Synthesis and characterization of a series of novel metal complexes of N-heterocyclic azo-colorants derived from 4-azo-pyrazol-5-one

Article abstract of DOI:10.1016/j.poly.2016.09.047

A series of novel metal complexes of Cu(II), Co(II), Ni(II), and Zn(II) with three N-heterocyclic derivatives of 4-azo-pyrazol-5-one were isolated and identified by FT IR, 1H NMR, ESR, and UV–Vis spectroscopy. The crystallographic data for a sodium salt a

Full text of DOI:10.1016/j.poly.2016.09.047

Accepted Manuscript
Synthesis and characterization of a series of novel metal complexes of N-het-
erocyclic azo-colorants derived from 4-azo-pyrazol-5-one
Olga V. Kovalchukova, Mikhael A. Ryabov, Pavel V. Dorovatovskii, Yan V.
Zubavichus, Andrey N. Utenyshev, Dmitry N. Kuznetsov, Oleg V. Volyansky,
Violetta K. Voronkova, Victor N. Khrustalev
PII:
S0277-5387(16)30477-6
DOI:
http://dx.doi.org/10.1016/j.poly.2016.09.047
POLY 12235
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
13 July 2016
9 September 2016
27 September 2016
Please cite this article as: O.V. Kovalchukova, M.A. Ryabov, P.V. Dorovatovskii, Y.V. Zubavichus, A.N.
Utenyshev, D.N. Kuznetsov, O.V. Volyansky, V.K. Voronkova, V.N. Khrustalev, Synthesis and characterization
of a series of novel metal complexes of N-heterocyclic azo-colorants derived from 4-azo-pyrazol-5-one,
Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.09.047
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Synthesis and characterization of a series of novel metal complexes of N-
heterocyclic azo-colorants derived from 4-azo-pyrazol-5-one
a*
a
b
Olga V. Kovalchukova , Mikhael A. Ryabov , Pavel V. Dorovatovskii , Yan V.
b
c
d
d
Zubavichus , Andrey N. Utenyshev , Dmitry N. Kuznetsov , Oleg V. Volyansky ,
e
f
Violetta K. Voronkova , Victor N. Khrustalev
a
Department of General Chemistry, Peoples’ Friendship University of Russia, Moscow, Russian
Federation, 117198
b
NRC Kurchatov Institute, Moscow, Russian Federation, 123182
c
Institute of Problems of Chemical Physics of the Russian Academy of Sciences (IPCP RAS),
Chernogolovka, Russian Federation, 142432
d
Department of Organic Chemistry, Moscow State University of Design and Technology,
Moscow, Russian Federation, 117997
e
Department of Chemical Physics, Kazan E. K. Zavoisky Physical-Technical Institute (KPhTI),
Kazan, Russian Federation, 420029
f
Department of Inorganic Chemistry, Peoples’ Friendship University of Russia, Moscow,
Russian Federation, 117198
*
Email: kovalchukova_ov@pfur.ru, Tel: +79161685829; Fax: +74959522644

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Abstract:
A series of novel metal complexes of Сu(II), Co(II), Ni(II), and Zn(II) with
three N-heterocyclic derivatives of 4-azo-pyrazol-5-one were isolated and
1
identified by FT IR, H NMR, ESR, and UV-VIS spectroscopy. The
crystallographic data for a sodium salt as well as Co(II) and Cu(II) complexes were
obtained. Formation constants of the complexes in ethanol aqueous solutions were
calculated. Structures of yet unstudied metal complexes were proposed based on
quantum-chemical modeling at the DFT/B3LYP level.
Keywords: Pyrazolone; Heteroaryl azo dyes; Metal complexes; Synthesis; X-ray
structural analysis; Spectroscopic characteristics; Quantum chemistry.
1
. Introduction:
Azo compounds are usually intensely colored, which promotes their
application as dyes and pigments in a variety of fields including textile and
polymer industries, etc. [1, 2]. Among such compounds, derivatives of 1-phenyl-3-
methyl-4-azopyrazole-5-one and their-based metal complexes play the major role
as colorants [3-5]. Typically, absorption spectra of these dyes show distinct
absorption maxima corresponding to colors ranging from yellow to orange due to
partly delocalized electrons in the heterocyclic rings [6, 7].
Within the last two decades, the emphasis in the synthesis and comparative
studies has been shifted towards heteroaryl substituted azopyrazolone derivatives

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[8-10]. They tend to show a bathochromic shift and deep color strength with a high
extinction coefficient as compared to analogous dyes derived from carbocyclic
aromatic systems [11-15]. Meanwhile, the introduction of an additional heterocycle
as an N-substituent in the pyrazolone ring or a coupling agent can induce some
new properties in the target azo-compounds and their-based metal complexes such
as antitumor and antioxidant activity [16], DNA affinity [17] optical properties for
use in high-density storage media [18] and others.
Recently [19, 20], we reported on the synthesis and crystal structure
determination of new heteroaryl derivatives of azopyrazolone and their metal
complexes. The purpose of this study is thus to investigate further some bis-ꢁ
heteroaryl monoazo dyes derived from pyrazole-5-one bearing two heterocyclic
rings per molecule (Fig. 1), in particular, their complexation behavior. The
structural and spectral characteristics, stability constants and stoichiometry of these
dyes as well as of their-based metal complexes were systematically elucidated by a
set of contemporary experimental techniques including quantum chemical
calculations.
2
2
. Experimental
.1. Materials
2
-Amino-4-nitro-phenol,
4-amino-1,5-dimethyl-2-phenyl-1,2-
dihydropyrazol-3-one, 5-methyl-2-phenyl-2,4-dihydropyrazol-3-one from Acros
Organics were of chemically pure grade. 2-(3,5-Dichloropyridin-2-yl)-5-methyl-

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2
,4-dihydropyrazol-3-one and 2-benzothiazol-2-yl-5-methyl-2,4-dihydropyrazol-3-
one were synthesized by the methods described elsewhere [21,22].
A facile route of synthesis of both free ligands and their metal complexes
was adopted, which is shown in Scheme 1 together with suggested structures.
1-3
2
2
.2. Synthesis of the ligands (HL ) and their metal complexes
1
.2.1. Synthesis of H2L
Diazotation. 1.1 g (7.2 mmol) of 2-amino-4-nitro-phenol was suspended in
10 mL of water, then 2 mL of 27.5 % HCl solution is added. The mixture was
o
stirred for 10-15 min and then 5 ml of water is added. After cooling down to 10 C,
2
.5 mL (7.2 mmol) of KNO solution (C 200 g/L) was quickly added and stirred
2
for 30 min.
Coupling. The azo-component was prepared by the dissolution of 1.76 g
7.2 mmol) of 2-(3,5-dichloropyridin-2-yl)-5-methyl-2,4-dihydropyrazol-3-one in
(
о
1
0 mL of ethanol heated to 80 С. After addition of 0.66 ml (7.4 mmol) of 33%
NaOH solution and 1.37 g (16.7 mmol) of sodium acetate, the solution was cooled
о
down to 5 С and the suspension of the diazo-compound was added within 15-20
min. The medium was maintained alkaline with 10% NaOH solution. The reaction
mixture was stirred for 1.5-2 hours. After the reaction is complete, the solution was
acidified to pH 5-6, and the precipitate formed was filtered off. Orange powder.
Mp 350ºС, Rf 0.63 (ethyl acetate). The spectroscopic characteristics and elemental
analysis are given in Table 1.

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2
2
.2.2. Synthesis of HL
Diazotation. 1.02 g (5 mmol) of 4-amino-1,5-dimethyl-2-phenyl-1,2-
dihydropyrazol-3-one was dissolved in 4.1 mL (12 mmol) of sulphuric acid
C 3 mol/L, cooled down to 5°C and slowly mixed with 1.72 mL (5 mmol) of
(
NaNO solution (C 200 g/L). The solution obtained ws stirred for 15 min.
2
Coupling. 0.87 g (5 mmol) of 5-methyl-2-phenyl-2,4-dihydropyrazol-3-one
was dissolved in 9 mL of water, and 0.2 g (5 mmol) of NaOH (aqueous solution)
was added under reflux till 50-60°C. To the formed brownish-yellow solution, 0.53
g (5 mmole) of Na CO was added. The solution was cooled to 8-10°C, and within
2
3
1
0 min., the solution of the diazonium salt was slowly added. The medium was
maintained alkaline with the 10% NaOH solution. Yellowish-orange powder. Mp.
15ºС, Rf 0.54 (ethanol). The spectroscopic characteristics and elemental analysis
are given in Table 1.
2
3
2
.2.3. Synthesis of HL
Diazotation. 1.02 g (5 mmol) of 4-amino-1,5-dimethyl-2-phenyl-1,2-
dihydropyrazol-3-one was dissolved in 4.1 mL (12 mmol) of sulphuric acid
C 3 mol/L and cooled down to 5°C. 1.72 mL (5 mmol) of NaNO solution (C 200
(
2
g/L) was slowly added within 15 min, and the solution obtained was stirred for 15
min more.
Coupling. 1.15 g (5 mmol) of 2-benzothiazol-2-yl-5-methyl-2,4-
dihydropyrazol-3-one was dissolved in 9 mL of water, and 0.2 g of NaOH
(aqueous solution) was added at stirring and reflux to 50-60°C. To the brownish-

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yellow solution formed, 0.53 g of Na CO was added. The reaction mixture was
2
3
cooled to 8-10°C, and the solution of the diazonium salt was slowly added within
0 min. The alkaline medium was maintained with NaOH solution. Yellow
1
powder. Mp. 192 °С, Rf 0.88 (ethanol). The spectroscopic characteristics and
elemental analysis are given in Table 1.
2
.2.4. Metallization
The resulting ligands (0.5 mmol) were dissolved in 40 mL of ethanol. After
heating up to reflux, the Сu(II), Ni(II), and Zn(II) chlorides (2.5 mmol) dissolved
in 20 mL of water were added dropwise under vigorous stirring. The solutions
were heated at 70°C for 20 min, and then 25 mL of water was added. The mixtures
were heated to boiling to afford suspensions of the metal complex dyes. The solids
precipitated were isolated by centrifugation, washed with water and then vacuum
dried to obtain the metal complexes. Specific details for each compound are given
in Table 1.
2
2
.2.5. Synthesis of [NaL (H O)] ⋅2H O
2
2
2
2
The saturated ethanol solution of HL was mixed with a diluted solution of
2
NaOH (C 2 mol/L) to pH 8. Colorless needles of [NaL (H O)] ⋅2H O were
2
2
2
precipitated from the solution at a room temperature after one week and isolated by
filtration.
1
2
2
.2.6. Growth of single crystals of CuL ·1.5H O and Co(L ) .
2
2

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1
2
Single crystals of CuL ·1.5H O and Co(L ) were obtained from the
2
2
powdered samples by recrystallization from ethanol. 0.01 g of the compounds were
dissolved in 5 ml of ethanol under reflux and then stored at room temperature for 2
1
2
days. Brownish needles of CuL ·1.5H O and reddish plates of Co(L ) were
2
2
collected by filtration and used without further purification.
2
.3. Instrument and methods
Elemental analysis of C, H and N was carried out on a Varian 735–OES
elemental analyzer. The Fourier Transform Infrared (FT-IR) spectra were obtained
-1
in KBr pellets on a Perkin Elmer FT-IR 1650 spectrometer in the 4000-400 cm
region at the Shared Research and Educational Center of Physico-Chemical
1
Studies of New Materials, Substances and Catalytic Systems, PFUR. H NMR
spectra (DMF solutions) were recorded on a JEOL JNM-ECA 600 Spectrometer in
DMF using TMS as an internal reference. The electrospray ionization (ESI) mass
spectra were acquired using a Surveyor LCQ spectrometer.
The UV-Visible spectra were measured using a Varian Cary 50 Scan
spectrophotometer. The EPR measurements of Cu-complexes were performed for
polycrystalline powders and DMF solutions using EMX/plus Brtuker techniques at
X-band, ν = 9.3938 Hz.
1
The X ray analysis on single crystals for CuL ·1.5H O was performed on a
2
Bruker APEX-II CCD automated diffractometer at 293(2) K (λ MoKα, graphite
monochromator, ϕ–ω scanning). The semi-empirical absorption correction based

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on equivalent reflections was applied. The structure was solved by direct methods
2
and refined by full-matrix least-squares technique on F with anisotropic
displacement parameters for non-hydrogen atoms. The hydrogen atoms of the
1
water molecules in CuL ·1.5H O were localized in the difference-Fourier map and
2
included in the refinement with fixed positional (riding model) and isotropic
displacement parameters [U (H) = 1.5U (O)]. The other hydrogen atoms were
iso
eq
placed in calculated positions and refined within riding model with fixed isotropic
displacement parameters [U (H) = 1.5U (C) for the methyl groups and 1.2U (C)
iso
eq
eq
for the other groups].
Unfortunately, the single-crystals of [NaL (H O)] ⋅2H O and Co(L ) were
2
2
2
2
2
2
of very low quality, and it was impossible to collect reflection intensities using
laboratory equipment (even in the case of microfocus Cu Kα tube). Therefore, X-
ray diffraction data for these complexes were collected on the ‘Belok’ beamline of
the Kurchatov Synchrotron Radiation Source (National Research Center
‘
Kurchatov Institute’, Moscow, Russian Federation) using a MAR CCD detector at
λ = 0.96990 Å. A total of 360 images were collected using an oscillation range of
.0° and ϕ scan mode. The data were indexed and integrated using the utility
1
iMOSFLM from the CCP4 suite [24] and then scaled and corrected for absorption
using the Scala program [25]. The main crystallographic data and characteristics
are presented in Table 2. The structures were determined by direct methods and
2
refined by full-matrix least square techniques on F with anisotropic technique for
2
non-hydrogen atoms. All four terminal phenyl substituents in Co(L ) are
2

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disordered over two positions each with equal occupancies. Hydrogen atoms of the
2
water molecules in [NaL (H O)] ⋅2H O were localized in the difference-Fourier
2
2
2
map and included in the refinement with fixed positional (riding model) and
isotropic displacement parameters [U (H) = 1.5U (O)]. Hydrogen atoms of the
iso
eq
2
coordinated water molecules within [NaL (H O)] ⋅2H O were disordered over two
2
2
2
positions with equal occupancies. Other hydrogen atoms in all compounds were
placed in calculated positions and refined within the riding model with fixed
isotropic displacement parameters [U (H) = 1.5U (C) for the methyl groups and
iso
eq
1
.2U (C) for other groups]. All calculations were carried out using SHELXTL
eq
program suite [26].
Quantum chemical modeling of molecular and electronic structures of H2L
1
3
–
HL , their ionic forms, ionic salts, and complexes with metals was performed
within the Density Functional Theory (DFT) using the hybride potential B3LYP
27] and a def2-SV(P) basis set [28] with the full geometry optimization. To
[
analyze the electronic structure, the Natural Bonding Orbitals (NBO) approach was
used [29]. All the calculations were performed with the Firefly 7.1.G software
package [30].
3
3
. Results and discussion
.1. Synthesis and characterization
1
3
The ligands H L – HL were synthesized by the diazo coupling reaction
2
according to a common procedure [6]. Their composition was proved using various

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1
techniques, such as IR, H NMR, UV-Vis, MS. In the IR spectra of all compounds,
-1
intense absorption bands of carbonyl stretchings at 1681 – 1664 cm were
1
observed, and the H NMR spectra reveal signals of NH protons (13.23 – 12.90
ppm) indicating the hydrazo tautomeric form of the organic dyes which is common
for the corresponding class of substances [6, 10, 11, 13-15].
The transition metal(II) complexes were synthesized by a reaction of the
relevant MCl salt with the free ligand in neutral aqueous ethanolic media. The
2
alkaline media initiated the metal hydroxide precipitation and the equimolar ratios
of the reactants provoked co-precipitation of complex compounds together with the
uncoordinated ligand. In order to increase the yields of the metal complexes, the 5-
time excess of the inorganic metal salts was used which was then removed by
washing with distilled water. The elemental analytical data and FT-IR spectra of
the metal(II) complexes agree well with their formulae. Both ML and ML2
complexes were isolated.
3
.2. Description of the crystal structures.
The organic ligands presented in this paper were synthesized by the diazo
coupling reaction. It is well known that among keto-hydrazone and azo-enol
tautomeric forms, the hydrazone form is energetically favorable for pyrazolone
derivatives both in solution and in the solid state [31, 32]. An intramolecular
hydrogen bridge linking the carbonyl groups to the NH-moiety of the hydrazone
unit was found to be a characteristic feature of this class of compounds. The

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formation of a six-membered intramolecular hydrogen-bonded ring is possible in
the keto-hydrazone tautomer as it was shown by several X-ray crystal structure
determinations of arylazo derivatives of pyrazol-5-one [33-35] and presented
below.
Recently [19], we reported on the molecular structure of 4-(2-(1,5-dimethyl-
2
3
-oxo-2phenylpyrazolynyl)hydrazono)-3-methyl-1-phenylpyrazol-5-one (HL ), in
which the organic molecule also adopted the hydrazone tautomeric form.
The azo group is coplanar to the C(1)C(2)C(3)N(1)N(2) ring, which supports
conjugation. The N(3)–N(4) and C(2)–N(3) bond lengths (1.309(2) and 1.329(3)
Å, respectively) correspond to the bond order of one and a half, and the
N(4)N(3)C(2)C(1) torsion angle is -1.0(2)°. The N(4)–C(11) bond between the
hydrazo group and the C(11)C(12)C(13)N(5)N(6) pyrazole ring is much longer
(1.393(3) Å), and the relative rotation of these fragments characterized by the
N(3)N(4)C(11)C(12) torsion angle of -166.7(2)°distorts the ideal planarity of the
central part of the molecule. The C(1)–O(1) and C(12)–O(2) bond lengths
(1.246(2) and 1.239(2) Å, respectively) lie between typical single and double CO
bonds and indicate a strong conjugation within the whole molecule. The hydrogen
atom of the hydrazo group forms the N(4)–Н(4N)ʜʜʜO(1) intramolecular hydrogen
bond (d(HʜʜʜO) 1.96(2) Å; d(NʜʜʜO) 2.744(2) Å; NHO angle 136(2) deg.).
2
3
.2.1. [NaL (H O)] ⋅2H O
2
2
2

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2
2
The sodium salt HL adopts a dimeric structure [NaL (H O)] (Fig. 2, Table
2
2
3
) as evidenced by X-ray crystallography. The N–N bond lengths of the ionized
hydrazo groups are 1.273(7) and 1.259(7) Å for two organic species of the dimer,
respectively. These values do not differ a lot from those determined for the non-
dissociated azo-dye (1.309(2) Å) [36] and lie between typical bond lengths of
single and double N=N bonds described elsewhere [34] (1.40(3) and 1.22(1)
respectively). The C–N bond lengths of the azo groups with two pyrazolone rings
2
(
1.422(10) and 1.356(11) Å) correlate with those determined for HL (1.393(3) and
2
-
1
.329(3) Å). These facts indicate that the tautomeric structure of the ionized (L )
anion in the sodium salt does not differ significantly from its neutral form. The C–
O bond lengths of the two organic anions in the dimer are 1.269(8) Å (C1–O1),
1
.249(7) Å (C12–O2), 1.253(7) Å (C22–O3) and 1.261(7) Å (C33–O4).
The pyrazole rings of both organic species are almost planar
(rms deviations are 0.003/0.009 and 0.005/0.010 Å, respectively). The two organic
anions in the dimer are arranged essentially one above the other in a head-to-tail
fashion (the distances between centroids of the pyrazole and phenyl rings are
3
.508(6) and 3.510(6) Å) and are roughly parallel to each other with the interplane
angle of 19.1°.
Each sodium atom adopts a distorted square-pyramidal configuration. The
two O and one N atoms of the tridentate organic ligand together with the O atom of
the coordinated water molecule form the basal plane of the pyramid, and an O
atom of the other organic ligand is located at its apex. The Na–O distances lie
within a range of 2.293(5)–2.466(5) Å, and the Na–N distances are 2.458(6) and

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2
.478(6) Å). Thus, the Na atoms are in the cis-position with Na…Na separation of
3
.241(4) Å.
2
In the crystal, the dimers [NaL (H O)] form H-bonded ribbons along the
2
2
[001] direction by the intermolecular O5−H5A…O1* and O6−H6B…O3**
hydrogen bonds (Table 4). The solvate water molecules link the ribbons into a
three-dimensional framework by the intermolecular O(w)−H…N and
O(w)−H…O(w) hydrogen bonds.
2
3
.2.3. Co(L )
2
2
In the centrosymmetric Co(L ) molecules, the Co atom is in the octahedral
coordination and is surrounded by four O atoms of the pyrazolone rings and two N
atoms of the deprotonated hydrazo group (the distances Co–O1 2.155(6); Co–O2
2
-
2
.010(5); Co–N3 2.105(6) Å) (Fig. 3, Table 5). The organic anions (L ) lie in the
perpendicular planes. They are not strictly planar, the pyrazole rings are tilted by
10.8(4)° relative to each other.
The crystallographic characteristics of the deprotonated hydrazo group
2
2
correlate well with those of HL , [NaL (H O)] ⋅2H O described above as well as of
2
2
2
2
previously reported Cu(L )Cl [20] (N3–N4 1.279(3), N3–C2 1.423(3), N4–C13
1
1
.364(3) Å), but the C–O distances are significantly elongated (1.300(8) and
.335(10) Å for C1–O1 and C12–O2, respectively), which indicates their single
rather than double character.
1
3
.2.4. CuL ·H O
2

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1
The organic ligand in CuL ·H O exists in the form of a dianion and is
2
coordinated to the metal atom via a tridentate chelating mode through the O atom
1
of the deprotonated nitro phenolate fragment of H L , carbonyl O atom of the
2
pyrazolone cycle and N atom of the deprotonated hydrazo group (Fig. 4, Table 6).
The additional coordination bond is formed by the N5A pyridine of the ligand to a
2
2-
neighboring complex. As a result, one (L ) anion is coordinated to two cupric
cations, i.e., acts as a bridging ligand and thus gives rise to a coordination polymer.
2
+
The coordination polyhedron of the Cu cation is a strongly distorted 4+2
octahedron. The equatorial plane includes atoms of the tridentate chelating ligand
(
O1, O2, N2) and the N5A atom of the pyridine ring from neighboring organic
ligand. The bond lengths in the equatorial plane are close to each other (Cu1–O1
.948(2), Cu1–O2 1.951(2), Cu1–N4 1.978(3), Cu1–N5A 2.033(3) Å), the bond
1
angles lie in a range 84.04(10)–95.90(10)° which corresponds to an nearly
undistorted square. A coordinated water molecule as well as the nitrogen N1A of
the pyrazolone fragment from the neighboring molecule occupy the axial positions.
The Cu1–O5 bond of 2.260(3) Å is almost perpendicular to the equatorial plane
(the angles between the axial and equatorial bonds lie in a range 92.32(12)–
9
6.01(12)°), whereas the Cu1-N1A bond is significantly inclined (73.15(13)-
6.31(12)°).
9
In the organic ligands, the pyrazolone and pyridine fragments are twisted by
5.6°. The angle between the planes of pyrazolone and nitrophenol rings is equal
4
to 9.3°.

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The С–О bond lengths of the phenol and pyrazolone fragments of the
organic anion are significantly equalized despite their original difference (1.310(4)
and 1.261(3) Å for C5–O2 and C1–O1 respectively). The N1–C2 (1.375(4) Å),
N2–C10 (1.394(4) Å), and N3–N4 (1.281(3) Å) distances together with the C–O
bond lengths also indicate the significant delocalization of the electron density over
the chelate rings.
1
In the lattice, the polymer chains of [Cu(L )H O] are joined together by the
2
n
hydrogen bonds with bridging water molecules to form a 3D-framework (Table 7).
3
.2.5. Structure overview
The coordination modes of pyrazolone-5 derivatives is of interest due to
their specific properties and prospective applications mentioned in the
introduction. Caesar & co-authors reviewed them in 2007 [5]. Unfortunately, high
dispersion of azo-dyes seriously limits the availability of their crystallographic
characteristics. The majority of them deals with coordination compounds of
bidentate ligands as well as there are only a few examples of structures of
tridentate coordination of azopyrazolones [20, 35, 36].
As it is reported here, the bond lengths within the chelating fragments of the
coordinated azopyrazolones are significantly equalized, which indicate a strong
delocalization of the electron density over the organic anion. Noteworthy, the C–O
distances in the deprotonated phenol and pyrazolone fragments are fairly similar
(1.268(4) – 1.353(4) Å) and are intermediate between typical single C–O and
double C=O bonds (1.33(2) and 1.21(1) Å, respectively [34]). In general, the

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complexation leads to an elongation of the C–O bonds in the organic anion and
shortening of the N–N distances of the deprotonated hydrazo group. The deviation
is however not so significant so we cannot say that the hydrazo tautomeric form of
an azo dye is transformed into its azo tautomer upon coordination as it was claimed
by several authors based on spectroscopic data [18, 37-42].
3
.3. Spectroscopic characteristics
Some spectroscopic characteristics of H L – HL and their-based metal
1
3
2
complexes are summarized in Table 1 and described below.
3
.3.1. IR spectroscopy.
The IR spectra of uncoordinated organic ligands H L – HL in the region of
1
3
2
the carbonyl stretching vibration are characterized by intense unresolved bands at
–
1
1
2
3
1
681, 1669, and 1664 cm for H L , HL , and HL , respectively (Table 1), which
2
can be assigned to the carbonyl groups of two pyrazolone rings involved into
strong intermolecular H-bonds. Upon complexation, the ν(C=O) absorption bands
shift to lower frequencies and this can be explained by their coordination to metal
-
1
ions. The ∆ν values (55 – 90 cm ) imply significant covalency of the M–O bonds.
1
3
.3.2. H NMR spectroscopy.
1
1
3
In the Н NMR spectra of H L – HL , multiple intense signals assigned to
2
the methyl groups and benzene rings lie in the ranges 2.0 – 2.6 and 6.5 – 8.5 ppm,
respectively. Weak singlets at 13.23 – 12.90 ppm are assigned to the NH groups of

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the hydrazone fragments of the molecules, and the signal at 12.28 ppm in the
1
spectrum of H L belongs to the proton of the OH group of the diazo-component of
2
the molecule.
Upon complexation, signals of the protons of the NH and OH groups of
1
3
H L – HL disappear. That indicates the ionization of the ligands and correlates
2
with the chemical analysis of the complexes and results of their structure
determination. The change of the position and character of splitting of the signals
of the methyl and phenyl protons can be explained by the change of geometry of
the organic ligands due to their flattening and change in the twisting angles of
benzene and pyridine rings.
3
.3.3. EPR spectroscopy.
The EPR spectroscopic investigations of Cu(II) complexes are widely used
for the determination of the types of coordination polyhedra and degree of their
2
distortion [44-47]. EPR spectrum of polycrystalline sample of CuL Cl is typical for
Cu ion and can be interpreted using spin Hamiltonian containing only Zeeman
interaction for S=1/2 with g||= 2.188 and g = 2.048, which indicate a square-
⊥
planar coordination of the Cu(II) cations. This type of coordination polyhedron
was previously detected for this complex and reported in [20]. The values g||= 2.30
1
and g =2.049 obtained from the EPR spectrum of CuL ·H O is related to a
⊥
2
slightly elongated octahedral structure of the central ion. The EPR spectrum of
3
CuL Cl recorded at a room temperature for polycrystalline sample contains one

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narrow signal described by g= 2.082, which splits to form a superfine structure of
g|| in frozen dimethyl sulfoxide solutions (T 4.9 and 190 K). The g|| value (2.38)
may be related to strongly distorted polyhedra of penta-coordinated Cu(II).
Unfortunately, the assignment of ESR spectra to specific coordination types
was unsuccessful for coordination compounds with other metal cations.
3
.3.4. UV-Vis spectroscopy.
The UV-Vis spectrum of ethanol solutions of H L corresponds to those of
1
2
other azopyrazolones aryl derivatives [47, 48]. It is characterized by an intense
absorption band at 427 nm, which shifts bathochromically both upon ionization
and complexation (Fig. 5).
2
3
On the other hand, the ionization of HL and HL is accompanied by a
hypsochromic shift of the long-wavelength absorption bands (Fig. 6a), and the
complexation with metal cations leads to an increase in the intensity of the band
with almost no change in its position (Fig. 6b).
Based on the results of spectrophotometric titrations, the composition of
complexes in solutions was determined and their formation constants were
calculated (Table 8).
3
.4. Theoretical modeling
Significant difficulties in the crystal growth processes of azo-dyes and their-
based metal complexes make theoretical modeling rather attractive for predicting
their molecular and electronic structures. For this purpose, we performed quantum

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2
chemical modeling of some metal complexes based on HL . The reliability of the
method was checked against available experimental data.
Together with the structural analysis presented in this paper (Fig. 3, 4) and
data reported earlier [20], the optimization of the structures of the complexes
2
-
indicated the tridentate coordination of the (L ) anion. The coordination involves
the О1, О2, and N2 atoms of the ligand, which gives rise to six- and five-membered
metallocycles (Scheme 2).
According to the DFT calculations of Cu(II) and Zn(II) complexes with the
2
M(L )X stoichiometry (X = Cl, OH), the coordination polyhedra can be described
as a square and distorted tetrahedron for Cu(II) and Zn(II), respectively (Fig. 7).
The distortion of the Zn(II) tetrahedral coordination is explained by the tendency to
keep the planarity of the ligand.
For the Co(II) and Ni(II) complexes of the same composition, the distorted
tetrahedral coordinations of high-spin complexes with spin multiplicities of Co(II)
and Ni(II) 4 and 3, respectively are energetically more stable than the square
coordinations of low-spin complexes with spin multiplicities of 2 and 1 for Co(II)
and Ni(II), respectively. The energy gains were calculated to be 28.89 and 23.45
kJ/mol for Co(II) and Ni(II), respectively.
2
In all the complexes with the M(L ) stoichiometry, the distorted octahedral
2
coordination of the metal cations were predicted by calculation. An essential
2
tetragonal distortion is expected only for the Cu(L ) complex. The high-spin
2
Co(II) and Ni(II) complexes were found to be more stable than the corresponding
low-spin complexes by 32.66 and 118.07 kJ/mol, respectively.

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A straightforward classification of the coordination polyhedra of the
complexes can be performed based on the valence angles (Table 9).
Experimentally determined and calculated bond lengths in the metal
2
complexes of HL are compared in Table 10. The good correspondence between
them justifies the reliability of the theoretical modeling.
The NBO analysis of atomic charges was performed to describe the
electronic structures of the complexes (Table 11).
It is clearly seen that the calculated atomic charges of the metal cations
differ significantly from the traditional +2 charge, which characterizes the degree
of covalency of the metal-to-ligand interaction. This fact indicates the transfer of
2
-
the electron density from the (L ) anion to the metal cations upon complex
formation, i.e., the metal-to-ligand interaction is of a strong covalent character,
which correlates to a large value of ∆ν(C=O) in the IR spectra of the metal
2
-1
complexes as compared to that of HL free ligand (55 – 90 cm ).
The classical theory of coordination bonds assumes that the electron density
is transferred from the electron donating atoms and groups of the ligand. In the
2
case of HL , these are О1, О2 and N2 atoms of the organic anion. However, the
NBO calculations indicate that the electron density on these atoms actually
increases, which provokes a decrease in their formal charges upon complexation of
2
-
(
L ) . This fact may be explained by the electron lability of a π-conjugated system
of the ligand. Thus, the electron donating ability of the ligand is redistributed along
the anion as a whole and cannot be assigned to any specific atom. The electron

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donating atoms just play the role of electron conductors defining donating
properties of the π-conjugated system of the ligand.
4
. Conclusions
We have described the synthesis, structure determination, and spectroscopic
1
characteristics (FT IR, UV-Vis, H NMR, ESR) of a series of novel metal
complexes of Сu(II), Co(II), Ni(II), and Zn(II) with three N-heterocyclic
derivatives of 4-azo-pyrazol-5-one. Both MLX (X = H O, OH, Cl) and ML
2
2
complexes were isolated. In all cases, the tridentate chelating coordination of the
organic ligands was observed. The existence of extra coordination sites (N atoms
of the pyridine and benzothiasole substituents) lead to the formation of polymeric
structures. It was indicated that the strong hypsochromic shift of ν(C=O) in the IR
spectra of the ligands upon complexation cannot be related to the azo-hydrazone
tautomeric transformation of the organic molecules but are due to the formation of
metal-to-ligand coordinate bonds of a strong covalent character. The formation of
high-spin complexes is preferred for Co(II) and Ni(II).
Appendix A. Supplementary data
CCDC 1473696; 1473697 and 1473698 contain the supplementary
1
2
2
crystallographic data for CuL ·1.5H O; [NaL (H O)] ⋅2H O and Co(L ) ,
2
2
2
2
2
respectively. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge

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Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgment
This research was supported by the Ministry of Education and Science of the
Russian Federation (project 4.143.2014–K).

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