has been discussed in reactions leading to ozone depletion,[14]
observed in pulse radiolysis experiments,[15] and proposed in
the iron-catalyzed decomposition of chlorite in aqueous
Experimental Section
All reactions were carried out in deionized water obtained from a
Millipore Milli-Q Academic TC water purification system. Phosphate
buffers were prepared by dissolving mono- and dibasic sodium
phosphate. 5,10,15,20-Tetrakis(pentafluorophenyl)porphyrin was
purchased from Scientific Frontier. Sodium chlorite was purchased
from Sigma and purified by crystallization. The purity was confirmed
by ion chromatography and UV/Vis. Chlorite samples even after
crystallization contained a few percent chloride and less than 2%
chlorate. Potassium monopersulfate (KHSO5 or oxone) was obtained
from Acros Organic Chemicals. UV/Vis spectra and kinetics were
recorded on a Shimadzu UV-2501PC scanning spectrophotometer
equipped with a temperature control cell holder. Ion chromatography
was performed on a Dionex DX-500 Liquid Chromatography System
equipped with a Dionex LC25 Chromatography Oven, a Dionex
ED40 Electrochemical Detector, and a Dionex Ion-Pac AS9-HC ion
exchange column. 9 mm Na2CO3 was used as eluant. Chromatography
calibration standards were prepared in the 0.5–60 mm concentration
range. Peaks were identified by comparison to standard samples, and
quantified by comparison of the integrals of the signals to standard
curves for the corresponding ion. Retention times and peak identities
were as follows: chlorite, 4.48 min; chloride, 5.23 min; and chlorate,
7.4 min. ESI mass spectra were obtained using a Finnigan LTQ Linear
Ion Trap Mass spectrometer in positive ion mode. Sample was
introduced by direct infusion from a syringe pump. (TF4DMAP)H2 or
(T(p-Me2N)F4PP)H2 was synthesized following a literature proce-
dure.[18] The complexes [Mn(TF4DMAP)] and [Mn(TF4TMAP)] were
prepared according to modified literature methods for their iron
counterparts as detailed below.[19] The anion for [Mn(TF4TMAP)]
was triflate.
medium.[13] ClO reacts with ClO2 rapidly to give ClO2 and
À
hypochlorite (ClOÀ). The hypochlorite byproduct reacts with
MnIII to give ClÀ and MnV–oxo. This reaction has been
demonstrated independently.[11] [MnV(O)]+ undergoes ET
with ClO2À to give ClO2 and MnIV(O). The potential for ClO2
À
oxidation to ClO2 is À1.071 V, and at pH 7.0 the potential for
an analogous MnV–oxo to MnIV(O) is 1.44 V.[16] Therefore,
this reaction is thermodynamically favorable and essentially
À
irreversible (KMnV
ꢀ106). MnIV(O) is also presumed to
=ClO2
react through ET with ClO2 to afford ClO2 and MnIII O ,
which reverts to MnIII through rapid protonation of the oxo
group. The reduction potential for MnIV(O) to MnIII at pH 7.0
based on an analogous porphyrin system is 1.08 V.[16] There-
À
À
À
À
fore, the reaction between MnIV(O) and ClO2 is an
À
equilibrium with KMnIV
ꢀ1.5. Hence, as ClO2 accumulates
=ClO2
MnIV(O) builds up and the reaction is inhibited. The rate-
determining step is essentially the reaction of MnIII and ClO2
À
and in combination with the reactions in Scheme 2 accounts
for most of the catalysis. As ClO2 and MnIV(O) build up, a
semi-equilibrium is reached and product inhibition is
observed. Indeed when ClO2 is removed by purging the
reaction with an inert gas (N2 for example), chlorite decom-
position resumes and quantitative conversion to ClO2 and ClÀ
is observed.
Synthesis of [Mn(TF4DMAP)]: [Mn(TF4DMAP)] was prepared
by refluxing 200 mg (0.17 mmol) of (T(p-Me2N)F4PP)H2 and 435 mg
(1.7 mmol) of Mn(OAc)2·4H2O in DMF for 12 h under N2 atmos-
phere. Solvent was removed under reduced pressure and the resulting
brown-yellow solid was chromatographed (neutral alumina) with
benzene as eluant. UV/Vis data [l, nm] acetonitrile 365, 474, 575, 606;
MS (ESI), m/z 1127.
Kinetic simulations of Scheme 2 with k1 = 2, k2 = fast
(ꢁ105), k3 = 20, k4 = 1000, k5 = 100, kÀ5 = 60mÀ1 sÀ1, and pro-
À
tonation of MnIII O being instantaneous provide reasonable
À
agreement with observed concentration changes. The relative
rates rather than absolute values of the reactions of MnV- and
MnIV–oxo determine how much ClO2 is formed and the
relative ratios of MnIII and MnIV–oxo in solution. For
example, the simulation predicts no accumulation of
MnV(O) and after around 2 h, 10% of the starting MnIII is
present as MnIV(O), which is in good agreement with
experiment.
Synthesis of [MnIII(TF4TMAP)](CF3SO3)5. [Mn(TF4TMAP)] was
prepared by reacting 100 mg (0.08 mmol) of [Mn(TF4DMAP)] and
200 mL of trifluoromethanesulfonate stirred in trimethyl phosphate at
608C for 12 h under N2 atmosphere. An immediate color change was
observed, brown to red. Methanol (1 mL) was added to destroy any
unreacted methyl trifluoromethanesulfonate. A red precipitate was
collected after slowly adding the reaction mixture to rapidly stirred
diethyl ether. The product was crystallized by vapor diffusion of
acetonitrile and diethyl ether. UV/Vis data [l, nm] 373, 464, 556.
EPR spectra were recorded on a Bruker ESP 300E EPR
spectrometer equipped with an HP 5350B microwave frequency
counter, an Oxford ITC4 temperature controller, and a VC40 gas flow
controller (for liquid He).
We have reported herein on the dismutation of chlorite
catalyzed by the fluorinated water-soluble manganese por-
phyrin [Mn(TF4TMAP)]. In contrast to the iron(III) [Fe-
(TF4TMAP)]-catalyzed reaction, dioxygen is not produced.
Instead, ClO2 is formed and MnIV–oxo accumulates concur-
À
rent with ClO2 consumption. The reaction comes to semi-
equilibrium at moderate chlorite conversion due to product
(ClO2) inhibition. The reaction can be shifted to give
quantitative product by removal of the ClO2 gas. Scheme 2
presents a series of reactions that account for all of the
experimental observations and is consistent with thermody-
namic considerations. Our findings present another pathway
for chlorite dismutation catalyzed by manganese porphyrin in
which ET initiates the catalysis followed by atom-transfer
reactions. Our results parallel many of the observations noted
for iron(III) and mercury(II)-mediated chlorite decomposi-
tion in aqueous medium through redox chemistry.[13,17]
Received: August 16, 2010
Revised: September 29, 2010
Published online: November 16, 2010
Keywords: chlorine dioxide · chlorite dismutation ·
.
electron transfer · manganese · porphyrinoids
[1] Perchlorate, Environmental Occurrence, Interactions and Treat-
ment (Eds.: B. Gu, J. D. Coates), Springer, New York, 2006,
p. 411.
[2] J. D. Coates, L. A. Achenbach in Perchlorate, Environmental
Occurrence, Interactions and Treatment (Eds.: B. Gu, J. D.
Coates), Springer, New York, 2006, pp. 279 – 291.
Angew. Chem. Int. Ed. 2011, 50, 699 –702
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
701