Experimental and quantum chemical studies of the structural and spectral properties of novel diazenyl formazans

Article abstract of DOI:10.1016/j.molstruc.2019.04.055

A new series of 3-(p-substitutedphenyl)-5-phenyl-1-(4-phenyldiazenyl)phenylformazans were synthesized by coupling p-substituted phenyl or pyridinylhydrazones with p-aminoazobenzene diazonium chloride. All compounds were characterized by FT-IR, UV–Vis, ¹H NMR, and ¹³C NMR spectroscopic techniques, and HR-MS. DFT was used to calculate the molecular structures and ¹H and ¹³C chemical shift values of the synthesized compounds with PBE1PBE functional and 6-311G(2d,2p)basis set combination. The IR spectra of the novel formazans were calculated using DFT at PBE1PBE/6-311G(d,p)level of theory. The electronic absorption spectra of the optimized structures were evaluated by TD-DFT method at PBE1PBE/6-311G(2d,2p)level of theory. The absorption spectra of the synthesized diazenyl formazans were investigated in three different solvents. A good correlation was established between the experimental data and calculated results.

Full text of DOI:10.1016/j.molstruc.2019.04.055

Accepted Manuscript
Experimental and quantum chemical studies of the structural and spectral properties
of novel diazenyl formazans
Habibe Tezcan, Hülya Şenöz, Nesrin Tokay
PII:
S0022-2860(19)30456-9
DOI:
https://doi.org/10.1016/j.molstruc.2019.04.055
MOLSTR 26427
Reference:
To appear in: Journal of Molecular Structure
Received Date: 15 January 2019
Revised Date: 10 April 2019
Accepted Date: 11 April 2019
Please cite this article as: H. Tezcan, Hü. Şenöz, N. Tokay, Experimental and quantum chemical studies
of the structural and spectral properties of novel diazenyl formazans, Journal of Molecular Structure
(2019), doi: https://doi.org/10.1016/j.molstruc.2019.04.055.
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ACCEPTED MANUSCRIPT
Experimental and quantum chemical studies of the structural and spectral properties of
novel diazenyl formazans
a,
b
b
Habibe Tezcan *, Hülya Şenöz , Nesrin Tokay
a
Gazi University, Faculty of Education, Department of Chemistry, 06500 Ankara, Turkey
b
Hacettepe University, Faculty of Sciences Department of Chemistry, 06800 Ankara, Turkey
ABSTRACT
A new series of 3-(p-substitutedphenyl)-5-phenyl-1-(4-phenyldiazenyl)phenylformazans were
synthesized by coupling p-substituted phenyl or pyridinylhydrazones with p-
aminoazobenzene diazonium chloride. All compounds were characterized by FT-IR, UV-Vis,
1
13
H NMR, and C NMR spectroscopic techniques, and HR-MS. DFT was used to calculate
1
13
the molecular structures and H and C chemical shift values of the synthesized compounds
with PBE1PBE functional and 6-311G(2d,2p) basis set combination. The IR spectra of the
novel formazans were calculated using DFT at PBE1PBE/6-311G(d,p) level of theory. The
electronic absorption spectra of the optimized structures were evaluated by TD-DFT method
at PBE1PBE/6-311G(2d,2p) level of theory. The absorption spectra of the synthesized
diazenyl formazans were investigated in three different solvents. A good correlation was
established between the experimental data and calculated results.
Keywords: Formazan, NMR, UV-Vis, substituent effect, DFT, TD-DFT

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1
. Introduction
Formazans are compounds containing the characteristic azohydrazone group:
It is the main skeleton of formazans. Although aromatic formazans were first described as
early as 1892 by von Pechmann and Bamberger [1,2] chemistry of formazans was not
,
the
developed until the 1940s when Kuhn and Jerchel found applications for tetrazolium salts in
biochemistry as indicators for oxidation-reduction processes [3,4]. Formazans exhibit a
variety of biological activities, such as antimicrobial [5-7], anti-inflammatory [5,8,9],
antitubercular [5,10-12], anticancer [13-15], anti-HIV [15,16], antifertility [17], and
antiproliferative [18,19] activities.
In addition, formazans are multidentate ligands; they exhibit a strong coordination and
form stable complexes with heavy metals [20-22]. In the field of coordination chemistry, the
interest in formazans is mainly focused on a description of their different coordination modes
to metals and the novel applications of such compounds.
Formazans and their complexes are widely used as dyes and applied for different purposes
as charge-generating compounds, biochemical markers for redox processes, photo- and
thermo-chromic materials [20,23,24], analytical reagents [20,22], and synthons for
heterocyclic compounds [20,25,26].
Theoretical quantum chemistry methods based on the density functional theory (DFT) are
1
13
widely used to determine the optimized geometries, IR spectra, H NMR, and C NMR
chemical shifts of compounds [27-29]. Furthermore, the time-dependent density functional
theory (TD-DFT) is used to predict the absorption spectra of dyes [30-32]. We have
previously reported the results of TD-DFT calculations of formazans [33-35].
2

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Scheme 1. Structures of the studied diazenyl formazans.
The aim of this work was dealt with the synthesis and characterization of 3-(p-
substitutedphenyl)-5-phenyl-1-(4-phenyldiazenyl)phenyl formazans (5a-5g), and 3-(p-
substitutedphenyl)-1-(4-phenyldiazenyl)phenyl-5-(pyridin-2-yl)formazans (5h,5i) (Scheme 1)
and to provide a complete description of the spectral and physicochemical features both
experimentally and theoretically. The structures of the novel hydrazone (3i) and diazenyl
1
13
formazans (5a–5i) were characterized using HR-MS, FT-IR, H NMR, C NMR, and UV-Vis
spectroscopy methods. Additionally, we modeled their geometrical parameters, IR spectra,
1
13
and H and C NMR chemical shifts using DFT and UV-Vis spectra by TD-DFT methods.
The frontier molecular orbitals, details of quantum molecular descriptors, and the molecular
electrostatic potential analysis of the new compounds have also been described.
2
. Experimental
2
.1. Materials and instruments
Commercially available substituted aldehydes, hydrazines, p-aminoazobenzene, and other
reagents were used. Organic solvents were purchased from Sigma-Aldrich and used without
further purification. The melting points of the synthesized compounds were recorded using an
Optimelt Automated melting point apparatus. The FT-IR absorption spectra were determined
3

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by an ATR (Nicolet iS10) spectrometer. The NMR spectra of the synthesized compounds
1
13
were recorded on a Bruker DPX 400 spectrometer (400 MHz for H and 100 MHz for C)
with TMS as an internal reference. Their coupling constants (J) were given in Hz. The
electronic spectra of the synthesized diazenyl formazans were determined in DMSO, MeOH,
and toluene by using a SHIMADZU UV-1800 UV-VIS spectrophotometer. Their mass
spectra were recorded on an Agilent 1200/6210 high resolution mass time-of-flight (TOF)
LC/MS spectrometer.
2
2
.2. Synthesis
.2.1. Synthesis of hydrazones (3a–3i)
The p-substituted phenyl or pyridinylhydrazones (3a–3i) were prepared according to
previously reported methods [36-38]. p-Substituted aldehydes (1a–1g) were dissolved in
EtOH and subsequently mixed with the hydrazines (2). The reaction mixtures were refluxed
for 10 min, and the formed precipitates were filtered and recrystallized from EtOH.
Compounds 3a–3h have been synthesized previously while 3i has not been reported before.
The general route is outlined in Scheme 2.
Scheme 2. Synthesis of hydrazones and diazenyl formazans.
4

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2
.2.1.1 Methyl 4-((2-(pyridin-2-yl)hydrazono)methyl)benzoate (3i)
–
1
White crystals; yield: 88%; mp: 255 °C. FT-IR (cm ): 3289 (N–H), 2949 (Ar–H), 2866
(C–H), 1705 (C=O), 1646 (C=C), 1598 (C=N), 1454 1423, 1267, 1107, 1081, 917, 862, 766,
1
6
8
1
1
1
1
97. H NMR(CDCl ) δ/ppm: 3.91 (s, 3H, OCH ), 6.74–6.77 (m, 1H, ArH), 7.32–7.34 (d, J =
3
3
.00 Hz, 1H, ArH), 7.55–7.61 (m, 1H, ArH), 7.65–7.67 (d, J = 8.00 Hz, 2H, ArH), 7.75 (s,
H, CH), 7.97–7.99 (d, J = 8.00 Hz, 2H, ArH), 8.09–8.10 (d, J = 4.00 Hz, 1H, ArH), 8.63 (s,
1
3
H, NH). C NMR(CDCl ) δ/ppm: 52.21 (OCH ), 107.67 (ArC), 116.36 (ArC), 126.13 (ArC),
3
3
29.98 (ArC), 137.50 (ArC), 138.26 (ArC), 139.22 (ArC), 147.67 (ArC), 156.23 (C=N),
66.80 (C=O). UV-Vis: ꢀꢁꢂꢃꢄ (DMSO): 364 nm,ꢅꢀꢁꢂꢃꢄ (MeOH): 353 nm, ꢀꢁꢂꢃꢄ
+
(
toluene):354 nm. HR-MS (ESI) m/z found: [M+H] 256.1086; molecular formula
+
C H N O requires [M+H] 256.1207.
14
13
3
2
2
.2.2. General procedure for the synthesis of formazans (5a–5i)
The diazenyl formazans (5a 5i) were prepared according to the literature procedure
Scheme 2) [39,40]. The synthesized hydrazone (5 mmol) was dissolved in pyridine (30 mL)
–
(
at 0°C, followed by the addition of 20% NaOH solution (10 mL). In order to prepare p-
aminoazobenzene diazonium chloride (4), sodium nitrite (5 mmol) was dissolved in water (5
mL) at 0°C and added slowly dropwise into a solution of p-aminoazobenzene (5 mmol) in
HCl(aq) (50% v/v) (30 mL) over the duration of 1 h. The mixture was stirred for an additional
1
h. The diazonium salt of the p-aminoazobenzene solution then was added dropwise to the
hydrazone solutions by continuous stirring for 1 h, and the reaction mixture was stirred for 3
h. Subsequently, it was poured into 250 mL of ice-cold water with continuous stirring. The
resulting solid was filtered, washed with water, and recrystallized from EtOH.
2
.2.2 1.3,5-diphenyl-1-(4-(phenyldiazenyl)phenyl)formazan (5a)
–
1
Dark solid; yield: 75%; mp:191 °C. FT-IR (cm ): 3052 (N–H), 3027 (Ar–H), 1597 (C=C),
1
506 (C=N), 1492, 1455, 1436 (N=N), 1409, 1348, 1314, 1223, 1179, 1127, 1097, 1072,
5

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1
1
7
8
041, 1016, 919, 832, 753, 683, 664, 631. H NMR(CDCl ) δ/ppm: 7.39–7.56 (m, 9H, ArH),
3
.67–7.69 (d, J = 8.00 Hz, 2H, ArH), 7.86–7.88 (d, J = 8.00 Hz, 2H, ArH), 7.92–7.94 (d, J =
.00 Hz, 2H, ArH), 8.01–8.03 (d, J = 8.00 Hz, 2H, ArH), 8.16–8.18 (d, J = 8.00 Hz, 2H,
1
3
ArH), 15.39 (s, 1H, NH). C NMR(CDCl ) δ/ppm: 117.13 (ArC), 120.95 (ArC), 122.79
3
(ArC), 124.68 (ArC), 126.12(ArC), 128.07 (ArC), 128.51 (ArC), 129.13 (ArC), 129.52(ArC),
1
1
30.11 (ArC), 130.79 (ArC), 137.02 (ArC), 141.86 (ArC), 147.51 (ArC), 149.96 (ArC),
+
50.37 (ArC), 152.84 (C=N). HR-MS (ESI) m/z found: [M+H] 405.1845; molecular formula
+
C H N requires [M+H] 405.4665.
25
20
6
2
.2.2.2 3-(4-fluorophenyl)-5-phenyl-1-(4-(phenyldiazenyl)phenyl)formazan (5b)
–
1
Claret red solid; yield: 78%; mp: 190 °C. FT-IR (cm ): 3050 (N–H), 3028 (Ar–H), 1596
(C=C), 1500 (C=N), 1455 (N=N), 1438, 1404, 1348, 1222, 1183, 1147, 1130, 1018, 919, 836,
1
7
67, 684, 627, 596. H NMR(CDCl ) δ/ppm: 7.13–7.17 (t, 2H, ArH), 7.43–7.56 (m, 6H,
3
ArH), 7.64–7.66 (d, J = 8.00 Hz, 2H, ArH), 7.84–7.86 (d, J = 8.00 Hz, 2H, ArH), 7.91–7.93
d, J = 8.00 Hz, 2H, ArH), 8.01–8.03 (d, J = 8.00 Hz, 2H, ArH), 8.12–8.16 (dd, J = 5.60 Hz,
(
1
3
J= 3.20 Hz, 2H, ArH), 15.34 (s, 1H, NH). C NMR(CDCl ) δ/ppm: 115.46 (ArC), 116.98
3
(
ArC), 117.08 (ArC), 119.50 (ArC), 120.91 (ArC), 122.76 (ArC), 124.66 (ArC), 127.91
ArC), 129.12 (ArC), 129.52 (ArC), 130.80 (ArC), 131.18 (ArC), 133.15 (ArC), 141.18
ArC), 147.37 (ArC), 149.96 (ArC), 152.79 (C=N). HR-MS (ESI) m/z found:
(
(
+
+
[
M+H] 423.1712; molecular formula C H FN requires [M+H] 423.1655.
25 19 6
2
.2.2.3 3-(4-chlorophenyl)-5-phenyl-1-(4-(phenyldiazenyl)phenyl)formazan (5c)
–
1
Dark purple solid; yield: 68%; mp: 212 °C. FT-IR (cm ): 3058 (N–H), 3025 (Ar–H), 1596
(C=C), 1506 (C=N), 1487 (N=N), 1454, 1397, 1350, 1227, 1183, 1130, 1091, 1031, 1007,
1
9
7
8
19, 833, 766, 683. H NMR(CDCl ) δ/ppm: 7.34–7.40 (m, 4H), 7.44–7.48 (m, 4H, ArH),
3
.58–7.60 (d, J = 8.00 Hz, 2H, ArH), 7.76–7.78 (d, J = 8.00 Hz, 2H, ArH), 7.85–7.87 (d, J =
.00 Hz, 2H, ArH), 7.94–7.96 (d, J = 8.00 Hz, 2H, ArH), 8.02–8.04 (d, J = 8.00 Hz, 2H,
6

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1
3
ArH), 15.36 (s, 1H, NH). C NMR(CDCl ) δ/ppm: 117.27 (ArC), 120.88 (ArC), 122.82
3
(
(
(
ArC), 124.67 (ArC), 127.31 (ArC), 128.63 (ArC), 129.14 (ArC), 129.55 (ArC), 130.17
ArC), 130.85 (ArC), 133.89 (ArC), 135.59 (ArC), 140.92 (ArC), 147.35 (ArC), 150.10
+
ArC), 150.14 (ArC), 152.82 (C=N). HR-MS (ESI) m/z found: [M+H] 439.1327; molecular
+
formula C H ClN requires [M+H] 439.1360.
2
5
19
6
2
.2.2.4 3-(4-cyanophenyl)-5-phenyl-1-(4-(phenyldiazenyl)phenyl)formazan (5d)
–
1
Titian crystal; yield:76%; mp: 210 °C. FT-IR (cm ): 3054 (N–H), 3031(Ar–H), 2221
(C≡N), 1596 (C=C), 1500 (C=N), 1454 (N=N), 1404, 1353, 1239, 1134, 1071, 1032, 919,
1
8
8
41, 765, 684, 628, 549. H NMR(CDCl ) δ/ppm: 7.43–7.56 (m, 6H, ArH), 7.69–7.74 (dd, J =
3
.00 Hz, J = 8.00 Hz, 4H, ArH), 7.82–7.84 (d, J = 8.00 Hz, 2H, ArH), 7.92–7.94 (d, J = 8.00
Hz, 2H, ArH), 8.03–8.05 (d, J = 8.00 Hz, 2H, ArH), 8.26–8.28 (d, J = 8.00 Hz, 2H, ArH),
1
3
1
1
1
1
5.70 (s, 1H, NH). C NMR(CDCl ) δ/ppm: 110.88 (ArC), 117.85 (ArC), 120.73 (ArC),
3
22.88 (ArC), 123.02 (ArC), 124.67 (ArC), 126.12 (ArC), 129.65 (ArC), 130.29 (ArC),
31.07 (ArC), 132.35 ( ArC), 139.90 (ArC), 141.53 (ArC), 147.20 (ArC), 149.53 (ArC),
+
50.66 (ArC), 152.75 (C=N). HR-MS (ESI) m/z found: [M+H] 430.1788; molecular formula
+
C H N requires [M+H] 430.1702.
26
19
7
2
.2.2.5 3-(4-nitrophenyl)-5-phenyl-1-(4-(phenyldiazenyl)phenyl)formazan (5e)
–1
Claret solid; yield: 70%; mp: 240 °C. FT-IR (cm ): 3065 (N–H), 3034 (Ar–H), 1593
(
C=C), 1505 (C=N), 1456 (N=N), 1405, 1335 (NO ), 1244, 1180, 1149, 1033, 1109, 1073,
2
1
1
7
7
035, 984, 922, 854, 840, 768, 752, 684, 667, 625. H NMR(CDCl ) δ/ppm: 7.44–7.57 (m,
3
H, ArH), 7.71–7.74 (d, J = 8.00 Hz, 2H, ArH), 7.84–7.86 (d, J = 8.00 Hz, 2H, ArH), 7.93–
.95 (d, J = 8.00 Hz, 2H, ArH), 8.04–8.06 (d, J = 8.00 Hz, 2H, ArH), 8.29–8.34 (m, 3H,
1
3
ArH), 15.79 (s, 1H, NH). C NMR(CDCl ) δ/ppm: 118.04 (ArC), 120.71 (ArC), 122.90
3
(
ArC), 123.95 (ArC), 124.68 (ArC), 126.19 (ArC), 129.17 (ArC), 129.68 (ArC), 131.10
ArC), 139.90 (ArC), 141.53 (ArC), 147.18 (ArC), 148.98 (ArC), 149.81 (ArC), 150.52
(
7

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+
(
ArC), 152.87 (C=N). HR-MS (ESI) m/z found: [M+H] 450.1648; molecular formula
+
C H N O requires [M+H] 450.1600.
25
19
7
2
2
.2.2.6 3-(4-isopropylphenyl)-5-phenyl-1-(4-(phenyldiazenyl)phenyl)formazan (5f)
–
1
Dark purple solid; yield: 57%; mp: 220 °C. FT-IR (cm ): 3064 (N–H), 3037 (Ar–H), 2931
(C–H), 1592 (C=C), 1504 (C=N), 1455 (N=N), 1404, 1335, 1242, 1148, 1132, 1034, 983,
1
8
2
7
53, 793, 683, 666, 626. H NMR(CDCl ) δ/ppm:1.34–1.36 (d, J = 8.00 Hz, 6H, (CH ) C),
3
3 2
.95–3.02 (m, 1H, CH), 7.32–7.34 (d, J = 8.00 Hz, 2H, ArH), 7.41–7.48 (m, 2H, ArH), 7.50–
.55 (m, 4H, ArH), 7.64–7.66 (d, J = 8.00 Hz, 2H, ArH), 7.85–7.87 (d, J = 8.00 Hz, 2H,
ArH), 7.91–7.93 (d, J = 8.00 Hz, 2H, ArH), 8.00–8.02 (d, J = 8.00 Hz, 2H, ArH), 8.06–8.08
1
3
(
d, J = 8.00 Hz, 2H, ArH), 15.24 (s, 1H, NH). C NMR(CDCl ) δ/ppm: 24.01 (CH ), 33.92
3 3
(CH), 116.84 (ArC), 116.94 (ArC), 120.98 (ArC), 124.68 (ArC), 126.26 (ArC), 126.30 (ArC),
1
1
26.59 (ArC), 129.11 (ArC), 129.49 (ArC), 130.07 (ArC), 130.71 (ArC), 134.60 (ArC),
47.55 (ArC), 148.98 (ArC), 149.81 (ArC), 150.52 (ArC), 152.87 (C=N). HR-MS (ESI) m/z
+
+
found: [M+H] 447.2286; molecular formula C H N requires [M+H] 447.2219.
2
8
26
6
2
.2.2.7 Methyl 4-(((4-(phenyldiazenyl)phenyl)diazenyl)(2-phenylhydrazono)methyl)benzoate
(5g)
–
1
Dark brown solid; yield: 78%; mp: 190 °C. FT-IR (cm ): 3050 (N–H), 3028 (Ar–H), 2966
C–H), 1716 (C=O), 1595 (C=C), 1498 (C=N), 1455 (N=N), 1405, 1348, 1222, 1181, 1147
(
1
(
C–O–C), 1072, 1016, 919, 834, 766, 684. H NMR(CDCl ) δ/ppm:3.96 (s, 3H, OCH ), 7.42–
3
3
7
.56 (m, 6H, ArH), 7.68–7.70 (d, J = 8.00 Hz, 2H, ArH), 7.83–7.85 (d, J = 8.00 Hz, 2H,
ArH), 7.92–7.94 (d, J = 8.00 Hz, 2H, ArH), 8.01–8.03 (d, J = 8.00 Hz, 2H, ArH), 8.10–8.12
1
3
(
d, J = 8.00 Hz, 2H, ArH), 8.21–8.23 (d, J = 8.00 Hz. 2H, ArH), 15.61 (s, 1H, NH). C
NMR(CDCl ) δ/ppm: 52.13 (CH ), 117.62 (ArC), 120.80 (ArC), 122.85 (ArC), 124.66 (ArC),
3
3
1
1
25.65 (ArC), 129.13 (ArC), 129.16 (ArC), 129.85 (ArC), 130.13 (ArC), 130.92 (ArC),
40.80 (ArC), 141.48 (ArC), 147.37 (ArC), 149.84 (ArC), 150.42 (ArC), 152.81 (C=N),
8

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+
1
67.13 (C=O). HR-MS (ESI) m/z found: [M+H] 463.1856; molecular formula C H N O
2
7
22
6
2
+
requires [M+H] 463.1804.
2
.2.2.8 3-(4-chlorophenyl)-1-(4-(phenyldiazenyl)phenyl)-5-(pyridin-2-yl)formazan (5h)
–
1
Brick red solid; yield: 50%; mp: 178 °C. FT-IR (cm ): 3039 (N–H), 2929 (Ar–H), 1588
(C=C), 1573, 1501 (C=N), 1480 (N=N), 1435, 1413, 1399, 1354, 1298, 1213, 1091, 1025,
1
1
8
009, 852, 828, 768, 687. H NMR(CDCl ) δ/ppm: 6.99–7.02 (t, 1H, ArH), 7.42–7.44 (d, J =
3
.00 Hz, 2H, ArH), 7.52–7.57 (m, 3H, ArH), 7.73–7.85 (m, 2H, ArH), 7.97–7.99 (d, J = 8.00
Hz, 2H, ArH), 8.07–8.14 (m, 4H, ArH), 8.16–8.18 (d, J = 8.00 Hz, 2H, ArH), 8.32–8.33 (d, J
1
3
=
4.00 Hz, 1H, ArH), 14.36 (s, 1H, NH). C NMR(CDCl ) δ/ppm: 109.02 (ArC), 119.07
3
(
(
(
ArC), 123.17 (ArC), 124.05 (ArC), 127.80 (ArC), 128.63 (ArC), 129.22 (ArC), 131.65
ArC), 134.20 (ArC), 135.02 (ArC), 138.38 (ArC), 141.36 (ArC), 148.33 (ArC), 152.73
+
ArC), 153.65 (ArC), 154.12 (ArC), 156.32 (C=N). HR-MS (ESI) m/z found: [M+H]
+
4
40.1376; molecular formula C H ClN requires [M+H] 440.1312.
24
18
7
2
.2.2.9 Methyl 4-(((4-(phenyldiazenyl)phenyl)diazenyl)-(2-(piridin-2-yl)hydrazono)methyl)
benzoate (5i)
–
1
Claret solid; yield: 55%; mp: 250 °C. FT-IR (cm ): 3074(N–H), 3004 (Ar–H), 2934 (C–
H), 1716, 1573 (C=C), 1503, 1406, 1279, 1214, 1143, 1099, 1043, 1027, 923, 852, 770, 685.
1
H NMR(CDCl ) δ/ppm:3.97 (s, 3H, OCH ), 6.76–6.80 (m, 1H, ArH), 7.04–7.07 (m, 2H,
3
3
ArH), 7.48–7.56 (m, 4H, ArH), 7.78–7.86 (m, 1H, ArH), 7.91–7.93 (d, J = 8.00 Hz, 2H,
ArH), 7.98–8.00 (d, J = 8.00 Hz, 2H, ArH), 8.09–8.13 (t, 2H, ArH), 8.15–8.17 (d, J = 8.00
1
3
Hz, 1H, ArH), 8.19–8.23 (t, 2H, ArH), 14.64 (s, 1H, NH). C NMR(CDCl ) δ/ppm: 52.23
3
(
(
(
CH ), 117.62 (ArC), 120.80 (ArC), 122.85 (ArC), 124.66 (ArC), 125.12 (ArC), 129.13
3
ArC), 129.46 (ArC), 129.62 (ArC), 129.85 (ArC), 130.13 (ArC), 131.66 (ArC), 140.91
ArC), 141.23 (ArC), 147.37 (ArC), 149.80 (ArC), 150.42 (ArC), 152.74 (C=N), 167.12
9

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+
(
C=O). HR-MS (ESI) m/z found: [M+H] 464.1669; molecular formula C H N O requires
2
6
21
7
2
+
[
M+H] 464.1757.
2
.3. Quantum chemical calculations
In our previous study [33], Perdew, Burke, and Ernzerhof (PBE) functional [41] with 6-
11G(2d,2p) basis set provided the best results for formazans. Therefore, the geometry
3
optimizations of the synthesized diazenyl formazans in the gas phase were also performed at
the DFT level using PBE1PBE/6-311G(2d,2p) without any symmetry restrictions. The
vibrational frequencies were calculated at the same level of theory for the optimized
structures. Vibrational frequency analysis confirmed that all optimized geometries
corresponded to minima on the potential energy surface by exhibiting all real frequencies.
However, in order to compare the experimental and calculated vibrational frequencies, the
geometry optimizations and vibrational frequencies were also calculated using the
PBE1PBE/6-311G(d,p) functional/basis set combination because the scale factor of this
combination is known (0.9593) [42].
The molecular structures of all compounds were optimized in three different solvents
(
DMSO, MeOH, and toluene) at the PBE1PBE/6-311G(2d,2p) level of theory. The
environmental effects were accounted for by using the polarized continuum model (PCM)
43-45].
For NMR calculations, the structures of 5a–5i were optimized with the same
[
functional/basis set combination in chloroform by using PCM. The theoretical calculations for
NMR chemical shifts were performed using the gauge-independent atomic orbital (GIAO)
method [46,47]. The NMR signals of the nuclei, as well as that of TMS reference for carbon
and hydrogen atoms, were calculated at the PCM-PBE1PBE/6-311G(2d,2p) level of theory in
1
chloroform. The H NMR chemical shifts were transformed to TMS scale by subtracting the
calculated absolute chemical shielding of TMS (31.6396 ppm) at PCM-PBE1PBE/6-
1
0

ACCEPTED MANUSCRIPT
1
3
3
11G(2d,2p) level. This factor was 188.817 ppm for the C NMR chemical shifts.
In order to calculate the excitation energies, maximum absorbances (ꢀꢁꢂꢃ), and oscillator
strengths of the compounds, TD-DFT [30,48-51] calculations were performed by using
optimized geometries at the same level theory as for the first twenty lowest singlet–singlet
transitions in the gas phase or in three different solvents (DMSO, MeOH, and toluene). In the
latter case, PCM was applied to quantify the impact of the environment.
All calculations were performed using Gaussian 09 software package [52].
3
. Results and Discussion
3
.1. Synthesis
The substituted hydrazones (3a–3i) were prepared by the condensation of the substituted
benzaldehydes (1a–1g) with phenylhydrazine or pyridinylhydrazine (2). Hydrazones 3a–3h
have been previously synthesized. The melting points of the synthesized hydrazones were in
agreement with the values reported in the literature [36-38]. Methyl 4-((2-(pyridin-2-
yl)hydrazono)methyl)benzoate (3i) is a novel pyridinylhydrazone. It was characterized by FT-
1
13
IR, H NMR, C NMR, and UV-Vis spectra, and HR-MS. Its spectra are given Fig. S1
(Supplementary data).
Several procedures for the synthesis of formazans have been reported in the literature
36,40,53,54]. Novel diazenyl formazans (5a–5i) were prepared via the coupling reaction of
[
substituted phenylhydrazones or pyridinylhydrazones with diazonium cation of p-
aminoazobenzene in a basic medium at 0–5ºC [39,40]. The synthetic route for the preparation
of 5a–5i is given in Scheme 2.
The diazenyl formazans (5a–5i) were recrystallized from EtOH in 50–78% yields. It was
found that the yields of the synthesized formazans were affected by the electron withdrawing
or electron donating substituents on the para position of 3-phenyl ring; the yields were higher
for the formazans containing electron withdrawing substituents such as –F, –Cl, –CN, –
1
1

ACCEPTED MANUSCRIPT
CO CH , and –NO and moderate for those containing the electron donating substituent (–
2
3
2
CH(CH ) ). In addition, the yields of pyridinyl formazans were lesser than those of the
3
2
corresponding diazenyl formazans.
3
.2. Optimized geometries
The molecular geometries of the synthesized molecules (5a–5i) were examined by DFT
calculations at PBE1PBE/6-311Gl(2d,2p) level of theory in the gas phase.
There were two possible conformations for 5g and 5h, and four for 5i. These
conformations are shown in Scheme S1 (Supplementary data). Their calculated heats of
formation and Gibbs free energies are given in Table S1 (Supplementary data). The data in
Table S1 demonstrates that conformation I of each compound, which had the lowest energy
in comparison with other conformers, was the most stable conformer. Henceforth, we used
only the most stable conformers of 5g, 5h, and 5i.
The optimized structures of the diazenyl formazans (5a–5i) and the Mulliken partial
charges of the substituents and pyridinyl groups are shown in Figs. 1 and S2 (Supplementary
data) and Table 1. They were visualized by using GaussView5 program [55]. In Table 2,
some selected optimized geometric parameters of the diazenyl formazans (5a–5i) and X-ray
data of 1-(4-bromophenyl)-3,5-diphenylformazan [56] are listed. It was assumed that the –Br
substituent of the crystal does not affect the geometric parameters of the skeletal structure of
formazans. The calculated geometric parameters were in good agreement with the X-ray data
of the reference crystal. It is worth noting that the optimized geometrical parameters were
found for the compounds in the gas phase whereas the experimental geometrical parameters
were obtained for the compounds in the solid phase. This partly explains the small differences
between the experimental and calculated geometric parameters.
Because of the pseudo six-membered ring with a hydrogen bond between H and N1 (the
lengths between N1 and H atoms, 1.780–1.826 Å, indicated the presence of a hydrogen bond
1
2

ACCEPTED MANUSCRIPT
0
.102
Fig. 1. Optimized structure of 5a computed at PBE1PBE/6-311Gl(d,p) level in the gas phase.
Mulliken partial charge (in e) for its R group is surrounded by dotted line.
in 5a-5i), the main skeletons of the diazenyl formazans were planar. This was evident from
the N1–N2–C3–N4 and N5–N4–C3–N2 dihedral angles of all studied formazans, which were
found to be nearly 0° (Table 2, Figs. 1 and S2). All structures were essentially planar. The 5-
phenyl groups were in the same plane as that of the main skeleton but the 1- and 3-phenyl
groups were out-of-plane by nearly 10° from the main skeleton (Fig. 1).
The Mulliken partial charge of R:H in 5a was calculated as 0.102e and it was used as a
reference compound to determine charge transfers of other compounds because it was the
Table 1
Calculated Mulliken partial charges of the
substituents (R) and charge transfers of 5a–5i.
Compound
q
∆q
5
5
a
0.102
-0.230
-0.068
-0.191
-0.394
0.029
0.037
-0,067
0.039
0.000
-0.332
-0.170
-0.293
-0.496
-0.073
-0.065
-0.169
-0.063
b
5
c
5
d
5
e
5
f
5
5
g
h
5
i
1
3

ACCEPTED MANUSCRIPT
Table 2
Selected optimized geometric parameters of diazenyl formazans.
Bond length (Å)
Compound
N1-N2
1.268
1.268
1.268
1.268
1.267
1.269
1.268
1.263
1.263
1.299
N2-C3
1.375
1.376
1.375
1.375
1.375
1.375
1.375
1.383
1.383
1.450
C3-N4
1.314
1.314
1.314
1.315
1.316
1.316
1.315
1.309
1.310
1.309
N4-N5
1.302
N5-H
1.025
1.025
1.025
1.026
1.027
1.026
1.026
1.022
1.023
0.860
N1⋯H
1.782
1.780
1.784
1.782
1.778
1.783
1.780
1.826
1.822
1.720
5
5
5
a
b
c
1.303
1.302
1.299
1.298
1.301
1.300
1.305
1.304
5
d
5
5
e
f
g
5
5
h
5
i
X-ray
1.360
Dihedral angle (º)
Compound
N1-N2-C3-N4
-1.8
-1.8
2.1
N5-N4-C3-N2
0.2
N2-N1-C1'-C6'
N2-C3-C1''-C6''
N4-N5-C1'''-C2'''
-3.2
5
5
a
b
-9.2
-9.0
9.9
-5.5
8.8
164.2
166.2
-0.1
-0.3
-0.5
-0.7
0.3
0.7
-1.1
-0.2
-3.6
3.1
-3.2
2.6
1.9
-2.6
-0.9
-0.8
5
c
-167.1
168.9
-170.6
-168.9
169.5
5
d
0.2
2.2
0.7
-2.3
1.3
5
e
5
f
4.7
5
5
g
h
-8.5
-4.1
-4.6
4.5
167.6
5
i
-0.1
-0.7
170.0
X-ray
-4.0
166.0
-0.3
simplest structure (i.e., there was no substituent on the para position of the 3-phenyl ring and
no N on the 5-phenyl ring). The calculated Mulliken partial charges of all substituents were
than that of 5a. It was observed that the presence of a pyridinyl group in the structure did not
affect the Mulliken partial charge of substituents as expected.
3
.3. Infrared spectra
The FT-IR spectrum of 3i is given in Fig. S1 in the Supplementary data and shows N–H,
aromatic and aliphatic C–H, C=N, and C=O stretching vibrations. All vibrational
–
1
wavenumbers of 3i are given in the experimental section. The band observed at 3289 cm
corresponded to N–H stretching vibration. The aromatic and aliphatic C–H stretching bands
–
1
–1
were observed at 2949 and 2866 cm , respectively. The band at 1705 cm was assigned to
–1
–1
C=O group. C=C and C=N stretching bands were found at 1646 cm and 1598 cm ,
respectively.
1
4

ACCEPTED MANUSCRIPT
The experimental and calculated IR spectra of the synthesized formazans can be seen in
Figure S3 (Supplementary data). The FT-IR spectra showed characteristic bands of N–H, C–H,
C=N, and N=N stretching vibrations. Shifting of these bands toward lower or higher
frequencies indicates whether the compound has a chelate or non-chelate structure, respectively
[
57]. All vibrational frequencies of the studied formazans are given in the experimental section.
Some selected experimental and calculated vibrational wavenumbers of 5a–5i are listed in
Table 3. The experimental and calculated IR spectra of 5a are shown in Fig. 2. The calculated
wavenumbers were scaled down to give up the rational with the observed wavenumbers. This is
because the calculated vibrational wavenumbers were higher than the experimental
wavenumbers as a result of discard of anharmonicity present in a real system. For the
PBE1PBE/6-311G(d,p) combination, the scaling factor is 0.9593 [42].
N–H vibrations
–
1
The N–H stretching vibrations are typically observed in the region 3350–3310 cm [58].
–1
In FT-IR spectra of 5a–5i, weak N–H stretching bands were observed at 3074–3039 cm .
–
1
The absorption bands with lower wavenumbers (3090–3011 cm ) implied an intramolecular
hydrogen bond and chelate structure [57]. The intramolecular hydrogen bonding in 5a can be
Table 3
–
1
Selected vibrational wavenumbers (cm ) for 5a–5i.
N-H
C-H(aromatic)
C-H(aliphatic)
C=N
C=C
Calc
N=N
Calc
C=O
Calc
C(=O)-O-C
Comp
Expt
3052
Calc
Expt
3027
Calc
3058
Expt
Calc
Expt
1506
Calc
Expt
1597
Expt
1436
Expt
Expt
Calc
5
5
a
3132
3132
3132
3122
3117
3131
3124
3205
3198
1533
1535
1534
1533
1533
1533
1533
1531
1582
1602
1614
1603
1611
1610
1615
1612
1609
1613
1367
1368
1368
1371
1370
1368
1370
1377
1379
b
3050
3050
3054
3065
3064
3050
3039
3074
3028
3031
3032
3034
3037
3028
2929
3004
3059
3059
3060
3107
3051
3060
3049
3063
1500
1506
1500
1505
1504
1498
1501
1503
1596
1596
1596
1593
1592
1595
1588
1573
1455
1486
1454
1456
1455
1455
1480
1436
5
c
5
d
5
e
5
f
2931
2996
3003
2942
5
5
g
1716
1716
1747
1748
1181
1214
1172
1172
h
5
i
2934
2942
1
5

ACCEPTED MANUSCRIPT
seen in Scheme 3. The calculated wavenumbers for N–H stretching vibrations of 5a–5i were
–
1
in the range of 3205–3117 cm .
C–H vibrations
For aromatic phenyl structures, the C–H stretching vibrations are generally observed in the
–1
range of 3100–3000 cm [58,59]. The aromatic C–H stretching wavenumbers for the
–
1
synthesized compounds were observed experimentally in the range of 3034–2929 cm while
–
1
the calculated wavenumbers were found in region 3115–3049 cm .
–
The stretching vibrations of the –CH group are expected in the range of 2900–3050 cm
3
1
[
60]. The experimental aliphatic C–H stretching vibrations of compounds 5a–5i were found
–
1
between 2934 and 2869 cm . The calculated spectra predicted that the C–H stretching mode
–
1
of the methyl group was in the region of 3003–2942 cm .
C≡N vibrations
Aromatic nitriles are characterized by medium absorption bands in the region between
–
1
–1
2
240–2222 cm [58]. The C≡N stretching band of 5d was observed at 2221 cm and the
–
1
calculated value was 2268 cm .
C=N vibrations
–
1
C=N stretching vibrations for hydrazines can be seen in the range of 1689-1471 cm [58].
–1
It was observed at 1598 cm for 3i.
(a)
(b)
Fig. 2. (a) Experimental and (b) calculated IR spectra of 5a at PBE1PBE/6-311G(d,p) level.
1
6

ACCEPTED MANUSCRIPT
–
1
Formazans show C=N stretching vibrations in the range of 1450–1600 cm . The C=N
–
1
stretching band at 1510–1500 cm typically implies a chelate structure [57]. On the other
hand, non-chelated structures typically have the C=N stretching band in the region of 1551–
1
1
565 cm [57]. Based on both experimental and calculated values of C=N stretching
vibrations, it was concluded that all diazenyl formazans were in the chelate form. These bands
–1
were observed experimentally in the range of 1506–1498 cm and their calculated
–
1
wavenumbers were found in the region of 1534–1528 cm .
C=C vibrations
In aromatic hydrocarbons, skeletal vibrations involving carbon–carbon stretching within
–
1
the ring absorb in the region between 1600 and 1585 cm [58-60]. The characteristic C=C
–
1
stretching bands were observed in the region of 1597–1573 cm for 5a–5i while the
–
1
calculated wavenumbers were found in the range of 1615–1602 cm .
N=N vibrations
–
1
The N=N bands of formazans are typically observed in the region between1400–1450 cm
–
1
[
57]. These bands were observed at 1436–1486 cm for 5a-5i and calculated to be in the
–
1
range of 1379–1367cm .
C=O vibrations
ꢀ
–
The C=O absorption bands of benzoate esters are generally in the region of 1730–1715 cm
1
–1
[
58]. The strong bands at 1716 cm in the FT-IR spectra of 5g and 5i were assigned to the
Scheme 3. Intramolecular hydrogen bonding of 5a.
1
7

ACCEPTED MANUSCRIPT
–
1
C=O stretching mode. The calculated C=O stretching modes at 1747 and 1748 cm were in
good agreement with the experimental values.
C–O–C vibrations
ꢀ
The C–O stretching vibrations of esters actually consist of two asymmetric coupled
vibrations, namely, C(=O)–O–C and O–C–C, of which the former is more important. These
–
1
bands typically appear in the region of 1300–1000 cm [58]. The corresponding symmetric
vibrations are of little importance. The C–C(=O)–O band is often broader and stronger than
–
1
the C=O stretching vibration bands [58]. These bands were observed at 1181 and 1214 cm
for 5g and 5i, respectively. The calculated wavenumbers of this vibration for both compounds
–
1
was 1172 cm .
NO vibrations
2
–1
–1
The NO band of 5e was found at 1335 cm experimentally and 1376 cm by calculation.
2
–
1
Generally, the symmetrical stretching of the NO is observed in the region of 1259-1389 cm
2
[
58].
There was good agreement between the experimental and calculated absorption
wavenumbers of the compounds. The calculated wavenumbers were generally greater than the
experimental values, except for N=N and C(=O)–O–C bands.
3
.4. NMR spectra
The details of the measured H and C NMR data and coupling constants for 3i and 5a–5i
1
13
are given in the experimental section.
1
In H NMR spectrum of 3i, the N–H signal was observed as a singlet at 8.63 ppm. The
aromatic protons were recorded between 6.74 and 8.10 ppm and the C–H signal was observed
1
3
as a singlet at 7.75 ppm. The methoxy protons also appeared as a singlet at 3.91 ppm. The C
1
8

ACCEPTED MANUSCRIPT
NMR chemical shifts of 3i were observed between 52.21 and 166.80 ppm. The carbon atom
of C=O was recorded downfield at 166.80 ppm while the methoxy carbon was observed at
1
13
5
2.21 ppm. These H and C NMR spectra can be seen in Fig. S1 (Supplementary data).
The NMR spectra of the diazenyl formazans were calculated at the PCM-PBE1PBE/6-
11G(2d,2p) level of theory in chloroform by using the GIAO method [46,47] and referenced
3
by TMS. The isotropic chemical shifts of the compounds with reference to TMS in the solvent
1
13
phase were calculated using PCM. H and C NMR chemical shifts of TMS at the PCM-
1
PBE1PBE/6-311G(2d,2p) level are 31.6396 ppm and 188.817 ppm, respectively. Thus, H or
1
3
C NMR isotropic chemical shift of any X atom was computed by using the relation CSX=
ISTMS–IS , where IS and IS are the isotropic chemical shifts of TMS and X, respectively.
x
TMS
X
1
13
The experimental and calculated H and C NMR spectra of 5a are shown in Fig. 3. The
full set of experimental and calculated NMR spectra can be seen in Fig. S4 (Supplementary
1
13
data), and the H and C-NMR signals of 5a and 5b–5i are given in Tables 4 and S2
(Supplementary data), respectively.
1
The observed H NMR chemical shift values of 5a–5i ranged from 1.34 to 15.79 ppm. The
1
calculated H NMR chemical shifts of the studied diazenyl formazans were found between
1
.28 and 17.52 ppm. The N–H signal in the NMR spectra of formazans was indicative in
evaluating the structure. While the N–H signal appearing downfield at 16 ppm indicated
intramolecular hydrogen bonding, the upfield shifts of this signal at 10 ppm indicated a
weakening of the same intramolecular hydrogen bond [60]. The N–H signals of 5a–5i were
observed as a singlet in the downfield region between 14.36 ppm and 15.79 ppm and
calculated as 16.47–17.52 ppm. Both experimental and calculated values verified the
existence of an intramolecular hydrogen bond in all of the studied diazenyl formazans (as
shown in Scheme 3).
1
9

ACCEPTED MANUSCRIPT
The chemical shifts of the aromatic protons were recorded in the range 6.76–8.34 ppm.
The calculated values for the aromatic protons were found between 7.40 and 9.00 ppm.
Owing to the different substituents on the 3-phenyl rings of all diazenyl formazans and N
atoms on the 5-pyridinyl rings of 5h and 5i, the aromatic protons showed a broad range of
NMR peaks. This was observed both experimentally and theoretically.
F atom has a spin number of ½ and couples strongly with protons [58]. In the case of the
formazan that contains a fluorine atom, 5b, the aromatic protons of the 3-phenyl ring
appeared as a doublet-doublet at 8.12–8.16 ppm, which was attributed to proton-fluorine
coupling. The corresponding calculated value was 7.44 ppm.
Methoxy protons were detected as a singlet at 3.96 and 3.97 ppm for 5g and 5i,
respectively, and the corresponding calculated values were 3.78 and 3.99 ppm. On the other
hand, the methyl protons were observed as doublets between 1.34–1.36 ppm and the
calculated value was 1.28 ppm. For 5f, the methine proton was recorded as a multiplet at 3.96
ppm and the calculated chemical shift was 2.88 ppm.
(a)
(b)
1
13
Fig. 3. (a) Experimental and (b) calculated H NMR (upper) and C NMR (lower) spectra of
5
a.
2
0

ACCEPTED MANUSCRIPT
Table 4
1
13
Theoretical H and C isotropic chemical shifts with
respect to TMS (in ppm) for 5a.
Calc
shift
17.34
8.79
8.79
8.79
8.79
8.66
8.66
8.59
8.59
8.50
8.10
7.99
7.90
7.90
7.90
7.90
7.82
7.77
7.59
7.44
Calc
shift
Atom
Expt
Atom
Expt
H(N5)
H(C3')
15.39
C1'
158.62
155.74
146.41
143.41
137.67
136.12
134.85
134.06
133.82
133.24
132.91
131.71
129.65
129.54
128.69
117.71
117.42
116.75
152.84
150.37
149.96
147.51
141.86
137.02
130.79
130.11
129.52
129.13
129.51
128.07
126.12
126.12
124.68
120.95
117.13
117.13
8.16-8.18 C4'
8.16-8.18 C1'''
8.01-8.03 C3
8.01-8.03 C4''''
7.92-7.94 C2'
7.92-7.94 C3'''
7.86-7.88 C5''''
7.86-7.88 C5'''
7.67-7.69 C3''''
7.67-7.69 C5''
7.39-7.56 C4''
7.39-7.56 C4'''
7.39-7.56 C2''
7.39-7.56 C6''
7.39-7.56 C6'
7.39-7.56 C2'''
7.39-7.56 C2''''
7.39-7.56
H(C2'')
H(C6')
H(C2''')
H(C6'''')
H(C6'')
H(C2')
H(C5')
H(C2'''')
H(C5'''')
H(C4'''')
H(C3'''')
H(C3''')
H(C3'')
H(C5'')
H(C5''')
H(C4'')
H(C4''')
H(C6''')
7.39-7.56
1
3
The C NMR chemical shifts of 5a–5i were recorded between 24.00 and 167.13 ppm,
whereas the calculated values were found between 25.64 and 171.24 ppm. The C=O chemical
shifts of 5g and 5i were the same and recorded downfield at 167 ppm. For these compounds,
the shifts were calculated as 171.24 ppm and 171.20 ppm, respectively. The carbon atoms in
characteristic C=N units appeared downfield between 147.17–156.32 ppm. In the calculated
NMR spectra, these chemical shifts were found between 141.24 and 143.69 ppm. The nitrile
carbon was observed at 120 ppm for 5d. The shift of the carbon in nitrile was calculated as
1
26.58 ppm. Methoxy carbons were experimentally recorded at 52.13 and 52.23 ppm for 5g
and 5i, while the calculated chemical shifts were 53.11 and 53.16 ppm, respectively. The
methyl and methine carbons were recorded upfield at 24.00 and 33.92 ppm, respectively, and
their calculated values were found in the range of 25.64–39.47 ppm.
2
1

ACCEPTED MANUSCRIPT
1
13
A comparison of the theoretical and experimentally observed H and C NMR chemical
shifts of the studied compounds revealed that the calculated chemical shifts were in good
compliance with the experimental findings (Fig. 3).
3
.5. Mass spectra
The mass spectral data of 3i and 5a–5i are given in the experimental section. Besides FT-
1
13
IR, H NMR, C NMR, and UV-Vis studies, mass spectra displayed the correct molecular
ions, for which the measured HR-MS data were in agreement with the calculated values of the
novel synthesized hydrazone (3i) and diazenyl formazans (5a–5i).
3
.6. Absorption spectra
The electronic absorption spectra of 5a–5i were recorded in DMSO, MeOH, and toluene.
The electronic properties, highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energies, absorption wavelengths, and oscillator strengths of 5a–5i
were calculated using the TD-DFT method [30-32] with PBE1PBE functional [41] and 6-
3
11G(2d,2p) basis set by using PCM [43-45]. The measured and calculated ꢀꢁꢂꢃꢄ and ꢀꢁꢂꢃꢆ
values are given in Table 5, which also presents the calculated ꢀꢁꢂꢃ values of 5a–5i in the
gas phase.
The experimental and calculated electronic absorption spectra of 5a–5i are given in Fig. 4.
The characteristic peak of formazan (ꢀꢁꢂꢃꢄ) is broad and generally observed in the range of
4
10–500 nm [36] This peak is due to the π→π* electronic transition in the formazan skeleton
.
(–NH–N=C–N=N–) [36,57,61].
The sharp peak in the range of 294–350 nm (ꢀꢁꢂꢃꢆ) was attributed to the π→π* transition
within the hydrogen chelate ring formed by the azo and hydrazo groups and intramolecular
charge transfer in the form of a hydrogen bond [61].
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2

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Table 5
Experimental and calculated ꢀꢁꢂꢃꢄand ꢀꢁꢂꢃꢆvalues (in nm) for all studied diazenyl formazans.
Solvent
MeOH
Expt Calc
Gas
DMSO
Expt Calc
Toluene
Expt Calc
Comp
Calc
Expt
Calc
Expt
Calc
Expt
ꢀꢁ
ꢂꢃꢄ
ꢀꢁ
ꢂꢃꢄ
ꢀ
ꢁꢂꢃꢄ
488
499
498
506
501
507
491
498
478
ꢀ
ꢁꢂꢃꢆ
344
339
377
373
357
350
373
370
368
ꢀ
ꢁꢂꢃꢆ
347
356
358
356
362
352
364
364
358
ꢀ
ꢁꢂꢃꢄ
496
497
497
499
499
496
498
501
502
ꢀ
ꢁꢂꢃꢄ
495
507
505
500
501
510
504
479
487
ꢀ
ꢁꢂꢃꢆ
344
339
376
371
356
350
372
369
366
ꢀ
ꢁꢂꢃꢆ
340
350
349
352
360
345
360
355
354
ꢀ
ꢁꢂꢃꢄ
558
563
555
540
533
566
545
505
505
ꢀ
ꢁꢂꢃꢄ
536
542
539
527
522
547
526
509
504
ꢀ
ꢁꢂꢃꢆ
351
341
379
373
381
356
386
369
367
ꢀ
ꢁꢂꢃꢆ
356
359
358
355
382
352
382
366
356
5
5
a
531
496
b
503
533
506
505
501
503
506
505
497
497
498
499
496
498
501
502
5
c
5
d
5
e
5
f
5
5
g
h
5
i
In our experimental study, the ꢀꢁꢂꢃꢄvalues of 5a–5i were observed in the ranges of 478–
07 nm in DMSO, 479–510 nm in MeOH, and 504–547 nm in toluene (Fig. 4). By
5
calculation, the ꢀꢁꢂꢃꢄvalues were found between 501–533 nm in the gas phase, 496–502 nm
in DMSO, 496–502 nm in MeOH, and 505–566 nm in toluene. The experimental ꢀꢁꢂꢃꢆ
values of 5a–5i were observed between 347–364 nm in DMSO, 340–355 nm in MeOH, and
3
52–382 nm in toluene (Table 5), and the calculated ꢀꢁꢂꢃꢆvalues were found between 339–
77 nm in DMSO, 338–377 nm in MeOH, and 341–385 nm in toluene (Table 5 and Fig.4).
3
Both experimental and calculated results were in accordance with the values reported in the
literature and indicated that chelate formation as a result of intramolecular hydrogen transfer
over a six-membered ring (Scheme 3).
The absorption spectra are influenced by both dipole moment of solvent and substituent
[
61]. The polarities of the solvents decrease from DMSO to toluene (Table 5). The ꢀꢁꢂꢃꢄ
absorption bands of diazenyl formazans shifted to the higher wavelengths with the decrease in
the solvent polarity. Fig. 4(a) presents the electronic absorption spectra of 5a in three
different solvents. The calculated electronic absorption spectrum of 5a in toluene is given in
Fig. 4(b). Fig. 4(c) shows the electronic absorption bands of 5a–5i in toluene. These results
2
3

ACCEPTED MANUSCRIPT
indicated that there is a hydrogen bond between the solute and DMSO and MeOH solvent
molecules. This type of hydrogen bond can complicate the chelation in the diazenyl
formazans. On the other hand, a hydrogen bond was not found between the solute and
toluene. Consequently, the ꢀꢁꢂꢃꢄabsorption bands of 5a–5i in toluene were at higher
wavelengths than that of all studied diazenyl formazans in DMSO and MeOH.
The substituent effect on the electronic absorption spectra of diazenyl formazans can be seen in
Tables 5 and 6, and Fig. 4(c). In order to discuss this effect, the ꢀꢁꢂꢃꢄvalues in toluene were
considered. The chemical shift values were estimated between the ꢀꢁꢂꢃꢄvalue of 5a and that of
substituted 5b–5i in toluene (Table 6). A hypsochromic shift is expected for a compound with
an electron withdrawing group [57]. The ꢀꢁꢂꢃꢄ values of 5d, 5e, and 5g, which contain
electron withdrawing groups –CN, –NO , and –CO CH , respectively, were shorter than that of
2
2
3
5
a. Therefore, hypsochromic effects were observed in the electronic absorption spectra of 5d,
e, and 5g. The ꢀꢀꢁꢂꢃꢄvalues of 5h and 5i, which contained the electron withdrawing
5
pyridinyl group (in addition to –Cl and –CO CH , respectively) were 27 and 32 nm
2
3
respectively. The shortest ꢀꢁꢂꢃꢄvalue of 5i could be correlated to the highest hypsochromic
shift (32 nm). Owing to its electron donating –CH(CH ) group, the ꢀ value of diazenyl
3
2
ꢁꢂꢃ
ꢄ
formazan 5f was shifted to a higher wavelength than that of 5a. It showed a bathochromic shift
–11 nm). Both 5b and 5c (with –F and –Cl substituents, respectively) showed slight
(
bathochromic shifts of –6 nm and –3 nm, respectively. These results were in good agreement
with the electron withdrawing inductive effect and electron donating resonance effect of –F and
–
Cl substituents.
Based on these results, it was evident that the calculated ꢀꢁꢂꢃvalues were in good
agreement with the experimental findings. This can be seen clearly in Fig. 5, which shows the
comparison of experimental and calculated ꢀꢁꢂꢃ in toluene for the studied diazenyl formazans.
2
4

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(a)
(b)
(c)
Fig. 4. (a) Electronic absorption spectra of 5a in DMSO, MeOH, and toluene. (b) Calculated
electronic absorption spectrum of 5a. (c) Electronic absorption spectra of 5a–5i in toluene.
There was a linear correlation between the experimental and calculated ꢀꢁꢂꢃvalues of the
2
compounds 5a–5i in toluene and the correlation coefficient r was 0.960.
Table 6
Experimental chemical shifts, ꢀꢀꢁꢂꢃꢄ(in nm), of 5a–5i in three solvents.
DMSO
MeOH
Toluene
Compound
X
R
ꢀꢁꢂꢃꢄ
488
499
498
506
501
507
491
498
478
Δꢀꢁꢂꢃꢄ ꢀꢁꢂꢃꢄ Δꢀꢁꢂꢃꢄ ꢀꢁꢂꢃꢄ Δꢀꢁꢂꢃꢄ
5
a
CH
CH
CH
CH
CH
CH
CH
N
p–H
p–F
p–Cl
p–CN
p–NO
p–CH(CH
p–CO
p–Cl
0
495
507
505
500
501
510
504
479
487
0
-12
-10
-5
536
542
539
527
522
547
526
509
504
0
5
b
-11
-10
-18
-13
-19
-3
-6
-3
5
c
5
d
9
5
e
2
-6
14
-11
10
27
32
5
f
3
)
2
-15
-9
5
g
2
CH
3
5
h
-10
10
16
8
5
i
N
p–CO
2
CH
3
2
5

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5
5
5
5
5
4
90
70
50
30
10
90
4
95
505
515
525
535
545
555
565
Experimental λmax1 (nm)
Fig. 5. Relationship between experimental and calculated ꢀꢁꢂꢃꢄ for 5a–5i in toluene.
The calculated electronic absorption spectra showed that the ꢀꢁꢂꢃ corresponded to the
electronic transition between the frontier molecular orbitals (i.e., from HOMO to LUMO).
calculated frontier molecular orbitals of 5a and of 5b–5i in toluene are given in Figs. 6 and S4
(Supplementary data), respectively. The HOMO and LUMO surfaces were visualized using
GaussView 05 program [56]. As observed in Figs. 6 and S4 (Supplementary data), the
electronic cloud distribution of the HOMOs of 5a–5i were centralized throughout the
molecules
Occupied MO
Virtual MO
-5.7087 eV
-3.0110 eV
Fig. 6. Frontier orbital pairs involved in the ꢀꢁꢂꢃꢄ excitation of 5a at TD-PCM-PBE1PBE/6-
11G(2d,2p) level in toluene.
3
2
6

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Table 7
Frontier orbital energies of the studied diazenyl formazans (in eV).
Frontier
molecular
orbital
Compound
Phase
5
a
5b
5c
5d
5e
5f
5g
5h
5i
HOMO
LUMO
-5.6652 -5.6826
-2.9628 -3.0199
-5.7566
-3.0613
2.6953
-5.7996
-3.0866
2.7130
-5.7988
-3.1125
2.6863
-5.7716
-3.0711
2.7005
-5.9806
-3.2055
2.7750
-5.9163
-3.1476
2.7688
-5.9172
-3.1476
2.7696
-5.9406
-3.1623
2.7783
-6.0298
-3.2205
2.8093
-5.9550
-3.1625
2.7925
-5.9558
-3.1628
2.7930
-5.9887
-3.1805
2.8082
-5.5879
-2.9386
2.6493
-5.7076
-3.0828
2.6248
-5.7055
-3.0793
2.6262
-5.6353
-2.9922
2.6431
-5.8089
-3.0635
2.7454
-5.8679
-3.1250
2.7429
-5.8665
-3.1236
2.7429
-5.8306
-3.0812
2.7495
-5.8744
-3.0831
2.7914
-5.9610
-3.1710
2.7900
-5.9588
-3.1685
2.7903
-5.9033
-3.1051
2.7982
-5.9302
-3.0817
2.8485
-6.0336
-3.1827
2.8510
-6.0315
-3.1805
2.8510
-5.9713
-3.1171
2.8542
Gas
∆
E
2.7024
2.6627
HOMO
LUMO
-5.7751 -5.7504
-3.0885 -3.1013
DMSO
MeOH
Toluene
S
o
l
v
e
n
t
∆
E
2.6866
2.6490
HOMO
LUMO
-5.7732 -5.7487
-3.0858 -3.0994
∆
E
2.6874
2.6493
HOMO
LUMO
-5.7087 -5.7057
-3.0110 -3.0482
∆
E
2.6978
2.6575
and those of their LUMOs were mainly localized on 1-phenyl ring, diazenyl groups, and the
main skeleton of the molecules. The energy gaps of the compounds were found to be in the
range of 2.6248–2.8542 eV from Table 7.
3
.7. Molecular electrostatic potential
The molecular electrostatic potential (MEP) surface is a very useful tool to study the
correlation between the structure and physiochemical properties of a molecule [62]. The MEP
may be used to predict reactive sites for electrophilic attack (electron rich region) and
nucleophilic attack (electron poor region) as the negative and positive regions of MEP are
related to electrophilic and nucleophilic reactivities, respectively. The red and blue regions
depict the electron rich and electron poor areas, respectively, while the green region in the
MEP suggests an almost neutral site.
The MEP surfaces of 5a and 5b–5i were generated from the optimized geometry using
GaussView 05 program [56] and are shown, respectively, in Figs. 7 and S5 (Supplementary
data). MEPs of 5a and 5f showed a considerable negative potential region (yellowish-red
area) around the 3-phenyl group, which was the binding site for electrophilic attack. For 5b
2
7

ACCEPTED MANUSCRIPT
and 5c, the MEP maps showed a modest negative potential region around the 3-phenyl groups
and the substituents.
The regions around the O atoms of 5e, 5g, and 5i and the regions around the N atoms of
the CN group in 5d and in the pyridinyl group of 5h and 5i were positive and preferred sites
for electrophilic attack. For all compounds, the H atoms of 5-phenyl and diazenyl groups bore
the maximum brunt of positive potential. Based on calculations, the other studied molecules
seemed to have almost neutral electrostatic potential. By considering the positive, negative,
and neutral sites, it is possible to predict the region of the compound that can participate in
intermolecular interactions and form metallic bonds.
!4.468e!2(
4.468e!2(
Fig. 7. Total electron density map of 5a with the molecular electrostatic potential surface.
2
8

ACCEPTED MANUSCRIPT
4
. Conclusions
Nine derivatives
of
3-(p-substitutedphenyl)-5-phenyl-1-(4-phenyldiazenyl)phenyl
1
13
formazans were synthesized and characterized by FT-IR, H NMR, C NMR, and UV–Vis
1
spectroscopic techniques, and mass analysis. The optimized geometric parameters, IR, H
13
NMR, and C NMR spectra were theoretically determined by the DFT method and compared
with the experimental data. There was an excellent agreement between the experimental and
calculated geometric parameters and spectroscopic data. The electronic absorption spectra of
the synthesized diazenyl formazans were also studied by the TD-DFT method. It was
concluded that the lowest singlet excited states of the molecules were mainly derived from the
HOMO–LUMO (π→π*) electron transitions. The observed and calculated UV-Vis spectra
indicated that the ꢀꢁꢂꢃ values of diazenyl formazans depended on solvent polarity. Theꢅꢀꢁꢂꢃ
values decreased with an increase in polarity of the solvent. Some diazenyl formazans that
contained electron withdrawing group such as –CN, –NO , and –CO CH showed a
2
2
3
hypsochromic effect in their electronic absorption spectra in toluene. The diazenyl formazans
that contained electron withdrawing groups such as –F and –Cl showed bathochromic shifts in
toluene as a result of two opposite effects (inductive electron withdrawing and resonance
electron donating effects). The diazenyl formazan that contained –CH(CH ) as the electron
3
2
donating group showed a bathochromic shift. Two of the synthesized diazenyl formazans that
contain both pyridinyl and electron withdrawing groups –Cl and –CO CH showed the highest
2
3
hypsochromic shifts. It was observed that the calculated ꢀꢁꢂꢃꢄand ꢀꢁꢂꢃꢆvalues were in good
agreement with the experimental findings.
Appendix A. Supplementary data
Supplementary data associated with this article can be found in XXX.
2
9