Kinetics and mechanistic study of the reduction of Mn III by oxalate in Salophen scaffold: relevance to oxalate oxidase

Article abstract of DOI:10.1007/s12039-018-1514-4

Abstract: The trans-Mn ^III(Salophen)(OH2)2+ and bioxalate (HOX ^-) in aqueous medium equilibrate rapidly to trans-Mn ^III(Salophen)(OH ₂) (HOX) followed by the acid dissociation equilibrium to the (aqua) mono oxal

Full text of DOI:10.1007/s12039-018-1514-4

J. Chem. Sci. (2018) 130:123
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1514-4
REGULAR ARTICLE
III
Kinetics and mechanistic study of the reduction of Mn by oxalate
in Salophen scaffold: relevance to oxalate oxidase
a
a,∗
, V RAO MUNDLAPATIb,
PRIYAMBADA JENA , ACHYUTA N ACHARYA
c,∗
b,∗
ANADI C DASH and HIMANSU S BISWAL
Department of Chemistry, College of Engineering and Technology, Bhubaneswar 751 029, India
School of Chemical Sciences, National Institute of Science Education and Research (NISER),
a
b
Bhimpur-Padanpur PO, Via-Jatni, Khurda, Bhubaneswar 752 050, India
c
Department of Chemistry, Utkal University, Bhubaneswar 751 004, India
E-mail: aacharya@cet.edu.in; acdash41@gmail.com; himansu@niser.ac.in
MS received 16 May 2018; revised 24 June 2018; accepted 30 June 2018
III
+
−
Abstract. The trans-Mn (Salophen)(OH2)2 and bioxalate (HOX ) in aqueous medium equilibrate rapidly
III
to trans-Mn (Salophen)(OH2)(HOX) followed by the acid dissociation equilibrium to the (aqua) mono oxalato
complex. The slow redox reactions of trans-Mn (Salophen)(OH2)(HOX/OX)
III
0/−
−
2−
with H2OX, HOX ,OX
III
II
obey second order kinetics satisfying 2:1 stoichiometry ([Mn ]T/[OX]T = 2/1). The products are Mn and
CO2. Acrylamide monomer has no effect on the rate constant and the reaction does not induce its polymerization.
The rate and activation parameters for the various rate limiting paths are reported. The intramolecular
reduction of Mn by the coordinated HOX and OX in trans-Mn (Salophen)(OH2)(HOX/OX)
not be detected. Contrary to our expectation, it is observed that H2OX is a better reducing agent than
III
−
2−
III
0/−
could
−
III
2−
HOX for trans-Mn (Salophen)(OH2)(HOX), the slowest being the redox reaction of OX with trans-
III
−
Mn (Salophen)(OH2)(OX) . The molecular modelling by DFT depicts the structural trans effect in the oxalato
III
−
complexes, it being maximum for trans-Mn (Salophen)(OH2)(OX) . The observed sequence of the redox
activity of the oxalato complexes reflects the potential role of non-covalent interaction i.e. H-bonding, governing
III
the proton controlled electron transfer process (PCET). The Mn (Salophen/Salen) complexes may turn out to
be good substitute candidates for Oxalo Oxidase (OXO) enzyme in alleviating the oxalate overload in plants
and animal biochemistry.
III
Keywords. Kinetics; Mn (Salophen); oxalate; oxidation; oxalo oxidase.
1
. Introduction
This is generally referred to as stabilization of metal
oxidation state by ligand binding. Besides we found
that in both cases (i.e. glyoxylate and oxalate oxida-
tion) the acids are better reductants than their mono
anions(andalsodianionforoxalicacid). Thisisanunex-
pected result considering the electron donor properties
of the acid molecules and their conjugate bases. How-
ever, the involvement of proton mediation in the redox
process was believed to explain this reverse trend of
reactivities and proton-coupled electron transfer process
Previously we have reported the redox reactions of
III
+
1
2
Mn (Salen) by glyoxylate and oxalate. The redox
process involves fast pre-equilibrium of the glyoxalate
and oxalate species followed by slow electron trans-
III
fer reactions between Mn (Salen)X (X = glyoxylate,
−
2−
HOX , OX ) and the corresponding acids, and their
anions. It turned out that the binding of ligand X
III
III
to Mn centre considerably stabilized Mn centre
for intramolecular electron transfer in Mn (Salen)X.
III
(
PCET) was suggested. In such a situation, we believe
thatthereisthepossibilityofhydrogenbondingofoxalic
and glyoxylic acid molecules with the weakly basic
coordinated phenoxide of Mn (Salen) which favoured
the PCET process. Attempts were made to shed light
on these aspects by molecular modelling at DFT level
*
For correspondence
This work is dedicated to Late Professor Rabindra Kumar
Nanda (ex-Professor & Head of the Department of
Chemistry, Utkal University, Bhubaneswar).
III
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1514-4) contains
supplementary material, which is available to authorized users.
0123456789().: V,-vol

123 Page 2 of 13
J. Chem. Sci. (2018) 130:123
3
with some degree of success. But this warrants further evaporated to ∼ 40 cm and then left to cool to room
studies. What will be the consequence of such redox temperature. The resulting product (black crystals) was fil-
tered under suction using a glass sintered funnel and recrys-
reactions when the salen moiety is manipulated? One of
III
+
tallized from methanol–water (50% v/v). The product was
air-dried and stored over silica gel in a desiccator being
protected from light and moisture. It may be noted that
such studies is presented herein using Mn (Salophen)
ꢀ
(
H salophen = N,N -bis(salicylidene) 1,2-diamino
2
benzene) as oxidant and oxalate as a reductant. The
salophen motif provides rigidity to the structure of the
Mn complex maintaining co-planarity of the donor
7
Yuan et al., mentioned the composition of this complex as
III
Mn (Salophen)(OH2) ClO4.
III
sites. We focus our attention on the following aspects: Caution ! Perchlorate salt of the MnI I I complex may be
(
i) How do the structure and electronic property of the potentially hazardous. As such only small amounts may be
prepared and handled with care.
salen motif, due to the changes made in the central
part of the di-imine skeleton, influence complex form-
ing ability of Mn with oxalate species? (ii) how is
III
Anal. Calcd. For [Mn (Salophen)(OH2)2]ClO4,H2O (F. wt.
5
III
22.8): C, 45.9; N, 5.36; H, 3.85%. Found: C, 45.4; N,
.29; H, 3.30%. ESI-MS from aqueous medium (Figure S1a,
Supplementary Information) m/z ; 369.044 (obs), 369.288
the intramolecular electron transfer processes affected?
5
+
(
iii) how is the proton controlled electron transfer pro-
III
−
+
+
cess between Mn and HOX /H OX mediated by this (calcd. for Mn(Salophen)), (m/z )+1; 370.046 (obs); m/z :
2
structural variation of the manganese complex? More- 837.033 (obs.), 837.068 {calcd. for {[Mn(Salophen)OCl(O2)
over, PCET is a fundamental process involved in many OMn(Salophen)]-H}. ESI-MS from acetronitrile solution
+
(
Figure S1b, Supplementary Information) : m/z : 368.76,
chemical reactions and continues to be an active area of
research.
+
3
369.76 and 370.76 9 (obs.); m/z : 368.288, 369.288 (calcd.
for Mn(Salophen)-H, and Mn(Salophen), respectively);
There is also another dimension to our research, i.e.
to find a suitable substitute for the enzyme Oxalate Oxi-
dase (OXO; E.C.1.2.3.4) which is widespread in nature
+
+
(
m/z ) + 1: 370.766 (obs.); m/z : 782.679 (obs.), 782.611
−1
;
(
calcd for Mn(Salophen)-CN-Mn(Salophen)OH2). IR (cm
KBr, Figure S2, Supplementary Information): νO-H, 3500–
4
5
6
particularly in plant tissues, fungi and bacteria and
functions as a catalyst for oxidation of oxalate, the
metabolic by-product. The humans and other animals
being deprived of this enzyme in their bio-system are
3
(
376, 1607; νC-H, 2922.4; νC=N, 1579, ν ClO4, 1146–1088
III
multiplet), νMnIII-O, 542.9; UV-Vis for Mn (Salophen)
+
(
OH2)2 (in aqueous medium, pH 5.62 ± 0.05) λmax/nm
3
−1
−1
(
ε /dm mol cm ): 245 (38663 ± 942), 300sh (20320 ±
seriously at risk of oxalate overload due to metabolic 567), 340 (26593 ± 563), 425sh (9601 ± 265).
failure. This is one of the causes of kidney stones and The GR grade (E. Merck) oxalic acid (H2OX), perchloric
renal failure, ultimately resulting in a threat to life. acid and sodium hydroxide were used as received. Man-
III
ganese(III) acetate dihydrate (Sigma Aldrich) and all other
Can Mn (Salen/Salophen) be a suitable alternative for
reagents were of highest grade purity available. Freshly pre-
taking care of oxalate overload, be it in plants or ani-
pared doubly distilled water as received from a borosilicate
mal kingdom? Not only the study of the mechanism
glass distillation setup was used for solution preparation; the
of oxidation of oxalate is important, it is this aspect
second distillation was made through alkaline KMnO4. Ionic
which encouraged us to undertake the present study in
strength was adjusted with NaClO4. The stock solution of
sequence to our earlier ones.
1
NaClO4 was prepared and standardized as described earlier.
−
3
−3
The stock solution of the complex (5×10 mol dm , pH 5,
1
0% v/v MeOH/water) was protected from light and stored in
a refrigerator at ∼20 C when not in use. It was not allowed
◦
2
. Experimental
to age for more than 24 h.
2.1 Materials and reagents
The tetradentate Schiff base ligand H2salophen was
synthesized by condensing ortho-phenylenediamine with sal-
icylaldehyde in 1:2 mole proportion in methanol. The solid
product was purified by recrystallization from methanol, air
dried and stored over silica gel in a desiccator avoiding expo-
sure to moisture and light.
2
.2 Physical measurements
A Perkin Elmer Lambda 25 UV-visible spectrophotometer
with a matched pair of 10 mm quartz cells were used for
all absorbance measurements. The IR measurements were
made on a Perkin Elmer FTIR spectrometer Spectrum 2
using KBr pellet. ESR measurement was done on a JEOL
III
2
.1a [Mn (Salophen)(OH ) ]ClO , H O:
For the (Japan) JES-FA 200 ESR spectrometer at room temperature
2
7
2
4
2
III
synthesis of the Mn complex, manganese(III) acetate dihy- operating in X-band mode (8.75–9.65 GHz, power 1.08 W,
−
9
drate (4 mmol) and the Schiff base (H2salophen) (4 mmol) sensitivity 7 × 10 spins/0.1 mT, resolution 2.35 μT). The
3
were mixed in ∼250 cm methanol to which 6 mmol of mass spectrum was recorded on a Macromass Q-TOF ESI-
3
1
13
LiClO4 in 80 cm of water was added. The solution was MS mass spectrometer. H and C NMR were performed

J. Chem. Sci. (2018) 130:123
Page 3 of 13 123
on a Bruker Avance-III 400 MHz NMR Spectrometer in desired solution was quickly transferred in to one of the
DMSO-D6. The DMSO signal (due to DMSO-H6 present optical cells housed in the thermostatted cell compartment
in the solvent) was split (3.720 and 2.492 ppm) indicat- of the spectrophotometer after rinsing. The other cell con-
ing the fast exchange of the coordinated H2O by DMSO tained water as a reference. The absorbance change as a
III
+
III
+
(
Mn (OH2) +2DMSO ꢀ Mn (DMSO)2 +2H2O, fast). function of time was recorded at the preset wavelength (340
2
A very broad signal at 26.521 ppm may be ascribed to that or 370 nm) at which the complex was the dominant absorb-
of the water protons while the H signals of the aromatic ing species. The concentration of the complex was varied as
1
−5
−3
salophen moiety are displayed at −17.359, −21.940 and (3.0−5.0)×10 mol dm . The methanol percentage in the
−
27.485 ppm (Figure S3a, Supplementary Information) as actual reaction medium due to the addition of the complex
8
observed for this complex in CD3OD by Ciringh et al. The solution (prepared in aqueous-methanol medium) was always
weak and broad imine proton resonance reported by Ciringh kept constant (0.2% MeOH v/v). The pH of the reaction mix-
8
et al., (at −398 ppm) lies far beyond the accessible range ture was varied by self-buffering due to oxalic acid in the
III
probedinthepresentwork. ThechemicalshiftsofMn Schiff presence of dilute HClO4 or NaOH. All rate measurements
−3
base complexes have been attributed to the contact term which were made at constant ionic strength (I = 0.3 mol dm
)
occurs through phenoxide oxygen and the imine group.9 adjusted by NaClO4.
The signals (Figure S3b, Supplementary Information) for
The observed rate constants (kobs) were calculated by fit-
the aromatic carbons are observed at 209.678, 201.103 and ting absorbance (At) –time (t) data to equation (1) by a
1
97.986 ppm and the one at 164.858 ppm is attributed to the nonlinear least squares computer program; (A0−A∞), kobs
13
imine carbons. The DMSO ( C) signal is also split: 41.731 and A∞ were used as adjustable parameters.
and 91.978 ppm, the former intense peak is assigned to the
free DMSO while the later (weak) due to the DMSO coor-
dinated to Mn centre signifies the fast exchange of DMSO
At = (A0− A∞) exp(−kobst) + A∞
(1)
III
with the H2O coordinated to the metal centre. Fluorescence The initial absorbance (A ) was in the range of 0.6–0.9 and
0
measurements were made on a JASCO fluorimeter model FP-
A
was ≤ 0.25 and close to the fitted value; σ(k )/k
∞
obs
−
obs
−1
5
8
200 using Xe/D2 light sources; bandwidth was set at 5 nm was ≤ ± 5%. The very slow reactions (k ≤ 4 × 10
s
)
obs
1
◦
and scan speed was 100 nm/min. Cyclic voltammetry was were followed by initial rate method. The rate data at 25 C
◦
performed using a Bioanalytical System Inc., USA epsilon are collected in Table 1. The rate data at (30–40) C and
electrochemical analyzer in conventional three-electrode con- some representative kinetic plots are presented in the sup-
figuration with bright platinum acting as the working elec- porting information in the Table S1(a,b) and Figure S5(a–c),
−
−3
trode and Ag/AgCl, Cl (3.5 mol dm KCl) as the reference Supplementary Information, respectively, in Supplementary
0
◦
10
electrode (E AgCl/Ag = 0.205 V at 25 C). The voltam- Information.
mograms (Figure S4 a,b, Supplementary Information) were
recorded under a nitrogen atmosphere for an aqueous solution
of the diaqua complex with tetrabutylammonium perchlorate
serving as the supporting electrolyte. The pH measurements
were made with Systronics (India) pH meter model 335 using
a glass-Ag/AgCl, Cl (3 mol dm NaCl) electrode CL 51.
NBS buffers of pH 4.01, 6.86 and 9.20 prepared using KHph-
thalate, Na2HPO4/KH2PO4, and Na2B4O7·10H2O, respec-
tively, were used to calibrate the pH meter.11 The measured
2.4 Product analysis and stoichiometry
III
+
The reaction mixture ([Mn (salophen)(OH2)2 ] = 4 ×
−3
−3
10
, [H2OX]T = 0.04 mol dm , ∼10% MeOH/water) was
−
−3
◦
set aside at 35 C for calc. 10 t1/2 and treated with a slight
excessofCaCl2 solutionfollowedbyaqueousNH3. Thewhite
precipitate obtained was allowed to coagulate, collected by
filtration, air dried and treated with dilute HCl. A colorless
gas with effervescence (characteristic of CO2) evolved. Our
attempt for quantitative estimation of unreacted oxalate by
KMnO4 titration in the acid medium after quantitatively pre-
cipitating CaC2O4 from an aqueous NH3 solution of the spent
reaction mixture proved unsuccessful due to the presence of
salophen ligand and the product manganese species.
+
pH values of the reaction medium were converted to p[H ]
+
(
= −log[H ] by a calibration curve using dilute HClO4 solu-
−2
+
−3
−5
tions (2.0×10 ≤ [H ]/mol dm ≤ 1.0×10 ) adjusted
−3
12
to I = 0.3 mol dm (NaClO4).
2.3 Kinetics
Figure 1(a) and 1(b) depict the six-line ESR spectrum of
The rate measurements were made under pseudo-first order the reaction mixture characteristic of MnII 1,2,13 establishing
◦
conditions at 25.0 ≤ T/ C ≤ 40.0. The reaction mixture that the reaction we followed under different conditions of pH
3
III
in a 25.0 cm measuring flask containing all components is in fact reduction of Mn in salophen scaffold by oxalate
except the complex and the complex solution of known con- species. This is in line with what we reported earlier for the
−
3
III
2
centration (ionic strength adjusted to 0.3 mol dm ) were corresponding Mn (Salen) complex. Based on these obser-
equilibrated separately in a water thermostat maintained at vations we assign the stoichiometry of the overall reaction as
the desired temperature. After thermal equilibrium, a known follows:
3
volume of the complex solution (0.5 cm ) was transferred
in to the reaction mixture and volume was made up to the
mark with distilled water at the same temperature and the
III
+
−
+
2
Mn (Salophen)(OH2)2 + C2O4H + 3H
II
=
2Mn + 2H2Salophen + 2CO2 + 4H2O
(2)

123 Page 4 of 13
J. Chem. Sci. (2018) 130:123
III
+
◦
a
Table 1. Rate data for the reduction of Mn (salophen)(OH2)2 by oxalate species at 25.0 ± 0.1 C.
[
OX]T/mol dm− pH 10 kobs/s
3
3
−1
10 kcal/s
3
−1
[OX]T/mol dm−3 pH 10 kobs/s
3
−1
10 kcal/s
3
−1
0.007
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.03
0.04
0.05
0.05
0.05
0.06
0.07
0.08
0.08
0.08
0.01
0.03
0.05
0.07
0.08
0.08
1.79
1.73
1.56
1.43
1.25
0.98
0.80
0.69
0.59
1.59
1.67
1.84
1.89
1.92
1.96
2.04
2.07
2.35
2.75
1.49
1.38
1.35
1.28
3.77
3.79
3.85
3.82
3.80
3.28
4.24
4.23
4.21
4.22
4.26
4.47
0.03
0.06
0.054
0.06
0.049
0.038
0.05
0.04
0.03
0.20
0.17
0.17
0.16
0.15
0.15
0.16
0.16
0.15
0.14
0.42
0.69
0.96
0.75
0.35
0.44
0.81
1.06
0.79
1.54
0.02
0.13
0.36
0.81
1.06
0.52
0.032
0.063
0.065
0.067
0.067
0.063
0.056
0.051
0.046
0.225
0.221
0.212
0.209
0.208
0.206
0.203
0.202
0.197
0.20
0.05
0.05
0.05
0.05
0.06
0.07
0.08
0.09
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.05
0.10
0.15
0.20
0.25
0.10
0.01
0.02
0.02
0.03
0.04
0.08
0.08
0.02
0.04
0.06
0.08
0.08
-
1.11
0.91
0.77
0.67
1.30
1.26
1.22
1.19
1.16
1.26
1.32
1.39
1.47
1.55
1.80
1.98
1.95
1.93
1.85
1.83
1.82
2.29
3.66
3.34
3.75
3.73
3.74
3.62
3.38
4.26
4.29
4.25
4.05
4.41
-
0.83
0.72
0.76
0.67
1.31
2.00
2.49
2.88
3.36
2.96
2.94
2.97
2.96
2.92
2.52
2.28
1.28
3.17
4.59
5.58
7.49
2.20
0.030
0.08
0.050
0.16
0.25
1.18
1.43
0.07
0.21
0.52
1.43
0.55
-
1.14
1.12
1.06
0.99
1.47
1.88
2.32
2.78
3.26
3.16
3.08
3.00
2.89
2.79
2.50
2.33
0.894
2.37
4.13
5.92
7.75
2.15
0.063
0.201
0.177
0.316
0.46
1.16
1.31
0.133
0.327
0.54
0.87
0.69
-
0.461
0.761
1.09
1.11
0.601
0.742
0.85
1.03
1.04
1.36
0.048
0.23
0.45
0.67
0.76
0.67
a
−5
−3
−3
+
λ /nm = 340, [complex]T = (4−5) × 10 mol dm , I = 0.3 mol dm ; pH = −log[H ]. pK1 =
1
.01, pK2 = 3.60 for H2OX: Martell A E and Smith R M 1977 Critical Stability Constants (New York:
Plenum Press) vol. 3, p.92.
2
.5 Test of free radical
earlier.14,15 In order to account for the 2:1 stoichiometry,
−
−.
we conclude that the radical C2O . (≡ CO + CO2) must
4
2
III
+
A few runs were made in the presence of acrylamide undergo fast oxidation by available Mn (Salophen) prior to
monomer. The purpose was to test the presence of rad- being scavenged by acrylamide, the congener of its polymer.
ical through polymerization of acrylamide monomer and
examine its influence on the reaction rate. The reaction car-
◦
−5
ried out at 40 C (I = 0.3, [complex]T = 4.0 × 10
,
2.6 DFT calculations
−3
[
OX]T = 0.03 mol dm , λ /nm = 340) in the presence
−3
of [acrylamide] = 0, 0.01, 0.02, 0.03 mol dm
(pH = The TURBOMOLE 6.4 package was adopted to perform
3
−1
16,17
1
1
.47± 0.01) yielded 10 kobs/s as 1.15± 0.03, 1.14± 0.03, all calculations using density functional theory (DFT).
.04± 0.02, 1.17± 0.06, respectively, indicating thereby neg- The structure optimization was accomplished by the BP86
ligible influence of the monomer. Further, we did not observe functionalanddef2-TZVPPbasissetalongwiththeresolution
any polymerization of acrylamide. A similar observation was –of-identity (RI) approximation18–20 (RI-BP86/def2-TZVPP
made by us in our previous study on oxalate oxidation by in brief). Further details of the energy, graphical presentations
III
2
−.
Mn (Salen). The formation of the oxalate radical(C2O ) and computation of the bond parameters of the complexes are
4
and its decomposition product, CO2 have been reported mentioned in our earlier work.2
−.

J. Chem. Sci. (2018) 130:123
Page 5 of 13 123
0
0
0
0
.90
.86
.81
.77
0
10
20
30
40
1 -
0 /[H ]/mol dm
8
+
-3
−
8
+
3
−1
Figure 2. Absorbance (A) vs. (10 /[H ])/dm mol
III
+
plot, λ = 340 nm; [Mn (Salophen)(OH2)2 ]total = 4.0 ×
−5
−3
−3
◦
10
mol dm , I = 0.3 mol dm , 30.0 C.
evidenced from the loss of reversibility in the repeat
scans (Figure S4 a,b in Supplementary Information).
III
+
3
.3 pK of Mn (Salophen)(OH )
M
2 2
Figure 1. (a) Six-line ESR spectrum (X-band, room tem-
perature) of the product Manganese(II) in solution after
◦
III
+
The pK of the diaqua complex was determined at 30 C
M
completion of the reaction of Mn (Salophen)(OH2)2 with
◦
H2OX at pH ∼1.5, 30 C. Intensity vs H (Gauss) plot, by measuring its absorbance (340 nm) of a constant
−5
−3
g = 2.0, hyperfine const. A = 96 G. (b) Six-line ESR concentration (= 4.0 × 10 , I = 0.3 mol dm ) as a
spectrum (X-band) of the product Mn(II) species at 100 K;
function of pH (5.65 ≤ pH ≤ 9.61, morpholine/HClO
4
III
Mn (Salophen) was allowed to react with oxalate species
buffer). The nature of the plot of absorbance (A) vs.
◦
at pH = 3.6 (40 C). Intensity vs H (Gauss) plot, g = 2.0,
−8
+
1
0 /[H ] (Figure 2) is consistent with the acidic
hyperfine const. A = 85 G.
dissociation of only one coordinated water molecule
(Eq. 3).
3
. Results and Discussion
III
+
2
Mn (salophen)(OH )
2
KM
III
+
3
.1 Preliminary observations
−−−−−ꢀ Mn (salophen)(OH)(OH ) + H
−−−−−
(3)
ꢁ
2
◦
The time-dependent spectral scans at pH 1.46 (35 C) for
the reaction mixture display an isosbestic point around
4
Accordingly, the absorbance (A) – pH data were ana-
lyzed by Eq. (4)
65nm(FigureS6ainSupplementaryInformation). The
ꢀ
+
+
maximum around 340 nm and the shoulders at 300 and A = (A
+ A K
/[H ])/(1 + K
/[H ])
(4)
0
M
M
4
25 nm characteristic of the parent complex tends to
ꢀ
where, A , A and A denote the absorbance of the com-
0
disappear with time due to its reduction by oxalate
species. A similar observation was also made when the
reaction was conducted at pH 2 (Figure S6 b,c, Supple-
mentary Information).
plex when it exists only in the diaqua form, as an
equilibrium mixture of the di-aqua and aqua-hydroxo
species at the experimental pH and as aqua–hydroxo
ꢀ
species, respectively. A , (A K ) and K were varied
0
M
M
and A-pH data were fitted to equation (4) by a nonlinear
−
3
least squares program. We obtained K /mol dm
=
3
.2 Cyclic voltammetry
M
−8
(
1.6± 0.2)×10 (pK = 7.80± 0.05), A = 0.889 ±
M
−3
0
ꢀ
−8
The CV scan for the diaqua complex shows discernible 0.003, A K /mol dm = (1.21 ± 0.16) × 10 .
M
peaks at −134 mV (cathodic) and +402 mV (anodic).
The value of pKM was further checked by fluo-
The system is, however, a quasi-reversible one as rescence intensity (F) measurements of the diaqua

123 Page 6 of 13
J. Chem. Sci. (2018) 130:123
H OX
2
+
(
- H ) K
1
Q1
MnIII(Salophen)(OH2)2+ + HOX-
MnIII(Salophen)(OH )(HOX)
2
+
(
-H )K
+
2
(
-H )K
d
Q2
MnIII(Salophen)(OH2)2+ + OX2-
MnIII(Salophen)(OH )(OX)
-
2
Scheme 1. Fast equilibria preceding the redox reaction.
−
5
−3
complex (1.0 × 10 mol dm ) at 450 nm (λ
=
exc
3
40 nm) which was enhanced significantly as pH was
raised from 3 to 9 (Figure S7, Supplementary Infor-
mation). Only one inflexion point in the Fobs vs. pH Figure 3. Fluorescene spectra of Mn (Salophen)(OH2)2
curve was observed consistent with the formation of
the monohydroxo species which displayed fluorescence
III
+
◦
in the absence and presence of oxalate at 27 C, pH = 3.81
−3
±
0.03, λexc. /nm = 340.0. [complex]T, [OX]T/mol dm
−5
(
curve no.): 0.00,0.001 (1); 0.00, 0.01 (2); 1.0 × 10 , 0 (3);
(
F ) higher than its diaqua analogue (F ). The fluo-
−5
−5
2
1
1
.0 × 10 , 0.001 (4); 1.0 × 10 , 0.01 (5).
rescence data were fitted to Eq. (5) (equivalent to Eq.
+
(
4) using 1/[H ] = exp(2.303pH)) and we obtained
−4
F = 348.25± 16.02, F K = (5.2± 1.9)×10 , K = where, A , A and A denote absorbances of the
1
2
M
M
0
1
2
−7
−3
(
7.13 ± 2.73) × 10 mol dm (pK = 6.15 ± 0.17 at di-aqua-, its bi-oxalato- and oxalato-complexes, respec-
M
◦
−3
2
6.0 C, I = 0.3 mol dm ).
tively, for the same total concentration and f
=
T
2
+
+ 2
+
K [H ]/([H ] + K [H ] + K K ). The A – [OX]
1
1
1
2
data were fitted to Eq. (6) by a nonlinear least squares
F + F K exp(2.303pH)
1
2
M
Fobs
=
(5) computer program. From the calculated values of [(1 +
1
+ K exp(2.303pH)
M
+
K /[H ])Q f ] we obtained K = (1.8 ± 0.2) ×
d
−
1
2
d
3
−3
3
−1
◦
1
0
mol dm and Q = 10.4 ± 0.3 dm mol (30 C)
1
(
see captions of Figure S8 a,b, Supplementary Informa-
3
.4 Equilibrium constants Q , and Kd
1
tion).
Figure 3 displays the enhancement of the fluores-
cence intensity of the complex in the presence of
oxalate. A check for the values of Q and K was
also performed by the fluorescence intensity measure-
ments of the complex in the presence of oxalate under
The 1H NMR measurement in DMSO-D (see
6
Experimental Section) indicates that the replacement
III
of the Mn bound aqua ligand by oxalate species
1
d
is a rapid and equilibrium controlled process. We
attempted to study the equilibria (Scheme 1) spec-
◦
varying conditions of pH and [OX] (2.05 ± 0.03 ≤
trophotometrically (370 nm) at 30 C; the concentra-
T
−4
−3
−
5
pH ≤ 5.06 ± 0.03, 1.0 × 10 ≤ [OX] /mol dm
≤
tion of the diaqua complex was fixed at 4.0 × 10
T
0
.1) at a constant concentration of the diaqua com-
and [OX] was varied at constant pH ([OX]
=
T
T
−
5
−3
−3
plex, 1.0 × 10 mol dm . The observed fluorescence
0
0
.005–0.1, and 0.002–0.08 mol dm at pH = 2.23 ±
.2 and 3.72 ± 0.02, respectively). Under these pH
intensity of the reaction mixture (F ) showed a
obs
2
−
−
decreasing trend with time at a relatively high con-
centration of oxalate indicating minor interference pre-
sumably due to the reduction of Mn centre (see
later). Hence all Fobs data were corrected for this
change by extrapolation to zero time (i.e. after 40 s
conditions the oxalate species are OX and HOX
◦
(
0
pK1 = 1.0, pK = 3.58 for H OX, 30 C, I =
2
2
III
−3
2
.3 mol dm ) and the redox reaction is too slow to
interfere with the equilibria depicted in Scheme 1. Rel-
evant data are shown in Figure S8(a,b) in Supplementary
Information.
The extrapolated zero time absorbance (A) of the
reaction mixtures (relative to the corresponding solu-
tion in absence of the complex) is given by Eq. (6):
of mixing) which invariably was of the form Fobs
=
a − bt for a small time span. The relevant data
are collected in Table S2, Supplementary Informa-
tion and further details of data analysis are presented
in Appendix A. The calculated values of Q (=
1
3
−1
3
−3
1
2
3.5± 8.2 dm mol )and10 K (= 3.8± 2.5 mol dm ,
d
+
A + [(A + A k /[H ])Q f ][OX]
◦
0
1
2
d
1
2
T
6 C) agree well with those obtained by the absorbance
A =
(6)
+
1
+ [(1 + k /[H ])Q f ][OX]
d
1
2
T
measurements.

J. Chem. Sci. (2018) 130:123
Page 7 of 13 123
3
−1
vs. pH plot at 25 ◦C, [OX]T/
Figure 5. 10 kobs/s
3
−1
3
◦
−
3
Figure 4. 10 kobs/s vs. [OX]T/mol dm plot at 25 C
mol dm = 0.02.
3
2
T
(
1.1 ≤ pH ≤ 2.07): 10 k
= P1[OX] /(1 + P2[OX]T),
obs
3
P1 = (1.65 ± 0.43) × 10 , P2 = 49.8 ± 15.3.
k
o
MnIII(Salophen)(OH
MnIII(Salophen)(OH
MnIII(Salophen)(OH
)(HOX)
)(HOX)
)(HOX)
2
k
1
OX -. + H+
+
HOX .
3.5 Analysis of the kinetic data
HOX-
+
2
k
2
+
2
H OX
A preliminary study indicated that the diaqua
complex is substantially stable to acid catalyzed ther-
mal decomposition (see Figure S9 in Supplementary
2
II
Mn (Salophen)(OH
2
)
2
+
(-H )K
d
+
k
3
HOX-
2
H OX
3
MnIII(Salophen)(OH
2
)(OX)-
(- H+)
K
1
Information). A typical plot of 10 k vs. [OX] (=
obs
T
−3
◦
+
k
4
0
.007–0.25 mol dm ; 1.1 ≤ pH ≤ 2.07) at 25 C
MnIII(Salophen)(OH2)(OX)-
HOX-
(- H )K
2
+
shows a nonlinear variation of k with greater than first
obs
k5
MnIII(Salophen)(OH
)(HOX) + OX2-
OX2-
order dependence on [OX] (Figure 4).
2
T
A similar trend is evident also at higher pH and at all
MnIII(Salophen)(OH
MnIII(Salophen)(OH
)(OX)-
OX2-
k
6
+
2
other temperatures. The pH dependence of k ,however,
obs
2
−
indicates that OX contrastingly is a much weaker
reducing species than HOX (Figure 5) in the present
2
)
2
+ (MnIII(Salophen)(OXH/OX)0/- + HOX . (OX -.
)
−
fast
context further supporting our earlier observation on the
III
2
Mn (Salen) + oxalate.
MnII(Salophen)(OH
+ (OXH-
OX2- + H ) + 2CO + H+
+
2
)
2
2
Scheme 2, in addition to Scheme 1, is proposed to
interpret the rate data (see Table 1 and Table S1a,b in
SupplementaryInformation). Accordingly, kobs takesthe
form:
III
Scheme 2. Reduction of Mn (Salophen) by oxalate
es
speci .
2
2
+
k Q f [OX] + k Q ( f [OX] ) + k Q f f [OX] + k K Q f [OX] /[H ]
0
1
2
T
1
1
2
T
2
1
1
2
T
2
3
d
1
2
T
ꢀ
2
+
+
+
k ( f [OX] ) /[H ] + k k K Q ( f [OX] /[H ])
2
T
6
d
2
1
2
T
kobs
=
(7)
+
+
1
+ K /[H ] + Q f [OX] (1 + K /[H] )
M
1
2
T
d
ꢀ
+ 2
+ 2
+ 2
+
where, k = (k K + k K )Q , f (= [H ] /([H ] + measurements and the analysis of the rate data using
4
d
5
2
1
1
+
K [H ]+ K K )) and f (= K [H]/([H ] + K [H ]+ the limiting forms of Eq. (7) at 0.5 ≤ pH ≤ 2.0
1
1
2
2
1
1
K K )) denote the fractions of [OX] as H OX and and 3.0 ≤ pH ≤ 4.5. The acid dissociation equilib-
1
2
−
T
2
+
HOX , respectively. The parameters (k Q ), (k Q ), rium of the diaqua complex was neglected (K /[H ]
0
1
1
1
M
ꢀ
III
+
(
k Q ), (k K Q ), k , (k K K Q ), Q and K were <<< 1). The reduction of Mn (salophen) (OH )
2 1 3 d 1 6 d 2 1 1 d 2
2
III
+
varied to fit the rate data to Eq. (7) by a nonlin- by H OX (Mn (Salen)(OH ) + H OX → Products;
2
2
2
2
ear least squares program. The initial inputs of the kH2OX) when included in the proposed Scheme 2,
parameters (Q , K ) were chosen from the equilibrium did not improve data fitting and gave the statistically
1
d

123 Page 8 of 13
J. Chem. Sci. (2018) 130:123
Table 2. Calculated values of the rate and equilibrium parameters.a,b
◦
◦
◦
◦
Temp.→
25.0 ± 0.1 C
30.0 ± 0.1 C
35.0 ± 0.1 C
40.0 ± 0.1 C
ko Q1
k1 Q1
k2 Q1
0.00
0.636 ± 0.031
2.20 ± 0.12
0.00
0.00
0.654 ± 0.073
2.70 ± 0.23
0.00
0.00
0.780 ± 0.078
3.85 ± 0.21
0.00
0.00
0.85 ± 0.21
7.00 ± 0.36
0.00
k3Kd Q1
ꢀ
−4
−4
−8
−4
−4
k Q1
(6.76 ± 1.05) × 10
(3.98 ± 1.19) × 10
22
(7.0 ± 6.0) × 10
(4.9 ± 5.4) × 10
15
(10.0 ± 5.1) × 10
(17.0 ± 4.7) × 10
−
8
−8
−8
k6Kd K2 Q1
(6.0 ± 4.7) × 10
(7.00 ± 4.27) × 10
Q1
KM
Kd
k0
k1
k2
16
16
−
7
3
−7
−7
−7
7.1 × 10
1.0 × 10
0.00
7.1 × 10
7.1 × 10
7.1 × 10
−
−3
−3
−3
1.5 × 10
1.2 × 10
1.0 × 10
0.00
0.00
0.00
−
2
2
−2
−2
−2
−2
(2.89 ± 0.14) × 10
(10.0 ± 0.54) × 10
0.00
(1.53 ± 0.24) × 10
(6.11 ± 0.95) × 10
(0.72 ± 0.21) × 10
2.85
(4.36 ± 0.49) × 10
(18.0 ± 1.53) × 10
0.00
(1.55 ± 1.33) × 10
(8.88 ± 7.60) × 10
(0.83 ± 0.93) × 10
18.04
(4.87 ± 0.49) × 10
(5.31 ± 1.31) × 10
−
−2
−2
(24.1 ± 1.3) × 10
(43.75 ± 2.25) × 10
k3
k4
k5
0.00
0.00
c
−2
−2
−2
−2
−2
−2
−2
−2
(2.60 ± 1.32) × 10
(5.31 ± 1.47) × 10
c
−2
−2
(11.1 ± 5.6) × 10
(1.11 ± 0.87) × 10
9.16
(18.0 ± 5.0) × 10
(1.48 ± 0.91) × 10
21.12
−2
−2
k6
χ
2
d
a
Rate constants (kis, i = 0–6) are not corrected for stoichiometry. Calcd. values of ko Q1 and k3KdQ1 are (0.003 ±
−3
−6
◦
−3
−7
◦
b
1
:
.7)×10 , (0.00035± 3.9)×10 (25 C); (1.8± 3.5)×10 , (0.0011± 1.32)×10 (30 C), respectively. units
−1
3
−1 −1
3
−1
−3
c
ꢀ
s
(k0, k3); dm mol
s
(k1, k2, k4, k , k6); dm mol (Q1), and mol dm (Kd, KM, K2). k4 = k /(2Kd);
5
ꢀ
d
3
2
k = k /(2K2). [10 (kcal–kobs)] .
5
insignificant value of the corresponding rate constant.
Hence, it was omitted. Also, the parameters (k Q ) and
o
1
(
k K Q ) turned out statistically insignificant (see foot-
3 d 1
note a of Table 2). Hence these two parameters were set
tozerointhefinalcalculation. Figure6andacomparison
of k with k in Table 1 are supportive of the proposed
cal
obs
Scheme 2 (see Table S1a,b and Figure S10a–c, Sup-
◦
plementary Information, for 30–40 C). The calculated
valuesoftheparametersarecollectedinTable2. Therate
ꢀ
constants k and k were calculated from the values of k
4
5
considering the cyclic Scheme 3 for which k K = k K
4
d
5
2
is valid through K = K /K and k = k K .
eq
d
2
5
4
eq
The activation parameters for the rate determining
paths are presented in Table 3.
3.6 Computed structures (DFT)
Figure 7 delineates the structures of the diaqua- and
III
different oxalate- complexes of Mn (Salophen) with
selected bond parameters listed in Table S3
3
−1
3
−1
Figure 6. Correlation plot (10 kcal/s vs. 10 kobs/s ) at
◦
Supplementary Information). The X-ray crystallogra- 25 C.
(
III
21
phy of [Mn (Salophen)(OH )(CH OH)](ClO ) and
2
3
4
III
22
[
Mn (Salophen)(C H OH) ](BPh ) have been re-
2 5 2 4
ported. The bond distances (Å), Mn-O(phenoxide) for the diaqua complex is significantly longer
2
1
22
21
22
(
1
1.859, 1.875 ; 1.85,1.85 ) and Mn-N(imine) (1.981, (0.126- 0.145 Å ; 0.137–0.147 Å ) than the
21
22
.987 ; 1.97,1.99 ) for these two complexes are corresponding (aqua)(methanol) and bis(ethanol) com-
in good agreement with those for the diaqua and plexes. All such complexes show the distorted octahe-
III
other (aqua)(oxalate) complexes (Table S3, Supple- dral arrangement of the ligands with Mn marginally
mentary Information) while the trans-di-axial bonds above the pseudo-square plane. While no hydrogen

J. Chem. Sci. (2018) 130:123
Page 9 of 13 123
K
eq
is favourable to enhance the acidity of the mono bonded
MnIII(Salophen)(OH
)(HOX) + OX2-
MnIII(Salophen)(OH )(OX) + HOX-
-
2
2
−
HOX as observed.
III
The intramolecular reduction of Mn by the coordi-
III
k
5
nated oxalate species for Mn (Salophen)(OH )
2
0
/−
k
4
(HOX/OX)
is not observed. Such reaction is also
statistically insignificant for the corresponding Salen
2
complex further demonstrating the effect of complex
formation on the redox stability of the Mn centre.
Products
III
Scheme 3. Evaluation of k4 and k5.
H OX alone is not a reducing species. The observed
2
reactivity trend: k > k ≥ k < k > k (k , k ∼ 0) is
2
1
4
5
6
0
3
worth noting. The kinetically most important reaction is
bonds between the coordinated phenoxide and apical
H O (and CH OH) were detected, intermolecular H-
III
III
the reduction of Mn in Mn (Salophen)(OH )(HOX)
2
◦
2
3
by H OX (k /k = 3–8 at 25–40 C, Table 2). The
2
2
1
bonding interaction was observed in the crystal of the
bis(ethanol) complex cation.22 No intramolecular H-
bonding was detected by us for the diaqua complex.
However, H-bonding with varying degree of stability
as indicated by H-bond distances could be observed
precursors of the k and k paths (structures E and F,
1
2
Figure 7) could not be identified by the equilibrium
measurements due to their low thermodynamic stabili-
III
ties. However, their subtle kinetic consequence in Mn
reduction is notable. A rate comparison for the k vs
1
III
in different oxalate species of Mn (Salophen) (B, C,
k2 paths for Salen and Salophen complexes in Table 4
clearly shows that the former complex is more sensi-
tive to reduction than the latter despite the fact that the
central aromatic ring, due to its relatively stronger elec-
tronwithdrawingeffect, islikelytofavourfacileelectron
D, E, F). The oxalate complexes may be scaled in
terms of their ground state complexation energy (E)
as E < F < G < B < C < D to which
both overall charge and hydrogen bonding presumably
contributes.
III
transfer to Mn centre through the bound HOX species
than the aliphatic ethylene bridge.
The basicity difference for the coordinated
phenoxides in these complexes, Mn (Salophen) <Mn
The reduction of Mn (Salophen) by oxalate species (Salen), appears to be a significant factor in controlling
is preceded by much faster equilibrium formation of the dynamics of electron transfer through H-bonding
its mono-oxalato complexes with HOX and OX . (see Figure 7 of ref 2). In other words, all these obser-
3
.7 Mechanism of redox reaction
III
III
III
−
2−
The values of the formation equilibrium constant for vations distinctly speak of the effect of hydrogen bond-
III
[
Mn (Salophen)(OH )HOX] obtained from equilib- ing and the involvement of proton controlled electron
2
rium and kinetic measurements are comparable but transfer process (PCET) as we proposed for the cor-
III
2
somewhat lower than the corresponding Mn (Salen) responding Salen complex. The activation parameters
3
−1
◦
2
species(Q = 42 dm mol 25–40 C). Also, itistobe for k
, k , k , and k paths are low and comparable with
1
1 4 5 6
III
ꢁ
=
−1
notedthatthe[Mn (Salophen)(OH )HOX]isastronger each other averaging to ꢂH av. = 41.6 ± 5.3 kJ mol
2
2
ꢁ
=
−1
−1
acid than its Salen counterpart (pK = 2.74 vs 3.33
and ꢂS av. = −136 ± 23 J K mol . This entails
◦
−3
at 30 C, I = 0.3 mol dm ). These differences might the fact that the overall energy demand in attaining
be related to the accessible H-bonded conformers (Fig- the transition state of the electron transfer process
III
2
ure 5, structures D – D for Mn (Salen)(OH )(HOX))
is low and virtually similar in magnitude despite the
1
4
2
due to the flexibility of the ethylene bridge which is not difference in the reductant oxalate species. The corre-
ꢁ
=
possible for its Salophen analogue due to the interven- sponding large negative ꢂS av. value supposedly point
ing rigid aromatic ring. In contrast, the electronic effect to the considerable ordering in assembling the transi-
of the conjugated aromatic rings for the latter complex tion state. In contrast, the substantially high values of
Table 3. Activation parameters for the rate controlling paths.
Paths →
k1
k2
k4
k5
k6
ꢁ
=
−1
ꢂH /kJ mol
36.2 ± 7.4 71.6 ± 6.0 57.7 ± 9.9 52.1 ± 3.1 33.7 ± 2.6
−153 ± 25 −24 ± 19 −86 ± 33 −93 ± 10 −173 ± 9
ꢁ
=
−1
−1
ꢂS /J K mol
a
ꢁ
=
ꢁ=
ki = (kBT/h)exp(−ꢂH /RT + ꢂS /R).

123 Page 10 of 13
J. Chem. Sci. (2018) 130:123
III
+
Figure 7. RI-BP86/def2-TZVPP optimized structures of trans-Mn (Salophen)(OH2)2 (A),
III
+
III
III
−
Mn (Salophen)(OH2)(H2OX) (B), Mn (Salophen)(OH2)(OXH) (C), Mn (Salophen)(OH2)(OX)
III
III
−
III
3−
(
D), Mn (Salophen)(HOX)(H2OX) (E), Mn (Salophen)(HOX)2 (F), Mn (Salophen)(OX)2
(G).
−
2−
−
−
−
HOX /OX : O-C(=O)CO2H/ O2C-CO .
2
the activation parameters for the k path (Table 3) is a was also made on the corresponding Salen complex.2
2
reflection of the reorganizational disorder in attaining The similarity in the mechanism of reduction via k and
1
the transition state in this path. A similar observation k2 paths is also indicated by the isokinetic relationship:

J. Chem. Sci. (2018) 130:123
Page 11 of 13 123
Table 4. Rate comparison for Mn (Salen)a and generated H O in the net enzymatic action of the
III
2
2
III
III
Mn (Salophen).
Mn (Salen/Salophen) in the oxalate oxidation may find
a useful application in plant biochemistry in terms of
cell-wall cross-linking, and fungicidal activity.23 It is
further pertinent to note that humans and many other
animals lacking in the enzyme OXO suffer from serious
health hazards like a chronic renal disease, liver damage,
etc., due to the improper and uncontrolled metabolism
◦
k1Salen/k1Salophen
k2Salen/k2Salophen
Temp., C
25.0
30.0
35.0
40.0
25.2 ± 4.3
19.9 ± 2.7
23.8 ± 2.8
32.6 ± 9.2
73.9 ± 11.0
57.8 ± 8.3
81.3 ± 8.3
61.0 ± 10.9
30
III
of oxalate. In that context, Mn (Salen/Salophen) type
of complexes may prove to be OXO mimetics consider-
aSee Table 2 of Ref.2.
ing their abilities to oxidize oxalate to CO while acting
as SOD and catalase mimics.
2
±
±
±
±
T /K = (ꢂH −ꢂH )/(ꢂS −ꢂS ) (= 274±
31
iso
2
1
2
1
2
9.7) where T denote the isokinetic temperature and
iso
the subscripts refer to the paths.
4
. Conclusions
3.8 Relevance to Oxalate Oxidase
III
+
2
The oxalate oxidation by Mn (Salophen)(OH ) pro-
2
Oxalate oxidase (OXO, EC 1.2.3.4)23,24 is a Mn con-
taining enzyme which catalyzes the oxidation of oxalate
II
III
ceeds with the prior equilibrium formation of Mn
Salophen)(OH )(HOX) which further undergoes acid
(
2
24
III
−
by oxygen. The crystallographic study of this enzyme
from barley has revealed its hexameric structure com-
prising a trimer of dimers with one active site Mn(II) per
protein subunit; Mn(II) has cis-octahedral coordination
environment with three histidines (ꢃ-N coordination),
dissociation to [Mn (Salophen)(OH )(OX)] (pK =
2
−3
◦
2
.7 at 25–40 C, I = 0.3 mol dm NaClO ). These
4
monooxalatocomplexesdonotshowsignofintramolec-
III
ular electron transfer from the oxalate moiety to Mn
centre. The redox reactions of these two mono bonded
complexes occur via second-order processes involving
H OX, HOX and OX ; the kinetic data have been
analyzed carefully and the rate and activation param-
eters of each of the paths have been reported. The
kinetic data reflect that the reduction of Mn centre in
one glutamate, and two H O molecules, the water
2
−
2−
molecules being in the cis disposition unlike in the
2
III
+
24
[
Mn (Salophen/Salen)(OH ) ] . Opaleye et al., pro-
2
2
posed a catalytic mechanism for OXO which involves
II
III
O for H O substitution at the Mn centre to which HOX
is vicinally coordinated. This is followed by intramolec-
ular electron transfer from Mn to the coordinated O
forming superoxide (still coordinated to Mn centre).
and transforming Mn to Mn . The Mn -HOX-O
intermediate then undergoes redox transitions follow-
ing several intramolecular paths finally resulting in
the oxidized products, CO and H O (Figure 6 of
ref. 24) and regenerating the enzyme. While the pro-
cess of oxalate oxidation by OXO is a catalytic one,
our system with distal coordination of HOX relative
to the coordinated H O is stoichiometric in nature in
vitro. But both involve Mn and Mn intermediates
2
2
III
Mn (Salophen)(OH )(HOX)byH OXisthefastestand
2
2
II
III
−
2−
2
of [Mn (Salophen)(OH )(OX)] by OX is the slow-
2
III
est. Molecular modeling and structure optimization by
DFT showed the involvement of hydrogen bonds in the
complexes which favour the assembling of the required
transition state. A comparative analysis of the rate con-
stants and activation parameters delineate the coupling
of proton transfer with electron transfer invoking the
proton controlled electron transfer (PCET) kinetics. The
II
III
III
2
2
2
2
−
III
+
relevance of trans-[Mn (Salophen/Salen)(OH ) ] to
2
2
2
the enzyme Oxalate Oxidase (OXO) in oxalate oxida-
tion is highlighted.
III
II
−
with mono dentate coordination of HOX to Mn cen-
tre and function under similar pH conditions (pH ∼ 4). Supporting Information (SI)
III
+
2
The trans-Mn (Salen/Salophen) (OH ) -oxalate sys-
2
Figures S1(a,b), S2(a,b), S3(a,b), S4(a,b), S5(a–c), S6(a–c),
S7, S8(a,b), S9, S10, Tables S1(a,b), S2 and S3, and Appendix
A are submitted as Supplementary Information. Supplemen-
tary Information is available at www.ias.ac.in/chemsci.
tem coupled with the superoxide activity (SOD) of these
synthetic enzymes can also function as a catalytic sys-
tem for oxalate oxidation as,
II
−.
+
Mn (Salen/Salophen) + O + 2H
2
Acknowledgements
III
→
Mn (Salen/Salophen) + H O
2
2
P J is thankful to MHRD, Govt. of India for a fellowship
under Technical Education Quality Improvement Program
tant Mn complex, while the toxic H O can be taken (TEQIP-II). Financial support to H S B and V R M from
is the possible path of the regeneration of the reac-
III
2
5–28
2
care of by their catalase activity.2
In addition, the the Department of Atomic Energy (DAE), Govt. of India is

123 Page 12 of 13
J. Chem. Sci. (2018) 130:123
gratefullyacknowledged. TheauthorsarethankfultoDr. R. K. 15. Kovács B, Vizvári B, Riedel M and Tóth J 2004
Behera of National Institute of Technology (NIT), Rourkela
for NMR measurements and to Prof. Gautam K. Lahiri, Indian
Institute of Technology, Mumbai for ESR measurement.
Decomposition of the permanganate/oxalic acid over-
all reaction to elementary steps on integer pro-
gramming theory Phys. Chem. Chem. Phys. 6
1236
1
6. Alrichs R, Bär M, Häser M, Horn H and Kölmel C 1989
Electronic Structure Calculations on Workstation com-
puters: The program system turbomole Chem. Phys. Lett.
References
162 165
1
. Kar A K, Acharya A N, Pradhan G C and Dash A C 17. Treutlor O and Ahlrichs R 1995 Efficient molecular
014 Glyoxylate as a reducing agent for manganese(III)
numerical integration schemes J. Chem. Phys. 102 346
in salen scaffold: A Kinetics and Mechanistic study J. 18. Vahtras O, Almlöf J and Feyereisen M W 1993 Inte-
2
Chem. Sci. 126 547
. Kar A K, Acharya A N, Mundlapati V R, Prad-
gral approximation for LCAO-SCF Calculations Chem.
Phys. Lett. 213 514
han G C, Biswal H S and Dash A C 2014 Ligand 19. Eichkorn K, Treutlor O, Öhm H, Häser M and Alrichs R
2
substitution and electron transfer reactions of trans-
diaqua)(salen)manganese(III) with oxalate: an experi-
1995 Auxiliary basis sets to approximate coulomb poten-
tials Chem. Phys. Lett. 240 283
(
mental and computational study RSC Adv. 4 58867
. Huynh M H V and Meyer T J 2007 Proton-coupled elec-
tron transfer Chem. Rev. 107 5004
20. Eichkorn K, Treutlor O, Öhm H, Häser M and Alrichs R
1995 Auxiliary basis sets to approximate coulomb poten-
tial Chem. Phys. Lett. 242 652
3
4
5
. Lane B G 1994 Oxalate, germin and the extracellular 21. Fukuda T, Sakamoto F, Sato M, Nakano Y, Tan
matrix of higher plants FASEB J. 8 294
X Shi and Fujii Y 1998 Photopromoted oxidative
cyclization of an o-phenylene-bridged Schiff base via
manganese(III) complex, leading to fluorescent com-
pound, 2-(2-hydroxyphenyl)benzimidazole Chem. Com-
mun. 1391
. (a) Sugiura M, Yamamura H, Hirano K, Sasaki M,
Morikawa M and Tsuboi M 1979 Purification and prop-
erties of Oxalate Oxidase from Barley Seedlings Chem.
Pharm. Bull. 27 2003; (b) Aguilar C, Urzua U, Koenig C
and Vicuna R 1999 Oxalate Oxidase from Ceriporiopsis 22. McAuliffe
C
A, Nabhan A, Pritchard
Watkinson M, Bermejo M and Sousa A 1994
[Mn(Salphen)(EtOH)2](BH4) [SalphenH N,
R
G,
subvermispora: biochemical and cytochemical studies
Arch. Biochem. Biophys. 366 275
. Koyama H 1988 Purification and characterization of
Oxalate Oxidase from pseudomonas Sp. Ox-53 Agri.
Biol. Chem. 52 743
=
2
ꢀꢀ
6
N -bis(salicylidene)-1,2-diaminobenzene], a further
example of photosyntheic model compound forming
dimers linked by hydrogen and π bonds Acta Cryst. C
50 1676
7
. Yuan M, Zhao F, Zhang W, Wang Z-Ming and Gao S
2
007 Azide–bridged one-dimensional MnI I I polymers: 23. Requena L and Bornemann S 1999 Barley (Horeum vul-
effects of side group of schiff base ligands on structure
and magnetism Inorg. Chem. 46 11235
gare) oxalate oxidase is a manganese-containing enzyme
Biochem. J. 343 185
8
. Ciringh Y, Gordon-Wylie Scott W, Norman R E, Clark 24. Opaleye O, Rose R-Sarah, Whittaker M M, Woo E
G R, Weintraub S T and Horwitz C P 1997 Multinuclear
paramagnetic NMR spectra and solid state X-ray crystal-
lographic characterization of manganese(III) Schiff-base
complexes Inorg. Chem. 36 4968
J, Whittaker J W and Pickersgill R W 2006 Struc-
tural and spectroscopic studies shed light on the
mechanism of oxalate oxidase J. Biol. Chem. 281
6428
9
. Bonadies J A, Maroney M J and Pecoraro V L 1989 25. Noritke Y, Umezawa N, Kato N and Higuchi T 2013
Structurally diverse manganese(III) Schiff base com-
plexes: solution speciation via paramagnetic proton
NMR spectroscopy and electrochemistry Inorg. Chem.
Manganese Salen complexes with acid-base catalytic
auxiliary: Functional mimetics of catalase Inorg. Chem.
52 3653
2
8 2044
26. Lanza V and Vechhio G 2009 New Conjugates of Super-
oxide dismutases/Catalases mimetics with cyclodextrins
J. Inorg. Biochem. 102 381
1
0. Sawyer D T and Roberts Jr. J L 1974 Experimental Elec-
trochemistry for Chemists (New York: Wiley-Intersc.) p.
4
2
27. Olivera V and Vecchio G 2011 A novel artificial super-
oxide dismutase: Non covalent conjugation of albumin
with a Mn -Salophen type complex Eur. J. Med. Chem.
46 961
some problems in determining the stoichiometric proton 28. Liu Shih-Yuan, Soper J D, Yang J Y, Rybak-Akimova E
1
1. Perrin D D and Dempsey B 1974 Buffers for pH and
Metal Ion control (London: Chapman and Hall) p. 120
2. Irving H M, Miles M G and Pettit L D 1967 A study of
III
1
dissociation constants of complexes by potentiometric
titrations using a glass electrode Anal. Chim. Acta 38
V and Nocera D G 2006 Mechanistic Studies of Hang-
man Salophen-mediated activation of O–O bonds Inorg.
Chem. 45 7572
475
1
3. Mcgravey B R 1966 In Transition Metal Chemistry, R L 29. Riley D P 1999 Functional mimics of Superoxide Dis-
Carlin (Ed.) (New York: Marcel Dekker Inc.) Vol. 3 p.
mutase Enzymes as Therapeutic Agents Chem. Rev. 99
2573
89
1
4. Jones T J and Noyes R M 1983 Mechanistic details of the 30. (a) Dahiya T and Pundir C S 2013 In vivo oxalate degra-
oxidation of oxalate by Manganese(III) J. Phys. Chem.
7 4686
dation by liposome encapsulated oxalate oxidase in rat
model of hyperoxaluria Indian J. Med. Res. 137 136; (b)
8

J. Chem. Sci. (2018) 130:123
Page 13 of 13 123
Gibson M I, Chen P Yang-Ting, Johnson A C, Piera E, 31. Signorella S, Palopoli C and Ledesma G 2018 Rationally
CanM, RagsdaleSWandDrennanCL2016OneCarbon
Chemistry of Oxalate Oxidoreductase Captured by X-
ray Crystallography PNAS 113 320, and references cited
therein
designed mimics for antioxidant manganoenzymes: Role
of structural features in the quest for catalysts with cata-
lase and superoxide dismutase activity Coord. Chem.
Rev. 365 75