A Perylene-Based Microporous Coordination Polymer Interacts Selectively with Electron-Poor Aromatics

Article abstract of DOI:10.1002/chem.201600526

The design, synthesis, and properties of the new microporous coordination polymer UMCM-310 are described. The unique electronic character of the perylene-based linker enables selective interaction with electron-poor aromatics leading to efficient separati

Full text of DOI:10.1002/chem.201600526

DOI: 10.1002/chem.201600526
Communication
&
Host–Guest Systems
A Perylene-Based Microporous Coordination Polymer Interacts
Selectively with Electron-Poor Aromatics
Ly D. Tran,[a] Jialiu Ma,[a] Antek G. Wong-Foy,[a] and Adam J. Matzger*[a, b]
selective separation of these compounds. As a secondary con-
Abstract: The design, synthesis, and properties of the
sideration, it is important to note that unlike typical silica-gel-
new microporous coordination polymer UMCM-310 are
based sorbents used in separations, the vast majority of MCPs
described. The unique electronic character of the pery-
employed as separation media contain relatively small pores
lene-based linker enables selective interaction with elec-
(less than 12 Š), which limits separations to low-molecular-
tron-poor aromatics leading to efficient separation of ni-
weight molecules. Herein, the new MCP UMCM-310 was de-
troaromatics. UMCM-310 possesses high surface area and
signed specifically to address these problems and to explore
large pore size and thus permits the separation of large
the potential of a ligand with unique electronic character to
organic molecules based on adsorption rather than size
affect a difficult class of separations.
exclusion.
Perylene, a polycyclic aromatic compound commonly found
in dye molecules and used as a donor in donor–acceptor com-
plexes, was chosen to construct UMCM-310.[5] Due to its large
Microporous coordination polymers (MCPs), a class of crystal-
line solids composed of metal clusters connected by organic
linkers, excel in a variety of applications, including gas sorp-
tion, separations, and catalysis due to their porosity, high sur-
face area, well-defined pore sizes/shapes, as well as tunable
chemistries. In particular, using MCPs as separation media for
liquid-phase separation has considerable potential for purifica-
tion of petrochemicals, structural isomers, and stereoisomers,
among other classes of adsorbates.[1] Such separations can be
strongly influenced by the presence and nature of coordi-
natively unsaturated metal sites,[2] sorbent pore size,[2a,3] and
linker electronic nature. For example, the separation of ethyl
benzene and styrene by using HKUST-1 as stationary phase is
remarkably efficient due to olefin–metal interaction causing se-
lective retention of styrene.[2a,c] An isomeric mixture of trans-
and cis-piperylene is resolved with MIL-96 due to the cage
structure and narrow cage windows, which selectively facilitate
the efficient packing of the trans isomers inside the pore.[3c]
Yet, examples of MCPs for liquid-phase separation based on
electronic interaction between the linkers and guest molecules
are rare.[4] Electronic interaction, particularly the p–p donor–ac-
ceptor, is one of the most predictable interactions between ar-
omatics with opposing electronic character and thus could act
as an orthogonal recognition element for electron-poor aro-
matics (e.g., nitroaromatics). Based on this hypothesis, a new
separation material can be designed specifically to allow the
conjugated p system, perylene frequently serves as a p donor,
and thus can have strong interactions with electron-deficient
nitroaromatics.[6] Keeping this in mind, the ligand 2,5,8,11-tetra-
kis(4-carboxyphenyl)perylene (H4L; Scheme 1) was designed
Scheme 1. Synthesis of 2,5,8,11-tetrakis(4-carboxyphenyl)perylene H4L
(cod=1,5-cyclooctadiene; dba=dibenzylideneacetone).
and synthesized based on the fact that the appended benzoic
acid moieties will significantly increase the size of the linker
and thus result in the formation of a MCP with large pores and
high surface area. It is important to note that the use of pery-
lene as a building unit for coordination polymers has not yet
succeeded, thus emphasizing the challenging of the linker syn-
thesis. Unlike pyrene, 2,5,8,11-tetra-substituted perylene cannot
be prepared simply by traditional halogenation reactions. Yet,
these specific CÀH bonds can be activated and borylated by
[a] Dr. L. D. Tran, J. Ma, Dr. A. G. Wong-Foy, Prof. A. J. Matzger
Department of Chemistry, University of Michigan
930 N. University Ave, Ann Arbor, MI 48109 (USA)
E-mail: matzger@umich.edu
[b] Prof. A. J. Matzger
Department of Macromolecular Science and Engineering
University of Michigan, Ann Arbor, MI 48109 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600526.
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a one-step Ir-catalyzed direct CÀH borylation reaction to form
tetra-borylated perylene (1; Scheme 1).[7] This tetraborylated
compound 1 was then subjected to Suzuki coupling with
butyl-4-iodobenzoate to give a tetra-ester intermediate (2),
which was hydrolyzed to form the desired linker H4L (see the
Supporting Information).
Solvothermal reaction of H4L with Cu(NO3)2 in the presence
of the co-linker 4,4’-bipyridine (bpy) gave the green crystalline
UMCM-310. The structure of UMCM-310 was evaluated by
single-crystal analysis. The selective interaction of UMCM-310
toward electron-poor aromatics was demonstrated through
separation experiments with nitroaromatics due to the fact
that these are an important class of industrial compounds and
have been widely used as explosives, dyes, insecticides, as well
as herbicides. Additionally, an example of employing UMCM-
310 as stationary phase for the separation of large organic
molecules is presented.
Figure 1. a) Structure of UMCM-310 with large pore (blue) and small pore
(green) indicated. b) Structure of mesoporous cage (large pore). c) Large-
pore window with CuÀCu distance measured to be 16.6 Š.
Due to the nature of H4L, a flat conjugated tetracarboxylic
acid, its reaction with CuII salts was predicted to form a 2D co-
ordination polymer. As a consequence, a 4,4’-bipyridine (bpy)
pillar linker is necessary to organize 2D sheets into a 3D struc-
ture. With that in mind, bpy was added as co-linker for the
MCP-formation reaction. UMCM-310 crystals were formed after
approximately 4.5 h incubation of the solution containing H4L,
bpy, and Cu(NO3)2 in NMP/DMF/ethanol/H2O/CF3COOH mixture.
Conducting the reaction with additional glass surfaces (see the
Supporting Information) for crystals to grow also induces the
formation of small particle size MCPs (10–90 mm; Figure S3 in
the Supporting Information), which are desirable to increase
the column-packing efficiency and thus separation per-
formance of the MCPs.[8] The isostructural MCP can be ob-
tained with zinc (Figure S4 in the Supporting Information).
Single-crystal X-ray diffraction of the green cuboctahedron-
shaped UMCM-310 revealed that the framework consists of Cu
paddlewheels linked together by perylene-based H4L linkers
and bpy. Each Cu paddlewheel is coordinated to four H4L link-
ers. The co-linker bpy coordinates to the axial position of two
Cu paddlewheels, making every Cu paddlewheel a five-con-
nected node in the framework—such coordination has been
rarely observed in Cu paddlewheel-based MCPs (Figure 1a). In
the ligand, the four carboxylate groups and phenyl rings are
twisted; the dihedral angle between the carboxylates and the
perylene plane is 38.58. The structure contains both mesopores
and micropores. The mesopores are constructed with 16 pad-
dlewheels and eight perylene linkers into a cuboctahedron
shape (Figure 1b). The internal diameter is 26 Š and the largest
pore window is 16.6 Š (Figure 1c). A smaller micropore (inter-
nal diameter 13 Š) was formed with four Cu paddlewheels in
a tetrahedral shape (Figure 1a). Topology analysis of UMCM-
310 revealed a new net. Most of the previously reported MCPs,
composed of Cu paddlewheels and tetracarboxylate linkers,
crystallize in the nbo net.[9] When bpy was used as co-linker, bi-
metallic paddlewheels and tetracarboxylate linkers coordinate
and form 2D sheets. These sheets are connected by bpy and
the obtained MCPs are fsc net.[10] On the other hand, UMCM-
310 crystalizes in a 2,4,5-c net, which has not been reported to
date. We designate the net LYT.
UMCM-310 was activated by flowing supercritical CO2 from
methanol or acetone-soaked crystals.[11] The Brunauer–
Emmett–Teller (BET) surface area was measured from the N2-
sorption isotherm and calculated to be in the range of 4511–
5110 m2gÀ1 depending on the sample (Figure 2); this value is
*
*
Figure 2. N2-sorption isotherm of UMCM-310 ( adsorption, desorption).
Insert: Pore-size distribution from NLDFT fitting of the Ar-sorption isotherm.
relatively high compared with other reported MCPs. Pore-size
distribution was evaluated though Ar-sorption experiment and
revealed that UMCM-310 has two types of pores correspond-
ing to the dimensions of 17 and 25 Š (Figure 2). Due to the
presence of the mesopores and large pore windows (Fig-
ure 1c), UMCM-310 can potentially be used for the separation
of large molecules. Despite the fact that UMCM-310 can be
fully activated to have high surface area, the framework gradu-
ally collapses after guest removal as the BET surface area drops
to approximately 2700 m2gÀ1 after nine days. However, the
UMCM-310 is stable in acetone or methanol, suggesting that
application in liquid-phase separations is feasible.
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The separation performance of UMCM-310 toward nitroaro-
matic compounds was initially studied with the nitrobenzene
series. A suspension of UMCM-310 (ca. 24 mg) in hexanes/di-
chloromethane (DCM) was packed into an HPLC column (50”
2.1 mm), and hexanes was chosen as the mobile phase. A mix-
ture of nitrobenzene (NB), o-dinitrobenzene (o-DNB), and 1,3,5-
trinitrobenzene (TNB) were fully separated with UMCM-310 as
stationary phase (Figure 3a). Similar baseline separation was
obtained with the series of NB, p-dinitrobenzene (p-DNB), and
TNB. The order of retention time is NB (1.2 min)<o-DNB
(4.5 min)ꢀp-DNB (5.6 min)<TNB (12 min). Calculation of reso-
lution (R) between these compounds gave the value of 1.32–
2.49 (see Table 3 in the Supporting Information). These high R
values demonstrate the ability of UMCM-310 to fully resolve
the mixture of nitrobenzenes. Overall, UMCM-310 showed
strong preference for compounds containing a higher number
of nitro groups consistent with a model, in which the more
electron-deficient aromatic systems interacts more strongly
with the perylene-based framework.
tially be used to purify TNT. A mixture of 2,4-dinitrotoluene
(2,4-DNT) and TNT was passed through a column packed with
UMCM-310 with hexanes as the eluent. A complete baseline
separation was obtained (Figure 4a). As expected, TNT (tR =
10.4 min) is more retained than 2,4-DNT (tR =3.5 min) due to
its higher level of nitration. Moreover, UMCM-310 successfully
separated the mixture of 2,4-DNT and 2,6-dinitrotoluene (2,6-
DNT; Figure 4b). As demonstrated in the chromatogram, 2,4-
DNT elutes first followed by 2,6-DNT (tR =9.5 min). In a higher-
performance column configuration, TNT would be easily sepa-
rable from the DNT isomers. Nonetheless, the difference in re-
tention times between the isomeric dinitroluenes indicates
that, in addition to purely electronic nature of the adsorbate,
other factors, such as the substitution pattern of nitration and
its influence on charge distribution and substituent conforma-
tion, influence the separation.
As mentioned previously, most of the examples using MCPs
as stationary phases explore small-molecule separations. For
example, for MIL-47 or MIL-53, 1,3,5-triisopropylbenzene is un-
retained and therefore was used to determine the bed void
time.[14,2b] The separation performance of UMCM-310 towards
large-molecule mixtures was evaluated through analyzing
a mixture of 1,3,5-triphenyl benzene and 1,3,5-tris(4-bromo-
phenyl)benzene, which have kinetic diameters of 11.8 and
13.6 Š, respectively.[2a] 1,3,5-Triphenyl benzene was unretained
in the columns packed with HKUST-1, and the 1,3,5-tris(4-bro-
mophenyl)benzene was unretained in MOF-5.[2a] A binary mix-
ture of these compounds was completely separated by UMCM-
310 (Figure 5). 1,3,5-Tris(4-bromophenyl)benzene has the
longer retention time, most likely because it is somewhat more
electron deficient and has stronger van der Waals interaction
with the framework. This example demonstrates the ability of
UMCM-310 to be used for the separation of mixtures of large
organic molecules.
TNT is the most prevalent explosive and is widely used for
both military and industrial purposes. In the manufacture of
TNT, after several nitration steps, crude TNT is produced conta-
minated with the meta (unsymmetrical isomers) TNT and dini-
trotoluenes (mostly 2,4-dinitrotoluene). Meta TNT can be re-
moved through the conventional sulphitation method.[12] How-
ever, this method is not capable of eliminating other contami-
nants. To minimize the amount of dinitrotoluenes (DNT), a rela-
tively harsh nitration condition is needed, which also facilitates
the formation of oxidation products. Recrystallization of crude
TNT in alcohol or nitric acid is effective, yet this method causes
loss of TNT.[12,13] These obstacles emphasize the need to search
for a new separation method to purify TNT from DNT. Because
UMCM-310 has demonstrated its ability to differentiate nitro-
benzenes based on the number of nitro groups, it can poten-
Figure 3. Liquid-phase separation of nitrobenzenes on a UMCM-310 packed column at a flow rate 1 mLminÀ1: a) mixture of nitrobenzene, o-dinitrobenzene,
and 1,3,5-trinitrobenzene (l=219 nm); and b) mixture of nitrobenzene, p-dinitrobenzene, and 1,3,5-trinitrobenzene (l=240 nm).
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Figure 4. Liquid-phase separation of nitrotoluenes on a UMCM-310 packed column at a flow rate 1.5 mLminÀ1: a) mixture of 2,4-dinitrotoluene and 2,4,6-trini-
trotoluene (l=224 nm); and b) mixture of 2,4-dinitrotoluene, 2,6-dinitrotoluene (l=227 nm).
Acknowledgements
This work was supported by the US Department of Energy
(DE-SC0004888). We acknowledge Dr. J.W. Kampf for the X-ray
crystallographic assistance and funding from NSF grant CHE-
0840456 for the Rigaku AFC10K Saturn 944+ CCD-based X-ray
diffractometer.
Keywords: adsorption
·
host–guest systems
·
liquid
chromatography · metal–organic frameworks · microporous
materials
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Received: February 3, 2016
Published online on March 2, 2016
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