Catalytic Water Oxidation by a Molecular Ruthenium Complex: Unexpected Generation of a Single-Site Water Oxidation Catalyst

Article abstract of DOI:10.1021/ic502755c

The increasing energy demand calls for the development of sustainable energy conversion processes. Here, the splitting of H₂O to O₂ and H₂, or related fuels, constitutes an excellent example of solar-to-fuel conversion schemes. The critical component in such schemes has proven to be the catalyst responsible for mediating the four-electron oxidation of H₂O to O₂. Herein, we report on the unexpected formation of a single-site Ru complex from a ligand envisioned to accommodate two metal centers. Surprising N-N bond cleavage of the designed dinuclear ligand during metal complexation resulted in a single-site Ru complex carrying a carboxylate-amide motif. This ligand lowered the redox potential of the Ru complex sufficiently to permit H₂O oxidation to be carried out by the mild one-electron oxidant [Ru(bpy)₃]³⁺ (bpy = 2,2′-bipyridine). The work thus highlights that strongly electron-donating ligands are important elements in the design of novel, efficient H₂O oxidation catalysts. (Chemical Equation Presented).

Full text of DOI:10.1021/ic502755c

Article
pubs.acs.org/IC
Catalytic Water Oxidation by a Molecular Ruthenium Complex:
Unexpected Generation of a Single-Site Water Oxidation Catalyst
Wangchuk Rabten,† Markus D. Karkas,*,† Torbjorn Åkermark,† Hong Chen,‡,§ Rong-Zhen Liao,∥
̈
̈
̈
Fredrik Tinnis,† Junliang Sun,‡,⊥ Per E. M. Siegbahn,† Pher G. Andersson,*,† and Bjorn Åkermark*,†
̈
†Arrhenius Laboratory, Department of Organic Chemistry, and Berzelii Centre EXSELENT on Porous Materials, Department of
‡
Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden
§Faculty of Material Science and Chemistry, China University of Geosciences, 430074, Wuhan, China
∥Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, School of Chemistry and Chemical
Engineering, Huazhong University of Science and Technology, 430074, Wuhan, China
⊥College of Chemistry and Molecular Engineering, Peking University, 100871, Beijing, China
S
* Supporting Information
ABSTRACT: The increasing energy demand calls for the
development of sustainable energy conversion processes. Here,
the splitting of H2O to O2 and H2, or related fuels, constitutes an
excellent example of solar-to-fuel conversion schemes. The critical
component in such schemes has proven to be the catalyst
responsible for mediating the four-electron oxidation of H2O to
O2. Herein, we report on the unexpected formation of a single-site
Ru complex from a ligand envisioned to accommodate two metal
centers. Surprising N−N bond cleavage of the designed dinuclear
ligand during metal complexation resulted in a single-site Ru
complex carrying a carboxylate−amide motif. This ligand lowered
the redox potential of the Ru complex sufficiently to permit H2O
oxidation to be carried out by the mild one-electron oxidant
[Ru(bpy)3]3+ (bpy = 2,2′-bipyridine). The work thus highlights that strongly electron-donating ligands are important elements in
the design of novel, efficient H2O oxidation catalysts.
significantly reduced redox potentials compared to WOCs
bearing neutral polypyridyl ligands.
INTRODUCTION
■
In attempts to store solar energy as chemical energy, such as
H2, artificial photosynthetic schemes are attractive. However,
the design of efficient catalysts for the oxidation of H2O to yield
O2, electrons, and protons (eq 1), where the two latter can
combine to give H2, has proved challenging.1−3
Incorporation of negatively charged amide24 and carbox-
ylate37 groups has previously proved to give viable candidates
for inclusion into ligand architectures of WOCs. It was
therefore envisioned that a combination of these motifs in a
single unit could create a suitable ligand environment. The
tailored ligand 1, which could be accessed through straightfor-
ward synthesis from readily available building blocks (Figure 1)
containing the hydrazide unit, seemed to be a promising
candidate for generation of a dinuclear Ru complex.
2H2O → 4H+ + 4e− + O2
(1)
Nature performs this complicated task by utilizing a
Mn4O5Ca cluster that efficiently carries out the four-electron
oxidation of H2O to O2.4 The lack of robust and efficient
artificial water oxidation catalysts (WOCs) thus hampers the
design of H2O splitting devices and has therefore been a hot
topic for researchers to pursue.5 Artificial molecular WOCs
based on Mn,6−10 Fe,11−14 Co,15−17 and Cu18−20 have been
constructed but Ir21−23 and, especially, Ru24−35 seem to be
superior and eventuate in robust molecular catalysts. The
incorporation of negatively charged functional entities into the
ligand scaffolds of WOCs has proved to be a promising strategy
for the controlled design of molecular Ru catalysts.36−39 The
reason is that the negatively charged ligands stabilize the metal
center(s) at high redox states via extensive electron
donation.40,41 This stabilizing effect further reflects in the
However, the single-site Ru complex 3 was unexpectedly
generated via N−N bond cleavage of ligand 1 (Figure 2). The
produced Ru complex 3 containing the tridentate 6-
carbamoylpicolinic acid ligand 4 (H3L) was also synthesized
independently and characterized and was found to possess
appealing catalytic properties which allowed H2O oxidation to
be driven by [Ru(bpy)3]3+ (bpy = 2,2′-bipyridine). Electro-
chemical measurements confirmed that the complex exhibited
lower redox potentials than Ru WOCs bearing neutral ligand
Received: November 21, 2014
© XXXX American Chemical Society
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Figure 1. Envisioned assembly of ligand 1.
Figure 2. Generation of the single-site Ru complex 3 from ligand 1 via N−N bond cleavage.
Scheme 1. Synthesis of Ligand 1
Scheme 2. Synthesis of Ligand 4
scaffolds. The introduction of amide functional groups to
WOCs could thus be a general strategy for constructing more
efficient catalysts with reduced onset potentials. In addition,
this work highlights that the inclusion of sensitive functional
motifs into ligand scaffolds could result in conversion to
unexpected structures. Care, therefore, has to be taken when
crafting ligand scaffolds as they might be subjected to further
transformation into unmapped metal complexes that are the
actual WOCs.
compound 8 with hydrazine hydrate (N2H4·H2O) converted 8
into the dimethyl ester 9. Finally, hydrolysis with LiOH gave
the dinuclear ligand 1 as a white solid.
Attempts to obtain a dinuclear Ru complex based on ligand 1
by refluxing the ligand with Ru(DMSO)4Cl2 as the metal
precursor only resulted in reductive cleavage of the N−N bond
to yield a single-site Ru complex (3). The structure of the
single-site complex was characterized as [Ru(HL)(pic)3]+ by
1H NMR spectroscopy (after reduction to RuII), mass
spectrometry, UV−vis spectroscopy, and X-ray crystallography
(vide infra). Independent synthesis of ligand 4, according to
Scheme 2, and Ru complex 3 further confirmed the structure of
the single-site Ru complex 3.
Suitable single crystals for X-ray diffraction of Ru complex 3
were obtained by cooling a solution of complex 3 in MeCN/
EtOAc (1:9), and the structure is presented in Figure 3. The
resolved crystal structure revealed that the RuIII is located in a
slightly distorted [RuN5O] octahedral configuration. In the
equatorial plane, three positions are occupied by two nitrogen
RESULTS AND DISCUSSION
■
Synthesis, Characterization and Crystal Structure of
Ru Complex 3. Ligand 1 was synthesized according to Scheme
1, starting from the readily available pyridine-2,6-dicarboxylic
acid (5). Refluxing compound 5 in MeOH in the presence of
H2SO4 resulted in the dimethyl pyridine-2,6-dicarboxylate ester
6. Selective hydrolysis of one of the ester moieties gave the
pyridine-2,6-dicarboxylate monomethyl ester 7, which was
subsequently transformed into acyl chloride 8. Treatment of
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Figure 4. Structure of the previously reported [RuII(pdc)(pic)3]
complex 10 (where H2pdc = 2,6-pyridinedicarboxylic acid) with
selected bond lengths.
ment.43 A similar hydrogen-bonding effect is also expected in
aqueous solutions for Ru complex 3, which houses two possible
proton handles, the amide and carboxyl units.
O2 Evolution Activity. The performance of Ru complex 3
to mediate H2O oxidation was evaluated at neutral conditions
(aqueous phosphate buffer; 0.1 M, pH 7.2) with the mild one-
electron chemical oxidant [Ru(bpy)3]3+. The amount of
produced O2 was monitored in real time by mass spectrometry.
Upon the addition of an aqueous solution containing complex
3 to the solid oxidant, O2 evolution could immediately be
detected (Figure 5). The low O2 evolution yields, i.e.,
conversion yields from oxidant to O2, in Table 2 are due to
decomposition of the [Ru(bpy)3]3+ oxidant in neutral aqueous
solutions and contribute to unproductive reaction pathways. In
the absence of any catalyst, the [Ru(bpy)3]3+ oxidant thus
spontaneously decomposes without generation of O2.
The initial rate of O2 evolution was shown to be proportional
to the initial concentration of complex 3 (Figure 6 and
Supporting Information). Assuming pseudo-first-order kinetics,
the initial rate of O2 evolution can be approximated by eq 2.
The measured rate constant kO2 (0.58) can subsequently be
converted to yield the turnover frequency (TOF), which equals
1.16 s−1.44 The calculated TOF of Ru complex 3, 1.16 s−1, is
approximately 1 order in magnitude greater than the TOFs
(using CeIV as the chemical oxidant) of the previously
developed [Ru(tpy)(bpy)(OH2)]2+-type complexes (tpy =
2,2′:6′,2″-terpyridine), which are based on neutral polypyr-
Figure 3. X-ray crystal structure of the single-site Ru complex 3.
Hydrogen atoms (except the N−H) have been omitted for clarity.
and one oxygen atoms from the tridentate 6-carbamoylpicolinic
acid ligand 4 and one position is contributed by the nitrogen
atom from a 4-picoline ligand. The two axial positions are both
occupied by 4-picoline ligands.
Compared to the crystal structure of [RuII(pdc)(pic)3] (10;
where H2pdc = 2,6-pyridinedicarboxylic acid), a previously
reported WOC,42 the open angle in Ru complex 3 [N(1)−
Ru(1)−O(2)] is close to the one found for this complex.
However, the corresponding Ru1−O2(carboxyl) and Ru−
N(pic) bonds of complex 3 are considerably shorter (cf. Table
1 and Figure 4). Quantum chemical calculations have
previously shown that a proton acceptor near the active site
of Ru WOCs favors proton-coupled electron-transfer (PCET)
processes. These PCET events are fundamental for accessing
high-valent Ru species since they avoid the formation of high-
energy intermediates via concerted proton−electron move-
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Ru Complex 3
Bond Lengths (Å)
Ru(1)−N(2)
Ru(1)−N(1)
Ru(1)−O(2)
1.958(3)
2.024(3)
2.043(3)
Ru(1)−N(4)
Ru(1)−N(5)
Ru(1)−N(3)
2.087(3)
2.095(3)
2.108(3)
Bond Angles (deg)
N(2)−Ru(1)−N(1)
N(2)−Ru(1)−O(2)
N(1)−Ru(1)−O(2)
N(2)−Ru(1)−N(4)
N(1)−Ru(1)−N(4)
O(2)−Ru(1)−N(4)
N(2)−Ru(1)−N(5)
N(1)−Ru(1)−N(5)
79.60(12)
80.44(12)
160.00(12)
92.37(12)
88.54(13)
91.10(12)
90.52(12)
93.40(13)
N(4)−Ru(1)−N(5)
N(2)−Ru(1)−N(3)
N(1)−Ru(1)−N(3)
O(2)−Ru(1)−N(3)
N(4)−Ru(1)−N(3)
N(5)−Ru(1)−N(3)
O(2)−Ru(1)−N(5)
176.77(12)
178.07(13)
100.19(12)
99.73(11)
85.70(12)
91.41(12)
87.95(11)
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Figure 6. Initial rate of O2 evolution plotted as a function of the
concentration of Ru complex 3. From the obtained kO2 (0.58), the
turnover frequency (TOF) can be calculated. This gives a TOF of
∼1.16 s−1 (for further information see Supporting Information).
significantly reduces the redox potentials of the metal complex
and hence lowers the catalytic onset potential at which H2O is
being oxidized. In order to generate the catalytically active
intermediate, Ru complex 3 has to undergo picoline−H2O
ligand exchange where one of the picoline ligands is replaced by
a solvent aqua ligand. Initial experiments in trying to detect the
1
important Ru−aqua species made use of H NMR. However,
1
the H NMR spectrum of the Ru complex 3 did not show any
Figure 5. Plots of O2 evolution vs time at various concentrations of Ru
complex 3. Reaction conditions: experiments were carried out by
addition of an aqueous phosphate buffer solution (0.1 M, pH 7.2, 0.50
mL) containing catalyst 3 in varying concentrations to the oxidant
[Ru(bpy)3](PF6)3 (3.6 mg, 3.6 μmol).
apparent peaks arising from the paramagnetic nature of the
RuIII center. The RuIII center was therefore reduced to RuII by
addition of ascorbic acid, a reductant, to a solution of complex
1
3 in CD3OD. The acquired H NMR spectrum now revealed
clear and sharp peaks. Analysis of the derived spectrum showed
integrals corresponding to the structure [RuII(HL)(pic)3]
(Supporting Information Figures S7−S9). It was further
demonstrated that “free 4-picoline” was absent in the reduced
sample (Supporting Information Figure S8). On the basis of
these observations, it is clear that substitution of the picoline
ligand by an aqua or solvent molecule does not occur to any
appreciable extent at the RuII state. Electrospray ionization
mass spectrometry (ESI-MS) was subsequently used to study
the ligand dissociation at the RuIII state. Analysis of an aqueous
solution of Ru complex 3 resulted in the appearance of a peak
at m/z 470.0544, corresponding to [RuIII(HL)(pic)2(OH2)]+
(Figure 7). This observation indicates that picoline−aqua
ligand exchange does occur for Ru complex 3 at the RuIII state,
enabling PCET and easy access to higher redox states, which
ultimately brings about the facile conversion of H2O into O2.
Quantum chemical calculations showed that the picoline−H2O
ligand displacement is favored at the equatorial position by 2.9
kcal mol−1.
a
Table 2. Summary of the Catalytic Data for Ru Complex 3
catalyst concn
TON
yield of O2
b
(μM)
(nmol O2/nmol cat.) (4 × nmol O2/nmol oxidant) (%)
9.0
110
114
134
140
155
185
280
220
235
250
225
230
55
51
52
47
45
41
35
14
8.0
7.0
6.1
5.2
4.0
2.25
1.12
0.75
0.55
0.45
0.22
9.8
7.8
5.6
2.8
a
Reaction conditions: An aqueous phosphate buffer solution (0.1 M,
pH 7.2, 0.50 mL) containing Ru complex 3 was added to the oxidant
[Ru(bpy)3](PF6)3 (3.6 mg, 3.6 μmol). Turnover numbers (TONs)
b
Electrochemical Properties. In order to get a deeper
understanding of the catalytic properties of Ru complex 3,
electrochemical measurements were carried out. It is important
to realize that the involvement of picoline−aqua ligand
exchange may complicate the interpretation of the electro-
chemical behavior of Ru complex 3. The electrochemistry of Ru
complex 3 was initially assessed by cyclic voltammetry (CV)
under neutral conditions, in an aqueous phosphate buffer
solution (0.1 M, pH 7.2) and is depicted in Figure 8. A distinct
rise in the current was observed at E = 1.17 and is ascribed to
the onset of a catalytic current for H2O oxidation. In order to
obtain the potentials for the different redox couples, differential
pulse voltammetry (DPV) was subsequently measured at
neutral pH. DPV revealed three peaks at 0.35, 0.72, and 0.92
V vs NHE, which were assigned to the RuIII/RuII, RuIV/RuIII,
were calculated from moles of produced O2/moles of catalyst.
idyl-type ligand scaffolds. The TOF value of Ru complex 3
(1.16 s−1) is also higher than the TOF value reported for the
related [Ru(pdc)(pic)3] complex (0.23 s−1)47 but is signifi-
cantly lower than the TOF values reported for the [Ru(bda)-
(pic)2]-type complexes33 (H2bda = 2,2′-bipyridine-6,6′-dicar-
boxylic acid) which, however, use CeIV as the chemical oxidant.
initial rate O2 evolution = kO2[3]
(2)
Ligand Exchange. The four-electron oxidation of H2O
requires the accumulation of multiple redox equivalents at the
catalytic entity. This can be regulated by the simultaneous
transfer of electrons and protons, enabling PCET,45,46 which
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Figure 7. (Upper) Experimental high-resolution mass spectrum of the corresponding RuIII−aqua species of Ru complex 3 ([RuIII(HL)-
(pic)2(OH2)]+) in positive mode and (lower) the simulated spectrum. pic = 4-picoline.
Figure 8. Cyclic voltammograms of Ru complex 3 (blue), [Ru-
(bpy)3]Cl2 (red) in an aqueous phosphate buffer solution (0.1 M, pH
7.2) together with pristine phosphate buffer (green). Inset: enlarged
voltammogram of [Ru(bpy)3]Cl2 in the range 0.80 < E < 1.60 V.
and RuV/RuIV redox couples, respectively (Figure 9). Table 3
summarizes the electrochemical results at pH 7.2.
The obtained redox potentials can be compared to the
potentials for the previously reported, and structurally related,
single-site [Ru(pdc)(pic)3] complex 10, which bears two
carboxylate moieties instead of having the carboxylate−amide
motif as in Ru complex 3. In an aqueous phosphate buffer
solution (pH 7.0) containing 10% acetonitrile, at a scan rate of
100 mV s−1, the [RuII(pdc)(pic)3] complex displayed three
Figure 9. (Upper) Differential pulse voltammogram of Ru complex 3
irreversible peaks at ∼0.46, ∼0.84, and ∼1.05 V vs NHE.42
at pH 7.2. (Lower) Enhanced view of the differential pulse
voltammogram of Ru complex 3 at pH 7.2 in the range 0.25 < E <
1.05 V. Conditions: voltammograms were recorded in aqueous
Although the authors did not speculate on the individual redox
events, in a subsequent paper the RuIV/RuIII couple was
phosphate buffer solutions (0.1 M, pH 7.2) containing Ru complex
calculated to occur at a potential of 0.78 V vs NHE at pH 7.0.47
3 (93 μM) with a scan rate of 0.1 V s−1, using the [Ru(bpy)3]3+/
[Ru(bpy)3]2+ couple as a standard (E1/2 = 1.26 V vs NHE).
The catalytic onset potential for the [Ru(pdc)(pic)3] complex
was found to occur at 1.26 V vs NHE at pH 7.2.42 When
comparing the redox potentials for the Ru complex 3 and the
related [Ru(pdc)(pic)3] complex 10 at neutral conditions, it is
clear that the amide motif in complex 3 is a better electron
donor and hence reduces the individual redox couples and the
catalytic onset potential of Ru complex 3 (see Table 3). This
highlights that incorporation of amide units into the ligand
frameworks of WOCs could be a general strategy for lowering
the onset potential for H2O oxidation.
chemical calculations were carried out at the B3LYP* level (see
the Experimental Section for detailed information). We started
off by calculating the redox potentials for the various Ru−
picoline species (Figure 10). Interestingly, it was found that the
amide motif could potentially coordinate either via the nitrogen
or the oxygen atom, which results in the two RuII isomers
II
II
3Ru O−pic and 3Ru N−pic, respectively, where the N-coordinated
isomer (3Ru N−pic) was found to be 3.2 kcal mol−1 higher in
II
Quantum Chemical Insight. In order to further confirm
the different redox events occurring at pH 7.2, quantum
energy. The redox events are thus expected to start with the
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associated with a potential of −0.08 V and is in good
agreement with our experimental measurement, in which the
oxidation and reduction were found to be irreversible. At the
RuIII state, picoline−aqua ligand exchange is expected to take
place, as observed experimentally, and generates the corre-
sponding RuIII−aqua complex.
Table 3. Summary of the Electrochemical Data for Single-
Site Ru Complex 3 and for the Related [Ru(pdc)(pic)3]
Complex
E1/2 (V vs NHE)
Ru complex 3
[Ru(pdc)(pic)3]
a
b
c
d
The different Ru−aqua/hydroxo/oxo species for Ru complex
3 are depicted in Figure 11 together with their respective redox
potentials. Picoline−aqua ligand exchange is thus expected to
redox couple
exptl
calcd
exptl
calcd
RuIII/RuII
RuIV/RuIII
RuV/RuIV
RuVI/RuV
0.35
0.72
0.92
e
0.36
0.76
0.84
1.30
0.46
0.84
1.05
0.60
0.78
1.38
generate the RuIII−aqua complex 3Ru
([RuIII(HL)-
IIIN
IIIO
(pic)2(OH)]), where the related 3Ru species was calculated
to be 3.6 kcal mol−1 higher in energy. The optimized structures
a
Electrochemical measurements were performed in an aqueous
III
III
of the two species 3Ru N and 3Ru O are presented in Figure 12.
phosphate buffer solution (0.1 M, pH 7.2). All potentials were
obtained from DPV and are reported vs normal hydrogen electrode
(NHE). Conditions: scan rate 0.1 V s−1, glassy carbon disk as working
electrode, a platinum spiral as counter electrode, and a saturated
calomel electrode (SCE) as reference electrode. Potentials were
converted to NHE by using the [Ru(bpy)3]3+/[Ru(bpy)3]2+ couple as
As is shown in Figure 9, the following redox transition, RuIV/
RuIII, results in the formation of 3Ru N ([RuIV(HL)(pic)2(O)])
IV
and was calculated to occur at a potential of 0.76 V, which is
close to the experimentally observed potential of 0.72 V. The
subsequent RuV/RuIV process is non-proton-coupled and was
b
calculated to occur at 0.84 V, and yields 3Ru N ([RuV(HL)-
a standard (E1/2 = 1.26 V vs NHE). Redox potentials were calculated
V
c
(pic)2(O)]+). The isomeric complex, 3Ru O, was found to lie
at the B3LYP* level. Electrochemical measurements were performed
V
in an aqueous phosphate buffer solution (0.05 M, pH 7.0) containing
10.8 kcal mol−1 higher in energy. The RuVI/RuV redox potential
was also calculated and was associated with a potential of 1.30
V. The low redox potential associated with the RuVI/RuV
d
10% acetonitrile, at a scan rate of 0.1 V s−1 (see ref 42). Redox
e
potentials were calculated at the M06//B3LYP level (see ref 47). The
RuVI/RuV redox couple could not be observed, most likely due to its
high reactivity.
transition suggests that the RuV species (3Ru N) is not
V
responsible for triggering O−O bond formation. Most likely
a further oxidation (not observed) to generate a species of
higher valency is needed to bring about O−O bond formation.
It should also be noted that the RuIII/RuII transition for 3Ru
/
III
N
II
III
II
3Ru N and 3Ru O/3Ru O were calculated to take place at low
redox potentials, −0.22 and 0.04 V, respectively. These
potentials differ significantly from the experimentally observed
potential, which suggests that the RuII−aqua complex is not
present in large quantities in solution at the RuII state. An
overview of the various redox processes that are presumed to
occur at neutral conditions, leading to O2 liberation is
presented in Scheme 3.
CONCLUSIONS
■
Herein we report on the unexpected cleavage of ligand 1,
resulting in the formation of the single-site Ru complex 3
bearing carboxylate and amide moieties. Complex 3 was also
independently synthesized and characterized, in order to
confirm the structure. The crystal structure of complex 3
revealed that the Ru center is in the RuIII state because of the
strong electron-donating ability of the 6-carbamoylpicolinic
acid ligand 4. This strong donor ability allowed chemical H2O
oxidation to be carried out with the mild one-electron oxidant
[Ru(bpy)3]3+. Compared to the single-site Ru complex 3,
WOCs based on neutral nitrogen containing heterocyclic
ligands are generally not compatible with the mild [Ru-
(bpy)3]3+ oxidant, thus highlighting the importance of
incorporating anionic groups into the ligand backbone of
WOCs. The beneficial effects of incorporating amide ligand 4
was further illustrated by electrochemical measurements where
Ru complex 3 displayed significantly lower redox potentials
than WOCs based on both neutral ligand scaffolds and the
dicarboxylate ligand of the related [Ru(pdc)(pic)3] complex 10.
Quantum chemical calculations further confirmed the reduced
redox potentials associated with Ru complex 3. This work also
highlights the caution that is needed when designing WOCs in
the future due to the unforeseen conversion of the envisioned
complexes into unexpected metal complexes that are the real
catalytic species.
Figure 10. Overview of the various Ru−picoline intermediates at pH
7.2 with the formal redox states highlighted. Calculated redox
potentials are shown in parentheses and are in volts vs normal
hydrogen electrode (NHE), and the relative energies are given in kcal
mol−1.
RuII−picoline complex 3Ru O−pic, which is oxidized to the
II
corresponding RuIII complex 3Ru
at a potential of 0.36 V
III
O−pic
vs NHE (Figure 10), in accordance with the experimentally
observed redox potential (0.35 V). This redox transition was
found to be non-proton-coupled, even though the RuII complex
possess a dissociable proton at the amide moiety, and originates
from the strong electron-donating nature of the carboxylate−
III
amide motif. The 3Ru O−pic species is then expected to undergo
an O → N isomerization with a subsequent deprotonation to
IIIN−pic
yield 3Ru
(3), which is the complex that is isolated. The
IIIN−pic
N-coordinated species 3Ru
(3) was calculated to be 7.0
kcal mol−1 lower in energy than the O-coordinated species
III
II
IIIN−pic
3Ru O−pic. The reduction of 3Ru
to 3Ru N−pic was
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Figure 11. Overview of the various Ru−aqua/hydroxo/oxo intermediates at pH 7.2 with the formal redox states highlighted. Calculated redox
potentials are shown in parentheses and are in volts vs normal hydrogen electrode (NHE), and the relative energies for each redox state are given in
kcal mol−1.
was performed using the CrysAlisPro program,48 and multiscan
adsorption correction was applied. Structure was solved by direct
method. Non-hydrogen atoms were located directly from difference
Fourier maps. All the hydrogen atoms are located directly in the
calculated position based on their geometry. Final structure refine-
ments were performed with the SHELX program49 by minimizing the
sum of the squared deviation of F2 using a full matrix technique.
Electrochemistry. Electrochemical measurements were carried out
with an Autolab potentiostat with a GPES electrochemical interface
(Eco Chemie), using a glassy carbon disk (diameter 3 mm) as the
working electrode and a platinum spiral as counter electrode. The
reference electrode was a saturated calomel electrode (SCE), and the
electrolyte used was an aqueous phosphate buffer (0.1 M, pH 7.2). All
potentials are reported vs normal hydrogen electrode (NHE), using
the [Ru(bpy)3]3+/[Ru(bpy)3]2+ couple (E1/2 = 1.26 V vs NHE) as a
reference.
Oxygen Evolution Measurements. A stock solution was made
of Ru complex 3 (93 μM) in CH3CN/H2O 1:9. The catalyst solutions
used in the experiments were then prepared by diluting the stock
solution with phosphate buffer (0.1 M, pH 7.2) to the desired
concentrations. The resulting solutions were then deoxygenated by
III
O
IIIN
bubbling with N2 for at least 15 min. In a typical run, the chemical
oxidant [Ru(bpy)3](PF6)3 (3.6 mg, 3.6 μmol) was placed in the
reaction chamber and the vessel was evacuated using a TRIVAC pump
(model D 2.5E) for 10 min. A pressure of 42 mbar of He was then
introduced into the system. After a few minutes the catalyst solution
(0.50 mL) was injected into the reaction chamber. The generated
oxygen gas was then measured and recorded by MS.
Computational Details. The geometry optimizations in the
present study were performed using the density functional B3LYP50 as
implemented in the Gaussian 09 package.51 The 6-31G(d,p) basis set
was used for the C, N, O, F, H elements and the SDD52
pseudopotential for Ru. In order to obtain more accurate energies,
single-point calculations using these optimized geometries were done
employing a larger basis set, where all elements, except Ru, were
described by 6-311+G(2df,2p) at the B3LYP* (15% exact exchange)
levels.53 Solvation effects from the water solvent were calculated using
the SMD54 continuum solvation model with the larger basis set at the
B3LYP* levels. Analytic frequency calculations were carried out at the
same level of theory as the geometry optimization to obtain the Gibbs
free energy corrections and to confirm the nature of the various
Figure 12. Optimized structures of the two isomers 3Ru and 3Ru
of Ru complex 3. Distances are given in angstroms, and the spin
densities on Ru are also indicated in italic.
EXPERIMENTAL SECTION
■
1
Materials and Methods. H NMR spectra were recorded on a
400 or 500 MHz Bruker UltraShield spectrometer (13C NMR spectra
at 100 MHz). Chemical shifts (δ) are reported in ppm using residual
solvent peak [CD3OD (δ(H) = 3.31 and (δ(C) = 77.16 ppm);
DMSO-d6 (δ(H) = 2.50 and (δ(C) = 39.52 ppm)] as standard. High-
resolution mass spectrometry (HRMS) measurements were recorded
on a Bruker Daltonics microTOF spectrometer with an electrospray
ionizer. UV−vis absorption spectra were measured on a CARY 300
Bio UV−visible spectrophotometer. Compounds were centrifuged on
a Thermo centrifuge CR3i multifunction at 4000 rpm for 15 min.
Single-Crystal X-ray Diffraction. Single-crystal X-ray diffraction
data were collected from a red-color needle-like crystal on a
SuperNova diffractometer at 100 K using Mo X-ray radiation source
(λ = 0.71073 Å) in φ scan mode at Peking University. Data reduction
G
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Article
Scheme 3. Summary of the Redox Events Proposed to Occur at pH 7.2
stationary points. Unless otherwise specified, the B3LYP*-D2 energies
are reported, including Gibbs free energy corrections from B3LYP and
dispersion corrections proposed by Grimme.55
and Et3N (0.20 mL, 1.5 mmol) in MeOH (5.0 mL) was added
Ru(DMSO)4Cl2 (0.16 g, 0.33 mmol). The solution was degassed with
N2 and refluxed overnight. After 16 h, an excess of 4-picoline (0.40
mL, 4.5 mmol) was added and the resulting solution was further
refluxed for another 48 h. The reaction mixture was then cooled to
room temperature and NH4PF6 (0.099 g, 0.61 mmol) was added, and
the resulting solution was stirred for another 30 min at room
temperature. To the mixture was added deionized H2O (15 mL). The
resulting suspension was centrifuged and the precipitate was discarded.
The supernatant was concentrated in vacuo, and EtOAc/CH3CN (4:1,
2.0 mL) was added then the mixture was left to stand at room
temperature for a couple of hours. The precipitate which formed was
centrifuged to give Ru complex 3 ([Ru(HL)(pic)3](PF6)) as red
crystals (0.060 g, 29%).
To calculate the redox potentials, the absolute redox potential of the
[Ru(bpy)3]3+/[Ru(bpy)3)]2+ couple (1.26 + 4.281 V) was used as the
reference,56 which corresponds to 127.8 kcal mol−1 for one-electron
oxidation and 407.9 kcal mol−1 for proton-coupled one-electron
oxidation at pH 7.2. For the latter case, the gas-phase free energy of a
proton is −6.3 kcal mol−1 and the experimental solvation free energy
of a proton (−264.0 kcal mol−1) was used.57
58
59
Synthesis. Ru(DMSO)4Cl2 and [Ru(bpy)3](PF6)3 were
prepared according to reported procedures. 6-Carbamoylpicolinic
acid (4, H3L) and methyl 6-(chlorocarbonyl)picolinate (8) were
synthesized according to previously reported procedures.60 2,6-
Pyridinedicarboxylic acid, hydrazine hydrate (Sigma-Aldrich) and
methanol (VWR, HPLC grade) were used as received without further
purification.
Synthesis of Ru Complex 3 from Ligand 4. To a mixture of 6-
carbamoylpyridine-2-carboxylic acid (4, 0.057 g, 0.35 mmol) and Et3N
(0.50 mL, 3.5 mmol) in MeOH (10 mL) was added Ru(DMSO)4Cl2
(0.17 g, 0.35 mmol). The solution was degassed with N2 and refluxed
overnight. After 16 h, an excess of 4-picoline (1.0 mL, 10.5 mmol) was
added, and the resulting solution was further refluxed for another 48 h.
The reaction mixture was then cooled to room temperature and
NH4PF6 (0.17 g, 1.05 mmol) was added, and the resulting solution
was stirred for another 30 min at room temperature. To the mixture
was added deionized H2O (15 mL). The resulting suspension was
centrifuged and the precipitate was discarded. The supernatant was
concentrated in vacuo, and EtOAc/CH3CN (4:1, 2.0 mL) was added
then the mixture was left to stand at room temperature for a couple of
hours. The precipitate which formed was centrifuged to give Ru
6-Carbamoylpicolinic Acid (4, H3L). 1H NMR (400 MHz,
DMSO-d6): δ = 8.31 (s, 1H), 7.88−8.04 (m, 3H), 7.71 (s, 1H). 13C
NMR (100 MHz, DMSO-d6): δ = 167.80, 166.51, 55.76, 149.09,
137.67, 125.95, 121.87. HRMS-ESI: calcd for C16H14N4O6 [M +
Na+]+, 189.0271; found, 189.0267.
Synthesis of Dimethyl 6,6′-(Hydrazine-1,2-dicarbonyl)-
dipicolinate (9). Compound 9 was prepared by dissolving compound
8 (1.00 g, 5.52 mmol) in dry CH2Cl2 (20 mL) followed by addition of
N2H4·H2O (120 μL, 2.50 mmol) and Et3N (4.60 mL, 33.1 mmol).
The mixture was then stirred at room temperature for 12 h. Aqueous
saturated Na2HCO3 solution (50 mL) was then added and the mixture
was extracted with CH2Cl2 (3 × 20 mL). The organic layer was
washed with 1 M HCl (2 × 20 mL) and dried over anhydrous Na2SO4.
Removal of the solvent in vacuo yielded the pure product as a white
solid (0.760 g, 40%). 1H NMR (400 MHz, CDCl3): δ = 10.26 (s, 2H),
8.39 (dd, J = 7.6, 0.8 Hz, 2H), 8.29 (dd, J = 8.0, 1.2 Hz, 2H), 8.05 (t, J
= 8.0 Hz, 2H), 4.03 (s, 6H). 13C NMR (100 MHz, CDCl3): δ =
164.96, 160.92, 148.61, 147.17, 138.82, 128.09, 125.82, 53.13. HRMS-
ESI: calcd for C16H14N4O6 [M + Na+]+, 381.0806; found, 381.0819.
Synthesis of 6,6′-(Hydrazine-1,2-dicarbonyl)dipicolinic Acid
(1). Compound 9 (0.100g, 0.28 mmol) was dissolved in THF/H2O
(3:2, 5 mL), and LiOH (0.016g, 0.70 mmol) was added. The mixture
was refluxed for 4 h and acidified with 1 M HCl. The formed
precipitate was filtered to give the title compound as a white solid
(0.086 g, 93%). 1H NMR (400 MHz, DMSO-d6): δ = 11.29 (brs, 2H),
8.35−8.27 (m, 6H). 13C NMR (100 MHz, DMSO-d6): δ = 164.67,
162.14, 148.05, 146.26, 140.28, 127.22, 125.84. HRMS-ESI: calcd for
C16H14N4O6 [M + Na+]+, 353.0493; found, 353.0485.
−
complex 3 (isolated as a mixture of the PF6 and Cl− salt,
1
[Ru(HL)(pic)3](PF6)0.5Cl0.5) as red crystals (0.12 g, 50%). H NMR
(500 MHz, CD3OD in the presence of ascorbic acid): δ = 8.64−8.63
(m, 2H), 8.36 (dd. J = 8.00, 1.00 Hz, 1H), 8.09 (dd, J = 8.00, 1.00 Hz,
1H), 7.86 (t, J = 8.00 Hz, 1H), 7.84−7.82 (m, 4H), 7.80 (s, 1H),
7.33−7.31 (m, 2H), 7.10−7.09 (m, 4H), 2.44 (s, 3H), 2.29 (s, 6H).
−
HRMS-ESI: calcd for C25H25N5O3Ru [M − PF6 ]+, 545.1002; found,
545.1006. Anal. Calcd (%) for C28H30.5Cl0.5F3N5.5O4P0.5Ru ([Ru(HL)-
(pic)3](PF6)0.5Cl0.5·0.5EtOAc·0.5CH3CN): C, 48.09%; H, 4.40%; N,
11.02%. Found: C, 48.03%; H, 4.54%; N, 10.82%.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR, 13C NMR, high-resolution mass spectrometry, UV−
vis spectra, and X-ray crystallographic data. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/ic502755c.
Synthesis of Ru Complex 3 from Ligand 1. To a mixture of
6,6′-(hydrazine-1,2-dicarbonyl)dipicolinic acid (1, 0.050 g, 0.15 mmol)
H
DOI: 10.1021/ic502755c
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Inorganic Chemistry
Article
(25) Zhang, G.; Chen, K.; Chen, H.; Yao, J.; Shaik, S. Inorg. Chem.
2013, 52, 5088−5096.
(26) Muckerman, J. T.; Kowalczyk, M.; Badiei, Y. M.; Polyansky, D.
E.; Concepcion, J. J.; Zong, R.; Thummel, R. P.; Fujita, E. Inorg. Chem.
2014, 53, 6904−6913.
(27) Li, T.-T.; Chen, Y.; Li, F.-M.; Zhao, W.-L.; Wang, C.-J.; Lv, X.-J.;
Xu, Q.-Q.; Fu, W.-F. Chem.Eur. J. 2014, 20, 8054−8061.
(28) Wang, Y.; Ahlquist, M. S. G. Phys. Chem. Chem. Phys. 2014, 16,
11182−11185.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: markusk@organ.su.se.
*E-mail: phera@organ.su.se.
*E-mail: bjorn.akermark@organ.su.se.
Notes
The authors declare no competing financial interest.
(29) Tamaki, Y.; Vannucci, A. K.; Dares, C. J.; Binstead, R. A.; Meyer,
T. J. J. Am. Chem. Soc. 2014, 136, 6854−6857.
(30) Isobe, H.; Tanaka, K.; Shen, J.-R.; Yamaguchi, K. Inorg. Chem.
2014, 53, 3973−3984.
ACKNOWLEDGMENTS
■
Financial support from the Swedish Research Council (621-
2013-4872 and 348-2014-6070), the Knut and Alice Wallen-
berg Foundation, and the Carl Trygger Foundation is gratefully
acknowledged.
́
(31) Neudeck, S.; Maji, S.; Lopez, I.; Meyer, S.; Meyer, F.; Llobet, A.
J. Am. Chem. Soc. 2014, 136, 24−27.
(32) Badiei, Y. M.; Polyansky, D. E.; Muckerman, J. T.; Szalda, D. J.;
Haberdar, R.; Zong, R.; Thummel, R. P.; Fujita, E. Inorg. Chem. 2013,
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