Temperature-dependent synthesis of metal-organic frameworks based on a flexible tetradentate ligand with bidirectional coordination donors

Article abstract of DOI:10.1021/ja0701917

Using a temperature-dependent synthetic approach, four distinct new silver(I) coordination polymers resulting from the different conformations adopted by the flexible ligand at different temperatures were obtained. Copyright

Full text of DOI:10.1021/ja0701917

Published on Web 03/24/2007
Temperature-Dependent Synthesis of Metal-Organic Frameworks Based on a
Flexible Tetradentate Ligand with Bidirectional Coordination Donors
Yu-Bin Dong,*,† You-Yun Jiang,† Jie Li,† Jian-Ping Ma,† Feng-Ling Liu,† Bo Tang,*,†
Ru-Qi Huang,† and Stuart R. Batten‡
College of Chemistry, Chemical Engineering and Materials Science, Engineering Research Center of Pesticide and
Medicine Intermediate Clean Production, Ministry of Education, Shandong Normal UniVersity, Jinan 250014,
People’s Republic of China, and School of Chemistry, Monash UniVersity, Victoria 3800, Australia
Received January 10, 2007; E-mail: yubindong@sdnu.edu.cn
Crystal engineering of metal-organic coordination frameworks
based on transition metals and organic spacers is currently attracting
significant attention.1 One of the most important motivations is that
programmable crystal engineering may provide a new approach to
the synthesis of functional nanoporous materials.2 In general, the
polymer topology can be modified by the chemical structure of
the ligands chosen, the coordination geometry preferred by the
metal, the inorganic counterions, the solvent system, and sometimes
the metal-to-ligand ratio.3 However, one controlling factor in
Figure 1. The chiral 1D chain substructure formed in 1. The intercalated
-
SbF6 anions are also shown. Carbon atoms are shown as green, oxygen
as red, nitrogen as blue, silver atoms as pink, antimony atoms as purple,
and fluorine atoms as yellow.
determining the ultimate topology of metal-organic frameworks,
which has not been widely employed to date, is temperature.4 It is
well-known that flexible organic molecules adopt different con-
formations under different conditions, especially at different tem-
peratures.5 Thus the reaction temperature is an additional important
factor that may be utilized in determining the framework topology
by controlling the ligand conformation.
interpenetrating 3D network. The structure crystallizes in the chiral
tetragonal space group I4122, and the asymmetric unit contains one
Ag(I) cation, one L ligand, and one SbF6- anion. Each ligand bonds
to four Ag cations via the nitrile nitrogen atoms (Ag-N )
2.240(10)-2.366(8) Å), while in turn each tetrahedral Ag(I) cation
coordinates to four ligands. The ligands bridge to create 1D chains
of cations connected by pairs of C(CN)2 moieties (Figure 1). The
silver atoms and C(CN)2 groups form squares; these squares share
Ag(I) corners to create the chain. The chains are helical and lie
about a 41 axis. These 1D chains are cross-linked by the backbones
of the ligands, which adopt a trans-conformation, such that each
chain is connected to four adjoining ones. All chains within the
structure have the same handedness, leading to an overall chiral
structure. The ligands cross-link the chains in π-stacked pairs
(closest C‚‚‚C contact is 3.55 Å) in which both ligands connect
the same chains, but one projects “forward” from the first chain to
the second, while the other projects “backward”, relative to the
Following this idea, we designed and synthesized a flexible
ethylene glycol ether-bridging tetradentate ligand L (Scheme 1),
Scheme 1. Synthesis of L, 1, and 2
-
direction of propagation of the chain. The SbF6 anions lie in
cavities within the network. The unusual topology of the network
may be simplified by considering the ligands and metal ions as
4-connecting nodes. This generates the 3D binodal 4-connected
network shown in Figure 2. This is a very unusual network (Schla¨fli
symbol (42.84)Ag(42.82.102)L); we are unaware of a precedent despite
the large number of 4-connected networks reported in the literature.
When viewed down the c-axis, very regular square channels
(crystallographic dimensions, ca. 10 × 10 Å) are evident in 1
(Figure 2).
When the reaction was carried out at 30 °C, compound AgLSbF6
(2) was generated, with a 2D (6,3) sheet structure (Supporting
information). It is noteworthy that, in 2, the L ligands adopt a cis-
conformation. From 1 to 2, the dramatic structural change observed
clearly results from the different conformations adopted by L at
different temperatures.
in which the dicyanomethylene groups act as the terminal coordina-
tion donors. The freedom of rotation around the central C-C single
bond gives rise to cis- or trans-conformations which may be frozen
at different temperatures. The cis- and trans-conformations of L
generate different donor orientations and thus different polymeric
patterns. In this contribution, we report four novel silver(I)
coordination polymers based on this temperature-dependent ap-
proach (Scheme 1).
As shown in Scheme 1, compounds 1-4 were synthesized based
on L and AgX (for 1-2, X ) SbF6-; for 3-4, X ) BF4-) in a
C6H6/CH2Cl2 mixed solvent system at 0 and 30 °C, respectively.
In AgLSbF6 (1), L adopts the trans-conformation and acts as a
tetradentate ligand to bind Ag(I) ions into a porous non-
To further investigate the effect of temperature on the structures
of Ag(I)-L coordination polymers, AgBF4 was used instead of
AgSbF6 to perform the reactions.
† Shandong Normal University.
‡ Monash University.
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J. AM. CHEM. SOC. 2007, 129, 4520-4521
10.1021/ja0701917 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
A particularly interesting and challenging subject in metal-organic
polymers is the design and construction of metal-organic nanotu-
bular structures. So far, for constructing nanotubular structures, the
coordination approach has been shown to be very effective, for
example, in the assembly of coordination helices or tape-like
secondary building blocks.6 Compound 4 provides a new approach
for the rational construction of polymeric coordination-driven
molecular tubes. That is, the polymeric metallatube is generated
by the combination of square planar metal ions with bent tetraden-
tate ligands which each possess two pairs of terminal coordination
groups with each pair of coordination donors facing opposite
directions.
To gain a deeper insight into the formation of compounds 1-4
based on the temperature-controlled approach, DFT calculations
were employed to examine the thermodynamics of the different
ligand conformations. The calculated results indicate that the trans-
conformation is more stable in solution. The transformation barrier
between trans- and cis-conformations is relatively low (13.25 kcal/
mol), and the transformation process is very fast (k ) 2.95 × 1010
s-1). Furthermore, the higher temperature clearly enhances the
tendency for the transformation from the trans- to cis-conformation,
which supports the current results and discussion.
Figure 2. Left: The overall network topology in the structure of 1; pink
spheres represent Ag(I) nodes, while the gray spheres represent the L ligand
nodes. Right: stick representation of the non-interpenetrating 3D network
containing different square-like channels.
Figure 3. The side and top views of the 1D tube motif in 4. Carbon atoms
are shown as green, oxygen as red, nitrogen as blue, and silver atoms as
pink.
In summary, we have synthesized four Ag coordination polymers
whose structures depend on the reaction temperature. This work
demonstrates that the temperature parameter can be used to control
the conformation of flexible ligands and, consequentially, the
topology of the metal-organic frameworks. Work is in progress to
obtain new metal-organic frameworks generated from other flexible
ligands and transition metals based on this approach.
Acknowledgment. We are grateful for financial support from
the National Natural Science Foundation of China (Nos. 20671060,
20371030, and 20335030), and Shandong Natural Science Founda-
tion (Nos. Z2004B01 and J06D05).
-
Figure 4. The packing of the tubes in 4 (left). Also shown are the BF4
counterions and the bis-η1-coordinated benzene molecules. One layer (right)
defined by the weak intermolecular Ag‚‚‚π interactions between the tubes
and the intercalated benzene molecules (highlighted in brown).
Supporting Information Available: Crystallographic data and CIF
files of 1-4, synthesis of L, 1-4, and structural description of 2 and
3 (PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
Compounds [Ag2L(H2O)](BF4)2 (3) and AgLBF4‚0.5(C6H6) (4)
were obtained by combination of L with AgBF4 at 0 and 30 °C,
respectively. In 3, the L ligands adopt a trans-conformation and
coordinate to four silver atoms to generate highly corrugated 2D
sheets (Supporting information).
The structure of 4 is remarkably different. In 4, the silver atom
acts as square planar 4-connecting nodes, while the ligands adopt
the cis-conformation, and 1D planar chains are formed. As shown
in Figure 3, two of these chains are connected to form a 1D tube
motif.
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