Synthesis, characterization, and spectroscopic properties of a symmetrical schiff base ligand and its bimetallic complexes

Article abstract of DOI:10.1080/15533174.2011.568431

This article presents the synthesis of new Schiff base ligand and preparation of its transition metal complexes with copper (II), nickel (II), zinc (II), and cobalt (II). Furthermore, absorption and fluorescence properties of ligand and its metal complexe

Full text of DOI:10.1080/15533174.2011.568431

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:484–490, 2011
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.568431
Synthesis, Characterization, and Spectroscopic Properties of
a Symmetrical Schiff Base Ligand and its Bimetallic
Complexes
1
1
2
Mevlu¨t Bayrakcı,1,2 S¸eref Ertul, Salih Zeki Bas¸, and Ibrahim Demir
1Department of Chemistry, Faculty of Science, Selc¸uk University, Konya, Turkey
2Department of Chemistry, Faculty of Science and Arts, Nigde University, Nigde, Turkey
˙
most abundant transition metal in the human body, plays many
roles in numerous cellular functions such as the regulation of
gene expression, apoptosis, co-factors in metalloenzyme catal-
ysis, anorexia, prevention of prematurely pregnancy, and neu-
rotransmission in biological systems.[11,12] The deficiency of
copper, a trace mineral that is essential to good health, can
cause growth retardation, anemia, osteoporosis, osteoarthritis,
rheumatoid arthritis, and pancytopenia in humans.[12] Cobalt
and nickel share right/left-sided cell receptors and are consid-
ered essential to human health, too. While a cobalt and vitamin
B12 relationship is well documented, a similar, less documented
affiliation applies to nickel and vitamin C.[12] The fluorescence
properties of the Schiff-base ligands with various metal ions
along with zinc ion, copper ion, cobalt ion, and nickel ion are
documented in the literature.[13–15] In consideration of the re-
ported fluorescence properties for salen and its transition metal
complexes, the authors report synthesis, characterization, and
spectroscopic properties of new salen type Schiff base and its
transition metal complexes with copper (II), nickel (II), zinc (II),
and cobalt (II).
This article presents the synthesis of new Schiff base ligand
and preparation of its transition metal complexes with copper
(II), nickel (II), zinc (II), and cobalt (II). Furthermore, absorp-
tion and fluorescence properties of ligand and its metal com-
plexes are investigated. The effect of the solvents, such as N,N-
dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol,
methanol, 1,4-dioxane on fluorescence properties of metal com-
plexes, are performed and obtained results are summarized. All
of the structures were characterized by using spectroscopic tech-
niques.
Keywords bimetallic complex, bissalophen, Schiff base, tetradentate
ligand
INTRODUCTION
Schiff bases are an important class of ligands in coordination
chemistry and find extensive application in different fields.[1–5]
Complexes of transition and non-transition metals with Schiff
bases are investigated extensively for many years because the
complexes are important in chemical applications.[6,7] These
metal Schiff-base complexes are frequently used as model com-
pounds for metallo-proteins, and have interesting catalytic, mag-
netic, spectral, chemical, electrochemical, and non-liner optical
properties that are often strongly dependent on the detailed lig-
and structure.[6,8] Also, metal ions using the complexation re-
action have a important role in biological processes such as
the functions of enzymes, the transport of oxygen, the health
of teeth, and diseases such as Alzheimer’s, Parkinson’s, Hunt-
ington’s, and Wilson’s diseases.[9,10] Zinc(II) ion, the second
EXPERIMENTAL
All of the reagents used in this study were obtained from
Merck or Fluka and used without further purification. Dry THF
was distilled from the ketyl prepared from sodium and ben-
zophenone. Thin layer chromatography (TLC) was performed
using silica gel on glass TLC plates (silica gel H, type 60,
Merck). Generally solvents were dried by storing them over
molecular sieves (Aldrich; 4 Å, 8–12 mesh). Melting points
were determined on a Gallenkamp apparatus. 1H and 13C NMR
spectra were obtained using a Varian 400 MHz spectrometer
operating at 400 MHz at room temperature using DMSO-d6
as solvent. IR spectra were recorded on a Perkin–Elmer 1605
FTIR spectrometer as KBr pellets. UV–Vis absorption and
fluorescence spectra were recorded on a Shimadzu UV-1700
spectrometer and on a Perkin–Elmer LS-55 spectrometer, re-
spectively. Elemental analyses were performed using a Leco
CHNS-932 analyzer. Conductivities were measured in DMF
using a LF 330 / SET conductivity meter and performed at
Received 6 August 2010; accepted 6 October 2010.
The authors gratefully acknowledge partial support of this study
by Research Laboratories of Nigde and Selc¸uk University, and also
thank the S.U. Foundation of Scientific Projects (BAP) for the financial
support provided.
Address correspondence to M. Bayrakcı, Department of Chemistry,
Faculty of Science, Selc¸uk University, 42031 Konya, Turkey. E-mail:
mevlutbayrakci@gmail.com
484

SPECTROSCOPIC PROPERTIES OF A SYMMETRICAL SCHIFF BASE
485
24◦C. Magnetic moments were measured by the Gouy method
OH
by using Hg[Co(SCN)4] as calibrant.
OH
HO
OH
Synthesis and Characterization of the Schiff Base Ligand
and its Bimetallic Complexes
NH2
N
NH2
N
O
H
The Schiff base ligand was prepared by adding dropwise a
solution of 3,3ꢁ-diaminobenzidine (0.214 g, 1 mmol) in THF (10
mL) to a THF solution (20 mL) of 2,4-dihydroxybenzaldehyde
(0.58 g, 4.2 mmol) and one drop of glacial acetic acid (catalytic
amount, not necessary) under stirring at room temperature. The
resultant reaction mixture was then refluxed for 9 h with an
exclusion of moisture. The resulting dark orange solution was
cooled to room temperature and evaporated to dryness. The
solid was recrystallized from THF and dried in vacuo. Yield:
OH
i
H2N
OH
N
NH2
N
HO
OH
1
2
HO
3
OH
1
90% of orange crystals. H NMR (400 MHz, DMSO-d6): δ
SCH. 1. Synthesis of binucleating tetradentate Schiff base ligand (i: THF,
reflux, 9 h).
13.50 (s, 4H, OH), 10.25 (s, 4H, OH), 8.85 (s, 4H, CH N),
7.35–7.65 (m, 8H, ArH), 6.20–6.35 (m, 10H, ArH); 13C NMR
(400 MHz, DMSO-d6): δ 163.9, 159.6, 147.8, 141.9, 137.1,
130.5, 127.1, 124.5, 122.2, 113.0, 109.9, 101.5. FT-IR (KBr
pellet, ν (cm−1)): 3400, (OH), 3100, 3050 (C-Har), 1645 (C
N), 1600, 1550, 1480 (C = C). MS (FAB), m/z: 695 [MH+,
[C40H31N4O8]+,%40]. Anal. calc.: C40H30N4O8. C, 69.16; H,
4.35; N, 8.07%. Found: C, 69.13; H, 4.31; N, 8.13%.
FT-Infrared Spectra
Significant frequencies were selected by comparing the IR
spectra of the free ligand 3 and its metal complexes. Since there
are no C = O and NH2 absorption in the IR spectra of the
ligand 3, these peaks indicate the formation of the expected
compound. In the IR spectra of the Schiff base ligand around
2700–2565 cm−1 range, the ligand exhibit broad medium in-
tensity bands that are assigned the intermolecular H-bonding
vibrations (O-H···N). This situation is common for aromatic
azomethine compounds containing o-OH groups.[16] In the IR
spectra of the bimetallic complexes, these bands disappear thor-
oughly. The infrared spectrum of the Schiff base exhibits a band
at 1645 cm−1 assignable to ν(C = N) of the azomethine. In the
Preparation of Bimetallic Complexes
All of the transition metal complexes were prepared as
follows. To a solution of 3,3ꢁ-diaminobenzidine (0.107 g,
0.5 mmol) and 2,4-dihydroxybenzaldehyde (0.29 g, 2.1 mmol)
in THF (20 mL) was added dropwise a solution of the corre-
sponding metal salts The metal complexes were prepared by
treatment of ligand with appropriate metal salts such as Cu(II),
Ni(II), Zn(II), and Co(II) chlorides (2.3 mmol) in THF (10 mL).
The suspension obtained was refluxed for 9 h, and then the
solvent was partially evaporated. The resulting precipitate was
filtered, washed several times with methanol, and then dried in
vacuo to yield the crude product.
OH
O
O
OH
M
N
N
N
RESULTS AND DISCUSSION
In this article, we describe the synthesis of Schiff base lig-
and 3 by the reaction of 3,3ꢁ-diaminobenzidine 1 with 2,4-
dihydroxybenzaldehyde 2 in boiling THF and the preparation of
its bimetallic complexes with copper (II), nickel (II), zinc (II),
and cobalt (II). The chemical equations concerning the forma-
tion of the Schiff base and the complexes represented as follows
(Scheme 1 and 2):
N
M
HO
O
O
The molecular structure of novel ligand 3 and its com-
plexes were characterized by FT-IR, UV-vis, 1H NMR spectra,
magnetic susceptibility measurements, and elemental analysis.
Table 1 shows the analytical and physical data for the ligand 3
and its metal complexes. The IR spectral data of the ligand 3
and metal complexes are listed in Table 2.
OH
SCH. 2. Structure of symmetric bimetallic salphen complexes (M: Cu, Ni, Zn
or Co).

˙
486
M. BAYRAKCI ET AL.
TABLE 1
Some analytical data and physical properties of newly synthesized ligand 3 and its metal complexes
Found (calcd)%
Yield
(%)
Compounds
Salphen
Empirical formula
C40H30N4O8
M.S. (FAB) m/z
Color
M.p. (◦C)
C
H
N
695 [MH+,%40] Orange
818 [MH+,%55] Dark
Green
155–156
90
40
69.13 (69.16) 4.31 (4.35) 8.13 (8.07)
58.57 (58.46) 3.59 (3.68) 6.89 (6.82)
Bis[Cu-(salphen)] C40H26N4O8Cu2
Bis[Ni-(salphen)] C40H26N4O8Ni2
Bis[Zn-(salphen)] C40H26N4O8Zn2
Bis[Co-(salphen)] C40H26N4O8Co2·2H2O
300d>
808 [MH+,%60] Dark
Green
300d>
300d>
300d>
33
30
38
59.05 (59.16) 3.69 (3.72) 6.94 (6.90)
58.27 (58.20) 3.77 (3.66) 6.85 (6.79)
54.49 (54.56) 3.77 (3.89) 6.46 (6.36)
821 [MH+,%35] Pale
Brown
844 [MH+,%55] Dark
Brown
dDecomposition points of the corresponding molecule.
IR spectra of the bimetallic complexes, this band shifts to lower ν(M–N) and ν(M–O) vibrations supporting the participation of the
frequencies and, at the same time, its intensity is lowered. This nitrogen atom of the azomethine group and oxygen atom of the
situation indicates that the imine nitrogen must be coordinated phenolic OH (orto position) group of the ligand in the complex-
to the metal ion. Additionally, the characteristic carbonyl peak ation with metal ions. The ratios of the ligand to metal (M/L =
is not seen around 1700 cm−1 in the IR spectra of the free lig- 2:1) are founded by means of both UV spectra[18] (Figures 1 and
and. A broad band in the 3430–3435 cm−1 region indicating the 2) and elemental analyses.
presence of water molecules is observed in the spectra of the
The elemental analysis data of the Schiff base 3 and its com-
Co (II) complex. This was also confirmed by elemental analy- plexes show the formation of 2:1 [M:L]. We found that the
sis of Co (II) complex. All of these data support the proposed theoretical values are in a good agreement with the found val-
structures of the complexes and free ligand. In the IR spectra ues. The purity of the Schiff base and its complexes are assured
of the free ligand and its complexes, the bands at 1110 cm−1 by the elemental analyses and TLC techniques.
and 1210–1220 cm−1 can be attributed to C-N and C-O bonds,
respectively. In the free ligand 3, a strong band at 1210 cm−1
due to C–O (phenolic) shifts to higher frequency by 20–43 cm−1
in the complexes indicating coordination of the phenolic oxy-
gen atom to the metal ion. The practically unchanged p(O–H)
at 3400–3480 cm−1 reveals that this group does not coordinate
to metal atoms by oxygen atoms.[17] The appearance of new
bands at 430–500 cm−1 and 640–680 cm−1, that attributed to
Magnetic Measurements and Mass Spectra
The magnetic measurements of the complexes are measured
at room temperature. From magnetic measurements, copper (II),
Ni (II), and Co (II) complexes are paramagnetic. The magnetic
moment of cobalt (II) complex is 4.61 B.M., which suggests
the high spin octahedral arrangement around the metal ion.
The copper (II) complex shows a magnetic moment 1.79 B.M.
TABLE 2
Illustration of IR data of newly prepared ligand 3 and its metal complexes (ν, cm−1
)
Compounds
Salphen
p-OH
C
Nimin
1645
C
CAr
C-HAr
Ar-O
1210
M-N
–
M-O
–
µeff (B.M.)
3400
1480
3100
3050
3100
3050
3100
3050
3100
3050
3100
3050
–
1550
1480
1549
1450
1580
1480
1550
1480
1550
Bis[Cu-(salphen)]
Bis[Ni-(salphen)]
Bis[Zn-(salphen)]
Bis[Co-(salphen)]
3420
3480
3455
3435b
1633
1635
1623
1645
1245
1230
1253
1240
420
480
440
410
560
540
580
520
1.79
2.83
Diam.
4.61
bbroad.

SPECTROSCOPIC PROPERTIES OF A SYMMETRICAL SCHIFF BASE
487
1.8
1,4
1,2
1
ligand
1.6
Cu(II) complex
Zn(II) complex
Co(II) complex
Ni(II) complex
1.4
1.2
1
0.8
0.6
0,8
0,6
0,4
Ni(II)
0.4
0.2
0
Co(II)
275
325
375
425
475
525
575
625
675
725
0
1
2
3
4
wavelength (nm)
M / L ratio
FIG. 3. Absorption spectra of the ligand and its complexes in DMF at room
temperature. (Figure is provided in color online.)
FIG. 1. Determination of metal to ligand ratio for Ni(II) and Co(II) by use of
absorbance values.
corresponding to one unpaired electron and magnetic suscep-
tibility of nickel (II) complex is 2.83 B.M consisting with its
proposed tetrahedral configuration. Zinc (II) complex is dia-
magnetic as expected for the d10 configuration. The FAB mass
spectra of Schiff base 3 and its transition metal complexes ex-
hibit the (M+) peaks, which are in agreement with the estimated
values for compounds. The mass spectral data for transition
metal complexes are consistent with deprotonation of four of
the eight hydroxyl groups of the ligand 3, giving neutral com-
plexes (Scheme 2).
1H and 13C NMR Spectra
1H NMR spectrum of the free ligand 3 clearly demonstrates
the presence of a (CH
N) environment at 8.80–8.85 ppm,
which also indicates the occurrence of the condensation reac-
1
tion. The H NMR spectra of 3 exhibits the absorption of the
phenolic hydrogens at low field, 13.50 ppm, which shows that
they are very acidic and the presence of intramolecular hydrogen
bonding. The absence of this peak in the zinc complex indicates
loss of OH proton due to complexation. Comparing the azome-
thine proton signal of free ligand 3 and Zn(II) complex, this peak
is shifted downfield in the Zn complex suggesting deshielding
of the azomethine group due to coordination with zinc. There
is no appreciable change in the other proton signals of this
complex.[19,20] Furthermore, the appearance of a new signal at
10.25 ppm may be assigned to the existence of –OH group (para
position) of the ligand 3, and aromatic C-H protons are observed
at 6.20–7.65 ppm. In the 13C NMR spectra of Schiff base ligand
3, it is obvious that compound 3 possesses certain symmetry el-
ements, and therefore the number of signals observed in the 13C
NMR is lesser than the number of C atoms in the Schiff base
ligand 3. Furthermore, in the 13C NMR for Schiff base com-
pound 3, one signal is hidden because of the overlapped peaks.
The molar conductance values of the synthesized binucleating
tetradentate Schiff base bissalophen 3 and its Cu (II), Ni (II),
Zn (II), and Co (II) complexes are in range from 2.5 to 33.5
Ω−1 cm2 mol−1 in DMF solutions indicating the non-electrolyte
nature of these compounds. These values provide some in-
dication to support the proposed structural conformation for
complexes.
1,2
1
0,8
0,6
0,4
Cu(II)
0,2
Zn(II)
0
0
1
2
3
4
M / L ratio
FIG. 2. Determination of metal to ligand ratio for Cu(II) and Zn(II) by using
absorbance values. (Figure is provided in color online.)

˙
M. BAYRAKCI ET AL.
488
nation of Co atom with the ligand 3. Figure 3 shows the change
in the absorption spectra for Zn (II) complex between 275 and
550 nm.
UV-visible Spectra
Electronic absorption spectral data show π-π∗ transitions re-
lated to benzene ring at 220–260 nm and imine π-π∗ transition
at 332–343 nm. Comparing the free ligand 3 and its transition
metal complexes, it can be seen that the imine π-π∗ transi-
tions are shifted to some extent because the imine nitrogen is
involved in coordination with metal ion. In the complexes, the
low intensity bands around the 630–500 nm range are consistent
with d→d transitions of the metal ions and intraligand n→π∗
transitions.[21] At the same time, around 430–355 nm range,
the more intense band for complexes may be due to the coin-
cidence of charge transfer d→π∗ and L→M transition.[22] The
UV-Vis spectrum of the Ni(II) and Cu(II) complexes exhibits ab-
sorption bands at 585 and 625 nm assignable to the transitions
As seen from Figure 3, new peak was observed at 394 nm.
The absorption peak at 348 nm in the ligand 3 is shifted to
316 nm in Zn (II) complex. The absorption spectra of Cu (II)
and Ni (II) complexes in diluted DMF solution show that the
spectra of Cu (II) and Ni (II) complexes are similar to each
other. Compared with the absorption peak of the ligand 3,
a corresponding absorption peak at 422 nm is observed in
Cu (II) complex. There is a blue shift by 14 nm in the 348
nm absorption band for Cu (II) complex. The new absorption
peaks at 316 nm and 393 nm are observed in Ni (II) com-
plex. The absorption peak at 348 nm in the ligand 3 is shifted
to 316 nm with formation of Ni (II) complex. Furthermore, a
new absorption peak is observed at 393–416 nm in Cu (II),
Ni (II), Zn (II), and Co (II) complexes, which is assigned to
the n–π∗ charge transfer transition from the filled π orbital
of the bridging phenolic oxygen to the vacant d-orbital of the
metal ions.[25]
3
3A2g→ T2g(F) for Ni(II) attributable tetrahedral complex,[17]
2
and 2B1g→ A1g for Cu(II), supporting square planar structure,
which is in agreement with the measured µeff values of the
Cu(II) complexes.[23,24] The electronic spectra of the high spin
Co(II) complex shows a broad band at 570 nm, which may be
4
4
assigned to T1g(F)→ T1g(P) transition compatible with octa-
hedral geometry.[19] The absorption spectra of the ligand 3 and
its corresponding Cu (II), Ni (II), Zn (II), and Co (II) complexes
in DMF solutions are shown in Figure 3. It can be seen that Fluorescence Properties
the absorption peaks of Co (II) complex are obviously different
from the ligand 3. In comparison with Co (II) complex, an im-
portant feature of the absorption spectrum of ligand 3 is shown
that one absorption peak is observed at 348 nm, which are ab-
sent in the spectrum of Co (II) complex. The other feature is
that the absorption peak at 348 nm in the ligand 3 is shifted to
345 nm in Co (II) complex, and a new absorption peak at 418
nm was observed in Co (II) complex, indicating that the coordi-
In Figure 4, the effects of the solvents such as N,N-
dimethylformamide (DMF), dimethyl sulfoxide (DMSO),
ethanol, methanol, 1,4-dioxane on the fluorescence spectra
of ligand 3 are shown. The ligand 3 in DMF display en-
hanced emission intensities compared to the other solvents when
excited with the same amount of energy. This result might
be related to the dipol moments and the viscosities of
solvents.
450
400
350
300
250
200
150
100
50
500
450
400
350
300
250
200
150
100
50
428,45
398,45
DMF
DMSO
Ethanol
Methanol
Dioxane
336,64
300,3
186,43
0
0
485
500
515
530
545
560
575
DMF
DMSO
Bhanol
Methanol
Dioxane
wavelength (nm)
FIG. 4. Emission spectra of the ligand in the different solvents at room temperature (ligand concentration: 5 × 10−5 M, λex : 460 nm). (Figure is provided in
color online.)

SPECTROSCOPIC PROPERTIES OF A SYMMETRICAL SCHIFF BASE
489
500
450
400
350
300
250
200
150
100
50
500
Zn
450
400
350
300
250
200
150
100
50
Ligand
Ni
Co
Cu
0
0
Zn(II)
Ligand
Ni(II)
Co(II)
Cu(II)
485
500
515
530
545
560
575
complex
complex complex complex
wavelength (nm)
FIG. 5. Emission spectra of the ligand and its complexes at room temperature (ligand concentration: 5×10−5 M, complex concentration: 5×10−5 M, λex : 460
nm). (Figure is provided in color online.)
Figure 5 shows the fluorescence spectra of the ligand 3 and ferent solvents with increasing dipol moments and decreasing
its Co (II), Cu (II), Ni (II), and Zn (II) complexes in DMF viscosities of the solvents in fluorometric experiments.
solutions. In comparison with the corresponding free ligand 3
(λem = 476 nm), Zn (II) complex exhibits blue-shift with λem
=
503 nm and shows a significant increase in fluorescence inten-
sity at 503 nm. This anomaly can be explained by the fact that
excited states resulting from complexes of Zn (II) are typically
ligand-centered in nature owing to the inability of the d10 metal
center to participate in low-energy charge transfer for metal-
centered transitions.[26–29] So, factors like an increased rigidity
in complex structure,[29] a restriction in the photoinduced elec-
tron transfer (PET),[22,27] etc. are assigned to the increase in the
fluorescence intensity.[25] A decrease of the fluorescence inten-
sity is observed for the complexes for Cu (II), Co (II), and Ni (II)
ions. A decrease of the fluorescence intensity caused by Cu (II),
Co (II), and Ni (II) ions is probably a result of the “heavy atom
effect” causing the increase of intersystem crossing rate con-
stant, ligand-to-ions electronic energy transfer, redox-activity,
and magnetic perturbation.[25,30–32]
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