Selectivity-Determining Steps in O2Reduction Catalyzed by Iron(tetramesitylporphyrin)

Article abstract of DOI:10.1021/jacs.9b13654

The oxygen reduction reaction (ORR) is the cathode reaction in fuel cells and its selectivity for water over hydrogen peroxide production is important for these technologies. Iron porphyrin catalysts have long been studied for the ORR, but the origins of their selectivity are not well understood because the selectivity-determining step(s) usually occur after the rate-determining step. We report here the effects of acid concentration, as well as other solution conditions such as acid pKa, on the H2O2/H2O selectivity in electrocatalytic ORR by iron(tetramesitylporphyrin) (Fe(TMP)) in DMF. The results show that selectivity reflects a kinetic competition in which the dependence on [HX] is one order greater for the production of H2O than H2O2. Based on such experimental results and computational studies, we propose that the selectivity is governed by competition between protonation of the hydroperoxo intermediate, FeIII(TMP)(OOH), to produce water versus dissociation of the HOO- ligand to yield H2O2. The data rule out a bifurcation based on the regioselectivity of protonation of the hydroperoxide, as suggested in the enzymatic systems. Furthermore, the analysis developed in this report should be generally valuable to the study of selectivity in other multi-proton/multi-electron electrocatalytic reactions.

Full text of DOI:10.1021/jacs.9b13654

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Communication
2
Selectivity-Determining Steps in O Reduction
Catalyzed by Iron(tetramesitylporphyrin)
Anna C. Brezny, Samantha I Johnson, Simone Raugei, and James M. Mayer
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b13654 • Publication Date (Web): 16 Feb 2020
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Selectivity-Determining Steps in O Reduction Catalyzed by
Iron(tetramesitylporphyrin)
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†
‡
‡
,†
Anna C. Brezny, Samantha I. Johnson, Simone Raugei, and James M. Mayer*
†Department of Chemistry, Yale University, New Haven, Connecticut 06520
‡Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, Richland, Washington 99352
*Corresponding author, james.mayer@yale.edu
Supporting Information Placeholder
P450 enzymes in which relative rates of proton delivery
dictate selectivity (Scheme 1). The branch point is suggested
to be the iron-hydroperoxo intermediate Fe -OOH:
ABSTRACT: The oxygen reduction reaction (ORR) is the
cathode reaction in fuel cells and its selectivity for water over
hydrogen peroxide production is important for these
technologies. Iron porphyrin catalysts have long been
studied for the ORR, but the origins of their selectivity are
not well understood because the selectivity-determining
step(s) usually occur after the rate-determining step. We
report here the effects of acid concentration, as well as other
III
protonation at the distal oxygen leads to O-O bond cleavage
and production of water, while protonation of the proximal
3,19
oxygen releases H
mutagenesis studies,
2
O
2
2
.
While this scenario is supported by
the origin of enzymic selectivity is
0-22
23-26
not viewed as a solved problem,
and is less well
understood in molecular systems. Recently, a correlation of
selectivity with the electrochemical overpotential (η) has
a 2 2 2
solution conditions such as of acid pK , on the H O /H O
selectivity in electrocatalytic ORR by iron
2 2
recently been proposed, with less H O being formed at
tetramesitylporphyrin Fe(TMP) in DMF. The results show
that selectivity reflects a kinetic competition in which the
dependence on [HX] is one order greater for the production
27
higher overpotentials.
Scheme 1. Canonical mechanism2–5,20–26,28,29 for H
2
2
O and
2 2 2
of H O than H O . Based on such experimental results and
H
2
O selectivity in cytochrome P450 and Fe(porphyrin)
computational studies, we propose that the selectivity is
governed by competition between protonation of the
systems. Expanded from ref. 30.
III
hydroperoxo intermediate, Fe (TMP)(OOH), to produce
–
water versus dissociation of the HOO ligand to yield to
2 2
H O . The data rule out a bifurcation based on the
regioselectivity of protonation of the hydroperoxide, as
suggested in the enzymatic systems. Furthermore, the
analysis developed in this study should be generally valuable
to the study of selectivity in other multiproton/multielectron
electrocatalytic reactions.
Selective electrocatalysis of multi-electron/multi-proton
reactions is key to the development of new technologies for
1
the interconversion of chemical and electrical energies. The
oxygen reduction reaction (ORR) is an important example,
being the cathode reaction in fuel cells. Most ORR
–
+
applications require selective 4e /4H reduction of O
2
to H
because the alternative 2e /2H path is less exoergic and the
product H is corrosive and hazardous. The ORR
selectivities of many soluble and surface-bound iron and
2
O
This report describes how the selectivity of the ORR by
iron tetramesityl-porphyrin Fe(TMP) in N,N-
dimethylformamide (DMF) is affected by changes of the
catalytic system such as the concentration and pK of acid.
Our previous mechanistic studies indicated a pathway of pre-
–
+
O
2 2
a
2
-7
cobalt catalysts have been reported, and strategies that
direct and modulate H and e delivery can partially control
Still, the mechanistic origins of
selectivity for soluble catalysts are not well understood.
+
–
equilibrium O binding and rate-determining proton transfer
2
5
-6,8-18
product formation.
to the iron superoxide complex for ORR catalysis by iron
3
0-33
tetraaryl-porphyrins with strong acids in organic media.
Kinetic studies in this work demonstrate that Fe(TMP)
follows the same mechanism, including general acid
Traditionally, discussions of iron-porphyrin ORR catalysts
have used the general mechanism proposed for cytochrome
34
2 2
catalysis. Measurements of the percent H O product as a
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a
function of the acid pK , acid concentration, and other
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parameters, yield the relative rates and rate laws for the
selectivity-controlling steps. The results are not consistent
with (i) the canonical distal vs. proximal protonation origin
of ORR selectivity or (ii) a simple correlation between
selectivity and η.
Rotating ring-disk voltammetry (RRDV) is the classic
7
method to measure %H
occurs at a central disk, H
and the %H is calculated from these currents. This
O
2 2
produced in the ORR. Catalysis
2
2
O is oxidized at an outer ring,
3
4
2 2
O
technique was developed for heterogeneous electrocatalysis,
but is often used for soluble catalysts. Ideally, the measured
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27,35-36
selectivity is independent of rotation rate.
Our system
did not reach rotation rate independence before the
maximum accessible rotation rate, but the relative effects of
pK
a
and [HX] are consistent across all rotation rates. The
Figure 1. A: Dependence of log(%H
exogenous acid (standard conditions, 100 mM buffered acid).
B: Correlation between log(%H ) and log([p-TsOH]) under
standard conditions, with either a 1:1 p-TsOH:p-TsONa buffer
blue circles) or with constant [p-TsONa] = 0.01 M (red
2 2 a
O ) on pK of the
conclusions below only depend on the relative effects, not on
3
4
the absolute values. Furthermore, stoichiometric UV-Vis
experiments generating H O and H from the
independently generated iron-peroxo complex support the
2 2
O
2
2 2
O
(
3
4
same conclusions as the RRDV results. Typical RRDV
diamonds).
conditions had 0.2 mM of the iron catalyst ([Fe(TMP)][OTf]),
1
00 mM [NBu
4 6 2
][PF ], 1 atm (3.1 mM) O , and 10-100 mM acid
a
Varying the overpotential by changing the acid pK gives
in DMF. All experiments were performed in buffered acid to
fix the overpotential.
the same selectivity trend observed by Nocera and coworkers
3
7,38
The reported selectivities are from
27,41
for dicobalt and Fe(porphyrin) catalysts (Figure 2A).
measurements at the fastest obtainable rotation rate (3000
RPM).
However, this thermodynamic relationship cannot be
complete. Changes in the concentration of 1:1 buffer do not
The pK
a
of the buffered acid was varied from –1.1 to 6 using
–
change the overpotential, because only the ratio of [HA]/[A ]
protonated DMF triflate ([H-DMF]OTf, –1.1), p-
toluenesulfonic acid (p-TsOH, 2.6), sulfuric acid (H
37
appears in the Nernst equation (Figure 2B, blue circles).
Therefore, the steep selectivity dependence on buffer
2 4
SO , 3.1),
39,40
and trifluoracetic acid (TFA, 6.0).
A very shallow
concentration reflects the kinetics of the bifurcation and not
the thermodynamics. This result contrasts with the simple
correlation between pK
strength of the acid (K
a
and selectivity was observed: as the
) decreases by 10 , only slightly more
7
27
a
overpotential-selectivity relationship previously proposed.
2 2
H O is produced, from 0.9 to 20% (Figure 1A). In contrast,
the selectivity shows a steep dependence on the
concentration of buffered acid. Decreasing the concentration
–
of 1:1 HA:A tenfold led to roughly ten times more H
2
O
2
(
Figure 1B, blue circles).
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Our analysis of the effects of protons on the H
2 2 2
O/H O
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selectivity uses the generic kinetic model in Scheme 2,
4
2
starting from the Fe(TMP)(OOH) intermediate. The step(s)
to generate water involve some dependence on the acid
(
aHX) and therefore has the rate law in eq 1. Analogously, the
rate of H formation involves bHX (eq 2). Because these
2 2
O
steps occur after the turnover-determining step, the 푘퐻2푂 and
푘퐻2푂2 and dependencies on [HX] cannot be determined
kinetically. However, the ratio of these equations gives an
expression for the selectivity, [H
2 2 2
O ]/[H O] (eq 3). Equation
3
can be manipulated to solve for the difference in reaction
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34
orders in acid, b–a (eq 4). Plotting our data according to eq
4
H
(Figure 3A) gives a straight line with slope b-a=–1. Thus the
pathway has a kinetic dependence on [HX]
2 2
O
b
(
(
rate [HX] ) that is one order lower than the H
rate [HX] ). This result contrasts with the lack of
2
O pathway
b+1
selectivity change reported for dicobalt ORR catalysts upon
changing [HX], so the two systems must have different
mechanisms.
27
Scheme 2. Kinetic analysis of the bifurcating pathways
from the Fe(TMP)(OOH) intermediate.
2 2
Figure 2. A: Reported correlation between %H O and
overpotential, adapted from ref. 27 (dicobalt catalysts, ;
prior Fe(porphyrin) catalysts, ) with the addition of
Fe(TMP) results from this work (violet triangles). B:
log(%H
with: 0.01-0.1 M 1:1 buffered p-TsOH:p-TsONa (blue circles);
.01-0.1 M p-TsOH buffered with 0.01 M p-TsONa (red
2 2
O ) versus overpotential under standard conditions
0
diamonds); 0.02 M p-TsOH buffered with 0.01-0.1 M p-
TsONa (green triangles).
We varied η based on the Nernst equation dependence on
HA]/[A ] by independently varying [p-TsOH] and [p-
–
[
TsONa]. Altering [p-TsOH] while holding [p-TsONa]
constant changes η, but led to the same selectivities as when
the 1:1 buffer concentration was varied (Figures 1B and 2B, red
diamonds). In contrast, there was no change in selectivity
observed when [p-TsOH] was held constant and [p-TsONa]
was varied, which also impacts η (Figure 2B green triangles).
These observations again show that the %H O produced is
2 2
not determined by overpotential, but instead by [p-TsOH].
The observed acid dependence rules out, at least for this
system, the commonly proposed origin of the H O/H O
2 2 2
selectivity. The traditional mechanism in Scheme 1, involving
protonation either at the proximal or the distal oxygen,
requires a first order dependence on [HX] for both pathways
from the Fe(TMP)(OOH) intermediate. An increase in [HX]
should increase the rates of H O and H O formation equally
2 2 2
and therefore lead to no change in selectivity. The data in
Figures 1&2 therefore require a different mechanism.
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the electrode.34 Our simulations showed that water facilitates
the heterolytic breaking of the Fe-OOH bond over the
homolytic bond breaking (∆G =12.5kcal mol ) by solvating
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‡
-1
–
–
HOO , resulting in a very exoergic release of HOO (∆G=-
4
-1
6.5 kcal mol in 0.02M water/DMF).
Scheme 3. Proposed mechanism for the bifurcation in
the ORR catalyzed by Fe(TMP).
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The shallow dependence of selectivity on pK
and 3B) is consistent with this mechanism. With the
conclusion that only the H O pathway is dependent on the
acid concentration, changing the pK only influences 푘퐻2푂
The change in a proton transfer rate constant with a change
a
(Figures 1A
2
a
.
4
3
a
in pK is given by the Brønsted equation, eq 5. The value of
훼 can indicate the asymmetry of the proton transfer reaction,
with 훼=0.5 suggesting a symmetrical process. Combining the
34
Brønsted equation and eq 3 above, with a=1, b=0, gives eq 6.
Plotting the data according to eq 6 (Figure 3B) gives 훼퐻2푂
=
0.ꢀ. This is consistent with distal protonation of
Fe(TMP)(OOH) because this exoergic step would typically
have 훼퐻2푂 < 0.5 based on Hammond’s postulate and because
Figure 3. Plots of log([H
the slope is the difference in the kinetic orders in [HX] for
-vs.-H O formation. B: versus pK ; the slope is the
difference in Brønsted 훼’s for H O and H production.
2 2 2
O ]/[H O]) A: versus log([p-TsOH]);
protonations of related superoxide intermediates have been
4
4
found to have a small α’s.
H O
2 2
2
a
2
2 2
O
Quantum chemical analysis based on density functional
theory (DFT) optimizations and hybrid DFT/molecular
mechanics free energy simulations provided insight into the
mechanisms that give rise to these dependencies on [HX] for
Several additional mechanistic experiments are consistent
–
3
4
with our proposal that HOO dissociates to generate H
2 2
O .
the two pathways. Calculations strongly indicate that
formation of H O is first order in acid (a=1), while formation
of H is zero order in acid (b=0), consistent with the
experimental constraint b–a=–1. Protonation of the distal
First, the selectivity of ORR using an extremely bulky proton
2
3
4,40,45-47
donor (2,6-di-tertbutyl pyridinium triflate, pKa ~ 1)
gave more H than expected based on the pK trend in
Figure 1A (12% versus ~3% H ). This is consistent with our
2 2
O
O
2 2
a
2 2
O
+
2
oxygen by H-DMF in DMF leads to a very exoergic H O
-
1
proposal because the very bulky acid (with a bulky
porphyrin) slows only the rate of water formation, not
release (∆G°=-48.0 kcal mol ) via a short-lived
+
-1
[Fe(TMP)(OOH
2
)] intermediate (∆G°=-5.7 kcal mol ). The
–
dissociation of OOH . If proximal protonation were required
kinetic instability of this cationic intermediate rules out H
formation via a mechanism that is second order in acid, and
implies that only one proton is required to produce H
Scheme 3, bottom; as in Scheme 1). This conclusion suggests
2
O
2 2 2 2
to form H O , the very bulky acid should give less H O than
a sterically accessible acid. Second, the ORR in acetonitrile is
an order of magnitude more selective for water than the
reaction under identical conditions in DMF. This, too, is
2
O
(
that H
O
2 2
formation proceeds via the unimolecular
consistent with a mechanism in which H
HOO dissociation, because this pathway should be
2
O
2
comes from
–
dissociation of HOO , which is quickly protonated by the
exogenous acid in solution (Scheme 3, top). Additional
–
suppressed in solvents that are less effective at solubilizing
2
computational analysis, including ca. 20 mM H O, revealed
4
8,49
–
‡
anions, such as MeCN compared to DMF.
Alternative
that the unimolecular dissociation of HOO (∆G =10.5 kcal
-1
mechanisms involving pre-equilibria or different orders in
acid were considered but are inconsistent with the
mol ) is kinetically considerably more favorable than the
2 2
release of H O by protonation of the proximal oxygen,
34
‡
-1
experimental data.
which is highly hindered sterically (∆G =29.1 kcal mol ).
Water is present in our dried DMF and is the major product
of catalysis; we conservatively estimate ca. 20 mM H O near
2
In summary, exploring the effect of reaction conditions on
selectivity enabled the study of steps that are kinetically
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invisible (that are after the rate-determining-step). The
analysis described here demonstrates that the rate law for
forming water from the Fe(TMP)(OOH) intermediate
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This kind of fundamental understanding of the origins of
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O
2
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heme enzymes in which regioselectivity of protonation
28
determines product selectivity. The conclusions here may
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technologies, to help optimize selectivity as well as
overpotential and TOF. More generally, the approach
developed here will be broadly applicable to unravelling the
origins of selectivities in multi-electron, multi-proton
catalytic reactions.
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ASSOCIATED CONTENT
Supporting Information
S.; Mayer, J. M. Distant protonated pyridine groups in water-soluble
iron porphyrin electrocatalysts promote selective oxygen reduction to
water. Chem. Commun. 2012, 48, 11100-11102.
Experimental and computational procedures and details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(15) Bhunia, S.; Rana, A.; Roy, P.; Martin, D. J.; Pegis, M. L.; Roy, B.;
Dey, A. Rational Design of Mononuclear Iron Porphyrins for Facile
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(
AUTHOR INFORMATION
Corresponding Author
2
of Cytochrome c Oxidase: Insights into Role of Proton Transfer
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*
james.mayer@yale.edu
Notes
The authors declare no competing financial interests.
(
18) Wang, L.; Gennari, M.; Cantu Reinhard, F. G.; Gutierrez, J.;
Morozan, A.; Philouze, C.; Demeshko, S.; Artero, V.; Meyer, F.; de
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ACKNOWLEDGMENT
A
This research was supported as part of the Center for
Molecular Electrocatalysis, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences. A.C.B. was
supported in part by a postdoctoral fellowship from the NIH
(Electro)catalytic Reduction of Dioxygen: Tuning the Selectivity
through Electron Delivery. J. Am. Chem. Soc. 2019, 141, 8244-8253.
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(
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