Synthesis and characterization of cobalt(III) complex of 1,5-bis(2-hydroxybenzamido)-3-aza pentane and its interaction with glutathione: A kinetic and spectrophotometric study

Article abstract of DOI:10.14233/ajchem.2019.22092

A new octahedral cobalt(III) complex of 1,5-bis(2-hydroxybenzamido)-3-aza pentane was synthesized and characterized by elemental analysis, UV-visible, IR, mass spectrometry and thermal analysis methods. Oxidative behaviour of the complex towards the physiologically abundant tripeptide, glutathione(GSH) was studied by UV-visible and FT-NMR techniques. Kinetics of electron transfer between the complex and GSH was studied at 25.0 ≤ t/°C ≤ 45.0, 0.025 ≤ [H⁺]/mol dm^-3 ≤ 0.20 and I = 1.0 mol dm^-3 (NaClO4). NMR spectra of the reaction mixture were taken as a function of time. Changes observed in the NMR-spectrum during the period of scanning are the initial disappearance of the fine structure and subsequent broadening of the signals of GSH protons. Peaks at (δ, ppm): 2.93 and 2.98, characteristics of Cys-CH2 δ protons in GSH get disappear slowly and new peaks develop at (δ, ppm): 3.33 and 3.38, corresponding to resonating frequencies of Cys-CH2 δ protons in GSSG. Likewise, the peak at (δ, ppm):2.93, characteristic of Cys-CH2 δ proton in GSH is replaced by the resurgence of two peaks at (δ, ppm): 4.3 and 4.1 ppm, commensurate with the resonating frequencies of same set of protons as that of GSH but in a dimeric environment. The time dependent NMR simulation of the reaction reveals that GSH reduces the Co^III moiety successfully, while it gets converted into the dimer, GSSG. The relatively fast transformation of GSH to GSSG as evident from both electronic spectra and NMR studies together with the activation parameters obtained from the temperature dependence of reaction rate suggest that there is no involvement of any bridging group for electron transfer and the redox process may be taking place via outer sphere mechanism.

Full text of DOI:10.14233/ajchem.2019.22092

Asian Journal of Chemistry; Vol. 31, No. 9 (2019), 2022-2028
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2019.22092
Synthesis and Characterization of Cobalt(III) Complex of 1,5-bis(2-Hydroxybenzamido)-3-aza
pentane and its Interaction with Glutathione: A Kinetic and Spectrophotometric Study
1
1,*
2
ARAVIND RUDRARAPU , GOURI SANKHAR BRAHMA and SUPRAVA NAYAK
1
2
Department of Chemistry, Faculty of Science and Technology, The ICFAI Foundation for Higher Education, Hyderabad-501203, India
Department of Chemistry, Government College, Sundergarh-770002, India
*
Corresponding author: E-mail: gsb_ad@ifheindia.org
Received: 22 March 2019; Accepted: 22 April 2019;
Published online: 31 July 2019;
AJC-19492
A new octahedral cobalt(III) complex of 1,5-bis(2-hydroxybenzamido)-3-aza pentane was synthesized and characterized by elemental
analysis, UV-visible, IR, mass spectrometry and thermal analysis methods. Oxidative behaviour of the complex towards the physiologically
abundant tripeptide, glutathione(GSH) was studied by UV-visible and FT-NMR techniques. Kinetics of electron transfer between the
+
-3
-3
complex and GSH was studied at 25.0 ≤ t/°C ≤ 45.0, 0.025 ≤ [H ]/mol dm ≤ 0.20 and I = 1.0 mol dm (NaClO ). NMR spectra of the
4
reaction mixture were taken as a function of time. Changes observed in the NMR-spectrum during the period of scanning are the initial
disappearance of the fine structure and subsequent broadening of the signals of GSH protons. Peaks at (δ, ppm): 2.93 and 2.98, characteristics
of Cys-CH
frequencies of Cys-CH
2
δ protons in GSH get disappear slowly and new peaks develop at (δ, ppm): 3.33 and 3.38, corresponding to resonating
2
δ protons in GSSG. Likewise, the peak at (δ, ppm):2.93, characteristic of Cys-CH δ proton in GSH is replaced
2
by the resurgence of two peaks at (δ, ppm): 4.3 and 4.1 ppm, commensurate with the resonating frequencies of same set of protons as that
of GSH but in a dimeric environment. The time dependent NMR simulation of the reaction reveals that GSH reduces the Co moiety
III
successfully, while it gets converted into the dimer, GSSG. The relatively fast transformation of GSH to GSSG as evident from both
electronic spectra and NMR studies together with the activation parameters obtained from the temperature dependence of reaction rate
suggest that there is no involvement of any bridging group for electron transfer and the redox process may be taking place via outer sphere
mechanism.
Keywords: Glutathione, Redox activity, Kinetics, Co(III) complex, Phenol-amide-amine coordination, NMR.
In recent times, cobalt complexes are mainly in the focal
point due to their role in biological domain [22] and also in
the point of interest in experimental cancer therapy research
because of their capacity to redox-dependent targeting the
malignant tissue of solid tumors [23]. These complexes are also
well suited for use as bio-reductive prodrugs, having a very
inert oxidized cobalt(III) state and very labile reduced cobalt(II)
state [24].
Of late, a series of substituted bis(thiosemicarbazone)
cobalt(III) complexes were synthesized [25] and characterized
by mass spectrometry, X-ray crystallography and multinuclear
NMR spectroscopy. These compounds were proved to be
potential anticancer agents in normoxic and hypoxic conditions
[26] and believed to act as chaperones for the cytotoxic com-
pounds [27].
INTRODUCTION
Synthesis and isolation of multidentate carboxamido
ligand containing N
4
O donor atoms in the form of amide,
2
amine and phenolate groups have remained a long standing
challenge [1-6] owing to their varied functionality during
complexation. These ligands have proved to be excellent comp-
III/II
III/II
lexing agents for several metal ions such as Mn [6], Fe
III
III/II
II
[
7-10], Cr [11], Co [12-14] and Cu [15], etc. It has been
shown that the amide ligands have allowed the preparation of
complexes with unusual geometries, oxidation and spin states
16,17]; and also these ligands improve the resistance of the
complex to undergo an oxidative degradation [12-14], which
makes them superior over other ligands. There has been a great
surge of interest in the recent years on the study of the amide
complexes of metal ions due to their ability to model active
sites present in some metalloproteins [18-21].
[
Glutathione reduced (GSH), a biological reducing agent
in thiol dependent enzymatic reactions has recently been
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Vol. 31, No. 9 (2019)
Synthesis and Characterization of Cobalt(III) Complex of 1,5-bis(2-Hydroxybenzamido)-3-aza pentane 2023
reported to have anticancer activity [28] analogous to the
Co(III) macrocyclic complexes. Novi [29] administered GSH
to rats bearing aflatoxin B1 liver tumors and observed falling
off of tumor growth leading to extended survival of the animals.
Further, it is believed that as GSH is fairly abundant in cells
throughout the body, the latter is instrumental in delayed
reoxidation of Co(III) drugs resulting in hyper activation of
same in hypoxic tissues. Several such Co(III) drugs have been
demonstrated to be reduced to Co(II) within the hypoxic tumor
tissue [30].As Co(II) complexes are more labile, the cytotoxic
ligands may be released from the “metal chaperon” and exert
their anticancer activity [31], thus inhibiting the proliferation
of malignant tumor cells.
kinetics measurements. The temperature of the cell compart-
ment was maintained (± 0.1 °C) by an electronically controlled
thermostatting device. The pH measurements were made using
an Elico digital pH meter model LI 120 equipped with a glass–
Ag/AgCl, Cl (3 mol dm NaCl) electrode, CL 51. NBS buffers
of pH 4.01, 6.86 and 9.20 were used to check the performance
of the pH meter.
Kinetic measurements: The rate measurements were made
spectrophotometrically under pseudo-first-order conditions at
λ = 390 nm, where the cobalt(III) complex is the main absorbing
species. The decrease in absorbance with time of the reaction
mixture was monitored against the corresponding solvent blank
–
-3
for at least 3 h.
III
In continuation of our earlier work on the redox reaction
of GSH and cobalt(III) complex of hexadentate phenol-amide
Synthesis of complex, [Co (HL)H O]: 50 % (v/v) EtOH
2
+ H
and LiOH·H
to make the deprotonation of phenolic OH groups. Then, the
metal salt CoCl ·6H O (2.38 g, 10 mmol) in solution phase is
2
O solution of ligand, LH
3
(structure 1) (3.4 g, 10 mmol)
ligand [32], we report herein the synthesis and characterization
2
O (0.5 g, 21 mmol) were mixed in 1:2 proportion
III
of a Co(III) complex, [Co (HL)(H
2
O)] with a pentadentate
L) and
ligand 1,5-bis(2-hydroxybenzamido)-3-aza pentane (H
3
2
2
its redox activity with GSH. The objective behind this study is
to explain the ligand substitution dynamics of [Co (HL)] with
added drop-wise to the ligand solution and was stirred to obtain
a uniform solution. The solution was allowed to undergo aerial
oxidation for 24 h. It was filtered off and the green colour
residue was collected and recrystallized from 50 % (v/v) EtOH
III
GSH, to distinguish the reactive species, characterize reaction
products and to establish the rate law and construe a plausible
mechanism. Further, since NMR is an ideal tool for studying
amino acids properties, an attempt has been made to use the
time dependent FT-NMR technique to establish the redox beha-
viour exhibited by the Co(III)/GSH redox couple in a chemi-
cally and magnetically discriminated proton environment.
+ H
the complex, but the yield was good i.e. 2.18 g (83 %). Anal.
calcd. for C18 Co (%): C, 51.67; H, 5.02; N, 10.05.
2
O solutions. Unfortunately, we did not get the crystal for
H
21
N O
3 5
Found: C, 50.938; H, 5.4; N, 10.15. The complex (structure 2)
is sparingly soluble in water but soluble in methanol.
EXPERIMENTAL
H
The pentadentate ligand, H
method as discussed in our earlier paper [4] and characterized
by H NMR and IR spectroscopic methods. Glutathione
3
L is synthesized by a known
O
O
H
N
1
reduced, GSH of Sigma-Aldrich, 99 % was used as received.
All other chemicals used in this experiment were of AnalaR
NH
grade. The NaClO
was prepared using standardized solutions of NaOH and
HClO . The stock solution of NaClO was adjusted to pH 6.0
4
solution used for ionic strength adjustments
4
4
HN
+
and estimated for Na by a combined ion exchange alkali-
C
+
metric procedure using Dowex 50W X8 resin in H form. All
-3
rate measurements were made at 1.0 mol dm ionic strength.
The GSH solution was always prepared fresh just before
starting the kinetic runs. The stock solution of complex was
cosseted from light and stored at low temperature (10 °C) in a
refrigerator.
O
O
H
3
Structure 1: Structure of ligand (LH )
H O
2
The mass spectrometric measurements were done by a
Varian Inc., USA; model 410-proster binary LC with IT PDA
Detectors. Direct infusion mass with ESI and APCI negative
and positive mode ionization, mass ranging from 50 to 2000
m/e. TG-DTA measurements of the sample was carried out by
Perkin Elmer USAA model Diamond TG/DTA in the tempe-
rature range 30-900 °C with a heating rate 10 °C/min. The
elemental analysis (C, H and N) was carried out with a Thermo
Finnigan, Italy, model Flash EA 1112 series elemental analyser.
Proton NMR time dependent spectra was taken in AV400-
Bruker 400 MHz High Resolution Multinuclear FT-NMR spec-
trometer.
NH
O
HN
C
Co
O
HN
O
C
O
A LABINDIA double beam spectrophotometer with a pair
of 10 mm matched Teflon stoppered quartz cells was used for
III
Structure 2: Structure of the complex [Co (HL)H O]
2

2
024 Rudrarapu et al.
RESULTS AND DISCUSSION
Asian J. Chem.
2
2
1
1
5
0
5
0
5
0
5
III
The synthesized [Co (HL)H O] is an octahedral cobalt
2
complex of a pentadentate ligand coordinating through two
phenolate, two amidate and one amine functional groups. The
sixth site of this complex is occupied by a H O molecule. This
tentative structure (Scheme-I) is proposed by taking into
account the following analytical evidences.
2
0
200
400
600
800
1000
-
III
Thermal analysis (TG-DTA) of [Co (HL)]·H O: The
2
-
10
15
20
III
TG curve of [Co (HL)(H O)], (Fig. 1a) shows three successive
2
-
degradation steps at 30-230, 230-500, 500-800 °C. Initial weight
loss of 11.59 % in the temperature range 30-230 °C is attributed
to two very close decomposition steps. The first 7 % weight loss
below 150 °C may be due to removal of lattice water. The next
-
Temperature (°C)
III
Fig. 1b. DTA curve of complex [Co (HL)(H
2
O)] at a heating rate 10 °C/min
in the range 30 to 900 °C
4
% weight loss is above 150 °C indicating the removal of one
III
coordinated water molecule from the complex. An observed
weight loss of 50.177 % and 13.058 % in the 2 step and 3
step, respectively of the thermogram is concomitant to the
metal coordinated ligand decomposition.
Proton NMR: Proton NMR spectrum of [Co (HL)(H
2
O)]
nd
rd
is shown in Fig. 2. Coordination of amide-amine-phenolate
moiety to Co(III) generates a set of three signals for aromatic
proton in the 6.5-8.0 ppm range. The assignment of these
aromatic proton signals reveals that Ha and Hc protons resonate
at 7.70 and 7.40 ppm, respectively. These two aromatic proton
signals are more deshielded as their position is ortho and para
with respect to electron withdrawing carboxamide group and
also meta to the electron donating phenolate group, whereas
the Hb and Hd protons, ortho and para to phenolate group
resonate at 6.89 ppm. The expected signals for aliphatic protons
are observed as an overlapping signal in the range 3.2 to 3.7
120
100
80
60
40
20
0
ppm. The non-equivalent CH
2
fragments protons (He and Hf)
0
200
400
600
800
1000
Temperature (°C)
Ha Hc H ,Hb
He
Aliphatic protons
Co
d
III
Fig. 1a. Thermogram of complex [Co (HL)(H
2
O)] at a heating rate 10 °C/
min in the range 30 to 800 °C
Aromatic protons
H
H
d
The ligand part of the complex is stable up to 240 °C and
decomposes in two stages up to 800 °C leaving cobalt oxide
and some carbon as residue. Ligand decomposition is fast in the
first stages with a weight loss of 50.177 % (calculated 49.52 %),
assigned to the removal of non-coordinated hydrocarbon part
of the ligand, while the actually coordinated part decomposes
later in the second stage at ~ 700 °C. An exo-effect with the
maximum centred at about 313 °C, associated with the decompo-
sition of the complex connected with the ligand’s oxidation,
is apparent from the DTA curves shown in Fig. 1b. Similar
observations are reported by Vanco et al. [33].
H
e
O
C
NH
g
NH
h
H
b
H
f
a
O He
He
H
f
Hh
Hf
8
.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 ppm
1
III
Fig. 2. H NMR spectrum of [Co (HL)H
methanol + water mixture
2
O] in 50 %(v/v) deuterated
NH2
O
H
N
HO
OH
N
H
NH2
O
O
O
CH2
S
O
H
2
×
HO
N
OH
[Ox]
Red]
N
H
S
[
O
O
CH2
SH
O
O
CH2
O
O
H
N
HO
N
H
OH
O
NH2
Scheme-I

Vol. 31, No. 9 (2019)
Synthesis and Characterization of Cobalt(III) Complex of 1,5-bis(2-Hydroxybenzamido)-3-aza pentane 2025
give rise to multiplets at ~ 3.2 ppm because of their diastereo-
tropic nature. The broad signal observed at around 3.65 ppm
is due to amine proton (Hh). Rest half of the ligand coordinated
to Co(III) in the complex resonate at same frequencies as that of
its symmetrical half, thus coalescing with the afore mentioned
peaks.
ions. The peak at 326 is also assigned to the ligand after its
inequitable fragmentation (LH –OH: m/z = 326.39). Thus, the
4
III
mass spectral data are consistent with reduction of the Co
II
centre by GSH resulting in a Co complex and subsequently
dissociate under acidic conditions to the free ligand and its
protonated form.
ESI-Mass: The mass spectrum of the complex was
scanned in the range: 100 ≤ m/z ≤ 2000 (Fig. 3a and 3b) at
FT-IR spectra: [Co(HL)(H O)] displays bands at 1639,
2
-1
1541 and 1487 cm due to νstr(C=O), νstr(C=N) of deprotonated
collision energy (CE) ≈ 3eV. The base peak appear at m/z
amide and νstr(C=C) of aromatic group, respectively. A strong
+
-1
3
44.16 corresponds to protonated ligand {H
3
L+ H } (f.w. of
band at 1639 cm is characteristic of νstr(C=O) of coordinated
H
5
3
L: 343). The peak at m/z 400 is for Co(HL) complex (f.w.
9 + 341). Peak at m/z 164 is showing the fragment ion of ligand
i.e. C (OH)(CONH)CH CH having f.w. 164. The spectrum
shows a low intense peak at m/z 743, which is attributed to
Co + HL + H L}. The peaks observed at m/z 562 and 605 are
attributed to {Co(HL)+C (O)(CONH)CH CH } and {Co(HL)
(O)(CONH)CH CH NHCH CH } moieties, respectively.
primary amide. νstr(O-H) of phenolic group and the same of
coordinated water together coalesce with the νstr(N-H) of sec-
-1
6
H
4
2
2
amide to result in a broad peak observed at 3442.04 cm . The
-1
-1
weak band at 2377 cm together with 2317 cm are attributed
to intra-molecular H-bonds of ligated water molecules and
band at 1541, partially overlapped with νstr(C=O) of (O=C-
{
3
6
H
4
2
2
+
C
6
H
4
2
2
2
2
NH ) owing to the bending mode of water molecules [34].
2
-1
-1
This fragmentation pattern followed by the complex is consis-
tent with its structure.
The bands at 1346 cm (w) and 1305 cm are ascribed to the
O-H bending and νstr(C=O) of co-ordinated phenolic groups
-1
and those at 833 and 750 cm are assigned to the (1,2)di-
substituted aromatic moiety. The bands at 1091, 1026, 914
3
44.16087
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
-
1
cm (CH
2
- rocking) and prominent peaks at 1487 and 1383
-1
cm due to CH deformation vibration are concomitant with
2
the unsymmetrical array of diene spacer unit [32].
UV-visible spectra: The electronic spectra of Co(III)
complex (Fig. 4) show two absorption bands each observed at
–
1
1
24,813 and 33,783 cm correspond to two transitions A →
1
g
3
1
1
T
1g (F) (ν
1
) and A1g (F) → T2g (P) (ν
of these two energies, {ν /ν } = 1.36, can be fitted to the modi-
2
), respectively. The ratio
2
1
fied Tanabe-Sugano diagram [35] and the corresponding Dq/
B value of 39 is obtained. The value of E/B′ for the lowest
energy transition can be read from the diagram as 37 which
605.19253
62.66653
7
43.22478
164.07077
400.07038
5
800.14231
986.29491
-1
lead to B′ value of 670 cm .
1
00
200
300
400
500
600
m/z
700
800
900 1000
III
Fig. 3a. ESI-Mass spectrum of the [Co (HL)(H
2
O)]
3.000
2
.248
2
1
1
1
1
1
4000000
3000000
2000000
1000000
0000000
1.495
0.742
-0.010
1
3
-10 at 15 min interval
9000000
8000000
7000000
6000000
5000000
4000000
3000000
2000000
1000000
200
300
400
Wavelength (nm)
500
600
O)]
III
Fig. 4. Repetitive UV-visible spectral scan of the mixture of [Co (HL)(H
2
-4
-3
and GSH. [Complex] = 2.5 × 10 M(1); [GSH] = 2.5 × 10 M (2)
and mixture after 30 s of mixing (3) (3-10) at 15 min interval of
time, at I = 1.0 M (NaClO ); pH ≈ 5.70 and temp. 25 °C
4
5
0
100 150 200 250 300 350 400 450 500 550 600 650 700 750
The apparent value of B′ in the complex is much smaller
m/z
3+
-1
Fig. 3b. ESI-Mass spectrum of 1:4 ratio mixture of Co(III)HL and GSH
than Co ion (1100 cm ) [36] is obviously due to the nephe-
lauxtic effect and is attributed to delocalization of the metal
ESI-MS spectra: The ESI-MS spectra (Fig. 3b) recorded
electrons over molecular orbitals that encompass both the metal
for the 1:4 ratio mixture of Co(III)HL and GSH gives intense
peak at m/z 308 indicating the presence of unreacted GSH in
protonated form. The appearance of peak at m/z = 613. (obs.)
against 612.57 (calc.) corresponds to the oxidized species
and the ligand.
III
Co (L)H O-GSH interaction study
2
UV-visible study: The Co(III) complex was mixed with
GSH in a 1:2 proportion and kept overnight at room tempe-
rature. Co(II) at the end of the reaction was estimated spectro-
photometrically following the method of Kitson [37]. To the
II
GSSG. The observed peak at 481 corresponds to a Co complex
after fragmentation of the ligand [Co + (GSH) + C
6
(OH)CN:
m/z = 481.22]. The most prominent peak at 344 is assigned to
+
given amount of reaction mixture, HClO
4
, NH NCS(50 %)
4
the mono-protonated ligand, LH
4
(m/z = 344). Other peaks at
and CH COCH were added. The absorbance of the deep blue
3
3
m/z 344, 308 etc. may be attributed to ligand peak and its fragment

2
026 Rudrarapu et al.
Asian J. Chem.
2
-
solution due to the formation of Co(CNS)
6
4
was measured at
disappearance of Cys-CH α proton peak intensity at ~ 4.5 ppm
with simultaneous appearance of new peaks at 4.3 and 4.1
ppm, respectively, also supports the conversion of GSH to
GSSG. The upfield shifting of the peak at 4.0 ppm towards
the 3.77 ppm peak and finally merging with each other supports
the transformation of GSH to GSSG. This transformation
followed by merger of the peaks is a consequence of dimeri-
zation and associated conformational changes that brings about
-1
-1
25 nm (εmax = 1023 mol L cm ). Concentration of Co(II)
was estimated by comparing with a calibration curve construc-
ted with standard Co(II) solution under identical conditions.
1
III
H NMR study: Co (HL)H
complex of a pentadentate ligand, H
complex is occupied by a H O molecule. In the other hand,
2
O is an octahedral cobalt
3
L. The sixth site of this
2
GSH is a tripeptide consisting of glutamic acid, cysteine and
glycine amino acids. It has several potential binding sites, i.e.
glutamyl (Glu) amino and carboxylate groups, glycinyl (Gly)
carboxylate, cysteine (Cys) thiol group and two amide groups.
The structure of glutathione is such that all its potential binding
sites cannot be simultaneously coordinated to the same metal
ion.Among these, thiol-S atom is the more prominating binding
the chemical shift of Gly-CH (a proton) and Glu-CH(f proton)
2
to converge and settle at 3.77 ppm. The relatively fast transfor-
mation of GSH to GSSG as evident from both electronic
spectra and NMR studies suggest that there is no involvement
of any bridging group for electron transfer and hence the redox
process may be taking place via outer sphere mechanism.
The observed shift of peak position of cysteine protons
side as it is a good donor atom.
III
The interaction between Co (HL)H
2
O and GSH was studied
by H NMR spectroscopy taking repetitive scan at different
intervals of time. Free GSH has six characteristic protons that
show NMR peak at chemical shift (a) 2.19-Glu CH
β, (d) 3.85- Gly CH
.0- Glu CH α and (f) 4.59-Cys CH α ppm [38]. The proton
NMR of GSH in presence of Co(III) complex at different time
interval from 1 min to 2 h are superposed and shown in Fig. 5
and the expanded spectrum taken at 1 min and 2 h after mixing
are shown in Fig. 5a and 5b, respectively. Changes observed
in the spectrum during the period of scanning are the initial
disappearance of the fine structure and subsequent broadening
of the signals of GSH. Reaction of GSH with Co(III)HL complex
led to the appearance of new peaks at 3.33 and 3.38 ppm (Cys-
i.e. Cys-CHα peak from 4.5 → 4.3 ppm and Cys-CH β peak
2
1
from 2.98→3.33 ppm is consistent with direct thiolate ligation
to the cobalt centre and after binding of GSH through S atom
the cysteine protons are conformationally adjacent to the cobalt
ion appeared to be strongly affected [40,41]. Similar observa-
tion also reported by Sun et al. [42] in their study of interaction
of Sb(III)tartarate with glutathione. As expected, during the
reaction, Co(III) complex was reduced by GSH to form a Co(II)
complex and a substantial amount of the total GSH was oxidized
2
β, (b)
α, (e)
2
4
.58- Glu CH
2
γ, (c) 2.96- Cys CH
2
2
1
to form GSSG as judged by H NMR spectroscopy. This transfor-
mation was further evidenced by the electronic spectroscopy
discussed earlier.
Kinetics of redox reaction: Oxidation of GSH leads to
formation of dimer GSSG [21].
CH
2
β) (Fig. 5a), while the same resonate at 2.93 and 2.98 ppm
+
–
2
GSH → GSSG + 2H + 2 e
(1)
for free GSH. This corresponds to formation of the oxidized
form of glutathione, GSSG [38,39]. The complex Co (HL)H O
also resonates at around 3.30 ppm and due to this, there is some
overlapping of its peak with the doublet of GSSG at 3.33 and
III
Estimation of unreacted GSH in the mixture suggested that
2
III
3+
2
mol of GSH consumed 2 mol of complex, [Co (HL)(H
such that
2[Co (HL)(H
2
O)]
III
3+
3
.38 ppm.
Another important change to be considered in the
2
O)] + 2GSH →
II
2+
2[Co (HL)(H
2
O)] + 2GSSG
(2)
(3)
spectrum is the reduction in the intensities and/or areas of
peaks of free GSH with time interval from 1 min to 2 h. This
observation is obviously due to decrease in the concentration
of free GSH during its reaction with complex.A slow but steady
The rate of the reaction is given by
a
b
Rate = k[GSH] [complex]
where k is the rate constant and a, b are the order of the reaction
with respect to the concentrations of GSH and complex, respec-
tively. It was found that the rate is dependent on the first power
of both oxidant and the complex, i.e. a ≈ b =1, in agreement
with previous study [43].
Since Co(III) complexes are all one-electron oxidants, the
oxidation of GSH would give the radical intermediate and the
following mechanism is proposed.
3.3 ppm GSSH peak
III
d+e
Co (HL)+GSH
b
a
c
2 h
f
30 min
–
+
GSH
GS + H
–
•
–
15 min
GS → GS + e
(4)
•
2
GS → GSSG
where the second step is the rate determining step. Under
pseudo-first order conditions, in which [GSH] >> [complex],
the concentration of GSH is practically constant throughout
the reaction.
5
min
b
c
d
e
a
f
1 min
For a first order dependence of reaction on [complex], the
observed absorbance time data pairs were fit to the equation:
8
.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5ppm
1
III
Fig. 5. Repetitive H NMR scan of 1:6 molar ratio mixture of [Co (HL)(H
2
O)]
A
t
= (A
o
– A
∞
) exp (– kobst) + A
∞
(5)
and GSH solution after 1 m, 5 m, 15 m, 30 m and 2 h of mixing

Vol. 31, No. 9 (2019)
Synthesis and Characterization of Cobalt(III) Complex of 1,5-bis(2-Hydroxybenzamido)-3-aza pentane 2027
where,A
o
,A
t
andA
∞
are the initial absorbance of the complex,
This enunciates the fact that the electron transfer between the
substrate and the reductant does not necessarily involve forma-
tion of any precursor complex to proceed via an inner sphere
mechanism. The structure of the complex is such that its
absorbance of reaction mixture at time “t” and final absorbance
of the reaction mixture (at time, t = ∞), respectively.
Therefore, the reaction rate is given by
“
close” or crowded octahedral geometry around the metal ion
Rate ≈ - d[complex]/dt = kobs[complex]
(6)
prohibits change in the coordination sphere for the formation
of precursor associative complex, compelling the electron
transfer process to proceed through an outer sphere mechanism
where, kobs is the observed rate of the reaction, given by
red
red
k
obs = k
0
+ k
1
[GSH]
T
(7)
[
45].
Conclusion
An octahedral cobalt complex of a pentadentate ligand
The rate data in Table-1 have been fitted to eqn. 7. It is
red
observed that k
0
is statistically insignificant, indicating
absence of formation of precursor complex. Outer sphere asso-
ciation complex formation is also not evident from the UV-
visible spectrum, due to non-appearance of any new band.
coordinating through two phenolate, two amidate and one
amine functional groups is synthesized and characterized by
different analytical, thermal and spectrometric techniques.
Redox reaction of the complex with tripeptide glutathione (GSH)
was studied as a function of time both by conventional UV-
visible and NMR technique. The time dependent FT-NMR
study reveals that the peaks of free GSH disappear and new
peaks of GSSG start appear supporting the fact that the former
undergoes oxidative dimerization to result the latter. Rate of
dimerization of GSH is found to be rapid and hence an outer-
TABLE-1
RATE DATA FOR THE REDUCTION REACTION
4
4
4
4
1
0 k
(
10 k
10 k
10 k
obs
obs
obs
obs
[
HClO4]
[GSH]
(mol
dm )
-1
-1
-1
-1
s )
(s )
(s )
(s )
(
mol
-3
-3
(25.0 ±
0
(35.0 ±
0.1 °C)
(40.0 ±
0.1 °C)
(45.0 ±
0.1 °C)
dm )
.1 °C)
0.16
0.35
0.61
0.72
0.86
0.21
0.39
0.57
0.70
0.83
0.18
0.38
0.53
0.67
0.82
0.14
0.33
0.56
0.64
0.79
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.025
.025
.025
.025
.025
.050
.050
.050
.050
.050
.100
.100
.100
.100
.100
.200
.200
.200
.200
.200
0.005
0.010
0.015
0.020
0.025
0.005
0.010
0.015
0.020
0.025
0.005
0.010
0.015
0.020
0.025
0.005
0.010
0.015
0.020
0.22
0.48
0.82
0.98
1.16
0.28
0.52
0.76
0.94
1.12
0.24
0.51
0.72
0.91
1.11
0.19
0.44
0.76
0.87
1.07
0.33
0.74
1.26
1.50
1.79
0.43
0.81
1.17
1.45
1.73
0.37
0.79
1.10
1.40
1.71
0.29
0.68
1.16
1.34
1.64
0.55
1.20
2.06
2.45
2.92
0.69
1.32
1.91
2.36
2.82
0.61
1.29
1.80
2.29
2.79
0.47
1.11
1.90
2.18
2.68
III
sphere electron transfer within the Co /GSH redox couple may
be predicted. The activation parameters obtained from the tempe-
rature dependence of reaction rates further corroborates the
outer sphere mode of electron transfer as evidenced by the
NMR technique.
ACKNOWLEDGEMENTS
One of the authors (AR), Research Scholar in Department
of Chemistry, FST, IFHE is thankful to the University for
necessary financial support.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests
regarding the publication of this article.
0.025
III
–4
-3
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CONSTANT AND ACTIVATION PARAMETERS
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