Light-induced water oxidation by a Ru complex containing a bio-inspired ligand

Article abstract of DOI:10.1002/chem.201003702

The new Ru complex 8 containing the bio-inspired ligand 7 was successfully synthesized and characterized. Complex 8 efficiently catalyzes water oxidation using Ce^IV and Ru^III as chemical oxidants. More importantly, this complex has a sufficiently low overpotential to utilize ruthenium polypyridyl-type complexes as photosensitizers. Water splitting: A bio-inspired ligand containing imidazole and phenol functionalities has been synthesized. The corresponding Ru complex (1) could efficiently catalyze water oxidation under visible light illumination using Ru polypyridyl-type complexes as photosensitizers. Copyright

Full text of DOI:10.1002/chem.201003702

FULL PAPER
DOI: 10.1002/chem.201003702
Light-Induced Water Oxidation by a Ru complex Containing a
Bio-Inspired Ligand
Markus D. Kꢀrkꢀs,[a] Eric V. Johnston,[a] Erik A. Karlsson,[a] Bao-Lin Lee,[a]
Torbjçrn ꢁkermark,[b] Mohammadreza Shariatgorji,[c] Leopold Ilag,[d] ꢂrjan Hansson,[e]
Jan-E. Bꢀckvall,[a] and Bjçrn ꢁkermark*[a]
Abstract: The new Ru complex 8 containing the bio-inspired ligand 7 was success-
fully synthesized and characterized. Complex 8 efficiently catalyzes water oxida-
tion using CeIV and RuIII as chemical oxidants. More importantly, this complex has
a sufficiently low overpotential to utilize ruthenium polypyridyl-type complexes as
photosensitizers.
Keywords: homogeneous catalysis ·
oxidation · photochemistry · ruthe-
nium · water splitting
Introduction
and iridium,[7] progress has been made in the field. Since the
pioneering work by Meyer, with the “blue dimer” ([cis,cis-
One of the most important challenges of the 21st century is
to provide clean and renewable fuels. An attractive option is
to model natural photosynthesis and use solar energy to
convert water into molecular oxygen and hydrogen based
fuels.[1–3] The development of efficient catalysts for water ox-
idation is essential for the creation of an artificial water-
splitting system. In the natural photosynthesis, a tetranuclear
manganese cluster located in the oxygen-evolving complex
(OEC) catalyzes the four-electron oxidation of water to mo-
lecular oxygen, thereby providing electrons for the reduc-
tion of carbon dioxide to biomass.[4]
(m-O)]4+) (bpy=2,2’-bipyridine),[8] a varie-
{RuACHTNUTGERG(NUNN bpy)2ACHTNURTGEG(NNUN H2O)}2ACHTNUGTRENNGUN
ty of different catalysts have been reported, ranging from
mononuclear homogeneous systems[9] to polynuclear hetero-
geneous catalysts.[10] However, the majority of these systems
depend on the use of stoichiometric amounts of strong oxi-
dants, such as CeIV, which has an oxidation potential of ap-
proximately +1.5 V versus the normal hydrogen electrode
(NHE). In contrast, light-induced water oxidation is rare,
there are only a few reported homogeneous catalysts which
are able to oxidize water photochemically.[11] Herein, we
would like to report a new type of bio-inspired complex 8,
which is capable of catalyzing light-driven water oxidation
in a homogeneous system at neutral pH.
Inspired by Nature, considerable efforts have been devot-
ed to finding a robust and efficient manganese-based water
oxidation catalyst. To date, this has resulted in limited suc-
cess.[5] However, by using noble metals such as ruthenium[6]
The introduction of negatively charged groups into the li-
gands of manganese and ruthenium complexes substantially
lowers the oxidation potentials relative to the complexes
with neutral ligands.[12] This is clearly demonstrated by the
dinuclear Ru complex 1,[12b] prepared from ligand
2
[a] M. D. Kꢀrkꢀs, E. V. Johnston, E. A. Karlsson, B.-L. Lee,
Prof. J.-E. Bꢀckvall, Prof. B. ꢁkermark
(Figure 1).
Department of Organic Chemistry, Stockholm University
Arrhenius Laboratory, 10691 Stockholm (Sweden)
Fax : +46 8 154908
E-mail: bjorn.akermark@organ.su.se
[b] Dr. T. ꢁkermark
Applied Electrochemistry
School of Chemical Science and Engineering
Royal Institute of Technology (KTH)
10044 Stockholm (Sweden)
[c] Dr. M. Shariatgorji
Medical Mass Spectrometry
Department of Pharmaceutical Biosciences
Uppsala University, 75123 Uppsala (Sweden)
[d] Assoc. Prof. L. Ilag
Figure 1. Structures of dinuclear Ru complex 1 and ligand 2.
Department of Analytical Chemistry, Stockholm University
Arrhenius Laboratory, 10691 Stockholm (Sweden)
[e] Dr. ꢂ. Hansson
Biophysics Group, Department of Chemistry
University of Gothenburg, P.O. Box 462
40530 Gothenburg (Sweden)
This complex has considerably lower redox potentials
from Ru2
II,II
up to Ru2III,IV, than the corresponding dinuclear
Chem. Eur. J. 2011, 17, 7953 – 7959
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Ru complex, in which the two terminal phenolic groups are
replaced by pyridines.
Although complex 1 turned out to be an interesting oxi-
dation catalyst, it failed to catalyze water oxidation. This is
probably due to oxidative degradation of the tertiary amine
group in the ligand that occurs at the high oxidation poten-
tial required for the oxidation of water. It was therefore de-
cided to synthesize ligand 7, in which the sensitive tertiary
amine function of the complex has been replaced by imida-
zole, which is more resistant towards oxidation and is also
present as a ligand in the OEC.
Results and Discussion
Ligand 7 was prepared according to Scheme 1. First, the di-
cesium salt of diacid 3 was allowed to react with bromoke-
tone 4 to give diester 5 in high yield. This was then treated
with ammonium acetate to give the bisimidazole compound
6. Deprotecting compound 6 with BBr3 afforded ligand 7 in
low yield (ca. 10%). However, demethylation with HI af-
forded ligand 7 as the hydroiodide salt, in high yield. Re-
Figure 2. UV/Vis absorption spectrum of complex 8 (0.25 mm) in CH2Cl2.
1
complete lack of H NMR signals is also compatible with a
III,III
Ru2
oxidation state.
Two additional elemental analyses were performed, one
sample in which compound 8 had been dried under reduced
pressure at 1008C overnight and one at 1508C. They indicat-
ed the loss of two picolines and four picolines, respectively.
The unusual affinity for water that complex 8 displays is not
easily explained.
The MALDI-TOF mass spectrum (Figure 3) shows two
major signals corresponding to the two halves of complex 8,
that is, complexes 9 (m/z 546) and 10 (m/z 637). A high reso-
fluxing 7 with triethylamine and [RuACTHNURGTENUNG(DMSO)4Cl2] in metha-
nol, followed by the addition of 4-picoline and continued
reflux gave, somewhat surprisingly, a complex with an over-
all ratio of 1:1 between ligand 7 and ruthenium instead of a
dinuclear complex, similar to 1. Unfortunately we have been
unable to obtain crystals suitable for X-ray crystallography
of the complex, but the elemental analysis suggests a struc-
ture such as 8, consisting of two mononuclear ruthenium
units linked by a bridging iodide ligand. The UV/Vis spec-
lution ESI-MS showed
a major signal at m/z 804.19
trum (Figure 2) displays a broad signal at approximately
(Figure 4). This could correspond to a compound in which
one of the picolines has been oxidized to the corresponding
aldehyde (11),[11a] or in which an imidazole or a picoline has
been oxidized to the corresponding N-oxide.[13]
III,III [12c]
550 nm, implying that the oxidation state is Ru2
.
The
Ru complex 8 was therefore studied with X-band EPR spec-
troscopy at 77 K. A frozen solution of complex 8 (1 mm) in
dichloromethane gave no EPR signal, thus confirming that
The catalytic activity of complex 8 in water oxidation was
III,III
complex 8 resides in the EPR-silent Ru2
oxidation state,
studied in acidic solution, using the CeIV oxidant [Ce
ACHTUNGTRENNUNG
with strong coupling between the ruthenium atoms. The
ACHTUNGTRENNUNG
Scheme 1. Synthesis of ligand 7 and complex 8.
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Chem. Eur. J. 2011, 17, 7953 – 7959

Light-Induced Water Oxidation by a Ru complex
FULL PAPER
stand overnight. Aliquots were
then taken and analyzed by
MALDI-TOF mass spectrome-
try. The mass spectra displayed
no significant difference, sug-
gesting that significant ligand
exchange does not occur during
water oxidation.
The cyclic voltammogram of
complex 8 in aqueous phos-
phate buffer at pH 7.2 shows a
current from catalytic oxygen
evolution starting at approxi-
mately 1.2 V versus NHE
(Figure 6). The electrochemistry
of complex 8 suggests that [Ru-
AHCTUNGTRENNUNG
A
,
Figure 3. MALDI-TOF mass spectrum of complex 8.
could be used to drive the
water oxidation.
Figure 5. Oxygen evolution catalyzed by complex 8 (1.4 mm/0.14 mm) em-
ploying [CeACHTNUTRGENNUG(NH4)2ACHTUNTGERN(NUGN NO3)6] (26 mm) as the oxidant in an aqueous
*
^
CF3SO3H solution (pH 1, 3.9 mL). =0.14 mm, =1.4 mm.
Figure 4. High-resolution ESI-MS spectrum of complex 8 in CH2Cl2.
In accordance with the results of the cyclic voltammetry
(CV), it was found that water oxidation was catalyzed by
added to an aqueous solution of 8 with pH 1 (0.1m
CF3SO3H). Immediate oxygen evolution was observed, with
an approximate turnover number (TON) of 1000/10 h based
on complex 8 (Figure 5). The TON is strongly dependent on
the concentration of the catalyst and the TON decreased
upon dilution of the catalyst.
One explanation could be that the complex dissociates at
lower concentration to give a mononuclear complex, which
is a less efficient catalyst. As in most published studies of
water oxidation, the oxygen evolution levels off after some
time, under our conditions after ca 6 hours. The major
reason is probably decomposition of the ligand backbone of
the catalyst, as indicated by the formation of carbon dioxide
during the water oxidation.
complex 8 using [RuACTHNGUTERNNUG
(bpy)3]3+ as the oxidant. In a typical ex-
periment, a deoxygenated solution of complex 8 in an aque-
ous phosphate buffer solution (pH 7.2) was added to
[RuACHTNUTGRNEUNG
(bpy)3]3+. This resulted in an instantaneous evolution of
O2, with a maximum TON of approximately 100/14 min
(Figure 7). The TON is, again, extremely depending on the
catalyst concentration and when this was decreased from
5.5 mm to 1.1 mm, the TON dramatically decreased to
ca. 8/13 min. The TON is considerably lower than with CeIV
as oxidant. One reason is clearly that [RuACTHNUTRGNEUNG
(bpy)3]3+ is unsta-
ble at neutral pH. Under our reaction conditions, in aqueous
phosphate buffer at pH 7.2 it decomposes, giving carbon di-
oxide as the main volatile product. Carbon dioxide was also
formed in the presence of catalyst 8. Interestingly, the TON
for carbon dioxide formation is only moderately depending
on the catalyst concentration (Figure 8).
In an attempt to obtain information about the mechanism
of water oxidation complex 8 was dissolved in an aqueous
solution (pH ca. 1, H3PO4 0.1m), in the presence and ab-
sence of CeIV (10 equiv). The reaction mixtures were left to
To ensure that H2O is the sole source of the oxygen
atoms in O2, isotopically labeled H2O (18O, 5.4%) was used.
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7955

B. ꢁkermark et al.
This resulted in proportionally enriched O2 (Figure 9). The
isotopomeric ratio of 16,18O2/16,16O2 (m/z ratio 34/32) was de-
termined to be 0.118 (theoretically 0.114=2·0.054/ACH-
T
ative error in m/z 36 was too large for a quantitative deter-
mination. These results confirm that H2O is the oxygen
source of the evolved O2.
Figure 6. Cyclic voltammogram in the presence (—) and absence (---) of
complex 8 (55 mm) in an aqueous phosphate buffer solution (pH 7.2,
0.1m) using a glassy carbon working electrode and an Ag/AgCl reference
electrode.
Figure 9. Chemical water oxidation catalyzed by complex 8 (5.5 mm) em-
ploying [RuACHTUNGTRENNUG(bpy)3]ACHTUNGTRNEN(UGN PF6)3 (40 mm) as the oxidant in an aqueous phosphate
buffer solution (0.1m, pH 7.2, 0.5 mL) containing 5.4% 18O.
=O2,
^
& =16,16O2,
=
16,18O2.
&
We also decided to study photocatalytic oxygen evolution.
In the initial experiments, [Ru(bpy)3]Cl2 was utilized as the
(bpy)3]3+ by
AHCTUNGTRENNUNG
photosensitizer. This should generate [Ru
ACHTUNGTRENNUNG
photo-electron transfer to persulfate, S2O82À, as the sacrifi-
cial electron acceptor. To our surprise no evolution of
oxygen was detected. However, when the anion in the pho-
À
tosensitizer was changed from ClÀ to PF6 , oxygen evolution
was observed with a TON of ca 30/20 min (Figure 10). This
dramatic difference could be due to the competing oxidation
of chloride to chlorine.[8]
Figure 7. Chemical water oxidation catalyzed by complex
8 (5.5 mm/
1.1 mm) employing [Ru(bpy)3](PF6)3 (40 mm) as the oxidant in an aqueous
A
ACHTUNGTRENNUNG
^
^
phosphate buffer (0.1m, pH 7.2, 0.5 mL). =5.5 mm, =1.1 mm.
By substituting [Ru
dizing photosensitizer, [Ru
bis(ethoxycarbonyl)-2,2’-bipyridine),
G
ACHTUNGTRENNUNG
E
G
ACHTUNGTRENNUNG
system rapidly generated O2 under illumination by visible
light (Figure 10). Oxygen evolution lasted for 20 min and
the TON was determined to ca 250/20 min. Addition of
more photosensitizer led to the generation of more O2, con-
firming that decomposition of the photosensitizer is a major
limitation.
Additional experiments were made in a weaker phosphate
buffer (0.01m instead of 0.1m), which after addition of cata-
lyst, sensitizer, and persulfate gave a pH of 5. Illumination
of this solution produced no oxygen. However, after neutral-
izing the pH to 7 with calcium hydroxide and upon illumina-
tion, oxygen evolution was observed. It is not clear if this
simply reflects the pH dependence of the oxidation poten-
tial of water or if a reasonable concentration of hydroxide is
required for the nucleophilic addition to an intermediate
ruthenium oxo-species. During the catalytic cycle the ruthe-
Figure 8. Carbon dioxide evolution during water oxidation, in the pres-
ence of complex 8 (5.5 mm/1.1 mm) and [Ru
ACHTUNGTRENUN(G bpy)3]AHCUTNGTREN(NNGU PF6)3 (40 mm) as the
oxidant in an aqueous phosphate buffer solution (0.1m, pH 7.2, 0.5 mL).
^
^
=5.5 mm, =1.1 mm.
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Light-Induced Water Oxidation by a Ru complex
FULL PAPER
oxygen. 3) Replacing catalyst 8 by RuO2 (same molar quan-
tity of Ru), a well-known water oxidation catalyst,[14] led to
the generation of only substoichiometric amounts of oxygen.
Conclusion
In conclusion, we have prepared a new Ru complex 8 based
on the bio-inspired ligand 7. Complex 8 was found to effi-
ciently catalyze water oxidation with CeIV as well as
[Ru
oxidation could also be driven by visible light, employing
[Ru(bpy)2A
(deeb)]2+ as photosensitizer, to give a high TON
(bpy)3]3+ as chemical oxidants. More importantly, water
ACHTUNGTRENNUNG
A
CHTUNGTRENNUNG
of approximately 250/20 min.
Future work will focus on linking the photosensitizer and
the water oxidation catalyst to improve the electron transfer
from the oxidation catalyst to the photooxidized photosensi-
tizer. Also attempts to attach this system to an acceptor sur-
face will be initiated.
Figure 10. Light-driven oxygen evolution catalyzed by complex
(5.4 mm), in an aqueous phosphate buffer solution (0.1m, pH 7.2, 1.0 mL),
8
~
*
in the presence of [RuACHTUNGTRENNUNG(bpy)2AHCNTURGTEGNN(UN deeb)]CAHTUNGTRENN(NUG PF6)2 ( ) or [RuACTHUGNTRENNG(NU bpy)3]ACHTUNGERTNNG(NU PF6)2 ( )
(99 mm) as photosensitizer and Na2S2O8 (20 mm) as sacrificial electron ac-
ceptor.
III,III
nium is presumably oxidized from the initial Ru2
state to
Ru2V, V. Earlier work by Meyer, with the “blue dimer”, shows
that the mechanism for water oxidation is kinetically very
complicated.[9d] This may also be true for catalyst 8, thus
partly explaining the sensitivity of catalyst 8 to the reaction
conditions in water oxidation.
To show that the oxygen is indeed derived from water,
also in the light-driven oxidation, an experiment with la-
beled water (4.5% 18O) was performed (Figure 11). The ex-
perimental ratio 16,18O2/16,16O2 was determined to 0.094,
which is exactly the theoretical ratio.
Experimental Section
Materials and general methods: [Ru
A
ACHTUNGTRENNUNG(bpy)3]-
[16]
AHCTUNGTRENNUNG
agents including solvents were obtained from commercial suppliers and
used directly without further purification. All solvents were dried by
standard methods when needed. 1H and 13C NMR spectra were recorded
at 400 MHz and at 100 MHz, respectively. Chemical shifts (d) are report-
ed in ppm, using the residual solvent peak [CDCl3 (d(H)=7.26 and
d(C)=77.16), [D6]DMSO (d(H)=2.50 and d(C)=39.52), CD3OD
(d(H)=3.31 and d(C)=49.00)] as internal standard. Splitting patterns
are denoted as s (singlet), d (doublet), t (triplet), q (quartet), m (multip-
let), and br (broad). Flash chromatography was carried out with 60 ꢁ
(particle size 35–70 mm) normal phase silica gel. High resolution mass
spectra measurements were recorded on a Bruker Daltonics microTOF
spectrometer with an electrospray ionizer. Cyclic voltammetric (CV)
measurements were carried out with an Autolab potentiostat with a
GPES electrochemical interface (Eco Chemie), using a glassy carbon
disk (diameter 3 mm) as the working electrode, and a platinum spiral as
counter-electrode. The reference electrode was an Ag/AgCl electrode
(3m KCl aqueous solution). The electrolyte used was a pH 7.2 phosphate
buffer (0.1m). All potentials are reported vs. NHE, using the
[Ru
standard. Half-wave potentials (E1/2) were determined by cyclic voltam-
metry as the average of the anodic and cathodic peak potentials (E1/2
(bpy)3]3+/[Ru(bpy)3]2+ couple (E1/2 =1.26 V vs. NHE) as an internal
ACHUTGTNRENNUG CAHTUNGTRENNUGN
=
(Epa+Epc)/2). IR spectra were recorded on a Perkin–Elmer Spectrum
One spectrometer, using samples prepared as KBr discs. The UV/Vis ab-
sorption spectra were measured on a CARY 300 Bio UV/Visible spectro-
photometer. Elemental analyses were carried out at Mikroanalytisches
Laboratorium Kolbe, Mꢄlheim an der Ruhr, Germany, and Analytische
Laboatorien, Prof. Dr. H. Malissa und G. Reuter GmbH, Lindlar, Ger-
many. EPR spectra were recorded at 77 K on a Varian E9 spectrometer
equipped with a quartz insert for liquid nitrogen. Measurements were
made with a microwave frequency of 9.12 GHz, a microwave power of
2 mW and a modulation amplitude of 2 mT.
Figure 11. Light-driven water oxidation catalyzed by complex 8 (4.8 mm),
in an aqueous phosphate buffer solution (0.1m, pH 7.2, 0.46 mL) contain-
ing 4.5% 18O, in the presence of [Ru
ACHTUGNTRENNN(UG bpy)2ACHTUNRTGEGN(NUN deeb)]ACHTNUGTRNEN(UGN PF6)2 (110 mm) as pho-
^
tosensitizer and Na2S2O8 (21 mm) as sacrificial electron acceptor. =O2,
& =16,16O2,
=
16,18O2.
&
2-Methoxyisophthalic acid (3): A solution of KOH (34.0 g, 606 mmol) in
H2O (200 mL) was added to a suspension of 2,6-dimethylanisole (25.0 g,
184 mmol), KMnO4 (191.5 g, 1.21 mol) and H2O (500 mL). After heating
at 808C overnight, the mixture was cooled to RT and the solid was fil-
tered off. Upon acidification of the filtrate with concentrated HCl to
pH 1, a white precipitate was formed and filtered off, washed with H2O
and Et2O, and dried under vacuum to obtain 3 as a white solid (13.3 g,
To demonstrate that water oxidation is indeed catalyzed
by 8 a series of control experiments were conducted: 1) In
the absence of catalyst 8, no oxygen was produced. 2) Irradi-
ation in the absence of sensitizer did not generate any
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7957

B. ꢁkermark et al.
37%). Spectral data were in accordance with those previously reported
in the literature.[17]
J=8.30, 7.65, 1.35 Hz, 2H), 7.20 (t, J=7.84 Hz, 1H), 7.04 (d, J=8.30 Hz,
2H), 6.98 ppm (t, J=7.65 Hz, 2H); 13C NMR (100 MHz, CD3OD): d=
157.8, 155.7, 142.8, 133.1, 131.2, 131.2, 128.0, 121.2, 120.9, 117.7, 117.2,
115.6, 114.4 ppm; IR (KBr): n˜max =3100, 1692, 1627, 1584, 1497, 1463,
2-(Bromoacetyl)anisole (4): 2-Methoxyacetophenone (0.75 g, 5.0 mmol)
and N-bromosuccinimide (0.89 g, 5.0 mmol) were triturated together with
p-toluenesulfonic acid (0.095 g, 0.50 mmol) in a porcelain mortar for
5 min to form a yellow viscous reaction mixture which was left to stand
at room temperature for 3 h. H2O (5 mL) was added, followed by extrac-
tion with CH2Cl2 (2ꢅ10 mL). The organic phases were combined and
washed with water (10 mL), dried over Na2SO4 and the solvent was
evaporated under reduced pressure. The pale grey oil was recrystallized
from EtOH to yield 4 as white crystals (0.97 g, 84%). 1H NMR (CDCl3,
400 MHz): d=3.95 (s, 3H; OCH3), 4.61 (s, 2H; CH2), 6.99 (dd, J=8.53,
0.76 Hz, 1H; Ph-H), 7.04 (ddd, J=8.37, 6.88, 0.95 Hz, 1H; Ph-H), 7.52
(ddd, J=8.52, 7.28, 1.85, 1H; Ph-H), 7.83 ppm (dd, J=7.74, 1.85 Hz, 1H;
Ph-H); 13C NMR (100 MHz, CDCl3): d=192.4, 158.9, 134.9, 131.6, 125.0,
128.3, 121.2, 111.7, 55.9, 37.8 ppm; HRMS (ESI): m/z calcd for
C9H9O2Br+Na+: 250.9678 [M+Na+]; found: 250.9681.
1233, 1210 cmÀ1
;
HRMS (ESI): m/z calcd for C24H19N4O3: 411.1452
[M+H+]; found: 411.1444; Anal. Calcd. (%) for C57H61I3N8O11
[2(7)·3HI·3(CH3)2CO·2H2O]: C 48.39, H 4.35, I 26.9, N 7.92; found:
AHCTUNGTRENNUNG
C 48.25, H 4.54, I 29.5, N 7.99.
Synthesis of Ru complex 8·MeOH·9H2O. Ligand 7 (67 mg, 0.095 mmol)
and Et3N (0.17 mL, 1.2 mmol) were dissolved in MeOH (2 mL). [Ru-
AHCTUNGRTEGN(NNU DMSO)4Cl2] (121 mg, 0.25 mmol) was added to this solution and the
mixture was refluxed for 24 h. Then 4-methylpyridine (0.34 mL,
3.5 mmol) was added, and the mixture was further refluxed for 48 h.
After cooling to RT, the reaction mixture was filtered to remove a yellow
precipitate. After addition of H2O (2 mL) to the filtrate, the complex pre-
cipitated as a dark-green solid. The precipitate was isolated by centrifu-
gation, washed with water (5ꢅ4 mL), and dried under vacuum to afford
8 (83 mg, quant.). IR (KBr): n˜max =3410, 3050, 2920, 1936, 1619, 1500,
Bis(2-(2-methoxyphenyl)-2-oxoethyl)-2-methoxyisophthalate (5): 2-Me-
thoxyisophthalic acid (18.3 g, 93.3 mmol) and Cs2CO3 (32.6 g, 93.3 mmol)
were suspended in EtOH (250 mL) and stirred vigorously at RT for 2 h.
The solvent was evaporated and the white solid was dried over phospho-
rus pentoxide before being suspended in DMF (300 mL). When com-
pound 4 (43.0 g, 186 mmol) was added, the reaction mixture turned light
brown. After stirring at RT overnight the solvent was evaporated, result-
ing in a pale brown solid which was extracted with CH2Cl2 (300 mL). The
white salt was removed by filtration and the solvent was evaporated to
give a brown solid. Recrystallization from EtOAc yielded 5 as pale
brown crystals (42.2 g, 92%). 1H NMR (400 MHz, CDCl3): d=3.98 (s,
6H; OCH3), 4.04 (s, 3H; OCH3), 5.49 (s, 4H; CH2), 7.04 (dd, J=8.35,
0.68 Hz, 2H; Ph-H), 7.06 (ddd, J=7.58, 7.29, 1.02 Hz, 2H; Ph-H), 7.24–
7.29 (m, 1H; Ph-H), 7.54 (ddd, J=8.41, 7.31, 1.85 Hz, 2H; Ph-H), 7.97
(dd, J=7.79, 1.85 Hz, 2H; Ph-H), 8.15 ppm (d, J=7.74 Hz, 2H; Ph-H).
13C NMR (100 MHz, CDCl3): d=55.7, 64.2, 70.7, 111.7, 121.2, 123.6,
124.43 126.4, 131.2, 135.1, 135.8, 159.7, 160.4, 165.2, 192.8 ppm. HRMS
(ESI): m/z calcd for C27H24O9+Na+: 515.1313 [M+Na+]; found:
515.1323.
1445, 1286, 1209, 1033, 812, 753 cmÀ1
;
Anal. Calcd (%) for
C73H81IN12O16Ru2 [Complex 8·MeOH·9H2O]: C 51.2, H 4.8, N 9.8, found:
C 51.8, H 4.8, N 9.5; 1008C; Anal. Calcd (%) for C60H63IN10O15Ru [Com-
plex 8–2 pic·9H2O]: C 48.3, H 4.2, N 9.4; found: C 47.9, H 4.2, N 9.1;
1508C: Anal. Calcd. (%) for C48H49IN8O15Ru2 [Complex 8–4 pic·9 H2O]:
C 44.1, H 3.8, N 8.6; found: C 43.9, H 3.8, N 8.7.
Oxygen evolution measurements: Oxygen evolution was measured by
mass spectrometry. We have previously used the MS technique to study
oxidation/reduction of water.[6a,19,20] In this study we have used the MS
technique to study chemical oxidation and light induced oxidation of
water. An aqueous stock solution X1 was made, containing 55 mm of com-
plex 8 and 0.05m of H3PO4. Solution X2 used in the experiments was
made by diluting the stock solution X1 ten times with a phosphate buffer
(pH 7.2, 0.1m). Solution X2 was then deoxygenated by bubbling with N2
for at least 5 min before being used in the experiments.
Chemical oxidation with CeIV: A CeIV-oxidant solution was made by dis-
solving [CeACHTNUTRGENNUG(NH4)2ACHTUNGTREN(NUGN NO3)6] (56 mg, 100 mmol) in water (3.7 mL, pH ca. 1
adjusted with CF3SO3H) and the solution was bubbled with N2 for at
least 5 min before being injected into the reaction chamber. Two experi-
ments were made with different solutions of the Ru complex. The first
solution was a mixture of X1 (0.01 mL, 0.55 nmol) and water (0.2 mL,
pH ca. 1 adjusted with CF3SO3H). The second solution was a mixture of
X1 (0.1 mL, 5.5 nmol) and water (0.1 mL, pH 1 adjusted with CF3SO3H).
These solutions were, one at the time, placed in the reaction chamber
and the reaction chamber was evacuated for ca. 10 min to remove O2
from the water solution. Ca. 30 mbar He was then introduced in to the
reaction chamber and after an additional 5 min the CeIV solution was in-
jected.
2,6-(2-Methoxyphenylimidazolyl)anisole (6): Compound 6 was prepared
according to a modified published procedure for conversion of ketoesters
into imidazoles.[18] The diester 5 (3.76 g, 7.6 mmol) and NH4OAc (17.66 g,
228 mmol) were suspended in o-xylene (35 mL) and the mixture was re-
fluxed using a Dean–Stark trap. Periodic aliquots were taken and ana-
lyzed by MS to determine the presence of the monocyclized intermedi-
ate. When reaction was complete, after a time period varying between
40 min and 2 h, the mixture was cooled to RT. The reaction mixture was
then diluted with EtOAc (50 mL) and washed with a saturated aqueous
solution of NaHCO3. The organic phase was dried with MgSO4 and the
solvents were evaporated under reduced pressure, resulting in a yellow
residue which was purified by flash chromatography. Elution with a gra-
dient from pentane (100%) to EtOAc (100%) gave compound 6 as a
yellow oily solid (1.12 g, 33%). 1H NMR (400 MHz, CDCl3): d=8.25 (d,
J=7.80 Hz; 2H), 7.88 (br, 2H), 7.65 (s, 2H), 7.35 (t, J=7.80 Hz, 1H),
7.28 (ddd, J=8.34, 7.54, 1.61 Hz, 2H), 7.08 (dt, J=7.54, 1.02 Hz, 2H),
7.03 ppm (d, J=8.34 Hz, 2H); 13C NMR (100 MHz, CDCl3): d=155.1,
153.4, 142.9, 134.3, 129.3, 128.3, 127.0, 126.1, 123.9, 121.8, 111.7, 62.2,
55.9 ppm; HRMS (ESI) m/z calcd for C27H25N4O3: 453.1921 [M+H+];
found: 411.1924.
Chemical oxidation with RuIII: [Ru
ACHTNUGTRENNG(U bpy)3]ACHTUNGTREN(NUGN PF6)3 (20.4 mg, 20.3 mmol) was
placed in the reaction chamber and the reaction chamber was evacuated
with a rough pump for 5 min. 43 mbar He was then introduced into the
system. Solution X2 (0.5 mL, 2.8 nmol) was then injected into the reac-
tion chamber (5 min after that He was introduced). The generated
oxygen was then measured and recorded versus time with the mass spec-
trometer.
18O-Isotopic labeling experiments: [Ru
ACHTNUGTRENNG(U bpy)3]ACHTUNTGREN(NUGN PF6)3 (20.4 mg, 20.3 mmol)
was placed in the reaction chamber. The reaction chamber was then
evacuated and after 5 min 44 mbar He was introduced into the system.
At the same time 97% 18O water (0.05 mL) was added to X2 (0.5 mL,
2.8 nmol). After 5 min the solution was injected into the reaction cham-
ber. The generated oxygen was then measured and recorded versus time
with the mass spectrometer. The 18O concentration in the water was mea-
sured with the mass spectrometer from the ratio between m/z 18 (H216O)
and 20 (H218O).
2,6-(2-Hydroxphenylimidazolyl)phenol (7)·1.5HI·1.5ACHTUNGTENR(UNNG CH3)2CO·H2O: A
suspension of compound 6 (1.81 g, 4.0 mmol) in hydriodic acid (57 wt%
in H2O, 40 mL) was refluxed under argon for 17 h. After cooling the re-
action mixture to RT the yellow precipitate was filtered off and washed
with water. Recrystallization from methanol/acetone gave the desired
compound as an orange solid (1.82 g, 64%). 1H NMR (400 MHz,
[D6]DMSO): d=14.67 (br, 2H), 14.11 (br, 2H), 10.52 (br, 2H), 8.14 (d,
J=7.84 Hz, 2H), 7.97 (s, 2H), 7.85 (dd, J=7.65, 1.35 Hz, 2H), 7.25 (ddd,
7958
www.chemeurj.org
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 7953 – 7959

Light-Induced Water Oxidation by a Ru complex
FULL PAPER
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Acknowledgements
We thank Dr. Y. Xu for a sample of [RuACHTUNGERTN(GUNN bpy)2ACHTUNTRGEG(NNUN deeb)]CAHTNUGTRNEN(UGN PF6)2, the Knut &
Alice Wallenberg Foundation, and The Swedish Energy Agency for fi-
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Received: December 22, 2010
Published online: May 26, 2011
Chem. Eur. J. 2011, 17, 7953 – 7959
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7959