Chemical and Photochemical Water Oxidation by [RuCl(NCNHCO)(DMSO)(py)]-Type Complexes

Article abstract of DOI:10.1002/cctc.201700049

In this work, the effects of substitutions in the backbone of imidazolylidene and axial ligands on the reactivity and stability of water-oxidation catalysts were investigated. Three pincer-type asymmetric imidazolium salts NC^NHCO, NC^NHC-4BrO, and NC^NHC-BO (NHC: N-heterocyclic carbene), of which the donating ability of the corresponding imidazolylidene decreases in the same sequence, were prepared. Their application in metalation afforded the corresponding Ru complexes 1 a, 1 b, 2, and 3. It was found that the complexes incorporating the stronger donor displayed lower potential for each redox couple and longer lifetimes, but relatively low reaction rates. Under acidic conditions, water oxidation driven by cerium ammonium nitrate resulted in turnover numbers (TONs) of 2322, 1728, 1928, and 2208 for 1 a, 1 b, 2, and 3, respectively. Complex 2 exhibited a good match of reactivity and stability with a TON of 136 in a typical three-component photocatalysis. Importantly, NHCs could be another powerful tool for tuning the reactivity and stability of water-oxidation catalysts, in addition to substituted pyridines. Rational orchestration of modified NHCs and pyridines could eventually result in water-oxidation catalysts exhibiting appreciable effectiveness and considerable robustness.

Full text of DOI:10.1002/cctc.201700049

Accepted Article
Title: Chemical and Photochemical Water Oxidation by
RuCl(NCNHCO)-(DMSO)(py) Type Complexes
Authors: Wei Su, Hussein A. Younus, Somboon Chaemchuen, Cheng
Chen, and Francis Verpoort
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
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To be cited as: ChemCatChem 10.1002/cctc.201700049
Link to VoR: http://dx.doi.org/10.1002/cctc.201700049
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ChemCatChem
10.1002/cctc.201700049
FULL PAPER
NHC
Chemical and Photochemical Water Oxidation by RuCl(NC O)-
(
DMSO)(py) Type Complexes
a
a
a
a
abc
Wei Su, Hussein A. Younus, Somboon Chaemchuen, Cheng Chen and Francis Verpoort
*
application
Abstract: In this present work, the effects of substitutions on the
backbone of imidazolylidene and axial ligands on the reactivity and
stability of water oxidation catalysts were discussed. Three pincer-
2
Cl-
n+
NHC
NHC-4Br
NHC-
2 Cl-
type asymmetric imidazolium salts NC O, NC
B
O and NC
N
N
N
N
N
N
N
N
N
O, of which the donating ability of the corresponding
imidazolylidene decreases in the same sequence, were prepared.
And their applications in metallation afforded corresponding Ru
Ru
N
Ru
N
[PF6]n
N
N
Ru
N
N
X
N
N
Iodo • bis(4-methyl pyridine) • (2,2';6,2''-terpyridine)
ruthenium hexafluorophospahte (C1: X=I, n=1)
Aqua • bis(4-methyl pyridine) • (2,2';6,2''-terpyridine)
ruthenium dihexafluorophospahte (C2: X=H2O, n=2)
(
2,2';6,2''-terpyridine) • 2-(quinol-8'-yl)-1,10-phenanthroline
Bis(2,2';6,2''-terpyridine) ruthenium(II) dichloride (B)
complexes 1a, 1b,
2 and 3. It was found that the complex
ruthenium (II) dichloride (A)
incorporating a stronger donor displayed lower potential for each
redox couple and longer life-time, but relatively low reaction rate.
Under acidic condition, water oxidation driven by CAN resulted in
Figureꢀ1.ꢀChemicalꢀStructuresꢀofꢀcomplexꢀA,ꢀBꢀandꢀC.ꢀ
TONs of 2322, 1728, 1928 and 2208 for 1a, 1b,
respectively. Complex 2 exhibited a good match of reactivity and
stability with TON of 136 in typical three-component
2 and 3,
and sustainability, many well-studied complexes are based on
a
a
[
7]
ruthenium owing to their superb activity and stability. And
furthermore, they provided rational mechanistic understandings
which shed light on further development of WOCs involving
other transition metals.
photocatalysis. Importantly, NHCs could be another powerful tool
tuning reactivity and stability of water oxidation catalysts in addition
to substituted pyridines. Rational orchestration of modified NHCs
and pyridines could eventually result in water oxidation catalysts
exhibiting appreciable effectiveness as well as considerable
robustness.
Apart from the redox-active metal centers, it is
acknowledged that ligands also play an important role in tuning
[
8]
performance of WOCs. Thummel
reported that
Ru(tpy)(phenq)] (tpy = 2,2’;6,2’’-terpyridine, phenq = 2-(quinol-
’-yl)-1-10-phenanthroline), which was prepared by replacement
2
+
[
8
Introduction
2+
of one tpy of inactive precursor [Ru(tpy)
2
IV
]
with phenq, could
decompose water in the presence of Ce (TON = 334 /24 h)
Figure 1, A and B). In addition, even ligand environment is
In the context of deteriorating energetic and environmental
issues, photocatalytic water oxidation is becoming more and
more attractive because it provides electrons and protons which
could be used to produce chemical fuels, such as dihydrogen as
(
slight different, it might lead to different mechanisms towards
water oxidation. It is surprising to find that C1 demonstrated a
higher initial rate than that of C2 which was generally considered
[
1]
well as alcohols (e. g. reduction of carbon dioxide). However,
catalysts for water oxidation reported to date are far from
practical use as most of them enable to generate dioxygen when
an excess of strong oxidant is added. Thus more efforts have to
be devoted to the design of water oxidation catalysts (WOCs),
aiming at attainment of robust and efficient WOCs which could
split water driven by light.
[9]
as the real catalyst (Figure 1, C). Presumably, C1 mediated
water oxidation by forming a reactive seven coordinated Ru-
aqua complex, of which the iodide reminded intact, rather than
formation of C2. Therefore, systematic investigation of the
influence of subtle ligand modification on the catalytic
performance is of great interest as well as significance.
Numerous
While molecular complexes based on various earth-
[
2]
[3]
[4]
abundant transition metals such as copper, cobalt, iron,
[
5]
[6]
manganese,
nickel
have been intensively investigated
towards water oxidation in recent years in term of large-scale
ꢀ
Supportingꢀinformationꢀforꢀthisꢀarticleꢀisꢀgivenꢀviaꢀaꢀlinkꢀatꢀtheꢀendꢀofꢀtheꢀ
document.((Pleaseꢀdeleteꢀthisꢀtextꢀifꢀnotꢀappropriate))ꢀ
Figureꢀ2.ꢀChemicalꢀStructuresꢀofꢀligandꢀprecursorsꢀandꢀcomplexꢀ1,ꢀ2ꢀandꢀ3.ꢀ
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ChemCatChem
10.1002/cctc.201700049
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efforts have been devoted to the comprehensive understandings
about how substitutions on ligands will influence the activity and
a
Table 1. Chemical shifts of ligand precursors and corresponding complex
[
10]
1
13
13
stability of WOCs.
However, as most of the reported WOCs
Compound
H
NCHN
C
NCN
C
C=O
are based on polypyridinyl (N-donor), little were known when
other types of donors are used in WOCs.
NHC
b
NC
O
9.23
137.6
184.5
184.3
138.9
187.4
144.0
199.6
166.3
169.9
169.9
164.9
169.2
165.9
169.5
Recently, we disclosed a pincer-type ruthenium WOC
b
1
a
-
(
Figure 2, 1a) based on a negatively charged asymmetric
[
11]
imidazolylidene (C-donor).
It is believed that introduction of N-
1b
-
9.41
-
heterocyclic carbenes (NHCs) is beneficial for enhancing the
properties of WOCs. The electronic property of NHCs could be
easily tuned by substitution of backbones of heterocyclic rings,
which will affect the catalytic performance of complexes by
modification of the electron density around metal centers. In this
work, synthesis and characterization of a series of ruthenium
NHC-4Br
NC
O
2
NHC-B
NC
O
9.90
-
3
NHC
complexes based on derivatives of NC O will be described
(
Figure 2, 1b, 2 and 3). The effects of different substitutions on
[
a] Data were collected by measurement of samples in DMSO-d
6
at 298K;
backbone of imidazole and at the pyridine ligands on the
electrochemistry and catalytic performance were investigated.
[b] Data were collected from ref. 11.
Results and Discussion
Synthesisꢀ andꢀ characterizationꢀ ofꢀ ligandꢀ precursorsꢀ andꢀ
complexesꢀ
Ligand precursors were prepared according to reported
[
11]
literature.
1H-imidazole derivatives were stepwise alkylated
with 2-chloromethyl pyridine and ethyl bromoacetate to afford
the ester compounds (Figure S1), which were consequently
hydrolyzed to give the corresponding carboxylate functionalized
imidazolium inner salts. Reaction of ligand precursors with Ag
followed by addition of [Ru(p-cymene)Cl and pyridine
derivatives resulted in the formation of complex 1b, 2 and 3,
2
O,
2 2
]
1
13
respectively. Selected H and C chemical shifts of ligand
precursors and complexes are tabulated (Table 1). Formation of
Figure 3. Molecular structures of 1a (left) and 3 (right) in the solid state with
ellipsoids at 50% of probability. Hydrogens and solvent molecules are omitted
for clarity.
Ru imidazolylidene complexes could be deduced from absence
1
of corresponding NCHN protons of the ligand precursors in
H
NMR spectra. The characteristic of these complexes is the
exhibition of AB patterns of methylene bridges, which indicated
Table 2. Selected bond lengths (Å) for crystals of 1a and 3
[
12]
the pincer-type configuration in the equatorial plane . Signals
a
1
3
1a
3
assigned to C NMR of the carbonyl group were slightly shifted
downfield due to coordination of carboxylates as well.
Additionally, large chemical shifts of NCN carbon indicated the
bonding of Ru-C (∆δ = 46.9, 46.7, 48.7and 55.6 ppm for 1a, 1b,
Ru1 - Cl1
Ru1 - S1
Ru1 - O1
Ru1 – N4
Ru1 – N1
Ru1 – C1
2.528(3)
2.225(3)
2.112(5)
2.13(1)
Ru1 - Cl1
Ru1 - S1
Ru1 - O1
Ru1 – N4
Ru1 – N3
Ru1 – C7
2.5449(6)
2.2320(6)
2.091(2)
2.085(2)
2.117(2)
1.930(2)
2
and 3, respectively). The chemical shifts of Ru-bonded carbon
indicated the donating ability of the corresponding
imidazolylidene in our complexes increases in an order 3 < 2 <
[
13]
1
a, which is consistent with reported literatures
differences in donor strength of these complexes are believed to
make difference in their electrochemistry and catalysis.
Additionally, four peaks in isotopic patterns at m/z = 489.06402,
75.03718, 568.95773 and 539.07355, assigned to the
. The
2.090(7)
1.953(8)
a
4
+
[a] Data were collected from ref. 11;
corresponding [M-Cl] species, were observed, which further
confirmed the formation of these complexes.
All the complexes were assumed to possess a slightly
distorted octahedral configuration like 1a (Figure 3, left). The
equatorial plane consists of the asymmetric imidazolylidene and
a chlorine which is trans to the NHC carbon. Dimethyl sulfoxide
(
DMSO), coordinated to Ru center via sulfur, is trans to pyridine
in the axial orientation. The structure of 3 was unambiguously
confirmed by its crystal in the solid state (Figure 3, right), and
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ChemCatChem
10.1002/cctc.201700049
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displayed an exact coordination configuration as 1a. Selected
bond lengths for crystals of 1a and 3 are listed in Table 2 for
comparison. The distances between Ru and imidazolylidene
carbons are shorter than those between Ru and nitrogens,
indicating the less possibility of dissociation of Ru - NHC prior to
that of Ru-pyridine, demonstrating the ability of NHCs in
maintaining the structure of WOCs. Another two differences in
bond length between 1a and 3 probably could make their
different performances rationalized. The bond length of Ru - Npic
transfer (MLCT) were observed presumably due to the lack of
extended conjugated systems. Importantly, these complexes did
not degrade under neutral conditions for hours. As analogues to
1a, pyridine/H
2
O exchange were observed for complexes 1b, 2
[
11, 15]
and 3 in acidic conditions tracked by UV-Vis
(Figure 4,
(Figure 5, S27 and S29).
[
11, 16]
Figure S26 and S28) and NMR
The presence of isosbestic points in the UV-Vis spectra implied
that no side products were produced during the exchange.
Peaks assigned to dissociated pyridines (aromatic and aliphatic
1
(
Ru1 – N3, 2.117 Å) in 3 is shorter than that (Ru1 – N4, 2.13 Å)
protons) were increasing in time-resolved H NMR spectra,
[
14]
in 1a, implying protonation of 4-picoline of 1a is easier , which
is in line with our findings from the UV-Vis study. Meanwhile, the
longer Ru – Cl (Ru1 – Cl1, 2.5449 Å) bond distance in 3 might
which definitely verified the occurrence of pyridine/H
exchange. Furthermore, the solutions of complexes in pH 1
HNO aqueous solution were analyzed by ESI-MS to show peak
2
O
3
account for the faster Cl/H
2
O
exchange under catalytic
in an isotopic pattern at m/z = 414.00571 and 414.02792 for 1a
+
conditions, causing a higher initial reaction rate.
and 1b (corresponding to [M-Cl] , calculated m/z = 414.00616),
The electron absorption spectra of these complexes were
then measured by UV-Vis spectroscopy in both pH 1 and 7
478.05944 and 505.09535 for 2 and 3 (corresponding to [M-
+
OH
2
-Cl+MeOH] , calculated m/z = 478.03746 and 505.93233,
aqueous solution. In the 10 mM phosphate buffer solution (pH 7), respectively), where M stands for the Ru aqua complex formed
all complexes displayed characteristic intensive bands in the
range between 250 and 400 nm, which were attributed to ligand
centered charge transfer (π-π* transition) (Figure S25). No
discernable absorption assigned to metal-to-ligand charger ꢀ
2
through pyridine/H O exchange. The Ru aqua complexes
generated in-situ due to axial pyridine dissociation were
considered to be the real catalysts.ꢀ
Figureꢀ4.ꢀTime-resolvedꢀUV-Visꢀspectralꢀchangesꢀofꢀcomplexꢀ1aꢀ(top)ꢀandꢀ3ꢀ(bottom)ꢀ
3
inꢀHNO ꢀsolutionꢀ(pHꢀ1).ꢀCondition:ꢀ[cat]ꢀ=ꢀ0.08ꢀmM.ꢀ
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1
[11]
Figureꢀ 5.ꢀ H-NMRꢀ changesꢀ ofꢀ 3ꢀ inꢀ D
2
Oꢀ (containingꢀ 70ꢀ equiv.ꢀ CF
3
SO
S
H)ꢀ inꢀ aromaticꢀ
on the reaction conditions (Figure 7 and Figure S37) . In pH 1,
scopeꢀ(top)ꢀandꢀaliphaticꢀscopeꢀ(bottom)ꢀoverꢀaꢀperiodꢀofꢀ68ꢀh.ꢀVerticalꢀdashꢀlinesꢀ
NHC
2
Ru(NC O) undergoes 4-picoline/H O exchange, affording the
1
correspondedꢀtoꢀtheꢀ Hꢀsignalsꢀofꢀ4-picolineꢀunderꢀtheꢀsameꢀconditions.ꢀ
corresponding Ru aqua complex (Figure 7, pH 1). The Ru aqua
V
complex is consequently oxidized to high-valent Ru =O complex,
which is the active intermediate in initiating water oxidation,
through one-electron oxidation and stepwise proton-coupled
a
Table 3. potentials for selected redox couples of the all the complexes
pH 1
pH 6
complex
IV/III
V/IV
IV/III
Ru
Ru
Ru
1
a
b
0.889
0.893ꢀ
0.907
0.942
1.089
1.098
1.154
1.105
0.704
0.731ꢀ
0.748
0.779
1
2
3
[
a] Data were collected from DPV under aqueous conditions.
Figure 6. Cyclic voltammetry of complexes (0.25 mM) in pH 6 phosphate
buffer solutions. Conditions: Glassy carbon as working electrode, SSCE as
reference electrode and platinum wire as counter electrode. Scan rate: 100
mV/s.
Electrochemistryꢀandꢀmechanismꢀstudyꢀ
The differences in electrochemical properties of these
complexes were analyzed by cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) in aqueous solutions. The
potentials reported here are versus Ag/AgCl electrode (SSCE).
3
In pH 1 HNO solutions, with the scan to positive potentials all
the complexes underwent current increase with a catalytic onset
of ca. 1.3 V, although the overall influence due to ligand
substitutions is smaller than expected (Figure S30). However,
the differences in potentials for redox couples could be easily
detected from their DPVs (Figure S31). Clearly, there is a trend
that the complex bearing a weaker donor often demonstrates a
higher potential for each redox couple (Table 3), which is
Figure 7. Proposed pathways of oxygen evolution employing
RuCl(NC O)(DMSO)(py) in pH 1 and 7 aqueous solution.ꢀ
NHC
[
10b, 10c]
consistent with the findings reported previously.
In phosphate buffer solutions (pH 6), catalytic water
oxidation started at ca. 1.0 V (Figure 6), indicating that the
V
formation of active high-valent Ru =O species is pH-dependent
in the scope from pH 1 to 6. Of great interest is that complex 2
displayed a peak current twice higher than those of 1a and 3,
implying that electron density is not the only factor that strongly
influences the property of WOCs. The key electrochemical
feature of these complexes is the occurrence of a peak at ca.
1
.2 V (Figure S32), which is associated with water oxidation.
The potentials for redox couples agreed with the trend observed
in acidic conditions. After 10 cycles CV scan, the catalytic
current either decreased due to the formation of oxygen bubbles
on the electrode (Figure S33-S36) or remained unchanged and
no films were observed. This implied the homogeneity and
chemical stability of these complexes as deposition of active
species would enhance the catalytic current with increasing scan
[
17]
time.
Previously, we have reported that 1a enabled to oxidize
water via formation of different Ru aqua complexes depending
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ChemCatChem
10.1002/cctc.201700049
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IV
(
2) intermediates related to high-valent Ru =O were detected by
[
18]
mass spectroscopy (Figure 8 and Figure S38)
.
Water oxidation
First of all, to rule out the oxidative decomposition of Ru
bonded imidazolylidene ligands, experiments oxidizing and
recovering the complex by addition of CAN and ascorbic acid
(
3
AA) in pH 0.5 HNO solution were carried out. Fortunately, the
imidazolylidene ligands were quite stable in the presence of
CAN under such a harsh condition (Figure S39~S41). Chemical
and photochemical water oxidation were then measured
employing these complexes. Under acidic conditions, a fresh
catalyst solution was injected into
a vessel containing a
deaerated aqueous solution of Cerium Ammonium Nitrate (CAN).
The oxygen generated was measured by a photosensitive
Ocean Optic probe positioned in the headspace of the vessel.
The total oxygen amount was calibrated by gas chromatography
at the end of reaction. Time profiles were illustrated in Figure 9,
and turnover numbers (TONs) were calculated. Over a period of
1
6 h, TONs of 2322, 1728, 1928 and 2208 for 1a, 1b, 2 and 3
were attained, respectively. The differences in reaction rate were
remarkable compared with these in TON. The weaker
imidazolylidene the complex incorporated, the higher initial
reaction rate and the shorter life-time it exhibited. Complex 3
reached the plateau around 6 h, while complex 1a was still
active with slightly reduced reactivity up to 16h. 1b displayed a
much longer induction time than that of 1a, which probably was
due to the slower
Figure 8. Experimental (top) and calculated (bottom) mass spectrum of high-
IV
+
valent intermediate [Ru = O(OH
2
)] generated by addition of a certain amount
of NaIO into 1b and its proposed structure.
4
Schemeꢀ 1.ꢀ Simplifiedꢀ schemeꢀ forꢀ aꢀ typicalꢀ three-componentꢀ photochemicalꢀ waterꢀ
oxidation.ꢀ
Figure 9. Water oxidation driven by CAN with complex 1, 2, 3 and 3-12h in
nitric acid solution (pH 1).ꢀ
electron transfers(PCETs). However, under neutral conditions,
applied potential and/or presence of an oxidant drives Cl/H
2
O
exchange, resulting in formation of the other Ru aqua complex
IV
(
Figure 7, pH 7). It is then oxidized to Ru =O through two
IV
PCETs. This Ru =O is oxidized to generate catalytically active
V
Ru =O species to mediate oxidation of water. These are general
NHC
reaction pathways concerning RuCl(NC O)(DMSO)(py) - type
complexes since (1) pyridine/H
2
O exchange was confirmed in all
1
complexes by UV-Vis and H NMR (see supporting information),
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Conclusions
In summary, we have discussed the effects of substitutions
on backbones of imidazolylidene and axial ligands on the
properties of WOCs employing Ru complexes which
incorporated negatively charged asymmetric imidazolylidenes. It
was found that introducing substituted imidazolylidenes in the
equatorial plane could be a powerful tool to tune the redox
NHC
potentials for RuCl(NC O)(DMSO)(py) - type WOCs. Among
this class of ruthenium complexes, those which comprise
stronger carbon donors exhibit lower redox couples (3 > 2 > 1).
In term of either chemical and photochemical water oxidation,
complexes incorporating weaker imidazolylidenes were
generally more active (3 > 2 > 1) but with a shorter lifetime (3 < 2
<
1). Mechanism study further proved that this class of
ruthenium WOCs mediates water oxidation through formation of
different Ru aqua complexes (e.g. Pyridine/H O exchange at pH
; Cl/H O exchange at pH 7).
Of importance is that substituted imidazolylidenes could be
Figure 10. Photochemcial water oxidation employing 1a, 2 and 3 in 10mM
2
phosphate buffer solution (pH 7).
1
2
an effective type of ligands in tuning reactivity and stability of
WOCs apart from substituted N-donors concerning that versatile
NHCs with various donating strengths are now available. It is
expected that effective and robust WOCs could be prepared
utilizing ligands in which substituted NHCs and N donors are
well-orchestrated.
pyridine/H
generated during the pyridine/H
catalyst, we arranged a controlled experiment in which a
deaerated acidic solution (pH 1, adjusted by HNO ) of 3 was
2
O exchange. Knowing that the Ru aqua complex
2
O exchange ought to be the real
3
allowed for exchange overnight (denoted as 3 – 12h) before
injection into CAN solution, to assure how it would influence the
reaction in term of reactivity and stability. Oxygen evolution was
immediately tracked by probe upon injection of 3
– 12h.
Experimental Section
Apparently, the Ru aqua complex was further evidenced to be
the real catalyst as suggested by the higher reaction rate and
Materials and methods
[
7a, 19]
TON
.
Chemicals in this contribution were purchased from Aladdin chemical Co.
LTD unless otherwise noted, and were used without further purification.
Ultrapure water with a resistivity of 18.25 MΩ●cm was used to prepare all
At pH 7, light-induced water oxidation was run employing
the well-established three-component system containing a WOC,
2
-
NHC
NHC-4Br
a photosensitizer and a sacrificial electron accepter (S
2
O
8
)
the aqueous solutions. THF applied in synthesis of NC Et, NC
Et
2
+
NHC-B
(
Scheme 1). Complex [Ru(debpy)(bpy)
diethylcarbonyl – 2,2’ – bipyridine, bpy = 2,2’ – bipyridine, E1/2
1.40 V vs. NHE) was used in this article due to its mild
2
]
(debpy = 4,4’ –
and NC
Et was dried and degassed by solvents purification system
(Innovative Technology, Puresolv PS-MD-5). CAN (>99.99% trace metals
2
+
2
basis) was purchased from SIGMA-ALDRICH Co. [Ru(debpy)(bpy) ]
=
[
20]
was attained following reported literature.
Column chromatography
oxidative power. Oxygen was accumulated and measured by
gas chromatography as depicted in Figure 10. At the first
hundred
1
13
was performed on silica gel (Haiyang Ltd. 0.044-0.074 mm). H and
C
NMR spectra were recorded at 500 MHz and at 126 MHz on Bruker
Avance III-HD, respectively, if not otherwise stated. High resolution mass
seconds, the reactivity decreases in an order 3 > 2 > 1a, is the
same as that in chemical water oxidation. However, the TONs
decreased in an order 2 (136) > 3 (80) > 1a (40). This is in
accordance with the electrochemistry under neutral conditions,
in which the catalytic current decreases in an order 2 > 3 > 1a as
displayed in Figure 6. Therefore, compared with the other two,
complex 2 reached a balance between reactivity and stability,
spectrum measurements were recorded on
a
Brukerꢀ Daltonics
microTOF-QII spectrometer with an electrospray ionization and time-of-
flight system. Fourier transform-infrared spectroscopy (FTIR) was
performed with a Bruker Vertex 80V FT-IR spectrometer. Elementary
analysis was performed using
a Vario EL III elementary analysis
instrument. UV-vis spectroscopy was measured with Shimazu UV-1800.
The electrochemical properties of all the complexes were measured with
a CHI660E potentionstate (Shanghai Chenhua Instrument Co. Ltd.)
employing a 3-electrode system. Glass carbon disk (diameter 3 mm,
CHI104) was used as working electrode, platinum wire (CHI115) as
counter electrode, and Ag/AgCl (SSCE, CHI111) as reference electrode.
Cyclic Voltammetry was carried out with a scan rate of 0.1 V/s. Potential
were reported vs SSCE (3M KCl aqueous solution as an electrolyte).
Tests at pH 7 were tested in an aqueous phosphate buffer solution
demonstrating an appreciable initial reaction rate and
a
considerable TON. Notably, the reaction system containing 2
and 3 deactivated when attempts were tried to enhance the
buffering ability by usage of more concentrated phosphate buffer
solution (up to 100 mM). However 1a was active in 100 mM
[
11]
phosphate buffers
, which might attribute to the stronger
(
2 4 2 4
NaH PO /Na HPO , 10 mM). Tests under acidic conditions were tested
donating ability of NHC in 1a. Further characterization regarding
in nitric acid solutions (pH 1).
to this is under study.
Chemical water oxidation was carried out in
a jacketed vessel
thermostated at 298 K. Typically, a 50 µL solution of 1 mM catalyst was
injected into a 5mL CAN solution (200 mM, pH 1, adjusted by HNO
3
)
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10.1002/cctc.201700049
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which was thoroughly deaerated, using an air tight syringe though the
rubber septum. The oxygen in the headspace was dynamically monitored
with an Ocean Optics Fluorescene-based oxygen sensor (Neofox,
FOSPOR-R1/16”) equipped with an Ocean Optics oxygen sensing
system (Neofox, NEOFOX-GT), and the end point of the reaction was
verified by GC analysis.
Subsequently, [Ru(p-cymene)Cl2]2 (0.12 mmol, 74.5 mg) was added,
which readily resulted in formation of AgCl. The suspension was stirred
for another 1h under the same condition. Upon addition of pyridine
O
derivatives, the mixture was allowed to heat overnight at 110 C toꢀafford
corresponding ruthenium complexes. Analytical pure products were
obtained by purifying with columnꢀ chromatography using specified
eluents for each ruthenium complexes
Photochemical oxygen evolution was carried out in a home-made glass
vessel with a water jacket, illuminated by 500 W xenon lamp (PL-
XQ500W) with a cut-off (420 nm). The temperature of the vessel was
maintained at 298 K by continuous flow of water connected with a
thermostat. Oxygen generated in the head space was analyzed with gas
chromatography (GC, FULI 9790II) equipped with a packed column
Synthesis of 1b
1
b was prepared following the general procedure, pyridine was added
instead of 4-picoline. Eluents: dichloromethane/methanol (20/1 ~ 10/1),
1
(
TDX-01). A mixture of catalyst (62.5 µM), photosensitizer (562.5 µM)
silica gel. Yield: 30 mg, 25 %. H NMR (500 MHz, DMSO-d
6
) δ 9.35 (d, J
and Na (20 mM) in 10 mM phosphate (2 mL) buffer was thoroughly
2
S
2
O
8
= 3.3 Hz, 1H), 7.91 (s, 2H), 7.83 (m, 2H), 7.64 (d, J = 7.3 Hz, 1H), 7.54 (s,
deaerated before illumination.
1
4
3
H), 7.28 (m, 2H), 7.24 (s, 1H), 7.18 (m, 1H), 5.61 (q, J = 15.6 Hz, 2H),
.32 (d, J = 16.5 Hz, 1H), 3.16 (d, J = 7.0 Hz, 1H), 2.98 (s ,3H), 2.92 (s,
Synthesis of ligand precursors
1
3
6
H). C NMR (126 MHz, DMSO-d ) δ 184.28, 158.43, 153.48, 137.70,
General procedure of ligand precursors: ligand precursors were prepared
NHC
NHC
136.87, 124.88, 121.72, 121.44, 53.56, 53.12, 46.00, 43.90. FT-IR (cm-
1): 1065.4, 1081.3 (S=O), 1609.6 (C=O). Anal. Calcd. for
C18H21ClN4O3RuS: C, 42.39; H, 4.15; N, 10.99; S, 6.29. Found: C,
42.62; H, 3.934; N, 11.19; S, 6.453. ESI-HR-MS: m/Z = 475.03718 [M-
in a similar method preparing NC Et and NC O. Alkylation of 1H-
imidazole (1 equiv.) derivatives with 2-chloromethylpyridine hydrochloride
(
1.2 equiv.) in the presence of KOH (2.5 equiv.) in reflux THF overnight
afforded 1-(pyrid-2-ylmethyl) imidazole. Reflux of 1-(pyrid-2-ylmethyl)
imidazole (1 equiv.) and ethyl bromoacetate (1.1 equiv.) in dry THF for 5h
+
Cl] , calcd: 475.0378.
NHC
NHC
gave NC Et as solid. Imidazolium precursor, NC O, was prepared
Synthesis of 2
2
by hydrolysis of the ester precursors with KOH in i-PrOH/H O (V/V:3/1).
NHC-4Br
NHC-4Br
2 was prepared following the general procedure employing NC
O.
Synthesis of NC
Et
Eluents: dichloromethane/methanol (10/1 ~ 8/1), silica gel. Yield: 25 mg,
NHC-4Br
NC
Et was synthesized following general procedure using 4-bromo-
1
1
7.2 %. H NMR (500 MHz, DMSO-d
6
) δ 9.41 – 9.23 (d, 1H), 7.87 – 7.73
1
1
H-imidazole (5 mmol, 700 mg). Yield:120 mg, 6 %. H NMR (500 MHz,
DMSO-d ) δ 9.52 (s, 1H), 8.57 (d, 1H), 8.22 (s, 1H), 7.92 (t, 1H), 7.54 (d,
J = 6.3 Hz, 1H), 7.43 (t, 1H), 5.69 (s, 2H), 5.35 (s, 2H), 4.25(q, 2H), 1.25
(
m, 4H), 7.63 (d, J = 7.3 Hz, 1H), 7.23 – 7.16 (t, 1H), 7.13 (s, 2H), 5.68 –
.50 (m, 2H), 4.25 (d, J = 15.7 Hz, 1H), 3.10 (d, J = 15.8 Hz, 1H), 2.97 (d,
6
5
1
3
1
3
J = 9.3 Hz, 6H), 2.29 (s, 3H). C NMR (126 MHz, DMSO-d ) δ 187.41,
6
(
t, 3H). C NMR (126 MHz, DMSO-d
6
) δ 166.35, 153.37, 150.11, 139.90,
1
38.08, 124.33, 123.20, 109.32, 62.73, 54.27, 49.38, 14.42.
169.19, 158.35, 157.80, 152.74, 149.78, 137.03, 126.09, 125.88, 125.11,
-
1
NHC-4Br
124.18, 122.13, 53.92, 51.33, 45.90, 43.64, 20.82. FT-IR (cm ): 1053.5,
Synthesis of NC
O
1
3
5
063.6 (S=O), 1589.2 (C=O). Anal. Calcd. for C19H22BrClN4O3RuS: C,
7.85; H, 3.68; N, 9.29; S, 5.32. Found: C, 37.76; H, 3.959; N, 9.490; S,
NHC-4Br
NHC-4Br
NC
O was prepared by hydrolysis of NC
Et (120 mg, 0.3
) δ 9.41 (s, 1H),
.57 (s, 1H), 8.02 (s, 1H), 7.89 (t, J = 7.2 Hz, 1H), 7.49 (d, J = 7.5 Hz,
1
mmol). Yield: 55 mg, 80 %. H NMR (500 MHz, DMSO-d
6
+
.567. ESI-HR-MS: m/Z = 568.95773([M-Cl] ), calcd: 568.96191.
8
1
d
1
1
3
Synthesis of 3
was prepared following the general procedure employing NC
H), 7.41 (s, 1H), 5.59 (s, 2H), 4.43 (s, 2H). C NMR (126 MHz, DMSO-
) δ 164.86, 153.86, 150.10, 138.94, 138.02, 124.18, 123.03, 122.96,
NHC-B
6
3
O
09.04, 53.95, 52.49.
(
62.5 mg, 0.23 mmol). Eluents: dichloromethane/methanol (10/1 ~ 8/1),
NHC-B
1
Synthesis of NC
Et
silica gel. Yield: 30 mg, 22.4 %. H NMR (500 MHz, DMSO-d
6
) δ 9.34 (d,
NHC-B
J = 5.6 Hz, 1H), 7.83 (m, 5H), 7.56 (d, J = 7.4 Hz, 1H), 7.33 (t, J = 7.6 Hz,
1H), 7.27 – 7.18 (m, 3H), 7.09 (d, J = 6.1 Hz, 2H), 6.14 (d, J = 15.8 Hz,
1H), 5.62 (d, J = 15.6 Hz, 1H), 4.72 (d, J = 16.7 Hz, 1H), 2.99 (d, J = 7.4
NC
Et was synthesized following general procedure using 1H-
1
benzimidazole (10 mmol, 1180 mg). Yield:1880 mg, 50 %. H NMR (500
MHz, DMSO-d ) δ 9.95 (s, 1H), 8.50 (s, 1H), 8.08 (d, J = 7.6 Hz, 1H),
.99 (d, J = 7.4 Hz, 1H), 7.92 (t, J = 7.0 Hz, 1H), 7.69 (d, J = 7.2 Hz, 3H),
.40 (s, 1H), 6.03 (s, 2H), 5.68 (s, 2H), 4.29 – 4.19 (m, 2H), 1.27 (t, J =
6
1
3
7
7
6
1
5
Hz, 6H), 2.26 (s, 3H). C NMR (126 MHz, DMSO-d
6
) δ 199.63, 169.47,
158.23, 158.02, 152.87, 149.78, 149.52, 137.13, 134.93, 134.57, 126.08,
1
3
6
.9 Hz, 4H). C NMR (126 MHz, DMSO-d ) δ 167.03, 153.28, 150.11,
1
4
25.90, 124.19, 122.56, 109.56, 109.38, 50.25, 49.97, 45.78, 43.55,
44.55, 138.07, 131.94, 131.33, 127.35, 124.25, 123.29, 114.40, 62.51,
-1
0.91, 20.79. FT-IR (cm ): 1065.4, 1081.3 (S=O), 1609.6 (C=O). Anal.
1.47, 48.08, 14.44.
Calcd. for C23H25ClN4O3RuS: C, 48.12; H, 4.39; N, 9.76; S, 5.58.
Found: C, 47.89; H, 4.303; N, 9.529; S, 5.699. ESI-HR-MS: m/Z =
NHC-B
Synthesis of NC
O
+
NHC-B
NHC-B
539.07355([M-Cl] ). calcd: 539.0691.
NC
O was prepared by hydrolysis of NC
Et (1180 mg, 3.1 mmol).
) δ 9.90 (s, 1H), 8.50
1
Yield: 670 mg, 82 %. H NMR (500 MHz, DMSO-d
6
Crystal structure determination
(
d, J = 4.4 Hz, 1H), 7.96 – 7.83 (m, 3H), 7.67 – 7.52 (m, 3H), 7.38 (dd, J
Slow evaporation of diethyl ether into
a
solution of
3
in
1
3
dichloromethane/methanol (V/V = 10/1) lead to suitable crystals of 3 for
X-ray diffraction. Diffraction data were collected with a Bruker APEX-II
CCD diffractometer with MoKα radiation (λ = 0.71073 Å). Numerical
absorption corrections were applied. The structure was solved by direct
methods and refined on F2 with anisotropic thermal parameters for all
non-hydrogen atoms. Proton atoms were refined on the calculated
=
7.0, 5.2 Hz, 1H), 5.95 (s, 2H), 4.83 (s, 2H). C NMR (126 MHz,
) δ 165.85, 153.80, 150.08, 144.01, 137.98, 132.35, 131.27,
26.53, 124.09, 123.06, 114.50, 113.90, 51.15, 50.88.
DMSO-d
6
1
Preparation of metal complexes
General procedure: complexes were prepared in a similar path preparing
positions using
a riding model. CCDC – 1525186 contains the
supplementary crystallographic data for this paper.
1
2
a. A mixture of ligand precursor (0.23 mmol) and Ag O (0.12 mmol, 26.7
o
mg) in DMSO was stirred at 50 C for 2h in the absence of light.
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ChemCatChem
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ChemCatChem
10.1002/cctc.201700049
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FULL PAPER
Effects of tuning the donating ability of
Water oxidation*
carbon donors by ligand substitutions
NHC
W. Su, H. A. Younus, S. Chaemchuen,
C. Chen, F. Verpoort *
at backbones of NHCs in Ru-NC
O
type complexes on water oxidation
were investigated.
Page No. – Page No.
Chemical and Photochemical Water
NHC
Oxidation by RuCl(NC O)(DMSO)-
(
py) Type Complexes
*
one or two words that highlight the emphasis of the paper or the field of the study
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