Redox-Active Ligand Assisted Multielectron Catalysis: A Case of CoIII Complex as Water Oxidation Catalyst

Article abstract of DOI:10.1021/jacs.8b00032

Water oxidation is the key step in both natural and artificial photosynthesis to capture solar energy for fuel production. The design of highly efficient and stable molecular catalysts for water oxidation based on nonprecious metals is still a great challenge. In this article, the electrocatalytic oxidation of water by Na[(L^4-)Co^III], where L is a substituted tetraamido macrocyclic ligand, was investigated in aqueous solution (pH 7.0). We found that Na[(L^4-)Co^III] is a stable and efficient homogeneous catalyst for electrocatalytic water oxidation with 380 mV onset overpotential in 0.1 M phosphate buffer (pH 7.0). Both ligand- and metal-centered redox features are involved in the catalytic cycle. In this cycle, Na[(L^4-)Co^III] was first oxidized to [(L^2-)Co^IIIOH] via a ligand-centered proton-coupled electron transfer process in the presence of water. After further losing an electron and a proton, the resting state, [(L^2-)Co^IIIOH], was converted to [(L^2-)Co^IV=O]. Density functional theory (DFT) calculations at the B3LYP-D3(BJ)/6-311++G(2df,2p)//B3LYP/6-31+G(d,p) level of theory confirmed the proposed catalytic cycle. According to both experimental and DFT results, phosphate-assisted water nucleophilic attack to [(L^2-)Co^IV=O] played a key role in O-O bond formation.

Full text of DOI:10.1021/jacs.8b00032

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Article
Redox Active Ligand Assisted Multi-Electron Catalysis:
III
A Case of CoComplex as Water Oxidation Catalyst
Hao-Yi Du, Si-Cong Chen, Xiao-Jun Su, Lei Jiao, and Ming-Tian Zhang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00032 • Publication Date (Web): 08 Jan 2018
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Redox Active Ligand Assisted Multi-Electron Catalysis: A
Case of Co Complex as Water Oxidation Catalyst
III
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Hao-Yi Du, Si-Cong Chen, Xiao-Jun Su, Lei Jiao* and Ming-Tian Zhang*
Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing, 100084, China
ABSTRACT: Water oxidation is the key step in both natural and artificial photosynthesis to capture solar energy for fuel production. The design of highly
efficient and stable molecular catalysts for water oxidation based on non-precious metals is still a great challenge. In this article, the electrocatalytic oxidation
4
-
III
of water by Na[(L )Co ], where L is substituted tetraamido macrocyclic ligand (TAML), have been investigated in aqueous solution (pH 7.0). We found
4
-
III
that Na[(L )Co ] is a stable and efficient homogenous catalyst for electrocatalytic water oxidation with 380 mV onset overpotential in 0.1 M phosphate
4
-
III
2-
buffer (pH 7.0). Both ligand- and metal-centered redox features are involved in the catalytic cycle. In this cycle, Na[(L )Co ] was first oxidized to [(L
III
2-
III
)
Co OH] via ligand-centered PCET process in the presence of water. After further losing an electron and a proton, the resting state, [(L )Co OH], was
converted to [(L )Co =O]. DFT calculations at the B3LYP-D3(BJ)/6-311++G(2df,2p)//B3LYP/6-31+G(d,p) level of theory confirmed the proposed
catalytic cycle. According to both experimental and DFT results, phosphate-assisted water nucleophilic attack (WNA) to [(L )Co =O] played a key role in
O-O bond formation.
2- IV
2
-
IV
INTRODUCTION
ty and stability of high oxidation state intermediates by using redox-
84,85
active ligand relative to metal center,
particularly for single site
Solar-to-fuel conversion chemistry is a highly active field in chemistry
and material science due to the rising global energy demands and cli-
catalysis. Although simple cobalt salts have been known to catalyze
86,87
mate changes.1 Inspired by nature, artificial photosynthesis, where
-4
5-7
water oxidation since the 1980s,
for example, the development of
cobalt-based molecular WOCs was boosted after in-situ generated
solar energy is employed to split H
an attractive way to convert solar energy into fuels. Water oxidation
2
O into O
2
and H
2
8-14
, is deemed to be
8
8
cobalt-based WOC (Co-Pi) was reported by Nocera and coworkers.
-
+
A series of cobalt complexes with different kinds of ligands such as
(
2H
2
O → O + 4e + 4H ), which provides the necessary reducing
2
corrole, polypyridine, salen, and porphyrin were applied for electro- or
equivalents for both hydrogen evolution and CO reduction, remains a
2
photo-catalyzed water oxidation.5
4-63
However, a challenge for Co-
bottleneck for solar-powered water splitting.1
1,15,16
Numerous molecu-
lar catalysts for water oxidation have been described,1
7-20
based homogeneous catalyst development is the intrinsic instability of
including
the high oxidation state intermediate from which the ligand easily lib-
robust and highly active ones that based on second- and third-row
89-92
transition metals, such as Ru2
1-37
38-42
erated and CoO formed.
x
and Ir . In contrast, the develop-
ment of molecular water oxidation catalysts (WOCs) based on earth-
abundant first-row transition metals that could operate under mild
1
8,43
conditions (neutral pH) remains a challenge.
Although examples
have been reported,
44-48
49-53
54-63
64,65
66-72
based on Mn, Fe,
Co,
Ni
and Cu
most of these catalysts require a formal high oxidation state, such as
73
74,75
76,77
68
Co(IV), Fe(V) , Fe(VI),
even Cu(IV) to drive water oxida-
tion. This led to relative liable catalytic systems and strong dependence
on the reaction conditions such as pH and type of buffer and oxidant,
compared to Ru based catalysts.43 Therefore, the rational design of
robust and efficient molecular water oxidation catalysts based on earth-
abundant first-row transition metals needs further exploration.
The intrinsic challenge of water oxidation catalysis is due to the multi-
electron catalytic process, which involved four steps of accumulative
4-
III
-
+
7
8,79
Figure 1. (a) Structure of the [(L1-3 )Co ] catalysts, where Na is the counter-
4- III - +
electron transfer.
This requires the catalyst to reach a high formal
cation; (b) Structure of the [(L )Co ] , where Na is the counter-cation; (c)
oxidation state, and thus increase the difficulty of keeping the catalyst
live and homogenous for a long time. In photosystem II (PSII), a
4
-
III
ORTEP representation of Na[(L
1
)Co ] (ellipsoids were drawn at 30% prob-
ability). Selected bond distance (Å) and angle (°): Co-N1 1.820(2)，Co-N2
.820(2), Co-N3 1.836(2), Co-N4 1.836(2)，N1-Co-N2 85.8(1), N1-Co-N3
6.5(1), N3-Co-N4 101.2(1); (d) ORTEP representation of Na[(L
ellipsoids were drawn at 30% probability). Selected bond distance (Å) and
angle (°):Co-N1 1.832(4), Co-N2 1.859(4), Co-N3 1.811(4), Co-N4 1.810(4),
N1-Co-N4 87.3(2), N1-Co-N2 99.6(2), N3-Co-N4 86.2 (2), N2-Co-N3 86.7
(2). Full crystallographic details are available in the Supporting Information.
Mn Ca-cluster in the oxygen evolution complex disperses the charges
4
1
8
at multiple metal sites to evade the charge accumulated on a single
4-
III
4
)Co ]
metal center and avoid reaching a high oxidation state.1
5,80,81
To avoid
(
charge accumulation, on the other hand, redox ligands were extensively
used in metalloenzymatic and organometallic multi-electron reactions
owing to synergistic ligand effect on reaching a formal high oxidation
8
2,83
state.
The latter strategy shows the possibility of tuning the reactivi-
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In this contribution, we report a family of cobalt-based water oxida-
tion catalysts supported by TAML ligands, where redox-active func-
tionalities are incorporated at equatorial positions. These cobalt cata-
lysts are capable of homogeneously electrocatalyzing water oxidation at
pH 7, where the redox ligand facilitates the multi-electron catalysis by
promoting synergistic accumulative electron-proton transfer. This
feature further illustrated that the ligand-centered redox process was
equally important to catalytic metal center for rational design of mo-
lecular catalyst for water oxidation.
The latter wave has a greatly enhanced underlying current compared to
the background (Figure 2a, red line). The i ~ v 1 graph in Figure 2b
/2
1
/2
showed that the first wave with Ep,a = 1.00 V is independent on v and
1
/2
the second wave with
Ep,a = 1.48 V is proportional to v , indicating
that the first wave relates to a diffusion-controlled process and the
second wave relates to a catalytic process. The onset potential for this
4-
catalytic process appears at ∼1.20 V vs NHE. In contrast to Na[(L1-3
III
4-
III
)Co ]
,
the complex with the non-redox ligand, Na[(L )Co ], only
4
displayed a non-catalytic irreversible wave at 1.308 V vs NHE (Figure
2a, blue dash line).
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Confirmation of Oxygen Evolution. Oxygen evolution was confirmed
by bulk electrolysis at +1.40 V (versus Ag/AgCl) on an ITO electrode
RESULTS AND DISCUSSION
Redox-active ligands have long been recognized not only for the intri-
guing electronic structure of the resulting metal complexes but also for
2
4-
III
with a large surface area (1 cm ), employing 1 mM Na[(L
1
)Co ] in
phosphate buffer (pH 7.0). The oxygen formed in the solution was
measured using a calibrated Ocean Optics FOXY probe (Figure 3).
The oxygen generated at the applied potential in the absence of catalyst
8
3
2-
their role as electron reservoirs. o-Diphenylenedicarboxamido(bpb )
is a classic example of redox-active ligand. Scheme 1 shows the three
possible redox forms of this ligand with o-diphenylenedicarboxamido
4
-
III
was negligibly small, while with 1 mM Na[(L
1
)Co ] added, the cata-
2
-
(bpb ) as its most reduced form, and o-dicarboxamido-semiquinonate
2
lytic current was sustained at a stable current density of 0.25 mA/cm
Figure S10). The dissolved O
to 256 µM in 1 h with a Faraday efficiency over 90% for the O
•
-
0
(bpb ) and o-benzoquinonedicarboxamido (bpb ) as intermediate and
(
2
in the solution phase increased from 65
evolu-
the most oxidized form, respectively.9
3-95
Tetraamido macrocyclic
2
ligand (TAML), developed by Collins and coworkers, is a derivative of
tion, indicating that the catalytic wave observed in Figure 2a is a water
oxidation process with an onset overpotential of 380 mV.
2
-
bpb ligand and is capable of stabilizing unusual formal high valent
metal ions such as Co(IV) and Fe(V), which were usually proposed as
key intermediate for water oxidation.4
9,75,96-99
Inspired by these pioneer
2
-
work, as well as the redox properties of bpb type ligand, we selected a
4-
III
series complexes, Na[(L1-3 )Co ] (Figure 1), as model complexes to
investigate the metal-ligand cooperation in water oxidation. Since both
metal and ligand have variable oxidation state, in principle, the catalyst
is more likely to reach a higher formal oxidation state and thus facilitate
the water oxidation. To clearly understand the importance of redox
4
-
III
ligand, an analog with non-redox ligand, Na[(L
4
)Co ] was selected as
reference. Based on the above considerations, we have conducted the
investigations presented as follows.
Scheme 1
O
O
R
R
O
O
R
R
O
O
R
R
N
N
e-
e-
N
N
- e-
+ e-
N
N
-
+
bpb2-
bpb0
bpb
4
-
III
Synthesis and Characterization of Na[(L )Co ]. Ligands (H
4
1-4
L )
listed in Figure 1 were synthesized through a modified literature pro-
cedure (the details were listed in SI).1
00,101
The ligands were deproto-
in THF under an argon
nated by NaHMDS and coordinated to CoCl
2
atmosphere. The desired Co(III) complexes were obtained under air
100
exposure. The cobalt complexes are negatively charged and the
+
counter-cation is Na . X-ray crystallography (Figure 1) showed that
Co(III) in solid state is in a square-planar coordination sphere com-
prised of four deprotonated N atoms of the ligand moiety, which is
1
00
similar to the reported structure. These complexes were character-
1
1
ized by ESI-MS and H NMR. Typical paramagnetically shifted H
III
NMR spectra of square planar Co (S = 1) systems (Figure S1-S7)
were observed for these complexes. DFT calculation showed these
Co(III) complexes still adopt four coordination without an aqua in
water, and the details will be discussed below.
4
-
III
Electrochemical Properties of Na[(L )Co ] in water at pH 7.0. Cyclic
4-
III
Voltammograms. The electrochemical behaviors of Na[(L )Co ]
were investigated in 0.1 M phosphate buffer solution at pH 7.0, and the
results are listed in Table 1 and illustrated in Figure 2 and Figures S8-
4-
III
4-
III
Figure 2. (a) CVs of 0.5 mM Na[(L
1
)Co (red line), 0.5 mM Na[(L
4
)Co
(blue dash line) and blank (black line) at glassy carbon (GC) electrode at 50
mV/s in 0.1 M pH = 7.0 phosphate buffer; (b) Normalized CVs of 0.5 mM
4
-
III
4-
III
S9. Figure 2 shows a typical CV i-E response of Na[(L
1
)Co ] and its
Na[(L
1
)Co ] in 0.1 M pH = 7.0 phosphate buffer at GC at different scan rates.
variation with scan rates (i vs v1 ). Two irreversible oxidation process-
/2
Proof for Homogeneous Catalysis. To distinguish homogeneous and
heterogeneous WOCs has received particular concern respect to the
es were observed at Ep,a = 1.001 V and 1.476 V (vs NHE), respectively.
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challenge of molecular catalyst stability. Given the fact that the robust
and effective heterogeneous catalyst, CoO , could be easily formed
during catalysis from simple aqueous Co or molecular pre-catalyst, we
thus investigated stability of Na[(L )Co ] WOCs under electrocata-
lytic conditions. Over multiple CVs at GC and ITO electrode, there
were insignificant changes in peak currents or wave shapes. Electrodes
after multiple CVs gave no catalytic response in a fresh, cobalt-catalyst-
free electrolyte at pH 7.0 (Figure S11-S12). XPS result showed no
were carried out in 0.1 M phosphate buffer solutions (Figure S15), and
the resulted E - pH diagram was shown in Figure 4. As shown in the CV
graph (Figure 2), there are two irreversible oxidation waves at Ep,a
x
2
+
=
4
-
III
1.00 and 1.48 V (vs NHE) at pH 7.0, respectively. The first irreversible
oxidation wave is pH-dependent from pH = 5 to 9 with a slope of 30
mV/pH, and the slope became 59 mV at pH > 9.0. Three lines of evi-
dences indicate this oxidation wave proceeds as a ligand-centered pro-
4-
III
2-
III
ton-coupled oxidation of Na[(L
1
)Co ] to[(L
1
)Co OH] as showed
signal of CoO
x
formed on the ITO electrode after bulk electrolysis
in Eq.1-2. At the 5 < pH < 9 range, it proceeded as shown in Eq. 1,
1
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1
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2
2
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2
2
2
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0
(Figure S13); SEM of ITO surface topography also showed no cobalt
oxide precipitated during electrolysis (Figure S14). The electrochemi-
ꢀꢁꢂ)ꢃꢄꢅꢅꢅ ꢆ+ ꢆꢇ ꢉ → (ꢀꢈꢂ)ꢃꢄꢅꢅꢅꢉꢇꢆ + ꢆ2ꢊ ꢆ+ ꢆꢇ ꢆꢆꢆꢆꢆꢆꢆꢆꢆ(1)ꢆꢆꢆꢆꢆꢆꢆꢆꢆ
ꢂ
ꢋ
(
ꢈ
and at pH > 9, the oxidation wave occurred as shown in Eq. 2
4
-
III
cal behaviors of Na[(L )Co ] strongly rely on the ligand (Figure S8),
which differs from the previously reported Co-Pi catalyst,88 which ex-
cludes the self-healing cobalt oxide catalysis. Taking these observa-
tions into account, the results provide strong evidences for the key role
(ꢀꢁꢂ)ꢃꢄꢅꢅꢅ ꢆ+ꢆꢇꢈꢉ → ꢆ(ꢀꢈꢂ)ꢃꢄꢅꢅꢅꢉ ꢆ+ ꢆ2ꢊ ꢆ+ ꢆ2ꢇ ꢆꢆꢆꢆꢆ(2)ꢆꢆꢆꢆꢆꢆꢆꢆꢆ
ꢂ
ꢂ
ꢋ
4-
III
of structurally intact Na[(L )Co ] in the electrocatalytic cycle and a
homogeneous catalysis process proceeded under the conditions used
here, despite the difficulty to definitively exclude a colloid material.
4
-
III
-
4-
III
-
Figure 5. Cyclic voltammograms of 0.5 mM [(L
1
)Co ] (black), [(L
2
)Co ]
4
-
III -
(
[
red), [(L )Co ] (blue), and the reference compound with non-redox ligand
3
4
-
III -
(L
4
)Co ] (green) in degassed anhydrous CH
as electrolyte). Boron-doped diamond (BDD) electrode was used as
working electrode and the scan rate is 50 mV/s.
3
CN solution (with 0.1 M
TBA·PF
6
Figure 3. Oxygen evolution during bulk electrolysis at the applied potential 1.40
4-
III
Table 1.
phate buffer (pH 7.0)
E1/2 values for four Na[(L )Co ] complexes in acetonitrile and phos-
4
-
III
V vs Ag/AgCl. Conditions: 1 mM Na[(L
1
)Co ]; 0.1 M phosphate buffer
2
solution (PBS) at pH 7.0; ITO electrode (1 cm );
+/0
a
b
CH
3
CN (V vs Fc
)
Water (V vs NHE)
Compd.
4-/2-
IV/III
III/II
4-/3-
IV/III -
L
Co
Co
L
Co
4
-
III
1.00
1.476
Na[(L
1
)Co ]
-1.17
0.49
0.89
4
4
4
-
-
-
III
Na[(L
Na[(L
Na[(L
a
2
3
4
)Co ]
-1.06
-1.23
-1.46
0.64
0.34
-
0.94
0.84
0.80
1.05
0.90
-
1.478
1.474
1.308
III
)Co ]
III
)Co ]
The CV shape is strongly dependent on the water content of the electrolyte solu-
tion. The solution was carefully degassed and its water content was kept less than 10
ppm during the measurement. 0.1 M phosphate buffer (pH 7.0) was used as electro-
b
lyte solution, the potentials were determined by differential pulse voltammetry (DPV)
method.
III
First, the ligand is easier to be oxidized than the Co center of Na[(L1-
4
-
III
4-
3
)Co ] in acetonitrile (ligand-centered oxidation). CVs of Na[(L1-3
III
III/II
)Co ] complexes employed in this study displayed a reversible Co
reduction wave at -1.06 to -1.23 V, two reversible one electron transfer
+
/0
oxidation waves at 0.34 to 0.64 V and 0.84 to 0.94 V (versus Fc ) in
CH CN (Figure 5), respectively. These two waves are in agreement
with the reported o-diphenylenedicarboxamido (bpb ) ligated Co
3
2
-
III
4
-
III
Figure 4. Pourbaix diagram of Na[(L
1
)Co ] in 0.1 M phosphate buffer solu-
complex.96 The electrochemical behavior of the complex with non-
tions (black and blue dots represent the potentials for the first and second wave
in CV, respectively). All data were determined by differential potential voltam-
metry method using a glassy carbon electrode as the working electrode.
4-
III
redox ligand Na[(L
4
)Co ] (Figure 5) displayed only two waves at -
1.46 V and 0.80 V, but without a reversible wave at 0.34 to 0.64 V.
Since the electrochemically generated monooxidized species, namely
Ligand-Centered Proton-Coupled Electron Transfer. The electro-
3
-
III
4-
III
[(L1-3 )Co ], are stable in acetonitrile, the UV-vis spectra was record-
chemical behaviors of Na[(L
1
)Co ] at full pH range from 5 to 11
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-
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ed. [(L
1
)Co ] species displayed intense new absorptions in visible
also observed. Additionally, the electrolysis at 1.2 V vs NHE resulted in
4
-
III
-
2-
III
and near-infrared regions (596 and 746 nm) that are characteristic of
imino)semiquinonato radical (bpb ) (Figure S16). The EPR spec-
trum of this electrochemically generated monooxidized species in ace-
tonitrile has an isotropic character with a g value of 1.997 (Figure 6),
indicating that the oxidation of [( L1-3 ) Co ] is ligand-centered, with
the formation of the corresponding (imino)semiquinonato organic
radical (bpb ). These results indicated that the reversible wave at 0.34
to 0.64 V(versus Fc ), which is strongly dependent on the substitu-
ents on the ligand, should be ascribed to the ligand-centered oxidation,
the oxidation of [(L
1
)Co ] to [(L
1
)Co -OH], leading to a loss of
•
-
96
intensity of the LMCT band at 230 and 265 nm and increasement of
the MLCT band at 338 and 412 nm, as well as the bleach at 600 nm.
These are probably due to the changes of d-d transition caused by the
(
4
-
III -
coordination environment transformed from square to quadrangular
4-
pyramid (Figure S22). X-ray photoelectron spectra (Figure 7) of [(L
1
III
-
2-
III
•
-
)Co ] and the [(L
1
)Co -OH] sample prepared by bulk electrolysis
+/0
at 1.2 V vs NHE display very similar binding energies (780.3 eV for Co
4
-
III -
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2p3/2 and 795.5 eV for Co 2p1/2 of [(L )Co ] ; 780.8 eV for Co 2p3/2
1
2
-
III
+
/0
and 796.3 eV for Co 2p1/2 of [(L
1
)Co -OH], the weak yellow satellite
while the wave at 0.84 to 0.94 V (versus Fc ) should be attributed to
the oxidation of Co .
III
peaks in both cobalt complexes’ Co 2p region probably were the shake-
up lines of Co(III))1
02-104
consistent with Co(III) oxidation state,
105-107
Second, the electrochemical behavior of the cobalt complex with the
non-redox ligand, Na[(L
versible oxidation wave around 1.0 V vs NHE of Na[(L1-3 )Co ]
would be a ligand-centered proton-coupled electron transfer process.
Interestingly, the electrochemical behavior of Na[(L
indicating that the oxidation process at 1.2 V vs NHE happened on the
ligand rather than Co(III) center. In contrast, the O 1s region of the
4
-
III
4
)Co ] (Figure 6), shows that the first irre-
4
-
III
2
-
III
XPS spectrum of [(L
1
)Co -OH] has an additional peak at 532.0 eV
4-
III -
compared with the spectrum of [(L
1
)Co ] (530.9 and 532.2 eV for
4
-
III
1
)Co ] in
4-
III
-
2-
III
[
(L
1
)Co ] ; 531.2, 532.0, and 532.6 eV for [(L
1
)Co -OH]). The
CH
3
CN/H
2
O was quite different. Upon gradually addition of water to
binding energy peak at 532.0 eV is consistent with an OH group bind-
4
-
III
acetonitrile solution of Na[(L
1
)Co ], the wave of ligand-centered
1
08-111
ing on the Co(III).
This matches well with the ligand-centered
oxidation at 0.49 V (versus Fc+ ) became irreversible and the anodic
/0
-
2-
III
PCET 2e oxidation involving a H
2
O and [(L )Co -OH] is the result-
current increased at 0.96 V, indicating an electrocatalytic oxidation
behavior (Figure S17-S19). Due to the existence of H O, the electro-
chemical behavior in buffer is different from the successive single elec-
tron and reversible redox behavior of [(L
)Co ]/[(L
)Co ] in aqueous solution, the H
the Co center and the second proton-coupled electron transfer occurs
simultaneously. The net result, formation of [(L
ed resting state for water oxidation catalysis. From these data, it is clear
2
that the ligand has undergone a PCET oxidation with H
the process (Eq. 1 and Eq. 2).
2
O involved in
4
-
III
-
3-
III
1
)Co ] /[(L
1
)Co ] and
3
4
-
-
IV
3-
III +
[(L
(L
1
1
)Co ] displayed in acetonitrile. Upon oxidation of
O molecular coordinates to
III
-
[
1
2
2
-
III
1
)Co -OH], is
-
+
equivalent to a combined “formal” (2e + H ) oxidation and hydroxide
binding. Figure 6b shows a typical electrochemical behavior of both
4-
III
4-
III
Na[(L
1
)Co ] and Na[(L
4
)Co ]. The former compound, which
has a redox-active ligand (L
wave around 1.0 V vs NHE, while for the latter with the non redox-
active ligand (L ), there is no electrochemical response around 1.0 V vs
NHE. This result, together with the E-pH relationship, supports that
the first irreversible oxidation wave around 1.0 V vs NHE should be
1
), displayed the an irreversible oxidation
4
4
-
III
ascribed to the ligand-centered PCET oxidation of Na[(L1-3 )Co ]
with the help of H O.
2
4
-
III
Figure 7. (a) Co 2p region of the XPS spectrum of Na[(L
1
)Co ], (b) O 1s
4
-
III
4-
III
Figure 6. (a) EPR spectrum of Na[(L
of 0.6 V vs Fc at 295 K. (b) Electrochemical behaviors of Na[(L
line) and Na[(L
pared with blank (black line) indicating the oxidative wave at 0.9 V vs NHE
belongs to the ligand-centered PCET oxidation.
1
)Co ] electrolyzed under the potential
region of XPS spectrum of Na[(L )Co ], (c) Co 2p region of the XPS spec-
1
+/0
4-
III
2-
III
1
)Co ](red
trum of [(L
1
)Co (OH)] synthesized by bulk electrolysis, (d) O 1s region of
4-
III
2-
III
4
)Co ] (blue line) in pH 7.0 phosphate buffer (0.1M) com-
the XPS spectrum of [(L
1
)Co (OH)] obtained by bulk electrolysis.
Water Oxidation Kinetics and Mechanistic Analysis. The second irre-
versible oxidation wave (Figure 2), which is an electrocatalytic water
oxidation process also displays pH dependence with a slope of 52
Third, as shown by ESI-HRMS, the electrolysis at 1.2 V vs NHE
caused the disappearance of the[(L )Co ] peak (m/z = 429.0792,
Figure S1) and the appearance of a new peak at m/z = 468.0886, which
was designated as {[(L
4
-
III -
1
2
-
mV/pH at 5 < pH < 9 range, indicative of a PCET oxidation of [(L
)Co -OH] (Eq. 3).
2
-
III
+
-
III
1
)Co -OH]+H
2
O-H } (Figure S20). When
4
-
III -
[
{
(L
3
)Co ] was treated with bulk electrolysis at 1.2 V vs NHE, the
)Co -OH]+H
ꢀꢈꢂ)ꢃꢄꢅꢅꢅꢉꢇꢆ → ꢆ(ꢀꢈꢂ)ꢃꢄꢅꢅꢅ − ꢉ ∙ꢆ+ꢆꢊꢂ + ꢇ ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ 3 ꢆꢆ
ꢋ
(
2
-
III
+ -
[(L
3
2
O-H } peak (m/z = 531.0259, Figure S21) was
4
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At pH>9, The second irreversible oxidation wave (Figure 2) displays
Randles-Svecik equation in Eq. 7,112 where n
= 2 is the number of of
electrons transferred in this diffusion controlled process, α = 0.5 is the
transfer coefficient of the catalyst.
d
2
-
III
pH independence since [(L )Co -OH](pK
indicative of a single electron oxidation of [(L )Co -O ] (Eq. 4).
a
= 9.3) was deprotonated,
2
-
III
-
(ꢀꢈꢂ)ꢃꢄꢅꢅꢅꢉꢂ → ꢆ(ꢀꢈꢂ)ꢃꢄꢅꢅꢅ − ꢉ ꢆ+ꢆꢊ ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ(4)
∙
ꢂ
ꢝ
ꢈ
ꢑꢛꢒꢚꢕꢖꢗ
ꢞꢟ
2
-
IV
2-
III
ꢍꢛ = 0.496ꢑꢛꢜꢝ/ꢈꢒꢓ ꢃꢄ
The resulting intermediate, [(L )Co =O] or [(L )Co -O·], might
react with water to form the O-O bond and enable the catalytic cycle of
water oxidation (Eq. 5).
ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ(ꢆ7)
could be
By dividing Eq. 6 by Eq. 7, a relationship between icat and i
d
obtained (Eq. 8) and the rate constants for water oxidation could be
evaluated using this equation.
(ꢀꢈꢂ)ꢃꢄꢅꢌ = ꢉꢆ + ꢇ ꢉ → ꢆ(ꢀꢁꢂ)ꢃꢄꢅꢅꢅ ꢆ+ꢆO ↑ +ꢆꢊ + 2ꢇ ꢆꢆꢆꢆꢆ(5)ꢆ
ꢂ
ꢋ
ꢈ
ꢈ
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ꢍꢎꢏꢐ
ꢑꢎꢏꢐ
ꢑꢛꢠ ꢈ
=
0.359
ꢔꢎꢏꢐ ꢜꢚꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ(8)
ꢍꢛ
Figure 8b showed the expected linear variation of icat/i
d
with ꢚ , and
-
1
-1
-1
the kcat, calculated from the slope, was 7.53 s , 7.58 s and 8.81 s for
4-
III
-
4-
III
-
4-
III -
[
(L
1
)Co ] , [(L
2
)Co ] , and [(L )Co ] , respectively (Figure 8,
3
S23-S29) . The catalytic rates of these catalysts compare well with
-1
those reported kcat around 1 s for the single site and polynuclear co-
5
6-58
balt-based WOCs.
Notably, no catalytic water oxidation behav-
ior was observed for the complex with the non redox-active ligand,
4
-
III -
[(L )Co ] , indicating that the redox-active ligand plays a critical role
4
in this multi-electron catalytic cycle .
Proposed mechanism. The catalytic peak current for water oxidation
4
-
III
i
cat varies linearly with the concentration of catalyst, Na[(L1-3 )Co ],
consistent with single-site cobalt catalysis (Figure 8a, S24, S27). CV
experiments were also performed in D O (pD = 7.0) phosphate buffer
Figure S30). Analysis of these data gave a kinetic isotope effect (KIE =
2
(
k
V
H2O/kD2O) of 2.0. As similar to the oxidation of water by Ru =O, this
kinetic isotope effect might be attributed to atom-proton transfer
(
APT) with O-O bond formation concerted with proton transfer to a
113,114
proton acceptor, such as base form of buffer.
The electrochemical
behaviors at different buffer concentrations were carried out at an ad-
justed ionic strength I = 0.462 M by NaClO
4
(Figures S31-S32) and
2
the dependence of (icat/i
d
) on phosphate concentrations (0 ~ 0.2 M)
indicated the buffer anion as proton acceptor could contribute to rate
IV
limiting water nucleophilic attack of Co =O. In terms of the available
experimental results, water oxidation mechanism in Scheme 2 was
proposed, in which O-O bond formation step is analogous to the
26,113,114
scheme proposed earlier for Ru polypyridyl catalysts.
In this
2-
III
catalytic cycle, the resting state, [(L )Co -OH] was formed by a
1
-
+
ligand-centered (2e + H ) involved PCET oxidation, and a further
2
-
IV
PCET oxidation generated the catalytically active [(L
1
)Co =O],
which reacts with water by rate limiting O-O bond formation step to
give an intermediate peroxide that could be further oxidized to release
oxygen and close the cycle.
4-
III
Figure 8. (a) CVs of different concentrations of Na[(L
1
)Co ] in 0.1 M PBS
buffer at pH = 7.0 (insert: linear regression of icat vs catalyst concentration,
-
1/2
[Cat.].); (b) Plot of linear regression of icat/i
d
vs v
.
-(2e- + H+)
(L2-)Co OH
2
H O
(L4-)CoIII
III
Kinetic Analysis. The catalytic current is linearly dependent on the bulk
concentrations of catalyst (Figure 8a), thus the peak current of this
O
2
1
12
catalytic process should obey the relationship displayed in Eq. 6,
where A is the electrode surface area, F is Faraday constants, i is the
-
(e- + H+)
(e- + H+)
-
d
peak current, icat is the peak current of the catalytic wave, [Co] is the
bulk concentration of catalyst, DCo is the diffusion coefficient of the
catalyst, and ncat = 4 is the number of electrons transferred in each cata-
L2-)CoIV=O
(L2-)CoIII-O
(
APT pathway
(L2-)Co OOH
II
H+
OH2
lytic cycle.
4-
III
Scheme 2. Proposed mechanism of Na[(L
tions (pH = 7.0)
1
)Co ] in phosphate buffer solu-
ꢘ
ꢍꢎꢏꢐ = ꢑꢎꢏꢐꢒꢓ ꢃꢄ ꢔꢎꢏꢐꢕꢖꢗ ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ(ꢆ6)ꢆꢆ
ꢙ
Scan rate normalized CVs (ꢍ ꢚ) at different scan rates shows that
the first oxidation wave is related to an irreversible diffusion limited
proton-coupled electron transfer (PCET) at the electrode. The peak
current (i ) varies linearly with ꢚ , which is in consistent with the
d
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ΔGsolv(298 K)
in kcal/mol
H
H
O
H
O
H
H
H
H
O
Reference: 1.40 V
O
30
O
H
Me
Me
O
N
Me
1.40 Å
O
Me
O
N
1.40 Å
1
N
Me
N
Co
Me
O
H
Me
Na
N
O
H
O
N
Me
Me Co
N
Me
.40 Å
N
H H H H
H
O
2
1
0
0
0
O
O
O
1.40 Å
O
O
H
O
TS'
(S = 3/2)
Me
O
1.40 Å
Me
Me
Me
O
N
Me
N
Co
Me
N
OH
P
N-Ar: 1.41 Å
N-Co: 1.82 Å
Me
14.0
N
O
H
3H
2
O
O
O
H
O
O
O
H
O
O
Na
Me
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Na[(L
(
4-)CoIII]
S = 1)
Me
1
Me
O
O
N
Me
N
Co
Me
N
Int1'
Me
(
S = 3/2)
O
N
0.0
H O
2
Me
4
H O
-4.8
PCET
0.90 V
2
TS
-10
(S = 3/2)
O
-12.0
Me
O
Me
O
H+ + 2e-
+ Na+
O
N
Me
N
Co
Me
N
PCET
Me
H
2
O +
-20
-
23.0
2-)CoIIIOH
S = 1)
-22.0
H+ + e-
N
1
.36 V
NaH PO
2
4
O
-
24.0
H+
(L12-)Co OOH
(S = 3/2)
II
O
(L
1
+
Me
(
(L
H+ + e- (S = 3/2)
1
2-)CoIV=O
-28.8
NaH
2
PO
4
-29.5
PCET
1.07 V
-30
Int1
S = 3/2)
Int2
L4-)CoIII•O2
(S = 0)
(
(
OH
1.42 Å
Me
O
OH
Me
Me
O
N
H
H
Me
O
N
Me
O
1.38 Å
O
H
O
N
-40
O
O
Me
N
Me
N
Co
Me
N
Me
N
Co
Me
O
Na+
O2
-54.8
Me
1.42 Å
Me
Me
Me
O
O
N
Me
N
Co
Me
O
Me
N
N
O
O
N
Me
N
N
Co
Me
N
O
O
P
Me
Na
1.38 Å
O
O
1.42 Å
O
HO
N
-50
Me
Me
O
O
Na[(L4-)CoIII
]
O
Me
O
N-Ar: 1.36 Å
N-Co: 1.88 Å
N-Ar: 1.36 Å
N-Co: 1.90 Å
Me
(S = 1)
a
(pK = 9.3)
-60
-
1
4-
III
Figure 9. Energy diagram (∆G298 K in kcal·mol ) for water oxidation catalyzed by Na[(L
1
)Co ], calculated at the B3LYP-D3(BJ)/6-311++G(2df,2p)//B3LYP/6-
3
1+G(d,p) level. The reference potential of 1.40 V vs NHE is used to setup the thermodynamics. The Gibbs free energies and the potentials for redox couples under pH
7 are reported.
further oxidized through another PCET process to produce the pro-
posed complex [(L
calculated structure revealed that, both the oxo moiety and the cobalt
center bear significant spin densities. Therefore the structure of the
complex is better described as [(L
2-
IV
2-
III
1
)Co =O] ↔ [(L
1
)Co -O∙]. Analysis of the
DFT calculation. To disclose more details of the water oxidation cata-
lytic cycle, in particular the oxygen-oxygen bond formation event, den-
sity functional theory calculation was performed at the B3LYP-
D3(BJ)/6-311++G(2df,2p)//B3LYP/6-31+G(d,p) level. For the
intermediates and the transition states, the solvation effect in water was
taken into account by applying the SMD model during single-point
energy calculation based on the gas-phase optimized structures. For the
2
-
III
1
)Co -O·] (see the Supporting
Information for details).
The oxidation potentials of these two PCET steps at pH 7 were cal-
culated to be 0.91 and 1.32 V vs NHE, respectively, in agreement with
the experimental CV diagram for Na[(L
two oxidation waves were found at 1.00 and 1.48 V vs NHE. This also
accounts for the experimental Pourbaix diagram (Figure 4), which
indicates a H + 2e process for the first wave, and a H + e process for
the second wave.
4
-
III
H
2
O molecule, proton, and sodium cation, the experimental solvation
1
)Co ] (Figure 2a), in which
free energies in water were used.
The model WOC employed in the DFT calculation is the triplet state
+
-
+
-
4
-
III
4-
complex Na[(L
1
)Co ] (Figure 9). In aqueous solution, Na[(L
1
III
+
-
)
Co ] combines one H
2
O molecule and undergoes PCET (H + 2e )
2
-
III
to give a triplet complex [(L
1
)Co -OH], in which a hydroxyl group
The crucial oxygen-oxygen bond formation step follows a water
nucleophilic attack (WNA) mechanism, in which the assistance of the
buffer anion plays an important role. First, [(L
NaH PO and a H
attached to one amide carbonyl group and the phosphate moiety,
together with the H
coordinates at the top of the complex and the redox-active tetraamido
macrocyclic ligand was oxidized (L4 to L ). The oxidation of the
-
2-
2-
III
1
)Co -O·] binds with
+
TAML moiety is clearly indicated by the changes in structural parame-
ters: in the starting complex Na[(L
the benzene ring is evenly distributed (around 1.40 Å), and the N-
aryl ring bond lengths are ca. 1.41 Å; in the complex [(L
2
4
2
O molecule to give intermediate Int1, in which Na
4-
III
1
)Co ], the C-C bond length of
+
2
O, complexed with Na . This process is exergonic
2
-
III
-
1
)Co -
by 4.4 kcal/mol. Then the H
2
PO
4
anion assisted the nucleophilic
+
-
2-
III
OH] after a PCET (H + 2e ), the C-C bonds in the former benzene
ring are elongated and shortened alternately (1.42 and 1.38 Å, respec-
tively), and the two N-aryl ring bonds shortened to 1.36 Å. This is in
accordance with the structural change in the oxidation of an o-
diphenylenedicarboxamido ligand to its benzoquinonedicarboxamido
form. The calculated natural bond orbital (NBO) charge distributions
attack of the H O molecule at the oxo group of [(L )Co -O·]: in the
2
1
-
WNA transition state TS, the H PO anion binds to the oxo group by
hydrogen bonding interaction (O∙∙∙H distance: 1.74 Å), and it
facilitates the attack of the oxygen in H O to the oxo group (O∙∙∙O
distance: 1.83 Å) by abstracting a proton from H O (Figure 9). The
WNA process has an activation free energy of 16.8 kcal/mol, and it
turns out to be the rate-limiting step in the catalytic cycle. An
alternative WNA pathway, which follows the established WNA model
for the Fe-TAML complexes involving multiple H O molecules, was
2
4
2
2
4-
III
2-
III
115
of Na[(L
1
)Co ] and [(L
1
)Co -OH] also support the ligand-
centered PCET, because the charge on the ligand moiety changed from
-1.885 to -0.482 during this process. The complex [(L
2
-
III
1
)Co -OH] is
2
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also calculated.76,77 It was found that, association of four H
O
2
ACKNOWLEDGMENTS
2
-
III
molecules with [(L
WNA transition state TS’, these water molecules form a hydrogen
bonding network and the nucleophilic attack of one H O to the oxo
group (O∙∙∙O distance: 1.78 Å) is facilitated by proton abstraction from
another H O molecule. However, this pathway has an overall
1
)Co -O·] is endogonic by 19.2 kcal/mol. In the
This work was supported by the National Natural Science Foundation of
China (NSFC Grant No. 21572113, 21661132006), Tsinghua University
Initiative Scientific Research Program (Grant No. 20151080404), National
Key Research and Development Program of China (2016YFB0401400)
and National Program for Thousand Young Talents of China. We
acknowledge the instruments platform of CBMS and the Tsinghua Xue-
Tang Talents Program for providing instrumentation and computational
resources.
2
2
activation free energy of 42.8 kcal/mol (from Int1), much higher than
that of the dihydrophosphate-assisted pathway. The significant
-
influence of H
2
PO
4
on the WNA process revealed by DFT calculation
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
3
3
3
3
4
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4
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4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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7
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
is in good agreement with the observed buffer effect.
Both WNA pathways affords the quartet hydroperoxo cobalt(II)
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-
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(
2
-
IV
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AUTHOR INFORMATION
(
Corresponding Author
M.-T. Zhang: mtzhang@mail.tsinghua.edu.cn
L. Jiao: leijiao@mail.tsinghua.edu.cn
(
(
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The authors declare no competing financial interests. We would like to dedicate
th
this work to Prof. Jin-Pei Cheng on the occasion of his 70 birthday.
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