Ferro/antiferromagnetic interactions transmitted by double bridged carboxylate groups in 2D mixed-ligand transition metal coordination polymers

Article abstract of DOI:10.1016/j.ica.2012.01.019

Three 2D coordination polymers with mixed ligands [M(bpe)(bba)]_n (M = Mn for 1, Co for 2 and Ni for 3, bpe = 1,2-bis(4-pyridyl)ethane, H ₂bba = isophthalic acid) have been synthesized and characterized by single crystal X-ray diffraction, PXRD, TGA and magnetic susceptibility measurements. The central metal ions of each have a distorted octahedral geometry and are bridged by μ₂-η¹:η¹ carboxylate groups to form binuclear units with metal-metal separations of 4.318 (1), 4.463 (2) and 4.551 (3), respectively. The binuclear units are linked by bba^2- to form 1D ribbons, which are further assembled through bpe ligands into a 2D grid structure. TGA analysis reveals that 1, 2, and 3 exhibit high thermal stability. The magnetic properties of 1, 2 and 3 were studied in the temperature range of 2-300 K. Analysis of the magnetic data reveals antiferromagnetic interactions in 1 as well as interesting ferromagnetic interactions in 2 and 3 between the anti-syn double carboxylate groups bridging metal ions.

Full text of DOI:10.1016/j.ica.2012.01.019

Inorganica Chimica Acta 387 (2012) 212–218
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Inorganica Chimica Acta
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Ferro/antiferromagnetic interactions transmitted by double bridged carboxylate
groups in 2D mixed-ligand transition metal coordination polymers
⇑
Tian Han, Peng Ren, Na Xu , Peng Cheng, Dai-Zheng Liao, Shi-Ping Yan
Department of Chemistry, Nankai University, Tianjin 300071, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 August 2011
Received in revised form 11 January 2012
Accepted 12 January 2012
Available online 28 January 2012
Three 2D coordination polymers with mixed ligands [M(bpe)(bba)]n (M = Mn for 1, Co for 2 and Ni for 3,
bpe = 1,2-bis(4-pyridyl)ethane, H2bba = isophthalic acid) have been synthesized and characterized by
single crystal X-ray diffraction, PXRD, TGA and magnetic susceptibility measurements. The central metal
ions of each have a distorted octahedral geometry and are bridged by l2- :g
g1 1 carboxylate groups to form
binuclear units with metal–metal separations of 4.318 Å (1), 4.463 Å (2) and 4.551 Å (3), respectively. The
binuclear units are linked by bba2ꢀ to form 1D ribbons, which are further assembled through bpe ligands
into a 2D grid structure. TGA analysis reveals that 1, 2, and 3 exhibit high thermal stability. The magnetic
properties of 1, 2 and 3 were studied in the temperature range of 2–300 K. Analysis of the magnetic data
reveals antiferromagnetic interactions in 1 as well as interesting ferromagnetic interactions in 2 and 3
between the anti–syn double carboxylate groups bridging metal ions.
Keywords:
Crystal structures
Transition metal
Magnetic interactions
Thermal stability
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
influencing the magnetic interactions. In this respect, the signifi-
cant magneto-structural relationship of binuclear Cu(II) complexes
The magnetic properties of coordination compounds have long
been a topical research field due to academic interest as well as
for potential application in high-density data storage and molecu-
lar switching [1–6]. When metal ions with unpaired electrons are
linked by bridging ligands to allow long range transmission of
magnetism, various magnetic properties may appear such as
ferro/antiferromagnetic interactions of spin carriers, slow relaxa-
tion of the magnetization, hysteresis of the magnetization, and
quantum tunneling in single-molecule magnets (SMMs) and sin-
gle-chain magnets (SCMs) [7–9]. Although several kinds of coordi-
nation compounds have been synthesized and magnetically
characterized, the rational design and synthesis of such com-
pounds with predictable magnetic properties remains a challenge.
Resolution of this problem lies in understanding magneto-struc-
tural correlations more thoroughly through experimental and
theoretical studies.
As an efficient three-atom magnetic coupler, the carboxylate
group is widely used to build magnetic units with transition metal
ions. The carboxylate group can adopt syn–syn, anti–syn, and anti–
anti coordination conformations, and the corresponding com-
pounds often show ferro- or antiferromagnetic behavior [10,11].
The binuclear complexes/units bridged by carboxylate groups
provide simple but useful examples to help understand factors
with S = 1/2 spin carriers has been well studied in the area of
molecular magnetism [2]. However, binuclear units with S > 1/2
spin carriers, especially those bridged only by l1,3 carboxylate
groups, have received less attention and deserve more investiga-
tion. As part of our research in molecule-based magnetic materials,
we have studied the assembly of paramagnetic transition metal
ions with aromatic acid ligands and the structural variations and
magnetic behaviors which resulted [12–14]. In this contribution,
isophthalic acid (H2bba) was selected due to its well-known coor-
dination chemistry and its ability to adopt a variety of coordination
modes. Further, we chose to employ 1,2-bis(4-pyridyl)ethane (bpe)
as a co-ligand, as the inclusion of electrically neutral linkers in the
synthetic system has been shown to result in new structures [15].
Three 2D mixed-ligand isomorphic coordination polymers
[M(bpe)(bba)]n (M = Mn for 1, Co for 2 and Ni for 3) were subse-
quently obtained and characterized. The central metal ions of
1–3 each adopt a distorted octahedral geometry and are double
bridged by l2- :
g1 g1 carboxylate groups in anti–syn mode to form
binuclear units. These units are linked via bba2ꢀ to form 1D rib-
bons, which are further assembled through bpe ligands into a 2D
grid structure with high thermal stability. It is noteworthy to men-
tion that, although the crystal structure of 2 was recently reported
by Hulvey et al. [16], it was also obtained independently by our
group. Analysis of the magnetic data reveals antiferro/ferromag-
netic behaviors for 1–3, depending on the electron configurations
of the corresponding spin carriers.
⇑
Corresponding author. Tel.: +86 22 23494881; fax: +86 22 23502458.
E-mail address: naxu@nankai.edu.cn (N. Xu).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2012.01.019

T. Han et al. / Inorganica Chimica Acta 387 (2012) 212–218
213
Table 1
2. Experimental
Crystal data and structure refinement for 1 and 3.
2.1. Materials and methods
Compound
1
3
Formula
Formula mass
Crystal system
Space group
a (Å)
C
20H16MnN2O4
C20H16NiN2O4
407.06
All reagents and solvents employed were commercially avail-
able and used as received without further purification.
Elemental analyses were performed on a Perkin-Elmer 240 ele-
mental analyzer. IR spectra (KBr) were performed on a Bruker TE-
NOR 27 spectrophotometer in the 4000–400 cmꢀ1 range. Powder
X-ray diffraction measurements were recorded on a D/Max-2500
403.29
triclinic
P1
triclinic
ꢀ
ꢀ
P1
10.062(5)
10.119(5)
10.154(5)
67.62(3)
79.95(3)
86.45(3)
941.3(3)
2
9.946(2)
9.990(2)
10.063(2)
68.87(3)
78.77(3)
85.63(3)
914.8(3)
2
b (Å)
c Å
a
(°)
b (°)
X-ray diffractometer using Cu K
a radiation. Thermogravimetric
c
(°)
analyses were measured by a standard Japan Science TG-DTA
instrument at 10 °C/min heating rate under air atmosphere. Vari-
able-temperature magnetic susceptibilities were measured with
an applied field of 1000 Oe on a Quantum Design SQUID MPMS
XL-7 magnetometer. Diamagnetic corrections were made with
Pascal’s constants for all the constituent atoms and sample holders.
V (Å3)
Z
F[000]
h range [°]
414
420
2.06–25.01
4761/3226
1.019
R1 = 0.0477,
wR2 = 0.1050
R1 = 0.0865,
wR2 = 0.1229
2.09–25.01
6924/3132
1.012
R1 = 0.0756,
wR2 = 0.2127
R1 = 0.1087,
wR2 = 0.2804
1.209/ꢀ2.914
Reflections collected/unique
Goodness-of-fit on F2
Final indices [I > 2
r(I)]
R indices (all data)
2.2. Preparations of [M(bpe)(bba)]n (M = Mn 1, Co 2, Ni 3)
Largest difference in peak/hole 0.352/ꢀ0.314
(e Åꢀ3
)
Compounds 1–3 were obtained by mixing H2bba (1.0 mmol,
0.166 g), bpe (0.1 mmol, 0.184 g), M(ClO4)2ꢁ6H2O (0.5 mmol,
M = Mn 0.181 g for 1, Co 0.183 g for 2 and Ni 0.183 g for 3), and
10 mL H2O in Teflon-lined vessel (23 mL) followed by heating at
180 °C for 3 days. After cooling to room temperature, single crys-
tals suitable for X-ray diffraction were isolated with yields of 55%
for 1, 48% for 2 and 53% for 3 (based on M), respectively. Anal. Calc.
for 1, C20H16MnN2O4 (403.29): C, 59.36; H, 4.00; N, 6.95. Found:
C, 58.90; H, 3.92; N, 6.78%. Anal. Calc. for 2, C20H16CoN2O4
(407.28): C, 58.98; H, 3.96; N, 6.88. Found: C, 58.66; H, 3.86; N,
6.51%. Anal. Calc. for 3, C20H16NiN2O4 (407.06): C, 59.01; H, 3.96;
N, 6.88. Found: C, 59.48; H, 4.02; N, 6.80%. IR (KBr) for 1: 3450(s,
b), 3059(m), 1613(s), 1574(s), 1544(s), 1475(m), 1449(s), 1424(s),
1394(s), 1337(m), 1229(w), 1076(w), 1018(m), 836(s), 753(s),
719(s), 545(m) cmꢀ1; for 2: 3431(s, b), 3063(m), 1614(s), 1565(s,
sh), 1541(s), 1478(s), 1453(s), 1424(s, sh), 1395(s), 1366(m),
1337(m), 1221(m), 1080(m), 1014(m), 835(s), 753(s), 711(s),
Table 2
Selected bond distances (Å) and bond angles (°) of 1 and 3.
1a
Mn(1)–O(3)#1
Mn(1)–O(1)
Mn(1)–N(1)
2.238(3)
2.209(2)
2.149(3)
Mn(1)–O(4)#2
Mn(1)–N(2)#3
Mn(1)–O(2)
O(1)–Mn(1)–O(3)#1
N(2)#3–Mn(1)–(3)#1
N(2)#3–Mn(1)–O(1)
N(1)–Mn(1)–O(4)#2
N(2)#3–Mn(1)–N(1)
O(4)#2–Mn(1)–O(2)
N(2)#3–Mn(1)–O(2)
2.220(3)
2.142(3)
2.315(3)
92.13(10)
84.96(11)
94.78(11)
100.08(12)
175.67(14)
87.61(10)
94.09(11)
O(4)#2–Mn(1)–(3)#1
O(1)–Mn(1)–O(4)#2
N(2)#3–Mn(1)–(4)#2
N(1)–Mn(1)–O(3)#1
N(1)–Mn(1)–O(1)
O(3)#1–Mn(1)–O(2)
O(1)–Mn(1)–O(2)
N(1)–Mn(1)–O(2)
115.48(9)
152.00(10)
83.76(11)
91.56(12)
82.77(11)
156.55(9)
64.56(10)
88.08(11)
3b
545(m) cmꢀ1
1537(s), 1482(s), 1449(s), 1424(s), 1406ꢂ1391(s, sh), 1337(m),
1238(w), 1072(w), 1018(m), 831(s), 753(s), 723(s), 549(m) cmꢀ1
; for 3: 3444(s, b), 3063(m), 1617(s), 1578(s),
Ni(1)–O(3)#1
Ni(1)–O(1)
Ni(1)–N(1)
2.017(6)
2.144(5)
2.134(6)
104.3(2)
155.5(2)
87.9(2)
Ni(1)–O(4)#2
Ni(1)–N(2)#3
Ni1—O2
O(3)#1–Ni(1)–O(1)
O(3)#1–Ni(1)–N(2)#3
N(2)#3–Ni(1)–O(1)
O(4)#2–Ni(1)–N(1)
N(1)–Ni(1)–N(2)#3
O(4)#2–Ni(1)–O(2)
N(2)#3–Ni(1)–O(2)
2.030(5)
2.136(6)
2.168(6)
99.33(19)
85.6(2)
87.6(2)
95.9(2)
175.6(2)
95.03(19)
90.7(2)
.
O(3)#1–Ni(1)–O(4)#2
O(4)#2–Ni(1)–O(1)
O(4)#2–Ni(1)–N(2)#3
O(3)#1–Ni(1)–N(1)
N(1)–Ni(1)–O(1)
O(3)#1–Ni(1)–O(2)
O(1)–Ni(1)–O(2)
N(1)–Ni(1)–O(2)
Caution! Perchlorate salts of metal complexes with organic
ligands are potentially explosive, and should be handled in small
amounts with care.
91.4(2)
89.8(2)
160.16(19)
60.99(18)
91.0(2)
2.3. X-ray crystallography
Diffraction data for the three compounds were collected at
293 K on a Rigaku 007 CCD diffractometer equipped with graph-
a
Symmetry codes: #1 ꢀx + 2, ꢀy + 2, ꢀz; #2 x, y ꢀ 1, z; #3 x + 1, y, z ꢀ 1.
Symmetry codes: #1 ꢀx + 1, ꢀy + 1, ꢀz + 1; #2 x, y, z + 1; #3 x ꢀ 1, y + 1, z.
b
ite-monochromated Mo K
a radiation (k = 0.71073 Å) by using the
x
-scan technique. Lorentz polarization and absorption corrections
of 1 is representatively described here. As shown in Fig. 1, the
asymmetric unit contains one Mn(II) ion, one bpe and one bba2ꢀ
anion. The Mn(II) ion is surrounded by four oxygen atoms from
bba2ꢀ and two nitrogen atoms from bpe in a slightly distorted
were applied. The structures were solved by the direct methods
and refined with the full-matrix least-squares technique using
the SHELXTL program package [17,18]. Anisotropic thermal parame-
ters were assigned to all non-hydrogen atoms. The hydrogen atoms
were set in calculated positions and refined as riding atoms with a
common fixed isotropic thermal parameter. The crystallographic
data and selected bond lengths and angles of 1 and 3 are listed
in Tables 1 and 2, respectively.
octahedral geometry. Each
Mn(II) ions and the two carboxylate groups are in l2
l
3-bba2ꢀ anion coordinates to three
g1 bridg-
-
g1
:
ing and chelating modes, respectively. Two Mn(II) ions are
connected by double bridging carboxylate groups to form a dimeric
[Mn(II)]2 unit with an intradimer Mn. . .Mn distance of 4.32 Å and
an interdimer Mn. . .Mn distance of 7.05 Å. The dimers are linked
via bba2ꢀ to form 1D ribbons, which are further connected through
bpe to construct 2D grids (Fig. 2). There are two types of C–H. . .O
hydrogen bonds between neighboring layers as shown in Fig. 3.
The hydrogen bond distances and angles (Table 3) are in accord
with a literature report of an H. . .O distance and C–H. . .O angle
in the range of 2.2–2.8 Å and 120–180°, respectively [19]. Offset
3. Results and discussion
3.1. Crystal structure of [M(bpe)(bba)]n (M = Mn 1, Co 2, Ni 3)
Single crystal X-ray diffraction analyses reveal that 1–3 are iso-
ꢀ
structural and crystallize in triclinic space group P1. The structure

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T. Han et al. / Inorganica Chimica Acta 387 (2012) 212–218
Fig. 1. Coordination environment of Mn(II) ion in 1. Hydrogen atoms are omitted
for clarity. Symmetry codes: (A) 1 + x, y, ꢀ1 + z; (B) x, ꢀ1 + y, z; (C) 2 ꢀ x, 2 ꢀ y, ꢀz.
pyridyl-pyridyl and benzene-benzene
p. . .p stacking interactions
exist between adjacent sheets with perpendicular distances of
3.864 and 3.109 Å, respectively. The centroid–centroid distances
are 4.570 and 4.471 Å with offset angles of 32.3° and 45.9°, respec-
tively. There is also edge-to-face
p. . .p stacking interactions
between adjacent sheets. The benzene rings are nearly perpendic-
ular to the pyridyl rings (angle = 88.4°) and are off-center by
Fig. 2. 1D ribbons of Mn(II) ions bridged via bba2ꢀ ligands (top) and the ribbons
pillared by bpe ligands into a 2D grid (bottom). Hydrogen atoms are omitted for
clarity.
0.243 Å from the mean plane with H. . .
distances of 2.897 and 4.930 Å, respectively. These hydrogen bonds
and . . . stacking interactions between 2D grids form a 3D supra-
molecular architecture.
p and centroid–centroid
p
p
Fig. 3. Packing diagram of 1 along the crystallographic c axis showing interlayer hydrogen bonds and p. . .p stacking interactions.

T. Han et al. / Inorganica Chimica Acta 387 (2012) 212–218
215
Table 3
Hydrogen bonds between 2D layers in 1.
Hydrogen bond H. . .O distance (Å) C. . .O distance (Å) C–H. . .O angle (°)
C6–H6B. . .O2
C9–H9. . .O2
2.700
2.611
3.6325(56)
3.5092(47)
161.30
162.64
Ni
Co
Mn
Simulation
Fig. 5. The TGA experimental curves of 1–3.
10
20
30
40
50
60
2 Theta
Fig. 4. The PXRD curves of 1–3. The bottom (black) curve is calculated from the
single crystal data of 2.
3.2. Powder X-ray diffraction and thermogravimetric analysis
The powder X-ray diffraction (PXRD) patterns of 1–3 were stud-
ied at room temperature to confirm the phase purity. As shown in
Fig. 4, patterns of 1–3 are in good agreement with the patterns
calculated from crystallographic data, implying purity of the bulk
materials. Thermogravimetric analysis (TGA) of 1–3 reveals high
thermal stability (Fig. 5). No major weight loss occurred until
388 °C for 1, 407 °C for 2 and 362 °C for 3, respectively. Above these
temperatures, a sharp weight loss was observed corresponding to
decomposition of the coordination networks and ligands.
Fig. 6. Temperature dependence of vM (s) and
corresponds to the best fit of the data.
vMT (4) for 1. The solid line
v0M ¼ ð2Nb2g2=kTÞf½x þ 5x3 þ 14x6 þ 30x10 þ 55x15ꢃ=½1 þ 3x
þ 5x3 þ 7x6 þ 9x10 þ 11x15ꢃg
3.3. Magnetic properties
ð1Þ
In expression (1), x = exp(J/kT), N, g and b are Avogadro’s num-
ber, the Lande factor and Bohr magneton, respectively. The J
parameter represents the intradimeric exchange coupling of
3.3.1. [Mn(bpe)(bba)]n (1)
The magnetic susceptibilities of 1 were measured in the temper-
ature range of 2–300 K, as shown in Fig. 6. At room temperature, the
the [Mn(II)]2 unit through double
l2-carboxylate bridges [20].
v
MT value is 8.89 cm3 K molꢀ1, which is close to the expected
The interdimeric magnetic interactions are further considered by
the mean field approximation with the parameter zJ0, and the total
magnetic susceptibility expression is shown in Eq. (2).
8.75 cm3 K molꢀ1 for two free high-spin Mn(II) ions (S = 5/2, g = 2).
Upon cooling, MT decreases gradually until ca. 50 K and then de-
v
creases rapidly at lower temperature till 1.09 cm3 K molꢀ1 at 2 K. A
maximum is observed in the magnetic susceptibility of
0.58 cm3 molꢀ1 at 4.5 K (Tmax), suggesting antiferromagnetic inter-
actions between Mn(II) ions. The estimation of magnitude of the
magnetic interaction is 1.08 cmꢀ1, obtained by applying the equa-
tion |J|/kTmax = 0.347 [2]. The data (9–300 K) obey the Curie–Weiss
vM
¼
v0M=½1 ꢀ ðzJ0=Nb2g2ÞvM0
ꢃ
ð2Þ
,
The best fit shown as the solid line in Fig. 6 gave J = ꢀ1.46 cmꢀ1
g = 2.04, zJ0 = ꢀ0.12 cmꢀ1 and R = 5.5 ꢄ 10ꢀ5 (R =
R[(vMT)obs
ꢀ (
v R[(v
MT)calc]2/ MT)obs]2). The negative and small J value confirms
law [
the
v
M = C/(T ꢀ h)] with C = 9.10 cm3 K molꢀ1 and h = ꢀ6.54 K. Both
the existence of weak antiferromagnetic interactions in the dimer,
while the zJ0 value indicates a very weak antiferromagnetic interdi-
meric interaction. The result shows that the double carboxylate
bridge in 1 is an antiferromagnetic pathway because it induces a
good overlap of magnetic orbitals [21,22].
vMT versus T curve and negative h value indicate the existence of
antiferromagnetic interactions between the Mn(II) ions. Currently,
an expression describing the relationship between magnetic sus-
ceptibility and temperature in such a 2D structure is not available,
hence a non-critical approximation was applied to estimate the
magnetic interactions. According to the crystal structure and corre-
sponding magnetochemistry, the magnetic exchange interaction of
3.3.2. [Co(bpe)(bba)]n (2)
The magnetic behavior of 2 in the form of
versus T is depicted in Fig. 7(a). The MT value at room temperature
is 6.46 cm3 K molꢀ1, which lies in the range for Co(II) complexes
vMT versus T and vM
the Mn(II) ions transmitted by the l2
should be larger than that of the
-
g1
:
g1 carboxylate group
v
l
2-bba2ꢀ ligand, while the bpe li-
gand possesses a bulky aromatic group and is unlikely to mediate
any significant interactions. The magnetic susceptibility data of 1
can be analyzed in terms of an isotropic exchange interaction by
using Eq. (1) derived by the Hamiltonian H = ꢀJSAꢁSB (SA = SB = 5/2).
with dimer structure. Interestingly, upon cooling, vMT first in-
creases smoothly to a maximum value of 6.90 cm3 K molꢀ1 at
90 K and then decreases to a minimum value of 6.22 cm3 K molꢀ1
at 16 K. At temperatures below 16 K, a sharp increase is observed

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T. Han et al. / Inorganica Chimica Acta 387 (2012) 212–218
3.2
(a) 8.0
(a)
4.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
3.0
2.8
2.6
2.4
2.2
2.0
χ
χ
χ
χ
0
50
100
150
T / K
200
250
300
0
50
100
150
T / K
200
250
300
5
(b)
(b)
6
5
4
3
2
1
0
4
3
2
1
0
0
10
20
30 40
H / kOe
50
60
70
0
10
20
30
40
50
60
70
H / kOe
Fig. 8. (a) Temperature dependence of vM (s) and
corresponds to the best fit of the data. (b) Field dependent magnetization of 3 at 2 K.
vMT (4) for 3. The solid line
Fig. 7. (a) Temperature dependence of vM (s) and
dependent magnetization of 2 at 2 K.
vMT (4) for 2. (b) Field
Dq values of 721 and 1091 cmꢀ1, respectively. Upon comparison
with the B parameter of free Co(II) ion (1030 cmꢀ1), the results
show that the d electrons of the Co(II) ions are partially shared
with the ligands. Although we have tried to model such a magnetic
behavior, it is impossible to obtain reasonable parameters. How-
to a maximum value of 7.88 cm3 K molꢀ1 at 2 K. All these features
indicate ferromagnetic interactions between Co(II) ions. The de-
crease of
vMT in the temperature region of 90–16 K is probably
due to depopulation of the high energy Kramers doublets as well
as the spin-orbit effects of the hexacoordinated high-spin Co(II)
ion [11]. The existence of ferromagnetic interaction is also sup-
ever, both the vMT versus T curve and the positive h value indicate
the existence of ferromagnetic interaction in 2. The magnetization
curve at 2 K (Fig. 7(b)) increases rapidly at external fields below 5
kOe and slowly thereafter without saturation, consistent with the
magnetic anisotropy and ferromagnetic interaction of Co(II) ions.
Field-cooled magnetization (FCM) and zero-field-cooled magneti-
zation (ZFCM) do not show evident divergence until 2 K, which
indicates paramagnetic behavior for 2.
ported by the fact that the minimum value of vMT at 16 K is much
higher than the calculated value for a magnetically isolated cobal-
t(II) ion (1.73 cm3 K molꢀ1 for Seff = 1/2 with g ꢅ 4.3) [23]. Fitting
the data with Curie–Weiss law gives the result of
C = 6.67 cm3 K molꢀ1 and h = 1.20 K. Though the magnetic ex-
change pathways are almost the same as 1, the first-order orbital
momentum of the hexacoordinated high-spin Co(II) ions prevents
analysis of the magnetic behavior by the spin Hamiltonian. As
the magnetic interaction in 2 is difficult to quantify, an approxima-
tion is necessary. Sakiyama et al. have shown that the magnetic
properties of homodinuclear Co(II) compounds can be described
by the following Hamiltonian [24]
3.3.3. [Ni(bpe)(bba)]n (3)
The
vMT versus T and vM versus T curves of 3 are shown in
Fig. 8(a). The value of v ,
MT at room temperature is 2.83 cm3 K molꢀ1
which is slightly larger than the expected value of 2.42 cm3 K molꢀ1
for two free Ni(II) ions (S = 1, g = 2.2) but lies in the observed range
for Ni(II) complexes. Upon cooling,
vMT decreases smoothly
H ¼ ꢀð3=2Þ
j
k ꢁ LS þ b½ꢀð3=2Þ L þ geSꢃ ꢁ H ꢀ JS1 ꢁ S2
j
to 2.77 cm3 K molꢀ1 at 120 K, then increases to 3.03 cm3 K molꢀ1
at 10 K. Further cooling leads to an abrupt decrease to
2.19 cm3 K molꢀ1 at 2 K. This feature suggests significant ferromag-
þ
D
½L2z ꢀ LðL þ 1Þ=3ꢃ
MAGSAKI(A) software developed by Sakiyama has been used to fit
the experimental data by using this Hamiltonian [25,26]. Before
the fitting, we attempted to derive crystal field parameters such
as the Racah parameter (B) and the crystal field splitting energy
(10 Dq) from UV–Vis data to optimize the fit, as five parameters
are used in the dinuclear anisotropy axial mode by the software.
Three peaks at 9761, 15070 and 19430 cmꢀ1 for 2 give the B and
netic behavior in 3. The decrease of
vMT at high temperature may
result from temperature-independent paramagnetism (TIP) of the
Ni(II) ion. Fitting of the data to the Curie–Weiss law gives
C = 2.81 cm3 K molꢀ1 and h = 0.04 K. The positive h value suggests
ferromagnetic interactions between Ni(II) ions. As with 1, the
intradimer magnetic interaction (J) and isotropic Hamiltonian

T. Han et al. / Inorganica Chimica Acta 387 (2012) 212–218
217
H = ꢀJSAꢁSB gives a molar magnetic susceptibility expression for the
Ni(II) dimer (SA = SB = 1) according to Eq. (3) [19]
ferromagnetic contribution (JF), leading to the antiferromagnetic
interactions observed in 1 and the ferromagnetic interactions ob-
served in 2 and 3. In addition, the interdimer interactions of 1–3
should be antiferromagnetic, as indicated by the results of 1 and 3.
v0M ¼ ð2Nb2g2=kTÞf½expðJ=kTÞ þ 5 expð3J=kTÞꢃ=½1 þ 3
ꢄ expðJ=kTÞ þ 5 expð3J=kTÞꢃg
ð3Þ
4. Conclusion
The interdimeric magnetic interactions are further considered
by the mean field approximation with the parameteꢀr1zJ0. The
In conclusion, three new isomorphic coordination polymers
containing binuclear magnetic transition metal units are reported.
Magnetic studies of 1–3 demonstrate the varying nature (ferrro/
antiferro) of the magnetic interactions transmitted by the anti–
syn double-bridged carboxylate groups: the Mn(II) ions in 1 are
antiferromagnetically coupled and the Co(II)/Ni(II) ions in 2/3 are
ferromagnetically coupled. In each case, the unique magnetic
behavior depends on the electron configurations of the central
metal ions.
best fit gives J = 3.91 cmꢀ1
,
g = 2.32, zJ0 = ꢀ0.62 cm
, TIP =
0.00016 cm3 molꢀ1 and R = 3.44 ꢄ 10ꢀ5. This result is in accord
with the fitting result obtained using the Curie–Weiss law.
The ferromagnetic interaction is further supported by the mag-
netization curve in 0–7 T at 2 K (Fig. 8(b)). The experimental line
lies between the two calculated curves that correspond to the Brill-
ouin functions for (i) two isolated Ni(II) centers with S = 1, g = 2.32,
and (ii) two ferromagnetic coupled Ni(II) centers with S = 2,
g = 2.32, which is in accord with ferromagnetic interactions.
Acknowledgements
3.3.4. Magneto-structural correlation
In general, a qualitative interpretation of magnetic interactions
in transition metal dimers can be afforded by the well-known
Goodenough–Kanamori rules, according to the interaction between
pairs of natural magnetic orbitals [27–30]. A three-atom bridge
with various coordination modes (syn–syn, syn–anti and anti–anti),
the carboxylate group can transmit ferro/antiferromagnetic inter-
actions between spin carriers with different magnitude based on
the nature of the electron configuration, bridge mode and bond an-
gle/distance. Binuclear complexes/units bridged by carboxylate
groups provide simple useful examples to understand the factors
which influence magnetic properties. It is known that the
syn–syn conformation always cause antiferromagnetic coupling,
as evidenced by the large number of magneto-structural studies
on acetato-bridged dicopper(II) complexes [2]. According to Kahn’s
model, the exchange integral (J) is composed of two terms, one fer-
romagnetic (JF) and one antiferromagnetic (JAF) [31]. The value of JAF
is proportional to the square of the integral overlap. For 1, the five
unpaired electrons of high-spin Mn(II) ion occupy the metal-cen-
tered orbitals with d character in a parallel fashion and the
value of overlap integral is nonzero, which leads to antiferromag-
netic coupling, as observed in many Mn(II) dimers bridged by l1,3
carboxylate groups independent of the bridge modes. It has been
reported that the strength of the magnetic interactions transmitted
This work was supported by the National Natural Science Foun-
dation of China (21151001, 21171100 and 90922032) and the
Fundamental Research Funds for the Central Universities.
Appendix A. Supplementary material
CCDC 801492 and 801494 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2012.01.019.
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