Carbene Character Research Articles

Deciphering the Influence of Alkylene Bridged and Chelating Mode on Pd—C and Pd—X (X = Cl, Br, and I) Bonding Interaction Within Bis‐(NHC)‐Palladium Complexes Using Quantum Chemistry Tools

ABSTRACTIn this paper, we have explored the influence of alkylene‐bridged length and coordination mode on the reactivity of a series of 24 alkylene‐bridged palladium complexes using computational tools (B3PW91/LANL2DZ//6‐31G(d) level) in gas phase and DMSO. These palladium complexes prefer a boat and chair configuration for methylene and ethylene bridge length, respectively. In addition, phenyl and nitro complexes present a higher activation for Pd—C bonds while the most stable Pd⋯C interactions are observed for abnormal mode with methylene bridge. According to the energy decomposition analysis (EDA), hydrogen and phenyl complexes in both chelation modes present a better electrostatic character. Moreover, Pd⋯C interactions are stronger compared to the Pd⋯X ones for ethylene‐bridged abnormal complexes (in gas phase). Finally, the donation/back‐donation ratio (d/b) values reveal the Fischer carbene character of these [bis(NHC)]⋯[PdX2] interactions. Concerning the hybridization around the metal cation for the Pd—C bond, the sp2d type is observed for methylene‐bridged palladium complexes while the sp3 one is observed for ethylene bridge complexes.

Carbon Suboxide, C2O3: A Hidden σ0π2 Carbene.

Electronic structure of carbon suboxide, C3O2, has been recently described as OC→C←CO having two lone pairs at the central carbon atom, thereby called as "Carbones". Although it has a linear geometry, the presence of two lone pairs comes to fore when its reactivity is analyzed with two protons. However, no attention had been paid on its alternating reactivity with hydride ions. Herein, detailed quantum chemical calculations predict that carbon suboxide can also have significant hydride ion affinity. This reactivity is in tune with σ0π2 carbene character of carbon suboxide. This study also shows that such a σ0π2 carbene character is also prevalent in carbodiphosphorane, C(PH3)2. This bonding situation has been hitherto unexplored in "Carbone" chemistry.

An Actinide Complex with a Nucleophilic Allenylidene Ligand.

The reaction of [Cp3Th(3,3-diphenylcyclopropenyl)] (Cp = η5-C5H5) with 1 equiv of lithium diisopropylamide (LDA) results in cyclopropenyl ring opening and formation of the thorium allenylidene complex, [Li(Et2O)2][Cp3Th(CCCPh2)] ([Li(Et2O)2][1]), in good yield. Additionally, deprotonation of [Cp3Th(3,3-diphenylcyclopropenyl)] with 1 equiv of LDA, in the presence of 12-crown-4 or 2.2.2-cryptand, results in the formation of discrete cation/anion pairs, [Li(12-crown-4)(THF)][Cp3Th(CCCPh2)] ([Li(12-crown-4)(THF)][1]) and [Li(2.2.2-cryptand)][Cp3Th(CCCPh2)] ([Li(2.2.2-cryptand)][1]), respectively. Interestingly, the complex [Li(Et2O)2][1] undergoes dimerization upon standing at room temperature, resulting in the formation of [Cp2Th(μ:η1:η3-CCCPh2)]2 (2), via loss of LiCp. The reaction of [Li(Et2O)2][1] with MeI results in electrophilic attack at the Cγ carbon atom, leading to the formation of a thorium acetylide complex, [Cp3Th(C≡CC(Me)Ph2)] (3), which can be isolated in 83% yield upon workup, whereas the reaction of [Li(Et2O)2][1] with benzophenone results in the formation of 1,1,4,4-tetraphenylbutatriene (4) in 99% yield, according to integration against an internal standard. Density functional theory (DFT) calculations performed on [1]- and 2 reveal significant electron delocalization across the allenylidene ligand. Additionally, calculations of the 13C NMR chemical shifts for the Cα, Cβ, and Cγ nuclei of the allenylidene ligand were in good agreement with the experimental shifts. The calculations reveal modest deshielding induced by spin-orbital effects originating at Th due to the involvement of the 5f orbitals in the Th-C bonds. According to a DFT analysis, the cyclopropenyl ring-opening reaction proceeds via [Cp3Th(η1-3,3-Ph2-cyclo-C3)]- (IM), which features a carbanion character at Cβ.

DFT and experimental characterization of 1-heteroatom-substituted alkylmagnesium chlorides

In addition to exhibiting nucleophilic reactivity, some alkylmagnesium chlorides that possess a heteroatom substituent at the 1-position can exhibit carbene-like reactivity. Herein, DFT calculations and experiments were performed to study the reactivity of several 1-heteroatom-substituted alkylmagnesium chlorides. The carbene character of 1-heteroatom-substituted 2-methylpropylmagneisum chlorides was evaluated using the geometric parameters in their DFT-optimized structures and the activation energies for the 1,2-hydrogen shift estimated by DFT calculations. 1-Heteroatom-substituted alkylmagnesium chlorides were generated from 1-heteroatom-substituted alkyl p-tolyl sulfoxides and isopropylmagnesium chloride via a sulfoxide/magnesium exchange reaction, and the reactivity of these species was evaluated based on the product ratio. The results revealed that 1-amino- and 1-thioalkylmagnesium chlorides were carbanions because the reaction yielded protonated products. In contrast, 1-haloalkylmagnesium chlorides were magnesium carbenoids because the reaction yielded alkene and cyclopropane without haloalkanes. Lastly, 1-pivaloxyalkylmagnesium chloride was a hybrid between carbanions and magnesium carbenoids. The order of carbene character estimated based on the theoretical results was consistent with the experimental results.

Modeling Electrochemical Vacancy Regeneration in Single-Walled Carbon Nanotubes.

Synthesis-induced defects in single-walled carbon nanotubes (SWCNTs) enable diverse catalytic reactions, but the nature of catalytic intermediates and how active species regeneration occurs are unclear. Using a quantum mechanics/molecular mechanics (QM/MM) hybrid methodology based on density functional theory (DFT) and a classical force-field, we explore the reactivity and electrochemical regeneration of a vacancy defect in a zigzag SWCNT. Our findings indicate that hydrolysis of the defect forms a ketone group on one carbon atom and C-H bonds on two adjacent carbons. Applying an electrochemical potential of ESHE = -0.740 V triggers a proton-coupled electron transfer (PCET), converting the ketone to a hydroxyl group. Further reduction at ESHE = -1.08 V induces another PCET, expelling the hydroxyl as water and forming an active carbon with carbene character that can react with hydrogen peroxide and perchlorate. The hydrogen atoms on neighboring carbons prevent further water dissociation, maintaining the catalytic vacancy.

Rare-earth metal complexes bearing electrophilic and nucleophilic carbon centres and their unique reactivity patterns towards pyridine derivatives.

The rare-earth metal dialkyl complexes (κ2-L1)RE(CH2SiMe3)2·(THF)2 [RE = Lu(1a), Yb(1b), Er(1c), Y(1d), Dy(1e)] (L1 = 1-(2-N-C5H10NCH2CH2)-3-(2,6-iPr2C6H3N[double bond, length as m-dash]CH)-C8H4N) and the rare-earth metal monoalkyl complexes (κ2-L1)2RE(CH2SiMe3)·(THF) n [n = 0, RE = Lu(2a), Yb(2b); n = 1, Er(2c), Y(2d), Dy(2e)], (κ2-L2)2RE(CH2SiMe3)·THF [RE = Yb(3a), Er(3b), Y(3c), Dy(3d), Gd(3e)] (L2 = 1-(2-N-C5H10NCH2CH2)-3-(AdN[double bond, length as m-dash]CH)-C8H4N) (Ad = adamantyl, C10H15) have been synthesized and fully characterized. These complexes feature chelate ligands having a conjugated system (-C[double bond, length as m-dash]C-C[double bond, length as m-dash]N) with an sp2 carbon, which enables both electrophilic and nucleophilic carbon centres to be directly connected to the highly electrophilic rare-earth metal ions. The reactions of these complexes with different pyridine derivatives have been systematically investigated with the discovery of reactivity patterns distinct from those of previously reported transition metal complexes. These unusual reactivity patterns include consecutive C-H activation/1,1-migratory insertion/C-N and C-H activation, C-H activation/1,1-migratory insertion/C-N bond activation, C-H activation/1,1-migratory insertion, and homolytic redox reactions. DFT calculation results together with experimental evidences support the key mechanistic proposal that the indol-2-yl carbon of the ligands exhibits electrophilic carbene character accounting for the subsequent 1,1-migratory insertion reaction after C-H activation.

On the Carbenic Nature of Nitrile Ylides: Experimental and Computational Characterization of Hydroxy and Amino Nitrile Ylides

AbstractNitrile ylides are 1,3‐dipolar species with structures that are commonly described by propargylic and/or allenic resonance forms. Nevertheless, certain nitrile ylides exhibit a tendency to form dimers, suggesting the existence of structures characterized by an important carbenic contribution. To address the nitrile ylide carbenic nature, here we investigate two derivatives with OH and NH2 electron‐donating substituents. These nitrile ylides were generated in cryogenic matrices by UV‐irradiation of 3‐hydroxy‐ and 3‐amino‐isoxazole precursors and were found to photoisomerize to the corresponding oxazoles. The IR spectral characterization of the photogenerated nitrile ylides reveals the absence of the characteristic strong νas(CNC) absorption, which is associated with allenic and propargylic type structures. Computed geometries for the two experimentally investigated nitrile ylides and for analogues with different substituents at C3 and C1, together with NBO and NRT electronic structure analyses, performed at the CCSD(T) level of theory, reveal substantial carbenic character exclusively for nitrile ylides bearing OH and NH2 substituents at C3. As a whole, the results shed light on the factors determining the structural features of nitrile ylides, which play a major role in defining the reactivity of these important elusive compounds.

Charge-Separated Reactive Intermediates from the UV Photodissociation of Chlorobenzene in Solution.

Although ultraviolet (UV)-induced photochemical cleavage of carbon–halogen bonds in gaseous halocarbons is mostly homolytic, the photolysis of chlorobenzene in solution has been proposed to produce a phenyl cation, c-C6H5+, which is a highly reactive intermediate of potential use in chemical synthesis and N2 activation. Any evidence for such a route to phenyl cations is indirect, with uncertainty remaining about the possible mechanism. Here, ultrafast transient absorption spectroscopy of UV-excited (λ = 240 and 270 nm) chlorobenzene solutions in fluorinated (perfluorohexane) and protic (ethanol and 2,2,2-trifluoroethanol) solvents reveals a broad electronic absorption band centered at 540 nm that is assigned to an isomer of chlorobenzene with both charge-separated and triplet-spin carbene character. This spectroscopic feature is weaker, or absent, when experiments are conducted in cyclohexane. The intermediate isomer of chlorobenzene has a solvent-dependent lifetime of 30–110 ps, determined by reaction with the solvent or quenching to a lower-lying singlet state. Evidence is presented for dissociation to ortho-benzyne, but the intermediate could also be a precursor to phenyl cation formation.

Crystalline monometal-substituted free carbenes

<h2>Summary</h2> The rich chemistry of transition-metal (TM) carbyne complexes has fueled their application in a wide range of fields, from synthetic chemistry to materials science. The isomeric forms of these complexes are monometal-substituted carbenes and have so far been detected only in the gas phase. In this article, we describe the synthesis of crystalline monometal-substituted free carbenes. To isolate these molecules, we selected a bulky π-donor substituent and a closed-shell electron-rich TM center as the substituents around the carbene center. This combination allows these metallocarbenes to exhibit a nucleophilic carbenic character with a high-lying highest occupied molecular orbital (HOMO) arising from the metal substituent. Additionally, we report a formal rearrangement of the metallocarbene to a zwitterionic species containing a carbyne anion, which makes these free compounds not only synthetic astonishments but also attractive for the development of novel subvalent carbon-transfer reactions.

Carbene Character in a Series of Neutral PCcarbeneP Cobalt(I) Complexes: Radical Carbenes versus Nucleophilic Carbenes

Carbene Character in a Series of Neutral PC_carbeneP Cobalt(I) Complexes: Radical Carbenes versus Nucleophilic Carbenes

Synthesis of MAuAg (M = Ni, Pd, or Pt) and NiAuCu Heterotrimetallic Complexes Ligated by a Tritopic Carbanionic N-Heterocyclic Carbene.

Heterotrimetallic complexes (NiAuAg, PdAuAg, PtAuAg, and NiAuCu) containing a tritopic N-heterocyclic carbene (NHC) have been synthesized for the first time through the deprotonation and metalation of heterodimetallic complexes and were structurally characterized by single-crystal X-ray diffraction. The carbene character of the donor groups in the tritopic NHC complexes was established on the basis of structural and NMR analyses.

Lithium Dicyclohexylamide in Transition-Metal-Free Fischer-Tropsch Chemistry.

Lithium dicyclohexylamide (Cy2NLi) reacts with syn-gas or CO to generate transient intermediates with carbene character, which are capable of reacting further with CO or H2, effecting sequential C-C and C-H bond formations from CO or H2, thus providing a transition-metal-free avenue to the fundamental reactions of the Fischer-Tropsch process. Further experimental and computational data indicate that reactions with CO and H2 are thermodynamically accessible, with a kinetic bias toward CO homologation.

Isolable dicarbon stabilized by a single phosphine ligand.

In contrast to naturally occurring F2, O2 and N2, diatomic C2 is an intriguing species that has only been observed indirectly in the gas phase, and because of its high reactivity has eluded isolation in the condensed phase. It has previously been stabilized in L→C2←L compounds but the bonding situation of the central C2 in this motif differs remarkably from that of free C2. Here we have prepared and structurally characterized diatomic C2 as a monoligated complex L→C2 using a bulky phosphine ligand bearing two imidazolidin-2-iminato groups (L is (NHCR=N)2(CH3)P, where NHCR is an N-heterocyclic carbene). The compound is stable in solution at ambient temperature and has also been isolated in the solid state. Reactivity studies, in combination with quantum chemical analysis, suggest that the two carbon atoms of the L→C2 complex both have carbene character. The complex underwent intermolecular C-H bond activation upon thermolysis and exhibited hydroalkoxylation-like reactivity with methanol.

The Influence of C(sp3)H–Selenium Interactions on the 77Se NMR Quantification of the π‐Accepting Properties of Carbenes

AbstractSelenium NMR has become a standard tool for scaling the π‐accepting character of carbenes. Herein, we highlight that non‐classical hydrogen bonding (NCHB), likely resulting from hyperconjugation, can play a significant role in the carbene–selenium 77Se NMR chemical shift, thus triggering a non‐linear behavior of the Se‐Scale.

The Influence of C(sp3 )H-Selenium Interactions on the 77 Se NMR Quantification of the π-Accepting Properties of Carbenes.

Selenium NMR has become a standard tool for scaling the π-accepting character of carbenes. Herein, we highlight that non-classical hydrogen bonding (NCHB), likely resulting from hyperconjugation, can play a significant role in the carbene-selenium 77 Se NMR chemical shift, thus triggering a non-linear behavior of the Se-Scale.

Multireference Ground and Excited State Electronic Structures of Free- versus Iron Porphyrin-Carbenes.

Iron porphyrin carbenes (IPCs) are important reaction intermediates in engineered carbene transferase enzymes and homogeneous catalysis. However, discrepancies between theory and experiment complicate the understanding of IPC electronic structure. In the literature, this has been framed as whether the ground state is an open- vs closed-shell singlet (OSS vs CSS). Here we investigate the structurally dependent ground and excited spin-state energetics of a free carbene and its IPC analogs with variable trans axial ligands. In particular, for IPCs, multireference ab initio wave function methods are more consistent with experiment and predict a mixed singlet ground state that is dominated by the CSS (Fe(II) ← {:C(X)Y}0) configuration (i.e., electrophilic carbene) but that also has a small, non-negligible contribution from an Fe(III)-{C(X)Y}-• configuration (hole in d(xz), i.e., radical carbene). In the multireference approach, the "OSS-like" excited states are metal-to-ligand charge transfer (MLCT) in nature and are energetically well above the CSS-dominated ground state. The first, lowest energy of these "OSS-like" excited states is predicted to be heavily weighted toward the Fe(III)-{C(X)Y}-• (hole in d(yz)) configuration. As expected from exchange considerations, this state falls energetically above a triplet of the same configuration. Furthermore, potential energy surfaces (PESs) along the IPC Fe-C(carbene) bond elongation exhibit increasingly strong mixings between CSS/OSS characters, with the Fe(III)-{C(X)Y}-• configuration (hole in d(xz)) growing in weight in the ground state during bond elongation. The relative degree of electrophilic/radical carbene character along this structurally relevant PES can potentially play a role in reactivity and selectivity patterns in catalysis. Future studies on IPC reaction coordinates should evaluate contributions from ground and excited state multireference character.

Sesquicarbene Complexes: Bonding at the Interface Between M–C Single Bonds and M═C Double Bonds

Allylic dimetalated complexes [M2C(vinyl)]+ (M = Au(IPr) and Cr(CO)5–) incorporate a new coordination mode of carbon. Digold complexes of this type have recently been detected experimentally. The intrinsic bond orbitals, partial charges, and structural parameters of the gold complexes and of chromium analogs were studied computationally and compared to those of the respective monometalated species and hydrocarbons. This showed that such digold complexes have a carbene character at both Au–C bonds comparable to typical carbene complexes of gold. Dichromium complexes with their stronger π-backdonation compete for interaction with carbon’s π-orbital; each of the chromium atoms partakes in double bonding that is significant but weaker than that in the carbene analogs. Containing two M–C bonds on the interface between single and double bonds, these bridged complexes can be conceived as “sesquicarbene complexes”. The π-system acted in a very adaptive manner and employed additional stabilization of the vinyl system only where needed. Significant carbene character is found simultaneously in both M–C bonds at the same carbon center. The discovery of these complexes with relatively strong double bond character between one carbon and two metal atoms could bring unusual single-carbon-centered organometallic cascade reactions to the horizon.

Synthesis, Structure, and DFT Analysis of the THF Solvate of 2‐Picolyllithium: A 2‐Picolyllithium Solvate with Significant Carbanionic Character

Previous studies of different solvates of 2‐methylpyridyllithium (2‐picolyllithium) have uncovered electronic structures corresponding to aza‐allyl and enamido resonance forms of the metallated pyridine‐based compounds. Here, we report the synthesis and characterization of [2‐CH2Li(THF)2C5H4N], a new THF solvate. X‐ray crystallographic studies reveal a dimeric arrangement featuring a non‐planar eight‐membered [NCCLi]2ring, in which the primary cation‐anion interaction is between the central Li atom and the C atom of the deprotonated methyl group [length, 2.285(2) Å], suggesting a new carbanionic resonance structure for this 2‐picolyllithium series. The significant carbanionic character of [2‐CH2Li(THF)2C5H4N] was confirmed by gas‐phase DFT calculations [B3LYP/6‐311+G(d)] with the calculated electron density interrogated by means of quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. For comparison these computational analyses were also performed on the literature structures of [2‐CH2Li(2‐Picoline)C5H4N] and [2‐CH2Li(PMDETA)C5H4N]. In a reactivity study, [2‐CH2Li(THF)2C5H4N] was found to undergo nucleophilic addition to pyridine to generate dipyridylmethane in a good yield.

Cycloaddition of Dialkylalumanyl Anion toward Unsaturated Hydrocarbons in (1+2) and (1+4) Modes.

The reactivity of dialkylalumanyl anion (1) towards naphthalene, anthracene, diphenylacetylene, and (E)/(Z)-stilbenes was investigated. The compound 1 reacts with naphthalene and anthracene through (1+4) cyclization, giving Al-containing norbornadiene derivatives. In the reaction of 1 with diphenylacetylene and (E)/(Z)-stilbenes, (1+2) cyclization proceeded to form Al-C-C three-membered rings. Cyclization toward (E)- or (Z)-stilbenes solely gave a trans-cycloadduct. DFT calculations revealed that the cycloaddition of 1 with (Z)-stilbene proceeds via a single transition state with a carbanion character, which results in the selectivity towards the trans-cycloadduct.

Trapping of a Highly Bent and Reduced Form of 2-Phosphaethynolate in a Mixed-Valence Diuranium-Triamidoamine Complex.

The chemistry of 2-phosphaethynolate is burgeoning, but there remains much to learn about this ligand, for example its reduction chemistry is scarce as this promotes P-C-O fragmentations or couplings. Here, we report that reduction of [U(TrenTIPS )(OCP)] (TrenTIPS =N(CH2 CH2 NSiPri 3 )3 ) with KC8 /2,2,2-cryptand gives [{U(TrenTIPS )}2 {μ-η2 (OP):η2 (CP)-OCP}][K(2,2,2-cryptand)]. The coordination mode of this trapped 2-phosphaethynolate is unique, and derives from an unprecedented highly reduced and highly bent form of this ligand with the most acute P-C-O angle in any complex to date (P-C-O ∡ ≈127°). The characterisation data support a mixed-valence diuranium(III/IV) formulation, where backbonding from uranium gives a highly reduced form of the P-C-O unit that is perhaps best described as a uranium-stabilised OCP2-. radical dianion. Quantum chemical calculations reveal that this gives unprecedented carbene character to the P-C-O unit, which engages in a weak donor-acceptor interaction with one of the uranium ions.