Proline functionalization of the mesoporous metal-organic framework DUT-32

Article abstract of DOI:10.1021/ic502380q

The linker functionalization strategy was applied to incorporate proline moieties into a metal-organic framework (MOF). When 4,4′-biphenyldicarboxylic acid was replaced with a Boc-protected proline-functionalized linker (H₂L) in the synthesis o

Full text of DOI:10.1021/ic502380q

Article
pubs.acs.org/IC
Proline Functionalization of the Mesoporous Metal−Organic
Framework DUT-32
Christel Kutzscher,† Herbert C. Hoffmann,‡ Simon Krause,† Ulrich Stoeck,†,§ Irena Senkovska,†
Eike Brunner,*,‡ and Stefan Kaskel*,†
‡
†Department of Inorganic Chemistry and Department of Bioanalytical Chemistry, Technische Universitat Dresden, Bergstraße 66,
D-01069 Dresden, Germany
̈
S
* Supporting Information
ABSTRACT: The linker functionalization strategy was
applied to incorporate proline moieties into a metal−organic
framework (MOF). When 4,4′-biphenyldicarboxylic acid was
replaced with a Boc-protected proline-functionalized linker
(H2L) in the synthesis of DUT-32 (DUT = Dresden
University of Technology), a highly porous enantiomerically
pure MOF (DUT-32-NHProBoc) was obtained, as could be
confirmed by enantioselective high-performance liquid chro-
matography (HPLC) measurements and solid-state NMR
experiments. Isotope labeling of the chiral side group proline
enabled highly sensitive one- and two-dimensional solid-state
13C NMR experiments. For samples loaded with (S)-1-phenyl-
2,2,2-trifluoroethanol [(S)-TFPE], the proline groups are
shown to exhibit a lower mobility than that for (R)-TFPE-loaded samples. This indicates a preferred interaction of the shift
agent (S)-TFPE with the chiral moieties. The high porosity of the compound is reflected by an exceptionally high ethyl
cinnamate adsorption capacity. However, postsynthetic thermal deprotection of Boc−proline in the MOF leads to racemization
of the chiral center, which was verified by stereoselective HPLC experiments and asymmetric catalysis of aldol addition.
alization of the linker before MOF synthesis was carried out by
Telfer et al.9 The reported homochiral material could be used
as heterogeneous chiral catalysts in asymmetric aldol reactions.
The important point in the synthesis of enantiopure (proline-
containing) MOFs is to avoid the racemization that can occur
particularly under thermal conditions. Therefore, stereo-
chemical investigation of the MOFs after synthesis plays an
important role in characterization of the materials with respect
to the desired applications.
In this work, we present the synthesis of the proline-
functionalized homochiral MOF DUT-32-NHProBoc (DUT -
Dresden University of Technology) by the incorporation of a
chiral proline side group into the ligand. Because the direct
MOF synthesis using a proline-substituted linker was not
successful, the N-(tert-butoxycarbonyl) (Boc) protective group
was introduced to prevent side reactions during the MOF
formation. The enantiomeric excess (ee) of DUT-32-
NHProBoc, as well as deprotection conditions, was studied
by enantioselective high-performance liquid chromatography
(HPLC) measurements, X-ray diffraction (XRD), and liquid-
and solid-state NMR spectroscopy. Selective 15N- and 13C-
INTRODUCTION
■
It has been widely accepted that enantiomers of the same chiral
compound may show quite different bioactivities, as well as
different physical, chemical, pharmacological, and toxicological
properties. Therefore, their preparation and especially analysis
are important in many fields of science. To obtain a chiral
heterogeneous selector/catalyst, usually the porous substrates
are coated or immobilized with proteins, enzymes, oligosac-
charides, or polysaccharides. Because important progress has
been made in the field of porous materials through the
development of metal−organic frameworks (MOFs), this
material class is also considered to be very promising for the
design and development of a new generation of chiral porous
materials. The incorporation of chiral groups by ligand or
cluster functionalization or the synthesis of chiral MOFs from
achiral building blocks generates homochiral materials. Inspired
by homogeneous chiral catalysis, the implementation of well-
established chiral catalytically active groups as linkers or as side
groups of the framework backbone has been reported for a
variety of MOFs.1−5 One common organocatalytic group for a
wide range of asymmetric reactions is proline. Up to now, there
are only a few homochiral MOFs with proline functionalities
reported. So far, the incorporation of proline could be
performed by coordination on open metal sites in MIL-101,6
postsynthetic click reaction,7 or postsynthetic amide coupling.8
Furthermore, the incorporation of proline by chiral function-
Special Issue: To Honor the Memory of Prof. John D. Corbett
Received: September 29, 2014
Published: December 9, 2014
© 2014 American Chemical Society
1003
dx.doi.org/10.1021/ic502380q | Inorg. Chem. 2015, 54, 1003−1009

Inorganic Chemistry
Article
Figure 1. (a) Linker used for the synthesis of DUT-32-NHProBoc. (b) Crystal structure of DUT-32-NHProBoc. (c) Simplified representation of
four pore types, highlighting bordering chiral ligand L2−.
isotope labeling of the chiral side group of DUT-32-NHProBoc
allows for comprehensive solid-state NMR spectroscopic
investigations. An enantioselective interaction between the
chiral MOF and the chiral guest molecule 1-phenyl-2,2,2-
trifluoroethanol (TFPE) could be detected, which results, for
example, in different mobilities for the two TFPE enantiomers.
In order to load DUT-32-NHProBoc with TFPE, an elaborate
exchange method was developed that enables fast and accurate
monitoring of the solvent exchange in order to ensure complete
replacement of N,N-dimethylformamide (DMF) by TFPE.
of Petushok et al.10 with slight modifications. The introduction
of the chiral group was performed by an amide coupling
reaction with the reagents N,N′-dicyclohexylcarbodiimide
(DCC) and hydroxybenzotriazole (HOBt). The proline
derivative N-(tert-butoxycarbonyl)-L-proline (Boc-Pro-OH)
was used as an enantiopure compound obtained from a
commercial source. Furthermore, the insertion of isotope-
labeled L-proline was carried out by the synthesis of the Boc-
Pro*-OH compound using L-proline-13C5,15N, giving H2L*.
The chiral MOF (further denoted as DUT-32-NHProBoc)
was synthesized using reaction conditions similar to those
applied by Grunker et al.11 for the synthesis of DUT-32. A
̈
RESULTS AND DISCUSSION
■
mixture of the trigonal ligand H3btctb and the linear ligand H2L
or H2L* in N,N-diethylformamide (DEF) was heated up to 100
°C for 48 h, giving rod-shaped crystals.
DUT-32-NHProBoc Synthesis and Characterization.
The highly porous MOF compound denoted as DUT-32
exhibits four different pores, including mesopores and micro-
pores with dimensions of 30 × 40, 28 × 32, 20 × 26, and 14 ×
18 Å (considering van der Waals radii of the atoms; Figure
1b).11 The high porosity and open character of the framework
are beneficial for the desired catalytic application. The structure
contains Zn4O clusters as well as two types of linkers: tritopic
4,4′,4′-[benzene-1,3,5-triyltris(carbonylimino)]trisbenzoate
(btctb3−) and ditopic 4,4′-biphenyldicarboxylate (bpdc2−). The
overall framework composition is given by the formula
Zn4O(btctb)4/3(bpdc).
Analysis of the product by single-crystal and powder XRD
confirmed the formation of a structure with a network
isoreticular to DUT-32. The crystal structure of DUT-32-
NHProBoc was determined using a combination of single-
crystal XRD and modeling. The unit cell parameters as well as
the space group were determined from the single-crystal XRD
data. Because of the homochirality of the framework, the P63
space group was chosen for the structure solution. Unfortu-
nately, only the atoms belonging to the DUT-32 backbone
could be localized from the diffraction data; therefore, the
NHProBoc groups in the S configuration were modeled on
each bpdc2− linker in the structure using Material Studio 5.0
because 1H NMR studies of the material after mild
decomposition in DCl demonstrated preservation of the side
group of the linker H2L during MOF synthesis.
Functionalization of the ditopic linker with a chiral
NHProBoc group (synthesis of H2L) was performed in three
steps starting from dimethyl biphenyl-4,4′-dicarboxylate in
analogy to the procedure described by Telfer et al.9 (Scheme
1). First, the amino group was introduced following the method
In order to investigate preservation of the enantiomeric
purity of proline during linker and MOF synthesis, an HPLC
method with a stereoselective column was established [see the
Supporting Information (SI), section 8]. The ee-value of H2L
after three steps of linker synthesis was determined as 99%. To
examine the influence of thermal treatment on the ee-value of
the linker during MOF synthesis, the supernatant solution after
MOF synthesis was investigated. The measurements gave an ee-
value beyond 90% for H2L, indicating little influence of the
synthetic conditions on the stereogenic center of the linker
itself.
Scheme 1. Synthesis of Ligand H2L in Three Steps: (a)
HNO3, H2SO4, 0 °C, 30 min; (b) Fe, HOAc, 50 °C, 12 h; (c)
Boc-Pro-OH or Boc-Pro*-OH, HOBt, DCC, DCM, rt, 4
days; (d) aqueous KOH, THF, 50 °C, 19 h
To identify the temperature range, where racemization can
by induced thermally, further experiments were carried out. A
solution of the linker H2L in DMF (c = 0.013 mol L−1) was
heated at 100, 120, and 140 °C, and the enantiomeric purity
1004
dx.doi.org/10.1021/ic502380q | Inorg. Chem. 2015, 54, 1003−1009

Inorganic Chemistry
Article
was monitored by HPLC. The measurements of the solutions
at 100 and 120 °C after defined treatment times show only low
variations in the ee-value. Temperatures of 140 °C cause a
distinctive decrease of the ee-value after 1 day or longer (Figure
2). These results confirm the assumption that the ee-value of
enantiomers of TFPE are on the order of 0.1−0.6 ppm. The
most pronounced differences are detected at the chiral center of
L2− and at its α position (C atom positions 4 and 2). The
differences are less pronounced for the C atom positions 1 and
3.
This demonstrates the local and regioselective character of
the host−guest interactions present between DUT-32-NH-
(13C5,15N)ProBoc and TFPE. (ii) Remarkably, the line widths
also exhibit pronounced differences for the samples loaded with
the two different enantiomers, (S)- and (R)-TFPE. All signals
are considerably narrower for the (S)-TFPE-loaded sample
than the (R)-TFPE-loaded sample. This effect indicates a more
defined state and/or lower mobility (stronger interaction) for
the chiral proline side groups with (S)-TFPE-loaded DUT-32-
NH(13C5,15N)ProBoc.
To substantiate the latter conclusion further, CP built-up
curves were measured (see Figure 4). CP is sensitive to the
presence of motional processes.20 Indeed, a slightly faster CP
built-up curve and a slower CP decay curve are observed for all
13C CP MAS NMR signals of DUT-32-NH(13C5,15N)ProBoc
loaded with (S)-TFPE. This behavior also points toward a less
mobile state of the chiral side groups in the presence of (S)-
TFPE compared with their state in (R)-TFPE-loaded samples.
However, the observed differences are relatively small.
Therefore, a further experiment was performed. 1H-driven
13C−13C spin-diffusion (PDSD) spectra could be measured
because the 13C-labeled proline side groups provide sufficient
signal intensity (see Figure 5). For a mixing time of 10 ms, a
significant difference between the (S)- and (R)-TFPE-loaded
samples occurs: Detectable cross peaks between atom positions
4 and 5 (chiral center and CO group) appear for the (S)-
TFPE-loaded sample. In contrast, no such cross peaks occur for
the (R)-TFPE-loaded sample. This difference is an additional
indication for a less mobile chiral L-proline side group in the
(S)-TFPE-loaded DUT-32-NH(13C5,15N)ProBoc. The lower
degree of mobility gives rise to stronger magnetic dipole−
dipole interactions in the case of DUT-32-NH(13C5,15N)-
ProBoc + (S)-TFPE, resulting in observable cross peaks in the
13C−13C PDSD spectrum.
Figure 2. ee-value evolution of H2L determined by enantioselective
HPLC measurements after heating in DMF at 100, 120, and 140 °C.
the incorporated H2L linker should be unchanged under the
synthesis conditions chosen for DUT-32-NHProBoc (c = 0.013
mol L−1, 100 °C, DEF).
Recently, a novel method for investigations of chirality in
MOFs by solid-state NMR spectroscopy was developed, based
on sample loading with chiral shift agents.12 Interactions of
enantiomerically pure shift agents with substrate isomers may
induce nonequivalent diastereomeric complexes depending on
the chirality of the substrates.13−19 The new method was
applied to DUT-32-NHProBoc, and the chiral solvating agent
TFPE was used. The 13C cross-polarization magic-angle-
spinning (CP MAS) NMR spectra of DUT-32-NH(13C5,
15N)ProBoc loaded with (S)- and (R)-TFPE (Figure 3) exhibit
two characteristic differences: (i) In analogy to previous
observations made on chirally modified UMCM-1,12 enantio-
selectively induced 13C chemical shifts are observable. The
chemical shift differences induced by the two different
Figure 3. 13C CP MAS NMR spectra of DUT-32-NH(13C5,15N)ProBoc loaded with (S)- and (R)-TFPE.
1005
dx.doi.org/10.1021/ic502380q | Inorg. Chem. 2015, 54, 1003−1009

Inorganic Chemistry
Article
Figure 5. 1H-driven 13C−13C PDSD spectra of DUT-32-NH-
(13C5,15N)ProBoc loaded with (S)- and (R)-TFPE.
Because the porosity and accessibility of the pores for
substrates in solution is of great importance for envisioned
applications, liquid-phase adsorption experiments were per-
formed. To probe the accessibility of the network for large
substrates, the dye adsorption experiments were carried out
using Nile Blue and Reichardt’s dye as probe molecules in
dichloromethane (DCM) as the solvent. Both compounds can
be adsorbed on DUT-32-NHProBoc (see the SI, section 2). To
quantify the specific pore volume, the procedure of Henschel et
al.21 determining the adsorption capacity of ethyl cinnamate in
n-heptane was applied to the DUT-32 and DUT-32-NHProBoc
materials and the adsorbed amounts were compared to the
uptake published for other MOFs. DUT-32-NHProBoc is able
to adsorb 610 mg g−1 ethyl cinnamate. This is comparable with
the uptake measured for DUT-32 of 447 mg g−1 (Figure 6).
Despite the lower pore volume, the chiral framework shows a
higher adsorption capacity than DUT-32, indicating additional
interactions of the chiral side groups of L2− with ethyl
cinnamate molecules. The effect is comparable to the effect
arising from additional interactions of the substrate with open
metal sites of the cluster in MIL-101 or Pd@MIL-101.
For catalytic applications, it is necessary to remove the
protecting Boc group from DUT-32-NHProBoc. Usually
deprotection can be performed acidolytically or thermally.
Because the Zn-based MOFs are not acid-tolerant, thermal
treatment was chosen as the deprotection method. Thermog-
ravimetric analysis of H2L indicates decomposition of the Boc
group at about 120 °C and decomposition of the DUT-32-
NHProBoc framework at temperatures beyond 300 °C (see the
SI). Therefore, a temperature range of 120−170 °C was tested
for deprotection of the material (see the SI, Table S1). Similar
to the method of Telfer et al.9 used for deprotection of
Figure 4. 1H−13C CP built-up curves of DUT-32-NH(13C5,15N)-
ProBoc loaded with (S)- and (R)-TFPE. tCP denotes the CP contact
time.
Porosity and Catalytic Activity. The pore sizes in the
DUT-32-NHProBoc framework in comparison to those of
DUT-32 are considerably influenced by the flexible proline
groups of the L2− because these groups protrude into the pores
of the structure. The amount of chiral groups per pore can vary
because of the flexibility of the side groups. Actually, each pore
(except for the smallest micropore) can comprise the chiral
group (Figure 1c). This ensures interaction of the side groups
with potential substrates in the pores, which is necessary for the
stereoselective properties of the material.
To measure the porosity of DUT-32-NHProBoc using N2
gas as the probe, the activation procedure optimized for DUT-
32 reported by Grunker et al.11 was applied to the chiral-
̈
functionalized material DUT-32-NHProBoc. The synthesis
solvent was exchanged to acetone or amyl acetate prior to
supercritical drying. The samples were soaked in liquid CO2
over 7 days (see the SI). Unfortunately, drying led to a loss of
crystallinity and low nitrogen uptake of about 150 cm3 g−1 for
the acetone-exchanged sample and 190 cm3 g−1 for the amyl
acetate exchanged sample at 77 K (see the SI, Figure S3).
Theoretical calculations show that the introduction of chiral
groups into DUT-32 should not lead to a significant decrease of
the porosity (ca. 14%). The theoretical pore volume of DUT-
32-NHProBoc is 2.76 cm3 g−1.
1006
dx.doi.org/10.1021/ic502380q | Inorg. Chem. 2015, 54, 1003−1009

Inorganic Chemistry
Article
anti adducts. These results additionally indicate that race-
mization occurs during the deprotection process of DUT-32-
NHProBoc.
CONCLUSIONS
■
A chiral MOF DUT-32NHProBoc could be synthesized using a
Boc-proline-functionalized linker. The structure shows a high
capacity in liquid-phase adsorption with ethyl cinnamate,
confirming the high porosity of the framework comparable to
that of achiral DUT-32. Stereoselective HPLC experiments
confirmed the linker and MOF synthesis while the chirality was
retained at the stereogenic center. Investigations on depro-
tection of the Boc group showed racemization effects due to
high temperatures during the reaction. An asymmetric aldol
reaction was used to verify these assumptions. Modification of
the linker synthesis using isotope-labeled Boc−proline gave a
13C5,15N-marked linker for MOF synthesis. In order to
investigate the interaction of chiral-isotope-labeled functional
groups in DUT-32-NHProBoc with chiral guest molecules, the
enantiomers of the chiral shift reagent TFPE were separately
infiltrated and examined by solid-state NMR methods, using
13C CP MAS NMR, CP built-up curves, and spin-diffusion
experiments. In summary, these solid-state NMR studies are
pointing toward a low degree of mobility of the chiral side
groups in (S)-TFPE-loaded samples, i.e., a preferred interaction
of (S)-TFPE with the functional groups of the MOF inner
surface, demonstrating the value and variable applicability of
this monitoring technique.
Figure 6. Adsorption of ethyl cinnamate in n-heptane on selected
MOFs (reported by Henschel et al.21) and on DUT-32 and DUT-32-
NHProBoc. [The total pore volume Vp for both DUT-32 systems was
calculated theoretically (see the SI).]
IRMOF-ProBoc, the Boc group of DUT-32-NHProBoc was
tested to be removed by microwave heating in DMF without
acidic or basic additives (see the SI, Table S1, trial 2). However,
1
the H NMR studies show just a low degree of deprotection
(about 30%), and structural changes caused by this treatment
were observed by powder XRD. Further deprotection
investigations were performed using conventional thermal
treatment. An increasing deprotection degree of chiral linker
L2− was found by increasing the temperature or thermal
treatment time in DMF and also by changing the structure of
the crystalline phase. Therefore, mesitylene was tested as an
alternative solvent. Using mesitylene, no change in the structure
was observed. Furthermore, an increasing degree of depro-
tection with increasing temperature could be achieved. The
optimum deprotection conditions to obtain pure DUT-32-
NHPro are heating at 170 °C for 72 h in mesitylene, which was
confirmed by 1H NMR studies and powder XRD measurement
(see the SI, Figures S6 and S7). Keeping in mind that
racemization of the protected linker H2L occurs at temper-
atures higher than 140 °C (see Figure 2), it is evident that the
enantiomeric purity of the MOF might be affected under such
deprotection conditions.
To test the enantioselectivity and catalytic activity of
deprotected DUT-32-NHPro, asymmetric aldol addition
between 4-nitrobenzaldehyde and cyclohexanone was per-
formed as a test reaction (Scheme 2). The reaction process was
monitored by qualitative chiral HPLC measurement (see the
SI). A filtration test (after 48 h of reaction time) suggests that
DUT-32-NHPro is a true heterogeneous catalyst for aldol
addition. Unfortunately, the material catalyzed the aldol
reaction with no specific stereoselectivity between syn and
EXPERIMENTAL SECTION
General Remarks. If not stated else, the reactions were carried out
under air. Chemicals were used as received without further
preparation.
■
Synthesis of Dimethyl-2-nitro-1,1′-biphenyl-4,4′-dicarboxy-
late (2a). To a solution of concentrated sulfuric acid (820 mL) was
added slowly dimethyl-1,1′-biphenyl-4,4′-dicarboxylate (76 g, 0.28
mol). Under stirring, the solution was cooled to −20 °C. Separately
nitration acid was prepared by mixing a nitric acid solution (26.9 g, 65
wt %) with concentrated sulfuric acid (120 mL) at 0 °C. The cooled
nitration acid was added to a precursor solution with a dropping
funnel in a time range of 3 h. The reaction temperature was kept under
−10 °C and stirred for 1 h after the complete addition of nitration
acid. The reaction was interrupted by mixing of the solution with 1 L
of ice water. The resulting white solid was dissolved in DCM, and the
aqueous phase was extracted by DCM several times. Washing of the
combined organic layers with saturated aqueous sodium hydrogen
carbonate, drying over magnesium carbonate, and evaporation under
reduced pressure gave a red solid. Purification by recrystallization in a
1:1 mixture of ethyl acetate and n-hexane completed the synthesis of a
white solid. Yield: 65 g (0.2 mol, 89%). 1H NMR (500 MHz, CDCl3):
3
3
δ (ppm) = 3.88 (s, 3H), 3.94 (s, 3H), 7.55 (dd, J1 = 1.89 Hz, J2 =
8.51 Hz, 2H), 7.76 (d, 3J1 = 7.88 Hz, 1H), 8.05 (dd, 3J1 = 1.89 Hz, 3J2
= 8.51 Hz, 2H), 8.30 (dd, J1 = 1.57 Hz, J2 = 9.77 Hz, 1H), 8.50 (d,
3J1 = 1.57 Hz, 1H).
3
3
Scheme 2. Asymmetric Aldol Addition between
Synthesis of Dimethyl-2-amino-1,1′-biphenyl-4,4′-dicarbox-
ylate (2b). 2a (10.3 g, 0.03 mol) was dissolved in 300 mL of acetic
acid (HOAc). To the solution was added iron powder (18.2 g, 0.32
mol, 11 equiv), and the mixture was stirred at 50 °C for 14 h. The
suspension was filtered, and the filtrate was partly dried in vacuum.
After washing with saturated aqueous sodium hydrogen carbonate, the
solution was extracted with DCM. The combined organic layers were
dried under vacuum and recrystallized in DCM for purification. Yield:
Cyclohexanone and 4-Nitrobenzaldehyde Catalyzed by
DUT-32-NHPro with 20 mol % Active Proline Side Groups
1
8.48 g (29.7 mmol, 91%). H NMR (500 MHz, DMSO-d6): δ (ppm)
= 3.83 (s, 3H), 3.87 (s, 3H), 5.25 (s, 2H), 7.13 (d, 3J1 = 7.88 Hz, 1H),
3
3
3
7.21 (dd, J1 = 1.89 Hz, J2 = 9.77 Hz, 1H), 7.44 (d, J1 = 1.89 Hz,
3
3
1H), 7.60 (dd, J1 = 1.89 Hz, J2 = 8.51 Hz, 4H).
1007
dx.doi.org/10.1021/ic502380q | Inorg. Chem. 2015, 54, 1003−1009

Inorganic Chemistry
Article
Synthesis of (S)-Me2L (3). 2b (542 mg, 1.90 mmol), HOBt (282
mg, 2.09 mmol, 1.1 equiv), and Boc-Pro-OH (465 mg, 2.16 mmol, 1.1
equiv) were suspended in 14 mL of anhydrous DCM and stirred at
room temperature in an argon flow. Afterward, the suspension was
cooled to 0 °C. Separately a solution of DCC (431 mg, 2.09 mmol, 1.1
equiv) in 5 mL of anhydrous DCM was prepared and added slowly to
the cooled reaction in a time range of 1 h. The reaction was heated to
room temperature and stirred for 4 days under an argon atmosphere.
The resulting suspension was filtered to remove a white solid. The
solvent was removed by evaporation under reduced pressure. Flash
chromatography of the raw product in ethyl acetate and hexane (1:1)
and drying under high vacuum afforded a pale-yellow solid. Yield: 825
mg (1.71 mmol, 90%). 1H NMR (500 MHz, CDCl3): δ (ppm) = 1.35
(s, 9H), 1.87−2.04 (m, 2H), 2.07−2.49 (m, 2H), 3.11−3.61 (m, 2H),
ASSOCIATED CONTENT
* Supporting Information
Additional X-ray data, HPLC spectra, physisorption isotherm,
thermal analysis, NMR details, and general procedures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
*E-mail: eike.brunner@tu-dresden.de. Tel: +49 351 463-32631.
Fax: +49 351 463-37188.
*E-mail: stefan.kaskel@chemie.tu-dresden.de. Tel: +49 351
463-33632. Fax: +49 351 463-37287.
■
3
3.95 (s, 3H), 3.97 (s, 3H), 4.23−4.41 (m, 1H), 7.33 (d, J1 = 7.9 Hz,
1H), 7.44 (d, 3J1 = 7.9 Hz, 2H), 7.89 (d, 3J1 = 7.9 Hz, 1H), 8.17 (d, 3J1
= 78.2 Hz, 2H), 8.92 (s, 1H).
Present Address
§U.S.: IFW Dresden, Helmholtzstraße 20, D-01069 Dresden,
Germany.
Synthesis of (S)-H2L. (S)-Me2L (825 mg, 1.71 mmol) was
dissolved in 19 mL of tetrahydrofuran (THF), and a 1 M aqueous
solution of KOH was added under stirring. The emulsion was stirred
at 50 °C overnight. The organic solvent was removed under reduced
pressure, and the resulting aqueous solution was cooled to 0 °C. The
addition of aqueous 1 M HCl to set a pH value of 4 afforded a white
precipitate, which was filtered and washed with water and ethanol. The
product was isolated by drying in high vacuum. Yield: 684 mg (1.50
mmol, 88%). 1H NMR (500 MHz, DMSO-d6): δ (ppm) = 1.30−1.40
(m, 9H), 1.62−1.80 (m, 3H), 1.97−2.11 (m, 1H), 3.20−3.32 (m, 2H),
4.09−4.18 (m, 1H), 7.47−7.53 (m, 2H), 7.53−7.60 (m, 1H), 7.82−
7.89 (m, 1H), 7.98−8.04 (m, 2H), 8.14 (br d, 3J = 37.2 Hz, 1H), 9.47
(br d, 3J = 74.4 Hz, 1H), 13.08 (br s, 2H). For HPLC, separation and
evaluation of the chiral ligand H2L was carried out with the following
parameters: mobile phase, 4:1 methanol/water (+0.05% trifluoroacetic
acid); flow, 0.2 mL min−1 [tR(S enantiomer) = 23.3 min; tR(R
enantiomer) = 21.6 min].
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors gratefully acknowledge financial support from the
Deutsche Forschungsgemeinschaft within the priority program
■
SPP 1362 “Porose metallorganische Gerustverbindungen”
̈
̈
(Grants KA 1698/15-2 and BR 1278/20-2).
REFERENCES
■
(1) Bradshaw, D.; Claridge, J. B.; Cussen, E. J.; Prior, T. J.;
Rosseinsky, M. J. Acc. Chem. Res. 2005, 38, 273−282.
(2) Yoon, M.; Srirambalaji, R.; Kim, K. Chem. Rev. 2012, 112, 1196−
1231.
(3) Liu, Y.; Xuan, W.; Cui, Y. Adv. Mater. 2010, 22, 4112−4135.
(4) Gedrich, K.; Senkovska, I.; Baburin, I. A.; Mueller, U.; Trapp, O.;
Kaskel, S. Inorg. Chem. 2010, 49, 4440−4446.
Synthesis of H3btctb. In an argon atmosphere, a solution of 4-
aminobenzoic acid (49.4 g, 0.36 mol, 3.8 equiv in 600 mL of
anhydrous acetone) was prepared. Dried potassium carbonate (45.4 g,
0.33 mol, 3.5 equiv) was suspended in the educt solution and rinsed
with 50 mL of anhydrous acetone. Separately, 1,3,5-benzenetricarbonyl
trichloride (25.0 g, 0.09 mol) was dissolved in 45 mL of anhydrous
acetone and added slowly to the educt suspension. Afterward, the
suspension was stirred at 80 °C under reflux conditions for 16 h. The
resulting yellow solid was filtered, washed with acetone, and stirred in
1 L of a 1 M aqueous HCl solution for 2 h. The solid was filtered and
washed with water and acetone. For further purification, the solid was
recrystallized in THF and dried in high vacuum. Yield: 49.5 g (0.09
(5) Gedrich, K.; Heitbaum, M.; Notzon, A.; Senkovska, I.; Frohlich,
̈
R.; Getzschmann, J.; Mueller, U.; Glorius, F.; Kaskel, S. Chem.Eur. J.
2011, 17, 2099−2106.
(6) Banerjee, M.; Das, S.; Yoon, M.; Choi, H. J.; Hyun, M. H.; Park,
S. M.; Seo, G.; Kim, K. J. Am. Chem. Soc. 2009, 131, 7524−7525.
(7) Zhu, W.; He, C.; Wu, X.; Duan, C. Inorg. Chem. Commun. 2014,
39, 83−85.
(8) Lili, L.; Xin, Z.; Shumin, R.; Ying, Y.; Xiaoping, D.; Jinsen, G.;
Chunming, X.; Jing, H. RSC Adv. 2014, 4, 13093.
(9) Lun, D. J.; Waterhouse, G. I. N.; Telfer, S. G. J. Am. Chem. Soc.
2011, 133, 5806−5809.
1
mol, 93%). H NMR (500 MHz, DMSO-d6): δ (ppm) = 7.96−8.04
(m, 12H), 8.79 (s, 3H), 11.0 (s, 3H).
(10) Olkhovik, V. K.; Vasilevskii, D. A.; Pap, A. A.; Kalechyts, G. V.;
Martveienko, Y. V.; Baran, A. G.; Halinouski, N. A.; Petushok, V. G.
Arkivoc 2008, ix, 69−93.
Synthesis of DUT-32-NHProBoc. Zn(NO3)2·4H2O (119 mg,
0.46 mmol, 4.6 equiv), H3btctb (49 mg, 0.09 mmol), and (S)-H2L (58
mg, 0.13 mmol, 1.5 equiv) were dissolved in 9.5 mL of DEF. The
mixture was thermally treated at 100 °C for 48 h, affording large
transparent rod-shaped crystals. These crystals were washed with DMF
(3 × 10 mL). To activate DUT-32-NHProBoc solvent exchange from
DMF in acetone or amyl acetate and additionally, exchange with CO2
was performed, followed by supercritical drying. For liquid-phase
adsorption experiments, the solvent DMF was exchanged by EtOH (5
× 10 mL), followed by n-heptane (5 × 10 mL) exchange. Mass
determinations of the samples for liquid-phase adsorption were carried
out by the pycnometer method using the crystallographic density of
the crystals and solvents (see the SI). Yield: 82.4 mg (78% referred to
the amount of H3btctb). Elem anal. Calcd for {Zn4O[(L)(btctb)4/3]}
[(C64H48N6O20Zn4)·5H2O]: C, 48.88; H, 3.72; N, 5.34. Found: C,
48.43; H, 3.82; N, 5.03. IR (cm−1): 3311 (br), 3071 (br), 2976 (br),
1932 (w), 1675 (m), 1605 (s), 1524 (s), 1401 (s), 1312 (m), 1248
(m), 1177 (w), 1157 (w), 1121 (w), 1089 (w), 1015 (w), 1007 (w),
955 (w), 913 (w), 859 (m), 780 (m), 729 (w), 698 (w), 633 (w), 531
(w), 504 (w), 446 (w).
(11) Grunker, R.; Bon, V.; Muller, P.; Stoeck, U.; Krause, S.; Mueller,
̈
̈
U.; Senkovska, I.; Kaskel, S. Chem. Commun. 2014, 50, 3450−3452.
(12) Hoffmann, H. C.; Paach, S.; Muller, P.; Senkovska, I.;
̈
Padmanaban, M.; Glorius, F.; Kaskel, S.; Brunner, E. Chem. Commun.
2012, 48, 10484−10486.
(13) Parker, D. Chem. Rev. 1991, 91, 1441−1457.
(14) Wenzel, T. J. Discrimination of Chiral Compounds Using NMR
Spectroscopy; Wiley: Hoboken, NJ, 2007.
(15) Wenzel, T. J.; Chisholm, C. D. Prog. Nucl. Magn. Reson.
Spectrosc. 2011, 59, 1−63.
(16) Wenzel, T. J.; Chisholm, C. D. Chirality 2011, 23, 190−214.
(17) Wenzel, T. J.; Wilcox, J. D. Chirality 2003, 15, 256−270.
(18) Lesot, P.; Lafon, O.; Zimmermann, H.; Luz, Z. J. Am. Chem. Soc.
2008, 130, 8754−8761.
(19) Aroulanda, C.; Zimmermann, H.; Luz, Z.; Lesot, P. J. Chem.
Phys. 2011, 134, 134502.
(20) Schulze, D.; Ernst, H.; Fenzke, D.; Meiler, W.; Pfeifer, H. J. Phys.
Chem. 1990, 94, 3499−3502.
1008
dx.doi.org/10.1021/ic502380q | Inorg. Chem. 2015, 54, 1003−1009

Inorganic Chemistry
Article
(21) Henschel, A.; Senkovska, I.; Kaskel, S. Adsorption 2011, 17,
219−226.
1009
dx.doi.org/10.1021/ic502380q | Inorg. Chem. 2015, 54, 1003−1009