Electrochemical synthesis and characterization of zinc(II) and cadmium(II) complexes of dianionic tetradentate schiff base ligand

Article abstract of DOI:10.1002/zaac.200700209

The electrochemical oxidation of anodic metal (zinc or cadmium) in acetonitrile solution of the potentially chelating Schiff base N,N(dithiodiethylenebis(faminylydenemethylydene)bis( 1,2-phenylene)ditosylamide (H₂L) afforded stable complexes of empirical formula [ML]. The crystal and molecular structures of [ZnL] CH₃CN (1) and [CdL] (2) have been determined by X-ray diffraction. In both complexes the metal atom is in a distorted tetrahedral environment with the Schiff base acting as a tetradentate N₄ donor. Spectroscopic data for the complexes (IR, ES MS and ¹H NMR spectra) are discussed and related to the structural information.

Full text of DOI:10.1002/zaac.200700209

DOI: 10.1002/zaac.200700209
Electrochemical Synthesis and Characterization of Zinc(II) and Cadmium(II)
Complexes of Dianionic Tetradentate Schiff Base Ligand
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Laura Rodrıguez, Elena Labisbal, Antonio Sousa-Pedrares, Jaime Romero, Jose-Arturo Garcıa Vazquez*, and
Antonio Sousa
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Santiago de Compostela/Spain, Departamento de Quımica Inorganica, Universidad de Santiago de Compostela
Received April 23rd, 2007.
th
˜
Dedicated to Professor Alfonso Castineiras on the Occasion of his 65 Birthday
Abstract. The electrochemical oxidation of anodic metal (zinc or
cadmium) in acetonitrile solution of the potentially chelating
Schiff base N,N-(dithiodiethylenebis(aminylydenemethylydene)-
bis(1,2-phenylene)ditosylamide (H2L) afforded stable complexes of
empirical formula [ML]. The crystal and molecular structures of
[ZnL]·CH3CN (1) and [CdL] (2) have been determined by X-ray
diffraction. In both complexes the metal atom is in a distorted
tetrahedral environment with the Schiff base acting as a tetraden-
tate N4 donor. Spectroscopic data for the complexes (IR, ES MS
1
and H NMR spectra) are discussed and related to the structural
information.
Keywords: Electrochemical synthesis; Schiff base complexes;
Sulfonamide complexes; Zinc; Cadmium; Crystal structure
1 Introduction
Considerable attention has been paid to N-donor metal
complexes in view of their interesting chemical properties
and potentially useful biochemical applications [1Ϫ3]. Of
particular interest are complexes of Schiff bases having
NNS donor sequences, owing to the industrial, carcinos-
tatic, antitumour, antiviral and antimalarial activity of the
complexes derived from these ligands [4Ϫ7]. However, the
synthesis of free Schiff base proligands containing thiol
groups can be problematic due to the formation of thiaz-
olines [8]. In order to overcome these difficulties, an electro-
chemical reductive cleavage of a disulfide bond present in
the preformed Schiff base has been used and the electro-
chemical synthesis and structural characterization of zinc
and cadmium complexes of the dianionic terdentate 2-
mercaptophenyliminophenolato ligand have been reported
[9, 10]. However, in some cases the reductive cleavage of the
SϪS bond did not occur [11].
Scheme 1
Although the difficulty in replacing the amide hydrogen
atom by a metal atom is well known [12], the electron-with-
drawing effect of the sulfonyl group increases the acidity of
the NϪH group and, in the deprotonated form, these
anionic systems are effective sigma-donor ligands. The co-
ordination process is facilitated by the presence of the ad-
ditional imine nitrogen donor atom on the ligand, allowing
the formation of stable six-membered chelate rings with the
metal ion. In this way, metal complexes of sulfonamide
ligands incorporating additional donor atoms from im-
inomethyl and phenol groups [13], iminomethyl and thio-
phenol groups [14] or pyridine groups [15Ϫ18] have been
investigated.
As part of our continuing interest in the coordination
chemistry of Schiff base ligands, we describe here the syn-
thesis and characterization of zinc and cadmium complexes
with the Schiff base derivative of bis(2-aminoethyl)disulfide
with 2-tosylaminobenzaldehyde a ligand characterized by
the presence of imine and amide donor groups (Scheme 1).
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* Profesor Dr. Jose Arturo Garcıa Vazquez
2 Results and Discussion
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Departamento de Quımica Inorganica
Universidad de Santiago de Compostela
E-15782-Santiago de Compostela
E-mail: qijagv@usc.es
The condensation of bis(2-aminoethyl)disulfide with 2-tosyl-
aminobenzaldehyde in a 1:2 ratio yielded the Schiff base
H2L, in good yield (see Experimental part).
FAX: ϩ34-981547102
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Z. Anorg. Allg. Chem. 2007, 633, 1832Ϫ1836

Zinc(II) and Cadmium(II) Complexes of Dianionic Tetradentate Schiff Base Ligand
The anodic oxidation of zinc, or cadmium, in the pres-
ence of H2L is a direct and efficient route to obtain com-
plexes for which the elemental analyses indicate that the
ligand reacts with the metals to afford the neutral com-
plexes [ZnL] or [CdL], where L represents the bi-depro-
tonated form of the ligand, without reductive cleavage of
the SϪS bond. This is supported by molar conductivity
measurements of the complexes in 10Ϫ3 M DMF solutions
which are in the close to 10 ΩϪ1 cm2 molϪ1 [19].
The metal compounds are moderately soluble in the reac-
tion medium and they are obtained as crystalline air-stable
solids at the bottom of the cell. The complexes are insoluble
in non-polar solvents, moderately soluble in chloroform
and soluble in polar solvents such as dimethylsulfoxide or
N,N-dimethylformamide. Crystallization from the mother
liquor afforded crystals of [ZnL].CH3CN and [CdL] suit-
able for X-ray studies.
The electrochemical efficiency (see Experimental part),
Fig. 1 Molecular structure of [ZnL].2CH3CN
defined as the amount of metal dissolved per Faraday of
charge, were in all cases close to 0.5 mol.FϪ1. This, and the
formation of hydrogen at the cathode, is consistent with the
following reaction schemes:
Cathode:
Anode:
H2L ϩ 2eϪ
M
Ǟ L2Ϫ ϩ H2
Ǟ M2ϩ ϩ 2eϪ
Overall: H2L ϩ M
M ϭ Zn, Cd.
Ǟ [ML] ϩ H2
2.1 The crystal structures of [ZnL]·CH3CN and
[CdL]
Crystallographic data and experimental details, refinement
results and details for the structure determination are listed
in Table 1. The molecular structures of [ZnL].CH3CN and
[CdL] are shown in Figures 1 and 2. Selected bond dis-
tances and angles are given in Tables 2 and 3.
Table 1 Crystal data and details of refinement for the complexes
Fig. 2 Molecular structure of [CdL]
[ZnL].CH3CN
[CdL]
˚
2 Selected bond distances /A and angles /deg for
[ZnL].CH3CN
Empirical formula
Molecular mass
Temperature /K
C34H35N5O4S4Zn
771.28
293(2)
0.71073
monoclinic
P21/c
21.714(5)
8.4350(18)
21.070(5)
110.067
3624.9(14)
4
C32H32CdN4O4S4
777.26
293(2)
0.71073
monoclinic
C2/c
34.526(7)
10.936(2)
17.788(3)
101.879
6572(2)
8
Table
˚
Wavelength /A
Zn(1)-N(1)
Zn(1)-N(2)
Zn(1)-N(3)
1.980(4)
2.033(5)
1.981(5)
Zn(1)-N(4)
Zn(1)-O(3)
Zn(1)-O(1)
2.049(5)
2.642(4)
2.634(4)
Crystal system
Space group
˚
a /A
˚
b /A
˚
N(2)-Zn(1)-N(1)
N(4)-Zn(1)-N(3)
N(4)-Zn(1)-N(1)
93.4(2)
93.0(2)
106.4(2)
N(4)-Zn(1)-N(2)
N(2)-Zn(1)-N(3)
N(3)-Zn(1)-N(1)
109.3(2)
110.5(2)
142.3(2)
c /A
β /°
3
˚
V /A
Z
Absorption coefficient /mmϪ1 0.952
0.962
Crystal size /mm
Reflections collected
Independent reflections
Final R ind [I > 2σ(I)]
All data
0.39 ϫ 0.13 ϫ 0.05
7378
7378 [R(int) ϭ 0.0000] 8028 [R(int) ϭ 0.0464]
R1 ϭ 0.0579
wR2 ϭ 0.0975
0.34 ϫ 0.24 ϫ 0.12
41494
Both compounds consist of molecular units in which the
metal atom is tetracoordinated by two imine nitrogen and
two amidate nitrogen atoms of a dianionic tetradentate li-
gand.
R1 ϭ 0.0377
wR2 ϭ 0.0826
Z. Anorg. Allg. Chem. 2007, 1832Ϫ1836
 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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L. Rodrıguez, E. Labisbal, A. Sousa-Pedrares, J. Romero, J.-A. Garcıa Vazquez, A. Sousa
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tance, 2.035(3) and 2.038(2) A for the 1 and 2, respectively,
Table 3 Selected bond distances /A and angles /deg for [CdL]
˚
are close to those found in free disulfide ligands (2.037 A
in 2-{(N,N-dimethylamino)ethyl}disulfide [28] and 2.066 A
in 2,2Ј-diaminodiphenyldisulfide [29] and also similar to
those found in zinc [11], manganese [30], iron [31] and
nickel [32] complexes with Schiff bases derived from 2,2Ј-
diaminodiphenyldisulfide and salicylaldehyde, with SϪS
˚
Cd(1)-N(4)
Cd(1)-N(3)
Cd(1)-O(3)
2.227(3)
2.232(3)
2.595(3)
Cd(1)-N(2)
Cd(1)-N(1)
Cd(1)-O(1)
2.239(3)
2.274(3)
2.652(3)
N(2)-Cd(1)-N(1)
N(4)-Cd(1)-N(2)
N(4)-Cd(1)-N(1)
82.41(11)
140.77(11)
124.55(11)
N(4)-Cd(1)-N(3)
N(2)-Cd(1)-N(3)
N(3)-Cd(1)-N(1)
85.40(11)
108.11(11)
117.62(11)
˚
bond distances in the range 2.046Ϫ2.066 A. Other struc-
tural parameters in the ligand are as expected, with CϪN
˚
bond lengths of 1.282(7) and 1.274(5) A (average values) for
The coordination polyhedron around the metal atom is
distorted tetrahedral, mainly due to the rigidity of chelate
rings, with bond angles of 93.2(2)° and 83.90(11)° (average
values) in [ZnL] and [CdL], respectively. The MϪN bond
distances in both complexes are as expected and close those
to found in other complexes containing similar ligands. For
1 and 2, respectively, in good agreement with the value of
1.30 A proposed for a CϭN bond [33]. The zinc compound
crystallizes with an acetonitrile molecule that is located
within the network but does not interact significantly with
the complex.
˚
˚
example, the ZnϪN(imine) bond distance 2.041(5) A (aver-
age value) is similar to that found in bis{2-[pyrrol-2-yl)me-
thydeneamino]thiophenolato-S,N}zinc(II), 2.057(3) A (av-
2.2 Spectroscopic studies
˚
erage value) [20], or in bis[N-(4-methylphenyl)salicylaldimi-
nato]Zn(II), 2.007(3) A [21]. Both of these complexes have
a distorted tetrahedral coordination around the metal atom.
Major vibration bands of the free ligands and metal com-
plexes are given in the experimental part. The ν(NH) band
was not observed in the IR spectra of any metal complex,
indicating deprotonation of the ligand and coordination. In
addition, the strong band in the ligand at 1630 cmϪ1, as-
signed to ν(CϭN), shows red shifts in the complexes. Fur-
thermore, the ν(CϪN) stretching frequency is shifted to
lower wavenumbers in the complexes. This behaviour is
compatible with the participation of both imine and amide
nitrogen atoms in the coordination to metal atoms. Finally,
two bands in the ranges 1249Ϫ1258 cmϪ1 and
1155Ϫ1158 cmϪ1 are assigned to the asymmetric and sym-
metric vibration modes of the SO2 group and are close to
those found in the free ligands [34, 35]. The ES mass spectra
(Experimental part) show peaks assigned to [ML]ϩ frag-
ments for both complexes, indicating ligand coordination
to the metal atom. Peaks assigned to [M2L2]ϩ fragments
could not be detected for any of the complexes, suggesting a
mononuclear nature for all of these compounds in solution.
The 1H NMR spectra of the complexes at room tempera-
ture (Experimental part) show the disappearance of the NH
protons present in the free ligand, a situation consistent
with the bis-deprotonation of the Schiff base. The imine
hydrogen atom undergoes an upfield shift on complexation,
indicating a withdrawal of charge from the imine group
upon coordination. Therefore, these imine nitrogen atoms
should be coordinated to the metal ions. The aromatic pro-
tons are slightly shifted to lower field, showing a drain of
charge from the phenyl ring. The aromatic proton signals
of the tosyl groups also shift on complexation. This could
be attributed to some interaction between the tosyl groups
and the metal atoms, as the solid-state X-ray structures of
Zn and Cd complexes seem to confirm.
˚
The mean ZnϪN(amide) bond distance is shorter
˚
1.980(5) A, but is also close to those found in other com-
plexes containing amidate nitrogen ligands and a zinc atom
˚
in a tetrahedral environment; for instance 1.942(8) A in
{[N,NЈ-bis(butanesulfonamide)-1,2-diaminocyclohexane]-
˚
(2,2Ј-bipyridyl)}zinc(II) [22] and 1.962(3) A in bis{N-[(2-
pyrrolyl)methylene]-NЈ-tosylbenzene-1,2-diaminato}-
zinc(II) [23]. In addition, the CdϪN(amide) bond distance,
˚
2.229(3) A (average value), is similar to those described in
K[Cd(ClC6H4SO2NCONH-n-Pr)3], in which the mean
˚
CdϪN(amide) bond distance is 2.21(2) A [24] and in
˚
[Cd(bipy)2(bsglyNO)2] [25], mean bond distance 2.215 A.
Weak contact between the metal atoms and the O(1) and
O(3) atoms of the two sulfonyl groups are also observed in
both compounds, with the MϪO distances being 2.634(4)
˚
˚
and 2.642(4) A for [ZnL] and 2.595(3) and 2.652 A for
[CdL], in both cases shorter than the sum of the van der
˚
Waals radii (2.90 and 3.10 A for ZnϪO and CdϪO) [26].
These contacts make the N(4)ϪMϪN(2) bond angle
142.3(2)° and 140.77(11)°, respectively, for 1 and 2 Ϫ longer
than one would expect for a regular tetrahedron. If this
interaction is taken into account, the metal atoms would
be in an [MN4O2] six-coordinate environment [4ϩ2]. Weak
interactions are usually observed in zinc and cadmium com-
plexes containing ligands with sulfonyl groups. Thus, in the
zinc compound with [(4-methylphenyl)sulfonyl]-1H-amido-
2-phenyl-2-oxazolines [27], a secondary contact between the
zinc atom and an oxygen of the sulfonyl group is observed,
˚
with a ZnϪO bond distance of 2.6801(14) A Ϫ similar to that
found in the compound reported here. In the same way, in
bis{[(4-methylphenyl)sulfonyl]-2-pyridylamide}cadmiun(II),
˚
in addition to CdϪO(sulfonyl) bond distances of 2.335(5) A,
3 Experimental Section
˚
longer CdϪO distances of 2.738(6) A are also observed and
are indicative of a weak interaction [16].
The sulfur atoms of the disulfide fragment of the ligands
are not coordinated to the metal atom. The SϪS bond dis-
Acetonitrile, aldehyde, amine and all other reagents were used with-
out further purification. Zinc and cadmium (Ega Chemie) were
used as 2 x 2 cm plates.
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 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2007, 1832Ϫ1836

Zinc(II) and Cadmium(II) Complexes of Dianionic Tetradentate Schiff Base Ligand
Preparation of the ligand H2L.
Physical measurements
Microanalyses were carried on a Fisons Instruments EA 1108
CHNS-O instrument. Infrared spectra were recorded, as KBr pel-
lets on a Mattson Galaxy FT-i.r.2020 spectrophotometer in the
400Ϫ4000 cmϪ1 range, NMR spectra in CDCl3 on a Bruker 300
AC spectrometer and ES mass spectra on a LC/MSD HP1100 spec-
trometer using CH2Cl2 as solvent.
To a vigorously stirred solution of bis(2-aminoethyl)disulfide (1 g,
6.6 mmol) in MeOH (40 mL) was added 2-tosylamidebenzaldehyde
(3.6 g, 13.1 mmol). The reaction mixture was heated under reflux
with a DeanϪStark condenser for 3Ϫ4 h. The precipitated product
was collected by filtration, washed thoroughly with MeOH (10 mL)
and dried in vacuo. 3.6 g (82 %). Anal. Calc. for C32H34N4O4S4: C,
57.63; H, 5.14; N, 8.40; found C 57.30, H 4.99, N 8.66 %.
Mass spectrometry 667.1, (HLϩ, 100 %). 1H NMR (CDCl3, ppm): 12.9 (s,
2H, -NH), 8.4 (s, 2H, NϭCH), 7.8Ϫ6.9 (m, 16H. aromatics), 3.9 (t, 4H, N-
CH2), 3.2 (t, 4H, S-CH2), 1.6 (s, 6H, CH3). IR spectroscopy (KBr, cmϪ1): ν
(NH) 3284 (m), ν (CϭN) 1630 (s), ν (C-N) 1338 (s), νas(SO2) 1285 (s),
νs(SO2) 1153 (s).
Crystal structure determination
X-ray data were collected on a Bruker SMART 1000 diffractometer
with graphite-monochromated Mo-Kα radiation (λ ϭ 0.71073 A).
˚
The data were collected at 293 K for all structures. The ω scan
technique was employed to measure intensities for all crystals. De-
composition of the crystals did not occur during data collection.
Corrections were applied for Lorentz and polarization effects and
for absorption.
The structures were solved by direct methods, missing atoms were
located in the difference Fourier maps and included in subsequent
refinement cycles. The structures were refined by full-matrix least-
squares refinement on F2, using anisotropic displacement param-
eters for all non-hydrogen atoms. In all cases, hydrogen atoms were
Electrochemical synthesis of metal complexes.
The complexes were obtained using an electrochemical procedure
[36]. The cell was a 100 ml tall-form baker fitted with a rubber
bung through the electrochemical leads entered. An acetonitrile
solution of the ligand was electrolysed using a platinum wire as the
cathode and the metal plate as the anode. Tetramethylammonium
perchlorate (10 mg) was added as
a supporting electrolyte.
˚
included using a riding model with CϪH distances of 0.93Ϫ0.97 A
(Caution: Although no problem has been encountered in this work,
all perchlorate compounds are potentially explosive and should be
handled in small quantities and with great care!). Direct current was
obtained from a purpose-built d.c. power supply. Applied voltages
of 10Ϫ20 V allowed sufficient current flow for smooth dissolution
of the metal. The cell can be summarized as:
and fixed isotropic thermal parameters. A weighting scheme of the
form ω ϭ 1/σ2(F) was introduced and the refinement proceeded
smoothly to convergence.
Crystallographic programs used for the structure solutions and re-
finement were those included in SHELX97 [37]. Atomic scattering
factors and anomalous dispersion corrections for all atoms were
taken from International Tables for X-ray Crystallography. The
crystal data and summary of data collection and structure refine-
ment for these compounds are given in Table 1, and ORTEP 3
drawings [39], along with the numbering scheme used, are shown
in Figures 1 and 2.
Crystallographic data for the structures reported in this paper have
been deposited at the Cambridge Crystallographic Data Centre as
supplementary publication numbers 643881 and 643882. Copies of
the data can be obtained free of charge on application to CCDC,
12 Union Road, Cambridge CB21EZ, UK [Fax: (internat)
ϩ 44-1223-336-033; E-mail: deposit@ccdc.cam.ac.uk]
Pt(Ϫ)/H2L ϩ MeCN/M(ϩ) M ϭ Zn, Cd
[ZnL].CH3CN. Electrochemical oxidation of a zinc anode in a solu-
tion of H2L (60.0 mg, 0.09 mmol) in acetonitrile (70 mL) contain-
ing about 10 mg of tetramethylammonium perchlorate as support-
ing electrolyte for 1 h with a current of 5 mA caused 6.0 mg of zinc
to be dissolved. Ef ϭ 0.49 mol FϪ1. During the electrolysis process,
hydrogen was evolved at the cathode and at the end of the reaction
a small quantity of an unidentified insoluble product was filtered
off. The mother liquor was left to stand for one week. Slow evapor-
ation of the solution yielded small white crystals of [ZnL].CH3CN
that were suitable for X-ray diffraction. Yield 48 mg (70 %). Anal.
Calc. for C34H35N5ZnO4S4: C, 52.94; H, 4.57; N, 9.08; S, 16.63;
found C, 53.00; H, 4.77; N, 9.17; S, 16.88 %. IR spectroscopy (KBr,
cmϪ1): ν(CϭN) xxx(x), ν(CϪN) xxx(x), νas(SO2) 1258(s), νs(SO2)
1153(s).
´
Acknowledgements. We thank the Ministerio de Educacion y
Cultura de Espan˜a and the Xunta de Galicia for financial support.
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