Efficient photochemical water oxidation by a dinuclear molecular ruthenium complex

Article abstract of DOI:10.1039/c4cc08606f

Herein is described the preparation of a dinuclear molecular Ru catalyst for H₂O oxidation. The prepared catalyst mediates the photochemical oxidation of H₂O with an efficiency comparable to state-of-the-art catalysts.

Full text of DOI:10.1039/c4cc08606f

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DOI: 10.1039/C4CC08606F
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ARTICLE TYPE
Efficient Photochemical Water Oxidation by a Dinuclear Molecular
Ruthenium Complex
a
a
a
a
a
Tanja M. Laine, Markus D. Kärkäs,* Rong-Zhen Liao,* Torbjörn Åkermark, Bao-Lin Lee, Erik A.
a
a
a
Karlsson, Per E. M. Siegbahn, and Björn Åkermark*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Herein is described the preparation of a dinuclear molecular
absorbing photosensitizer that can be regenerated and is stable at
neutral pH. An appealing approach is to drive H O oxidation with
[Ru(bpy) ] type oxidants, which can be photogenerated from the
3
Ru catalyst for H O oxidation. The prepared catalyst
2
2
3+
mediates the photochemical oxidation of H O with an
2
2+
8,9
wellꢀstudied [Ru(bpy) ] ꢀtype complexes. This can be realized
1
0
efficiency comparable to state-of-the-art catalysts.
3
45
by lowering the redox potentials of the WOCs, by introducing
3
f,g,10,11
The development of sources of renewable energy is an
increasingly acute problem for mankind. An attractive option
would be to utilize solar energy but in order to use its full
potential, means of storing it have to be developed. Solar driven
anionic ligands into the ligand frameworks of the WOCs.
We have recently reported on such a ligand (1), which contains
12
phenol and imidazole donor groups. However, reaction with
Ru(DMSO) Cl resulted in a dimeric Ru complex with a 1:1 ratio
4
2
1
2
2
3
3
4
5
0
5
0
5
0
splitting of H O into H and O provides an attractive way of 50 of Ru/ligand, instead of the anticipated dinuclear complex. Since
2 2 2
3
+
achieving this. However, this process is very intricate because it
involves multiꢀelectron transfer, extensive bond breaking and
this complex did indeed oxidize H
2
O, using [Ru(bpy)
3
]
as
oxidant, we decided to prepare a more open ligand that could
potentially give a dinuclear complex. Ligand 2, where the
bridging phenol unit has been replaced by a pyrazole moiety
bond formation. In the splitting of H O, the stumbling block is
2
the oxidation of H O to generate molecular oxygen, electrons,
2
and protons (Eq. 1). This is due to the fact that H O oxidation 55 provides such an open structure. As expected, coordination of
2
involves strongly oxidizing intermediates which leads to
decomposition of the catalysts. There is thus an urgent need to
two Ru centers yielded the dinuclear Ru complex 3 (Fig. 1 and
Fig. 2). Herein, we show that the dinuclear Ru complex 3
develop more robust and efficient water oxidation catalysts
catalyzes photochemical H
reaching turnover numbers (TONs; defined as produced moles of
O per mole catalyst) of ~900, which is, to the best of our
2
knowledge, among the highest obtained for a molecular catalyst
in a single run. The observed high catalytic activity is ascribed to
the ability of the designed ligand framework to stabilize the metal
centers in highꢀvalent redox states. This work thus highlights the
2
O oxidation with high activity,
1
(WOCs).
60
+
−
2
H O → O + 4 H + 4 e
(1)
2
2
During the past years a number of efficient Ru WOCs have
2
3
been presented, both dinuclear and mononuclear. Much work
has focused on mononuclear complexes because of the relative 65 importance of using negatively charged ligand scaffolds to carry
simplicity by which these complexes can be synthesized and
mechanistically studied, thus aiding the development of improved
WOCs. In addition to the widely developed Ruꢀbased WOCs,
earthꢀabundant metal WOCs based on Mn, Co, Fe and Cu
have been reported.
out the oxidation of H O, which could be a general strategy for
2
the design of future artificial WOCs with even higher catalytic
efficiencies.
4
5
6
7
Ligand 2 was synthesized according to Scheme S1, starting
70 from commercially available 3,5ꢀdimethyl pyrazole 4. The
II,III
II,III
The generation of ligands that can strongly bind and stabilize
metal centers in highꢀvalent oxidation states is of great interest.
dinuclear Ru
2
complex 3 ([(H
2
L)Ru
2
(pic)
6
](PF
6
)
2
) was
] and
2
obtained by refluxing the Ru precursor [Ru(DMSO)
Cl
4
Thus far, the majority of the developed WOCs require a powerful
ligand 2 followed by the addition of picoline. The structure of the
IV
sacrificial oxidant, such as Ce , to drive H O oxidation.
dinuclear Ru complex 3 was supported by several methods, such
2
1
However, a sustainable H O splitting system needs a lightꢀ 75 as H NMR, highꢀresolution mass spectrometry (HRMS),
2
elemental analysis and electron paramagnetic resonance (EPR).
1
a
Initial H NMR experiments of Ru complex 3 only resulted in
Department of Organic Chemistry, Arrhenius Laboratory,
II,III
Stockholm University, SE-106 91 Stockholm, Sweden. E-mail:
markusk@organ.su.se, rongzhen@organ.su.se,
bjorn.akermark@organ.su.se; Tel: +46 8 163223.
broad signals due to the paramagnetic nature of the Ru2
complex 3. However, addition of a reductant, sodium dithionite
(Na S O ), resulted in distinct signals, which supported the
proposed structure of the developed complex. HRMS
measurements of the complex in positive mode resulted in a peak
80
2
2
4
†
Electronic Supplementary Information (ESI) available: Experimental
details, NMR, electrochemistry, mass spectra and quantum chemical
calculations details. See DOI: 10.1039/b000000x/
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2
N
N
H
H
NH
HN
N
N
NH
HN
N
OH
N
N
N
N
N
N
N
HN
N
O
O
O
O
OH
HO
O
O
OH
Ru
N
Ru
N
HO
pic
pic
1
2
3
Previous work
This work
Fig. 1 Molecular structures of ligand 1, ligand 2 and the dinuclear Ru complex 3 (where pic = 4ꢀpicoline).
with a characteristic isotope pattern at m/z 1146.2155, which
II,III
2
2+
+ +
+ +
could be assigned to [{(H L)Ru
(pic)6} – H ] ([3 – H ] ).
The activity of complex 3 was initially evaluated for H O
2
2
The calculated structure of Ru complex 3 is depicted in Fig. 2.
The electrochemistry of Ru complex 3 was subsequently studied
by means of cyclic voltammetry (CV) and differential pulse 30 mild oneꢀelectron chemical oxidant (E1/2 Ru /Ru = 1.26 V vs.
oxidation under neutral conditions, in an aqueous buffered
3
+
solution (0.1 M, pH 7.2), using pregenerated [Ru(bpy) ] as a
3
III
II
5
0
5
0
voltammetry (DPV) in aqueous buffered solutions (phosphate
NHE). The evolved gaseous products were monitored and
quantified by realꢀtime mass spectrometry (MS). Indeed, when an
aqueous solution containing complex 3 was added to the
buffer; 0.1 M, pH 7.2). From CV, catalytic current
a
corresponding to the electrochemical oxidation of H O was
2
3+
observed with an onset potential of ~1.20 V vs. NHE, as shown in
Fig. S19. The DPV is depicted in Fig. S20 and revealed several 35 resulted in high TONs and initial turnover frequencies (TOFs;
[Ru(bpy)3] oxidant, O evolution was immediately triggered and
2
1
1
2
redox peaks, which were assigned to the formal redox processes
defined as produced moles of O per mol catalyst per unit time)
2
II,II
II,III
III,III
III,IV
IV,IV
IV,V
Ru2 → Ru2
→ Ru2
→ Ru2
→ Ru2
→ Ru2
,
(Fig.S17 and Table 1). The source of the oxygen atoms in the
evolved O was also studied, using isotopically labeled H O
where the latter state triggers OꢀO bond formation. The
electrochemistry was subsequently studied at pH 1 (Figs. S21ꢀ
S24). Also here, several redox events were observed, suggesting 40 were measured in realꢀtime with MS (Fig. S17). The ratio of the
that the dinuclear Ru complex 3 is able to accommodate a wide
variety of redox states. From the electrochemical measurements it
is clear that the use of ligand 2 in Ru complex 3 results in
lowering of the redox potentials and stabilizes the Ru centers in
2
2
18
18,18
16,18
16,16
(H2 O), whereby the different isotopes
O ,
2
O and
2
O2
different O isotopes was in agreement with the calculated values,
2
highlighting that H O is the sole source of the two oxygen atoms
2
in the evolved O2.
An important question was now if Ru complex 3 was able to
highꢀvalent states, suggesting that H O oxidation can be driven 45 mediate photochemical H O oxidation. The studied threeꢀ
2
2
3
+
2+
by photogenerated [Ru(bpy) ] ꢀtype complexes.
component catalytic system thus consisted of a [Ru(bpy) ] ꢀtype
3
3
complex as the lightꢀabsorbing component, sodium persulfate
(
Na S O ) as the sacrificial electron acceptor and Ru WOC 3. In
2 2 8
the Initial experiments, the stronger photosensitizer
2
+
III
II
5
5
6
6
0
5
0
5
[Ru(bpy) (deeb)] (E1/2 Ru /Ru = 1.40 V vs. NHE; deeb =
2
diethyl (2,2’ꢀbipyridine)ꢀ4,4’ꢀdicarboxylate) was used in aqueous
phosphate buffered solutions at pH 7.2 (Fig. 3).
However,
replacing
the
strong
photosensitizer
2+
2+
[
Ru(bpy) (deeb)] with the milder [Ru(bpy)3] photosensitizer
2
strongly decreased the TON (not shown), which can perhaps be
explained by the low thermodynamic driving force (E1/2 Ru /Ru
for [Ru(bpy) ] = 1.26 V and E = ~1.20 V vs. NHE). To
ensure that the reaction between the photosensitizer and
persulfate is not an important source of O the even milder
photosensitizer [Ru(dmb)3]
bipyridine), which generates a Ru complex with a redox
potential of 1.10 V vs. NHE, was also used. As was anticipated,
this potential is insufficient for H O oxidation and only traces of
O were generated (Fig. 3).
III
II
3
1/2 onset
2
2+
(dmb
=
4,4’ꢀdimethylꢀ2,2’ꢀ
III
2
2
It was therefore decided to continue using the stronger
2+
[
Ru(bpy) (deeb)] photosensitizer. Further experiments focused
2
Fig. 2
([(H L)Ru
Calculated structure of the dinuclear Ru complex 3
2+
II,III
on studying the effect of pH on the catalytic O evolution activity
25
2
2
6
(pic) ] ).
2
2
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DOI: 10.1039/C4CC08606F
since it was previously shown that the catalytic activity could be
13
pHꢀdependent. The photochemical experiments were therefore
performed at different pH, under otherwise unchanged conditions
It could subsequently be shown that pH 6.2 resulted in the best
activity (Fig. S18). Performing the photochemical experiments at
pH 6.2 resulted in increased catalytic activity to reach an
(Fig. S18).
2
+
4
4
5
0
5
0
impressive TON of ~900 when using [Ru(bpy) (deeb)] as
2
5
photosensitizer (Fig. S18 and Table 1). It is believed that at pH
6
.2, photosensitizer decomposition is reduced while still
maintaining a relatively high driving force for carrying out H O
2
oxidation. These two beneficial effects thus give rise to the
catalytic efficiency observed at pH 6.2. Collectively, this shows
that complex 3 is an unusually efficient donor of electrons to the
oxidized photosensitizer, thus resulting in high catalytic
efficiency, and highlights the ability of the customꢀsynthesized
ligand 2 to facilitate access to the required highꢀvalent redox
states.
Conclusions
To conclude, a novel dinuclear Ru complex based on ligand 2 has
been synthesized. The ligand scaffold was designed to stabilize
the Ru centers at high oxidation states, which is of importance in
55
H O oxidation catalysis. This approach was successful and
2
Fig. 3
2
Photochemical H O oxidation catalyzed by dinuclear Ru
resulted in an active WOC that could promote both chemical and
complex 3 at different catalyst concentrations. Reaction conditions:
Reactions were performed in an aqueous phosphate buffer solution (0.1
photochemical H O oxidation with mild oneꢀelectron
2
1
0
3+
[
Ru(bpy) ] ꢀtype oxidants. In the photochemical system, the
3
M, pH 7.2, 0.50 mL) containing Ru complex 3, [Ru(bpy)
Ru(dmb) ](PF as photosensitizer (0.60 mM) and Na
electron acceptor (23.5 mM).
2
(deeb)](PF
6
)
2
or
[
3
6
)
2
2
S
2
O
8
as sacrificial
developed Ru complex 3 showed to be an efficient WOC and
60 gave TONs comparable to stateꢀofꢀtheꢀart WOCs. Insight into the
catalytic features associated with complex 3 revealed that the
designed ligand architecture has an important role by stabilizing
the metal centers in a variety of different redox states, which is of
fundamental importance during the multiꢀelectron oxidation of
1
2
2
5
0
5
Carrying out the catalytic experiments at higher pH would
intuitively be expected to result in a higher catalytic activity due
to the higher driving force for H O oxidation at this pH.
65
H O. This highlights that the engineering of WOCs comprised of
2
2
anionic ligand scaffolds could be a general strategy for future
construction of more efficient WOCs.
Financial support from the Knut and Alice Wallenberg
Foundation, the Swedish Research Council, the Carl Trygger
Foundation and the Swedish Energy Agency is gratefully
acknowledged.
However, this was not what was observed. At pH 8.2, almost no
activity at all was observed, most likely due to decomposition of
the oxidized photosensitizer. Driving H O oxidation at lower pH
2
is hence associated with a lower driving force, however, it is
believed that photosenssitizer decomposition is reduced under
these conditions. At pH 5.5, the lower driving force thus reduced
the amounts of generated O2.
70
Notes and references
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a
b
Chemical Oxidation
Photochemical Oxidation
2
Concentration
pH 7.2 c
pH 6.2 c
pH 7.2
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[
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TON (TOF ) TON (TOF )
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3
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