Mechanism of water oxidation by [Ru(bda)(L)2]: The return of the blue dimer

Article abstract of DOI:10.1039/c4cc07968j

We describe here a combined solution-surface-DFT calculations study for complexes of the type [Ru(bda)(L)₂] including X-ray structure of intermediates and their reactivity, as well as pH-dependent electrochemistry and spectroelectrochemistry. These studies shed light on the mechanism of water oxidation by [Ru(bda)(L)₂], revealing key features unavailable from solution studies with sacrificial oxidants. This journal is

Full text of DOI:10.1039/c4cc07968j

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Mechanism of water oxidation by [Ru(bda)(L)2]:
the return of the ‘‘blue dimer’’†
Cite this: Chem. Commun., 2015,
51, 4105
Javier J. Concepcion,*a Diane K. Zhong,‡a David J. Szalda,b James T. Muckermana
and Etsuko Fujitaa
Received 11th October 2014,
Accepted 4th February 2015
DOI: 10.1039/c4cc07968j
www.rsc.org/chemcomm
We describe here a combined solution-surface-DFT calculations study
for complexes of the type [Ru(bda)(L)2] including X-ray structure of
intermediates and their reactivity, as well as pH-dependent electro-
chemistry and spectroelectrochemistry. These studies shed light on the
mechanism of water oxidation by [Ru(bda)(L)2], revealing key features
unavailable from solution studies with sacrificial oxidants.
Chart 1 A structures of [RuII(bda)(L)2] (L = pic, isq, and P2-py). See ESI† for
synthetic details.
Natural photosynthesis has sustained life on Earth for billions
of years. Water oxidation is a key component in this essential reported by Sun and coworkers have received particular attention,
process. It provides the redox equivalents and protons that Chart 1.4a,b These robust catalysts can oxidize water with turnover
ultimately lead to accumulation of solar energy in chemical frequencies reaching values close to those needed for practical
bonds. Many potential solutions to cover our growing energy applications. According to these reports, O–O bond formation takes
demands and to reduce damage to the environment involve place by bimolecular oxyl radical coupling of two [RuVQO]+ species
solar-driven water oxidation. But this requires efficient and generated by PCET oxidation of [RuIV–OH]+.4a
robust water oxidation catalysts.
We report here a combined solution-surface-DFT calculations
The first designed, well defined molecular water oxidation study of the [Ru(bda)(L)2] systems shown in Chart 1 that includes
catalyst was the Ru ‘‘blue dimer’’ reported by Meyer and coworkers, the X-ray structure of some of the intermediates in the catalytic
[(bpy)2(OH2)RuIIIORuIII(OH2)(bpy)2]4+ where bpy is 2,20-bipyridine, cycle as well as pH-dependent electrochemistry in solution and
([(OH2)RuIIIORuIII(OH2)]4+).1a,b Unfortunately, it undergoes with the catalyst anchored to metal oxide electrodes. These
rapid deactivation due to electron-transfer-induced anation, studies reveal that the bimolecular reaction takes place between
mainly at the [(OH2)RuIIIORuIV(OH2)]5+ stage.2 This process is two [RuIV–OH]+ species rather than two [RuVQO]+ molecules,
the result of the enhanced anion affinity of electron-deficient generating a blue-dimer-like intermediate that has a high molar
RuIV and the ease of oxidation of [(OH2)RuIIIORuIV(X)]4+ com- absorptivity and appears to be the active form of the catalyst.
pared to [(OH2)RuIIIORuIV(OH2)]5+
.
Fig. 1 shows cyclic voltammograms (CVs) for [Ru(bda)(pic)2] in
Detailed studies with the Ru ‘‘blue dimer’’2 created the basis for solution and for [Ru(bda)(P2-py)2] anchored on an FTO electrode in
the discovery that one side is enough for water oxidation catalysis, 0.1 M HClO4. Two waves with equivalent peak currents are observed
and pathways involving single-site catalysts have emerged.3a–e for [Ru(bda)(pic)2] followed by a catalytic water oxidation wave. These
Recently, catalysts of the type [Ru(bda)(L)2] (bda is 2,20-bipyridine- were previously assigned as [RuIII–OH2]+/[RuII–OH2], [RuIV–OH]+/
6,60-dicarboxylic acid; L is 4-picoline, pic, or isoquinoline, isq) [RuIII–OH2]+ and [RuVQO]+/[RuIV–OH]+ by Sun and coworkers.4a Note
that [RuII–OH2] and [RuIII–OH2] are 20 and 19-electron species,
respectively, and violate the 18-electron rule. For [Ru(bda)(P2-py)2]
the CV is strikingly different. The RuIII/II couple appears at lower
potential due in part to the pH-dependence introduced on this couple
by the phosphonic acid groups. The RuIV/III couple is kinetically
inhibited and there is no catalytic water oxidation wave. Kinetic
inhibition of RuIV/III couples is common, and it is particularly notice-
able with mono- or sub mono-layer coverage of complexes on planar
a Department of Chemistry, Brookhaven National Laboratory, Upton,
NY 11973-5000, USA. E-mail: jconcepc@bnl.gov
b Department of Natural Sciences, Baruch College, CUNY, New York, New York
10010, USA
† Electronic supplementary information (ESI) available: Experimental proce-
dures, computational details and additional data. CCDC 1028031 and 1028032.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4cc07968j
‡ Present address: BASF, 500 White Plains Road, Tarrytown, NY 10591-9005, USA. electrodes.3a,5 The lack of catalysis by [Ru(bda)(P2-py)2] in Fig. 1
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no catalysis. This result supports the involvement of a bimolecular
step or steps in the catalytic cycle. On a planar surface, with
site-isolated catalyst molecules, such pathways are inaccessible
or significantly inhibited.
For [Ru(bda)(pic)2] at low pH, the first redox process is a one-
electron oxidation from six coordinate, 18-electron 1[RuII] to six
coordinate, 17-electron 2[RuIII(k4-bda)(pic)2]+, Fig. S11 (ESI†).
Based on DFT calculations, the latter is in equilibrium with
2[RuIII(k3-bda)(pic)2(OH2)]+, which oxidizes at lower potential than
2[RuIII(k4-bda)(pic)2]+ and thus dominates the electrochemistry. This
is followed by a PCET process from six-coordinate, 17-electron
2[RuIII(k3-bda)(pic)2(OH2)]+ to seven coordinate, 18-electron
1[RuIV–OH]+, up to BpH 5. Coordination expansion is enabled
by a two-electron vacancy in d4 [RuIV]2+ compared to d6 [RuII]
and d5 [RuIII]+ and by the wide N–Ru–N angle. The structure
Fig. 1 CVs for [Ru(bda)(pic)2] (black trace) in solution with a glassy carbon
electrode and [Ru(bda)(P2-py)2] (blue trace) on an FTO electrode in 0.1 M
HClO4. The currents have been normalized for n1/2 and n, respectively, and
then for the RuIII/II peak current.
compared to [Ru(bda)(pic)2] can be explained in terms of and ground state of 1[RuIV–OH]+ has been confirmed by Sun
1
available mechanistic pathways for water oxidation catalysis and coworkers by single-crystal X-ray diffraction and H-NMR
for the two complexes. [Ru(bda)(P2-py)2] is site-isolated on the spectroscopy, respectively.4b The last redox process in Fig. S11
surface and most molecules only have access to single-site O–O (ESI†) has been previously assigned as the PCET oxidation of
bond formation for water oxidation, which is thermodynami- 18-electron 1[RuIV–OH]+ to 17-electron 2[RuVQO]+.4a A different
cally uphill for this catalyst. [Ru(bda)(pic)2], on the other hand, assignment is presented here, see below. An interesting transition
has access to bimolecular pathways between two catalyst mole- takes place in the Pourbaix diagram above pH 5 that also differs
cules as proposed previously.4a,b
from a previously reported result.4a The dominant species becomes
The X-ray structure of [Ru(bda)(pic)2] has been reported by Sun 3[RuIVQO] (more stable than 1[RuIVQO] by 23.9 kcal molꢀ1 based
et al.4b There is a six coordinate, 18-electron environment around on DFT calculations) instead of 1[RuIV–OH]+ and the second redox
the Ru center. It could be argued that [RuII(k3-bda)(pic)2(OH2)] is process becomes a 1eꢀ/2H+ process with a slope of Bꢀ118 mV per
in equilibrium with [RuII(k4-bda)(pic)2], although the 1H NMR for pH unit between pH 5 and 6. Around pH 6, another interesting
[Ru(bda)(P2-py)2] in D2O ([Ru(bda)(pic)2] is not soluble in pure transition takes place. The pH-dependent oxidation of 2[RuIII–OH2]+
D2O) is symmetric, consistent with only [RuII(k4-bda)(pic)2] in to 3[RuIVQO] crosses below pH-independent oxidation of 1[RuII] to
1
3
solution, see ESI.† On the other hand, [RuIII(k3-bda)(pic)2(OH2)]+ 2[RuIII]+ and a single 2eꢀ/2H+ process takes [RuII] to [RuIVQO]
is 1.2 kcal molꢀ1 lower in energy than [RuIII(k4-bda)(pic)2]+ + H2O with the slope again becoming ꢀ59 mV per pH unit. A significant
based on DFT calculations for [Ru(bda)(pic)2] (Fig. S7, see ESI† for implication of this transition is that 2[RuIII]+ becomes unstable with
details of DFT calculations). Bulk electrolysis in 0.1 M HClO4 past respect to disproportionation to 1[RuII] and 3[RuIVQO]. Solutions of
the first oxidation wave for [Ru(bda)(pic)2] in Fig. 1 resulted in 2[RuIII]+ in 0.1 M TFA (pH o 2) are stable for minutes to hours, but
passage of B1 eq. of charge, consistent with a one-electron redox increasing the pH to 7.0 by mixing with 0.2 M, pH 7.0 phosphate
process. The resulting RuIII intermediate was isolated as the per- buffer results in a very fast reaction (presumably disproportionation
2
chlorate salt and characterized by single-crystal X-ray diffraction of [RuIII]+) followed by slower competing reactions that lead to a
as [RuIII(bda)(pic)2](ClO4), Fig. S8 (ESI†). It is also a six coordinate mixture of products. Interestingly, the spectroscopic signature of
structure, with no coordinated water molecule. Apparently, the dominant product (lmax = 688 nm with high absorptivity)
an equilibrium exists between [RuIII(k3-bda)(pic)2(OH2)]+ and closely resembles the absorption spectrum of the III–III form of
[RuIII(k4-bda)(pic)2]+ + H2O in solution but less soluble [RuIII- the blue dimer, [(OH2)RuIIIORuIII(OH2)]4+, particularly that of the
(bda)(pic)2](ClO4) crystallizes preferentially. Table S1 (ESI†) shows carboxylated analogue with 2,20-bipyridine-4,40-dicarboxylic acid
a comparison of bond distances between [RuII(bda)(pic)2] and reported by Gratzel and coworkers (lmax = 678 nm).7 The culprit
¨
[RuIII(bda)(pic)2]+. The X-ray structure of [RuIII(bda)(isq)2](ClO4), in the generation of these species appears to be RuIV, either as
3
prepared by a similar procedure, is shown in Fig. S9 (ESI†).
1[RuIV–OH]+ (opH 5) or as [RuIVQO] (4pH 5, see the spectro-
A CV of [Ru(bda)(pic)2] in pH 8.0 phosphate buffer is shown in electrochemistry section below).
Fig. S10 (ESI†). A single reversible wave is followed by a catalytic
The Pourbaix diagram for [Ru(bda)(P2-py)2] on FTO is shown in
water oxidation wave. Note that the onset potential is significantly Fig. S12 (ESI†). It is topologically similar to the one for [Ru(bda)(pic)2]
lower and the catalytic current is significantly larger compared to pH with some minor differences. The first difference is the slight
1.0. The latter is indicative of base-assisted catalysis6a–c and arises pH-dependence in the RuIII/II couple. This behavior has been
because of the involvement of proton or proton-coupled electron observed before for phosphonated [Ru(bpy)3]2+ complexes8
transfer (PCET) in the rate determining step of the catalytic cycle. and for chromophore-catalyst assemblies incorporating phos-
Fig. S10 (ESI†) also shows a CV for [Ru(bda)(P2-py)2] on FTO under phonated [Ru(bpy)3]2+-like chromophores.9a–c Two pKa values
the same conditions, see figure caption for details. The inset shows (1.0 and 4.0) are discernible for the phosphonate groups of the
an expansion of the RuV/IV and RuIV/II waves. In contrast to the P2-py ligand. The second difference is the absence of a change
homogeneous version, the ‘‘heterogeneous’’ anchored catalyst shows in slope for the highest potential wave.
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reduction to RuIIIORuIII by RuII, see below. Spectral changes
for this process are shown in Fig. S13 (ESI†). Oxidation of RuIII
to ‘‘RuIV–OH’’ (wave II in Fig. 2 and 3, 75 to 110 s) leads to unexpected
spectral changes, Fig. S14 (ESI†). A decrease in absorption at 678 nm
is accompanied by an increase in absorption at 562 nm with the
appearance of intense absorption bands at 440 and 562 nm and
isosbestic points at 331, 387 and 640 nm. These spectral changes are
not consistent with the formation of a RuIV monomeric species, see
below and Fig. S15 (ESI†), but with the formation of a RuIV–O–RuIV
dinuclear species. TD-DFT calculations predict a band at 562 nm
for the latter with similar molar absorptivity to the experimentally
observed band (Fig. S15, ESI†). The last oxidation process (wave III in
Fig. 2 and 3, 110 to 140 s) results in a decrease in the absorbance at
562 nm and disappearance of the bands at 440 and 562 nm, Fig. S16
(ESI†). It also marks the onset potential for O2 generation, as
determined by rotating ring-disc experiments, Fig. S21 (ESI†).12
These bands are recovered in the first reduction of the reverse
scan from 141 to 170 s (return wave III) as shown by the growth
of the absorbance at 562 nm in Fig. 3. Their disappearance in the
following reduction step from 172 to 216 s (return wave II) is
accompanied by a steep increase in the absorbance at 678 nm
and the growth of an intense band at 680 nm with an isosbestic
point at 615 nm, Fig. S17 (ESI†). This intense band at 680 nm is
reminiscent of the absorption band observed for [Ru(bda)(pic)2]
in solution following disproportionation of the RuIII species. It
also resembles the spectrum of the III–III form of the blue dimer,
[(OH2)RuIIIORuIII(OH2)]4+ (lmax = 678 nm).7
Finally, the last reduction step (217 to 280 s, return wave I) leads
to a decrease of the absorbance at 678 nm and recovery of the
absorbance at 382 nm and the MLCT bands of monomeric RuII
with an isosbestic point at 588 nm. Spectral changes associated
with this process are shown in Fig. S18 (ESI†). Reduction of
[(OH2)RuIIIORuIII(OH2)]4+ in the blue dimer also leads to generation
of two equivalents of monomeric RuII. Nevertheless, the behavior
shown in Fig. 3 for [Ru(bda)(P2-py)2] is completely reversible and
the dinuclear species can be regenerated upon an anodic scan.
A very similar behavior is observed for [Ru(bda)(pic)2] in
solution. Controlled potential electrolysis at 1.2 V vs. NHE (just
past the RuIV/III couple) in 0.15 M TFA results in the formation
of a species I with relatively intense absorption bands in the
visible with lmax = 445 and 550 nm after passage of B2 equivalents
of charge, Fig. S19 (ESI†). As for [Ru(bda)(P2-py)2] on nano-ITO,
these absorption bands cannot be assigned to a monomeric RuIV
species because the latter display very weak absorptions in the
visible region. Instead, these features are consistent with
the formation of a dinuclear species structurally similar to the
RuIV–O–RuIV form of the blue dimer. DFT calculations show that
these dinuclear species are accessible without significant steric
encumbrance, Fig. S15 (ESI†). To corroborate the formation of a
dinuclear RuIV–O–RuIV species, controlled potential electrolysis was
carried out at +0.97 V, a potential more negative than the RuIV/III
couple but more positive than the RuIII/II couple. Passage of
B1 equivalent of charge resulted in a color change from purple
to blue-green with growth of an intense band at 672 nm. This is
consistent with reduction of the RuIV–O–RuIV form of the dinuclear
species to the corresponding RuIII–O–RuIII form, Fig. S20 (ESI†).
Fig. 2 CVs for [Ru(bda)(P2-py)2] on nano-ITO (black trace) and on FTO
electrodes (blue trace) in 0.1 M HClO4. The current has been multiplied by
a factor of 10 for the latter.
Fig. 2 shows CVs for [Ru(bda)(P2-py)2] on high surface area
nano-ITO10 and planar FTO in 0.1 M HClO4. For the latter the
current has been multiplied by a factor of 10 to account for
differences in current due to surface coverage. For the first wave, the
only noticeable difference between the two electrodes is the peak to
peak separation between the anodic and cathodic waves. This is a
reflection of the higher conductivity of the planar electrode. For the
second and third waves, the differences between the two electrodes
are remarkable, with significant current enhancements in the higher
surface area electrode. Current enhancements for the RuIV/III wave
on nano-ITO vs. planar electrodes have been reported previously.11
To obtain a better understanding at the mechanistic level of
the electrochemical behavior of these systems, we turned to
spectroelectrochemistry on nano-ITO.10,11 Fig. 3 shows spectro-
electrochemical results for [Ru(bda)(P2-py)2] on nano-ITO in
0.1 M HClO4. The potential was scanned in the anodic direction
at 10 mV sꢀ1 from 0.2 to 1.4 V and then cathodically back to
0.2 V with simultaneous absorption spectrum measurements
every second. The pink trace shows the CV expressed as current
vs. time. Absorbance changes vs. time at 3 selected wavelengths
(382, 562 and 678 nm) are shown in black, red and blue,
respectively. The absorbance at 382 and 562 nm decreases
sharply as the RuII is oxidized to RuIII (wave I in Fig. 2 and 3, 20
to 75 s) consistent with the disappearance of MLCT absorptions
associated with RuII. Simultaneously, the absorbance at 678 nm
increases with an isosbestic point at 617 nm. This absorption
band is not due to RuIII but rather to a RuIIIORuIII species. It is
formed by disproportionation of RuIII into RuII and RuIV–OH,
followed by generation of RuIVORuIV from 2RuIV–OH and
Fig. 3 Spectroelectrochemical data for [Ru(bda)(P2-py)2] on nano-ITO in
0.1 M HClO4. See text for explanation.
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comparing solution and surface studies and the importance
of catalysis studies with anchored catalysts in a true device
configuration. In addition, the results presented here provide
important insight for new catalysts design and protocols for
incorporation of existing catalysts into solar cell devices for
optimal performance.
This work was carried out at Brookhaven National Laboratory
and supported by the U.S. Department of Energy, Office of Science,
Division of Chemical Sciences, Geosciences, & Biosciences, Office
of Basic Energy Sciences under contract DE-AC02-98CH10886.
Scheme 1 Proposed water oxidation mechanism for [Ru(bda)(L2)].
Notes and references
1 (a) C. W. Chronister, R. A. Binstead, J. Ni and T. J. Meyer, Inorg.
Chem., 1997, 36, 3814–3815; (b) S. W. Gersten, G. J. Samuels and
T. J. Meyer, J. Am. Chem. Soc., 1982, 104, 4029–4030.
2 F. Liu, J. J. Concepcion, J. W. Jurss, T. Cardolaccia, J. L. Templeton
and T. J. Meyer, Inorg. Chem., 2008, 47, 1727–1752.
The combined solution and surface studies presented here
provide new and unexpected insight into the mechanism of
water oxidation by [Ru(bda)(L)2] and point to a very different
mechanism from that previously proposed.4a These results
are consistent with one-electron oxidation of six-coordinate
[RuII(k4-bda)(L)2] to six-coordinate [RuIII(k4-bda)(L)2]+. The latter is
in equilibrium with six-coordinate [RuIII(k3-bda)(L)2(OH2)]+ which
oxidizes preferentially to seven coordinate [RuIV(k4-bda)(L)2(OH)]+.
Generation of [RuIV–OH]+ leads to a bimolecular reaction (k2 in
Scheme 1) to give the RuIV–O–RuIV form of a blue-dimer-like
species. The proposed structure is based on spectroelectro-
chemistry, pH-dependent electrochemistry and DFT calculations.
The intense absorption band at 672 nm in solution and 680 nm on
nano-ITO can only be explained by invoking a strongly-coupled
RuIIIORuIII core.2 One-electron oxidation of this dinuclear species
will result in a RuIV–O–RuV species in a PCET process. As in the case
of the blue dimer, the RuIV–O–RuV intermediate can be further
oxidized to generate the corresponding RuV–O–RuV species, pre-
sumably followed by fast O–O coupling with oxygen release. The
steps with rate constants k2 and k3 in Scheme 1 can be accelerated
by bases owing to their PCET nature and this explains the base-
assisted catalysis experimentally observed.
The two catalysts considered in this study appear to be the
precursors of blue-dimer-like intermediates that are very active
towards water oxidation catalysis. The interconversion between
mono and dinuclear species is fast and reversible with [RuIV–OH]+
or [RuIVQO] being the species involved in the bimolecular step.
The anation problems responsible for deactivation of the
catalytic activity of the blue dimer are avoided by having
carboxylate groups around the metal centers that screen the
charges of the high oxidation states and donate electron density
to electron deficient RuIII, RuIV and RuV. These studies also
highlight both the mechanistic insight that can be gained by
3 (a) Z. Chen, J. J. Concepcion, J. W. Jurss and T. J. Meyer, J. Am.
Chem. Soc., 2009, 131, 15580–15581; (b) J. J. Concepcion, J. W. Jurss,
J. L. Templeton and T. J. Meyer, J. Am. Chem. Soc., 2008, 130,
16462–16463; (c) J. J. Concepcion, M.-K. Tsai, J. T. Muckerman and
T. J. Meyer, J. Am. Chem. Soc., 2010, 132, 1545–1557; (d) J. F. Hull,
D. Balcells, J. D. Blakemore, C. D. Incarvito, O. Eisenstein, G. W. Brudvig
and R. H. Crabtree, J. Am. Chem. Soc., 2009, 131, 8730–8731; (e) H.-W.
Tseng, R. Zong, J. T. Muckerman and R. Thummel, Inorg. Chem., 2008,
47, 11763–11773.
4 (a) L. Duan, F. Bozoglian, S. Mandal, B. Stewart, T. Privalov, A. Llobet
and L. Sun, Nat. Chem., 2012, 4, 418–423; (b) L. Duan, A. Fischer,
Y. Xu and L. Sun, J. Am. Chem. Soc., 2009, 131, 10397–10399.
5 Z. Chen, A. K. Vannucci, J. J. Concepcion, J. W. Jurss and T. J. Meyer,
Proc. Natl. Acad. Sci. U. S. A., 2011, 108, E1461–E1469.
6 (a) Z. Chen, J. J. Concepcion, X. Hu, W. Yang, P. G. Hoertz and
T. J. Meyer, Proc. Natl. Acad. Sci. U. S. A., 2010, 107, 7225–7229;
(b) Y. M. Badiei, D. E. Polyansky, J. T. Muckerman, D. J. Szalda,
R. Haberdar, R. Zong, R. P. Thummel and E. Fujita, Inorg. Chem.,
2013, 52, 8845–8850; (c) D. E. Polyansky, J. T. Muckerman,
J. Rochford, R. Zong, R. P. Thummel and E. Fujita, J. Am. Chem.
Soc., 2011, 133, 14649–14665.
7 P. Comte, M. K. Nazeeruddin, F. P. Rotzinger, A. J. Frank and
¨
M. Gratzel, J. Mol. Catal., 1989, 52, 63–84.
8 M. R. Norris, J. J. Concepcion, C. R. K. Glasson, Z. Fang, A. M.
Lapides, D. L. Ashford, J. L. Templeton and T. J. Meyer, Inorg. Chem.,
2013, 52, 12492–12501.
9 (a) D. L. Ashford, W. Song, J. J. Concepcion, C. R. K. Glasson, M. K.
Brennaman, M. R. Norris, Z. Fang, J. L. Templeton and T. J. Meyer,
J. Am. Chem. Soc., 2012, 134, 19189–19198; (b) J. J. Concepcion,
J. W. Jurss, P. G. Hoertz and T. J. Meyer, Angew. Chem., Int. Ed., 2009,
48, 9473–9476; (c) M. R. Norris, J. J. Concepcion, Z. Fang,
J. L. Templeton and T. J. Meyer, Angew. Chem., Int. Ed., 2013, 52,
13580–13583.
10 P. G. Hoertz, Z. Chen, C. A. Kent and T. J. Meyer, Inorg. Chem., 2010,
49, 8179–8181.
11 Z. Chen, J. J. Concepcion, J. F. Hull, P. G. Hoertz and T. J. Meyer,
Dalton Trans., 2010, 39, 6950–6952.
12 J. J. Concepcion, R. A. Binstead, L. Alibabaei and T. J. Meyer,
Inorg. Chem., 2013, 52, 10744.
4108 | Chem. Commun., 2015, 51, 4105--4108
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