Direct Observation of an Imidoylnitrene: Photochemical Formation of PhC(=NMe)?N and Me?N from 1-Methyl-5-phenyltetrazole

Article abstract of DOI:10.1002/anie.201712689

The imidoylnitrene 8, N-methyl-C-phenylimidoylnitrene, has been generated by laser photolysis of 1-methyl-5-phenyltetrazole 6 at 5 K and characterized by its ESR spectrum (|D/hc|=0.9602, |E/hc|=0.0144 cm^?1). In addition, the triplet excited sta

Full text of DOI:10.1002/anie.201712689

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: First Direct Observation of an Imidoylnitrene. Photochemical
Formation of PhC(=NMe)-N and Me-N from 1-Methyl-5-
phenyltetrazole
Authors: Curt Wentrup, Manabu Abe, Didier Bégué, Hugo Santos-
Silva, and Alain Dargelos
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201712689
Angew. Chem. 10.1002/ange.201712689
Link to VoR: http://dx.doi.org/10.1002/anie.201712689
http://dx.doi.org/10.1002/ange.201712689

10.1002/anie.201712689
Angewandte Chemie International Edition
First Direct Observation of an Imidoylnitrene. Photochemical Formation of
PhC(=NMe)-N and Me-N from 1-Methyl-5-phenyltetrazole
Manabu Abe, Didier Bégué, Hugo Santos-Silva, Alain Dargelos, and Curt Wentrup*
Abstract: The imidoylnitrene 8, N-methyl-C-phenylimidoylnitrene, has been generated by laser
photolysis of 1-methyl-5-phenyltetrazole 6 at 5 K and characterized by its ESR spectrum (|D/hc|
= 0.9602, |E/hc| = 0.0144 cm-1). In addition, the triplet excited states of 6 and of 2-methyl-5-
phenyltetrazole 11 were also observed by ESR spectroscopy in the 5 K matrices (6: |D/hc|=
0.123 cm-1, E/hc = 0.0065 cm-1, 11: |D/hc| = 0.126 cm-1, |E/hc| = 0.0056 cm-1). The
imidoylnitrene 8 is unstable both thermally (disappearing at 80 K) and photochemically
(disappearing on continued irradiation at 266 nm). Methyl(phenyl)carbodiimide is the end
product of photolysis.
Prof. Dr. Manabu Abe, Department of Chemistry, Graduate School of Science, Hiroshima
University, 1-3-1Kagamiyama, Higashi-Hiroshima Hiroshima 739-8526, Japan.
E-mail: mabe@hiroshima-u.ac.jp.
Prof. Dr. Didier Bégué, Hugo Santos Silva, Prof. Dr. Alain Dargelos, Institut
Pluridisciplinaire de Recherche sur l'Environnement et les Matériaux, Université de Pau et
des Pays de l'Adour, 64000 Pau, France. E-mail: didier.begue@univ-pau.fr.
[*] Prof. Dr. Curt Wentrup, School of Chemistry and Molecular Biosciences, The University
of Queensland, Brisbane, Queensland 4072, Australia. E-mail: wentrup@uq.edu.au.
Homepage: http://researchers.uq.edu.au/researcher/3606.
Supporting information and the ORCID identification number(s) for author(s) of this article
can be found under: https://doi.org/10.1002/anie...
1
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
Imidoylnitrenes RC(=NR’)-N 3 have been postulated frequently as reactive intermediates in
pyrolyses and photolyses, particularly of 1-aryltetrazoles 1.[1][2] The tetrazoles are expected to
undergo initial ring opening to imidoyl azides 2, which have been isolated or observed
spectroscopically in a few cases.[1,3] Although the imidoylnitrenes have never been observed
directly by any spectroscopic technique, evidence for their existence lies in the preparatively
valuable cyclization to form benzimidazoles 4[1,2] and 1,2-shifts forming carbodiimides 5[1,4,5]
[(Eq. (1)].
N
N
N
N3
Ph
N
hν
~R
N
(1)
N C N
R
R
N
R
N
Ph
R
5
Ph
3
Ph
2
1
H
N
R
N
4
Calculations indicate that imidoylnitrenes have triplet ground states.[5] The first excited
states are the open-shell singlets (OSS) which, depending on substituents, lie between ca. 6 and
15 kcal/mol above the triplets at the CASPT2 level. The second excited states are the closed-
shell singlets (CSS), lying more than 50 kcal/mol higher in energy. Very low calculated barriers
for the cyclization to 1H-diazirines and the rearrangement to carbodiimides 5 suggest that the
OSS nitrenes will be difficult to observe, and that it may be advantageous to generate the
nitrenes directly in the triplet ground states.
We now report that the photolysis of 1-methyl-5-phenyltetrazole 6 in a 5 K matrix in
fact generates the triplet excited state of the tetrazole, and photolysis of this species generates
the triplet N-methyl-C-phenylimidoylnitrene 8 as well as methylnitrene 9, both observed by
ESR spectroscopy.
2
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
N
N
8
N3
Ph
N
hν
N
N C N
Me
Ph
N
Ph
N
N
Ph
Me
Me
Me
6
7
10
hν
Ph CN
+
N N N
Me N
Me
9
Scheme 1. Formation of imidoylnitrene 8 and methylnitrene 9
Laser photolysis of 6 (10 mM in 2-methyltetrahydrofuran (MTHF)) at 266 nm
immediately generates a strong spectrum of the triplet excited tetrazole (Figure 1) characterized
by its Z1, X1Y1, Y2X2 and Z2 transitions with |D/hc|= 0.123 cm-1, |E/hc| = 0.0065 cm-1. The D
value corresponds to an average distance between the two radical spins of 2.8 Å.[6] The
calculated natural spin densities indicate that the two unpaired electrons are delocalized over
all the ring atoms with the highest spin densities at N2, N4, C1 of the benzene ring and the
ortho, meta, and para positions. A strong half-field signal at 1510 G, characteristic of triplet
diradicals was also observed (Figure 1). The strength of this signal is in agreement with the
size of the D value.[6] It is noteworthy that the triplet signals disappear immediately when the
irradiation is switched off. We have observed similar behaviour for other tetrazoles (see
below).
3
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
Figure 1. (a) ESR spectrum of the triplet excited state of tetrazole 6 (10 mM in MTHF)
recorded at 5 K during photolysis at 266 nm for 3 min. The signals from low to high field are
the half-field (Dms = 2) signal, 1510 G; Z1 signal, 2040 G; X1Y1 signals, 2525 and 2718 G; g =
2 signal at 3394 G due to adventitious doublet radicals; Y2X2 signals, 3852 and 4066 G; Z2
signal, 4678 G. Microwave frequency 9.4004 GHz. (b) Simulated spectrum using Bruker’s
Win-EPR SimFonia software. Ordinate: intensity in arbitrary units.
A transient triplet species was also observed by laser flash photolysis of 6 (0.9 mM in
nitrogen-purged CH3CN) at 266 nm (298 K). Several transitions were observed between 330
and 830 nm (Figure 2). The lifetime of the triplet was determined to be ~5-7 μs using a single
exponential fitting. The transient species was immediately quenched in the presence of air,
confirming that it is a triplet. The UV-Vis spectrum is not due to methylnitrene 9, which has
an absorption maximum at 314 nm and absorbs up to 350 nm.[7] The observed spectrum is in
good qualitative agreement with the calculated UV-Vis absorption spectrum of the triplet state
of 6, which shows weak absorptions up to 730 nm and a very weak absorption at 944 nm
(Figure S1, Supporting Information). However, the calculated UV-vis spectrum of the Z isomer
of triplet imidoylnitrene 8 also shows weak absorptions up to ca. 700 nm (Figures S2 and S3).
Therefore, UV-Vis spectroscopy does not clearly reveal which triplet species is being observed.
4
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
0.014
0.012
0.01
0.008
0.006
0.004
0.002
0
300
400
500
600
700
800
900
Wavelength (nm)
Figure 2. UV-Vis absorption spectrum during LFP of 6, 1.9 mM in CH3CN at 298 K, at 266
nm. Lifetimes under N2: 330 nm, 6.4 ± 0.03 µs; 480 nm, 5.1 ± 0.04 µs; 580 nm, 6.9 ± 0.05 µs.
All signals were completely quenched in aerated solution.
Continued photolysis of the MTHF matrix of 6 in the ESR cavity at 5 K and 266 nm
for 5 min generated a readily observed high-field signal at 8363 G (peak C in Figure 3) ascribed
to methylnitrene 9 with |D/hc| = 1.611 cm-1, |E/hc| = 0.0018 cm-1, which falls between the two
previously reported values (9 formed by sensitized photolysis of methyl azide in an organic
matrix: |D/hc| = 1.595, |E/hc| < 0.003 cm1),[8] and 9 produced by the action of metastable N2 on
methyl azide and trapped in N2 matrix at 10 K: |D/hc| = 1.72 cm-1, E = 0[7a,9]). Cycloreversion
of 6 to benzonitrile and methyl azide provides a logical route to methylnitrene (Scheme 1).
Two further signals at 6412 and 7111 G are assigned to the X2 and Y2 transitions of
imidoylnitrene 8 with |D/hc| = 0.9602, |E/hc| = 0.0144 cm-1 (peaks A and B in Figure 3).
Separate experiments revealed that 8 is photochemically unstable at 266 nm, so that the ESR
signals disappear on prolonged irradiation (halved in 2 h and disappearing in 10 h). In order to
obtain good intensities of the signals assigned to 8 it was advantageous to turn the ESR tube in
5
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
stages of 45-90o in order to expose fresh surfaces to the laser and photolyse for 5 minutes each
time. Thus, the spectrum shown in Figure 3 (a) was obtained after a total of 5 ´ 5 min of
irradiation. The calculated natural spin densities on the nitrene-N in the Z and E isomers of
triplet 8 (see structures in Figure 4) are 1.481 and 1.537 (UB3LYP/EPR-III). The linear
correlation of calculated spin densities with zero-field D values[1,10] then predicts D values of
0.84 and 0.96, respectively, which suggests that the observed nitrene exists predominantly as
the E isomer, which is also the isomer expected to be formed on fragmentation of azide 7
(Scheme 1). We usually find that DFT-calculated D values are too low by about 0.1 cm-1, and
that is also the case here, with calculated D values of 0.689 and 0.879 cm-1, respectively, at the
M06-2X/def2-TZVPP level of theory. The significant E value of 0.0018 cm-1 is in accord with
an unsymmetrical electronic structure with significant spin density on the imine nitrogen atom.
Thus, in addition to a spin density of 1.6 on the nitrene-N, which is responsible for the high D
value, there is also a spin density of 0.6-0.7 on the imine-N atom (see Table S1). Similar
separation of the X2 and Y2 lines and corresponding relatively high E values have been observed
in ESR spectra of acylnitrenes[11] and vinylnitrenes.[12],[13]
Figure 3. ESR spectra of nitrenes 8 and 9 resulting from 266 nm photolysis of tetrazole 6, 10
mM in MTHF. (a) After irradiation for 5 x 5 min at 5K. (b) After warming the sample in (a) to
6
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
80 K, keeping the sample at this temperature for 10 min, and re-cooling to 5K. Microwave
frequency 9.40166 GHz. A: 6412 G; B: 7111 G, assigned to the X2 and Y2 transitions of
imidoylnitrene 8; C: 8363 G, XY2 transition of methylnitrene 9. Ordinate: intensity in arbitrary
units.
Figure 4. E (left) and Z (right) isomers of triplet imidoylnitrene 8 (UB3LYP/6-311G(d,p)).
Warming of the nitrene matrix from 5 to 40 K caused gradual decrease of the ESR
signal intensities of both imidoylnitrene 8 and methylnitrene 9 as expected according to the
Curie law for species in their triplet ground states. The signals recovered fully on re-cooling to
5 K. However, after further warming to 60 K and re-cooling to 5 K the signals of 8 were
severely reduced, and after warming to 80 K for ca. 20 min (including the warming and cooling
periods), these signals disappeared completely and did not recover on re-cooling to 5 K,
whereas the methylnitrene signal was fully recovered (Figure 3 (b)). After further warming to
100 K and re-cooling to 5 K, the signal due to methylnitrene had also disappeared, and no
signals were left.
7
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
Figure 5. Energy profile corresponding to Scheme 1 calculated at the UB3LYP/6-
311G(d,p) level. A more extensive energy profile is available in Figure S7, Supporting
Information.
As good agreement between calculations for imidoylnitrens and related species at DFT
and CASPT2 levels was demonstrated previously,[5] we have carried out calculations of the
relevant energy surface at the UB3LYP/6-311G(d,p)) level (Figure 5). Structures and energies
of the open-shell imidoylnitrenes and the transition states for their reactions were also
performed at the CASPT2(8,8)/ 6-311+G(d,p) level (see Table S2, Supporting Information).
The triplet ground state of 8 can exist as Z and E isomers with a 2-5 kcal/mol energy difference,
whereby the DFT method predicts the Z isomer at the lowest energy, but this is reversed at the
CASPT2 level. The open-shell singlets lie 6-7 kcal/mol above the triplets, which is in good
agreement with the disappearance of the triplet signals at 80 K. The open-shell singlet nitrene
can rearrange to methyl(phenyl)carbodiimide 10, which is the known end product of both
matrix photolysis and flash vacuum pyrolysis, [4] with a barrier of about 9 kcal/mol (Scheme 1
and Figure 5). One could think that the E and Z isomers of the triplet nitrene 8 might
8
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
interconvert on warming. However, the calculated energy barrier for E-Z isomerization is 21
kcal/mol at the DFT level, which means that such interconversion will not happen before the
nitrenes react away to form the carbodiimide.
The photolysis of an isomeric starting material, 2-methyl-5-phenyltetrazole 11 (Scheme
2) was examined in the same way. Here, too, strong ESR signals ascribed to the triplet excited
diradical state of the tetrazole (Figure 6), as well as the XY2 signal of methylnitrene 9 (8363 G,
just like Figure 3) were observed immediately, but no signal due to imidoylnitrene 8 was
detectable after photolysis at 266 nm/5 K for 5 or 4 ´ 5 minutes, or after continuous irradiation
for 8 h. It is known from matrix IR spectroscopy that photolysis of 11 produces N-methyl-C-
phenylnitrile imine 12 very efficiently, and this molecule has a singlet ground state.[4] This was
also observed in an LFP experiment, whereby the UV-Vis absorption spectrum is dominated
by a long-lived (1.2 s at 298 K) oxygen-insensitive absorption at 315 nm (Figure S4), which is
ascribed to the nitrile imine 12 in agreement with previous reports on nitrile imines.[14,15]
Figure 6. (a) ESR spectrum of the triplet excited state of tetrazole 11 (10 mM in MTHF)
recorded at 5 K during photolysis at 266 nm for 3 min. The signals from low to high field are
the half-field (Dms = 2), Z1 signal, X1Y1 signals, g = 2 signal, Y2X2 signals, and Z2 signal (|D/hc|
= 0.126 cm-1; |E/hc| = 0.0056 cm-1). (b) Simulated spectrum. See caption to Figure 1 for further
details.
9
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
Further photolysis rapidly converts 12 to carbodiimide 10, presumably via the singlet
states of diazirine 13 and imidoylnitrene 8 (Scheme 2).[4] It then appears that singlet 8
rearranges to singlet 10 faster than its potential intersystem crossing to the triplet ground state
of 8, or, in other words, the concentration of triplet 8 is too low for detection. On the triplet
energy surface, the putative rearrangement of triplet 12 to triplet 8 is hampered by a calculated
energy barrier of 43 kcal/mol between 12 and 13. The full, calculated singlet and triplet energy
profiles, including tetrazoles 6 and 11, their azide and diazo valence tautomers, the nitrile imine
12, the diazirine 13, the nitrene 8 and carbodiimide 10 are available in Figures S6 and S7
(Supporting Information).
Ph
N
hν
PhCN + Me-N3
Me N
N
N
Me
N
9
11
-N2
hν
Ph
Ph
hν
~R'
Me
N
Me N
N C N Ph
C N N
Me
N
N
Ph
Me
12
13
10
8
Scheme 2. Formation of nitrile imine 12 and methylnitrene 9 (see also energy profiles
in Figures S6 and S7).
In conclusion, we have directly observed an imidoylnitrene, N-methyl-C-
phenylimidoylnitrene 8, for the first time and characterized it by its ESR spectrum. In addition,
the ESR spectra of the triplet excited states of two tetrazoles, viz 1-methyl-5-phenyltetrazole
6, and 2-methyl-5-phenyltetrazole 11 were also observed. The knowledge that imidoylnitrenes
can be observed directly, even though they are unstable both thermally and photochemically,
should permit more exacting investigations of reaction mechanisms involving putative
imidoylnitrenes as reactive intermediates.
10
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
Acknowledgements
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-in-Aid 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.
Conflict of interest
The authors declare no conflict of interest.
Keywords: ESR spectroscopy · matrix isolation · nitrenes · tetrazoles · triplet excited states
[1] C. Wentrup, Chem. Rev. 2017, 117, 4562-4623.
[2] a) D. Moderhack, J. Prakt. Chem. - Chem. Ztg. 1998, 340, 687-709. b) C. Wentrup, Adv.
Heterocycl. Chem. 1981, 28, 232-361. c) R. N. Butler, Tetrazoles. In Comprehensive
Heterocyclic Chemistry II, Storr, R. C., Ed.; Elsevier Science, Oxford, 1996; Vol. 4, Chapter
4.17, pp 621-678. d) M. I. L. Soares, T. M. V. D. Pinho e Melo, R. Fausto, Chem. Select.
2016, 1, 4591-45495.
[3] a) A. Subbaraj, O. S. Rao, W. Lwowski, J. Org. Chem. 1989, 54, 3945−3952.
b) H. A. Dabbagh, W. Lwowski, W. J. Org. Chem. 1989, 54, 3952−3957.
c) H. A. Dabbagh, S. Ghaelee, J. Org. Chem. 1996, 61, 3439−3445.
d) R. A. Evans, P. Lorenčak, T.-K. Ha, C. Wentrup, J. Am. Chem. Soc. 1991, 113, 7261-
7276.
[4] D. Bégué, G. G. Qiao, C. Wentrup, J. Am. Chem. Soc. 2011, 134, 5339-5350.
[5] D. Bégué, H. Santos-Silva, A. Dargelos, C. Wentrup, J. Phys. Chem. A 2017, 121,
8227−8235.
[6] M. Abe, Chem. Rev. 2013, 113, 7011-7088.
[7] a) R. F. Ferrante, J. Chem. Phys. 1987, 86, 25-32. b) P. G. Carrick, P. C. Engelking, J.
Chem. Phys. 1984, 81, 1661-1665.
[8] L. Barash, E. Wasserman, W. A. Yager, J. Am. Chem.. Soc. 1967, 89, 3931-3932.
[9] a) R. F. Ferrante, S. L. Erickson, B. M. Peek, J. Chem. Phys. 1087, 87, 2421-2425. b)
11
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
Carrick et al. estimated |D/hc| as 1.720 cm-1 based on gas phase emission spectra: P. G.
Carrick, C. R. Brazier, P. F. Bernath, P. C. Engelking, J. Am. Chem. Soc. 1987, 109, 5100-
5102.
[10] D. Kvaskoff, P. Bednarek, L. George, K. Waich, C. Wentrup, J. Org. Chem. 2006, 71,
4049-4058.
[11] a) R. Feng, Y. Lu, G. Deng, J. Xu, Z. Wu, H. M. Li, Q. Liu, N. Kadowaki, M. Abe, X.
Q. Zeng, J. Am. Chem. Soc. 2018, 140, 1-13.
b) Z. Wu, R. Feng, H. Li, J. Xu, G. Deng, M. Abe, D. Bégué, K. Liu, and X. Q. Zeng,
Angew. Chem. Int. Ed. 2018, 57. DOI: 10.1002/anie.201710307.
[12] S. K. Sarkar, O. Osisioma, W. L. Karney, M. Abe, A. D. Gudmundsdottir, J. Am. Chem.
Soc. 2016, 138, 14905-14914.
[13] A referee has enquired whether the formation of carbodiimides might be reversible, so
that photolysis of the carbodiimide could regenerate an imidoylnitrene. We have no evidence
that carbodiimides can form imidoylnitrenes, and a photolysis of diphenylcarbodiimide under
the matrix conditions described here did not produce any nitrene observable by ESR.
[14] a) H. Meier, W. Heinzelmann, H. Heimgartner, Chimia 1980, 34 504-506. b) N. H.
Toubro, A. Holm, J. Am. Chem. Soc. 1980, 102, 2093. c) P. Leitkauf, V. Lohse, C. Csongar,
G. Tomaschewski, J. Wilda, Z. Chem. 1985, 25, 266-267. d) K. Bhattacharyya, D. Ramaiah,
P. K. Das, M. V. George, J. Photochem. 1987, 36, 63-84.
[15] G. Bertrand, C. Wentrup, Angew. Chem. Int. Ed. Engl. 1994, 33, 527-545; Angew.
Chem. 1994, 106, 549-567.
12
This article is protected by copyright. All rights reserved.

10.1002/anie.201712689
Angewandte Chemie International Edition
Graphical abstract
The N-methyl-C-phenylimidoylnitrene 8 has been generated together with methylnitrene 9
and characterized in its triplet ground state by ESR spectroscopy at 5 K (|D/hc| = 0.9602,
|E/hc| = 0.0144 cm-1). Furthermore, the ESR spectra of the triplet excited states of its
precursor, 1-methyl-5-phenyltetrazole 6 as well as 2-methyl-5-phenyltetrazole 11 were also
observed.
N
N
N
hν
hν
N
Me N
Ph
N
5 K
5 K
N
Ph
9
Me
Me
5 K
8
6
hν
Triplet 6
Manabu Abe, Didier Bégué, Hugo Santos-Silva, Alain Dargelos, and Curt Wentrup*
Page No. –Page No.
First Direct Observation of an Imidoylnitrene. Photochemical Formation of
PhC(=NMe)-N and Me-N from 1-Methyl-5-phenyltetrazole
13
This article is protected by copyright. All rights reserved.