Modeling the mononuclear, dinuclear, and trinuclear copper(I) reaction centers of copper proteins using pyridylalkylamine ligands connected to 1,3,5-triethylbenzene spacer

Article abstract of DOI:10.1021/ic061509j

The structure and O₂-reactivity of copper(I) complexes supported by novel ligands, Pye2 (1,3,5-triethyl-2,4-bis((N-benzyl-N-(2-(pyridin-2-yl) ethyl)-)aminomethyl)benzene), Pye3 (1,3,5-triethyl-2,4,6-tris((N-benzyl-N-(2- (pyridin-2-yl)ethyl))aminomethyl)benzene), ^MePym2 (1,3,5-triethyl-2,4-bis((N-benzyl-N-(6-methylpyridin-2-ylmethyl))aminomethyl) benzene), and ^MePym3 (1,3,5-triethyl-2,4,6-tris((N-benzyl-N-(6- methylpyridin-2-ylmethyl))aminomethyl)benzene) have been examined. The ligands are designed to construct mono-, di-, and trinuclear copper(I) complexes by connecting two or three pyridylalkylamine metal-binding sites to a 1,3,5-triethylbenzene spacer. Thus, the reaction of the ligands with [Cu ^I(CH₃CN)₄]X (X = PF₆, CF ₃SO₃) or Cu^ICl gave the expected mononuclear copper(I) complexes [Cu^I(Pye2)(CF₃SO₃)] (1) and [Cu^I(Pye3)](CF₃SO₃) (2), dinuclear copper(I) complex [Cu^I₂(^MePym2)(Cl)]Cu^ICl ₂ (3), and trinuclear copper(I) complex [Cu^I ₃(^MePym3)(CH₃CN)₃](CF ₃SO₃)₃ (4), the structures of which were determined by X-ray crystallographic analysis. The mononuclear copper(I) complexes, 1 and 2, exhibit a distorted three-coordinate T-shape structure and a trigonal planar structure, respectively, which are very close to the coordination geometry of the CuA site of PHM (peptidylglycine α-hydroxylating monooxygenase) and the CuB site of CcO (cytochrome c oxidase). Notably, 1 and 2 showed a significantly high oxidation potential (990 mV vs SCE), thus showing virtually no reactivity toward O₂. On the other hand, the metal centers of the dinuclear and trinuclear copper(I) complexes, 3 and 4, exhibit a distorted trigonal planar geometry and a trigonal pyramidal geometry, respectively. In contrast to the mononuclear copper(I) complexes, these dinuclear and trinuclear copper(I) complexes reacted with O₂ to induce an aromatic ligand hydroxylation reaction involving an NIH-shift of one of the ethyl substituents on the benzene spacer. The NIH-shift of the alkyl substituent on the aromatic ring is strong evidence of the electrophilic aromatic substitution mechanism, although the active oxygen intermediate could not be directly detected during the course of the reaction. The biological relevance of the copper(I) complexes is also discussed on the basis of structure and O₂-reactivity.

Full text of DOI:10.1021/ic061509j

Inorg. Chem. 2006, 45, 10825−10835
Modeling the Mononuclear, Dinuclear, and Trinuclear Copper(I) Reaction
Centers of Copper Proteins Using Pyridylalkylamine Ligands Connected
to 1,3,5-Triethylbenzene Spacer
Hiromi Ohi, Yoshimitsu Tachi, and Shinobu Itoh*
Department of Chemistry, Graduate School of Science, Osaka City UniVersity, 3-3-138 Sugimoto,
Sumiyoshi-ku, Osaka 558-8585, Japan
Received August 10, 2006
The structure and O2-reactivity of copper(I) complexes supported by novel ligands, Pye2 (1,3,5-triethyl-2,4-bis((N-
benzyl-N-(2-(pyridin-2-yl)ethyl)-)aminomethyl)benzene), Pye3 (1,3,5-triethyl-2,4,6-tris((N-benzyl-N-(2-(pyridin-2-yl)-
ethyl))aminomethyl)benzene), MePym2 (1,3,5-triethyl-2,4-bis((N-benzyl-N-(6-methylpyridin-2-ylmethyl))aminomethyl)-
benzene), and MePym3 (1,3,5-triethyl-2,4,6-tris((N-benzyl-N-(6-methylpyridin-2-ylmethyl))aminomethyl)benzene) have
been examined. The ligands are designed to construct mono-, di-, and trinuclear copper(I) complexes by connecting
two or three pyridylalkylamine metal-binding sites to a 1,3,5-triethylbenzene spacer. Thus, the reaction of the ligands
with [CuI(CH3CN)4]X (X
)
PF6, CF3SO3) or CuICl gave the expected mononuclear copper(I) complexes [CuI(Pye2)(CF3-
SO3)] (1) and [CuI(Pye3)](CF3SO3) (2), dinuclear copper(I) complex [CuI2(MePym2)(Cl)]CuICl2 (3), and trinuclear
copper(I) complex [CuI3(MePym3)(CH3CN)3](CF3SO3)3 (4), the structures of which were determined by X-ray
crystallographic analysis. The mononuclear copper(I) complexes, 1 and 2, exhibit a distorted three-coordinate T-shape
structure and a trigonal planar structure, respectively, which are very close to the coordination geometry of the
CuA site of PHM (peptidylglycine
R-hydroxylating monooxygenase) and the CuB site of CcO (cytochrome c oxidase).
Notably, 1 and 2 showed a significantly high oxidation potential (990 mV vs SCE), thus showing virtually no reactivity
toward O2. On the other hand, the metal centers of the dinuclear and trinuclear copper(I) complexes, 3 and 4,
exhibit a distorted trigonal planar geometry and a trigonal pyramidal geometry, respectively. In contrast to the
mononuclear copper(I) complexes, these dinuclear and trinuclear copper(I) complexes reacted with O2 to induce
an aromatic ligand hydroxylation reaction involving an NIH-shift of one of the ethyl substituents on the benzene
spacer. The NIH-shift of the alkyl substituent on the aromatic ring is strong evidence of the electrophilic aromatic
substitution mechanism, although the active oxygen intermediate could not be directly detected during the course
of the reaction. The biological relevance of the copper(I) complexes is also discussed on the basis of structure and
O2-reactivity.
Introduction
involving mono-, di-, and trinuclear structures.1 To explore
the structures, physicochemical properties, and functions of
these copper active sites, a large number of model complexes
have so far been developed during the last two decades,
establishing biomimetic copper chemistry.2-4
Copper proteins are ubiquitous in nature, for which a wide
variety of physiological functions such as reversible dioxygen
binding, electron transfer, oxidase and oxygenase activities,
and superoxide dismutation are known.1 These functions are
accomplished at various types of copper reaction centers
For copper(I)/dioxygen chemistry, tridentate ligands such
as hydrotris(1-pyrazolyl)borate (HBpz3), 1,4,7-triazacy-
clononane (TACN), and bis[2-(2-pyridyl)ethyl]amine (PY2)
derivatives as well as tetradentate ligands such as tris(2-
pyridylmethyl)amine (TPA) and tris(aminoethyl)amine (tren)
derivatives have been examined in detail.2 Didentate ligands
such as ethylene diamine (en), 2-(2-pyridyl)ethylamine
* Author to whom correspondence should be addressed. E-mail: shinobu@
sci.osaka-cu.ac.jp.
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Wieghardt, K., Eds.; John Wiley & Sons: Chichester, 2001. (c)
Solomon, E. I.; Chen, P.; Metz, M.; Lee, S. K.; Palmer, A. E. Angew.
Chem., Int. Ed. 2001, 40, 4570-4590.
10.1021/ic061509j CCC: $33.50
Published on Web 11/29/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 26, 2006 10825

Ohi et al.
Chart 1
ligand system, attached to the 2,4,6-positions are enforced
to positioning the same side of the benzene ring of the spacer,
since the ababab configuration (a denotes ‘above’ and b
denotes ‘below’) is the most stable conformation due to the
steric repulsion between the neighboring substituents as
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(PY1), and â-diketiminate derivatives have also been studied
recently.2 The copper(I) complexes supported by these
capping ligands are usually very reactive toward O2, produc-
ing a series of copper-dioxygen complexes such as mono-
nuclear copper(II)-superoxo (both end-on and side-on),
copper(III)-peroxo (side-on), dinuclear copper(II)-µ-peroxo
(both end-on and side-on), bis(µ-oxo)dicopper(III), and bis-
(µ3-oxo)tricopper(II,II,III) complexes.2 Characterization and
reactivity studies of these copper-dioxygen complexes have
provided significantly important insights into the dioxygen
activation mechanisms at the mononuclear copper reaction
centers of peptidylglycine R-hydroxylating monooxygenase
(PHM) and dopamine â-monooxygenase (DâM) and the
dinuclear type-3 copper active sites of hemocyanin (Hc),
tyrosinase (Tyr), and catechol oxidase (CA).2
In this study, we have examined copper(I) coordination
chemistry of Pye2, Pye3, MePym2, and MePym3 (Chart 1)
in order to model the mononuclear, dinuclear, and trinuclear
copper(I) reaction centers in the biological systems. The
1,3,5-triethylbenzene derivatives have recently attracted much
attention as an efficient building block for the development
of receptors in molecular recognition chemistry and ligands
in supramolecular, biomimetic, and coordination polymer
chemistry.5-11 The 1,3,5-triethylbenzene spacer plays an
important role in controlling the steric configuration of the
functional groups introduced into the 2,4,6-positions. Namely,
the functional groups, the metal binding units in the present
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10826 Inorganic Chemistry, Vol. 45, No. 26, 2006

Copper(I) Reaction Centers of Copper Proteins
Electrochemical Measurement. The cyclic voltammetry (CV)
was performed on an ALS-630A electrochemical analyzer in
deaerated acetonitrile containing 0.1 M n-Bu4NPF6 as supporting
electrolyte. The Pt electrodes was polished with BAS polishing
alumina suspension and rinsed with H2O before use. The counter
electrode was a platinum wire. The measured potentials were
recorded with respect to a Fc/Fc+ (1.0 × 10-3 M) reference
electrode. The E values (vs Fc/Fc+) are converted to those versus
SCE by adding 0.33 V (Mann, K.; Barnes, K. K. Electrochemical
Reactions in Non-aqueous Systems; Marcel Dekker Inc.: New York,
1990). All electrochemical measurements were carried out at 25
°C under an atmospheric pressure of Ar in a glove box (Miwa Co.
Ltd.).
Figure 1. Schematic representation of the steric configuration of the
substituents of 1,3,5-triethylbenzene derivatives.
X-ray Structure Determination. The single crystal was mounted
on a glass fiber. Diffraction data of complexes 1, 3, 4, and 6 were
collected by a RIGAKU RAXIS-RAPID imaging plate two-
dimensional area detector using a graphite-monochromated Mo KR
radiation (λ ) 0.71075 Å) to a 2θmax of 55.0°. Diffraction data of
complex 2 were collected by a Rigaku AFC7/CCD Mercury area
detector using a graphite-monochromated Mo KR radiation (λ )
0.71070 Å) to a 2θmax of 55.0°. All the crystallographic calculations
were performed by using the Crystal Structure software package
from the Molecular Structure Corporation [Crystal Structure:
Crystal Structure Analysis Package, version 3.7.0; Molecular
Structure Corp. and Rigaku Corp. (2000-2005)]. The crystal
structures were solved by the direct method and refined by full-
matrix least-squares using SIR92 for 1, 2, and 4 and SHELX97
for 3 and 6. All non-hydrogen atoms and hydrogen atoms were
refined anisotropically and isotropically, respectively. Summary of
X-ray crystallographic data and selected bond lengths and angles
are given in Tables 1 and 2, respectively. Atomic coordinates,
thermal parameters, and intramolecular bond distances and angles
are deposited in the Supporting Information (CIF file format).
illustrated in Figure 1. This geometric feature of the 1,3,5-
triethylbenzene spacer may enable the molecules to adapt a
favorable conformation for the formation of biomimetic
copper(I) complexes and cause perturbation on the structure
and reactivity of the resulting complexes as discussed below.
So far, only a couple of examples of copper(I) complexes
supported by the similar triethylbenzene ligands have been
reported.11d,e
Experimental Section
General. All chemicals used in this study except the ligands
and the copper(I) complexes were commercial products of the
highest available purity and were further purified by the standard
methods, if necessary.12 FT-IR spectra were recorded with a
Shimadzu FTIR-8200PC or JASCO FT-IR-4100. UV-vis spectra
were obtained on a Hewlett-Packard 8453 photodiode array
spectrophotometer. 1H NMR spectra were recorded on a JEOL FT-
NMR Lambda 300WB, GX-400, or Bruker Advance 600 spec-
trometers. Mass spectra were obtained 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 analysis was carried out on a
Perkin-Elmer 240C or a Fisons instruments EA1108 elemental
analyzer.
Synthesis. 1,3,5-Triethyl-2,4-bis((N-benzyl-N-(2-(pyridin-2-yl)-
ethyl))aminomethyl)benzene (Pye2). To a mixture of N-benzyl-
N-(2-(pyridin-2-yl)ethyl)amine13a (4.0 g, 1.90 mmol) and K2CO3
(3.0 g, 21.7 mmol) in dry CH3CN (30 mL) was added a CH3CN
solution (40 mL) of 1,3-bis(bromomethyl)-2,4,6-triethylbenzene14
(2.0 g, 5.7 mmol) under anaerobic conditions (Ar), and the mixture
was stirred for 12 h at room temperature. The resulting precipitates
were removed by filtration and washed with CH2Cl2. The combined
organic layer was concentrated by evaporation to give a yellow
residue, from which ligand Pye2 was isolated by silica gel column
chromatography (eluent; ethyl acetate/CHCl3 ) 1:1, Rf ) 0.41) (1.7
g, 48%). IR (KBr, cm-1): 3061, 3025 (aromatic C-H), 2961, 2929,
2869, 2793 (aliphatic C-H), 1591, 1569, 1474, 1453, 1435, 744,
699 (aromatic), 1125, 1115 (C-N). 1H NMR (CDCl3, 400 MHz):
δ 0.95 (3 H, t, J ) 7.6 Hz, Ar-CH2CH3), 1.12 (6 H, t, J ) 7.6 Hz,
Ar-CH2CH3), 2.67 (4 H, q, J ) 7.6 Hz, Ar-CH2CH3), 2.82-
3.00 (8 H, m, Py-CH2CH2-), 3.01 (2 H, q, J ) 7.6 Hz, Ar-CH2-
CH3), 3.54 (4 H, s, -CH2-), 3.67 (4 H, s, -CH2-), 6.81 (2 H, d,
J ) 7.7 Hz, Py-3H), 6.84 (1 H, s, Ar-H), 7.01 (2 H, dd, J ) 7.0
and 4.8 Hz, Py-5H), 7.17 (10 H, bs, Ar-H), 7.38 (2 H, td, J )
7.7 and 1.6 Hz, Py-4H), 8.41 (2 H, d, J ) 4.8 Hz, Py-6H). HRMS
(FAB, pos): m/z ) 611.4107 calcd for ([M + H]+, C42H51N4
611.4114).
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Chemicals, 4th ed.; Butterworth-Heinemann: Oxford, 1996, pp 176
and 215.
(14) Walsdorff, C.; Saak, W.; Pohl, S. J. Chem. Res. 1996, (S) 282-283.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10827

Ohi et al.
Table 1. Summary of X-ray Crystallographic Data of Complexes 1-4 and 6.
1
2
3
4
6
formula
fw
cryst syst
space group
a, Å
C43H50CuF3N4O3S
823.49
C58H66CuF3N6O3S
1047.80
C42H50Cl3Cu3N4
907.88
triclinic
C66H75Cu3F9N9O9S 3
C60H78Cu12N4O4P2
1336.32
triclinic
1596.18
trigonal
P3 (No. 143)
24.4960(10)
24.4960(10)
10.4042(5)
90
90
120
5406.7(4)
3
monoclinic
P21/a (No. 14)
15.686(2)
15.410(4)
17.509(3)
90
130.437(9)
90
4116.4(13)
4
monoclinic
C2/c (No. 15)
21.6998(16)
12.9667(10)
39.095(3)
90
90.454(4)
90
11000.1(14)
8
P1h (No. 2)
9.4612(10)
10.8782(10)
22.414(2)
89.649(4)
78.527(4)
67.578(4)
2083.6(4)
2
P1h (No. 2)
11.318(4)
11.660(4)
12.129(4)
86.849(13)
78.394(15)
87.442(14)
1564.6(9)
1
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
F(000)
D
1728.00
1.329
4416.00
1.265
936.00
1.447
2466.00
1.471
694.00
1.418
calcd, g/cm-3
T, K
160
153
161
161
161
cryst size, mm3
µ(Mo KR), cm-1
2θmax, deg
0.35 × 0.33 × 0.32
0.21 × 0.16 × 0.07
0.30 × 0.22 × 0.21
0.22 × 0.22 × 0.21
0.21 × 0.20 × 0.18
6.382
4.938
17.436
10.472
8.416
55.0
37895
7347 ([I > 1.00σ(I)])
546
0.0399, 0.0415
1.013
55.0
40494
10628 ([I > 1.00σ(I)])
715
0.0764, 0.1570
0.992
54.8
19192
7843 ([I > 1.00σ(I)])
522
0.0366, 0.0400
1.020
55.0
50151
15964 ([I > 0.00σ(I)])
968
0.0435, 0.0479
0.978
54.5
14625
5927 ([I > 1.00σ(I)])
422
0.0479, 0.0607
1.023
no. reflns measd
no. reflns obsd
no. variables
b
R,a RW
GOF
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑wFo ]1/2
.
by following the same procedure as described above for Pye2 by
using 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene14 instead of 1,3-
di(bromomethyl)-2,4,6-triethylbenzene. Yellow powder precipitated
from the final reaction mixture was collected by filtration to give
Pye3 (1.50 g, 26%). IR (KBr, cm-1): 3060, 3026, 3007 (aromatic
C-H), 2957, 2925, 2867, 2792 (aliphatic C-H), 1591, 1568, 1474,
of 1,3-di(bromomethyl)-2,4,6-triethylbenzene. The eluent of silica
gel column chromatography was ethyl acetate (Rf ) 0.2) (3.60 g,
95%). IR (KBr, cm-1): 3060, 3026 (aromatic C-H), 2955, 2925,
2867, 2820, 2787 (aliphatic C-H), 1592, 1577, 1455, 1368, 796,
1
743, 700 (aromatic), 1125, 1114 (C-N). H NMR (CDCl3, 300
MHz): δ 0.64 (9 H, t, J ) 7.6 Hz, Ar-CH2CH3), 2.47 (9 H, s,
-CH3), 2.91 (6 H, q, J ) 7.6 Hz, Ar-CH2CH3), 3.51 (6 H, s,
-CH2-), 3.55 (6 H, s, -CH2-), 3.62 (6 H, s, -CH2-), 6.82 (3
H, d, J ) 7.6 Hz, Py-3H), 7.01 (3 H, d, J ) 7.6 Hz, Py-5H),
7.15-7.28 (18 H, m, Ar-H, Py-4H). HRMS (FAB, pos): m/z )
835.5413 calcd for ([M + H]+, C57H67N6 835.5427).
1
1453, 1434, 743, 699 (aromatic), 1126, 1116 (C-N). H NMR
(CDCl3, 400 MHz): δ 0.93 (9 H, t, J ) 7.6 Hz, Ar-CH2CH3),
2.79-2.91 (12 H, m, Py-CH2CH2-), 3.01 (6 H, t, J ) 7.6 Hz,
Ar-CH2CH3), 3.50 (6 H, s, -CH2-), 3.66 (6 H, s, -CH2-), 6.63
(3 H, d, J ) 7.6 Hz, Py-3H), 6.91 (3 H, ddd, J ) 7.6, 4.8, and 0.8
Hz, Py-5H), 7.09 (10 H, m, Ar-H), 7.15 (3 H, td, J ) 7.6 and
1.6 Hz, Py-4H), 8.37 (3 H, d, J ) 4.8 Hz, Py-6H). HRMS (FAB,
pos): m/z ) 835.5414 calcd for ([M + H]+, C57H67N6 835.5427).
1,3,5-Triethyl-2,4-bis((N-benzyl-N-(6-methylpyridin-2-ylme-
thyl))aminomethyl)benzene (MePym2). To a mixture of N-benzyl-
N-(6-methylpyridin-2-ylmethyl)amine13b (4.0 g, 1.90 mmol) and
K2CO3 (3.0 g, 21.7 mmol) in dry CH3CN (30 mL) was added a
CH3CN solution (40 mL) of 1,3-bis(bromomethyl)-2,4,6-triethyl-
benzene (2.0 g, 5.7 mmol) under anaerobic conditions (Ar), and
the mixture was stirred for 2 days at room temperature. The resulting
precipitates were removed by filtration and washed with CH2Cl2.
The combined organic layer was concentrated by evaporation to
give a yellow residue, from which ligand MePym2 was isolated by
silica gel column chromatography (eluent; ethyl acetate/CHCl3 )
1:9, Rf ) 0.2) (0.58 g, 17%). IR (KBr, cm-1): 3061, 3027 (aromatic
C-H), 2961, 2926, 2870, 2822, 2793 (aliphatic C-H), 1592, 1577,
1455, 1370, 787, 743, 699 (aromatic), 1123, 1111 (C-N). 1H NMR
(CDCl3, 400 MHz): δ 0.63 (3 H, t, J ) 7.6 Hz, Ar-CH2CH3),
1.07 (6 H, t, J ) 7.6 Hz, Ar-CH2CH3), 2.49 (6 H, s, -CH3), 2.65
(4 H, q, J ) 7.6 Hz, Ar-CH2CH3), 2.97 (2 H, q, J ) 7.6 Hz,
Ar-CH2CH3), 3.52 (4 H, s, -CH2-), 3.57 (4 H, s, -CH2-), 3.63
(4 H, s, -CH2-), 6.80 (1 H, s, Ar-H), 6.92 (2 H, d, J ) 7.6 Hz,
Py-3H), 7.17 (2 H, d, J ) 7.6 Hz, Py-5H), 7.20-7.31 (10 H, m,
Ar-H), 7.43 (2 H, t, J ) 7.6 Hz, Py-4H). HRMS (FAB, pos):
m/z ) 611.4108 calcd for ([M + H]+, C42H51N4 611.4114).
1,3,5-Triethyl-2,4,6-tris((N-benzyl-N-(6-methylpyridin-2-ylm-
ethyl))aminomethyl)benzene (MePym3). This compound was
prepared by following the same procedure as described above for
MePym2 using 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene instead
[CuI(Pye2)(CF3SO3)] (1). An acetone solution (3.0 mL) of Pye2
(101.2 mg, 0.166 mmol) was added slowly to an acetone solution
(0.5 mL) of [CuI(CH3CN)4]CF3SO315 (62.4 mg, 0.166 mmol) under
anaerobic conditions (in a glove box, [O2] < 1 ppm, [H2O] < 1
ppm). The suspension was turned to a yellow solution. After the
mixture was stirred for 12 h, insoluble materials were removed by
filtration, and the filtrate was concentrated under reduced pressure.
Addition of Et2O (15 mL) to the residue provided pale yellow
precipitates, which were isolated by decantation as pale yellow
powder (96.3 mg, 70%). Single crystals suitable for X-ray crystal-
lographic analysis were obtained by recrystallization from acetone/
CH2Cl2/hexane. IR (KBr, cm-1): 638, 1031, 1157, 1224, 1254,
1284 (CF3SO3-). HRMS (FAB, pos): m/z ) 673.3333 calcd for
([M + CuI]+, C42H50CuN4 673.3332). Anal. Calcd for [CuI(Pye2)(CF3-
SO3)], C43H50CuF3N4O3S: C, 62.72; H, 6.12; N, 6.80. Found: C,
62.85;H, 6.15; N, 6.86.
[CuI(Pye3)](CF3SO3) (2). A methanol solution (4.5 mL) of Pye3
(79.2 mg, 0.10 mmol) was added slowly to a methanol solution
(0.5 mL) of [CuI(CH3CN)4](CF3SO3) (35.9 mg, 0.10 mmol) in a
glove box ([O2] < 1 ppm, [H2O] < 1 ppm). The mixture turned
into pale yellow. After the mixture was stirred for 24 h, insoluble
materials were removed by filtration. The filtrate was then
concentrated under reduced pressure. Addition of Et2O (10 mL) to
the resulting residue gave pale yellow powder (78.4 mg, 79%).
Single crystals suitable for X-ray analysis were obtained by
recrystallization from acetone/hexane. IR (KBr, cm-1): 637, 1031,
1149, 1275 (CF3SO3-). HRMS (FAB, pos): m/z ) 897.4644 calcd
(15) Kubas, G. J. Inorg. Synth. 1979, 19, 90.
10828 Inorganic Chemistry, Vol. 45, No. 26, 2006

Copper(I) Reaction Centers of Copper Proteins
[CuI2(MePym2)Cl]CuCl2, C42H50Cl3Cu3N4: C, 55.56; H, 5.55; N,
6.17. Found: C, 55.63; H, 5.56; N, 6.10.
Table 2. Selected Bond Lengths (Å) and Angles (deg) of Complexes
1-5a
[CuI3(MePym3)(CF3SO3)(CH3CN)3](CF3SO3)2 (4). An acetone
solution (3.0 mL) of MePym3 (219.4 mg, 0.263 mmol) was added
slowly to an acetone solution (0.5 mL) of [CuI(CH3CN)4]CF3SO3
(297.0 mg, 0.788 mmol) under anaerobic conditions (in a glove
box, [O2] < 1 ppm, [H2O] < 1 ppm). The suspension turned into
a yellow solution. After the mixture was stirred for 30 h, insoluble
materials were removed by filtration. The filtrate was concentrated
under reduced pressure. Addition of Et2O (15 mL) to the resulting
residue gave pale yellow precipitates, which were isolated by
decantation as pale yellow powder (388.0 mg, 92%). Single crystals
suitable for X-ray crystallographic analysis were obtained by
recrystallization from acetone/CH2Cl2 (v:v ) 1:1)/hexane. IR (KBr,
cm-1): 2097 (CtΝ), 638, 1031, 1150, 1224, 1271 (CF3SO3-).
HRMS (FAB, pos): m/z ) 1321.2266 calcd for ([M + 3CuI +
2CF3SO3]+, C59H66Cu3F6N6O6S2 1321.2278). Anal. Calcd for
[CuI3(MePym3)(CH3CN)3](CF3SO3)3, C67H76Cu3F9N9O9S3: C, 49.66;
H, 4.74; N, 7.90. Found: C, 49.57; H, 4.74; N, 7.85.
1
Cu(1)-N(1)
1.8974(18) Cu(1)-N(3)
2.5972(19)
1.900(2)
Cu(1)-O(3)
N(1)-Cu(1)-N(3)
N(3)-Cu(1)-O(1)
165.73(8)
87.26(7)
N(1)-Cu(1)-O(1) 105.90(7)
2
Cu(1)-N(1)
Cu(1)-N(5)
N(1)-Cu(1)-N(3)
N(3)-Cu(1)-N(5)
2.008(5)
2.019(4)
118.28(19)
114.88(19)
Cu(1)-N(3)
2.022(4)
N(1)-Cu(1)-N(5) 124.6(2)
3
Cu(1)-Cl(1)
2.1765(7) Cu(2)-Cl(1)
2.1824(9)
2.065(2)
2.095(2)
Cu(1)-N(1)
2.040(2)
2.008(2)
3.1858(3)
Cu(2)-N(3)
Cu(2)-N(4)
Cu(1)-N(2)
Cu(1)-Cu(2)
Cl(1)-Cu(1)-N(1)
Cl(1)-Cu(1)-N(2)
N(1)-Cu(1)-N(2)
Cu(1)-Cl(1)-Cu(2)
129.98(7)
Cl(1)-Cu(2)-N(3) 128.58(6)
Cl(1)-Cu(2)-N(4) 148.23(5)
145.91(5)
83.34(8)
93.92(3)
N(3)-Cu(2)-N(4)
83.11(8)
Product Analysis. Oxygenation of 3 and Isolation of the
Modified Ligand MePym1-OH. Complex 3 (200.5 mg, 0.221
mmol) was dissolved in 5.0 mL of acetone under Ar. The solution
was then exposed to 16O2 for 24 h at -40 °C using an acetonitrile/
dry ice bath. After removal of the solvent by evaporation, the
resulting materials were dissolved into 20 mL NH3 aq (7%),
extracted with CH2Cl2 (10 mL × 3), and dried over K2CO3. By
removing CH2Cl2 by evaporation, a crude oily material was
obtained. The modified ligand MePym1-OH was isolated from the
crude oily material by silica gel column chromatography (eluent;
ethyl acetate, Rf ) 0.19) (74.7 mg, 84%). IR (KBr, cm-1): 3205
(O-H), 3063, 3030 (aromatic C-H), 2963, 2930, 2872 (aliphatic
C-H), 1593, 1577, 1506, 1456, 783, 743, 698 (aromatic), 1109,
4
molecule 1
molecule 2
Cu(1)-N(1)
2.021(3)
2.163(2)
1.883(3)
2.459(3)
Cu(2)-N(4)
Cu(2)-N(5)
Cu(2)-N(6)
Cu(2)-O(1)
2.008(3)
2.178(2)
1.879(2)
2.367(3)
5.9235(8)
82.00(12)
Cu(1)-N(2)
Cu(1)-N(3)
Cu(1)-O(1)
Cu(1)-Cu(1)*
N(1)-Cu(1)-N(2)
N(1)-Cu(1)-N(3)
N(2)-Cu(1)-N(3)
6.2338(8) Cu(2)-Cu(2)*
82.23(11)
137.62(15)
133.19(14)
N(4)-Cu(2)-N(5)
N(4)-Cu(2)-N(6) 138.62(16)
N(5)-Cu(2)-N(6) 133.23(14)
molecular 3
Cu(3)-N(7)
2.028(4)
2.158(4)
1.885(5)
Cu(3)-N(8)
Cu(3)N(9)
Cu(3)-O(3)
2.371(2)
Cu(3)-Cu(3)*
N(7)-Cu(3)-N(8)
N(7)-Cu(3)-N(9)
N(8)-Cu(3)-N(9)
5.8844(5)
82.32(15)
131.74(17)
136.72(15)
1
1061 (C-N). H NMR (CDCl3, 600 MHz): δ 1.14 (3 H, t, J )
7.6 Hz, -CH2CH3), 1.17 (3 H, t, J ) 7.5 Hz, -CH2CH3), 1.19 (3
H, t, J ) 7.5 Hz, -CH2CH3), 2.55 (3 H, s, Ar-CH3), 2.56 (2 H,
q, J ) 7.6 Hz, -CH2CH3), 2.58 (2 H, q, J ) 7.5 Hz, -CH2CH3),
2.68 (2 H, q, J ) 7.5 Hz, Ar-CH2CH3), 3.66 (2 H, s, Ar-CH2-),
3.74 (2 H, s, Py-CH2-), 3.81 (2 H, s, Ar-CH2-), 6.48 (1 H, s,
Ar-H), 7.01 (1 H, d, J ) 7.7 Hz, Py-5H), 7.15 (1 H, d, J ) 7.7
Hz, Py-3H), 7.24 (1 H, t, J ) 7.3 Hz Ar-H), 7.31 (2 H, t, J )
7.3 Hz, Ar-H), 7.34 (2 H, d, J ) 7.3 Hz, Ar-H), 7.53 (1 H, t, J
) 7.7 Hz, Py-4H), 11.21 (1 H, s, O-H). HRMS (FAB, pos): m/z
) 403.2744 calcd for ([M + H]+, C57H67N6 403.2749). Formation
of another product, N-benzyl-N-(6-methylpyridin-2-ylmethyl)amine,
was confirmed by 1H NMR and mass spectroscopy. 1H NMR
(CDCl3, 300 MHz): δ 2.40 (1 H, bs, N-H), 2.54 (3 H, s, -CH3),
3.86 (2 H, s, Ar-CH2), 3.90 (2 H, s, Py-CH2), 7.02 (1 H, d, J )
7.7 Hz, Py-3H), 7.12 (1 H, d, J ) 7.7 Hz, Py-5H), 7.21-7.38 (5
H, m, Ar-H), 7.52 (1 H, t, J ) 7.7 Hz, Py-4H). HRMS (FAB,
pos): m/z ) 213.1396 calcd for ([M + H]+, C14H17N2 213.1392).
18O-Labeling. The reaction of 3 (1.0 × 10-4 M, 3.0 mL) and
18O2 (introduced by gentle bubbling from 5.0 mL gastight syringe)
was carried out in anhydrous acetone at -40 °C. After 3 h, the
resulting brown solution was reduced in volume by evaporation.
Then, the residue was dissolved into 2 mL NH3 aq (14%), extracted
with CH2Cl2 (3.0 mL × 3), and dried over K2CO3. After removal
of K2CO3 by filtration, the solvent was removed by evaporation.
Formation of the 18O-labeled product, MePym1-18OH, was con-
firmed by ESI-MS (see Figure S3).
6
Cu(1)-N(1)
Cu(1)-O(1)
1.971(2)
Cu(1)-N(2)
1.998(2)
1.9623(19)
2.467(2)
1.9246(18) Cu(1)-O(1)*
3.0040(4) O(1)-O(1)*
Cu(1)-Cu(1)*
N(1)-Cu(1)-N(2)
N(2)-Cu(1)-O(1)
85.32(10)
93.45(9)
N(1)-Cu(1)-O(1)* 105.17(9)
O(1)-Cu(1)-O(1)* 78.78(8)
Cu(1)-O(1)-Cu(1 )* 101.22(9)
a Estimated standard deviations are given in parentheses. Symmetry code:
for complex 4; Cu(1)* ) (1 - x, 1 + x - y, z), Cu(2)* ) (1 - y, 2 + x
- y, z), Cu(3)* ) (2 - y, 2 + x - y, z), for complex 6; X* ) (- x, 1 -
y, 1 - z).
for ([L + CuI]+, C57H66N6CuI1 897.4645). Anal. Calcd for
[CuI(Pye3)](CF3SO3), C58H66Cu1F3N6O3S1: C, 66.48; H, 6.35; N,
8.02. Found: C, 66.25; H, 6.33; N, 7.81.
[CuI2(MePym2)(Cl)]CuCl2 (3). An acetone solution (3.0 mL) of
MePym2 (181.7 mg, 0.297 mmol) was added slowly to a suspension
of CuICl (58.9 mg, 0.595 mmol) in acetone (0.5 mL) under
anaerobic conditions (in a glove box, [O2] < 1 ppm, [H2O] < 1
ppm). The suspension turned into a yellow solution. After the
mixture was stirred for 18 h, insoluble materials were removed by
filtration. The filtrate was concentrated under reduced pressure.
Addition of Et2O (15 mL) to the residue gave pale yellow
precipitates, which were isolated by decantation as pale yellow
powder (175.0 mg, 65%). Single crystals suitable for X-ray
crystallographic analysis were obtained by recrystallization from
acetone/hexane. HRMS (FAB, pos): m/z ) 771.2313 calcd for
([2CuI + M + Cl]+, C42H50ClCu2N4 771.2316). Anal. Calcd for
Oxygenation of 4. Complex 4 (91.4 mg, 5.7 × 10-5 mmol) was
dissolved in deaerated acetone (10 mL), and the solution was cooled
to -40 °C using an acetonitrile/dry ice bath. Then, dry O2 gas was
introduced into the solution at this temperature. The resulting
Inorganic Chemistry, Vol. 45, No. 26, 2006 10829

Ohi et al.
Scheme 1
Mononuclear Copper(I) Complexes. The ligand carrying
two 2-(pyridin-2-yl)ethylamine metal binding sites, Pye2,
afforded mononuclear copper(I) complex 1 in the reaction
with [CuI(CH3CN)4](CF3SO3) as shown in Figure 2. The
crystallographic data and the selected bond lengths and angles
are summarized in Tables 1 and 2, respectively. The copper-
(I) complex exhibits a two-coordinate almost linear structure
with two pyridine nitrogen atoms of the ligand [angle N(1)-
Cu(1)-N(2) ) 165.7°]. Thus, the alkylamine nitrogen atoms
are free from coordination. On the other hand, one of the
oxygen atoms of CF3SO3- weakly interacts with the cuprous
ion (dCu(1)-O(1) ) 2.598 Å). Thus, the coordination geometry
of 1 can also be regarded as a distorted T-shape structure
(the deviation of Cu(1) from the trigonal plane consisted of
N(1), N(2), and O(3) is 0.084 Å). The aromatic ring of the
triethylbenzene spacer is placed just above the copper(I) ion,
although there is no direct interaction between them; the
distance between Cu(1) and one of the nearest carbon atom
of the aromatic ring is 3.694 Å. All the ethyl groups on the
benzene spacer point to the opposite face against the metal-
binding site (Figure 2). It should be noted that complex 1
exhibited a high oxidation potential at 990 mV versus SCE
(Figure S1). Thus, this complex showed virtually no reactiv-
ity toward O2 in contrast to ordinary copper(I) complexes
supported by the didentate ligands.2
solution was stirred for several hours at -40 °C. After the reaction,
evaporation of the solvent gave a crude product, which was
dissolved into 20 mL NH3 aq (7%), extracted with CH2Cl2 (10 mL
× 3), and dried over K2CO3. Identification and quantification of
1
the oxygenated product were carried out by H NMR (internal
Another mononuclear copper(I) complex, 2, was obtained
when Pye3 carrying three 2-(pyridin-2-yl)ethylamine metal
binding groups was treated with [CuI(CH3CN)4](CF3SO3).
The crystal structure of 2 is shown in Figure 3, and the
crystallographic data and the selected bond lengths and angles
are also listed in Tables 1 and 2, respectively. The whole
molecule of 2 exhibits a slightly distorted C3-like symmetry,
and the copper(I) center exhibits a three-coordinate trigonal
planar structure supported by the three pyridine donor groups
of Pye3. The deviation of Cu(1) from the trigonal plane
consisting of N(1), N(3), and N(5) is only 0.176 Å. In this
case as well, the alkylamine nitrogen atoms are free from
coordination, and all the methyl groups of the ethyl substit-
uents are located on the opposite side of the copper
coordination site with respect to the central benzene plane.
Notably, complex 2 also exhibits a high oxidation potential
at 990 mV versus SCE (Figure S2), thus exhibiting virtually
no reactivity toward O2 as in the case of 1. The copper(I)
coordination geometry of 2 and its less O2-reactivity are
similar to those of Riordan’s complex.11e
reference; 1,1,2,2-tetrachloroethane) and mass spectroscopy. Yield
of the oxygenated product, MePym2-OH, was 72% based on the
starting copper(I) complex. IR (KBr, cm-1): 3361 (O-H), 3055,
3001 (aromatic C-H), 2959, 2869, 2826 (aliphatic C-H), 1590,
1569, 1473,1438, 779, 757, 697 (aromatic), 1284, 1245, 1185, 1104
(C-N). 1H NMR (CDCl3, 600 MHz): δ 0.86 (3 H, t, J ) 7.5 Hz,
-CH2CH3), 0.93 (3 H, t, J ) 7.4 Hz, -CH2CH3), 1.13 (3 H, t, J
) 7.4 Hz, -CH2CH3), 2.48 (3 H, s, -CH3), 2.53 (3 H, s, -CH3),
2.64 (2 H, q, J ) 7.4 Hz, -CH2CH3), 2.75 (4 H, br q, -CH2CH3),
3.54 (2 H, s, Ar-CH2-), 3.55 (2 H, s, Ar-CH2-), 3.61 (2 H, s,
Ar-CH2-), 3.67 (2 H, s, Py-CH2-), 3.70 (2 H, s, Py-CH2-),
3.80 (2 H, s, Ar-CH2-), 6.91 (1 H, d, J ) 7.7 Hz, Py-5H), 6.99
(1 H, d, J ) 7.7 Hz, Py-5H), 7.14 (1 H, d, J ) 7.7 Hz, Py-3H),
7.18-7.34 (10 H, m, Ar-H), 7.22 (1 H, d, J ) 7.7 Hz, Py-3H),
7.47 (1 H, t, J ) 7.7 Hz, Py-4H), 7.51 (1 H, t, J ) 7.7 Hz, Py-
4H), 11.3 (1 H, bs, O-H). HRMS (FAB, pos): m/z ) 626.3962
calcd for ([M]+, C57H67N6 626.3985). Another product, N-benzyl-
N-(6-methylpyridin-2-ylmethyl)amine, was confirmed by 1H NMR
and mass spectroscopy as described above.
Results and Discussion
Biological Relevance of Mononuclear Copper(I) Com-
plexes 1 and 2. It is interesting to note that the trigonal planar
structure of 2 closely resembles the structure of the CuA
site of the reduced form of peptidylglycine R-hydroxylating
monooxygenase (PHM) shown in Figure 4, where the
copper(I) ion is ligated by three histidine imidazoles forming
a distorted trigonal planar geometry (deviation of CuA from
the trigonal plane consisting of three histidine imidazole
nitrogen atoms is 0.264 Å).16
Ligand Synthesis. The bis(didentate) ligands, Pye2 and
MePym2 (Chart 1), were prepared by the SN2 reaction
between 1,3-bis(bromomethyl)-2,4,6-triethylbenzene14 and
N-benzyl-N-(2-(pyridin-2-yl)ethyl)amine13a or N-benzyl-N-
(6-methylpyridin-2-ylmethyl)amine,13b respectively, in the
presence of potassium carbonate as a base in dry acetonitrile
(Scheme 1). The tris(didentate) ligands, Pye3 and MePym3
(Chart 1), were synthesized in a similar manner using 1,3,5-
tris(bromomethyl)-2,4,6-triethylbenzene14 instead of 1,3-bis-
(bromomethyl)-2,4,6-triethylbenzene. Treatment of the re-
spective ligands with [CuI(CH3CN)4]X (X ) CF3SO3 or PF6)
or CuICl under anaerobic conditions (Ar) gave the corre-
sponding copper(I) complexes 1-4 as follows.
The copper(I) ion at the CuB site in the fully reduced form
of cytochrome c oxidase (CcO) is also ligated by three
(16) Prigge, S. T.; Kolhekar, A. S.; Eipper, B. A.; Mains, R. E.; Amzel, L.
M. Nat. Struct. Biol. 1999, 6, 976-983.
10830 Inorganic Chemistry, Vol. 45, No. 26, 2006

Copper(I) Reaction Centers of Copper Proteins
Figure 2. (A) ORTEP drawing of [CuI(Pye2)(CF3SO3)] (1) showing 50% probability thermal ellipsoids. The hydrogen atoms are omitted for clarity. (B)
Expanded view of the copper(I) center of 1.
Figure 3. (A) ORTEP drawing of [CuI(Pye3)](CF3SO3) (2) showing 50% probability thermal ellipsoids. The counteranion and the hydrogen atoms are
omitted for clarity. (B) Expanded view of the copper(I) center of 2.
histidine imidazoles. In this case, the coordination geometry
is more like a distorted T-shape like complex 1 (deviation
of CuB from the trigonal plane consisted of three histidine
imidazole nitrogen atoms is 0.177 Å) (Figure 5).17
CuB-OOH. Judging from the high oxidation potential of
complex 2, we would suggest that the electron transfer from
CuA to O2-bound CuB might not take place so easily. Thus,
the recent mechanism involving CuB-OO• would be more
plausible as compared to the previous mechanism involving
CuB-OOH.
In the CcO system as well, the timing of electron-transfer
from reduced CuB to the O2-bound heme a3 is a mechanistic
issue in the multielectron reduction process of O2.4,17 In the
model systems, the reaction of FeII(Por)-CuI(L) model
compounds and dioxygen mostly provided peroxo-bridged
FeIII(Por)-O2-CuII(L) species. In those systems, TPA or
PY2 type capping ligands are employed to mimic the Cu-
binding site (CuB). Such ligands have been well demon-
strated to produce copper(I) complexes exhibiting high
reactivity toward O2.2 On the other hand, it has been reported
In the case of PHM, it is believed that one electron stored
at CuA is transferred to CuB where O2 is bound and activated
for the substrate oxygenation. One of the most important
but still unresolved mechanistic issues is the timing of
electron transfer from CuA to CuB. If the electron at reduced
CuA is transferred to the O2-bound CuB before the substrate
oxygenation, a copper(II)-hydroperoxo (CuB-OOH) inter-
mediate may be the reactive species.18 However, the recent
enzymatic and theoretical studies have suggested that the
electron transfer occurs after the C-H bond activation of
the substrate.19 In this case, a copper(II)-superoxo (CuB-
OO•) intermediate could be the real active species rather than
(17) Tsukihara, T.; Shimokata, K.; Katayama, Y.; Shimada, H.; Muramoto,
K.; Aoyama, H.; Mochizuki, M.; Shinzawa-Itoh, K.; Yamashita, E.;
Yao, M.; Ishimura, Y.; Yoshikawa, S. Proc. Natl. Acad. Sci. U.S.A.
2003, 100, 15304-15309.
(19) (a) Chen, P.; Solomon, E. I. J. Am. Chem. Soc. 2004, 126, 4991-
5000. (b) Francisco, W. A.; Wille, G.; Smith, A. J.; Merkler, D. J.;
Klinman, J. P. J. Am. Chem. Soc. 2004, 126, 13168-13169. (c)
Bauman, A. T.; Yukl, E. T.; Alkevich, K.; Ashley, L. McCormack,
A. L.; Blackburn, N. J. J. Biol. Chem. 2006, 281, 4190-4198.
(18) Klinman, J. P. Chem. ReV. 1996, 96, 2541-2561.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10831

Ohi et al.
Figure 6. ORTEP drawing of [CuI2(MePym2)(Cl)]CuICl2 (3) showing 50%
probability thermal ellipsoid. The counteranion (CuCl2-) and the hydrogen
atoms are omitted for clarity.
to His240, one of the ligands of CuB.20 Judging from the
geometric similarity between CuB of CcO and our model
complex 1 having the high oxidation potential (990 mV vs
SCE), the participation of CuB for the direct reduction of
O2 might be unlikely as suggested by the enzymatic
studies.17,20
Figure 4. (A) Active site structure of the reduced form of PHM (PDB ID
code 3PHM) and (B) expanded view of the reduced CuA center.
Dinuclear Copper(I) Complex. In contrast to the case
of Pye2 that gave mononuclear copper(I) complex 1 (Figure
2), bis(didentate) ligand MePym2 provided dinuclear copper-
(I) complex [CuI2(MePym2)(Cl)]CuCl2 (3) in the reaction with
CuICl.21 The crystal structure of 3 is shown in Figure 6, and
the crystallographic data and the selected bond lengths and
angles are summarized in Tables 1 and 2, respectively. The
copper(I) ions are bridged by chloride ion and exhibit largely
distorted trigonal planar structure with the N2Cl donor set
(deviation of Cu(1) and Cu(2) from the N2Cl trigonal plane
are 0.098 and 0.032 Å, respectively). The angle of Cu-Cl-
Cu is 93.9°, and the Cu-Cu distance is 3.186 Å.
So far, X-ray structures of the deoxy dicopper(I) forms of
type-3 copper proteins have been reported for Limulus
polyphemus hemocyanin (Hc),22 sweet potato catechol oxi-
dase (CO),23 and bacterial tyrosinase (Tyr).24 In all cases,
each copper(I) ion is ligated by three histidine imidazoles,
and the CuI-CuI distances are much longer (4.6, 4.4, and
4.1 Å, respectively).22-24 Thus, dinuclear copper(I) complex
3 could not be a precise structure model for those reduced
type-3 copper proteins. Nonetheless, complex 3 exhibited a
notable reactivity toward O2, providing a functional model
system of tyrosinase (vide infra).
Trinuclear Copper(I) Complex. Trinuclear copper(I)
complex [CuI3(MePym3)(CF3SO3)(CH3CN)3](CF3SO3)2 (4)
was obtained when tris(didentate) ligand MePym3 was
(20) Kitagawa, T.; Ogura, T. Prog. Inorg. Chem. 1997, 45, 431-479.
(21) The reaction of MePym2 and [CuI(CH3CN)4]PF6 also gave a similar
dicopper(I) complex [CuI2(MePym2)(CH3CN)2](PF6)2‚1/2CH2Cl2.
(22) Hazes, B.; Magnus, K. A.; Bonaventura, C.; Bonaventura, J.; Dauter,
Z.; Kalk, K. H.; Hol, W. G. J. Protein Sci. 1993, 2, 597-619.
(23) Klabunde, T.; Eicken, C.; Sacchettini, J. C.; Krebs, B. Nat. Struct.
Biol. 1998, 5, 1084-1090.
Figure 5. (A) Active site structure of the fully reduced CcO (PDB ID
code 1V55) and (B) expanded view of the reduced CuB center.
that CuB in the native enzyme is kept as a copper(I) reduced
state during the catalytic cycle, while the electrons are
supplied to heme a3 site through Tyr244 covalently bound
(24) Matoba, Y.; Kumagai, T.; Yamamoto, A.; Yoshitsu, H.; Sugiyama,
M. J. Biol. Chem. 2006, 281, 8981-8990.
10832 Inorganic Chemistry, Vol. 45, No. 26, 2006

Copper(I) Reaction Centers of Copper Proteins
Figure 9. Experimental and calculated peak envelopes in the positive-
ion electrospray mass spectra of of the monomeric copper(II) compound 5
(m/z ) 464) and the dimeric copper(II) compound 6‚Cl (m/z ) 965).
Figure 7. ORTEP drawings of (A) [CuI3(MePym3)(CF3SO3)(CH3CN)3]-
(CF3SO3)2 (4) (molecule 1) and (B) its closed view of the trinuclear copper-
(I) site showing 50% probability thermal ellipsoids. The noncoordinated
counteranions and hydrogen atoms are omitted for clarity.
Figure 10. ORTEP drawing of [CuII MePym1-O)2](PF6)2 (6) showing
(
2
50% probability thermal ellipsoids. The counteranions and the hydrogen
atoms are omitted for clarity.
Figure 8. Active site structure of the fully reduced form of ascorbate
oxidase (PDB ID code 1ASO).
∼2.4 Å). The trigonal arrangement of three copper(I) ions
in 4 resembles that of the trinuclear copper reaction center
of the fully reduced form of ascorbate oxidase (Figure 8),
even though the Cu-Cu distance of ∼6.0 Å in 4 (averaged
value of the three crystallographically independent molecules
1-3, see Table 2) is relatively longer than that in the enzyme
(Cu2-Cu3, 5.087 Å; Cu2-Cu4, 4.461 Å; Cu3-Cu4, 4.031
Å) (Figure 8).26 This complex showed a similar reactivity
toward O2 as discussed below.
Copper(I)-O2 Reactivity of Dinuclear and Trinuclear
Copper(I) Complexes 3 and 4. As noted in the above
section, mononuclear copper(I) complexes 1 and 2 showed
no reactivity toward O2 due to their high oxidation potential
employed. The crystal structure of 4 is presented in Figure
7, and the crystallographic data and the selected bond lengths
and angles are listed in Tables 1 and 2, respectively. The
molecule has C3 symmetry, and one of the counteranions,
-
CF3SO3 , is encapsulated in the molecular cavity constructed
by the three CuI(MePym)(CH3CN) units.25 Each copper(I) ion
exhibits a trigonal pyramidal structure with two nitrogen
atoms N(1) and N(2) of the ligand and one nitrogen atom
N(3) of the coordinated acetonitrile molecule occupying the
trigonal basal plane and one of the oxygen atoms O(1) of
-
the encapsulated CF3SO3 at the axial position (dCu(1)-O(1)
)
(25) Recently, Anslyn and coworkers developed a shape selective anion
binding using a similar type of receptor consisted of the trietylbenzene
spacer.5,6c,e,8b
(26) Messerschmidt, A.; Luecke, H.; Huber, R. J. Mol. Biol. 1993, 230,
997-1014.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10833

Ohi et al.
Scheme 2
(990 mV vs SCE). On the other hand, dinuclear copper(I)
complex 3 and trinuclear copper(I) complex 4 reacted with
O2 at -40 °C in acetone to induce an aromatic ligand
hydroxylation reaction as follows. In the reaction of 3, for
example, the ESI-MS of the final reaction mixture gave new
peaks at m/z ) 464 and 965, the peak positions and isotope
distribution patterns of which are consistent with the chemical
formulations of the mononuclear and dinuclear copper(II)
complexes of the hydroxylated ligand MePym1-O-, [CuII-
Chart 2
(
MePym1-O)]+ (5), and {[CuII (MePym1-O)2]Cl}+ (6‚Cl),
2
hydroxylation reaction in the dinuclear copper(I)-xylyl
complex by molecular oxygen,27 where electrophilic aromatic
substitution reaction by a (µ-η2:η2-peroxo)dicopper(II) in-
termediate has been proposed.28 Although we have not yet
succeeded in detecting the active oxygen intermediate, the
reaction mechanism could be the same as that proposed for
Karlin’s aromatic ligand hydroxylation reaction as illustrated
in Scheme 2. Namely, oxygenation of 3 may give an active
oxygen intermediate such as the (µ-η2:η2-peroxo)dicopper-
(II) complex a, from which electrophilic attack of the peroxo
group by the aromatic ring of the spacer occurs intramo-
lecularly to give cationic intermediate b. The 1,2-migration
of the ethyl group proceeds to give rearranged intermediate
c, from which one of the MePym metal-binding moieties is
released by the electron flow during the rearrangement shown
in Scheme 2. The NIH-shift of the ethyl group is the strong
evidence for the formation of the cationic intermediate b as
suggested in Karlin’s reaction.28
respectively (Figure 9). From the final reaction mixture, the
hydroxylated ligand MePym1-OH was isolated in 84% yield
by the workup treatment with aqueous ammonia (for detailed
MS and NMR analyses, see Experimental Section).
In addition, an isotope labeling experiment using 18O2
instead of 16O2 has clearly demonstrated that the oxygen atom
introduced into the hydroxylated ligand originated from
molecular oxygen as indicated in Figure S3. Furthermore,
single crystals of the dimeric copper(II) complex 6‚(PF6)2
suitable for the X-ray analysis have been isolated from the
final reaction mixture by allowing it to stand for a couple of
days. The crystal structure of 6‚(PF6)2 is shown in Figure
10 together with the selected bond lengths and angles
presented in Tables 1 and 2, respectively.
Complex 6 is a dimeric copper(II) complex of the modified
ligand MePym1-OH, where the deprotonated phenolate
oxygen atom introduced into the ligand by the oxygenation
reaction acts as a bridging ligand to form a rhombic Cu2O2
core structure. The compound has a center of symmetry in
the Cu2O2 core, and the cupric ion has a largely distorted
four-coordinate square planar structure with the N2O2 donor
A similar reaction (aromatic ligand hydroxylation ac-
companied by an NIH-shift of ethyl substituent) occurred,
when the reaction of the trinuclear copper(I) complex 4 and
O2 was carried out under the same experimental conditions.
Thus, the hydroxylated ligand MePym2-OH (Chart 2) was
obtained from the final reaction mixture by the ordinary
1
set. It should be noted that, as the H NMR suggested, one
of the ethyl groups of the trietylbenzene spacer migrated from
its original position to the neighboring carbon atom and the
hydroxylation occurred just on the carbon where the ethyl
group was originally attached. It is also obvious that one of
the MePym metal binding groups is detached from the
benzene spacer. Thus, the NIH-type alkyl rearrangement
occurred during the aromatic ligand hydroxylation reaction.
Thus, the overall chemistry of the present system is es-
sentially the same as that of Karlin’s aromatic ligand
(27) Nasir, M. S.; Cohen, B. I.; Karlin, K. D. J. Am. Chem. Soc. 1992,
114, 2482-2494.
(28) (a) Cruse, R. W.; Kaderli, S.; Karlin, K. D.; Zuberbu¨hler, A. D. J.
Am. Chem. Soc. 1988, 110, 6882-6883. (b) Karlin, K. D.; Nasir, M.
S.; Cohen, B. I.; Cruse, R. W.; Kaderli, S.; Zuberbu¨hler, A. D. J. Am.
Chem. Soc. 1994, 116, 1324-1336. (c) Mahapatra, S.; Kaderli, S.;
Llobet, A.; Neuhold, Y.-M.; Palanche´, T.; Halfen, J. A.; Young, V.
G. Jr.; Kaden, T. A.; Que, L. Jr.; Zuberbu¨hler, A. D.; Tolman, W. B.
Inorg. Chem. 1997, 36, 6343-6356.
10834 Inorganic Chemistry, Vol. 45, No. 26, 2006

Copper(I) Reaction Centers of Copper Proteins
Chart 3
reactive intermediate as recently demonstrated by Stack et
al.29 Unfortunately, we could not detect such active oxygen
intermediates during the course of the reaction.
It should be emphasized that the 1,3,5-triethylbenzene
spacer induces significantly large effects on the structure and
O2-reactivity of the copper(I) complexes. For example, we
have already demonstrated that the didentate ligands PyeR
(Chart 3) provided mononuclear copper(I) complexes, ex-
hibiting significantly high reactivity toward O2.30 In this case,
the oxygenated product was a bis(µ-oxo)dicopper(III) com-
plex, in which aliphatic hydroxylation occurred effectively
on the ligand sidearm.31 On the other hand, Pye2 and Pye3
containing 1,3,5-triethylbenzene spacer (Chart 1) afforded
the mononuclear copper(I) complexes with virtually no
reactivity toward O2. Moreover, the copper(I) complexes
supported by MePymR (Chart 3) also gave a bis(µ-oxo)-
dicopper(III) complex as the oxygenation product,32 while
MePym2 and MePym3 involving the triethylbenzene spacer
produced an active oxygen intermediate which exhibits a
peroxo-like reactivity (aromatic ligand hydroxylation, see
Scheme 2). The importance of the ethyl substituents on the
benzene spacer is also evident, since the PY2 type ligand
connected to the 1,3,5-positions of simple benzene spacer
(without the ethyl substituents) produced a complicated
hexanuclear copper(II) complex in the oxygenation reaction
of the copper(I) complex.33
workup treatment with NH4OH, in which one of the MePym
metal binding groups of MePym3 was removed and one of
the ethyl groups migrated to the next carbon where MePym
group was originally attached before the ligand modification
(for detailed product analysis, see Experimental Section).
Conclusion
In this study, we have developed a series of pyridylalky-
lamine ligands containing a 1,3,5-triethylbenzene spacer,
Pye2, Pye3, MePym2, and MePym3 (Chart 1). The ligands
consisting of the 2-(pyridin-2-yl)ethylamine didentate metal-
binding moiety Pye2 and Pye3 gave mononuclear copper(I)
complexes 1 and 2, exhibiting distorted three-coordinate
T-shape structure and trigonal planar structure, respectively.
These coordination geometries resemble those of CuA site
of PHM (peptidylglycine R-hydroxylating monooxygenase)
and CuB site of CcO (cytochrome c oxidase), providing good
structural models for those enzymes. Notably, copper(I)
complexes 1 and 2 showed high oxidation potential at 990
mV versus SCE, thus exhibiting virtually no reactivity toward
O2. These results suggest that the mononuclear copper site
in PHM and CcO may not be a good electron donor for the
multielectron reduction process of O2.
On the other hand, the ligands carrying the (6-methylpy-
ridin-2-yl)methylamine didentate metal binding sites such
as MePym2 and MePym3 provided the dinuclear and trinuclear
copper(I) complexes with a distorted trigonal planar geometry
and a trigonal pyramidal geometry, respectively. These
complexes showed moderate reactivity toward O2 to induce
an aromatic ligand hydroxylation reaction involving an NIH-
shift of one of the ethyl substituents on the aromatic spacer.
The NIH-shift is strong evidence of the electrophilic aromatic
substitution mechanism shown in Scheme 2. If so, the most
plausible active oxygen intermediate is (µ-η2:η2-peroxo)-
dicopper(II) complex a (Scheme 2) as demonstrated in
Karlin’s aromatic ligand hydroxylation reaction.28 However,
a bis(µ-oxo)dicopper(III) complex could also be a possible
Acknowledgment. This work was financially supported
in part by Grants-in-Aid for Scientific Research (No.
17350086, 18037062, and 18033045) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
Supporting Information Available: Crystallographic data in
CIF format, cyclic voltammograms, and ESI-MS spectra. This mater-
ial is available free of charge via the Internet at http://pubs.acs.org.
IC061509J
(29) Mirica, L. M.; Vance, M.; Rudd, D. J.; Hedman, B.; Hodgson, K. O.;
Solomon, E. I.; Stack, T. D. P. Science 2005, 308, 1890-1892.
(30) Taki, M.; Teramae, S.; Nagatomo, S.; Tachi, Y.; Kitagawa, T.; Itoh,
S.; Fukuzumi, S. J. Am. Chem. Soc. 2002, 124, 6367-6377.
(31) Itoh, S.; Taki, M.; Nakao, H.; Holland, P. L.; Tolman, W. B.; Que,
L., Jr.; Fukuzumi, S. Angew. Chem., Int. Ed. 2000, 39, 398-400.
(32) Taki, M.; Itoh, S. Unpublished results.
(33) Karlin, K. D.; Gan, Q. F.; Farooq, A.; Liu, S.; Zubieta, J. Inorg. Chem.
1990, 29, 2549-2551.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10835