Nickel(II) complexes of tpa ligands with 6-phenyl substituents (Ph ntpa). Structure and H2O2-reactivity

Article abstract of DOI:10.1246/bcsj.20090346

Nickel(II) complexes supported by a series of tpa {tris[(pyridin-2-yl) methyl]amine} ligand derivatives containing different numbers of phenyl substituent on the 6-position of pyridine donor group (Ph_ntpa, n = 03) have been prepared and structu

Full text of DOI:10.1246/bcsj.20090346

530
Bull. Chem. Soc. Jpn. Vol. 83, No. 5, 530–538 (2010)
© 2010 The Chemical Society of Japan
Nickel(II) Complexes of tpa Ligands with 6-Phenyl Substituents (Phntpa).
Structure and H2O2-Reactivity
Tetsuro Tano,1 Yoshitaka Doi,2 Masayuki Inosako,2 Atsushi Kunishita,1 Minoru Kubo,3
Hirohito Ishimaru,3 Takashi Ogura,3 Hideki Sugimoto,1 and Shinobu Itoh*1,4
1Department of Material and Life Science, Division of Advanced Science and Biotechnology,
Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871
2Department of Chemistry, Graduate School of Science, Osaka City University,
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585
3Picobiology Institute, Graduate School of Life Science, University of Hyogo,
3-2-1 Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297
4Institute for Molecular Science, National Institutes of Natural Sciences, 38 Nishigo-naka, Myodaiji, Okazaki 444-8585
Received December 28, 2009; E-mail: shinobu@mls.eng.osaka-u.ac.jp
Nickel(II) complexes supported by a series of tpa {tris[(pyridin-2-yl)methyl]amine} ligand derivatives containing
different numbers of phenyl substituent on the 6-position of pyridine donor group (Phntpa, n = 0-3) have been prepared
and structurally characterized. Ligand tpa and its derivatives with one or two 6-Ph substituent(s) (Ph0tpa, Ph1tpa, and
Ph2tpa) afforded nickel(II) complexes 1[0], 1[1], and 1[2] with a distorted octahedral geometry, whereas the ligand having
three 6-Ph groups (Ph3tpa) gave nickel(II) complex 1[3] with a five-coordinate trigonal bipyramidal structure. All the
complexes exhibited a high spin ground state (S = 1). Reactivity of the nickel(II) complexes 1[0] and 1[1] toward H2O2 was
very poor, whereas 1[2] and 1[3] readily gave bis(®-oxo)dinickel(III) complexes 2[2] and 2[3], respectively, in the reaction
with H2O2 at a low temperature. The bis(®-oxo)dinickel(III) complexes 2[2] and 2[3] gradually decomposed to cause an
aromatic hydroxylation reaction of one of the 6-Ph groups of the supporting ligands. The ligand substituent effects on the
formation and decomposition processes of the bis(®-oxo)dinickel(III) complexes are discussed based on the structures
and the redox potentials of the starting nickel(II) complexes.
R1
Tris[(pyridin-2-yl)methyl]amine (tpa), is one of the most
popular tetradentate tripodal ligands in coordination chemis-
try.1,2 Recently, several types of tpa derivatives have been
developed in order to tune up the structure and properties of the
supported complexes, where steric constraints and/or elec-
tronic effects including hydrogen-bonding interaction induced
by the substituents are crucial.1-9 Among the tpa derivatives,
aryl-appended ones are a relatively new class of chelate
ligands, being utilized to construct an aromatic-enriched
hydrophobic environment around the secondary coordination
sphere of the metal center.1a Notable examples are bioinorganic
modeling studies using nickel and iron complexes of the
aryl-appended tpa ligands carried out by Berreau et al. and
Que et al.1,4 We and others have also investigated the structure
and reactivity of copper complexes supported by a series
of tpa ligands with 6-phenyl substituents (Phntpa, Chart 1) to
find significant ligand effects on O2 and H2O2 chemistry.10-12
For instance, we have found that the reaction of H2O2 with
[CuII(Ph1tpa)(CH3CN)]2+ (n = 1) affords a copper(II)-hydro-
peroxo complex (LCuII-OOH), being analogous to the reac-
tivity of [CuII(tpa)(CH3CN)]2+ (n = 0) toward H2O2,13 whereas
the reaction of H2O2 with [CuII(Ph2tpa)(CH3CN)]2+ (n = 2)
gives a bis(®-oxo)dicopper(III) complex.12 Furthermore, the
same treatment of [CuII(Ph3tpa)(CH3CN)]2+ (n = 3) with H2O2
N
n
n
n
n
= 0; R1 = R2 = R3 = H
R3
N
= 1; R1 = Ph, R2 = R3 = H
= 2; R1 = R2 = Ph, R3 = H
= 3; R1 = R2 = R3 = Ph
N
N
PhnTPA
R2
Chart 1. Phntpa ligands.
has been found to cause reduction of the copper(II) complex to
the corresponding copper(I) complex.12 These differences in
H2O2-reactivity among the copper(II) complexes of Phntpa
have been attributed to steric as well as electronic effects of the
6-phenyl substituent(s) on the pyridine donor group(s).12
In this study, we have extended our research of the copper-
H2O2 chemistry to the nickel system (1[n], n = 0-3) using the
same series of ligands to gain further insights into the aromatic
substituent effects of the tpa ligands on the structure and
reactivity of the supported transition-metal complexes. Nickel/
active-oxygen complexes have also attracted much recent
attention due to their relevance to dioxygen activation
mechanism by transition-metal complexes, and the structure
Published on the web April 29, 2010; doi:10.1246/bcsj.20090346

T. Tano et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 5 (2010)
531
and reactivity of the nickel/active-oxygen complexes have
frequently been discussed in connection with those of the
related copper/active-oxygen complexes developed as the
active site models of copper monooxygenases and copper
oxidases.5,14-18 Thus, the present study will merit considerable
interest among researchers involved not only in coordination
chemistry but also bioinorganic chemistry.
[Ni(Ph1tpa)(CH3CN)2](ClO4)2 suitable for X-ray crystallo-
graphic analysis were obtained by slow diffusion of diethyl
ether into a CH3CN solution of the complex.
[Ni(Ph3tpa)(CH3CN)](ClO4)2 (1[3]): To a suspension of
Ph3tpa (136.4 mg, 0.26 mmol) in CH3CN (10 mL) was added
Ni(ClO4)2¢6H2O (96.2 mg, 0.26 mmol) in CH3CN (15 mL), and
the mixture was refluxed for 30 min. The resulting pale brown
solution was poured into diethyl ether (200 mL) to give the
titled compound as pale brown powder in 91% (194.8 mg).
Experimental
¹1
¹
Material and Apparatus. The reagents and the solvents
used in this study, except the ligands and the complexes, were
commercial products of the highest available purity and were
further purified by standard methods, if necessary.19 Ligands
Phntpa (n = 0-3) were synthesized according to reported
procedures,10,20-22 and [Ni(tpa)(CH3CN)(H2O)](ClO4)2 (1[0])
and [Ni(Ph2tpa)(CH3CN)(CH3OH)](ClO4)2 (1[2]) were prepared
according to reported methods.22,23 FT-IR spectra were record-
ed on a Shimadzu FTIR-8200PC or a Jasco FTIR-4100, and
UV-visible spectra were taken on a Jasco V-570 or a Hewlett
Packard 8453 photo diode array spectrophotometer equipped
IR (KBr): ¯ = 1057 and 622 cm (ClO4 ). HRMS: m/z =
675.1309, Calcd for Ni(Ph3tpa)(ClO4) (C36H30ClN4NiO4)
675.1315. Anal. Found: C, 55.78; H, 4.15; N, 8.58%.
Calcd for [Ni(Ph3tpa)(CH3CN)](ClO4)2 (C38H33Cl2N5NiO8):
C, 55.84; H, 4.07; N, 8.57%. UV-vis (CH3CN): -max (¾) =
617 (50) and 985 nm (90 M¹1 cm¹1). Single crystals of
[Ni(Ph3tpa)(CH3CN)](ClO4)2 suitable for the X-ray crystallo-
graphic analysis were obtained by slow diffusion of diethyl
ether into a CH3CN solution of the complex.
Product Analysis. [NiII(Ph2tpa-O)](ClO4) (3[2]): An
acetone solution (50 mL) of [Ni(Ph2tpa)(CH3CN)(CH3OH)]-
(ClO4)2 (1[2]) (77.3 mg, 0.1 mmol) was cooled to ¹78 °C using
a dry ice-acetone bath. Then, 30% H2O2 aqueous solution
(1 equiv) and Et3N (1 equiv) were added to the solution. The
resulting mixture was stirred for 1 h at ¹78 °C and then
gradually warmed up to room temperature. After stirring for
additional 30 min at room temperature, the solvent was
removed under reduced pressure to give a yellow residue, to
which diethyl ether (200 mL) was added. Allowing the mixture
to stand for several minutes resulted in precipitation of pale
yellow powder. The supernatant was then removed by decant-
ation, and the remaining green-brown solid was washed
with diethyl ether three times and dried to give 3[2] in 9%. IR
1
with a Unisoku thermostated cell holder USP-203. H NMR
spectra were recorded on a JEOL FT-NMR Lambda 300WB or
a JEOL FT-NMR GX-400 spectrometer. Mass spectra were
recorded on a JEOL JMS-700T Tandem MS-station mass
spectrometer. ESI-MS (electrospray ionization mass spectra)
measurements were performed on a PE SCIEX API 150 EX.
Elemental analyses were recorded with a Perkin-Elmer or
Fisons instruments EA1108 Elemental Analyzer.
Cyclic voltammetric measurements were performed on an
ALS-630A electrochemical analyzer in deaerated CH3CN
containing 0.10 M n-Bu4NClO4 as a supporting electrolyte. A
Pt working electrode (BAS) was polished with BAS polishing
alumina suspension and rinsed with acetone before use. The
counter electrode was a platinum wire. The measured potentials
were recorded with respect to an Ag/AgNO3 (0.01 M)
reference electrode. All electrochemical measurements were
carried out in a glove box filled with Ar gas at 25 °C.
Resonance Raman scattering was dispersed by a single
polychromator (Ritsu Oyo Kogaku, MC-100) and was detected
by a liquid nitrogen cooled CCD detector (CCD-1024 ©
256-OPEN-1LS, HORIBA Jobin Yvon). The resonance Raman
measurements were carried out using a rotating NMR tube
(outer diameter = 5 mm) thermostated at ¹90 °C by flashing
cold nitrogen gas. A 135° back-scattering geometry was used.
¹1
¹
(KBr): ¯ = 1091 and 703 cm (ClO4 ). HRMS (FAB, pos):
m/z = 515.1379, Calcd for Ni(Ph3tpa-O) (C30H25N4NiO)
515.1381. Anal. Found: C, 57.96; H, 4.03; N, 8.90%. Calcd
for [Ni(Ph2tpa-O)](ClO4)¢(1/2)H2O (C30H26ClN4NiO5.5): C,
57.68; H, 4.20; N, 8.97%. Single crystals of [Ni(Ph2tpa-O)]-
(ClO4)¢CH3CN¢CH3OH suitable for X-ray crystallographic
analysis were obtained by slow diffusion of diethyl ether into a
CH3CN/CH3OH solution of the complex.
[NiII(Ph3tpa-O)](ClO4) (3[3]): An acetone solution (50 mL)
of [Ni(Ph3tpa)(CH3CN)](ClO4)2 (1[3]) (24.5 mg, 30 ¯mol) was
cooled to ¹78 °C using a dry ice-acetone bath. Then, 30%
H2O2 aqueous solution (1 equiv) and Et3N (1 equiv) were
added to the solution. The resulting mixture was stirred for 1 h
at ¹78 °C and then gradually warmed up to room temperature.
After stirring for an additional 30 min at room temperature, the
solvent was removed under reduced pressure to give a brown
residue, to which diethyl ether (200 mL) was added. Allowing
the mixture to stand for several minutes resulted in precip-
itation of brown powder. The supernatant was then removed by
decantation, and the remaining green-brown solid was washed
with diethyl ether three times and dried to give 3[3]. IR (KBr):
Synthesis of Nickel(II) Complexes.
Caution! The
perchlorate salts prepared in this study are all potentially
explosive and should be handled with care.
[Ni(Ph1tpa)(CH3CN)2](ClO4)2 (1[1]): To a suspension of
Ph1tpa (128.3 mg, 0.35 mmol) in CH3CN (10 mL) was added
Ni(ClO4)2¢6H2O (128.0 mg, 0.35 mmol) in CH3CN (15 mL),
and the mixture was stirred for 30 min. The resulting pale
brown solution was poured into diethyl ether (200 mL) to give
the titled compound as pale brown powder in 81% (200.1 mg).
¹1
¹
¹1
¹
IR (KBr): ¯ = 1089 and 621 cm (ClO4 ). HRMS: m/z =
523.0669, Calcd for Ni(Ph1tpa)(ClO4) (C24H22ClN4NiO4)
523.0689. Anal. Found: C, 47.52; H, 3.96; N, 11.70%. Calcd
for [Ni(Ph1tpa)(CH3CN)2](ClO4)2 (C28H28Cl2N6NiO8): C,
47.62; H, 4.00; N, 11.90%. UV-vis (CH3CN): -max (¾) =
556 (30) and 936 nm (50 M¹1 cm¹1). Single crystals of
¯ = 1081 and 694 cm (ClO4 ). HRMS (FAB, pos): m/z =
591.1683, Calcd for C36H29N4NiO 591.1694. Anal. Found: C,
59.31; H, 4.28; N, 7.50%. Calcd for [Ni(Ph3tpa-O)](ClO4)¢
2H2O (C36H33ClN4NiO7): C, 59.41; H, 4.57; N, 7.70%.
Hydroxylated Ligand Ph3tpa-OH. After the reaction of
1[3] and H2O2 described above, the reaction mixture was

532
Bull. Chem. Soc. Jpn. Vol. 83, No. 5 (2010)
Nickel(II) Complexes of TPA Derivatives
(A)
(B)
Figure 1. ORTEP drawings of the cationic parts of (A) [Ni(Ph1tpa)(CH3CN)2](ClO4)2 (1[1]) and (B) [Ni(Ph3tpa)(CH3CN)](ClO4)2
(1[3]) showing 50% probability thermal ellipsoids. The hydrogen atoms are omitted for simplicity.
acidified to pH 1 by adding concd. HCl. Removal of the
solvent by evaporation gave an oily material, which was
dissolved into an NH3 aqueous solution. The aqueous solution
was then extracted with diethyl ether (20 mL © 5), and
the combined organic layer was dried over Na2SO4. After
removal of Na2SO4 by filtration, evaporation of the solvent
gave a yellow material, from which the hydroxylated ligand
Ph3tpa-OH was isolated as an oily material by silica gel column
Data Centre, 12, Union Road, Cambridge, CB2 1EZ, U.K.;
Fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
Kinetic Measurements.
The reaction of nickel(II)
complexes with H2O2 was performed in a 1.0 cm path length
UV-vis cell that was held in a Unisoku cryostat cell holder
USP-203. After an acetone solution containing the nickel(II)
complex (0.6 mM) and Et3N (0.6 mM) was kept at a desired
temperature for several minutes, H2O2 in acetone was injected
into the cell through a septum rubber cap with use of a micro-
syringe. The decomposition of the bis(®-oxo)dinickel(III)
complex was monitored by following the decrease of absorp-
tion at ca. 400 nm due to the bis(®-oxo)dinickel(III) complex.
1
chromatography (eluent: MeOH). H NMR (300 Hz, CDCl3):
¤ 4.05 (4H, s, -CH2-), 4.03 (2H, s, -CH2-), 6.89 (1H, t, J =
7.6 Hz), 7.05 (1H, d, J = 8.2 Hz), 7.31 (1H, dt, J = 1.5 and
7.7 Hz), 7.36-7.52 (7H, m), 7.59 (2H, d, J = 11.5 Hz), 7.61
(2H, d, J = 11.1 Hz), 7.72-7.80 (5H, m), 7.99 (4H, dd, J = 1.2
and 8.3 Hz), 14.70 (1H, s, ArOH). HRMS (FAB, pos): m/z =
535.2493, calcd for (C36H30ON4 + H) 535.2497.
Results
Characterization of Nickel(II) Complexes (Starting
Materials). The crystal structures of [Ni(tpa)(CH3CH2CN)-
(H2O)](ClO4)2 (1[0]¤ involving CH3CH2CN instead of CH3CN
as the co-ligand) and [Ni(Ph2tpa)(CH3CN)(CH3OH)](ClO4)2
(1[2]) have already been reported in the literature.22,23 The
X-ray structures of [Ni(Ph1tpa)(CH3CN)2](ClO4)2 (1[1]) and
[Ni(Ph3tpa)(CH3CN)](ClO4)2 (1[3]) have been determined in
this study as shown in Figure 1. The crystallographic data and
the selected bond lengths and angles of those complexes are
summarized in Tables 1 and 2, respectively.
Complex [Ni(Ph1tpa)(CH3CN)2](ClO4)2 (1[1]) having one
6-phenyl substituent (6-Ph) exhibits a distorted octahedral
geometry involving two acetonitrile co-ligands at cis-position
to each other (Figure 1A). The overall structure of the nickel
center resembles that of [Ni(tpa)(CH3CH2CN)(H2O)](ClO4)2
(1[0]¤) reported by Ito et al. previously, where CH3CH2CN and
H2O occupy the cis-positions instead of the two CH3CN
co-ligands in our complex 1[1].23 However, there are notable
differences in the nickel-nitrogen bond lengths between 1[0]¤
and 1[1]. Namely, the bond length of Ni(1)-N(2) (2.179(3) ¡) is
somewhat longer than those of Ni(1)-N(3) and Ni(1)-N(4)
(2.099(3) and 2.109(3) ¡) in 1[1], whereas the bond lengths
between the nickel ion and the three pyridine nitrogen atoms of
tpa ligand in 1[0]¤ are nearly identical (2.056(8), 2.076(10), and
2.089(8) ¡).23 The elongation of the Ni(1)-N(2) bond in 1[1]
The yield of hydroxylated product Ph3tpa-OH was deter-
mined to be 56% based on the nickel(II) starting material by
1
comparing an integral ratio in the H NMR spectrum between
the benzylic proton (-CH2-) at ¤ 4.05 of Ph3tpa-OH and that of
the original ligand Ph3tpa at ¤ 4.09.
X-ray Structure Determination.
Each single crystal
for 1[1], 1[3], and 3[2]¢CH3CN¢CH3OH was mounted on a
glass-fiber. Data of X-ray diffraction were collected by a
Rigaku RAXIS-RAPID imaging plate two-dimensional area
detector using graphite-monochromated Mo K¡ radiation (- =
0.71069 ¡). The structures were solved with SIR92.24 All non-
hydrogen atoms and hydrogen atoms except disordered atoms
were refined anisotropically. Hydrogen atoms were attached
ideal positions and refined using the rigid model. All the
crystallographic calculations were performed by using Crystal
Structure software package of the Molecular Structure Corpo-
ration25 [Crystal Structure: Crystal Structure Analysis Package
version 3.8.1, Molecular Structure Corp. and Rigaku Corp.
(2005)]. Crystallographic data have been deposited with
Cambridge Crystallographic Data Center: Deposition number
CCDC-737436, 737437, and 737438 for compounds 1[1], 1[3],
and 3[2]¢CH3CN¢CH3OH, respectively. Copies of the data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic

T. Tano et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 5 (2010)
533
Table 1. Summary of the X-ray Crystallographic Data of
Complexes 1[1] and 1[3]
Table 2. Selected Bond Lengths/¡ and Angles/° of
Complexes 1[1] and 1[3]
Complex
1[1]
1[3]
Complex 1[1]
Formula
Formula weight
Crystal system
Space group
a/¡
b/¡
c/¡
¡/°
¢/°
C28H28Cl2N6NiO8
706.17
C38H33N5NiCl2O8
817.31
Ni(1)-N(1)
Ni(1)-N(3)
Ni(1)-N(5)
2.093(3)
2.099(3)
2.082(5)
Ni(1)-N(2)
Ni(1)-N(4)
Ni(1)-N(6)
2.179(3)
2.109(3)
2.039(4)
monoclinic
P21/n (#14)
10.1684(7)
10.7955(8)
28.3903(18)
®
triclinic
ꢀ
P1 (#2)
N(1)-Ni(1)-N(2) 78.57(14)
N(1)-Ni(1)-N(4) 81.72(15)
N(1)-Ni(1)-N(6) 174.84(15) N(2)-Ni(1)-N(3) 101.11(14)
N(2)-Ni(1)-N(4) 159.02(15) N(2)-Ni(1)-N(5) 83.44(14)
N(2)-Ni(1)-N(6) 103.46(15) N(3)-Ni(1)-N(4) 83.28(15)
N(3)-Ni(1)-N(5) 175.32(14) N(3)-Ni(1)-N(6) 92.31(15)
N(1)-Ni(1)-N(3) 82.62(14)
N(1)-Ni(1)-N(5) 97.33(15)
10.931(7)
13.350(11)
13.404(11)
91.06(3)
94.19(3)
111.94(2)
1807.4(23)
2
89.801(5)
®
£/°
V/¡3
Z
N(4)-Ni(1)-N(5) 92.08(15)
N(5)-Ni(1)-N(6) 87.65(15)
N(4)-Ni(1)-N(6) 96.79(15)
3116.5(4)
4
F(000)
1456.00
1.505
844.00
1.502
¹3
Complex 1[3]
D
calcd/g cm
T/K
100
153
Ni(1)-N(1)
Ni(1)-N(3)
Ni(1)-N(5)
2.043(3)
2.090(3)
1.995(3)
Ni(1)-N(2)
Ni(1)-N(4)
2.062(3)
2.056(3)
Crystal size/mm3
0.23 © 0.20 © 0.20 0.40 © 0.30 © 0.20
8.524
55.0
¹1
® (Mo K¡)/cm
2ªmax/°
7.458
55.0
17833
N(1)-Ni(1)-N(2) 80.49(16)
N(1)-Ni(1)-N(4) 80.30(14)
N(2)-Ni(1)-N(3) 117.62(14) N(2)-Ni(1)-N(4) 113.37(14)
N(2)-Ni(1)-N(5) 97.81(14) N(3)-Ni(1)-N(4) 119.89(15)
N(1)-Ni(1)-N(3) 78.85(14)
N(1)-Ni(1)-N(5) 177.16(16)
No. of reflns measd 15551
No. of reflns obsd 6343(All reflections) 8817([I > 2.00·(I)])
No. of variables
407
520
Ra)
Rw
0.0800
0.2003
0.925
0.0558
0.0780
0.982
N(3)-Ni(1)-N(5) 100.05(12) N(4)-Ni(1)-N(5) 102.49(12)
b)
GOF
a) R = -««Fo« ¹ «Fc««/-«Fo«. b) Rw = [-w(«Fo« ¹ «Fc«)2/
−0.36
2 1/2
-wFo ]
.
could be attributed to steric repulsion between the 6-Ph
substituent on the pyridine donor group and the bound CH3CN
co-ligands.
1.0 µA
[Ni(Ph2tpa)(CH3CN)(CH3OH)](ClO4)2 (1[2]), reported by
Berreau et al., also exhibits a distorted octahedral geometry,
where the bond lengths between the nickel ion and the nitrogen
atoms of the pyridine donor groups having 6-Ph (2.218(5)
and 2.200(5) ¡) are also longer than that between the metal
ion and the nitrogen atom of the pyridine ring without 6-Ph
(2.065(4) ¡).22
E1/2 = −0.44
Interestingly, [Ni(Ph3tpa)(CH3CN)](ClO4)2 (1[3]) shows a
five-coordinate trigonal bipyramidal structure with the three
pyridine nitrogen atoms N(2), N(3), and N(4) occupying the
basal plane and the tert-amine nitrogen N(1) and that of the
CH3CN co-ligand N(5) at the axial positions as shown in
Figure 1B. Thus, the structure of 1[3] is exceptional among the
four nickel(II) complexes 1[n] (n = 0-3) with respect to the
coordination number (five vs. six) and geometry (trigonal
bipyramidal vs. octahedral). This could be attributed to the
steric restriction induced by the three 6-Ph groups in 1[3].
−0.51
−0.8
−0.4
0
E (vs. AgNO3) / V
Figure 2. Cyclic voltammogram of 1[0] (4 mM) in deaerated
acetone containing 0.01 M n-Bu4NClO4 at 25 °C; working
electrode Pt, counter electrode Pt, reference electrode
¹1
Ag/0.01 M AgNO3 in CH3CN, scan rate 0.05 V s
.
Berreau et al. reported the H NMR spectrum of 1[2] with a
nickel(II) complexes 1[n] exhibit a pair of redox peaks
corresponding to the reduction and re-oxidation of nickel(II).
Figure 2 shows a cyclic voltammogram (CV) of 1[0] as a
typical example and those of other complexes are presented in
Supporting Information (Figures S4-S6). The E1/2 = (Ered
Eoxp)/2 thus obtained are listed in Table 3.
Apparently, the E1/2 values of 1[0], 1[1], and 1[2], which
have a similar octahedral structure, are nearly identical
(¹0.41- ¹0.43 V vs. Ag/0.01 M AgNO3), whereas that of 1[3]
1
high spin ground state (S = 1) in CD3CN which exhibited
several isotropically shifted resonances spread over a chemical
shift range of ca. 60 ppm.26 Other nickel(II) complexes 1[0], 1[1],
and 1[3] also exhibit similar 1H NMR spectra as shown in
Figures S1-S3, clearly indicating that they also have a high-
spin ground state (S = 1) as in the case of 1[2].
+
p
Redox potential of the complexes is a good indicator of
the electronic effects of the ligand substituents (6-Ph).12 The

534
Bull. Chem. Soc. Jpn. Vol. 83, No. 5 (2010)
Nickel(II) Complexes of TPA Derivatives
M = Cu
M = Ni
CuII-OOH
NR
n
= 0
= 0
n
n
n
=
CuII-OOH
O
= 1
= 2
NR
O
NiIII
1
n
n
MII(Phntpa) + H2O2
= 2
n
NiIII
= 3
CuIII
CuIII
= 3
n
O
O
O
NiIII
NiIII
CuI
O
Scheme 1. H2O2 reactivity of NiII and CuII complexes of Phntpa.
Table 3. CV Data for the Electrochemical Redox Reaction
of 1[n] in Acetonitrile (V vs. Ag/0.01M AgNO3)
1.5
1.0
0.5
0
Complex
Eredp/V
Eoxp/V
¦Ep/V
E1/2/V
398
1[0]
1[1]
1[2]
1[3]
¹0.51
¹0.52
¹0.61
¹0.87
¹0.36
¹0.29
¹0.25
¹0.76
0.15
0.23
0.36
0.11
¹0.44
¹0.41
¹0.43
¹0.82
2[2]
showing the trigonal-bipyramidal geometry is significantly
negative (¹0.82 V). Such a large difference in E1/2 between
the former three and the later one could be attributed to the
different geometry of the complexes (octahedral vs. trigonal
bipyramidal).
1[2]
400
500 600
Wavelength / nm
700
800
H2O2 Reactivity. Reaction of the nickel(II) complexes and
H2O2 was then examined to see the reactivity difference
between the copper(II) complexes and the nickel(II) complexes
supported by the same series of the supporting ligands. As
stated in the introduction, the copper(II) complexes of Phntpa
exhibit quite different reactivity toward H2O2 depending on the
number of 6-Ph substituent on the pyridine donor group.12,13
Namely, [CuII(tpa)(CH3CN)]2+ (n = 0) and [CuII(Ph1tpa)-
(CH3CN)]2+ (n = 1) provided a copper(II) hydroperoxo com-
plex LCuII-OOH, whereas [CuII(Ph2tpa)(CH3CN)]2+ (n = 2)
gave a bis(®-oxo)dicopper(III) complex under the same reac-
tion conditions. On the other hand, [CuII(Ph3tpa)(CH3CN)]2+
(n = 3) was reduced to the copper(I) complex in the reaction
with H2O2 under the same experimental conditions (Scheme 1,
left).
Treatment of 1[0] and 1[1] (0.6 mM) supported by tpa and
Ph1tpa (n = 0 and 1), respectively, with H2O2 (0.6-12.0 mM)
in the presence of Et3N (3.0 mM) as a base in acetone or
in acetonitrile at a temperature range from ¹40 °C to room
temperature caused almost no change of the UV-vis spectrum.
Thus, it can be concluded that the H2O2-reactivity of the
nickel(II) complexes of these two ligands is significantly lower
than that of the copper(II) complexes supported by the same
ligands, which readily provided LCuII-OOH at a low temper-
ature (¹80 °C).12,13
Figure 3. Spectral change observed upon addition of H2O2
(3.0 mM) to an acetone solution of 1[2] (0.6 mM) in the
presence of triethylamine (0.6 mM) at ¹70 °C generating
bis(®-oxo)dinickel(III) complex 2[2]
.
spectrum is very close to those of the reported bis(®-
oxo)dinickel(III) complexes.14 Furthermore, the resonance
Raman spectrum of the reaction solution shown in Figure 4
exhibits a Raman band at 614 cm with H216O2 (solid line
¹1
spectrum) which is ascribable to the Ni2O2 core vibration.27
These spectroscopic features (UV-vis and resonance Raman)
clearly suggest that the generated brown species is a bis(®-
oxo)dinickel(III) complex 2[2] as shown in Scheme 2. Although
the detailed structure of 2[2] is not clear at present, each metal
center of the Ni2O2 core may have a square-planar geometry
with weakly coordinated pyridine donor groups having 6-Ph
in the axial positions as illustrated in Scheme 2. A similar
structure was reported by Suzuki et al. for the bis(®-oxo)-
dinickel(III) complex supported by Me2tpa (bis[(6-methyl-
pyridin-2-yl)methyl][(pyridin-2-yl)methyl]amine).14c In an iso-
tope labeling experiment using H218O2, a small resonance
¹1
Raman band appeared at ca. 580 cm (marked by asterisk in
Figure 4), but most of the resonance Raman band at 614 cm
¹1
On the other hand, treatment of 1[2] (0.6 mM, n = 2) with
H2O2 (3.0 mM) in acetone at ¹70 °C in the presence of Et3N
(0.6 mM) resulted in color change from pale purple to brown.
In Figure 3 is shown a spectral change for the reaction, where
an absorption band at 398 nm readily appeared. The final
remained unchanged (dotted line spectrum). This may be due to
rapid exchange of the oxygen atoms of the oxo groups of 2[2]
with that of water.28 A similar phenomena (no isotope shift in
the Raman measurement) was also reported in other bis(®-
oxo)dinickel(III) systems.14a,14c

T. Tano et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 5 (2010)
535
s
s
s
s
s
614
614
H218O2
H218O2
583
H216O2
H216O2
611
500
600
700
800
Raman Shift / cm−1
500
600
700
800
Raman shift / cm−1
Figure 4. Resonance Raman spectra of the reaction solution
of 1[2] with H216O2 (solid line) and with H218O2 (dotted
line) obtained with an excitation at -ex = 406.7 nm in
acetone at ¹90 °C: “s” denotes a solvent Raman band.
Figure 6. Resonance Raman spectra of 2[3] generated by the
reaction of 1[3] with H216O2 (solid line) and with H218O2
(dotted line) obtained with an excitation at -ex = 406.7 nm
in acetone at ¹90 °C: “s” denotes a solvent Raman band.
PyPh
NiIII
PyPh
2+
Py =
1
Py
N
0
O
O
N
NiIII
398
Ph
N
-1
Py
PhPy =
∞
A
-
N
PyPh
A
PyPh
-2
2[2]
Scheme 2. Presumed structure of bis(®-oxo)dinickel(III)
-3
0
5
10
15
complex 2[2]
.
2[2]
Time / s ( x 103)
1.5
397
3[2]
409
0
400 500 600 700 800
Wavelength / nm
Figure 7. Spectral change for the conversion of 2[2]
(0.6 mM) to 3[2] in acetone at ¹40 °C.
1.0
0.5
0
2[3]
at 611 cm (Figure 6, solid line spectrum). In this case, 2[3]
¹1
exhibited an isotope shift of the Raman band at 611 to
583 cm¹1 upon 18O-substitution using H218O2 (dotted line). The
observed isotope shift of ¦¯ = 28 cm¹1 is in the range of those
of the bis(®-oxo) dimetallic complexes.27 These spectroscopic
features suggest that complex 2[3] has a similar bis(®-oxo)-
dinickel(III) core. Enhanced steric bulkiness of Ph3tpa by the
additional 6-Ph groups in the equatorial position may protect
the Ni2O2 core from attack by water, thus preventing the
oxygen exchange reaction between the oxo groups and H2O.
Decomposition Product. Bis(®-oxo)dinickel(III) complex
2[2] was relatively stable at a low temperature (¹70 °C), but
gradually decomposed at a higher temperature (¹40 °C)
obeying first-order kinetics (Figure 7, kdec = 1.1 © 10¹4 s¹1).
In the UV-vis spectrum of the final reaction mixture shown in
1[3]
400
500 600
Wavelength / nm
700
800
Figure 5. Spectral change observed upon addition of H2O2
(3.0 mM) to an acetone solution of 1[3] (0.6 mM) in the
presence of triethylamine (0.6 mM) at ¹70 °C generating
bis(®-oxo)dinickel(III) complex 2[3]
.
The reaction of 1[3] and H2O2 under the same experimental
conditions provided a similar UV-vis spectrum (-max
409 nm) as shown in Figure 5, and a resonance Raman band
=

536
Bull. Chem. Soc. Jpn. Vol. 83, No. 5 (2010)
Nickel(II) Complexes of TPA Derivatives
Table 5. Selected Bond Lengths/¡ and Angles/° of
Complex 3[2]¢CH3CN¢CH3OH
Complex 3[2]¢CH3CN¢CH3OH
Ni(1)-N(1)
Ni(1)-N(3)
1.862(2)
1.902(3)
Ni(1)-N(2)
Ni(1)-O(1)
1.886(3)
1.817(2)
N(1)-Ni(1)-N(2) 170.82(16) N(1)-Ni(1)-N(3) 86.35(14)
N(2)-Ni(1)-N(3) 85.74(14) O(1)-Ni(1)-N(1) 95.22(13)
O(1)-Ni(1)-N(2) 92.80(13) O(1)-Ni(1)-N(3) 177.96(12)
2
-3
383
Figure 8. ORTEP drawing of the cationic part of 3[2]
CH3CN¢CH3OH showing 50% probability thermal ellip-
soids. The hydrogen atoms are omitted for simplicity.
¢
3[3]
∞
-4
A
-
A
409
-5
Table 4. Summary of the X-ray Crystallographic Data of
Complex 3[2]¢CH3CN¢CH3OH
1
-6
0
5
10
2[3]
Time / s ( x 103)
Compound
3[2]¢CH3CN¢CH3OH
Formula
Formula weight
C33H32N5NiClO6
688.80
Crystal system
Space group
a/¡
b/¡
c/¡
monoclinic
P21/a (#14)
11.820(9)
20.585(14)
12.922(12)
90
0
400 500 600 700 800
Wavelength / nm
¡/°
Figure 9. Spectral change for the conversion of 2[3]
(0.6 mM) to 3[3] in acetone at ¹40 °C.
¢/°
99.54(3)
90
3100(4)
4
1432.00
1.475
153
0.40 © 0.30 © 0.20
7.663
55.0
28864
5051([I > 0.40·(I)])
447
£/°
V/¡3
Z
complexes are summarized in Tables 4 and 5, respectively).
As clearly seen, one of the 6-Ph groups is hydroxylated at its
o-position to give a mononuclear NiII-phenolate complex 3[2],
where the nickel center exhibited a distorted square-planar
structure with a N3O donor set provided by the modified ligand
(L[2]-OH).
F(000)
¹3
D
calcd/g cm
T/K
Crystal size/mm3
¹1
® (Mo K¡)/cm
2ªmax/°
Bis(®-oxo)dinickel(III) product 2[3] decomposed more read-
ily, also obeying first-order kinetics (kdec = 1.3 © 10¹3 s
¹1
)
No. of reflns measd
No. of reflns obsd
No. of variables
Ra)
(Figure 9), and the ligand hydroxylated product having an
absorption band at 383 nm was produced as a major product
(56% yield determined by 1H NMR of the organic product
mixture after work-up, ca. 40% of original ligand was
recovered). Although the mechanistic details have yet to be
examined, the aromatic hydroxylation may proceed via electro-
philic aromatic substitution as previously demonstrated for the
NiII/H2O2 systems.14b,29,30
0.0429
0.0476
0.979
b)
Rw
GOF
a) R = -««Fo« ¹ «Fc««/-«Fo«. b) Rw = [-w(«Fo« ¹ «Fc«)2/
2 1/2
-wFo ]
.
Discussion
Figure 7, there is a small absorption band at 397 nm which is
due to the ligand hydroxylated product as described below (the
smaller absorbance is due to the lower yield of the hydroxy-
lated product, see below).
The hydroxylated complex was isolated as a minor product
(9% yield determined by 1H NMR of the organic product
mixture after work-up, ca. 90% of original ligand was
recovered) from the final reaction mixture in a preparative
scale, and its structure was determined by X-ray crystallo-
graphic analysis as indicated in Figure 8 (the crystallographic
data and the selected bond lengths and angles of those
In order to get insights into the substituent effects on the
structure and reactivity of the tpa-complexes, nickel(II) com-
plexes supported by a series of tpa ligands containing different
number of 6-Ph have been prepared and structurally charac-
terized. Phntpa with n = 0, 1, and 2 afforded the nickel(II)
complexes 1[0], 1[1], and 1[2] with a distorted octahedral
geometry, whereas Ph3tpa gave a nickel(II) complex with a
trigonal-bipyramidal structure. The sterically bulkier ligand
Ph3tpa may prevent the complex from having a six-coordinate
geometry with two additional external co-ligands, thus provid-

T. Tano et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 5 (2010)
537
ing the complex with a five-coordinate trigonal-bipyramidal
structure with only one external ligand.
tion, Culture, Sports, Science and Technology, Japan. We
also thank The Asahi Glass Foundation and The Mitsubishi
Foundation for their financial supports.
With respect to the reactivity, nickel(II) complexes 1[0] and
1[1] hardly reacted with H2O2 (Scheme 1, right). The poor
reactivity of the nickel(II) complexes of tpa and Ph1tpa toward
H2O2 is significantly different from the high H2O2-reactivity of
the copper(II) complexes of the same ligands, which afford
LCuII-OOH complexes (Scheme 1, left).12,13 Such a reactivity
difference between the nickel(II) and copper(II) complexes
could be attributed to a lower Lewis acidity of nickel(II) as
compared with copper(II). Namely, Lewis acidity of nickel(II)
may not be strong enough to stabilize the hydroperoxide
adduct. In fact, there have been few examples of mono-
nuclear nickel(II)-hydroperoxo complexes LNiII-OOH,
whereas many examples of LCuII-OOH complexes have so
far been reported.31
Supporting Information
1H NMR spectra of 1[0], 1[1], and 1[3] (Figures S1-S3) and
cyclic voltammograms of 1[1], 1[2], and 1[3] (Figures S4-S6).
This material is available free of charge on the Web at: http://
www.csj.jp/journals/bcsj/.
References
1
a) L. M. Berreau, Comments Inorg. Chem. 2007, 28, 123.
b) G. K. Ingle, R. W. Watkins, A. M. Arif, L. M. Berreau, J. Coord.
Chem. 2008, 61, 61.
2
3
A. G. Blackman, Eur. J. Inorg. Chem. 2008, 2633.
a) K. D. Karlin, S. Kaderli, A. D. Zuberbühler, Acc. Chem.
On the other hand, 1[2] and 1[3] supported by Ph2tpa
and Ph3tpa, respectively, provided the bis(®-oxo)dinickel(III)
complex 2[2] and 2[3] in the reaction with H2O2 at low
temperature (Scheme 1, right). Increasing the number of 6-Ph
groups of the ligands may cause a decrease of electron-donor
ability of pyridine(s), thus enhancing the Lewis acidity of
the nickel(II) center. As a result, reactivity of 1[2] and 1[3]
toward H2O2 is enhanced to afford the bis(®-oxo) complexes
presumably through a LNiII-OOH intermediate. Stability of the
higher oxidation state of nickel(III) may also help the formation
of the bis(®-oxo)dinickel(III) complexes.
Res. 1997, 30, 139. b) M.-A. Kope, K. D. Karlin, in Biomimetic
Oxidations Catalyzed by Transition Metal Complexes, ed. by B.
Meunier, Imperial College Press, London, 2000, pp. 309-362.
c) L. Q. Hatcher, K. D. Karlin, J. Biol. Inorg. Chem. 2004, 9, 669.
4
a) M. Costas, M. P. Mehn, M. P. Jensen, L. Que, Jr., Chem.
Rev. 2004, 104, 939. b) X. Shan, L. Que, Jr., J. Inorg. Biochem.
2006, 100, 421. c) L. Que, Jr., Acc. Chem. Res. 2007, 40, 493.
5
M. Suzuki, Acc. Chem. Res. 2007, 40, 609.
6
6, 34.
7
S. Yamaguchi, H. Masuda, Sci. Technol. Adv. Mater. 2005,
E. Y. Tshuva, S. J. Lippard, Chem. Rev. 2004, 104, 987.
L. M. Mirica, X. Ottenwaelder, T. D. P. Stack, Chem. Rev.
8
It should be also noted that the reactivity of the Ph3tpa
system is quite different between the nickel(II) and copper(II)
complexes; the nickel(II) complex provided the bis(®-oxo)-
dinickel(III) complex 2[3], whereas the copper(II) complex was
reduced to copper(I) (Scheme 1).12 Such a big difference in
reactivity in the Ph3tpa sysetm can apparently be attributed to
the difference in their redox potentials; E1/2 = ¹0.82 and
¹0.04 V vs. Ag/AgNO3 for [NiII(Ph3tpa)(CH3CN)](ClO4)2
(1[3]) and [CuII(Ph3tpa)(CH3CN)](ClO4)2, respectively.12
Namely, the reduced copper(I) complex is much more acces-
sible than the nickel(I) complex, which is the reason why the
copper(II) complex is easily reduced to the copper(I) complex.
Reactivity of the bis(®-oxo)dinickel(III) complexes toward
intramolecular ligand oxidation is also different from that of the
bis(®-oxo)dicopper(III) complex (Scheme 1); the bis(®-oxo)-
dinickel(III) complex induced the aromatic ligand hydroxyl-
ation at the o-position of 6-Ph via an electrophilic aromatic
substitution mechanism (Figure 8), whereas the bis(®-oxo)-
dicopper(III) complex resulted in an oxidative N-dealkylation
of the supporting ligand Ph2tpa through a hydrogen atom
abstraction and oxygen rebound mechanism or its concerted
variant.12 Thus, it becomes clear that the intrinsic reactivity of
the bis(®-oxo) complex between the nickel and copper systems
are quite different. These results obtained in this study will
provide significantly important insights into not only the
substituent effects on the tpa-coordination chemistry but also
the reactivity differences between the nickel and copper
complexes supported by the same series of ligands.
2004, 104, 1013.
9
E. A. Lewis, W. B. Tolman, Chem. Rev. 2004, 104, 1047.
10 a) C.-L. Chuang, K. T. Lim, Q. Chen, J. Zubieta, J. W.
Canary, Inorg. Chem. 1995, 34, 2562. b) C.-L. Chuang, K. Lim,
J. W. Canary, Supramol. Chem. 1995, 5, 39.
11 D. Maiti, H. R. Lucas, A. A. N. Sarjeant, K. D. Karlin,
J. Am. Chem. Soc. 2007, 129, 6998.
12 A. Kunishita, M. Kubo, H. Ishimaru, T. Ogura, H.
Sugimoto, S. Itoh, Inorg. Chem. 2008, 47, 12032.
13 T. Fujii, A. Naito, S. Yamaguchi, A. Wada, Y. Funahashi,
K. Jitsukawa, S. Nagatomo, T. Kitagawa, H. Masuda, Chem.
Commun. 2003, 2700.
14 a) S. Hikichi, M. Yoshizawa, Y. Sasakura, M. Akita, Y.
Moro-oka, J. Am. Chem. Soc. 1998, 120, 10567. b) S. Itoh, H.
Bandoh, S. Nagatomo, T. Kitagawa, S. Fukuzumi, J. Am. Chem.
Soc. 1999, 121, 8945. c) K. Shiren, S. Ogo, S. Fujinami, H.
Hayashi, M. Suzuki, A. Uehara, Y. Watanabe, Y. Moro-oka, J. Am.
Chem. Soc. 2000, 122, 254. d) B. S. Mandimutsira, J. L. Yamarik,
T. C. Brunold, W. Gu, S. P. Cramer, C. G. Riordan, J. Am. Chem.
Soc. 2001, 123, 9194. e) S. Itoh, H. Bandoh, M. Nakagawa, S.
Nagatomo, T. Kitagawa, K. D. Karlin, S. Fukuzumi, J. Am. Chem.
Soc. 2001, 123, 11168. f) R. Schenker, B. S. Mandimutsira, C. G.
Riordan, T. C. Brunold, J. Am. Chem. Soc. 2002, 124, 13842.
15 S. Yao, E. Bill, C. Milsmann, K. Wieghardt, M. Driess,
Angew. Chem., Int. Ed. 2008, 47, 7110.
16 a) K. Fujita, R. Schenker, W. Gu, T. C. Brunold, S. P.
Cramer, C. G. Riordan, Inorg. Chem. 2004, 43, 3324. b) M. T.
Kieber-Emmons, R. Schenker, G. P. A. Yap, T. C. Brunold, C. G.
Riordan, Angew. Chem., Int. Ed. 2004, 43, 6716. c) R. Schenker,
M. T. Kieber-Emmons, C. G. Riordan, T. C. Brunold, Inorg. Chem.
2005, 44, 1752.
This work was supported by Grant-in-Aid for Science
Research on Priority Areas (No. 200360044, Synergy of
Elements; No. 21108515, pi-Space) from Ministry of Educa-
17 a) M. T. Kieber-Emmons, J. Annaraj, M. S. Seo, K. M. V.
Heuvelen, T. Tosha, T. Kitagawa, T. C. Brunold, W. Nam, C. G.

538
Bull. Chem. Soc. Jpn. Vol. 83, No. 5 (2010)
Nickel(II) Complexes of TPA Derivatives
Crystallogr. 1994, 27, 435.
25 Crystal Structure: Crystal Structure Analysis Package
version 3.8.1, Molecular Structure Corp. and Rigaku Corp., 2005.
26 E. Szajna, P. Dobrowolski, A. L. Fuller, A. M. Arif, L. M.
Berreau, Inorg. Chem. 2004, 43, 3988.
Riordan, J. Am. Chem. Soc. 2006, 128, 14230. b) J. Cho, R.
Sarangi, J. Annaraj, S. Y. Kim, M. Kubo, T. Ogura, E. I. Solomon,
W. Nam, Nat. Chem. 2009, 1, 568.
18 S. Hikichi, H. Okuda, Y. Ohzu, M. Akita, Angew. Chem.,
Int. Ed. 2009, 48, 188.
19 W. L. F. Armarego, D. D. Perrin, Purification of Laboratory
Chemicals, 4th ed., Butterworth-Heinemann, Oxford, 1996.
20 Z. Tyeklár, R. R. Jacobson, N. Wei, N. N. Murthy, J.
Zubieta, K. D. Karlin, J. Am. Chem. Soc. 1993, 115, 2677.
21 M. P. Jensen, E. L. Que, X. Shan, E. Rybak-Akimova, L.
Que, Jr., Dalton Trans. 2006, 3523.
22 M. M. Makowska-Grzyska, E. Szajna, C. Shipley, A. M.
Arif, M. H. Mitchell, J. A. Halfen, L. M. Berreau, Inorg. Chem.
2003, 42, 7472.
27 L. Que, Jr., W. B. Tolman, Angew. Chem., Int. Ed. 2002, 41,
1114.
28 Since the bis(®-oxo)dinickel(III) complex 2[2]-16O was
generated in situ by adding a diluted H216O2 aqueous solution
(H216O) to 1[2] 16O/18O isotope exchange experiment by adding
,
H218O to a solution of 2[2]-16O containing a large excess of H216O
was unsuccessful.
29 A. Kunishita, Y. Doi, M. Kubo, T. Ogura, H. Sugimoto, S.
Itoh, Inorg. Chem. 2009, 48, 4997.
23 M. Ito, K. Sakai, T. Tsubomura, Y. Takita, Bull. Chem. Soc.
Jpn. 1999, 72, 239.
24 SIR92: A. Altomare, G. Cascarano, C. Giacovazzo, A.
Guagliardi, M. C. Burla, G. Polidori, M. Camalli, J. Appl.
30 K. Honda, J. Cho, T. Matsumoto, J. Roh, H. Furutachi, T.
Tosha, M. Kubo, S. Fujinami, T. Ogura, T. Kitagawa, M. Suzuki,
Angew. Chem., Int. Ed. 2009, 48, 3304.
31 S. Itoh, Curr. Opin. Chem. Biol. 2006, 10, 115.