A Dawson-type dirhenium(V)-oxido-bridged polyoxotungstate: X-ray crystal structure and hydrogen evolution from water vapor under visible light irradiation

Article abstract of DOI:10.1002/ejic.200701124

The synthesis and crystal structure of a Dawson-type dirhenium(V)-oxido- bridged polyoxotungstate are described. The potassium salt K ₁₄[O{Re^V(OH)(a₂-P₂W ₁₇O₆₁)}₂]·21H₂O (K-1) was obtained as analytically pure, homogeneous, black-purple crystals by the reaction of a monolacunary Dawson polyoxotungstate with [Re^IVCl ₆]²- in water, followed by crystallization from an aqueous HCl solution in the dark. Single-crystal X-ray structure analysis revealed that the dimeric dirhenium(V)-oxido-bridged structure has overall C_2h symmetry. Characterization of K-1 was also accomplished by elemental analysis, TG/DTA, and FTIR, UV/Vis absorption, ESR, and solution ³¹P NMR spectroscopy. Furthermore, the black-purple compound K-1 was grafted onto a TiO₂ surface by electrostatic binding to give TiO₂ with a cationic quaternary ammonium moiety (1-grafted TiO₂). Elemental analysis of 1-grafted TiO₂ indicated that the seven potassium counterions of K-1 were ion-exchanged with seven cationic ammonium moieties to form {TiO₂}₅₅₀₀[≡Si(CH₂) ₃N(CH₃)₃Cl]₇[≡Si(CH ₂)₃-N⁺(CH₃)₃] ₇(K₇[O{Re(OH)(a₂-P₂W ₁₇O₆₁)}₂]⁷-) and seven molecules of KCl. The diffuse reflectance (DR) UV/Vis spectrum of 1-grafted TiO₂ exhibited two sharp bands at 496 nm and 751 nm due to the Re^V→ W^VI intervalence charge transfer (IVCT) band and the d-d band of the rhenium(V) atom, respectively, in the visible light region, which suggested that polyoxoanion 1 was isolated on the TiO₂ surface. With 1-grafted TiO₂, hydrogen evolution from water vapor under irradiation with visible light (>400 nm and >420 nm) was achieved. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200701124

FULL PAPER
DOI: 10.1002/ejic.200701124
A Dawson-Type Dirhenium(V)-Oxido-Bridged Polyoxotungstate: X-ray Crystal
Structure and Hydrogen Evolution from Water Vapor under Visible Light
Irradiation
Chika Nozaki Kato,*[a] Kazunobu Hara,[a] Ayumi Hatano,[b] Kazuhiro Goto,[b]
Tomonori Kuribayashi,[b] Kunihiko Hayashi,[b] Akira Shinohara,[b] Yusuke Kataoka,[b]
Wasuke Mori,[b] and Kenji Nomiya[b]
Keywords: Polyoxometalates / Rhenium / X-ray diffraction / UV/Vis spectroscopy / Photocatalysis
The synthesis and crystal structure of a Dawson-type dirheni-
um(V)-oxido-bridged polyoxotungstate are described. The
potassium salt K14[O{ReV(OH)(α2-P2W17O61)}2]·21H2O (K-1)
was obtained as analytically pure, homogeneous, black-pur-
ple crystals by the reaction of a monolacunary Dawson poly-
oxotungstate with [ReIVCl6]2– in water, followed by crystalli-
zation from an aqueous HCl solution in the dark. Single-crys-
tal X-ray structure analysis revealed that the dimeric dirheni-
um(V)-oxido-bridged structure has overall C2h symmetry.
Characterization of K-1 was also accomplished by elemental
analysis, TG/DTA, and FTIR, UV/Vis absorption, ESR, and
solution 31P NMR spectroscopy. Furthermore, the black-pur-
ple compound K-1 was grafted onto a TiO2 surface by elec-
trostatic binding to give TiO2 with a cationic quaternary
ammonium moiety (1-grafted TiO2). Elemental analysis of 1-
grafted TiO2 indicated that the seven potassium counterions
of K-1 were ion-exchanged with seven cationic ammonium
moieties to form {TiO2}5500[ϵSi(CH2)3N(CH3)3Cl]7[ϵSi(CH2)3-
N+(CH3)3]7(K7[O{Re(OH)(α2-P2W17O61)}2]7–) and seven mole-
cules of KCl. The diffuse reflectance (DR) UV/Vis spectrum
of 1-grafted TiO2 exhibited two sharp bands at 496 nm and
751 nm due to the ReVǞWVI intervalence charge transfer
(IVCT) band and the d–d band of the rhenium(V) atom,
respectively, in the visible light region, which suggested
that polyoxoanion 1 was isolated on the TiO2 surface. With
1-grafted TiO2, hydrogen evolution from water vapor under
irradiation with visible light (Ͼ400 nm and Ͼ420 nm) was
achieved.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
material absorbs light at wavelengths below 535 nm),[2] vis-
ible-light-response TiO2 thin films prepared by a radio-fre-
quency magnetron sputtering deposition method,[3] NiOx
(partly oxidized nickel) promoted In0.9Ni0.1TaO4,[4] (AgIn)x-
Zn2(1–x)S2,[5] and (Ga1–xZnx)(N1–xOx) with RuO2, transi-
tion-metal mixed-oxides containing Cr, and noble-metal/
Cr2O3 (core/shell) nanoparticles as co-catalysts.[6] These
materials are powerful photocatalysts for water splitting un-
der visible light irradiation; however, the search for the use
of other materials as visible-light-response photocatalysts is
still an interesting objective.
Polyoxometalates (POMs) have attracted considerable at-
tention because of their extreme versatility and unique
range of properties, including catalytic and biological activi-
ties and/or photochemical, electrochromic, and magnetic
properties.[7] However, there is no report regarding the use
of POMs as photocatalysts for the water splitting reaction
under visible light irradiation. H3PW12O40 encapsulated
into the titanium-exchanged HY(TiHY) zeolite required
both UV and visible light for the water splitting reaction.[8]
To improve the visible light absorption of POMs, we fo-
cused on the synthesis of Dawson-type dirhenium(V)-
oxido-bridged POM, [O{ReV(OH)(α2-P2W17O61)}2]14– (1).
Then, we prepared a new type of POM-based photocatalyst
Introduction
The construction of an efficient device for the photosplit-
ting of water to hydrogen is one of the most important sub-
jects from the viewpoint of solar light energy utilization and
storage since Honda and Fujishima first demonstrated a
photoelectrochemical cell consisting of a TiO2 photoanode
and a Pt cathode to decompose water into hydrogen and
oxygen under ultraviolet (UV) irradiation with an external
bias.[1] The design and preparation of new photocatalysts
that are responsive in a similar manner to visible light are
a key target for the utilization of solar energy for hydrogen
production. To develop efficient photocatalysts that work
under visible light irradiation, a number of attempts have
been made, e.g., a chemically modified n-type TiO2 by con-
trolled combustion of Ti metal in a neutral gas flame (this
[a] Department of Chemistry, Faculty of Science, Shizuoka Uni-
versity,
836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
Fax: +81-54-237-3384
E-mail: sckatou@ipc.shizuoka.ac.jp
[b] Department of Materials Science, Faculty of Science, Kana-
gawa University,
Tsuchiya 2946, Hiratsuka, Kanagawa 259-1293, Japan
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
3134
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Eur. J. Inorg. Chem. 2008, 3134–3141

Dawson-Type Dirhenium(V)-Oxido-Bridged Polyoxotungstate
material by the electrostatic grafting of polyoxoanion 1
onto an anatase TiO2 surface through the cationic ammo-
nium moiety (1-grafted TiO2).
In this paper, we report the full details of the synthesis
and characterization of the potassium salt K14[O{ReV-
(OH)(α2-P2W17O61)}2]·21H2O (K-1) and the 1-grafted TiO2
material. Furthermore, the photocatalytic performance of
1-grafted TiO2 material in hydrogen formation from water
vapor under irradiation with visible light (Ͼ400 nm and
Ͼ420 nm) is investigated.
Results and Discussion
Synthesis, Characterization, and Molecular Structure of a
Potassium Salt of the Dawson-Type Dirhenium(V)-Oxido-
Bridged POM (K-1)
Figure 1. Molecular structure of K-1 with 50% probability ellip-
soids.
Table 1. Selected bond lengths [Å] and angles [°] for K-1 around
the Re–O–Re linkage.
The potassium salt of compound 1, K14[O{ReV(OH)(α2-
P2W17O61)}2]·21H2O (K-1), was synthesized by a slight
modification of the published method for the synthesis of
monomeric monorhenium(V)-substituted POM K7[α2-
ReVOP2W17O61]·2H2O.[9] Compound K-1 was formed by
heating at reflux the mixture of K2ReIVCl6 and monolacun-
ary Dawson POM [α2-P2W17O61]10– in an aqueous solution
in air followed by reprecipitation from hot water in the
dark. The pH of the solution changed from 3.40 to 1.54
during the reaction. The formation of K-1 is given by the
ionic balance shown in Equation (1), in which rhenium(IV)
is oxidized to rhenium(V).
Bond lengths
Re(1)–O(1M)
Re(1)–O(1H)
Re(1)–O(1)
1.7690(14) Re(1)–O(5A)
1.939(17)
1.917(19)
1.951(17)
2.369(14)
1.942(17)
Re(1)–O(9A)
Re(1)–O(10)
Bond angles
O(1M)–Re(1)–O(1H) 178.9(4)
O(1M)–Re(1)–O(1) 90.1(5)
O(1M)–Re(1)–O(5A) 90.5(5)
O(1M)–Re(1)–O(9A) 88.5(5)
O(1M)–Re(1)–O(10) 87.5(5)
O(9A)–Re(1)–O(1)
O(1)–Re(1)–O(10)
O(9A)–Re(1)–O(5A) 89.1(8)
O(5A)–Re(1)–O(10)
O(9A)–Re(1)–O(10)
W(1)–O(1)–Re(1)
W(2)–O(5)–Re(1A)
W(6)–O(9)–Re(1A)
W(7)–O(10)–Re(1)
90.9(8)
89.8(7)
90.3(7)
176.0(7)
153.6(10)
153.8(10)
167.2(11)
166.3(10)
O(1)–Re(1)–O(1H)
88.9(6)
O(5A)–Re(1)–O(1H) 90.5(6)
O(9A)–Re(1)–O(1H) 92.1(7)
O(10)–Re(1)–O(1H) 91.9(6)
2–
2 ReIVCl6 + 2 [α2-P2W17O61]10– + 3 H2O Ǟ
[O{ReV(OH)(α2-P2W17O61)}2]14– + 4 H+ + 12 Cl– (1)
O(5A)–Re(1)–O(1)
179.5(8)
Re(1A)–O(1M)–Re(1) 180
The crystallization of K-1 for the single-crystal X-ray
analysis was performed by slow evaporation from an aque-
The bond valence sum (BVS) value of the Re atom is
ous HCl solution (pH = 1.5) in the dark. Good crystals 5.782; which is in the range 5.556–6.864 that was observed
could not be obtained in water. The X-ray crystallographic for ReV–O–ReV complexes,[11] above the range of the BVS
structural analysis of K-1 exhibits a dimeric dirhenium(V)- values observed in ReIII–O–ReIII complexes (5.298–
oxido-bridged structure, as shown in Figure 1. The struc- 5.336),[13] but below the range of BVS values observed in
ture of K-1 is that of a (P2W17Re)2-O dimer with a Re– ReVII–O–ReVII complexes (7.557–7.920).[14] Therefore, on
O–Re linkage, which is similar to the structure of Dawson the basis of Re–oxygen bond lengths, valence sums, bulk
diruthenium(IV)-oxido-bridged POM [O{RuIV(Cl)(α2- diamagnetism, and charge balance from the analytical stoi-
10–
P2W17O61)}2]16– [10]
Both α2-P2W17O61 units act as tetra- chiometry, we assign the +5 oxidation state to the rhenium
.
dentate chelating and bridging ligands to the Re atoms and atoms. Note also that each Re was clearly 6- and not 7-
possess a center of symmetry for the anion. The two sets coordinate, as shown in Figure 1. The BVS values of the
of four oxygen atoms attached to the Re atoms in a plane 17W atoms are in the range 5.912–6.442 (average: 6.229),
perpendicular to the Re(1)–O(1M)–Re(1A) axis are eclipsed and those of the two P atoms are in the range 5.002–5.019
to give a C2h symmetry. The length of the Re(1)–O(1M) (5.011). These values correspond reasonably well to the for-
bond coordinated to the four oxygen atoms in the lacunary mal valances W6+ and P5+, respectively (see Tables S1 and
site is 1.7690(14) Å, which is smaller than that of ReV–O S2 in the Supporting Information).
in O=ReV–O–ReV=O (ca. 1.9 Å)[11] but similar to that in
The crystal structure of K-1 is consistent with the ele-
[O{RuIV(Cl)(α2-P2W17O61)}2]16– (1.773 Å) (see Table 1).[10] mental analysis, TG/DTA data, UV/Vis absorption, FTIR,
The Re–O(1H) bond length [2.369(14) Å; trans to the Re– ESR, and solution 31P NMR spectra. The elemental analy-
O–Re in 1] is longer than those of the ReV–OH bonds sis had to be performed for compound K-1 after drying it
(1.811–1.970 Å) in monomeric complexes of the type at room temperature at 10–3–10–4 Torr overnight. The result
[ReVO(OH)L4] (L = pyridine, CN–, cyclam, en).[12]
was consistent with the composition K14[O{ReV(OH)(α2-
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C. N. Kato et al.
FULL PAPER
P2W17O61)}2]·6H2O. The weight loss observed during the referenced to 25%H3PO4), indicating the complete coordi-
course of drying before analysis was 2.8% for K-1; this cor- nation of rhenium atoms by the monovacant site of [α2-
responds to 15–16 weakly solvated or adsorbed water mole- P2W17O61]10– and the high purity of K-1. It should be noted
cules. The TG/DTA measurement performed under ambi- that the spectral pattern of K-1 is quite similar to that of
ent conditions showed a weight loss of 3.93% without a the reported monomeric monorhenium(V)-substituted
clear endothermic point; this value corresponds to 21 water POM K7[α2-ReVOP2W17O61]·2H2O, but the chemical
molecules including both the intrinsic water of hydration shift was somewhat different from the values of –11.86
and the water adsorbed from the atmosphere. Thus, the and –12.86 ppm (referenced to 85%H3PO4) for K7[α2-
number of hydrated water molecules in compound K-1 is ReVOP2W17O61]·2H2O. The 31P NMR spectrum of the crys-
21. The elemental analysis of the crystalline sample of K-1 talline sample of K-1 crystallized from an aqueous HCl
also showed the same composition as the powder sample solution also showed the same chemical shifts (δ = –12.46
and the absence of chloride ion after the crystallization and –13.45 ppm; referenced to 25%H3PO4) as those of the
from an aqueous HCl solution.
powder sample. No ESR signals of solid K-1 at room tem-
The FTIR spectrum of compound K-1 measured by perature were detected over a wide range of magnetic fields,
using a KBr disk is shown in Figure 2. The positions of all also suggesting that K-1 is diamagnetic compound.
the bands (1091, 1018, 954, 907, 787, and 526 cm–1) in the
polyoxoanion region of this compound are similar to those
of [α-P2W18O62]6– (1091, 1020, 958, 912, 777, and 528 cm–1)
rather than to those of [α2-P2W17O61]10– (1086, 1054, 1016,
940, 885, 811, 741, 601, 527, and 467 cm–1), suggesting the
coordination of rhenium atoms by the monovacant site of
the [α2-P2W17O61]10–
.
Figure 3. 31P NMR spectrum of K-1 in D2O. The resonance at
0.0 ppm is due to the external reference: 25% H3PO4 in H2O.
Figure 2. FTIR spectrum, measured by using a KBr disk, in the
polyoxoanion region (1800–400 cm–1) of K-1.
The 31P NMR spectrum of K-1 in D2O at about 25 °C
has a clear two-line spectrum at δ = –12.38 and –13.39 ppm
(referenced to 25%H3PO4) and δ = –12.06 and –13.05 ppm
(referenced to 85%H3PO4), showing that K-1 is a diamag-
netic compound with an antiferromagnetic interaction be-
tween the two ReV atoms (see Figure 3). The downfield res-
onance was assigned to the phosphorus closest to the rhe-
nium sites and the upfield resonance to the phosphorus
closer to the W3 cap. These signals exhibit a shift from
those of [α2-P2W17O61]10– (δ = –6.96 and –14.10 ppm; refer-
Figure 4. UV/Vis spectrum of K-1 in H2O in the range 200–800 nm
enced to 25%H3PO4) and [α-P2W18O62]6– (δ = –12.75 ppm; (ϫ10–6 ). Inset: in the range 400–800 nm (ϫ10–4 ).
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Eur. J. Inorg. Chem. 2008, 3134–3141

Dawson-Type Dirhenium(V)-Oxido-Bridged Polyoxotungstate
The UV/Vis spectrum of K-1 in water has four absorp-
tion bands at 254 nm (ε = 9.39ϫ104 –1 cm–1), 298 nm (ε =
6.28ϫ104 –1 cm–1), 496 nm (ε = 6.61ϫ103 –1 cm–1), and
737 nm (ε = 4.02ϫ103 –1 cm–1), as shown in Figure 4. Two
large bands at 254 nm and 298 nm were assigned to the
charge transfer (CT) bands of W–O, and two small bands
at 496 nm and 737 nm were assigned to the ReVǞWVI inter-
valence charge transfer (IVCT) band and d-d band of the
rhenium(V) atom, respectively.[15,16]
Preparation, Characterization, and Photocatalytic
Performance of Anatase TiO2 Functionalized by a Cationic
Ammonium Moiety by Impregnation with 1
The black-purple compound K-1 that was prepared was
grafted onto a TiO2 surface by electrostatic binding to give
TiO2 functionalized with cationic quaternary ammonium
groups according to the published method.[17] This material
(abbreviated as TiO2–N+) was obtained by the addition of a
50% methanol solution of N-trimethoxysilylpropyl-N,N,N-
trimethylammonium chloride, (MeO)3Si(CH2)3N+(CH3)3-
Cl, to a TiO2 suspension in methanol at 25 °C. In this reac-
tion, the methoxy groups of the silane coupling reagent re-
acted cleanly with the hydroxy groups of TiO2 to eliminate
methanol and form the isolated surface species ϵSi(CH2)3-
N(CH3)3Cl. Then, polyoxoanion 1 was grafted onto TiO2–
N+ by employing the ion-exchange reaction of potassium
ion with a cationic ammonium moiety in water at 25 °C.
The product obtained is abbreviated as 1-grafted TiO2. Ac-
cording to the elemental analysis result, the loading of K-1
was found to be 2.2 µmolg–1 [the amount of the surface
cationic quaternary ammonium groups, ϵSi(CH2)2N(CH3)3-
Cl, was 31.7 µmolg–1]. The amount of potassium observed
in the elemental analysis of 1-grafted TiO2 was 0.032%, in-
dicating that the seven potassium counterions of K-1 were
ion-exchanged with the seven cationic ammonium moieties
to form {TiO2}5500[ϵSi(CH2)3N(CH3)3Cl]7[ϵSi(CH2)3N+-
Figure 5. Diffuse reflectance UV/Vis spectra of (a) as-prepared 1-
grafted TiO2, (b) 1-grafted TiO2 after H2 evolution from water
vapor (20 Torr) under irradiation with visible light (Ͼ400 nm), and
(c) 1-grafted TiO2 after H2 evolution from water (10 mL) in the
liquid-phase reaction containing Na2H2EDTA·2H2O under irradi-
ation with visible light (Ͼ400 nm).
(CH3)3]7(K7[O{Re(OH)(α2-P2W17O61)}2]7–)
and
seven
molecules of KCl. Thus, 49% of the surface silane coupling any additives in heterogeneous systems, as shown in
reagent of TiO2–N+ was covered by the grafting reaction Table 2. The reaction did not proceed when light irradiation
with K-1. The byproduct KCl can easily be removed by was stopped, even at 60 °C. With regard to the time course
washing with excess water.
of H2 evolution catalyzed by 1-grafted TiO2 under light ir-
The diffuse reflectance (DR) UV/Vis spectrum of 1- radiation (Ͼ400 nm), a linear increase in H2 with time was
grafted TiO2 (2.2 µmolg–1) showed two sharp bands at observed, as shown in Figure 6a. After 5 h, the amount of
496 nm and 751 nm in the visible light region (see Fig- H2 evolved was 19.8 µmol per g of catalyst [the rate, calcu-
ure 5a), suggesting that the surface species 1 is isolated on lated on the basis of the formula (molg–1 of H2)h–1, is
the TiO2 surface.[18] These bands might be assigned to the 3.96 µmolg–1 h–1]. After 55 h, the amount of H2 evolved
ReVǞWVI intervalence charge transfer (IVCT) band and reached 101.6 µmol per g of catalyst [the turnover number
the d–d band of rhenium(V) atom, respectively. The 31P (TON), calculated on the basis of the formula (mol of H2)/
NMR spectrum of solid 1-grafted TiO2 was not possible to (mol of 1 per g of catalyst), is 46.2] (Table 2, Entry 1). The
obtain because of the low loading of 1. The BET surface catalytic activities were reproducible on five repeated runs.
area was 39.4 m2 g–1 after the grafting of 1 onto the TiO2 19.5 µmol water vapor remained in the gas phase after 55 h,
surface, which was almost the same as that of the starting and some amounts of water vapor were adsorbed on the
TiO2 material (40.0 m2 g–1).
TiO2 surface. O2, CO2, CO, and CH4 were not detected
We initially examined the evolution of H2 from water under the present reaction conditions. The apparent quan-
vapor under light irradiation greater than 400 nm and tum yield (AQY) was quite low, i.e., Ͻ0.1% at 405 nm after
420 nm and catalyzed by 1-grafted TiO2 at 25 °C without 25 h; however, there is no report regarding AQY for H2
Eur. J. Inorg. Chem. 2008, 3134–3141
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C. N. Kato et al.
FULL PAPER
evolution from water vapor without any additives. No H2 as that of 1-grafted TiO2 (2.2 µmol 1 per g of sample,
evolution was detected in the absence of water vapor. A 4.4 µmol Re per g of TiO2) was used as a catalyst. An in-
mixture of K-1 (5.6 µmol) and TiO2 (1.0 g) in the absence duction period was not observed. After 5 h and 72 h,
of a silane coupling reagent induced 2.98 µmolg–1 and 53.8 µmolg–1 and 225 µmolg–1 of H2 were evolved, respec-
8.52 µmolg–1 of H2 evolution after 25 h and 50 h, respec- tively. These values are higher than those obtained with 1-
tively (rate: 1.19ϫ10–1 µmolg–1 h–1 after 25 h; TON: 3.9 af- grafted TiO2 (Table 2, Entry 10). These results suggested
ter 50 h) (Table 2, Entry 2). In addition, the photodecom- that both the rhenium site and TiO2 support were crucial
position of D2O vapor (20 Torr) was carried out under light for hydrogen production from water vapor under the pres-
irradiation (Ͼ400 nm). D2, H2, and HD were detected after
48 h, which were due to the decomposition of D2O vapor,
the decomposition of the surface silane coupling reagent,
and the H–D exchange reaction of D2 with H2, respectively.
These results indicate that H2 was certainly evolved from
water vapor; however, the surface silane coupling reagent of
1-grafted TiO2 was also decomposed. Under light irradia-
tion at a wavelength greater than 420 nm, a long induction
period was observed (see Figure 6b). After 93 h, the
amount of H2 production was 1.57 µmolg–1 of catalyst
(rate: 1.69ϫ10–2 µmolg–1 h–1). After 380 h, the amount of
H2 evolved reached 4.35 µmolg–1 of catalyst (TON: 1.98)
(Table 2, Entry 3), which was lower than that under light
irradiation at a wavelength greater than 400 nm. The induc-
tion period and the low activities under light irradiation
(Ͼ420 nm) might be due to the light absorption in the range
400–420 nm of 1-grafted TiO2, as shown in Figure 5a. In
control experiments under irradiation at a wavelength
greater than 400 nm, no reactions were observed when K-
1, anatase TiO2, a mixture of H3PW12O40 (5.6 µmol) and
TiO2 (1.0 g), tetra-n-butylammonium salt of 1 ([(n-C4H9)4-
N]14–xKx[O{ReV(OH)(α2-P2W17O61)}2], abbreviated as
TBA-1), and TiO2–N+ were used as catalysts (Table 2, En-
tries 4–8). In addition, 1-grafted amorphous SiO2 (11 µmol
1 per g of sample, 0.055 µmol 1 per m2 of amorphous SiO2)
prepared by the same method as that of 1-grafted TiO2
(2.2 µmol 1 per g of sample, 0.055 µmol 1 per m2 of TiO2)
exhibited no reaction (Table 2, Entry 9). In contrast, H2
evolution from water vapor under light irradiation
Figure 6. Time course for H2 evolution catalyzed by 1-grafted TiO2
under light irradiation (a) Ͼ400 nm and (b) Ͼ420 nm. Reaction
(Ͼ400 nm) was observed when the [ReIVCl6]2–-grafted TiO2
(4.4 µmol Re per g of sample) prepared by the same method conditions: see Table 2.
Table 2. Hydrogen evolution from water vapor catalyzed by 1-grafted TiO2 under irradiation with visible light.[a]
Entry
1
Catalyst
Light [nm]
Reaction time [h] H2 [µmolg–1]
Rate [µmolg–1 h–1][b]
1-grafted TiO2
Ͼ400
5
19.8
101.6
2.98
8.52
1.57
4.35
n.r.
3.96
(2.2 µmol of 1 per g of sample)
A mixture of 1 and TiO2
(5.6 µmol of 1 per g of sample)
1-grafted TiO2
55
25
50
93
380
30
30
30
2
3
Ͼ400
Ͼ420
1.19ϫ10–1
1.69ϫ10–2
(2.2 µmol of 1 per g of sample)
4
5
6
K-1[c]
Ͼ400
Ͼ400
Ͼ400
–
–
–
TiO2
n.r.
n.r.
A mixture of H3PW12O40 and TiO2
(5.6 µmol of H3PW12O40 per g of sample)
TBA-1[d]
7
8
9
Ͼ400
Ͼ400
Ͼ400
15
15
30
n.r.
n.r.
n.r.
–
–
–
TiO2–N+
1-grafted SiO2
(11 µmol of 1 per g of sample)
[ReIVCl6]2–-grafted TiO2
(4.4 µmol of Re per g of sample)
10
Ͼ400
5
72
53.8
225
10.8
[a] Reaction conditions: water (20 Torr; 257.3 µmol), catalyst (500 mg), light (Ͼ400 nm and Ͼ420 nm), 25 °C. [b] The rate was calculated
as H2 (molg–1) per reaction time (h). [c] 1 g K-1 was used. [d] 400 mg TBA-1 was used.
3138
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Dawson-Type Dirhenium(V)-Oxido-Bridged Polyoxotungstate
ent conditions, and the formation of ReIV by the electron cunary Dawson polyoxoanion. The characterization of
transfer reaction of the ReV site in 1-grafted TiO2 under compound K-1 has been accomplished by X-ray structure
light irradiation might accelerate the evolution of hydrogen analysis, elemental analysis, TG/DTA, and UV/Vis absorp-
from water with the decomposition of the surface silane tion, FTIR, ESR, and solution 31P NMR spectroscopy. The
coupling reagent on the TiO2 support.
crystal structure of K-1 reveals that the dimeric dirheni-
When photoreaction of water vapor (20 Torr) was carried um(V)-oxido-bridged structure has overall C2h symmetry.
out with twice the amount of 1-grafted TiO2 catalyst under Polyoxoanion 1 was grafted onto the TiO2 surface by elec-
light irradiation at a wavelength greater than 400 nm, the trostatic binding to give anatase TiO2 functionalized by a
production of hydrogen in the second run decreased to a cationic ammonium moiety. Elemental analysis indicated
third of that in the first run; however, the DR UV/Vis spec- that the seven potassium counterions of K-1 were ion-ex-
trum of the 1-grafted TiO2 after the photoreaction (Fig- changed with seven cationic ammonium moieties to form
ure 5b) showed the same spectral pattern as that of the as- {TiO2}5500[ϵSi(CH2)3N(CH3)3Cl]7[ϵSi(CH2)3N+(CH3)3]7-
prepared 1-grafted TiO2. In addition, elemental analysis for (K7[O{Re(OH)(α2-P2W17O61)}2]7–) (1-grafted TiO2) and
the 1-grafted TiO2 after the photoreaction revealed that the seven molecules of KCl. DR UV/Vis spectrum of 1-grafted
ratio Re/P was about 1:2. These results suggested that 1 was TiO2 exhibited two sharp bands at 496 nm and 751 nm,
stable under the present photoreaction conditions; however, which suggests that 1-grafted TiO2 absorbs visible light.
the change in the surface structure caused by the decompo- With the 1-grafted TiO2, the hydrogen evolution from water
sition of the surface silane species used as an electron donor vapor under visible light irradiation (Ͼ400 nm and
might influence the catalytic activities.
Ͼ420 nm) was achieved. To the best of our knowledge, the
To prevent the decomposition of the surface silane coup- Dawson dirhenium(V)-oxido-bridged POM [O{ReV(OH)-
ling reagent of 1-grafted TiO2, the liquid-phase photo- (α2-P2W17O61)}2]14– is the first example of the use of a
reaction of water (10 mL) in the presence of POM as a visible-light-response photocatalyst.
Na2H2EDTA·2H2O (111 mg, 30 m) as a sacrificial reagent
was repeated three times with the same 1-grafted TiO2 cata-
Experimental Section
lyst under light irradiation (Ͼ400 nm). For the first run,
5.80 µmolg–1 and 19.6 µmolg–1 of H2 were produced after
2 h and 4 h, respectively. In contrast, hydrogen production
in the second run was increased to 62.5 µmolg–1 and
131 µmolg–1, respectively, and the amounts of hydrogen
produced were almost the same in the third run
(60.5 µmolg–1 and 111.1 µmolg–1, respectively). The DR
UV/Vis spectrum of the 1-grafted TiO2 after the third pho-
toreaction (Figure 5c) also showed the same spectral
Materials: K10[α2-P2W17O61]·19H2O was prepared as described in
the literature.[20] The number of solvated water molecules was de-
termined by TG/DTA analysis. All the reagents and solvents were
obtained and used as received from commercial sources.
Instrumentation/Analytical Procedures: Elemental analyses were
carried out by Mikroanalytisches Labor Pascher (Remagen, Ger-
many). The samples were dried at room temperature at 10–3–
10–4 Torr overnight before analysis. The microanalysis of Re an K
pattern as that of the as-prepared 1-grafted TiO2. These elements was specially ordered for the 1-grafted TiO2 sample. Infra-
red spectra were recorded with a Jasco 4100 FTIR spectrometer by
using KBr disks at room temperature. Thermogravimetric analysis
(TGA) and differential thermal analysis (DTA) data were per-
formed with a Rigaku Thermo Plus 2 series TG/DTA TG 8120
instrument. TG/DTA measurements were performed in air with a
temperature ramp of 4 °C per min between 20 and 500 °C. The
31P{1H} NMR (161.70 MHz) spectra in solution were recorded in
5-mm-outer-diameter tubes with a JEOL JNM-EX 400 (Kanagawa
University) and a JEOL ECA-600 (Shizuoka University) NMR
spectrometer. The 31P{1H} NMR spectra were referenced to an ex-
results suggest that neither the surface polyanion 1 nor the
surface silane coupling reagent of 1-grafted TiO2 were de-
composed in the presence of a sacrificial reagent. The lower
activities in the first run relative to those in the second and
third run might be due to the different adsorption state of
water molecules on the surface of the catalyst under the
present liquid-phase reaction conditions.
With regard to the reaction mechanism, the excitation of
ReV to ReV* by irradiation with visible light and electron
transfer from ReV* to ReIV might occur with the decompo- ternal standard of 25% H3PO4 in H2O or 85% H3PO4 in a sealed
capillary. Chemical shifts were reported as negative on the δ scale
with resonances upfield of H3PO4 (δ = 0 ppm). Solution UV/Vis
spectra and diffuse reflectance (DR) UV/Vis spectra were recorded
with a Jasco V-570 spectrophotometer. For the DR UV/Vis mea-
surement, a Jasco diffuse reflectance attachment was used. Specific
surface areas were measured by the BET method by using an ASAP
2010 (Shimadzu) instrument. X-band EPR spectra were recorded
with a JEOL JES-RE Series EPR spectrometer at room tempera-
ture.
sition of the surface silane coupling reagent on TiO2 in the
absence of a sacrificial reagent.[19] In addition, the multie-
lectron collection and transfer for the reduction of H+ to
H2 were performed by TiO2. Further studies regarding the
reaction mechanism are in progress.
Conclusions
Synthesis and Characterization of K-1: A mixture of K2ReCl6
(0.701 g, 1.47 mmol) and K10[α2-P2W17O61]·17H2O (7.14 g,
1.47 mmol) in water (80 mL) was heated at reflux for 30 min in air.
During the refluxing, the color of the solution became a dark pur-
ple-black. The solvent was removed by rotary evaporation at 50 °C.
The synthesis and crystal structure of a Dawson-type di-
rhenium(V)-oxido-bridged polyoxotungstate is presented.
We have successfully obtained black-purple single crystals
of the water-soluble potassium salt K14[O{Re(OH)(α2-
P2W17O61)}2]·21H2O by treating [ReIVCl6]2– with a monola- The sample obtained was dissolved in hot water (ca. 9 mL) at 80 °C
Eur. J. Inorg. Chem. 2008, 3134–3141
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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C. N. Kato et al.
FULL PAPER
and placed in a refrigerator to yield a purple-black precipitate,
which was collected on a membrane filter (JG, 0.2 µm), and washed
with ethanol (3ϫ20 mL) and ether (3ϫ20 mL). Yield: 6.38 g
(89.7%). For K14[O{ReV(OH)(α2-P2W17O61)}2]·6H2O, H14K14O131-
P4Re2W34 (9404.277): calcd. H 0.15, Cl 0, K 5.82, P 1.32, Re 3.96;
found H 0.13, ClϽ0.1, K 5.91, P 1.24, Re 3.93. A weight loss of
2.8% (from K-1) was observed during the course of drying at room
temperature at 10–3–10–4 Torr overnight before analysis, suggesting
the presence of 15–16 weakly solvated or adsorbed water molecules.
TG/DTA data: a weight loss of 3.93% was observed below 500 °C;
calcd. 3.91% for x = 21 in K14[O{ReV(OH)(α2-P2W17O61)}2]·xH2O.
dried in the oven (at 50 °C) overnight (TiO2–N+ was obtained).
TiO2–N+ [1.5 g, 31.7 µmol of ϵSi(CH2)3N(CH3)3Cl groups per g]
was suspended in water (45 mL) for 10 min at 25 °C. Solid K-1
(1.0 g, 0.10 mmol) was added to this suspension. The resulting mix-
ture was stirred at 25 °C overnight. The solid product was collected
on a membrane filter (JG, 0.2 µm), washed with water (25 mLϫ3),
and then dried in the oven (at 50 °C) overnight. [Note: if the 1-
grafted TiO2 sample was contaminated by K-1 after the washing
with water, the catalytic activity decreased.] Yield: 1.38 g. For
[(TiO2)5500][Si(CH2)3N(CH3)3]4Cl]7[Si(CH2)3N(CH3)3]7(K7[O{ReV-
(OH)(α2-P2W17O61)}2])·(H2O)200 (2.2 µmol of 1 per g): calcd. C
IR (KBr): ν = 1091, 1018, 954, 907, 787, 527 cm–1. 31P NMR (in 0.22, H 0.14, K 0.06, Re 0.08, Si 0.09, Ti 58.00, found C 0.41, H
˜
D2O, at 25 °C, referenced to 25% H3PO4): δ = –12.38, –13.39 ppm.
31P NMR (in D2O, at 25 °C, referenced to 85% H3PO4): δ = –12.06,
–13.05 ppm. UV/Vis (water): λ (ε, Lmol–1 cm–1) = 254 (9.39ϫ104),
298 (6.28ϫ104), 496 (6.61ϫ103), 737 (4.02ϫ103) nm. ESR
(20 °C): silent.
0.20, K 0.032, Re 0.025, Si 0.16, Ti 58.2. DR UV/Vis spectrum in
the visible region: 496, 751 nm. BET surface area: 39.4 m2 g–1.
Catalytic Reaction Experiments: H2 evolution from water vapor
was carried out at 25 °C. A catalyst (500 mg) was placed into a
glass reaction vessel, which was connected to a conventional Pyrex
closed gas circulation system (238.8 cm3). Water vapor (20 Torr;
257.3 µmol) was introduced after evacuation at 25 °C, followed by
irradiation with a 500-W Xe lamp equipped with a cut-off filter
(λϾ400 nm and λϾ420 nm). H2, O2, CO, and CH4 were analyzed
by GC (TCD, Molecular Sieve 5A stainless steel columns), and
water and CO2 were analyzed by GC (TCD, Porapak Q stainless
steel columns), and assignments were made after comparing these
with the authentic samples analyzed under the same conditions.
D2, H2, and HD were detected with a quadruple mass spectrometer
(Pheiffer Vacuum Prisma QME200). The AQY, defined by the fol-
lowing equation, was measured by using combined band-pass and
cut-off (Kenko) filters and a photodiode (PM3; COHERENT).
AQY = (number of reacted electrons)/(number of incident pho-
tons)ϫ100 = (number of evolved H2 moleculesϫ2)/(number of in-
cident photons)ϫ100.[25]
Crystallization of K-1: The sample (1.0 g) was dissolved in aqueous
HCl solution (pH = 1.5; 3 mL) at Ͼ90 °C, and the purple-black
crystals were obtained by slow evaporation at 25 °C in the dark for
6 d. 31P NMR (D2O, 25 °C): δ = –12.46, –13.45 ppm. For
K14[O{ReV(OH)(α2-P2W17O61)}2]·6H2O,
H14K14O131P4Re2W34
(9404.277): calcd. H 0.15, Cl 0, K 5.82, P 1.32, Re 3.96; found H
0.17, ClϽ0.1, K 6.19, P 1.28, Re 4.00.
X-ray Crystallography: Crystals of compound K-1 were surrounded
by liquid paraffin to prevent their degradation. The crystal size
was 0.13ϫ0.06ϫ0.01 mm. Data collection was carried out with a
Bruker SMART APEX CCD diffractometer at 90 K. The intensity
data were automatically corrected for the Lorentz and polarization
effects during integration. The structure was determined by direct
methods (SHELXS-97),[21] followed by the subsequent difference
Fourier calculation, and refined by the full-matrix least-squares
procedure (SHELXL-97).[22] Absorption correction was performed
with SADABS (empirical absorption correction).[23] For complex
1, 34 tungsten atoms, two rhenium atoms, and four phosphorus
atoms were clearly identified. Thus, the main feature of the molecu-
lar structure of the polyoxometalate was clarified. However, the
resolution obtained for the structure of the salt was limited by the
poor quality of the available crystals and by the considerable disor-
der of the countercations and the solvent of crystallization. These
features are common in polyoxometalate crystallography.[24]
Supporting Information (see also the footnote on the first page of
this article): bond lengths, bond angles, and bond valence sums for
compound K-1 (Tables S1 and S2).
Acknowledgments
C. N. K. is grateful for the support of the IZUMI Science and
Technology Foundation, and the Grant-in Aid for Young Scientists
(B) from the Ministry of Education, Culture, Sports, Science and
Technology, Japan. K. N. acknowledges the support of the Grant-
in Aid for Scientific Research (C) No. 18550062 and a High-tech
Research Center Project, both from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. W. M. acknowl-
edges the support of a High-tech Research Center Project from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan. The research was partially carried out by using an instru-
ment at the Center for Instrumental Analysis of Shizuoka Univer-
sity. We thank Prof. S. Naito (Kanagawa Univ.) and Dr. T. Miyao
(Kanagawa Univ.) for the detection of the D2 produced from D2O
vapor.
¯
Crystal Data for K-1: Triclinic space group P1, purple-black granu-
lar crystals, a = 12.663(5) Å, b = 12.824(5) Å, c = 25.348(9) Å, α =
92.348(7)°, β = 103.666(7)°, γ = 114.919(6)°, and Z = 1. The data
were collected with a Bruker SMART APEX CCD sealed-tube dif-
fractometer with Mo-Kα (0.71073 Å) radiation [temperature =
90(2) K] in the range 0.84 Ͻ θ Ͻ 28.41°. The final cycle of refine-
ment, including the atomic coordinates, anisotropic thermal pa-
rameters (all W, Re, P, K atoms), and isotropic thermal parameters
(all O atoms) converged at R1 = 7.23% and wR2 = 15.87% (Fo Ͼ
2σFo). Further details of the crystal structure investigation may
be obtained from the Fachinformationszentrum Karlsruhe, 76344
Eggenstein-Leopoldshafen, Germany [Fax: +49-7247-808-666], e-
mail: crysdata@fiz-karlsruhe.de, on quoting the depository number
CSD-416410 for K-1.
[1] A. Fujishima, K. Honda, Nature 1972, 238, 37–38.
[2] S. U. M. Khan, M. Al-Shahry, W. B. Ingler Jr., Science 2002,
297, 2243–2245.
[3] M. Matsuoka, M. Kitano, M. Takeuchi, M. Anpo, J. M.
Thomas, Top. Catal. 2005, 35, 305–310.
[4] Z. Zou, H. Arakawa, J. Photochem. Photobiol. A: Chem. 2003,
158, 145–162.
[5] I. Tsuji, H. Kato, H. Kobayashi, A. Kudo, J. Am. Chem. Soc.
2004, 126, 13406–13413.
1-Grafted TiO2: TiO2 (anatase, 40 m2 g–1) was pretreated by being
dried in the oven (at 50 °C) overnight. The dried TiO2 support
(2.0 g) was dispersed in methanol (160 mL) at 25 °C. A 50% meth-
anol solution of (MeO)3Si(CH2)3N(CH3)3Cl (556 µL; 1.0 mmol)
was added to this. This mixture was heated at reflux for 6 h at
85 °C. The obtained white powder was collected on a membrane
filter (JG, 0.2 µm), washed with methanol (25 mLϫ3), and then
3140
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Eur. J. Inorg. Chem. 2008, 3134–3141

Dawson-Type Dirhenium(V)-Oxido-Bridged Polyoxotungstate
[6]
[7]
[15] F. Ortéga, M. T. Pope, H. T. Evans Jr., Inorg. Chem. 1997, 36,
2166–2169.
[16] F. Ortéga, M. T. Pope, Inorg. Chem. 1984, 23, 3292–3297.
[17] C. N. Kato, A. Tanabe, S. Negishi, K. Goto, K. Nomiya,
Chem. Lett. 2005, 34, 238–239.
[18] J. Jarupatrakorn, T. D. Tilley, J. Am. Chem. Soc. 2002, 124,
8380–8388.
[19] a) H. Hori, K. Koike, Energy Fuels 2005, 19, 2209–2213; b) E.
Amouyal, Sol. Energy Mater. Sol. Cells 1995, 38, 249–276.
[20] D. K. Lyon, W. K. Miller, T. Novet, P. J. Domaille, E. Evitt,
D. C. Johnson, R. G. Finke, J. Am. Chem. Soc. 1991, 113,
7209–7221.
[21] G. M. Sheldrick, Acta Crystallogr., Sect. A 1990, 46, 467–473.
[22] G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University of Göttingen, Germany, 1997.
[23] G. M. Sheldrick, SADABS, University of Göttingen, Germany,
1996.
[24] a) K. Nomiya, M. Takahashi, K. Ohsawa, J. A. Widegren, J.
Chem. Soc., Dalton Trans. 2001, 2872–2878; b) T. J. R. Weak-
ley, R. G. Finke, Inorg. Chem. 1990, 29, 1235–1241; c) Y. Lin,
T. J. R. Weakley, B. Rapko, R. G. Finke, Inorg. Chem. 1993, 32,
5095–5101; d) T. Yamase, T. Ozeki, H. Sakamoto, S. Nishiya,
A. Yamamoto, Bull. Chem. Soc. Jpn. 1993, 66, 103–108; e) K.
Nomiya, M. Takahashi, J. A. Widegren, T. Aizawa, Y. Sakai,
N. C. Kasuga, J. Chem. Soc., Dalton Trans. 2002, 3679–3685.
[25] I. Tsuji, H. Kato, H. Kobayashi, A. Kudo, J. Phys. Chem. B
2005, 109, 7323–7329.
X. Sun, K. Maeda, M. Le Faucheur, K. Teramura, K. Domen,
Appl. Catal. A: General 2007, 327, 114–121.
a) M. T. Pope, Heteropoly and Isopoly Oxometalates, Springer-
Verlag, Berlin, 1983; b) M. T. Pope, A. Müller, Angew. Chem.
Int. Ed. Engl. 1991, 30, 34–48; c) M. T. Pope, A. Müller (Eds.),
Polyoxometalates: From Platonic Solids to Anti-Retroviral Ac-
tivity, Kluwer Academic Publishers, Dordrecht, The Nether-
lands, 1994; d) Chem. Rev. 1998, 98, 1–390 (special issue on
polyoxometalates; guest editor: C. L. Hill).
S. Anandan, M. Yoon, J. Photochem. Photobiol. A: Chem.
2003, 160, 181–184.
A. Venturelli, M. J. Nilges, A. Smirnov, R. L. Belford, L. C.
[8]
[9]
Francesconi, J. Chem. Soc., Dalton Trans. 1999, 301–310.
[10] W. J. Randall, T. J. R. Weakley, R. G. Finke, Inorg. Chem. 1993,
32, 1068–1071.
[11] a) C. Tessier, F. D. Rocon, A. L. Beauchamp, Inorg. Chem.
2002, 41, 6527–6536; b) S. Fortin, A. L. Beauchamp, Inorg.
Chem. 2000, 39, 4886–4893; c) G. Battistuzzi, A. B. Corradi,
D. Dallari, M. Saladini, R. Battistuzzi, Polyhedron 1998, 18,
57–63 ; d) B. Machura, J. O. Dziegielewski, R. Kruszynski, T. J.
Bartczak, J. Kusz, Inorg. Chim. Acta 2004, 357, 1011–1022.
[12] J. M. Botha, A. Roodt, J. G. Leipoldt, S. Afr. J. Chem. 1995,
48, 120–126.
[13] H. Sugimoto, T. Takahira, T. Yoshimura, M. Shiro, M. Yama-
saki, H. Miyake, K. Umakoshi, Y. Sasaki, Inorg. Chim. Acta
2002, 337, 203–211.
[14] W. A. Hermann, H. Ding, F. E. Kühn, W. Scherer, Organome-
tallics 1998, 17, 2751–2757.
Received: October 17, 2007
Published Online: May 30, 2008
Eur. J. Inorg. Chem. 2008, 3134–3141
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3141