Structural evolution of the Ru-bms complex to the real water oxidation catalyst of Ru-bda: The bite angle matters

Article abstract of DOI:10.1039/c9dt04693c

Ru-Based complexes have advanced the study of molecular water oxidation catalysts (WOCs) both in catalysis and mechanism. The electronic effect has always been considered as an essential factor for the catalyst properties while less attention has been focused on the bite angle effect on water oxidation catalysis. The Ru-bda ([Ru(bda)(pic)₂]; bda^2- = 2,2′-bipyridine-6,6′-dicarboxylate; pic = 4-picoline) catalyst is one of the most active WOCs and it has a largely distorted octahedral configuration with an O-Ru-O bite angle of 123°. Herein, we replaced the carboxylate (-COO^-) groups of bda^2- with two methylenesulfonate (-CH₂SO₃^-) groups and prepared a negatively charged ligand, bms^2- (2,2′-bipyridine-6,6′-dimethanesulfonate), and the Ru-bms complex [Ru(bms)(pic)₂]. The O-Ru-O bite angle changed from 123° in Ru-bda to 84° in Ru-bms, leading to a dramatic influence on the catalytic behavior. Systematic analysis of the reaction intermediates suggested that Ru-bms transformed all the way to Ru-bdavia oxidative decomposition under Ce^IV-driven water oxidation conditions.

Full text of DOI:10.1039/c9dt04693c

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Structural evolution of the Ru-bms complex to the
real water oxidation catalyst of Ru-bda: the bite
angle matters†
Cite this: DOI: 10.1039/c9dt04693c
Jing Yang, Bin Liu and Lele Duan
*
Ru-Based complexes have advanced the study of molecular water oxidation catalysts (WOCs) both in cat-
alysis and mechanism. The electronic effect has always been considered as an essential factor for the
catalyst properties while less attention has been focused on the bite angle effect on water oxidation cata-
lysis. The Ru-bda ([Ru(bda)(pic)2]; bda2− = 2,2’-bipyridine-6,6’-dicarboxylate; pic = 4-picoline) catalyst is
one of the most active WOCs and it has a largely distorted octahedral configuration with an O–Ru–O bite
angle of 123°. Herein, we replaced the carboxylate (–COO−) groups of bda2− with two methyl-
enesulfonate (–CH2SO3−) groups and prepared a negatively charged ligand, bms2− (2,2’-bipyridine-6,6’-
dimethanesulfonate), and the Ru-bms complex [Ru(bms)(pic)2]. The O–Ru–O bite angle changed from
123° in Ru-bda to 84° in Ru-bms, leading to a dramatic influence on the catalytic behavior. Systematic
analysis of the reaction intermediates suggested that Ru-bms transformed all the way to Ru-bda via oxi-
dative decomposition under CeIV-driven water oxidation conditions.
Received 10th December 2019,
Accepted 26th February 2020
DOI: 10.1039/c9dt04693c
rsc.li/dalton
metal complexes involving Ru,5,6 Mn,7,8 Fe,9–11 Cu,12–14 Co15
and Ni16 have attracted considerable attention. Among these,
Introduction
Currently, the main global energy supply is provided by fossil fuels, both dinuclear and mononuclear Ru-based WOCs with poly-
and the heavy dependence on these finite resources has caused a pyridyl ligands,3,17 such as the blue dimer,5 the Ru-Hbpp cata-
severe energy crisis and environmental pollution.1,2 Considering lyst (Hbpp− = 3,5-bis(2-pyridyl)pyrazolato ligand)18 and Ru-bnp
that water can be split into O2 and H2 by solar energy, this so-called (bnp
=
4-tert-butyl-2,6-di([1′,8′]-naphthyrid-2′-yl)pyridine),6
water-splitting process is actually one of the most promising ways have achieved the greatest success, and even a metallosupra-
to meet the energy demands of humanity. Furthermore, two half- molecular macrocycle that gathers three Ru(bda) centers has
reactions are usually involved in the water-splitting process: water exhibited remarkable catalysis.19 By utilizing a strongly elec-
oxidation (2H2O → O2 + 4H+ + 4e−) and proton reduction (4H+ + tron-donating dianionic ligand, 2,2′-bipyridine-6,6′-dicarboxy-
4e− → 2H2); the former reaction is considered as the bottleneck of late (bda2−), Sun and co-workers reported a series of highly
the process due to the transfer of four electrons and O–O bond for- active Ru-bda WOCs, with turnover frequency (TOF) numbers
mation.3 Importantly, two different pathways of O–O bond for- up to 1000 s−1 under acidic conditions with cerium(IV)
mation, water nucleophilic attack (WNA) and interaction of two ammonium nitrate (CeIV) as a sacrificial chemical oxidant.20–23
metal oxo species (I2M), have been proposed.4
Homogenous systems possess superiority over hetero- and co-workers to give Ru-bpaH2 (bpaH2 = 2,2′-bipyridine-
geneous systems of bridging a rational designed molecular 6,6′-diphosphonate, O–RuIII–O: 112°) and Ru-bpHc (bpHc2−
Later on, phosphate groups were introduced by Concepcion
2−
=
structure with a well-defined reaction mechanism, which, in 6′-phosphono-[2,2′-bipyridine]-6-carboxylate, O–RuIII–O: 117°)
turn, affords catalyst optimization and innovation. In recent with the latter having a TOF of 100 s−1 (Fig. 1).24,25 The Llobet
years, water oxidation catalysts (WOCs) based on transition group developed the Ru-tda (tda2− = [2,2′:6′,2″-terpyridine]-
6,6″-dicarboxylate, O–RuIII–N: 142°) complex with one dan-
gling carboxylate group as an internal base, and this catalyst
displayed high electrochemical water oxidation efficiency
under neutral conditions, during which the tda2− ligand facili-
tates the formation of seven-coordinate Ru species.26
Minor modification of the ligand environments of Ru-bda
gives rise to significant differences toward catalytic water oxi-
dation.27 The replacement of axial ligands of Ru-bda from
Department of Chemistry, Shenzhen Grubbs Institute and Guangdong Provincial Key
Laboratory of Energy Materials for Electric Power, Southern University of Science and
Technology (SUSTech), Shenzhen, 518055, P. R. China.
E-mail: duanll@sustech.edu.cn
†Electronic supplementary information (ESI) available: X-ray crystallographic
data in table. CCDC 1968200. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c9dt04693c
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reactions was purified by a water purification system. cis-[Ru
(DMSO)4Cl2] was prepared according to the literature
method.31 The ligand disodium 2,2′-bipyridine-6,6′-dimetha-
nesulfonate (Na2bms) was synthesized on the basis of the lit-
erature procedures.32
Ru-bms: Na2bms (271.8 mg, 0.7 mmol) and cis-[Ru
(DMSO)4Cl2] (338.8 mg, 0.7 mmol) were placed in a mixed
solvent of methanol (24 mL) and water (12 mL), then degassed
with N2 and refluxed over 8 h. The solvent was removed and
replaced by methanol (20 mL). An excess of 4-picoline (0.7 mL)
was added and the reflux was continued for an additional 4 h.
Solvent was then removed, and the resulting mixture was puri-
fied by column chromatography on silica gel using mixed
methanol and dichloromethane as eluents (1/20 in volume),
yielding Ru-bms as a dark red solid (221 mg, yield = 50%).
1H NMR (400 MHz, d4-methanol, Fig. S1†): δ 8.63 (d, J =
7.7 Hz, 2H), 7.99–7.78 (m, 6H), 7.40 (d, J = 7.4 Hz, 2H), 7.05 (d,
J = 6.2 Hz, 4H), 4.02 (s, 4H), 2.32 (s, 6H). High-resolution mass
(Fig. S2†): m/z+ = 631.0242 (M + H+), calcd: 631.0254 Elemental
analysis: Calcd for C24H24N4O6RuS2·2H2O: C 43.30%, H 4.24%,
Fig. 1 Molecular structures of Ru-bms synthesized in the current work
(left) and the previously synthesized complexes Ru-bda, Ru-bpaH2, Ru-
bpHc, and Ru-tda (right).
4-picoline to isoquinoline resulted in a dramatic promotion of N 8.42%; found: C 43.44%, H 4.06%, N 8.21%.
the TOF from 32 s−1 to 303 s−1 21
By changing the equatorial
.
ligand of Ru-bda from a flexible bda2− ligand to a rigid pda2−
(H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid) ligand,
CeIV-driven water oxidation changes from the binuclear to the
Physical methods
mononuclear pathway.28 Notably, the large bite angle of O– 1H NMR spectra were recorded on a 400 MHz Bruker Advance
Ru–O is essential for direct coordination of a water molecule at spectrometer. Elemental analyses were acquired with
a
the seventh position to the highly valent Ru center, which is a Thermo-quest-Flash EA 1112 apparatus. Electrochemical
crucial step for the Ru-bda catalysts to access higher oxidation measurements were performed with a CHI760 electrochemical
states of the metal center through the proton-coupled electron workstation, using a glassy carbon disk (φ = 3 mm) as the
transfer (PCET) process.29,30 Especially, the seven-coordinate working electrode, a platinum column as the counter elec-
intermediate RuIV–OH has been confirmed by X-ray crystallo- trode, and an aqueous saturated Ag/AgCl electrode as the refer-
graphy for Ru-bda with a bite angle of 123°.20 Therefore, we ence electrode. All potentials reported herein were referenced
were motivated to investigate whether changing the large O– to NHE (E(Ag/AgCl) = 210 mV vs. NHE). All the buffer solutions
Ru–O cleft of Ru-bda from 123° to nearly 90° of an ideal octa- used in the electrochemistry study are phosphate buffers. The
hedral configuration would give any insight into the structure– generated oxygen was detected by a pressure transducer
mechanism–activity relationship.
(MIK-P300) driven at 10.00 V using a power supply (HY3005B)
Given the above considerations, we designed herein a nega- plus a data acquisition module (Omega OM-DAQ-USB-2401). A
tively charged ligand, 2,2′-bipyridine-6,6′-dimethanesulfonate solution of CeIV in 0.1 M CF3SO3H (3.0 mL) was added into the
(bms2−), and reported its Ru complex [Ru(bms)(pic)2] (Ru- flask, then an aqueous solution of the catalyst (1 mM) was
bms; Fig. 1) toward water oxidation. The Ru-bms complex dis- injected into the above solution under vigorous stirring at
played a smaller O–Ru–O bite angle than the Ru-bda complex. ambient temperature (25 1 °C).
The O–Ru–O bite angle has a dramatic effect on the water oxi-
Mass spectrometry measurements to capture the Ru inter-
dation activity. Under CeIV-driven water oxidation conditions, mediates were performed using a Thermo Scientific LCQ Fleet
the Ru-bms complex is a precatalyst and undergoes multiple mass spectrometer. In an acid solution of Ru-bms (0.67 mM),
oxidative decomposition steps to form the real water oxidation 10 equivalents of CeIV was added, and the reaction solution
catalyst, the Ru-bda catalyst. The O–Ru–O bite angle indeed was then directly injected into the mass spectrometer by a con-
plays a vital role in the transition metal complex-catalyzed tinuous injection syringe. High-resolution mass spectrometry
water oxidation reaction.
measurements were performed on a Thermo Scientific Q
Exactive mass spectrometer. The single-crystal X-ray diffraction
data were collected on a Bruker Smart Apex II CCD diffract-
ometer with graphite-monochromated Mo-Kα radiation (λ =
0.71073 Å) at 296 K. The structure was solved by direct
methods using SHELXS and refined by full-matrix least-
Experimental
Chemicals and starting materials
All solvents and chemicals are commercially available and squares on the |F2| algorithm (SHELXL) using the Olex2
were used without further purification. The water used in all program.
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Results and discussion
Synthesis and characterization
We initially attempted to prepare ([2,2′-bipyridine]-6,6′-diyl)dia-
cetic acid (L1) and its Ru complex. However, due to the synthetic
challenge of this ligand, we changed our plan and prepared
Na2bms instead of the diacetic ligand. The ligand Na2bms was
prepared according to the literature method and its synthetic
procedures are shown in Scheme 1. Deprotonation of 6,6′-
dimethyl-2,2′-bipyridine followed by chlorination affords 6,6′-bis
(chloromethyl)-2,2′-bipyridine, and then this intermediate is
treated with sodium sulphite to yield the desired ligand as a dis-
odium salt.32 The synthesis of Ru-bms was achieved by the reac-
tion of Na2bms and cis-[Ru(DMSO)4Cl2] (DMSO = dimethyl sulf-
oxide) in the presence of methanol and H2O, followed by
addition of excess 4-picoline (Scheme 1). The complex was
thoroughly characterized by X-ray crystallography, 1H NMR
Fig. 2 The X-ray crystal structure of Ru-bms with thermal ellipsoids at
the 50% probability level (hydrogen atoms are omitted for clarity). The
oxygen atoms O20, O21, O22, O23, and O24 are from solvate water
molecules.
spectroscopy, elemental analysis, mass spectrometry and coordinated oxygen atoms from sulfonate group hydrogen-
electrochemistry.
bonding to the lattice water molecules with the shortest non-
The X-ray crystallography of Ru-bms is depicted in Fig. 2. bonded O⋯O separation being about 2.756 Å, typical of
There are two ruthenium molecules and five solvate water O–H⋯O hydrogen-bonding.
molecules in the asymmetric unit of the crystal lattice. The
metal center of RuII adopts an octahedral geometry and the
Ligand exchange
bms2− ligand is in a tetradentate manner occupying the equa-
Ru-bms has strong resistance against ligand exchange, which
torial position of the Ru center while two 4-picoline moieties
was examined in various deuterated solvents. The 1H NMR
are situated at the axial positions. The Ru–O bond lengths of
spectrum of Ru-bms (Fig. 3a) in D2O/d4-methanol (1/1, v/v)
Ru-bms span from 2.137 to 2.161 Å, slightly smaller than those
agrees with the C2v symmetry of its proposed structure. In the
of Ru-bda (2.172 and 2.216 Å). Notably, the O11–Ru1–O12 bite
aromatic region, three peaks at 8.64 (d, 2H), 7.97 (d, 2H), and
angle of Ru-bms is 84°, close to the ideal 90° of an octahedral
7.44 (d, 2H) ppm represent the proton resonances of bms2−
,
configuration. This angle is much smaller than that of the pre-
viously reported Ru-bda catalyst with an O–Ru–O angle of
123°. Therefore, it would be difficult for Ru-bms to form seven-
coordinate Ru intermediates during the water oxidation
process, and thus the bite angle indeed has a dramatic influ-
ence on the catalytic activity. Additionally, there are more non-
and two doublets at 7.80 (d, 4H) and 7.09 (d, 4H) ppm are
attributed to the aromatic protons of two axial 4-picoline
ligands. With the use of D2O/d3-acetonitrile or d3-acetonitrile
(Fig. 3b and c), most of these peaks showed observable upfield
shifts but without the formation of new peaks. The C2v sym-
metry remains, suggesting that the coordinated methyl-
enesulfonate group cannot be replaced by acetonitrile in the
RuII state. However, in the case of Ru-bda in mixed D2O/d3-
acetonitrile, one carboxylate of the bda2− ligand could dis-
sociate from the Ru center to result in the product of a zwitter-
ionic, acetonitrile-coordinating complex, [Ru(κ3O,N,N-bda)
(pic)2(NCCH3)].33,34 Taking into consideration that Ru-bda
adopts a strongly distorted octahedral configuration with a
Fig. 3 Aromatic region of 1H NMR spectra of Ru-bms in different
deuterated media: (a) D2O/d4-methanol (1/1, v/v), (b) D2O/d3-aceto-
nitrile (1/3, v/v) and (c) d3-acetonitrile.
Scheme 1 (a) The initial goal of ligand L1 and (b) the synthetic routes
of Na2bms and Ru-bms in this work.
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very large O–Ru–O angle, this could trigger considerable steric ated, indicating that water is accessible to the metal center at
tension between the bda2− ligand and RuII. Accordingly, this high oxidation states. In pH 7.0 phosphate buffer solutions
appears to lengthen the distance of Ru–O bonds for Ru-bda so (Fig. 4b), the oxidation potential of RuIII/II remains at 0.98 V
as to weaken the coordination bond between O and RuII. In while the second and third oxidation waves (see the DPV
contrast, the strong binding of the equatorial bms2− ligand to curve) shifted to a lower oxidation potential. A dramatic
the Ru center is probably due to the fact that Ru-bms shows a enhancement in the catalytic current is achieved above 1.3
pseudo-octahedral structure that greatly facilitates decreasing V. In comparison, Ru-bda displays clearly separated RuIII/II and
the tension and makes the complex more stable against ligand RuIV/III waves prior to the water-oxidation onset,21,36 which is
substitution.
likely due to the relatively weaker electron-donating ability of
the methylenesulfonate group than the carboxylate group.
Meyer and co-workers have reported that the added base
Electrochemistry
2−
HPO4 plays an important role in electrochemical water oxi-
dation by Ru-based WOCs.34 Accordingly, an investigation was
Cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) were used to electrochemically characterize Ru-bms in
pH 1.0 and pH 7.0 aqueous solutions. Under acidic conditions,
as exhibited in Fig. 4a, one reversible peak appeared at 0.96 V
(vs. normal hydrogen electrode, NHE), corresponding to the
redox process of RuIII/II. The DPV of Ru-bms in the range of 1.6
to 1.8 V revealed two extra oxidation peaks that are not clearly
separated from each other and could be tentatively assigned to
the oxidation process to higher valent Ru states. We propose
that the RuvO species (most likely RuVvO) is generated at
such high applied potentials because we observed the C–H
bond oxidation of Ru-bms when CeIV (E(CeIV/III) = 1.61 V vs.
NHE)35 was used as a chemical oxidant (see below). A large
current enhancement is observed above 1.6 V, indicating the
presence of catalytic oxidation of water and/or ligand. Thereby,
although Ru-bms is stable against acetonitrile coordination in
the RuII state, its high valence oxo species could still be gener-
carried out by maintaining Ru-bms at 0.2 mM, a pH value of
7.0 and an ionic strength of 0.5 M with addition of NaNO3
−
while increasing the concentration of buffer (H2PO4
+
HPO42−) from 0.01 M to 0.20 M. Significant acceleration of
catalytic current toward water oxidation upon addition of
proton acceptor bases was observed, as shown in Fig. 5. As
indicated in Fig. 6, the CV measurements in pH 7.0 buffer
reveal a linear dependence of the ratio (icat/ip)2 on HPO4 in
2−
which ip is the peak current for the RuII → RuIII wave that is
measured at Ep = 1.03 V and icat is measured at Ep,a
=
1.35 V. This is in concert with the equation
ꢀ
ꢁ
2
icat
ip
2:07
ν
¼
ðkH O þ kB½BꢀÞ with k
2− = 2.86 0.16 M−1 s−1
,
HPO4
2
as illustrated by the slope, and kH O = 0.04 s−1, as elucidated
2
from the intercept, where kH O is the rate constant for unas-
2
sisted water oxidation and kB is the contribution from the
−
proton acceptors, H2PO4 and HPO42−. Kinetic enhancements
provided by the added proton bases originate from the involve-
ment of the PCET effect with electron transfer to the electrode
occurring along with proton transfer to the added base in the
rate-limiting step.
CeIV-Driven water oxidation
The CeIV-driven water oxidation by Ru-bms was further investi-
gated using CeIV as an oxidant in acid aqueous solutions.
Fig. 4 CVs (red curve) and DPVs (blue curve) of Ru-bms (1 mM) in (a)
pH 1.0 triflic acid aqueous solution with a scanning rate of 20 mV s−1
and (b) pH 7.0 0.2 M phosphate buffer solution (I = 0.5 M (NaNO3)) with
a scanning rate of 100 mV s−1. The diameter of the glassy carbon
working electrode is 3 mm. The blank data are shown by a black line.
Fig. 5 CVs of 0.2 mM Ru-bms at pH 7.0 in H2PO4−/HPO42− buffers, and
buffer concentrations are 0.01 M (black), 0.05 M (red), 0.10 M (blue),
0.15 M (magenta), and 0.20 M (green), I = 0.5 M (NaNO3), scan rate is
20 mV s−1
.
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2−
Fig. 6 Plots of kobs vs. buffer base concentration in H2PO4−/HPO4
buffer at pH 7.0. [Ru-bms] = 0.2 mM, I = 0.5 M (NaNO3), scan rate is
20 mV s−1
.
Fig. 8 (a) Oxygen evolution curve at various concentrations of catalyst.
[CeIV] = 0.083 M, [Cat.] = 12.7–63.5 μM. (b) O2 rate versus [Cat.] based
on the plots of oxygen evolution in the interval of 550–1050 s. All
experiments were carried out in pH 1.0 triflic acid aqueous solution at
298 K with a total volume of 3 mL.
Fig. 7 Oxygen evolution curve. [CeIV] = 0.083 M, [Cat.] = 12.7 μM, and V
= 3 mL, pH 1.0 triflic acid aqueous solution.
Fig. 7 depicts the plots of oxygen evolution versus time, and
the corresponding TON of Ru-bms was calculated to be 1171
after running for 39 hours. Specifically, Ru-bms displayed a
much longer lifetime than Ru-bda (less than an hour).
However, due to its small TOF number (0.05 s−1; see below),
Ru-bms gives a moderate TON. Nevertheless, the performance
of Ru-bms is comparable with other Ru-based WOCs other
than Ru-bda under CeIV-driven water oxidation conditions.
To get kinetic information on oxygen evolution, concen-
tration-dependent catalytic investigations were carried out. It
is worth noting here that there was an initial lag phase before
O2 evolution under water oxidation conditions (Fig. 8a), indi-
cating that Ru-bms is not the real catalyst and it has to evolve
to other real WOCs. After the induction period, O2 evolved lin-
early with time, and measurements of kinetic study revealed
that the rate of oxygen evolution under catalytic conditions
shows a nearly linear dependence on the catalyst concen-
tration, providing evidence for an apparent pseudo first-order
Fig. 9 (a) MS spectra of Ru-bms (0.67 mM) with additional 10 equiva-
lents of CeIV in pH 1.0 triflic acid aqueous solutions were recorded (a)
immediately, (b) after 35 min, and (c) after subsequent treatment with 2
equivalents of ascorbic acid after 70 min (the axial ligands of complexes
are omitted for clarity).
(Fig. 9). The mass spectra were obtained at different time inter-
vals after addition of 10 equivalents of CeIV in pH 1.0 triflic
acid aqueous solutions. Two major products were observed at
m/z = 630 and 646, which well fit the structures of [RuIII(SO3-
CH2-bpy-CH2-SO3)(pic)2]+ and [RuIII(SO3-CH2-bpy-CHOH-SO3)
(pic)2]+, respectively (Fig. 9a, 10a and b). Peaks with one
additional O atom are usually assigned as oxo species, such as
kinetic rate = kO [Cat.] ([CeIV] is in large excess; Fig. 8b). The
2
TOF of Ru-bms could be defined as the first-order rate con-
stant of O2 evolution, kO , which was determined as 0.05 s−1
.
2
Capture reaction intermediates
ESI-mass spectrometry was used to capture the reaction inter- [RuV(O)(SO3-CH2-bpy-CH2-SO3)(pic)2]+, but this is inapplicable
mediates during CeIV-driven water oxidation by Ru-bms in this case (see below). When the solution was kept for
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Fig. 10 The experimental (top) and calculated (bottom) isotopic patterns
of (a) [RuIII(SO3-CH2-bpy-CH2-SO3)(pic)2]+, (b) [RuIII(SO3-CH2-bpy-
CHOH-SO3)(pic)2]+, (c) [RuIII(SO3-CH2-bpy-CO2)(pic)2]+, (d) [RuII(SO3-
CH2-bpy-CHO)(pic)2]+ and (e) [RuIII(bda)(pic)2]+.
Fig. 11 Proposed oxygen generation pathways under the catalytic con-
ditions with CeIV as oxidant at pH 1.0. The dashed lines indicate the two
active species toward water oxidation.
35 min, its MS spectrum started to show changes. Most of the
original peaks reduced in intensity spontaneously while a new
peak at m/z = 580 appeared gradually and increased in inten-
sity where the RuIII species was assigned as the six-coordinate
[RuIII(SO3-CH2-bpy-CO2)(pic)2]+ with one carboxylate arm co-
ordinated instead of the original methylenesulfonate group
(Fig. 9b and 10c). Another peak at m/z = 530 with very weak
intensity was attributed to [RuIII(bda)(pic)2]+ (Fig. 9b and 10e).
Upon subsequent addition of 2 equivalents of ascorbic acid
after 70 min (Fig. 9c), the above solution exists predominantly
as a mixture of [RuII(SO3-CH2-bpy-CO2)(pic)2 + H]+ (m/z = 581)
and new species [RuII(SO3-CH2-bpy-CHO)(pic)2]+ (m/z = 565,
Fig. 10d) together with a little [RuIII(bda)(pic)2]+, whereas the
original species of [RuIII(SO3-CH2-bpy-CH2-SO3)(pic)2]+ and
[RuIII(SO3-CH2-bpy-CHOH-SO3)(pic)2]+ almost disappeared. If
the aforementioned signal at m/z = 646 is the RuV oxo species,
it should be reduced to [RuII(SO3-CH2-bpy-CH2-SO3)(pic)2 + H]+
by ascorbic acid but this signal remained. Thereby, this
species at m/z = 646 is ascribed to the C–H bond oxidized
species, [RuIII(SO3-CH2-bpy-CHOH-SO3)(pic)2]+. Additionally,
as shown in Fig. 10, all of their experimental and calculated
isotopic patterns match well with each other.
methylenesulfonate occurs to generate [RuIII(SO3-CH2-bpy-
CHOH-SO3)(pic)2]+. The oxidation of the Ru center occurs and
the RuVvO species [RuV(O)(SO3-CH2-bpy-CHOH-SO3)(pic)2]+ is
formed with a sulfonate arm uncoordinated. Intramolecular
oxidation of the C–S bond by the oxo group leads to the for-
mation of [RuIII(SO3-CH2-bpy-CHOH-O)(pic)2]+ and the
released SO3 further reacts with water to form H2SO4. The pro-
tonation and dehydration of [RuIII(SO3-CH2-bpy-CHOH-O)
(pic)2]+ give the aldehyde species [RuIII(SO3-CH2-bpy-CHO)
(pic)2]2+. Further oxidation of the aldehyde species gives the
intermediates
of
[RuIII(SO3-CH2-bpy-CO2)(pic)2]+
and
[RuIII(bda)(pic)2]+, which could be active species toward water
oxidation. Apparently, the formation of Ru-bda is slow, which
could explain the slow oxygen formation in the CeIV-driven
water oxidation. In addition, [RuIII(SO3-CH2-bpy-CO2)(pic)2]+
might also be active toward water oxidation but with low
activity, and it may catalyze the O–O bond formation via the
WNA pathway. Our hypothesis can explain the first-order kine-
tics of oxygen evolution with a low TOF value.
Conclusions
Evolution of Ru-bms
In summary, the mononuclear Ru-bms incorporating methyl-
On the basis of MS analysis together with the documented enesulfonate groups was prepared to study the bite angle effect
RuvO catalyzed C–H bond activation,37 the structural evol- on water oxidation. Decreasing the O–Ru–O bite angle reduced
ution of Ru-bms under CeIV-driven water oxidation conditions the tension in the bipyridine ring of Ru-bms and enhanced the
is proposed in Fig. 11. At pH 1.0, the first oxidation of the stability of Ru-bms against acetonitrile coordination. Upon chan-
initial [RuII]0 gives [RuIII]+; then, water nucleophilic attack on ging the O–Ru–O bite angle from 123° to 84°, Ru-bms became
[RuIII]+ by substituting a coordinated sulfonate ligand along inactive toward water oxidation and it transformed to a series of
with PCET oxidation steps affords a tentative RuVvO species Ru complexes, as evidenced by mass spectrometry experiments.
(RuIVvO is also possible but RuVvO is more common when Three intermediates, [RuIII(SO3-CH2-bpy-CHOH-SO3)(pic)2]+,
anionic ligands are used in the coordiantion sphere; we were [RuIII(SO3-CH2-bpy-CO2)(pic)2]+ and [RuIII(bda)(pic)2]+, were
not able to obtain any experimental evidence on the high detected when Ru-bms was oxidized by CeIV in acidic medium,
valent Ru species) with a dangling methylenesulfonate arm. where the latter two were proposed as the active species for water
The subsequent oxidation of the C–H bond of the dangling oxidation. This work sheds light on the bite angle effect on the
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structure–mechanism–property relationship and provides new 14 S. J. Koepke, K. M. Light, P. E. VanNatta, K. M. Wiley and
guidelines for the design of active water oxidation catalysts – the
bite angle matters.
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15 H. Y. Du, S. C. Chen, X. J. Su, L. Jiao and M. T. Zhang,
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Conflicts of interest
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17 L. P. Tong and R. P. Thummel, Chem. Sci., 2016, 7, 6591–
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There are no conflicts to declare.
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Acknowledgements
This work is supported by the National Natural Science Foundation
of China (21771098), the Science and Technology Innovation
Commission of Shenzhen Municipality (JCYJ20170817111548026),
the Guangdong Provincial Key Laboratory of Energy Materials for
Electric Power (2018B030322001) and the Shenzhen Nobel Prize
Scientists Laboratory Project (C17783101). The authors would like
to gratefully acknowledge Dr Xiaoyong Chang at SUSTech for the
X-ray crystal data refinement.
24 Y. Xie, D. W. Shaffer, A. Lewandowska-Andralojc,
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