Network flexibility: Control of gate opening in an isostructural series of Ag-mofs by linker substitution

Article abstract of DOI:10.1021/ic500908r

An isostructural series of 15 structurally flexible microporous silver metal-organic frameworks (MOFs) is presented. The compounds with a dinuclear silver core as secondary building unit (Ag₂N₄) can be obtained under solvothermal conditions from substituted triazolyl benzoate linkers and AgNO₃ or Ag₂SO₄; they exhibit 2-fold network interpenetration with lvt topology. Besides the crystal structures, the calculated pore size distributions of the microporous MOFs are reported. Simultaneous thermal analyses confirm the stability of the compounds up to 250 °C. Interconnected pores result in a three-dimensional pore structure. Although the porosity of the novel coordination polymers is in the range of only 20-36%, this series can be regarded as a model system for investigation of network flexibility, since the pore diameters and volumes can be gradually adjusted by the substituents of the 3-(1,2,4-triazol-4-yl)-5- benzamidobenzoates. The pore volumes of selected materials are experimentally determined by nitrogen adsorption at 77 K and carbon dioxide adsorption at room temperature. On the basis of the flexible behavior of the linkers a reversible framework transformation of the 2-fold interpenetrated network is observed. The resulting adsorption isotherms with one or two hysteresis loops are interpreted by a gate-opening process. Due to external stimuli, namely, the adsorptive pressure, the materials undergo a phase transition confirming the structural flexibility of the porous coordination polymer.

Full text of DOI:10.1021/ic500908r

Article
pubs.acs.org/IC
Network Flexibility: Control of Gate Opening in an Isostructural
Series of Ag-MOFs by Linker Substitution
Marcel Handke,† Hanna Weber,† Marcus Lange,‡ Jens Mollmer,‡ Jorg Lincke,† Roger Glaser,‡
̈
̈
̈
Reiner Staudt,§ and Harald Krautscheid*,†
†Fakultat fur Chemie und Mineralogie, Universitat Leipzig, Johannisallee 29, 04103 Leipzig, Germany
̈
̈
̈
‡Institut fur Nichtklassische Chemie e.V., Permoserstraße 15, 04318 Leipzig, Germany
̈
§Fakultat Maschinenbau und Verfahrenstechnik, Hochschule Offenburg, Badstraße 24, 77652 Offenburg, Germany
̈
S
* Supporting Information
ABSTRACT: An isostructural series of 15 structurally flexible micro-
porous silver metal−organic frameworks (MOFs) is presented. The
compounds with a dinuclear silver core as secondary building unit
(Ag2N4) can be obtained under solvothermal conditions from substituted
triazolyl benzoate linkers and AgNO3 or Ag2SO4; they exhibit 2-fold
network interpenetration with lvt topology. Besides the crystal structures,
the calculated pore size distributions of the microporous MOFs are
reported. Simultaneous thermal analyses confirm the stability of the
compounds up to 250 °C. Interconnected pores result in a three-
dimensional pore structure. Although the porosity of the novel
coordination polymers is in the range of only 20−36%, this series can be
regarded as a model system for investigation of network flexibility, since the
pore diameters and volumes can be gradually adjusted by the substituents
of the 3-(1,2,4-triazol-4-yl)-5-benzamidobenzoates. The pore volumes of selected materials are experimentally determined by
nitrogen adsorption at 77 K and carbon dioxide adsorption at room temperature. On the basis of the flexible behavior of the
linkers a reversible framework transformation of the 2-fold interpenetrated network is observed. The resulting adsorption
isotherms with one or two hysteresis loops are interpreted by a gate-opening process. Due to external stimuli, namely, the
adsorptive pressure, the materials undergo a phase transition confirming the structural flexibility of the porous coordination
polymer.
sudden increase in adsorption capacity.11,12 The phenomenon
INTRODUCTION
■
of structural flexibility is a response to external stimuli such as
Metal−organic frameworks (MOFs) or porous coordination
pressure or temperature.13 With substituted triazolyl benzoates
polymers (PCPs) have gained increasing interest in the last
containing anionic carboxylate and neutral 1,2,4-triazole donor
decades because of their distinct adsorption properties and high
groups14 porous and structurally flexible copper-based MOFs
diversity of possible framework structures.1 As highly porous
have already been reported.15
materials, MOFs possess great potential for applications in gas
separation and storage,2 as sensors,3 in heterogeneous
Herein we report on an isostructural series of 15 structurally
flexible microporous Ag-MOFs with 2-fold network inter-
catalysis,4 and in thermal energy storage.5 The potential of
penetration and closely related linkers whereupon we obtained
MOFs forming highly ordered porous structures has been
three types of crystallographically related structures. In contrast
impressively shown by the synthesis of, for example, the well-
to the most common dia topology16,17 especially for inter-
penetrated frameworks,18 the isostructural frameworks reported
here are described with the point symbol {42.84} and the less
common topology type lvt, first reported for MOFs by Boldog
et al.19
known HKUST-16 and MOF-57 as well as the MIL8 and DUT9
series. In addition to the frequently investigated pure
carboxylates, nitrogen-containing heterocycles such as pyr-
idines, pyrazoles, triazoles, and tetrazoles were found to be
valuable ligands in the synthesis of MOFs because of their
tunable coordination behavior.10 Regarding the different
behavior of MOFs during the activation procedure, the
materials are classified according to Kitagawa in three
generations.11 Typical for structurally flexible MOFs, stepwise
adsorption−desorption isotherms are noticeable. Such flexible
materials reveal a so-called gate-opening process in which the
framework undergoes a structural transformation along with a
Based on 15 substituted 3-(1,2,4-triazol-4-yl)-5-benzamido-
benzoate ligands (L1−−L15−), the pore diameters and volumes
of the isostructural Ag-MOFs 1−15 (Table 1) can be gradually
adjusted. The ligands bear substituents with different sterical
Received: April 22, 2014
© XXXX American Chemical Society
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Table 1. Linker Substitution Pattern of the Isostructural Ag-MOFs 3[Ag2(L)2] (1−15)
∞
Table 2. Unit Cells of 1−5 and 8−15.
1
2
3
4
5
8
structure type
II
III
III
III
III
I
space group
I41/acd
Iba2
Iba2
Iba2
Iba2
I41/acd
lattice constant [pm]
a = 2873.7(3)
a = 2340.7(5)
b = 2954.5(6)
c = 2917.1(6)
20174(7)
16
a = 2329.18(6)
b = 2715.41(8)
c = 3104.0(1)
19632(1)
16
a = 2330.24(5)
b = 2932.23(8)
c = 2970.87(9)
20299.4(9)
16
a = 2321.49(8)
b = 2756.6(1)
c = 3029.1(1)
19384(1)
16
a = 2856(1)
c = 6940(1)
57308(12)
48
c = 2303.9(9)
volume [106 pm3]
18788(15)
Z
16
9
10
11
12
13
14
15
III
III
III
III
III
III
I
Iba2
Iba2
Iba2
Iba2
Iba2
Iba2
I41/acd
a = 2318.95(8)
b = 2964.5(2)
c = 2951.6(1)
20291(2)
16
a = 2332.85(7)
b = 2799.2(1)
c = 3083.7(1)
20137(1)
a = 2339.87(6)
b = 2823.29(8)
c = 3081.2(1)
20355(1)
16
a = 2323.72(6)
b = 2850.6(1)
c = 2971.6(1)
19684(1)
16
a = 2299.14(8)
b = 2975.8(1)
c = 2878.8(1)
19696(2)
16
a = 2333.5(8)
b = 3037(1)
c = 2734(3)
19375(21)
16
a = 2866.1(2)
c = 2297.8(1)
18875(2)
16
16
demand, i.e., hydrogen, methyl, ethyl, and methoxy, at different
positions, and the size of the pores that are interconnected to a
three-dimensional pore structure depends on the linker
substituents. Therefore, this series of MOFs allows inves-
tigation of the influence of linker substituents on the gate-
opening behavior.
Besides the single-crystal structures, powder X-ray diffraction
(PXRD), temperature-dependent PXRD, thermogravimetry/
mass spectrometry (TG-MS) data, and adsorption properties
with N2 and CO2 as adsorptives are discussed.
correlate to a group−subgroup relationship of the space
groups.22 I41/acd (8, 15; structure type I) has a maximal
isomorphic subgroup with the lowest index of three I41/acd
with c′ = 3c (1; structure type II), resulting in a three times
larger unit cell (Table 2). Since Iba2 (2−7, 9−14; structure
type III) is a nonisomorphic subgroup of Ibca, I4 cd, and I4c2,
̅
1
and I41/acd again is a nonisomorphic supergroup of these space
groups, the structural relationship is confirmed (Figure S3).22
The crystallographic c axis of 1, 8, and 15 corresponds to the
crystallographic a axis of 2−7 and 9−14. According to the
group−subgroup relationship, the contents of the asymmetric
unit increase from half a formula unit [Ag2(L)2] in 8 and 15
(structure type I) to one and a half units in 1 (structure type II)
and two units in 2−5 and 9−14 (structure type III),
respectively. Representatively for the isostructural MOFs, the
crystal structures23 of compounds 1, 4, and 8 are discussed in
the following.
The silver atoms in the crystal structures of 1, 4, and 8 are
monodentately coordinated by two triazole groups and one
carboxylate group (Figure 1). Thus, the coordination of each
silver atom results in a trigonal planar N2O1 coordination
(bond lengths and bond angles for 1, 4, and 8 are given in
Table 3). The out-of-plane distances of the silver atoms are in
the range 1.0(5)−29.2(4) pm for 1, 4.9(1)−15.4(1) pm for 4,
and 8.1(1) pm for 8 caused by weak coordinative interactions
of the second carboxylate oxygen atoms (e.g., distance range for
4: 268.4(3)−281.6(4) pm). In each case two silver atoms are
bridged by two triazole groups to dinuclear Ag2N4 rings as
X-RAY CRYSTALLOGRAPHY
■
The coordination polymers 1−15 were obtained by solvother-
mal synthesis from either Ag2SO4 or AgNO3 and H(L1)−
H(L15) in H2O/MeCN (1:1, v/v). The detailed syntheses are
available in the Supporting Information. The crystal structures
of 1−5 and 8−15 were determined by X‑ray diffraction. The
crystals of compounds 1, 2, 8, and 14 exhibited only a low
scattering power using a laboratory X-ray source (Mo Kα, λ =
71.073 pm). Therefore, the crystal structure analyses were
performed using synchrotron radiation (λ = 82.657 pm) at the
beamline 14.2 of the synchrotron source BESSY-II.20,21
The differences in the linker substituents lead to different
crystal symmetry. 1, 8, and 15 crystallize in the centrosym-
metric tetragonal space group I41/acd (no. 142), whereas 2−7
and 9−14 crystallize in the noncentrosymmetric orthorhombic
space group Iba2 (no. 45). Nevertheless, the crystal structures
are closely related to each other, and the structural relationships
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Figure 1. Fragments of the crystal structures of 1, 4, and 8 (50% ellipsoids). I, II, and III label the structure types.
a
Table 3. Selected Bond Lengths and Angles of 1, 4, and 8
1
4
8
bond length [pm]
bond length [pm]
bond length [pm]
Ag1−N6a
Ag1−N10
Ag1−O1
Ag2−N9
Ag2−N5a
Ag2−O4
Ag3−N2c
Ag3−N1
Ag3−O7b
222.6(7)
225.7(6)
230.4(7)
225.6(5)
219.1(5)
221.8(6)
220.4(4)
225.2(4)
231.7(5)
Ag1−N14
Ag1−N10
Ag1−O1
Ag2−N9
Ag2−N13
Ag2−O4
Ag3−N6a
Ag3−N1
Ag3−O10c
Ag4−N2
Ag4−N5a
Ag4−O7b
222.0(4)
225.3(4)
Ag1−N2b
Ag1−N1c
Ag1−O1
219.1(3)
226.7(3)
228.3 (3)
234.2(4)
220.6(3)
225.2(3)
232.5(3)
221.6(4)
225.0(3)
231.9(4)
220.0(3)
224.7(4)
235.7(3)
bond angle [deg]
bond angle [deg]
bond angle [deg]
N10−Ag1−N6a
O1−Ag1−N6a
O1−Ag1−N10
N9−Ag2−N5a
O4−Ag2−N9
O4−Ag2−N5a
N1−Ag3−N2c
O7b−Ag3−N2c
O7b−Ag3−N1
125.6(2)
134.3(3)
100.1(2)
123.7(2)
128.9(2)
104.9(2)
119.5(1)
132.7(1)
107.8(2)
N10−Ag1−N14
O1−Ag1−N14
O1−Ag1−N10
N9−Ag2−N13
O4−Ag2−N9
O4−Ag2−N13
N1−Ag3−N6a
O10c−Ag3−N6a
O10c−Ag3−N1
N2−Ag4−N5a
O7b−Ag4−N2
O7b−Ag4−N5a
126.1(1)
132.9(1)
100.8(1)
123.4(1)
130.6(1)
104.6(1)
124.1(1)
128.9(1)
106.4(1)
123.1(1)
136.2(1)
100.4(1)
N1c−Ag1−N2b
O1−Ag1−N2b
O1−Ag1−N1c
125.7(1)
131.9(1)
102.0(1)
a
Symmetry codes for 1: a: 0.25+y, 1.75−x, 1.25−z; b: 1−x, 1.5−y, z; c: 1−x, 2−y, 1−z. Symmetry codes for 4: a: 0.5+x, −0.5+y, −0.5+z; b: 2−x, y,
−0.5+z; c: 1.5−x, −0.5+y, z. Symmetry codes for 8: a: 1.5−x, 1.5−y, 1.5−z; b: 0.25+y, 1.25−x, −0.25+z; c: 1.25−y, 0.25+x, 1.75−z.
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structural motifs. The distances between the silver atoms in the
Ag2N4 ring units are 341.0(6)−362.8(1) pm in 1, 344.6(1)−
348.4(1) pm in 4, and 342.3(1) pm in 8. Whereas the dinuclear
triazole-bridged silver units represent four-connected nodal
points, the ligands operate as two-connected bridges, resulting
in lvt topology24 with the point symbol {42.84}. The resulting
four- and eight-membered rings are responsible for the
formation of two different cavities (Figure 2). Viewing along
windows are 300 and 250 × 200 pm (viewed along the
a‑direction). The networks possess a 2-fold interpenetration of
type IIa with 2-fold rotation axes as full interpenetration
symmetry elements (FISE for 1, 8: 2[1,0,0], 2[0,1,0],
2[1,−1,0]; FISE for 4: 2[0,0,1]).17,24 The remaining solvent-
accessible pore volume25 of the three-dimensional networks
amounts to 36% in 1, 34% in 4, and 32% in 8. As shown in
Figure 3, the substituents on the triazole and benzoyl rings are
oriented toward the cavities. Comparing 1, 4, and 8 it is
obvious that the introduction of methyl groups to the benzoyl
ring (L4−) and the sterically demanding ethyl group on the
triazole ring (L8−) reduces the network porosity. The pore size
distributions are discussed in detail below.
PORE SIZE DISTRIBUTION
■
Figure 4 shows the calculated pore size distributions (PSD) of
1−5 and 8−15 based on their crystal structure data.26,27 In
general, the PSDs show significant differences in their
appearances, caused by the substituents of the 1,2,4-triazole
and benzoyl groups that are oriented toward the cavities
(Figure 3, Figure S6). This result is consistent with the IRMOF
concept,28 in which both pore size and shape can be influenced
by the choice of ligands with different sterical requirement. As
expected, compound 1, bearing ligand L1− with the lowest
sterical demand, has the largest pore diameter of up to 670 pm.
But the diameters of the most frequent pores of 1 are around
580 and 520 pm. By introduction of an ethyl group on the
triazole ring, the diameter of the most frequent pores is reduced
by 50 pm (520 pm in 8). With a second ethyl substituent on
the triazole ring this pore diameter decreases again to 470 pm
(15).
Figure 2. Left: Representation of the unit cell of 8 with lvt topology,24
point symbol {42.84} (H atoms are omitted). Ag2N4 units are
emphasized in red, black lines represent the four-membered rings, and
yellow lines represent an eight-membered ring. Right: Perspective view
of simplified networks of 8 with two interpenetrated nets (red and
blue) with view along [−105]. The four-membered rings create the
pore labeled A, whereas the eight-membered rings create pore B.
the crystallographic c-direction two pore windows with
diameters of 400 and 300 pm are visible in 1 as presented in
Figure 3. The diameters of both pore windows in 8 are 300 pm,
whereas in the crystal structure of 4 the equivalent pore
Introduction of a substituent in meta position of the benzoyl
group leads to a shift of the diameter of the most frequent
Figure 3. Space-filling diagrams of 1 (top), 4 (bottom left), and 8 (bottom right). Two interpenetrated nets (top center) and the position of the
substituents (yellow, green) are emphasized.
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Figure 5. X-ray powder diffraction patterns (λ(Cu Kα1) =
154.060 pm) of 1 obtained by solvothermal synthesis and after
postsynthetic treatments in comparison to the simulated X-ray powder
diffraction pattern based on single-crystal data.
in three different structure types. Furthermore, all presented
MOFs have the same topology (lvt, point symbol {42.84}) in
which the dinuclear triazole-bridged silver units are connected
by the substituted 3-(1,2,4-triazol-4-yl)-5-benzamidobenzoate
ligands. The differences in the unit cells, structures, and
symmetry lead to differences in the positions of the reflections
(Figure 6). In the case of the tetragonal structures 1, 8, and 15
(structure types I and II) the single 206 or 202 reflections
correspond to reflections 202 and 220 in the orthorhombic
compounds 2−7 and 9−14 (structure type III), caused by
different lengths of the lattice constants a and b. This
correlation is shown in detail in Figure S11.
It is to be noted that the single crystals of compounds 1−15
are breaking apart while drying in air. However, the PXRD
measurements confirm that the dried powder sample is still
crystalline. It is likely that the loss of solvent at room
temperature induces a phase transition (Figure S8−S10).
Figure 4. Pore size distribution26,27 and porosity25 of 1−5 and 8−15
calculated from single-crystal structure data.
pores to 460 and 590 pm for 2 and to 480 and 580 pm for 5. A
second substituent on the benzoyl group shifts the diameters of
these cavities to about 400 and 590 pm (4). In contrast to 2, 4,
and 5, which still show pore sizes up to 590 pm, 3, with a
methyl group in para position of the benzoyl group, has the
most abundant and largest cavities in the range of 450 and
530 pm. In this case L3− seems to realize a complete blocking
of pore sizes between 540 and 580 pm. However, in the case of
8−14 different positions of the ethyl groups at the triazole ring
can lead to differences in the orientations into the pores. This
results in two limiting forms of possible PSDs (Figures S4 and
S5). In general the frameworks 8−15 with the ethyl-substituted
triazole ligands show their maxima at smaller pore diameters
between 450 and 550 pm.
THERMAL STABILITY
■
In Figure 7 the temperature-dependent X-ray powder
diffraction pattern (TD-PXRD) of 1 is shown. The compound
undergoes a phase transition at 120 °C, which is associated with
the elimination of water (m/z = 18), confirmed in the TG-
DTA-MS analysis of 1 (Figure 8). The resulting phase
decomposes at about 250 °C. Above the decomposition
temperature the formation of elemental silver is detected. A
first TG-DTA-MS measurement of 1 was carried out with a
Soxhlet-extracted (MeOH), dry sample after vacuum activation.
This sample does not show a mass loss up to the
decomposition temperature. A second air-dried sample was
measured without any activation. Up to 120 °C a mass loss of
5.5% was found, which correlates with the calculated weight
loss for about three water molecules (5.7%). The decom-
position of 1 starting at 250 °C is endothermic and takes place
under release of carbon dioxide (m/z = 44).
X-RAY POWDER DIFFRACTION
■
Phase purity is confirmed by comparison of the X-ray powder
diffraction patterns with the simulated powder patterns based
on single-crystal data (Figure 5, Figures S8−S10). As a
representative example for the isostructural MOFs, 1 is
discussed in detail. Both the positions and the intensities of
the reflections of the air-dried product agree with those of the
simulated patterns. After Soxhlet extraction with methanol over
7 days and air drying, 1 has the same structure as the material
obtained by solvothermal synthesis. However, after vacuum
activation and after CO2 and N2 sorption experiments, a
completely different powder diffraction pattern was observed
for 1 at room temperature, indicating a phase transition. After
resolvation with methanol, the original powder diffraction
pattern of 1 is restored. The framework connectivity is
maintained, but its crystal structure changes due to the external
stimulus.29
SORPTION STUDIES
■
While single-crystal structures of compounds 1−5 and 8−15
could be determined from single-crystal diffraction data, 6 and
7 could be identified by X-ray powder diffraction. The similarity
of the PXRD data shown in Figure 6 confirms the presence of
the isostructural framework series. The compounds crystallize
Sorption measurements of compounds 1, 2, 4, 5, 8, 9, 12, 13,
and 15 were performed with nitrogen at 77 K and carbon
dioxide at 298 K. The obtained isotherms are illustrated in
Figure 9 (CO2) and Figure 10 (N2). These isotherms do not fit
to the recommended IUPAC classification30 because of a
E
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Figure 7. Temperature-dependent X-ray powder diffraction patterns
(λ(Cu Kα1) = 154.060 pm) (Guinier−Simon diagram) of 1.
In this study substituents constrict the pore apertures. As a
consequence, the pore diameter can be tuned. We expect that
the linkers L1−−L15− present in 1−15 have enough degrees of
freedom for reversible rotation and bending, which is important
for the expansion or shrinkage of the networks. On the basis of
this flexible nature of L1−−L15− a reversible framework
transformation is observed. We assume that the 2-fold network
interpenetration in 1−15 further supports structural framework
flexibility, leading to hysteresis of the sorption isotherm for
CO2 and N2 for 1−15. Interpenetrated networks have been
shown to form open and closed phases by interdigitation using
a glide movement.34 Thus, the various substituents on the
triazole and benzoyl ring with their orientation toward the
cavities (Figures S6, S7) and their different sterical demand are
able to influence this network movement and can affect both
the gate-opening pressure and the shape of the hysteresis loop
(Figures 9 and 10).
It becomes obvious that substitution with ethyl groups on the
triazole ring does not affect the gate-opening pressure for
carbon dioxide sorption too much (Figure 9a); for 1, 8, and 15
the gate-opening pressure is in a very close range between 0.43
and 0.64 MPa, although the materials show different sorption
capacities for the first sorption step before gate-opening. The
structural change depends on the external pressure for these
materials. A small additional step is observed for 8 with a
hysteresis loop at p = 0.02−0.2 MPa. On the other hand, by
introduction of methyl or methoxy groups on the benzoyl ring
the gate-opening pressure decreases (Figure 9b,c) for carbon
dioxide sorption at 298 K from 0.64 MPa for 1 without any
methyl group, to 0.16 MPa for 2 with one and 0.23 MPa for 4
with two methyl groups on the benzoyl ring (Figure 9b). This
might be a consequence of stronger interactions of CO2 and
the contracted pore walls of the evacuated material. If the
methyl group is replaced by a methoxy group, the gate opening
is slightly shifted to higher pressures, from 0.16 MPa for 2 to
0.3 MPa for 5 (Figure 9d). Again, the introduction of an ethyl
group on the triazole ring does not influence the gate-opening
pressure.
Figure 6. X-ray powder diffraction patterns (λ(Cu Kα1) =
154.060 pm) of 1−15 in contact with H2O/MeCN (1:1, v/v).
reproducible (Figure S14−S22) stepwise pore filling and a
sigmoidal curvature in logarithmic scale. In particular, low
carbon dioxide loadings (<2 mmol g−1) in the low-pressure
region (<0.64 MPa) are followed by a sudden steep increase at
a specific pressure, known as the gate-opening pressure.31−34 In
addition, desorption branches confirm this sorption behavior,
which is typical for flexible MOF materials.11,29 For the
investigated network compounds different gate-opening
pressures are observed depending on the substitution pattern
of the ligand. Fischer et al. also observed a relation between
substitution of the linker and gate-opening pressure.32,33
Sorption studies on functionalized [Zn2(bdc)2(dabco)]n
materials, which offer flexibility by introduction of alkyl ether
groups on the ligand, show that these groups act as molecular
gates32 for guest molecules and thus control the sorption
behavior.
Moreover, substitution influences the pore volume in
sorption measurements. In particular, by substitution of the
triazole ring the pore volume decreases from 0.33 cm3 g−1 for 1
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Figure 8. TG-DTA-MS analysis of 1 with and without previous activation under vacuum. The black traces represent the thermogravimetry (TG)
signals, while the green lines represent the differential thermal analysis (DTA), and the blue and red traces show the mass spectrometry (MS) signals.
Figure 9. Carbon dioxide sorption isotherms (298 K, closed symbols adsorption, open symbols desorption) for compounds 1, 2, 4, 5, 8, 9, 12, and
15. Influence of different substituents at the triazole group (a), at the benzoyl group (b), and variation of methyl and ethyl substitution on triazole
and different meta substitutions at the benzoyl ring (c, d).
to 0.28 cm3 g−1 for 8 with one and 0.22 cm3 g−1 for 15 with two
ethyl groups (Figure 9a). The same effect is found if the
benzoyl ring is methylated (Figure 9c, 2 and 9). Similarly, the
pore volume decreases with introduction of methyl or methoxy
groups on the benzoyl ring.
which a steep increase is observed. In contrast to CO2 (298 K),
the N2 sorption isotherms reveal large hysteresis loops in the
low-pressure region, typical for structurally flexible MOFs. In
case of 2, 4, 5, and 8 an additional step can be observed at low
relative pressures (p/p0 > 0.0005 for 4, p/p0 > 0.07 for 8, p/p0 >
0.4 for 2 and 5).
The nitrogen sorption isotherms (77 K) also show a stepwise
pore filling with low loadings up to a certain pressure, after
G
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Figure 10. Nitrogen sorption isotherms (77 K, p0 = 0.0972 MPa, closed symbols adsorption, open symbols desorption) of compounds 1, 2, 4, 5, 8, 9,
12, and 15. Representation of sorption isotherms of compounds with methyl- (a) and ethyl-substituted triazoles (b).
Again, the sorption isotherm curvature is influenced by
substitution of the triazole and benzoyl ring of the linker. The
introduction of one (8) or two ethyl groups (15) on the
triazole ring causes a decrease in pore volume, particularly for
15 with almost no nitrogen adsorbed. The ethyl groups with
higher sterical demand lead to smaller pore diameters.
However, 15 is a porous material shown by carbon dioxide
sorption, but nitrogen sorption at 77 K is kinetically hindered.
This observation is consistent with the calculated PSD shown
in Figure 4 with pore diameters smaller than 500 pm. The
modification on the benzoyl ring affects the sorption isotherm
shape more drastically. In particular, by introduction of
methoxy or methyl groups in meta position the sorption
isotherms offer a multistepwise pore filling: one methyl (2) or
methoxy group (5) in meta position presents an additional gate
opening (p/p0 > 0.4). Similarly, for 4 with two methyl groups
on the benzoyl ring, an additional gate-opening step appears at
low relative pressure (0.0005 ≤ p/p0 ≤ 0.002). Furthermore,
the methyl and methoxy groups reduce the pore volume (p/p0
= 0.3). But, in contrast, at higher pressures (p/p0 > 0.4)
additional transitions lead to higher pore volume compared to
unsubstituted materials.
Obviously the sterical demand of a substituent in meta
position of the benzoyl group causes a further framework
transformation, and the structure changes into an energetically
more favored form. However, this structural transformation
seems to be hindered by an additional sterically demanding
ethyl group on the triazole ring or another substituent on the
benzoyl group. Thus, compounds 2 and 5 show a second
hysteresis loop at higher pressures caused by the meta
substitution.
selected materials are determined by nitrogen adsorption
measurements at 77 K, resulting in a maximum pore volume
of up to 0.38 cm3 g−1 (49%) for 5. The adsorption studies with
nitrogen and carbon dioxide demonstrate the high flexibility of
the new materials, and the sorption experiments result in
isotherms with one or two hysteresis loops. The variations of
the sterical demand of the substituent of the triazole and the
benzoyl group adjust the pore volume to 0.18−0.33 cm3 g−1 for
carbon dioxide and 0.20−0.38 cm3 g−1 for nitrogen. 1−15 with
their 2-fold network interpenetration and structural flexibility
form a series of isostructural MOFs suitable for systematic
studies of the influence of linker substitution on network
porosity and gate-opening behavior.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details of ligand and MOF syntheses, detailed
characterization by X-ray diffraction, thermal analyses, calcu-
lated pore size distribution, IR spectroscopy, adsorption
measurements, as well as scanning electron microscopy. This
material is available free of charge via the Internet at http://
pubs.acs.org. CCDC 995524−995528, 996077−996080, and
996084−996087 contain the supplementary crystallographic
data. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Krautscheid@rz.uni-leipzig.de.
■
Notes
CONCLUSION
The authors declare no competing financial interest.
■
The Ag-based MOFs 1−15 form an isostructural series with lvt
topology and a thermal stability up to 250 °C. 1−15, obtained
under solvothermal conditions, consist of two interpenetrated
three-dimensional networks formed from the 3-(1,2,4-triazol-4-
yl)-5-benzamidobenzoates and dinuclear Ag2N4 ring units as
secondary building units. Their pore diameters and pore
volumes can be adjusted by linker substituents, and the
calculated porosity of 1−15 is in the range between 20% and
36%. The calculated PSDs of the microporous MOFs show
some essential differences in their appearances, caused by the
substituents of the 1,2,4-triazole and benzoyl groups that are
oriented toward the cavities. The pore volumes of several
ACKNOWLEDGMENTS
The Helmholtz-Zentrum Berlin (BESSY-II) is acknowledged
for granting beam time and travel support, in particular, Dr. U.
■
Muller and his team for assistance during data collection.
̈
Diffraction data have been collected on BL14.2 operated by the
Joint Berlin MX-Laboratory at the BESSY II electron storage
ring (Berlin-Adlershof, Germany). We thank D. Hirsch and D.
Poppitz (Leibniz-Institut fur Oberflachenmodifizierung e.V.)
̈
̈
for recording SEM images. We gratefully acknowledge financial
support by Deutsche Forschungsgemeinschaft (DFG SPP 1362,
Porose metallorganische Gerustverbindungen, STA 428/17-2,
̈
̈
H
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