Distant protonated pyridine groups in water-soluble iron porphyrin electrocatalysts promote selective oxygen reduction to water

Article abstract of DOI:10.1039/c2cc35576k

Fe(iii)-meso-tetra(pyridyl)porphyrins are electrocatalysts for the reduction of dioxygen in aqueous acidic solution. The 2-pyridyl derivatives, both the triflate and chloride salts, are more selective for the desired 4e ^- reduction than the iso

Full text of DOI:10.1039/c2cc35576k

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 11100–11102
www.rsc.org/chemcomm
COMMUNICATION
Distant protonated pyridine groups in water-soluble iron porphyrin
electrocatalysts promote selective oxygen reduction to waterw
a
Benjamin D. Matson,a Colin T. Carver,z Amber Von Ruden,b Jenny Y. Yang,yb
Simone Raugeib and James M. Mayer*a
Received 1st August 2012, Accepted 10th September 2012
DOI: 10.1039/c2cc35576k
Fe(III)-meso-tetra(pyridyl)porphyrins are electrocatalysts for the
reduction of dioxygen in aqueous acidic solution. The 2-pyridyl
derivatives, both the triflate and chloride salts, are more selective for
the desired 4eꢀ reduction than the isomeric 4-pyridyl complexes.
The inward-pointing pyridinium groups influence proton delivery
despite their distance from the iron centre.
Fig. 1 Electrocatalysts 1 and 2, triflate ions omitted.
Fuel cells are an attractive approach to improved energy efficiency
but their widespread application will require inexpensive catalysts
the iron centre (see below). To test the influence of these
rather than the current use of substantial amounts of platinum.
pyridyl groups, 1 is compared with the outward pointing
Catalysis of the oxygen reduction reaction (ORR), eqn (1), is
particularly challenging because of the complexity of the 4eꢀ/4H+
4-pyridyl isomer 2. These complexes, and their chloride ana-
logues 1Cl and 2Cl, are active and selective catalysts for the
reaction, the high thermodynamic potential, and the need for high
selectivity.1 The competing 2eꢀ/2H+ reduction of O2 to H2O2 is
ORR in acidic water, albeit at substantial overpotentials.
Despite the poor positioning of the 2-pyridyl substituents,
1 and 1Cl are significantly more selective for the desired 4eꢀ
reduction of O2 than 2/2Cl.
less exoergic and the H2O2 is potentially corrosive and hazardous.
O2 + 4H+ + 4eꢀ - 2H2O
(1)
The triflate derivatives 1/2 have been prepared by treatment of
1Cl/2Cl with AgOTf (see ESIw). These complexes have not been
previously examined as ORR electrocatalysts, although Kuwana
has studied the ORR by the tetra-N-methylated-4-pyridyl deriva-
tive.3 Compounds 1 and 2 are soluble in aqueous solutions that are
sufficiently acidic to protonate the pyridyl groups (pH o 2).
Cyclic voltammograms (CVs) of 1 or 2, 0.30 mM in pH 0.3
aqueous triflic acid (HOTf), have been obtained using a glassy
carbon working electrode (Ag/AgCl/sat. KCl reference electrode;
all potentials reported vs. NHE). Both compounds show a quasi-
reversible Fe(III/II) couple, at 250 mV and 150 mV,11 respectively
(Fig. 2 and Fig. S6 and S7, ESIw). In the presence of O2, CVs of
the same solutions show irreversible waves with much larger
currents, B20 and B50 times larger for 1 and 2 under 1 atm
O2. The currents are larger under 1 atm of pure O2 vs. under air
(1.3 vs. 0.27 mM O2).12 The onsets of the cathodic currents
correspond well with those of the corresponding Fe(III/II) couples.
Taken together, the loss of reversibility, the large increase in
reductive current, and the O2-dependence all indicate rapid
electrocatalytic oxygen reduction.
Described here are soluble iron meso-tetra(pyridyl)porphyrin
electrocatalysts for the ORR in acidic water. Iron-porphyrin
compounds have been widely used for the ORR.2,3 We have
included pyridyl groups with the goal of affecting proton
addition to O2-derived ligands at the iron centre. Proton relays
have less frequently been used for the ORR, from with the early
‘hangman’ porphyrin and corrole ORR electrocatalysts of
Nocera.4 The phosphine–amine ligands of DuBois are well-
positioned for O2 chemistry but are not oxidatively stable.5–7
Iron meso-tetra(2-carboxyphenyl)porphyrin is an unusually
selective ORR electrocatalyst, apparently due to the positioning
of the carboxylic acid groups.8
Fe(III)-meso-tetra(2-pyridyl)porphyrin (1, Fig. 1) is an
unlikely addition to this group of electrocatalysts because
the inward-pointing pyridyl nitrogens are quite distant from
a Department of Chemistry, University of Washington, Seattle,
WA 98195-1700, USA. E-mail: mayer@chem.washington.edu;
Fax: +1-206-553-2083; Tel: +1-206-553-2083
b Pacific Northwest National Laboratory (PNNL), Richland, WA,
USA. E-mail: Simone.Raugei@pnnl.gov
The traditional shape of the CV under N2 (Fig. 2 inset)
suggests a soluble electrocatalyst, and we tested for adsorption
on the glassy carbon electrode (as occurs with hydrophobic
iron-porphyrins13 but is less likely for the polycationic 1 and 2
in acid14). A scan was done under catalytic conditions, the
electrode was removed, rinsed with deionized water and placed
w Electronic supplementary information (ESI) available: Experimental
and computational details. See DOI: 10.1039/c2cc35576k
z Current address: Intel Corp. RA3-252 Hillsboro, OR 97124, USA.
y Current address: Joint Center for Artificial Photosynthesis,
California Institute of Technology, Mail Code 139-74, Pasadena,
CA 91125, USA. E-mail: jyy@caltech.edu
c
11100 Chem. Commun., 2012, 48, 11100–11102
This journal is The Royal Society of Chemistry 2012

View Article Online
wave for 1, B+0.4 V vs. NHE, is B0.8 V below the thermo-
dynamic potential. Complex 2 has a B100 mV higher over-
potential. High overpotentials are typical for iron porphyrin
ORR catalysts, with even cytochrome c modified electrodes
having Z 0.7 V onset overpotentials at pH 7.16
The selectivity of these dioxygen reduction catalysts was
probed using rotating ring-disk voltammetry (RRDV), following
Kuwana’s study of the closely related iron meso-tetra(N-methyl-4-
pyridinium porphyrin).3 The potential of the disk electrode was
scanned while the ring electrode was poised at +1.19 V to oxidize
the H2O2 formed. For 1 and 2, a ring current was observed that
closely mirrors the ORR current at the disk (Fig. 4), indicating
H2O2. Quantitative analysis of the currents and the collection
efficiency of the RRD (see ESIw) indicates that 1 produces 5%
H2O2. Complex 2 produces over twice this amount, 11% H2O2.
The chloride complexes 1Cl and 2Cl are also rapid ORR
electrocatalysts in acidic water (in this case 0.25 M HCl/0.5 M
KCl; see ESIw). RRDV experiments under 1 atm O2 show the
4-pyridyl isomer 2Cl produces B15% H2O2, similar to 2 in
HOTf. The 2-pyridyl isomer 1Cl, however, shows essentially
no onset of ring current as the disk is scanned through the
O2-reduction wave (Fig. 4 bottom left).
The high selectivities of these pyridyl catalysts for 4eꢀ
reduction of O2 are in marked contrast with Kuwana’s results
on the closely related meso-tetrakis(N-methyl-4-pyridinium)-
porphyrin catalyst.3 Essentially all of the O2 reduction with
that catalyst proceeded via H2O2, from pH 1–13 (in H2SO4,
phosphate or borate buffers, or NaOH). Their RRDV experi-
ments at pH 9 showed 95% H2O2.3b
Fig. 2 CVs of 1 (0.30 mM, 0.5 M HOTf) under: N2 (orange [bottom]),
air (blue [middle], 0.27 mM O2) and 1 atm of O2 (red [top], 1.3 mM O2).
Inset: CV under N2 enlarged.
in a fresh 0.5 M HOTf solution. Scans on these unpolished
electrodes showed an irreversible wave under air of similar shape
to that seen in the presence of dissolved 1 or 2, but with a
significantly lower current (Fig. S8, ESIw). Thus some adsorption
to the glassy carbon electrode may be occurring but it does not
appear to be the primary contributor to the electrocatalysis.
The turnover frequency (TOF) of a soluble electrocatalyst can
be calculated from the ratio of catalytic (ic) to non-catalytic
currents (ip) when the catalysis occurs under kinetic conditions
where substrate diffusion is not limiting15 (see ESIw). To achieve
kinetic conditions, CVs of 1 and 2 were obtained in a pressure
vessel under 68 atm O2 to achieve high substrate : catalyst ratios:
88 mM O2 : 500 mM H+ : 0.10 mM 1. Under these fairly
extreme conditions, the catalytic currents were independent of
scan rate between 25 and 100 mV sꢀ1 (Fig. 3), indicating kinetic
conditions. With the assumption that 1 is acting purely as a
soluble catalyst, the ic/ip of B110 +0.2 V at 100 mV sꢀ1 implies a
TOF of B600 sꢀ1 (see ESIw). In contrast, CVs of the 4-pyridyl
catalyst 2 under the same conditions are still limited by diffusion
(Fig. S15 and S16, ESIw). This implies that higher dioxygen
pressures would further increase the peak current, and indicates
that 2 is a faster electrocatalyst than 1.
It is remarkable that there is such a dramatic effect upon
+
changing from CH3 to H+ at the 4-pyridyl nitrogen that is
very distant from the iron centre. It is also interesting that the
2-pyridyl catalysts have higher selectivities than the 4-pyridyl
isomers. Electrocatalysis by 1 makes half as much H2O2 as
that by 2, and 1Cl makes essentially no H2O2 while 2Cl gives
While 1 and 2 are rapid electrocatalysts for the ORR, they
operate at quite high overpotentials. The onset of the reductive
Fig. 4 RRDVs under 1 atm O2, with the ring current magnified ꢁ10.
Top: 1 (left) and 2 (right) (0.3 mM, 0.5 M HOTf) with a rotation rate o
of 2000 rpm and a disk scan rate u of 250 mV sꢀ1. Bottom: 1Cl and 2Cl
(0.3 mM) in 0.5 M KCl/0.25 M HCl solutions, with o = 3000 RPM
Fig. 3 Background-corrected linear sweep voltammograms of 0.10 mM
1 in 0.5 M HOTf under 68 atm O2 at varying scan rates u. For non-
background-corrected versions, see Fig. S11–S20, ESI.w Inset: Plot of i at
+0.2 V vs. u1/2
.
and u = 250 mV sꢀ1
.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11100–11102 11101

View Article Online
the large distances of the pyridyl nitrogens from the iron centre.
Unlike previous electrocatalysts with proton relays, the pyridyls
are very poorly positioned to serve that function. The ability of
the protonated ligands to affect the proton delivery is especially
surprising because catalysis occurs in acidic water, pH o 1, where
proton transfer would be expected to be particularly facile. These
results point toward a more subtle role for groups introduced as
proton relays in multielectron/multiproton catalysis.
Fig. 5 Representative structure for [Fe{porphyrin(pyH)4}(O2)ꢂ
This research was supported as part of the Center for
Molecular Electrocatalysis, an Energy Frontier Research Center
funded by the US Department of Energy, Office of Science. AV
and JYY were supported by a Laboratory Directed Research
and Development program at Pacific Northwest National
Laboratory, which is operated by Battelle for the US Depart-
ment of Energy. Computational resources were provided at the
National Energy Research Scientific Computing Center (NERSC)
at Lawrence Berkeley National Laboratory.
(H2O)2]4+ in water.
B15% H2O2. These effects are likely due to differences in the
proton delivery to O2-derived intermediates in the catalytic
cycle. This is reminiscent of the effects of proton delivery to
FeOOH intermediates in cytochrome P450 enzymes.17 H+
delivery to the distal oxygen facilitates O–O bond cleavage,
while mutations allow protonation of the proximal oxygen
and lead to H2O2.
Proton delivery is kinetically significant under the high
pressure conditions discussed above: reducing the HOTf
concentration from 0.5 to 0.25 M reduces the catalytic current
by a roughly a factor of two (Fig. S17 and S18, ESIw).
In contrast, under 1 atm O2 when currents are limited by
diffusion of O2 to the electrode, changes in acid concentration
from pH 0 to 1 caused little change in the CVs of 1 or 2.
Computations have explored how the 2-pyridyl substituents
in 1 affect proton delivery. Density functional theory calculations
(see ESIw for relevant details) indicate that the tetraprotonated
Fe(II) derivative binds O2, and that the pyridinium protons are
much too far to hydrogen-bond to the O2 ligand. In a representative
conformer of [Fe{porphyrin(pyH)4}(O2)]4+, the closest pyH+ꢂꢂꢂO
distance is 3.80 A, with an NꢂꢂꢂO distance of 4.56 A. Complexes
with added water molecule(s) optimize to place H2O in between the
pyridinium and the bound O2; one such structure is shown in Fig. 5.
Even in this structure, the OHꢂꢂꢂO hydrogen bonding is weak, with
long HꢂꢂꢂO distances of 2.2 and 2.3 A. When more water molecules
are included, the pyridinium ions seem to organize them into a
cluster around the O2 ligand, but additional computations are
needed to better define the solvation structure around the O2 ligand.
In the iron(II) complex [Fe{porphyrin(pyH)4}]4+, the computed
pKa for the pyridinium groups is about 4.4. The protons needed
for the reduction of O2 thus may come from the acidic solution
(pH B 0.5) rather than from the pyridinium groups. This would
imply that the pyridinium cations act not as proton relays but in a
more subtle fashion, affecting other protons in the second coordi-
nation sphere. Computations to explore this issue are in progress.
Iron meso-tetra(pyridyl)porphyrin complexes are rapid and
selective electrocatalysts for the ORR. Under 68 atm O2
(88 mM) in 0.5 M HOTf, cyclic voltammograms for 1 and 2
suggest turnover frequencies of ca. 600 sꢀ1 or more. Electro-
catalysts 1 and 1Cl, with the pyridyl nitrogens pointing toward
the iron centre, are >95% selective for the desired 4eꢀ
reduction of O2 to H2O. The isomeric 4-pyridyl complexes
have somewhat lower selectivity (B85–89%) and the related
tetrakis-4-methyl-pyridinium derivative was found by Kuwana
to give almost exclusively H2O2.3
Notes and references
1 F. Jaouen, E. Proietti, M. Lefevre, R. Chenitz, J.-P. Dodelet,
´
G. Wu, H. T. Chung, C. M. Johnston and P. Zelenay, Energy
Environ. Sci., 2011, 4, 114.
2 Cf., (a) J. P. Collman, R. Boulatov, C. J. Sunderland and L. Fu,
Chem. Rev., 2003, 104, 561; (b) N. Kobayashi and W. A. Nevin,
Appl. Organomet. Chem., 1996, 10, 579; (c) S. Fukuzumi,
S. Mochizuki and T. Tanaka, Inorg. Chem., 1990, 29, 653;
(d) Z. Halime, H. Kotani, Y. Li, S. Fukuzumi and K. D. Karlin,
Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 13990; (e) S. Samanta,
K. Sengupta, K. Mittra, S. Bandyopadhyay and A. Dey, Chem.
Commun., 2012, 48, 7631.
3 (a) T. Kuwana, J. Electroanal. Chem., 1978, 88, 299–303;
(b) A. Bettelheim and T. Kuwana, Anal. Chem., 1979, 51,
2257; (c) P. A. Forshey and T. Kuwana, Inorg. Chem., 1983, 22,
699.
4 D. K. Dogutan, S. A. Stoian, R. McGuire Jr., M. Schwalbe,
T. S. Teets and D. G. Nocera, J. Am. Chem. Soc., 2011, 133,
131.
5 M. Rakowski DuBois and D. L. DuBois, Acc. Chem. Res., 2009,
42, 1974.
6 T. A. Tronic, M. Rakowski DuBois, W. Kaminsky, M. K.
Coggins, T. Liu and J. M. Mayer, Angew. Chem., Int. Ed., 2011,
50, 10936.
7 J. Y. Yang, R. M. Bullock, W. G. Doughtery, W. S. Kassel,
B. Twamley, D. L. DuBois and M. Rakowski DuBois, Dalton
Trans., 2010, 39, 3001.
8 C. T. Carver, B. D. Matson and J. M. Mayer, J. Am. Chem. Soc.,
2012, 134, 5444.
9 (a) T. Kaufmann, B. Shamsai, R. S. Lu, R. Bau and G. Miskelly,
Inorg. Chem., 1995, 34, 5073; (b) K. Kalyansundaram, Inorg.
Chem., 1984, 23, 2453.
10 A. D. Adler, F. R. Longo, F. Kampas and J. Kim, J. Inorg. Nucl.
Chem., 1970, 32, 2443.
11 For 1/2, DEp = 79/68 mV and ic/ia = 1.1/1.1 at 100 mV sꢀ1
.
12 E. Wilhelm, R. Battino and R. J. Wilcock, Chem. Rev., 1977, 77, 219.
13 K. Shigehara and F. C. Anson, J. Phys. Chem., 1982, 86, 2776.
14 Iron tetra(N-methyl-4-pyridinium porphyrin) does not adsorb on
glassy carbon electrodes: C. Shi and F. C. Anson, Inorg. Chem.,
1990, 29, 4298.
15 (a) J. M. Saveant, Chem. Rev., 2008, 108, 2348–2378;
´
(b) R. S. Nicholson and I. Shain, Anal. Chem., 1964, 36, 706.
16 (a) J. A. Cracknell, K. A. Vincent and F. A. Armstrong, Chem.
Rev., 2008, 108, 2439; (b) E. Katz, I. Willner and A. B. Kotlyar,
J. Electroanal. Chem., 1999, 479, 64.
17 Cf., (a) T. M. Makris, K. von Koenig, I. Schlichting and
S. G. Sligar, Biochemistry, 2007, 46, 14129; (b) For a recent
example: S. Sivaramakrishnan, H. Ouellet, H. Matsumura,
The substantial effects of the regiochemistry of the pyridi-
nium acids and of replacing H+ for CH3+ are surprising given
S. Guan, P. Moenne-Loccoz, A. L. Burlingame and P. R. Ortiz
¨
de Montellano, J. Am. Chem. Soc., 2012, 134, 6673.
c
11102 Chem. Commun., 2012, 48, 11100–11102
This journal is The Royal Society of Chemistry 2012