Syntheses, crystal structures and electronic properties of a series of copper(II) complexes with 3,5-halogen-substituted Schiff base ligands and their solutions

Article abstract of DOI:10.1016/j.poly.2005.06.018

We have systematically investigated the structural features, electronic properties, thermally-induced structural phase transitions and absorption spectra depending on the solvent for ten Cu(II) complexes with 3,5-halogen-substituted Schiff base ligands. Structural characterization of two new complexes, bis(N-R-1-phenylethyl- and N-R,S-2-butyl-5-bromosalicydenaminato- κ²N,O)copper(II), reveals that they afford a compressed tetrahedral trans-[CuN₂O₂] coordination geometry with trans-N-Cu-N = 159.4(2)° and trans-O-Cu-O = 151.7(3)° for the 1-phenylethyl complex and trans-N-Cu-N = 157.9(3)° and trans-O-Cu-O = 151.0(3)° for the 2-butyl one. All the complexes exhibit a structural phase transition by heating in the solid state regardless of their structures at room temperature. The absorption spectra of a series of ten complexes exhibit a slight shift of the d-d band at 16 000-20 000 cm^-1 and remarkable shift of the π-π* band at 24 000-28 000 cm^-1, which suggests that the dipole moment of the solvents presumably affects the conformation of the π-conjugated moieties of the ligands rather than the coordination environment. We have also attempted 'photochromic solute-induced solvatochromism' by a system of bis(N-R-1-phenylethyl-3,5- dichlorosalicydenaminato-κ²N,O)copper(II) and photochromic 4-hydroxyazobenzene in chloroform solution. We successfully observed a change of the d-d and π-π* bands of the complex in the absorption spectra caused by cis-trans photoisomerization of 4-hydroxyazobenzene.

Full text of DOI:10.1016/j.poly.2005.06.018

Polyhedron 24 (2005) 2933–2943
www.elsevier.com/locate/poly
Syntheses, crystal structures and electronic properties of a series
of copper(II) complexes with 3,5-halogen-substituted Schiff
base ligands and their solutions
*
*
Takashiro Akitsu , Yasuaki Einaga
Department of Chemistry, Faculty of Science of Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
Received 9 March 2005; accepted 3 June 2005
Available online 3 August 2005
Abstract
We have systematically investigated the structural features, electronic properties, thermally-induced structural phase transitions
and absorption spectra depending on the solvent for ten Cu(II) complexes with 3,5-halogen-substituted Schiff base ligands. Struc-
tural characterization of two new complexes, bis(N-R-1-phenylethyl- and N-R,S-2-butyl-5-bromosalicydenaminato-j2N,O)cop-
per(II), reveals that they afford a compressed tetrahedral trans-[CuN2O2] coordination geometry with trans-N–Cu–N = 159.4(2)ꢀ
and trans-O–Cu–O = 151.7(3)ꢀ for the 1-phenylethyl complex and trans-N–Cu–N = 157.9(3)ꢀ and trans-O–Cu–O = 151.0(3)ꢀ for
the 2-butyl one. All the complexes exhibit a structural phase transition by heating in the solid state regardless of their structures
at room temperature. The absorption spectra of a series of ten complexes exhibit a slight shift of the d–d band at 16000–
20000 cmꢀ1 and remarkable shift of the p–p* band at 24000–28000 cmꢀ1, which suggests that the dipole moment of the solvents
presumably affects the conformation of the p-conjugated moieties of the ligands rather than the coordination environment. We have
also attempted Ôphotochromic solute-induced solvatochromismÕ by a system of bis(N-R-1-phenylethyl-3,5-dichlorosalicydenami-
nato-j2N,O)copper(II) and photochromic 4-hydroxyazobenzene in chloroform solution. We successfully observed a change of
the d–d and p–p* bands of the complex in the absorption spectra caused by cis–trans photoisomerization of 4-hydroxyazobenzene.
ꢁ 2005 Elsevier Ltd. All rights reserved.
Keywords: Copper(II) complexes; Schiff base; Crystal structures; Chirality; Azobenzenes; Solvatochromism
1. Introduction
ors in this field, reliable and reasonable design of
photo-switching materials has not been established at
present. However, one of the promising ways to discover
desirable photo-switching materials may be examining
various types of chromic complexes, e.g. thermochromic
[10–12], piezochromic [13,14], solvatochromic [15–17]
and vapochromic [18,19], with irradiation of light.
Besides merely inorganic metal complexes, organic/inor-
ganic hybrid self-assemblies being composed of photo-
chromic organic compounds and inorganic metal
complexes may be useful for this purpose. Expected
advantages of such organic/inorganic hybrid systems
are as follows: (1) Reversible photoisomerization of
photochromic compounds can occur even at room
The developing interest attracted to photofunctional
materials of inorganic transition metal complexes [1,2]
that have azobenzenes as ligands [3,4], showing spin
crossover [5,6] and exhibiting long-lived metastable
states [7] is due not only to their fundamental aspects
but also their potential applications, for example
Fe–Co Prussian blue analogues [8] and one dimensional
organometallic complexes [9]. In spite of great endeav-
*
Corresponding author. Tel.: +81 45 566 1790; fax: +81 45 566
1697.
E-mail address: akitsu@chem.keio.ac.jp (T. Akitsu).
0277-5387/$ - see front matter ꢁ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.06.018

2934
T. Akitsu, Y. Einaga / Polyhedron 24 (2005) 2933–2943
temperature [20]. (2) Photo-switching of characteristic
properties of molecular compounds, for instance chiral
molecular recognition, spin crossover and conforma-
tional conversion, can be available in contrast to other
inorganic solid compounds such as Prussian blue
analogues.
In this context, flexible transition metal complexes are
suitable as a component of such photo-switching materi-
als. Classically, some Cu(II) or Ni(II) complexes incorpo-
rating Schiff base ligands [21] have been known to exhibit
flexibility from a square planar to a tetrahedral coordina-
tion environment by introduction of various substituents,
and show structural phase transitions by heating [22–30].
Especially, Ni(II) complexes exhibit not only flexibility of
the coordination environment [31–38] but also interesting
magnetic behavior; diamagnetic square planar complexes
change into paramagnetic tetrahedral complexes when a
complex adopts such coordination geometries in the solid
state, or when one is dissolved in solution, so called solu-
tion paramagnetism [39–41].
Herein, we report structural features, electronic proper-
ties, thermally-induced structural phase transition and
absorption spectra depending on the solvent for ten Cu(II)
complexes with3,5-halogen-substitutedSchiffbaseligands
(Scheme 1). Crystal structures of two new complexes,
bis(N-R-1-phenylethyl- and N-R,S-2-butyl-5-bromosali-
cydenaminato-j2N,O)copper(II), are also described. Fur-
thermore, we successfully observed a spectral change of
d–d and p–p* bands of the complex caused by cis–trans
photoisomerization of 4-hydroxyazobenzene (4HAZ)
for an organic/inorganic system involving bis-(N-R-1-
phenylethyl-3,5-dichlorosalicydenaminato-j2N,O)copper-
(II) and 4HAZ in chloroform solution.
2. Experimental
2.1. Materials
Chemicals of the highest commercial grade available
(Aldrich and Wako) were used as received without fur-
ther purification.
2.2. Physical measurements
Elemental analyses (C, H, N) were carried out on
an Elementar Vario EL analyser at Keio University.
Infrared spectra were recorded using Nujol mulls on a
BIORAD FTS-60A spectrophotometer in the range
4000–400 cmꢀ1 at 298 K. Thermal analysis was performed
on a SHIMADZU DSC-60 differential scanning calo-
rimeter (DSC), where the heating rate was 10 K minꢀ1
in the range of 313–673 K. Diffuse reflectance spectra
were measured on a JASCO V-560 spectrophotometer
equipped with an integrating sphere in the range 850–
220 nm at 298 K. Absorption spectra were measured
on a JASCO V-560 spectrophotometer in the range
900–200 nm at 298 K. Magnetic susceptibility was mea-
sured with a Sherwood Scientific magnetic susceptibility
balance at 298 K. Magnetic data were corrected for the
magnetization of diamagnetic contributions, which were
estimated from Pascal constants. The magnetic
properties were investigated with a Quantum Design
MPMS-5S superconducting quantum interference
device magnetometer (SQUID) at an applied field of
5000 Oe in a temperature range 5–250 K. Semi-empirical
molecular orbital calculations were performed with the
program ZINDO [42,43] in the CAChe software pack-
age based on the crystal structures determined.
R
2.3. Syntheses of complexes
H
X1
C
N
X2
2.3.1. Bis(N-R-1-phenylethyl-3,5-dichlorosalicydenami-
nato)copper(II) (1)
O
Cu
R
O
To a methanol solution (150 cm3) of copper(II) ace-
tate (0.91 g, 5.00 mmol), 3,5-dichlorosalicylaldehyde
(1.91 g, 10.0 mmol), and R-1-phenylethylamine (1.21 g,
10.0 mmol) were added and stirred at 45 ꢀC for 2 h. The
resulting brown precipitates were collected by filtration
and recrystallized from acetone/methanol. Yield 64.8%.
Anal. Calc. for C30H24N2Cl4CuO2: C, 55.44; H, 3.72; N,
2.31. Found: C, 55.37; H, 3.98; N, 4.09%. IR (KBr) mC@N
1623 cmꢀ1, mCu–N 445 cmꢀ1, mCu–O 419 cmꢀ1, m.p. 483 K
(decomposition). Magnetic moment leff/lBM = 1.73 at
298 K.
X2
X1
C
H
N
R
X1 X2
C6H5 Cl
C6H5 Br
Cl
Br
Cl
Br
Cl
Br
1
2
3
4
5
6
C6H5
C6H5
H
H
C2H5 Cl
C2H5 Br
7
8
9
C2H5
C2H5
H
H
Cl
Br
2.3.2. Bis(N-R,S-1-phenylethyl-3,5-dibromoosalicyden-
aminato)copper(II) (2)
The same procedure as for 1 afforded the condensa-
tion of 3,5-dibromosalicylaldehyde (2.80 g, 10.0 mmol)
and racemic 1-phenylethylamine (1.21 g, 10.0 mmol).
CH3
CH3
H
H
Cl
Br
10
Scheme 1. Molecular structures of 1–10.

T. Akitsu, Y. Einaga / Polyhedron 24 (2005) 2933–2943
2935
Yield 74.5%. Anal. Calc. for C30H24Br4CuN2O2: C,
racemic 2-butylamine (0.37 g, 5.00 mmol). Yield
46.4%. Anal. Calc. for C22H26Cl2CuN2O2: C, 54.49; H,
4.40; N, 5.78. Found: C, 54.46; H, 4.53; N, 5.72%. IR
43.64; H, 2.92; N, 3.38. Found: C, 43.77; H, 2.72; N,
3.25%. IR (KBr) mC@N 1617 cmꢀ1, mCu–N 444 cmꢀ1
,
mCu–O 424 cmꢀ1, m.p. 478 K (decomposition). Magnetic
moment leff/lBM = 1.47 at 298 K.
(KBr) mC@N 1618 cmꢀ1
,
mCu–N 459 cmꢀ1
,
mCu–O
436 cmꢀ1, m.p. 475 K (decomposition). Magnetic mo-
ment leff/lBM = 1.55 at 298 K.
2.3.3. Bis(N-R,S-1-phenylethyl-5-chlorosalicydenami-
nato)copper(II) (3)
2.3.8. Bis(N-R,S-2-butyl-5-bromosalicydenaminato)-
copper(II) (8)
The same procedure as for 1 afforded the condensa-
tion of 5-chlorosalicylaldehyde (1.57 g, 10.0 mmol) and
racemic 1-phenylethylamine (1.21 g, 10.0 mmol). Yield
49.6%. Anal. Calc. for C30H26Cl2CuN2O2: C, 62.02; H,
4.51; N, 4.82. Found: C, 62.07; H, 4.29; N, 4.70%.
IR (KBr) mC@N 1621 cmꢀ1, mCu–N 465 cmꢀ1, mCu–O
419 cmꢀ1, m.p. 474 K (decomposition). Magnetic mo-
ment leff/lBM = 1.66 at 298 K.
The similar procedure as for 1 afforded the condensa-
tion of 5-bromosalicylaldehyde (1.01 g, 5.00 mmol) and
racemic 2-butylamine (0.37 g, 5.00 mmol). Yield
49.7%. Anal. Calc. for C22H26Br2CuN2O2: C, 46.05; H,
4.57; N, 4.88. Found: C, 46.10; H, 4.84; N, 4.81%. IR
(KBr) mC@N 1616 cmꢀ1
,
mCu–N 459 cmꢀ1
,
mCu–O
426 cmꢀ1, m.p. 473 K (decomposition). Magnetic mo-
ment leff/lBM = 1.69 at 298 K. Brown plate-like single
crystals suitable for X-ray crystallography were grown
from the filtrate over a period of a few days at 298 K.
2.3.4. Bis(N-R-1-phenylethyl-5-bromosalicydenami-
nato)copper(II) (4)
The same procedure as for 1 afforded the condensation
of 5-bromosalicylaldehyde (2.01 g, 10.0 mmol) and R-1-
phenylethylamine (1.21 g, 10.0 mmol). Yield 24.4%.
Anal. Calc. for C30H26Br2CuN2O2: C, 53.79; H, 3.91; N,
4.18. Found: C, 53.60; H, 3.69; N, 4.11%. IR (KBr) mC@N
1620 cmꢀ1, mCu–N 463 cmꢀ1, mCu–O 418 cmꢀ1, m.p. 469 K
(decomposition). Magnetic moment leff/lBM = 1.91 at
298 K. Brown plate-like single crystals suitable for X-
ray crystallography were grown from the filtrate over a
period of a few days at 298 K.
2.3.9. Bis(N-2-propyl-5-chlorosalicydenaminato)
copper(II) (9)
The same procedure as for 1 afforded the condensation
of 5-chlorosalicylaldehyde (1.57 g, 10.0 mmol) and 2-pro-
pylamine (0.59 g, 10.0 mmol). Yield 50.9%. Anal. Calc.
for C20H22Cl2CuN2O2: C, 52.58; H, 4.85; N, 6.13. Found:
C, 52.39; H, 5.16; N, 6.21%. IR (KBr) mC@N 1626 cmꢀ1
,
Table 1
Crystallographic data for 4 and 8
2.3.5. Bis(N-S-2-butyl-3,5-dichlorosalicydenaminato)-
copper(II) (5)
4
8
Formula
C
669.88
30H26Br2CuN2O2
C22H26Br2CuN2O2
573.80
The similar procedure as for 1 afforded the condensation
of 3,5-dichlorosalicylaldehyde (0.96 g, 5.00 mmol) and
S-2-butylamine (0.37 g, 5.00 mmol). Yield 25.8%. Anal.
Calc. for C22H24Cl4CuN2O2: C, 47.71; H, 4.37; N, 5.06.
Found: C, 47.76; H, 3.94; N, 5.09%. IR (KBr) mC@N
1622 cmꢀ1, mCu–N 447 cmꢀ1, mCu–O 419 cmꢀ1, m.p. 484 K
(decomposition). Magnetic moment leff/lBM = 1.53 at
298 K.
Molecular weight
Crystal system
Space group
monoclinic
C2 (#5)
17.662(5)
8.557(3)
9.522(2)
102.28(2)
1406.2(7)
2
1.582
670.0
3.647
0.50 · 0.30 · 0.20
1724
monoclinic
C2/c (#15)
21.816(9)
10.577(3)
10.362(2)
96.61(3)
2375.1(13)
4
1.605
730.0
4.304
0.68 · 0.68 · 0.30
2733
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢀ)
3
˚
V (A )
Z
Dcalc (g cmꢀ3
F(0 0 0)
)
2.3.6. Bis(N-R,S-2-butyl-3,5-dibromosalicydenaminato)-
copper(II) (6)
l (mmꢀ1
)
Crystal size (mm)
Total unique
reflections
Observed reflections
(I > 2r(I))
No. of refined
parameters
R,a Rw,b
The similar procedure as for 1 afforded the condensa-
tion of 3,5-dibromosalicylaldehyde (1.40 g, 5.00 mmol)
and racemic 2-butylamine (0.37 g, 5.00 mmol). Yield
26.9%. Anal. Calc. for C22H24Br4CuN2O2: C, 36.12; H,
3.31; N, 3.83. Found: C, 36.50; H, 2.97; N, 3.84%. IR
1230
169
1171
132
(KBr) mC@N 1620 cmꢀ1, mCu–N 446 cmꢀ1, mCu–O 420 cmꢀ1
m.p. 459 K (decomposition). Magnetic moment
leff/lBM = 1.48 at 298 K.
,
S
0.0294, 0.0826, 1.028
ꢀ0.03(2)
0.0498, 0.2062, 1.100
Frack parameter
Minimum and
maximum residual
0.37, ꢀ0.37
ꢀ0.47, 0.48
3
˚
densities (e/A )
P
P
2.3.7. Bis(N-R,S-2-butyl-5-chlorosalicydenaminato)-
copper(II) (7)
The similar procedure as for 1 afforded the condensa-
tion of 5-chlorosalicylaldehyde (0.78 g, 5.00 mmol) and
a
R = iFoj ꢀ jFci/ jFoj.
P
P
b
2 1/2
Rw = ( w(jFoj ꢀ jFcj)2/ wjFoj )
,
w = 1/(r2(Fo) + (0.0286P)2
2
+ 1.2163P)
for
4,
w ¼ 1=ðr2ðF oÞ þ ð0.1PÞ Þ for 8; where P ¼
ðF 2o þ 2F 2c Þ=3.

2936
T. Akitsu, Y. Einaga / Polyhedron 24 (2005) 2933–2943
m
Cu–N 453 cmꢀ1, mCu–O 432 cmꢀ1, m.p. 469 K (decomposi-
ically for non-hydrogen atoms by full-matrix
least-squares methods with SHLEXL97 [45] on a teXsan
program package [46]. Empirical absorption corrections
were applied based on w scans. No significant decay in
the intensity of three standard reflections was observed
throughout the data collection. The hydrogen atoms
were located at geometrically calculated position with
tion). Magnetic moment leff/lBM = 1.55 at 298 K.
2.3.10. Bis(N-2-propyl-5-bromosalicydenaminato)
copper(II) (10)
The same procedure as for 1 afforded the condensa-
tion of 5-bromosalicylaldehyde (2.01 g, 10.0 mmol) and
2-propylamine (0.59 g, 10.0 mmol). Yield 54.1%. Anal.
Calc. for C20H22Br2CuN2O2: C, 44.01; H, 4.06; N,
5.13. Found: C, 44.01; H, 4.28; N, 5.05%. IR (KBr)
˚
C–H = 0.950 A and refined isotropically. All the non-
hydrogen atoms were refined anisotropically. Crystallo-
graphic data for 4 and 8 are summarized in Table 1. The
crystal structures of 1 [30] and 5 [29] have been already
reported elsewhere.
mC@N 1624 cmꢀ1, mCu–N 451 cmꢀ1, mCu–O 429 cmꢀ1
m.p. 481 K (decomposition). Magnetic moment leff/
BM = 1.54 at 298 K.
,
l
3. Results and discussion
2.4. X-ray crystallography
3.1. Description of the structure of 4
Intensity data of 4 and 8 were collected on a Rigaku
AFC-7R four-circle diffractometer at 297 K using
graphite-monochromated Mo Ka radiation (k =
The molecular structure of 4 is depicted in Fig. 1, and
selected bond distances and angles of 4 are given in
Table 2. Complex 4 adopts a compressed tetrahedral
trans-[CuN2O2] coordination environment, which is as-
cribed to bulky 1-phenylethyl moieties of the ligands.
˚
0.71073 A). The structures were solved by direct
methods using SIR92 [44] and expanded by Fourier
techniques. The structures were refined on F2 anisotrop-
Fig. 1. The molecular structure of 4 showing the atom-labelling scheme (a) top view and (b) side view. Displacement ellipsoids are drawn at the 30%
probability level. H atoms are omitted for clarity.

T. Akitsu, Y. Einaga / Polyhedron 24 (2005) 2933–2943
2937
Table 2
are 93.0(2)ꢀ and 92.0(2)ꢀ, respectively. The Cu1–N1
˚
Selected geometric parameters (A, ꢀ) for 4
˚
and Cu1–O1 bond distances are 1.890(4) and 1.972(4) A,
Cu1–O1
Cu1–N1
N1–C7
C6–C7
O1–C1
N1–C8
C8–C9
C8–C10
1.890(4)
1.972(4)
1.296(6)
1.423(8)
1.319(1)
1.449(7)
1.511(9)
1.514(9)
respectively, and the C7@N1 imine bond length is
1.296(6) A. There are no remarkable bond distances
˚
and angles for analogous compressed tetrahedral com-
plexes [27,30]. The central Cu(II) ion is located at the
center of symmetry along the C2 axis. Since only
R-enantiomers of 1-phenylethylamine were used for the
preparations, the absolute configuration was found to
be D(R,R) for the examined crystal of 4. As for the re-
lated bis[(N-R-1-phenylethyl)(3- or 5-halogen-)salicylide-
neaminato]M(II) complexes, 3,5-Cl-substituted Cu(II)
complex (1) of D(R,R) absolute configuration was re-
O1–Cu1–O1*
N1–Cu1–N1*
O1–Cu1–N1*
O1–Cu1–N1
Cu1–O1–C1
Cu1–N1–C7
Cu1–N1–C8
C7–N1–C8
O1–C1–C2
151.7(3)
159.4(2)
92.0(2)
93.0(2)
129.2(4)
124.0(4)
116.5(3)
119.4(4)
119.0(5)
122.7(5)
127.0(5)
115.2(4)
108.6(4)
111.0(5)
˚
˚
ported with Cu–N = 1.984(4) A, Cu–O = 1.890(3) A,
trans-N–Cu–N = 144.7(2)ꢀ and trans-O–Cu–O = 150.9
(1)ꢀ [30]. Unsubstituted Cu(II) [27], 3-EtO Cu(II) [30]
and 3-Cl Zn(II) complexes [47] were reported to be
K(R,R) absolute configuration. The pendant R-1-phenyl-
ethylamine groups of the ligands are on the same side of
the salicydenaminato plane, and the dihedral angle be-
tween the C1/C2/C3/C4/C5/C6 mean plane and the
C10/C11/C12/C13/C14 mean planes is 89.5(3)ꢀ. Intramo-
lecular and intermolecular hydrogen bonds could not be
detectable within van der Waals radii [48], crystal packing
as well as chiral molecular recognition of 4 are dominated
only by weak van der Waals forces during preparation or
crystallization.
O1–C1–C6
N1–C7–C6
N1–C8–C9
N1–C8–C10
C9–C8–C10
Atoms marked with *Õs are expanded by the symmetry operation
(ꢀx ꢀ 1, y, ꢀz ꢀ 2).
The trans-N1–Cu1–N1* and trans-O1–Cu1–O1* bond
angles are 159.4(2)ꢀ and 151.7(3)ꢀ, respectively, whereas
the cis-N1–Cu1–O1 and cis-N1*–Cu1–O1 bond angles
Fig. 2. The molecular structure of 8 showing the atom-labelling scheme (a) top view and (b) side view. Displacement ellipsoids are drawn at the 30%
probability level. H atoms are omitted for clarity.

2938
T. Akitsu, Y. Einaga / Polyhedron 24 (2005) 2933–2943
3.2. Description of structure of 8
3.3. IR spectroscopy
The molecular structure of 8 is depicted in Fig. 2, and
selected bond distances and angles of 8 are given in
Table 3. Complex 8 adopts a compressed tetrahedral
trans-[CuN2O2] coordination geometry similar to 4. This
structural feature suggests that steric hindrance of the
ethyl groups is sufficiently effective for distortion of
the coordination environment when electron-withdraw-
ing Br-groups are introduced. The central Cu(II) ion is
on the center of symmetry. The trans-N1–Cu1–N1*
and trans-O1–Cu1–O1* bond angles are 157.9(3)ꢀ and
151.0(3)ꢀ, respectively, whereas the cis-N1–Cu1–O1
and cis-N1*–Cu1–O1 bond angles are 93.3(2)ꢀ and
92.2(2)ꢀ, respectively. The Cu1–N1 and Cu1–O1 bond
In general, the IR spectra of the analogous free pro-
tonated ligands show an imine C@N band around
1635 cmꢀ1, and the disappearance of a C@O peak
around 1700 cmꢀ1 is indicative of a condensation reac-
tion giving Schiff base ligands [49]. The IR spectra of
all the complexes 1–10 show the imine C@N bands
around 1620 cmꢀ1, and it is found that the C@N bands
of the Cu(II) complexes are shifted by about 10 cmꢀ1 to
the lower energy regions compared to the free proton-
ated ligands [50]. In addition, the Cu–N and Cu–O
vibrations are found around 450 and 420 cmꢀ1, respec-
tively. These spectral characteristics are due to the coor-
dination of the imine nitrogens to the metal ions [51].
These peaks are in agreement with related complexes,
regardless of the degree of distortion of the trans-
[CuN2O2] coordination environment. The IR spectra
are not sensitive to distortion of coordination environ-
ment for the present complexes.
˚
distances are 1.966(6) and 1.865(4) A, respectively, and
˚
the C7@N1 imine bond length is 1.339(9) A. The degree
of distortion from square planar towards tetrahedral
environment of 8 is larger than that of 4, which leads
to elongate both Cu–N and Cu–O coordination bonds
of 8. The other geometric parameters in the Schiff base
ligands do not deviate significantly from the correspond-
ing common values. Selected geometrical parameters of
3.4. Diffuse reflectance electronic spectroscopy and
magnetic properties
˚
5 were reported to be Cu–N = 1.969(4) and 1.971(4) A,
˚
Cu–O = 1.886(3) and 1.892(3) A, trans-N–Cu–N =
The diffuse reflectance electronic spectra of 1–10
were recorded at room temperature, and the overall
spectral features are similar to that of analogous com-
plexes. The small differences of the spectral features
can be classified into five patterns: (i) 1 and 2, with
1-phenylethylamine moieties and 3,5-substituents, show
a broad d–d band in the region 17000–19000 cmꢀ1 and
a strong continuous band in the higher energy region
over 22000 cmꢀ1; (ii) 3 and 4, with 1-phenylethylamine
moieties and 5-substituents, show a broad d–d band in
the region 16000–19000 cmꢀ1 and a strong continuous
band in the higher energy region over 23000 cmꢀ1; (iii)
5 and 6, with 2-butylamine moieties and 3,5-substitu-
ents, show a strong d–d band around 15000 cmꢀ1
and a strong continuous band in the higher energy re-
gion over 23000 cmꢀ1; (iv) 7 and 8, with 2-butylamine
moieties and 5-substituents, show a broad d–d band in
the region 16000–19000 cmꢀ1 and a strong continuous
band in the higher energy region over 22000 cmꢀ1; (v)
9 and 10, with 2-propylamine moieties and 3,5-substit-
uents, show a strong d–d band around 14000 cmꢀ1 and
a continuous band in the higher energy region over
22000 cmꢀ1. In this way, d–d bands are more sensitive
to coordination geometries and electron-withdrawing
groups of the ligands than p–p* bands in the case of
solid state spectra. Generally, tetrahedral complexes
show a d–d band at lower energies, which shift to high-
er energies on moving from tetrahedral to square pla-
nar environments [52]. Apparently, the number of
halogen substituents (only 5- or both 3- and 5-) is more
effective than the kind of halogen (Cl or Br) substitu-
ents. The absence of pendant p-conjugated phenyl
158.4(2)ꢀ and trans-O–Cu–O = 158.7(1)ꢀ [29]. Both sides
of the 2-butyl pendant groups are on the apical side of
the umbrella molecular plane. Intramolecular and inter-
molecular hydrogen bonds are not detectable within van
der Waals radii [48] for 8.
Table 3
˚
Selected geometric parameters (A, ꢀ) for 8
Cu1–O1
Cu1–N1
N1–C7
C6–C7
O1–C1
N1–C8
C8–C9
C8–C11
1.865(4)
1.966(6)
1.339(9)
1.447(7)
1.284(8)
1.506(9)
1.43(1)
1.29(2)
O1–Cu1–O1*
N1–Cu1–N1*
O1–Cu1–N1*
O1–Cu1–N1
Cu1–O1–C1
Cu1–N1–C7
u1–N1–C8
C7–N1–C8
O1–C1–C2
O1–C1–C6
N1–C7–C6
N1–C8–C9
N1–C8–C10
C9–C8–C10
151.0(3)
157.9(3)
92.2(2)
93.3(2)
129.1(4)
124.1(4)
120.6(6)
115.2(7)
120.3(6)
123.9(5)
124.1(6)
115.0(8)
114.4(8)
130.5(9)
Atoms marked with *Õs are expanded by the symmetry operation
(ꢀx ꢀ 1, y, ꢀz ꢀ 1/2).

T. Akitsu, Y. Einaga / Polyhedron 24 (2005) 2933–2943
2939
groups in the Schiff base ligands results in the p–p*
region.
5 and 8, the effective magnetic moments demonstrate
the nature of the mononuclear complexes.
Theoretically, for Cu(II) complexes (effective symme-
2
try C2v point group) of the D state three transitions,
3.5. Structural phase transition
2A1ðdz2 Þ 2A1ðdx2ꢀy2 Þ; 2B1ðdzxÞ 2A1ðdx2ꢀy2 Þ and 2B2
ðdyzÞ 2A1ðdx2ꢀy2 Þ; are allowed, and a broad d–d band
is their convolution. A strong band in the UV region
is due to an overlap of the p–p* transition [47,53] and
phenolate O atoms of the ligand to Cu(II) metal charge
transfer transition. These transitions are related not only
to trans-[CuN2O2] coordination geometries but also the
net atomic charge on the coordinating atoms (O and N)
of the ligands.
The effective magnetic moments (leff) of mononuclear
Cu(II) complexes 1–10 at 298 K are in the range from
ca. 1.4 to 1.9 B.M., which is reasonable compared to
the theoretical spin-only value of 1.73 B.M. for S = 1/2
systems with a single unpaired electron sited in an
essentially dx2ꢀy2 orbital, the 2D state. In contrast to
the electronic spectra, the effective magnetic moment is
not sensitive to distortion of the trans-[CuN2O2] coordi-
nation environment for the present complexes. In order
to confirm that there is little or no intermolecular anti-
ferromagnetic coupling, we also investigated variable-
temperature (5–250 K) magnetic susceptibility behavior
for 1, whose crystal structure has been determined.
Complex 1 obeys the Curie–Weiss law approximately,
that is the vꢀM1 versus T plot is a linear graph. Thus, in
the absence of X-ray structural proof, except for 1, 4,
It has been investigated that the analogous Cu(II)
and Ni(II) complexes exhibit a thermally-induced struc-
tural phase transition by heating regardless of the coor-
dination geometries (square planar or tetrahedral) at
room temperature [21,37]. The temperature of the struc-
tural phase transition (endothermic peaks) and decom-
position (exothermic peaks) in the DSC curves for 1–10
are summarized in Table 4. In analogy with the related
Cu(II) complexes [21], the structural phase transition
may be ascribed to a transformation from a low tempera-
ture phase (compressed tetrahedral) into a high tempera-
ture phase (close to right tetrahedral). The relationship
between the transition temperature and chemical struc-
tures is unclear still. Because of the lability of the
compounds, we could not carry out quantitative measure-
ments. The structural phase transition seems to be irre-
versible for 1–10.
Furthermore, we have examined the possibility of a
photo-induced structural phase transition of microcrys-
talline samples for 1–10 with IR spectroscopy in the so-
lid state at room temperature. After irradiation of UV
or visible light for 5 min at room temperature, no spec-
tral changes could be detected for 1–10. On the other
hand, certain thermochromic Cu(II) complexes also
exhibited photochromism in the solid state at low tem-
perature [7,10]. Thus, at least, the present experimental
facts support the stability of the Cu(II) complexes
against UV or visible light at room temperature.
Table 4
Temperature (K) of the phase transition and decomposition by DSC
for 1–10
Phase transition
Decomposition
3.6. Calculation
1
2
419
484
474
412
396
410
412
400
444
446
556
547
547
542
557
532
548
544
542
554
3
A summary of the results of ZINDO calculations for
1, 4, 5 and 8, whose crystal structures are known, is
listed in Table 5. The dipole moment of the overall mol-
ecules were evaluated to be 6.029, 5.033, 4.166 and
4.215 D for 1, 4, 5 and 8, respectively. It can be expected
that both introduction of 3,5-halogen substituents and
deviation from square molecules as well as breaking
4
5
6
7
8
9
10
Table 5
Summary of the results of ZINDO calculations for 1, 4, 5 and 8
1
4
5
8
Heat of formation (kJ molꢀ1
Dipole moment (D)
)
325.2
6.029
295.9
5.033
255.4
4.166
251.8
4.215
HOMO–LUMO gap (eV)
Formal charge Cu
8.20
0.196
8.09
0.250
7.88
0.204
7.77
0.200
N
O
ꢀ0.129
ꢀ0.084
ꢀ0.143
ꢀ0.156
ꢀ0.456
ꢀ0.433
ꢀ0.483
ꢀ0.463
Calculated predominant p–p* transitions (cmꢀ1
)
32700
45700
51300
33100
36100
48300
31200
34400
45700
29700
32900
44400

2940
T. Akitsu, Y. Einaga / Polyhedron 24 (2005) 2933–2943
20
molecular symmetry lead to an increase of the magni-
tude of dipole moment. The numbers and kinds of 3,5-
halogen substituents are invariable factors for a given
molecule, whereas distortion of the overall molecular
structure can induce polarity depending on the molecu-
lar conformation. Thus, the dipole moment can interact
between the complexes and the surrounding species in
solution.
a
15
10
5
0
The formal charges on the Cu(II) ions are 0.196,
0.250, 0.204 and 0.200 for 1, 4, 5 and 8, respectively.
On the other hand, the averaged values on the coordina-
tion N atoms are ꢀ0.129, ꢀ0.084, ꢀ0.143 and ꢀ0.156,
and O atoms are ꢀ0.456, ꢀ0.433, ꢀ0.483 and ꢀ0.463
for 1, 4, 5, and 8, respectively. As indicated for these val-
ues, tetrahedral distortion of the coordination environ-
ment plays a role in distribution of charge from the
coordination atoms to the metal ions through coordina-
tion bonds. Although these electronic features are
advantageous for charge transfer transitions, it is diffi-
cult to estimate their contribution in electronic spectra.
Further, p–p* bands consist of several components in
the UV regions as calculations exhibit even for the
X-ray structures.
-5
5
b
4
3
2
1
12000
16000
20000
wavenumber / cm-1
24000
28000
32000
Fig. 3. The electronic (a) CD and (b) absorption spectra of 1 in
0.1 mmolꢀ1 dm3 CHCl3 solution at 298 K.
3.7. Electronic absorption spectroscopy in various solvents
Fig. 3 shows the electronic absorption spectra and
CD spectra of 1 in 0.1 mmol dmꢀ3 chloroform solution
at 298 K. The reason why 1 is employed for the follow-
ing investigation and discussion is that 1 displays the
largest dipole moment in the solid state and incorporates
chiral ligands whose CD spectrum is detectable.
Additionally, bulky 1-phenylethyl moieties and elec-
tron-withdrawing 3,5-Cl-substituents may enhance the
flexibility of 1. The absorption spectrum of 1 exhibits
a broad d–d band in the region 16000–20000 cmꢀ1
and a strong p–p* band in the region 24000–
28000 cmꢀ1. The corresponding CD spectrum of 1
shows a positive peak around 15000 cmꢀ1 and a nega-
tive peak around 18000 cmꢀ1 in the d–d region, and a
negative peak around 23000 cmꢀ1 and a positive peak
around 25000 cmꢀ1 in the p–p* region. The d–d band
can be ascribed to convolution of three allowed transi-
n2D0 ¼ 1.494) (the figures in parentheses denote dipole
moment, l(D), dielectric constant, e, and refractive in-
dex, n2D0) [15]. The predominant peaks of p–p* bands
(m cmꢀ1 and log e dm3 molꢀ1 cmꢀ1) are shown in Table 6.
The results show that the p–p* bands are sensitive to
the physical properties of the solvents for 1–10. How-
ever, the shifts of the d–d band, depending on solvents
or concentrations (1, 0.1, and 0.01 mmol dmꢀ3), are rel-
atively small for 1–10. Therefore, we can conclude that
the changes of polarity of the solvent lead to structural
changes of the complexes associated with conformation
of the p-conjugated pendant groups of the Schiff base
ligands.
In an attempt to produce solvatochromism induced by
a photochromic solute, we have measured the absorption
spectra of a 0.1 mmol dmꢀ3 chloroform solution of 1
containing equimolar 4-hydroxyazobenzene (4HAZ)
(Scheme 2). It is well known that irradiation with UV
and visible light results in reversible photoisomerization
of 4HAZ to the cis forms (l = ca. 3 D) and the trans
forms (l = almost 0 D). For a solution of only 4HAZ
(Fig. 4(a)), irradiation with UV and visible light for
5 min, which is enough time to saturate the photoiso-
merization reaction, to the solution results in reversible
spectral changes of the p–p* band of 4HAZ around
29000 cmꢀ1. On the other hand, for a solution contain-
ing 4HAZ and 1, very slight and relatively large spectral
changes could be observed at ca. 23000 cmꢀ1 and ca.
31000 cmꢀ1. Takingintoaccounttheabsorptionspectrum
2
2
tions, 2A1ðdx2ꢀy2 Þ ! A1ðdz2 Þ; 2A1ðdx2ꢀy2 Þ ! B1ðdxzÞ;
2
2A1ðdx2ꢀy2 Þ ! B2ðdyzÞ; in C2v symmetry. The sign of
the CD peaks seems to be reasonable with respect to
the D(R,R) absolute configuration of 1. The change of
coordination geometry of 1 and conformation of its li-
gands in solution may attribute to the difference between
absorption spectra and diffuse reflectance spectra.
The electronic absorption spectra were also recorded
for 1–10 in various solvents, N,N0-dimethylformamide
(DMF) (l = 3.82; e = 36.710; n2D0 ¼ 1.428), acetone
(l = 3.11; e = 20.700; n2D0 ¼ 1.356), methanol (l = 2.97;
e = 32.630; n2D0 ¼ 1.326), chloroform (l = 1.11; e =
4.806; n2D0 ¼ 1.444) and toluene (l = 0.37; e = 2.438;

T. Akitsu, Y. Einaga / Polyhedron 24 (2005) 2933–2943
2941
Table 6
Predominant p–p* peaks (m(cmꢀ1); log e (dm3 molꢀ1 cmꢀ1)) of the absorption spectra 0.1 mmol dmꢀ3 in various solvents at 298 K for 1–10
DMF
Acetone
Methanol
Chloroform
Toluene
1
2
26530(4.10)
26460(4.05)
26460(4.04)
33220(3.96)
26390(3.70)
33220(3.64)sh
26460(4.03)
32680(3.93)
26460(3.95)
32680(3.89)
26530(4.17)
33000(4.13)sh
26600(3.96)
33220(3.97)sh
26670(4.06)
32260(4.01)
26670(4.09)
32360(4.08)
26740(4.10)
32570(4.08)
26810(4.11)
32790(4.17)sh
26180(4.23)
33000(4.20)sh
26180(3.84)
33000(3.82)sh
26180(4.12)
32570(4.06)
26180(4.10)
32570(4.08)
26320(4.10)
33220(4.09)
26320(4.09)
32260(4.06)
26320(4.09)
32260(4.06)
26320(3.94)
32260(3.95)
26390(4.00)
32360(4.00)
26460(4.00)
32360(4.04)
26110(4.12)
26110(4.22)
26040(4.21)
26110(4.23)
26250(4.10)
26250(4.11)
26520(4.11)
26110(4.45)
26300(4.17)
26250(4.12)
26530(4.00)
26390(4.05)
26460(4.03)
26670(4.24)
26600(3.65)
26460(4.06)
26390(4.25)
26390(4.05)
26600(4.28)
26600(4.00)
26530(4.25)
26600(4.12)
26670(3.91)
26670(3.93)
3
4
5
6
7
26600(4.23)
32150(4.18)
26600(4.08)
32150(4.05)
26670(4.18)
8
9
10
26740(4.09)
sh
The figures marked with denotes a shoulder, and the remaining ones are bands.
of only 1 (Fig. 3(b)), the former band and the latter band
are ascribed to d–d and p–p* bands of 1. The spectral
change caused by the photoisomerization of 4HAZ
may be ascribed mainly to a conformational change of
the pendant 1-phenylethylamine moiety of the ligands
(Fig. 4(b)). The possibility of supramolecular helical
structures of 4HAZ induced by 1 as chiral dopants in
the solution can be denied, because the spectral changes
are detected not by CD spectra but by absorption spec-
tra. The fact that solvatochromic behavior mainly
affects the p–p* bands, as indicated in Table 6, and the
magnitude of the difference of dipole moment photoiso-
merization of 4HAZ may support this interpretation.
Although the other complexes also exhibit spectral shifts
depending on the solvent, we could not observe
photoisomerization caused by 4HAZ. The appropriate
pendant 1-phenylethylamine moiety and molecular flex-
ibility enhanced by electron-withdrawing substituents
are necessary for the complex served for this experiment.
1
a
vis UV
0.5
0
12000
16000
20000
24000
28000
32000
4
Cl
*
b
Cl
O
N
N
O
Cu
3
2
1
Cl
*
Cl
1
vis UV
vis
UV
UV
visible
0
12000
16000
20000
24000
28000
32000
HO
wavenumber / cm-1
HO
trans
cis
Fig. 4. The absorption spectra of (a) only 4HAZ and (b) 4HAZ and 1
in 0.1 mmolꢀ1 dm3 CHCl3 solution at 298 K before and after UV or
visible light irradiation for 5 min.
Scheme 2. Molecular structure of 1 and cis–trans photoisomerization
of 4HAZ.

2942
T. Akitsu, Y. Einaga / Polyhedron 24 (2005) 2933–2943
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This work was supported by Grant-in-Aid for the
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Professor Hidenari Inoue (keio University) for the use
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