Synthesis and structure of a novel mesomeric betaine 6,7-dimethyl-2H-pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine

Article abstract of DOI:10.1016/j.molstruc.2006.06.003

The synthesis and structural characterization by ¹H NMR, HRMS, elemental analysis and X-ray crystallography of a novel mesomeric betaine, 6,7-dimethyl-2H-pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine 8a are described. The compound 8a crystallizes in the monoclinic system in the space group P2₁/n with a = 11.766(2), b = 5.8097(12), c = 12.389(3) ?, β = 107.14(3)°, V = 809.2(3) ?³, Z = 4, D_calc = 1.561 gcm^-3, λ(Cu Kα) = 1.54178 ?, μ = 0.953 mm^-1 and final R = 0.0378 for 1526 reflections. The molecule as a whole has an almost planar conformation and the bond lengths and angles have the values characteristic for π-electron system. The molecular packing in the crystal is influenced by the van der Waals forces and presence of weak π-π interactions. The electronic properties (net charge and MEP distributions and dipole moment) of the molecule 8a are calculated using AM1 approximation. Twelve dipolar canonical forms for molecule 8a are presented and their contribution in mesomeric betaine structure of 8a is discussed (molecular mechanics calculations).

Full text of DOI:10.1016/j.molstruc.2006.06.003

Journal of Molecular Structure 829 (2007) 22–28
www.elsevier.com/locate/molstruc
Synthesis and structure of a novel mesomeric betaine 6,7-dimethyl-
2H-pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine ଝ
Zbigniew Karczmarzyk ¤, Mariusz Mojzych, Andrzej Rykowski
Department of Chemistry, University of Podlasie, ul. 3 Maja 54, 08-110 Siedlce, Poland
Received 20 April 2006; accepted 2 June 2006
Available online 17 July 2006
Abstract
1
The synthesis and structural characterization by H NMR, HRMS, elemental analysis and X-ray crystallography of a novel meso-
meric betaine, 6,7-dimethyl-2H-pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine 8a are described. The compound 8a crystallizes in the mono-
clinic system in the space group P21/n with a D 11.766(2), b D 5.8097(12), c D 12.389(3) Å, ꢀ D 107.14(3)°, V D 809.2(3) Å3, Z D 4,
Dcalc D 1.561 gcm¡3, ꢁ(Cu Kꢀ) D 1.54178 Å, ꢂ D 0.953 mm¡1 and Wnal R D 0.0378 for 1526 reXections. The molecule as a whole has an
almost planar conformation and the bond lengths and angles have the values characteristic for ꢁ-electron system. The molecular packing
in the crystal is inXuenced by the van der Waals forces and presence of weak ꢁ–ꢁ interactions. The electronic properties (net charge and
MEP distributions and dipole moment) of the molecule 8a are calculated using AM1 approximation. Twelve dipolar canonical forms for
molecule 8a are presented and their contribution in mesomeric betaine structure of 8a is discussed (molecular mechanics calculations).
© 2006 Elsevier B.V. All rights reserved.
Keywords: Mesomeric betaine; Pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine; X-ray structure
1. Introduction
from cyclization of an azido group into N-2 atom of 2 (see
Scheme 1).
Annulated 1,2,4-triazine derivatives are present as
important core structures in many biologically active com-
pounds, both naturally occurring and synthetic [2,3]. As a
part of an ongoing research program into synthesis of het-
eroaromatic analogues of this system, we were interested in
the formation of pyrazolo[4,3-e][1,2,4]triazines fused with
tetrazole ring. The nitrous acid oxidation of an easily avail-
able 5-hydrazino compound 1 [4] appeared to be the
method of choice [5]. The initial product of the oxidation,
the azido compound 2, can cyclize to form either linearly
fused compound 3a or angularly fused compound 3b. In
this study, the crystal structure of the tetrazole derivative
was clariWed by single crystal X-ray analysis [5] revealing
the predominant tautomeric structure as linear 3a, resulting
The propensity of 2 toward cyclization of the azido
group into N-2 rather than N-4 prompted us to examine
the behavior of 5-azido-2,3-dimethyl-2H-pyrazolo[4,3-e]
[1,2,4]triazine 7. In contrast to compound 2, the aromatic
sextet of ꢁ-electrons in 1,2,4-triazine part of the model com-
pound 7 is not retained, and in principle only one kind of
intramolecular ring closure of the azido group into N-4
atom leading to 8b could occur. However, it was found that
under thermal conditions compound 7 gave rise to linearly
fused betaine 8a. The structure of 8a was fully determined
by X-ray crystallographic analysis.
Compound 7 was conveniently prepared in the three-
step synthesis from 2,3-dimethyl-5-methylsulfanyl-2H-pyr-
azolo[4,3-e][1,2,4]triazine 4 by oxidation with potassium
manganate (VII), followed by treatment of the resulting
2,3-dimethyl-5-methylsulfonyl-2H-pyrazolo[4,3-e][1,2,4]tri-
azine 5 with anhydrous hydrazine. The nitrous oxidation of
5-hydrazino compounds 6 thus obtained gave the desired
compound 7 (see Scheme 2).
ଝ
See Ref. [1].
*
Corresponding author. Tel.: +48 25 643 10 17; fax: +48 25 644 20 45.
E-mail address: kar@ap.siedlce.pl (Z. Karczmarzyk).
0022-2860/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.06.003

Z. Karczmarzyk et al. / Journal of Molecular Structure 829 (2007) 22–28
23
Ph
Ph
Ph
N
N
N
N
N
N
N
N
N
HONO
NH-NH2
N
N
N
N
N
N
N
N3
N
N
CH3
CH3
CH3
3a
1
2
Ph
N
N
N
N
N
N
CH3
N
N
3b
Scheme 1.
N
N
N
N
N
N
N
KMnO4
H2N-NH2
N
N
N
H3C
N
H3C
N
H3C
N
N
NH-NH2
SCH3
N
SO2CH3
CH3
CH3
CH3
6
4
5
HONO
N
N
N
N
N
N
N
N
N
H3C
N
H3C
N
N
N3
N
CH3
CH3
8a
7
N
N
N
H3C
N
N
N
N
CH3
N
8b
Scheme 2.
During recrystallization of 7 from hot ethyl alcohol the
color of the ethanolic solution gradually changed from
yellow to red and a deep red precipitate of the tetrazole
derivative (8a or 8b) was formed. In order to establish
whether we are dealing with angularly fused compound 8b
or its linearly fused isomer 8a it became necessary to
conWrm its structure by X-ray.
analyzer and results for the indicated elements were within
0.3% of the calculated values. Compound 4 was prepared
according to the literature procedure [4]. Compound 6 was
used as a crude product.
2.2. Synthesis of 2,3-dimethyl-5-methylsulfonyl-2H-
pyrazolo[4,3-e][1,2,4]triazine 5
2. Experimental
To a solution of 4 (195mg, 1mmol) in 20 mL of benzene
were added water (30mL), potassium manganate (VII)
(474mg, 3mmol), catalytic amounts of tetrabutylammo-
nium bromide (65mg, 0.2 mmol) and 1.5mL of acetic acid.
The reaction mixture was stirred at rt for 1h. A saturated
solution of Na2S2O5 in water was added to the mixture
until the purple color disappeared. The organic layer was
separated and water phase was extracted with benzene
(3£ 10 mL). The combined extracts were dried over anhy-
drous MgSO4 and concentrated in vacuo. The crude prod-
uct was puriWed by column chromatography on silica gel
2.1. General methods
Melting points were determined on Boetius melting
point apparatus and are uncorrected. The 1H NMR spectra
were recorded on a Varian Gemini 200 MHz spectrometer
with TMS as internal standard in deuterated solvents. The
chemical shifts are given in ꢂ (ppm). Mass spectra were
measured on AMD 604 spectrometer. Elemental analysis of
C, H and N was recorded on Perkin-Elmer 2400-CHN

24
Z. Karczmarzyk et al. / Journal of Molecular Structure 829 (2007) 22–28
Table 1
using a mixture of chloroform-ethanol (40:1) as eluent and
recrystallized from ethanol to give 120mg (0.53 mmol, 53%)
Crystal data and structure reWnement for 8a
1
Empirical formula
Formula weight
Crystal system
Space group
Unit cell parameters
a (Å)
C6H6N8
190.19
Monoclinic
P21/n
of 5. Mp 164°C. H NMR (CDCl3) ꢃ: 2.87 (s, 3H), 3.58 (s,
3H), 4.42 (s, 3H). MS (EI 70eV, m/z, %): 227 (1) [M+], 199
(9), 136 (7), 79 (15), 56 (100); IR (KBr)cm¡1: 1320, 1130;
Anal. Calcd for C7H9N5SO2: C, 37.00; H, 3.96; N, 30.83.
Found: C, 37.11; H, 3.82; N, 31.05.
11.766(2)
5.8097(12)
12.389(3)
107.14(3)°
809.2(3)
b (Å)
c (Å)
2.3. Synthesis of 2,3-dimethyl-5-hydrazine-2H-pyrazolo
[4,3-e][1,2,4]triazine 6
ꢀ (deg)
V (Å3)
Z
4
Dcalc (g cm¡3
F(0 0 0)
)
1.561
392
1.54178
To a solution of 5 (227 mg, 1mmol) in dry THF (20mL)
cooled to 0–5°C anhydrous hydrazine (0.1 mL, 3 mmol)
was added. The reaction mixture was stirred at 0–5°C for
30 min and additional 3h at rt. After that time the solvent
was evaporated in vacuo to give 150 mg (0.84 mmol, 84%) of
the red solid. The crude product was used for the next step.
Mp 218–220 °C. 1H NMR (CD3OD) ꢃ: 1.65 (s, 2H), 2.59 (s,
3H), 4.18 (s, 3H), 6.92 (s, 1H). IR (KBr)cm¡1: 3300, 1550,
1450. MS (EI 70eV, m/z, %): 179 (10) [M+], 164 (5), 151 (4),
135 (10), 67 (5), 56 (100). HRMS (EI, m/z) 180.0984, Calcd
for C6H10N7 [M+H] 180.0992.
ꢁ (Cu Kꢀ) (Å)
Cell parameters from
ꢆ range for lattice parameters (°)
83 reXections
8.50–31.76
0.953
0.799/0.911
4.56–70.53
¡14/13, ¡7/7, ¡14/15
8609
1526 (Rint D 0.0214)
1474 with I >2 (I)
Full-matrix least-squares on F2
0.0378, 0.1194
1.099
Absorption coeYcient ꢄ (mm¡1
Tmin/Tmax
)
ꢆ range for data collection (°)
Index ranges h, k, l
No. of measured reXections
No. of independent reXections
No. of observed reXections
ReWnement method
Final R indices: R, wR(F2)
Goodness-of-Wt on F2, S
Data/parameters
2.4. Synthesis of 5-azido-2,3-dimethyl-2H-pyrazolo
[4,3-e][1,2,4]triazine 7
1526/146
Extinction coeYcient
0.0125(16)
+0.235 and ¡0.158
0.000
Largest diV. peak and hole (e Å¡3
(ꢇ/ )max
)
The solution of 6 (90 mg, 0.5 mmol) in CH3COOH/H2O
mixture (1:1, 8 mL) was cooled to 0–5°C and aqueous
NaNO2 (70 mg in 2 mL of water) was added dropwise so
that the temperature of mixture was kept below 5 °C. After
additional stirring for 15 min at 0–5°C the orange precipi-
tate was Wltered oV, washed with water and dried at room
temperature to give 92mg (0.48 mmol, 96%) of 7. Mp
nol solution. X-ray data were collected on the Bruker
SMART APEX CCD diVractometer at room tempera-
ture; crystal sizes: 0.30 £ 0.30 £ 0.10 mm, ꢄ scans. The
multiscan absorption correction was applied (SADABS
[6]). The structure was solved by direct methods using
SIR92 [7] and reWned by full-matrix least-squares with
SHELXL97 [8]. All hydrogen atoms were located from ꢃꢅ
map and their coordinates were reWned with isotropic dis-
placement parameters taken as 1.5 times those of the
respective parent atoms. All crystal and experimental data
are listed in Table 1. Molecular graphics were prepared
using ORTEP3 for Windows [9] and XP in SHELXTL-
Plus [10], PARST [11] and PLATON [12] were used for
geometrical calculations. All calculations were performed
using WINGX ver. 1.64.05 package [13].
1
140°C. H NMR (CDCl3) ꢃ: 3.02 (s, 3H), 4.36 (s, 3H). IR
(KBr)cm¡1: 2145 (N3). MS (EI 70 eV, m/z, %): 190 (35)
[M+], 78 (7), 57 (4), 56 (100), 54 (4). HRMS (EI, m/z)
190.07101, Calcd for C6H10N8 [M+] 190.07154.
2.5. Synthesis of 6,7-dimethyl-2H-pyrazolo[4,3-e]
tetrazolo[4,5-b][1,2,4]triazine 8a
The azido compound 7 (95mg, 0.5mmol) was dissolved in
boiling ethanol (5mL) and the resulting mixture was left to
slow crystallization at room temperature. The crystals and
dry residue after evaporation of the alcohol were combined
and puriWed on column chromatography using chloroform/
ethanol mixture 30:1 as eluent to give 8a (62mg, 0.32mmol,
64%) as a red solid. Mp 148°C. 1H NMR (CDCl3) ꢃ: 2.70 (s,
3H), 4.29 (s, 3H). MS (EI 70eV, m/z, %): 190 (27) [M+], 78 (6),
56 (100). HRMS (EI, m/z) 190.07189, Calcd for C6H10N8
[M+] 190.07154. Anal. Calcd for C6H10N8: C, 37.90: H, 3.18;
N, 58.92. Found: C, 38.08: H, 3.18; N, 58.65.
2.7. Theoretical calculations
The net charge distributions on the atoms, dipole moment
and the molecular electrostatic potential (MEP) for 6, 7-
dimethyl-2H-pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine 8a
were calculated at the restricted Hartree-Fock level (RHF)
using AM1 semiempirical SCF-MO method [14]. The molec-
ular mechanics studies (MM+force Weld [15]) were under-
taken to calculate the strain energy for all possible dipolar
canonical forms of 8a. All calculations were performed using
the program package HYPERCHEM rel. 4.5 [16]. The struc-
tures were fully optimized without any symmetry constraint
2.6. X-ray structure analysis
Red prismatic crystals of 8a suitable for X-ray diVrac-
tion analysis were grown by slow evaporation of an etha-

Z. Karczmarzyk et al. / Journal of Molecular Structure 829 (2007) 22–28
25
Fig. 1. A view of the molecule 8a with the atomic labelling. Non-H atoms are represented by displacement ellipsoids of 50% probability.
to a gradient norm of <0.1 and the initial geometries were
within 0.016(1)Å. The methyl substituents lie almost in the
built from crystallographic data of 8a.
plane of this ring with the displacements of 0.075(2)Å for
C13 and 0.016(2)Å for C14. The N–N and N–C bond
lengths are in the ranges 1.3167–1.3615 and 1.3237–
1.3830Å, respectively, and they have the values intermedi-
ate between expected single- and double-bond lengths char-
acteristic for ꢁ-electron system.
3. Results and discussion
3.1. X-ray structure analysis results
The X-ray analysis revealed that the linear tricyclic inner
salt with mesomeric betaine structure of the 6,7-dimethyl-
2H-pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine 8a is formed
via ring closure toward N-2 in 5-azido-2,3-dimethyl-2H-
pyrazolo[4,3-e][1,2,4]triazine 7. View of the molecule 8a
with numbering of atoms is shown in Fig. 1 and selected
geometrical parameters are listed in Table 2. The geometry
(bond lengths, angles and planarity) of the tricyclic pyrazol-
otetrazolotriazine skeleton is very similar in 8a and previ-
ously reported related structure of 7-methyl-5-phenyl-1H-
pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine [5]. The mole-
cule of 8a has a planar conformation as a whole. The pyraz-
olo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine ring is planar to
There are no classical hydrogen bonds present in the
crystal structure of 8a. The molecular packing in the crystal
(Fig. 2) is inXuenced by the van der Waals forces and pres-
ence of week ꢁ–ꢁ interactions. The pyrazolo[4,3-e]tetraz-
olo[4,5-b][1,2,4]triazine systems belonging to the inversion
related molecules partially overlap each other with the
shortest intermolecular contact 3.385(2)Å [symmetry code:
(i)D¡x, ¡y, ¡z].
3.2. Theoretical calculations results
According to the deWnition comprised in [17], “meso-
meric betaines are neutral compounds that can exclusively
be formulated as dipolar structures, in which the positive
and negative charges are delocalized within a common
ꢁ-electron system. They posses an even number of positive
and negative charges and no uncharged covalent structure
can be drawn”. The molecule of the title 6,7-dimethyl-2H-
pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine 8a fulWls all
requirements for mesomeric betaine structure. The pyraz-
olo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine ring system is fully
aromatic with 14 ꢁ-electrons. The small diVerentiation of
the bond lengths in the tricyclic ring system obtained from
X-ray investigation makes impossible precise localization
of the positive and negative charges on the atoms and
indicates on the delocalization of these charges within ꢁ-
electron system. The net charge on the atoms, the molecu-
lar electrostatic potential (MEP) distribution in the plane
of the molecule and the position of the vector of dipole
moment calculated using AM1 semiempirical method
show that negative charge is mainly delocalized on the N
atoms of the triazine and tetrazole rings, while the positive
charge is distributed in the pyrazole ring area (Fig. 3).
Table 2
Bond lengths (Å) and angles (°) for 8a
N(1)–C(6)
N(1)–N(2)
N(2)–N(12)
N(2)–C(3)
N(4)–C(5)
N(4)–C(3)
N(7)–C(6)
N(7)–N(8)
C(6)–N(1)–N(2)
N(1)–N(2)–N(12)
N(1)–N(2)–C(3)
N(12)–N(2)–C(3)
C(5)–N(4)–C(3)
C(6)–N(7)–N(8)
C(9)–N(8)–N(7)
C(9)–N(8)–C(14)
N(7)–N(8)–C(14)
N(11)–N(10)–C(3)
N(12)–N(11)–N(10)
N(11)–N(12)–N(2)
1.3256(16)
1.3348(15)
1.3441(15)
1.3830(17)
1.3237(16)
1.3330(17)
1.3459(16)
1.3615(16)
109.45(11)
122.84(11)
127.94(11)
109.22(11)
111.64(11)
102.38(10)
117.15(11)
125.21(13)
117.64(12)
105.86(12)
113.23(12)
104.79(12)
N(8)–C(9)
N(8)–C(14)
N(10)–N(11)
N(10)–C(3)
N(11)–N(12)
C(5)–C(9)
C(5)–C(6)
C(9)–C(13)
N(4)–C(3)–N(10)
N(4)–C(3)-N(2)
N(10)–C(3)–N(2)
N(4)–C(5)–C(9)
N(4)–C(5)–C(6)
C(9)–C(5)–C(6)
N(1)–C(6)–N(7)
N(1)–C(6)–C(5)
N(7)–C(6)–C(5)
N(8)–C(9)–C(5)
N(8)–C(9)–C(13)
C(5)–C(9)–C(13)
1.3406(17)
1.4571(17)
1.3362(19)
1.3406(17)
1.3167(19)
1.4109(17)
1.4518(17)
1.4779(19)
130.05(13)
123.05(11)
106.90(11)
131.23(11)
124.18(11)
104.59(10)
124.67(12)
123.72(11)
111.61(11)
104.27(11)
125.35(12)
130.33(12)

26
Z. Karczmarzyk et al. / Journal of Molecular Structure 829 (2007) 22–28
Fig. 2. Unit-cell packing in crystal of 8a.
Fig. 3. Net charge distribution on the atoms (e), MEP distribution in the plane of the fused tricyclic system (starting value: ¡0.12 eV, step: 0.02 eV) and
dipole moment vector (10.14 D) calculated for 8a (AM1 approximation).
There are twelve possible dipolar canonical structures
for the analysed molecule 8a, indicated on Scheme 3 as
n_m, where n and m are the atoms numbers with positive
and negative charges, respectively. One can see that the pos-
itive charge occurs only at N2 or N8 atoms, while the nega-
tive one can be localized in both causes at N1, N4, N7, C9,
N10 and N12 atoms. No common atoms for the delocaliza-
tion of the positive and negative charge exist. Thus, the 6,7-
dimethyl-2H-pyrazolo[4,3-e]tetrazolo[4,5-b][1,2,4]triazine
8a may be classiWed as cross-conjugated mesomeric betaine
CCMB [17].
using the molecular mechanics method. The strain energy
(Ei) for each structure was calculated and the percentage
(pi) of it in the hybrid mesomeric betaine structure was
estimated using a non-generate Boltzmann distribution
(Table 3). As it can be seen from Table 3, the ratio of the
contributions of the canonical forms positive charged on
N2 and N8 atoms is about 9:1 with clear predomination of
two canonical forms 2_1 and 2_4 exceeding 70%. This
result is to be expected, because the intramolecular ring clo-
sure of the azido group in 7 into N-2 atom leading to 8a
should give rise to charge migration mainly in the tetrazol-
otriazine part of the molecule retaining the aromatic sextet
of ꢁ-electrons in pyrazole ring. Similar eVects are observed
The energy minimization and geometry optimization for
all canonical structures of the molecule 8a were performed

Z. Karczmarzyk et al. / Journal of Molecular Structure 829 (2007) 22–28
27
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
H3C N+
N
H3C N+
N
H3C N+
N
N
N
8_7
N
N
8_1
N
N
8_12
N
H3C
H3C
H3C
N
N
N
N
N
N
N
N
N
N
N
H3C N+
N
H3C N+
N
H3C N+
C
N
8_ 9
N
N
8_4
N
N
H3C
H3C
H3C
8_10
N
N+
N+
N+
N
N
N
N
N
N
N
N
N
N
N
H3C
N
H3C
N
H3C
N
N
N
N
2_4
N
N
2_10
N
H3C
H3C
H3C
2_12
N
N+
N+
N+
N
N
N
N
N
N
N
C
N
N
N
H3C
N
N
H3C
N
N
H3C
N
N
N
N
H3C
H3C
H3C
2_9
2_7
2_1
Scheme 3.
Table 3
canonical forms of 8a those with positive charge on N2
atom give predominant contribution to the mesomeric
betaine structure providing for charge migration in the area
of the tetrazolotriazine part of 8a. The structure of 8a was
established by X-ray crystallography.
The strain energy (Ei, MMX molecular mechanics approximation) and
percentage (pi) for m_n canonical forms of molecule 8a
Canonical form
Ei (kJ mol¡1
)
pi (%)
8_7
8_1
8_12
8_10
8_4
155.54
152.11
154.16
155.16
149.17
155.12
149.72
150.85
144.28
149.97
146.41
143.69
0.29
1.26
0.54
0.36
4.20
0.37
3.36
2.11
31.35
3.03
13.09
39.97
5. Supplementary materials
CCDC-603862 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge at www.ccdc.cam.ac.uk/conts/retriev-
ing.html [or from the Cambridge Crystallographic Data
Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44(0) 1223 336 033; email: deposit@ccdc.cam.
ac.uk].
8_9
2_12
2_10
2_4
2_9
2_7
2_1
in angular [1,2,4]triazolo[3,4-c][1,2,4]benzotriazines and lin-
ear [1,2,4]triazolo[4,3-b][1,2,4]benzotriazines, where the
equilibrium between angular and linear isomers goes via
antiaromatic diazirines involving charge migration in the
heterocyclic rings area in the acting mechanism of valence
bond isomerization [18].
Acknowledgements
The authors thanks to Professor J. Karolak-
Wojciechowska from Institute of General and Ecological
Chemistry, Technical University in Góds, for her kind
consent to use the XP program for creating draft.
4. Conclusion
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