Spectroscopic and computational study of a new thiazolylazonaphthol dye 1-[(5-(3-nitrobenzyl)-1,3-thiazol-2-yl)diazenyl]naphthalen-2-ol

Article abstract of DOI:10.1016/j.molliq.2020.112713

A new thiazolylazo structure, 1-[(5-(3-nitrobenzyl)-1,3-thiazol-2-yl)diazenyl]naphthalen-2-ol (NBnTAN), has been synthesized for the first time. The geometric parameters and properties of the new dye were studied using spectroscopic, DFT and semiempirical

Full text of DOI:10.1016/j.molliq.2020.112713

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Spectroscopic and computational study of
thiazolylazonaphthol
a
new
dye
1
-[(5-(3-nitrobenzyl)-1,3-thiazol-2-yl)diazenyl]naphthalen-2-ol
Orest Fedyshyn, Yaroslav Bazeľ, Maksym Fizer, Vasyl Sidey,
Jan Imrich, Maria Vilkova, Oksana Barabash, Yurii Ostapiuk,
Oleksandr Tymoshuk
PII:
S0167-7322(19)37007-2
DOI:
https://doi.org/10.1016/j.molliq.2020.112713
MOLLIQ 112713
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
20 December 2019
11 February 2020
13 February 2020
Please cite this article as: O. Fedyshyn, Y. Bazeľ, M. Fizer, et al., Spectroscopic and
computational study of new thiazolylazonaphthol dye
-[(5-(3-nitrobenzyl)-1,3-thiazol-2-yl)diazenyl]naphthalen-2-ol, Journal of Molecular
Liquids(2020), https://doi.org/10.1016/j.molliq.2020.112713
a
1
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Spectroscopic and computational study of a new thiazolylazonaphthol dye 1-[(5-
(3-nitrobenzyl)-1,3-thiazol-2-yl)diazenyl]naphthalen-2-ol
a,b
b,c
c*
d
b
Orest Fedyshyn , Yaroslav Bazeľ , Maksym Fizer , Vasyl Sidey , Jan Imrich ,
b
a
a
a
Maria Vilkova , Oksana Barabash , Yurii Ostapiuk , Oleksandr Tymoshuk
a
Faculty of Chemistry, Ivan Franko National University of Lviv, Kyryla & Mefodiya
, 79 005 Lviv, Ukraine
6
b
Institute of Chemistry, Faculty of Science, University of Pavol Jozef Ńafárik in
Końice, Moyzesova 11, 041 80 Końice, Slovak Republic
c
Faculty of Chemistry, Uzhhorod National University, Pidhirna Str., 46, 88000
Uzhhorod, Ukraine
d
Research Institute for Physics and Chemistry of Solid State, Uzhhorod National
University, Pidhirna Str., 46, 88000 Uzhhorod, Ukraine
Corresponding author: M. Fizer, e-mail: max.fizer@uzhnu.edu.ua
1

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Abstract
new
A
thiazolylazo
structure,
1-[(5-(3-nitrobenzyl)-1,3-thiazol-2-
yl)diazenyl]naphthalen-2-ol (NBnTAN), has been synthesized for the first time. The
geometric parameters and properties of the new dye were studied using
spectroscopic, DFT and semiempirical methods through the computation of different
electronic descriptors (aromaticity, frontier molecular orbitals, dipole moments,
partial atomic charges). The solvation effect of different organic liquids on the
1
13
absorption spectra of the reagent has been studied. H, C, COSY, HSQC and
HMBC NMR, UV-Vis and IR spectra were used to confirm the reagent structure. It
has been shown that NBnTAN forms complex compounds with a number of
transition metals, allowing the development of selective methods for the
determination of these metals.
Keywords:
Azo dye; metal complex; spectroscopic characterization; partial charge; dipole
moment; aromaticity.
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1. Introduction
Azo dyes are a family of scientifically and commercially important organic
compounds widely used in spectrophotometric analysis and, owing to their color
fastness and fairly low price, in a variety of industrial applications [1]. The main
common structural feature of these compounds is the presence of a chromophoric azo
group (–N=N–) offering a wide spectrum of colors.
Thiazolylazo dyes, a subset of the azo dyes containing the thiazolyl group, have been
extensively used in analytical chemistry for spectrophotometric determinations,
because of good selectivity and sensitivity of these reagents over a wide pH range,
and also due to the relative simplicity of their synthesis and purification. It must be
noted, however, that for the conventional spectrophotometric analysis in aqueous
solution the typically low solubility of these azo compounds and their complexes
could be a serious problem [2]; so adding of some organic solvents or surfactants
could be necessary in order to overcome this problem. On the other hand, the low
solubility of the above compounds can be an advantage when the solid phase
spectrophotometry (SPS) technique is applied, as the reagent can be immobilized
through physical sorption on a properly selected support [3].
The list of applications of thiazolylazo dyes in the chemical operations includes
spectrophotometry, solid phase extraction, liquid chromatography, electrochemistry,
and liquid and cloud point extraction. The applications of thiazolylazo dyes in
spectrophotometry are based on the colored compounds resulting from their reaction
with most (especially with some transition) metals [1-5]. The simplest and most
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popular representative of the thiazolylazo dyes family is a spectrophotometric reagent
1-(2-thiazolylazo)-2-naphthol (TAN) which, despite its relatively low selectivity, is
commonly used for the spectrophotometric determination of various metal ions [6-
2
1
0]. TAN has also been successfully utilized in the extraction [14] and separation [15,
6] of metal ions. Thus, water samples of Mn(II), Ni(II), and Co(II) ions were
reportedly successfully determined via the flame atomic absorption spectrometry
using TAN as an extracting agent [17, 18]. Furthermore, the flow-injection solid
phase technique was successfully used for the determination of Ni(II), and Zn(II)
ions, where TAN was acting as a chelating agent [19, 20].
In order to continue the search for a new selective and sensitive chromogenic
thiazolylazo dye, and to study the influence of the substituent positions of a benzyl
group on the thiazole ring upon the characteristics of the reagent, earlier we
introduced a benzyl group into the 5-position of the thiazole ring of the TAN reagent
and thus synthesized a new compound, 1-[(5-benzyltiazol-2-yl)diazenyl]naphtalen-2-
ol (BnTAN) [21]. Thiazolylazonaphthol BnTAN was found to be an efficient
complexing reagent for the spectrophotometric and extraction-photometric
determination of a number of metals [22-24]. In view of the reactivity, selectivity and
sensitivity of the BnTAN reagent, a new derivative of this azo compound, 1-[(5-(3-
nitrobenzyl)-1,3-thiazol-2-yl)diazenyl]naphthalen-2-ol (NBnTAN), was synthesized
in which the nitro group was attached to the benzyl group. The decision about
introduction of the nitro group was dictated by the powerfulness, importance and
simplicity of this approach in molecular design [25, 26] for modification of chemical
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and physic-chemical properties [27, 28]. Thus, the main goal of the present work is
(
i) to study the structure and spectral properties of the new dye obtained; (ii) to
determine the effect of nitro group on the physico-chemical and electronic properties
of this azo dye; and (iii) to test the ability of this new reagent to form complex
compounds with certain transition and heavy metals, and thus its further potential
applicability in spectrophotometric analysis.
2
2
3
. Experimental details
.1. Synthesis
-(3-Nitrophenyl)-2-chloropropanal (2, C H ClNO ) was prepared by the method
9
8
3
described earlier [29]. A three-necked flask equipped with a stirrer, a dropping
funnel, and a gas-outlet tube (attached to a bubble counter) was charged with 0.2 mol
3
3
(
13.5 cm ) of acrolein, 10 g of CuCl ·2H O, and 50 cm of acetone. A cold aqueous
2
2
solution of 3-nitrophenildiazonium chloride 1 (prepared by diazotization of 27.6 g
0.2 mol) of 3-nitroaniline) was added dropwise under vigorous stirring. The
(
temperature was maintained within 0-5 °C. When the reaction was complete, the
organic layer was separated, and the aqueous layer was extracted with chloroform.
The extract was combined with the organic phase, dried over MgSO , evaporated and
4
the residue was distilled under reduced pressure. Yellow viscous oil, yield 41%
(
17.50 g), bp 150-153 °C (at pressure 3 mmHg).
Caution: Overheating of the product 2 can lead to an explosion, so the distillation of
should be carried out with care!
2
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2
-Amino-5-(3-nitrobenzyl)thiazole (3, C H N O S) was synthesized according to
10
9
3
2
Scheme 1. A mixture of 8.36 g (0.11 mol) thiourea and a3-(3-nitrophenyl)-2-
3
chloropropanal 2 21.36 g (0.1 mol) in 20 cm of ethanol was heated for 2 h under
3
reflux. The mixture was cooled, diluted with 500 cm of water, and made alkaline by
adding aqueous ammonia. The precipitate was filtered off and recrystallized from
1
toluene. Yellow crystals, yield 81% (18.80 g), mp 108-109 °C. H NMR (DMSO-d ,
6
4
00 MHz), δ (in ppm) 7.72-7.58 (4H, m, C H ), 6.88 (1H, s, thiazole), 6.77 (2Н, s,
6
4
NH ), 4.08 (2H, s, CH ). The spectroscopic data are in agreement with those reported
2
2
in the literature [30].
Scheme 1. Two-step synthesis of compound 3.
1
-[(5-(3-Nitrobenzyl)-1,3-thiazol-2-yl)diazenyl]naphthalen-2-ol (5, C H N OS) was
20
15
3
synthesized according to Scheme 2. 2-Amino-5-(3-nitrobenzyl)-1,3-thiazole 3 (2.35
3
g, 10 mmol) was dissolved in concentrated H SO (6 cm ) at –5 ºС. The solution was
2
4
3
diluted with 3 cm water. The suspension was warmed to 80 °C and a clear solution
was formed. The mixture was cooled to –15 °С, and crystalline sodium nitrite (1.73
g, 25 mmol) was added portion wise while keeping the temperature below –10 °С.
After 10 min, the resinous sediment was filtered and the clear solution was added
slowly with intensive stirring to the mixture of naphthalen-2-ol (1.44 g, 10 mmol),
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NaOH (0.40 g, 10 mmol), Na CO (20 g) and 50 cm of water. The temperature was
3
2
3
kept below 5 °С. After diazonium salt was added, the mixture was stirred 2 h at the
3
room temperature and diluted of water (200 cm ). The precipitate was filtered off and
recrystallized from EtOH-DMF (2:1). Purple-black crystals, yield 40 % (1.56 g,), mp
205-206 °C.
NMR signals of all atoms were assigned to individual protons and carbons of benzene
1
(
B), naphthalene (N), and thiazole (T) subunits. H NMR of NBnTAN (DMSO-d ,
6
6
1
5
7
1
00MHz), δ (in ppm): 4.41 (s, 2H, CH ), 7.02 (N, d, 1H, J = 9.3, H-3’), 7.44 (N, dd,
2
H, J = 7.8, 7.2, H-6’), 7.59 (N, dd, 1H, J = 7.8, 7.2, H-7’), 7.66 (B, t, 1H, J = 7.8, H-
), 7.79 (T, s, 1H, H-4’’, 7.80 (N, d, 1H, J = 7.8, H-5’), 7.83 (B, d, 1H, J = 7.8, H-6),
.98 (N, d, 1H, J = 9.3, H-4’), 8.12 (B, dd, 1H, J = 7.8, 1.5, H-4), 8.24 (B, d, J = 1.5,
13
H, H-2), 8.45 (N, d, 1H, J = 7.8, H-8’), 14.25 (N, br s, 1H, OH). C NMR (DMSO-
d6, 151 MHz), δ (in ppm): 32.04 (CH2), 121.35 (N, CH-8’), 121.79 (B, CH-4),
22.58 (N, CH-3’), 123.11 (B, CH-2), 125.53 (N, CH-6’), 127.89 (N, C-4a’), 128.82
N, CH-5’), 129.13 (N, CH-7’), 130.02 (N, C-1’), 130.22 (B, CH-5), 131.84 (N, C-
1
(
8a’), 135.37 (B, CH-6), 137.37 (T, C-5’’), 139.54 (N, CH-4’), 140.91 (T, CH-4’’),
141.95 (B, C-1), 147.94 (B, C-3), 162.69 (N, C-2’), 173.18 (T, C-2’’).
Elemental analysis, Found: C, 61.39; H 3.54; N 14.26%; molecular formula
C H N O S requires C, 61.53; H, 3.61; N, 14.35%.
20
14
4
3
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Scheme 2. Two-step synthesis of compound 5.
2.2. Materials and instrumentation
The stock solution of 1-[(5-(3-Nitrobenzyl)-1,3-thiazol-2-yl)diazenyl]naphthalen-2-ol
was prepared by dissolving the reagent in ethanol (96%). The reagent’s working
solutions were prepared by diluting the stock solution in the ethanol–water mixture.
All used chemicals were of analytical grade purity. Transition metals’ stock solutions
were prepared by dissolving their soluble salts in distilled water. To prevent metal
salts hydrolysis, a small amount of the acid corresponding to the salt anion was
added. The stock and working solutions were prepared using distilled water.
To maintain the desired pH values, the following solutions were used: HCl (pH = 1.0-
3
.0), CH COOH and CH COONa (pH = 4.0-7.0), NH OH and NH Cl (pH = 8.0-9.0),
3
3
4
4
NaOH (pH>10.0). The control of acidity was carried out by a pH-meter InoLab pH
Level 1 equipped with a combined glass electrode (WTW, Germany).
The measurements of absorbance were carried out on a computerized
spectrophotometer WPA Lightwave II UV/VIS (Biochrom, England) in 1.0 cm
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quartz cells in the wavelengths range of 350-900 nm with a resolution of 1 nm. The
organic solvents used for the extraction of the reagents were also used as the blanks.
®
For measurements of protolytic constants, a spectrophotometer Specord S 600 with
an optical probe (661,000 Hellma Analytics, Germany) was employed in the
wavelengths range of 400-900 nm with the step of 0.5 nm.
IR spectra were obtained by a FT-IR spectrometer model Nicolet 6700 from Thermo
SCIENTIFIC in ATR mode.
In the case of compound 2, NMR spectra were measured on NMR spectrometer
1
Bruker Avance 400 MHz spectrometer (at 400 MHz for H NMR). In the case of
compound 5, NMR spectra were measured on a 600 MHz Varian VNMRS
1
13
spectrometer (Palo Alto, USA) at 599.861 MHz ( H) and 150.835 MHz ( C) at room
temperature at a concentration of 16 mg of NBnTAN sample in DMSO-d (0.5 mL,
6
1
13
Sigma-Aldrich). H and C 1D NMR and 2D NMR methods (gCOSY, gHSQCAD,
gHMBCAD) were used to assign chemical shifts. The chemical shifts (δ scale, ppm)
1
13
were referenced to solvent signals ( H – 2.50 ppm, C – 39.50 ppm). Coupling
constants are given in Hz.
2.3. Computational methods
The structure of NBnTAN molecule was modelled based on previous XRD [21] and
DFT [31] studies of BnTAN molecule. DFT geometry optimizations and Hessian
calculations were performed at the PBE/6-311G(d,p) level of theory [32-34], as it
was shown that this method can accurately describe similar dyes [35, 36]. Total
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energy was computed at the M06-2X/6-311+G(2d,p) level [37], as this functional is
known to be superior for thermochemistry and non-covalent interactions (NCI) [38-
40]. Intramolecular NCI were analyzed using electron density calculated with the
PBE0/6-31++G(d,p) method [41]. NMR chemical shifts and NICS(0) aromaticity
indexes [42] were calculated with the gauge-independent atomic orbital (GIAO)
approach [43] at three different levels of theory: PBE/2 [44] in gas phase,
B3LYP/6-31G(d,p) [45] and PBE0/6-31G(d,p) with employment of the CPCM
solvent (DMSO) [46]. Choosing of the PBE functional was stipulated by the previous
successful NMR calculations [47-49]. In numerous works, it was shown that the
theoretical NMR shifts computed with the PBE [47-49], B3LYP [50] and PBE0 [51]
functionals demonstrate a good correlation with the experimental data; also,
consideration of the solvent effects is sometimes desirable [52, 53]. Dipole moments,
frontier molecular orbitals, Hirshfeld population analysis [54], ADCH [55], and CM5
[
56] net atomic charges were computed at the PBE0/6-31++G(d,p) level. Mulliken
57], Löwdin [58], and NPA [59] partial charges, which are good in prediction of
[
some physicochemical properties, such as partitioning coefficient logP [61], are
known for their inconsistency with the diffuse functions [60], so the non-augmented
6-31G(d,p) basis set was used in these calculations. The choice of PBE0 functional
was dictated by its adequate representation of dipole moments [62] and electron
density [63]. Taking into account the popularity of semiempirical quantum models
[64-67], we have studied the Mulliken [57] and Coulson [68] net charges calculated
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with the PM7 [69] and RM1 [70, 71] methods in gas phase and in the COSMO [72]
water solution.
All gas phase calculations employing the PBE functional were performed using the
PRIRODA 17 code [73]. Other DFT computations were done with the ORCA 4.1
package [74]. AIM critical points, reduced density gradient (RDG) [75] isosurface
and ADCH partial charges were calculated with the MultiWFN 3.6 software [76].
The JANPA 2.02 program was used for natural population analysis [77]. CM5
charges were calculated with the CM5charges code [78]. MOPAC2016 was used for
all calculation related to the semiempirical methods [79]. All manipulations with
geometric parameters and input files preparation were done using the Avogadro [80]
and the Gabedit [81] programs, respectively. Visualization and figures rendering
were performed with the VMD [82] and Jmol [83] programs. Resolution of identity
[84] and chain-of-spheres [85] techniques were used for speed-up the DFT
computations.
3
3
. Results and discussion
.1. Synthesis
Our
syntheses
of
the
target
1-[(5-(3-nitrobenzyl)-1,3-thiazol-2-
yl)diazenyl]naphthalen-2-ol 5 (NBnTAN) are illustrated in Schemes 1 and 2. At the
first step the acrolein was chloroarylated by diazonium salt in the presence of copper
(II) chloride under Meerwein arylation conditions. As mentioned above, distillation
of 2 requires care because overheating of this product can lead to an explosion. 3-(3-
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Nitrophenyl-2-chloropropanal 2 reacted with thiourea in ethanol at refluxing and
formed 2-amino-5-(3-nitrobenzyl)-1,3-thiazole 3 with a good yield (Scheme 1) [86].
Previously we have discussed the synthetic possibilities of 5-benzyl-2-aminothiazoles
during the synthesis of some derivatives as potential biologically active compounds
[87-90] and analytical reagents [21-23, 31, 91].
Herein we describe novel advantages of 2-amino-5-(3-nitrobenzyl)-1,3-thiazole
diazotization and the application of obtained diazonium salt in diazocoupling. 5-(3-
Nitrobenzyl)-2-aminothiazole is a weak base, so efficient diazotization can be
achieved only by using nitrosylsulfuric acid obtained from NaNO and concentrated
2
H SO .
2
4
5-(3-Nitrobenzyl)-1,3-thiazole-2-diazonium salt is unstable and readily participates in
the diazocoupling reaction. Diazocoupling of 5-benzyl-1,3-thiazole-2-diazonium with
naphthalen-2-ol was carried out in alkaline conditions forming 1-[(5-benzyl-1,3-
thiazol-2-yl)diazenyl]naphthalen-2-ol with a good yield (Scheme 2). It should be
concluded that the above described method can be an excellent synthetic route to new
azo dyes of the thiazolyl series.
3.2. General DFT computations of NBnTAN structure
Taking into account high similarity of NBnTAN with the previously described
thiazolylazo dye [21], starting geometry of NBnTAN was modeled based on BnTAN.
The four conformations related to m-nitrobenzyl fragment was checked at the M06-
2X/6-311+G(2d,p)//PBE/6-311G(d,p) level of theory (see Figure 1). The relative
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Gibbs free energies calculated for the four structures a, b, c, and d are 0.0, 0.3, 0.8,
and 0.7 kcal/mol, respectively. Off course, the difference is marginal and testifies
about the presence of all proposed structures simultaneously; however, to save
computational resources, it was necessary to choose one structure. Hence, the a
conformation was used as a basic structure in our further investigations.
Figure 1. Four considered conformations of NBnTAN with different position of m-
nitrobenzyl fragment.
In previous works related to the BnTAN molecule [31], four possible tautomeric
forms caused by possible different location of the hydrogen atom were investigated.
The same picture can be observed in the case of the NBnTAN molecule (Figure 2).
The non-covalent interactions (NCI) were studied with the RDG function and bond
critical points. Blue areas of the RDG isosurface on Figures 2A and 2B indicate the
presence of OHN and OHN hydrogen bonds, respectively.
Furthermore, CP68 and CP71 critical points also show the presence of strong
electrostatic attraction. Critical points CP70 and CP76 on Figures 2C and 2D are
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related to weak dispersion interaction (green RDG area); however, the neighboring
red areas indicate strong repulsion between the considered fragments. Hence, the C
and D tautomers are expected to be less stable.
Figure 2. Bond critical points (orange dots) and RDG function of four tautomeric
forms of NBnTAN. Blue areas of the RDG isosurface correspond to strong attraction,
green areas to weak interactions, and red areas to strong repulsion.
In order to find more accurate data about the stability of the tautomers, we have
carried out the calculation of thermodynamic properties at the M06-2X/6-
311+G(2d,p)//PBE/6-311G(d,p) level of theory (see Table 1). As can be seen, the
computed relative Gibbs free energy values and the relative enthalpy values are very
close for each considered tautomer-phase combination. Consequently, respective
Boltzmann distribution values relatively the same. We have to notice, that with the
increase of the solvents polarity (dielectric constant), the quantity of the forms B, C,
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and D is also increased, but the structure A is the most stable one in the all modeled
media; so all further calculations in the present work were based on this form.
Table 1. Relative Gibbs free energies of the A-D tautomeric forms of NBnTAN in
different media, and Boltzmann distribution of each tautomer.
Tautomer Gas phase Chloroform Ethanol
DMSO
0.2
Water
0.0
Relative
Gibbs free
energy,
A
B
C
D
A
B
C
D
A
B
C
D
A
B
C
D
15.3
17.2
3.4
4.6
0.5
1.6
1.2
1.0
30.8
12.3
7.5
6.8
6.6
kcal/mol
34.9
16.8
12.0
11.4
11.1
Boltzmann
distribution
according
to ΔG, %
95.99
4.01
88.53
11.47
2.8E-05
1.4E-08
3.4
85.49
14.51
7.0E-4
3.2E-07
0.5
85.05
14.95
1.1E-3
4.9E-07
0.2
84.86
15.14
1.3E-3
6.0E-07
0
4.2E-10
4.3E-13
15.3
Relative
enthalpy,
kcal/mol
17.1
4.6
1.5
1.1
1.0
32.2
13.7
8.9
8.2
8.0
35.6
17.6
12.8
12.2
11.9
Boltzmann
distribution
according
to ΔH, %
95.58
4.42
87.46
12.54
2.7E-06
3.6E-09
84.18
15.82
6.6E-05
8.4E-08
83.72
16.28
1.0E-4
1.3E-07
83.51
16.49
1.2E-4
1.6E-07
4.1E-11
1.1E-13
3.3. IR spectroscopy study
The experimental and calculated (at the PBE/6-311(d,p) level) IR spectra of
NBnTAN are presented on Figure 3. A full list of the calculated vibrations in the
NBnTAN molecule is presented in Supplementary Materials.
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Figure 3. Experimental (blue curve) and calculated (purple drop lines) IR spectra of
NBnTAN.
As the NBnTAN IR spectrum was overloaded with many bands, it was analyzed by
comparison with the IR spectra of the model compounds, 1-(5-benzyltiazol-2-
yl)azonaphtalen-2-ol and 1-(2-thiazolylazo)-2-naphthol. The assignment of selected
peaks in the IR spectrum of NBnTAN was supported by analysis of theoretical
calculations and corresponding types of vibrations listed in Table 2. A broad band in
-1
the range of 2500–3500 cm in the IR spectra of solid NBnTAN corresponds to the
-
1


(C–H) and (O–H) vibrations. Sharp medium peak at 3066 cm corresponds to the
(C–H) vibrations of the methylene group. The ν(C–C) vibrations are observed as
-1
-1
medium peak in the range of 1596-1615 cm . The strong band at 1523 cm
corresponds to the asymmetric ν(N–O) vibrations of the nitro group. Medium bands
-1
in the range of 1433-1478 cm are related to the ν(C=N), ν(C=C) and δ(C–O–H)
vibrations of 1,3-thiazole and naphthalen-2-ol moieties. Symmetrical N-O vibrations
-1
of the nitro group appear as the strong band at 1344 cm . The medium band at 1247
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-1
cm can be assigned to the ν(C–O) vibration in the naphthalen-2-ol fragment. In
general, the rest of the NBnTAN IR spectrum is very similar to the spectra of other
thiazolylazonaphthol dyes (e.g., TAN) [92].
As seen, there is a good correlation between the experimental and theoretical
wavenumbers, which additionally testifies the correctness of the selected
computational method. Some non-triviality of the O-H vibration observed for system
under study could be explained by the possible coexistence of a few tautomeric
NBnTAN forms, which was discussed earlier in detail for a similar molecule [6].
Table 2. Selected assigned experimental and theoretical IR vibrations of NBnTAN.
Experimental
wavenumbers, cm
Type of
vibration and
bond
Functional group
Calculated
wavenumbers, cm
-1
-1
2
3
500-3500w
066m
ν(O–H)
ν(C–H)
naphthalen-2-ol
benzyl
2966-3158 (C-H)
2967, 3009 (CH2)
2560 (OH)
1596-1615m
ν(C–C)
naphthalen
benzyl
1588-1623
1
1
523s
433-1478m
νas(N–O)
ν(C=N)
ν(C=C)
nitro
1,3-thiazole
naphthalen-2-ol
1544
1492-1506
δ(C–O–H)
νs(N–O)
ν(C–O)
1
1
344s
247m
nitro
naphthalen-2-ol
1314
1294
3.4. NMR spectroscopy study
The structure of the NBnTAN sample was unambiguously proven by NMR
13
spectroscopy. The 1H and C NMR signals of all atoms were assigned to individual
protons and carbons of benzene (B), naphthalene (N), and thiazole (T) subunits based
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on their chemical shifts, multiplicities, COSY, HSQC, and HMBC cross-peaks, and
substituent effects of the nitro, azo and hydroxyl substituents on the chemical shifts in
accord with literature and chemical expectations.
1
In the H NMR spectrum, four signals were observed for the benzene nucleus,
including two doublets H-4 and H-6, one triplet H-5 and one singlet H-2 as a result of
spin-spin interactions through 3-bonds (atom numbering according to Scheme 2). The
signals H-2 (8.24 ppm) and H-4 (8.12 ppm) were significantly deshielded by the
ortho-electron acceptor effect of the nitro group. Six aromatic signals were detected
for the naphthalene nucleus, of which 4 doublets belonged to protons 3’, 4’, 5’ and 8’
and two triplets belonged to protons 6’ and 7’. The high chemical shift value of the
OH proton at 14.25 ppm indicates its involvement in a strong hydrogen bond or
tautomerization. As expected, the OH group with a strong +M effect significantly
shifted the H-3’ doublet to a higher field, 7.02 ppm. The markedly deshielded H-8’
signal in naphthalene (8.45 ppm) appears to be due to the magnetic anisotropy of the
adjacent azo bond (N=N) on the C-1’ naphthalene carbon.
13
In the C NMR spectrum, all signals were observed at the expected chemical shifts,
similar to model spectra of m-nitrobenzyl, 2-naphthol and thiazole. It was easy to
assign more intense aromatic CH signals in a typical region of 121.35-139.54 ppm
and a CH group (32.04 ppm) using twelve one-bond HSQC correlations with the
2
identified protons. Quaternary carbon signals were mainly assigned using strong
3
HMBC cross peaks with protons spaced through 3 bonds ( J ≈ 8 Hz). Within the
CH
thiazole ring, the HSQC and HMBC transfers to all three carbons, N=C-2” (173.18
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ppm), CH-4” (140.91 ppm) and C-5” (137.37 ppm) from the proton H-4”, together
13
with C FID exponentially multiplied with a large line-broadening constant LB = 20,
allowed detection of the both quaternary carbons of a very low intensity (Figure 4).
Similarly, we found a very weak signal of the naphthalene HO-C-2’ carbon (162.69
ppm) by recording a spectrum with LB = 50.
Figure 4. 2D NMR HSQC spectra of the NBnTAN.
Comparison of the experimental and calculated chemical shifts was done for different
levels of theory. Particularly, the PBE/2 method without solvent model, the
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B3LYP/6-31G(d,p) and PBE0/6-31G(d,p) methods in combination with the CPCM
solvation model for taking into account the DMSO effects. As the reference standard
signals for comparison of computed versus experimental shifts, we have used the
shifts of the methylene group.
1
Table 3. Comparison of experimental and theoretical H chemical shifts (in ppm).
Numbering of atoms corresponds to Scheme 2.
Hydrogen
atom
CH2
H-3'
H-6'
H-7'
H-5
Experimental
shift
CPCM-B3LYP/6-
31G(d,p)
CPCM-PBE0/6- PBE/2
31G(d,p)
4.41
7.02
7.44
7.59
7.66
7.79
7.80
7.83
7.98
4.41
7.31
7.75
7.87
7.79
8.03
7.98
7.94
8.19
4.41
7.43
7.86
7.98
7.88
8.11
8.11
8.02
8.33
8.50
8.62
8.86
16.89
4.41
7.35
7.66
7.72
7.70
8.00
7.83
7.72
7.95
8.59
8.69
8.88
17.78
H-4"
H-5'
H-6
H-4'
H-4
H-2
H-8'
OH
8.12
8.24
8.45
8.43
8.53
8.78
14.25
Slope
Intercep2t
R
16.87
0.7675
1.5570
0.9906
0.7707 0.6996
1.4650 2.1098
0.9922 0.9803
It must be noted that inclusion of solvents influence considerably improve the
1
2
accuracy of predicted H shifts. Thus, the coefficient of determination R in the case
of PBE/2 model is only 0.9803, whereas the use of the CPCM approach with the
2
B3LYP/6-31G(d,p) and PBE0/6-31G(d,p) levels results in the R values of 0.9906
and 0.9922, respectively (see Table 3). For the best tested method, CPCM-PBE0/6-
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1G(d,p), the linear equation describing the correlation between the experimental and
3
calculated chemical shifts can be written as (exp.) = 0.7707 × (calc.) + 1.4650.
13
Table 4. Comparison of experimental and theoretical C chemical shifts (in ppm).
Numbering of atoms corresponds to Scheme 2.
Carbon
atom
CH2
Experimental
shift
CPCM-B3LYP/6-
31G(d,p)
CPCM-PBE0/6-
31G(d,p)
PBE/2
32.04
32.04
32.04
32.04
CH-8'
CH-4
CH-3'
CH-2
CH-6'
C-4a'
CH-5'
CH-7'
C-1'
CH-5
C-8a'
CH-6
C-5"
121.35
121.79
122.58
123.11
125.53
127.89
128.82
129.13
130.02
130.22
131.84
135.37
137.37
139.54
140.91
141.95
147.94
162.69
173.18
Slope
112.35
113.92
110.59
115.17
116.55
119.05
121.59
121.05
124.76
120.91
123.82
128.00
140.54
133.67
132.33
135.13
141.58
149.57
171.43
1.0179
4.7601
0.9804
113.83 108.85
115.77 116.60
112.35 114.37
116.93 117.45
118.42 118.66
119.67 126.18
123.31 120.92
123.11 121.84
125.54 128.82
122.76 123.94
124.70 127.42
130.00 129.42
140.00 140.74
135.92 133.43
134.09 135.30
135.90 140.47
142.51 146.35
150.81 153.86
171.73 170.62
1.0185 1.0010
3.3917 4.6795
0.9853 0.9814
CH-4'
CH-4"
C-1
C-3
C-2'
C-2"
Intercep2t
R
13
For computed C NMR chemical shifts, the solvent’s influence is less tremendous
2
(
see Table 4). Thus, the best correlation (R = 0.9853) was again got for the CPCM-
PBE0/6-31G(d,p) method. The PBE/2 level of theory outperforms the popular
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B3LYP/6-31G(d,p) method in each considered parameter: coefficient of
determination is 0.9814 versus 0.9804; the slope 1.0010 is closer to unity than
1.0179; the intercept 4.6795 is lower than 4.7601.
3.5. Comparison of electronic structure of NBnTAN and BnTAN
Figure 5. Frontier molecular orbitals and dipole moments (purple vectors) of
NBnTAN (left) and BnTAN (right).
Taking into account structural similarity of the NBnTAN and BnTAN molecules, we
decided to compare their electronic structures. The geometry of BnTAN was
optimized at the PBE/6-311G(d,p) level of theory [31]. Dipole moments vectors and
frontier molecular orbitals calculated for the above molecules at the PBE0/6-
31++G(d,p) level in gas phase are presented in Figure 5. Despite the different vector
directions found, the absolute values of the dipole moments are 2.72 D and 2.93 D for
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NBnTAN and BnTAN, respectively. However, the difference is more obvious for the
CPCM water solutions, with 3.05 D for NBnTAN and 4.16 D for BnTAN.
Frontier molecular orbitals of the two aforementioned dyes are quite similar. Thus,
the gas phase computed HOMO energies are –6.3 eV and –6.1 eV for NBnTAN and
BnTAN, respectively, while the respective LUMO energies are –3.2 eV and –3.0 eV.
In turn, in the water solution the difference between the energies of HOMOs and
LUMOs of the both dyes becomes negligible within the round-off error: in both cases
EHOMO = –6.2 eV and ELUMO = –3.2 eV.
Unfortunately, comparison of electronic structures of the above dyes through the
direct subtracting of electron densities is not possible due to the difference in their
geometries. Hence, it was decided to employ various population analysis schemes in
order to highlight the differences in the electron densities. The Hirshfeld, ADCH, and
CM5 partial charges calculated with the PBE0/6-31++G(d,p) method for gaseous and
water-dissolved states of NBnTAN and BnTAN are presented in Table 5. In all cases,
the differences between corresponding NBnTAN and BnTAN atomic partial charges
are less than 0.005 e, except the m-nitrobenzyl and benzyl fragments, where
differences in atomic partial charges values are about 0.01-0.02 e. This can be
explained by the relatively localized influence of the nitro group on the benzyl
moiety, rather than thiazole or naphthalene rings due to the absence of conjugation
between the benzyl and thiazole fragments. We have to emphasize the stable small
decrease in the partial charge of the N5 atom of NBnTAN in comparison to BnTAN,
which testify the lower nucleophilicity and lower proton-affinity of the N5 atom in
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the nitro-dye. Consequently, the basicity (pK value) of NBnTAN is expected to be
lower in comparison with BnTAN, which was proven experimentally.
Table 5. PBE0/6-31++G(d,p) Hirshfeld, ADCH, CM5 partial charges of BnTAN
(Bn) and NBnTAN (NBn) in the gas phase and in water. Numbering of atoms is
consistent with Figure 5.
Gas phase
ADCH
Bn NBn
Water
ADCH
Atom
Hirshfeld
Bn NBn
CM5
Bn NBn
Hirshfeld
Bn NBn
CM5
Bn NBn
Bn
NBn
S1
O2
H3
N4
N5
N6
C7
C8
0.048 0.047 0.060 0.059 0.031 0.030 0.048 0.050 0.063 0.065 0.030 0.033
-0.189 -0.186 -0.268 -0.264 -0.359 -0.357 -0.208 -0.207 -0.302 -0.300 -0.379 -0.377
0.113 0.114 0.175 0.176 0.317 0.318 0.111 0.112 0.176 0.176 0.315 0.315
-0.058 -0.060 0.021 0.019 -0.191 -0.193 -0.065 -0.066 0.016 0.014 -0.198 -0.199
-0.199 -0.198 -0.298 -0.296 -0.374 -0.373 -0.235 -0.234 -0.367 -0.365 -0.410 -0.409
-0.074 -0.072 -0.142 -0.139 -0.179 -0.178 -0.077 -0.076 -0.145 -0.143 -0.182 -0.182
0.112 0.114 0.122 0.123 0.136 0.138 0.109 0.110 0.124 0.125 0.133 0.134
0.013 0.014 0.053 0.054 0.089 0.090 0.012 0.012 0.054 0.054 0.087 0.088
0.003 0.003 -0.018 -0.018 0.005 0.005 -0.005 -0.004 -0.030 -0.030 -0.003 -0.003
-0.039 -0.038 -0.120 -0.120 -0.088 -0.088 -0.048 -0.048 -0.133 -0.132 -0.098 -0.097
0.040 0.040 0.114 0.114 0.101 0.101 0.041 0.041 0.117 0.118 0.102 0.102
-0.032 -0.030 -0.111 -0.109 -0.087 -0.085 -0.034 -0.034 -0.117 -0.116 -0.089 -0.088
0.045 0.047 0.136 0.138 0.100 0.101 0.055 0.056 0.153 0.153 0.110 0.110
-0.044 -0.042 -0.133 -0.132 -0.099 -0.097 -0.046 -0.046 -0.141 -0.141 -0.100 -0.100
0.044 0.045 0.134 0.136 0.098 0.100 0.054 0.055 0.152 0.152 0.109 0.109
-0.036 -0.034 -0.121 -0.120 -0.088 -0.086 -0.035 -0.035 -0.126 -0.126 -0.087 -0.087
0.043 0.044 0.131 0.132 0.099 0.099 0.056 0.057 0.152 0.152 0.112 0.112
-0.011 -0.010 -0.022 -0.023 -0.018 -0.017 -0.015 -0.015 -0.031 -0.031 -0.022 -0.022
-0.012 -0.009 -0.094 -0.091 -0.063 -0.060 -0.004 -0.003 -0.088 -0.087 -0.055 -0.054
0.051 0.053 0.142 0.144 0.107 0.108 0.067 0.068 0.168 0.168 0.123 0.123
-0.051 -0.050 -0.149 -0.148 -0.100 -0.099 -0.052 -0.052 -0.155 -0.155 -0.101 -0.101
0.054 0.055 0.144 0.146 0.112 0.113 0.062 0.062 0.157 0.158 0.120 0.120
0.089 0.092 0.107 0.110 0.257 0.260 0.082 0.084 0.107 0.108 0.250 0.252
-0.007 -0.005 -0.053 -0.053 0.017 0.018 -0.009 -0.006 -0.050 -0.048 0.015 0.018
0.053 0.054 0.145 0.147 0.116 0.117 0.065 0.067 0.165 0.167 0.129 0.130
-0.009 -0.013 -0.026 -0.034 0.000 -0.004 -0.001 -0.003 -0.011 -0.017 0.008 0.006
-0.036 -0.034 -0.133 -0.125 -0.130 -0.128 -0.033 -0.029 -0.091 -0.081 -0.127 -0.123
0.041 0.046 0.106 0.115 0.097 0.101 0.053 0.058 0.125 0.132 0.108 0.113
0.046 0.049 0.122 0.128 0.101 0.105 0.057 0.063 0.141 0.151 0.113 0.118
-0.001 0.009 -0.049 -0.048 -0.015 -0.004 -0.005 0.012 -0.186 -0.148 -0.018 -0.001
-0.043 -0.023 -0.038 0.060 -0.095 -0.075 -0.051 -0.017 0.449 0.492 -0.103 -0.069
0.039 0.048 0.073 0.032 0.095 0.104 0.045 0.058 -0.203 -0.214 0.101 0.115
-0.037 -0.025 -0.132 -0.113 -0.092 -0.079 -0.046 -0.023 -0.194 -0.138 -0.101 -0.077
C9
C10
H11
C12
H13
C14
H15
C16
H17
C18
C19
H20
C21
H22
C23
C24
H25
C26
C27
H28
H29
C30
C31
H32
C33
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H34
C35
H36
C37
C38
H39
0.044 0.053 0.135 0.151 0.098 0.108 0.050 0.066 0.155 0.177 0.105 0.120
-0.041 -0.024 -0.139 -0.131 -0.096 -0.070 -0.050 -0.024 -0.198 -0.177 -0.104 -0.070
0.043 0.055 0.133 0.141 0.097 0.118 0.050 0.060 0.145 0.147 0.104 0.122
-0.040 0.034 -0.128 0.019 -0.094 0.083 -0.047 0.031 -0.144 0.022 -0.101 0.080
-0.046 -0.031 -0.139 -0.134 -0.098 -0.075 -0.050 -0.028 -0.191 -0.180 -0.102 -0.072
0.039 0.050 0.124 0.130 0.094 0.114 0.049 0.057 0.142 0.140 0.104 0.120
H40/N40 0.043 0.231 0.133 0.373 0.098 0.071 0.051 0.235 0.147 0.392 0.105 0.076
O41
O42
-0.204
-0.205
-0.273
-0.275
-0.166
-0.168
-0.232
-0.232
-0.317
-0.317
-0.194
-0.195
Additionally, we have performed calculations of atomic partial charges through the
Mulliken, Löwdin, and natural population analyses at the PBE0/6-31G(d,p) level, and
the Mulliken and Coulson partitioning schemes with the PM7 and RM1
semiempirical methods. Selections of these semiempirical methods is dictated by
their good performance in the prediction of heterocycles reactivity [93]. In all these
cases, we have observed the same influence of the nitro group and very similar partial
charges in the thiazolylazo-2-naphthol fragments of the two dyes. Tables with the
PBE0/6-31G(d,p), PM7, and RM1 partial charges can be found in Supplementary
Materials.
Taking into account that NBnTAN and BnTAN contain few aromatic moieties
(benzyl substituent, thiazolyl cycle, and naphthalene fragment), aromaticity is an
important property of these two dyes. To quantitatively analyze and compare
aromaticity of the aforementioned parts of NBnTAN and BnTAN, we have calculated
the NICS(0) indexes at the three levels of theory: CPCM-PBE0/6-31G(d,p), CPCM-
B3LYP/6-31G(d,p), and PBE/2 (see Table 6). The more negative NICS(0) values of
benzyl (C30-C31-C33-C35-C37-C38), thiazole (S1-C23-N5-C24-C26), non-
substituted naphthalene (C9-C10-C12-C14-C16-C18) rings means the more evident
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aromaticity in comparison to naphthalene-2-ol ring (C7-C8-C9-C18-C19-C21). We
have to notice that at all tested computational methods the aromaticity of benzyl ring
is higher in NBnTAN than in BnTAN.
Table 6. Values of NICS(0) aromaticity indexes (in ppm) of NBnTAN (NBn) and
BnTAN (Bn). Numbering of atoms is consistent with Figure 5.
Ring
CPCM-B3LYP/6-31G(d,p) CPCM-PBE0/6-31G(d,p)
PBE/2
NBn
–9.97
–8.75
–5.99
–9.08
Bn
NBn
–10.06
–9.06
–6.03
–9.12
Bn
NBn Bn
C30-C31-C33-C35-C37-C38
S1-C23-N5-C24-C26
C7-C8-C9-C18-C19-C21
C9-C10-C12-C14-C16-C18
–9.41
–8.71
–6.13
–9.09
–9.40
–9.02
–6.17
–9.13
–8.76 –8.06
–8.39 –8.36
–4.89 –4.97
–7.45 –7.49
3.6. Investigation of UV-Vis properties
NBnTAN is an amorphous powder of red color, poorly soluble in water, but well
soluble in many organic solvents.
Figure 6. Absorption spectra of NBnTAN in different solvents. C(NBnTAN) =
–5
–1
2
.4∙10 mol∙L . 1 – amyl acetate, 2- acetone, 3 – amyl alcohol, 4 – triethylamine, 5 –
hexane.
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Absorption spectra of NBnTAN in some organic solvents are shown in Figure 6.
Absorption maxima in most solvents were found at 485-493 nm.
3
-
The molar absorption coefficients in different solvents ranges from 7.4 × 10 L × mol
1
-1
3
-1
-1
×
cm (hexane) to 12.6 × 10 L × mol × cm (amyl acetate). Increasing the
polarity of the solvent as a whole causes a bathochromic shift of the absorption
maximum and an increase in the value of ε, however, it is difficult to talk about the
regular nature of this tendency. In addition to polarity, the ability of polar in nature
solvents to deprotonate the dye and thus shift the equilibrium to the formation of
anionic form of NBnTAN. This is followed, for example, by a significant
bathochrome shift of the maxima to 505-511 nm as was observed after addition of
triethylamine, N,N-dimethylcyclohexylamine, dimethylformamide, which can be
explained by their strong basic properties. A less characteristic signal in the form of a
“
shoulder” was observed in the long-wave region at 570-590 nm. In low-polar
solvents such a “shoulder” is not observed.
Table 7. Spectral characteristics of NBnTAN in various organic solvents.
-3
-3
Organic solvent
ε∙10 ,
Organic solvent
ε∙10 ,
-1
-1
-1
-1
λ, nm L∙mol ∙cm
λ, nm L∙mol ∙cm
hexane
486
487
492
492
7.4
8.6
9.0
9.1
9.1
8.0
8.1
8.3
8.9
8.8
8.8
1-nonanol
493
486
486
488
485
487
488
8.9
7.2
8.8
9.2
9.9
12.6
9.2
9.1
9.3
9.4
8.8
cyclohexane
dichloromethane
chloroform
ethyl acetate
n-butyl acetate
acetonitrile
isobutyl acetate
amyl acetate
aceton
methyl isobutyl ketone 486
diethyl ether 485
methyl tert-butyl ether 485
tributyl phosphate 487
1,2-dichloroethane 492
tetrachlorethylene 488
methanol
ethanol
492
491
492
493
492
1-propanol
n-butanol
amylalkohol
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8.8 toluene
1-octanol
492
491
10.0
NBnTAN is well soluble in ethanol. There is a linear correlation between the
concentration of NBnTAN and absorption of solutions at maximum wavelength of
492 nm. The ethanol-water solutions allow avoiding the use of toxic organic solvents.
The results of the study of water-ethanol solutions of different composition showed
that increasing content of water in the water-alcohol solution decreased the molar
absorption coefficient (Table 8). In all cases, the dependence between NBnTAN
concentration and absorption of solutions is linear, with the correlation coefficient
close to 1.0, but the angle of inclination of the line (slope) and the intervals of
linearity are significantly different, probably due to different solubility of NBnTAN
in the respective media.
Table 8. Spectral characteristics of NBnTAN water-ethanol mixtures.
Volume fraction of ethanol
Spectral maximum, nm
96%
491
50%
492
10%
492
Molar absorbtion coefficient,
8
300
6500
4580
-1
L∙(mol∙cm)
Limits of linear dependence,
-5
-5
-5
-1
(2.1-8.6) ∙10
(3.2-8.6) ∙10
(0.8-4.4) ∙10
mol∙L
A = 0.0043 +
A = 0.0024 +
0.649∙10 ∙С
A = 0.0030 +
0.458∙10 ∙С
Calibration curve equation
4
4
4
0.831∙10 ∙С
Correlation coefficient, R
0.9997
0.9998
0.9996
3.7. Investigation of acid-base properties
Depending on the acidity of the medium, NBnTAN can exist in several forms. The
dissociation processes of BnTAN, on the basis of which this reactant was
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synthesized, are described in [21]. The nitro group effect significantly affects the
values of the NBnTAN acidity constants.
Depending on the acidity of the medium, NBnTAN in water-ethanol solutions can
exist in three forms. In a strongly acidic medium (pH<1), NBnTAN exists in the form
of a yellow compound with a maximum absorption at 446 nm. In a wide range of
acidities of the medium (pH = 1-9), NBnTAN dominates in the form of a compound
of orange-red color, with a maximum at 492 nm. In an alkaline medium (pH>9), a
purple colored compound with a maximum at 540 nm is formed.
Formation of these forms is conditioned by the protolytic equilibrium, which in
general can be written as shown in Scheme 3. Figure 7 shows the absorption spectra
of these forms in ethanol–water mixture (1:9, v/v) at different acidity values.
Scheme 3. Protonation and deprotonation of the NBnTAN molecule.
29