C O M M U N I C A T I O N S
Figure 4. Effect of pH on the electrochemical potential required to achieve
a specific turnover rate (ikin ) 32 µA cm-2).
and the reactant are deprotonated, the pH dependence is consistent
with one acid-base equilibrium, which is likely proton-coupled
reduction of (por)FeIIIOH to its ferrous form. In the intermediate
pH range, where the ferric catalyst is deprotonated but the ROOH
is not, a fractional order in protons is observed, suggesting a slow
step of deprotonation of the bound reactant preceding (or competing
with) the rds.
Figure 3. Possible mechanism consistent with the kinetic data for reduction
of ROOH by ferrous catalysts at different pH. Broken arrows refer to fast
steps after rds, which do not contribute to the kinetic law. pKa(FeIIIH2O) )
5.7, 7.7, and 8.4 for Fe(tpp), Fe(Ac), and Fe(NMe);10 pKa(ROOH) ) 8.2,
9.4, and 16.7 for PAA, KHSO5, and TBHP.
Rapid loss of catalytic activity in alkaline solutions precluded us
from obtaining Tafel plots at pH > 7.
The Koutecky-Levich slopes for electrocatalysis by ferrous
catalysts correspond to a 2e- process in accord with the expected
reaction (reaction 1). The stoichiometry of the catalysis with respect
to the reactant was 1 for all catalyst/reactant combinations, as
determined from the slopes of the log i vs log(1 - i/ilim) plots
(i and ilim are the catalytic and diffusion-limited currents, respec-
tively) at different rotation speeds and constant potential.7 The
kinetic currents determined from the Koutecky-Levich plots were
proportional to the amount of deposited catalyst up to the coverage
of ∼0.5 nmol cm-2. Depositing more catalyst did not increase
significantly the kinetic currents, in contrast to the reduction of O2
and H2O2, where much thicker films are catalytically active.10 This
difference suggests that steric factors may be important in governing
the kinetics of reactant transport within the film.
Catalytic reduction of KHSO5, PAA, and CPBA at ferric-Fe-
(NMe) and Fe(Ac) is substantially faster than reduction of H2O2,10
which is comparable to that of TBHP. This may be related to both
the better ligating power of ROOH and more facile O-O bond
heterolysis. As we have observed in catalytic reductions of H2O2,10
- 12
ClO2-, BrO3-, and IO4
,
an axial imidazole ligation of iron is
essential for the biologically relevant catalytic activity of ferric
porphyrins. In contrast, the effect of the distal organic moieties is
more limited. We expect that the presence of a distal metal (CuII,
FeII, and PdII) will enhance the ferrous porphyrins catalysis.
Acknowledgment. Work was supported by NIH (Grant 17880)
and NSF (Grant CHE-013206).
Supporting Information Available: Experimental details and
additional electrochemical data. This material is available free of charge
Under experimental conditions corresponding to the first-order
kinetics in both the reactant and the catalyst, the rate law is ikin
)
2FkredΓcROOH, and the apparent second-order rate constants, kred, at
a given pH and potential are estimated from the intercepts of the
Koutecky-Levich plots (Figure 2). The rates of ROOH reduction
at ferrous catalysts are independent of the nature of ROOH,
suggesting a change in the rate-determining step compared to that
in catalysis by the ferric form where the rate of catalytic turnover
correlates with the redox potential of the reactants.11
In the systems with no (or low) FeIII activity (Fe(tpp)/TBHP,
PAA, KHSO5; Fe(Ac)/TBHP; Fe(NMe)/TBHP), the Tafel slopes
are essentially independent of pH over the pH range 4-12 and
close to -120 mV/dec, which is the slope for a slow transfer of
the first electron9 (Figure 3). The systems showing FeIII catalysis
were not analyzed due to overlapping of the studied process with
residual catalysis by the ferric form.
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The pH dependence of potential at constant current derived from
Tafel plots indicates three distinct catalytic regimes with slopes of
0, -60, and -120 mV/pH (Figure 4), giving the chemical order in
protons, p ) (∂E/∂pH)/(∂E/∂logikin), of 0, 0.5, and 1. The pH
dependence arises from the complex acid-base equilibria of the
catalysts and reactants (Figure 3). In an acidic medium, where the
dominant acid-base forms of the catalyst and reactant are (por)-
FeOH2+ and ROOH, the catalytic currents are independent of pH,
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(11) Similar independence of kred on R in ROOH was observed in reaction of
HRP with ROOH (Baek, H. K.; Van Wart, H. I. J. Am. Chem. Soc. 1992,
114, 718-725), while calculated KM values increased with the size of R
in ROOH.
+
and the simplest mechanism involves reduction of (por)FeIIIOH2
(12) Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Shiryaeva, I. M.; Berg,
K. E. J. Am. Chem. Soc. 2002, 124, 10670-10671.
to the ferrous form, followed by coordination of ROOH and rate-
determining reductive O-O bond cleavage. When both the catalyst
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