Triplet States of Tetrazoles, Nitrenes and Carbenes from Matrix

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Triplet States of Tetrazoles, Nitrenes and Carbenes from Matrix Photolysis of Tetrazoles, and Phenylcyanamide as a Source of Phenylnitrene Manabu Abe, Didier Bégué, Hugo Santos Silva, Alain Dargelos, and Curt Wentrup J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b06960 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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The Journal of Physical Chemistry

Triplet States of Tetrazoles, Nitrenes and Carbenes from Matrix Photolysis of Tetrazoles, and Phenylcyanamide as a Source of Phenylnitrene Manabu Abe,† Didier Bégué,‡ Hugo Santos Silva,‡ Alain Dargelos‡ and Curt Wentrup§* †

Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1

Kagamiyama, Higashi-Hiroshima Hiroshima 739-8526, Japan. ‡

CNRS/Université de Pau et des Pays de l’Adour/E2S UPPA, Institut des Sciences Analytiques

et de Physicochimie pour l’Environnement et les Matériaux, UMR5254, 64000 Pau, France. §

School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane,

Queensland 4072, Australia.

Abstract: Photolysis of 1- and 5-aryltetrazoles at 5-10 K using a 266 nm laser immediately generates their triplet excited states, which are characterized by their electron spin resonance (ESR) spectra with zero-field splitting parameters D = 0.12-0.13 cm-1 and E = 0.002-0.008 cm-1. Further photolysis of all the aryltetrazoles affords arylnitrenes (D ≅ 1 cm-1), and in the case of 5aryltetrazoles also arylcarbenes (D ≅ 0.5 cm-1). The formation of arylnitrenes from 5aryltetrazoles, where no aryl-N bond is present, is explained by the photochemical rearrangement of initially formed nitrile imines ArCN+N−R to carbodiimides. The monosubstituted carbodiimide PhN=C=NH isomerizes to phenylcyanamide, PhNH-CN, and photolysis of the latter causes rapid elimination of HCN and formation of phenylnitrene. When N-methyl groups are present in the tetrazoles, methylnitrene, CH3-N, is formed too. In the case of 5phenyltetrazole, additional hydrogen shift and fragmentation afford cyano- and isocyanonitrenes, NCN and CNN.

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Introduction Nitrenes and carbenes are important intermediates in numerous thermal and photochemical reactions.1,2 The relationship between nitrile imines 4, 1H-diazirines 5, imidoylnitrenes 6, and carbodiimides 7, generated from tetrazoles 1 and 2, has been the subject of recent investigations. The nitrile imines 4 generated by either thermolysis or photolysis of 2,5-disubstituted tetrazoles 1 are frequently employed in 1,3-dipolar cycloadditions.3 They can be observed directly in matrixisolation experiments, where they have also been shown to rearrange to carbodiimides 7, both thermally (flash vacuum pyrolysis, FVP) and photochemically.4 This is believed to take place via 1H-diazirines 5, which have been characterized by IR spectroscopy.5,6,7 The further rearrangement to carbodiimides is believed to take place via imidoylnitrenes 6, but these species have been unknown until very recently, when the first observation by ESR spectroscopy of 6c, generated from 1-methyl-5-phenyltetrazole 2c, was reported.8 Imidoylnitrenes 6 have been postulated frequently as reactive intermediates in pyrolyses and in solution photolyses, particularly of 1-aryltetrazoles 1, which often afford benzimidazoles 9

9,10,11,12

and /or

carbodiimides 7 as major products (Scheme 1).2,3,13 In addition, the tetrazoles 2 are expected to undergo initial ring opening to imidoyl azides 3, which have been isolated or observed spectroscopically in a few cases.2,14,15,16,17


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Scheme 1. Primary photochemical and thermal reactions of tetrazoles.

We now report the formation of arylnitrenes and in some cases arylcarbenes as well as CNN and NCN on matrix photolysis of aryltetazoles as investigated by ESR spectroscopy. As a bonus, the triplet excited states of the aryltetrazoles themselves were also observed in several cases.

Experimental Section Matrix-Isolation ESR Spectroscopy. For measurements of organic glasses, 10 mM solutions of compounds in 2-methyltetrahydrofuran (MTHF) in quartz ESR tubes (4.0 mm I.D.) were degassed by the freeze−pump−thaw method before being flame sealed. ESR spectra were recorded at 4-100 K with the probe cooled by liquid helium using a Bruker ELEXSYS E500 spectrometer operating in the X-band (0-10000 G) and equipped with an Oxford digital temperature controller. Photolysis was performed using a pulsed Nd3+:YAG laser (266 nm, 5 or 15 mJ/pulse, Spectra-Physics INDI-40). Simulations were performed using Bruker’s Win-EPR


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SimFonia software. Transient absorption spectra and decay traces in the laser flash photolysis of tetrazoles (Abs266 = 1.0) were measured in acetonitrile using a 266 nm YAG laser (14 ns pulse, 8.2 mJ, LOTIS TII LS-2145TF).

Computational Methods. Ground-state geometries, energies and spin densities were determined at the DFT level using the (U)B3LYP exchange-correlation functional with the 6311G(d,p) basis set,18 which has proved to be reliable and yield results comparable to CASPT2 for calculations on related systems.8,13 Calculations were performed using Gaussian 09 and Molpro program packages.19,20 Excited-states calculations were performed within the TDDFT UB3LYP/6-311G(d,p) level of theory21 by using the Orca 3.0.3 software.22 The 30 low-lying electronic excited states were calculated in a space dimension maximum set to 300. The UV-vis spectra were obtained by summing the electronic transitions so obtained, which were individually enveloped by 20 nm of FWHM real-space Gaussian functions in the 100-800 nm range.

Results and Discussion 1. 5-Phenyltetrazole 1a/2a. Laser photolysis of 5-phenyltetrazole 1a/2a (10 mM in 2methyltetrahydrofuran (MTHF)) at 266 nm immediately generates a strong spectrum of the triplet excited state of the compound (Figure 1a) characterized by its Z1, XY1, XY2 and Z2 transitions, from which |D/hc|= 0.124 cm-1 and |E/hc| = 0.0024 cm-1 were derived. Excellent agreement with the simulated spectrum (Figure 1b) is observed. The D value corresponds to an average distance between the two radical spins of 2.8 Å.23 The calculated electron spin densities of triplet 1a and 2a are primarily in the C1, ortho and para positions of the benzene ring and N1


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and N3 of the tetrazole ring (see Supporting Information). The strong half-field signal at 1507 G is characteristic of triplet diradicals (Figure 1). The strength of this signal is in agreement with the magnitude of the D value.19 It is noteworthy that these triplet signals disappear immediately when the irradiation is switched off (but the g = 2 signal remains). The triplet state is calculated to lie ca. 75 kcal/mol above the singlet ground state tetrazole (see Supporting Information).

Figure 1. ESR spectrum of the triplet excited state of tetrazole 1a/2a (a) experimental (10 mM in MTHF) recorded at 5 K during photolysis at 266 nm (15 mJ/pulse) for 1 min. The signals from low to high field are A, half-field (∆ms = 2) signal, 1507; B, Z1 signal, 2036; C, XY1 signals, 2587 and 2636; D, g = 2 signal at 3370 G due to adventitious doublet radicals; E, XY2 signals, 3939 and 3991; F, Z2 signal, 4688 G. Microwave frequency 9.4004 GHz. (b): simulated spectrum. |D/hc| = 0.1240 cm-1, |E/hc| = 0.0024 cm-1. Ordinate: intensity in arbitrary units.

Further signals developed during the photolysis at 2164, 4954, and 5967 G and are assigned as the Z1, X2 and Y2 transitions of phenylcarbene 12a (|D/hc| = 0.5173, |E/hc| = 0.024 cm-1 (lit.:24,25 |D/hc| = 0.517, |E/hc| = 0.024 cm-1 (average)), and a further signal at 6905 G is ascribed to the XY2 transition of phenylnitrene 14a with |D/hc| = 1.000 cm-1, |E/hc| = 0.0015 cm-1 (lit.26 |D/hc| = 0.998, |E/hc| = 0.00 cm-1 in Ar matrix) (Figure 2). These signals were visible already after 1 minute, and the intensities increased with time of photolysis. They were stable in


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the dark, but warming of the matrix from 5 to 77 K caused decreasing ESR signal intensities of both phenylcarbene and phenylnitrene as the temperature increased, as expected according to the Curie law for species in the triplet ground state, and the signals were fully recovered after recooling to 5 K.

Figure 2. ESR spectrum arising from 1a/2a after 266 nm-photolysis (15 mJ/pulse) at 5 K for 16 min. PhCH 12a: 2164 (Z1) (not shown), A, 4954 (X2); B, 5967 G (Y2), |D/hc| = 0.5173, |E/hc| = 0.024 cm-1. PhN 14a: C, 6906 G (XY2), |D/hc| = 1.000 cm-1, |E/hc| = 0.0015 cm-1. D, CNN: 7285 G. E, NCN: 8194 G; this signal develops more slowly than the one due to CNN. Microwave frequency 9.40058 GHz. Ordinate: intensity in arbitrary units.

Two additional signals at 7285 and 8194 G, |D/hc| = 1.150 and 1.532 cm -1, respectively, and |E/hc| = 0.0008 and 0.0009 cm-1, respectively, are also seen in Figure 2. The ZFS parameters identify the carriers as triplet isocyanonitrene CNN and cyanonitrene NCN.27,28 The high-field transition ascribed to NCN developed more slowly. The mechanisms of formation of phenylcarbene and phenylnitrene are described in Scheme 2.


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Scheme 2. Formation of arylcarbenes 11 and arylnitrenes 13 from 5-aryltetrazoles. Numbers in normal font are energies (kcal/mol, 0K) for the singlet (S) or triplet (T) species for R = Ph at the UB3LYP/6-311G(d,p) level.

5-Monosubstituted tetrazoles exist in the 1H tautomeric form 1 in the solid state29 and predominantly in the 2H form 2 in the gas phase, where the 2H form is favored by 2-3 kcal/mol.30 A solvent-dependent mixture of the two forms may exist in liquid solution.30 Calculations indicate a barrier of 49 kcal/mol for the interconversion of the 1H- and 2H-


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tautomers and 55 kcal/mol for formation of the 5H tautomer 10.30 The three tautomers may interconvert both photochemically and thermally (FVP), but the 2H-tautomer is the main species observed in the matrix. It is known that FVP of 1a/2a yields a small amount of phenyldiazomethane 11a and products derived from phenylcarbene 12a, e.g. fulvenallene.4,9 The ground state tautomerization of nitrile imine 4 to 11 has a high calculated barrier, but it is possible that such a tautomerization could take place in an excited state. At any rate, phenylcarbene 12a is formed and gives rise to the observed ESR signals. The formation of phenylnitrene 14a from 1a/2a is at first sight surprising, since there is no C-N connectivity. However, the well-established reaction sequence13 4→ →5→ →6→ →7 generates the NH-carbodiimide 7a, which

can interconvert with the lower-energy tautomer

phenylcyanamide 13a, as has been observed in matrix photolysis.5 As shown in the following Section 2, phenylcyanamide is indeed a photochemical source of phenylnitrene. Triplet isocyanonitrene CNN and cyanonitrene NCN may form via tautomerization to 10a and 15 (Scheme 3). Ring opening of 15 can generate compounds 16 and 17, which after loss of N2 and benzene yield CNN and NCC. These species can also be formed from tetrazol-5ylidene 18, which is predicted to have a triplet ground state (3B1).31 The energetics of these transformations on the triplet energy surface are summarized in Figure 3. It is known that CNN and NCN are interconvertible photochemically,32 but it is usually the rerrangement of NCN to CNN that is reported in the wavelength range we have used.27,28,32 Since we observed that CNN was formed first in the photolysis at 266 nm, it is likely that separate routes to CNN and NCN are occurring, in accordance with Scheme 3 and Figure 3.


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We have seen the formation of CNN and NCN in other tetrazole photolyses too, e.g. by elimination of phenol from 5-phenoxytetrazole, which will be the subject of a separate investigation. Scheme 3. Possible mechanism of formation of triplet CNN and NCN.


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Figure 3. Calculated pathways for the formation of CNN and NCN from 5-phenyltetrazole 2a initiated by isomerization to 10a on the triplet energy surface (U-B3LYP/6-311G(d,p); blue: singlets, black: triplets).

2. Phenylcyanamide 13a as a source of Phenylnitrene. The formation of phenylnitrene 14a from 5-phenyltetrazole 1a/2a requires an explanation. It has been reported that nitrile imines can dissociate to nitriles and nitrenes photochemically,33 but such a reaction of the nitrile imines 5 (Schemes 1 and 2) would not yield arylnitrenes. In the rearrangements of nitrile imines and imidoylnitrenes, carbodiimides are obtained as end-products of both pyrolysis and photolysis.4 Monosubstituted carbodiimides RN=C=NH are


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known to tautomerize to the lower-energy cyanamides RNH-CN on matrix photolysis,5 e.g. in the case of 1a/2a, phenylcarbodiimide 7a (2130, 2167 cm-1), rearranges slowly to phenylcyanamide 13a (3400 and 2256 cm-1) on irradiation at 266 nm. Therefore, we subjected phenylcyanamide 13a to 266-nm photolysis in MTHF matrix at 5 K. Indeed, phenylnitrene 14a was observable almost immediately, and the spectrum shown in Figure 4 was obtained after 10 minutes. PhN 14a was the only observable triplet species.

Figure 4. ESR spectrum of phenylnitrene 14a formed by photolysis of phenylcyanamide 13a (266 nm, 15 mJ/pulse, MTHF, 5 K) for 10 min. Microwave frequency = 9.4016 GHz. PhN: 6910 G, |D/hc| = 1.000 cm-1; |E/hc| = 0.0014 cm-1. Ordinate: intensity in arbitrary units.

3. 5-(p-Tolyl)tetrazole 1b/2b. Laser photolysis of this compound at 266 nm in MTHF at 4 K was completely analogous to that described above, immediately yielding strong signals for the triplet tetrazole during the photolysis (Figure 5a), and there was excellent agreement with the simulated spectrum (Figure 5b). In addition, weaker signals for p-tolylcarbene 12b (|D/hc| = 0.511; |E/hc| = 0.0204 cm-1) (lit.34 D = 0.516, E = 0.024 in Ar) and p-tolylnitrene 14b (|D/hc| = 0.96, E = 0.0007 cm-1) (lit.26 D = 0.96, E = 0.000 cm-1) developed in the course of 14 hours of


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photolysis (Figure 6). The formation of 12b and 14b is in accord with Scheme 2. It is noted that the p-tolylcarbene may undergo carbene-carbene rearrangements2,34 under these experimental conditions, but no direct evidence for such rearrangements was obtained in these experiments.

Figure 5. ESR spectrum of triplet 5-(p-tolyl)tetrazole 1b/2b obtained during photolysis in MTHF at 266 nm (15 mJ/pulse) for 4 min at 4 K. A, half-field (∆ms = 2) signal, 1523; B, Z1 signal, 2073; C, XY1 signals, 2620 and 2704; D, g = 2 doublet signal; E, XY2 signals, 3899 and 3987; F, Z2 signal, 4659 G. Microwave frequency 9.4004 GHz. (b): simulated spectrum. The calculated spin densities are predominantly at N2 and in the benzene ring (see Supporting Information). Microwave frequency = 9.4004 GHz. |D/hc| = 0.12077, |E/hc| 0.00254 cm-1. Ordinate: intensity in arbitrary units.

Figure 6. ESR spectrum resulting from the 266-nm photolysis (5 mJ/pulse) of 1b/2b in MTHF at 4 K for 14 h showing signals for p-tolylcarbene 12b (A, B), |D/hc| = 0.511; |E/hc| = 0.0204 cm-1 and p-tolylnitrene 14b (C) (|D/hc| = 0.96, E = 0.0007 cm-1). Ordinate: intensity in arbitrary units.


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The photolysis of 1b/2b was also examined by laser flash photolysis (LFP) (0.9 mM in CH3CN, N2, 298 K, 266 nm, 14 ns pulse width, 8.2 mJ). A broad absorption maximum at ~550 nm showed rapid first-order decay with a lifetime of 10-11µs (Figure S1, Supporting Information). It may be due to the triplet excited state of the tetrazole corresponding to the ESR observation. A similar UV-vis absorption was observed for 1-methyl-5-phenyltetrazole 19.8 5Phenyltetrazole 1a/2a was also examined by LFP in the same way, but no clear transient decay was observed; photolysis products were formed within the duration of the laser pulse. Calculated UV-vis spectra are available in the Supporting Information.

4. 1-Methyl-5-Phenyltetrazole 19 and 2-Methyl-5-Phenyltetrazole 24. The matrix photochemistry of these two compounds was reported recently.8 The ESR spectra of the triplet tetrazoles 19 and 24 as well as the imidoylnitrene 20 and methylnitrene 23 were observed in the 5 K matrices. Two routes were followed in each case, one being a cycloreversion to benzonitrile and methyl azide 22, which gives rise to methylnitrene. In the case of 19 the other route leads to the imidoylnitrene 20, whereas in the case of 24 the nitrile imine 25 is formed first. In both cases, the final photolysis product is methyl(phenyl)carbodiimide 21 (Scheme 4).


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Scheme 4. Photochemistry of 19 and 24

These experiments were performed with 5 mJ laser pulses at 266 nm.8 We have now repeated and fully confirmed these experiments using the more powerful 15 mJ laser pulses. While the same spectra as before were observed, an additional, weak signal ascribed to phenylnitrene 14a was seen between the A and B signals of imidoylnitrene 20 (|D/hc| = 0.9602, |E/hc| = 0.0144 cm-1) in the photolysis of 19 (Figure S2). The signal matches the XY2 transition of phenylnitrene from other sources. As there is no Ph-N bond in the starting material 19, a rearrangement is required. In the cases of 1a/2a and 1b/2b the photolysis of phenylcyanamide explained the formation of phenylnitrene. In the case of 19 the excited triplet state 21T of the carbodiimide may cleave to Ph-N 14a and methyl isocyanide 27 (Scheme 4) with a barrier of ca. 27 kcal/mol, but this does not appear to be a general reaction of carbodiimides: photolysis of diphenylcarbodiimide under the same conditions (266 nm, 15 mJ) for up to 14 hours did not produce any triplet ESR signal. Other potential photochemical rearrangements of 21T include


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1,3-H and 1,4-H shift to PhNCHNCH2 and PhNH-C≡N+-CH2-, but these have higher activation barriers of 47-50 kcal/mol, and they are even higher for the corresponding singlet state reactions (73-83 kcal/mol). At any rate, since PhN is formed from 21, then also the matrix photolysis of 24 should yield PhN, because we know from IR spectroscopy that 21 is formed rapidly.5 Indeed, the 6 K photolysis of 24 at 266 nm (15 mJ/pulse) afforded a weak signal due to phenylnitrene along with a stronger methylnitrene signal (Figure S3). These results call for further studies of the photolysis of carbodiimides.

5. 1-Phenyltetrazole 2d. Photolysis of this compound again afforded the ESR spectrum of the triplet 1-phenyltetrazole 2d (Figure 7a) in excellent agreement with the simulated spectrum (Figure 7b). In this case, the major calculated spin densities (0.5-0.7) are at C1 and Cpara of the phenyl ring with smaller spin densities (0.1-0.2) at Cortho and at N2, N3 and C5 of the tetrazole ring (see Supporting Information). In addition, a strong XY2 transition of phenylnitrene 14a developed rapidly (|D/hc| = 0.999; |E/hc| = 0.0007 cm-1) (Figure 8).


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Figure 7. ESR spectrum of the triplet state of 1-phenyltetrazole 2d (MTHF, 5 K) during irradiation at 266 nm (5 mJ/pulse). (a) Experimental, (b) simulated. A, 1478 (half-field signal), B, 1944, C, 2447, D, 2658, E (g = 2 doublet signal), F, 3886, G, 4124, H, 4778 G. Microwave frequency: 9.4018 GHz. |D/hc| = 0.1324 cm-1; |E/hc| = 0.00795 cm-1. Ordinate: intensity in arbitrary units.

Figure 8. ESR spectrum of phenylnitrene 14a from photolysis of 2d at 266 nm (5 mJ/pulse, MTHF, 5 K) for 2 h. Ordinate: intensity in arbitrary units.


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No evidence for an imidoylnitrene 6d or any other nitrenes or carbenes was found in the ESR or IR spectra, but IR spectroscopy in Ar matrix at 10 K showed the formation of phenylcarbodiimide 7d (2129, 2167 cm-1) as the main product (Figure S4), and bands due to phenylcyanamide 13 at 3404 and 2256 cm-1 increased as a function of photolysis time. Therefore, cyanamide 13 is likely to be the main source of phenylnitrene 14a in the photolysis of 2d (Scheme 5). The calculated activation energy for the 1,2-H shift 6d → 7d is 15.6 kcal/mol for the singlet state, and 48.7 kcal/mol for the triplet state (see Supporting Information). Phenylnitrene may also be derived from phenyl azide, itself formed from 2d by elimination of HCN in a cycloreversion reaction (Scheme 5). The presence of a trace of phenyl azide was suggested by a weak absorption at 2087 cm-1 in the Ar matrix IR spectrum at 10 K following photolysis of 2d at 266 nm.

Scheme 5. Photochemistry of 1-phenyltetrazole.

Other 1-aryltetrazoles behaved similarly, e.g. 5-amino-1-phenyltetrazole, which developed a phenylnitrene signal in the course of 2 hours of photolysis at 266 nm (|D/hc| = 0.998; |E/hc| = 0.0008 cm-1) (Figure S5, Supporting Information).


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6. 5-(3-Pyridyl)tetrazole. The triplet excited state of tetrazole 28 was observed immediately when irradiation at 266 nm started (Figure 9a) and, as usual, a good agreement with the simulated spectrum is evident (Figure 9b). The D value (|D/hc| = 0.1223) is very similar to that of 5-phenyltetrazole (Figure 1), but the E value (|E/hc| = 0.00853 cm-1) is higher (for the calculated spin densities, which are largely in the pyridine ring, see the Supporting Information).

Figure 9. ESR spectrum of triplet 5-(3-pyridyl)tetrazole 328. (a) experimental, Ar, 5 K, 266 nm, 15 mJ/pulse). A, 1509 (half-field signal); B, 2045; C, 2506; D, 2747, E, g = 2 doublet signal; F, 3813; G, 4090; H, 4666 G. Microwave frequency = 9.4005 GHz. (b) Simulated spectrum. |D/hc| = 0.1223, |E/hc| = 0.00853 cm-1.

Figure 10. ESR spectrum (4500-8000 G region) resulting from photolysis of 5-(3pyridyl)tetrazole 28 for 2 h (266 nm, 15 mJ/pulse) showing 3-pyridylcarbene 30 at 4947 and 5935 G, |D/hc| = 0.5114 and |E/hc| = 0.0239 cm-1 (a weak Z1 signal at 2130 G is not shown), and


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PhN 14a at ~6900 G (see text). Microwave frequency = 9.4005 GHz. Ordinate: arbitrary intensity units.

Signals due to 3-pyridylcarbene 30 were clearly identifiable already after 10 minutes of irradiation (Figure 10). In addition, a signal at 6820-6900 G with |D/hc| = 0.998 and |E/hc| = 0.002 cm-1 is ascribed to the XY2 transition of phenylnitrene 14a, formed by the efficient rearrangement of 3-pyridylcarbene (Scheme 6).9,35

However, this nitrene signal is broad,

indicating that two species are present (Figure 10). The higher-field component is assigned to 3pyridylnitrene 37, formed in a reaction analogous to Scheme 2, with |D/hc| = 1.013, |E/hc| = 0.001 cm-1 in agreement with the literature value.36

Scheme 6. Photochemistry of 5-(3-pyridyl)tetrazole.

The isomeric 5-(4-pyridyl)tetrazole is highly insoluble in all common solvents, with the consequence that only very tentative transitions of 4-pyridylcarbene could be discerned in the


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low temperature ESR spectra. ESR spectra of 4-pyridylcarbene (|D/hc| = 0.531; |E/hc| = 0.0247 cm-1)35,37 and 4-pyridylnitrene (|D/hc|) 1.107; |E/hc| ~ 0 cm-1)38 have been generated previously from the diazo compound and azide, respectively. It is known that FVP of 5-(4-pyridyl)tetrazole results in a rearrangement to phenylnitrene.9,37

Conclusion The formation of arylnitrenes Ar-N by matrix photolysis of tetrazoles 1 and 2 is a widespread reaction, taking place by different mechanisms depending on substitution. The matrix photolysis of 5-aryltetrazoles 1 generates arylnitrile imines, ArCN+N−R (4 and 25) rapidly, and these have been shown to rearrange to carbodiimides in all cases.4 The tautomerization of NHcarbodiimides ArN=C=NH to cyanamides ArNHCN has now been identified as a source of arylnitrenes Ar-N. Thus, photolysis of phenylcyanamide 13a is an efficient source of phenylnitrene 14a. In 1-aryltetrazoles 2 a cycloreversion to the aryl azide is one of the sources of the nitrene, but the arylcyanamide route is the major one. The photolyses of 5-phenyl-1methyltetrazole 19 and 5-phenyl-2-methyltetrazole 24 give rise to transitions due to PhN and MeN in the ESR spectra. Imidoylnitrenes 6, Ar-C(=NR)-N, are postulated as transient intermediates in many of these reactions4,13 and were observed directly by ESR spectroscopy in the case of PhC(=N-Me) 20 (|D/hc| = 0.9602, |E/hc| = 0.0144 cm-1).8

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca….. Additional ESR, UV-vis and IR spectra, computational details, mechanistic pathways and Cartesian coordinates (PDF).


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■ AUTHOR INFORMATION Corresponding Author: C. Wentrup, *E-mail: [email protected]. ORCID Manabu Abe: 0000-0002-4822-3321 Didier Bégué: 0000-0002-4553-0166 Hugo Santos-Silva: 0000-0002-7075-0101 Curt Wentrup: 0000-0003-0874-7144

Notes The authors declare no conflict of interest. Acknowledgments This work was supported by the Queensland Cyber Infrastructure Foundation at The University of Queensland and the Mésocentre de Calcul Intensif Aquitain of the Université de Bordeaux and the Université de Pau et des Pays de l’Adour. MA acknowledges financial support by a Grant-inAid for Science Research on Innovative Areas “Stimuli-responsive Chemical Species (No.2408)” (JSPS KAKENHI Grant Number JP24109008) and thanks Mr. K. Hagioka for supplying liquid Helium. CW also thanks the JSPS for a Fellowship and Hiroshima University for a Visiting Professorship, which made this work possible. We are grateful to Mr Norito Kadowaki for the synthesis of phenylcyanamide.

References

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TOC Graphic


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Triplet States of Tetrazoles, Nitrenes and Carbenes from Matrix

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