Single- and mixed-linker Cr-MIL-101 derivatives: A high-throughput investigation

Article abstract of DOI:10.1021/ic4005328

New single- and mixed-linker Cr-MIL-101 derivatives bearing different functional groups have been synthesized. The influence of the reaction parameters, such as metal source (CrO₃, CrCl₃, and Cr(NO₃)₃·9H₂O) or linker composition, on product formation have been investigated using high-throughput methods. Highly crystalline Cr-MIL-101 materials were obtained with CrCl₃ as the metal source with either 2-bromoterephthalic (TA-Br) or 2-nitroterephthalic (TA-NO₂) acid as one of the mixed-linker components. On the basis of these results, numerous new mixed-linker Cr-MIL-101 derivatives containing -NH₂, -NO₂, -H, -SO₃H, -Br, -OH, -CH ₃, and -COOH have been synthesized. The use of TA-NH₂ and TA-OH were shown, under the same reaction conditions, to lead to decarboxylation and the formation of 3-amino- and 3-hydroxybenzoic acid, respectively. Furthermore, we were also able to directly synthesize single-linker Cr-MIL-101-X derivatives with X = F, Cl, Br, CH₃. Postsynthetic modification was used to selectively reduce the mixed-linker compound Cr-MIL-101-Br-NO ₂ to Cr-MIL-101-Br-NH₂. To establish the successful incorporation of the linker molecules and possible decomposition of certain starting materials, ¹H NMR spectra of dissolved reaction products were recorded.

Full text of DOI:10.1021/ic4005328

Article
pubs.acs.org/IC
Single- and Mixed-Linker Cr-MIL-101 Derivatives: A High-Throughput
Investigation
Martin Lammert, Stephan Bernt, Frederik Vermoortele, Dirk E. De Vos, and Norbert Stock*,†
†
†
‡
‡,§
†
Institut fu
̈
r Anorganische Chemie, Christian-Albrechts-Universitat
Centre for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Arenbergpark 23, B-3001 Leuven, Belgium
S Supporting Information
̈
zu Kiel, Max-Eyth-Straße 2, D-24118 Kiel, Germany
‡
*
ABSTRACT: New single- and mixed-linker Cr-MIL-101
derivatives bearing different functional groups have been
synthesized. The influence of the reaction parameters, such
as metal source (CrO , CrCl , and Cr(NO ) ·9H O) or linker
3
3
3 3
2
composition, on product formation have been investigated
using high-throughput methods. Highly crystalline Cr-MIL-
1
01 materials were obtained with CrCl as the metal source
3
with either 2-bromoterephthalic (TA-Br) or 2-nitroterephthalic
TA-NO ) acid as one of the mixed-linker components. On
(
2
the basis of these results, numerous new mixed-linker Cr-MIL-
01 derivatives containing -NH , -NO , -H, -SO H, -Br, -OH,
1
2
2
3
-
CH , and -COOH have been synthesized. The use of TA-NH and TA-OH were shown, under the same reaction conditions, to
3
2
lead to decarboxylation and the formation of 3-amino- and 3-hydroxybenzoic acid, respectively. Furthermore, we were also able
to directly synthesize single-linker Cr-MIL-101-X derivatives with X = F, Cl, Br, CH . Postsynthetic modification was used to
3
selectively reduce the mixed-linker compound Cr-MIL-101-Br-NO to Cr-MIL-101-Br-NH . To establish the successful
2
2
1
incorporation of the linker molecules and possible decomposition of certain starting materials, H NMR spectra of dissolved
reaction products were recorded.
INTRODUCTION
For the possible applications of MOFs, high thermal and
chemical stability is preferable. The compound Cr-MIL-101,
■
In recent years, metal−organic frameworks (MOFs) have
gained widespread attention due to their considerable physical
and chemical properties. By the choice of the organic building
[
Cr (OH)(H O) (μ -O)(O CC H CO ) ]·nH O, is outstand-
3 2 2 3 2 6 4 2 3 2
1
ing for its high thermal stability up to 270 °C as well as its
17
2
3
chemical stability. It forms a zeotypic MTN network as
observed in the zeolite ZSM-39 with two types of mesoporous
unit (linker), the pore size, specific surface area, and chemical
4
functionality can be varied. MOFs have been studied in
5
6
12
cages: dodecaedric cages (5 ) with a 29 Å diameter and
different research areas such as catalysis, gas storage, gas
7
8
12 4
separation, drug release, and heat transformation applica-
hexacaidecaedric cages (5 6 ) with a 34 Å diameter (Figure 1).
The cages are accessible through five-membered (12 Å) and
six-membered (16 Å) windows.
9
tions. It has been shown that the introduction of different
10
functional groups influences the specific surface area of MOFs
4
and their breathing behavior, such as in Al-MIL-53-X, Fe-MIL-
1
1
12
5
3-X, and Fe-MIL-88-X (X = functional group). The
properties can be fine-tuned by using mixtures of linker
molecules bearing different functional groups, leading to mixed-
1
3
linker MOFs. Although some functionalized MOFs can be
obtained directly by replacing the unfunctionalized linker by
the desired functionalized linker, this is not always easily
accomplished. Especially in the case of compounds that are
synthesized at high temperatures, some functional groups
cannot be incorporated because the starting material
decomposes or functional groups are substituted during the
synthesis. These groups can be introduced and further modified
Figure 1. Two types of cages observed in Cr-MIL-101: dodecaedric
cages containing five-membered rings (left) and hexacaidecaedric
cages containing five-membered and six-membered rings (right).
17
14
via postsynthetic modification (PSM). This has previously
been shown by the introduction of amino groups, which were
reacted with alkyl isocyanates to obtain the corresponding urea
Received: March 1, 2013
Revised: June 19, 2013
Accepted: June 19, 2013
1
5
derivative or with phosgene/thiophosgene to yield the
16
isocyanate/thioisocyanate derivative.
©
XXXX American Chemical Society
A
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Although Cr-MIL-101 exhibits these striking properties, just
a few functionalized derivatives of MIL-101 are known: the
chromium source and 125 μmol of the terephthalic acids, or equimolar
mixtures adding up to 125 μmol, were introduced. The Teflon reactors
were initially filled with the solids (terephthalic acid derivatives, CrCl3)
1
5,18−20
amino and nitro derivatives,
tive, and the sulfate derivative. Very recently, the PSM of
the sulfonic acid deriva-
2
1
22
followed by the liquids (metal solution (Cr(NO ) ·9H O or CrO ),
3
3
2
3
water (1 mL), HCl (250 μmol)). The reactor block was covered with
Teflon foil, sealed with the top plate of the reactor, and heated in an
oven at 180 °C for 96 h. After the reaction time, the solid was filtered
and washed with 500 μL of demineralized water and 500 μL of
ethanol. Afterward, the solid was dried in air at 70 °C. The samples
were automatically characterized using a STOE HT X-ray powder
diffractometer. Details of the high-throughput metal screening
experiment are given in the Supporting Information (Tables S1−S3).
Cr-MIL-101-NH to the iodo and fluorine derivatives and the
2
2
3,24
introduction of photoswitchable groups was described.
comparison to the large number of known derivatives for MIL-
3 and MIL-88, the number of incorporated functional groups
In
5
is limited. In addition, no mixed-linker Cr-MIL-101 compounds
have been reported up until now. This is probably due to the
unpredictable synthesis conditions that have to be established
for every new linker or mixed-linker system.
High-Throughput Synthesis of Br- and NO -Containing
2
Mixed-Linker Cr-MIL-101 Derivatives. For all syntheses, a reactor
block with 24 Teflon inserts (2 mL volume each) was used. The
starting materials 125 μmol of CrCl3 and 125 μmol of ligand
An efficient way for the simultaneous investigation of a wide
25
parameter range is the high-throughput (HT) methodology.
It is based on the miniaturization of conventional hydrothermal
reactions where reactions take place in a reactor block with 24
(equimolar mixture of two derivatives) were introduced into the
Teflon liners. After the addition of 1 mL of demineralized water, the
reactor block was sealed and heated to 180 °C within 1 h. After
holding the temperature for 120 h, the reactor block was cooled to
room temperature within 6 h. The green solid was centrifuged and
redispersed in 20 mL of water (this process was repeated two times
with water and three times with ethanol). The resulting green solid
(2 mL volume each) or 48 (300 μL volume each) Teflon
inserts. This process allows the simultaneous investigation of
several synthesis parameters, such as stoichiometry of reactants
or variation of pH. On the other hand, the amount of reactants
that are used is drastically reduced compared to that of
conventional hydrothermal syntheses. Therefore, HT methods
are useful for the discovery of new phases and the subsequent
synthesis optimization.
Here, we report the high-throughput investigation of single-
and mixed-linker systems in order to get new functionalized Cr-
MIL-101 derivatives.
was dried in air at 70 °C. Details of the experiments using CrCl as the
3
metal source are given in the Supporting Information (Table S4).
Synthesis Scale-Up of the Mixed-Linker Cr-MIL-101 Deriv-
atives Cr-MIL-101-Br-COOH and Cr-MIL-101-NO -COOH. The
2
starting materials CrCl (198 mg, 1.25 mmol) and 1.25 mmol of ligand
3
(equimolar mixture of the two derivatives) were introduced into a 40
mL Teflon reactor. After the addition of 5 mL of demineralized water,
the reactor was sealed and heated to 180 °C within 1 h. The
temperature was held for 96 h and cooled to room temperature within
6 h. The green solid was centrifuged and redispersed in 20 mL of water
(this process was repeated two times with water and three times with
ethanol). The resulting green solid was dried in air at 70 °C. PXRD
patterns of the scaled-up products are given in the Supporting
Information (Figure S1).
EXPERIMENTAL SECTION
■
Materials. Chromium nitrate (99%, Cr(NO ) ·9H O, Honeywell
3
3
2
Riedel-de Hae
̈
n), chromium oxide (99%, CrO , Merck), and
3
chromium chloride (99%, CrCl , Aldrich) were used as obtained.
3
Terephthalic acid (98%, TA-H, Aldrich), 2-aminoterephthalic acid
(
98%, TA-NH , Aldrich), 2-nitroterephthalic acid (98%, TA-NO ,
Optimized Synthesis Conditions for Single-Linker Cr-MIL-
2
2
Fluka), 2-bromoterephthalic acid (95%, TA-Br, Aldrich), 2-hydrox-
yterephthalic acid (97%, TA-OH, Aldrich), monosodium-2-sulfoter-
ephthalic acid (98%, TA-SO Na, ABCR), 2-methylterephthalonitrile
101 Derivatives. Synthesis of Cr-MIL-101-CH . For the synthesis, a
3
reactor block with 24 Teflon inserts (2 mL volume each) was used.
The starting materials CrCl (19.8 mg, 125 μmol) and TA-CH (22.5
3
3
3
(
98%, Aldrich), dimethyl-2-fluoroterephthalate (96%, Aldrich),
mg, 125 μmol) were introduced into the Teflon liner. After the
addition of 1 mL of demineralized water, the reactor block was sealed
and heated to 180 °C within 1 h. After holding the temperature for 96
h, the reactor block was cooled to room temperature within 3 h. The
green solid was centrifuged and redispersed in 20 mL of water (this
process was repeated two times with water and three times with
ethanol). The resulting green solid was dried in air at 70 °C.
dimethyl-2-iodoterephthalte (98%, Aldrich), 2-chloro-1,4-dimethyl-
benzene (99%, Aldrich), and sodium hydroxide were used without
any further purification. 1,2,4-Benzenetricarboxylic acid (98%, TA-
COOH) was received from TCI, and tin(II) chloride (99%, SnCl2)
and sodium nitrite (99%, NaNO ) were received from Merck.
̈
Potassium iodide (99%, KI) was received from Grussing.
2
The linker molecules TA-CH , TA-F, TA-Cl, and TA-I were
Synthesis of Cr-MIL-101-F. For the syntheses, a reactor block with
24 Teflon inserts (2 mL volume each) was used. The starting materials
3
synthesized as described in the Supporting Information.
NMR spectra were measured on a Bruker DRX 500 spectrometer.
Sorption experiments were performed using a Bel Japan, Inc.
Belsorpmax instrument. IR spectra were measured on a Bruker
ALPHA-FT-IR A220/D-01 spectrometer equipped with an ATR-
unit. PXRD data was recorded on a Stadi P Combi diffractometer with
CrCl (19.8 mg, 125 μmol) and TA-F (23.0 mg, 125 μmol) were
3
introduced into the Teflon liners. After the addition of 950 μL of
demineralized water and 50 μL of a 5 M NaOH solution, the reactor
block was sealed and heated to 180 °C within 1 h. The temperature
was held for 96 h and then cooled to room temperature within 6 h.
The green solid was centrifuged and redispersed in 20 mL of water
(this process was repeated two times with water and three times with
ethanol). The resulting green solid was dried in air at 70 °C.
Synthesis of Cr-MIL-101-Cl and Cr-MIL-101-Br. The starting
materials CrO3 (198 mg, 1.25 mmol) and TA-Cl/TA-Br (1.25
mmol) were introduced into a 40 mL Teflon reactor. After the
addition of 5 mL of demineralized water and 76.5 μL (2.50 mmol) of
concentrated HCl, the reactor was sealed and heated to 180 °C within
1 h. The temperature was held for 120 h and then cooled to room
temperature within 6 h. The green solid was centrifuged and
redispersed in 20 mL of water (this process was repeated two times
with water and three times with ethanol). The resulting green solid
was dried in air at 70 °C.
Cu Kα radiation equipped with an image-plate detector system and an
1
xy-stage.
Most of the syntheses were carried out in HT reactors with a
25
maximum volume of 2 mL each. The synthesis scale-up of selected
single- and mixed-linker Cr-MIL-101 derivatives to reactors with a
volume of 40 mL was also accomplished, but not all syntheses were
scaled up. We are aware of the fact that the scale-up of reactions is not
easily accomplished, and for some reported syntheses, procedure
adjustment of the reaction parameters will probably be necessary.
High-Throughput Metal-Source Screening. For the systematic
investigation of the influence of the metal sources on the product
2
5
formation, a reactor block with 24 Teflon inserts was used. The
terephthalic acid derivatives were always used in a molar ratio of 1:1.
Cr(NO ) ·9H O and CrO were used as aqueous solutions (1.25 M),
3
3
2
3
Postsynthetic Modification of Cr-MIL-101-Br-NO to Cr-MIL-
2
and CrCl was used as a solid. In each Teflon insert, 125 μmol of the
101-Br-NH . Cr-MIL-101-Br-NO (60 mg) was suspended in 15 mL
3
2
2
B
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of ethanol, and 1.2 g of SnCl ·2H O was added. The mixture was
2
2
stirred for 6 h at 70 °C. The green suspension was centrifuged and
redispersed in 20 mL of concentrated HCl. The solid was centrifuged
again and redispersed in 20 mL of demineralized water (this process
was repeated two times with water and three times with ethanol). The
resulting green solid was dried in air at 70 °C.
Sample Treatment Prior to NMR Measurements. Due to the
paramagnetic properties of the chromium species in the Cr-MIL-101
derivatives, the direct solid-state NMR measurement is not easily
accomplished. To establish the molar ratio of the incorporated linker
molecules and possible decomposition products, solution NMR
spectroscopy was carried out. After the decomposition of the Cr-
MIL-101 compounds in aqueous NaOH, the precipitated Cr(OH)3
was removed by filtration. After the pH value was adjusted to pH ≈ 3,
the terephthalic acid species were extracted with diethyl ether. After
the solvent was removed in vacuo and dissolved the solid in 5%
1
Figure 3. Results of the high-throughput metal screening experiment
NaOD/D O solution, the H NMR spectra were recorded. Systems
2
with CrO as the metal source (green = Cr-MIL-101, black = X-ray
3
containing sulfoterephthalic acid were treated slightly different. After
amorphous, red = Cr-MIL-53, blue = Cr-MIL-88). The shaded fields
denote the single-linker systems.
the Cr(OH) was removed by filtration, the pH was adjusted to pH ≈
3
3
5
, and the water was evaporated at 70 °C. The solid was dissolved in
% NaOD/D O for solution NMR measurements. The relative
2
amounts of linker molecules incorporated into the final Cr-MIL-101
derivative were calculated from the integrals of the H NMR signals,
thalic acid did not form Cr-MIL-101-SO H as described
3
21
1
elsewhere. Although the original synthesis was carried out in a
100 mL autoclave, the syntheses in this study are carried out in
which are characteristic for each terephthalic acid derivative.
2
mL reactors. This unexpected behavior could be due to the
RESULTS AND DISCUSSION
change of the reactor size because different surface to volume
ratios lead to changes in the heating profile, which also affects
the solubility of the starting materials.
■
High-Throughput Screening. For the synthesis of the
new functionalized Cr-MIL-101 derivatives, three different
metal sources were used: Cr(NO ) ·9H O from which Cr-MIL-
Using Cr(NO ) ·9H O as the metal source gave eight pure
3
3
2
3 3 2
17
1
01 originally was obtained, CrCl from which Cr-MIL-101-
crystalline Cr-MIL-101 phases (Figure 4 and Supporting
3
15
NO can be synthesized, and CrO which was reported as the
2
3
2
1
metal source for Cr-MIL-101-SO H. Initially, a high-
3
throughput metal screening experiment was carried out in
order to determine where the most crystalline Cr-MIL-101
phases can be found. On the basis of previous results on
functionalized Cr-MIL-101 derivatives, long reaction times
1
5,21
(
96−120 h) were chosen.
In addition to single-linker
systems, mixed-linker systems containing equimolar mixtures of
functionalized terephthalic acid derivatives were also inves-
tigated (Figure 2).
Using CrO as the metal source yielded predominantly X-ray
3
amorphous products (Figure 3 and Supporting Information
Table S1). Besides Cr-MIL-53 and Cr-MIL-88, four pure Cr-
MIL-101 phases and mixtures thereof were also formed. It is
notable that the system with pure monosodium-2-sulfotereph-
Figure 4. Results of the high-throughput metal screening experiment
with Cr(NO ) ·9H O (green = Cr-MIL-101, black = X-ray
3
3
2
amorphous, red = Cr-MIL-53). The shaded fields denote the single-
linker systems.
Information Table S2). Similar to the results using CrO , X-
3
ray amorphous products and Cr-MIL-53, but no Cr-MIL-88,
were observed. The effect of employing miniaturized reactors is
also observed in the single-linker system using terephthalic acid.
In addition to Cr-MIL-101, small amounts of Cr-MIL-53 were
observed (Supporting Information Figure S1). This could also
be due to the extended reaction time of 96 h. In the original
synthesis of the Cr-MIL-101, a reaction time of 8 h was used.
Using CrCl as the metal source resulted in a large number of
3
single- and mixed-linker Cr-MIL-101 derivatives (Figure 5).
Furthermore, the number of functionalized terephthalic acids
used was extended, and methylterephthalic acid (TA-CH ) and
3
trimellitic acid (TA-COOH) were also employed. In this
system, the majority of products were crystalline Cr-MIL-101
derivatives (Figure 5, Supporting Information Table S3). The
Figure 2. Schematic representation of terephthalic acid derivatives and
chromium sources used in the high-throughput screening to obtain
single- and mixed-linker Cr-MIL-101 compounds.
C
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patterns, they are present only in small amounts or they are not
crystalline.
Because we started with equimolar mixtures of TA-X linker
molecules in the synthesis, the relative amount of linker
molecules incorporated into the final Cr-MIL-101 structure
gives information on the reactivity of these derivatives. Solution
1
H NMR spectroscopy of single- and mixed-linker Cr-MIL-101
samples was carried out to quantify the relative amounts of
linker molecules incorporated and to determine if the
decomposition of the linker molecules had taken place under
the harsh reaction conditions. These results are summarized in
Figure 7, Supporting Information Table S6, and Supporting
Figure 5. Results of the high-throughput metal screening experiment
using anhydrous CrCl as the metal source (green = Cr-MIL-101,
3
black = X-ray amorphous, red = Cr-MIL-53). The shaded fields denote
the single-linker systems. In contrast to the results of using CrO or
3
Cr(NO ) ·9H O as the metal source, mainly Cr-MIL-101 derivatives
3
3
2
were obtained.
mixed-linker systems with TA-Br and TA-NO yielded highly
2
crystalline Cr-MIL-101 derivatives (Supporting Information
Figure S2, Figure 6). Although the use of TA-NO resulted
2
Figure 7. Molar ratios of the incorporated linker molecules in the Cr-
MIL-101 derivatives obtained when CrCl was used as the metal
3
1
source. Molar ratios are based on the results of solution H NMR
experiments (orange = presence of 2-chloroterephthalic acid, blue =
presence of 3-hydroxybenzoic acid, gray = single-linker systems). Full
details are given in Supporting Information Table S6.
Information Figure S14−S20. In some systems, small amounts
(
ca. 1−2%) of terephthalic acid (TA-H) were detected, which is
Figure 6. PXRD patterns of reaction products obtained in the high-
throughput screening with TA-NO2 compared to the simulated
pattern of Cr-MIL-101 (8, black): Cr-MIL-101-NO -NH (1, violet),
due to the presence of TA-H impurities in the commercially
available functionalized terephthalic acid derivatives (Support-
ing Information Table S5).
2
2
Cr-MIL-101-NO -H (2, light green), Cr-MIL-101-NO -SO H (3,
2
2
3
In comparison to the recently reported results on the
red), Cr-MIL-101-NO -Br (4, blue), Cr-MIL-101-NO -OH (5, pink),
2
2
formation of Cr-MIL-101-NH , which was feasible using an
2
Cr-MIL-101-NO -CH (6, dark green), and Cr-MIL-101-NO -COOH
2
3
2
OH-assisted hydrothermal synthesis carried out at 150 °C for
(
7, orange).
18
1
2 h, the use of pure TA-NH or TA-OH in the synthesis of
2
Cr-MIL-101 leads exclusively to X-ray amorphous products
under the studied reaction conditions. Their use of mixed-
linker Cr-MIL-101 derivatives in the formation does not (TA-
exclusively in the formation of Cr-MIL-101 derivatives (Figure
), very small impurities of Cr-MIL-53 derivatives were
6
NH ) or does only in small amounts (TA-OH) lead to the
2
observed when TA-Br was employed (Supporting Information
Figure S2).
incorporation of these linker molecules in the final structure.
Therefore, control experiments without adding the metal
source to the reaction mixture were carried out. These show
SEM measurements were carried out to determine the
morphology of the reaction products. Cr-MIL-101 crystals are
easily recognized by their octahedral shape; thus, byproducts
can easily be detected (Supporting Information Table S5). The
observed crystal shapes support, in general, the results of the
PXRD measurements. Mostly phase-pure products are
obtained. The products that contain a byproduct according to
the PXRD data exhibit crystals of different morphologies. In the
that TA-NH and TA-OH undergo a decomposition reaction,
2
and 3-aminobenzoic acid and 3-hydroxybenzoic acid, respec-
tively, are formed (Figure 8). This reaction has been reported
already in 1877 for “oxodicarboxylic acids” and can be
explained by the decarboxylation at elevated temperatures
26
during the synthesis. Additionally, in the systems TA-Br/TA-
NH and TA-Br/TA-OH, the chlorine derivative TA-Cl was
2
micrographs of Cr-MIL-101-Cl, Cr-MIL-101-CH , and Cr-
MIL-101-Br-H, particles of different morphology are observed.
observed in the final Cr-MIL-101 derivative. This occurrence
3
must be due to the use of CrCl as the metal source and the
3
Because no crystalline impurities were detected in the PXRD
partial substitution in acidic reaction milieu.
D
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incorporated and did not undergo any site reactions
(
Supporting Information Figure S3). Additionally, in the IR
spectrum, the methyl C−H stretching vibrations can be found
between 2900 and 2974 cm− (Supporting Information Figure
S4).
1
Also, halogenated Cr-MIL-101 derivates were directly
obtained from the corresponding functionalized terephthalic
acid derivatives. Cr-MIL-101-F was obtained from a reaction
mixture of CrCl and TA-F upon addition of sodium hydroxide
3
and using the 2 mL reactor system. The addition of sodium
hydroxide leads to deprotonation of TA-F and, thus, increases
the solubility. In contrast, Cr-MIL-101-F was recently reported
to be accessible by a two-step procedure starting with Cr-MIL-
Figure 8. Decarboxylation (top) and nucleophilic substitution
(
bottom) of 2-aminoterephthalic acid and 2-hydroxyterephthalic acid
taking place at a reaction temperature of 180 °C.
24
1
01-NH and carrying out a Schiemann reaction. PXRD
2
Synergistic effects are observed when TA-COOH is used in
the synthesis of mixed-linker Cr-MIL-101 derivatives. Although
the use of TA-COOH as single-linker leads only to X-ray
amorphous products, the use of equimolar mixtures with TA-H,
measurements of the directly synthesized Cr-MIL-101-F
reaction product demonstrate the phase purity (Figure 9, 2).
Using IR spectroscopy, we found the characteristic C−F
−1
stretching vibration at 1234 cm (Supporting Information
27
TA-NO , TA-Br, or TA-CH leads to the incorporation of both
2
3
Figure S5). To demonstrate that TA-F is incorporated, we
employed solution H NMR spectroscopy. In addition to the
linker molecules in an almost equimolar ratio each.
1
The use of TA-SO Na resulted in the formation of mixed-
3
TA-F, only traces of unfunctionalized terephthalic acid (2%)
were found (Supporting Information Figure S6). Cr-MIL-101-
Cl was obtained in large amounts in the 40 mL reactor system
linker Cr-MIL-101 derivatives when TA-H, TA-Br, TA-NO2,
and TA-CH were employed. According to the results of the
3
NMR measurements, the incorporated molar ratio deviates
substantially from the one used in the reaction mixture. With
starting from CrO , TA-Cl, and concentrated HCl. No
3
crystalline byproducts were formed (Figure 9, 3), and the
the exception of the synthesis using TA-CH , the final products
3
characteristic C−Cl stretching vibration is observed at 1050
2
7
contain ca. 1/3 TA-SO H, and no side reactions of the linker
−1
3
cm in the IR spectrum (Supporting Information Figure S7).
molecules could be detected (Supporting Information Table
S6). Two possible reasons are the very good solubility of the
monosodium-2-sulfoterephthalic acid or its lower reactivity.
Single-Linker Cr-MIL-101 Derivatives. In addition to
mixed-linker Cr-MIL-101 derivatives, the direct synthesis of
single-linker derivatives was also investigated. Therefore, the
1
H NMR measurements show the sole incorporation of TA-Cl
(
Supporting Information Figure S8). Cr-MIL-101-Br could also
be obtained in large amounts in the 40 mL reactor system
starting from CrO , TA-Br, and concentrated HCl. PXRD
3
measurements reveal that Cr-MIL-101-Br was obtained without
crystalline impurities (Figure 9, 4). Employing IR spectroscopy,
we observed the characteristic C−Br stretching vibration at
terephthalic acid derivatives TA-X with X = CH , F, Cl, Br, and
3
I were studied. The PXRD patterns of the reaction products are
presented in Figure 9. These newly incorporated functional
groups could be valuable for PSM reactions.
−1
27
1
1
042 cm (Supporting Information Figure S9). Solution H
NMR spectroscopy demonstrated the presence of small
amounts of terephthalic acid (2%). (Supporting Information
Figure S10).
The attempt to synthesize Cr-MIL-101-I directly from
Cr(NO ) ·9H O, TA-I, and concentrated HCl resulted in the
3
3
2
formation of a crystalline compound denoted Cr-MIL-101-I.
No crystalline impurities could be detected (Figure 9, 5). The
detailed characterization of the incorporated linker molecules
1
by H NMR measurements revealed an unexpected linker
composition (Figure 10). In addition to TA-I, the majority of
linker molecules were shown to be TA-NO , although TA-Cl
2
and TA-H were also found (molar ratio TA-I/TA-NO /TA-Cl/
2
TA-H = 0.18/1/0.25/0.06). In the IR spectrum of Cr-MIL-101-
−1
I, the C−I stretching vibration (1049 cm ) and the C−Cl
−1
stretching vibration (1073 cm ) were found (Supporting
27
Information Figure S11).
The incorporation of the different linker molecules can be
explained by side reactions taking place during the synthesis.
Iodine is known to be a very good leaving group, and the in situ
formation of TA-Cl and TA-NO2 is due to nucleophilic
substitution reactions occurring at the elevated reaction
temperature. Cr-MIL-101-I has recently been shown to be
accessible by postsynthetic modification starting with Cr-MIL-
Figure 9. Powder pattern of the single-linker Cr-MIL-101 compounds
compared to the simulated pattern of Cr-MIL-101 (6, black): Cr-MIL-
1
01-CH (1, red), Cr-MIL-101-F (2, blue), Cr-MIL-101-Cl (3, pink),
3
Cr-MIL-101-Br (4, green), and Cr-MIL-101-I (5, orange).
Cr-MIL-101-CH was obtained from CrCl and TA-CH
3
3
3
24
using the 2 mL reactor system. PXRD measurements
demonstrated the formation of crystalline Cr-MIL-101 without
any crystalline impurity (Figure 9, 1). After decomposition of
the framework structure and extraction of the linker molecules,
101-NH via Sandmeyer reaction.
2
Postsynthetic Modification. Because the direct incorpo-
ration of TA-NH in mixed-linker Cr-MIL-101 derivatives has
2
not been successful, the postsynthetic modification route was
chosen. On the basis of the results of the synthesis of Cr-MIL-
1
H NMR spectroscopy showed that only TA-CH3 was
E
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Inorganic Chemistry
Article
Figure 12. 500 MHz 1H NMR spectra of the extracted linker
molecules from Cr-MIL-101-Br-NH (red) and Cr-MIL-101-Br-NO
2
2
(black). The signal which is characteristic for terephthalic acid is
marked with a star, and the signals of TA-NH are marked with
2
diamonds.
Figure 10. 500 MHz 1H NMR spectra of the extracted linker
molecules from Cr-MIL-101-I obtained from the direct synthesis using
TA-I (red). In addition, the NMR spectra of pure 2-nitroterephthalic
acid (blue), 2-iodoterephthalic acid (green), and 2-chloroterephthalic
acid (black) are presented. The signal of the unfunctionalized
terephthalic acid is marked with a star.
1
01-NH starting from Cr-MIL-101-NO , the mixed-linker
2
2
derivative Cr-MIL-101-Br-NO was used for a postsynthetic
2
15
modification reaction. In cases where all nitro groups are
accessible for the reduction, a Cr-MIL-101 derivative
containing only TA-Br and TA-NH should be achievable.
2
PXRD measurements prove the preservation of the crystallinity
during these harsh reaction conditions (Figure 11).
Figure 13. N adsorption isotherms of the single-linker Cr-MIL-101
2
derivatives: Cr-MIL-101-F (blue diamonds), Cr-MIL-101-Cl (pink
triangles), Cr-MIL-101-Br (green stars), and Cr-MIL-101-CH (red
3
squares).
Figure 11. PXRD pattern of Cr-MIL-101-Br-NH compared to that of
2
Cr-MIL-101-Br-NO and the simulated pattern of Cr-MIL-101.
2
Solution 1H NMR measurements show the successful
modification of the nitro groups (Figure 12). In the spectrum
of the extracted linker molecules, only signals of TA-Br and TA-
Figure 14. N adsorption isotherms of the mixed-linker Cr-MIL-101
derivatives: Cr-MIL-101-Br-NH2 (violet circles), Cr-MIL-101-Br-
2
COOH (orange squares), Cr-MIL-101-Br-NO (light green triangles),
2
Cr-MIL-101-Br-H (red diamonds), Cr-MIL-101-Br-SO H (blue stars),
NH can be found; additionally, a small signal corresponding to
3
2
and Cr-MIL-101-Br-CH (dark green pentagons).
the presence of terephthalic acid is observed. It can thus be
3
concluded that all nitro groups in Cr-MIL-101-Br-NO are
2
accessible and were quantitatively reduced.
N Sorption Measurements. The results of the sorption
measurements of the single- and selected mixed-linker Cr-MIL-
01 derivatives are presented in Figures 13 and 14, Table 1, and
the Supporting Information (Figures S12 and S13). Literature
values of previously reported Cr-MIL-101 compounds are given
for comparison. The observed SBET values of the single- and
mixed-linker Cr-MIL-101 derivatives fall within the range of
values previously reported. An exception is the small value
2
step curve shape is observed, which is due to the filling of two
17
cages of different sizes. The SBET values of the halogen-
functionalized Cr-MIL-101-X derivatives decrease with increas-
ing weight, although the values observed for the -Cl and -Br
differ only slightly. There is no clear trend comparing the
single-linker with the corresponding mixed-linker systems. The
1
S
BET values for Cr-MIL-101-Br-NH
the single-linker systems, whereas for Cr-MIL-101-Br-SO H
3
falls between the values of
2
observed for Cr-MIL-101-Br-CH , which is due to the presence
3
of small amounts of impurities observed in the PXRD and SEM
measurements. In all sorption isotherms, the characteristic two-
and Cr-MIL-101-Br-NO , the values observed are below and
above, respectively, the corresponding single-linker systems.
2
F
dx.doi.org/10.1021/ic4005328 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Specific Surface Areas (SBET) and Micropore
ASSOCIATED CONTENT
Supporting Information
■
Volumes of the Single-Linker Cr-MIL-101-X Derivatives
*
S
a
with TA-X and Mixed-Linker Cr-MIL-101-Br-Y Derivatives
Tables with ratios of incorporated linker molecules of mixed-
linker Cr-MIL-101-derivatives, PXRD patterns, and IR spectra.
b
with TA-Y
1
Cr-MIL-101 SBET [m g−1] Vmic [cm g−1] Vtot [cm g−1]
2
3
3
Solution H NMR spectra of selected reaction products. This
reference
e
material is available free of charge via the Internet at http://
pubs.acs.org.
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
H
2367−4100
2.0(1)
15, 21, 29
this work
24
c
c
F
2282
1.20
d
d
d
d
F
1619
1.26
1.19
2.26
c
c
AUTHOR INFORMATION
Corresponding Author
E-mail: stock@ac.uni-kiel.de.
Cl
1720
0.87
this work
this work
24
■
c
c
Br
1631
0.87
d
*
I
1431
c
c
CH3
NH2
NH2
NO2
1467
0.68
this work
15
Notes
c
c
2313
1.09
The authors declare no competing financial interest.
d
§
2070
24
E-mail: dirk.devos@biw.kuleuven.be.
c
c
1425
0.75
15
SO H
1915
21
ACKNOWLEDGMENTS
We appreciate support from the Deutsche Forschungsgemein-
schaft (DFG, SPP 1362) and Prof. Dr. So
of Kiel) for NMR measurements.
3
■
c
c
Br-NH2
Br-COOH
Br-NO2
Br-H
1900
1.03
this work
this work
this work
this work
this work
this work
c
c
1870
0.89
̈
nnichsen (University
c
c
1696
0.80
c
c
1574
0.74
c
c
Br-SO H
1549
0.70
3
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f
c
c
■
Br-CH3
991
0.45
(
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CONCLUSIONS
■
́
We have demonstrated the successful formation of new single-
and mixed-linker functionalized Cr-MIL-101 derivatives. A
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̀
́
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9,20
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(
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thermal synthesis due to nucleophilic substitution reactions
taking place during the synthesis. A number of mixed-linker Cr-
MIL-101 compounds were obtained. A synergetic effect was
observed during the synthesis of Cr-MIL-101-COOH-Y
compounds. Only certain combinations with other functional
groups yielded the desired reaction product, and the
corresponding single-linker compound Cr-MIL-101-COOH
could not be obtained. In the future, the new single-linker as
well as the mixed-linker compounds could be used for further
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́
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dx.doi.org/10.1021/ic4005328 | Inorg. Chem. XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/ic4005328 | Inorg. Chem. XXXX, XXX, XXX−XXX