In situ synthesis of an imidazolate-4-amide-5-imidate ligand and formation of a microporous zinc-organic framework with H2-and CO 2-Storage Ability

Article abstract of DOI:10.1002/anie.200906188

"(Figure Presented)" Narrow channels with polar walls are the structural and functional features responsible for the high capacity of a zinc-organic framework based on an imidazolate-amide-imidate ligand for the uptake of H₂ and CO₂ (see structure: orange Zn, blue N, red O, dark gray C, light gray H). The rigid and stable chelating ligand was synthesized in situ by partial hydrolysis of a dicyanoimidazole compound.

Full text of DOI:10.1002/anie.200906188

Communications
DOI: 10.1002/anie.200906188
Metal–Organic Frameworks
In Situ Synthesis of an Imidazolate-4-amide-5-imidate Ligand and
Formation of a Microporous Zinc–Organic Framework with H2-and
CO2-Storage Ability**
Franziska Debatin, Arne Thomas, Alexandra Kelling, Niklas Hedin, Zoltan Bacsik,
Irena Senkovska, Stefan Kaskel, Matthias Junginger, Holger Mꢀller, Uwe Schilde, Christian Jꢁger,
Alwin Friedrich, and Hans-Jꢀrgen Holdt*
Metal–organic frameworks (MOFs) are an emerging class of
materials with various potential applications in gas adsorption
and separation,[1] catalysis,[2] ion exchange,[3] sensors,[4] photo-
voltaics,[5] and drug release.[6] Besides a distinct porosity, many
of these applications demand a MOF with high chemical and
thermal stability, as well as organic linkers with designated
chemical or physical functionalities.[7]
A recent advance has been the development of a class of
MOFs known as zeolitic imidazole frameworks (ZIFs),[8] in
which tetrahedral coordinated metal atoms, such as zinc, are
linked through N atoms by ditopic imidazolate linkers to form
neutral frameworks. These compounds have high chemical
and thermal stabilities. Yaghi and co-workers used function-
alized imidazolate linkers for synthesizing ZIFs with varying
pore diameters and surface polarities. Gas-sorption experi-
ments with the ZIFs revealed high capacities for selective CO2
uptake; this capacity is influenced primarily by the polar
functionalities (NO2, CN, Br, Cl).[8b,h] Recently, ZIFs were
used as microporous metal–organic frameworks (MMOFs)
for the kinetic separation of propane and propene.[8a]
Imidazolate-4,5-dicarboxylate (IDC3À) is a rigid planar
ligand containing, besides the two imidazolate donor
N atoms, two carboxylate groups as coordination sites.
IDC3À is beneficial, for example, for the construction of
anionic zeolite-like metal–organic frameworks (ZMOFs).[9]
IDC3À, as well as its mono- and diprotonated forms HIDC2À
and H2IDCÀ, respectively, can coordinate multiple metal ions
to form a series of MOFs with different structures and
interesting topologies and properties.[10]
In situ ligand synthesis is of growing interest as a new
approach for the synthesis of coordination polymers.[11]
Herein, we present the synthesis of the first zinc–imidazo-
late-4-amide-5-imidate framework. The new imidazolate
chelate ligand, 2-methylimidazolate-4-amide-5-imidate (2),
was generated in situ by partial hydrolysis of 4,5-dicyano-2-
methylimidazole (1)[12] under solvothermal conditions in N,N-
dimethylformamide (DMF) in the presence of Zn-
(NO3)2·4H2O (Scheme 1). Ligands 2 link with Zn2+ ions and
form the neutral microporous imidazolate metal–organic
framework (IMOF) [Zn(2)]n (3) with 1D hexagonal channels.
[*] F. Debatin, A. Kelling, M. Junginger, H. Mꢀller, Prof. Dr. U. Schilde,
Dr. A. Friedrich, Prof. Dr. H.-J. Holdt
Institut fꢀr Chemie, Anorganische Chemie, Universitꢁt Potsdam
Karl-Liebknecht-Strasse 24–25, 14476 Golm (Germany)
Fax: (+49)331-977-5055
E-mail: holdt@chem.uni-potsdam.de
Prof. Dr. A. Thomas
Institut fꢀr Chemie, Fakultꢁt II, Technische Universitꢁt Berlin
10623 Berlin (Germany)
Scheme 1. Synthesis of the IMOF [Zn(2)]n (3).
Dr. N. Hedin, Dr. Z. Bacsik
The chelate ligand 2 shows a strong structure-directing
effect: the combination of amide/imidate and imidazolate
groups causes a strong tendency for coordination and
generates a permanent porosity in the coordination polymer
3. The rigidity and stability of 2 also impart excellent thermal
and chemical stability to the IMOF. Moreover, the imidazo-
late–amide–imidate linker and its hydrogen bonds polarize
the walls of the microporous channels. The microporous and
polarized channels have significant capacity for the capture of
H2 and CO2.
Department of Physical, Inorganic and Structural Chemistry
Arrhenius Laboratory, Stockholm University
10691 Stockholm (Sweden)
Dr. I. Senkovska, Prof. Dr. S. Kaskel
Anorganische Chemie, Chemie und Lebensmittelchemie
Technische Universitꢁt Dresden, 01062 Dresden (Germany)
Prof. Dr. C. Jꢁger
BAM, Federal Institute for Materials Research and Testing
Division I.3, 12489 Berlin (Germany)
[**] Financial Support from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged (SPP 1362 “Porous metal-organic frame-
works”, HO 1706/7-1). We thank Dr. Guoying Zhao for a discussion
on CO2 adsorption, and Prof. V. A. Blatov for providing help in
describing the underlying net topology.
IMOF 3 is best prepared by treating 1 with an equal
amount of Zn(NO3)2·4H2O at 1208C for 48 h under autog-
enous pressure (Scheme 1). Under these reaction conditions,
the cyano groups of 1 are partially hydrolyzed to an amide and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906188.
À
=
an imidate (-C(O ) NH) group to form the chelating linker 2.
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The hydrolysis of 1 is conducted in situ and is directly affected
by the acidity of the solution in DMF. At intermediate
pH values (when the reaction is carried out with an initial
pH value of 6.1), the coordination polymer 3 with the amide–
imidate–imidazolate ligand 2 is formed. In more acidic
solutions (when HCl is added to establish an initial pH value
of 3.5), 2-methylimidazole-4,5-diamide is formed.
in the formation of two five-membered chelate rings. One ring
is formed by the coordination of a Zn2+ ion to the N2
(imidazolate) atom and the O1 (amide) atom. The second
chelate ring is generated by the coordination of the N1
(imidazolate) atom and the N4 (imidate) atom to a second
Zn2+ ion. The negatively charged O2 (imidate) atom is
coordinated to a third Zn2+ ion. Moreover, the structure of 3 is
additionally stabilized by three hydrogen bonds, one “inner
ligand” between an amide NH group and an imidate O atom
(H3A···O2I) and “intermolecular” hydrogen bonds between
an amide NH group and an amide O atom (H3B···O1II) as
well as between an imidate NH group and an imidazolate
N atom (H4···N1III).
The in situ hydrolysis of the CN groups of the reactant 1 to
amide and imidate moieties can be observed by IR spectros-
copy. Thus, the IR spectrum of 3 shows no CN bands. Instead,
the typical absorption bands for an amide are observed
between 3350 and 3100 cmÀ1 and at 1664 and 1562 cmÀ1 (see
Figure S1 in the Supporting Information). X-ray crystallog-
raphy[13] of 3 revealed that the compound crystallizes in the
IMOF 3 is a rare example of MOFs in which the Zn2+ ion
is pentacoordinated.[10,14] In these systems, imidazolate-4,5-
dicarboxylate (IDC3À), its monoprotonated form (HIDC2À),
and other ligands are coordinated to the Zn2+ ion. However,
IMOF 3 is the first example of an MOF in which fivefold
coordination to the Zn2+ ion by only one ligand type is
observed.
To the best of our knowledge, the coordination polymer 3
is the first example of an imidate–metal complex. Imidates are
usually unstable. The imidate group in 3 appears, however, to
be stabilized by coordination to two Zn2+ ions and by
participation in two hydrogen bonds.
ꢀ
high symmetrical trigonal space group R3. The asymmetric
unit of 3 contains one Zn2+ ion and the bridging ligand 2
divided into three parts (see Figure S4 in the Supporting
Information). The coordination environment of the Zn2+ ion
and the bridging mode of 2 are shown in Figure 1. The Zn2+
ion is pentacoordinated by donor atoms of three ligands 2 to
form a distorted environment with a trigonal-bipyramidal
character (Figure 1). In this arrangement, the two imidazolate
N atoms (N1 and N2) occupy the axial positions, and the
imidate N and O and amide O atoms reside in the equatorial
positions (for selected geometric parameters, see Table S2 in
the Supporting Information). Such an arrangement is a
precondition for the Zn atom to be a triconnected node, as
described below. The ligand 2 functions as a pentadentate
linker that coordinates three Zn2+ ions (Figure 1).
In 3, each zinc ion is connected to three ligands, and
through these ligands to six other zinc ions. The underlying
net consists of two types of nodes: Zn atoms and imidazolate
ligands. This net is inherently uninodal; for example, both Zn
and the ligand play the same topological role. It is the three-
coordinated etb net (3/8h1).[15] If one considers only Zn atoms
and simplifies this net by contracting the ligands, the structure
can be described as the six-coordinated smg net (6/3h16).[16]
The topological analysis was performed with TOPOS[17]
including the Reticular Chemistry Structure Resource
(RCSR) database.[16] With the program OLEX,[18] in which
all atoms are taken into consideration, IMOF 3 was assigned
the long topological vertex symbol 3.4.10.10.12.14.142.
The 3D coordination polymer 3 forms 1D hexagonal
channels running along the [001] direction (Figure 2). An
open channel of IMOF 3 with a view of the channel wall lying
in the bc plane is shown in Figure 3. The wall of the channel in
3 is essentially formed by the rigid and planar pentadentate
imidazolate–amide–imidate linkers 2. The Zn2+ ions are
located almost on the edges of the hexagonal channels.
They are bridged by the linkers 2 through coordination with
both N atoms of the imidazolate heterocycle. The imidate and
amide functional groups of the ligand 2 are embedded in the
wall of the channel. The imidate group bridges two zinc ions
of two channel edges through its N and O atoms. The three
different types of hydrogen bonds are all confined to the
channel wall as well. The localization of the imidate and
amide groups and the hydrogen bonds in the channel walls of
the IMOF 3 polarize and functionalize the coordination space
of this material. Hence, guest molecules can be hydrogen
bonded by potential donors and acceptors. Moreover, Lewis
acids can interact with the nitrogen atom of the amide moiety.
The hydrophilic character of the surface of the channel is
expected to be decreased by the methyl groups of the ligands
The amide and imidate groups, formed by the in situ
hydrolysis of cyano groups, enable each ligand 2 to participate
Figure 1. Portion of the crystal structure of [Zn(2)]n (3) showing the
coordination environment of ZnII, the bridging mode of linker 2, and
the hydrogen bonds (dotted lines). For the symmetry codes and for
details of the hydrogen bonds, see the Supporting Information.
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1
signals for DMF. By monitoring the H und 13C MAS NMR
spectra we found that IMOF 3 is solvent-free after an
activation procedure involving heating at 2508C and
10À3 mbar for 2 days.
TGA/DTA/MS measurements of as-synthesized 3 indi-
cated a loss of water and DMF up to 3008C on the basis of a
weight loss of 17–18% (see Figure S3 in the Supporting
Information). After the loss of the water and DMF molecules,
no further weight loss was observed until 3808C. X-ray
powder diffraction (PXRD; see Figure S10 in the Supporting
Information) demonstrated that the framework structure of
the IMOF 3 is maintained upon the removal of solvent
molecules and is stable up to 3808C.
Along with its thermal stability, 3 showed remarkable
chemical stability in boiling water or methanol for 7 days. The
PXRD patterns recorded for 3 after such extensive treatment
were unchanged (see Figures S7–S9 in the Supporting Infor-
mation).
Figure 2. Hexagonal channels in 3; view along the c axis (orange Zn,
blue N, red O, dark gray C, light gray H).
The N2 isotherm of 3 was measured at 77 K (Figure 4). We
obtained an apparent surface area (Brunauer–Emmett–Teller
(BET) model) of 802 m2 gÀ1 by using the data points on the
adsorption branch of the N2 isotherm in the pressure region p/
p0 = 0.02–0.1 and calculated
a micropore volume of
0.34 cm3 gÀ1.
Figure 3. Open channel in IMOF 3; view of the channel wall lying in
the bc plane (dotted lines are hydrogen bonds).
2; these methyl groups protrude into the open channels
(Figure 2). We assume that the methyl group of 2 has a
structure-directing effect for the formation of the channels in
the IMOF 3.
The accessible diameter of the channel in the structure 3
was estimated to be 3.82 ꢀ by considering van der Waals
radii.[19] Calculations with the PLATON toolkit indicated that
IMOF 3 contains 41% void space.[20]
Figure 4. Nitrogen gas sorption isotherm of 3 at 77 K. STP=standard
temperature and pressure.
The channels of the as-synthesized IMOF 3 contain
significant amounts of solvent molecules. The presence of
water could be concluded from single crystals of 3. Within the
channels, significant residual electron density was found. The
solvent-accessible void volume and the number of electrons
were calculated by the program PLATON,[20] which revealed
a content of two or three H2O molecules per formula unit (for
details, see the Supporting Information). Solid-state NMR
spectroscopy and thermogravimetric analysis (TGA)/differ-
ential thermal analysis (DTA)/mass spectrometry of the as-
synthesized material also provided evidence for the presence
of water molecules as well as some DMF.
IMOF 3 reversibly adsorbs 1.5 wt% H2 at 77 K and 1 bar
(Figure 5). This capacity is typical for MOFs, which show
capacities in the range of 0.57–2.59 wt%;[21] however, the H2
capacity is slightly higher than that of ZIFs (ZIF 8,
1.29 wt%,[8k] ZIF 11, 1.37 wt%,[8k] ZIF 20, 1.1 wt%[8i]).
ZIF 8, ZIF 11, and ZIF 20 consist of cages with diameters of
11.6–15.4 ꢀ that are interconnected by small windows (with
apertures of 2.8–3.4 ꢀ). The IMOF 3, on the other hand, has
no cages. Its pore structure consists of microporous channels
only. The absence of large pores in 3 enhances the interaction
potential with H2 in comparison with that in the ZIFs. Such an
enhancement of the interaction with H2 has been observed for
MOFs.[22] We presume that the narrow channels in the IMOF
cause the enhanced uptake of H2, as compared with that of the
ZIFs. In the narrow channels, the interaction potential with
H2 is enhanced because of the overlap of the potential fields
of the pore walls.[22]
Solid-state magic-angle-spinning (MAS) 13C and 1H NMR
spectra of as-synthesized and activated 3 (see Figure S2 in the
1
Supporting Information) showed the H and 13C MAS NMR
signals of the ligand 2 coordinated in 3 by three Zn2+ ions.
Moreover, the 1H MAS NMR spectrum of the as-synthesized
material showed a water signal at d = 3.8 ppm, and the 1H and
13C MAS NMR spectra of as-synthesized 3 exhibited typical
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Chemie
and CO2/CO, which are commercially relevant mixtures. We
intend to study these potential applications further.
Experimental Section
4,5-Dicyano-2-methylimidazole (100 mg, 0.76 mmol) and zinc(II)
nitrate tetrahydrate (198 mg, 0.76 mmol) were dissolved in DMF
(6 mL) in a sealed tube (Ace, type A). The sealed tube was closed,
and the mixture was heated at 1208C for 48 h and then cooled to room
temperature. [Zn(2)]2 (3; 132 mg, 75% based on Zn(NO3)2·4H2O)
was obtained by filtration as fine pale-yellow crystals and dried in air.
It was activated by drying at 2508C and 10À3 mbar. M.p. > 3808C;
elemental analysis: calcd (%) for C6H6N4O2Zn: C 31.12, H 2.61, N
24.2, Zn 28.24; found: C 30.80, H 2.74, N 23.81, Zn 27.76; IR (KBr
~
pellet): n = 3342 (m), 3129 (m), 1664 (s), 1562 (vs), 1462 (m), 1439 (m),
Figure 5. Hydrogen gas sorption isotherm of 3 at 77 K.
1266 (m), 1115 (m), 813 cmÀ1 (m).
Full experimental details are presented in the Supporting
Information.
IMOF 3 captures CO2 well. Figure 6 shows the CO2-
adsorption isotherms for 3 at 253, 298, and 343 K. The uptake
of CO2 by 3 at 298 K and 1 bar is about 48 cm3 gÀ1. A similar
high uptake was recently found by Yaghi and co-workers for
ZIF 78 and ZIF 82, which were synthesized by using imida-
zolates containing the functional groups NO2 and CN.[8b] We
Received: November 3, 2009
Published online: January 18, 2010
Keywords: gas sorption · metal–organic frameworks ·
.
microporous materials · N,O ligands · solvothermal synthesis
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5132.2(7) ꢀ3, Z = 18, 1calcd = 1.663 gcmÀ1; m(MoKa) = 2.17 mmÀ1
(l = 0.71073 ꢀ), T= 210 K; 2Vmax = 49.988, 11122 reflections
measured, 2017 unique (Rint = 0.0594), R = 0.0314, wR = 0.0825
(I > 2s(I)). For details of data collection and structure solution
and refinement, see the Supporting Information. CCDC 732233
contains 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.
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