Carbene Character Research Articles

Theoretical study of the Si2NO potential energy surface

AbstractThe structures, spectroscopies, and stabilities of the doublet Si2NO radical are explored at the density functional theory (DFT) and ab initio levels. Seventeen isomers are located, connected by 26 interconversion transition states. At the CCSD(T)/6‐311+G(2df)//QCISD/6‐311G(d)+ZPVE level, three low‐lying isomers are predicted, that is, one bent species SiNSiO 3 (5.1 kcal/mol) containing the important SiN triple bonding and two four‐membered ring isomers including cyclic cSiNSiO 1 (0.0) with SiSi cross‐bonding with C2v symmetry and puckered cSiNSiO 1′ (11.9) with divalent carbene character. Three low‐lying isomers 1, 1′, and 3 have reasonable kinetic stabilities and might be observable either experimentally or astrophysically. The possible formation strategies of 1, 1′, and 3 in laboratory and in space are discussed in detail. The calculated vibrational frequencies and possible formation processes of 3 are consistent with recent experimental observations. In light of the fact that no cyclic nitrogen‐containing species have been detected in space, two cyclic isomers 1 and 1′ could be promising candidates. Furthermore, the bonding nature of three isomers 1, 1′, and 3 is analyzed. The calculated results are also compared with those of the analogue C2NO radical. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

N‐Heterocyclic Carbene Adducts of High‐Oxidation‐State Metal Halides: An Unexpected Instance of Lewis Acidic Carbene Character

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

A Versatile Conformational Switch Regulates Reactivity in Human Branched-Chain α-Ketoacid Dehydrogenase

The dehydrogenase/decarboxylase (E1b) component of the 4 MD human branched-chain alpha-ketoacid dehydrogenase complex (BCKDC) is a thiamin diphosphate (ThDP)-dependent enzyme. We have determined the crystal structures of E1b with ThDP bound intermediates after decarboxylation of alpha-ketoacids. We show that a key tyrosine residue in the E1b active site functions as a conformational switch to reduce the reactivity of the ThDP cofactor through interactions with its thiazolium ring. The intermediates do not assume the often-postulated enamine state, but likely a carbanion state. The carbanion presumably facilitates the second E1b-catalyzed reaction, involving the transfer of an acyl moiety from the intermediate to a lipoic acid prosthetic group in the transacylase (E2b) component of the BCKDC. The tyrosine switch further remodels an E1b loop region to promote E1b binding to E2b. Our results illustrate the versatility of the tyrosine switch in coordinating the catalytic events in E1b by modulating the reactivity of reaction intermediates.

Synthesis, structures and electrochemical properties of ruthenium (II) complexes bearing bidentate 1,8-naphthyridine and terpyridine analogous (N,N,C)-tridentate ligands

1,8-Naphthyridine (napy) and terpyridine-analogous (N,N,C) tridentate ligands coordinated ruthenium (II) complexes, [Ru L(napy-κ 2 N, N′) (dmso)](PF 6) 2 ( 1: L= L 1= N″-methyl-4′-methylthio-2,2′:6′,4″-terpyridinium, 2: L = L 2 = N″-methyl-4′-methylthio-2,2′:6′,3″-terpyridinium) were prepared and their chemical and electrochemical properties were characterized. The structure of complex 1 was determined by X-ray crystallographic study, showing that it has a distorted octahedral coordination style. The cyclic voltammogram of 1 in DMF exhibited two reversible ligand-localized redox couples. On the other hand, the CV of 2 shows two irreversible cathodic peaks, due to the Ru–C bond of 2 containing the carbenic character. The IR spectra of 1 in CO 2-saturated CH 3CN showed the formation of Ru-(η 1-CO 2) and Ru–CO complexes under the controlled potential electrolysis of the solution at −1.44 V (vs. Fc/Fc +). The electrochemical reduction of CO 2 catalyzed by 1 at −1.54 V (vs. Fc/Fc +) in DMF-0.1 M Me 4NBF 4 produced CO with a small amount of HCO 2H.

Theoretical Study on Structures and Stability of Si2CP Isomers

The structures, energetics, spectroscopies, and isomerization of various doublet Si2CP species are explored theoretically. In contrast to the previously studied SiC2N and SiC2P radicals that have linear SiCCN and SiCCP ground states, the title Si2CP radical has a four-membered-ring form cSiSiPC 1 (0.0 kcal/mol) with Si-C cross-bonding as the ground-state isomer at the CCSD(T)/6-311G(2df)//B3LYP/6-311G(d)+ZPVE level, similar to the Si2CN radical. The second low-lying isomer 2 at 11.6 kcal/mol has a SiCSiP four-membered ring with C-P cross-bonding, yet it is kinetically quite unstable toward conversion to 1 with a barrier of 3.5 kcal/mol. In addition, three cyclic species with divalent carbene character, i.e., cSiSiCP 7, 7' with C-P cross-bonding and cSiCSiP 8 with Si-Si cross-bonding, are found to possess considerable kinetic stability, although they are energetically high lying at 44.4, 46.5, and 41.4 kcal/mol, respectively. Moreover, a linear isomer SiCSiP 5 at 44.3 kcal/mol also has considerable kinetic stability and predominantly features the interesting cumulenic /Si=C=Si=P/* form with a slight contribution from the silicon-phosphorus triply bonded form /Si=C*-Si[triple bond]P/. The silicon-carbon triply bonded form *Si[triple bond]C-Si[triple bond]P/ has negligible contribution. All five isomers are expected to be observable in low-temperature environments. Their bonding nature and possible formation strategies are discussed. For relevant species, the QCISD/6-311G(d) and CCSD(T)/6-311+G(2df) (single-point) calculations are performed to provide more reliable results. The calculated results are compared to those of the analogous C3N, C3P, SiC2N, and Si2CN radicals with 17 valence electrons. Implications in interstellar space and P-doped SiC vaporization processes are also discussed.

Post-Hartree−Fock Studies on the Structure of Bis(ortho-substituted phenyl)methylenes

Long-lived triplet bisarylmethylenes are now well-known. Experimental data (primarily ESR hyperfine parameters) suggest that the carbon framework of long-lived bisarylmethylenes approaches D(2)(d)() symmetry, as ortho-substitution forces the central angle to approach 180 degrees . According to DFT modeling, the approach of the central angle to 180 degrees is accompanied by a dramatic shortening of the central CC bonds and severe quinoid distortion of the phenyl rings. In contrast, X-ray investigation of bis(2,4,6-trichlorophenyl)methylene shows a structure closer to the carbene valence bond representation with less seriously distorted phenyl rings, a more acute central angle, and a longer bond from the methylene carbon to the aryl substituent. We address the difficulty of achieving a balance of cumulene and carbene character, treating the model systems diethynylmethylene, dicyanomethylene, and diisocyanomethylene by post-Hartree-Fock methods CAS and CCSD as well as DFT models, and applying the perturbation-corrected CAS methods to the chloro and methyl ortho-substituted bisphenyl carbenes.

Bridging Methylene to Bridging Acyl Conversion in Heterobinuclear Rh/Ru Complexes: Models for Adjacent-Metal Involvement in Bimetallic Catalysts

Protonation of the methylene-bridged complex [RhRu(CO)4(μ-CH2)(dppm)2][CF3SO3] (1) at −80 °C yields an asymmetrically bridged methyl complex in which the methyl ligand is σ-bound to Ru while being involved in an agostic interaction with Rh. Warming to −40 °C results in migration of the methyl group to a terminal site on Rh, and further warming to 0 °C results in migratory insertion to give the acetyl-bridged complex [RhRu(OSO2CF3)(CO)2(μ-C(CH3)O)(μ-CO)(dppm)2][CF3SO3] (4), in which the acetyl carbon is bound to Rh and its oxygen is bound to Ru. At ambient temperature one carbonyl is lost from 4 to give [RhRu(OSO2CF3)(CO)2(μ-C(CH3)O)(dppm)2][CF3SO3] (5). Carbon-13 NMR spectroscopy and the X-ray structures of both acetyl-bridged products suggest significant carbene character in the acyl carbon. Attempts to obtain methyl and subsequently acetyl species by methylene insertion into metal−hydride bonds of the appropriate precursors [RhRu(X)(CO)4(μ-H)(dppm)2][X] (X- = CF3SO3-, BF4-) failed, with these hydride sp...

Nitrile ylide dimerization: investigation of the carbene reactivity of nitrile ylides.

A series of novel hexaaryl diazatrienes 5 ("nitrile ylide dimers") were synthesized directly from the corresponding diaryl ketimines 12 and dichlorotoluenes 13 in a facile one-pot synthesis. The carbene character of the nitrile ylides was investigated by varying the substituents on the aromatic ring adjacent to the carbene center. The isolation of the corresponding carbene dimers as stable crystalline materials with absorption maxima (lambda(max)) from 363 to 422 nm was shown to be promoted by the absence of strongly electron-withdrawing substituents. The crystal structures indicate that the E-isomers were isolated when phenyl, 3-methylphenyl, and 3-chlorophenyl substituents are present at the carbene carbon; the Z-isomer was isolated when the more sterically hindered 2,4,6-trimethylphenyl substituent (Mes) is present. The (1)H NMR spectra of the E-isomers demonstrate the nonequivalence of the aromatic rings, in which two of the aromatic rings of the imine moiety are pseudoaxial and the remaining aromatic rings are pseudoequatorial. The reactions proceed via the intermediate nitrile ylides 1 generated by the base-promoted 1,1-elimination of HCl from the intermediate chloroimine 14. The nitrile ylide was also generated by 1,3-elimination of HCl from the imidoyl chloride 18, confirming common pathways via the nitrile ylide as the dimer products obtained from these different routes were identical. The strongly electron-withdrawing 4-nitrophenyl substituent promotes the linear carbanion character of the 1,3-dipole and no dimer is formed.

Oxidative and nonoxidative decarboxylation of N-Alkyl-N-phenylglycines by horseradish peroxidase. Mechanistic switching by varying hydrogen peroxide, oxygen, and solvent deuterium.

Horseradish peroxidase (HRP) typically oxidizes aniline derivatives using hydrogen peroxide as the oxidant. The action of HRP on N-alkyl-N-phenylglycine derivatives 1b-1e (PhN(R)CH(2)COOH; R = Me, Et, n-Pr, i-Pr, respectively) is highly unusual if not unique. Under standard peroxidatic conditions (HRP/H(2)O(2)/air), the major product (ca. 70%) is the secondary aniline 2b-2e (PhNHR) resulting from the expected oxidative decarboxylation process, but a significant amount (ca. 30%) of the related tertiary aniline PhN(CH(3))R (3b-3e) arises from an unexpected nonoxidative decarboxylation process. Under anaerobic, peroxide-free conditions only the tertiary anilines 3b-3e are formed in a reaction that is extremely rapid compared to those in which H(2)O(2), molecular oxygen, or both are present. In D(2)O buffers, the product is exclusively the monodeutero tertiary aniline PhN(CH(2)D)R and the reaction is much slower (k(H(2)O)/k(D(2)O) = 5.7), suggesting that a proton transfer step is substantially rate-limiting in turnover. It is proposed that ferric HRP oxidizes 1 to a cation radical, which then decarboxylates to an alpha-amino radical having carbanion character on carbon; protonation of the latter, followed by electron capture from ferrous HRP, completes the cycle. Hydrogen peroxide and oxygen slow turnover by diverting ferric HRP toward the compound I/compound II forms or toward compound III, respectively. Finally, under peroxidatic conditions, 1a (R = cyclopropyl) inactivates HRP with concurrent formation of 2a but not N-phenylglycine, but under anaerobic, peroxide-free conditions, 1a inactivates HRP almost instantly with no detectable product formation.

Synthesis and structures of four crystalline lithium β-diketiminates derived from [Li{CH(SiMe 3)(SiMe 2OMe)}] 8 and PhCN or Bu tCN and PhCN

Treatment at ambient temperature in diethyl ether of one equivalent of [Li{CH(SiMe 3)(SiMe 2OMe)}] 8 ( A) with (i) two equivalents of PhCN, or (ii) successive equivalent portions of Bu tCN and PhCN gave from (i) ▪ ( 1) and ▪ ( 2) as well as the known lithium 1-azaallyl [Li{N(SiMe 2OMe)C(Ph)CH(SiMe 3)}] 3, and from (ii) ▪ ( 3) The latter was also obtained from the known [Li{N(SiMe 2OMe)C(Bu t)CH(SiMe 3)}] 2 and PhCN under similar conditions. Recrystallisation of 2 from THF/hexane afforded ▪ ( 2 ′). X-ray diffraction data on 2, 2 ′ and 3 are presented; data for 1 were only adequate to confirm its gross tetrameric structure. The particularly novel feature of these transformations of the lithium alkyl A into lithium β-diketiminates 1, 2, 2 ′ or 3 is that while the first 1,3-carbon to nitrogen shift from the α-carbon of A is both silicotropic and regiospecific (in so far as SiMe 2OMe > SiMe 3 in migratory aptitude), the second migration is indiscriminate: silicotropy yielding 1 or 3 but prototropy giving 2 (or 2 ′). Consistent with these observations, the central carbon atom of the β-diketiminato ligand has significant carbanionic character in 2 or 2 ′, attributed to its stabilisation by the exocyclic Me 3Si at C2, whereas in 1 or 3 there is π-delocalisation.

The electronic structure of nitrilimines revisited.

A combination of density-functional theory and natural resonance theory has been used to show that a complete description of the electronic structure of nitrilimines, R(1)CNNR(2), requires four resonance structures (propargylic, allenic, 1,3-dipolar and carbenic); appropriate substituents were shown to enhance the carbene character of nitrilimines to the point where they may be considered stable carbenes.

Ligand Displacement Reaction of Ru(η4-1,5-COD)(η6-1,3,5-COT) with Lewis Bases

Reactions of Ru(η4-1,5-COD)(η6-1,3,5-COT) (1) (COD = cyclooctadiene, C8H12; COT = cyclooctatriene, C8H10) with a series of Lewis bases are studied. Treatments of 1 with P(OMe)3, P(OEt)3, P(OMe)2Ph, P(OEt)2Ph, P(OiPr)3, P(OMe)Ph2, or P(OEt)Ph2 lead to the formation of Ru(η4-1,5-COD)(η4-1,3,5-COT)L (2) followed by preferential liberation of the 1,5-COD ligand to give COT complexes formulated as Ru(6-η1:1−3-η3-COT)L3 (3) and Ru(η4-1,3,5-COT)L3 (4). Reaction of 1 with P(OPh)3 also gives 2, but further treatment of this system leads to the liberation of both 1,5-COD and 1,3,5-COT to give a mixture of Ru(η4-1,5-COD)L3 (5) and Ru(6-η1:1−3-η3-COT)L3 (3) and finally gives (6h). In the reaction of 1 with tert-butylisonitrile, 1,3,5-COT is initially liberated with concomitant formation of Ru(η4-1,5-COD)(CNtBu)3 (5j), which further reacts with the liberated 1,3,5-COT, giving eventually Ru(6-η1:1−3-η3-COT)(CNtBu)3 (3j). The molecular structure of 5j determined by X-ray structure analysis reveals that one of the isonitrile ligands has a considerable carbene character. In the presence of excess isonitrile, the homoleptic complex Ru(CNtBu)5 (7) is formed. On the other hand, 1 does not react with poor π-accepting ligands such as NEt3, pyridine, and (dimethylamino)pyridine as well as bulky phosphorus ligands such as triphenylphosphine and tricyclohexylphosphine. These results suggest that the initial coordination of electron-donating ligands to Ru essentially encourages the dissociation of the 1,5-COD ligand, but the π-accepting ability of the ligand induces the dissociation of the 1,3,5-COT ligand.

An η2-vinyl pathway may explain net trans hydrosilylation via transition metal catalysis even in cyclic cases

Recently published results seem to rule out the accepted η2-vinyl pathway for net trans addition in alkyne hydrosilation in cyclic cases. We now suggest how these results can be rationalized on the basis of the accepted mechanism using two concepts: the electrophilic carbene character of η2-vinyl groups and the tendency of silyl groups to migrate to electron-deficient centers.

Diphosphorus‐Containing Unsaturated Three‐Membered Rings: Comparison of Carbon, Nitrogen, and Phosphorus Chemistry

AbstractFor Abstract see ChemInform Abstract in Full Text.

Bimetallic Reductive C−C Coupling Reaction Induced by Chemical Oxidation: Formation of a μ3-C3 Ring on a Triruthenium Cluster

Oxidation of the triruthenium cluster 2 bearing a face-bridging allyl fragment results in the formation of a μ3-C3 ring on the Ru3 plane as a consequence of bimetallic reductive C−C coupling. Both structural and spectral data of 5 indicate carbenic character of the carbon atoms within the newly formed face-bridging C3 ring.

Terpyridine-Analogous (N,N,C)-Tridentate Ligands: Synthesis, Structures, and Electrochemical Properties of Ruthenium(II) Complexes Bearing Tridentate Pyridinium and Pyridinylidene Ligands

The cyclometalated complexes [RuL(terpy)][PF6]2 (3, L = N‘ ‘-methyl-4‘-methylthio-2,2‘:6‘,4‘ ‘-terpyridinium; 4, L = N‘ ‘-methyl-4‘-methylthio-2,2‘:6‘,3‘ ‘-terpyridinium) with a (N,N,N)(N,N,C)-coordination mode were synthesized in good yields and fully characterized by X-ray crystallographic, spectroscopic, and electrochemical measurements. 13C{1H} NMR and electronic spectra revealed that the Ru−C bond of complex 4, which has a quaternized N-Me unit at the para-position of the carbon atom bonding to the metal center in the terminal ring of the tridentate ligand, involves carbenic (RuC) character in solution.

Tuning low-coordinate metal environments: high spin d5–d7 complexes supported by bis(phosphinimino)methyl ligation

Treatment of FeCl2 with the lithium derivative of [CH2(Ph2PNC6H2Me3-2,4,6)2] in THF and crystallisation from Et2O gave the ‘ate’ complex [{CH(Ph2PNC6H2Me3-2,4,6)2}Fe(μ-Cl)2Li(THF)(OEt2)] in which the iron is four-coordinate and the chlorides bridge to lithium. Treatment of [M{N(SiMe3)}2] (M = Mn, Fe, Co) with [CH2(Ph2PNC6H2Me3-2,4,6)2] in toluene afforded the complexes [{CH(Ph2PNC6H2Me3-2,4,6)2}MN(SiMe3)2] which all feature essentially planar N(1)–N(2)–N(3)–M coordination. The carbanionic character of the bis(phosphinimino)methyl ligand results in close C(1)–M contacts in both the Mn and Co complexes effectively raising the coordination number to four. The analogous iron compound is strictly three-coordinate as demonstrated by X-ray crystallography and 57Fe Mossbauer. However protonolysis of this compound with Ph3COH yields the iron alkoxide [{CH(Ph2PNC6H2Me3-2,4,6)2}FeOCPh3] that features a close C(1)–Fe contact of 2.375 A. Although the iron centre in this complex is only marginally pyramidalised by this contact, its 57Fe Mossbauer spectrum indicates a significant perturbation to the local electronic environment at the metal.

Reactivity of the Amido Complex [Re(NHpTol)(CO)3(bipy)] toward Neutral Organic Electrophiles

The amido complex [Re(NHpTol)(CO)(3)(bipy)] (1) reacts with pTolNCS and with Ph2CCO giving the products of the insertion of these heterocumulenes into the Re-N and N-H bonds, the complexes [Re{SC(NpTol)(NHpTol)}(CO)(3)(bipy)] (2) and [Re{N(pTol)C(O)CHPh2}(CO)(3)-(bipy)] (3), respectively. The reaction of 1 with activated acetylenes containing at least one alkoxycarbonyl group afforded the compounds [Re{N(pTol)C(R')C(R)C(OH)}(CO)(3)(bipy)] (4a-c). The formation of the metallacycle present in these compounds involves the activation of one of the carbonyl ligands of 1. The structures of the complexes 2, 3, and 4b have been determined by single-crystal X-ray diffraction. C-13 NMR and crystallographic data indicate that the metal-bonded carbon atom of the metallacycle possesses carbene character.

Unusual Approaches to Organophosphorus Compounds: The Surprising Reactivity of Bis(methylene)phosphoranes and Related Phosphoranylidene Carbenoids

The reactivity of the isomeric LiX-phosphoranylidene carbenoids E-, Z-[P]C(X)Li ([P] = Mes*PCTms2; X = Cl, Br) as well as the synthesis and reactivity of the corresponding E-[P]C(F)Li carbenoid was examined. The carbenoids exhibit a pronounced carbanion character, and differences in their reactivity mainly stem from the configuration of the carbenoid centers and/or the nature of the halogen substituent. Reaction of the dibromo-substituted bis(methylene)phosphorane, [P]CBr2, with an excess of n-BuLi proceeded through a dilithio intermediate ([P]CLi2), which was trapped with MeI and H2O. Endo-methylated bis(methylene)phosphoranes ([P]C(Me)X) exhibit a surprising acidity that, depending on the substitution pattern, induces rearrangement and/or elimination reactions. Rearrangement reactions accompanied by the loss of LiX afford a phosphirene that was used as a ligand in a nickel complex. Fluoro-substituted bis(methylene)phosphoranes ([P]C(F)R) show weak interactions with an adjacent γ-silyl group, leading to configurationally stable Z-systems and exhibit carbenoid character in the case of the α-silyl-substituted bis(methylene)phosphorane.

Formation of cis-enediyne complexes from rhenium alkynylcarbene complexes.

Dimerization of the alkynylcarbene complex Cp(CO)(2)Re=C(Tol)C(triple bond)CCH(3) (8) occurs at 100 degrees C to give a 1.2:1 mixture of enediyne complexes [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)CC(CH(3))=C(CH(3))C(triple bond)CTol] (10-Eand 10-Z), showing no intrinsic bias toward trans-enediyne complexes. The cyclopropyl-substituted alkynylcarbene complex Cp(CO)(2)Re=C(Tol)C(triple bond)CC(3)H(5) (11) dimerizes at 120 degrees C to give a 5:1 ratio of enediyne complexes [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)C(C(3)H(5))C=C(C(3)H(5))C(triple bond)CTol] (12-E and 12-Z); no ring expansion product was observed. This suggests that if intermediate A formed by a [1,1.5] Re shift and having carbene character at the remote alkynyl carbon is involved, then interaction of the neighboring Re with the carbene center greatly diminishes the carbene character as compared with that of free cyclopropyl carbenes. The tethered bis-(alkynylcarbene) complex Cp(CO)(2)Re=C(Tol)C(triple bond)CCH(2)CH(2)CH(2)C(triple bond)CC(Tol)= Re(CO)(2)Cp (13) dimerizes rapidly at 12 degrees C to give the cyclic cis-enediyne complex [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)CC(CH(2)CH(2)CH(2))=CC(triple bond)CTol] (15). Attempted synthesis of the 1,8-disubstituted naphthalene derivative 1,8-[Cp(CO)(2)Re=C(Tol)C(triple bond)C](2)C(10)H(6) (16), in which the alkynylcarbene units are constrained to a parallel geometry, leads to dimerization to [Cp(CO)(2)Re](2)(eta(2),eta(2)-1,2-(tolylethynyl)acenaphthylene] (17). The very rapid dimerizations of both 13 and 16 provide compelling evidence against mechanisms involving cyclopropene intermediates. A mechanism is proposed which involves rate-determining addition of the carbene center of A to the remote alkynyl carbon of a second alkynylcarbene complex to generate vinyl carbene intermediate C, and rearrangement of C to the enediyne complex by a [1,1.5] Re shift.