Mechanistic studies on the oxidation of glyoxylic and pyruvic acids by a {Mn3O4}4+ core in aqueous media

Article abstract of DOI:10.1002/kin.20488

In aqueous media, the Mn^lV trimer (Mn^lV ₃(μ.-O)₄(phen)₄(H2O)2]⁴⁺ (1, phen = 1,10- phenanthroline) equilibrates with its deprotonated from (Mn ₃(μ.-O)₄(phen)₄(H₂O)(OH)] ³+ (2). Among the several synthetic multinuclear oxo- and/or carboxylato-bridged manganese complexes known to date containing metal-bound water, to the best of our knowledge, 1 is one of the rare examples that deprotonates (1 ? 2 + H⁺;pK_a = 4.00 (±0.15) at 25.0°C, I = 1.0 mol dm^-3, maintained with NaNO₃) at physiological pH. In aqueous media (pH 2-4), 1 oxidizes both glyoxylicand pyruvic acids to formic and acetic acid, respectively, along with the formation of CO₂, the end manganese state being Mn^II. Kinetic studies suggest that the species 1,its deprotonated form 2, the reducing acids (HA), and their conjugate bases (A^-) all take part in the reaction. The oxidant 1 is found to be more reactive than its conjugate base 2, and HA reacts faster than A- in reducing 1 or 2. The gem-diol form of the α-oxo acids (especially for glyoxylic acid) is the possible reducing species. The Mn^IV₃ to Mn^II transition in the present observation proceeds through the intermediate generation of the spectrally characterized mixed-valent Mn^IIIMn^IV dimer that quickly collapses to Mn^II. The observed rates of glyoxylic or pyruvic acid oxidation do not depend on the variation of 1,10-phenanthroline content of the solution, indicating the absence of any phen-releasing preequilibrium of the title complex in solution. The reactions rates were found to be lowered in media enriched with D₂O in comparison to that in H₂O and a rate-limiting one electron one proton (1e, 1H⁺) electroprotic mechanism is proposed.

Full text of DOI:10.1002/kin.20488

Mechanistic Studies on the
Oxidation of Glyoxylic and
4
+
{
}
Pyruvic Acids by a Mn O
3
4
Core in Aqueous Media
PULAK CHANDRA MANDAL, SURANJANA DAS, SUBRATA MUKHOPADHYAY
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Received 1 July 2009; revised 19 November 2009, 29 December 2009; accepted 8 January 2010
DOI 10.1002/kin.20488
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: In aqueous media, the MnIV trimer [MnIV3(μ-O)4(phen)4(H2O)2] (1, phen = 1,10-
4+
3+
phenanthroline) equilibrates with its deprotonated from [Mn3(μ-O)4(phen)4(H2O)(OH)] (2).
Among the several synthetic multinuclear oxo- and/or carboxylato-bridged manganese com-
plexes known to date containing metal-bound water, to the best of our knowledge, 1 is one of
+
◦
the rare examples that deprotonates (1 ꢁꢀ 2 + H ; pKa = 4.00 (± 0.15) at 25.0 C, I = 1.0 mol
−3
dm , maintained with NaNO3) at physiological pH. In aqueous media (pH 2–4), 1 oxidizes
both glyoxylic and pyruvic acids to formic and acetic acid, respectively, along with the formation
II
of CO2, the end manganese state being Mn . Kinetic studies suggest that the species 1, its
−
deprotonated form 2, the reducing acids (HA), and their conjugate bases (A ) all take part in
the reaction. The oxidant 1 is found to be more reactive than its conjugate base 2, and HA reacts
−
faster than A in reducing 1 or 2. The gem-diol form of the α-oxo acids (especially for glyoxylic
IV
II
acid) is the possible reducing species. The Mn 3 to Mn transition in the present observation
proceeds through the intermediate generation of the spectrally characterized mixed-valent
Mn Mn dimer that quickly collapses to Mn . The observed rates of glyoxylic or pyruvic acid
oxidation do not depend on the variation of 1,10-phenanthroline content of the solution, in-
dicating the absence of any phen-releasing preequilibrium of the title complex in solution.
The reactions rates were found to be lowered in media enriched with D2O in comparison to
III
IV
II
+
that in H2O and a rate-limiting one electron one proton (1e, 1H ) electroprotic mechanism is
proposed. ꢀC 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 323–335, 2010
INTRODUCTION
Photosystem II (PSII), the metalloenzyme found in
green plants, cyanobacteria, and in some algae, catal-
yses the light-driven water oxidation reaction to gen-
erate dioxygen. The active site of PSII may contain
a Mn3Ca cluster, which is linked with a fourth Mn
atom with a mono μ-oxo bridge [1,2]. However, owing
to the limited resolution and problems with radiation
damage [3,4], the crystal structures so far established
can provide only limited information on the precise
Correspondence to: Subrata Mukhopadhyay; e-mail:
smukhopadhyay@chemistry.jdvu.ac.in, ju subrata@yahoo.co.in.
Contract grant sponsor: Council of Scientific and Industrial
Research (CSIR), New Delhi, India.
Contract grant number: 01(2182)/07/EMR-II.
Some representative first-order rate constants for the oxidation
of glyoxylic acid (Table SIA) pyruvic acid (Table SIB) by the title
−
3
◦
−3
complex (0.1 mmol dm ) at T = 25.0 C and I = 1.0 mol dm
NaNO ) are available as Supporting Information in the online issue
at www.interscience.wiley.com.
c 2010 Wiley Periodicals, Inc.
(
3
ꢀ

3
24
MANDAL, DAS, AND MUKHOPADHYAY
4
+
transfer. PCET is a key feature in the S-state transi-
tions in PSII [17].
N
N
N
N
In this report, we describe the kinetics of the oxida-
tion of two small α-keto carboxylic acids of biological
significance [18,19], viz. glyoxylic (HC(O)CO2H) and
Mn
O
O
O
O
H O
OH2
2
3 2
pyruvic acid (H C C(O)CO H), by the trinuclear core
Mn
N
Mn
N
of 1. We also try to resolve here the actual species of
the reducing agents involved in the redox as these α-
keto acids or their anions coexist in equilibrium with
N
N
Figure 1 Schematic drawing of [MnIV3(μ-O)4(phen)4-
H2O)2] . N N is 1,10-phenanthroline.
∗
4+
>
their hydrated geminal-diol forms in aqueous solution
(
[20] and particularly for pyruvic acid or its anion tau-
tomeric enolization [21] makes a more complex reduc-
ing system. A conclusion regarding the involvement
of the actual reducing species of these acids would be
a fertile endeavor, which is often overlooked in the
mechanistic studies of oxidation of these substrates in
aqueous media.
structure of the Mn4Ca cluster though recent EXAFS
5], pulsed EPR data [6], and DFT calculations [7] sup-
port a topology where a fourth Mn center is predicted
to be connected to a trinuclear unit forming a “3 + 1”
structural motif.
[
Synthetic chemists have prepared an array of trin-
uclear (Mn3) clusters as a target in modeling the ac-
tive site of water oxidase. Although the spectroscopic
properties of these models are quite different to that of
the PSII active site, recent X-ray crystallographic data
support a structure particularly for the S1 state of the
water oxidation complex (WOC), where a Mn triangle
is linked to a fourth Mn atom [1,8].
EXPERIMENTAL
Materials
The complex salt hydrate [Mn3(μ-O)4(phen)4(H2O)2]-
(
NO3)4·2.5H2O was prepared according to a known
II
procedure [9] that involves oxidation of Mn -acetate
by Ce
IV
A well-characterized trinuclear Mn complex,
4+
−3
HNO3 containing
in ca. 1.6 mol dm
IV
4+
[
Mn 3(μ-O)4(phen)4(H2O)2] (Fig. 1), could be a
1,10-phenanthroline, and the purity of the prod-
putative one that could be synthesized from aqueous
medium [9]. It has some important features common
to the WOC of PSII: [10–12] oxo-bridged Mn ions
in high oxidation states (oxo-bridged manganese clus-
ters containing N- and O-donor ligands are sometimes
called “molecular bricks” for the WOC in PSII) [1,2],
uct was checked by elemental analysis (found for
C48H41N12O20.5: C 43.9, H 3.35, N,12.9 (%). Calcd.
C 44.2, H 3.20, N 13.0 (%)).
Preparation, standardization, and storage of gly-
oxylic acid, pyruvic acid, and sodium nitrate
were described earlier [16a,22]. 1,10-Phenanthroline
˚
two sets of Mn· · ·Mn contacts, ca. 2.7 and 3.3 A (iden-
(E. Merck, Darmstadt, Germany) was used as received.
tical to the Mn· · ·Mn distances in the Mn4Ca clusters
in PSII) and two fully protonated (or undeprotonated)
water molecules coordinated to two different Mn cen-
ters [2]. Although the functional and structural data
so far available do not allow any conclusive decisions
to be reached regarding the mechanisms proposed for
the water oxidation to dioxygen but the key to all the
mechanistic proposals involve water molecules bind-
ing at two neighboring Mn sites and then subsequent
deprotonation steps followed by O O bond formation
D2O (99.9%) was purchased from Sigma (St. Louis,
MO, USA) or E. Merck. All other chemicals were of
reagent grade. Doubly distilled, deionized, and then
freshly boiled H2O was used throughout.
Equilibrium Measurements
The acid dissociation constants of the Mn trimer and of
the reducing acids, viz. glyoxylic and pyruvic acids in
both H2O and in 95% D2O and 5% H2O mixtures were
determined using a Metrohm (736 GP Titrino) autoti-
trator. In a series of (at least 10) experiments, solutions
[13]. The title oxidant (1) thus meets several notable
criteria for a synthetic analogue of the Mn cluster in the
WOC. In some model complexes, water molecules co-
ordinated to Mn centers enhance the reactivity of their
hosts [14]. Thus, the role they could play in controlling
the reactivity of 1 appears to be interesting. An added
interest in the redox reactions of multinuclear high-
valent Mn complexes is to examine the possibility of
occurrence of proton-coupled electron transfer (PCET)
−
3
of 5.0–10.0 mL Mn complex (0.05–0.1 mmol dm
)
−3
in 1.0 mol dm NaNO3 (pH of all these solutions
were initially made at ∼2.0 by adding HNO3) were
∗
The ranges of hydration constants of the reducing species as
found from the representative references in [54–65] are as follows:
2
3
glyoxylic acid 1.0 × 10 to 1.4 × 10 , glyoxylate 6.5–260, pyruvic
acid 0.61–3.1, and pyruvate 6.4 × 10− to 0.21.
2
[15,16] in their mechanistic pathways of electron
International Journal of Chemical Kinetics DOI 10.1002/kin

MECHANISTIC STUDIES ON THE OXIDATION OF GLYOXYLIC AND PYRUVIC ACIDS
325
titrated with 1.0–2.0 mmol dm 3 freshly prepared,
−
Theoretical Studies
−3
carbonate-free NaOH solutions in 1.0 mol dm
The C C bond lengths in glyoxylic or pyruvic acid and
in their hydrated forms were estimated by semiempir-
ical calculations using the HYPERCHEM 5.01 [28] at
AM1, PM3, and ZINDO/1 levels.
◦
NaNO3 at 25.0 ± 0.1 C. Similar procedures were
−3
adopted (using 0.05–0.1 mmol dm α-keto acids) for
the determination of acid dissociation constants of gly-
oxylic and pyruvic acids [16a].
Stoichiometric Measurements
RESULTS AND DISCUSSION
Equilibrium Measurements
The stoichiometry of the reactions was measured in ki-
IV
netic conditions (THA > 3[Mn 3], pH 2.5–4.0, where
HA defines glyoxylic or pyruvic acid and THA is
The built-in program of the autotitrator yielded acid
the analytical concentration of the acids defined as
ionization constants pK1 = 4.00 (± 0.15) and pK1 =
−
THA = [HA] + [A ]). Reaction mixtures containing
4
.70 (± 0.20) for the trinuclear Mn complex (1) in H2O
−3
6
–15 times HA over the oxidant (0.05–0.5 mmol dm )
and in 95% D2O and 5% H2O mixed media, respec-
tively. The measured pKa values of glyoxylic and pyru-
vic acids are 3.13 and 2.50 in H2O and 3.40 and 2.75
in 95% D2O and 5% H2O media, respectively [16a].
These values are averages of at least 10 independent
measurements with associated uncertainty well within
were allowed to react completely (until absorbance be-
came less than 0.01 at 420 nm). Formation of formic
and acetic acids, respectively, as the oxidation product
of glyoxylic and pyruvic acids was established with
the aid of chromotropic acid and the La(NO3)3/I2 test
2+
[
23], where no interference was caused by Mn ion
0.10 units. The decrease in acidity of 1 or of the re-
and 1,10-phenanthroline. The generation of gaseous
CO2 as common oxidation product of both glyoxylic
and pyruvic acids was quantitatively established by GC
analysis as described earlier [24].
ducing acids in medium that is relatively poor in H2O
content is in accord with the fact that weak acids be-
come weaker when solvent H2O is replaced by D2O
[16a, 29]. D2O has a smaller autoprotolysis constant
than H2O by a factor of ca. 5, and thus it is less ba-
sic than H2O [30], which means that anions of weak
acids will be able to compete with the solvent for the
deuteron in D2O more effectively than for the proton
in H2O. An increase in pKa values is thus expected in
D2O-enriched media. It is to be noted that the title trinu-
clear Mn oxidant is one of the rare examples known so
far among the synthetic multinuclear Mn complexes
containing metal-bound water [31] that deprotonates
at physiological pH. Currently, the only other example
Physical Measurements and Kinetics
All absorbance and optical spectra were recorded
with a Shimadzu (1610 PC) spectrophotometer using
1.00-cm quartz cells. The kinetics was monitored in
situ in the “kinetic mode” of the instrument at 420 nm
◦
in the electrically controlled thermostated (25 ± 0.1 C)
cell housing (CPS-240A). The ionic strength was nor-
−3
mally maintained at 1.0 mol dm with NaNO3. Excess
+
IV
1
,10-phenanthroline concentration, Cphen (= [Hphen ]
known is the pKa value of the water bound to Mn
−3
III/IV
+
[phen]), in the range 1–50 mmol dm , was used in
in [(terpy)2Mn
(μ-O)2(H2O)2](NO3)3 nearly at 4
ꢁ
ꢁ
ꢁꢁ
all the kinetic runs. Both 1,10-phenanthroline and the
reducing acid, present in large excess over the Mn oxi-
dant, acted as good buffers for the title redox studied in
(terpy = 2,2 :6 ,2 -terpyridine) [32].
The acidity values for the glyoxylic and pyruvic
acids compare well with the literature data [33,34].
Apparently, it is expected that because of the electron-
pushing effect of the methyl group, glyoxylic acid
should behave as a stronger acid than pyruvic acid
(cf. formic acid vs. acetic acid). The observed reversal
in the expected order of acid strength is in line with the
exclusive hydration of glyoxylic acid in aqueous solu-
tion leading to the gem-diol species HC(OH)2CO2H,
whereas such aquation is at best weak for pyruvic acid
(see page 324 footnote). Interestingly, it might be noted
that progressive replacement of α-H atom of acetic acid
increases the acidity of the species thus formed, viz.
CH3CO2H (pKa = 4.57) [35], CH2(OH)CO2H (pKa =
3.62) [35], and HC(OH)2CO2H (pKa = 3.13, this work;
+
the pH range 2.0–4.1 (pKa of Hphen = 5.12, [25]. The
pKas of glyoxylic acid and pyruvic acid are, respec-
tively, 3.13 and 2.50 in H2O, vide infra) measured with
an Orion–Ross combined electrode system (model 81-
02), and a maximum of only 0.04 unit drift in pH
was observed during the reaction. The electrode cali-
+
bration was described earlier to read −log10[H ] [26]
directly from a calibration curve constructed by plot-
+
ting pH-meter reading against calculated −log10[H ].
For reaction in D2O, pD was calculated as pHmeasured
+
0.40 [27]. Excess reducing agent (at least 20
times over [Mn 3]) was maintained in all the kinetic
runs.
IV
International Journal of Chemical Kinetics DOI 10.1002/kin

3
26
MANDAL, DAS, AND MUKHOPADHYAY
◦
−3
ꢁ
all pKa values are at 25.0 C, I = 1.0 mol dm ). It may
thus be concluded that the acid dissociation originates
from the hydrated form of glyoxylic acid, whereas the
dehydrated form of pyruvic acid dissociates in aque-
ous solution. The two –OH groups in HC(OH)2CO2H
possibly render the bonded C atom less electropositive,
and the pKa value is therefore a proof of the diol form.
tion (bipy = 2,2 bipyridine) [36]. The difference in
ꢁ
structural rigidity between 2,2 -bipyridine and 1,10-
phenanthroline may be responsible for this observa-
tion. Bipyridine is known to open up to give mon-
odentate form [37,38], and when there is an excess
of a potential ligand such as water, the aromatic rings
to go to the monodentate bipyridine can be entirely
replaced by water ligands. Although bipyridine can ro-
ꢁ
tate around the 2,2 single bond, phenanthroline com-
Stoichiometric Analysis
plexes can not do so [38,39]. The presence of a rigid
aromatic backbone in phenanthroline hinders ligand
rotation, and, thus, blocks its opening by preventing
dissociation of one bite side of the ligand. It is known
that dissociation of one of the bipyridine ligands from
Fe (bipy)3] occurs upon dissolution, and the pro-
cess is acid-catalyzed [37,38], whereas such dissoci-
ation of phenanthroline from [Fe (phen)3] occurs
only very slowly in solution [40].
Quantitative (94 ± 5)% evolution of CO2 was found
in both the α-keto acid oxidations, along with HCO2H
and CH3COOH, respectively, for glyoxylate and pyru-
II
vate redox. The likely Mn species under the reaction
II
2+
II
†
[
conditions are Mn -phen complexes. Equations (1)
and (2) thus represent the overall reactions, respec-
tively, for glyoxylic and pyruvic acid oxidations.
II
2+
Mn3(μ-O)4(phen)4(H2O)2]4+ + 3CHOCO2H
The acid-induced (pH ∼ 2.5) conversion of
[
III
IV
3+
IV
[
(
Mn Mn (μ-O)2(bipy)4] to [Mn 3(μ-O)4(bipy)4-
+
2+
+
+
2H → 3Mn + 4phen + 3CO2
3HCO2H + 3H2O
4+
H2O)2] has been proposed [41] to be initialized
by the opening of a bipyridine chelate ring, the mon-
odentate bipyridine thus produced accepts a proton
that traps it as a monodentate ligand and allows sub-
(1)
4+
[
Mn3(μ-O)4(phen)4(H2O)2] + 3CH3COCO2H
III
IV
+
2+
2 3
sequent release to generate [Mn Mn (μ-O) (bipy) -
+2H → 3Mn + 4phen + 3CO2
3CH3CO2H + 3H2O
3+
(
H2O)2] intermediate on a millisecond time scale
that slowly disproportionates to [Mn 3(μ-O)4(bipy)4-
H2O)2] and Mn . The process is extremely slow
+
(2)
IV
4+ 2+
(
under similar conditions when bipyridine is replaced
by the ligand phenanthroline [41] and the phenanthro-
line analog is, indeed, prepared in a different way (see
the Experimental section).
The trinuclear Mn oxidant is a weak monobasic acid
as evidenced from its pKa value where acidity gener-
Kinetics
In the presence of excess reducing agents, the reactions
followed excellent first-order kinetics for at least four
half-lives whence the first-order rate constants (kobs)
were determined. At a fixed reducing agent concentra-
tion, kobs increases with increasing acidity for the oxi-
dations of both the reducing agents. In either reaction at
a particular acidity, no influence of added phen (phen =
ates from either of the two Mn bound aqua molecules
IV
(
Eq. (3)). The trinuclear Mn species is synthesized
in a highly acidic media (see the Experimental section)
where no oxo-bridge protonation was realized. De-
1,10-phenanthroline) on the reaction rate removes any
possibility of phen-releasing preequilibrium of the title
complex in aqueous solution. Some representative kobs
values are listed in Table SI (in the Supporting Infor-
mation). Averages of the kobs values (Tables SIA and
SIB, both in the Supporting Information) from at least
three independent runs were taken, and the average
coefficients of variation for these measurements was
within 3%.
IV
protonation should thus originate from a Mn -bound
water molecule.
K1
Mn3(μ-O)4(phen)4(H2O)2]4 ꢁꢀ
+
[
+
[
Mn3(μ-O)4(phen)4(H2O)(OH)]3 + H+ (3)
It might be noted here that the bipy analog of the
The observed first-order rate constants, kobs, in-
crease with increasing acidity of the reaction media
for both the redox. On decreasing pH of the reaction
media, the concentration of 1 increases with expect-
edly superior redox reactivity as protonated metal oxi-
dants generally react quicker than their conjugate bases
[16a,22,42]. On the contrary, increasing acidity pro-
IV
4+
title oxidant, [Mn 3(μ-O)4(bipy)4(H2O)2] , releases
3
IV
bipy in solution though d Mn is inert to substitu-
†
◦
The stability constants (25.0 C, I = 0.1 M) for the formation
2
+
of 1:1, 1:2, and 1:3 complexes of Mn and phen are, respectively,
4
−1
7
−2
10
−3
◦
1
× 10 M , 2 × 10 M , and 2 × 10
a
M , and the pK (25.0 C,
+
−
I = 1.0 M) of Hphen is 5.12 (see [26].)
duces more HA at the cost of their conjugate bases, A ,
International Journal of Chemical Kinetics DOI 10.1002/kin

MECHANISTIC STUDIES ON THE OXIDATION OF GLYOXYLIC AND PYRUVIC ACIDS
327
K
a
A + H+
(4)
(5)
(6)
(7)
(8)
HA
4
+
k1
[
Mn ( -O) (phen) (H O) ]
+ HA
+ A-
Products
Products
3
4
4
2
2
4
+
k2
[
[
[
Mn ( -O) (phen) (H O) ]
3
4
4
2
2
3+
k3
Mn ( -O) (phen) (H O)(OH)]
+ HA
+ A-
Products
3
4
4
2
3+
k4
Mn ( -O) (phen) (H O)(OH)]
Products
3
4
4
2
Scheme 1 Possible parallel reactions for the redox; HA is glyoxylic or pyruvic acid and THA = [HA] + [A−].
and in many instances superior redox reactivity of an-
ions over their conjugate acids was observed [29b,43].
The kinetic observation stated above may be best
fitted by Eqs. (4)–(8), shown in Scheme 1, which leads
to the rate law equation (9).
charges, viz. anions of the glyoxylic acid and pyruvic
acid are reacting with 1 and 2, and suggesting a defi-
nite contribution of the k2 and k4 paths; in fact, the k4
path is found to be operating (Table I). The inequality
1
0
8
6
4
2
0
+
+
kobs(Ka + [H ])(K1 + [H ])
+
2
=
k1[H ]
THA
+
+
(k2Ka + k3K1)[H ] + k4K1Ka
(9)
Scheme 1, along with the equilibria (3) and (4),
reflects the observed kinetics. The left-hand side of
Eq. (9) when plotted against [H ] resulted in good
A
+
+
polynomial fits (Fig. 2), second order in [H ], for
both the redox. The nonlinear least-squares solution
of Eq. (9), viz. k1, (k2Ka + k3K1), and k4 as found is
reported in Table I. The rate constants and their com-
posite thus evaluated regenerate all the experimental
kobs values quite satisfactorily (within 7%).
0
1
2
3
4
5
6
3
+
–3
10 [H ], mol dm
The two α-oxo acids and their anions are extensively
hydrated in aqueous medium [20] forming gem-diol
species, the extent of hydration decrease in the order
glyoxylic acid > glyoxalate > pyruvic acid > pyruvate.
However, as the spread of the reported values for
the hydration of each species is sufficiently high (see
page 324 footnote), no attempt was made to compare
their relative reactivities.
3.0
2.5
2.0
1
1
.5
.0
Although, in general, protonated oxidants are
quicker reacting than their deprotonated forms
B
[
16a,22,42], a comparison of reactivities of 1 and 2
−
0.5
0.0
with either HA or A is not possible as k2 and k3 paths
are proton ambiguous. However, it is found that k1 >
(
3
−1 −1
k2)max (as (k2)max = 11.88 dm mol
s
for gly-
3
−1 −1
oxylic acid reaction and (k2)max = 0.73 dm mol
s
0
2
4
6
8
for pyruvic acid reaction, assuming k3 path is absent
for both the redox) that unambiguously concludes a
faster reaction of HA with 1 (k1 path) than 1 reacts
3
+
–3
1
0 [H ], mol dm
+
−3
Figure 2 Plot of left-hand side of Eq. (9) vs. [H ], [com-
−
3
plex] = 0.1 mmol dm , Cphen = 5.0 mmol dm , T =
−
with A (k2 path) for both redox. Purely on this ba-
sis (k1 vs. k2), a quicker reaction of 2 with HA than
with A is anticipated. Furthermore, the kobs values
increase with decreasing ionic strength of the reaction
media (Table SI), indicating reaction between opposite
◦
−3
2
5.0 C, and I = 1.0 mol dm (NaNO3). A: HA = gly-
oxylic acid; B: HA = pyruvic acid. Points in circles are calcu-
lated from experimentally observed first-order rate constants,
whereas the solid curvature is the best fit of the left-hand side
−
+
of Eq. (9) vs. [H ] (see the text).
International Journal of Chemical Kinetics DOI 10.1002/kin

3
28
MANDAL, DAS, AND MUKHOPADHYAY
Table I Rate Constants for the Reduction of the Title Complex by the Reducing Acids at C
T = 25.0 C, and I = 1.0 mol dm (NaNO3)
= 5.0 mmol dm−3,
phen
◦
−3
Glyoxylic Acid
H2O Medium
Pyruvic Acid
H2O Medium
Glyoxylic Acid
D2O Medium
Pyruvic Acid
D2O Medium
Reaction Path
k1 (dm3 mol
−1
s
−1
)
33.4 ± 3.0
(8.8 ± 0.9) × 10
5.5 ± 0.4
3.8 ± 0.3
(2.3 ± 0.3) × 10
0.32 ± 0.03
27.0 ± 2.0
(1.2 ± 0.1) × 10
1.3 ± 0.1
2.9 ± 0.2
(4.0 ± 0.5) × 10
0.14 ± 0.01
−
1
−3
−3
−3
−4
k2Ka + k3K1 (s
)
3
−1 −1
k4 (dm mol
s )
(
k3)max > k1 (Table I), however, does not conclude
to proceed through the intervention of the mixed-valent
Mn Mn dimer.
III
IV
anything about the relative contributions of the k1 and
k3 paths in the redox. However, as (k3)max is much
higher than k4 for both the redox (Table I), one can
anticipate a faster reaction of 2 with HA than with
4[Mn3(μ-O)4(phen)4(H2O)2]4 + 3RCOCOOH
+
+
III,IV
3+
4
−
+ 2H + 8phen → 6[Mn
2
2
(μ-O) (phen) ]
A (like k1 vs. k2). Although it is difficult to explain
the higher reactivity of the reducing acids (HA) than
their anions with 1 or 2, stabilization of hydrogen-
bonded species formed through the oxo-bridges of 1
+
3RCOOH + 3CO2 + 9H2O
(10)
The trinuclear Mn oxidant is involved in a pH- and
−
or 2 with the reducing acids (HA instead of A ) that
+
buffer-dependent (phen/phenH ) transformation to the
makes close proximity of the redox agents, may be
a reason. It has been shown [9] that the coordinated
water molecules in 1 participate in hydrogen bond-
ing with lattice water molecules and nitrate anions.
In [Mn2(μ-O)2(μ-OAc)(H2O)2(bipy)2](ClO4)3·H2O
also, crystallographic data strongly suggest that
the coordinated water molecules participated in
dinuclear mixed-valent species [41] upon dissolution
in water, particularly at pH ≥ 4.5 where free ligand
phenanthroline acts as reducing agent [9]. This con-
version, as noted here, is only less than 5% within 10 h
when the media pH was kept within ca. 2.0–4.1 even in
the presence of excess 1,10-phenanthroline. This trinu-
IV
III
IV
clear Mn to binuclear Mn Mn transformation thus
had no contribution to the observed kinetics reported
here.
a
three-dimensional hydrogen-bonding network
[31a]. In a tetranuclear Mn complex, [Mn4(μ-
3+
O)2(TPHPN)2(H2O)2(CF3SO3)2] , the intramolecu-
It was further observed that an aqueous solution
lar μ-oxo· · ·H2O (Mn-bound) distance becomes con-
III
IV
3+
of the dinuclear [(phen)2Mn (μ-O)2Mn (phen)2]
˚
siderably short (within 2.6 A) due to hydrogen-bonding
reacts with glyoxylic or pyruvic acid with a rate com-
parable or only slightly faster than the observed rate
(
HTPHPN = N,N,N’,N’-tetrakis(2-pyridylmethyl)-2-
hydroxypropane-1,3-diamine [31b].
IV
of reduction of the trinuclear Mn species (1) un-
der comparable conditions of pH and reducing acid
concentrations. Yet, a purely first-order decay of the
IV
Mn 3 species in the presence of excess reducing acids
Mechanism
as observed in this investigation confirms that the in-
termediate dinuclear species formed from the trinu-
clear species decays quickly in a nonrate limiting step
The overall six-electron transaction in the title redox
must proceed through several steps, and in an at-
tempt to detect any intermediate Mn complex, 1 was
mixed separately with slightly less than (1:THA = 1:2)
and slightly more than (1:THA = 1:5) stoichiometric
amounts (Eq. (1) or (2)) of both the reducing agents,
and the spectral changes with time were recorded when
progressive growth and decay of a narrow band at
possibly by reacting with radicals produced from one-
IV
electron oxidation of these reducing acids by Mn
3
species in addition to the reducing acids. Formation of
radical in the reaction media is supported by the ob-
served polymerization when the reactions are carried
out in the presence of 6% (v/v) acrylonitrile solution for
both the redox studied. Radicals produced from one-
electron oxidation of glyoxylate, i.e. glyoxylate radi-
cal, are already established by EPR experiments [45],
and pyruvate radicals have also been proposed else-
where [46]. Moreover, it might be that the intermedi-
684 nm with a shoulder near at 555 nm was observed.
Both bands clearly represent the mixed-valent dimer
III
IV
3+
[44], [(phen)2Mn (μ-O)2Mn (phen)2] , and typi-
cally with 1:THA = 4:3, nearly quantitative (96 ± 2%),
generation of the mixed-valent dimer (Fig. 3) in accor-
IV
II
III
IV
dance with Eq. (10) was noticed. The Mn 3 to Mn
transition in the present system, therefore, is believed
ate Mn Mn dimer first produced is not [Mn2(μ-O)2-
3+
(phen)4] ; it is rather a bis(phenanthroline) species
International Journal of Chemical Kinetics DOI 10.1002/kin

MECHANISTIC STUDIES ON THE OXIDATION OF GLYOXYLIC AND PYRUVIC ACIDS
329
0.20
0.15
0.10
0.05
0.00
(
i)
684 nm
(
vii)
5
55 nm
450
500
550
600
650
700
750
Wavelength (nm)
−3
−3
Figure 3 Spectrum (i) is obtained immediately after mixing 0.075 mmol dm THA (HA = glyoxylic acid) with 0.1 mmol dm
IV
−3
◦
−3
Mn 3 complex at pH 4.04, Cphen = 5.0 mmol dm , T = 25.0 C, and I = 1.0 mol dm (NaNO3). Spectra (ii)–(vii) show
change of (i) at 8, 14, 30, 45, 60, and 420 min, respectively. Spectrum (vii), shown in blue, did not change after a further 5 h and
III
IV
indicates quantitative formation (>96%) of dinuclear Mn Mn (see the text). [Color figure can be viewed in the online issue,
which is available at www.interscience.wiley.com.]
+
ꢁ
[
Mn2(μ-O)2(phen)2(H2O)4]3 , which changes to an
equilibrium mixture of the corresponding tris-
phenanthroline) and tetrakis(phenanthroline) com-
(L = 1,10-phenanthroline or 2,2 -bipyridyl). The equi-
librium mixture oxidizes nitrite, hydrogen peroxide,
or thiosulfate, where the tetrakis(L) species was not
found to be reactive [47a,b]. In the present study, more
acidic environment (pH 2.3–4.1) may result in further
aquation, generating more reactive bis(phenanthroline)
species. The trend that aquated species are more re-
active, however, is not a specialty for the binuclear
complexes; it is also well documented for higher va-
lent mononuclear manganese complexes [48]. The ob-
served kinetics could be best fitted by Eq. (11) of
Scheme 2.
(
plexes. These hydrolytic bis(phenanthroline) and
tris(phenanthroline) species are more aquated than
[
Mn2(μ-O)2(phen)4]3 and are expected to be kinet-
+
ically more reactive [47] and reduced rapidly. These
species have similar spectra [44,47b], and interconver-
sions are likely to be rapid in the time scale for spectral
3+
measurements. [Mn2(μ-O)2(L)4] equilibrates with
its tris(L) species [Mn2(μ-O)2(L)3(H2O)2]3 in ligand
+
buffer (pH 4–5), and this aquation is acid catalyzed
IV
IV
Slow
IV
III
Mn
+
R
Mn 2Mn
+
Rox
(11)
(12)
(13)
(14)
(15)
3
III
Fast
Fast
IV
III
2
Mn 2Mn
+
R
R
Mn Mn
+
Rox
Rox
IV
III
IV
III
II
Mn Mn
+
Mn Mn Mn
+
2
IV
IV
III
II
Fast
III
IV
Mn
+
Mn Mn Mn
3Mn Mn
3
III
IV
Fast
Mn Mn
Products
−
where R = HA or A
Scheme 2
International Journal of Chemical Kinetics DOI 10.1002/kin

3
30
MANDAL, DAS, AND MUKHOPADHYAY
In Scheme 2, the first electron transfer act (Eq. (11))
nucleophilic attack of periodate to the carbonyl group
of the α-oxo acids where the hydrated forms were non-
reactive [55]. However, a reverse trend was found in
this study—an increase in the rate with decreasing pH
that might be due to weaker reactivity of the reducing
anions in comparison to their parent acids. Further-
more, it was noted that glycolic acid (CH2(OH)CO2H)
does not practically react with the Mn oxidant that
lends support the gem-diol forms of the reducing α-
oxo acids could be the actual reactive species. More-
over, it is supposed that enolization of pyruvic acid is
not apparent here, which becomes prominent when
pyruvic acid is used at much higher concentration
and at pH higher than 6 [55], conditions far from the
present experimental conditions. The absence of oxalic
acid in the product solution for pyruvate redox further
confirmed the nonreactivity of enol pyruvate in this
reaction [56].
is rate determining; one presumes all subsequent
steps to be rapid and kinetically silent, involving
one-electron changes with retention of the nuclearity
(
{Mn3O4} core), until at Eq. (14) when the dinuclear
III IV
Mn Mn species is formed. It might be noted here
that instead of reactions (13) and (14) one may propose
reaction (16) as an alternative route for the generation
III
IV
of the mixed valent Mn Mn dimer.
Mn Mn 2 + MnIV2MnIII → 3Mn Mn
IV
III
III
IV
(16)
Cooper and Calvin estimated a very high com-
proportionation constant (101 ) for the reaction (III,
III) + (IV, IV) = 2(III, IV) from electrochemical
studies [44], and thus one can expect a high driving
force for reaction (16) and a very fast redox is thus
expected. Moreover, in aqueous acidic environment
5.6
II
clusters containing Mn is not stable—it quickly re-
Glyoxylate oxidation by high-valent metal centers
is generally faster [16a,55–57] than pyruvate oxida-
tion. The relative strength of C(O) COOH bonds in
these two reducing species may be responsible for this
difference in the rate. The presence of an electron-
pushing methyl group in pyruvic acid increases the
electron density at the adjacent C atom of the C O
group, which reduces similar charge repulsion between
the C O and the COOH C atoms. The calculated
C C bond distances, however, in glyoxylic and pyru-
vic acids, estimated with semiempirical calculations at
the AM1, PM3, and ZINDO/1 level showed a reverse
result; the bond length in glyoxylic acid was found as
II
leases Mn [49]. The reaction shown in Eq. (16) is
III
IV
thus much preferred for the formation of Mn Mn
intermediate [50]. Stepwise one-electron transfer and
retention of nuclearity as proposed above are sup-
ported by the fact that different oxidation levels of
the binuclear catalase enzyme models preserve nu-
clearity [51], the various oxidation levels (S states)
of the manganese cluster in PSII also apparently re-
tain bond connectivity [2] and high valent manganese
has a propensity to form (μ-oxo) complexes in weakly
acidic media [52]. The formation of proposed trinu-
IV
III
IV
III
clear species in Scheme 2 (Mn 2Mn , Mn Mn
2
IV
III
II
˚
and Mn Mn Mn ) was further supported by the
kinetic evidence for the formation of a trinuclear
Mn 2Mn intermediate while studying dismutation
of H2O2 by a {Mn2O2} core [53] and synthetic com-
plexes of Mn 2Mn and Mn Mn Mn are available
31b,54]. The redox decay of the mixed-valent interme-
diate Mn Mn to Mn should also proceed through a
sequence of one-electron transfers. We propose the re-
1.505 (AM1), 1.524 (PM3), and 1.449 A (ZINDO/1),
whereas for pyruvic acid the values were 1.510 (AM1),
IV
III
˚
1.530 (PM3), and 1.458 A (ZINDO/1). These calcula-
3+
tions, when applied to the hydrated form of glyoxylic
acid, results in a bond length of 1.527 (AM1), 1.550
III
IV
IV
III
II
˚
[
(PM3), 1.469 A (ZINDO/1). Similarly, hydrated pyru-
III
IV
II
vic acid gives rise to values 1.534 (AM1), 1.553 (PM3),
˚
and 1.473 A (ZINDO/1). These results indirectly sup-
III
IV
III
II
III
action sequence Mn Mn → Mn2 → Mn Mn
→
port the involvement of hydrated glyoxylic and non-
hydrated pyruvic acid as reactive species, respectively.
The lower C(O) COOH bond energy in hydrated gly-
oxylic acid compared to that in regular pyruvic acid
(dehydrated) may, thus, be a driving force for faster
initial electron transfer leading to faster overall oxida-
tion of glyoxylic acid.
The initial decarboxylation of glyoxylic acid to form
oxalic acid followed by the oxidation of oxalic acid to
CO2 seems unlikely in the present investigation as it
was verified that oxalic acid react with the title Mn
complex with a rate at least 10 times slower than the
oxidation of the glyoxylic acid under comparable con-
dition. Again, it is known that acetaldehyde on oxida-
tion by metal ion in mineral acid yields both formic and
II
2
Mn as suggested by Ghosh and coworkers [15a]
−
for the reaction of the said dimer with HSO , hy-
3
droquinone, and ascorbate. The above sequence is
preferred over two other possible alternatives, viz.
III
IV
II
IV
III
IV
Mn Mn → Mn Mn → etc. and Mn Mn
→
III
II
Mn Mn → etc. The fact that electrochemical reduc-
III
IV
3+
tion of [(bipy)2Mn (μ-O)2Mn (bipy)2] and similar
3+
complexes with a {Mn2O2} core is known to pro-
III
duce an Mn 2 dimer [31d,46] inclined us to propose
III
the transfer of a single electron at a time forming Mn
from Mn Mn .
2
III
IV
The oxidation rates of glyoxylic and pyruvic acids
by periodate were increased on increasing pH, and the
authors suggested the reactions proceed through the
International Journal of Chemical Kinetics DOI 10.1002/kin

MECHANISTIC STUDIES ON THE OXIDATION OF GLYOXYLIC AND PYRUVIC ACIDS
331
6.0 × 10–2
acetic acids [58,59]. In the present investigation, how-
ever, only acetic acid as pyruvic acid oxidation product
was found. Initial decarboxylation of pyruvic acid fol-
lowed by oxidation of acetaldehyde is thus ruled out.
Moreover, it was also noted that acetaldehyde does not
react with the Mn oxidant when used in concentrations
similar to that used for the present reducing agents.
Redox reactions of multinuclear higher valent oxo-
bridged Mn and Fe complexes are often associated with
proton transfers as the oxo-bridges in the reduced metal
clusters must have increased basicity [22,60] compared
to their oxidized forms, and a fast proton transfer from
the bulk solvent is expected. To further explore the
electron-transfer mechanism, the kobs values were mea-
sured in media enriched with D2O and computed the
second-order rate constants or their composites (Ta-
ble I) when kinetic isotope effects were observed, albeit
small, in both the redox. Variations in the rate resulting
from substitution of D2O for solvent H2O are expected
to be slight for simple electron transfer reactions,
whereas substantial rate retardation is expected when
such transfer is coupled with the movement of a proton
that is in equilibrium with solvent protons [61]. Low-
ering of rate constants in D2O media indicates an elec-
troprotic mechanism (proton-coupled electron transfer,
PCET) operates in the rate step of the title redox [62].
An increase in oxo-bridge basicity of the one-electron
reduced species formed in the rate-determining step
.8 × 10–2
5
5
5
5
5
4
.6 × 10–2
.4 × 10–2
.2 × 10–2
pH/pD = 2.50
A
.0 × 10–2
.8 × 10–2
4.6 × 10–2
.4 × 10–2
4
0
20
40
60
80
100
mol-% D O
2
.2 × 10–3
1
1
1
9
8
7
6
5
.1 × 10–3
.0 × 10–3
.0 × 10–4
pH/pD = 4.04
B
.0 × 10–4
.0 × 10–4
.0 × 10–4
.0 × 10–4
0
20
40
60
80
100
mol-% D O
2
(Eq. (11)) should cause a simultaneous proton transfer
Figure 4 Effect of the amount of D2O (mol-%) on
from the bulk solvent to the oxo-bridge. Substantial
increase in oxo-bridge basicity of the reduced (μ-oxo)
manganese species compared to their oxidized forms
is well established [60a—d] in the multinuclear high-
valent manganese chemistry. Notably, the decrease in
the rate in H2O–D2O mixtures for both the redox
studied here was found to be linearly related (Fig. 4;
r > 0.98), suggesting transfer of a single proton in
the slow steps [61b]. From the pH-dependence elec-
trochemical conversion of the closely related bipyri-
−3
◦
kobs. [complex] = 0.1 mmol dm , T = 25.0 C, I =
−3
−3
1
.0 mol dm , and Cphen = 5.0 mol dm . A: glyoxylic
−3
acid, THA = 2.0 mmol dm ; B: pyruvic acid, THA =
−3
2
.0 mmol dm
.
system, formation of a acyl radical (almost surely hy-
drated acyl radical, HC(OH)2, for glyoxylic acid and
possibly dehydrated acyl radical, H3CC(O), for pyru-
vic acid) accompanied by one-electron reduction of the
oxidant, and simultaneous one proton transfer to the
oxo-bridge of one electron reduced oxidant, may de-
fine the rate-determining step. Formation of acyl radi-
cals by one-electron oxidants like Mn [63] or V [64]
has been proposed earlier. Formation of one-electron
reduced oxidant is also supported by the kinetic ev-
˙
˙
IV
4+
dine analogue of 1, [Mn 3(μ-O)4(bipy)4(H2O)2]
,
III
IV
III
to the mixed valent Mn Mn dimer, [(bipy)2Mn (μ-
O)2Mn (bipy)2] , Thorp et al. [49b] proposed an
initial one-electron proton-coupled reduction of the
IV
3+
III
V
IV
III
4+
trimer [Mn 2Mn (μ-O)3(μ-OH)(bipy)4(H2O)2] as
an intermediate. PCET is a characteristic feature in
the S-state transitions in PSII [17] where owing to
changes in the valence state of Mn atoms, a difference
in the basicities of the oxo-bridges connecting the Mn
centers resulting in protonation/deprotonation of the
oxo-bridges [7] and of the nearby amino acid residues
as well as the simultaneous movements of proton and
electrons within the WOC is evident [13a].
IV
III
idence for the formation of a trinuclear Mn 2Mn
intermediate while studying the dismutation of H2O2
3
+
by a {Mn2O2} core [53]. The trimer undergoes step-
wise one electron transfer and retains its nuclearity
III
IV
until the dinuclear Mn Mn is formed, which decays
rapidly to the final product.
IV
Among the three six coordinated Mn centers in
Although it is difficult to exactly pinpoint a sin-
gle act in the rate-determining step of the title redox
1, ligand environment of one (that shown at the top of
Fig. 1) is different from the two other similar Mn .
IV
International Journal of Chemical Kinetics DOI 10.1002/kin

3
32
MANDAL, DAS, AND MUKHOPADHYAY
This unique MnIV is ligated with two oxo ligands,
whereas each of the two other equivalent Mn are co-
by oligomerization, mechanisms of reactions lead-
ing to higher nuclearity necessary in the metallopro-
teins, the relation between μ-oxo Mn and Fe com-
plexes which often exhibit striking resemblances and
ultimately to design redox catalysts for various bio-
IV
ordinated with three oxo ligands. The positive charge
IV
density on the Mn centers is thus not expected to
IV
be similar. The unique Mn that binds with two oxo
ligands should bear more positive charge on it than
the two other Mn centers due to the binding of three
logical transformations. Recently, two dinuclear Mn
IV
III/IV
complexes, one oxo-bridged Mn
2 species [70]
III
IV
strongly electron-donating oxo ligands in each. The
rate step as shown in Eq. (11) should thus involve the
[Mn Mn (μ-O)2(terpy)2(H2O)2](NO3)3 and another
II/II
bis(carboxylate)-bridged Mn 2 dimer [71], have
IV
above-described unique Mn . Moreover, the presence
been reported to catalyze homogeneous water oxida-
tion to dioxygen via the formation of higher valent
reactive species. Although these Mn oxidants require
additional oxidizing agents for their performance, it ap-
pears to be a step toward the goal of making artificial
water splitting catalysts.
IV
of two π-accepting phen ligands in this Mn instead
IV
of one each in the two other Mn should favor electron
acceptance of the former.
CONCLUSIONS
BIBLIOGRAPHY
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ever, a central mechanistic theme for most of the re-
actions of these models including the present ones is
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is a common feature in bioenergetic transformations
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