Synthesis, spectral identification, electrochemical behavior and theoretical investigation of new zinc complexes of bis((E) 3-(2-nitrophenyl)-2- propenal)propane-1,2-diimine

Article abstract of DOI:10.1016/j.saa.2012.05.011

Synthesis, spectroscopic, electrochemical behavior and theoretical investigation of some zinc complexes of a new Schiff base ligand of bis((E) 3-(2-nitrophenyl)-2-propenal)propane-1,2-diimine (L) with a general formula of ZnLX₂(X = Cl^-, Br^-, I^-, SCN ^- and N3-) are described. The ligand and its complexes were identified by elemental analysis, molar conductivity, UV-Visible spectra, FT-IR spectra, ¹H NMR and ¹³C NMR spectra. The complexes were found to be molecular and non-electrolyte based on conductivity measurement. The spectral data confirm coordination of ligand and anions(X^-) to zinc ion center. Electrochemical behavior of ligand and complexes were investigated by cyclic voltammetry technique exhibiting different redox behavior of complexes with respect to free ligand so that the ligand and complexes showed quasi-reversible and irreversible electron transfer processes respectively. Molecular structures of the ligand and complexes have been optimized at the UB3LYP/LANL2MB level of theory. Accordingly some theoretical thermodynamical and/or structural parameters such as HF-energy, Gibbs free energy, enthalpy, selected bond distances, bond angles and torsion angles of optimized structures are presented.

Full text of DOI:10.1016/j.saa.2012.05.011

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 70–76
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectral identification, electrochemical behavior and theoretical
investigation of new zinc complexes of bis((E) 3-(2-nitrophenyl)-2-propenal)
propane-1,2-diimine
⇑
Morteza Montazerozohori , Maryam Sedighipoor
Department of Chemistry, Yasouj University, Yasouj 75918-74831, 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
"
Synthesis of some new zinc
complexes of a new asymmetric
Schiff base is reported.
"
"
"
"
The complexes were molecular and
non-electrolyte.
Electrochemical behavior of ligand
showed quasi-reversible process.
Complexes exhibited irreversible
redox processes.
Pseudo-tetrahedral was suggested
for complexes based on the UB3LYP/
LANL2MB⁄ level.
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis, spectroscopic, electrochemical behavior and theoretical investigation of some zinc complexes
of a new Schiff base ligand of bis((E) 3-(2-nitrophenyl)-2-propenal)propane-1,2-diimine (L) with a gen-
eral formula of ZnLX2(X = Clꢀ, Brꢀ, Iꢀ, SCNꢀ and Nꢀ3 ) are described. The ligand and its complexes were
identified by elemental analysis, molar conductivity, UV–Visible spectra, FT-IR spectra, 1H NMR and
13C NMR spectra. The complexes were found to be molecular and non-electrolyte based on conductivity
measurement. The spectral data confirm coordination of ligand and anions(Xꢀ) to zinc ion center. Elec-
trochemical behavior of ligand and complexes were investigated by cyclic voltammetry technique exhi-
biting different redox behavior of complexes with respect to free ligand so that the ligand and complexes
showed quasi-reversible and irreversible electron transfer processes respectively. Molecular structures of
the ligand and complexes have been optimized at the UB3LYP/LANL2MB⁄ level of theory. Accordingly
some theoretical thermodynamical and/or structural parameters such as HF-energy, Gibbs free energy,
enthalpy, selected bond distances, bond angles and torsion angles of optimized structures are presented.
Ó 2012 Elsevier B.V. All rights reserved.
Received 14 February 2012
Received in revised form 19 April 2012
Accepted 3 May 2012
Available online 12 May 2012
Keywords:
Schiff base
Complex
Spectral
NMR
Voltammetry
Theoretical
Introduction
been extensively introduced in a wide variety of applications
including biological activities [1–4], clinical [5] and analytical fields
A large number of Schiff bases as coordinating chelates have
been studied due to easily formation of their complexes with vari-
ous transition metal ions. Schiff bases and their complexes have
[6–8]. In organic synthetic point of view, some Schiff base com-
plexes catalyze a variety of organic reactions including oxidation
[9], epoxidation [10], water photolysis [11] and decarboxylation
[12]. Various types of Schiff base ligands have been widely used
for the synthesis of coordination compounds [13–15]. Among many
other coordination compounds, zinc Schiff base complexes have
been found to show the luminescence properties [16–17]. In recent
⇑
Corresponding author. Tel./fax: +98 7412223048.
E-mail addresses: mmzohori@mail.yu.ac.ir, mmzohory@yahoo.com (M. Monta
zerozohori).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.05.011

M. Montazerozohori, M. Sedighipoor / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 70–76
71
0
years, in addition to other properties, the electrochemical behavior
of complexes is of interest for chemists because of its sensitivity and
relatively short analysis time. Sometimes redox data give some cru-
cial information on structural features, chemical and biochemical
activity [18–23]. Accordingly the electrochemical studies of some
Schiff base complexes are reported [24–26]. A literature survey
shows that the reports on electrochemical behaviors of N2-Schiff
base ligands and their complexes are rare. Therefore in continuation
of our program [27–29] on synthesis of zinc group complexes, in this
work we wish to report synthesis, spectral identification, electro-
chemical behaviors and theoretical investigation of bis((E) 3-(2-
nitrophenyl)-2-propenal)propane-1,2-diimine (L) as a N2-donor
Schiff base and its new zinc complexes.
1HNMR (in DMSO-d6): 8.11(d, 1He J = 8.85 Hz), 8.09(d, 1He ,
0
0
J = 8.85 Hz), 7.91(d, 2Hkk , J = 8.15 Hz), 7.79(d, 2Hhh , J = 7.85 Hz),
7.70(t(dd), 2Hii , J = 6.76 Hz), 7.56(t(dd), 2Hjj , J = 7.75 Hz and J =
7.70 Hz), 7.34(d,2Hgg , J = 15.90 Hz), 6.93(dd, 1Hf, J = 15.80 Hz and
J = 8.75 Hz), 6.92(dd, 1Hf , J = 15.80 Hz and J = 8.62 Hz), 3.66(bd,
0
0
0
0
1Hb, J = 7.30 Hz), 3.53(m, 2Hac), 1.19(d, 3Hd, J = 5.70 Hz) ppm.
13CNMR(in DMSO-d6 ): 163.18(C2), 161.14(C2 ), 147.92 (C10,10 ),
0
0
0
0
0
135.37(C4), 135.21 (C4 ), 133.49(C9,9 ), 132.54(C5), 132.40(C5 ),
130.17(C3), 130.11(C3 ), 129.79(C6), 129.17 (C6 ), 128.39(C7,7 ),
0
0
0
0
0
124.42(C8,8 ) 67.27(C1 ), 65.83(C1), 20.37 (C11).
Synthesis of zinc complexes
The zinc complexes were synthesized by gradually addition of
ethanolic solution of ligand (0.5 mmol in 10 mL) to zinc halide, thi-
ocynate or azide salts in absolute ethanol (0.5 mmol in 10 mL) un-
der severe stirring for 2–3 h at room temperature. After this time,
the ZnLX2 complexes were obtained as precipitate that were filtered
off and washed with ethanol several times. The complexes were
purified via recrystallization from dichloromethane/ ethanol mix-
ture (1:1) and dried at (80–100 °C) under vacuum and kept in a des-
iccator over silica-gel. Some important physical and spectral (IR and
UV–Visible) data have been collected in Table 1 and 2. The 1H and
13C NMR data of zinc complexes based on scheme 1 are suggested
as in below:
Experimental
Materials and methods
2-Nitrocinnamaldehyde, 1,2-propanediamine, zinc halides and
other chemicals were purchased from Aldrich, Merck or BDH
Chemicals. Zinc thiocynate and azide were synthesized based on
our previous report [27]. FT/IR spectra of compounds were re-
corded on JASCO-FT/IR680 sepectrophotometer in the range of
the 4000–400 cmꢀ1as KBr pellets. Electronic spectra were obtained
in DMF solutions by use of a JASCO-V570 sepectrophotometer. A
Brucker DPX FT/NMR spectrometer at 500 MHz was used for
recording of 1H and 13C NMR spectra in DMSO-d6. CHN analysis
was performed using an elemental analyzer. BUCHI B-545 melting
point instrument was applied for recording of melting points.
Metrohm-712 conductometer with a dip-type conductivity cell
made of platinum black was applied for measurement of molar
conductivities of 10ꢀ3 M solutions of the ligand and its complexes
in CHCl3 at room temperature. Cyclic voltamograms of compounds
have been recorded on a BHP 2063 Potentiostat Galvanostat instru-
ment. Theoretical calculations were performed on the Schiff-base
ligand and its complexes at the UB3LYP/LANL2MB⁄ level of theory.
[ZnLCl2]: 1HNMR(in DMSO-d6): 8.32(t(2d), 2He,e , J = 8.65 Hz and
0
0
0
J = 8.30 Hz), 8.03(d, 2Hk,k , J = 8.15 Hz), 7.83(d, 2Hhh , J = 7.65 Hz),
0
0
7.77(t(dd), 2Hii , J = 7.45 Hz and J = 7.55 Hz), 7.63 (t(dd), 2Hj,j
,
0
J = 7.75 Hz and J = 7.70 Hz), 7.56(d, 1Hg, J = 15.95 Hz), 7.55 (d, 1Hg ,
J = 15.75 Hz), 7.00(dd, 1Hf, J = 15.72 Hz and J = 8.97 Hz), 6.98(dd,
0
1Hf , J = 15.72 Hz and J = 8.97 Hz), 3.80 (m, 2Hb,c), 3.61(b dd, 1Ha,
J = 16.78 Hz and J = 4.60 Hz), 1.22(d, 3Hd, J = 6.20 Hz) ppm.
13CNMR(in DMSO-d6): 165.97(C2), 163.83(C2 ), 147.96(C10,10 ),
0
0
0
0
0
138.48(C4), 138.22(C4 ), 133.81(C9,9 ), 131.14(C5), 130.86(C5 ),
0
0
0
130.40(C3), 130.35(C3 ), 129.98(C6), 129.93(C6 ), 128.40(C77 ),
0
0
124.64(C8,8 ),65.33(C1 ), 64.10 (C1), 19.87(C11) ppm.
[ZnLBr2]: 1HNMR(in DMSO-d6): 8.43(t(2d), 2Hee , J = 8.40 Hz and
0
0
0
0
J = 7.50 Hz), 8.05(d, 2Hkk , J = 8.10 Hz), 7.81(m, 4Hhh ii ), 7.65(t,
0
0
0
4Hjj gg ) detailed as [7.66 (t(dd), 2Hjj , J = 7.05 Hz and J = 7.80 Hz),
Cyclic voltammetry
0
0
7.65(d, 2Hgg , J = 14.85 Hz], 7.06(sixtet, 2Hff ) detailed as [7.08
0
(dd,1Hf, J = 17.10 Hz and J = 9.05 Hz), 7.05(dd, 1Hf , J = 15.98 Hz
All cyclic voltammograms were recorded in a cell containing
three electrodes: glassy carbon as working, platinum disk as sup-
porting and silver wire as reference electrodes at room temperature.
Scan rate was of 0.1 V/S. For recording of cyclic voltammograms,
10ꢀ3 M of ligand and its zinc complexes in dry acetonitrile as well
as tetrabutylammonium hexafluorophosphate as supporting elec-
trolyte were used. All solutions were deoxygenated by passing a
stream of pre-purified N2 into the solution for at least 10 min prior
to recording the voltammograms. All chemical potentials were
modified using ferrocene/ferrocenium as internal standard
electrode.
and J = 9.00 Hz)], 3.89(m, 2Hbc), 3.64(dd, 1Ha, J = 20.10 Hz and
J = 6.90 Hz), 1.26(d, 3Hd, J = 10.40 Hz) ppm. 13CNMR(in DMSO-d6):
0
0
0
167.22(C2), 165.16(C2 ), 147.95(C10,10 ), 139.98(C4), 139.76(C4 ),
133.96(C9,9 ), 130.68(C5), 130.64 (C5 ), 130.37 (C3), 130.05(C3 ),
0
0
0
0
0
0
0
129.91(C6), 129.86(C6 ), 128.34(C7,7 ), 124.76(C8,8 ), 64.34(C1 ),
63.30(C1), 19.90(C11) ppm.
[ZnLI2]: 1HNMR(in DMSO-d6): 8.31(t(2d), 2Hee , J = 8.80 Hz and
0
0
0
J = 8.75 Hz), 8.02(d, 2Hkk
,
J = 8.05 Hz), 7.84(d, 2Hhh
7.77(t(dd),2Hii , J = 7.45 Hz and J = 7.65 Hz), 7.62(t(dd), 2Hjj , J =
8.05 Hz, J = 7.45 Hz), 7.53(d, 2Hgg , J = 15.75 Hz), 7.06(sixtet, 2Hff )
detailed as [7.06 (dd,1Hf, J = 15.62 Hz and J = 10.80), 7.04(dd, 1Hf ,
,
J = 8.05),
0
0
0
0
0
J = 15.65 Hz and J = 9.30 Hz)], 3.79(bd, 2Hb,c, J = 9.05), 3.06(dd,
1Ha, J = 13.87 Hz and J = 7.1 Hz), 1.22(d, 3Hd, J = 6.15 Hz) ppm.
Synthesis of ligand (L)
13CNMR(in DMSO-d6): 165.90(C2), 163.96(C2 ), 147.95(C10,10 ),
0
0
0
0
0
Bis((E) 3-(2-nitrophenyl)-2-propenal)propane-1,2-diimine as a
bidentate Schiff base ligand was prepared by condensation reac-
tion of propane-1,2-diamine, (2 mmol) with 2-nitrocinnamalde-
hyde (4 mmol) in absolute methanol (25 mL) under severe
stirring for 1.5 h in an ice bath. After completion of the reaction,
the reaction mixture was poured into cooled water (50 mL). Then
the yellowish-white precipitate was filtered, washed and recrystal-
lized from ethanol to obtain pure ligand in 75% [30]. Some impor-
tant physical and spectral (IR and UV–Visible) data have been
collected in Table 1 and 2. The 1H and 13C NMR data of ligand based
on scheme 1 are as following:
138.72(C4,4 ), 133.81(C9,9 ), 131.08(C5), 130.86(C5 ), 130.41(C3),
0
0
0
0
130.38(C3 ), 130.00(C6), 129.65(C6 ), 128.33(C7,7 ), 124.65(C8,8 ),
0
65.27(C1 ), 64.18(C1), 20.04(C11) ppm.
[ZnL(NCS)2]: 1HNMR(in DMSO-d6): 8.20(d, 1He, J = 8.95 Hz),
0
0
0
8.18(d, 1He J = 9.00 Hz), 8.00(d, 2Hkk , J = 8.15 Hz),7.92(d, 2Hhh
J = 7.80 Hz), 7.73(t(dd), 2Hii , J = 7.55 Hz and 7.70 Hz), 7.59(t(dd),
2Hjj , J = 7.75 Hz and J = 7.70 Hz), 7.44 (d, 1Hg, J = 15.85 Hz),
,
0
0
0
0
7.43(d, 1Hg , J = 15.80), 7.03 (septet, 2Hff ) detailed as [7.037 (dd,
0
1Hf, J = 15.25 Hz and J = 8.80 Hz), 7.04(dd, 1Hf , J = 15.55 Hz and
J = 9.35 Hz)], 3.73(dd, 1Hb, J = 12.05 and J = 4.50 Hz), 3.65(sixtet,
1Hc, J = 6.10 Hz), 3.54(dd, 1Ha, J = 11.95 Hz and J = 6.40 Hz),

72
M. Montazerozohori, M. Sedighipoor / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 70–76
Table 1
Analytical and physical data of the Schiff base ligand (L) and its Zn(II) complexes.
Compounds
Color
Decomposed point. (°C)
Yield (%)
Found (Calcd.) (%)
C
KM(cm2 ꢀ1 Mꢀ1
X )
H
N
L
Light yellow
Cream
Cream
Cream
Cream
74
75
61
60
80
72
55
64.42(64.28)
47.83(47.70)
41.1(40.84)
35.10(35.44)
48.30(48.13)
46.7(46.55)
5.21(5.14)
3.73(3.81)
3.1(3.26)
2.89(2.83)
3.42(3.51)
3.6(3.72)
14.18(14.28)
10.71(10.60)
9.2(9.07)
7.80(7.87)
14.60(14.64)
26.1(25.85)
0.045
0.030
0.035
0.162
0.055
0.096
ZnLCl2
ZnLBr2
ZnLI2
ZnL(NCS)2
ZnL(N3)2
260
282
265
220
195
Cream
Table 2
Vibrational (cmꢀ1) and electronic (nm) spectral data of the Schiff base (L) and its zinc (II) complexes.
Compound
L
m
CHimine
m
SCN
m
N3
m
C=N
m
C–N
m
C=C
m
M–N
m
NO2
kmax (e )
, cmꢀ1 Mꢀ1
2828(w)
2864(w)
2853(w)
2854(w)
–
–
1635(m), 1616
(m)
1634(vs),1619(s)
1155(m) 1476(w),
1443(w)
1513(vs),
1350(vs)
1527(vs),
1346(vs)
1518(vs),
1342(vs)
1521(vs),
1343(vs)
1522(vs ,
1342(w)
1516(vs),
1341(vs)
216, 258, 326(sh)
ZnLCl2
–
–
–
–
–
–
–
1169(s)
1171(s)
1173(s)
1174(S)
1176(s)
1480(w),1440(m) 551(w), 512(w)
1476(w),1441(w) 517(w),486(w)
1476(w),1442(m) 512(w),489(w)
1476(w),1442(w) 489(w)
222, 258,
283(sh),326(sh)
ZnLBr2
ZnLI2
1638(vs)
1636(vs)
1635(vs)
222, 267, 286, 326(sh)
227, 265, 285, 326(sh)
218, 264, 294, 322(sh)
224, 267, 283, 326(sh)
ZnL(NCS)2
ZnL(N3)2
2829(w) 2069(VS)
2860(w)
–
2067(vs) 1636(vs)
1462(w),1442(w) 519(w),478(w),
458(w)
1.19(d, 3Hd, J = 6.35 Hz(ppm. 13CNMR(in DMSO-d6): 164.72(C2),
Infra-red spectral data
0
0
0
162.64(C2 ), 147.95(C10,10 ), 136.93(C4), 136.69(C4 ), 135.20(CSCN),
0
0
0
133.61(C9,9 ), 131.84(C5), 131.62(C5 ), 130.09(C3), 130.04(C3 ),
Some important IR absorption frequencies of Schiff base ligand
and its zinc complexes have been summarized in the Table 2. The
IR spectrum of the Schiff base shows several weak peaks at 3079,
3022, 2988 and 2828 cmꢀ1 that are attributed to aromatic, olefinic,
aliphatic and iminic C–H groups respectively. They have small
changes on binding of ligand to zinc ion. In another region, the
ligand IR spectrums exhibits two medium peaks at 1635 and
1616 cm–1 confirming the formation of azomethine linkages that
may be assigned to asymmetric and symmetric stretching vibra-
tions, respectively [27–33]. After binding the ligand to zinc ion cen-
ter, the intensity of azomethine asymmetric peak is increased and
appeared as very strong peak in all IR spectra of zinc. The position
of this peak is unchanged in the case of zinc thiocyanate complex;
shifts to lower wave number by 2 cmꢀ1 in the case of zinc chloride
complex and ultimately shifts to higher energy by 1–3 cmꢀ1 in the
cases of zinc bromide, iodide and azide complexes. On the other
hand, azomethine symmetric peak is shifted to higher wave num-
bers by 3–20 cmꢀ1 and merged with asymmetric peak in the case
0
0
0
0
128.48(C6,6 , C7,7 ), 124.52(C8,8 ), 66.31(C1 ), 64.93(C1), 20.08(C11
)
ppm.
[ZnL(N3)2]: 1HNMR(in DMSO-d6): 8.29(t(2d), 2Hee , J = 9.15 Hz
and J = 11.20 Hz), 8.02(d, 2Hkk
J = 7.85 Hz),7.75(t(dd), 2Hii , J = 7.50 Hz and J = 7.65 Hz), 7.61(t(dd),
2Hjj , J = 7.50 Hz and 7.80 Hz), 7.51(d, 1Hg, J = 16.05 Hz), 7.50(d,
1Hg , J = 15.80 Hz), 7.04(dd, 2Hff , J = 15.75 Hz and J = 8.95 Hz),
3.75(m, 2Hbc), 3.57(dd, 1Ha, J = 18.55 Hz and J = 5.35), 1.19(d, 3Hd,
J = 6.20 Hz(ppm. CNMR(in DMSO-d6): 165.90(C2), 163.76(C2 ),
147.96(C10,10 ), 138.07(C4), 137.80(C4 ), 133.70(C9,9 ), 131.35(C5),
0
0
0
,
J = 8.15 Hz), 7.88(d, 2Hhh ,
0
0
0
0
13
0
0
0
0
0
0
0
131.06(C5 ), 130.29(C3), 130.23(C3 ), 130.03(C6), 129.97(C6 ),
0
0
0
128.45(C7,7 ), 124.59(C8,8 ), 65.56(C1 ), 64.11(C1), 19.73(C11) ppm.
Results and discussion
Physical and analytical data
All the analytical, physical and spectroscopic data of the Schiff
base and its zinc complexes have been collected in Table 1 and 2.
The analytical data are in agreement with the chemical formula
of present ligand and its zinc coordination compounds (ZnLX2;
X = Crꢀ, Brꢀ, Iꢀ, SCNꢀ and Nꢀ3 ) (Scheme 1). All zinc complexes are
soluble in most of common organic solvents such as chloroform,
dichloromethane, dimethylformamide and dimethylsulfoxide but
are non-soluble in alcohols such as ethanol and methanol. Melting
point of Schiff base ligand is 74 °C but the zinc complexes are
decomposed in the range of 195–282 °C. All complexes were found
to have molecular structure based on their molar conductivities
d,11
CH3
Ha
Hc
Hb
1
1'
2
1
2',e'
2,e
N
N
4,g
3,f
4',g'
3',f'
O2N
NO2
Zn
10
10`
5
1
5`
2
X
X
9`,k`
8',j'
6,h
6',h'
9,k
that were in the range of 0.03–0.162 cm2
X
ꢀ1 Mꢀ1 in chloroform
solvent at room temperature [24,29]. Conductivity values confirm
coordination of Schiff base and Xꢀ ions to zinc center collectively
in the same coordination sphere.
8,j
7,i
7',i'
Scheme 1. Structural formula of ligand and its zinc complexes.

M. Montazerozohori, M. Sedighipoor / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 70–76
73
Fig. 1. Cyclic voltammograms of ligand, (a) ZnLCl2, ZnLBr2, ZnLI2, ZnL(NCS)2 and ZnL(N3)2, (b) 1–5, respectively.
of zinc bromide, iodide, thiocyanate and azide complexes indicat-
ing coordination of the Schiff base to zinc ion through the iminic
nitrogen(s) in all complexes.
phatic hydrogens of b, (a, c) and d as a broad doublet(bd), a multi-
plet(m) and a doublet(d) respectively at 3.66, 3.53 and 1.19 ppm.
The 1H NMR spectra of the zinc complexes of the ligand showed
the proton signals of ligand as well as distinguishable changes.
After coordination, two doublet signals of imine hydrogens are
merged in the zinc chloride, bromide, iodide and azide complexes
and observed as a downfielded triplets(t(2d)) at 8.29–8.43 ppm ex-
cept for zinc thiocyanate complex that only downfielded of two
separate doublet signals are seen at 8.20 and 8.18 ppm. Other spec-
tral changes include downfielding of aromatic hydrogens of kk0, ii0
and jj0 to 8.00–8.05, 7.73–7.77 and 7.59–7.63 ppm respectively and
upfielding of hydrogens of hh0 except for zinc thiocyanate complex.
The g and g0 hydrogens are separated and downfielded in zinc chlo-
ride, thiocyanate and azide complexes but only are downfieled in
zinc bromide and iodie complexes with respect to free ligand.
The hydrogen signals of f and f0 shift to down field in zinc chloride,
bromide iodide and thiocyanate complexes while they are merged
and appeared at the same chemical shift in the zinc azide complex
spectrum. All the aliphatic hydrogens signals of b, (a, c) and d are
shifted to down field after coordination. Also in comparison with
the free ligand, the signals of a and c hydrogens are observed sep-
arately. The 13C NMR spectrum of the ligand is suggested to show
two different azomethine carbons signals at 163.18(C2) and
In the ligand spectrum, asymmetric and symmetric stretching
of NO2 groups are present at 1513 and 1350 cmꢀ1 that are shifted
by 3–14 and 4–9 cmꢀ1 to higher and lower wave numbers
respectively after complexation [34]. Stretching vibrations of M–
N in the complexes are seen as weak absorption frequencies at
450–550 cm–1 [35] that confirm the coordination of two azome-
thine nitrogens of Schiff base ligand, -NCS and -N3 ions. Also bind-
ing of azide and thiocyanate ions to zinc ion are certainly proved by
appearance of very strong peaks assigned to stretching vibrations
of coordinated azide and N-coordinated thiocyanate ions at 2067
and 2069 cm–1 respectively [36–38].
,
UV–Visible Spectra
The electronic spectral data of ligand and its zinc complexes in
chloroform have been presented in Table 2. In UV–visible spectrum
of ligand, three bands including a band at 216 nm, a shoulder band
at 326 nm and a very intensive band at 258 nm are observed that
may be related to the
p–
p⁄ transition of benzene rings, alkene
and imine bounds respectively. After binding of ligand to zinc
ion, the first band is red shifted at all complexes by 2–8 nm. The
third band is also red shifted and smoothly converted to two inten-
sive bands at the ranges of 258–267 and 283–294 nm after
coordination. These changes in electronic spectra are due to coor-
0
161.14(C2 ) ppm, respectively [28,29]. These signals were found
to be appeared at 165.64–168.55 and 162.54–165.70 ppm after
binding of ligand to zinc ion center. The suggested carbon signals
for C(100, 10), C(40,4), C(90,9), C(60,6) and C(80,8) in ligand spectrum
are shifted to down field but the proposed signals for C(50,5), C(10,1)
and C(11) smoothly are appeared at up field in the zinc complexes.
The carbon signals that may be assigned to C(70,7) and C(30,3) are
also changed after coordination but with different trends in zinc
complexes.
dination of ligand to zinc ion and then the p- back bounding from
d-electrons of zinc to p⁄ orbitals on ligand molecular in the com-
plexes structures.
1H and 13C NMR spectra
The 1H and 13C NMR spectra of Schiff base ligand and all its zinc
complexes (data brought in experimental section) provide required
evidences for confirming of suggested structures in scheme 1. The
1H NMR spectrum of the ligand shows the imine protons (He and
Electrochemical behavior
Cyclic voltammetry is generally used to study the electrochem-
ical properties of an analyte in solution. Accordingly cyclic voltam-
mograms of present ligand and its zinc complexes were recorded
in dry acetonitrile solutions on glassy carbon electrode with scan
rate of 0.1 V/S and are illustrated in Fig. 1. The redox potential data
have been summarized in Table 3. The ligand is redox active within
the experimental potential window of acetonitrile. It displays a
semi-reversible cyclic voltammogram. With running toward nega-
tive voltages, Schiff base ligand is reduced in two negative poten-
tials of ꢀ0.98 and ꢀ1.56 V. These reductive processes may be
attributed to reduction of nitro groups to nitro radical anions
[41]. In a reverse sweeping of voltage, the radical anions that ap-
peared in reductive processes are oxidized at anodic potentials of
ꢀ1.21, ꢀ0.81 and ꢀ0.57 V. Cyclic voltammograms of all zinc
0
He ) signals as two doublets(d) at 8.11 and 8.09 ppm with the same
coupling constants of 8.85 Hz [39]. The signals assigned to aro-
matic hydrogens(kk0 and hh0) of the ligands are seen as two dou-
blets(d) at 7.91 and 7.79 ppm with coupling constants of 7.85
and 6.76 Hz, respectively [40]. It is expected that two doublet of
doublet signals(dd) be appeared for ii0 and jj0 but due to unification
of two inside lines, two distinct triplets(t) observed at 7.70 and
7.56 ppm are attributed to ii0 and jj0 hydrogens respectively. Other
hydrogen signals of ligand are appeared as: g and g0 hydrogens due
to coupling with f and f0 as a doublet at 7.34 ppm; f and f0 hydro-
gens as two separate doublet of doublets (dd) due to coupling with
g and g0 protons and then with e and e0 at 6.93 and 6.92 ppm; ali-

74
M. Montazerozohori, M. Sedighipoor / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 70–76
Table 3
Electrochemical data of ligand and complexes.
a
b
Compounds
EPC1(EPC/2), EPC2(EPC/2
)
D
EP1
D
EP2
EPa1(EPa/2), EPa2(EPa/2), EPa3(EPa/2)
L
ꢀ0.98(ꢀ0.88), ꢀ1.56(ꢀ1.42)
ꢀ0.86(ꢀ0.73)
ꢀ0.10
ꢀ0.14
ꢀ0.13
ꢀ0.09
ꢀ0.09
ꢀ0.14
ꢀ0.15
ꢀ1.21(ꢀ1.32), ꢀ0.81(ꢀ0.88), ꢀ0.57(ꢀ0.62)
ZnLCl2
ZnLBr2
ZnLI2
ZnL(NCS)2
ZnL(N3)2
–
–
–
–
–
–
–
–
–
–
ꢀ0.58(ꢀ0.49)
ꢀ0.46(ꢀ0.37)
ꢀ0.76(ꢀ0.62)
ꢀ0.64(ꢀ0.49)
a
D
D
EP1 = EPC1 ꢀ EPC/2
EP2 = EPC2 ꢀ EPC/2
.
.
b
Table 4
Theoretical parameters of optimized structures including HF-energy, Gibbs free energy and enthalpy (in kJ/mol).
Thermodynamic parameters(theoretical)
L
ZnLCl2
ZnLBr2
ZnLI2
ZnL(NCS)2
ZnL(N3)2
HF energy
ꢀ1314.903
0.399
0.383
ꢀ1314.569
ꢀ1314.585
ꢀ1314.477
ꢀ1314.520
ꢀ1314.476
ꢀ1410.605
0.404
0.388
ꢀ1410.271
ꢀ1410.288
ꢀ1410.169
ꢀ1410.216
ꢀ1410.168
ꢀ1410.184
ꢀ1407.044
0.403
0.387
ꢀ1406.714
ꢀ1406.730
ꢀ1406.609
ꢀ1406.657
ꢀ1406.608
ꢀ1406.624
ꢀ1403.485
0.403
0.387
ꢀ1403.157
ꢀ1403.172
ꢀ1403.050
ꢀ1403.098
ꢀ1403.049
ꢀ1403.065
ꢀ1584.093
0.421
0.406
ꢀ1583.746
ꢀ1583.762
ꢀ1583.635
ꢀ1583.688
ꢀ1583.634
ꢀ1583.651
ꢀ1704.599
0.424
0.408
ꢀ1704.252
ꢀ1704.2667
ꢀ1704.139
ꢀ1704.191
ꢀ1704.138
ꢀ1704.154
Zero temperature energy
Corrected zero temperature energy
Gibs free energy
Corrected Gibs free energy
Total electron energy
Corrected total energy
Enthalpy
Corrected Enthalpy
Table 5
Selected bond distances, bond angles and torsion angles of optimized structures.
Selected data
L
ZnLCl2
ZnLBr2
ZnLI2
ZnL(SCN)2
ZnL(N3)2
Bond length(Å)
M–N(1)
M–N(2)
M–X(1)
M–X(2)
–
–
–
–
2.080
2.091
2.417
2.440
1.328
1.330
2.075
2.086
2.647
2.619
1.328
1.331
2.071
2.072
2.886
2.852
1.330
1.333
2.072
2.062
2.594
2.610
1.333
1.332
2.127
2.131
1.987
1.955
1.326
1.327
C(2)=N(1)
C(20)=N(2)
1.325
1.324
Bond angle(°)
N(2)–M–N(1)
N(2)–M–X(1)
N(2)–M–X(2)
N(1)–M–X(1)
N(1)–M–X(2)
X(1)–M–X(2)
–
–
–
–
–
–
83.67
84.02
84.30
84.57
115.34
114.35
113.81
104.02
118.89
80.90
115.50
101.37
102.64
106.10
136.10
112.20
113.39
110.78
106.28
123.14
114.46
111.65
104.56
111.88
122.99
112.99
112.78
109.41
107.97
122.48
Torsion angle(°)
N(1)–C(1)–C(10)–N(2)
N(1)–C(2)–C(3)–C(4)
N(2)–C(20)–C(30)–C(40)
C(3)–C(4) –C(5)–C(6)
C(30)–C(40)–C(50)–C(60)
ꢀ67.13
ꢀ179.64
ꢀ0.48
ꢀ49.21
179.06
ꢀ30.91
ꢀ1.71
ꢀ48.82
179.72
ꢀ30.24
ꢀ1.46
ꢀ47.25
179.27
28.32
ꢀ45.26
ꢀ177.29
33.33
10.40
ꢀ166.73
ꢀ47.37
178.52
36.25
0.26
167.36
0.21
ꢀ0.82
164.03
ꢀ164.60
ꢀ164.86
ꢀ171.62
Fig. 2. Optimized structure of ligand (left) and zinc ZnLCl2 (right).
complexes show only a reductive irreversible electrochemical
wave. In the complexes voltammograms, one cathodic reduction
in the range of ꢀ0.46 to ꢀ0.86 V is seen that may be assigned to
the first reduction of ligand at them. In comparison with free

M. Montazerozohori, M. Sedighipoor / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 70–76
75
ligand, the reduction potential is changed to more positive values
due to the coordination of ligand to the zinc(II) ion center. As
shown in Table 3, positive shift of reduction wave is increased in
the order of ZnLI2 > ZnLBr2 > ZnLCl2 for halide complexes and
ZnL(N3)2 > ZnL(NCS)2 for two pseudo-halides complex.
and theoretical data suggest pseudo-tetrahedral geometry for all
complexes. Electrochemical behavior of ligand and complexes were
investigated by cyclic voltammetry technique. The cyclic voltam-
mogram of ligand showed semi-reversible redox processes while
the zinc complexes exhibited irreversible electrochemical behavior
so that only a reductive wave is observed in their cyclic voltammo-
grams. The ligand and its zinc complexes were subjected to
molecular modeling at the UB3LYP/LANL2MB⁄ level of theory. Opti-
mization suggested pseudo-tetrahedral geometry for all zinc
complexes. Accordingly some theoretical thermodynamic and
structural parameters such as HF-energy, Gibbs free energy, enthal-
py, selected bond distances, bond angles and torsion angles of opti-
mized structures were presented.
Overall after binding of ligand to zinc center, the iminic nitro-
gens of ligand obtain positive formal charge. This positive charge
is induced to the rest of ligand structure. Naturally more strength
bonding to metal center leads to more induction of positive charge
on the ligand surface. This causes easier reduction namely at less
negative potential (or more positive). As it can be seen from theo-
retical calculation (Table 5), the bond length of Zn–N(iminic) of ha-
lide complexes is decreased as ZnLCl2 > ZnLBr2 > ZnLI2 and/or in
other phrase bond strength is ordered in contrary direction. It
seems that the observed trend for ease of reduction in zinc halide
complexes is in agreement with this explanation. But about two
pseudo-halide zinc complexes namely zinc azide and thiocyanate,
the observed trend for reduction viz. ZnL(N3)2 > ZnL(NCS)2 is in
contrast with before explanation. In this case it is suggested that
another more effective agent may be responsible. We think more
positive inductive property of azide with respect to thiocyanate
on zinc indirectly reinforces positive charge on the ligand structure
that leads to easier reduction of azide complex and observed trend.
Acknowledgement
Partial support of this research by Yasouj University is
acknowledged.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.05.011.
Molecular modeling and analysis
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group. The similarity of some important structural parameters ob-
tained for zinc chloride complex with X-ray structural ones in a
similar complex suggests relative usefulness of the method for
the complexes [42]. For instance, the optimized structures of li-
gand and its zinc chloride complex are depicted in fig. 2.
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