Synthesis, spectroscopic characterization, first order nonlinear optical properties and DFT calculations of novel Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes with 1,3-diphenyl-4-phenylazo-5-pyrazolone ligand

Article abstract of DOI:10.1016/j.molstruc.2017.10.002

Novel Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) metal ions with 1,3-diphenyl-4-phenylazo-5-pyrazolone (L) have been prepared and characterized using different analytical and spectroscopic techniques. 1:1 Complexes of Mn(II), Co(II) and Zn(II) are distorted octahedral whereas Ni(II) complex is square planar and Cu(II) is distorted trigonal bipyramid. 1:2 Complexes of Mn(II), Co(II), Cu(II) and Zn(II) are distorted trigonal bipyramid whereas Ni(II) complex is distorted tetrahedral. All complexes behave as non-ionic in dimethyl formamide (DMF). The electronic structure and nonlinear optical parameters (NLO) of the complexes were investigated theoretically at the B3LYP/GEN level of theory. Molecular stability and bond strengths have been investigated by applying natural bond orbital (NBO) analysis. The geometries of the studied complexes are non-planner. DFT calculations have been also carried out to calculate the global properties; hardness (η), global softness (S) and electronegativity (χ). The calculated small energy gap between HOMO and LUMO energies shows that the charge transfer occurs within the complexes. The total static dipole moment (μ_tot), the mean polarizability (<α>), the anisotropy of the polarizability (Δα) and the mean first-order hyperpolarizability (<β>) were calculated and compared with urea as a reference material. The complexes show implying optical properties.

Full text of DOI:10.1016/j.molstruc.2017.10.002

Accepted Manuscript
Synthesis, spectroscopic characterization, first order nonlinear optical properties
and DFT calculations of novel Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes
with 1,3-diphenyl-4-phenylazo-5-pyrazolone ligand
Samir A. Abdel-Latif, Adel A. Mohamed
PII:
S0022-2860(17)31333-9
10.1016/j.molstruc.2017.10.002
MOLSTR 24373
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
03 August 2017
29 September 2017
01 October 2017
Please cite this article as: Samir A. Abdel-Latif, Adel A. Mohamed, Synthesis, spectroscopic
characterization, first order nonlinear optical properties and DFT calculations of novel Mn(II), Co(II),
Ni(II), Cu(II) and Zn(II) complexes with 1,3-diphenyl-4-phenylazo-5-pyrazolone ligand, Journal of
Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.10.002
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ACCEPTED MANUSCRIPT
Highlights
 New azopyrazolone complexes were prepared and characterized.
 Spectroscopic, magnetic moments and molar conductance studies were carried
out.
 Optimized geometry were computed by DFT theory and compared with
experimental data.
 EHOMO and ELUMO and the energy gap were calculated.

Nonlinear optical parameters are computed using B3LYP/GEN level of
theory.

LUMO plot
LUMO plot
(First excited state)
(First excited state)
ELUMO = -0.0881 au
Egap 3.2803 eV
EHOMO = -0.2103 au
ELUMO = -0.0835 au
Egap 3.2803 eV
EHOMO = -0.2041 au
=
=
HOMO plot
(ground state)
HOMO plot
(ground state)
HOMO
LUMO
HOMO
LUMO
Mn-L 1:1 complex
Mn-L 1:2 complex

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Synthesis, spectroscopic characterization, first order nonlinear
optical properties and DFT calculations of novel Mn(II), Co(II),
Ni(II), Cu(II) and Zn(II) complexes with 1,3-diphenyl-4-phenylazo-5-
pyrazolone ligand
Samir A. Abdel-Latif*, Adel A. Mohamed
Chemistry Department, Faculty of Science, Helwan University, Cairo, Egypt
________________________
E-mail:saalatif@yahoo.co.uk (S. A. Abdel-Latif)
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Synthesis, spectroscopic characterization, first order nonlinear
optical properties and DFT calculations of novel Mn(II), Co(II),
Ni(II), Cu(II) and Zn(II) complexes with 1,3-diphenyl-4-phenylazo-5-
pyrazolone ligand
Samir A. Abdel-Latif*, Adel A. Mohamed
Chemistry Department, Faculty of Science, Helwan University, Cairo, Egypt
ABSTRACT
Novel Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) metal ions with 1,3-diphenyl-4-
phenylazo-5-pyrazolone (L) have been prepared and characterized using different
analytical and spectroscopic techniques. 1:1 Complexes of Mn(II), Co(II) and Zn(II)
are distorted octahedral whereas Ni(II) complex is square planar and Cu(II) is
distorted trigonal bipyramid. 1:2 Complexes of Mn(II), Co(II), Cu(II) and Zn(II) are
distorted trigonal bipyramid whereas Ni(II) complex is distorted tetrahedral. All
complexes behave as non-ionic in dimethyl formamide (DMF). The electronic
structure and nonlinear optical parameters (NLO) of the complexes were investigated
theoretically at the B3LYP/GEN level of theory. Molecular stability and bond
strengths have been investigated by applying natural bond orbital (NBO) analysis.
The geometries of the studied complexes are non-planner. DFT calculations have
been also carried out to calculate the global properties; hardness (η), global softness
(S) and electronegativity (). The calculated small energy gap between HOMO and
LUMO energies shows that the charge transfer occurs within the complexes. The total
static dipole moment (μtot), the mean polarizability (<α>), the anisotropy of the
polarizability (Δα) and the mean first-order hyperpolarizability (<β>) were calculated
and compared with urea as a reference material. The complexes show implying
optical properties.
Keywords:
Pyrazolone, synthesis, complexes, spectroscopic studies, DFT calculations, nonlinear
optics
________________
* Corresponding Author E-mail: saalatif@yahoo.co.uk (S.A. Abdel-Latif)
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1. Introduction
Azole is the suffix used for five-membered ring containing two or more hetero-
atoms, at least one of which is nitrogen e.g. 3-methyl-1-phenylpyrazolone. This
compound on methylation gives antipyrine (phenazone), which is exploited in
medicine as a antipyretic compound [1,2]. The applicability of 5-pyrazolones as
potential ligands for a large number of metal ions have gathered great attention in
coordination compounds [3-6] due to their analytical and biological applications [7,8]
of the formed metal complexes. 4-arylazo derivatives of 5-pyrazolones as well as their
metal complexes were widely used in dye industry and as analytical reagents for
micro determination of metals [9-13]. Electronic and IR spectra of some
arylazopyrazolones were obtained [14,15] in organic solvents of differing polarities
and buffer solutions and correlated to its molecular structure[16]. The studied
absorption and fluorescence spectra of some arylazopyrazolone dyes indicated that the
emission was appointed to their hydrazone form [17]. Nuclear magnetic resonance
was utilized to determine the tautomeric forms of arylazo -3-, -4- and -5-pyrazolones
and several related azo heterocycles in chloroform [18]. Mass spectra of the
phenylazopyrazolones was found to occur primarily through rupture of the phenyl N-
bond and less readily via rupture of the pyrazole-oxocyclic N bond [19] Kinetic
studies of the thermal decomposition of metal chelates of substituted
hydrazopyrazolone using TG, DTG and DTA were performed [20]. Structural studies
of some transition metal complexes with some substituted-arylazo-5-pyrazolone
compounds have been synthesized and characterized [21,22]. Spectroscopic, TG,
DTG, DTA and molar conductivity studies have been carried out on chelates of
Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) with 3-methyl and 3-phenyl-4-nitroso-5-
pyrazolones [23]. Nonlinear optical (NLO) materials have variety of optical
phenomena such as the generation of new light frequencies or the alteration of the
materials optical properties. Nonlinear optical materials play a main role in nonlinear
optics and with great effect on information technology and many industrial
applications. The widespread range of applicability of single crystals is apparent in
several fields such as solid state lasers, semiconductors, infrared detectors, non-linear
optics, polarizers, piezoelectric, acousto-optics, photosensitive constituents and
crystalline thin films for computer productions and microelectronics [24].
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In this paper, the metal complexes were synthesized and their structures were
elucidated by different analytical and spectroscopic techniques to get more
information on the formed bonds and to elucidate their molecular structures. The
geometrical parameters (bond lengths, bond angles and dihedral angles), NBO
analysis (natural charges, natural population and natural configuration) and
electrostatic potential of these complexes were computed using B3LYP/GEN level of
theory. The electronic dipole moment (µ) and first order hyperpolarizability (β) values
have been analyzed at the same level to study the NLO properties. Global reactivity
descriptors; electronegativity (), hardness (η), global softness (S) were also
estimated and analyzed.
2. Experimental
All chemicals used were of the highest degree of purity available from BDH
chemicals and included manganese chloride (MnCl2.4H2O), cobalt chloride
(CoCl2.6H2O), nickel chloride (NiCl2.6H2O), copper chloride (CuCl2.2H2O), zinc
chloride (ZnCl2), aniline, sodium hydroxide, ammonium hydroxide (30% NH3),
ammonium chloride, disodium salt of ethylenediamine tetraacetic acid dihydrate
(EDTA), sodium chloride, silver nitrate (AgNO3), sulphuric acid (H2SO4), hydrogen
peroxide (H2O2, 30%) and hydrochloric acid (37% HCl). The solvents used, ethanol,
dimethylsulphoxide (DMSO) and dimethyl formamide (DMF), were purified by
distillation [25]. The water used was doubly distilled.
2.1. Synthesis of 1,3-diphenyl-5-pyrazolone
A mixture of ethylbenzoyl acetate (9.6g; 0.05mol) and phenyl hydrazine (5.4g;
0.05mol) was heated at 100ºC in water bath for one hour. The resulting oil was cooled
and stirred with ether (50 ml) until solidification occurred. The crude product was
then filtered off. The final product was crystallized from 50% aqueous ethanol and
collected as white powder [26].
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2.2. Synthesis of 1,3-diphenyl-4-phenylazo-5-pyrazolone (L)
A well-stirred solution of aniline 0.01mole in 40 mL ethanol and 20 mL of 2 M
HCl was cooled in an ice salt bath and diazotized with aqueous sodium nitrite solution
(20 mL, 0.01 mol). The cooled (0-5 °C) diazonium solution was added slowly to a
well-stirred solution of 0.01 mole 1,3-diphenyl-5-pyrazolone in 100 mL ethanol
containing sodium hydroxide (10 g). The reaction mixture was stirred for one hour at
room temperature and then acidified with diluted HCl (100 mL, 2.5 M) to neutralize
the reaction mixture and precipitate the ligand [25]. The product was recrystallized
from ethanol and subjected to elemental analyses as shown in Table 1; the structural
formula is presented in Scheme 1.
2.3. Synthesis of solid complexes
The solid complexes were synthesized by mixing 0.001 mole of metal ion
dissolved in hot ethanol with the required amount of the ligand to form 1:1 or 1:2
(M:L) complexes. The pH of the solution was then maintained at a value of 6.5-7.5 by
addition of dilute (10% v/v) 30% ammonia solution [27]. The reaction mixture was
heated on a steam bath with continuous stirring for 4 hours and evaporated until
dryness. The produced complexes were then dissolved in ethanol to remove unreacted
species. It was then filtered off by suction and rewashed with ethanol until a colorless
filtrate was obtained, filtered and then finally kept in a vacuum desiccator. The
prepared complexes were subjected to elemental analyses of their C, H, N and the
results are shown in Table 1. The metal ions of the prepared solid complexes were
determined by titration with EDTA using recommended indicators and also the metal
content of the complexes were determined [28].
2.4. Physical measurements
The FTIR spectra were performed on Shimadzu FTIR-8201 PC spectrophotometer
1
applying the KBr disc technique in the range 400-4000 cm-1. H NMR spectra were
acquired by using a Varian Gemini 200 MHz NMR spectrometer using deutrated
DMSO (d6-DMSO) as solvent. The spectra were extended from 0-14 ppm using TMS
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as internal standard. Mass spectra were done on GC-Mass 2b 1000 Ex mass
spectrophotometer at 70 eV and MA energy using a direct insertion probe at
temperature 220 ºC. Thermal studies were determined using Shimadzu, Type TG-50
and DTA-50 derivatograph thermal analyzer (Japan). The TG curves were obtained
by the weight loss percent from ambient temperature up to 800 ºC with a heating rate
of 10 ºC per minute. Molar conductance of the solid complexes in DMF was
measured using a conductivity meter type (Philips, PW 9526 digital conductivity
meter). Magnetic susceptibility measurements were obtained at room temperature
(25) using Faraday’s method with a model MKI, MSBI/24093/6232 Sherwood
Scientific, Magnetic Susceptibility Balance (Cambridge, U.K.). The electron spin
resonance (ESR) spectra of copper complexes were recorded by means of EMS ESR
spectrometer (Bruker) 1998 Y.
3. Computational details
Due to the absence of a crystal structure and to get the molecular conformation of
the ligand and its complexes, energy minimization studies were carried out using
Gaussian 09W software package [29]. The ground state geometrical structures were
optimized using the DFT [30] method with the B3LYP exchange correlation
functional [31] approach. The 6-311G(d,p) and LANL2DZ were used as the basis set
for C, N, O, H atoms and metal ions [32-34], respectively.
The geometry of the studied systems was fully optimized in gas phase without
any symmetry constraints. The vibrational frequency calculations were performed to
ensure that the obtained optimized geometries represent local minima. The spin
density difference map calculations were also performed to explain their optical
properties. The Gauss View 5 software [35] was used to prepare figures of MOs.
Natural bond orbital (NBO) calculations were performed with the NBO code included
in Gaussian 09 to understand different second order interaction between the filled
orbital of one subsystem and empty orbital of another subsystem which is measure of
the molecular delocalization or hyperconjugation. The total static dipole moment (µ),
the mean polarizability <α>, the anisotropy of the polarizability ∆α and the mean
first-order hyperpolarizability <β> using the x, y, z components were determined by
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using the B3LYP method of theory [36-38] in order to estimate the nonlinear optical
properties.
4. Results and discussion
The solid complexes of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) with the
investigated ligand 1,3-diphenyl-4-phenylazo-5-pyrazolone (L) were prepared as
described under the experimental section. The resulting complexes were subjected to
elemental analyses for their C, H, N, Cl and metal content [28], infrared spectral
studies (FTIR), nuclear magnetic resonance (1H NMR), magnetic susceptibility,
thermogravimetric analyses (TG and DTA), electron spin resonance (ESR) and molar
conductance. The results of elemental analyses listed in Table 1 are in good
agreement with those postulated by the proposed formula where 1:1 and 1:2 (M:L)
solid complexes are formed.
4.1. Spectroscopic results
4.1.1. Infrared spectra of the free ligand and its solid complexes
The infrared absorption spectrum of the free ligand (L) presented in Table 2 and
Figure 1 shows the presence of a broad band at 3429 cm-1 which corresponds to the
stretching vibration of the OH group results from the azo-hydrazo tautomerism (CH-
N=N to C=N-NH) followed by hydrogen bonding with C=O group of the pyrazolone
ring. The appearance as well as the broadness and the low frequency of the band
denote the presence of intramolecular hydrogen bonding [39]. The NH band appears
as a medium broad one at 3294 cm-1. The C=C band appeared at 1550 cm-1 while the
N=N band is observed at 1496 cm-1. The band observed at 1654 cm-1 is assigned to
C=O stretching for the carbonyl group [40,41]. The infrared spectra of the solid
complexes display interesting changes which may give a sufficient idea about the
structure of these complexes as shown in Table 2 and Figure 1. The IR spectra of the
metal complexes showed that the N=N are shifted to lower or higher wavenumber
compared to that of the free ligand suggesting a center of chelation. The C=O band
disappeared indicating that it is a center of chelation as it loses its double bond
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character (-C-O-M-) which is confirmed by the appearance of a new band
corresponding to C-O in the range 1211-1265 cm-1 for all the complexes. For 1:1 and
1:2 complexes, the OH stretching frequency appears in the spectra of the complexes
as a broad band in the range 3355-3494 cm-1 which is ascribed to coordinated water
molecules and chelated OH- ion neutralizing the charge on the metal ion. The spectra
of the metal complexes exhibits bands in the ranges 510-590 and 416-489 cm-1 which
may assign to (M←N) and (M←O) stretching frequencies [42], respectively. These
bands are possibly due to the formation of coordination bonds between the donor
atoms (O and N) and the central metal ion.
4.1.2. 1H NMR spectra of the free ligand and its Zn complexes
1H NMR technique is used for the purpose of establishing the possible azo-
hydrazo [43] or keto-enol tautomerism. The signals observed at 8.19-7.23 ppm are
assigned to the protons of the aromatic ring [41] while the signal observed at 13.7
ppm is assigned to the NH proton as shown in Table 3 and Figure 2. The appearance
of this signal indicates the involvement of this ligand in azo-hydrazo tautomerism
followed by hydrogen bonding as shown in Scheme 2.
For Zn-L (1:1) complex, the signal observed at 16.6 ppm is assigned to the proton
of the coordinated OH-. The signals observed within the range 8.19-6.71 ppm are
assigned to the protons of the aromatic ring. For Zn-L (1:2) complex, the signals
observed within the range 8.18-7.05 ppm are assigned to the protons of the aromatic
ring. The upfield shift of these signals may be due to complexation. For 1:1 and 1:2
complexes, the signal observed at 3.32 and 3.37 ppm is assigned to protons of
coordinated H2O molecules.
4.1.3. Mass spectrum of the investigated ligand
The mass spectrum of the investigated ligand as indicated in Figure 3 was in good
agreement with the proposed structure and shows a molecular ion peak at m/z = 339
(Calc. M. wt. = 340) with a relative intensity near to 75% which is equal to its
molecular mass. The other peaks appeared in the mass spectrum (abundance range 1–
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100%) are assigned to the fragmentation of the ligand molecule obtained from the
rupture of different bonds in the molecule. In addition, fragment peaks observed at
m/z 311, 263, 234,194 and 105 are attributed to cleavage of C20H15N4, C15H11N4O,
C14H10N4, C13H10N2 and C6H5N2, respectively.
4.1.4. Thermogravimetric analysis (TG and DTA)
To estimate the stability of the complexes, thermogravimetric measurements for
Mn (1:1 and 1:2) complexes were carried out at the atmosphere of nitrogen in the

temperature range 25-800 C. The TG curve of 1:1 shows a weight loss of 11.51%
(Calc. 11.61%) from 150-378 ºC, corresponding to the removal of three coordinated
water molecules. In the temperature range 378-400 ºC a weight loss of 3.23% (Calc.
3.60%) is observed corresponding to the loss of OH-. Then the decomposition of the
ligand anion was followed in the temperature range 400-550 ºC with a weight loss of
72.32% (Calc. 72.91%) corresponding to the departure of C21H15N4O (ligand).
Constant weight is attained at around 550 ºC. The end product, estimated as MnO, has
an observed mass of 15.88% (Calc. 15.26%) which is in good agreement with the
calculated values obtained by determination of the metal content after decomposition
of the complex applying the method described by Macdonald [28] and with the results
of elemental analysis (Table 1). 1:2 Complex exhibits a continuous weight loss in the
temperature range of 223-800C. The first weight loss (2.26%) in the range of 223-
319 C corresponds to the departure of one water molecule. The weight loss is
consistent with the calculated value (2.39%).The decomposition of the ligand anions
was followed in the temperature range of 319-557 C. The observed mass loss (90.66)
agrees with the calculated value (90.29%) due to the loss of the two ligands. The final
residue is qualitatively proved to be MnO, the observed mass of 9.62% as compared
to the calculated value of 9.45%. Differential thermal analysis curves (DTA) are
characterized by the presence of a large and sharp exothermic peak on the DTA
curves at the temperature range 437-438 for Mn-L1 (1:1 and 1:2) complexes. At these
temperatures a phase change is occur due to the change in crystal structure of the
complex, i.e. crystallographic phase transition. By increasing the temperature above
these values the metallic oxide residue is formed.
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4.1.5. Molar conductance measurements
The molar conductance values of the complexes in DMF (10-3 mol L-1 solution at
25 C) are found in the range 4.78-7.73 ohm-1 cm2 mol-1. These values were small for
the ionic complexes of the divalent metal ions [44,45]. These low conductivity
ascribed to the absence of chloride ions rather than ionic association to the metal ions
during complex formation indicating that the complexes are non-electrolytes in nature
[46,47]. The conductivity values for all of the complexes are listed in Table 1.
4.1.6. Magnetic susceptibility measurements
The calculated magnetic moments of 1:1 and 1:2 (M:L) complexes for Mn(II)
metal ion are 5.89 and 5.90 B.M., respectively. It indicates the presence of 5 unpaired
electrons in its d-orbital (µeff = 5.92 B.M) and shows a high spin d5 configuration
3
(
-
2) [48] and supports the octahedral geometries of these complexes [49]. For
푡2푔 푒푔
Co(II) 1:1 and 1:2 complexes, the calculated magnetic moments of the complexes are
3.89 and 3.85 B.M., respectively. It shows the presence of three unpaired electrons
per metal ion (eff = 3.87 B.M.) which supports the octahedral geometries of these
complexes [49], Table 1. For Ni(II) 1:1 and 1:2 complexes, the eff are 0.32 and 0.45
B.M. representing no unpaired electrons (eff = 2.83 BM for paramagnetic) and
showing diamagnetic properties and supports a low spin Ni(II) ion in square planar
geometry [48]. The room temperature magnetic moment of Cu(II) complexes (1:1 and
1:2) are 1.77 and 1.78 BM, respectively, which is close to one electron
paramagnetism (µeff = 1.73 BM) of a mononuclear Cu(II) ion [50]. The Cu(II) ion
center is five-coordinated [51] and this is typical for square pyramid or distorted
square pyramid geometries. Thus, the geometry about the Cu(II) center in the
complexes is closer to square pyramid. The data obtained are consistent with the
reported literature for distorted square pyramidal Cu(II) complexes [52] For Zn 1:1
and 1:2 complexes, the calculated magnetic moment are 0.44 and 0.54 B.M.
indicating the absence of unpaired electrons and Zn(II) complexes has a diamagnetic
properties and Zn(II) ion centers are six-coordinated [51]
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4.1.7. Electron spin resonance (ESR)
The solution spectra were carried out in DMF solution in capillary tube and recorded
to confirm that the complexes do not undergo structural change in solution. The solution
ESR spectra of Cu(II) complexes as illustrated in Figure 4 were studied and Hamiltonian
parameters g//, g were also calculated. The X-band ESR spectra of 1:1 Cu complexes at
room temperature generally show two broad signals (g// and g ) at 2.0559 and 2.0678.
For 1:2 complexes, the signals appear at 2.0381 and 2.0714, respectively. For Cu(II)
complexes, g// indicates covalence with g// < 2.3 for covalent and g// ≥ 2.3 for ionic
complexes [53,54]. The nature of ligand is calculated from G which is the gauss value.
The parameter G measures the exchange interaction between the metal centers in a
polycrystalline solid [55] and can be measured using the equation G = (g// − 2)/(g⊥−2).
According to Hathaway et al. [56] if G > 4, the exchange interaction is negligible, while
for G < 4 indicates considerable exchange interaction in the solid complexes. For G < 4.0,
the ligand forming the complex is regarded as a strong field ligand in copper complexes
[53] which can be attributed to trigonal bipyramid or distorted square pyramid around the
Cu(II) ions. Moreover, the apparent broadening of the ESR signals may be due to an
interaction between Cu(II) ions which are probably present in nonequivalent lattice
position. The positive shift of g values from that of the free electron (2.0023) indicates a
high covalent character of the bonding between the Cu(II) ions and the ligands [57,58],
indicating a dx2-y2 ground state which is consistent with the electronic absorption
spectroscopic assignments [59].
Based on the above results the structures of the complexes are formulated as seen
below in Scheme 3. The proposed geometrical structures for the investigated metal
complexes (1:1) suggest octahedral geometry for Mn, Co and Zn; square planar for Ni
and square pyramidal structure for Cu. The proposed stereochemical structures for 1:2
complexes suggest square pyramidal structures and tetrahedral for Ni complex.
4.2. Molecular orbital treatment
4.2.1. Geometry of the Ligand
Geometry structures of the ligand L was optimized at the B3LYP/6-311G(d,p)
level and the results are given in Table 4. The ketoamine H-N structure of L was
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found to be the most stable tautomer while H-O and H-C tautomers have 23.31 and
34.04 kcal/mol higher than H-N structure, which is explained by the hydrogen bond
O…H-N stabilizing H-N tautomer as seen in Figure 5. Experimental and
computational data for similar ligand revealed that ketoamine H-N tautomeric form
predominates for the azomethines of 1-phenyl-3-methyl-4-formylpyrazole-5-one both
in solutions and in the solid state [60,61].
Our obtained results show that the ligand L has a complete coplanar structure as
indicated by the dihedral angles of the phenyl rings attached to the pyrazolone ring.
On complexation, L loses the hydrogen atom converting to the anion form L-. The
calculated natural charges of the expected chelating centers of L are given in Table 5.
It is clear from this presented data that the natural charge on O6 atom is -0.641, while
on N7 is -0.245 and on N8 is -0.297 which indicates the suitability of O6 and N8
atoms as chelating centers for coordination to the positively charged metal ions.
4.2.2. Geometry of the complexes
As mentioned before, the ligand is coordinated to the metal ions as a bidentate one
through pyrazolonic-oxygen O6, and azo-nitrogen N8, forming a six membered ring.
The coordination number of each metal ion is completed by hydroxyl group and
oxygen of the water molecules. This perception was experimentally confirmed by the
results of the IR spectra of both free ligand and the complexes, Table 2, as indicated
by the shift of N=N band, the disappearance of C=O band and appearance of a new
band corresponding to C-O on complex formation. Also, the presence of broad band
in the range 3355-3494 cm-1 corresponding to coordinated water molecules and
coordinated OH- ion as well as new bands which may assigned to (M←N) and
(M←O) stretching frequencies [42] confirming that they are center of chelation.
The thermodynamic stability of the various chelated structures of 1:1 M:L
(OH)(OH2)n, n=1,2 or 3, and 1:2 complexes were calculated using the B3LYP method
and the mixed basis sets 6-311G(d,p) and LANL2DZ. The calculated results showed
that, the best optimized geometry located of the Zn-L (OH)(OH2) complex is the
tetrahedral structure and the octahedral one, Zn-L (OH)(OH2)3. Nevertheless the
trigonal bipyramid or square pyramid structures were not located as local minima.
The tetrahedral form was obtained with the second water molecule existing in the
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hydration sphere, the distance H2O…Zn = 3.187Å. The two structures, tetrahedral and
octahedral, are energetically stable i.e. as two minima at this level but the latter one
formed, isolated and has the highest negative ∆Gf, therefore it will be considered as
the lonely complex in this study. The same results were also found in case of Mn-L
(OH)(OH2)3 and Co-L(OH)(OH2)3 complexes. On the other hand the calculations at
the same level showed that the most stable structure of copper complex, Cu-L
(OH)(OH2)2 , is the trigonal bipyramid one while the distorted square planar structure
was obtained for nickel complex, Ni-L(OH)(OH2). In the latter two cases the distance
between the third water molecule and Cu ion is 3.191 Å and in case of Ni the
distances are 3.308 and 3.644 Å, respectively, implying that these water molecules
subsist in the hydration sphere. The above obtained complexes are stabilized by the
formation of six membered rings and existence of hydrogen bonds of different
strengths with the coordinated OH- and H2O; 1.704 Å and 1.821 Å, respectively. The
geometrical parameters of the most stable structures of the ligand and the investigated
1:1 and 1:2 complexes are quoted in Table 4. For all complexes the C=O6 bond
distance does not appreciably affected while the calculated N7-N8 bond distance for
Zn, Co and Mn complexes elongates by 0.01 Å while for Cu and Ni complexes it
shortens comparing with the free ligand. In our calculations, the computed M–N8 and
M–O6 bond lengths are predicted near 2.24 and 1.91 Å, respectively, which are in
good agreement with the available experimental values [51,62,63], Table 4. One also
notices that, the ligand moiety became non-planar where the phenyl groups rotate out
of the pyrazolone moiety plane.
In case of 1:1 octahedral structure, Mn, Co and Zn metal ions are coordinated in
the equatorial plane to the O6, azo-nitrogen N8, O28 of the hydroxyl group and
oxygen of the water molecule, O30, while the oxygen of the other two water
molecules O29 and O31 are positioned in the apical site, Figure 5. Also, it can be
noted that metal ion deviates out of the equatorial plane by the angle ∠C1N7N8M ≈
6.7-8.7° and ∠C1C5O6M ≈ 11.7–16.5° leading to a distorted octahedral structure.
The same finding is also confirmed by the angle value of ∠O30MO29 which has a
value of 76.46°. The effect of metal ion size on the geometrical parameters of the
chelation is well noticed by the variation of the values of M-X bond distances. Zn
complex has the longest distances. The presence of H-bond interactions of different
strengths with the O28 atom of the hydroxyl group influences the bond lengths around
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the metal ion. In the case of the trigonal bipyramid structure of copper complex, [Cu-
L(OH)(OH2)2], the calculated results showed that the basal plane is occupied by the
O6, azo-nitrogen N8, and O28 of hydroxyl group coordinated to copper ion while the
two oxygen of the water molecules O29 and O31 are located in the apical site.
Finally, the bond lengths of the distorted square planar structure obtained for nickel
complex are shorter than those of the other complexes.
In case of 1:2 [M-L(OH2)n, n=0,1], the B3LYP results show that also the metal
ions coordinated with each L- via oxygen atoms of the carbonyl groups, O6 and O28,
and α-N atoms of the azo groups, N8 and N29, Figure 5. The thermodynamical
findings indicated that, the best optimized geometry located of the Ni-2L complex is
the distorted tetrahedral structure while the other studied metal complexes behave as
distorted trigonal bipyramid M-2L(OH2). In each case two six membered rings
stabilize the complexes and the phenyl group substituents rotate out to avoid the steric
hindrance. Table 4 displays the geometrical parameters of these complexes. In
comparison of 1:1 with 1:2 complexes, one notices that the maximum difference
between the corresponding parameters of the coordination sphere, ∆r is 0.04Å. The
basal plane of the trigonal bipyramid M-2L(OH2) is occupied by the O6, azo-
nitrogens N8, and N28 coordinated to metal ion while O28 and oxygen of water
molecule O54 are positioned in the apical site. Our computed values are in very good
agreement with the available experimental values [51,62-63].
It is well known that the study of the molecular orbital compositions and energy
levels of the molecule is very important in describing the electronic characteristics of
molecular systems. EHOMO characterizes its electron donating ability, while ELUMO
energy characterizes the electron withdrawing ability. Moreover, the difference
between EHOMO and ELUMO, Energy gap Eg, depicts the molecular chemical stability
and electron conductivity thus it is an essential factor in determining molecular
electrical transport. The spatial distribution of frontier MOs, HOMO and LUMO
acidic L form of the ligand is given in Figure 6. The calculated values of EHOMO and
ELUMO of L are respectively, -0.21806 and -0.10758 au, thus the corresponding energy
gap value Eg is 3.01 eV. The HOMO of L resides on pyrazolone moiety and the two
attached phenyl groups while the LUMO is mainly localized on pyrazolone and the
phenyl azo moiety. Table 6, Figures 7 and 8 list the partial frontier molecular orbital
compositions and energy levels of the investigated complexes in the ground state.
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These results show that the HOMO of the 1:1 complexes is lying within the range
5.55-5.94 eV. Comparing with the values corresponding to the acidic form of ligand,
it can be noted that the chelation process does not appreciably affect EHOMO but leads
to destabilization of the LUMO except of Zn case. Cu and Zn complexes have the
most stable HOMO whereas Mn complex is the lowest stable. On the other hand, the
LUMO values are 2.27-3.35 eV whereas Zn complex has the most stable LUMO and
the lowest stable one is found for Mn complex. The ionization potential of the
investigated complexes can be explicated as IP ~ -EHOMO, and the electron affinity as,
EA ~ -ELUMO. The computed ionization potential values of the free ligand and its
complexes can be arranged as: Mn <Co < Ni < L ~ Zn ~Cu, whereas the electron
affinity values have the order: Mn< Ni < Cu < Co < L< Zn. The electron density in
the HOMO of Co complexes is nearly extended overall the molecule except of water
molecule. In case of Ni, Cu and Zn complexes, the electron density in the HOMO is
mainly localized on pyrazolone and the two attached phenyl groups, azo moiety as
well as the metal ion. While the electron density in the LUMO is mainly localized on
the pyrazolone ring and phenyl group attached to the N=N bond along with some
metal d orbital.
4.2.3. Global reactivity descriptors
The energy difference between the HOMO and LUMO, Eg, of the studied
complexes occurs in the range 3.54-2.60 eV. The energy gap for Cu complex is the
maximum (3.54 eV) while Zn complex has the minimum (2.59 eV) value. It is also
worth noting that Eg values of Mn and Co complexes are of the same magnitude.
Similar observation is also noticed for Cu and Ni complexes. As a result, charge
transfer and polarization can easily occur within the Zn complex than other complexes
with more reactivity. The chemical hardness, η, electronegativity, χ, chemical
potential, μ, and global softness, S, were calculated using HOMO and LUMO
energies and listed in Table 6. Zn complex has the lowest η and maximum S values
which means that the charge transfer occurs easily in this complex and has a lower
chemical hardness. It can be concluded that the large Eg gap indicates a hardness of
the molecule, while smaller Eg gap is a characteristic for a soft and reactive molecule.
The collected data in Table 6 show that the HOMO of the 1:2 complexes are less
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stable than that of the 1:1 complexes and the acidic form of ligand which is reflected
on their IP values. Thus, the ionization potential values of the 1:2 complexes are less
than those of 1:1 complexes, except for Mn. On the other hand, LUMO of 1:2
complexes of Co, Cu and Zn are destabilized while that of Mn and Ni are stabilized
comparing with the values of the 1:1 complexes. Therefore, their electron affinity
values have the order: Co < Cu < Zn < Mn < Ni. Consequently, the obtained energy
gap Eg, for 1:2 Co complex has the maximum value (3.48 eV) while that of Ni
complex is the minimum (2.83 eV). Therefore, charge transfer and polarization can
easily occur within the Ni complex than the other complexes due to its higher
reactivity. The computed reactivity parameters shown in Table 6 reveals that Ni
complex has the lowest η and maximum S values which means that the charge
transfer occurs in this complex and has lower chemical hardness.
4.2.4. Natural charges and natural population
The calculation of atomic charges is a very important in describing the electronic
characteristics of the molecular systems. Natural atomic charges of the metal ions and
those atoms where coordinated to them for the studied complexes are collected in
Table 7. It can be noticed that the calculated charge on the metal ion is considerably
lower than its formal charge of +2. This is due to significant charge donation from the
O6, N7, N8 and the pyrazolone ring. The charge on the N8-donor center is
significantly smaller and less negative than the charge on the oxygen atom of C=O6
group, indicating that there is higher electron density delocalization from the N-donor
center to the metal ion. The highest observed natural positive charge (1.252) of Zn
ion is attributed to the completely filled d10 orbital, while the lowest natural positive
charge (0.684) is observed in case of Mn ion. It is evident from Table 7, the central
metal ion coordinates with the coordinating atoms via mostly the 3d, 4s and 4p
orbitals, e.g. the electronic arrangement of Mn ion is [core]4s0.113d3.344p0.204d0.01. The
contribution of 4d and 5p orbitals is negligible except of Zn ion where the electron
number of 5p orbital is 0.50. Both oxygen and nitrogen atoms of the ligands are
bonded through 2s and 2p orbitals. The natural charge distribution of metal ion and
the donating atoms of 1:2 complexes are similar as in case of 1:1 complexes. The
calculated natural positive charge on each metal ion observed in case of 1:1
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complexes is lower than that of 1:2 complexes suggesting that lower donation in case
of the later. This variation in natural charges distribution on the different centers of
the two types of complexes is reflected on their dipole moments. 1:2 complexes have
higher values of dipole moment except of Ni one.
4.2.5. Nonlinear optical properties (NLO)
The distribution of the atomic charges in the complexes is also valuable in the
determination of the magnitude and direction of its moment vector which depends on
the centers of negative and positive charges. The dipole moment, the mean
polarizability, the anisotropy of the polarizibility and the first-order
hyperpolarizibility for the studied free ligand and all complexes as well as urea was
calculated using the same level and the obtained values are tabulated in Table 8. The
table also contains the experimental values of urea. Our calculated dipole moment
value of the free ligand in the gas phase is 2.52 D. The three complexes Mn, Co and
Zn in the octahedral form have higher dipole moment values than L while those of Cu
and Ni complexes are lower which is attributed to that the dipole moment vectors of
different moieties of the first complexes are of the same direction. Table 8 also shows
that 1:2 complexes have higher dipole moment values than the corresponding 1:1
complexes except of Ni. On the other hand the calculated values of the polarizability
of 1:1 complexes have the range 24.10-26.63x10-24 (esu). Ni complex has the lowest
calculated value and Mn complex has the highest value. In case of 1:2 complexes the
values are lower than that of 1:1 complexes. Compared with urea [64] as a reference
compound, all the studied complexes have higher polarizability and first-order
hyperpolarizability values indicating that they are expected to be an effective
candidate for NLO materials.
5. Conclusion
From the experimental data obtained for the ligand and its complexes, 1:1 and 1:2
complexes are isolated and characterized. The metal complexes of the ligand were
prepared and subjected to different analytical and spectroscopic techniques such as
1
elemental analysis, IR, H NMR, mass spectra, Thermal analysis (TG and DTA),
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magnetic susceptibility, molar conductance, electron spin resonance and Density
Function Theory (DFT). 1:1 complexes of Mn(II), Co(II) and Zn(II) are distorted
octahedral whereas Ni(II) complex is square planar and Cu(II) is distorted trigonal
bipyramid stryctures. 1:2 complexes of Mn(II), Co(II), Cu(II) and Zn are distorted
trigonal bipyramid whereas Ni(II) complex is distorted tetrahedral. All complexes
behave as non-ionic in dimethyl formamide (DMF). The molecular geometry of the
studied 1:1 and 1:2 complexes in the ground state have been calculated by using DFT-
B3LYP/GEN level of theory. The optimized structures of the studied complexes are
nonlinear with the metal ion and are not in the same plane as the donating sites. The
HOMO-LUMO energy gap helped in analyzing the chemical reactivity, hardness,
global softness, chemical potential and electronegativity. The computed ionization
potential values of the free ligand and its complexes are arranged as: Mn <Co < Ni <
L ~ Zn ~Cu, whereas the electron affinity values have the order: Mn< Ni < Cu < Co <
L< Zn. The electron affinity values have the order: Co < Cu < Zn < Mn < Ni.
Consequently, the obtained energy gap Eg, for 1:2, Co complex has the maximum
value (3.48 eV) while that of Ni complex is the minimum (2.83 eV). Natural charge
distribution of the studied complexes was studied which indicated the electronic
charge distribution in the complexes. The calculated dipole moment and first-order
hyperpolarizability results indicate that the complexes have acceptable good non-
linear optical behavior.
Acknowledgement
The authors are grateful to thank Chemistry Department, Faculty of Science,
Helwan University which supporting this study.
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Wavenumber (cm-1)
Fig. 1. FT-IR spectra of L and its Mn complexes.

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Fig. 2. NMR spectra of L and its Zn complexes.