Kinetic study of the complex reaction between copper(II) and 2-(2′-hydroxy-3′-methoxyphenyl)benzothiazole

Article abstract of DOI:10.1016/j.saa.2009.04.017

2-(2′-Hydroxy-3′-methoxyphenyl)benzothiazole reacts with copper(II) in an ethanol/water mixture to form an O,S chelate which exhibits the remarkable property of changing the chelation site above a pH of ca. 5.0, to the O,N site. The detailed kinetics of t

Full text of DOI:10.1016/j.saa.2009.04.017

Spectrochimica Acta Part A 74 (2009) 30–35
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Kinetic study of the complex reaction between copper(II) and
ꢀ
ꢀ
2
-(2 -hydroxy-3 -methoxyphenyl)benzothiazole
a
b
c
d
a,∗
Wolfhardt Freinbichler , Ahmed Soliman , Reginald F. Jameson , Guy N.L. Jameson , Wolfgang Linert
a
Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9-163AC, A-1060 Vienna, Austria
Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt
Department of Chemistry, The University, Dundee, Scotland, United Kingdom
Department of Chemistry, University of Otago, Dunedin, New Zealand
b
c
d
a r t i c l e i n f o
a b s t r a c t
ꢀ
ꢀ
Article history:
Received 16 October 2008
Accepted 16 April 2009
2-(2 -Hydroxy-3 -methoxyphenyl)benzothiazole reacts with copper(II) in an ethanol/water mixture to
form an O,S chelate which exhibits the remarkable property of changing the chelation site above a pH of ca.
.0, to the O,N site. The detailed kinetics of this reaction in an ethanol/water mixture (3:1) at a temperature
of 25 C was investigated using a stopped-flow spectrophotometric technique employing a wavelength of
00 nm. The initial complex, Cu(O,S), is formed via a fast, reversible second-order complex formation step
whereupon the formation of the Cu (O,N) follows first order kinetics. The Cu(O,N) complex is, however,
unstable towards internal electron exchange and after the reaction is complete, a black polymeric material
very slowly precipitates out of solution. Rate and equilibrium constants for the postulated reactions are
presented and discussed.
5
◦
Keywords:
4
ꢀ
ꢀ
2
-(2 -Hydroxy-3 -
methoxyphenyl)benzothiazole
Copper(II)
Kinetics
Change of coordination side
©
2009 Elsevier B.V. All rights reserved.
1. Introduction
Polyfunctional ligands based on benzazoles have been inten-
sively investigated due to their biological activity as fungicides,
antibiotics, pesticides and neuroprotectors [1–7]. In addition they
have been studied because of their fluorescent and luminescent
properties [8,9] and the possibility to give rise to supramolec-
ular arrangements [10,11]. Compounds with transition metal
ions of benzazole derivatives, such as 2-aminobenzothiazole, 2-
aminothiazole, and 2-aminobenzoxazole, have shown that bonding
occurs through the nitrogen atom in a monodentate form, regard-
less of the metal ion or the heteroatom (S,O) present in the
molecule. In the absence of the amino group, a bidentate coordina-
tion has been observed and a delocalized system may be stabilized
[
12–16].
When 2-(2 -hydroxy-3 -methoxyphenyl)benzothiazole (I), is
ꢀ
ꢀ
dissolved in a 75% ethanol and 25% water mixture and then added
to a copper(II) acetate solution (in the same mixed solvent) a
yellow colour appears rapidly. This colour is stable in the pH range
2. Experimental
2.1. Materials and methods
<5.0. In the pH range 5.0–6.0 the solution becomes more intense in
colour. The latter step lasts approx. 100 s. After that the colour very
slowly fades, accompanied by precipitation, over a time interval of
ca. 3000 s (Fig. 1). This intriguing phenomenon encouraged us to
study the reaction by stopped-flow kinetic experiments.
2
.1.1. Chemicals
The chemicals used were of analytical grade. These include 2-
hydroxy-3-methoxybenzaldehyde (o-vanilin) (Sigma–Aldrich,
Vienna, Austria), 2-aminothiophenol (2-mercaptoanilin)
Sigma–Aldrich, Vienna, Austria), Cu(II)-acetate xH O (Merck,
(
2
Vienna, Austria), buffer solutions (Riedel de Haen, Vienna, Austria),
ethanol (Sigma–Aldrich, Vienna Austria), and EDTA (Sigma–Aldrich,
Vienna, Austria). De-ionized water was used throughout the work.
∗ Corresponding author. Tel.: +43 1 58801 15350; fax: +43 1 58801 15399.
E-mail address: wlinert@mail.zserv.tuwien.ac.at (W. Linert).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.04.017

W. Freinbichler et al. / Spectrochimica Acta Part A 74 (2009) 30–35
31
Fig. 1. Absorption changes at 400 nm for the whole reaction (the dashed line in (B) indicates how the formation would continue in the absence of any further reaction).
2
.1.2. Preparation of the ligand
2
Before putting the reaction mixtures into the stopped-flow
machine, the pH values were adjusted using diluted NaOH and
acetic acid. The pH was measured of both the copper and the lig-
ꢀ
ꢀ
-(2 -Hydroxy-3 -methoxyphenyl)benzothiazole (I) was syn-
thesized by reacting 2-hydroxy-3-methoxybenzaldehyde and
-aminothiophenol [17,18]. The reagents (in 1:1 molar ratio) were
2
and solutions, and the pHc value was calculated from the average
+
stirred in polyphosphoric acid used as the reaction medium for
2
and neutralized with 50% NaOH. The crude product was then crys-
tallized from ethanol. Identity and purity of the product were
H
concentration; the measured values being reproducible within
◦
–3 h, at 180–200 C. The reaction mixture was diluted with water
± 0.02 units. The solutions were de-oxygenated with argon and
injected into the stopped-flow unit.
Absorbance time curves, measured relative to de-ionized water,
were analyzed using either the internal software of the instrument
or Microsoft Excel.
◦
1
confirmed by comparison of its melting point (162 C) and H NMR
spectrum with the literature data.
1H NMR CdCl3 ı [ppm]: 8.02 (2, 1H, J = 1.61 Hz, H4), 7.91 (2, 1H,
J = 7.91 Hz, H7), 7.52 (4, 1H, J = 7.89 Hz, J = 8.05 Hz, H5), 7.42 (4, 1H,
1
2
2.1.4. UV–vis difference spectra
J = 7.87 Hz, J = 7.90 Hz, H6), 7.33 (2, 1H, J = 7.90 Hz, H3), 7.03 (2, 1H,
Difference spectra, using the ligand as reference, were used to
investigate the intermediate complexes. Spectra were recorded on
a PerkinElmer Lambda 900 UV-VIS-NIR machine.
1
2
J = 7.89 Hz, H1), 6.93 (4, 1H, J = 7.88 Hz, J = 8.00 Hz, H2), 3.80 (1, 3H,
H8).
1
2
2.1.3. Kinetic measurements
2.1.5. IR measurements
All experiments were carried out using a stopped-flow SX17.MV
spectrometer (Applied Photophysics) either in the single wave-
length (for the complex formation) or in the diode array mode
IR-spectra were recorded on PerkinElmer 16 PC MID-IR FT-IR
(PerkinElmer, Vienna, Austria) using a circular cell which allows
direct injection of a liquid sample. The ligand as well as cop-
per was dissolved in ethanol, and mixed with a 10 mM acetate
buffer solution to give an ethanol-to-water ratio of 3:1, whereby
the ligand concentration was 8 mM and the copper concentration
1 mM. The solutions were mixed to give a final volume of 50 ml.
After 200 s the solution was mixed with carbon tetrachloride to
extract the complex. The organic phase was concentrated to 10 ml
using a rotary evaporator. The blank was prepared by adding CCl4
to an ethanol (75%)–water (25%) mixture, and then treated like
the sample. The blank was then used as the background of the
IR-spectrum; CCl4 was chosen to reduce interference by the back-
ground.
(
2
for the rearrangement reaction). The temperature was kept at
◦
5 ± 0.2 C using a Lauda RM6 (Lauda-Königshofen, Germany) ther-
mostat. The pH-measurements were made with a PH-196 pH-meter
manufactured by WTW and using their Senix 21-3 electrodes.
Because the activity of a 75% alcoholic solution is not well defined
+
the concentration of H was used for all calculations. In order to cal-
ibrate the electrode, the measured pH values were plotted against
the logarithm of several known concentrations of perchloric acid
and the resulting straight line was used to calculate the electrode
in terms of the H+ concentration. The resulting conversion fac-
tor used in this work was pHc = (pH
pHc = −log10[H ] is used in order to distinguish it from pH.
+ 1.085)/1.381 in which
measured
+
+
Fig. 3. Typical complex formation curve: Cu(O,S)+; [Cu] = 0.1 mM; [L]t = 2 mM;
Fig. 2. Spectrum of the Cu–(O,S) chelate, Cu(O,S) ; [Cu] = 0.1 mM; [L]t = 1 mM;
pHc = 5.48. Data measured after 100 ms.
pHc = 5.11.

3
2
W. Freinbichler et al. / Spectrochimica Acta Part A 74 (2009) 30–35
The UV–vis spectrum of the complex exhibits three charge trans-
fer absorptions at 375, 405(sh) and 456 nm (the solutions were
far too dilute for any d–d transitions to be observable). There-
fore all data were recorded at the shoulder of the initial complex
spectrum at 400 nm (Fig. 2), and in order to work under pseudo
first-order conditions the ligand concentration was always at least
10 times higher than that of copper. A typical complex formation
curve is illustrated in Fig. 3 which obeys a (pseudo)first order rate
law.
The variation of the rate constants with ligand concentration at
constant pHc values leads to straight lines that pass through the
origin, which indicates the expected second-order rate of complex
formation. The slopes of these straight lines yielding the second-
Fig. 4. Dependence of kobs/[LT] on pHc (theoretical curve based on Eq. (13)
order rate constants. Fig. 4 illustrates the observed rate constant,
2
−1 −1
6
−1 −1
obs
and the calculated rate constants, k+1 = 2.79 × 10
M
s
; k−1 = 1.69 × 10
M
s
;
k
, as a function of the pHc value for [L]T = 0.004 M (Table 1). It
9
−1 −1
obs
k2 = 1.86 × 10
M
s
).
can be seen that k
passes through a minimum which indicates a
reversible reaction for high, and an irreversible reaction for lower
+
As traces of water and ethanol caused a low signal-to-noise ratio
H
concentrations. As is described later, one can use these data to
−1
spectrum only a range from 1500 to 1800 cm could be used for
analysis. In this area the spectrum contains only one band, which
was identified as the C N stretch.
obtain values of the rate and equilibrium constants (k+1, k−1, k2, and
M
1
K
).
Below a pHc of ca. 5.0 and in the absence of oxygen the initial
complex is stable and, in fact the monocomplex (with a dimethyl
amino group meta to the sulphur) can be isolated as a crystalline
solid [19]. Chelation is via the O,S site (see II) and hence is written
2
.1.6. Re-extraction experiments
To investigate the possibility that the thiazole ring was open-
+
ing, the complex was extracted into CCl4 as described above, but
then copper was re-extracted into the aqueous phase (indicated by
the appearance of a blue colour) by using a 10 mM EDTA solution,
as [Cu(O,S) ] in formulae.
leaving the free ligand in the organic phase. The organic phase was
1
dried with NaSO , filtered and finally analyzed by H NMR.
4
3. Results
3.1. Formation of the initial complex
To examine whether the ligand coordinates via the O,N or
O,S donor atoms, the IR-spectra of the free ligand and the com-
plex were compared at a pHc value of 4.5. The free ligand, as
−
1
well as the complex, showed a clearly visible band at 1643 cm
,
3.2. The rearrangement reaction to Cu(O,N)
which is ascribed to the C N stretch. As there was no shift
detected upon complexation with copper it was concluded that
coordination was achieved via the oxygen and the sulphur donor
atoms.
At pHc values above about 5.0 there is a sequential reaction that
takes place over ca. 100 s. The spectrum of the product is illus-
trated in Fig. 5; it has charge transfer bands at 380 and 400 nm
Table 1
Typical results for the formation of the Cu–(O,S) chelate, Cu(O,S)+ ([L]T = 0.004 M; [Cu2+] = 0.10 mM; K OH = 1010.45).
7
+
5
H
+
kobs/s
10 kobs/[L]T
−3
pH(meas.)
pHc
10 [H ]/M
10 /(1 + K [H ])
Max. abs.
0.28
O
4.60
4.68
4.71
4.80
4.90
4.97
5.10
5.27
5.40
5.47
5.80
5.97
6.05
6.26
6.49
6.62
6.73
6.98
7.25
7.40
7.50
7.80
4.12
4.17
4.20
4.26
4.33
4.39
4.48
4.60
4.70
4.75
4.98
5.11
5.17
5.32
5.48
5.58
5.66
5.84
6.03
6.14
6.22
6.43
766
670
632
549
465
410
333
251
200
178
104
78.1
68.3
48.1
32.8
26.2
21.8
14.4
9.24
7.20
6.09
3.69
0.046
0.053
0.056
0.065
0.076
0.087
0.107
0.142
0.177
0.199
0.342
0.455
0.519
0.737
1.08
1.35
1.63
2.47
3.84
4.93
5.82
9.60
132
119
110
95
87
71
74
54
49
47
33.00
29.75
27.50
23.75
21.75
17.75
18.50
13.50
12.25
11.75
14.50
13.30
16.25
18.00
23.29
30.00
31.25
47.00
75.00
0.32
0.36
0.50
0.56
0.57
0.59
0.56
0.53
0.43
0.47
0.48
0.27
0.19
0.11
0.13
0.12
0.08
0.04
58
53
65
72
93
120
125
188
300

W. Freinbichler et al. / Spectrochimica Acta Part A 74 (2009) 30–35
33
Even with small amounts of oxygen present, after the rear-
rangement reaction there follows an extremely slow decrease in
absorption that is accompanied by the precipitation of a black poly-
meric material, [CuL]n. As the rate increases with the number of
shots in one experiment, it is evident that the reaction is triggered
by oxygen leaking into the system. The reaction could be avoided
by cleaning the system between each shot with argon saturated
ethanol/water mixture.
However, sufficient material for analysis could be obtained by
allowing a reaction mixture to stand for about an hour, and analysis
confirmed the formulation [CuL]n (analytical results are: C = 52.50%
(52.41); H = 3.23% (3.45); N = 4.42% (4.36); S = 9.90% (9.99)). No
attempt was made to further characterise this material. However,
from the colour, it also seems certain that some electron exchange
between copper(II) and sulphur atoms is involved thus leading to
a free-radical condensation reaction, which could be triggered by
Fig. 5. Spectrum of the –O,N copper(II) chelate, Cu(O,N); [Cu] = 0.1 mM; [L]t = 1 mM;
pHc = 5.48. Data measured after 100 s.
O . Furthermore, since this species is highly insoluble, once started,
this reaction will proceed to completion.
and therefore all kinetic data for this compound were collected at
this wavelength.
2
IR measurements of the resulting complex showed a shift of the
N stretch vibration from 1643 to 1582 cm indicating a weak-
−
1
C
4. Discussion
ening of the C N bond due to complexation.
A possible ring opening was investigated by re-extracting the
copper with EDTA solution to the aqueous phase, and comparing
the NMR spectra of the remaining free ligand with the ones of the
original ligand. As no differences could be found between these two
any structural changes of the ligand can be excluded. Therefore the
compound was assigned to be the –O,N chelated equivalent of II
and hence can be written as [Cu(O,N)] (see III). The bis-complex of
III could be crystallized from ethanolic solution [20–22].
4.1. Formation of the initial complex
In order to establish that it was safe to measure rate constants
using a single wavelength, it was first established that k was con-
obs
stant over the absorbance range of 370–470 nm; the reaction was
then followed at 400 nm.
The appearance of the coloured complex is a measure of the
consumption of Cu2+ and hence the experimental rate law can be
expressed as Eq. (1):
d[coloured complex]
=
kobs{[Cu2+] − [Cu2+]eq
}
(1)
dt
Furthermore, the pHc-dependence of kobs, which is illustrated
obs
in Fig. 4, taken in conjunction with the fact that k
is linearly
dependent on [L]T, contains all the information required to solve
the kinetics of initial complex formation.
Above a pHc of about 5 the reversible part of the reaction is no
longer present and we further assume that both protonated and
unprotonated species react:
As is illustrated in Fig. 6, the formation curve of Cu(O,N) has
the typical shape of a single exponential function (and it can be
perfectly fitted with it) indicating a first order reaction. The rate
was found to be independent of pH and both the ligand and the
copper concentrations.
+
d[Cu(O,S) ]
+
−
2+
=
k
[LH][Cu2 ] + k [L ][Cu
]
(2)
+
1
2
dt
At these proton concentrations the de-protonation of the lig-
and is very small, and hence [LH] can be equated with [L]T. Use of
H
O
−
the protonation constant*, K , enables [L ] to also be expressed in
terms of [L]T. The protonation reaction is
H
O
K
−
+ H ꢀLH log K OH = 10.45
+
L
(3)
(4)
which leads to:
[
L]T
H
[
L−] =
+
1
+ K [H ]
O
and thus, from Eqs. (1) and (4) one obtains:
kobs
k2
=
k
+
(5)
+
1
H
+
[
L]T
1 + K [H ]
O
Plotting of kobs/[L]T vs. 1/(1 + K [H ]) yields a straight line
H
+
O
9
−1 −1
s with an
(
Fig. 7), the slope of which yields k = 1.86 × 10 M
2
2
−1 −1
s .
intercept of k+1 = 2.79 × 10 M
If one considers the reversible reaction at pHc < 4.7, the following
relations are valid:
k
Cu2 + LH ꢀ Cu(O,S) + H+
+
+1
+
(6)
Fig. 6. The first order formation of the complex Cu(O,N). The rate constant was
k
obs
−1
−1
determined to k = 0.028 s
.

3
4
W. Freinbichler et al. / Spectrochimica Acta Part A 74 (2009) 30–35
Fig. 8. Formation and decay of Cu(O,S)+ as a function of pHc. The absorbance mea-
sured after 50 ms is plotted.
Fig. 7. Plot of kobs/[L]T vs. 1/(1 + K [H ]); straight line based on a least square fit.
H
+
O
9
−1 −1
2
−1 −1
Slope = k2 = 1.86 × 10
M
s
; intercept = k+1 = 2.79 × 10
M
s
.
4.2. The rearrangement reaction
for which:
+
+
k
k
[Cu(O,S) ]eq[H ]
+
−
1
1
As explained above, the Cu(O,S) complex is stable only at pH
K1
=
=
(7)
[Cu2 ]eq[LH]eq
+
values lower than approx. 5.5, whereas above this pHc a slow
transformation reaction is observable. As the IR results indicate
an involvement of the N-atom in the finally formed complex the
only product with can be formulated without breaking the ring
structure is the Cu(O,N) bidentate complex. It is evident that the
transformation reaction must lead via an intermediate complex and
this equilibrium is influenced by pHc. A proper explanation for the
The rate expression is given by (8)
+
d[Cu(O,S) ]
[Cu2+][L] − k [Cu(O,S) ][H ]
+
+
=
=
k
+1
T
−1
dt
kobs([Cu] − [Cu]eq
)
(8)
Making use of the relationship [Cu(O,S) ]eq = [Cu2+]0 − [Cu2+]eq
+
−
data is that Cu(O,S) loses a proton of a Cu bound H2O or gains OH
means that Eq. (7) at constant pH can be expressed in terms of
to form the relatively unstable Cu(O,S)OH complex which is then
converted into Cu(O,N) bidentate complex by the rotation of the
molecule.
2+
[
Cu ]eq, which can be rearranged to:
+
+
k
+
1[Cu]eq[L] + k [Cu]eq[H ] = k [Cu] [H ]
with the relation [Cu(O,S) ] = [Cu2+]0 − [Cu2+] (8) can be expressed
(9)
T
−1
−1
0
The transformation of the Cu(O,S) complex into Cu(O,N) through
the formation of Cu(O,S)OH can be deduced from the decay of the
absorbance of the Cu(O,S) species as a function of pHc.
It can also be concluded from Fig. 8, that the disappearance is
+
as
+
d[Cu(O,S) ]
[Cu2 ][L] + k [Cu ][H ] − k [Cu] [H ]
+
2+
+
+
−
=
k
at the basic side proportional to [OH ] concentration, hence the
+1
T
−1
−1
0
dt
−
transformation reaction requires the addition of an OH or the de-
(
10)
protonation of a bound H O. Thus an unstable species Cu(O,S)OH
2
+
Substituting now the term −k−1[Cu] [H ] with the left expres-
is formed which then converts via rotation of the benzothiazole
molecule to the Cu(O,N) species.
0
sion in (9) leads to
Evidence for the above rearrangement could also be found by
measuring the full spectrum of the reaction, subtracting the ligand
spectrum and following the rearrangement reaction with a two-
beam spectrometer (Fig. 9). It can be seen in Fig. 9 that the charge
+
d[Cu(O,S) ]
[L] {[Cu2+]−[Cu]eq}+k−1[H ]{[Cu2+]−[Cu2+]eq
+
}
=
k
+1
T
dt
=
kobs{[Cu] − [Cu]eq
}
(11)
That directly yields
kobs
[H ]
+
=
k
+ k−1
(12)
+1
[
L]T
[L]T
and therefore, by taking Eq. (5) into account, the expression appli-
cable to the whole pH range studied is given by Eq. (13):
kobs
[H ]
+
k2
=
k
+ k−1
+
(13)
+1
H
+
[
L]T
[L]T
(1 + K [H ])
O
Since k+1 and k2 have been obtained, k can be obtained by
−1
curve fitting to the original data. The result of this operation is illus-
6
−1 −1
s was accepted.
trated in Fig. 4 and a value of k 1 = 1.69 × 10 M
−
Cu(O,S)
The complex stability constant, K1
, is given by Eq. (14):
Cu(O,S)+
K
1
+
Cu2 + L−
+
ꢀ
Cu(O,S)
(14)
and this means that:
+
k
k
Cu(O,S)
H
+1
H
O
K1
= K KO
=
K
(15)
1
−1
Fig. 9. Absorption spectrum of the complex reaction: [L] = 0.0504 mM,
[Cu] = 0.0490 mM; pHc = 6.00; spectra taken after 30, 190 and 600 s; measured
against [L] = 0.0454 mM.
+
Substitution in Eq. (15) the stability constant of the Cu(O,S)
complex species is found to be log K 1C u(O,S) = 6.67.

W. Freinbichler et al. / Spectrochimica Acta Part A 74 (2009) 30–35
35
transfer (CT) band of Cu(O,S)OH at 351 nm is hidden behind the
strong ligand CT-band and therefore could not be used for stopped-
flow studies. The appearance of the Cu(O,N) species is represented
by the rise of the absorbance at 394 nm, whereas the disappear-
ance of the Cu(O,S)OH species can be seen at 351 nm. Moreover,
and in a similar study reported [19], in alkaline ethanolic solution
the Cu(O,S)OH complex could indeed be isolated and characterised.
As the absorbance after re-arrangement stayed constant
N17) and to the “Hochschuljubiläumstiftung der Stadt Wien”
(Project H-01684/2007).
References
[
[
[
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(
approx. 0.85) over the whole measured pHc area above 5.5 it is
concluded that no other complex species are formed.
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the following equations:
[5] P. Jimonet, J. Med. Chem. 42 (1999) 2828–2843.
[
[
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7] R.C. Schnur, A.F.J. Fliri, S. Kajiji, V.A. Pollack, J. Med. Chem. 34 (1991) 914–918.
Cu + LH → Cu(O, S)+ + H+
(16)
(17)
+
+
−
+
Cu(O,S) + H O ꢀ Cu(O,S) OH + H
[8] E. Koyama, G. Yang, S. Tsuzuki, K. Hiratani, Eur. J. Org. Chem. (2002) 1996–2006.
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93–297.
2
[
1
[
[
−
Cu(O, S)OH → Cu(O, N) with (OH ) bound to the copper
(18)
2
[
[
12] A. Decken, R.A. Gossage, J. Inorg. Biochem. 99 (2005) 664–667.
13] D.E. Lynch, H.L. Duckhouse, Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 57
5
. Conclusion
(
2001) 1036–1037.
From the presented considerations it can be concluded that
[14] L. Sieron, M. Bukowska-Strzyzewska, Acta Crystallogr., Sect. C: Cryst. Struct.
Commun. 56 (2000) 19–21.
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16] E.S. Raper, R.E. Oughtred, I.W. Nowell, L.A. March, Acta Crystallogr., Sect. B: Cryst.
Struct. Chem. 38 (1982) 2044–2046.
the ligand indeed changes its coordination site from O,S to O,N
coordination depending on its possibility to form a Cu(O,S)(OH)
[
[
+
intermediate. In the more acidic area the meta-stable Cu(O,S) com-
plex is stabilized, because the O,S coordination is achieved quicker
than the O,N one and the high activation energy of the rotation
effectively hinders the molecule to change into its thermodynami-
cally stable state.
[17] R.D. Haugwitz, R.G. Angel, G.A. Jacobs, B.V. Maurer, V.L. Narayanan, L.R.J. Szanto,
J. Med. Chem. 25 (8) (1982) 969–974.
[
18] A.W. Addison, T.N. Rao, Curtis G. Wahlgren, J. Heterocyclic Chem. 20 (1983)
481–1484.
[19] M.N.H. Moussa, A.M. Shallaby, F.I.M. Taha, Egypt J. Chem. 16 (1973) 471–479.
1
[
20] R.D. Lorenz, M.T. Barbara, R.J. Wasson, Inorg. Nucl. Chem. Lett. 12 (1) (1976)
6
5–71.
Acknowledgements
[
21] S.P. Ghosh, S.K. Gupta, L.K. Mishra, J. Indian Chem. Soc. 56 (2) (1979) 206–208.
[
22] M.F. El-Shazly, T. Salem, M.A. El-Sayed, S. Hedewy, Inorg. Chim. Acta 29 (2)
Thanks for financial support are due to the “Fonds zur Förderung
der Wissenschaftlichen Forschung in Österreich” (Project 19335-
(1978) 155–163.