Four cobalt(III) Schiff base complexes – Structural, spectroscopic and electrochemical studies

Article abstract of DOI:10.1016/j.ica.2017.03.001

Four new complexes of the general formula [Co(L_n)Cl] (where L_nis dianion of pentadentate Schiff base ligand N,N′-bis(2-hydroxybenzylidene)-1,6-diamino-3-azahexane?=?H₂L₁, N,N′-bis(2-hydroxy-3-methoxy-benzylidene

Full text of DOI:10.1016/j.ica.2017.03.001

Accepted Manuscript
Four Cobalt(III) Schiff base complexes – structural, spectroscopic and electro-
chemical studies
Luká š Pogány, Ján Moncol, Miroslav Gál, Ivan Šalitroš, Roman Boča
PII:
S0020-1693(16)30946-X
http://dx.doi.org/10.1016/j.ica.2017.03.001
ICA 17460
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
25 November 2016
21 February 2017
1 March 2017
Please cite this article as: L. Pogány, J. Moncol, M. Gál, I. Šalitroš, R. Boča, Four Cobalt(III) Schiff base complexes
– structural, spectroscopic and electrochemical studies, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/
10.1016/j.ica.2017.03.001
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Four Cobalt(III) Schiff base complexes – structural, spectroscopic and electrochemical studies
Lukáš Pogány, a*Ján Moncol,a Miroslav Gál,a Ivan Šalitroš,a and Roman Bočaa,b
a) Institute of Inorganic Chemistry, Slovak University of Technology, Bratislava SK-81237, Slovakia,
*e-mail: lukas.pogany@stuba.sk
b) Department of Chemistry (FPV), University of SS Cyril and Methodius, 91701 Trnava, Slovakia
Abstract
Four new complexes of the general formula [Co(Ln)Cl] (where Ln is dianion of pentadentate Schiff
base ligand N,N’-bis(2-hydroxybenzylidene)-1,6-diamino-3-azahexane=H2L1, N,N’-bis(2-hydroxy-3-
methoxy-benzylidene)-1,6-diamino-3-azahexane=H2L2, N,N’-bis(3-ethoxy-2-hydroxy-benzylidene)-
1,6-diamino-3-azahexane=H2L3 or N,N’-bis(2-hydroxybenzylidene)-1,7-diamino-4-methyl-4-
azaheptane=H2L4) have been synthesized and characterized by elemental analyses, IR, UV–VIS. The
redox properties have been studied by cyclic voltammetry. Moreover, the crystal structures of 1, 2
and 3 have been determined by X-ray diffraction and 1H NMR spectrum of 4 has been obtained.
Keywords: Schiff base; Cobalt(III) complexes; Crystal structure; Spectral properties; Cyclic
voltammetry
1. Introduction
Schiff base ligands with imine groups (–RC=N–) capable to coordinated with metal ions have
many advantages and they are very popular up to now. These ligands can be easily prepared by a
one-pot condensation reaction of an aldehyde or ketone with primary amines usually in an alcohol as
a solvent. Generally, even a small modification of the ligand structure can significantly change
properties of the corresponding complexes [1-3]. However, simplicity of preparation of Schiff base
ligands allows one a fast and systematic preparation of a wide range of ligands with various
substituents and tuning of physical properties can be studied easier. Schiff bases represent a very
important class of compounds that are applied in many fields of research [4-10]. However, they are
often coordinated to various transition metal ions in order to improve or modify their properties.
Cobalt(III) complexes with pentadentate N3O2 ligands and their reactivity and structural properties
have been extensively investigated for their miscellaneous applications, such as antifungal and
antimicrobial agents as well as hypoxia-activated transporter of bioactive small molecules [11-17],
production of redox-active sensors [18], ionic ferroelectrics [19], highly efficient catalysts in various
fields of synthesis, as a precursor for preparation of Co3O4 nanoparticles and other useful reactions
[20-25].
In the case when pentadentate N3O2 Schiff base is coordinated to cobalt(II) ion there is a
tendency of this ion to oxidise and one arrives to cobalt(III) Schiff base complex. In order to prevent

this oxidation an inert atmosphere has to be involved in synthesis [26]. In comparison to
paramagnetic cobalt(II) Schiff base complexeswhich can exhibits even the single molecule magnetism
[27], cobalt(III) complexes are, from magnetochemical point of view, usually low-spin and
diamagnetic [28].
However also cobalt(III) core may become interesting if such a moiety forms a building block of
Prussian blue analogues where it is linked to iron(II) via CN bridge. In this case a pair of diamagnetic
centres {FeIILS(t2g6,S=0)-(μ-CN)CoIIILS(t2g6,S=0)} is reversibly converted into paramagnetic
2
{FeIIILS(t2g5,S=1/2)-(μ-CN)CoIIHS(t2g5eg S=3/2)} unit by electron transfer induced either thermally
,
(heating above 150 K) or by light (blue light irradiation). This phenomenon is possible also within
compounds with higher nuclearity, e.g. in the octanuclear {Fe4Co4} species [29-31].
Herein we report the synthesis of four cobalt(III) complexes with general formula [Co(Ln)Cl],
where Ln is a dianion of pentadentate Schiff base ligand and n=1-4. These compounds have been
characterized by elemental analyses, IR, UV–VIS. The X-ray crystal structures of 1, 2 and 3 have been
determined. The structure of 4 has been confirmed by 1H NMR spectroscopy . The electrochemical
behaviour and redox properties of these complexes are also reported and discussed.
2. Experimental
2.1. Materials and physical measurements
All solvents and chemicals were purchased from commercial sources and used without further
purification. Elemental analyses were performed by using Eager 300 (Carlo Erba) elemental analyser.
UV–VIS spectra were obtained on a Specord 250 plus (nujol technique), Analytical Jena
spectrophotometer. Infrared spectra (ATR technique) were measured on a Magna FT- IR 750, Nicolet
spectrophotometer. 1H NMR spectrum was recorded with a Unity Inova spectrometer (300 MHz).
Proton chemical shifts are reported in ppm relative to CD3OD as an internal standard. The redox
properties of the complexes were studied by cyclic voltammetry. LiCl (anhydrous) and absolute
ethanol (≥99.8%) were supplied by Sigma–Aldrich. The indifferent electrolyte TBAPF6 was dried under
vacuum. Ethanol was dried over activated molecular sieves. Electrochemical measurements were
performed using AUTOLAB instrument PGSTAT 302N (Metrohm). An electrochemical data from cyclic
voltammetry were analyzed using AUTOLAB software. A three-electrode electrochemical cell was
used. The reference electrode (RE), Ag|AgCl|1 M LiCl, was separated from the test solution by a salt
bridge. The working electrode (WE) was Pt disk (d=0.8 mm). Prior to each experiment the surface of
the WE was polished by 1 μm alumina and subsequently by 0.3 μm alumina. The auxiliary electrode
was platinum net. The scan rate for the cyclic voltammetry was in the range 16-1024 mV s-1. Oxygen
was prior to each experiment removed from the solution by passing a stream of argon saturated with
vapours of the solvent.

2.2. Synthesis and spectral properties
Herein reported Schiff base ligands have been synthesized without isolation, purification or
characterisation. „In situ“ prepared ligands have been coordinated with cobalt(II) chloride in the
presence of air, which allowed spontaneous oxidation to cobalt(III) complexes (Fig. 1). Complex 3 has
already been prepared previously by slightly different procedure [32,33]. However, it has not been
isolated and thus studied, characterised and described in details.
R2
(CH2)m
(CH2)n
O
N
N
N
H2N
(CH2)n
(CH2)m
H
R2
N
+
acetone,
50 °C, 1h
OH
OH
HO
H2N
R1
R1
R1
ligand L
R2
(CH2)n
R2
(CH2)m
(CH2)m
(CH2)n
N
N
N
N
N
N
CoCl2.6H2O
air
CoIII
acetone,
rfx, 1h
OH
O
HO
O
Cl
R1
R1
R1
R1
ligand L
complex
Fig. 1 Scheme of syntheses of ligands (upper) and complexes (bottom). For 1 (and corresponding
ligand H2L1) R1 = R2= H , n=2, m=3. For 2 (and corresponding ligand H2L2) R1=OCH3, R2=H, n=2, m=3.
For 3 (and corresponding ligand H2L3) R1=OCH2CH3, R2=H, n=2, m=3. For 4 (and corresponding ligand
H2L4) R1=H, R2=CH3, n=m=3.
Synthesis of complex 1[Co(L1)Cl]:Solution of salicylaldehyde (0.520 g, 4.26 mmol, 2 eq.) in acetone
(30 mL) was heated to 50 °C and combined with a solution of N-(2-aminoethyl)-propane-1,3-diamine
(0.249 g, 2.13 mmol, 1 eq.) in acetone (25 mL) subsequently. The reaction mixture was stirred at 50
°C for 1 hour, then combined with solution of cobalt(II) chloride hexahydrate CoCl2.6H2O (0.507 g,
2.13 mmol, 1 eq.) in acetone (30 mL) and reaction mixture was refluxed for another 1 hour. One
spoon of decolourising charcoal was then added to the reaction mixture, reflux continued for
another 10-15 minutes and solution was filtered into the beaker with small amount of acetone (5
mL). Solution was set for crystallisation at 5°C and after few days were selected single crystals or

polycrystalline powder. Yield 45 %. Elemental analysis for C19H21ClCoN3O2.H2O: Found % (Calc %): C
52.46 (52.37); H 5.36 (5.32); N 9.05 (9.64). FT-IR (ATR, cm-1) νmax: 3599 (m, H2O), 3421 (m, H2O), 3201
(m, N–H), 1645 (s, C=N). Solid state UV–VIS: λmax(nm) =697, 663, 620, 501, 402, 271.
Synthesis of complex 2 ([Co(L2)Cl]): Complex 2 has been prepared via procedure similar to that of
complex 1, however salicylaldehyde has been substituted with 3-methoxysalicylaldehyde (0.498 g,
3.27 mmol, 2 eq.). Further procedure continued in the same manner as in the synthesis of complex 1.
Yield 55 %. Elemental analysis for C21H25ClCoN3O4.C3H6O: Found % (Calc %): C 53.46 (53.79); H 5.92
(5.83); N 7.84 (7.84). FT-IR (ATR, cm-1) νmax: 3203 (m, N–H), 1690 (s, C=O; acetone), 1644 (s, C=N).
Solid state UV–VIS: λmax(nm)=834, 705, 509, 390, 251.
Synthesis of complex 3([Co(L3)Cl]): Synthesis of complex 3 has started with 3-ethoxysalicylaldehyde
(0.502 g, 3.02 mmol, 2. eq.) instead of salicylaldehyde. Further procedure continued in the same
manner as in the synthesis of complex 1. Yield 53 %. Elemental analysis for C23H29ClCoN3O4.H2O:
Found %(Calc %): C 52.08 (52.73); H 5.91 (5.96); N 7.88 (8.02). FT-IR (ATR, cm-1) νmax: 3462 (m, H2O),
3163 (m, N-H), 1718 (s, C=O; acetone), 1650 (s, C=N). Solid state UV–VIS: λmax(nm)=695, 664, 629,
510, 400, 261.
Synthesis of complex 4([Co(L4)Cl]): Complex 4 has been prepared via procedure similar to that of
complex 1, but in this case a solution of N-methyl-N,N-bis(3-aminopropyl)amine (0.305 g, 2.10 mmol,
1 eq.) instead of N-(2-aminoethyl)-propane-1,3-diamine was used. Further procedure continued in
the same manner as in the synthesis of complex 1. Yield 50 %. Elemental analysis for
C21H25ClCoN3O2.acetone: Found % (Calc %): C 56.93 (57.20); H 6.33 (6.20); N 8.52 (8.34). FT-IR (ATR,
cm-1) νmax: 1712 (s, C=O; acetone), 1628 (s, C=N). Solid state UV–VIS: λmax(nm)=773, 594, 500, 393,
262. 1H NMR (300 MHz, CD3OD, 25 °C, δ/ppm):8.26 (s, 1H, CH=N), 8.06 (s, 1H, CH=N), 7.36-7.33 (d,
2H, Har), 7.24-7.15 (m, 4H, Har), 6.65-6.54 (m, 2H, Har), 6.11-6,04 (m, 2H, Har), 3.81-3.74 (t, 1H, Halif),
3.65-3.59 (m, 1H, Halif), 3.65-3.59 (m, 1H, Halif), 3.12-3.02 (m, 1H, Halif) 2.37-2.13 (m, 6H, Halif), 1.54 (s,
3H, N-CH3), 1.37-1.16 (m, 3H, Halif).
2.3. X-Ray Crystallography for 1, 2 and 3
Data collection and cell refinement of 1-2 were carried out using a κ-axis diffractometer Xcalibur
SCCD (Oxford Diffraction) with graphite monochromated MoKα radiation. The diffraction intensities
were corrected for Lorentz and polarization factors. The data collection for 3 at 100 K was made by
StoeStadiVari diffractometer using Pilatus 300K HPAD detector and microfocus source Xenocs FOX3D
with CuKα. The structures were solved by direct methods using SHELXT [34] or SUPERFLIP [35] and

refined by the full-matrix least-squares procedure with SHELXL (version 2016/4) [36]. The absorption
corrections were made by using SCALE3 ABSPACK in CRYSALIS-RED. Geometrical analyses were
performed with SHELXL. The structures were drawn using the OLEX2 package [37]. Crystal data and
conditions of data collection and refinement are reported in Table 1.
Tab. 1 Crystallographic data for the reported compounds
Compound
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
1
2
3
C19H23ClCoN3O3
435.78
C24H31ClCoN3O5
535.91
C23H29ClCoN3O4
505.87
293(1)
293(1)
100(1)
0.71073
monoclinic
P21/n
0.71073
monoclinic
P21/c
1.54186
trigonal
R–3
8.7080(4)
12.5195(6)
17.7401(9)
90
8.9154(4)
19.7800(10)
13.7207(6)
90
27.0224(7)
27.0224(7)
16.7206(4)
90
b (Å)
c (Å)
α (°)
β (°)
100.338(5)
90
92.081(4)
90
90
γ (°)
V (Å3)
Z, ρcalc g.cm-3
Radiation type
μ (mm-1)
120
1902.63(16)
4, 1.521
Mo-Kα
2418.00(19)
4, 1.472
Mo-Kα
10573.8(6)
18, 1.430
Cu-Kα
1.067
0.861
7.058
F (0 0 0)
904
1120
4752
Crystal size (mm)
2θ Ranges (°)
Final R indices [I > 2σ(I)]
0.06x0.20x0.40
8.154 to 52.744
R1=0.0519
wR2=0.0694
R1=0.0927
wR2=0.0795
1.100
0.08x0.16x0.50
8.184 to 52.744
R1=0.0673
wR2=0.0988
R1=0.1139
wR2=0.1147
1.182
0.09x0.11x0.50
6.496 to 142.554
R1=0.0327
wR2=0.0691
R1=0.0481
wR2=0.0725
0.903
R indices (all data)
S
CCDC number
1507220
1507221
1510657
3. Results and discussion
3.1. Syntheses and spectral characterisation
Schiff base ligands H2L1 – H2L4 were prepared by the literature method [38] via the condensation
of salicylaldehyde (H2L1, H2L4), 3-metoxysalicyladehyde (H2L2) or 3-etoxysalicyladehyde (H2L3) with
1,6-diamino-3-azahexane (H2L1, H2L2 and H2L3) or 1,7-diamino-4-methyl-4-azaheptane (H2L4) in
acetone The yellow solutions of ligands were prepared „in situ“ and used for the further
complexation without any purification and characterisation. The new complexes (1–4) were prepared

through the oxidation of corresponding Co(II)intermediates by stirring in aerobic conditions. In case
of 1, 2 and 3 dark brown crystals suitable for the diffraction analysis were obtained after several days
of controlled evaporation of solution at lowered temperature (45 - 55 % yields). The crystallisation of
compound 4 failed therefore the polycrystalline powder has been used for the spectral and
electrochemical investigation. Its structure was confirmed by 1H NMR spectroscopy.
In the infrared spectroscopy (see supporting information Fig. S1) one can observe vibrational
bands of secondary amino group –N – H at 3201, 3203 and 3163 cm-1 in 1 – 3, respectively. This band
is missing in the spectrum of 4 because aliphatic part of related Schiff base ligand contains tertiary
amino group. In case of imino group –C=N bands located at 1645, 1644 and 1650 cm-1 are observed
in IR spectra of complexes 1 – 3, respectively. It reaches maximum at 1628 cm-1 in case of complex 4.
However, it is combined with a band of skeletal vibrations of aromatic rings IR spectrum of 1 exhibits
bands 3599 and 3421 cm-1 that can be assigned to those of symmetric and antisymmetric vibration
bands of crystal water molecules. Similarly, also in the IR spectrum of 3 a band at 3462 cm-1 of crystal
water was found. On the other hand bands of acetone were found at 1690 and 1712 cm-1 in 2 and 4,
respectively.
The solid state electronic absorption spectra of 1 – 4 exhibit very similar features (see supporting
information Fig. S2). One can observe bands at 251 – 271 nm that belong to π →π* transitions of
Schiff base ligands. The charge transfer transitions appear in 390–402 nm region. Bands in the visible
region at 500-510 nm and 594-620 nm correspond to d-d transitions (1A1g → 1T1g and1A1g → 1T2g ).
Compound 4 was purified by column chromatography (silica, 1% methanol in CH2Cl2) prior to 1H NMR
experiments. In the 1H NMR spectrum of 4 (see supporting information Fig. S3) two imine protons of
the ligand appear as two separate singlet peaks at 8.26 and 8.06 ppm. This is in agreement with
thenon-similarity of the imine protons environment in octahedral coordination sphere of complex 4.
The aromatic protons of benzene rings are except doublet between 7.36 and 7.33 ppm found as
complex multiplets appearing between 6.92 and 8.01 ppm. Signals of the aliphatic protons of the
ethylene and propylene linkages in ligands appear except triplet in range 3.81-3.74 ppm as multiplets
in the ranges 6.11-6.04, 3.65-3.59, 3.12-3.02, 2.37-2.13, 1.37-1.16 ppm. Aliphatic methyl group
attached to the tertiary amino nitrogen atom was found as singlet at 1.54 ppm.
3.2. Description of crystal structures
Complexes 1 and 2 crystallize in centrosymmetric monoclinic space groups P21/n and P21/c for 1
and 2, respectively. On the other hand, complex 3 crystallizes in centrosymmetric trigonal space
group R–3. Selected crystallographic parameters are listed in the Table 1. Asymmetric unit contains in
both monoclinic crystal structures one molecule of complex and either one molecule of water (1) or
acetone (2) and four of them can be found in the unit cell (Fig. 2). The independent part of cell in
crystal structure of 3 contains one complex molecule and undefined disordered solvent molecules of

water around special positions. Coordination polyhedra can be expressed as deformed tetragonal
bipyramid (for selected bond angles see Tab. S1) and distortion parameters acquire comparable
values (Σ ≈ 28°, 29°and 23° for 1, 2 and 3 [39] with similar iron(III) low-spin analogues [40,41], thus
one can predict the same spin state for 1, 2 and 3 as well. The low-spin state is evident also from
values of Co1–N2(imino), Co1–N1(amino) and Co1–N3(amino) bond distances which vary in the
ranges 1.97–1.98Å, 1.92–1.94 and 1.88–1.89Å, respectively (Tab. 2).The Co1–O1 and Co1–O2
distances are the shortest within the coordination core and in the compound 1 are the same 1.88Å,
on the other hand in both cases 2 and 3 they reach the bonds values 1.88 and 1.89Å. The axial Co-Cl
distances are the longest and weakest with distances 2.30, 2.27 and 2.29Å for 1, 2 and 3,
respectively.
The equatorial planes are in both complexes formed by one phenolate oxygen donor atom, two
imino and one amino nitrogen donor atoms. The second phenolate oxygen atom and chlorido
terminal ligand anion occupy axial positions. Two oxygen atoms are mutually arranged in cis position;
on the other hand the three nitrogen atoms form a meridional conformation (Fig. 2-Fig. 4).

Fig. 2 Asymmetric unit of 1
Fig. 3 Asymmetric unit of 2
Fig. 4 Asymmetric unit of 3
Tab. 2 Selected bond lengths of 1, 2 and 3
3
bond lengths
Co1-O1
1
2
1.8994(14)
1.8969(14)
1.9392(17)
1.884(2)
1.884(2)
1.928(3)
1.898(3)
1.881(3)
1.942(3)
Co1-O2
Co1-N1

1.9792(17)
1.8919(17)
2.2892(5)
Co1-N2
Co1-N3
Co1-Cl1
1.970(3)
1.888(3)
2.3029(10)
1.973(3)
1.890(3)
2.2746(13)
Fig. 5 Formation of 1D supramolecular chain in crystal structure of complexes 1
Fig. 6 Formation of 1D supramolecular chain in crystal structure of complexes 3
Molecules of complex [Co(L1)Cl] and lattice water form supramolecular hydrogen bounded 1D
chain within the crystal structure of 1. The oxygen atom of water molecule creates non-covalent
contacts with amine nitrogen atoms of the first neighbouring [Co(L1)Cl] molecule (N2∙∙∙O1W distance
of 2.97Å) on the one site and with chlorine atom of the second [Co(L1)Cl] molecule (O1W-H2W∙∙∙Cl1
with O1W∙∙∙Cl1 distance of 3.23Å) on the other site, as is presented inFig. 5. In the crystal structure of
2 (Fig. 6) , there is a single hydrogen bond between a hydrogen atom attached to amine nitrogen and
oxygen atom of acetone molecule (N3∙∙∙O5 distance of 3.01 Å). The crystal structure of 3 forms spiral
supramolecular chains from connecting complex molecules through hydrogen bonds between imine
nitrogen groups and chloride ligands of neighbouring [Co(L3)Cl] molecule (N2-H2...Cl with N2∙∙∙Cl1
distance of 3.29Å).

3.3. Electrochemical studies
The redox behaviour of cobalt(III) complexes was studied by cyclic voltammetry in 0.1 M LiCl solution
in dry ethanol. Voltammetric measurements confirmed a reversible one-electron transfer reaction
which is in good agreement with previous studies. The expected direct relationship between both the
electron-withdrawing and electron-donating character of the individual substituents and its
reduction potential was recorded. In the next figure (Fig. 4) cyclic voltammograms are plotted.
Fig. 4 Cyclic voltammogram of scan rate 128 mV s-1; compound 1– blue line, 2– black line, 3 – red
line, 4– green line.
All curves are characterized by the presence of single oxidation and reduction peak. The position of
both oxidation and reduction peak depends on the structure of the respective compounds.
Moreover, the broadness and separation of oxidation and respective reduction counter peak also
depends on the structure of individual compounds. The lowest and the sharpest reduction potential,
Ered was observed in case of compound 4, Ered = -0.015 V vs. RE followed by the reduction potential of
compounds 2 and 3, Ered ≅ -0.330 V vs. RE and compound 1, where Ered = -0.450 V vs. RE was
recorded. Furthermore, reduction potential of the compound 1 is much broader compare to the
compound 4. The most positively on the potential axis is situated the oxidation peak of the
compound 4. Compounds 1 and 2 are oxidized at approximately the same potentials, Eox ≅ 0.015 V vs.
RE. The oxidation of compound 3 occurs at the lowest potential, Eox ≅ -0.075 V vs. RE compare to the
previous compounds.
In case of compound 4 the smallest separation ca ∆E = 90 mV between oxidation and reduction
peaks was detected. This value of the peak separation is slightly higher compare to the potential
difference ∆E = 65 mV found for ferrocene/ferrocenium redox couple. In case of the compounds 2

and 3 the separation of the respective oxidation and reduction peaks are almost ∆E = 250 mV. The
ratio between the anodic and the cathodic peak (ipa/ipc ratio) is in case of the compound
4approximately unity, in case of compounds 2 and 3 approximately 0.9. However, if the scan rate is
lowered the ipc/ipa ratio reached unity. The position of oxidation and reduction peaks are somewhat
dependent of the scan rate for all measured compounds, which indicate slower electron transfer
and, therefore, quasi reversible behaviour. Little bit different situation is recorded in case of
compound 1. The peak separation is more than 350 mV (Figure 4) and the peak ration is ca 0.6. The
position of respective oxidation and reduction peaks depends on the scan rate similarly as in case of
previous compounds 2 – 4. It seems that the electron transfer rate is for compound 1 slower than for
the previous compounds 2 – 4. One can conclude that even small changes in the structure of
individual studied compounds 1 – 4 influence the electrochemical behaviour of all compounds.
The presence of alkoxy groups in case of compounds 2 and 3 shifts both reduction and oxidation
potential to the more negative values compare to the compound 4. More energy is required for the
reduction and oxidation of the respective compounds. The differences between voltammograms of
compound 1 and 4 are caused by the presence of methyl group and propylene link between nitrogen
atoms in the structure of compound 1.
In the next Fig. 5 the dependence of the height of oxidation and reduction peak of the compound 4
on the scan rate is plotted.
Fig 5 Dependence of the height of the (9) oxidation and (.) reduction peak of the compound 4 on the
scan rate.
The reaction is controlled by the diffusion, which is demonstrated by the linear dependence of the
peaks height, ipa and ipc on the square root of the scan rate, v1/2. Moreover, the dependence of
logarithm of anodic peak current vs. logarithm of scan rate gave a straight line with a slope of 0.45,

which is close to the theoretical value of 0.5 for a purely diffusion controlled process. The current
peak analysis of several voltammograms recorded at different scan rates gave, according to the
Randles-Sevcik equation, the number of electrons exchanged during redox reaction, n = 0.9 0.1. This
means that during negative scan direction Co(III) is reduced to Co(II) and, vice versa, during reversal
scan direction, Co(II) is quasi reversibly reoxidized to Co(III).
4.
Conclusion
Spectroscopic (IR and UV-VIS) and redox properties (cyclic voltammetry) of four new complexes of
the type [Co(Ln)Cl], where Ln is dianion of pentadentate Schiff base ligand and n=1-4, crystal structure
of 1-3 and 1H NMR of 4 have been investigated. IR and UV-VIS spectra follow very similar features for
all of them and all differences are fully in agreement with expectations based on the variation of
structure. The crystal structures of complexes 1-3 are similar: low-spin diamagnetic cobalt(III) atom is
surrounded by pentadentate Schiff base ligand and chlorido anion that form deformed tetragonal
bipyramid environment. However, from supramolecular point of view one can found 1D infinite chain
of alternating and H-bound water and complex molecules in 1, a single H-bond between complex
molecule and acetone molecule in 2 and spiral supramolecular chains from H-bond connected
complex molecules in 3. Structure of 4 was confirmed by 1H NMR. A reversible one-electron transfer
reaction was confirmed by voltammetric measurements. In case of compound 4 was observed the
lowest and the sharpest reduction potential Ered = -0.015 V vs. RE, followed by the compounds 2 and
3 with reduction potential Ered ≅ -0.330 V vs. RE and finally was recorded compound 1, where
Ered = -0.450 V vs. RE. One can thus conclude that even small changes in the structure of individual
studied compounds 1 – 4 influenced the electrochemical behaviour of all compounds. Slower
electron transfer and thus quasi reversible behaviour was indicated by observation that the positions
of oxidation and reduction peaks were somewhat dependent of the scan rate for all measured
compounds. The dependence of the height of oxidation and reduction peak of the compound 4 on
the scan rate was also studied. It was found that reaction is controlled by the diffusion and according
to the Randles-Sevcik equation, the number of electrons exchanged during redox reaction is
n = 0.9 0.1. The change between cobalt (II) and cobalt(III) thus took place.
Acknowledgements
Grant Agencies (Slovakia: APVV-14-0078, APVV-14-0073, VEGA 1/0522/14) and STU Grant
scheme for Support of Young Researchers are acknowledged for the financial support.
This article was created with the support of the MŠVVaŠ of the Slovak Republic within the
Research and Development Operational Programme for the project "University Science Park of STU
Bratislava" (ITMS project no. 26240220084) co-funded by the European Regional Development Fund.

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Graphical Abstract
Four Cobalt(III) Schiff base complexes – structural, spectroscopic and electrochemical studies
Lukáš Pogány, a*Ján Moncol,a Miroslav Gál,a Ivan Šalitroš,a and Roman Bočaa,b
c) Institute of Inorganic Chemistry, Slovak University of Technology, Bratislava SK-81237, Slovakia,
*e-mail: lukas.pogany@stuba.sk
d) Department of Chemistry (FPV), University of SS Cyril and Methodius, 91701 Trnava, Slovakia
Four new mononuclear Co(III) complexes with
pentadentate Schiff base ligands have been
synthesized, spectrally and structurally
characterised. In addition, electrochemical study
revealed significant dependence of redox
properties on the subtitle modification of Schiff
base ligand structure.

Highlights
•
•
•
Four mononuclear Co(III) complexes with Schiff base ligands have been prepared-
Structural investigation confirmed miscellaneous intermolecular interactions-
Redox properties are strongly dependent on the Schiff base ligand skeleton.