Modulation of coordination chemistry in copper(I) complexes supported by bis[2-(2-pyridyl)ethyl]amine-based tridentate ligands

Article abstract of DOI:10.1021/ic010625d

Structure and physicochemical properties of copper(I) complexes of the tridentate ligands L² (N,N-bis[2-(6-methylpyridin-2-yl)ethyl]phenethylamine) and L³ (N,N-bis[2-(2-pyridyl)ethyl]-β-methylphenethylamine) have been examined to obtain deeper insights into modulation of the coordination chemistry of copper(I) complexes. [Cu^I(L²)(CH₃CN)](CIO₄) (2·CH₃CN) has a distorted tetrahedral geometry, which consists of three nitrogen atoms of the ligand and one nitrogen atom of the bound CH₃CN. Steric repulsion between the 6-methyl group on the pyridine nucleus of L² and the metal ion of the complex prevents the cuprous complex from adaptation to a three-coordinate geometry which must have a shorter Cu-N(pyridine) distance (@O1.88 A). Thus, the four-coordinate copper(I) complex (2·CH₃CN) with a longer Cu-N bond (1.98@O2.13 A) becomes favorable, resulting in rather strong binding of CH₃CN to the metal ion. In [Cu^I(L³)](CIO₄) (3), there is a Cu^I-π interaction between the cuprous ion and the phenyl group of the ligand sidearm. Such a copper(I)-arene interaction is essentially weak, but is significantly stabilized in complex 3. The methyl group at the benzylic position of L³ reduces the degree of freedom of sidearm rotation to make the phenyl group stick on the cuprous ion. Thus, the reactivity of the copper(I) complexes of L² and L³ toward dioxygen is significantly diminished, showing sharp contrast to the high reactivity of the copper(I) complex supported by a similar tridentate ligand L¹ (N,N-bis[2-(2-pyridiyl)ethyl] phenethylamine).

Full text of DOI:10.1021/ic010625d

6604
Inorg. Chem. 2001, 40, 6604-6609
Modulation of Coordination Chemistry in Copper(I) Complexes Supported by
Bis[2-(2-pyridyl)ethyl]amine-Based Tridentate Ligands
Takao Osako,† Yoshimitsu Tachi,† Masayasu Taki,‡ Shunichi Fukuzumi,*,‡ and
Shinobu Itoh*,†
Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138, Sugimoto,
Sumiyoshi-ku, Osaka, 558-8585, Japan, and Department of Material and Life Science, Graduate School
of Engineering, Osaka University, CREST, Japan Science and Technology Corporation,
2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
ReceiVed June 12, 2001
Structure and physicochemical properties of copper(I) complexes of the tridentate ligands L2 (N,N-bis[2-(6-
methylpyridin-2-yl)ethyl]phenethylamine) and L3 (N,N-bis[2-(2-pyridyl)ethyl]-â-methylphenethylamine) have been
examined to obtain deeper insights into modulation of the coordination chemistry of copper(I) complexes. [CuI-
(L2)(CH3CN)](ClO4) (2‚CH3CN) has a distorted tetrahedral geometry, which consists of three nitrogen atoms of
the ligand and one nitrogen atom of the bound CH3CN. Steric repulsion between the 6-methyl group on the
pyridine nucleus of L2 and the metal ion of the complex prevents the cuprous complex from adaptation to a
three-coordinate geometry which must have a shorter Cu-N(pyridine) distance (∼1.88 Å). Thus, the four-coordinate
copper(I) complex (2‚CH3CN) with a longer Cu-N bond (1.98∼2.13 Å) becomes favorable, resulting in rather
strong binding of CH3CN to the metal ion. In [CuI(L3)](ClO4) (3), there is a CuI-π interaction between the
cuprous ion and the phenyl group of the ligand sidearm. Such a copper(I)-arene interaction is essentially weak,
but is significantly stabilized in complex 3. The methyl group at the benzylic position of L3 reduces the degree
of freedom of sidearm rotation to make the phenyl group stick on the cuprous ion. Thus, the reactivity of the
copper(I) complexes of L2 and L3 toward dioxygen is significantly diminished, showing sharp contrast to the
high reactivity of the copper(I) complex supported by a similar tridentate ligand L1 (N,N-bis[2-(2-pyridiyl)ethyl]-
phenethylamine).
Introduction
per(II) complex (A in Scheme 1), generated by treating
[CuI(L1)]+ with O2 or [CuII(L1)]2+ with H2O2 at a low
temperature (-80 °C), decomposes, leading to benzylic hy-
droxylation of its ligand sidearm to give L1-OH (Chart 1).
Mechanistic studies have suggested that a bis(µ-oxo)dicopper-
(III) intermediate (B in Scheme 1), formed from the peroxo
complex (A) by O-O bond homolysis, is the actual active
oxygen species for the aliphatic C-H bond activation.5 Re´glier
et al recently investigated a closely related system.6
The structure and reactivity of copper complexes has been
demonstrated to alter significantly by introducing a small change
in the supporting ligands. One of the most remarkable examples
is found in copper/dioxygen chemistry with the TMPA [tris(2-
pyridylmethyl)amine] ligand system (Chart 1). The copper(I)
complex of TMPA itself afforded a (µ-1,2-peroxo)dicopper(II)
Great success has been brought about in copper/dioxygen
chemistry using a variety of bis[2-(2-pyridyl)ethyl]amine-based
tridentate ligands.1 Karlin and co-workers first developed a series
of dinuclear copper(I) complexes supported by dinucleating
ligands containing two bis[2-(2-pyridyl)ethyl]amine units, and
they succeeded in mimicking the structures and functions of
the active sites of hemocyanin and tyrosinase (reversible O2
binding and aromatic ligand hydroxylation).2 Such pioneering
works by Karlin and co-workers attracted many researchers in
the related area to open the new field of copper/dioxygen
bioinorganic chemistry.1,3 Our contribution in this field is finding
quantitative aliphatic ligand hydroxylation with a mononuclear
copper complex of ligand L1 (N,N-bis[2-(2-pyridiyl)ethyl]-
phenethylamine, Chart 1).4 Namely, a (µ-η2: η2-peroxo)dicop-
(3) (a) Kitajima, N.; Moro-oka, Y. Chem. ReV. 1994, 94, 737-757. (b)
Tolman, W. B. Acc. Chem. Res. 1997, 30, 227-237. (c) Blackman,
A. G.; Tolman, W. B. In Metal-Oxo and Metal-Peroxo Species in
Catalytic Oxidations; Meunier, Ed.; Springer: Berlin, 2000; pp 179-
211. (d) Mahadevan, V.; Gebbink, R. K.; Stack, T. D. P. Curr. Opin.
Chem. Biol. 2000, 4, 228-234.
(4) Itoh, S.; Kondo, T.; Komatsu, M.; Ohshiro, Y.; Li, C.; Kanehisa, N.;
Kai, Y.; Fukuzumi, S. J. Am. Chem. Soc. 1995, 117, 4714-4715.
(5) (a) Itoh, S.; Nakao, H.; Berreau, L. M.; Kondo, T.; Komatsu, M.;
Fukuzumi, S. J. Am. Chem. Soc. 1998, 120, 2890-2899. (b) 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.
(6) (a) Blain, I.; Bruno, P.; Giorgi, M.; Lojou, E.; Lexa, D.; Re´glier, M.
Eur. J. Inorg. Chem. 1998, 1297-1304. (b) Blain, I.; Giorgi, M.; De
Riggi, I.; Re´glier, M. Eur. J. Inorg. Chem. 2000, 393-398. (c) Blain,
I.; Giorgi, M.; De Riggi, I.; Re´glier, M. Eur. J. Inorg. Chem. 2001,
205-211.
* To whom correspondence should be addressed.
† Osaka City University.
‡ Osaka University.
(1) (a) Kopf, M.-A.; Karlin, K. D. In Biomimetic Oxidations Catalyzed
by Transition Metal Complexes; Meunier, B., Ed.; Imperial College
Press: London, 2000; pp 309-362. (b) Karlin, K. D.; Zuberbu¨hler,
A. D. In Bioinorganic Catalysis, 2nd ed.; Reedijk, J., Bouwman, E.,
Eds.; Marcel Dekker: New York, 1999; pp 469-534. (c) Karlin, K.
D.; Kaderli, S.; Zuberbu¨hler, A. D. Acc. Chem. Res. 1997, 30, 139-
147. (d) Fox, S.; Karlin, K. D. In ReactiVe Oxygen in Biochemistry;
Valentine, J. S., Foote, C. S., Greenberg, A., Liebman, J. F., Eds.;
Chapman & Hall: Glasgow, 1995; pp 188-231. (e) Schindler, S. Eur.
J. Inorg. Chem. 2000, 2311-2326.
(2) (a) Karlin, K. D.; Haka, M. S.; Cruse, R. W.; Gultneh, Y. J. Am. Chem.
Soc. 1985, 107, 5828-5829. (b) Karlin, K. D.; Gultneh, Y.; Hutch-
inson, J. P.; Zubieta, J. J. Am. Chem. Soc. 1982, 104, 5240-5242.
10.1021/ic010625d CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/13/2001

Modulating Coordination Chemistry in Copper(I) Complexes
Inorganic Chemistry, Vol. 40, No. 26, 2001 6605
Chart 1
tion in the ligand triggers a drastic change in the oxidation state
of metal (CuII versus CuIII) and oxygen (peroxo versus oxo).
Such a drastic change in the structure of copper complexes
induced by the 6-methyl substituent has also been found in a
copper(I)/disulfide system, where ligand L4 (Chart 1) affords a
disulfide-dicopper(I) complex (type D in Scheme 1), and
reductive cleavage of the S-S bond of the ligand occurs to
give a bis(µ-thiolato)dicopper(II) complex (type E in Scheme
1) in the case of ligand L5 (Chart 1).9 Such prominent effects
of the 6-methyl substituent of the pyridyl ligands have also been
observed in the related iron chemistry.10
Effects of the alkyl linker chain in the tetradentate ligand
system (methylene in TMPA versus ethylene in TEPA, tris(2-
pyridylethyl)amine, Chart 1) have also been investigated to
demonstrate that the longer ethylene linker of TEPA can adapt
its cuprous complex to a tetrahedral geometry, stabilizing the
Cu(I) state of the complex.11 Thus, the copper(I) complex of
TEPA does not react with O2; this is in sharp contrast to the
high reactivity of the copper(I) complex of TMPA.7 In the
copper(I)/disulfide case as well, ligand L6 (Chart 1) with the
ethylene linker between the tertiary amine nitrogen and the
pyridine nucleus affords a different type of disulfide-dicopper-
(I) complex (type F in Scheme 1).9
In our continuing effort to clarify the ligand effects on the
copper/dioxygen reactivity, we report herein the substituent
effects on the structure and reactivity of copper(I) complexes
supported by bis[2-(2-pyridyl)ethyl]amine-based tridentate ligands,
L2 and L3 (Chart 1). Only one methyl substitution either at the
6-position of the pyridine nucleus or at the benzylic position of
the ligand sidearm of L1 resulted in a drastic change in the
structure and reactivity of the copper(I) complexes, providing
further insight into the ligand control of copper/dioxygen
chemistry.
Results and Discussion
Synthesis of Ligands and Copper(I) Complexes. Ligands
L2 and L3 were prepared by a Michael addition of phenethyl-
amine to 6-methyl-2-vinylpyridine and of â-methylphenethyl-
amine to 2-vinylpyridine, respectively, in refluxing methanol
under acidic conditions. The copper(I) complexes of L2 and L3
were obtained by treating the ligand with an equimolar amount
of [CuI(CH3CN)4](ClO4) in CH2Cl2 under anaerobic conditions
(in a glovebox). All compounds gave satisfactory results in the
elemental and MS analyses (see Experimental).
Scheme 1
Crystal Structure. Crystal structures of the copper(I) com-
plexes of ligands L2 and L3, [CuI(L2)(CH3CN)](ClO4) (2‚CH3-
CN) and [CuI(L3)](ClO4) (3), have been determined by X-ray
crystallographic analysis, as shown in Figures 1 and 2. The
crystallographic data and selected bond lengths and angles are
summarized in Table 1 and Tables 2 and 3, respectively. Despite
our great efforts, single crystals of the copper(I) complex of
ligand L1, [CuI(L1)](ClO4) (1),5a suitable for X-ray crystal-
lographic analysis have not been obtained so far. Thus, we used
complex (C in Scheme 1) in the reaction with O2 at a low
temperature,7 while introduction of 6-methyl group into two of
the three pyridine nuclei of the ligand (Me2TMPA:bis(6-methyl-
2-pyridylmethyl)(2-pyridylmethyl)amine, Chart 1) resulted
in formation of a bis(µ-oxo)dicopper(III) complex (B) under
similar experimental conditions.8 In this case, a small perturba-
(9) Itoh, S.; Nakagawa, M.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123,
4087-4088.
(10) (a) Zang, Y.; Elgren, T. E.; Dong, Y.; Que, L., Jr. J. Am. Chem. Soc.
1993, 115, 811-813. (b) Que, L., Jr.; Dong, Y. Acc. Chem. Res. 1996,
29, 190-196. (c) Zheng, H.; Zang, Y.; Dong, Y.; Young, V. G., Jr.;
Que, L., Jr. J. Am. Chem. Soc. 1999, 121, 2226-2235. (d) Chen, K.;
Que,L., Jr. Angew. Chem., Int. Ed. 1999, 38, 2227-2229. (e) Costas,
M.; Tipton, A. K.; Chen, K.; Jo, D.-H.; Que, L., Jr. J. Am. Chem.
Soc. 2001, 123, 6722-6723.
(7) Jacobson, R. R.; Tyeklar, Z.; Farooq, A.; Karlin, K. D.; Liu, S.; Zubieta,
J. J. Am. Chem. Soc. 1988, 110, 3690-3692.
(8) Hayashi, H.; Fujinami, S.; Nagatomo, S.; Ogo, S.; Suzuki, M.; Uehara,
A.; Watanabe, Y.; Kitagawa, T. J. Am. Chem. Soc. 2000, 122, 2124-
2125.
(11) (a) Karlin, K. D.; Sherman, S. E. Inorg. Chim. Acta 1982, 65, L39-
L40. (b) Schatz, M.; Becker, M.; Thaler, F.; Hampel, F.; Schindler,
S.; Jacobson, R. R.; Tyekla´r, Z.; Murthy, N. N.; Ghosh, P.; Chen, Q.;
Zubieta, J.; Karlin, K. D. Inorg. Chem. 2001, 40, 2312-2322.

6606 Inorganic Chemistry, Vol. 40, No. 26, 2001
Osako et al.
Table 1. Summary of X-ray Crystallographic Data
[CuI(L2)(CH3CN)]ClO4
[CuI(L3)]ClO4
empirical formula
formula weight
crystal system
space group
a, Å
b, Å
c, Å
C26H32N4O4ClCu
563.56
triclinic
C23H27N3O4ClCu
508.49
triclinic
P1 (#1)
P-1 (#2)
15.621(2)
17.202(2)
8.2593(9)
92.232(6)
90.706(4)
87.545(4)
2215.6(4)
4
11.956(1)
12.073(1)
10.379(1)
106.494(2)
106.108(5)
99.266(3)
1332.5(2)
2
R, deg
â, deg
γ, deg
V, Å3
Z
F(000)
D
588.00
1.405
1056.00
1.524
calc, g/cm3
Figure 1. ORTEP drawing of the cationic part of [CuI(L2)(CH3CN)]-
ClO4 (molecule 1) showing 50% probability thermal ellipsoids. The
counteranions and hydrogen atoms are omitted for clarity.
T, °C
-115
-100
cryst size, mm
µ (Mo KR), cm-1
diffractometer
0.30 × 0.30 × 0.20
0.20 × 0.20 × 0.20
9.58
Rigaku
11.42
Rigaku
RAXIS-RAPID
RAXIS-RAPID
radiation
Mo KR
Mo KR
(0.71069 Å)
55.0
no. of reflns measd 9468
(0.71069 Å)
55.0
13360
2θmax, deg
no. of reflns obsd
no. of variables
Ra
4862 [I > 1.5σ(I)]
6018 [I > 3.0σ(I)]
714
638
0.042
0.064
0.046
0.072
b
Rw
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw){∑w(|Fo| - |Fc|)2/∑wFo }1/2
.
2
Table 2. Selected Bond Lengths (Å) and Angles (deg) of
[CuI(L2)(CH3CN)]ClO4
Molecule 1
Molecule 2
Cu(1)-N(1)
2.216(7)
2.054(6)
2.009(7)
1.930(8)
98.3(3)
Cu(2)-N(5)
2.152(6)
1.996(7)
2.133(6)
2.005(6)
98.3(3)
Cu(1)-N(2)
Cu(2)-N(6)
Figure 2. ORTEP drawing of the cationic part of [CuI(L3)]ClO4
(molecule 1) showing 50% probability thermal ellipsoids. The coun-
teranions and hydrogen atoms are omitted for clarity.
Cu(1)-N(3)
Cu(1)-N(4)
Cu(2)-N(7)
Cu(2)-N(8)
N(1)-Cu(1)-N(2)
N(1)-Cu(1)-N(3)
N(2)-Cu(1)-N(3)
N(1)-Cu(1)-N(4)
N(2)-Cu(1)-N(4)
N(3)-Cu(1)-N(4)
N(5)-Cu(2)-N(6)
N(5)-Cu(2)-N(7)
N(6)-Cu(2)-N(7)
N(5)-Cu(2)-N(8)
N(6)-Cu(2)-N(8)
N(7)-Cu(2)-N(8)
96.3(2)
96.9(3)
the reported crystal structures of copper(I) complexes supported
by similar tridentate ligands containing the same bis[2-(2-
pyridyl)ethyl]amine moiety for comparison.12,13
122.0(3)
113.7(3)
122.4(3)
101.4(3)
123.3(3)
114.3(3)
124.8(3)
96.6(3)
The unit cell of [CuI(L2)(CH3CN)](ClO4) (2‚CH3CN) consists
of two crystallographically independent CuI-complex ions,
including two N-bound acetonitrile molecules and two ClO4
Table 3. Selected Bond Lengths (Å) and Angles (deg) of
[CuI(L3)]ClO4
-
ions. The cuprous ion in 2‚CH3CN has a distorted tetrahedral
geometry with an N4 donor set, one from the bound CH3CN
and the others from the tridentate ligand L2. Although bond
distances and angles between the two molecules (molecule 1
and molecule 2) are different slightly from each other, structural
parameters (Cu-N distances and N-Cu-N angles) of 2‚CH3-
CN are fairly similar to those of the reported dinuclear CuI
complex, [CuI2(N4PY2)(CH3CN)2](ClO4)2 (4‚CH3CN), where
each copper ion has a pseudotetrahedral geometry supported
by the bis[2-(2-pyridyl)ethyl]amine unit and one molecule of
CH3CN (Cu-N distances, 1.945-2.151 Å; N-Cu-N angles,
98.1-122.8°) (Scheme 2).13 Thus, the 6-methyl group of the
pyridine nucleus of L2 induces little effect on the structure of
the four-coordinate copper(I) complex.
Molecule 1
Molecule 2
Cu(1)-N(1)
2.113(4) Cu(2)-N(4)
2.113(3)
2.024(4)
1.980(4)
2.242(4)
2.189(4)
1.370(7)
1.403(6)
1.405(7)
1.366(6)
1.413(6)
1.404(6)
100.6(1)
100.1(1)
111.4(2)
98.2(2)
Cu(1)-N(2)
2.031(4) Cu(2)-N(5)
Cu(1)-N(3)
2.003(4) Cu(2)-N(6)
Cu(1)-C(19)
2.251(4) Cu(2)-C(42)
2.375(5) Cu(2)-C(43)
1.367(8) C(38)-C(39)
1.389(7) C(38)-C(43)
1.383(8) C(39)-C(40)
1.368(8) C(40)-C(41)
1.397(7) C(41)-C(42)
1.382(7) C(42)-C(43)
102.2(2) N(4)-Cu(2)-N(5)
100.1(1) N(4)-Cu(2)-N(6)
113.7(2) N(5)-Cu(2)-N(6)
111.9(2) N(4)-Cu(2)-C(42)
Cu(1)-C(20)
C(15)-C(16)
C(15)-C(20)
C(16)-C(17)
C(17)-C(18)
C(18)-C(19)
C(19)-C(20)
N(1)-Cu(1)-N(2)
N(1)-Cu(1)-N(3)
N(2)-Cu(1)-N(3)
N(1)-Cu(1)-C(19)
N(2)-Cu(1)-C(19)
N(3)-Cu(1)-C(19)
N(1)-Cu(1)-C(20)
N(2)-Cu(1)-C(20)
N(3)-Cu(1)-C(20)
C(19)-Cu(1)-C(20) 34.6(2)
On the other hand, Karlin and co-workers have also reported
the nearly T-shaped three-coordinate copper(I) complex 5
supported by N,N-bis[2-(2-pyridyl)ethyl]benzylamine (Scheme
3),12 where the Cu-Npy distances are significantly shorter
97.9(2)
129.5(2) N(6)-Cu(2)-C(42)
82.5(2)
N(5)-Cu(2)-C(42)
139.8(2)
99.8(2)
87.5(1)
108.9(2)
136.6(2)
N(4)-Cu(2)-C(43)
124.3(2) N(5)-Cu(2)-C(43)
120.0(2) N(6)-Cu(2)-C(43)
C(42)-Cu(2)-C(43) 36.9(2)
(12) Blackburn, N. J.; Karlin, K. D.; Concannon, M.; Hayes, J. C.; Gultneh,
Y.; Zubieta, J. Chem. Commun. 1984, 939-940.
(13) Karlin, K. D.; Haka, M. S.; Cruse, R. W.; Meyer, G. J.; Farooq, A.;
Gultneh, Y.; Hayes, J. C.; Zubieta, J. J. Am. Chem. Soc. 1988, 110,
1196-1207.
(1.873, 1.893 Å) than those in 2‚CH3CN and 4‚CH3CN. In this
study, however, such a three coordinate copper(I) complex could
not be obtained when ligand L2 was employed. This is probably

Modulating Coordination Chemistry in Copper(I) Complexes
Inorganic Chemistry, Vol. 40, No. 26, 2001 6607
Scheme 2
Scheme 3
Figure 3. Spectral change for the titration of 1 (1.0 × 10-4 M) with
CH3CN in CH2Cl2 at -20 °C. Inset: Plot of (A - A0)/(A∞ - A) versus
[CH3CN] based on the absorption change at A ) 290 nm.
Scheme 4
Scheme 5
due to the steric effect of the 6-methyl group in L2, which
prohibits formation of such a three-coordinate copper(I) complex
having the shorter Cu-Npy distance. It has previously been
demonstrated that the donor ability of the pyridine nucleus
carrying the 6-methyl group is diminished due to the steric
repulsion between the methyl group and metal ion.10a,14 Thus,
only the four-coordinate tetrahedral geometry with the longer
Cu-N distance is available in the L2 ligand system. Conse-
quently, removal of the bound CH3CN from 2‚CH3CN becomes
difficult. This point will be discussed later in more detail.
The copper(I) complex [CuI(L3)](ClO4) (3) also affords two
crystallographically independent CuI-complex ions in the crystal
(see Table 3). Notably, the phenyl ring of the ligand sidearm
of L3 comes closer to the cuprous ion, making a coordinative
interaction in a η2 fashion (Figure 2). Thus, the cuprous ion in
3 also adapts a distorted tetrahedral geometry, consisting of the
three nitrogen atoms of the ligand and the C(19)-C(20) moiety
of the phenyl ring (C(42)-C(43) in molecule 2). This bonding
interaction is unsymmetrical, Cu(1)-C(19), 2.251(4) Å; Cu-
(1)-C(20), 2.375(5) Å in molecule 1 and Cu(2)-C(42), 2.242-
(4) Å; Cu(2)-C(43), 2.189(4) Å in molecule 2, and these values
are comparable to the Cu-C distances in the known CuI-η2-
benzene complexes (2.09-2.30 Å)15,16 and in the recently
reported CuI-η2-naphthalene 6 (2.129, 2.414 Å)17 and CuI-
η2-indole 7 (2.228, 2.270 Å)18 complexes (Scheme 4). Such an
interaction between the cuprous ion and the aromatic group of
the ligand sidearm is not seen in complex 2 in which CH3CN
molecule occupies the fourth coordination site (Figure 1). In
the case of complex 6 as well, coordination of CH3CN to the
metal center prevents the copper(I) complex from making such
a CuI-aromatic interaction,17 and the CuI-aromatic interaction
is easily broken when complex 7 is dissolved in CH3CN.18 Thus,
the coordinative interaction between the cuprous ion and the
aromatics is essentially weak.
Physicochemical Properties and Reactivity in Solution.
UV-visible spectra of [CuI(L1)](ClO4) (1),5a [CuI(L2)(CH3CN)]-
(ClO4) (2‚CH3CN), and [CuI(L3)](ClO4) (3) in CH2Cl2 are
presented in Figure S1.19 The complex 3 exhibits an absorption
band at 290 nm (ꢀ ) 9700 M-1 cm-1) that can be assigned to
the metal-to-phenyl charge transfer (MLCT), as suggested for
complex 7 (metal-to-indole) by Shimazaki et al (308 nm, ꢀ
) 18000 M-1 cm-1).18,20 A similar absorption band at 290 nm
is also observed with complex 1 (Figure S1). This suggests that
there is a similar CuI-phenyl interaction within 1 when it is
dissolved in a nonpolar solvent, such as CH2Cl2. On the other
hand, however, complex 2‚CH3CN does not show such an
absorption band around 300 nm in CH2Cl2 (Figure S1),
indicating that the coordination of CH3CN in 2‚CH3CN is rather
strong so as to prevent the CuI-phenyl interaction as discussed
above.
To confirm this idea, titration of 1 by CH3CN was carried
out in CH2Cl2 at -20 °C (Scheme 5). Figure 3 shows a spectral
change of the titration. The absorption band at 290 nm due to
the CuI-phenyl interaction decreases while increasing the added
CH3CN concentration, and the final spectrum of the titration
resembles the spectrum of 2‚CH3CN in CH3CN. The association
(14) Nagao, H.; Komeda, N.; Mukaida, M.; Suzuki, M.; Tanaka, K. Inorg.
Chem. 1996, 35, 6809-6815.
(15) Turner, R. W.; Amma, E. L. J. Am. Chem. Soc. 1966, 88, 1877-
1882.
(16) Dines, M. B.; Bird, P. H. J. Chem. Soc., Chem. Commun. 1973, 12.
(17) Striejewske, W. S.; Conry, R. R. Chem. Commun. 1998, 555-556.
(18) Shimazaki, Y.; Yokoyama, H.; Yamauchi, O. Angew. Chem., Int. Ed.
1999, 38, 2401-2403.
(19) See Supporting Information.
(20) The CuI-phenyl interaction in the cuprous complex is now being
investigated in more detail using a series of p-substituted phenyl
derivatives of ligand L1.5a

6608 Inorganic Chemistry, Vol. 40, No. 26, 2001
Osako et al.
constant Kas ) [1‚CH3CN]/[1][CH3CN] at -20 °C has been
determined as 6.4 M-1 by analyzing the absorption change, as
indicated in the inset of Figure 3. The conformational change
induced by the CH3CN coordination has also been indicated
by 1H NMR. Namely, sharp 1H NMR peaks due to the ethylene
linker of 1 (-CH2-CH2-Ph) appear at δ ) 2.82 and 3.18 (both
triplet, J ) 6.1 Hz) in CD2Cl2, while those peaks become a
broad singlet (δ ) 3.0) in CD3CN. Thus, the two methylene
groups of the ethylene linker are nonequivalent as a result of
the coordination of the phenyl group in CD2Cl2, while such a
CuI-phenyl interaction is broken in CD3CN, letting the two
methylene groups be nearly magnetically equivalent. In addition,
the 6-proton of the pyridine nucleus (Hpy-6) at δ ) 8.07 in CD2-
Cl2 shifts to δ ) 8.46 in CD3CN. Such a downfield shift of
Hpy-6 may be due to the disappearance of a ring current effect
by the phenyl group, as shown in Scheme 5, where the phenyl
group in conformation (a) sits right above the Hpy-6 to induce
the upfield shift of Hpy-6, whereas such a ring current effect by
the phenyl ring disappears in conformation (b). The CuI-phenyl
interaction in complex 1 is further supported by 13C NMR in
CD2Cl2. Namely, the 13C NMR peaks of the phenyl ring of 1
appear at 123.1 (C4), 127.43 (C2,6), 129.36 (C3,5), and 138.92
(C1) ppm, which are rather similar to those of 3 (121.20, 127.60,
129.44, and 138.93 ppm in CD2Cl2), which has the CuI-phenyl
interaction, but are relatively different from those of 2 (127.09,
128.55, 129.12, and 140.20 ppm in CD2Cl2) in which such an
interaction is absent.
Scheme 6
The Kas value of 3 has also been determined as 0.21 M-1 in
the same way. The significantly smaller Kas value of 3 as
compared with that of 1 clearly demonstrates that the CuI-
phenyl interaction, which is essentially weak in 1, is enhanced
significantly in complex 3. The methyl group of the ligand
sidearm of L3 may occupy a less crowded space in complex 3
(conformation (a) in Scheme 5), placing the phenyl group on
the cuprous ion to stabilize the CuI-phenyl interaction entropi-
cally.20
Complex 1 is known to react with O2 rapidly to generate a
peroxo species at low temperature (-80 °C), as illustrated in
Scheme 6.4,5a In sharp contrast to this, neither 2‚CH3CN nor 3
reacts with dioxygen at ambient temperature (Scheme 6). The
high oxidation potential of 2‚CH3CN (0.30 V versus Fc/Fc+)
as compared with 1 (0.08 V vs Fc/Fc+) is certainly an important
factor for the stability toward dioxygen. The strong binding of
CH3CN to CuI in 2‚CH3CN may also prohibit the initial
coordination of O2 to the metal center, which is required for
formation of the peroxo species from 1 and O2.5a In the case of
complex 1, O2 can easily bind to the copper ion in place of the
weak CuI-phenyl bonding, as suggested by the larger Kas value.
In the case of complex 3, the stronger CuI-phenyl interaction,
as demonstrated by the smaller Kas value, may be the main factor
for prohibiting the direct interaction of the metal ion with
dioxygen, since the redox potential of 3 (0.07 V versus Fc/
Fc+) is essentially the same as the value of 1 (0.08 V versus
Fc/Fc+).
In conclusion, the small perturbation induced by the methyl
substitution at the 6-position of the pyridine nucleus in L2 and
at the benzylic position of the ligand sidearm in L3 leads to a
drastic change in the structure and reactivity of the copper(I)
complexes. Because of a repulsive interaction between the
6-methyl group of L2 and the metal ion, the cuprous complex
resists having a three-coordinate geometry with a shorter Cu-N
bond length, such as found in complex 5 (∼1.88 Å). Thus, the
copper(I) complex of L2 tends to have a four-coordinate
tetrahedral geometry with a longer Cu-N distance (1.98∼2.13
Å) in which CH3CN occupies the fourth coordination site. Such
a steric effect by the 6-methyl group of L2 makes the binding
of CH3CN to the metal ion rather strong. Thus, the reactivity
of 2‚CH3CN toward O2 is almost lost. On the other hand, the
methyl group at the benzylic position of L3 reduces the degree
of freedom of the sidearm movement to enhance the stability
of the CuI-phenyl interaction. In this case as well, a direct
interaction between the cuprous ion and O2 is prohibited due
to the stable CuI-phenyl interaction. These results will provide
Complex 2‚CH3CN exhibited a reversible redox couple at
0.30 V versus Fc/Fc+ (ferrocene/ferrocenium) in CH2Cl2, as
shown in Figure S2.19 The good reversibility of the cyclic
voltammetry indicates that the coordination of CH3CN in 2‚
CH3CN is maintained during the electrochemical redox process.
In other words, the coordination of CH3CN in 2‚CH3CN is
strong enough to keep the four-coordinate copper ion, as
discussed above. On the other hand, 1 and 3 provided quasi-
reversible redox couples at lower potentials (0.08 and 0.07 V
versus Fc/Fc+, respectively), but the reversibility of their cyclic
voltammetry became worse (∆E1/2 ) 0.53 and 0.35 V, respec-
tively) as compared with the case of complex 2‚CH3CN (∆E1/2
) 0.17 V). This suggests that a larger conformational change
occurs during the electrochemical redox process in the cases of
1 and 3. In the previous study, we reported the crystal structure
of the copper(II) complex of ligand L1, in which there is no
interaction between the cupric ion and the aromatic pendant.4
Thus, the conformational changes in 1 and 3 could be largely
attributed to the change of position of the ligand sidearm, i.e.,
CuI-phenyl interaction in the copper(I) state while little
interaction between CuII and the phenyl group in the copper(II)
state. The higher redox potential of 2‚CH3CN (0.30 V) as
compared with those of others (0.08 and 0.07 V) can be
attributed to the bound CH3CN, which stabilizes the copper(I)
state of the complex. The 6-methyl group in L2 may also result
in increase of E1/2, since Tanaka and co-workers have demon-
strated that introduction of the 6-methyl group induces a positive
shift in E1/2 of the copper(II) complexes of TMPA ligands.14

Modulating Coordination Chemistry in Copper(I) Complexes
Inorganic Chemistry, Vol. 40, No. 26, 2001 6609
three times and dried (41% yield). All procedures were done in a
glovebox ([O2] < 0.1 ppm). 1H NMR (600 MHz, acetone-d6): δ 2.04
(3 H, s, bound CH3CN), 2.73-2.80 (4 H, m, -CH2-CH2-Ph), 2.90
(6 H, s, -CH3), 3.20 (4H, br, -CH2-CH2-Py or -CH2-CH2-Py),
3.31 (4 H, t, -CH2-CH2-Py or -CH2-CH2-Py), 7.06-7.21 (5 H,
m, aromatic H), 7.47 (2 H, t, J ) 7.7 Hz, Hpy-3 or Hpy-5), 7.54 (2 H,
d, J ) 7.7 Hz, Hpy-3 or Hpy-5), 7.97 (2 H, t, J ) 7.7 Hz, Hpy-4). FT-IR
(KBr): 1105, 1086, and 625 cm-1 (ClO4-). FAB-MS (pos), m/z 422.28
(M+). Anal. for [CuI(L2)‚CH3CN]ClO4. Calcd for C26H32O4N4CuCl: C,
55.41; H, 5.72; N, 9.94. Found: C, 55.28; H, 5.66; N, 9.67.
[CuI(L3)]ClO4 (3). This compound was prepared in a manner similar
to that described above using ligand L3 (172.7 mg, 0.5 mmol) and
[CuI(CH3CN)4]ClO4 (160.1 mg, 0.5 mmol); 52% yield. All procedures
were done in a glovebox ([O2] < 0.1 ppm). 1H NMR (300 MHz,
acetone-d6): δ 1.20 (3 H, d, J ) 6.6 Hz, -CH3), 2.65-3.71 (11 H, m,
-CH- and -CH2-CH2-), 7.29-7.47 (9 H, m, aromatic H, Hpy-3, and
important insights into modulation of the structure and reactivity
of copper(I) complexes.
Experimental Section
General. All chemicals used in this study except the ligands and
the complexes were commercial products of the highest available purity
and were further purified by the standard methods, if necessary.21
6-Methyl-2-vinylpyridine was kindly supplied by Koei Chemical Co.
Ltd, and was purified by fractional distillation. Synthetic procedures
of ligand L1 and its copper(I) complex were reported previously.5a FT-
IR spectra were recorded with a Shimadzu FTIR-8200PC. UV-vis
spectra were measured using a Hewlett-Packard HP8453 diode array
spectrophotometer with a Unisoku thermostated cell holder designed
for low-temperature measurements. Mass spectra were recorded with
a JEOL JMS-700T Tandem MS station. 1H NMR spectra were recorded
on a JEOL FT-NMR Lambda 300WB or a Bruker Advance 600.
The cyclic voltammetry (CV) measurements were performed on an
ALS-630A electrochemical analyzer in anhydrous CH2Cl2 containing
0.1 M NBu4ClO4 as supporting electrolyte. The Pt working electrodes
were 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/ferricenium (Fc/Fc+) couple as a reference. All electro-
chemical measurements were carried out at 25 °C under an atmospheric
pressure of Ar in a glovebox (Miwa Co. Ltd.).
H
py-5), 7.49 (2 H, br, Hpy-4), 8.16-8.32 (2 H, br, Hpy-6). FT-IR
(KBr): 1121, 1089, and 625 cm-1 (ClO4-). FAB-MS (pos.), m/z
408.28 (M+). Anal. for [CuI(L3)]ClO4. Calcd for C23H27O4N3CuCl: C,
54.33; H, 5.35; N, 8.26. Found: C, 54.24; H, 5.30; N, 8.27.
Caution! The perchlorate salts used in this study are all potentially
explosive and should be handled with care.
X-ray Structure Determination. Single crystals of 2‚CH3CN and
3 suitable for X-ray structural analysis were obtained by vapor diffusion
of ether into a CH2Cl2 solution of the complex. In the case of 2‚CH3-
CN, a few drops of CH3CN was added to the CH2Cl2 solution of 2‚
CH3CN. The single crystal was mounted on a glass fiber. Data of X-ray
diffraction were collected by a Rigaku RAXIS-RAPID imaging plate
two-dimensional area detector using graphite-monochromated Mo KR
radiation (λ ) 0.71070 Å) to 2θ max of 55.0°. All the crystallographic
calculations were performed by using the Crystal Structure software
package of the Rigaku Corporation and Molecular Structure Corporation
(version 1.01, 2000). The crystal structure was solved by direct methods
and refined by full-matrix least squares using SIR-92. All non-hydrogen
and hydrogen atoms were refined anisotropically and isotropically,
respectively. Summary of the fundamental crystal data and experimental
parameters for structure determinations is given in Table 1. The
experimental details including data collection, data reduction, structure
solution and refinement, the atomic coordinates, and Biso/Beq; anisotropic
displacement parameters and intramolecular bond distances and angles
have been deposited in the Supporting Information.
Synthesis of Ligands. Ligands L2 and L3 were prepared by a
Michael addition of phenethylamine to 6-methyl-2-vinylpyridine and
of â-methylphenethylamine to 2-vinylpyridine, respectively, in refluxing
methanol containing acetic acid, and the products were purified by flash
column chromatography (SiO2) as reported previously.5a The structure
1
of the products were confirmed by H NMR.
N,N-Bis[2-(6-methylpyridin-2-yl)ethyl]phenylethylamine (L2). Pale
brown oil; 1H NMR (300 MHz, CDCl3): δ 2.52 (6 H, s, -CH3), 2.67-
2.83 (4 H, m, -CH2-CH2-), 2.85-3.00 (8 H, m, -CH2-CH2-), 6.83
(2 H, d, J ) 7.5 Hz, Hpy-3 or Hpy-5), 6.95 (2 H, d, J ) 7.5 Hz, Hpy-3
or Hpy-5), 7.10-7.27 (5 H, m, C6H5), 7.42 (2 H, t, J ) 7.5 Hz, Hpy-4).
N,N-Bis[2-(2-pyridyl)ethyl]-â-methylphenylethylamine (L3). Pale
1
brown oil; H NMR (300 MHz, CDCl3): δ 1.12 (3 H, d, J ) 6.6 Hz,
-CH3), 2.53-2.97 (11 H, m, -CH- and -CH2-CH2-), 7.05-7.28 (7
H, m, aromatic H), 7.49 (2 H, dt, J ) 1.7 and 7.7 Hz, Hpy-4), 8.52 (2
H, d, J ) 4.3 Hz, Hpy-6).
Synthesis of Copper(I) Complexes. [CuI(L2)‚CH3CN]ClO4 (2‚
CH3CN). Ligand L2 (126.1 mg, 0.35 mmol) was treated with [CuI(CH3-
CN)4]ClO4 (112.3 mg, 0.35 mmol) in CH2Cl2 (5 mL) under Ar
atmosphere. After stirring for 30 min at room temperature, the insoluble
material was removed by filtration. Addition of ether (100 mL) to the
filtrate gave a pale yellow powder that was precipitated by allowing
the mixture to stand for several minutes. The supernatant was then
removed by decantation, and the remaining pale yellow solid was
dissolved in CH3CN (5 mL). Addition of ether (100 mL) to the filtrate
gave a pale yellow powder that was precipitated by allowing the mixture
to stand for several minutes. The supernatant was then removed by
decantation, and the remaining pale yellow solid was washed with ether
Acknowledgment. This work was partially supported by
Grants-in-Aid for Scientific Research on Priority Area (Nos.
11228205, 11228206) and Grants-in-Aid for Scientific Research
(No. 13480189) from the Ministry of Education, Science,
Culture, and Sports, Japan. The authors also acknowledge Ms.
Masumi Doe of Osaka City University for her help in obtaining
measurements on the 600 MHz NMR.
Supporting Information Available: UV-vis spectra of the cuprous
complexes in CH2Cl2 (Figure S1), cyclic voltammogram of 2‚CH3CN
in CH2Cl2 (Figure S2), and X-ray crystallographic files for 2‚CH3CN
and 3 in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
(21) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 4th Edition; Pergamon Press: Elmsford, NY,
1996.
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