The synthesis of new triazole ligands and determination of complex stability constants with transition metal cations in aqueous media

Article abstract of DOI:10.1007/s10953-008-9331-x

Six new triazole compounds were synthesized. These compounds containing the substituted benzylidenamino group were obtained by reaction of 3-(pyridine-4-yl)-5-p-tolyl-4-amino-4H-l,2,4-triazole 1 with the corresponding aldehyde. The reduced forms were prepared with NaBH₄ in methanol. The structures of the compounds were determined by IR, ¹H NMR, and ¹³C NMR spectral data, and their interaction with cations such as Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Ba²⁺, Sr²⁺, Ca²⁺, Cu²⁺, Cr ³⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd ²⁺, Pb²⁺ and Ag⁺ were investigated by using UV-visible spectrophotometry. Of the tested metal cations, Cu²⁺, Cr³⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd ²⁺, Pb²⁺ and Ag⁺ complexed with the ligands. The complex stability constants (log₁₀ K) were measured in slightly acidic aqueous media at 25.0±0.1°C. These stability constants were determined by measuring the increase in solubility of the nearly insoluble ligand molecule due to complex formation with a soluble cation, and this method is discussed. It was found that the position of chlorine atoms on the benzene ring strongly affects the complexation of Cu²⁺ ion with these ligands.

Full text of DOI:10.1007/s10953-008-9331-x

J Solution Chem (2008) 37: 1489–1497
DOI 10.1007/s10953-008-9331-x
The Synthesis of New Triazole Ligands
and Determination of Complex Stability Constants
with Transition Metal Cations in Aqueous Media
Miraç Ocak · Nurhan Gümrükçüog˘lu ·
Ümmühan Ocak · Hans-Jurgen Buschmann ·
Eckard Schollmeyer
Received: 14 April 2008 / Accepted: 4 June 2008 / Published online: 13 September 2008
© Springer Science+Business Media, LLC 2008
Abstract Six new triazole compounds were synthesized. These compounds containing
the substituted benzylidenamino group were obtained by reaction of 3-(pyridine-4-yl)-5-
p-tolyl-4-amino-4H-l,2,4-triazole 1 with the corresponding aldehyde. The reduced forms
were prepared with NaBH4 in methanol. The structures of the compounds were determined
by IR, 1H NMR, and 13C NMR spectral data, and their interaction with cations such as Li+,
Na+, K+, Rb+, Cs+, Ba2+, Sr2+, Ca2+, Cu2+, Cr3+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+ and Ag+
were investigated by using UV-visible spectrophotometry. Of the tested metal cations, Cu2+
,
Cr3+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+ and Ag+ complexed with the ligands. The complex sta-
bility constants (log10 K) were measured in slightly acidic aqueous media at 25.0 0.1 °C.
These stability constants were determined by measuring the increase in solubility of the
nearly insoluble ligand molecule due to complex formation with a soluble cation, and this
method is discussed. It was found that the position of chlorine atoms on the benzene ring
strongly affects the complexation of Cu2+ ion with these ligands.
Keywords Triazole · Synthesis · Complexes · Stability constant · Ligand solubility
1 Introduction
A large number of heterocyclic compounds containing the 1,2,4-triazole ring are associated
with diverse pharmacological properties, such as antiinflammatory, antifungal, antimicro-
bial, anticonvulsant, antidepressant, antiviral and antitumor activity [1–10]. Some synthetic
thiazoles also exhibit a range of biological activities. In addition, it is well known that Schiff
M. Ocak ( ) · Ü. Ocak
ꢀ
Department of Chemistry, Faculty of Arts and Sciences, KTÜ, Trabzon, Turkey
e-mail: mircak04@gmail.com
N. Gümrükçüog˘lu
Department of Chemistry, Faculty of Arts and Sciences, Giresun University, Giresun, Turkey
H.-J. Buschmann · E. Schollmeyer
Deutsches Textilforschungszentrum Nord-West e.V., Adlerstrasse 1, 47798 Krefeld, Germany

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J Solution Chem (2008) 37: 1489–1497
bases possess potent activities including antimicrobial, anticonvulsant, antiviral and anti-
inflammatory [11–14]. Recently, a series of thiazolyl Schiff bases bearing the 1,2,4-triazole
ring were synthesized and investigated for their antitumor activity against three human can-
cer cell lines, HL-60 (leukemia), BGC-823 (stomach) and HEP-2 (larynx cancer) [15].
It has been found that metal-ligand complexes of some thiosemicarbazone ligands have
increased biological activity [16]. Forming the palladium complex of a Schiff base bearing
the thiosemicarbazone moiety enhances the ligand’s antitumor activity [17]. Similarly, the
antioxidant effect of thiosemicarbazone derivative Schiff bases and their metal complexes
was studied on rats, and it was shown that the zinc complex behaved as an antioxidant [18].
Similarly, the influences of metal complexes with 1,2,4-triazoles on biological activity are
the subject of great interest in clinic chemistry [19].
In view of this, several articles devoted to the syntheses and biological activity of
4-arylidenamino-4H-1,2,4-triazoles have been published recently [8–10]. Due to their
structural features, the 4-arylidenamino-4H-1,2,4-triazoles are important as being potential
biologically active compounds [8–10]. The synthesis of certain 4-arylidenamino-4H-1,2,4-
triazoles 2–4 was carried out in the present study by treating 3-(pyridine-4-yl)-5-p-tolyl-
4-amino-4H-l,2,4-triazole 1 with selected aldehydes. Compound 1 was obtained from the
reaction of ethyl p-tolylbenzoate (pyridine-4-yl) hydrazone with hydrazine by using a pub-
lished method [9]. In general, reduction of an imine-type compound is possible [8–10, 20].
However, attempts to reduce the imines 2–4 may also result in reduction of the hetero ring.
For this reason, selective reduction of only the imino group of compounds 2–4, without af-
fecting the hetero ring, was carried out in the present study. Thus, a general and convenient
method was established for the synthesis of 4-arylmethylamino-4H-l,2,4-triazoles 5–7 in
good yields by using NaBH4 as a selective reducing agent.
Recently, metal complexes of Schiff base containing triazole ligands have been studied
in the solid state by X-ray crystallography [21]. The copper(I) complex of N-benzylidene-
3,5-di(2ꢀ-pyridyl)-4-amino-1,2,4-triazole was characterized by UV-vis, luminescence, and
1H NMR spectroscopy and by cyclic voltammetry in a dichloromethane solution [22]. In
the present study we investigated the complexation behavior between novel 1,2,4-triazole
ligands with a substituted benzylideneamino group and various transition metal cations, and
also with Pb2+, in aqueous solution. The extent of complexation was evaluated by measuring
the stability constants.
The most common experimental techniques for determining stability constants of com-
plexes are potentiometry, conductometry, polarography, UV-vis and fluorescence spec-
troscopy, NMR spectroscopy, mass spectrometry, kinetic measurement and calorimetry
[23–29]. Due to the low solubility of triazoles, no common experimental methods can be
used to study their complex formation in aqueous solution. Some years ago a technique was
developed to study reactions between nearly insoluble ligands and salts [30]. Thus, it was
shown that it is possible to calculate the stability constants of the complexes formed be-
tween nearly insoluble ligands and cations under certain conditions, based on the increase
in solubility of these ligands in aqueous solution [31, 32]. We applied this method to the
complexation of novel triazole ligands with cations in aqueous media. Revealing the com-
plexation properties of the novel Schiff bases bearing a triazole ring, with metal cations in
aqueous media, is important for the further investigation of the biological activity of these
ligands.

J Solution Chem (2008) 37: 1489–1497
2 Experimental
1491
2.1 Chemicals and Apparatus
All chemicals used in the synthesis of the ligands were purchased from Merck and Fluka.
The starting compound 1 was synthesized by the method reported earlier [9]. The anhy-
drous salts LiCl, NaCl, KCl, RbCl, CsCl, BaCl2, SrCl2, CoCl2, CuCl2, ZnCl2, CaCl2, NiCl2,
CdCl2, Pb(NO3)2, CrCl3, and AgNO3 (all from Merck) were of the highest purity available
and were used without further purification. Doubly distilled water was used as the solvent.
Melting points were determined on a Büchi oil-heated melting-point apparatus and are
uncorrected. 1H NMR and l3C NMR spectra were recorded on a Varian 200 A spectrometer
using tetramethylsilane as the internal reference. IR spectra were recorded on a Perkin-Elmer
1600 FTlR spectrophotometer using KBr pellets. Absorption spectra of the solutions were
recorded using a Varian Cary 5E spectrophotometer.
2.2 Measurements
The solid ligands were added to slightly acidic aqueous solutions of the salts (2 × 10−3 to
2 × 10−2 mol·L−1). The amount of ligand added was high enough to ensure formation of
saturated solutions. The solutions were thermostatted at 25 ◦C and shaken at intervals. Sam-
ples of the solutions were analyzed after three days. The solutions were centrifuged and the
resulting clear solutions were filtered through a membrane filter (polycarbonate, 0.4 µm) to
remove the undissolved ligand. The absorption spectra of the solutions were then recorded.
Changes in the absorption of the solutions due to complex formation were evaluated at the
absorbance maximum wavelength. The wavelengths used for the measurement of the sta-
bility constants were 264, 575, 511, 394, 300, 302, 300 and 301 nm for Cu2+, Cr3+, Co2+
Ni2+, Zn2+, Cd2+, Pb2+ and Ag+, respectively, with all of the ligands.
,
2.3 Synthesis
2.3.1 General Method for the Synthesis of Compounds 2–4
The corresponding aldehyde (0.005 mole) was added to a solution of compound 1
(0.005 mole) in 20 mL of glacial acetic acid and the mixture was refluxed for 4 h. After
cooling, the mixture was poured into a beaker containing 100 mL of ice water. The formed
precipitate was separated by filtration. After being dried in vacuo, the product was recrys-
tallized from an appropriate solvent to give the desired compound.
3-(Pyridin-4-y1)-5-(p-tolyl)-4-(2-chlorobenzylidenamino)-4H-1,2,4-triazole 2 was re-
crystallized from ethanol. The yield was 75%, m.p. 161–162 ◦C; IR (KBr): 1608 and
1
1590 cm−1 (ν2C=N); H NMR (DMSO-d6): δ = 2.38 (s, 3H, CH3), Ar-H: [7.37 (d, 3H),
7.54–7.74 (m, 6H), 7.86 (s, 1H), 8.10–8.16 (m, 1H), 8.45 (d, 1H)], 8.80 (s, 1H, N=CH);
13C NMR (DMSO-d6): δ = 165.97 (N=CH), δ = 150.34 (triazole C3), 150.01 (triazole C5),
Ar-C: [142.73 (CH), 139.90 (CH), 139.54 (C), 135.10 (C), 130.27 (C), 129.99 (CH), 129.38
(2CH), 129.05 (2CH), 128.82 (2CH), 128.37 (2CH), 128.22 (CH), 127.33 (CH), 123.60 (C),
122.10 (C)], 20.85 (ar-CH3).
3-(Pyridin-4-yl)-5-(p-toly1)-4-(2,4-dichlorobenzylidenamino)-4H-1,2,4-triazole 3 was
recrystallized from ethanol. The yield was 86%, m.p. 162–163 ◦C; IR (KBr): 1601 and
1
1583 cm−1 (ν2C=N); H NMR (DMSO-d6): δ = 2.37 (s, 3H, CH3), Ar-H: [7.21–7.39 (m,
3H), 7.62–7.83 (m, 6H), 8.05–8.21 (m, 1H), 8.35–8.45 (m, 1H)], 8.75 (s, 1H, N=CH);

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J Solution Chem (2008) 37: 1489–1497
13C NMR (DMSO-d6): δ = 164.92 (N=CH), δ = 150.70 (triazole C3), 150.33 (triazole C5),
Ar-C: [148.18 (CH), 139.94 (CH), 139.58 (C), 138.63 (C), 135.91 (C), 133.76 (CH), 129.91
(C), 129.52 (2CH), 129.40 (2CH), 128.49 (2CH), 127.63 (2CH), 127.35 (CH), 123.02 (C),
122.13 (C)], 20.86 (ar-CH3).
3-(Pyridin-4-yl)-5-(p-tolyl)-4-(2,6-dichlorobenzylidenamino)-4H-1,2,4-triazole 4 was
recrystallized from ethanol. The yield was 82%, m.p. 162–163 ◦C; IR (KBr): 1601 and
1
1583 cm−1 (ν2C=N); H NMR (DMSO-d6): δ = 2.37 (s, 3H, CH3), Ar-H: [7.33–7.39 (m,
2H), 7.63–7.70 (m, 3H), 7.77–7.85 (m, 2H), 8.06–8.18 (m, 2H), 8.69–8.75 (m, 2H)], 8.78 (s,
1H, N=CH); 13C NMR (DMSO-d6): δ = 164.92 (N=CH), δ = 150.69 (triazole C3), 150.33
(triazole C5), Ar-C: [139.93 (CH), 139.57 (CH), 138.64 (C), 135.92 (C), 133.75 (C), 129.92
(CH), 129.63 (C), 129.52 (2CH), 129.39 (2CH), 128.55 (2CH), 128.48 (2CH), 128.85 (CH),
123.53 (C), 122.14 (C)], 20.86 (ar-CH3).
2.3.2 General Method for the Synthesis of Compounds 5–7
The corresponding compound 2–4 (0.005 mole) was dissolved in 50 mL of dried methanol
and NaBH4 (0.005 mole) was added in small portions to this solution. The mixture was
refluxed for 20 min and then allowed to cool. After concentration at 25 to 30 ◦C under
reduced pressure, the resulting solid residue was washed with cold water. After drying in
vacuo, the solid product was recrystallized from an appropriate solvent to yield the desired
compound.
3-(Pyridin-4-yl)-5-(p-tolyl)-4-(2-chlorobenzylamino)-4H-1,2,4-triazole 5 was recrystal-
lized from ethyl acetate. The yield was 89%, m.p. 162–163 ◦C; IR (KBr): 3309 (νNH), 1619
and 1608 cm−1 (ν2C=N); 1H NMR (DMSO-d6): δ = 2.38 (s, 3H, CH3), 3.86 (d, 2H, CH2),
6.83 (t, 1H, NH) Ar-H: [7.05–7.34 (m, 4H), 7.37–7.43 (m, 4H), 7.75–7.94 (m, 4H)]; 13C
NMR (DMSO-d6): δ = 150.34 (triazole C3), 150.01 (triazole C5), Ar-C: [142.73 (CH),
139.90 (CH), 139.54 (C), 135.10 (C), 130.27 (C), 129.99 (CH), 129.38 (2CH), 129.05
(2CH), 128.82 (2CH), 128.37 (2CH), 128.22 (CH), 127.33 (CH), 123.60 (C), 122.10 (C)],
51.39 (CH2), 20.85 (ar-CH3).
3-(Pyridin-4-yl)-5-(p-tolyl)-4-(2,4-dichlorobenzylamino)-4H-1,2,4-triazole 6 was re-
crystallized from ethyl acetate. The yield was 86%, m.p. 162–163 ◦C; IR (KBr): 3329
1
(νNH), 1602 and 1598 cm−1 (ν2C=N); H NMR (DMSO-d6): δ = 2.39 (s, 3H, CH3), 3.85
(d, 2H, CH2), 6.80 (t, 1H, NH), Ar-H: [7.07 (d, 1H), 7.22–7.47 (m, 6H), 7.76–7.88 (m,
4H)]; 13C NMR (DMSO-d6): δ = 153.64 (triazole C3), 151.84 (triazole C5), Ar-C: [149.66
(CH), 139.87 (CH), 139.25 (C), 134.45 (2CH), 133.80 (2CH), 133.11 (2CH), 132.34 (2CH),
129.19 (CH), 128.89 (C), 128.35 (2CH), 127.71 (C), 126.85 (C), 124.07 (C), 123.56 (C)],
50.92 (CH2), 20.85 (ar-CH3).
3-(Pyridin-4-yl)-5-(p-tolyl)-4-(2,6-dichlorobenzylaınino)-4H-1,2,4-triazole 7 was re-
crystallized from ethyl acetate. The yield was 84%, m.p. 162–163 ◦C; IR (KBr): 3327 (νNH),
1
1601 and 1598 cm−1 (ν2C=N); H NMR (DMSO-d6): δ = 2.41 (s, 3H, CH3), 3.86 (d, 2H,
CH2), 6.80 (t, 1H, NH), Ar-H: [7.05–7.10 (m, 1H), 7.23–7.36 (m, 4H), 7.40–7.48 (m, 1H),
7.77–7.89 (m, 5H)]; 13C NMR (DMSO-d6) : δ = 153.64 (triazole C3), 151.83 (triazole C5),
Ar-C: [149.66 (CH), 139.87 (CH), 139.25 (C), 134.45 (2CH), 133.78 (2CH), 133.31 (2CH),
132.62 (2CH), 129.19 (CH), 128.90 (C), 128.35 (2CH), 127.75 (C), 126.85 (C), 124.07 (C),
121.66 (C)], 50.92 (CH2), 20.93 (ar-CH3).

J Solution Chem (2008) 37: 1489–1497
3 Results and Discussion
1493
3.1 Treatment of the Experimental Data
The formation of a 1:1 complex between a ligand L and a cation Mn+ can be described by:
L + Mn+ ꢀ LMn+
(1)
The corresponding stability constant is defined as:
[LMn+
]
K =
(2)
[L][Mn+
]
If only the ligand and the complex formed absorb at a given wavelength, then the exper-
imentally measured absorptivity Aꢀ for an optical path length d is given by Eq. 3:
A = (Aꢀ/d) = ε1[L] + ε2[LMn+
]
(3)
The molar absorptivities of the ligand and complex are εı and ε2, respectively. The salt
solution is saturated with the ligand so the first term in Eq. 3 is constant. As a result one
gets:
Ao = ε1[L]sat
(4)
where [L]sat denotes the solubility of the ligand in the pure solvent. Equations 5 and 6 de-
scribing the material balances,
cM = [Mn+] + [LMn+],
(5)
(6)
cL = [L]sat + [LMn+
]
where cM and cL are the total concentrations of the ion and the ligand in solution, respec-
tively. Equation 3 can be transformed into Eq. 7 using Eq. 2:
(A/Ao) − 1 = [(ε2/ε1)Kcsalt]/(1 + K[L]sat)
(7)
Plotting (A/Ao) − 1 as a function of the total salt concentration csalt, one gets a straight line
with the slope b. From this slope the stability constant of the complex can be calculated
according to Eq. 8:
K = b/[(ε2/ε1) − b[L]sat]
(8)
If both molar absorptivities are nearly equal and the solubility of the ligand is low, then
the stability constant K is given by the slope b. The correctness of these assumptions has
already been established for some macrocyclic ligands such as cryptands, cucurbit[n]urils,
aza crown ethers and benzo crown ethers [26, 30–35].
3.2 The Synthesis and Characterization of Ligands
The synthetic pathways to the new ligands (2–7) are summarized in Scheme 1. Many sub-
stituted 4-amino-3,5-diaryl-4H-1,2,4-triazoles have been prepared by using known methods
[8–10]. 4-Amino-3-phenyl-5-pyridinyl-4H-1,2,4-triazole 1 was synthesized according to a

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J Solution Chem (2008) 37: 1489–1497
Scheme 1 The synthetic pathways for the triazole ligands used in the present study
literature method [9]. Schiff bases 2–4 were prepared by the condensation of 4-amino-3-
phenyl-5-pyridinyl-4H-1,2,4-triazole 1 with various aldehydes in acetic acid. During the
reduction of compounds 2–4, formation of multiple products was possible since reduction
of the hetero ring might occur. However, the reductions were performed on the imino group
of 5–7 without affecting the hetero ring by using NaBH4 as a selective reducing agent.
IR spectra showed C=N bands of 2–4 in the 1583–619 cm−1 region. The 1H NMR char-
acteristic signals of 2–4 were observed at δ = 8.55–8.80 ppm (s, 1H, N=CH). The 13C NMR
signals for the –N=CH– group of compounds 2–4 were recorded at δ = 164–166 ppm. Aryl-
methylamino derivatives 5–7 showed IR absorption bands around 3309–3290 cm−1 (νNH).
The 1H NMR characteristic signals of 5–7 were observed as a triplet at δ = 6.80–6.83 ppm
(t, 1H, NH) and as a doublet at δ = 3.85–3.86 ppm (d, 2H, CH2); the –NH–CH2– carbon
signal of compounds 5–7 were recorded between δ = 50 and 52 ppm.
3.3 The Stability Constants of Metal-Ligand Complexes
Any experimental method for the determination of the total concentration of a ligand in a
salt solution, e.g. UV-Vis spectroscopy or TOC measurements, allow the determination of
the stability constant of the complex formed in solution. It is not necessary to know the exact
relation between the measured signals and the ligand concentrations, and the intensities of
the signals only need to be directly proportional.
In the present study Cu2+, Cr3+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+ and Ag+ cations caused
a small change in the absorption spectra of all the ligands. Therefore, we can say that the
molar absorptivities of the ligand and complex, εı and ε2, are nearly the same. The measure-
ments were carried out at an appropriate wavelength in which the increments of absorbance
are regular functions of increasing metal concentration. There was no regular correlation
between (A/A0) − 1 and csalt in the case of other tested metal ion such as Li+, Na+, K+,
Rb+, Cs+, Ba2+, Sr2+ and Ca2+. Because of this, the corresponding stability constant could
not be calculated for those cations.
For the Cu2+, Cr3+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+ and Ag+ cations, straight lines were
found in accordance with Eq. 7 for all of the studied ligands. Owing to complex forma-
tion, the total concentration of the dissolved ligand in solution increased. The change in the
total concentration of the dissolved ligand was monitored with spectrophotometric mea-
surements. The complex stability constant (K) was calculated according to the present
method, which was previously used for various macrocyclic ligands [30–35]. The quantity
(A/A0) − 1 is plotted versus csalt, and the stability constant is given by the slope. For exam-
ple, the linear response of (A/A0) − 1 versus cCu2+ for ligand 7 with Cu2+ is represented by
the equation y = 3546.2x − 6.43 with a correlation coefficient of 0.9913.

J Solution Chem (2008) 37: 1489–1497
1495
Table 1 Stability constants (log K) at infinite dilution for 1:1-complex formation of different new ligands
10
◦
with metal salts in aqueous solution at 25.0 C
a
Cation
log
K
10
2
3
4
5
6
7
2+
3+
Cu
2.46 0.07
3.12 0.01
3.17 0.01
3.14 0.01
2.49 0.01
2.60 0.02
2.61 0.02
2.65 0.02
2.53 0.06
3.11 0.01
3.09 0.01
3.08 0.01
2.73 0.01
2.66 0.01
2.69 0.01
2.65 0.02
3.13 0.02
3.22 0.02
3.04 0.01
3.09 0.01
2.75 0.01
2.78 0.01
2.77 0.01
2.66 0.01
3.42 0.04
3.31 0.01
3.08 0.01
3.04 0.0l
3.04 0.03
2.84 0.0l
2.88 0.0l
2.46 0.0l
2.82 0.05
3.23 0.01
2.83 0.0l
3.10 0.01
2.76 0.01
2.91 0.01
2.95 0.01
2.89 0.01
3.80 0.04
3.16 0.01
3.02 0.01
3.18 0.01
2.75 0.01
2.87 0.01
2.92 0.01
2.76 0.01
Cr
2+
Co
2+
2+
Ni
Zn
2+
Cd
2+
Pb
+
Ag
a
Average values and standard deviations calculated from five independent experiments. The confidence level
is 95%
Table 1 shows the stability constants (as log10 K) determined by means of the present
method for Cu2+, Cr3+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+ and Ag+ ions with all of the ligands.
As seen from the table, the stability constants increase when the ligand changes from 2
to 3 for Cu2+, Zn2+, Cd2+ and Pb2+ ions. These results show that the number of chlorine
atoms has a strong effect on the complexation. Namely, increasing the number of chlorine
atoms increases the stability constant for these cations. This situation may be explained by
the increasing electron density on the azomethine bond that increases the electron donor
properties of the nitrogen atom. Consequently, more stable complexes form with ligand 3
compared with ligand 2. Similarly, the stability constants increase when the ligand changes
from 3 to 4 for all of the metal cations except for Co2+. This result shows that the position
of the second chlorine atom is also important for the complexation for these cations with the
Schiff base ligands. In ligand 4 the second chlorine atom, which is located on the 6-position,
increases the electron donor properties of the nitrogen atom because of its nearer position
to the azomethine group that is attached to the benzene ring. This produces more stable
complexes with the ligand having the 2-6-dichloro-phenyl group with the mentioned metal
cations. The position effect of the second chlorine atom is observed most strongly for Cu2+
.
The value of log10 K increases from 2.53 to 3.13 when the ligand changes from 3 to 4. The
smallest position effect of the chlorine atom is seen on the complexation of Ni2+ and Ag+
cations for ligands, 3 and 4. When the ligand changes from 3 to 4 the log10K value remains
constant at 2.65–2.66 and 3.08–3.09 for Ag+ and Ni2+, respectively.
As seen from Table 1, all of the Schiff base ligands and their reduced forms give stable
complexes having log10K values that are higher than 3.04 with the Cr3+ and Ni2+ cations.
A similar situation occurs for Co2+ except for ligands, 6 and 7. However, the values of
log10K for the Cd2+, Pb2+ and Ag+ ions with all of the ligands are below 2.95. A similar
situation occurs for Zn2+ except for ligand 5. It is interesting to note that complexation with
the Cu2+cation is affected most by the structure of the ligands. It is clear that Cu2+ gives
the most stable complex with ligand 7 among the ligands. The corresponding log10K value
is 3.80 in this case. Consequently, Table 1 shows that moderately soft metal cations (Cu2+
,
Cr3+, Co2+, Ni2+, Zn2+) form more stable complexes with the triazole ligands compared to
soft metal cations such as Cd2+ and Ag+. We can say that the nitrogen atoms of the triazole
ligands are effective for metal complexation. The nitrogen atom belonging to an azomethine
group may also participate in complexation. As is known, the nitrogen atom is a moderately

1496
J Solution Chem (2008) 37: 1489–1497
soft donor atom in coordination compounds. Therefore, one expects that the present triazole
ligands form more stable complexes with moderately soft transition metal cations such as
Cu2+, Cr3+, Co2+, Ni2+, Zn2+, compared with soft metal cations such as Cd2+ and Ag+.
The results presented here clearly demonstrate the possibility of quantitatively studying
complexation reactions between triazole ligands and metal cations even when the ligand is
nearly insoluble in water. Due to the fact that nearly insoluble triazole ligands were used,
very small samples of them are sufficient to perform these measurements. Therefore the
present method requires only very small samples of the ligand for the study of complex for-
mation. The method is rapid and simple. Moreover, the ligand sample can be recovered after
the determination. Because of low solubility properties of triazole compounds in aqueous
media, there is no convenient method described in the literature for measuring complexation
properties of these types of ligands with metal cations. The presented method is therefore
important in this respect.
Acknowledgements The authors thank the Deutsches Textilforschungszentrum Nord-West and The Sci-
entific and Technical Research Council of Turkey (TUBITAK) for the financial support of this study.
References
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