Synthesis, characterization and electrochemical behavior of Co II, NiII and CdII complexes with N 2O2 donor ligands derived from 4,4′-diaminobiphenyl and 2-hydroxybenzaldehyde or 2,4-dihydroxybenzalde

Article abstract of DOI:10.1007/s10953-010-9512-2

The condensation of 2-hydroxybenzaldehyde or 2,4-dihydroxybenzaldehyde with 4,4′-diaminobiphenyl in absolute EtOH in a molar ratio 2:1 gave, respectively, the Schiff base ligands H₂L1 or H₂L2. The bases served as tetradentate ligands

Full text of DOI:10.1007/s10953-010-9512-2

J Solution Chem (2010) 39: 511–521
DOI 10.1007/s10953-010-9512-2
Synthesis, Characterization and Electrochemical
Behavior of CoII, NiII and CdII Complexes with N2O2
Donor Ligands Derived from 4,4ꢀ-Diaminobiphenyl
and 2-Hydroxybenzaldehyde
or 2,4-Dihydroxybenzaldehyde
Lakhdar Sibous · Embarek Bentouhami · Amor Maïza ·
Gilles M. Bouet · Mustayeen Ahmed Khan
Received: 9 July 2009 / Accepted: 28 October 2009 / Published online: 26 March 2010
© Springer Science+Business Media, LLC 2010
Abstract The condensation of 2-hydroxybenzaldehyde or 2,4-dihydroxybenzaldehyde with
4,4ꢀ-diaminobiphenyl in absolute EtOH in a molar ratio 2:1 gave, respectively, the Schiff
base ligands H2L1 or H2L2. The bases served as tetradentate ligands to coordinate CoII,
NiII and CdII chlorides, leading to complexes where the metal:ligand ratio is 2:1. All of the
compounds were characterized by elemental analysis, infrared, electronic and mass spec-
troscopy, 1H-n.m.r. and d.s.c. The cyclic voltammograms of the ligands and their complexes
in DMF are discussed.
Keywords Schiff bases · Cobalt(II) complexes · Nickel(II) complexes · Cadmium(II)
complexes · Cyclic voltammetry
1 Introduction
Since the first report of their metal complexes, Schiff base compounds have become an im-
portant class of ligands that have been extensively studied in coordination chemistry, mainly
due to their facile synthesis and easily tunable steric, electronic and catalytic properties [1].
They are also used in constructing supramolecular structures [2–4].
Various transition metal complexes with bi- and tridentate Schiff bases containing ni-
trogen and oxygen donor atoms play an important role in biological systems and represent
interesting models for metallo-enzymes, which efficiently catalyze the reduction of dinitro-
gen and dioxygen [5, 6]. Furthermore, macrocyclic derivatives of these Schiff bases were
L. Sibous · E. Bentouhami · A. Maïza
Département de Génie des Procédés, Faculté des Sciences de l’Ingénieur, Université Ferhat Abbas,
Sétif 19000, Algeria
G.M. Bouet · M.A. Khan (
)
ꢀ
Chimie de Coordination, Faculté de Pharmacie, Université d’Angers, 16 Boulevard Daviers,
49045 Angers Cedex 01, France
e-mail: mustayeen.khan@univ-angers.fr

512
J Solution Chem (2010) 39: 511–521
found to have many fundamental biological functions such as photosynthesis and transport
of oxygen in mammalian and other respiratory systems [7–9].
On the other hand, there is a great amount of research interest in the chemistry of the
metal complexes of tetradentate N2O2 Schiff bases due to their ability to form adducts with
dioxygen and thus may catalyze reduction of dioxygen. The transition metal ions in the
complexes form adducts with dioxygen via charge transfer. These complexes will thus be
expected to show catalytic effects. For instance, complexes of chromium and ruthenium
with a wide variety of Schiff bases having donor atoms such as N2O2 and N4 around the
metal ion have been used as catalysts for carbonylation, hydrogenation, hydroformylation
and epoxydation reactions [10–12]. The cobalt and manganese complexes that coordinate
through N2O2 donor atoms have been extensively studied as oxygen carriers and also as
catalysts for water splitting systems [13, 14].
Macrocyclic ligands also have coordination properties and have attracted much atten-
tion [15–17]. Cadmium is an environmental pollutant that inhibits ribonucleic acid (RNA)
polymerase activity in vivo [18, 19] and reacts readily with proteins and other biological
molecules. Consequently, the macrocyclic chemistry of cadmium has recently been devel-
oped [20].
Thus, in view of the above preceding research, it was considered worthwhile to pre-
pare some new metal-based macrocyclic skeletons bearing multiple donor sites that could
probably be used for the binding of some guest molecules. The present work deals with
the synthesis, characterization and cyclic voltammetry of two Schiff-base ligands de-
rived from the condensation of 4,4ꢀ-diaminobiphenyl with 2-hydroxybenzaldehyde or 2,4-
dihydroxybenzaldehyde and their Co2+, Ni2+ and Cd2+ complexes.
2 Experimental
2.1 Materials
All materials and solvents were analytical reagent grade and were used without further pu-
rification. 2-Hydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde and 4,4ꢀ-diaminobiphenyl
were purchased from Aldrich. All metals were used in the form of their hydrated chlorides
and were used as received.
2.2 Measurements
The 1H-n.m.r. spectra of the ligands and the diamagnetic complexes were recorded on a Jeol
GSX WB spectrometer at 270 MHz in DMSO-D6; the chemical shifts are given in p.p.m.,
using tetramethylsilane (TMS) as the internal reference. The IR spectra were recorded with
a Perkin-Elmer 1000 series FT-IR spectrophotometer using KBr disks. The UV-vis spectra
were obtained in DMF with a UNICAM UV-300 spectrophotometer. The electron impact
mass spectra (ESI) were recorded on a Bruker Daltonics Data Analysis 3.1 spectrometer.
The melting points were determined with a Kofler bench and are uncorrected. Differential
scanning calorimetry (d.s.c.) diagrams were recorded in the 25–400 °C range with a Mettler
DSC 822e unit, with the help of Mettler Toledo STARe SW 8.10 System software. The
heating rate was 10 °C per minute, and all measurements were made in 40 mm3 closed
aluminum crucibles.
The elemental analyses were carried out with an EL III–ELEMENTAR. All of these
measurements were performed in France. Electrochemical measurements were recorded on

J Solution Chem (2010) 39: 511–521
513
a Radiometer VOLTALAB 32 (DEA 332 type): the working electrode was a 2 mm diameter
Pt rotating disk and the auxiliary electrode was a Pt wire. A saturated calomel electrode
(SCE) was used as the reference electrode and measurements were carried out at 27 °C.
DMF was used as the solvent, and the ionic strength was maintained at 0.1 mol·L−1 with
Bu4NClO4 (TBAP) as supporting electrolyte. The concentrations of individual chemical
species were in the 2.5 ×10−3 to 5 ×10−3 mol·L−1 range. The sweep speed was 100 mV·s−1
unless otherwise indicated.
2.3 Preparation of the Ligands
The ligands were prepared by condensation of 4,4ꢀ-diaminobiphenyl with 2-hydroxybenzal-
dehyde for H2L1, and with 2,4-dihydroxybenzaldehyde for H2L2. The reactions were
carried out in EtOH (1:2 molar ratio) [21, 22]. To a solution of 4,4ꢀ-diaminobiphenyl
(0.43 mol·L−1) in absolute EtOH was added a solution of 2-hydroxybenzaldehyde or
2,4-dihydroxybenzaldehyde (0.87 mol·L−1). The mixture was stirred under a nitrogen at-
mosphere and refluxed for 3 h. The ligand was recovered by filtration, washed twice with
EtOH (10 mL) and dried under vacuum.
2.4 Preparation of the Complexes
All of the complexes were prepared using a synthetic pathway described in the literature
[23–26], by mixing appropriate amounts of ligand H2L1 or H2L2 (0.1 mol·L−1) and a hy-
drated metal chloride in 1:2 molar ratio in absolute EtOH. After standing under reflux con-
ditions for 8 h, the mixture was then kept overnight at room temperature. The complex that
precipitated was removed by filtration, washed several times with EtOH and dried under vac-
uum. Their purity was verified using thin layer chromatography (t.l.c.) with CH2Cl2/EtOH
(7:3 v/v) as eluent.
3 Results and Discussion
3.1 Synthesis and Characterization
The Schiff base ligands H2L1 and H2L2 were synthesized according to Fig. 1 and serve as
tetradentate ligands in all six complexes reported in this account. The ligands employ two
Fig. 1 Synthesis pathway for the Schiff base ligands

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J Solution Chem (2010) 39: 511–521
Elemental analysis, found (Calcd.) (%)
Table 1 Main analytical data for the ligands and their complexes
+
−1
Compound
Color
Yield (%)
M
/g·mol
C
H
N
H L1
Yellow
Yellow
73
60
57
392.0
650.0
–
79.36 (79.51) 5.30 (5.10) 6.76 (7.13)
47.91 (47.99) 3.05 (2.77) 4.42 (4.31)
45.11 (45.52) 3.32 (3.20) 4.23 (4.08)
41.50 (41.22) 2.55 (2.37) 3.85 (3.69)
73.40 (73.51) 5.02 (4.71) 6.34 (6.59)
46.05 (45.76) 2.96 (2.64) 3.91 (4.10)
46.01 (45.78) 2.81 (2.64) 4.25 (4.10)
39.37 (39.55) 2.50 (2.28) 3.84 (3.55)
2
(CoCl ) L1
2 2
[(NiCl ) L1]·2H O Yellow
2 2
2
(CdCl ) L1
Orange 45
757.0
424.0
–
2 2
H L2
2
Tan
51
60
46
50
(CoCl ) L2
Brown
Green
Brown
2 2
(NiCl ) L2
681.6
–
2 2
(CdCl ) L2
2 2
imine nitrogens and two phenoxo oxygens to bind the metals. All complexes were synthe-
sized in good yield by the reaction of the hydrated metal chlorides of CoII, NiII and CdII
with H2L1 or H2L2 in a molar ratio of 2:1, using ethanol as solvent at 50 °C. Analytically
pure compounds were obtained in all cases as indicated by elemental analysis, with melt-
ing points higher than 260 °C, and are consistent with the proposed molecular formulae that
show the ratio of metal to ligand to be 2:1 in all cases. The main analytical data for the
ligands and their complexes are given in Table 1.
3.2 Mass Spectrometry
The molecular weights of all prepared compounds were determined from their spectra using
electron impact (EI) conditions. Parent ion peaks M+ corresponding to the ligands and some
of the complexes were present. The spectra also showed peaks attributed to (M+ + 1H) and
(M+ +2H), together with prominent breakdown peaks that corresponded to a loss of chlorine
atoms in the cases of the nickel(II) and cobalt(II) complexes. These results agree with the
expected structures and are mentioned in Table 1.
3.3 Infrared Spectra
In the IR spectra of the ligands, the broad bands in the 3232–3625 cm−1 range are attributed
to the ν(O–H) stretching vibration of the phenolic groups [27]. In the case of the nickel(II)
complex with ligand H2L1, this broad band results from the presence of water molecules
according to the d.s.c. results. The bands in the 1282, 1284, 1620, and 1631 cm−1 regions are
assigned to the stretching vibrations ν(C–O) and ν(C=N) of H2L1 and H2L2, respectively
(Table 2). The ν(C=N) band of the ligands undergoes small shifts to lower frequencies in the
spectra of the complexes (1600–1572 cm−1), indicating coordination of the imine nitrogen
[28–31]. Additional evidence for coordination of the nitrogen is the presence of a ν(M–N)
band in the 437–497 cm−1 region.
Coordination through the oxygen atom is inferred from the shifts to lower wavenumbers
of the ν(C–O) band as compared to their position in the ligands [32], and the appearance
of a new band in the 528–586 cm−1 region assigned to ν(M–O), in agreement with earlier
reported results [33–36].

J Solution Chem (2010) 39: 511–521
515
Table 2 Main spectroscopic data for the ligands and their complexes
−1
UV-vis
(nm)
(DMF solution)
Compound
Infrared (cm
)
−1
−1
λ
max
[ε/L·mol ·cm
]
ν(O–H) ν(C=N) ν(C–O) ν(M–O) ν(M–N)
H L1
3454
–
1620
1572
1599
1600
1631
1595
1595
1596
1282
1250
1220
1238
1284
1242
1271
1247
–
–
366
370
370
368
368
380
376
378
[13600]
[20361]
[5388]
2
(CoCl ) L1
572
528
574
–
437
493
438
–
2 2
[(NiCl ) L1]·2H O 3360
2 2
2
(CdCl ) L1
–
[18373]
[19204]
[21936]
[25744]
[27412]
2 2
H L2
2
3363
3625
3288
3232
(CoCl ) L2
586
563
584
497
489
491
2 2
(NiCl ) L2
2 2
(CdCl ) L2
2 2
3.4 Electronic Spectra
The band maxima (λmax) and molar absorptivities (ε) obtained from the electronic spectra
of the ligands and complexes were recorded in DMF and the relevant data are presented in
Table 2. In general, the UV-visible region of the electronic spectra of these compounds was
dominated by intense intra-ligand π–π∗ transitions.
All of the complexes exhibit an absorption band in the 370–380 nm range, which is
assigned to a metal-ligand charge transfer band.
In the case of the cobalt(II) complex of ligand H2L2, two weak bands from d–d tran-
sitions, not mentioned in Table 2, were observed at 606 and 654 nm and correspond to a
tetrahedral environment around the Co2+ ion [37, 38].
3.5 1H-n.m.r. Spectra
The observations obtained from the previous analysis were further substantiated by the com-
parison of 1H-n.m.r. spectra of the free symmetrical ligands H2L1 and H2L2 and their com-
plexes, which were recorded in DMSO-D6 solutions.
The aromatic protons of the ligands showed multiplets at δ = 7.7–6.92 and 7.9–
7.2 p.p.m., assigned to both phenyl and bipyridyl protons of H2L1 and H2L2, respectively.
These protons, in each case, could not be distinguished from each other. The spectra also
contained absorptions at δ = 9.3 and 8.9 p.p.m. that are ascribed to the protons of the azome-
thine groups, while the singlets at δ = 13.1 and 13.6 p.p.m. are typical of hydroxyl pro-
tons. The remaining –OH absorption of H2L2 occurred at δ = 10.95 p.p.m. These chemical
shifts may be compared with those of literature [39, 40] where the hydroxyl proton of 2-
hydroxybenzaldehyde or 2-hydroxyacetophenone is reported to resonate at δ = 12.05 p.p.m.
This deshielding, in fact, is due to the formation of intramolecular hydrogen bonding. For
H2L1 and H2L2 the interaction is exclusively with the nitrogen atoms of the azomethine
groups.
This difference becomes more important in the corresponding nickel(II) complexes NiL1
and NiL2, for example, where the aromatic protons in each case gave rise to a multiplet
at δ = 8–7.25 p.p.m., while the protons of the azomethine groups become deshielded and
appeared at δ = 10.2 and 9.1 p.p.m., respectively, for the same reasons as those previously
indicated for the hydroxyl protons. This fact is supported by a higher structural rigidity
leading to the coplanarity of the different sp2 systems constituting the complex, according
to the square planar geometry generally observed for nickel complexes [41].

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J Solution Chem (2010) 39: 511–521
Table 3 Voltammetric results (anodic E , cathodic E ) at room temperature in DMF at the ionic strength
pa
pc
−1
−1
0.1 mol·L (TBAP) reported in V versus SCE; sweep speed 100 mV·s
Compound
E
E
E
E
E
E
ꢀE
E
1/2
pa1
pa2
pa3
pc1
pc2
pc3
H L1
2
1.05
0.68
1.60
0.96
1.24
0.32
0.31
−0.73
0.685
1.170
(CoCl ) L1
0.46
0.47
0.45
0.56
0.52
0.66
0.52
−1.46
−1.50
−1.43
2 2
[(NiCl ) L1]·2H O
1.10
−0.14
2 2
2
(CdCl ) L1
2 2
H L2
2
1.03
0.74
0.81
−0.25
−0.14
0.20
0.435
1.130
0.910
(CoCl ) L2
1.20
1.06
1.01
−1.50
−1.68
−1.50
2 2
(NiCl ) L2
2 2
(CdCl ) L2
2 2
In addition to this, the disappearance of the hydroxyl protons at δ = 13.1 and 13.6 p.p.m.
in the 1H-n.m.r. spectra of the complexes obviously indicates that the –OH groups becomes
deprotonated during coordination. The absorption that is observed at δ = 10.53 p.p.m. in the
NiL2 spectrum is due to the remaining hydroxyl group in position four of the azomethine
group.
3.6 Thermal Analysis
The d.s.c. diagrams show significant differences among the ligands H2L1, H2L2 and their
complexes. The decomposition of all these compounds occurs at temperatures higher than
300 °C and is generally followed by several exothermic peaks due to this decomposition.
The d.s.c. diagram of the nickel(II) complex derived from H2L1 shows many peaks at
the 130–200 °C range that are due to the loss of water molecules.
The cobalt(II) and cadmium(II) complexes of ligand H2L1 or H2L2 are very stable ther-
mally and show endothermic peaks in the 360–390 °C range. These results are in good agree-
ment with literature values [42].
3.7 Electrochemical Behavior of the Ligands and the Complexes
The main electrochemical results for the ligands and the complexes in this study are sum-
marized in Table 3 and some representative voltammograms are shown in Fig. 2.
3.8 Ligands
In the −1.8 to +1.8 range, the cyclic voltammogram of the ligand H2L1 (Fig. 2a) shows two
waves on the anodic side, with peak potentials of +1.05 and +1.60 V that can be ascribed to
the irreversible oxidations of the phenol moieties, and one cathodic peak at +0.32 V due to
the reduction of the oxidized species of the ligand (oxidation of the azomethine moiety) [43].
In the case of H2L2 (Fig. 2b), the cyclic voltammogram shows two irreversible waves
on the anodic side with only one cathodic peak. The potentials of the anodic peaks are
at +0.56 and +1.03 V. During the reduction sweep, a broad cathodic peak was obtained at
+0.31 V resulting from reduction of the imino group. This ligand seems to have an increased
tendency towards oxidation due to the presence of the electron donating groups (OH) in
the meta-positions of the phenol moieties, leading to lower oxidation potentials. Electron
withdrawing substituents usually yield shifts to more positive potentials [44].

J Solution Chem (2010) 39: 511–521
517
−1
Fig. 2 Representative cyclic voltammograms in DMF (25 °C, ionic strength of 0.1 mol·L , Bu NClO ,
4
4
−1
II
II
II
II
II
i in µA, v = 100 mV·s ) of: H L1 (a), H L2 (b), Co L1 (c), Co L2 (d), Ni L1 (e), Ni L2 (f), Cd L1 (g),
2
2
II
and Cd L2 (h)
3.9 Complexes
Because the ligands used in this electrochemical examination are not reversibly oxidized
or reduced in the applied potential range, the behavior of the complexes showed similar
responses that could be considered as the sum of the individual response of the metal center

518
J Solution Chem (2010) 39: 511–521
and of the ligands. For example, there are no Epa1 and Epc1 peaks for H2L1, whereas for its
nickel complex there is a new oxidation peak Epa1 at +0.47 V and a new reduction peak
Epc1 at +1.10 V. This clearly indicates that there is an effect from the metal center on the
electrochemical properties. The voltammetric responses of the cobalt(II) complexes show
cathodic peaks at −1.46 V corresponding to the reduction of complexed CoII to Co0 for
H2L1 (Fig. 2c) and −1.50 V and +1.06 V for H2L2 (Fig. 2d). The electrooxidation of Co0
to CoII in these species, in addition to those of the ligands (observed at nearly the same
potential values as the corresponding H2L1 and H2L2), occur at +0.46 and +0.52 V. These
values agree with those previously published for cobalt complexes in the same solvent [45].
In the case of nickel(II) complexes, the cyclic voltammograms on the negative scan show
four reduction peaks at −1.50 and +1.10 V for Ni(II)L1 (Fig. 2e), and −1.68 and +1.01 V
for Ni(II)L2 (Fig. 2f). The cathodic waves at −1.50 V and −1.68 V correspond to the reduc-
tions of complexed NiII to Ni0 whereas, on the reverse sweep, up to three oxidation peaks
occur in each case. The peaks that are absent in the cyclic voltammograms of the free lig-
ands are located at +0.47 V (ligand H2L1) and +0.66 V (ligand H2L2). These peaks can be
associated to the irreversible oxidation of Ni0 to NiII. The electrochemical responses of the
Ni(II) compounds are essentially the same with minor shifts in the potential values (Table 3),
and agree with those previously described for nickel complexes in the same solvent [46].
In addition, a linear dependence was observed between ip and v1/2 and Ep and ip as the
voltage scan rate (v) was increased (i.e., at 100, 200, 300, and 500 mV·s−1), indicating a
diffusion controlled electron exchange reaction. The plot of ipa for the oxidation peak po-
tential of Ni(II)L1 and that of Ni(II)L2, each as a function of the square root of the scan rate
(not shown), gave a straight line parallel to the horizontal axis (log10 v) indicating a quasi-
reversible electrode exchange reaction [42]. Furthermore, the ratio ipc/ipa was independent
of scan rate and equal to unity for scan rates varying from 100 mV·s−1 to 1 V·s−1
.
Finally, the cadmium(II) complexes also undergo reduction processes located at −1.43 V
for Cd(II)L1 (Fig. 2g) and −1.50 V for Cd(II)L2 (Fig. 2h). These peak values correspond
to the reduction of CdII to Cd0. The reoxidations at +0.45 V for Cd(II)L1 and +0.52 V for
Cd(II)L2 are due to the oxidation of Cd0 back to coordinated CdII .
When the scan rate changes, this last single reduction peak (at −1.50 V) sometimes
is decomposed into two waves and the second peak is often more distinct than the first.
These two successive reduction peaks are observed in the cyclic voltammogram of Cd(II)L2
at various scan rates (100 to 10 mV·s−1), but Cd(II)L1 gives only one peak in each case.
A similar behavior has been observed for other symmetric Schiff bases [45, 47].
3.10 Proposed Structures
The proposed structures are given in Fig. 3. The cobalt(II), cadmium(II) and nickel(II) com-
pounds exhibit the same structure. These species are binuclear complexes and the coor-
Fig. 3 The proposed structures
for the complexes R = H (H L1)
2
or OH (H L2)
2

J Solution Chem (2010) 39: 511–521
519
dination of the metal ions is through nitrogen and oxygen atoms. The cobalt(II) and the
cadmium(II) compounds present a distorted tetrahedral geometry while the nickel(II) com-
plexes exhibit a typical square planar structure [48]. In addition, two water molecules are
present in the case of the nickel(II) complex (ligand H2L1) as indicated by infrared spec-
troscopy and thermal analysis.
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