A mixed ligand route for the construction of tetrahedrally coordinated porous lithium frameworks

Article abstract of DOI:10.1039/c1dt10859j

Reported here are two 4-connected 3D frameworks based on monomeric lithium nodes, which are synthesized through a mixed ligand route. By combining a negatively charged imidazole ligand and a neutral bis(imidazolyl)methane ligand (or one of its derivatives), neutral frameworks adopting chiral quartz-dual and diamond topologies have been obtained. These materials have low framework density and can reversibly adsorb hydrogen gas. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10859j

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COMMUNICATION
A mixed ligand route for the construction of tetrahedrally coordinated porous
lithium frameworks†
a
a
b
a
Xiang Zhao, Tao Wu, Xianhui Bu and Pingyun Feng*
Received 7th May 2011, Accepted 18th June 2011
DOI: 10.1039/c1dt10859j
˚
Reported here are two 4-connected 3D frameworks based
on monomeric lithium nodes, which are synthesized through
a mixed ligand route. By combining a negatively charged
imidazole ligand and a neutral bis(imidazolyl)methane ligand
The short B–N bond (ª1.5 A) in BIFs, however, leads to a
closer contact and stronger steric repulsion between imidazolate
bridges, making it challenging to tune the framework topology
by introducing imidazolates with various substituent groups (e.g.,
benzimidazole). In this work, we aim to develop imidazolate
frameworks with only 4-connected lithium nodes (Li–N bond ª
˚
.0 A). However, we noticed that the total charge of the resulting
-connected framework will become negative if B /Li is replaced
(
or one of its derivatives), neutral frameworks adopting chiral
quartz-dual and diamond topologies have been obtained.
These materials have low framework density and can re-
versibly adsorb hydrogen gas.
2
4
3
+
+
+
+
with Li /Li and no change is made on the imidazolate ligands.
Thus, we have developed the mixed ligand route to balance
the charge of the framework (Scheme 1). By replacing half of
the negatively charged imidazolate ligands with neutral ligands,
the resulting 3D framework will be neutral. To execute this
strategy, any neutral ditopic ligands can be examined. However, we
are particularly interested in neutral ligands whose coordination
geometry and bonding features closely mimic the imidazolate in
a way similar to those in ZIFs and BIFs. In this work, we used a
bis(imidazolyl)methane type ligand which contains an imidazole-
like ring on each side, but is neutral in charge.
The synthesis of crystalline porous frameworks has long been
of great interest because of their many possible applications.
1–5
Materials based on imidazolates, such as zeolitic imidazolate
frameworks (ZIFs) which are a family of 3D porous crystalline
materials, have caught the attention of many researchers in recent
6–11
years.
Typically, ZIFs are simply built by using divalent metal
2
+
2+
2+
cations (such as Zn , Co , or Cd ) with imidazolate bridges.
In these materials, all the cations are tetrahedrally coordinated
and the final 3D frameworks are neutral. The topology can
be controlled by applying different substitution groups on the
imidazolate module. In this way, about ten zeolitic topologies have
been obtained and their gas sorption studies have showed some
exciting results.
Introducing lightweight elements into the construction of
metal–organic frameworks is one strategy to enhance the gas
12–18
sorption properties by directly reducing the framework density.
Recently, a method based on the mixed tetrahedral nodes has
3
+
been developed by utilizing tetrahedrally coordinated B and
+
18
3+
+
Li . Since the total charge of B and Li equals that of two
2
+
Zn ions, the resulting frameworks (known as boron imidazolate
framework or BIF) are still neutral. These BIFs exhibit similar
zeolitic topologies as ZIFs, however, the framework boron and
lithium ions are much lighter than zinc.
Scheme 1 Schematic illustration of the mixed valent ligand strategy to
build a tetrahedrally coordinated lithium framework.
a
Department of Chemistry, University of California, Riverside, CA, 92521,
Here, we report the synthesis and structures of two 3D networks
USA. E-mail: pingyun.feng@ucr.edu; Fax: (+1) 951-827-4713; Tel: (+1)
in LiL
and L
as MVLIF-1 and MVLIF-2 with formula Li(bim)[CH
Li(im)[CH (mim) ], respectively (MVLIF = Mixed Valent ligand
1
L
2
type (L
is a neutral bis(imidazolyl)methane type ligand) denoted
(im) ] and
1
is a negatively charged imidazole type ligand
9
51-827-2042
b
2
Department of Chemistry and Biochemistry, California State University,
1
8
250 Bellflower Blvd., Long Beach, CA, 90840, USA; Fax: (+1) 562-985-
557; Tel: (+1) 562-985-4843
2
2
2
2
†
Electronic supplementary information (ESI) available: Experiment de-
Lithium Imidazolate Framework). In contrast to the inability
to prepare 3D BIFs from benzimidazole, the 3D framework of
benzimidazole-containing MVLIF-1 demonstrates the effect of
the longer Li–N distance (as compared to the B–N distance).
1
tails, H NMR spectroscopy of synthesized ligands, powder XRD, TGA
analysis and CIF files. CCDC reference numbers 794129 and 794130. For
ESI and crystallographic data in CIF or other electronic format see DOI:
1
0.1039/c1dt10859j
8
072 | Dalton Trans., 2011, 40, 8072–8074
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Bis(imidazolyl)methane ligands were synthesized through a
liquid–liquid phase transfer reaction according to the literature
report with minor modifications (Scheme S1, ESI†). In brief,
a mixture of sodium hydroxide, tetraethyl-ammonium bromide
N atoms from different imidazole rings (Fig. 1). Specifically,
the lithium ion is connected to two negatively charged benzim-
idazole ligands and two neutral bis(imidazolyl)methane ligands
in MVLIF-1, while the lithium ion is connected by two nega-
tively charged imidazole ligands and two neutral bis(2-methyl-
imidazolyl)methaneligands inMVLIF-2. Suchabondingmodeby
mixed charge-complementary ligands follows exactly our original
synthetic strategy. However, an obvious difference between the two
structures is that, in MVLIF-1, two neutral ligands attached to the
same Li site are oriented away from each other, while in MVLIF-2,
the two neutral ligands are oriented towards each other (Fig. 1). In
addition, unlike the typical case in which the M–N bond between
the metal ion and imidazole is located within the imidazole plane,
in MVLIF-1, the structure is slightly distorted with one of the
Li–N bonds located off the benzimidazole plane.
(
TEAB), and imidazole in a mixed solvent of methylene chloride
and water was reacted under reflux conditions. After about 24
h, the final white powder product was obtained by extraction of
the reaction mixture with methylene chloride, followed by rotating
evaporation. To examine the structure-directing role of substitu-
tion groups on the ligands, the bis(2-methyl-imidazolyl)methane
was also synthesized by the similar method. The identity and the
1
purity of the products were confirmed by H NMR spectroscopy
(
Fig. S1, ESI†).
The typical synthesis of the LiL
by solvothermal reactions of excess Li
1
L
2
compounds was performed
S and equal molar amount
of both types of ligands (HIm type and CH (Im) type) in
2
2
◦
2
acetonitrile. Keeping the reaction mixture at 120 C for 48 h leads
to the formation of high quality colorless crystals for single-crystal
X-ray diffraction measurement. The purity of the compounds was
confirmed by the powder XRD patterns (Fig. S5 and S6, ESI†).
In fact, the synthesis of such lithium compounds has proven to be
challenging, mainly in two aspects: (1) the choice of the solvent and
(
2) the choice of the lithium source. As is well known, lithium has a
strong affinity with oxygen-containing species. However, there are
only Li–N bonds in our target structures, whose affinity is weaker
than that between lithium and oxygen. Thus we chose acetonitrile
as the solvent to eliminate the strong solvation effect that is
likely with oxygen-containing solvents such as amides, alcohols,
+
Fig. 1 Coordination environment of Li in (a) MVLIF-1 and (b)
MVLIF-2. (purple: Li; blue: N; black: C).
or water. Similarly, we used Li
the possible affect from oxygen-containing anions. In addition, it
is essential to maintain the basic environment by using excess Li
to fully deprotonate the imidazole ligand during the synthesis.
Other salts such as LiNO have been found to be ineffective.
2
S as the metal source to exclude
Similar to ZIFs and BIFs, the steric effects of substituent groups
exhibit strong structure-directing effects. Through two unique
combinations between neutral and charged ligands (bim and
CH (im) in MVLIF-1 vs. im and CH (mim) in MVLIF-2), two
2
S
3
2
2
2
2
The crystal structures of both compounds have been solved
by single-crystal X-ray crystallography (Table S1, ESI†). In both
compounds, lithium sites are tetrahedrally coordinated with four
totally different chiral frameworks (Fig. S2, ESI†) were obtained.
The topological analysis shows that MVLIF-1 adopts the quartz
dual (qzd) topology (Fig. 2c), which is an intrinsically chiral net. In
Fig. 2 (a) Segment of Li(bim) helical chain in MVLIF-1, (b) Segment of Li(im) zigzag chain in MVLIF-2, and illustration of framework topologies in
MVLIF-1 (c) and MVLIF-2 (d).
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comparison, MVLIF-2 adopts the single diamond (dia) topology
Fig. 2d), also with a chiral symmetry. The structure of MVLIF-
can be understood as parallel helical chains of Li(bim) inter-
connected by the neutral bis(imidazolyl)methane (Fig. 2a). In
MVLIF-2, zig-zag chains of Li(im) are further bridged by the
neutral bis(2-methyl-imidazolyl)methane ligand (Fig. 2b) into the
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1
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D framework.
The gas sorption properties of both compounds have also been
studied, and both compounds showed the ability to reversibly
adsorb hydrogen gas (Fig. 3). The volumetric uptake capacity is
1
233.
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-3
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-3
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2
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This work was supported by the Department of Energy-Basic
Energy Sciences under Contract No. DE-SC0002235. We thank
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074 | Dalton Trans., 2011, 40, 8072–8074
This journal is © The Royal Society of Chemistry 2011