Oxidation of glyoxylic acid by a mononuclear manganese(IV) complex of 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane: A kinetics and mechanistic study

Article abstract of DOI:10.1016/j.poly.2011.03.032

The oxidation of glyoxylic acid (HGl) by Mn^IVL {L^4- = tetra deprotonated 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane} was investigated in the pH range 1.67-10.18, at 25-45 °C and 0.5 M ionic strength. The reaction exhibited biphasic kinetics with Mn^IIIL ^- as the reactive intermediate. Mn^IV was reduced to Mn^II. The products of oxidation of HGl were identified as formic acid and CO₂ in acidic medium, and oxalate in basic medium, consistent with the stoichiometry: -Δ[Mn^IV]/-Δ[HGl] = 1. In acidic medium, both Mn^IVL and Mn^IIIL^- formed outer-sphere adducts with the neutral HGl {HC(OH)₂COOH} molecule, with an association constant Q_av of 28 and 70 M^-1, respectively. A similar adduct formation was not observed for the glyoxylate mono anion {Gl^-, CH(OH)₂(CO₂^-)} and glyoxylate dianion {Gl^2-, CH(OH)(O^-)CO₂ ^-}. The rate and activation parameters for the various paths are reported and an outer-sphere electron transfer mechanism is suggested.

Full text of DOI:10.1016/j.poly.2011.03.032

Polyhedron 30 (2011) 1637–1645
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Polyhedron
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Oxidation of glyoxylic acid by a mononuclear manganese(IV) complex of
1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane: A kinetics and mechanistic study
b
a,
a
c
Suprava Nayak a, , Gouri S. Brahma , K. Venugopal Reddy , K. Veera Reddy , Anadi C. Dash
⇑
⇑
a Department of Chemistry, Osmania University, Hyderabad 500007, India
b Faculty of Science and Technology, ICFAI University, Dehradun 248197, India
c Department of Chemistry, Utkal University, Bhubaneswar 751004, India
a r t i c l e i n f o
a b s t r a c t
The oxidation of glyoxylic acid (HGl) by MnIVL {L4ꢀ = tetra deprotonated 1,8-bis(2-hydroxybenzamido)-
3,6-diazaoctane} was investigated in the pH range 1.67–10.18, at 25–45 °C and 0.5 M ionic strength.
The reaction exhibited biphasic kinetics with MnIIILꢀ as the reactive intermediate. MnIV was reduced
to MnII. The products of oxidation of HGl were identified as formic acid and CO2 in acidic medium, and
Article history:
Received 25 February 2011
Accepted 15 March 2011
Available online 31 March 2011
oxalate in basic medium, consistent with the stoichiometry: ꢀ
D
[MnIV]/ꢀ
D[HGl] = 1. In acidic medium,
both MnIVL and MnIIILꢀ formed outer-sphere adducts with the neutral HGl {HC(OH)2COOH} molecule,
Keywords:
Kinetics
Manganese(IV)
1,8-Bis(2-hydroxybenzamido)-3,6-
diazaoctane
with an association constant Qav of 28 and 70 Mꢀ1, respectively. A similar adduct formation was not
observed for the glyoxylate mono anion {Glꢀ, CH(OH)2(CO2ꢀ)} and glyoxylate dianion {Gl2ꢀ
,
CH(OH)(Oꢀ)CO2ꢀ}. The rate and activation parameters for the various paths are reported and an outer-
sphere electron transfer mechanism is suggested.
Glyoxylic acid
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
there are several reports on the oxidation of HGl [11,18–25] and
other similar carboxylic acids [26–29], a kinetics and mechanistic
Studies on high-valent manganese complexes, irrespective of
their nuclearity, are of high importance because of their potential
uses in various biological as well as non-biological redox processes
[1–3], and in photosynthesis [4–6]. Since manganese in its +4 oxi-
dation state is involved in most of the processes, the kinetics and
mechanistic studies of the redox reactions of MnIV complexes are
important in coordination and biomimetic chemistry.
In the recent past, there have been several reports on the mech-
anistic studies of redox reactions of multinuclear MnIV complexes
[7–11], whereas the parallel studies on mononuclear complexes
are relatively scarce. The present work focuses on the redox reaction
of a mononuclear manganese(IV) complex, MnIVL {L4ꢀ = tetra depro-
tonated 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane} (Fig. 1), a
study of the reaction of HGl with a mononuclear manganese(IV)
complex in such a non-porphyrinic N4O2 coordination environ-
ment has not been investigated so far to the best of our knowledge.
Also, the idea of using HGl as one of the reactants has come from
recent studies, where it was reported that organic acids such as
oxalic acid and oxo carboxylic acids were found to be secondary or-
ganic aerosols (SOA) in the aqueous phase of clouds. It is further
expected that water mediated aldehyde chemistry has important
consequences in the formation of SOA [30–33]. The oxidative path-
ways of HGl involving the formation of these SOA remain highly
speculative, leading to uncertainties in the predictions of atmo-
spheric models. Compelled by above facts, we herein have made
an attempt through this investigation to (i) identify the reaction
products of HGl and elucidate the oxidative pathways followed
by the reactants under varying pH conditions, (ii) determine the
rate and activation parameters for all major reaction paths, and
(iii) develop a kinetic model for the MnIVL–HGl system.
structurally well-characterized species [12] having a strongly
r-
bonded tetra anionic binding pocket of the ligand to metal center
through di-[N-amidate, O-phenolate, N-amine) coordination.
In continuation of our current interest on the redox reactions of
MnIVL [13,14], we have chosen glyoxylic acid, HGl (a
a-keto acid)
as the reductant due to its importance in metabolic processes such
as glycine catabolism [15] and plant physiology [16,17]. Although
2. Experimental
2.1. Materials and reagents
⇑
Corresponding authors. Tel.: +91 9949127293; fax: +91 01353003080
(S. Nayak), tel.: +91 9989019038 (K.V. Reddy).
The complex MnIVLꢁ4H2O was prepared and characterized as
E-mail
addresses:
suprava7107@gmail.com,
nayaksuprava@yahoo.com
(S. Nayak), kvgr1951@gmail.com (K.V. Reddy).
reported earlier [13]. Glyoxylic acid (AR, Aldrich, 98%), MeOH
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.03.032

1638
S. Nayak et al. / Polyhedron 30 (2011) 1637–1645
2.3. Kinetics
The kinetics of the reaction between MnIVL and HGl was moni-
tored spectrophotometrically under pseudo first order conditions
using 0.001 6 [HGl], M 6 0.15 and 4.75 6 105 [MnIVL], M 6 8.05
{1.6% (v/v) MeOH + H2O medium}. [HGl] P 0.15 M was not used
to avoid complications due to the dimerisation of HGl [35]. The
progress of the reaction was monitored at four different tempera-
tures, 25, 35, 40 and 45 °C by observing the decrease of absorbance
at 469 nm, where only MnIVL has an absorption maximum
(10ꢀ4 = 5.85 0.055 Mꢀ1 cmꢀ1) and HGl or its oxidized products
e
and MnII species do not absorb significantly. The reaction was stud-
ied in the pH range 1.67–10.18. Carbonate/hydrogen carbonate and
acetic acid/sodium acetate buffers were used to adjust the pH of
solutions in the range 9.11ꢀ10.18 and 3.70ꢀ5.70, respectively,
and the kinetic runs made at pH 1.67–3.70 were adjusted by HClO4
or NaOH. The buffers had no effect on the absorbance of the com-
plex and also on the reaction rate. HGl was used as its sodium salt
in the runs made in buffer medium in order to avoid a pH shift. The
ionic strength (I) was maintained at 0.5 M (NaClO4), unless men-
tioned otherwise. The absorbance (At) versus time data were col-
lected up to 80–95% of completion of reaction for most runs
Fig. 1. Structure of MnIVL {L4ꢀ = tetra deprotonated 1,8-bis(2-hydroxybenzamido)-
3,6-diazaoctane}.
(G.R., Merck, 99.8%), sodium carbonate (AR, BDH, England) and ace-
tic acid (AR, Qualigens) were used as received. All other chemicals
were of the highest purity available. The water used was doubly
distilled, the second distillation being made from alkaline KMnO4
in all glass (Borosil) distillation apparatus. A stock solution of gly-
oxylic acid was prepared by dissolving solid glyoxylic acid mono-
hydrate in water, whereas the complex solution was prepared by
dissolving the appropriate amount of the complex in a minimum
volume of methanol which resulted in 16% (v/v) MeOH + H2O sol-
vent composition in the stock solution. The solutions were pro-
tected from light and stored at low temperature (10 °C) in a
refrigerator. The HGl solution was standardized potentiometrically
using standard NaOH. The NaClO4 used to maintain the ionic
strength (I) was prepared by mixing freshly prepared and stan-
dardized stock solutions of NaOH and HClO4. The stock solution
of NaClO4 was adjusted to pH 6.0 and was estimated for Na+ by a
combined ion-exchange alkali-metric procedure using Dowex
50W X8 cation exchange resin in the H+ form.
(ꢀ Aobs ꢂ 0.4). The infinity time absorbance (A1) was virtually
D
zero under all experimental conditions. This was further estab-
lished by measuring the absorbance of the reaction mixture con-
taining MnCl2, LH4 and all other components in appropriate
concentrations.
The decrease of absorbance (At) with time (t, s) was monotonic
and fitted well to a bi-exponential decay (Fig. 2S) [36] with the
kinetic profile described by a set of consecutive first order
reactions:
kfobs
ksobs
MnIVL
INT
P
ꢀꢀ!
ꢀꢀ!
where INT was taken to be MnIIILꢀ.
At ¼ A1 þ C1 expðꢀkf tÞ þ C2 expðꢀkosbstÞ
ð1Þ
obs
In Eq. (1), C1 = (eMnIVL ꢀ eMnIII kof bs
/D
k) [MnIVL]T + d1, C2 = {eMnIII
L
L
½MnIVLꢃTkfobs
/
D
k} ꢀ d2, where
D
k = kfobs ꢀ kosbs, d1 = A kosbs
/D
k and
1
d2 = A kf
/D
k; [MnIVL]T is the total initial concentration of com-
1
obs
plex and eMnIV
, eMnIII denote the molar extinction coefficients of
L L
MnIVL and MnIIILꢀ species, respectively. Absorbance versus time
data were fitted to Eq. (1) by a non-linear least squares computer
program assigning unit weight to each data point and yielded
2.2. Physical measurements
A Shimadzu (UVPC-1601) double beam spectrophotometer with
a pair of 10 mm matched quartz cells was used to record all UV–Vis
spectra and to carry out the kinetic measurements. The tempera-
ture was maintained ( 0.1 °C) by an electronically controlled
thermostated cell-housing of the instrument (CPS-240A). The pH
measurements were done using an Elico digital pH meter model
LI 120 equipped with a glass–Ag/AgCl, Clꢀ (3 M NaCl) electrode,
CL 51. NBS buffers of pH 4.01, 6.86 and 9.20 were used to check
the performance of the pH meter. The pH values were converted
into p[H+] = ꢀlog[H+] (1.67 6 pH 6 5.26) by a calibration curve, as
reported earlier [34]. For the alkaline pH conditions (pH >8), no
such calibration was attempted and pH = ꢀlog[H+] was used. The
gaseous product of the oxidation of HGl, CO2, was qualitatively
confirmed by gas chromatographic analysis using a GC-17A Shima-
dzu gas chromatograph equipped with a thermal conductivity
detector (TCD) and a porapak-T coated with molecular sieves
13XWCOT column. A split-less mode of injection of the analyte
was done by maintaining the temperature of the column, injector
and detector at 40, 43 and 100 °C, respectively. The carrier helium
gas flow was adjusted to 30 ml minꢀ1. The IR spectrum was re-
corded on KBr pellet using a Tensor 27 (Bruker optics, Germany)
FT-IR spectrometer.
kof bs and kosbs with
r(kobs) 6 6% and 61%, respectively. The number
of data points for a run was at least 70 and at the most 160. Each
entry of kobs in Tables 1 and 2 is from a single run. Replicate exper-
iments reproduced the observed rate constants within the errors
quoted as above. The kinetics runs at 350 and 320 nm indicated
that there was no wavelength dependence of the observed rate
constants (see Tables 1 and 2).
2.4. Product analysis and stoichiometry
2.4.1. Detection of unreacted glyoxylic acid
The reaction mixtures containing MnIVL and a small excess of
HGl at different pHs were allowed to stand at 30 °C for completion
of the reaction (i.e. 10t1/2 for the ks path). The unspent HGl was
obs
estimated by the method of Kramer et al. [37]. A freshly prepared
2.5 ml phenyl hydrazine hydrochloride (1% solution in aqueous
medium) was added to 2.5 ml of each reaction mixture. The resul-
tant solutions were incubated at ꢂ110 °C for 8 min and then cooled
to room temperature (ꢂ25 °C), followed by successive addition of
2.5 ml each of concentrated HCl and 1% K3Fe(CN)6. The mixtures
were allowed to stand for 1 min for quantitative formation of a

S. Nayak et al. / Polyhedron 30 (2011) 1637–1645
1639
Table 1a
Rate data for the reduction MnIVL by glyoxylic acid in acidic medium.a
pHb
[HGl]T
104kf (sꢀ1
)
pHb
[HGl]T
104kf (sꢀ1
)
pHb
[HGl]T
104kf (sꢀ1
)
pHb
[HGl]T
104kf (sꢀ1
)
M
obs
cal
M
obs
cal
M
obs
cal
M
obs
cal
25 °C
35 °C
40 °C
45 °C
1.85
2.07
2.11
2.42
2.45
2.73
2.87
2.96
3.12
3.16
3.37
3.55
3.96
4.18
4.54
4.85
5.05
3.25
3.12
3.01
2.84
2.83
2.96
2.96
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.01
0.02
0.03
0.07
0.10
0.12
0.15
8.78
7.07
6.86
4.28
4.83
3.56
3.14
2.56
2.47
2.40
2.51
1.85
0.74
0.70
0.34
0.15
0.10
0.60
1.21
1.28
3.05
3.70
3.85
3.83
9.10
7.15
6.24
4.39
4.26
3.35
3.00
2.80
2.46
2.38
1.95
1.59
0.89
0.62
0.33
0.19
0.14
0.71
1.43
2.07
3.48
3.88
3.75
3.94
1.67
1.86
2.01
2.17
2.39
2.54
2.76
2.90
3.04
3.07
3.23
3.58
3.87
4.32
4.92
3.77
3.60
3.35
3.17
3.11
3.05
3.05
3.05
3.06
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.001
0.004
0.008
0.02
0.03
0.06
0.09
0.10
0.12
19.8
14.4
12.1
10.1
7.94
7.42
7.18
6.70
6.79
5.41
4.55
3.07
1.75
1.01
0.37
0.10
0.56
1.51
3.08
4.13
6.56
8.79
9.25
8.86
18.9
14.8
12.5
10.8
1.90
2.05
2.15
2.27
2.37
2.59
2.69
2.87
3.22
3.54
3.57
3.83
4.33
4.44
4.92
5.05
5.26
3.13
3.11
3.04
3.00
3.05
3.05
3.02
3.01
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.02
0.03
0.05
0.07
0.08
0.10
0.12
0.15
22.5
18.2
17.0
16.4
15.3
12.3
10.0
8.99
7.86
5.16
4.76
2.23
2.03
1.52
0.83
0.77
0.56
4.54
5.87
8.35
11.3
12.6
11.5
17.4
18.4
22.3
18.9
17.3
15.8
14.7
12.6
11.8
10.4
1.87
1.90
1.90
2.02
2.11
2.15
2.85
3.50
3.70
4.00
4.08
4.26
4.26
4.26
4.28
4.66
5.12
3.09
3.03
3.00
3.02
3.00
2.98
3.00
3.00
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.01
0.02
0.03
0.03
0.05
0.07
0.08
0.10
56.2
52.9
51.7
46.8
37.8
40.1
18.1
7.80
5.03
4.57
3.53
2.85
2.87c
3.00d
2.69
2.55
1.58
4.56
9.11
11.2
10.6
15.8
20.7
21.3
25.7
55.5
53.1
53.1
45.0
40.1
38.2
18.8
9.10
8.22
7.10
6.44
5.77
5.63
4.87
3.28
2.23
1.21
0.69
0.06
0.31
0.91
2.58
3.80
6.40
7.95
8.36
9.00
8.59
7.60
6.41
4.08
3.64
2.84
2.84
2.84
2.77
1.84
1.38
3.68
7.47
5.17
4.96
3.40
1.60
1.37
0.82
0.75
0.66
4.05
5.79
9.04
10.9
11.6
10.6
16.1
20.6
21.9
25.0
12.0
13.6
15.1
16.8
a
b
c
I = 0.5 M, 1.6% (v/v) MeOH + H2O medium, 469 nm.
pH = ꢀlog[H+].
320 nm.
350 nm
d
Table 1b
Rate data for the reduction MnIIILꢀ by glyoxylic acid in acidic medium.a
pHb
[HGl]T
104ks (sꢀ1
)
pHb
[HGl]T
104ks (sꢀ1
)
pHb
[HGl]T
104ks (sꢀ1
)
pHb
[HGl]T
104ks (sꢀ1
)
M
obs
cal
M
obs
cal
M
obs
cal
M
obs
cal
25 °C
35 °C
40 °C
45 °C
1.85
2.07
2.11
2.42
2.45
2.73
2.87
2.96
3.12
3.16
3.37
3.55
3.96
4.18
4.54
4.85
5.05
3.25
3.12
3.01
2.84
2.83
2.96
2.96
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.01
0.02
0.03
0.07
0.10
0.12
0.15
3.06
2.57
2.03
1.95
2.22
1.99
1.80
1.78
1.35
1.28
1.32
0.91
0.46
0.37
0.13
0.10
0.06
0.30
0.56
0.64
0.98
1.12
1.37
1.41
3.03
2.51
2.26
1.76
1.73
1.48
1.38
1.32
1.22
1.20
1.05
0.91
0.58
0.42
0.22
0.12
0.08
0.48
0.85
1.10
1.50
1.60
1.56
1.60
1.67
1.86
2.01
2.17
2.39
2.54
2.76
2.90
3.04
3.07
3.23
3.58
3.87
4.32
4.92
3.77
3.60
3.35
3.17
3.11
3.05
3.05
3.05
3.06
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.001
0.004
0.008
0.02
0.03
0.06
0.09
0.10
0.12
6.20
4.70
4.30
3.87
3.14
2.79
2.24
2.24
1.79
1.69
1.12
0.66
0.50
0.24
0.14
0.024
0.2 6
0.58
1.06
1.35
1.77
1.81
2.09
2.28
6.31
4.78
3.96
3.33
2.75
2.47
2.16
1.99
1.82
1.79
1.60
1.16
0.80
0.37
0.11
0.03
0.15
0.41
1.03
1.38
1.93
2.18
2.23
2.31
1.90
2.05
2.15
2.27
2.37
2.59
2.69
2.87
3.22
3.54
3.57
3.83
4.33
4.44
4.92
5.05
5.26
3.13
3.11
3.04
3.00
3.05
3.05
3.02
3.01
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.02
0.03
0.05
0.07
0.08
0.10
0.12
0.15
6.18
5.66
5.36
4.84
4.56
3.77
2.57
2.61
1.50
1.15
1.33
0.75
0.56
0.36
0.29
0.17
0.14
1.50
1.55
1.96
2.79
2.75
2.81
3.12
3.21
6.88
5.59
4.93
4.32
3.91
3.23
3.00
2.64
2.03
1.48
1.43
1.01
0.42
0.34
0.12
0.09
0.06
1.31
1.72
2.33
2.71
2.74
2.92
3.10
3.26.
1.87
1.90
1.90
2.02
2.11
2.15
2.85
3.50
3.70
4.00
4.08
4.26
4.26
4.26
4.28
4.66
5.12
3.09
3.03
3.02
3.00
3.00
2.98
3.00
3.00
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.01
0.02
0.03
0.03
0.05
0.07
0.08
0.10
12.8
11.1
11.7
12.4
11.0
10.1
5.64
1.95
1.51
0.81
0.65
0.85
0.97c
0.94d
0.55
0.63
0.43
1.45
2.05
2.72
2.75
4.62
5.13
5.57
6.57
12.9
12.4
12.4
10.7
9.71
9.31
5.22
2.65
1.97
1.17
1.00
0.70
0.70
0.70
0.67
0.30
0.11
1.31
2.48
3.33
3.40
4.61
5.51
5.75
6.27
a–d
Same as under Table 1(a).
pink colored formazan derivative of HGl, that was measured at
520 nm; the concentration of the unreacted HGl was directly read
from a calibration curve constructed using a known amount of HGl
(reported value 17 870 Mꢀ1 cmꢀ1 [37]). A blank correction was also
made while estimating HGl. We obtained [HGl]/
[MnIVL] = 1.0 0.05 as the average consumption ratio of HGl
for the MnIV ? MnII transformation.
ꢀD
ꢀD
which gave e
520nm = 17 094 Mꢀ1 cmꢀ1 for the formazan derivative

1640
S. Nayak et al. / Polyhedron 30 (2011) 1637–1645
Table 2a
Rate data for the reduction of MnIVL by glyoxylic acid (HGl) in basic medium.a,b
pH
[HGl]T
104kf (sꢀ1
)
pH
[HGl]T
104kf (sꢀ1
)
pH
[HGl]T
104kf (sꢀ1
)
pH
[HGl]T
104kf (sꢀ1
)
M
obs
cal
M
obs
cal
M
obs
cal
M
obs
cal
25 °C
35 °C
40 °C
45 °C
9.37
9.65
0.05
0.05
0.30
0.36
0.19
0.36
9.57
9.84
0.05
0.05
1.70
2.95
1.63
3.03
9.63
9.63
0.01
0.02
0.71
1.03
0.73
1.47
9.35
9.40
9.40
9.60
9.73
9.87
10.05
10.13
9.88
9.82
9.79
9.82
9.85
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.03
0.04
0.07
0.09
0.10
6.18
7.14
8.61
10.9
15.1
18.6
27.8
35.9
12.8
11.0
22.3
32.1
37.0
6.54
7.22
7.22
9.90
9.89
10.06
10.02
10.10
0.05
0.10
0.05
0.05
0.05
0.63
1.03
0.89
0.95
1.20
0.65
1.27
0.94
0.86
1.03
9.90
10.14
10.21
9.86
9.85
9.84
9.87
9.84
10.06
10.18
0.05
0.05
0.05
0.005
0.01
0.05
0.07
0.09
0.05
0.05
3.40
5.90
7.10
0.26
0.82
2.80
3.50
6.12
5.67
6.66
3.48
6.05
7.11
0.32
0.62
3.03
4.55
5.46
5.03
6.63
9.71
9.66
9.63
9.73
9.72
9.39
9.38
9.11
9.40
9.97
0.03
0.05
0.06
0.08
0.10
0.05
0.05
0.05
0.05
0.05
2.07
4.05
4.44
6.89
9.56
1.92
2.07
1.69
2.29
7.13
2.58
3.90
4.40
7.18
8.79
2.33
2.29
1.47
2.37
7.41
10.9
14.3
19.4
28.8
34.5
11.9
13.9
22.8
31.3
37.1
a
2ꢀ
I = 0.5, [MnIVL]T = (4.7–8.05) ꢄ 10ꢀ5 M, 1.6% (v/v) MeOH + H2O; HCO3ꢀ/CO3 buffer; 469 nm.
104kfobs, sꢀ1 (I, pH): 4.05 (0.135, 9.90), 3.12 (0.135, 10.07), 5.26 (0.135, 10.18), 3.31 (0.935, 9.66), 4.49 (0.83, 9.78), 1.95 (0.335, 9.82) at 40 °C.
b
Table 2b
Rate data for the reduction of MnIIILꢀ by glyoxylic acid (HGl) in basic medium.a,b
pH
[HGl]T
104ks (sꢀ1
)
pH
[HGl]T
104ks (sꢀ1
)
pH
[HGl]T
104ks (sꢀ1
)
pH
[HGl]T
104ks (sꢀ1
)
M
obs
cal
M
obs
cal
M
obs
cal
M
obs
cal
25 °C
35 °C
40 °C
45 °C
9.37
9.65
0.05
0.05
0.071
0.14
0.09
0.18
9.57
9.84
0.05
0.05
0.56
0.70
0.77
1.31
9.63
9.63
0.01
0.02
0.37
0.54
0.24
0.49
9.35
9.40
9.40
9.60
9.73
9.87
10.05
10.13
9.88
9.82
9.79
9.82
9.85
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.03
0.04
0.07
0.09
0.1
1.72
2.85
2.91
2.53
5.29
4.93
9.50
12.2
2.55
3.04
5.53
6.25
6.89
1.54
1.73
1.73
2.74
3.69
5.10
7.71
9.27
3.13
3.63
5.93
8.17
9.73
9.90
9.89
10.06
10.02
10.10
0.05
0.10
0.05
0.05
0.05
0.32
0.64
0.44
0.41
0.36
0.31
0.60
0.41
0.54
0.36
9.90
10.14
10.21
9.86
9.85
9.84
9.87
9.84
10.06
10.18
0.05
0.05
0.05
0.005
0.01
0.05
0.07
0.09
0.05
0.05
1.73
2.35
2.70
0.13
0.41
1.43
1.76
2.65
2.59
2.60
1.48
2.47
2.87
0.14
0.27
1.31
1.95
2.35
2.07
2.69
9.71
9.66
9.63
9.73
9.72
9.39
9.38
9.11
9.40
9.97
0.03
0.05
0.06
0.08
0.10
0.05
0.05
0.05
0.05
0.05
1.05
1.28
1.51
2.31
2.86
0.57
0.74
0.67
0.75
2.51
0.86
1.29
1.46
2.38
2.91
0.77
0.75
0.48
0.78
2.46
a
Same as under Table 2(a).
104ksobs, sꢀ1 (I, pH): 0.886 (0.135, 9.90), 0.827 (0.135, 10.07), 0.935 (0.135, 10.18), 0.689 (0.935, 9.66), 1.32 (0.83, 9.78), 0.89 (0.335, 9.82) at 40 °C.
b
2.4.2. Detection of free ligand (LH4)
on a glass sintered funnel and dried over silica gel in a desiccator
after thoroughly washing with water. A part of this precipitate,
on heating with diphenylamine, gave a blue coloration, indicating
the presence of oxalate [41,42]. The FT-IR spectrum of that precip-
itate displayed absorption bands at 1604, 1417, 1329, 1134, 939,
873, 839, 783, 712, 652 and 627 cmꢀ1 (Fig. 5S), characteristic of
several modes of vibrations of a metal oxalate [43,44].
Based on these facts, the overall stoichiometry of the reactions
in acidic and basic medium is given by Eqs. (2) and (3),
respectively.
After complete reduction of MnIV to MnII, the reaction mixtures
of MnIVL and HGl at three different pHs, viz. 9.9, 4.34 and 3.53,
were acidified to pH 1.0 and diluted to a constant volume. The
UV spectra of these solutions were compared with the ligand
(LH4) solution under the same conditions. More than 98% of free li-
gand was observed (Fig. 3S) in the reaction medium after complete
reduction of MnIV
.
2.4.3. Detection of oxidation products
MnIVL þ CHðOÞCO2H þ H2O ¼ MnIIL2ꢀ þ HCO2H þ CO2 þ 2Hþ ð2Þ
Under acidic conditions (1.67 6 pH 6 5.26), CO2 and formic acid
were the oxidation products of glyoxylic acid. CO2 was detected
qualitatively by GC analysis [38] (Fig. 4S) and formic acid by the
chromotropic acid test [39,40]. It may be noted that glyoxylic acid
containing a formyl group which developed a red coloration in the
chromotropic acid test. This, however, did not cause any serious
difficulty in detecting formic acid qualitatively due to a significant
color difference.
MnIVL þ CHðOÞCO2ꢀ þ H2O ¼ MnIIL2ꢀ þ C2O2ꢀ þ 3Hþ
ð3Þ
4
However, under acidic conditions MnIIL2ꢀ exists in multiple
2+
stages of proteolytic equilibria, ultimately leading to Mn(OH2)6
and LHi(4ꢀi)+ (i = 1–6).
In basic medium, oxalate was confirmed to be the oxidation
product of glyoxylic acid by multiple tests. The reaction mixture
2.5. Test for free radicals
2ꢀ
in HCO3ꢀ/CO3 buffer, after completion of the reaction, was acid-
When the reaction was carried out in acidic medium (pH 3.5), in
the presence of 10% (w/v) of acrylamide, a viscous solution was no-
ticed within 5–10 min and the solution then turned into a thick gel
after ꢂ2 h. Blank experiments were conducted without MnIVL or
ified with perchloric acid. CO2 was removed and then calcium chlo-
ride solution was added, followed by dilute NH3 until the solution
became ammoniacal. A white precipitate formed. This was filtered

S. Nayak et al. / Polyhedron 30 (2011) 1637–1645
1641
HGl, which gave no detectable polymer. In contrast, no redox in-
duced polymerization of acrylamide was observed in basic med-
ium (pH ꢅ 9.5). From the above results, following observations
were made:
medium. In alkaline medium, a further shift of the equilibrium oc-
curs either due to deprotonation of the alcoholic OH group of the
gem-diol or by fast nucleophilic addition of OHꢀ at the aldehydꢀic
carbonyl center, generating the dianion, Gl2ꢀ, (HC(OH)(Oꢀ)CO2
[35,48].
)
(i) The MnIVL + HGl redox couple generated free radicals in
acidic medium which survived long enough to initiate poly-
merization of acrylamide.
The reported data on the acid dissociation constant of glyoxylic
acid show a wide variation with a small temperature coefficient
(pK1 = 2.98–3.46, 0 6 I, M 6 1.0, 20–25 °C,
D
H = ꢀ0.6 K cal molꢀ1
)
(ii) A radical of glyoxalate (Glꢀꢀ) formed in the one-electron
reduction of MnIVL and MnIIILꢀ in basic medium did not
build up to a reasonable concentration level for inducing
polymerization. It is, however, pertinent to note that under
these conditions the free radical (a congener of the product
C2O42ꢀ) generated in the transition state of the first step
(MnIVL ? MnIIILꢀ) must have diffused very fast into the bulk
phase for the survival of MnIIILꢀ so that the second phase of
the reaction could be discernible.
[50,51]. In the present study, the pH titration measurements in
1.6% (v/v) MeOH + H2O medium gave a value of pK1 = 3.00 0.05
at 25 °C and I = 0.5 M.
3.3. Effect of pH on the molar extinction coefficients of MnIVL and
MnIIILꢀ species
The values of C1, C2, A and the relevant rate constants were
1
used to calculate eMnIV and eMnIII from the relationships: eMnIV =
L
Lꢀ
L
(C1 + C2 + A )/[MnIVL]T and eMnIII
=
(C2Dk/kfobs þ A )/[MnIVL]T
Lꢀ
1
1
3. Results and discussion
(see Section 2.3). We obtained eMnIV = (6.10 0.51) ꢄ 103 and
L
eMnIII = (4.94 0.94) ꢄ 103 Mꢀ1 cmꢀ1 (k = 469 nm) in the pH range
Lꢀ
3.1. Preliminary observations
1.67–10.18 using the relevant data at 25 °C. Hence we conclude
that both the MnIVL and MnIIILꢀ species are not significantly pro-
tonated under the experimental conditions of pH considering the
fact that the protonation of MnIVL and MnIIILꢀ would have resulted
in a significant variation in their molar extinction coefficients.
The repetitive spectral scans of the reaction mixture displayed a
steady decrease of absorbance with time in the range 400 6 k
6 700 nm in both acidic as well as basic media (Fig. 6S); the kmax
around 287 nm was shifted to 300 nm with no isosbestic point in
acidic medium (Fig. 6Sa). In contrast there was a large red shift
(287 ? 326 nm) with clear isosbestic points at 309 and 345 nm
which were retained in alkaline medium (Fig. 6Sb), but ultimately
started to drift and disappear on a much longer timescale. In the
acidic pH range, the isobestic point started to build up (see
Fig. 6Sa) but could not develop properly due to the relatively faster
second phase reaction and the dissociation of the intermediate
MnIIILꢀ complex to MnIIL + H2L. The reduction of MnIV to MnII
was evident, as the deep brown color of the reaction mixture of
3.4. Reaction in acidic medium (1.67 6 pH 6 5.26)
The rate data for the MnIV ? MnIII and MnIII ? MnII reactions
fðsÞ
are collected in Tables 1(a) and 1(b) respectively. k
decreased
obs
with an increase of pH, levelling off to a value close to zero around
pH 6.0 (Fig. 8). Since there was no evidence of protonation of MnIVL
and MnIIILꢀ, the observed trend is rationalized in terms of the reac-
tivity difference between HGl and Glꢀ.
fðsÞ
At a constant pH (3.05), k
for both MnIVL and MnIIILꢀ in-
obs
MnIVL and HGl became colorless and the band at 469 nm of MnIV
L
creases with an increase in [HGl]T and tends to attain a saturation
vanished completely at the end of the reaction. Further the final
spectrum of the reaction mixture after prolonged standing virtu-
ally agreed with the spectrum of the mixture of MnCl2 and the li-
gand (LH4) at the same pH. This is attributed to the reduction of
MnIVL to MnII via MnIIILꢀ (as an intermediate species). The time
dependent formation and decay of the intermediate MnIIILꢀ, decay
of MnIVL and the formation of the MnII species (Fig. 7Sa) clearly de-
pict the profile of consecutive reactions. The absorption spectra of
MnIVL, the MnIIILꢀ intermediate (generated from the spectrum of
the reaction mixture for 60% conversion of MnIVL to MnIIILꢀ) and
the final product, Mn(II) + H2L, are presented in Fig. 7Sb. The spec-
limit, (Fig. 9) indicating the equilibrium pre-association of the
fðsÞ
reactants (Scheme 2), for which k is given by Eq. (4).
obs
fkf1ðsÞ
Q
fðsÞ þ kf2ðsÞK1=½Hꢃþ þ k3fðsÞQfðsÞ½Hꢃþg ꢄ ½HGlꢃT
1 þ K1=½Hꢃþ þ QfðsÞ½HGlꢃT
fðsÞ
k
¼
ð4Þ
obs
O
H+
+
_
CO2
H
trum of MnIIILꢀ displayed kmax, nm ( , dm3 molꢀ1 cmꢀ1) as 287
e
_
Gl1
(13249), 325 (12870), 465 (4275) at pH 9.46 and 287 (14863),
320sh (7866), 465 (4325) at pH 4.28. There was no indication of
protonation of MnIVL or MnIIILꢀ from the analysis of the kinetic
data. Thus the small differences under the pH conditions is attrib-
uted to the formation of the hydrogen bonded adducts of MnIVL
and MnIIILꢀ at pH 4.28, as the reaction was run at a high concentra-
tion of HGl (=0.05 M) (see later).
In the nearly neutral pH region (5.5–8.0), the complex is quite
stable to reduction under ambient conditions. Hence we did not
extend our measurements to this pH range.
+
O
H2O
H+
Kh
+
H
CO2H
OH
OH
HGl1
OH
_
_
Kd
C
+
H2O
O
C
H+
+
H
CO2
_
'
_
Kh
H
CO2
Gl2
_
OH
Gl2
3.2. Equilibria of glyoxylic acid
OH
C
The hydration–dehydration and ionization equilibria of glyoxy-
lic acid [35,45–51] in aqueous medium are presented in Scheme 1.
It has been established by 1H NMR spectroscopy [45,46] and
from hydration equilibrium constant data [47] that >99% of HGl
and 93–95% of Glꢀ are present in their gem-diol form in aqueous
H
CO2H
HGl2
Scheme 1. Hydration and ionization equilibria of glyoxylic acid.

1642
S. Nayak et al. / Polyhedron 30 (2011) 1637–1645
f
Q f
k1
_
MnIVL + HGl
{MnIVL,HGl}
_
MnIII
L
+ H+ + Gl.
K1
f
_
k2
MnIVL + Gl
+
MnIIIL
+
Gl.
H+
f
k3
_
Gl. + 2H+
+
{MnIVL,HGl}
+ H+
MnIIIL
_
fast
H2O
MnIVL + Gl.
MnIIIL
+
HCO2H + CO2
s
Q s
_
k1
_
_
MnIIL2
+ + Gl.
+ H
MnIII
L
+ HGl
{MnIIIL ,HGl}
_
s
_
_
k2
MnIII
L + Gl
MnIIL2
+
Gl.
s
k3
_
_
+ H+
MnIIL2 + Gl. + 2H+
_
{MnIIIL , HGl}
_
fast
H2O
MnIIIL + Gl.
MnIIL2
+
HCO2H + CO2
fðsÞ
Fig. 8. pH dependence of k
for the oxidation of glyoxylic acid. Conditions:
obs
_
[MnIVL] = (6.0–8.0) ꢄ 10ꢀ5 M, [glyoxylic acid] = 0.05 M and I = 0.5 M (NaClO4) at
40 °C; (1) 104kof bs, (2) 104kosbs (ꢁ – experimental points, the calculated best fit points
are shown as a solid curve).
Mn(H2O)62+ + LH4
MnIIL2 + 4H+
H2O
HGl = HC(OH)2CO2H + HC(=O)CO2H
_
_
_
Gl = HC(OH)2CO2
+ HC(=O)CO2
Scheme 2. Reaction of glyoxylic acid with MnIVL and MnIIILꢀ at pH 1.67–5.26.
where a is a constant {=k1f(s) + k2f(s)K1/(Qf(s)[H+]) + k3f(s)[H+]}. The
fðsÞ
gradient and intercept of the plots of 1/k
versus (1 + K1/[H+])/
obs
[HGl]T yielded Qf (Qs)Mꢀ1 as 39.3 8.7 (78.9 7.9), 13.2 5.5
(45.5 16.0), 11.2 2.6 (31.9 13.0) and 22.4 3.1 (40.5 8.1)
(pK1 = 3.0) at 25, 35, 40 and 45 °C, respectively. Using these values
of Qf(s) and the known value of pK1, the kfðsÞ data were fitted to
obs
Eq. (4) by a non-linear least squares computer program. It turned
out that pK1 = 3.0 (25–40 °C) and 2.9 (45 °C) fitted best. The calcu-
lated values of the rate and equilibrium constants are collected (Ta-
ble 3).
3.5. Effect of acetic acid
Effect of acetic acid/acetate buffer on the reaction of
MnIVL + HGl was studied by varying the concentration of acetic
acid from 0.01 to 0.1 M at pH 2.00 (I = 0.5 M, 40 °C). This experi-
ment was planned to examine if this monobasic acid has any influ-
ence on the reduction rate as a consequence of its association with
fðsÞ
Fig. 9. Dependence of k
on glyoxylic acid, at pH 3.05 ( 0.1), [MnIVL] = 8.0 ꢄ
obs
10ꢀ5 M, I = 0.5 M (NaClO4) and T = 35 °C; (1) 104kfobs, (2) 104ksobs. Experimental
points are represented by dots and the calculated best fit points are represented by
a solid line.
MnIVL and MnIIILꢀ species. There was no significant change in the
fðsÞ
values of k with a variation [CH3CO2H] when the pH was held
obs
constant.
where [HGl]T = [glyoxylic acid]total = [HGl]free + [Glꢀ]free (neglecting
the depletion of [HGl]free due to its association with MnIVL and
MnIIILꢀ as [MnIVL]T 6 [HGl]T was maintained) and superscripts ‘‘f’’
and ‘‘s’’ stand for the reactions of MnIVL and MnIIILꢀ, respectively.
k1, k2 and k3 are not corrected for stoichiometry (i.e. ki corr = ki/2, i
= 1–3, see scheme 2). Further no distinction is made between the
hydrated and unhydrated forms of glyoxylic acid and also its conju-
gate base in terms of their reactivities under a condition when both
of them exist in the hydrated form to >93%.
3.6. Reaction in basic medium (9.11 6 pH 6 10.18)
Rate constants for the reactions of MnIVL and MnIIILꢀ with Glꢀ at
9.11 6 pH 6 10.18 are collected in Tables 2a and 2b respectively.
The kfðsÞ versus [Glꢀ]T plots at pH 9.67 0.04 reveals no association
obs
of [Glꢀ] with MnIVL and MnIIILꢀ (Fig. 10).
fðsÞ
Also k
versus 1/[H+] plots at constant [Glꢀ]T (Fig.11S) are
obs
reconciled with (i) negligible reaction between MnIVL(MnIIILꢀ)
and Glꢀ (linear plot with positive slope and zero intercept) and,
(ii) its dianion (Gl2ꢀ) must be a reactive species. Considering the
At constant pH (=3.05 0.1) Eq. (4) can be linearized to Eq. (5):
ꢀ
hydration and ionization equilibria of HC(@O)CO2 in basic med-
1
1 þ K1=½Hꢃþ
1
a
¼
þ
ð5Þ
ium (see Scheme 1) and Gl2ꢀ {(HC(Oꢀ)(OH)CO2ꢀ} as the reactive
species, in addition to Glꢀ (see Scheme 2):
fðsÞ
aQfðsÞ½HGlꢃT
k
obs

S. Nayak et al. / Polyhedron 30 (2011) 1637–1645
1643
Table 3
Calculated values of the rate and equilibrium constants and activation parameters for the oxidation of glyoxylic acid by [MnIVL] ([MnIIIL]ꢀ).
Qfa (Qs)a (Mꢀ1
)
f
s
f
s
f
s
0f
b
0s
a
a
a
104k1 (104k1)a (sꢀ1
)
104k2 (104k2)a (Mꢀ1 sꢀ1
)
102k3 (102k3)a (Mꢀ1 sꢀ1
)
1012k4 (1012k4 )b (sꢀ1
)
Temperature (°C)
25.0
50 (110)
4.25 0.19 (1.63 0.13)
14.1 0.5 (2.68 0.14)
1.0 3.4c (0.0002 3.5)c
0.006 2.7d (0.08 1.4)d
9.8 7.5c (0.001 3.9)c
0.066 6.7d (2.8 4.5)d
10.5 8.6c (0.011 3.2)c
10.3 5.4d (3.3 1.7)d
22.5 7.9c (0.08 5.6)c
6.19 0.33 (1.41 0.25)
0.16 0.032 (0.077 0.016)
0.87 0.07 (0.337 0.046)
35.0
25 (70)
9.65 0.55 (2.58 0.16)
40.0
45.0
20 (60)
18 (40)
26.0 1.0 (3.54 0.18)
49.4 1.4 (8.98 0.58)
15.7 1.7 (4.61 0.35)
54.4 1.6 (8.20 0.67)
1.48 0.11 (0.492 0.034)
4.97 0.21 (1.37 0.29)
17.1 14.4d (0.000 20.0)d
e
D
D
H
S
(kJ molꢀ1
)
94.7 2.4 (60.7 20.3)
88.9 19.6 (77.6 12.0)
137 14 (96.6 16.2)
ꢀ33 46 (ꢀ173 51)
e
(J Kꢀ1 molꢀ1
)
+7.7 8.0 (ꢀ116 65)
P
+28 62 (ꢀ23 38)
P
Based on pK1 = 3.0 (25–40 °C); 2.9 (45 °C); [104(kfcal ꢀ kof bs)]2 = 2.28 (25 °C), 7.60 (35 °C), 22.0 (40 °C) and 23.1 (45 °C); [104(kcsal kosbs)]2 = 1.99 (25 °C),1.57 (35 °C),
a
2.59 (40 °C) and 10.2 (45 °C).
[104(kfcal ꢀ kof bs)]2 = 0.109 (25 °C), 2.08 (35 °C), 1.50 (40 °C) and 16.6 (45 °C);
R
[104((kscal ꢀ kosbs)]2 = 0.028 (25 °C), 0.949(35 °C), 0.145(40 °C) and 29.7 (45 °C).
b
R
c
d
e
From acidic medium.
From basic medium.
Parenthesized values are for MnIIILꢀ. ln(ki/T) = [ln(kB/h) +
D
S /R] ꢀ
DH /RT.
3.7. Effect of ionic strength
The effect of ionic strength on kfobsðkosbsÞ was studied at constant
pH (2.30 0.09), at 0.135 6 I, M 6 0.935 ([MnIVL]T = 8.0 ꢄ 10ꢀ5
,
[HGl]T = 0.05 M, 40 °C). Under these conditions, HC(OH)2CO2H (hy-
drated HGl) is the dominant reactant. Both the rate constants (kof bs
and ksobs) are virtually insensitive to ionic strength variation
(Fig. 12S), consistent with the coulombic concept that the reacting
species are either uncharged {MnIVL and hydrated HGl} or only one
of them bears a negative charge (MnIIILꢀ).
The effect of the ionic strength on k04fðsÞ was also investigated.
fðsÞ
The k
values collected at 9.11 6 pH 6 10.18), 0.01 6 [Glꢀ]T,
obs
M 6 0.10 and 0.135 6 I, M 6 0.935 ([Glꢀ]T = 0.025 M) {40 °C, see
footnote b of Tables 2a and 2b} could be iteratively fitted to Eq.
(8), based on the Debye-Hückel model [52]:
ꢁ
ꢂ
log kfðsÞ=½GlꢀꢃT ꢀ k2fðsÞ ꢀ pH ¼ logk400fðsÞ þ S½I1=2=ð1 þ I1=2
Þ
obs
þ C0Iꢃ
ð8Þ
where k400fðsÞ (¼ k04fðsÞKd0) denotes the parameter at zero ionic strength.
The value of ks2 used in this calculation was also ionic strength cor-
rected using the value of ks2 at I = 0.5 M (see Table 3) as a reference
fðsÞ
Fig. 10. Dependence of
k
on glyoxylic acid, at pH 9.67 ( 0.04), [MnIVL] =
obs
8.0 ꢄ 10ꢀ5 M, I = 0.5 M (NaClO4) and T = 40 °C; (1) 104kfobs, (2) 104ksobs. Experimental
points are represented by dots and the calculated best fit points are represented by
a solid line.
ks2(I)/ks2(I = 0.5) =
C
(I)/
C
(0.5),
C
¼
cGl2ꢀ cMn
=
c–ðMnIII
and the
IIILꢀ
LꢁGlÞ3ꢀ
activity coefficient relationship, log
c
i = ꢀ0.5Zi2I1/2/(1 + I1/2). We ob-
tained log k040ðsÞ = ꢀ13.2 0.2, S = 2.5 0.6 for C0 = ꢀ0.12 ( corr.
Coeff. = 0.716 for the least squares best line plot of {log
(ksobs=½GlꢀꢃT ꢀ k2s ) – pH} versus {I1/2/(1 + I1/2) + C0I}. Similar analysis
k4f
MnIVL þ HACðOꢀÞðOHÞCO2ꢀ ! MnIIILꢀ þ HACðOꢀÞðOHÞCOꢀ2
ð6aÞ
ð6bÞ
for kfobs yielded log k040f = ꢀ11.1 0.2, S = ꢀ3.6 0.7 for C0 = ꢀ0.4 (corr.
coeff. = 0.776) (Fig. 13S). The positive value of the slope (S) is satis-
factorily reconciled with the reaction between two negatively
charged species, MnIIILꢀ and Gl2ꢀ. This is in contrast with a negative
value of S for the reaction between the uncharged MnIVL (a species
carrying no net charge, but charges are distributed at several centers
in the molecule) and the same di-negative reductant.
fast
MnIVL þ HACðOꢀÞðOHÞCOꢀ2 ! MnIIILꢀ þ C2O2ꢀ þ 2Hþ
4
k4s
MnIIILꢀ þ HACðOꢀÞðOHÞCO2ꢀ ! MnIIL2ꢀ þ HACðOꢀÞðOHÞCOꢀ2
ð6cÞ
ð6dÞ
fast
MnIIILꢀ þ HACðOꢀÞðOHÞCOꢀ2 ! MnIIL2ꢀ þ C2O2ꢀ þ 2Hþ
4
fðsÞ
4. Mechanism
and k takes the form :
obs
The reduction of MnIVL to MnIIL by glyoxylic acid involves two
successive one-electron transfer steps via MnIIILꢀ as the reactive
intermediate. Glyoxylic acid (HGl) and its mono anion (Glꢀ) and
dianion (Gl2ꢀ) were identified kinetically as the reactive reduc-
tants. The Gl2ꢀ species resulted from the ionization of the gem-diol
form of Glꢀ in basic medium. Szammer et al. [53] has reported,
fðsÞ
k
¼ ½k4fðsÞKd=fKd þ ½Hꢃþð1 þ Kꢀhy1dÞg þ kf2ðsÞꢃ ꢄ ½GlꢀꢃT
¼ ðk04fðsÞ=½Hꢃþ þ k2fðsÞÞ½GlꢀꢃT
obs
ð7Þ
where k04fðsÞ ¼ kf4ðsÞKd (for 1 >> Kꢀ1 and [H+] >> Kd). The calculated
hyd
values of k40fðsÞ and also kf2ðsÞ are collected in Table 3.
Gl2ꢀ to be the reducing species in the reduction of MnO4 in
ꢀ

1644
S. Nayak et al. / Polyhedron 30 (2011) 1637–1645
strongly alkaline medium ([OHꢀ] = 0.1–2.0 M) with oxalate identi-
fied as the product of the oxidation.
The redox reactions of MnIIILꢀ were found to be slower than
that of MnIVL. This is similar to our earlier observations on
MnIVL + SIV [13] and MnIVL +
L-ascorbic acid/oxalic acid [14] reac-
tions. In addition, these findings are also analogous with the re-
ports on the reduction of MnO2 by organic reductants [54] and
the oxidation of bi-functional organic substrates including glyoxy-
lic acid by soluble MnIV phosphate [55] in which MnIII has been
implicated as the reactive intermediate.
Jaky [55] reports a value of 12.7 Mꢀ1 (25 °C, I = 3 M) for the sta-
bility constant of the inner-sphere glyoxylic acid complex,
MnIII(HGl)(HPO4)(H2PO4). The metal ion in MnIVL and its congener,
MnIIILꢀ is coordinatively saturated by the hexadentate ligand (Fig
1). Their association with HGl is purely of the outer-sphere type
and is controlled by hydrogen bonding interactions. In that context
it is worth noting that MnIIILꢀ has marginally greater associative
propensity for glyoxylic acid than MnIVL (Qs > Qf, see footnote a
in Table 3). Although this difference was not evident in our earlier
work, we nevertheless observed a strong outer-sphere association
of MnIVL and MnIIILꢀ with oxalic acid and ascorbic acid
[Q = 3.9 ꢄ 103 (1.6 ꢄ 103) and 9.4 ꢄ 102 (6.84 ꢄ 102) Mꢀ1 for
Fig. 14. Plot of ꢀlogkMnðIVÞ vs. ꢀlogkMnðIIIÞ at 25 °C (see Table 4) (corr. coeff. = 0.956).
L
Lꢀ
MnIVL(MnIIILꢀ)] [14]. The glyoxylate anions HC(OH)2CO2 and
ꢀ
Intercept on logkMnIV axis = ꢀ0.47 0.19, slope = 0.90 0.091.
L
HC(OH)(Oꢀ)CO2ꢀ, however, did not prove to be good hydrogen
bond donors or acceptors for associating with MnIVL and MnIIILꢀ
to a substantial extent. We are led to believe that these differential
characteristics most likely rest on the preferential hydrogen bond-
ing of the glyoxylic acid (via the carboxylic acid function) to the
deprotonated amide site of the complex ions.
5. Conclusion
The oxidation of glyoxylic acid by MnIVL (L4ꢀ = tetra deproto-
nated 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane) exhibited
biphasic kinetics, MnIIILꢀ being the reactive intermediate. The oxi-
dation products of glyoxylic acid were identified as formic acid and
CO2 in acidic medium and oxalate in basic medium. The ligand
frame (L4ꢀ) remained unchanged but reversibly bound to MnII,
and was in different stages of protonation depending on the pH
of the medium in the final product state. The glyoxylic acid
(HGl), and glyoxalate ions (Glꢀ and Gl2ꢀ) in their hydrated forms
are the reactive reductants. Substantial associations between
MnIVL, MnIIILꢀ and HGl were evident, but no such effect was ob-
served for the glyoxylate ions. This was evidence in favor of a
hydrogen bonding interaction between HGl and MnIVL, MnIIILꢀ.
Polymerization of acrylamide was observed in the redox reaction
in acidic medium, but not in alkaline medium. The electron trans-
fer involved an outer-sphere mechanism.
The generation of radicals by the one-electron oxidation of gly-
oxylic acid is well established by EPR spectroscopy [56]. In the
present study, the formation of two different radicals must be as-
sumed to account for the products, i.e. HCO2H and CO2 in acidic
medium and oxalate in basic medium (see Section 2.5). As no oxa-
late could be detected in acidic medium, we discounted the possi-
bility of inter-conversion of the radicals.
The second order rate constants for the reduction of MnIVL and
MnIIILꢀ by HGl, -ascorbic acid, oxalic acid and HSO3ꢀ/SO3ꢀ via dif-
L
ferent paths reflect wide variations (see Table 4). A linear plot of
ꢀlogkMnIV versus ꢀlogkMnIII with a slope = 0.90 0.091(corr. coeff:
L
Lꢀ
0.956, Fig. 14) is consistent with a common mechanism, which is
essentially reminiscent of the outer-sphere electron transfer [57].
The values of the activation parameters for the different reac-
tion steps of the MnIV/IIIL0/ꢀ + glyoxylic acid redox system (see
Table 3) appear to be sensitive to the oxidation state of the metal
Acknowledgements
ion. However, it is apparent that a higher value of
D
H– is associ-
ated with a more positive value of
D
S–, thus reflecting a compen-
Financial assistance from the U.G.C., New Delhi through the Dr.
D. S. Kothari fellowship (Scheme No. F.4-2/2006(BSR)/13-47/
sation effect normally expected for a common mechanism.
Table 4
Comparative listing of rate constants for the reduction of MnIVL and MnIIILꢀ
.
kMnIV (Mꢀ1 sꢀ1
)
kMnIII (Mꢀ1 sꢀ1
)
Reactions
Conditions
Ref.
L
Lꢀ
(1) (MnIV/IIIL)0/ꢀ + H2Asca
(2) {(MnIV/IIIL)0/ꢀꢁH2Asc} + H2Asc
(3) (MnIV/IIIL)0/ꢀ + HAscꢀa
(4) (MnIV/IIIL)0/ꢀ + H2OXb
3.92
0.285
31.6
1.0
0.15
15.5
0.85
0.0026
0.0179
0.014
8 ꢄ 10ꢀ6
0.77
25.8 °C/I = 0.5 M/1.5% (v/v) MeOH + H2O
[14]
[14]
[14]
[14]
17.3
b
(5) (MnIV/IIIL)0/ꢀ + HOXꢀ
0.096
0.021
0.062
1 ꢄ 10ꢀ4
1.6
[14]
(6) (MnIV/IIIL)0/ꢀ + HGl
(7) ((MnIV/IIIL)0/ꢀꢁHGl) + H+
(8) (MnIV/IIIL)0/ꢀ + Glꢀ
(9) (MnIV/IIIL)0/ꢀ + Gl2ꢀc
25 °C/I = 0.5 M/1.6% (v/v) MeOH + H2O
25 °C/I = 0.3 M/10% (v/v) MeOH + H2O
this work
this work
this work
this work
[13]
ꢀ
(10) (MnIV/IIIL)0/ꢀ + HSO3
0.0105
0.0958
0.0056
0.0568
(11) (MnIV/IIIL)0/ꢀ + SO3
[13]
2ꢀ
a
b
c
H2Asc/HAscꢀ:
L
-ascorbic acid/its mono anion.
H2OX/HOXꢀ: oxalic acid/its mono anion.
From k04 value assuming Kd M = 10ꢀ13 (see text).

S. Nayak et al. / Polyhedron 30 (2011) 1637–1645
1645
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Appendix A. Supplementary data
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