Solvent effect on the structure and topology of metal-organic frameworks with the rigid tripodal star ligand 1,3,5-tris-(1-imidazolyl)benzene and lead(II) nitrate

Article abstract of DOI:10.1039/b306876p

Reaction of 1,3,5-tris(1-imidazolyl)benzene with Pb(NO₃) ₂ in DMF affords a novel metal-organic framework with 4.8 ² topology in which the DMF molecules are attached to the channel walls, while in the reaction carried out in methanol, a one-dimensional coordination polymer was obtained.

Full text of DOI:10.1039/b306876p

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Solvent effect on the structure and topology of metal-organic
frameworks with the rigid tripodal star ligand 1,3,5-tris-
(1-imidazolyl)benzene and lead(II) nitratey
Jian Fan,a Mou-Hai Shu,b Taka-aki Okamura,c Yi-Zhi Li,a Wei-Yin Sun,*a Wen-Xia Tanga
and Norikazu Ueyamac
L e t t e r
a
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry,
Nanjing University, Nanjing 210093, China. E-mail: sunwy@nju.edu.cn
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University,
Shanghai 200240, China
b
c
Department of Macromolecular Science, Graduate School of Science, Osaka University,
Toyonaka, Osaka 560-0043, Japan
Received (in New Haven, CT, USA) 12th May 2003, Accepted 27th June 2003
First published as an Advance Article on the web 24th July 2003
Reaction of 1,3,5-tris(1-imidazolyl)benzene with Pb(NO3)2 in
DMF affords a novel metal-organic framework with 4.82 topo-
logy in which the DMF molecules are attached to the channel
walls, while in the reaction carried out in methanol, a one-dimen-
sional coordination polymer was obtained.
Interest in porous metal-organic frameworks (MOFs) has been
driven by the prospect of generating a wide range of materials
with useful properties for applications such as adsorption, ion-
exchange and catalysis.1–3 The stability of porous MOFs is a
prerequisite for their use as such materials.4 Typically, rigid
ligands such as 4,4-bipyridine5 and benzene-1,3,5-tricarboxylic
acid have been deliberately chosen to construct such porous
MOFs.6
On the other hand, lead(II) compounds have been increas-
ingly studied owing to their possible applications in different
fields, especially in environmental protection due to the toxi-
city of lead and in biological systems for its diverse interactions
with biological molecules. In this paper, we report novel
MOFs [Pb(tib)(DMF)(NO3)2] (1) and [Pb(tib)(NO3)2] (2) con-
structed from the assembly of the rigid tripodal star ligand
1,3,5-tris(1-imidazolyl)benzene (tib) with lead(II) nitrate in
DMF (N,N-dimethylformaimde) and methanol, respectively,
which provide nice example of structures and topologies of
MOFs determined by the solvent.
The tib ligand was prepared from 1,3,5-tribromobenzene
and imidazole using the Ullmann condensation reaction.7
Fig. 1 (a) Coordination environment around the lead(II) atom in 1.
(b) View of the 2D 4.82 network of 1, the DMF molecules are repre-
sented by a space-filling model, the hydrogen atoms and counter-ions
are omitted for clarity.
Complex [Pb(tib)(DMF)(NO3)2] (1) was synthesized by slow
diffusion of diethyl ether into a solution of tib and Pb(NO3)2
in DMF. The 2D network structure of 1 was revealed by a sin-
gle crystal X-ray diffraction study. In the repeating unit of 1,
there are two nitrate ions, one lead(II) cation, one DMF mole-
cule and one tib ligand as shown in Fig. 1(a). Each lead(II)
center has a eight-coordinated environment in a holo-directed
geometry, according to the discussions by Shimoni-Livny
et al.,8 and each nitrate anion is coordinated with Pb(II) in a
bidentate mode with the Pb–O distances ranging from
from different tib ligands have different Pb–N distances ran-
˚
ging from 2.538(6) to 2.579(6) A, which are comparable with
those previously reported in complex [Pb(timpt)(NO3)2]ꢀ
(Et2O)0.417(H2O)0.167 {timpt ¼ 2,4,6-tris[4-(imidazol-1-ylmethyl)-
phenyl]-1,3,5-triazine}.9 In 1, both the Pb(II) atoms and
tib ligands serve as three-connected nodes, resulting in the
formation of 2D network with 4.82 topology as illustrated in
Fig. 1(b). Compared with the (6,3) network, the 4.82 topology
was not well-known up to now.3b,9–10 Due to the relatively
small coordination angle of N–Pb–N, ranging from 80.0(2)ꢁ
˚
2.678(6) to 2.823(15) A. It is notable that the O atom of the
DMF molecule is coordinated to Pb(II) with a Pb1–O31 dis-
˚
tance of 2.720(15) A. Three N atoms (N11, N31 and N51)
y Electronic supplementary information (ESI) available: crystal pack-
ing diagram of 1. See http://www.rsc.org/suppdata/nj/b3/b306876p/
DOI: 10.1039/b306876p
New J. Chem., 2003, 27, 1307–1309
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to 83.2(2)ꢁ, the tib ligands coordinated to the same Pb(II) atom
extend within a narrow space. As a result, the imidazole
groups are twisted by large angles (30.7ꢁ, 65.4ꢁ and 71.9ꢁ) with
respect to the core benzene ring and the 2D network is heavily
corrugated. The 2D sheet contains two kinds of metallacycles
and the DMF molecules are positioned within the larger one
[Fig. 1(b)]. The most striking feature of this complex is that
the 2D sheets stack in a parallel mode to generate an open fra-
mework (see Fig. S1 in the Electronic supplementary informa-
tion) and the solvent DMF molecules are attached the channel
wall via a weak Pb–O coordination bond with a length of
˚
2.720(15) A. The remarkable openness of this MOF structure
is indicated by the presence of a wide, square aperture of ca.
2
˚
15ꢂ 6 A . After removing the DMF molecules, a Platon calcula-
tion showed that the framework has 25.2% void volume.
To examine the mobility of the solvent molecules within this
MOF, a thermogravimetric analysis for 1 was carried out. A
weight loss corresponding to the liberation of the DMF mole-
cules was observed below 160 ꢁC and no weight loss was
observed over the temperature range of 160–330 ꢁC. The pow-
der X-ray diffraction pattern of the complex without DMF
obtained by heating complex 1 to 160 ꢁC for 2 h under N2 is
sharp and essentially the same as that of complex 1 (Fig. 2).
This result indicates that the porosity of 1 is retained in the
absence of the DMF molecules. The stability of porous materi-
als requires a robust framework and the rigidity of the ligand
tib is responsible for the stability of complex 1, even after
removing the DMF molecules. Furthermore, when complex
1, after removal of the DMF molecules, was immersed in an
aqueous solution for one day, the resulting complex shows a
weight loss in two steps: a weight loss of 2.6% below 70 ꢁC
and a further weight loss of 2.5% below 160 ꢁC, corresponding
to the liberation of two water molecules per formula unit
(calcd 5.3%). As a result, one water molecule is proposed to
coordinate to the Pb(II) atom in the place of the DMF mole-
cule. No absorption was observed when 1, after removing
the DMF molecules, was immersed in tetrahydrofuran, prob-
ably due to steric hindrance.
In order to investigate the effect of solvent DMF molecules
on the self-assembly process, complex [Pb(tib)(NO3)2] (2) was
synthesized in a methanol–water system and structurally char-
acterized. As shown in Fig. 3(a), each lead(II) was six-coordi-
nated in a hemi-directed geometry, with two nitrate anions
in bidentate and monodentate binding modes. There is an
identifiable void trans to the Pb–N32, which can be seen
clearly from the polyhedron presentation [Fig. 3(b)]. Although
with the same metal-to-ligand ratio as complex 1, complex 2
is composed of 1D chains, termed as a ‘‘tubular ribbon’’.11
Divalent lead is known to exhibit variable coordination num-
bers from 2 to 10. Obviously, the coordination of the DMF
molecule to Pb(II) in 1 changes the disposition of the ligands
around the lead(II). It has been concluded that small cycles
are favored over larger ones for entropic reasons.12 In complex
1, due to the presence of DMF, four tib ligands, each using two
Fig. 3 (a) Coordination environment around the lead(II) atom in 2.
(b) One-dimensional polymeric chain representation of complex 2.
of their three arms, are linked by four lead(II) atoms to form a
large metallacycles. In complex 2, two kinds of metallacycles
are found to result in the formation of the 1D chain, with both
metallacycles being composed of two lead(II) atoms and two
tib ligands.
In conclusion, two novel MOFs were obtained by assembly
reactions of the rigid tripodal star ligand 1,3,5-tris(1-imidazo-
lyl)benzene with lead(II) nitrate. The structural difference indi-
cate that the solvents greatly affect the self-assembly process in
this system. This approach of incorporating solvent molecules
into MOFs provides opportunities for the design of new
MOFs.
Experimental
Syntheses
Synthesis of 1. Pb(NO3)2 (16.5 mg, 0.05 mmol) and tib (13.8
mg, 0.05 mmol) were added to the DMF solution (10 ml). The
mixture was stirred for half an hour and filtered. Colorless
crystals were obtained by slow diffusion of diethyl ether into
the above filtrate over several weeks. Yield 56%. Anal. calcd
for C18H19N9O7Pb: C, 31.77; H, 2.81; N, 18.52; found: C,
31.65; H, 2.89; N, 18.64.
Synthesis of 2. A methanol solution (10 ml) of tib (13.8 mg,
0.05 mmol) was layered over an aqueous solution (2 ml) of
Pb(NO3)2 (16.5 mg, 0.05 mmol). Colorless crystals were iso-
lated by filtration after several weeks. Yield 63%. Anal. calcd
for C15H12N8O6Pb: C, 29.66; H, 1.99; N, 18.44; found: C,
29.77; H, 2.10; N, 18.49.
X-Ray crystallography
The data collection for complex 1 was made on a Bruker Smart
Apex CCD with graphite monochromated Mo-Ka radiation
(l ¼ 0.71073 A) at 293 K. The structures was solved by direct
Fig. 2 X-Ray powder diffraction diagram of (a) 1 after removing the
DMF molecules, (b) of 1 and (c) the simulated diagram of 1.
˚
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methods using SHELX-97 and refined by full-matrix least-
squares method anisotropically for non-hydrogen atoms.
Calculations were performed on a PC-586 computer with the
Siemens SHELXTL program package. The data collection
for 2 was performed on a Rigaku RAXIS-RAPID Imaging
Plate diffractometer with graphite monochromated MoKa
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Crystal data for [Pb(tib)(DMF)(NO3)2] (1). Monoclinic,
b ¼ 11.7744(11),
P21/c,
M ¼ 680.61,
a ¼ 8.5468(8),
3
ꢁ
˚
˚
6
7
c ¼ 23.277(2) A, b ¼ 96.143(2) , U ¼ 2329.0(4) A , Z ¼ 4,
Dc ¼ 1.941
g
cmꢃ3
,
m ¼ 0.7303 mmꢃ1
,
R1 ¼ 0.0536,
wR2 ¼ 0.1468 (4221 reflections).
Crystal data for [Pb(tib)(NO3)2] (2). Triclinic, P ꢃ 1,
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˚
M ¼ 607.52, a ¼ 8.6278(6), b ¼ 9.5870(7), c ¼ 11.5759(8) A,
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3
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˚
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Acknowledgements
The authors are grateful for funding from the National
Natural Science Foundation of China (Grant No. 20231020)
for financial support of this work.
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