Hydrogen generation by hangman metalloporphyrins

Article abstract of DOI:10.1021/ja202136y

A cobalt(II) hangman porphyrin with a xanthene backbone and a carboxylic acid hanging group catalyzes the electrochemical production of hydrogen from benzoic and tosic acid in acetonitrile solutions. We show that Co^IIH is exclusively involved i

Full text of DOI:10.1021/ja202136y

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pubs.acs.org/JACS
Hydrogen Generation by Hangman Metalloporphyrins
Chang Hoon Lee, Dilek K. Dogutan, and Daniel G. Nocera*
Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-
4307, United States
S Supporting Information
b
Scheme 1
ABSTRACT: A cobalt(II) hangman porphyrin with a
xanthene backbone and a carboxylic acid hanging group
catalyzes the electrochemical production of hydrogen from
benzoic and tosic acid in acetonitrile solutions. We show
that CoIIH is exclusively involved in the generation of H2
from weak acids. In a stronger acid, a CoIIIH species is
observed electrochemically, but it still needs to be further
reduced to CoIIH before H2 generation occurs. Overpoten-
tials for H2 generation are lowered as a result of the
hangman effect.
Chart 1
ydrogen generation from carbon-neutral sources is an
H
important part of a multifaceted strategy to meet growing
global energy demands.1,2 Accordingly, renewed interest in H2
catalyst discovery has led to the creation of a variety of complexes
that electrocatalyze Hþ reduction.3ꢀ9 A particularly fascinating
design element of emergent catalysts is the incorporation of a
proton relay from a pendant acidꢀbase group proximate to the
metal center where H2 production occurs.3,4,10,11 These catalysts
are akin to the active sites of hydrogenases, which feature pen-
dant bases positioned near the metal centers that are postulated
to play a role in enzyme catalysis.12 The benefits of a pendant
proton relay are consistent with the early proposal of H2
generation via the pathway shown in Scheme 1A: reduction of
a CoII center to CoI followed by Hþ attack to yield a hydridic
CoIIIH species that yields H2 upon protonolysis or bimetallic
reaction. However, this mechanism has been recently reconsid-
ered in view of the contention that CoIIIH centers are not
sufficiently basic to drive protonolysis, and it has been suggested
that more reduced cobalt species must be attained before
protonolysis can occur (Scheme 1B).13,14 The inability to control
proton stoichiometry in most catalytic cycles has made it difficult
to distinguish mechanisms and thus discern which intermediate
is involved in catalysis. On this count, we realized the utility of
hangman active sites for providing insight into the mechanism of
H2 evolution by stoichiometric generation of a key intermediate
as a result of the hangman effect. In the hangman construction, an
acidꢀbase functionality is positioned from a xanthene or furan
spacer over the face of a redox-active macrocycle such as
porphyrin,15,16 salen,17,18 or corrole.19 The acidꢀbase hanging
group permits the facile transfer of a single proton to or from a
substrate bound to a metal macrocycle. With the ability to control
proton stoichiometry from the hanging group, we undertook
studies to examine H2 generation at CoHPX-CO2H (1-Co),
shown in Chart 1. Comparison of the electrochemistry of 1-Co
to that of a macrocyclic analogue in which the hanging group has
been removed, CoHPX-Br (2-Co, Chart 1), establishes the
hangman effect (via a reduced overpotential) and that the Co
center produces H2 beyond reduction potentials exceeding the
CoI oxidation state. Our results are consistent with the genera-
tion of CoIIH as a key intermediate in H2 electrocatalysis at the
hangman cobalt porphyrin active sites.
Hangman porphyrins can be obtained in appreciable quanti-
ties, in short synthesis times, and in high yields.20,21 1-Co and 2-
Co were synthesized following these methods. As shown in
Figure 1a, 1-Co and 2-Co exhibit reversible waves for the CoII/I
couple at almost the same potentials (ꢀ1.08 V vs the ferrocene/
ferrocenium couple for 1-Co and ꢀ1.10 V for 2-Co). The cyclic
voltammogram (CV) of 2-Zn shows redox waves at ꢀ1.52 and
ꢀ1.92 V (Supporting Information (SI) Figure S9) and confirms
that each electrochemical feature of 1-Co and 2-Co has sig-
nificant cobalt character. For simplicity, the reduction potentials
will be formally ascribed to Co, though we believe that there is
significant electron density on the porphyrin ring at very redu-
cing potentials.
Whereas 2-Co shows a reversible wave for CoI/0 at ꢀ2.14 V,
interestingly, 1-Co produces an irreversible wave for the reduc-
tion of CoI, and the wave is positively shifted by ∼200 mV. The
only structural difference between 1-Co and 2-Co is the hanging
carboxylic acid group, and accordingly the irreversible process of
Received: March 8, 2011
Published: May 10, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja202136y J. Am. Chem. Soc. 2011, 133, 8775–8777
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during the electrolysis was determined by gas chromatography
after 15 C of charges had passed. Faradaic efficiencies for H2
production were ca. 80% and 85% for 1-Co and 2-Co, respec-
tively; no other gaseous product is detected in the experimental
condition. On the basis of TLC, mass spectra, and UVꢀvis
measurements, the decomposed product in bulk electrolysis in
the presence of 2-Co does not correspond to a demetalated
porphyrin or other porphyrin product.
In the presence of the stronger tosic acid (pKa = 8.3 in
acetonitrile22), both 1-Co and 2-Co exhibit catalytic cathodic
waves at ∼ ꢀ1.5 V (Figures 1c and SI Figures S4 and S5). The
similarity of the CVs with regard to current and the onset of
electrocatalysis suggests that the stronger acid overwhelms the
chemistry of the system and the hangman effect is obviated. As
observed for benzoic acid, electrocatalysis for 1-Co and 2-Co
occurs at potentials negative of the CoII/I couple. However, there
is one significant difference between the benzoic acid and tosic
acid data: unlike the situation for benzoic acid, the CoII/I wave
becomes irreversible in the stronger tosic acid for both 1-Co and
2-Co (Figures 1c, S4, and S5). This indicates that CoI is
protonated by the tosic acid. But the observation that catalysis
occurs well past the CoII/I reduction event indicates that a CoIIIH
species, when formed, needs to be further reduced to CoIIH for
H2 generation to occur. One determinant of the metal basicity is
the presence of meso groups on the macrocycle periphery. The
electron-withdrawing C6F5 groups will attenuate the metal
center basicity and make the metal less reactive to protons, as
has previously been observed.6,23
In summary, the hangman porphyrin provides mechanistic
insight into Hþ reduction owing to the ability to control proton
equivalency precisely via the hanging group. The irreversibility
and positive shift of the reduction of CoI in 1-Co together with a
lowered overpotential for H2 production are results of the
hangman effect. For the case of weak acids, H2 is produced upon
reduction to Co0, followed by protonation (middle bracket,
Scheme 1B). For stronger acids, CoI is first protonated, and
electron reduction follows it (top bracket, Scheme 1B). Regard-
less of the strength of the acid, these results are consistent with
H2 production being mediated by CoIIH. Further reduction of
the metal is needed for the effective protonation of the hydride to
produce H2.
Figure 1. (a) CVs of 0.5 mM 1-Co (black line), 2-Co (red line), and
2-Co in the presence of 0.5 mM benzoic acid (green line). (b) CVs of
0.5 mM 1-Co in the presence of 2.5 mM benzoic acid (black line) and
0.5 mM 2-Co in the presence of 3.0 mM benzoic acid (red line). (c) CVs
of 0.8 mM 1-Co (black line) and 2-Co (red line) in the presence of
10 mM tosic acid. Scan rate, 100 mV/s; 0.1 M NBu4PF6 in acetonitrile.
Glassy carbon working electrode, Ag/AgNO3 reference electrode, and
Pt wire counter electrode.
1-Co is ascribed to the hangman effect, where the reduction of
CoI to Co0 is followed by immediate proton transfer from the
hanging group to produce CoIIH. The second wave in the CV of
2-Co also becomes irreversible upon the addition of external
benzoic acid. At 1 equiv of benzoic acid, the wave begins to
exhibit irreversibility, also indicating protonation of the Co0
species. Complete irreversibility of the wave is observed only
upon addition of >1 equiv of benzoic acid; this observation is
consistent with the hangman effect in 1-Co.
In the presence of excess benzoic acid (pKa = 20.7 in aceto-
nitrile),22 1-Co and 2-Co exhibit catalytic cathodic waves (Figure 1b
and SI Figures S2 and S3). Whereas the overpotential for catalysis is
large (∼800 mV), the catalysis performance is not our interest; the
CV features of the electrocatalysis uncover essential mechanistic
details of the hydrogen evolution reaction (HER) at cobalt macro-
cycles. The CoII/I reduction feature is not affected much by the
presence of acid (SI Figures S2, S3, and S8), but the second
reduction wave exhibits pronounced catalytic activity. These results
indicate that benzoic acid is too weak an acid to protonate the CoI
center, and hence catalytic H2 production is observed only upon
further reduction to Co0. The overpotential for proton reduction of
1-Co is ∼120 mV lower than that of 2-Co at 3 mM acid
concentration. Moreover, the potential of the second reduction
wave of 1-Co is the same in the presence and absence of acid
(Figure S2). This is not the case for 2-Co; with increasing acid
concentration, the wave shifts to more positive potential by 80 mV
(Figure S3). These results are also indicative of the hangman effect
since in 1-Co, proton transfer is not rate-determining for catalysis
(hence the insensitivity of the reduction wave to proton con-
centration), whereas in 2-Co, the proton transfer is a determinant
of the mechanism (hence the shift to more positive potential with
increasing acid). For either case, H2 catalysis is initiated from the
CoIIH.
’ ASSOCIATED CONTENT
S
Supporting Information. Full synthetic details and char-
b
acterization for 2, 2-Co, and 2-Zn; CVs of 1-Co and 2-Co in the
presence of benzoic and tosic acid; CVs of benzoic acid and tosic
acid; and CV of 2-Zn. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
nocera@mit.edu
’ ACKNOWLEDGMENT
We thank the Office of Basic Energy Sciences of the DOE
(DE-FG02-05ER15745) for support of this work. Grants from
the NSF also provided instrument support to the DCIF at MIT
(CHE-9808061, DBI-9729592). We thank Yogesh Surendranath
and Kwabena Bediako for helpful discussions.
Bulk electrolysis was performed in acetonitrile solutions of
0.4 mM 1-Co at ꢀ2.05 V and of 0.5 mM 2-Co at ꢀ2.20 V in the
presence of 15 mM benzoic acid. The amount of H2 gas produced
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