Syntheses, structural characterization and photophysical properties of 4-(2-pyridyl)-1,2,3-triazole rhenium(i) complexes

Article abstract of DOI:10.1039/b718538c

Novel chelators, i.e., 4-(2-pyridyl)-1,2,3-triazole derivatives, were synthesized by means of Cu(i)-catalyzed 1,3-dipolar cycloaddition and used to prepare luminescent Re(i) complexes [ReCl(CO)₃(Bn-pyta)], [ReCl(CO)₃(AcGlc-pyta)] and

Full text of DOI:10.1039/b718538c

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PAPER
www.rsc.org/dalton | Dalton Transactions
Syntheses, structural characterization and photophysical properties of
4-(2-pyridyl)-1,2,3-triazole rhenium(I) complexes†
Makoto Obata,*a Asuka Kitamura,a Akemi Mori,a Chiaki Kameyama,a Justyna A. Czaplewska,a
Rika Tanaka,b Isamu Kinoshita,c Toshiyuki Kusumoto,d Hideki Hashimoto,d Masafumi Harada,e
Yuji Mikata, f Takuzo Funabikig and Shigenobu Yano*a
Received 30th November 2007, Accepted 8th April 2008
First published as an Advance Article on the web 22nd May 2008
DOI: 10.1039/b718538c
Novel chelators, i.e., 4-(2-pyridyl)-1,2,3-triazole derivatives, were synthesized by means of
Cu(I)-catalyzed 1,3-dipolar cycloaddition and used to prepare luminescent Re(I) complexes
[ReCl(CO)3(Bn-pyta)], [ReCl(CO)3(AcGlc-pyta)] and [ReCl(CO)3(Glc-pyta)] (Bn-pyta =
1-benzyl-4-(2-pyridyl)-1,2,3-triazole, AcGlc-pyta = 2-(4-(2-pyridyl)-1,2,3-triazol-1-yl)ethyl
2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside, Glc-pyta = 2-(4-(2-pyridyl)-1,2,3-triazol-1-yl)ethyl
b-D-glucopyranoside). X-Ray crystallography of Bn-pyta and Glc-pyta indicated an azocompound-like
structure while the 1,2,4-triazole isomer has an azine character. [ReCl(CO)3(Bn-pyta)] crystallized in
the monoclinic system with space group P21/n. Bn-pyta ligand coordinates with the nitrogen atoms of
the 2-pyridyl group and the 3-position of 1,2,3-triazole ring, which is a very similar coordinating
fashion to that of the 2,2ꢀ-bipyridine derivative. The glucoconjugated Re(I) complexes
[ReCl(CO)3(AcGlc-pyta)] and [ReCl(CO)3(Glc-pyta)] hardly crystallized, and were analyzed by
applying extended X-ray absorption fine structure (EXAFS) analysis. The EXAFS analyses suggested
that the glucoconjugation at the 1-position of the 1,2,3-triazole makes no influence to the coordinating
fashion of 4-(2-pyridyl)-1,2,3-triazole. [ReCl(CO)3(Bn-pyta)] showed a blue-shifted maximum
absorption (333 nm, 3.97 × 103 M−1 cm−1) compared with [ReCl(CO)3(bpy)] (371 nm, 3.35 ×
103 M−1 cm−1). These absorptions were clearly assigned to be the mixed metal–ligand-to-ligand charge
transfer (MLLCT) on the basis of time-dependent density functional theory calculation. The
luminescence spectrum of [ReCl(CO)3(Bn-pyta)] also showed this blue-shifted feature when compared
with that of [ReCl(CO)3(bpy)]. The luminescence lifetime of [ReCl(CO)3(Bn-pyta)] was determined to
be 8.90 ls in 2-methyltetrahydrofuran at 77 K, which is longer than that of [ReCl(CO)3(bpy)] (3.17 ls).
The blue-shifted electronic absorption and elongated luminescence lifetime of [ReCl(CO)3(Bn-pyta)]
suggested that 4-(2-pyridyl)-1,2,3-triazole functions as an electron-rich bidentate chelator.
and molecular biology.1–4 Organic fluorophores and luminescent
Introduction
lanthanide chelates were the first to be developed,5 but the use of
The development of non-radioactive biological labels is a recent
these reagents has faced limitations relating to photodegradation
important research topic, and has attracted a great deal of
rates, pH dependence, fluorescence lifetimes, Stokes shifts and
attention in fields including photochemistry, medicinal chemistry
design of new chelators. In an effort to overcome these limitations,
luminescent transition metal complexes such as rhenium(I), ruthe-
aGraduate School of Humanities and Sciences, Nara Women’s University,
Kitauoyanishimachi, Nara, 630-8506, Japan. E-mail: mobata@cc.nara-
wu.ac.jp, yano@cc.nara-wu.ac.jp; Fax: +81 742 203392; Tel: +81 742
203392
nium(II) and iridium(III) polypyridines have been developed, and
ꢀ
a,a -diimines such as 2,2ꢀ-bipyridine (bpy) have extensively been
used as chelators.6–14 These complexes show intense and long-
lived photoluminescence in the visible region with the emission
energy affected by chelators.15 Thus, the synthesis of new versatile
chelating ligands is essential for the development of tailor-made
luminescent metal labels.
bGraduate School of Engineering, Osaka City University, Osaka, 577-8502,
Japan
cDepartments of Chemistry and Material Sciences, Graduate School of
Science, Osaka City University, Osaka, 577-8502, Japan
dDepartments of Physics, Graduate School of Science, Osaka City University,
Osaka, 577-8502, Japan
The photophysical and photochemical properties of lumines-
cent metal complexes can be modulated by controlling the elec-
eFaculty of Human Life and Environment, Nara Women’s University, Nara,
630-8506, Japan
ꢀ
tronic structure of a,a -diimine ligands, which act as electron ac-
f
ceptors from a metal center. Introduction of electron-donating or
KYOUSEI Science Center, Nara Women’s University, Nara, 630-8506,
ꢀ
Japan
electron-withdrawing groups to the a,a -diimine ligands effectively
gBiomimetics Research Center, Doshisha University, Kyo-Tanabe, 610-0321,
Japan
adjusts the LUMO level of the complexes. In systematic studies on
the photophysics of rhenium(I), osmium(II) and rhodium(III) com-
plexes with polypyridyl ligands,16–18 it was found that electronic
† Electronic supplementary information (ESI) available: Crystal structure
data. See DOI: 10.1039/b718538c
3292 | Dalton Trans., 2008, 3292–3300
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absorption and luminescence spectra of rhenium(I) complexes
[ReCl(CO)3(4,4ꢀ-X-bpy)] are highly dependent on the nature of
the 4,4ꢀ-substituents (X) of 2,2ꢀ-bipyridine.16 The blue-shifts of ab-
sorption and emission maxima by electron-donating groups such
as amino groups are explained by the “energy-gap law”.19–22 Five-
membered heterocyclic ligands such as imidazole, pyrazole and
triazole have the greater tunability of the p-electronic structure and
the synthetic versatility of luminescent metal complexes. Multiple
coordination sites, however, may enable these azole ligands to form
complicated multinuclear complexes with or without the electronic
interaction between metal ions. To eliminate this complexity,
ligands with an additional coordination site (such as a 2-pyridyl
group) have been developed. Thus, imidazole, pyrazole and 1,2,4-
triazole with a 2-pyridyl group have been used as ligands for these
luminescent complexes.23–39 Compared with the rich coordination
chemistry of 1,2,4-triazole derivatives, 1,2,3-triazole derivatives
have been little-studied except in antitumor dinuclear platinum(II)
complexes,40–42 dinuclear ruthenium(II) complexes,43,44 and spin-
crossover iron(II) complexes.45–47
After the mixture had cooled down, THF was taken off under
reduced pressure. The resulting precipitate of the Cu complex was
decomposed by addition of small portions of aqueous ammonia.
The organics were extracted with CHCl3, and washed with water,
sat. NaHCO3 (aq) and brine. After drying over Na2SO4, the solvent
was taken off under reduced pressure. The crude product was
purified by column chromatography (silica gel, ethyl acetate) to
1
give a white solid (0.720 g). Yield 80%. H NMR (400 MHz,
3
4
5
CDCl3): d/ppm = 8.53 (ddd, J = 4.9 Hz, J = 1.8 Hz, J =
3
4
1.0 Hz, 1H, 6-pyH), 8.17 (dt, J = 7.9 Hz, J = 1.1 Hz, 1H, 3-
pyH), 8.04 (s, 1H, 5-triazoleH), 7.76 (dt, 3J = 7.8 Hz, 4J = 1.8 Hz,
3
1H, 4-pyH), 7.40–7.31 (m, 5H, PhH), 7.20 (ddd, J = 7.5 and
4.9 Hz, 4J = 1.2 Hz, 1H, 5-pyH), 5.58 (s, 2H, -CH2-). 13C NMR
(100 MHz, CDCl3): d/ppm = 150.05 (C), 149.15 (6-pyC), 148.56
(C), 136.75 (4-pyC), 134.17 (C), 129.05 (PhC), 128.73 (PhC),
128.19 (PhC), 122.74 (5-pyC), 121.78 (5-triazoleC), 120.12 (3-
pyC), 54.45 (-CH2-). Anal. calcd. for C14H12N4: C, 71.17; H, 5.12;
N, 23.71. Found: C, 70.92; H, 4.98; N, 23.95. IR (KBr): m/cm−1
1605, 1597, 1421, 1225, 1045, 787, 727. UV-vis (CH3CN): k/nm
(e × 10−3/M−1 cm−1) = 280 (8.53), 241 (15.3).
=
The aim of this study is to develop a convenient synthesis of
ꢀ
4-(2-pyridyl)-1,2,3-triazole (pyta) derivatives as new a,a -diimine-
like ligands for luminescent metal complexes. The regioselective
cycloaddition of azides and terminal alkynes, now well-known
as “click chemistry”, is a convenient method of synthesis of
1,2,3-triazole derivatives,48 and was first applied to install the
coordination site on biomolecules for complexation to rhenium
and technetium.49 This methodology is now rapidly growing in
the field of coordination chemistry, and was recently applied to
the synthesis of palladium(II) and platinum(II) complexes,50 and
iron(II), ruthenium(II) and europium(III) complexes.51 We here
report “click chemistry” syntheses of 1-benzyl-4-(2-pyridyl)-1,2,3-
triazole (Bn-pyta), 2-(4-(2-pyridyl)-1,2,3-triazol-1-yl)ethyl 2,3,4,6-
tetra-O-acetyl-b-D-glucopyranoside (AcGlc-pyta) and 2-(4-(2-
pyridyl)-1,2,3-triazol-1-yl)ethyl b-D-glucopyranoside (Glc-pyta)
and the structures and photophysical properties of rhenium(I)
complexes having these ligands.
2-(4-(2-Pyridyl)-1,2,3-triazol-1-yl)ethyl 2,3,4,6-tetra-O-acetyl-
b-D-glucopyranoside (AcGlc-pyta). 2-Azidoethyl 2,3,4,6-tetra-
O-acetyl-b-D-glucopyranoside (1.87 g) and 2-ethynylpyridine
(0.499 g) were dissolved in the mixture of THF (50 mL) and water
(50 mL). To the solution was added 1 M sodium ascorbate (aq)
(0.9 mL) and 7.5 wt% CuSO4(aq) (1.5 mL). The mixture was
stirred at 50–60 ◦C for 2 days in the dark. After concentration
under reduced pressure, the products were extracted with ethyl
acetate. After drying over Na2SO4, the solvent was taken off under
reduced pressure. The crude product was purified by silica gel
column chromatography followed by recrystallization from ethyl
acetate–hexane to give colorless needle-like crystals (1.71 g). Yield
1
3
73%. H NMR (400 MHz, CDCl3): d/ppm = 8.57 (ddd, J =
4.9 Hz, J = 1.8 Hz, J = 0.93 Hz, 1H, 6-pyH), 8.17 (s, 1H, 5-
triazoleH), 8.14 (ddd, J = 7.7 Hz, J = 1.2 Hz, J = 0.93 Hz,
4
5
3
4
5
3
4
1H, 3-pyH), 7.76 (dt, J = 7.7 Hz, J = 1.8 Hz, 1H, 4-pyH),
Experimental
3
4
7.22 (ddd, J = 4.9 and 7.7 Hz, J = 1.2 Hz, 1H, 5-pyH), 5.16
(t, 3J = 9.4 Hz, 1H, 3-GlcH), 5.07 (t, 3J = 9.8 Hz, 1H, 4-GlcH),
5.02 (dd, 3J = 9.6 Hz, 3J = 8.0 Hz, 1H, 2-GlcH), 4.69 (ddd, 2J =
General information and materials
All reagents were commercial products and used with-
out further purification. 2-Azidoethyl 2,3,4,5-tetra-O-acetyl-b-D-
glucopyranoside was prepared according to the literature.52 1H and
13C NMR spectra were recorded using a JEOL AL-400 instrument.
Electronic absorption spectra were recorded on JASCO V-670
or JASCO V-570 spectrophotometers. Elemental analyses were
carried out using a Perkin-Elmer PE2400 Series II CHNS/O
Analyzer (Nara Institute of Science and Technology). IR spectra
were recorded on a Shimadzu FT-IR 8700 as KBr disks. Steady-
state luminescence spectra were recorded on a JASCO FP-6600
spectrofluorometer. Electron-spray ionization time-of-flight (ESI-
TOF) mass spectra were recorded on a JEOL JMS-T100LC
instrument.
3
3
14.5 Hz, J = 3.0 Hz, J = 2.0 Hz, 1H, -O-CH2-CHH-N-), 4.57
(ddd, 2J = 14.5 Hz, 3J = 9.0 Hz, 3J = 3.0 Hz, 1H, -O-CH2-CHH-
N-), 4.49 (d, 3J = 7.9 Hz, 1H, 1-GlcH), 4.30 (ddd, 2J = 10.5 Hz,
3J = 3.0 Hz, 3J = 2.0 Hz, 1H, -O-CHH-CH2-N-), 4.24 (dd, 2J =
12.4 Hz, 3J = 4.7 Hz, 1H, 6-GlcHH), 4.13 (dd, 2J = 12.3 Hz, 3J =
2
3
2.3 Hz, 1H, 6-GlcHH), 3.96 (ddd, J = 10.5 Hz, J = 9.0 Hz,
3J = 3.0 Hz, 1H, -O-CHH-CH2-N-), 3.68 (ddd, 3J = 7.0 Hz, 3J =
3
4.6 Hz, J = 2.3 Hz, 5-GlcH), 2.08 (s, 3H, CH3), 2.02 (s, 3H,
CH3), 1.98 (s, 3H, CH3), 1.90 (s, 3H, CH3). 13C NMR (100 MHz,
=
=
=
CDCl3): d/ppm = 170.41 (C O), 169.93 (C O), 169.25 (C O),
=
169.15 (C O), 149.95 (C), 149.24 (6-pyCH), 148.20 (C), 136.66
(4-pyCH), 123.16 (5-triazoleCH), 122.71 (5-pyCH), 119.97 (3-
pyCH), 100.45 (1-GlcC), 72.52 (3-GlcC), 71.95 (5-GlcC), 70.80 (2-
GlcC), 68.13 (4-GlcC), 67.78 (-O-CH2-CH2-N-), 61.68 (6-GlcC),
50.20 (-O-CH2-CH2-N-), 20.85 (CH3), 20.70 (CH3), 20.52 (CH3).
ESI-TOF HRMS: m/z for C23H28O10N4Na ([M + Na]+) calcd.
543.17031, found 543.17126 (error 0.95 mmu, 1.75 ppm). Anal.
calcd. for C23H28O10N4: C, 53.07; H, 5.42; N, 10.76. Found: C,
1-Benzyl-4-(2-pyridyl)-1,2,3-triazole (Bn-pyta). Benzyl azide
(0.508 g) and 2-ethynylpyridine (0.419 g) were dissolved in a
mixture of THF (40 mL) and water (40 mL). To the solution
was added 1 M sodium ascorbate (aq) (0.8 mL) and 7.5 wt%
CuSO4 (aq) (1.3 mL). The mixture was stirred at 50 ◦C overnight.
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52.99; H, 5.39; N, 10.74. IR (KBr): m/cm−1 = 1753 (mC=O). UV-vis
(CH3CN): k/nm (e × 10−3/M−1 cm−1) = 279.5 (7.44), 242.0 (13.2).
6.78. Found: C, 37.53; H, 3.23; N, 6.84. IR (KBr): m/cm−1 = 2027,
1926, 1900, 1875, 1747, 1371, 1234, 1045, 781. UV-vis (CH3CN):
k/nm (e × 10−3/M−1 cm−1) = 332 (3.97). FL (CH3CN, kex
280 nm): k/nm = 530.
=
2-(4-(2-Pyridyl)-1,2,3-triazol-1-yl)ethyl
b-D-glucopyranoside
(Glc-pyta). AcGlc-pyta (298 mg) was dissolved in a mixture
of EtOH and water (1 : 1 v/v) (14 mL). To the solution was
added 1.7 g of DOWEX 550OA OH− anion exchange resins. The
mixture was stirred at 60 ◦C for 2 days. After removal of the
resin by filtration, the filtrate was evaporated to afford Glc-pyta
[ReCl(CO)3(Glc-pyta)]. Glc-pyta (83 mg), [ReCl(CO)5]
(94 mg) and MeOH (15 mL) were placed in a 100 mL flask and
stirred at 60 ◦C for 2 days. The resulting mixture was concentrated
and cooled to 4 ◦C. The precipitate was collected by filtration
and washed with hexane to give a pale yellow powder (117 mg).
Yield 76%. ESI-TOF HRMS: m/z for C18H20O9N4Re ([M − Cl]+)
calcd. 623.07878, found 623.07755 (error −1.23 mmu, −1.97
ppm). Anal. calcd. for C18H20O9N4ClRe: C, 32.85; H, 3.06; N,
1
as a white powder (140 mg). Yield 69%. H NMR (400 MHz,
DMSO-d6, 40 ◦C): d/ppm = 8.61 (s, 1H, 5-triazoleH), 8.58 (ddd,
4
5
3J = 4.9 Hz, J = 1.7 Hz, J = 0.98 Hz, 1H, 6-pyH), 8.01 (d,
3J = 8.1 Hz, 1H, 3-pyH), 7.87 (dt, 3J = 7.7 Hz, 4J = 1.9 Hz, 1H,
4-pyH), 7.32 (ddd, 3J = 4.8 and 7.4 Hz, 4J = 1.2 Hz, 1H, 5-pyH),
4.98 (d, 3J = 4.9 Hz, 1H, OH), 4.84 (d, 3J = 4.6 Hz, 1H, OH), 4.83
(d, 3J = 5.1 Hz, 1H, OH), 4.65 (t, 3J = 5.2 Hz, 2H, -CH2-), 4.45
(t, 3J = 6.0 Hz, 1H, 6-GlcOH), 4.26 (d, 3J = 7.8 Hz, 1H, 1-GlcH),
4.17–4.11 (m, 1H, -CHH-), 4.02–3.96 (m, 1H, -CHH-), 3.71–3.66
(m, 1H, 6-GlcHH), 3.47–3.41 (m, 1H, 6-GlcHH), 3.18–3.11 (m,
2H, GlcH), 3.07–2.95 (m, 2H, GlcH). 13C NMR (100 MHz,
8.51. Found: C, 32.63; H, 3.01; N, 8.32. IR (KBr): m/cm−1
=
3443 (mO–H), 2023 (msym,C O), 1933 (masym,C O), 1898 (masym,C O). UV-vis
≡
≡
≡
(CH3CN): k/nm (e × 10−3/M−1 cm−1) = 407 (0.918), 331 (3.87).
FL (CH3CN, kex = 331 nm): k/nm = 538.
Photophysical measurements
◦
DMSO-d6, 40 C): d/ppm = 149.75 (C), 149.15 (6-pyC), 146.70
Time-resolved fluorescence measurements were performed for
the 2-methyltetrahydrofuran solution of [ReCl(CO)3(bpy)] and
[ReCl(CO)3(Bn-pyta)] at 77 K. A quartz test-tube (6 mm diameter)
with sample solutions (OD355 = 0.075 for [ReCl(CO)3(bpy)] and
OD355 = 0.182 for [ReCl(CO)3(Bn-pyta)]) was placed in a cold-
finger filled with liquid nitrogen. The sample solutions (glassy
solids at 77 K) were irradiated with the third harmonics (355 nm,
10 ns pulse duration, 10 mW, 10 Hz) of a Nd:YAG laser (Con-
tinuum, Surelite) without focusing. The luminescence signal was
detected at right angles to the excitation laser beam, with a silicon
PIN photo-diode (Hamamatsu, S7911) after passing through
a sharp cut filter (HOYA, Y49). The temporal dependence of
the luminescence intensity after photoexcitation was determined
by detecting the output electric signal from the photo-diode
with a digital oscilloscope (Sony Tektronix, TDS380P). Time-
dependent fluorescence profiles obtained by the exposure of
256 excitation laser pulses were averaged. The time-resolution
of the measurement system was determined to be as short as
10 ns. Inspection of the absorption spectra before and after
the time-resolved luminescence measurements confirmed that no
degradation of the sample solutions was induced by the 355 nm
laser irradiation.
(C), 136.76 (4-pyC), 123.60 (5-triazoleC), 122.50 (5-pyC),
119.14 (3-pyC), 102.75 (1-GlcC), 76.81 (GlcC), 76.47 (GlcC),
73.18 (GlcC), 69.94 (GlcC), 67.02 (CH2), 60.99 (6-GlcC), 49.83
(CH2). ESI-TOF HRMS: m/z for C15H20O6N4Na ([M + Na]+)
calcd. 375.12805, found 375.12910 (error 1.05 mmu, 2.80 ppm).
Anal. calcd. for C15H20O6N4·0.5H2O: C, 49.86; H, 5.86; N, 15.50.
Found: C, 49.43; H, 5.81; N, 15.60. IR (KBr): m/cm−1 = 3400
(mO–H). UV-vis (CH3OH): k/nm (e × 10−3/M−1 cm−1) = 280 (7.69),
240 (11.8).
[ReCl(CO)3(Bn-pyta)]. The ligand Bn-pyta (101 mg) and
[ReCl(CO)5] (139 mg) were dissolved in MeOH (42 mL) and stirred
for 24 h at 60 ◦C. After standing at room temperature, yellow
fine needle-like crystals were collected by filtration. Yield 0.110 g
(53%). 1H NMR (400 MHz, CD3CN): d/ppm = 8.93 (ddd, 3J =
4
5
5.6 Hz, J = 1.5 Hz, J = 0.91 Hz, 1H, 6-pyH), 8.55 (s, 1H, 5-
triazoleH), 8.10 (dt, 3J = 7.8 Hz, 4J = 1.5 Hz, 1H, 4-pyH), 7.98
3
4
5
(ddd, J = 8.0 Hz, J = 1.4 Hz, J = 0.88 Hz, 1H, 3-pyH), 7.49
(ddd, 3J = 7.6 and 5.6 Hz, 4J = 1.4 Hz, 1H, 5-pyH), 7.47–7.42 (m,
5H, PhH), 5.73 (s, 2H, -CH2-). 13C NMR (100 MHz, CD3CN):
d/ppm = 153.64 (6-pyC), 149.74 (C), 149.53 (C), 140.90 (4-
pyC), 134.36 (ipso-PhC), 129.92 (PhCH), 129.89 (PhCH), 129.44
(PhCH), 126.85 (5-pyC), 125.66 (5-triazoleC), 123.30 (3-pyC),
56.33 (-CH2-). ESI-TOF HRMS: m/z for C17H12O3N4ClReNa
([M + Na]+) calcd. 565.00531, found 565.00312 (error −2.19 mmu,
−3.88 ppm). Anal. calcd. for C17H12O3N4ClRe: C, 37.67; H, 2.23;
Emission quantum yield (U) was determined by a relative
2+
method using aqueous Ru(bpy)3 solution (Ustd = 0.042) as a
standard.53
N, 10.34. Found: C, 37.45; H, 1.94; N, 10.40. IR (KBr): m/cm−1
=
X-Ray structural determination
2027 (msym,C O), 1909 (masym,C O), 1880 (masym,C O). UV-vis (CH3CN):
≡
≡
≡
Single-crystal X-ray diffraction data were recorded on Rigaku
Mercury CCD and AFC/Mercury CCD diffractometers using
graphite-monochromated Mo Ka radiation. All data sets were
corrected for Lorentzian polarization effects and for absorption.
All structures were solved by the direct method (SIR9254 or
SIR9755). Hydrogen atoms were placed into calculated positions,
and refined with isotropic thermal parameters riding on those
of the parent atoms. Refinement of the non-hydrogen atoms
was carried out with the full-matrix least-squares technique
(SHELXL-9756). Crystal and refinement data are summarized in
Table 1.†
k/nm (e × 10−3/M−1 cm−1) = 333 (3.97). FL (CH3CN, kex
=
333 nm): k/nm = 538.
[ReCl(CO)3(AcGlc-pyta)]. AcGlc-pyta (160 mg), [ReCl(CO)5]
(109 mg) and MeOH (30 mL) were placed in a 100 mL flask,
and stirred at 62 ◦C for 2 days. The resulting mixture was
concentrated and cooled to 4 ◦C to afford a diastereomeric mixture
of [ReCl(CO)3(AcGlc-pyta)] as a pale yellow powder (160 mg).
Yield 64%. ESI-TOF HRMS: m/z for C26H28O13N4Re ([M −
Cl]+) calcd. 791.12104, found 791.11663 (error −4.41 mmu, −5.57
ppm). Anal. calcd. for C26H28O13N4ClRe: C, 37.80; H, 3.42; N,
3294 | Dalton Trans., 2008, 3292–3300
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Table 1 Summary of crystal data information and collection/refinement parameters for Bn-pyta, Glc-pyta and [ReCl(CO)3(Bn-pyta)]
Bn-pyta
Glc-pyta
[ReCl(CO)3(Bn-pyta)]
Formula
FW
C14H12N4
236.28
C15H20N4O6
352.35
C17H12N4O3ClRe
541.96
Crystal system
Space group
Orthorhombic
Fdd2
28.4093(11)
29.0074(15)
5.7657(2)
90
4751.4(3)
16
1984
Monoclinic
P21
Monoclinic
P21/n
˚
a/A
11.560(4)
5.4449(17)
13.267(5)
100.247(4)
821.8(5)
2
370
1.424
Mo Ka
0.111
6.6557(5)
20.5809(15)
13.0676(11)
98.876(4)
1768.6(2)
4
1032
2.035
Mo Ka
7.047
˚
b/A
˚
c/A
b/◦
V/A
Z
3
˚
F(000)
D
calc/g cm−3
Wavelength
1.321
Mo Ka
0.0831
l/mm−1
T/K
173
173
193(2)
h limits/◦
3.15–27.47
−36/36, −37/36, −7/7
11 614
1503
1388
Numerical
1.028
0.0294
0.021
0.0353
3.1–27.5
−14/15, −6/7, −17/15
6404
2063
1765
Numerical
1.004
0.0380
0.025
0.0506
4.19–27.48
−8/5, −19/26, −15/16
14 028
4011
3716
Multi-scan
1.007
0.0452
0.0336
0.0774
Min/max h, k, l
Collected reflns
Unique reflns
Reflns with I > 2r(I)
Absorption correction
GOF
R [F2 > 2r(F2)]
Rint
wR (all data)
Max, min Dq/e A
CCDC No.
−3
˚
0.57, −0.25
0.20, −0.21
0.77, −0.82
633046
669575
633047
XAS Data acquisition and analysis
in the range from −10 to 10 eV. The fits were performed on the
˚
inverse Fourier transform space for the region between 1.2 to 3.5 A
The X-ray absorption spectra were recorded at the beam line 9C
of the Photon Factory of the High Energy Acceleration Research
Organization (KEK-PF; Tsukuba, Japan). The ring current was
300–450 mA, and the storage ring was operated with an electron
energy of 2.5 GeV. The experiments at the Re LIII edge (10 531 eV)
were carried out at room temperature in the transmission mode.
The spectra were collected at room temperature with a Si(111)
double-crystal monochromator, and 30% harmonic rejection was
achieved by detuning the two crystal from parallel alignment. The
samples were finely ground with boron nitrate (BN), and pressed
to afford a pellet.
The EXAFS oscillations v(k) were extracted from the X-ray
spectra using the automated background subtraction algorithm
(AUTOBK) developed by Newville et al.57 The k3-weighted
EXAFS oscillations, k3v(k), are theoretically given by following
equation:
using the IFEFFIT program suite.59
Computational methodology
Geometry optimization of [ReCl(CO)3(bpy)] and [ReCl(CO)3(Me-
pyta)] was performed using the PBE1PBE functional and the
6-31(G) basis set for C, H, N, O and Cl atoms, and the
Stuttgart/Dresden pseudo-core potential (SDD) for Re atoms,
implemented in the Gaussian03 program package.60 The sta-
tionary point was established by the analysis of the harmonic
vibrational frequencies. The electronic structure and transition
energies were recalculated using the conductor-like polarizable
continuum model (CPCM) and Dunning’s correlation-consistent
basis function, with the cc-pvdz basis set for C, H, N, O and Cl
atoms instead of the 6-31(G) basis set.
Results and discussion
(1)
Synthesis and characterization
where S20, ri, Ni, Fi(k), Ui(k) and ri represent the amplitude
reduction factor, the half-path-length, the degeneracy, the back-
scattering amplitude, the phase shift, and the Debye–Waller factor
of the ith path, respectively, and k is the photoelectron wave
vector defined as k = [(2m/h2)(E − E0 + DEi)]1/2 with E0 being
the threshold energy and DEi the energy shift of the ith path.
Theoretical amplitude Fi(k) and phase shift Ui(k) functions for
the single and multiple scattering pathways were calculated by
means of the FEFF 8.2 program.58 The S02 value was optimized in
the range from 0.8 to 1. The DEi value for each path was optimized
1,2,3-Triazole has two possible coordination sites, viz. the 2-
and 3-nitrogen atoms, but introduction of a 2-pyridyl group at
the 4-position results in the preferred bidentate sites at the 3-
triazole nitrogen and the 2-pyridyl nitrogen. By applying the
copper(I)-catalyzed 1,3-dipolar cycloaddition (a well-known ex-
ample of “click chemistry”), the 4-(2-pyridyl)-1,2,3-triazole (pyta)
derivatives were readily synthesized from azide derivatives and 2-
ethynylpyridine (Scheme 1). Thus, the reaction of benzyl azide
with equimolar 2-ethynylpyridine in the mixture of THF and
water in the presence of CuSO4 and sodium ascorbate produced
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◦
˚
Table 2 Selected bond lengths (A) and angles ( ) of Bn-pyta and Glc-pyta
Bn-pyta
1.318(2)
1.353(2)
1.3442(19)
1.374(2)
1.365(2)
1.473(2)
108.65(13)
107.10(13)
111.02(12)
104.71(14)
108.53(13)
Glc-pyta
N(2)–N(3)
N(3)–N(4)
N(4)–C(7)
C(7)–C(6)
C(6)–N(2)
C(5)–C(6)
C(6)–N(2)–N(3)
N(2)–N(3)–N(4)
N(3)–N(4)–C(7)
N(4)–C(7)–C(6)
C(7)–C(6)–N(2)
1.322(3)
1.352(2)
1.337(3)
1.367(3)
1.366(3)
1.466(4)
108.9(2)
106.6(2)
111.2(2)
105.3(2)
108.1(2)
Scheme 1 Synthesis of 4-(2-pyridyl)-1,2,3-triazole ligands and their
rhenium(I) complexes. Reagents and conditions: (a) CuSO4, 1 M sodium
ascorbate, THF–H2O (1 : 1 v/v), 50–60 ◦C, 16 h; (b) DOWEX 550OA
OH− anion exchange resins, EtOH–H2O (1 : 1 v/v), 60 ◦C, 2 days;
(c) [ReCl(CO)5], MeOH, 60 ◦C, 24 h.
colorless crystals of 1-benzyl-4-(2-pyridyl)-1,2,3-trizole (Bn-pyta)
after overnight stirring at 50 ◦C in 80% yield. The reaction is
highly selective to give the 4-(2-pyridyl) isomer, although 4- and 5-
(2-pyridyl) isomers are possible.48 Fig. 1 shows the crystal structure
of Bn-pyta. Selected bond lengths and angles are listed in Table 2.
The angle between least-square planes of triazole and pyridine
rings is determined to be 15.46◦, which is larger than that of
typical 3-(2-pyridyl)-1,2,4-triazoles such as 4-amino-5-methylthio-
3-(2-pyridyl)-1,2,4-triazole (9.77◦).61
Fig. 2 Schematic representation of crystal structure of 1,2,3-tria-
zole and 1,2,4-triazole rings. Bond lengths are those of Bn-pyta and
4-amino-5-methylthio-3-(2-pyridyl)-1,2,4-triazole61 as typical examples of
1,2,3- and 1,2,4-triazole rings.
is shorter than the adjacent N(2)–C(6) and N(3)–N(4) bonds,
while the corresponding N(2)–N(3) bond distance of 1,2,4-triazole
˚
(1.395(3) A) is longer than those of adjacent two bonds. This
structural feature suggests that 1,2,3-triazole has an azo character
while 1,2,4-triazole has an azine character.
The rhenium(I) complex, [ReCl(CO)3(Bn-pyta)], was prepared
by the◦reaction of [ReCl(CO)5] with Bn-pyta in methanol overnight
at 60 C. Yellow needle-like crystals were obtained in 53% yield
and characterized by elemental analysis, ESI-MS (detection of
[M − Cl]+ ion peak identified by its characteristic isotope-pattern),
1H and 13C NMR (large shifts by coordination to Re(I)), IR (three
≡
C O peaks, indicating three facially coordinated carbonyl groups;
mC≡O = 2025 cm−1 (msym), 1915 and 1890 cm−1 (masym)), and electronic
absorption spectroscopies. Fig. 3 shows the ORTEP drawing of
[ReCl(CO)3(Bn-pyta)].
The coordinating fashion of Bn-pyta to the Re(I) center
is very similar to that of the 2,2ꢀ-bipyridine derivative.62 The
selected bond distances and bond angles are listed in Table 3.
˚
The bond distance of Re–N(triazole) is 2.127(5) A, which is
˚
shorter than that of Re–N(pyridine) (2.197(4) A). A similar
distortion is known in the Re(I) complexes of azole ligands
having a 2-pyridyl group such as 2-(2-pyridyl)benzimidazole,23 2-
(2-pyridyl)benzothiazole,63 2-(2-pyridyl)benzoxazole33 and a 3-(2-
pyridyl)-1,2,4-triazole derivative.32 Interestingly, the bond distance
of Re–N(triazole) is the shortest in the Re(CO)3 complexes of
pyridine–azole conjugates, probably due to the strong electron-
donating property of the 1,2,3-triazole ring. The bite angle N–Re–
N keeps constant at ca. 74.5◦ in these complexes.
Fig. 1 ORTEP drawings of Bn-pyta (a) and Glc-pyta (b) with thermal
ellipsoids shown at 50% probability.
A significant difference between the 1,2,3-triazole and 1,2,4-
triazole rings was found in the bond distances as shown in Fig. 2;
˚
the N(2)–N(3) bond distance of the 1,2,3-triazole ring (1.318(2) A)
3296 | Dalton Trans., 2008, 3292–3300
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1
1
and H and 13C NMR spectroscopies. H and 13C NMR spectra
indicated that the product is a mixture of two diastereomers due
to the chirality of sugar moieties and the asymmetric octahedral
Re(I) complex. The diastereomeric ratio is close to unity, indicating
no stereoselectivity in the reaction. Since these glycoconjugated
complexes hardly crystallized, the coordination geometries were
examined by extended X-ray absorption fine structure (EXAFS)
analysis at the Re LIII edge (Fig. 4). The resulting EXAFS oscil-
lations (k3v(k)) of [ReCl(CO)3(AcGlc-pyta)] and [ReCl(CO)3(Glc-
pyta)] resembled that of [ReCl(CO)3(Bn-pyta)], as shown in the
insets of Fig. 4. The Fourier transform of k3v(k) shows three
˚
peaks at 1.6, 2.2 and 2.6 A (before phase-shift correction). The
EXAFS oscillations were fitted with the contributions from 4
single scattering paths between Re and neighboring N, C, Cl
≡
and O atoms and 2 multiple scattering paths for the Re–C O
linear triatomic system (Fig. 5). The fitting results are listed in
Table 4. The resulting structural parameters, e.g., half-path-lengths
ri, are quite close to those derived from the crystal structure of
[ReCl(CO)3(Bn-pyta)], indicating the similar coordination struc-
tures of [ReCl(CO)3(AcGlc-pyta)] and [ReCl(CO)3(Glc-pyta)].
Fig. 3 ORTEP drawing of [ReCl(CO)3(Bn-pyta)] with thermal ellipsoids
shown at 50% probability.
◦
˚
Table 3 Selected bond lengths (A) and angles ( ) of [ReCl(CO)3(Bn-pyta)]
(X-ray crystallography) and [ReCl(CO)3(Me-pyta)] (DFT calculation)
[ReCl(CO)3(Bn-pyta)]a
[ReCl(CO)3(Me-pyta)]b
Re–Cl
Re–N(1)
Re–N(2)
Re–C(15)
Re–C(16)
Re–C(17)
N(1)–Re–N(2)
Cl–Re–C(15)
N(1)–Re–C(17)
N(2)–Re–C(16)
C(16)–Re–C(17)
2.4713(18)
2.196(5)
2.127(5)
1.974(8)
1.922(7)
1.916(7)
74.5(2)
174.9(2)
170.5(2)
170.6(2)
91.6(3)
2.4845
2.2203
2.1698
1.9187
1.9264
1.9314
74.12
175.66
170.40
170.07
90.45
a Determined by X-ray crystallography. b Determined by DFT calculation.
In order to validate the feasibility of this methodology for
syntheses of the pyta-ligating moiety, we synthesized glycoconju-
gated pyta ligands and their Re(I) complexes. 2-Azidoethyl 2,3,4,6-
tetra-O-acetyl-b-D-glucopyranoside (AcGlc-N3) was prepared
by the method reported previously.52 The copper(I)-catalyzed
1,3-dipolar cycloaddition of AcGlc-N3 with 2-ethynylpyridine
afforded 2-(4-(2-pyridyl)-1,2,3-triazol-1-yl)ethyl 2,3,4,6-tetra-O-
acetyl-b-D-glucopyranoside (AcGlc-pyta) without formation of its
isomer. Saponification of AcGlc-pyta using DOWEX 550A OH−
anion exchange resins afforded the deprotected ligand, 2-(4-(2-
pyridyl)-1,2,3-triazol-1-yl)ethyl b-D-glucopyranoside (Glc-pyta).
These ligands were characterized by elemental analysis, ESI-MS,
and 1H, 13C NMR and electronic absorption spectroscopies. Glc-
pyta crystallized in the monoclinic system, space group P21, from
a mixture of ethanol and hexane. The crystal structure of Glc-pyta
is shown in Fig. 1 and selected bond lengths and angles are listed
in Table 2. The least-square planes of the triazole and pyridine
rings are twisted relative to each other with a dihedral angle of
10.94◦ to form the (S)-configuration in the crystal due to the
chirality of sugar unit. No significant structural differences in the
pyta moieties was found between Bn-pyta and Glc-pyta. The Re(I)
complexes of these ligands were prepared under similar conditions
to Bn-pyta and characterized by elemental analysis, ESI-MS,
Fig.
4
Fourier transform of k3-weighted EXAFS oscillation (be-
fore phase-shift correction) (k3v(k)) of [ReCl(CO)3(Bn-pyta)] (a),
[ReCl(CO)3(AcGlc-pyta)] (b) and [ReCl(CO)3(Glc-pyta)] (c) in BN pellets
at the Re LIII edge. The insets are the corresponding Fourier-filtered k3v(k).
Broken line represents observed data and solid line is the best-fit with
standard EXAFS equation.
Photophysical properties
Fig. 6 shows the electronic absorption and luminescence spec-
tra of [ReCl(CO)3(Bn-pyta)] together with [ReCl(CO)3(bpy)]
in acetonitrile at 25 ◦C. [ReCl(CO)3(Bn-pyta)] showed
a
blue-shifted electronic absorption at 333 nm (e = 3.97 ×
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Fig. 5 Important single and multiple scattering paths.
103 M−1 cm−1) compared with [ReCl(CO)3(bpy)] (371 nm,
e = 3.35 × 103 M−1 cm−1). A density functional theory (DFT)
calculation was performed for [ReCl(CO)3(Me-pyta)] (Me-pyta
is 1-methyl-4-(2-pyridyl)-1,2,3-triazole as a reduced model for
the ligand Bn-pyta) and [ReCl(CO)3(bpy)] to analyze these
electronic transitions. Equilibrium geometries of [ReCl(CO)3(Me-
pyta)] and [ReCl(CO)3(bpy)] were obtained by using the PBE1PBE
functional and Stuttgart/Dresden pseudo-core potential (SDD)
for the Re atom and the 6-31(G) basis set for other atoms, and
confirmed by normal coordination analysis. The selected bond
lengths and bond angles are listed in Table 3. The DFT calcula-
tion of [ReCl(CO)3(Me-pyta)] reproduced well the coordination
structure of [ReCl(CO)3(Bn-pyta)]. The electronic transitions of
[ReCl(CO)3(Me-pyta)] and [ReCl(CO)3(bpy)] were calculated by
time-dependent formalism of DFT calculation according to the
Vlcˇek and Za´lisˇ method14,64,65 using the PBE1PBE functional and
Dunning’s correlation-consistent basis function, the cc-pvdz basis
set for C, H, N, O and Cl atoms and SDD for the Re atom
using the conductor-like polarizable continuum model (CPCM).
The significant oscillator strength f was found at 340.4 nm
(3.6424 eV, f = 0.0867) and 380.7 nm (3.2564 eV, f = 0.0819)
for [ReCl(CO)3(Me-pyta)] and [ReCl(CO)3(bpy)], respectively. The
Fig.
6
Electronic absorption and normalized luminescence spec-
tra (inset) of [ReCl(CO)3(bpy)] (a), [ReCl(CO)3(Bn-pyta)] (b) and
[ReCl(CO)3(Glc-pyta)] (c) in acetonitrile. Bars in the electronic ab-
sorption spectra represent the oscillator strength (f , right axis) using
TD-DFT calculation (PBE1PBE/SDD + cc-pvdz) of [ReCl(CO)3(bpy)]
and [ReCl(CO)3(Me-pyta)].
calculation reproduced the experiments well, giving slightly larger
kmax values than the observed values for [ReCl(CO)3(Bn-pyta)]
and [ReCl(CO)3(bpy)]. Fig. 7 shows the energy diagrams with
HOMO-1 and LUMO orbitals of [ReCl(CO)3(Me-pyta)] and
[ReCl(CO)3(bpy)] in acetonitrile.
The HOMO-1s of [ReCl(CO)3(Me-pyta)] and [ReCl(CO)3(bpy)]
predominantly localize at the Re(I) and Cl atoms, which form
a p-antibonding orbital. On the other hand, the LUMOs are
essentially the p* orbital of pyta and bpy ligands. Hence the
electronic transition at the lowest energy region is clearly assigned
to the mixed metal–ligand-to-ligand charge transfer (MLLCT)
in both complexes. The same energy level of HOMO-1 in both
Table 4 Least-square fitting results for EXAFS data of [ReCl(CO)3(Bn-pyta)], [ReCl(CO)3(AcGlc-pyta)] and [ReCl(CO)3(Glc-pyta)]a
Ligand
Bn-pyta
AcGlc-pyta
Glc-pyta
c
e
f
2
e
f
2
e
f
2
Pathb
nleg
Nd
r /A
r /A
r /A
r /A
r /A
˚
˚
˚
˚
˚
˚
r /A
1
2
3
4
2
2
2
2
3
4
—
3
1
2
3
5
3
—
1.90(6)
2.2(10)
2.5(3)
3.08(8)
3.08(8)
3.08(8)
0.015
0.003
0.002
0.004
0.001
0.003
0.013
—
1.90(6)
2.2(12)
2.4(2)
3.08(8)
3.08(8)
3.08(8)
0.014
0.004
0.001
0.003
0.002
0.003
0.014
—
1.90(9)
2.2(11)
2.5(4)
3.08(9)
3.08(9)
3.08(9)
0.015
0.003
0.001
0.005
0.001
0.003
0.015
—
5
6
Rg
a
3
The structural model shown in Fig. 5 is used to fit Fourier-filtered k v(k) data (range 1.2 < r < 3.5 A). b Path number defined in Fig. 5. c Number of legs.
˚
d Degeneracy. e Half-path-length (interatomic distance for single scattering). f Debye–Waller factor. g R-factor, defined as R[v(k)obs − v(k)model]2/R[v(k)obs]2.
3298 | Dalton Trans., 2008, 3292–3300
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Fig. 7 Energy diagram (au) and HOMO-1 and LUMO orbitals of
[ReCl(CO)3(bpy)] (left) and [ReCl(CO)3(Me-pyta)] (right).
Table 5 Emission maxima, quantum yield and luminescence lifetime of
[ReCl(CO)3(bpy)] and [ReCl(CO)3(Bn-pyta)]
kmaxa/nm
Ua
sb/ls
Fig.
8
Luminescence decay curve of [ReCl(CO)3(bpy)] (a) and
[ReCl(CO)3(Bn-pyta)] (b) in 2-methyltetrahydrofuran at 77 K. The dotted
line and the solid line represent observed data and the best-fit with a single
exponential function, respectively.
[ReCl(CO)3(bpy)]
[ReCl(CO)3(Bn-pyta)]
633
538
0.0027
0.0033
3.17
8.90
a Determined in acetonitrile at 298 K. b Determined in 2-methyltetra-
hydrofuran at 77 K.
Conclusions
We have developed novel azole ligands with a 2-pyridyl group
affording an additional coordination site, namely 4-(2-pyridyl)-
1,2,3-triazole (pyta) ligands. The pyta ligands are readily and
easily prepared by means of copper(I)-catalyzed 1,3-dipolar
cycloaddition of commercially available 2-ethynylpyridine and
azide compounds. The reaction proceeds under relatively mild
conditions and affords the ligands selectively in fairly good yields.
X-Ray crystallography clearly indicates that the 1,2,3-triazole
has an azo-like character while its isomer, 1,2,4-triazole, has an
azine nature. The coordinating ability of pyta was established
on the ReCl(CO)3 core. The blue-shift in electronic absorption
and luminescence spectra of the resulting rhenium(I) complex
clearly indicated that pyta acts like a bipyridine mimic with
strongly electron-donating substituents. The photophysical and
photochemical properties of pyta were theoretically clarified by
DFT calculations.
Enhanced luminescence intensity and lifetime could be highly
advantageous in the application of pyta metal complexes as
luminescence probes in time-gated imaging. The ready formation
of the sugar-attached pyta complexes in aqueous conditions
indicates that the present approach leads to a powerful method
to install the bipyridine-like ligands and their photofunctional
metal complexes into biomolecules, e.g. saccharides, proteins and
nucleic acids.
complexes and the significantly higher energy level of LUMO in
[ReCl(CO)3(Me-pyta)] than [ReCl(CO)3(bpy)] semiquantitatively
explain the blue-shift of the MLLCT band of the former.
According to the “energy-gap law”,16,17,65 this blue-shift in the
ligand pyta suppresses a radiationless deactivation to afford the
long-lived excited state. The glycoconjugated Re(I) complexes
[ReCl(CO)3(AcGlc-pyta)] and [ReCl(CO)3(Glc-pyta)] also show
the blue-shifted MLLCT band at 332 and 331 nm, respectively.
This explains the following luminescence properties.
[ReCl(CO)3(Bn-pyta)] exhibited the blue-shifted luminescence
spectrum compared with 633 nm of [ReCl(CO)3(bpy)] (insets
of Fig. 6). Table 5 lists the maximum emission wavelength
(kmax) and emission quantum yield (U) in acetonitrile and
the luminescence lifetime (s) in 2-methyltetrahydrofuran (vide
infra) of [ReCl(CO)3(bpy)] and [ReCl(CO)3(Bn-pyta)]. Fig. 8
shows the luminescence decay curves of [ReCl(CO)3(Bn-pyta)]
and [ReCl(CO)3(bpy)] in 2-methyltetrahydrofuran at 77 K. The
luminescence lifetime of [ReCl(CO)3(Bn-pyta)] (8.90 ls) is al-
most three-times longer than that of [ReCl(CO)3(bpy)] (3.17 ls)
and comparable to [ReCl(CO)3(4,4ꢀ-diamino-2,2ꢀ-bipyridine)] (ca.
11 ls).16 The U value of [ReCl(CO)3(Bn-pyta)] also greater
than that of [ReCl(CO)3(bpy)] even at ambient temperature in
acetonitrile. These results also indicate that the ligand Bn-pyta
can function as an electron-rich bipyridine-like ligand, affording
the long-lived luminescence with the higher quantum yield. The
glycoconjugated Re(I) complexes [ReCl(CO)3(AcGlc-pyta)] and
[ReCl(CO)3(Glc-pyta)] exhibited similar photophysical properties
to that of [ReCl(CO)3(Bn-pyta)], showing FL kmax at 532 nm
for the AcGlc-pyta complex and at 538 nm for the Glc-pyta
complex, respectively. The long-lived luminescence properties of
Re(I) complexes with the ligand pyta could be useful for time-gated
imaging, especially in biological labeling systems.
Acknowledgements
The authors are grateful to Prof. Dr Chikara Ohtsuki (Nagoya
University) and Prof. Dr Masao Tanihara (Nara Institute of
Science and Technology) for elemental analysis. This work was
supported by Grants-in-Aid for Scientific Research (No. 19350031
and 19614009) from the Ministry of Education, Culture, Sport,
Science and Technology (MEXT) of the Japanese Government,
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Japan–German exchange program supported by Japan Society
for the Promotion of Science (JSPS), Nara Women’s University
Intramural Grant for Project Research, and grants from Osaka
Gas and the San-EiGen Foundation for chemical research. The X-
ray absorption spectral study was performed under the approval
of the Photon Factory Program Advisory Committee (Proposal
No. 2004G290).
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