{[Ru(bda)]:XLy}n cross-linked coordination polymers: Toward efficient heterogeneous catalysis for water oxidation in an organic solvent-free system

Article abstract of DOI:10.1039/c7nj04330a

Recently, the development of polymeric catalysts for water splitting has received an increasing amount of attention. In this study, we successfully developed a few novel cross-linked coordination polymers (CCPs) with the formula {[Ru(bda)]_xL_y}_n as efficient heterogeneous catalysts for water oxidation in an organic solvent-free system, where Ru(bda) represents the catalytic center. Detailed water oxidation catalytic kinetics studies suggested that single-site water nucleophilic attack (WNA) is the general mechanism applied to these polymeric catalysts, which is different to the small-molecular reference, [Ru(bda)(pic)₂] (pic = 4-picoline). The experimental evidence also indicated the importance of interfacial wettability and the existence of the Ru(bda)-macrocyclic fragments in the polymer network in determining the overall catalytic activity. More interestingly, end-capping modification via the pyridine/DMSO exchange reaction further removed the residual Ru(DMSO)_x moieties on the surfaces of the polymer network, which leads to the improved performance with an impressive TOF of ～4.6 s^-1 and TON of ～750 in an organic solvent-free system, which are superior to [Ru(bda)(pic)₂]. The rate of catalysis is among the highest for a heterogeneous system reported to date. An electrochemical study showed the polymeric catalysts were also promising electrode materials for electrocatalytic water oxidation and an electrode based on CCP/Nafion/ITO maintained ～73% of its initial activity after 27 cycles under the optimal conditions.

Full text of DOI:10.1039/c7nj04330a

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Volume 40 Number
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January 2016 Pages 1–846
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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Page 1 of 27
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{
[Ru(bda)] L }
Cross-linked Coordination Polymers: Toward
x
y n
Efficient Heterogeneous Catalysis for Water Oxidation in Organic
Solvent-free System
Tao Zheng and Lianwei Li*
Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics,
University of Science and Technology of China, Hefei, Anhui 230026, China
Abstract: Development of polymeric catalysts for water splitting has receiving increasing
attentions recently. In this work, we successively developed few novel crossꢀlinked
x y n
coordination polymers (CCPs) with a formula {[Ru(bda)] L } as efficient heterogeneous
catalysts for water oxidation in organic solventꢀfree system, where Ru(bda) represents the
catalytic center. Detailed water oxidation catalytic kinetics studies suggested that the
singleꢀsite water nucleophilic attack (WNA) is a general mechanism applied to these
polymeric catalysts, different with the smallꢀmolecular reference [Ru(bda)(pic)
2
] (pic =
4ꢀpicoline). The experimental evidence also indicated the importance of interfacial wettability
and the existence of Ru(bda)ꢀmacrocyclic fragments of polymer network in determining the
overall catalytic activity. More interestingly, the endꢀcapping modification via pyridine/dmso
x
exchange reaction further removed the residual Ru(dmso) moieties on the surfaces of
ꢀ1
polymer network, which led to improved performance with impressive TOF ~ 4.6 s and
TON ~ 750 in organic solventꢀfree system, even superior to [Ru(bda)(pic) ]. The rate of
2
catalysis is among the highest heterogeneous systems reported so far. Electrochemical study
showed these polymeric catalysts were also promising electrode materials for electrocatalytic
water oxidation, and the electrode based on CCP/Nafion/ITO maintained ~ 73% of its initial
activity after 27 cycles under optimal condition.
*
To whom corresponding should be addressed. llw@mail.ustc.edu.cn

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Introduction
The catalytic water oxidation to molecular oxygen involves fourꢀelectronꢀtransfer process and
1
ꢀ3
is the bottleneck of the half reaction for water splitting. One of the most extensively studied
homogeneous water oxidation catalysts (WOCs) are based on ruthenium complexes, which
2
have been shown to exhibit high turnover numbers (TONs) and turnover frequencies (TOFs).
An important development is the discovery of highly active family [Ru(bda)(L)
2
] (bda=
2,2'ꢀBipyridineꢀ6,6'ꢀdicarboxylate, and L: donor ligand) by Sun and coꢀworkers, which was
ꢀ1
shown to be among the first to reach activities (TON > 8000 and TOF > 300 s ) comparable
ꢀ1 4
to that of the natural oxygenꢀevolving cluster (TOF ~ 100ꢀ400 s ).
In spite of excellent performance for [Ru(bda)(L) ] catalyst family, large amounts of
2
organic solvents such like acetonitrile or dimethylformamide were normally needed as
cosolvent to increase the solubility of catalyst in water, especially when the axial ligand is
4
, 5
composed of large πꢀextended aromatic ring such like isoquinoline. From a green chemistry
perspective, the utilization of nonecoꢀfriendly solvent is highly undesired. Thus, ligand
modification is generally adopted to solve the solubility issue and lower the volume ratio of
organic solvent used in catalysis. For instance, Würthner et al. recently reported a highly
efficient homogeneous system based on metallosupramolecular macrocycle [Ru(bda)(bpb)]
3
ꢀ1
(
TON ~ 7400, TOF ~ 150 s ), where triethylene glycol and protonable tertiary amines were
6
, 7
incorporated into the bridging ligands to increase the solubility.
However, sophisticated
synthesis, timeꢀcosting purification and low yield are generally unavoidable.
Heterogeneous catalysts show advantages over homogenous ones for practical applications
due to their low cost, facile preparation and ease in separation/recovery. Moreover, the
solubility issue is no longer a key consideration. Recently, the metal–organic frameworks

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(MOFs) have been applied as the heterogeneous hosts for encapsulating molecular WOCs,
3
+
+ 8ꢀ10
such as [(terpy)Mn(ꢁꢀO)
2
Mn(terpy)] and [Cp*Ir(bpy)Cl] .
Meanwhile, the linear
Unfortunately, most of
11, 12
coordination polymers were reported as heterogeneous WOCs.
these WOCs, though very promising in their approaches, showed only limited catalytic
ꢀ1
activities (TOF < 0.1 s and TON < 100). There is still an urgent demand for the development
of highly efficient heterogeneous WOCs.
In contrast, metalꢀcontaining porous polymer catalytic systems are much less explored for
water oxidation application, but they are intriguing because of their diverse synthetic
modularity, which allows their structural properties to be tailored systematically for catalytic
13
water splitting. In our previous work, we have shown that, with proper structural design and
manipulation, crossꢀlinked porous conjugated polymer materials can act as efficient
14, 15
heterogeneous photocatalysts for hydrogen evolution reaction.
We also established the
structureꢀactivity correlation and clarified the critical role of residual palladium for efficient
catalysis.
Here we report the first example of using crossꢀlinked coordination polymers (CCPs) with
x y n
a formula {[Ru(bda)] L } as heterogeneous catalysts for water oxidation in ecoꢀfriendly
water system (Fig. 1). These Ru(bda)ꢀbased polymeric catalysts are designed based on the
following rationales: 1) the 3D polymeric network suppresses the selfꢀoxidation and ligand
dissociation, and further improves the storageꢀ and reactionꢀstability of catalysts; 2) the
network porosity and protonable triazine core (CCP3) prompt the preorganization and
diffusion of water and oxidant molecules; 3) the semiconducting ligand facilitates the
electronꢀtransfer during the redox process; 4) the polymeric framework ensures the isolation
of individual Ruꢀsites for mechanistic study; 5) the insolubility of polymeric catalysts in water

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renders CCPs viable electrode materials for aqueous electrochemical oxidation.
(
a)
(
b)
Fig. 1 (a) Chemical structures of ligands L0ꢀL3 and [Ru(bda)(dmso)
2
]. (b) Chemical
structures of LCP and CCPs.
We carried out detailed kinetics studies on these polymeric catalysts in organic solventꢀfree
aqueous solution by using cerium(IV) ammonium nitrate (CAN) as chemical oxidant. The
experimental results supported the water nucleophilic attack (WNA) mechanism for these
CCP catalysts, which was attributed to the local confinement effect. Structureꢀproperty
correlation study indicated that the hydrophilic nature/interface wettability, existence of
Ru(bda)ꢀmacrocyclic fragments, and good preꢀdispersion of CCP catalysts are dominant
factors affecting their catalysis performance. It’s also found that the 3D crossꢀlinked network
is beneficial for the storageꢀ and reactionꢀstability of catalysts. More interestingly, the
postꢀmodification of polymeric catalyst via pyridine/dmso exchange reaction could remove
the residual dmso moieties and further led to ~ 6 times of enhancement in catalytic activity
ꢀ1
with impressive TOF ~ 4.6 s and TON ~ 750 in organic solventꢀfree aqueous solution. The
present study also demonstrated that these polymeric catalysts are promising electrode
materials for electrochemical water oxidation due to their insolubility in water.

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5
(
a)
CCP3
*
*
*
*
*
L3
CCP2
L2
*
*
*
*
CCP1
*
L1
*
LCP
*
*
*
L0
*
[
Ru(bda)(dmso)2]
dmso
*
*
3000
2500
2000
1500
1000
ꢀ1
Wavenumber / cm
(b)
c,d,e
CCP1
c
b
a
e
a, b
CH Cl2
d
2
c,d
CCP3
e
e
d
300 250 200 150 100 50
0
ꢀ50
σ
/ ppm
1
13
Fig. 2 (a) FTIR spectra of LCP, CCPs and the ligand precursors. (b) Solidꢀstate Hꢀ C
CP/MAS NMR spectra of CCP1 and CCP3.
Results and Discussion
Synthesis and Characterization of the CCP Heterogeneous WOCs. Three CCPs were
2
synthesized by ligandꢀexchange polycondensation reaction between [Ru(bda)(dmso) ] and
various multiꢀpyridyl monomers (L0ꢀL3) (Fig. 1). Ligands L1 and L3 own the same
planeꢀtriangle configuration but different core structure (benzene and triazine). The
incorporation of spiroꢀlinkage into L2 is in order to increase the porosity of resultant CCP
1
6
network due to its superior networkꢀsupporting effect. Ligands L0ꢀL2 were synthesized
through typical Suzukiꢀcoupling procedure (Fig. S1 and S2, see experimental part). During
the polycondensation process, the CCP products typically started to precipitate out from
solution phase after ~ 2 h of reaction (Fig. S3). After 24 h of polymerization, the insoluble

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products were collected by ultracentrifugation and further purified by successive solvent
extraction (methanol, acetone, THF and dichloromethane). It is worth noting that these CCPs
are highly insoluble in water and various organic solvents, which is the basic premise for their
heterogeneous application. Meanwhile, one linear coordination polymer (LCP) was prepared
as nonꢀporous analogue for comparison.
The resultant CCP powders typically exhibited darkꢀbrown color. FTIR spectra (Fig. 2a)
show the skeleton stretching vibration bands of benzene and pyridine rings (1450 ~ 1580
ꢀ1
ꢀ1
cm ), accompanied with the feature vibration bands of carboxylic ester (1560 ~ 1720 cm )
ꢀ1
for all these CCPs and LCP. The unique bands of triazine ring (~ 1370 and ~ 1520 cm )
ꢀ1
were observed for CCP3. The vibration peaks of S=O (dmso) located at ~ 1080 cm and ~
ꢀ1
1
410 cm show great reduction after polycondensation, indicating the high conversion of
1
3
reaction. The solid state CꢀNMR spectroscopic results further confirmed the structure as
proposed (Fig. 2b), namely, the broad peaks ranging from 110 to 160 ppm correspond to
signals of aromatic carbons of benzene and pyridine; the carbon signal located at ~ 170 ppm
(e) is attributed to carbons of triazine ring from CCP3; the signals ranging from 180 to 240
ppm are attributed from bda units. ICPꢀMS results quantitatively demonstrate the successful
incorporation of Ru(bda) units into these CCPs (Fig. 3a). The measured Ru contents were
found to be slightly lower than the theoretical values, probably due to the incompletion of the
polymerization. TGA measurements indicate that all these CCPs are thermally stable up to
150 °C (Fig. 3b).

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(
a) 25
Calculated
Measured
20
15
10
5
0
18.5
1
8.5
1
7.7
1
17.8
17.2
16.9
1
6.1
5.8
LCP
CCP1
CCP2
CCP3
b)
LCP
1.0
0.9
0.8
0.7
CCP1
CCP2
CCP3
1
00 150 200 250 300 350 400 450
T / °C
Fig. 3 (a) Measured (ICPꢀMS) and calculated mass fractions (Wt%Ru) of ruthenium element
in LCP and CCPs. (b) TGA spectra of the LCP and CCPs in nitrogen.
Morphology Study of the CCP Catalyst Particles. Morphology study combines SEM,
powderꢀXRD, DSC and gas adsorption measurements. The SEM images show the irregular
nanoparticle morphologies with primary particle sizes in the range from 50 to 200 nm for all
these CCPs (Fig. 4a), while CCP2 and CCP3 catalyst particles show much stronger trend in
interꢀparticle aggregation. The nanoscaled morphology is supposed to be beneficial to
heterogeneous catalysis. PowderꢀXRD measurements (Fig. 4b) indicate the semiꢀcrystalline
feature for CCP1 (multiple sharp diffraction peaks, 10º ~ 35º), whereas the introduction of
spiroꢀlinkages renders CCP2 amorphous. The observed broad diffraction peaks for CCP3
indicate some degree of ordered structure. However, no phase transition was observed in the
DSC scans for all these CCPs (Fig. 5) from ꢀ50 ºC to 120 ºC. The nitrogen adsorption
2
measurements (Fig. 4c) show moderate BET surface areas (SABET) of 24, 55 and 12 m /g for

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8
CCP1, CCP2 and CCP3, respectively (Figure 2c). As expected, LCP shows nonꢀporous
feature.
(
a)
60
50
40
30
20
10
0
(
b)
(c)
2
LCP, 2 m /g
2
CCP1, 24 m /g
CCP1
CCP2
CCP3
2
CCP2, 55 m /g
2
CCP3, 12 m /g
10
20
30
40
50
60
70
0.0
0.2
0.4
0.6
0.8
1.0
2θ / degree
Relative pressure
(P/P0)
Fig. 4 (a) SEM images, (b) powder XRD patterns and (c) nitrogen sorption isotherms of CCP
particles in solidꢀstate.
(a)
(b)
LCP
CCP1
Cooling
Cooling
First cycle
First cycle
Second cycle
Second cycle
Heating
Heating
ꢀ
50
0
50
100
ꢀ50
0
50
100
o
o
T / C
T / C
(c)
(d)
CCP2
CCP3
Cooling
Cooling
First cycle
First cycle
Second cycle
Second cycle
Heating
Heating
ꢀ
50
0
50
100
ꢀ50
0
50
100
o
o
T / C
T / C
Fig. 5 DSC scans of LCP and CCPs from ꢀ50 ºC to 120 ºC.

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(a)
(b) 500
LCP
Ce4+
C
Ru
56.4
2.6
1.3
4
400
O2
1
6
3
ꢀM
ꢀM
ꢀM
3
00
00
2
4Ce3+
CCP3
2
H O
100
0
2
0
50
100 150 200 250 300
Crossꢀlinked coordination polymer for
Time / s
water oxidation
750
c)
250
(
CCP1
CRu
(d)
CCP2
C
Ru
2
1
1
00
50
00
1
70.5
68.1
4.1
ꢀ
ꢀM
ꢀM
M
159.5
3.8
31.9
ꢀM
ꢀM
6
5
2
00
50
0
ꢀ
M
3
50
0
0
50 100 150 200 250 300
0
50
100 150 200 250 300
Time / s
Time / s
1
1
1
500
250
000
2
(
e)
10
CCP3
(f)
~ C1.87
~ C1.11
C
ν
ν
Ru
70.50
35.25
17.63
ꢀ
ꢀ
ꢀ
M
M
M
1
ν
ν
~ C0.92
~ C1.05
1
0
0
8
.82 ꢀM
7
5
2
50
00
50
0
CCP3
CCP1
ν
~ C0.95
0
1
LCP
CCP2
[Ru(bda)(pic)2]
0
50
100
150
200
250
300
1
2
10
10
Time / s
CRu / ꢀM
Fig. 6 (a) Schematic illustration of chemically driven water oxidation by using CCP3 as
IV
catalyst and Ce as oxidant. (b)ꢀ(e) Reaction time (t) dependence of oxygen concentration
IV
([O
2
]) by using LCP and CCPs as WOCs ([Ce ] = 100 mM, pH = 1.0). (f) Catalyst
i
concentration (CRu) dependence of initial oxygen evolution rate (v ) in logꢀscale coordinate.
Chemical Water Oxidation Using CCPs as Heterogeneous Catalysts: Kinetics Study. The
catalytic water oxidation in pure water was performed by using cerium(IV) ammonium nitrate
(CAN) as sacrificial electron acceptor (Fig. 6a and Fig. S4). The reaction kinetics was inꢀsitu
monitored with an oxygenꢀsensor, and the evolved amount of O in gas space was analyzed by
2
GC (Fig. S5). In a typical experiment, 0.5 mL of CCP water suspension was degassed and
IV
injected into O ꢀfree HNO solution (9.5 mL, pH 1.0, Ce 100 mM) in a sealed vial with
2
3
rubber septum under constant stirring. Each CCP suspension was prepared by diluting a
mother suspension (1.0 mg/mL, Fig. S4) which was previously stirred vigorously for 24 h to

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0
maximize the specific surface area of catalyst particles (Table S1). Fig. 6b-e show the
reaction timeꢀ and catalyst concentrationꢀdependent oxygen evolution curves for LCP and
CCPs. Obviously, the oxygen evolution rate increases with the molar concentration of Ru
(C
Ru), where CRu represents the total molar concentration of Ru in the final water suspension
(10 mL) used for catalysis. Control experiment by using the filtrate of CCP3 suspension as
catalyst solution showed no oxygen evolution (Fig. S6), indicating no dissolution of
ruthenium complex from polymer backbones into solution phase.
According to Henry's law, the produced molecular oxygen in solution phase is supposed to
nucleate, fuse and further diffuse into gas phase to reach the thermodynamic equilibrium.
Thus, the observed turning points from increase to decrease in the evolution curves reflect
that the O
2
diffusion rate starts to exceed the O
2
evolution rate (Fig. 6b-e). To obtain reliable
) were calculated only based on the data
kinetics results, the initial oxygen evolution rates (v
i
within the first few seconds of catalysis, and the exact values of CRu were calculated
according to the previous ICPꢀMS results (Table S1). A few quasiꢀlinear relationships
α
between v and C were observed in logꢀscale, i.e., v ~ C . The fitting scaling exponent α is
i
Ru
i
Ru
0
.92, 0.95 and 1.11 for CCP1, CCP2 and CCP3, respectively (Fig. 6f), whereas the exponent
is 1.05 for LCP and 1.87 for the rederence [Ru(bda)(pic) ] (Fig. S7).
The quasiꢀfirstꢀorder kinetics for CCPs was also confirmed by monitoring the decay of
2
IV
IV
Ce at 360 nm (Fig. S8 and S9) on the basis of the following fact, i.e., ꢀd[A]/dt ~ ꢀd[Ce ]/dt
~
vi. Based on the first 10 data points (Fig. S9), the scaling exponent α is estimated to be 1.11
and 1.05 for CCP1 and CCP3, respectively (Fig. S8), consistent with the values obtained
from oxygenꢀsensor measurements. The above experiments seem to imply that these CCPs
and LCP heterogeneous catalysts oxidize water via direct singleꢀsite water nucleophilic attack

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1
(
WNA) (Fig. S10), instead of via the interaction between two metal oxide units (I2M) for
Ru(bda)(pic) ].
Recent electrochemical study on Ru(bad)ꢀsupported electrodes by Sun et al. demonstrated
[
2
1
7
the significant influence of Ruꢀsite microenvironment on the kinetic isotope effect (KIE),
implicating the possible mechanism switch from I2M to WNA due to the interface
confinement effect; however, no other experimental evidence was provided to further support
such a structureꢀmechanism correlation. Without doubt, directly immobilizing Ru(bda)ꢀsites
into these CCP rigid polymeric framework can be an alternative to impose spatial
confinement on catalytic sites. Our observation implied that the local confinement originated
from the network rigidity induces the catalytic mechanism switch from I2M to WNA for
these polymeric catalysts.
V
Note that the intramolecular dimerization of two neighboring Ru =O species could also
lead to an apparent firstꢀorder kinetics process accompanied with a radical coupling
18, 19
mechanism.
To clarify this point, we further investigated how the initial reaction rate
depends on the oxidant concentration. The results show that the value of v for CCP1 and
i
IV
CCP3 increases almost linearly with [Ce ] under stoichiometric amounts of CAN (Fig. S11
IV
and S12). The reaction kinetics is quasiꢀfirstꢀorder on Ce concentration based on
quantitatively fitting results (Fig. S11), which is consistent with the observation by Würthner
6
,7
et al. in their macrocycleꢀsystems. To further address whether O−H bond cleavage is
involved in the rateꢀdetermining step, the kinetic isotope effect (KIE) of CCP3 was tested by
2
using D O as reaction solvent. The KIEH/D value is expected to be > 2.0 if the protonꢀcoupled
process is involved; while it is generally between 0.7 ~ 1.5 if the catalytic mechanism is
2
0
through the dimerization of two metal oxide. The measured KIEH/D value for CCP3 is 2.4

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2
2
(Fig. S13), which is much higher than the reported value (0.9) for [Ru(bda)(pic) ]
6
, 7
homogeneous catalyst, further supporting the WNA mechanism.
All the above evidences ambiguously confirm that these polymeric catalysts catalyze water
oxidation through the singleꢀsite WNA mechanism. For nonꢀporous LCP catalyst, the
observed WNA mechanism is reasonable because the mobility of active Ru sites of catalyst
particles is greatly hindered due to the insoluble characteristics and compact interchain
stacking (nonporous feature). Whereas for CCP WOCs, the features of porosity and 3D
crossꢀlinking are more likely to promote the thermal fluctuation and dimerization of
neighboring Ru centers, resulting in CCP catalysts going through I2M mechanism. This is
exactly what observed by Sun and his coworkers in their recent study on Ru(bad)ꢀbased
17
crossꢀlinked polymer film.
The film was prepared by radical polymerization of
[
Ru(bda)(4ꢀvinylpyridine) ] monomers on electrode, and the KIE study revealed that the I2M
2
mechanism was applied to the Ru(bad)ꢀbased polymer catalyst, which was attributed to the
close spatial distance and flexibility of carbonꢀcarbon linkage between Ru centers. Therefore,
our results clear clarify that it is the space confinement effect imposed by network rigidity that
ensures the isolation of individual Ru sites and dominates the WNA catalytic mechanism.
Correlation between Catalytic Performance and Catalyst Structure. In addition to
mechanism study, we also tried to understand how the polymer network parameter affects the
i
catalytic performance. Fig. 7a summarizes the experimental values of initial TOF (v /CRu) and
TON for these polymer catalysts. As shown, the overall catalytic performance follows the
order as: CCP3 > CCP1 > LCP > CCP2. The highest activity was achieved for CCP3 with
ꢀ1
TOF ~ 0.85 s (CRu = 70.5 ꢁM) and TON ~ 290 (CRu = 70.5 ꢁM), but it is kind of poorer than

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ꢀ1
the performance of reference [Ru(bda)(pic)
70 (CRu = 19.0 ꢁM) in water. However, it has to be noted that the higher performance of
Ru(bda)(pic) ] was achieved by the usually much faster I2M mechanism, whereas the CCPs
reported here work via WNA mechanism.
2
] with TOF ~ 3.5 s (CRu = 76.0 ꢁM) and TON ~
6
[
2
4
00
500
00
(
a)
TOF
TON
(b)
0
1
2
0
1
2
10
TOF
TON
1
0
0.85
4
2
90 300
ꢀ
10
300
200
2
1
0
00
ꢀ
0.099
1
0
1
50
1
39
ꢀ
10
00
0
.028
0.026 80
1
0
00
ꢀ
ꢀ3
1
0
10
0
10
20
30
40
50
60
LCP
CCP1
CCP2
CCP3
2
SABET / (m /g)
5
4
00
00
(c) 120
ο
10
(d)
H O
0
1
ο
2
ο
1
ο
ο
ο
105
10
TOF
TON
1
03
00
103
102
HNO3
ο
83
9
6
3
0
0
0
0
ο
ꢀ1
69
10
300
2
1
0
00
00
ꢀ
2
1
0
ꢀ
3
10
80
85
90
95 100 105 110 115
LCP
CCP1
CCP2
CCP3
ο
Contact Angle /
IV
Fig. 7 (a) Plot of experimental TOF and TON values for LCP and CCPs, where [Ce ] = 100
mM, pH = 1.0 (HNO ), and CRu = 62.6, 68.1, 63.8, and 70.5 ꢁM for LCP, CCP1, CCP2 and
3
CCP3, respectively. (b) BET surface area (SABET) dependence of TOF and TON values for
LCP and CCPs. (c) Experimental contact angles for LCP and CCPs. (d) Contact angle
dependence of TOF and TON values for LCP and CCPs.
In addition, no correlation between the catalytic activity and SABET was observed (Fig. 7b),
2
namely, CCP2 with the highest SABET (55 m /g) presented the lowest activity, which goes
against our original conjecture. It is noted that when catalysis is performed inside porous
materials, there is always a question as to whether the catalysis is only limited to the
8
ꢀ10
outermost surface of catalyst particles. Thus, we carried out contact angle measurement on
these insoluble CCP materials to explore the CCP/water interfacial property (Fig. 7c). The
results demonstrate that LCP, CCP1 and CCP2 are highly hydrophobic, reflected in the large

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1
4
contact angles (103° ~ 110°), whereas CCP3 is much less hydrophobic (83°). The
replacement of pure water by HNO (aq, pH ~ 1.0) further led to increased interfacial
wettability. Especially for CCP3, the great difference in contact angles between pure water
83°) and HNO (69°) can be rationalized by the fact that the protonation of triazine units can
3
(
3
significantly enhance the hydrophilicity. Apparently, the catalytic activity negatively
correlates with the contact agnle/interfacial energy (Fig. 7d). The above evidences imply that
the diffusion process of reactant molecules (CAN and water), inside the porous network,
might suffer great hindrance due to the large CCP/water interfacial energy. Thus, the catalysis
is supposed to be limited only on the macroscopic surface of catalyst particles for CCP1 and
CCP2, whereas the catalysis is even probably allowed to occur in the outermost few porous
layers near the surface for CCP3. Construction of fully hydrophilic crossꢀlinked network is
probably a good solution for the further enhancement of CCP/water interfacial wettability.
On the other hand, we noticed the the importance of incorporation of Ru(bda)ꢀmacrocyclic
fragments into the CCP network for efficient catalysis. Namely, the best two polymeric
catalysts are CCP1 and CCP3, and both of them are constructed by ligands with
planeꢀtriangle configuration. Recent studies by Würthner et. al. have already demonstrated the
cooperative effect of multiꢀRu centers for efficient homogeneous catalysis, where a few
metallosupramolecular macrocycle [Ru(bda)(bpb)]
3
WOCs were prepared by incorporating
three Ru(bda) fragments into a cyclic arrangement using bpb (1,4ꢀbis(pyridꢀ3′ꢀyl)benzene) as
6
, 7
the bridging ligand. It was found that the [Ru(bda)(bpb)] macrocycle could facilitate the
3
preorganization of water molecules in the cavity and decrease the reaction activation energy,
which was beneficial for the protonꢀcoupled process. Similarly, such a preorganizationeffect
is also supposed to exist for CCP WOCs containing Ru(bda)ꢀmacrocyclic fragments on

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1
5
surface, which helps partially explain why CCP1 and CCP3 presented higher activities than
CCP2 and LCP.
1
000
CeIV] = 100 mM
pH = 1.0 (HNO
C
CCP3
[CeIV] = 100 mM
pH = 1.0 (HNO3)
C
Ru
CCP3
(a)
[
Ru
7.63
(b)
400
300
200
100
0
3
)
1
ꢀ
M
35.26
N2 on
ꢀM
N2 on
N on
2
750
N off
2
N2 off
N2 on
5
00
50
0
2
N off
2
N2 off
0
500
1000
1500
2000
0
500
1000
1500
2000
Time / s
Time / s
1
1
1
500
250
000
1000
750
500
250
0
(c)
[Ru(bda)(pic)
2
]
(d)
C
Ru
38.0 ꢁM
νi = 33.3 ꢀΜ/s, t = 48h
CCP3
C
Ru
35.25 ꢁM
[CeIV] = 100 mM
[
CeIV] = 100 mM
pH = 1.0 (HNO3)
νi = 48.0 ꢀΜ/s, t = 0h
νi = 20.8 ꢀΜ/s, t = 24h
7
5
2
50
00
50
0
3
pH = 1.0 (HNO )
νi = 27.4 ꢀΜ/s, t = 24h
t = 0h t = 24h
t = 24h t = 48h
0
20
40
60
80
100
0
20
40
60
80
100
Time / s
Time / s
Fig. 8 (a)ꢀ(b) Longꢀterm oxygen evolution curves by using different amounts of CCP3 as
WOCs. (c) Oxygen evolution curves by using [Ru(bda)(pic) ] WOC under different storage
2
time. (d) Oxygen evolution curves by using CCP3 WOC under different storage time.
Stability of CCP WOCs during Storage and Catalysis. To further examine the stability of
CCP catalysts under reaction condition, we attempted to monitor the longꢀterm oxygen
evolution by using CCP3 as catalyst. As shown in Fig. 8a and 8b, after bubbling nitrogen gas
into the solution phase, oxygen evolution was clearly observed within 2000 s, and the
presented activity showed no obvious catalyst concentration dependence. In addition to
reaction stability, the storage stability is also important to catalyst activity. It is known that the
selfꢀoxidation and ligand dissociation are considered to be the main degradation pathways for
[
Ru(bda)L
air and needed to be used immediately after its preparation. Fig. 8c shows that, after the
Ru(bda)(pic) ] stock solution was stirred for 24 h, the solution color changed from brown to
2 2
] catalyst family. Therefore, [Ru(bda)L ] catalyst solution is normally sensitive to
[
2

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6
green, which was also reflected in the shift in UVꢀvis spectra (Fig. S14). Accordingly, the
catalytic rate dramatically decreased from 48.0 to 20.8 ꢁM/s. Whereas for CCP3 (Fig. 8d),
2
4
0% of enhancement in activity was even observed when the stirring time was prolonged to
8 h. This can be rationalized that the prolonged strong shearing could further break the CCP
particles into smaller sizes and expose more active Ruꢀcenters. More importantly, the
diffusion reflectance UVꢀvis spectra showed no obvious spectrum shift (Fig. S14), indicating
that the structure of CCP3 was intact after prolonged mechanical stirring.
All in all, the good stability in catalysis and storage is attributed to the 3D crossꢀlinked
structure, which prevents the selfꢀoxidation and ligand dissociation. The best activity of
CCP3 is mainly attributed to the optimal balance between various factors, such as the high
interfacial wettability, the moderate porosity and the existence of waterꢀmolecule
preorganization effect.
2
Though the apparent reactivity of smallꢀmolecule reference [Ru(bda)(pic) ] appears slightly
higher than CCP3, such comparison must be viewed with caution because not all of the
catalytic centers in polymeric catalyst are accessible during the catalysis in present system (as
stated above). Thus, model calculation was further carried out to estimate the percentage (x%)
of active Ruꢀsites in the total number of Ruꢀsites in heterogeneous catalysis, by assuming only
the Ruꢀsites on the surface of the nanoscaled catalyst particles are account for the catalytic
activity (Fig. S15, and calculation details in Supporting Information). One specific case was
considered, i.e., the particle surface is totally occupied by wellꢀdefined Ru(bda)ꢀmacrocyclic
fragments. By assuming the packing density of CCP particles (0.6 g/mL) and the average
particle size (50 nm), the estimated x% is ~ 2.0%, indicating that only a small portion of
Ruꢀsites is able to take part in the catalysis. This qualitative estimation is further consistent

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17
with the following observation.
(
a)
CCP3 Ligand exchange CCP3M
Endꢀcapping
Fragmentization
1
1
1
500
250
000
CCP3M (c)5.0
M
1000
00
800
00
100
(d)
(b)
C
Ru
5.25
7.63
ꢀ
ꢀ
4.6
TOF
TON
9
83ο
85
ο
3
1
ꢀ
ꢀ
M
M
M
4.0
3.0
750
8.82
75
50
7
4.41
CeIV
600
500
400
[
] = 100 mM
3
pH = 1.0 (HNO )
7
5
2
50
00
50
0
2
.0
.0
290
300
2
5
0
1
0.85
200
100
0.0
0
0
50 100 150 200 250 300
CCP3
CCP3M
CCP3 CCP3M
Time / s
Fig. 9 (a) Schematic illustration of postꢀmodification of CCP3 via pyridine/dmso exchange
reaction. (b) Reaction time (t) dependence of oxygen concentration ([O ]) by using CCP3 as
WOCs. (c) Experimental TOF and TON values for CCP3 and CCP3 , where CRu = 70.5 ꢁM
CCP3) and CRu = 17.63 ꢁM (CCP3 ) were used in experiments. (d) Experimental contact
angles for CCP3 and CCP3M.
2
M
M
(
M
Post-modification of CCP Network via Pyridine/dmso Exchange Reaction. For
heterogeneous catalysis, the number of catalytic centers located at the most surficial structure
8
, 21, 22
of network has been proven to be crucial.
a trace amount of inactive Ru(dmso) moieties remain on the network surface. To activate
these potential Ruꢀsites, CCP3 was further treated with pyridine/methanol solution to endꢀcap
the dmso sites (Fig. 9a). Surprisingly, the postꢀmodified CCP3 catalysts (CCP3 ) showed
After polycondensation reaction, there is still
x
M
ꢀ1
dramatically enhancement in activity with impressive TOF ~ 4.6 s and TON ~ 750 in pure
water (Fig. 9b and 9c), which is even superior to the performance of reference
[
2
Ru(bda)(pic) ]. Preliminary kinetics study implies that the catalytic mechanism for the

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8
modified CCP3 WOC is still based on the singleꢀsite WNA (Fig. S16).
The successful insertion of pyridine into network is reflected in the enhanced vibration
ꢀ1
peak located at 1150 cm in FTIR spectra (Fig. S17). Diffusion reflectance UVꢀvis spectra
also show slight redꢀshift (Fig. S18) for CCP3 , probably due to the enhanced
M
metalꢀtoꢀligand charge transfer interaction. Meanwhile, the interfacial wettability of
CCP3/water seems to be unchanged after modification (Fig. 9d).
In addition to the endꢀcapping effect, the possibility of dynamic ligandꢀexchange between
free pyridine and L3 ligand cannot be excluded at present stage, which will disrupt the
polymer network and expose more active centers for catalysis (Fig. 9a). The structural
evolution during the ligand exchange process needs to be further investigated. On the other
hand, the substituted pyridineꢀderivatives, such as 4ꢀpyridine carboxylate and
hydroxypyridine, might act as better modification reagents to regulate the interfacial
wettability and enhance the activity of the resultant CCP. Further investigation of the
postꢀmodification of CCP catalysts is undergoing in our lab.
Electrochemical Study of CCP/nafion/ITO Film. The insolubility of CCPs in water makes
these polymeric catalysts viable electrode materials for electrocatalytic water oxidation
studies. Electrochemical activities of working electrodes based on CCP/nafion/ITO film were
further investigated (Fig. S19). Working electrode was typically made by dropꢀcasting 100 ꢁL
of suspension containing 0.4 mg of CCP catalyst and 4 ~ 64 ꢁL of nafion solution (5%wt)
onto an ITO electrode. The mass ratio of nafion and CCP (wtnafion/wtCCP) varied from 0.5 to
2
8
.0. The electroactive area (1.5 cm ) was controlled with Kapton tape mask.

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1
9
(a)
0.6
(
b)
0.5
CCP1
Cycle 1
Nafion/CCP
1/1 (wt/wt)
CCP3
Cycle 1
Nafion/CCP
1/1 (wt/wt)
0
.4
.3
6%
0.4
0.2
0.0
11%
Cycle 2
Cycle 3
Cycle 4
Cycle 5
0
Cycle 2Working electrode
Cycle 3
Cycle 4
0.2
.1
Cycle 5
0
0.0
0.1
ITO/Nafion
ꢀ
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Potential (V vs Ag/AgCl)
Potential (V vs Ag/AgCl)
0
0
0
0
0
0
0
0
.70
.60
.50
.40
.30
.20
.10
.00
RuV
/RuIV
(
c)
0.04 ITO/Nafion/CCP1
Cycle 1
(
d)
CCP1
CCP3
Cycle 2
RuIII/RuII
0
.02
.00
Cycle 3
Cycle 4
Cycle 5
RuIV
/RuIII
0
Nafion/CCP
0.5/1 (wt/wt)
ꢀ0.02
0
1
1
0
10
0.0
0.2
0.4
0.6
0.8
1.0
Wtnafion/WtCCP
Potential (V vs Ag/AgCl)
Fig. 10 (a) and (b) Repetitive cyclic voltammograms (CVs) for CCP1/nafion/ITO and
CCP3/nafion/ITO electrodes at pH = 8.0 up to 1.50 V (100 mV/s). (c) Mass ratio of nafion
and CCP (wtnafion/wtCCP) dependence of catalytic current at 1.50 V. (d) Repetitive cyclic
voltammograms for CCP1/nafion/ITO electrode at pH = 8.0 up to 1.00 V (100 mV/s).
Fig. 10a and 10b show the typical cyclic voltammograms using CCP1 and CCP3 as active
materials. Weak redox waves for Ruꢀspecies between 0.4 to 1.0 V (vs Ag/AgCl) were first
observed, and followed by a large catalytic water oxidation wave with the onset potential
Eonset = ~1.0 V. Control experiment by using bare nafion/ITO as electrode shows much higher
overpotential and weaker electrocatalytic waves at 1.5 V (black curve, Fig. 10b). Under
2
optimal nafion loading, catalytic currents as high as 0.55 and 0.53 mA/cm can be achieved
for CCP1 and CCP3, respectively (Fig. 10c and S19). Excess loading of nafion binder leads
to thicker films and prevents the mass transport of water molecules.
2
3
In recent work reported by Antoni Llobet et al., they observed a decrease by ~ 20% in the
III/II
intensity of electrocatalytic wave (Ru
couple) and an increase by ~ 40% in the intensity at
]ꢀmodified glass carbon electrode, which
active species from the surface of electrode and the
0
.9 V after 20 scan cycles for [Ru(bda)(pyridine)
2
implies the depletion of the Ru−OH
2

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20
formation of more active RuO
2
species. When scan was carried out from 0.0 to 1.0 V for
III/II
CCP1/nafion/ITO (Fig. 10d), the Ru
couple at ~ 0.50 V can be more clearly seen as well
IV/III
V/IV
as the electrocatalytic waves associated with the Ru
(0.7 V) and Ru
(0.9 V) couples.
IV/III
V/IV
The weak signals for Ru
and Ru
couples are due to the slow protonꢀcoupled electron
10
transfer process in heterogeneous film. But most of all, the intensities at different potentials
are almost constant after 5 cycles, indicating the good stability of CCP catalysts.
The durability of the hybrid electrode at high potential (1.5 V) was also evaluated. The
catalytic current at 1.5 V decreases by 11% and 6% for CCP1 and CCP3 after 5 cycles (Fig.
1
0a and 10b), respectively. CCP3/nafion/ITO maintains ~ 73% of initial intensity after 27
cycles (Fig. S20) under optimal condition. It is known that the main degradation pathway of
Ru(bda)L ] catalyst family is through the dissociation of axial pyridyl ligands and diffusion
[
2
24
of the cleaved fragments into the solution; thus, the design idea of incorporating the
catalytic site into crossꢀlinked polymeric network helps impart a better stability through the
cooperative chelate effect.
Conclusions
In summary, as a proof of concept, we demonstrated the feasibility of utilizing crossꢀlinked
coordination polymers (CCPs) with [Ru(bda)] complex for heterogeneous water oxidation.
These porous polymeric catalysts oxidize water via the singleꢀsite water nucleophilic attack.
Postꢀmodification via pyridine ligand exchange reaction allows CCP catalyst to reach high
–
1
TON and TOF values of > 750 and > 4.6 s . Meanwhile, these CCP catalysts also present
good electrochemical activities on ITO electrodes, and the durability of electrode was greatly
improved due to their unique network structure. The results clearly indicate that crossꢀlinked

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1
coordination polymers could preserve catalytic Ru sites under harsh oxidative conditions,
among other benefits of the heterogeneous catalysis. Our investigation also reveals the
potential for further improving the catalyst efficiency and active site utilization through
rational design of polymeric catalyst at both molecular and morphological levels.
Experimental Section
Unless otherwise stated, all chemicals obtained from commercial suppliers were used without
further purification. [Ru(bda)(dmso)
2
] and [Ru(bda)(pic)
2
] were prepared according to
2
5, 26
14
literatures.
Ligands L0ꢀL2 were synthesized through typical Suzukiꢀcoupling procedure,
and the synthesis detailed can be found in the Supporting Information. FTIR spectra were
recorded on Nicolet™ iS™10 FTIR Spectrometer as KBr pellets. UV−vis absorption spectra
were measured on a Shimadzu UVꢀ3600 device. Diffuse reflectance accessory was used for
1
3
the measurement of solid samples. Solidꢀstate CꢀNMR spectra were recorded on a Bruker
DRXꢀ500 spectrometer. TGA measurement of the polymers was performed using a TA Q600
instrument under nitrogen with a heating rate of 15 °C/min. DSC was performed on a Q2000
differential scanning calorimetry from TA Instruments under a nitrogen flow rate of 100
o
mL/min with a scanning rate of 10 C/min. The contact angle measurements were performed
using a KSV (Helsinki, Finland) CAM 200 contact angle goniometer, and the CCP powders
were compressed in to thin discs for measurement. SEM images were recorded on FEI
NanoSEM. Wide angle powder Xꢀray diffraction (XRD) patterns were collected using a
Bruker D8 diffractometer with a Cu Kα Xꢀray source (λ = 0.154 nm) operating at 40 kV and
40 mA by depositing fineꢀpowder on glass substrate. Nitrogen BET surface areas were
measured using a Micromeritics ASAP 2010 system.

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22
Synthesis of LCP and CCPs through Ligand Exchange Reaction. The synthesis procedure
for CCP3 is as follows: [Ru(bda)(dmso) ] (625 mg, 1.25 mmol) and L3 ligand monomer (261
mg, 0.835 mmol) were dissolved in a degassed mixture of chloroform and methanol (60 mL +
0 mL) and heated for 24 h at 65 °C under N . The mixture was cooled to room temperature,
2
3
2
separated by centrifugation, and extracted successively with methanol, acetone, THF and
dichloromethane. The final product was dried in vacuum for 12 h at 45 °C to give CCP3
.
(yield 95%) The synthesis procedures for LCP (yield 75%), CCP1 (yield 85%) and CCP2
(yield 93%) are identical with that of CCP3.
Post-modification of CCP3 Network via Pyridine/dmso Exchange Reaction. In a glove
box, the asꢀprepared CCP3 (100 mg) was suspended in anhydrous methanol (20 mL), and an
excess of distilled pyridine (500 uL) was further added. The vial was sealed and the mixture
was stirred at 50 °C for 12 hrs. The insoluble polymer was recycled by centrifugation, and
further washed with methanol, acetone and dichloromethane. The product was dried in
M
vacuum for 12 h at 45 °C to give CCP3 (80 mg).
Kinetics Study on the Chemically Driven Water Oxidation Reaction Monitored by an
IV
Oxygen Sensor. The Ce driven water oxidation reactions were performed under ambient
conditions in a 25 mL glass vial with a rubber stopper. Before measurement, the degassed
water suspension of CCP (1.0 mg/mL) was stirred vigorously for 24 h in a sealed vial. Water
suspension with different CRu was prepared by diluting the mother suspension. In each
experiment, 548 mg of cerium(IV) ammonium nitrate (CAN) was dissolved in 9.5 mL of a
HNO
3
solution (pH 1.0) in a 25 mL glass vial capped with a rubber stopper. The CAN
solution was bubbling with N for 10 min. Then, 0.5 mL of airꢀfree CCP water suspension
2

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23
was injected into the reaction vial through a septum via an airꢀtight syringe under vigorous
stirring. The produced O in solution phase was recorded inꢀsitu by an oxygen sensor
Unisense, OXꢀN). Calibration of oxygen sensor was carried out before each measurement.
2
(
For turnover number (TON) measurement, the reaction was stopped after 6 h, and 300 ꢁL of
the gas in head space were taken out with a gasꢀtight syringe and injected into the gas
chromatograph with a thermal conductivity detector using argon as carrying gas. A few
injections of known amount of O
2 2
were used as calibration standards. The O concentration
was obtained based on 2 injections.
Kinetics Study on the Chemically Driven Water Oxidation Reaction Monitored by
IV
UV-vis Spectrometer. The water oxidation kinetics with respect of the concentrations of Ce
IV
and CCP catalyst was also studied by monitoring the loss of Ce under pH 1.0 using UVꢀvis.
The reaction kinetics depending on the concentration of CCP catalyst was measured under
IV
the condition as follows: the concentration of Ce was kept 1.12 mM for each run, but the
concentration of CCPs was changed from 2.5 ~ 20.0 ꢁM. The reaction kinetics depending on
IV
the concentration of Ce was measured under the condition as follows: the concentration of
CCP catalyst was kept at 20.0 ꢁM (CCP1) and 10 ꢁM (CCP3) for each run, but the
IV
concentration of Ce was changed from 1.12 mM, 1.96 ꢁM, 2.80 mM, to 3.60 mM. Typically,
IV
the CCP water suspension was rapidly injected into the quartz cell containing 3 mL of Ce
under vigorous stirring. The UVꢀvis spectrum was recorded after the solution was vigorously
stirred for ~ 3 s.
Electrochemical Study. Electrochemical measurements were performed on a commercial
instrument with a standard threeꢀelectrode configuration: reference electrode: Ag/AgCl (3 M

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2
4
2
KCl aqueous solution), working electrode: ITO, auxiliary electrode: Pt slice (~ 2.5 cm ). 150
mL of 0.1 M K HPO solution (pH = 8.0) was used as electrolyte solution, and 3 M HNO
was used to adjust the pH value. Scan rate was fixed at 100 mV/s. Before measurement, a
water suspension of CCP (4.0 mg/mL) in H O was stirred vigorously for 24 h in a sealed vial.
2
4
3
2
Working electrode was made by dropꢀcasting 100 ꢁL of suspension containing 0.4 mg of
CCP catalyst and 4 ~ 64 ꢁL of nafion solution onto an ITO electrode. Kapton tape functions
2
as mask to control the electroactive area to be 1.5 cm .
Acknowledgements
The National Natural Scientific Foundation of China Projects (21774116 and
51703216) are gratefully acknowledged.
Notes and references
1
2
3
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L. Duan, F. Bozoglian, S. Mandal, B. Stewart, T. Privalov, A. Llobet and L. Sun, Nat.
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M. Schulze, V. Kunz, P. D. Frischmann and F. Würthner, Nat. Chem., 2016, 8, 576ꢀ583.
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1
1
1
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For table of content used only
4
Ce4+
O2
4Ce3+
2
H O
2
Cross-linked coordination polymer for water oxidation
x y n
Cross-linked coordination polymers (CCPs) with a formula {[Ru(bda)] L } were
developed as heterogeneous catalysts for water oxidation in organic solvent-free
system.