Structure and dioxygen-reactivity of copper(I) complexes supported by bis(6-methylpyridin-2-ylmethyl)amine tridentate ligands

Article abstract of DOI:10.1039/b500202h

The structure and dioxygen-reactivity of copper(I) complexes 2^R supported by N,N-bis(6-methylpyridin-2-ylmethyl)-amine tridentate ligands L2^R [R (N-alkyl substituent) = -CH₂Ph (Bn), -CH ₂CH₂Ph (Phe) a

Full text of DOI:10.1039/b500202h

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F U L L P A P E R
Structure and dioxygen-reactivity of copper(I) complexes supported
by bis(6-methylpyridin-2-ylmethyl)amine tridentate ligands†‡
Takao Osako,a Shohei Terada,b Takehiko Tosha,c Shigenori Nagatomo,c Hideki Furutachi,b
Shuhei Fujinami,b Teizo Kitagawa,c Masatatsu Suzukib and Shinobu Itoh*a
a Department of Chemistry, Graduate School of Science, Osaka City University,
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka, 558-8585, Japan.
E-mail: shinobu@sci.osaka-cu.ac.jp
b Department of Chemistry, Faculty of Science, Kanazawa University, Kakuma-machi,
Kanazawa, Ishikawa, 920-1192, Japan
c Okazaki Institute for Integrative Bioscience, National Institutes of Natural Sciences,
Myodaiji, Okazaki, 444-8787, Japan
Received 6th January 2005, Accepted 15th March 2005
First published as an Advance Article on the web 23rd September 2005
The structure and dioxygen-reactivity of copper(I) complexes 2R supported by N,N-bis(6-methylpyridin-2-ylmethyl)-
amine tridentate ligands L2R [R (N-alkyl substituent) = –CH2Ph (Bn), –CH2CH2Ph (Phe) and –CH2CHPh2 (PhePh)]
have been examined and compared with those of copper(I) complex 1Phe of N,N-bis[2-(pyridin-2-yl)ethyl]amine
tridentate ligand L1Phe and copper(I) complex 3Phe of N,N-bis(pyridin-2-ylmethyl)amine tridentate ligand L3Phe
.
Copper(I) complexes 2Phe and 2PhePh exhibited a distorted trigonal pyramidal structure involving a d–p interaction with
1
an g -binding mode between the metal ion and one of the ortho-carbon atoms of the phenyl group of the N-alkyl
substituent [–CH2CH2Ph (Phe) and –CH2CHPh2 (PhePh)]. The strength of the d–p interaction in 2Phe and 2PhePh was
2
1
weaker than that of the d–p interaction with an g -binding mode in 1Phe but stronger than that of the g d–p interaction
in 3Phe. Existence of a weak d–p interaction in 2Bn in solution was also explored, but its binding mode was not clear.
Redox potentials of the copper(I) complexes (E1/2) were also affected by the supporting ligand; the order of E1/2 was
1Phe > 2R > 3Phe. Thus, the order of electron-donor ability of the ligand is L1Phe < L2R < L3Phe. This was reflected in
the copper(I)–dioxygen reactivity, where the reaction rate of copper(I) complex toward O2 dramatically increased in
the order of 1R < 2R < 3R. The structure of the resulting Cu2/O2 intermediate was also altered by the supporting
2
2
ligand. Namely, oxygenation of copper(I) complex 2R at a low temperature gave a (l-g :g -peroxo)dicopper(II)
complex as in the case of 1Phe, but its O–O bond was relatively weakened as compared to the peroxo complex derived
from 1Phe, and a small amount of a bis(l-oxo)dicopper(III) complex co-existed. These results can be attributed to the
higher electron-donor ability of L2R as compared to that of L1Phe. On the other hand, the fact that 3Phe mainly
afforded a bis(l-oxo)dicopper(III) complex suggests that the electron-donor ability of L2R is not high enough to
support the higher oxidation state of copper(III) of the bis(l-oxo) complex.
Introduction
Bis(pyridin-2-ylalkyl)amine tridentate ligands have been playing
very important roles in copper/dioxygen (Cu/O2) chemistry.1–5
It has been well established that copper(I) complexes of
bis[2-(pyridin-2-yl)ethyl]amine tridentate ligands (L1R, Chart 1)
2
2
predominantly afford (l-g :g -peroxo)dicopper(II) complexes A
(Chart 2) in the reaction with O2 at a low temperature.1–4 The
peroxo complexes A supported by L1R-type ligands have been
extensively studied as structural and functional models of oxy-
hemocyanin and oxy-tyrosinase on their ability of reversible
dioxygen binding and aromatic hydroxylation reaction.6–10
Recently, Itoh and co-workers have also demonstrated that
adoption of the bis(pyridin-2-ylmethyl)amine tridentate ligand
L3R [R = –CH2CH2Ph (Phe), Chart 1] instead of L1R induces
O–O bond cleavage of the peroxo complex A to give a bis(l-
oxo)dicopper(III) complex B in the reaction of the copper(I)
complex and O2 under similar reaction conditions.11 The shorter
alkyl linker chain (methylene) in L3R affords a smaller five-
membered chelate ring with increasing electron donor ability
Chart 1
Chart 2
of pyridine,12 thus enhancing the O–O bond cleavage and
stabilizing the higher oxidation state of copper(III) in B. The
electron-donor ability of pyridine can be also tuned by the C-4
substituents on the pyridine nucleus of L1R-type ligand, affecting
the equilibrium position between A and B in solution, where the
electron-donating substituent such as NMe2 weakens the O–O
bond and increases the ratio of B in the equilibrium.13
† Based on the presentation given at Dalton Discussion No. 8, 7–9th
September 2005, University of Nottingham, UK.
In the case of TPA tetradentate ligand system (TPA =
tris(pyridin-2-ylmethyl)amine), on the other hand, the pres-
ence of a methyl substituent at the 6-position of pyridine
nucleus has been well demonstrated to induce large effects not
‡ Electronic supplementary information (ESI) available: Spectral change
for the titration of 2Phe by CH3CN (Fig. S1) and Arrhenius plots
for the oxygenation reaction of 2Phe and 2PhePh (Fig. S2). See DOI:
10.1039/b500202h
3 5 1 4
D a l t o n T r a n s . , 2 0 0 5 , 3 5 1 4 – 3 5 2 1
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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only on Cu/O2 chemistry but also on iron/dioxygen (Fe/O2)
chemistry.2,14–16 The 6-methyl group has been shown to cause
a decrease of electron-donor ability of pyridine due to steric
repulsion between the substituent and the bound metal ion.17
We have recently found a similar effect of 6-methylpyridine in
the L1R-type tridentate ligand system, where the reactivity of
copper(I) complex [CuI(L1Phe)(MeCN)]+ (Phe = –CH2CH2Ph)
power used was 10 mW at the sample point. All measurements
were carried out with a spinning cell (1000 rpm) at −80 to
−100 ◦C. Raman shifts were calibrated with indene and acetone,
and the accuracy of the peak positions of the Raman lines
was 1 cm−1.
X-Ray structure determination
toward O2 is significantly diminished when a methyl group is
Single crystals of copper(I) complexes 2Phe and 2PhePh for X-ray
structural analysis were obtained by vapor diffusion of ether
into a CH2Cl2 solution of the complex. The single crystal was
mounted on a CryoLoop (Hamptom Research Co.). Data of
X-ray diffraction were collected by a Rigaku RAXIS-RAPID
imaging plate two-dimensional area detector using graphite-
18
introduced into the 6-position of the pyridine nucleus of L1Phe
.
In this case, the weaker donor ability of the 6-methylpyridine
induces stronger binding of MeCN to copper(I), thus prohibiting
the reaction of copper(I) toward O2.
As our continuing efforts to understand the factors that
control the copper(I)–dioxygen reactivity in the pyridylalky-
lamine tridentate ligand system, we herein investigated the
structure and reactivity of the copper(I) complexes of L2R,
bis(6-methylpyridin-2-ylmethyl)amine tridentate ligands [R = –
CH2Ph (Bn), –CH2CH2Ph (Phe) and –CH2CHPh2 (PhePh),
Chart 1]. The results demonstrated that not only the structure
of the copper(I) complex but also strength of the O–O bond of
side-on peroxo complex is significantly affected by the 6-methyl
substituent.
˚
monochromated Mo-Ka radiation (k = 0.71070 A) to 2hmax of
55.0◦. All the crystallographic calculations were performed by
using Crystal Structure software package of the Rigaku Corpo-
ration and Molecular Structure Corporation [Crystal Structure:
Crystal Structure Analysis Package version 2.0, Rigaku Corp.
and Molecular Structure Corp. (2001)]. The crystal structures
were solved by direct methods and refined by full-matrix
least squares using SIR-92 or SHELX-97. All non-hydrogen
atoms and hydrogen atoms were refined anisotropically and
isotropically, respectively.
Data collection of copper(I) complexes 2Bn·MeCN and 2Bn·CO
were carried out on a Rigaku R-axis IV imaging plate area
detector with graphite-monochromated Mo-Ka radiation (k =
Experimental
General
˚
All chemicals used in this study except the ligands and the com-
plexes were commercial products of the highest available purity
and were further purified by the standard methods, if necessary.19
FT-IR spectra were recorded with a Shimadzu FTIR-8200PC or
a Horiba FT-200 spectrophotometer. UV-vis spectra were mea-
sured using a Hewlett Packard HP8453 diode array spectropho-
tometer equipped with a Unisoku thermostated cell holder
designed for low-temperature measurements (USP-203) or a
Shimadzu diode array spectrometer Multispec-1500 equipped
with the same cell holder. Mass spectra were recorded with a
JEOL JMS–700T Tandem MS station or a PE SCIEX API
150EX (for ESI-MS). ESI-TOF/MS spectra were measured with
a Micromass LCT spectrometer. NMR spectra were recorded on
a Bruker Avance 600 spectrometer or a JEOL JNM-LM400. 1H
NMR spectra were referenced to the residual proton resonance
of the solvent and 13C NMR spectra to the solvent resonance
(CD2Cl2: 1H, d 5.32, 13C, d 53.8). Complete peak assignments in
0.71070 A). A single crystal was mounted on the tip of a glass
rod. The structures were solved by direct method (SHELX-86
for 2Bn·MeCN and SIR92 for 2Bn·CO) and expanded using a
Fourier technique. The structures were refined by a full-matrix
least-squares method by using the teXsan crystallographic
software package (Molecular Structure Corporation). Non-
hydrogen atoms were refined with anisotropic displacement
parameters. Hydrogen atoms were positioned at calculated
˚
positions (0.95 A). They were included, but not refined, in the
final least-squares cycles.
A summary of the fundamental crystal data and experimental
parameters for all the structure determinations are given in
Table 1 and 2.
CCDC reference numbers 259907–259910.
See http://www.rsc.org/suppdata/dt/b5/b500202h/ for cry-
stallographic data in CIF or other electronic format.
1
the H and 13C NMR spectra of ligands L2Phe and L2PhePh and
Kinetic measurements
their copper(I) complexes have been accomplished by employing
2D NMR techniques (COSY, NOESY, HMQC and HMBC).
Reaction of the copper(I) complex and O2 was carried out in
a 1 cm path length UV-vis cell that was held in a Unisoku
thermostated cell holder USP-203 (a desired temperature can
be fixed within 0.5 ◦C). After the deaerated solution of the
copper(I) complex (2.0 × 10−4 M) in the cell was kept at a
desired temperature for several minutes, dry dioxygen gas was
continuously supplied by gentle bubbling from a thin needle.
Formation of the dicopper–dioxygen complex was monitored
by following an increase in the absorption at 355 nm. The
reaction obeyed second-order kinetics and the second-order rate
constants (kobs) were obtained as the slopes of linear lines of the
second-order plots, (A − A0)/{(A∞ − A)[Cu]0} vs. time, where
A0 and A∞ are the initial and final absorption at 355 nm and
[Cu]0 is the initial concentration of the copper(I) complex.
Electrochemical measurements
The cyclic voltammetry (CV) measurements were performed on
an ALS–630A electrochemical analyzer in anhydrous CH2Cl2
containing 0.1 M NBu4ClO4 as a supporting electrolyte. The
Pt working electrode was polished with a polishing alumina
suspension and rinsed with CH2Cl2 before use. The counter-
electrode was a Pt wire. A silver pseudo-reference electrode was
used, and the potentials were determined using the ferrocene–
ferrocenium (Fc/Fc+) couple as a reference. All electrochemical
measurements were carried out at 25 ◦C under an atmospheric
pressure of Ar in a glove box (Miwa Co. Ltd.).
Visible resonance Raman measurements
Syntheses
The 514.5 nm line of an Ar+ laser (Model GLG3200, NEC) was
used as the exciting source. Visible resonance Raman scattering
was detected with a liquid nitrogen cooled CCD detector (Model
LN/CCD-1340 × 400PB, Princeton Instruments) attached to
a 1 m single polychromator (Model MC-100DG, Ritsu Oyo
Kogaku). The slit width and slit height were set to be 150 lm
and 20 mm, respectively. The spectral slit width is 5.7 cm−1.
The wavenumber per single channel is 0.69 cm−1. The laser
N,N-Bis(6-methylpyridin-2-ylmethyl)-2-phenylethylamine
(L2Phe). Acetic acid (1.20 g, 20 mmol) was added to a
methanol solution (200 mL) containing 2-phenylethylamine
(1.21 g, 10 mmol) and 6-methyl-2-pyridinecarbaldehyde (2.42 g,
20 mmol), and the solution was stirred for 1 h at room
temperature. NaBH3CN (1.26 g, 20 mmol) was then added
slowly to the solution, and the mixture was stirred for three days
at room temperature. The reaction was quenched (pH = 1) by
D a l t o n T r a n s . , 2 0 0 5 , 3 5 1 4 – 3 5 2 1
3 5 1 5

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Table 1 Summary of the X-ray crystallographic data for complexes 2Phe, 2PhePh, 2Bn·CH3CN and 2Bn·CO
2Phe
2PhePh
2Bn·CH3CN
2Bn·CO
Empirical formula
Formula weight
Crystal system
C22H25N3CuClO4
494.46
C28H29N3CuClO4
570.55
Triclinic
C23H26N4CuPF6
567.00
Monoclinic
P21/c (14)
14.380(2)
7.196(2)
C22H23N3O5CuCl
508.43
Monoclinic
P21/n (14)
11.7088(3)
14.6661(5)
12.4843(4)
Triclinic
¯
Space group (no.)
P1 (1)
P1 (2)
˚
a/A
7.715(4)
8.887(6)
9.361(6)
83.77(3)
88.29(2)
75.93(3)
618.9(6)
1
11.159(2)
17.147(2)
12.387(2)
88.88(1)
109.24(1)
91.35(1)
2237.0(6)
4
˚
b/A
˚
c/A
25.291(4)
a/◦
b/◦
c /◦
V/A
Z
93.147(1)
99.58(1)
3
˚
2140.6(1)
4
1024.00
1.534
2580.6(9)
4
1160.00
1.459
F(000)
296.00
1.531
1048.00
1.510
Dc/g cm−3
T/◦C
−115
−115
22
−120
Crystal size/mm
l(Mo-Ka)/cm−1
Diffractometer
0.20 × 0.20 × 0.10
11.80
0.40 × 0.10 × 0.10
10.32
0.30 × 0.25 × 0.15
0.40 × 0.25 × 0.25
11.35
9.70
Rigaku
Rigaku
Rigaku
R-axis IV
Mo-Ka (0.71070)
51.4
Rigaku
R-axis IV
Mo-Ka (0.71070)
51.6
RAXIS-RAPID
Mo-Ka (0.71069)
54.9
RAXIS-RAPID
Mo-Ka (0.71075)
55.0
˚
Radiation (k/A)
◦
2hmax
/
No. reflns. measd.
No. reflns. obsd.
No. variables
Ra,c
20832
3791 [I > 3.0r(I)]
305
0.044
0.072
1.48
5544
2636 [I > 0.1r(I)]
364
0.058
0.063
1.098
4365
3616 [I > 3.0r(I)]
317
0.087
0.132
1.91
7193
6582 [I > 3.0r(I)]
577
0.044
0.073
1.39
b,d
Rw
GOF
ꢀ
ꢀ
ꢀ
ꢀ
2
2
2
a R = [|Fo| − |Fc|]/ |Fo| (I ≥ 3.0r(I) for 2Phe and I ≥ 0.1r(I) for 2PhePh). b Rw = { w(|Fo| − |Fc|)2/ wFo
}
1/2; w = 1/[0.001|Fo| + 3.0r|Fo| +
ꢀ
ꢀ
ꢀ
ꢀ
0.10] for 2Phe and 2PhePh
.
c R = [|Fo| − |Fc|]/ |Fo| (I ≥ 2.0r(I) for 2Bn·MeCN and I ≥ 3.0r(I) for 2Bn·CO). d Rw = [ w(|Fo| − |Fc|)2/ w|Fo| ]1/2
;
2
2
w = 1/[r2(Fo) + p2|Fo| /4] (p = 0.081 for 2Bn·MeCN and p = 0.098 for 2Bn·CO).
adding conc. HCl and the solvent was removed by evaporation.
To the resulting material was added 15% NaOH aqueous
solution (100 mL), and the organic materials were extracted
with CHCl3 (50 mL × 3). After drying over anhydrous K2CO3,
evaporation of the solvent gave a brown residue, from which
ligand L2Phe was isolated as a yellow oily material by SiO2
column chromatography. Yield: 2.9 g (88%). 1H NMR (600 Hz,
CD2Cl2): d 2.49 (6 H, s, –CH3), 2.77 (2 H, dd, J = 8.0 and
above for the synthesis of L2Phe using benzylamine instead of
2-phenylethylamine. Yield: 42%. 1H NMR (400 MHz, CDCl3):
d 2.51 (6 H, s, –CH3), 3.68 (2 H, s, –CH2Ph), 3.78 (4 H, s,
–CH2Py), 6.99 (2 H, d, J = 7.5 Hz, HPy-5), 7.19–7.33 (3 H, m,
HPh), 7.42 (2 H, d, J = 7.5 Hz, HPy-3), 7.45 (2 H, d, J = 8.1 Hz,
HPh), 7.55 (2 H, t, J = 7.5 Hz, HPy-4). Anal. Calc. for C21H23N3:
C, 79.46; H, 7.30; N, 13.24%. Found: C, 79.49; H, 7.37; N,
13.13%. FAB-MS (+): m/z 318 (L + H). FT-IR/cm−1 (KBr
=
=
6.9 Hz, –NCH2CH2Ph), 2.85 (2 H, dd, J = 8.0 and 6.9 Hz,
–NCH2CH2Ph), 3.81 (4 H, s, –NCH2Py), 6.99 (2 H, d, J =
7.6 Hz, HPy-5), 7.12 (2 H, d, J = 7.5 Hz, HPh-2 and HPh-6), 7.17 (1
H, t, J = 7.5 Hz, HPh-4), 7.20 (2 H, d, J = 7.6 Hz, HPy-3), 7.24
(2 H, t, J = 7.5 Hz, HPh-3 and HPh-5), 7.48 (2 H, t, J = 7.6 Hz,
HPy-4). 13C NMR (600 Hz, CD2Cl2): d 24.53 (–CH3), 33.86
(–NCH2CH2Ph), 56.45 (–NCH2CH2Ph), 60.76 (–NCH2Py),
119.87 (CPy-3), 121.44 (CPy-5), 126.15 (CPh-4), 128.51 (CPh-3 and
CPh-5), 129.28 (CPh-2 and CPh-6), 136.75 (CPy-4), 141.22 (CPh-1),
157.86 (CPy-6), 159.74 ppm (CPy-2). FAB-MS (+); m/z 332.19
(L + H).
disk): 1591 (C C, aromatic), 1578 (C C, aromatic).
CAUTION! The perchlorate salts employed in this study are all
potentially explosive and should be handled with care.
[CuI(L2Phe)]ClO4 (2Phe). Ligand L2Phe (99.4 mg, 0.3 mmol)
was treated with [CuI(CH3CN)4]ClO4 (96.1 mg, 0.3 mmol) in
CH2Cl2 (5 mL) under Ar atmosphere (in a glove box). After
stirring for 30 min at room temperature, insoluble materials
were removed by filtration. Addition of ether (100 mL) to the
filtrate gave a white powder that was precipitated by standing the
mixture for several minutes. The supernatant was then removed
by decantation, and the remained pale brown solid was washed
with ether three times, and dried. Yield: 114 mg (77%). All
procedures were done in a glove box (DBO-1KP, Miwa Co. Ltd.)
([O2] < 0.1 ppm). 1H NMR (600 MHz, CD2Cl2): d 2.56 (6 H, s, –
CH3), 2.83 (2 H, t, J = 6.2 Hz, –NCH2CH2Ph), 3.13 (2 H, d, J =
6.2 Hz, –NCH2CH2Ph), 3.88 (2 H, d, J = 16.0 Hz, –NCHHPy),
4.20 (2 H, d, J = 16.0 Hz, –NCHHPy), 6.95 (2 H, d, J = 7.3 Hz,
HPh-2 and HPh-6), 7.24 (2 H, d, J = 7.7 Hz, HPy-3), 7.28 (2 H, d,
J = 7.7 Hz, HPy-5), 7.33 (2 H, t, J = 7.3 Hz, HPh-3 and HPh-5), 7.43
N,N-Bis(6-methylpyridin-2-ylmethyl)-2,2-diphenylethylamine
(L2PhePh). This ligand was prepared by the same procedure
described above for the synthesis of L2Phe using 2,2-
diphenylethylamine instead of 2-phenylethylamine. Yield:
37%. 1H NMR (600 Hz, CD2Cl2): d 2.50 (6 H, s, –CH3), 3.18 (2
H, d, J = 7.8 Hz, –NCH2CHPh2), 3.80 (4 H, s, –NCH2Py), 4.36
(1 H, t, J = 7.8 Hz, –NCH2CHPh2), 6.89 (2 H, d, J = 7.6 Hz,
HPy-3), 6.99 (2 H, d, J = 7.6 Hz, HPy-5), 7.12 (4 H, d, J = 7.2 Hz,
HPh-2 and HPh-6), 7.19 (2 H, t, J = 7.2 Hz, HPh-4), 7.25 (4 H, t, J =
7.6 Hz, HPh-3 and HPh-5), 7.40 (2 H, t, J = 7.6 Hz, HPy-4). 13C NMR
(600 Hz, CD2Cl2): d 24.54 (–CH3), 49.84 (–NCH2CHPh2),
59.84 (–NCH2CHPh2), 61.06 (–NCH2Py), 120.26 (CPy-3), 121.46
(CPy-5), 126.54 (CPh-4), 128.58 (CPh-3 and CPh-5), 128.75 (CPh-2 and
CPh-6), 136.66 (CPy-4), 144.16 (CPh-1), 157.71 (CPy-6), 159.37 ppm
(CPy-2). FAB-MS (+): m/z 408.2 (L + H).
(1 H, t, J = 7.3 Hz, HPh-4), 7.75 (2 H, t, J = 7.7 Hz, HPy-4). 13
C
NMR (600 Hz, CD2Cl2): d 27.36 (–CH3), 33.94 (–NCH2CH2Ph),
56.27 (–NCH2CH2Ph), 59.70 (–NCH2Py), 121.80 (CPy-3), 122.61
(CPh-2 and CPh-6), 124.59 (CPy-5), 127.80 (CPh-3 and CPh-5), 128.29
(CPh-4), 138.12 (CPh-1), 139.27 (CPy-4), 157.26 (CPy-2), 157.94 ppm
(CPy-6). FT-IR/cm−1 (KBr disk): 1109, 1090 and 625 (ClO4−).
ESI-MS (+): m/z 394.4 (M+). Anal. Calc. for C22H26O4.5N3CuCl:
C, 52.48; H, 5.21; N, 8.35. Found: C, 52.85; H, 5.05; N, 8.46%.
N,N-Bis(6-methylpyridin-2-ylmethyl)benzylamine
(L2Bn ).
This ligand was prepared by the same procedure described
3 5 1 6
D a l t o n T r a n s . , 2 0 0 5 , 3 5 1 4 – 3 5 2 1

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◦
Phe
˚
Anal. Calc. for C28H30O4.5N3CuCl: C, 58.03; H, 5.22; N, 7.25.
Found: C, 58.30; H, 5.09; N, 7.46%.
Table 2 Selected bond lengths (A) and angles ( ) for complexes 2
,
2
PhePh, 2Bn·CH3CN and 2Bn·CO
[CuI(L2Bn)(CH3CN)]PF6 (2Bn·MeCN). Treatment of ligand
L2Bn (0.314 g, 0.99 mmol) and [CuI(CH3CN)4](ClO4) (0.327 g,
1.00 mmol) in ethanol (20 mL) under anaerobic conditions (in
a Schlenk flask) gave a yellow solution, to which an aqueous
solution (10 mL) of NH4PF6 (0.489 g, 3.00 mmol) was added.
The precipitated pale yellow materials were dissolved into the
solution again by heating, and the solution was kept standing at
room temperature to give yellow crystals. Yield: 0.413 g (72.8%).
1H NMR (400 MHz, acetone-d6): d 2.40 (3 H, s, CH3CN), 2.81
(6 H, s, –CH3), 3.81 (2 H, d (br), J = 14.9 Hz, –NCHHPy), 4.03
(2 H, s, –CH2Ph), 4.14 (2 H, d (br), J = 14.9 Hz, –NCHHPy),
7.27–7.36 (5 H, m, HPh and HPy-3), 7.40 (2 H, d, J = 7.8 Hz,
HPh), 7.48 (2 H, m, HPy-5), 7.81 (2H, t, J = 7.7 Hz, HPy-4). FT-
Complex 2Phe
Cu(1)–N(1)
Cu(1)–N(3)
Cu(1)–C(18)
C(17)–C(18)
C(19)–C(20)
C(21)–C(22)
2.202(3)
2.010(4)
2.148(4)
1.416(7)
1.397(7)
1.379(7)
Cu(1)–N(2)
Cu(1)–C(17)
Cu(1)–C(19)
C(18)–C(19)
C(20)–C(21)
C(17)–C(22)
2.058(3)
2.627(4)
2.500(5)
1.416(7)
1.389(7)
1.408(7)
N(1)–Cu(1)–N(2)
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–C(18)
N(1)–Cu(1)–C(19)
N(3)–Cu(1)–C(19)
81.6(1)
116.3(1)
103.3(2)
129.7(1)
124.9(2)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–C(18)
N(3)–Cu(1)–C(18)
N(2)–Cu(1)–C(19)
C(18)–Cu(1)–C(19)
84.4(1)
96.0(2)
139.9(2)
111.2(1)
34.4(2)
IR/cm−1 (KBr disk): 1603 (C C, aromatic), 1578, 839 and 557
=
Complex 2PhePh
(PF6−). Anal. Calc. for C23H26N4CuPF6: C, 48.72; H, 4.62; N,
9.88. Found: C, 48.62; H, 4.64; N, 9.89%.
Cu(1)–N(1)
Cu(1)–N(3)
Cu(1)–C(18)
C(17)–C(18)
C(18)–C(19)
C(20)–C(21)
2.179(6)
2.051(5)
2.158(8)
1.381(9)
1.40(1)
Cu(1)–N(2)
Cu(1)–C(17)
Cu(1)–C(19)
C(17)–C(22)
C(19)–C(20)
C(21)–C(22)
1.975(6)
2.629(7)
2.441(8)
1.376(9)
1.37(1)
[CuI(L2Bn)(CH3CN)]ClO4 was prepared similarly but without
the salt exchange reaction with NH4PF6. Yield: 82%. FT-
IR/cm−1 (KBr disk): 1603 (C C, aromatic), 1576, 1086, 794
=
and 623 (ClO4−). FAB-MS (+): m/z 380 (M+). Anal. Calc. for
C23H26N4O4CuCl: C, 52.97; H, 5.03; N, 10.74. Found: C, 52.64;
H, 4.99; N, 10.77%.
1.37(1)
1.38(1)
N(1)–Cu(1)–N(2)
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–C(18)
N(1)–Cu(1)–C(19)
N(3)–Cu(1)–C(19)
84.8(2)
115.6(2)
131.8(2)
125.9(3)
114.4(2)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–C(18)
N(3)–Cu(1)–C(18)
N(2)–Cu(1)–C(19)
C(18)–Cu(1)–C(19)
81.3(2)
91.2(3)
111.1(3)
124.3(2)
34.9(3)
[CuI(L2Bn)(CO)]PF6 (2Bn·CO). Treatment of [CuI(L2Bn)-
(CH3CN)]PF6 (1.14 g, 2.01 mmol) in ethanol (4 mL) under CO
atmosphere over night gave white powder which was collected by
filtration. Yield: 1.04 g (93.0%). 1H NMR (400 MHz, acetone-
d6): d 2.85 (6 H, s, –CH3), 3.99 (2 H, d, J = 15.9 Hz, –CHHPy),
4.31 (2 H, s, –CH2Ph), 4.47 (2 H, d, J = 15.9 Hz, –CHHPy),
7.31–7.40 (5 H, m, HPh and HPy-3), 7.49 (2 H, d, J = 7.8 Hz, HPh),
7.54 (2 H, m, HPy-5), 7.90 (2 H, t, J = 7.7 Hz, HPy-4). FT-IR/cm−1
Complex 2Bn·CH3CN
Cu(1)–N(1)
Cu(1)–N(3)
2.215(4)
2.051(5)
Cu(1)–N(2)
Cu(1)–N(4)
2.050(5)
1.918(6)
=
(KBr disk): 2088 and 2100 (C≡O), 1606 (C C, aromatic), 1578,
841 and 557 (PF6−). Anal. Calc. for C22H23N3OCuPF6: C, 47.70;
H, 4.19; N, 7.59. Found: C, 47.75; H, 4.16; N, 7.60%.
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(4)
N(2)–Cu(1)–N(4)
80.3(2)
119.1(2)
120.8(2)
N(1)–Cu(1)–N(3)
N(2)–Cu(1)–N(3)
N(3)–Cu(1)–N(4)
82.1(2)
115.1(2)
122.4(2)
−
Single crystals of the copper(I) complex with ClO4 counter
anion, [CuI(L2Bn)(CO)]ClO4, were obtained by the same treat-
ment of [CuI(L2Bn)(CH3CN)]ClO4 with CO in a 81% yield.
Complex 2Bn·CO
FT-IR/cm−1 (KBr disk): 2096 and 2085 (CO), 1606 (C C,
=
Molecule 1
Cu(1)–N(1)
Cu(1)–N(3)
aromatic), 1578, 1091, 791 and 623 (ClO4−). Anal. Calc. for
C22H23N3O5CuCl: C, 51.97; H, 4.56; N, 8.26. Found: C, 51.73;
H, 4.52; N, 8.43%.
2.131(3)
2.063(3)
Cu(1)–N(2)
Cu(1)–C(22)
2.042(2)
1.806(3)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–C(22)
N(2)–Cu(1)–C(22)
83.73(9) N(1)–Cu(1)–N(3)
80.51(10)
105.22(9)
117.6(1)
[CuI(L2Bn)]PF6 (2Bn). This complex was prepared by remov-
ing CO from [CuI(L2Bn)(CO)]PF6. Thus, a methanol solution
of the CO-complex was heated on hot water-bath (75 ◦C)
for 10 min, and then the solvent was removed under reduced
127.7(1)
129.0(1)
N(2)–Cu(1)–N(3)
N(3)–Cu(1)–C(22)
Molecule 2
Cu(2)–N(4)
Cu(2)–N(6)
1
pressure to give yellow powder of 2Bn. H NMR (400 MHz,
2.124(3)
2.051(3)
Cu(2)–N(5)
Cu(2)–C(44)
2.054(3)
1.800(3)
acetone-d6): d 2.83 (6 H, s, –CH3), 3.97 (2 H, s, –CH2Ph), 3.8–
4.3 (4 H, br s, –CH2Py), 7.25–7.37 (7 H, m, HPh and HPy-3), 7.45
(2 H, d, J = 7.7 Hz, HPy-5), 7.86 (2 H, t, J = 7.7 Hz, HPy-4). FT-
N(4)–Cu(2)–N(5)
N(4)–Cu(2)–C(44)
N(5)–Cu(2)–C(44)
83.68(10) N(4)–Cu(2)–N(6)
80.6(1)
108.0(1)
122.2(1)
129.0(1)
121.8(1)
N(5)–Cu(2)–N(6)
N(6)–Cu(1)–C(44)
IR/cm−1 (KBr disk): 1610 (C C, aromatic), 1577, 849 and 557
=
(PF6−). EI-MS (CH2Cl2): m/z 380.1 (M+). A satisfactory result
of elemental analysis could not be obtained due to instability of
the complex.
[CuI(L2PhePh)]ClO4 (2PhePh). This compound was prepared in
a manner similar to the synthesis of complex 2Phe using ligand
L2PhePh (103.6 mg, 0.3 mmol) instead of L2Phe. Yield 139 mg
(81%). 1H NMR (600 Hz, CD2Cl2): d 2.58 (6 H, s, –CH3), 3.46
(2 H, dd, J = 8.5 Hz, –NCH2CHPh2), 4.05 (2 H, d, J = 16.2 Hz,
–NCHHPy), 4.14 (1 H, t, J = 8.5 Hz, –NCH2CHPh2), 4.36 (2 H,
d, J = 16.2 Hz, –NCHHPy), 7.00 (4 H, dd, J = 6.3 and 1.1 Hz,
HPh-2 and HPh-6), 7.26–7.34 (10 H, m, HPh-3, HPh-4, HPh-5, HPy-3 and
HPy-5), 7.81 (t, J = 7.7 Hz, HPy-4). 13C NMR (600 Hz, CD2Cl2):
d 27.61 (–CH3), 49.87 (–NCH2CHPh2), 60.95 (–NCH2CHPh2),
61.20 (–NCH2Py), 121.85 (CPy-5), 124.73 (CPy-3), 125.50 (CPh-2
and CPh-6), 128.49 (CPh-4), 128.64 (CPh-3 and CPh-5), 139.57 (CPy-4),
140.81 (CPh-1), 157.77 (CPy-2), 158.01 ppm (CPy-6). FT-IR/cm−1
(KBr disk): 1097 and 623 (ClO4−). ESI-MS (+): m/z 470.3 (M+).
Results and discussion
Characterization of copper(I) complexes
The tridentate ligands [L2R; R = –CH2Ph (Bn), –CH2CH2Ph
(Phe) and –CH2CHPh2 (PhePh)] were prepared by reductive
coupling of the corresponding amines (RNH2) and 2 equiv of
6-methyl-2-pyridinecarbaldehyde in the presence of NaBH3CN
as the reductant. Treatment of the ligand with an equimolar
amount of [CuI(CH3CN)4]ClO4 under anaerobic conditions (Ar)
−
gave the corresponding copper(I) complexes 2R as ClO4 salts.
In the case of 2Bn, single crystals of the CH3CN-containing
−
complex, 2Bn·CH3CN, were obtained as a PF6 salt which was
prepared by the salt exchange reaction of the ClO4−-complex
D a l t o n T r a n s . , 2 0 0 5 , 3 5 1 4 – 3 5 2 1
3 5 1 7

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with NH4PF6. 2Bn·CH3CN can be easily converted into air-
stable carbonyl complex 2Bn·CO by treating it with CO gas.
Furthermore, the CO ligand in 2Bn·CO can be easily removed
◦
by heating it at 75 C for 10 min to give complex 2Bn without
any external ligand. Crystal structures of [CuI(L2Phe)]ClO4
(2Phe), [CuI(L2PhePh)]ClO4 (2PhePh), [CuI(L2Bn)(CH3CN)]PF6
(2Bn·CH3CN) and [CuI(L2Bn)(CO)]ClO4 (2Bn·CO) have been
determined by X-ray crystallographic analysis as shown in
Fig. 1. The crystallographic data of the complexes are presented
in Table 1, and their selected bond lengths and angles around
the copper ion are summarized in Table 2.
Fig. 2 d–p Interaction in complexes 1R and 3Phe
.
In fact, the distances from the copper(I) ion to the aromatic
carbon atom and to the ligand nitrogen atoms in 2Phe and 2PhePh
are similar to those in 3Phe
.
In the UV-vis spectra, copper(I) complexes 2Phe and 2PhePh
exhibit a characteristic absorption band around 270 nm
(Table 3), which has been tentatively assigned to an MLCT
(metal-to-ligand charge transfer) band through the d–p interac-
tion as in the case of other copper(I) complexes exhibiting a d–p
interaction.11,18,20,21 The relative strength of the d–p interaction
can be evaluated by the competitive binding of CH3CN to
the cuprous ion (titration of 2R by CH3CN) in a non-polar
solvent such as CH2Cl2.11,18,20 The MLCT band around 270 nm
gradually decreased, when CH3CN was added into a CH2Cl2
solution of 2R. Fig. S1 (ESI‡) shows the spectral change for the
titration of complex 2Phe by CH3CN in CH2Cl2 at −20 ◦C as
a typical example. A decrease of the absorption band around
270 nm may be due to the ligand exchange reaction between the
phenyl ring of the d–p interaction and the added CH3CN. The
association constant of CH3CN to the copper(I) ion of 2Phe
,
Kas = [CuIL(CH3CN)]/[CuIL][CH3CN], was then determined as
622 15 M−1 by analyzing the absorption change as indicated
in the inset of Fig. S1, and the Kas values for 2PhePh and 2Bn were
determined similarly, as listed in Table 3, where the Kas values
for 1Phe and 3Phe are also included.
The Kas value may reflect the strength of the d–p interaction
in the copper(I) complexes. Namely, the smaller the Kas value
(thus the weaker the CH3CN-binding), the stronger the d–p
interaction. As clearly seen in Table 3, the Kas values of 2Phe and
Fig.
1
ORTEP drawings of (A) [CuI(L2Phe)]ClO4 (2Phe), (B)
[CuI(L2PhePh)]ClO4 (2PhePh), (C) [CuI(L2Bn)(CH3CN)]PF6 (2Bn·CH3CN)
and (D) [CuI(L2Bn)(CO)]ClO4 2Bn·CH3CN) showing 50% probability
thermal ellipsoids. The counter anion and hydrogen atoms are omitted
for clarity.
2
2PhePh are much larger than that of 1Phe having the g -binding
interaction, but are smaller than that of 3Phe. Thus, the strength
of d–p interaction decreases in the order of 1Phe > 2 R > 3Phe (the
strength of d–p interaction in 2Bn is discussed below). Although
the theoretical aspects of the d–p interaction have yet to be
addressed in detail, the order of strength of the d–p interaction
seems to be correlated to the order of the electron-donor ability
of ligand; 1R < 2R < 3R (the order of electron-donor ability
of pyridine is discussed below). Namely, the d–p interaction
becomes stronger (1R > 2R > 3R) as the electron-donor ability
d–p Interaction in 2R. Copper(I) complexes 2Phe and 2PhePh
containing –CH2CH2Ph (Phe) and –CH2CHPh2 (PhePh)
sidearms, respectively, exhibit a copper(I)–arene interaction in
the crystals, where the phenyl ring of the N-alkyl substituent
(ligand sidearm) exists just above the cuprous ion as shown in
Fig. 1(A) and (B), respectively. The cuprous ion of each complex
adapts to a trigonal pyramidal geometry consisting of the two
pyridine nitrogen atoms N(2) and N(3) and one of the ortho-
carbon atoms C(18) of the phenyl ring occupying the trigonal
basal plane and the tertiary alkyl amine nitrogen atom N(1)
R
of ligands becomes weaker (1R < 2 < 3R). In other words,
the d–p interaction becomes stronger (1R > 2R > 3R) as the
bonding interaction between the cuprous ion and the pyridine
nitrogen becomes weaker (1R < 2R < 3R). On the other hand,
the meaningful difference in Kas between 2Phe and 2PhePh can be
attributed to a steric effect of the benzylic substituent (Ph) in
as the axial ligand. Deviation of the copper(I) ion from the
basal plane is 0.076 and 0.163 A in 2 and 2PhePh, respectively.
Phe
˚
It should be noted that the distances between Cu and C(18)
(2.148 A in 2 and 2.165 A in 2PhePh) are much shorter than
Phe
˚
˚
the Cu–C(17) distances (2.629 A in 2 and 2.639 A in 2PhePh).
Table 3 The UV-vis data and the equilibrium constants Kas for the
Phe
˚
˚
a
titration of copper(I) complexes with CH3CN in CH2Cl2
Thus, the copper(I)–arene interaction in these complexes can be
1
described as an g -binding interaction.
kmax/nm (e/M−1 cm−1
)
Kas/M−1
We have already demonstrated that copper(I) complexes 1R
(R = –CH2CH(X)Ph where X = H, Me and Ph) supported by
bis[2-(pyridin-2-yl)ethyl]amine tridentate ligands L1R with the
longer ethylene linker exhibit a distorted tetrahedral geometry
Complex
1Pheb
2Phe
290 (8820)
270 (13500)
270 (13100)
285 (20900)d
290 (6010)
6.4 0.1
622 15
230 6.0
4470 330
3360 17
2PhePh
2Bn
2
involving a d–p interaction with an g -binding mode, while
3Phec
complex 3Phe with bis(pyridin-2-ylmethyl)amine ligand L3Phe
shows a trigonal pyramidal structure involving a d–p interaction
a At −20 ◦C. b The data were taken from the literature.20 c The data were
with an g -binding mode as illustrated in Fig. 2.11,18,20 Thus, the
1
taken from the literature.11 d Shoulder.
copper(I)–arene interaction in 2R resembles that in complex 3Phe
.
3 5 1 8
D a l t o n T r a n s . , 2 0 0 5 , 3 5 1 4 – 3 5 2 1

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2
PhePh, which prohibits free rotation of the ligand sidearm, thus
ring.12 Then, it becomes apparent that the 6-methyl group in 2Phe
somewhat reduces the electron-donor ability of pyridine to cause
the negative shift of E1/2 as compared to that of 3Phe. However,
the electron-donor ability of pyridine in 2Phe is still higher than
that in 1Phe, making the order of E1/2 as 1Phe > 2Phe > 3Phe. The fact
that the E1/2 value of 2PhePh is somewhat higher than that of 2Phe
suggests that the stronger d–p interaction stabilizes the lower
oxidation state of copper(I) more than the copper(II) oxidation
state. A similar trend was seen in the 1R system, where the E1/2
value of 1PhePh is higher than that of 1Phe (DE1/2 = 0.08 V).20
stabilizing the d–p interaction.20
Copper(I) complexes of L2Bn. Crystal structures of
2Bn·CH3CN and 2Bn·CO are shown in Fig. 1(C) and (D),
respectively. Copper(I) complex 2Bn·CH3CN exhibits a distorted
trigonal pyramidal structure with an N4 donor set, where two
pyridine nitrogen atoms N(2) and N(3) and acetonitrile nitrogen
N(4) occupy the trigonal basal plane and tertiary amine nitrogen
N(1) acts as an axial ligand. The axial ligand N(1), however,
largely slips out of the ideal position of the apex of trigonal
pyramidal geometry. Apparently, there is no interaction between
the cuprous ion and the phenyl group of the ligand sidearm
(–CH2Ph) in 2Bn·CH3CN.
Copper(I)–dioxygen reactivity
The unit cell of copper(I) complex 2Bn·CO consists of two
crystallographically independent molecules. Both of the cuprous
centers of 2Bn·CO also have a distorted trigonal pyramidal (or
a significantly distorted tetrahedral) geometry where CH3CN
ligand in 2Bn·CH3CN is replaced by a CO molecule. In this
complex as well, there is no d–p interaction between the metal
center and the phenyl group of the N-benzyl substituent.
The CO ligand in 2Bn·CO can be easily removed by heating it
in methanol to give 2Bn involving no external ligand. Although
structural examination of 2Bn has yet to be accomplished due
to its instability in solution, complex 2Bn exhibits a shoulder
absorption band at 285 nm in CH2Cl2. This absorption band
disappeared when acetonitrile was added into the solution of
2Bn, and the final spectrum of the titration was identical to that
of 2Bn·CH3CN [kmax = 313 nm (e = 7700 M−1 cm−1)]. Thus, the
285 nm band of 2Bn can be also attributed to a MLCT transition
of a d–p interaction, although the binding mode of copper(I)
to the phenyl group is not clear at present. As clearly seen in
Table 3, the Kas value of 2Bn is significantly larger than those of
2R (R = Phe and PhePh) and fairly close to that of 3Phe. This
means that the d–p interaction in 2Bn is much weaker than that
in 2Phe and 2PhePh and is comparable to that in 3Phe. The weaker
interaction could be attributed to the shorter methylene linker
between the amine nitrogen atom and the phenyl ring of the
ligand sidearm. Namely, the methylene linker may be too short
to construct a stable d–p interaction.
Ligand effects on the copper(I)–dioxygen reactivity were also
examined. As noted in Introduction, the copper(I) complexes of
2
2
L1R-type ligands (1R) mainly afford (l-g :g -peroxo)dicopper(II)
complex A in the reaction with O2 at a low temperature, whereas
the copper(I) complex of L3R (3R) predominantly affords bis(l-
oxo)dicopper(III) complex B in a similar reaction. Thus, it
is of interest to know what is obtained in the oxygenation
reaction of 2R.
Treatment of copper(I) complexes 2Phe with O2 in anhy-
drous acetone at −80 ◦C readily afforded a brown solution
which exhibited an intense absorption band at 355 nm (e =
18700 M−1 cm−1) together with a relatively weak band at
511 nm (770 M−1 cm−1) as shown in Fig. 3. Similar spectra
were obtained in the reaction of 2PhePh and 2Bn under the same
experimental conditions (see Table 4 and Fig. 4). After a short
lag phase (0–10 s), the reaction obeys second-order kinetics,
and the second-order rate constant (kobs) was obtained from
the slope of the line of the second-order plot shown in the
Redox potentials of the copper(I) complexes. Copper(I) com-
plexes 2Phe and 2PhePh exhibited a quasi-reversible redox couple
due to one-electron oxidation–reduction of the copper center
in CH2Cl2. The redox potentials (E1/2 vs. Fc/Fc+) of these
complexes are listed in Table 4 together with those of 1Phe and
3
Phe. The redox behavior of 2Bn was not so simple probably due
to instability of the complex under the present experimental
conditions.
As clearly seen in Table 4, the E1/2 values of 1Phe, 2Phe and
3Phe decrease in this order (E1/2 = 0.07, −0.06 and −0.20 V
vs. Fc/Fc+, respectively). The higher E1/2 value of 1Phe has
been attributed to the lower electron-donor ability of pyridine
of ligand L1Phe as compared to that of other ligands, which
can be simply attributed to the chelate ring size effect that is
normally in the order of six-membered ring < five-membered
Fig. 3 Spectral change observed upon introduction of O2 gas into
acetone solution of 2Phe (2.0 × 10−4 M) at −80 ◦C. Inset: second-order
plot based on the absorption change at 355 nm.
Table 4 Redox potentials (E1/2)a of the copper(I) complexes, UV-vis and resonance Raman data of the Cu2/O2 complexes and second-order rate
constants (kobs) for the formation of Cu2/O2 complexes at −80 ◦C in acetone
Cu2/O2 complex
16 O–18
Complex
E
1/2, V vs. Fc/Fc+a
UV-vis, kmax/nm (e/M−1 cm−1
)
Raman, mO–O (Dm
)
O
Formation rate, kobs/M−1 s−1
1Pheb
2Phe
0.07
362 (15400) ∼520 (770)
355 (18700) 511 (770)
356 (16700) ∼505 (770)
353 (23500) 529 (970)
385 (6540)
746 (42)
726 (36)
716 (37)
714 (39)
—
4.1
130.0
33.6
−0.06
2PhePh
2Bn
0.01
—
−0.20
29.2
3Phec
59000d
a The electrochemical measurements were performed in CH2Cl2 containing 0.1 M tetrabutylammonium perchlorate (TBAP) at a scan rate of 10–50
mV s−1 at 25 ◦C. b The data are taken from the literature.20 c The data are taken from the literature.11 d At −94 ◦C in acetone.
D a l t o n T r a n s . , 2 0 0 5 , 3 5 1 4 – 3 5 2 1
3 5 1 9

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Scheme 1
Fig. 4 Absorption spectra of the oxygenated products of 2Phe, 2PhePh and
2Bn in acetone at −80 ◦C. The initial concentration of 2Phe, 2PhePh and 2Bn
is 2.0 × 10−4 M.
inset of Fig. 3. The second-order kinetics clearly indicates that
two molecules of the copper complex are involved in the rate-
determining step of the Cu2/O2 complex formation process.
It can be presumed that the bimolecular reaction between a
mononuclear (superoxo)copper(II) complex, generated by the
reaction of 2R and O2, and another molecule of copper(I)
complex is the rate-determining step as indicated in Scheme 1.
The initial lag phase shown in Fig. 3 may be due to the
time required to attain the pre-equilibrium reaction between
the copper(I) complex and O2 to give the (superoxo)copper(II)
intermediate (see Scheme 1). From the dependence of the rate
constants on the reaction temperature shown as Arrhenius
plots in Fig. S2 (ESI‡) were obtained the activation enthalpy
(DH=) of 5.8 0.3 and 8.2 0.3 kJ mol−1 and the activation
entropy (DS=) of −140.1 1.6 and −139.1 1.5 J K−1 mol−1
for the oxygenation reaction of 2Phe and 2PhePh, respectively.
The significantly large negative DS= values are consistent with
the proposed mechanism (Scheme 1), where the bimolecular
reaction between the monomeric (superoxo)copper(II) complex,
Fig. 5 Resonance Raman spectra of the oxygenated products of (A)
2
PhePh, (B) 2Phe and (C) 2Bn obtained with 514.5 nm excitation in acetone
at −90 ◦C. The label ‘s’ denotes the solvent peak.
peroxo)dicopper(II) complex A. In addition, Cu–Cu stretching
2
2
vibrations in the (l-g :g -peroxo)dicopper(II) core were observed
at 312 and 280 cm−1 for 2PhePh and 314 and 282 cm−1 for 2Phe (data
not shown in Fig. 5), further confirming the formation of side-
on peroxo dicopper(II) complex A.24§ Notably, the frequency
of the O–O bond stretching vibration of A supported by L2R
(714–726 cm−1) is relatively lower than that of the L1Phe-complex
(746 cm−1, see Table 4). The results clearly suggest that the O–O
bond in the L2R-complexes is relatively weakened as compared
to that in the L1R-complex. This could be also attributed to the
higher electron-donor ability of pyridine in L2R as compared to
L1R, as discussed below.
•
[CuII(L2R)(O2 −)]+, and another Cu(I) starting compound is the
As stated above, the UV-vis spectra (Fig. 4) suggested co-
existence of bis(l-oxo)dicopper(III) complex B in solution. This
was confirmed by the resonance Raman data. Namely, weak
Raman bands due to bis(l-oxo)dicopper(III) complex B were
detected at 588 and 563 cm−1 (Fermi doublet) for 2PhePh, 592 and
561 cm−1 for 2Phe, and 590 and 567 cm−1 for 2Bn. The Fermi
doublet signals of 2PhePh in the 16O2-derivative shift into one
band at 553 cm−1. Thus, the isotope shift was calculated to be
23 cm−1 by subtracting the frequency (553 cm−1) of the 18O2-
derivative from the average frequency (578 cm−1) of the 16O2-
derivative. These results are consistent with the well-established
Raman data of the bis(l-oxo)dicopper(III) complexes.25 It is also
apparent that the content of bis(l-oxo)dicopper(III) complex B
increases in going from 2Bn to 2PhePh. Thus, the Raman peaks
of the 18O2-derivatives derived from 2Bn and 2Phe were too
weak to be detected (see Fig. 5 (B) and (C)). These results
are also consistent with the UV-vis data shown in Fig. 4,
rate determining step.22
In all cases, frozen CH2Cl2 solutions of the oxygenated
product were ESR silent at −150 ◦C. The UV-vis features shown
in Fig. 4 as well as the ESR silence strongly suggest that the
2
2
oxygenated product is (l-g :g -peroxo)dicopper(II) complex A.
It should be noted, however, that intensity of the LMCT bands
at ∼355 and ∼510 nm due to the side-on peroxo complex A is
somewhat different among the three ligand systems L2R (R =
Bn, Phe and PhePh, decreasing in this order) and the shoulder
∼400 nm seems to grow in going from L2Bn to L2PhePh. The
spectral feature around 400 nm can be attributed to co-existence
of a bis(l-oxo)dicopper(III) complex B. This was confirmed by
the resonance-Raman studies described below.
Fig. 5 shows the resonance-Raman spectra of the oxygenated
product of 2Bn, 2Phe and 2PhePh obtained with 514.5 nm excitation
in acetone at –90 ◦C. In all cases, a relatively intense Raman band
at 714–726 cm−1 with an isotope shift of 36–39 cm−1 with 18O2
was obtained. These Raman features have been ascribed to the
O–O bond stretching vibration of the side-on peroxo ligand.23
§ In the reaction of 2Bn, the Cu–Cu stretching vibration was not detected,
2
2
Thus, the data unambiguously support the formation of (l-g :g -
the reason for which is not clear at present.
3 5 2 0
D a l t o n T r a n s . , 2 0 0 5 , 3 5 1 4 – 3 5 2 1

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where content of the bis(l-oxo)dicopper(III) species somewhat
increases in going from 2Bn to 2PhePh, although the reason for
this phenomenon is not clear at present. There could be some
sort of interaction between copper(III) ion in B and the aromatic
group of the ligand sidearm of 2PhePh, somewhat stabilizing the
bis(l-oxo)dicopper(III) species.
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Acknowledgements
This work was financially supported in part by Grants-in-Aid
for Scientific Research (No. 15350105 for S. I. and No. 16350030
for M. S.) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan and by Research Fellowships of
Japan Society for the Promotion of Science for Young Scientists.
D a l t o n T r a n s . , 2 0 0 5 , 3 5 1 4 – 3 5 2 1
3 5 2 1