Singlet Carbene Research Articles

Carbenes as Electron-Pair Donors To CO2 for C···C Tetrel Bonds and C-C Covalent Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to identify stable complexes and molecules and the transition structures that interconvert them on the potential surfaces of ten singlet carbene bases acting as electron-pair donors to CO2. The carbene bases include cyclic C(NHCH)2 or NHC, C(NH2)2, an oxygen heterocyclic carbene C(OCH)2 or OHC, C(OH)2, C(CH3)2, cyclic C3H2, CCCH2, CCl2, CCH2, and CF2. Carbene:CO2 complexes stabilized by C···C tetrel bonds have been found on all potential surfaces, whereas carbene-CO2 molecules stabilized by C-C covalent bonds have been found on eight surfaces. Three of these molecules have open structures with C2v symmetry, whereas the remaining have cyclic three membered C-O-C rings with Cs symmetry. The transition structures which connect the complex and the molecule are bound on three of the potential surfaces. Whether the transition structure is bound or unbound relative to the carbene and CO2 depends on the relationship among C-C distances at the three stationary points on the surface. Charge-transfer interactions stabilize carbene:CO2 complexes. The primary charge transfer in complexes arises from electron donation from the carbene lone-pair to the CO2 molecule. There is also back-donation of charge from CO2 to the carbene in three complexes. Systematic changes in bonding properties occur as complexes go through transition structures and become molecules. EOM-CCSD inter- and intramolecular C-C and C-O spin-spin coupling constants have been computed and compared for complexes and molecules. A search of the CSD database found the (NH2)2C-CO2 structure and 17 NHC-CO2 derivatives. Computed bond distances and angles have been compared with experimental data.

Carbenes as Electron-Pair Donors for P⋅⋅⋅C Pnicogen Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations were performed on the P⋅⋅⋅C pnicogen-bonded complexes of the singlet carbene molecules C(NH2 )2 , C(OH)2 , and cyclic C(OCH)2 [OHC] with H2 XP molecules, with X=F, Cl, NC, OH, CH3 , CN, CCH, and H. The H2 XP:C(NH2 )2 and H2 XP:C(OH)2 complexes have Cs symmetry and two different structures: one in which the symmetry plane of the complex and the local symmetry plane of the carbene are non-coplanar, and the other in which they are coplanar. The non-coplanar H2 XP:C(NH2 )2 and H2 XP:C(OH)2 complexes arise only when X is one of the more electronegative substituents. Coplanar H2 XP:C(NH2 )2 complexes form when X is one of the more electropositive substituents, whereas coplanar H2 XP:C(OH)2 complexes exist for all X. H2 XP:C(NH2 )2 and H2 XP:C(OH)2 are stabilized by covalent P-C bonds or P⋅⋅⋅C pnicogen bonds, but co-planar H2 (CH3 )P:C(OH)2 and H3 P:C(OH)2 are stabilized by O-H⋅⋅⋅P hydrogen bonds. The H2 XP:OHC complexes have non-coplanar structures that are also stabilized by P-C covalent bonds or pnicogen bonds. The H2 (CH3 )P:OHC and H3 P:OHC complexes in which the symmetry plane of the complex and the local symmetry plane of the carbene are perpendicular are stabilized by P⋅⋅⋅π bonds with P acting as the electron-pair donor to the OHC π system. The H2 XP:C(NH2 )2 , H2 XP:C(OH)2 , and H2 XP:OHC complexes are described in terms of their binding energies, charge-transfer energies, electron density properties, and equation-of-motion coupled cluster singles and doubles spin-spin coupling constants.

Carbene–aerogen bonds: an ab initio study

ABSTRACTThrough the use of ab initio calculations, the possibility of formation of σ-hole interaction between ZO3 (Z = Ar, Kr and Xe) and carbene species is investigated. Since singlet carbenes show a negative electrostatic potential on their divalent carbon atom, they can favourably interact with the positive electrostatic potential generated by the σ-hole of Z atom of ZO3. The characteristic of this interaction, termed as ‘carbene–aerogen’ bond, is analysed in terms of geometric, interaction energies and electronic features. The energy decomposition analysis indicates that for all complexes analysed here, the electrostatic energy is more negative than the polarisation or dispersion energy term. According to the electron density analysis, some partial covalent character can be ascribed to Xe⋅⋅⋅C interactions. In addition, the carbene–aerogen bond exhibits cooperative effects with the H⋅⋅⋅O hydrogen-bonding interaction in ternary complexes where both interactions coexist. For a given carbene, the amount of these cooperative effects increases with the size of the Z atom. The results obtained in this work may be helpful for the extension and future application of σ-hole intermolecular interactions as well as coordination chemistry.

Effects of Solvation and Hydrogen Bond Formation on Singlet and Triplet Alkyl or Aryl Carbenes.

Formation of hydrogen-bonded complexes involving singlet and triplet alkyl or aryl carbenes and the impacts of solvation and hydrogen bonding upon the carbene singlet-triplet gaps have been investigated using computational methods. Single-point CCSD(T)-F12 and MRCI+Q methodologies have been employed with aug-cc-pVDZ and aug-cc-pVTZ basis sets to determine accurate singlet-triplet gaps of carbenes and hydrogen-bonded complexes involving carbenes, with geometries and vibrational frequencies obtained at the B3LYP-D3/aug-cc-pVTZ level. Using the PCM continuum solvent method and density functional theory (B3LYP/aug-cc-pVTZ), the singlet-triplet gaps of the carbenes are found to exhibit significant solvent effects; due its higher polarity, the singlet carbene is stabilized to a greater degree than the corresponding triplet carbene, impacting the singlet-triplet gap by as much as 4.4 kcal/mol. In addition, water and methanol, acting as hydrogen bond donors, form hydrogen bonds with all the singlet and triplet carbenes studied in this work. Singlet carbenes form relatively strong hydrogen bonds with binding energies in the range 3-9 kcal/mol; triplet carbenes form weaker hydrogen bonds with binding energies in the range 1-4 kcal/mol. NBO analysis demonstrates that the singlet carbene hydrogen bonds are stabilized in typical fashion, through donation of electron density from the lone pair orbital on carbon into the O-H antibonding orbital. This stabilizing interaction also is present in triplet carbene hydrogen bonds; however, a back-donation from the O-H bonding orbital into the carbon lone pair orbitals also is observed, which leads to reduced charge transfer in the triplet carbene hydrogen-bonded complexes. With the exception of methylene, hydrogen bond formation is strong enough to reverse the ordering of the singlet and triplet states for the carbenes possessing triplet ground states.

A Prediction of Proton-Catalyzed Hydrogenation of Ketones in Lewis Basic Solvent through Facile Splitting of Hydrogen Molecules.

A ketone's carbonyl carbon is electrophilic and harbors a part of the lowest unoccupied molecular orbital of the carbonyl group, resembling a Lewis acidic center; under the right circumstances it exhibits very useful chemical reactivity, although the natural electrophilicity of the ketone's carbonyl carbon is often not strong enough on its own to produce such reactivity. Quantum chemical calculations predict that a proton shared between a ketone and the Lewis basic solvent molecule (dioxane or THF) activates carbonyl carbon to the point of enabling a facile heterolytic splitting of H2 . Proton-catalyzed hydrogenation of a ketone in Lewis basic solvent is the result. The mechanism involves the interaction of H2 with the enhanced Lewis acidity of a carbonyl carbon and the free Lewis basic solvent molecule polarizes H2 and enables the hydride-type attack on carbonyl carbon, which is very strongly influenced by the proton shared between a ketone and solvent. The hydride-type attack on carbon is reminiscent of the splitting of H2 by singlet carbenes except that, in this case, a Lewis base from the surrounding environment (solvent) is necessary for polarization of H2 and acceptance of the proton resulting from the heterolytic splitting of H2 .

Transition-Metal-like Behavior of Main Group Elements: Ligand Exchange at a Phosphinidene.

(Phosphino)phosphaketenes (>P-P═C═O) behave as (phosphino)phosphinidene-carbonyl adducts (>P-P←:C═O). CO scrambling was observed using 13C labeled CO, and exchange reactions with phosphines afford the corresponding (phosphino)phosphinidene-phosphine adducts (>P-P←:PR3). The latter react with isonitriles and singlet carbenes giving (phosphino)phosphinidene-isonitrile (>P-P←:CNR) and -carbene adducts (>P-P←:C<). Based on experimental results and DFT calculations, it is shown that these "ligand" exchange reactions occur via an associative mechanism as classically observed with transition metal complexes.

An Experimental and Computational Investigation of (α-Methylbenzylidene)carbene.

Photolysis of 1-(1-phenylethylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, in C6H6 (or C6D6), at ambient temperature, produces (α-methylbenzylidene)carbene which undergoes a facile Fritsch-Buttenberg-Wiechell (FBW)-type rearrangement to 1-phenylpropyne. The alkyne results exclusively from a 1,2-phenyl shift as evident from the use of a (13)C-labeled precursor. This experimental result is consistent with CCSD(T)/cc-pVTZ//B3LYP/6-31+G* calculations which reveal that a 1,2-phenyl shift in the singlet carbene needs to overcome a barrier of only 3.8 kcal/mol whereas the 1,2-methyl shift has to surmount a much larger barrier of 11.9 kcal/mol. The alkyne remains the predominant product when the photolysis is carried out in cyclohexene but the carbene-alkene cycloadduct could be detected, albeit in low yield, in the photolysate.

Singlet (Phosphino)phosphinidenes are Electrophilic.

A room-temperature stable (phosphino)-phosphinidene reacts with carbon monoxide, stable singlet carbenes, including the poor π-accepting imidazol-2-ylidene, and phosphines giving rise to the corresponding phosphaketene, phosphinidene-carbene and phosphinidene-phosphine adducts, respectively. Whereas the electronic ground-state calculations indicate a PP multiple bond character in which the terminal phosphorus is negatively charged, the observed reactivity clearly indicates that (phosphino)phosphinidenes are electrophilic as expected for an electron-deficient species. This is further demonstrated by competition experiments as well as by the results of Fukui function calculations.

ChemInform Abstract: Silicon Based Radicals, Radical Ions, Diradicals and Diradicaloids

AbstractReview: [93 refs.

Light and Temperature Control of the Spin State of Bis(p-methoxyphenyl)carbene: A Magnetically Bistable Carbene.

Bis(p-methoxyphenyl)carbene is the first carbene that at cryogenic temperatures can be isolated in both its lowest energy singlet and triplet states. At 3 K, both states coexist indefinitely under these conditions. The carbene is investigated in argon matrices by IR, UV-vis, and X-band EPR spectroscopy and in MTHF glasses by W-band EPR and Q-band ENDOR spectroscopy. UV (365 nm) irradiation of the system results in formation of predominantly the triplet carbene, whereas visible (450 nm) light shifts the photostationary equilibrium toward the singlet state. Upon annealing at higher temperatures (>10 K), the triplet is converted to the singlet; however, cooling back to 3 K does not restore the triplet. Therefore, depending on matrix temperature and irradiation conditions, matrices containing predominantly the triplet or singlet carbene can be generated. Controlling the magnetic and chemical properties of carbenes by using light of different wavelengths might be of general interest for applications such as information storage and radical-initiated polymerization processes.

Carbene-derived α-acyl formamidinium cations: organic molecules with readily tunable multiple redox processes

A series of α-acyl formamidinium ions and their corresponding 1-electron reduced neutral radicals were synthesized, and their electrochemical properties were evaluated. These cations exhibit multi-electron redox processes that are highly electrochemically reversible at rapid scan rates (100 mV s(-1)), and the redox potentials were readily tailored by up to ∼1.0 V, making them ideal candidates for organic radical-based charge storage materials.

Intermolecular Reactions of a Foiled Carbene with Carbonyl Compounds: The Effects of Trishomocyclopropyl Stabilization.

endo-Tricyclo[3.2.1.0(2,4)]oct-8-ylidene is a foiled carbene reaction intermediate. It was generated by thermolyzing Δ(3)-1,3,4-oxadiazoline precursors dissolved in benzaldehyde and acetophenone. The products appear to stem from direct insertion of the carbene's divalent C atom into the α-bonds of the carbonyl compounds; however, this is only superficial. The strict stereochemistry observed is due to the topologies of the reaction intermediates of the proposed two-step mechanism. Bimolecular nucleophilic addition generates bent 1,3-zwitterions. The neutral reaction intermediates undergo pinacolic rearrangements to form the observed adducts. Product ratios reflect the migratory aptitudes of the carbonyl compounds' α-substituents. The carbene reaction was modeled using DFT. The singlet carbene's bicoordinate C atom bends 31° toward the endo-fused cyclopropane bond, elongating it to r = 1.69 Å. The resulting trishomocyclopropyl HOMO{-1} is a three-center two-electron bond responsible for the electron-deficient carbene's nucleophilicity. Its calculated properties are consistent with this assertion: (1) singlet-triplet (ΔE(S-T)) energy gap of -25 kcal/mol, (2) gas-phase proton affinity (PA) value of 272 kcal/mol, (3) hard and soft acid and base (HSAB) ΔN value of -0.2 in its initial reaction with the carbonyl compounds, and (4) negative frontier orbital interaction values ΔΔE(PhC(O)H) = -4.38 eV and ΔΔE(PhC(O)Me) = -3.97 eV.

Bond Energies and Thermochemical Properties of Ring-Opened Diradicals and Carbenes of exo-Tricyclo[5.2.1.0(2,6)]decane.

Exo-tricyclo[5.2.1.0(2,6)]decane (TCD) or exo-tetrahydrodicyclopentadiene is an interesting strained ring compound and the single-component high-energy density hydrocarbon fuel known as JP-10. Important initial reactions of TCD at high temperatures could cleave a strained carbon-carbon (C-C) bond in the ring system creating diradicals also constrained by the remaining ring system. This study determines the thermochemical properties of these diradicals (TCD-H2 mJ-nJ where m and n correspond to the cleaved carbons sites) including the carbon-carbon bond dissociation energy (C-C BDE) corresponding to the cleaved TCD site. Thermochemical properties including enthalpies (ΔH°f298), entropies (S(T)), heat capacities (Cp(T)), and C-H and C-C BDEs for the parent (TCD-H2 m-n), radical (TCD-H2 mJ-n and m-nJ), diradical (TCD-H2 mJ-nJ), and carbene (TCD-H2 mJJ-n and m-nJJ) species are determined. Structures, vibrational frequencies, moments of inertia, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) level of theory. Standard enthalpies of formation in the gas phase for the TCD-H2 m-n parent and radical species are determined using the B3LYP density functional theory and the higher level G3MP2B3 and CBS-QB3 composite methods. For singlet and triplet TCD diradicals and carbenes, M06-2X, ωB97X-D, and CCSD(T) methods are included in the analysis to determine ΔH°f298 values. The C-C BDEs are further calculated using CASMP2(2,2)/aug-cc-pvtz//CASSCF(2,2)/cc-pvtz and with the CASMP2 energies extrapolated to the complete basis set limit. The bond energies calculated with these methods are shown to be comparable to the other calculation methods. Isodesmic work reactions are used for enthalpy analysis of these compounds for effective cancelation of systematic errors arising from ring strain. C-C BDEs range from 77.4 to 84.6 kcal mol(-1) for TCD diradical singlet species. C-H BDEs for the parent TCD-H2 m-n carbon sites range from 93 to 101 kcal mol(-1) with a similar range seen for loss of the second hydrogen to generate the diradical singlet species. A wider range for C-C BDEs is seen for the carbenes from about 77 to 100 kcal mol(-1) as compared to the diradicals. Results from the DFT methods for the parents, radicals, diradicals, and carbenes are in good agreement with results from the composite methods using our sets of work reactions.

Photodissociation dynamics of cyclopropenylidene, c-C3 H2.

In this joint experimental and theoretical study we characterize the complete dynamical "life cycle" associated with the photoexcitation of the singlet carbene cyclopropenylidene to the lowest lying optically bright excited electronic state: from the initial creation of an excited-state wavepacket to the ultimate fragmentation of the molecule on the vibrationally hot ground electronic state. Cyclopropenylidene is prepared in this work using an improved synthetic pathway for the preparation of the precursor quadricyclane, thereby greatly simplifying the assignment of the molecular origin of the measured photofragments. The excitation process and subsequent non-adiabatic dynamics have been previously investigated employing time-resolved photoelectron spectroscopy and are now complemented with high-level ab initio trajectory simulations that elucidate the specific vibronic relaxation pathways. Lastly, the fragmentation channels accessed by the molecule following internal conversion are probed using velocity map imaging (VMI) so that the identity of the fragmentation products and their corresponding energy distributions can be definitively assigned.

ChemInform Abstract: Diazo‐ and Transition‐Metal‐Free C—H Insertion: A Direct Synthesis of β‐Lactams.

AbstractA novel and unprecedented C‐H insertion of β‐ketoamides is presented.

Theoretical Designs for Fullerene Carbenes, C60–E–C60 and C70–E–C70 (E = Group 14 Elements): A Target for Experimental Studies

A density functional study of singlet and triplet state fullerene carbenes is performed, using the M06-2X/CRENBL ECP, M06-2X/Def2-SV(P), and M06-2X/LANL2DZ levels of theory. The structures of two model carbenes (C60–E–C60 and C70–E–C70; E = C, Si, Ge, Sn, and Pb) in their closed-shell singlet and open-shell triplet states are obtained. The theoretical computations suggest that the formation of C60–E–C60 should be energetically more feasible than that of C70-E-C70 (E = C, Si, and Ge). In particular, the C70–E–C70 species with tin and lead should be relatively difficult to produce, from the viewpoint of bonding dissociation energy. The theoretical evidence also suggests that the bulky substituents that occupy the inner side of C60–E–C60 and C70–E–C70 should make the triplet ground state easily achievable. These theoretical findings show that both the relativistic effect and steric effect play an essential role in determining the electronic states of fullerene carbenes.

Absolute Reactivity of (N-Methyl-3-pyridinium)chlorocarbene

(N-Methyl-3-pyridinium)chlorocarbene tetrafluoroborate (MePyr(+)CCl BF4(-), 4) is generated by laser flash photolysis (LFP) of the corresponding diazirine (5) and reacted with tetramethylethylene, cyclohexene, 1-hexene, 2-ethyl-1-butene, methyl acrylate, and acrylonitrile. Absolute rate constants are measured for these carbene-alkene addition reactions, and activation parameters are obtained for additions of MePyr(+)CCl BF4(-) to tetramethylethylene, cyclohexene, and 1-hexene. MePyr(+)CCl BF4(-) is computed to be a highly reactive, electrophilic, singlet carbene, and experiments are in accord with expectations. Its activation parameters are compared with those of CF3CCl, CCl2, CClF, and CF2. In all cases, enthalpy-entropy compensation is observed, with ΔH(‡) and ΔS(‡) decreasing in tandem as carbenic stability decreases. A qualitative explanation is offered for this phenomenon.

Activation of Molecular Hydrogen by a Singlet Carbene through Quantum Mechanical Tunneling

AbstractCarbenes are among the few metal‐free molecules that are able to activate molecular hydrogen. Whereas triplet carbenes have been shown to insert into H2 through a two‐step mechanism that at low temperature is assisted by quantum mechanical tunneling (QMT), singlet carbenes insert in concerted reactions with considerable activation barriers, and are thus unreactive towards H2 at cryogenic temperatures. Here we show that 1‐azulenylcarbene with a singlet ground state readily inserts into H2, and that QMT governs the insertion into both H2 and D2. This is the first example that shows that QMT can also be important for singlet carbenes inserting into dihydrogen.

Activation of molecular hydrogen by a singlet carbene through quantum mechanical tunneling.

Carbenes are among the few metal-free molecules that are able to activate molecular hydrogen. Whereas triplet carbenes have been shown to insert into H2 through a two-step mechanism that at low temperature is assisted by quantum mechanical tunneling (QMT), singlet carbenes insert in concerted reactions with considerable activation barriers, and are thus unreactive towards H2 at cryogenic temperatures. Here we show that 1-azulenylcarbene with a singlet ground state readily inserts into H2 , and that QMT governs the insertion into both H2 and D2 . This is the first example that shows that QMT can also be important for singlet carbenes inserting into dihydrogen.

Tris(pyrazolyl)methanides of the alkaline earth metals: influence of the substitution pattern on stability and degradation.

Trispyrazolylmethanides commonly act as strong tridentate bases toward metal ions. This expected coordination behavior has been observed for tris(3,4,5-trimethylpyrazolyl)methane (1a), which yields the alkaline-earth-metal bis[tris(3,4,5-trimethylpyrazolyl)methanides] of magnesium (1b), calcium (1c), strontium (1d), and barium (1e) via deprotonation of 1a with dibutylmagnesium and [Ae{N(SiMe3)2}2] (Ae = Mg, Ca, Sr, and Ba, respectively). Barium complex 1e degrades during recrystallization that was attempted from aromatic hydrocarbons and ethers. In these scorpionate complexes, the metal ions are embedded in distorted octahedral coordination spheres. Contrarily, tris(3-thienylpyrazolyl)methane (2a) exhibits a strikingly different reactivity. Dibutylmagnesium is unable to deprotonate 2a, whereas [Ae{N(SiMe3)2}2] (Ae = Ca, Sr, and Ba) smoothly metalates 2a. However, the primary alkaline-earth-metal bis[tris(3-thienylpyrazolyl)methanides] of Ca (2c), Sr (2d), and Ba (2e) represent intermediates and degrade under the formation of the alkaline-earth-metal bis(3-thienylpyrazolates) of calcium (3c), strontium (3d), and barium (3e) and the elimination of tetrakis(3-thienylpyrazolyl)ethene (4). To isolate crystalline compounds, 3-thienylpyrazole has been metalated, and the corresponding derivatives [(HPz(Tp))4Mg(Pz(Tp))2] (3b), dinuclear [(tmeda)Ca(Pz(Tp))2]2 (3c), mononuclear [(pmdeta)Sr(Pz(Tp))2] (3d), and [(hmteta)Ba(Pz(Tp))2] (3e) have been structurally characterized. Regardless of the applied stoichiometry, magnesiation of thienylpyrazole 3a with dibutylmagnesium yields [(HPz(Tp))4Mg(Pz(Tp))2] (3b), which is stabilized in the solid state by intramolecular N-H···N···H-N hydrogen bridges. The degradation of [Ae{C(Pz(R))3}2] (R = Ph and Tp) has been studied by quantum chemical methods, the results of which propose an intermediate complex of the nature [{(Pz(R))2C}2Ca{Pz(R)}2]; thereafter, the singlet carbenes ([:C(Pz(R))2]) dimerize in the vicinity of the alkaline earth metal to tetrapyrazolylethene, which is liberated from the coordination sphere as a result of it being a very poor ligand for an s-block metal ion.