Two novel high-dimensional iron(ii) coordination polymers modeled by semi-rigid tetrapyridines

Article abstract of DOI:10.1039/c2ce25607j

Two new iron(ii) coordination polymers with the formula [Fe(TPOM)(SCN) ₂]·2(HN(CH₃)₂)·4(H₂O) (1, TPOM = tetrakis(4-pyridyloxymethylene)methane) and [Fe(TPOM2)(SCN) ₂]·2(CH₃OH) (2, TPOM2 = tetrakis(3- pyridyloxymethylene)methane), have been synthesized and characterized by elemental analysis, IR spectroscopy, TGA, magnetic measurements and X-ray single-crystal diffraction. Compound 1 crystallizes in the orthorhombic space group Cccm, and exhibits a 3D porous pts framework in which the Fe ^II(NCS)₂ units are linked by the tetra-topic TPOM ligands. In contrast, compound 2 crystallizes in the monoclinic space group C2/c, whose crystal structure consists of a 2D sheet formed by the connection of Fe ^II(NCS)₂ units and tetra-topic TPOM2 ligands. Interestingly, every 2D sheet interleavingly stacks over each other to totally construct a [4] tiling structure with a plane net signature of [122] ₄[4·4·4·4]. Temperature-dependent SQUID under variable external pressure and X-ray diffraction measurements indicate that the iron(ii) ions in 1 and 2 remain in the high spin state over the temperature range of 2-300 K.

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PAPER
Two novel high-dimensional iron(II) coordination polymers modeled by
semi-rigid tetrapyridines{
Fan Yu*ab and Bao Li*c
Received 20th April 2012, Accepted 14th June 2012
DOI: 10.1039/c2ce25607j
Two new iron(II) coordination polymers with the formula [Fe(TPOM)(SCN)2]?2(HN(CH3)2)?4(H2O)
(1, TPOM = tetrakis(4-pyridyloxymethylene)methane) and [Fe(TPOM2)(SCN)2]?2(CH3OH)
(2, TPOM2 = tetrakis(3-pyridyloxymethylene)methane), have been synthesized and characterized by
elemental analysis, IR spectroscopy, TGA, magnetic measurements and X-ray single-crystal
diffraction. Compound 1 crystallizes in the orthorhombic space group Cccm, and exhibits a 3D
porous pts framework in which the FeII(NCS)2 units are linked by the tetra-topic TPOM ligands. In
contrast, compound 2 crystallizes in the monoclinic space group C2/c, whose crystal structure consists
of a 2D sheet formed by the connection of FeII(NCS)2 units and tetra-topic TPOM2 ligands.
Interestingly, every 2D sheet interleavingly stacks over each other to totally construct a [4] tiling
structure with a plane net signature of [122]4[4?4?4?4]. Temperature-dependent SQUID under variable
external pressure and X-ray diffraction measurements indicate that the iron(II) ions in 1 and 2 remain
in the high spin state over the temperature range of 2–300 K.
as connecting nodes.5 However, the predicted and controlled
Introduction
synthesis of the final products is still a great challenge at this
Coordination polymers (CPs) have attracted great interest in the
stage because the self-assembly of coordination polymeric
fields of supramolecular chemistry and crystal engineering, not
frameworks with unique structures and functions are frequently
only for their intriguing variety of molecular topologies and
influenced by many factors, such as the nature of organic
architectures,1 but for their potential applications as functional
ligands, the coordination geometry of metal ions, and the pH
materials in diverse areas such as magnetism, heterogeneous
catalysis, gas storage, non-linear optics and chirality.2 In this
value of the solution, etc.6–9 Therefore, the proper design and
selection of organic linkers with modifiable backbone and high
regard, many synthetic efforts have been devoted to the design
connectivity assembled with iron(II) ions would be a facile route
and synthesis of 2D or 3D iron(II) coordination compounds with
to modulate novel iron(II) network topologies with anticipated
3
the typically octahedral FeN6
,
motivated by their potential
properties.10 Among the reported studies, the bidentate and
tridentate N-ligands are commonly utilized to connect isolated
iron(II) ions to promote the formation of high dimensional
coordination frameworks with fascinating SCO properties.11 In
contrast, high dimensional iron(II) CPs constructed from the
more extended N-ligands that possess four or more coordination
arms and resulting conformation have received little attention,
which might be ascribed to the rare selection of multi-dentate
ligands and the difficulty of crystallization of iron(II) networks.
Among these N-containing ligands, TPOM, an important
flexible tetra-dentate ligand, is a good candidate for the
construction of CPs with diverse structures. Several structural
topologies including pts, pcu, bbf and qtz, etc., built from the
connection of TPOM and variable metal ions have been
reported,12,13 which are commonly seen for CPs based on
4-connected organic linkers. However, rare CPs constructed of
TPOM and iron(II) ions have been reported, while no CPs with
FeN6 octahedral have been presented.14 Herein, as an effort to
explore the potential of such a versatile ligand, we tried our best
to synthesize high dimensional iron(II) CPs with typical FeN6
properties of spin crossover (SCO) behaviours and proposition
that direct covalent linking of the active coordination sites
through chemical bridges may improve cooperative communica-
tion between SCO centers in extended systems compared to
mononuclear ones.4 Therefore, it is of great interest and
essentiality to investigate every new 2D or 3D iron(II) CPs with
the typically octahedral FeN6 .
Generally, the construction of high dimensional iron(II) CPs
with charming structural topologies greatly depends on organic
ligands with appropriate functional groups and metal ions acting
aKey Laboratory of Optoelectronic Chemical Materials and Devices of
Ministry of Education, Jianghan University, Wuhan, 430056, P. R. China.
E-mail: yufan0714@163.com
bSchool of Chemistry and Environmental Engineering, Jianghan
University, Wuhan, Hubei, 430056, P. R. China
cSchool of Chemistry and Chemical Engineering, Huazhong University of
Science and Technology, Wuhan, Hubei, 430074, P. R. China.
E-mail: libao@mail.hust.edu.cn
{ CCDC reference numbers of 1 and 2, 875827–875828. For crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
c2ce25607j
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octahedra assembled with TPOM and SCN2. In addition, the
simplest structural type of pts network is known to be constructed
of 4-connected tetrahedral and square-planar shapes, and it is very
interesting for us to question the possibility whether the
appearance of a pts network would be achieved by the combination
of tetrahedral TPOM ligands and neutral Fe(NCS)2 units. In
accordance with our hypothesis, the simplified pts topological
structure of [Fe(TPOM)(SCN)2]?2(HN(CH3)2)?4(H2O) (1) is truly
presented. Furthermore, with the consideration of the key role of
the organic linkers in directing the self-assembly of final structure
of CPs, a similar tetra-topic ligand TPOM2 was generated.
Compared to the flexibility of TPOM, TPOM2 would adopt
much more variable conformations because of the additional
rotation of the pyridinate rings, illustrated in Scheme 1, leading
Synthesis of [Fe(TPOM)(SCN)2]?2(HN(CH3)2)?4(H2O) (1)
FeSO4?7H2O (0.2 mmol) and KSCN (0.4 mmol), with a small
amount of ascorbic acid (5 mg) were stirred in 8 mL methanol
for 5 min, then the solution was filtered and 1 mL water was
added to the filtrate. The resulting solution was placed in a test
tube, above which was layered 1 mL methanol, and 10 mL
methanol containing 0.05 mmol TPOM was layered on the top.
After one week, glittering yellow crystals were obtained (30%
based on TPOM). Anal. Calcd for dehydrated C27H24FeN6O4S2:
C, 52.6%, H, 3.92%, N, 13.63%; found C, 53.1%, H, 3.66%, N,
13.98%. IR (KBr, cm-1): 3437, 2071, 1607, 1571, 1482, 1434,
1301, 1158, 1101, 1011, 754.
Synthesis of [Fe(TPOM2)(SCN)2]?2(CH3OH) (2)
to much more unpredictable CPs.
A new iron(II) CP,
The synthetic route was similar to 1, excepted for TPOM2. Anal.
Calcd for C29H32FeN6O6S2: C, 51.18%, H, 4.74%, N, 12.35%;
found C, 50.45%, H, 5.15%, N, 13.11%. IR (KBr, cm-1): 3441,
2066, 1600, 1570, 1485, 1432, 1301, 1154, 1102 1011, 757.
[Fe(TPOM2)(SCN)2]?2(CH3OH) (2), with [4] tiling structure was
concomitantly obtained. These two new compounds are char-
acterized by X-ray crystallography, topological analysis, elemental
analysis, IR spectra, TGA and magnetic susceptibility, and
discussed in detail.
X-Ray structural determination
Experimental sections
Diffraction data for 1 (0.5 6 0.3 6 0.15 mm) and 2 (0.5 6 0.3
6 0.2 mm) were collected using an Oxford Gemini S Ultra
˚
diffractometer using Mo-Ka (l = 0.71073 A) radiation at
Materials and general methods
All starting materials were obtained commercially and were used
without further purification. Elemental analysis for C, H, N were
performed on a Perkin-Elmer 240Q elemental analyzer. The IR
spectra were recorded in range of 400–4000 cm21 on a Nicolet
5DX spectrometer (KBr pellets). Magnetic susceptibility mea-
surements were carried out in the temperature range of 2–300 K
with a magnetic field of 5000 Oe on a Quantum Design MPMS
XL-7 magnetometer. The ligand TPOM was synthesized by a
literature method.15 The magnetic measurements under pressure
were performed by using an EasyLab Mcell 10 hydrostatic
pressure cell, which is specially designed for the MPMS setup
and made of hardened beryllium bronze with silicon oil as the
pressure transmitting medium. The applied pressure was
measured by using the pressure dependence of the super-
conducting transition temperature of a built-in pressure sensor
made of high-purity tin. Magnetic data were calibrated with the
sample holder and the diamagnetic contributions. Three tripodal
ligands were synthesized by a literature method.
123 K. The structures of complexes were solved by direct
methods, and the non-hydrogen atoms were located from the
trial structure and then refined anisotropically with SHELXTL
using a full-matrix leastsquares procedure based on F2 values.16
The hydrogen atom positions were fixed geometrically at
calculated distances and allowed to ride on the parent atoms.
Structure refinement after modification of the data for the non-
coordinate lattice solvent molecules with the SQUEEZE
routine of PLATON led to better refinement and data
convergence. CCDC-875827 (1), and CCDC-875828 (2) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/
datarequest/cif.
Table 1 Crystal data of 1–2
1
2
Chemical formula
Formula mass
Crystal system
Space group
C31H48FeN8O8S2
778.71
Orthorhombic
Cccm
C29H32FeN6O6S2
680.56
Monoclinic
C2/c
˚
a/A
8.9846(5)
22.9562(11)
24.0229(12)
90.00
4954.8(4)
123(2)
Cccm
16.5531(9)
11.7181(7)
17.4935(8)
103.372(5)
3301.2(3)
123(2)
C2/c
4
3233
˚
b/A
˚
c/A
b (u)
Unit cell volume/A
T/K
Space group
3
˚
Z
4
2503
No. of reflections measured
Rint
Final R1 values (I . 2s(I))
Final R1 values (all data)
0.0000
0.0541
0.0787
0.0223
0.0312
0.0396
Scheme 1 The flexibility of the tetra-pyridinate ligands TPOM and
TPOM2.
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17
˚
˚
NSCN bond lengths are 2.219 and 2.120 A respectively, in
accordance with the typical values of a high-spin iron(II) state.
The connections between iron ions and NCS2 groups are bent
with a C–N–Fe angle of 166.1(2)u. Due to centrosymmetry, the
angles of N–Fe–N are almost around 90 and 180u, indicating a
regular octahedral geometry of the coordinated iron ion. The
axial positions are occupied by two trans-disposed NCS2
groups, while the equatorial positions are occupied by four
pyridyl rings that are cis-disposed to the opposite ring. The
octahedral distortion parameter, S, which is used to quantify
the angular deviation from the ideal octahedral geometry,18 is
11.3u, indicating a more regular octahedron shape compared to
other 3D iron(II) SCO compounds possessing similar FeN6
connectivity.
Table 2 Selected bond lengths [A] and angles [u] for 1 and 2. ,dFe-N. is
the average Fe–N distance. SFe is the sum of deviations of cis N–Fe–N
angles from 90u
1
2
Fe(1)–N(1)
Fe(1)–N(2)
2.219(2)
2.120(3)
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(3)
2.239(2)
2.218(2)
2.109(2)
2.189(2)
12.1
˚
,dFe-N. [A]
˚
2.169(2)
11.3
8.528(2)
,dFe-N. [A]
SFe [u]
Fe Fe distance [A]
SFe [u]
Fe Fe distance [A]
…
…
˚
˚
8.511(2)
Results and discussion
Synthesis of CPs 1 and 2
Each central iron(II) ion connects four distinct TPOM ligands,
while the tetradentate ligand TPOM connects four independent
Yellow crystalline samples of 1 and 2 were synthesized by the
diffusion method at room temperature with different layers
containing Fe(NCS)2 and TPOM or TPOM2. Both of the two
samples are very unstable when removed from the mother liquid.
X-Ray single crystal analyses at 123 K show that CPs 1 and 2 are
constituted of metal isothiocyanate and tetrapyridinate ligand.
The presence of the SCN groups is also supported by the strong
vibration peak around 2050 cm21 in the FT-IR spectrum of each
compound. The crystal data and structural parameters of 1 and 2
are summarized in Table 1 and 2.
…
…
iron(II) ions. The N Ccentral N angles of the flexible TPOM
ligand range from 93.09(2)u to 152.90(2)u, and the torsion angles
of Ccentral–O–CH2–Cpyridyl equal to 179.05(2)u, indicating that
the ligand deviates slightly from tetrahedral geometry and
adopts a fully extended configuration. In this way, the ligands
in such conformations serve as quaternary bridges to link four
iron(II) ions into an infinite 3D crystal structure with three
…
directional channels, as shown in Fig. 2. The shortest Fe Fe
˚
distance is about 8.985(4) A. The isolated aqua molecules and
(CH3)2NH in situ generated from DMF molecules during the
synthesis process of organic ligand19 are housed inside these
pores. Calculated using the PLATON routine, the solvent
accessible volume in the dehydrated structure of 1 is about
54.2%. Considering the central iron atoms and TPOM ligands as
four-connecting nodes, the overall structure of 1 topologically
possesses a 4,4-connected 2-nodal pts net20 with the point
(Schla¨fli) symbol {42?84} calculated with TOPOS software
(Fig. 3).21
Crystal structure of 1
The X-ray crystallographic analyses reveal that compound 1
crystallizes in the orthorhombic crystal system Cccm. The
asymmetric unit contains an independent SCN group, a quarter
iron atom and a quarter TPOM ligand (Fig. 1). The central iron
atom is coordinated by 6 N atoms. The Fe–Npyridine and Fe–
Crystal structure of 2
In contrast to the crystal structure of 1, the X-ray crystal-
lographic analyses reveals that compound 2 crystallizes in the
monoclinic crystal system C2/c. The asymmetric unit contains
half an iron atom, an independent SCN group, and a half
TPOM2 ligand containing N1 and N2 atoms (Fig. 4). The
central iron atom is also coordinated by 6 N atoms. The average
˚
Fe–Npyridine and Fe–NSCN bond lengths are 2.229 and 2.109 A,
respectively, also indicating the high-spin iron(II) state in 2. The
similar regular octahedron of the iron(II) ions of 2 could be also
confirmed by the S value of 12.1u. The axial positions are
Fig. 2 The 3D crystal packing mode of 1 along a, b, and c axis,
respectively, which show the three directional channels in the whole
crystal structure.
Fig. 1 The asymmetric unit of 1(a) and the octahedral coordination
environment of Fe atom (b).
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Fig. 5 (a) The 2D sheet crystal packing mode of 2 in the bc plane; (b)
the cavities left between the adjacent layers.
156.25(2)u. While the torsion angles of Ccentral–O–CH2–Cpyridyl
are 2 6 137.90(2)u and 2 6 167.41(2)u, indicating the not fully
extended conformation. The infinite 2D sheet structure in the bc
plane (Fig. 5(a)) is constructed by the connection of such
distorted quaternary bridges and four-connected iron(II) ions.
Tiny channels are found between adjacent 2D sheets (Fig. 5(b)),
which are occupied by the isolated methanol molecules. Within a
Fig. 3 View of the pts network of 1 in a simplified ball-and-stick model
(the purple and gray balls represent the centers of iron atoms and
4-connected TPOM, respectively).
…
˚
sheet, the shortest Fe Fe distance is about 8.767(2) A. The
adjacent sheets stack over each other to block the voids and form
the 3D strucutre (Fig. 5(b)). Calculated using the PLATON
routine, the solvent accessible volume in the dehydrated
structure of 2 is about 20.4%.
Considering the central iron atoms as four-connecting nodes
and TPOM2 ligands as another four-connecting nodes, the 2D
sheet of 2 topologically possesses a 4-connected uninodal sql/
Shubnikov tetragonal plane22 net with the point (Schla¨fli) symbol
{44?62} calculated with TOPOS software (Fig. 6(a)).21 Thus
interleaved sql/Shubnikov tetragonal planes net stack over each
other along the a axis direction. Interestingly, a [4] tiling
structure with plane net signature of [122]4[4?4?4?4] is further
presented, which not only tends to decrease the pore volume, but
stabilize the whole crystal structure (Fig. 6(b)) through weak
interactions.
Thermal stability
The thermogravimetric (TG) analysis was performed in N2
atmosphere on fresh polycrystalline samples of 1 and 2 and the
TG curves are shown in Fig. 7. The TG curve of 1 shows the first
weight loss of 11% in the temperature range of 25–90 uC, which
indicates the loss of lattice molecules per formula unit. The
Fig. 4 The asymmetric unit of 2(a) and the octahedral coordination
environment of Fe atom (b).
occupied by two nearly linear NCS2 groups, while the equatorial
positions are occupied by four pyridyl rings that are also cis-
disposed to the opposite ring. The connections between the iron
ions and NCS2 groups are bent with an C–N–Fe angle of
175.0(2)u.
Fig. 6 (a) View of the sql/Shubnikov tetragonal plane net of 2 in a
simplified ball-and-stick model; (b) the [4] tiling of sql/Shubnikov plane
net (the red/purple octahedrons and blue/green balls represent the centers
of iron atoms and 4-connected TPOM2, respectively).
Each tetradentate TPOM2 ligand adopts the highly-distorted
tetrahedral geometry to connect four independent iron(II) ions
…
…
with the range of N Ccentral N angles from 58.46(2)u to
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accordance with the tiling structure of 2 compared to the porous
structure of 1.
Magnetic properties of 1 and 2
The variable-temperature magnetic susceptibilities of 1 and 2
were measured on crystalline samples under 5 KOe in the range
of 2–300 K and shown as xMT versus T plots in Fig. 8. For 1, at
T = 300 K and P = 1 bar, the xMT value is 3.54 cm3 mol21
K
featuring the high spin state of Fe(II) atoms. As the temperature
is lowered, xMT values practically remain constant, the dropping
of xMT values at temperatures below 25 K might be caused by
the occurrence of zero-field splitting of the high spin iron(II) ions.
The magnetic data was fitted with the Curie–Weiss expression (C
= 3.267(2) cm3 mol21 K and h = 21.47(3) K). With the
…
consideration of the Fe Fe distance and the low values of the
Fig. 7 TGA analysis of 1 and 2.
Weiss constant, the decrease below 50 K might be also attributed
to very weak antiferromagnetic interactions or the presence of
zero-field splitting or both. The presence of guest molecules in
the crystal lattice of porous iron(II) CPs is known to play an
important role in affecting or triggering SCO behavior.
However, for 1, no thermal-driven SCO behavior could be
observed for the desolvated sample or samples immersed in
different solvents such as ethanol, acetonitrile, and benzene, etc.
In addition, to further track the possibility of pressure-induced
SCO behavior, magnetic properties of 1 were also investigated
under external hydrostatic pressures up to 8.88 Kbar in the
temperature range of 2–300 K, but no obvious spin transition
was observed from the xMT values. Similar magnetic properties
of 2 under different external pressures and guest molecules could
be observed, also indicating the absence of SCO behavior.
Compared to the distorted octahedron of FeN6 in other SCO
compounds,5 for 1 and 2, the regular octahedron of high-spin
iron(II) state was exhibited, mostly caused by the centrosym-
metry and expanded low density structure.3 In particular, the
octahedron of the low spin state is more regular than the high
spin state. Therefore, in 1 and 2, the high spin iron(II) ions could
not be switched either by hydrostatic pressure or thermally in the
temperature region of 2–300 K. Future efforts might be focused
on increasing the steric hindrance of the pyridyl rings around
central iron ions, which might be helpful to form irregular
octahedra at high temperature ranges, and subsequently trigger
the occurrence of spin transition.
deviation between the observed and calculated values was mostly
caused by the quick loss of guest molecules of a fresh sample in
the air. The metal–organic framework retains until 300 uC, and
then starts to decompose accompanying the loss of organic
ligands. For 2, the weight loss attributed to the gradual release of
two methanol molecules per formula unit is observed in the
range of 25–150 uC (obsd: 8.53% and calcd: 8.83%), then the
framework retains in the temperature range of 150–350 uC, and
then decomposes. The higher thermostability of 2 over 1 is in
Conclusions
In summary, two new iron(II) coordination polymers modeled by
the distinct tetra-pyridine ligands have been synthesized and
structurally characterized. The structural results show that
utilizing the flexible tetra-pyridinate ligands is a good approach
to achieve high dimensional iron(II) CPs structures with FeN6
octahedra. 1 has a 3D crystal structure with typical pts topology,
while 2 has a 2D crystal structure with a [4] tiling structure. The
variable coordination position of the pyridyl rings must be
responsible for assembling of the distinct structures. Further, the
stiffness and low density of the crystal structure, as well as
the regular octahedron shape, might play a crucial role in the
absence of the spin transition. More similar and systematic
attention are needed to build upon on the successful construction
Fig. 8
xMT versus T plots of 1(a) and 2(b) in the temperature range of
2–300 K under 5 kOe and variable external pressures.
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7 (a) Y. T. Wang, H. H. Fan, H. Z. Wang and X. M. Chen, Inorg.
Chem., 2005, 44, 4148; (b) L. Carlucci, G. Ciani, D. M. Proserpio and
S. Rizzato, New J. Chem., 2003, 27, 483; (c) G. H. Cui, J. R. Li, J. L.
Tian, X. H. Bu and S. R. Batten, Cryst. Growth Des., 2005, 5, 1775;
(d) I. S. Lee, D. MokShin and Y. K. Chung, Chem.–Eur. J., 2004, 10,
3158.
of high dimensional iron(II) CPs modeled by the other new
multi-pyridine ligands.
Acknowledgements
8 (a) M. L. Tong, S. Kitagawa, H. C. Chang and M. Ohba, Chem.
Commun., 2004, 418; (b) P. M. Forster, A. R. Burbank, C. Livage, G.
Fe´rey and A. K. Cheetham, Chem. Commun., 2004, 368; (c) Y. B. Go,
X. Q. Wang, E. V. Anokhina and A. J. Jacobson, Inorg. Chem., 2005,
44, 8265; (d) Y. B. Dong, Y. Y. Jiang, J. Li, J. P. Ma, F. L. Liu, B.
Tang, R. Q. Huang and S. R. Batten, J. Am. Chem. Soc., 2007, 129,
4520.
9 (a) S. T. Wu, L. S. Long, R. B. Huang and L. S. Zheng, Cryst. Growth
Des., 2007, 7, 1746; (b) X. Q. Lu, J. J. Jiang, C. L. Chen, B. S. Kang
and C. Y. Su, Inorg. Chem., 2005, 44, 4515; (c) D. M. Shin, I. S. Lee,
D. Cho and Y. K. Chung, Inorg. Chem., 2003, 42, 7722; (d) B. Zheng,
H. Dong, J. F. Bai, Y. Li, Sh. Li and M. Scheer, J. Am. Chem. Soc.,
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We gratefully acknowledge the National Natural Science
Foundation of China and the Fundamental Research Funds
for the Central Universities for financial support (nos. 21101066,
2011QN112), and the Analytical and Testing Center, Huazhong
University of Science and Technology for analysis and spectral
measurements. Prof. Jun Tao in Xiamen University is also
thanked for his great guidance.
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