Pyrido triazin-nucleus synthesis and theoretical studies: 2,3,6-trioxo-8-aryl-1,3,4,6-tetrahydro-]2H[pyrido]1,2-b] [1,2,4[triazin-7,9-dicarbonitryl derivatives

Article abstract of DOI:10.1016/j.molstruc.2020.129032

An efficient method for the synthesis of potentially biologically [2H]-pyrido [1,2-b]1,2,4-triazines derivatives is described via reaction of 1,6-diamino-2-oxo-4-phenyl-1,2-dihydropyridine-3,5-dicarbonitrile derivatives with oxalyl chloride in DMF in the presence of pyridine as a base. The intramolecular hydrogen bonding (IMHB) interactions have been investigated at M06-2X/6-311++G(d,p) level of theory. The eight compounds (5a-h) characterized by geometries and energies. The Natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) were performed to explore the nature of the hydrogen-bonding interactions in these compounds. The IMHB formed between the C[dbnd]O and N[sbnd]H groups. The theoretical calculations showed that the IMHB strength increases in the presence of electron-donor substituents. An opposite behavior was observed for electron-acceptor substituents.

Full text of DOI:10.1016/j.molstruc.2020.129032

Journal Pre-proof
Pyrido triazin-nucleus synthesis and theoretical studies:
2,3,6-trioxo-8-aryl-1,3,4,6-tetrahydro-]2H[pyrido]1,2-b]
[1,2,4[triazin-7,9-dicarbonitryl derivatives
Maryam Tahmasby , Ali Darehkordi , Marziyeh Mohammadi ,
Farzaneh Nejadkhorasani
PII:
S0022-2860(20)31355-7
DOI:
Reference:
https://doi.org/10.1016/j.molstruc.2020.129032
MOLSTR 129032
To appear in:
Journal of Molecular Structure
Received date:
Revised date:
Accepted date:
11 June 2020
22 July 2020
5 August 2020
Please cite this article as: Maryam Tahmasby , Ali Darehkordi , Marziyeh Mohammadi ,
Farzaneh Nejadkhorasani , Pyrido triazin-nucleus synthesis and theoretical studies: 2,3,6-trioxo-
8-aryl-1,3,4,6-tetrahydro-]2H[pyrido]1,2-b] [1,2,4[triazin-7,9-dicarbonitryl derivatives, Journal of Molec-
ular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.129032
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Highlights
 synthesis and Theoretical studies an important series of Pyrido triazin-nucleus.
 Biologically important N-heterocycles.
 Simple procedure and purification in good yields.
1

Graphical Abstract
O
O
O
O
H
N
NC
NH2
NH2
NC
O
O
H
N
CN
N
Pipyridine
N
Cl
Cl
+
NC
NH2
CN
2a-j
EtOH
Reflux,4 h
Pyridine, DMF
12 h, 100oC
N
H
O
R
CN
CN
R
1
R
3a-j
5a-j
R= H, CH3, OCH3, Cl, NO2, N(CH3)2, F, Br, OH
Pyrido triazin-nucleus synthesis and theoretical studies: 2,3,6-trioxo-8-aryl-1,3,4,6-
tetrahydro-]2H[pyrido]1,2-b[ ]1,2,4[triazin-7,9-dicarbonitryl derivatives
Maryam Tahmasby, Ali Darehkordi, Marziyeh Mohammadi*, Farzaneh Nejadkhorasani
Department of Chemistry, Faculty of Science, Vali-e-Asr University of Rafsanjan,
Rafsanjan
77176, Iran
*Corresponding author. Tel.: +98 3431312423; fax: +98 343132429.
E-mail address:m.mohammadi@.vru.ac.ir
Abstract:
An efficient method for the synthesis of potentially biologically [2H]-pyrido [1,2-
b]1,2,4-triazines derivatives is described via reaction of 1,6-diamino-2-oxo-4-phenyl-
1,2-dihydropyridine-3,5-dicarbonitrile derivatives with oxalyl chloride in DMF in the
presence of pyridine as a base. The intramolecular hydrogen bonding (IMHB)
interactions have been investigated at M06-2X/6-311++G(d,p) level of theory. The
eight compounds (5a-5h) characterized by geometries and energies. The Natural bond
orbital (NBO) and quantum theory of atoms in molecules (QTAIM) were performed to
explore the nature of the hydrogen-bonding interactions in these compounds. The
IMHB formed between the C=O and N-H groups. The theoretical calculations showed
that the IMHB strength increases in the presence of electron-donor substituents. An
opposite behavior was observed for electron-acceptor substituents.
Keyword: Oxalyl chloride, Trioxo-pyrido-triazine, Diamino-2-oxo-4-phenyl-
dihydropyridine-3,5-dicarbonitrile, Theoretical studies, Intramolecular hydrogen bond
2

1. Introduction
Substituted triazines represent an important class of nitrogen-containing heterocycles.
The 1,2,4-triazine core is a versatile synthetic platform to access a wide range of
condensed heterocyclic ring systems via intramolecular Diels–Alder reactions with a
vast array of dienophiles [1]. More recently, numerous small heterocyclic compounds
containing 1,2,4-triazine ring have been shown to exhibit a great variety of biological
activities. Also, the triazine ring system is a key component of commercial dyes,
herbicides, insecticides. Many researches have been reported on the application of these
compounds. Therefore, the biological activities of 1,2,4-triazines have attracted the
attention of many chemists because numerous 1,2,4-triazines are biologically active [2-
8] and are used in medicine, especially as anti-AIDS agents, anticancer agents [9,10],
antitubercular agents [11], potent CRF receptor antagonists [12], cathepsin K inhibitors
[13], and for their anti-anxiety and anti-inflammatory activities [14,15], as well as in
agriculture [16-19]. They also form complexes with metal ions which are used for metal
determination. This interest is reinforced by the development of new drugs (e.g., the
effective anticonvulsant lamotrigine and anticancer drug tirapazamine), luminescent
materials, dyes, specific ligands for complexation with metals, and other compounds
based on 1,2,4-triazines. Fused 1,2,4-triazine systems have also attracted considerable
interest in their biological activities. For example 1,2,4-triazolo[5,1-c][1,2,4]triazinones
and their sodium salts, along with azoloannelated 1,2,4-triazines, express high activity
against different kinds of viruses, including influenza and bird flu (culture H5N1) [20,
21], and also several pyrrolotriazine derivatives were identified as potentially active
anticancer agents acting on vascular endothelial growth factor receptor (VEGFR)
tyrosine kinases [22].
Intramolecular hydrogen bonds (IMHBs) have a basic role in biochemistry and
chemistry. They can influence the electronic distribution, molecular geometry,
configuration, and structure of systems. Also, IMHBs extremely affect molecular
properties, biological activity, and interactions [23−27]. Furthermore, the break or
formation of IMHBs is studied by medicinal chemists to regulate desired biological and
chemical properties. For example, the IMHBs can stabilize the bioactive compound of a
certain ligand. This decreases the translational and conformational entropy upon
binding and lead to stronger connection [28−31] Several theoretical approaches can be
applied to study HBs, for example atoms in molecules (AIM) methodology [32,33]. The
AIM theory of Bader [34] was applied to analyze the topological properties of the bond
3

critical point (BCP) of the hydrogen bond. Besides, the natural bond orbitals (NBO)
methodology is widely used as a technique to study IHBs. The NBO [34] analysis was
also performed to obtain more precise information on this bond. Because of importance
triazine compounds in medicinal chemistry and also high effects of inter- and
intramolecular hydrogen bonding in biological properties in this investigation, firstly
we have synthesized a new series of 2,3,6-trioxo-8-aryl-1,3,4,6-tetrahydro-
]2H[pyrido]1,2-b] [1,2,4[triazin-7,9-dicarbonitryl derivatives. Then the strength of
intramolecular hydrogen bond formed between C=O and N–H groups for compounds
5a-5h have been investigated by theoretical calculations.
2-Experimental Section
2-1-General Methods
All chemicals and solvents were purchased from commercial sources and used without
further purification unless otherwise stated. Melting points were obtained by a
Bamslead Electrothermal 9200 apparatus and are uncorrected. Infrared spectra were
recorded on a Thermo Scientific, Nicolet iS10 Fourier transform IR spectrometer
(Thermo Fisher Scientific, Waltham, MA) in KBr with absorption in cm-1. All of the
13
NMR spectra were recorded on a Varian model UNITYInova 500 MHz (1H: 500 C:
1
125 MHz) NMR spectrometer. Chemical shifts of H is reported in parts per million
(ppm) from tetramethylsilane (TMS) Chemical shifts are given in ppm relative to TMS
as an internal standard in DMSO-d6 as a solvent, the coupling constants J are given in
Hz and spectra were recorded in parts per million (ppm).
General procedure for the synthesis of 2,3,6-trioxo-8-phenyl-1,3,4,6-tetrahydro-2H-
pyrido[1,2-b][1,2,4]triazine-7,9-dicarbonitrile (5a-j)
A mixture of oxalyl dichloride (0.002 mmol), 1,6-diamino-2-oxo-4-aryl-1,2-
dihydropyridine-3,5-dicarbonitrile (0.002 mmol) and Pyridine (0.003 mmol) in DMF (5
ml) was stirred at reflux conditions for 12 hours. The progress of the reaction was
monitored by TLC. Upon completion the reaction, precipitate was filtrated and washed
with water and then water-ethanol several times to afford the pure 2,3,6-trioxo-8-
phenyl-1,3,4,6-tetrahydro-2H-pyrido[1,2-b][1,2,4]triazine-7,9-dicarbonitrile.
4

2,3,6-trioxo-8-phenyl-1,3,4,6-tetrahydro-2H-pyrido[1,2-b][1,2,4]triazine-7,9-
dicarbonitrile (5a)
Brown solid, m.p 280-285 °C. FT-IR (KBr) υmax (cm-1) 2213 (CN), 1596 (C=O), 1467
1
(C=C) cm-1. H NMR (DMSO-d6, 500 MHZ): δ 8.47 (s, 1H. NH), 7.54 (t, 3H, arom),
7.48 (d, 2H, arom) 5.63 (s, 1H, NH), 13C NMR (DMSO-d6, 125 MHz): 160.0, 159.6,
157.1, 135.0, 130.6, 129.0, 128.7, 128.4, 116.8, 115.9, 86.8, 74.7 ppm.
2,3,6-trioxo-8-(p-tolyl)-1,3,4,6-tetrahydro-2H-pyrido[1,2-b][1,2,4]triazine-7,9-
dicarbonitrile (5b)
Brown solid, m.p 315-320 °C. FT-IR (KBr) υmax (cm-1) 2219 (CN), 1642, 1595 (C=O),
1
1459 (C=C) cm-1. H NMR (DMSO-d6, 500 MHZ): δ 8.41 (s, 1H. NH), 7.37 (d, 2H,
J=8.0 Hz), 7.34 (d, 2H, J=8.0 Hz), 5.61 (s, 1H, NH), 2.07 (s, 3H, Me), 13C NMR
(DMSO-d6, 125 MHz): 160.0, 159.7, 157.0, 140.4, 132.1, 129.5, 129.3, 128.4, 116.8,
116.0, 86.7, 74.7, 21.4 ppm.
8-(4-methoxyphenyl)-2,3,6-trioxo-1,3,4,6-tetrahydro-2H-pyrido[1,2-b][1,2,4]triazine-
7,9-dicarbonitrile (5c)
Brown solid, m.p 298-302 °C. FT-IR (KBr) υmax (cm-1) 2213 (CN), 1664, 1609 (C=O),
1
1472 (C=C), 1174 (CO) cm-1. H NMR (DMSO-d6, 500 MHZ): δ 8.41 (s, 1H. NH),
7.45 (d, 2H, J=10.0 Hz), 7.09 (d, 2H, J=10.0 Hz), 5.64 (s, 1H, NH), 3.83 (s, 3H, OMe),
13C NMR (DMSO-d6, 125 MHz): 161.1, 159.7, 157.2, 157.0, 130.2, 126.9, 117.0,
116.1, 114.4, 86.7, 74.6, 55.8 ppm.
8-(4-chlorophenyl)-2,3,6-trioxo-1,3,4,6-tetrahydro-2H-pyrido[1,2-b][1,2,4]triazine-
7,9-dicarbonitrile (5d)
Brown solid, m.p 290-296 °C. FT-IR (KBr) υmax (cm-1) 2220 (CN), 1636, 1595 (C=O),
1
1466 (C=C) cm-1. H NMR (DMSO-d6, 500 MHZ): δ 8.51 (s, 1H. NH), 7.62 (d, 2H,
13
J=7.0 Hz), 7.52 (d, 2H, J=7.0 Hz), 5.66 (s, 1H, NH), C NMR (DMSO-d6, 125 MHz):
160.7, 160.0, 159.8, 157.1, 130.3, 125.2, 117.1, 116.2, 115.6, 86.5, 74.5 ppm.
8-(4-hydroxyphenyl)-2,3,6-trioxo-1,3,4,6-tetrahydro-2H-pyrido[1,2-b][1,2,4]triazine-
7,9-dicarbonitrile (5e)
Brown solid, m.p 310-314 °C. FT-IR (KBr) υmax (cm-1) 3301 (OH), 2212 (CN),
1
1655,1606 (C=O), 1551,1519 (C=C), H NMR (DMSO-d6, 500 MHZ): δ 8.38 (s, 1H.
NH), 7.34 (d, 2H, J=8.5 Hz), 6.89 (d, 2H, J=8.5 Hz), 5.62 (s, 1H, NH), 4.63 (s, 1H,
OH), 13C NMR (DMSO-d6, 125 MHz): 160.0, 159.8, 159.7, 157.1, 130.3, 125.2,
117.1, 116.6, 116.2, 115.7, 86.6, 74.5ppm.
5

8-(4-fluorophenyl)-2,3,6-trioxo-1,3,4,6-tetrahydro-2H-pyrido[1,2-b][1,2,4]triazine-7,9-
dicarbonitrile (5f)
Brown solid, m.p 319-324 °C. FT-IR (KBr) υmax (cm-1) 2216 (CN), 1664, 1617 (C=O),
1
1467 (C=C), 1238 (CF) cm-1. H NMR (DMSO-d6, 500 MHZ): δ 8.49 (s, 1H. NH),
13
7.56 (d, 2H, J=8.0 Hz), 7.55 (d, 2H, J=8.0 Hz), 5.66 (s, 1H, NH), C NMR (DMSO-d6,
125 MHz): 164.4, 162.7, 162.4, 159.6, 159.0, 157.0, 131.4, 131.0, 116.7, 116.0, 115.8,
86.9, 74.9 ppm.
8-(4-bromophenyl)-2,3,6-trioxo-1,3,4,6-tetrahydro-2H-pyrido[1,2-b][1,2,4]triazine-
7,9-dicarbonitrile (5g)
Brown solid, m.p 298-302 °C. FT-IR (KBr) υmax (cm-1) 2219 (CN), 1595 (C=O), 1465
1
(C=C) cm-1. H NMR (DMSO-d6, 500 MHZ): δ 8.56 (s, 1H. NH), 7.77 (d, 2H, J=10.0
Hz), 7.49 (d, 2H, J=10.0 Hz), 5.64 (s, 1H, NH), 13C NMR (DMSO-d6, 125 MHz):
159.5, 158.8, 157.0, 134.2, 132.1, 131.8, 131.2, 130.6, 124.2, 116.6, 115.8, 86.7, 74.6
ppm.
8-(4-(dimethylamino)phenyl)-2,3,6-trioxo-1,3,4,6-tetrahydro-2H-pyrido[1,2-
b][1,2,4]triazine -7,9-dicarbonitrile (5h)
Brown solid, m.p 317-320 °C. FT-IR (KBr) υmax (cm-1) 2211 (CN), 1602 (C=O), 1461
(C=C), 1292-1194 (NCH3) cm-1. 1H NMR (DMSO-d6, 500 MHZ): δ 8.31 (s, 1H. NH),
7.36 (d, 2H, J=10.0 Hz), 6.80 (d, 2H, J=10.0 Hz), 4.77 (s, 1H, NH), 2.09 (s, 6H,
NCH3), 13C NMR (DMSO-d6, 125 MHz): 160.1, 157.1, 151.9, 130.0, 127.1, 126.1.
121.2, 117.4, 116.6, 111.6, 111.4, 85.9, 74.1 ppm.
8-(3-chlorophenyl)-2,3,6-trioxo-1,3,4,6-tetrahydro-2H-pyrido[1,2-b][1,2,4]triazine-
7,9-dicarbonitrile (5i)
Brown solid, m.p 315-319 °C. FT-IR (KBr) υmax (cm-1) 2219 (CN), 1637 (C=O), 1509
1
(C=C) cm-1. H NMR (DMSO-d6, 500 MHZ): δ 8.58 (s, 1H. NH), 8.44 (s, 1H, arom),
8.06 (s, 1H, NH), 7.88 (d, 1H, arom), 7.58 (d, 1H, arom), 7.48 (m, 1H, arom), 13C NMR
(DMSO-d6, 125 MHz): 164.2, 158.7, 156.6, 153.3, 146.2, 143.0, 138.4, 133.3, 130.7,
128.8, 127.4, 118.5, 117.0, 83.3, 77.9 ppm.
8-(2-nitrophenyl)-2,3,6-trioxo-1,3,4,6-tetrahydro-2H-pyrido[1,2-b][1,2,4]triazine-7,9-
dicarbonitrile (5j)
Brown solid, m.p 310-316 °C. FT-IR (KBr) υmax (cm-1) 2218 (CN), 1669, 1593
-1. 1H NMR (DMSO-d6, 500 MHZ): δ 8.62 (s,
1H. NH), 8.33 (d, 1H, J=10.0 Hz), 7.99 (t, 1H, arom), 7.86 (t, 1H, arom), 7.69 (d, 1H,
6

J=10.0 Hz), 5.27 (s, 1H, NH), 13C NMR (DMSO-d6, 125 MHz): 159.2, 158.0, 156.9,
146.7, 135.4, 132.2, 131.2, 129.9, 125.6, 124.4, 115.9, 115.1, 86.5, 74.4 ppm.
2.2 Computational methods
The all structures were fully optimized at the M06-2X/6-311++G(d,p) level of theory
with the Gaussian 09 program package [35]. The harmonic vibrational frequency
calculations were performed at the same level. The frequency calculations confirmed
that the resulting structures were as minima.
The nature of the intramolecular hydrogen bond has been vividly characterized with the
help of Bader’s quantum theory of atoms in molecules (QTAIM). The AIM [36]
analysis at the M06-2X /6-311++G(d,p) level was done using AIM 2000 software [37].
Natural bond orbital (NBO) analysis [38] has been performed at the M06-2X /6-
311++G(d,p) level of theory using NBO3.1 program as implemented in the Gaussian 09
package on the optimized structures.
3. Results and Discussion
3.1 Synthesis
The synthesis of triazine compounds due to its extensive use is developing every day
and has been widely developed in recent years. Therefore, the development of efficient
and clean approaches using new catalysts for the synthesis of 2,3,6-trioxo-8-aryl-
1,2,4]triazine-7,9-dicarbonitrile derivative is an important task for organic chemists.
At the present work firstly, the synthesis of a new series 2,3,6-trioxo-8-aryl-1,3,4,6-
tetrahydro-]2H[pyrido]1,2-b[ ]1,2,4[ triazin-7,9-dicarbonitryl derivatives has been
investigated. The intermediate 4-(aryl)-1,6-diamino-2-oxo-1,2- dihydropyridine-3,5-
dicarbonitrile derivatives were synthesized [39] from cyclocondensation of
cyanoacetohydrazide with benzilidenemalononitriles under reflux conditions in absolute
ethanol in the presence of piperidine (Scheme1).
Scheme 1: Synthesis of 1,6-diamino-2-oxo-4-phenyl-1,2-dihydropyridine-3,5-
dicarbonitrile derivative
7

These compounds have two amino groups nucleophile which can be participate in
nucleophilic reactions and more favored to the ring closure reactions.
The 1,6-diamino-2-oxo-4-aryl-1,2-dihydropyridine-3,5-dicarbonitrile derivatives (3a-j)
were reacted with oxalyl chloride (2) in the presence of pyridine and
dimethylformamide as solvent. The reaction carried out smoothly in
dimethylformamide at reflux conditions to afford the corresponding 2,3,6-trioxo-8-aryl-
1,3,4,6-tetrahydro-2H-pyrido[1,2-b][1,2,4] triazine-7,9-dicarbonitrile derivatives (5a-j)
in 82% yield after 12 h. The synthetic pathway is shown in Schemes 2.
O
O
H
N
Pyridine
DMF
NC
NH2
NH2
NC
O
O
O
O
N
N
+
12h, Reflux
Cl
Cl
N
H
CN
3a-j
CN
R
R
2
5a-j
R= H, Me, OMe, Cl, NO2, N(CH3), F, Br, OH
Scheme 2: Synthesis of 2,3,6-trioxo-8-phenyl-1,3,4,6-tetrahydro-2H-pyrido[1,2-
b][1,2,4]triazine-7,9-dicarbonitrile derivative (5a-j)
The scope and generality of this procedure is illustrated with different 1,6-diamino-2-
oxo-4-phenyl-1,2-dihydropyridine-3,5-dicarbonitriles (Table 1). All products were fully
characterized and confirmed by NMR, IR spectra.
The IR spectrum of 2,3,6-trioxo-8-phenyl-1,3,4,6-tetrahydro-2H-pyrido[1,2-
b][1,2,4]triazine-7,9-dicarbonitrile (5a) showed absorption band at 2213, 2361 and
2342 cm-1 corresponding to -CN groups stretching. Also absorption bands of C=O
groups appeared at 1596, 1523 cm-1. The 1H-NMR spectrum of 5a showed a signal at δ
8.45 ppm corresponding to the NH group, a multiplet at δ 7.54 ppm, and a doublet at δ
7.48 ppm due to the aromatic protons, δ5.63 ppm corresponding to the NH group. The
13C-NMR spectrum of 5a displayed downfield signals at δ 160.0 ppm carbonyls groups
and δ 159.6 and 157.1 ppm for carbon nitrogen transplant and at δ135.5, 130.6, 129.5,
128.7, 128.4, 116.8, 115.9 ppm for aromatic carbons and δ86.8 and 74.7 ppm for two
1
groups of cyanide. The IR, H-NMR and 13C NMR data for all of the synthesized
compounds have been provided in the experimental section.
A
plausible mechanism for reaction of 1,6-diamino-2-oxo-4-phenyl-1,2-
dihydropyridine-3,5-dicarbonitrile with oxalyl shown in scheme 3.
8

O
O
O
H
H
O
NC
N
OH
NC
N
NC
NH2
NH2
O
O
N
O
N
N
Cl
NH2 Cl
O
+
-HCl
Cl
NH2
Cl
Cl
CN
CN
CN
R
R
R
2
3
B
A
O
O
H
H
NC
N
O
O
NC
N
O
N
N
-HCl
OH
Cl
N
H
N
H
CN
CN
R
R
5
C
Scheme 3. A plausible reaction pathway
Table 1: Synthesis of 2,3,6-trioxo-8-aryl-1,3,4,6-tetrahydro-]2H[pyrido]1,2-b]
[1,2,4[triazin-7,9-dicarbonitryl (5a-j)
Entry
1,2-Diamino derivatives (3a-j)
Products (5a-j)
Yield
(%)
O
O
H
N
NC
NC
NH2
NH2
NC
NC
O
O
N
N
77
80
1
N
H
CN
CN
5a
3a
O
O
H
N
O
O
NH2
NH2
N
N
2
3
N
H
CN
CN
H3C
H3C
3b
5b
O
O
H
NC
NH2
NH2
NC
N
O
N
N
82
N
H
O
CN
CN
H3CO
H3CO
3c
5c
9

O
O
H
N
NC
NH2
NH2
NC
O
O
N
N
4
5
N
H
85
75
74
CN
CN
Cl
Cl
3d
5d
O
O
H
N
NC
NH2
NH2
NC
O
O
N
N
N
H
CN
CN
HO
HO
3e
5e
O
O
H
NC
NH2
NH2
NC
N
O
N
N
6
N
H
O
CN
CN
F
F
3f
5f
O
O
H
N
NC
NH2
NH2
NC
O
O
N
N
7
8
N
H
70
83
CN
CN
Br
Br
3g
5g
O
O
H
NC
NH2
NC
N
O
N
N
NH2
N
H
O
H3C
CN
H3C
CN
5h
N
N
3h
CH3
CH3
O
H
N
O
NC
O
O
N
9
NC
NH2
N
67
Cl
N
H
Cl
NH2
CN
CN
3i
5i
10

O
O
H
N
NC
NH2
NH2
NC
O
O
N
N
O2N
O2N
10
89
N
H
CN
CN
5j
3j
3.2. Theoretical Calculations
The optimized structural parameters which were optimized at M06-2X /6-311++G(d,p)
level of theory are gathered in Table 2.
Table 2. The most important optimized parameters (in Å) and estimated H-bond energy (in kJ mol-1)
rO···H
rN-H
q1
q2
EHB
2.0598
2.0596
2.0595
2.0619
2.0601
2.0616
2.0620
2.0569
1.01912
1.01915
1.01913
1.01920
1.01908
1.01910
1.01921
1.01926
-0.5203
-0.5202
-0.5202
-0.5214
-0.5205
-0.5213
-0.5214
-0.5188
3.0789
3.0788
3.0786
3.0811
3.0792
3.0807
3.0812
3.0762
-26.604
-26.596
-26.599
-26.501
-26.581
-26.514
-26.512
-26.719
5a
5b
5c
5d
5e
5f
5g
5h
The H-bond length is often considered as a relative strength of H-bond [40]. The results
show that the O···H bond length decreases by increasing electron-withdrawing power
of substituents. If a hydrogen bond is considered as A-H···B, a hydrogen bridge forms
between two atoms A and B. The parameters q1 and q2 are defined as follows:
q1=0.5(r1-r2)
q2=r1+r2
which r1 and r2 are the distance between hydrogen atom with A and B atoms,
respectively. The q1 and q2 are very useful for studying hydrogen bonding phenomena.
The q1 and q2 values are collected in Table 2. The hydrogen bond strength increases by
reducing the amount of q2. It is observed that the |q1| and q2 are decreased on going
from electron-withdrawing substituents to electron-donating ones.
11

In 1998, Espinosa, Molins and Lecomte [41] estimated the energies of the
intermolecular H-bonds by the formula that could be extracted from the potential
electron density at the BCPs of the H-bonds:
EHB = 0.5 V (rBCP
)
This equation is valid for all types of Van der Waals interactions, hydrogen bonds and
weak junctions such as H···H and CH···O interactions. The Table 2 indicates that the
value of |EHB| increases/decreases in the presence of electron donor/electron acceptor
substituents. Also, the EHB values versus rO···H are shown in Fig. 1. As can be seen,
there is a good linear relationship between EHB and hydrogen bond length which the
correlation coefficient is approximately equal to 0.99.
-26.45
-26.5
R2 = 0.99
-26.55
-26.6
-26.65
-26.7
-26.75
2.056
2.057
2.058
2.059
2.06
2.061
2.062
2.063
rO···H (Å)
Fig. 1. The linear correlation between EHB and O···H bond length
The formation of a bond path between the two atoms indicates that the atoms are
bonded to one another. The relative strength of hydrogen bonding is evaluated by the
electronic density () at the bond critical point and it's Laplacian (2). The Laplacian
of electron density (∇2ρ) is an important parameter in the specification of chemical
bonding. The magnitude of ∇2ρ in the critical point is a measure of the concentration of
electron density in the interatomic space. In covalent and polar bonds, the sign of ∇2ρ
(rc) is negative and the accumulation of charge points interaction line. In ionic or van
der Waals bonds, the sign of ∇2ρ (rc) is positive and the charge is depleted in the
interatomic space.
12

∇2ρ (rc) at CPs is related to kinetic energy (G (rc)) and potential energy (V (rc)) by the
following equation:
1/4∇2ρ (rc) = 2G (rc) + V (rc)
The total electron energy density (H (rc)) is obtained from the sum of V (rc) and G (rc):
H (rc) = V (rc) + G (rc)
The values of ρ, 2ρ and H at the BCP of O···H interaction were calculated by means
of the AIM method on the wave functions obtained at M06-2X/6-311++G(d,p) level of
theory (see Table 3). A typical molecular graph is represented in Fig. 2. It is obvious
from this Figure, a bond critical point was found between the hydrogen atom and the
oxygen atom. A higher value of  demonstrates a stronger hydrogen bond. It is
observed that from Table 3, the amount of electron density increases by increasing of
electron-donating strength of substituents. This order indicates that the hydrogen bond
is strengthening by electron-donating substituents.
Fig. 2. A typical molecular graph. Small red spheres, small yellow spheres, and lines represent bond
critical points (BCP), ring critical points (RCP), and bond paths, respectively.
Table 3. Topological electron density properties at O···H bond critical point in atomic unit

2.476
2.477
2.478
2.466
2.475

11.603
11.595
11.589
11.581
11.589
H102
0.4371
0.4364
0.4356
0.4383
0.4362
5a
5b
5c
5d
5e
13

2.467
2.467
2.491
11.583
11.586
11.603
0.4381
0.4385
0.4327
5f
5g
5h
Rozas et al. [42] suggested that the hydrogen bonds are classified into three groups:
i) Weak hydrogen bonds: ∇2ρ (rBCP) > 0 and HBCP > 0
ii) Medium hydrogen bond: ∇2ρ (rBCP) > 0 and HBCP < 0
iii) Strong hydrogen bond: ∇2ρ (rBCP) < 0 and HBCP < 0
According to the results, the O···H hydrogen bond in all complexes is classified as
weak H- bond. Also, a correlation can be found between ρO···H and EHB with R2=0.99
(Fig. 3).
-26.45
-26.5
-26.55
R2 = 0.99
-26.6
-26.65
-26.7
-26.75
2.46
2.465
2.47
2.475
2.48
2.485
2.49
2.495
 102 (e/au3)
Fig. 3. The linear correlation between EHB and HB.
The Bader theory [43-45] can be used to study the nature of intramolecular hydrogen
bond. On the other hand, the NBO method has been known as a powerful tool to gain
knowledge into orbital interactions, such as stabilization energies caused by electron
transfer and hyperconjugation stabilization energies [46,47]. The Bader theory is also
applied for the unconventional hydrogen bond. The NBO analysis has been performed
on the studied complexes at M06-2X /6-311++G(d,p) level. The natural charges on the
O and H atoms obtained using NBO calculations at the M06-2X /6-311++G(d,p) level
of theory are gathered in Table 4. These results show that the natural charge on O and H
atoms involved in the intramolecular hydrogen bond is negative and positive value,
14

respectively. Therefore more charge density is focused on O atom and is electron-rich.
The interaction between filled and antibonding orbitals represents the deviation of the
molecule from the Lewis structure and can be used as a measure of delocalization due
to the presence of hydrogen bonding interaction [34]. The NBO calculations show that
the most important donor-acceptor interaction is Lp(O) → σ*(N-H). The energy values of
this interaction are reported in Table 4. This interaction is the most interaction
responsible for the intramolecular hydrogen bond. It can be noticed that the stabilization
energy E(2) is strengthened by increasing electron-donating power of substituents. .
Table 4. The natural atomic charges and second order perturbation energies E(2) (kcal mol-1) for
LPO*NH related to hydrogen bond formation obtained using NBO calculations at M06-2X/6-
311++G(d,p) level of theory.
q(O)
q(H)
LP(1) O BD*N- H
0.77
LP(2) O BD*N- H

-0.6008
-0.6023
-0.6052
-0.5985
-0.6029
-0.5991
-0.5984
-0.6103
0.4649
0.4646
0.4644
0.4649
0.4648
0.4651
0.4652
0.4642
2.49
2.5
3.26
3.27
3.29
3.21
3.26
3.22
3.21
3.36
5a
5b
5c
5d
5e
5f
0.77
0.78
2.51
2.45
2.49
2.46
2.45
2.56
0.76
0.77
0.76
0.76
5g
5h
0.80
4. Conclusion
We have described an efficient method for the preparation of potentially biologically
[2H]-pyrido [1,2-b]1,2,4-triazines derivatives via reaction of 1,6-diamino-2-oxo-4-
phenyl-1,2-dihydropyridine-3,5-dicarbonitrile derivatives with oxalyl chloride. The
structures of 5a-5h compounds were calculated using DFT calculations. The energies of
the intramolecular hydrogen bonds were evaluated by the EML formula. The estimated
EHB values decrease/increase by electron-withdrawing/ electron-donating groups.
Analyses of the second order stabilization NBO energies (ΔE (2)) shows that the main
contribution to stabilization energy is correspond to LPO→σ*N-H. Topological
parameters at bond critical points (BCP) are calculated to analyze the strength and
15

nature of intramolecular interactions in 5a-5h compounds by AIM theory. The results
from AIM theoretical analysis are in qualitative agreement with EHB and NBO results.
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credit author statement
Marziyeh Mohammadi: Conceptualization, Validation, Resources, Writing-Original
draft preparation
Ali Darehkordi: Supervision
Maryam Tahmasby: Methodology and Investigation
Farzaneh Nejadkhorasani: Data curation
Declaration of interests
■ The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in this
paper.
21