Carbene Structures Research Articles

Theoretical Investigation of Carbon–Sulfur Triple Bonds

By means of first principle calculations we have investigated a set of molecules that are presumed to contain carbon-sulfur triple bonds, namely HCSOH, H(3)SCH, cis-FCSF, F(3)CCSF(3), and F(5)SCSF(3). For HCSOH, FCSF, and H(3)SCH we used the CCSD(T) methodology and the correlation-consistent basis sets. On the other hand, F(3)CCSF(3) and F(5)SCSF(3) were studied at the B3LYP, M06-2X, MP2, and G3 levels of theory. We found that none of these molecules display a carbon-sulfur adiabatic bond dissociation energy (ABDE) as strong as diatomic CS (170.5 kcal mol(-1)), or a diabatic bond dissociation energy (DBDE) larger than the one found in SCO (212.0 kcal mol(-1)), although the DBDE of FCSF comes quite close at 208.3 kcal mol(-1). The CS ABDEs of F(3)CCSF(3), F(5)SCSF(3), and H(3)SCH are comparable to that of a single C-S bond. In contrast with the experimental results, F(3)CCSF(3) and F(5)SCSF(3) are predicted to be linear with C(3v) and C(s) symmetry, respectively, at the B3LYP/6-311+G(3df,2p) level. MP2/6-311+G(2df,2p) calculations support the C(3v) symmetry for F(3)CCSF(3), despite F(5)SCSF(3) not having a perfect linear structure; the CSC angle is 174.6°, which is nearly 20° larger than the experimental value. The analysis of the carbene structures of HCSOH and H(3)SCH revealed that they are not significant, because the triplet state is dissociative in these cases. However, for F(3)CCSF(3) and F(5)SCSF(3) , the carbene triplet states lie 0.81 and 0.77 eV above the singlet state, respectively. In the same vein, our investigation supports the presence of a strong double bond for HCSOH. The conflicting evidence available for F(3)CCSF(3) and F(5)SCSF(3) makes it very difficult to determine the nature of the CS bonds. However, the bond dissociation energies and the singlet-triplet splittings clearly suggest that these compounds should be considered as masked sulfinylcarbenes. The analysis of the bond dissociation energies challenges the existence of a triple bond in these five molecules, but from a strictly thermodynamic standpoint, cis-FCSF is found to be the candidate most likely to exhibit triple-bond character.

Novel Intramolecular CAryl–S Bond Activation by an Electron Rich, Ring-Expanded-NHC-Rh centre: A Combined Experimental and DFT Study

The reaction of (o-MeSPh)-N-functionalized tetrahydropyrimidinium salts with KN(SiMe3)2 and [Rh(COD)Cl]2 in THF leads to the formation of a novel dimeric RhIII bis-carbene complex. The reaction involves the unexpected cleavage/oxidative addition of the aryl-sulfur bond to give dimeric metallated RhIII with bridging MeS< moieties. This unusual reaction is probably a consequence of the sterically imposing structure and strong donor capacity of ring-expanded N-heterocyclic carbenes (RE-NHCs). An X-ray structure of the [(Ph,DIPP-NHC)Rh(Cl)(SMe)2] product complex has been obtained, and DFT studies were undertaken to gain an insight into the reaction pathway.

Theoretical characterization of a Ru N-Heterocyclic Carbene derivative of a polyoxometalate. Enhanced π-interaction in oxide supported TM-organic linkages

The electronic structure of the N-heterocyclic carbene (NHC) functionalized polyoxometalate [K(PW(9)O(34))(2)(cis-WO(2))(cis-RuLMe(2))](12-) (1) is studied computationally; and its Ru-NHC bonding situation is compared with other monometallic Ru-NHC complexes, as well as other representative carbene complexes. The DFT analysis further supports the low spin configuration, S = ½, as the ground state, as the unoccupied d ruthenium orbitals are high in energy and mixed with the d-type orbitals of W. We gauge the TM-carbene bonding nature by means of atoms in molecules (AIM) quantum theory and a modified version of energy decomposition analysis (EDA) based on orbital deletion. According to the calculations, the NCH ligand in POM derivative 1 is more tightly bonded than in its analogous neutral monometallic Ru-NHC complexes. This is manifested in a higher electron density ρ(r) at the bond critical point (bcp) between Ru and the carbene carbon atoms. The energetic contribution of σ-donation is similar for all Ru-NHC complexes, and significantly higher than for Fischer-type carbene complex, in agreement with the description of the N-heterocyclic carbene ligand as a strong σ-donor. Among the studied Ru-NHC complexes, the POM derivative 1 shows the largest π-back-donation ability, explaining the observed short Ru-C(carbene) bond distance and the calculated high interaction energy. The POM framework acting as ligand makes the ruthenium d electrons more loose via orbital mixing and promotes the back-donation to the ligand.

Heterocyclic carbenes of diverse flexibility: A theoretical insight

A systematic density functional investigation has been carried out on the structure, stability and reactivity of heterocyclic carbenes of diverse flexibility, i.e., carbenes with different modes of binding specially normal and remote mode of binding. Ligating properties of these carbenes have been assessed by virtue of their HOMO energies and verified further by inspection of the symmetric CO frequencies of their respective palladium carbonyl complexes. All the remote carbenes were found to have higher σ-donating abilities compared to their normal analogs. N-heterocyclic carbenes 1 and 5 are found to be electrophilic in nature while the remote carbene 3 and P-heterocyclic carbene 6 are found to be nucleophilic. Quantum theory of atoms in molecules (QTAIM) reveals significant covalent character in the C carbene–Pd bonds.

Stereodivergent Quaternization of 2‐Alkyl‐2‐p‐tolylsulfinylacetonitriles: NMR Spectroscopic Evidence of Planar and Pyramidal Benzylic Carbanions

Enantiomerically enriched alpha,alpha-disubstituted phenylacetonitriles have been readily prepared by stereoselective quaternization of 2-alkyl-2-[2-(p-tolylsulfinyl)phenyl]acetonitriles with different alkylating electrophiles in the presence of bases. The use of potassium hexamethyldisilazane (KHMDS)/[18]crown-6 ether and NHMDS with alkyl halides afforded S,S(S) and R,S(S) diastereoisomers, respectively, in high enantiomeric purities, thus providing stereodivergent processes for synthesizing both isomers. The dependence of the stereochemical course of the reactions on the experimental conditions (mainly on the counterion) has been rationalized by assuming a planar or pyramidal structure for the benzylic carbanions. This hypothesis has been supported by NMR spectroscopic studies, which permit one to assign a chelated pyramidal structure to the sodium benzylic carbanions and an almost planar naked carbanionic structure to the potassium benzylic carbanions generated in the presence of [18]crown-6 ether.

Effect of substituents at the heteroatom on the structure and ligating properties of heterocyclic carbene, silylene, germylene and abnormal carbene: A theoretical study

The effect of substituents at the heteroatom on the electronic structures of different N-heterocyclic carbenes (1, 2 and 3), silylene (4) and germylene (5) are examined using Density Functional Theory. The kinetic and thermodynamic stabilities of these molecules are assessed by examining the HOMO-LUMO gap and hydrogenation energies, respectively. The extent of cyclic electron delocalization present in these five-membered ring systems are quantified with the help of NICS calculations. The ligating properties of 1-5 and the recently synthesized free abnormal carbene 6 (Bertrand et al., Science, 2009, 326, 556-559) are examined by looking at the energies of the sigma symmetric electron-donating orbital of the respective molecules. Among the systems considered, 6 is found to have the strongest sigma-donating ability. A comparative study of the ligating properties between the two isomeric carbenes 1 and 6 is performed by calculating the carbonyl stretching frequencies of some iridium carbonyl complexes of these two tautomeric carbenes.

Resonance structures of N-heterocyclic carbenes

Electronic structure of N-heterocyclic carbenes with five-member rings and two nitrogen atoms was studied using the natural resonance theory (NRT) in the framework of density functional theory. Also the role of the hydroxyl substituents on the carbon atoms was investigated. The analysis provides a consistent picture of chemical bonding, even in cases where no single Lewis structure can be drawn. It interprets the observed stabilities and aromaticities of the different isomers. A carbene lone pair on C 1 atom was identified in all cases, while the electron donation to the formally empty p z orbital of this atom was found to be a crucial effect in all cases.

HgCl 2 sorption on lignite activated carbon: Analysis of fixed-bed results

Factors that influence kinetic reactivity and equilibrium between elemental mercury, carbon, and flue gas components have been the focus of numerous studies. This study pertains to recent bench-scale fixed-bed tests in which activated carbon was exposed to HgCl 2 in a flue gas composition typical of an unscrubbed eastern bituminous coal. Results are discussed in light of a refined binding site model based on the zigzag carbene structures recently proposed for electronic states at the edges of the carbon graphene layers.

Novel Expanded Ring N-Heterocyclic Carbenes: Free Carbenes, Silver Complexes, And Structures

The addition of aromatic formamidines to dihaloalkanes proceeds in air under mildly basic conditions and in polar solvents to afford the corresponding amidinium salts in high yields. To demonstrate the wide scope of this methodology, several five-, six-, and seven-membered saturated N-heterocyclic amidinium salts have been prepared. The free carbenes 7-Mes, 7-Xyl, 7-Pri, 6-Mes, and 6-Xyl have also been isolated as stable white solids, and for the first time the solid state structures of free seven-membered N-heterocyclic carbenes have been determined (7-Mes and 7-Xyl). The syntheses and characterization of their Ag(I) halide complexes are also described and the X-ray structures for Ag(6-Pri)Br, Ag(7-Mes)Br, Ag(7-Xyl)I, and Ag(Xyl7-Mes)Br determined. On the basis of NMR data the silver halide species form either neutral, Ag(NHC)X, or ionic, [Ag(NHC)2][AgX2] complexes in solution. A feature of the expanded ring carbenes is the extremely large NCN angle (≥120°) and the consequential steric impact of the N-substituents on the metal center. For the free seven-membered ring carbenes the 13C NMR shifts are 258−260 ppm, well down field from those observed for the six- and five-membered ring carbenes.

New Routes for the Functionalization of P4

New Routes for the Functionalization of P₄

Coupled Cluster Study of Polycyclopentanes: Structure and Properties of C5H2n, n = 0-4

The title hydrocarbons have been examined by the CCSD(T)/cc-pVTZ (singlets) and UMP2/cc-pVTZ (triplets) methods. They were confirmed to represent local minima on the singlet potential energy surface, while 1,3-biradical, 1,4-biradical, or carbene structures were found on the triplet surface, including an intermediate for the triplet energy transfer from one to the other double bond of 1,4-pentadiene. Bonding is discussed in terms of Weinhold's NBO theory and the absence of a simple correlation between bond strength and bond length in these highly strained systems is pointed out. Predictions of NMR, IR, and Raman spectra are provided.

N-Heterocyclic Carbenes with Inorganic Backbones: Electronic Structures and Ligand Properties

The electronic structures of known N-heterocyclic carbenes (NHCs) with boron, nitrogen, and phosphorus backbones are examined using quantum chemical methods and compared to the experimental results and to the computational data obtained for a classical carbon analogue, imidazol-2-ylidene. The sigma-donor and pi-acceptor abilities of the studied NHCs in selected transition-metal complexes are evaluated using a variety of approaches such as energy and charge decomposition analysis, as well as calculated acidity constants and carbonyl stretching frequencies. The study shows that the introduction of selected heteroatoms into the NHC backbone generally leads to stronger metal-carbene bonds and therefore improves the ligand properties of these systems. The backdonation of pi-electron density from the metal to the ligand is found to be strong in complexes of the studied NHCs with group 11 metals, where it constitutes up to nearly 35% of the total orbital interaction energy. The ligand properties of the aluminum analogues of some of the reported NHCs with boron backbones are also assessed.

On the electron delocalization in cyclopropenylidenes – An experimental charge-density approach

The extent and nature of cyclic electron delocalization in free and coordinated cyclopropenylidene carbenes has been analyzed by combined experimental and theoretical charge-density studies. The significant asymmetry of the C–C bond lengths in substituted cyclopropenylidene carbenes was identified as cooperative effect which depends on contributions of both σ- and π-bonding. We show that analyses of (i) the topology of the Laplacian of the electron density distribution and (ii) the out-of-plane atomic quadrupole moments – the charge-density analogues of p π occupation – allow to distinguish between the influence of σ- and π-electrons on cyclic electron delocalization. These studies hint for pronounced electron localization in the carbene lone pair region which dominates the electronic structure of free cyclopropenylidene carbenes and hinders the establishment of true aromaticity. We further investigated the electron donating/withdrawing ability of cyclopropenylidene ligands relative to N-heterocyclic carbenes. The experimental benchmark systems LCr(CO) 5 (L = 2,3-diphenylcyclopropenylidene and 1,2-dimethylimidazol-2-ylidene) show that the cyclopropenylidene ligand clearly displays the higher π-acceptor capability relative to N-heterocyclic carbenes.

Electronic spectroscopy, lifetimes, and barrier to linearity in the [formula omitted] system of dibromocarbene

Using an improved discharge recipe for the production of dibromocarbene (CBr 2), we recently reassigned the electronic origin of the A ˜ 1 B 1 ← X ˜ 1 A 1 system, based on an extensive data set of isotope shifts for the pure bending transitions [C. Tao, C. Mukarakate, D. Brusse, Y. Mishchenko, S. A. Reid, J. Mol. Spectrosc. 240 (2006) 139–140]. In this study, we report the complete analysis of the fluorescence excitation spectrum of the A ˜ 1 B 1 ← X ˜ 1 A 1 system in the 525–650 nm region, obtained at a rotational temperature of ∼10 K. A total of 32 cold bands involving the pure bending levels 2 0 n with n = 2–19 and combination bands 1 0 1 2 0 n ( n = 1–16) in the A ˜ 1 B 1 ← X ˜ 1 A 1 system were observed; a number of these are reported here for the first time. Rotational analysis typically yielded A rotational constants and band origins for all three bromine isotopomers (C 79Br 2, C 79Br 81Br, C 81Br 2), and Dunham expansion fits yielded an extensive set of vibrational parameters for each. The isotope shifts are in good agreement with the product rule, and, when plotted vs. bending quantum number, the measured A constants follow a linear trend except for the highest bending levels, where an abrupt increase is observed, indicative of the approach to linearity. This is mirrored in the vibrational intervals, which also change abruptly in this region, and the estimated barrier height of ∼18 800 cm −1 above the vibrationless level of the X ˜ 1 A 1 state is in excellent agreement with the ab initio prediction of Sendt and Bacskay [K. Sendt, G.B. Bacskay, J. Chem. Phys. 112 (2000) 2227–2238]. We also report fluorescence lifetimes as a function of vibrational level and K a ′ ; the lifetimes decrease rapidly with energy, but display no dependence on K a ′ over the measured range. The implications of these results for understanding the excited state structure of this prototypical carbene are emphasized.

Computational Studies on Stable Triplet States of Heteroacetylenes and the Effects of Halogen Substituents

This paper describes theoretical studies of halogen-substituted heteroacetylenes (XCMY, M = Si and Ge; X, Y = H, Cl and F) performed at the QCISD(T)/6-311G//QCISD/6-31G level of theory. The electronegative halogen substituents destabilize the singlet state such that the triplet state tends to become favorable. The triplet state has the bifunctional electronic structure of a triplet carbene joined to a heavy singlet carbene. We found that the substituents effectively reduce the energy of the donor-acceptor interactions (E(D-A)) between the two in-plane lone pairs of electrons of the singlet state; therefore, the remaining pi bond is less favorable energetically than the triplet state with a sigma bond. A related phenomenon occurs for the homonuclear heavy acetylenes in singlets in which the lead compound RPbPbR switches to a Pb-Pb sigma bond from the pi bonds observed for the lighter acetylenes.

Quantitative Structure−Activity Relationships of Ruthenium Catalysts for Olefin Metathesis

A quantitative structure-activity relationship (QSAR) model is presented in which both the independent and dependent (response) variables are derived from density functional theory (DFT) calculations on a large set of 14-electron complexes, LCl(2)Ru=CH(2), with different dative ligands, L. The multivariate model thus correlates the properties of the 14-electron complexes with a calculated measure of activity, with modest computational cost, and reproduces the experimental order of activity for the Grubbs ruthenium catalysts for olefin metathesis. The accuracy and applicability of the model is to a large extent due to the use of highly specific geometric and electronic molecular descriptors which establish a direct connection between activity and chemically meaningful donor ligand properties. The ligands that most efficiently promote catalytic activity are those that stabilize the high-oxidation state (+4) metallacyclobutane intermediate relative to the ruthenium-carbene structures dominating the rest of the reaction pathway. Stabilization of the intermediate is ensured, among others, through ligand-to-metal sigma donation, whereas metal-to-ligand pi back-donation destabilizes the intermediate and lowers catalytic activity. A bulky dative ligand drives the reaction toward the less sterically congested metallacyclobutane species and thus contributes to catalytic activity. The multivariate model and the high-level descriptors furthermore provide practical handles for catalyst development as exemplified by the suggestion of several new donor ligands predicted to give more active and functional group tolerant ruthenium-based catalysts. The present strategy holds great promise for broader screenings of olefin metathesis catalysts as well as for development of homogeneous transition metal catalysts in general.

Reactions of [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru(COD) with Lewis bases: Synthesis, structure, and electrochemistry of ruthenium amine, nitrile, carbene, phosphite and phosphine complexes

Treatment of [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru(COD) ( 1) with phosphites, phosphines, amines or N-heterocyclic carbene in THF afforded the COD displacement complexes [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru[P(OEt) 3] 2 ( 2), [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru[PPh 2(OEt)] 2 ( 3), [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru[NH 2CH 2CH 2Pr i ] 2 ( 4), [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru(NH 2Pr n ) 2 ( 5), [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru (η 2-NH 2CH 2CH 2NH 2) ( 6), [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru[η 2-NH(CH 3)CH 2CH 2NH(CH 3)] ( 7) or [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru[NHC] 2 ( 8, NHC = 1,3,4,5-tetramethylimidazol-2-yilidene), respectively. Ruthenium–amine complexes were much more labile than 1. Upon exposure to moisture, 5 was converted into [{η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)}Ru(μ-H 2O)] 2 ( 9). Reactions of 5 with PR 3 (R = PPh 3, Cy), TMEDA (TMEDA = N, N, N′, N′-tetramethylethylenediamine) and CH 3CN afforded the corresponding amine replacement products[η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru(NH 2Pr n )(PPh 3) ( 10), [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru(NH 2Pr n )(PCy 3) ( 11), [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru(TMEDA) ( 12) and [η 5:σ-Me 2C(C 5H 4)(C 2B 10H 10)]Ru(NCCH 3) 2 ( 13). These results indicated that the steric factor dominated these substitution reactions. The electrochemical studies showed that the electron richness of the Ru atom decreased in the order L 2Ru(NHC) 2 > L 2Ru(amine) 2 > L 2Ru(NCMe) 2 > L 2Ru(P) 2. All of these complexes were fully characterized by various spectroscopic techniques and elemental analyses. The molecular structures of 2, 3, 5– 10, 12 and 13 were further confirmed by single-crystal X-ray analyses.

Electron delocalization in mixed-valence butadienediyl-bridged diruthenium complexes

We report electrochemical and spectroelectrochemical investigations on the butadienediyl-bridged diruthenium complexes [{Ru(PPh3)2(CO)Cl}2(μ-C4H4)] (1), [{Ru(PEt3)3(CO)Cl}2(μ-C4H4)] (2), and [{Ru(PPh3)2(CO)Cl(NC5H4COOEt-4)}2(μ-C4H4)] (3). All these complexes are oxidized in two consecutive one-electron steps separated by 315 to 680 mV, depending on the co-ligands. The first oxidation is a chemically and electrochemically reversible process whereas the second varies from nearly reversible to irreversible at room temperature. We have generated and investigated the mixed-valence monocations and observed CO band shifts of ca 25 cm−1 and the appearance of new bands in the visible regime at ca 720 to 800 and 430 to 450 nm. The lower-energy band which tails into the near infrared has been assigned as a charge-resonance (or intervalence charge-transfer) absorption and used to estimate the electronic coupling parameter H AB. Our investigations point to valence delocalization for 2 + , and nearly delocalized behavior for 1 + and 3 + . Even the complex with the smallest potential splitting is, however, fully delocalized on the longer ESR timescale, as is evident from the coupling pattern of the solution spectrum. Overall IR band shifts on full oxidation and the hyperfine splittings for 1 + argue for charge and spin delocalization onto the bridging C4H4 ligand. This issue has also been addressed by quantum chemical calculations employing DFT methods. Geometry optimizations at each oxidation level reveal inversion of the C–C bond pattern from a short–long–short to a long–short–long alteration and a bis(carbenic) structure at the dication stage. All spectroscopic features such as IR band shifts, average g-values and g-tensor anisotropies are fully reproduced by the calculations.

A Modular Approach to Main-Chain Organometallic Polymers

A highly efficient route to a new class of organometallic polymers containing difunctional N-heterocyclic carbenes has been developed. Bis(imidazolium) halides and divalent group X metals were copolymerized to afford organometallic polymers in up to quantitative yields and with molecular weights up to 10(6) Da, depending on the structure of the N-heterocyclic carbene and the incorporated transition metal. Enhanced solubilities were demonstrated through post-polymerization ligation with phosphines. Finally, selective end-group functionalization and excellent molecular weight control was achieved through the inclusion of monofunctional chain transfer agents during the polymerization.

Connectivity and Reactivity of Gaseous CrC3H4O+

Gaseous CrC3H4O+ is prepared by reaction of Cr+ with monofluoracetone and investigated by means of Fourier transform ion-cyclotron resonance mass spectrometry. CrC3H4O+ only slowly dehydrogenates alkanes larger than propane, but efficiently reacts with alkenes to afford dehydrogenation, ligand loss, and C−C bond cleavage. These reactions can be explained by assuming the metal carbene structure 1 for CrC3H4O+, which reacts with alkenes by an initial [2+2] cycloaddition. Consecutive cycloreversion results in metathesis products that undergo alkylidene-olefin conversion and elimination of the C−C cleaved fragments. For allene, the corresponding vinylidene-acetylene transformation appears to be particularly favorable. In addition, the experimental findings suggest the occurrence of initial hetero-Diels−Alder reactions that involve the carbonyl group of 1. At elevated energies, such as upon complexation by benzene, 1 can rearrange to form the chromaoxacyclobutene 2 and eliminate propyne.