A three-dimensional porous metal-organic framework [Fe2L3·(DMF)7·(C4 H10O)0.5] constructed from triple-helices {L = bis[2,4-dihydroxybenzaldehyde]hydrazone}

Article abstract of DOI:10.1039/b206702a

A novel 3-D large porous metal-organic framework was achieved from triple metal helices via π-π stacking interactions between the aromatic groups of the helices.

Full text of DOI:10.1039/b206702a

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A three-dimensional porous metal–organic framework
[
Fe L ؒ(DMF) ؒ(C H O) ] constructed from triple-helices
2
3
7
4
10
0.5
{L ؍ bis[2,4-dihydroxybenzaldehyde]hydrazone}
Mo Hong, Guo Dong, Duan Chun-ying,* Li Yu-ting and Meng Qing-jin
Coordination Chemistry Institute, The State Key Laboratory of Coordination Chemistry,
Nanjing University, Nanjing, 210093, P. R. China. E-mail: duancy@nju.edu.cn
Received 10th July 2002, Accepted 13th August 2002
First published as an Advance Article on the web 23rd August 2002
A novel 3-D large porous metal–organic framework
was achieved from triple metal helices via ꢀ–ꢀ stacking
interactions between the aromatic groups of the helices.
molecules in the pores, which is reported to be important for the
inclusion chemistry of such open frameworks.
Ligand H L is readily prepared by mixing 2,4-dihydroxy-
2
benzaldehyde and hydrazine hydrate in methanol. The porous
complex 1 was obtained by assembling H L and Fe(NO ) .†
2
3 3
Recently, intense research activities have been directed towards
the development of open porous frameworks including the
assembly of inorganic metal clusters, coordination complexes,
and organic molecules into extended motifs held together either
by strong metal–ligand bonding or by weaker bonding forces
Crystallographic study of complex 1‡ has unequivocally con-
firmed that the porous networks are constructed from triple
helical molecules (Fig. 1). Complex 1 crystallizes in a centro-
1–3
such as hydrogen-bonding and π–π interactions. The use of
discrete molecular units in the assembly of open porous frame-
works is an attractive synthetic approach since the structural
integrity of the building units can be maintained through-
out the reaction, and the desired physical properties can be
4,5
imparted to solid-state materials. While a number of open
frameworks with new structural aspects have been assembled
by the reactions of metal ions with organic molecules such as
di-, tri-, and poly-topic N-bound organic linkers or carboxylate
6
linkers such as 4,4Ј-bipyridine (BPY) and benzene-1,3,5-
7
tricarboxylate (BTC), large porous open frameworks sustained
8
by π–π interactions are still rare or lacking.
On the other hand, helicity is a fundamental structural
feature of great current interest in inorganic and coordination
chemistry, and much effort has been devoted to the design of
metal complexes exhibiting helical architectures, due to their
potential applications in some emerging fields of asymmetric
catalysis, non-linear optical materials and aesthetically appeal-
9,10
ing topology of helical compounds.
Although the basic
Fig. 1 Molecular structure of complex 1 showing the triple helical
arrangement. Selected bond lengths (Å): Fe(1)–O(3) 1.922(3), Fe(1)–
O(2) 1.931(3), Fe(1)–O(1) 1.906(3), Fe(1)–N(1) 2.144(3), Fe(1)–N(3)
features of the design necessary to assemble such a helix are
now fairly well established, exerting sufficient control over this
aggregation to afford porous open metal–organic frameworks
2
.182(3), Fe(1)–N(2) 2.129(4).
1
1,12
remains challenge.
To the best of our knowledge the use of
helical metal–organic architectures as building blocks for the
formation of 3-D networks has not been explored before.
Herein we report a novel metal–organic framework (MOF)
with large pores achieved from triple metal helices, and the
exploitation of π–π stacking interactions between the aromatic
groups of the helices to sustain and generate porosity. The
ligand used to assemble the triple helix is one of the simplest
imine-based ligands (Scheme 1). The ease of synthesis of these
symmetric space group C2/c, consequently the molecules occur
as a racemic mixture of Λ and ∆ configuration enantiomers,
whereby the equivalent fragments are inter-related by the C2
axis which runs perpendicular to the N(1) to N(1A) bond. Each
iron center is bound to three ON binding units to attain a
distorted octahedral coordination geometry with an Fe ؒ ؒ ؒ Fe
separation of ca. 3.91 Å. Each ligand loses two protons and
coordinates to two metal centers as a bis(bidentate) ligand to
complete the helical arrangement.
The most intriguing structural feature of the complex in the
solid state is that it forms a three-dimensional porous frame-
work into which guest molecules are adsorbed (Fig. 2). The
13
phenyl rings I and II stack to their symmetry-related ones
IB and IIC from other helices (symmetry code B: Ϫx, 2 Ϫ y,
1
Ϫ z; C: Ϫ0.5 Ϫ x, 1.5 Ϫ y, 1 Ϫ z), respectively, to form
two-dimensional networks. The π–π stacking interactions
between aromatic rings are characterized by short inter-planar
atom ؒ ؒ ؒ atom separations of 3.35 [C(4) ؒ ؒ ؒ C(6B)] and 3.48 Å
[C(12) ؒ ؒ ؒ C(12C)] for the parallel rings in each stack pair,
respectively. Those two-dimensional sheets are connected
together using the C–H ؒ ؒ ؒ O hydrogen bonds of the types
C(18)–H(18A) ؒ ؒ ؒ O(6ЈD) and C(18Ј)–H(18B) ؒ ؒ ؒ O(6D)
(symmetry code D: Ϫx, 1 Ϫ y, 1 Ϫ z) to produce a very open
Scheme 1
ligand systems has allowed us to systematically probe the effects
of modifications to the ligand backbone through which the
precise topography or macroarchitecture of the arrays should
be controlled. The hydroxyl groups in the ligand helices have
the potential to form classical hydrogen bonds with the guest
3
422
J. Chem. Soc., Dalton Trans., 2002, 3422–3424
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b206702a

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Fig. 2 Crystal structure of complex 1, with large apertures and voids
achieved by π–π stacking interactions.
porous MOF. The D ؒ ؒ ؒ A separations are 2.62 and 2.50 Å,
and D–H ؒ ؒ ؒ A angles are 157 and 176Њ, for the two kinds of
hydrogen bonds, respectively. Seven DMF molecules and half
a diethyl ether molecule per formula unit are found to fill the
Fig. 4 Powder X-ray diffraction pattern of complex 2 (a) and the
pattern calculated from the single-crystal data for complex 1 (b).
assembly of helical units into a metal–organic porous open
framework is interesting both in terms of controlled aggre-
gation of helices and porous materials. The absence of counter
ions and the significant large internal void make these and
related materials exciting new candidates for examination of
their potential utilities in, for example, catalysis or separation
processes. It is also noteworthy that although the energy of π–π
3
-D body. The oxygen atoms of the DMF molecules form
hydrogen bonds with the p-hydroxy groups stabilizing the
inclusion compound. The O ؒ ؒ ؒ O separations range from 2.61
to 2.83 Å and the O–H ؒ ؒ ؒ O angles range from 161 to 175Њ,
respectively. The remarkable openness of the MOF structure is
indicated by the presence of a wide square aperture of 7–9 Å,
and spherical internal voids of diameter ca. 13–15 Å. The
volume of a van der Waals sphere that would just fit inside the
Ϫ1
interactions is only 2–20 kJ mol , this kind of non-covalent
bond has the potential to assemble smaller and simpler frag-
ments into the desired cavities under favorable conditions,
which is important in host–guest chemistry and has applica-
tions in chemistry, biology and materials science.
3
14
voids is ca. 940 Å , a value which exceeds that observed for
most open zeolites.
15
To evaluate the mobility of the guests within the framework,
we examined the as-synthesized crystals by thermal gravimetric
techniques (Fig. 3). In flowing nitrogen, a crystalline sample was
Acknowledgements
This work was supported by the National Natural Science
Foundation of China. We also thank Mr Liu Yong-jiang for
collecting the crystal data.
Notes and references
†
Synthesis of H L. To a 25 mL methanol solution containing 2,4-
2
dihydroxybenzaldehyde (1.38 g, 10 mmol), hydrazine hydroxide (0.31 g,
mmol) was added. After refluxing for 4 hours, a yellow solid (H L)
5
2
was formed which was isolated by filtration and dried over P O
2
5
under vacuum. Yield: 84%. Anal. calc. for C H N O : C, 61.8; H, 4.4;
1
4
12
2
4
Ϫ1
N, 10.3%. Found: C, 61.8; H, 4.5; N, 10.4%. IR (cm , KBr disk) 3251
(
(
ν
), 2852, 1619, 1589, 1514, 1465 (νC᎐C, νC᎐N), 1263 (νN᎐N). δH
C–H
2
2
4
[ H ]DMSO) 11.33 (2H, s, HOH), 10.14 (2H, s,
H
OH
3
), 8.68 (2H, s,
6
6
5
HCH᎐N), 7.34(2H, d, H ), 6.32(2H, d, H ), 6.25(2H, s, H ).
bz
bz
bz
Synthesis of 1. To a 25 mL methanol solution containing H L
2
Fig. 3 EG and TGA for complex 1 from 28.6 to 500 ЊC.
(0.41 g, 1.5 mmol), Fe(NO ) ؒ9H O (0.41 g, 1 mmol) was added. After
3
3
2
finishing the addition, the solution was stirred for 1 hour at room
temperature. Black solid was isolated by filtration and dried over P O
under vacuum. Yield: 62%. Dark-red crystals (complex 1) suitable for
X-ray structural determination were obtained by slow diffusion of
diethyl ether into a DMF solution.
Ϫ1
2
5
heated at a constant rate of 5 ЊC min . A rapid weight loss of
3
5% was observed below 200 ЊC corresponding to the liberation
of all seven DMF and the half molecule of diethyl ether
per formula unit (calc. 37%); over the temperature range of
‡
Crystal data: (C H N O Fe )(C H NO) (C H O)0.5, monoclinic,
42 30 6 12 2 3 7 7 4 10
2
00–250 ЊC almost no weight loss was observed. Above this
space group, C2/c, a = 23.682(3), b = 23.465(3), c = 14.912(2) Å, β =
112.501(2)Њ, T = 293(2) K, V = 7656.0(18) Å , Z = 4, D = 1.276 g cm
F(000) = 3092, µ = 0.453 mm , 18224 measured reflections, 6590
independent reflections (Rint = 0.0975). Final refinement converged at
R = 0.074, wR = 0.127, respectively. CCDC reference number 186191.
3
Ϫ3
temperature, a weight loss step between 250 and 350 ЊC was
observed and is attributed to decomposition of the framework.
A powder X-ray diffraction pattern of the sample (complex 2),
which is obtained by heating complex 1 to 200 ЊC for 2 hours,
shows that the positions of the most intense lines remain
unchanged relative to the simulated pattern based upon the
single-crystal data of complex 1 (Fig. 4). The good agreement
between the peaks in both diagrams demonstrates that the
porous framework is retained in the absence of guest molecules
,
c
Ϫ1
1
2
See http://www.rsc.org/suppdata/dt/b2/b206702a/ for crystallographic
data in CIF or other electronic format.
8
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in the pores.
In conclusion, the present study shows the first example of
-D porous frameworks assembled from triple helices. The
3
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