Coordination Chemistry Reviews 257 (2013) 2232–2249
Contents lists available at SciVerse ScienceDirect
Coordination Chemistry Reviews
journal homepage: www.elsevier.com/locate/ccr
Review
Interpenetration control in metal–organic frameworks for
functional applications
Hai-Long Jiang a,b , Trevor A. Makal a , Hong-Cai Zhou a,∗
a
Department of Chemistry, Texas A&M University, College Station, TX 77843, USA
Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, University of Science
and Technology of China, Hefei, Anhui 230026, PR China
b
Contents
1.
2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2232
Structural Interpenetration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2233
2.1.
Polycatenation, polythreading and polyrotaxane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2233
2.2.
Polyknotting or self-penetrating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2234
2.3.
Interpenetration based on 1D chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2236
2.4.
Interpenetration based on 2D layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2236
2.5.
Interpenetration based on 3D networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2237
2.6.
Interpenetration of networks with different dimensionalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2238
Interpenetration control and related functional applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2238
3.1.
Reaction temperature and concentration control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2239
3.2.
Template-directed control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2240
3.3.
Ligand design/modification-induced control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2243
3.4.
Coordinated or uncoordinated solvent removal/addition-triggered control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2245
3.5.
Layer-by-layer assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2246
3.6.
Relationship between structural interpenetration and functional applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2246
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2247
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2247
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2247
a r t i c l e
i n f o
Article history:
Received 8 November 2012
Accepted 13 March 2013
Keywords:
Interpenetration
Metal–organic framework
Coordination polymer
Surface area
Hydrogen uptake
a b s t r a c t
Interpenetration in metal–organic frameworks (MOFs) is an intriguing phenomenon with significant
impacts on the structure, porous nature, and functional applications of MOFs. In this review, we provide an
overview of interpenetration involved in MOFs or coordination polymers with different dimensionalities
and property changes (especially gas uptake capabilities and catalysis) caused by framework interpenetration. Successful approaches for control of interpenetration in MOFs have also been introduced and
summarized.
Published by Elsevier B.V.
1. Introduction
Metal–organic frameworks (MOFs) are porous organic–
inorganic hybrid materials, often regarded as a subclass of coordination polymers, constructed from metal ions or clusters and
organic ligands linked via coordination bonds to form infinite
∗ Corresponding author. Tel.: +1 979 845 4034; fax: +1 979 845 4719.
E-mail address: zhou@mail.chem.tamu.edu (H.-C. Zhou).
0010-8545/$ – see front matter. Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.ccr.2013.03.017
systems [1]. MOFs are becoming one of the most rapidly developing fields in chemical and materials sciences, not only due to the
intriguing structural topologies but also because of their potential
as functional materials in structure-dependent applications, such
as gas storage and separation, sensing, catalysis, and drug delivery,
as well as various proof-of-concept demonstrations [2–7]. MOFs
are generally constructed by inorganic vertices (metal ions or
clusters) and organic linkers via metal–oxygen or metal–nitrogen
coordination bonds. The most attractive features of MOFs are
their crystalline nature, high and permanent porosity, uniform
H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
pore sizes in the nanoscale range (from several angstroms up to
10 nm), as well as high surface area (typically, BET surface area of
1000–3000 m2 /g, and the Langmuir surface area record is more
than 10,000 m2 /g) [8]. Additionally, the chemical versatility and
structural tailorability provide a significant level of tunability
to the physical and chemical properties of MOFs. The judicious
combination of metal ions and predesigned organic ligands under
suitable reaction conditions afford various kinds of MOF structures
with desired functionalities.
Interpenetration, sometimes also referred to as catenation, can
be expressed as polymeric analogs of catenanes and rotaxanes,
and is the most common form of entanglement [9,10]. The occurrence of structural interpenetration is currently becoming very
common with significant increases in the number of reported
MOFs. While there are no chemical bonds between the interpenetrated networks, they cannot be separated without the breakage
of bonds. Largely, it seems that the formation of structurally interpenetrated systems is hardly anticipated and is discovered rather
serendipidously. Generally speaking, porous materials minimize
the systematic energy through optimal filling of void space, and
thus structural interpenetration may occur only if the pore space
of an individual net is sufficiently large to accommodate an additional net. When two or more networks (less guest species) are
generated from the same combination of ligand and metal clusters, varying only in the degree of interpenetration, they may be
described as interpenetration isomers, a specific type of framework
isomer [11]. Interpenetrated motifs that minimize the empty space
could significantly enhance the stability of frameworks, not only
through filling of void space but also in the formation of repulsive forces that serve to prevent one net from collapsing in on
itself. Therefore, the use of elongated organic linkers in attempts to
synthesize MOFs with expected large pores is generally a very challenging task, as the formation of interpenetrated frameworks may
be preferred to increase the stability of the framework. Undoubtedly, structural interpenetration, which is closely associated with
pore character (such as size, environment, etc.), plays a crucial role
in the functional applications of MOFs.
It should be noted here that a great number of interpenetrated
frameworks have been reported in coordination polymers with 1D
or 2D networks due to their remarkable structural flexibility and
diversity [9]. Generally, the term “coordination polymer” constitutes extended structures based on metal ions and organic ligands
that link into an 1D chain, 2D sheet, or 3D architecture, and the
term “MOF” is widely used to describe 3D porous coordination
networks, but is seldomly used to describe 1D or 2D structures.
However, the boundary between coordination polymer and MOF
is still confusing, as neither term has been rigidly used in previous
reports [12]. In this contribution, we maintain that both terms may
be used interchangeably, preferring the term MOF, no matter the
dimension of the structures. In the first section, we briefly review
interpenetration involved in MOFs with different dimensions. The
differences in the concepts of interpenetration, polycatenation,
polythreading, polyrotaxanes and polyknotting (self-penetrating)
that are frequently employed to describe structures (especially for
flexible coordination polymers) have also been discussed. In the
second section, successful strategies for interpenetration control
in the syntheses of interpenetration isomers and other noninterpenetrated frameworks have been introduced, highlighting the
differences in functional applications (esp. gas adsorption and
catalysis) resulting from structural interpenetration in these MOFs.
2. Structural Interpenetration
As the most investigated type of entanglement, interpenetration
is very common in coordination polymers. Along with increasing number of coordination polymers with very flexible structures
2233
Fig. 1. Schematic illustration shows “assembly procedure” for the 2-fold interpenetrated framework involving the polycatenation of 0D cages to 3D architectures that
are interpenetrated.
Adapted with permission from Ref. [13a]. Copyright 2010, Nature Publishing Group.
reported, interpenetration readily accompanies new and more
complex types of entanglement being recognized, such as, polycatenation, polythreading and polyknotting, which are reminiscent
of molecular catenanes, rotaxanes, and knots, respectively, and will
be discussed based on their respective features. Following that, we
present an introduction of structural interpenetration based on the
dimension of single interpenetrating units (0D, 1D chain, 2D layer
or 3D network), as quite a few reviews examining interpenetration in flexible coordination polymers have been published [9,10].
Interpenetration among 0D structures (also termed as catenane,
borromean, etc.) only occurs in very flexible coordination polymers
[9a,d] and will not be discussed in detail.
2.1. Polycatenation, polythreading and polyrotaxane
It is almost unavoidable to mention polycatenation when
describing interpenetration because of their close relation,
especially in flexible structures. In fact, the strict/detailed classification of entanglements indicates polycatenation has significantly
different features from interpenetration [9d]. Generally, for polycatenation: (1) the motifs could be the same or different types in
0D, 1D or 2D nets that contain closed loops to be interlocked; (2)
the number of entangled motifs can be finite or infinite; (3) all the
constituent motifs have lower dimensionality than that of resultant
architectures; (4) each individual motif is catenated only with the
surrounding ones but not with all the others, etc. In contrast, for
interpenetration systems: (1) all individual motifs with an identical topology are usually extended 2D or 3D networks; (2) the
number of interpenetrated motifs is finite; (3) the single network
and final structure have the same dimensionalities; (4) each single network is interlaced with all the other ones to afford the final
structure. To date, polycatenation has been reported in many flexible coordination polymers [13]. One vivid example is a recent 2-fold
interpenetrated MOF exhibiting two identical 3D polycatenated
architectures, each of which is achieved by mechanical interlocking
of 0D adamantane-like molecular cages as building blocks in three
directions (Fig. 1) [13a]. The first polycatenated array of 1D nanotubes has been obtained by interlocking of each nanotube aligned
in parallel with the two nearest neighboring ones, giving rise to
the final 2D polycatenated layer [13b]. Recently, a coordination
polymer consisting of 2-fold interpenetrated layers was reported,
in which the interpenetrated layers are further catenated to the
two adjacent such sheets in parallel fashion to afford an overall
unique (2D → 3D) polycatenated framework, further revealing the
common co-existence of interpenetration and polycatenation in a
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H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
Fig. 2. (a) Perspective view of the polycatenated structure and (b) temperature dependence of the magnetic susceptibility curves. The solid red line shows the Curie–Weiss
fitting.
Adapted with permission from Ref. [13i]. Copyright 2011, The Royal Society of Chemistry.
structure [13e]. It is noticeable that, just like MOF and coordination
polymer, the terms polycatenation and interpenetration are sometimes not rigidly used as the strict definitions above. The terms as
they appeared in the original works are maintained in this review.
Some entangled frameworks with polycatenation character show interesting magnetic properties. Two polycatenated
complexes have been assembled by appropriately combining
Co(II) with long, linear bidentate N-donor ligands and the antiinflammatory drug olsalazine, which display a new 2D → 3D
parallel polycatenation of undulating (4,4) layers and an unusual
2D → 3D inclined polycatenation of (6,3) layers, respectively. The
magnetic properties of both compounds have been studied by measuring their magnetic susceptibility in the temperature range of
2–300 K. For the compound with parallel polycatenated structure,
the m T remains roughly constant from 300 to 50 K, and then
decreases upon further cooling, which is attributed to the zero-field
splitting (ZFS). The Curie–Weiss fitting of 1/m in the temperature range of 2–300 K gives a good result with C = 2.33 cm3 K/mol
and = −1.57 K. For the compound with inclined polycatenated
structure, the 1/m T vs. T plot in the range of 21–300 K follows
the Curie–Weiss law with C = 6.67 cm3 K/mol and = −18.64 K. The
deviation of the fitted curve from the experimental data at lower
temperature suggests the ZFS effect still plays a role [13h]. An
interesting MOF with 2D + 2D → 3D inclined polycatenation based
on a mixed N-donor and carboxylate ligands has been assembled
(Fig. 2a). Cobalt(II) ions are first bridged by carboxylate ligands to
form Co3 clusters, which are further linked by the N-donor ligands to afford 2D 44 -square lattice (sql) layers with large rhomboid
grids. Magnetic investigation indicates that paramagnetism and
canted antiferromagnetism coexist with Tc of 48 K. The reciprocal molar magnetic susceptibility data obey the Curie–Weiss law
in the high temperature region of 58–300 K with a Curie constant
of C = 3.53 cm3 K/mol and a Weiss constant of = −14.76 K, which
indicates that dominant interactions between the Co(II) ions are
antiferromagnetic. When the temperature is lowered the m T value
increases abruptly to a maximum of 5.41 cm3 K/mol at 48 K, and
then drops quickly to a minimum of 3.29 cm3 K/mol at 5 K (Fig. 2b).
This behavior indicates the occurrence of a ferromagnetic coupling
below 64 K, which may be attributed to spin-canting in a nonlinear
antiferromagnet or a ferrimagnetic transition [13i].
Polythreaded architectures are polymeric analogs of molecular rotaxanes and pseudorotaxanes, where numerous fascinating
topologies have been observed [9d,10f]. Both polyrotaxanes and
polypseudorotaxanes contain closed 2-membered rings/loops and
rod/string elements that thread through the loops. The only difference between them is that both ends of the rod/string have
capping groups (similar to a dumbbell) in the former system,
making the motifs inseparable without breaking links, whereas
the rod/string may slip off and disentangle due to the absence
of capping groups in the latter. The constituent units could have
0D, 1D or higher dimensionalities and the resultant network can
present the same or an increased dimensionality compared to
that of the polythreaded motifs. Quite a few interesting coordination polymers exhibiting polythreading network architectures
have been reported so far [14], in which polythreading with
finite components is relatively rare. Some reported compounds
involve polythreaded 0D rings with side arms that afford 1D or
2D arrays [15], and molecular ladders with dangling arms, leading
to 1D → 2D or 1D → 3D [16] polythreaded arrays. The 3D polythreading coordination polymers based on 2D motifs was first
reported in Zn(Hbtc)(4,4 -bpy) (btc = 1,2,4-benzenetricarboxylate;
4,4 -bpy = 4,4 -bipyridyl, Scheme 1), in which 2D motifs with side
arms lead to polythreaded network exhibiting a 2-fold interpenetrated 3D array, when H-bonds are taken into account (Fig. 3)
[17]. Since then, several polythreading networks assembled from
2D motifs were constructed [18,13b].
In addition, there are also some entangled networks with
coexistence of polycatenation, polythreading and polyrotaxane
characters [19]. A novel entangled framework incorporating
functional nanosized polyoxoanions in the presence of both
polycatenane and polyrotaxane has been reported, in which loopcontaining 2D layers are catenated to a 3D network of primitive
cubic (pcu) topology [19a]. Subsequently, a Cu-based MOF based on
in situ generated ligand involves chemically and structurally different 2D square grids and irregular layers in a unique 3D framework
that presents polycatenation and polythreading features [19c].
Recently, the long rigid bisimidazole ligand, 4,4 -bis(imidazol-1yl)biphenyl, and long flexible pimelic acid have been used for
constructing a 3D nickel complex with T-shaped bilayer units.
The structure represents an unprecedented 2D → 3D network containing both polycatenation and polythreading [19d], representing
another unprecedented polythreaded framework featuring 3D
entangled motif constructed from 2D polyrotaxane layers. It is the
first 2D → 3D polythreaded framework with both polyrotaxane and
polypseudorotaxane character [19g].
2.2. Polyknotting or self-penetrating
Besides polycatenation and polythreading mentioned above, as
well as the extensively studied interpenetration, self-penetrating
networks (also called self-catenating, self-entangling or polyknotting), are single nets that have the special feature that the smallest
H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
2235
Scheme 1. The organic ligands/linkers/precursors and related abbreviations mentioned in this review.
Fig. 3. The structure of a single 2D motif with dangling arms (left) and schematic illustration showing the mutual polythreading of the 2D sheets (middle) and two
interpenetrating motifs with rutile topology (right).
Reproduced with permission from Ref. [17]. Copyright 2004, The Royal Society of Chemistry.
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H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
Fig. 4. (a) Parallel interlacing of the chains in a polyrotaxane layer of
[Zn2 (bix)3 (SO4 )2 ]·8H2 O (top) and a schematic illustration for the chain entanglement (bottom). (b) Schematic view of two inclined polyrotaxane motifs involving
1D infinite interlaced polymer that results in a 2D polythreaded layer.
Adapted with permission from Ref. [22]. Copyright American Chemical Society,
2005.
topological rings are passed through (catenated) by other components of the same network [9c,20]. To build binodal high-connected
self-penetrating networks, a strategy involving the construction
of two distinct metal clusters with suitable coordination geometries with the help of appropriate ligands has been investigated.
With this strategy, the first (6,8)-connected self-penetrating MOF
has been obtained using an asymmetric neutral ligand, dinuclear zinc clusters as six-connected nodes and trinuclear zinc
clusters as eight-connected nodes [20a]. A rare ten-connected selfpenetrating MOF on the basis of tetranuclear cobalt clusters is the
highest connected uninodal network topology in self-penetrating
systems so far [20b]. The first binodal (5,12)-connected 3D selfpenetrating MOF, constructed by dinuclear [Ba2 (␮2 -OH2 )2 ] core
as 12-connected node and flexible 4,4 -sulfonyldibenzoate ligand
(Scheme 1) as 5-connected node, represents the highest connected
self-penetrating topology in MOFs [20c]. Complicated structures
sometimes involve both interpenetration and self-penetrating
character. With the combination of long and rigid dicarboxylate
linkers with a flexible V-shaped pyridylamide derivative, two Cobased MOFs built by amide derivative and organodicarboxylate
co-ligands, display 3-fold interpenetration of 65 ·8-mok nets which
are 4-connected self-penetrating nets described theoretically in the
early nineties [20e].
2.3. Interpenetration based on 1D chains
The prerequisite of interpenetration between 1D chains is that
the individual chains must contain rings. In addition, various
weak supramolecular forces (H-bonding, ␲· · ·␲ aromatic stacking interactions, and van der Waals forces) are believed to play
important roles in the formation of interpenetrated structures.
Interpenetration between 1D chains containing both rings and
rods generally occurs in a manner similar to that of catenanes or
rotaxanes. Usually, simple 1D chains are interpenetrated/entangled
in parallel to give new 1D structures. A Zn complex with 1D
chain structure is constructed by alternating rings and rods, in
which the rods pass through the center of the rings and are
formed via Br Br interactions [21]. Whether the interpenetrated
1D chains are in parallel or inclined (not colinear), either form
of entanglement could lead to higher dimensional frameworks.
Zinc(II) sulfates have been reacted with the flexible ligand 1,4bis(imidazol-1-ylmethyl)benzene (bix, Scheme 1) to afford the
novel coordination network Zn2 (bix)3 (SO4 )2 , containing 1D polymeric motifs of alternating rings and rods. This framework shows
extended rotaxane-like mechanical links generating 2D sheets via
parallel interlacing modes of the chains (Fig. 4a) [22]. The compound [Ag2 (bix)3 ](NO3 )2 has been reported in which inclined 1D
Fig. 5. (a) Schematic representation of the 2-fold parallel interpenetration between
2D layered structures. (b) Side-on view of the 3-fold parallel interpenetration from
2D to 3D. (c) View of the 3D array fabricated by interpenetration of the three sets of
layers with different colors down the a-axis.
Adapted with permissions from Refs. [26,29b]. Copyrights 2009, The Royal Society
of Chemistry and 2002, American Chemical Society, respectively.
chains are interpenetrated to give a 2D square sheet [23]. As
schematically illustrated in Fig. 4b, only rotaxane-like interactions
are involved in such network, with each ring of each net containing
a rod from another net. In addition, the complicated interpenetration of 1D chains or ladders has even been demonstrated to be
able to afford 3D networks [24]. Most of the known 1D → 3D transformations occur through 2-fold inclined catenation, with each
square interlocked by the other two ladders [24a–c]. An unusual 1D
ladder-like silver(I) complex exhibits interesting 3D plywood-like
stacking arrays. When Ag· · ·O interactions are considered, a novel
5-fold interpenetrated framework is observed with a unique [3+2]
catenation [24d].
2.4. Interpenetration based on 2D layers
Similar to that of 1D chains mentioned above, interpenetration between 2D layers also has two types: parallel and
inclined. The majority of interpenetrated 2D frameworks are
primarily based on either (4,4) or (6,3) topological nets. Parallel interpenetration gives either a new 2D layered or a 3D
structure, while a dimensionality increase from 2D layers to
an overall 3D network inevitably occurs in the case of inclined
interpenetration. A layered MOF, Cu(hfipbb)(H2 O) (H2 hfipbb = 4,4 (hexa-fluoroisopropylidene)bis(benzoic acid), Scheme 1) features
2-fold parallel interpenetration of 2D layers (Fig. 5a). Each 2D
layer has an undulating net with a rhombic window, in which
each dinuclear Cu(II) paddlewheel secondary building unit (SBU)
connects with four neighboring SBUs by the bent hfipbb ligands
[25]. CoSO4 ·7H2 O has been assembled with two long V-shaped
ligands of 1,3-bis(4-carboxy-phenoxy)propane (pcp, Scheme 1)
and 1,3-bis(4-pyridyl)propane (bpp, Scheme 1) to provide 3-fold
parallel interpenetrating networks showing 2D to 3D motifs
[26]. The mean planes in the interpenetrating layers are parallel;
however, these mean planes are offset but not coincident (Fig. 5b),
and thus generate an overall 3D entanglement structure (3-fold 2D
to 3D parallel interpenetration). To present such interpenetration
mode, each sheet rotates roughly 90◦ relative to its two adjacent
layers, as displayed in Fig. 5b. It is interesting to note that the
length of ligand and/or the distance between two metal centers in
H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
a structure could have intrinsic influence on the final number of
parallel interpenetration; the longer length of a ligand may result
in a larger number of identical interpenetrating nets, which has
been well demonstrated. For example, a 3-fold 2D → 2D parallel
interpenetrated framework with (4,4) topology was increased to a
structure with 5-fold interpenetration when the distance between
metal centers was increased [27].
In addition to parallel interpenetration commonly reported in
MOFs [28], inclined interpenetration between 2D nets has also
been reported to produce 3D interlocked structures. In inclined
interpenetration, there are two stacks of layers, with one stack
inclined compared to the other. Any particular 2D layer has an infinite number of inclined layers passing through it. In a dicopper
paddlewheel-based MOF, the Cu(II) ion has an elongated octahedral environment with four nitrogen atoms of 4,4 -bipyridine
(4,4 -bpy) ligands in the equatorial plane and two oxygen atoms of
H2 O molecules in the axial sites. The Cu(II) centers are bridged by
4,4 -bpy ligands to give a 2D sheet with square grids. The 2D sheets
lying in the (a–b)c and (b–a)c planes present an inclined 2-fold
interpenetration way to afford a 3D framework with microporous
channels along the c-axis that are filled by free SiF6 2− dianions
[29a]. The structure of [Cu(bpp)2 Cl]Cl·1.5H2 O consists of (4,4) layers on the basis of five-coordinated copper ions in a geometry of
square-pyramid. The three similar, but crystallographically independent sets of layers (shown in red, green, and blue in Fig. 5c), lie
in the ac (red) and ab (green and blue) planes and present inclined
interpenetration to give an overall 3D architecture [29b]. In the
structure of In(bdc)1.5 (2,2 -bipy) (H2 bdc = 1,4-benzendicarboxylic
acid, 2,2 -bipy = 2,2 -bipyridyl, Scheme 1), the bdc connector plays
two different roles. One bdc, located in the general position, coordinates two In(III) ions to afford 1D [In(bipy)(bdc)]+ infinite zigzag
chains along the b-axis. The other bdc ligand, the centroid of which
is located on an inversion center, interconnects with these chains
along the a-axis to result in a (6,3) hexagonal layer. Two identical sets of parallel hexagonal layers, extending from two different
stacking directions, are interlocked in an inclined way, giving rise
to an overall 3D entangled network [29c].
It should be pointed out that polyrotaxane network entanglement, which was described above as the extended periodic
structure of rotaxane motifs, plays a very important role in such
2D interpenetration. Since Robson and coworkers reported the first
unusual 2D to 2D polyrotaxane network [Zn(bix)2 (NO3 )2 ]·4.5H2 O,
in which the independent 2D polymeric layer contains bix rods
and Zn2 (bix)2 loops [30a], there have been many MOFs exhibiting parallel interpenetration of 2D layers to 2D or 3D polyrotaxane
architectures [19b,30], and also inclined interpenetration of 2D layers to 3D polyrotaxane networks [31].
2.5. Interpenetration based on 3D networks
Compared to MOFs with lower dimensionality, interpenetration in 3D MOFs is more common. Generally, MOFs constructed
with longer ligands usually have larger voids, which make
them unstable. Thus, interpenetration reasonably occurs to
reduce pore space in order to meet the systematic stability
requirement in MOFs. Different degrees of interpenetration,
for example, 2-fold [32], 3-fold [32b,33], 4-fold [33d,34], 5-fold
[18e,35], 6-fold [34b,36], 7-fold [37], 8-fold [38], 9-fold to beyond
10-fold [39], and even recently reported the highest 54-fold
interpenetration [40], have been widely investigated. Among the
interpenetrated structures, MOFs with diamond-type structural
topology, a 4-connected network with tetrahedral nodes and linear linkers are the most commonly observed structures and have
very high propensity for the formation of highly interpenetrated
structures [39c–e]. A 2-fold interpenetrated MOF as a luminescent
host has been developed for molecular decoding by embedding
2237
Fig. 6. Low-pressure CO2 isotherms for Zn-MOF at temperatures from 263 K to
298 K. Closed symbols, adsorption; open symbols, desorption. Lines between data
points are intended to guide the eye.
Reproduced with permission from Ref. [32c]. Copyrights 2010, Wiley-VCH Verlag
GmbH & Co. KGaA.
naphthalenediimide into the entangled MOF framework that
exhibits flexible structural dynamics. After encapsulation of a
class of aromatic compounds, an intense turn-on emission can be
observed and the chemical substituent of the aromatic compounds
strongly affects the observed luminescent color. It is proposed
that an enhanced naphthalenediimide–aromatic guest interaction contributes to the unprecedented chemoresponsive and
multicolor luminescence, according to the observed induced-fit
structural transformation of the entangled framework [32d]. In
addition, quite a few MOFs with interpenetrated frameworks
exhibit unique gas sorption properties [41]. A microporous doubly
interpenetrated MOF that features a primitive cubic net has
been rationally designed with small pore cavities of around 3.6 Å
interconnected by pore openings of 2.0 Å × 3.2 Å, exhibiting highly
selective sorption behavior toward certain gases [32a].
A 2-fold interpenetrated MOF with pillared paddlewheel framework based on Zn(II) coordinated to tetratopic carboxylate ligands
and linear dipyridyl ligands has been developed. Interestingly, this
MOF exhibits dramatic steps in the adsorption and hysteresis in the
desorption of CO2 (Fig. 6). This type of behavior has been observed
previously in MOF materials, but is generally only observed at
higher pressures (generally above 5–10 bar). Careful characterization shows that structural changes possibly occur in the MOF, with
interpenetrated frameworks moving with respect to each other
upon CO2 sorption. In contrast to most other MOFs with dynamic
framework behavior, the interconversions here are robust, with the
material retaining essentially all its porosity even after more than a
dozen cycles, which could be attributed to the absence of torque or
other strain on individual chemical bonds in this Zn-MOF [32c].
A MOF with a 4-fold interpenetrating diamondoid network has
been developed in which the MOF shows selective gas adsorption
behavior upon activation and potential applications in gas separation technologies for H2 , CO2 , and O2 over N2 and CH4 [34a].
Recently, a flexible 6-fold interpenetrated MOF with hydrophobic
pores has been fabricated (Fig. 7a and b). The MOF is able to adsorb
a wide variety of common gases, while water molecules cannot be
adsorbed even under 100% humidity at room temperature. Moreover, the framework is flexible enough to transform in response
to adsorption of O2 , N2 , and CO2 to exhibit stepwise adsorption
isotherms (Fig. 7c) [36].
Most recently, a unique MOF, NOTT-202, displaying partially
2-fold interpenetrated structure in which the dominant network
2238
H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
Fig. 7. (a) Topological view of the 6-fold interpenetrated diamondoid network. (b) A perspective view showing the channels running along the c-axis involved in the 6-fold
interpenetrated structure. Cu: turquoise, C: grey, H: white, N: blue, O: red. (c) Adsorption isotherms for N2 (77 K), O2 (77 K), CO2 (195 K), and H2 O (298 K). Filled and open
shapes represent adsorption and desorption, respectively. The saturation pressure (P0 ) equals 760 Torr for N2 at 77 K and for CO2 at 195 K, 155.73 Torr for O2 at 77 K, and
23.57 Torr for H2 O at 298 K.
Adapted with permission from Ref. [36]. Copyrights 2011, Wiley-VCH Verlag GmbH & Co. KGaA.
is fully present whereas the secondary partially formed network
shows an occupancy of only 0.75, has been reported. Such MOF represents a new class of dynamic material that undergoes pronounced
framework phase transition upon desolvation. The temperaturedependent adsorption/desorption hysteresis in desolvated form,
NOTT-202a, responds selectively to CO2 . The CO2 isotherm shows
interesting three steps in the adsorption profile at 195 K, and
stepwise filling of pores generated within the observed partially
interpenetrated structure has been modeled by grand canonical
Monte Carlo (GCMC) simulations. Adsorption of N2 , CH4 , O2 , Ar
and H2 exhibit reversible isotherms without hysteresis under the
same conditions, allowing capture of gases at high pressure, but
selectively leaving CO2 trapped in the nanopores at low pressure
[41g].
2.6. Interpenetration of networks with different dimensionalities
As mentioned above, the interpenetration between networks
with chemically and topologically identical structure, termed
homo-interpenetrating nets, is quite common because the same
molecular fragments favor the same periodicity. In contrast, the
interpenetration of chemically and/or crystallographically different structures, termed hetero-interpenetrating nets, is not very
common, especially those with different dimensionalities. Systems
based upon interpenetration of networks with different dimensionalities, such as 0D + 1D, 1D + 2D, 1D + 3D, and 2D + 3D, are still rare,
in spite of the intrinsic attraction of these topologies for chemists
[9d,42]. An interpenetrated MOF has been fabricated by mixed
bdc and 4,4 ,4 -benzene-2,4,6-triyltribenzoate (btb, Scheme 1)
ligands and low-nuclear metal–carboxylate SBUs. Two different
frameworks of 2D 63 bilayer and rare (3,5)-connected 3D hexagonal mesoporous silica (hms) net are involved in the interesting
2D + 3D framework, as a rare example of interpenetration among
2D bilayers and a 3D hms net [42e]. An unprecedented 2D + 3D
interpenetrated MOF has also been constructed, that features a
3D pcu net entangled by (4,4)-connected 2D layers, by introducing a large flat anthracene group in the organic ligand as structure
controlling unit to direct the whole structure [42f]. In addition to
hetero-interpenetrating structures with different dimensionalities,
there are also very few hetero-interpenetrating nets containing different chemical compositions or topologies reported [43].
Recently, an unprecedented 3D + 3D hetero-interpenetrating MOF
with different chemical compositions and topologies has been
developed. An exceptional acentric Cd-based MOF material with
2-fold hetero-interpenetrated nets consisting of a 3D diamond network and a 3D CsCl framework, which present two different nodes
(4- and 8-connected), different chemical compositions [mononuclear Cd(CO2 )4 node and trinuclear Cd3 (CO2 )8 node], and different
topologies of networks (66 and 424 ·64 ) and three 6-, 7-, and 8coordinated Cd2+ ions, has been constructed (Fig. 8). The resultant
MOF with acentric structure displays a high thermal stability (up
to 420 ◦ C) and weak SHG activity (ca. 0.8 times that of urea) [43e].
3. Interpenetration control and related functional
applications
As discussed above, the use of long linkers for the design of
frameworks often affords interpenetrated MOFs with smaller pores
[44]. Although interpenetration usually makes MOFs robust, it negatively affects the porosity of open frameworks by reducing the
size of open pores. Highly interpenetrated frameworks typically
have low porosity (<20%) and surface area, and high density, negatively affecting the potential applications of such materials since
high surface area and porosity are generally most desired in porous
materials. Taking MOF-5 as an example, it has been subjected
to numerous studies and reports in the past several years and
reported to exhibit adsorption properties with significant differences (specific surface areas in the range from 570 to 3800 m2 /g
and H2 uptakes from 1.3 to 7.1 wt.%). The Long group has found the
time period of MOF-5 exposure to air/water is a key point to its
phase purity and gas sorption capabilities [45a]. Around the same
time, the Lillerud group has employed single-crystal XRD to investigate structural differences between the MOF-5 with low and high
surface areas. The MOF-5 sample with low surface area includes
two different types of crystals. One of the phases has lower gas
adsorption capacity and surface area than the anticipated values
because it is composed by 2-fold interpenetrated MOF-5 networks.
In contrast, the cavities in the other phase are partially occupied by
Zn(OH)2 species, which inevitably makes the hosting cavity, and
possibly also adjacent cavities, inaccessible, and thus reduces the
pore volume of the material [45b].
Therefore, it is necessary to develop strategies to suppress the
interpenetration in order to construct highly porous MOFs with
high surface area. The Yaghi group has introduced the use of
H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
2239
Fig. 8. (a) The 2-fold 3D/3D hetero-interpenetrated networks in a Cd-MOF (blue, green, and purple polyhedra represent 8-, 7- and 6-coordinated Cd2+ , respectively; the MeO
groups and H atoms are omitted for clarity); (b) the Cd3 (CO2 )8 cluster-based CsCl-like framework; (c) the Cd(CO2 )4 unit-based diamond net; (d) a simplified CsCl unit from
the 8-connected node; (e) a simplified diamond unit from the 4-connected node; (f) simplified 2-fold 3D/3D hetero-interpenetrated nets in the MOF (blue: diamond unit;
red: CsCl unit).
Adapted with permission from Ref. [43e]. Copyrights 2012, The Royal Society of Chemistry.
infinite SBUs to address this issue [46]. They have constructed 4,4 biphenyldicarboxylate (bpdc, Scheme 1)-based Zn-MOFs bearing
Zn O C columnar units running along one direction. The structural topology is the same as that of the Al net in SrAl2 . However,
compared to original SrAl2 , all these MOFs have an open structure in which Zn O links within the SBUs and C6 H4 C6 H4 links
between the SBUs expand the Al net. The intrinsic arrangement of
these rods in the structure has effectively avoided interpenetration.
Subsequently, they demonstrated the usefulness of the concept of
rod SBUs in the design and synthesis of a series of MOFs [47]. Such
methodology may be applicable to longer linkers than bpdc with
the same width (one benzene ring), which could be used in identical
syntheses to afford the same framework but with accordingly larger
pores. However, the controllable construct of rod SBUs is relatively
subtle and many factors are associated with the final MOF structures. Many other groups have also devoted work to controlling
framework interpenetration in MOFs by regulating the synthetic
parameters or approaches in the last decade [48–68].
3.1. Reaction temperature and concentration control
Reaction parameters, such as, temperature and concentration,
have been found to be important in the determination of the
framework interpenetration of MOFs. It has been found that longer
organic linkers readily lead to 2-fold interpenetrating structures
during their synthesis of the IRMOF series (IRMOF = isoreticular
MOF). However, more dilute reaction solutions are prone to afford
noninterpenetrating MOFs with larger pores. Along with this
research strategy, pairs of interpenetration isomers for IRMOF10, -12, -14 and -16 have been obtained, each pair composed of
one noninterpenetrated and the other with 2-fold interpenetrated
structure [48].
The variation of temperature and concentration for controllable syntheses of a noninterpenetrated form of [Cd(4,4 bpy)(bdc)]·3DMF·H2 O (compound 1) and its previously reported
2-fold interpenetrated form, [Cd(4,4 -bpy)(bdc)] (compound 2,
Table 1) have been systematically investigated [49]. The noninterpenetrated 1 has been obtained by reacting Cd(NO3 )2 ·4H2 O,
4,4 -bpy, and H2 bdc in a 1:1:1 molar ratio in DMF/DEF (2:1, v/v) at
85 ◦ C. Compound 1 features a 3D network bearing a pillared-layer
structure, in which the layers, constructed by Cd2 N4 O8 clusters
and bdc ligand, are linked by 4,4 -bpy as pillars. The resultant
3D framework has 8.1 Å × 11.7 Å square channels along the a-axis
and 12.5 Å × 12.5 Å channels in rhombic shape along the c-axis
which are occupied by disordered DMF and H2 O solvents. The
noninterpenetrated structure of 1 is surprisingly similar to the
previously reported single net of the doubly interpenetrated compound 2. To rationalize the influence of reaction conditions for
the framework interpenetration, the authors carefully adjusted the
temperature and concentration parameters and the results clearly
showed that both variables subtly affected the interpenetration and
product purity (Table 1). Based on these studies, they concluded
that the interpenetrated isomer was preferentially produced at elevated temperature, whereas lowering the concentration of starting
materials in a certain range reduced the possibility of forming a
sublattice in the voids of noninterpenetrated structures.
Recently, on the basis of the same reaction starting materials, nonporous to microporous MOFs have been prepared by
interpenetration control through decreasing reaction temperature, and micropores have been further enlarged to mesopores
by simply decreasing reactant concentrations and reducing reaction time [50]. As displayed in Fig. 9, solvothermal reactions of
the same amounts of Cd(NO3 )2 ·4H2 O, 4,4 -bpy, and 2-amino-1,4benzenedicarboxylic acid (H2 abdc, Scheme 1) in DMF yielded
Cd(abdc)(4,4 -bpy) (3), Cd(abdc)(4,4 -bpy)·4H2 O·2.5DMF (4), and
Cd(abdc)(4,4 -bpy)·4.5H2 O·3DMF (5) with the same framework formula but different guest solvents. All these MOFs are framework
isomers with hierarchical pores based on the same dicadmium(II)
SBU and mixed ligands. The nonporous 3 has a 3D 2-fold interpenetrated network, in which each network has a pillared-layer
structure with 4,4 -bpy as pillars and the planar channel size of
13 Å × 17 Å composed by Cd(II) and abdc ligands. After interpenetration, the channels almost disappear, with free volume of only
3.4%. By only decreasing the reaction temperature from 160 to
105 ◦ C, microporous 4, with a noninterpenetrated structure, was
obtained. It has a framework structure similar to the single network
Table 1
Summary of the products obtained at different temperatures and concentrations by
reaction combination of Cd(NO3 )2 ·4H2 O, 4,4 -bpy, and H2 bdc [49].
◦
85 C
95 ◦ C
105 ◦ C
115 ◦ C
125 ◦ C
0.2 M
0.1 M
0.05 M
0.025 M
0.0125 M
0.00625 M
Unknown
Unknown
Unknown
Unknown
Unknown
1
1
1+2
2
2
1
1
1+2
2
2
1
1
1
1+2
1+2
1
1
1
1
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown
2240
H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
Fig. 9. Schematic illustration for the synthesis of MOF isomers 3–5 with hierarchical pores and related applications for 4 and 5. The structures are all presented in space-filling
mode for clearly showing the pore space. The 2-fold interpenetrated networks in structure 3 are in blue and purple, respectively.
Adapted with permission from Ref. [50]. Copyrights 2010, American Chemical Society.
of 3, but slight distortions inevitably occur in order to meet the systematic stability. The resultant pillared-layer framework of 4 with
4,4 -bpy as pillars has planar channel size of 11 Å × 19 Å surrounded
by Cd(II) and abdc ligands. Upon controlling the interpenetration,
4 possesses a free volume of 61.2%, much higher than that of 3
(3.4%). Strikingly, in contrast to 4, pillared-layer mesoporous 5 with
larger hexagonal channel sizes of 18 Å × 23 Å can be prepared with
lower concentrations of reactants. Similar to that in 3 and 4, the
4,4 -bpy linker still acts as a pillar in 5 to connect Cd-abdc layers,
leading to a 3D open-framework with enormous open channels
and a very high free volume of 68.2%. The above results, in agreement with Zaworotko’s proposition, suggest that high temperature
and concentration favor interpenetrated frameworks, while low
temperature and concentration prefer noninterpenetrated phases
and even frameworks with very large mesopores. For the possible
mechanism, multiple independent frameworks are self-assembled
at the same time to give rise to interpenetrated structures due
to faster crystal growth during MOF synthesis at higher temperatures. On the other hand, lowering the temperature reduces the
rate at which deprotonation of the carboxylic acids and nucleation
occurs. As a result, fewer independent frameworks would form,
thus substantially decreasing the possibility of the formation of
interpenetrated MOFs.
Under the excitation wavelength of ex = 362 nm, compound
3 exhibits weak photoluminescence (PL) at ∼435 and ∼525 nm,
which could be assigned to the ligand-to-metal charge transfer (LMCT) and intraligand fluorescent emission, respectively. In
comparison, 4 displays strong emission at ∼435 nm attributed
to LMCT. However, after desolvation, the PL of compound 4 is
almost quenched and shows similar emission bands as those in
3, possibly due to the distortion of framework. Interestingly, the
PL spectra of desolvated 4 in different solvent emulsions exhibit
excellent fluorescence sensing for small molecules (Fig. 9). The
emission intensity is strongly dependent on the solvent molecule:
when dispersed in acetonitrile, the fluorescence intensity gradually increases with the addition of H2 O. It is assumed that the
restoration from the distorted framework of 4 in different solvents
is responsible for the fluorescence enhancement. The result reveals
desolvated 4 could be a promising luminescent probe for detecting
small molecules. In addition, 5 has been demonstrated to be effective for size-selective dominant liquid chromatographic separation
of Rhodamine 6G (smaller than pores of 5) and Brilliant Blue R-250
(larger than pores of 5) dyes.
3.2. Template-directed control
The use of hard templates has been widely employed to construct porous materials. It is reasonable to introduce a template in
order to construct noninterpenetrated MOFs with larger pores. It is
expected that the MOF will grow around the surface of the template
and thus prevent the interpenetration of multiple nets.
The Zhou research group first conducted template-directed
interpenetration control for MOFs and, most significantly, they
have been able to make the direct comparison of hydrogen uptake
between the interpenetrated and noninterpenetrated MOF counterparts and found that structural interpenetration can effectively
enhance the hydrogen uptake capacity [51]. Starting with a planar
ligand 4,4 ,4 -s-triazine-2,4,6-triyltribenzoate (tatb, Scheme 1) and
Cu(NO3 )2 ·2.5H2 O, a 2-fold interpenetrated MOF, Cu3 (tatb)2 (H2 O)3
(PCN-6), was firstly synthesized. PCN-6 involves dicopper tetracarboxylate paddlewheel SBUs with aqua ligands that are linked by
tatb bridges in the equatorial plane to afford a network, in which
the distance between opposite corners of the involved cuboctahedral cage is as large as 38 Å (Fig. 10a). The overall structure contains
two identical interpenetrated nets, with the second net generated
by a translation of the first by c/5 in the [001] direction (Fig. 10b).
By introducing oxalate as a template, the noninterpenetrated MOF
isomer, Cu6 (H2 O)6 (tatb)4 ·DMA·12H2 O (PCN-6 ), was successfully
synthesized. As shown in Fig. 10b and c, the structure of PCN-6 can
be generated by two identical interpenetrated nets of PCN-6 , and
thus PCN-6 and 6 are the first pair of interpenetration isomers. The
authors have attempted to modulate the synthetic parameters such
as temperature and solvent, while controlled experiment results
revealed that none of these can determine the final phase but only
the template can account for the presence or absence of interpenetration. To demonstrate the generality of the template strategy,
another large trigonal-planar ligand htb (s-heptazine tribenzoate)
was also employed to react with Cu(NO3 )2 ·2.5H2 O under reaction
conditions similar to those of PCN-6 and PCN-6 , and the results
further confirmed that the addition of oxalic acid as template can
control the framework interpenetration.
The Langmuir surface area of PCN-6 (2700 m2 /g, pore volume
1.045 mL/g) is lower than that of PCN-6 (3800 m2 /g, pore volume
1.453 mL/g), indicating a 41% enhancement in Langmuir surface
area upon interpenetration (Fig. 10d), although PCN-6 has a higher
solvent-accessible volume (86%) than PCN-6 (74%). Such counterintuitive surface area increase could be attributed to the generation
H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
2241
Fig. 10. (a) Cuboctahedral cage with the inner void highlighted with large red sphere. (b) Space-filling model of the interpenetrated nets in PCN-6 and (c) noninterpenetrated
net in PCN-6 . (d) N2 sorption isotherms of PCN-6 and PCN-6 at 77 K upon activation at 50 ◦ C. (e) Excess hydrogen sorption isotherms of PCN-6 and PCN-6 at 77 K (red) and
298 K (black): circles, PCN-6; squares, PCN-6 ; solid symbols, adsorption; open symbols, desorption.
Adapted with permissions from Refs. [51b,c]. Copyrights 2007 and 2008, American Chemical Society.
of new adsorption sites and some small pores upon interpenetration. In addition to increasing the surface area, the presence of small
pores is beneficial to strengthen the overall interaction between
gas molecules and the pore walls. It should be noted that the open
channels of PCN-6 with such small pores are still large enough to
accommodate gas molecules, which guarantees high surface area.
However, other examples of interpenetration isomers have shown
that the overall surface area may drop significantly in the case that
the open channels are blocked as a result of interpenetration, as in
the MOF-5 example provided above.
Hydrogen uptake has been carefully studied for the pair of
interpenetration isomers [51b]. To clearly differentiate the contributions by coordinatively unsaturated metal sites (CUSs) and
interpenetration to hydrogen uptake, the samples were subjected
to activation at 50 ◦ C and 150 ◦ C, related to removal of guest
molecules and axial aqua ligands on the Cu centers, respectively.
As shown in Table 2, PCN-6 exhibits 133% enhancement in volumetric and 29% in gravimetric hydrogen uptake compared to
PCN-6 due to the structural interpenetration when both samples
were activated at 50 ◦ C. When activated at 150 ◦ C, hydrogen uptake
capacities of both MOFs are improved, attributed to CUSs. The
smaller improvement of hydrogen uptake upon CUS activation in
PCN-6 than that in PCN-6 suggests that some of the CUSs in PCN-6
could be blocked due to the structural interpenetration whereas the
CUSs remain accessible in PCN-6 . Inelastic neutron scattering (INS)
studies showed that the adsorbed H2 molecules first occupy the
open Cu centers of the paddlewheel units with comparable interaction energies in the two isomers, while the H2 molecules adsorb
mainly on or around the organic linkers at high H2 loadings. Understandably, the interaction between H2 molecule and framework
in interpenetrated PCN-6 with smaller pores is found to be substantially stronger than that in noninterpenetrated PCN-6 , leading
to much higher H2 uptake in the interpenetrated isomer. Hydrogen sorption measurements at high pressures up to 50 bar have
further demonstrated that framework interpenetration in MOFs is
beneficial to the enhancement of hydrogen adsorption (Table 2)
[51c].
Furthermore, there is additional evidence supporting that
interpenetration increases hydrogen uptake, especially at low
temperature [52]. In a series of Zn4 O carboxylates studied, the
interpenetrated IRMOF-11 material showed the greatest hydrogen
uptake at 77 K [52a]. The influence of framework interpenetration
for hydrogen storage in MOFs has also been examined with GCMC
simulations [52c]. The investigations have shown that interpenetrated frameworks, with more metal-corner sites per unit volume
and increased heats of adsorption due to smaller pore size, exhibit
higher hydrogen uptake at low pressure regime and low temperatures, while noninterpenetrated MOFs, with larger free volume for
adsorbed molecules, generally present greater hydrogen storage
capacities at higher temperatures and pressures.
Most recently, PCN-6 and PCN-6 framework isomers have
been reproduced by a sonochemical approach and demonstrated
that interpenetrated PCN-6 has higher CO2 adsorption capacity
(189 mg/g) than that of PCN-6 (156 mg/g) and excellent selectivity over N2 (>20:1) at 298 K and 1 atm. The adsorption capacity of
PCN-6 can be well retained over 5 adsorption–desorption cycles
over the course of 800 min with high purity CO2 in a flow system, and can be regenerated at 75 ◦ C under He flow. The work
has also determined that PCN-6 has high CO2 adsorption capacity (1200 mgCO2 /gadsorbent ) under high-pressure of 30 bar at 298 K
[53].
The Lin group has reported that solvents, especially DEF and
DMF with different molecular sizes, can be employed as molecular
templates for effectively controlling framework interpenetration
[54]. They have firstly realized framework interpenetration control
in homochiral MOFs. The solvothermal reaction of Cu(NO3 )2 and
racemic tetratopic carboxylic acid L in DMF/H2 O mixed solvents
at 80 ◦ C afforded a 3D MOF, meso-[LCu2 (H2 O)2 ]·(DMF)8 ·(H2 O)4 ,
with doubly interpenetrated networks. It crystallizes in the centrosymmetric space group I 41 /a and features two interpenetrating
nets of opposite chirality. Although large pores exist in the single network, the MOF exhibits relatively small open channels with
the largest dimension of ∼14 Å, due to the interpenetration of
the two enantiomeric networks. Interestingly, the interpenetration
2242
H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
Table 2
Hydrogen uptake data of PCN-6 and PCN-6 .
PCN-6
PCN-6
PCN-6
PCN-6
Activated at 50 ◦ C
(77 K, 1 bar)
Activated at 150 ◦ C
(77 K, 1 bar)
Excess (mg/g,
298 K, 50 bar)
Total (mg/g,
298 K, 50 bar)
Enthalpy
(kJ/mol)
1.74 wt.%
1.35 wt.%
1.9 wt.%
1.62 wt.%
9.3
4.0
15
8.1
6.2–4.5
6.0–3.9
Excess (mg/g,
77 K, 50 bar)
Excess (g/L,
77 K, 50 bar)
Total (mg/g,
77 K, 50 bar)
Total (g/L, 77 K,
50 bar)
Deliverable
(77 K,
1.5–50 bar)
40.2
11.8
95
58
53.0
13.2
75 mg/g (41.9 g/L)
42 mg/g (11.8 g/L)
72
42
isomerism can be controlled by solvent molecules with different sizes as templates. Reaction of Cu(NO3 )2 and racemic L in
DEF/H2 O mixed solvents at 80 ◦ C gave noninterpenetrating rac[LCu2 (H2 O)2 ]·(DEF)12 ·(H2 O)16 , which is isostructural to one of the
interpenetrated networks of the meso-structure. The DMF and DEF
solvents are proposed to play the roles of templates during the MOF
growth by coordinating to the Cu centers that are on the surface
of growing single crystals. In this case, with larger molecular size,
DEF solvent does not favor the formation of interpenetrating MOF
since the formation of smaller pores is not permitted with the larger
solvent [54a].
Subsequently, on the basis of systematically elongated dicarboxylate struts derived from chiral Mn-salen catalytic subunits,
the Lin group has also synthesized a family of isoreticular chiral MOFs (CMOFs 1–5) with Zn4 O SBUs [54b]. Similarly, DMF and
DEF as reaction solvents have successfully controlled the structural interpenetration of CMOFs 1 and 2 and CMOFs 3 and 4 pairs
with the same Zn4 O SBUs, in which CMOFs 1 and 3 present interpenetrated while 2 and 4 have noninterpenetrated structures. The
open channel sizes in these CMOFs have been systematically tuned
by using different lengths of organic linkers and combination of
the framework interpenetration control with the solvent template.
CMOF-5 features a 3-fold interpenetrated structure with very long
Mn-salen-derived dicarboxylate strut. Considering that pore size
in MOFs plays important roles in transportation of substrates and
products to facilitate asymmetric catalysis, catalytic activities of
CMOFs 1–5 and the dependence of reaction rates on the open pore
sizes were examined for the asymmetric epoxidation of unfunctionalized olefins. The results have shown that the conversion
rate decreases in the order of CMOF-1 > CMOF-5 > CMOF-3 > CMOF2 > CMOF-4, in agreement with the decrease of open channel sizes.
It is proposed that the reaction rates are highly dependent on
diffusion of the bulky alkene and oxidant reagents and the epoxide product when the CMOF channels are small. However, For
CMOFs-2 and -4 with large open channels, the catalytic activities
are dominated by the intrinsic reactivity of the catalytic molecular building blocks and are comparable to homogeneous control
catalyst.
Recently, following a similar research line to generalize such a
strategy, they have again obtained a pair of interpenetrated and
noninterpenetrated chiral MOFs featuring pcu topology based on
redox active Ru(III)/salen-based bridging ligands and Zn4 O SBUs
[54c]. As shown in Fig. 11, Zn(NO3 )2 ·6H2 O reacted with [Ru(LH2 )(py)2 ]Cl in DBF/DEF/EtOH (DBF = N,N-dibutylformamide) or in
DMF/DEF/EtOH at 80 ◦ C afforded compounds 6 and 7, respectively. Compound 6 crystallizes in the R32 space group and forms
a 2-fold interpenetrated 3D network with 54.5% void space, in
which the largest cavities are 8 Å in diameter and are interconnected by 4 Å × 3 Å windows. In comparison, compound 7
crystallizes in the R3 space group with a noninterpenetrated structure. The structures of 6 and 7 have the same asymmetric unit,
the same metal–ligand connectivity and network topology, while
the framework 7 has only the single net of 6. Therefore, the
noninterpenetrated 3D structure of 7 has 78.8% void space and
much larger open channel sizes of 14 Å × 10 Å and a cavity diameter
of 17 Å
Remarkably, compounds 6 and 7 can be reduced in a singlecrystal to single-crystal fashion by the treatment with strong
reducing agents of LiBEt3 H or NaB(OMe)3 H. The reduction converts the Ru(III) to Ru(II) in the MOFs, along with a color change
from dark green to dark red. The resultant MOFs 6R and 7R maintain the same space groups and similar cell parameters as those of 6
and 7, revealing their identical framework topology. Interestingly,
the reduced frameworks can be oxidized back in air to yield crystals
of 6 and 7, respectively (Fig. 11). More importantly, the reduction of
the Ru(III) centers in compound 7 turned on the catalytic activity,
making 7R highly active for the asymmetric cyclopropanation
Fig. 11. Synthesis and single-crystal to single-crystal reduction/oxidation of compounds 6 and 7. Typical colors and morphologies of 7 (green) and 7R (red) are shown
in the right-side photographs.
Adapted with permission from Ref. [54c]. Copyrights 2011, Wiley-VCH Verlag GmbH
& Co. KGaA.
H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
2243
Fig. 12. (a) Schematic illustration for the control of degree of interpenetration and structural flexibility in 8 and 9 with benzene as a template. C gray, N blue, O red, S yellow,
H light blue. (b) CO2 sorption of 8 at 195 K. (c) CO2 sorption of 9 at 195 K. A: amount adsorbed at STP (standard temperature and pressure). P/P0 is relative pressure; P0 of CO2
at 194.7 K is 101.3 kPa.
Adapted with permission from Ref. [56]. Copyrights 2011, Wiley-VCH Verlag GmbH & Co. KGaA.
of substituted alkenes with very high diastereo- and enantioselectivities. In contrast, compounds 6 and 6R are interpenetrated
structures with small channels that are unable to transport the
reaction substrates, thus resulting in the inactivity of the catalyst.
Similarly, Guo and Sun have synthesized two 3D Zn-based
MOFs with the same framework formula of Zn3 (L1 )2 (L2 ) (guest
solvents are not considered, L1 = 4-[3-(4-carboxyphenoxy)2-[(4-carboxyphenoxy)methyl]-2-methyl-propoxy]benzoate,
L2 = 1,4-bis(1-imidazolyl)benzene, Scheme 1) under almost the
same reaction conditions except for different solvents used [55].
Due to the flexible coordination environments of L2 and extended
lengths of both ligands, the resultant two MOFs feature 2- and
3-fold interpenetrated structures in which DMF and acetonitrile
serve as templates, respectively.
A reaction between Zn(NO3 )2 ·6H2 O, 2,2 -bithiophene-5,5 dicarboxylic acid (H2 btdc, Scheme 1), and 4,4 -bpy with solvent
of DMF only or mixed DMF/benzene (1:1) yielded 3-fold interpenetrated 8 and 2-fold interpenetrated 9, respectively (Fig. 12a) [56].
The benzene molecule with larger size in the reaction could play a
templating role, which prevents the dense packing of frameworks
during the synthesis process, leading to the resultant 9 with a 2-fold
interpenetrated structure. By replacing benzene with other similar
solvents, such as toluene or cyclohexane, the degree of interpenetration can also be controlled. However, introduction of much larger
solvent molecules, such as naphthalene or mesitylene, proved
unsuccessful to control the structural interpenetration. The structural analyses showed that the compositions of both frameworks
are the same, indicating that they are a pair of interpenetration isomers. Due to the different degrees of framework interpenetration, 8
features a rigid framework whereas 9 has a rather flexible structure.
As a result, both compounds display entirely different CO2 sorption
behaviors: the CO2 sorption in 8 indicates that its rigid 3-fold interpenetrated framework remains intact during the CO2 molecule
accommodation (Fig. 12b), while 9 shows much higher CO2 uptake
and dynamic features involving framework transformations (sliding motions and/or shrinkage/expansion), further verifying the
high flexibility of the framework (Fig. 12c).
The interpenetration modulation has also been investigated by
assembling a long rigid ligand, 2,5-bis(4 -(imidazol-1-yl)benzyl)3,4-diaza-2,4-hexadiene (ImBNN, Scheme 1), and M(CF3 SO3 )2
(M = Cd or Mn) in the absence or presence of aromatic molecules
[57]. Without aromatic guest molecules, the 3-fold interpenetrated
networks were fabricated with closely packed layered structures;
when aromatic guest molecules, such as, o-xylene, naphthalene,
phenanthrene, benzene, p-xylene, and pyrene, are introduced, the
2-fold interpenetrated networks were obtained with the inclusion of the aromatic molecules. The results have clearly revealed
that the aromatic molecules act as templates that control the
degree of structural interpenetration, and the 2-fold interpenetrated networks display strong preference for aromatic guest
inclusion likely due to ␲–␲ interactions between ligand and aromatic guest, but less selectivity toward shape and size differences
of these guest molecules.
3.3. Ligand design/modification-induced control
Similarly to the template effect, the presence of stericly bulky
groups on the organic ligand could be an alternative approach
to effectively prohibit framework interpenetration. Therefore, the
goal could be achieved by selective modification of a ligand
with pendant groups. For example, the hydrothermal reaction
of dicarboxylate ligand bdc with uranyl cation (UO2 2+ ) led to a
layered structure with a doubly interpenetrated (6,3) net. The
structural interpenetration can be effectively prevented by the
pre-introduction of bulky substituents on the bdc ligand, but the
framework structure is fixed to be (6,3) net topology [58].
Interpenetration has been suppressed through simple ligand
design and succeeded in preparation of a series of interpenetratednoninterpenetrated framework isomers [59]. As shown in Fig. 13,
Zn-L5 or Zn-L6 form 2D sheets within the xy-plane and are pillared by a variety of dipyridyl ligands to afford similar pillared-layer
MOFs, although the sizes of dipyridyl ligands are widely tailored
from L1–L4 [59a]. Remarkably, all MOFs constructed from L6 are
noninterpenetrated whereas 2-fold interpenetrated structures are
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H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
Fig. 13. (Top) Organic linkers involved in the syntheses of MOFs. (Bottom) Single
crystal X-ray structures: 2-fold interpenetrated structures (10, 12, 14, and 16) with
building block H4 L5 and noninterpenetrated structures (11, 13, 15, and 17) with
building block H4 L6.
Adapted with permission from Ref. [59a]. Copyrights 2010, American Chemical
Society.
obtained with L5 under identical reaction conditions. The only difference between L5 and L6 is the change from aryl-H to aryl-Br
moieties furnished on the tetra-topic carboxylate ligand, which
has been demonstrated to be efficient for interpenetration control. The CO2 sorption experiments demonstrate the differences
of porosity and sorption properties between interpenetrated 10
and noninterpenetrated 11. Furthermore, The potential dynamic
structural behavior in interpenetrated 16 gives rise to the steps
in the isotherm at P/P0 ≈ 0.022 upon activation and guest adsorption, while the behavior disappears in 17 with a noninterpenetrated
network. All these results illustrate that interpenetration plays significant roles in structural dynamics and gas sorption properties in
MOFs.
Subsequently, with standard biphenyl dicarboxylate linkers
with one or two pendant azolium moieties, two cubic Zn-based
MOFs have been obtained with interpenetrated and noninterpenetrated structures, respectively. Results have clearly indicated that
the introduction of different numbers of bulky azolium moieties
(one vs. two) onto the linear dicarboxylate linkers is able to control
structural interpenetration in MOFs [59b].
Reactions of bdc, 4,4 -bpy, and cobalt(II) salt in DMF gave
a nonporous, doubly interpenetrated MOF, Co2 (bdc)2 (4,4 -bpy)2 .
In contrast, similar reaction conditions using 2-amino-1,4benzene dicarboxylate realized the formation of a porous doubly
pillared-layer MOF without interpenetration, [Co2 (abdc)2 (4,4 bpy)2 ]·8DMF, with permanent porosity. The results have further
demonstrated that introduction of bulky substituents on the
organic linkers could be a general method for interpenetration control [60].
Four structurally similar rod-like ligands (Scheme 1), 1,4bis(benzoimidazol-1-yl)-phenyl, 1,4-bis(imidazol-1-yl)-benzene,
4,4 -bis(imidazol-1-yl)-biphenyl, and 4,4 -bis(benzoimidazol-1yl)biphenyl have been employed to react with Co(II) or Cd(II) salts
to produce 3D MOFs with different degrees of interpenetration
under similar conditions [61]. Structural analyses confirmed that
both coordination mode and spacer length play significant roles in
the determination of the degree of interpenetration of MOFs, which
also could be tunable by ligand modification, such as varying the
ligand spacer or terminal group.
The successful syntheses of noninterpenetrated and interpenetrated Cu-MOFs with expanded sodalite-type network by
respectively employing benzene- and triazine-centered versions
of an elongated triangular N-donor ligand have been reported
[62]. It is concluded that the reason for the interpenetration
manipulation for the structures is not only because of the three
different atoms (C or N) involved in the two ligands, but also
mainly due to the presence of some degree of torsion between
the central and outer phenyl rings in the benzene-centered ligand,
whereas such strain does not exist in the triazine-centered one.
It is found that the noninterpenetrated structure, although it
affords larger pore space, is prone to collapse upon desolvation,
while interpenetration can stabilize the other framework, leading
to increases in both surface area and hydrogen storage capacity. This result shows that interpenetration control could be an
important factor in the synthesis of hydrogen storage materials.
Most recently, interpenetration control in MOFs with similar net
topologies can be manipulated via a simple change in the ligand
by replacing a C C bond with a C C bond [63]. The reactions
of Zn(NO3 )2 ·6H2 O and 4-(2-carboxyvinyl)benzoic acid (H2 cvb) or
4-(2-carboxyethyl)benzoic acid (H2 ceb), respectively afford the
noninterpenetrated MOF [Zn4 O(CVB)3 ]·13DEF·2H2 O and the 2-fold
interpenetrated MOF [Zn4 O(CEB)3 ]·6DEF·H2 O under almost identical reaction conditions (Fig. 14). Gas sorption experiments for the
two MOFs indicate both structure- and pressure-dependent gas
sorption properties and the results are almost in agreement with
previous reports mentioned above [51]. Under higher pressures,
the MOF with noninterpenetrated structure and larger pore volume
(pore size: ca. 9.0 × 9.0 Å) can accumulate more gas molecules and
has much higher gas adsorption capacities regardless of the temperature. At pressures lower than 1 atm, the interpenetrated MOF
with smaller pores (ca. 2.5 × 2.5 Å) has higher H2 and CH4 sorption capacities, possibly due to stronger interactions between gas
molecules and the framework, while it adsorbs less N2 at 77 K and
CO2 at 195 K. The interpenetration control for both MOFs could also
be attributed to the ligand conformations that are generally different in the structures involving C C and C C bonds. Based on the
above examples, it can be envisioned that not only ligand modification with bulky groups but also the conformation of organic linkers
affects structural interpenetration in MOFs.
The first interpenetration control for lanthanide-based
MOFs has also been realized [64]. Isostructural MOFs of
Ln(bdc)1.5 (DMF)(H2 O) (Ln = Er, 18; Tm, 19) have been synthesized under the solvothermal reactions between H2 bdc and
ErCl3 ·6H2 O or Tm(NO3 )3 ·3H2 O in DMF/EtOH/H2 O. The isostructural MOFs feature 2-fold interpenetrated 3D frameworks with
limited porosity. Careful analysis of the structure of 18 has revealed
that the coordinated H2 O and DMF molecules locate at adjacent
positions with OH2 O Er ODMF angle of 78.56◦ . Therefore, in
order to suppress interpenetration and improve the porosity, the
following two strategies have been identified to achieve great
success: (a) use of other chelating ligands such as phen to replace
the coordinated H2 O and DMF, (b) modified bdc ligand with a
hindrance group. Using both of these strategies independently,
and then together, generated three other MOFs, 20–22, that
possess the same topology as 19, but remain noninterpenetrated.
Compared to MOF 18, chelating phen ligand has been employed
to replace DMF and H2 O molecules for the synthesis of MOF 20, as
the first strategy. MOF 21 is constructed by tbdc (H2 tbdc = 2,3,5,6tetramethyl-1,4-benzenedicarboxylic acid, Scheme 1), instead of
H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
2245
Fig. 14. (a) Ligands used for the MOF synthesis and crystal structures of (b) noninterpenetrated and (c) 2-fold interpenetrated MOFs. The interpenetrated networks are
represented in red and blue in (c).
Adapted with permissions from Ref. [63]. Copyrights 2012, Wiley-VCH Verlag GmbH & Co. KGaA.
bdc, but the coordinated solvates are unchanged, based on the
2nd strategy. Finally, the two strategies have been combined to
synthesize MOF 22, in which both bdc and coordinated solvents
are replaced by tbdc and chelating phen ligands, respectively. The
coordination modes of carboxylate groups of tbdc remain the same
as those of bdc in MOFs 20 and 21 and the steric hindrance of the
methyl groups in tbdc ligand as well as the large terminal phen
ligand are proposed to be responsible for the interpenetration
control. Gas sorption examinations for MOF 22 have shown that
it selectively adsorbs CO2 and H2 over N2 and Ar based on the
size-exclusive selectivity due to the limited pore opening sizes of
3.3–3.54 Å
The Telfer group has succeeded in interpenetration control by
the pre-modification of a tert-butylcarbamate (NHBoc) group on
biphenyldicarboxylate ligand before MOF synthesis. After incorporation of the modified ligand in the MOF, the amino group can
be unmasked by complete removal of the protective group via a
simple thermolytic reaction that does not contain any external
reagents and results in no non-volatile species (Fig. 15a). Due to
the steric hindrance of the large NHBoc group, framework interpenetration has been suppressed and the postsynthetic process
further expands the cavities/pores in the MOF [65a]. Following
this research line, they have incorporated organocatalytic moieties
into MOFs by introducing thermolabile protecting groups that can
be released to create accessible catalytic sites within the pores
by simple thermal treatment process after MOF synthesis. Due
to the interpenetration control induced by the bulky group binding to the ligand, the removal of thermolabile protecting group
to expose organocatalytic moieties makes the highly porous MOF
catalytically active for asymmetric aldol reactions with relatively
large substrates [65b]. Most recently, they reported that photolabile bulky groups can be grafted onto the biphenyldicarboxylate
ligand and subsequently introduced into a cubic Zn-based MOF.
The framework interpenetration in the MOF has been successfully prevented in the presence of a 2-nitrobenzyl ether group
and such group can be quantitatively cleaved by photolysis to
expose a hydroxyl group (Fig. 15b) [65c]. It should be noted that
the resultant Zn-MOF bearing unmasked hydroxyl group cannot
be directly synthesized because the hydroxyl group is reactive and
readily coordinates to metal centers during the solvothermal reaction process. Therefore, the work not only affords a new strategy
for suppressing framework interpenetration but also opens up new
perspectives to produce tailored pore surfaces with desired functional groups in MOFs for possible applications.
3.4. Coordinated or uncoordinated solvent
removal/addition-triggered control
Solvent molecules have been demonstrated to be able to
influence the interpenetration not only during the synthetic
process as described above, but also after the formation of
MOFs. The guest solvent desorption/absorption have been found
to cause the reversible rearrangement between 5- and 6-fold
interpenetrated 3D networks [66]. Slow evaporation of an ammonia solution of 3-amino-1,2,4-triazole (Hatz, Scheme 1) affords
[Ag6 Cl(atz)4 ]OH·6H2 O, which features parallel 5-fold interpenetrated networks possessing square 1D channels with diameters of
ca. 8.5 Å running along the c-axis. This MOF undergoes guest water
desorption in a very slow stream of dry air or by heating at 375 K
for 3 h to give partially desovated product [Ag6 Cl(atz)4 ]·OH·xH2 O
(x ≤ 2) at 293 K. Compared to the original MOF, the framework of the
Fig. 15. (a) Schematic showing the incorporation of a bulky NHBoc group in the
organic linkers anticipated to suppress the framework interpenetration in MOFs.
The post-synthetic cleavage of the Boc group by thermolysis expands the void
space within the framework and also exposes reactive amino functional groups. (b)
MOF synthesis with ligand H2 1 affords a noninterpenetrated cubic MOF, [Zn4 O(1)3 ],
followed by the photolytic deprotection of the hydroxyl group to produce noninterpenetrated [Zn4 O(2)3 ]. Ligands H2 2 or H2 3 in the MOF synthesis give the
interpenetrated frameworks.
Reproduced with permissions from Refs. [65a,c]. Copyrights 2010, Wiley-VCH Verlag
GmbH & Co. KGaA and 2012, The Royal Society of Chemistry, respectively.
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H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
Fig. 16. (a) MOF-123 is converted into the interpenetrated crystals of MOF-246 by the removal of coordinated DMF (large pink spheres). C black, N blue, O red, Zn blue
polyhedra. The interpenetrated framework is shown in green. (b) N2 adsorption isotherms measured at 77 K for the samples, MOF-123, intermediates (a–e), and MOF-246.
The samples have been prepared by heating MOF-123 at (a) 70 ◦ C, (b) 90 ◦ C, (c) 140 ◦ C, (d) 180 ◦ C, (e) 220 ◦ C, and 260 ◦ C under vacuum.
Adapted with permission from Ref. [67]. Copyrights 2012, Wiley-VCH Verlag GmbH & Co. KGaA.
desolvated one is 6-fold interpenetrated and drastically distorted,
but still retains the same topology with elliptical 1D channels
(4.3 Å × 10.4 Å). Significantly, the conversion between the original
and desolvated structures is reversible, as confirmed by both powder and single crystal XRD analyses of samples of the desolvated
product after exposure to saturated water vapor for 1 day.
Similar to that for uncoordinated solvents, the removal and
addition of coordinated solvent molecules in MOFs can also trigger
the transformation between interpenetrated and single nets [67].
The solvothermal reactions of zinc(II) nitrate and H2 nbd (nbd = 2nitrobenzene-1,4-dicarboxylate, Scheme 1) in DMF/methanol
yielded MOF-123 with formula Zn7 O2 (NBD)5 (DMF)2 , which features a noninterpenetrated 3D porous structure with DMF residing
in the pores as terminal ligands coordinated to Zn centers. The
coordinated DMF ligands can be completely removed below 270 ◦ C
by simple heating. Remarkably, single-crystal X-ray diffraction
analysis for the heated MOF-123 indicated that the coordination
mode of Zn(II) ions changed upon removal of the coordinated
DMF molecules and a doubly interpenetrated structure, designated as MOF-246, with the same backbone as MOF-123 was
formed (Fig. 16a). It is interesting and quite rare that the transformation between MOF-123 and MOF-246 has been proven to
be reversible and even can be recycled many times by addition
of DMF and simple heating.Dinitrogen sorption was employed to
track the changes in porosity of the system as it underwent gradual transition from MOF-123 to MOF-246. As shown in Fig. 16b,
MOF-123 shows typical Type I isotherm with BET and Langmuir surface areas of 1200 and 1340 m2 /g, respectively. The two
intermediates a and b exhibit similar maximum N2 uptake and
surface areas as that of MOF-123, which could be understandable since partial interpenetration offsets the space created by
the removal of DMF. The pore space gradually decreases when
the interpenetration proceeds to MOF-246, which shows no N2
uptake.
3.5. Layer-by-layer assembly
Framework interpenetration suppression has also been elegantly demonstrated by using liquid-phase epitaxy on an organic
monolayer modified surface followed by a layer-by-layer growth
approach [68]. It has been shown that the pillared-layer MOF-508
can be fabricated in a noninterpenetrated form via this synthetic
route. In contrast, conventional solvothermal synthesis always produced 2-fold interpenetrated networks [69]. Such unconventional
approach to generate noninterpenetrated MOF-508 takes advantage of the separate ethanolic solutions of the components. Zinc
acetate and a mixture of the organic components (H2 bdc and
4,4 -bpy) were separately stayed in two beakers, then a properly
functionalized organic surface was alternately immersed in the two
solutions with intermittent rinsing. The organic surface was based
on an Au substrate, on which a self-assembled monolayer (SAM)
of 4,4-pyridyl-benzenemethanethiol (PBMT) was fabricated. It is
assumed that the second, interpenetrating network in the surfacemounted MOF (SURMOF) cannot match the pyridine-terminated
organic surface that acts as a nucleation template (Fig. 17), and
therefore such second lattice cannot nucleate at the surface and
the interpenetration can be successfully suppressed.
3.6. Relationship between structural interpenetration and
functional applications
As previously mentioned, interpenetration in MOFs is now
numerous and extensively studied and originates from the presence of large free voids in a single network. The most prominent
influence of interpenetration should be the decrease of pore size
and pore volume, which are directly associated with gas sorption
properties and selective catalysis, as summarized above. Generally, for the small pore size and volume in an interpenetrated
structure, the interaction between framework and gas molecules
H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232–2249
Fig. 17. Schematic showing that two identical interpenetrated networks (colored
red and green) are usually formed by conventional synthesis, while using liquidphase epitaxy, the equivalence of these two networks is lifted by the presence of
the substrate (dotted line in the schematic diagram on the right) and the formation
of interpenetrated networks is suppressed, generating SURMOFs with only one net.
Reproduced with permission from Ref. [68]. Copyright 2009, Nature Publishing
Group.
(H2 , CO2 , etc.) could be strong, which makes gas sorption capacity considerable, at low pressure. In contrast, for the large pore
size and volume in a non-interpenetrated MOF, the gas sorption capacity at low pressure could be lower whereas the storage
capacity should be higher at high pressure than that for an interpenetrated MOF because the latter has smaller pore volume, as
clearly demonstrated in previous pairs of MOFs [51–53,63], typically for PCN-6 and -6 . The results reveal the respective merits
of interpenetration and non-interpenetration in gas sorption and
storage applications. Meanwhile, the pore size limitation in an
interpenetrated MOF will not allow large catalytic substrates and
products to go through, which, of course, is not definitely a disadvantage because this point can be reasonably employed for
selective catalytic reactions in some cases [54b]. Various catalytic
substrates and products with different sizes may readily access
the pore space in a non-interpenetrated structure with large pore
sizes, which is desirable for many catalytic applications [54c,65b].
Hence, both interpenetrated and non-interpenetrated frameworks
can find their special advantages in catalytic applications. In addition to the main applications in gas sorption and catalysis, variation
in interpenetration may also affect other functional applications,
for example, interpenetrated structures with better stability allow
studies for molecular sensing and recognition [50], interpenetrated
structures could potentially induce stronger magnetic interactions due to closer distances between magnetic metal centers,
and so on.
4. Conclusions
Framework interpenetration and interpenetration control have
long been topics of interest in MOF research. Though we have
primarily focused on interpenetration in systems built upon rigid
linkers, it is also frequently observed in MOFs with flexible organic
ligands and presents diverse and fantastic structures [9d]. Furthermore, we also wish to note that in this review no distinction has
been made between interpenetration and interweaving, although
some distinctions have previously been identified between them.
The term of interweaving tends to be used when the interlocking
of multiple networks enhances the stability of a single network,
but blocks potential adsorptive sites. Interpenetration is more
often used when the pore size is decreased without blockage of
2247
adsorptive sites, thus maximizing the exposed surfaces of each
network.
We have summarized several routes reported to realize
interpenetration control in MOFs: reaction temperature or concentration control, template-directed control, ligand design/
modification-induced control, coordinated or uncoordinated solvent removal/addition-triggered control as well as layer-by-layer
assembly. So far, it is still hard to conclude some general rules to
control framework interpenetration because there are too many
factors that can influence the formation of the final structures of
MOFs. While the first three strategies have been demonstrated
to work well by different research groups or in various MOFs, no
method has been identified that may universally apply, though it
seems the layer-by-layer approach should be able to serve for interpenetration control in quite a variety of cases, though it would
be relevant only for small-scale MOF fabrication. These reported
routes and strategies may be used or modified for future efforts
to control framework interpenetration. Generally speaking, interpenetration can be used for practical advantage if one desires a
MOF with small pore sizes, which could be beneficial to H2 and
CO2 uptakes [51–53]. In addition, the interaction of interpenetrated
networks can lead to interesting dynamical behavior and the guestinduced displacement of networks with respect to each other can
give rise to hysteretic sorption, which could also be beneficial to gas
storage or separation [32c,56,70]. In contrast, noninterpenetrated
frameworks with lower densities offer larger pore sizes and pore
volumes, which could be favorable for gravimetric gas uptake and
surface area, as well as gas storage capacities under high pressure.
The large pores involved in MOFs could allow large substrates to be
accessible to catalytically active sites in catalytic reactions. Therefore, interpenetration and noninterpenetration have their own
advantages and the existence of interpenetration is not necessarily negative. With continued efforts of scientists and development
of the MOF field, we expect to see more rational approaches to
manipulate interpenetration, and thus to serve for enhanced performances of targeted MOFs.
Acknowledgements
The authors thank Prof. A.B.P. Lever and the reviewers for their
valuable comments and suggestions. This work was supported as
part of the Center for Gas Separations Relevant to Clean Energy
Technologies, an Energy Frontier Research Center funded by the
U.S. Department of Energy (DOE), Office of Science, Office of Basic
Energy Sciences under Award Number DE-SC0001015. H.-L.J. was
supported by U.S. Department of Energy (DE-FC36-07GO17033)
and University of Science and Technology of China. T.A.M. was
supported by the Welch Foundation (A-1725). H.-C.Z. gratefully
acknowledges support from the U.S. Department of Energy (DEAR0000073).
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