Matrix Isolation, Spectroscopic Characterization, and Photoisomerization of <i>m</i>-Xylylene

A new efficient synthesis of m-xylylene 1 is reported. The diradical 1 was trapped in argon matrices at 10 K and characterized by IR, UV−vis, and EPR spectroscopy. The syntheses reported before only allowed generation of 1 in organic glasses, and the spectroscopic identification was limited to fluorescence and EPR spectroscopy. Diradical 1 proved to be highly photolabile, and irradiation results in the formation of three isomeric hydrocarbons 7, 9, and 11 which could be identified by comparison of their IR spectra with the results of DFT calculations

Matrix Isolation, Zero-Field Splitting Parameters, and Photoreactions of Septet 2,4,6-Trinitrenopyrimidines

The key intermediates of decomposition of high-energy 2,4,6-triazidopyrimidine and its 5-chloro-substituted derivative, the detonation of which is used for preparation of carbon nitrides, were investigated using electron paramagnetic resonance (EPR) spectroscopy in combination with quantum chemical calculations. The decomposition of the triazides was carried out photochemically, using the matrix isolation technique. The photodecomposition of both triazides with 254 nm light in argon matrices at 5 K occurred selectively to subsequently give the corresponding triplet 4,6-diazido-2-nitrenopyrimidines, quintet 4-azido-2,6-dinitrenopyrimidines, and septet 2,4,6-trinitrenopyrimidines. The latter were photochemically unstable and decomposed to form triplet nitrenes NCN and NNC as well as triplet carbenes NCCCN, HCCN, and HCCCCN. The results obtained provide important information about exchange interactions in high-spin nitrenes with the pyrimidine ring and the mechanism of the formation of carbon nitrides during thermolysis of 2,4,6-triazidopyrimidine

Tunneling Rearrangement of 1‑Azulenylcarbene

1-Azulenylcarbene was synthesized by photolysis of 1-azulenyldiazomethane in argon or neon matrices at 3–10 K. The highly polar singlet carbene is only metastable and undergoes a tunneling rearrangement to 8-methylene-bicyclo[5.3.0]­deca-1,3,5,6,9-pentaene. After substitution of the 4 and 8 positions with deuterium, the rearrangement is completely inhibited. This indicates a very large kinetic isotope effect, as expected for a tunneling reaction

Photochemistry of Matrix Isolated (Trifluoromethyl)sulfonyl Azide, CF₃SO₂N₃

The photochemistry of matrix isolated (trifluoromethylsulfonyl) azide, CF₃SO₂N₃, has been studied at low temperatures. Upon ArF laser irradiation (λ = 193 nm), the azide eliminates N₂ and furnishes triplet [(trifluoromethyl)­sulfonyl]­nitrene, CF₃SO₂N, which has been characterized by IR and EPR spectroscopy. Upon subsequent UV light irradiation (λ = 260–400 nm) the nitrene converts to CF₃NSO₂ and CF₃S­(O)­NO through a Curtius-type rearrangement. Further two new species CF₂NSO₂F and FSNO were identified together with CF₂NF, SO₂, F₂CO, CF₃NO, and SO as side products. In addition, triplet nitrene CF₃N was detected by its EPR and IR spectra. The complex stepwise photodecomposition of matrix isolated CF₃SO₂N₃ is discussed in terms of the observed photolysis products and quantum chemical calculations

C–H Bond Amination by Photochemically Generated Transient Borylnitrenes at Room Temperature: A Combined Experimental and Theoretical Investigation of the Insertion Mechanism and Influence of Substituents

A number of azidoboranes having substitution patterns that are derived from catechol (3), pinacol (4a), 1,2-diaminoethane (4b,c), 1,2-ethanedithiol (4d), and 1,2,4,5-tetrahydroxybenzene as well as acyclic dialkoxy species (5) were synthesized and, in the case of 4c (N,N′-ditosyl-2-azido-1,3,2-diazaborolane), also structurally characterized. The azidoboranes were photolyzed in cyclohexane solvent in order to investigate the tendency of the generated borylnitrenes to undergo intermolecular C–H insertion reactions. The yields of intermolecular insertion products ranged from very good (4a) to vanishingly small, depending on the substitution of the azidoborane. For a number of borylnitrenes the zero-field splitting parameter D was measured in organic glasses at 4 K. The small primary kinetic isotope effect (kH/kD = 1.35) measured for 4a in mixtures of [H12]­cyclohexane and [D12]­cyclohexane suggests that the insertion reaction is concerted and involves the singlet state of the borylnitrene. Computations at the CBS-QB3 and CCSD­(T)/TZ2P levels of theory show that the relative energies of singlet and triplet states of a wide variety of borylnitrenes and even their nature as minima or saddle points depend strongly on the substituents. Photolysis of the most reactive azidoborane, 4a, in methane in a flow reactor at atmospheric pressure produces an intermolecular insertion product in low yields, in agreement with the expectation of intersystem crossing to the less reactive triplet state of the borylnitrene

Strain Effects in Electron Spin Resonance Spectroscopy of Quintet 2,6-Bis(4′-nitrenophenyl)-4-phenylpyridine

Photolysis of 2,6-bis(4′-azidophenyl)-4-phenylpyridine in 2-methyltetrahydrofuran (2MTHF) glass at 7 K leads to quintet 2,6-bis(4′-nitrenophenyl)-4-phenylpyridine as a mixture of rotational isomers. The electron spin resonance (ESR) spectrum of this mixture of rotamers shows a considerable broadening of many transitions in the range of 0–5000 G and cannot be reproduced by computer simulations solely based on the tuning of the spin Hamiltonian parameters g, DQ, and EQ alone or on predictions of DFT calculations. The best modeling of the experimental ESR spectrum is obtained only when the large line-broadening parameter of Γ(EQ) = 1200 MHz along with the spin Hamiltonian g = 2.003, DQ = 0.154 cm–1, and EQ = 0.050 cm–1 is used in the spectral simulations. The most accurate theoretical estimations of the magnetic parameters of the dinitrene in a 2MTHF glass are obtained from the B3LYP/6-311+G(d,p)+PBE/DZ/COSMO calculations of the spin–spin coupling parameters DSS and ESS. Such calculations overestimate the EQ and DQ values of the dinitrene just by 1% and 10%, respectively, demonstrating that contributions of the spin–orbit coupling parameters DSOC and ESOC to the total DQ and EQ values are negligibly small. The research shows that ESR studies of polynuclear high-spin nitrenes, obtained by photolysis of rotational isomers of the starting azides, can only be successful if large EQ strain effects are taken into account in the spectral simulations

C–H Bond Amination by Photochemically Generated Transient Borylnitrenes at Room Temperature: A Combined Experimental and Theoretical Investigation of the Insertion Mechanism and Influence of Substituents

A number of azidoboranes having substitution patterns that are derived from catechol (<b>3</b>), pinacol (<b>4a</b>), 1,2-diaminoethane (<b>4b</b>,<b>c</b>), 1,2-ethanedithiol (<b>4d</b>), and 1,2,4,5-tetrahydroxybenzene as well as acyclic dialkoxy species (<b>5</b><b></b>) were synthesized and, in the case of <b>4c</b> (<i>N</i>,<i>N</i>′-ditosyl-2-azido-1,3,2-diazaborolane), also structurally characterized. The azidoboranes were photolyzed in cyclohexane solvent in order to investigate the tendency of the generated borylnitrenes to undergo intermolecular C–H insertion reactions. The yields of intermolecular insertion products ranged from very good (<b>4a</b>) to vanishingly small, depending on the substitution of the azidoborane. For a number of borylnitrenes the zero-field splitting parameter <i>D</i> was measured in organic glasses at 4 K. The small primary kinetic isotope effect (<i>k</i>_H/<i>k</i>_D = 1.35) measured for <b>4a</b> in mixtures of [H₁₂]­cyclohexane and [D₁₂]­cyclohexane suggests that the insertion reaction is concerted and involves the singlet state of the borylnitrene. Computations at the CBS-QB3 and CCSD­(T)/TZ2P levels of theory show that the relative energies of singlet and triplet states of a wide variety of borylnitrenes and even their nature as minima or saddle points depend strongly on the substituents. Photolysis of the most reactive azidoborane, <b>4a</b>, in methane in a flow reactor at atmospheric pressure produces an intermolecular insertion product in low yields, in agreement with the expectation of intersystem crossing to the less reactive triplet state of the borylnitrene

Molecular Structure and Magnetic Parameters of Septet 2,4,6-Trinitrenotoluene

Septet 2,4,6-trinitrenotoluene is the major paramagnetic product formed during the photolysis of 2,4,6-triazidotoluene in cryogenic matrices. This trinitrene displays different electron paramagnetic resonance (EPR) spectra in solid argon and in 2-methyltetrahydrofuran (2MTHF) glass, corresponding to septet spin states with the zero-field splitting (ZFS) parameters DS = −0.0938 cm−1, ES = −0.0040 cm−1 and DS = −0.0934 cm−1, ES = −0.0015 cm−1, respectively. Analysis of these parameters shows that the molecular and electronic structure of the septet trinitrene derived from the EPR spectrum in argon is in good agreement with the expectations from DFT calculations. The very small parameter ES in 2MTHF glass is explained by significant changes of the spin densities on the three nitrene units due to interactions of the nitrogen atom with surrounding 2MTHF molecules

A Perimidine-Derived Non-Kekulé Triplet Diradical

6,9-Di(tert-butyl)-1-methyltetrazolo[1,5-a]perimidine (1) has been synthesized from naphthalene in seven steps. The EPR spectra, recorded after irradiation of 1 in a butyronitrile matrix at 77 K (λ = 351 nm) and in Ar and Xe matrixes at 4.6 K (λ ≥ 345 nm), showed a six-line, high-field signal (ΔmS = ± 1), centered at 3350 G in butyronitrile, along with a half-field signal (ΔmS = ± 2), which is characteristic for triplets. Simulation of the observed EPR spectra gave values for the zero-field splitting parameters of |D/hc|/cm−1 = 0.0105, |E/hc|/cm−1 = 0.0014 in butyronitrile and |D/hc|/cm−1 = 0.0107, |E/hc|/cm−1 = 0.0016 in Ar. These EPR parameters are consistent with the diradical 5,8-di(tert-butyl)-2-(N-methylimino)perimidine-1,3-diyl (32) as source of the EPR spectra. Linearity of the Curie−Weiss plot and UB3LYP and (14/14)CASPT2 calculations of the singlet−triplet energy difference (ΔEST ≈ 8−10 kcal/mol) indicate that the triplet is the ground state of 2, as predicted for such a nondisjoint diradical

A Perimidine-Derived Non-Kekulé Triplet Diradical

6,9-Di(tert-butyl)-1-methyltetrazolo[1,5-a]perimidine (1) has been synthesized from naphthalene in seven steps. The EPR spectra, recorded after irradiation of 1 in a butyronitrile matrix at 77 K (λ = 351 nm) and in Ar and Xe matrixes at 4.6 K (λ ≥ 345 nm), showed a six-line, high-field signal (ΔmS = ± 1), centered at 3350 G in butyronitrile, along with a half-field signal (ΔmS = ± 2), which is characteristic for triplets. Simulation of the observed EPR spectra gave values for the zero-field splitting parameters of |D/hc|/cm−1 = 0.0105, |E/hc|/cm−1 = 0.0014 in butyronitrile and |D/hc|/cm−1 = 0.0107, |E/hc|/cm−1 = 0.0016 in Ar. These EPR parameters are consistent with the diradical 5,8-di(tert-butyl)-2-(N-methylimino)perimidine-1,3-diyl (32) as source of the EPR spectra. Linearity of the Curie−Weiss plot and UB3LYP and (14/14)CASPT2 calculations of the singlet−triplet energy difference (ΔEST ≈ 8−10 kcal/mol) indicate that the triplet is the ground state of 2, as predicted for such a nondisjoint diradical