Kinetics and mechanism of the redox reaction of N,N′-phenylenebis-(salicylideneiminato)iron(III) with oxalic acid in mixed aqueous medium

Article abstract of DOI:10.1007/s11243-018-0291-8

The kinetics of electron transfer between N,N′-phenylenebis-(salicylideneiminato)iron(III), hereafter referred to as [Fe(Salphen)]⁺, and oxalic acid was studied in mixed aqueous medium (DMSO:H₂O; 1:4 v/v) under pseudo-first-order con

Full text of DOI:10.1007/s11243-018-0291-8

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0291-8
Kinetics and mechanism of the redox reaction of N,N′‑phenylenebis‑(s
alicylideneiminato)iron(III) with oxalic acid in mixed aqueous medium
1
1
1
2
I. Ibrahim · S. O. Idris · I. Abdulkadir · A. D. Onu
Received: 3 September 2018 / Accepted: 15 November 2018
©
Springer Nature Switzerland AG 2018
Abstract
The kinetics of electron transfer between N,N′-phenylenebis-(salicylideneiminato)iron(III), hereafter referred to as
+
[
Fe(Salphen)] , and oxalic acid was studied in mixed aqueous medium (DMSO:H O; 1:4 v/v) under pseudo-ꢀrst-order
2
2
−3
conditions at 26± 1 °C, I=0.2 coulomb mol dm (NaCl) and λ =435 nm. The reaction was found to be second order
max
overall and acid independent, and displayed zero Brønsted–Debye salt eꢁect. There was no evidence for the formation of an
intermediate complex or free radicals during the reaction. Overall, the kinetic data suggest an inner-sphere mechanism for
the reaction, which is ꢀrst order in both reactants. A plausible reaction mechanism is proposed.
Introduction
In spite of these and numerous other uses, the
+
redox reaction of the complex, [Fe(Salphen)] (sal-
Metallo-salen complexes have been extensively used as cata-
lysts for a broad range of transition-metal-catalyzed reac-
tions including epoxidation of oleꢀns, hydroxylation, lactide
polymerization and asymmetric ring opening of epoxides
phen=bis(salicylidene)phenylenediamine), with oxalic acid
has not yet been reported. The present study has therefore
been carried out to obtain kinetic data with a view to gaining
insight into the mechanism of this redox reaction.
[
1]. The oxidative nature of metallo-salens has also been
exploited for the development of novel chemical nucleases
2–6]. Various metallo-salen complexes are also capable of
[
Materials and methods
hydrolytic cleavage of DNA and RNA [7]. As DNA-inter-
acting molecules ꢀnd potential applications in anti-tumor
therapy, intense research eꢁorts are currently being invested
toward the development of novel DNA/RNA modiꢀers and
understanding their molecular mechanisms of action [8–10].
Oxalic acid is the simplest dicarboxylic acid, with the for-
mula HO CCO H. It is a much stronger acid than acetic acid
All the reagents used were of Analar grade. Reaction rates
were monitored by following the decrease in absorbance of
the reaction mixture at 435 nm on a CORNING colorimeter
253. Conductivity measurements were taken with a HANNA
HI 4321 conductivity meter. Oxalic acid (JHD) was used as
the reducing agent, while sodium chloride (M&B) was used
to maintain the ionic strength of the reaction medium.
Bis(salicylidene)phenylenediamine and the N,N′-phenyl
enebis(salicylideneiminato)iron(III) complex [Fe(Salphen)]
Cl were synthesized and characterized according to the
published procedures [16]. The structure of the complex is
shown in Scheme 1. The Schiꢁ base was prepared by reꢂux-
ing o-phenylenediamine (BDH, 1.71 g, 15.8 mmol) with
salicylaldehyde (Merck, 3.3 ml, 31.6 mmol) in methanol
2
2
and is also a strong reducing agent [11]. Its conjugate base,
2
−
the oxalate dianion (C O ), is a chelating agent for many
2
4
metals, for example platinum(II) in the drug oxaliplatin [12].
Oxalic acid and oxalates can be oxidized by permanganate
in an autocatalytic reaction [13]. Redox reactions involving
oxalic acid have been studied [11, 14, 15].
(
Merck, 30 ml) for 1 h. The precipitate was collected by
*
I. Ibrahim
ꢀ
ltration after cooling, washed with methanol and dried in
ibrahimmismaila@gmail.com
a desiccator: yield 4.29 g (86%).
1
2
Department of Chemistry, Ahmadu Bello University, Zaria,
Nigeria
The complex was prepared by stirring a mixture of the
Schiꢁ base (0.78 g, 2.5 mmol) with anhydrous ferric chlo-
ride (SureChem, 0.4 g, 2.5 mmol) in methanol (20 ml)
Department of Chemistry, Federal College of Education,
Zaria, Nigeria
Vol.:(0
1
123456789)
3

Transition Metal Chemistry
′
Scheme 1 Structure of N,N -
phenylenebis(salicylideneimin
ato)iron(III)
N
N
O
Fe
N
N
O
Fe
H
2
O
-
Cl
+
Cl
O
O
+
at 60 °C for 30 min, then keeping the mixture at room
temperature overnight to precipitate out the complex. The
product was recrystallized from methanol and dried in a
desiccator: yield 0.47 g (47%).
were determined at constant [Fe(Salphen) ], [H C O ] and
2 2 4
ionic strength.
Spectra of the reaction mixture were recorded after com-
mencement of the reaction and were compared with the
spectra of the complex alone over a wavelength range of
The molar conductivity of the complex was deter-
−
4
−3
mined in 2.0 × 10 mol dm (DMSO:H O; 1:4 v/v)
400–700 nm. A Michaelis–Menten-type plot of 1/k versus
2
1
2
−1
solution as 145 S cm mol with speciꢀc conductance
1/[H C O ] was also made. A test for free radicals was made
2
2
4
−
6
−1
of 29.0 × 10 S cm , consistent with a 1:1 electrolyte.
by the addition of acrylamide followed by excess methanol
+
+
The existence of [Fe(salen)] species in DMSO–H O (4:1
to partially reacted mixtures of [Fe(Salphen)] and H C O
2
2
2
4
v/v) and CH CN–H O (1:1 v/v) solvent systems has been
[22].
3
2
reported previously [17, 18]. Furthermore, Kurahashi and
co-workers reported that the ESI mass spectrum of H O
2
coordinated iron(III)salen perchlorate in solution gave a
Results and discussion
+
single signal corresponding to [Fe(salen)] with loss of
−
both H O and ClO [19].
From the stoichiometric studies, the mole ratio of the reac-
tion was found to be 2:3 (Fig. 1) and can therefore be repre-
sented by the equation:
2
4
The stoichiometry of the reaction was determined by
spectrometric titration using the mole ratio method. The
stoichiometry was evaluated from a plot of absorbance
against mole ratio [20]. The kinetic studies were carried
out under pseudo-ꢀrst-order conditions with [H C O ] in
ꢀ
ꢁ+
2 Fe(Salphen) + 3H C O + O
2
2
2
4
ꢀ
ꢁ
(2)
+
2
2
4
→ 2 Fe(Salphen) + 6CO + 2H O + 2H
2 2
+
2
excess over [Fe(Salphen) ] at 435 nm, I = 0.2 coulomb
−
3
mol dm , T = 26 ± 1 °C. Pseudo-ꢀrst-order rate plots of
log (A − A ) versus time were drawn (where A and A
A stoichiometry of 1:4 has been reported in the reaction
between chromic and oxalic acids [11].
t
∞
∞
t
are the absorbance at the end of the reaction and at time
In the kinetic analysis, a plot of log(A − A ) against time
t
∞
t), and from the slopes of the plots, the pseudo-ꢀrst-order
t gave a straight line graph, suggesting that the reaction is
+
rate constants (k ) were determined. The second-order rate
ꢀrst order with respect to [Fe(Salphen) ] (Fig. 2). The order
1
constants (k ) were obtained from Eq. 1.
of the reaction with respect to [H C O ] was determined
2
2
2
4
ꢀ
ꢁ
by plotting log k against log [H C O ]. The slope of the
1 2 2 4
k = k ∕ H C O
(1)
The effect of [H ] on the reaction rate was inves-
2
1
2
2
4
resulting straight line was 1.04 (Fig. 3). The value of the
+
+
−5
tigated by varying the [H ] between 1.0 × 10 and
−
4
−3
+
1
.0 × 10 mol dm (using HCl), while [Fe(Salphen) ]
0.4
0.35
0.3
−
4
−3
and [H C O ] were kept constant at 2.0 × 10 mol dm
2
2
4
−
3
−3
and 6.0 × 10 mol dm , respectively, at 26 ± 1 °C and
2
−3
0.25
I = 0.2 coulomb mol dm [21]. The effect of vary-
ing the ionic strength of the reaction medium on the
rate of the reaction was investigated in the range of
0
.2
.15
.1
0
0
2
−3
0
.18–0.40 coulomb mol dm , while the concentrations
0.05
0
of the reactants were kept constant at 26 ± 1 °C. The
eꢁects of added cation and anion were investigated for
0
0.5
1
1.5
2
2.5
3
3.5
4
+
Mole Ratio ([FeSalphen] : H C O )
2
2
4
−
3
−3
2+
−
[
X] = 1.0–6.0 × 10 mol dm ([X] = Mg or AcO ) at
+
constant [Fe(Salphen) ], [H C O ] and ionic strength. The
2
2
4
Fig. 1 Plot of Absorbance versus mole ratio for the redox reaction of
inꢂuence of temperature on the reaction rates was studied
+
+
−4
−3
[
Fe(Salphen)] and H C O at [Fe(Salphen) ]=2.0×10 mol dm ,
2 2 4
2 −3 °
in the range of 298–313 K, and thermodynamic parameters
µ=0.2 coulomb mol dm , T=26± 1 C and λ =435 nm
max
1
3

Transition Metal Chemistry
ꢀ
me, s
Table 1 Pseudo-ꢀrst-order
and
second-order
rate
con-
0
+
stants for the reaction of [Fe(Salphen)] with H C O at
2
2
4
0
100
200
300
400
500
600
700
-
-
-
-
0.2
0.4
0.6
0.8
+
−4
−3
°
[
Fe(Salphen) ]=2.0×10
mol dm
,
T=26± 1
C
and
λ
=435 nm
max
3
2
10 k (s−1)
3
3
1
0 [H C O ]
10 µ, coulomb
10 k (dm
2
2
4
1
2
−
3
−3
−1 −1
(
mol dm
)
(mol dm
)
mol
s )
-
1
-
-
-
1.2
1.4
1.6
4
5
6
7
8.0
9.0
11.0
6
6
.0
.0
.0
.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.8
2.0
2.4
2.8
3.2
3.6
4.0
2.86
3.64
4.26
5.07
5.96
6.79
8.06
4.28
4.26
4.29
4.27
4.30
4.29
4.26
7.15
7.28
7.10
7.24
7.45
7.54
7.33
7.13
7.10
7.15
7.12
7.17
7.15
7.10
Fig. 2 Typical pseudo-ꢀrst-order plot for the redox reaction of
+
+
−4
−3
−3
[
Fe(Salphen)] and H C O at [Fe(Salphen) ]=2.0×10 mol dm
,
,
2
2
4
−
3
−3
2
H C O =6.0×10
mol dm
,
µ=0.2 coulomb mol dm
2
2
4
°
T=26± 1 C and λ =435 nm
max
.0
.0
log [H C O ]
6.0
2
2
4
-
1.9
6
6
6
6
.0
.0
.0
.0
-
2.4
-2.35
-2.3
-2.25
-2.2
-2.15
-2.1
-2.05
-2
-2
-
-
-
-
-
-
2.1
2.2
2.3
2.4
2.5
2.6
Table 2 Eꢁect of acid concentration on the rate of reaction of
+
+
−4
[
Fe(Salphen)]
with H C O
at [Fe(Salphen) ]=2.0×10
2
2
4
−
3
3
−3
−3
µ=0.2 coulomb2
mol dm
,
H C O =6.0×10
mol dm
,
2
2
4
−
°
mol dm , T=26± 1 C and λ =435 nm
max
Fig. 3 Plot of log k versus log [H C O ] for the redox reaction of
1
2
2
4
+
+
−4
−3
10 [H ] (mol dm−3)
5
+
10 k1 (s−1)
3
[
Fe(Salphen)] and H C O at [Fe(Salphen) ]=2.0×10 mol dm
,
10 k2
2
2
4
2
−3
°
(dm mol s−1
3
−1
µ=0.2 coulomb mol dm , T=26± 1 C and λ =435 nm
)
max
0
.0
4.27
4.29
4.28
4.26
4.29
4.27
4.28
7.12
7.15
7.13
7.10
7.15
7.12
7.13
second-order rate constant k was fairly constant for diꢁerent
1.0
2.0
4.0
6
8
2
[
H C O ] and ionic strengths (Table 1). The rate equation
2 2 4
for the reaction can be represented by Eq. 3.
ꢁꢀ
ꢀ
ꢁ
ꢀ
ꢁ
.0
.0
+
d Fe(Salphen) ∕dt = k Fe(Salphen) H C O
+
−
(3)
2
2
2
4
−
1
3
−1 −1.
where k =7.29± 0.157×10 dm mol
s
2
10.0
The rate of reaction was found to be independent of
+
[
H ], as shown in Table 2. This observation suggests that
the undissociated oxalic acid HO CCO H is involved in the
2
2
a Michaelis–Menten-type plot of 1/k versus 1/[H C O ] was
1 2 2 4
reaction [11, 14]. The results in Table 1 show that variations
linear with a positive intercept, suggesting the participation
of an intermediate complex (Fig. 4) and a possible inner-
sphere mechanism.
in the ionic strength of the reaction medium had no eꢁect
on the rate. A plot of log k against √I gave a slope of zero,
2
suggesting a negligible Brønsted–Debye salt eꢁect [23]. This
implies that the reaction proceeds via an interaction between
uncharged forms of the reactants [24].
Addition of acrylamide to the partially reacted solution to
serve as a radical scavenger in the presence of large excess
of methanol did not produce a gelatinous precipitate. This
indicates that the involvement of free radicals in the reaction
is unlikely.
The reaction rate was also unaꢁected by the presence of
2
+
−
added ions (Mg and AcO , Table 3). This result suggests
that the reaction follows an inner-sphere mechanism.
We next considered whether an intermediate com-
The results of temperature dependence experiments are
ǂ
presented in Table 4. A large negative value of ∆S indicates
plex is involved in the reaction. There was no shift in λ
max
that the species in the activated complex are more ordered,
which is evidence for an associative mechanism and an
inner-sphere mechanism of electron transfer.
(
435 nm) when the spectrum of the reaction mixture was
+
compared with that of [Fe(Salphen) ]. The lack of spec-
trophotometric evidence for the formation of intermediate
complex may suggest an outer sphere mechanism. However,
We have analyzed the reaction products as follows. On
2
+
completion of the reaction, the presence of Fe as the
1
3

Transition Metal Chemistry
+
Table 3 Eꢁect of added ions on the rate of reaction of [Fe(Salphen)]
presence of CO , the oxidation product of H C O , was con-
2
2
2
4
+
−4
−3
with H C O
at [Fe(Salphen) ]=2.0×10
mol dm
,
,
2
2
4
ꢀrmed using lime water (Ca(OH) ) which turned milky [26].
−
3
−3
2
−3
2
H C O =6.0×10
mol dm
,
µ=0.2 coulomb mol dm
2
2
4
°
On the basis of the results obtained from this investi-
gation, the following reaction scheme is proposed for this
reaction:
T=26± 1 C and λ =435 nm
max
3
10 k (s−1)
3
3
[
X]
10 [X]
10 k (dm
1
2
(
mol dm−3)
mol
−1
s )
−1
ꢀ
ꢁ+
Fe(Salphen) + H C O ⟶ Fe(Salphen) ∕∕H C O
k
ꢀ
ꢁ
Mg2+
3
+
0.0
4.26
4.30
4.29
4.29
4.26
4.27
4.28
4.28
4.26
4.29
4.27
4.30
4.29
4.26
7.10
7.17
7.15
7.15
7.10
7.12
7.14
7.13
7.10
7.15
7.12
7.17
7.15
7.10
2
2
4
2
2
4
1.0
2.0
3.0
4.0
5.0
6.0
(
4)
ꢀ
ꢁ
k
ꢀ
ꢁ
4
Fe(Salphen) ∕∕H C O ⟶ Fe(Salphen) + 2CO + 2H + e
+
+
−
2
2
4
2
(
5)
ꢀ
ꢁ+
−
Fe(Salphen) + 2H C O + O + e
2
2
4
2
−
CH COO
0.0
(6)
3
k
ꢀ
ꢁ
5
1.0
2.0
3.0
4.0
6.0
8.0
⟶ Fe(Salphen) + 4CO + 2H O
2 2
For this mechanism,
ꢀ
ꢁ
+
Rate = k Fe(Salphen) ∕∕H C O
4
2
2
4
(7)
Applying the steady-state approximation for the interme-
+
diate complex [Fe(Salphen) //H C O ],
2
2
4
ꢀ
+ꢁꢀ
k Fe(Salphen) H C O − k−3 Fe(Salphen) ∕∕H C O
4
ꢁ
ꢀ
ꢁ
+
3
2
2
4
2
2
ꢀ
ꢁ = 0
k Fe(Salphen) ∕∕H C O
4 2 2 4
(8)
4
3
3
2
2
1
1
00
50
00
50
00
50
00
+
−
Then
ꢀ
ꢁ
ꢀ
+ꢁꢀ
ꢁ ꢂ
ꢃ
+
Fe(Salphen) ∕∕H C O = k Fe(Salphen) H C O ∕ k + k4
2
2
4
3
2
2
4
−3
(
9)
Substituting Eq. (9) into Eq. (7),
ꢁꢀ
ꢀ
ꢁ ꢂ
ꢃ
+
Rate = k k Fe(Salphen) H C O ∕ k + k4
(10)
5
0
3
4
2
2
4
−3
0
Equation (10) is analogous to Eq. (3), where k = k k /
2
3 4
5
0
100
150
200
3
250
300
−
1
3
−1 −1.
(
k−3 +k )=7.29± 0.157×10 dm mol
s
-1
4
1
/[H C O ], dm mol
2 2 4
Fig. 4 Michaelis-Menten plot for the redox reaction of
+
+
−4
−3
Conclusion
[
Fe(Salphen)] and H C O at [Fe(Salphen) ]=2.0×10 mol dm ,
2 2 4
2 −3 °
µ=0.2 coulomb mol dm , T=26± 1 C and λ =435 nm
max
The redox reaction between N,N′-phenylenebis-
(
salicylideneiminato)iron(III) and oxalic acid in mixed
reduction product of Fe3+ was conꢀrmed by mixing the
aqueous medium (DMSO:H O; 1:4) showed a stoichi-
2
reaction solution with KMnO solution. The formation of a
ometry of 2:3. The reaction is second order overall. The
4
2
+
brown precipitate indicated the presence of Fe [25]. The
rate of the reaction is acid independent and displayed zero
Table 4 Temperature
dependence of the rate
constants and activation
103 k (s−1)
10 k (dm mol s−1
3
−1
T (K)
)
1
2
2
3
99
03
4.29
7.15
7.57
8.23
9.08
parameters for the reaction of
4.46
4.94
5.45
8.23
+
[
Fe(Salphen)] with H C O
2
2
4
+
−4
−3
2
307
311
at [Fe(Salphen) ]=2.0×10
−
3
mol dm , H C O =6.0×10
2
2
4
−
3
mol dm , µ=0.2 coulomb
3
∆
15
10.15
−
3
mol dm and λ =435 nm
max
ǂ
−1
∆S =− 198.756 J mol K−1
ǂ
−1
ǂ
−1
H =+14.693 kJ mol
∆G =+74.121 kJ mol
at 299 K
E =+17.224 kJ mol−
1
a
1
3

Transition Metal Chemistry
Brønsted–Debye salt eꢁect. Kinetic investigations showed
evidence for the formation of an intermediate complex.
Based on these observations, an inner-sphere mechanism
is proposed as the most plausible mechanistic pathway for
this reaction.
13. Kovacs KA, Grof P, Burai L, Riedel M (2004) Revising the
mechanism of the permanganate/oxalate reaction. J Phys Chem
A 108(50):11026–11031
1
4. Bakore GV, Jian CL (1969) Chromic acid oxidation of oxalic acid:
kinetic investigation of the uncatalyzed oxidation of oxalic acid
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1
5. Subba-Rao PV, Krishna-Rao GSR, Ramakrishna K, Murthy PSN
(
1991) Kinetics of chromic acid–oxalic acid reaction catalyzed
by manganese(II)—kinetic analysis of the consecutive pathway.
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