatic products were detected until iron carbide was formed demonstrated that carburized Fe was the active species for the aromatization of methane. Under the same reaction conditions, both metallic Fe and carburized Fe were highly effective for activation of methane, but they catalyzed dehydrogenation of methane to carbonaceous materials and to aromatics, respectively. Difference in the catalytic properties of those two Fe species suggested that carburized Fe could stabilize carbene, the primary intermediate of activated methane, possibly via formation of a metal–carbene complex, CFeCH2, so that carbene was able to dimerize into ethylene, which was then oligomerized to aromatics on the acid sites of the zeolite.

Remote Control by π-Conjugation of the Emissive Properties of Fischer Carbene-BODIPY Dyads.

The synthesis, structure, and complete characterization of mono- and bimetallic dyads joining Fischer carbene complexes and BODIPY chromophores are reported. In these organometallic species, the Fischer carbene complex is attached to the BODIPY moiety through a p-aminophenyl group linked at the C8 carbon atom of the BODIPY core. The photophysical properties, namely the corresponding UV/vis absorption and emission spectra of these new metal-carbene complexes, are analyzed and discussed. It is found that whereas the absorption of the considered dyads strongly resembles that of the parent 4-anilinyl-substituted BODIPY, the fluorescence emission is significantly reduced in these species, very likely as a result of a Förster-type energy transfer mechanism. At variance, the replacement of the pentacarbonyl-metal(0) fragment by a carbonyl group leads to high fluorescence emission intensity. In addition, the emissive properties of the BODIPY core in these organometallic dyads can be tuned by remote groups by means of π-conjugation, as supported by density functional theory calculations.

Bond Activation by Metal-Carbene Complexes in the Gas Phase.

"Bare" metal-carbene complexes, when generated in the gas phase and exposed to thermal reactions under (near) single-collision conditions, exhibit rather unique reactivities in addition to the well-known metathesis and cyclopropanation processes. For example, at room temperature the unligated [AuCH2](+) complex brings about efficient C-C coupling with methane to produce C2Hx (x = 4, 6), and the couple [TaCH2](+)/CO2 gives rise to the generation of the acetic acid equivalent CH2═C═O. Entirely unprecedented is the thermal extrusion of a carbon atom from halobenzenes (X = F, Cl, Br, I) by [MCH2](+) (M = La, Hf, Ta, W, Re, Os) and its coupling with the methylene ligand to deliver C2H2 and [M(X)(C5H5)](+). Among the many noteworthy C-N bond-forming processes, the formation of CH3NH2 from [RhCH2](+)/NH3, the generation of CH2═NH2(+) from [MCH2](+)/NH3 (M = Pt, Au), and the production of [PtCH═NH2](+) from [PtCH2](+)/NH3 are of particular interest. The latter species are likely to be involved as intermediates in the platinum-mediated large-scale production of HCN from CH4/NH3 (the DEGUSSA process). In this context, a few examples are presented that point to the operation of co-operative effects even at a molecular level. For instance, in the coupling of CH4 with NH3 by the heteronuclear clusters [MPt](+) (M = coinage metal), platinum is crucial for the activation of methane, while the coinage metal M controls the branching ratio between the C-N bond-forming step and unwanted soot formation. For most of the gas-phase reactions described in this Account, detailed mechanistic insight has been derived from extensive computational work in conjunction with time-honored labeling and advanced mass-spectrometry-based experiments, and often a coherent description of the experimental findings has been achieved. As for some transition metals, in particular those from the third row, the metal-carbene complexes can be formed directly from methane, coupling of the so-generated [MCH2] species with an inert molecule such as CH4, CO2, or NH3 constitutes a route to activate and functionalize methane under ambient conditions. Clearly, while these gas-phase studies cannot be translated directly to formally related processes in solution or those that occur at a surface, they nevertheless provide a conceptual mechanistic understanding and permit researchers to probe directly the remarkable intrinsic features of these elusive molecules and, in a broader context, help to identify the active site of a catalyst, the so-called "aristocratic atoms".

Inside Cover: Facile Synthesis of Functionalized Carbene Metal Complexes from Coordinated Isonitriles (Chem. Asian J. 3/2016)

Inside Cover: Facile Synthesis of Functionalized Carbene Metal Complexes from Coordinated Isonitriles (Chem. Asian J. 3/2016)

Beyond Fischer and Schrock carbenes: non-heteroatom-stabilized group 6 metal carbene complexes – a general overview

Non-heteroatom-stabilized group 6 metal carbene complexes: versatile reagents or intermediates in stoichiometric or catalytic reactions in organic synthesis.

Recent Progress of Supported N-Heterocyclic Carbene Catalyst in Organic Reactions

The supported N-heterocyclic carbene (NHC) catalysts have been extensively applied in the catalysis of different organic reactions due to its unique characteristic such as high reactivity, easy separation, purification and recyclability. In this paper, the recent progresses in synthesis and application of supported N-heterocyclic carbene metal complexes based on various types of supports, such as polymer, magnetic nanoparticles, carbon and silica material have been reviewed.

BIAN N-Heterocyclic Gold Carbene Complexes induced cytotoxicity in human cancer cells via upregulating oxidative stress.

Nanoparticles of gold and silver are offering revolutionary changes in the field of cancer therapy. N-heterocyclic carbene (NHC) metal complexes possess diverse biological activities and are being investigated as potential chemotherapeutic agents. The purpose of this study was to examine the cytotoxicity and possible mechanisms of action of two types of newly synthesized nanofiber composites containing BIAN N-heterocyclic gold carbene complexes in two types of human cancer cells, namely breast cancer (MCF7) and liver cancer (HepG2) cells and also in normal human embryonic kidney cells (HEK 293). Cytotoxicity was assessed by MTT cell viability assay and oxidative stress by checking the total glutathione level. Both compounds affected the cell survival of the tested cell lines at very low concentrations (IC50 values in the micro molar range) as compared to a well-known anti-cancer drug, 5 fluorouracil. A 60-80% depletion in total glutathione level was detected in treated cells. Reduction in total glutathione level is one of the biochemical pathways for the induction of oxidative stress which in turn could be a possible mechanism of action by which these compounds induce cytotoxicity in cancer cell lines. The in vitro toxicity towards cancer cells found here means that these molecules could be potential anticancer candidates.

Synthesis, structure and DFT study of cymantrenyl Fischer carbene complexes of group VI and VII transition metals

Bi- and trimetallic carbene complexes of group VI and VII transition metals (Cr, Mo, W, Mn and Re), with CpMn(CO)3 as the initial synthon, have been synthesised according to the classical Fischer methodology. Crystal structures of the novel carbene complexes with general formula [Mx(CO)y-1{C(OEt)(MnCp(CO)3)}], where x = 1 then y = 3 or 6; x = 2 then y = 10, of the complexes are reported. A density functional theory (DFT) study was undertaken to determine natural bonding orbitals (NBOs) and conformational as well as isomeric aspects of the polymetallic complexes. Application of the second-order perturbation theory (SOPT) of the natural bond orbital (NBO) method revealed stabilizing interactions between the methylene C–H bonds and the carbonyl ligands of the carbene metal moiety. These stabilization interactions show a linear decrease for the group VI metal carbene complexes down the group.

Biological Screening of Newly Synthesized BIAN N-Heterocyclic Gold Carbene Complexes in Zebrafish Embryos.

N-Heterocyclic carbene (NHC) metal complexes possess diverse biological activities but have yet to be extensively explored as potential chemotherapeutic agents. We have previously reported the synthesis of a new class of NHC metal complexes N-heterocyclic with acetate [IPr(BIAN)AuOAc] and chloride [IPr(BIAN)AuCl] ligands. In the experiments reported herein, the zebrafish embryos were exposed to serial dilutions of each of these complexes for 10–12 h. One hundred percent mortality was observed at concentrations ≥50 µM. At sub-lethal concentrations (10–30 µM), both compounds influenced zebrafish embryonic development. However, quite diverse categories of abnormalities were found in exposed embryos with each compound. Severe brain deformation and notochord degeneration were evident in the case of [IPr(BIAN)AuOAc]. The zebrafish embryos treated with [IPr(BIAN)AuCl] exhibited stunted growth and consequently had smaller body sizes. A depletion of 30%–40% glutathione was detected in the treated embryos, which could account for one of the possible mechanism of neurotoxicity. The fact that these compounds are capable of both affecting the growth and also compromising antioxidant systems by elevating intracellular ROS production implies that they could play an important role as a new breed of therapeutic molecules.

A DFT and structural investigation of the conformations of Fischer carbene complexes

A set of different Fischer carbene complexes of group VI and VII metals, with varied heteroatom and heteroaromatic substituents on the carbene carbon atom, was studied. Density functional theory as well as single crystal diffraction techniques were employed to investigated the most stable conformation of these complexes. The complexes studied, [M(CO)4L{C(X)Z}], with L = PPh3 or CO, X = ethoxy (-OCH2CH3) or amino (-NH2 or NHCy) substituents as the heteroatom carbene substituents, Z = 2-furyl (-C4H3O), 2-thienyl (-C4H3S), 2-(N-methyl)pyrrolyl (-C4H3NCH3) as the second carbene substituent had their substituents varied systematically to give all the possible conformations of these complexes. The conformations of the complexes, in particular the relative orientations of the heteroatoms in the molecule (syn vs. anti), E/Z isomerism in the aminocarbene complexes and cis/trans isomerism in the ligand substituted complexes as well as various combinations of these aspects, were studied. In general, it was found that the most stable conformation theoretically as well as in the solid state for most of the complexes preferred the syn conformation. The Z-isomer is generally preferred over the E isomer while the cis is more predominant than the trans isomer. Using DFT and NBO calculations, explanations for the preferred conformations were explored. It was concluded that both steric and electronic factors influence the conformations of the carbene complexes, with the extent of contribution of these two factors varying for each of the different carbene substituents.

Facile Synthesis of Functionalized Carbene Metal Complexes from Coordinated Isonitriles.

The scope and limitations of the isonitrile-based NHC template synthesis were investigated with a series of precursors containing a nucleophilic amine in combination with tethered electrophiles. In the case of alkynes and phosphonic esters as electrophiles no ring closure was observed and new functionalized NAC gold complexes were obtained. By the use of unsaturated esters and phosphonic esters as Michael acceptors in the amine precursors, ester-modified gold and palladium NHC complexes were accessible in high efficiency.

N-heterocyclic carbene metal complexes as bio-organometallic antimicrobial and anticancer drugs.

Late transition metal complexes that bear N-heterocyclic carbene (NHC) ligands have seen a speedy growth in their use as both, metal-based drug candidates and potentially active homogeneous catalysts in a plethora of C-C and C-N bond forming reactions. This review article focuses on the recent developments and advances in preparation and characterization of NHC-metal complexes (metal: silver, gold, copper, palladium, nickel and ruthenium) and their biomedical applications. Their design, syntheses and characterization have been reviewed and correlated to their antimicrobial and anticancer efficacies. All these initial discoveries help validate the great potential of NHC-metal derivatives as a class of effective antimicrobial and anticancer agents.

ChemInform Abstract: Intramolecular Cyclopropanation and C—H Insertion Reactions with Metal Carbenoids Generated from Cyclopropenes

AbstractReview: 81 refs.

H–D exchange in metal carbene complexes: Structure of cluster (μ-H)(μ-OCD3)Os3(CO)9{:C(CD3)NC2H8O}

X-ray and spectroscopic data for the new complex (μ-H)(μ-OCH3)Os3(CO)9{:C(CD3)NC2H8O} (2) obtained in the reaction of the (μ-H)(μ-Cl)Os3(CO)9{:C(CH3)NC2H8O} (1) with NaOCD3 in CD3OD solution are reported. It is shown that cluster 1 has the property of CH-acidity inherent of Fisher type carbenes. This had demonstrated using hydrogen deuterium exchange reaction in the presence of a strong base. Bridging chlorine to metoxide ligand substitution takes place during the reaction. The molecular structure of 2 is compared with known analogues.

Intramolecular cyclopropanation and C-H insertion reactions with metal carbenoids generated from cyclopropenes.

Activation of unsaturated carbon-carbon bonds by means of transition metal catalysts is an exceptionally active research field in organic synthesis. In this context, due to their high ring strain, cyclopropenes constitute an interesting class of substrates that displays a versatile reactivity in the presence of transition metal catalysts. Metal complexes of vinyl carbenes are involved as key intermediates in a wide variety of transition metal-catalyzed ring-opening reactions of cyclopropenes. Most of the reported transformations rely on intermolecular or intramolecular addition of nucleophiles to these latter reactive species. This Account focuses specifically on the reactivity of carbenoids resulting from the ring-opening of cyclopropenes in cyclopropanation and C-H insertion reactions, which are arguably two of the most representative transformations of metal complexes of carbenes. Compared with the more conventional α-diazo carbonyl compounds, the use of cyclopropenes as precursors of metal carbenoids in intramolecular cyclopropanation or C-H insertion reactions has been largely underexploited. One of the challenges is to devise appropriately substituted and readily available cyclopropenes that would not only undergo regioselective ring-opening under mild conditions but also trigger the subsequent desired transformations with a high level of chemoselectivity and stereoselectivity. These goals were met by considering several substrates derived from the readily available 3,3-dimethylcyclopropenylcarbinols or 3,3-dimethylcyclopropenylcarbinyl amines. In the case of 1,6-cyclopropene-enes, highly efficient and diastereoselective gold(I)-catalyzed ring-opening/intramolecular cyclopropanations were developed as a route to diversely substituted heterocycles and carbocycles possessing a bicyclo[4.1.0]heptane framework. The use of rhodium(II) catalysts enabled us to widen the scope of this transformation for the synthesis of medium-sized heterocyclic scaffolds incorporating an eight-membered ring. The reactivity of rhodium(II) carbenoids generated from 3,3-dimethylcyclopropenylcarbinols was also investigated in intramolecular C(sp(3))-H insertions. Despite their low electrophilic character, these purely donor rhodium(II) carbenoids underwent remarkably efficient diastereoselective 1,5- or 1,6-C-H insertions allowing access to a wide variety of substituted cyclopentanols, cyclohexanols, bicycloalkanols, and tetrahydropyrans with high level of diastereoselectivity and with complete tolerance of a free hydroxyl group. The products arising from the gold(I)- or rhodium(II)-catalyzed ring-opening/intramolecular cyclopropanation or C-H insertion of 3,3-dimethylcyclopropenylcarbinols or 3,3-dimethylcyclopropenylcarbinyl amines always incorporate an isopropylidene moiety, which can potentially undergo subsequent oxidative cleavage into a carbonyl group without epimerization. By virtue of this operation, the 3,3-dimethylcyclopropenyl group formally behaves as a valuable surrogate for an α-diazoketone, with obvious advantages considering the ease of access to the corresponding substrates and that no hazardous reagents are involved in their preparation. These studies have set a useful basis for the development of other reaction pathways involving metal carbenoids generated from these readily available families of substituted cyclopropenes, including the investigation of the yet underexploited synthetic potential of purely donor rhodium(II) carbenoids.

Synthesis of Homoleptic and Heteroleptic Bis-N-heterocylic Carbene Group 11 Complexes

A straightforward synthetic route has been developed permitting the formation of homoleptic and heteroleptic bis-N-heterocyclic carbene metal complexes. The methodology has proven efficient for all group 11 members. These complexes are easily synthesized using [M(Cl)(NHC)] with different NHC salts in the presence of inexpensive bases such as sodium hydroxide. These systems were fully characterized and displayed high stability even in the presence of light.

Two-step solid-state synthesis of PEPPSI-type compounds.

The two-step mechanochemical preparation of carbene-pyridine complexes of palladium and platinum is reported. The organometallic products, which represent a class of commercially available catalysts, are rapidly formed in excellent yield proving solvent-free synthesis to be a viable synthetic alternative even in the case of NHC-containing compounds.

Frustrated Lewis pair-like reactions of nucleophilic palladium carbenes with B(C6F5)3.

The reactions of two nucleophilic palladium carbene complexes with the strong Lewis acid B(C6F5)3 afforded two zwitterionic products. One of them features a remote nucleophilic attack at the para-carbon of the supporting ligand, while the other indicates C-F activation of B(C6F5)3. Both behaviours are reminiscent of the reactivity of frustrated Lewis pairs due to the steric inaccessibility of the nucleophilic carbon center, but are unprecedented for transition metal carbene complexes. Furthermore, when those reactions are carried out in the presence of H2, products resulting from H2 splitting are observed.