Two-dimensional metal-organic framework constructed from 4,4′-bipydine and 1,2,4-benzenetricarboxylate: Synthesis, structure and magnetic properties

Article abstract of DOI:10.1016/j.jssc.2005.08.014

Two new copper coordination polymers generated from mixed organic ligands 4,4′-bipydine and 1,2,4-benzenetricarboxylate [Cu₃(1,2,4-BTC) ₂(4,4′-bipy)(H₂O)₄] 1 and [Cu(1,2,4-HBTC)(4,4???-bipy)] 2 (1,2,4-BTC=1,2,4-benzenetricarboxylate) have been prepared hydrothermally and characterized by single crystal X-ray diffraction. Compound 1 crystallizes in the noncentrosymmetric space group Fdd2, with a=19.004(3)A, b=26.711(4)A, c=11.0770(15)A, V=5622.8(14)A3, and Z=8. Its structure contains two different one-dimensional (1-D) chains for the Cu ions, namely [Cu2(1,2,4-BTC)2]n2n- and [Cu(4,4′-bipy)]n2n+; the carboxylate of 1,2,4-BTC ligands links the two different 1-D chains into an interesting steplike neutral framework. Compound 2 crystallizes in the monoclinic space group P21/n, with a=9.9806(8), b=16.5512(16), c=11.3109(7)A, β=99.375(5)°, V=1843.5(3)A3, Z=4. Its structure contains novel paddle-wheel clusters Cu₂(CO ₂)₄(4,4′-bipy)₂ as SBUs which are further linked by monoprotonated 1,2,4-BTC ligands into a 2-D neutral framework. The magnetic behaviors of both compounds have been studied.

Full text of DOI:10.1016/j.jssc.2005.08.014

ARTICLE IN PRESS
Journal of Solid State Chemistry 178 (2005) 3359–3365
www.elsevier.com/locate/jssc
Two-dimensional metal-organic framework constructed from
4,40-bipydine and 1,2,4-benzenetricarboxylate: Synthesis, structure
and magnetic properties
Ã
Lei Wang, Ming Yang, Zhan Shi, Yan Chen, Shouhua Feng
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China
Received 5 July 2005; received in revised form 21 August 2005; accepted 23 August 2005
Available online 27 September 2005
Abstract
Two new copper coordination polymers generated from mixed organic ligands 4,40-bipydine and 1,2,4-benzenetricarboxylate
[Cu3(1,2,4-BTC)2(4,40-bipy)(H2O)4] 1 and [Cu(1,2,4-HBTC)(4,40-bipy)] 2 (1,2,4-BTC ¼ 1,2,4-benzenetricarboxylate) have been prepared
hydrothermally and characterized by single crystal X-ray diffraction. Compound 1 crystallizes in the noncentrosymmetric space group
(
(
(
( 3
Fdd2, with a ¼ 19:004ð3Þ A, b ¼ 26:711ð4Þ A, c ¼ 11:0770ð15Þ A, V ¼ 5622:8ð14Þ A , and Z ¼ 8. Its structure contains two different one-
2nÀ
dimensional (1-D) chains for the Cu ions, namely ½Cu2ð1; 2; 4 À BTCÞ2Š
and ½Cuð4; 40 À bipyފ2nþ; the carboxylate of 1,2,4-BTC ligands
n
n
links the two different 1-D chains into an interesting steplike neutral framework. Compound 2 crystallizes in the monoclinic space group
(
( 3
P21=n, with a ¼ 9:9806ð8Þ, b ¼ 16:5512ð16Þ, c ¼ 11:3109ð7Þ A, b ¼ 99:375ð5Þ1, V ¼ 1843:5ð3Þ A , Z ¼ 4. Its structure contains novel
paddle-wheel clusters Cu2(CO2)4(4,40-bipy)2 as SBUs which are further linked by monoprotonated 1,2,4-BTC ligands into a 2-D neutral
framework. The magnetic behaviors of both compounds have been studied.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Neutral framework; Hydrothermal synthesis; Coordination polymer; Crystal structure
1. Introduction
[Cu4{(1,4-C6H4(COO)2)}3(4,40-bipy)2]n. Chen et al. [13–15]
reported a series of transition metal (II)-based neutral
frameworks, such as [Zn4O(ip)3(4,40-bipy)] and [Ni(ip)(4,40-
bipy)], etc., which were constructed by mixed ligands of
4,40-bipy and 1,3-benzenedicarboxylate.
Studies of the synthesis of metal-organic frameworks
by copolymerization of organic molecules with metal
ions are of great interest, mostly owing to their intrigu-
ing structural features and enormous range of applica-
tions in catalysis, absorbent, non-linear optics and
molecular recognition [1–4]. Ligands with pyridyl or
carboxylate groups are employed the most often in the
design and synthesis of these frameworks [5–8]. With an
aim to search for novel materials and rational design
and synthesis of these compounds, we and others in this
field have focused largely on the combination of pyridyl
and carboxylate ligands, and find it has a great ability
to form neutral porous networks [9–11]. Williams
et al. [12] reported a stable metal-organic framework
with mixed-valence [Cu2]3+ dimer subunits formulated as
In our previous studies, we obtained several novel
coordination polymers, such as [Mn(maleate)(4,40-bi-
py)] ꢀ 0.5H2O and [Mn(fumareate)(4,40-bipy)] ꢀ 0.5H2O(4,40-
bipy), in which a structural transformation from 2D to 3D
through conformation change of carboxylate ligands was
observed [16]. A study of the literature about metal-organic
frameworks shows that 1,2,4-benzenetricarboxylate (1,2,4-
BTC) is a good candidate for assembly of these compounds.
Scheme 1 illustrates the versatile coordination abilities
of the organic ligand; therefore we extended our research
into the metal-4,40-bipy-1,2,4-BTC system [17–25]. As the
results in this system, in this paper we report the synthesis,
structure and magnetic properties of two new copper
coordination polymers [Cu3(1,2,4-BTC)2(4,40-bipy)(H2O)4]
1 and [Cu(1,2,4-HBTC)(4,40-bipy)] 2.
Ã
Corresponding author. Fax: +86 431 5168624.
E-mail address: shfeng@mail.jlu.edu.cn (S. Feng).
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.08.014

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L. Wang et al. / Journal of Solid State Chemistry 178 (2005) 3359–3365
3360
M
O
M
O
M
O
M
O
M
M
O
M
M
M
M
O
M
O
O
O
O
O
O
O
O
O
O
M
M
M
M
M
O
O
O
O
O
O
O
O
O
O
O
O
O
O
M
M
M
M
M
(a)
(b)
(c)
(d)
(e)
M
M
M
O
M
O
M
O
M
O
M
M
M
O
M
O
O
M
M
O
M
O
O
O
M
O
O
O
O
M
M
O
O
M
O
O
OH
O
OH
O
O
O
O
O
O
O
O
M
M
M
M
M
M
M
M
(f)
(g)
(h)
(i)
(j)
Scheme 1. Coordination modes of 1,2,4-BTC in the compounds reported in the literature: a, [17]; b, [18]; c, [19]; d, [20]; e, [21]; f, [22]; g and h, [23]; i,
(see compound 1); j, (see compound 2).
2. Experimental
The ICP analysis and elemental analyses were consistent
with the theoretical values. Calcd for 1: Cu, 22.88; C, 40.33;
H, 2.64; N, 3.36%. Found: Cu, 22.17; C, 40.44; H, 2.51; N,
3.29%. The absence of the expected characteristic bands at
1730–1690 cmÀ1 atributed to the protonated carboxylate
groups indicates the complete deprotonation of 1,2,4-BTC
on the reactions with copper ion. The TGA curve of 1
exhibits two continuous weight loss stages in the tempera-
ture ranges 120–230 and 250–340 1C, corresponding to the
concomitant release of composed water and 4,40-bipy and
1,2,4-BTC groups. The residue was CuO. The whole weight
loss (71.74%) is in good agreement with the calculated
(71.36%).
2.1. General methods
The elemental analyses were performed on a Perkin-
Elmer 2400 element analyzer. The inductively coupled
plasma (ICP) analysis was carried out on a Perkin-Elmer
Optima 3300DV ICP instrument. The infrared (IR)
spectrum was recorded within the 400–4000 cmÀ1 region
on a Nicolet Impact 410FTIR spectrometer using KBr
pellets. Thermogravimetric analysis (TGA) was carried out
on a Perkin-Elmer TGA 7 thermogravimetric analyzer in
air with a heating rate of 10 1C minÀ1. Magnetic suscept-
ibility data were collected over the temperature range
2–300 K at a magnetic field of 5 KG on a Quantum Design
MPMS-7 SQUID magnetometer.
2.3. Synthesis and characterization of 2
In a typical synthetic procedure, compound 2 were
prepared from a mixture of Cu(NO3)2 (0.242 g, 1.5 mmol),
1,2,4-BTC (0.210 g, 1 mmol), 4,40-bipy (0.192 g, 1 mmol),
1-butanol (4 mL) and distilled water (6 mL) with a molar
composition of 3:2:3:88:666. The mixture was sealed in a
23 mL capacity of Teflon-lined stainless-steel autoclave under
autogenous pressure, and heated at 160 1C for 120h. After
cooling blueness crystals for single-crystal X-ray diffraction
were obtained. The yield was 72%, based on Cu.
The ICP analysis and elemental analyses were consistent
with the theoretical values. Calcd for 2: Cu, 14.84; C, 53.29;
H, 2.80; N, 6.54%. Found: Cu, 15.12; C, 52.82; H, 2.39; N,
6.13%. Despite their synthesis from a mixed solvothermal
condition, samples of this material are stable in air and
2.2. Synthesis and characterization of 1
All chemicals and solvents used in the syntheses were of
reagent grade and used as purchased without further
purification. Compound 1 were prepared from a mixture
of Cu(NO3)2 (0.242g, 1.5 mmol), 1,2,4-BTC (0.210g,
1 mmol), 4,40-bipy (0.096g, 0.5 mmol), NaOH (0.080g,
2 mmol) and distilled water (10 mL) with a molar composi-
tion of 3:2:1:2:1100. The mixture was sealed in a 23mL
capacity of Teflon-lined stainless-steel autoclave under
autogenous pressure, and heated at 160 1C for 120 h. After
cooling blueness crystals for single-crystal X-ray diffraction
were obtained. The yield was 63%, based on Cu.

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3361
Table 1
Crystal data and structure refinement parameters for 1
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
a (A)
C28H22N2O16Cu3
833.10
C19H12N2O6Cu
427.85
293(2) K
0.71073 A
293(2) K
0.71073 A
Monoclinic
P21=n
9.9806(8)
16.5512(16)
11.3109(7)
99.375(5)
1843.5(3)
4
Orthorhombic
Fdd2
19.004(3)
26.711(4)
b (A)
c (A)
11.0770(15)
90
5622.8(14)
b (deg)
Volume (A3)
Z
8
1.968
Calculated density (g/cm3)
1.542
1.224
Absorption coefficient (mmÀ1
F(0 0 0)
Crystal size (mm)
)
2.338
3352
868
0.32 ꢁ 0.27 ꢁ 0.25
2.20–24.99
0.35 ꢁ 0.26 ꢁ 0.23
2.26–30.02
y range (deg)
Limiting indices
À26php26, À37pkp24, À15plp15
9655/3833 [RðintÞ ¼ 0:0451]
99.9%
Full-matrix least-squares on F2
3833/5/240
0.912
À11php11, À19pkp10, À12plp13
9991/3230 [RðintÞ ¼ 0:0715]
99.6%
Full-matrix least-squares on F2
3230/12/253
0.979
Reflections collected/unique
Completeness to y ¼ 30:02
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I42sðIÞ]
R indices (all data)
R1 ¼ 0:0357, wR2 ¼ 0:0687
R1 ¼ 0:0529, wR2 ¼ 0:0730
0.410 and À0.454
R1 ¼ 0:0543, wR2 ¼ 0:1380
R1 ¼ 0:1034, wR2 ¼ 0:1580
0.851 and À0.700
Largest diff. peak and hole (e. AÀ3
)
insoluble in water and common organic solvents such as
alcohol, toluene, THF, DMF and DMSO. The presence of
the expected characteristic bands at 1711 cmÀ1 (IR)
attributed to the protonated carboxylate groups indicates
that the carboxylates of the1,2,4-BTC ligand are not
completely deprotonated. The TGA curve of 2 exhibits
one continuous weight loss stage in the temperature ranges
210À420 1C, corresponding to the concomitant release of
composed 4,40-bipy and 1,2,4-BTC groups. The residue was
also CuO. The whole weight loss (81.67%) is in good
agreement with the calculated (81.40%).
compound 1 and and P21=n for compound 2 by direct
methods and refined on F2 by full-matrix least-squares
using SHELXTL97 [27]. All non-hydrogen atoms were
easily found from the difference Fourier map and refined
anisotropically. The hydrogen atoms on the ligand were
placed geometrically. The crystallographic data and details
on the refinements for compounds 1 and 2 are summarized
in Table 1. Selected bond distances and angles are listed in
Table 2.
3. Results and discussion
2.4. Structure determination
3.1. Crystal structure of [Cu3(1,2,4-BTC)2(4,40-
bipy)(H2O)4] 1
A suitable blueness single crystal with dimensions
0.35 ꢁ 0.27 ꢁ 0.23 mm for 1 and 0.50 ꢁ 0.30 ꢁ 0.25 mm for
2 was glued to a thin glass fiber and mounted on a Siemens
Smart CCD diffractometer equipped with a normal-focus,
2.4-kW sealed-tube X-ray source (graphite-monochromatic
Single-crystal X-ray diffraction study performed on 1
revealed an extended 2-D coordination network that
crystallizes in the noncentrosymmetric space group Fdd2.
The asymmetric unit of 1 (Fig. 1) consists of two copper
centers, one 1,2,4-BTC anions, one 4,40-bipy molecule, and
two coordinated water molecules. The Cu1 center coordi-
nates to two carboxylate oxygen atoms of two separated
1,2,4-BTC ligands (Cu1-O3 1.975(2) A) and two nitrogen
atoms of two 4,40-bipy (Cu(1)–N(1) 2.001(5), Cu(1)–N(2)
1.986(5) A) to furnish a distorted square geometry; while
the Cu2 center coordinates to three carboxylate oxygen
atoms of three separated 1,2,4-BTC ligands (Cu(2)–O(1)
1.941(3), Cu(2)–O(4) 2.234(2), Cu(2)–O(5) 1.960(3) A), and
two water molecules (Cu(2)–O(7) 1.950(3), Cu(2)–O(8)
1.982(2) A) , exhibits a distorted trigonal bipyramidal
geometry with the O–Cu–O angles in the range from
(
MoKa radiation (l ¼ 1:5418 A) operating at 50 kV and
40 mA. The experiment was collected at room temperature.
Date processing was performed using the SAINT proces-
sing program [26]. The total number of measured reflec-
tions and observed unique reflections were 9655 and 3833
for 1, for 2 were 9991 and 3230, respectively. For
compound 1, intensity data of 3833 independent reflections
(À26php26, À37pkp24, À15plp15) were collected in
the o scan mode. For compound 2, intensity data of
3230 independent reflections (À11php11, À19pkp10,
À12plp13) were collected in the o scan mode. The
structure was solved in the space group Fdd2 for

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3362
Table 2
Selected bond lengths (A) and angles (deg) for 1
Compound 1
Cu(1)–O(3)#1
Cu(1)–O(3)
Cu(1)–N(2)
1.975(2)
1.975(2)
1.986(5)
2.001(5)
1.941(3)
1.950(3)
1.960(3)
1.982(2)
2.234(2)
177.46(15)
88.73(8)
88.73(8)
91.27(8)
O(3)–Cu(1)–N(1)
N(2)–Cu(1)–N(1)
O(1)–Cu(2)–O(7)
O(1)–Cu(2)–O(5)
O(7)–Cu(2)–O(5)
O(1)–Cu(2)–O(8)
O(7)–Cu(2)–O(8)
O(5)–Cu(2)–O(8)
O(1)–Cu(2)–O(4)
O(7)–Cu(2)–O(4)
O(5)–Cu(2)–O(4)
O(8)–Cu(2)–O(4)
91.27(8)
180.000(4)
83.81(12)
168.44(14)
94.04(12)
93.71(12)
167.67(12)
85.97(12)
97.48(11)
86.59(10)
93.73(11)
105.73(9)
Cu(1)–N(1)
Cu(2)–O(1)
Cu(2)–O(7)
Cu(2)–O(5)
Cu(2)–O(8)
Cu(2)–O(4)
O(3)#1–Cu(1)–O(3)
O(3)#1–Cu(1)–N(2)
O(3)–Cu(1)–N(2)
O(3)#1–Cu(1)–N(1)
Compound 2
Cu(1)–O(2)
Cu(1)–O(1)
1.955(4)
1.976(4)
1.961(4)
O(2)–Cu(1)–O(6)
O(5)–Cu(1)–O(6)
O(1)–Cu(1)–O(6)
O(2)–Cu(1)–N(1)
O(5)–Cu(1)–N(1)
O(1)–Cu(1)–N(1)
O(6)–Cu(1)–N(1)
90.98(19)
166.56(16)
88.88(19)
100.08(18)
99.13(17)
93.63(17)
94.19(17)
Cu(1)–O(5)
Cu(1)–O(6)
Cu(1)–N(1)
1.979(4)
2.154(5)
O(2)–Cu(1)–O(5)
O(2)–Cu(1)–O(1)
O(5)–Cu(1)–O(1)
88.43(19)
166.26(16)
88.53(19)
Symmetry transformations used to generate equivalent atoms: #1
Àx+1,Ày+1, z.
2nÀ
Fig. 2. (a) A viewof the infinite double ½Cu2ð1; 2; 4 À BTCÞ2Š
chain and
chain; (c) the two-dimensional neutral
n
2nþ
(b) the infinite ½Cuð4; 40 À bipyފ
n
framework along the b-axis. Hydrogen atoms are omitted for clarity.
carboxylate moieties are 4.91 (C7/O1/O2), 89.51 (C8/O3/
O4), and 4.61 (C9/O5/O6), respectively.
The most interesting structure feature of 1 is that it
exhibits two different 1-D chains along the b-axis, as shown
in Fig. 2: the 4,40-bipy molecule links the Cu1 center to
2nþ
form an infinite ½Cuð4; 40 À bipyފ
chain, while the 1,2,4-
BTC ligand links the Cu2 center to form an infinite double
n
2nÀ
½Cu2ð1; 2; 4 À BTCÞ2Š
chain. Thus, the 2-D neutral
framework can be viewed as constructed by the later
n
2nÀ
½Cu2ð1; 2; 4 À BTCÞ2Š
chain connecting the ½Cuð4; 40 À
n
2nþ
bipyފ
chain through the carboxylate (C8/O3/O4) of
n
1,2,4-BTC linkers. The adjacent 2-D steplike layers are
parallel with each other and construct a 3-D noncentry-
symmetric framework via hydrogen bonding interactions
between the two coordinated water molecules and the
carboxyl groups of 1,2,4-BTC. The typical hydrogen
bonds are Oð7Þ2Hð7AÞ ꢀ ꢀ ꢀ Oð3Þ 2.842(3) A, Oð7Þ2Hð7BÞ
ꢀ ꢀ ꢀ Oð6Þ 2.675(4) A, Oð8Þ2Hð8AÞ ꢀ ꢀ ꢀ Oð6Þ 2.725(4) A, and
Oð8Þ2Hð8BÞ ꢀ ꢀ ꢀ Oð2Þ 2.600(4) A.
Fig. 1. The building block unit including the asymmetric unit present in
crystalline 1 (50% thermal ellipsoids).
83.81(12)1 to 168.44(14)1. The carboxylic O–Cu bond
distances range from 1.941(3) to 2.234(2) A for Cu1 and
Cu2, where O1, O3, and O5 are tightly bonded, while O4 is
weakly bonded. The 1,2,4-BTC ligand shows a new
coordination mode connecting four coppers through its
three asymmetry ÀCOOÀ groups, two of which in a
monodentate mode, while the third in a bridging mode;
this simple coordination mode is not seen in other metal
1,2,4-BTC compounds (Scheme 1i). The dihedral angles
between benzene rings (C1/C2/C3) and the three different
3.2. Crystal structure of [Cu(1,2,4-HBTC)(4,40-bipy)] 2
Single-crystal X-ray diffraction study performed on 2
reveals an extended 2-D coordination network. The
fundamental unit is shown in Fig. 3. There is only one

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3363
Fig. 3. The building block unit including the asymmetric unit present in
crystalline 2 (50% thermal ellipsoids).
crystallographically unique Cu center in the crystal
structure. The Cu center coordinates to four carboxylate
oxygen atoms of four separated 1,2,4-BTC ligands and one
nitrogen atoms of 4,40-bipy to furnish a pseudo-octahedral
geometry. The O–Cu–O angles are in the range from
83.81(12)1 to 168.44(14)1 and the O–Cu–N angels are in the
range from 93.63(17)1 to 100.08(18)1, respectively. The
average O–Cu bond lengths is 1.967 A and the N–Cu bond
length is 2.154(5) A. The 1,2,4-BTC ligand shows a new
coordination mode (Scheme 1j) connecting four coppers
through its three asymmetry carboxylate groups, two of
which in a di-monodentate mode; while the third group,
C(8)/O(3)/O(4), is a carboxylic complex in which the O(3)
atom is the acidic oxygen, in agreement with the IR
spectrum (1711 cmÀ1) and the observed C(8)–O(3) bond
length (1.284 A). The 4,40-bipy molecule here as a mono-
dentate ligand is little seen in coordination polymers, but
can be seen in some hybrid metal phosphates reported by
Lii [28–30].
Fig. 4. Crystal structure of 2 viewed along the c-axis. Hydrogen atoms are
omitted for clarity.
compound 2, the axial positions on Cu are occupied by
4,40-bipy ligands which serve to hold together via the p– p
interactions, and no water or solvent guests reside in the
1-D channels but the organic ligand itself.
3.3. Magnetic studies
Temperature dependence of the magnetic susceptibility of
1 was investigated in the temperature range 2–300 K at
5 kOe (Fig. 5). The effective magnetic moment at 300 K,
calculated from meff ¼ 2:828ðXMTÞ1=2, is 1.90mB per Cu,
which is in the range of the experimentally observed value
(1.70–2.20mB) for Cu(II) ions. The susceptibility data
above 8 K can be fitted by Curie–Weiss law, XM
¼
As shown in Fig. 4, the 2-D framework can be viewed as
constructed form paddle-wheel clusters Cu2(CO2)4(4,40-
bipy)2 as SBUs bridged by 1,2,4-BTC linkers. The distance
between the dimeric cupper centers is 2.689 A. The SBU
paddle-wheel cluster M2(CO2)4(L)2 (where M ¼ Cu or Zn,
L ¼ H2O or pyridine) is known in many compounds, such
as interesting Zn(1,4-BDC)(DMF)(H2O), [Cu3(TMA)2
(pyridine)3], etc., but to our knowledge this is the first
instance of a SBU where L ¼ 4,40-bipy is reported [31–35].
In the packing arrangement of 2, the adjacent 2-D
supramolecurlar layers are parallel with each other and
construct a 3-D supramolecular framework via the weak
p– p interactions between the aromatic groups of 4,40-bipy
and 1,2.4-BTC ligands. The average distance of the
aromatic is about 11.176 A between the 1,2.4-BTC ligands
and 11.390 A between the 4,40-bipy groups.
One intriguing feature of the compound is its close
structural relation to the known 2-D MOF Zn(1,4-
BDC)(DMF)(H2O) [35]. The main difference of the two
structures is that, in the case of Zn(1,4-BDC)(DMF)(H2O),
the axial positions on Zn are occupied by water ligands
which serve to hold together the sheets by mutual hydrogen
bonding to the carboxylate oxygen on an adjacent sheet.
The sheets stack in registry to allowthe formation of 1-D
channels where DMF guests reside. But in the instance of
C=ðT À yÞ, w hereC is the Currie constant, and y the Weiss
constant, resulting in parameters C ¼ 0:44cm3 kmolÀ1 and
y ¼ þ6:6 K. The positive Weiss constant indicates a
predominantly ferromagnetic interaction between the cop-
per centers. This is confirmed by the continuous increases of
wMT value upon cooling. According to the structural data,
the shorter Cu1 ꢀ ꢀ ꢀ Cu2 distance between the two different
1-D chains (4.846 A) bridged by the carboxylate can
contribute to the ferromagnetic interaction.
The magnetic behavior of compound 2 is shown in
Fig. 6. The effective magnetic moment at 300 K, calculated
from meff ¼ 2:828ðXMTÞ1=2, is 1.99 mB per Cu, which is in
the range of the experimentally observed value
(1.70–2.20 mB) for Cu(II) ions. The wMT versus T exhibits
a value of 0.99 emu kOeÀ1 molÀ1 at 300 K and continuously
increases on cooling to a value of 1.36 emu kOeÀ1 molÀ1 at
21 K. This behavior of the wMT curve shows that there exist
ferromagnetic interactions in 2. However, the curve drops
abruptly below15 K, indicating that an antiferromagnetic
interaction exists in 2 at lower temperatures. The Weiss
constant, determined in the temperature range 80–300 K, is
+30.15 K, suggesting a strong predominantly ferromag-
netic interaction between the copper centers. According to
the structural data, compound 2 contains interesting
paddle-wheel clusters as SBUs which are further linked

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3364
construct a 3-D supramolecular framework via the weak
p– p interactions. The magnetic studies for both com-
pounds showa dominant ferromagnetic exchange between
the Cu(II) ions. Further research of this system may focus
on the replacement of copper and 4,40-bipy with other
transition metals and multidentate pyridyl ligands to
prepare novel polymeric compounds and explore their
valuable properties.
0.4
1.5
1.0
0.5
0.0
0.2
0.0
Acknowledgments
This work was supported by the National Science
Foundation of China (No. 20071013 and 20301007), and
the State Basic Research Project of China (G2000077507).
0
50
100
150
200
250
300
Temperature (K)
Fig. 5. Temperature dependence of magnetic susceptibility of 1.
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