Synthesis and characterisation of zinc, cadmium and mercury complexes of benzilbisthiosemicarbazone. Structure of cadmium derivative

Article abstract of DOI:10.1016/S0020-1693(01)00607-7

Complexes [MLH₆(NO₃)₂] (M = Cd, Hg) (1 and 2), [MLH₄] (M = Hg, Zn) (3 and 5) and [ZnLH₅NO₃] (4) have been synthesised by reaction of benzilbisthiosemicarbazone LH₆ with cadmium, mercury and zinc nitrates in ethanol. The X-ray diffraction study of cadmium complex 1 shows that the azomethine nitrogen atoms, the sulfur atoms and the nitrate groups are bonded to the cadmium atom giving, the first time for this kind of ligands, a distorted octahedral geometry. Complexes have been characterised by elemental analyses and mass spectra as well as by IR, ¹H, ¹³C and ¹¹³Cd NMR and ¹³C and ¹¹³Cd CP/MAS NMR spectroscopies. In all complexes LH₆ acts as a tetradentate N₂S₂ ligand. The electrochemical behaviour of complexes, studied by cyclic voltammetry, shows metal centred and ligand reduction processes for all of them.

Full text of DOI:10.1016/S0020-1693(01)00607-7

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Inorganica Chimica Acta 323 (2001) 130–138
Synthesis and characterisation of zinc, cadmium and mercury
complexes of benzilbisthiosemicarbazone. Structure of cadmium
derivative
Elena Lo´pez-Torres a, Ma Antonia Mendiola a,*, Jesu´s Rodr´ıguez-Procopio b,
Ma Teresa Sevilla b, Enrique Colacio c, Jose´ Ma Moreno c, Isabel Sobrados d
a Departamento de Qu´ımica Inorga´nica, Uni6ersidad Auto´noma, 28049 Madrid, Spain
b Departamento de Qu´ımica Anal´ıtica y Ana´lisis Instrumental, Uni6ersidad Auto´noma, 28049 Madrid, Spain
c Departamento de Qu´ımica Inorga´nica, Uni6ersidad de Granada, 18071 Granada, Spain
d Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain
Received 14 February 2001; accepted 6 August 2001
Abstract
Complexes [MLH6(NO3)2] (M=Cd, Hg) (1 and 2), [MLH4] (M=Hg, Zn) (3 and 5) and [ZnLH5NO3] (4) have been synthesised
by reaction of benzilbisthiosemicarbazone LH6 with cadmium, mercury and zinc nitrates in ethanol. The X-ray diffraction study
of cadmium complex 1 shows that the azomethine nitrogen atoms, the sulfur atoms and the nitrate groups are bonded to the
cadmium atom giving, the first time for this kind of ligands, a distorted octahedral geometry. Complexes have been characterised
by elemental analyses and mass spectra as well as by IR, 1H, 13C and 113Cd NMR and 13C and 113Cd CP/MAS NMR
spectroscopies. In all complexes LH6 acts as a tetradentate N2S2 ligand. The electrochemical behaviour of complexes, studied by
cyclic voltammetry, shows metal centred and ligand reduction processes for all of them. © 2001 Elsevier Science B.V. All rights
reserved.
Keywords: Thiosemicarbazone complexes; Crystal structures; Cadmiun complexes; Zinc complexes; Mercury complexes
In the last few years, a large number of works have
tried to design compounds highly selective and sensitive
to heavy metals, in order to detect and if it is possible,
to remove them. Many different strategies have been
proposed, such as the preparation of fluorescent or
electrochemical sensors [24–26].
In previous works [27–31], we have been interested
in the synthesis and characterisation of transition metal
complexes with macroligands containing thiosemicarba-
zone moieties. Benzilbisthiosemicarbazone (LH6) is a
suitable ligand because it has donor atoms (nitrogen
and sulfur) which bond to mercury, cadmium and zinc.
LH6 complexes are very stable because of its chelating
character, yielding pseudomacrocyclic complexes. Fur-
thermore, LH6 copper complexes have been applied in
potentiometric copper sensors [32]. Now, we have ex-
tended our interest in cadmium and mercury complexes
because of their potential application as electrochemical
sensors of these metals.
1. Introduction
Many studies on complexes of transition metal ions
with macroligands have been reported due to their
relationship with biological molecules, most of them
contain tetradentate SNNS ligands [1–6]. However,
complexes of metals of Group 12 constitute an espe-
cially attractive topic in view of the marked differences
among these metals as regards both chemical behaviour
and biological activity. Zn(II) is an essential ion be-
cause of its presence in certain metalloenzymes [7–9]
while cadmium and mercury, present in the environ-
ment, are toxic but only recent studies have considered
the reactivity of macroligands containing sulfur with
zinc, cadmium and mercury [10–23].
* Corresponding author. Tel.: +34-91-397-4844; fax: +34-91-397-
4833.
E-mail address: antonia.mendiola@uam.es (M.A. Mendiola).
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00607-7

E. Lo´pez-Torres et al. / Inorganica Chimica Acta 323 (2001) 130–138
131
The high sensitivity of the 113Cd chemical shift to the
co-ordination environment (donor atom, co-ordination
number and geometry) enables to apply 113Cd NMR
spectroscopy as a probe of the metal in binding sites in
biological system and co-ordination compounds. When
cross polarisation magic angle spinning (CP/MAS)
113Cd NMR spectroscopy is combined with X-ray crys-
tallography, an excellent opportunity is provided to
study and correlate metal ion geometry with the chemi-
cal shifts. 113Cd NMR signals in solid or solution are
able to describe the ligand and its geometry about the
cadmium atom [17,33–35]. The aim of this work is to
prepare new benzilbisthiosemicarbazone complexes
with zinc, cadmium and mercury nitrates and to char-
acterise them in solution and in solid state by infrared,
nuclear magnetic resonance of proton, carbon and cad-
mium spectroscopies, conductivity measurements, mass
spectrometry together with X-ray data for the cadmium
nitrate derivative. We also explore the electrochemical
behaviour of the new compounds, which is the first
requirement to apply them as electrochemical sensors.
nitrate monohydrate (0.38 g, 1.11 mmol) in ethanol (40
cm3). The mixture was stirred for 12 h at room temper-
ature (r.t.). The yellow solid formed was filtered off and
then partially dissolved in ethanol (filtering again to
remove the insoluble fraction) and concentrated in
vacuo until a yellow solid was obtained. Yield 42%,
m.p. 180 °C with decomposition. lH (300 MHz,
(CD3)2SO) 8.4 (2H, s, NH), 7.5 (4H, s, NH2), 7.1–7.3
(10H, m, HPh). m/z 557 (M−2NO3, 8%) \M=66 cm2
V
−1 mol−1 in DMF, \M=16 cm2 Vmol−1 in CH3CN.
2.2.3. [HgLH4] (3)
After complex 2 was maintained in ethanol during
several days an orange solid was obtained, which was
filtered off and dried in vacuo. M.p. 190 °C. lH (300
MHz); solvent (CD3)2SO, 7.5 (4H, s, NH2), 7.1–7.3
(10H, m, HPh), m/z 557 (M++1, 15%). \M=17 cm2
V−1 mol−1 in DMF.
2.2.4. [ZnLH5NO3] (4)
To a suspension of LH6 (0.50 g, 1.58 mmol) in
ethanol (40 cm3) was added zinc nitrate hexahydrate
(0.50 g, 1.88 mmol) in the same solvent (10 cm3). The
mixture was stirred under reflux for 4 h. The yellow–
orange solid was filtered off, washed with ethanol and
dried in vacuo. Yield 50%, m.p. 260 °C. Anal. Found:
C, 39.65; H, 3.15; N, 20.17; S, 13.18. C16H15N7S2ZnO3
requires C, 39.80; H, 3.11; N, 20.31; S, 13.27%. m/z 419
(M−NO3, 100%). \M=51 cm2 Vmol−1 in DMF.
2. Experimental
2.1. Reagents
Cadmium, mercury and zinc nitrates were commer-
cial products of highest chemical grade (Fluka). Sol-
vents were purified according to standard procedures.
2.2.5. [ZnLH4] (5)
2.2. Synthesis
An orange solid was formed from the filtrate of the
previous synthesis after cooling it during 2 months at
−20 °C. The solid was filtered off and dried in vacuo.
M.p. 260 °C. Found: C, 45.64; H, 3.34; N, 20.01, S,
14.76. C16H14N6S2Zn requires C, 45.78, 3.60; N, 19.78;
S, 14.66%. m/z 419 (M++1, 10%).
Benzilbisthiosemicarbazone, LH6 was prepared fol-
lowing the procedure previously reported [36].
2.2.1. [CdLH6(NO3)2] (1)
A solution of cadmium nitrate hexahydrate (0.52 g,
1.70 mmol) in ethanol (25 cm3) was added to a suspen-
sion of LH6 (0.60 g, 1.70 mmol) in the same solvent (25
cm3). The mixture, which immediately changed to yel-
low, was: stirred at room temperature for 6 h or
refluxed with stirring for 6 h. In both procedures the
yellow solid formed was filtered off and then treated
with ethanol with stirring for 15 min. The residual solid
was separated by filtration. The filtrate was concen-
trated until a pale-yellow solid was obtained. Yield
62%, m.p. 220 °C. Anal. Found: C, 32.48; H, 2.76; N,
18.78; S, 10.75. C16H16N8S2CdO6 requires C, 32.41; H,
2.70; N, 18.91; S, 10.80%. m/z 569 (M−2NO3, 10%).
\M=62 cm2 V−1 mol−1 in DMF, \M=13 cm2
Vmol−1 in CH3CN.
2.3. Physical measurements
Microanalyses were carried out using a Perkin–
Elmer 2400 II CHNS/O elemental analyser. IR spectra
were recorded as KBr pellets on a Bomen-100 spec-
trophotometer in the range 4000–400 cm−1 and be-
tween 550 and 200 cm−1 on a Bruker IFS 66V.
UV–Vis spectra in solution were registered on a Ati-
Unicam UV2 spectrophotometer. Conductivity data
were measured using freshly prepared DMF and
CH3CN solutions (ca. 10−3 M) at 25 °C with a
Metrohm Herisau model E-518 instrument. 1H, 13C and
113Cd NMR spectra in solution were recorded on a
spectrophotometer Bruker AMX-300 using dimethyl-
sulfoxide-d6 or methanol-d4 as solvents and TMS or
0.1M aqueous Cd(ClO4)2 as references. 113Cd CP/MAS
NMR spectrum was obtained at 298 K in a Bruker
MSL-400 spectrometer, using a standard cross-polarisa-
2.2.2. [HgLH6(NO3)2] (2)
To a suspension of LH6 (0.40 g, 1.11 mmol) in
ethanol (30 cm3) was added a suspension of mercury

132
E. Lo´pez-Torres et al. / Inorganica Chimica Acta 323 (2001) 130–138
tion pulse sequence [37–39]. The external magnetic field
was 9.4 T, and the sample was spun at 4–4.5 kHz
around an axis inclined 54°44% with respect to this field.
Spectrometer frequencies were set to 88.76 MHz. For
recorded spectrum a contact time of 10 ms was used.
The number of scans ranged 4000. 13C CP/MAS NMR
spectra were obtained in the same spectrophotometer
with the frequency set to 100.63 MHz and a contact
time of 1ms. Chemical shifts values were referenced to
the same solution. The analysis of 113Cd CP/MAS
NMR spectrum was done with the WINFIT Program.
In this program the principal values of the chemical
shift anisotropy tensor were determined from the inten-
sity of sidebands with Herzfeld and Berger’s method
[40]. Fast atom bombardment mass spectra were
recorded on a VG Auto Spec instrument using Cs as
the fast atom and m-nitrobenzylalcohol (mNBA) as the
matrix.
cally while hydrogen atoms were included in calculated
positions and treated as riding atoms.
2.5. Electrochemical properties
Electrochemical measurements were performed with
a BAS CV 27 voltammograph and a BAS A-4 XY
register using a glassy carbon (f 5 mm) as working
electrode, a platinum wire as auxiliary, and a double
junction, with porous ceramic wick, Ag/AgCl as refer-
ence electrode, standardised by the redox couple fer-
ricinium/ferrocene (E1/2= +0.48 V, DEp=60 mV).
Cyclic voltammetry studies of the ligand and complexes
were carried out on 0.01 M solutions in dimethylfor-
mamide containing 0.1 M [NBu4][PF6] as supporting
electrolyte. The range of potential studied was between
+1 and −2.2 V. All solutions were purged with
nitrogen steam for 5 min before measurement and the
working electrode was polished before each experiment
with diamond paste. The procedure was performed at
room temperature and nitrogen atmosphere was main-
tained over the solution during the measurements.
2.4. Crystallography
Single crystals of CdLH6(NO3)2 (1) were grown by
vapour diffusion of Et2O into a 2:1 methanol: acetoni-
trile solution. Crystal data: CdC16H16N8O6S2, M=
3. Results and discussion
592.89, orthorhombic, a=14.522(3), b=11.161(2),
3
,
,
c=27.754(6) A, U=4498(2) A , T=293(2) K, space
group Pbca, Z=8, v(Mo Ka)=1.208 mm−1, 7518
reflections measured, 3902 unique (Rint=0.0316). The
final indices were R1(I\2|(I))=0.034, wR2=0.0728
and S=0.988 for 298 parameters.
Reactions of LH6 with cadmium nitrate only give
complex 1. From mercury nitrate two complexes 2 and
3 are isolated as well as from zinc nitrate, where
complexes 4 and 5 are isolated.
Analytical data for complexes indicate a variable
grade of deprotonation in the ligand depending on the
metal ion. For complex 1, data are in agreement with
the presence of two nitrate groups, with LH6 acting as
a neutral molecule. Complexes from zinc nitrate show
variable grade of deprotonation: complex 4 contains a
nitrate group and the ligand acts as a monoanion but in
complex 5 benzilbisthiosemicarbazone is a dianionic
ligand. Conductivity data in DMF of complexes indi-
cate that all of them are non-electrolyte compounds, so
the nitrates are bonded to the metal ion. Data for
complexes 1 and 2 are close to the limit values for 1:1
electrolytes, probably due to a partial substitution of
one of the nitrate groups by solvent molecules [42].
The unit cell parameters were determined and the
data collected on a Siemens STADI4 diffractometer.
The data were corrected for Lorentz-polarisation effects
and for absorption (empirical scan). The structure was
solved by direct methods and refined (full-matrix least-
squares on F2) by using the SHELXTL/PC v5.0 package
[41]. All non-hydrogen atoms were refined anisotropi-
3.1. X-ray structure of complex 1
The crystal structure of 1 consists of discrete
[CdLH6(NO3)2] molecules, held together in the crystal
packing by hydrogen bonds. A perspective view of the
complex with the atom labeling scheme is given in Fig.
1 whereas selected bond lengths and angles are in Table
1. Within the complex the cadmium(II) ion is hexa-co-
ordinated in a distorted octahedral arrangement toward
a symmetry pseudo-C26. In this octahedral description
four of the co-ordination positions belong to the unde-
Fig. 1. An ORTEP drawing of complex [CdLH6(NO3)2] (1).

E. Lo´pez-Torres et al. / Inorganica Chimica Acta 323 (2001) 130–138
133
Table 1
angles are similar to those found in similar Cd(II)
complexes and do not deserve further comments. The
presence of two distances for the Cd–O and Cd–S
bonds agrees with the distorted octahedral geometry.
The pseudo-macrocyclic co-ordination mode of the
ligand afford three five-membered chelate rings. The
bisthiosemicarbazone core can be considered planar
with a maximum deviation from the mean plane of
,
Bond lengths (A) and bond angles (°) for 1
Bond lengths
Cd(1)–N(4)
Cd(1)–N(7)
Cd(1)–S(2)
N(1)–C(2)
C(2)–N(3)
C(2)–S(2)
2.437(3)
2.438(3)
2.5348(13)
1.314(5)
1.365(5)
1.685(4)
1.353(4)
1.291(5)
Cd(1)–S(9)
Cd(1)–O(23)
Cd(1)–O(32)
C(9)–N(10)
N(8)–C(9)
C(9)–S(9)
2.5492(12)
2.436(3)
2.491(3)
1.320(5)
1.369(5)
1.685(4)
1.359(4)
1.289(5)
,
0.2059 A for S(9), while the phenyl rings form, with
N(3)–N(4)
N(4)–C(5)
N(7)–N(8)
C(6)–N(7)
respect to this plane, dihedral angles of 69.2 and 66.8°
for C(51)–C(56) and C(61)–C(66), respectively. Thus,
from the three limit conformations for the phenyl rings:
both rings co-planar with equatorial co-ordination
plane, one co-planar and the other perpendicular and
both rings perpendicular, the system adopts the latter
one which is the disposition less sterically hindered. On
the other hand, in bisthiosemicarbazone ligands con-
taining a single phenyl group [46] the ring, less sterically
constrained, adopts a closer co-planar arrangement
with a dihedral angle in the range of 20–30°, while in
pyridinethiosemicarbazone the aromatic ring is, of
course, co-planar with the basal co-ordination plane
[17]. Thus, the dihedral angle between the mean basal
co-ordination plane and the aromatic ring seems to be
a balance of two opposite factors: the steric crowding
and the electronic delocalization of the p-system of the
thiosemicarbazone branch and the aromatic ring.
Therefore, in bisthiosemicarbazone ligands containing a
single phenyl ring the electronic factor might be pre-
dominant leading to a nearly co-planar conformation
while in compounds having two neighboring phenyl
rings, these tend to minimize the steric hindrance
adopting a greater value of the dihedral angle, being, in
addition, both rings parallel to each other.
Bond angles
S(2)–Cd(1)–S(9)
S(2)–Cd(1)–N(4)
N(4)–Cd(1)–S(9)
146.50(4)
74.30(8)
139.18(8)
O(23)–Cd(1)–S(2)
O(23)–Cd(1)–N(4)
O(23)–Cd(1)–N(7)
O(23)–Cd(1)–S(9)
95.74(9)
87.62(11)
75.03(11)
88.57(9)
N(4)–Cd(1)–O(32) 84.27(11)
N(7)–Cd(1)–N(4)
N(7)–Cd(1)–S(9)
N(7)–Cd(1)–S(2)
65.27(11)
74.49(8)
138.68(8)
O(23)–Cd(1)–O(32) 159.44(10)
O(32)–Cd(1)–S(2)
O(32)–Cd(1)–S(9)
N(10)–C(9)–N(8)
N(10)–C(9)–S(9)
N(8)–C(9)–S(9)
N(7)–N(8)–C(6)
N(7)–N(8)–C(9)
N(7)–C(6)–C(5)
100.18(8)
85.32(8)
114.4(4)
120.7(3)
124.8(3)
120.2(3)
119.9(3)
115.7(3)
N(7)–Cd(1)–O(32) 84.42(10)
N(1)–C(2)–N(3)
N(1)–C(2)–S(2)
N(3)–C(2)–S(2)
N(4)–N(3)–C(2)
C(5)–N(4)–N(3)
N(4)–C(5)–C(6)
114.9(4)
120.9(3)
124.2(3)
120.2(3)
119.4(3)
115.8(3)
protonated bisthiosemicarbazone ligand which occupies
the equatorial positions through the imine nitrogen
atoms N(4) and N(7) and the thione groups S(2) and
S(9), as could be expected for this kind of ligands. The
two remaining positions in the octahedral geometry are
the axial ones which are occupied by two nitrate an-
ions. It should be noted that, to the best of our
knowledge, the hexa-co-ordination is not frequent for
Cd(II) complexes with thiosemicarbazone ligands [43].
Thus, major part of these complexes show a co-ordina-
tion number of five or seven [16,18,44–47], and only
three could be considered as hexa-co-ordinated
[45,48,49]. Nevertheless this complex represents the first
example of a hexa-co-ordinated Cd(II) complex with a
bisthiosemicarbazone ligand.
In accordance with the symmetry of the ligand bond
distances and angles in both thiosemicarbazone moi-
eties are very similar. Bond lengths agree well with an
imine-thione form of the ligand, as does the NMR
spectroscopy (see below), but with a considerable elec-
tronic delocalization through the thiosemicarbazone
backbone. Thus, the thione bond distances of ca. 1.69
Finally, the molecules are held together in the crystal
packing through an extended network of intermolecular
hydrogen bonds involving the amino groups, the proto-
nated N(3) and N(8) atoms and the nitrate anions (see
Table 2).
3.2. Spectroscopy
The NMR signal assignments for the free ligand were
1
previously reported [28]. H NMR spectra of mercury
complexes 2 and 3, show different number of acidic
hydrogen atoms. The spectrum of 2 indicates the pres-
ence of hydrogens from the primary and secondary
amine groups, while the spectrum of complex 3 does
not contain the signals corresponding to the hydrazinic
protons. In both spectra the thioamidic proton signals
are shifted to high field. These data indicate that LH6 is
in its neutral form in complex 2 but acts as a dianionic
ligand, owing to the deprotonation, in complex 3. From
the proton resonance data the formulae of mercury
complexes 2 and 3 must be equivalent of complex 1 and
complex 5, respectively.
,
A (Table 1) are intermediate between a theoretical C–S
,
single and double bond (1.82 and 1.56 A, respectively)
,
[50] as occur with the N–N bonds (ca. 1.35 vs. 1.44 A
calculated for a single N–N) and the three C–N bonds
of each thiosemicarbazone branch. Furthermore the
bond angles in the bisthiosemicarbazone ligand of ap-
proximately 12095° are also compatible with the elec-
tronic delocalization. Remaining bond distances and

134
E. Lo´pez-Torres et al. / Inorganica Chimica Acta 323 (2001) 130–138
Table 2
groups in complexes 1, 2 and 4 and peaks correspond-
Relevant hydrogen bonds for 1
ing to the molecular ion in complexes 3 and 5. In Fig.
2 the mass spectrum with the theoretical and the exper-
imental isotopic pattern of complex 4 is showed.
,
Hydrogen bond
N···O (A)
NHO (°)
13C NMR spectra of complexes in DMSO-d6 solution
(Table 3) reveal the thione form of the ligand and
co-ordination by the azomethine nitrogens in all com-
plexes [16]. The high field shift of the thione carbon
atom (Table 3) in complexes 2 and 3 indicates the
co-ordination of the sulfur atoms to the mercury
[14,18]. However, the spectrum of complex 1 in
methanol shows this signal slightly shifted to low field.
The 13C CP/MAS NMR spectrum of 1 exhibits aro-
matic carbon atoms, and the same number of imine and
thione carbon atoms (148.9 and 178.9 ppm) than in
solution. The thione carbon is slightly shifted to high
field indicating that LH6 acts as a N2S2 ligand, as the
X-ray structure has established. Because of the low
solubility of zinc complexes for NMR studies in solu-
tion, the 13C CP/MAS NMR of complex 4 was
recorded. The spectrum shows signals in the aromatic
carbon region and two corresponding to imine carbon
at 141.5 and 149.5 ppm and one signal attributable to
the thione carbon at 181.0 ppm. These data agree with
the asymmetric behaviour of the ligand in this complex,
showing the peak at higher field due to the reduction of
the double bond character after deprotonation of one
NH group. For comparison with the spectral data of
complex 1, the co-ordination sphere of the zinc atom
must be formed by two azomethine nitrogen, two sulfur
and one oxygen atoms, giving a pentaco-ordinated
environment for the metal ion, although the tetrahedral
geometry is the most common for this metal. This fact
can be explained by the planar disposition of the SNNS
ligand observed in the cadmium complex, being the
fifth position occupied by the nitrate group [15,22].
113Cd NMR spectrum of complex 1 in methanol
exhibits a peak at 177.8 ppm, which agrees with the
cadmium co-ordination sphere formed by nitrogen, sul-
fur and oxygen atoms [17,33]. The sideband pattern of
113Cd CP/MAS NMR spectrum of complex 1 traces out
the envelope of a shielding tensor with an orthorhom-
bic symmetry according to the value p=0.50. The
anisotropy value (−270.37 ppm) correlate well with a
distortion from regular polyhedron due to the presence
of three different donor atoms bonded to the cadmium
N(1)–H(1A)···O(31)i
N(1)–H(1B)···O(31)ii
N(3)–H(3A)···O(31)i
N(8)–H(8A)···O(21)iii
N(10)–H(10A)···O(23)iii
N(10)–H(10A)···O(21)iv
2.993
3.019
3.232
3.191
2.945
2.959
162.6
159.3
145.3
155.8
142.0
153.7
i=1−x, y−1/2, 1.5−z; ii=x−1/2, y, 1.5−z; iii=1−x, −y, 1−z;
iv=x+1/2, 1/2−y, 1−z.
Fig. 2. FAB mass spectrum of complex [ZnLH5(NO3)] (4).
Mass spectra of all complexes show peaks for the
cation [C16H15N6S2M]+ revealing the loss of nitrate
Table 3
13C NMR data in DMSO-d6 for LH6 and their complexes
CꢀS
CꢀN
C1
C2
C3
C4
LH6
179.1
181.4
173.7
170.8
140.5
149.7
147.7
148.0
133.1
131.9
132.4
135.4
130.1
130.8
129.5
129.7
128.9
130.7
129.0
127.8
126.8
129.4
128.0
127.6
a
1
LH6Cd(NO3)2
2 LH6Hg(NO3)2
3 LH4Hg
a Methanol-d4 as solvent.

E. Lo´pez-Torres et al. / Inorganica Chimica Acta 323 (2001) 130–138
135
Table 4
Relevant IR spectral data and assignments of LH6 complexes
w(NH)
w(NH2)
w(CꢀN)
l(NH2) Thioamide II Thioamide IV w(M–S) w(M–N)
w(M–O3N)
LH6
3415, 3342, 3384, 3328, 1610
1585
1465
848
3236
3145
1 H6Cd(NO3)2
2 LH6Hg(NO3)2 3460, 3250
3 LH4Hg
4 LH5ZnNO3
3415, 3248
3288, 3150
3320, 3141
3267
1628
1613
1575
1573
1433
1489
1441
1439
804
802
812
816
315
321
342
341
494
459
459
447
297
292
3433, 3127
3440, 3268, 3421, 3169
3111
1617, 1590 1607
1629, 1576 1615
341
5 LH4Zn
3495, 3266
3327, 3152
1595
1567
1496
816
357
438
ion (two N, two S and two O) as the crystal structure
has shown. The isotropic chemical shift of |iso=285
ppm is in the range expected for an octahedron. NMR
parameters and crystallographic data of solid cadmium
complex correlate well. The orthorhombic distortion
observed at the metal centre of geometrical constraint is
due to the crystal geometry. The different chemical shift
values observed from cadmium resonance in solution
and in solid state are not surprising, since the same fact
was described from the carbon resonance studies. In
solution, the cadmium co-ordination sphere change
with respect to the solid state due to the displacement
of the sulfur atoms by the methanol molecules.
The most significant IR bands of LH6 and their
complexes are listed in Table 4. The absence of any
bands in the 2600 cm−1 region suggest the absence of
any thiol tautomer in the solid state [51]. Spectra of
complexes 1 and 2 show several bands in the 3460–
3050 cm−1 region, corresponding to NH stretching
vibrations, thus indicates that the ligand is in its neutral
form. The nitrate ion co-ordinates to a metal as a
unidentate, symmetric and asymmetric chelating biden-
tate, and bridging ligand. It is rather difficult to differ-
entiate these structures by vibrational spectroscopy
since the symmetry of the nitrate ion differs very little
among them (C26 or Cs). In general the separation of
the two highest-frequency bands is larger for bidentate
than for unidentate co-ordination if the complexes are
similar. Both spectra show bands corresponding to the
nitrate ion co-ordinated to the metal ion, but due to the
presence of multiple bands in this region it is difficult to
differentiate the co-ordination mode [52,53]. The three
bands observed in the spectra of complexes 3 and 5
between 3500 and 3150 cm−1, together with the ab-
sence of bands from the nitrate group, indicate the total
deprotonation of the ligand. The spectrum of 4 shows
an intermediate situation, with the presence of a nitrate
group and a partial loss of the hydrazinic protons in the
ligand. The band at 1585 cm−1 assigned in the free
ligand to w(CꢀN) is shifted in all complexes to lower
wave number. The new bands observed between 500
and 200 cm−1 assigned to w(MN) confirm the co-ordi-
nation through the azomethine nitrogen in all com-
plexes and the oxygen atom from the nitrate in
complexes 1, 2 and 4. The co-ordination by the sulphur
atoms is observed by a decrease in the frequency of the
w(CS) in the spectra of all the complexes. These new
bonds are confirmed by the appearance of new bands
assignables to w(MS) [46].
The UV–Vis spectrum of LH6 has a band at 28.985
cm−1 (m=4.3×104 dm3 mol−1 cm−1) associated with
the insaturations present in the molecule. The electronic
spectra of complexes 1, 2 and 3 show a p“p* centred
band at 30.959 cm−1 (m=1.9×104 dm3 mol−1 cm−1),
32.051 cm−1 (m=1.3 104 dm3 mol−1 cm−1) and 31.446
cm−1 (m=1.3×104 dm3mol−1cm−1), respectively, and
an additional band in the visible region at 21.929 cm−1
(m=1.1×104 dm3 mol−1 cm−1), 24.570 cm−1(m=
1.5×103 dm3 mol−1 cm−1) and 22.522 cm−1 (m=
3.8×103 dm3 mol−1 cm−1), respectively, that are
assignable to charge transfer transitions.
3.3. Cyclic 6oltammetry
The cyclic voltammogram of the ligand in DMF only
shows two irreversible cathodic waves in the negative
margin (−1.440 and −1.746 V), which can be at-
tributed to the reduction of the imine and thioamide
groups present in the ligand [54].
The cyclic voltammograms of all complexes show
metal centred reduction processes and waves corre-
sponding to the ligand.
The voltammetric response of complex 1 (Fig. 3)
shows an irreversible wave at −1.125 V which can be
attributed to a cadmium reduction process, since
Cd(NO3)2·4H2O shows a peak at −0.667 V. Moreover
other two peaks corresponding to the ligand shifted to
more negative potential values can be observed. Be-
cause of the position of the first peak it can be deduced
the bigger difficulty to reduce the Cd(II) ion in the
complex.
The cyclic voltammogram of complex 2 (Fig. 4)
exhibits in the cathodic scan peaks at −0.460, −1.469
and −1.950 V. In the reverse scan, a peak at +0.320
V associated with the one at −0.460 V is observed.
The more negative waves correspond to ligand reduc-

136
E. Lo´pez-Torres et al. / Inorganica Chimica Acta 323 (2001) 130–138
tion processes. The redox couple is associated to the
process involving the mercury due to the fact that
Hg(NO3)2·H2O in DMF solution shows peaks at −
0.162 and +0.570 V.
Voltammetric response of complex 3 shows a similar
pattern to that of complex 2, but the reduction process
of the metal ion is shifted to positive values, +0.111 V.
Potential values for both complexes indicate that is
easier the reduction process of the mercury ion for this
complex than for complex 2.
The cyclic voltammogram pattern of complex 4 (Fig.
5) depends on the potential range studied. In the range
between +1 and −2.2 V two irreversible waves in the
negative margin at −1.444 and −1.950 V correspond-
Fig. 5. Cyclic voltammogram for complex [ZnLH5(NO3)] (4), 6=100
mV s−1
.
ing to ligand reductions are observed. However, in the
range from 0 to −2.2 V additional peaks are observed.
4. Conclusions
Although the reactions have been carried out follow-
ing the same procedure for all metal ions, the com-
plexes obtained present different stoichiometry and
geometries in accordance with the preferences of each
metal ion. The distorted octahedral geometry for the
cadmium complex, deduced from the IR spectrum and
resonance studies, shows a good correlation with that
from the crystallographic data. In the absence of crystal
structure data for mercury and zinc complexes, we have
proposed those that agree with the spectroscopical
data.
Fig. 3. Cyclic voltammogram for complex [CdLH6(NO3)2] (1), 6=
100 mV s−1
.
Structural characteristics and reactivity of a ligand
are valuable information for its potential use as base of
a sensor for metals. In this respect, the simplicity to get
complexes, their stability and structural characteristics
are the most important features for a co-ordination
compound to be applied as a sensor. For electrochemi-
cal sensors is necessary that the complexes show metal
centred redox activity. In previous work, copper com-
plexes of LH6 have been used as base for potentiomet-
ric sensors. From our results, cadmium and mercury
complexes have also the requirements to be used as a
base of potential modified electrodes for determination
and speciation of these metals. In fact the redox proper-
Fig. 4. Cyclic voltammogram for complex [HgLH6(NO3)2] (2), 6=
100 mV s−1
.

E. Lo´pez-Torres et al. / Inorganica Chimica Acta 323 (2001) 130–138
137
Liberta, C. Maichle-Mo¨ssmer, M.M. Salberg, D.X. West, Eur. J.
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ties of LH6 and of the complex 2 have allowed us to
incorporate this ligand to a carbon paste electrode as
an amperometric sensor for mercury in natural waters
[55]. On the other hand, the N2S2 co-ordination sphere
for the zinc atom could make it suitable as a metalloen-
zyme mimic complex.
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5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Center, CCDC No. 171085. Copies of this infor-
mation may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2
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