Impact and correlation of p: K a and dn electrons of some selected thiosemicarbazone Schiff base metal Co, Ni, Cu complexes: A study of electrochemical behavior, excitation and optical energies

Article abstract of DOI:10.1039/c7nj00073a

The electron-transfer and coordination mechanism of Schiff bases with metal ions, correlation of the electrochemical and optical properties for their potential applications in various fields of chemistry and biochemistry are underexplored. Thus, detailed

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Impact and correlation of pKa and dn electrons of selected thiosemicarbazone Schiff
base metal Co, Ni, Cu complexes: Electrochemical behavior, excitation and optical
energies study
M. S. ElꢀShahawi1*, W. Ahmad1, G.I. Mohammed2, Y.M. Moustafa2 A.A. ElꢀAsmy3, G.A Alꢀ
Hazmi4
1Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203,
Jeddah 21589, Saudi Arabia
2Faculty of Applied Science, Umm AlꢀQura University, P.O.Box 16722, Makkah 21955,
Saudi Arabia
3Department of Chemistry, Faculty of Science, Mansoura University, Mansoura, Egypt
4Department of Chemistry, Faculty of Science, King Khaled University, P.O. Box 9004,
Abha, Saudi Arabia
*Corresponding author. Tel: 009662ꢀ6952000 ext. 644422.; Fax: 009662ꢀ6952292,
Email: malsaeed@kau.edu.sa; mohammad_el_shahawi@yahoo.co.uk
On sabbatical leave from the Department of Chemistry, Faculty of Science, Damiatta
University, Damiatta, Egypt.
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Abstract
The electron transfer and coordination mechanism of Schiff base with metal ions, correlation
of electrochemical and optical properties for their potential applications in various fields of
chemistry and biochemistry are underexplored. Thus, detailed assignment of redox behavior
of some Co2+, Ni2+ and Cu2+ thiosemicarbazone complexes in tetrabutylammonium
ꢀ
tetrafluoroborate (TBA+ BF4 )ꢀ dimethylformamide (DMF) was investigated by bulk cyclic
voltammetry (CV) and controlled potential coulometry (CPC). The redox behavior of the
electrode couples (M+/M2+ and M2+/M3+, M = Co, Ni or Cu) was dependent on the electron
donating (or withdrawing) substituent on thiosemicarbazone moiety (N1H and/or N4H),
Schiff base pka and dn electrons of metal ions. On increasing the concentration of the
ꢀ
complex species in (TBA+ BF4 )ꢀDMF, the cyclic voltammograms of the complexes showed
no significant variations in the values of the diffusion coefficient(D) (0.27 ꢀ0.29)×10ꢀ6 cm2sꢀ1.
The Epa and Epc of selected electrode couples were satisfactorily correlated with the remote
effect of N1H and N4H substituent, Hammett parameters (σ) and dn electrons of the metal
ions. The optical band energies, HOMO and LUMO were obtained from electronic spectra
and cyclic voltammogram, respectively. The mechanism describing the electron transfer of
the electroactive species and stability of the complex species towards oxidation (M3+) and/ or
reduction (M+) was discussed.
Keywords: Thiosemicarbazone Schiff bases; Redox behavior of Co2+, Ni2+ and Cu2+
complexes; Ligand pKa; Electrode mechanism; HOMO and LUMO.
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1. Introduction
In coordination chemistry, Schiffꢀbases have often been utilized as chelating ligands [1].
Their potential applications in chemistry, biochemistry, medicine have laid the foundation
for its widespread synthesis [2, 3]. They have been used as antiꢀpyretic, antiꢀdiabetic, antiꢀ
bacterial, anti cancer, antiꢀinflammatory, antiꢀHIV and as chemosensors [4]. Several Schiffꢀ
bases with potential donors N, O and S are helpful in determining transformation mechanism
in biological system due to its structural resemblance [5].
Multidentate ligands of thiosemicarbazide and its derivatives containing sulphur and nitrogen
have received considerable attention. Their structural, spectral, chemical and redox
properties, photochromism, complexing towards heavy metals, catalytic activity and
antimicrobial activities are well known [6ꢀ13]. Its binding with metal ions may give rise to
various coordination modes, thus transforming their biological properties [14]. Regardless of
the extensive literature on Schiff bases [6ꢀ17], there are still unexplored areas about their
coordination mechanism with metal ions, detailed correlation of their electrochemical and
optical properties with transition metal ions for their potential applications in various fields.
Thus, further studies continued for improving the present day knowledge oriented mainly
towards their coordination tendency and biological properties. The redox couples CoI/CoII,
CoII/CoIII, NiI/NiII and NiII/NiIII are markedly affected by the structure of the ligand,
stereochemistry of the complex, background electrolyte and nature of solvent [18].
The redox behavior of cobalt (II) and nickel (II) model complexes have special significance
as the electron transfer processes in these metals are also involved in many biological
systems [19ꢀ26]. In continuation to our previous work on thiosemicarbazone Schiff bases and
their chelates with Co2+, Ni2+ and Cu2+ [19ꢀ25,27], the present study is focused on: i)
Studying the crucial role of pka of the Schiff bases on the redox behavior of the observed
electrode couples(M2+/ M+, M2+/ M3+, where M=Co, Ni or Cu) ; ii) Evaluating the effect of
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the of the substituent N1H and/or N4H in thiosemicarbazone moiety of the Schiff bases on the
redox properties of metal complexes; iii) Highlighting the propertiesꢀstructural relationships
with the excitation and optical energies of the observed electrode couples of the metal
complexes; iv) Investigating the influence of dn ꢀ electrons on the Epa, Epc, HOMO, LUMO
and optical energies of the observed couples; assigning the most probable reduction
mechanism and finally showing the most probable electron transfer mechanism of the
electrode couples. This study will be useful for the researchers in understanding the role of
the thiosemicarbazone Schiff bases and their metal complexes in biosystems. The study
also helps the scientists in assigning the semiconducting properties and the feasibility
materials.
2. Experimental
2.1. Reagents and materials
All chemicals and solvents were of A.R. grade. Tetrabutylammonium ꢀtetrafluoroborate
−
(TBA+ BF4 ) and N, Nꢀdimethylformamide (DMF) were purchased from BDH (Poole,
England). The chemical structures are shown in Fig. 1 while the pKa of the prepared
thiosemicarbazone Schiff bases are given in Table 1. The Schiff bases and their cobalt (II),
nickel (II) and copper (II) complexes have been prepared and characterized as reported
earlier [17ꢀ19]. DMF was chosen in the present study as a proper solvent due to its suitable
potential window and significant solubility of complexes in this solvent.
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Table. 1 Abbreviations, IUPAC names and pKa values of the of the prepared
thiosemicarbazone Schiff bases*
Abbreviation
HAT
Name of compound
pKa value
11.29
1ꢀAcetophenonethiosemicarbazone
HAET
1ꢀAcetophenoneꢀ4ꢀethylthiosemicarbazone
1ꢀActophenoneꢀ4ꢀphenylthiosemicarbazone
1ꢀAcetophenoneꢀ4ꢀpꢀchlorophenyl thiosemicarbazone
9.90
HAPT
6.50
HApClPT
8.72
12.39
H2ST
1ꢀSalicylaldehydethiosemicarbazone
(10.05)
10.8 (6.1)
10.3
H2SET
H2SPT
1ꢀSalicylaldehydeꢀ4ꢀethylthiosemicarbazone
1ꢀSalicylaldehydeꢀ4ꢀphenylthiosemicarbazone
1ꢀSalicylaldehydeꢀ4ꢀpꢀchlorophenylthiosemicarbazone.
(7.85)
11.3 (8.3)
H2SpClPT
1ꢀ(2ꢀHydroxyꢀ4ꢀmethoxybenzophenone) 4ꢀphenylthioꢀsemiꢀ
carbazone.
11.5 (4.8)
H2HyMBPT
11.36
(9.3)
1ꢀ(2ꢀHydroxyꢀ4ꢀmethoxybenzophenone) 4ꢀpꢀchlorophenyl
thiosemicarbazone.
H2HyMBpClPT
11.20
(Not
HBT
1ꢀBenzophenonethiosimecarbazone.
available)
* pKa value of the ligands are taken from reference 14. The values of pK2 computed by the
least square method are given in parentheses.
R'
R
4
H
N
2
NHR''
2
NH
R
N
S
N
S
4
NHR''
R'
The abbreviations of R, R´ and R´´in the abbreviated Schiff bases are as follows:
HAT : R=CH3, R´,R´´=H ;
HAET: R=CH3, R´=H, R´´= ꢀCH2CH3
HApClPT: R=CH3, R´=H, R´´= 4ꢀClC6H4
H2SET : R=H, R´=OH, R´´= CH2CH3
H2SpClPT: R=H, R´=OH, R´´= 4ꢀClC6H4
HAPT: R=CH3, R´=H, R´´= ꢀC6H5;
H2ST : R=H, R´=OH, R´´=H;
H2SPT : R=H, R´=OH, R´´= Ph;
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HBT : R, R´, R´´= H;
H2HyMBPT: R=OH, R´= ꢀOCH3, R´´= ꢀC6H5)
H2HyMBpClPT (R=OH, R´=OCH3, R´´= 4 ClC6H4)
Fig. 1 Chemical structures of the prepared thiosemicarbazone Schiff bases.
2.2. Physical measurements
Cyclic voltammograms were recorded at room temperature (298 K) on a Potentiostat Wave
Generator (Oxford electrode) equipped with Phillips PM 8043 XꢀY recorder. A three
compartment electrochemical cell configuration consists of Pt wires of 0.5 mm diameter as
working and counter electrodes and Ag/AgCl as a reference electrode was used. Anodic (ipa)
and cathodic (ipc) peak currents were evaluated [28, 29]. The number of electrons transferred
in each electrode processes was determined using controlled potential electrolysis (CPC) in
−
the same supporting electrolyte (DMFꢀTBA+BF4 ) at Pt sheet working electrode.
2.3. General preparation of thiosemicarbazone Schiff bases and their complexes (Co, Ni, Cu)
The Schiff bases have been prepared according to the method reported earlier [19ꢀ21] as
follows: An accurate weight of thiosemcarbazide (9.1 g, 100 mmol) in ethanol was mixed
with equimolar amounts of acetonphenone, benzophenone and salicylaldehyde individually
followed by a reflux for 2ꢀ3 h with addition of glacial acetic acid. The precipitates were
filtered, recrystallized from ethanol and dried with anhydrous CaCl2 in desiccators. The
stoichiometries and mass data of the prepared Schiff bases are in agreement with the
proposed structures (Fig. 1) [19]. Similarly the complexes of cobalt(II), nickel(II) and
copper(II) were prepared by adding stoichiometric amounts of ligands and metal acetates
followed by reflux for 4ꢀ6 h. The formed precipitates were filtered and washed with hot
water, ethanol and dried with anhydrous CaCl2 in desiccators and characterized as reported
earlier [19ꢀ21].
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The purity of the Schiff bases and their Co2+, Ni2+ and Cu2+ complexes was first tested by
TLC using different solvent systems e.g. dioxaneꢀwater, acetoneꢀwater and petroleum ether
at various compositions. Good results were achieved with petroleum ether where well –
separated spots were noticed. The sample solution was preconcentrated from 1.0 mL to 10
µL fractions for injection in gas chamber of GCꢀMS. The products were identified by GCꢀ
MS. Total ion chromatogram of the electrolyzed species showed the corresponding
significant peak fragments which are most likely corresponding to the proposed chemical
structures. The Schiff bases and their complexes were fully characterized as reported earlier
[19ꢀ24]. The proposed structures were supported by molecular weight determination of the
Schiff bases and their complexes. The recorded mass spectra for most of the complexes and
the molecular ion peaks confirmed the proposed formulae.
3. Results and discussion
The electrochemical studies for a series of cobalt(II), nickel(II) and copper(II) [24] of
complexes thiosemicarbazone Schiff bases have been discussed to highlight the crucial
role of pKa, dn electrons of the metal ions and structural relationships [18,19].
Considerable advances have been made on studying the biological activity of the
thiosemicarbazone Schiff base complexes of the title metal ions. [19, 20]. Thus, most of
these complexes have witnessed incredible growth in medicinal chemistry owing to its
coordination number, geometries and kinetic properties [17, 20]. These complexes were
studied for their redox behavior and it confirmed that they can be used efficiently for
treatment of tuberculosis (TB) [22].
3.1. Redox behavior of cobalt (II) complexes
The electrochemical parameters of mono and biꢀnuclear cobalt (II) complexes in DMFꢀ
ꢀ
TBA+.BF4 are given in Table 2. Representative cyclic voltammograms of the binuclear
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complex [Co2 (SET)2].C2H5OH is also shown in Fig.2. In general, the CVs of the cobalt(II)
complexes reveal diffuse, irreversible and successive two one –electron reductions coupled
with two one –electron oxidation. The irreversible nature of the cathodic peaks was
confirmed by the large potentialꢀpotential difference (ꢁ=EpcꢀEpa ) between two counter peaks
or the absence of well defined peaks in the reverse scan. By increasing the potential scan
rates (υ) from 0.01 to 1 V sꢀ1, response were achieved in which the (ip,a/ip,c) ratio for the
observed couples (Co2+/Co3+ and Co2+/Co+) progressively decreased. Thus, the mass transfer
is limited and the species that initially formed in the electrode process may also react further
to give products that are not reoxidized at the same potential as the first formed species. At υ
> 1.0 V sꢀ1, the responses were typical of an uncomplicated oneꢀelectron charge transfer
whose nature appeared not typical reversible from the CVs explored at high sweep rate.
The CVs of the mononuclear cobalt(II) complexes: [Co(AET)2(H2O)2], [Co(APT)2(H2O) 2],
[Co(SPT)(H2O)], [Co(SpClPT)(H2O)2].H2O and Co(HyMBPT)(H2O)3].H2O displayed two
electrode couples. The cyclic voltammograms showed two cathodic peaks (Epc) in the range
ꢀ0.98 – 0.07 V (Epc1) and ꢀ0.21– 0.92 V (Epc2) coupled with two anodic peaks (Epa) in the
range ꢀ0.3 ꢀ0.14 (Epa1, couple I) and 0.04 – 1.02 V (Epa2, couple II), respectively. The ꢁEp
values (where ꢁ=EpcꢀEpa) of the couples were found higher than 60 mV except for
Co(HyMBPT)(H2O)3].H2O (Table 2) suggesting the irreversible nature of the electrode
couples. The cyclic voltammogram of the complex Co(HyMBPT)(H2O)3].H2O showed Epc =
ꢀ0.36V coupled with Epa= ꢀ0.30 V with ꢀEp ≅ 60 mV indicating the reversible nature of the
couple. By analogues with the cyclic voltammograms of other cobalt(II) complexes [20ꢀ23],
the first couple (couple I) was safely assigned to Co+/Co2+ couple whereas the second couple
(couple II) couple was assigned to Co2+/Co3+. In the same potential window (ꢀ1.0ꢀ1.0 V), the
CVs of the Schiff bases were also recorded to rule out the possible presence of redox peaks
of imine and thiocarbonyl groups present in the ligands. The CVs results revealed no Epa or
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Epc in the employed potential window at the used scan rate (50ꢀ100 mVsꢀ1) confirming the
observed electrode couples of cobalt (Co2+/Co3+ and Co2+/Co+).
¯
Fig. 2 Cyclic voltammogram of [Co2(SET)2]C2H5OH in TBA+ BF4 ꢀDMFꢀ at 100 mVsꢀ1 vs.
Ag/AgCl reference electrode.
Controlled potential coulometry (CPC) at Pt net electrode of the complex
[Co(HyMBPT)(H2O)3].H2O was recorded at the potential of the limiting current plateau of
the cathodic peak under N2 atmosphere. The number of electron transfer was calculated from
the charge – time curve using the following equation [30]:
QF = QT ꢀQB = n FVC
(1)
where Q = charge in coulomb, Q F = the faradic charge required for complete electrolysis of
the complex in solution, QT = the faradic charge required for complete electrolysis of the
test solution which was measured by extrapolating the linear of the curve to zero time, QB =
the faradic charge required for complete electrolysis of the supporting electrolyte only, V =
volume of the test solution in the cell in liter, C = the concentration of the solution, mol Lꢀ1
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and F = Faraday’s number (96485 C/equiv). At the potential of the limiting current plateau of
Epc of Co2+/Co+ couple of [Co (HyMBPT)ꢀ(H2O)3].H2O, the data confirmed one electron
transfer/one cobalt centre in the complex.
The linear dependence of Epc vs. scan rate (log υ) for the couple Co2+/Co+ of the complex of
[Co(AET)2(H2O)2] (Fig. 3) confirmed the irreversible nature of the reduction step. From the
slope of this plot, the product of the number (n) of electrons involved in the reduction step
and the corresponding charge transfer coefficient (α) can be determined. Assuming n=1, and
α value of 0.68 is obtained. The values of Epa and Epc of this couple (Co2+/Co+) were also
shifted to more positive and negative values, respectively adding further support to the
irreversible nature of the couple [29,31].
Fig. 3 Plot of Ep,c vs. log υ for the electrode couple Co2+/Co+ of the complex
[Co(AET)2(H2O)2], vs. Ag/AgCl electrode.
The peak current (ip,c) of the couple Co2+/Co+ is directly proportional to the square root of
the potential sweep (υ1/2) and the plot of ipc vs. υ was linear and passing through the origin of
coordinates.
At the temperature used and according to the CV theory [28, 29], this electrode process is
controlled by diffusion up to the polarization of sweep rate. Based on these results and the
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peak current of other cobalt(II) complexes [22,23], couple I was safely assigned to the
irreversible couple Co2+/Co+. The current function (ip,c/υ1/2) of Co2+/Co+ couple decreased
linearly on increasing concentration of the complex ion. On the other hand, similar trend was
also noticed on raising the sweep rate confirming an EC type mechanism in which an
irreversible first order reaction is proposed [31, 32].
The cyclic voltammogram of [Co(HyMBPT)(H2O)3].H2O showed another couple with Epa in
the range of 0.04ꢀ1.2 V coupled with an ill defined peak Epc in the range of ꢀ0.21 to +0.92 V
vs. Ag/AgCl electrode. CPC measurement at the potential of the limiting current plateau of
Ep,c of this couple showed an indication of one electron transfer/cobalt center. Thus, the
couple was safely assigned to the irreversible one electron oxidation (Co2+/Co3+). The value
of ꢀ
Ep of this couple was found in the range 60 ± 7 mV at υ <100mVsꢀ1 and it increased at υ
≥100 mVsꢀ1. On raising the sweep rate, the complex species that are initially formed during
the reduction process may be changed to some other forms (or decomposed to products) that
are not reꢀoxidized at the same potential [31ꢀ33]. The kinetics of follow up chemical
reactions at υ > 100 mVsꢀ1 may also lead to different products of electrogenerated Co3+
complex species that influence the peak current ratio [31, 32].
The cyclic voltammograms of the binuclear complexes [Co2(AT)3(OH)(H2O)2]2.5H2O,
[Co2(ApClPT)(OAc)(OH)2(H2O)]H2O, [Co2(ST)2(H2O)2]0.5H2O, [Co2(SET)2 ]C2.H5OH and
[Co2 (HyMBpClPT)(OH)2 (H2O)3] (Table 2) showed two successive two one ꢀ electron
reduction processes. In the reverse scan, two successive one electron oxidation couples were
easily observed. Based on the sequential one electron transfer for Co2+/Co+ and Co2+/ Co3+
couples of other similar binuclear cobalt(II) complexes [32ꢀ35], the observed trend in the
complex [Co2 (ST)2(H2O)2]0.5H2O can be assigned to the sequential one electron transfer
reactions as demonstrated in scheme I.
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Co(II)ꢀCo(II)
Co(II)ꢀCo(II)
Co(II)ꢀCo(I)
Co(II)ꢀCo(III)
Co(I)ꢀCo(I)
Co(III)ꢀCo(III)
Scheme I. Proposed sequential one electron transfer mechanism for the binuclear cobalt (II)
complexes.
The chemical structures of Co+ or Co3+ complex species, hence can easily be deduced by
adding and/ or withdrawing one electron in the form of H+ or Hꢀ to cobalt (II) in the complex
species (Fig. 4), respectively.
NHR
H2O
S
O
N
S
N
N
H3C
Co
N
S
NH2
H2N
Ni
Co
O
N
S
N
N
OH2
CH3
N
RHN
I
II
Fig.4. Proposed chemical structures of the complexes [Co2 (ST)2(H2O)2]½H2O, I and
[Ni(AET)2], II.
The values of Epc of the couple Co2+/Co+ of the binuclear cobalt(II) complexes were found
less negative than that of the mononuclear ones in agreement with data reported for other
cobalt(II) complexes [33, 34] (Table. 2). Thus, the ability of the mononuclear complex
species to accept electron is much easier compared to the binuclear ones and these Epc values
are in agreement with the data reported for other cobalt(II) complexes [33, 34]. The
constancy of the ip,c/ip,a ratios at the employed scan rate (υ =20ꢀ200 mV/s) confirmed the
stability of the single oneꢀelectron reduction of the couple Co2+/Co+ of the binuclear ones.
The variation of the current function (ip,c/υ1/2) of this couple decreased continuously with the
scan rate (>500.0 mVsꢀ1), revealing that, the reduction process favors ECE (chemical
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reaction coupled between two charge ꢀ transfer processes) type mechanism and adsorption of
the electroactive species is limited [27ꢀ29].
Table. 2 Electrochemical data (V) of cobalt (II) complexes in DMFꢀTBA+ BF4¯ vs. Ag/AgCl
reference electrode at 100 mVsꢀ1*
Electrode couple, I
Electrode couple, II
Complex
Epa
Epc
ꢁEp
1.08
0.10
Epa
Epc
ꢁEp
0.24
0.14
0.46
ꢀ0.17
ꢀ0.62
+0.07
0.60
0.84
0.36
0.70
[Co2(AT)3(OH)(H2O)]2.5H2O
[Co(AET)2(H2O)2]
[Co(APT)2(H2O)2]
ꢀ0.20
ꢀ1.27
ꢀ0.67
1.07
0.45
0.75
0.49
0.26
ꢀ0.22
ꢀ0.54
ꢀ0.26
0.62
1.02
0.10
0.63
1.38
0.72
0.88
0.80
0.92
0.04
0.57
0.34
0.64
0.80
ꢀ0.21
0.10
0.05
0.06
1.04
0.08
0.08
1.01
ꢀ0.60 ꢀ 0.06
[Co2(ApClPT)(OAc)(OH)2(H2O)]H2O
[Co2(ST)2(H2O)2]0.5H2O
0.33
0.07
1.13
ꢀ0.51
ꢀ0.50
ꢀ0.20
0.14
ꢀ0.98 ꢀ 0.48
[Co2(SET)2]C2H5OH
0.26
0.06
0.98
[Co(SPT)(H2O)]
ꢀ0.84
[Co(SpClPT)(H2O)2]H2O
[Co(HyMBPT)(H2O)3]H2O
ꢀ0.18
ꢀ0.96
0.78
0.88
0.03
0.85
ꢀ0.30
ꢀ0.38
+0.01
ꢀ0.36
ꢀ0.44ꢀ
0.05
0.06
0.06
0.06
0.04
0.54
0.86
ꢀ0.06
0.34
0.80
0.10
0.20
0.06
[Co2(HyMBpClPT)(OH)2(H2O)3]
*ꢁEp = Epc ꢀ Epa
The influence of varying concentrations of [Co2 (AT)3(OH)(H2O)2]2.5H2O (as a
representative example of cobalt(II) binuclear complexes) on the values of Epc and diffusion
coefficient (D) of the couple Co2+/Co was also studied. On raising the concentrations from
1.0×10ꢀ4 to 4.0×10ꢀ3 mole/dm3, the value of Epc was shifted cathodically adding further
support to the irreversible nature of the couple. The value of D of the couple was also
calculated using the equation [36, 37]:
ip,c= 2.72x 105n3/2 D1/2 Co υ 1/2
(2)
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where ip,c = cathodic peak current (A cmꢀ2), n=number of electrons involved in the
electrochemical step, υ is the scan rate (V/sꢀ1) and Co is the concentration of the depolizer
complex (mmole/dm3). On raising the concentration of the depolizer, the computed values of
D were found in the range of (0.27 ꢀ0.29) ×10ꢀ6 cm2 sꢀ1. These variations were not very
significant suggesting no aggregation behavior of the complex species in the solution [36ꢀ
38]. The trend of other cobalt(II) complexes may also be rationalized in a similar approach.
A comparison between mononuclear cobalt(II) mononuclear complexes [Co(AET)2(H2O)2],
[Co(APT)2(H2O)2], [Co(SpClPT)(H2O)2]H2O and [Co (SPT)(H2O)] is given in Table 2.The
comparison was based on taking two variables into consideration i.e the influence of the
nature of the bulky group and the influence of the position of the bulky group on both the
complexes. The data given in Table 2 revealed that, the electronꢀwithdrawing group (pꢀ
ClC6H4) makes the former complex ([Co(SpClPT)(H2O)2]H2O) more positively charged and
easy to be reduced (Table 2) more than the latter one ([Co (SPT)(H2O)]) containing electron
donating group (phenyl) (Table2). Hence, cobalt(I) species are stabilized by electronꢀ
withdrawing groups whereas cobalt(III) species are stabilized by electronꢀdonating groups.
The influence of the Schiff base pka, on the values of Epa and Epc of the couple Co2+/Co+ of a
series of selected cobalt(II) complexes with different substituent groups on the
thiosemicarbazone moiety N1H and/ or N4H was critically determined. The values of Epa and
Epc are not simply correlated to the pKa values (where Ka=acid dissociation constant) of the
ligands. The influence of the change of the coordination number of the ions upon the redox
potentials may account for this trend [39]. The Epa and Epc values of an electrochemically
irreversible system depends on the chemical kinetics involved in the electron transfer
process. Thus, the value of E1/2 i.e. (Epa+Epc)/2 has no thermodynamic meanings [27ꢀ29]. The
Epa and Epc values were not very sensitive to the change in the nucleophilic properties of the
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azomethine nitrogen atom of the thiosemicarbazone moiety N1H and/ or N4H substituent in
the ligand (Table 2). The substitution by the ethyl or phenyl group on N4H on
[Co(AET)2(H2O)2] and [Co(APT)2(H2O)2] complexes the makes both oxidation and
reduction processes more difficult than other complex species. This effect may be explained
by a steric effect of the ethyl or phenyl group at the reduction centre, which would hinder the
approach of the reduction centre to the surface of the Pt electrode [40]. The potential
generated by the electron density of the donor atoms on the antiꢀbonding d orbital is also
participated and responsible for the observed trend [33, 38]. Thus, cobalt chelates of ligands
of high electron density on their donor atoms showed lower Epa and Epc values compared to
other Schiff bases of lower pka.
3.2. Redox behavior of nickel (II) complexes
The cyclic voltammograms of the square planar nickel(II) thiosemicarbazones complexes in
the potential range of ꢀ1.2 to +1.2V vs. Ag/AgCl at 100 mV sꢀ1 displayed two electrode
couples. The electrochemical parameters are given in Table. 3 and representative cyclic
voltammogram of the complex [Ni(H2SpClPT) (OAc)2(H2O)] at 100 mV sꢀ1 scan rate is
shown in Fig. 5. CPC at potential lower than Ep,c (ꢀ0.45 to ꢀ1.25 V) revealed that, the species
are reduced via one electron/ nickel center. The one electron nature of this couple (Ni2+/Ni+)
was also established by comparison with other nickel (II) analogs [34].
Electrochemical data for Nickel (II) complexes of the Schiff bases H2ST, H2SET, H2SPT,
H2SpClPT, H2HyMBPT and H2HyMBpClPT (Table 3) showed similar trend. Changing the
scan rate from υ = 20 to 200 mV sꢀ1, showed an ill (not well resolved) defined cathodic peak
with Ep,c in the range ꢀ0.82 – to1.09 V coupled with one anodic peak (Epa) in the ꢀ0.04 – ꢀ
0.67 V with ∆Ep = 0.31 – 0.79 V. The values of Ep,,c and Ep,,a shifted cathodically and
anodically, respectively on increasing the potential sweep confirming the irreversible nature
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and occurrence of slow electrochemical reaction following the electrode process [41]. The
influence of the sweep rate on the anodic peak current of cyclic voltammogram of the
complex [Ni (HHyMBPT)(OH)(H2O)] was demonstrated in Fig. 5. In the cyclic
voltammogram the ipa increased linearly on raising the sweep rate (ν1/2) indicating that, the
electrochemical oxidation process is diffusion controlled electrochemical process [24].
ꢀ
Fig. 5 Cyclic voltammograms of the complex [Ni (HHyMBPT)(OH)(H2O)] inTBA+BF4 ꢀ
DMF at 100 mVsꢀ1 scan rate in the potential range ꢀ1.2 ꢀ +1.2 (A); 0.4ꢀ 1.0 (B) and ꢀ1.4ꢀ ꢀ0.4
V (C) vs. Ag/AgCl reference electrode at 298 K.
The one electron nature of Ni2+/Ni+ couple was also confirmed from the CPC data. The
couple with Epa (0.80 – 1.14 V) and Ep,c (0.51 – 0.83 V) with ∆Ep = 0.12 – 0.39 V vs.
Ag/AgCl was safely assigned to the irreversible Ni2+/Ni3+ (with E1/2 values from ꢀ0.48 to ꢀ
1.25 V) couple by comparison with similar nickel(II) analogous [34, 35]. The observed trend
of Ni2+/Ni+ in nickel(II) complexes can be explained by adding (or withdrawing) one
electron as shown for [Ni (HHyMBPT)(OH)(H2O)] in Fig. 6. The plot of Epc vs. log υ was
linear supporting the irreversible nature of the couples. CPC measurement at fixed potential
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corresponding to the second oxidation peak (Couple II) showed one electron transfer/one
nickel center corresponding to Ni2+/Ni3+ [34, 35].
Fig.6. The plot of ip,a vs. square root of the sweep rate (ν) of the couple Ni2+/Ni3+ of the
complex [Ni (HHyMBPT)(OH)(H2O)].
The irreversible nature of the observed electrode couples was further confirmed from the
dependency of the linear plot of the Epc1 and Epc2 potentials vs. log ν. The charge transfer
coefficient (α) and the product of the number (n) of electrons i.e. α nα of the redox species
involved in the reduction step can be computed from the slope of the linear plot of Ep,c vs. log
υ (Fig. 3)employing the following equation:
ꢀ
ꢀ
Epc / log ν = ꢀ 29.58 / α n
(3)
For the couple Co2+/Co+ and assuming n=1, the computed value of α from the slope of the
linear plot of Epc vs. log υ (Fig. 3) was found equal 0.59 confirming the irreversible nature of
the couple [42, 43]. On the other hand, the ∆Ep of the Ni2+/Ni3+couple also shifted towards
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more negative and also on raising the sweep rate (υ). This added further support to the
irreversible nature and slow chemical reaction [41]. Thus, it can be concluded that, the
species that initially formed may react further to give other Ni2+ complex products that are
not reꢀoxidized at the same potential as in the first formed species of Ni2+/Ni3+couple [24,
39,41].
The substituents on the thiosemicarbazone moiety (Table 3) have significant effect on the
values of Epa and Epc of the couples Ni+/Ni2+ and Ni2+/Ni3+. The methoxy group on the
complex [Ni(HHyMBPT)(OH)(H2O)] decreases the ability of Ni2+/Ni+ couple to be oxidized
whereas the pꢀClC6H4 group in the [Ni(H2SpClPT)(OAc)2(H2O)] (Table 3) increases the
ability of Ni3+/Ni2+ couple to be reduced. Similar behavior was also noticed on comparing
[Ni (HHyMBPT)(OH)(H2O)] with [Ni2(H2HyMBpClPT)(OAc)4(H2O)3] and [Ni
(H2SpClPT)ꢀ(OAc)2(H2O)] with [Ni(SET)(H2O)]1.5H2O.C2H5OH. The group pꢀClC6H4
stabilized nickel(I) species, while the NH2 or C2H5NH group favor oxidation to Ni3+ species.
The complex species containing the electron attracting group pꢀClC6H4 decreases the
electron density at the reduction center. Thus, it become more positively charged and more
easily to be reduced to nickel(I) species [37, 41]. On contrary, group e.g. NH2 or C2H5NH
makes the complexes less positively charged and more easily to be oxidized to Ni3+ species.
The Epa and Epc values of the couple Ni2+/Ni+ of the complexes were slightly sensitive to
substituent on N4H and ligand pka. The fact that, the change of the coordination number of
the metal ions upon the redox potentials may account for the observed trend [39]. The Epc
and Epa peaks are from different chemical species i.e. are not to the complexes of interest and
are undefined chemically as reported [39] and the chemical kinetics involved in the electron
transfer process may account for the trend observed. Electron withdrawing group (pꢀClC6H4)
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stabilizes Ni+ complexes, while electronꢀdonating group (NH2 or C2H5NH) favors oxidation
to Ni3+ species.
¯
Table. 3 Electrochemical data of nickel (II) complexes in TBA+BF4 ꢀDMF at 100 mVsꢀ1*
Electrode couple, I
Epa Epc ꢁEp
Electrode couple, II
Complex
Epa
Epc
ꢁEp
0.34
0.05
[Ni(AT)2]H2O
[Ni(AET)2]
ꢀ1.12 ꢀ1.39 0.27
ꢀ0.80 ꢀ1.20 0.40
1.28
1.19
0.94
1.14
[Ni2(APT)3(OH)(H2O)]H2O
[Ni(ApClPT)2]
ꢀ0.20 ꢀ0.78 0.58
ꢀ0.72 ꢀ1.08 0.36
ꢀ0.67 ꢀ1.08 0.41
ꢀ0.65 ꢀ1.07 0.42
ꢀ0.60 ꢀ1.08 0.48
ꢀ0.62 ꢀ1.09 0.47
ꢀ0.51 ꢀ0.82 0.31
0.86
1.20
0.87
0.80
1.12
0.96
1.14
0.95
0.74
0.78
0.98
0.75
0.51
0.83
0.73
0.83
0.56
0.62
0.08
0.22
0.12
0.29
0.29
0.23
0.31
0.39
0.12
[Ni(HST)(OAc)]
[Ni(SET)(H2O)]1.5H2O.C2H5OH
[Ni(HSPT)(OAc)(H2O)2]
[Ni(H2SpClPT)(OAc)2(H2O)]
[Ni(HHyMBPT)(OH)(H2O)]
[Ni2(H2HyMBpClPT)(OAc)4(H2O)3] ꢀ0.04 ꢀ0.83 0.79
[Ni(BT)(OAc)(H2O)2]H2O
*ꢁEp = Epc ꢀ Epa
ꢀ0.12 ꢀ0.78 0.66
The Epa and Epc of the couple Ni2+/Ni+ of the complex [Ni(AT)2]H2O and [Ni(ApClPT)2] are
more sensitive to the changes in the nucleophilic properties of the N1H moiety; where ꢀpka =
2.57 is associated with an increase of Epa and Epc of Ni+/Ni2+ couple. For Ni2+/Ni3+ couple of
[Ni (ApClPT)2] and [Ni(H2SpClPT)(OAc)2(H2O)] , the same trend was also noticed where,
when ꢀpka was 2.57, an increase in the Epc and Epa was achieved (Table 3) in agreement
with the results reported for Ru3+/Ru.2+ couple [43, 44]. The spherical potential generated by
the electron density of the donor atoms and the antiꢀbonding dꢀorbital’s may account for the
observed trend [33].Thus, complexes of ligands having high electron density on their donor
atoms (high pka) showed lower the Epc and Epa values than those of weaker bases. The plot of
the Hammett constants (σ) of the substituents in N1H or N4H moiety vs. Epc and Epa of the
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couple Ni2+/Ni+ or Ni2+/Ni3+ revealed evidence of the substituents in N1H or N4H moiety
on the values of Epa and Epc where the slope of the linear plot in most cases was > 0.1 [43].
3.3. Influence of ligand pka and dn electron on the Epc
The variations of the Epc values of the couples M2+/M+; M2+/M3+ (where M = Co, Ni or Cu)
[23] may also be correlated with the nature of orbital’s involved in the redox processes [27].
HOMO orbital is consisting predominately of dx2–y2 orbital while LUMO consisting of the
antiꢀbonding πꢀ orbital of the thiosemicarbazone moiety. Thus, the low values of Epc for
Ni2+/Ni+ couple and the high values for Ni2+/Ni3+(Table 3) couple compared to Co2+/Co+ and
Co2+/Co3+ couples (Table 2) and Cu2+/Cu+ and Cu2+/Cu3+ [27] of the same ligand can be
explained in terms of the higher energy of d x2 –y2 orbital in nickel (II) compared to cobalt
(II) and copper(II), respectively. These data are in good agreement with the data reported for
Pd2+ and Pt2+complexes [45,46].The different extent of the back bonding from d πꢀ orbital of
the metal to the anti bonding πꢀ orbital of the corresponding ligand may also account for the
observed behavior [46, 47].
The influence of pka of the ligands (Table 1) and dn electron of metal ions of t2g configuration
was also studied. The plots of dn electrons for Co2+, Ni2+ and Cu2+ complexes of the same
ligand and geometry versus Epa and Epc for the couple M+/M2+ are shown in Figs. 7 and 8.
Good correlation between Epa and Epc values of the complexes and dn electrons in metal ions
was noticed. The energy difference arising from the interaction between the nonꢀbonding dn
electrons of the metal ion and the lone pair electrons of the coordinating sites of the Schiff
base may account for the observed trend [38, 47]. This interaction raises the energy of t2g
metal HOMO electrons [48]. Based on the results given in Tables 2 and 3 and the data
reported for other copper (II) complexes of the same ligands [28, 29], it can be concluded
that, ligand of pka = 12.39 enforced metal ions to be oxidized in the following order:
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Co2+ > Cu2+ > Ni2+
Probably, the steric of electronꢀdonating group at the reduction center hindered the approach
of reduced centre to the electroactive surface [46, 47]. Thus, metal ions of high dn electrons
have Epc lower than metal ions of low dn electrons in the complexes i.e. the more the number
of d electrons the less reduction on the electrode surface. Ligand of pka = 10.8 also enforced
metal ions to be reduced in the order: Co2+ > Ni2+ > Cu2+. Thus, high value of Epc and Epa for
Co2+/Co3+ couple compared to Ni2+/Ni3+ or Cu2+/Cu3+ couples of the same ligand was
explained in terms of higher energy of d x2ꢀy2 orbital in Ni2+ or Cu2+ than in Co2+ [45, 47].
Thus, Co2+ ions became more positively charged and more difficult to be oxidized.
Fig. 7 Plots of Epa of the electrode couple M+/M2+ (M = Co, Ni or Cu) vs. dnꢀelectrons of
selected complexes.
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Fig. 8 Plots of Ep,c of the electrode couple M+/M2+ (M = Co, Ni or Cu) vs. dnꢀelectrons of
selected complexes
3.4. Energy levels approximation by cyclic voltammetry
Energy level characterization i.e HOMO/LUMO approximations were successfully
computed for some of the Co2+, Ni2+ and Cu2+ complexes. The optical band gap energy was
calculated from the electronic spectra of each complex. The cross point of absorption onset
line and corrected base line was used to calculate energy band gap [49]. The HOMO and
LUMO processes are calculated from the cyclic voltammogram from their respective
oxidation onset and reduction onset potentials. The equations including the value of
ferrocene ꢀ4.4 were used as following [50]:
E (HOMO) =ꢀe [Eoxonset + 4.4]
(4)
The optical energy band gap was calculated using the following equation from the longest
absorption wavelength λonset
Eg = 1242/λonset
:
(5)
The combination of these two methods would deduce the feasibility of donorꢀacceptor
system. The HOMO/LUMO energy levels and optical band gap energy are deduced using the
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equations 3ꢀ5 as summarized in Table 4. As shown in Table 4 the transition of an electron
from HOMO to LUMO upon photoꢀexcitation at LUMO of the acceptor would eventually be
collected at the respective electrode. This phenomenon is attributed to the fact that the
excitation binding energy is lower than the electron affinity of the acceptor and potential
difference between ionization potential of the donor. These characterizations would estimate
whether the materials are suitable for use in photovoltaic devices and solar cells.
Table. 4 Calculation of optical band gap energies and excitation binding energies from
electronic spectra and cyclic voltammetry
E0
onset vs.
Ag/AgCl
(V)
E0ox (V)
Ag/AgCl
(V)
HOMO E0red (V)
LUMO
level
(eV)
Optical
Energies
(Eg)
ox
Complex
level
(eV)
Ag/AgCl
(V)
[Co(APT)2(H2O)2]
ꢀ0.22
ꢀ0.20
ꢀ0.30
ꢀ0.60
ꢀ0.03
ꢀ0.05
ꢀ0.20
ꢀ0.52
ꢀ0.55
ꢀ0.43
4.37
4.35
4.2
ꢀ0.67
ꢀ0.26
ꢀ0.36
ꢀ1.08
ꢀ1.09
ꢀ0.6
2.54
2.75
2.55
1.33
1.72
2.3
1.83
1.6
[Co2(SET)2]C2H5OH
[Co(HyMBPT)(H2O)3]H2O
[Ni(HSPT)(OAc)(H2O)2]
1.65
2.55
2.13
1.67
3.88
3.85
3.97
[Ni(H2SpClPT)(OAc)2(H2O)] ꢀ0.62
[Cu(AT)(OH)(H2O)3]
ꢀ0.25
4. Conclusion
Substituents on the thiosemicarbazone moiety (N1H and/or N4H) have significant effect on
the redox properties of the redox couples (M2+/M+ and M2+/M3+). Co2+ or Ni2+ chelates
undergo single one electron reduction and oxidation to the corresponding M+ and M3+
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species, respectively. No significant variations in the D values on raising the concentrations
of the complex species in solution. The changes in E1/2 for the copper (II) complexes are
strongly related to changes in the nucleophilic properties of substituents on C=N1 and N4H
(Table 2, 3). Furthermore, the values of E1/2 for the couples CuI/CuII and CuII/CuIII suggest
that these couples are more sensitive to changes in the nucleophilic parameters of the N4H
substituents than the changes in C=N1 derivatives. The correlation between E1/2 of the
complexes and the pKa of the ligands involved has been attributed to the spherical potential
generated by the electron density of the donor atoms in the anti bonding d orbital’s as
reported by Lintvedt and Fenton [33]. Most of published Schiff base metal complexes of Co,
Ni and Cu suffer from many drawbacks e.g. no correlation on the influence of metal ion dn
electrons and pka of the Schiff base on the Epc of the redox couples, while in the present
manuscript a correlation between the redox characteristics of the complexes with differently
substituted of the thiosemicarbazone moiety N1H and/ or N4H was noticed. The redox
potentials are also not simply related to the pKa values of the ligands (pka). Excitation
energies i.e HOMO/LUMO and optical band gap energies were successfully computed for
some of the Co2+, Ni2+ and Cu2+ complexes. The difference between the potentials of the
redox couples and the absorption band energy for irreversible electron transfer processes is
most likely assigned to the thermodynamic and kinetics parameters. The work is continuing
for studying the spectroelectrochemical behavior of the various redox couples at optically
transparent thin layer electrochemical cell.
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