Synthesis and spectroscopic properties of a copper(II) binuclear complex of a novel tetradentate asymmetrical Schiff base ligand and its DFT study

Article abstract of DOI:10.1016/j.poly.2015.10.019

A new asymmetrical Schiff base ligand, N,N′-(salicylidene)-5-bromo-2-hydroxybenzaldehyde-p-phenylenediamine (SBHBP), and its binuclear Cu(II) complex have been synthesized. The molecular structures and spectroscopic properties of the ligand and its complex were experimentally characterized by elemental analysis, FT-IR, NMR and UV-Vis spectroscopic techniques and computationally by the density functional theory (DFT) method. The molecular geometry and vibrational frequencies of the SBHBP ligand in the ground state have been calculated using the B3LYP/6-31G(d,p) and 6-31++G(d,p) basis sets. A potential surface scan study was carried out and the most stable geometry of the SBHBP ligand was confirmed. The calculated results show that the optimized geometry can well reproduce the structural parameters, and the theoretical vibrational frequencies are in good agreement with the experimental values. On calculation of the electronic absorption spectra, TD-DFT calculations were carried out in both the gas and solution phases. The energetic behavior of the SBHBP ligand has been examined in solvent media using the B3LYP method with the 6-31G(d,p) and 6-31++G(d,p) basis sets by applying the Onsager method and polarizable continuum model (PCM). Additionally, Mulliken and NBO atomic charges, NMR analysis, molecular electrostatic potential (MEP) and frontier molecular orbitals (FMO) analysis of the SBHBP ligand were investigated using theoretical calculations.

Full text of DOI:10.1016/j.poly.2015.10.019

Accepted Manuscript
Synthesis and spectroscopic properties of a copper(II) binuclear complex of a
novel tetradentate asymmetrical Schiff base ligand and its DFT study
Iman Rajaei, Seyed Nezamoddin Mirsattari
PII:
S0277-5387(15)00600-2
DOI:
http://dx.doi.org/10.1016/j.poly.2015.10.019
POLY 11604
Reference:
Polyhedron
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Accepted Date:
2 June 2015
6 October 2015
Please cite this article as: I. Rajaei, S.N. Mirsattari, Synthesis and spectroscopic properties of a copper(II) binuclear
complex of a novel tetradentate asymmetrical Schiff base ligand and its DFT study, Polyhedron (2015), doi: http://
dx.doi.org/10.1016/j.poly.2015.10.019
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Synthesis and spectroscopic properties of a copper(II) binuclear
complex of a novel tetradentate asymmetrical Schiff base ligand and
its DFT study
Iman Rajaeia, Seyed Nezamoddin Mirsattaria,b
*
aDepartment of Chemistry, Shahreza Branch, Islamic Azad University, P.O Box 311-86145, Shahreza, Isfahan,
Iran
bRazi Chemistry Research Center (RCRC), Shahreza Branch, Islamic Azad University, Isfahan, Iran
Abstract
A
new
asymmetrical
Schiff
base
ligand,
N,N'-(salicylidene)-5-bromo-2-
hydroxybenzaldehyde-p-phenylenediamine (SBHBP), and its binuclear Cu(II) complex have
been synthesized. The molecular structures and spectroscopic properties of the ligand and its
complex were experimentally characterized by elemental analysis, FT-IR, NMR and UV–Vis
spectroscopic techniques and computationally by the density functional theory (DFT) method.
The molecular geometry and vibrational frequencies of the SBHBP ligand in the ground state
have been calculated using the B3LYP/6-31G(d,p) and 6-31++G(d,p) basis sets. A potential
surface scan study was carried out and the most stable geometry of the SBHBP ligand was
confirmed. The calculated results show that the optimized geometry can well reproduce the
structural parameters, and the theoretical vibrational frequencies are in good agreement with
the experimental values. On calculation of the electronic absorption spectra, TD-DFT
calculations were carried out in both the gas and solution phases. The energetic behavior of
the SBHBP ligand has been examined in solvent media using the B3LYP method with the 6-
31G(d,p) and 6-31++G(d,p) basis sets by applying the Onsager method and polarizable
continuum model (PCM). Additionally, Mulliken and NBO atomic charges, NMR analysis,
molecular electrostatic potential (MEP) and frontier molecular orbitals (FMO) analysis of the
SBHBP ligand were investigated using theoretical calculations.
Keywords: Asymmetric; Schiff base; Binuclear complex; DFT; MEP; Tautomer
1. Introduction
1

Compounds with the structure XC=NY are known as Schiff bases, and these compounds are
usually synthesized by the condensation of primary amines and active carbonyl groups. Schiff
bases are used as starting materials in the synthesis of important drugs such as antibiotic,
antiallergic, antitumor and antifungal drugs because of their biological activities [1,2]. They
have also been widely used as ligands in the field of coordination chemistry [3,4]. Transition
metal Schiff base complexes have many applications in industrial [5,6], pharmaceutical [7,8],
anti-microbial, antifouling, antiviral agent, antioxidant [9,10] and chemical fields [11,12].
Schiff bases with two imine groups in an aromatic ring can be synthesized by the
condensation of diamines with aldehydes or ketones [13,14], or condensation of dialdehydes
or diketones with amines [15,16], but a stable compound involving both –NH2 and –CHO
groups in an aromatic ring cannot be synthesized since these groups react with each other.
Therefore, Schiff bases including two asymmetric imino groups attached to the same aromatic
ring (∼N=HC–Ar–N= CH∼) cannot be synthesized directly. Thus, asymmetric diimines of
these types have not been investigated yet, while much literature concerning symmetrical
diimines has been published. Herein, we report the synthesis and characterization of a new
asymmetric diimine Schiff base N,N'-(salicylidene)-5-bromo-2-hydroxybenzaldehyde-p-
phenylenediamine (SBHBP) and its Cu(II) complex. Also, geometry optimizations and
theoretical assignment of the IR, UV-Vis and NMR spectra of the ligand have been performed
using the DFT method, which is widely used as a remarkable method in many areas of
computational chemistry, such as kinetics and mechanism studies, spectroscopic assignments
and thermodynamic parameters [17-19]. In addition the molecular structure, frontier
molecular orbital energies, molecular electrostatic potential, and natural bond orbital (NBO)
were investigated. These calculations are valuable for providing insight into the molecular
properties of Schiff base compounds.
2. Experimental section
2.1. Materials and measurements
All of the chemicals and reagents were purchased from Merck chemical company and used
without further purification. The NMR spectrum was recorded on a Bruker Avance 400 MHz
spectrometer using d6-DMSO as the solvent. FT-IR (KBr pellet, 400-4000cm-1) spectra were
taken with a Perkin Elmer 65 FT/IR-4000 spectrometer. The electronic spectra were recorded
on a T70 UV/Vis spectrophotometer PG instruments Ltd. Microanalyses (C, H and N) of the
ligand and its complex were carried out on a Perkin–Elmer 240C elemental analyzer. An
analyst 300 Perkin Elmer atomic adsorption spectrometer was used for the determination of
2

the Cu(II) concentration. Melting points were determined on a Gallen kamp melting point
apparatus.
2.2. Synthesis of starting Schiff base ligand (SB1-NO2)
The starting Schiff base ligand 2-{[(4-nitrophenyl)imino]methyl}phenol (SB1-NO2) was
prepared by refluxing a mixture of a solution containing 4-nitro aniline (1.0 g, 7.24 mmol) in
60 ml ethanol and a solution containing 2-hydroxybenzaldehyde (0.88g, 7.24 mmol) in
ethanol (60 ml). The reaction mixture was stirred for 2 h under reflux and allowed to cool.
Yield: 90% (dark orange; Elemental Anal. Calc. for C13H10N2O3 (242.23 g/mol): C, 64.46; H,
4.16; N, 11.56. Found: C, 63.85; H, 4.43; N, 12.15 %. FTIR (cm-1): υ(O-H) 3437.84; υ(C=N)
1612; υ(NO2) 1564.03.
2.2.1. Synthesis of the asymmetric diimine
The asymmetric Schiff base was prepared as shown in Scheme 1. The SBHBP ligand was
prepared with a two-step method. 1.0 mmol of the starting Schiff base (0.24 g for SB1-NO2)
was dissolved in an ethanol:water, 1:1 mixture (60 ml) at 70 ºC. 5.0 mmol (0.87 g) of solid
sodium dithionite was slowly added to the solution in small portions over 1 h and stirred for 2
h at 50 ºC for completion of the reducing process. Consequently, the amino derivative of the
starting Schiff base (SB1-NH2) was formed in the solution. 1.14 mmol of the appropriate
aldehyde in 10 ml ethanol was added to this solution dropwise over a period of 1 h with
stirring, and the resulting solution was refluxed for 4 h at 60 ºC. The yellow precipitate that
formed was removed by filtration. Slow evaporation of the mother solution yielded a yellow-
white precipitate. Yield: 50.4%. M.p.: 295 ºC. Analytical data, UV-Vis and the main IR bands
of the SBHBP ligand are summarized in Table 1.
Scheme 1. Synthetic route to the asymmetric Schiff base ligand (SBHBP).
2.2.2. Synthesis of the Cu(II) complex
3

Copper(II) acetate monohydrate (0.05 g, 0.25 mmol) in EtOH (15 ml) was added, with
stirring, to an ethanolic solution (35 ml) of the asymmetric Schiff base ligand (0.10 g,
0.25mmol) and refluxed for 6 h. After removing the solvent, the product was separated,
washed with ethanol and dried in vacuum. The probable structure of the complex is shown in
Scheme 2. Color: dark green. Yield: 45%. M.p.: 368 ºC. Anal. Calc. for C40H26Br2Cu2N4O4
(913.56 g/mol): C, 52.59; H, 2.87; N, 6.13; Cu, 13.91. Found: C, 52.75; H, 2.79; N, 6.10; Cu,
13.15 %. IR (cm-1): υ(O-H) 3415.3; υ(C=N) 1608.8. A summary of these results are contained
in Table 1.
OH
N
N
OH
Br
(CH3COO)2Cu.H2O
C2H5OH, 6h reflux
Br
Br
O
O
N
N
N
Cu
N
O
O
Cu
Scheme 2. Synthetic route to the binuclear Cu(II) complex
2.3. Computational details
The first task for the computational work was to determine the optimized geometry of the
asymmetrical ligand. The hybrid B3LYP [20,21] method based on Becke's three parameter
functional of DFT and 6-31G(d,p) and 6-31++G(d,p) basis sets level were chosen. All
calculations were performed using the Gaussian 09 package program with a molecular
visualization program [22,23]. The absence of imaginary frequency modes for the optimised
structure of the ligand at the DFT levels confirms a true minimum on the potential energy
surface. The most widely used technique for calculating NMR shielding tensors is the GIAO
1
(Gauge Including Atomic Orbital) method [24,25]. This method was used for calculating H
NMR chemical shifts at the B3LYP level with the 6-31G(d,p) and 6-31++G(d,p) basis sets. In
order to evaluate the energetic and atomic charge behavior of the ligand in solvent media, we
carried out theoretical calculations in five kinds of solvent (ε = 78.3553 - water; ε = 32.613
methanol; ε = 24.852 - ethanol; ε = 10.125 - dichloroethane; ε = 4.7113 - chloroform). The
4

methodologies used in this investigation were centered on two solvation methods: Onsager’s
reaction field theory [26] of electrostatic solvation and the polarized continuum model (PCM)
[27]. The molecular electrostatic potential (MEP) of the optimized structure and the
representation of the HOMO, LUMO energy distributions and HOMO–LUMO energy gap
[28] density calculations were done at the same level of theory and plotted using the Gauss
view program. The B3LYP method has also been used to calculate the dipole moment µ
(Debye) and the atomic charges by natural bond orbital (NBO) analysis.
3. Results and discussion
Condensation of aromatic dialdehydes with primary amines or aromatic diamines with aldehydes
give symmetrical diimines, N=C–Ar–C=N and C=N–Ar–N=C, respectively. Herein, the C=N–Ar–
N=C′ asymmetric diimine SBHBP was prepared and characterized by elemental analysis, UV-Vis,
IR and NMR spectroscopy. Elemental analyses (Table 1) are in agreement with the chemical
formula of the compound. The synthesized asymmetric ligand has been used to prepare a binuclear
copper(II) complex in the present study. The microanalytical (C, H, N and Cu), IR and UV-Vis
data, as collected in Table 1, correspond to the expected composition of this complex.
Table 1. Analytical and selected spectral data of the asymmetric Schiff base ligand and its Cu(II)
complex.
Found (Calcd)%
N
υ(O-H) υ(C=N)
(cm-1) (cm-1)
Compound
λmax (nm)
C
H
Cu
SBHBP
60.42(60.78) 3.54(3.83)
7.00(7.09) ––
6.10(6.13) 13.15(13.91)
381,277,244
3438.8 1614.03
[Cu2(SBHBP)2] 52.75(52.59) 2.79(2.87)
660,387,262,211 ––
1608.8
3.1.1. FT-IR study
The starting Schiff base (SB1-NO2) shows a strong band at 3437.84 cm-1 (Fig. 1 (a)) assigned to
the (O-H...N) intramolecular hydrogen bonding due to the phenolic OH group of the phenol-imine
form [29]. The observation of a sharp and strong band at 1612 cm-1 can be assigned to the
stretching band of the (C=N) Schiff base imino group. The spectrum of SB1-NH2 (Fig. 1 (b))
contains some characteristic bands of the stretching vibrations of the NH2 and C=N groups. The
5

IR spectrum of SB1-NH2 shows two bands at 3374.53 and 3271.79 cm-1 due to N-H stretching
vibrations. These observations show that SB1-NO2 was reduced into the amino derivative (SB1-
NH2). The strong band at 1618.79 cm-1 corresponds to the stretching vibrations of the C=N bond.
The IR spectrum of the asymmetric Schiff base ligand (SBHBP) shows a strong band at 3438.80
cm-1 which is assigned to the υ(O-H) stretching vibration (Fig. 1 (c)). In addition the C=N
stretching vibration was observed at 1614.03 cm-1.
The IR spectrum of the complex was analyzed in comparison with that of the free ligand SBHBP
(Fig. 1 (d)). A strong band observed in the spectrum of the complex at 1608.8cm-1 is attributable
to the azomethine groups. The shift of this band to a lower wavenumber, in comparison to the free
ligand, indicates coordination of the azomethine nitrogen atom to the copper(II) ion [30].
Furthermore, the OH stretching band of the phenolic hydroxyl groups of the free ligand
disappears in the IR spectrum of the complex, supporting the deprotonation of the phenolic
hydroxyl groups of the ligand after complexation.
(b)
(a)
(c)
(d)

Fig. 1. Experimental IR spectra of (a) SB1-NO2, (b) SB1-NH2, (c) SBHBP and (d) the
[Cu2(SBHBP)2] complex
3.1.2. NMR spectral studies of the ligand
1
Fig. 2 shows the H NMR spectrum of the Schiff base ligand (SBHBP) in DMSO-d6.
Experimentally, nine peaks were observed. The synthesis of the SBHBP ligand as a new
asymmetric Schiff base was confirmed by the exhibition of three different signals in the range δ
9.00-9.04 ppm, attributed to the asymmetric imines due to different chemical environments. The
1H NMR spectrum of the SBHBP ligand also exhibits an OHphenolic proton resonance at δ 13.05
ppm. The broadening of these absorption peaks may occur due to the involvement of the protons
in intra- and intermolecular hydrogen bonding interactions [31]. The multiplets in the δ 6.96-7.90
ppm range can be assignable to the protons of the aromatic rings in the ligand.
Fig. 2. Experimental 1H NMR spectrum of the SBHBP ligand
3.1.3. Electronic spectra
7

The UV-Visible spectrum of the free ligand consists of three strong, sharp discernible bands
at around 244, 277 and 381 nm, and these absorptions are in consistent with π-π* and n-π*
transitions [32,33]. These observations show a blue shift in the first and second absorption
bands in the complex, indicating involvement of the azomethine nitrogen and phenolic
oxygen atoms in the coordination. Further, the electronic spectrum of the copper complex
shows a sharp charge transfer band at around 387 nm and a d-d transition band in the range
660 nm, Fig. 3 (b), the later one is broadened due to the overlap of the d-d transitions with the
charge transfer transitions.
(a)
(b)
Fig. 3. Experimental UV-Visible spectra of the (a) SBHBP ligand and (b) the
Cu2(SBHBP)2]complex
[
3.2. Theoretical section
3.2.1. Molecular geometry and potential energy surface scan
The optimised geometry and the atom numbering scheme of the asymmetric ligand (SBHBP),
under investigation is presented in Fig. 4. The ground state optimized structural parameters,
such as bond length, bond angle and dihedral angle values using the B3LYP method with 6-
31G(d,p) and 6-31++G(d,p) basis sets are presented in Table 2. The global minimum energy
of the asymmetric ligand (SBHBP) is calculated to be about -98042.6955 eV at B3LYP/6-
31++G(d,p) levels of theory. The total dipole moment of the SBHBP molecule is 2.662
Debye. DFT calculations reveal that the aldehyde rings are essentially non-planar and make

dihedral angles of 150.81 and 177.13º with the p-phenylenediamine ring (Fig. 4). The phenol-
imine and keto-amine tautomeric forms of N,N'-(salicylidene)-5-bromo-2-
hydroxybenzaldehyde-p-phenylenediamine (SBHBP) have been also optimized in gas media
employing the B3LYP/6-31++G(d,p) level of theory, and the optimized tautomers are shown
in Fig.5. Harmonic vibrational frequencies have been obtained at the same level of theory,
and they clearly indicate that all the optimized structures of the tautomers of SBHBP are at
stationary points on the potential energy surface without any imaginary frequencies. The
present calculations show that among the two tautomers of SBHBP, the phenol-imine form is
more stable than keto-amine form in all media. In the gas phase, the dienol form is more
stable by 13.45 kcal mol-1 with respect to the least stable diketo form.
Fig. 4. Molecular structure with the atom numbering of SBHBP calculated at B3LYP/6-
31++G(d,p) level.
Table 2. The calculated geometric parameters of SBHBP, bond lengths in angstrom (Å) and angles in
degrees (◦).
Parameters
Bond lengths (Å)
6- 6-
31++G( 31G(d,p
Parameters
Bond angles (◦)
6- 6-
31++G( 31G(d,p
Parameters Dihedral angle (◦)
6- 6-
31++G(d 31G(d,
d,p)
1.342
1.293
1.406
)
d,p)
117.68
123.59
121.68
)
,p)
p)
C(14)-
C(15)- N(16)
C(12)-
C(15)- N(16)
C(15)-
N(16)- C(17)
C(18)-
C(17)- N(16)
C(17)- C(18)-
C(21)-O(24)
C(23)- C(22)-
C(21)-O(24)
C(21)- C(18)-
C(17)-N(16)
C(11)- C(12)-
C(15)-N(16)
C(21)- O(24)
C(17)-N(16)
N(16)- C(15)
1.337
1.293
1.405
117.63
123.68
121.72
121.87
0.11
0.13
179.93
-0.54
179.90
-0.63
-
C(17)- C(18)
C(20)- C(19)
1.451
1.383
1.449
1.382
121.87
119.27
-178.65
177.29
178.91
177.34
C(21)-
119.24 C(15)- N(16)-
9

C(18)- C(19)
C(17)-C(18)
C(14)-
C(15)- C(12)
C(21)-
C(12)- C(15)-
N(16)-C(17)
C(19)- C(18)-
C(17)-N(16)
C(15)- C(12)-
C(11)-C(10)
C(20)- C(19)-
C(18)-C(17)
Br(25)-
C(11)- C(12)
C(11)- C(10)
1.390
1.406
1.906
2.627
1.388
1.405
1.912
2.620
118.64
107.03
121.89
147.35
118.62
107.03
121.99
148.03
-32.33
179.35
-1.06
-31.58
179.63
-0.98
O(24)- H(40)
C(20)-
Br(25)
C(18)-
C(21)- O(24)
O(24)-
O(24)-N(16)
179.88
179.91
H(40)-N(16)
N(16)..
H(40)
C(19)-
C(20)-Br(25)
-
1.731
1.719
119.88
119.95
C(20)-C(19)-
-179.84
179.82
C(18)
(a)
(b)
Fig. 5. The optimized geometries of (a) the phenol-imine and (b) the keto-amine tautomeric
forms of SBHBP ligand at the B3LYP/6-31++G(d,p) level of theory.
In order to investigate all the possible conformations of the SBHBP ligand, a detailed
potential energy surface (PES) scan for the C11–C10–N9–C8 dihedral angle was performed
(Fig. 6). The scan was performed by minimizing the potential energy for all geometrical
parameters by changing the dihedral angle every 30º for a 360º rotation around the bond. The
shape of the potential energy as a function of the dihedral angle is given in Fig. 6. The
conformational energy profile shows two maxima near the 120º and 300º dihedral angles. It is

clear from Fig. 6, there is one local minimum observed at 0º or 360º for the C11–C10–N9–C8
dihedral angles. Therefore, the most stable conformer is for the 0º and 360º dihedral angles.
Fig. 6. Potential energy surface scan for the C11–C10–N9–C8 dihedral angle of the SBHBP
ligand.
3.2.2. Atomic charges of the Schiff base ligand (SBHBP)
The NBO and Mulliken atomic charges [34] of the SBHBP ligand were also analyzed
according to the results calculated at the B3LYP 6-31G(d,p) and 6-31++G(d,p) levels, and
the results are listed in Table 3. The NBO charge distribution of the SBHBP ligand is
determined by the B3LYP/6-31++G(d,p) method with respectable accuracy, as shown in Fig
7. Most of the carbon atoms contain negative charges and the NBO charges on the carbon
atoms range from -0.122 to -0.288 atomic charge unit (a.ch.u.). The carbon atom (C5) is the
most negative carbon atom. The Mulliken and NBO charge populations show that there is a
high positive charge on the hydrogen atoms (H30, H40), which could suggest that the reaction
corresponds to a proton transfer. This is in agreement with previously reported examples in
the literature that have shown that the hydrogen bonding leads to charge rearrangement
[35,36]. The negativity of the oxygen atoms increases significantly in the O-H···N hydrogen
11

bond in comparison with the corresponding negativity in the OH group of the phenol unit. At
the same time the hydrogen atoms (H30, H40) become more positive upon hydrogen bonding.
The population of charge for the donor oxygen (O7, O24) is much larger than that of the
acceptor nitrogen (N9, N16). The N9 and N16 atoms have negative NBO charges of -0.519 and
-0.508 respectively, suggesting they act as possible donors to metal ions. On the other hand,
the O7 and O24 atoms of the hydroxyl groups have a significant negative NBO charge (-0.701
and -0.699 a.ch.u.) and they also serve as another coordinating site to metal ions. The NBO
and Mulliken atomic charges clearly indicate that the SBHBP ligand can act as a tetradentate
ligand through the imine N and hydroxyl O atoms.
Table 3. Selected NBO and Mulliken charges of SBHBP at the B3LYP/6-
31G(d,p) and 6-31++G(d,p) levels.
NBO
Mulliken
Atom
6-
6-
6-31G(d,p)
6-31G(d,p)
31++G(d,p)
31++G(d,p)
O7
-0.701
-0.691
-0.689
-0.521
-0.514
0.520
0.520
0.390
0.386
0.052
-0.503
-0.566
-0.565
-0.600
-0.601
0.354
0.352
0.311
0.317
-0.133
O24
N9
-0.699
-0.519
-0.508
0.528
0.528
0.376
0.373
0.063
-0.505
-0.129
-0.130
0.505
0.506
-0.336
-0.479
0.062
N16
H40
H30
C6
C21
Br25
12

Fig. 7. NBO atomic charges for the SBHBP Schiff base ligand at the 6-31++G(d,P) level of
theory
3.2.3. DFT vibrational analysis of the SBHBP ligand
Harmonic vibrational frequencies of the SBHBP ligand were calculated using the
DFT/B3LYP method with the 6–31G(d,p) and 6-31++G(d,p) basis sets [37]. At the optimized
structure of the examined specie, no imaginary wavenumber modes were obtained, proving
that a true minimum on the potential surface was found [38]. In order to facilitate assignment
of the observed peaks we analyzed some vibrational frequencies and the calculated results of
the SBHBP ligand were compared with their experimental ones, as shown in Table 8 [37]. In
general, B3LYP/6-31G(d,p) and 6-31++G(d,p) levels of calculation with the scaling factors
[39] methodology used in this study provided reasonable agreement with the experimental
findings. The correlation value between the experimental and calculated vibrational
wavenumbers are found to be 0.9986(υ) and 0.9984(υ) respectively, as presented in Fig. 9.
The results obtained in this study also indicate that the B3LYP/6-31G(d,p) and 6-31++G(d,p)
methods are reliable and they are helpful for the understanding the vibrational spectra and
structural parameters of prepared Schiff base compounds.
13

Fig. 8. Calculated, at 6-31++G(d,P) level, and experimental wavenumbers of the
SBHBP ligand
B3LYP/6-31++G(d,p)
R2=0.0.9984
B3LYP/6-31G(d,p)
R2=0.9986
Fig. 9. Correlation graphics of calculated vs. experimental wavenumbers of the SBHBP
ligand.
Table 4. Comparison of the experimental and calculated vibrational frequencies
(cm−1).
Basis Set
Experimental
Assignments
6–
6–31G(d,p)
31++G(d,p)
3040.465
1614.03
1567.28
1474.19
3173.06
1667.09
1613.35
1541.35
3184.40
1673.73
1623.07
1550.53
υs (O-H)
υs (C=N)
υs(O–H)
υs(O–H)
β aliphatic(HCN) +
υ(NC)
1350.90
1435.28
1462.79
1275.53
979.77
1319.82
962.89
1337.40
986.32
υ(COH) + β(HCC)
υs(C–H)

βR2(CCC) +
βR1(CCC)
828.98
790.56
885.61
υ, stretching; β, in-plane bending; s, symmetric; Abbreviations: R1
phenylenediamine ring; R2, aldehyde rings.
3.2.4. Solvent effects
In order to evaluate the energetic behavior of the SBHBP ligand in solvent media, we carried
out calculations in five kinds of solvent with different dielectric constants (ε = 78.355 - water;
ε = 32.613 - methanol; ε = 24.825 - ethanol; ε = 10.125 - dichloroethane; ε = 4.7113
chloroform). All the geometries allowed relaxing after changing the solvents. The total
energies and dipole moments were calculated in solvent media by the B3LYP method with 6-
31G(d,p) and 6-31++G (d,p) levels using Onsager and PCM models, and the results are
presented in Table 5. According to Table 5, it can be concluded that obtained total energies of
the SBHBP ligand by Onsager and PCM decrease with increasing polarity of the solvent, and
the stability of the compound increases on going from the gas phase to the solution phase
[40]. The energy difference between gas phase and solvent media is given in Fig. 10, for both
methods. As can be seen from Fig. 9, the PCM method (6-31++G (d,p)) provides a more
stable structure than Onsager's method (5.362 kCal/mol average).
Table 5. Total energies and dipole moments of the SBHBP ligand in different
solvents
εa
Method/Solvent
∆Eb(kcal/mol)
µ (D)
B3LYP(gas)
1
2.5721
6–
6–
6–
6–
Onsager
31++G(d,p) 31G(d,p)
31++G(d,p) 31G(d,p)
Chloroform
Dichloroethane
Ethanol
4.7113
10.125
24.825
32.613
78.3553
-0.3130
-0.4232
-0.5000
-0.5145
-0.5431
-0.2831
-0.3799
-0.4460
-0.4583
-0.4825
3.9061
4.4509
4.8622
4.9423
5.1041
3.7089
4.1812
4.5257
4.5914
4.7232
Methanol
Water
PCM
Chloroform
Dichloroethane
Ethanol
4.7113
10.125
24.825
32.613
-5.2218
-6.4690
-7.2028
-7.3298
-4.4659
-5.5266
-6.1494
-6.2570
3.1832
3.3192
3.4040
3.4190
3.0093
3.1125
3.1743
3.1851
Methanol
15

Water
78.3553
-7.5728
-6.4629
3.4483
3.2059
aε = dielectric constant, b∆E = ESolvation-EGas
B3LYP/6-31G(d,p)
B3LYP/6-31++G(d,p)
Fig. 10. Energy difference between the gas phase and solvent media by PCM and Onsager
methods at the B3LYP/6-31G(d,p) and 6-31++G(d,P) levels of theory.
3.2.5. Molecular electrostatic potential and contour maps
Molecular electrostatic potential (MEP) mapped surfaces illustrate the charge distributions of
molecules three dimensionally which give a clear signature of the interactions of the
molecules. The electrostatic potential is used to find the reactive sites of a molecule. The
molecular electrostatic potential (MEP) is related to the electronic density and is a very useful
descriptor in understanding sites for electrophilic attack and nucleophilic reactions as well as
hydrogen bonding interactions [41,42]. The MEP mapped surface of the molecule was
calculated by the B3LYP method with the 6-31++G(d,p) basis set at 0.02 a.u isovalues for the
MO surface and 0.004 a.u isodensity surface values. The different values of the electrostatic
potential at the surface are represented by different colors. The potential increases in the
order: red < orange < yellow < green < blue. The color codes of these maps are in the ranges
between -1.0229 eV (deepest red) to +1.0229 eV (deepest blue) in the compound. In the color
scheme of the MEP plot, the red region show atoms with lone pairs or negative electrostatic
potentials; the intensity of the color is proportional to the absolute value of the potential
energy. The positive electrostatic potentials are shown in the blue/yellow color region and
characterize the polar hydrogen in O–H bonds. The green areas cover parts of the molecule
where the electrostatic potentials are close to zero (C–C, C–N and C–Br bonds). As can be

seen from the MEP map (Fig. 11) of the SBHBP ligand, this molecule has several possible
sites for electrophilic attack. The negative regions are mainly over the oxygen atoms on each
of the OH groups. The negative potential surface map values are -1.0068 eV for oxygen
atoms and -0.380 eV for the N atom in imine groups. However, the oxygen atoms have larger
negative potential values than the nitrogen atoms. The hydrogen atoms bear the maximum
region of positive charge and the most positive regions were observed around the hydrogen
atoms of imine groups [43].
-3.759e-2
3.759e-2
Fig. 11. Molecular electrostatic potential (MEP) map calculated at the B3LYP/6-31++G(d,p)
level
3.2.6. Frontier molecular orbitals (FMOs)
The FMOs play an important role in the electric and optical properties, as well as in UV-Vis
spectra and chemical reactions [44,45]. In order to evaluate the energetic behavior of the
SBHBP ligand, we carried out calculations in ethanol, methanol and in the gas phase. The
energies of four important molecular orbitals of SBHBP: the highest and the second highest
occupied MO's (HOMO and HOMO−1), the lowest and the second lowest unoccupied MO's
(LUMO and LUMO+1) were calculated using B3LYP/6-31G(d,p) and 6-31++G(d,p) and the
results are presented in Table 6. The pictorial illustration of the frontier molecular orbitals
and their respective positive and negative regions using B3LYP/6-31++G(d,p) are shown in
Fig. 12. The positive and negative phases are represented in red and green colours,

respectively. As seen in Fig. 12, the HOMO is mainly localized on the ring carbon atoms and
the bromine atom and the lowest-lying unoccupied molecular orbital (LUMO) is localized
mainly over the entire molecule. While the HOMO−1 is localized on the Br atom and the
LUMO+1 is localized over the whole structure. The values of the energy separation between
the HOMO and LUMO are 3.5539, 3.5531 and 3.4929 eV in methanol, ethanol and gas
phase, respectively [45]. The HOMO→LUMO transition implies that an intramolecular
charge transfer takes place within the molecule [46]. The energy gap (Eg) of SBHBP has
been calculated from the HOMO and LUMO levels. Molecules with a large energy gap are
chemically stable molecules. Hard molecules have a large energy gap and they are more
stable than soft molecules having a small energy gap. Since soft molecules have a small
energy gap, they are more polarizable and more reactive than hard molecules. The value of
the energy separation between the HOMO and LUMO for SBHBP ligand is 3.4929eV (in the
gas phase) and this low energy gap indicates that the thermodynamics of the SBHBP
structure are not stable. The HOMO–LUMO energy separation has been used as a simple
indicator of kinetic stability [47]. The higher the energy of the HOMO, the easier it is for the
HOMO to donate electrons, whereas it is easier for the LUMO to accept electrons when the
energy of the LUMO is low. The dipole moment in a molecule is an important electronic
property which results from a non-uniform distribution of charges on the various atoms in a
molecule. Based on the predicted dipole moment values, it was found that in going to the
solvent phase from the gas phase, the dipole moment value increases (Table 6).
Table 6. Calculated energies and Dipole moment values of the SBHBP ligand in methanol,
ethanol and in the gas phase.
B3LYP
Methanol
6-
Ethanol
6-31
Gas
6-
6-
6-
6-
Etotal(eV)
-
-
-
-
-
-
EHOMO(eV)
ELUMO(eV)
-5.7526 -6.0291
-2.1932 -2.4752
-5.7520 -6.0291
-2.1932 -2.4760
-5.8914 -6.0729
-2.2580 -2.5800
∆EHOMO-LUMO
EHOMO-1(eV)
ELUMO+1(eV)
∆EHOMO-1-LUMO+1
3.5594
3.5539
3.5588
3.5531
3.6334
3.4929
-6.1632 -6.4607
-1.5279 -1.8316
-6.1616 -6.4593
-1.5279 -1.8322
-6.1007 -6.4258
-1.5764 -1.9184
4.6353
4.6291
3.4190
4.6337
3.1743
4.6271
3.4040
4.5243
2.5721
4.5074
2.6628
Dipole moment (µ) 3.1851
33
18

Fig. 12. Molecular orbital surfaces and energy levels given in parentheses for the HOMO−1,
HOMO, LUMO and LUMO+1 of the SBHBP ligand in the gas phase computed at the
B3LYP/6-31++G(d,p) level
3.2.7. Ultraviolet spectra analysis
The ultraviolet spectra of the SBHBP ligand have been investigated in the methanol, ethanol
and gas phase by theoretical calculations. On the basis of the fully optimized ground-state
structure, the time-dependent DFT (TD-DFT) method at the B3LYP6-31G(d,p) and 6-
31++G(d,p) levels of theory have been used to determine the low-lying excited states of the
SBHBP ligand. The theoretical electronic excitation energies, absorption wavelength and
oscillator strengths are listed in Table 7. Calculations of the molecular orbital geometry show
that the maximum absorption wavelength corresponds to the electronic transition from the
→ LUMO,
HOMO
and other electronic transitions with a smaller contribution correspond to
→ LUMO → LUMO
the HOMO – 1
from the HOMO
and HOMO - 2
→ LUMO is n → π*
transitions. The observed transition
.
The calculated absorption maxima values were found using the B3LYP/6-31++G(d,p)
method as 401.56, 362.16, 338.51 nm for gas phase, 402.78, 350.72, 333.00 nm for methanol
19

solution and 403.54, 351.06, 333.23 nm for ethanol solution. As can be seen, the calculations
performed for methanol and ethanol are very close to each other when compared with the gas
phase. Also it can be observed that the calculated peaks in the gas phase are in good
agreement with the experimental peaks, Fig. 13.
Fig.13. Experimental and calculated UV-Visible spectra of the SBHBP ligand at the 6-
31++G(d,P) level in thegas phase
Table 7. Experimental and calculated absorption wavelengths (λ), excitation energies (E) and oscillator
strengths (ƒ) of the SBHBP ligand in methanol, ethanol and in the gas phase.
TD- B3LYP/6-31G(d,p)
Gas
Experimental
Gas
Methanol
Ethanol
Major
Assignment
State
contributiona
(nm)λ E (eV)
E (eV)
(nm)λ
(ƒ)
E (eV)
(nm)λ
(ƒ)
E (eV)
(nm)λ
(ƒ)
Percentage (%)
3.254
3.111
5
398.4
7
3.098
9
400.1
0
3.093
1
400.8
4
Single
381
5
1.061
1.292
1.301
n→π*
π→π*
H → L (98%)
t
4.476
3.432
6
361.1
9
3.534
1
350.8
2
3.530
9
351.1
4
Single
→ L
(96%)
277
5
0.013
0.227
0.008
0.131
0.009
0.132
H-1
t
5.081
3.700
7
335.0
3
3.744
8
331.0
8
3.742
4
331.3
0
Single
t
→π*
n
244
9
H-2 → L (93%)
TD- B3LYP/6-31++G(d,p)
Methanol
Gas
Experimental
Gas
Ethanol
Major
Assignment
Single
contributiona
3.254
3.087
6
401.5
6
3.078
2
402.7
8
3.072
4
403.5
4
381
5
1.077
0.023
1.312
0.016
1.321
0.016
n→π*
H → L (98%)
t
4.476
3.423
5
362.1
6
3.535
1
350.7
2
3.531
7
351.0
6
Single
t
277
5
π→π*
H-1 → L (96%)
20

5.081
9
3.662
6
338.5
1
3.723
3
333.0
0
3.720
6
333.2
3
Single
t
244
0.185
0.097
0.097
n→π*
H-2 → L (93%)
a H: HOMO; L: LUMO.
3.2.8. NMR analysis
For a comparison between experimental and theoretical NMR data, which may be helpful in
making correct assignments and understanding the relationship between the chemical shift
1
and molecular structure, H NMR chemical shifts calculations for further clarification of the
synthesized Schiff base ligand are reported. The "gauge independent atomic orbital" (GIAO)
method [41] has proven to be quite accepted and accurate. Density functional theory (DFT)
shielding calculations are rapid and applicable to large systems. To provide an explicit
1
assignment and analysis of the H NMR spectrum, theoretical calculations on the chemical
shifts of the SBHBP ligand were carried out with the GIAO method at the B3LYP 6-31G(d,p)
1
and 6-31++G(d,p) levels. The H NMR theoretical and experimental chemical shifts,
isotropic shielding constants and the NMR spectral assignments are presented in Table 8. To
clarify the relation between the theoretical and experimental values of the NMR chemical
shift constants, the experimental data were plotted versus the computed values. As can be
seen, the results obtained by the theoretical are in reasonable agreement with the
experimental values, with only slight differences. As shown in Fig. 14, there are good linear
relationships between the experimental and theoretical B3LYP 6-31G(d,p) and 6-31++G(d,p)
chemical shifts.
B3LYP/6-31++G(d,p)
R2=0.9885
B3LYP/6-31G(d,p)
R2=0.9846
Fig. 14. Theoretical vs. experimental 1H NMR chemical shifts of the SBHBP Schiff base.

1
Table 8. The experimental and theoretical H isotropic chemical
shifts (ppm) with respect to TMS for the SBHBP ligand
Theoretical δ (ppm)
1H
Experimental δ
6–
Assignment
(ppm)
6–31G(d,p)
31++G(d,p)
H40, H30
H36, H31
H39, H28
H35, H32
H34, H33
H37, H29
H38, H27
13.37
8.90
7.80
7.72
7.65
7.37
7.24
13.28
8.56
7.54
7.47
7.43
7.13
7.01
13.05
9.04
7.90
7.89
7.55
6.98
6.96
4. Conclusion
In this study, some experimental investigations and theoretical results were used to clarify the
identification of a novel tetradentate asymmetrical Schiff base ligand and its Cu(II) binuclear
complex. The ligand structural parameters, IR wavenumbers and activities of vibrational
bands were calculated with the B3LYP method and 6-31G(d,p) basis set. The applied method
is in good accordance with the experimental values. Quantum chemical descriptors, such as
EHOMO, ELUMO, the energy gap between EHOMO and ELUMO, dipole moments, NBO and
1
Mulliken atomic charges, H NMR spectrum and molecular electrostatic potential were
calculated. The value of the energy gap between the HOMO and LUMO and the chemical
hardness of the ligand also increase with increasing solvent polarity. The MEP map shows
that the negative potential sites are on electronegative atoms (N and O atoms) while the
positive potential sites are around the hydrogen atoms. These sites give information about the
region from where the compound can undergo non-covalent interactions. Natural bond orbital
analysis indicates strong intramolecular interactions and are in agreement with the
experimental intramolecular hydrogen bonding results. Any differences observed between the
experimental and the calculated wavenumbers could be due to the fact that the calculations
have been performed for a single molecule in the gaseous state, contrary to the experimental
values recorded in the presence of intermolecular interactions. We hope the results of this
study will help researchers in the design and synthesis new asymmetric ligands.
22

References
[1] R.H. Lozier, R.A. Bogomolni, W. Stoeckenius, Biophys. J. 15 (1975) 955.
[2] E.M. Hodnett, W.J. Dunn, J. Med. Chem. 13 (1970) 768.
[3] M. Calligaris, L. Randaccio, Comprehensive Coordination Chemistry, vol. 2,Pergamon
Press, London, 1987, pp. 715.
[4] A.D. Garnovski, A.L. Nivorozhkin, V.I. Minkin, Coord. Chem. Rev. 126 (1993) 1.
[5] C.A. McAuliffe, R.V. Parish, S.M. Abu-El-Wafa, R.M. Issa, Inorg. Chim.Acta 115 (1986)
91.
[6] A. Prakash, D. Adhikari, Chem. Tech. Res. 3 (2011) 1891.
[7] S. Zolezzi, E. Spodine, A. Decinti, Polyhedron 21 (2002) 55.
[8] V. Ambike, S. Adsule, F. Ahmed, Z. Wang, Z. Afrasiabi, E. Sinn, F. Sarkar, S.Padhye, J.
Inorg. Biochem. 101 (2007) 1517.
[9] T. Rosu, E. Pahontu, C. Maxim, R. Georgescu, N. Stanica, G. L. Almajan, A. Gulea,
Polyhedron 29 (2010) 757.
[10] J. Rahchamani, M. Behzad, A. Bezaatpour, V. Jahed, G. Dutkiewicz, M. Kubicki, M.
Salehi, Polyhedron 30 (2011) 2611.
[11] T.W. Hambley, L.F. Lindoy, J.R. Reimers, P. Turner, W. Wei, A.N.W. Cooper, J. Chem.
Soc., Dalton Trans. (2001) 614.
[12] S. Chandra, L.K. Cupta, Trans. Met. Chem. 30 (2005) 630.
[13] A.R. Fakhari, A.R. Khorrami, H. Naeimi. Talanta 66 (2005) 813.
[14] J. Liu, B. Wu, B. Zhang, Y. Liu. Turk. J. Chem., 30 (2006) 41.
[15] I. Cianga, M. Ivanoiu. Eur. Polym. J. 42 (2006) 1922.
[16] M. Dolaz, M. Tümer, M. Diğrak. Trans. Met. Chem. 29 (2004) 528.
[17] H. Eshtiagh-Hosseini, M.R. Housaindokht, S.A. Beyramabadi, S. Beheshti, A.A.
Esmaeili, M. Javan-Khoshkholgh, A. Morsali, Spectrochim. Acta A 71 (2008) 1341.
23

[18] H. Eshtiagh-Hosseini, M.R. Housaindokht, S.A. Beyramabadi, S.H. MirTabatabaei, A.A.
Esmaeili, M. JavanKhoshkholgh, Spectrochim. Acta A 78 (2011) 1046.
[19] H. Eshtiagh-Hosseini, H. Aghabozorg, M. Mirzaei, S.A. Beyramabadi, H. Eshghi, A.
Morsali, A. Shokrollahi, R. Aghaei, Spectrochim. Acta A 78 (2011) 1392.
[20] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[21] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[22] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G.
Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato,X. Li,
H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M.
Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark,
J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K.
Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M.
Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W.
Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J.
Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J.
Cioslowski, D.J. Fox, Gaussian 09, Revision A. 02, Gaussian, Inc.,Wallingford, CT,
2009.
[23] R. Dennington II, T. Keith, J. Millam, GaussView, Version 4.1.2, Semichem,
Inc.,Shawnee Mission, KS, 2007.
[24] K. Wolinski, J.F. Hilton, P. Pulay, J. Am. Chem. Soc. 112 (1990) 8251.
[25] J.L. Dodds, R. McWeeny, A.J. Sadlej, Mol. Phys. 41 (1980) 1419.
[26] L. Onsager, J. Am. Chem. Soc. 58 (1936) 1486.
[27] J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 105 (2005) 2999.
[28] D.C. Young, Computational Chemistry: A Practical Guide for Applying Techniquesto
Real World Problems (Electronic), John Wiley & Sons Inc., New York, 2001, p.267.
[29] H. Nazır, M. Yıldız, H. Yılmaz, M.N. Tahir, D. Ülkü, J. Mol. Struct. 524 (2000)241.
24

[30] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds,
John Wiley & Sons Inc., New York, 1978, p.339.
[31] W. Kemp, Organic Spectroscopy, John Wiley & Sons Inc., New York, 1975, p. 248.
[32] B.S. Creaven, M. Devereux, A. Foltyn, S. Mcclean, G. Rosair, V. R. Thangella, M.
Walsh, Polyhedron 29 (2010) 813.
[33] D. Nartop, P. Gurkan, N. Sari, S. Cete, J. Coordination. Chem. 61 (2008) 3516.
[34] A.E. Reed, R.B. Weinstock, F. Weinhold, J. Chem. Phys. 83 (1985) 735.
[35] Y. Dimitrova, Spectrochim. Acta A 69 (2008) 517.
[36] S. Sebastian, S. Sylvestre, D. Jayarajan, M. Amalanathan, K. Oudayakumar, T.
Gnanapoongothai, T. Jayavarthanan, Spectrochim. Acta A 101 (2013) 370.
[37] A.D. Becke, J. Chem. Phys. 98 (1993) 5648-5652.
[38] H. Tanak, Comput. Theor. Chem. 967 (2011) 93.
[39] K. Balcı, S. Akyuz, Vib. Spectrosc.48 (2008) 215.
[40] Y.B. Alpaslan, G. Alpaslan, A.A. Agar, N.O. Iskeleli, E. Oztekin, J. Mol. Struct. 995
(2011)58.
[41] F.J. Luque, J.M. Lopez, M. Orozco, Theor. Chem. Acc. 103 (2000) 343.
[42] T.U. Devi, S. Priya, S. Selvanayagam, K. Ravikumar, K. Anita, Spectrochim. Acta A 97
(2012) 1063.
[43] M. Karabacak, E. Kose, A. Atac, Spectrochim. Acta A 91 (2012) 83.
[44] I. Fleming, Molecular Orbitals and Organic Chemical Reactions: Student Edition, John
Wiley & Sons, Ltd,Chichester, 2009, p. 61.
[45] U. Rani, M. Karabacak, O. Tanriverdi, M. Kurt, N. Sundaraganesan, Spectrochim. Acta
A 92 (2012) 67.
[46] R.G. Pearson, Proc. Natl. Acad. Sci. 83 (1986) 8440.
[47] K.H. Kim, Y.K. Han, J. Jung, Theor. Chem. Acc. 113 (2005) 233.
25

Graphical Abstract:
A new asymmetrical Schiff base ligand (SBHBP) and its binuclear Cu(II)
complex have been synthesized. The molecular structures and spectroscopic
properties of the ligand and its complex were experimentally characterized by
spectroscopic techniques and computationally by the density functional theory
(DFT) method. A potential surface scan study was carried out and the most
stable geometry of the SBHBP ligand was confirmed. The theoretical
vibrational frequencies are in good agreement with the experimental values.
Additionally, Mulliken and NBO atomic charges, NMR analysis, molecular
electrostatic potential (MEP) and frontier molecular orbitals (FMO) analysis of
the SBHBP ligand were also investigated using theoretical calculations.
27