H2O2-reactivity of copper(II) complexes supported by tris[(pyridin-2-yl)methyl]amine ligands with 6-phenyl substituents

Article abstract of DOI:10.1021/ic801568g

The structure and H₂O₂-reactivity of a series of copper(II) complexes supported by tris[(pyridin-2-yl)methyl]amine (TPA) derivatives having a phenyl group at the 6-position of pyridine donor group(s) [(6-phenylpyridin-2-yl)-methyl]bis[(pyridin-2-yl)methyl]amine (Ph ₁TPA), bis[(6-phenylpyridin-2-yl)methyl][(pyridin-2-yl)methyl]amine (Ph₂TPA), and tris[(6-phenylpyridin-2-yl)methyl]amine (Ph ₃TPA) have systematically been examined to get insights into the aromatic substituent (6-Ph) effects on the coordination chemistry of TPA ligand system. The X-ray crystallographic analyses have revealed that [Cu ^II(TPA)(CH₃CN)](ClO₄)₂ (CuTPA) and [Cu^II(Ph₃TPA)(CH₃CN)](ClO₄) ₂ (3) exhibit a trigonal bipyramidal structure, whereas [Cu ^II(Ph₁TPA)(CH₃CN)](ClO₄) ₂ (1) shows a slightly distorted square pyramidal structure and [Cu^II(Ph₂TPA)(CH₃CN)](ClO₄) ₂ (2) has an intermediate structure between trigonal bipyramidal and square pyramidal. On the other hand, the UV-vis and ESR data have suggested that all the copper(II) complexes have a similar trigonal bipyramidal structure in solution. The redox potentials of CuTPA, 1, 2, and 3 have been determined as E_1/2 = -0.34, -0.28, -0.16, and -0.04 mV vs Ag/AgNO₃, respectively, demonstrating that introduction of each 6-Ph group causes positive shift of E_1/2 about 0.1 V. Notable difference in H₂O ₂-reactivity has been found among the copper(II) complexes. Namely, CuTPA and 1 afforded mononuclear copper(II)-hydroperoxo complexes CuTPA-OOH and 1-OOH, respectively, whereas complex 2 provided bis(μ-oxo)dicopper(III) complex 2-oxo. On the other hand, copper(II) complex 3 was reduced to the corresponding copper(I) complex 3^red. On the basis of the H ₂O₂-reactivity together with the X-ray structures and the redox potentials of the copper(II) complexes, the substituent effects of 6-Ph are discussed in detail.

Full text of DOI:10.1021/ic801568g

Inorg. Chem. 2008, 47, 12032-12039
H2O2-Reactivity of Copper(II) Complexes Supported by
Tris[(pyridin-2-yl)methyl]amine Ligands with 6-Phenyl Substituents
Atsushi Kunishita,† Minoru Kubo,‡ Hirohito Ishimaru,‡ Takashi Ogura,‡ Hideki Sugimoto,†
and Shinobu Itoh*,†,§,⊥
Department of Material and Life Science, DiVision of AdVanced Science and Biotechnology, Graduate
School of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan, Department
of Chemistry, Graduate School of Science, Osaka City UniVersity, 3-3-138 Sugimoto, Sumiyoshi-ku,
Osaka 558-8585, Japan, Institute for Molecular Science, National Institutes of Natural Sciences, 38
Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan, and Research Institute of Picobiology, Graduate
School of Life Science, UniVersity of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun,
Hyogo 678-1297, Japan
Received August 17, 2008
The structure and H2O2-reactivity of a series of copper(II) complexes supported by tris[(pyridin-2-yl)methyl]amine
(TPA) derivatives having a phenyl group at the 6-position of pyridine donor group(s) [(6-phenylpyridin-2-yl)-
methyl]bis[(pyridin-2-yl)methyl]amine (Ph1TPA), bis[(6-phenylpyridin-2-yl)methyl][(pyridin-2-yl)methyl]amine (Ph2TPA),
and tris[(6-phenylpyridin-2-yl)methyl]amine (Ph3TPA) have systematically been examined to get insights into the
aromatic substituent (6-Ph) effects on the coordination chemistry of TPA ligand system. The X-ray crystallographic
analyses have revealed that [CuII(TPA)(CH3CN)](ClO4)2 (CuTPA) and [CuII(Ph3TPA)(CH3CN)](ClO4)2 (3) exhibit a
trigonal bipyramidal structure, whereas [CuII(Ph1TPA)(CH3CN)](ClO4)2 (1) shows a slightly distorted square pyramidal
structure and [CuII(Ph2TPA)(CH3CN)](ClO4)2 (2) has an intermediate structure between trigonal bipyramidal and
square pyramidal. On the other hand, the UV-vis and ESR data have suggested that all the copper(II) complexes
have a similar trigonal bipyramidal structure in solution. The redox potentials of CuTPA, 1, 2, and 3 have been
determined as E1/2 ) -0.34, -0.28, -0.16, and -0.04 mV vs Ag/AgNO3, respectively, demonstrating that
introduction of each 6-Ph group causes positive shift of E1/2 about 0.1 V. Notable difference in H2O2-reactivity has
been found among the copper(II) complexes. Namely, CuTPA and 1 afforded mononuclear copper(II)-hydroperoxo
complexes CuTPA-OOH and 1-OOH, respectively, whereas complex 2 provided bis(µ-oxo)dicopper(III) complex
2-oxo. On the other hand, copper(II) complex 3 was reduced to the corresponding copper(I) complex 3red. On the
basis of the H2O2-reactivity together with the X-ray structures and the redox potentials of the copper(II) complexes,
the substituent effects of 6-Ph are discussed in detail.
Introduction
this ligand have been prepared, and their structure, physi-
cochemical properties, and reactivity have been explored in
detail.1,2 Recently, several types of TPA derivatives have
also been developed to tune the structure and properties of
the supported complexes, where steric constraints and/or
electronic effects induced by the substituents are crucial.1-9
For instance, Suzuki, Tanaka, and co-workers systematically
investigated the structures and the redox properties of
TPA, tris[(pyridin-2-yl)methyl]amine, is one of the most
popular tetradentate tripodal ligands in coordination chem-
istry.1,2 A large number of transitional metal complexes of
* To whom correspondence should be addressed. E-mail: shinobu@
mls.eng.osaka-u.ac.jp.
†
Osaka City University.
University of Hyogo.
Osaka University.
Institute for Molecular Science.
‡
§
⊥
(3) (a) Karlin, K. D.; Kaderli, S.; Zuberbuhler, A. D. Acc. Chem. Res.
1997, 30, 139–147. (b) Kope, M.-A.; Karlin, K. D. In Biomimetic
Oxidations Catalyzed by Transition Metal Complexes; Meunier, B.,
Ed.; Imperial College Press: London, 2000; pp 309-362. (c) Hatcher,
L. Q.; Karlin, K. D. J. Biol. Inorg. Chem. 2004, 9, 669–683.
(1) (a) Berreau, L. M. Comments Inorg. Chem. 2007, 28, 123–171. (b)
Ingle, G. K.; Watkins, R. W.; Arif, A. M.; Berreau, L. M. J. Coord.
Chem. 2008, 61, 61–77.
(2) Blackman, A. G. Eur. J. Inorg. Chem. 2008, 17, 2633–2647.
12032 Inorganic Chemistry, Vol. 47, No. 24, 2008
10.1021/ic801568g CCC: $40.75  2008 American Chemical Society
Published on Web 11/08/2008

H2O2-ReactiWity of Copper(II) Complexes
Chart 1
[CuII(MenTPA)(Cl)]+ (MenTPA, n ) 1-3, see Chart 1) to
find that structural perturbation induced by the 6-Me sub-
stituent(s) leads to positive shift of the redox potential of
the copper(II) complexes (E1/2 ) -0.33, -0.20, and -0.04
mV vs Ag/AgCl, respectively, in H2O (pH 7)).10 The results
have been attributed mainly to steric repulsion between the
6-methyl substituent and the bound metal ion, which causes
reduction of electron-donor ability of the pyridine donor
groups.10 Such effects of the 6-Me group(s) of MenTPA have
also been evaluated in the iron system, by which not only
E1/2 but also the spin state of the metal center can be altered
to control the reactivity of the supported iron complexes.4
More recently, TPA ligands with 6-Ph substituent(s)
(PhnTPA, Chart 1) have also been investigated in anticipation
of larger and/or different types of effects on the coordination
chemistry of TPA. For instance, Canary and co-workers have
examined the substituent effects of the 6-Ph groups on the
structures and redox properties of Cu(I) and Cu(II) complexes
using Ph3TPA (Chart 1), where the 6-Ph substituents causes
a positive shift of the redox potential by 300-480 mV
relative to that of the copper(II) complex of TPA itself
depending on the solvent used.11 The positive shift of the
redox potential has been attributed to an electron withdrawing
effect by 6-Ph and a reduction of the local dielectric character
because of steric shielding in the metal center.11 However,
little has been discussed about the aromatic substituent effects
on the reactivity of PhnTPA complexes. In this study, we
have systematically investigated the Cu(II)-H2O2 reactivity
in the PhnTPA ligand system to disclose the aromatic
substituent effects on the redox reactivity of the copper-TPA
complexes.
hydroperoxo complex LCuII-OOH,12 which is one of the
important active oxygen intermediates involved in copper
oxidases and copper monooxygenases as well as in numerous
copper-catalyzed oxidation reactions.13 Karlin and his co-
workers have recently demonstrated that the LCuII-OOH
intermediate generated by using a Ph1TPA derivative having
a p-tert-butyl group on 6-Ph has an ability to induce an
aromatic ligand hydroxylation in acetone.14 Here we have
found that the copper(II) complexes supported by Ph2TPA
and Ph3TPA exhibit totally different reactivity toward H2O2,
providing more insights into the substituent effects on the
TPA coordination chemistry.
Experimental Section
General Procedures. The reagents and the solvents used in this
study, except the ligand and the copper complexes, were commercial
products of the highest available purity and were further purified
by the standard methods, if necessary.15 Ligands TPA and PhnTPA
were synthesized according to the reported procedures.11,16-18 The
copper(II) complexes of [CuII(TPA)(CH3CN)](ClO4)2 (CuTPA) and
[CuII(Ph3TPA)(CH3CN)](ClO4)2 (3) were prepared as reported
previously.11,12 FT-IR spectra were recorded on 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 with
1
a Unisoku thermostatted cell holder USP-203. H NMR spectra
were recorded on a JEOL FT-NMR Lambda 300WB. Electron spin
resonance (ESR) spectra were recorded on a BRUKER E-500
spectrometer at -150 °C. Mass spectra were recorded on a JEOL
JMS-AX500 mass spectrometer and JMS-700T Tandem MS-station
mass spectrometer. ESI-MS (electrospray ionization mass spectra)
measurements were performed on a PE SCIEX API 150EX or a
Micromass LCT spectrometer. Resonance Raman scattering was
excited at 406.7 nm from a Kr+ laser (Spectra Physics, BeamLok
2060). Resonance Raman scattering was dispersed by a single
It has been reported that the reaction of the copper(II)
complex of TPA and H2O2 gives a mononuclear copper(II)
(4) (a) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. ReV.
2004, 104, 939–986. (b) Shan, X.; Que, L., Jr. J. Inorg. Biochem.
2006, 100, 421–433. (c) Que, L., Jr. Acc. Chem. Res. 2007, 40, 493–
500.
(5) Suzuki, M. Acc. Chem. Res. 2007, 40, 609–617.
(6) Yamaguchi, S.; Masuda, H. Sci. Technol. AdV. Mater. 2005, 6, 34–
47.
(7) Tshuva, E. Y.; Lippard, S. J. Chem. ReV. 2004, 104, 987–1012.
(8) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. ReV. 2004,
104, 1013–1045.
(9) Lewis, E. A.; Tolman, W. B. Chem. ReV. 2004, 104, 1047–1076.
(10) Nagao, H.; Komeda, N.; Mukaida, M.; Suzuki, M.; Tanaka, K. Inorg.
Chem. 1996, 35, 6809–6815.
(12) Fujii, T.; Naito, A.; Yamaguchi, S.; Wada, A.; Funahashi, Y.;
Jitsukawa, K.; Nagatomo, S.; Kitagawa, T.; Masuda, H. Chem.
Commun. 2003, 2700–2701.
(13) Itoh, S. Curr. Opin. Chem. Biol. 2006, 10, 115–122.
(14) Maiti, D.; Lucas, H. R.; Sarjeant, A. A. N.; Karlin, K. D. J. Am. Chem.
Soc. 2007, 129, 6998–6999.
(15) Armarego, W. L. F.; Perrin, D. D. In Purification of Laborator
Chemicals, 4th ed.; Butterworth-Heinemann: Oxford, 1996; pp 176
and 215.
(16) Tyeklar, Z.; Jacobson, R. R.; Wei, N.; Murthy, N. N.; Zubieta, J.;
Karlin, K. D. J. Am. Chem. Soc. 1993, 115, 2677–2689.
(17) Jensen, M. P.; Que, E, L.; Shan, X.; Rybak-Akimova, E.; Que, L., Jr.
Dalton. Trans. 2006, 3523–3527.
(11) (a) Chuang, C.-L.; Lim, K. T.; Chen, Q.; Zubieta, J.; Canary, J. W.
Inorg. Chem. 1995, 34, 2562–2568. (b) Chuang, C.-L.; Lim, K. T.;
Canary, J. W. Supramol. Chem. 1995, 5, 39–43.
(18) Makowska-Grzyska, M. M.; Szajna, E.; Shipley, C.; Arif, A. M.;
Mitchell, M. H.; Halfen, J. A.; Berreau, L. M. Inorg. Chem. 2003,
42, 7472–7488.
Inorganic Chemistry, Vol. 47, No. 24, 2008 12033

Kunishita et al.
polychromator (Ritsu Oyo Kogaku, MC-100) and was detected by
a liquid nitrogen cooled CCD detector (Roper Scientific, LNCCD-
1100-PB). The resonance Raman measurements were carried out
using a rotated cylindrical cell thermostatted at -80 °C or a rotating
NMR tube (outer diameter ) 5 mm) thermostatted at -40 °C by
flashing cold nitrogen gas. A 135° back-scattering geometry was
used. Cyclic voltammetric measurements were performed on an
ALS-630A electrochemical analyzer in deaerated CH3CN containing
0.10 M NBu4ClO4 as a supporting electrolyte. A Pt working
electrode (BAS) was polished with BAS polishing alumina suspen-
sion and rinsed with acetone before use. The counter electrode was
a platinum wire. The measured potentials were recorded with respect
to a Ag/AgNO3 (0.01 M) reference electrode. All electrochemical
measurements were carried out in a glovebox filled with Ar gas at
25 °C. Elemental analyses were recorded with a Perkin-Elmer or a
Fisons instruments EA1108 Elemental Analyzer.
Figure 1. Oak Ridge Thermal Ellipsoid Plot (ORTEP) drawing of 1
showing 50% probability thermal ellipsoids. The counteranion and the
hydrogen atoms are omitted for clarity.
Synthesis of Copper(II) Complexes. Caution! The perchlorate
salts in this study are all potentially explosiVe and should be
handled with care.
[CuII(Ph1TPA)(CH3CN)](ClO4)2 (1). CuII(ClO4)2 ·6H2O (148.0
mg, 0.40 mmol) was added to a CH3CN solution (5 mL) of ligand
Ph1TPA (146.4 mg, 0.40 mmol). After stirring for 10 min at room
temperature, insoluble material was removed by filtration. Addition
of ether (100 mL) to the filtrate gave a blue powder that was
precipitated by standing the mixture for several minutes. The
supernatant was then removed by decantation, and the remained
blue solid was washed with ether three times and dried to give
complex 1 in 69%. Single crystals of 1 were obtained by vapor
diffusion of ether into a CH3CN solution of the complex. FT-IR
(KBr) 1109, 1094, and 623 cm-1 (ClO4-); HR-MS (FAB, pos.)
m/z ) 429.1145 calcd for C24H22N4Cu: 429.1140; Anal. Calcd for
[CuII(Ph1TPA)(CH3CN)](ClO4)2 (C26H25CuCl2N5O8): C, 46.61; H,
3.76; N, 10.45. Found: C, 46.64; H, 3.69; N, 10.32.
[CuII(Ph2TPA)(CH3CN)](ClO4)2 (2). This compound was pre-
pared by following the same procedures as described for 1 in 89%.
Single crystals of 2 were obtained by vapor diffusion of ether into
a CH3CN solution of the complex. FT-IR (KBr) 1091 and 623 cm-1
(ClO4-); HR-MS (FAB, pos.) m/z ) 505.1448 calcd for
C30H26N4Cu: 505.1453; Anal. Calcd for [CuII(Ph2TPA)(CH3CN)]-
(ClO4)2 ·0.5H2O (C21H31CuCl2N4O8.5): C, 50.90; H, 4.00; N, 9.28.
Found: C, 50.62; H, 3.81; N, 9.27.
Figure 2. ORTEP drawing of 2 showing 50% probability thermal ellipsoids.
The counteranion and the hydrogen atoms are omitted for clarity.
of the Molecular Structure Corporation [Crystal Structure: Crystal
Structure Analysis Package version 3.8.1, Molecular Structure Corp.
and Rigaku Corp. (2005)]. The structures were solved with SIR92
and refined with CRYSTALS. All non-hydrogen atoms and
hydrogen atoms were refined anisotropically and isotropically,
respectively. Atomic coordinates, thermal parameters, and intramo-
lecular bond distances and angles are deposited in the Supporting
Information (CIF file format).
Kinetic Measurements. The reaction of copper(II) complexes
with H2O2 was performed in a 1 cm path length UV-vis cell that
was held in a Unisoku cryostat holder USP-203. After CH3CN or
CH3CH2CN solutions of copper(II) complexes (0.6 mM) in the cell
was kept at a desired temperature for several minutes, H2O2 (10
equiv) and then Et3N (2 eq) were injected into the cell though a
septa cap with use of a microsyringe. The reaction was monitored
by following an increase of the characteristic absorption band of
the Cu(II)-H2O2 adduct.
[CuII2(Ph2TPA)2(µ-OH)2](CF3SO3)2(4). CuII(CF3SO3)2 (109.0
mg, 0.30 mmol) and Et3N (1eq) were added to an MeOH solution
(2 mL) of ligand Ph2TPA (132.6 mg, 0.30 mmol). After stirring
the solution for 10 min at room temperature, insoluble material
was removed by filtration. Addition of ether (100 mL) to the
filtrate gave blue powder that was precipitated by standing the
mixture for several minutes. The supernatant was then removed
by decantation, and the remaining blue solid was washed with
ether three times and dried to give complex 4 in 71%. Single
crystals of 4 were obtained by vapor diffusion of ether into a
CH3CN solution of the complex. FT-IR (KBr) 3600 cm -1 (OH-),
1222, 1165, 1032, and 638 cm-1 (CF3SO3-); ESI-MS (pos.) m/z
) 523.4 [CuII(Ph2TPA)(OH)]+ and m/z ) 1193.5 [CuII (Ph2TPA)2-
2
(OH)2(CF3SO3)]+; Anal. Calcd for [CuII2(Ph2TPA)2(µ-OH)2]-
(CF3SO3)2 · H2O (C62H56Cu2F6N8O9S2): C, 54.66; H, 4.14; N,
8.22. Found: C, 54.62; H, 4.03; N, 8.19.
Results
Structural Characterization of Copper(II) Complexes.
First of all, the structures of the copper(II) complexes of TPA
and PhnTPA (Chart 1) have been examined. The crystal
structures of CuTPA and 3 have already been reported in
the literature,11,12 and those of 1 and 2 have newly been
determined in this study as shown in Figures 1 and 2. The
crystallographic data of 1 and 2 are listed in Table 1, and
X-ray Structure Determination. Each single crystal 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 KR radiation (λ )
0.71069 Å) to 2θmax of 55°. All the crystallographic calculations
were performed by using the Crystal Structure software package
12034 Inorganic Chemistry, Vol. 47, No. 24, 2008

H2O2-ReactiWity of Copper(II) Complexes
Table 1. Summary of the X-ray Crystallographic Data of Compound 1
and 2
Table 2. Selected Bond Lengths (Å) and Angles (deg) of Compounds
CuTPA, 1, 2, and 3a
compound
1
2
Compound CuTPA12
Cu(1)-N(1)
2.027(4)
2.007(4)
1.960(4)
117.95(17)
123.60(17)
82.65(17)
99.15(18)
177.92(19)
Cu(1)-N(2)
2.054(4)
2.045(4)
formula
C26H25N5CuCl2O8
669.96
C32H30N5CuCl2O8
747.07
monoclinic
P21/n (No. 14)
14.696(8)
11.748(6)
18.660(14)
90
99.98(2)
90
3172.9 (33)
4
1536.00
1.564
153
Cu(1)-N(3)
Cu(1)-N(4)
formula weight
crystal system
space group
a, Å
b, Å
c, Å
Cu(1)-N(5)
monoclinic
P21/c (No. 14)
10.585(6)
10.625(5)
24.121(19)
90
95.73(4)
90
2699.2 (30)
4
N(1)-Cu(1)-N(2)
N(1)-Cu(1)-N(4)
N(2)-Cu(1)-N(3)
N(2)-Cu(1)-N(5)
N(3)-Cu(1)-N(5)
N(1)-Cu(1)-N(3)
N(1)-Cu(1)-N(5)
N(2)-Cu(1)-N(4)
N(3)-Cu(1)-N(4)
N(4)-Cu(1)-N(5)
82.46(16)
97.57(17)
113.19(17)
81.97(17)
96.31(19)
R, deg
Compound 1
ꢀ, deg
γ, deg
Cu(1)-N(1)
1.962(2)
2.042(2)
1.986(2)
164.59(8)
100.53(8)
82.69(8)
94.26(9)
171.86(8)
Cu(1)-N(2)
Cu(1)-N(4)
1.989(2)
2.330(2)
V, Å3
Cu(1)-N(3)
Z
Cu(1)-N(5)
F(000)
1372.00
1.649
153
N(1)-Cu(1)-N(2)
N(1)-Cu(1)-N(4)
N(2)-Cu(1)-N(3)
N(2)-Cu(1)-N(5)
N(3)-Cu(1)-N(5)
N(1)-Cu(1)-N(3)
N(1)-Cu(1)-N(5)
N(2)-Cu(1)-N(4)
N(3)-Cu(1)-N(4)
N(4)-Cu(1)-N(5)
83.67(8)
98.28(9)
84.79(8)
82.57(7)
104.78(8)
Dcalcd, g/cm-3
T, K
crystal size, mm
µ(Mo KR), cm-1
2θmax, deg
no. of reflns measd
no. of reflns obsd
no. of variables
Ra
0.50 × 0.50 × 0.30
0.30 × 0.20 × 0.10
10.679
9.176
55.0
25191
5071([I > 2.00σ(I)])
404
0.0500
0.0632
0.995
55.0
28922
4744 ([I > 0.8σ(I)])
460
Compound 2
Cu(1)-N(1)
2.232(2)
2.000(2)
1.953(2)
108.27(9)
103.14(9)
80.18(10)
97.36(10)
175.89(9)
Cu(1)-N(2)
Cu(1)-N(4)
2.067(2)
2.010(2)
Cu(1)-N(3)
Cu(1)-N(5)
0.0343
0.0422
0.947
2
N(1)-Cu(1)-N(2)
N(1)-Cu(1)-N(4)
N(2)-Cu(1)-N(3)
N(2)-Cu(1)-N(5)
N(3)-Cu(1)-N(5)
N(1)-Cu(1)-N(3)
N(1)-Cu(1)-N(5)
N(2)-Cu(1)-N(4)
N(3)-Cu(1)-N(4)
N(4)-Cu(1)-N(5)
78.28(9)
b
Rw
105.68(10)
140.69(9)
83.74(10)
96.29(10)
GOF
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑wFo ]1/2
.
their selected bond lengths and angles are summarized in
Table 2 together with those of reported CuTPA12 and 311
for comparison. The copper center in 1 exhibits a slightly
distorted square pyramidal geometry (τ ) 0.12),19 where two
pyridine nitrogen atoms N(1) and N(2), tertiary amine
nitrogen atom N(3), and the acetonitrile nitrogen atom N(5)
occupy each corner of the equatorial plane and the pyridine
nitrogen atom N(4) of PhPy (6-phenylpyridin-2-yl) group
exists at the axial position. On the other hand, copper(II)
complex 2 carrying two PhPy groups exhibits an intermediate
structure between square pyramidal and trigonal bipyramidal
(τ ) 0.58) as shown in Figure 2. Since other copper(II)-
complexes CuTPA and 3 show trigonal bipyramidal structure
(τ ) 0.90 and 1.0, respectively),11,12 it is obvious that the
6-Ph substituent(s) largely affect the solid state structure of
the copper(II) complexes.
It should be noted that the average distance between Cu
and PhPy-nitrogen (2.159 Å) is significantly longer than the
average distance between Cu and Py-nitrogen (2.015 Å). This
can be attributed to the steric repulsion between the bound
CuII ion and the 6-Ph and/or that between the bound CH3CN
coligand and the 6-Ph.
The ESR parameters of the copper(II) complexes deter-
mined by the computer simulation are summarized in Table
3. The ESR spectra themselves are presented in Supporting
Information, Figures S1-S4. In spite of having the different
structures in the solid state [square pyramidal (SP) vs trigonal
bipyramidal (TBP), see above], all copper(II) complexes
exhibit relatively similar ESR parameters in frozen CH3CN
solution. This clearly indicates that they adopt a similar
structure in solution. Judging from the relatively small A1
values (90-130 G), they may have a trigonal bipyramidal
structure as in the case of CuTPA and 3 in crystal. This
Compound 311
Cu(1)-N(1)
2.108(5)
1.996(6)
1.935(6)
117.3(2)
118.6(2)
81.3(2)
Cu(1)-N(2)
2.124(7)
2.091(6)
Cu(1)-N(3)
Cu(1)-N(4)
Cu(1)-N(5)
(1)-Cu(1)-N(2)
N(1)-Cu(1)-N(4)
N(2)-Cu(1)-N(3)
N(2)-Cu(1)-N(5)
N(3)-Cu(1)-N(5)
N(1)-Cu(1)-N(3)
N(1)-Cu(1)-N(5)
N(2)-Cu(1)-N(4)
N(3)-Cu(1)-N(4)
N(4)-Cu(1)-N(5)
80.0(2)
99.1(2)
116.0(2)
80.1(2)
101.2(2)
98.2(2)
178.7(2)
a The date are taken from the literatures; ref 12 for CuTPA and ref 11
for 3.
Table 3. Redox Potential, UV-vis, and ESR Parameters of CuTPA, 1,
2, and 3
E1/2
∆Ep
λmax
complex
(V)a
(V)b (ε, M-1 cm-1
)
ESR parametersc
CuTPA -0.34 0.10
628 nm (85)
855 nm (300)
g1 ) 2.170 A1 ) 100 G
g2 ) 2.068 A2 ) 30 G
g3 ) 2.145 A3 ) 30 G
g1 ) 2.235 A1 ) 100 G
g2 ) 2.043 A2 ) 5 G
g3 ) 2.090 A3 ) 30 G
g1 ) 2.230 A1 ) 130 G
g2 ) 2.040 A2 ) 18 G
g3 ) 2.140 A3 ) 18 G
g1 ) 2.185 A1 ) 90 G
g2 ) 2.110 A2 ) 2 G
g3 ) 2.120 A3 ) 22 G
1
2
3
-0.28 0.08
-0.16 0.08
-0.04 0.08
625 nm (95)
820nm (180)
630 nm (100)
800 nm (300)
650 nm (65)
865 nm (380)
a In CH3CN containing 0.1 M tetrabutylammonium perchlorate (TBAP)
at 25 °C; working electrode Pt, counter electrode Pt, reference electrode
Ag/0.01 M AgNO3, sweep rate 50 mV s-1 b Peak separation. c In CH3CN
.
at 128-129 K, microwave frequency 9.40-9.41 GHz, modulation frequency
100 kHz, modulation amplitude 5 G, microwave power 0.32-0.33 mW.
notion is supported by the absorption spectral data presented
in Table 3. Namely, two distinctive d-d bands at ∼630 nm
and ∼850 nm of all complexes in CH3CN are indicative of
a large contribution of trigonal bipyramidal (TBP) geometry
in solution.20
(20) Mukherjee, R. In ComprehensiVe Coordination Chemistry II; Mc-
Cleverty, J. A.; Meyer, T. J., Eds.; Elsevier: Amsterdam, 2004; Vol.
6, pp 747-910, and references cited therein.
(19) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C.
J. Chem. Soc., Dalton Trans. 1984, 1349–1356.
Inorganic Chemistry, Vol. 47, No. 24, 2008 12035

Kunishita et al.
Redox Potential of Copper(II) Complexes. The redox
potential of the complex is an important indicator of the
electron-donor ability of the supporting ligand. The cyclic
voltammograms of the complexes were measured under the
same experimental conditions (Supporting Information,
Figures S5-S8). In all cases, a reversible CuII/CuI redox
couple was obtained, from which the redox potentials (E1/2)
of the copper(II) complexes were determined as the midpoint
of the oxidation and reduction peaks as summarized in Table
3. Apparently, the redox potential of the complexes largely
increases in the order of CuTPA < 1 < 2 < 3. This can be
attributed to the weaker electron-donor ability of the PhPy
group. Namely, the steric repulsion between the bulky 6-Ph
substituent and the bound copper(II) ion and/or the repulsion
between the 6-Ph substitutes and the CH3CN coligand
weakens the coordinative interaction between the metal ion
and PhPy-nitrogen. In addition, electron-withdrawing nature
of 6-Ph may also reduces the electron donor ability of PhPy.
Thus, increasing number (n) of PhPy resulted in the reduction
of the overall electron-donating nature of the TPA deriva-
tives. The reduction in the local dielectric character because
of the steric shielding of the metal center may also cause
the positive shift in E1/2 as discussed previously.11
Reaction of Copper(II) Complexes and H2O2. Masuda
and co-workers reported that the reaction of CuTPA and
H2O2 gave a mononuclear copper(II)-hydroperoxo complex
CuTPA-OOH in the presence of triethylamine (eq 1).12 The
hydroperoxo complex exhibited a ligand-to-metal charge
transfer (LMCT) band at 379 nm (ε ) 1700 M-1 cm-1)
together with d-d bands at 668 nm (170 M-1 cm-1) and 828
nm (200 M-1 cm-1),12 and the ESR parameters of g| ) 2.01,
g⊥ ) 2.19, A| ) 87 G, and A⊥ ) 99 G. These spectral data
indicate that CuTPA-OOH has a trigonal bipyramidal
structure as in the case of the starting material.12 Furthermore,
CuTPA-OOH showed a resonance Raman band at 847 cm-1
assignable to the 16O-16O stretching vibration, which shifted
to 792 cm-1 upon 18O-substitution.12
Figure 3. Spectral change for the reaction of 1 (0.6 mM) with H2O2 (6.0
mM) in the presence of NEt3 (1.2 mM) in CH3CN at -40 °C. Inset:
Resonance Raman spectrum of 1-OOH (6 mM) generated by the reaction
of 1 and H216O2 in CH3CN at -40 °C (λex ) 406.7 nm); s denotes the
solvent bands.
CuTPA-OOH, confirming that the generated species is also
a copper(II)-hydroperoxo complex 1-OOH (eq 2).
The ESR spectrum of 1-OOH shown in Supporting
Information, Figure S9 exhibits a larger A1 value (166 G) as
compared with that of the stating material 1 (100 G),
suggesting that the trigonal bipyramidal structure of 1 in
solution becomes a distorted square pyramidal structure in
1-OOH. This notion is supported by the UV-vis spectral
change shown in Figure 3, where the relatively intense d-d
band at 820 nm of 1, which is a characteristic d-d band for
copper(II) complexes with trigonal bipyramidal structure, is
diminished in 1-OOH. Binding of the stronger electron-
donating ligand like HOO- to copper(II) center shifts its
position from the axial of trigonal bipyramidal geometry to
the equatorial of square pyramidal structure as illustrated in
eq 2. Moreover, double integration of the ESR spectrum of
1-OOH demonstrated that 99% of the spin remained,
confirming the mononuclearity of 1-OOH.
The reaction of 2 and H2O2 was then examined in
propionitrile at -70 °C in the presence of Et3N. The spectral
change of the reaction is shown in Figure 4, where an intense
absorption band at 379 nm (ε ) 6200 M-1 cm-1) readily
appears.21 Notably, the spectrum is different from that of
the mononuclear copper(II) hydroperoxo complex 1-OOH
shown in Figure 3, and the generated intermediate was ESR
silent. The resonance Raman spectrum measured at -80 °C
with λex ) 406.7 nm exhibited a characteristic peak at 607
cm-1 that shifted to 577 cm-1 upon H218O2 substitution (inset
of Figure 4). All these spectral characteristics are very close
In this study, reactions of other copper(II) complexes and
H2O2 have been examined under similar experimental
conditions. Figure 3 shows the spectral change for the
reaction of complex 1 and H2O2 (10 equiv) in CH3CN at
-40 °C in the presence of 2 equiv of Et3N, where an
absorption band at 384 nm (ε ) 1550 M-1 cm-1) appears
together with a d-d band at 620 nm (170 M-1 cm-1). In this
case, the resonance Raman spectrum of the resulting solution
exhibited a peak at 840 cm-1 attributable to the 16O-16O
stretching vibration of a hydroperoxo species, although the
Raman band of the 18O-derivative was difficult to detect
because of its weak intensity (inset of Figure 3). The UV-vis
and the resonance Raman data are very close to those of
(21) The reaction of H2O2 and 2 in CH3CN at -40°C gave the same
spectrum as shown in Supporting Information, Figure S10.
12036 Inorganic Chemistry, Vol. 47, No. 24, 2008

H2O2-ReactiWity of Copper(II) Complexes
Table 4. Summary of the X-ray Crystallographic Data of Compound 4
compound
4
formula
C31H26N4CuF3O4S
671.17
formula weight
crystal system
space group
a, Å
b, Å
c, Å
triclinic
j
P1 (No. 2)
11.700.(8)
12.320(9)
12.710(10)
106.76(2)
107.09(2)
112.149(20)
1447.9(18)
2
R, deg
ꢀ, deg
γ, deg
V, Å3
Z
F(000)
688.0
1.539
153
Dcalcd, g/cm-3
T, K
Figure 4. Spectral change for the reaction of 2 (0.6 mM) and H2O2 (6.0
mM) in the presence of NEt3 (1.2 mM) in CH3CH2CN at -70 °C. Inset:
Resonance Raman spectra of 2-oxo (6 mM) generated by using H216O2
(solid line) and H218O2 (dashed line) with excitation at λex ) 406.7 nm in
CH3CH2CN at -80 °C; s denotes the solvent bands.
crystal size, mm
µ(Mo KR), cm-1
2qmax, deg
no. of reflns measd
no. of reflns obsd
no. of variables
Ra
0.30 × 0.30 × 0.20
8.906
55.0
14207
4252([I > 0.50s(I)])
423
0.0450
0.0497
b
Rw
GOF
0.974
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑wFo ]1/2
.
2
Table 5. Selected Bond Lengths (Å) and Angles (deg) of Compound 4
Compound 4
Cu(1)-O(1)
Cu(1)-N(1)
Cu(1)-N(3)
Cu(1)-Cu*(1)
1.929(4)
2.924(6)
2.049(5)
2.9983(10)
Cu(1)-O*(1)
Cu(1)-N(2)
Cu(1)-N(4)
1.932(3)
2.754(5)
1.996(4)
O(1)-Cu(1)-O*(1)
O(1)-Cu(1)-N(4)
O(1)*-Cu(1)-N(4)
78.13(17)
100.1(2)
176.76(19)
O(1)-Cu(1)-N(3)
O(1)-Cu(1)-N(3)
N(3)-Cu(1)-N(4)
173.97(15)
97.04(19)
84.6(2)
lengths and angles of 4 are presented in Table 4 and Table
5, respectively.
Complex 4 consists of a centrosymmetric Cu2(µ-OH)2 core
supported by two Ph2TPA ligands. Each copper ion has a
square planar geometry consisting of a N2O2 donor set with
the pyridyl nitrogen atom N(4), tert-amine nitrogen atom
N(3), and two bridging hydroxo oxygen atoms O(1) and
O(1)*, whereas the two pyridine donor groups having the
6-Ph substituent (PhPy) interact with the copper ion very
weakly from the axial position (Cu-N(1) and Cu-N(2) are
2.754 Å and 2.924 Å, respectively). Thus, the ligand Ph2TPA
seems to behave as a bidentate ligand in this particular
system. The overall structure of 4 is similar to that of
[Cu2(OH)2(Me2-TPA)2](ClO4)2 reported by Suzuki and co-
workers,23 but the average distances of Cu-Naxial (2.839 Å)
and Cu · · · Cu* (2.998 Å) bonds are substantially elongated
as compared with those of Suzuki’s complex [Cu-Naxial
(2.596 Å), and Cu · · · Cu* (2.936 Å)].23 The larger values of
Cu-Naxial and Cu · · ·Cu* bond distances in 4 relative to those
of [Cu2(OH)2(Me2-TPA)2]2+ reflect the larger steric bulkiness
of 6-Ph as compared with that of 6-Me.
Figure 5. ORTEP drawing of 4 showing 50% probability thermal ellipsoids.
The counteranion and the hydrogen atoms are omitted for clarity.
to those of the reported bis(µ-oxo)dicopper(III) complexes,22
strongly suggesting the formation of a similar complex 2-oxo
as indicated in (eq 3).
At first glance, it seems to be difficult to assess such a
doubly bridged dinuclear copper complex using the sterically
demanding ligand Ph2TPA, but such a possibility has been
realized by the formation of bis(µ-hydroxo)dicopper(II)
complex 4 (Figure 5) in the treatment of 2 with Et3N in
MeOH. The crystallographic data and the selected bond
Interestingly, complex 3 behaved quite differently in the
reaction with H2O2. Namely, copper(II) complex 3 was
reduced to the corresponding copper(I) complex 3red. The
(22) (a) Que, L., Jr.; Tolman, W. B. Angew. Chem., Int. Ed. 2002, 41,
1114–1137. (b) Henson, M. J.; Mukherjee, P.; Root, D. E.; Stack,
T. D. P.; Solomon, E. I. J. Am. Chem. Soc. 1999, 121, 10332–10345.
(c) Holland, P, L.; Cramer, C, J.; Wilkinson, E, C.; Mahapatra, S.;
Rogers, K, R.; Itoh, S.; Taki, M.; Fukuzumi, S.; Que, L., Jr.; Tolman,
W. B. J. Am. Chem. Soc. 2000, 122, 792–802.
(23) Hayashi, H.; Fujinami, S.; Nagatomo, S.; Ogo, S.; Suzuki, M.; Uehara,
A.; Watanabe, Y.; Kitagawa, T. J. Am. Chem. Soc. 2000, 122, 2124–
2125.
Inorganic Chemistry, Vol. 47, No. 24, 2008 12037

Kunishita et al.
The Cu(II)-H2O2 reactivity is drastically altered by the
supporting ligands. CuTPA and 1 supported by TPA itself
and Ph1TPA, respectively, afforded the mononuclear cop-
per(II)-hydroperoxo complex CuTPA-OOH and 1-OOH. It
should be noted here that no ligand modification (neither
N-dealkylation nor aromatic hydroxylation) took place from
these copper(II)-hydroperoxo species, even though the reac-
tion mixture was warmed to room temperature before the
workup treatment. This is in contrast to Karlin’s system with
a Ph1TPA having p-tert-butyl substituent on 6-Ph, where a
aromatic ligand hydroxylation reaction was observed
(20%).14 This can be attributed to the existence of the
electron-donating tert-butyl substituent on 6-Ph in Karlin’s
system, which may increase the susceptibility of the aromatic
substituent toward the electrophilic attack by the Cu-OOH
moiety.
Incorporation of an extra 6-Ph group to Ph1TPA to give
Ph2TPA resulted in totally different reactivity of the sup-
ported copper(II) complex toward H2O2. In this case, bis(µ-
oxo)dicopper(III) complex 2-oxo was generated. Replace-
ment of two pyridine (Py) groups of TPA by the bulkier
PhPy groups may enforce the TPA derivative to act as a
bidentate ligand using the non-substituted Py group and the
tert-amine nitrogen as the donor groups. Such a bidentate
2-pyridylmethylamine (PyCH2NR2) type ligand has been
well-demonstrated to stabilize the bis(µ-oxo)dicopper(III)
complex.25 This may be the reason why Ph2TPA affords
2-oxo. Suzuki and his co-workers reported that the reaction
of [CuI(Me2TPA)]+ and O2 also gave a bis(µ-oxo)dicop-
per(III) complex, in which the copper(III) center exhibited
a four-coordinate square planar geometry with a N2O2 donor
set (consisting of Py nitrogen, tert-amine nitrogen, and two
oxo groups) and the remaining MePy groups interact with
copper(III) ion very weakly from the axial positions.23
In the case of 2-oxo, N-dealkylation reaction took place
to give 6-phenyl-2-pyridinecarboxaldehyde in 36%. How-
ever, we did not see any ligand modification on the 6-Ph
aromatic group. This is in contrast to Suzuki’s Me2TPA
system, where they observed peroxygenation of the 6-Me
substituent in the reaction of [CuII(Me2TPA)]2+ complex and
H2O2.26
Figure 6. Spectral change for the reaction of 3 (0.6 mM) with H2O2 (6.0
mM) in the presence of Et3N (1.2 mM) in CH3CN at -40 °C.
d-d bands at 650 and 865 nm of the copper(II) complex 3
completely disappeared (see Figure 6) and the resulting
solution became ESR silent. The ESI-MS data and the
elemental analysis on the isolated product confirmed the for-
mation of 3red (Supporting Information, Figure S11). The
reduction of copper(II) to copper(I) in 3 may proceed via
CuII-O bond homolysis of an initially formed copper(II)-
hydroperoxo intermediate 3-OOH as previously reported in
a related system.24
Discussion
In this study, substituent effects of 6-Ph on the structure,
physicochemical properties, and H2O2-reactivity of the
copper(II) complexes supported by PhnTPA ligands (see,
Chart 1) have systematically been examined. The copper(II)
centers of CuTPA and 3 (Ph3TPA-complex) exhibit trigonal
bipyramidal structure in the crystal, whereas complex 1
supported by Ph1TPA shows a slightly distorted square
pyramidal geometry (Figure 1). Complex 2 supported by
Ph2TPA, on the other hand, takes an intermediate structure
between trigonal bipyramidal and square pyramidal in the
solid state. However, the UV-vis and ESR data suggest that
all copper(II) complexes exhibit a similar trigonal bipyra-
midal geometry in solution. Thus, the difference in the solid-
state structures of the complexes can be attributed to the
crystal packing force.
Copper(II) complexes CuTPA, 1, 2, and 3 have redox
potential at -0.34, -0.28, -0.16, and -0.04 V vs Ag/
AgNO3, respectively. Thus, incorporation of each 6-Ph group
causes ∼0.1 V increase of E1/2. The positive shift of E1/2
has been attributed to the steric repulsion between the bulky
6-Ph substituent and the cupric ion and/or the repulsion
between the 6-Ph and the CH3CN coligand, which cause
reduction of the coordinative interaction between the pyridine
donor groups and the copper(II) ion. In fact, the average
distance between Cu and PhPy-nitrogen (2.159 Å) is sub-
stantially elongated as compared to the average distance
between Cu and Py-nitrogen (2.015 Å) (vide ante). The
electron-withdrawing effect by 6-Ph as well as a reduction
of the local dielectric character because of the steric shielding
in the metal center may also contribute to the positive shift
of E1/2.
Because of the high E1/2 of the Ph3TPA system, on the
other hand, the copper(II) complex 3 was easily reduced to
the corresponding copper(I) complex by the reaction with
H2O2. The reaction may proceed via Cu-O bond homolysis
of an initially formed CuII-OOH intermediate.
Although the factors that control the H2O2-reactivity of
the supported copper(II) complexes have yet to be clarified
completely, the results reported herein will provide important
insights into the aromatic substituent effects on the redox
chemistry of the TPA-complexes.
(24) Osako, T.; Nagatomo, S.; Tachi, Y.; Kitagawa, T.; Itoh, S. Angew.
Chem., Int. Ed. 2002, 41, 4325–4328.
(25) (a) Holland, P. L.; Rodgers, K. R.; Tolman, W. B. Angew. Chem.,
Int. Ed. 1999, 38, 1139–1142. (b) Shimokawa, C.; Teraoka, J.; Tachi,
Y.; Itoh, S. J. Inorg. Biochem. 2006, 100, 1118–1127.
(26) Mizuno, M.; Honda, K.; Cho, J.; Furutachi, H.; Tosha, T.; Matsumoto,
T.; Fujinami, S.; Kitagawa, T.; Suzuki, M. Angew. Chem., Int. Ed.
2006, 45, 6911–6914.
12038 Inorganic Chemistry, Vol. 47, No. 24, 2008

H2O2-ReactiWity of Copper(II) Complexes
Supporting Information Available: The ESR spectra (Figures
S1-S4) and the cyclic voltammograms (Figures S5-S8) of the
copper(II) starting materials, the ESR spectrum of 1-OOH (Figure
S9), UV-vis spectrum of 2-oxo generated by using H2O2 in CH3CN
(Figure S10), and ESI-MS of 3red generated by the reaction of 3
and H2O2 in CH3CN. (Figure S10). This material is available free
of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was financially supported
in part by Grants-in-Aid for Scientific Research on Priority
Area (Nos. 19020058, 20036044, and 20037057 for S.I.) and
Global Center of Excellence (GCOE) Program (“Picobiology:
Life Science at Atomic Level” to T.O.) from MEXT, Japan
and by Grants-in-Aid for Scientific Research (No. 20350082
for S.I.) from JSPS, Japan. The authors also thank Asahi
Glass Foundation for the financial support and JSPS Research
Fellowship (for A.K.).
IC801568G
Inorganic Chemistry, Vol. 47, No. 24, 2008 12039