Glyoxylate as a reducing agent for manganese(III) in salen scaffold: A kinetics and mechanistic study

Article abstract of DOI:10.1007/s12039-014-0605-0

The kinetics of oxidation of glyoxylic acid (HGl) by Mn ^III(salen)(OH₂)₂⁺ ((H ₂salen = N,N′- bis(salicylidene)ethane-1,2-diamine) is investigated at 30.0-45.0°C, 1.83 ≤ pH ≤ 6.10, I = 0.3 mol dm ^-3(NaClO₄). The products are identified as formic acid, CO₂ and Mn^II with the reaction stoichiometry, |Δ[Mn^III]/Δ[HGl]| = 2. The overall reaction involves fast equilibrium pre-association of Mn^III(salen)(OH₂) ₂⁺ with HGl and its conjugate base Gl^- forming the corresponding inner sphere complexes (both HGl and Gl^- being the monohydrate gem-diol forms) followed by the slow electron transfer steps. In addition, the second order electron transfer reactions involving the inner-sphere complexes and HGl/Gl^- are also observed. The rate, equilibrium constants and activation parameters for various steps are presented. Mn^III(salen)(OH₂)(Gl) is virtually inert to intra molecular electron transfer while the process is facile for Mn ^III(salen)(OH₂)(HGl)⁺ (10⁵ k _et = 2.8 ± 0.3 s^-1 at 35.0°C) reflecting the involvement of proton coupled electron transfer mechanism in the latter case. A computational study of the structure optimization of the complexes, trans-Mn^III(salen)(OH₂)₂⁺, trans-Mn^III(salen)(OH₂)(Gl), and trans- Mn ^III(salen)(OH₂)(HGl)⁺ (all high spin Mn ^III(d⁴) systems), reveals strongest axial distortion for the (aqua)(Gl) complex; HGl bound to Mn ^III centre by the C=O function of the carboxyl group in the (aqua)(HGl) complex facilitates the formation of a hydrogen bond between the proton of the carboxyl group and the coordinated phenoxide moiety ((O-H ...O hydrogen bond distance 1.745 A) and the gem-diols are not involved in H-bonding in either case. A rate comparison for the second order paths: Mn^III(salen)(OH ₂)(HGl)/Gl)^+/0+ HGl/Gl^-→ products, shows that HGl for the (aqua)(HGl) complex is a better reducing agent than Gl ^- for the (aqua)(Gl) complex (k _HG ～ 5 k _Gl). The high values of activation enthalpy (ΔH ^≠= 93-119 kJ mol^-1) are indicative of substantial reorganization of the bonds as expected for inner-sphere ET process.

Full text of DOI:10.1007/s12039-014-0605-0

c
J. Chem. Sci. Vol. 126, No. 3, May 2014, pp. 547–559. ꢀ Indian Academy of Sciences.
Glyoxylate as a reducing agent for manganese(III) in salen scaffold:
A kinetics and mechanistic study
AKSHAYA K KARa, ACHYUTANANDA ACHARYAb, GURU C PRADHANa and
ANADI C DASHa,∗
aDepartment of Chemistry, Utkal University, Bhubaneswar 751 004, India
bDepartment of Chemistry, College of Engineering and Technology, Bhubaneswar 751 003, India
e-mail: aacharya@cet.edu.in; gurucpradhan@yahoo.co.in; acdash1@rediffmail.com
MS received 27 August 2013; revised 4 December 2013; accepted 7 December 2013
Abstract. The kinetics of oxidation of glyoxylic acid (HGl) by MnIII(salen)(OH2)+2 ((H2salen = N,Nꢁ-
bis(salicylidene)ethane-1,2-diamine) is investigated at 30.0–45.0◦C, 1.83 ≤ pH ≤ 6.10, I = 0.3 mol
dm−3(NaClO4). The products are identified as formic acid, CO2 and MnII with the reaction stoichiometry,
|ꢀ[MnIII]/ꢀ[HGl]| = 2. The overall reaction involves fast equilibrium pre-association of MnIII(salen)(OH2)2+
with HGl and its conjugate base Gl− forming the corresponding inner sphere complexes (both HGl
and Gl− being the monohydrate gem-diol forms) followed by the slow electron transfer steps. In addi-
tion, the second order electron transfer reactions involving the inner-sphere complexes and HGl/Gl− are
also observed. The rate, equilibrium constants and activation parameters for various steps are presented.
MnIII(salen)(OH2)(Gl) is virtually inert to intra molecular electron transfer while the process is facile for
MnIII(salen)(OH2)(HGl)+ (105ket = 2.8 0.3 s−1 at 35.0◦C) reflecting the involvement of proton coupled
electron transfer mechanism in t+he latter case. A computational study of the structure optimization of the com-
plexes, trans-MnIII(salen)(OH2)2 , trans-MnIII(salen)(OH2)(Gl), and trans- MnIII(salen)(OH2)(HGl)+ (all high
spin MnIII(d4) systems), reveals strongest axial distortion for the (aqua)(Gl) complex ; HGl bound to MnIII
centre by the C=O function of the carboxyl group in the (aqua)(HGl) complex facilitates the formation of a
hydrogen bond between the proton of the carboxyl group and the coordinated phenoxide moiety ((O-H. . .O
hydrogen bond distance 1.745 Å) and the gem-diols are not involved in H-bonding in either case. A rate com-
parison for the second order paths: MnIII(salen)(OH2)(HGl)/Gl)+/0+ HGl/Gl− → products, shows that HGl
for the (aqua)(HGl) complex is a better reducing agent than Gl− for the (aqua)(Gl) complex (kHG ∼ 5 kGl).
The high values of activation enthalpy (ꢀH= = 93–119 kJ mol−1) are indicative of substantial reorganization
of the bonds as expected for inner-sphere ET process.
Keywords. Glyoxylate; MnIII(salen); electron transfer; kinetics; DFT.
undergo redox cycling of MnIII and MnII states, get
transformed to the oxo-MnIV species under oxidative
stress and implicated as possible therapeutic agents
for neurodegenerative conditions in Alzheimer’s, and
Parkinson’s diseases and multiple sclerosis.6 Several
enzymes in which Mn is a cofactor are also well-known
and the importance of this metal ion in +II state on
human physiology has been well-recognized.7 Though
an essential element it is also a potential toxicant at high
concentration level causing degeneration of neurons in
brain even in +II state and more so in higher oxidation
states.8 The redox sensitivity of this metal ion in higher
oxidation states has been exploited in several synthetic
and limited kinetics studies in the past.9 However, every
new complex formed by Mn in +III/IV state offers
challenges in respect of understanding the mecha-
nistic aspects of reactions with reducing/oxidizing
agents as rates and paths of reactions are likely to be
1. Introduction
Manganese is known to exist in different formal oxi-
dation states ranging from −I to +VII. Note worthy is
the fact that it is an essential catalyst in +III and +IV
oxidation states in the Nature’s tetra manganese clus-
ter of the oxygen evolving system (OEC) of photosys-
tem II (PS II) available in Plants’ domain; the involved
catalytic water splitting reaction entails redox cycling
of MnIV and MnIII.1–4 Recently a novel binuclear MnIII
complex, [Mn2III,III(tpdm)2(µ-O)(µ-OAc2)]2+ (tmpd =
tris(2-pyridyl)methane) has been modelled as Nature’s
water oxidation catalyst.5 However, there is little suc-
cess in this regard. The MnIII(salen) (H2salen = N,Nꢁ-
bis(salicylidene)ethane-1,2-diamine) and several simi-
lar complexes, in their SOD and catalase mimicry,
∗For correspondence
547

548
Akshaya K Kar et al.
characteristic features of the reacting partners.9 We
have been investigating the redox and ligand substi-
tution reactions of MnIII/MnIV species keeping that in
mind.10,11 In that context, we report here a detailed and
elaborate study of the kinetics and mechanism of the
reaction of MnIII(salen) with glyoxylate, a reductant
of considerable importance due to its involvement in
glycine catabolism12 and plant physiology.13 To the best
of our knowledge there is no report of this work earlier,
although similar works are available in the literature for
the sake of comparison.
were made on a Perkin Elmer FTIR spectrometer,
model Spectrum 2 using KBr pellet. ESR measure-
ment was performed on a JEOL (Japan) JES-FA 200
ESR spectrometer at room temperature operating in X-
band mode (8.75–9.65 GHz, power 1.08 W, sensiti-
vity 7 × 109 spins/0.1 mT, resolution 2.35 µT). The
mass spectrum of the MnIII complex (aqueous solu-
tion) was recorded on a Macromass Q-TOF ESI-MS
mass spectrometer. The pH measurements were made
with a Systronics (India) pH meter model 335 using a
glass-Ag/AgCl, Cl− (3 mol dm−3 NaCl) electrode CL
51. NBS buffers of pH 4.01, 6.86 and 9.20 prepared
from KHphthalate, Na2HPO4/ KH2PO4, and Na2B4O7,
10H2O, respectively were used to calibrate the pH
meter. The measured pH of the reaction medium was
converted to p[H+] (= -log[H+]) by the relationship
p[H+] = (1.09 0.02)pH - 0.318 0.075 established
by a calibration curve using dilute HClO4 solutions
(1.98 × 10−2 ≤ [H+] / mol dm−3 ≤ 1.00 × 10−5).17
2. Experimental
2.1 Materials and reagents
MnIII(salen)Cl,H2O was prepared by the published
method essentially as described by Sharpe and cowork-
ers except that LiCl was used in place of KCl.14,6d
The elemental analysis was in satisfactory agreement
with this formulation. IR (cm−1, KBr): 1598.4(C=N), 2.3 Kinetics
1292.8(C-O of the phenoxide moiety), 3394 (OH
The rate measurements were made by batch sam-
of H2O). Similar observation was also made by
Banerjee et al.15 for [MnIII(salen)ClH2O]H2O (vC=N,
1598; vC-O, 1286; vOH, 3404–3448 cm−1); for
MnIII(salen)(OH2)+2 (generated in situ, pH 4, aque-
ous medium) λmax, nm (ε, dm3 mol−1 cm−1): 234.8
(40,243), 279 (18,625); lit.15: 234 (37, 600), 279
(17,100). Glyoxylic acid monohydrate, HGl (AR,
Sigma Aldrich, purity 98%) was used without further
purification. The concentration of the acid in the stock
solution was further checked by potentiometric titra-
tion using standard NaOH. All other reagents were AR
grade. Water was doubly distilled. The second distilla-
tion was made from alkaline KMnO4 in a standard joint
fitted borosilicate glass distillation apparatus. Freshly
distilled water was used to prepare solutions for kinetic
runs. The stock solution of the complex (5 × 10 −3 mol
dm−3) was protected from light and stored in a refrige-
rator at ∼20◦C. Ionic strength adjustment was done
with NaClO4 which was prepared by mixing standard
solutions of NaOH and HClO4 in requisite volumes.
The pH of the stock NaClO4 solution was adjusted to
6 and the concentration checked by a combined ion-
exchange alkalimetric procedure using Dowex 50W X8
resin in the H+ form.16
pling technique under pseudo first order conditions
at 30.0 ≤ t, C ≤ 45.0. The reaction mixture con-
◦
taining all components except the complex was equi-
librated in a 50 or 100 cm3 measuring flask in a
water thermostat maintained at the desired tempera-
ture ( 0.1◦C). The complex solution was also thermally
equilibrated separately. It may be noted that the com-
plex, MnIII(salen)(OH2)Cl, instantaneously aquates10
to [MnIII(salen)(OH2)2]+ (m/z+ : 392.97(obs), 392.79
(cal) for [Mn(salen)(OH2)2,Cl]) and hence the reac-
tion we studied is the reaction of the di-aqua com-
plex with glyoxylate. After thermal equilibrium was
reached, a known volume of the complex (1 cm3) was
quickly transferred in to the reaction mixture and vol-
ume was made up. Samples (5 cm3) were withdrawn
from the reaction mixture in to a clean and dry test
tube kept at 10◦C to quench the reaction by cooling.
Immediately absorbance (A) was measured at 280 nm
against solvent blank. The concentration of the com-
plex, [MnIII(salen)(OH2)2]+ was varied as (3–6) × 10−5
and that of [Gl]T (= total glyoxylic acid concentra-
tion) in the range of 0.001–0.1 mol dm−3. The ionic
strength of the medium was fixed at 0.3 mol dm−3
(NaClO4) unless otherwise quoted. The ionic strength
variation, to examine the effect of ionic back ground
on rate, was made in the range of 0.01–0.4 mol dm−3
2.2 Physical measurements
A Systronics (India) model 118, and a Perkin Elmer at 40◦C, constant pH (=2.21
0.05) and [complex]T
Lambda 25 UV-visible spectrophotometers with a (=2.805 × 10−5 mol dm−3). The pH of the reac-
matched pair 10 mm quartz cells were used for tion mixture varied in the range of 1.83–6.10 by
all absorbance measurements. The I R measurements self-buffering due to glyoxylic acid/glyoxylate which

Glyoxylate as a reducing agent for manganese(III)
549
ꢀ
ꢁ
where Cꢁ = A0[1 − (A∞/A0)]/ 1 − (A∞/At) (forAt ∼
could be achieved by addition of standard solution of
NaOH/HClO4 to glyoxylic acid solution.
The observed rate constants (kobs) were calculated by
fitting the absorbance (At) – time (t) data to equation (1)
A0 and A∞ ∼ 0, Cꢁ → A ); σ k /k
(
)
from data
0
obs
obs
fitted to equation (1) was generally better than 2%
while the same to equation (2) was 6%. The rate data
are presented in tables 1 and 2.
At = (A0 − A∞) exp (−kobst) + A∞.
(1)
2.4 Product analysis and stoichiometry
A was close to zero for the completion of the reaction
∞
The batches of reaction mixtures of MnIII(salen) +
excess HGl at different pHs (1.8–2.5) were set aside
at 40◦C till the brown colour of the complex was
fully discharged (10t1/2) indicating complete reduc-
tion of MnIII. The unreacted HGl was then estimated
by the method of Kramer et al.18 which involved the
formation of a pink coloured formazan derivative of
HGl using phenyl hydrazine and K3Fe(CN)6 and mea-
suring absorbance at 520 nm (ε520 nm = 17870 dm3
mol−1 cm−1). The details of the procedure adopted
has been described by Nayak and co-workers.19 Our
which was further verified by simulating the reaction
mixture at complete reaction with appropriate solutions
made out of MnII acetate, glyoxylic acid and other com-
ponents at the same pH. The initial absorbance was in
the range of 0.5–0.8. For very slow reactions (kobs
∼
10−5–10−6 s−1) the rate constants were evaluated by the
method of initial rate. In this case At - time(t) data were
fitted to equation (2), a limiting form of equation (1),
At = Cꢁ exp (−kobst) = Cꢁ − Cꢁkobst,
(2)
Table 1. Rate data forthe reduction of MnIII(salen)(OH2)2+ by Glyoxalate (Gly) at 30.0◦ and 35.0◦C.a
[Gly]T / mol dm−3
pHa
105kobs/s−1
105kcal/s−1
[Gly]T /mol dm−3
pHa
105kobs/s−1
105kcal/s−1
30.0 0.1◦C
0.001
0.004
0.007
0.010
35.0 0.1◦C
0.001
0.004
0.007
0.010
0.010
0.010
0.020
0.020
0.030
0.030
0.035
0.040
0.050
0.060
0.060
0.080
0.100
0.020
0.020
0.020
0.020
2.38
2.30
2.28
2.23
0.076
0.14
0.23
0.44
0.033
0.15
0.28
0.44
2.36
2.34
2.30
2.27
3.06
3.00
2.11
2.23
2.06
2.96
3.10
2.02
1.98
1.96
2.08
2.01
1.97
1.83
2.23
2.72
2.90
0.086
0.25
0.44
0.77
0.31
0.55
1.07
1.11
2.18
1.24
1.20
3.18
4.39
5.82
5.06
7.24
10.1
1.80
1.40
1.03
0.85
0.048
0.21
0.39
0.58
0.31
0.33
1.39
1.33
2.30
1.19
1.20
3.30
4.37
5.47
5.27
7.79
9.95
1.48
1.33
0.98
0.81
0.015
0.020
0.025
0.025
2.18
2.15
2.15
2.11
0.51
1.07
0.98
0.94
0.74
1.07
1.42
1.44
0.030
0.040
0.050
0.060
0.080
0.100
0.080
0.100
0.020
2.11
2.07
2.02
2.05
1.98
1.97
1.98
1.95
3.00
1.87
3.03
4.30
4.51
6.77
7.77
6.11
8.94
0.32
1.81
2.65
3.57
4.42
6.45
8.46
6.45
8.52
0.48
0.020
0.020
0.020
0.020
0.020
0.020
0.020
3.09
3.20
3.49
3.63
3.89
4.18
5.16
0.35
0.31
0.26
0.24
0.17
0.22
0.14
0.43
0.36
0.24
0.21
0.17
0.16
0.14
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
3.21
3.30
3.42
3.56
3.74
4.00
4.51
5.20
6.00
0.65
0.55
0.56
0.46
0.33
0.200
0.15
0.14
0.21
0.56
0.50
0.43
0.37
0.32
0.27
0.23
0.22
0.22
aI = 0.3 mol dm−3; -log[H+] = 1.09 pH - 0.318

550
Akshaya K Kar et al.
Table 2. Rate data for the reduction of MnIII(salen)(OH2)2+ by Glyoxalate (Gly) at 40.0◦C and 45.0◦C.a
[Gly]T /mol dm−3
pHa
105kobs/s−1
105kcal/s−1
[Gly]T /mol dm−3
pHa
105kobs/s−1
105kcal/s−1
40.0 0.1◦C
0.001
0.004
0.007
0.010
0.015
0.020
0.020
0.025
45.0 0.1◦C
0.001
0.004
0.007
0.010
0.015
0.020
0.025
0.030
0.040
0.050
0.05
2.36
2.83
2.29
2.25
2.16
2.13
2.15
2.10
2.10
2.08
0.084
0.21
0.45
0.65
1.06
1.39
1.30
2.47
2.36
3.37
0.052
0.23
0.42
0.66
1.11
1.60
1.60
2.14
2.71
2.73
2.38
2.33
2.30
2.23
2.18
2.16
2.12
2.10
2.05
2.05
2.12
2.00
1.98
2.01
1.95
2.67
2.78
2.90
3.00
3.07
3.14
3.33
3.53
3.71
3.85
4.00
4.10
4.28
4.39
4.79
5.34
0.15
0.44
0.66
1.03
1.41
2.06
2.72
3.34
4.31
8.34
7.63
11.6
17.2
15.8
22.2
1.45
1.44
1.42
1.35
1.16
1.11
0.97
0.85
0.75
0.75
0.67
0.68
0.59
0.50
0.44
0.43
0.055
0.25
0.49
0.79
1.38
2.06
2.87
3.75
5.80
8.03
7.80
10.7
16.3
16.1
22.6
1.48
1.33
1.18
1.05
0.98
0.91
0.78
0.70
0.66
0.65
0.65
0.65
0.65
0.65
0.65
0.65
0.030
0.030
0.040
0.040
0.050
0.050
0.060
0.080
0.100
0.100
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
2.04
2.05
2.01
2.01
1.99
1.97
1.93
1.90
2.72
2.76
2.90
2.98
3.05
3.14
3.63
3.82
3.94
4.63
6.13
4.03
3.69
5.57
5.59
6.82
10.1
14.4
13.6
1.12
1.02
0.97
0.88
0.88
0.80
0.55
0.45
0.35
0.33
0.29
4.03
4.02
5.46
5.46
6.99
10.2
13.9
14.0
1.08
1.04
0.89
0.81
0.78
0.68
0.45
0.42
0.41
0.39
0.39
0.060
0.080
0.080
0.100
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
aSame as under table 1
result,–ꢀ[HGL]/ –ꢀ [MnIII(salen)] = 1.99 0.08, indi-
cated that the average consumption of MnIII per mol of
HGl was 2. The product manganese species was MnII
(IN = 5/2) as evidenced from the 6 line ESR spectrum
of the spent reaction mixture (see figure 1).
The other products of oxidation are formic acid and
CO2; the former was established qualitatively by chro-
motropic acid test20,21 and the latter by precipitating
as a white solid by CaCl2 under ammoniacal condi-
tion and performing the conventional test for CO2 on
treatment with dilute HCl. Accordingly the overall sto-
ichiometry of the reaction of glyoxylic acid monohy-
drate (CH(OH)2CO2H) in acidic medium is given by
equation (3).
2.5 Test of free radical
The reaction carried out at pH = 2.08 (adjusted with
HClO4) with [HGl]T = 0.03, [MnIII(salen)]T = 2.805
× 10−5, I = 0.3 mol dm−3, 40◦C in the presence of
[acrylamide]T = 0.0, 0.01, 0.02, and 0.03 mol dm−3
yielded 105kobs/s−1 as 2.25, 1.90, 1.73, and 1.61, respec-
tively. This small decreasing trend of kobs distinctly
shows the involvement of a radical intermediate from
glyoxylic acid which reacted with MnIII(salen) almost
instantaneously to maintain the stoichiometry as shown
in equation (3) but was scavenged by acrylamide caus-
ing a decrease in the overall rate constant (kobs). How-
ever, the competitive effectiveness of acrylamide in
influencing the radical scavenging was not so signifi-
cant under these experimental conditions as we did
not observe any polymer formation in contrast to the
polymerization of this monomer in acidic medium by
the redox couple HGL + MnIV complex reported by
Nayak et al.19
2MnIII (salen) (OH2)2+ + CH (OH)2 CO2H
= 2MnII (salen) (OH2)2 + HCO2H + CO2 + 2H+.
(3)

Glyoxylate as a reducing agent for manganese(III)
551
3. Results and discussion
3.1 Preliminary observations
Figure 2 displays the repetitive spectral scans of the
reaction mixture (250 ≤ λ, nm ≤ 500 nm) at 40◦C,
pH = 2.05 over extended time. There is a steady
decrease of absorbance at all wavelengths with an iso-
sbestic point at 265 nm. The final spectrum (curve 12)
shows a maximum at 322 nm which corresponds to
that of a freshly prepared solution of MnII+ glyoxylic
acid + H2salen under similar conditions (except that the
simulated solution is 2% v/v MeOH/water as methano-
lic solution of H2salen is used). Hence the spectral
behaviour is in conformity with the reduction of MnIII
by glyoxylic acid.
3.2 Equilibrium measurements
Figure 1. Six line ESR spectrum of the product man-
ganese(II) in solution of MnIII(salen)(OH2)2+ + HGl after
completion of the reaction. Intensity versus H/G (Gauss)
plot at room temperature; g = 2.028, hyperfine constant a =
95.8 G.
The fast hydration/dehydration equilibria of glyoxylic
acid and its conjugate base in water has been well-
studied and discussed at length by Nayak et al.19 The
acid and its anion exist in the monohydrate form (gem-
diol) to the extent of >99% and 93–95%, respectively.
We, therefore, consider the protolytic equilibrium of
HGl in terms of the gem-diols:
K1
fast
HC (= 0) CO2H + H2O −−→ HC (OH)2 CO2H ꢀ HC (OH)2 CO−2 + H+
H2O
HGI (gem-diol)
GI− (gem-diol)
ꢀ
ꢁ
so that Gl− = f1 [Gl]T , [HGl] = f2 [Gl]T where
ꢂꢀ
ꢁ
ꢃ
ꢀ
ꢁ ꢂꢀ
ꢁ
ꢃ
f1 = K1/ H+ + K1 , f2 = H+ / H+ + K1 and
ꢀ
ꢁ
[Gl]T = [HGl] + Gl− . The acid dissociation con-
stant (pK1) of HGl was determined by pH titration.
We obtained pK1 =3.01
0.05 at 28.0◦C and I
= 0.3 mol dm−3 in satisfactory agreement with the
reported values (lit. values : pK1 = 2.98–3.46, 0 ≤
I / mol dm−3 ≤ 1.0, 20–25◦C; ꢀH/kJ mol−1
−2.51).22,23
=
There was a small but measurable instantaneous
spectral change (ꢀA) (see figure 3) at 280–320 nm
for the mixture of MnIII(salen)(OH2)+2 + HGl rela-
tive to that of the sum of the absorbance of the
complex and HGl at the same pH indicating fast
complex formation between the two. At constant
[MnIII(salen)(OH2)2+] (6.00 × 10−5 mol dm−3), vary-
ing [Gl]T (0.005–0.15 mol dm−3) (I = 0.3 mol dm−3),
virtually constant pH (4.34–4.09) at which HGl exists
predominantly as Gl−, the equilibrium (4) is valid.
Figure 2. Repetitiv+e spectral scans of the reaction mixture
of MnIII(salen)(OH2)2 + glyoxylic acid at 40◦C. [complex]T
= 2.855 × 10−5, [Gl]T = 0.08 mol dm−3, pH = 2.05; time in
minutes (curve no.): 0(1), 7 (2), 18 (3), 31(4), 48 (5), 63 (6),
79 (7), 101 (8), 131 (9), 166 (10), 216 (11), 1440 (12).

552
Akshaya K Kar et al.
Figure 3. ꢀA versus [Gl]T/ mol dm−3 plot at 280.1 (1), Figure 4. 105 kobs / s−1 vs. [Gl]T /mol dm−3 plot, 40◦C,
290.2 (2), 310.3 (3+), and 319.9 nm (4); 4.34 ≤ pH ≤ 4.01, pH = 2.13 0.27; (b inset) 105kobs/s−1 vs. pH plot, [Gl]T =
[MnIII(salen)(OH2)2 ]T = 6.0 × 10−5, I = 0.3 mol dm−3
,
0.02 mol dm−3, 45◦C.
27 1◦C.
KM
MnIII(salen)(OH2)2+ +Gl− ꢀ MnIII(salen)(OH2)(Gl) .
evidence for the greater than first order dependence of
kobs on [HGl]T (at constant pH 2.13) emerges. Simi-
lar trend is observed at other temperatures. Under this
condition, the major reductant species is HGl (88%).
However, kobs at constant [Gl]T (=0.02 mol dm−3)
reflects a steadily decreasing trend with the increase
of pH at all temperatures (see figure 4b, inset). This
indicates that Gl−, in contrast to our expectations, is
not a superior reducing agent (kinetically) than its con-
jugate acid, HGl. This trend is rarely reported in the
literature. Interestingly, reduction of MnIV in [Mn3IV(µ-
O)4(Phen)(OH2)2]4+ (Phen = 1,10 ortho phenanthro-
line) by glyoxylate follows a similar trend as reported
by Mandal et al.24, Das et al.25 interpreted the observed
(4)
–
The absorbance data (ꢀA = AGIT + AMnIII
A
mixedsolution), analysed by equation (5) under the
c
= (ꢀε)−1 + (ꢀεKM)−1 × (f1 [Gl]T)−1
,
(5)
ꢀA
ꢀ
condition, [Gl]T >>> c, where c = MnIII(salen)
ꢁ
(OH2)+2 T , ꢀε = ε1 – ε2, ε1 and ε2 being th+e
molar extinction coefficients of MnIII(salen)(OH2)2
and MnIII(salen)(OH2)(Gl), respectively, yielded
103(ꢀε)−1/mol dm−3 cm and 105(ꢀε KM)−1
/mol2 dm−6 cm as (1.378
0.11), (4.78
0.15)
(280.1 nm); (1.226 0.18), (5.521 0.25) (290.2 nm); increasing trend of kobs with decreasing pH for the
(1.534
0.089), (3.046
0.12) (310.3 nm); reduction of a MnIV tetramer [Mn4(µ-O)6(bipy)6]5+
(1.617 0.119), (4.541 0.016) (319.9 nm) (Corr. (bipy = 2,2ꢁ bipyridyl) in terms of relatively higher reac-
Coeff. 0.992–0.996), respectively. The weighted tivity of the protonated form of the complex with HGl.
average value of KM turned out as 33.2 5.3 dm3 In another recent study of oxidation of glyoxylic by
mol−1.
tris(biguanide)manganese(III) ([(big)3MnIII]4+), Dhar
et al.26 report that it is the acid form, HGl, which has
kinetic significance in the redox process but not the
conjugate anion, Gl−.
3.3 The slow reaction+of HGl
with MnIII(salen)(OH2)2
Under our experimental conditions the +dissociation
The slow reaction of HGl with MnIII(salen)(OH2)2+ was of the di-aqua complex, MnIII(salen)(OH2)2 to the cor-
identified as redox reaction (see experimental section) responding (aqua) (hydroxo) form can be neglected
succeeding the initial fast equilibrium complexation of {pKMnIII(salen)(OH2)2 = 7.7(10◦C), 7.4 (25◦C) I = 0.3 mol
MnIII(salen) by HGl/Gl−. The pseudo-first order rate dm−3 10; 7.79, 7.34 (kinetic data) at 30◦C, I = 0.2 mol
constants (kobs) at varying [HGl]T , pH (1.83–6.10) and dm−3}.15 Further, its absorption spectrum in mod-
temperatures (30–45◦C) are presented in tables 1 and erately acidic media (10−5 ≤ [H+]/ mol dm−3
≤
2. Figure 4(a) displays the variation of kobswith [HGl] 0.1, figure S1) is essentially acid independent indi-
at constant pH (= 2.13 0.27) at 45.0◦C. A clear cut cating thereby that the complex does not undergo

Glyoxylate as a reducing agent for manganese(III)
protonation at least up to pH 1. The plausible reac-
553
tions in tune with these observations are depicted in
scheme 1.
Accordingly kobs is given by equation (6) where
k0Q1f1 [Gl]T + k1Q2f2 [Gl]T + k2Q1f12 [Gl]T2 + k3Q2f22 [Gl]2T + kꢁf1f2 [Gl]T2
1 + Q1 (f1 + Rf2) [Gl]T
kobs
=
kꢁ = k4Q1 + k5Q2,
(6)
+
MnIII(salen)(OH2)2
+
K1
HGl
Gl-+ H+
Q1
Q2
K1
MnIII(salen)(HGl)+
H+ + MnIII(salen)(OH2)(Gl)
H2O k1
k0 H2O
H+ + Gl + MnII(salen)(OH2)2
Gl- +
HGl
H+ + Gl + MnII(salen)(OH2)2
H2O
HGl
H2O
Gl-
k3
k2
+
+
K1
MnIII(salen)(OH2)(HGl)+
H+ + MnIII(salen)(OH2)(Gl)
+
+
HGl
Gl-
H2O k5
HGl
k4
H2O
H+ + Gl + MnII(salen)(OH2)2
Gl- +
fast
MnIII(salen)(OH2)2+ + Gl
MnII(salen)(OH2)2 + CO2 + HCO2H + H+
Scheme 1. Reduction of MnIII(salen)(OH2)2+ by glyoxylate species, HC(OH)2CO2H (HGl) and HC(OH)2CO−2 (Gl−).

554
Akshaya K Kar et al.
dm−3 (40◦C) and 0.01 ≤ I / mol dm−3 ≤ 0.4. Under
this condition >85% of [Glyoxalic acid]total exists in the
form of HGl. We observe a small rate retardation of
the overall rate with the increase of ionic strength {105
kobs/s−1(I, mol dm−3) = 1.57 (0.01), 1.49(0.02), 1.41
(0.05), 1.35 (0.10), 1.29 (0.20), 1.30 (0.30), 1.15 (0.40)}
consistent with involvement of oppositely charged ions
(see scheme 1).
R = Q2/Q1 ; f1, f2 , [Gl]T are as mentioned earlier and all
other terms are as defined in scheme 1 (note ki (corr.) =
ki/2, 2 stands for stoichiometry, subscript i = 0–5).
3.4 Analysis of the rate data (kobs
)
The rate constants (kobs) are fitted to equation (6) by
a nonlinear least squares program by varying all the
parameters with the restriction that the value of R (=
Q2/Q1) is < 1 and pK1 (= 3.0 at 30.0 ≤ t/◦C ≤ 45.0)
is temperature independent (reflected by the low value
of the associated ꢀH, see above). The calculated rate
parameters are given in table 3. It is evident that both
the terms, k0Q1 and kꢁ are statistically insignificant (see
table 3).
3.6 Molecular modelling and structure optimization
The ground state geometries of all the complexes were
optimized using density functional theory (DFT). The
BP86 functional and def2-TZVPP basis set within the
resolution-of-the-identity (RI) approximation27,28 (RI-
BP86/def2-TZVPP in short) was employed for the opti-
mization procedure. Frequency calculations of the opti-
mized structures were done to ensure that they were
true minima not the transition states. The DFT calcula-
tions were accomplished with the TURBOMOLE pro-
gram package (Version-6.4).29 The ‘freeh’ script of tur-
bomole was used to calculate the free energies. The
free energies of the complexes were computed at 25◦C
(298 K) and 1 atmospheric pressure. For the graphical
presentation and the bond distance and angle measure-
ments Mercury 3.0 was used.30
The relevant activation parameters obtained by
weighted least squares fit of kis (wi = (1/σki)2) to the
ꢂ
Eyring equation, ki
=
(kBT/h) exp −ꢀH =/RT+
ꢃ
ꢀ S=/R are also collected in table 3. A comparison
of kcal with kobs (see tables 1 and 2) shows the validity
of equation (6) in relation to the proposed scheme.
Neglecting the (k0Q1) and kꢁ terms the representative
ꢀ
plots of 105 kobs (1 + Q1 (f1 + Rf2) [Gl]T) − k2Q1f12
ꢁ
[Gl]2T / (f2 [Gl]T) vs. 102f2 [Gl]T (see figures S2 and
S3) at 30◦C and 45◦C for the linearized form of equation
(6) also bear this fact.
3.5 Effect of ionic strength on kobs
3.7 Analysis of results
The reaction is relatively complex in the sense that it
involves several rate and equilibrium steps. As such Our results indicate that internal electron trans-
a detailed analysis of ionic strength effect is not fer from the coordinated glyoxylate to MnIII in
attempted. However, limited rate measurements are MnIII(salen)(OH2)(Gl) is not a favourable process as
reported at pH = 2.23
0.07, [Gl]T = 0.020 mol compared to the same for its conjugate acid analogue,
Table 3. Calculated rate and equilibrium constants.a,b
Parameters
30.0 0.1◦C 35.0 0.1◦C 40.0 0.1◦C 45.0 0.1◦C ꢀH= / kJ mol−1 ꢀS∗/J K−1 mol−1
105k0Q1/dm3 mo−1 s−1
0.064 12.3 0.82 5.54 0.56 7.73 0.93 9.60
0.032 0.01 1.27 0.50 0.12 0.50 0.24 0.77
–
–
102kꢁ/ dm6 mol−2 s−1
105k1Q2 / dm3 mo−1 s−1 38.1 10.1
54.6 5.2
60.1 5.0
62.0 9.8
103k2Q1 / dm6 mol−2 s−1 5.87 7.07 8.60 2.93 14.15 4.40 21.7 5.4
102k3Q2/ dm6 mo−2 s−1
Q1(Q2)c/ mol dm−3
R
2.54 0.15 2.80 0.09 2.61 0.07 3.89 0.15
33.0 (19.8)
0.6
33.0 (19.8)
0.6
25.0 (10.0)
0.4
18 (7.2)
0.4
–
–
105k0/s−1
0.002 0.37 0.025 0.17 0.022 0.31 0.051 0.53
1.92 0.51 2.76 0.26 6.01 0.50 8.61 1.36
1.78 2.14 2.61 0.88 5.66 1.76 12.05 3.04
1.28 0.08 1.41 0.05 2.61 0.07 5.40 0.21
105k1 / s−1
93 11
119
93 18
−28 36
+75 20
+3 60
104k2 / dm3 mol−1 s−1
103k3 / dm3 mol−1 s−1
104k4 / dm3 mol−1 s−1d
F e
6
0.06 0.02
2.10
2.4 0.95
0.76
0.34 1.4
1.42
0.96 3.0
4.96
aI = 0.3 mol dm−3. berrors of (k0Q1 ) and kꢁ estimated after fixing the values of other parameters and vice versa. cQ2 = R
ꢀ
ꢃꢁ
ꢄ
2
× Q1. dk4 = k5 = kꢁ/(Q1 + Q2). eF =
105 (kcal − kobs)

Glyoxylate as a reducing agent for manganese(III)
555
Figure 5. RI-BP86/def2-TZVPP optimized structures of MnIII(salen)
(OH2)2+ (A), MnIII(salen)(H2O)(HGl)+ (B) and MnIII(salen)(H2O)(Gl)
(C) with atom labelling, 1 denotes MnIII.

556
Akshaya K Kar et al.
Table 4. Selected bond distances (Å) and bond angles (deg) for the RI-BP86/def2-TZVPP optimized structures of
complexes, trans- MnIII(salen)(OH2)2 + (A), MnIII(salen)(OH2)(HGl)+ (B), and MnIII(salen)(OH2)(Gl) (C).
A
B
C
Bond distances
Mn-O(2 W) 2.424
Mn-O(2 W) 2.466
Mn-O(2 W) 2.996
Mn-O(39 W) 2.424
Mn-O(3 phenoxide) 1.880
Mn-O(4 phenoxide) 1.880
Mn-N(5 imine) 1.985
Mn-N(6 imine) 1.985
Mn-O(40 O=C-OH) 2.353
Mn-O(3 phenoxide) 1.914
Mn-O(4 phenoxide) 1.868
Mn-N(5 imine) 1.976
Mn-N(6 imine) 1.985
O(3). . .H(42) 1.745
Mn-O(40 −O-C=O) 2.047
Mn-O(3 phenoxide) 1.914
Mn-O(4 phenoxide) 1.897
Mn-N(5 imine) 1.995
Mn-N(6 imine) 1.990
Bond Angles
O(2)-Mn-O(39) 176.5
O(3)-Mn-O(39) 90.4
O(4)-Mn-O(39) 87.1
N(5)-Mn-N(6) 82.6
O(4)-Mn-N(6) 90.8
O(3)-Mn-N(5) 90.8
O(4)-Mn-N(5) 173.3
O(3)-Mn-N(6) 173.3
O(3)-Mn-O(4) 95.8
O(3)-Mn-O(2) 87.3
O(4)-Mn-O(2) 90.4
N(5)-Mn-O(39) 93.5
N(6)-Mn-O(39) 89.2
Mn-N(6)-C(15) 113.2
Mn-O(4)-C(22) 130.6
Mn-O(3)-C(7) 130.6
O(2)-Mn-O(40) 175.7
O(3)-Mn-O(40) 87.5
O(4)-Mn-O(40) 89.4
N(5)-Mn-N(6) 82.9
O(4)-Mn-N(5) 91.4
O(3)-Mn-N(6) 91.4
O(4)-Mn-N(5) 171.4
O(3)-Mn-N(6) 173.5
O(3)-Mn-O(4) 94.6
O(3)-Mn-O(2) 89.4
O(4)-Mn-O(2) 87.9
N(6)-Mn-O(40) 97.0
N(5)-Mn-O(40) 90.2
Mn-N(6)-C(13) 124.7
Mn-O(4)-C(22) 131.5
Mn-O(3)-C(7) 130.0
O(2)-Mn-O(40) 164.8
O(2)-Mn-O(40) 96.2
O(4)-Mn-O(40) 99.2
N(5)-Mn-N(6) 82.2
O(4)-Mn-N(6) 90.7
O(3)-Mn-N(5) 89.4
O(4)-Mn-N(5) 162.6
O(3)-Mn-N(6) 162.6
O(3)-Mn-O(4) 93.1
O(3)-Mn-O(2) 68.7
O(4)-Mn-O(2) 81.3
N(6)-Mn-O(40) 100
N(6)-Mn-O(2) 95.2
Mn-N(5)-C(13) 124.7
Mn-O(4)-C(22) 131.2
Mn-O(3)-C(7) 126.1
MnIII(salen)(H2O)(HGl)+ (i.e., k0 << k1). The val-
ues of Q1 and Q2, however, reflect that HGL forms a
much weaker complex than GL− with MnIII(salen)+.
Thus the redox activity is inversely related to the ther-
modynamic stability. The nature of the two complexes
can be judged considering the outer-sphere association
of Gl− and HGl with MnIII(salen)(OH2)+2 on statistical
considerations and theory of diffusion31,32 according to
which
corresponding values obtained from the equilibrium
study and kinetic data fitting (see table 3). This supports
the inner sphere nature of such complexes which are
believed to be generated by replacement of the coordi-
nated H2O by HGL and Gl−. We have made structural
characterization of these adducts (inner sphere com-
plexes) along with the corresponding diaqua complex,
MnIII(salen)(OH2)+2 , by a computational study (see
figure 5). Some selected bond distances (Å) and bond
angles (deg) for the optimized structures of trans-
MnIII(salen)(OH2)+2 (A), MnIII(salen)(OH2)(HGl)+ (B),
and MnIII(salen)(OH2)(Gl) are given in table 4.
Notable fact is that there is no hydrogen bonding
of the OH groups of the gem-diol moiety and Gl−
acts as a mono dentate ligand for the metal centre.
Contrastingly, the HGl moiety binds MnIII(salen) vir-
tually as a bi-dentate ligand, with one bond to MnIII
via the −C = O function and the other via a hydrogen
bond involving the carboxyl proton and the bound
phenoxide {O(3). . .H(42) in figure 5B, H-bond dis-
ꢂ
ꢃ
ꢀ
ꢂ
ꢃꢁ
Qout = 4πNa3/3000 exp (b) exp −bkpa/ 1 + kpa
,
(7)
where b = |Z1Z2|e2/DakBT, a is the distance (cm)
of closest approach between the participating
species carrying charges Z1 and Z2, e is elec-
tronic charge (esu), D is the bulk dielectric con-
stant of the medium, kB is the Boltzmann con-
ꢅ
ꢆ
ꢀꢂ
ꢃ ꢁ
1/2
stant (ergs), kp
=
8πNe2/1000DkBT I
is
the Debye Huckel ion atmosphere parameter and tance = 1.745 Å, see table 4}. This imparts sub-
T is the absolute temperature. At 25◦C the cal- stantial thermodynamic stability to the complex,
culated values of Qout, using a constant value of MnIII(salen)(OH2)(HGl)+as reflected by the value of
a = 5 Å (I = 0.3 mol d+m−3), are 0.7 and 0.3 dm3 mol−+1 Q2. A simple calculation shows that the acid dissocia-
for MnIII(salen)(OH2) , Gl− and MnIII(salen)(OH2) , tion constant of the coordinated HGl (Kꢁ = K1Q1/Q2)
2
2
1
HGL, respectively which are very much lower than the is only marginally (<10 times) higher than that of free

Glyoxylate as a reducing agent for manganese(III)
557
in favour of inner sphere binding of Gl− (Eq. 8)
but not HGl in the second step despite relatively
stronger ground state structural trans effect of Gl−.
We must mention here that the equilibria (8) and (9)
could not be detected under the experimental con-
ditions. However, our computational strategy attempts
to differentiate the mechanistic path ways of reduc-
tion of MnIII(salen)(OH2)(Gl) by Gl− and MnIII(salen)
(H2O)(HGL)+ by HGL; the former involves inner
sphere mechanism unlike the latter for which the outer
sphere mechanism might prevail (k2 and k3 paths in
scheme 1).
A comparison of the rate constants (kis) given
in table 3 shows that the intra molecular reduction
of MnIII in MnIII(salen)(OH2)(Gl) (k0 path) is sta-
tistically insignificant as for MnIII(salen)(OH2)(HQ)
and MnIII(salen)(OH2)(Hcat) (H2Q and H2cat
denote hydroquinone and catechol, respectively)15
and MnIII(salen)(H2O)(SO3)−10 while the same for
MnIII(salen)(OH2)(HGl)+ is moderately facile (k1
path); both steps are likely to involve inner sphere
mechanism as electron transfer occurs through the
bound ligands, Gl− and HGl. For the k1 path ꢀH= is
substantially high and ꢀS= is close to zero (or small
negative, see table 3) as expected for an intra molecu-
lar process with some degree of bond rearrangement.
Further, the observed differential rates (k1 vs k0), and
the activation parameter data make us inclined to think
that H-bond in MnIII(salen)(OH2)(HGl)+ as also the net
positive charge at the MnIII centre play a dominant role
to consider this ‘electro-protic reaction’ as a distinct
case of ‘proton coupled electron transfer’ process (see
figure 6).
HGl. This is attributed to the locking of the acidic
proton in the hydrogen bond.
The optimized structures and the computed structure
parameters (i.e., bond distances and bond angles) for
the complexes, trans-MnIII(salen)(OH2)(X) (X=H2O,
HGl, G−) are provided in tables S1, S2 and S3
(supplementary information). MnIII is tightly held
by the salen scaffold being slightly displaced above
the plane of N,N(imines) and O,O (phenoxides)
(∼0.03 Å for MnIII(salen)(OH2)(HGl)+, and ∼0.27 Å
for MnIII(salen)(H2O)(Gl)). Significant axial distortion
is noted for these two complexes (see table 4). A
comparison of the MnIII-X bond length trans to the
MnIII-OH2 in the complexes, MnIII(salen)(H2O)(X):
2.6209 15, 2.424, 2.353, 2.407 Å and similarly the
15
MnIII-OH2 bond length trans to MnIII-X: 2.3329
,
2.424, 2.466, and 2.996 Å for X = Cl−, H2O,
HGl and Gl−, respectively demonstrates that the
ground state structural trans effect (GSTE) is the
strongest for MnIII(salen)(H2O)(Gl). However, the
MnIII-O(phenoxide) and MnIII-N(imine) bonds in all
such complexes (see ref. 15 and table 4) do not reveal
substantially different axial ligand dependent distortion
of the N1,N2,O1,O2 square plane. In order to assess
how the GSTE influences the equilibria involving the
replacement of the second water molecule from trans
MnIII(salen)(OH2)(Gl/HGl)0/+ by Gl−/HGl (Eqs 8, 9),
Q3
MnIII (salen) (OH2) (Gl) + Gl− ꢀ
MnIII (salen) (Gl)− + H2O,
(8)
Q4
MnIII (salen) (OH2) (HGI)+ + HGI− ꢀ
MnIII (salen) (HGI)2+ + H2O.
(9)
A comparison of the second order rate constants
We made an attempt to compute theoretically the rele- (k4 < k2 < k3, see table 3) also reveals that HGl reduces
vant ꢀG0 values at 25◦C. ꢀG0 for equilibrium (8) much faster than Gl− unlike in several other cases where
turned out negative (∼−79.5 kJ mol−1) while the reverse sequence has been reported for anions and their
same for (9) was positive (∼8.6 kJ mol−1). This goes conjugate acids reducing various MnIII complexes.33
OH
OH
H
H
O
HO
+
O
+
HO
H
H
O
O
O
O
O
O
MnIII
MnII
N
N
N
O
N
O
H
H
H
H
Figure 6. Proton coupled electron transfer in MnIII(salen)(OH2)(HGl)+.

558
Akshaya K Kar et al.
that the proton of the carboxylic acid function is hydro-
Interestingly, the results of Das and co-workers25
gen bonded to one of the coordinated phenoxide func-
tion while both the diol-OH groups are not involved
in H-bonding in the state of coordination of HGl and
Gl− to MnIII. There is remarkable kinetic stability of
MnIII(salen)(H2O)(Gl) towards intra-molecular electron
transfer in contrast to moderate rate of intra-molecular
reduction of MnIII centre by the coordinated HGl. This
electro-protic reaction through the influence of H-bond,
is considered to be a clear case of ‘proton coupled
intra-molecular electron transfer’ process. However,
MnIII(salen)(H2O)(Gl) further undergoes reduction
of MnIII centre by Gl− in a second order process as
also MnIII(salen)(H2O)(HGl)+ by HGl; computational
study, however, indicates the possibility of adduct for-
mation between MnIII(salen)(OH2)(Gl) and Gl− but
not between MnIII(salen)(OH2)(HGl)+ and HGl. We
conclude that the intimate reduction steps follow inner-
sphere mechanism possibly except in the second order
path of the reaction between MnIII(salen)(OH2)(HGl)
and HGl.
are in agreement with ours where they show that gly-
oxylic acid (HGl) very likely reduces MnIV in [Mn4(µ-
O)6(bipy)6]4+ (I4+) appreciably faster than its conj−u1-
gate anion (Gl−) (I4++HGl →, k
> 50 dm3 mol
red
s−1 ; I4+ + Gl−
→, kred
= 0.68 dm3 mol−1
s−1 at 25◦C, I = 1.0 mol dm−3). However, proton
ambiguity in that case is a serious shortcoming as
pointed out by them. The reduction of MnIV in [Mn3IV(µ-
O)4(Phen)(OH2)2]4+ by HGl/Gl−have been shown by
Mandal and co-workers24 to follow second order kine-
tics with the reactivity trend, kred(HGl) > kred(Gl−), an
observation similar to ours but for the proton ambi-
guity in their case. In a recent study of the oxi-
dation of glyoxylic acid by MnIVL and MnIII L {L
= tetra deprotonated 1,8-bis(2-hydroxybenzamido)-3,6-
diazaoctane} Nayak and co-workers19 have unambigu-
ously demonstrated the higher reactivity of HGl rela-
tive to that of its conjugate base, Gl− and attributed this
reactivity order to the preferential hydrogen bonding
effect of HGl.
The activation enthalpy for the k2 path is marginally
higher than the same for the k3 path (ꢀH=(k2) −
ꢀH=(k3) = 26
19 kJ mol−1) and the correspond-
Supplementary information
ing difference in the activation entropy is also sim-
ilar in magnitude (ꢀS=(k2) − ꢀS=(k3) = +72
63 J K−1 mol−1). Our computational study shows that
unlike HGl, Gl− can still bind to MnIII centre via
H2O replacement of MnIII(salen)(H2O)(Gl) forming a
precursor (MnIII(salen)(Gl)−2 ) preceding electron trans-
fer via inner sphere mechanism (k2 path). Hence, in
this case the overall activation enthalpy and entropy
will have contributions from the equilibrium precur-
sor formation and the redox step. On the contrary, no
such inner sphere precursor complex between HGl and
MnIII(salen)(H2O)(HGl)+ appears to be formed preced-
ing the electron transfer step via k3 path. Hence it is not
unlikely that the activation parameters for the k2 and
k3 paths will differ by a small margin. However, the
large values of ꢀH=(93–119 kJ mol−1) reflect substan-
tial reorganization energy which is expected for inner
sphere ET process.
Figure S1 showing the lack of [H+] depen-
dence of the +Uv-Vis absorption spectra of
MnIII(salen)(OH2)2 (pH = 1 – 4); figures S2 and
S3 showing the plots of 105[kobs(1+Q1[Gl]T(f1+
f2R) – k2Q1f21 [Gl]T2 ]/(f2[Gl]T ) (=Y) vs. 102f2[Gl]T
at 30◦C and 45◦C, respectively; Tables S1, S2 and
S3 presenting the optimized structures and cor-
responding bond+ distances and bond angles for
MnIII(salen)(OH2)2 (A), MnIII(salen)(OH2)(HGl)+ (B),
and MnIII(salen)(OH2)(Gl) (C), respectively are pro-
vided as supplementary materials (see www.ias.ac.in/
chemsci).
Acknowledgements
Financial support from the University Grants Commis-
sion (UGC), New Delhi in terms of a Teacher Fel-
lowship to AKK (Ref. T.F.OU3-007-1/10-11 (ERO))
is acknowledged. AKK thanks the Odisha Educa-
tion Department and the authority of Sishu Ananta
4. Conclusion
The electron transfer react+ion between glyoxylic Mahavidyalaya, Khurda, Odisha, India for granting
acid and MnIII(salen)(OH2)2 involves equilibrium study leave. Authors thank Prof. Gautam K Lahiri,
pre-association of the reactants yielding inner- Indian Institute of Technology, Mumbai for ESR mea-
sphere
complexes,
MnIII(salen)(OH2)(Gl)
and surement. We also thank Dr. Himansu S Biswal, School
MnIII(salen)(OH2)(HGl)+, where HGl and Gl− are the of Chemical Sciences, National Institute of Science
gem-diols of the acid and the conjugate base forms of Education and Research (NISER), Bhubaneswar for the
the reductant, respectively. Computational study shows quantum chemical calculations and discussions.

Glyoxylate as a reducing agent for manganese(III)
559
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