Selective adsorption properties of cationic metal-organic frameworks based on imidazolic linker

Article abstract of DOI:10.1021/cg301347t

In this work we propose an approach for the controlled synthesis of cationic frameworks by the use of imidazolium salts as linker molecules. The imidazolium salt linker H₂ImidCl was prepared in five steps and converted with zinc and copper nitrate, respectively, to give the two topologically different cationic frameworks [Cu(Imid)(H₂O)] ⁺ and [Zn₄(Imid)₅]³⁺. The framework charge is compensated by anions located in the pores. [Cu(Imid)(H ₂O)]⁺ shows a selective adsorption of organic molecules either bearing a carboxylic group or having an ionic nature. Furthermore, adsorbed molecules featuring a carboxylic group are captured inside the pores, whereas ionic substances can be reversibly adsorbed and desorbed, which was followed by UV/vis spectroscopy.

Full text of DOI:10.1021/cg301347t

Article
pubs.acs.org/crystal
Selective Adsorption Properties of Cationic Metal−Organic
Frameworks Based on Imidazolic Linker
Georg Nickerl,† Andreas Notzon,‡ Maja Heitbaum,‡ Irena Senkovska,† Frank Glorius,*,‡
and Stefan Kaskel*,†
†Department of Inorganic Chemistry, Dresden University of Technology, Bergstraße 66, D-01062 Dresden, Germany
‡Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat Munster, Corrensstraße 40, D-48149 Munster, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: In this work we propose an approach for the
controlled synthesis of cationic frameworks by the use of
imidazolium salts as linker molecules. The imidazolium salt
linker H2ImidCl was prepared in five steps and converted with
zinc and copper nitrate, respectively, to give the two
topologically different cationic frameworks [Cu(Imid)(H2O)]+
and [Zn4(Imid)5]3+. The framework charge is compensated by
anions located in the pores. [Cu(Imid)(H2O)]+ shows a
selective adsorption of organic molecules either bearing a
carboxylic group or having an ionic nature. Furthermore,
adsorbed molecules featuring a carboxylic group are captured
inside the pores, whereas ionic substances can be reversibly
adsorbed and desorbed, which was followed by UV/vis spectroscopy.
However, the imidazolium salt is not only bearing catalytic
INTRODUCTION
■
potential, it is also an ideal linker for the construction of
cationic frameworks. In contrast to zeolites, where only cation
exchange is feasible due to the anionic nature of zeolite
frameworks, MOFs can undergo both anion and cation
exchange depending on their framework charges.28 Cationic
MOFs are usually formed when the positive charge of the metal
ions is not completely balanced by the charge of organic linkers.
In this case the positive charge of the network should be
compensated by guest anions, usually the anions from the metal
salt used during the synthesis.29 However, the targeted
synthesis of such compounds is difficult, since the charge
balance cannot be easily controlled during the synthetic
procedure. The use of a salt as a linker opens the possibility
for such controlled design of positively charged frameworks.
Recently, charged metal−organic frameworks have attracted
considerable attention due to the possibility to perform
postsynthetic modification by ion exchange.30−34 Pore size
and functionality of the material can be tuned in an elegant way
by the contribution to these properties of the ionic guest.
Additionally, the selective adsorption can be achieved in such
ion containing MOFs, since only ionic species can be
exchanged, preventing adsorption of neutral species due to
reduced pore space.
Metal−organic frameworks (MOFs), a class of highly porous
materials, exhibit high specific surface areas and allow a
controlled pore design.1,2 The careful selection of building
blocks enables the targeted construction as well as incorpo-
ration of active sites into the MOF.3,4 Furthermore these sites
are well ordered and can be precisely localized due to the
crystallinity of the material. The integrated sites can be active,
e.g., for catalysis as intrinsic sites, or used for postfunctionaliza-
tion.5,6 Postfunctionalization can be done by attaching
molecules to the linkers or by adsorbing (encapsulation)
functional molecules. However, adsorption and desorption are
in equilibrium and thus noncovalent integration of functional
sites may cause leaching of encapsulated species.7 We focused
our investigations on imidazolium salts as building blocks for
MOF synthesis. Imidazolium salts are precursors for N-
heterocyclic carbenes (NHC), which are known for their
attractive properties, being electron-rich and sterically demand-
ing.8−14 Several attempts were made to immobilize the
imidazolium-based NHCs in different materials with high
specific surface areas including nanoparticles,15,16 poly-
mers,17−19 or SBA-15.20 Recently, we have developed the
synthesis of a porous NHC-based element organic framework
(EOF) material, which shows good performance as heteroge-
neous organocatalyst in the NHC catalyzed conjugated
umpolung of unsaturated cinnamic aldehyde and trifluoroace-
tophenone.21 Furthermore incorporation of functionalized
imidazolium salts and Pd(II)−NHC organometallic complexes
as the organic building block into MOFs has also attracted
considerable interest.22−27
Here, we report two MOFs incorporating an imidazolium
salt linker and copper or zinc as metals, respectively. The
Received: September 14, 2012
Revised: November 6, 2012
© XXXX American Chemical Society
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Adsorption Experiments. Gas Adsorption. Prior to the gas
physisorption measurements the samples were activated using
supercritical CO2 and additionally evacuated at room temperature
for 12 h. N2 physisorption isotherms were measured at 77 K up to 1
bar using a Quantachrome Autosorb 1C apparatus.
imidazolium salt was synthesized in four steps from 2,6-
dimethylaniline (Figure 1). The accessibility of the C2 atom of
the imidazolium moiety and ion exchange properties of the
MOFs were studied.
Liquid Phase Adsorption. The crystals of the investigated
compounds were placed into corresponding dye-containing ethanolic
solutions. After three days, the crystals were inspected under the
microscope. For desorption experiments the dye loaded crystals were
placed in fresh ethanol in a cuvette and the dye release was followed in
situ by UV/vis spectroscopy.
Figure 1. 1,3-Bis(4-carboxy-2,6-dimethylphenyl)-1H-imidazolium
chloride (5) was synthesized in four steps starting from 2,6-
dimethylaniline (1).
RESULTS AND DISCUSSION
■
For the linker synthesis 2,6-dimethylaniline (1) was brominated
to 4-bromo-2,6-dimethylaniline (2) followed by a copper
catalyzed reaction with sodium cyanide to 4-amino-3,5-
dimethylbenzonitrile (3) in an excellent yield. In the next
step the resulting intermediate 3 was treated with paraformal-
dehyde and glyoxal solution to generate the imidazolium salt
(4). Finally, hydrolysis with concentrated HCl gave the product
1,3-bis(4-carboxy-2,6-dimethylphenyl)-1H-imidazolium chlor-
ide (H2ImidCl) (5).
Conversion of linker 5 with copper nitrate in ethanol at 80
°C yielded [Cu(Imid)(H2O)](Cl)x(NO3)1−x (6) as turquoise
crystals. The reaction with zinc nitrate in ethanol at 90 °C
results in yellow crystals with the formula [Zn4(Imid)5]-
(Cl)x(NO3)3−x (7). The chemical compositions of the
compounds were determined from representative synthetic
batches by means of elemental and thermal analysis as
[Cu(Imid)(H2O)](Cl)0.5(NO3)0.5(H2O)0.5(EtOH)0.5 for 6
and [Zn4(Imid)5](NO3)3(H2O)12 for 7, respectively. It should
be noted that slight variations in composition are probable, due
to the possible anion exchange between chloride of the linker
and nitrate of metal nitrate used as metal source during the
synthesis. Therefore the formulas for the neutral solvent free
compounds are formulated as [Cu(Imid)(H2O)]-
(Cl)x(NO3)1−x for 6 and [Zn4(Imid)5](Cl)x(NO3)3−x for 7.
Using similar reaction conditions, two different structural
building units (SBUs) were obtained and thus two structurally
and topologically different MOFs.
The secondary building unit in 7 is a frequently observed
assembly in MOF chemistry, a binuclear paddle-wheel cluster
consisting of two copper atoms being bridged by four
carboxylate groups (Figure 2a). The crystallographic center of
symmetry is located between two copper atoms of the paddle-
wheel. The Cu−O distances are in the range 1.958(6)−
1.979(5) Å.
As a result, two-dimensional cationic layers with uninodal sql
topology are formed, in which the five membered planar
imidazolium ring lies in the plane of the layer (Figure 2b). The
counterions could not be located crystallographically. The
distance between two C2 atoms of imidazolium units inside the
mesh of the net is 21.186 Å. The structure consists of three
EXPERIMENTAL SECTION
■
A detailed description of the synthesis of 1,3-bis(4-carboxy-2,6-
dimethylphenyl)-1H-imidazolium chloride (5) (H2ImidCl) starting
from 4-bromo-2,6-dimethylaniline (1) can be found in section 2 of the
Supporting Information.
Synthesis of [Cu(Imid)(H2O)](Cl)x(NO3)1−x (6). 1,3-Bis(4-car-
boxy-2,6-dimethylphenyl)-1H-imidazolium chloride (17.0 mg, 0.04
mmol) and Cu(NO3)2·3H2O (14.5 mg, 0.06 mmol; 1.5 equiv) were
dissolved in 6 mL of ethanol (99%). The resulting solution was heated
to 80 °C. After 48 h the mixture was cooled down to room
temperature. The turquoise crystals were collected and washed with
ethanol. Yield: 73%.
Synthesis of [Zn4(Imid)5](Cl)x(NO3)3−x (7). 1,3-Bis(4-carboxy-2,6-
dimethylphenyl)-1H-imidazolium chloride (17.0 mg, 0.04 mmol) and
Zn(NO3)2·4H2O (15.7 mg, 0.06 mmol; 1.5 equiv) were dissolved in 6
mL of ethanol (99%). The resulting solution was heated to 90 °C.
After 48 h colorless crystals were formed. The resulting solid was
collected and washed with ethanol. Yield: 54%.
Crystal Structure Determination. A crystal of [Cu(Imid)-
(H2O)](Cl)x(NO3)1−x (6) or [Zn4(Imid)5](Cl)x(NO3)3−x (7),
respectively, was sealed in a glass capillary with a small amount of
solvent. X-ray data for [Zn4(Imid)5](Cl)x(NO3)3−x were collected at
20 °C by using Mo Kα (λ = 0.71073 Å) radiation with a STOE IPDS
imaging plate diffractometer. The data for [Cu(Imid)(H2O)]-
(Cl)x(NO3)1−x were collected at 20 °C using synchrotron radiation
(λ = 0.88561 Å; 14.0 eV) on beamline 14.2 of Freie Universitat Berlin
̈
at BESSY (Berlin, Germany) with a MX-225 CCD detector (Rayonics,
Illinois).35 The data were integrated and scaled with the XDS software
package.36 The structures were solved using direct methods with the
help of SHELXS-97 and refined by full-matrix least-squares techniques
using SHELXL-97.37 Due to the high disorder and low residual
−
electron density it was impossible to locate counterions (NO3 or
Cl−), EtOH, and H2O guest molecules. Non-hydrogen atoms were
refined with anisotropic temperature parameters. The hydrogen atoms
were positioned geometrically and refined using a riding model.
CCDC 860563 (for 6) and CCDC 860564 (for 7) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. The topology of the
networks was analyzed with the program package TOPOS.
Crystal Data for [Cu(Imid)(H2 O)](Cl)x (NO3 )1 − x
.
different intersected 2D layers running perpendicular to [110],
[010], and [100] crystallographic directions.
̅
C H Cu N O : trigonal, P3c1 (No. 165), a = 27.597(4) Å; c =
̅
42 34
2
4
10
21.871(4) Å, V = 14425(4) Å3, T = 293(2) K, Z = 6, θmax = 35.63°,
reflections collected 115363, unique 9952 [Rint = 0.0484], final R
indices, R1 = 0.0594, wR2 = 0.1818; R1(all data) = 0.0642, wR2 (all
data) = 0.1882, GOF = 1.088.
The centers of the mesh are occupied by the knots of the
next net (Figure 2c) in such way, that a catenated 3D structure
is formed. The framework contains two different kinds of
channels along the c-axis: smaller trigonal channels and larger
hexagonal channels 18.8 Å in size (Figure 2d). The total solvent
accessible volume calculated using PLATON is 65.4% of the
unit cell volume. According to the orientation of the linker in
the structure, the imidazolium unit is hardly accessible.
Crystal Data for [Zn4(Imid)5](Cl)x(NO3)3−x. C105H95N10O20Zn4:
monoclinic, P2/n (No. 13), a = 12.036(2) Å, b = 16.838(3) Å, c =
40.383(8) Å, β = 90.46(3)°, V = 8184(3) Å3, T = 293(2) K, Z = 2,
θmax =20.79°, reflections collected 8101, unique 8101 [Rint = 0.0859],
final R indices, R1 = 0.0483, wR2 = 0.1097; R1(all data) = 0.0744,
wR2(all data) = 0.1139, GOF = 0.991.
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Figure 2. Structures of [Cu(Imid)(H2O)]+ (6) and [Zn4(Imid)5]3+ (7) (color scheme: Cu, green; Zn, light blue; O, red; N, dark blue; C, gray;
hydrogen atoms are omitted): (a) paddle-wheel cluster of 6; (b) two-dimensional layer with sql topology of 6 (view along b); (c) four catenated
layers of 6; (d) view on the structure of 6 along c; (e) secondary building unit of 7; (f) view along the double layers of 7; (g) double layer of 7; (j)
intercalation of double layers in the structure of 7.
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Figure 3. Adsorption of dyes from ethanolic solution on [Cu(Imid)(H2O)]+. Nile blue can be ad- and desorbed from the framework, whereas
fluorescein and methyl red are adsorbed irreversibly. No adsorption of Nile red was observed.
Interestingly, Chun et al. synthesized a similar copper based
compound (with isopropyl substituent instead of methyl in the
linker) using N,N-dimethylformamide as solvent.22 Similar sql
layers are present in the structure, but the layers are stacked
along one direction resulting in a two-dimensional layered
structure. The authors also report the formation of organo-
metallic NHC−copper(I) units during the MOF synthesis,
which is obviously not the case during the synthesis in ethanol.
[Zn4(Imid)5](Cl)x(NO3)3−x consists of binuclear zinc
paddle-wheel units being coordinated by three bridging
carboxylate groups (Zn2(COO)3). The coordination sphere is
completed by two more linkers coordinating the zinc atoms in a
monodentate fashion on the axial positions of the paddle-
wheels (Figure 2e). The bridging dicarboxylates interconnect
the SBUs to form layered structural motifs (Figure 2f)
interconnected by the monodentate linker molecules to give
double layers (Figure 2g). The double layers intercalate to give
an extended three-dimensional structure (Figure 2j). The total
solvent accessible volume amounts to 49.8% as calculated using
PLATON. 7 contains small channels along the a-axis with 9.2 Å
in diameter (taking van der Waals radii into account). Similar to
6, the imidazolium units do not point into the open space of
the channel, but rather are positioned parallel to the channel
walls. In both compounds the conformation of the NHC-linker
is indicative for no carbene formation during the synthesis of
MOFs. According to the literature, the N−C−N angle in the
molecular carbenes is about 101.4°.38 In 7 this angle is in the
range 107.39° and 109.63° and in 6 it is 106.55°. These angle
values correspond to the values reported for the imidazolium
salt.39
Since the linker 5 contains an imidazolium unit, the
transformation to an imidazole-2-thione would prove the
formation of a carbene (imidazol-2-ylidene) inside the
MOF.40 Imidazole-2-thiones are commonly prepared by
reaction of an imidazolium ion with sulfur in the presence of
a base, via carbene intermediates. However, in the described
reaction of compounds 6 and 7 with sulfur, neither in the
supernatant solution nor after digesting the MOF in hydro-
chloric acid could the derivate of imidazole-2-thiones be
detected. This finding confirms the inaccessibility of the C2
atom, expected from crystal structure examination.
To evaluate the porosity of the compounds, nitrogen (at 77
K) and liquid phase (at 298 K) adsorption experiments were
performed. Unfortunately, the structures of the compounds
undergo significant changes during the sample evacuation at
293 K (Figure S1 in the Supporting Information, ESI). The
specific BET surface area calculated from nitrogen adsorption
(Figure S7 in the Supporting Information, ESI) isotherm
measured at 77 K on an activated and thus structurally changed
sample of 6 is only 170 m2 g−1. For activated 7 no significant
nitrogen uptake was observed.
In order to demonstrate pore accessibility without structural
transformation and ion exchange properties of 6 and 7, dye
adsorption/desorption experiments were performed with a
range of different dyes. 7 showed no dye adsorption under the
described conditions regardless of the dye used, which is in
accordance with the small pore size of 9.2 Å estimated from
crystal structure.
In contrast, 6 adsorbs and desorbs dyes selectively depending
on the nature of the dye molecule. [Cu(Imid)(H2O)]+ (6)
readily adsorbs fluorescein, methyl red and Nile blue to give
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Author Contributions
fluorescein@[Cu(Imid)(H2O)], methyl_red@[Cu(Imid)-
(H2O)] and Nile_blue@[Cu(Imid)(H2O)] composites, re-
spectively. Interestingly, Nile red is not adsorbed at all,
although it is structurally very similar to Nile blue (Figure 3).
The main difference is the ionic nature of Nile blue
compared to Nile red (Figure 3). Apparently the ionic nature
of the dye is important to enter the channels of 6 which present
an ionic surrounding themselves. Nevertheless upon adding
fresh ethanol the Nile blue@[Cu(Imid)(H2O)] composite
releases Nile blue. The desorption process was followed via
UV/vis spectroscopy (Figure S9 in the Supporting Information,
ESI) until a final equilibrium concentration of dye was reached
after 24 h. Also from the solution containing both dyes (Nile
blue as well as Nile red) compound 6 selectively adsorbs only
Nile blue (Figure S8 in the Supporting Information, ESI).
Methyl red and fluorescein can be adsorbed on compound 6
as well, but cannot be desorbed through exposure to solvent
excess. This was proven by monitoring the supernatant solvent
via UV/vis spectroscopy. As distinguished from Nile blue,
methyl red and fluorescein contain a carboxylic group.
Obviously the corresponding anion, formed by deprotonation,
is trapped inside the channels by electrostatic interaction or can
replace the terminal water molecules coordinated to the copper
paddle-wheels. Such coordination leads to the formation of a
neutral framework with the composition [Cu(Imid)(dye)]. In
this case the dye becomes an integral part of the framework and
cannot be desorbed. Such behavior could be useful for
noncovalent but nevertheless permanent postfunctionalization
by attaching a carboxylic group to the desired functional
molecule. In addition, postfunctionalization can be done via a
simple adsorption from liquid phase. Furthermore, trapping
and thus removal of toxic or undesirable acids, for example 2,4-
dichlorophenoxyacetic acid, a widely used herbicide, is an
interesting field of application as well.
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding
The work was financially supported by the German Research
Foundation (DFG) within the priority program “Porous
Metal−Organic Frameworks” (SPP 1362, MOFs) and by the
Fonds der Chemischen Industrie (M.H.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful to the BESSY staff (Dr. U. Mueller, Dr.
M. S. Weiss) for support during the measurements and the
Helmholtz Centre Berlin for financing the travel costs to
BESSY II. We also thank Basudev Sahoo (International NRW
Graduate School of Chemistry) for some experimental
contributions to this work.
ABBREVIATIONS
■
MOF, metal−organic framework; NHC, N-heterocyclic
carbene; SBA, Santa Barbara amorphous type material; EOF,
element organic framework; H2ImidCl, 1,3-bis(4-carboxy-2,6-
dimethylphenyl)-1H-imidazolium chloride; SBU, secondary
building unit
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CONCLUSION
■
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