Synthesis, characterization and properties of some divalent metal(II) complexes: Their electrochemical, catalytic, thermal and antimicrobial activity studies

Article abstract of DOI:10.1016/j.saa.2006.09.009

In this study, we synthesized the amine compound 2-(2-aminoethyliminomethyl)phenol (H₃A) as the starting material, and then we prepared the polydentate Schiff base ligands from the reactions of the amine compound (H₃A) with phtaldialdehyde (H₂L), 4-methyl-2,6-di-formlyphenol (H₃L¹) and 4-t-butyl-2,6-di-formylphenol (H₃L²) in the ethanol solution. Moreover, the complexes Cd(II), Cu(II), Co(II), Ni(II), Zn(II) and Sn(II) of the ligands H₂L, H₃L¹ and H₃L² have been prepared. All compounds have been characterized by the analytical and spectroscopic methods. In addition, the magnetic susceptibility and molar conductance measurements have been made. The catalytic properties of the mono- and binuclear Co(II) and Cu(II) complexes have been studied on the 3,5-di-tert-butylcatechol (3,5-DTBC) and ascorbic acid (aa) as a substrate. The oxidative C-C coupling properties of the Co(II) and Cu(II) complexes have been investigated on the sterically hindered 2,6-di-tert-butylphenol (dtbp). The antimicrobial activity properties of the ligands and their mono- and binuclear complexes have been studied against the bacteria and fungi. The results have been compared to the antibacterial and fungi drugs. The TGA curves show that the decomposition takes place in three steps for all complexes. Electrochemical properties of the complexes Cu(II) and Ni(II) have been investigated for the first time in acetonitrile by cyclic voltammetry.

Full text of DOI:10.1016/j.saa.2006.09.009

Spectrochimica Acta Part A 67 (2007) 916–929
Synthesis, characterization and properties of some divalent
metal(II) complexes: Their electrochemical, catalytic,
thermal and antimicrobial activity studies
a,∗
b
b
a
¨
¨
Mehmet Tumer , Duygu Ekinci , Ferhan Tumer , Akif Bulut
a University of Kahramanmaras¸ Su¨tc¸u¨ Imam, Faculty of Science and Arts, Department of Chemistry, 46100 Kahramanmaras¸, Turkey
b Atatu¨rk University, Faculty of Science and Arts, Department of Chemistry, Erzurum, Turkey
Received 5 January 2006; received in revised form 31 August 2006; accepted 11 September 2006
Abstract
In this study, we synthesized the amine compound 2-(2-aminoethyliminomethyl)phenol (H3A) as the starting material, and then we prepared
the polydentate Schiff base ligands from the reactions of the amine compound (H3A) with phtaldialdehyde (H2L), 4-methyl-2,6-di-formlyphenol
(H3L1) and 4-t-butyl-2,6-di-formylphenol (H3L2) in the ethanol solution. Moreover, the complexes Cd(II), Cu(II), Co(II), Ni(II), Zn(II) and Sn(II)
of the ligands H2L, H3L1 and H3L2 have been prepared. All compounds have been characterized by the analytical and spectroscopic methods. In
addition, the magnetic susceptibility and molar conductance measurements have been made. The catalytic properties of the mono- and binuclear
Co(II) and Cu(II) complexes have been studied on the 3,5-di-tert-butylcatechol (3,5-DTBC) and ascorbic acid (aa) as a substrate. The oxidative
C–C coupling properties of the Co(II) and Cu(II) complexes have been investigated on the sterically hindered 2,6-di-tert-butylphenol (dtbp). The
antimicrobial activity properties of the ligands and their mono- and binuclear complexes have been studied against the bacteria and fungi. The
results have been compared to the antibacterial and fungi drugs. The TGA curves show that the decomposition takes place in three steps for
all complexes. Electrochemical properties of the complexes Cu(II) and Ni(II) have been investigated for the first time in acetonitrile by cyclic
voltammetry.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Schiff base; Electrochemical; Catalytic; Thermal and antimicrobial activity
1. Introduction
these complexes would be obstructed by the steric constraint
due to the bulky i-Pr group.
The metal complexes with Schiff bases as ligands have been
playing an important role in the development of coordination
chemistry as a whole. However, it was not until the 1950s that
concrete and rapid advances in this field became evident. In
the early days, the main efforts were directed toward synthesis
and characterization of rather fundamental complexes, which do
not look striking nowadays but were strongly needed in those
days. Some of typical examples included metal complexes of the
following types: M(II)(X–Sal–NR)2, M(III)(X–Sal–NR)3, and
so on, where X–Sal–NR denotes bidentate Schiff bases [1]. To
cite another example, it was regarded as significant to synthesize
complexes of the formula Co(X–Sal–NR)3, R: i-Pr, where X: 5-
Br and 5-NO2 [2]; it had been believed that the formation of
A wide variety of cobalt(II) complexes are known to bind
dioxygen more or less reversibly and are therefore frequently
studied as model compounds for natural oxygen carriers and
for their use in O2 storage, as well as in organic synthesis due
to their catalytic properties under mild conditions [3]. In this
respect, Co(II)complexeswithN-donorligandscontainingbind-
ing units suitable either for the coordination of a single metal
ion or for assembling dimetallic centres have been shown to
be particularly useful [4]. The catechol oxidases (EC 1.10.3.1)
are type 3 copper enzymes containing a dinuclear copper centre
[5]. Well-known representatives of these type 3 copper proteins
are hemocyanin [6,7], the dioxygen carrier for arthropods and
mollusks, and tyrosinase [8]. Catechol oxidase belongs, like
tyrosinase, to the polyphenol oxidases which oxidize pheno-
lic compounds to the corresponding quinones in the presence
of oxygen. Whereas tyrosinase (EC 1.14.18.1) catalyzes the
hydroxylation of tyrosine to dopa (cresolase activity) and the
∗
Corresponding author. Tel.: +90 344 2191093; fax: +90 344 2191042.
E-mail address: mtumer@ksu.edu.tr (M. Tu¨mer).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.09.009

M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
917
oxidation of dopa to dopaquinone (catecholase activity) with
electron transfer to dioxygen, catechol oxidase exclusively cat-
alyzes the oxidation of catechols to quinones without acting on
tyrosine [9].
Numerous electrochemical studies have been made for a
fairly large number of acyclic and macrocyclic copper(II) com-
plexes derived from Schiff bases. These investigations revealed
that the redox properties of copper(II) complexes are markedly
influenced by structural and electronic factors [10,11].
The present study deals with the synthesis, characterization,
antimicrobial activity, catalyses, thermal and electrochemical
properties of the Schiff bases and their metal complexes.
ate diamine, with aldehydes. To the vigorously stirred and cool
dilute solution (T = 5–10 ◦C) of the ethylenediamine (20 mmol)
in the absolute ethanol (100 mL) under argon, was added drop-
wise a cooled solution of the salicylaldehde (15 mmol) in the
absolute ethanol (80 mL). After the addition was complete,
the mixture was stirred for 15–30 min and then refluxed for
15–60 min. The resulting solution was evaporated under vacuum
to remove the solvent and excess diamine was extracted by ben-
zene and was used for the next step without further purification.
H3A: Yield (40%), color: yellow, m.p. 105 ◦C. Found (%
calc.): C: 65.80 (65.83), H: 7.40 (7.37), N: 17.10 (17.06).
UV–vis: (λmax, nm, EtOH as solvent): 407, 342, 315, 290, 268,
255. i.r.: (KBr, cm−1): 3377 [␯(OH)], 2872 [␯(CH2)], 2640
1
2. Experimental
[␯(O–H· · ·N)], 1633 [␯(CH N)], 1377 [␯(C–OH)]. H NMR:
(CDCl3 as solvent, δ in ppm): 10.5(s, OH), 6.66–7.99(Ar–H,
2.1. Material
M), 8.68(CH N), 4.52(t, NH2), 3.22; 3.23(t, CH2, p, CH2).
All organic compounds and the solvents were purchased from
Fluka and Merck. The metal salts CuCl2·2H2O, CoCl2·6H2O,
CdCl2·2H2O, NiCl2·6H2O, SnCl2·2H2O and ZnCl2 were
obtained from Fluka and used for complex preparation. 4-
Methyl-2,6-di-formylphenol and 4-t-butyl-2,6-di-formylphenol
were obtained according to the literature method [12].
2.4. Synthesis of the Schiff bases H2L, H3L1 and H3L2
The unsymmetrical Schiff bases were obtained by conden-
sation of the half units and the appropriate aldehydes. To the
stirred solution of the precursor (half unit, H3A) in anhydrous
ethanol was added a solution of phtaldialdehyde, 4-methyl-
2,6-di-formylphenol and 4-t-butyl-2,6-di-formylphenol in anhy-
drous ethanol. The mixture was concentrated in vacuum by
evaporation of the solvent until a colored solid precipitated. The
product was filtered, washed with cold solvent and recrystal-
lized from the appropriate solvents (methanol/hexane mixture
(1:1) for H2L, H3L1 and H3L2) to give colored crystals. General
formulae of the ligands are shown in Fig. 1.
2.2. Instrumentation
Elemental analyses (C, H, N) were performed using a Carlo
Erba 1106 elemental analyser. Infrared spectra were obtained
using KBr discs (4000–400 cm−1) on a Shimadzu 8300 FT-IR
spectrophotometer. The electronic spectra in the 200–900 nm
range were obtained on a Shimadzu UV-160 A spectropho-
tometer. Magnetic measurements were carried out by the Gouy
method using Hg[Co(SCN)4] as calibrant. Molar conductances
of the Schiff base ligands and their transition metal complexes
were determined in MeOH (∼10−3 M) at room temperature
using a Jenway Model 4070 conductivity meter. Mass spectra
of the ligands were recorded on a VG ZabSpec GC–MS spec-
trophotometer with fast atom bombardment. 1H and 13C NMR
spectra were recorded on a Varian Gemini XL-200 instrument.
TMS was used as internal standard and deuteriated CDCl3 as
solvent. The metal contents of the complexes were determined
according to the literature methods [13]. Chlorine was deter-
mined gravimetrically as silver chloride. The thermal analyses
studies of the complexes were performed on a Perkin-Elmer
Pyris Diamond DTA/TG Thermal System under nitrogen atmo-
sphere at a heating rate of 10 ◦C/min.
The spectral data of the ligands are given below:
H3L1:λmax(nm), hexane:362, 322, 298, 249;toluene:476, 364,
350, 324; EtOH: 461, 411, 368, 350, 319, 258, 252; MeOH:
407, 365, 315, 286, 280, 253. FT-IR (cm−1, KBr): ␯(OH) 3422,
␯(CH3) 2945, ␯(O–H· · ·N) 2741, ␯(CH N)x 1634, ␯(CH N)y.
H3L2: λmax(nm), hexane: 363, 324, 318, 256; toluene: 438,
367, 362, 324; EtOH: 431, 414, 349, 309, 274, 285, 253;
MeOH: 424, 362, 318, 280, 254. FT-IR (cm−1, KBr): ␯(OH)
3420, ␯(CH3) 2959, ␯(O–H· · ·N) 2590, ␯(CH N)x 1633,
␯(CH N)y 1612.
H2L: λmax(nm), hexane: 389, 350, 334, 274, 259; toluene: 462,
450, 395, 350, 334, 286; EtOH: 431, 414, 349, 309, 274, 285,
253; MeOH: 410, 349, 332, 295, 277. FT-IR (cm−1, KBr):
␯(OH) 3370, ␯(CH3) 2948, ␯(O–H· · ·N) 2590, ␯(CH N)x
1631, ␯(CH N)y 1620.
Catalysis experiments were monitored on a Shimadzu 160-
A UV–vis spectrophotometer. The change in the concentration
of the TTBD was monitored by optical spectroscopy. For the
catalytic activity of the mono- and binuclear complexes Co(II)
and the [Cu(H2L)]Cl2 complex, a similar procedure was adopted
for reactions with the aa, the d–d band being monitoring.
x and y are salicylidene and diformyl fragments, respectively.
2.5. Preparation of the complexes
The complexes were prepared by similar methods. A solution
of the metal salt (1 mmol for the ligand H2L, 2 mmol for the
ligands H3L1 and H3L2) in the absolute MeOH (25 mL) was
added to a solution of the ligands H2L, H3L1 and H3L2 (1 mmol)
in the absolute MeOH (20 mL) and the mixture was boiled under
reflux for 6–7 h. At the end of the reaction, determined by t.l.c.,
the precipitate was filtered off, washed with EtOH and Et2O,
and dried in vacuo.
2.3. Synthesis of the 2-(2-aminoethyliminomethyl)phenol
(H3A)
Theprecursorwaspreparedaccordingtothemodifiedmethod
described earlier [14,15] by mono condensation of the appropri-

918
M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
Fig. 1. General formulae of the ligands H3A, H2L, H3L1 and H3L2.
Cd2(L1)Cl: λmax(nm), EtOH: 361, 281, 221.
Λ
[Cd(H2L)]Cl2: λmax(nm), EtOH: 422, 358, 263.
Λ
(ꢀ−1 cm2 mol−1): 57.7. FT-IR (cm−1, KBr): ␯(OH) 3372,
␯(CH2) 2904, ␯(CH N)x 1614, ␯(CH N)y 1597, ␯(M–O) 532,
␯(M–N) 434. [Cu(H2L)]Cl2: λmax(nm), EtOH: 613, 377, 284,
250. Λ (ꢀ−1 cm2 mol−1): 61.3. FT-IR (cm−1, KBr): ␯(OH)
3370, ␯(CH2) 2900, ␯(CH N)x 1618, ␯(CH N)y 1600, ␯(M–O)
534, ␯(M–N) 442. μeff (B.M.): 1.79. [Co(H2L)]Cl2: λmax(nm),
EtOH: 643, 382, 291, 248. Λ (ꢀ−1 cm2 mol−1): 60.4. FT-IR
(cm−1, KBr): ␯(OH) 3372, ␯(CH2) 2893, ␯(CH N)x 1620,
␯(CH N)y 1598, ␯(M–O) 490, ␯(M–N) 468. μeff (B.M.):
4.27. [Ni(H2L)]Cl2: λmax(nm), EtOH: 610, 376, 288, 254.
Λ (ꢀ−1 cm2 mol−1): 58.9. FT-IR (cm−1, KBr): ␯(OH) 3375,
␯(CH2) 2907, ␯(CH N)x 1608, ␯(CH N)y 1600, ␯(M–O) 518,
␯(M–N) 475. [Sn(H2L)]Cl2: λmax(nm), EtOH: 432, 390, 278,
260. Λ (ꢀ−1 cm2 mol−1): 54.6. FT-IR (cm−1, KBr): ␯(OH)
3370, ␯(CH2) 2908, ␯(CH N)x 1615, ␯(CH N)y 1594, ␯(M–O)
507, ␯(M–N) 470. [Zn(H2L)]Cl2: λmax(nm), EtOH: 398, 373,
260, 245. Λ (ꢀ−1 cm2 mol−1): 57.1. FT-IR (cm−1, KBr): ␯(OH)
3372, ␯(CH2) 2905, ␯(CH N)x 1625, ␯(CH N)y 1598, ␯(M–O)
507, ␯(M–N) 450.
(ꢀ−1 cm2 mol−1): 17.2. FT-IR (cm−1, KBr): ␯(CH3) 2948,
␯(CH2) 2904, ␯(CH N)x 1610, ␯(CH N)y 1590, ␯(M–O) 532,
␯(M–N) 434. Cu2(L1)Cl: λmax(nm), EtOH: 612, 581, 361, 281,
221. Λ (ꢀ−1 cm2 mol−1): 18.9. FT-IR (cm−1, KBr): ␯(CH3)
2947, ␯(CH2) 2897, ␯(CH N)x 1620, ␯(CH N)y 1595, ␯(M–O)
503, ␯(M–N) 444. μeff (B.M.): 1.10. Co2(L1)Cl: λmax(nm),
EtOH: 652, 468, 390, 350, 335, 280, 256. Λ (ꢀ−1 cm2 mol−1):
16.5. FT-IR (cm−1, KBr): ␯(CH3) 2926, ␯(CH2) 2896,
␯(CH N)x 1641, ␯(CH N)y 1600, ␯(M–O) 473, ␯(M–N) 444.
μeff (B.M.): 4.18. Ni2(L1)Cl: λmax(nm), EtOH: 557, 393, 345,
317, 240. Λ (ꢀ−1 cm2 mol−1): 17.0. FT-IR (cm−1, KBr):
␯(CH3) 2926, ␯(CH2) 2900, ␯(CH N)x 1625, ␯(CH N)y 1604,
␯(M–O) 543, ␯(M–N) 471. Sn2(L1)Cl: λmax(nm), EtOH: 364,
352, 345, 301, 241. Λ (ꢀ−1 cm2 mol−1): 18.3. FT-IR (cm−1
,
KBr): ␯(CH3) 2997, ␯(CH2) 2930, ␯(CH N)x 1623, ␯(CH N)y
1595, ␯(M–O) 510, ␯(M–N) 475. Zn2(L1)Cl: λmax(nm), EtOH:
471, 348, 275, 255. Λ (ꢀ−1 cm2 mol−1): 15.9. FT-IR (cm−1
,
KBr): ␯(CH3) 2922, ␯(CH2) 2900, ␯(CH N)x 1639, ␯(CH N)y
1605, ␯(M–O) 497, ␯(M–N) 467.
Cd2(L2)Cl: λmax(nm), EtOH: 383, 324, 303, 256. Λ
(ꢀ−1 cm2 mol−1): 18.4. FT-IR (cm−1, KBr): ␯(CH3) 2905,
␯(CH2) 2874, ␯(CH N)x 1605, ␯(CH N)y 1605, ␯(M–O) 563,
␯(M–N) 428. Cu2(L2)Cl: λmax(nm), EtOH: 562, 549, 359, 28,
220. Λ (ꢀ−1 cm2 mol−1): 16.8. FT-IR (cm−1, KBr): ␯(CH3)
2955, ␯(CH2) 2898, ␯(CH N)x 1613, ␯(CH N)y 1604, ␯(M–O)
562, ␯(M–N) 444. μeff (B.M.): 1.11. Co2(L2)Cl: λmax(nm),
EtOH: 655, 430, 391, 352, 331, 293, 255. Λ (ꢀ−1 cm2 mol−1):
17.2. FT-IR (cm−1, KBr): ␯(CH3) 2961, ␯(CH2) 2900,
␯(CH N)x 1643, ␯(CH N)y 1600, ␯(M–O) 532, ␯(M–N) 475.
μeff (B.M.): 4.15. Ni2(L2)Cl: λmax(nm), EtOH: 565, 450,
2.6. Catalytic oxidation of 2,6-di-tert-butylphenol
In a dichloromethane/methanol solvent mixture (1:1, 50 mL),
theCo(II)andCu(II)complexesweredissolved. Toit2,6-di-tert-
butylphenol was added and the solution was stirred for 48–60 h.
The solution was then filtered and the filtrate was evaporated
to dryness. The 5 mL of methanol were added to dissolve the
excess 2,6-di-tert-butylphenol.
For measuring the progress of the reaction, 50 ␮L from
the aliquot was passed through an Amberlyst cationic ion-
exchanger and washed with 10 mL (2 × 5) dichloromethane.
The change in the concentration of product 3,3ꢀ-5,5ꢀ-tetra-tert-
butyl-4,4ꢀ-diphenoquinone (TTBD) was monitored by optical
spectroscopy.
392, 345, 321, 255. Λ (ꢀ−1 cm2 mol−1): 17.3. FT-IR (cm−1
,
KBr): ␯(CH3) 2961, ␯(CH2) 2904, ␯(CH N)x 1627, ␯(CH N)y
1594, ␯(M–O) 536, ␯(M–N) 470. Sn2(L2)Cl: λmax(nm), EtOH:
446, 359, 329, 294, 289, 231. Λ (ꢀ−1 cm2 mol−1): 15.9.
FT-IR (cm−1, KBr): ␯(CH3) 2995, ␯(CH2) 2960, ␯(CH N)x
1610, ␯(CH N)y 1590, ␯(M–O) 514, ␯(M–N) 460. Zn2(L2)Cl:
λmax(nm), EtOH: 426, 350, 338, 276. Λ (ꢀ−1 cm2 mol−1):
18.1. FT-IR (cm−1, KBr): ␯(CH3) 2986, ␯(CH2) 2957,
␯(CH N)x 1627, ␯(CH N)y 1597, ␯(M–O) 529, ␯(M–N)
432.
2.7. Catecolase activity studies
The reaction of the Co(II) complexes and the [Cu(H2L)]Cl2
complex with 3,5-di-tert-butylcatechol (3,5-DTBC) and ascor-
bic acid (aa) was monitored as follows: in a typical exper-

M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
919
iment, 100 mm3 of a solution of the complex in methanol
([C]o = 2.4 × 10−5 mol dm−3) was added to a 1 cm path length
cell containing 3 cm3 of O2-saturated methanol at 25 ◦C. The
reaction was initiated by the addition of 100 cm3 of catechol
solution ([3,5-DTBC]o = 1.0 × 10−3 to 1.0 × 10−4 mol dm−3).
In a separate set of experiments, the kinetic determinations were
performed without the catalyst. The UV–vis spectra of the orig-
inal solution, of the solution directly after the addition and after
5, 10, 15 and 20 min were recorded and corrected for volume
changes.
previously been inoculated with the above organisms. In addi-
tion, blank paper discs treated with ampicillin, streptomycin
and nystatin-saturated antibiotics were used as positive con-
trols. Afterwards the plates combined with the discs were left
at 4 ◦C for 2 h, the plates injected with yeast were incubated
at 25 0.1 ◦C for 24 h, and ones injected with bacteria were
incubated at 37 0.1 ◦C for 24 h. After 24 h, inhibition zones
appearing around the discs were measured and recorded in mm.
The initial number of microorganisms in the suspension was
determined for the total yeasts and bacterial count during 24 h
at 37 ◦C for bacteria and 48 h 25 ◦C for yeasts [16,17].
2.8. Electrochemical experiments
3. Results and discussion
Electrochemical studieswerecarried outwithaBAS100B/W
Electrochemical Workstation equipped with a low current mod-
ule (BAS PA-1) at a platinum disk electrode (area 0.68 cm2).
Cyclic voltammetric measurements were made at room temper-
ature in a single compartment cell (BAS model C-3 cell stand)
with a platinum counter electrode and an Ag/Ag+ reference elec-
trode (BAS). All potentials are reported with respect to Ag/Ag+.
The tetrabutylammonium perchlorate (TBAClO4) as a sup-
porting electrolyte was used as received. Acetonitrile (MeCN)
was purified by drying with calcium hydride, followed by dis-
tillation from phosphorus pentoxide. It was kept over molecular
Analytical data of the ligands and their complexes are given
in Table 1 and are in well agreement with the expected values.
To prepare the ligands H2L, H3L1 and H3L2, firstly, we pre-
pared the starting material 2-(2-aminoethyliminomethyl)phenol
(H3A). One of a major difficulties in obtaining the Schiff
base H3A is due to its bis product formation. Other prob-
lem is low yield of the compound H3A. The compound
2-(2-aminoethylimino-methyl)phenol (H3A) is prepared from
salicylaldehyde and ethylenediamine in a 1:1 molar ratio in
an ethanolic solution, with subsequent removal of the excess
diamine with extraction into benzene. The reaction has been
carried out using highly dilute conditions. The reaction product
has not been crystallized. Because, both bis product [2-[({2-[(2-
hydroxybenzylidene)amino]ethyl}imino)methyl]phenol] and
the half unit compound H3A soluble in the common organic sol-
vents. Moreover, as the bis product has not free primary amine
group, it does not react with carbonyl compounds. But, the
starting material H3A has the free –NH2 functional group. This
free –NH2 group may easily react with carbonyl compounds as
the amines. Therefore, this starting material, immediately, has
been used without further purification. Also, its spectroscopic
and analytical data confirm the resulting of the compound H3A.
To prepare the ligands H2L, H3L1 and H3L2, the carbonyl
compounds phtaldialdehyde, 4-methyl-2,6-di-formylphenol
and 4-t-butyl-2,6-di-formyl-phenol with the amine compound
H3A have been used in 1:2 molar ratio (carbonyl:amine)
in ethanolic solution. Condensation between aldehydes and
amines is realized in different reaction conditions, and in
different solvents. In this study, ethanol, at room temperature
or in refluxing conditions, is also a valuable solvent for the
preparation of Schiff bases. Degradation of the Schiff bases can
occur during the purification step. Chromatography of Schiff
bases on silica gel can cause some degree of decomposition of
the Schiff bases, through hydrolysis. In this case, it is better
to purify the Schiff base by crystallization. If the Schiff bases
are insoluble in hexane or cyclohexane, they can be purified by
stirring the crude reaction mixture in these solvents, sometimes
adding a small portion of a more polar solvent (Et2O, CH2Cl2),
in order to eliminate impurities. In general, Schiff bases are
stable solids and can be stored without precautions. Conden-
sation of salicylaldehydes or salicylaldehyde derivatives with
1,2-diamines leads to the formation of one extremely important
class of ligands, generally known as “Salens”. Salicylaldehydes
˚
sieves(3 AMerck)inordertoeliminateitswatercontentasmuch
as possible. The solutions for electrochemical measurements
contained 3 × 10−3 M complex in 0.1 M TBAClO4/MeCN.
The solutions were deoxygenated by passing dry nitrogen
through the solution for 30 min prior to the experiments, and
during the experiments the flow was maintained over the
solution.
2.9. Preparation of microorganism cultures
The growth inhibitory activity of the chemical matter was
tested against thirteen bacteria [Corynebacterium xerosis UC
9165, Bacillus brevis NRS, Bacillus megaterium DSM 32, Bacil-
lus cereus EU, Mycobacterium smegmatis CCM 2067, Staphy-
lococcus aureus Cowan 1, Pseudomonas aeruginosa ATCC
27853, Klebsiella pneumoniae FMC 5, Enterococcus faecalis
ATTC 15753, Micrococcus luteus LA 2971, Yersinia entero-
colitica CMC 120 and Escherichia coli DM] and three yeasts
[Kluvyeromyces fragilis A 230, Saccharomyces cerevisiae WET
136andCandidaalbicansATCC1023]. Theantimicrobialactiv-
ities of the ligands and their metal complexes were determined
using the agar-disc diffusion method as will be described below.
The bacteria were first incubated at 37 0.1 ◦C for 24 h in nutri-
ent broth (Difco), and the yeasts were incubated in sabouraud
dextrose broth (Difco) at 25 0.1 ◦C for 24 h. The cultures of
the bacteria and yeast were injected into the petri dishes (9 cm)
in the amount of 0.1 mL. And then, mueller hinton agar and
sabouraud dextrose agar (sterilized in a flask and cooled to
45–50 ◦C) were homogenously distributed onto the sterilized
petri dishes in the amount of 15 mL. Subsequently, the steril-
ized blank paper discs of 6 mm diameter were saturated with
1200 ␮g of the ligands and their metal complexes per disc. The
discs thus treated were placed onto the agar plates, which had

920
M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
Table 1
Analytical and physical data for the Schiff Base ligands and their Cu(II) and Cd(II) complexes
Compound
Color
Yield (%)
m.p. (◦C)
Found (calculated) (%)
C
N
M
H
H3L1
Light brown
Brown
Brown
Black
Dark red
Light red
Red
Yellow
Brown
Brown
Black
Light red
Yellow
Yellow
Red orange
Black
85
72
65
71
68
70
75
80
71
68
74
77
65
68
90
73
70
72
69
70
75
115
>250
>250
>250
>250
>250
>250
97
>250
>250
>250
>250
>250
>250
72–74
>250
>250
>250
>250
>250
>250
71.07 (71.03)
52.65 (52.63)
45.46 (45.42)
53.44 (53.43)
53.49 (53.46)
55.35 (55.32)
44.60 (44.63)
72.22 (72.27)
54.78 (54.75)
47.71 (47.67)
55.54 (55.52)
55.56 (55.55)
54.47 (54.44)
47.93 (46.88)
73.29 (73.26)
55.71 (55.66)
51.25 (51.20)
55.54 (55.52)
55.56 (55.55)
54.47 (54.44)
47.93 (46.88)
6.23 (6.18)
4.04 (4.06)
3.54 (3.51)
4.14 (4.12)
4.10 (4.12)
4.07 (4.04)
3.46 (3.44)
6.90 (6.87)
4.69 (4.71)
4.16 (4.13)
4.76 (4.78)
4.77 (4.78)
4.72 (4.69)
4.08 (4.04)
6.15 (6.10)
4.69 (4.64)
4.22 (4.27)
4.76 (4.78)
4.77 (4.78)
4.72 (4.69)
4.08 (4.04)
12.31 (12.27)
9.12 (9.09)
7.88 (7.85)
9.21 (9.24)
9.28 (9.24)
9.01 (9.04)
7.74 (7.71)
11.30 (11.24)
8.55 (8.52)
7.46 (7.41)
8.67 (8.64)
8.66 (8.64)
8.50 (8.47)
7.27 (7.30)
13.18 (13.15)
10.03 (9.99)
9.15 (9.19)
8.67 (8.64)
8.66 (8.64)
8.50 (8.47)
7.27 (7.30)
–
[Cu2(L1)]Cl
[Cd2(L1)]Cl
[Co2(L1)]Cl
[Ni2(L1)]Cl
[Zn2(L1)]Cl
[Sn2(L1)]Cl
H3L2
20.71 (20.64)
31.57 (31.52)
19.48 (19.43)
19.47 (19.38)
21.19 (21.12)
32.74 (32.70)
–
19.40 (19.33)
29.81 (29.76)
18.25 (18.18)
18.20 (18.13)
19.84 (19.77)
30.99 (30.92)
–
12.71 (12.66)
11.70 (11.65)
18.25 (18.18)
18.20 (18.13)
19.84 (19.77)
30.99 (30.92)
[Cu2(L2)]Cl
[Cd2(L2)]Cl
[Co2(L2)]Cl
[Ni2(L2)]Cl
[Zn2(L2)]Cl
[Sn2(L2)]Cl
H2L
[Cu(H2L)]Cl2
[Cd(H2L)]Cl2
[Co(H2L)]Cl2
[Ni(H2L)]Cl2
[Zn(H2L)]Cl2
[Sn(H2L)]Cl2
Orange
Black
Green
Yellow
Orange
bearing different substituents are obtained by the introduction
of a formyl group, using a simple and well established reaction,
into the corresponding phenol derivatives (Fig. 1). The Schiff
base ligands obtained from the condensation reactions soluble
in polar organic solvents such as, EtOH, MeOH, CHCl3,
(C2H5)O, etc.
ative amount of hydrocarbon character increases, the polarity
decreases. Note that hexane, which is 100% hydrocarbon, is the
least polar solvent. For example, in hexane and toluene, the lig-
and H3L1 shows eight bands in the 476–249 nm range. However,
in ethanol and methanol, the ligand H3L1 has the bands in the
461–252 nm range. In non-polar solvents, the bands observed in
the 476–322 nm range may be assigned to the ketoamine tau-
tomer and the latter to the enolimine tautomer of the Schiff
base ligands [18]. However, in polar solvents, the bands at
approximately the same region can be attributed to the enolimine
tautomeric forms. In the toluene and hexane solvents, the free
ligands have the ketoamine form. However, in EtOH and CHCl3
solvents, the enol-imine forms have been observed [19]. Other
ligands have also electronic transitions in polar and non-polar
solvents.
Electronic spectra of the ligands and their metal complexes
have been measured in EtOH and the numerical data are given
in Section 2. All the complexes show an intense band in the
324–303 nm range which is assigned to a ␲–␲* transition asso-
ciated with the azomethine linkage [20]. The spectra of the
complexes show intense bands in the high-energy region in
the 487–358 nm range which can be assigned to charge trans-
fer L → M bands [21]. The bands observed in the 628–549 nm
region can be attributed to d–d transitions of the metal ions. In the
spectra of the Cu(II) complexes, the d–d transitions are observed
in the 628–549 nm range. These values are of particular impor-
tance since they were highly dependent on the geometry of the
molecule. It is known that the transitions from a square-planar
structure to a deformed tetrahedral structure leads to a red shift
of absorption in the electronic spectra [22]. Thus, the smaller
value of the wavelength of the band corresponding to the transi-
tions is resemblance between the geometry of the complex and
that of square-planar complex.
In the case of the ligands and their metal complexes, it is
particularly important to establish whether the molecules retain
the imine character of their phenol precursor. The most useful
techniques to investigate the tautomeric forms of these ligands
are UV and NMR spectroscopy, while IR seems of limited value
here because location of the ␯(C O) and ␯(C–O) stretches in
the spectra is obscured by the abundance of aromatic skeletal
modes. In case of Schiff bases calculations of related forms of
proton transfer (pt) tautomer are possible because pt stationary
states are formed. Tautomeric forms of the polydentate ligands
H3L1 and H3L2 are shown in Fig. 2. In order to investi-
gate the keto-enol tautomeric forms of the free ligands, the elec-
tronic spectra were measured in heptane, chloroform, toluene
and ethanol. The UV spectral data of the ligands are given in
Section 2.
The stronger solute/solvent hydrogen bonding by the
hydroxyl or azomethine groups, as well as the increasing impor-
tance of the solvent polarity/polarizability in the stabilization of
the electronic excited state lead to a hypsochromic shift with
both increasing solvent hydrogen bond acceptor basicity and
solvent polarity/polarizability. This suggests that most of the
solvatochromism in the ligands is due to the solvent polarity
and basicity rather than to the solvent acidity. As the num-
ber of CH2 groups in ROH increases, the dielectric constant
decreases. If the molecules contain a polar component (OH)
and a non-polar component (R), then the polarity of a compound
reflects the balance between these two components. As the rel-

M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
921
Fig. 2. Tautomeric forms of the Schiff base ligands in the organic solvents.
complexes of the ligands H3L1 and H3L2 were found to be in
the range 15.9–18.9 ꢀ−1 cm2 mol−1 suggesting that the com-
plexes belong to the 1:1 electrolytes. This indicates that the
chloride ion in the complexes is not coordinated to the metal
ions.
IR data of the ligands and their complexes are given in
Section 2. In the spectra of the starting compound H3A with
the final compounds H3L1and H3L2, the broad bands in the
3370–3422 cm−1 range can be attributed to the ␯(OH) cm−1
vibration. In the transition metal complexes, the bands due to the
OH modes are no longer observed, denoting that all the hydroxyl
protons are displaced by MII ions (Cd, Cu, Co, Ni, Sn and Zn)
leading to covalent ␯(M–O) bonding with the ligands. In the lig-
ands, the broad bands in the 2741 and 2590 cm−1 comes from
the intramolecular hydrogen bonding along the fenolic –OH and
azomethine –CH N– groups, these bands are disappear as com-
plexation between the oxygen and nitrogen atoms with metals
ions. The ␯(CH N) band (1634–1605 cm−1) is shifted to the
lower wave numbers denoting that the nitrogen atom of the
azomethine group is coordinated to the metal(II) ions. The bond-
ing of the metal(II) ions to the ligands through the nitrogen and
oxygen atoms is further supported by the presence of new bands
in the 563–473 and 475–428 cm−1 range due to the ␯(M–O) and
␯(M–N), respectively.
The molar conductances data of the ligands and their com-
plexes are listed in Section 2. Conductance measurements were
carried out in EtOH using 10−3 M solutions. Equivalent con-
ductance of the complexes of the ligand H2L were found in
the range 54.6–61.3 ꢀ−1 cm2 mol−1 suggesting that the com-
plexes belong to 1:2 electrolytes [23]. Two chloride ions are
outside the coordination sphere. That is, they do not coordinate
to the metal ion. The equivalent conductance of the binuclear
The magnetic moments (as B.M.) of the complexes were
measured at room temperature and data are given in Section 2.
The Cd(II), Sn(II) and Zn(II) complexes of the ligands have the
diamagnetic character. Analytical and spectroscopic evidences
show that the metal ions Cd(II), Sn(II) and Zn(II) have four coor-
dinations. Therefore, we can suggest that the geometry around
the metal centres for these metal ions is the tetrahedral. The
room temperature magnetic moment measurements show that
the nickel–Schiff base complexes, are diamagnetic, suggesting
low-spin (dsp2) square-planar geometries as expected for the
d8 system. The electronic absorption spectra of all nickel(II)
complexes display two spin-allowed d–d transitions correspond-
ing to electronic excitation from the three lower levels to the
empty dx2 − y2 orbital in the square-planar geometry of nickel(II)
complexes with a Ni2–N4O3 chromophore. In addition to the
spin-allowed d–d transitions, the spectra are characterized by
strong absorption bands within the 393 and 392 nm which can
be attributed to intramolecular charge transfer (CT) interactions
and to ␲–␲* internal ligand transitions. All cobalt(II) complexes
of the ligands H2L, H3L1 and H3L2 have magnetic moments val-
ues in the 4.20, 4.19 and 4.27 B.M. range, and these values are
consistent with the tetrahedral geometry [24].

922
M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
In the binuclear complexes, the magnetic susceptibility stud-
the OH proton. When D2O is added to the solution of the lig-
ands, the OH peaks disappear. Other ligand shows also similar
spectral properties. In the spectrum of the Cd(II) complex of the
H3L1 ligand, signals due to the hydrogen atom of the azomethine
groups of the ligand shifted to the downfield according to the
free ligand. This shows that the nitrogen atom of the azomethine
group is coordinated to the metal ions. The OH signals observed
for the ligands had disappeared confirming that the phenolic
C–OH group is coordinated to the transition metal ions. In the
complex, the signals of the methylene groups also shifted to the
downfield.
ies measured at room temperature (298 K) are substantially
lower than the spin only value of 1.73 B.M., which may be due
to the presence of antiferromagnetic coupling. Room tempera-
ture susceptibilities gave μeff values at the 1.10 and 1.11 B.M.
for the complexes [Cu2(L1)]Cl and [Cu2(L1)]Cl, respectively.
The reason for such an observation is due to the quenching
of the unpaired spin of the metal ion by the unpaired spin
on the adjacent Cu(II) ion, coupled by superexchange interac-
tion (Cu–O–Cu). In other words, the complex [Cu(L)]2Cl has a
magnetic moment value (1.79 B.M.) and this indicates that the
geometry around the Cu(II) ion is tetrahedral. The coordination
geometry in the Co(II) dimers is most likely distorted tetrahe-
dral, as suggested by the observed room temperature μeff values
per Co atom (4.2–4.5 ␮B), in the range reported for several tetra-
hedral Schiff bases Co(II) complexes [25].
13C NMR spectra of the ligands and their Cd(II) complexes
were recorded in CDCl3 and are a good diagnostic tool for
determining the mode of bonding. The 13C NMR spectra of the
ligands and their complexes demonstrate similar properties. In
the spectrum of the ligand H3L1, signals due to the azomethine
carbon atoms are observed in the 168.48 and 163.05 ppm. In the
Cd(II) complex, these signals are shifted downfield, suggesting
the involvement of the nitrogen lone pair in coordination with the
metal ion. The signals of the carbon atoms of the benzene rings
areobservedapproximatelyinthe118.95–157.75 ppmrangeand
the signals observed at 61.79 and 62.50 ppm may be attributed
to the carbon atoms of the ␣-CH2 and ␣ꢀ-CH2, respectively. The
signal at the 21.65 ppm can be attributed to the methyl carbon
atom of the benzene ring. However, in the spectra of the Cd(II)
complex, the signals due to these groups are observed at the
60.05, 62.21 and 21.12 ppm.
The formulation of the ligands are deduced from analyt-
1
ical data, H and 13C NMR and further supported by mass
spectroscopy. The relatively low intensities of the molecular
ion peaks, [M]+, are indicative of the ease of fragmentation
of the compounds, and this may reflect the number of het-
eroatoms present in each structure. The spectra of the ligands
H2L, H3L1 and H3L2 show peaks at m/e 426, 455 and 497,
respectively. These peaks can attributed to the molecular ion
peaks [M]+. All the ligands decompose in a similar way. In the
spectra of the ligands, the highest intensity peaks are at m/e 144
(100%), 160 (100%) and 202 (100%). They may be assigned
to the [C8H6N2O]2+, [C9H8N2O]2+ and [C12H14N2O]2+ ions
which are formed by the loss of other parts (salicylidene moiety
together with daimine fragment) of the molecular ions.
3.1. Catecholase activity
The 1H and 13C NMR spectra of the ligands were recorded
in CDCl3 and data are given in Table 2. The ligands have two
different surroundings. These are the alkylformylphenol and sal-
icylidene moiety. The chemical shifts of these are different. In
two ligands, there are two azomethine groups in the different
centres. In addition, there are methylene chains as bridge. In
the methylene chain, there are two CH2 groups as ␣-CH2 and
␣ꢀ-CH2. These are shown in the different regions. The ligands
show approximately the same peaks. In the 1H NMR spectrum,
the triplets in the 1.26–1.18 ppm range can be attributed to the
protons of the ␣-CH2 and ␣ꢀ-CH2 groups. The protons of the
methyl group of the aromatic ring are observed at 2.29 ppm as
a singlet. The multiplets in the 6.62–7.33 ppm range may be
assigned to the protons of benzene rings. The azomethine pro-
tons of the ligand are observed in the 8.36 and 8.42 ppm range
as the singlet. A singlet in the 10.45 ppm may be attributed to
The ability of the complexes to oxidise acid and catechol
substrates using molecular oxygen was examined using as sub-
strates aa and 3,5-DTBC. Catalytic studies were performed in
methanol solution owing to the good solubility of the complexes
as well as the substrate in methanol. The oxidation product
3,5-di-tert-butyl-o-banzoquinone (3,5-DTBQ) is considerably
stable and exhibits a strong absorption at 400 nm. Therefore, the
activities and reaction rates can be determined using electronic
spectroscopy by following the appearance of the absorption
maximum of the quinone. Phenolic compounds have a specific
reactivity due to the interaction of the hydroxyl groups and the
aromatic ring not found in either the aromatic or hydroxy group
alone. This is due to the freedom of two electrons from the oxy-
gen atom being able to partially shift. The phenolic molecule
becomes more reactive due to this electron shift. In particu-
lar, the withdrawal of electron charge from the oxygen tends
Table 2
The 1H (13C) NMR data (as ppm) for the Schiff base ligands and their diamagnetic complexes using CDCl3 as solvent
Compound
␣CH2
␣ꢀCH2
CH3
t-Bu
Ar
CH
N
OH
H3L1
1.26 (61.79)
1.43 (61.42)
1.28 (60.50)
1.35 (60.05)
1.57 (60.82)
1.68 (61.77)
1.18 (62.50)
1.41 (61.75)
1.30 (61.95)
1.24 (62.21)
1.58 (59.25)
1.52 (62.63)
2.29 (21.65)
–
–
2.32 (21.12)
–
–
–
6.62–7.33 (118.95–157.75)
6.80–7.85 (115.33–158.39)
6.74–7.65 (112.45–158.38)
6.43–7.72 (115.56–159.48)
6.19–7.75 (112.74–159.80)
6.44–7.87 (113.22–156.44)
8.36 (168.48)
8.40 (168.47)
8.65 (164.33)
8.63 (164.92)
8.73 (166.76)
8.85 (165.80)
10.45
10.60
10.55
–
H3L2
1.20 (33.21)
–
–
1.37 (32.75)
–
H2L
Cd2(L1)
Cd2(L2)
[Cd(L4)]2Cl
–
10.53
␣CH2 coupling constant J ∼ 6 Hz, CH3 coupling constant J ∼ 3 Hz.

M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
923
to make the –OH group more likely to form a hydronium ion,
thus increasing the acidity of the molecule. The electron shift
also leads to more hydrogen bonding possibilities. This hydro-
gen bonding is what gives the phenolic its ability to bind with
proteins via the weak hydrogen bonding force. When a phe-
nolic ring has two adjacent hydroxyl groups, it may oxidise
to a quinone. Once formed the quinones may also react with
electron rich compounds (negative charge). The simplest phe-
nolic compound substrate of polyphenol oxidases is catechol
since it has the basic o-di-hydroxyphenol structure, yet, this
does not mean that it is the best substrate. Any substitution on
the ring (position and nature of the substituent) affects the rate
of the enzymatic reaction. As an example, the oxidation rate
for 3,4-di-hydroxyphenolic compounds was higher than the rate
of oxidation for 2,3-di-hydroxyphenolic compounds [26]. The
presence of an electron donating group at position 4 increases the
rate of the oxidation reaction, whereas an electron withdrawing-
group reduces substrate reactivity [27]. The electron-pushing
groups make the catalyst more active by increasing the electron
density on the imine-nitrogen, which thus positively influences
the hydrogen bond-formation with the substrate, as has been pro-
posed in the pre-equilibrium step prior to oxidation (Scheme 1).
On the other hand, substitute groups on the benzene ring also
decrease the affinity for the enzyme possibly due to steric hin-
drance [28]. In addition to, there are also other factors that need
to be considered, such as electrochemical properties, the geom-
etry imposed by the ligands on the metal ion and the nature of
any exogenous donors. While the ligand H3L2 has the tert-butyl
group at para position of the di-formyl ring, the ligand H3L1
has the methyl group in the same position. The tert-butyl group
gives to the ring more electron density than the methyl group.
Therefore, the reactivity of the complex increases.
The two-electron oxidation of 3,5-DTBC to quinone was
investigated since this is the reaction that the cobalt-containing
enzyme catechol oxidase catalyses. In order to compare the dif-
ferencebetweenmononuclearanddinuclearcomplexes, wehave
studied our cobalt complexes for catecholase activity, using the
changes in the electronic spectrum after the addition of the 3,5-
DTBC substrate. The catecholase activity of the [Cu(H2L)]Cl2,
[Co(H2L)]Cl2, [Co2(L1)]Cl and [Co2(L2)]Cl complexes has
been studied with the help of electronic spectroscopy by moni-
toring the appearance of the absorption maximum of the substi-
tuted quinone product (λmax = 400 nm, εmax = 1900 M−1 cm−1).
In binuclear cobalt(II) complexes, larger magnetic exchange
interaction has been observed when the planarity of the metal
centre increases. Therefore, binuclear systems with larger mag-
netic interaction should exhibit greater catalytic activity. The
catecholase activity studies of a few of the present complexes
reveal that only the complexes of the ligands H3L1 and H3L2
have significant catalytic activity with respect to the aerial oxi-
dation of 3,5-DTBC to its corresponding quinones compared to
complexes of the ligand H2L. This behavior can be explained by
the fact that two cobalt centres must be nearby to link two phe-
nolic oxygen atoms of catechol to mediate the redox reaction.
The complex [Co(H2L)]Cl2 shows little catalytic activity as it is
mononuclear. In other words, the complex [Cu(H2L)]Cl2 does
not show the any activity. The Co(II) complexes proved to be
significantly more efficient, generally effecting efficient conver-
siontotheo-quinonewithin24 handwithexcellentrecyclability.
The overall stoichiometry is given by the equation:
DBCatH2 + 0.5O2 → DTBQ + H2O
Suggested catalytic cycle for the 3,5-di-tert-butylcatechol by the
Co(II) complexes is shown in Scheme 1.
Scheme 1. L: The oxygen atom of the di-formylphenol fragment, X: the oxygen atom of the salicylidene moiety, M: Co(II). Suggested catalytic cycle for 3,5-DTBC.

924
M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
Reaction of the ascorbic acid with the mono- and binuclear
Co(II) and the [Cu(H2L)]Cl2 complexes were examined under
nitrogen. The reduction process was monitored by measuring
the changes in absorbance of the d–d bands of the complexes.
The d–d bands of the complexes [Co2(L1)]Cl, [Co2(L2)]Cl
and [Co(H2L)]Cl2 in the range 655–643 and 613 nm for
[Cu(H2L)]Cl2 disappearonadditionofascorbicacidundernitro-
gen, reappearing when the solution is exposed to air. It has been
shown that only CuII complex with the tetrahedral structure are
reduced to CuI by ascorbic acid. Planar CuII complexes are not
reduced by these two-electron donors. Reduction of the Co(II)
and Cu(II) complexes by ascorbic acid are therefore indirect
evidence for the tetrahedral structure of the complexes. This
has been inferred from spectral data. Moreover, the reduction
process with ascorbic acid depend on the structure of the com-
plexes. Binuclear and mononuclear Co(II) complexes are more
active than the [Cu(H2L)]Cl2 complex. Also, the ligand H3L2
has more wide chelate ring (n = 3) than the other ligands (n = 2).
Therefore, the complex [Co2(L2)]Cl has highest activity against
to the ascorbic acid. For the complexes, the activity order is:
Fig. 3. Proposed structure of the intermediate X.
2CoII + O2 + DBCatH2 → CoIII(DBCatH) + CoIIIO2H (5)
2CoIIIO2H → 2CoIIIOH + O2
(6)
CoIIIOH + DBSQ•− + H+ → CoII + DTBQ + H2O
(7)
CoIIIOH + CoIII(3, 5-DBCatH)
→ CoII + CoIII(DBSQ•−) + H2O
(8)
(9)
CoIII(DBSQ•−) → CoII + DTBQ
The kinetic behavior requires the formation of intermediate X
(Fig. 3), which decomposes in the rate-determining H-atom
abstraction step.
[Co2(L2)]Cl > [Co2(L1)]Cl > [Co(H2L)]Cl2
> [Cu(H2L)]Cl2.
In the steady state following the fast initial phase, the over-
all stoichiometry requires that O2 reduced to H2O, rather than
H2O2, therefore, the hydroperoxo Cosalen (CoIIIO2H) formed
in step (3) should undergo disproportionation, regenerating one-
half of the O2 absorbed and producing the hydroxoCosalen
(CoIIIOH) (step (6)).
The hydrogen peroxide formation during the catalytic reac-
tion was determined by iodometric titration [29]. Also, for a
sustained catalytic cycle to occur in the steady state, a route to
the product DTBQ and a path regenerating the Co(II) catalyst are
necessary. These requirements of a catalytic cycle are fulfilled
by addition of electron transfer steps (7) and (8) to the mecha-
nism. The kinetics and mechanisms of the oxidations catalyzed
by the Co(II) complexes are very similar. The kinetic results
obtained for the systems studied are collected in Table 3. The
3.2. Kinetics of 3,5-di-tert-butylcatechol oxidation
The kinetics of the oxidation of 3,5-DTBC were determined
by the method of initial rates by monitoring the growth of
the 400 nm band of the product 3,5-DTBQ. To determine the
dependence of the rates on the substrate concentration, solutions
of the complexes [Cu(H2L)]Cl2, [Co(H2L)]Cl2, [Co2(L1)]Cl
and [Co2(L2)]Cl were treated with increasing amounts of 3,5-
DTBC. A first order dependence was observed at low con-
centrations of the substrate, whereas a saturation kinetic was
found for all three compounds [Co(H2L)]Cl2, [Co2(L1)]Cl and
[Co2(L2)]Cl at higher concentrations. A treatment on the basis of
the Michaelis–Menten model, originally developed for enzyme
kinetics, was applied. In our study, we can also propose a pre-
equilibrium of free complex and substrate on the one hand,
and a complex–substrate adduct on the other hand. The irre-
versible conversion into complex and quinone can be imagined
as the rate-determining step. Although a much more compli-
cated mechanism may be involved, the results show this sim-
ple model to be sufficient for a kinetic description. For the
determination of the kinetic parameters for the three com-
pounds studied the substrate rate was varied in the range from
1 × 10−3 to 1 × 10−4 mol dm−3 at a complex concentration of
plot of log (A /A∞ − At) versus time for the catecholase activ-
∞
ity of the complexes are shown in Fig. 4. The reactivity order
(relative rate) for the Co(II) complexes is:
[Co2(L2)]Cl (32.74) > [Co2(L1)]Cl (30.58)
> [Co(H2L)]Cl2 (3.72).
We have also examined the catalytic activity of the
mono- and binuclear complexes [Co(H2L)]Cl2, [Co(H2L)]Cl2,
Table 3
1 × 10−4 mol dm−3
.
Kinetic properties of the mono- and binuclear complexes of the ligands H2L,
H3L1 and H3L2
The observed kinetic behavior is consistent with the reaction
mechanism (1)–(9).
a
Complex
Turnover-number (h−1
)
CoII + O2 → CoIIIO2
(1)
(2)
(3)
(4)
[Cu(H2L)]Cl2
[Co(H2L)]Cl2
[Co2(L1)]Cl
[Co2(L2)]Cl
No activity
3.72
30.58
CoIIIO2 + DBCatH2 → X
32.74
X → CoIIIO2H + DBSQ•− + H+
CoII + DBSQ•− → CoIII(DBCatH)
a
Turnover number is given as the number of moles converted or produced per
unit weight and unit time.

M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
925
Fig. 6. Electronic spectra of the eluted solutions at different time intervals for
the aerial oxidation of dtbp to TTBD by using complex [Cu2(L2)]Cl as catalyst.
Fig. 4. Oxidation of catechol by the mono- and binuclear Co(II) complexes: (a)
[Co2(L1)]Cl, (b) [Co2(L2)]Cl and (c) [Co(L)]2Cl.
for the sterically hindered phenol. This different situation can
be explained by the properties of the ligands and complexes.
The Cu(II) complex of the ligand H2L is mononuclear and has
not any activity against the phenol. The reactivity order (relative
rate) for the Co(II) and Cu(II) complexes is:
[Co2(L1)]Cl, [Co2(L2)]Cl, [Cu2(L1)]Cl and [Cu2(L2)]Cl for
oxidation of 2,6-di-tert-butylphenol by air O2 to 3,3ꢀ-5,5ꢀ-tetra-
tert-butyl-4,4ꢀ-diphenolquinone (TTBD) in air saturated DCM
according to the reaction in Fig. 5. All complexes in the catal-
ysis reaction are soluble in the DCM-methanol solvent mix-
ture. If they are not also soluble in this mixture, there is not
any problem. Because, this type of reactions can occur at the
heterogen or homogen phase in the similar way. As the oxida-
tive carbon–carbon coupling of the sterically hindered phenol
2,6-di-tert-butylphenol leading to diphenoquinones is radical
mediated reactions, we have studied the mono- and binuclear
Co(II) and Cu(II) complexes for oxidative studies with 2,6-di-
tert-butylphenol. When the phenol (2 mmol) is added to the
complexes (0.01 mmol) in DCM–methanol (1:1) solvent mix-
ture (50 mL), the dark brown or black colors of the solution turn
to red or light red in presence of air. It was found that after 48 h,
the phenol was converted to the 3,3ꢀ-5,5ꢀ-tetra-tert-butyl-4,4ꢀ-
diphenolquinone.
[Co2(L2)]Cl > [Co2(L1)]Cl > [Cu2(L2)]Cl > [Cu2(L1)]Cl
> [Co(H2L)]Cl2.
The spectral properties of the complexes are shown in Fig. 5.
The course of the reaction was followed by optical spectroscopy,
as TTBD shows a characteristic peak in the electronic spectrum
at 425 nm, as seen also from Fig. 6. Such changes in the elec-
tronic spectra of the complexes at different time intervals were
monitored by removing the catalyst from the reaction solution.
At the end of the oxidative coupling reaction, the properties of
the complexes changed.
3.3. Electrochemical studies
The binuclear complexes are better oxidant than the mononu-
clear complexes. In addition, interestingly, the correspond-
ing dicopper(II) complexes containing four radicals exhibit
much less catechol oxidase activity than that of the corre-
sponding dicobalt(II) complexes. For the compound 2,6-di-tert-
butylphenol, the best oxidant is the complex of the ligand H3L2.
Moreover, the complex of the ligand H2L is the worst oxidant
A typical cyclic voltammograms of the complexes
[Ni2(L1)]Cl, [Ni2(L2)]Cl and [Cu2(L1)]Cl are shown in
Figs. 7–9 in MeCN containing 0.1 M TBAClO4. For all the
cyclic voltammograms at 100 mV s−1 scan rate, reduction at
negative potential is the usual trend observed for phenoxo cop-
per and nickel complexes because of the electronegativity and
hard nature of the phenoxide atoms in the ligand. The initial
Fig. 5. Oxidative carbon–carbon coupling of the sterically hindered phenol 2,6-di-tert-butylphenol.

926
M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
Fig. 7. Cyclic voltammogram for the reduction of the complex [Cu2(L2)]Cl in
MeCN containing 0.1 M TBAClO4 at Pt disk electrode, scan rate 100 mV s−1
.
Fig. 9. Cyclic voltammogram of the complex [Ni2(L2)]Cl in MeCN containing
0.1 M TBAClO4 at Pt disk electrode, scan rate 100 mV s−1
.
cathodic scan of the complex [Cu2(L2)]Cl (Fig. 7) shows two
reduction peaks, I (−680 mV) and II (−1200 mV). No oxidation
peak corresponding to these waves were observed at this scan
rate. This complex undergoes stepwise reductions at different
potentials. This is similar to the behavior observed for related
complexes [30]. In subsequent positive scans, two new redox
couples were observed at the potentials 420 and 1020 mV (III
and IV).
Coulometric experimentsconducted atapotential −1150 mV
than the second reduction wave indicate the consumption of two
electrons per molecule. This indicates that the electron transfer
may occur as follows:
the binuclear complexes may be assigned to the metal ion in the
first binding site, that is the ethylene compartment. The second
reduction potentials are comparable to that of the metal in the
6-coordination (N4O2) site.
For the binuclear complexes [Ni2(L1)]Cl and [Ni2(L2)]Cl,
similar voltammograms have been obtained in anhydrous ace-
tonitrile solution containing 0.1 M TBAClO4 at a platinum disc
electrode. In the complex [Ni2(L1)]Cl (Fig. 8), one irreversible
reduction peak (I) at −480 mV versus Ag/AgCl could be seen. In
the positive sweep, two quasireversible oxidation peaks appear
at 1275(II) and 1433(III) mV. Fig. 9 shows the cyclic voltammo-
gram of complex [Ni2(L2)]Cl. This cyclic voltammogram has
a similar voltammetric behavior as the [Ni2(L1)]Cl complex.
However, a new irreversible wave at −1235 mV is appeared. In
both cases, the complexes exhibit cathodic reduction peaks in the
range of 450–480 mV versus Ag/AgCl, probably corresponding
to a process involving Ni0 at the electrode surface. These reduc-
tion potentials, however, do differ the reported value of +360 mV
versus SCE for the enzyme tyrosinase isolated from mushrooms.
The cyclic voltammogram of the complex [Cu(H2L)]Cl2
shows a irreversible reduction at Epc = −0.700 and −0.842 V
versus Ag/AgCl, which can be ascribed to the Cu2+ → Cu+
process. The reduction process is irreversible for the Cu(II) com-
plex, and leads to a deposit of copper(0) on the electrode surface,
as judged by the observation of a sharp oxidation peak during
the reverse scan.
+e−
+e−
Cu(II)–Cu(II)−→ Cu(II)–Cu(I)−→ Cu(I)–Cu(I)
These new peaks may be due to a chemical change occurring
with the electron transfer. It depends on the structural reorga-
nization in coordination sphere. The reduction potentials of the
metal centers are influenced by the chelate ring size.
The electrochemical studies of similar types of complexes
with hexa- and heptadentate coordination sites reported in the
literature [31], suggest that the formal reduction potentials of
3.4. Antimicrobial studies
The biological activity of the three Schiff base ligands (H2L,
H3L1 and H3L2) and their complexes were tested against the
bacteria and yeast because bacteria can achieve resistance to
antibiotics through biochemical and morphological modifica-
tions [30]. The organisms used in the present investigations
included C. xerosis, B. brevis, B. megaterium, B. cereus, M.
smegmatis, S. aureus, M. luteusandE. faecalis(asGram-positive
bacteria) and P. aeruginosa, K. pneumoniae, E. coli, Y. entero-
colitica, Kluyveromyces fragilis, S. cerevisiae, C. albicans (as
Fig. 8. Cyclic voltammogram of the complex [Ni2(L1)]Cl in MeCN containing
0.1 M TBAClO4 at Pt disk electrode, scan rate 100 mV s−1
.

M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
927
Table 4
Antimicrobial activity data for the selected some Schiff base complexes
Microorganisms
CFUa/mL inoculum
Inhibition zone (mm)
1
2
3
4
5
6
7
8
A10b
S10c
N100d
Gram-positive bacteria
Corynebacterium xerosis
Bacillus brevis
Bacillus megaterium
Bacillus cereus
Mycobacterium smegmatis
Staphylococcus aureus
Micrococcus luteus
Enterococcus faecalis
f
6.9 × 108
7.6 × 108
8.3 × 107
5.8 × 108
6.9 × 108
9.7 × 107
5.5 × 108
6.6 × 108
24e
20
35
34
27
33
30
25
7
30
27
28
30
24
27
30
27
20
25
28
30
29
30
27
29
10
27
17
30
28
20
28
26
24
23
29
24
30
24
30
28
12
10
17
7
22
22
11
23
12
14
11
15
19
25
33
16
10
16
17
18
15
22
–
NT
NT
NT
NT
NT
NT
NT
NT
20
18
18
20
18
17
23
17
15
20
24
20
22
16
17
Gram-negative bacteria
Pseudomonas aeruginosa
Klebsiella pneumoniae
Escherichia coli
Yersiniaenterocolitica
Kluyveromyces fragilis
Saccharomyces cerevisiae
Candida albicans
5.6 × 108
8.7 × 108
9.6 × 107
6.4 × 108
7.6 × 108
5.0 × 108
6.9 × 108
35
33
24
30
28
30
30
18
15
18
12
13
16
30
26
25
30
26
30
30
30
28
27
24
23
15
28
30
30
25
26
–
31
28
27
20
22
21
14
16
20
14
26
25
28
25
16
30
30
8
11
10
17
13
13
NT
NT
NT
13
16
17
17
NT
NT
NT
NT
NT
NT
NT
15
18
14
15
12
14
19
a
Number of colony forming units.
Ampicillin (10 ␮g/disc).
Streptomycin (10 ␮g/disc).
Nystatin 100 Units (10 ␮g/disc).
Inhibition zone (mm).
b
c
d
e
f
No inhibition zone is determined. Blanks mean not investigated. NT: not tested. 1: [Cd2(L1)]Cl, 2: [Ni2(L1)]Cl, 3: [Co2(L1)]Cl, 4: [Cu2(L1)]Cl, 5: [Cu2(L2)]Cl,
6: [Ni2(L2)Cl], 7: [Co2(L2)Cl], 8: [Cd2(L2)]Cl.
Gram-negative bacteria). The diffusion agar technique was used
to evaluate the antibacterial activity of the synthesized com-
plexes.
the Schiff base ligands and their metal complexes increases as
the concentration increases because it is a well known fact that
concentration plays a vital role in increasing the degree of inhibi-
tion. Almost all compounds (except the complexes [Cu2(L2)]Cl
and [Ni2(L2)]Cl have not activity against the Y. enterocolitica
and C. xerosis, respectively) have activity against the bacteria
andyeasts. Thecomplex[Cd2(L1)]Clhashighestactivityagainst
the B. megaterium and P. aeruginosa (bacteria) and S. cerevisiae
and C. albicans (yeast). In other words, the antibiotic nystatin
is the standard antifungal. Therefore, it did not test against the
all bacteria. The nystatin was only tested for the three yeasts
and obtained results are low. In like manner, the ampicillin and
streptomycin are antibacterial drugs and have been only tested
againstthe bacteria. Thestreptomycindoesnot showanyactivity
against the M. luteus.
The results of the bactericidal screening of the synthesized
compounds are recorded in Table 4. The Schiff base ligands
H2L, H3L1 and H3L2 have moderate activity in comparison
with Staphylococcus aureus, Escherichia coli and less active
in comparison with P. aeruginosa. The remarkable activity of
the Schiff base ligands may be arise from the hydroxyl groups,
which may play an important role in the antibacterial activity,
as well as the presence of four imine groups which imports in
elucidating the mechanism of transformation reaction in bio-
logical systems. The activity of the two Schiff base ligands and
their complexes increases as the concentration increases because
it is a well known fact that concentration plays a vital role in
increasing the degree of inhibition. The results indicate that the
complexes show more activity and the ligands do not have any
activity against same microorganisms under identical experi-
mental conditions. This would suggest that the chelation could
facilitate the ability of a complex to cross a cell membrane and
can be explained by Tweedy’s chelation theory [32]. Chelation
considerably reduces the polarity of the metal ion because of
partial sharing of its positive charge with donor groups and
possible electron delocalization over the whole chelate ring.
Such a chelation could enhance the lipophilic character of the
central metal atom, which subsequently favors its permeation
through the lipid layer of the cell membrane. The variation in the
effectivenessofdifferentcompoundsagainstdifferentorganisms
depends either on the impermeability of the cells of the microbes
or on differences in ribosome of microbial cells. The activity of
3.5. Thermogravimetric studies
In order to give more insight into the structure of the com-
plexes, the thermal studies of the complexes have been carried
out using thermogravimetry (DTA-TG) techniques. The ther-
mogravimetric studies have been made in the temperature range
20–850 ◦C. All complexes have a similar decomposition pro-
cess. Thermal decomposition takes places in two steps. In the
first step, the thermogravimetric analysis (TGA) indicates loss
of one chloride ion in the range of 180–220 ◦C for the com-
plexes (in the binuclear complexes 1 mol chloride ion and in
the mononuclear complexes 2 mol of chloride ion). Loss of
chloride ion as HCl at the low temperatures indicates that the
chloride ions are not coordinated to the metal ions. This is in

928
M. Tu¨mer et al. / Spectrochimica Acta Part A 67 (2007) 916–929
Fig. 10. Thermal analyses curves of the binuclear complex [Cu2(L1)]Cl.
agreement with C, H, N analysis presented in Table 1. The ther-
mogravimetric analysis revealed that the complex loses mass
between 180 and 220 ◦C, corresponding to 13.64% of the total
mass. In the second step, the remaining metal complexes by
loss of the outside chloride ion(s) decompose to the metal
oxide [(MxOy) (x: 3, y: 4 for Co(II) and Ni(II); x: 1, y: 1
for Cu(II)] at the higher temperatures up to 850 ◦C. The per-
centage ratio of the metal oxides fits well with the proposed
formula.
Differential thermal analyses of the complexes have similar
properties. In all complexes, the loss of the chloride ion as HCl
is endothermic process and metal oxide formation process is
exothermic. In Fig. 10, the thermal decomposition steps of the
binuclear complex [Cu2(L1)]Cl is shown. As seen from Fig. 10,
the complex does not contain any hydrated or adsorbed water
molecules. Decomposition process starts approximately at the
∼180 ◦C temperature and continues to the higher temperatures.
The final product of the decomposition process is the metal
oxide.
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Acknowledgements
¨
¨
This work was supported by K. Maras Sutc¸u Imam Univer-
sity, Head of Research Unit (Project code: 2003/7-5). Thanks
are also due to the referees for their valuable comments. We
thank Dr. Metin Dıgrak (K. Maras¸ Sutc¸u Imam University,
Biology Department) for the antimicrobial activity tests. We
also thank the Cukurova University for thermal analyses and
˘
¨ ¨
¨
the Ataturk University for NMR and electrochemical studies,
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