Dissection of the mechanism of manganese porphyrin-catalyzed chlorine dioxide generation

Article abstract of DOI:10.1021/ic201430v

Chlorine dioxide, an industrially important biocide and bleach, is produced rapidly and efficiently from chlorite ion in the presence of water-soluble, manganese porphyrins and porphyrazines at neutral pH under mild conditions. The electron-deficient manganese(III) tetra-(N,N-dimethyl)imidazolium porphyrin (MnTDMImP), tetra-(N,N-dimethyl)benzimidazolium (MnTDMBImP) porphyrin, and manganese(III) tetra-N-methyl-2,3-pyridinoporphyrazine (MnTM23PyPz) were found to be the most efficient catalysts for this process. The more typical manganese tetra-4-N-methylpyridiumporphyrin (Mn-4-TMPyP) was much less effective. Rates for the best catalysts were in the range of 0.24-32 TO/s with MnTM23PyPz being the fastest. The kinetics of reactions of the various ClO_x species (e.g., chlorite ion, hypochlorous acid, and chlorine dioxide) with authentic oxomanganese(IV) and dioxomanganese(V)MnTDMImP intermediates were studied by stopped-flow spectroscopy. Rate-limiting oxidation of the manganese(III) catalyst by chlorite ion via oxygen atom transfer is proposed to afford a trans-dioxomanganese(V) intermediate. Both trans-dioxomanganese(V)TDMImP and oxoaqua-manganese(IV)TDMImP oxidize chlorite ion by 1-electron, generating the product chlorine dioxide with bimolecular rate constants of 6.30 × 10 ³ M^-1 s^-1 and 3.13 × 10³ M^-1 s^-1, respectively, at pH 6.8. Chlorine dioxide was able to oxidize manganese(III)TDMImP to oxomanganese(IV) at a similar rate, establishing a redox steady-state equilibrium under turnover conditions. Hypochlorous acid (HOCl) produced during turnover was found to rapidly and reversibly react with manganese(III)TDMImP to give dioxoMn(V)TDMImP and chloride ion. The measured equilibrium constant for this reaction (K_eq = 2.2 at pH 5.1) afforded a value for the oxoMn(V)/Mn(III) redox couple under catalytic conditions (E′ = 1.35 V vs NHE). In subsequent processes, chlorine dioxide reacts with both oxomanganese(V) and oxomanganese(IV)TDMImP to afford chlorate ion. Kinetic simulations of the proposed mechanism using experimentally measured rate constants were in agreement with observed chlorine dioxide growth and decay curves, measured chlorate yields, and the oxoMn(IV)/Mn(III) redox potential (1.03 V vs NHE). This acid-free catalysis could form the basis for a new process to make ClO₂.

Full text of DOI:10.1021/ic201430v

ARTICLE
pubs.acs.org/IC
Dissection of the Mechanism of Manganese Porphyrin-Catalyzed
Chlorine Dioxide Generation
Thomas P. Umile, Dong Wang, and John T. Groves*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
S Supporting Information
b
ABSTRACT: Chlorine dioxide, an industrially important biocide
and bleach, is produced rapidly and efficiently from chlorite ion in
the presence of water-soluble, manganese porphyrins and por-
phyrazines at neutral pH under mild conditions. The electron-
deficient manganese(III) tetra-(N,N-dimethyl)imidazolium por-
phyrin (MnTDMImP), tetra-(N,N-dimethyl)benzimidazolium
(MnTDMBImP) porphyrin, and manganese(III) tetra-N-meth-
yl-2,3-pyridinoporphyrazine (MnTM23PyPz) were found to be
the most efficient catalysts for this process. The more typical
manganese tetra-4-N-methylpyridiumporphyrin (Mn-4-TMPyP)
was much less effective. Rates for the best catalysts were in the
range of 0.24ꢀ32 TO/s with MnTM23PyPz being the fastest. The kinetics of reactions of the various ClO species (e.g., chlorite ion,
x
hypochlorous acid, and chlorine dioxide) with authentic oxomanganese(IV) and dioxomanganese(V)MnTDMImP intermediates were
studied by stopped-flow spectroscopy. Rate-limiting oxidation of the manganese(III) catalyst by chlorite ion via oxygen atom transfer is
proposed to afford a trans-dioxomanganese(V) intermediate. Both trans-dioxomanganese(V)TDMImP and oxoaqua-manganese-
3
(
M
IV)TDMImP oxidize chlorite ion by 1-electron, generating the product chlorine dioxide with bimolecular rate constants of 6.30 ꢁ 10
ꢀ
1
ꢀ1
3
ꢀ1 ꢀ1
s
and 3.13 ꢁ 10 M s , respectively, at pH 6.8. Chlorine dioxide was able to oxidize manganese(III)TDMImP to
oxomanganese(IV) at a similar rate, establishing a redox steady-state equilibrium under turnover conditions. Hypochlorous acid (HOCl)
produced during turnover was found to rapidly and reversibly react with manganese(III)TDMImP to give dioxoMn(V)TDMImP and
chloride ion. The measured equilibrium constant for this reaction (K = 2.2 at pH 5.1) afforded a value for the oxoMn(V)/Mn(III)
eq
0
redox couple under catalytic conditions (E = 1.35 V vs NHE). In subsequent processes, chlorine dioxide reacts with both
oxomanganese(V) and oxomanganese(IV)TDMImP to afford chlorate ion. Kinetic simulations of the proposed mechanism using
experimentally measured rate constants were in agreement with observed chlorine dioxide growth and decay curves, measured chlorate
yields, and the oxoMn(IV)/Mn(III) redox potential (1.03 V vs NHE). This acid-free catalysis could form the basis for a new process to
make ClO2.
’
INTRODUCTION
stream such as chlorine and chlorate ion can cause corrosion of
2,5
equipment or toxicity concerns. Moreover, the instability of
Chlorine dioxide has emerged as the preferred agent for
ClO mandates that all of these process issues be addressed
2
microbial decontamination, for wood pulp bleaching and for
the detoxification of sites contaminated by biological warfare
directly at the point-of-use. Clearly, there is a need for a simpler,
6
7
1
,2
more reliable and more green process for the production of
ClO2.
agents such as anthrax. As a broad-spectrum biocide, chlorine
dioxide has advantages over chlorine because of its higher
potency against bacteria, viruses, and parasites while having
a decreased tendency to produce haloacetic acids and chloro-
form. Both gaseous and aqueous solutions of chlorine dioxide,
sometimes in concert with paraformaldehyde, have proven to be
effective in the decontamination of Bacillus anthracis spores in
buildings and on machinery. Despite these advantages, the
commercial production of chlorine dioxide (ClO ) continues
to be problematic because of its instability for long-term storage.
Industrial processes to produce ClO typically involve the
Recently, we reported the discovery that ClO can be pro-
2
duced rapidly and efficiently using a highly electron-deficient,
8
water-soluble manganese porphyrin catalyst. Key advantages of
this system are the mildly acidic to neutral reaction conditions,
full conversion of the chlorite starting material, high activity on a
3
solid support, and the fact that ClO production occurred within
2
seconds. A more typical cationic manganese porphyrin has been
2
reported to give significantly slower rates and incomplete conver-
9
sion. For both of these manganese porphyrin systems, the pro-
2
2
4
duction of ClO contrasts markedly from the oxygen-producing
2
oxidation of chlorite ion or the reduction of chlorate ion using
strong acids and/or oxidizing/reducing agents leading to inven-
tory storage issues and complications arising from inconsis-
Received: July 6, 2011
Published: September 21, 2011
1
tent product purity. In particular, contaminants in the product
r 2011 American Chemical Society
10353
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|Inorg. Chem. 2011, 50, 10353–10362

Inorganic Chemistry
ARTICLE
Table 1. Turnover Frequencies of ClO Generation
2
initial turnover
ꢀ1
catalyst
frequency (s
)
mol % cat.
a
a
MnTM4PyP
MnTM2PyP
0.01
0.24
0.40
0.48
32.0
0.25
0.25
0.25
0.50
a
MnTDMImP
a
MnTDMBImP
b
MnTM23PyPz
0.27
a
b
Figure 1. Manganese porphyrins and porphyrazine studied as catalysts.
pH 6.8 50 mM phosphate buffer, T = 25 °C, 2.0 mM NaClO
0 mM acetate buffer, T = 25 °C, 1.8 mM NaClO .
2
. pH 4.5
5
2
Figure 2. (a) Rapid appearance of ClO
MnTDMImP-catalyzed disproportionation of ClO
2
(359 nm) from the
ꢀ
Figure 3. Effect of temperature on the evolution of ClO
2
upon mixing
2
(10 mM) at pH
ꢀ
III
2
.0 mM ClO2 and 10 μM Mn TDMImP. ClO concentration is
6.8 with 0.1 mol % catalyst (445 nm) showing the first 30 s of reaction,
2
measured by the change in absorbance at 359 nm. Subtle changes in the
catalyst spectrum at this wavelength over very long reaction times
1
scan/s. (b) Similar reaction with MnTM23PyPz (9 mM sodium
chlorite, 10 μM MnTM23PyPz, pH 4.7 100 mM acetate buffer) showing
the first 30 s of reaction, 1 scan/s.
resulted in an overestimate of ClO decay at 35 °C. Experimental
2
conditions: 100 mM pH 6.8 phosphate buffer.
10
pathway catalyzed by the heme protein chlorite dismutase and
11
synthetic iron porphyrins.
Here we describe a dissection of the mechanism of chlorite ion
The appearance of ClO using MnTDMImP was studied
2
in 100 mM acetate buffer (pH 4.7 and 5.7) and 100 mM
oxidation to ClO catalyzed by this active manganese tetra-(N,N-
2
phosphate buffer (pH 6.8). Initial turnover frequencies observed
ꢀ
dimethyl imidazolium) porphyrin (MnTDMImP). The goals of
this effort were to determine which properties of the manganese
complex are essential for effective catalysis and to elucidate which
of several likely pathways are involved. The well-characterized
under these conditions at 25 °C for 2 mM ClO and 10 μM
2
III
Mn TDMImP were 1.00, 1.03, and 0.42 at pH 4.7, 5.7, and 6.8,
respectively. However, the turnover frequency was influenced
more by buffer composition than by pH. Increasing buffer con-
centration and ionic strength (with added sodium perchlorate)
was found to inhibit the MnTDMImP-catalyzed reaction by a
factor of 5 in the range 5 mMꢀ100 mM (Supporting Informa-
tion, Figure S1) while at a constant buffer concentration, the
turnover frequency did not change from pH 5.9 to 7.01 (Sup-
porting Information, Figure S2).
IV
V
oxoMn and trans-dioxoMn oxidation states of this particular
1
2ꢀ14
porphyrin
have facilitated the direct observation of each
individual step in this catalysis. In addition we describe remark-
ably high catalytic activity for chlorine dioxide production by a
cationic manganese porphyrazine.
ꢀ
III
When ClO2 was rapidly mixed with Mn TDMImP at 25 °C,
’
RESULTS
the spectroscopically observed concentration of ClO produced
2
ꢀ
2
Catalytic Generation of ClO from ClO . Several cationic,
rose quickly and reached a plateau within 3 min. The product
2
water-soluble manganese porphyrins and a similar porphyrazine
complex catalyzed the rapid production of chlorine dioxide from
chlorite ion under mild conditions (Figure 1). The evolved ClO2
could be monitored from the appearance of the characteristic
ClO absorbance subsequently decayed in a slower process over
2
the course of about 15 min. The maximum ClO concentration
2
was not affected by temperature (Figure 3), although the reaction
rate increased markedly from 5 to 35 °C (Supporting Informa-
tion, Figure S3).
15
chromophore at 359 nm (Figure 2). During turnover, the
III
V
IV
ꢀ
2
visible spectra of the Mn catalysts remained largely unchanged,
Reduction of oxoMn and oxoMn TDMImP by ClO . The
V IV
demonstrating the resistance of the catalyst to bleaching as well
as providing mechanistic insight (vide infra). The initial turnover
rates for each of the studied catalysts are reported in Table 1.
Extremely high activity (32 TO/s) was observed for the manganese-
reactions of both oxoMn TDMImP and oxoMn TDMImP
with ClO2 were studied by double-mixing, stopped-flow spec-
ꢀ
V
IV
troscopy. In the first push, authentic oxoMn or oxoMn was
III
generated by mixing Mn TDMImP with 1 equiv of oxone or
1
2ꢀ14
(III) porphyrazine, MnTM23PyPz.
tBuOOH, respectively, in 10 mM pH 8.0 phosphate buffer.
1
0354
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Inorganic Chemistry
ARTICLE
Figure 4. (a) Time-resolved UVꢀvis spectra of the reaction between
V
ꢀ
5
μM oxoMn TDMImP and 125 μM ClO in 100 mM pH 4.7 acetate
Figure 5. (a) Time-resolved UVꢀvis spectra of the reaction between
2
IV
ꢀ
buffer over 0.3 s. A single isosbestic point was observed at 437 nm. Inset:
10 μM oxoMn TDMImP and 1.0 M ClO in 100 mM pH 4.7 acetate
2
V
Plot of pseudo-first order rate constant (kobs) of the decay of oxoMn
buffer over 3 s. A single isosbestic point is observed at 433 nm. Inset: Plot
ꢀ
IV
versus initial [ClO2 ]. The apparent second-order rate constant (kapp) is
of pseudo-first order rate constant (kobs) of the decay of oxoMn versus
5
ꢀ1 ꢀ1
ꢀ
(
6.77 ( 0.08) ꢁ 10 M
s
; (b) Time-resolved spectra of the reaction
initial [ClO2 ]. The apparent second-order rate constant (kapp) is
V
ꢀ
3
ꢀ1 ꢀ1
between 10 μM oxoMn TDMImP and 1.0 mM ClO
2
in 100 mM
(6.03 ( 0.06) ꢁ 10 M
s
. (b) Time-resolved UVꢀvis spectra of
IV
ꢀ
pH 6.8 phosphate buffer over 3 s. Two, time-separated isosbestic points
were observed, first at436 nm(0ꢀ225ms) andthenat 432 nm(225 msꢀ
the reaction between 10 μM oxoMn TDMImP and 1.0 M ClO in
2
100 mM pH 6.8 phosphate buffer over 1.5 s. A single isosbestic point is
3
.0 s). Inset: Plot of pseudo-first order rate constant (kobs) of the decay
observed at 431 nm. Inset: Plot of pseudo-first order rate constant (kobs)
V
ꢀ
IV
ꢀ
of oxoMn during the first phase of reaction versus initial [ClO
2
]. The
of the decay of oxoMn versus initial [ClO2 ]. The apparent second-
3
ꢀ1
3
ꢀ1 ꢀ1
apparent second-order rate constant (k ) is (6.30 ( 0.62) ꢁ 10 M
order rate constant (kapp) is (3.13 ( 0.12) ꢁ 10 M
s .
app
III
ꢀ
1
V
IV
s
. Self-decay of Mn and Mn to Mn results in nonzero intercepts.
point between the Soret bands at 437 nm. By contrast, at pH 6.8
V
IV
V
ꢀ
III
In the second push, oxoMn or oxoMn was mixed with an
the reaction of oxoMn with ClO2 afforded Mn (Figure 4,
panel b). The presence of two time-separated isosbestic points
(436 and 432 nm) during this reaction demonstrated the
ꢀ
excess of ClO2 in 100 mM pH 4.7 acetate or pH 6.8 phosphate
buffer. This “pH jump” experiment permitted observation of the
reaction between each of the high-valent manganese species with
IV
V
intermediacy of oxoMn in the reaction between oxoMn and
ꢀ
ꢀ
ClO2 at lower pH values.
ClO2 . Under these conditions, the apparent second-order rate
V
The decay of oxoMn (as measured by the characteristic Soret
absorbance at 425 nm ) was pH dependent and followed pseudo-
first order kinetics when mixed with an excess of ClO2 . This
constant kapp was determined by first obtaining a pseudo-first
order rate constant (kobs) during the first phase of the reaction
(where only the 436 nm isosbestic was observed).
13
ꢀ
IV
ꢀ
decay was fitted to a modeled exponential decay curve to obtain
the observed pseudo-first order rate constant (kobs), which was
The reaction between oxoMn and an excess of ClO2 was
IV
similarly found to result in a pseudo-first order decay of oxoMn
to Mn (as measured by the characteristic Soret absorbance of
oxoMn at 422 nm) at pH 4.7 and 6.8 (Figure 5). A single
ꢀ
III
plotted as a function of [ClO2 ]. The apparent second-order rate
ꢀ
IV
constants (k ) calculated from the slope of k vs [ClO2 ] are
app
obs
5
3
ꢀ1 ꢀ1
6
.8 ꢁ 10 and 6.3 ꢁ 10 M
s
at pH 4.7 and 6.8, respectively.
isosbestic point was observed at 433 nm between the Soret bands
V
ꢀ
IV
III
At pH 4.7, the 1-electron reduction of oxoMn by ClO was
fast and allowed the direct observation of the reduced oxoMn
TDMImP catalyst (Figure 4, panel a) with a single isosbestic
of oxoMn and Mn . The apparent second-order rate constants
2
IV
IV ꢀ 3
measured for the reduction of oxoMn by ClO are 6.0 ꢁ 10
2
3
ꢀ1 ꢀ1
and 3.1 ꢁ 10 M
s
at pH 4.7 and 6.8, respectively.
1
0355
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Inorganic Chemistry
ARTICLE
Figure 7. Determination of the apparent second-order rate constant of
III
oxo-transfer from HOCl to Mn TDMImP. The slope of each line is
III
equal to the product of k and the concentration of Mn (k
=
).
app
ꢀ1 ꢀ1
app 4.7
ꢀ1 ꢀ1
s
4
6
(6.64 ( 0.23) ꢁ 10 M
s
, kapp 6.8 = (1.71 ( 0.27) ꢁ 10 M
III
Figure 8. Comparison of the reaction of ClO with 5 μM Mn TD-
2
MImP at pH 6.8 and 4.7. The ClO
59 nm.
2
concentration was measured at
3
Measurement of k for oxo-Transfer between HOCl and
app
III
Mn . The reaction between hypochlorous acid (HOCl) and
III
Mn TDMImP was studied by single-mixing stopped-flow spec-
trometry. At pH 6.8, the reaction resulted in complete formation
V
of oxoMn in <100 ms (Figure 6). An apparent second-order
rate constant for the reaction was determined by measuring
V
kobs of oxoMn appearance (425 nm) as a function of [HOCl]
(
4
6
ꢀ1 ꢀ1
V
1
gave a slower rate constant of 1.47 ꢁ 10 M
elicited with even a 50-fold excess of HOCl (Figure 6c). Never-
4
theless, the apparent second-order rate constant (k = 6.6 ꢁ 10
4
ꢀ
1
ꢀ1
M
s ) under these conditions was calculated from a plot of
kobs for the decrease in Mn absorbance (444 nm) versus
III
[
HOCl]. A similar analysis using the method of initial rates
3
ꢀ1 ꢀ1
provided a lower calculated k = 4.22 ꢁ 10 M
s
. After a
4
quick reaction time (<0.1 s at pH 4.7), the concentrations of
III
V
Mn and oxoMn reached an apparent steady-state equilibrium.
This steady-state observation was better demonstrated at slightly
III
Figure 6. UVꢀvis spectral changes for the reaction of Mn TDMImP
less-acidic conditions of pH 5.1.
with HOCl at (a) pH 6.8, (b) 5.1, and (c) 4.7. Buffer conditions: pH 6.8,
III
Reaction of ClO with Mn TDMImP. To account for the
1
00 mM phosphate buffer; pH 4.7 and 5.1, 100 mM acetate buffer.
2
III
III
Reaction conditions: (a) 10 μM Mn , 5 equiv of HOCl; (b) 5 μM Mn ,
observed decay of ClO (Figure 3), the reaction of ClO with
2
2
III
III
20 equiv of HOCl; (c) 5 μM Mn , 50 equiv of HOCl.
Mn TDMImP was investigated. The ClO absorbance at 360 nm
2
1
0356
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Inorganic Chemistry
ARTICLE
ꢀ
Table 2. Measured Concentrations of ClO Produced
3
ꢀ
III
(
[ClO3 ]obsd) from the Reaction of 10 μM Mn TDMImP
with a Given Amount of Substrate (ClO or ClO ) at pH 6.8
ꢀ
a
2
2
3ꢀ
3ꢀ
]pred (mM)
added substrate
[ClO
]obsd (mM)
[ClO
1
1
2
mM ClO
mM ClO
mM ClO
2
0.7
0.6
1.7
0.8
0.7
1.4
2ꢀ
2ꢀ
a
3ꢀ
3ꢀ
Also shown are the ClO concentrations predicted ([ClO
]
pred) by
a kinetic model of the proposed mechanism (vide infra) given the same
substrate/catalyst starting conditions.
Scheme 1. Proposed Mechanism for ClO Evolution from
2
ꢀ
ClO2
V
Figure 9. (a) Pseudo-firstorder analysis ofthe reaction between oxoMn
V
and ClO
versus ClO
kapp 6.8 = (1.61 ( 0.12) ꢁ 10 M
2
. The observed first-order decay of oxoMn (kobs) is plotted
4
ꢀ1 ꢀ1
excess buffer anions coeluted with chloride anion, the quantifica-
tion of chloride produced by the catalytic decomposition of
2
concentration. (kapp 4.7 = (2.24 ( 0.14) ꢁ 10 M
s
,
4
ꢀ1 ꢀ1
s ). (b) Pseudo-first order analysis of
the reaction between oxoMn and ClO . The observed first-order decay
2
IV
ꢀ
ClO2 could not be accomplished.
IV
of oxoMn (kobs) is plotted versus ClO
2
concentration. (kapp 4.7
=
).
3
ꢀ1 ꢀ1
4
ꢀ1 ꢀ1
(
7.90 ( 0.53) ꢁ 10 M
s
, kapp 6.8 = (1.19 ( 0.06) ꢁ 10 M
s
’
DISCUSSION
We have studied the catalytic oxidation of ClO2 to chlorine
III
ꢀ
quickly disappeared in the presence of Mn TDMImP at pH 6.8,
but not pH 4.7 (Figure 8). In both cases, the only porphyrin
dioxide by water-soluble manganese porphyrins and a manga-
nese porphyrazine. Large amounts of chlorine dioxide, represent-
ing many turnovers of the catalysts, were observed within
seconds for the tetra-dimethylimidazolium porphyrin MnTDMI-
mP, the corresponding benzimidazolium porphyrin, TDMBImP,
and the cationic manganese porphyrazine MnTM23PyPz. This
latter catalyst proved to react so rapidly with chlorite ion (32
TO/s) that it was difficult to analyze in detail. Those efforts are
ongoing. By contrast, the reaction of chlorite ion with MnTD-
MImP was particularly well behaved, allowing a complete kinetic
deconvolution of the primary steps in this catalysis. Given the
III
oxidation state observed in solution was Mn .
Electron Transfer between oxoMn , oxoMn , and ClO .
IV
V
2
Stopped-flow techniques were again used to observe how
V
IV
oxoMn TDMImP and oxoMn TDMImP reacted with ClO .
2
V
IV
Both oxoMn and oxoMn reacted with ClO via 1-electron
2
IV
transfer. Although oxoMn did not build up in the reaction of
V
oxoMn with ClO , the observation of two, time-separated isos-
2
IV
bestic points indicated the intermediacy of oxoMn in the
reaction. The product of 1-electron oxidation of ClO is formally
2
ꢀ
a chlorine(V) species, shown below to be chlorate (ClO3 ). The
1,2
apparent second-order rate constants (k ) for the reduction of
industrial significance of ClO2, the need for better methods for
its production, and the variety of differing known heme- and
app
V
4
ꢀ1 ꢀ1
4
ꢀ1 ꢀ1
oxoMn by ClO were 1.6 ꢁ 10 M
s
and 2.2 ꢁ 10 M
s
2
8,10,11,17ꢀ19
at pH 6.8 and 4.7, respectively (Figure 9, panel a). Calculated k
porphyrin-mediated reactions of chlorite ion,
it is of
app
and
IV
4
ꢀ1 ꢀ1
for the reduction of oxoMn by ClO are 1.2 ꢁ 10 M
s
interest to understand the mechanisms of these reactions and to
elucidate which catalyst properties favor each of these specific
reactivities.
2
3
ꢀ1 ꢀ1
7
.9 ꢁ 10 M
s
at pH 6.8 and 4.7, respectively (Figure 9,
panel b).
ꢀ
ClO3 Determination by HPLC. The observation that ClO2
itself acted as a 1-electron reductant of oxoMn TDMImP and
oxoMn TDMImP suggests that some chlorine(V) species was
produced. An indirect UV-detection HPLC method was there-
fore employed to confirm and quantify the presence of chlorate
General Mechanism for Manganese Catalyzed ClO Gen-
2
IV
eration.Werecentlyproposed themechanismshowninScheme 1
V
8
to account for the appearance of ClO in these reactions. The key
2
1
6
initiating step in this catalytic cycle was suggested to be oxygen
III
atom transfer from chlorite to the Mn catalyst to form trans-
ꢀ
III
V
in reaction mixtures of either ClO2 or ClO with Mn TD-
dioxoMn TDMImP. The thermodynamic driving force for
2
ꢀ
MImP (Table 2). Buffer, catalyst, ClO2 , and ClO stock solu-
oxo-transfer from chlorite ion is only slightly less than that of
2
ꢀ
ꢀ
ꢀ1 20
tions were free of ClO3 prior to reaction. A measured amount of
either ClO2 or ClO was added to solutions of MnTDMImP
hypobromite ion (BrO ) (ΔΔG° = +16.7 kJ mol ). As we
ꢀ
have reported, hypobromite is a very facile oxo-transfer agent
2
III
13
and stirred for about 1 h before being analyzed. Because the
in its reaction with Mn porphyrins, with a rate constant for
1
0357
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Inorganic Chemistry
ARTICLE
III
Scheme 2. Reversible Reactions of Mn TDMImP with (1)
HOCl and (2) ClO2
Table 3. Calculated Thermodynamic Parameters (K , E) for
the Reversible Oxo-Transfer from HOCl to Mn TDMImP
eq
III
pH
K
eq
ΔE (mV)
EMn(V)/Mn(III) (V, vs NHE)
ꢀ
3
1
4
.7
.1
.6
(5.1 ( 0.5) ꢁ 10
(2.2 ( 0.7) ꢁ 10
12.0
- 68 ( 1
- 20 ( 5
+ 32
+ (1.411 ( 0.001)
+ (1.352 ( 0.005)
+ 1.285
ꢀ
5
5
a
a
(
errors represent standard deviation for multiple experiments with
varying [HOCl], except for pH 5.6 where equilibrium was only observed
spectroscopically for one experiment (50 μM HOCl, 5 μM MnPor).
III
5
ꢀ1 ꢀ1
oxo-transfer from HOBr to Mn TDMImP ∼10 M
s
at
neutral pH. For the reaction with chlorite ion, the fact that the
Mn oxidation state of the catalyst persisted during turnover
III
(
Figure 2) requires that any change in the porphyrin oxidation
III III
state from Mn be slow relative to Mn -forming reactions.
Therefore, we conclude that the oxidation of Mn by ClO2
must be the rate-determiningstep of the overall cycle. Accordingly,
a slower 2-electron oxidation of Mn by ClO , generating HOCl
and trans-dioxoMn is consistent with these observations. Inter-
III
ꢀ
III
ꢀ
2
V
estingly, the pH-independence of the turnover rate (Supporting
Information, Figure S2) implies that the oxygen transfer step to
V
afford the Mn species is pH independent and occurs at about the
III
same rate as the analogous heterolysis of HOO-Mn TDMImP,
V 12
which also affords Mn .
As can be seen in the data in Table 1, higher turnover rates
were observed for porphyrins with the most electron-withdraw-
ing meso-substituents (TDMImP and TDMBImP). Although
porphyrin electronics could potentially influence the binding of
Figure 10. Plot of calculated MnTDMImP EOMn(V)/Mn(III) redox
potential as a function of pH from HOCl equilibrium data (black
squares) and HOBr equilibrium data (black circles). Nernst equation
III
chlorite ion to the Mn center, porphyrin substituent effects
plots for HOBr/Br- (blue), HOCl/Cl- (green) and H
are superimposed for reference.
2 2 2
O /H O (black)
have been shown to have little effect on the normally fast axial
21
ligand exchange rate. However, electron-withdrawing substit-
III
uents on porphyrins have been shown to increase the Mn /
IV
Mn redox potential, to stabilize trans-dioxomanganese(V)
observation with respect to the proposed mechanism is that
V
porphyrins by decreasing the basicity of the terminal oxo-groups,
HOCl and trans-dioxoMn TDMImP are observed to be present
and to lower the energy of the porphyrin a highest occupied
together over the entire period of the measurement. Thus, hydro-
2u
13,22,23
V
molecular orbital (HOMO).
If oxidation of the manganese-
III) catalyst to oxoMn is the rate-determining step of this reac-
gen abstraction from HꢀOCl by OdMn dO to afford the chlo-
V
IV
(
roxy radical and oxoMn must be slow or thermodynamically
tion, the accessibility of this intermediate may be the reason the
more electron-deficient porphyrins are faster catalysts.
unfavorable.
III
ꢀ
V
The equilibrium between HOCl/Mn and Cl /dioxoMn
(Figure 6, panels b and c) allows us to directly calculate K
V
IV
The observed reduction of oxoMn and oxoMn TDMImP
eq
24
V
by chlorite ion (E° = +1.068 V) to afford ClO is highly ana-
logous to the well-studied reduction of such species by nitrite ion
E° = +1.04 V). The similar reduction of Compounds I and II
of horseradish peroxidase by chlorite ion to generate ClO is also
for the reversible oxo-transfer from dioxoMn to chloride ion
(Scheme 2, reaction 1). This steady state could be observed at pH
4.7, 5.1, and 5.6, and the concentrations of both porphyrin
species could be directly measured from the steady-state UVꢀvis
spectrum. From these values and the known HOCl and chloride
ion concentrations, an equilibrium constant Keq was calculated
for this oxo-transfer reaction (Table 3). This value allowed a fur-
2
2
2
(
2
1
8
V
known. The measured k for reduction of oxoMn TDMImP
app
3
ꢀ1 ꢀ1
by chlorite ion at pH 6.8 (6.30 ( 0.62 ꢁ 10 M
s ) is signi-
22
ficantly slower than those previously reported for the related
V
V
III
cationic N-methylpyridinium porphyrins, oxoMn TM4PyP and
ther determination of the redox potential E(OMn /Mn ), for
oxo-transfer from oxoMn TDMImP. The calculated potentials
V
7
4
ꢀ1 ꢀ1
V
oxoMn TM2PyP with nitrite (1.5 ꢁ 10 and 2 ꢁ 10 M
s
,
13
respectively) at pH 7.4. The rate of chlorite oxidation by
as a function of pH are plotted in Figure 10. As can be seen,
there is a pronounced pH effect on the thermodynamics of halide
oxygenation, with bromide ion oxygenation being facile at neu-
tral pH and chloride ion oxidation becoming accessible at pH 5.
The facile oxygenation of chloride ion observed here also sug-
gests that the in situ generation of hypochlorite may be useful in
other manganese porphyrin catalyzed reactions, such as in the
unusual methylene-selective CꢀH chlorination we have recently
V
oxoMn TDMImP increased by approximately 2 orders of mag-
nitude over a ∼2 pH range, indicating that a single protonation of
14,25
the trans-dioxomanganese(V) species is required for activation.
Hypochlorous acid is proposed to be a product of the initial
chlorite-Mn(III) reaction (Scheme 1). In this regard, the results
III
for the reaction of Mn TDMImP with HOCl were very reveal-
V
ing. At pH 6.8, the reaction generated oxoMn completely within
5
ꢀ1 ꢀ1
26
3
00 ms (kapp 6.8 = (2.23 ( 0.11) ꢁ 10 M
s
) (Figure 6).
reported. Interestingly, Figure 10 indicates that water oxidation
III
However, at pH 4.7 and 5.1, Mn TDMImP and trans-diox-
oMn TDMImP were apparently in rapid, reversible equilibrium
could become accessible at pH 3. That reaction is currently under
investigation.
V
(
Scheme 2, reaction 1), similar to the behavior of bromide/
As shown in Figure 2, ClO is generated in an initial fast reac-
tion upon mixing solutions of ClO2 with the catalysts MnTDMImP
2
13
ꢀ
hypobromite with this porphyrin. The significance of this
1
0358
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Inorganic Chemistry
ARTICLE
ꢀ
Scheme 3. Mechanism of ClO Dismutation by
MnTDMImP
is consistent with an initial rate analysis of the same data (Sup-
porting Information).
2
a
Kinetic Simulation of the Mechanism. To evaluate the
IV
proposed scheme (and the reversibility of oxoMn reduction
ꢀ
by ClO2 ), the mechanism in Scheme 3 was modeled by kinetic
simulation. Experimentally determined rate constants for all
known reactions were used, (Table 4) and the model was fitted
to ClO growth and decay curves such as those in Figure 3 and
2
Figure 8. The rate constant k 3, corresponding to the oxidation
ꢀ
III
of Mn by ClO was left as an unconstrained parameter. Two
2
computational fits of the model to the data provided calculated
2
2
ꢀ1 ꢀ1
k
values of 3.04 ꢁ 10 or 6.64 ꢁ 10 M
s
(Figure 11). The
ꢀ3
predicted k value (when taken with the experimentally ob-
ꢀ
3
served k ) allows calculation of a K to be ∼7.5, in agreement
3
eq
IV
with our prediction of a reversible step near pH 6.8. The Mn /
III
Mn reduction potential that can be calculated from these
equilibrium values is between +1.0 V and +1.13 V, depending
upon which of several standards is used for the oxidation
potential of chlorite ion, essentially the same as the measured
a
The bold arrows indicate high-flux pathways. Because of the pH range
27
of this study (4.7ꢀ6.8), hypochlorite is written in protonated form.
value (1.03 V) and those reported for other cationic manganese
28ꢀ30
IV
III
porphyrins.
Thus, the Mn /Mn reduction potential of
and MnTM23PyPz. The ClO concentration was observed to
rise quickly to a plateau and then to decrease with minimal
catalyst bleaching in a slower subsequent phase of the reaction.
MnTDMImP is well matched to that of chlorite ion such that
2
IV
Mn does not accumulate during catalysis.
Activation Parameters for Oxo-transfer from ClO2 to
ꢀ
To account for the further decay of ClO during the slower
Mn(III)TDMImP. Using the data from Figure 3, it was possible
to extract second-order rate constants for the oxidation of Mn
2
III
second phase of this catalysis, the reactions of ClO with authen-
2
V
IV
III
ꢀ
tic oxoMn , oxoMn , and Mn TDMImP were examined.
by ClO
2
at pH 6.8 over the temperature range 5ꢀ35 °C (Sup-
Traditional stopped-flow spectroscopy indicated 1-electron re-
porting Information, Figure S3a). Using these values, thermo-
dynamic activation parameters could be determined for the initial
V
IV
dox chemistry between ClO and both oxoMn and oxoMn
2
ꢀ
q
ꢀ1
q
(
Figure 9) to produce ClO3 . Indeed, chlorate ion was detected
oxo-transfer reaction; ΔH = +44.4 kJ mol and ΔS = ꢀ60.7 J
ꢀ ꢀ1
1
as a product by indirect ion chromatography HPLC (Table 2).
Chlorine dioxide also decomposed in the presence of aqua-
mol
K .
q
q
Positive, moderate ΔH values and negative ΔS values have
III
IV
III
31
hydroxoMn TDMImP at pH 6.8 (Figure 8). The Mn /Mn
been associated with heterolytic OꢀO bond cleavage of iron,
32
12
redox potential could be observed by square wave voltammetry
chromium, and manganese peroxides, leading to metal-oxo
IV
q
tobe 1.03 V at thispH. Thus, thereductionpotential foroxoMn /
formation. The calculated ΔH value is similar to that reported
III
Mn TDMImP is either equal to or slightly less than that of
for oxygen atom transfers to phosphites and phosphonites from
dioxomolybdenum and dioxotungsten complexes (+34 to +58 kJ
ꢀ
ClO /ClO in the vicinity of pH 6.8, confirming that ClO2
2
2
III
ꢀ1 33
should be capable of oxidizing Mn . Of course, this is the
mol ). Heterolysis of the OꢀCl bond in a chloritomanga-
ꢀ
IV
V
reverse reaction of the observed oxidation of ClO2 by oxoMn
Scheme 2, reaction 2), and we conclude therefore that this step
is reversible during catalysis near neutral pH.
nese(III) adduct leading to oxoMn would be analogous to the
(
concerted and entropically unfavorable OꢀO cleavage in HOO-
III
12
Mn TDMImP (Scheme 4). A negative activation entropy
term would also arise from the bimolecular nature of the pro-
posed manganese porphyrin oxidation by chlorite ion. Another
similarity between these two heterolytic reactions is the pH
independence that is observed, suggesting a similar pushꢀpull
protonationꢀdeprotonation scenario in both mechanisms.
Chlorite as a 1- or 2-Electron Oxidant. The kinetic data re-
By accounting for the new results described here, we propose a
more complete mechanism of this manganese porphyrin-chlorite
reaction as shown in Scheme 3. All but two of the elementary
steps of this catalytic cycle have been observed and analyzed
directly by stopped-flow methods. The endergonic oxidation of
III
ꢀ
Mn by ClO2 is the rate-limiting step for this system (k ) and
1
ꢀ
III
cannot be directly observed because of the rapid subsequent
ported here argues that ClO2 reacts with Mn via oxo-transfer
III
IV
V
steps. Similarly, the oxidation of Mn to oxoMn by ClO (k
)
to generate dioxoMn TDMImP and HOCl. Further, the ob-
2
ꢀ3
could not be directly observed, as the reverse reaction (reduction
of oxoMn by ClO2 ) is shown to progress fully under the
conditions studied.
served steady-state equilibrium shown in Figure 6 shows that
IV
ꢀ
V
HOCl does not reduce dioxoMn TDMImP at a detectable rate.
Another conceivable pathway from an initial chloritomanganese-
Although we could not directly observe the 2-electron oxida-
(III) adduct would lead via ClꢀO bond homolysis to an oxo-
III
ꢀ
IV
•
tion of Mn by ClO2 , the rate constant of this step was inferred
Mn intermediate and chloroxy radical (ClO ), as suggested by
ꢀ
9
by monitoring the first-order decay of ClO2 ion during turn-
Abu-Omar et al. A similar homolysis mechanism has been
3
4
over. A steady-state approximation (Supporting Information) f
or the mechanism in Scheme 1 suggests that the rate constant
implicated computationally for chloritoiron(III) porphyrins.
Chlorite is known to act as both a 1- and 2-electron oxidant with
ꢀ
III
35
(
kapp) for ClO2 consumption is equal to 5 k [Mn ], where k
transition metal systems. As discussed above, the rationale for
1
1
ꢀ
is the apparent second-order rate constant for oxo-transfer from
our proposal of ClO2 as a 2-electron oxidant is the similar ΔG°
ꢀ
ꢀ
III
ꢀ
ꢀ
20
ClO2 to Mn . Indeed, at pH 6.8 the first-order conversion of
for oxo-transfer from both ClO2 and hypobromite (BrO ),
ClO by 8.6 μM MnTDMImP is constant ((2.92 ( 0.19) ꢁ
the latter a known oxo-transfer reagent in manganese porphyrin
2
ꢀ
3 ꢀ1
ꢀ1 ꢀ1
ꢀ
1
0
s
) and predicts a k equal to 68 ( 4.4 M
s
. This value
chemistry. Further, ClO2 has been demonstrated to be an
1
1
0359
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Inorganic Chemistry
ARTICLE
ꢀ
Table 4. Collected Apparent Second-Order Rate Constants for the Elementary Steps in the Decomposition of ClO by
MnTDMImP
2
a
ꢀ1
ꢀ1
kapp (M
s )
reaction
pH 4.7
pH 6.8
III
2ꢀ
V
ꢀ
2
5
3
1
k
k
k
k
k
k
k
k
1
Mn + ClO f oxoMn + ClO
(2.50 ( 0.13) ꢁ 10
(6.77 ( 0.08) ꢁ 10
(6.03 ( 0.06) ꢁ 10
n.d.
(6.80 ( 0.14) ꢁ 10
(6.30 ( 0.62) ꢁ 10
(3.13 ( 0.12) ꢁ 10
(4.59 ( 2.37) ꢁ 10
(1.71 ( 0.27) ꢁ 10
V
2ꢀ
IV
3
2
oxoMn + ClO f oxoMn + ClO
2
IV
2ꢀ
III
3
3
oxoMn + ClO f Mn + ClO
2
2ꢀ
III
IV
2b
6
ꢀ3
4
Mn + ClO
2
f oxoMn + ClO
III
ꢀ
V
ꢀ
4
Mn + ClO f oxoMn + Cl
(6.64 ( 0.23) ꢁ 10
V
ꢀ
III
ꢀ
7c
2c
ꢀ4
5
oxoMn + Cl f Mn + ClO
1.30 ꢁ 10
1.42 ꢁ 10
V
IV
3ꢀ
4
4
oxoMn + ClO
2
f oxoMn + ClO
(2.24 ( 0.14) ꢁ 10
(7.90 ( 0.53) ꢁ 10
(1.61 ( 0.12) ꢁ 10
(1.19 ( 0.06) ꢁ 10
IV
III
3ꢀ
3
4
6
oxoMn + ClO
2
f Mn + ClO
a
ꢀ
All errors represent standard errors of linear fit analyses as described in the text; For simplicity, both HOCl and ClO are represented in reactions 1, 4,
ꢀ
b
and ꢀ4 as ClO ; n.d, not determined. k reported as the average of two values determined from computational fit of mechanism to data (see text);
ꢀ
3
c
k
4
ꢀ4 calculated from measured k and Keq;
III
Figure 11. Modeling data for the mechanism in Scheme 3. Empirically determined rate constants were used for all steps except Mn + ClO
2
(kꢀ3). The
2
ꢀ1 ꢀ1
kinetic simulations varied parameter kꢀ3 to determine the best fit of data. (a) Data from Figure 3, 25 °C. Model calculates kꢀ3 = 6.64 ꢁ 10 M
s
.
2
ꢀ1 ꢀ1
(
b) Data from Figure 8. Model calculates k = 3.04 ꢁ 10 M
s .
ꢀ
3
Scheme 4. Proposed Mechanism for pH-Independent O-X
Heterolysis for the Formation of trans-Dioxomanganese(V)
oxo-transfer “shunt” oxidant for manganese porphyrins in or-
V
17
ganic solvents, generating presumed oxoMn intermediates.
Substrate oxygenations and oxygen production were reported in
that case.
III
IV
V
Figure 12. Hypothetical equilibrium between oxoMn and oxoMn as
a result of heterolytic or homolytic cleavage of the OꢀCl bond from a
chloritomanganese(III) adduct.
One can consider the two proposed oxidations of Mn by
ꢀ
ClO2 as competing homolysis and heterolysis of the OꢀCl
bond in an inferred chloritomanganese(III) adduct (Figure 12).
A chloritoiron(III) complex has been detected in an iron-cat-
27
alyzed oxidation of chlorite ion at very low pH. Although both
While we have not directly measured the redox potential for
III
V
IV
of these plausible reactions are endergonic for Mn TDMImP,
trans-dioxoMn TDMImP/oxoMn TDMImP, this value can
be determined using the thermodynamics of other measured re-
actions. The redox potential for the two-electron reduction of
thermochemical considerations show that the formation of
V
IV
ꢀ1
oxoMn over oxoMn is favored by ∼25 kJ mol at pH 6.8.
1
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Inorganic Chemistry
ARTICLE
V
III
trans-dioxoMn TDMImP to give Mn TDMImP has been pre-
solution of NaClO
2
and bubbled through deionized water in a chilled
0
13
viously determined to be E
The redox potential for the one-electron reduction of oxo-
manganese(IV) to give manganese(III) was measured electro-
chemically to be E
= +1.13 V vs NHE.
amber bottle. Aliquots of the resulting solution were frozen at ꢀ30 °C
Mn(V)/Mn(III)
for prolonged storage. The concentration of ClO was checked im-
2
ꢀ1
ꢀ1 15
mediately prior to use (ε359 nm = 1230 cm
from all experiments was neutralized with sodium iodide before being
disposed of in general waste. Mn TDMImP was synthesized as the
M ). Leftover ClO
2
0
= +1.03 V vs NHE for this
Mn(IV)/Mn(III)
III
0
porphyrin at pH 6.8. From these two values (E
Mn(V)/Mn(III)
1
3
0
chloride salt using reported procedures, and MnTDMBImP was pre-
pared analogously from N-methyl-2-benzimididazole-carboxaldehyde.
MnTM2PyP and MnTM4PyP were purchased from Mid Century and
purified by double precipitation. MnTM23PyPz was prepared following
the method of W €o hrle et al. Briefly, manganese diacetate (170 mg)
and pyridine-2,3-dicarbonitrile (500 mg) were heated in an unsealed
reaction tube to 200 °C with mechanical stirring for 4 h. The deep
blue solid product (a mixture of isomers) was washed with acetone,
isolated by filtration, suspended in 50 mL of dimethylformamide
and E
), the reduction potential for the couple
Mn(IV)/Mn(III)
IV
V
0
oxoMn /oxoMn can be determined to be E
=
Mn(V)/Mn(IV)
+
1.23 V at this pH (Supporting Information). Given the very high
oxidation potential for hypochlorous acid (E°(ClO /ClOH) =
1.86 V vs NHE), the heterolytic oxygen atom transfer from
chlorite ion in Figure 12 appears to be strongly favored for
Mn TDMImP.
•
3
8
3
6
+
III
’
SUMMARY AND CONCLUSIONS
(
DMF), and tetra-methylated using excess dimethyl sulfate at 120 °C
for 12 h. The product was precipitated with acetone and purified by
double precipitation.
We have investigated the mechanism of a manganese por-
phyrin-catalyzed chlorine dioxide production from chlorite ion.
Instrumentation. UVꢀvis spectroscopic measurements were taken
using a Hewlett-Packard 8453 diode array spectrophotometer equipped
with a temperature-controlled cell housing, VWR 1140 thermostat bath
and a Hi-Tech SFA Rapid Kinetics Accessory. Stopped-flow experiments
for fast reactions were carried out using a Hi-Tech SF-61 double-mixing
instrument with a 1 cm path length equipped with an ISOTEMP 1016 S
thermostat bath. Ion chromatography was accomplished with an HPLC
system consisting of a Waters 600 controller, Hamilton PRP X-100 col-
umn, and Waters 996 photodiode array detector.
This process involves rate-limiting oxidation of the manganese-
ꢀ
(
III) catalyst by ClO2 to afford high-valent manganese species.
We argue on kinetic, electrochemical, and thermodynamic
grounds that this initial intermediate is the same trans-
V
dioxoMn TDMImP species that we have previously observed
1
3
in reactions of this manganese porphyrin with hypobromite
12
V
IV
ꢀ
2
and hydrogen peroxide. Both oxoMn and oxoMn oxidize ClO
ꢀ
2
directly to ClO . Interestingly, both the oxidation of ClO by
oxoMn and the oxo-transfer from HOCl to Mn were found to
2
IV
III
ꢀ
III
Stopped-Flow Experiments. Reactions of ClO2 with Mn
porphyrins were studied using traditional UVꢀvis and rapid mixing
be fast and reversible near pH 6.8. The ClO evolved from this
2
ꢀ
V
catalysis is itself further oxidized to ClO3 by oxoMn and
oxoMn in a slower subsequent phase of the reaction. The entire
39
ꢀ
IV
techniques. Solutions of catalyst and ClO were prepared in buffered
2
solutions and mixed 1:1. All rate calculations were based on final con-
catalytic process can be modeled via kinetic simulation, in very
ꢀ
centrations resulting from this dilution. The oxidations of ClO
2
and
good agreement with empirical ClO growth and decay curves
V
IV
2
2
ClO by oxoMn TDMImP and oxoMn TDMImP were studied in
ꢀ
and the measured ClO3 that is produced.
III
double mixing mode using diode array detection. Solutions of Mn TD-
MImP and oxidant (oxone, tBuOOH) were prepared in weak pH = 8.0
phosphate buffer (10 mM) and mixed 1:1 in a first push. After a short
aging time to ensure complete conversion to the high-valent species
The fast and efficient evolution of ClO catalyzed by MnTD-
2
MImP and MnTM23PyPz suggest that a viable, scalable, and
acid-free process could be developed for chlorine dioxide pro-
duction using these catalysts. As we have previously demon-
(
2ꢀ150 s, fine-tuned for each experiment), the porphyrin solution was
ꢀ
strated, the evolved ClO can be efficiently removed from the
2
mixed 1:1 with the substrate (ClO or ClO ) prepared in a higher
2
2
reaction vessel via simple sparging or air stripping during turn-
over. Further, the catalysts are active on a solid support, sug-
gesting their use in a flow system, a cartridge, or a trickle-bed
strength buffer (100 mM) at the pH to be studied (4.7 or 6.8). All
concentrations used in subsequent rate calculations accounted for the
4-fold and 2-fold dilutions inherent to the double-mixing technique.
Each reaction was run in duplicate or triplicate at T = 25 °C, and most
errors in measured rate constants were < ( 2.0%. In many cases in the
text, the error bars in the reported data are smaller than the data point
marker. Averaging of the runs and analysis of the data was accomplished
using the KinetAsyst 3 software package. Kinetic simulations of the
overall mechanism were performed with the Berkeley-Madonna soft-
ware package.
8
reactor. The mechanism of this catalytic process now suggests
methods for greatly enhancing the rate of ClO -generation under
2
mild, neutral conditions. Efforts to address the practical imple-
mentation of manganese porphyrin-generatedClO areunder way.
2
’
EXPERIMENTAL SECTION
2ꢀ
2
Reagents. Sodium chloride, sodium chlorate, t-butyl hydroperoxide
Ion Chromatography Experiments. Aliquots of ClO or ClO
III
(70% aqueous solution), and Oxone were purchased from Aldrich and
solutions were added to buffered solutions of Mn TDMImP under
ꢀ
used as received. Sodium chlorite was obtained from Aldrich as >80%
technical grade and recrystallized twice from ethanol/water (>95%
final). All oxidant stock solutions were prepared fresh daily and standard-
ized by iodometric titration before use. Dilute (0.5ꢀ10.0 mM) chlo-
rite solutions were standardized spectrophotometrically (ε260 nm =
mechanical stirring at ambient temperatures. ClO
3
was quantified
1
6
using an indirect ion chromatography method. In brief, an aqueous
solution of 4-aminosalicylic acid (4 mM) was pH adjusted to pH =
6.0 and employed as the mobile phase. Reaction samples were injected
directly without any modification. The eluent was monitored at 320 nm,
and a decrease in absorbance was observed as anions were eluted.
Concentration of analyte was calculated directly from total area of
the peak using a concentration curve prepared daily using prepared
standards.
ꢀ
1
ꢀ1 15
1
54 cm
acetic acid/sodium acetate (pH = 4.7, 5.7) or potassium phosphate
monobasic)/potassium phosphate (dibasic) (pH = 6.8, 8.0) and pH-
M ). Buffers were prepared fresh each day using either
(
adjusting no more than 0.1 units using perchloric acid or sodium hydro-
ꢀ
xide. ClO was prepared from ClO using a previously reported proce-
Temperature-Dependence on ClO Evolution. Buffered so-
2
2
2
37
III
ꢀ
dure. Briefly, a 2.5% w/w solution of NaClO
2
was acidified with
lutions (100 mM) of Mn TDMImP and ClO
2
were mixed 1:1 and
sulfuric acid under an argon flow in the dark. The evolved gas was carried
by the argon flow through a gas scrubbing tower containing a 2.5% w/w
monitored in the UVꢀvis. The cell holder and mixing accessory were
equilibrated to the set temperature for 30 min before the measurement.
1
0361
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Inorganic Chemistry
ARTICLE
’
ASSOCIATED CONTENT
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(31) Groves, J. T.; Watanabe, Y. J, Am. Chem. Soc. 1988, 110, 8443.
(32) Pestovsky, O.; Bakac, A. J. Chem. Soc., Dalton Trans. 2005, 556.
S
Supporting Information. Ionic strength and pH rate
b
plots, temperature effect and Arrhenius plot, derivation of ki-
netics and Nernst equations, initial rate analysis. This material is
available free of charge via the Internet at http://pubs.acs.org.
(33) Tucci, G. C.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 1998,
37, 1602.
(
(
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AUTHOR INFORMATION
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77.
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(
Corresponding Author
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38) W €o hrle, D.; Gitzel, J.; Okura, I.; Aono, S. J. Chem. Soc., Perkin
*Phone: (609) 258-3593. Fax: (609) 258-0348. E-mail: jtgroves@
princeton.edu.
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6
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ACKNOWLEDGMENT
We are grateful for support of this research by the National
Science Foundation (CHE 0616633). We thank Christina Chang
for technical assistance.
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