Carbene Structures Research Articles

Cooperative B-H Activation: Ligand Diversity in Groups 6 and 8 Transition Metal κ2-N,E-Chelated Complexes (E = S or Se).

The synthesis, structure, and reactivity of the N-heterocyclic carbene (NHC)-supported κ2-N,S-chelated ruthenium complex [(IMe)(PPh3)Ru{κ2-N,S-(mbz)}2] (IMe = 1,3-dimethylimidazol-2-ylidene, mbz = 2-mercaptobenzothiazolyl (NC7H4S2)), 2 has been established. Complex 2 is generated from the reaction of [Ru(PPh3)2Cl2], 1 with IMe followed by the addition of [(mbz)K]. The room-temperature reaction of 2 with [BH3·THF] led to the formation of the dihydridoborate complex, [(Ph3P)Ru{κ3-H,S,S'-(NH2BSBH2N)(S2C7H4)2}], 3. Further, the synthesis and structure of Cp*-supported κ2-N,S-chelated tungsten complexes, [Cp*W(CO)2{κ2-N,S-(NC7H4S2)}], 5 and [Cp*W(CO)2{κ2-N,S-(NC7H4S2)}{κ2-N,S-(N(CH3)C5H4S)}], 6, generated from the reaction of [Cp*W(CO)3Me], 4 and [Na(H3B-NC7H4S2)] have been described. In a similar fashion, pyridine-2-thiolato/selenolato ligand-supported Cp*-based κ2-N,E-chelated (E = S and Se) tungsten complexes, [Cp*W(CO)2{κ2-N,E-(NC5H4E)}] (7a: E = S (pyridine-2-thiolato), 7b: E = Se (pyridine-2-selenolato)), have been isolated from the reaction of [Cp*W(CO)3Me], 4 and [Na(H3B-NC5H4E)] ligands. The reaction of bulky borane ArFBH2 (ArF = 3,5-bistrifluoromethylbenzyl) with 5 led to the isolation of a borate species, [Cp*W(CO)2{κ2-S,H-(H2ArFB-NC7H4S2)}], 10. The complex [Cp*Mo(CO)2{κ2-N,Se-(NC5H4Se)}], 9 generated from the salt elimination reaction of [Cp*Mo(CO)3Cl], 8 and [K(NC5H4Se)] reacts with [BH3·THF] to yield the borate complex [Cp*Mo(CO)2{κ2-Se,H-(H3B-NC5H4Se)}], 11. To investigate the effect of the ligand diversities on the reactivity of all of the κ2-N,E-chelated complexes, density functional theory (DFT)-based calculations were performed.

Structure-Induced Reversing Basicity of α-Anionic Fischer Carbenes.

Group VI metal carbenes and their α-anionic complexes play an important role in organic synthesis, yet structural characterizations and thermodynamic studies for reactive α-anions remain challenging. This work investigated the molecular structures and thermodynamics of α-anionic Fischer carbenes and their conjugated acids. The results clarify the negative charge localized on the M(CO)5 moiety and the C═X (X = C, N, or O) double bond in all anionic structures. The conjugation between the π-donating group and the M(CO)5 moiety significantly lowers the energy of neutral complexes, resulting in a reversing acidity between α-methyl, amino, and hydroxyl groups. This study deepens our understanding of the molecular structure, electronic structure, and thermodynamic property of Fischer carbenes.

Simultaneous Thermally Stimulated Delayed Phosphorescence (TSDP) and Thermally Activated Delayed Fluorescence (TADF) in a Two-Coordinated Au(I) Bimetallic Complex Featuring a Tandem Carbene Structure.

A bimetallic, two-coordinated carbene-metal-amine (cMa) Au(I) complex featuring a twisted tandem carbene structure (NHC1-Au-NHC2-Au-carbazolyl) was synthesized. The molecular structure in single crystals revealed a large dihedral angle between the two carbene ligands, while the bridged carbene NHC2 and carbazolyl (Cz) ligands were coplanar. A bluish green thermally stimulated delayed phosphorescence (TSDP) was observed in crystals with an emission lifetime over 70 μs, which can be attributed to the spin allowed diabatic population of a high-lying emissive triplet state from the 3LE characterized low-lying ones. The small rotation energy barrier of Cz along the coordination bond allowed conformers with large dihedral angles between NHC2 and Cz. The ICT characterized S1 state was consequently stabilized to achieve a thermally accessible energy gap to facilitate ISC between triplets and the S1, leading to the thermally activated delayed fluorescence (TADF). Simultaneous TSDP and TADF dual emission can be recorded in its doped polymer film owing to the coexistence of these different conformers.

Elucidating the Lability and Diversity of Self-Assembled Structures of N-Heterocyclic Carbenes at the Liquid/Metal Interface

Elucidating the Lability and Diversity of Self-Assembled Structures of N-Heterocyclic Carbenes at the Liquid/Metal Interface

Tailored Monolayers of N-Heterocyclic Carbenes by Kinetic Control.

Despite the substantial success of N-heterocyclic carbenes (NHCs) as stable and versatile surface modification ligands, their use in nanoscale applications beyond chemistry is still hampered by the failure to control the carbene binding mode, which complicates the fabrication of monolayers with the desired physicochemical properties. Here, we applied vibrational sum-frequency generation spectroscopy to conduct a pseudokinetic surface analysis of NHC monolayers on Au thin films under ambient conditions. We observe for two frequently used carbene structures that their binding mode is highly dynamic and changes with the adsorption time. In addition, we demonstrate that this transition can be accelerated or decelerated to adjust the binding mode of NHCs, which allows fabrication of tailored monolayers of NHCs simply by kinetic control.

IR spectroscopic characterization of 3d transition metal carbene cations, FeCH2+ and CoCH2+: periodic trends and a challenge for DFT approaches.

A combination of IR multiple-photon dissociation (IRMPD) action spectroscopy and quantum chemical calculations was employed to investigate the [M,C,2H]+ (M = Fe and Co) species. These were formed by reacting laser ablated M+ ions with oxirane (ethylene oxide, c-C2H4O) in a room temperature ion trap. IRMPD spectra for the Fe and Co species are very similar and exhibit one major band. Comparison with density functional theory (DFT) and coupled cluster with single and double excitations (CCSD) calculations allows assignment of the spectra to MCH2+ carbene structures. For these 3d transition metal systems, experimental IRMPD spectra compare relatively poorly with DFT calculated IR spectra, but CCSD calculated spectra are a much better match primarily because the M-C stretch gains significant intensity. The origins of this behavior are explored in some detail. The present results are also compared to previous results for the 4d and 5d congeners and the periodic trends in these structures are evaluated.

Recent advances in the chemistry of isolable carbene analogues with group 13-15 elements.

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics.

Transition-metal acyclic carbene complexes: building blocks for luminescent, stimuli-responsive, bioactive materials and catalysts

Acyclic carbene complexes exhibit exceptional performance and functional properties comparable to their NHC counterparts, and their environmentally sensitive open acyclic carbene structure makes them ideal for developing smart materials and sensors.

Synthesis, structure and reactivity of a boron-containing spirocycle carbanion

The crystal structure, electronic characteristic, as well as the formation process, and nucleophilic reactivity of an unexpected boron-containing carbanion are presented. Its structure consists of a discrete cation and a spirocycle structure, in which the sp 3 hybrid C7 atom connects the mesityl substituted oxaborole unit and the puckered anthracene moiety. • A novel boron-containing carbanion with distinct structural characteristics was synthesized, which was composed of a discrete [K(cryptand)] + ion and a spirocycle anion. • The formation and electronic structure of carbanion were explored through DFT calculations. • The nucleophilic reactivity of carbanion with some chlorosilane or halogenated alkanes in THF solution was evaluated. Carbanion ( 2 ) was synthesized by the deprotonation strategy through treatment of (2-(anthracen-9-yl)phenyl)(hydroxy)(mesityl)borane ( 1-H ) with KN(SiMe 3 ) 2 in the presence of [2,2,2]-cryptand in tetrahydrofuran (THF) solution followed by crystallization of the crude product from cooled THF. Its crystal structure and electronic properties are presented, as well as the formation process and nucleophilic reactivity with chlorosilane or halogenated alkanes.

Bond Dissociation Energies of Carbene-Carbene and Carbene-Main Group Adducts.

A range of carbene structures and their adducts with one another and with a selection of small-molecule electrophiles and nucleophiles were examined at the composite correlated molecular orbital theory G3MP2 level to explore ground-state "carbenic" structures, their stabilities, and reactivities. Differences between carbene general classification as a singlet electrophilic carbene or singlet nucleophilic carbene and their given reactivity are discussed. A key quantity is the carbon-carbon bond dissociation energy for carbene dimers or the carbene-adduct dissociation energy for other species. The carbene dimer bond dissociation energies span a wide range from 10 to 170 kcal/mol. The hydrogenation energies and singlet-triplet splitting were found to correlate best with the carbene's self-dimerization energy, whereas other descriptors do not. The proton and fluoride affinities of the carbenes alone prove inadequate for classifying reactivity among classes of carbenes. The self-dimerization bond dissociation energy, hydrogenation energy, and singlet-triplet splitting of various carbenes, despite sometimes large differences in proton affinity and other indicators of reactivity, provide usable metrics to correlate substantial amounts of thermodynamic and kinetic (reactivity) information regarding these structures.

Odd-Number Cyclo[n]Carbons Sustaining Alternating Aromaticity.

Cyclo[n]carbons (n = 5, 7, 9, ..., 29) composed from an odd number of carbon atoms are studied computationally at density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) levels of theory to get insight into their electronic structure and aromaticity. DFT calculations predict a strongly delocalized carbene structure of the cyclo[n]carbons and an aromatic character for all of them. In contrast, calculations at the CASSCF level yield geometrically bent and electronically localized carbene structures leading to an alternating double aromaticity of the odd-number cyclo[n]carbons. CASSCF calculations yield a singlet electronic ground state for the studied cyclo[n]carbons except for C25, whereas at the DFT level the energy difference between the lowest singlet and triplet states depends on the employed functional. The BHandHLYP functional predicts a triplet ground state of the larger odd-number cyclo[n]carbons starting from n = 13. Current-density calculations at the BHandHLYP level using the CASSCF-optimized molecular structures show that there is a through-space delocalization in the cyclo[n]carbons. The current density avoids the carbene carbon atom, leading to an alternating double aromaticity of the odd-number cyclo[n]carbons satisfying the antiaromatic [4k+1] and aromatic [4k+3] rules. C11, C15, and C19 are aromatic and can be prioritized in future synthesis. We predict a bond-shift phenomenon for the triplet state of the cyclo[n]carbons leading to resonance structures that have different reactivity toward dimerization.

Functional Fiber with Bis(N-Heterocyclic Carbene) Structure Prevents Pd(0) Nanoparticle Agglomeration for an Efficient and Recyclable Heck Reaction

Functional Fiber with Bis(N-Heterocyclic Carbene) Structure Prevents Pd(0) Nanoparticle Agglomeration for an Efficient and Recyclable Heck Reaction

Structure and electronic properties of benzimidazole and cycloheptaimidazole gold N-heterocyclic carbenes

Two related families (one previously experimentally reported and another theoretically proposed) of gold benzimidazole (family 1) and cycloheptaimidazole (family 2) N-heterocyclic carbenes were studied due to the potential application in medicinal chemistry and the resurgence of Au based catalysis. Both families show interesting aromatic properties, which were calculated computationally using the Nucleus Independent Chemical Shift (NICS) indexes, Electron Localization Function (ELF) and the Electrostatic Potential Surface (EPS) maps. The calculation of the NICS indexes, the ELF values and EPS showed that there is a small difference between both families of studied complexes, where family 1 is slightly more aromatic that family 2. Both families showed that the carbene ring has σ aromaticity, while the six or seven membered rings showed π aromaticity. To study their spectroscopic properties, Time-Dependent Density Functional Theory (TD-DFT) calculations were performed. These calculations showed that there is almost no contribution from the metal to the observed UV–Vis transitions (all of them showed Ligand to Ligand Charge Transfer (LLCT) character), due to the high stability that the molecular orbitals (MOs) of the gold atom have in the complex.

N‐Phosphine Imide‐Substituted Imidazolylidenes

AbstractA series of novel multifunctional carbenes of the type N‐phosphine imide‐substituted imidazolylidenes (PimIms) has been synthesized and characterized. Substituents including (+)‐10‐camphorsulfonyl and pyroglutamate groups were introduced on the N‐phosphinimidoyl moiety to diversify available structures of isolable carbenes. The impact that the rotation of the N‐phosphinimidoyl groups has on the spatial environment surrounding the carbene carbon atoms was evaluated theoretically. This analysis suggested that the shape as well as the volume of the reaction field around the carbene carbon atoms varies drastically during this rotation.

N-Heterocyclic Carbenes–Cu

The electronic structures of N-heterocyclic carbenes (NHC) imidazolinylidene, thiazolinylidene, imidazolylidene, thiazolylidene, and 1,2,4-triazolylidene and their complexes with cuprous halides (CuX, X = Cl, Br, I) were investigated theoretically at the B3LYP/def2-SVP level. In contrast to other NHCs, imidazolylidene and 1,2,4-triazolylidene do not dimerize owing to the negligible coefficient of the vacant p-orbital at the carbene centre in their respective LUMOs. This is further supported by their greater thermodynamic and kinetic stabilities revealed by greater activation free energies and smaller standard free energies for their dimerization. Second-order perturbation interactions in the natural bond orbital (NBO) analysis of the NHCs indicate that six π electrons are delocalized in imidazolylidene, thiazolylidene, and 1,2,4-triazolylidene, conferring aromatic character and thereby enhancing their thermodynamic stability. NBO analysis reveals the existence of effective back bonding from a d orbital of Cu to the NHC, increasing the Wiberg bond index of the C–Cu bond to ~1.5. Owing to the large electronic chemical potential (μ) and high nucleophilicity indices, NHCs are able to transfer their electron density effectively to the cuprous halides having low μ values and high electrophilicity indices to yield stable NHC–CuI complexes. Large values of the Fukui function f(r) at the carbene centre of the NHCs and Cu atom of the NHC–CuI complexes indicate their softness. Imidazolylidene was found to be the softest, rationalizing wide use of this class of NHCs as ligands. The coordination of the NHCs to cuprous halides is either barrierless or has a very low activation free energy barrier. In the A3 reaction wherein NHC–Cu(I) complexes are used as catalyst, the reaction of NHC–CuI with phenylacetylene changes the latter into acetylide accompanied by raising the energy level of its HOMO considerably compared with the level of the uncomplexed alkyne, making its reaction with benzaldehyde barrierless.

Coordination’s preference and electronic structure of N-heterocyclic carbene–monometallic complexes: DFT evaluation of σ-bonding and π-backbonding interactions

The geometrical parameters, the electronic structures and the nature of the chemical bonding for complexes of the type (NqMes)M[Cl(CO)2] (NqMes = naphtoquinone-annulated NHC ligand and M = V, Mn, Re, Co, Rh, Cu) which possess even electron counts have been investigated at the BP86 and B3LYP levels of theory using orbital molecular and energy decomposition analyses. The coordination of the six-membered ring is strongly disfavoured (structures A), while the structures B1 and B2 which differ by the orientation of the M[Cl(CO)2] unit are in competition with regard to the metal’s nature and the spin state. The carbene–metal bonds are governed chiefly by electrostatic attractions contributing 65–75% of the bonding attractive interactions. The carbene–metal bonds are strong in electron-moderately rich Re element and weak in electron-rich Cu metal. In the most cases, orbital interactions are more important in symmetrical species than in the unsymmetrical ones, which are mainly described by σ-bonding with the π-backbonding part less than 20%.

Reactions of Transition-Metal Carbyne Cations with Ethylene in the Gas Phase

The reactions of iridium- and osmium-carbyne hydride cations [HIrCH]+ and [HOsCH]+ with ethylene have been studied using mass spectrometry with isotopic-labeling in the gas phase. The carbyne reactivity is compared with that of the rhodium, cobalt, and iron analogues [TMCH2]+ (TM = Fe, Co, and Rh), which were determined to have the carbene structures. Besides the cycloaddition/dehydrogenation reaction in forming the [TMC3H4]+ + H2 (TM = Ir and Os) products, a second reaction pathway producing the [TMC2H2]+ ion and CH4 via triple hydrogen atom transfer reactions to the carbyne carbon is observed to be the major channel. The latter channel is not observed in the rhodium, cobalt, and iron carbene cation reactions. Quantum-chemical calculations indicate that the distinct reactivity is not due to different initial structures of the reactants. Both reaction channels are predicted to be thermodynamically exothermic and kinetically facile for the carbyne cations, and the reactions proceed with the initial formation of a carbene intermediate via hydride-carbyne coupling. The latter channel is also exothermic but kinetically unfavorable for the rhodium, cobalt, and iron carbene cations.

ФІЛЬНІСТЬ КАРБЕНІВ. НОВИЙ ПОГЛЯД

The electronic properties of carbenes including thermodynamic parameters such as new electronic philicity indices Ie, Ph, and for comparizon chemical hardnesses η, proton affinities (РA) calculated by DFT method (B3LYP5/6-31G*/RHF for definition of electronic indices and B3LYP5/3-21G/RHF, B3LYP5/3-21G/UHF for definition of chemical hard¬nesses) have been discussed in the paper. With their help, the estimation of philicities, electron-donating and electron-withdrawing abilities of a wide range of carbenes of both nucleophilic and electrophilic type was carried out. It was established that the philicities of carbenes according to electronic indices Іе, Ph depend on the carbenic structure (the backbone of the molecule and steric effects of substituents) and also on the reagent structure, particularly its steric effect. For typical nucleophilic carbenes, the Ph is in the range of 1–3,0, for neutral carbenes 1–1,5, IeH 8,5–22,2 eV, for neutral carbenes IeH 8,5–10,5, for typical electrophilic – in the intervals of PhH –0,3–0,5, IeH –3,4–3,4 eV. The intermediate values (Ph 0,5–1,0, IeH 3,4–8,5 eV) are characteristic for typical ambiphilic carbenes. In the evaluation of electron-donating and electron-withdrawing properties, the values of ED and EA should be taken into account (maximal EDH values were found for neutral carbene 20 (13,8 eV) and for superbasic anionic carbenes (for 22 18,4 eV)). The highest electron acceptability EAH was found for cationic carbene 29 (11,8 eV). In the reactions with carbon ions, the values of the IeH, PhH indices decrease significantly, and the electron acceptability increases. Increasing the steric effects leads to «inversion» of philicities for nucleophilic carbenes (Ph up to 0,1), and the properties of electrophilic carbenes become even more pronounced (Ph up to –2,7). The found dependences of the electronic properties of carbenes allow regulating the structure of carbenes to achieve certain characteristics, which together with stability factors can be used in the design of structures for synthesis and practical application.

The electronic properties of carbenes

For carbenes as ambiphilic compounds there is no single scale for estimating their electron properties.Aim. To consider the known methods of estimating the electron-donating and electron-withdrawing properties of carbenes, first of all, created by the authors of the article, and show their possibilities in predicting the properties of carbenes.Materials and methods. The studies were performed using the DFT (B3LYP5/6-311G/RHF) method to estimate proton affinity, DFT (B3LYP5/3-21G/RHF and B3LYP5/3-21G/UHF) to determine chemical hardness and electronic indices.Results and discussion. The electronic properties of carbenes, including thermodynamic parameters, such as proton affinity (PA), chemical hardness η, and new electronic indices Ie, are discussed in the paper. With their help, the electron-donating and electron-withdrawing ability of a wide range of carbenes of both nucleophilic and electrophilic type has been estimated. It has been shown quantitatively that the electronic properties of carbenes depend both on the backbone of the molecule (for example, the type of the heterocyclic nucleus) and on substituents. The above data show the ways of regulating the structure of carbenes to achieve certain characteristics, which together with stability factors can be used in the design of structures for the synthesis and practical application.Conclusions. The author’s results of estimating proton affinity, chemical hardness and electronic indices for the design and use of carbene compounds are considered. Electronic indices have been shown to have some advantages over others for determining the nature (electron-donating and electron-withdrawing) of carbenes.

Computational Investigations of Heme Carbenes and Heme Carbene Transfer Reactions.

Engineered heme proteins and biomimetic iron porphyrins have been found to possess excellent catalytic properties for numerous carbene transfer reactions. Computational studies, including the use of DFT calculations and molecular dynamics simulations, have been employed to help understand some important mechanistic aspects of heme carbene transfer reactions. This review summarizes advances in the computational results published in the following two areas: 1) the electronic and geometric structures of heme carbenes; spectroscopic properties; and effects of carbene substituent, porphyrin substituent, axial ligand, and spin state on heme carbene formation; and 2) mechanisms of heme carbenoid X-H (X=C, Si, B, N, S) insertions and cyclopropanation, including effects of heme carbene structural components and protein environment, as well as oxidation state and spin state. A brief outlook of future development is also addressed.