Bi- and trinuclear cobalt based coordination polymers constructed from aromatic polycarboxylic acids and 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene: Crystal structure and magnetic properties

Article abstract of DOI:10.1016/j.molstruc.2009.12.052

Two new metal-organic frameworks (MOFs) [Co(btx)(bdc)(H₂O)]_n 1 and [Co₃(btx)₃(btc)₂(H₂O)₂]_n 2, (btx = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene; H₂bdc = 1,4-

Full text of DOI:10.1016/j.molstruc.2009.12.052

Journal of Molecular Structure 967 (2010) 196–200
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Bi- and trinuclear cobalt based coordination polymers constructed from aromatic
polycarboxylic acids and 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene:
Crystal structure and magnetic properties
a
Xiaoyan Xing a, Xueyan Song a, Peipei Yang a, Ruina Liu a, Licun Li a,b, , Daizheng Liao
*
a Department of Chemistry, Nankai University, Tianjin 300071, China
b State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new metal–organic frameworks (MOFs) [Co(btx)(bdc)(H2O)]n 1 and [Co3(btx)3(btc)2(H2O)2]n 2,
(btx = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene; H2bdc = 1,4-terephthalate acid; H3btc = 1,3,5-benzene-
tricarboxylate acid) have been synthesized and characterized by single-crystal X-ray diffraction studies
and low temperature magnetic measurements. Compounds 1 and 2 are three-dimensional compounds
whose structures can be described as layers of Co2 units (1) and Co3 (2) units with the carboxylate groups
acting as mono or bidentate ligand, which are further connected through the flexible bis(triazole) ligand.
The study of the magnetic properties of 1 and 2 in the temperature range 2–300 K shows the occurrence
of weak ferromagnetic interactions in 1 due to the anti–syn carboxylate bridge and weak antiferromag-
netic interactions in 2 between the high-spin Co(II) ions.
Received 2 September 2009
Received in revised form 29 December 2009
Accepted 29 December 2009
Available online 11 January 2010
Keywords:
Metal–organic frameworks
Three-dimensional
Magnetic properties
Anti–syn
Ó 2010 Elsevier B.V. All rights reserved.
High-spin
1. Introduction
applications as functional materials. Many transition-metal clusters
with very high ground-state spins, can be used as magnetic building
Crystal engineering of metal–organic frameworks (MOFs) have
received rather considerable attention in recent years not only
from their variety of intriguing architectures and topologies [1],
but also from their potential applications in materials science
and industrial technologies [2]. The chemistry of MOFs network
formation is highly sensitive to the conditions of the reaction,
which includes the nature of organic ligands, the metal ions, the
counterions, and sometimes the ratio between metal salt and li-
gand [3]. Among the reported studies, organic ligands with aro-
matic polycarboxylate groups are especially attractive because
the carboxyl group cannot only bridge two or more metal centers
to produce polymers with versatile structural motifs, but also can
adopt various types of bridging modes which affect the magnetic
properties [4]. Aromatic polycarboxylic derivatives such as ben-
zenedicarboxylic acid, 1,3,5-benzenetricarboxylic acid, 1,2,4,5-
benzenetetracarboxylic acid, and so on, belong to the important
family of polycarboxylate O-donor ligands, which have been exten-
sively used to prepare MOFs [5].
units to construct multi-functional coordination polymers combin-
ing magnetism with porosity. To date, some highly connected net-
works based on metallic clusters have been reported [3d,6].
All these have prompted us to synthesize new MOFs with novel
structural diversity and interesting magnetic properties using some
polycarboxylates in combination with other flexible ligand. The
choice of terephthalate and benzenetricarboxylate arises as both
possess versatile coordination modes and have the ability to form
large and tightly bound metal clusters, and the long flexible bis
(triazole) ligand can link discrete clusters into an extended network
via its bridging ability and should be feasible to form high-connected
3D networks. As expected, we successfully isolated two novel
3D Co(II)-organic frameworks [Co(btx)(bdc)(H2O)]n
1
and
[Co3(btx)3(btc)2(H2O)2]n 2, (btx = 1,4-bis(1,2,4-triazol-1-ylmethyl)-
benzene; H2bdc = 1,4-terephthalate; H3btc = 1,3,5-benzenetricarb-
oxylate). Magnetic data reveal weak ferromagnetic coupling for 1
and weak antiferromagnetic coupling for 2.
On the other hand, the introduction of polynuclear clusters into
the coordination polymers has increased the range of their possible
2. Experimental
2.1. Materials and physical measurements
* Corresponding author. Address: Department of Chemistry, Nankai University,
Tianjin 300071, China.
Commercially available reagents were used as received
without further purification. The ligand 1,4-bis(1,2,4-triazol-1-
E-mail address: llicun@nankai.edu.cn (L. Li).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.12.052

X. Xing et al. / Journal of Molecular Structure 967 (2010) 196–200
197
Table 1
ylmethyl)benzene was prepared according to the reported proce-
Selected bond lengths [Å] and angles [°] for complexes 1 and 2.
dures [7]. Elemental analysis (EA) for C, H, and N was performed
on a Perkin–Elmer 240 analyzer. IR spectra using KBr pellets were re-
corded on a Bruker Tensor 27 FT-IR spectrophotometer. Variable-
temperature magnetic susceptibilities were measured on a SQUID
MPMS XL-7 magnetometer performed on a SQUID MPMS XL-7 Mag-
netometer. X-ray crystallography: the crystal structures were deter-
mined using a Rigaku SCX mini diffractometer.
1
Co1–O1A
Co1–O2
Co1–O4
Co1–O5
Co1–N1
Co1–N5B
2.026217
2.089417
2.132017
2.107718
2.1382
O1A–Co1–O2
O5–Co1–O4
O5–Co1–N1
O2–Co1–N5B
O4–Co1–N5B
N1–Co1–N5B
93.84(7)
85.69(7)
93.83(8)
95.39(8)
82.05(8)
169.83(8)
2.1402
2
Co1–O3B
Co1–O6C
Co1–O1
Co1–N1
Co2–O5
Co2–O5E
Co2–O7
2.022(3)
2.062(3)
2.133(3)
2.158(4)
2.115(3)
2.115(3)
2.091(3)
Co2–N5
2.121(4)
O3B–Co1–O1
O3B–Co1–N1
O1–Co1–N1
O7–Co2–O5
O5–Co2–N5
O7–Co2–N5
155.03(12)
89.51(13)
89.65(13)
93.24(12)
93.43(14)
93.88(14)
2.2. Synthesis of compound 1
2.2.1. [Co(btx)(bdc)(H2O)]n
1
A mixture of CoCl2ꢀ6H2O (0.0476 g, 0.2 mmol), H2bdc (0.0332 g,
0.2 mmol), and btx (0.048 g, 0.2 mmol) was dissolved in 10 mL of
distilled water. The pH value was then adjusted to 6.0 with 1 M
NaOH solution. The resulting mixture was transferred and sealed
in a 25 mL Teflon-lined stainless steel vessel, and heated at
160 °C for 2 days. After the reaction system was slowly cooled to
room temperature purple crystals were filtered off, washed with
distilled water, and dried in air. Yield: (76% based on CoCl2ꢀ6H2O).
Anal. Calcd. for C20H16CoN6O5 (%): C 50.12; H 3.36; N 17.53. Found
Symmetry transformations used to generate equivalent atoms in 1: A ꢁx + 2, ꢁy + 2,
ꢁz + 1, B x + 1, ꢁy + 3/2. Symmetry transformations used to generate equivalent
atoms in 2: B ꢁx + 3/2, y ꢁ 1/2, ꢁz + 1/2, C x + 1, yꢁ1, z, E xꢁ1, y + 1, z.
Table 2
Crystallographic data and details of structure refinements for complexes 1 and 2.
(%): C 50.22; H 3.72; N 17.49. IR: (m
/cmꢁ1) 3414 (vs), 3111 (s), 1616
Complex
1
2
(w), 1517 (s), 1384 (vs), 1280 (s), 1130 (m), 1017 (m), 987 (m) 752
(m), 687 (m), 645(m), 481 (m).
Formula
Formula weight
Temperature (K)
Crystal system
Space group
b (°)
C20H16CoN6O5
479.32
296(2)
Monoclinic
P21/c
100.43(3)
12.133(2)
9.6093(2)
17.102(3)
1961.0(6)
4
1.624
0.924
0.924
0.0367
4496/0/289
0.71073
1.129
0.0468
0.0990
C27H21Co1.5N9O7
671.93
296(2)
Monoclinic
P21/n
92.30(3)
9.992(2)
10.064(2)
27.233(5)
2736.3(9)
4
1.636
0.986
0.986
0.0814
4811/0/403
0.71073
1.106
0.0613
0.1208
2.3. Synthesis of compound 2
a (Å)
b (Å)
2.3.1. [Co3(btx)3(btc)2(H2O)2]n
2
c (Å)
A mixture of CoCl2ꢀ6H2O (0.0476 g, 0.2 mmol), H3btc (0.042 g,
0.2 mmol), and btx (0.048 g, 0.2 mmol) was dissolved in 10 mL of
distilled water. The pH value was then adjusted to 6.0 with 1 M
NaOH solution. The resulting mixture was then transferred and
sealed in a 25 mL Teflon-lined stainless steel vessel, and then
heated at 160 °C for 3 days. After the reaction system was slowly
cooled to room temperature purple crystals were filtered off,
washed with distilled water, and dried in air. Yield: (72% based
on CoCl2ꢀ6H2O). Anal. Calcd. for C27H21Co1.5N9O7 (%): C 48.12; H
V (Å3)
Z
qcalc (g cmꢁ3
)
l
(cmꢁ1
)
Absorption coefficient (mmꢁ1
Rint
)
Data/restraints/parameters
k(Mo-K
a
) (Å)
GOF
a
R1 [I > 2
r
(I)]
b
3.44; N 18.71. Found (%): C 47.63; H 3.78; N 19.17. IR: (m )
/cmꢁ1
wR2 (all data)
P
P
a
3445 (vs), 3070 (s), 1615 (w), 1518 (s), 1540 (vs), 1441 (vs), 1360
(vs), 1277 (s), 985 (m) 772 (m), 686 (m), 647 (m), 519 (m).
R1
=
(||F0| ꢁ |Fc||)/ |F0|.
P
P
wR2 = { w(|F0|2 ꢁ |Fc|2)2/ w(|F0|2)2}1/2
.
b
3.1. Crystal structure of compound 1
2.4. X-ray measurements
The structure determination reveals that the 3D framework of 1
(Fig. 1b) consists of [Co(bdc)(H2O)]n layers and btx pillars. The coor-
dination sphere around the six-coordinate Co(II) cation is com-
posed of three bdc ligands, one water and two btx ligands. The
best equatorial plane is formed by two bis-bidentate bdc oxygen
atoms (O2 and O1C), one bis-monodentate bdc oxygen atom(O4)
and one oxygen atom from a water molecule (O5). The apical posi-
tions are then occupied by two nitrogen atoms (N1 and N1D) from
btx as depicted in Fig. 1a. The Co–O/N bond lengths vary from 2.026
to 2.140 Å. Two cobalt(II) ions are bridged by double carboxylate
groups in anti–syn conformation from two different bdc ligands to
form the binuclear Co unit (Co1ꢀꢀꢀCo1C 4.929 Å), which is linked
X-ray diffraction data of compounds 1 and 2 were collected on a
Rigaku SCX mini diffractometer using Mo
Ka radiation
(k = 0.71073 Å). The structure was solved by direct methods with
SHELXS-97 [8] and refined by full matrix least squares on F2 with
SHELXL-97 [9]. All non-hydrogen atoms were refined with aniso-
tropic thermal parameters. Hydrogen atoms were added theoreti-
cally and refined with riding model and fixed isotropic thermal
parameters. Detailed data collection and refinement of the com-
pounds 1 and 2 are summarized in Table 1. Selected bond lengths
and angles are listed in Table 2.
by the bis-bidentate l4
-
g1
:
g1
:
g1
:g
1 bdc ligands to form a 1D chain
g1 bdc bridge
and further connected by a bis-monodentate l2
-
g1
:
3. Results and discussion
to generate a 2D layer (Fig. 1c). The flexible btx ligands link adjacent
layers to form a 3D framework with CoꢀꢀꢀCo separation of 14.09 Å.
Compounds 1 and 2 were synthesized by the hydrothermal
method. It should be noted here that the synthesis of compounds
is sensitive to the reaction temperature and pH: above 160 or be-
low 150 °C, no product is obtained; if pH value was higher than
6 or lower than 5, then no crystal was obtained. The reported pH
value in Section 2 is the best choice, based on the yield.
3.2. Crystal structure of compound 2
The crystal structure of 2 can also be described as layers of anti–
syn carboxylate-bridged trinuclear Co(II) ions extended in the bc

198
X. Xing et al. / Journal of Molecular Structure 967 (2010) 196–200
Fig. 1. (a) The structure of 1 showing the coordination environment around the Co(II) center. (b) A view of 3D network of 1. (c) 2D Co/BDC layer.
plane, which are linked through btx ligands to build the 3D struc-
ture (Fig. 2b). The asymmetric unit of 2 consists of two indepen-
dent six-coordinate Co(II) centers (Co1, Co2). The Co1 center is in
a distorted octahedral coordination sphere completed by four oxy-
gen atoms (O1, O2, O3B, O6C) of btc ligands in the equatorial posi-
tions and two nitrogen atoms (N1, N1D) as the apices, these
oxygen atoms come from three different btc ligands. While Co2
atom is coordinated by two nitrogen atoms (N5, N5E) of btx ligands
at the axial positions, two oxygen atoms (O7, O7E) of water and
two carboxylate oxygen atoms (O5, O5E) from two different btc li-
gands in equatorial positions (Fig. 2a). The Co1 and Co2 atoms are
bridged by the carboxylate groups in anti–syn mode to form a tri-
nuclear cluster (Co1ꢀꢀꢀCo2 5.105 Å). The trinuclear clusters are con-
nected through the btc ligands to generate an infinite 2D network
a
high-spin Co(II) center (1.87 cm3 K molꢁ1 for S = 3/2 and
g = 2.0). This larger value is the result of contributions to the sus-
ceptibility from orbital angular momentum at high temperatures.
For complex 1, it can be seen that with decreasing temperature,
the
vMT value continuously decreases to reach a minimum of
4.00 cm3 K molꢁ1 at 12.0 K and then it sharply increases to reach
a value of 4.48 cm3 K molꢁ1 at 2.0 K. Concerning the possibility of
the occurrence of a magnetic interaction in 1, three exchange
pathways can be considered: one is the carboxylate group of
bdc bridge in the anti–syn coordination mode; another is through
the aromatic ring of the bridging bdc; the third is through btx
bridge ligand. However, the magnetic coupling through the last
two exchange pathways should be very weak and can be ignored.
Thus, from a magnetic point of view, complex 1 can be treated as
binuclear cobalt(II) units. As well known, a detailed quantitative
analysis of the susceptibility data for cobalt(II) complexes is com-
plicated by the fact that the orbital moment, spin–orbit coupling,
distortions from regular stereochemistry, electron delocalization,
and crystal field mixing of excited states into the ground-state
affect the magnetic properties in addition to a possible magnetic
interaction. Very recently, Lloret et al. [10] have proposed an ap-
proach to analyzing the magnetic data of compounds with six-
coordinated high-spin Co(II) ions that is valid in the limit of
the weak magnetic coupling as compared to the spin–orbit cou-
pling, |J/k| < 0.1. This approach allows the Co(II) ion in axial sym-
metry to be treated as an effective spin doublet (Seff = 1/2), which
is related with the real spin (S = 3/2) by Seff = (3/5)S. Following
this approach, we can use the expression for the magnetic sus-
ceptibility of interacting two spin doublets (Eq. (1)) to analyze
the magnetic susceptibility of 1, where the g factor has been re-
placed by the G(T,J) function.
(Fig. 2c). Each btc ligand acts as a
atoms, in which one carboxylate group adopts a l2
l
4-bridge linking four cobalt
-
g1
:g
1 bridging
coordination mode connecting two cobalt atoms, while other two
carboxylate groups adopt a monodentate mode and a bidentate
mode coordinating to two cobalt atoms. The 2D networks are fur-
ther pillared by flexible btx ligands to form a 3D supramolecular
architecture with CoꢀꢀꢀCo separation of 14.18 Å.
3.3. Magnetic properties
The magnetic susceptibilities of complexes 1 and 2 were mea-
sured in the range 2–300 K in an external magnetic field of
2000 Oe. The plots of
vMT (vM being the magnetic susceptibility
per Co(II) ion) vs. T of the two compounds 1 and 2 are shown in
Figs. 3 and 4, respectively. At room temperature, the experimen-
tal
tively, which are larger than expected for the spin-only value for
v
MT values of 1 and 2 are 2.96 and 3.11 cm3 K molꢁ1, respec-

X. Xing et al. / Journal of Molecular Structure 967 (2010) 196–200
199
Fig. 2. (a) The structure of 2 showing the coordination environment around the Co(II) center. (b) A view of 3D network of 2. (c) 2D Co/BTC layer.
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
6.0
5.5
5.0
4.5
4.0
3.5
3.0
0
50
100
150
200
250
300
0
50
100
150
200
250
300
T/K
T/K
Fig. 3. Temperature dependence of the
vMT product of 1.
Fig. 4. Temperature dependence of the vMT product of 2.
2
2Nb2½GðT; JÞꢂ
1
ground-state Kramers doublets (Eqs. (4) and (5)), with n being the
number of CoꢀꢀꢀCo interactions of each binuclear Co(II) unit (n = 1
in the present case) and Jeff = (25/9)J. k is the spin–orbit coupling
vM
¼
ð1Þ
ð2Þ
kT
n
½3 þ expðꢁ25J=9kTÞꢂ
GðT; JÞ ¼ GðTÞ þ
D
gP0
constant and
a is defined as a = jA, with A being a parameter whose
2
value depends on the strength of the crystal field [its value varies
between 1.5 (weak crystal field) and 1 (strong crystal field)] and
where G(T) represents the magnetic behavior of a magnetically iso-
lated Co(II) ion, which can be calculated using Eq. (3), g and P0
being given by Eqs. (4) and (5), respectively (see Ref. [7]). gP0 ac-
counts for the variation of g0 (corresponding to the magnetically
isolated Co(II) ion) due to the magnetic interactions between the
j
being the orbital reduction factor (0 6 j 6 1). D is the axial split-
D
ting parameter, being the energy gap between the singlet 4A2 and
doublet 4E levels issued from the splitting of the orbital triplet
4T1g ground-state under an axial distortion.
D

200
X. Xing et al. / Journal of Molecular Structure 967 (2010) 196–200
4k
2
References
½GðTÞꢂ ¼
v
MT
ð3Þ
Nb2
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,
D
= 752.8 cmꢁ1
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a
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vMT value continuously decreases upon cooling
to reach a value of 1.71 cm3 K molꢁ1 at 2.0 K. This value is close the
expected one for a magnetically isolated Co(II) ion, which implies
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system. Following the previous approach in 1, complex 2 can be
viewed as linear three interacting spin doublets. The expression
of magnetic susceptibility for such magnetic system is described
by Eq. (6).
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(2008) 2532.
Nb2½GðT; JÞꢂ 1 þ expð25J=9kTÞ þ 10 expð75J=18kTÞ
vM
¼
ð6Þ
4kT
1 þ expð25=9kTÞ þ 2 expð75J=18kTÞ
[6] (a) Y. Zhou, L. Han, J.-G. Pan, X. Li, Y.-Q. Zheng, Inorg. Chem. Commun. 11
(2008) 1107;
where the Landé factor g has been replaced by the G(T,J) function
with n = 4/3 and Jeff = (25/9)J. The best fit gave the parameters
(b) T.T. Luo, H.L. Tsai, S.L. Yang, Y.H. Liu, R.D. Yadav, C.C. Su, C.H. Ueng, L.G. Lin,
K.L. Lu, Angew. Chem., Int. Ed. 44 (2005) 6063;
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Chem. Mater. 17 (2005) 2866;
(d) C.-B. Ma, M.-Q. Hu, H. Chen, C.-N. Chen, Q.-T. Liu, Eur. J. Inorg. Chem. (2008)
5274;
J = ꢁ0.23 cmꢁ1, k = ꢁ138.62 cmꢁ1
,
D
= ꢁ763.5 cmꢁ1
, a = 1.43 with
R = 4.25 ꢃ 10ꢁ4. The magnetic coupling in 2 is very weak, as ex-
pected, and the values of the k, a, and D parameters lie within the
range of those observed in other six-coordinated high-spin cobal-
(e) D. Li, T. Wu, X.-P. Zhou, R. Zhou, X.-C. Huang, Angew. Chem., Int. Ed. 44
(2005) 4175;
t(II) complexes [3f,11].
(f) J. Zhang, Y. Kang, J. Zhang, Z.J. Li, Y.Y. Qin, Y.G. Yao, Eur. J. Inorg. Chem.
(2006) 2253;
(g) Y.-H. Wen, Q.-W. Zhang, Y.-H. He, Y.-L. Feng, Inorg. Chem. Commun. 10
(2007) 543;
4. Conclusion
(h) R.-Q. Zou, R.-Q. Zhong, M. Du, T. Kiyobayashi, Q. Xu, Chem. Commun. (2007)
2467;
(i) S.-Q. Ma, H.-C. Zhou, J. Am. Chem. Soc. 128 (2006) 11734;
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Martín, T. López, F. Lloretd, M. Julved, CrystEngComm 6 (2004) 106;
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(2003) 280;
We have reported the preparation of three metal polycarboxyl-
ate containing the ligand 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene
by a hydrothermal conditions. Compound 1 displays a 3D network
based on a dimer through bdc and btx bridge. Compound 2 is a 3D
network based on a trinuclear Co linked by btc and btx ligand. The
different structures of 1 and 2, mainly caused by the different poly-
carboxylate ligands, lead to the formation of the different frame-
works. Their magnetic properties revealed the occurrence of very
weak ferromagnetic for 1 and weak antiferromagnetic for 2 inter-
actions between the Co(II) ions through carboxylate bridges with
the anti–syn conformation. A detailed quantitative analysis of the
susceptibility data for complex 1 is complicated.
(c) H. Kumagai, Y. Oka, K. Inoue, M. Kurmoo, Dalton Trans. (2002) 3442;
(d) E.W. Lee, Y.J. Kim, D.K. Jung, Inorg. Chem. 41 (2002) 501;
(e) J.M. Rueff, N. Masciocchi, P. Rabu, A. Sironi, A. Skoulios, Eur. J. Inorg. Chem.
(2001) 2843.
Acknowledgments
[12] (a) V. Mishra, F. Lloret, R. Mukherjee, Inorg. Chim. Acta 359 (2006) 4053;
(b) J.-M. Rueff, C. Paulsen, J. Souletie, M. Drillon, P. Rabu, Solid State Sci. 7
(2005) 431.
This work was supported by the National Science Foundation of
China (Nos. 50672037, 20971072 and 90922032) and the NSF of
Tianjin (No. 09JCYBJC05600).