Tungsten Carbene Complexes Research Articles

N-heterocyclic carbene and cyclic (alkyl)(amino)carbene complexes of molybdenum(IV) and tungsten(IV).

The synthesis and characterization of N-heterocyclic carbene (NHC) and cyclic (alkyl)(amino)carbene (cAAC) complexes of molybdenum(IV) and tungsten(IV) chloride is reported. Reaction of two equivalents of the NHCs IMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), IDipp (1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) and IiPrMe (1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene) with [MCl4(SEt)2] (M = Mo, W) in toluene afforded the bis-NHC complexes [MCl4(NHC)2] (M = Mo: NHC = IMes 1, NHC = IMes 2; M = W: NHC = IMes 3, NHC = IDipp 4, NHC = IiPrMe5). Mono-carbene complexes [MCl4(NHC)(SEt2)] 6-9 (M = Mo: NHC = IMes 6, NHC = IMes 7; M = W: NHC = IMes 8, NHC = IDipp 9) are available via ligand dismutation of 1-4 with [MCl4(SEt)2]. Complexes 1-9 were characterized by using elemental analysis, IR- and multinuclear NMR spectroscopy and X-ray diffraction for 1-8. The reactivity of the mono-NHC complexes 6-9 towards the cyclic (alkyl)(amino)carbene cAACMe as well as the two electron donor ligands trimethylphosphine, triphenylphosphine, pyridine (py) and N,N-dimethylpyridin-4-amine (DMAP) was studied, which led to the synthesis of the mixed substituted complexes [MCl4(NHC)(cAACMe)] (M = Mo: NHC = IMes 10, NHC = IDipp 11; M = W: NHC = IMes 12, NHC = IDipp 13), [MCl4(NHC)(PR3)] (M = W: NHC = IMes, R = Me 14, NHC = IMes, R = Ph 15, NHC = IDipp, R = Me 16, NHC = IDipp, R = Ph 17), [MCl4(NHC)(py)] (M = Mo: NHC = IMes 18, NHC = IDipp 20; M = W: NHC = IMes 22, NHC = IDipp 24) and [MCl4(NHC)(DMAP)] (M = Mo: NHC = IMes 19, NHC = IDipp 21; M = W: NHC = IMes 23, NHC = IDipp 25).

The Effect of Remote Donor Substituents on the Properties of Alkoxy and Amino Fischer Carbene Complexes of Tungsten

AbstractTungsten Fischer ethoxy‐ and dimethylaminocarbene complexes [W{C(X)(C6H4‐4‐R}(CO)5] (X=OEt: series a; or X=NMe2: series b) are synthesized from phenyl substrates containing remote tertiary amino substituents R’2N with R’=Me (2), Ph (3) or C6H4Br‐4 (4). The π‐delocalization and carbene‐stabilizing effects of the distant tertiary amine donor substituent are investigated by NMR and IR spectroscopy, electrochemistry and quantum chemical calculations. A significant transfer of electron density from the remote 4‐R’2N substituent to the carbene C atom in the ethoxycarbene complexes is supported by NMR data and the solid‐state structure of 2 a. This is strongly attenuated in the amino‐substituted carbene complex 2 c and irrelevant in the dimethylaminocarbene complexes of series b, where the electron demand of the electrophilic carbene center is satisfied by the directly attached dimethylamino substituent. Quantum chemical calculations and IR spectroelectrochemistry on complexes 1 a‐‐4 a verify carbene‐centered reductions and a ligand based oxidation of complex 2 b.

Synthesis and Reactivity of Metallocarbene-Containing Polymers.

Metallopolymers are an emerging class of materials with potential utility as semiconductors, catalysts, optical device components, and stimuli responsive networks. While polymer frameworks have been decorated with an array of organometallic moieties, the incorporation of metallocarbenes has been largely overlooked. Here, we report ring-opening metathesis polymerization as a strategy for the synthesis of Fischer carbene-containing polymers. High degrees of polymerization were observed (>800 repeats), and the isolated materials exhibited exceptional solubility and thermal stability. The tungsten carbene subunits were readily incorporated into block copolymers and could be modified through subsequent transformations. Moreover, the metallocarbene polymers were found to release carbon monoxide upon exposure to light or oxygen, which is unusual for tungsten carbene complexes. These metallocarbene-containing polymers could represent new platforms for the development of functional materials.

Correction: Triphenylstibine-substituted Fischer carbene complexes of tungsten(0): synthesis, structure, DFT and electrochemistry

Correction for ‘Triphenylstibine-substituted Fischer carbene complexes of tungsten(0): synthesis, structure, DFT and electrochemistry’ by Marilé Landman et al., New J. Chem., 2018, 42, 7301–7313.

On the mechanism of imine elimination from Fischer tungsten carbene complexes.

(Aminoferrocenyl)(ferrocenyl)carbene(pentacarbonyl)tungsten(0) (CO)5W=C(NHFc)Fc (W(CO)5(E-2)) is synthesized by nucleophilic substitution of the ethoxy group of (CO)5W=C(OEt)Fc (M(CO)5(1Et)) by ferrocenyl amide Fc-NH– (Fc = ferrocenyl). W(CO)5(E-2) thermally and photochemically eliminates bulky E-1,2-diferrocenylimine (E-3) via a formal 1,2-H shift from the N to the carbene C atom. Kinetic and mechanistic studies to the formation of imine E-3 are performed by NMR, IR and UV–vis spectroscopy and liquid injection field desorption ionization (LIFDI) mass spectrometry as well as by trapping experiments for low-coordinate tungsten complexes with triphenylphosphane. W(CO)5(E-2) decays thermally in a first-order rate-law with a Gibbs free energy of activation of ΔG‡298K = 112 kJ mol−1. Three proposed mechanistic pathways are taken into account and supported by detailed (time-dependent) densitiy functional theory [(TD)-DFT] calculations. The preferred pathway is initiated by an irreversible CO dissociation, followed by an oxidative addition/pseudorotation/reductive elimination pathway with short-lived, elusive seven-coordinate hydrido tungsten(II) intermediates cis(N,H)-W(CO)4(H)(Z-15) and cis(C,H)-W(CO)4(H)(Z-15).

Computational chemistry insights in the REDOX Behaviour of Cr and W Fischer carbene complexes

An electrochemical study of a series of Fischer carbene complexes containing a hetero-aryl group showed that Cr and W carbenes exhibit different electrochemical behaviour. The Cr carbenes are oxidized in two one electron oxidation processes, namely Cr(0) to Cr(I) and Cr(I) to Cr(II). On the contrary, Fischer carbene complexes of tungsten are directly oxidized from W(0) to W(II). The first reduction process observed for both W- and Cr- carbenes, is a one electron process. A density functional theory (DFT) computational chemistry study of the electronic structure of the Cr- and W-carbenes, showed that the oxidation is metal based and the reduction is located on the carbene ligand. The DFT calculations further showed that the Cr(II) species is a triplet and the W(II) species a closed shell singlet. The DFT calculated energies of the HOMO and LUMO of the neutral carbenes relate linearly to the experimental oxidation and reduction potential, respectively. These mathematical relationships obtained can be used to predict experimentally measured potentials of related Fischer carbene complexes.

Negishi Cross-Coupling Reaction as a Simple and Efficient Route to Functionalized Amino and Alkoxy Carbene Complexes of Chromium, Molybdenum, and Tungsten

Synthesis of functionalized phenyl, 2-thienyl, and N-methyl-pyrrol-2-yl Fischer carbene complexes of chromium and tungsten was achieved via a Negishi cross-coupling reaction of metalated aminocarbenes in the presence of a palladium catalyst. The reverse approach, i.e., coupling bromocarbenes with organozinc reagents, is also feasible and is particularly useful with alkoxycarbene complexes, metalation of which is not possible. Both these procedures are also suitable for the cross-coupling of molybdenum amino- and alkoxycarbene complexes. The presented methodology enables an access to the new amino and alkoxy carbene complexes bearing aldehyde, ketone, nitrile, and ester functionality.

Intermolecular and regioselective access to polysubstituted benzo- and dihydrobenzo[c]azepine derivatives: modulating the reactivity of group 6 non-heteroatom-stabilized alkynyl carbene complexes.

We highlight the versatility of non-heteroatom-stabilized tungsten-carbene complexes 3 synthesized in situ, which have been used in a modular approach to access 2-benzazepinium isolable intermediates 5. By employing very mild conditions, benzazepinium derivatives 5 have been obtained in high yield from simple compounds, such as acetylides 2, Fischer-type alkoxycarbenes 1, and phenylimines 4. The process, involving a formal [4+3] heterocycloaddition, occurs in a totally regioselective manner, which differs from the approach previously observed in similar procedures for other carbene analogues. This work, which involves three components, reveals a control of the reactivity of non-heteroatom-stabilized carbene complexes 3 ([4+3] vs. [2+2]-heterocycloaddition reactions) depending on the acetylide substitution pattern. The influence of the substitution pattern in the behavior of the complexes has been computationally analyzed and rationalized. Finally, elaboration of the 2-benzazepinium intermediates allows access to 3H-benzo[c]azepines 6 and 3H-1,2-dihydrobenzo[c]azepines 7-9 with high control of the substitution of the nine positions of the heterocycle.

Chemistry of transition metal carbene complexes: nucleophilic substitution reactions of cyanamide anion to Fischer carbene complexes

Rate constants for the reactions of cyanamide anion N≡C-NH(-) with Fischer carbene complexes of the type (CO)(5)M=C(XR)C(6)H(4)Z with M = Cr and W, XR = SMe and OMe, Z = NMe(2), OMe, Me, H, F, Cl and CF(3) in 50% MeCN-50% water (v/v) at 25 °C are reported. N≡C-NH(-) shows a much higher reactivity towards these carbene complexes than OH(-), primary aliphatic amines (e.g. n-butylamine) or secondary alicyclic amines (e.g. piperidine) but is slightly less reactive than thiolate ions (e.g. HOCH(2)CH(2)S(-)). The alkoxy carbene complexes were found to react faster than the thiomethyl derivatives, consistent with previous findings for alkoxide ion, CH(CN)(2)(-), OH(-), amine and thiolate ion nucleophiles. Hammett ρ values are 3.00 ± 0.08 (k(1)) and 2.98 ± 0.08 (k(2)) for Cr-OMe-Z-N≡C-NH(-) reactions and 0.94 ± 0.05 (k(1)) for Cr-SMe-Z-N≡C-NH(-) reactions. The ρ values for the reaction of Cr-OMe-Z and Cr-SMe-Z with CH(CN)(2)(-) and DABCO (1,4-diazabicyclo[2.2.2]octane) (in 50% MeCN-50% H(2)O (v/v) are comparable to the present reactions. The much higher reactivity and hence much higher ρ value for methoxy carbene complexes over the corresponding thiomethyl derivatives is best explained by considering the substituent effects not only on the transition state (TS) but also on the reactant, and consistent with the previously observed pattern. Higher k(1)(W)/k(1)(Cr) ratios for (thiomethyl)carbene complexes as well as methoxy and ethoxycarbene complexes are related to the intrinsic rate constant which is higher for tungsten-carbene complexes than the corresponding Cr ones resulting in an enhanced ratio. This can also be explained by considering the electronegativity of Cr and W, which is higher for the latter; as a result the negative charge on the central metal atom is more localized in case of W causing destabilization of the TS and hence higher reactivity.

ChemInform Abstract: Alkylation of Acylmetalates with Alkyl Halides to Prepare Fischer Carbene Complexes: An Improved Protocol.

AbstractAlkoxy Fischer carbene complexes of chromium, tungsten and molybdenum are synthesized by alkylation of lithium acylmetalates with alkyl halides in the presence of catalytic amounts of tetrabutylammonium bromide under biphasic conditions.

A General Route to Perfluoroalkyl‐ and Trifluorovinyl‐Substituted Fischer Carbene Complexes of Tungsten and Chromium – Syntheses, Characterisation and Structures

AbstractHexacarbonyltungsten and ‐chromium [M(CO)6; M = W, Cr] react smoothly with perfluoroalkyl (Rf = CF3, C2F5) and trifluorovinyl (Rf = CF=CF2) silanes in the presence of equimolar amounts of fluoride ions with exclusive attack of one of the CO ligands to form the corresponding salts, Cat[(CO)5MC(Rf)O] (Cat = [Me4N]+, [Cs(15‐crown‐5)2]+) which after methylation represent the first hitherto unknown perfluoroorgano‐substituted Fischer carbene complexes, (CO)5MC(Rf)(OCH3), of those metals. The methoxy group can be easily replaced by amino functional groups as shown for the corresponding carbonyltungsten compound. The molecular structures of (CO)5WC(CF3)(OCH3) and (CO)5WC(CF3)[N(C2H5)2] have been elucidated by XRD measurements.

Multimetal Fischer carbene complexes of Group VI transition metals: synthesis, structure and substituent effect investigation

Fischer carbene complexes of tungsten with substituents containing up to two additional different transition metals, with all the metals in electronic contact with the carbene carbon atom, were synthesised and studied both in solution and in the solid state. For the complexes of the type [W(CO)(5){C(OR')R}], the substituents chosen were heteroaromatic 2-benzo[b]thienyl (2-BT), or 2-BT π-bonded to a chromium tricarbonyl fragment ([Cr(CO)(3)(2-η(6)-BT)]) or ferrocenyl (Fc) as the R-substituent, while the OR'-substituent was systematically varied between an ethoxy or a titanoxy group, to yield the complexes 1b (R' = Et, R = 2-BT), 2b (R' = Et, R = [Cr(CO)(3)(2-η(6)-BT)]), 3b (R' = TiCp(2)Cl, R = 21-BT), 4b (R' = TiCp(2)Cl, R = [Cr(CO)(3)(2-η(6)-BT)]), 5b (R' = Et, R = Fc) and 6b (R' = TiCp(2)Cl, R = Fc). The structural features and their relevance to bonding in the multimetal carbene compounds of both these tungsten and the analogous chromium complexes were investigated as they represent indicators of possible reactivity sites in multimetal carbene assemblies. The possibility of using DFT calculations to quantify the effect of metal-containing substituents on the carbene ligands was tested and correlated with experimental parameters by employing methods such as vibrational spectroscopy, molecular orbital analysis, and cyclic voltammetry.

Reactions of (Silyl)(silylene)tungsten and -molybdenum Complexes with Sulfur Reagents

Reaction of (silyl)(silylene)tungsten complex Cp*(OC)2W(SiMe3)(═SiMes2) (1) with sulfur reagents, S8 and ethylene sulfide 4, afforded cyclic carbene tungsten complex Cp*(S)W{═C(SiMe3)C(═O)OSi(Mes)2S} (3) via C═O bond fission of a carbonyl ligand and 1,2-migration of the SiMe3 group from W to the resulting carbene carbon. In contrast to the tungsten complex, reaction of (silyl)(silylene)molybdenum complex Cp*(OC)2Mo(SiMe3)(═SiMes2) (2) with 4 gave η2-thiocarbonyl complex Cp*(S)Mo[{η2-S═C(SiMe3)}C(═O)OSi(Mes)2S] (5), which would be formed by addition of a sulfur atom to the Mo═C bond of a molybdenum analogue of 3.

Competitive Pathways in the Reaction of Lithium Oxy‐ortho‐quinodimethanes and Fischer Alkoxy Alkynyl Carbene Complexes: Synthesis of Highly Functionalised Seven‐Membered Benzocarbocycles

Up to four different outcomes have been found for the reaction between 1-oxy-ortho-quinodimethanes (oQDMs) and alkoxy alkynyl Fischer carbene complexes (FCCs). The product formed depends on the structure of both reagents and on the reaction solvent. The pathways can be topologically classified as a [4C+2C], a [3(2C+O)+3C], and two different [4C+3C] processes and, in all these sequences, 1-oxy-oQDMs behave as enolates or as vinylogous enolates. The reaction of Choy and Yang's unsubstituted oQDM 1 with tungsten alkynyl FCCs is solvent controlled; thus, selective formation of benzocycloheptenones can be achieved in THF, whereas exclusive synthesis of benzocycloheptene ketals is reached in diethyl ether. On the other hand, THF is the solvent of choice to form benzocycloheptene ketals when an alkyl or aryl group is placed at position 1 of the oQDM in its reaction with tungsten carbene complexes; however, a pyranylidene carbene complex is formed when a chromium carbene complex is used. Alternatively, the presence of bulky alkoxy groups in the FCC component favours a Diels-Alder aromatisation sequence, which leads to 1-naphthyl FCCs. Furthermore, the isolation and the characterisation of several deuterated compounds by labelling experiments have provided some insight into the reaction pathways, and mechanisms consistent with those findings have been established and several reaction intermediates have been identified.

Multicomponent Synthesis of Pyran-2-ylidene Chromium(0) and Tungsten(0) Fischer Carbene Complexes

A new reaction between chromium(0) and tungsten(0) carbene complexes 1 and alkynyl esters or ketones 2 in the presence of NMO to yield pyran-2-ylidene complexes 3 has been explored. Here, we discuss the scope of the reaction and propose a plausible mechanistic pathway. The method is a new multicomponent approach for the synthesis of pyranylidene carbene complexes.

Sequential Five‐Component Construction of the Cyclopenta[e][1,3]oxazine Skeleton using Stable 2‐Azetine Derivatives

Small-ring heterocycles are of prominent importance because of their potential as bioactive compounds and synthetic building blocks. Whilst the chemistry of three-membered nitrogen heterocycles has been widely reported, studies on their four-membered counterparts have focused primarily on 2-azetidinones and, to a much lesser extent, azetidine rings. The 2-azetine system 1,2-dihydroazete, which has a strained cyclic enamine ring, is particularly elusive. Most 2-azetine compounds undergo spontaneous electrocyclic ring opening to afford their 1-azadiene analogues; as such, there are few examples of stable 2-azetines in the literature and those reported require electron-withdrawing substituents, for example, carbonyl, carboxyl, sulfonyl, or nitro groups, attached to the nitrogen atom. As a consequence, the chemistry of 2azetines remains largely unexplored, and has been predominantly focused on their potential as a precursor to azadienes. Moreover, there are no general synthetic routes to 2azetines. Although, the [2+2] cycloaddition of alkynes and imines appears to be a convenient route to this type of heterocyclic framework, only a few specific examples have been reported; in these cases, electron-rich alkynes, as ynamines, alkynyl selenides, or alkynyl sulphides, are able to form the expected 2-azetine skeleton, which is not isolated but rapidly opens to the azadiene system. The [2+2] cycloaddition of electron-poor alkynes and imines has only previously been suggested as an intermediate in the cyclization reaction between an alkynyl(ethoxy)carbene of tungsten and imine fluorenones to afford pyrroline derivatives. As part of our interest in Fischer-type metal carbenes, we have recently considered more-electrophilic metal carbenes, the so-called non-heteroatom-stabilized carbenes. These compounds are particularly useful in organic synthesis, and our preliminary results have revealed significant differences in the reactivity of both heteroatomand non-heteroatomstabilized systems. Herein, we report the [2+2] cycloaddition reaction of alkynyl-substituted (pentacarbonyl)chromium or -tungsten carbene complexes with imines as a suitable procedure for accessing stable 2-azetine derivatives. Moreover, the conjugation of the resulting azetine unit with the metal carbene allows for a facile synthesis of novel cyclopenta[e][1,3]oxazines involving treatment with alkynes. This threecomponent process (azetine, alkyne, and CO ligand) features cleavage of the azetine C3 C4 bond, rather than the expected N C4 bond, and the formation of three C C bonds and one C O bond (Scheme 1).

Asymmetric C2−C3 Cyclopentannulation of the Indole Ring

Chiral nonracemic alkynyl(alkoxy)carbene complexes of tungsten(0) undergo the [3 + 2] cyclization toward alpha-methylindoles and 1,6-dimethyl-1,2,3,4-tetrahydropyridine to provide 2,3-indoline-fused cyclopentenone and 2,3-piperidine-fused cyclopentenone skeletons, respectively, with very high enantiomeric purities (96-99% ee).

The Noncarbonylative Photochemistry of Group 6 Fischer Carbene Complexes

AbstractThe different photochemical reaction mechanisms of group 6 Fischer carbene complexes have been studied by a joint experimental and density functional theory (DFT) computational study. This study unifies the photochemcal behavior of group 6 metal carbene complexes (M = Cr or W), which may occur by three different photoreactions, namely photocarbonylation, stepwise 1,2‐dyotropic rearrangement, and α‐fragmentation. The metal atom and the substituent on the nitrogen atom attached to the carbene carbon atom influence the topography of the potential energy surface, which leads to two different activated species, diradicals or metallacyclopropanones. Complexes having a hydrogen substituent at the nitrogen atom experience a stepwise dyotropic migration with no significant differences between the chromium‐ and tungsten–carbene complexes. The excitation of complexes having substituents other than hydrogen at the nitrogen atom leads to two different T1 species, and hence to the formation of mixtures of products arising from α‐fragmentation (noncarbonylation) and amino esters (photocarbonylation). The two triplet states are isoenergetic in the Cr derivatives, but the T1 species that leads to α‐fragmentation is strongly favored for the W derivatives. Finally, both the chromium‐ and tungsten–carbene complexes are photoreactive in spite of the longstanding claim that tungsten–carbene complexes were photochemically inert. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The reactivity of a nucleophilic nickel acylate complex

The reactivity of a nucleophilic nickel acylate complex with a tungsten carbene complex, Fe(CO) 5, Cr(CO) 6, PPh 3, and CO was investigated. With the tungsten carbene complex, a methyl transfer occurred. With the metal carbonyl complexes, the acylate group on the nickel and a carbonyl on the iron or chromium traded places. With the PPh 3 and CO, the acylate anion was replaced by the phosphine or CO ligand.

Easy synthesis of heterocyclic carbene complexes by activation of chalcogenopyrones and benzopyrones to pyrylium salts and subsequent addition of carbanion of methoxy(methyl)pentacarbonyltungsten carbene complex

Methylenechalcogenopyran and benzopyran Fischer carbene complexes are easily obtained from commercially available chalcogenopyrones or benzopyrones and carbanion of methoxy(methyl)carbene tungsten complex. The key of the heterocyclic carbene formation is the activation of the carbonyl group by alkylation with alkyl trifluoromethanesulfonate reagent.