An unprecedented 2D → 3D metal-organic polyrotaxane framework constructed from cadmium and a flexible star-like ligand

Article abstract of DOI:10.1039/c0cc04724d

An unprecedented 2D → 3D metal-organic polyrotaxane framework, based on a new star-like tri(4-imidazolylphenyl)amine ligand, has been synthesized and characterized, which represents the first example of 2D → 3D polyrotaxane entangled in a parallel fashion.

Full text of DOI:10.1039/c0cc04724d

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An unprecedented 2D - 3D metal–organic polyrotaxane framework
constructed from cadmium and a flexible star-like ligandw
Hua Wu,ab Hai-Yan Liu, Ying-Ying Liu, Jin Yang,* Bo Liu and Jian-Fang Ma*
a
a
a
a
a
Received 1st November 2010, Accepted 23rd November 2010
DOI: 10.1039/c0cc04724d
4
An unprecedented 2D - 3D metal–organic polyrotaxane
framework, based on a new star-like tri(4-imidazolylphenyl)-
amine ligand, has been synthesized and characterized, which
represents the first example of 2D - 3D polyrotaxane
entangled in a parallel fashion.
involving loops and rods (Scheme S1w). In particular, only
one example of 2D - 3D polyrotaxane has been reported very
4
e
recently.
Since the pioneering work by Sauvage and Stoddart,
admirable synthetic strategies initiated by Kim and Robson
et al. have been developed to assemble rotaxanes into
polyrotaxanes with highly repeated structures in the field of
Entangled systems of metal–organic frameworks (MOFs) have
undergone revolutionary growth over the past decades
because of their undisputed aesthetic topological structures,
fascinating properties such as molecular machines and sensor
2
metal–organic polyrotaxane framework. Nevertheless, the
construction of 3D polyrotaxane remains a challenging and
unexplored issue in coordination chemistry. As mentioned in
previous works, the synthetic strategy for the polyrotaxane
networks is mainly dependent on the ligands selected with
both flexible and long rigid characters. In these cases,
conformationally flexible ligands have the ability to give
unusual entanglements involving a loop, while long rigid
ligands are good candidates for a rod in the assembly, and
insertion of a linear rod through those loops. Typically,
conformationally flexible 1,4-bis(imidazol-1-ylmethyl)benzene
1
devices, and potential biological applications. Polyrotaxane,
as an intriguing branch of the entanglement system, is
regarded as particularly significant, which has provided a
2
long-standing fascination for chemists. As elucidated in
several comprehensive reviews, the polyrotaxane family can
be described as regularly ordered infinite versions of the finite
2
molecular motifs. Actually, by means of ‘ideal’ continuous
deformations, we could separate the components of any finite
3
portion of a polyrotaxane. Although the extended polymeric
0
(bix), 1,3-bis(4-pyridyl)propane, 1,2-bis(4,4 -bipyridinium)-
0
analogues of the species can be characterized by the presence
of the rings as stoppers and side arms as the rods, this is at
ethane and 4,4 -bis(1,2,4-triazol-1-ylmethyl)biphenyl have
4
been used to construct polyrotaxane architectures. Taking
3
,4
present a largely unexplored area of chemical topology.
inspiration from the aforementioned points, in this work, we
synthesized a conformationally flexible star-like tridentate
ligand, tri(4-imidazolylphenyl)amine (Tipa) (Scheme S2,
ESIw). The Ph-imidazole arms of the Tipa ligand can rotate
freely and adjust itself sterically around the central N moiety
when coordinating to the metal centers. Thus, the star-like
Tipa ligand is a good candidate for the construction of
metal–organic polyrotaxane frameworks. Herein, we report
On the other hand, as mentioned by Ciani and coworkers in
their reviews, pseudo-rotaxanes (Scheme 1), which can be
entangled via rotaxane-like links and the rods that can be
withdrawn from the loops, have been investigated widely from
3
,5
1
D to 3D frameworks.
However, the real polyrotaxane
4
arrays are exceedingly rare. As far as we know, so far, only
a few fascinating examples showing 1D - 1D, 1D - 2D and
2
D - 2D polyrotaxane associations have been documented
an intriguing 2D - 3D framework [Cd(Tipa)Cl
2
]
n
ꢀ1.07nH
2
O
(
1) based on the star-like Tipa ligand. Remarkably, this
framework represents the first example of 2D - 3D
polyrotaxane entangled in a parallel fashion.
Compound [Cd(Tipa)Cl
solvothermal reaction of CdCl
Tipa (0.044 g, 0.1 mmol), CH OH (8 mL) and water (2 mL) at
50 1C for three days.w Single-crystal X-ray diffraction reveals
2
]
n
ꢀ1.07nH
2
O (1) was prepared by
2
ꢀ2.5H
2
O (0.068 g, 0.3 mmol),
3
1
that 1 is a unique 2D - 3D polyrotaxane entangled in a
parallel fashion. The asymmetric unit of 1 contains one unique
Cd(II) atom, one unique Tipa ligand, one point zero seven
uncoordinated water molecules and two unique chlorine
anions, where the O2W atom lies on an inversion center
(Fig. S1, ESIw). The Cd(II) ion is five-coordinated by three N
atoms from three Tipa ligands and two chlorine anions in a
Scheme 1 (I) Schematic representations of the finite molecular motifs
of rotaxane with the presence of bulky stoppers; (II) the presence of
the rings as stoppers; (III) pseudorotaxane with no stopper.
a
Key Lab of Polyoxometalate Science, Department of Chemistry,
Northeast Normal University, Changchun 130024,
People’s Republic of China. E-mail: yangjinnenu@yahoo.com.cn,
jianfangma@yahoo.com.cn; Fax: +86 431 85098620;
Tel: +86 431 85098620
Heilongjiang Agricultural College of Vocational Technology,
Jiamusi, 154007, People’s Republic of China
6
distorted square pyramidal geometry with a t value of 0.11.
˚
The Cd–N bond lengths vary from 2.276(2) to 2.346(2) A, and
b
N–Cd–N angles range from 94.95(8)1 to 153.05(8)1. Each Tipa
ligand bridges three Cd(II) centers to generate a highly
˚
undulant 2D sheet with a thickness of ca. 9.53 A (Fig. 1).
w Electronic supplementary information (ESI) available: Crystallo-
graphic data and complementary drawings for crystal structures.
CCDC 799006. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc04724d
From a topological viewpoint, the sheet can be considered as a
1
818 Chem. Commun., 2011, 47, 1818–1820
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Fig. 1 A view of a single sheet in 1.
2
3
-connected network with a Schla
ESIw). There are two kinds of large windows of Cd
and Cd (Tipa) in the resulting layers. The Cd (Tipa)
¨
fli symbol of 4ꢀ8 (Fig. S2,
(Tipa)
window
2
2
4
4
2
2
is built up by two Cd atoms and four Ph-imidazole arms of
˚
two different Tipa ligands with the dimension of 9.52 ꢁ 9.75 A,
4 4
while the Cd (Tipa) unit is built up by four Cd atoms and
eight Ph-imidazole arms of four ligands with the dimension of
˚
1
8.56 ꢁ 16.02 A (Fig. S3, ESIw). It is noteworthy that every
one Cd (Tipa) unit of each layer is threaded through
2
2
simultaneously by two armed rods of the ligands from two
adjacent layers in a parallel fashion as highlighted in Fig. 2.
Each green armed rod of the above layer passes through one of
the Cd
2
(Tipa)
2
unit (blue one), and each orange armed rod of
(Tipa) unit
the below layer passes through the same one Cd
2
2
(
(
Fig. 2).
All sheets are identical, and entangled in an ABC fashion
Fig. S4, ESIw). As a result, an unprecedented 2D - 3D
Fig. 3 (top) A view of the three entangled nets in 1 (blue sheet
lies in middle, and green and orange sheets lie above and below,
respectively); (middle) schematic representation of the three entangled
nets in a parallel mode in 1 and (below) schematic representation of
the 2D - 3D polyrotaxane in 1.
entanglement is generated, originating from the three adjacent
polymeric sheets at one time (Fig. 3). Actually, if we regard
each Cd (Tipa) unit as a loop and one arm of the ligand
2
2
as a single rod, undoubtedly, the entanglement in 1 is an
unquestionable polyrotaxane. The way the networks
entangled, however, is very different from previous reported
examples, and is in fact the first example of its particular
4
entangled topology.
2 2
Fig. 2 (top) One blue Cd (Tipa) unit is threaded through by two
Notably, further study into the nature of the intricate
armed rods of the adjacent layers, where all chloride ions are omitted
for clarity and (bottom) schematic representation of one ring threaded
by two rods.
architecture of 1 is that all loops of Cd
2 2
(Tipa) are equivalent.
However, the planes of adjacent Cd (Tipa) rings within the
2
2
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Chem. Commun., 2011, 47, 1818–1820 1819

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layer are almost close to perpendicular with the dihedral angle
being 89.431. Without a doubt, the results afford a high
feasibility of the entangled 2D - 3D polyrotaxane array.
On the other hand, another important factor for the construction
of this intriguing polyrotaxane is the large Cd (Tipa) loops of
framework from a star-like ligand. The synthetic strategy
presents a progressive evolution for the construction of high
dimensional polyrotaxane frameworks.
We thank the National Science Foundation of China
(21001023 and 21071028), Program for Changjiang Scholars
and Innovative Research Teams in Chinese University, the
Specialized Research Fund for the Doctoral Program of
Higher Education, the Science Foundation of Jilin Province
(20090137 and 20100109), the Training Fund of NENU’S
Scientific Innovation Project, and the Fundamental Research
Funds for the Central Universities for support.
2
2
a single network. Thus, every two rods from two independent
sheets can pass through one ring of another sheet
simultaneously and every one ring encircles two rods of two
adjacent sheets, thus giving an unprecedented type of 3D
polyrotaxane by one ring with four stoppers rather than a
3
common rotaxane with two stoppers.
So far, only a few 1D and 2D polyrotaxane frameworks
4
have been reported. Robson and co-workers have reported
Notes and references
the first unusual 2D
-
2D polyrotaxane network
1
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2
4
3 2
2
2
b
benzene). In that reported example, there are two independent
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and the Zn (bix) loops. The bix rods pass through the loops
of another layer in a parallel fashion to give an unusual
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2
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example, several polyrotaxanes of 2D - 2D based on
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2
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f,g
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D - 3D polyrotaxane [Cd (bix) (bpea) ]ꢀ4H O (bpea =
4
4
4
2
0
biphenylethene-4,4 -dicarboxylate), involving Cd (bix) loop
2
2
4e
and bpea rod, has been documented, where one loop of an
independent layer is penetrated by one rod of the adjacent
layer. In these modes, each 2D network is entangled with four
inclined networks, giving an aesthetic 3D polyrotaxane in an
inclined fashion. It should be pointed out that although both
1
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2
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the reported [Cd
the 2D - 3D polyrotaxanes, their entangled modes are
2
completely different. The sheets of [Cd (bix) (bpea) O
4
(bix)
4
(bpea)
4
]ꢀ4H
2
O and compound 1 show
3
4
L. Carlucci, G. Ciani and D. M. Proserpio, Coord. Chem. Rev.,
2
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4
4
4
]ꢀ4H
are entangled in an inclined fashion, while the ones of 1 are
entangled in a parallel fashion. In addition, only one rod
passes through one loop in [Cd (bix) (bpea) ]ꢀ4H O, whereas,
4
4
4
2
(
d) D. M. L. Goodgame, S. Menzer, A. M. Smith and
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D. J. Williams, Angew. Chem., Int. Ed. Engl., 1995, 34, 574;
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The emission spectrum of 1 exhibits an intense emission at
2
008, 2233; (f) X.-Y. Cao, Y.-G. Yao, S. R. Batten,
4
26 nm in the blue region upon excitation at 320 nm, which is
a slightly red-shifted 6 nm compared with the Tipa ligand
ex = 365 nm, lem = 420 nm) (Fig. S5, ESIw). Therefore, the
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(
l
emission peak of 1 may be assigned to the intraligand
fluorescent emission. The red-shift of luminescence may be
attributed to the ligand coordinating to the metal center,
which effectively increases the rigidity of the ligand and
reduces the loss of energy by radiationless decay.
5
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(
1
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In summary, an unprecedented 2D - 3D polyrotaxane
framework based on the star-like Tipa ligand has been
prepared by using Cd (Tipa) as the loop and the long
2
714; (f) M.-L. Tong, Y.-M. Wu, J. Ru, X.-M. Chen, H.-C. Chang
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2
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(
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Ph-imidazole arm as the rod. The entangled modes observed
in 1 are unique in the metal–organic polyrotaxane framework.
Also, it is the first time for the construction of a polyrotaxane
1
6
1349.
1
820 Chem. Commun., 2011, 47, 1818–1820
This journal is c The Royal Society of Chemistry 2011