Synthesis, crystal structure, theoretical calculations and thermal characterization of two heterodinuclear NiII–ZnII complexes prepared from ONNO-type symmetrical Schiff base and its reduced derivative

Article abstract of DOI:10.1007/s10973-019-08630-w

Two complexes in Ni^II–Zn^II heterodinuclear form were prepared in DMF medium by the use of an ONNO-type symmetrical Schiff base bis-N,N′(salicylidene)-1,3-diaminopropane (LH₂) and the reduced derivative of this ligand bis-N,N′(2-hydroxybenzylidene)-1,3-diaminopropane (L^HH₂). The fact that the complexes are in [NiL·ZnCl₂·DMF₂] and [NiL^H·ZnCl₂·(DMF)₂] stoichiometry was verified with elemental and thermogravimetric analyses and IR spectroscopy. The structures of the complexes were determined by the use of X-ray diffraction. The two complexes were very similar, almost isostructure, and it was observed that Ni(II) ions in both complexes coordinated with two phenolic oxygens and two iminic nitrogen of organic ligand and formed an octahedral coordination between two DMF molecules. On the other hand, the Zn(II) ion was observed to be located in a tetrahedral coordination sphere coordinated with two phenolic oxygens between two halides. Although the molecular structures of the complexes are very similar, their thermal properties are quite different from of each other. The decomposition of [NiL·ZnCl₂·(DMF)₂] was observed between 140 and 190?°C by the removal of coordinative DMF molecules, leaving a residue of a mixture of NiL and ZnCl₂ behind. The complex of the reduced ligand [NiL^H·ZnCl₂·(DMF)₂] was observed to be stable up to 250?°C. After this temperature, the coordinative DMF molecules rapidly leave the structure before the degradation of NiL^H. That is why the activation energies of the thermal reactions were evaluated by the use of isothermal and nonisothermal kinetic models: Coats–Redfern, Ozawa, Ozawa–Flynn–Wall and Kissinger–Akahira–Sunose. Also the thermal differences between these two complexes were examined by the use of theoretical programs included in Gaussian 09 package. The ground-state energies calculations were carried out by the use of density functional theory method 631G(d) basis set. The calculated theoretical bond energies and angles were observed to be different compared with the experimental data. The HOMO and LUMO values of the complexes were also calculated. The difference between these two complexes was evaluated.

Full text of DOI:10.1007/s10973-019-08630-w

Journal of Thermal Analysis and Calorimetry
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Synthesis, crystal structure, theoretical calculations and thermal
characterization of two heterodinuclear NiII–ZnII complexes prepared
from ONNO-type symmetrical Schiff base and its reduced derivative
Arda Atakol1 Hasan Nazir2 Ingrid Svoboda3 M. Levent Aksu4 Orhan Atakol2
•
•
•
•
Received: 31 August 2018 / Accepted: 24 July 2019
´
´
Ó Akademiai Kiado, Budapest, Hungary 2019
Abstract
Two complexes in NiII–ZnII heterodinuclear form were prepared in DMF medium by the use of an ONNO-type sym-
metrical Schiff base bis-N,N0(salicylidene)-1,3-diaminopropane (LH2) and the reduced derivative of this ligand bis-N,N0(2-
hydroxybenzylidene)-1,3-diaminopropane (LHH2). The fact that the complexes are in [NiLÁZnCl2ÁDMF2] and [NiLH-
ZnCl2Á(DMF)2] stoichiometry was verified with elemental and thermogravimetric analyses and IR spectroscopy. The
structures of the complexes were determined by the use of X-ray diffraction. The two complexes were very similar, almost
isostructure, and it was observed that Ni(II) ions in both complexes coordinated with two phenolic oxygens and two iminic
nitrogen of organic ligand and formed an octahedral coordination between two DMF molecules. On the other hand, the
Zn(II) ion was observed to be located in a tetrahedral coordination sphere coordinated with two phenolic oxygens between
two halides. Although the molecular structures of the complexes are very similar, their thermal properties are quite
different from of each other. The decomposition of [NiLÁZnCl2Á(DMF)2] was observed between 140 and 190 °C by the
removal of coordinative DMF molecules, leaving a residue of a mixture of NiL and ZnCl2 behind. The complex of the
reduced ligand [NiLHÁZnCl2Á(DMF)2] was observed to be stable up to 250 °C. After this temperature, the coordinative
DMF molecules rapidly leave the structure before the degradation of NiLH. That is why the activation energies of the
thermal reactions were evaluated by the use of isothermal and nonisothermal kinetic models: Coats–Redfern, Ozawa,
Ozawa–Flynn–Wall and Kissinger–Akahira–Sunose. Also the thermal differences between these two complexes were
examined by the use of theoretical programs included in Gaussian 09 package. The ground-state energies calculations were
carried out by the use of density functional theory method 631G(d) basis set. The calculated theoretical bond energies and
angles were observed to be different compared with the experimental data. The HOMO and LUMO values of the
complexes were also calculated. The difference between these two complexes was evaluated.
Keywords ONNO-type Schiff base Á Reduced Schiff base Á Dinuclear complex Á Thermogravimetry (TG) Á
Theoretical calculation Á 631G(d) Á DFT
Introduction
Bis-N,N0(salicylidene)-1,3-diaminopropane (LH2) and its
& Orhan Atakol
reduced derivative bis-N,N0(2-hydroxybenzylidene)-1,3-
atakol@science.ankara.edu.tr
diaminopropane (LHH2) are the ligands used in coordina-
1
Turkish Standards Institution, C¸ ayirova, Gebze, Kocaeli,
Turkey
tion chemistry for diversified purposes [1–4]. These ligands
are very popular due to their great tendency to give
2
Department of Chemistry, Faculty of Sciences, Ankara
University, 06100 Bes¸evler, Ankara, Turkey
polynuclear coordination compounds. The first dinuclear
complex synthesized with the use of LH2 ligand was
3
Strukturforschung, FB11, TU-Darmstadt, Alarich Weiss
Street 2, 64287 Darmstadt, Germany
reported in 1976, and the first trinuclear complex was
obtained in 1990 [5, 6]. These ligands give a vast number
4
Faculty of Education, Gazi University, Teknik Okullar,
of homo- and heteropolynuclear complexes with many
Bes¸evler, Ankara, Turkey
123

A. Atakol et al.
transition metals. They form homotrinuclear complexes
with Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) [7–20].
In addition to these, there are a vast number of researches
about their heterodinuclear [21–24], heterotrinuclear
[25–31], homotetranuclear and heterotetranuclear [32–38]
and polynuclear complexes [39] in the literature.
prepared are depicted in Fig. 1a, b. The prepared com-
plexes were characterized by elemental analysis, IR spec-
troscopy, X-ray diffraction methods and simultaneous
thermogravimetric and differential thermal analysis.
Although these complexes have very similar structures,
their thermal behaviors especially their thermal features are
markedly different from each other.
In some dinuclear, trinuclear and tetranuclear com-
plexes, the solvent molecules such as N,N0-dimethylfor-
mamide (DMF), dimethylsulfoxide (DMSO) or dioxane
enter the coordination sphere [10, 40]. LH2 ligand Ni(II)
and Cu(II) may form mononuclear NiL and CuL complex
structures which are known since 1985 [41]. These
mononuclear complexes form dinuclear complexes with
metal halides in DMF, DMSO or dioxane in a two-stage
process as shown in Scheme 1 [10, 17, 35]. These dinu-
clear complexes could be prepared in a single stage with
template method [13, 14, 22] (Scheme 2).
DMF molecules have been identified in the respective
complexes which were also verified by the mass loss cor-
responding to two DMF molecules. This was also the case
in thermogravimetric analyses of such complexes prepared
with LH2 in the literature [43]. In contrast, the mass losses
of dinuclear complexes of the reduced LHH2 do not give a
clear picture about the thermal properties of the complexes
[22]. There are marked differences especially between the
thermal properties of these complexes, and in the present
work, the thermal behaviors of the complexes prepared
with the reduced and unreduced ligands were compared
and interpreted. For this purpose, thermokinetic analysis
and theoretical programs were employed.
Similar reactions can also take place with CuL complex;
however, the coordination sphere of Cu(II) and Ni(II)
central ions is different.
LH2 ligand can easily be reduced with NaBH4 in alco-
holic media giving a phenol–amine structured ONNO-type
tetradentate ligand, bis-N,N0(2-hydroxyphenyl)-1,3-di-
aminopropane (LHH2) (Scheme 3) [14, 22, 42].
The net mass loss between 120 and 190 °C which cor-
responds to the removal of coordinated DMF or DMSO
groups in the complex prepared with LH2 is in good
accordance with complex stoichiometry. However, this is
not the case for the complex prepared with LHH2 which
displays a very irregular mass loss at 240–250 °C. In
addition, thermogravimetric analysis of the NiL mononu-
clear complex was performed in order to help in explaining
the TG curves, but due to the inability of obtaining the
NiLH complex the thermal study of this structure could not
be performed.
Unfortunately it was not possible to isolate the
mononuclear NiLH complex with the use of LHH2 and there
is no report about it in the literature. However, dinuclear
complexes prepared with this LHH2 ligand by template
synthesis and their structures have been reported in the
literature, and the structures of these dinuclear complexes
are similar as given in Fig. 1 [22]. For this reason, the two-
step reaction shown in Scheme 1 with the reduced ligand
did not take place, but the dinuclear complexes were
obtained by template reactions (Scheme 4).
The theoretical programs were used to explain differ-
ence between two similar complexes, and in addition,
thermokinetic analysis was performed. Algorithms in
Gaussian 09 software were used as theoretical program
[44]. The charge densities, HOMO and LUMO levels,
Mulliken charges and theoretical bond lengths of the atoms
present in the coordination spheres of [NiLÁZnCl2Á(DMF)2]
and [NiLHÁZnCl2Á(DMF)2] complexes were calculated by
the use of DFT method and 631G(d) basis set, and Lanl2dz
algorithm embedded in Gaussian 09 software in order to
explain this difference. Also electron densities and the
relative energies gained by the d orbitals of the central
atoms as a result of overlapping were deduced by the use of
nbo (natural bond orbital) program. The nbo program was
applied twice by the use of both the theoretical and X-ray
diffraction data, and the results were comparatively
evaluated.
In this work, Ni(II)–Zn(II) dinuclear complexes were
prepared with unreduced LH2 and reduced LHH2 ligands in
DMF medium and the general formulas of the complexes
in MeOH or EtOH
Ni2+ + LH2
NiL + MX2
NiL
(mononuclear complex)
step 1
step 2
–2H+
in DMF (or DMSO)
X = Cl , Br or I
.
.
[NiL ZnCl2 (DMF)2]
M = Co(II), Zn(II), Cd(II)
Scheme 1 Two-stage preparation of dinuclear complexes
in DMF (or DMSO)
.
.
Ni2+ + LH2 + MX2
[NiL MX2 (DMF)2]
⋅
⋅
The thermal kinetic analyses of the complexes were
carried out by the use of isothermal and nonisothermal
methods. Due to its user-friendliness, the Coats–Redfern
(CR) method was proffered as the nonisothermal method.
In this kinetic thermal analytic method, the average
–2H+
M = Co(II), Zn(II), Cd(II)
X = Cl , Br or I
Scheme 2 Preparation of the complexes given in Scheme 1 with
template synthesis
123

Synthesis, crystal structure, theoretical calculations and thermal characterization…
Scheme 3 Reduction of LH2
H
H
H
H
NaBH4
in MeOH
N
C
C
N
C
N
N
C
H
H
H
H
OH
HO
HO
OH
LHH2
phenol-amine
LH2
phenol-imin
Fig. 1 a Open formula of the
complex prepared with LH2.
b Open formula of the complex
prepared with the reduced
ligand, LHH2
(a)
(b)
S
S
H
H
H
C
N
N
C
NH
HN
C
C
H
H
Ni
Ni
O
O
O
O
Zn
Zn
Cl
Cl
S
S
Cl
Cl
S = DMF
S = DMF
in DMF (or DMSO)
and N analyses were performed on EuroVector 3018 C, H, N,
S analyzer. Metal analyses were recorded on GBC Avanta
PM Model atomic absorption spectrometer using FAAS
mode. Complex (2–3 mg) was dissolved in 1 mL HNO3
(63%) with heating, dilutedto 100 mL and given to nebulizer
of atomic absorption spectroscopy for metal analysis. The
mass spectra of the ligands were obtained by Shimadzu 2010
plus with direct inlet (DI) unit with an electron impact ion-
izer. DI temperature was varied between 40 and 140 °C, and
ionization was carried out with electrons with 70 eV energy.
The NMR spectra were recorded on the Bruker Ultrashield
300 MHz NMR spectrometer. d6—DMSO solution was the
solvent. The thermogravimetric analyses were performed by
Shimadzu DTG-60H. In thermogravimetric analyses, tem-
perature was varied between 30 and 600 °C. These analyses
were performed at 5, 10, 15, 20 and 25 °C min-1 rates and
under N2 atmosphere in Pt pans. Calibration of the instru-
ment was done with metallic In and Zn.
Ni2+ + LHH2 + MX2
[NiLH MX2 (DMF)2]
.
.
–2H +
M = Co(II), Zn(II), Cd(II)
X = Cl , Br or I
Scheme 4 Preparation of dinuclear complex with reduced ligand
activation energy (Ea) and Arrhenius pre-exponential factor
(A) values were evaluated by the use of average heating
scan rates of 5, 10, 15, 20 and 25 °C min-1. The same
values were also computed by the graphical calculations
with Kissinger–Akahira–Sunose (KAS) and Ozawa–
Flynn–Wall (OFW) methods, two of the most popular
nonisothermal–isoconversional methods in the literature.
Due to the fact that the equipment used for the thermal
analyses contains a single kinetic calculation program
which only works according to Ozawa method, the same
calculations were carried out according to this method. As
it is known, Ozawa method is the base of all the non-
isothermal techniques [44, 45].
X-ray crystallography
A single crystals of [NiLÁZnCl2Á(DMF)2] and [NiLH-
ZnCl2Á(DMF)2] were analyzed on an Oxford Diffraction
Xcalibur single-crystal X-ray diffractometer with a sap-
Experimental
Materials and apparatus
˚
phire CCD detector using MoKa radiation (k = 0.71073 A)
operating in x/2h scan mode. The unit-cell dimensions
were determined and refined by using the angular settings
of 25 automatically centered reflections in 2.37° B h
B 30.57° range for [NiLÁZnCl2Á(DMF)2] and 2.46°–26.37°
The reagents supplied from Merck or Fluka companies used
without further purification. In this study, Shimadzu Infinity
FTIR spectrometer equipped with three reflectional ATR
units was used for IR spectra with 4 cm-1 accuracy. The C, H
123

A. Atakol et al.
for [NiLHÁZnCl2Á(DMF)2]. The data for [NiLÁZnCl2-
(DMF)2] complex were collected at 293(2) K, and for other
complexes, they were collected 302(2) K. The empirical
absorption corrections were applied by the semi-empirical
method via the CrysAlis CCD software [46]. The model
was obtained from the results of the cell refinement, and
the data reductions were carried out using the solution
software SHELXL-97 [47]. The structure of the complex V
was solved by direct methods using the SHELXS-97 soft-
ware implemented in the WinGX package [48]. Supple-
mentary material for structure has been deposited to the
Cambridge Crystallographic Data Center as CCDC no:
1503400 and 1503401 (deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk).
and the resulting solution was heated up to 50 °C adding
solid NaBH4 in small portions under rigorous stirring until
it becomes colorless [31–34]. After 10 min of stirring,
300 cm3 of ice water was added to it. The final mixture was
kept on the bench for 24 h. The resulting white precipitate
was filtered off and dried in air. The product bis-N,N0 (2-
hydroxybenzyl)-1,3-diaminopropane (LHH2) was recrys-
tallized from hot EtOH:H2O (2:1, v/v). Yield: 55–60%,
mp: 107 °C (determined by DTA).
Elemental analysis, C17H22N2O2:
Expected %, C: 71.30, H: 7.74, N: 8.01; found %, C:
70.86, H: 6.69, N: 8.37.
IR (cm-1): mN–H: 3307, mC–H (Ar): 3055–3023, mC–H (Aliph)
:
2967–2823, mC=C (ring): 1606–595, mC–O (Phenol): 1253–1099,
dC–H (Ar): 752.
Preparation of the ligands
1H NMR data in d6-CH3SOCH3 (d, ppm): 13.22 (s),
7.12 (m), 6.67 (m), 4, 73(broad), 3.80 (m), 2, 56 (m), 1,15
Preparation of N,N0-bis (2-hydroxyphenylidene)-1,3-
diaminopropane (LH2)
(m)
13C NMR data in d6-DMSO (d, ppm): 155.41, 151.23,
143.27, 126.92, 128.56, 124.85, 65.43, 61.17, 18.62.
MS (m/z): 286 (molecular peak), 179 [HO-C6H4-CH2-
NH-CH2-CH2-CH2-NH]?, 163 [HO-C6H4-CH2-NH-CH2-
CH2-CH2]?, 150 [HO-C6H4-CH2-NH-CH2-CH2]?, 134
[HO-C6H4-CH2-NH-CH2]?, 122 [HO-C6H4-CH2-NH]?,
107 [HO-C6H4-CH2]? (base peak), 90 [C6H4-CH2]?, 77
[C6H5]?.
This Schiff base was prepared via condensation reaction in
EtOH under hydrothermal conditions using 2-hydroxy-
benzaldehyde and 1,3-diaminopropane. 2-Hydroxyben-
zaldehyde (100 mmol, 12.20 g) was dissolved in 120 cm3
of warm EtOH, Then 50 mmol (3.70 g) of 1,3-diamino-
propane was added to it, and the resulting solution was
heated up to the boiling point. After cooling, yellow
crystals were filtered off and dried in air. Yield: 90–95%,
mp: 58 °C (determined by DTA).
Preparation of mononuclear complex (NiL)
Elemental analysis, C17H18N2O2:
Mononuclear complex, NiL, was prepared by ammonia in
ethanol solution of LH2ÁNiCl2Á6H2O and was prepared as
outlined in the literature [7]. A quantity of 10 mmol LH2
(2.82 g) was dissolved in 100 mL hot EtOH under stirring.
10 mL concentrated ammonia (20%) solution was added to
this ethanol solution, and the mixture was heated up to
boiling temperature. To this mixture was added a solution
of 10 mmol NiCl2.6H2O (2.36 g) in 30 mL hot water.
After setting the mixture, it was left on the bench for an
hour and the precipitate of the light green complex of
NiL.NH3 was filtered off and dried in an oven at 150 °C for
4–5 h. The light brown complex was recrystallized in
EtOH–dioxane mixture (v/v = 50:50).
Expected %, C: 72.32, H: 6.43, N: 9.92; found %, C:
71.95, H: 6.33, N: 10.09.
Important IR data (cm-1): mO–H: 2627, mC–H(Ar)
3021–3019, mC–H(Aliph): 2929–2862, mC=N: 1629, mC=C(ring)
1608, mC–O(Phenol): 1274–1151, dC–H(Ar): 762.
:
:
kmax = 243 nm, e = 7045 dm3 mol-1 cm-1 in DMSO,
kmax = 242 nm, e = 7865 dm3 mol-1 cm-1 in MeOH.
1HNMR data in d6-CH3COCH3 (d, ppm): 13.51 (s) (O–
H), 8.60 (s) (–CH=), 7.43 (d) (HAr), 7.32(t) (HAr), 6.88
(t) (HAr), 3.68 (t) (N-CH2–), 2.01 (p) (–CH2–).
13CNMR data in d6-CH3COCH3 (d, ppm): 166.6, 161.1,
132.7, 132.1, 119.1, 118.9 (CAr), 116.9 (–C=N), 58.5 (N-
CH2–), 31.9 (–CH2–).
Elemental analysis, C17H16N2O2Ni:
m/z: 282 [M]?, 161 [HO-C6H4-CH=N-CH2-CH2-
CH2]?, 148 [HO-C6H4-CH=N-CH2-CH2]? (BP), 134
[HO-C6H4-CH=N-CH2]?, 120 [HO-C6H4-CH=N]?, 107
[HO-C6H4-CH2]?, 77 [C6H5]?.
Expected %, C: 60.28, H: 4.76, N: 8.27, Ni: 17.33;
found %, C: 60.55, H: 3.17, N: 7.93. Ni: 17.19
IR (cm-1): mC–H
:
3061–3030, mC–H
:
:
(Ar)
(Aliph)
2922–2866, mC=N: 1607, mC=C (ring): 1589–1541, dC–H (Aliph)
1475, mC–O (Phenol): 1228–1124, dC–H (Ar): 725–744.
Preparation of N,N-bis (2-hydroxybenzyl)-1,3-
MS (m/z): 340 (isotope peak, because of 60Ni isotope),
338 (molecular peak), 219 [Ni–O–C6H4–CH=NH–CH2–
CH2–CH2]?, 205 [Ni–O–C6H4–CH=NH–CH2–CH2]?,
179 [Ni–O–C6H4–CH=NH]?, 134 [O–C6H4–CH = NH–
CH2]?, 107 [HO–C6H4–CH2]?, 58 [Ni]?
diaminopropane (LHH2)
3.0 g of bis-N,N0 (salicylidene)-1,3-diaminopropane (LH2)
was dissolved in 70.0 cm3 of MeOH by rigorous stirring,
123

Synthesis, crystal structure, theoretical calculations and thermal characterization…
Preparation of [NiLÁZnCl2Á(DMF)2]
Preparation of [NiLÁZnCl2Á(DMF)2] with template method
This complex can be prepared by two different methods. In
the first method, the first step was via the preparation of the
mononuclear NiL complex which is then converted into the
dinuclear complex in DMF solvent. The second method is
the template method where the Ni2? ligand is reacted with
ZnCl2 in DMF medium.
A quantity of 2 mmol (0.565 g) LH2 was dissolved in
60 mL hot DMF under stirring and heated approximately
to 100–110 °C. A solution of 2 mmol (0.476 g) NiCl2-
6H2O in 20 mL MeOH and 1.0 mL Et3N was added to this
solution. Finally a solution of 2 mmol ZnCl2 (0.272 g) in
20 mL hot MeOH was added to this mixture. The final
mixture was rigorously stirred and left on the bench for
2–6 days. The crystals formed were filtered off, washed
with MeOH and finally dried in air (Table 1).
Preparation of [NiLÁZnCl2Á(DMF)2] with the first method
1 mmol (0.340 g) NiL was dissolved in 50 mL hot DMF
under stirring and heated up to 100–110 °C. A solution of
1 mmol ZnCl2 (0.136 g) in 20 mL hot MeOH was added to
this solution. The mixture was kept on the bench for
2–4 days at room temperature. The precipitated crystals
were filtered off and dried in air.
Elemental analysis, C23H30N4O4NiZnCl2:
Expected %, C: 44.48, H: 4.87, N: 9.01, Ni: 9.45, Zn:
10.53, Cl: 11.42; found %, C: 44.75, H: 4.39, N: 8.77 Ni:
8.92, Zn: 10.33, Cl: 10.69.
IR (cm-1): mC–H
2924–2852, mC=O(DMF): 1644, mC=N: 1626, mC=C
:
3038–3012, mC–H
:
:
(Ar)
(Aliph)
(ring)
1593–1551, dC–H (Aliph): 1477, mC–O (Phenol): 1300–1109,
C–H (Ar): 756–735.
d
Table 1 Crystal data and data collection values
[NiLÁZnCl2Á(DMF)2]
[NiLHÁZnCl2Á(DMF)2]
Formula mass/g mol-1
621.29
625.52
T/K
293(2)
302(2)
Crystal color
Crystal system
Space group
Green–blue
Monoclinic
Cc
Blue
Orthorhombic
Pbca
˚
a/A
10.530(5)
15.165(5)
17.203(5)
90.00
13.5709(8)
˚
b/A
19.539(2)
˚
c/A
20.887(2)
Alpha
Beta
90.00
98.751(5)
90.00
90.00
Gamma
90.00
3
˚
V/A
2715.1(18)
2
5538.4(8)
Z
8
Calc. density/g cm-3
l/mm-1
1.520
1.500
1.809
1.774
F (000)
1280
2592
˚
Radiation wavelength/A
0.71073
2.37–30.57
0.71073
h range/°
Index ranges
2.46–26.37
- 15 B h B 15, - 21 B k B 21, - 24 B l B 24
- 10 B h B 16, - 24 B k B 24, - 26 B l B 14
Reflections collected
Reflections unique
8263
5655
7337
3536
´
R1, wR2 (2o)
0.0359–0.0756
0.0430–0.0793
8263/336
1.094
0.0345–0.0627
0.0804–0.0724
5655/326
0.996
R1, wR2 (all)
Data/parameters
GOOF of F2
-3
˚
Largest difference peak hole/e A
0.333 to (- 0.455)
1503400
0.303 to (- 0.283)
1503401
CCDC no.
123

A. Atakol et al.
Preparation of [NiLHÁZnCl2Á(DMF)2]
Figures 2 and 3 show that the structures of these two
complexes were very similar. The Ni(II) central ions in
both complexes are located between two nitrogen and two
oxygen atoms of the organic ligand. The central Ni(II) ion
is settled in a O4N2 distorted octahedral coordination
sphere coordinated by the oxygen atoms of two DMF
molecules. On the other hand, Zn(II) ion is located in a
distorted tetrahedral coordination sphere O2Cl2 coordinated
by two Cl and two phenolic oxygens. Phenolic oxygens
form l-bridges. There is a significant similarity between
these two complexes. The only difference was that
[NiLHÁZnCl2Á(DMF)2] molecule includes four more
hydrogen atoms. The coordination spheres are approxi-
mately the same. Ni–Zn distances in [NiLÁZnCl2Á(DMF)2]
This complex was prepared by template method using
reduced Schiff base and NiCl2 in DMF solvent. A quantity
of 2 mmol (0.570 g) LHH2 was dissolved in 40 mL hot
DMF under rigorous stirring and heated approximately up
to 110–120 °C. A solution of 2 mmol (0.476 g) NiCl2-
6H2O in 20 mL hot MeOH and 1.0 mL Et3N and then a
solution of 2 mmol ZnCl2 (0.272 g) in 20 mL hot MeOH
were added to it. The final mixture was rigorously stirred
and left at the bench for 5–10 days. The blue crystals
formed were filtered off, washed with EtOH and finally
dried in air.
Elemental analysis, C23H34N4O4NiZnCl2:
H
˚
Expected %, C: 44.16, H: 5.47, N: 8.95, Ni: 9.38, Zn:
10.45, Cl: 11.33; found %, C: 44.37, H: 4.93, N: 8.95, Ni:
8.92, Zn: 9.96, Cl: 10.58.
IR (cm-1): mN–H: 3278–3216, mC–H (Ar): 3043–3012,
mC–H (Aliph): 2929–2860, mC=O(DMF): 1658, mC=N: 1636,
and [NiL ÁZnCl2Á(DMF)2] molecules are 3.0864 A and
˚
3.0546 A, respectively. The bond lengths around the Ni(II)
ion in [NiLÁZnCl2Á(DMF)2] molecule are measured to be
˚
2.018, 2.021, 2.0214 and 2.032 A, and the bonds between
Ni(II) ion and oxygens of the coordinated DMF oxygens
˚
were found to be 2.137 and 2.138 A. The corresponding
mC=C (ring): 1597–1571, dC–H (Aliph): 1485, mC–O
1274–1111, dC–H (Ar): 756.
:
(Phenol)
distances were measured to be 2.080, 2.070, 2.073, 2.086,
H
˚
2.1062 and 2.0698 A in [NiL ÁZnCl2Á(DMF)2] molecule.
There are no significant differences between these bond
lengths and angles listed in Table 2. However, it is
observed that the bonds between organic ligand are shorter,
but the bonds between the DMF molecules are longer in
[NiLÁZnCl2Á(DMF)2] molecule. In [NiLHÁZnCl2Á(DMF)2]
Results and discussion
The pluton illustrations of the molecular models of the
complexes based on X-ray data are presented in Figs. 2 and
3. The crystallographic data were tabulated together with
collection conditions, and selected bond lengths and angles
in Table 2.
˙
molecule, the six bonds around the NI(II) ion are close to
each other, but the distances between the Ni(II) ion and
organic ligand donors are longer but the bands established
Fig. 2 Pluton structure
[NiLÁZnCl2Á(DMF)2] complex
C20
C9
C10
N3C8
C1804
N2
C11
N1
C7
C13
C12
NL
03
02
C1
C17
C14
C2
01
C6
C21
C15
C16
C3
CL1
C5
N4
Zn
C4
C23
C22
CL2
123

Synthesis, crystal structure, theoretical calculations and thermal characterization…
Table 2 Selected bond lengths and angles of around coordination
sphere of the complexes prepared
Table 2 (continued)
Complex
Bond lengths
Bond angles
O2 Zn1 Ni1 42.66(5)
Complex
Bond lengths
Bond angles
O1 Zn1 Ni1 42.33(5)
Cl2 Zn1 Ni1 123.28(3)
Cl1 Zn1 Ni1 121.70(3)
[NiLÁZnCl2Á(DMF)2]
Zn O1 1.9892(19) O1 Zn O2 79.94(7)
Zn O2 2.0064(19) O1 Zn Cl2 112.16(7)
Zn Cl2 2.2066(10) O2 Zn Cl2 116.65(6)
Zn Cl1 2.2342(11) O1 Zn Cl1 111.66(6)
Ni N1 2.018(2)
Ni O1 2.021(2)
O2 Zn Cl1 111.58(6)
Cl2 Zn Cl1 118.57(4)
Ni O2 2.0214(19) N1 Ni O1 90.37(9)
Ni N2 2.032(2) N1 Ni O2 169.19(8)
Ni O3 2.1376(19) O1 Ni O2 78.84(7)
C15
C16
C14
C13
Ni O4 2.138(2)
Ni Zn 2.0864(8)
N1 C7 1.287(4)
N2 C11 1.278(4)
N1 Ni N2 100.08(10)
O1 Ni N2 169.35(9)
O2 Ni N2 90.73(9)
N1 Ni O3 90.30(9)
O1 Ni O3 90.79(8)
O2 Ni O3 89.34(8)
N2 Ni O3 91.22(9)
N1 Ni O4 88.68(9)
O1 Ni O4 91.79(8)
O2 Ni O4 92.14(8)
N2 Ni O4 86.43(9)
O3 Ni O4 177.23(8)
N2 Ni1 O3 85.60(8)
N2 Ni1 O1 172.54(8)
O3 Ni1 O1 91.69(7)
N2 Ni1 N1 95.36(9)
C20
C17
Zn1
C12
CL2
C19
N3
C11
02
C18
03
CL1
N2
C23
01
C2
C1
C22
N4
04
C3
N1
C6
C4
[NiLHÁZnCl2Á(DMF)2] N1 Ni1 2.080(2)
N1 H1A 0.84(2)
C5
C7
C8
N2 Ni1 2.070(2)
Fig. 3 Pluton structure [NiLHÁZnCl2Á(DMF)2] complex
N2 H2A 0.83(3)
O1 Zn1 1.999(17) O3 Ni1 N1 94.37(8)
O1 Ni1 2.0730(16) O1 Ni1 N1 91.77(8)
O2 Zn1 1.9866(17) N2 Ni1 O2 92.99(8)
O2 Ni1 2.0861(17) O3 Ni1 O2 86.16(7)
O3 Ni1 2.0698(17) O1 Ni1 O2 79.89(7)
O4 Ni1 2.1062(18) N1 Ni1 O2 171.65(8)
Cl1 Zn1 2.2493(8) N2 Ni1 O4 88.92(8)
Cl2 Zn1 2.2197(9) O3 Ni1 O4 172.40(7)
Ni1 Zn1 3.0546(5) O1 Ni1 O4 93.09(7)
with DMF oxygens are shorter. This shows the fact that the
overlapping in the establishment of the coordination bonds
between the donor atoms and the central Zn(II) ion is more
strained than the other complex. The biggest structural
difference between these two complexes is the chelate
rings around the coordination sphere. The Ni–N1–C8–C9–
C10–N2 chelate ring conformation of the [NiLÁZnCl2-
(DMF)2] complex is different than that of [NiLHÁZnCl2-
(DMF)2] complex. In [NiLÁZnCl2Á(DMF)2] complex, the
angle between N1–Ni–N2 and N1–C8–C10–N2 planes was
found to be below 5° and the angle between N1–C8–C10–
N2 and C8–C9–C10 was 62.87°. The corresponding angles
in [NiLHÁZnCl2Á(DMF)2] complex, on the other hand, were
measured to be 30.28° and 62.22° [49]. These results show
that the chelate ring in [NiLHÁZnCl2Á(DMF)2] complex is
closer to chair conformation and [NiLÁZnCl2Á(DMF)2]
complex has been found to have semi-chair conformation.
As known, the semi-chair conformation is an unsta-
ble structure [50]. The nitrogen atoms of the reduced ligand
have a seconder amine structure and are richer than the
iminic nitrogens with regard to electrons. Therefore, the
donor power of the nitrogens of the reduced ligand is
expected to be higher. In addition to these, the chelate ring
C7 N1 1.482(3)
C10 N2 1.482(3)
N1 Ni1 O4 91.39(8)
O2 Ni1 O4 88.87(7)
N2 Ni1 Zn1 132.12(7)
O3 Ni1 Zn1 82.43(5)
O1 Ni1 Zn1 40.49(5)
N1 Ni1 Zn1 131.60(6)
O2 Ni1 Zn1 40.19(5)
O4 Ni1 Zn1 97.43(5)
O2 Zn1 O1 84.13(7)
O2 Zn1 Cl2 119.15(6)
O1 Zn1 Cl2 117.78(5)
O2 Zn1 Cl1 109.67(5)
O1 Zn1 Cl1 106.99(5)
Cl2 Zn1 Cl1 114.93(3)
123

A. Atakol et al.
formed with Ni(II) is closer to the chair formation. In this
case, the complex prepared with the reduced ligand is
expected to be more stable. But in reality the situation is
absolutely reverse. The NiL mononuclear complex was
prepared, but it was not possible to synthesize NiLH
mononuclear complex.
place in the range of 125–240 °C depending on the heating
rate. It is probable that the residue of [NiLÁZnCl2Á(DMF)2]
complex left after this degradation process is the mixture of
NiL complex and ZnCl2 because the TG curve indicates the
loss of two DMF molecules in the thermal reaction. The
TG–DTA curve of NiL complexes is illustrated in Fig. 6.
Table 3 lists the thermodynamic data of NiL complexes.
As shown in Figs. 4 and 6, after 300 °C, the TG curve of
the residue of [NiLÁZnCl2Á(DMF)2] complex and the TG
curve of the mononuclear NiL complex are in similar
forms. NiL complexes are degraded with a 33% mass loss
in 356–420 °C temperature range depending on the heating
rate. On the other hand, the residue of first thermal reaction
of [NiLÁZnCl2Á(DMF)2] complex is decomposed between
380 and 440 °C depending on the heating rate giving a
similar TG curves with NiL. The mass loss after the second
thermal reaction of [NiLÁZnCl2Á(DMF)2] complex is
approximately 18%. Looking at the fraction of NiL in
[NiLÁZnCl2Á(DMF)2] complex, 18% mass loss corresponds
to nearly 33% for pure NiL. Between these two reactions,
there is a 20 °C difference between the thermal decom-
position temperatures. The other difference is the
endothermic melting signal observed at 311 °C for the NiL
complex. This is an acceptable outcome since the residue
left behind is a mixture of NiL and ZnCl2, because we do
not know the interaction between NiL and ZnCl2 in the
residue.
This situation is apparent in thermogravimetric curves of
the two complexes in Figs. 4 and 5. Two endothermic
reactions are observed in [NiLÁZnCl2Á(DMF)2] complex
and a single endothermic reaction [NiLHÁZnCl2Á(DMF)2]
complex. The thermodynamic data of this thermal reaction
are given in Table 3. In [NiLÁZnCl2Á(DMF)2] complex, the
mass of the first reaction was found to be 23.54 0.23%.
The theoretical mass loss corresponding to two DMF
molecules is 23.50%. The first mass loss from the
[NiLÁZnCl2Á(DMF)2] complex belongs to the removal of
two coordinative DMF molecules. This degradation takes
100
80
60
40
20
0
There is an endothermic signal appeared in the TG–
DTA curve of the [NiLHÁZnCl2Á(DMF)2] complex starting
from 250 °C, showing an highly erratic mass loss. A
similar situation has been reported in the literature [22]. It
is highly probable that in [NiLHÁZnCl2Á(DMF)2] complex it
is the DMF molecules which are effective in the formation
of dinuclear complex; therefore, it is easily decomposed
when the DMF molecules leave the structure. As men-
tioned earlier, NiLH complex could not be isolated and
there is no study related to its preparation in the literature.
When DMF molecules thermally dissociates, the unsta-
ble NiLH starts to decompose. The [NiLHÁZnCl2Á(DMF)2]
complex can only be prepared in DMF medium with a
dinuclear structure. In [NiLHÁZnCl2Á(DMF)2] complex,
NiLH unit which was formed after the insertion of the DMF
molecules in the coordination sphere and as soon as the
DMF molecules leave the structure, the unstable NiLH
mononuclear unit starts to decompose. Due to this reason,
an irregular mass loss is observed in the TG curve of the
[NiLHÁZnCl2Á(DMF)2] complex.
0
100
200
300
400
500
600
700
Temperature/°C
Fig. 4 Thermogravimetric comparison of the complexes prepared,
black: [NiLÁZnCl2Á(DMF)2], red: [NiLHÁZnCl2Á(DMF)2]
50
0
Is this difference between complexes observed in kinetic
analyses? The complexes were subjected through thermal
kinetic analyses for comparison purposes.
– 50
0
100
200
300
400
500
Temperature/°C
The complexes were subjected to nonisothermal Ozawa,
Flynn–Ozawa–Wall (FOW), Kissinger–Akahira–Sunose
(KAS) and isothermal Coats–Redfern (CR) methods for the
Fig. 5 DTA curves of the two complexes prepared, black:
[NiLÁZnCl2Á(DMF)2], red: [NiLHÁZnCl2Á(DMF)2]
123

Synthesis, crystal structure, theoretical calculations and thermal characterization…
123

A. Atakol et al.
40
20
0
In this equation, 1/T was plotted against lnb, ln[b/T2]
and ln[g(a)/T2] and the activation energy (Ea) and Arrhe-
nius pre-exponential factor (A) values calculated from the
slope and intercepts of these graphs. The results are tabu-
lated in Table 4.
100
80
The first thermal reaction of [NiLÁZnCl2Á(DMF)2]
complex is the removal of two DMF molecules from the
structure leaving two stable products behind:
60
Â
Ã
NiL Á ZnCl2 Á ðDMFÞ2
! NiLsolid þ ZnCl2ðsolidÞ
40
solid
þ 2DMFgas
The products are known compounds and the reaction is
:
– 20
0
100 200 300 400 500 600 700
Temperature/°C
homogenous. As given in Table 4, that is why the results
found by four different methods are comparable with each
other. However, the situation is different for [NiLH-
ZnCl2Á(DMF)2] complex. As mentioned above, this com-
plex starts to degrade at 250 °C, but contrary to the above
in this reaction the NiLH unit starts to decompose by the
loss of two DMF molecules. That is why there are marked
differences between the data measured by the isothermal
and nonisothermal methods. This is the case for the second
thermal equation of [NiLÁZnCl2Á(DMF)2] complex. The
second reaction is the thermal degradation of the NiL
residual mononuclear complex. This is a thermal degra-
dation reaction leaving no net residue behind. That is why
the isothermal and nonisothermal methods give different
results. Table 4 lists the data obtained from NiL thermal
degradation of the mononuclear complex. These data are
different from those obtained from the second thermal
reaction of the dinuclear complexes. The residue of the first
thermal reaction of [NiLÁZnCl2Á(DMF)2] complex left after
the removal two DMF molecule is not pure NiL; it is
NiL ? ZnCl2 mixture. That is why one cannot expected
similar results from this NiL complex. The fact that the
data obtained by the use of three nonisothermal–isocon-
versional techniques are comparable with each other veri-
fies the ICTAC proposal. Why are the thermal behaviors
much different in spite of the fact that coordination systems
and molecular structures of these two complexes are very
similar? As mentioned above, this difference is originated
from the difference in their thermal stabilities. The residue
left from thermal degradation of the [NiLÁZnCl2Á(DMF)2]
complex after the coordinative DMF molecules leave the
structure is NiL ? ZnCl2 mixture. Since NiL is stable at
this temperature, the residue remains intact until the
degradation of NiL. On the other hand, the coordinative
DMF molecules keep the dinuclear [NiLHÁZnCl2Á(DMF)2]
complex stable. Therefore, as soon as the removal of the
DMF molecules in complex [NiLHÁZnCl2Á(DMF)2], the
NiLH unit begins to decompose.
Fig. 6 TG–DTA curve of NiL complex. The signal observed at
approximately 300 °C is due to melting of NiL complex
kinetic analyses. These methods are graphical methods,
and the thermal calculations were carried out by the use of
the following formulas:
The application of the Ozawa method, the most popular
nonisothermal–isoconversional method, was carried out by
TA60WS version 0.1 software present in the TG equipment
using the following formula [44]:
ꢀ
ꢁ
AEa
bEa
RT
ln b ¼ ln
þ a þ
gðaÞR
a ¼ À 2:315; b ¼ À 0:4567 for 0:20 ꢀ gðaÞ ꢀ 0:60:
However, most workers opt for Ozawa–Flynn–Wall
(OFW) equation rather than Ozawa equation [51–55]. The
following equation was applied manually using the TG
data:
0:0048AEa
RgðaÞ
Ea
ln b ¼ ln
À 1:0516
:
RT
Parallel to the OFW equation based on the same foun-
dation but different is Kissinger–Akahira–Sunose (KAS)
equation [51–55]. This equation is commonly employed in
the literature:
b
AEa
Ea
ln ¼ ln
T2
À
:
RgðaÞ RT
Nonisothermal kinetic analysis methods are more pop-
ular than its isothermal counterpart since International
Confederation for Thermal Analysis and Calorimetry
(ICTAC) stated that nonisothermal methods give more
accurate results [56]. In this study, the kinetic parameters
of these two complexes were determined by the use of
nonisothermal Coats–Redfern (CR) equation given below
[57]:
gðaÞ
AR
Ea
ln
¼ ln
À
:
The complexes were subjected a theoretical investiga-
tion in order to clarify this issue. As mentioned above,
T2
bEa RT
123

Synthesis, crystal structure, theoretical calculations and thermal characterization…
123

A. Atakol et al.
123

Synthesis, crystal structure, theoretical calculations and thermal characterization…
Table 5 Theoretically
˚
Bond lengths/A
Complex
Bond angles/°
calculated bond lengths and
angles of the complexes, (a) by
optimization, (b) using X-ray
data around the coordination
spheres
Atoms
Lengths
Atoms
Angles
(a)
NiL
Ni–O1
1.840
1.840
1.899
1.899
1.828
1.828
1.939
1.939
1.880
1.878
1.896
1.894
2.752
3.888
3.037
2.050
2.044
2.238
2.219
1.889
1.884
1.944
1.935
2.758
3.534
3.065
2.021
2.026
2.232
2.246
95.354
91.077
82.387
91.078
99.042
87.439
85.728
87.448
96.775
91.436
80.442
91.304
101.131
101.400
124.008
110.966
72.686
Ni–O2
N1–Ni–O2
O2–Ni–O3
O3–Ni–N2
N1–Ni–N2
N1–Ni–O2
O2–Ni–O3
O3–Ni–N1
N1–Ni–N2
N1–Ni–O2
O1–Ni–O6
O2–Ni–N2
Ni–O1–Zn
Ni–O2–Zn
Cl–Zn–Cl2
Cl1–Zn–O1
O1–Zn–O2
Ni–N1
Ni–N2
NiLH
Ni–O1
Ni–O2
Ni–N1
Ni–N2
[NiLÁZnCl2Á(DMF)2]
Ni–O1
Ni–O2
Ni–N1
Ni–N2
Ni–O3 (DMF)
Ni–O4 (DMF)
Ni–Zn
Zn–O1
Zn–O2
Zn–Cl1
Zn–C2
[NiLHÁZnCl2Á(DMF)2]
Ni–O1
N1–Ni–N2
N1–Ni–O2
O1–Ni–O6
O2–Ni–N2
Ni–O1–Zn
Ni–O2–Zn
Cl–Zn–Cl2
Cl1–Zn–O1
O1–Zn–O2
98.467
90.315
Ni–O2
Ni–N1
79.881
Ni–N2
90.547
Ni–O3 (DMF)
Ni–O4 (DMF)
Ni–Zn
103.188
103.173
123.014
112.061
73.526
Zn–O1
Zn–O2
Zn–Cl1
Zn–Cl2
(b)
[NiLÁZnCl2Á(DMF)2]
Ni–O1
2.020
2.021
2.018
2.032
2.138
2.137
3.064
1.990
2.006
2.234
2.207
N1–Ni–N2
N1–Ni–O1
O1–Ni–O2
O2–Ni–N2
Ni–O1–Zn
Ni–O2–Zn
Cl1–Zn–Cl2
Cl1–Zn–O1
O1–Zn–O2
100.089
90.356
Ni–O2
Ni–N1
78.838
Ni–N2
90.729
Ni–O3 (DMF)
Ni–O4 (DMF)
Ni–Zn
100.039
100.638
118.571
111.659
79.943
Zn–O1
Zn–O2
Zn–Cl1
Zn–Cl2
123

A. Atakol et al.
Table 5 continued
˚
Bond lengths/A
Complex
Bond angles/°
Atoms
Lengths
Atoms
Angles
[NiLHÁZnCl2Á(DMF)2]
Ni–O1
2.046
2.046
2.094
2.094
2.120
2.141
3.146
2.015
2.015
2.253
2.275
N1–Ni–N2
N1–Ni–O1
O1–Ni–O2
O2–Ni–N2
Ni–O1–Zn
Ni–O2–Zn
Cl1–Zn–Cl2
Cl1–Zn–O1
O1–Zn–O2
97.015
92.594
Ni–O2
Ni–N1
77.696
Ni–N2
92.597
Ni–O3 (DMF)
Ni–O4 (DMF)
Ni–Zn
101.587
101.586
117.218
113.094
79.129
Zn–O1
Zn–O2
Zn–Cl1
Zn–Cl2
using the density functional theory (DFT) method
631G(d) basis set in Gaussian 09 software their optimized
bond lengths, bond angles, dipole moments, the HOMO
and LUMO energies in the case of probable overlaps, and
their charge distributions, the electron charges and energies
in d orbitals of the central ions have been determined. Nbo
program was both applied directly in theoretical way and
semi-experimentally to the X-ray data. Table 5a lists the
theoretically optimized bond lengths and bond angles and
5b includes the same parameters calculated by the use of
X-ray data. The HOMO and LUMO images of NiL, NiLH,
[NiLÁZnCl2Á(DMF)2] and [NiLHÁZnCl2Á(DMF)2] com-
plexes are depicted in Fig. 7a, b using theoretical
optimization.
As mentioned above, since the nitrogen donors of the
reduced ligand are of amine structure the charge densities
of the nitrogen donors are expected to be higher. That is
why the formation of NiLH mononuclear complex is an
expected outcome. However, the geometries of the struc-
tures of the overlapping molecular orbitals may change
after the reduction process. The molecular structures of
NiL and CuL complexes were reported in the literature
[41]. In NiL complex, Ni(II) is located in square planar
coordination sphere. However, the Cu(II) ion in CuL is in a
squashed tetrahedral or highly distorted square–planar
coordination sphere. As shown in Fig. 6a, the phenolic
oxygens and iminic nitrogen atoms of the ligand overlap
with dx2–y2 orbital of the central Ni(II) ion. However, this
is not the case for the structure depicted in Fig. 6b since the
nitrogen atoms of the reduced ligand do not have a suit-
able geometry to overlap the dx2–y2 orbital of the central
Ni(II) ion. It is most probably the reason for not obtaining
the NiLH molecule. But in DMF medium the unpaired
electrons of the oxygen atoms of DMF molecules overlap
with the dz2 orbitals of the Ni(II) ion. The X-ray data
clearly illustrate this situation. The distances between the
DMF oxygens and central Ni(II) ion of the [NiLÁZnCl2-
The Mulliken charges of the central ions and the donor
atoms are tabulated in Table 6, and the numerical values of
the HOMO and LUMO levels, dipole moments, elec-
tronegativities and softness of the complexes are given in
Table 7. The occupancy values of the d orbitals of the
central ions and their relative ratios of the central atoms
obtained by the help of natural bond orbital (nbo) program
with and without the use of the X-ray data are presented in
Table 8.
˚
The situation is little bit different in the calculation
carried out by the X-ray data without optimization in din-
uclear complexes. The HOMO and LUMO levels obtained
after X-ray data are shown in Fig. 8.
(DMF)2] complex are 2.1376 and 2.1380 A, and the cor-
˚
responding values are 2.0698 and 2.1062 A in
[NiLHÁZnCl2Á(DMF)2] complex. Although the nitrogen
molecules of the reduced ligand posses higher electron
density in [NiLHÁZnCl2Á(DMF)2] complex, the oxygens of
DMF molecules supply more electrons to the central Ni(II)
ion because the Ni(II) ion does not receive sufficient
electrons from the aminic nitrogens. In reality, a com-
pletely reverse situating is present since the electron den-
sity of aminic nitrogen is higher than their iminic
counterparts. This shows the fact that during the opti-
mization process no overlap of DMF oxygens is observed
From the computed values of HOMO and LUMO
energy values for the prepared complexes, the electroneg-
ativity and chemical hardness can be calculated as follows:
v = (IP ? EA)/2 (electronegativity), l = -(IP ? EA)/2
(chemical
& (IP - EA)/2 (hardness), x = l2/2g (electrophilicity
index), S = 1/g (softness), IP = -EHOMO and
potential),
g = 1/2(ELUMO - EHOMO)-
EA = -ELUMO, respectively, as shown in Table 7.
123

Synthesis, crystal structure, theoretical calculations and thermal characterization…
Fig. 7 Determined HOMO and
(a)
LUMO images of the mono
nuclear, a stable NiL and its
dinuclear [NiLÁZnCl2Á(DMF)2],
b unstable NiLH and its
stable dinuclear
[NiLHÁZnCl2Á(DMF)2] complex
by 631G(d) basis set
NiL HOMO
NiL LUMO
[NiL·ZnCl2·(DMF)2] LUMO
[NiL·ZnCl2·(DMF)2] HOMO
(b)
NiLH LUMO
NiLH HOMO
[NiLH·ZnCl2·(DMF)2] HOMO
[NiLH·ZnCl2·(DMF)2] LUMO
in dinuclear complexes. However, the effect oxygens must
be definitely present since they are coordinately bonded to
the DMF molecules. This situation is illustrated in Fig. 8
which shows HOMO and LUMO levels obtained by the use
of X-ray data. If you look at these illustrations carefully,
you can see the possibility of the DMF oxygens overlap-
ping with the dz2 orbital. Apart from that, the d orbitals
affected by the organic ligand and its reduced form’s donor
atoms are different. Although the optimization studies
show no difference between two complexes, the experi-
mental data give more realistic results. This is especially
apparent in bond lengths. Table 5a, b shows the bond
lengths and angles deduced by optimization process and
X-ray data, respectively. The data listed in Table 5b are
much closer to the data listed in Table 2.
Almost there are no differences between the experi-
mentally determined and theoretically calculated coordi-
native bonds of Ni(II) ion. However, there are big
differences between the theoretically found and experi-
mentally determined bond lengths. This shows that the
method and the basic set employed are not compatible with
the calculation of bond lengths. It is possible to obtain
more accurate bond lengths, but it is doubtful that other
data will be accurate.
The nbo data are listed in Table 8. According to these
data, the orbital with the higher energy is observed to be dxy
123

A. Atakol et al.
Table 6 Mulliken charges of
the atoms around the
coordination sphere of the
complexes
Atom
NiL
0.691
NiLH
0.581
[NiÁZnCl2Á(DMF)2]
[NiLHÁZnCl2Á(DMF)2]
Ni
0.831
- 0.779
- 0.780
- 0.484
- 0.483
- 0.488
- .512
0.682
- .797
O1
- 0.626
- 0.626
- 0.492
- 0.492
- 0.660
- 0.660
- 0.641
- 0.641
O2
- 0.804
- 0.652
- 0.690
- 0.511
- 0.538
0.762
N1
N2
O3 (DMF)
O4 (DMF)
Zn
0.745
Cl1
- 0.521
- 0.489
- 0.538
- 0.495
Cl2
Table 7 Calculated frontier
orbital energies,
Atom
NiL
NiLH
[NiÁZnCl2Á(DMF)2] [NiLHÁZnCl2Á(DMF)2]
electronegativity, hardness and
softness of complex
E/opt, a.u.
l/debye
- 2425.64876993 - 2428.02602188 - 5622.388728
- 5624.79581512
12.256
5.618
- 4.949
- 1.421
3.528
9.497
- 4.626
- 0.702
3.924
9.296
- 5.672
- 1.818
3.854
E
HOMO/eV
LUMO/eV
- 0.19979 a.u.
- 0.05141 a.u.
0.14838 a.u.
0.19979 a.u.
0.05141 a.u.
- 0.1256 a.u.
0.1256 a.u.
E
DE(Elumo–Ehomo)
I/eV
4.949
4.626
5.672
A/eV
1.421
0.702
1.818
l/eV
- 3.185
3.185
- 2.664
2.664
- 3.745
3.745
v/eV
g/eV
4.238
4.275
4.763
0.1741 a.u.
S/eV-1
0.236
0.234
0.210
0.0005744 a.u.
x/eV
for both the NiL and NiLH complexes which is lowest
unoccupied occupied orbital (LUMO). However, if both
these complexes are converted into dinuclear state the
electron distribution of d orbitals of Ni(II) ion will change
due to the contribution of the DMF oxygens in the case the
LUMO of Ni(II) changes as dyz. If the nbo program is
applied in a totally theoretical manner, there is no apparent
difference found between these two complexes. However,
thermal behaviors of two complexes are distinctively dif-
ferent. That is why the nbo program is applied again by the
use of X-ray data. The data obtained from this application
for the Ni(II) and Zn(II) central atoms are given in Table 9.
The results obtained in Table 9 are similar to the data
listed in Table 8, and the biggest difference is the highest
energy orbital of Ni(II) which is found to be different. In
purely theoretical calculations, there are no difference
between these two dinuclear complexes where the highest
energy semi-occupied orbital was found to be dzx for both
[NiLÁZnCl2Á(DMF)2] and [NiLHÁZnCl2Á(DMF)2] com-
plexes. However, the difference between two complexes
becomes clear when X-ray data are used. Since X-ray data
are experimental values and that bond lengths and bond
angles found by the application of 631G(d) basis set are
almost the same, the experimental values prove that the
data listed in Table 9 are logical. Because the orbitals
directly facing DMF molecules are dz2 orbitals, they can
easily overlap with the oxygen atoms of DMF molecules
which decrease their energies. In reduced Schiff bases, the
geometry is quite suitable for two oxygens to directly
overlap with dx2–y2 and dxy orbitals. Under this situation,
the energy of the orbital located between x and z axis is
increased and the most energetic orbital becomes dxz. In
[NiLHÁZnCl2Á(DMF)2] complex, two nitrogen atoms have a
suitable geometry to overlap with the dx2–y2 orbital of the
central ion. Under this situation, the dyz orbital with the
least overlapping capacity becomes the most energetic
orbital. The basis sets embedded in theoretical programs
cannot explain all these features. As seen, they can suc-
cessfully explain one feature, while they fail to say any-
thing about the others. However, these programs are
123

Synthesis, crystal structure, theoretical calculations and thermal characterization…
Table 8 Calculated electron
Complex
Orbital
Central atoms
Ni
occupied values and relative
energies of the d orbitals of the
central atoms
Zn
Charge/e
Relative energy/eV
Charge/e
Relative energy/eV
NiL
dxy
0.3286
- 0.14287
- 0.22360
- 0.22521
- 0.20962
- 0.27000
- 0.13594
- 0.22548
- 0.22451
- 0.19986
- 0.26640
- 0.23146
- 0.24076
- 0.17048
- 0.26710
- 0.24498
- 0.22570
- 0.20358
- 0.17920
- 0.26232
- 0.23806
dzx
1.86985
1.90097
1.86360
1.96873
0.69687
1.91310
1.95203
1.84261
1.96573
1.74573
1.91215
0.88892
1.92471
1.90727
1.88908
1.46521
1.10823
1.92327
1.97783
dyz
dz2
dx2–y2
dxy
NiLH
dzx
dyz
dz2
dx2–y2
dxy
[NiLÁZnCl2Á(DMF)2]
1.99610
1.98680
1.98891
1.98895
1.98336
1.98848
1.98755
1.98802
1.98784
1.99542
- 0.42413
- 0.42060
- 0.42025
- 0.42107
- 0.42161
- 0.40924
- 0.40738
- 0.40764
- 0.40797
- 0.41135
dzx
dyz
dz2
dx2–y2
dxy
[NiLHÁZnCl2Á(DMF)2]
dzx
dyz
dz2
dx2–y2
Fig. 8 HOMO and LUMO
levels of the dinuclear
[NiLÁZnCl2Á(DMF)2] and
[NiLHÁZnCl2Á(DMF)2]
complexes determined by
631G(d) basis set using X-ray
data without optimization
[NiL·ZnCl2·(DMF)2] LUMO
[NiL·ZnCl2·(DMF)2] HOMO
[NiLH·ZnCl2·(DMF)2] HOMO
[NiLH·ZnCl2·(DMF)2] LUMO
123

A. Atakol et al.
Table 9 Calculated electron
occupied value and relative
energy results of the d orbitals
of the central atoms obtained by
the use nbo program using
X-ray data
Complex
Orbital
Central atoms
Ni
Zn
Charge/e
Relative energy/eV
Charge/e
Relative energy/eV
[NiÁZnCl2Á(DMF)2]
dxy
1.96662
0.57099
1.84540
1.96297
1.93723
1.80305
1.97565
0.59942
1.97946
1.97163
- 0.20904
- 0.13126
- 0.20477
- 0.20510
- 0.24688
- 0.19696
- 0.20523
- 0.12728
- 0.20973
- 0.24019
1.99531
1.98620
1.98623
1.98535
1.98913
1.99554
1.98574
1.98826
1.98614
1.98884
- 0.39522
- 0.39119
- 0.39228
- 0.39306
- 0.39306
- 0.40858
- 0.40565
- 0.40688
- 0.40688
- 0.40609
dzx
dyz
dz2
dx2–y2
dxy
[NiLHÁZnCl2Á(DMF)2]
dzx
dyz
dz2
dx2–y2
Acknowledgments Funding was provided by Ankara Universitesi
(TR) (Grant No. 13L42.40018).
constantly being improved. In this study, the behavioral
differences between the two strictly similar dinuclear
complexes have been investigated by the use of theoretical
calculations. The use of experimental data reveals the
difference between these complexes. However, the opti-
mization is also quite successful and gives the idea why the
NiLH complex is unstable. Figure 6b shows that the
nitrogen atoms in NiLH mononuclear complex do not make
any overlapping which may be the prime reason for the
failure of the synthesis of these complexes. This also
explains the difference in the thermal behaviors of these
two dinuclear complexes. In spite of the concerted efforts,
it is not possible to synthesize NiLH mononuclear complex
and there were no report in the literature about it. It can be
prepared by the use of other metals since the change in
d electrons changes the probability of overlapping.
Recently synthesis of CuLH mononuclear complex has
been reported [58].
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