Mechanistic studies on oxidation of l-ascorbic acid by an oxo-bridged diiron complex in aqueous acidic media

Article abstract of DOI:10.1039/b615593f

[Fe₂(-O)(phen)₄(H₂O)₂] ⁴⁺ (1) (Fig. 1, phen = 1,10-phenanthroline) equilibriates with [Fe₂(-O)(phen)₄(H₂O)(OH)]³⁺ (2) and [Fe₂(-O)(phen)₄(OH)₂]²⁺ (3) in aqueous solution in the presence of excess phen, where no phen-releasing equilibria from 1, 2 and 3 exist. 1 quantitatively oxidizes ascorbic acid (H₂A) to dehydroascorbic acid (A) in the pH range 3.00-5.50 in the presence of excess phen, which buffers the reaction within 0.05 pH units and ensures complete formation of end iron product ferroin, [Fe(phen) ₃]²⁺. The reactive species are 1, 2 and HA^- and the reaction proceeds through an initial 1: 1 inner-sphere adduct formation between 1 and 2 with HA^-, followed by a rate limiting outer-sphere one electron one proton (electroprotic) transfer from a second HA^- to the ascorbate-unbound iron(III). The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b615593f

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PAPER
www.rsc.org/dalton | Dalton Transactions
Mechanistic studies on oxidation of L-ascorbic acid by an oxo-bridged diiron
complex in aqueous acidic media†
Jhimli Bhattacharyya, Suranjana Das and Subrata Mukhopadhyay*
Received 23rd November 2006, Accepted 31st January 2007
First published as an Advance Article on the web 8th February 2007
DOI: 10.1039/b615593f
4
+
[
[
Fe
Fe
2
(l-O)(phen)
(l-O)(phen)
4
(H
(H
2
2
O)
2
]
(1) (Fig. 1, phen = 1,10-phenanthroline) equilibriates with
(l-O)(phen) (OH) (3) in aqueous solution in the presence
3
+
2+
2
4
O)(OH)] (2) and [Fe
2
4
2
]
of excess phen, where no phen-releasing equilibria from 1, 2 and 3 exist. 1 quantitatively oxidizes
ascorbic acid (H A) to dehydroascorbic acid (A) in the pH range 3.00–5.50 in the presence of excess
phen, which buffers the reaction within 0.05 pH units and ensures complete formation of end iron
2
2
+
−
product ferroin, [Fe(phen)
3
] . The reactive species are 1, 2 and HA and the reaction proceeds through
−
an initial 1 : 1 inner-sphere adduct formation between 1 and 2 with HA , followed by a rate limiting
outer-sphere one electron one proton (electroprotic) transfer from a second HA to the
−
ascorbate-unbound iron(III).
Introduction
Binuclear oxo-bridged iron species have received much atten-
tion in bio-inorganic chemistry due to the wide occurrence of
such unit(s) in different organisms and their performance of a
range of biological activities such as the storage and transport
1
of dioxygen (in hemerythrin, Hr), hydroxylation of alkanes
(
4+
2 4 2 2
Fig. 1 Schematic drawing of [Fe (l-O)(phen) (H O) ] . The NꢀN
ligand is 1,10-phenanthroline.
2
in methylmonoxygenase, MMOH), phosphate ester hydroly-
3
sis (in purpleacidphosphatase, PAP) and DNA synthesis (in
4
5
ribonucleotidereductase, RNR). A series of model systems have
also been developed in parallel to contribute better understanding
of the mechanism of action of the bio-molecules and eventually to
mimic their activities. Complexes capable of acting as functional
model for the diiron non-heme proteins must have either a free
coordination site or a ligand (unsaturated metal center), which
can be replaced easily by an incoming substrate molecule.
agent. Ascorbic acid, a weak dibasic acid in aqueous solution, has
a rich redox chemistry and simultaneous proton transfer reactions
in ascorbate redox introduces an additional subtlety. The kinetics
10
of ascorbate reductions for a wide range of metal complexes are
available, where particular attention has been paid to outer-sphere
11
processes because they are most commonly observed. In some
of its reactions, ascorbate binds with the metal centres prior to
Despite the fact that a handful of structural models for oxo-
bridged diiron proteins have appeared in literature during the last
12
electron transfer, when the metal centres allow rapid substitution
of ligands like the water molecules in 1. The title complex is not
6
7
◦
two decades, reactivity and mechanistic studies of these model
III
III
II
a powerful oxidant (E red for Fe
2
/Fe Fe = −0.18 V versus
complexes are not so common. The title l-oxo diiron(III,III)
7c
NHE), yet it rapidly oxidises ascorbic acid. To examine if
8
complex, [Fe
2
(l-O)(phen)
4
(H
2
O)
2
](NO
3
)
4
.5H
2
O, (1) (Fig. 1) with
the increased basicity of the l-oxo bridge of the reduced 1 is
responsible for simultaneous electron proton transfer that could
two labile water ligands, is an excellent Raman spectroscopic
model for the binuclear iron site in RNR and metHr (oxidized
5a,13
reduce the activation barriers
for the apparently unfavourable
8
◦
•
form of Hr). The hydroxo-aquo diiron(III,III) base of (1), [Fe
2
(l-
−
oxidation of ascorbic acid (E red for HA /HA = 0.70 V and that
3
+
O)(phen)
4
(H
2
O)(OH)] , (2) is a possible functional model of
+
for (A + 2H )/H
2
A = 0.40 V (both versus NHE; H
2
A = ascorbic
9
PAP. The diiron complex (1) is freely soluble in water, retains
its dinuclearity in aqueous solution and is stable against self-
decomposition in the acidity range pH 3.0–7.0 in presence of
added phen. The title complex 1 thus meets several criteria for
examining its mechanistic pathways of electron transfer.
−
10,14
acid, HA = ascorbate mono anion, A = dehydroascorbic acid)
is an obvious question. We also note that the mechanistic aspects
15
16
of mononuclear iron(III) (aquated or complexed ) oxidation of
L-ascorbic acid are of considerable interest to chemists due to their
mechanistic versatility. How dimerisation affects the mechanistic
17
Aqueous solutions of ascorbic acid have been widely used by
chemists because of the convenience of ascorbate as a reducing
features of monomeric metal complexes is also a potential interest
of this study.
The intricate interaction between iron and ascorbic acid in food
or in digestive systems have been of considerable significance for
investigation because it directly imparts on the availability and
Department of Chemistry, Jadavpur University, Calcutta 700 032, India.
E-mail: smukhopadhyay@chemistry.jdvu.ac.in
†
Electronic Supplementary Information (ESI) available: Estimation of
18
utilization of these nutrients in human and/or animal bodies. It
the maximum possible value of the pre-equilibrium constant for the inner-
sphere adduct (precursor complex) formation between 1 and HA− and its
is also known that ascorbic acid significantly enhances the iron
19
evidence. See DOI: 10.1039/b615593f
uptake in the intestine. The dinuclear iron(III,III) ascorbate
1
214 | Dalton Trans., 2007, 1214–1220
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redox may thus have a potential biochemical significance and
this study, to the best of our knowledge, is the first of its kind
describing the reduction of an oxo-bridged diiron(III,III) complex
by ascorbic acid.
Experimental
Materials
All solutions were prepared in doubly distilled, freshly
boiled water. Crystals of the title complex [Fe
O) ](NO .5H
2
(l-O)(phen)
O were prepared using a reported method and
4
-
8
(
H
2
2
3
)
4
2
gave satisfactory CHN analyses and identical optical spectra as
8
reported. Stock solutions of L-Ascorbic acid (G. R., E. Merck) of
known concentrations were prepared by accurately weighing out
the required amount and dissolving this quickly in deoxygenated
water. Fresh solutions were prepared prior to each experiment
and diluted as per need. Solutions of recrystalised NaNO
E. Merck) and NaClO (G. R., E. Merck) were standardised
as described earlier. 1,10-phenanthroline (G. R., E. Merck)
and D O (Aldrich, 99.9 atom% D) were used as received. All
other chemicals were of reagent grade. Chromium(II) scrubbed
dinitrogen gas was used for deoxygenating the reacting solutions
while studying the effect of dissolved oxygen on the reaction rate.
3
(G. R.,
Fig. 2 Plot of absorbance versus [H
titration at 510 nm with varying [H A] .
2 T
2
A]
T
during spectrophotometric
4
20
2
Results and discussion
Stoichiometry and reaction products
All the reported kinetic and equilibrium data are at 25.0 (± 0.1)
Spectrophotometric titration (Fig. 2) showed that the stoichiome-
try of the title reaction obeys eqn. (1) where HA is the ascorbate
mono anion and A is the dehydroascorbic acid.
◦
−
C and at I = 1.0 M maintained by NaNO
3
.
III
4+
−
+
[
Fe
2
(l-O)(phen)
4
(H
2
O)
2
]
]
+ HA + H + 2phen
+ A + 3H
Physical measurements and kinetics
2
+
→
2[Fe(phen)
3
2
O
(1)
The kinetics were monitored using a Biologic SFM-3/QS
Stopped-flow spectrophotometer with a DT-2801 data transmis-
sion system at 510 nm—the visible absorption maximum of the
The UV-vis spectra of the product solution confirmed quan-
2
+
titative (99 ± 2)% formation of [Fe(phen)
3
]
as the only iron
end-product with absorption maxima at 510 nm (e = 1.11 ×
2
+ 21
product iron species, [Fe(phen) ] . Spectrophotometric titration
3
4
−1
−121
1
0 M cm ).
and spectral observations were recorded with a Shimadzu (1601
PC) spectrophotometer using 1 cm quartz cells in the electrically-
Although the dominance of ascorbate redox results in the
formation of dehydroascorbic acid, in certain situations oxalate,
tryhydroxy butyric acid or even carbon dioxide are formed as the
◦
controlled thermostatted (25.0 ± 0.1) C cell housing (CPS-
2
40A). Solution pH values (3.00–5.50) were measured with an
24
final oxidation products. The present study conclusively shows
that the oxidation of ascorbic acid by the title oxidant leads
to the formation of dehydroascorbic acid as the sole oxidation
product. The title complex, an overall two-electron oxidant,
oxidises ascorbic acid through a free radical mechanism (vide
infra). No oxalate could be detected in the product solutions (by
Orion pH meter (model 710 A) with calibrated electrodes as
2
2
23
described earlier. The relation, pD = pHmeasured + 0.40 was used
while working in D O media, where pHmeasured is the pH meter
2
reading in D
2
O. Excess (at least ten-fold) reducing agent ([H
2
A]
T
=
=
−
[
[
H
2
A] + [HA ]), 0.50–5.0 mM and 1,10-phenanthroline (Cphen
+
Hphen ] + [phen]), 3–10 mM were used over the complex
CaCl
2
at pH ∼ 4), which further supports the non-occurrence of
concentration (mostly 0.05 mM) in all the kinetic runs.
a single two-electron transfer in the title redox.
Equilibrium studies
Equilibrium studies
The acid dissociation constant of ascorbic acid in 95% D O media
2
The acid dissociation constant of ascorbic acid was determined
by pH-metric titration using a Metrohm (736 GP Titrino)
was found to be 4.28 ± 0.10 at 25.0 ◦C, I = 1.0 M while the
25
7c
reported value in H O media is 4.03.
2
autotitrator in 95% D O media as described earlier.
2
Kinetics
Stoichiometry and reaction products
An isosbestic point (at ∼388 nm) exists among the family of
spectra (Fig. 3) of the equilibrium mixtures of the title complex
with several stoichiometrically deficient amounts of ascorbic acid
probably suggests formation of precursor 1 : 1 complex between
ascorbic acid and the diiron(III,III) complex. The spectrum of
The stoichiometry of the overall reaction was determined by
spectrophotometric titration (Fig. 2) at 510 nm with 0.05 mM
complex solution and varying concentrations of ascorbic acid
under the same conditions under which kinetics were studied.
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−
1
III
Fig. 4 Plot of k
C
the fit of eqn. (9).
0
(s ) versus pH at [Fe
2
] = 0.05 mM, [H
2
A]
T
= 0.50 mM,
Fig. 3 Spectrum of 0.05 mM diiron(III,III) complex solution at Cphen
=
◦
◦
phen = 3.0 mM, T = 25.0 C, I = 1.0 M (NaNO
3
). The solid line represents
3
.0 mM, I = 1.0 M (NaNO
3
), T = 25.0 C is shown in (i). (ii)–(vi)
represent spectra of equilibrium mixtures of the complex with several
stoichiometrically deficient amount of ascorbic acid concentration ([H A]
2
T
◦
7c
2
5.0 C, I = 1.0 M (NaNO
3
) ). The bell-shaped k versus pH plot
0
in mM = 0.025, 0.030, 0.035, 0.040 and 0.045 respectively) at pH 3.85,
◦
reflects the effect of these protic equilibria (eqn (2)–(4), vide infra).
C
phen = 3.0 mM, I = 1.0 M (NaNO
3
), T = 25.0 C. (vii) is the spectrum of
These and other proposed reaction steps are shown in Scheme 1.
equilibrium mixture of stoichiometric amounts of complex and ascorbic
At a fixed pH the k
the plot of k versus [H
Fig. 5) indicating involvement of two ascorbate species at the rate
step.
0
values increase with increase in [H
2
A]
T
and
acid solutions (both at 0.05 mM) at Cphen = 3.0 mM, I = 1.0 M (NaNO
3
)
◦
2
and T = 25.0 C.
0
2
A]
T
is an excellent straight line (r ≥ 0.98)
(
equilibrium mixture of stoichiometric amounts of ascorbic acid
and the diiron(III,III) complex (which results in quantitative
2
+
formation of [Fe(phen)
3
] ) also passes through the isosbestic at
∼
388 nm. However, the spectrum of the straw yellow-coloured
solution of the pure complex does not pass through this isosbestic,
which suggests the said isosbestic point sets up between the
precursor diiron(III,III)–ascorbate complex and the final reaction
2
+
2+
product, [Fe(phen)
3
] . [Fe(phen) ] has a peak at 510 nm and is
3
the dominant colour present. There are additional contributions
from the diiron(III,III) complexes and small amounts of Fe(III)
species formed during the reaction, which explains why the
spectrum shown in Fig. 3(i) is displaced from the well defined
isosbestic point.
The whole reaction course followed good first order kinetics—
the log10(A
r ≥ 0.99) until the completion of reaction, where A
stand, respectively, for the absorbances of the final [Fe(phen)
l
–A ) versus time plots were found to be linear
t
(
l
and A
t
2
+
]
Scheme 1
3
2
+
(
=2[1]Total ) and [Fe(phen)
3
]
t
(=2[1]consumed at t). The observed first-
order rate constants, k
linear plots. Presence of excess phen (pK
that of H
0
, were evaluated from the slopes of these
The reaction rate remains invariant when Cphen is varied in the
range 3.0–10.0 mM, revealing absence of any phen dissociation
equilibrium from the title complex in aqueous solution. The
added excess of phen confirms the quantitative formation of
tris(phenanthroline) complexes from any possible transient bis
+
26
a
of Hphen = 5.12 and
2
+
26
25
2
phen = 1.9 ) and ascorbic acid (pK
a
= 4.03) control
any pH drift during the reactions well within 0.05 units. Presence
or absence of dissolved oxygen did not alter the observed rate
significantly (within 5%).
intermediates. Substantially higher k values were yielded (Table 1)
0
In the investigated pH range (3.00–5.50), k
0
increases sharply
when the reactions were carried out at lower ionic strengths (I <
with increase in pH at lower pH level but this increase is restricted
0.1 M) pointing towards reaction between oppositely charged
ions like 1 and its hydrolytic derivatives with HA . Although the
−
at higher pH and at pH ca. ≥ 4.2 significant rate retardation
−
was found (Fig. 4). On increasing pH the concentration of HA
ionic strengths employed in these experiments lie well outside
the Debye–H u¨ ckel region, the ionic strength dependence is
25
increases (pK
ionization constant of H
its conjugate acid H A due to its anionic charge, whereas the less
reactive deprotonated form of the oxidant continually increases
pKa1 and pKa2 of 1 are 3.71 ± 0.05 and 5.28 ± 0.10 respectively at
a
of H
2
A is 4.03 at I = 1.0 M where K
a
is the first
27
2
A) with expected superior reactivity over
qualitatively as expected for the charges involved. The k values
0
2
remained unchanged when the reactions were carried out with
varied complex concentration (0.025–0.10 mM) in the presence of
2
+
(
added [Fe(phen)
3
]
(upto 0.3 mM) and at different monitoring
1
216 | Dalton Trans., 2007, 1214–1220
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a
Table 2 Composite rate constants for the oxidation of ascorbic acid by
◦
the dinuclear iron(III,III) complex at T = 25.0 C, I = 1.0 M (NaNO
3
),
C
phen = 3.0 mM
Composite rate constant
Composite rate constant
−
2
−1
−2 −1
Reaction path
in H
2
O/M
s
in D
2
O/M
s
−
7
5
6
4
k
k
1
K
K
1
2
(I
(I
1
2
+ HA )
(1 ± 0.05) × 10
(1 ± 0.07) × 10
(7.7 ± 0.1) × 10
(5.9 ± 0.4) × 10
−
2
+ HA )
a
The k
1
2
and k values are for overall reaction and are therefore equal to
twice the rate constant for eqn (12) shown in Scheme 2.
Non-reactivity of 3 in the redox is probably due to absence of
any labile water molecules that inhibit any kind of pre-equilibrium
adduct formation prior to the electron transfer.
+
A plot of left hand side of eqn. (9) versus 1/[H ] resulted in
7
a good linear fit (r ≥ 0.98) with (k
1
K
1
) = 1 × 10 and (k
2
K
2
) =
5
−2 −1
2
III
1 × 10 (both in M s ) (Table 2). These values regenerate the
Fig. 5 Plot of k
C
0
versus [H
2
◦
A]
T
at [Fe
2
] = 0.05 mM, pH = 4.11 ± 0.02,
).
experimental k values within ± 15%. We wish to note here that
0
phen = 3.0 mM, T = 25.0 C, I = 1.0 M (NaNO
3
the reaction of H
2
A with 2 (not shown in Scheme 1) is proton-
−
ambiguous with the reaction of HA with 1 and thus they are
Table 1 Some representative first-order rate constants for the oxidation
a
not separable. However, overwhelming dominance of reactivity of
of ascorbic acid by the dinuclear iron(III,III) complex at Cphen = 3.0 mM,
◦
−
HA over H
2
A in outer sphere reactions (the rate steps k
1
and k
2
T = 25.0 C, I = 1.0 M (NaNO
3
)
shown in Scheme 1 as eqn (7) and (8) are outer sphere, vide infra)
4
−1
b
16b,f ,29
pH
10 [H
2
A]
T
/M
k
0
/s
CV (in %)
is well-established
and the so-far-known reactivity order of
7c,7d,7e
the protolytic species of the title oxidant is 1 > 2 > 3.
It thus
3
3
3
4
4
5
5
4
4
4
3
5
5
5
4
4
.00
5
5
5
5
5
5
5
10
25
50
2.5
5
0.020
0.086
0.189
0.245
0.208
0.079
0.029
0.786
5.102
23.01
0.005c
0.046
4.5
4.7
4.9
4.6
4.8
4.8
4.7
4.5
4.3
4.4
4.3
4.9
4.6
4.7
4.6
4.5
appears that the H
than HA reaction with 1 towards their combined rate.
2
A reaction with 2 might be less contributive
.45
.86
.11
.55
.08
.47
.09
.10
.13
.01
.45
.44
.45
.52
.54
−
+
2
+
+
k
0
([H ] + Ka1[H ] + Ka1
K
a2)(K
a
+ [H ])
1
= k
1
K
1
+ k
2
K
2
K
a1
+
)2
+
(
[H ][H
2
A]
T
K
a
[H ]
(
9)
III
2+
III
Rate = −d[Fe
2
]/dt = 2d[Fe(phen)
3
] = k
0
[Fe
2
]
(10)
d
5
5
5
5
0.062e
Mechanism
The observed trend in composite constants, k
0.030f
0.208
g
1
K
1
> k
2
2
K , is
0.206
indicative of superior reactivity of protonated oxidant over their
a
III
b
[
Fe
2
] = 0.05 mM unless otherwise stated. CV was measured using
7,30
conjugate deprotonated form. In addition to the charge effect,
the relation: CV = sd × 100/x, where sd = standard deviation of
c
a lesser statistical probability of replacing one H
ascorbate in 2 than in 1 is also responsible for K being less than
is 0.1 (see ESI†) and the value
2
O ligand by
the measurements of k
0
and x = average k
0
. See ref. 28. At I =
d
e
0
.50 M (NaNO
3
). At I = 0.05 M (NaNO
3
g
). In presence of 0.20 mM
2
2+
f
III
[
Fe(phen)
3
] . In presence of 6.0 mM phen. [Fe
2
] = 0.025 mM.
K
1
. An estimated upper limit of K
1
of K
by Fe aq
2
is expected to be much lower. Oxidation of ascorbic acid
3
+
15a
(with six replaceable H O molecules) also proceeds
2
wavelengths (440–530 nm). Averages of k
0
values (Table 1) from at
through an initial 1 : 1 pre-equilibrium adduct formation between
the redox agents—the equilibrium constant for adduct formation
least three independent runs were taken and the average coefficient
28
3
of variation (CV) for these measurements were within 5%.
The kinetic observations stated above can be well understood
by the reactions (2)–(8) shown in Scheme 1, and it leads to the rate
law given by eqn. (9), where rate is defined by eqn. (10). In deriving
lies at ∼10 level, where ascorbate acts as a bidentate ligand,
much higher than the estimated value for the adduct formation
between Fe(III,III) and ascorbate (acting as monodentate) in the
present investigation. The dinuclear complex 1 has only one water
molecule on each Fe(III) center. Consequently, little chance is
left for the formation of an ascorbate chelate, but only a mono-
associated complex at either Fe(III) center appears feasible. The
distance between the two oxygen atoms of the two water molecules
eqn. (9), we approximated both K
1
and K
2
much less than unity.
An estimated maximum value of K
1
lies at around 0.1 (see the
ESI†) and thus K
2
should be even less than this purely on charge
grounds.
−
8
In the proposed scheme, we have considered only HA as the
in 1 is 524.4 pm (calculated from the crystal structure of 1 using the
−
active reducing species. Remarkable superior reactivity of HA
programme G. M. SHELXL-93; a programme for crystal structure
refinement, University of Gottingen, Gottingen, Germany, 1993)
whereas the bite distance of the coordinating oxygen atoms in
1
2c,16b,f ,29
over H
2
A in redox has many precedences
and in the acidity
2
−
range employed in the present investigation [A ] is negligibly small
−
25
31
(
pK
a
of HA is 11.93).
ascorbate is nearly 300 pm, which does not allow ascorbate to
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View Article Online
act as a bidentate fashion of binding with 1. The thermodynamic
stability of I and I should thus be much smaller than that
for the [Fe(HA)] . The upper limit estimated for K and K
A subsequent fast electron transfer from the iron(III) bound
ascorbate would lead to the products (Scheme 2).
1
2
2
+
1
2
thus appears reasonable for inner-sphere binding. Moreover, the
III
first coordination sphere of Fe centers in 1 are already nearly
saturated with strong electron-donating ligands.
III 2+
It has been shown that the solution of [L
4
Cl
2
(l-O)Fe
2
]
III 4+32
rapidly aquates to generate [L
stantaneously produces [L (SCN)
(L = 2,2 -bipyridine or 1,10-phenanthroline) that
clearly indicates labile nature of the H O ligands bound to high-
4
(H
2
O)
(l-O)Fe
2
(l-O)Fe
2
]
and also in-
III 2+
4
2
2
] on adding KSCN
32,33
ꢀ
solution
2
5
34
spin d Fe(III). In some instances, spectral evidence was collected
m+
for the generation of a transient blue intermediate [Fe(HA)
n
]
aq
(
n = 1, m = 2 or n = 2, m = 1) during oxidation of ascorbic
3+ 15a,b,c
2 6
acid by [Fe(H
O)
]
in highly acidic media.
Also, a coloured
Scheme 2
1
: 1 adduct formation between l-oxo binuclear Fe(III) species,
2
−
15b
[Cl
3
Fe–O–FeCl
3
]
with ascorbate has been reported.
We propose the rate determining steps to be one-electron
transfer from a free HA to the HA -unbound iron(III) centers
and I to form the respective Fe (HA)–O–Fe dimers,
which quickly collapse either by aquation or by further reduc-
In an attempt to detect the existence of any such likely precursor
−
−
complex, we rapidly mixed reagent solutions, all pre-equilibrated
III
II
◦
in I
1
2
at pH 3.00 (± 0.02) and at 25.0 C, I = 1.0 M, in quartz cells
kept in the cell compartments so that the desired concentrations
3
5
−
4
−5
tion to Fe(II) and Fe(III) monomers. Martell and coworkers,
(
1
2.5 × 10 M in ascorbic acid, 5 × 10 M in diiron complex, 3 ×
36
4+
−
3
and subsequently many authors, explained that the {Fe
2
O}
0
M in phen) are reached after mixing. The absorbance data
5
core unit gains stability from the superexchange of two d high-
spin Fe(III) centers linked by an oxo-bridge. However, the high-
spin iron(II) and iron(III) are probably less strongly bound to
oxide since both the oxidation states having two antibonding
electrons are directed towards the formal bond axes. This would
were then collected immediately after mixing. The process was
executed at various wavelengths ranging between 350–600 nm.
The plot of the absorbances thus found at these very initial times
with k when compared to the spectrum of the pure diiron(III,III)
complex (Fig. 6) reflects a noticeable difference, albeit small, but
the position of the isosbestic point (388 nm, Fig. 3) was maintained
II
III
impart weaker Fe –O–Fe bonds, both of which should be
rapidly broken by aquation, or the bound ascorbate quickly
(
not shown in Fig. 6, see ESI†). These observations clearly infer
III
−
reduces Fe . The HA bound iron(III) centers in I
should bear less formal positive charge than the other iron(III)
center as HA is much more basic than H
1
and I
2
the formation of a precursor complex and the isosbestic at 388 nm
is due to this precursor and the end iron product, [Fe(phen) ] ,
the UV-vis spectrum of which maintains this isosbestic (Fig. 3).
2
+
3
−
37
2
O
and thus the
−
HA -unbound iron(III) in I
1
and I is expected to be reduced
2
2
+
first. The iron(III) monomer [Fe(phen)
reduced to the iron(II) monomer [Fe(phen)
HA —the process is expected to occur rapidly as the reduction of
2
(HA)(H O)] is further
2
2
+
2
(H
2
O) ] by the bound
2
−
3
+
−
9
−1 −1 16a
[
Fe(phen)
Formation of two moles of [Fe(phen)
by two successive (slow and fast) one-electron reductions at
3
] by HA is known to be very fast (k > 10 M s ).
2
+
2
(H
2
O)
2
]
from I and
1
I
2
the two iron centers thus seems feasible. Excess phenanthroline,
present in the reaction media rapidly forms the end iron product,
2
+
[
Fe(phen)
3
] (eqn. 16).
kf
2
+
2+
10
−1
[
Fe(phen)
2
(H
2
O)
2
] phen −ꢂ−−ꢁ− ][Fe(phen)
3
]
K = k
f
/k
b
= 10 M
kb
(
16)
1
0
−1
The equilibrium constant (K = 10 M ) for reaction (16)
38
can be calculated from the known formation constants of
2
+
11
−2
2+
21
−3
[
Fe(phen)
2
(H
Fe aq and phen. The formation rate constant (k
from its bis analogue and phen could be calculated as 7.5 ×
0 M from the reported k s ). Compared
with the value of k
2
O)
2
]
(10 M ) and [Fe(phen)
3
] (10 M ) from
2
+
2+
f
) of [Fe(phen) ]
3
Fig. 6 The plot of the absorbances at initial times (dotted line) at various
wavelengths (see text) shows a noticeable change when compared to the
spectrum of the pure diiron(III,III) complex (solid line).
5
−1 −1
−5
−1 −1 39
1
s
b
(7.5 × 10
M
8
−2 −1
1
(∼10 M s , Table 2, assuming (K
1
)
max ≈ 0.1,
vide supra), it appears at a first approximation that formation of
the end product [Fe(phen) ] from its bis analogue should limit the
2
+
The title redox reaction is initialized by rapid inner-sphere 1 :
3
1
adduct formation between the oxo-bridged binuclear complex
rate of the title redox, and the observed rate should be independent
on ascorbate concentration. The observed dependency of rate on
and ascorbate followed by a slow rate-limiting outer-sphere
electron transfer step involving another ascorbate anion forming
one-electron-reduced unstable [Fe –O–Fe ] and HA radical.
ascorbate concentration along with a purely first-order nature
III
II 3+
•
III
of observed rate (when [Fe
2
] ꢁ [H
2
T
A] ) rules out any rate
1
218 | Dalton Trans., 2007, 1214–1220
This journal is © The Royal Society of Chemistry 2007

View Article Online
determining step of the reaction that does not involve ascorbate.
is thus mechanistically reminiscent to the deoxygenation process
for the oxygen binding to hemerythrin. Reversible oxygenation
to deoxyhemerythrin, disproportionation/reduction of mixed-
2
+
The rate of formation of [Fe(phen)
Rate = k [Fe(phen) (H
Assuming at least 1 × 10 M (1/5th of the initial di-
iron(III) complex) [Fe(phen) (H O) is formed from the col-
lapse of the [Fe (HA)–O–Fe ] intermediate, the formation
3
] by eqn. (16) will be:
2
+
f
2
2
O)
2
] [phen]
(17)
45a,48a,b,49
valent forms of diiron sites in hemerythrin,
reaction of
−
5
50
51
52
NO, HNO
2
or H
2
O
2
with deoxyhemerythrin unambiguously
2
+
2
2
2
]
demonstrates that oxo-bridge protonation is an essential prereq-
uisite for the above noted redox processes.
III
II 2+
2
+
rate of [Fe(phen)
(
3
]
under the chosen experimental condition
−
3
−1
[phen]minimum = 1 mM) would be 7.5 × 10 M s . It thus appears
2+
3
Acknowledgements
that the formation of 0.1 mM [Fe(phen)
]
(most of the kinetic
III
runs were performed with initially [Fe
2
] = 0.05 mM) requires
Both J. B. and S. D. gratefully acknowledge Council of Scientific
and Industrial Research (CSIR), New Delhi for the award of
Senior Research Fellowship (SRF) to both of them by CSIR. We
also thankfully acknowledge Professor Soumen Basak, Head,
Chemical Sciences Division, Saha Institute of Nuclear Physics
−
3
only around 1/100th of a second. (d[ferroin] = 0.1 × 10
= 7.5 × 10 M s × 1/75 s)). The time requirements for the
observed reactions are much larger (∼0.1–400 seconds). A fast
much faster than the rate step of the title redox) formation of
M
−
3
−1
(
(
2
+
[Fe(phen)
3
] is therefore conceived. Thus, use of excess Cphen was
(
SINP), Salt Lake, Calcutta 700 064 and Dr. Kalyan Giri of SINP
essential also to ensure rapid and quantitative formation of the
for permitting and providing kind help in using the Stopped Flow
Spectrophotometer at SINP.
3
+
2+
tris(phenanthroline) complexes, [Fe(phen)
3
]
and [Fe(phen) ] ,
3
from any transient bis intermediates. Additionally, it has been
4
0
41
established that chlorite, peroxydiphosphate and hydrogen
42
2+
4+
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D O (Table 2) is reduced considerably, suggesting an electroprotic
2
45
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and/or kinetic barriers to further reduction. In this context we
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2
O media
47
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2
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