A "strongly" self-catenated metal-organic framework with the highest topological density among 3,4-coordinated nets

Article abstract of DOI:10.1021/ic4018757

A new type of 3D "strongly" self-catenated metal-organic framework (SDU-9) has been constructed from [Cu₂(COO)₄] paddlewheel secondary building units and a tripodal carboxylate linker. SDU-9 ([Cu₆(H₂O)₆L₄]·24H ₂O, where [H₃L = 4,4′,4″-(hydroxysilanetriyl) tris(triphenyl-4-carboxylic acid), represents a rare example of a highly symmetrical coordination network and extremely tight self-catenation. To the best of our knowledge, SDU-9 has the highest topological density among all known 3,4-coordinated nets.

Full text of DOI:10.1021/ic4018757

Communication
pubs.acs.org/IC
A “Strongly” Self-Catenated Metal−Organic Framework with the
Highest Topological Density among 3,4-Coordinated Nets
Huiqing Ma,† Di Sun,† Liangliang Zhang,‡ Rongming Wang,‡ Vladislav A. Blatov,*,§,∥ Jie Guo,†
and Daofeng Sun*,†,‡
†Key Lab for Colloid and Interface Chemistry of Education Ministry, School of Chemistry and Chemical Engineering, Shandong
University, Jinan 250100, People’s Republic of China
§Department of Chemistry, Samara State University, Ac. Pavlov St. 1, Samara 443011, Russia
‡College of Science, China University of Petroleum (East China), Qingdao Shandong 266555, People’s Republic of China
∥Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
S
* Supporting Information
html), a few examples of self-catenated networks with nodes of
different connectivity such as 4,4-,13 3,5-,14 3,6-,15 and 3,12-
ABSTRACT: A new type of 3D “strongly” self-catenated
connections16 have been observed; however, reports on a self-
catenated 3,4-coordinated network are quite rare. Mu et al.
reported an unusual 3,4-coordinated self-catenated network that
can be viewed as the cross-linking of a 2D + 2D → 3D inclined
polycatenation.17 Similarly, most of the self-catenated networks
can be represented as interweaved arrays of 2D or 3D individual
equivalent components after breaking some edges,18 which we
call the “weakly” self-catenated nets, in order to discriminate the
net reported here. Generally, most reported self-catenated nets
are constructed from flexible organic ligands; reports on rigid
carboxylate ligands are somewhat rare. To construct such self-
catenated nets, the conformation and linking mode of the organic
ligand seem very important.
Recently, we reported a series of porous 3,24-coordinated rht-
type MOFs based on C3v-symmetry silicon-based hexatopic
carboxylate linkers.19 According to the TOPOS TTD collec-
tion,11b in total there are 20 examples of rht-type coordination
networks, the first of which were synthesized in 2008.
Developing our previous results, in this work, we have
synthesized a new tripodal organic ligand, 4,4′,4″-
(hydroxysilanetriyl)tris(triphenyl-4-carboxylic acid) (H3L). Sur-
prisingly, the solvothermal assembly of H3L and copper
paddlewheel secondary building units (SBUs) resulted in the
formation of a novel “strongly” self-catenated MOF
([Cu6(H2O)6L4]·24H2O, which is quite different from other
MOFs based on tripodal carboxylate ligands and paddlewheel
SBUs.
metal−organic framework (SDU-9) has been constructed
from [Cu2(COO)4] paddlewheel secondary building units
and a tripodal carboxylate linker. SDU-9 ([Cu6(H2O)6L4]·
24H2O, where [H3L = 4,4′,4″-(hydroxysilanetriyl)tris-
(triphenyl-4-carboxylic acid), represents a rare example
of a highly symmetrical coordination network and
extremely tight self-catenation. To the best of our
knowledge, SDU-9 has the highest topological density
among all known 3,4-coordinated nets.
etal−organic frameworks (MOFs) have caused an
Mupsurge of interest not only for their potential
applications in gas adsorption, catalysis, ion exchange, bio-
medical applications, etc.,1−6 but also for their intriguing variety
of architectures and topologies.4a,7 The network topological
approach proposed by Wells8 is an important and essential aspect
of the analysis, comparison, and design of MOFs by reducing
multidimensional structures to simple node-and-connection
reference nets. In the past decade, many structural types and
entanglement topological features unprecedented in the world of
inorganic compounds and minerals have been unmasked in the
MOF field.9 Recently, Yang and co-workers described a silver
coordination polymer with a record 54 interpenetrating
networks, which provide a new template for future highly
interpenetrated or self-catenated structures.10
Among the different types of entanglements, such as
interpenetration, polycatenation, polythreading, and self-cate-
nation (other equivalent terms are self-penetration or polyknot-
ting), interpenetrating networks are the most abundant and most
comprehensively studied objects, as shown in the extensive
reviews.11 However, the study of self-catenated networks
considered as extended periodic equivalents of molecular knots
remains largely unexplored.11a These structures are single
networks with regions in which chains from the same net pass
through smallest topological circuits in a fashion similar to that of
interpenetrating systems. Recently, uninodal (with one kind of
node) self-catenated coordination networks were reviewed by Ke
et al.12 According to the list given by Batten in a web site (http://
www.chem.monash.edu.au/staff/sbatten/interpen/examples7.
The solvothermal reaction of Cu(NO3)2·6H2O with H3L
(Figure 1a) in N,N-dimethylformamide/ethanol/water [1:1:1
(v/v/v), 10 mL] at 75 °C for 2000 min yields cubic green crystals
of SDU-9. Single-crystal X-ray diffraction analysis reveals that
SDU-9 crystallizes in the chiral cubic space group F432. Two
copper ions are bridged by four carboxylates to form the well-
known paddlewheel [Cu2(COO)4] SBU (Figure 1b). Each SBU
connects four organic ligands, and each ligand binds three SBUs
to form a 3D 3,4-coordinated self-catenated MOF containing
right-handed helices along the [1,0,1] direction (Figure 1d). The
Received: July 23, 2013
Published: September 23, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
complete framework. Because the underlying net is edge-
transitive (all edges are equivalent by symmetry), it cannot be
naturally split into separate interpenetrating 2D or 3D nets; after
breaking any edge, one obtains an unconnected structure. Hence,
we call such self-catenation “strong”. The self-catenation is
extremely tight: each 12-ring is crossed by more than 100 other
12-rings. Figure 3 demonstrates the unprecedented complexity
Figure 1. (a) Organic ligand used in this work. (b) Coordination
environment of CuII ions in SDU-9 with thermal ellipsoids at the 50%
probability level. Hydrogen atoms were omitted for clarity. (c)
Connection type between the helical chains (viewed along the [0,1,−
1] direction). (d) Perspective view of the helical chain along the [1,0,1]
direction in SDU-9 (top) and the 1D channel (bottom). (e) 3D packing
of SDU-9 along [1,0,0] direction.
Figure 3. (left) One of the 12-rings (12b, in blue) catenated by 176
other 12-rings in the underlying net sdf1 of SDU-9. (right) One of the
174 simple links between the 12-rings and one of two multilinks.
connection type between the helices is different (Figure 1c): one
is connected through the [Cu2(COO)4] SBU, and the other is
linked through the silicon atom.
To perform topological analysis of the 3D architecture, we
have used the TOPOS program package.20 Topologically, by
regarding each Cu2-SBU as a 4-coordinated node (Figure 2) and
of self-catenation: all depicted nodes of the underlying net belong
to the 176 12-rings that catenate the same (blue) 12-ring. Besides
simple (Hopf-type) links between 12-rings, there are a few
multilinks (also known as Solomon links)22 that are rare in self-
catenated coordination networks (Figure 3). This tight self-
catenation causes a high topological density23 of the net, TD10 =
3245; according to the RCSR database,24 to the best of our
knowledge, this is the highest topological density among all
known 3,4-coordinated nets.
On the basis of the calculation from the PLATON/VOID
routine,25 the total solvent-accessible volume for the desolvated
framework after removal of guest solvents and coordinated water
molecules is estimated to be 35.9%. To check the permanent
porosities of SDU-9, the freshly prepared samples were soaked in
methanol and dichloromethane to exchange the less volatile
solvent molecules. A color change from bright-blue to blue-
green, indicating that open CuII sites similar to those observed for
other frameworks have been generated.18 As shown in Figure 4,
desolvated crystal SDU-9 displays a typical type IV adsorption
Figure 2. Schematic representation of the 3D framework of SDU-9,
where the purple balls are the simplified paddlewheel units.
each L as a 3-coordinated node (Figure 2), the overall 3D net can
be rationalized as a binodal 3,4-coordinated edge-transitive (with
one kind of edge) underlying net (i.e., the net of centroids of
structural groups) with the point symbol21 of (123)4(126)3
(Figure 2). It is striking that this net has collisions; i.e., some
of its vertices have coordinates that are the average of the
coordinates of their neighbors.11c,d This feature is quite rare for
the underlying nets.11c Topological classification with the
TOPOS TTD collection revealed that such a net has never
been found in crystals; we have deposited it into the TTD
collection under the name sdf1. Further analysis showed that
there are four kinds of 12-rings in this 3D framework (Figure S4
in the Supporting Information). Each ring consists of six Cu2-
SBUs that are connected by six halves of organic ligands.
The most fascinating and peculiar structural feature of SDU-9
is that all of the rings are catenated together, coming to a
Figure 4. N2 sorption isotherms at 77 K (solid circles, adsorption; open
circles, desorption).
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Communication
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In summary, a 3D (123)4(126)3 coordination network with
high topological density and extremely tight self-catenation of
12-rings has been obtained based on [Cu2(COO)4] paddlewheel
SBUs and a tripodal carboxylate linker. The self-catenation is
“strong”; i.e., the network cannot be transformed into an array of
interpenetrating nets by breaking any chemical bond.
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ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data in CIF format, detailed synthetic
procedures, IR and TGA for the compounds, and additional
graphics. This material is available free of charge via the Internet
at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: blatov@samsu.ru.
*E-mail: dfsun@sdu.edu.cn.
Funding
This work was supported by the NSFC (Grants 21001115 and
21271117), NCET-11-0309 and the Shandong Natural Science
Fund for Distinguished Young Scholars (JQ201003), and the
Fundamental Research Funds for the Central Universities
(Grants 13CX05010A and 13CX02006A).
Notes
The authors declare no competing financial interest.
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