Formation Of Carbene Complexes Research Articles

Color modulation in organometallic dyes. Purple-colored acyclic carbenes derived from 2-isocyanoazulene gold(I) complexes.

This study reports new carbene azulene gold(I) complexes [AuCl{C(NHAz)(NR2)}] (R = Me (1) and nBu (2)) and [Au(C6F4OC10H21){C(NHAz)(NR2)}] (R = Me (3) and nBu (4)) prepared by reaction of 2-isocyanoazulene gold(I) complexes [AuX(CNAz)] (X = Cl and C6F4OC10H21) with the corresponding secondary amines. Their photophysical properties were investigated by absorption and emission spectroscopies, and by TD-DFT calculations. All the compounds display an intense coloration based on HOMO-LUMO transitions, dominated by the azulene core. Gold-isocyanide complexes show a slight bathochromic shift related to azulene, while a hypsochromic shift was observed after formation of carbene complexes. Thus, the transformation of the gold-isocyanide group into the gold-carbene functionality produces a substantial color change from blue to deep purple. This different electronic behavior is mainly due to the stabilization of the LUMO orbital in the isocyanide complexes, and to the stabilization of the HOMO in the carbene derivatives. These carbene gold complexes show fluorescence in solution associated with the azulene core. This work illustrates how the synthetically easy isocyanide-carbene transformation open new perspectives to a fine color modulation in organometallic azulene dyes.

Carbene complex formation versus cyclometallation from a phosphoryl-tethered methanide ruthenium complex

An oxophosphoryl-substituted methanide ligand system for transition metal complexes has been synthesized and isolated as the sodium salt Na[Ph2P(O)–C(H)–SO2Ph]. This ligand features structural components known to enable the isolation of nucleophilic late transition metal carbene complexes. The corresponding ruthenium(cymene) chlorido complex was readily available by simple salt metathesis reaction. However, in contrast to previously reported thio- and iminophosphoryl-tethered ligand systems, dehydrohalogenation of the chlorido complex led to the formation of a cyclometallated ruthenium complex instead of the carbene complex. All compounds have been characterized in solution and solid state. Additional density functional theory (DFT) studies have been performed to elucidate the mechanism of the observed cyclometallation and to shed light on the effects of different P(V) groups in the ligand system on the stability and reactivity of the corresponding carbene complexes. The calculations show that the weaker coordination of the PO compared to the PS or PN moiety is responsible for the more facile C–H activation.

Raman spectroscopy along with Principal Component Analysis for the confirmation of Silver(I)-N-heterocyclic carbene complex formation.

In this study Raman spectroscopy is employed for the characterization of two different ligands called as S1 and S2 and their respective co-ordinate complexes called C1 and C2. Specific Raman spectral signatures are observed for each of these Silver(I)-N-heterocyclic carbene complexes Ag(I)-(NHCs), which can be associated with the imidazolium ring, part of both of the ligands, indicating the formation of new coordinate covalent bond. For the detailed analysis, Raman spectral data of these ligands and complexes is analyzed by multivariate data analysis technique, Principal Component Analysis (PCA) which is found very helpful to differentiate two ligands and complexes from each other. The significant Raman peaks with higher intensities in the complexes as compared to the respective ligands are associated with imidazole ring which can be attributed to the enhanced polarizability of this ring on complex formation. Moreover, the spectral features associated with (AgC) bond are observed with higher intensity at 360 in (C1) and 383 in (C2). This study indicates the potential of Raman spectroscopy for the characterization and confirmation of formation of organometallic complexes and other chemical products.

ChemInform Abstract: Fischer Carbene Complexes Remain Favourite Targets, and Vehicles for New Discoveries

AbstractReview: 117 refs.

Fischer carbene complexes remain favourite targets, and vehicles for new discoveries.

Exciting new variations in Fischer-type carbene complex composition and reactivity have been realised by following or modifying well-established synthetic approaches such as metal carbonyl functionalization and modification of existing carbene ligands. The formation of targeted complexes for organic synthesis, carbene-containing chelates, and polynuclear carbene complexes, by employing 'click chemistry', warrants discussion. Transmetallation and α,α-dehydrogenation of ethers and amines have come into their own as viable synthetic methods to access carbene complexes with unique properties and activities. Successful mediation of carbene complex formation with pincer ligands has proved its worth. Quantum chemistry has become essential for supporting or initiating mechanistic proposals, but heuristic approaches such as invoking the vinylology principle to describe substituted phenylcarbene complexes are still valuable in the interpretation of bonding properties and the classification of complex types. Electrochemical studies now also constitute a powerful part of the experimental characterization tool kit.

Structure, Bonding, and Reactivity of Room-Temperature-Stable Lithium Chloride Carbenoids

Electronic stabilization of the negative charge by a thiophosphinoyl and pyridyl/quinolyl substituent allows for the isolation of two lithium chloride carbenoids at room temperature. Molecular structure analysis by X-ray crystallography and multinuclear NMR spectroscopy reveal no direct lithium–carbon interaction in the solid state and in solution. This leads to remarkable thermal stability but also to a reduced ambiphilic character of the compounds. Thus, properties typically observed for nonstabilized Li/Cl carbenoids are less pronounced. Nevertheless, computational studies still show that despite the charge delocalization within the compound a high negative charge remains at the carbenoid carbon atom. Preliminary reactivity studies confirm this nucleophilic character and show that the carbenoids can still be used as a “carbene” source for the formation of carbene complexes.

Mechanistic insights into the Pt(II)-catalyzed rearrangements from homopropargylic alcohols to furans: Hydrogen vs tert-butyldimethylsilyl group migrations

By carrying out B3LYP calculations, we have performed a detailed mechanism study on the reaction of platinum catalyzed rearrangements of homopropargylic alcohols derivatives into isomeric polysubstituted furans in THF and toluene solutions, respectively. The reaction steps include α,β-unsaturated carbene complex formation, and H atom or Si (Si = tert-butyldimethylsilyl) group migration. In the step of α,β-unsaturated carbene complex formation, the methoxy group is the optimal leaving group. Our calculations are consistent with the experimental results that Si-migration is favored in toluene while H-migration is favored in THF. Based on the Cl-aided H-migration mechanism postulated by Ferreira et al., we proposed a H2O and Cl co-aided H-migration mechanism in THF, in which H2O and Cl both act as the medium to transfer H.

Density Functional Study on the Mechanism of Nickel-Mediated Diazo Carbonylation

The mechanism of diazo activation as well as the carbonylation of the resulting carbene complexes has been investigated by means of DFT calculations at the PBE0/TZVP level of theory. The free energy profile of all elementary steps of the reaction, i.e., diazo coordination, dinitrogen extrusion, carbene–CO coupling, CO coordination, and ketene elimination, have been elucidated for diazomethane and ethyl diazoacetate as substrates and for Ni(CO)3, Ni(CO)2(PH3), and Ni(dtbpe)(CO) (dtbpe = 1,2-bis(di-tert-butylphosphino)ethane) as precursors. The reaction rate is determined by the formation of the coordinatively unsaturated precursor followed by the dinitrogen extrusion, regardless of the initial diazo compound and the catalyst precursor. For the homoleptic precursor Ni(CO)3 the free energy of activation for diazomethane and ethyl diazoacetate (EDA) is 16.9 and 22.4 kcal/mol, respectively. The replacement of one carbonyl ligand with PH3 results in a decrease in the activation barrier, modifying the energies to 15.7 and 20.5 kcal/mol, respectively. The activation free energy of the diazo extrusion step promoted by Ni(dtbpe)(CO) is 24.3 kcal/mol for diazomethane and 28.1 kcal/mol for EDA. The formation of carbene complexes is slightly endergonic starting from the homoleptic precursor and exergonic for the phosphine-substituted complex. The carbene–CO coupling, resulting in coordinatively unsaturated ketene complexes, is a fast and highly exergonic process with reaction free energies between −32.3 and −42.1 kcal/mol, depending on the substituents. Under a carbon monoxide atmosphere the ketene complexes may uptake one CO, forming coordinatively saturated ketene complexes via low barriers in exergonic reactions. The final step, i.e., the dissociation of ketene or ethoxycarbonylketene from the saturated ketene complex, regenerates the corresponding nickel–carbonyl precursor. The azine formation side reaction for the simplest case between Ni(CO)3 and diazomethane has been also examined and found to be highly exergonic; however, the large free energy barrier prevents the formation of benzophenone-azine when CO is also present.

Normal and abnormal carbene complexes derived from thiazole: Preparation and a preliminary investigation of their relative catalytic performance

Readily prepared 2-, 4- and 5-bromo-3-methyl thiazolium triflates react by oxidative substitution with M(PPh 3) 4 (M = Ni or Pd) to furnish five of the expected normal and abnormal cationic thiazolylidene complexes ( 1a, 1b, 2a, 2b, and 3b). Carbene complex formation is accompanied by a ca. 40 ppm downfield shift of the α-N carbene carbons in Pd complexes 1 and 2 in their 13C NMR spectra but the chemical shift of C(carbene) in the abnormal 3b ( δ 135.7) is particularly low. Crystal and molecular structures of complexes 1a, 2b, and 3b all indicate a square planar arrangement of the ligands around the central metal atoms. The new complexes catalyse Suzuki–Miyaura aryl coupling.

Ionic liquid effects on Mizoroki–Heck reactions: more than just carbene complex formation

Reaction profiles for a Mizoroki-Heck reaction in either an ionic liquid or a molecular solvent with different palladium sources demonstrate that the rate enhancements observed in ionic liquids cannot be solely attributed to Pd-carbene complex formation.

The acyclic diene metathesis (ADMET) polymerization approach to silicon containing materials

The utility of acyclic diene metathesis (ADMET) polymerization in the synthesis of silicon–carbon hybrid materials is reviewed. Silicon containing macromolecules are synthesized from organosilicon α,ω-dienes (except those which are divinyl substituted). As an extension of olefin metathesis, ADMET is catalyzed by many transition metal complexes which either initiate the formation of carbene complexes (“ill-defined” catalysts) or by metallacarbene complexes (“well-defined” catalysts). Constant developments of catalytic systems have made this methodology more viable by allowing incorporation of numerous silicon containing functionalities into the polymer's backbone or pendant to it. Divinyl substituted organosilicon derivatives are inert to ADMET conditions; they undergo silylative coupling (SC) polycondensation catalyzed by ruthenium and rhodium complexes yielding metathesis-like products, although this transformation proceeds via a non-metallacarbene mechanism.

Benzothiazolin-2-ylidene Complexes of Iridium(I)

The reaction of allyl bromide with benzothiazole under neat conditions furnished 3-(2-propenyl)benzothiazolium bromide, (H-1)Br, in high yield. Attempts to synthesize the corresponding carbene dimer by deprotonation of (H-1)+ led to the isolation of the rearrangement product 2,3-di(2-propenyl)-2‘,3‘-dihydro-2,2‘-bisbenzothiazole (2). The reaction of [Ir(μ-OMe)(cod)]2 with the salt (H-1)Br unexpectedly afforded [IrBr(cod)(benzothiazole)] (3) (cod = 1,5-cyclooctadiene), which contained an N-coordinated unsubstituted benzothiazole ligand. The formation of carbene complexes of IrI with the benzothiazolin-2-ylidene ligand could be achieved via precoordination of the allyl substituent of (H-1)+ to [Ir(cod)(MeCN)2]BF4 and subsequent deprotonation at the C2 position of (H-1)+ by addition of base. The use of NaH as external base yielded the square planar IrI complex 4 with an N-propyl-substituted carbene ligand, while deprotonation with KOtBu gave the five-coordinated IrI complex [IrBr(cod)(η2-1)], 5. Displacement...

Unprecedented ROMP Activity of Low‐Valent Rhenium–Nitrosyl Complexes: Mechanistic Evaluation of an Electrophilic Olefin Metathesis System

The reaction of [Re(H)(NO)2(PR3)2] complexes (1 a: R = PCy3; 1 b: R = PiPr3) with [H(OEt2)2][BAr(F)4] ([BAr(F)4] = tetrakis{3,5-bis(trifluoromethyl)phenyl}borate) in benzene at room temperature gave the corresponding cations [Re(NO)2(PR3)2][BAr(F)4] (2 a and 2 b). The addition of phenyldiazomethane to benzene solutions of 2 a and 2 b afforded the moderately stable cationic rhenium(I)-benzylidene-dinitrosyl-bis(trialkyl)phosphine complexes 3 a and 3 b as [BAr(F)4]- salts in good yields. The complexes 2 a and 2 b catalyze the ring-opening metathesis polymerization (ROMP) of highly strained nonfunctionalized cyclic olefins to give polymers with relatively high polydispersity indices, high molecular weights and over 80 % Z configuration of the double bonds in the chain backbone. However, these complexes do not show metathesis activity with acyclic olefins. The benzylidene derivatives 3 a and 3 b are almost inactive in ROMP catalysis with norbornene and in olefin metathesis. NMR experiments gave the first hints of the initial formation of carbene complexes from [Re(NO)2(PR3)2][BAr(F)4] (2 a and 2 b) and norbornene. In a detailed mechanistic study ESI-MS/MS measurements provided further evidence that the carbene formation is initiated by a unique reaction sequence where the cleavage of the strained olefinic bond starts with phosphine migration forming a cyclic ylide-carbene complex, capable of undergoing metathesis with alternating rhenacyclobutane formation and cycloreversion reactions ("ylide" route). However, even at an early stage the ROMP propagation route is expected to merge into an "iminate" route by attack by the ylide function on one of the N(NO) atoms followed by phosphine oxide elimination. The formation of phosphine oxide was confirmed by NMR spectroscopy. The proposed mechanism is supported further by detailed DFT calculations.

Dinitrosyl rhenium complexes for ring-opening metathesis polymerization (ROMP)

Abstract The treatment of benzene solutions of the cations [Re(NO)2(PR3)2][BArF 4] (R = Cy and R = i Pr; [BArF 4] = tetrakis{3,5-bis(trifluoromethyl)phenyl}borate) with phenyldiazomethane afforded the moderately stable cationic rhenium(I) benzylidene dinitrosyl bis(trialkyl) phosphine complexes as [BArF 4]- salts in good yields. The cationic rhenium dinitrosyl bisphosphine complexes catalyze the ring-opening metathesis polymerization (ROMP) of highly strained nonfunctionalized cyclic olefins to give polymers with relatively high polydispersity indices, high molecular weights, and Z configurations of the double bonds in the polymer chain backbones of over 80 %. The benzylidene derivatives are almost inactive in ROMP catalysis with norbornene and in olefin metathesis. NMR experiments gave first hints for the initial formation of carbene complexes when [Re(NO)2(PR3)2][BArF 4] was treated with norbornene. The carbene formation is initiated by an unique reaction sequence where the cleavage of the strained olefinic bond starts with phosphine migration forming a cyclic ylid carbene complex. The [2+2] addition of a norbornene molecule to the Re=C bond leads to the rhenacyclobutane complex, which is expected to be converted into an iminate complex by attack of the ylid function onto one of the NNO atoms followed by Wittig-type phosphine oxide elimination. The formation of phosphine oxide was confirmed by NMR spectroscopy. This species is thought to drive the ROMP metathesis with alternating rhenacyclobutane formations and cycloreversions. The proposed mechanism is supported by density functional theory (DFT) calculations.

Synthesis and some properties of 14 group element-containing alkylidene complexes of molybdenum and tungsten

Alkylidene complex of molybdenum Ph 3Si–CH Mo(NAr)(OCMe 2CF 3) 2 ( 1) ( Ar = 2 , 6 - Pr 2 i C 6 H 3 ) has been prepared by the reaction of Ph 3SiCH CH 2 with known alkylidene compounds Alkyl-CH Mo(NAr)(OCMe 2CF 3) 2 (Alkyl = Bu t , PhMe 2C). According to X-ray diffraction studies the geometry of the Mo atom in ( 1) can be described as a distorted tetrahedron. Reactions of heteroelement-containing imido alkyl compounds of molybdenum and tungsten (ArN) 2M(CH 2EMe 3) 2 ( Ar = 2 , 6 - Pr 2 i C 6 H 3 ; M = Mo. W; E = Si, Ge, Sn) with triflic acid were shown to yield a complex mixture of products among which carbene complexes have not been observed. The formation of carbene complexes of tungsten in low yields was observed by 1H NMR spectroscopy in the reactions of (ArN) 2W(CH 2EMe 3) 2 (E = Si, Ge, Sn) with hydrogen chloride in THF- d 8. Catalytic properties of heteroelement-containing alkylidene complexes of molybdenum R 3E–CH Mo(NAr)(OCMe 2CF 3) 2 (E = Si, Ge; R = Me, Ph) and also hydrocarbon analogs Alkyl-CH Mo(NAr)(OCMe 2CF 3) 2 (Alkyl = Bu t , PhMe 2C) in metathesis of 1-hexene were investigated. The catalytic activity of alkylidene complexes was shown to depend essentially on the nature of substituent bonded to carbene carbon atom.

The reaction of amidinato(pyridine) complexes of molybdenum and tungsten with triethylborane adduct of N-heterocyclic carbene (NHC · BEt 3): Investigation on the reactivity of NHC · BEt 3 as carbene precursor toward transition metal complexes

The reaction of triethylborane adduct of N-heterocyclic carbene, NHC · BEt 3, (NHC = I i Pr = 1,3-diisopropylimidazol-2-ylidene (I i Pr · BEt 3; 1a), NHC = IMes = 1,3-dimesitylimidazol-2-ylidene (IMes · BEt 3; 1b)), which was prepared by the reaction of the corresponding imidazolium salt with one equivalent of LiBEt 3H, with amidinato(pyridine) complex, [M(η 3-allyl){η 2-(NPh) 2CH}(CO) 2(NC 5H 5)] (M = Mo; 2-Mo M = W; 2-W), was investigated. The reaction of compound 1 with complex 2 under toluene-reflux conditions resulted in the formation of carbene complex [M(η 3-allyl){η 2-(NPh) 2CH}(CO) 2(NHC)] (M = Mo, NHC = I i Pr; 3a-Mo, M = Mo, NHC = IMes; 3b-Mo, M = W, NHC = I i Pr; 3a-W, M = W, NHC = IMes; 3b-W). These complexes were characterized spectroscopically as well as by X-ray analyses. Complex 3a-Mo was formed in various solvents such as 1,2-dimethoxyethane (DME), 1,2-dichloroethane, and acetonitrile under refluxing conditions for 3 h. In toluene, 3a-Mo was obtained in a good yield by heating at 70 °C for only 20 min. Employment of NHC · BEt 3 ( 1) was found to afford convenient route for the introduction of the carbene ligand to the transition metal complexes.

Metallacarbenes from diazoalkanes: an experimental and computational study of the reaction mechanism.

PCP ligand (1,3-bis-[(diisopropyl-phosphanyl)-methyl]-benzene), and PCN ligand ([3-[(di-tert-butyl-phosphanyl)-methyl]-benzyl]-diethyl-amine) based rhodium dinitrogen complexes (1 and 2, respectively) react with phenyl diazomethane at room temperature to give PCP and PCN-Rh carbene complexes (3 and 5, respectively). At low temperature (-70 degrees C), PCP and PCN phenyl diazomethane complexes (4 and 6, respectively) are formed upon addition of phenyl diazomethane to 1 and 2. In these complexes, the diazo moiety is eta(1) coordinated through the terminal nitrogen atom. Decomposition of complexes 4 and 6 at low temperatures leads only to a relatively small amount of the corresponding carbene complexes, the major products of decomposition being the dinitrogen complexes 1 and 2 and stilbene. This and competition experiments (decomposition of 6 in the presence of 1) suggests that phenyl diazomethane can dissociate under the reaction conditions and attack the metal center through the diazo carbon producing a eta(1)-C bound diazo complex. Computational studies based on a two-layer ONIOM model, using the mPW1K exchange-correlation functional and a variety of basis sets for PCP based systems, provide mechanistic insight. In the case of less bulky PCP ligand bearing H-substituents on the phosphines, a variety of mechanisms are possible, including both dissociative and nondissociative pathways. On the other hand, in the case of i-Pr substituents, the eta(1)-C bound diazo complex appears to be a critical intermediate for carbene complex formation, in good agreement with the experimental results. Our results and the analysis of reported data suggest that the outcome of the reaction between a diazoalkane and a late transition metal complex can be anticipated considering steric requirements relevant to eta(1)-C diazo complex formation.

Carbene complexes derived from lithiated heterocycles, mainly azoles, by transmetallation

Heterocyclic carbene complex formation can be achieved by lithiation of CH-acidic azoles, transmetallation involving a variety of transition metal complexes and, finally, protonation or alkylation. This article describes the synthetic methodology involved with a special emphasis on unexpected or (presently) unusual features of the reactions or products. The procedure can be extended to allow carbene complex formation by reaction at remote heteroatoms and also for diorgano(carbene) complex formation. Certain azolyls do not substitute but add to coordinated carbonyls and the resulting anionic Fischer-type carbene complexes can function as bidentate ligands. With gold(I) as central metal, many azolyls as well as carbene complexes participate in homoleptic rearrangement.

Carbene complexes of titanium group metals—formation and reactivity

Abstract The principles in the formation of carbene complexes of titanium group metals are summarized. Characteristic examples of isolated as well as carbene complexes as intermediates are given. Selected structures and reactivity patterns are discussed. Starting from the Tebbe-type chemistry, new aspects of carbene complexes with only one or without cyclopentadienyl ligands are illustrated. In that direction, the formation of carbene complexes with N and O-donor ligands is described. Applications in co-ordination chemistry, organic syntheses and material sciences are summarized.

Mechanistic test for carbocationic alkene cyclization initiated by cationic iron carbene complex formation

A key mechanistic test is performed to determine whether cationic iron carbene complexes may function as initiators of cationic alkene cyclization reactions. Generation of a cationic iron carbene complex 15 within a framework containing a 1,5-diene system results in the formation of a bicyclic product 16, thus indicating the potential utility of these carbene complexes as polycyclization initiators.