Selective gas sorption property of an interdigitated 3-D metal-organic framework with 1-D channels

Article abstract of DOI:10.1039/b712485f

An interdigitated 3-D metal-organic framework, [Cd₃(OH) ₂L₄(H₂O)₂], with 1-D channels was prepared using 4-aminophenyl-1H-tetrazole (HL) and Cd(II) ions, where the host framework shows selective gas s

Full text of DOI:10.1039/b712485f

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Selective gas sorption property of an interdigitated 3-D metal–organic
framework with 1-D channels{
Yang Zou, Seunghee Hong, Mira Park, Hyungphil Chun and Myoung Soo Lah*
Received (in Cambridge, UK) 14th August 2007, Accepted 28th September 2007
First published as an Advance Article on the web 11th October 2007
DOI: 10.1039/b712485f
An interdigitated 3-D metal–organic framework, [Cd3(OH)2-
L4(H2O)2], with 1-D channels was prepared using 4-amino-
phenyl-1H-tetrazole (HL) and Cd(II) ions, where the host
framework shows selective gas sorption behavior that is based
on the different nature of the interactions between the gas and
the framework rather than on the size-exclusion effect of the
micropores.
Scheme 1 4-Aminophenyl-1H-tetrazole (HL).
Considerable effort has been devoted to the synthesis and
[Cd3(OH)2L4(H2O)2]?DMF (1) was prepared by solvothermal
reaction of equivalent amounts of Cd(NO3)2?3H2O and the ligand
HL in DMF–H2O mixed solvent (see ESI{). The crystal structure
of 1{ (see ESI{) is shown in Fig. 1. Two types of Cd2+ centers and
two 4-aminophenyl-1H-tetrazolate ligands (Fig. S1, ESI{) lead to a
3-D network with 1-D channels. The coordination geometry
around Cd1, which is on a crystallographic inversion center, is
octahedral with four N atoms of the tetrazole groups from four
different ligands occupying the corners of a square and two O
atoms from two m3-OH2 groups at the apical positions (Table S2,
ESI{). Cd2 is in a distorted octahedral environment and is
coordinated to two nitrogen atoms of tetrazole groups from two
different ligands, one nitrogen atom from the amino group of a
ligand, one oxygen atom from water, and two oxygen atoms from
two different m3-OH2 groups. The m3-OH2 groups link Cd1 and
Cd2 to form a 1-D chain, {[Cd3(OH)2]4+}n, running along the
a-axis (Fig. 1(a)). The two ligands in the asymmetric unit of 1 are
in two different coordination modes. One is a tridentate ligand and
characterization of microporous metal–organic frameworks
(MOFs) composed of transition metal ions and bridging organic
ligands because of their potential applications in many areas,
including storage, separation, and exchange processes.1 In contrast
to conventional microporous materials such as zeolites and
activated carbon materials, MOFs have pore surfaces that can
be rationally designed and functionalized by their components.1,2
Several MOFs that exhibit selective gas sorption properties have
been reported,2a,3 and most of the selectivities are believed to be
based on the size-exclusion effect.4 Modulation of the sorption
profiles of MOFs5 is a challenging but attractive topic (because of
their potential in practical applications such as specific sensing and
separation of gas molecules). Recently, it was reported that an
adsorption system for specific target molecules can be realized if
there are multiple sites for specific interactions (hydrogen bonding
and van der Waals forces) at suitable positions on the micro-
pores.2a,6 It is also known that adsorption will be affected by a
change in the polarizabilities of guest molecules or the polarities of
frameworks.6c,7 The interactions between guest molecules and the
MOF backbone (such as dipole–dipole interaction, donor–
acceptor affinity, etc.) may make some contributions to the
selective gas sorption. In order to test this hypothesis, a suitable
MOF needs to be constructed. We selected 4-aminophenyl-1H-
tetrazole (HL) (see ESI{) as a ligand for the following reasons: (1)
it can act as a rigid building block to form robust MOFs with
metal ions; (2) the amino group of the ligand can participate in
either coordination or hydrogen bonding, which will play an
important role in the assembly of MOFs;5f,8 (3) the tetrazolate
group and the amino group connected through a conjugated
p-electron reservoir might provide polar environment in the
resulting MOFs (Scheme 1).
Fig. 1 The crystal structure of 1. The solvent molecules in the 1-D
channels and hydrogen atoms are omitted for clarity. (a) A 1-D
{[Cd3(OH)2]4+}n chain along the a-axis. (b) A 2-D sheet was constructed
by the 1-D {[Cd3(OH)2]4+}n chains interconnected by the tridentate ligands
(blue), and the bidentate ligands (red) coordinated with 1-D chains
forming ridge parts in the 2-D sheet along the c-axis. (c) Bidentate ligands
in a 2-D sheet (pink) are interdigitated with those of neighboring 2-D
sheets (differentiated in green) using hydrogen bonds (blue dotted line) and
p–p interactions to form a 3-D structure.
Department of Chemistry and Applied Chemistry, College of Science and
Technology, Hanyang University, Ansan, Kyunggi-Do, 426-791, Korea.
E-mail: mslah@hanyang.ac.kr; Fax: 82 31 436 8100; Tel: 82 31 400 5496
{ Electronic supplementary information (ESI) available: Synthesis and
characterization including detailed X-ray crystallographic analysis of the
ligand, 1 and 1a, TGA, temperature dependent PXRD of 1. See DOI:
10.1039/b712485f
5182 | Chem. Commun., 2007, 5182–5184
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uses the two nitrogen atoms (N2, N3) of its tetrazolate group to
bridge two adjacent Cd atoms (Cd1, Cd2) in the 1-D chain and the
nitrogen atom (N5) of the amino group to link atom Cd2 in the
adjacent chain to form a 2-D sheet structure (Fig. 1(b) and Fig. S2,
ESI{). The other as a bidentate ligand uses the two nitrogen atoms
(N7, N8) of its tetrazolate group to bridge another two adjacent
Cd atoms (Cd1, Cd2) in the 1-D chain and acts as the ridge part of
the 2-D sheet motif (Fig. 1(b) and (c)). The ridges are interdigitated
with those of neighboring 2-D sheets by hydrogen bonds to form a
3-D network with 1D channels. The hydrogen atoms of the amino
group (N10) in the bidentate ligand act as hydrogen donors
towards the nitrogen atoms (N1, N4) of the tetrazole residue, while
the nitrogen atom (N10) of the amino group also acts as a
hydrogen acceptor to form a hydrogen bond with a m3-OH2 group
(Fig. 1(c) and Table S3, ESI{).8d,9 As shown in Fig. 1(c), the two
neighboring bidentate ligands, which are almost parallel to each
other, also contribute to the formation of the interdigitated 3D
structure via the p–p interaction between phenyl and tetrazole
groups (Fig. S5 and Table S4, ESI{). The 1-D channels are along
Fig. 3 PXRD patterns: (a) calculated 1; (b) as-synthesized 1; (c) 1a
prepared by heating 1 at 200 uC; (d) 1a prepared by soaking 1 in
acetonitrile and desolvating.12
2
˚
the a-axis with a cross section of 5.4 6 5.4 A (the channel size is
measured after considering van der Waals radii for constituting
3
˚
atoms) (Fig. 2). The cavity volume is 208.5 A per unit cell, which
is ca. 20.2% of the total crystal volume (calculated by PLATON).11
Thermogravimetric analysis (TGA) of 1 (Fig. S3, ESI{) reveals a
weight loss of 4.7% in the region 80–250 uC. This weight loss
corresponds to 0.75 DMF molecules per formula unit (calc. 5.0%)
and is consistent with the elemental analysis. Decomposition of the
framework is observed above 300 uC. The solvent-free framework,
1a, was obtained by heating a crystal of 1 at 200 uC for about
20 min.12 The crystal structure of 1a{ is similar to that of 1, except
that the DMF and water molecules in the solvent channels are
removed from the framework (see ESI{). The integrity of the
microporous framework was also confirmed by measuring the
powder X-ray diffraction (PXRD) pattern of 1a, which shows
sharp diffraction peaks similar to those of 1 even without solvent
molecules (Fig. 3). The temperature-dependent PXRD patterns
suggest the stability of the framework up to 300 uC (Fig. S4, ESI{),
which is consistent with the TGA result.
Fig. 4 Gas sorption isotherms of N2 (77 K), H2 (77 K) and CO2 (195 K);
$ and # represent the adsorption and desorption of CO2; m and n H2,
and & and % N2, respectively.
consistent with the single-crystal structure analysis. The pore
volume for 1a (0.056 mL g21) measured from H2 isotherm
indicates that less than a half of the available pore volume
(0.120 mL g21, calculated using PLATON) was filled at 77 K. The
BET surface areas estimated from the CO2 and H2 sorption
isotherms are 210 and 90 m2 g21, respectively (see ESI{). The
difference in the surface areas indicates that the effect of the
framework polarity is significant. The framework host, which
contains Cd2+ ions, ligated water molecules, the polar residues and
the p-electrons of the ligand, gives rise to an electric field that
induces a dipole in CO2. Besides such dipole-induced dipole inter-
actions, the quadrupole moment of CO2 (3.3–3.4 6 10226 e.s.u.)
would interact with the electric field gradient, providing a further
contribution to the potential energy of adsorption.5f,7 In addition,
there may be some donor–acceptor affinity between the CO2
molecules and the Lewis acidic Cd2+ ions.4c
The host framework, 1a, was subjected to gas sorptions using
CO2, H2 and N2. It was observed that 1a adsorbs CO2 and H2, but
not N2 (Fig. 4). A steep rise was observed at relatively low pressure
in the adsorption branch of CO2, this early uptake indicating the
presence of permanent micropores in 1a. Calculations based on the
T-method13 from H2 isotherm gave a pore opening of ca. 5.5 A,
˚
Hysteresis was observed in 1a gas sorption isotherms for H2
at 77 K, and for CO2 at 195 K. Similar hysteresis behavior
was also observed for various MOFs.4f,4g,14 The mechanism for
hysteresis in the adsorption/desorption isotherms of MOFs is not
well understood currently. However, guest-directed framework
rearrangements have been proposed for some coordination
networks.9a,14c,15,16 It is unlikely that this mechanism operates in
the hysteresis described here, since 1a keeps its robust framework
up to 300 uC. The hindrance of some residual solvent molecules
Fig. 2 The top view of 1 with 2 6 2 arrays of the 1-D channels.10 The
outside surfaces of the channels appear in dark grey, and the inside
surfaces in cyan.
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and metal hydroxide fragments at the pores for the diffusion of the
gas molecules is also unlikely to be the cause of the hysteresis,4f
because the sample for sorption study was prepared by repeatedly
soaking and desolvating 1 in low-boiling acetonitrile, and by
subsequent drying under vacuum at 100 uC overnight. We
speculate that such behavior is a result of the increased sorbate–
sorbent interactions as the gas molecules access pore region around
the metal centers.14a
Most of the reported gas sorption selectivities in MOFs are
related to the size-exclusion effect.4 In principle, guests with kinetic
diameters smaller than the pore windows can diffuse through the
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window and reach any point of the pore in the framework.
17
Although the kinetic diameter of N2 (3.54 A) is much smaller
˚
than that of the effective pore window of 1a (5.5 A), N2 sorption at
˚
77 K was not observed. This sorption behavior is not due to lower
kinetic energy of N2, because the result at 193 K was similar to that
at 77 K (Fig. S12, ESI{). Such a phenomenon is rare for micro-
porous MOFs.18 A similar behavior was recently reported in
Kitagawa’s microporous Ni-MOF.18a It is likely that some
nitrogen molecules interact very strongly with the pore windows
around the metal centers having polar environment, which
prevents other nitrogen molecules from entering the micropores,
because the framework has no additional open channels along the
a- and b-axes (Fig. 2).19
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We have synthesized Cd-containing interdigitated 3-D MOF 1
using a rigid tetrazole ligand. We observed that the solvent
molecules in the channels of 1 can be removed without disturbing
the framework structure, yielding microporous MOF 1a. The
framework is highly robust, retaining its stability up to 300 uC. 1a
adsorbs H2 and CO2, but does not adsorb N2, not because of size
selectivity but because of the different nature of the host–guest
interactions.
We gratefully acknowledge the financial assistance offered
by KRF (KRF–2005–070–C00068), KOSEF (R01–2007-000-
10167-0), and CBMH (R11-2003-019-02001-0).
Notes and references
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{ CCDC 656711–656713. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b712485f
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5184 | Chem. Commun., 2007, 5182–5184
This journal is ß The Royal Society of Chemistry 2007