An unprecedented (6,8)-connected self-penetrating network based on two distinct zinc clusters

Article abstract of DOI:10.1039/b709835a

The first (6,8)-connected self-penetrating metal-organic framework has been constructed using an asymmetric neutral ligand based on dinuclear zinc clusters, as six-connected nodes, and trinuclear zinc clusters as eight-connected nodes, representing the hi

Full text of DOI:10.1039/b709835a

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An unprecedented (6,8)-connected self-penetrating network based on
two distinct zinc clusters{
Ya-Qian Lan, Xin-Long Wang, Shun-Li Li, Zhong-Min Su,* Kui-Zhan Shao and En-Bo Wang*
Received (in Cambridge, UK) 28th June 2007, Accepted 24th August 2007
First published as an Advance Article on the web 19th September 2007
DOI: 10.1039/b709835a
As shown above, it is a great challenge to achieve self-
penetrating MOFs based on binodal high-connected nets in
coordination chemistry. Recently, metal clusters with substituted
single metal atoms have been used as nodes for high-connected
metal–organic frameworks.7b A feasible approach to building
binodal high-connected self-penetrating networks is to choose two
distinct metal clusters with suitable coordination geometries and to
rationally design the appropriate ligands. According to previous
literature, multi-carboxylate10 and long flexible9b ligands with
different characters are excellent candidates for the construction of
novel high-connected formworks based on the following con-
siderations: (i) the synthesis of metal clusters is usually by the
controlled hydrolysis of metal salts with the aid of carboxylate
ligands,11 because the carboxylate group may induce core
aggregation and it should be feasible to link discrete clusters into
an extended network via its bridging ability; (ii) an asymmetric
ligand may induce dissymmetric building blocks by coordinating
to different metal centers or metal clusters with different
coordination atoms (N and O), which benefits the formation of
mix-connected frameworks;12 and (iii) long flexible ligands are
selected for constructing MOFs because they benefit the formation
of self-penetrating networks.8a
The first (6,8)-connected self-penetrating metal–organic frame-
work has been constructed using an asymmetric neutral ligand
based on dinuclear zinc clusters, as six-connected nodes, and
trinuclear zinc clusters as eight-connected nodes, representing
the highest-connected binodal network topology presently
known for self-penetrating systems.
Known metal–organic frameworks (MOFs) are rapidly increasing
in number because of their wide potential applications as
functional materials and intriguing structures.1 The topological
analysis of multitudinous networks has become a topical research
area, and is not only an important tool for simplifying complicated
compounds but also plays an instructive role in the rational design
of certain functional materials with desirable properties.2 The
classification of structures by Wells laid the foundation for our
general understanding of some natural prototypes in metallic or
binary inorganic solids.3 To date, a variety of uninodal network
topologies have been realized. The majority are based on three-,
four- and six-connected topologies.1b,4 Examples of seven-, eight-
and twelve-connected metal–organic coordination frameworks are
extremely rare5 due to the limited coordination numbers of metal
centers and steric hindrance issues of the most commonly-used
organic ligands. On the other hand, some metal–organic frame-
works with mixed-connected topologies have been reported, for
example: Pt3O4, boracite, twisted boracite, PtS, rutile, pyrite and
anatase.6 However, there is an unfavorable lack of investigation on
binodal high-connected structures, such as (3,8)-, (4,8)- and (3,9)-
connected frameworks.7 The (6,8)-connected framework, com-
pared to the above-mentioned nets, is more difficult to achieve
owing to it containing two kinds of high-connected nodes. Until
now, this net had not been observed in metal–organic frameworks.
Of concurrent interest has been the construction of entangled
systems, as discussed in several excellent reviews.1g,8 Of the many
reported types of entanglement in polymeric architectures, the
phenomenon of self-penetration (self-catenation or polyknotting)
has attracted much attention. These species are single nets, having
the peculiarity that the smallest topological rings are catenated by
other rings belonging to the same network.8a Hitherto, only a
limited number of self-penetrated nets have been observed in
metal–organic frameworks.9
Taking these points into account, we designed and synthesized
1-((49-((pyridin-3-yloxy)methyl)biphenyl-4-yl)methyl)pyridinium-3-
olate (L) as an asymmetric neutral bridging ligand, and selected
1,4-benzenedicarboxylic acid (H2bdc) as a multi-carboxylic brid-
ging ligand. Fortunately, compound [Zn2.5L(bdc)2.5]?H2O (1) was
isolated by hydrothermal methods.{ The product is very stable in
air, and the framework only begins to decompose when heated to
more than 346 uC (Fig. S1{). To the best of our knowledge, it
represents the first example of a (6,8)-connected self-penetrating
network based on two distinct zinc clusters as nodes. Single crystal
X-ray analysis§ revealed that compound 1 is constructed from
three kinds of ZnII cations, three kinds of bdc anions and one L
ligand (Fig. S2{). Atom Zn1 is five-coordinated and exhibits a
twisted trigonal bipyramidal geometry [ZnNO4], which is
surrounded by four carboxylic oxygen atoms (Zn1–O 1.983(4)–
2.096(7) s) and one nitrogen atom (Zn1–N 2.016(6) s) from the L
ligand. Meanwhile, Zn2 shows a tetrahedral geometry [ZnO4],
which is completed by three carboxylic oxygen atoms and one
oxygen atom (Zn2–O 1.942(3)–1.976(3) s) from a hydroxyl group
of the L ligand. Zn3 lies at an inversion center, is coordinated by
four carboxylic oxygen atoms and two oxygen atoms (Zn3–O
2.035(3)–2.159(3) s) from two hydroxyl groups of different L
ligands, and shows a distorted octahedral geometry [ZnO6]. All the
bond distances and angles are within normal range.9a,b,13 The L
ligand exhibits an anti-bridging coordination mode that
Institute of Functional Material Chemistry, Key Lab of
Polyoxometalate Science of Ministry of Education, Faculty of
Chemistry, Northeast Normal University, Changchun, 130024, China.
E-mail: zmsu@nenu.edu.cn; wangenbo@public.cc.jl.cn;
Tel: +86 431 85099108
{ Electronic supplementary information (ESI) available: Details of the
synthesis, additional figures, TGA and luminescent spectra. See DOI:
10.1039/b709835a
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Chem. Commun., 2007, 4863–4865 | 4863

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coordinates to one Zn1 atom with one N atom and bridges two Zn
atoms (Zn2 and Zn3) with one O atom. It is interesting to note
that the three types of bdc anions (bdc1, bdc2 and bdc3) show
different coordination modes (Scheme S1{).
Two carboxylic groups from the bdc1 ligand bridges two
equivalent Zn1 trigonal bipyramids to generate
a twisted
…
octahedral subunit [Zn2(CO2)4N2] (Fig. S3a{) with a Zn1 Zn1
distance of 4.427(5) s, which further extends to a 2D (4,4) square-
grid with a rhombic window. The dimensions of the net are
11.313 6 11.313 s, corresponding to the distance between
adjacent centers of dinuclear zinc subunits at their corner
(Fig. 1(a)). Meanwhile, the Zn3 octahedron, at a crystallographic
inversion center, connects two crystallographically-equivalent Zn2
tetrahedra in a vertex-sharing mode to form a [Zn3(CO2)6O2]
dodecahedral cluster by six carboxylic groups from four bdc2 and
two bdc3 ligands, and two oxygen atoms from distinct L ligands
Fig. 2 (a) Schematic view of the geometrical relationship between the
six-connected nodes and the net formed of eight-connected nodes. (b)
Schematic representation of the (6,8)-connected self-penetrating frame-
work of (412?5?62)(420?52?66) topology (pink and blue spheres represent the
six- and eight-connected nodes, respectively).
…
with a non-bonding Zn2 Zn3 distance of 3.264(3) s (Fig. S3b{).
Each [Zn3(CO2)6O2] cluster is further linked by four bdc2 and two
bdc3 ligands to form a 3D framework (Fig. S4a{), showing a
distorted a-Po (pcu) net with dimensions of 16.984 6 11.313 6
11.313 s (Fig. 1(b)). It is interesting that each bdc3 ligand,
coordinating to two [Zn3(CO2)6O2] clusters, penetrates the 2D
sheet formed by the [Zn2(CO2)4N2] clusters and bdc1 ligands along
the a-axis, forming
a 2D/3D interpenetrating composition
(Fig. S4b{).14 The 2D (4,4) and 3D (pcu) nets are connected by
asymmetric L ligands coordinating to di- and trinuclear zinc
clusters. Therefore, each dinuclear zinc cluster, as a six-connected
node (Fig. S5{), lies at the center of a distorted cube formed by
eight trinuclear zinc clusters as eight-connected nodes (Fig. S6{).
One body diagonal of a distorted cube therefore passes through
the dinuclear zinc cluster (Fig. 2(a)). The overall structure of 1 is a
3D (6,8)-connected self-penetrating framework (Fig. 2(b)). The
Schla¨fli symbol for 1 is (412?5.62)(420?52?66) (Fig. S7{).
According to a new approach to the analysis of high-connected
frameworks proposed by Hill et al.,15 namely the visualization of
structures as combinations of interconnected 2D subnet tectons,
the familiar six-connected net (bcu, sometimes called the CsCl net,
Fig. 3(a)1j,5e,16) can be thus described such that the (4,4) nets (I)
(orange) are cross-linked by zigzag chains (green), and the zigzag
chain in the inter-layer region bridges across the diagonal of a
single window in the (4,4) net. In comparison to a bcu net,
compound 1 contains eight- and six-connected nodes. The net
formed by all eight-connected nodes in 1 exhibits the same
skeleton, which includes the similar (4,4) net (I) and zigzag chain
Fig. 3 Schematic representations of the topologies observed in (a) the
bcu network and (b) the (6,8)-connected self-penetrating framework in 1.
(dark green sticks in Fig. 3(b)). Different to the bcu net, an
additional series of (4,4) nets (II) are formed by six-connected
nodes, which are at the midpoint of the diagonals, and all the
angles between these diagonals and the nets (I), formed by eight-
connected nodes, are approximately 40u. Based on the above
connection mode, the other kind of diagonal, with an angle of
approximately 77u (corresponding angle formed by the line of the
diagonal and the plane (I)), penetrates net (II). As a result,
catenations of two types of four-membered shortest-circuits,
formed by six- and eight-connected nodes, respectively, are
observed in the 3D net (black and light green rings in Fig. 3(b)).
The resulting array is therefore a (6,8)-connected self-penetrating
network. Offering further insight into the nature of this intricate
architecture, it can also be considered as a distorted bcu net that is
inserted by another kind of (4,4) net (II) along the crystallographic
bc plane.
To date, (6,8)-connected frameworks have not been observed in
MOFs. O’Keeffe et al. have enumerated a (6,8)-connected net
(ocu)17 based on an octahedron, as a six-connected node, and a
cube, as an eight-connected node (Fig. S8{). Compound 1 is
clearly different to the ocu network, and is a self-penetrating
framework constructed from a distorted octahedron, as a six-
connected node, and a twisted dodecahedron, as an eight-
connected node. Therefore, the net in 1 defines a new topology
for (6,8)-connected coordination networks that is not only
unobserved but also unenumerated, and represents the highest
connected binodal network topology for self-penetrating systems.
The photoluminescence properties of compound 1 and free
neutral ligand L have been studied at room temperature in
Fig. S9.{ It can be observed that a maximum emission wavelength
occurs at 410 nm, with an additional shoulder peak at 454 nm
Fig. 1 Ball-and-stick representations of (a) one (4,4) layer formed by
Zn1 and bdc1 ligand, and (b) pcu nets by Zn2, Zn3, bdc2 and bdc3
ligands.
4864 | Chem. Commun., 2007, 4863–4865
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(lex = 430 nm) appears at 513 nm. The origin of the emission for
compound 1 might be attributed to a ligand-to-metal charge
transfer and intraligand p* A p transitions of the neutral ligand.18
In summary, we have prepared and characterized the first (6,8)-
connected self-penetrating network using an asymmetric neutral
ligand based on dinuclear zinc clusters, as six-connected nodes,
and trinuclear zinc clusters, as eight-connected nodes. The
successful isolation of this species not only provides an intriguing
example of chemical topology but also confirms the significant
potential of constructing new binodal high-connected frameworks
from different metal clusters. Appropriate choices of asymmetric
ligands with different coordinating conditions, as well as spatial
linkers, should lead to the discovery of a large variety of new
mixed-connected topological structures and types in the near
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We are thankful for financial support from the Program for
Changjiang Scholars and Innovative Research Team in University,
the National Natural Science Foundation of China (no.
20573016), and the Science Foundation for Young Teachers of
Northeast Normal University (no. 20070309).
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Notes and references
{ Synthesis of 1: A mixture of Zn(NO3)2?6H2O (297 mg, 1.0 mmol), H2bdc
(166 mg, 1.0 mmol), L (184 mg, 0.5 mmol) and water (10 mL) was heated
at 150 uC for 72 h in a 23 mL Teflon reactor under autogenous pressure
and then cooled to room temperature at a rate of 10 uC h21. Colorless
crystals of 1 were obtained in 76.2% yield based on Zn(NO3)2?6H2O.
Elemental analysis calc. for C44H32N2O13Zn2.5 (M = 960.14): C, 55.03; H,
3.56; N, 2.92. Found: C, 55.01; H, 3.59; N, 2.91%. IR (/cm21): 3412 (s),
3173 (m), 1598 (s), 1502 (s), 1384 (s), 1321 (s), 1143 (m), 1012 (w), 823 (s)
and 750 (m).
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§ Crystallographic data for 1 (C44H32N2O13Zn2.5), M = 960.14, monoclinic,
space group P21/c, a = 16.9840(18), b = 16.3420(19), c = 15.648(2) s, b =
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