Electrochemical synthesis and crystal structure of iron(II), cobalt(II), nickel(II) and copper(II) complexes of dianionic tetradentate N4 Schiff base ligand

Article abstract of DOI:10.1016/j.ica.2010.01.034

The electrochemical oxidation of anodic metal (iron, cobalt, nickel and copper) in an 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 compounds obtained have been characterized by microanalysis, IR spectroscopy and ES-MS mass spectrometry. The crystal and molecular structures of [FeL]·CH₃CN (1) [CoL]·CH₃CN (2), [NiL]·CH₃CN (3) and [CuL]·CH₃CN (4) have been determined by X-ray diffraction in all complexes, the metal atom is in a distorted tetrahedral environment with the Schiff base acting as a tetradentate N4 donor.

Full text of DOI:10.1016/j.ica.2010.01.034

Inorganica Chimica Acta 363 (2010) 1284–1288
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Electrochemical synthesis and crystal structure of iron(II), cobalt(II), nickel(II)
and copper(II) complexes of dianionic tetradentate N4 Schiff base ligand
*
Laura Rodríguez, Elena Labisbal, Antonio Sousa-Pedrares, José Arturo García-Vázquez ,
Jaime Romero, Antonio Sousa
Departamento de Química Inorgánica, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 May 2009
Received in revised form 19 January 2010
Accepted 23 January 2010
Available online 6 February 2010
The electrochemical oxidation of anodic metal (iron, cobalt, nickel and copper) in an acetonitrile solution
of the potentially chelating Schiff base N,N(dithiodiethylenebis-(aminylydenemethylydene)-bis(1,2-phe-
nylene)ditosylamide (H2L) afforded stable complexes of empirical formula [ML]. The compounds
obtained have been characterized by microanalysis, IR spectroscopy and ES-MS mass spectrometry.
The crystal and molecular structures of [FeL]ꢀCH3CN (1) [CoL]ꢀCH3CN (2), [NiL]ꢀCH3CN (3) and
[CuL]ꢀCH3CN (4) have been determined by X-ray diffraction in all complexes, the metal atom is in a dis-
torted tetrahedral environment with the Schiff base acting as a tetradentate N4 donor.
Ó 2010 Elsevier B.V. All rights reserved.
Dedicated to Professor Jonathan R. Dilworth.
Keywords:
Iron(II), cobalt(II), nickel(II) and copper(II)
complexes
Electrochemical synthesis
Schiff base ligand
Crystal structure
1. Introduction
effect of the sulfonyl group increases the acidity of the N–H
group and, in the deprotonated form, these anionic systems are
Metal complexes with Schiff base ligands have played an impor-
tant role in the development of coordination chemistry since the
early days due to their preparative accessibility and structural
variety [1–4]. A great deal of work has been done not only from
an inorganic and biological point of view [5–7], but also in catalysis
[8–10].
In particular, transition metal complexes with N4 Schiff bases
containing sulfonamide groups have been used as catalysts for dif-
ferent processes. Thus, Moberg described the preparation of tetra-
dentate N4 ligands from chiral aziridines bearing sulfonamido
functionalities and their titanium complexes catalyze the addition
of diethylzinc to benzaldehyde with high enantioselectivity (80%
ee) [11]. Noyori demonstrated that the use of chiral sulfonamide li-
gands prepared from chiral diamines gave active and enantioselec-
tive ruthenium catalysts for hydrogen-transfer reductions of
ketones [12]. In other studies, high ee values have been achieved
with bis(sulfonamide) complexes of zinc (cyclopropanation) [13]
or aluminium (Diels–Alder cycloadditions) [14].
effective sigma-donor ligands. The coordination process is facili-
tated by the presence of the additional imine nitrogen donor
atom on the ligand, allowing the formation of stable chelate
rings with the metal ion. Over the past 10 years, there have been
many reports on transition metal complexes with sulfonamide
Schiff base [16–18]. The research mainly focused on two types
of sulfonamide Schiff bases, one derived from the condensation
of 2-tosylaminobenzaldehyde and diamines [19,20], and the
other by the condensation of N-tosyl-diamine and aldehydes
[20–24].
Electrochemical procedures have been widely used for the
synthesis of metal complexes, especially in cases where the or-
ganic ligands containing weak acidity groups [25]. The condi-
tions employed in this experimental methodology have proved
suitable for deprotonating amide functions and thus it provides
an easy and inexpensive synthetic procedure whereby com-
plexes may be formed in the absence of external coordinating
species.
Although the difficulty in replacing the amide hydrogen atom
by a metal atom is well known [15], the electron-withdrawing
In a continuation of our earlier studies on the coordination
chemistry of Schiff base ligands, herein we report synthesis and
structural studies of cobalt, nickel, copper and iron with a poten-
tially tetradentate dianionic Schiff base, N,N(dithiodiethylenebis-
(aminylydenemethylydene)-bis(1,2-phenylene)ditosylamide (H2L)
(see Scheme 1).
* Corresponding author. Tel.: +34 (9) 81 563100x14950; fax: +34 (9) 1547102.
E-mail address: josearturo.garcia@usc.es (J.A. García-Vázquez).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.01.034

L. Rodríguez et al. / Inorganica Chimica Acta 363 (2010) 1284–1288
1285
2.4.1. Synthesis [FeL]ꢀCH3CN (1)
A solution of the ligand (0.062 g, 0.093 mmol) and tetramethyl-
ammonium perchlorate (ca. 10 mg) in acetonitrile (80 cm3) was
electrolyzed at 8 V and 5 mA during 1 h; 6.0 mg of iron metal
was dissolved from the anode, Ef = 0.48 mol Fꢂ1. After the electrol-
ysis the clear solution was filtered to remove any solid impurities
and red crystals of [FeL]ꢀCH3CN (1) suitable for X-ray studies were
obtained by air concentration. Elemental analysis: Calc. (%) for
C34H35FeN5O4S4: C, 53.61; H, 4.63; N, 9.19; S, 16.84. Found: C,
53.61; H, 4.63; N, 9.19; S, 16.84. IR (KBr, cmꢂ1
)
m
(C@N) 1607 (s),
(SO2)s 1158 (s). EI MS m/z:
ꢂ1 cm2 molꢂ1): 8.
(BM):
m
(C–N) 1280 (s), m(SO2)as 1249 (s), m
Scheme 1.
720 (FeL+, 100%). Molar conductance (
5.0.
X
l
Air concentration of the resulting solution yielded an orange
crystalline solid of 1 suitable for X-ray studies.
2. Experimental
2.1. General considerations
2.4.2. Synthesis of [CoL]ꢀCH3CN (2)
Electrochemical oxidation of a cobalt anode in a solution of H2L
(0.062 g, 0.093 mmol) in acetonitrile (80 cm3) at 10 V and 5 mA for
1 h caused 6.0 mg of cobalt to be dissolved, Ef = 0.50 mol Fꢂ1. Dur-
ing the electrolysis process, hydrogen was evolved at the cathode.
The resulting solid was filtered off, washed with acetonitrile and
ether and characterized as [CoL]ꢀCH3CN. Elemental analysis: Calc.
(%) for C34H35CoN5O4S4: C, 53.39; H, 4.61; N, 9.16; S, 16.77. Found:
Acetonitrile, aldehyde, amine and all other reagents were used
without further purification. Iron, cobalt, nickel and copper (Ega
Chemie) were used as plates (ca. 2 ꢁ 2 cm).
2.2. Physical measurements
Elemental analyses were performed using Carlo-Erba EA 1112
microanalyser. IR spectra were recorded on KBr discs using a Bru-
ker IFS 66 V spectrophotometer. 1H and 13C NMR spectra were re-
corded on Bruker AMX350 MHz instrument using CDCl3 as solvent
and chemical shifts were determined against TMS. Electrospray
mass spectral data were obtained with 5 ꢁ 10ꢂ4 M solutions
(MeOH or CH2Cl2) by flow injection into a Hewlett–Packard 1100
series MSD. The carrier solvent was 49% MeOH/49% H2O/2% formic
acid or 98% CH2Cl2/2% formic acid. Electrospray ionization condi-
tions were as follows: nitrogen drying gas flow 10.0 l minꢂ1; neb-
ulizer pressure, 40 psig; drying gas temperature 350 °C; capillary
voltage, 4 kV. The capillary exit voltage was varied from 0 to
150 V. Magnetic measurements at room temperature were
performed using a Sherwood Scientific Magnetic Susceptibility
Balance, calibrated with mercury tetrakis(isothiocyanato)
cobaltate(II).
C, 53.40; H, 4.65; N, 9.10; S, 16.07. IR (KBr cmꢂ1
(C–N) 1286 (s), (SO2)as 1258 (s), (SO2)s 1155 (s); EI MS m/z: 723
(CoL+, 100%). Molar conductance ( ꢂ1 cm2 molꢂ1): 8.
(BM): 4.3.
) m(C@N) 1625 (s),
m
m
m
X
l
Air concentration of the resulting solution yielded an orange
crystalline solid of 2 suitable for X-ray studies.
2.4.3. Synthesis of [NiL]ꢀCH3CN (3)
An experiment similar to that described above, with nickel as
the anode and a solution of H2L (0.062 g, 0.093 mmol) and tetra-
methylammonium perchlorate (ca. 10 mg) in acetonitrile
(80 cm3), at 5 mA and 6 V for 1 h, dissolved 6.0 mg of nickel
(Ef = 0.49). A green solid characterized as [NiL]ꢀCH3CN (3) was ob-
tained. Elemental analysis: Calc. (%) for C34H35NiN5O4S4: C, 53.41;
H, 4.61; N, 9.16; S, 16.77. Found C, 53.45; H, 4.66; N, 9.16; S,
16.80. IR (KBr, cmꢂ1):
1249 (s),
(SO2)s 1158 (s); EI MS m/z: 722 (NiL+, 100%). Molar con-
ductance ( (BM): 3.6.
ꢂ1 cm2 molꢂ1): 10.
m(C@N) 1619 (s), m(C–N) 1281 (s), m(SO2)as
m
X
l
2.3. Preparation of ligand
Crystals of 3 suitable for X-ray studies were obtained by crystal-
lization from acetonitrile/dichloromethane.
The ligand H2L was obtained by a classical Schiff condensation
between 2-(tosylamino)benzaldehyde and bis(2-aminoethyl)disul-
fide as previously reported by our group [19]. The compound was
characterized by elemental analyses, melting point, IR, 1H NMR
spectroscopy and EI mass spectroscopy.
2.4.4. Synthesis of [CuL]ꢀCH3CN (4)
Electrolysis of a solution of the ligand (0.062 g, 0.093 mmol) and
tetramethylammonium perchlorate (ca. 10 mg) in acetonitrile
(80 cm3) at 8 V and 5 mA for 1 h dissolved 11.4 mg of copper from
the anode, Ef = 0.96 mol Fꢂ1. The solid obtained in the cell was col-
lected by filtration washed with acetonitrile and ether and charac-
terized as [CuL]ꢀCH3CN (4). Elemental analysis: Calc. (%) for
C34H35CuN5O4S4: C, 53.07; H, 4.58; N, 9.10; S, 16.67. Found: C,
2.4. Electrochemical synthesis of the metal complexes
The complexes were obtained using an electrochemical proce-
dure [25]. The cell consisted of a tall-form beaker (100 mL) fitted
with a rubber bung through which the electrochemical leads en-
tered. An acetonitrile solution of the ligand, containing a few mg
of tetramethylammonium perchlorate as a current carrier, was
electrolyzed using a platinum wire as the cathode and a metal
plate as the sacrificial anode (Caution: Although problems were
not encountered in this work, all perchlorate compounds are
potentially explosive, and should be handled in small quantities
and with great care!). The applied voltages (10–20 V) allowed suf-
ficient current flow for smooth dissolution of the metal. The cur-
rent was maintained at 5 mA for 1 h. In all cases, during the
electrolysis hydrogen was evolved at the cathode. Under these con-
ditions the cell can be summarized as:
53.07; H, 4.58; N, 9.10; S, 16.67. IR (KBr, cmꢂ1
)
m
(C@N) 1618 (s),
(SO2)s 1158 (s). EI MS m/z:
ꢂ1 cm2 molꢂ1): 9.
(BM):
m
(C–N) 1285 (s), m(SO2)as 1249 (s), m
769 (CuL+, 100%). Molar conductance (
2.1.
X
l
Slow air evaporation of the solvent from mother liquor provided
red brown crystals of 4 suitable for X-ray studies.
2.5. X-ray crystallographic studies
Intensity data for compounds 1, 2 and 4 were collected with the
use of a Smart-CCD-1000 Bruker diffractometer (Mo Ka radiation,
k = 0.71073 Å) equipped with a graphite monochromator. Intensity
data for compound 3 were collected using a CAD4 Enraf Nonius dif-
MðþÞ=H2L þ CH3CN=PtðꢂÞ
:
M ¼ Co; Ni; Cu; Fe
ð1Þ
fractometer (Cu Ka radiation, k = 1.54180 Å) equipped with a

1286
L. Rodríguez et al. / Inorganica Chimica Acta 363 (2010) 1284–1288
Table 1
graphite monochromator. Compounds 1 and 4 were measured at
120 K and compounds 2 and 3 were collected at 293 K. The scan
Summary of crystallographic data and structure refinement.
x
technique was employed to measure intensities in all crystals. No
decomposition of the crystals occurred during data collection.
The intensities of all data sets were corrected for Lorentz and
polarization effects. Absorption effects in all compounds except
in 3 were corrected using the program SADABS [26]; absorption in
Compound
2
4
Empirical formula
Formula weight
Crystal size (mm)
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C34H35CoN5O4S4
764.84
C34H35CuN5O4S4
769.45
0.37 ꢁ 0.33 ꢁ 0.03
0.32 ꢁ 0.18 ꢁ 0.05
293(2)
120(2)
0.71073
Monoclinic
P21/c
0.71073
Monoclinic
P21/c
3 was corrected using semiempirical
w scans. The crystal struc-
tures of all compounds were solved by direct methods. Crystallo-
graphic programs used for structure solution and refinement
were those of SHELX97 [27] installed on a PC. Scattering factors were
those provided with the SHELX program system. Missing atoms were
located in the difference Fourier map and included in subsequent
refinement cycles. The structures were refined by full-matrix
least-squares refinement on F2, using anisotropic displacement
parameters for all non-hydrogen atoms. Hydrogen atoms were
placed geometrically and refined using a riding model, including
free rotation about C–C bonds for methyl groups, with C–H dis-
tances of 0.93–0.97 Å. For all compounds, hydrogen atoms were re-
fined with Uiso constrained at 1.2 (for non-methyl groups) and 1.5
(for methyl groups) times Ueq of the carrier C atom. In the last cy-
cles of refinement of all structures a weighting scheme was used,
with weights calculated using the following formula w = 1/
21.652(2)
8.4654(9)
21.021(2)
90
109.966(2)
90
3621.4(6)
4
0.749
21.403(3)
8.3001(13)
20.833(3)
90
110.357(3)
90
3469.8(9)
4
0.916
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Volume (Å3)
Z
l
(mmꢂ1
)
Number of reflections collected
Number of independent
reflections
Data/restraints/parameters
Goodness-of-fit
31256
7408
33019
8369
[Rint = 0.0743]
7408/0/434
1.020
[Rint = 0.0736]
8369/0/436
1.076
R1 = 0.0429
wR2 = 0.0778
Final R indices [I > 2
r
(I)]
R1a = 0.0486
wR2b = 0.0905
[r
2(Fo2) + (aP)2 + bP], where P = (Fo2 + 2Fc2)/3.
a
R1
wR2 = [|
=
R
[|Fo| ꢂ |Fc|/
R|Fo].
Crystals of compounds 1 and 3 were very weak diffractors and
b
2
R
(Fo ꢂ Fc2)/
R .
(Fo2)]1/2
their refinements show several important problems. In compound
1 the maximum and minimum residual electron density peaks in
the final difference map were +4.75 and ꢂ1.32 e Åꢂ3, respectively.
The high maximum density peak is located between the two para-
toluensulfonic groups, too close to both the iron atom (2.31 Å) and
the oxygen atom O1 (1.52 Å) to be important from a chemical point
of view. Crystals of compound 3 were very weak diffractors and we
were only able to measure 38.3% of the total reflections (complete-
ness to h = 74.33°). Due to this small number of reflections, only the
nickel and sulfur atoms were refined anisotropically. In conclusion,
in the case of compounds 1 and 3 the X-ray studies have only pro-
vided the unit cell dimensions and the connectivity around the me-
tal centres. [Compound 1: monoclinic, space group P21/c, unit cell
dimensions: a = 21.581(5) Å, b = 8.360(2) Å, c = 20.989(5) Å,
b = 110.590(4)°; compound 3: monoclinic, space group P21/c, unit
cell dimensions: a = 21.614(2) Å, b = 8.4002(5) Å, c = 20.997(4) Å,
b = 110.028(12)°].
Table 2
Selected bond lengths (Å) and angles (°) for the complexes.
2
4
M(1)–N(2)
M(1)–N(4)
M(1)–N(3)
M(1)–N(1)
M(1)–O(1)
M(1)–O(3)
S(1)–S(2)
1.985(3)
1.990(3)
2.018(4)
2.022(4)
2.589(3)
2.579(3)
2.031(2)
1.281(5)
1.281(5)
1.974(3)
1.977(2)
2.025(3)
2.035(3)
2.570(2)
2.569(2)
2.0355(13)
1.281(4)
1.279(4)
N(1)–C(3)
N(3)–C(19)
N(2)–M(1)–N(4)
N(2)–M(1)–N(3)
N(4)–M(1)–N(3)
N(2)–M(1)–N(1)
N(4)–M(1)–N(1)
N(3)–M(1)–N(1)
144.09(15)
109.44(15)
92.18(14)
92.25(15)
107.15(14)
110.17(15)
145.19(11)
109.31(11)
92.24(10)
92.50(11)
106.92(10)
107.81(10)
Table 1 summarizes pertinent details of the data collections and
structure refinements, while Table 2 lists important geometrical
data for all compounds. Further details regarding the data collec-
tions, structure solutions and refinements are included in the Sup-
porting Information. ORTEP3 [28] drawings with the numbering
scheme used is shown in Fig. 1.
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. Crystalliza-
tion from the mother liquor afforded crystals of [FeL]ꢀCH3CN,
[CoL]ꢀCH3CN and [CuL]ꢀCH3CN suitable for X-ray studies. Crystalli-
zation from acetonitrile/dichloromethane afforded also crystal of
[NiL]ꢀCH3CN suitable for these studies.
3. Results and discussion
3.1. Synthesis and characterization of the metal complexes
In the synthesis of iron, cobalt and nickel complexes, the elec-
trochemical efficiency values, Ef, were close 0.5 mol Fꢂ1 (see Sec-
tion 2) and these are consistent with the following reaction
scheme:
The new metal complexes were obtained by electrochemical
oxidation of the appropriate iron, cobalt, nickel and copper metal
in a cell containing an acetonitrile solution of the ligand. Elemental
analysis shows that metal ions react with the ligands in a 1:1 M ra-
tio to afford the neutral complexes [ML] where L represents the bi-
deprotonated form of the ligand, without reductive cleavage of the
S–S bond. This is supported by molar conductivity measurements
of the complexes in 10ꢂ3 M DMF solutions which are close to
10 Oꢂ1 cm2 molꢂ1 [29].
Cathode : ½H2Lꢃ þ 2eꢂ ! H2 þ ½L2ꢂꢃ
ð2Þ
Anode : M ! M2þ þ 2eꢂ
Overall : ½H2Lꢃ þ M ! ½MLꢃ þ H2
M ¼ Fe; Co and Ni:
ð3Þ
ð4Þ
ð5Þ
The compounds appear to be stable in the solid state and in
solution. They are moderately soluble in the reaction medium
and they are obtained as crystalline air-stable solids at the bottom
In the synthesis of the copper complex, an Ef value close to
1.0 mol Fꢂ1 is indicative that the anionic oxidation leads initially
to the Cu(I). However, the analytical data show that the final

L. Rodríguez et al. / Inorganica Chimica Acta 363 (2010) 1284–1288
1287
metal centers [from 92.18(14)° to 144.09(15)° for 2 and from
92.24(10)° to 145.19(11)° for 4] also shows the deviation from
the ideal tetrahedral geometry.
The M–N distances in all complexes are as expected and close to
those found in other complexes containing similar ligands. For
example, the Co–N(imine) bond distance 2.020(4) Å (average va-
lue) is similar to that found in N,N0-bis(2-tosylaminobenzylid-
ene)-1,3-diaminopropane]cobalt(II), 2.020(11) Å (average value),
or N,N0-bis(2-tosylaminobenzylidene)-1,4-diaminobutane Co(II),
2.007(3) Å [31]. Both of these complexes have a distorted tetrahe-
dral coordination around the metal atom. The average Co–
N(amide) bond distance is shorter 1.988(3) Å, but is also close to
those found in other complexes containing amidate nitrogen li-
gands and a cobalt atom in a tetrahedral environment; for instance
1.962(6) Å and 1.962(3) Å in the compounds before mentioned,
respectively [31]. In addition, the Cu–N(amide) bond distance,
1.976(2) Å (average value), is similar to those described N,N’-
bis(2-tosylaminobenzylidene)1,3-diaminopropane]cooper(II)
in
which the mean Cu–N (amide) bond distance is 1.975(5) Å [31].
Weak contact between the metal atoms and the O(1) and O(3)
atoms of the two sulfonyl groups are also observed in all com-
pounds, with the Mꢀ ꢀ ꢀO distances being 2.589(3) and 2.579(3) Å
for [CoL] and 2.570(2) and 2.569(2) Å for [CuL]. In the complexes,
those distances are shorter than the sum of the van der Waals radii
[32] and therefore, we considered that these Mꢀ ꢀ ꢀO (tosyl) dis-
tances are too long to be considered as true co-ordinated bonds
and should be best described as secondary intramolecular interac-
tions. These contacts make the N(4)–M–N(2) bond angle
149.09(15)° and 145.19(11)°, respectively, for 2 and 4 longer than
one would expect for a regular tetrahedron. If this interaction is ta-
ken into account, the metal atoms would be in an [MN4O2] six-
coordinate environment [4+2]. Weak interactions are usually ob-
served in cobalt and copper complexes containing ligands with sul-
fonyl groups [31b].
The sulfur atoms of the disulfide fragment of the ligands are not
coordinated to the metal atom. The S–S bond distance, 2.043(3)
and 2.031(2) Å for the 2 and 4, respectively, are close to those
found in free disulfide ligands (2.037 Å in 2-{(N,N-dimethyl-
amino)ethyl}disulfide [33] and 2.066 Å in 2,20-diaminodiphenyldi-
sulfide [34] and also similar to those found in manganese [35], iron
[36] and nickel [37] complexes with Schiff bases derived from 2,20-
diaminodiphenyldisulfide and salicylaldehyde, with S–S bond dis-
tances in the range 2.046–2.066 Å. Other structural parameters in
the ligand are as expected, with C–N bond lengths of 1.281(5)
and 1.281(4) Å (average values) for 2 and 4, respectively, in good
agreement with the value of 1.30 Å proposed for a C@N bond
[38]. All complexes crystallize with an acetonitrile molecule that
is located within the network but does not interact significantly
with the complexes.
Fig. 1. ORTEP diagram of the molecular structure of 2 with 50% thermal ellipsoid
probability.
compound is [CuL]. This suggests a subsequent oxidation in solu-
tion from Cu(I) to Cu(II) as soon as it is formed, according to the fol-
lowing reaction scheme:
Cathode : ½H2Lꢃ þ eꢂ ! 1=2H2ðgÞ þ ½HLꢂꢃ
Anode : Cu ! Cuþ þ eꢂ
ð6Þ
ð7Þ
ð8Þ
Solution : ½HLꢂꢃ þ Cuþ ! ½CuLꢃ þ 1=2H2ðgÞ
This behavior has been observed previously in the synthesis of
other Cu(II) complexes with related ligands using an electrochem-
ical procedure [30].
3.2. X-ray diffraction studies
All metal compounds were studied by X-ray diffraction. The
crystal parameters and experimental details for data collection
are given in Table 1. A selection of bond distances and angles for
the compounds are given in Table 2. The structure determination
of 1 and 3 was undertaken in order to establish the chemical con-
nectivity, although it was clear from the outset that the data would
not be of high quality and any numbers (bond length or angles)
could be extract from them.
3.3. Spectroscopic studies
Given the great similarities between the complexes, Fig. 1
shows the molecular structure of 2 and all of them and they will
be discussed together in a comparative manner.
The m(NH) band was not observed in the IR spectra of any metal
complex, indicating deprotonation of the ligand and coordination.
The X-ray structure shows that all complexes (1–4) consist of
mononuclear neutral ML molecules with acetonitrile as solvate.
The ligand uses its four nitrogen atoms (two imine and two amide)
to link the metal atom. Therefore, the metal centre is in a tetraco-
ordinated environment and the geometry about the metal ions
could be described as pseudo-tetrahedral, although the two termi-
nal bidentate fragments are not mutually perpendicular.
The dihedral angle h between the two N1–M–N3 and N2–M–N4
terminal planes is 79.89(14)° and 79.55(14)° for 2 and 79.99(10)°
and 81.08(10)° for 4. This shows deviations from tetrahedral to
square-planar (h = 0 for square-planar geometry and h = 90 for tet-
rahedral geometry). The wide range of angles observed around the
In addition, the IR spectra of all complexes show a strong band at
ca. 1620 cmꢂ1, assigned to
m
(C@N). This supposes a negative shift
of 5–23 cmꢂ1 respect to the free ligand. Furthermore, the
m
(C–N)
stretching frequency is also shifted to lower wavenumbers in the
complexes. This behavior is compatible with the participation of
both imine and amide nitrogen atoms in the coordination to metal
atoms. Finally, two bands in the ranges 1249–1258 cmꢂ1 and
1155–1158 cmꢂ1 are assigned to the asymmetric and symmetric
vibration modes of the SO2 group and are close to those found in
the free ligands [39,40]. The ES mass spectra show peaks assigned
to [ML]+ fragments for the complexes, indicating ligand coordina-
tion to the metal atom. Peaks assigned to [M2L2]+ fragments could

1288
L. Rodríguez et al. / Inorganica Chimica Acta 363 (2010) 1284–1288
[5] M. Lanznaster, A. Neves, A.J. Bortoluzzi, A.M.C. Assumpcao, I. Vencato, S.P.
Machado, S.M. Drechsel, Inorg. Chem. 45 (2006) 1005.
[6] T. Ueno, T. Koshiyama, M. Ohashi, K. Kondo, M. Kono, A. Suzuki, T. Yamane, Y.
Watanabe, J. Am. Chem. Soc. 127 (2005) 6556.
not be detected for any of the complexes, suggesting a mononu-
clear nature for all of these compounds.
[7] T. Ueno, T. Koshiyama, S. Abe, N. Yokoi, M. Ohashi, H. Nakajima, Y. Watanabe, J.
Organomet. Chem. 692 (2007) 142.
3.4. Magnetic properties
[8] K.C. Gupta, A.K. Sutar, Coord. Chem. Rev. 252 (2008) 1420.
[9] N.S. Venkataramanan, G. Kuppuraj, S. Rajagopal, Coord. Chem. Rev. 249 (2005)
1249.
[10] T. Katsuki, Coord. Chem. Rev. 140 (1995) 189.
[11] F. Lake, C. Moberg, Eur. J. Org. Chem. 3179_3188 (2002), and references
therein.
Magnetic measurements at room temperature of the iron,
cobalt, nickel and copper complexes were performed. The value
for the copper complex does not allow drawing any other conclu-
sion. However, the magnetic moment of the Ni complex is 3.6 BM,
in agreement with a high spin d8 system in a tetrahedral environ-
ment. The magnetic moments of [FeL] and [CoL] are in agreement
with high-spin M(II) tetrahedral complexes, in the range of 5.0–
5.4 BM for Fe(II) and 4.3–4.8 BM for Co(II). Thus, in view of the for-
mulation of the complexes and their magnetic moments, it seems
quite reasonable to propose a tetrahedral environment around the
metal atom for all complexes, as confirmed by the crystal struc-
tures of 1–4.
[12] S. Hashiguchi, A. Fujii, J. Takehara, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 117
(1995) 7562.
[13] Review of cyclopropanation: W.A. Donaldson, Tetrahedron 57 (2001) 8589.
[14] E.J. Corey, S. Sarshar, D.-H. Lee, J. Am. Chem. Soc. 116 (1994) 12089.
[15] (a) M. Akbar Ali, C.M. Haroon, M. Nazimuddin, S.M.M.H. Ma-jumder, M.T.H.
Tarafder, M.A. Khair, Transition Met. Chem. 17 (1992) 133;
(b) S.M.M.H. Majumder, M. Akbar Ali, F.E. Smith, M.A.U. Mridha, Polyhedron 7
(1988) 2183.
[16] M. Bernal, J.A. García-Vázquez, J. Romero, C. Gómez, M.L. Durán, A. Sousa, A.
Sousa-Pedrares, D.J. Rose, K.P. Maresca, J. Zubieta, Inorg. Chim. Acta 295 (1999)
39.
[17] A.M. García-Deibe, J.S. Matalobos, M. Fondo, M. Vázquez, M.R. Bermejo, Inorg.
Chim. Acta 357 (2004) 2561.
[18] J. Sanmartín, A.M. García-Deibe, M. Fondo, F. Novio, N. Ocampo, M.R. Bermejo,
Inorg. Chim. Acta 359 (2006) 3156.
4. Conclusion
The designed N-donor Schiff base ligand seems suitable for
yielding stable complexes of empirical formula [ML]. The electro-
chemical method employed in the synthesis of the complexes is
a good alternative to a classical chemical approach in this case.
The crystal and molecular structures of [FeL]ꢀCH3CN (1),
[CoL]ꢀCH3CN (2), [NiL]ꢀCH3CN (3) and [CuL]ꢀCH3CN (4) have been
determined by X-ray diffraction. In all complexes the metal atom
is in a distorted tetrahedral environment with the Schiff base act-
ing as a tetradentate N4 donor.
[19] L. Rodríguez, E. Labisbal, A. Sousa-Pedrares, J. Romero, J.A. García-Vázquez, A.
Sousa, Z. Anorg. Allg. Chem. 633 (2007) 1832.
[20] E. Labisbal, L. Rodriguez, A. Vizoso, M. Alonso, J. Romero, J.A. García-Vázquez,
A. Sousa-Pedrares, A. Sousa, Z. Anorg. Allg. Chem. 631 (2005) 2107.
[21] H.-W. Yang, Y.-Z. Li, X. Pan, J.-T. Sun, C.-J. Zhu, Acta Crystallogr., Sect. E 62
(2006) o1982.
[22] E. Labisbal, L. Rodríguez, A. Sousa-Pedrares, M. Alonso, A. Vizoso, J. Romero, J.A.
García-Vázquez, A. Sousa, J. Organomet. Chem. 691 (2006) 1321.
[23] S.-N. Li, Q.-G. Zhai, M.-C. Hu, Y.-C. Jiang, Inorg. Chim. Acta 362 (2009) 2217.
[24] S.-N. Li, Q.-G. Zhai, M.-C. Hu, Y.-C. Jiang, J. Coord. Chem. 62 (16) (2009) 2709.
[25] C. Oldham, D.G. Tuck, J. Chem. Educ. 59 (1982) 420.
[26] G.M. Sheldrick, SADABS: Program for Absorption Correction using Area Detector
Data, University of Göttingen, Göttingen, Germany, 1996.
[27] G.M. Sheldrick, SHELX97 [Includes SHELXS97, SHELXL97, CIFTAB]. Programs for Crystal
Structure Analysis (Release 97-2), Institüt für Anorganische Chemie der
Universität, Tammanstrasse 4, D-3400 Göttingen, Germany, 1998.
[28] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
Acknowledgements
This work was supported by Xunta de Galicia (Spain) (grant
PGIDIT07PXIB203038PR) and by the Ministerio de Ciencia and
Tecnología (Spain) (grant CTQ2006-05298BQU).
[29] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81.
[30] (a) L. Rodríguez, E. Labisbal, A. Sousa-Pedrares, J.A. García-Vázquez, J. Romero,
M.L. Durán, J.A. Real, A. Sousa, Inorg. Chem. 45 (2006) 7903;
(b) I. Beloso, J. Borrás, J. Castro, J.A. García-Vázquez, P. Pérez-Lourido, J.
Romero, A. Sousa, Eur. J. Inorg. Chem. (2004) 635;
(c) J. Sanchez-Piso, J.A. García-Vázquez, J. Romero, M.L. Durán, A. Sousa-
Pedrares, E. Labisbal, O.R. Nascimento, Inorg. Chim. Acta 328 (2002) 111.
[31] (a) M. Vázquez, M.R. Bermejo, M. Fondo, A. García-Deibe, A.M. González, R.
Pedrido, Eur. J. Inorg. Chem. (2002) 465;
(b) M.R. Bermejo, J. Sanmartıín, A.M. García-Deibe, M. Fondo, F. Novio, D.
Navarro, Inorg. Chim. Acta 347 (2003) 53.
[32] A. Bondi, J. Phys. Chem. 68 (1964) 441.
[33] T. Ortensen, L.G. Warner, Acta Crystallogr., Sect. B 29 (1973) 2954.
[34] J.D. Lee, M.W.R. Bryant, Acta Crystallogr., Sect. B 25 (1969) 2497.
[35] D.P. Kessissoglou, W.M. Buttler, V.L. Pecoraro, Inorg. Chem. 26 (1987) 495.
[36] J.A. Bertrand, J.L. Breece, Inorg. Chim. Acta 8 (1974) 495.
[37] C. Manzur, C. Bustos, R. Schrebler, D. Carrillo, C.B. Knobler, P. Gouzerh, Y.
Jeanning, Polyhedron 8 (1989) 2321.
Appendix A. Supplementary material
CCDC 733750, 733751, 733752 and 733753 contains the sup-
plementary crystallographic data for 1–4. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2010.01.034.
References
[38] J.N. Brown, R.L. Towns, L.M. Trefonas, J. Am. Chem. Soc. 92 (1972) 7436.
[39] M. Morioka, M. Kato, H. Yoshida, T. Ogata, Heterocycles 45 (1997) 1173.
[40] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, John Wiley and Sons, New York, 1997.
[1] H. Schiff, Ann. Suppl. 3 (1864) 343.
[2] A.D. Garnovskii, A.L. Nivorozhkin, V.I. Minkin, Coord. Chem. Rev. 126 (1993) 1.
[3] V. Alexande, Chem. Rev. 95 (1995) 273.
[4] S. Yamada, Chem. Rev. 190–192 (1999) 537.