Synthesis of radicals underlain by 1,3-bis(4,5-diphenylimidazol-2-yl) benzene

Article abstract of DOI:10.1134/S1070428011110121

A dimer of a monoradical was obtained proceeding from 1,3-bis(4,5- diphenylimidazol-2-yl)benzene and the rate constants were measured of the dissociation of dimers of 1,3-bis(4,5-diphenylimidazol-2-yl)benzene mono- and biradicals.

Full text of DOI:10.1134/S1070428011110121

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 11, pp. 1723−1726. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © B.S. Tanaseichuk, M.K. Pryanichnikova, A.A. Burtasov, A.V. Dolganov, O.A. Tsyplenkova, A.M. Lukin, 2011, published in Zhurnal
Organicheskoi Khimii, 2011, Vol. 47, No. 11, pp. 1687−1690.
Synthesis of Radicals Underlain
by 1,3-Bis(4,5-diphenylimidazol-2-yl)benzene
B. S. Tanaseichuka, M. K. Pryanichnikovaa, A. A. Burtasova,
A. V. Dolganova, O. A. Tsyplenkovaa, and A. M. Lukina
aOgarev Mordovskii State University, Saransk, 430005 Russia
e-mail: mordorgchem@mail.ru
bNesmeyanov Institute of Organoelemental Compounds, Russian Academy of Sciences, Moscow
Received February 11, 2011
Abstract—A dimer of a monoradical was obtained proceeding from 1,3-bis(4,5-diphenylimidazol-2-yl)benzene
and the rate constants were measured of the dissociation of dimers of 1,3-bis(4,5-diphenylimidazol-2-yl)benzene
mono- and biradicals.
DOI: 10.1134/S1070428011110121
Bisimidazoles and radicals based thereon up till
now remain a poorly studied division of the chemistry
of stable free radicals. The oxidation of 1,4-bis(4,5-
diphenylimidazol-2-yl)benzene is known to yield a qui-
none [1–4], 1,4-bis(4,5-diphenylimidazol-2-ylidene)
cyclohexa-2,5-diene, that according to the statement
of Sakaino et al. [2] is partially converted into a biradi-
cal on heating. Similarly the oxidation of 1,4-bis(2,5-
diphenylimidazol-4-yl)benzene also gives a quinoid
system [5]. However when in the benzene ring that
connects two imidazole rings the hydrogen atoms are
replaced by fluorine the quinone formed by the oxidation
of the bisimidazole can exist in the equilibrium with the
biradical [6]. This was found at the oxidation of 1,4-bis-
(4,5-diphenylimidazol-2-yl)-2,3,5,6-tetrafluorobenzene.
Thus obtained greenish-blue quinone on the crystalliza-
tion from benzene gave a colorless dimer, and this would
be possible only at the formation of a biradical [6]. The
formation of quinoid systems is common for compounds
where two imidazole rings are in the conjugation, and
the connecting central ring may be a benzene as well as
a five-membered heterocycle [4].
In keeping with the previously advanced hypothesis
connecting the relative stability of a radical and its
existence in the solution with the solvating ability of the
given solvent [8] it is presumable that if the oxidation
would be carried out in a solvent where the formed radical
is more soluble than the initial imidazole the oxidation
would result in the bisimidazolyl radical. For instance, the
oxidation of 1,3-bis(4,5-diphenylimidazol-2-yl)benzene
(I) with potassium ferricyanide in the alkaline medium
in the two-phase system benzene–water provided the
corresponding biradical II isolated and characterized
in the form of a dimer IIa [9]. The colored biradical II
formed by the oxidation of imidazole I is unstable and
becomes colorless giving dimer already in the course of
its formation.
If the formed radical is less soluble under the reaction
conditions than initial imidazole I the oxidation of
a single imidazole ring should result in the formation of
monoradical III insoluble under the given conditions that
would not suffer oxidation at the second imidazole ring.
To prove this assumption we synthesized monoradical
III by the oxidation of bisimidazole I [7] in the alkaline
water-alcohol solution of potassium ferricyanide. The
formed in the course of oxidation monoradical III
precipitated in the form of the so-called “piezochromic
dimer” IIIа [10, 11] with the appearance of a fine
crystalline violet powder whose color was due to the
presence of free imidazolyl radicals III. On dissolving
However when the two imidazole rings are not
conjugated it is possible to obtain both mono- and
biradical system depending on the conditions of the
imidazole oxidation. This can be demonstrated by an
example of 1,3-bis(4,5-diphenylimidazol-2-yl)benzene
(I) [7].
1723

TANASEICHUK et al.
1724
Scheme 1.
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
N
N
N
N
N
Ph
Ph
N
N
N
N
N
N
K3[Fe(CN)6]
KOH, H2O, benzene
H
N
N
HN
Ph
N
N
Ph
Ph
Ph
Ph
Ph
IIа
I
II
the “piezochromic dimer” IIIа in benzene the solution
got crimson and quickly lost the color originating from
the dissociation of dimer IIIа into radicals III followed
by their dimerization into a “photochromic dimer” IIIb
which was isolated in the crystalline form on evaporation
of the benzene solution (Scheme 2). “Piezochromic
dimer” is the product of the dimerization of imidazolyl
radicals by the atoms with the highest density of the
unpaired electron. The “photochromic dimer” originates
from the donor-acceptor dimerization [8].
structure of “photochromic” dimer IIa was established
earlier [9].
Dimer IIIb is light-green (λmax 301 nm) and gives
a weak singlet ESR signal both in solid state and in
benzene solution thus showing the presence of a small
concentration of nondimerized radicals III. It is likely to
be due to the spatial hindrances arising at the formation
of the corresponding dimers.
The formation of dimer IIIb was proved by its mass
spectrum, and the structure suggested basing on the data
of [9, 11–14] was confirmed by 13С NMR spectrum where
alongside with the signals characteristic of carbon atoms
of the benzene and imidazole rings signals appeared at
112.25 ppm belonging to atom С2 of 2Н-imidazole ring
The structures of dimers IIIa, IIIb we suggest by
the analogy to the structure of “piezochromic” and
“photochromic” dimers of triphenylimidazolyl [11,
12] and the XRD data for “photochromic” dimer of
4,5-diphenyl-2-о-chlorohenylimidazolyl [13, 14]. The
Scheme 1.
Ph
Ph
Ph
N
N
N
Ph
N
N
Ph PhN
K3[Fe(CN)6]
KOH, H2O, EtOH
H
HN
Ph
N
HN
Ph
N
HN
Ph
N
Ph
Ph
Ph
IIIа
I
Ph
N
Ph
Ph
N
N
N
Ph
Ph
N
Ph
N
H
N
N
Ph
Ph
Ph
IIIb
HN
Ph
N
N
HN
Ph
Ph
III
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 11 2011

SYNTHESIS OF RADICALS UNDERLAIN BY 1,3-BIS(4,5-DIPHENYLIMIDAZOL-2-YL)BENZENE
1725
Thermodynamic parameters of dissociation in toluene of radical dimers obtained from 1,3-bis(4,5-diphenylimidazol-2-yl)benzene
ΔS≠,
Rate constants of dissociation k × 105 s–1
ΔН≠, kJ mol−1
Еа,
kJ mol−1
J mol–1 K–1
Dimer nature
log А
55°С 60°С 65°С 70°С 75°С 80°С 85°С
70°С
Dimer of monoradical
IIIb
–
9.5
4.2
–
–
13.8 15.2 16.8
–
–
28.1
64.4
0.40
5.77
25.3
61.5
238.4
135.7
Dimer of biradical IIa
3.5
9.5
–
–
–
Dimer of 2,4,5-triphenyl-
imidazolyl
–
–
19.7 35.0 60.3
133.3
95.9
11.13
93.0
32.9
existing in the sp3–hybridization, and at 166.89 ppm
characteristic of С2 atom of the imidazole ring linked to
the benzene ring [9].
with KBr. UV spectra in visible region were taken on
spectrophotometers Specord UV vis and SF-16. ESR
spectra were registered on an instrument EPA 2M. Mass
spectra MALDI-TOF of positive ions were obtained
on an instrument Bruker Autoflex (UV laser, 336 nm).
The samples were placed on a nickel plate, matrix
was 2,5-dihydroxybenzoic acid. 13С NMR spectrum
was registered on a spectrometer Bruker AMX-400
(100 МHz) in DMSO-d6.
In order to estimate the effect of the structure of dimers
IIa and IIIb on the rate of their dissociation into radicals
we studied the kinetics of this process by the method of
radical traps using diphenylpicrylhydrazine (DPPH-Н)
[15] (see the table).
As seen from the table, the rate constants of the
dissociation of dimers IIa and IIIb as well as the
activation energies and the pre-exponential factors are
significantly smaller than the corresponding values for the
dimer of 2,4,5-triphenylimidazolyl [16]. This may be due
only to considerable steric hindrances from the presence
of two imidazole rings in the meta-position of the central
benzene ring distorting the planarity of the system of the
dimer molecule as is confirmed by the XRD data for dimer
IIa: The imidazole rings are turned with respect to the
central benzene ring by 82–83 deg [9].
1,3-Bis(4,5-diphenyl-1H-imidazol-2-yl)benzene (I)
and dimer 2,2'-(m-phenylene)bis(4,5-diphenylimidazol-
1-yl)biradical (IIa) were obtained by procedures [7, 9]
respectively.
2,2'-Bis[3-(4,5-diphenyl-1H-imidazol-2-yl)-
phenyl]-4,4',5,5'-tetraphenyl-4H,4'H-4,4'-bisimidazol
(“piezochromic” dimer of monoradical) (IIIа). To
a solution of 0.5 g (1 mmol) of bisimidazole I and 0.6 g
(1.5 mmol) of sodium hydroxide in 50 ml of ethanol at
constant stirring was added dropwise a solution of 1 g
(3 mmol) of potassium ferricyanide in 100 ml of water
within 1 h at 5–10°С. Then the reaction mixture was
stirred for 0.5 h at room temperature. The separated
light-violet fine crystalline precipitate was filtered off
and washed on the filter with water till neutral washings.
The product was dried in air. Yield 0.3 g (60%). Light-
violet crystals, mp 203–205°С. IR spectrum, ν, cm–1:
3630 (NH), 1604 (imidazole), 1550, 1535, 1527, 1512
(4H-imidazole).
To estimate the role of the steric factors and their
effect on the dissociation rate of dimers IIa and IIIb
thermodynamic parameters of this process at 70°С
were calculated. As seen from the table, the ΔS≠ values
indicate the governing influence of the steric factors on
the dissociation rate of the dimers. At the same time the
steric factor is more important for dimer IIIb than for
dimer IIa and triphenylimidazolyl dimer.
2-[3-(1-{2-[3-(4,5-Diphenyl-1H-imidazol-2-yl)-
phenyl]-4,5-diphenyl-2H-imidazol-2-yl}-4,5-diphenyl-
1H-imidazol-2-yl]phenyl)-4,5-diphenyl-1H-imidazol
(“photochromic” dimer of monoradical) (IIIb). A so-
lution of 0.51 g (0.5 mmol) of «piezochromic» dimer of
monoradical IIIа in 25 ml of benzene was evaporated in
a vacuum to the volume of 10 ml. The separated gray-
EXPERIMENTAL
The reaction products were analyzed by TLC
on Silufol UV-254 plates (eluents petroleum ether,
acetone, dichloromethane), development in iodine
vapor under UV irradiation IR spectra were recorded
on a spectrophotometer Infra-Lyum FT-02 from pellets
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 11 2011

TANASEICHUK et al.
1726
vol. 109, p. 19448.
6. Kikuchi, A., Iwahori, F., and Abo, J., J. Am. Chem. Soc.,
2004, vol. 126, p. 6526.
7. Krieg, B. and Maeckl, G., Macromol. Chem., 1967,
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8. Tanaseichuk, B.S., Zh. Org. Khim., 2004, vol. 40, p. 629.
9. Okada, K., Imamura, K., Oda, M., Kozaki, M.,
Morimoto, K.Y., Ishino, R., Tashiro, K., Chem. Lett.,
1998, 891.
green precipitate was filtered off and dried in air. Yield
0.5 g (100%). Gray-green crystals, mp 212–214°С. IR
spectrum, ν, cm–1: 3406 (NH), 1604 (2H-isoimidazole),
1558, 1500 (C2–N1 of imidazoles). 13С NMR spectrum,
δ, ppm: 112.25, 122.58, 125.18, 126.67, 126.77, 127.22,
127.43, 128.40, 128.82, 129.21, 129.71, 131.12, 131.94,
134.38, 135.39, 137.38, 137.54, 138.14, 138.58, 145.08,
145.32, 145.58, 148.06, 148.46, 166.89. Mass spectrum:
m/z 1027. Found, %: С 84.41; Н 5.02; N 10.31. C36H25N4.
Calculated, %: C 84.07; Н 4.81; N 10.89. М 1027.22.
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vol. 43, p. 429.
11. White, D.M., Sonnenberg, J. J. Am. Chem. Soc., 1966,
vol. 88, p. 3825.
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 11 2011