Electocatalytic water oxidation by cobalt(III) hangman β-octafluoro corroles

Article abstract of DOI:10.1021/ja202138m

Cobalt hangman corrole, bearing β-octafluoro and meso- pentafluorophenyl substituents, is an active water splitting catalyst. When immobilized in Nafion films, the turnover frequencies for the 4e ^-/4H⁺ process at the single cobalt ce

Full text of DOI:10.1021/ja202138m

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Electocatalytic Water Oxidation by Cobalt(III) Hangman
β-Octafluoro Corroles
Dilek K. Dogutan,† Robert McGuire, Jr.,† and Daniel G. Nocera*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307,
United States
S Supporting Information
b
Scheme 1
ABSTRACT: Cobalt hangman corrole, bearing β-octa-
fluoro and meso-pentafluorophenyl substituents, is an active
water splitting catalyst. When immobilized in Nafion films,
the turnover frequencies for the 4eꢀ/4Hþ process at the
single cobalt center of the hangman platform approach
1 sꢀ1. The pH dependence of the water splitting reaction
suggests a proton-coupled electron transfer (PCET) cata-
lytic mechanism.
Hangman corroles can be synthesized concisely from easily
n attractive approach to meeting the energy demands of a
available starting materials, in two steps, in good yields, and
A
growing global population1ꢀ3 is to capture solar energy and
with abbreviated reaction times.30 In order to boost the
oxidizing power of the corrole subunit, we modified the
macrocycle with electron-withdrawing groups. Introduction
of ancillary fluorinated phenyl groups onto the 5 and 15 meso-
positions of the framework can increase the oxidizing power of
the macrocycle by more than 0.4 V,31,32 and an additional
0.5ꢀ0.6 V33 is gained upon fluorination of the β-pyrrolic
positions of the macrocycle.34,35 The β-octafluoro hangman
corrole was synthesized in 23% overall yield. 19F NMR (Figure
S1g) establishes that the trans-A2B isomer is obtained. The
relatively high yield in part is due to the use of microwave
irradiation, which efficiently drives metal insertion and de-
protection of the hanging methyl ester group.26,30,36 Synth-
eses and characterization details of new compounds are
provided in the Supporting Information.
store it in the form of chemical fuels.4ꢀ6 A prevailing solar-to-
fuels process is the splitting of water to produce hydrogen,7
which may be used directly or combined with carbon dioxide to
produce a more conventional fuel. The water splitting process is a
4eꢀ/4Hþ process, thus providing an imperative for the discovery
of new catalysts that bridge the 1eꢀ/1Hþ capture process of
most light-harvesting systems, including semiconductors, to
the 4eꢀ/4Hþ process of water splitting. We recently reported
that heterogeneous Co-phosphate (Co-Pi)8ꢀ10 and Ni-borate
(Ni-Bi)11 films form self-healing12 catalysts that promote the
efficient splitting of water under benign conditions. XAS studies13,14
have established that the Co-Pi is a molecular cobaltate catalyst that
is a structural relative of the oxygen-evolving complex Photosystem
II.15 With this discovery, we have turned our attention to exploring
molecular cobalt catalysts in homogeneous solution because char-
acterization of the catalytic species, the ability to tune catalytic
properties, and the mechanism of action is in principle more easily
investigated. Several molecular catalysts have been reported recently
that demonstrate impressive OER activity.16ꢀ25 A recent report
from our laboratory reveals that Co(II) hangman porphyrins
promote the 4eꢀ/4Hþ reduction of oxygen to water.26 The oxygen
reduction reaction (ORR) is the reverse of the oxygen-evolving
reaction (OER). Moreover, the oxygen atoms from two water
molecules may be situated within the hangman cleft; one water is in
the primary coordination sphere of the metal, whereas another is
held in the secondary coordination sphere via hydrogen bonding to
the hanging group.27 These two observations taken together piqued
our interest in the possibility that Co hangman corroles may be
developed as OER catalysts. Our interest was further bolstered by
the report that Mn corroles bearing nitroaromatic meso groups28 and
pentadentate CoII complexes29 are OER catalysts. We now report
the synthesis of β-octafluoro hangman corrole (Scheme 1),
CoHβFCX-CO2H, and we show that it is the most active OER
catalyst among cobalt corroles.
Samples for electrocatalysis were prepared by dissolving the
corrole in a 10:2:1 mixture of THF/ethanol/Nafion solution,
to give a final concentration of 0.5% Nafion and 1 mM catalyst.
A 30 μL drop of solution was then deposited onto an FTO-
coated glass slide and allowed to slowly evaporate. The resulting
film contained 30 nmol of catalyst per cm2.
A cyclic voltammogram (CV) of CoHβFCX-CO2H in
CH2Cl2 is shown in Figure S3f. The previously published
CVs of Co(C6F5)3 and CoHCX-CO2H are also reproduced in
Figure S3f for convenience.30 CoHβFCX-CO2H exhibits a
reduction wave at 0.33 V and an oxidation event at 0.87 V vs
ferrocene. Addition of water to a DMF solution of the corrole
reveals a catalytic peak positive of the reversible oxidation
process (Figure S3f) prompting us to examine the electro-
chemistry in aqueous solutions (0.1 M phosphate buffer,
pH = 7). Figure 1 shows that CoHβFCX-CO2H exhibits
Received: March 8, 2011
Published: May 23, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja202138m J. Am. Chem. Soc. 2011, 133, 9178–9180
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Figure 1. Cyclic voltammograms of FTO background (black) and
Nafion films containing 30 nmol/cm2 of Co(C6F5)3 (red, 9),
CoHCX-CO2H (green, b) and CoHβFCX-CO2H (blue, 2) deposited
on FTO glass in 0.1 M KPi buffer, pH 7. Scan rate, 0.1 V/s. Inset: Tafel
plots of the same electrodes.
Figure 3. pH dependence of cyclic voltammograms at pH 1, 3, 7, 10,
and 14 from left to right. Scan rate is 100 mV/s. Inset: Potential
(measured at 40 μA) vs pH with a slope of 88 mV/pH unit.
The Faradaic efficiency of the CoHβFCX-CO2H catalyst was
measured by using a fluorescence-based O2 sensor. Electrolysis
was performed in aqueous solutions (0.1 M phosphate buffer,
pH = 7) in a gastight electrochemical cell under an N2 atmo-
sphere with the sensor placed in the headspace. After initiating
electrolysis at 1.4 V, the percentage of O2 detected in the
headspace rose in accord with what was predicted by assuming
that all of the current was caused by 4eꢀ oxidation of water to
produce O2 (Figure 2 inset). Thus only O2 is produced during
catalytic turnover by CoHβFCX-CO2H. The gaseous products
evolved during electrolysis at constant potential (1.4 V vs Ag/
AgCl, pH = 7) were also measured by mass spectrometry and the
results are shown in Figure 2. By this measurement, CoHβFCX-
CO2H is also observed to be the most efficient catalyst of the
series as nearly twice as much oxygen is detected as compared to
CoHCX-CO2H and Co(C6F5)3 (Figure S5). Importantly, dur-
ing the course of the electrolysis, we observe no CO2, which may
result if the corrole framework were to be oxidized. As further
testament to catalyst stability, the CoHβFCX-CO2H electrode
was immersed in THF upon the completion of electrolysis,
and the solution was concentrated. UVꢀvis (Figure S4a), LD-
MS MALDI-TOF (Figure S4b), and high-resolution ESI-MS
(Figures S4c and S4d) all indicate only the presence of the
cobalt hangman corrole. Finally the pH dependence of the
OER reaction (Figure 3) shows that the overpotential for OER
decreases with increasing pH. A plot of the potential as
measured at constant current (40 μA) versus pH shows a
monotonic decrease from pH = 14 to pH = 1 (Figure 3, inset).
This indicates that decomposition of the catalyst to cobalt
oxide is unlikely, as such species are unstable in very acidic
solutions. Finally, the onset potentials for the three catalysts
are unique and the overpotential for OER is also inconsistent
with cobalt oxide-type species.38
With OER established, the turnover frequency (TOF) may be
calculated by measuring the current density for the 4eꢀ/4Hþ
OER process of CoHβFCX-CO2H. At 1.4 V vs Ag/AgCl,
the TOFs per Co atom for CoHβFCX-CO2H is 0.81 sꢀ1. This
number compares favorably with regard to other cobalt-based
water oxidation catalysts.39,40
In summary, hangman cobalt corroles are OER catalysts with
β-octafluoro CoIII xanthene hangman corrole, bearing 5,15-bis-
(pentafluorophenyl) substitutients as the most effective OER
catalyst. The catalyst is stable under operating conditions and
evolves only oxygen as the OER product at modest overpotential.
The ability of the hanging group to preorganize water within the
hangman cleft appears to be beneficial for the OꢀO bond-
forming reaction of OER.
Figure 2. Mass spectrometric detection of O2 and CO2 during electrolysis
of a bare electrode (ꢀ, black) and CoHβFCX-CO2H (ꢀ, blue) at 1.4 V vs
Ag/AgCl. Inset: O2 production measured by fluorescent sensor (ꢀ, blue)
and the theoretical amount of O2 produced (ꢀ, black) assuming a
Faradaic efficiency of 100%.
greater current and earlier onset of catalytic current as
compared to its β-nonfluorinated congener, and both hang-
man compounds exhibit greater current than the cobalt
corrole possessing meso-C6F5 groups but lacking the hanging
group (Co(C6F5)3). The onset of the catalytic current (1.25 V
vs Ag/AgCl) occurs ∼0.6 V beyond the thermodynamic
potential for water oxidation at pH = 7 (0.62 V vs Ag/AgCl);
this value is nearly 100 mV higher than that of recently
reported molecular cobalt catalysts.17 The inset in Figure 1
shows the Tafel plots of the three corrole compounds. A slope
of ∼120 mV/decade indicates that an electron transfer step is
the rate-determining step.37 The curvature of the Tafel plot
indicates a potentially more complex mechanism; however,
this curvature is more likely due to the mediation of electron
and proton transfer by the Nafion film and not to mechanistic
details associated with the intrinsic activity of the catalyst. We
note that the “CoIV” hangman corroles are isolable species. We
have shown that the spectroscopic properties and DFT
calculations of cobalt corrole axially ligated by chloride are
consistent with a “CoIV” species that is better described as a
CoIII site strongly antiferromagnetically coupled to the S = 1/2
of the monoradical dianion corrole, [CoIIICl-corroleþ•].30 We note
that β-fluorination will make the corrole more difficult to oxidize,
and therefore, the exact oxidation states of the metal and corrole
remain ambiguous. Regardless of the exact nature of the precatalytic
state, water oxidation occurs more positive of the first oxidation
event of CoIIIHβFCX-CO2H to produce formally CoIIIHþ•βFCX-
CO2H or CoIVHβFCX-CO2H. These results indicate that the active
catalytic species is CoIVHþ•βFCX-CO2H.
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’ ASSOCIATED CONTENT
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S
Supporting Information. Full synthetic details and char-
b
acterization for Co(C6F5)3, CoHCX-CO2H, and CoHβFCX-
CO2H; additional electrochemical data for Co(C6F5)3 and
CoHβFCX-CO2H; and characterization after electrolysis of
CoHβFCX-CO2H. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
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Corresponding Author
nocera@mit.edu
Author Contributions
†These authors contributed equally to this work.
(29) Wasylenko,D.J.;Ganesamoorthy,C.;Borau-Garcia,J.;Berlinguette,
C. P. Chem. Commun. 2011, 47, 4251.
’ ACKNOWLEDGMENT
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This work was performed under the auspices of the Division of
Chemical Sciences, Geosciences, and Biosciences, Office of Basic
Energy Sciences of the U.S. Department of Energy through
Grant DE-FG02-05ER15745. New synthetic methods were
performed under the sole sponsorship of Eni S.p.A under the
Eni-MIT Alliance Solar Frontiers.
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