Structure investigations of Schiff bases derived from 3-amino-1H-1,2,4-triazole

Article abstract of DOI:10.1016/j.molstruc.2019.02.027

In the present paper, twelve Schiff bases derived from 3-amino-1H-1,2,4-triazole (ATz) and various benzaldehydes, and salicylaldehydes were synthesized. The ¹H, ¹³C, and ¹⁵N NMR data are discussed in relation to the structure of ATz and its imine products. In addition, X-ray, ATR-FTIR, and UV–Vis analytical techniques are used for structure elucidation of ATz-based Schiff bases. It was found that the starting material, 3-amino-1H-1,2,4-triazole, exists as tautomeric mixture of three forms (Graphical Abstract) in solution, whereas in the solid state (¹³C and ¹⁵N CPMAS data) potentially tautomeric proton is located on nitrogen atom traditionally marked as N-2 (Graphical Abstract, 2N–H structure). All investigated Schiff bases derived from salicylaldehydes exist in both phases as tautomeric equilibrium mixtures, where enol-imine forms are dominated structures. The positions of those equilibria only very slightly depend on substituents in phenol ring. Generally, the contributions of keto-amine forms in the solid state are higher comparing with DMSO solutions.

Full text of DOI:10.1016/j.molstruc.2019.02.027

Accepted Manuscript
Structure investigations of Schiff bases derived from 3-amino-1H-1,2,4-triazole
Beata Kołodziej, M. Morawiak, Wojciech Schilf, Bohdan Kamieński
PII:
S0022-2860(19)30151-6
DOI:
https://doi.org/10.1016/j.molstruc.2019.02.027
MOLSTR 26184
Reference:
To appear in: Journal of Molecular Structure
Received Date: 14 November 2018
Revised Date: 30 January 2019
Accepted Date: 7 February 2019
Please cite this article as: B. Kołodziej, M. Morawiak, W. Schilf, B. Kamieński, Structure investigations of
Schiff bases derived from 3-amino-1H-1,2,4-triazole, Journal of Molecular Structure (2019), doi: https://
doi.org/10.1016/j.molstruc.2019.02.027.
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Graphical Abstract

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Structure investigations of Schiff bases derived from 3-amino-1H-1,2,4-triazole
Beata Kołodzieja*, M. Morawiakb, Wojciech Schilfb, and Bohdan Kamieńskib,c
a- West Pomeranian University of Technology, Szczecin; Faculty of Chemical
Technology and Engineering; Department of Inorganic and Analytical Chemistry, Al.
Piastów 42, 71-065 Szczecin, Poland
b- Institute of Organic Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-
224 Warsaw, Poland
c- Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-
224 Warsaw, Poland
*- corresponding author
Key words
aminotriazole, Schiff bases, 1H, 13C, and 15N NMR, FT-IR-ATR, UV-Vis, and X-ray
spectroscopic methods, intramolecular hydrogen bond.
Abstract
In the present paper, twelve Schiff bases derived from 3-amino-1H-1,2,4-triazole
1
13
(ATz) and various benzaldehydes, and salicylaldehydes were synthesized. The H, C, and
15N NMR data are discussed in relation to the structure of ATz and its imine products. In
addition, X-ray, ATR-FTIR, and UV-Vis analytical techniques are used for structure
elucidation of ATz-based Schiff bases. It was found that the starting material, 3-amino-1H-
1,2,4-triazole, exists as tautomeric mixture of three forms (Graphical Abstract) in solution,
whereas in the solid state (13C and 15N CPMAS data) potentially tautomeric proton is located
on nitrogen atom traditionally marked as N-2 (Graphical Abstract, 2N-H structure). All
investigated Schiff bases derived from salicylaldehydes exist in both phases as tautomeric
equilibrium mixtures, where enol-imine forms are dominated structures. The positions of
those equilibria only very slightly depend on substituents in phenol ring. Generally, the
contributions of keto-amine forms in the solid state are higher comparing with DMSO
solutions.

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Introduction
Imines derived from o-hydroxy aromatic carbonyls are of great interest because of
their ability to form the asymmetric intramolecular hydrogen bond between oxygen atom of
hydroxyl group and nitrogen atom of imine group [1-3]. This ability has a decisive impact on
biological and thermo- or photochromic properties of o-hydroxy aromatic Schiff bases [4-7]
and makes them very useful compounds in chemistry, biochemistry, medicine, and
technology [8-11]. Therefore, the very important issue is determining the positions of
tautomeric equilibria in these compounds. Consequently, various instrumental research
techniques are used to provide insight into the structure of molecules of studied o-hydroxy
Schiff bases [7, 12-13]. Determination of the position of the hydrogen atom involved in
hydrogen bonding can be achieved by using a combination spectroscopic techniques (i.e.,
ATR-IR, UV-Vis, and heteronuclear NMR spectroscopies) and x-ray diffractometery [14-16].
Very convenient, but not always exact method of proton position estimation in
intramolecular hydrogen bridge is proton NMR measurement. By this method one can obtain
quick information about presence of such bonds. The signal position of proton involved in the
hydrogen bonding is strongly downfield shifted to the range from 10 to even 21 ppm.
Unfortunately, the chemical shift by itself does not provide reliable information about
13
geometry of hydrogen bridge. To obtain such data it is necessary to use C NMR data. The
most valuable parameter is chemical shift of formally OH substituted aromatic carbon atom.
For enol-imine structure the typical position of this signal is in the range from 155 to 165
ppm, whereas for keto-amine structure this signal is shifted close to 180 ppm [17].
Very useful in investigations of such systems is nitrogen NMR technique [18-19]. This
is because of two reasons:
- high sensitivity of nitrogen chemical shift of formally imine atom (δNim) on proton position
in H-bond [20] (it is well known that δNim for pure imine form is about -50 ÷-60 ppm and for
pure enamine form is about -220 ÷-240 ppm),
- low sensitivity of δNim on the substitution effects connected with different substituents in
salicylaldehyde unit [21-22].
Infrared spectroscopy is a method very readily used in order to confirm the existence
of an imine group in Schiff bases [23]. In the case of simple imines there is no problem in
assigning the bandwidth of the binding site. However, if the imine has in its structure
additional groups (for example –COOH, -C=O) giving bands in the same area, this task
becomes more difficult [24].

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UV-Vis spectroscopy is recognized as a very good and sensitive method to
determining the tautomeric equilibria of o-hydroxy Schiff bases in various solvents [25-26].
Generally, if the compound exists mainly in enol-imine form, in UV-Vis spectrum strong
band below 400 nm is expected. The position of the band in the spectrum above 400 nm
suggests shifting of tautomeric equilibrium to keto-amine form [27]. UV-Vis spectroscopy in
the solid state (UV-Vis-DR) is much less common than in solution, and there are few works
about analysis of Schiff bases by this method [28-29]. One of the reason of rare use of this
method is probably the shape of the obtained spectra, where bands are very often widened and
overlapping.
In the present paper we describe synthesis and instrumental analysis of twelve (three
new) Schiff bases derived from 3-amino-1H-1,2,4-triazole (Amitrole, ATz), which is a
heterocyclic primary amine having in the five-membered ring three nitrogen atoms potentially
involved in tautomeric exchange (Fig.1) [4, 30].
Fig.1
This amine is a very good substrate for synthesis of biologically active compounds. It
is well known that 1,2,4-triazole ring (blue part in Fig.1) has a strong interaction with heme
iron, and the substituted triazoles are potent for the inhibition of ergosterol, which is the main
constituent of fungal cell membrane [4, 31]. Furthermore, Amitrole inhibits production of
imidazoleglycerol-phosphate dehydratase (gene HIS3) in Saccharomyces cerevisiae [30, 32].
Triazoles are considered as good corrosion inhibitors due to the presence of three nitrogen
atoms acting as adsorption centers and due to their environmental acceptability [33]. A
literature survey revealed that triazolyl Schiff bases are of great interest in medicinal
chemistry because of their capability to form stable intermediates which are connected with
diverse biological activity [4].
Experimental
Synthesis of Schiff bases
All compounds were synthesized by standard procedure [34] slightly modified for the
present reaction.
To a methanolic solution of ATz (2.3 mmol) a methanolic solution of suitable
aldehyde (2.3 mmol) was added. The mixture was stirred and refluxed for 30 min, allowed to
cool to ambient temperature and was left undisturbed for 24 hours.

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The resulting precipitate was filtered off in a water pump filter kit, washed with cold
methanol and air dried.
The authenticity and purity of obtained compounds were examined by proton NMR
measurements.
For two imines suitable for X-ray measurements single crystals have been obtained.
NMR measurements
The all NMR experiments in DMSO-d6 solution were performed on Varian-Agilent
600 MHz VNMRS instrument using 5 mm inverse probehead equipped with Z-gradient coil.
The chemical shifts assignment in proton and carbon spectra was done on the base of 2D
experiments (GCOSY, GHSQC and GHMBC). The acquisition of data and spectra processing
was done using standard Agilent software. Internal TMS was applied as the standard
compound for chemical shifts of proton and carbon spectra. For nitrogen-15 experiments (2D
GHMBC), external nitromethane was applied as the chemical shift standard (0 ppm).
The solid state NMR spectra were done using Bruker Avance 500 MHz spectrometer
equipped with 4mm CPMAS broadband probehead. The standard experimental parameters for
carbon CPMAS spectra were: spectral width 31 kHz, acquisition time 20 ms, contact time 2
ms, spin rate 12 kHz. The short contact time experiments (SCT) were applied to differentiate
protonated and quaternary carbon atoms. For those experiments contact time equal 40 µs was
applied. Originally, the carbon solid state spectra were referenced to the glycine sample and
then the obtained chemical shifts values were recalculated to TMS scale. For nitrogen spectra
similar procedure was applied to recalculate chemical shifts values to nitromethane as the
standard. For nitrogen CPMAS experiments the following acquisition parameters were
applied: spectral width 25 kHz, acquisition time 20 ms, spin rate 6-12 kHz, contact time for
spin-lock 4ms, relaxation delay 10 to 120 s depending on relaxation condition for particular
sample, the values parameter were estimated from carbon measurement. For SCT nitrogen
measurements the contact time was reduced to 0.1 ms.
X-ray structure analysis
Crystals of two of ATz-based Schiff bases were obtained from methanol: colourless
crystals of 4BrbenzATz, and light yellow crystals of 4BrsalATz. Crystal data for these
compounds were collected at room temperature, on Bruker X8 APEXII diffractometer using
Cu-Kα radiation (λ = 1.54178 Å) (Table 1 (Suppl. Mat.)). Frames were integrated with the
Bruker SAINT [35] software package using a narrow-frame algorithm. The structure was

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solved and refined using the Bruker SHELXTL Software Package [36]. Data were corrected
for absorption effects using the face-indexed numerical method (SADABS) [37]. Hydroxyl
group hydrogens and N-hydrogen were found from the difference electron density maps and
refined with an anisotropic thermal motion model. Other hydrogen atoms were placed in
calculated positions and refined as riding on their parent atoms with Uiso = 1.2 Ueq. The
structure was solved by direct methods SHELXS-2014 [38] and refined with full-matrix least-
squares calculations on F2 using SHELX-2014 [38]. All non-hydrogen atoms were refined
anisotropically.
Crystallographic data have been deposited at the Cambridge Crystallographic Data
Centre, 12 Union Road, 129 Cambridge CB2 1EZ, UK, and copies can be obtained on
request, free of charge, by quoting the publication citation and the deposition number CCDC
1867614 for 4BrbenzATz and CCDC 1867615 for 4BrsalATz.
ATR-FTIR measurements
ATR-FTIR spectra were recorded on a Bruker Alpha Fourier Transform IR (FTIR)
spectrometer equipped with a platinum ATR single reflection diamond-sampling module
(Bruker Optics).
The infrared spectra were collected as an average of 24 scans per sample between the
wavenumber range of 4000–360 cm-1 at a resolution of 4 cm-1, controlled by Optics User
Software (OPUS) version 7.5 (Bruker Optics). Air was used as reference background spectra.
The ATR diamond surface was cleaned with acetone before each sample was scanned.
UV-Vis measurements
a) in solution
UV-Vis spectra were recorded at room temperature on the double-beam UV-Vis absorption
spectrometer, Jasco V-670, using a 1.0 cm matched silica cells. The UV-Vis spectra measured
in CHCl3, CH3CN, and (CH3)2SO solutions were collected in the wavelength range of 190-
800 nm.
b) in the solid state (UV-Vis-DR measurements)
Diffuse reflectance (DR) spectra were recorded on a Jasco V-670 spectrometer equipped with
an integrating sphere accessory at room temperature.
The powdered compounds were each packed into the sample holder, which was placed in the
next step in the sample space of the instrument.

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The UV-Vis-DR spectra were collected between the wavelength range from 190 to 800 nm,
controlled by JASCO Spectra Analysis. Spectralon was used as a reference background
spectra. Cuvette was cleaned with acetone before each sample was scanned.
Results and discussion
1. NMR spectroscopy
The structures and atom numbering in investigated ATz-based Schiff bases are
presented in Fig.2, and the full set NMR measurements results in DMSO-d6 solution are
collected in Table 2.
Fig.2
Table 2
The proton and carbon atoms signals assignment has been done on the base of both
homo- and heteronuclear 2D spectra (COSY, GHSQC and GHMBC). The differentiation of
carbon signals (C-3’ and C-5’) of triazolyl ring was done applying one-bond correlation with
protons in H-5’ position (Fig.2). This correlation was observed for some derivatives, where
those proton signals were relatively sharp (typical chemical shift of H-5’ signal close to 8.5
ppm). Generally the C-3’ signals were found by correlation with H-7 proton signals.
Unfortunately, due to dynamic character of triazolyl ring, which causes severe broadening of
both proton signals in triazolyl ring (H-1’ and H-5’), we could not detect in DMSO-d6
solution any nitrogen atoms signals from this part of molecules. We could measure only Nim
signals obtained by correlation with very characteristic H-7 proton signals. The positions of
OH and NH protons were defined by comparison of salicylaldehydes derivatives spectra with
those of benzaldehydes derivatives, where OH signals do not exist. The carbon signals
assignment in CPMAS spectra has been done by comparison of regular 13C solid state spectra
with short contact time SCT experiments (only protonated carbon signals are present), and by
analysis of characteristic position of signals in both solution (Table 2) and solid state (Table
3) spectra. The nitrogen signals assignment in solid state spectra has been performed using
SCT experiments to verify positions of N-1’ signals. In all cases the upfield signals close to -
170 ppm were detected as NH signals. To decide how to assign those signals to proper
positions in triazolyl ring it is necessary to analyze nitrogen chemical shifts of aminotriazole
and some of its methyl derivatives, where tautomeric equilibrium is eliminated (Fig.3).
Fig.3

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If we compare the nitrogen chemical shifts of three methyl derivatives (Fig.3, I) [39-
40], where tautomeric exchange is eliminated, with those obtained for ATz in the solid state,
we can decide that in this phase hydrogen atom in ATz is located on nitrogen atom close to
NH2 group (Fig.3, II), similar to A structure (Fig.3, IA). In nitrogen CPMAS spectra of o-
hydroxy Schiff bases the only one NH signal was found in each compound. In all discussed
cases the NH signals are located in relatively narrow spectral range close to -170 ppm. It
means that triazolyl part in studied imines does not correspond to the A structure. The
chemical shifts patterns observed for all Schiff bases in the solid state (Table 3) are very
similar to the B structure, which is postulated for those compounds. It means that we need to
assign them to different ring position comparing with ATz, so we can assume that in all
salicylaldehyde derivatives in the solid state the triazolyl ring has structure similar to
compound B. Consequently, the remaining signals from lowfield region (close to -100 ppm)
should be assigned to the atoms from –N-N- moiety and finally the signals from -150 to -170
ppm range should be assigned to N-4’ position. Unfortunately, in the lowfield range (-100 ÷ -
120 ppm) the imine nitrogen signals can be located making exact signal assignment
impossible.
For imines obtained from ATz and salicyldehydes two major structure problems have
to be solved. The first problem is possible tautomeric equilibrium in triazolyl rings and the
second one is presence or not of the intramolecular hydrogen bonds between OH hydrogen
atoms and formally imine nitrogen atoms. If such bonds are present it is necessary to
determine the position of hydrogen atom in such bridge.
The tautomeric equilibrium in triazolyl ring can be detected by analysis of dynamic
broadening of appropriate proton and carbon signals. In all investigated imines the very
similar carbon chemical shifts in C-3’ and C-5’positions respectively were measured in both
phases. It means that in DMSO-d6 solution and in the solid state the same tautomeric
equilibria exist. Since we excluded location of hydrogen atom in position 4’ in solid state,
consequently in DMSO-d6 we should do the same. It reduces tautomeric equilibrium process
to exchange proton position between N-1’ and N-2’ site. In all imines we observed single
broadened signals (position 3’ and 5’) indicating fast on NMR time scale but close to
coalescence exchange. The different broadening of mentioned signals of each compound
informs that substituents in aromatic rings influence significantly the exchange rates of
observed process but do not change the mechanism of it.
Different picture was found in proton and carbon spectra of ATz in DMSO-d6
solution: in both spectra slow on NMR time scale tautomeric exchange between

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nonequivalent positions takes place. The proportion of tautomers, estimated from both spectra
is close to 8:2. Unfortunately, in proton-nitrogen correlations only NH (-203.3 ppm) and NH2
(-331.9 ppm) signals of major component were measured. The position of NH signal tells us
that the major tautomer in DMSO-d6 solution has the same molecular structure as mentioned
above methyl derivative A (Fig.3).
To solve the second structural problem: presence or absence of intramolecular
hydrogen bonding, it is necessary to analyze the nitrogen chemical shift (Nim) of potentially
imine nitrogen atom and carbon chemical shifts of atoms in positions 2 and 7. Some
information about presence or not of this bonding can be obtained from hydrogen chemical
shift of proton in the hydrogen bridge. Generally, large lowfield shift of this signal confirms
presence of intramolecular hydrogen bond but does not determine exact proton position.
Much better estimation of proton position in the bridge can be done by analysis of chemical
shifts of Nim and C-2 atoms [21, 41-47]. In some cases, where the proton positions in the
bridge (OH or NH form) are extremely different, position of signal of C-7 atom can give some
valuable information. However, the nitrogen chemical shift seems to be the best spectral
parameter for this estimation due to the highest sensitivity on proton position and the lowest
sensitivity on substitution in aldehyde ring.
The proton positon in hydrogen bridge is determined by three factors: substituent in
aromatic ring (all substituents, which increase the acidity of OH groups promote proton
transfer from oxygen to nitrogen site), amine used for condensation (aliphatic amines shift
proton to nitrogen atom comparing with aromatic amines), phase of analyzed sample (in the
solid state proton is generally more shifted to nitrogen atom than in the solution). One more
factor, temperature, has to be mentioned here: in elevated temperature the NH form is
preferred [41-43].
Analyzing the NMR data included in Table 2 one can state, that for all salicylaldehyde
derivatives studied in DMSO-d6 solution very similar structure (tautomeric equilibria where
mainly OH form exists) was found. This conclusion is based on the fact that all interesting
spectral parameters are located in very narrow ranges. Only in 4-OCH3sal derivative some
higher contribution of NH form should be mentioned (upfield shift of imine signal). The
differences in chemical shifts of C2 atoms are less diagnostic since this parameter can be
affected by aromatic ring substitution effects.
Table 3
The nitrogen chemical shifts of imine atoms in the solid state (Table 3) show that in
this phase more NH form contribute in hydrogen bond structure and still the differences in the

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structures between solution and solid state of all investigated salicylaldehyde derivatives are
smaller comparing with simple aliphatic amine derivatives [45]. Again for 4-OCH3sal
compound nitrogen signal is the most upfield shifted in this series indicating the highest
contain of NH form. The spectral data for 5-OCH3sal and 4-Brsal derivatives are close to 4-
OCH3sal substituted compound but proton transfer processes for those compounds are not so
advanced. The exact analysis of 15N solid state data for remaining compounds is more
complicated because the spectral ranges typical for imine atoms and N-2’ atoms are
overlapping. But since they are located in very narrow band, the possible error in structure
estimation connected with missed assignment is small and does not change the general
conclusion. The amount of signals in the solid state spectra also indicates that some
polymorphic phenomena can exist. The signals of possible polymorphs are located very close
one to another so one can conclude that structure of hydrogen bonds of them are very similar
and observed differentiation is connected with crystal structure phenomena.
2. X-ray diffractometry
Molecular structure of compounds 4BrbenzATz and 4BrsalATz (Fig.4) show the
expected bond lengths and angles, which are summarized in Table 4 (Suppl. Mat.).
Fig.4
The 4-bromobenzaldehyde derivative forms hydrogen bonds only with adjacent
molecules (Fig.5), while the 4-bromosalicylaldehyde derivative, as could be predicted,
additionally forms an intramolecular hydrogen bond (Fig.6). Details of the hydrogen bonds
are given in Table 5.
Fig.5
Fig.6
Table 5
The bond lengths and angles in 4BrsalATz are close related to structures of 4-Bromo-
2-[(E)-(1H-1,2,4-triazol-3-ylimino)methyl]phenol [48]. In the structure of the compound
synthesized in the Chohan’s group, hydrogen bonds with bromine (C5–H5···Br1) are not
formed, which in our case determines the packing of molecules in the elementary cell (Fig. 7
(Suppl. Mat.)).
An intramolecular O1—H2···N1 hydrogen bond in 4BrsalATz, a characteristic bond
for Schiff bases, leads to the formation of a S(6) six-membered ring. Additional forming of
this bond causes the molecule to flatten. The dihedral angle between the plane of the 1,2,4-

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triazolyl ring and the plane of the phenyl ring decreases from 19.9° for 4BrbenzATz to 4.2°
for the salicylic derivative. (Fig. 8 (Suppl. Mat.)).
The differences between geometry of 4BrbenzATz and 4BrsalATz molecules are
illustrated in Fig. 9 (Suppl. Mat.), showing the overlay of molecules. The conformations of
these molecules described the torsion angles φ3 = N1–C7–C1–C2 and φ6 = N2–C8–N1–C7,
which are of 9.1(5)° and 10.2(5)° for 4BrbenzATz, 1.8(6)° and 1.1(6)° for 4BrsalATz,
respectively.
3. ATR-FTIR spectroscopy
In analysis of infrared spectra of Schiff bases two major spectral ranges should be
considered: 3500-2700 cm-1 and 1770 – 1550 cm-1.
The first one from 3500 to 2700 cm-1 is usually treated as a very useful region in
purity determination, as well as in hydrogen bond structure study.
In purity determination one can expect to find at least two effects:
- disappearance of the bands attributable to the N-H stretching vibrations of the primary
amino group of amines (pair of two bands at about 3400 and 3300 cm-1 [22]),
- disappearance of the bands attributable to the C-H stretching vibrations of -CHO group of
aldehydes (pair of weak bands, one at 2860-2700 cm-1 and the other at 2760-2700 cm-1 [22]).
To verify presence of hydrogen bond it is necessary to analyze the shape of
appropriate band as well as its position. IR spectra of o-hydroxy aromatic imines based on
heterocyclic amines in this region show broad to very broad bands with maxima in relatively
broad range 3500-2700 cm-1 (3300-2700 [49]; 3500-3200 cm-1 [7]; 3200-2500 [50]). This
phenomenon is treated as a proof of the hydrogen bond existence [49].
ATR-FTIR spectrum of ATz shows two bands characteristic for presence of primary
amino group in the compound (Fig.10, red line, 3402 and 3322 cm-1). These bands are absent
in spectra of ATz-based Schiff bases (Fig.10, blue line). In the range of 3500-2700 cm-1 in the
spectra of o-hydroxy aromatic ATz-based Schiff bases, one can observe weak broad bands
(Fig.10. blue line) assigned to stretching vibrations of O-H bond engaged in intermolecular
(for example HO···H-C) and intramolecular (O-H···N) hydrogen bonding. This conclusion is
in good agreement with those taken from NMR and X-ray studies.
In the spectrum of ATz the band of 3199 cm-1 with a shoulder on the left side was
found. This is very typical for amines with N-H···N hydrogen bonds [50].
Fig.10

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The range from 2860 to 2700 cm-1 seems to not very useful in assessing the absence of
the aldehyde group in the studied Schiff bases. The spectrum of salicylaldehyde (Fig.10,
green line) shows two bands in this region: 2846 and 2751 cm-1, which confirms the presence
of aldehyde group in investigated compound. However, in the same region there are bands
(Fig.10 red line, 2837-2712 cm-1), which one probably can assign to the stretching vibrations
of N-H bonds in the ring of ATz [51-52]. This situation makes it very difficult to
unambiguously state that there is no unreacted aldehyde in the product.
The next range (1700-1450 cm-1) is always considered during analysis of Schiff bases
structures. It is very useful for study of:
i) presence of various intensity bands caused by stretching vibrations of C=N bonds (1645-
1592 cm-1 for Ar-CH=N-ATz [53-57],
ii) presence of various intensity bands of stretching vibrations of C=O bond in o-hydroxy
aromatic Schiff bases occurring in keto form with HB (C=O···H-N) (bands at higher
wavenumbers), and C=N bond in enol form with HB (O-H···N=C) (bands at lower
wavenumbers) [58] (Fig.11),
iii) absence of strong intensity bands of stretching vibrations of C=O bond connected directly
with phenyl in the region 1700-1660 cm-1 for benzaldehydes [22],
iiii) absence of strong intensity bands of stretching vibrations of C=O bond in o-hydroxy
aldehydes occurring in keto form (doublet in the range from 1730 to 1695 cm-1), and in enol
form (one band observed in the region 1640-1570 cm-1) [22] (Fig.11).
Fig. 11
Fig. 11 presents fragment of spectra of aldehyde and its imine derivative. In the 5-
bromosalicylaldehyde spectrum (Fig.11 red line) there are three bands in the region
characteristic for stretching vibrations of C=O in aldehydes existed in two forms: keto form
(1670 and 1651 cm-1) and enol form (1610 cm-1). The last band is probably a result of
combination of two vibrations: ν(C=O) of enol form of aldehyde and ν(C=C) of aromatic
ring.
In the discussed range of ATR-FTIR spectra of ATz-based Schiff base products there
are bands assigned as follows:
1. C=N stretching vibrations (ν(C=N)) of the Schiff bases without intramolecular hydrogen
bonds (1597-1585 cm-1) (Table 6),
2. stretching vibrations of C=N bond in enol-imine form or C=O bond in keto-amine form
of the Schiff bases having intramolecular hydrogen bonds (1639-1602 cm-1) (Table 6).

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It is known that IR spectra of heterocyclic Schiff bases without intramolecular
hydrogen bonds have typical bands near 1600 cm-1 assigned to stretching vibrations of C=N
bonds. Spectra of all considered p-substituted benzaldehyde derivatives have two bands in this
region (weak or medium at 1618-1613 cm-1 and medium or strong at 1597-1585 cm-1). This
may be due to possible polymorphic forms existence or due to possible intermolecular
hydrogen bonds involving imine group and nitrogen atom in the ring (see NMR and X-ray
consideration), or because of the type of substitution in the phenyl ring (according to the
Bellamy [50] di-substituted (in para position) aromatic compounds influence the shifting of
band assigned to v (C=C) towards higher frequencies).
Table 6 presents wavenumbers of bands which one can assign to:
a) stretching vibrations of C=NꢀꢀꢀH-O in enol-imine form (lower wavenumber) and C=OꢀꢀꢀH-
N in keto-amine form (higher wavenumber) bonds in the proton transfer forms of o-hydroxy
aromatic Schiff bases,
b) stretching vibrations of C=N bond engaged (higher wavenumbers) or not engaged (lower
wavenumbers) in the intermolecular hydrogen bond in the spectra of Schiff bases derived
from p-substituted benzaldehydes,
c) comparison of results with the wavenumbers of these bands in the literature (KBr pellet
method).
Table 6
As we know from CPMAS NMR study, o-hydroxy aromatic Schiff bases obtained
from ATz exist in the solid state as a tautomeric equilibrium of two containing intramolecular
hydroden bonds forms: enol-imine and keto-amine, where the first of them is the dominant
one (Table 3). The bands assigned to ν(C=N) are shifted to higher wavenumbers comparing
with compounds without intramolecular H-bonds.
In some cases two separated bands were found: first close to 1600 cm-1 and the second
at 1639-1624 cm-1. According to the rule that for o-hydroxy Schiff bases two bands appear in
this region of IR spectra: the band in higher wavenumbers (1639-1624 cm-1) for imine existed
in the keto-amine form (C=O···H−N), and the band in lower wavenumbers (close to 1600 cm-
1) for imine existed in the enol-imine form (C-O-H···N) (Fig.12.) [58], some estimation of
contents of tautomeric equilibrium was done and compared with CPMAS NMR data (Table
7). This estimation was done by means of intensity of transmittance of proper bands.
Fig. 12.

ACCEPTED MANUSCRIPT
It is well visible relation between tautomeric form contents with nitrogen chemical
shifts of imine atoms. Higher content of keto-amine form is connected with upfield shift of
imine atom, which is in agreement with literature data [43].
Table 7
4. UV-Vis spectroscopy
In the case of heterocyclic molecules, their electronic spectra are usually complex
because of combinations of π→π* and n→π* transitions. The common method of studying
derivatives of heterocyclic molecules is to compare their spectra to the spectra of the parent
heterocyclic systems [22].
The electronic absorption spectra of the ATz-based Schiff bases were recorded in three
organic solvents of different polarities (chloroform, acetonitrile, and dimethyl sulfoxide), and
in the solid state.
In the UV-Vis spectra recorded during measurements in the solution, bands around
200-300 nm are usually of strong intensity with values of molar absorption coefficients going
as high as 104 dm3ꢀmol-1ꢀcm-1. This is indicative of π→π* electronic transition characteristics
of the aromatic system [59].
The two long-wavelength bands, which are correlated with the presence of tautomeric
equilibrium of o-hydroxy aromatic Schiff bases (OH-form bands and NH-form bands) in the
literature are defined as the CT electronic transitions [56, 60] (Fig. 13). Unfortunately, in
some spectra (Table 8, “ ”) of investigated derivatives the OH-form bands are overlapped
with π→π* of C=C and C=N transitions making exact analyses more complicated.
Fig. 13
Table 8 lists the λmax and εmax values of π→π* C=C, and C=N, and CT electronic
transitions of the studied Schiff’s bases.
Table 8
In the case of o-hydroxy aromatic Schiff bases studied in this work, the correlation
between polarity of the solvent used and CT band position is visible very well. The increase
of solvent polarity causes the hipsochromic shift of this band. It is typical for the compounds
which are polar in the ground state [62]. Additionally, for some compounds (4BrsalATz,
4OCH3salATz, 3NO2salATz, and 5NO2salATz), increase in polarity of solvent generates a
new, low intensity band, assigned to NH-form. Since the molar extinction coefficient for pure
NH form of imine derivatives of ATz is unknown, we were not able to perform the
quantitative analysis and could only estimate percentage of tautomeric forms considering

ACCEPTED MANUSCRIPT
molar integral intensities (Table 8). In these compounds the small integral intensity of NH-
form band indicates the equilibrium between two forms OH and NH, which is strongly shifted
to OH form. This conclusion is only partly consistent with NMR data, but we have to
remember that, apart from mentioned above reason, the concentrations of the samples for both
measurements methods are significantly different, which can influence different tautomeric
equilibria positions.
On the base of literature data concerning Schiff base structure investigation in the
solution, one can assume that in UV-Vis-DR spectra the bands above 400 nm should be
assigned to keto-amine form and those below 400 nm to enol-imine tautomer. Unfortunately,
the problem is more complicated, since in the electronic spectra in the solid state the pattern
of overlapping bands was observed. Anyway, some correlation between position of the band
below 400 nm (Fig. 14) and positions of tautomeric equilibria taken from mentioned above
spectral methods were found (Table 9). Generally, for the compounds with the same
substituent in different positions in phenyl ring, the upfield shift of nitrogen signal of formally
imine signal is correlated with CT band shift to higher wavelengths.
Fig.14
For the 3Brsal derivative we could not properly assign the chemical shift of imine
nitrogen atom (see NMR consideration) because of signal overlapping with signals of
nitrogen atoms in triazolyl ring. Now, comparing with UV-Vis-DR data we can assume, that
this signal should be found close to -123 ppm value.
Table 9
Conclusions
Twelve ATz-based Schiff bases were synthesized and analyzed by various analytical
techniques (NMR, X-ray, IR, UV-Vis) in the solution and in the solid state. First of all we
verified the structure of 3-amino-1,2,4-triazole in the solid state. In DMSO-d6 solution this
compound is known as tautomeric mixture of three species. In the solid state only one
structure with proton on nitrogen atom N-2’ was found. The structure of triazolyl ring in
13
15
investigated Schiff bases was defined by analysis of C and N NMR data. Unfortunately,
due to tautomeric processes in DMSO-d6 solutions we could not detect any nitrogen signals
from this part of molecules, but comparison of carbon signals suggest that in both phases the
same structure is present. The crystal structures of two imines confirm that proton in triazolyl
ring is located in position N-1’. Analyzing the combined spectroscopic data we found out that
in all imines derived from various salicylaldehydes the enol-imine form is the dominant one.

ACCEPTED MANUSCRIPT
The position of proton in intramolecular hydrogen bridge only in some extend depends on
substituent in phenyl ring, which is in contract to Schiff bases obtained from aliphatic amines.
In the solid state the tautomeric equilibria of investigated o-hydroxy Schiff bases are shifted
to keto-amine form. This effect was observed in aliphatic Schiff bases but again in ATz-based
imines it is much weaker. Even application of 5-nitrosalicylaldehyde, which is known to shift
strongly tautomeric equilibrium to keto-amine form in the case of ATz derivatives it does not
work. To determine the position of tautomeric equilibria the nitrogen chemical shifts of imine
atoms seems to be the best choice. In some cases, where no signal overlapping takes place, the
IR and UV-Vis data can be valuable source of quantitative estimation.
This work was partly supported by the National Science Centre (No. 2017/01/X/ST4/01132).
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ACCEPTED MANUSCRIPT
Table 2. Results of NMR measurements in DMSO solution.
1
-
2
3
4
-
5
6
7
N-H
14.0
320Hz
O-H
Nim
-
5’
8.33
27Hz
3’
-
OCH3
-
1H
7.92
7.72
7.72
7.92
9.18
-
4BrbenzATz
4OCH3benzATz
4NO2benzATz
salATz
146
300Hz
8.40
13C/15N 134.9 131.4
1H
13C/15N 128.8 131.6
1H
13C/15N 141.4 130.6
1H
132.5
7.07
126.5
-
132.5
7.07
131.4 163.3
7.93 9.11
131.6 163.1
8.23 9.32
130.6 161.7
7.77 9.43
-
-
-
-
-82.1#
-
**
-
13.9
30Hz
-
7.93
3.82
120Hz
143.7
50Hz
8.52
80 Hz
144.7
13Hz
8.44
no
signal
166.3
50Hz
115.0
8.33
163.2
-
115.0
8.33
56.0
14.1
50 Hz
-
8.23
-
-
-
166.1
12Hz
124.6
149.6
7.42
124.6
-
-69.6
-
6.96
ov.
6.96
ov.
14.1
12.4
300Hz 270Hz
-
-
-
-
-
58Hz
146.0
170Hz 200Hz
163.0
13C/15N 119.7 160.8 117.3* 134.8 119.9* 132.8 165.3
-107.6
-
14.20
76Hz
13.88
120Hz
8.57
60Hz
1H
13C/15N 110.7 157.6
1H
13C/15N 127.7 161.3
1H
13C/15N 121.9 159.7
1H
13C/15N 119.6 150.9
1H
13C/15N 113.1 163.5
1H
13C/15N 119.7 155.0
1H
13C/15N 121.9 155.4
1H
13C/15N 120.3 165.7
-
-
-
7.75
137.3
-
6.95
121.1
7.15
123.2
-
7.79
133.7 165.9
7.75 9.39
133.7 163.9
8.04 9.42
133.8 163.2
7.37 9.39
123.9 165.5
7.65 9.27
134.8 165.0
7.37 9.40
114.4 164.5
8.15 9.50
138.9 164.9
8.79 9.51
127.2 162.0
9.40
-
-
3BrsalATz
145.0
10Hz
8.53
80Hz
144.8
24Hz
8.55
90Hz
144.8
17Hz
8.43
163.7
9Hz
120.5
7.19
120.0
6.97
119.7
-
-110.0
-
14.1
40Hz
12.67
110Hz
-
-
-
-
-
4BrsalATz
164.6
24Hz
119.3
7.59
136.8
7.14
116.6
-
-103.6
-
14.16
53Hz
12.4
120Hz
-
-
-
-
-
-
5BrsalATz
164.7
12Hz
110.9
6.90
119.5
6.55
107.9
-
-100.2
14.1
130Hz 220Hz
12.4
-
-
-
-
3.80
56.3
3.79
56.1
3.72
56.0
-
74Hz
145.3
3OCH3salATz
4OCH3salATz
163.8
148.4
6.50
101.4
6.90
118.2
-
-107.4
̴
̴
13.98
50Hz
13.12
110Hz
-
-
-
-120.2
-
90Hz
144.6
90Hz
8.43
100Hz
144.2
30Hz
8.63
11 Hz
145.6
32Hz
8.57
56Hz
145.0
90Hz
7.29
164.9
30Hz
164.8
7.04
122.2
8.12
130.0
8.25
129.3
14.07
54Hz
11.72
140Hz
-
-
-
5OCH3salATz
164.0
40Hz
152.6
7.14
119.4
-
-103.2
-
14.25
19Hz
14.4
v.br
-
-
-
3NO2ATz
163.3
38Hz
139.4
7.14
118.3
-109.6
-
-
14.2
52Hz
13.0
390Hz
-
-
-
-
5NO2salATz
164.5
100Hz
140.5
-97.2
-
11.9
80%
12.7
20%
80%
7.96
20%
149.2
80%
1H
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
ATz
156.4
80%
163.8
20%
13C/15N
141.9
20%
*-assignment can be changed
**-probably very broad signal close to 163 ppm overlapped with C-7 signal
#-value measured at 80
̊C
ov. – overlapped signals
for ATz chemical shift of nitrogen atom of NH is -203.3 ppm

ACCEPTED MANUSCRIPT
Table 3. Nitrogen and carbon chemical shifts of selected salicylaldehyde derivatives in solid
state from CPMAS experiments.
N-1’
C-1
C-2
C-3
C-4
C-5
C-6
C-7
C-3’
C-5’
Nim
N-2’
N-4’
NO2
CH3
(NH)
-117.7$
-122.0$
-123.3$
-135.2
-172.6
-170.2
-168.4
-174.4
-110.5$
-114.3$
-116.0$
-90.0
106.0
137.4
broad
131.1
134.1
-152.6
162.6
118.9
120.8
161.7
157.7
144.4
3BrsalATz
v.broad
-149.6?
116.8
120.2
155.3
159.7
119.6
117.2
124.9*
139.4
112.9
124.9*
111.7
broad
135.7
136.0
163.0
162.5
162.9
161.2
150.6
142.5
-165.4
-151.7
-
-
4BrsalATz
5BrsalATz
-121.7
-170.2
-171.1
-177.6
-112.7
150.9
152.5
162.4
163.1#
147.1
149.7
122.0
123.4
135.4
136.9
162.1
162.7
142.9
146.2
-115.1%
-119.5%
-147.4
-104.7%
-112.4%
117.5
116.9
162.6
156.8
-153.6
-167.5
-
-
-
56.4
54.9
3OCH3salATz
4OCH3salATz
5OCH3salATz
113.0
116.8
109.7
111.5
97.3
167.2#
126.2
164.9
170.1
149.3
148.5
-97.4
broad
-175.1
broad
-96.9
-95.3
-167.7
-166.2
54.3
56.0
117.3*
156.4*
117.3*
151.9
112.7
156.4*
-136.6
-119.6
157.4
158.3
129.3
132.7
119.7
121.3
143.5
141.2
-176.8
-169.2
-162.3
-153.7
-12.8
-8.1
@
134.3
161.8*
161.8*
-117.1?
-100.8?
-
-
3NO2salATz
144.2#
143.5#
117.3
-
163.3#
-
121.3
-
129.3*
-
139.6
-
129.3*
-
163.3
-
165.4#
156.6
145.1
147.9
-108.7*
NH2
-173.2
-206.0
-108.7*
-116.7
-153.6
-176.5
-11.3
5NO2salATz
ATz
-325.7
@-quaternary C-1 signal completely overlapped by C-5 signals
%-four close signals, no arguments to distinguish them
*- overlapped signals
#- assignment can be reversed
$-six close signals, no arguments to assign them properly

ACCEPTED MANUSCRIPT
Table 5. Hydrogen bond distances (Å) and angles (°).
D-H ···A
D-H
H···A
D···A
4BrbenzATz
2.961(4)
Angle
Symmetry codes
N3–H1···N1
C9–H9···N4
0.78(5)
0.92(5)
2.20(5)
2.53(4)
167(5)
127(3)
[x + 1/2, -y + 3/2, z – 1/2]
3.176(5)
4BrsalATz
2.592(2)
O1–H2···N1
C5–H5···Br1
C9–H9···O1
0.99(6)
1.06(5)
0.93
1.74(6)
3.03(5)
2.43
143(5)
131(3)
152.1
3.187(4)
[-x + 2, -y, z – 1/2]
3.283(5)
[-x + 3/2, y + 1, z – 1/2]

ACCEPTED MANUSCRIPT
Table 6. Chosen ATR-FTIR spectral data of ATz-based Schiff bases.
Wavenumber [cm-1]
Compound
Observed (ATR)
Literature data (KBr)
4BrbenzATz
1613 (m), 1585 (m)a
1592 [53]
4OCH3benzATz 1613 (w), 1597 (s)a
1595 [53]; 1601 [56]
4NO2benzATz
salATz
1618 (w), 1595 (m)a
1599 [56]; 1605 [53]; 1638 [34]
1610 (s)
1610 [56]; 1620 [53]; 1636 [52]; 1645 [55]
3BrsalATz
1639 (vw), 1607 (s)a
1624 (w), 1604 (s)a
1611 (s)
-
4BrsalATz
-
5BrsalATz
1632 [54]
3OCH3salATz
4OCH3salATz
5OCH3salATz
3NO2salATz
5NO2salATz
1611 (s)
1630 [52]
1639 (m), 1602 (s)a
1633 (vw), 1617 (s)a
1620 (s)
-
1638 [55]
-
1615 (m)
1635 [54]; 1643 [55]
s – strong intensity; m – medium intensity; w – weak intensity, vw – very weak intensity
a –there are two bands in the range characteristic of bands assigned to ν(C=N).
ATz : band of N-H bending vibrations (primary amino group and N-H bond in the ring) –
observed at 1635 (s); found in the literature [57] at 1639 (s). In the spectra of Schiff bases
strong bands of C=O and C=N stretching vibrations usually overlap bands assigned to
bending vibrations of N-H bond in the ring [58].

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Table 7. Estimated percentage of tautomeric forms in o-hydroxy aromatic Schiff bases based
on ATz.
Compound
Estimated percentage of tautomeric forms
in investigated compounds [%]
δNim [ppm]
(Table 3)
Enol-imine form
Keto-amine form
3BrsalATz
4BrsalATz
90
78
76
63
10
22
24
37
-110.0 ÷ -123.0
-135.2
5OCH3salATz
4OCH3salATz
-136.6
-147.4

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Table 8. Electronic absorption bands of π→π* and CT electronic transitions of ATz-based
Schiff’s bases in various solvents and in the solid state.
λmax [nm] (εmaxꢀ104) [dm3ꢀmol-1ꢀcm-1]
λmax
DR
Electron
Compound
transition
π→π*
π→π*
π→π*
π→π*
CT
CHCl3
294.4 (b)
CH3CN
284.2 (b)
(CH3)2SO
304.3 (0.53)
4BrbenzATz
4OCH3benzATz
4NO2benzATz
358.0
370.0
368.0
335 c
385.5 c
323c
a
312.5 (2.18)
312.0 (b)
316.6 (2.42)
313.9 (b)d
300 ( 3.10)c
347.4 (1.66)d
305 ( 1.70) c
350.8 (0.91)
303.1 (2.14)
324.3 (1.84)d
300 ( 2.80) c
341.6 (0.44)d
310 ( 1.90) c
342.7 (1.19)
300 ( 1.30) c
340.6 (0.97)
305 ( 1.40) c
343.0 (0.81)
301.3 (2.90)
salATz
π→π*
3BrsalATz
CT
π→π*
371.0
323c
306.4 (1.84)
4BrsalATz
340.8 (2.10) 98.6%
448.2 (0.029) 1.4%
310 ( 0.80) c
CT
π→π*
CT
345.9 (1.71)
338.3 (2.47)
305 ( 1.00) c
351.3 (0.82)
399.0
315c
368.0
490
5BrsalATz
a
351.6 (0.87)
π→π*
CT
π→π*
303.9 (2.10)
358.4 (0.48)
300 ( 1.00) c
350 ( 0.20) c
302.7 (2.12)
338.6 (3.11)
305.2 (2.56)
350 ( 0.70) c
307.1 (1.74)
325c
3OCH3salATz
370.5
334.0
4OCH3salATz
a
341.6 (2.95) 99.2%
497.9 (0.024) 0.8%
CT
405 ( 0.039)
385.5
c
π→π*
CT
π→π*
310 ( 1.00) c
381.7 (0.67)
305 ( 1.00) c
310 ( 1.00) c
373.1 (0.71)
310 ( 1.00) c
300 ( 1.30) c
374.0 (0.81)
290 ( 1.30) c
334c
380.5
330c
5OCH3salATz
3NO2salATz
346.8 (1.06) 75.8%
477.2 (0.34) 24.2%
311.3 (1.32)c
CT
π→π*
CT
356.0 (0.95)
283.9 (1.85)
327.6 (0.99)
349.2 (0.96)
283.7 (2.52)
324.9 (1.40)
388.5
336c
350 ( 0.70) c
5NO2salATz
a – insoluble
377.0
432.2 (0.66)
b – concentration of sample is unknown due to its very low solubility
c – broad band; there are probably few bands which are overlapped each other
d – the literature data for 4NO2benzATz [61]: 313.0 nm (CHCl3) and 320.0 nm (DMSO), and
304.0 nm (CH3OH) [56]; for salATz: 348.0 nm (CHCl3), 345.0 nm (DMSO), and 344.0 nm
(CH3OH) (εmax = 1.49) [56]; for 4OCH3benzATz: 316.0 (CH3OH) (εmax = 2.4) [56]

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Table 9. Comparison of results obtained from 15N NMR measurements with UV-Vis data in
the solid state.
Aldehyde part
15N NMR [ppm]
UV-Vis-DR [nm]
3Brsal
close to
-123.0
371.0
5Brsal
-121.0
368.0
4Brsal
-135.2
399.0
3OCH3sal
-115.1
5OCH3sal
4OCH3sal
3NO2sal
-117.1
-119.6
388.5
5NO2sal
-108.7
377.0
-136.6
-147.4
-119.5
370.5
380.5
385.5

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Fig.1. 3-amino-1H-1,2,4-triazole (Amitrole, ATz)

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3600
3400
3200
3000
Wavenumber [cm-1]
2800
2600
2400
Fig.10. ATR-FTIR spectra of ATz (red line), sal (green line), and salATz (blue line) in the
range from 3600 to 2400 cm-1

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Fig. 11. ATR-FTIR spectra of 5-bromosalicylaldehyde (red line) and Schiff base derived from
5Brsal and ATz (blue line) in the range of 1700-1450 cm-1.