Solvatochromism of heteroaromatic compounds: XIV. 5-Amino-1-methyl-4-nitropyrazole and its analogs

Article abstract of DOI:10.1023/A:1015412205674

The solvatochromism of H complexes of 5-amino-1-methyl-4-nitropyrazole and 2-nitroaniline derivatives in aprotic protophilic media was described on a quantitative level with the aid of Kamlet-Taft empirical parameters. Specific solvation affects only one of the two long-wave bands, namely that corresponding to an electronic transition involving orbital electron density transfer from the H-bound nitrogen atom. When such transfer does not occur, which is typical of the second transition, specific solvatochromic effect is either weak or absent.

Full text of DOI:10.1023/A:1015412205674

Russian Journal of General Chemistry, Vol. 72, No. 3, 2002, pp. 449 455. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 3, 2002,
pp. 483 489.
Original Russian Text Copyright
2002 by Vokin, Shulunova, Krivoruchka, Krylova, Lopyrev, Turchaninov.
Solvatochromism of Heteroaromatic Compounds:
1
XIV. 5-Amino-1-methyl-4-nitropyrazole and Its Analogs
A. I. Vokin, A. M. Shulunova, I. G. Krivoruchka, O. V. Krylova,
V. A. Lopyrev, and V. K. Turchaninov
Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences, Irkutsk, Russia
Received July 18, 2000
Abstract The solvatochromism of H complexes of 5-amino-1-methyl-4-nitropyrazole and 2-nitroaniline
derivatives in aprotic protophilic media was described on a quantitative level with the aid of Kamlet Taft
empirical parameters. Specific solvation affects only one of the two long-wave bands, namely that correspond-
ing to an electronic transition involving orbital electron density transfer from the H-bound nitrogen atom.
When such transfer does not occur, which is typical of the second transition, specific solvatochromic effect is
either weak or absent.
Electronic absorption spectra of ortho-aminonitro-
pyrazoles, in particular of 4-amino-1-methyl-5-nitro-
pyrazole, are very similar to the spectrum of 2-nitro-
aniline. On the basis of this analogy Perevalov et al.
tributes most to the total charge on atoms of the
heteroring (the orbital electron density distribution,
calculated by the MNDO method, was given in [3]).
Taking into account that both long-wave absorption
bands of ortho-aminonitropyrazoles are well-resolved
[2, 3] assigned the absorption bands and established
the nature of the coorresponding electronic transitions
with a sufficient reliability. The data of IR spectros-
copy indicated formation of intramolecular hydrogen
and fairly intense, we have analyzed spectral
*
effects of specific solvation on
transitions
involving different intramolecular charge distribution
bond in the ground state of ortho-aminonitropyrazoles. patterns, using 5-amino-1-methyl-4-nitropyrazole (I)
The H bond turned out to be weaker than the intra-
molecular hydrogen bond in 2-nitroaniline [2]. The
difference was explained by reduction of bond angles
as example. The amino and nitro groups in mole-
cule I are separated by a bond having a higher order
than the corresponding bond in 2-nitroaniline (II).
in going from a six-membered ring to five-membered Therefore, their solvatochromic parameters can be
and increased basicity of the amino group in pyra-
compared directly.
*
zoles. According to [2, 3], the long-wave
-
NHR1
transition involves redistribution of the orbital electron
density from the amino to nitro group. Therefore, the
strength of the intramolecular hydrogen bond should
change in going to the Franck Condon excited state.
A question arises so as to how does vertical transition
to the first excited state affect the intramolecular
hydrogen bond formed by ortho-aminonitropyrazoles
with a protophilic solvent S.
O N
O N
2
2
N
NO2
H N
2
N
H N
2
N
N
R2
Me
Me
VII
I
II VI, VIII
NHR1
NHR1
*
The second
transition does not involve the
amino group. It occurs from that MO which con-
R2
NO2
O
NO2
N
O
IX XIV
XV, XVI
N
H
H
N
N
R
1
1
1
II V, IX, XIII, XV, R = H; VI, R = NH ; VIII, X, R =
2
S
1
1
Me; XI, XIV, XVI, R = Et; XII, R = i-Pr; II, VIII XII,
2
2
2
R = H; III, XIII, XIV, R = H; IV, R = OMe; V, VI,
1
2
For communication XIII, see [1].
R
= NO2.
1
070-3632/02/7203-0449$27.00 2002 MAIK Nauka/Interperiodica

4
50
VOKIN et al.
method, are given in Table 1. Analysis of these data
*
8
0.49
*
shows that the assignment of
transitions made
m+2
0
.07
in [2, 3] in terms of -electron approximation is con-
firmed by the results of a more accurate procedure.
*
According to the latter, the
transition in the
6
7
UV spectra of ortho-aminonitropyrazoles and ortho-
nitroanilines results in decrease of the orbital electron
density not only on the amino nitrogen atom, but also
*
7
0.42
2
on the N atom of the heteroring, as well as in some
m+1
0.79
redistribution of the orbital electron density over
carbon atoms of the six-membered ring. The next
*
transition, in keeping with the AM1(CI) data,
is not one-electron. The excited state thus formed is
the result of interaction of two singly excited con-
figurations, only one of which corresponds to a locally
excited state with intramolecular charge transfer to
the nitro group (see figure). Nevertheless, the second
*
transition can also be sufficiently sensitive
m
9
.07
to nonspecific solvation, for the predominant con-
figuration is
tions at
m + 1
and the electron density distribu-
6
m
1
9.48
*
MO and
MO are essentially different. In
5
7
order to substantiate this assumption on an empirical
level, we examined nonspecific solvatochromic effects
for 2-nitro-4-methylaniline (III), 2-nitro-4-methoxy-
aniline (IV), 2,4-dinitroaniline (V), 2,4-dinitrophenyl-
hydrazine (VI), and 5-amino-1-methyl-4-nitroimid-
azole (VII). Solvatochromic parameters of compounds
I III and V VII (as regards their long-wave absorp-
tion bands) are given in Tables 2 and 3. With the goal
of examining effects of both nonspecific and specific
solvatation we used aprotic inert and protophilic
solvents (containing no aromatic fragments). Multiple
linear regression analysis with the aid of Kamlet
Taft solvatochromic parameters [4, 5] was applied.
5
m
1
1
0.53
10.53
N
N
Me
O N
O2N
2
H N
H2N
2
Orbital correlation diagram for 2-nitroaniline and
-amino-1-methyl-4-nitropyrazole, according to AM1
calculations. The orbital energies are given in eV.
5
=
+ b + s * + e .
max
0
The figure shows the orbital correlation diagram for
compounds I and II, which was plotted on the basis
* = = 0; is a solvent
=
Here,
=
at
0
max
of AM1(CI) calculations. The parameters of two low-
parameter characterizing its proton-acceptor power in
H complex with the solute;
*
*
energy
transitions, calculated by the same
is a parameter charac-
terizing the ability of solvent to stabilize dipolar
molecules or ions; and is an approximate covalent
constituent of the coordination bond. The regression
coefficients and free terms of the above equation,
which is valid for both initial compounds and
H complexes derived therefrom, are given in Table 4.
m + 1
*
Table 1. Parameters of long-wave
transitions in
compounds I and II, according to AM1(CI) semi-
empirical calculations
Transition
moment
Comp. Transi-
,
Configurational
compositiona
1
no.
tion
cm
The crucial role of the
configuration in the
1
m
(CNDO/S)
second electronically excited state of pyrazole (I)
is confirmed by analysis of the low-energy part of its
photoelectron spectrum reported in [3]. Let us assume
m+1
*
7
I
31600
3100
28300
0.45
0.30
0.54
0.45
0.61
6
6
m
m+1
m+2
*
*
3
0.45
0.60
0.37
, 0.37
, 0.54
m 1
m
*
that both long-wave
electron (
transitions in I are one-
m+1
*
7
(1)
(2)
II
m
corresponds to
). The difference in their energies is
m +1
, and
, to
m
m +1
m+1
m+2
m
32600
m 1
m
1
equal to the difference in the energies of ionization
from and MOs. According to the first two
a
2
i
0
j
j
i
The transition
corresponds to the configuration
.
m
m 1
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 3 2002

SOLVATOCHROMISM OF HETEROAROMATIC COMPOUNDS: XIV.
451
Table 2. Solvatochromic parameters of compounds I III in aprotic protophilic solvents
Kamlet Taft parametersa
I
II
III
Solvent
1
2
1
2
1
2
,
,
,
,
,
,
max
*
max
max
max
max
max
1
1
1
1
1
1
cm
cm
cm
cm
cm
cm
Triethylamine
0.71
0.46
0.47
0.49
0.37
0.45
0.55
0.77
0.31
0.40
0.25
1.05
0.69
0.77
0.76
0.14
0.24
0.27
0.27
0.55
0.55
0.58
0.72
0.75
0.83
0.85
0.87
0.88
0.92
1.00
1.00
0.20
0.20
0.20
0.20
0.00
0.00
0.20
0.10
0.00
0.00
0.20
0.00
0.00
0.00
30350
30750
30600
30650
30000
30050
29900
25650b
25650b
25650
25550b
Dibutyl ether
Diethyl ether
Diisopropyl ether
Dioxane
Ethyl acetate
THF
Triethyl phosphate
Acetonitrile
Propylene carbonate
Nitromethane
HMPA
38100
38100
38250
37350
36650b
36650
36650b
36250
36350d
36200
24800c
24700
36200
36250
36200
35850
35900
35850
24700c
25250
24400
d
24350c
24200c
25200c
37350
25000
24650b
23850
b
b,e
36100b
35950
24150e
23800
23950
23200
29650
29350
29600
29050
29000
28900
28600
36900
36650
25000
24650
24900
23950b
24350
35650
35550
36750
36600
36550
36250
35750
35400
b
DMF
23550c
c
1
-Methyl-2-pyrrolidone
24250
35750
23450
35400
35250
DMSO
24100b
35700c
23350c
a
b
c
d
The parameters were taken from [4, 5]. Data of [6 8]. Average value of
*
from [7, 8] and the present work. Given are
max
1
max
1
e
1
values for methyl acetate ( = 0.42,
= 0.60) from [6]; in ethyl acetate, for
pounds II and III, measured in acetone ( = 0.48,
= 25300 cm [7]. The
values for com-
max
*
1
= 0.71), are 24850 and 24050 cm , respectively.
Table 3. Solvatochromism of the long-wave absorption bands of compounds V VII in aprotic protophilic solvents
Kamlet Taft parametersa
1
max
, cm 1
Solvent
*
Vb
VIb
VII
d
Triethylamine
0.71
0.14
0.24
0.27
0.27
0.50
0.53
0.55
0.55
0.58
0.61
0.70
0.75
0.83
0.83
0.87
0.87
0.88
0.88
0.92
1.00
1.00
30450
30750
30700
30850
30050
29000c
29100c
29250c
29350c
28900
28750
28750
28600
28500
28600
28150
28050
27850
27550
27450
d
d
Dibutyl ether
Diethyl ether
Diisopropyl ether
Butyl acetate
0.46
0.47
0.49
0.45
0.41
0.37
0.45
0.55
0.25
0.35
0.31
0.40
0.80
1.05
0.49
0.69
0.76
0.77
0.76
0.20
0.20
0.20
0.00
0.20
0.20
0.00
0.00
0.00
0.00
0.10
0.00
0.00
0.20
0.00
0.00
0.00
0.00
0.00
29500
~
~
29500
d
28650
28450
28500
28650
28350
1
,2-Dimethoxyethane
Dioxane
Ethyl acetate
THF
Ethyl trichloroacetate
Ethyl chloroacetate
Acetonitrile
Propylene carbonate
Tetramethylurea
HMPA
30000
29900
29750
30200
29800
29650e
29300
28800
28550
29100
28900
28700
28700
28450
28250
27950
27800
27700
27650
27750
27550
27550
27550
27250
Butyrolactone
DMF
Dimethylacetamide
27450
27400
27250
27050
1
-Methyl-2-pyrrolidone
DMSO
a
c
b
The parameters were taken from [4, 5]. The long-wave absorption band is split into two components due to the presence of two
1
nitro groups; the data are given for the short-wave component; the second component appears as a shoulder at 24000 26000 cm
The observed absorption maximum is a superposition of bands belonging to the monomeric species and self-associates, which exist
in a dynamic equilibrium; the given values were not taken into account while calculating solvatochromic parameters. Measure-
.
d
1
max
ment of
was difficult because of the very poor solubility; the approximate values were not used in the calculation of coeffici-
e
*
1
max
1
ents for the solvatochromic equation.
In acetone ( = 0.48,
= 0.71),
= 29700 cm
.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 3 2002

4
52
VOKIN et al.
Table 4. Solvatochromic parameters of ortho-nitro compounds I VII in aprotic protophilic solvents and dipole moments
of their H complexes with dioxane
Comp.
no.
,a
D
, cm 1
0
b, cm 1
s, cm 1
e, cm 1
R
s
n
g
I
6.06
4.62
31820 90
8780 60
26590 50
37080 50
25580 50
750 100
2550 110
2480 90
1710 60
1210 70
1620 60
1210 40
1720 170
2710 120
3360 130
2720 120
610 120
0.987
0.994
0.987
0.982
0.988
0.992
0.982
0.988
0.991
0.985
80
80
60
50
50
30
70
80
60
60
14
11
17
11
15
11
8
20
15
15
3
II
1040 80
260 80
850 70
140 50
500 260
1220 110
410 90
320 100
b
b
b
4
.68 , 4.36
III
4.61
36600 30
IVc
V
VI
VII
24140 110
32080 80
30710 70
30220 90
b
6.53 , 6.48
6.90
380 120
720 190
a
b
c
In dioxane at 25 C.
Published data [9 12].
Experimental data from [7, 8].
ionization potentials of pyrazole I, given in [3], this
solvent is accompanied by a red shift of the absorption
bands. Hence the intermolecular hydrogen bond
becomes stronger in going to the Franck Condon
excited state. The specific solvatochromic effect
1
difference is about 7200 cm ; in keeping with the
(
2)
(1)
0
UV spectral data, the difference
6
is equal to
0
960 cm 1 (Table 4).
(
= b ), as well as nonspecific, depends on the
H
Taking into account that spectral shifts are additive,
structure of proton donor. For example, it ranges from
.5 to 3.5 kcal/mol for the first absorption band of
compounds I VII in DMSO ( = 0.76) (Table 4).
Compounds VI and VII are characterized by minimal
the nonspecific solvatochromic effect for a particular
0
*
compound is determined by the coefficient s ( = s ).
As follows from Table 4, the values of s for two low-
energy electronic transitions are almost similar. This is
explained by the fact that in both cases orbital electron
density transfer to the nitro group occurs mainly from
the nitrogen atom either of the amino group or of the
pyrazole ring (see figure and Table 1). The first and
second long-wave absorption bands of 2-nitroanilines
show different sensitivities to nonspecific solvation
values. The structure of 2,4-dinitrophenylhydra-
H
zine (VI) assumes formation of a bifurcation (three-
center) hydrogen bond with a protophilic solvent S.
O
O
N O
N O
H S
H S
(
Table 4), since the second electronic transition in-
O N
2
N
N
volves charge transfer from the ring containing no
heteroatoms. Thus, any general relations holding in
the effect of nonspecific solvation on absorption spec-
trum in the series of 4-nitropyrazoles and 2-nitroani-
lines may be found only for those transitions which
involve considerable change of electron density on the
heteroatom. In particular, we observed the expected
NH2
Me
VI
VIII
However, this assumption disagrees with the results
of treatment of the experimental data reported in [15]
on solvatochromic properties of N-methyl-2-nitro-
increase of nonspecific solvatochromic effect (which aniline (VIII), which is also capable for analogous
1
is expressed by the regression coefficient s) for the
long-wave absorption band with increasing dipole
moment of the H complexes with dioxane in the
ground state (Table 4). Analogous pattern was ob-
served previously for isomeric nitropyrazoles [13] and
some other compounds [14].
interactions (in addition, we obtained
in dibutyl
max
1
and diethyl ethers: 24150 and 24100 cm , respec-
tively). In aprotic protophilic solvents, the following
equation is valid:
1
(1610 120) *;
= (24640 70)
(140 200)
max
The spetcific solvatochromic effect is determined
by the coefficient b. Its absolute value for the first
absorption band of compound I is considerably larger
than for the second band (Table 4). The negative sign
of b means that the formation of H complex with the
R = 0.987, s = 50, n = 9.
This equation suggests that the spectral shifts of the
long-wave absorption band of compound VIII are
determined mainly by nonspecific solvation (cf. [8]).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 3 2002

SOLVATOCHROMISM OF HETEROAROMATIC COMPOUNDS: XIV.
453
a
Table 5. Solvatochromic parameters of para- and meta-nitroanilines in aprotic protophilic solvents and dipole moments
of their 1: 1 H complexes with dioxane
Comp.
no.
,b
D
, cm 1
0
b, cm 1
s, cm 1
R
s
n
g
IX
X
XI
XII
XIII
XIV
XV
XVI
6.80
7.12
30890 80
29060 70
28990 70
28760 90
30890 80
29040 70
29160 70
27300 40
2480 130
890 120
810 120
640 150
2470 160
990 130
3150 110
1650 70
3330 90
3350 90
3490 90
3490 110
3050 120
3100 110
1720 90
2110 50
0.988
0.993
0.993
0.988
0.993
0.991
0.991
0.995
110
70
80
100
80
80
27
18
18
18
17
17
19
19
5.24
5.51
80
50
a
b
The experimental data were taken from [6,15 23].
Published data for dioxane [24].
Therefore, the most probable hypothesis is that the
formation of H complex through the terminal amino
group of VI leads to some variation of the state of
the intramolecular hydrogen bond. The formation of
an intermolecular H complex between 2,4-dinitro-
phenylhydrazine and protophilic medium is supported
by the very strong effect of the chemical nature of the
solvent on the dipole moment of VI in the ground
state ( g 5.59 D in benzene and 6.90 D in dioxane
Franck Condon excited states. The specific solvato-
chromic effect for compounds in which the amino
group is located ortho with respect to the nitro group
(Table 4) strongly depends on the structure of the
ring, and in some cases it is comparable with that
found for N-alkyl-4-(or 3-)nitroanilines. Probable
reasons are both variation of the electronic effect of
the nitro group (cf. 3- and 4-nitroanilines) and forma-
tion of intramolecular hydrogen bond. The two
reasons are likely to be closely interrelated, for change
of the substituent and ring structure leads to consider-
able variations in the specific solvatochromic effect
in the series of ortho isomers (Table 4). In particular,
the coefficient b for 2-nitroaniline is larger than that
for 5-amino-1-methyl-4-nitropyrazole whose intra-
molecular hydrogen bond in the ground state is less
stronger [2].
at 25 C).
Before considering specific solvatochromic effects
in the series of ortho-nitro derivatives under study,
it seems reasonable to give some information on
meta- and para-nitroanilines, whose solvatochromic
properties were studied in detail in [6, 15 23]. Table 5
contains the results of analysis of the average
max
values, measured in aprotic protophilic media, with
the aid of the Kamlet Taft parameters. Let us em-
phasize once more that protophilic solvents having
an aromatic fragment were not included. The data in
Table 5 lead us to the following conclusions. First,
the specific solvatochromic effect arising from forma-
tion of intermolecular hydrogen bond for meta- and
para-nitroanilines is not always additive (cf. IX and
The weakening of the effect of specific solvation
on the solvatochromism of orto-nitro compounds in
the series V > II > III > I > IV > VI > VII is ex-
plained primarily by reduced degree of orbital electron
density transfer from the amino group upon excitation.
This directly follows from the dependence of
on
H
the para substituent in 2-nitroanilines (NO > H >
2
Me > OMe) and from the considerably lower specific
solvatochromic effect for the second absorption band
of compounds I III (as compared to the first one;
X XII, XIII and XIV, XV and XVI, and data of
2
[
8, 18]). Second, the regression coefficient b does not
remain constant in the series of analogous 3- and
-isomers (Table 5). Different b values are also ob-
*
Table 4). As we already noted, the second
4
transition does not appreciably change of the orbital
electron density on the amino nitrogen atom. The
above stated may be regarded as evidence of the
applicability of the concept of electrostatic nature of
hydrogen bonding to nonequilibrial excited charge-
transfer states of unsym-metrical complexes with a
normal intramolecular hydrogen bond. At least, there
is no need of invoking the concept of three-center
four-electron interaction (3c4e) [25] for qualitative
analysis of the revealed relations.
tained for 4-isomers differing by branching of the
alkyl group on the amino nitrogen atom. This means
that such variations of the H-donor structure change
the relation between the energies of the corresponding
H complexes with the solvent in the ground and
2
The nonadditivity is masked when all kinds of aprotic solvents
are analyzed jointly [8].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 3 2002

4
54
VOKIN et al.
5. Abraham, M.H., Grellier, P.L., Abboud, J.-L.M.,
EXPERIMENTAL
The electronic absorption spectra were recorded on
Doherty, R.M., and Taft, R.W., Can. J. Chem., 1988,
vol. 66, no. 11, pp. 2673 2686.
a Specord UV-Vis spectrophotometer at 22 25 C.
6
7
8
9
. Lutskii, A.E. and Bocharova, V.V., Zh. Obshch.
Khim., 1975, vol. 45, no. 12, pp. 2724 2730.
Quantum-chemical calculations of the orbital structure
*
and energies of
transitions for compounds I
. Yokoyama, T., Taft, R.W., and Kamlet, M.J., J. Am.
Chem. Soc., 1976, vol. 98, no. 11, pp. 3233 3237.
and II were performed in the AM1 approximation
[
(
26] with allowance for configurational interactions
200 configurations arising from electron excitation
. Yokoyama, T., Taft, R.W., and Kamlet, M.J., Aust.
J. Chem., 1983, vol. 36, no. 4, pp. 701 709.
from six occupied molecular orbitals to six vacant
MOs were considered).
. Lutskii, A.E. and Kondratenko, B.P., Zh. Fiz. Khim.,
1
959, vol. 33, no. 9, pp. 2017 2023.
0. Richards, J.H. and Walker, S., Trans. Faraday Soc.,
961, vol. 57, pp. 418 424.
Compound I was synthesized by the procedure
reported in [27]. Its melting point was 264 266 C,
which is consistent with the data for the same com-
pound prepared as described in [28]. The product was
purified by recrystallization first from methanol and
then from hot water.
1
1
11. Vasil’ev, V.G. and Syrkin, Ya.K., Zh. Fiz. Khim.,
1941, vol. 15, no. 2, pp. 254 255.
1
1
2. Smith, J.W. and Walshaw, S.M., J. Chem. Soc.,
957, no. 11, pp. 4527 4531.
1
5-Amino-1-methyl-4-nitroimidazole (VII). Tri-
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4
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