New 3,4-diaminobenzoic acid Schiff base compounds and their complexes: Synthesis, characterization and thermodynamics

Article abstract of DOI:10.1016/j.saa.2013.10.103

Some new tetradentate Schiff base ligands (H₃L) were prepared via condensation of 3,4-diaminobenzoic acid with 2-hydroxybenzaldehyde derivatives, such as 3,4-bis((E)-2,4-dihydroxybenzylideneamino)benzoic acid (H₃L¹), 3,4-b

Full text of DOI:10.1016/j.saa.2013.10.103

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 179–185
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
New 3,4-diaminobenzoic acid Schiff base compounds and their
complexes: Synthesis, characterization and thermodynamics
⇑
⇑
Khosro Mohammadi , Mahmood Niad , Tahereh Jafari
Chemistry Department, Faculty of Sciences, Persian Gulf University, Bushehr 75169, Islamic Republic of Iran
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
Some new tetradentate Schiff base ligands (L) and their metal complexes were prepared via condensation
of 3,4-diaminobenzoic acid with 2-hydroxybenzaldehyde derivatives, Khosro Mohammadi, Mahmood
Niad and Tahereh Jahfari
ꢀ
Schiff base ligands of 3,4-
diaminobenzoic acid derivaties (L)
have been synthesized and
characterized.
ꢀ
ꢀ
The their metal complexes of Co(II),
Ni(II), Cu(II) and Zn(II) were prepared.
The formation constants are
increased according to the sequence:
Cu2+ > Ni2+ > Co2+ > Zn2+
.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2013
Received in revised form 27 October 2013
Accepted 31 October 2013
Available online 14 November 2013
Some new tetradentate Schiff base ligands (H3L) were prepared via condensation of 3,4-diaminobenzoic
acid with 2-hydroxybenzaldehyde derivatives, such as 3,4-bis((E)-2,4-dihydroxybenzylideneamino)ben-
zoic acid (H3L1), 3,4-bis((E)-2-hydroxy-3-methoxybenzylideneamino)benzoic acid (H3L2) and 3,4-bis((E)-
5-bromo-2-hydroxybenzylideneamino)benzoic acid (H3L4). Additionally, a tetradentate Schiff base ligand
3,4-bis((E)-2-hydroxybenzylideneamino)benzoic acid (H3L3) and its complexes were synthesized. Their
metal complexes of Co(II), Ni(II), Cu(II) and Zn(II) were prepared in good yields from the reaction of
the ligands with the corresponding metal acetate. They were characterized based on IR, 1H NMR, Mass
spectroscopy and UV–Vis spectroscopy. Also, the formation constants of the complexes were measured
by UV–Vis spectroscopic titration at constant ionic strength 0.1 M (NaClO4), at 25 °C in dimethylformam-
ide (DMF) as a solvent.
Keywords:
3,4-Diaminobenzoic acid
Salicylaldehyde derivatives
Tetradentate Schiff base
Complexation
Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
Formation constant
Introduction
play an important role in the formation of metal complexes, and
that Schiff base compounds show photochromism and thermo-
Schiff bases have been used extensively as ligands in the field of
coordination chemistry. Some of the reasons are that the intramo-
lecular hydrogen bonds between the (O) and the (N) atoms, which
chromism in the solid state by proton transfer from the hydroxyl
(O) to the imine (N) atoms [1]. Schiff bases derived from aromatic
amines and aromatic aldehydes and their metal complexes are
widely applicable in inorganic, biological and analytical chemistry.
Schiff bases have improved conservative reactions such as allylic
alkylation, the hydrosilation of acetophenones, the decomposition
⇑
Corresponding authors. Tel.: +98 7714222340; fax: +98 7714541494.
E-mail addresses: khmohammadi@pgu.ac.ir, khmohammadi@yahoo.com (K.
of
hydrogen
peroxide,
Michael
addition,
annulations,
Mohammadi).
1386-1425/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.10.103

180
K. Mohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 179–185
carbonylation, Heck reaction, benzylation of alanine, amidation
and the aziridination of hydrocarbons, the isomerization of norbor-
nadiene to quadricyclane, the addition of cyanides to imine, cyclo-
Lambda 25 UV–Vis spectrophotometer. Melting points were mea-
sured in capillary tubes using a Buchi 535 melting point apparatus.
propanation,
the
silylcyanation
of
aldehydes,
the
Synthesis of Schiff bases (H3L)
desymmetrization of meso compounds, Diels–Alder reaction, and
aldol condensation reaction[2–5].
Schiff bases are an important class of compounds in medicinal
and pharmaceutical field. They show biological applications
including antibacterial [6,7], antifungal [8] and antitumor activity
[9].
The main purpose of the present article is to study the struc-
tural characterization of Schiff base ligands and their metal com-
plexes and the behavior of Schiff bases towards Co(II), Ni(II),
Cu(II) and Zn(II) ions.
All the new tetradentate Schiff base ligands (H3L1, H3L2 and H3L4)
weresynthesizedby condensing 1 mmolof 3,4-diaminobenzoicacid
and 2 mmol of substituted aldehyde [(2,4-dihydroxybenzaldehyde,
2-hydroxy-3-methoxybenzaldehyde, 5-bromo-2-hydroxybenzal-
dehyde] in methanol (30 mL) at reflux temperature for 2–3 h (See
Scheme 1). The products were then poured by wash well water
and chloroform and filtered. The purity was checked by TLC in a mix-
tureof n-hexaneandethylacetatesolventsystemand finallydriedat
70 °C in an oven. The H3L3 Schiff base ligand and its complexes were
prepared according to a previously published method [10] (See
Scheme1). ThedetailsforthesynthesisofthenewSchiffbaseligands
(H3L1, H3L2 and H3L4) are presented in Table 1.
Experimental
Materials and reagents
Synthesis of the metal complexes
All the chemicals used were of the analytical reagent grade (AR),
and of highest purity available. They included 3,4-diaminobenzoic
acid, 2-hydroxybenzaldehyde, 5-bromo-2-hydroxybenzaldehyde,
2-hydroxy-3-methoxybenzaldehyde, 2,4-dihydroxybenzaldehyde,
nickel (II) acetate tetrahydrate, cobalt (II) acetate tetrahydrate,
copper (II) acetate monohydrate and zinc (II) acetate dihydrate.
All the materials and organic solvents including absolute methanol,
ethyl acetate, n-hexane, chloroform and dimethylformamide were
commercially obtained from Merck, Aldrich or Fluka.
The synthesis methods of the new metal complexes are as fol-
low: Metal (II) acetate (1 mmol) dissolved in methanol (20 ml)
was reacted with a methanol solution (20 ml) of Schiff base ligands
(H3L1 and H3L2) (1 mmol) and Metal (II) acetate (1 mmol) dissolved
in DMSO (20 ml) was reacted with a DMSO solution (20 ml) of
Schiff base ligand (H3L4) (1 mmol) by refluxing for 1 h. The solid
products were formed. Analytically pure products can be obtained
by being washed well with methanol (to remove metal), filtered
and dried in vacuum. The details for synthesis of the new metal
complexes are presented in Table 1.
Physical measurements
Purity of the products was checked by TLC in a mixture of ethyl
acetate and n-hexane solvent system. Infrared spectra were
measured from 4000 to 400 cmꢁ1 as KBr pellets on a Shimadzu
FTIR-8300 spectrophotometer. The NMR spectra were scanned on
a Bruker Avance DPX-400 spectrometer by using DMSO-d6 as a sol-
vent and TMS as an internal standard at 400 MHz. Mass spectra of
the compounds were scanned on a Shimadzu LCMS-2010EV instru-
ment. UV–Vis measurements were carried out in Perkin–Elmer
Results and discussion
1H NMR spectra
The chemical shifts of the different types of protons in the 1H
NMR spectra of the H3L ligands and the metal complexes of
[M(HL)] were reported in Table 2. For all the complexes, the
coordination of the phenolic oxygens to metal was confirmed by
CO2H
CO2H
NH2
CHO
OH
Methanol
Reflux
+
N
N
NH2
X
OH HO
X
X
Abbreviation
Name
H3L1 (X=4-OH)
3,4-bis ((E)-2,4-dihydroxybenzylideneamino)benzoic acid
3,4-bis ((E)-2-hydroxy-3-methoxybenzylideneamino)benzoic acid
3,4-bis ((E)-2-hydroxybenzylideneamino)benzoic acid
H3L2 (X=3-OMe)
H3L3 (X=H)
H3L4 (X=5-Br)
[Co(HL1)] (1)
[Ni(HL1)] (2):
3,4-bis ((E)-5-bromo-2-hydroxybenzylideneamino)benzoic acid
(3,4-bis ((E)-2,4-dihydroxybenzylideneamino)benzoic acid)cobalt (II)
(3,4-bis ((E)-2,4-dihydroxybenzylideneamino)benzoic acid)nickel (II)
[Cu(HL1)] (3)
[Zn(HL1)] (4)
[Co(HL2)] (5)
[Ni(HL2)] (6)
(3,4-bis ((E)-2,4-dihydroxybenzylideneamino)benzoic acid)copper (II)
(3,4-bis ((E)-2,4-dihydroxybenzylideneamino)benzoic acid)zinc (II)
(3,4-bis ((E)-2-hydroxy-3-methoxybenzylideneamino)benzoic acid)cobalt (II)
(3,4-bis ((E)-2-hydroxy-3-methoxybenzylideneamino)benzoic acid)nickel (II)
(3,4-bis ((E)-2-hydroxy-3-methoxybenzylideneamino)benzoic acid)copper (II)
(3,4-bis ((E)-2-hydroxy-3-methoxybenzylideneamino)benzoic acid)zinc (II)
[Cu(HL2)] (7)
[Zn(HL2)] (8)
[Co(HL3)] (9)
[Cu(HL3)] (10)
[Zn(HL3)] (11)
(3,4-bis ((E)-2-hydroxybenzylideneamino)benzoic acid)cobalt (II)
(3,4-bis ((E)-2-hydroxybenzylideneamino)benzoic acid)copper (II)
(3,4-bis ((E)-2-hydroxybenzylideneamino)benzoic acid)zinc (II)
[Co(HL4)] (12)
[Ni(HL4)] (13)
[Cu(HL4)] (14)
[Zn(HL4)] (15)
(3,4-bis ((E)-5-bromo-2-hydroxybenzylideneamino)benzoic acid)cobalt (II)
(3,4-bis ((E)-5-bromo-2-hydroxybenzylideneamino)benzoic acid)nickel (II)
3, 4-bis ((E)-5-bromo-2-hydroxybenzylideneamino)benzoic acid)copper (II)
(3,4-bis ((E)-5-bromo-2-hydroxybenzylideneamino)benzoic acid)zinc (II)
Scheme 1.

Table 1
The physical properties of the ligands and their complexes.
Compounds
(H3L1)
Molecular formula (empirical formula)
C22H16N2O6 (C11H8NO3)
Molecular weight (g/mol)
402.36
Color
Weight of starting materials mmol (g)
Yield% (g)
Melting point (°C)
Dark-yellow
1 mmol (0.15 g)
73%
176
3,4-Diaminobenzoic acid and 2 mmol (0.28 g)
2,4-Dihydroxybenzaldehyde
(0.294 g)
(H3L2)
(H3L4)
C24H18N2O6 (C12H9NO3)
430.42
528.16
Aream
Aream
1 mmol (0.15 g)
3,4-Diaminobenzoic acid and 2 mmol (0.30 g)
2-Hydroxy-3-ethoxybenzaldehyde
76%
(0.327 g)
159
153
C22H12N2O4Br2 (C11H6NO2Br)
1 mmol (0.15 g)
63%
3,4-Diaminobenzoic acid and 2 mmol (0.40 g)
5-Bromo-2-hydroxybenzaldehyde
(0.294 g)
[Co(HL1)] (1)
[Ni(HL1)] (2)
[Cu(HL1)] (3)
[Zn(HL1)] (4)
[Co(HL2)] (5)
C22H14N2O6Co, (C22H14N2O6Co)
C22H14N2O6Ni (C22H14N2O6Ni)
C22H14N2O6Cu (C22H14N2O6Cu)
C22H14N2O6Zn (C22H14N2O6Zn)
C24H16N2O6Co (C24H16N2O6Co)
459.28
459.06
463.89
465.72
487.33
Dark brown
Dark brown
Dark brown
Light brown
Dark brown
1 mmol (0.25 g)
33%
(0.152 g)
>250
>250
>250
>250
>250
Cobalt(II) acetate tetrahydrate and 1 mmol (0.40 g) H3L1
1 mmol (0.18 g)
46%
(0.211 g)
Nickel(II) acetate tetrahydrate and 1 mmol (0.40 g) H3L1
1 mmol (0.20 g)
51%
(0.237 g)
Copper(II) acetate monohydrate and 1 mmol (0.40 g) H3L1
1 mmol (0.22 g)
42%
(0.196 g)
and 1 mmol (0.40 g) H3L1
1 mmol (0.25 g)
50%
Cobalt(II) acetate tetrahydrate and
(0.244 g)
1 mmol (0.43 g) H3L2
[Ni(HL2)] (6)
[Cu(HL2)] (7)
C24H16N2O6Ni (C24H16N2O6Ni)
C24H16N2O6Cu (C24H16N2O6Cu)
487.11
491.95
Light green
Brown
1 mmol (0.18 g)
55%
(0.268 g)
>250
>250
Nickel(II) acetate tetrahydrate and 1 mmol (0.43 g) H3L2
1 mmol (0.20 g)
47%
(0.231 g)
Copper(II) acetate monohydrate and 1 mmol (0.43 g) H3L2
1 mmol (0.22 g)
[Zn(HL2)] (8)
[Co(HL3)] (9)
C24H16N2O6Zn (C24H16N2O6Zn)
C22H12N2O4Co, (C22H12N2O4Co)
493.77
427.28
Aream
Brown
Zinc(II) acetate dihydrate and 1 mmol (0.43 g) H3L2
36% (0.178 g)
>250
>250
1 mmol (0.25 g)
43%
Cobalt(II) acetate tetrahydrate and
(0.184 g)
1 mmol (0.37 g) H3L3
[Cu(HL3)] (10)
C22H12N2O4Cu (C22H12N2O4Cu)
431.89
Dark brown
1 mmol (0.20 g)
56%
(0.242 g)
>250
Copper(II) acetate monohydrate and 1 mmol (0.37 g) H3L3
[Zn(HL3)] (11)
[Co(HL4)] (12)
C22H12N2O4Zn (C22H12N2O4Zn)
433.72
585.07
Brown
1 mmol (0.22 g)
43%
(0.186 g)
46%
>250
>250
Zinc(II) acetate dihydrate and 1 mmol (0.37 g) H3L3
1 mmol (0.25 g)
C22H10N2O4Br2Co (C22H10N2O4Br2Co)
Burly wood
Cobalt(II) acetate tetrahydrate and 1 mmol (0.53 g) H3L4
(0.269 g)
[Ni(HL4)] (13)
[Cu(HL4)] (14)
[Zn(HL4)] (15)
C22H10N2O4Br2Ni (C22H10N2O4Br2Ni)
C22H10N2O4Br2Cu (C22H10N2O4Br2Cu)
C22H10N2O4Br2Zn (C22H10N2O4Br2Zn)
584.85
589.69
591.51
Green
Green
Aream
1 mmol (0.18 g)
51%
(0.298 g)
>250
>250
>250
Nickel(II) acetate tetrahydrate and 1 mmol (0.53 g) H3L4
1 mmol (0.20 g)
53%
(0.330 g)
Copper(II) acetate monohydrate and 1 mmol (0.53 g) H3L4
1 mmol (0.22 g)
57%
(0.337 g)
Zinc(II) acetate dihydrate and 1 mmol (0.53 g) H3L4

182
K. Mohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 179–185
Table 2
The 1H NMR data of the ligands and their complexes.
Compounds
H3L1
ArAH
HC@N
OH
COOH
AOCH3
6.41(s, 2H), 6.46(d, 2H, J = 8 Hz), 7.62(d, 1H, J = 4 Hz), 7.65(d, 1H, J = 4 Hz),
7.84(d, 2H, J = 8 Hz), 7.87(s, 1H)
6.95–6.99(dd, 2H, J = 8, 8 Hz), 7.11(d, 2H, J = 8 Hz), 7.64(d, 2H, J = 8 Hz),
7.72(s, 1H), 7.95(d, 1H, J = 4 Hz), 7.97(d, 1H, J = 4 Hz)
7.04(d, 2H, J = 8 Hz), 7.54(d, 2H, J = 8 Hz), 7.73(s, 2H), 7.89(d, 1H, J = 4 Hz),
7.91(d, 1H, J = 4 Hz), 8.23(s, 1H)
6.10(s, 2H), 6.14(d, 2H, J = 4 Hz), 7.49(d, 1H, J = 1.6), 7.51(d, 1H, J = 1.6),
7.69(d, 2H, J = 4 Hz), 7.89(s, 1H)
6.53–7.83(m, 9H)
8.14 (s, 2H)
10.10(s, 2H),
12.82(s, 2H)
12.79(s, 2H)
13.11(s, 1H)
3.83 (s, 6H)
H3L2
8.24 (s, 2H)
8.29 (s, 2H)
8.00 (s, 2H)
13.39(s, 1H)
13.35(s, 1H)
13.07(s, 1H)
H3L4
12.87(s, 2H)
9.53(s, 2H)
[Zn(HL1)] (4)
[Zn(HL2)] (8)
[Ni(HL4)] (13)
7.82 (s, 2H)
8.05 (s, 2H)
3.80 (s, 6H)
6.81(d, 2H, J = 4), 7.02(d, 2H, J = 4 Hz), 7.19(d, 1H, J = 1.6), 7.20(d, 1H, J = 1.6),
7.41(s, 2H), 7.92(s, 1H)
6.72–8.07(m, 9H)
13.49(s, 1H)
13.47 (s, 1H)
[Zn(HL4)] (15)
8.17 (s, 2H)
Fig. 2. The IR spectra of H3L4 ligand and [Zn(HL4)] complex.
group, a band at 3320–3335 cmꢁ1related to OAH group in ligands
which disappears at complexes and the bands at 1250–1282 cmꢁ1
range can be related to the phenolic (CAO) group vibrations [11].
The band observed at 1620–1685 cmꢁ1 is due to acidic (C@O)
group vibrations. The broad band appearing at 2400–3700 cmꢁ1
is attributed to the acidic (OAH) group of the ligands and their
complexes. The ring skeletal vibrations (C@C) were in the region
of 1400–1560 cmꢁ1 [12]. A comparison of the complexes data with
Fig. 1. The 1H NMR spectra of the ligand, H3L1 and the its zinc complex, [Zn(HL1)],
in DMSO-d6.
the absence of 12.79–12.87 ppm signals in the 1H NMR spectra.
The presence of a singlet for the C(H)@N proton in ligands and their
complexes (7.90–8.29 ppm) clearly indicates that the magnetic
environment is equivalent for all such protons. The proton signals
of acidic OH proton are observed at 13.07–13.49 ppm. The aro-
matic ring proton signals appeared in the range of 6.10–7.97 ppm.
In 1HNMR spectra of H3L1 and [Zn(HL1)] (4), the proton peaks of
OH situated in the para position appeared in 10.10 and 9.53 ppm.
In 1HNMR spectra of H3L2 and [Zn(HL2)] (8), the protons peaks of
O-CH3 appeared in 3.80 and 3.83 ppm regions. The 1H NMR spectra
of H3L1 and [Zn(HL1)] (4) compounds are shown in Fig. 1.
the corresponding ligands indicates that the m(C@N) band, m(CAO)
band, and m(C@C) band are shifted to lower energy [13,14]. Fig. 2
shows IR spectra of H3L4 and [Zn(HL4)] (15) compounds. The infra-
red spectral data of the ligands and their complexes are presented
in Table 3.
The electronic spectra of the compounds
Electronic absorption spectral data of H3Lx are very similar to
each other because of their structural identities. The absorption
band below 260 nm is assigned to intraligand
p ?
p* transitions
of the phenolic chromophores. The absorption band observed in
all ligands spectra within the range of 299–363 nm is most proba-
bly due to the transition of n ? p* of imine groups [15]. The
charge-transfer bands are expected for copper, cobalt, nickel and
zinc complexes within the range of 240–424 nm [16]. The dAd
IR spectra
In the FT-IR spectral data of the ligands and their complexes, a
band around 1538–1640 cmꢁ1 is assigned to an azomethine (C@N)

K. Mohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 179–185
183
Table 3
The infrared spectral data of the ligands and their complexes.
Compounds
IR (KBr, cmꢁ1
)
(mOH
)
(mOH
)
(
mC@O
)
(
mC@N
)
(mC@C
)
(
mCAO
)
(
mNAM
)
(mOAM)
acid
(H3L1)
2400–3700
2400–3700
2400–3700
2400–3700
2400–3700
2400–3700
2400–3700
2400–3800
2400–3700
2400–3800
2400–3700
2500–3700
2500–3700
2500–3700
2400–3800
2400–3700
2400–3700
2400–3700
1630
1695
1658
1620
1620
1620
1620
1620
1620
1620
1610
1615
1604
1603
1660
1640
1640
1650
1610, 1580
1620, 1590
1620, 1580
1600, 1540
1600, 1540
1600, 1560
1600, 1540
1600, 1560
1600, 1580
1600, 1555
1600, 1580
1528, 1460
1556, 1532
1560, 1481
1600, 1560
1600, 1560
1600, 1540
1610, 1580
1550, 1480
1520, 1480
1520, 1485
1480, 1460
1480, 1400
1540, 1475
1530, 1475
1540, 1480
1560, 1480
1520, 1480
1560, 1480
1441, 1370
1528, 1478
1458, 1401
1540, 1480
1520, 1480
1480, 1420
1560, 1480
1260
1282
1282
1255
1255
1255
1255
1255
1250
1255
1250
1193
1251
1249
1260
1260
1258
1255
(H3L2)
3335
3320
(H3L4)
[Co(HL1)] (1)
[Ni(HL1)] (2)
[Cu(HL1)] (3)
[Zn(HL1)] (4)
[Co(HL2)] (5)
[Ni(HL2)] (6)
[Cu(HL2)] (7)
[Zn(HL2)] (8)
[Co(HL3)] (9)
[Cu(HL3)] (10)
[Zn(HL3)] (11)
[Co(HL4)] (12)
[Ni(HL4)] (13)
[Cu(HL4)] (14)
[Zn(HL4)] (15)
575
573
571
575
550
553
550
555
540
550
545
538
540
535
535
445
423
440
440
467
462
445
460
450
440
438
440
440
445
440
Table 4
The electronic spectral data and the mass fragmentations of the ligands and their complexes.
Compounds
UV–Vis: (kmax, nm, DMF)
Mass spectra (ESI) m/z(%)
H3L1
329, 339.
299, 309.
322, 334.
267, 364.
266, 372.
270, 363.
270, 362.
269, 363.
270, 371.
270, 365.
265, 362.
294, 398.
309, 424.
300, 407.
270, 368.
270, 367.
270, 373.
267, 361.
392 [M]+, 282, 267, 254, 149, 134, 121
H3L2
420 [M]+, 372, 324, 297, 254, 226, 121, 105, 91, 77
518 [M]+, 499, 454, 175, 148, 120
H3L4
[Co(HL1)] (1)
[Ni(HL1)] (2)
[Cu(HL1)] (3)
[Zn(HL1)] (4)
[Co(HL2)] (5)
[Ni(HL2)] (6)
[Cu(HL2)] (7)
[Zn(HL2)] (8)
[Co(HL3)] (9)
[Cu(HL3)] (10)
[Zn(HL3)] (11)
[Co(HL4)] (12)
[Ni(HL4)] (13)
[Cu(HL4)] (14)
[Zn(HL4)] (15)
bands were not observed, as they were masked by the CT bands.
The electronic spectral data and the mass fragmentations of the li-
gands and their complexes are presented in Table 4.
The stoichiometry evaluation of ligands and the metal ions
UV–Vis spectroscopy titration method can help to determine
the stoichiometry of formed complexes. One of the methods is mo-
lar ratio (MR) [17] method. The curve plotted for this method is
characterized by one break located at the molar ratio [M]/[H3L]
which equals to ꢂ1. This behavior clearly indicates that the stoichi-
ometry of the different complexes formed in the solution from the
reaction of the metal ions (Co2+, Ni2+, Cu2+ and Zn2+) with ligands
(H3L1AH3L4) is 1:1. Fig. 3 shows the stoichiometry curve of Cu2+
ion with H3L4 ligand.
Another method is the comparison of UV–Vis spectrum of a
synthesized complex with the final spectrum of the formed
complex obtained from titration. If the UV–Vis spectrum of the
synthesized complex is the same as the titration final spectrum,
it can indicate that the structures are the same. The electronic
spectra of the complexes formed during the titrations were the
same as those of the separately synthesized complexes which
can show that the synthesized complexes are correct (Fig. 4) .
Fig. 3. The molar ratio curve of Cu2+ ion and H3L4 ligand in DMF at 25 °C.
Thermodynamic studies by UV–Vis absorption spectroscopy
and SQUAD computer program were used to find the formation
constants of complexes and the metal and ligand effects on them.
Formation constant measurements have been determined by UV–
Vis absorption spectroscopy through titration of the ligands with
various concentrations of metal ions at constant ionic strength
(0.10 M NaClO4 at 25 °C ( 0.1 °C)). In a typical measurement,
2.5 mL solution of the ligands (3 ꢃ 10ꢁ5–2 ꢃ 10ꢁ3 M) was
transferred into thermodynamic cell compartment of UV–Vis

184
K. Mohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 179–185
Table 5
The logKf for the metal ions with the tetradentate Schiff base ligands in DMF at 25 °C
and at I = 0.10 M NaClO4.
Log kf
Compounds
Co2+
Ni2+
Cu2+
Zn2+
H3L1
H3L2
H3L3
H3L4
4.86( 0.05)
4.76( 0.03)
4.35( 0.01)
4.29( 0.01)
4.93( 0.02)
4.86( 0.03)
4.57( 0.02)
4.38( 0.01)
5.96( 0.15)
5.02( 0.07)
4.91( 0.03)
4.71( 0.01)
4.74( 0.07)
4.37( 0.01)
4.12( 0.04)
3.98( 0.22)
in one row, the amount of effective nucleic charge increases with
an increase in the atomic number, and then we can expect that
Cu2+ and Ni2+ions have more formation constants.
Fig. 4. UV–Vis spectra of the ligand, H3L2 (1), the end point of the titration of the
ligand with Cu(II) ion (2), and the synthesized [Cu(HL2)] (3) in DMF.
The above trend is also confirmed by considering the crystal
field stabilization energy (CFSE). The tendency for coordination to
the ligand was increased with the increasing of the CFSE. Cu2+
and Ni2+ have more CFSE but Zn2+ and Co2+ have a smaller one
which is according to the formation constant sequence.
Moreover, the ionic radius of the metals, 0.68, 0.67, 0.65, 0.63
for zinc, cobalt, copper and nickel, respectively [19], helps us to
interpret this sequence that each metal ion which has ionic radius
according to the size of chelate ligand can interact better with
ligand.
In the end, the factor which can influence the sequence of the
formation constant is the structure and shape of the ligand and
metal ions. If they have the same structure and shape, they can
interact in a good manner with each other. Cu2+ and Ni2+ions prefer
the square planar form but Zn2+ and Co2+ ions like the tetrahedral
form. The ligands like to have square planar structure, therefore
Cu2+ and Ni2+ ions form more stable complexes with the ligands,
so they have more formation constants.
instrument and titrated by the metal ion solution. The titration was
performed with aliquots of the metal ion with Hamilton syringe to
the ligand. The absorption measurements were carried out at
various wavelengths where the difference in absorption was the
maximum after equilibrium. The variation of the electronic spectra
for H3L4 titrated with various concentrations of Cu(II) acetate
monohydrate at 25 °C in DMF is shown in Fig. 5. The same changes
are valid for other systems.
The formation constants, Kf, were calculated using the SQUAD
computer program [15] designed to calculate the best value of
the formation constants of the proposed equation model (Eqs. (1)
and (2)) by employing a non-linear, least-squares approach.
MðOAcÞ þ H3LX ꢀ ½MðHLXÞꢄ þ 2HOAc Kf
ð1Þ
ð2Þ
2
Kf ¼ ½MðHLXÞꢄ=ð½M2þꢄ½H3LXꢄÞ
The formation constants, Kf, are presented in Table 5.
The effects of substituent of Schiff base ligands
The metal effect
The substituted effects on Schiff base ligands were studied by
changing the different substituents on ligands. The formation con-
stants of the metal ions with the different ligands decrease accord-
ing to the following sequence:
With respect to the same ligand, the specific formation constant
value generally runs according to the following sequence:
ZnðIIÞ < CoðIIÞ < NiðIIÞ < CuðIIÞ
H3L1ðOHÞ > H3L2ðOMeÞ > H3L3ðHÞ > H3L4ðBrÞ
This is in agreement with the general order of the stability of the
complexes of these metal ions which has been reported before
[18]. The increasing of the formation constant is according to differ-
ent factors such as: effective nucleic charge, crystal field stabiliza-
tion energy (CFSE) and ionic radius, structure of complex.
This sequence can be related to two factors of ligands: electronic
and steric effects. This is somewhat in agreement with decreasing
electron-releasing character of the substituents in the same direc-
tion, which results in a decrease in the basicity of the azomethine
nitrogen, and the phenolic oxygen groups of the ligand. Conse-
quently, the tendency toward complex formation is expected to
be decrease. For example, H3L1 ligand (which has two OH substi-
tuted groups) has the tendency to act as a good electron donor as
the result of the high electron-releasing power of the (OH) groups
relative to the substituted (OMe) in H3L2 ligand and electron with-
drawing (Br) group in H3L4. The withdrawing functional groups
make the Schiff base a poor donor ligand and decrease the forma-
tion constants while the electron donor groups increase the forma-
tion constants. Therefore, the ligand with the (Br) groups has the
smallest formation constant while the ligand with (OH) has the
highest (Table 5) [20,21].
The trend for the stability of the metal ions increases as the
effective nucleic charge increases. In transition metals of d-block
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Conclusions
290
315
340
365
390
415
440
465
In this work, some new tetradentate Schiff base ligands contain-
ing the N2O2 donor group have been synthesized and spectrally
characterized. Then a series of copper, cobalt, nickel and zinc com-
plexes of these tetradentate Schiff base ligands have been synthe-
sized and characterized by several spectroscopic methods. At the
Wavelength(nm)
Fig. 5. The variation of the electronic spectra of H3L4 titrated with various
concentrations of Cu (II) acetate at constant ionic strength (I = 0.10 M NaClO4)
and at 25 °C in DMF.

K. Mohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 179–185
185
[4] A. Elmali, M. Kabak, Y. Elerman, J. Mol. Struct. 477 (2000) 151–158.
[5] P.R. Patel, B.T. Thaker, S. Zele, Indian J. Chem. Sec. A 38 (1999) 563–567.
[6] K. Shanker, R. Rohini, V. Ravinder, P.M. Reddy, Y.P. Ho, Spectrochim. Acta Part A
73 (2009) 205–211.
end, the formation constants show that with increasing in ten-
dency to have a square planar structure, the amount of formation
constants increase as follows:
[7] S.K. Bharti, G. Nath, R. Tilak, S.K. Singh, Eur. J. Med. Chem. 45 (2010) 651–660.
[8] O.M. Walsh, M.J. Meegan, R.M. Prendergast, T.A. Nakib, Eur. J. Med. Chem. 31
(1996) 989–999.
[9] S. Ke, X. Qian, F. Liu, N. Wang, Q. Yang, Z. Li, Eur. J. Med. Chem. 44 (2009) 2113–
2121.
[10] S.S. Mandal, U. Varshney, N. Vinaykumar, J. Inorg. Biochem. 8 (1997) 798–812.
[11] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, fifth ed., John Wiley & Sons, New York, 1997.
[12] G.G. Mohamed, Z.H.A.E. Wahab, Spectrochim. Acta Part A 61 (2005) 1059–
1068.
Cu2þ > Ni2þ > Co2þ > Zn2þ
The electron donor group increases the formation constant while a
withdrawing functional group decreases it. The trend of the forma-
tion constants is as follows:
½MðHL1Þꢄ > ½MðHL2Þꢄ > ½MðHL3Þꢄ > ½MðHL4Þꢄ; M ¼ Cu; Zn; Co; Ni:
[13] G.G. Mohamed, M.M. Omar, A.M.M. Hindy, Spectrochim. Acta Part A 62 (2005)
1140–1150.
[14] H. Koksal, M. Tumer, S. Serin, Synth. React. Inorg. Met. Org. Chem. Nano-Met.
Chem. 26 (1996) 1577–1588.
Acknowledgements
[15] D.L. Leggett, Computational Methods for the Determination of Formation
Constant, Plenum Press, New York, 1985.
[16] S. Zolezzi, A. Deciniti, E. Spondine, Polyhedron 18 (1999) 897–904.
[17] R.C. Felicio, E.T.G. Canalherio, E.R. Dockal, Polyhedron 20 (2001) 261–268.
[18] H.H. Jaffe, M. Orehin, Theory and Application of Ultroscopy, John Wiley and
Sons, New York, 1982.
The authors thank Persian Gulf University of Bushehr Research
Councils for financial support of this work.
References
[19] L.H. Ahrens, Cosmochim. Acta 2 (1976) 155–169.
[20] M. Asadi, A.H. Sarvestani, M. Abbasi, J. Chem. Res. 56 (2007) 56–59.
[21] M. Asadi, M. Mohammadikish, Kh. Mohammadi, Cent. Eur. J. Chem. 8 (2)
(2010) 291–299.
[1] S.M. Abdallah, G.G. Mohamed, M.A. Zayed, M.S. Abou El-Ela, Spectrochim. Acta
Part A 73 (2009) 833–840.
[2] P. Singh, R.L. Goel, B.P. Singh, J. Indian Chem. Soc. 52 (1975) 958–959.
[3] B.F. Perry, A.E. Beezer, R.J. Miles, B.W. Smith, J. Miller, M.G. Nascimento,
Microbois 45 (1988) 181–191.