2
00
K. Raouf-Benchekroun et al. / Journal of Molecular Structure 476 (1999) 191–201
2
ϩ
respectively. The isotopic pattern of these peaks is
consistent with these assignments (see Fig. 6). Thus,
the weak variation with Zn , only the two other
2ϩ
complexes have been studied. With Hg (Fig. 6a),
the obtaining of the L Hg complex and mercuric salts
of the thiooxamic acid can be expected.
The i.r. spectrum of the grey solid obtained by
precipitation from the acetonitrile solution shows
bands at 3438 (coordinated water), 3263 (n(NH)),
a clear break in the graph is observed for r 0.5 at
2
l 326 nm indicating a ML complex formation.
2
However, whereas the formation of a new maximum
was seen near 275 nm, an accurate formation constant
for the ML complex (bML2) cannot be calculated with
2
1
7
742 (n(CyO)), 1670 (n (CO )), 1543 (TA I) and
the STAR program; only an approximate value of
as
2
Ϫ1
2ϩ
19 (n(TA IV)) cm . The carbonyl stretching vibra-
logbML2 13.4 ^ 0.6 is obtained. With Cd (Fig.
Ϫ1
tion at 1742 cm is very close to those found for zinc
and cadmium complexes (1746 cm ), undergoing
the same frequency decrease versus ligand; it can be
assigned to the L Hg complex. Besides this product,
the mercuric salt of the thiooxamic acid is character-
ized by the asymmetric carboxylate stretching vibra-
tion at 1670 cm , here also in agreement with the
same bands for zinc and cadmium salts. Other
bands, such as TA I and TA IV, present respectively
an increase of 20 cm and a decrease of 16 cm ,
characteristic of chelation by the sulfur atom of the
thiooxamate.
5b), isosbestic points are observed at 298, 307 and
390 nm. On the other hand, slope changes of the OD
versus r curves are found, for instance, at 340 nm for
r 0.5 and at 230 nm for r 0.3, suggesting respec-
tively ML and ML complexes. Taking into account
Ϫ1
2
2
3
these two complexes, two formation constants are
Ϫ1
found, i.e. logbML2 10.0 ^ 0.1 and logb
ML3
14.0 ^ 0.3. The species distribution along 0 Ͻ r Ͻ
1 indicates that the ML complex does not exceed 5%.
3
Ϫ1
Ϫ1
Moreover, the absorption curves of the pure species
show that the complexes absorb near 295 nm.
Finally, the ML complex formation constant with
2
The L Hg complex is less soluble in acetonitrile
Hg(II) is higher than that with Cd(II) by three loga-
rithmic units, according to the affinity difference of
these two metallic ions for the thioamide function of
the ligand. This affinity difference can be also corre-
lated to the lmax shift of the complexes (51 and 31 nm
respectively). In each case, it is an hypsochromic and
hypochromic shift related to a lesser conjugation of
2
than the other two metallic complexes; so was
obtained a solid which is composed of the two entities,
complex and salt, without noticeable mercuric
compound in the acetonitrile layer at the i.r. maximum
concentration (0.05 M).
The variation of the absorption spectrum of the
thiooxamate by progressive addition of mercuric
perchlorate in acetonitrile shows (Fig. 5a) that, at
the electronic system in the two ML complexes.
2
Ϫ4
the u.v. concentration ( ϳ 10 M), the complexed
thiooxamate is the unique entity in the medium.
This is in accordance with its expected greater solu-
bility than mercuric salts and also with the results of
mass spectrometric analysis.
4. Conclusion
In this study, we found that the potential enzyme
inhibitor t-butyl N-(4-chlorophenyl)thiooxamate
presents a different conformational pattern than the
ethyl thiooxamate of known structure, i.e. anti
3.6. Spectrophotometric study of the complexes
…
carbonyl versus thiocarbonyl groups and H
O
Ϫ3
Aliquots of metallic perchlorates (2.4 × 10 to
hydrogen bonds instead of syn configuration and
Ϫ3
…
3
.4 × 10 M) were added to a acetonitrile solution
H
S bonds in the ethyl thiooxamate due to favour-
of t-butyl N-(4-chlorophenyl)thiooxamate (6.1 ×
able electrostatic and torsional interactions in that
conformation of the t-butyl derivative. This behaviour
explains, for instance, the great decrease of the ester
carbonyl stretching band near 1700 cm .
The action of a rough metalloenzyme model like
Ϫ5
1
0
M) at 25ЊC. After dilution correction, spectra
at different r M/L values are gathered in Fig. 5.
Ϫ1
We note that, for 0 Ͻ r Ͻ 1, the OD variation at
2ϩ
lmax 326 nm is strong with Hg (0.36), lesser
2ϩ
2ϩ
with Cd (0.23), and weak with Zn (0.05).
The OD variation as a function of r is represented
in Fig. 5 for all the complexes. Taking into account
ZnBr on the t-butyl derivative led to the characteriza-
2
tion of three entities: a Zn(II) complex of the thioox-
amate; the thiooxamic acid obtained by isobutene