Hydrogen Evolution Catalysis by a Sparsely Substituted Cobalt Chlorin

Article abstract of DOI:10.1021/acscatal.7b00969

A sparsely substituted chlorin macrocycle containing a Co(II) center (1-Co) has been synthesized and structurally characterized. The Co(II) atom resides in a square planar coordination environment and induces significant out-of-plane distortion of the chlorin macrocycle. The paramagnetic Co(II) center resides in the macrocycle in a S = 1/2 spin state, which displays an axial doublet signal (g_⊥ ≈ 2.3 and g_∥ ≈ 2.03) in the X-band EPR spectrum. The open-shell d-orbital configuration is manifest in the transient absorption spectrum, which reveals an excited-state lifetime of 8.6 ± 0.2 ps for 1-Co. The Co(II) chlorin exhibits a rich oxidation-reduction chemistry with five reversible one-electron waves (three oxidative processes and two reductive processes) observed in the cyclic voltammogram. The reduction processes of 1-Co drive hydrogen evolution catalysis. Electrochemical kinetics analysis of HER by 1-Co in trifluoroacetic acid reveals a hydrogen evolution mechanism that proceeds by an ECEC mechanism. Benchmarking the catalytic activity, 1-Co exhibits higher HER activity at low overpotentials, versus its porphyrin congeners. (Graph Presented).

Full text of DOI:10.1021/acscatal.7b00969

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Article
Hydrogen Evolution Catalysis by a Sparsely Substituted Cobalt Chlorin
Andrew G. Maher, Guillaume Passard, Dilek Kiper Dogutan, Robert L. Halbach, Bryce L. Anderson,
Christopher J. Gagliardi, Masahiko Taniguchi, Jonathan Sidney Lindsey, and Daniel G. Nocera
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00969 • Publication Date (Web): 10 Apr 2017
Downloaded from http://pubs.acs.org on April 10, 2017
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Andrew G. Maher,a,‡ Guillaume Passard,a,‡ Dilek K. Dogutan,a Robert L. Halbach,a Bryce L.
Anderson,a Christopher J. Gagliardi,a Masahiko Taniguchi,b Jonathan S. Lindseyb,* and Daniel G.
Noceraa,*
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aDepartment of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138
bDepartment of Chemistry, North Carolina State University, Raleigh, North Carolina 27695
ABSTRACT: A sparsely substituted chlorin macrocycle containing a Co(II) center (1-Co) has been synthesized and
structurally characterized. The Co(II) atom resides in a square planar coordination environment and induces significant
out-of-plane distortion of the chlorin macrocycle. The paramagnetic Co(II) center resides in the macrocycle in a S = ½
spin state, which displays an axial doublet signal (g  2.3 and g‖  2.03) in the X-band EPR spectrum. The open shell d-
orbital configuration is manifest in the transient absorption spectrum, which reveals an excited state lifetime of 8.6  0.2
ps for 1-Co. The Co(II) chlorin exhibits a rich oxidation-reduction chemistry with five reversible one-electron waves (two
oxidative processes and three reductive processes) observed in the cyclic voltammogram. The reduction processes of 1-Co
drive hydrogen evolution catalysis. Electrochemical kinetics analysis of HER by 1-Co in trifluoroacetic acid reveals a
hydrogen evolution mechanism that proceeds by an ECEC mechanism. Benchmarking the catalytic activity, 1-Co exhibits
higher HER activity at low overpotentials versus its porphyrin congeners.
Keywords: electrocatalysis, macrocycle, chlorin,
cobalt, hydrogen evolution reaction, solar fuels
Chart 1. π-Electron Networks in Chlorins and
Porphyrins
INTRODUCTION
The macrocyclic ring of chlorins is reduced by two
electrons and two protons relative to its parent porphyrin
structure. The reduction occurs at the -pyrrolic
periphery of the macrocycle and thereby preserves an
aromatic 18 π-electron conjugated pathway (Chart 1).
Structural data of chlorin macrocycles are consistent with
this electronic formulation, as the -bond external to
the 18 π-electron network exhibits a CC distance that is
consistent with a more localized π bond.1 Hence, the
macrocyclic core of chlorins is electronically similar but
chemically reduced relative to porphyrins. In view of the
exceptional catalytic activity of porphyrins in promoting
reductive processes such as the hydrogen evolution
reaction (HER) and the oxygen evolution reaction (ORR),2
chlorins presented an interesting new class of catalysts for
the activation of small molecules of energy consequence.
The extensive literature on the redox properties and
reaction chemistry of porphyrins2‒5 is contrasted by a
limited number of studies on the electrochemistry of
nickel6,7 and iron8,9 chlorins. A cobalt chlorin featuring
chlorophyll-like substituents is reported to catalyze the
two-electron reduction of dioxygen (O2) to produce
hydrogen peroxide (H2O2) chemically at activities higher
than that observed for cobalt porphyrins,10,11 which
typically drive ORR to H2O as opposed to H2O2. The same
cobalt chlorin has been reported to produce H2
photochemically at the potential of the photoproduced
Ru(bpy)3+ reductant.12 The potential dependence of the
HER catalysis of chlorins remains undefined, especially
Chart 2. Free Base Chlorin 1 and Co Chlorin 1-Co
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Figure 1. Temperature-dependent EPR spectra (35 K, ▬; 50
K, ▬; 70 K, ▬) of 1-Co.
Figure 2. Solid-state structure of 1-Co with thermal
ellipsoids (drawn using SHELX/XP) shown at the 50%
probability level. Selected interatomic distances (Å) and
angles (deg): d(Co1–N1) = 1.9445(17), d(Co1–N2) = 1.9660(17),
d(Co1–N3) = 1.9465(18), d(Co1–N4) = 1.9901(18), d(C7–C8) =
1.356(3), d(C17–C18) = 1.520(4), ∠(C1–N1–C4) = 104.66(18),
∠(C6–N2–C9) = 104.67(17), ∠(C11–N3–C14) = 104.82(18),
∠(C16–N4–C19) = 107.27(19).
with regard to how catalytic activity is influenced by
potential.
The study of chlorins has been facilitated by advances
in the synthesis of the macrocycle that feature various
substituents in specific patterns.13 The sparsely
substituted chlorin macrocycle 1 (Chart 2) is available
from a concise synthesis in appreciable yields14 to afford
material on scales that permit catalysis studies to be
undertaken. Moreover, the gem-dimethyl group in the
pyrroline ring bestows stability to the chlorin by
preventing adventitious dehydrogenation to the
porphyrin. We now report the insertion of Co into the
chlorin to furnish 1-Co (Chart 2), which has been
structurally characterized. Inasmuch as there are few
structurally characterized simple metal chlorin
complexes,15 a comparison of the crystal structure of 1-Co
with that of sparsely substituted chlorin macrocycles
furnishes insight to the structural perturbation induced
by a centrally chelated metal ion. 1-Co exhibits a rich
redox chemistry, and the various redox species have been
correlated to their steady-state and transient electronic
spectra. We establish that the redox chemistry of 1-Co is
manifest to an active HER catalysis in the presence of
trifluoroacetic acid (TFA). The complex is stable to
oxygen, but is unstable in the presence of acid as a
promoter of ORR. 1-Co exhibits higher turnover
frequency for HER catalysis at low overpotentials than its
porphyrin congeners, likely due to the increased basicity
of the chlorin macrocycle.
CH2Cl2/CH3OH (4:1) mixtures under anaerobic conditions
in a glove box. The metalation of the chlorin is evident by
1
the paramagnetic broadening of the H NMR resonances
(Figure S1a), which stand in contrast to the sharp
1
resonances observed in the H NMR spectrum of 1 (Figure
S1b), and the FTIR spectrum. For the latter, the N–H
stretching vibration at 3343 cm‒1 in free base chlorin 1,
which is similar to that for free base porphyrins (3310–
3326 cm–1),18 disappears upon cobalt insertion (Figure S3).
On the basis of similar assignments in free base and
nickel porphyrin derivatives,18 two weak bands between
2750 and 3000 cm‒1 are assigned to the C–H stretching
bands of the meso-carbons, intense vibrations at 1595 and
1620 cm‒1 are assigned to C=C stretching vibrations, and
bands at 1350 cm‒1 are assigned to C–N stretching
vibrations. The formulation of 1-Co has also been
confirmed by high resolution LC-MS in the positive ion
mode (Figure S4) with m/z = 605.2211 (for M+, where M =
C38H34CoN4, calcd 605.2115).
The paramagnetic properties of 1-Co, indicated by the
1
broad peaks in the H NMR spectrum, are verified by the
EPR spectrum of the complex (Figure 1). Under anaerobic
conditions, 1-Co exhibits typical EPR signatures for a
Co(II) ion in the tetragonal field of the chlorin
macrocycle. The X-band EPR spectrum exhibits an axial
doublet signal with a quasi-axial g tensor of g  2.3 and
g‖  2.03. These g values are consistent with a low spin, S
= ½ spin system with the unpaired electron occupying the
RESULTS AND DISCUSSION
Preparation and characterization of 1-Co. The free
base chlorin 1 is delivered by a de novo synthetic route.14
The de novo route builds the chlorin from pyrrolic
constituents rather than relying on the more prevalent
dz orbital. Strong and well-resolved 59Co hyperfine
2
route wherein a porphyrin is converted to a chlorin.16
A
couplings of 98 G are observed for A‖, consistent with
other Co(II) macrocycles.19,20
modified procedure was used to insert Co(II) into the
macrocycle.17 The metalation proceeds quantitatively over
48 h upon treatment of 1 with 82 mol excess Co(OAc)2 in
Dark green crystals of 1-Co were obtained by cooling a
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Figure 3. The inner core of sparsely substituted chlorin in
the presence and absence of a metal.
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○
saturated solution of 1-Co in pentane at –20 C. Figure 2
displays the x-ray crystal structure for which the details of
structure determination are listed in Table S1. The central
cobalt atom resides in a square planar coordination
environment defined by the chlorin macrocycle, which
deviates from an ideal planar system. The average Co–N
bond distance of the pyrrolic rings is 1.952(3) Å whereas
the Co–N bond distance of the pyrroline unit is 1.9901(18)
Å. The mesityl substituent is nearly perpendicular (82°) to
the mean plane of the macrocycle whereas the p-tolyl
substituent is less twisted (51°) (Figure 2).
••▬ ••▬
Figure 4. Absorption spectra of 1 (
) and 1-Co (▬) at
room temperature in CH2Cl2. The spectra are normalized
with respect to the Soret band intensities.
insertion causes the core composed of the four nitrogens
to assume a more square-like geometry, there is much
greater out-of-plane distortion resulting from the metal
insertion.
Another notable structural metric is the bond distance
(Table S6) of the pyrrolic Cβ–Cβ bond C7–C8, which is the
double bond external to the 18 π-electron conjugated
pathway shown in Chart 1. The C7–C8 double bond of 1-
Co is 1.356(3) Å, which is statistically indistinguishable
from the other Cβ–Cβ bond distances of 1-Co (C2–C3 =
1.344(3) Å, C12–C13 = 1.345(3) Å) because the differences in
bond lengths are not more than three times greater than
The chlorin macrocycle of 1-Co undergoes significant
out-of-plane distortion upon metalation, in contrast to
the structure of a related free base chlorin,1 FbC, which
exhibits a nearly planar macrocycle. The distortions from
planarity can be accounted for by the sum of the absolute
values of the positive and negative displacements of the
nitrogen atoms of the chlorin from a plane (defined by
the four nitrogen atoms).1 The average out-of-plane
distortion of FbC is 0.044 Å, whereas the average
distortion of 1-Co is 0.261 Å. Table S2 compares the
average deviation from planarity to other free base
chlorins as well; in all cases 1-Co is more distorted.
Metalation also alters the structural metrics of the
pyrroline ring, as shown in Table S3; there is an increase
in the Cα–Cβ (C16–C17) bond distance of the pyrroline ring
and a decrease in the Cα–Cβ (C17–C18) and Cα–Cβ (C18–
C19) bond distances of 1-Co as compared to FbC. The
most notable change in the bond angles between 1-Co
and FbC is observed at C17 and C18 where the
hybridization changes from sp2 to sp3. The bond angles of
C16–C17–C18 (103.13(19)°) and C17–C18–C19 (100.27(19)°) in
1-Co are narrowed by 3.7° and 6.8°, respectively,
compared to the corresponding bond angles in FbC.
Compensation for these narrowed angles (~10.5° in total)
in the 5-membered ring occurs by widening of the
remaining three angles: C16–N4–C19 (~2.1°), N4–C16–C17
(~1.5°), and C18–C19–N4 (~1.7°). Metalation also leads to a
change in the Cα–N–Cα framework bond angles (Table S4)
as well as the core shape and core size of the macrocycle
(Table S5). As shown in Figure 3, the core size of 1-Co is
contracted relative to the free base species FbC and
becomes more symmetric upon insertion of Co into the
chlorin macrocycle, almost assuming a square as opposed
to the more trapezoidal core of FbC. Thus whereas metal
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the estimated standard deviations added in quadrature.
A similar comparison of the C7–C8 bond with respect to
the other Cβ–Cβ bonds has also been observed in nickel
tetramethylchlorin (C7–C8 = 1.362 Å, C2–C3 = 1.353 Å,
C12–C13 = 1.351 Å).15 These results stand in contrast to the
trend seen in the free base FbC, in which the C7–C8 bond
distance of 1.3588(19) Å is shorter than the other Cβ–Cβ
distances (C2–C3 = 1.3704(19) Å, C12–C13 = 1.3680(19) Å) to
a statistically significant extent. The shortening of the C7-
C8 bond in the free base chlorin could be a manifestation
of the exclusion of that bond from the extended π-
electron network in order to avoid an anti-aromatic 20 π-
electron count. In this regard, the lack of shortening of
the C7–C8 bond in the metalated chlorins suggests that
the presence of the metal center may lessen the anti-
aromaticity of the 20 π-electron network.
The absorption spectra of 1 and 1-Co, shown in Figure
4, exhibit the signature B(Soret)- and Q-bands common
to tetrapyrrole macrocycles as well as the enhanced Q-
band intensity in the red region particular to chlorins,22
though the two spectra differ significantly from each
other. With respect to the absorption spectrum of 1, 1-Co
exhibits a ~10 nm hypsochromic shift of the B(0,0) Soret
band and a ~40 nm hypsochromic shift of the Qy(0,0)
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Figure 6. CVs of 1-Co (▬) and 1 (▬) (1 mM) in CH3CN (0.1
M TBAPF6) at a scan rate of 0.1 V s–1 under an Ar atmosphere
with a glassy carbon working electrode (3 mm).
Figure 5. Transient absorption spectra of 1-Co at room
temperature in toluene at time delays ranging from < 1 ps
(black) to 60 ps (red) using an excitation wavelength of 400
nm. Inset: single wavelength kinetics trace monitored at 604
nm.
2Q(,*) state. Rather, the fast component may reflect
vibrational cooling, intersystem crossing to either a
doublet 2T(,*) or quartet 4T(,*) excited state, or
relaxation to a (d,d) or charge-transfer excited state. The
8.6 ps overall excited-state lifetime of the 1-Co in toluene
is similar to those obtained for other Co(II) tetrapyrrole
macrocycles such as porphyrins, which typically have
lifetimes shorter than 20 ps, attributable to the formation
of a low-lying ligand-to-metal charge-transfer (LMCT)
state.26,27 The very short lifetime of 1-Co is in contrast to
the free base and Zn chlorin analogs, which have singlet
excited state lifetimes of 10.7 ns and 1.6 ns respectively,23
underscoring the influence of the open-shell cobalt on the
electronic structure of the complex.
Oxidation Reduction Chemistry. Chlorins 1 and 1-Co
exhibit a rich redox chemistry. The CVs of 1 and 1-Co,
shown in Figure 6, reveal a number of reductive and
oxidative redox processes, all of which are
electrochemically reversible in the case of 1-Co. From the
resting state, 1-Co shows two reduction waves at standard
potentials of 퐸10− = −1.401 and 퐸20− = −2.460 V vs. Fc+/Fc
(Figure 6, red line). The compound also exhibits three
oxidation waves centered at 퐸10+ = −0.167, 퐸20+ = 0.430
and 퐸30+ = 0.796 V vs. Fc+/Fc. A comparison of the CV of
1-Co to that of 1 (Figure 6, blue line) shows that the two
most negative reduction processes and two most positive
oxidative processes are largely associated with the redox
chemistry of the chlorin. The 퐸20+ oxidation process for 1-
Co occurs at a similar potential to the first oxidation of 1
as well as that of other free base and zinc chlorins,28
further supporting the assignment of a ligand-centered
oxidation. It is interesting to note that while the 퐸20+ and
퐸30+ oxidative processes for 1-Co occur at nearly identical
potentials as the two most positive oxidations of 1, the
processes are quasi-reversible for free base 1 but reversible
for the metal-containing 1-Co chlorin. The irreversibility
of these oxidation processes in chlorins and
isobacteriochlorins has been suggested to stem from
oxidative dehydrogenation leading to porphyrin
species,7,29 but in our case the presence of geminal methyl
groups at the 18 position of the macrocycle makes
band. The split in the Soret band of 1, which is common
in free base chlorins, is due to the partial resolution of the
x- and y-polarized components. The split Soret is not
observed for 1-Co, consistent with reports of coalescence
of Bx and By bands in other metallochlorins, though the
exact nature of the relative positioning of the Bx and By
bands in chlorins is not yet fully understood.23–25
Transient Absorption Spectroscopy. The excited
state of 1-Co is short-lived as expected for an open shell d-
orbital metal ion in a π-aromatic system. In the absence
of emission, the excited-state dynamics were examined by
picosecond transient absorption (TA) spectroscopy.
Figure 5 displays the visible light picosecond TA spectrum
of 1-Co in toluene upon excitation with a 400 nm laser
pulse at a variety of time delays between < 1 ps and 60 ps.
Samples of 1-Co are indefinitely stable under TA
excitation conditions (Figure S5). The transient difference
spectrum shows a prompt broad absorption growth
(between 425 nm and 550 nm) and bleaches of the ground
state Q bands. Additionally, an absorption band slightly
to the red of the intense 604 nm ground state bleach
grows in over the first few picoseconds. By 60 ps, the
transient spectrum has returned to baseline, indicating
that excited 1-Co completely relaxes back to its electronic
ground state during that time interval. A biexponential
least-squares fit of the single-wavelength trace at 604 nm
(Figure 5 inset) yields time constants of 700  100 fs for
the short component decay and 8.6  0.2 ps for the long
component decay. A monoexponential fit of the fast
growth process at 618 nm yields a time constant of 660 
40 fs, which matches the faster component of the
biexponential bleach decay at 604 nm. The faster
dynamical process is observable for both 400 nm and 600
nm excitation wavelengths, indicating that it is not due to
2
internal conversion from the B(,*) excited state to the
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Figure 7. (a) Thin-layer UV-vis spectroelectrochemistry of 1-Co in CH3CN and 0.1 M TBAPF6 in a nitrogen-atmosphere glovebox
at room temperature (a) Potential held at 0.04 V vs Fc+/Fc. The Co(III/II) couple is shown with Co(II) (▬) and Co(III) (▬). (b)
Potential held at –1.51 V vs. Fc+/Fc. The Co(II/I) couple is shown with Co(II) (▬) and the tentatively assigned Co(I) (▬).
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porphyrin formation unlikely, especially in the absence of
strong acid. The reversibility of the oxidations of 1-Co
suggests that replacing the two N-pyrrolic protons with a
metal center prevents a ligand-based chemical step, such
as deprotonation of the N-pyrrolic protons upon
oxidation. This contention is supported by the
observation that the first oxidation of the analogous Zn
chlorin complex, 1-Zn, which occurs at a similar standard
potential (0.30 V vs. Fc+/Fc) to that of 1, is also
reversible.24 Concerning the reductive processes, we note
that an overlay of the CVs (Figure 6) reveals that the
reductive processes (퐸10− and 퐸20−), in contrast to the two
most positive oxidative processes, are perturbed
considerably by the presence of the Co ion within the
chlorin core, with the first reduction shifting in the
positive direction and the second reduction shifting to a
more negative potential with respect to 1. Thus, the
reduction processes likely involve molecular orbitals with
considerable admixtures of the Co d-orbitals with the
chlorin frontier orbitals. For simplicity and the purposes
of the work described herein, we will use a metal
oxidation state formalism and describe the reductions as
Co(II/I) and Co(I/0) processes with the caveat that these
nm bathochromically shifts to λmax = 425 nm with a
concomitant increase in molar absorption coefficient. The
Qy(0,0) band also undergoes a bathochromic shift,
moving 10 nm from 597 nm to 607 nm while maintaining
well-anchored isosbestic points (Figure 7a). The shifts of
the Soret and Q bands are consistent with spectral
changes expected for oxidation of the Co(II) ion to Co(III)
within the chlorin macrocycle. In addition, no absorption
is observed in the 450-500 nm region of the
spectroelectrochemical difference spectrum of Figure 6a;
broad growths in this region are observed for -cations of
macrocycles such as porphyrins.31,32 Indeed, in the TA
(Figure 5), we do see growths in this region, consistent
with an LMCT transition. Thus, the absence of
absorptions in this region of the difference
spectroelectrochemistry provides further support for an
oxidation process occurring largely at the metal center.
Consistent with the electrochemical reversibility of this
redox couple, the spectrum cleanly reverts to that of
Co(II) when the potential is held at open circuit.
The reduced form of the Co(II) chlorin was
interrogated similarly, by applying an electrochemical
potential of –1.51 V vs Fc+/Fc. Upon application of
potential, the Soret band decreases in intensity and splits,
with the new bands forming at 363 nm and 421 nm. The
Qy(0,0) band shifts 14 nm to 611 nm (Figure 7b). Such
spectral shifts likely correspond to the Co(II/I) redox
couple; the shifts are reversible upon returning cleanly
back to Co(II) when held at open circuit potential.
reductions have
macrocycle.
a parentage involving the chlorin
We ascribe the unique CV wave of 1-Co at 퐸10+
=
−0.167 V to the Co(III/II) redox process. This value for
the first oxidation of 1-Co falls within the range of
reported values for the Co(III/II) couple of cobalt
porphyrins, though the reduction potential for the
Co(III/II) couple of cobalt tetraphenylporphyrin is highly
variant (–0.40 to +0.60 V vs Fc+/Fc) and depends
markedly on the solvent and electrolyte used.30 The
Co(III/II) couple of 1-Co was further interrogated using
thin-layer UV-vis spectroelectrochemistry by applying an
electrochemical potential of 0.04 V vs. Fc+/Fc. Upon
application of the potential, the Soret band at λmax = 399
The spectral changes observed upon controlled
potential electrolysis of 1-Co at 0.04 V and –1.51 V,
especially for the Soret bands, are similar to those
observed upon the formation of Co(III) and Co(I)
porphyrins, respectively,33,34 thus supporting the
assignment of Co(III) for the one-electron oxidized
chlorin complex and of a formal Co(I) chlorin for the one-
electron reduced complex. In particular, the split Soret
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band we observe for the Co(I) chlorin has also been attribute this catalysis to the decomposition product of 1-
1
observed for Co(I) porphyrins, and in the latter case the
origin of the higher energy band has been attributed to a
double excited state of charge-transfer character on the
basis of magnetic circular dichroism experiments.33,35 The
similarity between the spectra of the Co(I) chlorin and
those reported for Co(I) porphyrins suggests that the first
reduction of both species are similar in nature with regard
to the site of electron transfer. The first reduction of
Co(II) porphyrins has conventionally been assigned as
metal-centered because the resulting complexes are
diamagnetic.36,37 However, in light of recent reports
detailing the antiferromagnetic coupling of non-innocent
macrocycle ligand radicals to metal centers with odd d-
electron counts,38 diamagnetism alone is insufficient to
definitively assign whether the nature of the reduction is
purely metal-centered, purely ligand centered, or a
combination of the two.
Electrocatalysis. With access to 1-Co on scales
approaching 20-100 mg per synthesis, we were able to
undertake exploratory studies of 1-Co as a small molecule
reduction catalyst with emphasis on ORR and HER. With
regard to the former, the addition of O2 has no effect on
the Co(III/II) (퐸10+) process, which remains fully reversible
and unchanged in potential (Figure 8a) during CV
cycling. In the presence of both oxygen and acid,
however, the catalyst decomposes to a yet unidentified
product. This chemical instability is evident by the
prompt decrease in the current of the Co(III/II) CV wave
upon the addition of 1 mM chloroacetic acid (ClAcOH) to
an oxygenated solution of 1-Co. A catalytic wave is
observed to more negative potentials under these
conditions, and the current of this wave varies over time.
A stable current and reproducible CV is achieved only
after 30 min (Figure 8b). Addition of acid results in an
increase in catalytic ORR current (Figure 8c). We
Co. The decomposition may be reproduced chemically
when 1-Co is exposed to humid air over long periods of
time. The CV waves of 1-Co in air-decomposed samples
are negligible (Figure S6a). Notwithstanding, we observed
high ORR activity from the decomposed product (Figure
S6b) that is very similar to that shown in Figure 8c, as is
evident when the traces are overlaid (Figure S6c). We
therefore conclude that 1-Co decomposes in the presence
of oxygen and acid, most likely due to a slow reaction
with oxygen that is accelerated via an acid-catalyzed
process. It is this decomposition product that gives rise to
the apparent ORR activity of 1-Co. We note the recent
reports of Co chlorins as selective ORR catalysts for H2O2
production from O2; the chlorins are naturally derived
and differ structurally from 1-Co.10,11 The generality of the
decomposition phenomenon observed herein for 1-Co is
not known across other Co chlorins but warrants
investigation.
Whereas the chlorin framework of 1-Co appears to be
unstable in an environment containing both acid and
oxygen, it is stable in the presence of acid alone with the
caveat that acids that are too strong will lead to
demetalation of the chlorin. For example, a CV wave (Ep =
–1.3 V vs Fc+/Fc) appears to potentials positive of the
Co(II/I) couple when 1-Co is titrated with a strong acid
such as p-toluenesulfonic acid (pTSA, pKa = 8.6 in
CH3CN),39 as shown in Figure S7. Bulk electrolysis
performed at potentials < –1.3 V vs Fc+/Fc confirms
demetalation of 1-Co, as the absorption spectrum of the
solution following bulk electrolysis shows the presence of
1 (Figure S8); thin layer chromatography (Figure S9) and
HR-MS (Figures S10 and S11) also establish the presence of
demetalated chlorin. Conversely, bulk electrolysis of 1-Co
performed in the presence of weaker acids such as
ClAcOH (pKa = 15.3 in CH3CN)39 and TFA (pKa = 12.7 in
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Figure 8. (a) CVs of 퐸10+ process of 1-Co (1 mM) in CH3CN (0.1 M TBAPF6) at a scan rate of 0.1 V s–1 with a glassy carbon working
electrode (3 mm) under an Ar atmosphere (▬) and in a saturated O2 solution (8.1 mM) (▬). (b) CVs of 1-Co (1 mM) in CH3CN
(0.1 M TBAPF6) at a scan rate of 0.1 V s–1 with a glassy carbon working electrode (3 mm) in a saturated O2 solution (8.1 mM) in the
presence of 1 mM chloroacetic acid. Evolution with time: from 0 min (▬) before the addition of acid until 30 min (▬). Around 5
min elapsed between each CV. (c) CVs of 1-Co (1 mM) in CH3CN (0.1 M TBAPF6) at a scan rate of 0.1 V s–1 with a glassy carbon
working electrode (3 mm) in a saturated O2 solution (8.1 mM). Addition of chloroacetic acid: 0 (▬), 1, 2, 5, 10, 20 and 50 mM (▬).
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ACS Catalysis
Scheme 1. Proposed HER Mechanism for by 1-Co
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mechanism in which the second electron transfer is more
difficult than the first.42 This is not observed in the CVs
(Figure 9), so this case is also ruled out. An ECCE
mechanism in which the second electron transfer is easier
than the first is equivalent to an EECC mechanism in
terms of the dependency of the half-wave potential on the
rate constants for proton transfer.42 Conceptually, one
could view this type of ECCE mechanism from the Co(II)
chlorin as equivalent to an EECC mechanism from the
Co(III) chlorin. In such an ECCE or EECC case the half-
wave potential of the catalytic wave could only be shifted
positive of the standard potential of the catalyst by a
Figure 9. CVs of 1-Co (1 mM) in CH3CN (0.1 M TBAPF6) at a
scan rate of 0.1 V s–1 under an Ar atmosphere with a glassy
carbon working electrode (3 mm). Addition of trifluoroacetic
acid: 0 (▬), 1, 2, 5, 10, 20, 50 (▬) mM.
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CH3CN)39 show no presence of 1 in absorption spectra of
bulk-electrolyzed solutions. Accordingly, to avoid
complications arising from demetalation, we undertook
the study of HER catalysis by 1-Co under these weaker
acid conditions. In considering the choice of the weaker
acid, reduced Co tetrapyrrole macrocycles are known to
be strong nucleophiles and thus HER reactions may be
complicated by competing SN2 reactions of weak C–X
bonds. Such reactivity has been observed for Co
porphyrins and cobalamin complexes, which perform
catalytic reductive dechlorination of alkyl chlorides.40,41
Indeed, the use of ClAcOH as a weak acid in the presence
of reduced Co chlorin species showed the presence of
chloride formation in competition with HER.
Accordingly, studies of HER focused on the use of TFA,
which is less prone to nucleophilic attack by the reduced
Co chlorin center owing to the presence of stronger C–F
vs C–Cl bonds.
Titration of TFA to 1 -Co gives rise to a significant
increase in current of the Co(II/I) (퐸10−) wave (Figure 9),
indicating catalysis. GC analysis of the headspace
following bulk electrolysis in this case showed H2
evolution (Figure S12), with a faradaic efficiency of 67%.
Analysis of the catalytic CV traces in Figure 9 provides
insight into the HER mechanism. Specifically, the half-
wave potential of the catalytic wave, which is the
potential at which the current is at half of its peak height,
may be used to determine the sequence of
electrochemical (E) and chemical (C) steps in the
mechanism (Scheme 1). Possible sequences include ECEC,
EECC, and ECCE mechanisms. Beginning with the Co(II)
chlorin, an EECC mechanism would require two one-
electron reductions prior to a chemical step. As the
catalytic wave occurs after one one-electron reduction,
near the Co(II/I) reduction potential and well positive of
the Co(I/0) potential, an EECC mechanism from the
Co(II) chlorin appears unlikely. An irreversible one-
electron wave would be observed in the case of an ECCE
maximum of 푅푇 ln2 = 17.6 mV, regardless of the values of
퐹
the rate constants for electron transfer or proton
transfer.42,43 For 1-Co, the half-wave potential is constant
and at a potential positive of the standard potential of the
Co(II/I) couple by 99 mV, as shown in Figure 10b. The
position of the half-wave potential is therefore
inconsistent with an ECCE mechanism with Co(II) chlorin
as the initial species. Accordingly, HER catalysis by 1-Co is
most consistent with an ECEC mechanism in which
sequential electrochemical reduction and chemical steps
occur.
The S-shape of the catalytic wave at high acid
concentration (≥ 5 mM) in Figure 9 is consistent with
HER limited only by the kinetics of the catalytic reaction
as opposed to the diffusion of the substrate (the acid).44
Under such conditions, the plateau current provides
invaluable mechanistic insight, particularly with regard to
the nature of H2 production. The two consensus
mechanisms for H2 generation are “heterolytic” or
“homolytic” reaction pathways where the former involves
protonation of a metal hydride (a hetero-coupling event)
and the latter involves homolytic dissociation of the two
metal hydride bonds.45 Costentin, Dridi, and Savéant have
shown that these two pathways may be distinguished by
analysis of the plateau current and the location of the
half-wave potential. In the case of the titration of 1-Co
with TFA, the plateau current is proportional to the
square root of the acid concentration (Figure 10a). This
observation, in conjunction with the aforementioned
observation that the half-wave potential is constant and
more positive than the standard potential, is consistent
with the prevalence of a heterolytic pathway for H2
45
production.
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The mechanism for HER by 1-Co proposed in Scheme 1
Page 8 of 12
1
reflects a heterolytic pathway that conforms to an ECEC
mechanism. In this proposed mechanism, one-electron
reduction of the Co(II) species is followed by protonation
of Co(I) to deliver a Co(III)H hydride. Subsequent one-
electron reduction of Co(III)H to Co(II)H produces an
intermediate with increased hydricity, thus driving a
second protonation to produce hydrogen and close the
catalytic ECEC cycle. We do not see H2 production from
the Co(III)H as has been postulated for a more elaborately
substituted Co chlorin.12 That the catalysis occurs at the
first reduction wave rather than at a more negative
potential indicates that the second electron transfer
occurs at a potential positive of the first (퐸20 > 퐸10).45
Thus, the catalysis appears to occur at the Co(II/I) wave,
as the second CV wave for the second reduction shifts
inside (more positive) the first reduction wave upon
protonation following the first reduction process. These
results are consistent with the chlorin exhibiting
increased basicity as compared to porphyrins, which
exhibit HER upon a second reduction that is more
negative than the first reduction (퐸10 > 퐸20).46 Moreover, 1-
Co is also distinct from Nature’s cobalt tetrapyrrole,
cobalamin, which is reduced at too positive a potential for
HER, with no further reductions observed, thus resulting
in cobalamin being incapable of HER from weak acids.40
We also note that ligand involvement in the delivery of
protons to the metal center to either form or protonate a
metal hydride has been proposed in mechanisms for HER
catalysts possessing pendant amines,47 pendant carboxylic
acids,46,48 or even carbon atoms in aromatic ligands.49
Although protonation of the meso or β carbon atoms of
macrocyclic ligands has been proposed,50 our present
studies of 1-Co have uncovered no evidence for the
chlorin ligand directly participating in proton transfer
steps.
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Figure 10. Analysis of the data in Figure 9a: (a) Variation of
the normalized plateau current (●) with the acid
concentration and least-squares fitting (▬) of the
experimental data to Eq. (1) and expected normalized plateau
currents (■) using Eq. (1) and k1 = 5.7 × 106 M–1 s–1 and k2 = 2.6
× 103 M–1 s–1. (b) The half-wave potential (●) with the square
root of the acid concentration; average value (▬).
10a and Eq. (2) to the average half-wave potential of the
data shown in Figure 10b. The details of the fitting
procedure are described in Section F of the Supporting
Information to furnish the rate constants k1 = 5.7 × 106 M–1
s–1 and k2 = 2.6 × 103 M–1 s–1. Using these calculated rate
constants, values of the normalized plateau current from
Eq. (1) agree well with the experimental data (green
squares in Figure 10a).
The HER catalytic activity of 1-Co can be benchmarked
by the methods of Savéant and Artero42,43 to furnish
“catalytic Tafel plots”, which represent the evolution of
the turnover frequency (TOF) as a function of the
overpotential (η) of the catalytic reaction. This approach
is independent of experimental factors, such as cell
configuration, and includes in the calculation of the TOF
only the amount of active catalyst present in the reaction
diffusion layer, thereby providing a rational method of
benchmarking intrinsic catalytic activity. An efficient
catalyst will exhibit a high turnover frequency at low
overpotential and consequently is situated in the top left
corner of Figure 11. The TOF as related to HER is given
by,42,43
The plateau current also gives direct access to the
apparent rate constant of the catalytic reaction.42 In the
context of an ECEC mechanism, Savéant and Artero have
shown that the rate constants k1 and k2 of the first and
second chemical steps (proton transfers in Scheme 1) may
be deduced from the plateau current (ipl), which is
normalized by the current of the one-electron wave in the
absence of acid, 푖푝0, and the half-wave potential (E1/2),
according to the following,43
푇푂퐹
푚푎푥
푇푂퐹 =
(3)
(4)
F
1 + exp [ (퐸H0
R푇
F
R푇
− 퐸1/2)] exp (−
휂)
+
/H
2
푘1푘2
[ ]
AH
푇푂퐹
=
max
푘1 + 푘2
[
]
푖푝푙
푖푝0
R푇 √푘1푘2 퐴퐻
where TOFmax is the maximal value of the TOF. From this
equation, the previously determined values of k1 and k2
yield TOFmax for a given concentration of acid. The
“catalytic Tafel plots” (Figure 11) are constructed by
inputting a concentration of [AH] = 1 M for each catalyst
into Eq (4). The TOF scales with overpotential, η, as
described by Eq. (3). The thermodynamic standard
√
(1)
(2)
= 4.48
F푣
푘 +
푘
√
√
1
2
R푇
푘1
푘2
퐸1/2 = 퐸10 +
ln (1 +
)
√
F
where v is the scan rate and 퐸10 = 퐸10− = −1.401 V vs.
Fc+/Fc in this case. The values of the two rate constants
can be determined from Eqs. (1) and (2) by parametrically
fitting Eq. (1) to the experimental data shown in Figure
potential for hydrogen generation, 퐸H0
, from TFA at a
+
/H
2
concentration of 1 M is calculated to be –0.62 V vs. Fc+/Fc
using the method established by Fourmond et al.51 This
value of
퐸H0
takes into account the effect of
+
/H
2
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ACS Catalysis
homoconjugation, which is the association of an acid with
its conjugate base via hydrogen bonding. The equilibrium
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constant for homoconjugation of TFA in CH3CN is 7.9 ×
103 M–1.39 E1/2 is determined from the CVs in Figure 9a to
be –1.302 V vs. Fc+/Fc. From these values, the plot of TOF
vs. η for HER for 1-Co can be constructed, and is
presented in Figure 11 together with the activity curves of
other reported HER catalysts,52–56 including an Fe
porphyrin.52 1-Co is a superior HER catalyst as compared
to porphyrins at all η, and especially in the low η region.
Inasmuch as energy conversion cycles seek to perform
catalysis at low overpotential, the connection of η to TOF
is the critical metric in catalyst design.
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Figure 11. Catalytic Tafel plots for the different
electrocatalysts listed in Table 1 of ref. 43 and for 1-Co based
on the data in Table 1: FeTPP in DMF and Et3NH+(▬);
Co(dmgH)2py in DMF and Et3NH+ (▬); [Ni(P2PhNPh)2]2+ in
CONCLUSION
Insertion of a Co(II) ion into a sparsely substituted
chlorin (1-Co) induces significant out-of-plane distortion
of the macrocycle. Structural comparisons between
metal-free and Co chlorins reveal a constriction of the
C
H
4
CH3CN and DMFH+ (▬); [Ni(P2PhN2
X)2]2+ in CH3CN and
6
TfOH with X = H (▬) and in CH3CN and DMFH+ with X =
CH2P(O)(OEt)2 (▬ ▬); 1-Co in CH3CN and TFA (▬).
macrocycle core and
a possible mitigation of the
destabilizing effect of anti-aromaticity in the 20 π-
electron conjugated pathway due to the presence of the
Co(II) ion. EPR, electronic absorption and time-resolved
absorption spectra are consistent with the presence of an
open-shell, S = ½, metal center residing within the chlorin
ligand environment. The electrochemistry of 1-Co speaks
to significant coupling of the Co d-orbitals with those of
the electron-rich chlorin macrocycle. Efforts to perform
the electrocatalytic oxygen reduction reaction (ORR) with
1-Co revealed that the Co chlorin is unstable to the
combined presence of oxygen and acid, which are
requisite for ORR chemistry. However, the Co chlorin
decomposition product formed under such conditions is
competent for ORR. 1-Co is an authentic electrocatalyst
for the hydrogen evolution reaction (HER), with the
caveat that acids that are too strong will cause
demetalation. With weaker acids such as trifluoroacetic
acid, 1-Co performs HER via a proposed mechanism
involving the protonation of Co(II) hydride to afford H2.
The ability of the Co chlorin to perform HER via an
intermediate that is reduced at a standard potential
positive of that of the first reduction of the complex is in
contradistinction to Co porphyrins, which catalyze HER
from acids, regardless of strength, at potentials well
negative of the first reduction (therefore leading to higher
overpotentials).46 The origin for this disparate behavior
does not arise from the redox properties of the
macrocycle, as 1-Co and Co tetraphenylporphyrin
undergo multiple reduction and oxidations in similar
potential ranges.34 The increased activity of 1-Co as
compared to porphyrins appears to be derived from the
increased electron density of the chlorin macrocycle, thus
leading to a more basic Co(I) metal center, in turn
promoting hydride formation and subsequent reduction
to Co(II) hydride. The importance of cobalt basicity on
such a Co(I) → Co(III)H → Co(II)H reaction sequence for
HER has been theoretically established.57 Examination of
the electrochemical kinetics of 1-Co has revealed HER to
proceed by an ECEC mechanism, and allowed us to obtain
the key chemical rate constants (k1 and k2 in Scheme 1) of
the ECEC catalytic cycle. From these rate constants, the
catalytic activity as related by the TOF for HER has been
determined. Upon benchmarking the catalytic activity of
molecular HER catalysts, we find that at low
overpotentials the Co chlorin 1-Co exhibits superior HER
activity as compared to its porphyrin congeners.
Electronic Supplementary Information (ESI) available:
materials, experimental methods, description of data analysis
and instrumentation, X-ray crystallographic information,
mass spectrometry, gas chromatography, and additional UV-
vis and cyclic voltammetry data. See DOI: XXXXXXXX
* E-mail: jlindsey@ncsu.edu (JSL)
* E-mail: dnocera@fas.harvard.edu (DGN)
‡ These authors contributed equally to this work
We thank Seung Jun Hwang for assistance with gas
chromatography. This material is based upon work
supported under the Solar Photochemistry Program of the
Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences of the U. S. Department of
Energy (DGN) and by a grant DE-FG02-05ER15651 from the
Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, of the U. S. Department of
Energy (JSL).
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