Insights into Ru-based molecular water oxidation catalysts: Electronic and noncovalent-interaction effects on their catalytic activities

Article abstract of DOI:10.1021/ic302687d

A series of Ru-bda water oxidation catalysts [Ru(bda)L₂] (H ₂bda = 2,2′-bipyridine-6,6′-dicarboxylic acid; L = [HNEt₃][3-SO₃-pyridine], 1; 4-(EtOOC)-pyridine, 2; 4-bromopyridine, 3; pyridine, 4; 4-methoxypyridine, 5; 4-(Me₂N)- pyridine, 6; 4-[Ph(CH₂)₃]-pyridine, 7) were synthesized with electron-donating/-withdrawing groups and hydrophilic/hydrophobic groups in the axial ligands. These complexes were characterized by ¹H NMR spectroscopy, high-resolution mass spectrometry, elemental analysis, and electrochemistry. In addition, complexes 1 and 6 were further identified by single crystal X-ray crystallography, revealing a highly distorted octahedral configuration of the Ru coordination sphere. All of these complexes are highly active toward Ce^IV-driven (Ce^IV = Ce(NH₄) ₂(NO₃)₆) water oxidation with oxygen evolution rates up to 119 mols of O₂ per mole of catalyst per second. Their structure-activity relationship was investigated. Electron-withdrawing and noncovalent interactions (attraction) exhibit positive effect on the catalytic activity of Ru-bda catalysts.

Full text of DOI:10.1021/ic302687d

Article
pubs.acs.org/IC
Insights into Ru-Based Molecular Water Oxidation Catalysts:
Electronic and Noncovalent-Interaction Effects on Their Catalytic
Activities
Lele Duan,† Lei Wang,† A. Ken Inge,‡ Andreas Fischer,† Xiaodong Zou,‡ and Licheng Sun*,†,§
†Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm,
Sweden
‡Berzelii Center EXSELENT on Porous Materials and Department of Materials and Environmental Chemistry, Stockholm University,
SE-106 91 Stockholm, Sweden
§State Key Lab of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of
Technology (DUT), 116024 Dalian, China
S
* Supporting Information
ABSTRACT: A series of Ru-bda water oxidation catalysts
[Ru(bda)L2] (H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid; L
= [HNEt3][3-SO3-pyridine], 1; 4-(EtOOC)-pyridine, 2; 4-
bromopyridine, 3; pyridine, 4; 4-methoxypyridine, 5; 4-
(Me2N)-pyridine, 6; 4-[Ph(CH2)3]-pyridine, 7) were synthe-
sized with electron-donating/-withdrawing groups and hydro-
philic/hydrophobic groups in the axial ligands. These
complexes were characterized by 1H NMR spectroscopy,
high-resolution mass spectrometry, elemental analysis, and
electrochemistry. In addition, complexes 1 and 6 were further
identified by single crystal X-ray crystallography, revealing a highly distorted octahedral configuration of the Ru coordination
sphere. All of these complexes are highly active toward CeIV-driven (CeIV = Ce(NH4)2(NO3)6) water oxidation with oxygen
evolution rates up to 119 mols of O2 per mole of catalyst per second. Their structure−activity relationship was investigated.
Electron-withdrawing and noncovalent interactions (attraction) exhibit positive effect on the catalytic activity of Ru-bda catalysts.
INTRODUCTION
■
Splitting water to hydrogen and oxygen, driven by solar energy,
has long been perceived as a promising way to replace fossil
fuels. One of the obstacles preventing the application of water
splitting devices comes from the lack of efficient (high activity
and low overpotential), robust and cost-effective catalysts for
water oxidation which produces protons and electrons as
starting materials for hydrogen production. Many transition
metal complexes including Ir-,1 Ru-,2 Cu-,3 Mn-,4 Co-,5 and Fe-
complexes6 have been reported capable of catalyzing water
Figure 1. Selected Ru-bda water oxidation catalysts A−E.
oxidation.
We recently reported a family of Ru-bda (H2bda = 2,2′-
bipyridine-6,6′-dicarboxylic acid) water oxidation catalysts
mechanism is represented in Scheme 2.7b In contrast, other
mononuclear Ru water oxidation catalysts usually form six-
coordinate Ru−OH/RuO species at the Ru +IV and +V
states, and the nucleophilic attack of a water molecule on a
RuVO (or RuIVO, see ref 2u) species forms the O−O
bond (Scheme 1B).2j From the mechanistic point of view, Ru-
bda water oxidation catalysts are largely different from other
mononuclear water oxidation catalysts. It is interesting to
explore this family of catalysts to gain more insights in their
[Ru(bda)L2] (L = N-donor ligands; see Figure 1 for selected
Ru-bda water oxidation catalysts A−E) that effectively catalyze
CeIV-driven (CeIV = Ce(NH4)2(NO3)6) water oxidation, with
turnover frequencies (TOFs) up to 300 per second and
turnover numbers (TONs) up to 55400.7 Mechanistic studies
have revealed that (i) seven-coordinate ruthenium species are
involved in the catalytic cycle, as evidenced by the isolation of a
seven-coordinate RuIV dimer, and (ii) Ru-bda catalysts catalyze
water oxidation via a bimolecular coupling pathway, in which
the O−O bond formation step involves the coupling of two
RuVO species (Scheme 1A). The proposed reaction
Received: December 7, 2012
Published: June 28, 2013
© 2013 American Chemical Society
7844
dx.doi.org/10.1021/ic302687d | Inorg. Chem. 2013, 52, 7844−7852

Inorganic Chemistry
Article
synthesis and characterization of a series of mononuclear Ru
complexes Ru(bda)L2 (L = [HNEt3][3-SO3-pyridine], 1; 4-
(EtOOC)-pyridine, 2; 4-bromopyridine, 3; pyridine, 4; 4-
methoxypyridine, 5; 4-(Me2N)-pyridine, 6; 4-[Ph(CH2)3]-
pyridine, 7; Figure 2), employ their electrochemistry in
Scheme 1. Illustrations of the Intermolecular Coupling
Pathway versus the Water Nucleophilic Attack Pathway and
the 7-Coordinate RuVO versus the 6-Coordinate RuVO,
Respectively
Figure 2. Molecular structures of complexes 1−7.
Scheme 2. Proposed Water Oxidation Mechanism by Ru-bda
Catalysts with H2O as the Seventh Ligand (Ref 7b)
aqueous solutions, examine their catalytic activities toward
CeIV-driven water oxidation, and reveal the structure−activity
relationship of the Ru-bda family.
SYNTHESIS AND CHARACTERIZATION
■
Complexes 1−7 were synthesized by procedures similar to
those used for complexes A and B.7b Complexation of cis-
[Ru(dmso)4Cl2] and H2bda in the presence of triethylamine,
followed by adding excess of the free axial ligands, yielded the
corresponding mononuclear Ru-bda catalysts 1−7. All of the
nature. Our focus in this paper is to develop efficient Ru-based
water oxidation catalysts and understand the structure−activity
relationship.
We have noticed that the equatorial bda ligand is essential for
Ru-bda catalysts being efficient. When the bda ligand is
replaced by a pda ligand (H2pda = 1,10-phenanthroline-2,9-
dicarboxylic acid), the resulting [Ru(pda)L2] complexes
catalyze water oxidation via the water nucleophilic attack
pathway instead of the bimolecular coupling pathway.2p In
addition, the reactivity of Ru-bda catalysts is particularly
dependent on the axial ligands, and a minor change on the
axial ligands could have dramatic influence on the reactivity of
Ru-bda catalysts. For instance, one of the catalysts, [Ru(bda)-
(isoq)2] (A; isoq = isoquinoline), exhibits a turnover frequency
of over 300 s−1 using CeIV as oxidant while [Ru(bda)(pic)2] (B;
pic = 4-picoline) shows only a TOF of 32 s−1 under the same
reaction conditions.7b The increased TOF of the former
isoquinoline-containing catalyst was due to the intermolecular
π−π stacking of the axial ligands, which facilitates the coupling
of two RuVO species in the O−O bond formation step. The
second observation is from the comparison between [Ru(bda)-
(dmso)(IMBr)] (C; dmso = dimethyl sulfoxide and IMBr = 5-
bromo-N-methylimidazole) and [Ru(bda)(IMBr)2] (D).7d The
introduction of dmso leads to 40 fold increase in the reactivity
in comparison with complex D. The third observation comes
from [Ru(bda)(ptz)2] (E; ptz = phthalazine).7c The major
degradation of Ru-bda catalysts is the axial ligand dissociation.
Replacing two isoquinoline ligands by two phthalazine ligands
leads to conspicuous increase of the lifetime of E and its
turnover number (55400 for E versus ca. 8400 for A). These
observations motivated us to systematically study the axial
ligand effects, for instance the electronic effect and the
hydrophobic effect, with the view to improving their efficiency
toward water oxidation. In aqueous solutions, the hydrophobic
effect will help to bring molecules together and thereby favors
the second order reactions, for instance the O−O bond
formation of the Ru-bda catalysts. Herein, we report the
1
complexes were characterized by H NMR, high resolution
mass spectrometry, and elemental analysis. Because of the
strong electron donating ability of bda2−, these RuII complexes
could be oxidized to their RuIII forms by molecular oxygen in
solution, which leads to a problem in obtaining sharp signals in
1H NMR spectra. However, this problem could be solved by
adding a small amount of ascorbic acid as a reductant to the
NMR samples. The C2v symmetry of these complexes reduces
the complexity of each 1H NMR spectrum. Take complex 3 for
instance (Figure 3). Three peaks at 8.51 (dd, 2H), 8.05 (dd,
1
Figure 3. H NMR spectrum (400 MHz, methanol-d4 + dichloro-
methane-d2 with a small amount of ascorbic acid) of complex 3.
2H), and 7.88 (t, 2H) ppm represent the proton resonances of
bda2−; two doublets at 7.64 (d, 4H) and 7.37 (d, 4H) ppm are
assigned to the aromatic protons of two 4-Br-pyridine ligands.
Acetonitrile reacts readily with [Ru(bda)(pic)2] in aqueous
solutions. Complex B in either methanol-d4 or D2O/acetone-d6
1
(9:1 v/v) remains its C2v symmetry according to the H NMR
spectra (Figure 4). When D2O/CD3CN (9:1 v/v) was used as
solvents, four broad proton resonance peaks of bda2− were
observed with a ratio of 1:1:3:1, implicating the break of C2v
7845
dx.doi.org/10.1021/ic302687d | Inorg. Chem. 2013, 52, 7844−7852

Inorganic Chemistry
Article
bda)(pic)2(OH2)] (named as RuII−OH2). The sharp proton
resonance peaks of complex B in the mixed D2O/acetone-d6
(Figure 4, middle) indicate the fast structural exchange (Figure
5, bottom). However, we could not exclude the possibility that
the RuII−OH2 species is seven coordinated. Further inves-
tigation of the coordination sphere of RuII-bda complexes is in
progress. Nevertheless, oxidation of B to higher valent states
leads to stronger binding of carboxylate to the Ru cation and
the formation of seven-coordinate Ru species, as evidenced by
the isolation of the seven-coordinate RuIV species of B. It is
well-known that the carboxylate is a “hard” ligand and binds
more strongly to the high valent Ru cations than to the low
oxidation state Ru cations. Thereby, the formation of seven-
coordinate RuIV species in a κ4O,O,N,N-bda fashion is reasonable
although the binding between the RuII cation and carboxylate is
weak.
The crystal structures of complexes 1 and 6 have been
resolved and are depicted in Figure 6. The selected bond
1
Figure 4. H NMR spectra (500 MHz) of complex B in different
solutions: (top) methanol-d4, (middle) D2O/acetone-d6 (9:1 v/v), and
(bottom) D2O/CD3CN (9:1 v/v).
symmetry (Figure 4, bottom). We propose that the resulting
product is six-coordinate [Ru(κ3O,N,N-bda)(pic)2(MeCN)], one
carboxylate dissociating from the Ru center (Figure 5, top).
Figure 6. X-ray crystal structures of complexes 1 (left) and 6 (right)
with thermal ellipsoids at 50% probability (hydrogen atoms, solvated
MeOH molecules, and counterions are omitted for clarity).
Table 1. Selected Bond Lengths [Å] and Angles [deg] for
Complexes 1 and 6
Complex 1
Complex 6
N1−Ru1
N1′−Ru1
N2−Ru1
N2′−Ru1
O1−Ru1
O1′−Ru1
N1−Ru1−O1
N1−Ru1−N1′
N1′−Ru1−O1′
O1−Ru1−O1′
1.923(3)
1.923(3)
2.080(2)
2.080(2)
2.185(3)
2.185(3)
78.3(1)
N1−Ru1
N2−Ru1
N3−Ru1
N5−Ru1
O2−Ru1
O3−Ru1
N1−Ru1−O2
N2−Ru1−N1
N2−Ru1−O3
O2−Ru1−O3
1.923(4)
1.915(4)
2.095(4)
2.087(4)
2.188(4)
2.196(4)
77.72(16)
82.16(18)
77.99(16)
122.12(13)
Figure 5. Schematic illustration of the reactions between complex B
and solvents (top: acetonitrile; bottom: water).
Accordingly, the coordination between carboxylate and
ruthenium(II) is weak. The slow structural exchange of the
resulting complex makes the proton resonance peaks of bda2−
broad (Figure 4, bottom).
82.1(1)
78.3(1)
121.39(9)
Previous study on the Pourbaix diagram of complex B
suggests that dissolution of complex B in aqueous solutions
yields a Ru-bda aqua complex.7b Unfortunately, this Ru aqua
species could not be captured by the mass spectrometer, and
only the [M + H]+ species was observed, implying weak
coordination between the aqua ligand and the ruthenium(II)
lengths and angles are listed in Table 1. The Ru−N bonds fall
in the range of 1.915−2.095 Å; equatorial Ru−N bonds are
slightly shorter than the axial ones. The octahedral coordination
configuration of Ru atom is strongly distorted with the O1−
Ru1−O1′ angle of 121.39° for 1 and the corresponding O2−
Ru1−O3 angle of 122.12° for 6, which is similar to the
previously reported configuration of complex B (O2−Ru1−O3
cation. Considering the weak coordination between carboxylate
O,N,N
and RuII, we propose the aqua complex as [Ru(κ3
-
7846
dx.doi.org/10.1021/ic302687d | Inorg. Chem. 2013, 52, 7844−7852

Inorganic Chemistry
Article
angle, 122.99°).7a These angles are significantly larger than the
O−Ru−O angle (90°) in an ideal octahedron, which plays a
critical role for this type of catalysts to form seven-coordinate
Ru intermediates with water molecules.
upon addition of CF3CH2OH (Supporting Information, Figure
S2). Complex 1 in the pH 1.0 aqueous solution has a maximum
MLCT (metal-to-ligand charge-transfer) absorption at 376 nm.
This band was shifted to 370 nm in the mixed pH 1.0/
CF3CH2OH (5:1 v/v) aqueous solution. This small change did
not reflect a big structural change of complex 1. Next, proton
NMR spectra of complex 1 in various concentrations of
CF3CD2OD aqueous solutions were recorded (Supporting
Information, Figure S3). Two peaks were shifted obviously
upon addition of CF3CD2OD. However, no decomposition of
complex 1 was observed: (i) the C2v symmetry was not broken,
and (ii) no axial ligand dissociation occurred. Unfortunately, it
is not clear whether the spectral change in both UV−vis and
NMR spectra is due to the solvation effect or the coordination
of CF3CH2OH to the Ru cation.
ELECTROCHEMISTRY
■
The potential/pH diagram (Pourbaix diagram) of complex B
has been studied previously and revealed the proton transfer
sequence of B at pH 1 upon oxidation as follows: RuII−OH2 →
RuIII−OH2 → RuIV−OH → RuVO.7b Herein, complex 1
bearing two sulfonate substitutes is water-soluble and is a good
candidate for studying its potential change versus pH in
aqueous solutions. Its Pourbaix diagram is shown in Figure 7.
The oxidation of CF3CH2OH was examined by means of
electrochemistry. The cyclic voltammograms of complex 2 in
aqueous pH 1.0 solutions with various concentration of
CF3CH2OH were recorded and depicted in Supporting
Information, Figure S4. If CF3CH2OH can be easily oxidized
by RuO species of our catalyst 2 or the electrode, addition of
CF3CH2OH in the aqueous solution of complex 2 will result in
the growth of the catalytic current. As shown in Supporting
Information, Figure S4, addition of CF3CH2OH did not lead to
increase of the catalytic current, suggesting that negligible
oxidation of CF3CH2OH, if any, occurs. Contrarily, the current
decreases upon addition of CF3CH2OH, showing the inhibiting
ability of CF3CH2OH toward water oxidation.
Figure 8 (top) representatively depicts the cyclic voltammo-
gram (CV) and the differential pulse voltammogram (DPV) of
Figure 7. Pourbaix diagram of complex 1. The potential values were
obtained from its differential pulse voltammograms at various pH
(Supporting Information, Figure S1). Working electrode: pyrolytic
grahite electrode (basal plane). The RuIV/III peaks are broad shoulder
peaks in the differential pulse voltammograms and thereby the error of
the estimated E(RuIV/III) values are large.
The oxidation of RuII−OH2 involves only electron transfer at
pH < 4.8 and becomes proton-coupled at 4.8 < pH < 7.5 (slope
≈ 45 mV per pH unit). At pH > 7.5, deprotonation of RuII−
OH2 occurs, generating RuII−OH, the oxidation of which gives
RuIII−OH without proton transfer. The pKa values of RuII−
OH2 and RuIII−OH2 are 7.5 and 4.8, respectively. The
oxidation of RuIII−OH2 and RuIII−OH is coupled with one
proton transfer (slope ≈ 50 mV per pH unit). The peak of
RuV/IV could not be assigned clearly because of the overlap of
this peak with the catalytic peak. The Pourbaix diagram of 1 is
in good agreement with the early observation about complex B.
Considering the structural similarity of complexes 1−7 and B,
we assume that the redox processes observed at pH 1.0
conditions correspond to RuII−OH2 → RuIII−OH2 → RuIV−
OH → RuVO for all of these Ru-bda complexes.
Since not all of complexes 1−7 are water-soluble,
CF3CH2OH was used to increase the solubility of complexes
2−7 in the aqueous electrolytes. The introduction of
CF3CH2OH brought two issues to the electrochemistry
measurements: (i) are the Ru-bda complexes stable in the
present of CF3CH2OH? (ii) can CF3CH2OH be oxidized by a
high energy electrode, for example electrode at 1.6 V, or by
electrochemically generated Ru oxo species?
Figure 8. Top: the CV (black) and DPV (blue) of complex 3. Bottom:
normalized DPVs of complexes 1−7. Conditions: 1−6 in the mixed
CF3CH2OH/pH 1.0 aqueous solution (1:2 v/v); 7 in the mixed
CF3CH2OH/pH 1.0 aqueous solution (2:1 v/v). Oxidation peak at 0.4
V in the DPV of 7 is attributed to the impurity of the electrolyte (see
more in Supporting Information, Figure S5).
First, we tried to address the first issue by monitoring the
UV−vis spectral change of complex 1 in an aqueous solution
7847
dx.doi.org/10.1021/ic302687d | Inorg. Chem. 2013, 52, 7844−7852

Inorganic Chemistry
Article
complex 3. At the range 0.2−1.6 V, three redox waves were
observed at 0.67 V, 1.09 V, and 1.30 V, corresponding to RuIII−
OH2/RuII−OH2, RuIV−OH/RuIII−OH2, and RuVO/RuIV−
OH processes, respectively. A catalytic current started arising
after RuV was generated, implying RuV triggers water oxidation.
From the cathodic scan, a reduction peak at about −0.3 V was
observed, corresponding to the reduction of molecular O2
generated electrochemically from the anodic scan. Owing to the
strong electron-donating ability of bda2−, the oxidation
potentials of 3, especially for the oxidation steps only involving
electron transfer,2q are significantly lower than those of other
Ru water oxidation catalysts with neutral ancillary ligands. For
instance, under pH 1.0 conditions, the well-studied [Ru(tpy)-
(bpy)(OH2)]2+ (tpy = 2,2′:6′,2″-terpyridine; bpy = 2,2′-
bipyridine) complex exhibits three oxidation waves at 1.04 V
for RuIII−OH2/RuII−OH2, 1.23 V for RuIVO/RuIII−OH2,
and 1.80 V for RuVO/RuIVO,2b, and Meyer’s [Ru(tpy)-
(bpm)(OH2)]2+ (bpm = 2,2′-bipyrimidine) complex shows a
two-electron RuII−OH2/RuIVO wave at about 1.18 V and a
RuVO/RuIVO wave at 1.65 V.2i Besides Ru-bda aqua
complexes, many other transition metal complexes carrying
negatively charged ligands (R-COO−, Ph-O−, RR′N−, and
HO−/O2−) possess low oxidation potentials, and their high
oxidation states are stabilized dramatically. For example, the
blue dimer F, trans-oxoaquoruthenium(IV) complex G
(E1/2(RuIV/III) = 0.86 V vs NHE (0.62 vs SCE)) and
mononuclear MnV-oxo complex H are of the representative
complexes (Figure 9).8 Stabilization of the high valent metal
Figure 10. Linear potential sweep voltammograms (i-E curves) of
complexes 1−6 in the mixed CF3CH2OH/pH 1.0 aqueous solution
(1:2 v/v).
the lowest catalytic current. Two reasons may cause the low
efficiency of 1. (i) The hydrophilic effect of the sulfonate
substituted axial ligand reduces the entropy contribution to the
O−O bond coupling step in comparison with other hydro-
phobic axial ligands. Note that the hydrophobic/hydrophilic
effects originate from the disruption of hydrogen bonding of
water molecules by the solute. To minimize the surface tension
of water molecules surrounding the nonpolar solute molecules,
the solute molecules tend to get together and minimize the
surface area exposing to water. This is so-called hydrophobic
effect, which is mainly controlled by the entropy change. The
hydrophilic effect has an opposite effect. (ii) The stereo-
repulsion raised by sulfonate groups might slow down the
coupling step. Generally, the electron-withdrawing groups have
positive effect on the electrochemical water oxidation activity.
This is probably because electron deficient substitutes
thermodynamically destabilize the RuVO (or the cationic
radical RuIV−O• species), make the oxo species more reactive,
and eventually enhance the O−O bond formation. The trend of
electronic effect also reflects that the electron density of the Ru
center is developing from RuVO to the transition state
(RuV−O···O−RuV) of the rate determining step.
Figure 9. Molecular structures of complexes F−H.
oxo species eventually could reduce the oxidative damage of the
ancillary ligands of metal complexes while it would decrease the
oxidizing power of the metal oxo species and the reactivity
toward oxidation reactions.
To dissolve complex 7 and test its electrochemical catalytic
activity, we increased the ratio of CF3CH2OH/pH 1.0 to 2/1
(v/v). The CV of complex 7 together with that of B are
depicted in Figure 11. Note that the C−H (py−CH2−) bond of
complex 7 is stable against electrochemical oxidation under the
given conditions (Supporting Information, Figure S6).
Complex B in pH 1/CF3CH2OH (1:2 v/v) showed a smaller
catalytic current than its pH 1/CF3CH2OH (2:1 v/v)
solution.7b The decrease of the catalytic current most likely is
caused by the coordination of CF3CH2OH, which inhibits the
access of water to the catalyst. Coordination of CF3CH2OH to
Meyer’s Ru catalyst has been documented.9 Nevertheless,
under the same conditions pH 1/CF3CH2OH (1:2 v/v),
complex 7 shows a better electrochemical catalytic activity than
complex B. Since complexes 7 and B have almost identical
E(RuIII/II), the electronic effect could be ruled out as the major
contribution to the activity difference. We believe that this is
due to the influence of hydrophobic chains of complex 7, which
contributes to the entropy change and facilitates the radical
coupling step (2 RuVO → RuIV−O−O−RuIV) when two
molecules couple to each other in the aqueous solution.
Figure 8 (bottom) shows the normalized DPVs of complexes
1−7 in the mixed CF3CH2OH/pH 1.0 aqueous solutions (only
RuIII/II peaks are shown). The potentials of E1/2(RuIII−OH2/
RuII−OH2) of these complexes are apparently dependent on
the nature of axial pyridine ligands. As expected, E1/2(RuIII−
OH2/RuII−OH2), spanning from 0.47 to 0.75 V, decreases
when the axial ligand is varied from the electron withdrawing
one to the electron donating one. The difference in E1/2(RuIV−
OH/RuIII−OH2) of these complexes is within 50 mV with an
exception of complex 1 (not shown herein); the onset
potentials of catalytic curves are close to each other.
Apparently, the electronic effect is less pronounced on the
higher Ru oxidation states (+IV and +V) of Ru-bda complexes.
The linear potential sweep voltammograms of complexes 1−
6 in the mixed CF3CH2OH/pH 1.0 (1:2 v/v) solutions are
represented in Figure 10 (complex 7 is poorly soluble in above
mixed solvent). It was observed that complexes 2 and 3 with
electron deficient axial ligands gave higher catalytic current in
comparison with complexes 4, 5, and 6 with electron rich
ligands. However, the most electron deficient complex 1 raised
7848
dx.doi.org/10.1021/ic302687d | Inorg. Chem. 2013, 52, 7844−7852

Inorganic Chemistry
Article
Figure 11. CVs of complexes 7 and B in the mixed CF3CH2OH/pH
1.0 aqueous solution (2:1 v/v).
Figure 12. Plots of oxygen evolution versus time. Conditions: 3.6 mL
of solution, catalyst (8 × 10−7 mol, 2.16 × 10−4 M) and CeIV (1.18 ×
10−3 mol, 0.327 M).
WATER OXIDATION
■
As is known, water oxidation is favored at high pH conditions
while proton reduction to dihydrogen is favored at low pH
conditions. To promote the hydrogen production, it becomes
interesting to consider the catalytic activity of water oxidation
catalysts under acidic environments. Commonly, CeIV is a
widely used as a sacrificial electron acceptor in homogeneous
water oxidation study although it has to be conducted at low
pH solutions, usually pH 1.0, which is too much for practical
application. For convenience, we still use CeIV as oxidant in pH
1.0 to evaluate the catalytic ability of complexes 1−7. Generally,
a catalyst in the mixed acetonitrile/pH 1.0 was injected in an
acidic aqueous solution containing CeIV. Bubbles were observed
immediately. The oxygen evolution was recorded by monitor-
ing the pressure change of the gas phase. Although several
groups have observed significant CO2 production by using
other water oxidation catalysts, the CO2 production in our
system is negligible. It is most likely because (i) the RuVO
species of our Ru-bda catalysts are much less oxidizing than
other Ru water oxidation catalysts, as can be seen from the low
redox potentials of RuV/IV, and (ii) the rapid O−O bond
formation (2 RuVO → RuIV−O−O−RuIV) suppresses the
oxidative decomposition markedly. We thereby assume that the
pressure change in our case is attributed to oxygen generation.
Since the oxygen evolution is second order in the
concentrations of our catalysts, the high catalyst concentration
[catalyst] favors the oxygen evolution. To obtain a high oxygen
evolving rate, we chose high catalyst loading. The concen-
ances than those with electron-donating groups, such as 4, 5,
and 6. Complex 1 with two hydrophilic and bulk sulfonate
groups showed a relatively low oxygen evolution rate, which is
in agreement with its electrochemical catalytic activity.
Under above reaction conditions, CeIV was consumed totally
within one minute, and thereby the turnover numbers are
limited by the amount of CeIV. We thereby decreased the
[catalyst] to evaluate their durability. In this case, the
concentrations of catalysts and CeIV are respectively 1.20 ×
10−5 M and 0.365 M ([catalyst]/[CeIV] ≈ 3.3/100000). The
TONs are respectively 360 for 1, 4800 for 2, 4500 for 3, 580 for
4, 760 for 5, and 790 for 6 (Figure 13). Because of the solubility
issue, complex 7 was not examined.
trations of catalysts and CeIV are respectively 2.16 × 10−4
M
and 0.327 M ([catalyst]/[CeIV] ≈ 6.6/10000). The plots of
oxygen evolution versus time were represented in Figure 12.
The fastest two water oxidation catalysts are 2 and 3 with
oxygen evolution rates being 119 and 115 mols of O2 per mole
of catalyst per second, respectively. Complexes 4, 5, and 6 rank
in the middle with respective rates of 25, 25, and 14 mols of O2
per mole of catalyst per second, then followed by complex 1
(9.8 mols of O2 per mole of catalyst per second). Since the
solubility of complex 7 is very poor in water, it precipitated out
immediately when a solution of 7 was injected in the operating
system. Thereby the rate obtained for 7 (4 mols of O2 per mole
of catalyst per second) is not comparable with others.
According to the electronic effect observed in the electro-
chemistry study, it is reasonable that complexes with electron-
withdrawing groups, such as 2 and 3, exhibit better perform-
Figure 13. Plots of oxygen evolution versus time. Conditions: 3.22 mL
of solution, catalyst (4 × 10−8 mol, 1.20 × 10−5 M) and CeIV (1.18 ×
10−3 mol, 0.365 M).
Besides, we have found that strong noncovalent interactions
display more prominent effects on the catalytic activity in
comparison with the electronic effect. For example, complexes
4 and A (see ref 7b) have almost identical redox potentials of
RuII/III (E = 0.62 for 4 and 0.63 for A) but very distinct activity.
Under the same conditions, complex A exhibits a TOF of 300
per second while complex 4 gives only 25 per second. It is
proposed, in the early work, that the intermolecular π−π
stacking of the axial isoquinoline ligands of complex A
7849
dx.doi.org/10.1021/ic302687d | Inorg. Chem. 2013, 52, 7844−7852

Inorganic Chemistry
Article
Complex 1. A mixture of 2,2′-bipyridine-6,6′-dicarboxylic acid
(H2bda) (100 mg, 0.41 mmol), cis-[Ru(dmso)4Cl2] (200 mg, 0.41
mmol), and NEt3 (0.8 mL) in methanol (30 mL) was degassed with
N2 and refluxed over 3 h. An excess of 3-pyridyl sulfonic acid (0.52 g,
3.27 mmol) was added and the mixture was heated to reflux overnight.
The solvent was removed, and the solid was redissolved in about 2 mL
of methanol, which was then placed in the fume hood stationarily over
two weeks. Crystals were filtered out, washed with the mixed MeOH/
Et2O (1:1 v/v), and dried under vacuum. Complex 1·2MeOH was
obtained as dark crystals (110 mg, yield: 31%). More crystals could be
promotes the O−O bond formation step and makes A far
superior to 4. We have also reported that strong binding of the
axial ligand to the Ru center could result in a robust Ru-bda
catalyst. In other words, strong electron donating ligands could
enhance the longevity of Ru-bda catalysts. The electronic effect
on the longevity is contrary to that on the reactivity. Of course,
a good catalyst should have both high reactivity and longevity
besides the low cost and the low overpotential. Then, how to
solve this paradoxical effect? Our observation of the non-
covalent interaction effect could solve it just because the
noncovalent interactions are more effective than the electronic
effect on the reactivity (A versus 4). Thereby, to design a more
robust and active Ru-bda catalyst requires the axial ligands
more electronically rich and more “attractive” to each other.
1
obtained from the filtrate by recrystallization over weeks. H NMR
(500 MHz, D2O with a small amount of (+)-sodium L-ascorbate): δ =
8.57 (d, 2H), 8.20 (d, 2H), 8.01−7.93 (m, 8H), 7.35 (t, 2H), 3.16 (q,
12H), 1.23 (t, 18H). HRMS (ESI): m/z+ = 682.9103 (M + Na+),
calcd: 682.9098; Found: C 46.21, H 5.79, N 9.15. Calc. For
C34H46N6O10RuS2·2CH3OH: C 46.59, H 5.86, N 9.06%.
CONCLUSIONS
■
A series of Ru-bda complexes 1−7 have been prepared,
characterized, and examined as water oxidation catalysts under
typical CeIV-driven water oxidation conditions. At the RuII state,
the carboxylate (bda2−) is labile while it binds to higher valent
Ru cations more strongly. The Pourbaix diagram of complex 1
was obtained and revealed an oxidation sequence of RuII−OH2
→ RuIII−OH2 → RuIV−OH → RuVO at pH 1.0, which is in
agreement with the observations of complex B. In comparison
with neutral ligands, such as bpy and tpy, the strong electron
donating ability of bda2− and HO−/O2− make the Ru-bda aqua
complexes readily access high valent Ru states up till +V at
relatively low potentials. The electronic effect on the water
oxidation activity has been examined by installing different axial
ligands: electron-withdrawing and -donating pyridyl ligands.
Electron withdrawing groups enhance the catalytic activity of
Ru-bda water oxidation catalysts while electron donating ones
give negative effect. By comparison of complex 7 with complex
B, it is concluded that the hydrophobic effect can enhance the
activity of Ru-bda catalysts.
Complex 2. Yield: 57%. 1H NMR (500 MHz, methanol-d4
EXPERIMENTAL SECTION
■
General Procedures. 2,2′-Bipyridine-6,6′-dicarboxylic acid
(H2bda) was purchased from Jinan Henghua Sci. & Tec. Co. Ltd.,
and cis-[Ru(dmso)4Cl2] was prepared according to the literature
method.10 All other chemicals are commercially available. All solvents
1
are reagent grade and predeoxygenated. The H NMR spectra were
dichloromethane-d2 with a small amount of ascorbic acid): δ = 8.39 (d,
2H), 8.02 (d, 2H), 7.74 (t, 2H), 7.16 (d, 4H), 6.30 (d, 4H), 2.88 (s,
12H); HRMS (ESI): m/z+ = 589.1174 (M + H+), calcd: 589.1137;
Found: C 50.31, H 4.75, N 13.99. Calc. For C26H26N6O4Ru·1.6H2O:
C 50.66, H 4.77, N 13.63%.
recorded with either 400 or 500 MHz of Bruker Avance spectrometer.
High resolution mass spectrometry (HR-MS) measurements were
performed on a Q-Tof Micro mass spectrometry. Elemental analyses
were performed with a Thermoquest-Flash EA 1112 apparatus. Cyclic
voltammetric (CV) measurements were carried out with either
Autolab potentiostat with a GPES electrochemical interface (Eco
Chemie), with pyrolytic graphite electrode (either basal or edge plane)
as working electrodes, Pt wire as auxiliary, and measured versus SCE
reference electrode. The scan rate is 0.1 V/s. All potentials reported
here are converted to their corresponding value versus NHE, using an
internal reference [Ru(bpy)3]2+ (E1/2(RuIII/II) = 1.26 V versus NHE).
The oxygen evolution was recorded with a pressure transducer
(Omega PX138−030A5 V) driven at 8.00 V using a power supply
(TTi-PL601) plus a data acquisition module (Omega OMB-DAQ-
2416; running at 8 Hz for our measurements) and the final amount of
oxygen was calibrated by GC (GC-2014 Shimadzu; equipped with a
thermal conductive detector, a 5 Å molecular sieve column and with
He as carrier gas) in a separate experiment. The pressure transducer
was calibrated by injecting a certain amount of gas, and the response of
the transducer is fast and the data is reproducible. Note: for curves
related to complexes 1, 4, 5, and 6 in Figure 13, the final amount of
oxygen was not calibrated by GC. The general procedures about
oxygen measurements were reported previously.7b
Complex 7. Yield: 51%. 1H NMR (500 MHz, methanol-d4
+
CDCl3 with a small amount of (+)-sodium L-ascorbate): δ = 8.54 (d,
2H), 8.03 (d, 2H), 7.85 (t, 2H), 7.63 (d, 4H), 7.18 (t, 4H), 7.08−7.06
(m, 6H), 6.98 (d, 4H), 2.56−2.51 (m, 8H), 1.85−1.79 (m, 4H).
HRMS (ESI): m/z+ = 761.1668 (M + Na+), calcd: 761.1678; Found:
C 62.90, H 5.21, N 6.95. Calc. For C40H36N4O4Ru·1.5H2O: C 62.81,
H 5.14, N 7.33%.
Single crystals of complex 1 were obtained by recrystallization of 1
in methanol. Single crystal X-ray diffraction data were collected on an
Oxford Xcalibur 3 with molybdenum radiation (λ = 0.71073 Å). Data
reduction and absorption correction were applied with CrysAlis. The
structure (Table 2) was solved by SUPERFLIP11 in the WinGX
package.12 Hydrogen atoms were located in the Fourier difference
+
maps, except for those on the carbon atoms of the disordered HNEt3
which were geometrically positioned on parent atoms using a riding
model. All non-hydrogen atoms were refined with anisotropic
displacement parameters with SHELXL97 using a full-matrix least-
squares technique on F2.13 The data resolution for 1 was cut at 0.70 Å
7850
dx.doi.org/10.1021/ic302687d | Inorg. Chem. 2013, 52, 7844−7852

Inorganic Chemistry
Article
Table 2. Summary of the Crystal Data for Complexes 1 and 6
ACKNOWLEDGMENTS
■
This work is supported by the Swedish Research Council, the
Knut & Alice Wallenberg Foundation, the Swedish Energy
Agency, China Scholarship Council (CSC), the Basic Research
Program of China (2009CB220009), and the National Natural
Science Foundation of China (21120102036, 91233201).
1
6
formula
C36H54N6O12RuS2
928.04
C26H26N6O4Ru
587.60
formula weight
space group
C2/c (No. 15)
21.2732(7)
8.1023(2)
26.1598(5)
90.00
P1 (No. 2)
̅
a/Å
8.211(4)
11.037(8)
15.89(2)
70.23(8)
78.96(6)
77.86(5)
1314
b/Å
c/Å
REFERENCES
■
α/deg
(1) (a) McDaniel, N. D.; Coughlin, F. J.; Tinker, L. L.; Bernhard, S. J.
Am. Chem. Soc. 2008, 130, 210. (b) Hull, J. F.; Balcells, D.; Blakemore,
J. D.; Incarvito, C. D.; Eisenstein, O.; Brudvig, G. W.; Crabtree, R. H. J.
Am. Chem. Soc. 2009, 131, 8730. (c) Lalrempuia, R.; McDaniel, N. D.;
β/deg
112.677(2)
90.00
γ/deg
V/Å3
4160
Z
4
2
Muller-Bunz, H.; Bernhard, S.; Albrecht, M. Angew. Chem., Int. Ed.
̈
Dc/g cm−3
1.482
1.486
2010, 49, 9765. (d) Joya, K.; Subbaiyan, N.; DʼSouza, F.; de Groot, H.
Angew. Chem., Int. Ed. 2012, 51, 9601. (e) Petronilho, A.; Rahman, M.;
Woods, J. A.; Al-Sayyed, H.; Muller-Bunz, H.; Don MacElroy, J. M.;
Bernhard, S.; Albrecht, M. Dalton Trans. 2012, 41, 13074. (f) Schley,
N.; Blakemore, J.; Subbaiyan, N.; Incarvito, C.; DʼSouza, F.; Crabtree,
R.; Brudvig, G. J. Am. Chem. Soc. 2011, 133, 10473. (g) Blakemore, J.
D.; Schley, N. D.; Olack, G. W.; Incarvito, C. D.; Brudvig, G. W.;
Crabtree, R. H. Chem. Sci. 2011, 2, 94. (h) Blakemore, J.; Schley, N.;
Balcells, D.; Hull, J.; Olack, G.; Incarvito, C.; Eisenstein, O.; Brudvig,
G.; Crabtree, R. J. Am. Chem. Soc. 2010, 132, 16017.
T/K
293
299
F(000)
1936
600
μ(MoKR)/mm−1
refl. collected
goodness-of-fit on F2
R1 [I > 2 σ (I)] (all)
wR2 [I > 2 σ (I)] (all)
0.71073
6141
0.71073
4581
1.09
1.04
0.057
0.078
0.126
0.119
(2) (a) Wasylenko, D. J.; Ganesamoorthy, C.; Koivisto, B. D.;
Berlinguette, C. P. Eur. J. Inorg. Chem. 2010, 3135. (b) Wasylenko, D.
J.; Ganesamoorthy, C.; Henderson, M. A.; Koivisto, B. D.; Osthoff, H.
D.; Berlinguette, C. P. J. Am. Chem. Soc. 2010, 132, 16094. (c) Wada,
T.; Tsuge, K.; Tanaka, K. Angew. Chem., Int. Ed. 2000, 39, 1479.
(d) Muckerman, J. T.; Polyansky, D. E.; Wada, T.; Tanaka, K.; Fujita,
E. Inorg. Chem. 2008, 47, 1787. (e) Xu, Y.; Åkermark, T.; Gyollai, V.;
Zou, D.; Eriksson, L.; Duan, L.; Zhang, R.; Åkermark, B.; Sun, L. Inorg.
Chem. 2009, 48, 2717. (f) Chen, Z.; Concepcion, J. J.; Meyer, T. J.
Dalton Trans. 2011, 40, 3789. (g) Concepcion, J. J.; Tsai, M.-K.;
Muckerman, J. T.; Meyer, T. J. J. Am. Chem. Soc. 2010, 132, 1545.
(h) Concepcion, J. J.; Jurss, J. W.; Brennaman, M. K.; Hoertz, P. G.;
Patrocinio, A. O. T.; Murakami Iha, N. Y.; Templeton, J. L.; Meyer, T.
J. Acc. Chem. Res. 2009, 42, 1954. (i) Concepcion, J. J.; Jurss, J. W.;
Templeton, J. L.; Meyer, T. J. J. Am. Chem. Soc. 2008, 130, 16462.
(j) Romain, S.; Vigara, L.; Llobet, A. Acc. Chem. Res. 2009, 42, 1944.
(k) Mola, J.; Dinoi, C.; Sala, X.; Rodríguez, M.; Romero, I.; Parella, T.;
Fontrodona, X.; Llobet, A. Dalton Trans. 2011, 40, 3640. (l) Maji, S.;
Vigara, L.; Cottone, F.; Bozoglian, F.; Benet-Buchholz, J.; Llobet, A.
Angew. Chem., Int. Ed. 2012, 51, 5967. (m) Bernet, L.; Lalrempuia, R.;
Ghattas, W.; Mueller-Bunz, H.; Vigara, L.; Llobet, A.; Albrecht, M.
Chem. Commun. 2011, 47, 8058. (n) Duan, L.; Tong, L.; Xu, Y.; Sun,
L. Energy Environ. Sci. 2011, 4, 3296. (o) An, J.; Duan, L.; Sun, L.
Faraday Discuss. 2012, 155, 267. (p) Tong, L.; Duan, L.; Xu, Y.;
Privalov, T.; Sun, L. Angew. Chem., Int. Ed. 2011, 50, 445. (q) Tong, L.;
Wang, Y.; Duan, L.; Xu, Y.; Cheng, X.; Fischer, A.; Ahlquist, M. S. G.;
because of weak reflections at higher 2θ values. CheckCIF on complex
1 raises one level A alert. This is due to the large difference in thermal
parameters for hydrogen atoms. This is expected as the complex is well
ordered and thus hydrogen atoms of the complex have low thermal
parameters, while hydrogen atoms on the HNEt3 cations have large
thermal parameters. The large thermal parameters occur as the carbon
atoms are disordered on HNEt3 cations. As a riding model was used
for hydrogen atoms on HNEt3 cations, these hydrogen atoms have
relatively larger thermal parameters. All other hydrogen atoms were
ordered and were located in Fourier difference maps.
Single crystals of complex 6 were obtained by diffusion of an ether
layer on the top of a methanolic 6 solution. Single crystal X-ray
diffraction data were collected on a Bruker-Nonius KappaCCD with
molybdenum radiation (λ = 0.71073 Å). Data reduction was done with
EvalCCD and absorption correction with SADABS. The structure was
solved by Direct Methods (SHELXS97). Hydrogen atoms were placed
at calculated positions and refined using a riding model. All non-
hydrogen atoms were refined with anisotropic displacement
parameters with SHELXL97 using a full-matrix least-squares technique
on F2.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Differential pulse voltammograms (DPVs), UV−vis, and H
Sun, L. Inorg. Chem. 2012, 51, 3388. (r) Tong, L.; Gothelid, M.; Sun,
̈
NMR spectra of complex 1 (Figure S1−S3), cyclic voltammo-
grams of complex 2 under various [CF3CH2OH] (Figure S4),
and DPVs of complex 7 (Figure S5). This material is available
free of charge via the Internet at http://pubs.acs.org.
L. Chem. Commun. 2012, 48, 10025. (s) Kaveevivitchai, N.; Chitta, R.;
Zong, R.; El Ojaimi, M.; Thummel, R. P. J. Am. Chem. Soc. 2012, 134,
10721. (t) Kaveevivitchai, N.; Zong, R.; Tseng, H.-W.; Chitta, R.;
Thummel, R. P. Inorg. Chem. 2012, 51, 2930. (u) Polyansky, D. E.;
Muckerman, J. T.; Rochford, J.; Zong, R.; Thummel, R. P.; Fujita, E. J.
Am. Chem. Soc. 2011, 133, 14649. (v) Boyer, J. L.; Polyansky, D. E.;
Szalda, D. J.; Zong, R.; Thummel, R. P.; Fujita, E. Angew. Chem., Int.
Ed. 2011, 50, 12600. (w) Yoshida, M.; Masaoka, S.; Abe, J.; Sakai, K.
Chem.Asian. J. 2010, 5, 2369. (x) Yoshida, M.; Masaoka, S.; Sakai, K.
Chem. Lett. 2009, 38, 702. (y) Masaoka, S.; Sakai, K. Chem. Lett. 2009,
38, 182. (z) Yagi, M.; Tajima, S.; Komi, M.; Yamazaki, H. Dalton
Trans. 2011, 40, 3802. (aa) Yamazaki, H.; Hakamata, T.; Komi, M.;
Yagi, M. J. Am. Chem. Soc. 2011, 133, 8846. (ab) Xu, Y.; Fischer, A.;
Duan, L.; Tong, L.; Gabrielsson, E.; Åkermark, B.; Sun, L. Angew.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: lichengs@kth.se.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Chem., Int. Ed. 2010, 49, 8934. (ac) Karkas, M.; Åkermark, T.;
̈
̈
Notes
Johnston, E.; Karim, S.; Laine, T.; Lee, B.-L.; Åkermark, T.; Privalov,
The authors declare no competing financial interest.
T.; Åkermark, B. Angew. Chem., Int. Ed. 2012, 51, 11589.
7851
dx.doi.org/10.1021/ic302687d | Inorg. Chem. 2013, 52, 7844−7852

Inorganic Chemistry
Article
(3) (a) Barnett, S. M.; Goldberg, K. I.; Mayer, J. M. Nat. Chem. 2012,
4, 498. (b) Zhang, M.-T.; Chen, Z.; Kang, P.; Meyer, T. J. J. Am. Chem.
Soc. 2013, 135, 2048. (c) Chen, Z.; Meyer, T. J. Angew. Chem., Int. Ed.
2013, 52, 700.
(4) (a) Yagi, M.; Wolf, K. V.; Baesjou, P. J.; Bernasek, S. L.;
Dismukes, G. C. Angew. Chem., Int. Ed. 2001, 40, 2925. (b) Najafpour,
M.; Ehrenberg, T.; Wiechen, M.; Kurz, P. Angew. Chem. Int. Ed. 2010,
49, 2233. (c) Poulsen, A. K.; Rompel, A.; McKenzie, C. J. Angew.
Chem. Int. Ed. 2005, 44, 6916. (d) Kurz, P. Dalton Trans. 2009, 6103.
(e) Gao, Y.; Åkermark, T.; Liu, J.; Sun, L.; Åkermark, B. J. Am. Chem.
Soc. 2009, 131, 8726. (f) Yagi, M.; Narita, K. J. Am. Chem. Soc. 2004,
126, 8084. (g) Hocking, R. K.; Brimblecombe, R.; Chang, L.-Y.; Singh,
A.; Cheah, M. H.; Glover, C.; Casey, W. H.; Spiccia, L. Nat. Chem.
2011, 3, 461. (h) Limburg, J.; Vrettos, J. S.; Liable-Sands, L. M.;
Rheingold, A. L.; Crabtree, R. H.; Brudvig, G. W. Science 1999, 283,
1524. (i) Karlsson, E.; Lee, B.-L.; Åkermark, T.; Johnston, E.; Karkas,
̈
̈
̈
M.; Sun, J.; Hansson, O.; Backvall, J.-E.; Åkermark, B. Angew. Chem.,
̈
Int. Ed. 2011, 50, 11715.
(5) (a) Wasylenko, D. J.; Ganesamoorthy, C.; Borau-Garcia, J.;
Berlinguette, C. P. Chem. Commun. 2011, 47, 4249. (b) Dogutan, D.
K.; McGuire, R.; Nocera, D. G. J. Am. Chem. Soc. 2011, 133, 9178.
(c) Kanan, M. W.; Nocera, D. G. Science 2008, 321, 1072. (d) Yin, Q.;
Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.;
Luo, Z.; Hardcastle, K. I.; Hill, C. L. Science 2010, 328, 342. (e) Rigsby,
M. L.; Mandal, S.; Nam, W.; Spencer, L. C.; Llobet, A.; Stahl, S. S.
Chem. Sci. 2012, 3, 3058. (f) Leung, C.-F.; Ng, S.-M.; Ko, C.-C.; Man,
W.-L.; Wu, J.; Chen, L.; Lau, T.-C. Energy Environ. Sci. 2012, 5, 7903.
̀ ́
(6) (a) Fillol, J. L.; Codola, Z.; Garcia-Bosch, I.; Gomez, L.; Pla, J. J.;
Costas, M. Nat. Chem. 2011, 3, 807. (b) Ellis, W. C.; McDaniel, N. D.;
Bernhard, S.; Collins, T. J. J. Am. Chem. Soc. 2010, 132, 10990.
(c) Chen, G.; Chen, L.; Ng, S.-M.; Man, W.-L.; Lau, T.-C. Angew.
Chem., Int. Ed. 2013, 52, 1789.
(7) (a) Duan, L.; Fischer, A.; Xu, Y.; Sun, L. J. Am. Chem. Soc. 2009,
131, 10397. (b) Duan, L.; Bozoglian, F.; Mandal, S.; Stewart, B.;
Privalov, T.; Llobet, A.; Sun, L. Nat. Chem. 2012, 4, 418. (c) Duan, L.;
Araujo, C. M.; Ahlquist, M. S. G.; Sun, L. Proc. Natl. Acad. Sci. U.S.A.
2012, 109, 15584. (d) Wang, L.; Duan, L.; Stewart, B.; Pu, M.; Liu, J.;
Privalov, T.; Sun, L. J. Am. Chem. Soc. 2012, 134, 18868.
(8) (a) Gilbert, J. A.; Eggleston, D. S.; Murphy, W. R., Jr.; Geselowitz,
D. A.; Gersten, S. W.; Hodgson, D. J.; Meyer, T. J. J. Am. Chem. Soc.
1985, 107, 3855. (b) Che, C. M.; Tang, W. T.; Wong, W. T.; Lai, T. F.
J. Am. Chem. Soc. 1989, 111, 9048. (c) Collins, T. J.; Gordon-Wylie, S.
W. J. Am. Chem. Soc. 1989, 111, 4511.
(9) Chen, Z.; Concepcion, J. J.; Luo, H.; Hull, J. F.; Paul, A.; Meyer,
T. J. J. Am. Chem. Soc. 2010, 132, 17670.
̀
(10) Duliere, E.; Devillers, M.; Marchand-Brynaert, J. Organometallics
2003, 22, 804.
(11) Palatinus, L.; Chapuis, G. J. Appl. Crystallogr. 2007, 40, 786.
(12) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(13) Sheldrick, G. Acta Crystallogr., Sect. A: Found. Crystallogr. 2007,
64, 112.
7852
dx.doi.org/10.1021/ic302687d | Inorg. Chem. 2013, 52, 7844−7852