Unusual high thermal stablility of a 2D → 3D polycatenated Fe(II) metal-organic framework showing guest-dependent spin-crossover behavior and high spin-transition temperature

Article abstract of DOI:10.1021/cg3007876

A novel three-dimensional (3D) Fe(II) metal-organic framework (MOF), [Fe₂(pptp)₄2H₂O]_n (1), constructed from the rigid 2-(3-(4-(pyridin-4-yl)phenyl)-1H-1,2,4-triazol-5-yl)pyridine (Hpptp) ligand was synthesized

Full text of DOI:10.1021/cg3007876

Communication
pubs.acs.org/crystal
Unusual High Thermal Stablility of a 2D → 3D Polycatenated
Fe(II) Metal−Organic Framework Showing Guest-Dependent
Spin-Crossover Behavior and High Spin-Transition Temperature
Xiu-Tang Zhang,† Di Sun,*,‡ Bao Li,*,§ Li-Ming Fan,† Bin Li,† and Pei-Hai Wei†
†Advanced Material Institute of Research, Department of Chemistry, Qilu Normal University, Jinan, 250013, China
‡Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering,
Shandong University, Jinan, Shandong, 250100, China
§School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China
S
* Supporting Information
ABSTRACT: A novel three-dimensional (3D) Fe(II) metal−organic
framework (MOF), [Fe2(pptp)4·2H2O]n (1), constructed from the rigid
2-(3-(4-(pyridin-4-yl)phenyl)-1H-1,2,4-triazol-5-yl)pyridine (Hpptp) ligand
was synthesized and structurally characterized. Complex 1 exhibits a two-
dimensional (2D) → 3D polycatenated framework based on inclined
interlocked 2D 44-sql grids. Variable-temperature magnetic susceptibility
measurements reveal that 1 is a rare example exhibiting a thermal-driven
dehydration and consequently accompanied by a dramatic change of spin
crossover (SCO) behaviors. Its spin transition temperature is as high as
390 K with ∼20% high-spin character. Moreover, the framework of 1 shows
unusual thermal stability to nearly 500 °C.
architectures. So design and synthesis of novel ligand to
construct and modulate SCO compounds are still the hot
topics in this field. Recently, Tong et al., designed a dipyridinyl
triazole ligand, 2-(3-(pyridin-4-yl)-1H-1,2,4-triazol-5-yl)-
pyridine, and used it to construct a series of Fe(II) compounds
showing tunable SCO behaviors accompanying the cis-to-trans
transformation behavior and supramolecular isormer phenom-
enon.10
Following the above ideas and our previously work on SCO
compounds,11 we designed and synthesized a longer dipyridinyl
triazole ligand, 2-(3-(4-(pyridin-4-yl)phenyl)-1H-1,2,4-triazol-
5-yl)pyridine (Hpptp), which could provide a suitable ligand
field supporting the occurrence of SCO and could be used to
construct high-dimensional interpenetrated networks,12 thus
enhancing cooperation between SCO sites. In this contribution,
we report the crystal structure and physical properties of a
2D → 3D polycatenated Fe(II) MOF based on inclined inter-
locked 2D 44-sql grids. This MOF is a rare example exhibiting a
thermal-driven dehydration and consequently accompanied by
dramatic change of SCO behaviors. Our focus is directed
toward a fundamental insight of the structural and magnetic
properties relationship to elucidate the solvent effect on the
spin transition behavior.
he spin crossover (SCO) phenomenon mainly observed
T
in octahedrally coordinated first-row d4−d7 transition
metal (most commonly Fe(II), 3d6) complexes has drawn great
attention due to its bistable nature of spin states, namely, high
spin (HS) and low spin (LS), which can be triggered by
external stimuli such as light irradiation and variation in
temperature or pressure.1 Since the first observation of a SCO
compound in 1931,2 many fascinating spin-transition com-
pounds showing room-temperature hysteresis,3 large hysteresis
loops,4 and photo- and pressure-induced spin transitions5 have
been obtained. Among them, discrete mononuclear Fe(II) or
low-dimensional species historically dominated SCO research;
however, organic ligands linked SCO centers in polynuclear
Fe(II) or high-dimensional framework are rare.5 On the other
hand, the search for spin transition (ST) materials with a
transition temperature (T1/2) around room temperature with a
large hysteresis loop has continued since the first revelation of a
“ST based display device” by Kahn.6 Although recent years have
witnessed a great advance in the elevation of transition
temperature, compounds with T1/2 in the range of room
temperature and above are still scarce.7 As we know, T1/2, is not
only affected by local coordination of metal ions but also by
counteranions and sometimes even by solvent molecules.8
According to previous reports, three-dimensional (3D) SCO
frameworks could increase the molecular cooperativity,9 and
T1/2 might be shifted to the higher temperature region,
accompanying a simultaneously widened hysteresis loop, which
may be due to the enhanced rigidity in higher dimensional
Received: June 10, 2012
Revised: July 1, 2012
© XXXX American Chemical Society
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Ligand Hpptp was synthesized using Suzuki cross-coupling
reaction of 4-pyridinylboronic acid and 2-(3-(4-bromophenyl)-
1H-1,2,4-triazol-5-yl)pyridine (Scheme 1 and see Supporting
a
Scheme 1. Synthetic Procedures of the Hpptp
a
(a) Na/CH3OH; (b) 2-picolinyl hydrazide; (c) Pd(PPh3)4, K3PO4,
1,4-dioxane, 4-pyridinylboronic acid.
Information). Black crystals of 1 were synthesized by
solvothermal reaction of iron(II) sulfate and Hpptp in the
presence of sodium hydroxide in DMF-H2O (14 mL, v:v = 1:6,
170 °C). Repeated attempts to synthesize the complex 1 using
different solvents resulted only in intractable precipitate. Phase
purity of is sustained by the powder X-ray diffraction patterns
(Figure S1 in the Supporting Information). For 1, the most
peak positions of simulated and experimental patterns are in
good agreement with each other. The dissimilarities in intensity
may be due to the preferred orientation of the crystalline
powder samples.
The crystal structure of 1 was determined by single-crystal
X-ray diffraction analysis at 298 K (see Supporting Information).
Compound 1 crystallizes in the orthorhombic space group of
Pbcn and is a neutral 3D framework based on inclined
interlocked 2D 44-sql grids. In the asymmetric unit, there is one
crystallographically independent Fe(II) ion located on an
inversion center, one anionic pptp ligand and one lattice water
molecule passed through by a 2-fold axis. As shown in Figure
1a, the Fe(II) ion is coordinated by two chelating pptp ligands
and two bridging pptp ligands at equatorial and apical
directions, respectively, as a result, a perfect FeN6 octahedron
is formed imposed by crystallographic inversion center (Fe1−
N1 = 2.006(3), Fe1−N2 = 1.953(2) and Fe1−N5ii = 2.000(2)).
The average Fe−N bond length is 1.986(3) Å within a
1.953(2)−2.006(3) Å range, in accordance with the typical low
spin state Fe(II)−N bond length.13 The pptp ligands use the
4-position N-donor to connect the Fe(II) centers to form a 2D
44-sql network incorporating square grids with a dimension of
14.46 × 14.46 Å based in Fe···Fe separation (Figure 1b). The
ligand pptp is very similar to that of 2-(3-(pyridin-4-yl)-1H-
1,2,4-triazol-5-yl)pyridine;10 however, it has longer dimension
and is more rigid in a bent conformation. Structurally
equivalent 2D grids are interlocked at an angle of 46° (Figure
2a), resulting in a 2D → 3D polycatenated framework (Figure
2b).14 This structure is geometrically similar and topologically
identical to nanoporous MOF [Fe2(azpy)4(NCS)4·(EtOH)]
(azpy is trans-4,4′-azopyridine and EtOH is ethanol),9a which
exhibits an interesting change in magnetic behavior upon
absorption−desorption of guest molecules in the crystal lattice,
and such a phenomenon is reversible and highly dependent on
the guest molecules.
Figure 1. (a) Octahedral coordination geometry around the Fe center
of 1; thermal ellipsoids are at 30% probability (symmetry codes:
(i) −x, −y, −z; (ii) x − 1/2, y − 1/2, −z + 1/2; (iii) −x + 1/2, −y + 1/2,
z − 1/2). (b) Perspective view of the 2D network in 1.
Figure 2. (a) A diagrammatic representation of the polycatenation
of infinite Fe2(pptp)4 square grids. (b) Schematic representation of
simplified 3D polycatenated networks.
An appealing feature of 1 is its unusual high thermal stability.
Thermogravimetric analysis of 1 was conducted under a
B
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nitrogen atmosphere (Figure 3) and showed that the first
weight loss of 2.91% in the range of 65−108 °C corresponds to
increase successively and get to the value of 0.33 cm3 K mol−1.
According to the TG trace, we found that 1 becomes a
dehydrated phase above 375 K. Therefore, the increased χMT
values might be ascribed to the spin transition of the iron(II)
ions in the solvent-free framework. Staying at 390 K for about
10 min, the χMT value reaches a maximum value of 0.7 cm3 K
mol−1. Then cooling the temperature, the χMT value decreases
gradually and reaches 0.12 cm3 K mol−1 at 300 K. After heating
the temperature to 390 K, the χMT values repeat again and no
obvious hysteresis could be observed. The whole tendency of
the χMT values indicates the occurrence of SCO behavior of the
solvent-free sample with T1/2 larger than 390 K. Compared to
the χMT value of 3.50 cm3 K mol−1 attributed to the HS state of
Fe(II), only about 20% HS Fe(II) was captured at 390 K. A
higher percentage of trapped HS Fe(II) species may be possible
here with the use of a higher temperature.
In summary, we have successfully prepared a novel 3D Fe(II)
metal−organic framework based on inclined interlocked 2D 44-
sql grids. This MOF is a rare example exhibiting a thermal-
driven dehydration and consequently accompanied by a
dramatic change of SCO behaviors. The magnetic measure-
ments of 1 reveal spin transitions at a temperature higher to
390 K with 20% high-spin character. Because of the increasing
cooperativity through hydrogen bonds, or in the synergy of
dehydration and spin transition, lattice water molecule plays a
dominant role in the SCO behaviors. Moreover, the framework
of 1 shows unusual thermal stability to nearly 500 °C.
Figure 3. The TG-DSC plots of 1.
two H2O molecule per formula unit (calcd. 2.69%) with a small
exothermic peak observed at 95 °C. After that, a plateau
appears and the dehydrated sample is thermally stable up to
490 °C. The framework starts to collapse with the loss of pptp
ligands at 490 °C accompanying one strong exothermic peak at
500 °C. Generally speaking, degradation of the organic
components of porous MOFs typically starts at moderate
temperatures (200−350 °C),15 leading to the decomposition of
the frameworks. There are only limited MOFs which are
reported to be stable near 500 °C.16 To the best of our
knowledge, the present MOF is a rare example with both high
thermal stability, which may highly facilitate their functional
applications. It is assumed that the polycatenated networks
tighten the backbone of the ligands and enhance their
resistance to pyrolysis.
ASSOCIATED CONTENT
* Supporting Information
■
S
The preparation of ligand and 1, Tables S1−S2, IR, and powder
XRD patterns. X-ray crystallographic file in cif format for 1.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: dsun@sdu.edu.cn (D.S.); libao@mail.hust.edu.cn
(B.L.).
■
Variable temperature magnetic susceptibility measurements
of 1 revealed a relatively abrupt one-step incomplete spin
transition (Figure 4) induced by dehydration. First, the χMT
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Natural Science Foundation of
China (Grant No. 21101097), Natural Science Foundation of
Shandong Province (ZR2010BQ023), Independent Innovation
Foundation of Shandong University (2011GN030), the Special
Fund for Postdoctoral Innovation Program of Shandong
Province (201101007), and the China Postdoctoral Science
Foundation (2012M511010).
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