Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

Article abstract of DOI:10.1021/jacs.5b08396

Artificial photosynthesis (AP) promises to replace societys dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the Earths crust; unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxides activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O₂ and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co(III)₄] cubane (Co₄O₄(OAc)₄py₄, py = pyridine, OAc = acetate), that can be oxidized to the [Co(IV)Co(III)₃] state. Upon addition of 1 equiv of sodium hydroxide, the [Co(III)₄] cubane is regenerated with stoichiometric formation of O₂. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichiometric OER, implying that terminal oxo ligands are responsible for forming O₂. The OER is also examined with stopped-flow UV-visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O₂ formation requires disproportionation of the [Co(IV)Co(III)₃] state to generate an even higher oxidation state, formally [Co(V)Co(III)₃] or [Co(IV)₂Co(III)₂]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems.

Full text of DOI:10.1021/jacs.5b08396

Article
pubs.acs.org/JACS
Mechanistic Investigations of Water Oxidation by a Molecular Cobalt
Oxide Analogue: Evidence for a Highly Oxidized Intermediate and
Exclusive Terminal Oxo Participation
Andy I. Nguyen,†,‡ Micah S. Ziegler,† Pascual Ona-Burgos,§ Manuel Sturzbecher-Hohne,‡ Wooyul Kim,∥
̃
Donatela E. Bellone,† and T. Don Tilley*,†,‡
†Department of Chemistry, University of California at Berkeley, Berkeley, California 94720-1460, United States
‡Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
§Department of Chemistry and Physics, University of Almería, Carretera de Sacramento s/n, 04120 Almería, Spain
∥Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: Artificial photosynthesis (AP) promises to
replace society’s dependence on fossil energy resources via
conversion of sunlight into sustainable, carbon-neutral fuels.
However, large-scale AP implementation remains impeded by
a dearth of cheap, efficient catalysts for the oxygen evolution
reaction (OER). Cobalt oxide materials can catalyze the OER
and are potentially scalable due to the abundance of cobalt in
the Earth’s crust; unfortunately, the activity of these materials
is insufficient for practical AP implementation. Attempts to
improve cobalt oxide’s activity have been stymied by limited
mechanistic understanding that stems from the inherent
difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt
oxide revealed the intermediacy of the unusual Co(IV) oxidation state, much remains unknown, including whether bridging or
terminal oxo ligands form O2 and what the relevant oxidation states are. We have addressed these issues by employing a
homogeneous model for cobalt oxide, the [Co(III)4] cubane (Co4O4(OAc)4py4, py = pyridine, OAc = acetate), that can be
oxidized to the [Co(IV)Co(III)3] state. Upon addition of 1 equiv of sodium hydroxide, the [Co(III)4] cubane is regenerated with
stoichiometric formation of O2. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during
this stoichiometric OER, implying that terminal oxo ligands are responsible for forming O2. The OER is also examined with
stopped-flow UV−visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O2 formation requires
disproportionation of the [Co(IV)Co(III)3] state to generate an even higher oxidation state, formally [Co(V)Co(III)3] or
[Co(IV)2Co(III)2]. The mechanistic understanding provided by these results should accelerate the development of OER
catalysts leading to increasingly efficient AP systems.
Mechanistic details of this catalysis, including that of the natural
process in photosystem II, is limited but expected to provide
important design principles that will enable optimization of
catalyst efficiency.9,10 Much of the mystery associated with such
mechanisms concerns the O−O bond-forming event, although
several possibilities seem plausible.8,11
Cobalt oxide materials are among the most promising
catalysts for water oxidation, due to their robust structures,
inherent activity, and most importantly, the natural abundance
of cobalt.13−16 Unfortunately, thorough structural character-
izations and mechanistic studies of the active species for cobalt
oxide catalysts are inherently difficult, given their heteroge-
INTRODUCTION
■
Society’s current and future energy demands require sustainable
resources that do not contribute to global climate change.1
While the sun provides abundant energy, conversion of solar
energy into a storable, transportable, easily accessible form is
not yet economically feasible.2,3 An ideal system would
artificially replicate natural photosynthesis, which involves the
endergonic four-electron oxidation of water to O2 and four
protons (eq 1), and the subsequent storage of captured solar
2H2O → 4H+ + 4e− + O2
(1)
energy in the chemical bonds of a fuel.4 However, without a
catalyst this transformation is inefficient; therefore, research on
solar fuel production has focused heavily on the discovery of
increasingly efficient water oxidation catalysts (WOCs).5−8
Received: May 3, 2015
Revised: August 8, 2015
© XXXX American Chemical Society
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neous nature.17,18 For heterogeneous catalysts like this,
information about the structure and reactivity of the active
site can be gained by studying appropriately designed,
molecular models. However, while a few soluble, molecular
cobalt compounds are reported to behave as active WOCs
under photochemical or electrochemical conditions, their
associated mechanisms remain poorly defined.8,19−21 Such
investigations are complicated by the difficulty in establishing
the integrity of molecular cobalt catalysts under operating
conditions, as these compounds may simply serve as precursors
to a heterogeneous cobalt oxide catalyst.22,23 Thus, to date the
mechanisms available to soluble and insoluble cobalt-based
WOCs are not well understood.
may mediate water oxidation. These should provide a basis for
the rational design of more efficient and robust cobalt-based
catalysts.
ELECTROCHEMISTRY AND REACTION CHEMISTRY
■
Cubane complex 1 was purified by column chromatography to
remove impurities from the crude product obtained using the
published synthesis.29,34 In addition, compound 1 was treated
with tetrasodium diaminoethanetetracarboxylate (Na4EDTA)
in the workup to remove any trace Co(II) impurities. To more
thoroughly investigate the stability and chemistry of cubane 1
in aqueous solution, cyclic voltammetry (CV) was performed
over the pH range from 0.0 to 12. From pH 4 to 10, a 1 mM
Herein, we provide experimental evidence for the mechanism
of water oxidation as catalyzed by a molecular cobalt oxide
cluster. This cluster, Co4O4(OAc)4py4 (1), was first synthesized
in 1998 by Beattie et al., and is an attractive model compound
due to its resemblance to a subunit of extended cobalt oxides
(Figure 1).24 The Nocera and Britt groups have employed 1 as
solution of 1 displays a fully reversible redox couple at E1/2
=
1.25 V (vs NHE), corresponding to a one-electron [Co(III)4]/
[Co(III)3Co(IV)] redox couple (1/1+) (Figure 2A). In
Figure 2. Cyclic voltammograms (100 mV/s) of 1 mM 1 in H2O with
0.1 M Na2SO4 electrolyte at (A) pH 0.0−7.0 and (B) pH 9.0−12.0.
The vertical dashed line in panel B shows the 1/1+ redox couple.
Figure 1. Structural comparison of cobalt oxyhydroxide (CoOOH)12
with cobalt cubane (1). Red, green, blue, and gray spheres or ellipsoids
represent O, Co, N, and C, respectively.
agreement with the results of Nocera and co-workers, purified
1 does not exhibit noticeable catalysis over this pH range,
indicating that these samples do not contain Co(II)
impurities.29 This reversible couple shifts linearly with pH to
more oxidizing potentials below pH 4, at a rate (63 mV/pH
unit) that indicates a 1H+, 1e− redox process (Figure S2). The
peak potential remains constant below pH 0.7 (to 0), and this
behavior is consistent with protonation of cubane 1 to 1H+,
with pKa = 3.5 for 1. For comparison, the dicationic cobalt
cubane complex [Co4O4(OAc)2(bipy)4]2+ exhibits similar
behavior, with pKa = 3.15.35
Increasing the pH to 12 results in a significant rise in current
density for the anodic wave and bubble formation that is
consistent with the catalytic oxidation of hydroxide to oxygen
(Figure 2B). By 1H NMR spectroscopy, a solution of 1 (10 mM
in D2O) at pD 12 shows partial conversion to a new species
(vide infra), but the total integration of all cobalt species
decreased by only 0.9% after 2.5 h, implying that there is only
minor decomposition into insoluble or paramagnetic com-
pounds (Figure S3). This electrocatalysis corresponds to an
overpotential η of 728 mV at a current density of ∼2 mA cm−2
(TOF = 0.2 s−1, Supporting Information). The catalytic wave
was mostly retained in the presence of 0.25 mM Na4EDTA,
suggesting that catalytic activity is not due to Co(II), though it
should be noted that EDTA4− is readily oxidized at E > 1.1 V vs
NHE, which may account for slight differences in the shapes of
the catalytic waves in the presence of Na4EDTA.36,37
Furthermore, rinsing the glassy carbon electrode with water
removed all catalytic species that might have been on the
electrode, as evidenced by the lack of catalysis in the cyclic
voltammogram in a solution containing only electrolyte at pH
12 (Figure S1a). The electrocatalytic activity of 1 is also
a model for the cobalt phosphate (CoPi) WOC in electron
paramagnetic resonance spectroscopy studies.25 Its promise as a
functional model for water oxidation catalysis has been
examined by the groups of Dismukes, Scandola, Sun, and
Nocera.26−29 In support of 1 as a possible WOC candidate,
density functional theory (DFT) calculations by Siegbahn and
co-workers demonstrated the existence of an energetically
feasible pathway for O−O bond formation by 1 via a Co(V)
intermediate, but their results do not have experimental
support.30 Similarly, theoretical calculations on other cobalt
systems suggest that Co(V) is likely the true active
intermediate, although the lack of experimental evidence for
such species renders it controversial.31−33 The investigations
described below involve a combination of electrochemistry,
stoichiometric reactivity, spectroscopy, and kinetic experiments
to observe and identify many key intermediates and reaction
steps of catalyzed water oxidation involving 1. In particular, the
conversion of hydroxide to oxygen is shown to occur at a
molecular [Co4O4] cubane center, and the O−O bond
formation is demonstrated via a clean stoichiometric reaction
from an isolated Co(IV) intermediate. Kinetic studies reveal an
unusual mechanism implicating formation of a formal [Co-
(III)3Co(V)] or [Co(III)2Co(IV)2] intermediate prior to O2
release. To our knowledge, this is the first example of an
isolated WOC intermediate shown to cleanly form O2 in the
absence of exogenous oxidant. Taken together, these data form
a clearer picture of how water oxidation is mediated by 1, and
also gives broader implications of how cobalt oxide materials
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Scheme 1. Observed Stoichiometric Intermediates Starting with 1
significantly different from that of Co(II) under these
conditions (Figure S1a). From the CV and control experi-
ments, it seems that very pure samples of 1 exhibit true
homogeneous water oxidation catalysis at pH 12. To acquire a
better understanding of the chemical behavior of complexes 1
and 1+ at different pH values, attempts were made to isolate
and/or observe the species responsible for the electrochemistry
described above.
The pH-dependent behavior of 1 suggests that the
protonated cubane, 1H+, is present at lower pH values, and
the existence of this complex was established by its isolation
and characterization. Addition of 1 equiv of 4-toluenesulfonic
acid (TsOH) to 1 in methanol afforded [1H]OTs in
quantitative yield, and the structure of [1H]OTs·4MeCN was
determined by X-ray crystallography (Figure S8a). With respect
to 1, the Co−OH bonds of [1H]OTs are elongated to an
average length of 1.897(2) Å, while the remaining Co−O bond
lengths remain unchanged with an average of 1.865(2) Å (1 has
a Co−O bond length range of 1.860−1.876 Å).34 A close
contact of 2.586(2) Å between an oxygen atom of the cubane
(O(4)) and the tosylate oxygen atom O(13) is consistent with
O···H···O hydrogen bonding. The 1H NMR spectrum of
[1H]OTs in CD3CN at room temperature suggests that the
molecule is Cs symmetric with broadened resonances,
suggesting slow proton exchange on the NMR time scale.
Furthermore, the infrared spectrum contains a broad peak at
3400 cm−1, assigned to an O−H stretching mode.
reaction A). Recrystallization from CH2Cl2/hexane afforded
analytically pure single crystals of [1]PF6·CH2Cl2 for X-ray
diffraction (XRD) analysis (Figure S8b). The Co−O bond
distances in the cubane core of [1]PF6 are in agreement with
those reported in the literature.29,38
The oxidized cubane [1]PF6 is soluble in a mixture of neutral
water and acetonitrile (9:1 by volume), but appears to form an
acidic aqua complex under these conditions. Thus, a decrease in
the observed pH from 8.2 to 5.9 (with 0.4 mM [1]PF6) is
consistent with acidification of H2O via coordination to cobalt,
1
that is 1(H2O)+ ⇌ 1(OH) + H+. The H NMR spectrum of
[1]PF6 in pure CD3CN consists of four very broad resonances
corresponding to equivalent acetate (10.17 ppm) and pyridine
(6.62, 6.15, and 2.62 ppm) resonances. For a sample of [1]PF6
dissolved in 9:1 D2O/CD3CN, the same broad 1H NMR
resonances for [1]PF6 are observed, along with a set of four
new resonances that correspond to a 23% conversion of [1]PF6
to a new species (Figure S4). The 8:8:4:12 ratio for these
resonances is consistent with the presence of a new cubane
species with four acetate and four pyridine ligands. The new
resonances are paramagnetically shifted, as indicated by a
pyridine-H chemical shift at 2.50 ppm, and an acetate-CH3
resonance at 3.50 ppm. These results are consistent with the
presence of a new Co(IV)-containing cubane cluster with an
acidic aqua ligand, 1(H2O)+. Thus, an aqueous solution of
[1]PF6 contains 1+ and 1(H2O)+ in equilibrium, and the acidity
of the latter species also implies the presence of 1(OH)
(Scheme 1, reaction B).
The presence of a fully reversible redox couple over a wide
pH range indicates that the chemical oxidation of 1 (or 1H+) to
1+ is feasible in water. Indeed, the oxidized cubane [1]PF6 has
previously been isolated from the oxidation of 1 in
acetonitrile.29 In water, the reaction of 1 with 1 equiv of ceric
ammonium nitrate, followed by addition of NH4PF6, resulted in
precipitation of [1]PF6 as a black solid in 75% yield (Scheme 1,
The species responsible for the anodic wave observed at pH
12 was targeted in the stoichiometric reaction of 1 equiv of
NaOH with 1 in D2O. By 1H NMR spectroscopy, this reaction
results in an equilibrium involving 1, free acetate, and a new
complex. Since many pyridine ligand resonances for the new
1
complex overlap, H TOCSY NMR spectroscopy was used to
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resolve them, and the resulting assignments are consistent with
a C2-symmetric cubane complex possessing two types of
pyridine ligands in a 1:1 ratio (Figure S5). Furthermore, two
new acetate ligand resonances are present in a 6:3 ratio, and
integration of the pyridine and acetate peaks shows that the
new species possesses four pyridine and three acetate ligands.
This is confirmed by 1H pulsed field gradient spin−echo
diffusion NMR spectroscopy, which shows that the new acetate
and pyridine resonances belong to the same species having a
hydrodynamic radius that is similar to that of 1 (rH = 7.0 Å for
1, and 6.8 Å for the new species). This information is most
consistent with a [Co(III)4]−dihydroxide complex, [Co4O4-
(OAc)3py4(OH)2]− (2−), formed by substitution of an acetate
ligand by two hydroxide ions (Scheme 1, reaction C). The
chemical identity of 2− and the presence of hydroxide ligands
was furthered evidenced by addition of 1 equiv of acetic acid to
the reaction mixture, which reformed 1 in quantitative yield by
1H NMR spectroscopy. Note that the structural syn-dihydroxide
motif assigned to 2− has been proposed in the literature as an
active site for O2 evolution in cobalt oxide materials.17,30,31,39
Thus, an appealing mechanism for O2 formation observed in
the cyclic voltammogram involves electrochemical oxidation of
2− to the [Co(III)3Co(IV)]−dihydroxide state, followed by
formation of the O−O bond. The possibility of a Co(IV)
hydroxide complex as a catalyst for water oxidation prompted
us to pursue the observation or isolation of such a species.
A conceivable pathway to a Co(IV) hydroxide complex in
this system is by the associative addition of hydroxide to 1+.
Interestingly, reaction of [1]PF6 with 1 equiv of NaOH(aq) in
CD3CN quantitatively produced 1 (by 1H NMR spectroscopy),
indicating a net one-electron reduction of 1+ by hydroxide
anion. On a larger scale, this reaction produced noticeable
amounts of gas, suggesting the possibility of O2 as a product of
hydroxide oxidation by 1+. Quantitative measurement by an O2-
sensing probe revealed a ∼75% yield of O2 with respect to 1,
though the yield is most likely higher since a nontrivial amount
of unmeasured bubbles adhered to the wall of the flask (Figure
3A). Mass spectrometric measurement of the head space
confirmed the presence of 32O2, and when ∼97% enriched
Na18OH (in H218O) was employed, 90% 36O2 (94%
theoretical) and 10% 34O2 (5.9% theoretical) were detected
(Figure 3B). High-resolution electrospray ionization mass
spectrometry (ESI-MS) analysis of the solution after reaction
with Na18OH showed no measurable incorporation of 18O into
1, meaning that the O-atoms within the cluster do not exchange
with OH−/H2O on the time scale of the experiment
(Supporting Information). Significantly, this isotopic labeling
experiment reveals that the O−O bond is formed solely from
OH− and not from the μ-O-atoms within the cuboidal core of
1+. Note that while oxygen isotopic labeling studies with a
cobalt oxide (CoPi) catalyst could not conclusively distinguish
between mechanisms involving terminal versus bridging oxo/
hydroxide ligands,39 the results in this work favor mechanisms
for which only terminal oxo/hydroxide ligands are involved in
O−O bond formation. Furthermore, the reaction of 1+ with
hydroxide supports the importance of the cobalt(IV) oxidation
state in O2 evolution by cobalt catalysts. The quantitative
reduction of 1+ by hydroxide to regenerate 1 and give O2
completes a conceivable catalytic cycle, for which each
stoichiometric step of the cycle has been observed (Scheme
1, reaction D).
KINETICS AND PROPOSED MECHANISM
■
To gain further information on the mechanism of the reaction
between [1]PF6 and NaOH, a kinetics analysis of the reaction
by stopped-flow UV−vis spectroscopy was conducted in 9:1
H2O:MeCN. The reaction progress was monitored by decay of
an absorption for [1]PF6 at 280 nm. Two isosbestic points, at
335 and 362 nm, were observed for the reaction (Figure S6).
These isosbestic points show that the decay of 1+ and growth of
product 1 are nearly symmetrical with no measurable buildup
of an intermediate. A striking feature of all kinetic experiments
is the slightly sigmoidal decay of 1+ over the course of the
reaction. The least-squares fit of the dependence of [1+]0 on the
initial rate at 10 mM [OH−] gives a mixed-order rate law
containing a second-order term (kobs2 = 21 000 4000 M−1
s−1) and a first-order term (kobs1 = 0.8 0.1 s−1) (Figure 4A, eq
2).
Figure 4. Kinetic plots. (A) Initial rate versus [1+] (with 10 mM
[OH−]). (B) Initial rate versus [OH−] (blue squares) and [pyridine]
(with 5 mM [OH−]) (red triangles). (C,D) Numerical kinetic
modeling. The black solid lines are the fits from the model, and the
points represent the measured values.
Figure 3. Measurement of O2 from the reaction of [1]PF6 and NaOH.
(A) Quantification by fluorescence probe. (B) Isotopic quantification
by mass spectrometry.
D
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2
saturation behavior of hydroxide in this system, and the first-
order term in the rate law (vide supra), catalyst generation likely
results from formation of 1(OH), by deprotonation of
1(H2O)+ or by addition of hydroxide ion to 1+. The second-
order term in [1+]0 is explained by the participation of 1+ as an
oxidant in a proton-coupled electron transfer process with
1(OH), represented in Scheme 2 by two steps (deprotonation
followed by electron transfer) that generate the formally
[Co(III)3Co(V)] species 1(O). Note, however, that the order
of deprotonation and electron transfer in the proposed
mechanism is not distinguishable with these data. This
unusually high oxidation state for cobalt is predicted to be
favored over the [Co(III)2Co(IV)2] state by several theoretical
calculations; the oxidation state assignment is discussed in the
following section.30,32,33
v = kobs1[1+]0 + kobs2[1+]0
(2)
i
The second-order term strongly implicates the involvement
of two molecules of 1+ in the pathway to O−O bond formation,
while the first-order term is consistent with either an activation
of 1+ or a concurrent pathway involving 1+. Also, the rate of the
reaction increased with increasing concentration of NaOH, but
reached an asymptote at about 110 equiv of NaOH (6 mM,
Figure 4B). This saturation behavior for NaOH suggests that
OH− reacts with an intermediate species whose concentration
is limited. Four observationsthe symmetry of growth and
decay, the sigmoidal shape of the kinetic profile, the first- and
second-order [1+] terms in the rate law, and the saturation
behavior of [OH−]are most consistent with a chain reaction
propagated by a small amount of non-steady-state catalytic
intermediate.40 In this type of chain reaction mechanism, the
reactant is both a pre-catalyst and a participant in a catalytic
cycle. The precatalyst activation step gives rise to the first-order
term, and the reaction of the precatalyst with a catalytic
intermediate gives the second-order term. Saturation behavior
in [OH−] is thus explained by reaction(s) of OH− with catalytic
intermediate(s). Pyridine has only a very weak inhibitory effect
at high concentrations (16% rate decrease at 300 equiv of
pyridine), implying that the reaction does not require
dissociation of a pyridine ligand (Figure 4B). At 298 K, the
activation free energy ΔG⧧ can be estimated from kobs2 to be 12
kcal/mol. When adjusted for the three steps involving H+
transfer at pH 12 (−1.36ΔpH kcal/mol per deprotonation
step) and a redox potential of 1.25 V (the redox potential for 1/
1+), Siegbahn’s calculated activation energy for O−O bond
formation starting from Co(III) is 11.2 kcal/mol, which is very
close to the value obtained from the kinetics.30
The reactions described above are consistent with the DFT
calculations by Siegbahn for water oxidation, which indicate
that 1(O)− is unable to engage in O−O bond formation by
reaction with water; however, the higher oxidation state of
1(O) should allow attack by water to produce the hydroperoxy
species 1(OOH)−.30 Peroxide intermediates for cobalt catalysts
have been speculated to exist based on observations by the
groups of Frei and Stahl.17,41 Finally, the OOH− ligand may be
readily oxidized to superoxide (1(O2 )−), then finally to O2.
•
The plausibility of the peroxide oxidation was tested by the
reaction of [1]PF6 with hydrogen peroxide, which gave O2 and
the reduced and protonated cubane, [1H]PF6, characterized by
1H NMR spectroscopy and single-crystal XRD (Figure S8).
Kinetic measurements on the reaction in 9:1 MeCN:H2O by
stopped-flow UV−vis indicate clean first-order behavior for
[1+] and [H2O2], with a second order rate constant of k = 78
3 M−1 s−1 at 25 °C. Replacement of 1(OH) by 2− in the
catalytic cycle should give the analogous intermediates, with the
only difference being that an OAc− ligand is replaced by an
OH− ligand.
The proposed mechanism of Scheme 2 is consistent with all
the experimental results described above. On the basis of the
Scheme 2. Proposed Mechanism for the Reduction of 1+ to 1 by Hydroxide
E
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Figure 5. DFT-calculated molecular orbitals of 1(O)·H2O using B3LYP/6-31G+d,p: (a) singlet-state frontier orbitals and (b) triplet-state frontier
orbitals (β-spin).
d[1+]
dt
A possible related mechanism (Scheme S1) could involve
−
= k1[1+][OH−] +
displacement of acetate from 1(OH) by hydroxide, to produce
the dihydroxide 2. This reaction would be analogous to that
observed between 1 and OH−, which leads to 2− and acetate
(vide supra). The more oxidized dihydroxide 2, with a
[Co(III)3Co(IV)](OH)2 diamond core moiety, would then
have to be oxidized (by 1+) to a [Co(III)2Co(IV)2] or
[Co(III)3Co(V)] analogue before engaging in O−O bond
formation across one face of the cubane. DFT calculations
indicate that dicobalt centers like this may undergo O−O bond
formation from the Co(IV)Co(IV) state, perhaps via
deprotonation to an oxo−hydroxy intermediate.31,42 Note
that Siegbahn’s DFT calculation indicates that a direct coupling
mechanism of this type is less preferred in cubanes, by ∼3 kcal/
mol.30
This complex mechanism was modeled numerically with
global kinetics analysis, which successfully reproduced the
initial rates, as well as the entire kinetic profile (Figure 4C,D).
The model utilized eight rate constants (see Supporting
Information); however, a sensitivity analysis revealed that
only the values of k1, k3, and k7 were major contributors for a
good fit. Not surprisingly, those rate constants (k1, k3, and k7)
represent the key features of the mechanism: catalyst activation,
oxidation to a formal Co(V) by disproportionation, and catalyst
turnover. Similar conclusions about the rate equation could also
be derived analytically, as follows. A rate law for the initial rates
with excess [OH−] can be derived for the proposed mechanism
using the steady-state approximation for all intermediates
except 1(OH) (see Supporting Information), which describes
the experimental observations: the first- and second-order
terms in [1+]0, and the saturation behavior in [OH−] (eq 3).
2k2([1+]0 − [1+] − [1])[OH−]
[OH−]
k3 + k7
k−7[1]
k7[1+]
k2
k4
k2
k5[1+]
k2
k6[1+]
k2
+
+
+
+
+ 1
(3)
[1+]
k3k7
(
)
At very high concentrations of [OH−], low concentrations of
product ([1] ≈ 0), and for [1+] = f [1+]0 (where 0 < f < 1), the
rate law reduces to eq 2, with the expressions for kobs1 and kobs2
given in eqs 4 and 5.
kobs1 = k1f[OH−]
(4)
2k3k7
k3 + k7
kobs2
=
(f − f2 )
(5)
Again, it is shown that the kinetics (under excess [OH−]
conditions) depend only on k1, k3, and k7, in good agreement
with the numerical fitting. Additionally, it can be shown that the
exact expression of [1+](t) over all time is a sigmoidal curve (eq
S12 and Figure S7). The consistency of this model with all the
observations is compelling evidence for the proposed
mechanism.
NATURE OF THE HIGHLY OXIDIZED
INTERMEDIATE
■
The kinetic experiments discussed above suggest that the
formal [Co(III)3Co(IV)] oxidation state disproportionates into
the [Co(III)4] state and a higher oxidation state, [Co(III)2Co-
(IV)2] or [Co(III)3Co(V)]. Of course, the concept of oxidation
state is merely a formalism having a defined set of rules,43 and
extrapolation of this formalism to actual charge distributions
must be made with caution. Nonetheless, it may be instructive
to determine the location of the electron hole in the oxidized
cluster. Significantly, Co(V) has never been observed
spectroscopically but it has often been suggested by
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Article
calculations. If the mechanism suggested by the kinetic data is
correct, it would not be possible to build up significant
concentrations of the formal [Co(III)3Co(V)] species 1(O) for
typical spectroscopic investigations. Therefore, we chose to
investigate the nature of this intermediate species with DFT
calculations.
including the non-involvement of cubane oxo ligands in the
formation of O2, the viability of intermediates 1(H2O)+ and
1(OH), and the dependence of the rate on [OH−].
Furthermore, the experimentally determined activation energy
is consistent with theoretical values reported in the literature.
This study helps solidify the role of cobalt cubane complexes as
WOCs, and as useful structural and functional models for
cobalt oxide materials. Significantly, this work suggests that O−
O bond formation on cobalt oxide occurs via terminal oxo,
rather than bridging oxo ligands present at edge sites.
These results provide several guidelines relevant to future
catalyst design strategies. The most relevant formal oxidation
states for cobalt WOCs appear to be +3, + 4, and +5, while the
+2 oxidation state does not appear to be necessary in the
catalytic cycle. Thus, supporting frameworks should be
designed to stabilize the higher oxidation states such as Co(IV)
and Co(V). Multimetallic oxido systems, such as the cobalt
cubane described herein, are well suited to stabilize such states,
as they can relieve the burden of an unstable oxidation state by
sharing an electron hole throughout the cluster. Ancillary
ligands may play a key role in tuning a cluster’s redox potentials
to those appropriate for efficient water oxidation. In addition,
catalyst design must allow for water or hydroxide to bind to the
cobalt center, as this is the first step toward generating the
putative high-valent cobalt−oxo intermediate. Finally, since
bridging oxo ligands do not appear to play a direct role in O2
formation, design elements that allow formation of terminal
cobalt−oxo moieties, while stabilizing the [Co4O4] core, may
prove advantageous to water oxidation catalysis.
First, the feasibility of the Co(V) oxidation state may be
considered on the basis of simple ligand-field arguments. While
it has been demonstrated that the electron hole in the
[Co(III)3Co(IV)] complex (1+) is completely delocalized over
the cubane (each Co is better described with an “oxidation
state”/charge of +3.125),25 introduction of the strongly π-basic
terminal oxo ligand at one cobalt center in the cubane (1(O)−)
is expected to raise the energy of the corresponding t2g-like
orbitals to effectively localize the electron hole at that particular
cobalt ion. Stated another way, the electron hole should be
most stable on the cobalt ion associated with the π-basic
terminal oxo ligand. Thus, the terminal oxo ligand should cause
a shift from [Co(+3.125)4] toward [Co(III)3Co(IV)]. By the
same argument, the [Co(III)3Co(V)] state should also be a
better description than [Co(III)2Co(IV)2] in the 1(O)
complex, as the presence of the terminal oxo ligand should
also cause preferential localization of the hole. These ligand-
field rationalizations provide an intuitive albeit rough guide for
possible oxidation state assignments.
DFT can potentially provide a more quantitative description
of the oxidation state. Complex 1(O) has been described by
Siegbahn30 to have an S = 1 ground state, with an S = 0 excited
state 5.6 kcal/mol higher in energy. Inspection of the molecular
orbitals in both the triplet and singlet states indicates significant
localization of the SOMO and LUMO, respectively, on the
terminal Co−oxo moiety (Figure 5). In the triplet state, the β-
spin wave functions reside predominately on the Co−O unit,
while the α-electron wave functions are more delocalized
throughout the cubane. Similarly, in the singlet state, the
LUMO is localized on the Co−O unit. These results reiterate
the spin-density calculations by Siegbahn, and are also more
consistent with the formal [Co(III)3Co(V)] oxidation state. In
summary, both ligand-field arguments and DFT calculations
support the formal oxidation state in 1(O) as being described
as [Co(III)3Co(V)] rather than [Co(III)2Co(IV)2].
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b08396. CIF
files can also be obtained free of charge from the Cambridge
Crystallographic Data Centre under reference numbers CCDC-
1054644, CCDC-1054645, and CCDC-1054646.
Full experimental details and spectroscopic data (PDF)
X-ray crystallographic data for [1H]PF6·2H2O (CIF)
X-ray crystallographic data for [1]PF6 (CIF)
X-ray crystallographic data for [1H]OTs (CIF)
CONCLUSIONS
■
The characterization of species implicated by electrochemical
experiments over a wide pH range provides important insights
into the reactivity of a cobalt cubane cluster. The [Co(III)4]
cubane 1 reacts with H+ as well as OH− to give observable
products. Although it is difficult to establish the identity of the
electrocatalyst implicated by CV studies, stoichiometric
reactivity demonstrates that cobalt cubane 1 reacts with OH−
to produce a likely WOC, and the [Co(III)3Co(IV)] cubane
([1]PF6) reacts with OH− to produce O2 with quantitative
reduction to 1. These reactions strongly suggest that there
exists at least one molecular, homogeneous pathway for O2
evolution by cobalt cubane complexes. Kinetic experiments on
the stoichiometric oxygen evolution from 1+ provide strong,
direct evidence for the homogeneous nature of the reaction,
especially as it obeys a rate law consistent with the proposed
mechanism. The second-order term in [1+] indicates that a
more oxidized state for the cubane, formally [Co(III)3Co(V)],
must be reached via disproportionation of the [Co(III)3Co-
(IV)] state, before O2 evolution. The participation of a terminal
cobalt−oxo intermediate is supported by several observations,
AUTHOR INFORMATION
Corresponding Author
*tdtilley@berkeley.edu
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Heinz Frei for help with the mass spectrometry
measurements of O2, Anthony T. Iavarone for ESI-MS (QB3
Chemistry Mass Spectrometry Facility), and Kurt van Allsburg,
Gavin R. Kiel, Patrick W. Smith, Robert G. Bergman, and
Richard G. Finke for helpful discussions. We also thank
Antonio DiPasquale for assistance with solving X-ray diffraction
structures, and the NIH Shared Instrumentation Grant S10-
RR027172. This work was supported by the Director, Office of
Science, Office of Basic Energy Sciences of the U.S.
Department of Energy under contract no. DE-AC02-
05CH11231. P.O.-B. acknowledges grant PIOF-GA-2011-
299571 (7th FP, People Marie Curie Actions) for funding.
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