Synthesis, spectroscopy, electrochemistry and thermogravimetry of copper(II) tridentate Schiff base complexes, theoretical study of the structures of compounds and kinetic study of the tautomerism reactions by ab initio calculations

Article abstract of DOI:10.1016/j.saa.2012.12.010

Attempts to spectroscopic and structural study of copper complexes, some Cu(II) Schiff base complexes were synthesized and characterized by means of electronic, IR, ¹HNMR spectra and elemental analysis. The thermal analyses of the complexes wer

Full text of DOI:10.1016/j.saa.2012.12.010

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 374–382
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectroscopy, electrochemistry and thermogravimetry of copper(II)
tridentate Schiff base complexes, theoretical study of the structures of
compounds and kinetic study of the tautomerism reactions by ab initio calculations
b
c
d
Ali Hossein Kianfar a, , Shapour Ramazani , Roghaye Hashemi Fath , Mahmoud Roushani
⇑
a Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
b Department of Chemistry, Yasouj University, Yasouj, Iran
c Young Researchers Club, Gachsaran Branch, Islamic Azad University, Gachsaran, Iran
d Department of Chemistry, Ilam University, Ilam, Iran
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
The new complexes were
synthesized by a simple method.
Article contains some scopes like ab
initio calculations of ligands and
complexes.
"
"
"
"
The ligands exist as keto-amine/
enol-imine tautomeric forms.
Cu(II) ion is in square-planar NO3
coordination geometry.
Paper contains Inorganic, organic,
analytical and physical chemistry.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 October 2012
Received in revised form 2 December 2012
Accepted 6 December 2012
Available online 28 December 2012
Attempts to spectroscopic and structural study of copper complexes, some Cu(II) Schiff base complexes
were synthesized and characterized by means of electronic, IR, 1HNMR spectra and elemental analysis.
The thermal analyses of the complexes were investigated and the first order kinetic parameters were
derived for them. The cyclic voltammetric studies in acetonitrile were proposed a monomeric structure
for complexes. The structures of compounds were determined by ab initio calculations. In the solid state,
the ligands exist as keto-amine/enol-imine tautomeric forms with an intramolecular hydrogen bond
(NAHꢀ ꢀ ꢀO) between amine and carbonyl group. The kinetic studies of the tautomerism and equilibrium
constant of the reactions were calculated using transition state theory. The optimized molecular geom-
etry and atomic charges were calculated using MP2 method with 6-31G(d) basis set for H, C, N and O
atoms and LANL2DZ for the Cu atom.
Keywords:
Schiff base ligands
Copper Schiff base complexes
Electrochemistry: thermogravimetry
Ab initio calculations
The results suggested that, in the complexes, Cu(II) ion is in pseudo square-planar NO3 coordination
geometry. Also the bond lengths and angles were studied and compared.
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
states. The p-system in a Schiff base often imposes a geometrical
constriction and affects the electronic structure as well. Transition
metal Schiff base complexes have been studied as catalysts in
organic redox and electrochemical reduction reactions [1–5].
Transition metal complexes are important in many areas of
science, including catalysis, medicine, modifier of nanostructure
compounds, design of high value materials, analytical chemistry.
In addition, they are used as model compounds of the structure
Schiff bases have been widely used as ligands because of the
high stability of coordination compounds with different oxidation
⇑
Corresponding author. Tel.: +98 311 3913251; fax: +98 311 3912350.
E-mail addresses: akianfar@cc.iut.ac.ir, asarvestani@yahoo.com (A.H. Kianfar),
ramazani@mail.yu.ac.ir (S. Ramazani).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.12.010

A.H. Kianfar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 374–382
375
and function of metalloproteins [6–10]. The keto-amine/enol-
imine tautomerism is reported for Schiff bases derived from b-
diketones [11,12]. The theoretical study of Schiff bases with tau-
tomerism forms were investigated previously [13,14].
(metrom glassy carbon, 0.0314 cm2) were employed for the elec-
trochemical studies. Voltammetric measurements were performed
at room temperature in acetonitrile solution with 0.1 M tetrabutyl-
ammonium perchlorate as the supporting electrolyte.
The present study describes the synthesis of Schiff base ligands
derived from the reactions of acetylacetone with 2-aminophenol
and 2-amino-4-chlorophenol and the reaction of benzoylacetone
with 2-aminophenol. The copper(II) complexes of synthesized li-
gands were prepared in methanol (Figs. 1 and 2) and identified
by IR, NMR, UV–Vis spectroscopy and elemental analysis. The ther-
mal analyses of the studied complexes were investigated. From
thermal decomposition data, the kinetic’s parameters were calcu-
lated using Coats and Redfern [15] method. The structures of com-
pounds were determined by ab initio calculations. The keto-amine/
enol-imine conversion was investigated by ab initio quantum
chemical calculations in order to reveal the stability of the different
tautomers and the possible formation of order conformers. The
conjugation of the different tautomers was examined in order to
understand the differences in their relative stability. The kinetic
studies of the tautomerism and equilibrium constant of the reac-
tions were calculated using transition state theory. The optimized
molecular geometry and atomic charges were calculated using
MP2 method with 6-31G(d) basis set for H, C, N and O atoms and
LANL2DZ for the Cu atom.
Synthesis of Schiff base ligands
Synthesis of 2-(3-hydroxy-1-methyl-but-2-enylideneamino)-phenol
(H2L1)
The Schiff base ligand, H2L1, was prepared by adding a metha-
nolic solution of 2-aminophenol (0.546 g, 5 mmol) to acetylacetone
(0.514 ml, 5 mmol) in methanol and refluxed for 4 h. The reaction
mixture was filtered, reduced to one-third of its original volume,
and left to crystallize at room temperature. The light yellow crys-
tals were isolated from the solution and dried in vacuum [10].
Yield: 82%, m.p ꢂ 183 °C. IR (KBr pellets, cmꢁ1): 3300 (
mNH), 3000
(mOH), 1600 (m
C@N/C@O). 1HNMR (DMSO, dH): 12.14 (s, 1H, NH/OH),
9.91 (s, 1H, phenilic OH), 6.77–7.16 (m, 4H, Aromatic protons),
5.18 (s, 1H, C(3)AH), 1.99 and 1.94 (s, 6H, CH3), ppm., kmax (nm)
(e
, L molꢁ1 cmꢁ1) (Ethanol): 206 (10000), 233 (3900), 320 (11500).
Synthesis of 2-(3-hydroxy-1-methyl-but-2-enylideneamino)-4-
chlorophenol (H2L2)
The Schiff base ligand, H2L2, was prepared by adding a metha-
nolic solution of 2-amino-4-chlorophenol (0.718 g, 5 mmol) to ace-
tylacetone (0.514 ml, 5 mmol) in methanol and refluxed for 6 h.
The light green crystals were isolated from the solution at room
temperature and dried in vacuum. Yield: 79%, m.p ꢂ 160 °C. IR
Experimental
Chemicals and apparatus
(KBr pellets, cmꢁ1): 3100–2400 ( C@N/C@O). 1HNMR
mNH/OH), 1600 (m
All of the chemicals and solvents used for the synthesis were of
commercially available reagent grade and they were used without
purification. Infrared spectra were recorded as KBr discs on a FT-IR
JASCO-680 spectrophotometer in the 4000–400 cmꢁ1. The elemen-
tal analysis was determined on a CHN-O-Heraeus elemental anal-
(DMSO, dH): 12.14 (s, 1H, NH/OH), 10.24 (s, 1H, phenolic OH),
6.87–7.26 (m, 3H, Aromatic protons), 5.22 (s, 1H, C(3)AH),
2.01and 1.96 (s, 6H, CH3), ppm., kmax (nm) (e
, L molꢁ1 cmꢁ1) (Eth-
anol): 207 (7300), 238 (3300), 326 (7300).
yzor. UV–Vis spectra were recorded on
a
JASCO V-570
spectrophotometer in the 190–900 nm. The 1HNMR spectra were
recorded in DMSO-d6 on DPX-400 MHz FT-NMR. Thermogravime-
try (TG) and differential thermoanalysis (DTA) were carried out on
a PL-1500. The measurements were performed in air atmosphere.
Synthesis of 2-(3-hydroxy-1-methyl-3-phenyl-prop-2-
enylideneamino)-phenol (H2L3)
The Schiff base ligand, H2L3, was prepared by adding a metha-
nolic solution of 2-aminophenol (0.546 g, 5 mmol) to benzoylace-
tone (0.811 g, 5 mmol) in methanol and refluxed for 2 h. The
yellow crystals were isolated from the solution at room tempera-
ture and dried in vacuum. Yield: 84%, m.p ꢂ 166 °C. IR (KBr pellets,
The heating rate was kept at 10 °C minꢁ1
.
Cyclic voltammograms were performed using an autolab mode-
lar electrochemical system (ECO Chemie, Ultrecht, The Nether-
lands) equipped with a PSTA 20 module and driven by GPES
(ECO Chemie) in conjunction with a three-electrod system and a
personal computer for data storage and processing. An Ag/AgCl
(saturated KCl)/3 M KCl reference electrode, a Pt wire as counter
electrode and a glassy carbon electrode as working electrode
cmꢁ1): 3100–2400 ( C@N/C@O). 1HNMR (DMSO, dH):
mNH/OH), 1600 (m
12.80 (s, 1H, NH/OH), 10.03 (s, 1H, phenolic OH), 6.96–7.91 (m,
9H, Aromatic protons), 6.02 (s, 1H, C(3)ꢁH), 2.13 (s, 3H, CH3),
ppm., kmax (nm) (e
, L molꢁ1 cmꢁ1) (Ethanol): 206 (25300), 244
(13600), 355 (25700).
Fig. 1. Structure of Schiff base ligands and their tautomerism equilibrium: keto-amine and enol-imine.

376
A.H. Kianfar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 374–382
Fig. 2. Proposed molecular structure of metal complexes (D–F).
Metal complexes synthesis
of 2400–3300 cmꢁ1 and disappeared in the spectra of the com-
plexes. The strong band at 1600 cmꢁ1 can be related to C@N and/
or C@O bonds [18]. The IR spectra of the complexes show a strong
band at 1470–1540 cmꢁ1 due to (C@N) [22,23]. The C@C vibra-
tional frequencies appeared in the range of 1470–1500 cmꢁ1. These
bands are shifted towards lower frequencies in the spectra of metal
complexes compared with the free Schiff bases indicating the
involvement of the azomethine nitrogen in chelation with the me-
tal ions, the coordination of nitrogen to the metal ion would be ex-
pected to reduce the electron density of the azomethine link and
thus causes a shift in the m(C@N) group [24]. The vibrational peak
A methanolic solution of the appropriate amount of cop-
per(II)acetate monohydrate was added to a hot methanolic solu-
tion of the Schiff base ligand. The resulting mixture was stirred
under reflux for 30 min during which the metal complexes were
precipitated. The precipitate was filtered off and washed with
methanol and dried in vacuum.
[CuL1(H2O)] (D) is prepared from Cu(OAc)2ꢀH2O (0.4 g, 2 mmol)
and H2L1 (0.382 g, 2 mmol). Yield: 67%. m.p ꢂ 282 °C. Anal. Calc. for
C
11H13NO3Cu: C, 49.82; H, 4.28; N, 6.19. Found: C, 48.79; H, 4.83;
N, 5.17%. IR (KBr pellets, cmꢁ1): 1560 (
mC@N), 526 (
mCuAO), 482
of coordinated water is in the range of 3400–3450 cmꢁ1
.
(m
CuAN). kmax (nm) (
e
, L molꢁ1 cmꢁ1) (Ethanol): 217 (16700), 242
(9900), 275 (9900), 315 (4500), 372 (12900).
Electronic absorption spectra
[CuL2(H2O)] (E) was prepared from Cu(OAc)2ꢀH2O (0.4 g,
2 mmol) and H2L2 (0.451 g, 2 mmol). Yield: 65%. m.p ꢂ 294 °C.
Anal. Calc. for C11H12NO3ClCu: C, 44.06; H, 3.38; N, 4.36. Found:
The electronic spectra of free ligands under study in ethanol
solution were characterized mainly by three absorption bands in
the region of 900–200 nm assigned to
complexes, these transitions were found to be shifted to lower or
higher energy region compared to the free ligands transitions con-
firming the coordination of the ligands to metal ions [17]. In addi-
tion, a d–d transition appeared in the range of 315–333 nm region.
C, 43.28; H, 4.28; N, 4.58%. IR (KBr pellets, cmꢁ1): 1570 (
m
C@N),
p–
pꢃ transition. In metal
540 (mCuAO), 480 (mCuAN). kmax (nm) (e
, L molꢁ1 cmꢁ1) (Ethanol):
222 (22500), 245 (16400), 277 (14400), 321 (8400), 377 (18100).
[CuL3(H2O)] (F) was prepared from Cu(OAc)2ꢀH2O (0.4 g,
2 mmol) and H2L3 (0.506 g, 2 mmol). Yield: 70%. m.p ꢂ 280 °C. Anal.
Calc. for C16H15NO3Cu: C, 58.90; H, 3.98; N, 5.19. Found: C, 57.73; H,
4.53; N, 4.20%. IR (KBr pellets, cmꢁ1): 1580 (
m
e
sy(C@N)), 1560
1HNMR spectra
(m
asy(C@N)), 520 (mCuAO), 490 (mCuAN). kmax (nm) (
, L molꢁ1 cmꢁ1
)
(Ethanol): 212 (14900), 254 (15200), 278 (12800), 332 (8200),
407 (17100).
The chemical shift (d, ppm) of different protons of the Schiff
base ligands is presented in Sections ‘Synthesis of 2-(3-hydroxy-
1-methyl-but-2-enylideneamino)-phenol (H2L1), Synthesis of 2-
(3-hydroxy-1-methyl-but-2-enylideneamino)-4-chlorophenol
(H2L2) and Synthesis of 2-(3-hydroxy-1-methyl-3-phenyl-prop-2-
enylideneamino)-phenol (H2L3)’.
Computational calculations
Ab initio calculations were carried out using the Gaussian 03
program [16]. The geometries of all the stationary points were
optimized at the MP2 [16] level with the 6-31G(d) basis set for
H, C, N and O atoms and LANL2DZ for the Cu atom. Harmonic vibra-
tional frequencies were obtained at B3LYP/6-31G(d) level in order
to characterize stationary points as local minima or first-order sad-
dle points. The number of imaginary frequencies (0 or 1) indicate
whether a minimum or a transition state was located. The atomic
charges were calculated at the MP2 method with 6-31G(d) basis
set.
The 1H NMR of ligands is in line with the preposed structures.
The NH/OH proton was seen in the range of 12.14–12.80 ppm as
singlet. The phenolic hydrogen was seen in the range of 9.91–
10.03 ppm as singlet. The proton of alkene was seen in the range
of 5.18–6.02 ppm as singlet. The aromatic hydrogens were seen
in 6.77–7.91 ppm region as multiplet. The hydrogens of methyl
group were seen in 1.96–3.38 ppm region as singlet.
Thermogravimetric analyses
Results and discussion
The thermal decomposition of the copper complexes showed
characteristic pathways, depending on the nature of the ligands,
as can be seen from the TG/DTA curves in Fig. 3. The absence of
weight loss up to 80 °C indicates that there is no water molecule
in the crystalline solid. In these complexes, the TG showed weight
loss up to 250 °C indicates the presence of solvent molecule coor-
dinated with complexes [25–27].
IR spectra
The mode of binding Schiff base ligands to the metal ions was
elucidated by recording the IR spectra of the complexes as com-
pared with the spectra of the free ligands [17]. A keto-amine/
enol-imine structure for Schiff-base ligands derived from diketones
and diamines was proposed and shows that the hydrogen is
bonded to both oxygen and nitrogen simultaneously (NH/OH)
[18–21]. The vibrational band of (NH/OH) is observed in the range
The complex of [CuL1(H2O)] decomposes in one step. The
decomposition occurs up to about 250 °C and is attributed to the
release of water and C11H11NO (TG = 69.30; calc. = 70.60%) resulted
in the formation of the CuO at 349 °C (TG = 30.70%; calc. = 29.40%).

A.H. Kianfar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 374–382
377
Fig. 3. TG and DTG curve of: CuL2(H2O).
W
f
logWf
T2
ꢁW
Fig. 4. Coats–Redfern plots of [CuL2(OH2)] complex, step 1, A ¼
.
The complex of [CuL2(H2O)] decomposes in two steps. The first
step occurs up to about 256 °C and is attributed to the release of
water and C10H10N (TG = 50.13; calc. = 51.20%). The second step
of thermal decomposition which occurred in the range of 367–
452 °C (TG = 9.50%; calc. = 11.10%), resulted in the formation of a
mixture of CuO and CuCl2 at 452 °C.
The complex of [CuL3(H2O)] decomposes in three steps. The first
step occurred up to about 278 °C and was attributed to the release of
water and C6H4NO (TG = 35.70%; calc. = 37.30%). The second ther-
mal event was observed between 335 and 370 °C (TG = 17.20%;
calc. = 15.60%). The third mass loss between 375 and 444 °C was
assigned to the rest of the ligand (TG = 21.70%; calc. = 23.10%) with
the formation of the CuO (TG = 25.10%; calc. = 24.20%).
Fig. 5. Optimized structures of H2L1 (A).
Kinetics aspects
All the well-defined stages were selected to study kinetics
decomposition of the complexes. The kinetics parameters (the acti-
vation energy E and the pre-exponential factor A) were calculated
using the Coats–Redfem equation [15],
ꢀ
ꢁ
ꢀ
ꢁ
gð
a
Þ
AR
/E
2RT
Ea
Ea
log
¼ log
1 ꢁ
ꢁ
ð1Þ
T2
2:303RT
Table 1
Thermal and kinetics parameters for copper complexes.
Compound
D
T (°C)a
Percent (%)b
Eꢃ (kJ/mol)
Aꢃ (sꢁ1
)
Sꢃ (kJ molꢁ1 Kꢁ1
)
Hꢃ (kJ/mol)
Gꢃ (kJ/mol)
CuL1(H2O)
CuL2(H2O)
250–349
256–319
367–452
278–331
335–370
375–444
69.30(70.60)
50.13(51.20)
9.50(11.10)
35.70(37.30)
17.20(15.60)
21.70(23.10)
89.37
117.73
22.22
201.79
81.02
2.83 ꢄ 105
2.00 ꢄ 108
4.25 ꢄ 100
9.66 ꢄ 1015
1.23 ꢄ 106
3.35 ꢄ 102
ꢁ141
ꢁ85
86.87
115.33
18.81
199.29
78.11
129
140
115
181
124
142
ꢁ236
61
CuL3(H2O)
ꢁ130
63.89
ꢁ199
60.51
a
The temperature range of decomposition pathways.
Percent of weight lose found (calculated).
b

378
A.H. Kianfar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 374–382
Fig. 7. Optimized structures of H2L3 (C).
Fig. 6. Optimized structures of H2L2 (B).
Ea ¼ H– þ RT;
G– ¼ H– ꢁ TS–
ð3Þ
ð4Þ
where g(
a
) = [Wf)/(Wf ꢁ W)]. In the present case, a plot of L.H.S (left
hand side) of this equation againest 1/T gives straight line (Fig. 4)
whose slope and intercept are used to calculate the kinetic param-
eters by the least square method. The goodness of fit was checked
by calculating the correlation coefficient. The other systems and
their steps show the same trend.
The various kinetics parameters calculated are given in Table 1.
The activation energy (Ea) in the different stages are in the range of
22.22–117.73 kJ molꢁ1. The respective values of the pre-exponen-
tial factor (A) vary from 4.25 ꢄ 100 to 9.66 ꢄ 1015 sꢁ1. The corre-
sponding values of the entropy of activation (S–) are in the range
of ꢁ236 to 61 J molꢁ1. The corresponding values of the enthalpy
of activation (H–) are in the range of 18.81–199.39 kJ molꢁ1. The
corresponding values of the free energy of activation (G–) are in
the range of 115–181 kJ molꢁ1. Due to the unstable intermediate
the activation energy of the later stages for [CuL2(H2O)] and
[CuL3(H2O)] were smaller than that of the first one. The negative
values of entropy of activation indicate that the activated complex
has a more ordered structure than the reactants [28,29].
The entropy of activation S– was calculated using the following
equation
–
kTs
S
R
A ¼
e
ð2Þ
h
where k, h and Ts are the Boltsman’s constant, the Planck’s constant
and the peak temperature, respectively. The enthalpy and free en-
ergy of activation were calculated using Eqs. (3) and (4).

A.H. Kianfar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 374–382
379
Table 3
Significent bond lenghts and bond angles calculated at the MP2 method with 6-
31G(d) basis set for H, C, N and O atoms and LANL2DZ for the Cu atom.
Structural parameter
CuL1(H2O)
CuL2(H2O)
CuL3(H2O)
Bond lenghts (Å)
O(1)ACu(13)
N(5)ACu(13)
O(12)ACu(13)
O(14)ACu(13)
O(1)AC(2)
1.86
1.90
1.88
1.93
1.42
1.34
1.88
1.88
1.84
1.95
1.36
1.36
1.87
1.89
1.88
1.97
1.36
1.36
C(11)AO(12)
Bond angles (°)
O(1)ACu(13)AN(5)
O(1)ACu(13)AO(14)
N(5)ACu(13)AO(12)
O(12)ACu(13)AO(14)
O(1)ACu(13)AO(12)
N(5)ACu(13)AO(14)
91.94
77.39
91.74
98.95
162.84
169.20
98.84
77.50
95.72
87.94
165.44
176.25
104.36
79.01
93.22
83.41
162.42
176.38
Molecular structure and analysis of bonding modes
The structures of the ligands were determined by ab initio cal-
culations. The different tautomerism conversion was investigated
to compare the stability of the different tautomers and the best
structure conformers for the ligands and the geometry of copper
complexes.
The geometries of the Schiff base ligands and complexes are
represented in Figs. 5–8. Significant bond lengths and bond angles
are also reported in Tables 2 and 3. It is shown that in the com-
plexes, Cu(II) ion is in square-planar NO3 coordination geometry.
Because of the different character of the atoms forming around
the Cu(II) ion, the coordination square is distorted. The bond angle
of O(1)ACu(13)AO(14) is smaller than O(1)ACu(13)AN(5),
N(5)ACu(13)AO(12) and O(12)ACu(13)AO(14). It is may be due
to the intramolecular hydrogen bonds between the enolate oxygen
with the hydrogens of the H2O molecule in the proposed structure.
In this article, we also investigated the reactions R1, R2 and R3
which are hydrogen abstraction reaction by oxygen or nitrogen
atoms. Schematic potential energy diagram at the B3LYP/6-
31G(d) level is shown in Fig. 9. The calculated relative energies
at theory level mentioned above are shown in Fig. 10. In the reac-
tion R1 oxygen number 1 in the A-1 structure receives hydrogen
from NAH bond to produce A-2. This reaction proceeds through
transition states TS1 with imaginary frequency 1150.5i cmꢁ1. As
demonstrated in Figs. 9 and 10, the barrier energy for this reaction
is 15.4 kJ molꢁ1. It was shown that the structure of A-1 is
12.9 kJ molꢁ1 more stable than the structure of A-2 for H2L1 ligand.
There are similar pathways in R2 and R3 reactions, with transition
state in the top of minimum energy path, TS2 and TS3 for which the
imaginary frequencies are 1125.8i cmꢁ1 and 1249.4i cmꢁ1 and the
Fig. 8. Optimized structures of complexes.
Table 2
Significant bond lenghts and bond angles calculated at the MP2/6-31G(d) level.
Structural parameter
H2L1
A-1
H2L2
B-1
H2L3
C-1
TS1
A-2
X-raya
TS2
B-2
TS3
C-2
Bond lenghts (Å)
O(1)AC(2)
C(4)AN(5)
N(5)AC(6)
C(11)AO(12)
1.26
1.34
1.34
1.27
1.34
1.41
1.37
1.26
1.27
1.34
1.26
1.34
1.34
1.37
1.42
1.36
1.31
1.42
1.37
1.31
1.41
1.37
1.37
1.42
1.36
1.34
1.41
1.36
1.31
1.41
1.36
1.37
1.43
1.37
1.32
1.42
1.37
1.31
1.41
1.37
Bond angles (°)
C(4)AN(5)AC(6)
N(5)AC(6)AC(7)
C(6)AC(11)AO(12)
C(10)AC(11)AO(12)
123.77
122.23
123.01
117.71
118.65
122.07
120.75
118.98
122.22
124.32
120.58
119.09
131.50
124.72
115.94
123.60
123.78
121.73
123.30
117.68
128.77
122.52
121.84
117.11
122.22
123.92
120.68
119.16
122.73
122.14
120.89
119.06
118.70
122.06
120.79
118.97
122.53
124.45
120.65
119.00
a
X-ray data for H2L1 taken from Ref. [10].

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A.H. Kianfar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 374–382
Table 5
Energy of activation for forward reaction Ea,f (kJ molꢁ1), energy of activation for
reverse reaction Ea,r (kJ molꢁ1), standard free energy for forward reaction
DG0f ðkJ molꢁ1Þ, standard free energy for reverse reaction DG0r ðkJ molꢁ1Þ, equilibrium
constant for forward reaction Kf and equilibrium constant for reverse reaction Kr for
R1, R2 and R3 reactions.
Energy
Rn
R1
R2
20.1
R3
21.5
Ea,f
Ea,r
15.4
2.5
8.8
2.6
D
Gf0
Gr0
14.0
11.1
19.5
ꢁ14.0
ꢁ11.1
0.01
89.59
ꢁ19.5
D
Kf
Kr
3.50 ꢄ 10ꢁ3
3.85 ꢄ 10ꢁ4
285.75
259.12
The relative energies were corrected with zero point energies. Zero
point energies and dipole moments of the individual isomers of li-
gands are presented in Table 4.
Standard free energies, activation energies and equilibrium con-
stants for the forward and reverse of R1, R2 and R3 reactions were
reported in Table 5.
The Arhinus equation is
k ¼ A expðꢁEa=RTÞ
ð5Þ
where k is the rate constant, Ea is the activation energy, A is preex-
ponential factor, R is the gas constant and T is temperature. Accord-
ing to this equation and the above results (Table 5), k(R1) in the
forward and reverse directions are greater than k(R2) and k(R3).
Equilibrium constant for these reactions are also calculated to
show the ratio of concentration of products to concentration of
reactants.
Natural bond orbital (NBO) analysis yields valuable information
concerning the electronic structure of ligands and their metal com-
plexes. In ligand H2L1, N(5)AH, O(1)AH and the phenolic O(12)AH
are polar covalent bonds with ꢁ0.70, ꢁ0.79 and ꢁ0.76 charges,
respectively. In the complex CuL1(H2O), Cu(13)AN(5), Cu(13)AO(1)
and Cu(13)AO(12) bonds are ionic with ꢁ0.52, ꢁ0.98 and ꢁ0.98
charges on the coordinated nitrogen and oxygens, respectively.
The negative charge on nitrogen coordinated to metal was less
than the free liagand. The other systems show the same trends.
The atomic charges are represented in Table 6 which are from nat-
ural population analysis (NPA) for ligands and the metal complexes
with NO3 coordination. It could be argued that the amine form of
Fig. 9. Potential energy surface for R1, R2 and R3 reactions.
barrier energies are 20.1 kJ molꢁ1 and 21.5 kJ molꢁ1, respectively.
Relative energies of different species which were corrected with
zero point energies are shown in these structures. The results show
that keto-amine (B-1 and C-1) structures are more stable than
enol-imine (B-2 and C-2) for H2L2 and H2L3 ligands, respectively.
Fig. 10. Relative energies of different species for reactions R1, R2 and R3 in kJ molꢁ1 at the MP2/6-31G(d) level. All values are corrected with zero point energies.
Table 4
Relative energies (with zero point energy correction)
level using a 6-31G(d) basis set.
D
E (kJ molꢁ1), zero point energys ZPE (kJ molꢁ1) and dipole moments
l (D) of the isomers of ligands calculated at the MP2
H2L1
D
E
ZPE
l
H2L2
D
E
ZPE
l
H2L3
D
E
ZPE
l
A-1
TS1
A-2
0.0
15.4
12.9
584.5
582.5
583.6
3.36
3.03
3.45
B-1
TS2
B-2
0.0
20.1
11.3
559.4
559.3
558.1
3.27
2.79
3.89
C-1
TS3
C-2
0.0
21.5
19.0
726.5
722.7
724.1
2.29
3.26
3.58

A.H. Kianfar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 374–382
381
Table 6
Atomic charges (e) calculated from natural population analysis at the MP2/6-31G(d) level.
Atom
Compound
A-1
A-2
D
B-1
B-2
E
C-1
C-2
F
N(5)
O(1)
O(12)
O(14)
Cu(13)
ꢁ0.70
ꢁ0.78
ꢁ0.76
–
ꢁ0.70
ꢁ0.79
ꢁ0.76
–
ꢁ0.52
ꢁ0.98
ꢁ0.98
ꢁ1.08
1.67
ꢁ0.71
ꢁ0.77
ꢁ0.76
–
ꢁ0.70
ꢁ0.78
ꢁ0.76
–
ꢁ0.69
ꢁ1.01
ꢁ0.99
ꢁ1.06
1.66
ꢁ0.74
ꢁ0.74
ꢁ0.76
–
ꢁ0.69
ꢁ0.78
ꢁ0.76
–
ꢁ0.62
ꢁ1.02
ꢁ0.96
ꢁ1.02
1.66
–
–
–
–
–
–
Table 7
Redox potential data of Cu(II) complexes in acetonitrile solution.
Compound
Epa (II ? III)
Epc (III ? II)
Epc (II ? I)
Epa (I ? II)
CuL1(H2O)
CuL2(H2O)
CuL3(H2O)
0.58
0.63
0.82
–
–
0.64
ꢁ0.34
ꢁ0.40
ꢁ1.04
ꢁ0.25
ꢁ017
–
case, the corresponding anodic peak is observed in the range from
ꢁ0.17 to ꢁ0.25 V, except for [CuL1(H2O)] complex, which shows to-
tally irreversible behaviour.
The anodic and cathodic process contain one peak in oxidation
and reduction area and show that, the synthesized complexes con-
tains one cupper atom in a monomer form [30,31].
Conclusions
Considering the spectroscopy, thermogravimmetrical and ab
initio calculations over the Schiff base ligands and copper com-
plexes showed the following results.
1. The IR, 1HNMR and elemental analysis confirmed that the syn-
thesised ligands and complexes contain the proposed formula.
2. The thermal decomposition pathways of the studied Schiff base
complexes are first order in all steps.
3. The keto-amine is more stable than the enol-imine form of the
ligands.
4. Keto-amine and enol-amine forms can convert to each other via
a transition state.
5. In the complexes, Cu(II) ion has square-planar NO3 coordination
geometry.
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