Self-assembled arrays of single-walled metal-organic nanotubes

Article abstract of DOI:10.1002/anie.200904501

[Chemical equation presented] Dragon's lair: A large, single-walled metal-organic nanotube (MONT) with an exterior diameter of up to 3.2 nm and an interal channel diameter of 1.4 nm is presented. The tube can be depicted as hexastranded helices consisting

Full text of DOI:10.1002/anie.200904501

Angewandte
Chemie
DOI: 10.1002/anie.200904501
Metal–Organic Nanotubes
Self-Assembled Arrays of Single-Walled Metal–Organic Nanotubes**
Tzuoo-Tsair Luo, Huang-Chun Wu, Yu-Chen Jao, Sheng-Ming Huang, Tien-Wen Tseng, Yuh-
Sheng Wen, Gene-Hsiang Lee, Shie-Ming Peng, and Kuang-Lieh Lu*
Many recent advances in the field of metal–organic frame-
work (MOF) materials have been reported, not only from the
standpoint of the potential applications, ranging from gas
storage to catalysis and drug delivery, but also because of their
intriguing architectures and framework topologies.[1,2] Con-
ceptually, endless structures can be produced by assembling
judiciously selected molecular building blocks.[2] Just as the
notable saying in crystal engineering goes “the limits are
mainly in our imagination”,[2a] any conceivable MOF might be
obtained in the future, although it is all currently imagination.
Since the first discovery of carbon nanotubes (CNTs) by
Iijima in 1991,[3] discrete hollow tubular structures such as
various CNTs and other synthetic nanotubes (SNTs) pre-
pared from inorganic, organic, or biological precursors have
been successfully developed, because they possess useful
functionalities and can serve as molecular capillaries, sieves,
and biological models.[4–6] In theory, the curling-up or rolling-
up mechanism of topology transformations from 2D flat
sheets to 1D hollow tubes is achievable.[4d] Thanks to effective
design and synthesis strategies, many porous MOFs with
various interesting network topologies have been reported
over the past decade.[2] Compared with the focus on CNTs
and SNTs, it is surprising that significantly less effort has been
directed to the preparation of metal–organic nanotubes
(MONTs).[7–9] In particular, discrete MONT structures are
extremely rare to date.[9]
and an interior channel diameter of 1.4 nm. These MONTs
are held together by alkaline cations to form 3D nanotubular
supramolecular arrays (Figure 1). To the best of our knowl-
edge, a single-walled MONT with such a large diameter is
unprecedented.[9]
Compounds MAS-21–23 were synthesized by reaction of
cadmium perchlorate, 4-amino-3-[(pyridin-4-ylmethylene)a-
mino]benzoic acid (Hapab), and MOH (MI = CsI, KI, and NaI,
respectively) in an EtOH/THF/H2O solvent diffusion system
at 48C through a single-step, self-organization process
(Scheme 1). The appropriate choice of an organic ligand
with specific functional groups and geometry is a major factor
in achieving these large nanotube-based structures. The
multifunctional Schiff base ligand of Hapab was designed
deliberately and possesses a bending angle of 1208 between
the pyridyl and carboxylate groups. Unlike similar banana-
shaped organic linkers, the use of the apab scaffold favors the
formation of a tubular structure, rather than a spherical
network.[11]
As part of our ongoing efforts in the design and synthesis
of functional crystalline materials,[2f,8d,10] we wish to report
herein on a unique type of MOF of [{[Cd(apab)2(H2O)]3-
(MOH)·G}n] (MAS-21–23, MI = CsI, KI, NaI, respectively; for
MAS-22, G = 18H2O·6C2H5OH·3C4H8O; apab = 4-amino-3-
[(pyridin-4-ylmethylene)amino]benzoate; MAS = materials
of Academia Sinica), all of which consist of a large single-
walled metal–organic nanotube of [{Cd(apab)2(H2O)}3n]
(MONT-A1) with an exterior wall diameter of up to 3.2 nm
[*] Dr. T.-T. Luo, H.-C. Wu, S.-M. Huang, Dr. Y.-S. Wen, Prof. Dr. K.-L. Lu
Institute of Chemistry, Academia Sinica, Taipei 115 (Taiwan)
Fax: (+886)2-2783-1237
E-mail: lu@chem.sinica.edu.tw
Dr. G.-H. Lee, Prof. Dr. S.-M. Peng
Department of Chemistry, National Taiwan University, Taipei 107
(Taiwan)
Y.-C. Jao, Prof. Dr. T.-W. Tseng
Department of Chemical Engineering, National Taipei University of
Technology, Taipei 106 (Taiwan)
[**] We are grateful to the Academia Sinica and the National Science
Council, Taiwan for financial support of this research.
Figure 1. a) Assembly of MONTs-A1, which are held together by
potassium cations in MAS-22; K yellow. b) Top view of MONT-A1; Cd
green, C white, O pink, N blue, H tan. c) Side view of (a).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904501.
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Scheme 1. Synthesis of MAS-22 by a one-step self-organization pro-
cess.
Single-crystal X-ray diffraction analyses revealed that
MAS-21–23 are all isomorphous and crystallize in the trigonal
space group P31c.[12] Therefore, only the structure of MAS-22
ꢀ
is discussed in detail. The asymmetric unit of MAS-22 consists
of one half of the CdII center, one apab ligand, one third of the
KI center, and one coordinated water molecule; other
disordered anions and guest molecules are not crystallo-
graphically well-defined. Analysis of the local symmetry of
the metal cations showed that the CdII center resides on a
special position (site occupancy factor (SOF) = 0.5) contain-
ing a twofold axis of rotation, and the KI center resides on the
other special symmetry site (SOF = 1/3) containing a three-
fold axis of rotation and other perpendicular twofold axes of
rotation. Each CdII center is bound to one water molecule and
two pyridyl nitrogen atoms, four carboxylate oxygen atoms in
a heptacoordinate manner. Each KI center is octahedrally
bound to six carboxylate oxygen atoms of the six separate
apab ligands (Figure S1 in the Supporting Information).
The most intriguing feature of MAS-22 is that there is a
large single-walled metal–organic nanotube formed from a
four-connected pseudo-square-planar CdII node and an angu-
lar bidentate organic linker of the apab ligand (Figure 2a).
The top view of the open-ended, hollow nanotube indicates
that it is an undulated hexanuclear {Cd6(apab)12(H2O)6}
metallamacrocycle with a very large 72-membered ring
(72MR) consisting of six CdII atoms and six apab ligands
(6Cd, 48C, 12N, and 6O) with S6 symmetry (Figure 2b).
Every alternating Cd center is coplanar, and each cadmium–
cadmium separation, bridged by the apab ligand, is
13.45(2) ꢀ. It is noteworthy that the MONT has a large
cross section with an exterior wall diameter of 3.2 nm, and an
interior channel diameter of 1.4 nm (calculated from the van
der Waals radii of the relevant atoms; Figure 1c).
To illustrate the unique structure of MONT-A1, each Cd
center, which is connected to four apab ligands, is represented
by a distorted square four-connected node, and the apab
linker is simplified by an angular bar with a bending angle
close to 1208 (Figure 2c). In Figure 2d,e, the unusual skeleton
of MONT-A1 with the only topology having all equivalent
links and four-connected pseudo-square-planar nodes is
apparent. This cylinder topology is potentially of great
interest, but has been not addressed previously.[9,13]
Conceptually, when a selected 2D layer is rolled up and its
adjacent edges glued appropriately, an open-ended hollow
tube with variable diameter will be generated.[6] Certainly,
MONT-A1 can be regarded as a nanotube that is folded from
a (4,4)-topology sheet (Figure 3a).[9a,b] In general, the manner
in which the graphene sheet are wrapped is represented by a
pair of indices (n,m).[4a] Analogous to the three types of
Figure 2. Structures of MONT-A1 in MAS-22: a) A four-connected
pseudo-square-planar cadmium center. b) The cross section of the
nanotube showing 72-MR with S6 symmetry. c) Simplified view of a
four-connected cadmium center and the angular apab ligand. d) Side
view and e) top view of the simplified nanotube structure. Cd green, C
white, O pink, N blue, H aqua.
single-walled CNTs, it is interesting to see how the (4,4)-
square layer is rolled up to make the nanotubes. Thus, the
index of MONT-A1 is defined as (6,6). Its tube axis is along a
direction diagonal to the (4,4)-square layer (Figure 3c). The
tube is not chiral and can be depicted as hexastranded helices
consisting of three right-handed and three left-handed helical
chains with a 61-screw axis and a 65-screw axis, respectively.
The Cd–Cd translation per helix turn is 63.17 ꢀ (3 ꢁ c).
Interestingly, as shown in Figure 3a, this huge and magnifi-
cent column-shaped nanostructure of MONT-A1 resembles a
rolled-up dragon column, which can be found in Chinese
palaces and temples. (Rolled-up dragons are a type of ancient
Chinese totem representing nobleness and authority; see
Figure 3b.)
The large nanotubes are all held together by potassium
ions and are closely packed in a hexagonal manner to form a
3D framework (Figure 3d). As shown in Figure 1c, this array
of MONTs in MAS-22 is analogous to a packet of densely
packed straws. An analysis using the PLATON[14] software
tool indicates that the extra-framework volumes per unit cell
for MAS-21, -22, and -23 are approximately 63, 61, and 59%,
respectively. These large free spaces within the frameworks
cause the structures to be highly porous and have low
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Figure 4. a) Coordination environment of the {KO6Cd3} cluster. b) The
six-connected trigonal prismatic building units of the {KO6Cd3} cluster.
c) Top view of the MAS-22 framework. d) The 49.66-acs net.
linked by 12 separate ditopic apab ligands (Figure 4b,c). The
double antiparallel apab ligands serve as linkers to bridge to
two {KO6Cd3} clusters. The overall array of the framework
results in a rare 49.66-acs topology, a high-symmetry uninodal
six-connected semiregular net (Figure 4d).[16,17]
In conclusion, we have prepared a new type of MOF
consisting of a large single-walled metal–organic nanotube
(MONT). These large MONTs are held together by alkaline
cations, in an analogous way to a packet of densely packed
straws, leading to unique supramolecular nanotubular arrays.
Our innovative results highlight an important research topic
and provide vision for the development of discrete metal–
organic nanotube-based materials that have been largely
unexplored. We believe that this type of MONT should be
isolated and will have great potential in the future. Further
research is currently underway.
Figure 3. a) Ball-and-stick representation of the MONT-A1 with helical
strands individually colored. b) A curling-up dragon column. c) Top-
ology relationship between a (4,4)-layer and the nanotube (colors are
the same as in (a)). d) Packing array of the nanotubes (nanotubes
individually colored).
calculated densities (in the absence of all guests) of 0.72, 0.75,
and 0.81 gcmÀ3, respectively. A side view of the framework
reveals other triangular channel openings with effective
dimensions of 9 ꢁ 9 ꢀ along the a and b axes (Figure 1c).
The overall array contains a 3D intersecting channel system
and large open windows. This structure is likely to be a major
factor in the unusually high instability of these three frame-
works upon the loss of guest moieties. Nitrogen sorption
studies reflect this structural characteristic (Figure S12 in the
Supporting Information).
Interpenetration is currently a major barrier to the design
and synthesis of low-density MOFs.[2b,15] Interestingly, MAS-
21–23 are all non-interpenetrating, despite the large 3D
intersecting channel systems within the frameworks, mainly
because the large single-wall MONTs are tightly held
together by alkaline cations. Thus, this is likely to be an
effective strategy for creating non-interpenetrated frame-
works through direct discrete tube-to-tube adherence,
because interpenetration is truly forbidden in such cases
(Figure 3d).
Experimental Section
MAS-22: A solution of Cd(ClO4)2·4H2O (0.10 mmol) in C2H5OH
(5 mL) was carefully layered on top of a bilayer solution comprised of
a solution of KOH (0.25 mmol) and 4-amino-3-[(pyridin-4-ylmethy-
lene)amino]benzoic acid (Hapab, 0.20 mmol) in H2O (7 mL) on the
bottom and a blank buffer solvent of THF (8 mL) on the top. Caution:
perchlorate salts are potentially explosive and should be handled with
care. The system was allowed to stand at 48C for 4 weeks, after which
time deep orange hexagonal crystals had formed in 18% yield (based
on CdII). The solid product was washed with deionized water and
ethanol, and only slightly dried in air prior to elemental analysis.
Elemental analysis (%) for C102H163Cd3KN18O43: calcd C 45.28, H
6.07, N 9.32; found C 45.71, H 6.31, N 9.96. The formula {[Cd(apab)2-
(H2O)]3(KOH)}·18H2O·6C2H5OH·3C4H8O was assigned by elemen-
tal microanalysis, thermogravimetric analysis, IR spectroscopy, and
single-crystal X-ray diffraction studies. MAS-21 and -23 were also
synthesized under similar reaction conditions using the same molar
ratios of reactants, except that CsOH or NaOH were used instead.
Further analysis of the frameworks reveals the MONT-
based structure as a suitable candidate. In considering the
weaker supramolecular interactions between the K+ center
+
À
and carboxylate oxygen atoms (K O 2.644(1) ꢀ), the K
center is surrounded by six carboxylate oxygen atoms of six
separate apab ligands in an octahedral manner. Thus, a six-
connected tetranuclear supramolecular cluster with the for-
mula {KO6Cd3} is formed (Figure 4a,b). Each {KO6Cd3}
cluster serves as a trigonal prismatic building block that is
Received: August 12, 2009
Published online: November 10, 2009
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Communications
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Keywords: crystal engineering · microporous materials ·
nanotubes · self-assembly · topochemistry
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