Electron spin and electron nuclear double resonances of the stable 1-(4-nitrophenyl)-3-phenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl free radical

Article abstract of DOI:10.1023/A:1021331930462

The ¹H and ¹⁴N hyperfine structure constants for the stable 1-(4-nitrophenyl)-3-phenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl free radical obtained by the oxidation of 1-(4-nitrophenyl)-3-phenyl-1,4-dihydro-1,2,4-benzotriazine or N-phen

Full text of DOI:10.1023/A:1021331930462

1796
Russian Chemical Bulletin, International Edition, Vol. 51, No. 10, pp. 1796—1799, October, 2002
Physical Chemistry
Electron spin and electron nuclear double resonances of the stable
1ꢀ(4ꢀnitrophenyl)ꢀ3ꢀphenylꢀ1,4ꢀdihydroꢀ1,2,4ꢀbenzotriazinꢀ4ꢀyl
free radical
ꢀ
M. K. Kadirov, B. I. Buzykin, and N. G. Gazetdinova
A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Research Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420083 Kazan, Russian Federation.
Fax: +7 (843 2) 75 2253. Eꢀmail: kmk@iopc.kcn.ru
1
The Н and 14N hyperfine structure constants for the stable 1ꢀ(4ꢀnitrophenyl)ꢀ3ꢀphenylꢀ
1,4ꢀdihydroꢀ1,2,4ꢀbenzotriazinꢀ4ꢀyl free radical obtained by the oxidation of 1ꢀ(4ꢀnitrophenyl)ꢀ
3ꢀphenylꢀ1,4ꢀdihydroꢀ1,2,4ꢀbenzotriazine or Nꢀphenylbenzamide 4ꢀnitrophenylhydrazone
were determined by the ESR and 1Н electron nuclear double resonance methods.
Key words: electron spin resonance, 1H electron nuclear double resonance, stable free
radical, 1ꢀ(4ꢀnitrophenyl)ꢀ3ꢀphenylꢀ1,4ꢀdihydroꢀ1,2,4ꢀbenzotriazinꢀ4ꢀyl, 1ꢀ(4ꢀnitrophenyl)ꢀ
3ꢀphenylꢀ1,4ꢀdihydroꢀ1,2,4ꢀbenzotriazine, amidrazone, oxidation.
The 1,3ꢀdiphenylꢀ1,4ꢀdihydroꢀ1,2,4ꢀbenzotriazinꢀ
4ꢀyl radical (1), its selectively isotopeꢀsubstituted anaꢀ
logs, and 1ꢀ(4ꢀchlorophenyl) and 3ꢀtertꢀbutyl derivatives
have previously been studied by the ESR method.1—3
The electron nuclear double resonance (ENDOR)
spectra of radical 1 in the mineral oil were recorded for
1
the first time in our previous work.4 The highꢀquality Н
and 15N electron nuclear triple resonance spectra of radiꢀ
cal 1 and its isotopeꢀsubstituted analogs in toluene have
also been measured.5 This paper is devoted to the ESR
1
and H ENDOR study of spectral characteristics of the
stable 1ꢀ(4ꢀnitro)ꢀ3ꢀdiphenylꢀ1,4ꢀdihydroꢀ1,2,4ꢀbenzoꢀ
triazinꢀ4ꢀyl free radical (2).
The new compound 2 was synthesized according to
Scheme 1.
Experimental
IR spectra were recorded using a URꢀ20 instrument for a
suspension in Nujol between KBr plates. UV spectra were meaꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1650—1653, October, 2002.
1066ꢀ5285/02/5110ꢀ1796 $27.00 © 2002 Plenum Publishing Corporation

ESR and 1H ENDOR spectra
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 10, October, 2002
1797
Scheme 1
sured with a Specord UVꢀVIS spectrometer. 1H NMR spectra
were recorded using Varian Tꢀ60 and Bruker WMꢀ250 specꢀ
trometers using Me4Si as the internal standard. The reaction
course and the purity of the synthesized compounds were moniꢀ
tored by TLC on Silufol UVꢀ254 plates using an Et2O—hexꢀ
ane—methanol (6 : 3 : 1) system as the eluent.
1ꢀ(4ꢀNitrophenyl)ꢀ3ꢀphenylꢀ1,4ꢀdihydroꢀ1,2,4ꢀbenzotriazine
(5). Freshly prepared silver oxide (5 g) was added to a solution of
amidrazone 3 (1.66 g, 5 mmol) in THF (150 mL), and the
suspension was stirred for 3 days in a nitrogen atmosphere. The
slime was thoroughly separated, and the solvent was removed in
vacuo without heating. Dark green needles of benzotriazine 5
were obtained in 63% yield (1.05 g), m.p. 216—218 °C (decomp.,
Boetius stage). The substance is well soluble in benzene, diꢀ
oxane, and acetone and is moderately soluble in alcohols.
Found (%): С, 69.42; Н, 4.25; N, 16.58. С19H14N4O2. Calcuꢀ
lated (%): С, 69.09; Н, 4.24; N, 16.97. UV (PriOH), λ/nm (logε):
254 (4.42), 470 (4.32). IR (Nujol), ν/cm–1: 3365, 1650, 1598,
1530 w, 1512, 1498. 1Н NMR (DMSO, δ: 9.47 (s, 1 H, NH);
8.07 (d, 2 H, C6H4NO2, J = 9 Hz); 7.52 (d, 2 H, C6H4NO2, J =
9 Hz); 7.46 (m, 3 Н, H(5)—H(7)); 7.93 (m, 1 Н, H(8)); 7.00
(m, 5 Н, Ph).
1ꢀ(4ꢀNitrophenyl)ꢀ3ꢀphenylꢀ1,4ꢀdihydroꢀ1,2,4ꢀbenzotriazinꢀ
4ꢀyl (2). A solution of amidrazone 3 in ethanol or dioxane was
refluxed for 5—6 days without oxidants or for 1—2 days in the
presence of AgO or HgO (methods А and B), or solutions of
benzotriazine 5 were refluxed in ethanol or treated similar to the
treatments described in methods А and B. After the slime was
separated and solvents were distilled off, dark violet, almost
black needles of compound 2 with m.p. 179—181 °C (decomp.,
Boetius stage) were obtained. They are well soluble in diethyl
ether and acetone and are moderately soluble in benzene and
alcohols. The yield of compound 2 was 40—50%. Found (%):
С, 68.57; Н, 4.06; N, 17.11. С19H13N4O2. Calculated (%):
С, 69.30; Н, 3.95; N, 17.02. UV, λ/nm (logε), Et2O: 273
(4.83), 370 (4.10), 492 (3.98), 565 (3.75); CCl4, 370, 489, 590.
IR (Nujol), ν/cm–1: 1598, 1521, 1485, 1409, 1359, 1310.
Spectroscopic studies. Product 2 was dissolved in toluene in
a concentration of 10–3 mol L–1. The freezing—thawing proꢀ
cedure was performed three times to remove oxygen from
the sample. ESR and ENDOR spectra were recorded using
a BrukerꢀER200D radiospectrometer (Germany) using an
ENDOR/TRIPLE attachment and a BꢀVTꢀ1000 temperature
variator.
NꢀPhenylbenzamide 4ꢀnitrophenylhydrazone (3). A solution
of 4ꢀnitrophenylhydrazone benzoyl chloride (1.38 g, 5 mmol),
freshly distilled aniline (0.47 g, 5 mmol), and triethylamine
(0.7 mL, 5 mmol) in ethanol (200 mL) was stirred in a nitrogen
atmosphere for 15 h at 18—20 °C. After the solution was stored
for 27 days, amidrazone 3 (1.5 g, 90%) precipitated. Fast recrysꢀ
tallization from ethanol by small portions produced brickꢀred
fine needles of compound 3 with m.p. 193—195 °C (decomp.,
Boetius stage; cf. Ref. 6: 180—181 °C)* without admixtures of
azoimine 4, benzotriazine 5, and benzotriazinyl 2. A similar
result was obtained for the reaction in dioxane. An increase in
the reaction temperature results in a mixture of products 2, 3, 4,
and 5, whose ratio depends, as a rule, on the duration and
temperature of the reaction and the method used for the isolaꢀ
tion of the target product. It is fairly difficult to separate this
mixture, because oxidative transformations of compounds 3 and
5 and heterocyclization of azoimine 4 occur in any solvent (see
Refs. 7 and 8). Found (%): С, 68.54; Н, 4.72; N, 16.77.
С19H16N4O2. Calculated (%): С, 68.67; Н, 4.82; N, 16.86. UV,
λ/nm (logε), EtOH: 250 (4.31), 293 (4.09), 427 (4.54); dioxane:
406 (4.48); CCl4: 282, 395. IR (Nujol), ν/cm–1: 3394, 3292,
1608, 1544, 1512, 1348, 1323, 1307, 1280. 1Н NMR, δ, HMFA:
10.53 (s, 1 H, N(1)H); 8.97 (s, 1 H, N(3)Н); 8.23 (d, 2 H,
C6H4NO2, J = 9 Hz); 7.80—6.70 (w.m, 12 Н, 2 Ph + 2 H,
C6H4NO2); DMSO: 10.03 (s, 1 H, N(1)H); 8.29 (s, 1 H, N(3)Н);
8.01 (d, 2 H, C6H4NO2, J = 9 Hz); 7.60—6.50 (w.m, 12 Н,
2 Ph + 2 H, C6H4NO2); 1,4ꢀdioxane: 8.73 (s, 1 H, N(1)H);
8.01 (d, 2 H, C6H4NO2, J = 9 Hz); 7.70—6.50 (w.m, 13 Н,
2 Ph + N(3)Н + 2 H, C6H4NO2).
* The reaction of aniline with αꢀnitrobenzaldehyde 4ꢀnitroꢀ
phenylhydrazone in ethanol, which was carried out by Ponzio
and coꢀauthors,6 should yield the oxidant (the nitrite anion).
The melting point of the reaction product decreases due to oxiꢀ
dative processes (m.p. 179—181 °C). Perhaps, in this case, the
reaction product is radical 2 rather than amidrazone 3.
The HFS constants for product 2, which were obtained from
1
the Н ENDOR and ESR spectra and by the calculation of the
theoretical ESR spectra, are presented in Table 1. The program
for ESR spectra simulation produced the theoretical spectrum
in the isotropic or anisotropic medium with the chaotic distribuꢀ

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Russ.Chem.Bull., Int.Ed., Vol. 51, No. 10, October, 2002
Kadirov et al.
Table 1. Absolute values of the HFS constants (in G) of
benzotriazinyl radicals 1 and 2
Magnetic
nuclei
1 6
2
Magnetic
nuclei
1 6
2
N(1)
N(2)
N(4)
H(5)
H(6)
H(7)
H(8)
7.51 6.8
4.78 5.4
5.05 5.6
1.36 1.40
1.19 0.95
1.84 1.99
0.87 0.95
H(2´), H(6´) 0.75 0.55
H(3´), H(5´) 0.51 0.55
N(4´)
0.61 0.66
H(2″), H(6″) 0.23 0.25
H(3″), H(5″) 0.14 0.13
H(4″)
0.23 0.35
10 G
Fig. 1. ESR spectrum of a 10–3 М solution of compound 2 in
toluene at 298 К.
tion of paramagnetic species. The resonance fields and probꢀ
abilities of transitions were calculated using the theory of firstꢀ
order perturbations.
insignificant effect on the spin density distribution. The
spin density is mainly concentrated in the triazine cycle
of both radical 1 and 1ꢀ(4ꢀchlorophenyl)ꢀ3ꢀphenylꢀ1,4ꢀ
dihydroꢀ1,2,4ꢀbenzotriazinꢀ4ꢀyl (6).1—4
We examined and calculated the spectra assuming the
free rotation of the aryl substituents around the N—C and
C—C bonds in the ESR time scale.2—4 The N(1) atom has
a considerable spin density and аN(1) is by an order of
magnitude higher than аH(2´), aH(6´), аH(3´), aH(5´), and
аN(4´). The observed splittings from the protons of the
Results and Discussion
The ESR spectrum of radical 2 in toluene is shown in
Fig. 1. Despite a very low concentration (10–3 mol L–1
of radical 2, we could not achieve a satisfactory resolution
of the ESR spectrum.
The study of radical 2 in toluene at 193 К by
1H ENDOR spectroscopy was more successful. The
ENDOR spectra (Fig. 2) distinctly demonstrate the
splittings from protons of the aryl rings bound to both the
nitrogen atom in position 1 and the carbon atom in poꢀ
sition 3.
A comparison of the spectral characteristics of comꢀ
pounds 1 and 2 shows that the introduction of the nitro
group to position 4 of the 4 Nꢀphenyl fragment has an
)
3ꢀphenyl ring are insignificant: аH(4″) = 0.35 G, аH(2″)
aH(6 ) = 0.25 G, аH(3 ) = aH(5 ) = 0.13 G.
=
″
″
″
Taking into account the stereochemical peculiarities
of radical 2, we can assume that in radical 2, like in
radicals 1 and 6, the steric interaction between the H(8)
proton and H(2´) and H(6´) protons is hindered due to
νp
2´,6´,3´,5´
5
6,8
2´,6´,3´,5´
6,8
7
7
3″,5″
2″,6″
5
4″,2″,6″
4″
3″,5″
1.0 MHz
Fig. 2. 1Н ENDOR spectrum of a 10–3 М solution of compound 2 in toluene at 193 К.

ESR and 1H ENDOR spectra
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 10, October, 2002
1799
the coplanar arrangement of the Nꢀaryl substituent and
1,4ꢀdihydroꢀ1,2,4ꢀbenzotriazinyl entity. The nonplanar
arrangement of the fragments impedes the spin density
delocalization in the Nꢀaryl fragment, and the character
of orbital interactions hinders the spin density delocalizaꢀ
tion in the Сꢀphenyl fragment. The assumption about
steric hindrances in the molecules and 1,4ꢀdihydroꢀ1,2,4ꢀ
benzotriazinyl radicals5 was experimentally confirmed.
The Xꢀray diffraction studies of 1ꢀ(4ꢀchlorophenyl)ꢀ3ꢀ
phenylꢀ1,4ꢀdihydroꢀ1,2,4ꢀbenzotriazinꢀ4ꢀyl3 (6), radiꢀ
cal 1,9 and 1,3,5,8ꢀtetraphenylꢀ1,2,4,5,7,8ꢀhexazaanthraꢀ
ceneꢀ4,6ꢀdiyl10 (7) showed that the phenyl rings bound to
the nitrogen atoms are substantially noncoplanar, unlike
the Сꢀphenyl rings. For example, in radical 1 the turning
angle of the phenyl ring about the Ph—N(1) bond is –54°,
and that about the Ph—С(3) bond is only 8°. In radical 6,
the corresponding angles are 56.9 and 15.9°, and the bonds
of the N(1) nitrogen atom have the planar trigonal conꢀ
figuration, which is not characteristic of nitrogen atoms in
8pπꢀheterocycles,11 including 1,4ꢀdihydroꢀ1,2,4ꢀbenzoꢀ
triazine. The N(2)—C(3) and C(3)—N(4) bonds are equal
to 1.336(2) and 1.333(2) Å in radical 1 and 1.327(2) and
1.315(3) Å in radical 6, respectively. It is likely that the
same situation is retained in radical 2 under study.
Based on the results obtained in this work, we can
conclude that the complex use of different methods of
magnetic resonance allows one to identify free radicals
and obtain a detailed information on HFS constants of
free radicals with bulky and lowꢀsymmetry structures.
References
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The available data1—5,9,10 indicating that the spin denꢀ
sity is mainly delocalized in the amidrazonyl fragment of
the 1,4ꢀdihydroꢀ1,2,4ꢀtriazinyl cycle, regardless of the
nature of substituents in the 1ꢀphenyl fragment, suggest
that resonance structure 8 is most preferential for radicals
of the 1,4ꢀdihydrobenzoꢀ1,2,4ꢀtriazinyl series. Structure 8
is close to structure 9, which was considered12 as a posꢀ
sible resonance structure.
Received February 11, 2002;
in revised form June 28, 2002