Synthesis and crystal structures of the flexible Schiff base complex bis(N-1,2-diphenylethyl-salicydenaminato-κ2N,O)copper(II) (methanol): A rare case of solvent-induced distortion

Article abstract of DOI:10.1016/j.poly.2005.07.048

A new mononuclear copper(II) complex incorporating a bulky Schiff base ligand, bis(N-1,2-diphenylethyl-salicydenaminato-κ²N,O) copper(II) (green form) and its co-crystal containing methanol solvent molecules (brown form) have been synthesized and characterized by X-ray crystallography, XRD, IR and diffuse reflectance spectra in the solid state. This is a rare case in that both solvent free and containing crystals can be isolated and characterized. Both forms adopt a compressed tetrahedral trans-[CuN ₂O₂] coordination geometry with coordination bond angles of trans-N-Cu-N = 157.0(4)° and trans-O-Cu-O = 151.1(3)° for the brown form and trans-N-Cu-N = 141.3(3)° and trans-O-Cu-O = 139.9(2)° for the green form. The methanol solvent molecules force the complex of the brown form to adopt a more planar coordination environment in the solid state. Semi-empirical calculations (ZINDO) revealed the magnitude of the dipole moment of both molecules and that the energy of the green form is 5.7 kJ mol ^-1 higher than that of the brown form. In addition, absorption spectra in various organic solvents were measured and a relationship between absorption spectra and polarity of the solvents is discussed.

Full text of DOI:10.1016/j.poly.2005.07.048

Polyhedron 25 (2006) 1089–1095
www.elsevier.com/locate/poly
Synthesis and crystal structures of the flexible Schiff base complex
bis(N-1,2-diphenylethyl-salicydenaminato-j2N,O)copper(II)
(methanol): A rare case of solvent-induced distortion
*
*
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 5 February 2005; accepted 27 July 2005
Available online 11 October 2005
Abstract
A new mononuclear copper(II) complex incorporating a bulky Schiff base ligand, bis(N-1,2-diphenylethyl-salicydenaminato-
j2N,O)copper(II) (green form) and its co-crystal containing methanol solvent molecules (brown form) have been synthesized and char-
acterized by X-ray crystallography, XRD, IR and diffuse reflectance spectra in the solid state. This is a rare case in that both solvent free
and containing crystals can be isolated and characterized. Both forms adopt a compressed tetrahedral trans-[CuN2O2] coordination
geometry with coordination bond angles of trans-N–Cu–N = 157.0(4)ꢀ and trans-O–Cu–O = 151.1(3)ꢀ for the brown form and trans-
N–Cu–N = 141.3(3)ꢀ and trans-O–Cu–O = 139.9(2)ꢀ for the green form. The methanol solvent molecules force the complex of the brown
form to adopt a more planar coordination environment in the solid state. Semi-empirical calculations (ZINDO) revealed the magnitude
of the dipole moment of both molecules and that the energy of the green form is 5.7 kJ molꢀ1 higher than that of the brown form. In
addition, absorption spectra in various organic solvents were measured and a relationship between absorption spectra and polarity of the
solvents is discussed.
ꢁ 2005 Elsevier Ltd. All rights reserved.
Keywords: Copper(II) complexes; Crystal structures; Schiff base; Solvent
1. Introduction
lideneimine)copper(II) complexes incorporating Schiff
base ligands [13] exhibit flexible structural changes of
coordination spheres from square planar to tetrahedral
depending on steric and/or electronic effects of the ligands,
such as bulky substituents [14–17] and chirality [18–22].
Furthermore, they also exhibit structural isomers [23],
thermally induced structural phase transitions [24–28]
and solvatochromism [13]. Indeed, some other copper(II)
complexes incorporating monodentate amine ligands easily
vary their coordination geometries depending on system-
atic steric effects of ligands such as 1,2-diphenylethylamine
[29] and chirality, for example of 1-phenylethylamine
[30,31] or 1-cyclohexylethylamine [30]. However, so far it
has been rare that structural distortion is induced by guest
molecules into host mononuclear copper(II) complexes
[32,33]. We report here a structural comparison of the
bis(N-1,2-diphenylethyl-salicydenaminato-j2N,O)copper(II)
complex (green form) and its co-crystal containing metha-
In recent years, functional materials of transition metal
complexes whose physical properties can be controlled by
various external perturbations, such as light irradiation,
have been attracting great interest [1,2]. A general strategy
to achieve such photo-switching materials has not been
established as yet. One of the hypotheses may be examining
candidates which exhibit structural isomerism or structural
phase transitions, induced by other external conditions,
with a small energy barrier, for example, temperature,
pressure, solvents and so on. Thus, thermochromic [3–5],
piezochromic [6,7], solvatochromic [8–10] and vapochro-
mic [11,12] complexes may be suitable for this purpose.
It has been well known that a series of bis(N-alkyl-salicy-
*
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.2005.07.048

1090
T. Akitsu, Y. Einaga / Polyhedron 25 (2006) 1089–1095
(100 mL) at 318 K for 3 h gave rise to brown crude precip-
itates. Recrystallization from an acetone solution at 298 K
over a period of several days yielded green plate-like
crystals suitable for X-ray crystallography, green form
(1). Anal. Calc. for C42H36CuN2O2: C, 75.94; H, 5.46; N,
4.22. Found: C, 75.68; H, 5.37; N, 4.17%. IR (KBr):
1610 cmꢀ1 (imine C@N stretching band). M.p. 572 K
(decomposition). leff = 1.52 BM at 298 K. On the other
hand, recrystallization from methanol–acetone mixed solu-
tion (1:1, v/v) at 298 K over a period of several days yielded
a brown precipitate. Anal. Calc. for C42H36CuN2O2: C,
75.94; H, 5.46; N, 4.22. Found: C, 75.84; H, 5.46; N,
4.12%. IR (KBr): 1609 cmꢀ1 (imine C@N stretching band).
M.p. 559 K (decomposition). leff = 1.43 BM at 298 K.
Well-grown brown prismatic single crystals suitable for
X-ray crystallography contain methanol molecules, brown
form (2).
O
N
N
O
Cu
1
2 = 1 • CH3OH
Scheme 1.
nol solvent molecules (brown form). Guest methanol
molecules force the complex to adopt a more planar coor-
dination environment in the solid state. In addition,
absorption spectra in various organic solvents have been
measured and a relationship is found between the features
of absorption spectra and polarity of solvents as chemical
external perturbations (Scheme 1).
2.3. X-ray crystallography
Intensity data were collected on a Rigaku AFC-7R four-
circle diffractometer. The structures were solved by direct
methods using SIR92 [34] and expanded by Fourier tech-
niques. The structures were refined on F2 anisotropically
for non-hydrogen atoms by full-matrix least-squares meth-
ods with SHLEXL-97 [35] of the teXsan program package
[36]. Empirical absorption corrections were applied based
on w scans; transmission factors for 1 and 2 were 0.726–
0.933 and 0.919–0.999, respectively. No significant decay
in the intensity of three standard reflections was observed
throughout the data collection. The hydrogen atoms were
located at geometrically calculated positions, C–H =
2. Experimental
2.1. General
Chemicals of the highest commercial grade available
(Aldrich and Wako) were used as received without further
purification. Elemental analyses (C, H, N) were carried out
on an Elementar Vario EL analyser at Keio University.
Infrared spectra were recorded as KBr pellets on a JASCO
FT-IR 660 plus spectrophotometer in the range of 4000–
400 cmꢀ1 at 298 K. Thermal analysis was performed on
a SHIMADZU DSC-60 differential scanning calorimeter
(DSC), where the heating rate was 10 K minꢀ1 in the range
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 measured with a Sherwood
Scientific magnetic susceptibility balance at 298 K. Mag-
netic data were corrected for the magnetization of diamag-
netic contributions, which were estimated from Pascal
constants. XRD were recorded on a Rigaku RAD-C dif-
˚
0.950 A, and refined isotropically. All the non-hydrogen
atoms were refined anisotropically. As for 2, C8(H6A)–
C9A(H7A, H8A) and C8(H6B)–C9B(H7B, H8B) were
treated as a positional disorder with 0.5 occupancy. Crys-
tallographic data for 1 and 2 are summarized in Table 1.
3. Results and discussion
3.1. Description of structures
The molecular and crystal structures of 1 and 2 are de-
picted in Figs. 1–4, respectively. Selected bond distances
and angles, and torsion angles of 1 and 2 are given in
Tables 2 and 3, respectively.
The green form (1) affords a compressed tetrahedral
trans-[CuN2O2] coordination geometry, in which the biden-
tate chelate ligands coordinate to the copper ion through
phenolate oxygen atoms and imine nitrogen atoms. The
˚
fractometer with Cu Ka radiation (k = 1.5418 A). Semi-
empirical molecular orbital calculations were performed
the program ZINDO in a CAChe software package based
on the crystal structures determined.
˚
Cu–N bond distances are 1.959(10) and 1.990(9) A, while
˚
the Cu–O bond distances are 1.877(7) and 1.885(8) A.
2.2. Synthesis
These values are comparable to related non-planar or dis-
torted tetrahedral complexes [16,17,19,22]. The trans-N–
Cu–N and trans-O–Cu–O bond angles are 157.0(4)ꢀ and
151.1(3)ꢀ, respectively. The dihedral angle between the
O1/Cu1/N1 plane and the O2/Cu1/N2 plane is 36.46(1)ꢀ,
Treatment of an equimolar amount of copper(II) acetate
(0.91 g, 5.00 mmol), salicylaldehyde (1.22 g, 10.0 mmol)
and 1,2-diphenylethylamine (1.97 g, 10.0 mmol) in ethanol

T. Akitsu, Y. Einaga / Polyhedron 25 (2006) 1089–1095
1091
Table 1
Crystallographic data for 1 and 2
1
2
Formula
C42H36CuN2O2
664.28
C43H40CuN2O3
696.32
Molecular weight
Crystal system
Space group
monoclinic
P21/a (No. 14)
19.06(2)
10.363(8)
17.49(1)
103.22(7)
3362(5)
monoclinic
P21 (No. 4)
20.668(5)
9.329(2)
9.281(4)
91.06(3)
1789.2(10)
2
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢀ)
3
˚
V (A )
Z
4
Dcalc (g cmꢀ3
F(000)
)
1.312
1388
0.689
1.293
730
0.652
l (mmꢀ1
)
Crystal size (mm)
Temperature (K)
0.50 · 0.40 · 0.10
298
0.30 · 0.30 · 0.30
298
h
min, hmax (ꢀ)
5.10, 27.55
0.71073
5.05, 27.50
0.71073
Wavelength (Mo Ka)
Total reflections
7781
4737
Observed reflections (I > 2r(I))
Number of refined parameters
Ra, Rwb, S
2627
425
3127
407
0.0957, 0.3814, 1.036
ꢀ0.82, 0.57
0.0539, 0.1744, 0.936
ꢀ0.41, 1.60
3
˚
Minimum and maximum residual densities (e/A )
P
P
a
R ¼ kF oj ꢀ jF ck= jF oj.
P
P
2
2 1=2
b
Rw ¼ ð wðjF oj ꢀ jF cj Þ = w jF oj Þ , w = 1/(r2(Fo) + (0.1469P)2 + 10.7070P) for 1, and w = 1/(r2(Fo) + (0.102P)2 + 0.0866P) for 2, where
P ¼ ðF 2o þ 2F 2c jÞ=3.
Fig. 1. The molecular structure of 1 showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 50% probability level.
Fig. 2. Crystal packing of 1 viewed from the crystallographic b axis.
which results from two skewed ligands at opposite sides.
This distorted coordination geometry is mainly ascribed
to the steric hindrance of the bulky 1,2-diphenylethylamine
moieties [29]. One of two 1,2-diphenylethylamine moieties
of 1 is folded up, being close to the inner side of the mole-
cule, while another one is expanded on the outside of the
molecule. The corresponding torsion angles are N1–
C8–C9–C16 = 65(1) and N2–C29–C30–C37 = ꢀ160.9(8)ꢀ.
The imine C@N double bond lengths are 1.27(1) and

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T. Akitsu, Y. Einaga / Polyhedron 25 (2006) 1089–1095
Table 2
˚
Selected geometric parameters (A, ꢀ) for 1
Cu1–O1
Cu1–N1
N1–C7
C1–C7
O1–C2
C8–C9
1.877(7)
1.959(10)
1.27(1)
1.44(1)
1.31(1)
1.34(3)
1.56(3)
1.55(2)
Cu1–O2
Cu1–N2
N2–C28
C22–C28
C23–O2
C29–C30
C29–C31
C30–C37
1.885(8)
1.990(9)
1.28(1)
1.45(2)
1.32(1)
1.50(1)
1.55(1)
1.54(2)
C8–C10
C9–C16
O1–Cu1–O2
O1–Cu1–N1
O2–Cu1–N1
Cu1–O1–C2
Cu1–N1–C7
C7–N1–C8
C1–C7–N1
151.1(3)
N1–Cu1–N2
O1–Cu1–N2
O2–Cu1–N2
Cu1–O2–C23
Cu1–N1–C8
C28–N2–C29
C22–C28–N2
157.0(4)
94.2(3)
92.1(3)
126.9(8)
117.8(7)
116.7(8)
126.1(9)
92.8(3)
92.3(4)
128.2(7)
125.7(8)
115(1)
125(1)
N1–C8–C9–C16
C7–N1–C8–C10
65(1)
ꢀ44(1)
N2–C29–C30–C37
C28–N2–C29–C31
ꢀ160.9(8)
80.5(10)
Table 3
˚
Selected geometric parameters (A, ꢀ) for 2
Cu1–O1
Cu1–N1
N1–C7
C1–C7
O1–C2
C8–C9A
C8–C10
C9A–C16
1.906(5)
1.997(5)
1.259(9)
1.433(10)
1.313(7)
1.18(2)
Cu1–O2
Cu1–N2
N2–C28
C22–C28
C23–O2
C29–C30
C29–C31
C30–C37
1.875(4)
1.973(5)
1.286(8)
1.445(8)
1.303(7)
1.541(9)
1.539(7)
1.519(7)
Fig. 3. The molecular structure of 2 showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 50% probability level. Disor-
dered atoms are omitted for clarity.
1.54(2)
1.57(2)
O1–Cu1–O2
O1–Cu1–N1
O2–Cu1–N1
Cu1–O1–C2
Cu1–N1–C7
C7–N1–C8
C1–C7–N1
139.9(2)
N1–Cu1–N2
O1–Cu1–N2
O2–Cu1–N2
Cu1–O2–C23
Cu1–N1–C8
C28–N2–C29
C22–C28–N2
141.3(3)
95.9(2)
95.6(2)
126.9(4)
119.6(6)
121.3(5)
126.9(6)
92.6(2)
101.7(2)
127.9(4)
123.6(5)
113.3(6)
127.4(6)
N1–C8–C9A–C16
C7–N1–C8–C10
120(1)
116.0(8)
N2–C29–C30–C37
C28–N2–C29–C31
165.6(5)
ꢀ22.9(8)
the two 1,2-diphenylethylamine moieties face the inner
side of the crystal lattice. A parallel arrangement of the
molecules attribute to form molecular alignment along
the crystallographic a axis. It should be noted that there
is no space to include additional small molecules.
In contrast to 1, the brown form (2) contains methanol
molecules as crystalline solvent. Individual molecules of 2
also afford a compressed tetrahedral trans-[CuN2O2] coor-
dination geometry. The Cu–N bond distances are 1.977(5)
Fig. 4. Crystal packing of 2 viewed from the crystallographic b axis.
˚
1.28(1) A, which indicates that there is a sufficient decrease
˚
of the electronic distribution of dp–pp character through
the p-conjugated system to cause distortion towards a tet-
rahedral environment [37]. The rest of the geometric
parameters are unremarkable among analogous copper(II)
complexes. No specific intramolecular hydrogen bonds
could be detectable within van der Waals radii [38].
On the other hand, its crystal packing does not reveal
remarkable intermolecular interactions, hence the crystals
are estimated to be aggregated by weak van der Waals
forces. In the crystal of 1, the phenyl groups of one of
and 1.973(5) A and the Cu–O bond distances are 1.906(5)
˚
and 1.875(4) A. These values are also comparable to the re-
lated non-planar complexes [16,17,19,22]. The trans-N–
Cu–N and trans-O–Cu–O bond angles are 141.3(3)ꢀ and
139.9(2)ꢀ, respectively. The dihedral angle between the
O1/Cu1/N1 plane and the O2/Cu1/N2 plane is 54.46(1)ꢀ.
These figures suggest that the degree of tetrahedral distor-
tion deviated from square planar of 2 is larger than that
of 1. Although the main part of 1 and 2 is identical, the
structural differences are ascribed to methanol solvent

T. Akitsu, Y. Einaga / Polyhedron 25 (2006) 1089–1095
1093
and conformation and arrangement of the ligands, espe-
cially the 1,2-diphenylethylamine moieties. As for 2, all
phenyl groups of 1,2-diphenylethylamine moieties in the li-
gands are expanded outside of the molecule. The torsion
angles are N1–C8–C9A–C16 = 120(1) and N2–C29–C30–
C37 = 165.6(5)ꢀ. The imine C@N double bond lengths of
the slight difference in distortion of coordination environ-
ment between 1 and 2 is not detectable by IR spectra.
The diffuse reflectance electronic spectra of 1 and 2 were
recorded in the solid state. Both complexes exhibit a broad
band in the region 15000–20000 cmꢀ1 and another strong
band in the region 21000–24000 cmꢀ1. The former broad
d–d bands are attributed to a ligand field transition of
the 3d9 copper(II) ion. In general for copper(II) complexes
in C2 symmetry, the free copper(II) ion of 3d9 configura-
˚
1.259(9) and 1.286(8) A are similar to the corresponding
values for 1 regardless of the degree of distortion [26–28].
Nevertheless, 2 contains methanol molecules as solvent,
neither intramolecular nor intermolecular hydrogen bonds
could be detectable in the crystals. However, a pair of phe-
2
2
2
2
2
tion with the D state splits into A1, A2, B1 and B2
2
levels. For this splitting, three transitions, 2A1 ! A1,
2
2
2
nyl groups, with a distance between them of about 4.2 A,
2A1 ! B1 and A1 ! B2, can be expected to be allowed,
and the broad band is due to a combination of the three al-
lowed transitions. A slight difference in the spectral features
and apparent colors of 1 and 2 are caused by the ligand
field splitting derived from the distortion of the coordina-
tion environment. Indeed, there is a tendency that tetrahe-
dral complexes show a d–d band at lower energies, which
shifts to higher energies on distorting from tetrahedral to
square planar [23]. On the other hand, the strong band
found at higher energies is due to overlap of a p–p* transi-
tion [49,50] and ligand (phenolate O) to copper(II) metal
charge transfer (LMCT) transition. The peaks of these
transitions are related to not only trans-[CuN2O2] coordi-
nation geometries but also the net atomic charge on the
coordinating atoms (O and N) of the ligands.
˚
are faced as if they formed p–p stacking. The methanol sol-
vent molecules are included in the void space between these
quasi-stacking phenyl groups and its parent salicydenami-
nano moiety to stabilize crystal packing.
Recently, metal-organic frameworks for nano-scaled
pores [39–43] and structural tuning by containing guest sol-
vent molecules [44–48] have been studied widely in the field
of supramolecular chemistry. In contrast to many of these
metal-organic frameworks, the present complexes, 1 and 2,
are flexible mononuclear complexes without bridging li-
gands or intermolecular interactions. Interestingly, 1 (and
2) may be a rare example which can vary its individual
molecular structure as well as overall crystal structure to
fit, or be induced by, solvent methanol molecules. The crys-
tal lattice differences are also supported by XRD patterns
for 1 and 2 (Fig. 5). The results are also consistent with
simulated XRD patterns derived from the analyses of sin-
gle crystal structures qualitatively.
3.3. Calculations
The main results of semi-empirical calculations (ZIN-
DO) [51,52] are summarized in Table 4. Single point calcu-
lations of heat of formation of the crystal structures
determined show a difference of 5.7 kJ molꢀ1 between 1
and 2. The fact that the energy of the green form (1), close
to tetrahedral, is higher than that of the brown one (2),
close to square planar. This indicates that the structural
conformation of the ligands in individual molecules of 2
is more stable thermodynamically. In other words, provid-
ing external energy may be necessary to convert from the
stable form 1 into the unstable form 2.
3.2. IR and electronic spectra in the solid state
The IR spectra of 1 and 2 show bands at 1610 and
1609 cmꢀ1, respectively, which are ascribed to a stretching
vibration of the imine C@N groups. The IR spectra of both
complexes studied are in agreement with previous reports
of analogous complexes (around 1620 cmꢀ1). However,
The formal charges on the coordinating O and N
atoms of the ligands and copper(II) ions have been cal-
culated and the net atomic charges thus obtained have
been averaged. The formal charges on copper(II) ions
are 0.190 and 0.141 for 1 and 2, respectively, which indi-
cates that the more tetrahedrally distorted coordination
environment prevents charge distribution through the
coordination bonds. This tendency is qualitatively consis-
tent with conventional interpretation; distortion from
regular planar to tetrahedral leads to a weakening of
both r- and p-characters of coordination bonds. As
the ligands of 1 and 2 are identical, the difference can
be ascribed to purely coordination geometries. However,
the correlation between the average negative charge of
the coordinating atoms and the peaks may not be simple,
because the electronic spectra consist of several types of
transitions.
o
x 1
o
2
o
o
o
x
x
o
o
o
x
o
x
o
x
x
o
x
o
10
15
20
25
30
2θ/Þ
Fig. 5. XRD patterns of 1 and 2.

1094
T. Akitsu, Y. Einaga / Polyhedron 25 (2006) 1089–1095
Table 4
Summary of calculated values of several properties for 1 and 2
1
2
Heat of formation (kJ molꢀ1
)
358.1
352.4
Dipole moment (Debye)
4.96
6.96
Formal charge Cu
0.190
0.141
N
O
ꢀ0.173
ꢀ0.237
ꢀ0.477
ꢀ0.230
The lowest p–p* transition
29200
26100
Calculated (cmꢀ1
)
(LUMO ! HOMO ꢀ 1)
(LUMO + 4 ! HOMO ꢀ 1)
The dipole moments of 1 and 2 were evaluated to be
4.96 and 6.96 Debye, respectively. These values can be con-
sidered as an indication of the polarity of the molecules in
their given conformations. Since 1 and 2 are distortion iso-
mers, the larger dipole moment of 2 can be attributed to the
conformation of the ligands.
are: acetone (l = 3.11, e = 20.7), chloroform (l = 1.11,
e = 4.81), methanol (l = 2.97, e = 32.6), toluene (l =
0.37, e = 2.44) and DMF (l = 3.82, e = 36.7). The p–p*
bands exhibit a slight tendency of negative solvatochro-
mism, namely the peaks of the bands shift to higher ener-
gies in solvents of increasing polarity. The results of the
molecular orbital calculations above suggest that tetrahe-
drally distorted molecules indicate larger polarity. How-
ever, weak d–d bands are less sensitive to the polarity
of solvents.
In view of the theory of dielectric polarization [8], nega-
tive solvatochromism is expected for transitions from a
higher dipole moment state to a smaller one. Generally,
the LMCT transition moment lies antiparallel to the
ground state dipole moment, when the ground state is more
polar than the excited state. In this case, the ground state is
stabilized by polar solvents and its transition energies are
increased. On the other hand, in the cases where the peak
of a band for a certain type of transition can shift in accor-
dance with intermolecular interactions between the dipole
moment of solutes and solvents, we should consider in-
duced short-range specific interactions such as hydrogen
bonds and p–p stacking.
3.4. Absorption spectra in various solvents
As shown in Fig. 6, the absorption spectra of 1 in the
range 18000– 30000 cmꢀ1 are characterized by a strong
band around 27000 cmꢀ1 band, and a weak shoulder-like
band around 24000 cmꢀ1. The former is assigned to a
p–p* transition in the ligands, while the latter is a d–d
band reflecting the coordination environment. The p–p*
peaks (wavenumber) and their intensity (loge) are solvent
dependent, as well as occasionally allowing the slight
appearance of the shoulder-like band shape of the d–d re-
gion to shift. The shift of the p–p* region is as follows:
acetone (27000 cmꢀ1
,
4.16), chloroform (26900 cmꢀ1
,
,
4.08), methanol (27100 cmꢀ1, 4.04), toluene (27000 cmꢀ1
4.17) and DMF (26900 cmꢀ1, 4.06). The dipole moment
(l, Debye) and dielectric constant (e) of the solvents used
For the present case, however, the correlation between
the dipole moment of the complexes and the physical prop-
erties of solvents do not allow us to draw a conclusion that
the various solvents attribute to the shift of the LMCT
transitions of the complexes. We consider only the di-
pole–dipole part of the van der Waals solute–solvent inter-
actions, neglecting dipole-induced dipole interactions and
LondonÕs dispersion force, which is a complicated matter
that we should study furthermore.
4.2
toluene
3.8
acetone
CHCl3
DMF
3.4
4. Concluding remarks
MeOH
3
The syntheses, structural characterization and spectro-
scopic data have been reported for both the green form of
solvent free and the brown form containing methanol sol-
vent of the present copper(II) complex. It is revealed that
methanol molecules as solvents in the crystals play an
important role in tuning not only crystal packing but also
the individual molecular conformation of this flexible
mononuclear copper(II) complex without remarkable inter-
molecular interactions. The ZINDO calculations based on
the crystal structures elucidated thermodynamical stability
and difference in energy for both forms. The absorption
2.6
2.2
18000
22000
26000
30000
wavenumber/cm-1
Fig. 6. Absorption electronic spectra of 1 in 1 mM solutions (acetone,
chloroform, methanol, toluene and DMF) at room temperature.

T. Akitsu, Y. Einaga / Polyhedron 25 (2006) 1089–1095
1095
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Crystallographic data for the structural analyses are
deposited with the Cambridge Crystallographic Data Cen-
tre, CCDC with deposition numbers 260175 and 260176
for 1 and 2, respectively. Copies of this information can
be obtained from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
+44 1223 336033; deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
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Acknowledgements
This work was supported by Grant-in-Aid for the 21st
Century COE program ÔKEIO Life Conjugate ChemistryÕ
form the Ministry of Education, Culture, Sports, Science
and Technology, Japan. The authors thank Professor
Yohru Yamada and Dr. Taketo Ikeno (Keio University)
for use of the DSC apparatus.
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