A new tridentate Schiff base Cu(II) complex: Synthesis, experimental and theoretical studies on its crystal structure, FT-IR and UV-Visible spectra

Article abstract of DOI:10.1016/j.saa.2013.03.008

A new Cu(II) complex [Cu(L)(NCS)] has been synthesized, using 1-(N-salicylideneimino)-2-(N,N-methyl)-aminoethane as tridentate ONN donor Schiff base ligand (HL). The dark green crystals of the compound are used for single-crystal X-ray analysis and measur

Full text of DOI:10.1016/j.saa.2013.03.008

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 124–129
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A new tridentate Schiff base Cu(II) complex: Synthesis, experimental and
theoretical studies on its crystal structure, FT-IR and UV–Visible spectra
a
a
b
c
Vahid Saheb a, , Iran Sheikhshoaie , Nasim Setoodeh , Hadi Amiri Rudbari , Giuseppe Bruno
⇑
a Department of Chemistry, Shahid Bahonar University, Kerman, Iran
b Faculty of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
c Dipartimento di Chimica Inorganica, Chimica Analitica Chimica Fisica Università di Messina, Vill. S. Agata, Salita Sperone 31, 98166 Messina, Italy
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A
novel Cu(II) complex [Cu(L)(NCS)] has been synthesized, using 1-(N-salicylideneimino)-2-(N,N-
ꢀ
ꢀ
A new Cu(II) complex complex has
been synthesized.
It is used for single-crystal X-ray
analysis and measuring FTIR and
UV–Visible spectra.
methyl)-aminoethane as tridentate ONN donor Schiff base ligand (HL) and characterized by IR and
UV–Visible spectroscopy and single-crystal X-ray analysis. Quantum chemical methods are used to repro-
duce the structure, UV–Visible, FTIR of the compound.
ꢀ
ꢀ
A scaling factor of 1.015 is obtained
for vibrational frequencies computed
at the B3LYP/6-311G(d,p).
The UV–Visible spectrum of the
complex is computed by TD-DFT
method.
a r t i c l e i n f o
a b s t r a c t
Article history:
A new Cu(II) complex [Cu(L)(NCS)] has been synthesized, using 1-(N-salicylideneimino)-2-(N,N-methyl)-
aminoethane as tridentate ONN donor Schiff base ligand (HL). The dark green crystals of the compound
are used for single-crystal X-ray analysis and measuring Fourier Transform Infrared (FT-IR) and UV–Vis-
ible spectra. Electronic structure calculations at the B3LYP and MP2 levels of theory are performed to
optimize the molecular geometry and to calculate the UV–Visible and FT-IR spectra of the compound.
Vibrational assignments and analysis of the fundamental modes of the compound are performed.
Time-dependent density functional theory (TD-DFT) method is used to calculate the electronic transitions
of the complex. A scaling factor of 1.015 is obtained for vibrational frequencies computed at the B3LYP
level using basis sets 6-311G(d,p). It is found that solvent has a profound effect on the electronic absorp-
tion spectrum. The UV–Visible spectrum of the complex recorded in DMSO and DMF solution can be cor-
rectly predicted by a model in which DMSO and DMF molecules are coordinated to the central Cu atom
via their oxygen atoms.
Received 5 November 2012
Received in revised form 15 January 2013
Accepted 3 March 2013
Available online 14 March 2013
Keywords:
Tridentate Schiff base
Cu(II) complex
UV–Visible
FT-IR
Quantum chemical calculations
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
Schiff bases have played an important role in the development
of coordination chemistry as they readily form stable complexes
with most of the transition metals. In the area of bioinorganic
⇑
Corresponding author. Tel.: +98 9173165106.
E-mail address: vahidsaheb@mail.uk.ac.ir (V. Saheb).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.03.008

V. Saheb et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 124–129
125
functional (B3LYP); and a Hartree–Fock calculation followed by a
Møller–Plesset correlation energy correction (MP2 method)
[20,21]. The theoretically predicted spectra of the compound en-
able us to assign each absorption band to the corresponding
transition.
Experimental section
Materials and measurements
All of the chemicals and reagents were purchased from Fluka
and Merck chemical companies and were used without further
purification. FT-IR (KBr pellet, 450–4400 cmꢁ1) spectrum was ta-
ken with Perkin–Elmer Model RX-I FT-IR spectrometer. The elec-
tronic spectra were recorded on a Beckman DU-7000 UV–Visible
spectrophotometer. Microanalyses (C, H, and N) of the complex
were carried out on a Heracuse CHN rapid analyzer.
Synthesis
Fig. 1. A view of the molecular structure of the title compound, showing the atomic
numbering scheme and the thermal displacement ellipsoids drawn at the 50%
probability level.
The complex was prepared by the similar method for synthesis
of the azido adduct reported in the literature [11]. To a ethanolic
solution (20 ml) of N,N-methyl-ethylenediamine (1 mmol, 0.
0.088 g) was added a methanolic solution (20 ml) of salicylalde-
hyde (1 mmol, 0.122 g). The reaction mixture was then stirred for
30 min. Copper (II) chloride (1 mmol, 0.134 g) dissolved in 5 ml
ethanol was added and was stirred for 10 min. Finally, 5 ml aque-
ous solution of KSCN (2 mmol, 0.194 g) was added drop-wise to the
mixture with continuous stirring and the resulting solution was
then filtered. Dark green single crystals suitable for structure
determination were obtained from ethanolic solution within
2 weeks (Scheme 1). (Yield ca. 60%). Anal. Calcd. for C12H15CuN3OS
(MW = 312.87 g molꢁ1): C, 46.06; H, 4.83; N, 13.43%. Found: C,
chemistry interest in Schiff base complexes has centered on the
role of such complexes in providing synthetic models for the me-
tal-containing sites in metallo-proteins and metallo-enzymes
[1,2]. 2-Hydroxy Schiff base ligands derived from the reaction of
derivatives of salicylaldehyde with amines and their complexes
have been extensively studied in great details for their various
crystallographic, structural and magnetic features [3]. Recently,
interest has grown in the chemistry of the copper (II) complexes
because of their utility in the molecular design of magnetic mate-
rials and models for the active sites of metalloenzymes [3–16]. Azi-
do derivatives of Schiff base copper (II) complexes could form
dimmers or one-dimensional infinite chain structures in which
copper (II) ions are bridged by single azido groups in end-to-end
manner [7–16].
46.00; H, 4.78; N, 13.49%. IR (KBr):
t ; t(CN,
(CN, NCS), 2097 cmꢁ1
HC@N), 1635 cmꢁ1 (CS, NCS), 845 cmꢁ1
;
t
.
X-ray diffraction data of the copper(II) complex
In the present study, a new Cu(II) complex (Fig. 1), using the tri-
dentate Schiff base ligand 1-(N-salicylideneimino)-2-(N,N-
methyl)-aminoethane, has been synthesized and characterized by
UV–Visible, IR spectroscopy and single-crystal X-ray analysis. This
research is mainly aimed at spectral properties of the complex. In
the recent years, electronic structure calculations such as Density
Functional Theory (DFT) and ab initio methods have been used
extensively to calculate a wide variety of molecular properties such
as equilibrium structure, charge distribution, UV–Visible, FT-IR and
NMR spectra [17]. Here, two electronic structure calculation
methods are used to predict the structure, FT-IR, and UV–Visible
spectra of the title compound. The methods include Beck’s three-
parameter exchange functional [18] with Lee et al. [19] correlation
Single-crystal data collection was performed at 296 K on a Bru-
ker–Nonius X8 ApexII diffractometer equipped with a CCD area
detector by using graphite-monochromated Mo Ka radiation
(k = 0.71073 Å) generated from a sealed tube source. Data were col-
lected and reduced by SMART and SAINT software [22] in the Bru-
ker package. The structure was determined by the direct methods
routines in the SHELXS program and refined by full-matrix least-
squares, on F2s, in SHELXL [23]. The non-hydrogen atoms were re-
fined with anisotropic thermal parameters. Hydrogen atoms were
included in idealized positions. The H-atom Uiso values were set
to ride on the Ueq values of the parent carbon atoms. The molecular
structure and crystallographic numbering scheme are illustrated in
the PLATON [24] drawing, Fig. 1.
H3C
N
CH3
H3C
C2H5OH
N
CuCl
+
SCN
+
2
N
reflux
CH3
N
OH
Cu
O
NCS
Scheme 1. Schematic representation of the synthesis of Cu(II) Schiff base complex.

126
V. Saheb et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 124–129
Theoretical methods
Results and discussion
The geometry of the complex was optimized at the B3LYP/6-
311G(d,p) and MP2/6-31G(d) levels of theory. The harmonic vibra-
tional frequencies and their relative intensities were calculated by
B3LYP/6-311G(d,p) method and the results were compared with
experimental FT-IR spectra. Time-dependent density functional
theory (TD-DFT) was used to compute excitation energies and
oscillator strengths for electronic transitions from ground to ex-
cited states [25–27]. All calculations were carried out with the
GAUSSIAN03 software [28].
A Monoclinic group P2(1)/n was determined by X-ray crystal-
lography from single-crystal data of this complex. The details of
experimental condition, data collection and refinement are sum-
marized in Table 1. The molecular structure and crystallographic
numbering scheme are illustrated in Fig. 1. Some selected experi-
mental bond lengths and bond angles are given in Table 2 in com-
parison with calculated values at the B3LYP/6-311G(d,p) and MP2/
6-31G(d) levels. As shown in Table 2, the calculated bond lengths
and bond angles at both levels of theory are in reasonable agree-
ment with the values determined by crystallographic analysis.
The molecule is optimized in gas phase and the deviations in
molecular geometries are attributed to intramolecular interactions
in the crystalline structure. The geometry around Cu ion is a regu-
lar square planar arrangement. The Schiff base coordinates via O1,
N2 and N3 atoms to the Cu atom. The fourth position is occupied
by SCN through nitrogen atom. The optimized geometry and
charge distribution (Mulliken population and NBO analyses) of
the complex is given in Fig. 1S in the Supplementary information.
The high positive charge density (about 1.3) concentrated on cop-
per atom reveals the potential catalytic activity of this site of the
present complex. The catalytic activity of the Cu(II) complexs are
proved experimentally .
The experimentally observed Fourier transform infrared (FT-IR)
peaks of the complex are given in Table 3 and compared with the
most intense harmonic vibrational frequencies calculated at the
B3LYP/6-311G(d,p) level of theory. The vibrational frequencies
are often overestimated by theoretical methods due to the errors
arising from vibrational anharmonicity and incomplete treatment
of electron correlation [29,30]. Therefore, the computed vibrational
frequencies are often multiplied by a scaling factor so that the
computed frequencies reproduce the corresponding experimental
Table 1
Crystal and refinement data for the complex.
Empirical formula
Formula mass
C12H15CuN3OS
312.87
Crystal symmetry
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P2(1)/n
7.0424(2)
19.0302(4)
10.1561(2)
97.7910(10)
1348.54(5)
4
1.541
644
1.765
15,753
b (°)
V (Å3)
Z
Dcalcd. (g cmꢁ3
F(000)
)
l
(Mo K
Collected refl.
Unique refl. (Rint = 0.0356)
R1,wR2 (I > 2 (I))
a
) (mmꢁ1
)
3926
r
0.0336, 0.0725
0.0573, 0.0802
1.011
0.438/ꢁ0.343
R2, wR2 (all data)
S (goodness of fit)
Min./max. res (e Åꢁ3
)
Table 2
Selected bond lengths (Å), angles (°) and torsion angles in comparison with calculated values.
Parameters
X-ray analysis
B3LYP/6-311G(d,p)
MP2/6-31G(d)
Bond distances
Cu(1)AO(1)
Cu(1)AN(2)
Cu(1)AN(1)
Cu(1)AN(3)
S(1)AC(1)
1.9047(14)
1.9331(15)
1.9645(17)
2.0889(16)
1.6270(19)
1.149(2)
1.91355
1.94947
1.90869
2.12495
1.61677
1.18224
1.86619
1.94216
1.89044
2.05894
1.61044
1.20260
N(1)AC(1)
Bond angles
O(1)ACu(1)AN(2)
O(1)ACu(1)AN(1)
N(2)ACu(1)AN(1)
O(1)ACu(1)AN(3)
N(2)ACu(1)AN(3)
N(1)ACu(1)AN(3)
C(2)AO(1)ACu(1)
C(1)AN(1)ACu(1)
C(8)AN(2)ACu(1)
C(9)AN(2)ACu(1)
C(12)AN(3)AC(11)
C(12)AN(3)ACu(1)
N(1)AC(1)AS(1)
N(3)AC(10)AH(10A)
93.60(6)
90.29(7)
166.24(8)
173.33(7)
83.64(6)
92.10824
94.07062
170.67840
171.66441
83.70193
93.05735
96.95618
159.65678
160.23911
84.14547
91.03(7)
91.00580
92.07036
126.93(12)
173.50(18)
125.66(13)
115.35(12)
109.26(18)
115.99(14)
179.5(2)
129.23780
158.39750
126.02464
114.45992
109.90496
106.10559
177.68025
111.14489
128.69288
157.33179
126.32269
113.84768
108.82650
110.62817
177.72212
110.98937
109.6
Torsion angles
N(2)ACu(1)AO(1)AC(2)
N(1)ACu(1)AO(1)AC(2)
O(1)ACu(1)AN(2)AC(8)
O(1)ACu(1)AN(2)AC(9)
N(2)ACu(1)AN(3)AC(12)
N(2)ACu(1)AN(3)AC(10)
Cu(1)AO(1)AC(2)AC(7)
Cu(1)AN(2)AC(8)AC(7)
Cu(1)AN(3)AC(10)AC(9)
ꢁ7.33(18)
ꢁ174.13(18)
3.42(18)
ꢁ173.88(14)
144.21(15)
23.83(13)
8.2(3)
ꢁ2.92682
ꢁ175.99251
2.56134
ꢁ177.48117
137.14618
16.24636
4.60292
ꢁ157.65595
ꢁ1.89984
171.04040
136.69571
17.29711
ꢁ2.56828
ꢁ2.94048
ꢁ41.08129
2.05968
ꢁ0.3(3)
ꢁ1.45150
ꢁ43.33(18)
ꢁ38.24059

V. Saheb et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 124–129
127
Table 3
the present study. Regarding the theoretical prediction of vibra-
tional frequencies, most of the previous studies have been focused
at closed-shell systems. In 2001, in a systematic study of 33 small
radical molecules, Byrd et al. [32] have shown that the accuracy of
B3LYP is roughly comparable, or better, to CCSD and at much re-
duced computational cost. In the most recent study on the geom-
etries and vibrational frequencies of small radicals, it is reported
that B3LYP method produces reliable results, only slightly worse
than CCSD methods [33]. To correct discrepancies between exper-
imental harmonic frequencies and those calculated at the CCSD/
PCVTZ level, a scaling factor of 0.965 is proposed. To the best of
Vibrational frequencies of the complex in comparison with the calculated values the
B3LYP/6-311G(d,p) level of theory.
Experiment
B3LYP/6-311G(d,p)
Assignmenta
620
642
733
755
845
622
655
763
780
880
IB (C@C; ring)
IB (C@C; ring)
OB (HCCH)
BS (CAN)
BS (CS; NCS)
IB (C@C; ring)
BS (CAN)
906
921
1016
1033
1077
1125
1142
1193
1251
1322
1348
1397
1449
1465
1531
1600
1635
2097
2916
2974
3008
3047
1033
1048
1096
1156
1177
1234
1265
1366
1373
1426
1477
1492
1556
1650
1663
2134
3003
3013
3056
3098
BS (CAC)
BS (CAN; CAN@C)
IB (CCH; ring)
IB (CCH; ring)
IB (CCH; ring)
T (HCH)
W (HCH)
IB (CCH; ring)
W (HCH)
SC (HCH)
SC (HCH)
BS (CC, ring)
BS (CC, ring)
BS (CN, HC@N)
BS (CN; NCS)
BS (CH; methyl)
BS (CH; methyl)
BS (CH; HC@N)
BS (CH; methyl)
a
BS: Bond stretching, SC: scissoring, R: rocking, W: wagging, T: twisting, IB: in-
plane bending, OB: out-of-plane bending.
values. For a set of 3474 independent vibrational frequencies, Irik-
ura et al. have reported a scaling factor of 0.9669 0.0205 for
vibrational fundamentals predicted by the B3LYP/6-311G(d,p)
method [30]. A study involving 125 molecules has yielded an opti-
mum scaling factor of 0.9679 for vibrational frequencies computed
at the B3LYP level using basis sets 6-311G(d,p) or larger [31]. In the
present study, to clarify the relation between the theoretical and
experimental values of the vibrational frequencies, the observed
FT-IR peaks are plotted versus the most intense frequencies com-
puted at the B3LYP/6-311G(d,p) level. As shown in Fig. 2, there is
a good linear relationship between the experimental and theoreti-
cal values. Surprisingly, the scaling factor of 1.015 is obtained in
Fig. 2. A plot of experimental versus theoretical vibrational frequencies at the
Fig. 3. The experimental (A) and predicted (B) UV–Visible spectrum in DMSO
B3LYP/6-311G(d,p) level.
solution.

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V. Saheb et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 124–129
our knowledge, the present work is the first study suggesting a
scaling factor for a large open-shell molecule. It is revealed that
systematic studies as performed by Refs. [30,31] are necessary to
reproduce the experimental vibrational frequencies from theoreti-
cal values. Using the fitted linear equation, each absorption peak
could be assigned to the corresponding normal mode. The results
of the vibrational bands assignments are given in the third column
of Table 3. The metal complex showed two prominent bands at 845
and 2097 cmꢁ1 assigned to C@S and C@N stretching of n-thiocya-
nato ligand.
The UV–Visible spectrum of the complex recorded in dimethyl
sulfoxide (DMSO) and N,N-dimethylformamide (DMF) displayed
two maxima in the range 350–390 and 550–700 nm (Fig. 3). Spe-
cial attempts were made to model the observed UV–Visible spec-
tra. First, starting from the crystallographic structure, the
geometry of the complex was optimized at the B3LYP/6-
311G(d,p) level and the excitation energies and oscillator strengths
was computed by TD-DFT at the B3LYP level along with two stan-
dard basis sets of DGDZVP and 6-311G(d,p). The computed elec-
tronic absorption spectra by these two DFT methods are given in
the Supplementary information (Figs. 2S and 3S). It was revealed
that none of the predicted absorption maxima are in accordance
with the observed UV–Visible spectrum. It is found that solvent
has a profound effect on the electronic absorption spectrum. In
the second model, solvent effect is accounted for by considering
parison with the experimental spectrum. As can be seen, there is a
good accordance between experimental and predicted UV–Visible
spectrum. According to latter model, two absorption maxima could
be assigned to ligand-to-metal Cu(d
p) O(p) charge transfer tran-
sitions. Four important electronic transitions are visualized by iso-
surfaces of the corresponding orbitals in Fig. 6S. The excitation
energies and oscillator strengths of the complex in DMF solution
was also calculated by the same procedure as third model.
Fig. 7S in the Supplementary Information shows the optimized
geometry of the complex with a DMF molecule being attached to
the Cu atom via its oxygen atom. Solvent (other DMF molecules)
was considered as a uniform dielectric constant (e = 37.22). The
computed electronic spectrum by TD-DFT at the B3LYP/6-
311G(d,p) level represent strong absorptions at 328, 365, 417
and 608 nm. As can be seen from Fig. 5, the simulated UV–Visible
spectrum in DMF solution is in agreement with the experimental
spectrum.
the solvent (DMSO) as a uniform dielectric constant (e = 46.7)
and Polarizable Continuum Model (PCM) [34] was employed. The
predicted electronic absorption spectra are demonstrated in
Figs. 4S and 5S in the Supplementary information. This model can-
not also reproduce the observed UV–Visible spectrum. In the third
model, it is supposed that one DMSO molecule is coordinated to
the central Cu atom via its oxygen atom. The optimized geometry
of the suggested complex formed in the DMSO solution is depicted
in Fig. 4. The short length of the CuAO bond shows that the DMSO
molecule is strongly attached to the Cu center. In addition, solvent
(other DMSO molecules) was considered as a uniform dielectric
constant and PCM was employed to calculate the electronic
absorption spectrum. The computed excitation energies and oscil-
lator strengths by TD-DFT at the B3LYP/6-311G(d,p) level represent
strong absorptions at 331, 410 and 636 nm. The simulated UV–Vis-
ible spectrum in DMSO solution is demonstrated in Fig. 3 in com-
Fig. 4. The geometry of the complex with a DMSO molecule being coordinated to
Fig. 5. The experimental (A) and predicted (B) UV–Visible spectrum in DMF
the Cu atom.
solution.

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single-crystal X-ray analysis and measuring Fourier Transform
Infrared (FT-IR) and UV–Visible spectra. The electronic structure
methods such as B3LYP and MP2 are employed to compute geo-
metrical parameters and predict the FT-IR and UV–Visible spectra
of the compound. The bond lengths, bond angles and dihedral an-
gles computed by two theoretical methods are in agreement with
the X-ray crystallographic data. The normal mode vibrational fre-
quencies calculated at the B3LYP/6-311G(d,p) level of theory is
compared with the recorded FT-IR spectrum and vibrational bands
assignments and analysis of the fundamental modes of the com-
pound are performed. The scaling factor of 1.015 obtained for
vibrational frequencies suggests future systematic studies for
reproducing the experimental vibrational frequencies for large
open-shell molecules. To predict the UV–Visible spectrum of the
complex recorded in DMSO and DMF solution, a model is used in
which a solvent molecule (DMSO or DMF) is coordinated to the
central Cu atom via their oxygen atoms.
Supplementary data
CCDC 862128 contains the supplementary crystallographic data
for this article. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found in
the online version.
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Acknowledgment
We are grateful to Shahid Bahonar University of Kerman Re-
search Council for the financial support of this research.
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