Cyclobutane and thiazole substituted Schiff base ligands and their cobalt(II), copper(II), nickel(II), and zinc(II) complexes

Article abstract of DOI:10.1081/SIM-120020330

Two novel bidentate Schiff base ligands containing cyclobutane and 2,4-disubstituted thiazole rings, 4-(1-methyl-1-p-xylylcyclobutane-3-yl)-2- (2-hydroxybenzylidenehydrazino)thiazole (L¹H), 4-(1-methyl-1-p-xylylcyclobutane-3-yl)-2- (2-hydroxy-5-bromobenzylidenehydrazino)thiazole (L²H), and their mononuclear complexes with a 1:2 metal:ligand ratio have been prepared with acetate salts of Co(II), Cu(II), Ni(lI) and Zn(II) in ethanol. The authenticity of the ligands and their complexes have been established by microanalyses, IR, ¹H and ¹³C NMR spectra, and by magnetic susceptibility measurements. The thermal properties of all complexes have been studied by TGA and DSC techniques. All the complexes were found to be mononuclear.

Full text of DOI:10.1081/SIM-120020330

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Cyclobutane and Thiazole Substituted Schiff Base
Ligands and Their Cobalt(II), Copper(II), Nickel(II), and
Zinc(II) Complexes
İbrahim Yilmaz a & Alaaddin Çukurovali a
a Chemistry Department, Faculty of Arts and Sciences, Firat University, 23119, Elazig, Turkey
Available online: 15 Feb 2007
To cite this article: İbrahim Yilmaz & Alaaddin Çukurovali (2003): Cyclobutane and Thiazole Substituted Schiff Base Ligands
and Their Cobalt(II), Copper(II), Nickel(II), and Zinc(II) Complexes, Synthesis and Reactivity in Inorganic and Metal-Organic
Chemistry, 33:4, 657-668
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 4, pp. 657–668, 2003
Cyclobutane and Thiazole Substituted Schiff Base
Ligands and Their Cobalt(II), Copper(II),
Nickel(II), and Zinc(II) Complexes
*
I_brahim Yilmaz and Alaaddin C¸ ukurovali
Chemistry Department, Faculty of Arts and Sciences,
Firat University, Elazig, Turkey
ABSTRACT
Two novel bidentate Schiff base ligands containing cyclobutane and 2,4-
disubstituted thiazole rings, 4-(1-methyl-1-p-xylylcyclobutane-3-yl)-2-
(2-hydroxybenzylidenehydrazino)thiazole (L1H), 4-(1-methyl-1-p-xylyl-
cyclobutane-3-yl)-2-(2-hydroxy-5-bromobenzylidenehydrazino)thiazole
(L2H), and their mononuclear complexes with a 1:2 metal:ligand ratio
have been prepared with acetate salts of Co(II), Cu(II), Ni(II) and Zn(II)
in ethanol. The authenticity of the ligands and their complexes have been
established by microanalyses, IR, 1H and 13C NMR spectra, and by
*
Correspondence: Alaaddin C¸ ukurovali, Chemistry Department, Faculty of Arts and
Sciences, Firat University, 23119, Elazig, Turkey; E-mail: acukurovali@firat.edu.tr.
657
DOI: 10.1081/SIM-120020330
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

658
Yilmaz and C¸ ukurovali
magnetic susceptibility measurements. The thermal properties of all
complexes have been studied by TGA and DSC techniques. All the
complexes were found to be mononuclear.
Key Words: Cyclobutane; Thiazole; Schiff base; Complexes.
INTRODUCTION
As a part of our research program concerning the chelating behaviour
of 2,4-disubstituted thiazoles, we have examined previously the coordina-
tion properties of some 2,4-disubstituted thiazoles in several transition
metal complexes.[1–4] Metal complexes of some 2,4-disubstituted thiazoles
derivatives have been found in some instances to have enhanced or mo-
dified their activity in comparison to the uncomplexed ligand. Thiosemi-
carbazones, cyclobutanes, thiazoles and Schiff bases are of great importance
for the preparation of various pharmaceuticals and are used in many other
areas of chemistry as starting materials. Schiff bases derived from sali-
cylaldehydes are well known polydentate ligands.[5] It has been a sugges-
tion shown that Schiff base complexes derived from 4-hydroxysalicyl-
aldehyde and amines have strong anticancer activity.[6] Some thiazole
derivatives containing the tetralin moiety have been synthesized and used in
biological activity studies.[7] Recently, there has been considerable interest
Figure 1. Synthesis of the ligands.

Complexes of Schiff Base Ligands
659
in the chemistry of Schiff base compounds containing thiosemicarbazones
and their metal complexes due to their biological activities[8] and non-linear
optical properties.[9] Research has concentrated on different properties of
poly-substituted cyclobutane derivatives for years.[10] On the other hand,
our group has been heavily engaged in the synthesis of novel 1,1,3-
trisubstituted cyclobutane thiazoles and their Schiff base derivatives.[11]
These compounds, containing cyclobutane, thiazole and Schiff base
functions in their molecules, seem to be suitable candidates for further
chemical modifications and may be pharmacologically active and use-
ful ligands.
Since these ligands have not been reported in the literature, this paper
deals with their preparation (Figure 1) and characterization, as well as their
complexes with cobalt(II), copper(II), nickel(II), and zinc(II) acetates.
RESULTS AND DISCUSSION
Synthesis and Characterization of the Ligands
Synthetic pathways for the preparation of the ligands are shown in
Figure 1. Two of the starting substances, 1-(2-hydroxybenzylidene) thio-
semicarbazide (1) and 1-(2-hydroxy-5-bromobenzylidene) thiosemicarbazide
(2) were synthesized by condensation of thiosemicarbazide, and salicylal-
dehyde or 5-bromosalicylaldehyde in absolute ethanol at 60–70 °C. The
new ligands, 4-(1-methyl-1-p-xylylcyclobutane-3-yl)-2-(2-hydroxybenzyli-
denehydrazino)thiazole (3), and 4-(1-methyl-1-p-xylylcyclobutane-3-yl)-2-
(2-hydroxy-5-bromobenzylidenehydrazino)-thiazole (4) were then prepared
by a one-step reaction of 1-methyl-1-p-xylyl-3-(2-chloro-1-oxoethyl)cyclo-
butane, which was prepared according to the previously published pro-
cedure,[12] with (1) or (2). The compounds are very soluble in CHCl3,
Me2CO, DMSO and DMF and partially soluble in organic solvents such
as EtOH and MeOH. Furthermore, hot solutions of the ligands were used
for the preparation of the complexes. The synthetic process for the for-
mation of the ligands is shown in Figure 1. It is necessary to say that
these ligands are the product of difficult, tedious and time-consuming
organic syntheses.
The elemental analysis results (Table 1) of (3), (4) and their metal
derivatives were found to agree with their proposed constitutions. The
characterization of (1) and (2) has been published elsewhere.[1] The most
important infrared spectral bands of the ligands and their complexes are
provided in Table 2. In the IR spectrum of (3) and (4), stretching vibrations
for NH2 and C=S were not observed, but there are new peaks at 1600 and
2978–2927 cm–1 for C=N (thiazole) and aliphatics, respectively.

660
Yilmaz and C¸ ukurovali

Complexes of Schiff Base Ligands
661

662
Yilmaz and C¸ ukurovali
Observation of the absence of C=O, C–Cl, C=S and NH2 absorptions
in the IR spectra of the ligands L1H and L2H, and existing characteristic
peaks indicate the formation of the expected compounds.
The strong and sharp band observed at 3113 and 3131 cm–1 for the
ligands, respectively, can be attributed to –NH– group vibrations. Both
ligands exhibit a broad, medium-intensity band in the 2700–2560 cm–1
range which is assigned to the intramolecular H-bonding vibrations
(O–HÁ Á ÁN). As is known, this situation is common for aromatic azome-
thine compounds containing o-OH groups.[13] The azomethine group
vibration of the free ligands occurs at 1619 and 1625 cm–1, respectively.
In the free ligands, the band at 3182 and 3275 cm–1 can be attributed to
the phenolic (O–H) group vibration,[13] respectively, for L1H and L2H.
Characteristic bands for C–O of the ligands are observed at 1286 and
1140 cm–1, respectively. The aliphatic group vibrations indicating that the
compounds contain cyclobutane are in the 2978–2927 cm–1 range for
both ligands. The C=N group vibrations in the thiazole ring are observed
at 1558 cm–1 for L1H and 1529 cm–1 for L2H. Another group, C–S–C,
which may be affected by the complexation, exhibits bands at 637 and
656 cm–1 for the ligands L1H and L2H, respectively.
1
The H and 13C NMR spectra of the ligands were recorded in CDCl3.
1
The H NMR and 13C NMR assignments are given in detail in the Ex-
1
perimental section. It is important to emphasize that the H NMR resonance
of the O–H group at 10.75–11.45 and 8.60–9.80 ppm for the L1H and L2H
ligands, respectively, as broad singlet, is due to the presence of intra-
molecular hydrogen bonding.[14] These two broad singlets also contain the
–NH– signals of the molecules. The 1H NMR signal observed for the
protons of C–OH and –NH– disappeared upon addition of D2O to the
1
solution. The singlet proton resonances in the H NMR spectra of these
ligands at 8.12 and 8.06 ppm have been assigned to the azomethine group
protons. The aromatic ring resonances are observed at 6.80–7.31 and 6.77–
7.00 ppm as multiplets for the ligands L1H and L2H, respectively. The
1
detailed H NMR spectral data of the ligands are given in the Experimental
section, and a more detailed spectral investigation of a similar cyclobutane
compound can be found in the literature.[4] The 13C NMR spectral data of
1
the ligands confirm the H NMR spectral results.
Metal Complexes of L1H and L2H Ligands
Mononuclear complexes were obtained by the reactions of the ligands
with Co(II), Cu(II), Ni(II) and Zn(II) acetates. In general, the reactions of
the ligands L1H, L2H, with the metal acetates were rapid and gave good

Complexes of Schiff Base Ligands
663
yields of the complexes corresponding to the general formula M(L)2; they
are stable at room temperature and soluble in Me2CO, DMF, DMSO and
CHCl3. In every complex compound, one metal ion is coordinated to two
azomethine nitrogen atoms and two oxygen atoms of two ligand molecules
(Figure 2). The IR spectra of all mononuclear complexes, the copper
complexes are dimers (Fig. 3), are very similar to those of their ligands
except for the disappearance of the OH stretching frequency and the shift
of the azomethine stretching of the ligands. The azomethine stretching
vibration of the ligands are shifted to the lower frequency of 1606 cm–1
for all of the metal complexes of the ligand L1H. A similar situation was
observed for the L2H ligand complexes; their azomethine vibrations are at
1604 cm–1. At the same time, the C–O stretching vibrations of the ligands
are also shift to higher frequencies at 1292, 1292, 1303 and 1303 cm–1,
respectively, for L1H and 1176, 1182, 1176 and 1176 cm–1 for L2H for
the above metal ion complexes. These results indicate the high
effectiveness of two functional groups of the ligands by complexation
with metal salts. Despite their presence in the ligands and the possibility of
complexation by the C=N group and sulfur atoms in the thiazole ring, the
unchanged band positions for these groups in the IR spectra of the
complexes indicate that the thiazole ring does not complex with the metal.
In the complexes, the bands in the 556–491 and 457–428 cm–1 range can
Figure 2. Suggested structure of the complexes.

664
Yilmaz and C¸ ukurovali
Figure 3. Suggested structure of the Cu(II) complexes.
be attributed to the n(M–N) and n(M–O) modes for the L1H complexes,
and the 584–497 and 482–434 cm–1 range for n(M–N) and n(M–O)
modes for the L2H complexes.
The metal contents were determined by FAAS, and the TGA curves
were studied in the temperature range 20–900 °C. The weight losses in the
TGA and the metal contents as determined by FAAS of the complexes have
been found to be approximately the same as the percentages calculated
stoichiometrically from their chemical formulas given in Table 1. All of
these complexes undergo complete decomposition to the corresponding
metal oxides, CuO, Co3O4, NiO or ZnO.
As is known, magnetic susceptibility measurements provide informa-
tion regarding the structure of the complexes. The magnetic moments of the
complexes were measured at room temperature. The cobalt(II), copper(II)
and nickel(II) complexes of both ligands are paramagnetic and their mag-
netic susceptibilities are 4.15, 0.49 and 3.81 B.M., respectively, for the L1H
complexes and 4.13, 0.96 and 3.80, respectively, for the L2H complexes.
The zinc(II) complexes of the present ligands are diamagnetic. These data
indicate two unpaired electrons for Ni(II) and three for Co(II). The mag-
netic moments of the Co(II) complexes of both ligands at room temperature
fall in the range of 4 to 5 B.M., which is characteristic for mononuclear,
high-spin, tetrahedral Co(II) complexes. The magnetic moment values of
the Ni(II) complexes of the ligands are also consistent with a tetrahedral
geometry.[15]
Cu(II) complexes exhibit a wide range of geometries, often with low
symmetry and in most geometries the electronic spectra exhibit a very
broad band with a maximum, which contains all the expected transitions.
On the other hand, the copper complexes of the present ligands have
unexpected magnetic moments, 0.49 and 0.96 B.M. (nearly diamagnetic),
respectively, for the ligands L1H and L2H. This means that the copper
complexes have a dimeric structure. Clearly the single unpaired electron on
the copper atoms interacts, or ‘‘couples’’, antiferromagnetically, to produce

Complexes of Schiff Base Ligands
665
a low-lying singlet (diamagnetic).[16] For all of these complexes, the
elemental analysis results are in good agreement with the proposed struc-
ture. On the basis of the spectral and magnetic data, the cobalt, nickel and
zinc complexes have tetrahedral geometry, while the copper complexes are
in a square-planar structure.
As mentioned in the Introduction, our group has been heavily engaged
in the synthesis of novel 1,1,3-trisubstituted cyclobutanethiazoles and their
Schiff base derivatives. Many more thiazole derivatives, containing sub-
stituted cyclobutane, were synthesized, characterized in detail and used for
the complexation with some transition metal salts. The effect of functional
groups, such as benzene and mesitylene, on the complexes have no effect.
These functional groups are very far from the pendants taking part in the
complexation. On the other hand, in the case of p-xylene as a functional
group in the molecule, the copper complexes showed low magnetic mo-
ments. This situation may be interpreted as a structural effect of p-xylene-
bearing molecules on the complexes. But it is very difficult to explain the
reason for this effect.
EXPERIMENTAL
Thiosemicarbazide, 2-hydroxybenzaldehyde, 2-hydroxy-5-bromobenzal-
dehyde and metal acetates were purchased from Merck (pure) and were
usedwithoutfurtherpurification.1-Methyl-1-p-xylyl-3-(2-chloro-1-oxoethyl)-
cyclobutane was prepared according to a known procedure[12] and was
purified by column chromatography prior to use. Solvents were of analy-
tical grade and purified by standard methods where necessary.
Physical Measurements
Elemental analysis were performed on a LECO CHNSO-932 auto ele-
mental analysis apparatus. IR spectra were recorded on a Mattson 1000
FT-IR spectrometer as KBr pellets. 1H and 13C NMR spectra were re-
corded on a Varian-Gemini 200 MHz at 50.34 MHz or JEOL FX-90Q
spectrometer. Magnetic susceptibilities were determined on a Sherwood
Scientific magnetic susceptibility balance (Model MK1) at room temper-
ature (20 °C) using Hg[Co(SCN)2] as calibrant. Melting points were
determined on a Gallekamp melting point apparatus and checked by dif-
ferential scanning calorimetry (DSC) and are uncorrected. Thermogravi-
metric curves were recorded on a Shimadzu TG-50 thermobalance. Metal
contents of the complexes were determined by an Ati Unicam (model 929)
atomic absorption spectrophotometer in solutions prepared by decompos-

666
Yilmaz and C¸ ukurovali
ing the compounds in concentrated HCl and subsequent dilution with 0.01
M HCl.
Synthesis of the Ligands (3) and (4)
To a suspension of 1-(2-hydroxybenzylidene) thiosemicarbazide (1.95
g, 10 mmol), or 1-(2-hydroxy-5-bromobenzylidene)thiosemicarbizide (2.74
g, 10 mmol), in absolute ethanol (30 mL), a solution of 1-methyl-1-p-xylyl-
3(2-chloro-1-oxoethyl)cyclobutane (2.505 g, 10 mmol) in absolute ethanol
(20 mL) was added dropwise at ca. 30–40 °C with continuous stirring and
monitoring the reaction with IR. Excess of NaHCO3 was added to the
reaction mixture in order to neutralize the liberated HCl. After completing
the addition of the a-haloketone, the temperature of the mixture was raised to
50–55 °C. Monitoring the presence of the carbonyl group of 1-methyl-1-p-
xylyl-3-(2-chloro-1-oxoethyl)cyclobutane is easily done by IR and thus it is
very easy to determine when the reaction is complete. The solution was then
made alkaline (if necessary) with an aqueous solution of NH3 (5%) and
the yellow solids of L1H and L2H separated. The precipitate was filtered,
washed with aqueous NH3 solution several times, dried in air and crystallized
from ethanol, dried and stored in a desiccator over CaCl2.
Characteristic 1H NMR peaks for L1H, (CDCl3, TMS, d ppm):
6.80 À 7.31 (m, 7H, Ar–H), 1.47 (s, 3H, CH3), 2.19 (s, 3H, ortho-CH3 of p-
xylyl), 2.29 (s, 3H, meta CH3 of p-xylyl), 2.55 (d, 4H, J =9 Hz, –CH2– of
cyclobutane), 3.54 (q, 1H, J=7 Hz, > CH–), 6.07 (s, 1H, =CH–S), 8.12 (s,
1H, N=CH), 10.75–11.45 (broad singlet, 2H, –NH– plus –OH, D2O-
exchangeable); Characteristic 13C NMR peaks for L1H, (CDCl3, TMS, d
ppm): 120.05 (C1), 159.83 (C2), 118.86 (C3), 133.17 (C4), 121.51 (C5),
132.26 (C6), 150.82 (C7), 171.02 (C8), 101.73 (C9) 153.39 (C10), 32.14
(C11), 41.86 (C12), 43.06 (C13), 29.52 (C14), 149.83 (C15), 133.26 (C16),
132.96 (C17), 128.22 (C18), 137.29 (C19), 129.81 (C20), 21.49 (C21), 22.99
(C22) ppm.
1
Characteristic H NMR peaks for L2H, (CDCl3, TMS, d ppm): 6.77–
7.00 (m, 3H, Ar–H) and 7.24–7.36 (m, 3H, Ar–H), 1.45 (s, 3H, CH3),
2.18 (s, 3H, ortho-CH3 of p-xylene), 2.28 (s, 3H, meta CH3 of p-xylene),
2.52 (d, J =9 Hz, 4H, –CH2–), 3.48 (q, 1H, J= 7 Hz,
>CH–), 6.02 (s, 1H, =CH–S), 8.06 (s, 1H, N=CH), 8.60–9.80 (br, 2H, –
NH– plus –OH, D2O-exchangeable); Characteristic 13C NMR peaks for L2H,
(CDCl3, TMS, d ppm): 121.82 (C1), 158.84 (C2), 120.73 (C3), 133.64 (C4),
113.12 (C5), 133.45 (C6), 148.69 (C7), 171.40 (C8), 101.53 (C9), 152.18
(C10), 31.88 (C11), 41.89 (C12), 42.98 (C13), 29.45 (C14), 150.43 (C15),
135.41 (C16), 133.21 (C17), 128.14 (C18), 137.30 (C19), 128.67 (C20), 21.50
(C21), 23.00 (C22).

Complexes of Schiff Base Ligands
Synthesis of the Complexes
667
The ligand L1H (0.1955 g, 0.50 mmol), or L2H (0.235 g, 0.50 mmol)
was dissolved in absolute ethanol (15–20 mL). A solution of 0.25 mmol
of the metal salt [Co(AcO)2Á4H2O (0.0623 g), Cu(AcO)2ÁH2O (0.0499 g),
Ni(AcO)2Á4H2O (0.0623 g) Zn(AcO)2Á2H2O (0.0549 g)] in ethanol
(10 mL) was added drop-wise with continuous stirring. In the case of
the cobalt(II) complexes, a slow stream of nitrogen was passed through
the solution. Every mixture was refluxed for 1 hour and then left to stand
overnight at room temperature. The complexes, precipitated as micro-
crystals, were filtered, washed with cold ethanol and water several times
and dried in vacuum at 60 °C (over P4O10) and stored in a desiccator
over CaCl2. Yields, melting points, elemental analysis results and
characteristic IR bands (NaCl cell) are given in Tables 1 and 2. The
1H NMR spectra of the Zn(II) complexes of the ligands showed the same
resonances as those of L1H and L2H except for the absence of the OH
proton resonance and a small shift was observed for the azomethine
proton resonance.
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Received: March 29, 2002
Accepted: January 12, 2003
Referee I: K. Moedritzer
Referee II: L. E. Roecker