A New Set of Isoreticular, Homochiral Metal-Organic Frameworks with ucp Topology

Article abstract of DOI:10.1021/acs.chemmater.5b03723

A new isoreticular series of metal-organic frameworks, called UHM-25 (UHM: University of Hamburg Materials), based on the copper paddle wheel motif and a novel set of homochiral linkers has been synthesized. Starting from amino acids available from the chiral pool a synthesis procedure was established that allows a straightforward, multigram scale synthesis of homochiral linkers in 4-5 steps. These linkers carry substituents that have been proven useful in stereoselective organic chemistry, such as the Evans auxiliary or chiral amino alcohols. The resulting MOFs only differ in the chiral moiety provided by the amino acid starting material. The structure of UHM-25 is composed of cuboctahedral cages of Cu₂ paddle wheel motifs connected by the isophthalate moieties of the linker. These cages are linked via the bent backbone of the linker resulting in a primitive cubic arrangement, giving rise to the rare underlying (3,4)-c binodal net ucp. MOFs of the UHM-25 series show surface areas up to S_BET = 1900 m²/g. Postsynthetic modification reactions with excellent conversion rates confirmed the accessibility to the chiral groups. Furthermore, UHM-25-Pro bearing a prolinol functionality was used in a self-directed, enantioselective aldol addition of acetaldehyde, demonstrating the potential of the UHM-25 series with regard to heterogeneous, stereoselective catalysis.

Full text of DOI:10.1021/acs.chemmater.5b03723

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Article
A New Set of Isoreticular, Homochiral Metal-
Organic Frameworks with UCP Topology
Michael Sartor, Timo Stein, Frank Hoffmann, and Michael Fröba
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03723 • Publication Date (Web): 17 Dec 2015
Downloaded from http://pubs.acs.org on December 20, 2015
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Chemistry of Materials
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A New Set of Isoreticular, Homochiral Metal-Organic Frame-
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Michael Sartor, Timo Stein, Frank Hoffmann, and Michael Fröba*
Institute of Inorganic and Applied Chemistry, Department of Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-
2
0146 Hamburg, Germany
Metal-Organic Frameworks, Homochirality, Amino Acids, Enantioselective Catalysis
ABSTRACT: A new isoreticular series of metal–organic frameworks, called UHM-25 (UHM: University of Hamburg Materials), based on
the copper paddle wheel motif and a novel set of homochiral linkers has been synthesized. Starting from amino acids available from the chiral
pool a synthesis procedure was established that allows a straightforward, multi-gram scale synthesis of homochiral linkers in 4–5 steps. These
linkers carry substituents that have been proven useful in stereoselective organic chemistry, such as the “Evans auxiliary” or chiral amino
alcohols. The resulting MOFs only differ in the chiral moiety provided by the amino acid starting material. The structure of UHM-25 is
composed of cuboctahedral cages of Cu paddle wheel motifs connected by the isophthalate moieties of the linker. These cages are linked via
2
the bent backbone of the linker resulting in a primitive cubic arrangement, giving rise to the underlying (3,4)-c binodal net ucp, which was
2
hitherto only theoretically described. MOFs of the UHM-25 series show surface areas up to SBET = 1900 m /g. Post-synthetic modification
reactions with excellent conversion rates confirmed the accessibility to the chiral groups. Furthermore, UHM-25-Pro − bearing a prolinol
functionality − was used in a self-directed, enantioselective aldol addition of acetaldehyde demonstrating the potential of the UHM-25 series
with regard to heterogeneous, stereoselective catalysis.
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INTRODUCTION
iary ligands to the inorganic building unit of the MOF.
Other
approaches to synthesize chiral MOFs employ linker molecules
The inversion of the absolute configuration of a useful pharma-
ceutical and its metabolites can render them useless or even harm-
ful, which has led regulatory authorities to impose producers of
drugs on the requirement to provide stereochemically pure com-
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2,23,47–56
with axial chirality
or precursor molecules that are obtained
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from chiral pool starting materials.
Moreover, isoreticular approaches may be chosen by modifying a
linker that has been established in other MOFs with enantiopure
1
pounds, whenever possible. Therefore, the preparation of enanti-
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omerically pure compounds and the separation of racemic mixtures
play important roles in the pharmaceutical industry. Solid-
supported reagents that can provide so-called chiral information to
substrates are highly interesting to pharmaceutical
substituents, such as oxazolidinones or pyrrolidines.
MOFs
that include the latter may serve as asymmetric heterogeneous
organocatalysts in which secondary amines represent the catalyti-
cally active site. These heterogeneous systems show great resem-
blance to proline-based organocatalysts and considerable stereo-
chemical induction has been reported for these MOF-based sys-
However, in some cases only a general catalytic effect is
documented or a stereoselective catalysis is supposedly caused by
adsorbed proline species released from the MOF, which has been
pointed out by Canivet and Farrusseng. The proline-functional-
ized MOF DUT-32 in its Boc protected form was proven to be able
to interact stereospecifically with (S)-/(R)-1-phenyl-2,2,2-trifluo-
roethanol that was used a chiral shift agent in solid-state NMR
measurements, however, an asymmetric catalytic aldol test reaction
with the deprotected DUT-32-NHPro MOF showed no specific
stereoselectivity, probably because racemization occurred during
2,3
manufacturing.
Metal-organic frameworks (MOFs) have gained substantial sci-
entific interest over the last two decades and underwent an impres-
sive development resulting in a wide variety of possible applica-
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tems.
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catalytic applications.
Homochiral MOFs may be eligible for uses in stereoselective
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processes.
They can provide a very high density of accessible
functional groups that can serve for example as stereoselective
transition metal catalysts.
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Furthermore, the pore structure of
MOFs may provide a chiral environment that accounts for stere-
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oselectivity and asymmetric reactions may benefit from rigid
surroundings in the reaction step that defines the configuration of
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the deprotection process.
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In the field of organocatalysis the proline motif has been im-
proved by modifying the basic pyrrolidine structure with hydrogen
donors and/or bulky substituents on the exocyclic carbon atom of
the proline motif. One example of this improvement is the α,α-
diphenyl prolinol-system developed by Jørgenson and Hayashi.
the stereocenter.
To obtain homochiral MOFs, some approaches employ crystal-
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lization from chiral solvents
or in the presence of chiral
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additives
or they rely upon the addition of enantiopure auxil-
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So far, a α,α-diaryl prolinol organocatalyst has not been included in
a MOF.
Our approach to obtain homochiral MOFs uses molecules from
group as a hydrogen bond acceptor may be beneficial for chiral
recognition in MOFs.
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the chiral pool as starting materials and transforms them into tetra-
carboxylic acid linkers. We chose amino acids for three reasons.
First, they provide a carboxylic carbon atom that can serve as an
electrophilic target for addition-elimination reactions. Second,
numerous amino acids with a broad variety concerning their steri-
cal properties and functional groups, are readily available in nature
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or can be easily synthesized. Last, the nitrogen atom, inherent to
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amino acids, enables these compounds and their derivatives to act
as stereoselective organocatalysts by providing a functional group
that can bind substrates via enamine or imine bonds, thereby teth-
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ering the substrate in a chiral environment.
This is for example
the case in reactions catalyzed by the α,α-diaryl prolinol system.
Here, we present a reaction pathway for new homochiral MOFs
that employ diisophthalate linkers, which contain chiral amino acid
substituents. This pathway yielded UHM-25, a new series of cop-
per(II)-based, isoreticular MOFs. Materials from this UHM-25
series have been evaluated regarding their potential for post-
synthetic modification (PSM) and enantioselective organocataly-
sis.
RESULTS AND DISCUSSION
Syntheses of the Linkers. A general approach to obtain
the desired organic building units on a multi-gram scale was devel-
oped. The reagents from the chiral pool were modified to accom-
modate two aryl halides that offer the possibility of subsequent
cross-coupling reactions to furnish tetracarboxylate linkers.
Based upon their individual molecular structure, the linker mole-
cules used herein can be assigned to three types shown in Figure 1.
Type 1 includes N-Boc (Boc: tert-butyloxycarbonyl) protected
amino alcohol linkers, whereas type 2 comprises free amino alcohol
linkers. Linker molecules featuring the Evans auxiliary motif are
assigned to type 3. In the following, the reaction sequence for the
synthesis of tetracarboxylic acid linkers starting from L-alanine is
given. This procedure is representative for the preparation of the
linkers used for preparation of the UHM-25 series. Detailed reac-
tion schemes are given in the Supporting Information (SI).
Figure 1. Overview of the different types of linkers used in the UHM-
25 MOFs. Linkers and MOFs are named with the three-letter code of
the respective amino acid used for the preparation of the linker, ta
stands for tetracarboxylic acid. Boc appendix indicates the presence of
the N-Boc protecting group (dashed lines mark the connection points
to the framework host).
Following Scheme 1, the carboxylic acid functional group of L-
alanine was subjected to esterification and the amino group was
subsequently protected with di-tert-butyl dicarbonate (Boc O) to
2
give compound 2a. An excess of 4-bromophenyllithium (prepared
from 1,4-dibromobenzene and n-butyllithium) was reacted with
the N-Boc protected methyl ester, resulting in the N-Boc protected
amino alcohol 3a.
In the next step, a Suzuki-Miyaura cross-coupling was performed
between the α,α-diaryl amino alcohol 3a and two equivalents of 3,5-
di(methoxycarbonyl)phenylboronic acid. This reaction, shown in
Scheme 2, provided the tetramethyl ester 4a.
The tetramethyl ester 4a was saponificated in a mixture of aque-
ous potassium hydroxide solution and tetrahydrofuran (THF). The
reaction product was precipitated under mild acidic conditions.
This furnished the first type of linker (5a) bearing an N-Boc pro-
tecting group. N-Boc groups can be cleaved thermolytically, thus
Scheme 1. Conversion of L-alanine to its protected derivative 2a
and further reaction to the α,α-diaryl substituted, N-Boc protected
tertiary amino alcohol 3a.
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releasing the free amine after the MOF synthesis. This strategy
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can be used to prevent interpenetration or may allow MOF syn-
theses where free amino groups would otherwise inhibit the for-
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NMR spectroscopy, mass spectrometry and optical rotation meas-
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Scheme 2. Suzuki-Miyaura cross-coupling reaction of the α,α-di(4-
bromophenyl) substituted, protected L-alaninol 3a to synthesize
the tetramethyl ester of the linker molecule 4a.
To obtain the second type of linkers the N-Boc protected tetra-
carboxylic acids were deprotected using a mixture of trifluoroacetic
acid (TFA) in dichloromethane according to Scheme 3. The result-
ing amine was precipitated as the corresponding TFA adduct 6a.
This salt was later used in the synthesis of a MOF without further
purification.
Figure 2. Powder X-ray diffractograms for all MOFs of the UHM-25
series. Structures of the chiral substituents are given next to the diffrac-
tograms (dashed lines mark the connection points to the framework
host).
Scheme 3. Deprotection procedure for the primary amino alcohol
4
a: First, tetramethyl esters were saponificated to give 5a, then the
UHM-25-Ala-Boc crystallizes in the cubic space group P432 (a = b
= c = 28.9416 Å). The tetracarboxylate linker serves as a 4-
coordinated organic building unit. Copper paddle wheel motifs
constitute square-planar inorganic secondary building units
amino group was released acidolytically to yield 6a.
The linker bearing the Evans-type auxiliary constitutes the third
type of linker (Evans-Val-ta). This type of linker was synthesized
similar to the linkers described above (see SI for details).
(SBUs). Due to the 120° angle between the carboxylate groups on
the isophthalate moieties, cuboctahedral metal-organic polyhedra
(MOP) with copper paddle wheel units being located at each
Synthesis and Structure of the MOFs. In a typical syn-
thesis of a UHM-25 series material, a solution of linker molecule in
N,N-dimethylformamide (DMF) was acidified with nitric acid, an
aqueous solution of copper(II) nitrate was added, and the reaction
mixture was placed in a screw-capped flask in an oven for 72 h at
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corner are formed as subunits in this MOF. These cuboctahedral
MOP build a primitive cubic arrangement and each MOP is con-
nected to every neighboring MOP via four linkers (see Figure 3).
Topological Considerations. Following the recommenda-
5
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tions of O'Keeffe and Yaghi, the branching points of the linker
were suitable for single-crystal X-ray diffraction experiments for
UHM-25-Ala-Boc and UHM-25-Pro (further details on the deter-
mination of the crystal structure are given in the SI). MOFs were
obtained as crystalline materials from each of the linkers depicted
in Figure 1. Powder X-ray diffraction data of the MOFs shows
phase purity and an isostructural relationship between the differ-
ently substituted MOFs (see Figure 2). To investigate and prove
the stability of the chiral linkers after the MOF synthesis, the
UHM-25 MOFs were digested in diluted hydrochloric acid. The
were explicitly included in the topological analysis. Accordingly, the
inorganic SBU constitutes a square-planar 4-c node and the linker
was considered as a set of two joint 3-c nodes instead as a single 4-c
node. The topology of the UHM-25 MOF series was classified with
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the ToposPro suite. The underlying topology of UHM-25 is the
binodal (3,4)-c net ucp, which has been theoretically described as a
86
possibility to connect cuboctahedral MOP by O’Keeffe and Yaghi
but has – to the best of our knowledge – not been found in a MOF
so far.
1
integrity of the reisolated linkers was independently verified by H
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Figure 3. a) Representative linker Ala-Boc-ta used in the synthesis of UHM-25 MOFs and its topological representation as a set of two connected
triangles, b) cuboctahedral MOP built from isophthalates and copper paddle wheels (the yellow sphere serves as a guide to the eye) c) cubic primitive
arrangement of the cuboctahedral MOP giving rise to the (3,4)-c binodal net ucp, here shown in its augmented version ucp-a; d) view of the crystal
structure of UHM-25-Ala-Boc along a crystallographic axis; the unit cell is indicated as a black square; for the linkers outside the unit cell only the
isophthalate moieties are shown; hydrogen atoms as well as any disorder were omitted for clarity.
Interconnected MOP like those observed in UHM-25 have
is present. Figure 4 shows the connectivity of neighboring cubocta-
hedra in UHM-25-Val.
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been described by either treating them as cuboctahedra
or as
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rhombic cuboctahedra. However, the treatment of the MOPs as
rhombic cuboctahedra neglects the role of the copper paddle wheel
as a 4-c node and leads to the description of an underlying 5-c net,
whereas neither the linker nor the paddle wheel show this connec-
tivity. Hence, Yaghi and O’Keeffe have proposed to treat such
Apart from different synthesis conditions (solvent, temperature),
the structural properties of the linkers are particularly decisive for
the formation of certain topologies. The zmj, zhc, and ucp topolo-
gies have been observed only, if V-shaped tetracarboxylate linkers
are used to build MOFs. Each of the three types of connections (a–
c) between the cuboctahedra requires the linker to adopt a certain
conformation with respect to the isophthalate moieties. These
conformations can be discerned by the torsion of the isophthalate
moieties and are depicted in Figure 5. First, for a connection in
manner (a), the isophthalates adopt a non-coplanar conformation
where both isophthalates adopt torsion angles of 90°. Second, for a
connection in manner (b), the isophthalates are in a coplanar ori-
entation (torsion angles: 0°, 0°). Third, for a connection in manner
(c) the isophthalates are in a non-coplanar orientation and adopt
torsion angles of 0° and 90°, respectively.
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systems as interconnected cuboctahedra. There are at least three
ways to connect two neighboring cuboctahedra:
(a) In an arrangement where two faces of the cuboctahedra are
pointing at each other.
(
b) In an arrangement where the corners of the cuboctahedra are
pointing at each other.
c) In an arrangement where a corner of one cuboctahedron is
(
pointing at the face of the neighboring cuboctahedron.
Figure 5. Three conformations of V-shaped diisophthalate linkers that
are required to interconnect MOP in a specific manner (see text); the
conformations are distinguished by the torsion angle of the two
isophthalate moieties, which is exemplarily highlighted in blue for
connection type (a).
The fact that only type (a) connections are observed in the
UHM-25 MOFs suggests that the necessary conformation of the
isophthalates is preferred over the other two arrangements shown
in Figure 5. This is possibly due to the hydroxy and chiral substitu-
ents at the tetrahedral carbon atom, inducing the observed confor-
mation of the biphenyl systems.
Figure 4. Interconnection of cuboctahedra in UHM-25-Val. a) Sche-
matic representation of the conformation adopted by the linker in the
MOF. b) Three-dimensional representation of the linker. c) Neighbor-
ing MOP in UHM-25-Val are connected only via their faces but not via
their corners.
The realization of MOFs with ucp topology is important with
respect of the so-called principle of minimal transitivity. It states
that the vast majority of MOFs and other related network com-
pounds build structures based on nets that have a minimum num-
ber of different kind of vertices p and for a particular value of p a
Three nets that represent interconnected cuboctahedra have
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been documented by Yaghi and O’Keeffe. In the zmj net, the
arrangements (a) and (b) can be observed. In the zhc net the ar-
rangements (a) and (c) are found. The ucp net of UHM-25 consti-
tutes the simplest of these nets as only the arrangement of type (a)
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minimum number of different kind of edges q. zhc (the net of
PCN-12, ref. 90) is one of the most complicated nets that has been
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realized so far, with pq = 89, and zmj (realized for instance in ref.
8) – being still a fairly complicated 4-nodal net – has the transitivi-
physisorption measurement for UHM-25-Val-Boc shows a typical
type I(b) isotherm and is depicted in Figure 7.
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ty pq = 45. Both nets have been known for a relatively long time.
However, the simplest net consisting of two differently coordinated
species (3-c and 4-c) to connect cuboctahedra consistent with the
principle of minimal transitivity (pq = 22), namely ucp, was real-
ized only now.
Figure 6 depicts the natural tiling of the ucp net, which aids the
understanding of the arrangement of the pores in UHM-25. How-
ever, it should be noted that this representation neglects the size
relationship between the different pore types.
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There are four distinct types of pores in MOFs of the UHM-25
series: First, the spherical volume inside the MOP with a diameter
of ~12 Å which is represented as a yellow tile in Figure 6. The space
framed by the “concave” sides of the linker forms a second set of
pores, represented by the red tiles, which are located on the edges
of the unit cell. These pores connect the MOP to each other. The
“
convex” sides of the linker form an octapodal pore at the center of
Figure 7. Nitrogen physisorption isotherm of activated UHM-25-Val-
Boc measured at 77 K.
the unit cell. These pores include the chiral substituents and are
represented by blue tiles. A fourth pore type connects the pores
with the chiral substituents to each other. The centroids of these
pores coincide with those of the faces of the unit cell. This type of
pore is represented as a green tile in Figure 6. The voids in the
UHM-25 MOFs form a three-dimensional, interconnected pore
system.
As a general trend, it can be observed that the MOFs bearing an
N-Boc protecting group show higher surface areas than their non-
protected counterparts.
In order to assess the thermal stability of the UHM-25 MOFs we
carried out thermogravimetric analysis in an Ar/O
2
atmosphere
(80:20) coupled with mass spectrometry (TGA-MS). Thermal
stability might be an importance aspect considering potential catal-
ysis reactions under non-ambient conditions. After evaporation of
remaining solvents (water/THF, m/z = 18/42) up to temperatures
of approx. 220 °C the decomposition of the unprotected UHM-25
MOFs proceeds in two consecutive steps between ~ 250–440 °C
(see SI for details). The cleavage of the Boc group from the respec-
tive N-protected MOFs occurs in a well-separated step that can be
clearly distinguished from the combustion of the MOF itself and
takes place at temperatures at around 180 °C, which can be moni-
tored by the characteristic isobutene cleavage product (m/z = 56).
Figure 6. Topological representation of UHM-25 as a natural tiling and
the underlying ucp net. Left: 2 × 2 × 2 supercell of the tiling. Right: Part
of the ucp net that is carried by its respective tiling; four-coordinated
nodes are blue, three-coordinated nodes are red. Only two of the four
types of tiles are shown for clarity. Yellow tiles represent the MOP, blue
tiles represent the pore space in which the amino acid substituents are
located.
As shown by Telfer and coworkers, it should be principally pos-
sible to cleave N-Boc protecting groups within MOFs by heating
7
0
the material to ~ 150 °C. Therefore, we applied similar conditions
to evaluate a possible N-Boc deprotection for the UHM-25 MOFs.
Unfortunately, and in contrast to Telfer’s experience, thermal
deprotection of the N-Boc protected UHM-25 MOFs resulted in
X-ray amorphous materials. However, please note, in order to
obtain deprotected MOFs, it was not necessary to perform the
synthesis with the N-Boc protected linkers. Unlike in Telfer’s find-
ings, it was no problem to synthesize the MOFs from the ammoni-
um salts of the linker molecules 6a–c. We did not observe any
implications on the synthesis of the UHM-25 MOFs out of linkers
containing unprotected amino groups.
Porosity and thermal analysis. In order to evaluate the
porosity of the UHM-25 MOF series, nitrogen physisorption
measurements were carried out. Activation of the MOFs involved a
stepwise solvent-exchange with DMF, THF, and amyl acetate
followed by supercritical drying with carbon dioxide. Even when
applying this mild activation procedure, satisfying specific surface
areas could not be inferred in all cases, indicating a limited robust-
ness of the framework with respect to complete solvent removal.
Post-synthetic modification and catalysis. Aiming at
realizing systems for heterogeneous, asymmetric applications, we
incorporated (a) a 1,3-oxazolidin-2-one (the so-called Evans auxil-
2ꢀ −1
However, a specific surface area of SBET = 1922 m g (calculated
from the adsorption branch and in the relative pressure interval
from 0.009 to 0.020) and a micropore volume of Vpore = 1.00
93,94
iary ) and (b) a α,α-diaryl prolinol residue into the organic build-
3
−1
cm ꢀg (calculated at p/p
0
= 0.20) could be achieved for UHM-25-
ing unit of the UHM-25 MOF platform. In the case of the oxazoli-
dinone, we want to establish a chiral auxiliary system in the solid
state. Consequently, we have performed a post-synthetic modifica-
tion that constitutes the first step of a typical reaction sequence
commonly associated with the Evans auxiliary in stereoselective
Val-Boc (for a full overview of the values for all samples, see SI),
2
−1
compared to a theoretical value of Stheor. = 2200 m ꢀg of a com-
pletely activated sample evaluated with the Atom Surface and
9
1
Volume tool of the Accelrys Materials Studio suite. The nitrogen
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Page 6 of 13
synthesis. Further reaction steps, such as the formation of a chiral hyde 9 to the respective acetal 10. The conversion of the aldehyde
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enolate are still under investigation.
facilitates work-up and analysis by decreasing the volatility of 9.
After evaporation of methanol, the resulting liquid reaction product
that was obtained with a yield of 45 % was analyzed with enantiose-
In order to use such a MOF as a solid-state Evans auxiliary, the
NH function of the carbamate has to be acylated. Post-synthetic
acylation is a straightforward process for MOF-bound amines.
18,34
lective GC/MS (H as the carrier gas, heptakis-(2,3-di-O-methyl-6-
2
O-tert-butyldimethylsilyl)-β-cyclodextrin on fused silica as the
stationary phase, see SI for details). The total ion chromatogram
(TIC) of this separation is shown in Figure 8. The absolute config-
uration of the reaction products was assigned according to litera-
ture. The (R)-enantiomer is the main product and the enantio-
meric ratio was determined to be 70:30.
However, amides are far less nucleophilic and thus less reactive
than amines and require activation before acylation can take place.
Common procedures to obtain acylated oxazolidinones in ho-
mogeneous syntheses such as deprotonation of the NH function
with n-butyllithium and the subsequent reaction with an acyl elec-
trophile or using lithium chloride as an acylation catalyst failed to
provide an acylated derivative of UHM-25-Val-Evans. However,
the acylation of the MOF-bound oxazolidinone was achieved by
9
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applying a procedure that uses 4-(dimethylamino)pyridine
95,96
(
DMAP) as a catalyst, see Scheme 4. DMAP reacts with propi-
onic anhydride and the resulting reagent serves as an efficient acyl
donor to the oxazolidinone in UHM-25-Val-Evans to give UHM-
2
5-Val-Evans-PSM. Stirring the reaction mixture at room tempera-
Scheme 5. Self-directed aldol addition of acetaldehyde catalyzed by
UHM-25-Pro and the subsequent conversion of 3-hydroxybutanal
(9) to 1,1-dimethoxy-3-hydroxybutane (10).
ture led to a 37 % N-acylation of the MOF-bound carbamate func-
tional groups within 24 hours. Extending the reaction to 72 h led to
conversion rates of 90 %. To determine the conversion rates, the
MOF samples were digested after modification using diluted hy-
drochloric acid to isolate the mixture of the linker Evans-Val-ta and
its N-acylated derivative, whose relative proportions were estimat-
1
ed by H NMR spectroscopy (for a detailed description, see SI).
To assert the chemical stability of the MOF, powder X-ray diffrac-
tograms of the MOF before and after the PSM were compared and
show retention of the crystal structure of the MOF (see SI for
details).
Figure 8. Total ion chromatogram from an enantioselective GC/MS
separation of the product from the self-directed aldol addition of acet-
aldehyde catalyzed by UHM-25-Pro and the subsequent conversion to
1
0.
Scheme 4. Post-synthetic acylation of UHM-25-Val-Evans with
propionic acid anhydride gave UHM-25-Val-Evans-PSM (the
diamond shape indicates the framework host of the chiral substitu-
ent).
To estimate the performance of the heterogeneous reaction es-
tablished with UHM-25-Pro, an experiment using (S)-1,1-diphe-
nyl-2-pyrrolidinemethanol was performed under homogeneous re-
action conditions. This led to the aldol product with an enantio-
meric ratio of 75:25, being slightly higher than the ratio of the
respective heterogeneous reaction.
Apart from the post-synthetic modification of UHM-25-Val-
Evans, we also wanted to assess the use of a UHM-25 MOF in
heterogeneous catalysis. We chose UHM-25-Pro, which includes a
α,α-diaryl prolinol motif. This group is known as a homogeneous
catalyst in aldol additions but has, to the best of our knowledge, not
The induction of stereochemical information in the reaction
product proves the chiral nature and the catalytic activity of UHM-
25-Pro. The observed enantiomeric ratios are lower than those for
76,97
been included in a MOF so far.
As a catalysis test reaction sys-
97
homogeneous systems documented in the literature. However,
tem we chose the self-directed aldol reaction of acetaldehyde. This
reaction is depicted in Scheme 5.
these reactions are highly dependent on the nature of the α,α-diaryl
substituents on the prolinol catalyst as well as solvent effects. Bulky,
electron-withdrawing α-aryls may stabilize important reaction in-
To prepare the MOF for the reaction, UHM-25-Pro was pre-
treated with THF, then with dichloromethane to replace the sol-
vent of the MOF synthesis with the solvent of the catalysis. Thus
treated UHM-25-Pro was suspended in dichloromethane and acet-
aldehyde was added to yield a 10 % (v/v) solution. The reaction
mixture was stirred at room temperature for five days. Afterwards,
UHM-25-Pro was removed by filtration and the filtrate was treated
with methanol and Amberlyst 15. This procedure converted alde-
9
8
termediates. Therefore, we see room for improvement for the
heterogeneous catalyst system of UHM-25-Pro by introducing
substituents that significantly reduce the electron density on the
geminal aromatic rings.
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CONCLUSIONS
Chemistry of Materials
ASSOCIATED CONTENT
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Amino acids are a valuable source of chiral information in the
synthesis of MOFs. The functional groups of the amino acids can
be retained during the steps necessary to construct a MOF. The
results presented above prove the success of a strategy based on
chiral pool starting materials to synthesize a series of isoreticular,
porous materials with accessible chiral substituents. The integrated
functionality in the materials can be used in a stereoselective reac-
tion. Compounds from the UHM-25 series are – to the best of our
knowledge – the first MOFs with a catalytically active α,α-diaryl
prolinol unit. Furthermore, UHM-25-Val-Evans provides an acces-
sible Evans auxiliary. In contrast to previously published MOFs in
which the auxiliary is bound to the framework via the nitrogen
Supporting Information. Experimental procedures and analytical data
for the organic intermediates. experimental procedures, nitrogen physi-
sorption isotherms and TGA-MS plots of the UHM-25 MOFs, crystal-
lographic information of UHM-25-Ala-Boc and UHM-25-Pro, TIC
and MS traces from enantio-GC/MS of the enantioselective reaction.
This material is available free of charge via the Internet at
http://pubs.acs.org. Crystallographic information is deposited at the
Cambridge Crystallographic Data Centre (CCDC) for UHM-25-Ala-
Boc (CCDC No. 1415562) and UHM-25-Pro (CCDC No. 1415563).
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AUTHOR INFORMATION
Corresponding Author
69
atom, the oxazolidinone moiety can be acylated in a post-synthe-
tic modification reaction. This could potentially be utilized in ste-
reoselective syntheses on solid supports.
*E-mail: froeba@chemie.uni-hamburg.de. Phone: +49-40-42838-
3101. Fax: +49-40-42838-6348.
ACKNOWLEDGEMENT
Finally, the geometry of the linker molecule allows the for-
mation of the simplest topology of interconnected cuboctahedral
MOP (ucp), which can be regarded as a confirmation of the validity
of the principle of minimal transitivity.
The authors thank Prof. Dr. Ulrich Behrens for helpful discus-
sions regarding the crystal structure determination of the UHM-25
9
9
MOFs. Figures 3 and 4 were produced with VESTA.
M. I.; Lee, J. S.; Gygi, D.; Howe, J. D.; Lee, K.; Darwish, T. A.; James,
M.; Peterson, V. K.; Teat, S. J.; Smit, B.; Neaton, J. B.; Long, J. R.;
Brown, C. M. Comprehensive Study of Carbon Dioxide Adsorption
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