Triplet Carbene Research Articles

Ultrafast UV−Visible and Infrared Spectroscopic Observation of a Singlet Vinylcarbene and the Intramolecular Cyclopropenation Reaction

Ultrafast UV-vis/IR spectroscopies were used to study the photochemistry of a vinyl diazo ester PhCH=CHCN2CO2CH3 (1) in solution. The results indicate that singlet styrylcarbomethoxy carbene ((1)2) is produced from the excited state of diazo precursor (1*). It is concluded that vinyl singlet carbene ((1)2) undergoes an intramolecular cyclopropenation reaction to produce the cyclopropene product (3), and undergoes intersystem crossing to ground triplet carbene ((3)2). The predictions of DFT calculations are consistent with the observations.

Ultrafast time‐resolved studies of the photochemistry of diazo carbonyl compounds

AbstractThe rich photochemistry of diazo carbonyl compounds in solution has been extensively studied by ultrafast transient absorption spectroscopy with UV–Vis and mid‐IR detection. Both, stepwise (carbene mediated) and concerted Wolff rearrangement (WR) mechanisms have been confirmed for cyclic and acyclic diazo carbonyl precursors. Cyclic diazo‐carbonyl compounds are structurally locked in a syn conformation and mainly follow the concerted mechanism, however, a stepwise pathway does slightly contribute to the overall WR process, contrary to Kaplan's rule. With acyclic diazo ester carbonyl compounds, the WR process is dominated by the stepwise mechanism. A carbene–carbene interconversion process and singlet to triplet carbene intersystem crossing (ISC) processes are discussed in this review. The growth of secondary species (cations, ylides, OH insertion product) emerging from an interaction between a singlet carbene and solvent molecules has been characterized by picosecond time‐resolved UV–Vis transient absorption spectroscopy. Carbene reactivity results in shortening of the singlet carbene lifetime relative to more inert solvents. The quantum yield of diazo decomposition can be deduced from partial ground state bleach recovery using ultrafast time‐resolved IR spectroscopy. Copyright © 2009 John Wiley & Sons, Ltd.

Racemizations of diazacycloheptatetraenes through singlet diazacycloheptatrienylidenes at theoretical levels

The topology of nitrogen atoms is found to play an important role in the racemization of non-planar diazacycloheptatetraenes through their corresponding singlet diazacycloheptatrienylidene transition states, at B3LYP/6-311++G∗∗, MP2/6-31G∗, MP4(SDTQ)/6-31G∗//B3LYP/6-311++G∗∗, CCSD(T)/6-31G∗//B3LYP/6-311++G∗∗, and QCISD(T)/6-31G∗//B3LYP/6-311++G∗∗ levels of theory. Two types of singlet carbenes are encountered: σ 2 and p 2. The σ 2 (- 1A) carbenes are aromatic and include 2,3-; 2,4-; 2,5-, and 3,4-diazacycloheptatrienylidenes, while p 2 (- 1A′) carbenes turn out antiaromatic and include 1,6-; 1,2-; 1,3-; 1,4-; and 1,5-diazacycloheptatrienylidenes. All singlet carbenes showed a negative force constant and except for the 1,6-isomer the rest appear of higher energy than their corresponding triplet states. Nevertheless, all allenes emerged more stable than their corresponding singlet and triplet carbenes. Due to higher energy barriers, racemization is unlikely for allenic structures including 2,3-; 2,4-; 2,5-; and 3,4-diazacyclohepta-1,2,4,6-tetraenes. In contrast, carbene-like reactivity is expected from ketenimine and carbodiimide (1,2-; 1,3-; 1,4-; 1,5-; 1,6-diazacyclohepta-1,2,4,6-tetraenes) which go through N-heterocyclic transition states that are mostly of rather lower energy.

Spectroscopy and Photochemistry of Triplet Methylpentadiynylidene (Me−C≡C−C̈−C≡C−H)

Triplet carbene methylpentadiynylidene, MeC(5)H (1), was investigated in cryogenic matrices by IR, UV/vis, and EPR spectroscopy. Broadband irradiation (lambda > 497 nm) of the isomeric diazo compounds, 1-diazo-hexa-2,4-diyne (2) or 2-diazo-hexa-3,5-diyne (3), generates triplet carbene 1. EPR spectra yield zero-field splitting parameters (|D/hc| = 0.62 cm(-1), |E/hc| < 0.0006 cm(-1)), which are typical for a triplet carbene with axial symmetry. The electronic spectrum of triplet 1 is characterized by a weak absorption in the near-UV and visible region (350-430 nm) with vibronic progressions corresponding to excitations of the acetylenic stretching and the terminal C[triple bond]C-H bending modes. Chemical trapping of triplet 1 in an O(2)-doped matrix affords carbonyl oxides derived predominantly from attack at C-3. Upon irradiation at lambda > 399 nm, triplet 1 undergoes photochemical 1,2-hydrogen migration to form hex-1-ene-3,5-diyne (6).

Observation and Characterization of the CH3S(O)CH− and CH3S(O)CH−·H2O Carbene Anions by Photoelectron Imaging and Photofragment Spectroscopy

We report the observation of the CH(3)S(O)CH(-) and CH(3)S(O)CH(-) x H(2)O carbene anions formed upon overall H(2)(+) abstraction from dimethyl sulfoxide by O(-). Photoelectron spectroscopy reveals singlet and triplet carbenes for the remaining neutral, with the singlet state assigned as the ground state. Although some formation of the distonic CH(2)S(O)CH(2)(-) radical anion is also expected, no conclusive evidence of the presence of this isomer is found. The photoelectron spectrum of HCSO(-) is also reported for the first time. Photofragmentation of CH(3)S(O)CH(-) with 532 nm light reveals two main types of anionic products: a dominant HCSO(-) fragment, resulting from methyl elimination, and a less intense SO(-) product. For the monohydrated anion, an additional SO(-) x H(2)O fragment is observed. Intriguingly, both the SO(-) x H(2)O and SO(-) products are produced with much higher yields in the fragmentation of CH(3)S(O)CH(-) x H(2)O, compared to the SO(-) yield from the dissociation of the bare CH(3)S(O)CH(-) anion. Two possible pathways are proposed as likely mechanisms for the SO(-)-based photoproducts, both involving a photoinduced intramolecular rearrangement and the formation of a C-C bond.

Photoarylation/Alkylation of Bromonaphthols

The photochemistry of 6-bromo-2-naphthols has been studied in acetonitrile, aqueous acetonitrile, and isopropyl alcohol in the absence and in the presence of triethylamine by product distribution analysis, laser flash photolysis (LFP), fluorescence, phosphorescence, electrochemical measurements, and DFT calculations. Hydrobromic acid loss in the presence of Et(3)N occurs from the triplet state of 6-bromo-2-naphthol, generating an electrophilic carbene intermediate, which has been successfully trapped by oxygen, allyltrimethylsilane, 2,3-dimethylbut-2-ene, pyrrole, acrylonitrile, 1,4-dimethoxybenzene, and also pyridine. The generation and the reactivity of a triplet carbene intermediate has been supported by LFP, with the detection of 2,6-naphthoquinone-O-oxide (530 < lambda < 650 nm) in the presence of O(2). The electrophilic diradical character of the carbene has been supported by DFT calculations, using the B3LYP, PBE0, and MPWB1K functionals, with the 6-31+G(d,p) basis set and PCM solvation model.

Photochemical generation, intramolecular reactions, and spectroscopic detection of oxonium ylide and carbene intermediates in a crystalline ortho-(1,3-dioxolan-2-yl)-diaryldiazomethane

Photochemical excitation of a diaryldiazomethane with an ortho-acetal both in solution and in crystals led to products that originate from the expected diarylcarbene and form an intramolecular oxonium ylide. While crystals strongly favored the formation of a benzocyclobutane by intramolecular hydrogen abstraction in the triplet carbene, reactions in benzene led exclusively to products that derive from the oxonium ylide. Studies in methyl cyclohexane glasses and in mixed crystals at 77 K led to the spectroscopic detection of triplet carbene (3)C, which partially transformed into a new species that we assign as the sought-after oxonium ylide Y. The formation of a formally ionic intermediate suggests that the scope of reactions by reactive intermediates in crystalline solids may be broader than it is generally assumed.

A ligand knowledge base for carbenes (LKB-C): maps of ligand space

We describe the development of a ligand knowledge base designed to capture the properties of C-donor ligands coordinating to transition metal centres, LKB-C. This knowledge base has been developed to describe both singlet (Arduengo and Fischer) and triplet (Schrock) carbenes, as well as related neutral monodentate C-donor ligands. The descriptors evaluated and used have been derived from a range of coordination environments to maximise their transferability and hence utility for the investigation of such ligands. These descriptors have been analysed with different statistical approaches, both individually to determine their chemical context, and collectively by principal component analysis thereby allowing the derivation of maps of ligand space for different ligand sets. The utility of such maps for investigating ligand similarity and identification of target areas for future ligand designs has been discussed. In addition, linear regression models have been fitted for the prediction of a calculated response variable, highlighting further potential applications of such a knowledge base.

Kinetic Topochemical Effect on Methanol Insertion into Phenyl-α-ketocarbene

The matrix isolation of methanol insertion has been performed into phenyl-α-ketocarbene (PKC) along with temperature decrease from the room temperature to the cryogenic temperature and the critical point of structure-spin inversion has been obtained by the kinetic results. The matrix isolation spectroscopic technique has been employed and the rate constants have been obtained by the laser flash photolysis and FT-IR and UV/ vis spectroscopic measurements. The results show that the singlet carbene reacts through a proton abstraction in the high temperature range, 254-312 K, whereas the non-Arrhenius behavior and abnormal temperature effect on the magnitude of the Hammett ρ indicates that the triplet carbene reaction occurs in the low temperature range, 77-154 K. The spin inversion was found to occur at about 254 K. A strong relationship exists between the spin state of the carbene and the geometry and reactivity of the carbene. It was found that the colder matrices of hexafluorobenzene and methanol are more rigid and permits a smaller range of motion for the PKC, and the reaction is controlled by topochemical inversion.

Influence of Solvent on Carbene Intersystem Crossing Rates

The influence of coordinating solvents on singlet-to-triplet carbene intersystem crossing (ISC) rates has been studied with diphenylcarbene (DPC) and para-biphenyltrifluoromethylcarbene (BpCCF 3) by using ultrafast time-resolved spectroscopy. DPC has a triplet ground state in all of the solvents considered, and the concentration of singlet carbene at equilibrium is too small to be measured. It is found that the lifetime of (1)DPC is extended in acetonitrile, benzene, tetrahydrofuran, dichloromethane, and halobenzene solvents relative to cyclohexane. The solvent effect does not well correlate with bulk measures of solvent polarity. The singlet-triplet energy separation of BpCCF 3 is close to zero. The data demonstrates that BpCCF 3 has a triplet ground state in benzene, fluorobenzene, and hexafluorobenzene. Halogenated solvents are found to dramatically retard the rate of ISC in (1)BpCCF 3. We postulate that the empty p orbital of a singlet carbene coordinates with a nonbonding pair of electrons of a halogen atom of the solvent to form a pseudoylide solvent complex, stabilize the singlet carbene, and decrease the singlet-triplet (S-T) energy gap. The "golden rule" of radiationless transitions posits that the smaller the energy gap between the two states, the faster their rate of interconversion. To explain the apparent violation of the golden rule of radiationless transitions for the carbene ISC processes monitored in this study, we propose that the significantly different specific solvation of the singlet and triplet carbenes imposes a Franck-Condon-like factor on the ISC process. Those solvents that most solvate the singlet carbene will also have the greatest structural difference between singlet carbene-solvent complex and their triplet spin isomer-solvent complex, the smallest S-T gap, and the slowest ISC rate. Alternatively, one can propose that a highly solvated singlet carbene must desolvate prior to ISC, and that this requirement decelerates the radiationless transition.

On the mechanism of air nitrogen fixation on activated carbon surface in water

High-performance liquid chromatography and gas chromatography were used to reveal formation of hydroxyl radicals in air-saturated water containing activated carbon. Probable mechanisms of OH· formation are considered. The role of OH· radicals in the formation of carbene centers on the activated carbon surface is discussed. For interpreting the experimental data on atmospheric nitrogen fixation on activated carbon surface, probable mechanisms of nitrogen activation are explored by quantum-chemical methods. Highly reactive OH· radicals are unable to react with molecular nitrogen under mild conditions. A high reactivity of singlet and triplet carbenes of different nature on activated carbon surface with respect to N2 molecules was revealed by density functional theory methods.

S,C-Sulfonium Ylides from Thiophenes: Potential Carbene Precursors

Photolysis of S,C-sulfonium ylides derived from thioanisol, thiophene, benzothiophene, or dibenzothiophene provides products deriving from dicarbomethoxycarbene. In methanol, no rearrangement of the carbene to the ketene derivative is observed. Formation of dibenzothiophene and benzothiophene is quantitative. For the thiophene-based ylide, insertion of the carbene into the alpha-CH bond of thiophene is observed. Evidence is presented that supports formation of both singlet and triplet carbene.

Electronic Stabilization of Ground State Triplet Carbenes

Based on systematic ab initio (CCSD(T)/cc-pVDZ) studies of substituent effects, we present a concept for the construction of electronically stabilized triplet ground state carbenes with singlet-triplet energy separations (DeltaEST) exceeding that of methylene. Sterically demanding and conjugating substituents were excluded from the selection of model compounds under investigation, as these either destabilize both the singlet and the triplet states or delocalize unpaired spins away from the carbene carbon. Negative partial charges on the carbene center allow for the prediction of the electronic stabilization of substituted carbenes. To decrease carbene reactivity, we chose beta-substituents with strong polar bonds. Among them, highly electronegative elements such as fluorine and oxygen enlarge the DeltaEST value with respect to hydrogen, while chlorine does not due to p-orbital participation.

Divalency switch from carbenes to germylenes at theoretical levels

Acyclic germylenes with X C C Ge X formula, and singlet ( X s ) and triplet ( X t ) states are generated at ab initio and DFT levels, through free optimization of their corresponding cyclic singlet and triplet carbenes: 2,3-di(X)-2-germacyclopropenylidenes, 1 s and 1 t , respectively (X = F, Cl, and Br). The calculated frontier molecular orbitals, bond orders, NBO charges, and dipole moments are consistent with the X C C Ge X structure. Their relative stabilities and the singlet (s)–triplet (t) energy splitting (Δ E s–t) indicate singlet states ( X s ) as the global minima for all the acyclic germylenes scrutinized.

Ring flips of allenes (C 9H 7X) over triplet carbenes at ab initio and DFT levels (X = H, F, Cl, Br)

Ring flips of cyclic C 9H 7X allenes over triplet carbenes are compared and contrasted for X = H, F, Cl and Br. Three series of conjugated non-planar allenes including 4-, 5-, and 6-X-1,2,4,6,8-cyclononapentaenes, turn into their mirror images; through their corresponding triplet planar conjugated carbenic transition states: 3-, 4-, and 5-X-2,4,6,8-cyclononatetraenylidenes, at ab initio and DFT levels of theory. The halogens are deliberately chosen not to be directly attached to the allenic motif. For all three series, the relative ease of racemizations is proportional to the electro-negativity of the employed halogen: F > Cl > Br. All the optimized transition states appear planar, electrophilic, and achiral.

From Persistent Triplet Carbenes to Persistent High‐Spin Polycarbenes.

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Carbene Stabilization by Aryl Substituents. Is Bigger Better?

The geometries and relative stabilities of the singlet and triplet states of phenyl- (Cs), diphenyl- (C2), 1-naphthyl- (Cs), di(1-naphthyl)- (C2), and 9-anthryl-substituted (Cs) carbenes were investigated at the B3LYP/6-311+G(d,p) + ZPVE level of density functional theory. The singlet-triplet energy separations (DeltaEST), 2.7, 2.9, 3.4, 3.7, and 5.7 kcal/mol, respectively, after including an empirical correction (2.8 kcal/mol) based on the error in the computed singlet-triplet gap for methylene versus experiment, are in good agreement with available experimental values. Consistent with literature reports, triplet di(9-anthryl)carbene has a linear, D2d symmetrical, allene structure with 1.336 A C=C bond lengths and considerable biradical character. B3LYP favors such cumulene biradical structures and triplet spin states and predicts a large (>15 kcal/mol) "di(9-anthryl)carbene" singlet-triplet (biradical) energy gap. The resonance stabilization of both singlet and triplet carbenes increases modestly with the size of the arene substituent and overall, (di)arylcarbenes, both singlet and triplet, are better stabilized by bigger substituents. For example, methylene is stabilized more by a naphthyl than a phenyl group (singlets, 26.6 versus 24.4; and triplets, 20.9 versus 18.1 kcal/mol, respectively). The carbene geometries are affected by both steric effects and arene-carbene orbital interactions (sigma-p and p-pi). For instance, the central angles at the carbene are widened by a second arene group, which leads to increased s-character and shorter carbene bond lengths (i.e., C-C, C-H). In general, the aromaticity of the substituted rings in triplet carbenes is most affected by the presence of the unpaired electrons.

Reaction Pathways for the Additions of Triplet Carbene, Silylene, Germylene and Nitrene onto the Diamond (100) Surface: A Theoretical Investigation

The additions of triplet carbene (CH2), silylene (SiH2), germylene (GeH2), and nitrene (NH) onto the diamond (100) surface have been investigated by means of density functional theory in conjunction with effective cluster models. The calculated reaction energies and reaction pathways have demonstrated that the reaction profiles for the additions of triplet species onto the diamond (100) surface is clearly different from those for singlet ones. Namely, triplet CH2, SiH2, GeH2, and NH are inclined to form the one-end-sided species by attaching to only one carbon of the surface dimer of diamond (100). Hence, the other carbon of the surface dimer will be open, which can be potentially used for anchoring other functional groups. This is in distinct contrast to the profile for the additions of singlet ones onto diamond (100) in which the as-formed surface species feature a three-membered ring structure. The results illustrate that the addition profiles of CH2s, SiH2s, GeH2s, and NHs onto the diamond (100) surface could be appropriately tuned depending on their spin states.

From Persistent Triplet Carbenes to Persistent High-Spin Polycarbenes

Abstract Triplet carbenes are regarded as one of the most effective spin sources for organic ferromagnetic materials since the magnitude of the exchange coupling between the neighboring centers is large. However, those systems have two disadvantages that prevent their use as practical magnetic materials. First, a triplet carbene unit is highly unstable and lacks the stability for practical applications under ambient conditions. Second, diazo groups, which are precursors for triplet carbenes, are generally labile and, hence, cannot be used as building blocks to prepare a more complicated poly(diazo) compound. After great efforts to stabilize the triplet carbenes, we have succeeded in preparing fairly stable triplet carbenes that survive for days. We found that a diazo precursor for a persistent triplet carbene was also persistent and, hence, can be further modified into a more complicated diazo compound that can generate persistent high-spin polycarbenes. In this account, we would like to summarize our efforts along this line. We employed the following three approaches to prepare poly(diazo) compounds; (1) synthesis of dendritic molecules having peripheral diazo groups starting from diazo compounds, (2) preparation of a poly(phenylacetylene) with a diazo unit using Rh-complex-catalyzed polymerization of diazo compounds with p-ethynyl substituents, and (3) preparation of a polymer chain as a result of complexation of the pyridine ligand on diazo compounds with coordinatively unsaturated metal ions. We also characterized the magnetic properties of photoproducts obtained by photolysis of those poly(diazo) compounds. Although spin-states observed for those photoproducts were not so high, the results suggest that our approach using persistent triplet carbenes will eventually lead us to a persistent high-spin polycarbenes by taking advantage of the stability of our diazo compounds.

Photochemistry of 1,2-Dihydronaphthalene Oxide: Concurrent Triplet and Singlet Processes via Singlet Excitation

The photochemistry of 1,2-dihydronaphthalene oxide (254 nm) was reexamined and indan was found to be a primary photoproduct, as well as the traditionally assumed secondary photoproduct. Quenching studies demonstrated that indan, as a primary photoproduct, is derived from a triplet pathway, competing with a singlet route, back to the ground state surface. CASSCF calculations strongly suggest that the triplet pathway consists of a dissociation of the oxirane moiety to give a triplet carbene and aldehyde, which via hydrogen abstraction-decarbonylation-ISC recloses to give indan. Conical intersections corresponding to the presumed 1,2-hydrogen shift and 1,2-alkyl shift to give 2-tetralone and 1-indancarbaldehyde, respectively, were located computationally.