Paddle Wheel Based Triazolyl Isophthalate MOFs: Impact of Linker Modification on Crystal Structure and Gas Sorption Properties

Article abstract of DOI:10.1021/acs.inorgchem.5b02921

Syntheses and comprehensive characterization of two closely related series of isomorphous metal-organic frameworks (MOFs) based on triazolyl isophthalate linkers with the general formula _∞³[M₂(R¹-R²-trz-ia)₂] (M = Cu, Zn) are presented. Using solvothermal synthesis and synthesis of microcrystalline materials on the gram scale by refluxing a solution of the starting materials, 11 MOFs are readily available for a systematic investigation of structure-property relationships. The networks of the two series are assigned to rutile (rtl) (1-4) and α-PbO₂ (apo) (5-9) topology, respectively. Due to the orientation of the triazole substituents toward the cavities, both the pore volume and the pore diameter can be adjusted by choice of the alkyl substituents. Compounds 1-9 exhibit pronounced microporosity with calculated porosities of 31-53% and show thermal stability up to 390 °C as confirmed by simultaneous thermal analysis. Systematic investigation of adsorption properties by CO₂ (298 K) and N₂ (77 K) adsorption studies reveal remarkable network flexibility induced by alkyl substituents on the linker. Fine-tuning of the gate opening pressure and of the hysteresis shape is possible by adjusting the substitution pattern and by choice of the metal ion.

Full text of DOI:10.1021/acs.inorgchem.5b02921

Article
pubs.acs.org/IC
Paddle Wheel Based Triazolyl Isophthalate MOFs: Impact of Linker
Modification on Crystal Structure and Gas Sorption Properties
†
‡
†
†
†,‡
†
†
†
Merten Kobalz, Jo
̈
rg Lincke, Karolin Kobalz, Oliver Erhart, Jens Bergmann, Daniel Lassig,
̈
‡
§
,†
Marcus Lange, Jens Mo
̈
̈
llmer, Roger Glaser, Reiner Staudt, and Harald Krautscheid*
†
Fakultat
̈
fu
Institut fur Nichtklassische Chemie e.V., Permoserstraße 15, D-04318 Leipzig, Germany
Maschinenbau und Verfahrenstechnik, Hochschule Offenburg, Badstraße 24, D-77652 Offenburg, Germany
S Supporting Information
̈
̈
r Chemie und Mineralogie, Universitat Leipzig, Johannisallee 29, D-04103 Leipzig, Germany
‡
̈
§
Fakultat
̈
*
ABSTRACT: Syntheses and comprehensive characterization
of two closely related series of isomorphous metal−organic
frameworks (MOFs) based on triazolyl isophthalate linkers
3
1
2
with the general formula ∞ [M (R −R −trz−ia) ] (M = Cu,
2
2
Zn) are presented. Using solvothermal synthesis and synthesis
of microcrystalline materials on the gram scale by refluxing a
solution of the starting materials, 11 MOFs are readily
available for a systematic investigation of structure−property
relationships. The networks of the two series are assigned to
rutile (rtl) (1−4) and α-PbO (apo) (5−9) topology,
2
respectively. Due to the orientation of the triazole substituents
toward the cavities, both the pore volume and the pore diameter can be adjusted by choice of the alkyl substituents. Compounds
−9 exhibit pronounced microporosity with calculated porosities of 31−53% and show thermal stability up to 390 °C as
confirmed by simultaneous thermal analysis. Systematic investigation of adsorption properties by CO (298 K) and N (77 K)
1
2
2
adsorption studies reveal remarkable network flexibility induced by alkyl substituents on the linker. Fine-tuning of the gate
opening pressure and of the hysteresis shape is possible by adjusting the substitution pattern and by choice of the metal ion.
INTRODUCTION
coordination polymers are most promising to understand
structure−property relationships. Thereby, variation of a given
MOF system is possible either by linker functionalization or
■
Due to their structural diversity and tunable properties, porous
coordination polymers (PCPs) or metal−organic frameworks
MOFs) have been intensively studied in the past 15 years.
This unique class of highly porous solids contains promising
adsorbents for gas separation and storage
liquid phase separation. Further applications in sensor design,
1
,2
5
,14,15,17,19,21,25−27
(
metal substitution.
With knowledge of gate
opening mechanisms on the atomic level, the targeted
adjustment of adsorption properties, e.g., gate opening pressure
and hysteresis shape, will be possible.
3
4,5
6
as well as fo7r
8
9
heterogeneous catalysis, and as proton conductors illustrate
the variety of these materials with targeted functionalities. To
ensure accessibility of the pore system for guest molecules
removal of incorporated solvent molecules is essential.
Regarding their behavior during this activation procedure,
MOFs are classified in three generations according to Kitagawa
Herein, we present the syntheses, crystal structures, and
thermal and gas adsorption properties of two closely related
28,29
series of MOFs based on triazolyl isophthalate linkers
and
the well-known paddle wheel motif. Seven 5-(1,2,4-triazol-4-
1
2
2−
2
,10,11
yl)isophthalate ligands ((R −R −trz−ia) ) were used to
et al.
In recent years, more and more research has focused
3
1
synthesize 11 MOFs with the general formula [M (R −
R −trz−ia) ] (M = Cu, Zn). The ligands differ in the sterical
demand of the triazole substituents R and R , i.e., hydrogen,
methyl, ethyl, and n-propyl. This allows a comprehensive
investigation of the impact of gradual changes in the linker,
combined with the choice of metal ion, on gas adsorption
properties.
∞
2
on the investigation and possible applications of MOFs
belonging to the third generation.
12−17
2
This particular class
2
1
2
of porous materials, also called soft porous crystals, consists of
structurally flexible, porous coordination polymers possessing
the ability to reversibly respond to external stimuli like
1
8−21
22,23
24
pressure,
temperature,
or light. Their structural
flexibility causes stepwise adsorption and a characteristic
adsorption/desorption hysteresis, which is described as a gate
opening process or breathing, respectively. In order to
investigate the origin of structural transformations occurring
in such gate opening processes, isostructural series of flexible
Received: December 18, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02921
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Inorganic Chemistry
Article
SYNTHESIS AND X-RAY CRYSTALLOGRAPHY
focus on the systematic investigation of both paddle wheel
based triazolyl isophthalate MOF series, including crystal
structures and thermal and adsorption properties.
■
The crystal structures of 1, 2, and 4−9 were determined by
single crystal X-ray diffraction. Suitable crystals were obtained
starting from the appropriate metal salt and the protonated
ligands via solvothermal synthesis and via diffusion methods
Considering the unit cell parameters of 2 (mP) and 5 (oP),
the close structural relationship of both series becomes obvious.
Whereas the axes a and b of 5 correspond to b and c of 2, the c
axis of 5 is related to the doubled a axis of 2 (Table 2). This
structural similarity is illustrated by comparison of the unit cell
packings of 2 and 5, respectively (Figure 2 and Figure S1). Both
MOF series contain the well-known dinuclear paddle wheel
using H O/MeCN or DMF/EtOH (1:1, v/v) as solvents.
2
Cu
Furthermore, compounds 1, 2, 3, 4, and 8 are easily
accessible in gram scale by refluxing a solution of the starting
materials. Detailed MOF synthesis protocols and crystal
structure data are reported in the Supporting Information.
Compounds 1−4 and 5−9 are members of two closely related
series of isomorphous MOFs (Figure 1). While 1, 2, and 4
27,29,33−36
motif as a secondary building unit (SBU).
The
bridging coordination of the carboxylate groups results in a
square planar MO (M = Cu, Zn) environment for the metal
4
ion. Through coordination of N1 (triazole group) in the apical
position, the NO coordination sphere is completed (Figure 1).
4
27,35
This structural motif is known for M = Cu, Zn, Co, and Ni.
Within the two series, the M···M distance varies for M = Cu
from 267.9(1) pm to 274.49(4) pm (Table 3), which is in good
agreement with other paddle-wheel-based MOF struc-
30−33,36
tures.
Interestingly, disubstituted triazole groups cause
a small extension of the M1···M1a distance compared to linkers
with only one substituent on the triazole ring, which is probably
Zn
due to the enhanced electron density of N1. In the cases of 7
Zn
and 8 , the M1···M1a distance is significantly larger, i.e.,
3
09.38(3) pm and 307.83(5) pm, respectively. In addition, the
2
+
O−M1 and N1d−M1 bond lengths differ between the Zn
Figure 1. Fragments of the crystal structures of 4 (rtl-topology, left)
2+
Cu
and Cu paddle wheel units. For the latter, O−M1 distances in
the narrow range of 196.6(3)−200.1(2) pm are observed. Due
and 8 (apo-topology, right): coordination sphere of the paddle
wheel motif (50% ellipsoids). Symmetry codes, 4: (a) −x, 1 − y, 1 − z;
2+
(
8
1
b) x, 0.5 − y, 0.5 + z; (c) −x, 0.5 + y, 0.5 − z; (d) 1 − x, 1 − y, 1 − z.
to Jahn−Teller distortion of the Cu coordination sphere, the
N1d−Cu1 bond length is elongated (214.7(6)−217.3(3) pm).
In contrast, no stretching of the Zn1−N1d bond is observed for
Cu
: (a) 1 − x, y, 1.5 − z; (b) 0.5 − x, 0.5 + y, z; (c) 0.5 + x, 0.5 + y,
.5 − z; (d) 0.5 + x, 1.5 − y, 1 − z.
Zn
Zn
10
7
and 8 . As expected for a d metal ion, the coordinative
bonds to Zn1 are almost uniform in length (203.1(2)−
06.7(2) pm). As described earlier for similar paddle-wheel-
based MOFs, the elongated Zn1···Zn1a distance is partially
crystallize in the monoclinic space group P2 /c (no. 14, abbr. as
1
2
mP) with two formula units per unit cell, 5−9 crystallize in the
orthorhombic space group Pbcn (no. 60, abbr. as oP) with four
formula units per unit cell (Tables 1 and 2). Interestingly, the
37
compensated by the shortened Zn1−N1d bond. Note that in
all crystal structures of 1, 2, and 4−9, the triazole ring
coordinates exclusively via N1, which is the sterically less
hindered donor atom. In the cases of 2 and 5, the methyl group
is found to be disordered on both possible sides of the triazole
ring with minor probability near N1.
2
−
use of the monosubstituted ligands (H−Me−trz−ia) and
2
−
(
H−Et−trz−ia) leads to both, monoclinic and orthorhombic
paddle wheel based MOFs (2 and 5; 3 and 6, respectively). In
case of the disubstituted ligands, only the orthorhombic
structure type is observed.
In context of isoreticular platforms, the crystal structure of
While the paddle wheel units represent six-connected nodes,
the linker molecules act as three-connected nodes, due to their
coordination via both carboxylate groups and the triazole group
to different Cu ions. Through bridging of paddle wheel units
by triazolyl isophthalate ligands, a three-dimensional network is
built up. Topological analysis of the networks resulted in two
3
30
the monoclinic [Cu (trz−ia) ] (1) was described earlier. In
∞
2
2
addition, the crystal structures and gas adsorption properties of
3
31
2+
orthorhombic
[Cu (trz−ia) ]
2
and monoclinic
∞
2
2
3
∞
32
[
Cu (tetrazole−ia) ] were reported previously, which
implied the potential of this class of MOFs. In this work, we
2
Table 1. Overview 1−9: Metal Ion and Linker Substitution Pattern
B
DOI: 10.1021/acs.inorgchem.5b02921
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Table 2. Unit Cell Parameters of Paddle Wheel Based 3∞ [M (R −R −trz−ia) ] MOFs 1, 2, and 4−9
Article
1
2
2
2
1
2
4
5
6
7Cu
7Zn
8Cu
8Zn
Pbcn
9
space group
a/pm
P2 /c
P2 /c
P2 /c
Pbcn
Pbcn
Pbcn
Pbcn
Pbcn
Pbcn
1
1
1
1096.8(2)
1220.3(1)
1436.6(2)
110.30(1)
1803.3(5)
2
1085.9(1)
1087.8(2)
1257.9(2)
1411.4(2)
110.02(2)
1814.5(5)
2
1298.7(1)
1406.2(1)
2040.4(2)
90
1345.9(1)
1320.5(1)
2018.4(2)
90
1340.1(1)
1337.94(8)
2028.1(1)
90
1349.52(7)
1324.45(6)
2044.0(1)
90
1343.44(9)
1334.9(1)
2018.1(2)
90
1356.3(1)
1333.7(1)
2033.7(2)
90
1333.4(2)
1365.5(1)
2004.2(2)
90
b/pm
1278.6(2)
1402.6(2)
110.508(8)
1823.9(4)
2
c/pm
β/deg
6
3
V/10 pm
3726.3(5)
4
3587.3(5)
4
3636.3(4)
4
3653.4(4)
4
3619.0(5)
4
3678.9(6)
4
3649.1(6)
4
Z
Figure 2. Comparison of the unit cells of 2 (rtl topology, left, view in b direction) and 5 (apo topology, right, view in a direction). H atoms are
omitted for clarity.
Table 3. Selected Atom Distances and Angles of 1, 2, and 4−9
1
2
4
5
6
7Cu
7Zn
8Cu
8Zn
9
atom distance/pm
M1···M1a
M1−O1
M1−O2a
M1−O3b
M1−O4c
M1−N1d
269.58(9)
198.2(4)
197.9(4)
197.8(3)
197.0(3)
216.9(4)
268.98(8)
196.6(3)
197.1(3)
196.9(3)
197.7(3)
215.0(3)
267.9(1)
198.3(6)
198.5(6)
197.0(5)
197.6(5)
214.7(6)
270.84(4)
198.6(2)
199.0(2)
198.4(2)
199.1(2)
216.6(2)
270.07(4)
196.9(2)
274.49(4)
199.3(2)
197.2(2)
197.5(2)
200.1(2)
217.2(2)
309.38(3)
205.3(2)
203.1(2)
204.5(2)
205.1(2)
203.4(2)
273.51(3)
197.6(2)
196.8(2)
197.1(2)
198.7(2)
215.9(2)
307.83(5)
205.5(2)
204.6(2)
206.7(2)
205.3(2)
203.7(3)
272.03(5)
197.5(2)
197.9(2)
197.6(2)
198.3(2)
217.3(3)
197.2(2)
197.7(2)
198.1(2)
214.9(2)
bond angle/deg
99.94(9)
O1−M1−N1d
O3b−M1−N1d
O2a−M1−O3b
O3b−M1−O4c
92.5(1)
94.3(1)
90.2(1)
166.9(1)
94.1(1)
96.4(1)
90.6(1)
166.7(1)
92.8(2)
94.4(2)
90.9(2)
167.4(2)
96.64(8)
94.81(8)
166.89(7)
90.36(7)
103.00(8)
91.89(8)
169.13(8)
90.05(8)
107.13(6)
94.68(6)
161.08(6)
88.83(6)
103.01(6)
92.45(6)
169.44(6)
90.11(6)
108.04(9)
94.13(9)
163.30(9)
88.70(9)
103.6(1)
91.4(1)
167.5(1)
90.8(1)
93.19(9)
167.72(9)
90.39(9)
closely related network types. Whereas 1, 2, and 4 possess rtl
interconnected three-dimensional pore system for both
isomorphous MOF series (Figure 4). Even in the structure of
2
2 10 3
(
rutile) topology with point symbol {4.6 } {4 .6 .8 }, the
2
1
2
networks of 5−9 are of apo (α-PbO ) topology with point
9 (R = R = Et), very narrow nonlinear pore channels are
visible. For 1−4 and 5−9, the void fractions are calculated to be
2
2
2
9 4
symbol {4.6 } {4 .6 .8 }. Thus, the networks of the two
2
3
8
isomorphic series differ only slightly in the number of six-
and eight-membered circuits, which illustrates their close
relationship.
in the range of 53−35% and 50−31%, respectively (Table 4).
For a detailed analysis of the pore system, pore size
distributions (PSDs) were calculated on the basis of the crystal
40
For both framework types, narrow pore channels are visible
along the a (1, 2, 4) or c direction (5−9), accordingly. The
pore surface is built up by aromatic rings of the linkers, whereas
structures (Figure 5). In case of 3, the PSD was calculated on
the basis of the crystal structure of 4 (replacing n-propyl with
ethyl group) in combination with lattice constants as
determined from the X-ray powder diffraction pattern of 3.
The pore diameters in 1−9 vary in the range of 0.25 nm up to
0.60 nm, which illustrates their pronounced microporosity.
Interestingly, compound 1 does not show the largest pore
2
the R substituents are arranged inside the pore windows
(Figure 3). In this fashion, the choice of the substituent
influences the solvent accessible volume of the framework. A
detailed view inside the crystal structures reveals an
C
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Inorganic Chemistry
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Figure 3. Representation of the 3D networks of 1 (top left), 4 (top right), 5 (bottom left), and 9 (bottom right). The alignment of the substituents
2
1
is emphasized (green, R ; yellow, R ).
Table 4. Calculated Pore Volumes for CO Adsorption at
2
45
2
98 K According to Gurvich Equation (p = 6.4121 MPa)
0
calculated from X-ray structure
data
after first pore
filling
after second
pore filling
VPore3
/
VPore3
/
VPore/
3
ρ/g
cm
cm
cm
−1
−
3
−1
−1
porosity/%
cm
g
p/p0
g
p/p0
g
1
2
52.6
1.086
1.124
0.48
0.43
0.796
0.015
0.49
0.01
0.29
0.05
0.03
0.26
0.22
0.22
0.15
0.10
47.1/50.4
0.664
0.40
0
.309
a
a
a
3
4
6
7
8
8
9
39.9
1.197
1.233
1.195
1.179
1.236
1.223
1.277
0.33
0.28
0.32
0.35
0.29
0.30
0.24
0.118
0.045
0.377
0.341
0.241
0.054
0.115
0.867
0.869
0.882
0.798
0.844
0.931
0.793
0.34
0.29
0.34
0.28
0.28
0.32
0.26
35.0
38.5
40.7
35.9
36.1
30.8
Figure 4. Visualization of the 3D interconnected pore system (gray, 2
39
×
2 × 2 super cell) of 1 (left) and 9 (right).
Cu
Cu
Zn
diameter of the monoclinic MOF series although the network is
1
2
built up by the ligand with the smallest substituent (R = R =
H). Instead, the network of 2 containing the methyl substituted
linker shows the largest pore size of 0.49 nm. In addition, in the
cases of 1 and 2, a smaller aperture is present at 0.33 and 0.32
nm, respectively. Replacement of the methyl by an ethyl group
leads to a unimodal PSD of 3 with a maximum frequency at
a
Calculated based on unit cell dimensions determined by PXRD.
multimodal, and their shape is influenced by the choice of the
linker and, to a smaller extent, by the metal ions used. The
networks of 7 and 7 , containing the disubstituted ligand
Cu
Zn
0.34 nm. Hence, the bulkier ethyl group induces a blocking of
2
−
the larger pore. By further elongation of the alkyl chain (n-
propyl, 4) the pore diameter is reduced to 0.32 nm, which is in
good agreement with the orientation of the substituent toward
the cavities (Figure 3). In contrast to 1−4, PSDs of 5−9 are
(Me −trz−ia) , form the largest pores of the orthorhombic
2
MOF series with maximum pore diameters of 0.56 and 0.57
nm, respectively. It can be concluded that an increase of the
sterical demand of one triazole substituent results in reduction
D
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indexing the powder diffraction pattern (Table S3). Indexing of
the as-synthesized PXRD of 4, which shows minor deviations in
comparison to the simulated one, including broadening of
individual reflections, revealed a monoclinic unit cell similar to
the single crystal data (Figure S4 and Table S4). The
broadening of the 111 reflection at 2θ = 11.5° (1 and 4) was
̅
30
observed earlier for 1 and for an isostructural MOF based on
32
a tetrazolyl isophthalate ligand. Interestingly, the PXRD
pattern of as-synthesized 6 shows deviations from the simulated
one. An explanation for this deviation could be a slightly
smaller orthorhombic unit cell identified by indexing of the
experimental pattern (Figure S5 and Table S5). As discussed
below, the thermal behavior of 6 fits perfectly to the
orthorhombic MOF series (Figure S10). The compilation of
the as-synthesized powder patterns of 1−4 and 6−9 is shown in
Figure S7 and illustrates the structural similarity within the two
isomorphous series.
In order to achieve a complete solvent exchange prior to
thermal stability and gas adsorption studies, Soxhlet extraction
with methanol was carried out over 7 days. After this
postsynthetic treatment, the PXRD patterns remain unchanged
Cu
except in case of 7 , indicating a response of this framework to
Figure 5. Pore size distribution of 1−9 calculated from single-crystal
structure data (3: structure model based on 4; solid and dashed lines
correspond to the two limiting cases due to disorder of the substituent
solvent exchange. However, after adsorption experiments with
N and CO including activation procedures and multiple
2
2
(
2, 5)).
adsorption/desorption cycles, the powder diffraction patterns
change as a response to the sorption stress. After resolvation
with methanol, the pristine PXRD patterns and, thus, the
of the maximum pore diameter by 50 pm (8Cu and 8 ). This
observation is consistent with the concept of isoreticular MOFs
Zn
Cu Zn
crystal structures are restored in the cases of 2, 3, 7 , 8 , and
9, indicating unchanged network connectivities during
activation and adsorption/desorption processes. For 1, 4, 6,
(
IR-MOFs), indicating that pore size, shape, and functionality
41
Cu
can be tuned systematically. All in all, the PSDs show cavities
with varying diameters that can be fine-tuned via linker
substitution. Thus, the two series are promising candidates for
the systematic investigation of linker influence on gas
adsorption properties.
and 8 , the PXRD patterns (Figures S2−S6) after adsorption
experiments and resolvation do not fit to the pattern of the
Soxhlet extracted sample. However, as discussed below, the
adsorption/desorption isotherms are fully reproducible.
Prior to adsorption experiments the thermal stability of
Soxhlet extracted samples was investigated by simultaneous
thermal analysis (TG-DTA-MS) and temperature dependent
X-ray diffraction (TD-PXRD). In contrast to all other analyzed
samples, the TD-PXRD pattern of 1 shows no phase transition,
although solvent molecules are evaporated from the pore
system (Figures S8 and S11). Interestingly, even after Soxhlet
extraction over 7 days with methanol, a great portion of water
remains in the pores. After desolvation, the framework collapses
X-RAY POWDER DIFFRACTION AND THERMAL
STABILITY
■
The comparison of X-ray powder diffraction (PXRD) patterns
of 1−4 and 6−9 with simulated powder patterns based on
single crystal data confirm phase purity of all samples used for
adsorption studies (Figures S2−S6). Exemplarily for the
monoclinic and orthorhombic MOF series, the PXRD patterns
of 2 and 9 are shown in Figure 6. In general, the experimental
powder patterns of the as-synthesized samples are in good
agreement with the simulated ones. As mentioned earlier, 3 is
not accessible as single crystals but could be identified by
at 250 °C associated with the release of CO from the organic
2
linker. This observation is in good agreement with the TD-
PXRD pattern showing no reflections above this temperature.
For all other compounds, a phase transition is observed due to
Figure 6. X-ray powder diffraction patterns (λ(Cu−K ) = 154.060 pm) of 2 (left) and 9 (right) obtained by several synthetic methods and the
α1
powder patterns after postsynthetic treatments.
E
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solvent evaporation. The peculiarity of 1 showing no phase
transition indicates that the absence of triazole substituents is
related to the observed rigidity, and, on the other hand,
introduction of alkyl substituents in the linker allows the
framework to respond to solvent evaporation with structural
changes. This key impact of linker side chains on the thermal
behavior was also discussed for a series of MOFs based on
temperature phases show reflections up to 250 °C, in the case
of 8 even up to 390 °C. The thermal decomposition
Zn
accompanied by release of CO starts at 250 °C (9) to 280 °C
2
Cu
Zn
(4, 6, 7 ) and in the case of 8 at 390 °C.
ADSORPTION STUDIES
■
22
In order to evaluate their adsorption characteristics, high
pressure CO (298 K) and low pressure N (77 K) adsorption
alkoxy functionalized bdc-linkers. It also appears to be related
to the structural flexibility observed during CO adsorption
2
2
2
Cu
Cu
Zn
measurements with 1, 2, 3, 4, 6, 7 , 8 , 8 , and 9 were
experiments (see below).
Exemplarily for the monoclinic and orthorhombic MOF
series, the TD-PXRD patterns of 2 and 9 are shown in Figure 7.
carried out. Although these frameworks exhibit calculated pore
3
−1
38
volumes of up to 0.48 cm g (PLATON), N is not
2
Cu Zn
Cu
adsorbed by 2, 8 , 8 , and 9 at 77 K. For 1, 3, 4, 6, and 7 ,
only a small uptake of N is observed (Figures S16−S18). In
2
contrast, CO (298 K) is adsorbed in all investigated MOF
2
materials. Whereas 1 shows a type I isotherm with a saturation
loading of 9 mmol g− (Figure S16), the CO isotherms of 2−9
1
2
do not correspond to any type of the recommended IUPAC
42
classification of physisorption isotherms. Instead, S-shaped
isotherms originating from a stepwise pore filling are observed
in combination with broad adsorption/desorption hystereses
(
Figures S16−S18). This type of isotherm is characteristic of
flexible MOFs showing a steep increase in the adsorption
branch at the so-called gate opening pressure.
1
1,20,21,26
This
flexibility is based on flexible linkers and/or deformation of
12
secondary building units (SBUs). In particular, paddle-wheel-
based MOFs are well-known for their high degree of structural
flexibility.
Figure 7. Temperature dependent PXRD patterns (Guinier-Simon
diagrams) of 2 (left) and 9 (right) (λ(Cu−K ) = 154.060 pm).
α1
5
,14−16,27,37,43
As discussed before, the PXRD patterns of several members
of the two series after adsorption experiments and resolvation
with methanol do not fit to the patterns of the Soxhlet extracted
samples. However, the shape of all isotherms is reproducible,
demonstrating the reversibility of structural changes occurring
during the gate opening process and the robustness of the
adsorbents against sorption stress. Again, the pore diameters
can be fine-tuned by choice of the ligand. Hence, the pore
volume is dependent on the sterical demand of the substituent
(Table 4). For the monoclinic MOF series, this relationship is
Whereas 9 shows a distinct change in reflection positions at 110
°
C, for 2 a gradual structural change is observed up to 130 °C
followed by two phase transitions. With the exception of 4 and
, the simultaneous thermal analysis of all samples reveals the
endothermic loss of methanol and small amounts of water up to
80 °C, which indicates that in the cases of 4 and 9 all solvent is
9
1
already removed during sample preparation and premeasure-
ment evacuation in the instrument (Figures S11−S15). As
shown in Figure S10, the high temperature phases of 2 and 3
are very similar, whereas the pattern of 4 is different. During the
phase transition, the monoclinic symmetry is preserved
associated with a cell volume contraction by 36% (Table S6).
Contraction of the unit cell volume during activation is
probably the reason for the decay of single crystals as observed
in SEM images of 2 (Figure S23). The high temperature phases
proven experimentally, as the CO saturation loading decreases
2
with elongated alkyl chain (Figure 8). The experimentally
determined pore volumes of 1−4 are in good agreement with
the calculated ones based on single crystal data. Thus, CO is
2
able to access the entire pore system (Table 4). It becomes
obvious that the substituent not only influences the accessible
pore volume but also affects the gate opening pressure. For 2, a
steep increase is observed at 0.3 MPa followed by a plateau
of the orthorhombic MOF series are in accordance except for
Zn
8
. Nevertheless, the volumes of the orthorhombic unit cells
determined at 200 °C are reduced by 30%. These high
Figure 8. CO adsorption isotherms (298 K; closed symbols, adsorption; open symbols, desorption; lines are to guide the eyes) for 1−4 (left) and
2
6−9 (right) in semilogarithmic plot.
F
DOI: 10.1021/acs.inorgchem.5b02921
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 9. CO adsorption isotherms (298 K; closed symbols, adsorption; open symbols, desorption; lines are to guide the eyes) for 3 and 6 (left)
2
Cu
Zn
and 8 and 8 (right) in semilogarithmic plot.
indicating a complete filling of the accessible pore system (p/p
9 already adsorb some amount of CO , 2−4 are nearly
0
2
3
−1
=
0.309; 0.29 cm g ). At 2.0 MPa another gate opening
nonporous before gate opening. This crucial difference is
illustrated by comparison of the isotherms of 3 and 6 (Figure
9). Both materials contain the ethyl substituted linker and differ
only in the type of network (rtl and apo, respectively). Both
show the same saturation loading, and their desorption
branches are identical. Though, the adsorption branches differ
considerably. Therefore, the gate opening process is influenced
by the topology of the framework.
occurs, generating new sorption sites. The saturation loading of
1
3
−1
7
mmol g− corresponds to a pore volume of 0.40 cm g ,
which is in quite good agreement with the theoretical value of
3
−1
0
.43 cm g . Up to 0.2 MPa, almost no CO is adsorbed (<0.5
2
−
1
mmol g ), indicating that after evacuation a nearly nonporous
phase is present. By further increase of CO pressure, the
2
framework is opened to form an intermediate porous phase
Another possibility to manipulate the gate opening behavior
(first pore filling). The second gate opening is interpreted as
Cu
Zn
is given by choice of the metal ion. Although 8 and 8 are
isomorphous with the same geometric properties and differ
another structural transformation resulting in the completely
opened framework (second pore filling). This gradual process is
reversible as the desorption shows a stepwise shape, too.
Substitution of the methyl by an ethyl group (3) leads to a one-
step gate opening at 1.0 MPa which is consistent with the
only in the sort of metal ion used, the CO adsorption
2
isotherms deviate from each other (Figure 9). In the low
pressure region, the adsorption branches show an identical
Zn
slope. At 0.25 MPa, the gate opening pressure of 8 is reached,
second step in the CO isotherm of 2. Due to the smaller pore
2
−1
followed by a steep increase resulting in the opened form. In
volume, the saturation loading is reached at 6 mmol g . Again,
starting from a nonporous framework, the structural trans-
Cu
contrast, for 8 , a two-step gate opening is observed with a
second steep increase at 1.5 MPa. Thus, a higher CO pressure
formation of 3 and 4 during CO adsorption generates the
2
2
Cu
Zn
is required to open the framework of 8 than for 8 . This
finding is consistent with DFT calculations of copper and zinc
paddle wheel systems, which predict a higher degree of
flexibility for the latter one because of orbital directing effects in
opened structure in a one-step fashion. Interestingly, the gate
opening pressure of 4 (0.4 MPa) is smaller than that of 3. An
explanation for this observation could be stronger interactions
of CO with the tighter pore system of 4 resulting from the
2
37
the case of copper. Interestingly, also the desorption branches
longer alkyl chain.
In contrast to the distinct gate opening in case of the
Cu
proceed in a different manner. Whereas 8 shows a broad
Zn
hysteresis down to 0.01 MPa, the desorption branch of 8
monoclinic MOF series, CO isotherms of 6−9 show a more
2
reaches the adsorption branch already at 0.1 MPa. This
observation illustrates that for flexible MOFs not only the type
of network or choice of linker but also the sort of metal ion has
a huge influence on the isotherm shape. Therefore, it is possible
to adjust these three parameters to design an adsorbent with
cumulative shape (Figure 8). For 6, a nearly continuous uptake
without a distinct plateau is observed up to 2.5 MPa. Further
dosing of CO causes a steep increase in uptake due to gate
2
3
−1
opening, resulting in a pore volume of 0.34 cm g . The
adsorption branch of 7 (R = R = Me) proceeds quite similar
Cu
1
2
specific CO adsorption characteristics (saturation loading, gate
2
to 6 with a gate opening at 2.1 MPa. However, CO is not able
2
opening pressure, hysteresis shape). Obviously, by introduction
of triazole substituents in the parent rigid framework 1,
structural flexibility is induced (2−9). This phenomenon
proves that the alkyl substituents play a key role for interaction
to generate the entire pore volume (Table 4). Instead, the
Cu
Cu
saturation loading of 7 is equal to 8 , although the latter
contains the bulkier ethyl substituent. As discussed above, the
Cu
PXRD pattern of 7 after Soxhlet extraction does not match
of the framework with CO molecules. As the substituents are
2
the as-synthesized pattern, indicating a structural change, which
could be responsible for the smaller experimental pore volume.
It becomes obvious that introduction of a second substituent
arranged toward the pores (Figure 3), they act as a gating group
and thus have a major impact on the gate opening pressure.
Similar structure−property relationships were found for other
(
7−9) causes a closer adsorption/desorption hysteresis loop
14,15,44
systems like MOFs with ether functionalized linkers,
21
than for the monosubstituted linker (6). By integration of two
ethyl groups (9), the gate opening pressure is reduced to 0.7
MPa, which, again, could be a consequence of contracted pores
26
MIL-53(Fe), or a series of interpenetrated silver MOFs.
Cu
CONCLUSION
compared to 8 . The main difference between the monoclinic
■
and orthorhombic MOF series is the slope of the adsorption
branch before reaching the gate opening pressure. Whereas 6−
The paddle-wheel-based triazolyl isophthalate MOFs 1−4 and
5−9 form two closely related isomorphous series with rtl and
G
DOI: 10.1021/acs.inorgchem.5b02921
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
apo topology, respectively. The thermal stability of these
frameworks reaches up to 390 °C (8 ). Due to orientation of
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i
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H
DOI: 10.1021/acs.inorgchem.5b02921
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Inorganic Chemistry
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