Triarylverdazyl radicals as promising redox-active components of rechargeable organic batteries

Article abstract of DOI:10.1007/s11172-020-2905-5

A novel design of electroactive components of rechargeable organic batteries based on stable verdazyl radicals bearing various substituents is proposed. 3-Positioned aromatic substituents at the verdazyl moiety affect the reduction potentials and almost do not affect the oxidation potential, while 1-positioned aromatic substituents affect contrariwise the oxidation potential of this radical without any influence on the reduction potential. The acquired electrochemical data allowed us to reveal the structure—potential relationship for the cathodic and anodic processes, which provided the design of triarylverdazyl radicals possessing record-breaking parameters of the “electrochemical gap”.

Full text of DOI:10.1007/s11172-020-2905-5

Russian Chemical Bulletin, International Edition, Vol. 69, No. 7, pp. 1321—1328, July, 2020
1321
Triarylverdazyl radicals as promising
redox-active components of rechargeable organic batteries*
S. G. Kostryukov, O. Yu. Chernyaeva, B. S. Tanaseichuk, A. Sh. Kozlov, M. K. Pryanichnikova, and A. A. Burtasov
N. P. Ogarev Mordovia State University,
68 ul. Bolshevistskaya, 430005 Saransk, Russian Federation.
Fax: +7 (834) 224 2444. E-mail: kostryukov_sg@mail.ru
A novel design of electroactive components of rechargeable organic batteries based on
stable verdazyl radicals bearing various substituents is proposed. 3-Positioned aromatic sub-
stituents at the verdazyl moiety affect the reduction potentials and almost do not affect the
oxidation potential, while 1-positioned aromatic substituents affect contrariwise the oxidation
potential of this radical without any influence on the reduction potential. The acquired elec-
trochemical data allowed us to reveal the structure—potential relationship for the cathodic and
anodic processes, which provided the design of triarylverdazyl radicals possessing record-
breaking parameters of the "electrochemical gap".
Key words: verdazyl radicals, cyclic voltammetry, electron-donating substituent, electron-
withdrawing substituent, electrode potential, electrochemical gap.
Verdazyl (tetrazinyl) radicals first reported in 1964 by
Kuhn and Trischman1 are "electron amphoteric", i.e.
capable of both oxidizing and reducing2 to give stable
cations and anions (Scheme 1).
Until quite recently, mostly magnetic4—7 and complex-
ing properties8—10 of verdazyls were considered. The redox
properties of these radicals remain much less explored. In
particular, electrochemical investigations of a verdazyl
series aimed to find a correlation between their redox
properties and structure11 have revealed such a correlation
between the distribution of electron spins and redox prop-
erties. The boundary orbitals that are involved in the redox
processes are localized in different parts of the molecule:
the HOMO of nitrogen atom at the position 1 makes
the largest contribution to the oxidation process, while
a carbon atom at the position 3 makes the most signifi-
cant contribution to the reduction process proceeding
via LUMO.
Scheme 1
As a material for the storage of charge, 3-phenyl-1,5-
di(p-tolyl)verdazyl has been applied since it can be re-
versibly oxidized and reduced in single-electron pro-
cesses12 and employed it in a symmetric all-organic
non-aqueous redox battery of the flow-type. The op-
portunity to utilize verdazyl radicals as materials for
the electricity storage has been also mentioned in the
review.13
Red is reduction, and Ox is oxidation.
Results and Discussion
Verdazyls are the only family of neutral radicals, whose
stability approaches to that of the known nitroxyl radicals.
The favorable properties of verdazyls include tolerance to
dimerization, as well as resistance to air and moisture.3
The unique structure of verdazyl radicals allows one
to finely tune the redox properties by varying the nature
of substituents at the N(1) and C(3) atoms of the verdazyl
moiety. In the present work, we report on the synthesis
and electrochemical properties of verdazyl radicals 1a—m
bearing substituents of the various nature.
* Based on the materials of the XXI Mendeleev Congress
on General and Applied Chemistry (September 9—13, 2019,
St. Petersburg, Russia).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1321—1328, July, 2020.
1066-5285/20/6907-1321 © 2020 Springer Science+Business Media LLC

Kostryukov et al.
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Russ. Chem. Bull., Int. Ed., Vol. 69, No. 7, July, 2020
the LUMO located at the C(3) atom. Therefore, the com-
putational data confirms the individual effect of substitu-
ents at the different positions on the redox processes.
Taking into account the calculated data, we selected
a set of verdazyls 1a—m bearing substituents of the differ-
ent natures at para-positions of the benzene moieties at-
tached to the positions 1 and 3 of verdazyl ring, whose
"structure—potential value" relationship was estimated for the
cathodic and anodic processes. Cyclic voltammetry (CV)
was used as the express method for evaluating values of the
"electrochemical gap" for the obtained verdazyl radicals.
Radicals 1a—m were prepared from the corresponding
formazans 2a—m according to the classical approach.
Compounds 2a—m were obtained by the reaction of
phenylhydrazones 3a—e of substituted benzaldehydes
4a—e with aryldiazonium salts 5a—e synthesized from
para-substituted anilines 6a—e (Scheme 2). Formazans
2a—m were cyclized with formaldehyde in the presence
of potassium hydrogen sulfate and subsequently oxidized
with atmospheric oxygen under alkaline conditions to give
verdazyls 1a—m. The duration of air bubbling through the
reaction mixture was monitored by changing its color from
red-violet to green.
Compounds 1 R1
R2
H
H
H
H
H
Br
MeO
Compounds 1 R1
R2
NO2
CN
a
b
c
d
e
f
H
Br
MeO
NO2
CN
H
h
i
H
H
j
k
l
MeO NO2
MeO CN
NO2 MeO
CN MeO
m
g
H
It has been assumed11 that substituents in the 3-posi-
tioned aromatic ring of the verdazyl moiety would strongly
affect the reduction potential and cause no significant
effect on the oxidation potential, while substituents in the
1-positioned aromatic ring of the verdazyl moiety would
affect the oxidation potential, but not the reduction one.
To verify this hypothesis, we computed the electron den-
sity distribution for the boundary orbitals of the considered
compounds within the density functional theory (DFT)
method using the B3LYP hybrid functional14 and the
standard 6-31G basic set15 implemented in the Firefly soft-
ware package.16 Figure 1 shows the localization of boundary
orbitals for verdazyl radical 1j, where one can see the HOMO
localized at the nitrogen atoms of the verdazyl moiety and
This approach to verdazyls is used quite successfully.
Some limitations may arise, e.g., in the case of the pres-
ence of strong electron-withdrawing substituents at the
benzaldehyde aromatic ring, while yields of the inter-
mediate formazans are usually low.11 Using the synthesis
of 1,5-diphenyl-3-arylverdazyls as the example, it has been
revealed that the yields of formazans are increased upon
using a DMF—pyridine mixture as the solvent.17
The structure of formazans 2a—m was confirmed by
1H and 13C NMR spectroscopy and mass spectrometry.
a
b
H
C
O
N
Fig. 1. LUMO (a) and HOMO (b) for compound 1j.

Triarylverdazyl radicals
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 7, July, 2020
1323
Scheme 2
3—6: R1, R2 = H (a), Br (b), MeO (c), NO2 (d), CN (e)
Compounds 1, 2
R1
H
Br
MeO
NO2
CN
H
R2
H
H
H
H
H
Br
MeO
Compounds 1, 2
h
i
j
k
l
R1
H
H
MeO
MeO
NO2
CN
R2
NO2
CN
NO2
CN
MeO
MeO
a
b
c
d
e
f
m
g
H
Reagents and conditions: i. PhNHNH2, EtOH, AcONa, 20 C; ii. NaNO2, HCl, 0 C; iii. DMF, Py, 0—25 C; iv. CH2O, KHSO4,
DMF, H2O, 20 C; v. 1) NaOH, 2) O2, 20 C.
Verdazyl radicals 1a—m were characterized using IR,
UV—Vis, and EPR spectroscopy and mass spectrometry. In
general, all verdazyls 1a—m are fine-crystalline substances
colored in various shades of green. Their IR spectra con-
Electrochemical properties of radicals 1a—m were
evaluated using the CV method in solutions in MeCN.
Completely reversible oxidation and reduction processes
were observed for almost all the verdazyls, except for the
nitro derivatives. The oxidation potentials of verdazyls
1a—m are in the range from 0.18 to 0.41 V (Ag/AgCl/
KCl). The reduction of these radicals proceeds at negative
potentials varying from –0.63 to –0.87 V (Ag/AgCl/KCl).
The electrochemical parameters of verdazyl radicals 1a—m
are given in Table 1.
tain vibration bands in the range of ~1150—1160 cm–1
,
which is characteristic of verdazyl radicals. The benzene
solutions of radicals 1a—m exhibit typical time-stable EPR
spectra containing nine broad lines with the intensity
ratio of 1 : 4 : 10 : 16 : 19 : 16 : 10 : 4 : 1 caused by a cou-
pling of the unpaired electron with four spectroscopically
equivalent nitrogen atoms of the tetrazine ring. Figure 2
exemplifies the EPR spectrum of verdazyl 1d. Although there
are two chemically nonequivalent pairs of the nitrogen
atoms (N(1), N(5) and N(2), N(4)), the hyperfine coupling
constants for radicals 1a—m have close values of ~5.6 G.
Table 1. Electrochemical parameters for verdazyl
radicals 1a—m
Verdazyl
EOx
–ERed
Eec
5.6 G
V
1a
1b
1c
1d
1e
1f
1g
1h
1i
0.31
0.30
0.26
0.35
0.34
0.25
0.20
0.41
0.38
0.36
0.34
0.18
0.19
0.81
0.80
0.87
0.61
0.65
0.81
0.85
0.65
0.67
0.87
0.86
0.63
0.64
1.12
1.10
1.13
0.96
0.99
1.06
1.15
1.06
1.05
1.23
1.20
0.81
0.83
1j
1k
1l
3280 3300 3320 3340 3360 3380 3400 B/G
Fig. 2. EPR spectrum of verdazyl 1d.
1m

Kostryukov et al.
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Russ. Chem. Bull., Int. Ed., Vol. 69, No. 7, July, 2020
As one can see from Table 1, the nature of substituent
at the para-position of aromatic ring attached at the posi-
tion 3 of the verdazyl affects slightly the oxidation poten-
tial (EOx). Thus, the EOx value is slightly shifted to the
cathodic region in the case of compounds bearing the
electron-withdrawing substituents, viz., 1d (R1 = NO2, R2
= H) and 1e (R1 = CN, R2 = H), while that for compound
with the electron-donating substituent, 1с (R1 = МеО, R2
= H), is shifted to the anodic region. At the same time, a
fairly strong influence of the substituent R1 on the reduc-
tion potential (ERed) was observed. Thus in the cases of
compounds 1d and 1e, the electron-withdrawing nitro and
cyano groups shift ERed to the cathodic region by 0.2 and
0.16 V relative to 1a, respectively. A bromine atom causes
no significant effect, while a methoxy group slightly shifts
the ERed value to the anodic region (by 0.06 V as compared
to compound 1a).
In the case of simultaneous presence of electron-do-
nating (at the position 3) and electron-withdrawing (at
the position 1) substituents at the verdazyl moiety, a more
pronounced increase in the "electrochemical gap" value
was observed due to shifting the EOx value to the cathodic
region by the MeO-substituted aromatic ring at the posi-
tion 3 of verdazyl with simultaneous shifting the EOx value
to the anodic region by the NO2- and CN-substituted
aromatic rings, thereby increasing the difference between
the highest oxidation and reduction potential values.
A reverse "distribution" of the substituents results in nar-
rowing the "electrochemical gap", since the electron-
withdrawing groups (NO2 and CN) at the aromatic ring
linked to the position 3 of verdazyl shift the EOx value to
the anodic region, while the electron-donating methoxy
group at the aromatic ring shifts it to the cathodic region,
thereby decreasing the difference between the highest
oxidation and reduction potential values.
The substituent at the aromatic ring linked to the posi-
tion 1 of the verdazyl moiety causes a more pronounced
effect on the oxidation and much weaker one on the reduc-
tion. At this end, electron-donating substituents facilitate
the oxidation process, shifting EOx to the cathodic region
by 0.06 and 0.11 V as compared to compound 1a in the
cases of 1f (R1 = H, R2 = Br) and 1g (R1 = H, R2 = МеО),
respectively. Electron-withdrawing substituents hinder the
oxidation, shifting EOx to the anodic region by 0.10 and
0.07 V relative to 1a in the cases of 1h (R1 = H, R2 = NO2)
and 1i (R1 = H, R2 = CN). The bromine atom does not
affect the reduction process, while the methoxy group
slightly shifts the ERed value to the anodic region (by 0.05
V as compared to compound 1a). The electron-withdraw-
ing groups facilitate the reduction, shifting ERed to the
cathodic region by 0.16 and 0.14 V as compared to 1a for
1h (R1 = H, R2 = NO2) and 1i (R1 = H, R2 = CN).
Considering all the acquired results, a magnitude of the
"electrochemical gap" (Eec = |EOx – ERed|) has been only
insignificantly changed for verdazyl radicals 1b—i as com-
pared to radical 1a. Figure 3 shows the CV curves recorded
for verdazyls 1a,k,m.
In summary, in the present work the disubstituted
triarylverdazyl radicals exhibiting record values of the
"electrochemical gap" were synthesized. The magnitude
of "electrochemical gap" affects a capacity of the battery
based on neutral radicals,18 thus the radicals possessing
a significant difference between the electrochemical po-
tentials of oxidation and reduction can be employed in
organic batteries, allowing one to significantly increase
the capacity of energy storage devices.
Experimental
The solvents were dried and distilled before use. All the re-
were agents were purchased from Sigma-Aldrich Rus and Merck
and used as received. IR spectra were recorded in KBr pellets
using an InfraLUM FT-02 Fourier spectrometer. 1H and 13C NMR
spectra were recorded in CDCl3 on a JNM-ECX400 spectrometer
(JEOL) operating at the frequencies of 400.1 and 100.6 MHz,
respectively.
Elemental analysis was performed using a Vario MICRO
CHNS-analyzer. Analytical TLC was performed on Silufol
UV-245 plates using benzene or dichloromethane—ethyl acetate
(2 : 1) as the eluents; the spots of the compounds were visualized
by iodine vapors. Column chromatography was carried out on
L40/100  silica gel using benzene as the eluent. Melting points
were measured in sealed glass capillaries on a MP-50 apparatus
(Mettler Toledo, Switzerland). Electron impact mass spectra
were recorded on a Konik RBK-HRGC 5000B—MSQ12 spectro-
meter (Spain); ionization potential was 70 eV. UV–Vis spectra
were recorded on a Shimadzu UV-2600 spectrophotometer (in
benzene), and EPR spectra were recorded on an ESP 70-03 XD/2
EPR spectrometer.
The molecular geometry was completely optimized without
any symmetry restrictions, taking into account the influence of
solvent (MeCN) within the framework of polarized continuum
model (PCM),14 wherein a cavity containing the dissolved par-
ticle is an ensemble of the intersected atomic spheres of certain
radius. The dielectric constant inside the cavity is equal to the
dielectric constant in vacuo and outside the cavity, to that of the
solvent. Data for the solvent consideration (dielectric constant,
I/A
20
1k
10
1m
0
1a
–10
–20
–1.5
–1.0
–0.5
0
0.5 E/V
РFig. 3. CV curves recorded for compounds 1a,k,m. The solvent
was MeCN, the supporting electrolyte was 0.1 М Bu4NBF4, the
potential scan rate was 200 mW s–1
.

Triarylverdazyl radicals
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 7, July, 2020
1325
molecule radius) were taken from the online source (https://
www.scm.com/doc/ADF/Input/COSMO.html). To confirm the
achievement of true minimum during the geometry optimization,
the frequencies of normal vibrations were calculated. The absence
of imaginary frequencies in the vibrational spectrum of the op-
timized structure indicates that the resulting structure corresponds
to a minimum on the full surface of potential energy.
300 [M]+ (30), 285 (25), 272 (33), 246 (34), 194 (100), 167 (40),
103 (40), 63 (50). Found (%): C, 76.16; H, 5.54; N, 18.36.
C19H16N4. Calculated (%): C, 75.98; H, 5.37; N, 18.65.
3-(4-Bromophenyl)-1,5-diphenylformazan (2b). Yield 2.12 g
(56%), m.p. 201—202 C (MeOH). 1H NMR (CDCl3), : 15.41
(s, 1 H, NH); 8.21 (d, 2 H, J = 8.9 Hz); 8.02 (d, 2 H, J = 8.9 Hz);
7.75 (d, 4 H, J = 8.0 Hz); 7.60—7.25 (m, 6 H). 13C NMR
(CDCl3), : 153.0, 147.1, 145.0, 137.4, 132.1, 131.7, 129.2, 129.2,
127.1, 124.7, 124.0, 122.2, 114.5. IR (KBr), /cm–1: 1589 (s),
1488 (s), 1396 (s), 1232 (s), 1019 (s), 756 (s). UV–Vis (CH2Cl2),
max/nm (): 274 (18100), 310 (25000), 492 (16400). MS (EI,
70 eV), m/z (Irel (%)): 380 [M + 1]+ (22), 378 [M – 1]+ (22),
350 (25), 352 (25), 299 (80), 272 (100), 270 (98), 223 (38),
103 (80), 65 (60). Found (%): C, 76.16; H, 5.54; N, 18.36.
C19H15BrN4. Calculated (%): C, 75.98; H, 5.37; N, 18.65.
3-(4-Methoxyphenyl)-1,5-diphenylformazan (2с). Yield 1.48 g
(45%), m.p. 172—173 C (MeOH). 1H NMR (CDCl3), : 15.45
(s, 1 H, NH); 8.31 (d, 2 H, J = 8.9 Hz); 8.02 (d, 2 H, J = 8.9 Hz);
7.76 (d, 4 H, J = 8.0 Hz); 7.58—7.27 (m, 6 H); 3.85 (s, 3 Н,
CH3O). 13C NMR (CDCl3), : 160.3, 154.1, 148.2, 145.4, 131.8,
129.9, 129.2, 128.3, 127.0, 124.8, 123.1, 120.4, 113.6, 55.8. IR
(KBr), /cm–1: 1597.2, 1512.1, 1454.2, 1358.7, 1250.2, 1226.0,
1172.4. UV–Vis (CH2Cl2), max/nm (): 279 (20700), 308
(27400), 522 (15900). MS (EI, 70 eV), m/z (Irel (%)): 330 [M]+
(10), 315 (37), 238 (40), 226 (58), 224 (55), 93 (100), 65 (30).
Found (%): C, 72.36; H, 5.51; N, 17.01. C20H18N4O. Calculat-
ed (%): C, 72.71; H, 5.49; N, 16.96.
3-(4-Nitrophenyl)-1,5-diphenylformazan (2d). Yield 1.52 g
(44%), m.p. 207—209 C (MeOH). 1H NMR (CDCl3), : 15.80
(s, 1 H, NH); 8.41 (d, 2 H, J = 8.9 Hz); 7.77 (d, 2 H, J = 8.0 Hz);
7.58—7.27 (m, 10 H). 13C NMR (CDCl3), : 155, 152, 149, 144,
141, 130, 129, 128, 125, 129, 121, 114. IR (KBr), /cm–1: 1597.2,
1527.1 (NO2), 1454.5, 1350.7 (NO2), 1242.2, 740.2. UV–Vis
(CH2Cl2), max/nm (): 266 (20200), 419 (17000), 490 (20900).
MS (EI, 70 eV), m/z (Irel (%)): 345 [M]+ (56), 328 (46), 299 (66),
241 (100), 105 (62). Found (%): C, 65.96; H, 4.41; N, 20.21.
C19H15N5O2. Calculated (%): C, 66.08; H, 4.38; N, 20.28.
3-(4-Cyanophenyl)-1,5-diphenylformazan (2e). Yield 1.30 g
(41%), m.p. 177—178 C (MeOH). 1H NMR (CDCl3), : 15.78
(s, 1 H, NH); 8.37 (d, 2 H, J = 8.9 Hz); 7.65 (d, 2 H, J = 8.0 Hz);
7.58—7.27 (m, 10 H). 13C NMR (CDCl3), : 150.9, 148.3, 145.5,
141.4, 139.7, 127.8, 126.5, 125.3, 123.5, 121.6, 120.8, 119.6,
118.6 (CN), 113.1. IR (KBr), /cm–1: 2255 (CN), 1598, 1451,
1243, 741. UV–Vis (CH2Cl2), max/nm (): 272 (30200), 338
(27000), 488 (21000). MS (EI, 70 eV), m/z (Irel (%)): 325 [M]+
(28), 297 (31), 271 (22), 219 (100), 192 (40), 105 (40). Found (%):
C, 73.96; H, 4.61; N, 21.41. C20H15N5. Calculated (%): C, 73.83;
H, 4.65; N, 21.52.
Electrochemical data were acquired by the cyclic voltammetry
in acetonitrile solution (0.1 M Bu4NBF4 as the supporting elec-
trolyte) in an electrochemical cell (5 mL) using a Gamry po-
tentiostat (Canada); the concentration of test compounds was
0.001 mol L–1. A glassy carbon (GC) electrode with S = 0.125 cm2
was used as the working electrode. The electrode was carefully
polished and washed before measurements. A platinum electrode
was the auxiliary one, and a standard silver chloride electrode
(E = 0.33 V in MeCN vs. the Fc/Fc+ couple) was the reference.
All the solutions were deaerated before use by passing an Ar flow.
Phenylhydrazones 3a—e (general procedure). A solution of
the corresponding aldehyde 4a—e (0.02 mol) in 1,4-dioxane
(25 mL) was added by small portions under stirring to a solution
of phenylhydrazine hydrochloride (4.34 g, 0.03 mol) and an-
hydrous sodium acetate (2.46 g) in water (70 mL). The mixture
was stirred at room temperature for ~20—60 min. The precipitate
formed was filtered off, washed on the filter with water, and dried
in air. The target products were purified by recrystallization. The
melting points and spectral characteristics of the obtained com-
pounds 3a—e coincide with those reported previously.
1-Benzylidene-2-phenylhydrazine (3a). Yield 4.19 g (73%),
m.p. 155—156 C (EtOH).19
1-(4-Bromobenzylidene)-2-phenylhydrazine (3b). Yield 5.86 g
(71%), m.p. 116—117 C (EtOH).19
1-(4-Methoxybenzylidene)-2-phenylhydrazine (3c). Yield 4.42 g
(65%), m.p. 119—120 C (EtOH).19
1-(4-Nitrobenzylidene)-2-phenylhydrazine (3d). Yield 5.43 g
(75%), m.p. 158—159 C (EtOH).20
1-(4-Cyanobenzylidene)-2-phenylhydrazine (3e). Yield 4.72 g
(71%), m.p. 177—178 C (EtOH).19
Formazans 2a—m (general procedure). Aryldiazonium salts
5a—d were synthesized preliminarily as follows. An aqueous
solution of NaNO2 (3 mL, 0.011 mol) and concentrated hydro-
chloric acid (3.5 mL) were subsequently added to the correspond-
ing aniline 6a—e (0.011 mol) at 0—5 C. The resulting suspension
of aryldiazonium salt 5a—e was added in small portions to a stirred
and cooled (–5–10 C) solution of phenylhydrazone 3a—e
(0.01 mol) in a mixture of pyridine (5 mL) and DMF (10 mL),
at the addition rate to maintain the temperature of the reaction
mixture below 5 C. The mixture became intensively dark
cherry colored. The mixture was kept at ~5 C for 3 h and at
room temperature for 12 h, diluted with water (20 mL), and
extracted with CH2Cl2 (320 mL). The combined organic ex-
tracts were washed with water (3×10 mL), passed through
a layer of neutral Al2O3, and the solvent was evaporated in vacuo.
The resulting product was triturated with methanol.
1,3,5-Triphenylformazan (2a). Yield 1.97 g (66%), m.p.
148—150 C (MeOH). 1H NMR (CDCl3), : 15.38 (s, 1 H, NH);
8.15 (d, 2 H, J = 8.0 Hz); 7.73 (d, 4 H, J = 8.0 Hz); 7.60—7.25
(m, 9 H). 13C NMR (CDCl3), : 148.5, 141.7, 138.0, 130.4,
130.0, 129.7, 129.6, 129.5, 129.0, 128.4, 128.2, 128.1, 126.4,
122.7, 119.3. IR (KBr), /cm–1: 1597 (s), 1512 (s), 1493 (s),
1235 (s), 1018 (s), 752 (s). UV–Vis (CH2Cl2), max/nm (): 270
(17250), 300 (22000), 488 (14250). MS (EI, 70 eV), m/z (Irel (%)):
1-(4-Bromophenyl)-3,5-diphenylformazan (2f). Yield 2.20 g
(58%), m.p. 189—190 C (MeOH). 1H NMR (CDCl3), : 15.38
(s, 1 H, NH); 8.21 (d, 2 H, J = 8.0 Hz); 8.05 (d, 2 H, J = 8.9 Hz);
7.65 (d, 2 H, J = 8.9 Hz); 7.55—7.25 (m, 8 H). 13C NMR
(CDCl3), : 153.3, 147.2, 144.8, 136.4, 134.7, 131.6, 130.4, 129.3,
128.1, 125.7, 124.0, 122.2, 113.9. IR (KBr), /cm–1: 1588, 1488,
1393, 1230, 1010, 746. UV–Vis (CH2Cl2), max/nm (): 240
(15100), 276 (18300), 320 (29600), 490 (16000). MS (EI, 70 eV),
m/z (Irel (%)): 380 [M + 1]+ (12), 378 [M – 1]+ (12), 352 (22),
350 (22), 299 (40), 194 (100), 167 (43), 91 (65). Found (%):
C, 76.16; H, 5.54; N, 18.36. C19H15BrN4. Calculated (%):
C, 75.98; H, 5.37; N, 18.65.

Kostryukov et al.
1326
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 7, July, 2020
1-(4-Methoxyphenyl)-3,5-diphenylformazan (2g). Yield 1.33 g
(40%), m.p. 119—120 C (MeOH). 1H NMR (CDCl3), : 15.40
(d, 1 H, NH); 8.15 (d, 2 H, J = 8.0 Hz); 7.92 (d, 2 H, J = 8.9 Hz);
7.60—7.15 (m, 10 H); 3.82 (s, 3 Н, CH3O). 13C NMR (CDCl3),
: 160.6, 153.1, 144.2, 140.4, 135.4, 131.8, 129.6, 128.8, 128.0,
125.0, 122.1, 115.1, 113.6, 55.7 (CH3O). IR (KBr), /cm–1: 1598
(s), 1513 (s), 1454, 1347, 1249, 1225, 1176. UV–Vis (CH2Cl2),
max/nm (): 280 (20200), 310 (25100), 520 (14700). MS (EI,
70 eV), m/z (Irel (%)): 330 [M]+ (10), 303 (25), 276 (20), 195 (55),
122 (100), 104 (51), 93 (70). Found (%): C, 72.36; H, 5.51;
N, 17.01. C20H18N4O. Calculated (%): C, 72.71; H, 5.49;
N, 16.96.
1-(4-Nitrophenyl)-3,5-diphenylformazan (2h). Yield 1.86 g
(54%), m.p. 197—198 C (MeOH). 1H NMR (CDCl3), : 15.40
(s, 1 H, NH); 8.33 (d, 2 H, J = 8.9 Hz); 7.79 (d, 2 H, J = 8.0 Hz);
7.58—7.27 (m, 10 H). 13C NMR (CDCl3), : 154.8, 153.3, 148.2,
144.2, 135.7, 130.1, 128.8, 127.9, 126.8, 124.7, 123.6, 122.4,
113.9. IR (KBr), /cm–1: 1598, 1538 (NO2), 1453, 1351 (NO2),
1244, 741. UV–Vis (CH2Cl2), max/nm (): 266 (20800), 424
(17100), 490 (20200). MS (EI, 70 eV), m/z (Irel (%)): 345 [M]+ (10),
317 (12), 299 (14), 224 (19), 195 (100), 123 (12). Found (%):
C, 65.96; H, 4.41; N, 20.21. C19H15N5O2. Calculated (%):
C, 66.08; H, 4.38; N, 20.28.
1-(4-Methoxyphenyl)-3-(4-nitrophenyl)-5-phenylformazan
(2l). Yield 1.01 g (27%), m.p. 168—169 C (MeOH). H NMR
1
(CDCl3), : 14.75 (s, 1 H, NH); 8.23 (d, 2 H, J = 8.9 Hz); 7.70
(d, 2 H, J = 8.9 Hz); 7.65 (d, 2 H, J = 8.5 Hz); 7.55 (d, 2 H,
J = 8.0 Hz); 7.35 (t, 1 H, J = 7.1 Hz); 7.12 (d, 2 H, J = 8.0 Hz);
6.99 (d, 2 H, J = 8.5 Hz); 3.82 (s, 3 H, CH3O). 13C NMR
(CDCl3), : 160.3, 155.8, 150.3, 146.2, 142.1, 132.5, 129.4, 127.5,
124.7, 123.6, 114.9, 113.9, 55.6 (CH3O). IR (KBr), /cm–1
:
1599, 1528, 1454, 1351, 1248, 1226, 741. UV–Vis (CH2Cl2),
max/nm (): 269 (22400), 424 (16000), 490 (19200). MS (EI,
70 eV), m/z (Irel (%)): 375 [M]+ (11), 360 (38), 347 (24), 254 (34),
241 (52), 121 (100). Found (%): C, 63.92; H, 4.55; N, 18.64.
C20H17N5O3. Calculated (%): C, 63.99; H, 4.56; N, 18.66.
3-(4-Cyanophenyl)-1-(4-methoxyphenyl)-5-phenylformazan
(2m). Yield 1.10 g (31%), m.p. 135—137 C (MeOH). 1H NMR
(CDCl3), : 14.65 (s, 1 H, NH); 8.20 (d, 2 H, J = 8.5 Hz);
7.58—7.69 (m, 6 H); 7.30—7.45 (m, 3 H); 7.05 (d, 2 H, J = 8.5 Hz);
3.80 (s, 3 H, CH3O). 13C NMR (CDCl3), : 160.2, 155.1, 150.4,
145.3, 133.5, 131.2, 129.3, 127.0, 124.7, 123.1, 118.6 (CN),
114.8, 114.0, 106.4, 55.5 (CH3O). IR (KBr), /cm–1: 2224 (CN),
1598, 1509, 1454, 1244, 1223, 742. UV–Vis (CH2Cl2), max/nm
(): 273 (12900), 424 (10500), 490 (15800). MS (EI, 70 eV), m/z
(Irel (%)): 355 [M]+ (10), 340 (22), 327 (25), 234 (49), 220 (56),
129 (55), 121 (100), 93 (71). Found (%): C, 70.88; H, 4.89;
N, 19.65. C21H17N5O. Calculated (%): C, 70.97; H, 4.82;
N, 19.71.
Verdazyls 1a—m (general procedure). A mixture of the cor-
responding formazan 2a—m (1 mol), DMF (50 mL), potassium
hydrosulfate (1.36 g, 0.01 mol), and paraformaldehyde (1.5 g)
was stirred at room temperature for 24 h. The reaction mixture
was filtered, and formaldehyde solution (37%, 12 mL) and 2 M
NaOH solution (10mL) were subsequently added dropwise to
the filtrate under stirring. The resulting mixture was stirred at
50 C for 1 h and cooled down to room temperature, and 2 M
NaOH solution (~10 mL) was added to pH 10. The mixture was
stirred under continuous air bubbling for 0.5—24 h (depending
on the nature of starting formazan) and then diluted with water
(100 mL). The product was extracted with benzene (350 mL);
the combined extracts were washed with water (350 mL), dried
over anhydrous MgSO4, and filtered through a layer of silica gel.
The solvent was evaporated in vacuo. The resulting product was
crystallized from methanol.
1,3,5-Triphenylverdazyl (1a). The duration of air oxidation
was 0.5 h. Yield 0.173 g (55%), m.p. 139—140 C (MeOH). IR
(KBr), /cm–1: 1585, 1489, 1323, 1265, 1207, 1146, 752. UV–Vis
(CH2Cl2), max/nm (): 273 (62300), 405 (7500), 716 (3600).
MS (EI, 70 eV), m/z (Irel (%)): 313 [M]+ (32), 299 (15), 286 (16),
209 (60), 104 (100). Found (%): C, 76.92; H, 5.51; N, 17.74.
C20H17N4. Calculated (%): C, 76.65; H, 5.47; N, 17.88.
3-(4-Bromophenyl)-1,5-diphenylverdazyl (1b). The duration
of air oxidation was 2 h. Yield 0.197 g (50%), m.p. 162—163 C
(MeOH). IR (KBr), /cm–1: 1597, 1505, 1486, 1374, 1234, 1207,
1151, 826. UV–Vis (CH2Cl2), max/nm (): 280 (65300), 410
(8500), 720 (3900). MS (EI, 70 eV), m/z (Irel (%)): 391 [M – 1]+
(27), 367 (60), 291 (55), 219 (67), 183 (82), 121 (80), 106 (100).
Found (%): C, 61.32; H, 4.21; N, 14.41. C20H16BrN4. Calculat-
ed (%): C, 61.24; H, 4.11; N, 14.28.
1-(4-Cyanophenyl)-3,5-diphenylformazan (2i). Yield 1.79 g
(55%), m.p. 173—175 C (MeOH). 1H NMR (CDCl3), : 15.40
(s, 1 H, NH); 8.27 (dd, 2 H, J = 8.9 Hz, J = 1.5 Hz); 8.09 (dd,
2 H, J = 8.0 Hz, J = 1.5 Hz); 7.65—7.30 (m, 10 H). 13C NMR
(CDCl3), : 155.1, 150.3, 146.2, 136.2, 131.9, 129.1, 128.5, 127.3,
125.4, 124.1, 123.0, 118.4 (CN), 113.9, 106.3. IR (KBr), /cm–1
:
2254 (CN), 1598, 1528, 1453, 1244, 741. UV–Vis (CH2Cl2),
max/nm (): 273 (33400), 340 (6000), 489 (21500). MS (EI,
70 eV), m/z (Irel (%)): 325 [M]+ (22), 310 (30), 297 (43), 271 (30),
220 (61), 194 (100), 167 (40), 130 (50). Found (%): C, 73.91;
H, 4.68; N, 21.47. C20H15N5. Calculated (%): C, 73.83; H, 4.65;
N, 21.52.
3-(4-Methoxyphenyl)-1-(4-nitrophenyl)-5-phenylformazan
1
(2j). Yield 2.14 g (57%), m.p. 171—172 C (MeOH). H NMR
(CDCl3), : 14.75 (s, 1 H, NH); 8.23 (d, 2 H, J = 8.9 Hz); 7.70
(d, 2 H, J = 8.9 Hz); 7.65 (d, 2 H, J = 8.5 Hz); 7.55 (d, 2 H,
J = 8.0 Hz); 7.35 (t, 1 H, J = 7.1 Hz); 7.12 (d, 2 H, J = 8.0 Hz);
6.99 (d, 2 H, J = 8.5 Hz); 3.82 (s, 3 H, CH3O). 13C NMR
(CDCl3), : 160.3, 155.8, 150.3, 146.2, 142.1, 132.5, 129.4, 127.5,
124.7, 123.6, 114.9, 113.9, 55.6 (CH3O). IR (KBr), /cm–1
:
15989, 1528, 1454, 1351, 1244, 1226, 741. UV–Vis (CH2Cl2),
max/nm (): 270 (20300), 422 (16500), 490 (19600). MS (EI,
70 eV), m/z (Irel (%)): 375 [M]+ (12), 360 (40), 347 (22), 238 (35),
224 (52), 138 (100). Found (%): C, 63.96; H, 4.61; N, 18.72.
C
20H17N5O3. Calculated (%): C, 63.99; H, 4.56; N, 18.66.
1-(4-Cyanophenyl)-3-(4-methoxyphenyl)-5-phenylformazan
(2k). Yield 1.66 g (45%), m.p. 142—144 C (MeOH). 1H NMR
(CDCl3), : 14.66 (s, 1 H, NH); 8.20 (d, 2 H, J = 8.5 Hz);
7.58—7.69 (m, 6 H); 7.30—7.45 (m, 3 H); 7.05 (d, 2 H, J = 8.5 Hz);
3.80 (s, 3 H, CH3O). 13C NMR (CDCl3), : 160.2, 155.1, 150.4,
145.3, 133.5, 131.2, 129.3, 127.0, 124.7, 123.1, 118.6 (CN),
114.8, 114.0, 106.4, 55.5 (CH3O). IR (KBr), /cm–1: 2223 (CN),
1598, 1509, 1455, 1245, 1223, 743. UV–Vis (CH2Cl2), max/nm
(): 273 (12900), 424 (10500), 490 (15800). MS (EI, 70 eV), m/z
(Irel (%)): 355 [M]+ (12), 340 (38), 327 (35), 250 (45), 238 (37),
226 (68), 118 (90), 93 (100). Found (%): C, 70.88; H, 4.89;
N, 19.65. C21H17N5O. Calculated (%): C, 70.97; H, 4.82;
N, 19.71.
3-(4-Methoxyphenyl)-1,5-diphenylverdazyl (1c). The dura-
tion of air oxidation was 3 h. Yield 0.197 g (50%), m.p. 122—
123 C (MeOH). IR (KBr), /cm–1: 1590, 1489, 1401, 1234,
1207, 1155, 826. UV–Vis (CH2Cl2), max/nm (): 283 (27300),

Triarylverdazyl radicals
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 7, July, 2020
1327
323 (11500), 390 (7300), 735 (3400). MS (EI, 70 eV), m/z
(Irel (%)): 343 [M]+ (25), 315 (43), 301 (45), 277 (38), 241 (61),
212 (44), 136 (100). Found (%): C, 73.51; H, 5.51; N, 16.29.
C21H19N4O. Calculated (%): C, 73.45; H, 5.58; N, 16.32.
3-(4-Nitrophenyl)-1,5-diphenylverdazyl (1d). The duration
of air oxidation was 5 h. Yield 0.161 g (45%), m.p. 153—155 C
(MeOH). IR (KBr), /cm–1: 1590, 1534 (NO2), 1497, 1349
(NO2), 1207, 1154, 737. UV–Vis (CH2Cl2), max/nm (): 290
(47300), 345 (13500), 740 (9400). MS (EI, 70 eV), m/z (Irel (%)):
358 [M]+ (19), 334 (50), 332 (46), 275 (49), 261 (42), 219 (35),
151 (100). Found (%): C, 67.11; H, 4.53; N, 19.49. C20H16N5O2.
Calculated (%): C, 67.03; H, 4.50; N, 19.54.
3-(4-Cyanophenyl)-1,5-diphenylverdazyl (1e). The duration
of air oxidation was 5 h. Yield 0.149 g (44%), m.p. 137—138 C
(MeOH). IR (KBr), /cm–1: 2243 (CN), 1587, 1495, 1210, 1152,
739. UV–Vis (CH2Cl2), max/nm (): 288 (35300), 344 (11500),
735 (8400). MS (EI, 70 eV), m/z (Irel (%)): 338 [M]+ (30),
312 (70), 234 (60), 212 (42), 131 (100), 121 (45). Found (%):
C, 74.61; H, 4.79; N, 20.62. C21H16N5. Calculated (%): C, 74.54;
H, 4.77; N, 20.70.
1-(4-Bromophenyl)-3,5-diphenylverdazyl (1f). The duration
of air oxidation was 3 h. Yield 0.157 g (40%), m.p. 145—146 C
(MeOH). IR (KBr), /cm–1: 1598, 1508, 1484, 1373, 1236, 1209,
1156, 824. UV–Vis (CH2Cl2), max/nm (): 278 (35300), 405
(9500), 715 (4100). MS (EI, 70 eV), m/z (Irel (%)): 391 [M – 1]+
(10), 367 (45), 290 (37), 261 (27), 183 (44), 121 (40), 106 (100).
Found (%): C, 61.32; H, 4.21; N, 14.41. C20H16BrN4. Calculat-
ed (%): C, 61.24; H, 4.11; N, 14.28.
1-(4-Cyanophenyl)-3-(4-methoxyphenyl)-5-phenylverdazyl
(1k). The duration of air oxidation was 24 h. Yield 0.1915 g (52%),
m.p. 83—84 C (MeOH). IR (KBr), /cm–1: 2241 (CN), 1588,
1494, 1205, 1158, 820. UV–Vis (CH2Cl2), max/nm (): 282
(48400), 330 (11500), 393 (8300), 742 (4000). MS (EI, 70 eV),
m/z (Irel (%)): 368 [M]+ (14), 344 (60), 342 (85), 340 (76), 326
(54), 237 (74), 146 (63), 133 (86), 131 (100), 106 (75). Found (%):
C, 71.81; H, 4.88; N, 19.09. C22H18N5O. Calculated (%):
C, 71.72; H, 4.92; N, 19.01.
1-(4-Methoxyphenyl)-3-(4-nitrophenyl)-5-phenylverdazyl
(1l). The duration of air oxidation was 24 h. Yield 0.113 g (29%),
m.p. 111—113 C (MeOH). IR (KBr), /cm–1: 1590, 1534
(NO2), 1496, 1350 (NO2), 1210, 1155, 822. UV–Vis (CH2Cl2),
max/nm (): 285 (46500), 325 (13800), 392 (7200), 735 (3500).
MS (EI, 70 eV), m/z (Irel (%)): 388 [M]+ (11), 372 (28), 362
(81), 360 (100), 343 (60), 291 (46), 284 (49), 257 (48), 136 (60).
Found (%): C, 64.88; H, 4.71; N, 18.09. C21H18N5O3. Calculat-
ed (%): C, 64.94; H, 4.67; N, 18.03.
3-(4-Cyanophenyl)-1-(4-methoxyphenyl)-5-phenylverdazyl
(1m). The duration of air oxidation was 24 h. Yield 0.141 g (41%),
m.p. 91—93 C (MeOH). IR (KBr), /cm–1: 2223 (CN), 1592,
1489, 1401, 1234, 1207, 1155, 826. UV–Vis (CH2Cl2), max/nm
(): 286 (37500), 330 (12500), 390 (7500), 730 (4200). MS (EI,
70 eV), m/z (Irel (%)): 368 [M]+ (14), 342 (78), 326 (51), 300 (38),
236 (69), 151 (49), 131 (100), 121 (84), 106 (89). Found (%):
C, 71.77; H, 4.98; N, 19.08. C22H18N5O. Calculated (%):
C, 71.72; H, 4.92; N, 19.01.
This work was financially supported by the Russian
Foundation for Basic Research (project No. 18-43-
130002).
1-(4-Methoxyphenyl)-3,5-diphenylverdazyl (1g). The dura-
tion of air oxidation was 3 h. Yield 0.197 g (50%), m.p. 71—72 C
(MeOH). IR (KBr), /cm–1: 1589, 1489, 1400, 1234, 1208, 1106,
826. UV–Vis (CH2Cl2), max/nm (): 280 (28400), 319 (10500),
388 (7300), 730 (3200). MS (EI, 70 eV), m/z (Irel (%)): 343 [M]+
(15), 317 (100), 315 (50), 301 (60), 267 (50), 242 (70), 151 (44),
136 (90). Found (%): C, 73.51; H, 5.51; N, 16.29. C21H19N4O.
Calculated (%): C, 73.45; H, 5.58; N, 16.32.
1-(4-Nitrophenyl)-3,5-diphenylverdazyl (1h). The duration
of air oxidation was 10 h. Yield 0.147 g (41%), m.p. 120—122 C
(MeOH). IR (KBr), /cm–1: 1590, 1532 (NO2), 1495, 1350
(NO2), 1209, 1152, 741. UV–Vis (CH2Cl2), max/nm (): 293
(45300), 350 (14500), 738 (8400). MS (EI, 70 eV), m/z (Irel (%)):
358 [M]+ (21), 334 (100), 308 (48), 275 (59), 261 (80), 257 (82),
151 (60), 106 (100). Found (%): C, 67.11; H, 4.53; N, 19.49.
C20H16N5O2. Calculated (%): C, 67.03; H, 4.50; N, 19.54.
1-(4-Cyanophenyl)-3,5-diphenylverdazyl (1i). The duration
of air oxidation was 10 h. Yield 0.121 g (36%), m.p. 117—119 C
(MeOH). IR (KBr), /cm–1: 2241 (CN), 1590, 1495, 1209, 1152,
741. UV–Vis (CH2Cl2), max/nm (): 287 (34300), 334 (12500),
741 (9400). MS (EI, 70 eV), m/z (Irel (%)): 338 [M]+ (17), 324
(80), 312 (86), 286 (35), 237 (74), 146 (48), 131 (85), 106 (100).
Found (%): C, 74.48; H, 4.73; N, 20.59. C21H15N5. Calculated
(%): C, 74.54; H, 4.77; N, 20.70.
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m.p. 90—92 C (MeOH). IR (KBr), /cm–1: 1590, 1522, 1495,
1346, 1208, 1155, 824. UV–Vis (CH2Cl2), max/nm (): 280
(58400), 325 (13500), 390 (7300), 740 (4200). MS (EI, 70 eV),
m/z (Irel (%)): 388 [M]+ (10), 372 (30), 360 (100), 343 (69), 317
(42), 291 (46), 244 (50), 224 (60), 134 (70), 106 (50). Found (%):
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Received August 28, 2019;
in revised form November 5, 2019;
accepted January 9, 2020