A ferromagnetic chain of imidazolato-bridged one-dimensional copper(ii) complex

Article abstract of DOI:10.1246/bcsj.82.467

A mononuclear copper(II) complex, [2-ethoxy-6-{[3-(2-methyl-4- imidazolylmethyleneamino)propyl]iminomethyl}- phenolato]copper(II) perchlorate [CuHL]C104 and its deprotonated complex at the imidazole moiety, [CuL] _n, were prepared. The structure

Full text of DOI:10.1246/bcsj.82.467

© 2009 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 82, No. 4, 467–471 (2009)
467
A Ferromagnetic Chain of Imidazolato-Bridged
One-Dimensional Copper(II) Complex
Tetsuya Yoshida,1 Masaaki Towatari,1 Tetsuya Sato,1 Naohide Matsumoto,*1
Nazzareno Re,2 and Jerzy Mrozinski3
1Department of Chemistry, Faculty of Science, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555
2Facolta di Farmacia, Università degli Studi “G. D’Annunzio,” I-66100 Chieti, Italy
3Faculty of Chemistry, University of Wroclaw, 14, F. Joliot-Curie, 50-383 Wroclaw, Poland
Received November 18, 2008; E-mail: naohide@aster.sci.kumamoto-u.ac.jp
A mononuclear copper(II) complex, [2-ethoxy-6-{[3-(2-methyl-4-imidazolylmethyleneamino)propyl]iminomethyl}-
phenolato]copper(II) perchlorate [CuHL]ClO4 and its deprotonated complex at the imidazole moiety, [CuL]n, were
prepared. The structure of [CuHL]ClO4 consists of a cation [CuHL]+ exhibiting a four-coordinated square-planar
¹
geometry and an anion ClO4 , while [CuL]n assumes an imidazolato-bridged zigzag-chain structure, in which the
copper(II) ion is coordinated by the imidazolato nitrogen atom of the adjacent molecule with Cu-N = 2.258(5) ¡ and the
two adjacent CuL units assumes a perpendicular orientation. The temperature-dependent magnetic susceptibilities from
2.0 to 300 K and field-dependent magnetization at 2.0 K demonstrate that [CuHL]ClO4 is a magnetically isolated
copper(II) molecule while [CuL]n is a ferromagnetic chain. The magnetic susceptibility data of [CuL]n were analyzed by
Baker’s model based on H = ¹2J-SAjSAj+1 for chain of equally spaced copper(II) ions to give the best-fit parameters of
¹1
g = 2.08 and J = +0.63 cm
.
In the last three decades, the field of molecular-based
magnets has shown a spectacular advance, in which the concept
of magnetic orthogonality introduced by Kahn has played a
leading role for development.1 Owing to this concept, one can
easily predict the magnetic properties between two adjacent
magnetic centers. As the simplest case consisting of one
unpaired electron per metal ion, the magnetic properties of
binuclear copper(II) complexes have been extensively studied.2
However, ferromagnetic coupling is rarely observed. Hatfield,
Hodgson, et al. investigated the magneto-structural correlation
of planar hydroxo-bridged copper(II) binuclear complexes and
found that the magnetic coupling constant J is correlated
linearly to the bridging angle ¡ of Cu-O-Cu.3 The ferromag-
netic coupling observed at ¡ < 97.5° is rationalized by
accidental orthogonality.1 Ferromagnetic coupling has been
also observed in alkoxo-bridged cubane-type tetranuclear
copper(II) complexes, where each copper(II) ions assume a
square-pyramidal coordination geometry and the two adjacent
equatorial coordination planes are nearly perpendicular to each
other.4 Due to the perpendicular arrangement of the two
adjacent coordination planes, a strict orthogonality is preserved
so that ferromagnetic coupling is always expected. If such a
perpendicular arrangement is extended from two adjacent
molecular units to one-dimensional (1D) infinite lattice, the
resulting 1D chain would be a ferromagnetic chain. From a
structural viewpoint, a copper(II) complex with the 1:1:1
condensation product of 2-methylimidazole-4-carbaldehyde,
1,3-propanediamine, and 3-ethoxysalicylaldehyde would show
such properties.5 As schematically drawn in Figure 1, the
copper(II) complex [CuHL]+ can play the role of a self-
complementary building block to form a self-assembled 1D
chain structure, since the complex potentially involves not only
an acceptor coordination ability at the vacant axial position but
also a donor coordination ability at the imidazolato nitrogen
atom. When the proton of the imidazole moiety is dissociated,
the self-assembly reaction (the imidazolato nitrogen atom of a
molecule generated by the deprotonation axially coordinates to
copper(II) ion of the adjacent molecule) is triggered and leads to
the assembly structure. The protonated copper(II) complex
assumes a square-planar coordination geometry with N3O donor
atoms, whereas the deprotonated copper(II) species assumes a
square-pyramidal coordination geometry with N4O donor
atoms, where the axial coordination site is occupied by an
imidazolato nitrogen atom of an adjacent molecular unit to give
an imidazolato-bridged 1D structure. Although twenty years
ago, one of the present authors investigated the structures and
magnetic properties of several such complexes, at that time he
could not confirm the ferromagnetic chain properties, due to
the very weak magnetic interaction and the magnetic suscepti-
bility data from room to liquid nitrogen temperature are not
enough for detection. In this study, the analogous copper(II)
complex, [2-ethoxy-6-{[3-(2-methyl-4-imidazolylmethylene-
amino)propyl]iminomethyl}phenolato]copper(II) perchlorate
[CuHL]ClO4 and its deprotonated complex [CuL]n, are newly
prepared and their structures and magnetic properties are
investigated and it is found that [CuL]n is a ferromagnetic chain.
Results and Discussion
Syntheses and Characterizations of [CuHL]ClO4 and
[CuL]n. The copper(II) complexes with the 1:1:1 condensa-
tion product of 2-methylimidazole-4-carbaldehyde, 1,3-pro-
panediamine, and 3-ethoxysalicylaldehyde were prepared by a
Published on the web April 11, 2009; doi:10.1246/bcsj.82.467

468
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
Copper(II) Ferromagnetic Chain
+
EtO
EtO
EtO
H+
Me
Me
Me
O
O
O
self-assembly
N
N
N
Cu
Cu
N
Cu
N
N
N
N
N
HN
N
N
n
Figure 1. (top) Self-assembly reaction from [CuHL]+ to its deprotonated 1D complex [CuL]n. (bottom) The schematic drawing of
the assembly reaction. The imidazole hydrogen is drawn by a cap, and the donor and acceptor coordination sites are drawn by
and , respectively.
Table 1. X-ray Crystallographic Data for [CuHL]ClO4 and
[CuL]n
Table 2. Selected Bond Distances (¡) and Angles (°) for
[CuHL]ClO4 and [CuL]n
[CuHL]ClO4
[CuL]n
[CuHL]ClO4
[CuL]n
Formula
Fw
Crystal system
Space group
a/¡
b/¡
c/¡
¢/°
V/¡3
C17H21ClCuN4O6
476.38
C17H20CuN4O2
375.92
Cu-O(1)
Cu-N(1)a)
Cu-N(2)
Cu-N(3)
Cu-N(4)
1.881(2)
1.923(3)
2.258(5)
2.016(4)
2.042(3)
1.975(5)
monoclinic
P21/c (No. 14)
7.027(4)
12.235(4)
23.267(9)
92.512(17)
1998.4(14)
4
1.583
1.26
0.0446
0.0337
orthorhombic
Pbca (No. 61)
9.318(4)
15.310(7)
23.043(8)
90
3287(2)
8
1.519
1.999(2)
2.006(2)
1.968(2)
O(1)-Cu-N(2)
O(1)-Cu-N(4)
N(2)-Cu-N(3)
N(3)-Cu-N(4)
O(1)-Cu-N(3)
N(2)-Cu-N(4)
90.25(9)
93.45(9)
81.58(10)
94.54(10)
171.59(10)
174.14(11)
90.31(19)
91.7(2)
81.6(2)
92.3(2)
162.84(19)
164.76(19)
Z
¹3
D
calcd/Mg m
¹1
®(Mo K¡)/mm
1.34
0.0524
0.1102
Ra)
a) The symmetry operation of ¹1/2 + x, 1/2 ¹ y, ¹z.
wRb)
2
Data with a) R = -««Fo« ¹ «Fc««/-«Fo«. b) wR = [-w(«Fo « ¹
[CuHL]ClO4 and a methanolic solution of triethylamine under
diluted conditions at ambient temperature. Reflectance spectra
showed a maximum in the visible region at 512 and 535 nm
for [CuHL]ClO4 and [CuL]n, respectively, demonstrating the
different coordination geometry of the copper(II) ion in these
two complexes.
2
2 2 1/2
«Fc «)2/-w«Fo « ]
.
step-by-step procedure according to methods reported pre-
viously.5b,5c At first, a precursor complex with the 1:1
condensation product of 3-ethoxysalicylaldehyde and 1,3-
propanediamine,
[bis{6-[(3-aminopropyl)iminomethyl]-2-
Structural Descriptions of [CuHL]ClO4 and [CuL]n.
The crystal structures of [CuHL]ClO4 and [CuL]n were
determined by single-crystal X-ray diffraction analyses. Crys-
tallographic data and relevant bond distances and angles are
given in Tables 1 and 2, respectively.
ethoxyphenolato}]nickel(II), was synthesized by adopting the
method of Elder.6 A methanolic solution of the 1:1 ligand was
obtained by refluxing the nickel(II) complex and two equiv-
alents of dimethylglyoxime for 2 h in methanol and then
by filtering out deep red colored bis(dimethylglyoximato)-
nickel(II). To the yellow colored filtrate of the 1:1 ligand was
added a methanol solution of one equivalent of 2-methylimi-
dazole-4-carbaldehyde. To the resulting 1:1:1 ligand solution, a
methanolic solution of copper(II) acetate monohydrate and then
a methanolic solution of sodium perchlorate were added. Green
fine crystals precipitated were collected and then recrystallized
from a mixture of N,N-dimethylformamide and methanol to
give deep-green rod-like crystals with the formula [CuHL]-
ClO4. The deprotonated complex, [CuL]n, was obtained as
light-green platelet crystals by mixing a methanolic solution of
Figures 2 and 3 show an ORTEP drawing with the selected
atom numbering scheme and the packing diagram of [CuHL]-
ClO4, respectively. The crystal structure consists of a complex
¹
cation [CuHL]+ and an anion ClO4 , as shown in Figure 3. The
shortest distance between an oxygen atom of the perchlorate
ion and the CuII ion is Cu-O5 = 2.709(2) ¡, a distance too
long for an axial coordination bond. Thus, the coordination
geometry of copper(II) ion can be described as square planar
with N3O donor atoms of the quadridentate ligand HL with
the coordination bond distances of Cu-O1 = 1.881(2), Cu-
N2 = 1.999(2), Cu-N3 = 2.006(2), and Cu-N4 = 1.968(2) ¡.

T. Yoshida et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
469
Figure 2. ORTEP drawing of [CuHL]ClO4 with the atom
numbering scheme.
Figure 4. ORTEP drawing of [CuL]n with the atom
numbering scheme, the same atom numbering of [CuHL]-
ClO4. The axial coordination atom N1* (* denotes the
symmetry operation of ¹1/2 + x, 1/2 ¹ y, ¹z) and the
coordination bond of Cu-N1* are drawn.
Figure 3. Packing diagram of [CuHL]ClO4, showing that
¹
the cation [CuHL]+ and anion ClO4 exist separately.
The copper(II) ion is positioned almost in the plane, with the
deviation of 0.056 ¡ from the plane of N3O.
Figure 4 shows an ORTEP drawing of [CuL]n with the atom
numbering scheme, where the same atom numbering scheme
of [CuHL]ClO4 was adopted. The copper(II) ion assumes a
square-pyramidal coordination geometry with N3O donor
atoms of the tetradentate ligand in the equatorial plane and
an imidazolato nitrogen atom of the adjacent molecular unit at
the axial position. The coordination bond distances are Cu-
O1 = 1.923(3), Cu-N2 = 2.016(4), Cu-N3 = 2.042(3), and
Cu-N4 = 1.975(5) ¡ for the equatorial plane and Cu-N1* =
2.258(5) ¡ for the axial coordination (* denotes the symmetry
operation of ¹1/2 + x, 1/2 ¹ y, ¹z). The copper(II) ion is
displaced by 0.265 ¡ from the equatorial plane toward the axial
coordination atom N(1)*. Compared with the coordination
bond distances of [CuHL]ClO4 in the equatorial plane, the
coordination bond distances of [CuL]n are longer than the
corresponding values of [CuHL]ClO4. As the result of the axial
coordination of Cu-N1*, zigzag-chain structure is formed,
where the two adjacent molecules within a chain are related
by a symmetry operation of a two-fold screw axis along the a
axis. Figure 5 shows a 1D zigzag-chain structure constructed
by the imidazolato-bridges of Cu-N1* = 2.258(5) ¡. The
dihedral angle between the two adjacent coordination planes
is 75.23°, where the coordination plane is defined by the atoms
of Cu, O1, N2, N3, and N4. The coordination plane of the
adjacent unit almost lies on the coordination bonds of Cu-N2
and Cu-N4.
Figure 5. Zigzag chains of [CuL]n running along the a axis.
The two adjacent chains run along the opposite directions.
Magnetic Properties of [CuHL]ClO4 and [CuL]n. The
temperature dependences of the magnetic susceptibilities of
[CuHL]ClO4 and [CuL]n were measured in the temperature
range of 2.0-300.0 K under an external magnetic field of
0.5 T. Figure 6 shows the plots of »MT vs. T for [CuHL]ClO4
and [CuL]n, where »M is the molar magnetic susceptibility
per Cu unit and T is the absolute temperature. The »MT vs.
T plots of [CuHL]ClO4 are nearly constant with a value of
¹1
0.41 cm3 K mol over the whole temperature range, a value
¹1
consistent with that of 0.414 cm3 K mol expected for one
copper(II) (S = 1/2) ion and g = 2.10.
The »MT vs. T plot of [CuL]n shows a constant increase on
lowering the temperature from 300.0 to 2.0 K. The increase of
the »MT value indicates the operation of a ferromagnetic
interaction between two adjacent copper(II) ions through the
imidazolato group. The Curie-Weiss plots of 1/»M vs. T from
2.0 to 40.0 K follow the Curie-Weiss equation of 1/»M
=
(T ¹ ª)/C with a positive Weiss constant of ª = +0.8 K and a
Curie constant of C = 0.409 cm3 K mol¹1. It should be noted
that both »M and »MT values under 0.5 T continue to increase
on lowering the temperature down to 1.8 K, indicating the lack
of significant antiferromagnetic interchain interaction.

470
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
Copper(II) Ferromagnetic Chain
Figure 6. Plots of »MT vs. temperature for [CuHL]ClO4
( ) and [CuL]n ( ). The solid line represents the
Figure 7. Field dependence of the magnetization of M/N¢
at 2.0 K as a function of the applied magnetic field H (tesla)
for [CuHL]ClO4 ( ) and [CuL]n ( ). The experimental
data of [CuHL]ClO4 ( ) was reproduced by the Brillouin
function for S = 1/2 and g = 2.10. The upper dotted lines
correspond to the theoretical values per Cu atom calculated
with the Brillouin functions for the ferromagnetic ground
states of isolated dinuclear and trinuclear linear copper(II)
complexes (S = 1 and S = 3/2) with g = 2.08.
theoretical curve for [CuL]n derived from the eq 1 with
¹1
the parameters of g = 2.08, J = +0.63 cm
.
The X-ray analysis of [CuL]n demonstrates that the present
copper(II) complex is the simplest case of a magnetic chain
constituted by equally spaced copper(II) ions with SCu = 1/2
local spins.1 For the present case, the spin Hamiltonian in zero-
field adapted to describe the isotropic interaction between
nearest neighbor ions is H = ¹2J-SAjSAj+1, where the
summation runs over the n sites of the chain. When n tends
to infinite, there is no analytical method that can be used to
determine the energies of the low-lying states and the magnetic
susceptibility. The problem may be solved numerically by
considering ring chains of increasing size. The method was first
applied by Bonner and Fisher in 1964,7 but the method is only
applicable for antiferromagnetic case J < 0.7 Alternatively,
a high-temperature series expansion, valid for both positive
and negative J values, has been proposed by Baker et al.8
This method has been applied to the present complex. The
expression is given below (1), where the coupling constant J
and g value are the best-fit parameters to be determined. The
indicating ferromagnetic coupling as shown by the magnetic
susceptibility analysis. The magnetization data of [CuL]n are,
however, smaller than the values per copper(II) calculated with
the Brillouin function for the ferromagnetic ground state of the
trinuclear linear copper(II) complex (S = 3/2), and close to
those of the Brillouin function for the S = 1 ferromagnetic
ground state of the dinuclear copper(II) complex, consistently
with the small value of ferromagnetic coupling constant
determined from the magnetic susceptibility analysis.
There are several examples of copper(II) complexes exhibit-
ing ferromagnetic chain character, such as 1D azido-bridged
ferromagnetic chain.9 To the best of our knowledge, this is the
first example of ferromagnetic coupling between two copper(II)
ions via imidazolato bridging ligands. Indeed, the observed
solid line in Figure 6 shows the calculated curve with the best-
¹1
¹1
fit parameters of g = 2.08, J = +0.63 cm
.
ferromagnetic coupling constant of +0.63 cm is very differ-
2
ent from those reported for imidazolato-bridged polynuclear
copper(II) complexes, in the range ¹40 to ¹100 cm¹1, in which
the imidazolato group bridges both copper(II) ions at their
equatorial sites.10 These fairly large antiferromagnetic inter-
actions result from a superexchange mechanism in which
the unpaired electrons of adjacent copper(II) ions occupy the
dx2ꢁy2 orbitals lying in the basal coordination planes interact
through the imidazolato bridge.11 On the other hand, when the
imidazolato group bridges a copper(II) ion at its equatorial site
with another at the axial site, the unpaired electrons occupy,
respectively, the orthogonal dx2ꢁy2 and dz2 orbitals and a small
ferromagnetic coupling is expected.
2=3
»
¼ ðNg2¢ =4kTÞ½N=Dꢀ with
A
N ¼ 1:0 þ 5:7979916y þ 16:902653y2
þ 29:376885y3 þ 29:832959y4 þ 14:036918y5;
D ¼ 1:0 þ 2:7979916y þ 7:0086780y2
þ 8:6538644y3 þ 4:5743114y4; and y ¼ J=2kT
ð1Þ
The field dependence of the magnetization at 2.0 K was
measured from 0 to 5 T. The result is plotted in Figure 7 in the
form of M/N¢ vs. H and compared with the theoretical values
per Cu atom calculated with the Brillouin function for an
isolated copper(II) ion (S = 1/2) and for the ferromagentic
ground states of dinuclear and trinuclear linear copper(II)
complexes (S = 1 and 3/2). The experimental data of [CuHL]-
ClO4 are consistent with the calculated Brillouin function of
g = 2.10 and S = 1/2 expected for the magnetically isolated
copper(II) molecule. The experimental data of [CuL]n are larger
than the calculated Brillouin function of g = 2.08 and S = 1/2,
Concluding Remarks
The deprotonation of [CuHL]ClO4 motivates a self-assembly
reaction to give [CuL]n. The [CuL]n complex assumes an
imidazolato-bridged zigzag-chain structure. The temperature-
dependent magnetic susceptibilities from 2.0 to 300 K and

T. Yoshida et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
471
and [CuL]n were obtained using a Rigaku R-Axis Rapid dif-
fractometer at 296 K. The structures were solved by direct methods
and expanded using the Fourier technique. The non-hydrogen
atoms were refined with anisotropic thermal parameters. Hydrogen
atoms were fixed at calculated positions and refined using a riding
model. All calculations were performed using the CrystalStructure
crystallographic software package.12 Cystallographic data in CIF
format for compounds [CuHL]ClO4 and [CuL]n has been deposited
at the deposition numbers 712693 and 712694 of CCDC.
field-dependent magnetization at 2.0 K demonstrate that [CuL]n
is a ferromagnetic chain. The magnetic susceptibility data
were analyzed by Baker model for chains of equally spaced
copper(II) ions to give the best-fit parameters of g = 2.08 and
¹1
J = +0.63 cm
.
Experimental
General and Materials. All chemicals and solvents, obtained
from Tokyo Kasei Co., Ltd., and Wako Pure Chemical Industries,
Ltd., were of reagent grade and were used for the syntheses
without further purification. All the synthetic procedures were
carried out in the open atmosphere.
This work was supported in part by a Grant-in-Aid for
Science Research (No. 16205010) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
[CuHL]ClO4. The precursor complex [bis{6-[(3-aminopro-
pyl)iminomethyl]-2-ethoxyphenolato}]nickel(II) was prepared by
the method of Elder.5 A methanolic solution of the ligand was
obtained by refluxing the NiII complex (1.504 g, 3 mmol) and two
equivalents of dimethylglyoxime (0.697 g, 6 mmol) in 100 mL of
methanol for 2 h and cooled to room temperature. Deep red
fine crystals of bis(dimethylglyoximato)nickel(II) were removed
by suction filtration. To the yellow filtrate of the 1:1 ligand
was added one equivalent of 2-methylimidazole-4-carbaldehyde
(0.661 g, 6 mmol) in 50 mL of methanol and the mixture was
warmed at 50 °C for 1 h and cooled to ambient temperature. To the
resulting solution was added a solution of copper(II) acetate
monohydrate (1.198 g, 6 mmol) in 50 mL of methanol and then a
solution of sodium perchlorate (0.735 g, 6 mmol) in 10 mL of
methanol. Moss-green fine crystals precipitated were collected by
suction filtration. Crude product, 1.5 g (52%). Recrystallization
from a mixture of N,N-dimethylformamide and methanol gave
deep green rod-like crystals. Anal. Calcd for C17H21O6N4CuCl: C,
42.86; H, 4.44; N, 11.76%. Found: C, 43.19; H, 4.56; N, 11.79%.
[CuL]n. To a solution of [CuHL]ClO4 (281 mg, 0.59 mmol) in
40 mL of methanol was diffused a solution of triethylamine
(59 mg, 0.59 mmol) in 20 mL of methanol. Several days later, light
green platelet crystals precipitated and they were collected by
suction filtration, washed with a small amount of water and dried
in air. Yield, 143 mg (64%). Anal. Calcd for C17H20O2N4Cu: C,
54.32; H, 5.36; N, 14.90%. Found: C, 53.80; H, 5.37; N, 14.89%.
CAUTION: The perchlorate salts of metal complexes with organic
ligands are potentially explosive. Only small quantities of the
compound should be prepared, and they should be handled with
much care.
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X-ray Diffraction Analysis. Diffraction data of [CuHL]ClO4