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The silylenes Si(tBu2bzam)R (tBu2bzam=N,N′‐bis(tertbutyl)benzamidinate; R=mesityl, CH2SiMe3) attack the Ccarbene atom of the Fischer alkynyl(ethoxy)carbene complex [W(CO)5{C(OEt)C2Ph}] to give, after a striking rearrangement, zwitterionic σ‐allenyl complexes in which the original carbene C atom forms part of the allene C3 fragment and also of a Si‐C‐N‐C‐N five‐membered ring after insertion into a Si−N bond of the original amidinatosilylene. These remarkable allenyl products, which contain two stereogenic groups, are selectively formed as single diastereomers. A striking stereoselective rearrangement, which ends in unprecedented zwitterionic allenyls having one end of the allene C3 fragment inserted into a Si−N bond, takes place in reactions of amidinatosilylenes with a Fischer alkynylcarbene. [ABSTRACT FROM AUTHOR]

The formation of the five‐membered‐ring germylene complexes [M(CO)5{Ge(tBu2bzamC(OEt)Me)tBu}] (3M; M=Cr, W), which occurs readily at room temperature from the germylene Ge(tBu2bzam)tBu (1tBu) and Fischer carbenes [M(CO)5{C(OEt)Me}] (2M; M=Cr, W), has been found to be reversible. Upon heating at 60 °C, complexes 3M undergo epimerization to an equilibrium mixture of 3M and 3′M. At that temperature, the chromium epimers (but not the tungsten ones) release CO to end in the mixed germylene–Fischer carbene complexes [Cr(CO)4{C(OEt)Me}{Ge(tBu2bzam)tBu}] (cis‐4Cr and trans‐4Cr). The latter decompose at 120 °C to [Cr(CO)5{Ge(tBu2bzam)tBu}] (6Cr). Because the formation of cis‐4Cr and trans‐4Cr from 3Cr or 3′Cr requires the presence of free 1tBu and 2Cr in the reaction solutions, the reactions of 1tBu with 2M to give 3M (and 3′M at 60 °C) should be reversible. This proposal has been proven by germylene‐exchange crossover reactions in which free 1tBu and [M(CO)5{Ge(tBu2bzamC(OEt)Me)CH2SiMe3}] (5′M; M=Cr, W) were formed when complexes 3M were treated at room temperature with the germylene Ge(tBu2bzam)CH2SiMe3 (1tmsm). A clear differential behavior between N‐heterocyclic carbenes (NHCs) and amidinatogermylenes (1tBu and 1tmsm) in their reactivity against group 6 metal Fischer carbene complexes is demonstrated. The higher electron‐donor capacity of amidinatogermylenes with respect to NHCs and the bias of the former to get involved in ring expansion processes are responsible for this differential behavior. Reversible ring expansion: The formation of the five‐membered‐ring germylene complexes 3M (M=Cr, W) from germylene 1tBu and electrophilic Fischer carbenes (2M) is reversible (see graphic). In fact, complex 3Cr leads to the CO substitution products cis‐4Cr and trans‐4Cr when it is heated at 60 °C. [ABSTRACT FROM AUTHOR]

Group 6 alkynyl Fischer carbene complexes behave as electrophiles with highly reactive masked dienes such as anthrone and 3‐hydroxy‐2‐pyrone, which also can react as bidentate nucleophiles. Both Cr0 and W0 alkynyl carbene complexes require tertiary amines to promote the formation of 1,4‐C‐addition products, but a marked dependence of the reactivity on the metal was observed. Although W0 complexes form exclusively the 1,4‐C‐adducts in the presence of a catalytic amount of base, Cr0 complexes lead to new anthrone‐derived bis‐carbene complexes, obtained by a sequential 1,4‐C‐ and 1,4‐O‐addition process with high diastereoselectivity and only in the presence of an excess of base. DFT computational studies show that the gap between the LUMO of the complex and the HOMO of the anthrone determines the differences in reactivity and that the stability of the intermediate metal allenyls formed after the initial 1,4‐addition is decisive for the outcome of the reaction. Group 6 alkynyl Fischer carbene complexes act as electrophiles with highly reactive masked dienes in the presence of tertiary amines to form different metal‐dependent products. DFT shows that the HOMOcomplex–LUMOdiene gap determines the reactivity of the Cr and W complexes, the stability of the intermediate metal allenyls formed in the initial 1,4‐addition being decisive for the reaction outcome. [ABSTRACT FROM AUTHOR]

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The insertion of carbene ligands into Ge−N (three examples) and Si−N (one example) bonds has been achieved for the first time by treating Fischer carbene complexes (M=W, Cr) with bulky amidinatotetrylenes (E=Ge, Si). These reactions, which start with a nucleophilic attack of the amidinatotetrylene heavier group 14 atom to the carbene C atom, proceed through a stereoselective insertion of the carbene fragment into an E−N bond of the amidinatotetrylene ENCN four-membered ring, leading to [M(CO)5L] derivatives in which L belongs to a novel family of tetrylene ligands comprising an ECNCN five-membered ring. [ABSTRACT FROM AUTHOR]

The addition of NHCs to α,β-unsaturated Cr0 and W0 (Fischer) carbene complexes is strongly dependent on the electrophilicity of the carbene carbon. Electrophilic alkoxy-carbene complexes quantitatively react with NHCs to yield stable zwitterionic (racemic) Cr0- and W0-alkenyls with total regio- and E-stereoselectivity. Less electrophilic aminocarbenes react with NHCs to promote the displacement of a CO ligand and yield 'mixed' NHC/Fischer biscarbenes in a process that is unprecedented in group 6 metal-carbene chemistry. In fact, the compounds prepared, are some of the scarce examples of Fischer bisylidenes reported in the literature. The electrochemistry of the zwitterionic Cr0- and W0-alkenylcomplexes made, show that these compounds have a strong reductor character, which is demonstrated in their reactions towards [Ph3C][PF6]. The oxidation processes lead to new types of cationic Fischer mono- and biscarbene complexes having a charged NHC fragment in their structures, in a new example of the use of electron-transfer reactions as a method to prepare novel group 6 (Fischer) carbene complexes. [ABSTRACT FROM AUTHOR]

The synthesis, structure, and photophysical properties of novel BODIPY–Fischer alkoxy-, thio-, and aminocarbene dyads are reported. The BODIPY chromophore is directly attached to the carbene ligand by an ethylenic spacer, thus forming donor–bridge–acceptor π-extended systems. The extension of the p-conjugation is decisive in the equilibrium geometries of the dyads and is clearly reflected in the corresponding absorption and emission spectra. Whereas the BODIPY fragment is mainly isolated in aminocarbene complexes, it is fully conjugated in alkoxycarbene derivatives. The former thus exhibit the characteristic photophysical properties of BODIPY units, whereas complete suppression of the BODIPY fluorescence emission is observed in the latter, as a direct consequence of the strong electron-accepting character of the (CO)5M=C moiety. As the π-acceptor character of the metal–carbene group can be modified, the electronic properties of the conjugated BODIPY can be tuned. Density functional calculations have been carried out to gain insight into the photophysical properties. [ABSTRACT FROM AUTHOR]

The synthesis, structure and electronic properties of novel Group 6 Fischer alkoxy-bis(carbene) complexes are reported. The UV/Vis spectra of these species display two main absorptions at approximately 350 and 550 nm attributable to a ligand-field (LF) and metal-to-ligand charge-transfer (MLCT) transitions, respectively. The planarity of the system and the cooperative effect of both pentacarbonyl metal moieties greatly enhance the conjugation between the group at the end of the spacer and the metal carbene fragment provoking dramatic changes in the LF and MLCT absorptions. This is in contrast to related push-pull Fischer monocarbenes, where the position of the MLCT band remains mostly unaltered regardless the substituent attached to the donor fragment. In addition, the MLCT maxima can be tuned with subtle modifications of the electronic nature of the central aryl fragment in the novel A-π-D-π-A (A=acceptor, D=donor) systems. DFT and time-dependent (TD) DFT quantum chemical calculations at the B3LYP/def2-SVP level have also been performed to determine the minimum-energy molecular structure of this family of compounds and to analyse the nature of the vertical one-electron excitations associated to the observed UV/Vis absorptions as well as to rationalise their electrochemical behaviour. The ability of tuning up the electronic properties of the compounds studied herein may be of future use in material chemistry. [ABSTRACT FROM AUTHOR]

Alkynylamino Cr0 and W0 Fischer carbenes undergo a CuAAC reaction with a diverse range of di-, tri-, and tetra-azides to produce polymetallic chromium(0) and tungsten(0) (Fischer)-carbene complexes in good-to-excellent yields. This method is simple, versatile, and is suitable for the preparation of a diverse range of structures with a high level of symmetry. Moreover, the resulting polymetallic carbene complexes are suitable partners for the peripheral functionalization of the metal nuclei, whilst retaining the metal fragment. This fact has been demonstrated in a simultaneous Pauson-Khand reaction, which, in some cases, allows for the generation of four bicyclic [5,5] rings on the periphery of a tetrametallic molecule in a process that involves the formation of 12 new CC bonds, with four simultaneous CO-insertion processes. The electrochemistry of the polymetallic Fischer carbenes show completely independent behavior for each nucleus, as well as an anomalous observation of the reversible oxidation of the allyl substituents, which has not been reported before in this class of chemistry. [ABSTRACT FROM AUTHOR]

The thermal Diels-Alder reaction between alkenylmetal(0) Fischer carbenes and 1 ,3-dienes (isoprene and cyclopentadiene) has been studied computationally within the density functional theory framework. The selectivity of the [4 + 2] cycloadditions between alkenyl-group 6(Fischer) carbene complexes and isoprene is similar to the selectivity computed for the reactions involving Lewis acid complexed acrylates. The experimentally observed complete endo selectivity in the [4 + 2] cycloadditions of alkenyl-group 6 (Fischer) carbene complexes with cyclopentadiene, which takes place under kinetic control, may be due in part to the presence of stabilizing secondary orbital interactions. These interactions are stronger than the analogues in the metal-free processes. The [4 + 21 cycloadditions between alkenyl-group 6 (Fischer) carbene complexes and neutral dienes occur concertedly via transition structures which are more asynchronous and less aromatic than their non-organometallic analogues, a behavior which is extensible to the reactions between Lewis acid complexed acrylates. [ABSTRACT FROM AUTHOR]

The thermal cycloaddition between alkynyl metal(O) Fischer carbenes and nitrones has been studied computationally within the Density Functional Theory framework. It is found that the [3 + 2] cycloaddition takes place via transition structures that are more asynchronous and less aromatic than their nonorganometallic analogues. These reactions are also found to be completely regioselective in favor of the cycloadduct possessing the Fischer carbene moiety and the oxygen atom of the nitrone in a 1,3-relative disposition. These results are consistent with the role of the Fischer carbene moiety as an electron withdrawing group that enhances the electrophilic character of the alkyne group acting as a Michael acceptor as a dipolarophile. In terms of the isolobal analogy model, it can be concluded that alkynylalkoxy metal(O) carbene complexes act in this reaction as organometallic analogues of organic alkyl-propiolates with enhanced electrophilic character. [ABSTRACT FROM AUTHOR]

Examines the transfer reaction of the group six metal-carbene complexes in inter- and intramolecular carbene. Effect of additives on the nature and the isometric ratio of the reaction products; Preparations of mono- and bicyclic derivatives; Formation of other alkyl complexes to a new mononuclear carbene complexes.

Examines the diverse di- and polymetallic carbene complexes. Synthesis of di- and polymetallic metal carbenes; Metal centers of Class III di- and polymetallic metal carbenes; Reactivity and applications to organic synthesis of carbenes; Conclusions.

Discusses a light-induced aminocarbene to imine dyotropic rearrangement in a chromium(0) center. Usefulness of the light-induced reactions of chromium(0)carbene complexes; Preparation of the complexes by intramolecular ligand displacement on aminocarbene complexes; Transformation of chromium(0)carbenes upon irradiation into N-metalated imines.

A two-step synthesis of gold mesoionic carbene complexes containing estrone moieties has been developed. The method uses the methylation of the triazole nucleus, followed by the treatment of the triazolium salt with Ag2O and transmetalation with [AuCl(SMe2)]. Mono-, bi-, tri-, and tetrametallic gold complexes can be obtained depending on the structure of the starting triazolium salts. Tetrametallic gold carbene embedded in a macrocylic stereoidal cavity containing four estrone nuclei has been also prepared. Additionally, the mono- and bimetallic silver carbene complexes containing triazole-steroid ligands have been isolated and characterized. These complexes resulted to be stable and have been characterized by spectroscopic and HRMS techniques. The gold and silver complexes having triazole-steroid ligands are unprecedented in the literature and the method reported here to access to these compounds is easy and efficient. Preliminary results regarding the catalytic activity of some of the gold-carbenes prepared in the insertion of diazoalkanes into alcohols are presented. [ABSTRACT FROM AUTHOR]

The systematic investigation of the effect of different catalysts and additives in the reaction of self-dimerization of alkoxychromium(0) (Fischer) carbene complexes resulted in the selection of Pd(PtBu3)2 to effect this transformation with good to excellent E selectivities and acceptable to excellent chemical yields. This catalyst will allow the control of the geometry in the synthesis of polyconjugated olefins, one emerging application of these catalytic reactions. [ABSTRACT FROM AUTHOR]

The formal [8 + 2] cycloaddition reaction between alkynyl Fischer carbene complexes and tropothione leads to the regioselective formation of novel 3aH-cyclohepta-[b]thiophene carbene complexes. Computational DFT calculations indicate that the process proceeds stepwise via antiaromatic zwitterionic intermediates. [ABSTRACT FROM AUTHOR]

New and well-characterized Ag-bis(1,2,3-triazolylidene) complexes having enantiopure (S)-sulfoxides upon sequential treatment with alcohols and Au(I) form separable mixtures of regioisomeric C-unsubstituted Au-1,2,3-triazolylidene complexes. Mechanistic studies and DFT calculations support a desulfinylation process for in situ generated free triazolylidene salts. [ABSTRACT FROM AUTHOR]

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 p-conjugation, as supported by density functional theory calculations. [ABSTRACT FROM AUTHOR]

A series of stable chiral (racemic), formally neutral, zwitterionic mono- and bimetallic M(CO)5[C(OEt)-C-CR(NHC)] (M = Cr, W) σ-allenyls are ready available by the addition of N-heterocyclic carbenes (NHCs) to Cr(0) and W(0) alkynyl Fischer carbene complexes. Different classes of NHCs, (e.g., 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene, and their six- and seven-membered analogues and 1,3-bis(dimethyl)imidazol-2-ylidene) were employed as nucleophiles in these C-C bond-forming reactions yielding the novel complexes in essentially quantitative yields. A systematic experimental and computational study of the electronic properties of the Cr- and W-allenyls shows that their UV-vis spectra are directly influenced by the structure of the heterocyclic moiety derived from the NHC (ring size, substituents on the N atoms) and by the nature of the metal fragment (Cr/W). The electron-releasing nature of these complexes allows them to participate in electron-transfer reactions in the ground state, leading to a type of charged α,β-unsaturated Fischer carbenes that incorporate an NHC fragment in their structure. [ABSTRACT FROM AUTHOR]

The syntheses of the starting vinylaziridines and vinylazetidines are described as well. [ABSTRACT FROM AUTHOR]

The joint computational (TD-DFT) experimental study of the UV-vis spectroscopy of alkoxychromium(0) carbene complexes accurately assigns the vertical transitions responsible for the observed spectra of these compounds. Both the LF and the MLCT band have a remarkably π-π* character, which has been demonstrated by the strong dependence of the absorptions with the donor/acceptor nature of the substituent in p-substituted styrylchromium(0) carbene complexes. The effect of the substituent is also related with the equilibrium geometry of the complexes and the occupations of the p atomic orbital of the carbene carbon atom. Additionally, the ferrocenyl moiety behaves in chromium(0) (Fischer) carbene complexes as a π-donor group. [ABSTRACT FROM AUTHOR]

By means of deuterium-labeling experiments, we have carried out a systematic ESI-MS study to determine the mechanism of ESI ionization of alkenyl and alkynyl group 6 Fischer carbene complexes. These compounds can be ionized under ESI conditions only in the presence of additives such as hydroquinone (HQ) or tetrathiafulvalene (TTF). Our results demonstrate that in the ESI source an anion-radical is formed after the initial HQ- or TTF-mediated electron transfer to the metallic carbene complex. For alkenyl carbene complexes, this species evolves by extrusion of a hydrogen radical to form an allenylchromium anion that is detected as the [M - H]- ion in the mass spectrum. The preference for this mechanistic pathway could be rationalized by DFT calculations. In the case of alkynyl carbene complexes, experiments combining deuterated substrate, additive, and solvent demonstrate that the previously proposed allene-anion carbene complex is not formed. Instead, the H transfer from the ethoxy group in the anion radical, followed by extrusion of a hydrogen radical, leads to an allenyl anion that is detected in the ESI-MS as [M - H - CO]-. [ABSTRACT FROM AUTHOR]

The reduction of α,β-unsaturated cyclic group 6 metal-carbene complexes strongly depends on the electronic profile of the groups attached both to the carbene and the β-carbon and occurs with exquisite regioselectivity. Thus, for complexes 8 the reduction does not take place at the carbene carbon but exclusively at the γ-carbonyl group. The resulting alkoxide 20 evolves to a tricyclic epoxide structure 21, which precludes additional hydride transfers. Complex 9 experiences the exclusive 1,4-reduction because of the imino character of the β-carbon (due to the participation of the aromatic resonance form 22). In contrast, monocyclic carbene complexes 10 behave as their acyclic congeners and experience 1,2-hydride addition followed by the 1,3-migration of the metal center. In this case, the participation of ηsup3;-Cr(CO)[sub 5] species 31 allows us to understand the labeling pattern found in the final products. [ABSTRACT FROM AUTHOR]