Preparation, structural diversity and characterization of a family of Cd(ii)-organic frameworks

Article abstract of DOI:10.1039/c3dt51118a

Two Cd(ii)-organic frameworks based on 5-iodoisophthalate (IIP ^2-), {[Cd(IIP)(bte)(H₂O)]·H₂O} _n (1) and [Cd(IIP)(bpp)(H₂O)]_n (2), were obtained either at an ambient temperature or under solvothermal conditions at 120°C in the presence of 1,4-bis(1,2,4-triazol-1-yl)ethane (bte) and 1,3-bis(4-pyridyl)propane (bpp) as auxiliary ligands, respectively. 1 is a novel discrete single-walled Cd(ii)-organic tube (SWCOT) which further extends into a 3D supramolecular interdigitated microporous columnar architecture supported by C-I...I halogen bonds and hydrogen bonds, while 2 exhibits an interesting two-fold interpenetrated 3D diamond network architecture. When the auxiliary ligands bte or bpp were replaced by a longer spacer ligand with more flexibility, 1,4-bis(triazol-1-ylmethyl)benzene (bbtz), the unique discrete single-walled Cd(ii)-organic nanotube (SWCONT), {[Cd(IIP)(bbtz)(H ₂O)]·H₂O}_n (3), which further extends into a 3D supramolecular microporous framework supported by face-to-face π...π stacking interactions and hydrogen bonds, was generated at room temperature. Under solvothermal conditions at 120°C, an interesting two-fold 2D embracing (4,4) topological network, [Cd(IIP)(bbtz)(H₂O)] (4), which further extends into a two-fold 3D embracing supramolecular framework through O-H...O hydrogen bonds, is obtained. 4 loses crystallinity in air, leading to the formation of [Cd(IIP)(bbtz)]·0.5H₂O (4A) evidenced by elemental analysis, thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD). Remarkably, in situ rapid and reversible dehydration-rehydration in static air occurs in 1-3, indicating their potential applications as water absorbent and sensing materials. Dehydrated 1 and 3 show selective gas adsorption of CO ₂ over N₂ and dehydrated 3 can adsorb methanol and ethanol vapors strongly. These compounds exhibit blue fluorescence in the solid state.

Full text of DOI:10.1039/c3dt51118a

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Page 1 of 41
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DOI: 10.1039/C3DT51118A
Preparation, structural diversity and characterization of a family of
Cd(II)-organic frameworks
Ye Deng,[a] Chuan-Tao Hou,[a] Richard I. Walton,[b] Jiaqian Tang,[a] Peizhi Zhu,[a]
Kou-Lin Zhang,∗[a,b] Seik Weng Ng[c]
[a] College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou
225002, P.R. China
[b] Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK
[c] Department of Chemistry, University of Malaya, 50603, Kuala Lumpur, Malaysia
Two Cd(II)-organic frameworks based on 5-iodoisophthalate (IIP2-),
{[Cd(IIP)(bte)(H2O)]·H2O}n (1) and [Cd(IIP)(bpp)(H2O)]n (2), were obtained either at
ambient temperature or under solvothermal condition at 120 °C in the presence of
1,4-bis(1,2,4-triazol-1-yl)ethane (bte) and 1,3-bis(4-pyridyl)propane (bpp) as auxiliary
ligands, respectively. 1 is a novel discrete single-walled Cd(II)-organic tube (SWCOT)
further extending into a 3D supramolecular interdigitated microporous columnar
architecture supported by C-I…I halogen bonds and hydrogen bonds, while 2 exhibits
an interesting two-fold interpenetrated 3D diamond network architecture. When the
auxiliary ligands bte or bpp were replaced by a longer spacer ligand with more
flexibility, 1,4-bis(triazol-1-ylmethyl)benzene (bbtz), the unique discrete
single-walled Cd(II)-organic nanotube (SWCONT), {[Cd(IIP)(bbtz)(H2O)]·H2O}n (3),
which further extends into a 3D supramolecular microporous framework supported by
face-to-face π…π stacking interactions and hydrogen bonds, was generated at room
temperature. Under solvothermal condition at 120 °C, an interesting two-fold 2D
“embracing” (4, 4) topological network, [Cd(IIP)(bbtz)(H2O)] (4), which further
∗Corresponding author. Tel /Fax: +86 514 87975244. E-mail: klzhang@yzu.edu.cn; koulinzhang2002@yahoo.com
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extends into a two-fold 3D “embracing” supramolecular framework through O-H…O
hydrogen bonds, is obtained. 4 loses crystallinity in air, leading to the formation of
[Cd(IIP)(bbtz)]·0.5H2O (4A) evidenced by elemental analysis, thermographic analysis
(TG) and powder X-ray diffraction pattern (PXRD). Remarkably, in situ rapid and
reversible dehydration-rehydration under static air occurs in 1 - 3, indicating their
potential application as water absorbent and sensing materials. Dehydrated 1 and 3
show selective gas adsorption of CO2 over N2 and dehydrated 3 can adsorb methanol
and ethanol vapors strongly. These compounds exhibit blue fluorescence in the solid
state.
Introduction
Recently, the rational design and synthesis of metal-organic frameworks (MOFs) have
created much interest due to the intriguing topologies of the materials and their
potential applications in adsorption, catalysis and other technologies.1-4 The
architectures of MOFs depend on several factors, such as the chemical structure of the
ligand, preferred coordination geometry of the metal ions and a number of
experimental variables such as the solvent, temperature and templates, etc.5-10
Although carbon nanotubes have been intensively studied since their discovery in
1991,11 only a few MOFs with nanotubular structure have as-yet been synthesized.12
This may be because it is difficult to prepare discrete metal-organic nanotubes
(MONTs) owing to the possibility of interlocks or interpenetration. Compared to 3D
nanotubular MOFs, discrete MONTs may have different adsorption properties and
even new applications in nanotechnology.13-14 Therefore, it is still a particularly
2

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challenging subject to design and construct discrete MONTs.
From a design perspective, to assemble MOFs, the linking mode of the ligand
should meet with the coordination geometry of the metal ion. Some specific strategies
have been used to attempt to construct discrete MONTs. For instance, it has been
reported that 0D squares can be connected from their four vertices by bridging
organic ligands to generate novel interpenetrating or noninterpenetrating MONTs.13a,c
Previously, we performed a systematic study focusing on the temperature and
auxiliary ligand-controlled assembly in a series of Zn(II)-organic frameworks based
on 5-iodoisophthalic acid (H2IIP) in the presence of btp as auxiliary ligand; we thus
successfully obtained the first discrete single-walled Zn(II)-organic tube with
dimensions of 0.92 × 0.90 nm2 based on the 0D elliptical dimeric Zn(II) subunit.15
This gives us an indication that d10 metal ions when connected by flexible organic
ligands with appropriate length and intramolecular interdigitated angle generate a
nanosized elliptical dimeric unit, which then the rigid organic ligand links the
elliptical subunits from the two vertexes to generate a discrete single-walled
Cd(II)-organic nanotube (SWCONT) (Chart 1).
In this paper, we use H2IIP as a rigid ligand and 1,4-bis(1,2,4-triazol-1-yl)ethane
(bte), 1,3-bis(4-pyridyl)propane (bpp) and 1,4-bis(triazol-1-ylmethyl)benzene (bbtz)
as auxiliary flexible ligands to assemble Cd(II) frameworks under different reaction
conditions. Our strategy is based chiefly on the following considerations: (1) The
iodide group is a potential interaction site for creating C-I…I/N/O halogen bonds
which may help to extend the structure into a high-dimensional supramolecular
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framework due to its specific directional nature and relatively high halogen bonding
energy;16 and (2) The auxiliary flexible ligands chosen each has two different
conformations: T (trans) and G (gauche) for bte,17 TG (trans-gauche) and GG
(gauche-gauche) for bpp and bbtz,18 so their coordination modes may be influenced
by the reaction conditions. The G conformation of bte and GG conformation of bpp
and bbtz have the potential to form elliptical dimeric units with the d10 metal ion,
which may further be connected by the rigid ligand IIP2- leading to the formation of a
discrete single-walled metal-organic tube. Thus, a structural prediction of the resulting
framework may be possible to some extent.
Herein,
a
novel discrete single-walled Cd(II)-organic tube (SWCOT)
{[Cd(IIP)(bte)(H2O)]·H2O}n (1) and an interesting two-fold interpenetrated 3D
framework with diamond topology [Cd(IIP)(bpp)(H2O)]n (2) were obtained with IIP2-
in the presence of bte and bpp, respectively (Scheme 1). When bte and bpp were
replaced by a more flexible one with longer spacer, bbtz, a unique discrete
single-walled Cd(II)-organic nanotube, {[Cd(IIP)(bbtz)(H2O)]·H2O}n (3), and an
interesting two-fold 2D “embracing” (4, 4) topological network, [Cd(IIP)(bbtz)(H2O)]
(4), were obtained at ambient temperature and under solvothermal condition at 120 °C,
respectively. 4 loses crystallinity in the air and transforms into [Cd(IIP)(bbtz)]·0.5H2O
(4A). Thermal stability and solid state fluorescent properties of 1-3 and 4A have been
studied. Dehydration-rehydration behavior of 1-3 had been investigated. The gas and
vapor adsorption of 1 and 3 had also been performed.
Results and discussion
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Synthesis of the complexes
The discrete SWCOT 1 as well as the two fold interpenetrated 3D diamond
topological network 2 could be obtained at either room temperature or under
solvothermal condition at 120 °C, while at these two corresponding temperatures, a
pair of temperature-dependent frameworks, the discrete SWCONT 3 and 4, were
obtained. Interestingly, the molar ratio of H2IIP:NaOH is very important for the
growth of single crystals. During the synthesis of 1 and 2, the molar ratio of
H2IIP:NaOH (1:0 for 1 and 1:2 for 2) was used at room temperature, while the molar
ratio of H2IIP:NaOH (1:0.5 for 1 and 1:1 for 2) was used under solvothermal
condition at 120 °C although the synthesis of both 1 and 2 is temperature-independent.
At the higher temperature, however, polycrystals or cotton-like solids were obtained.
The molar ratio of H2IIP:NaOH (1:2) was used in attempts to synthesize 3 and 4. It
should be noted that 3 and 4 were not obtained without the presence of small
quantities of N, N’-dimethylformamide (DMF) and ethanol, respectively. This
indicates that the cosolvent has great influence on the crystal growth of these MOFs.
There are great differences in the coordination modes of the auxiliary ligand bbtz
(Scheme 1), which exert an important influence on the crystalline architectures of 3
and 4 as described below.
Structural description of the tube {[Cd(IIP)(bte)(H2O)]·H2O}n (1)
The SWCOT 1 crystallizes in the monoclinic system with space group C2/c and
exhibits a novel 1D single-walled tubular chain structure. The asymmetric unit
contains one Cd(II) ion, one IIP2- anion, one bte ligand, one coordinated water and
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one lattice water molecule. The central Cd(II) ion is heptacoordinated by four
carboxylate oxygen atoms from two different IIP2- ligands, two nitrogen atoms from
two different bte ligands and one water molecule in a pentabipyramidal coordination
geometry with average Cd–Ocarboxylate and Cd–Nbte distances of 1.952 Å and 2.008 Å,
respectively (Fig. 1A, Table 1). The auxiliary flexible ligand bte acts as a bidentate
bridging ligand showing a G(gauce) conformation (Scheme 1) with a dihedral angle
of 65.17° between the two triazole rings and inramolecular N1…N6 distance of 5.169
Å, linking the Cd(II) nodes into an elliptical dimeric unit [Cd(bte)2Cd]. The dimeric
subunits [Cd(bte)2Cd] are further connected by the main rigid ligand IIP2- with the
(
κ2)-(κ2)-ꢀ2 bridging mode to generate a novel discrete single-walled tube with the
dimensions of 0.9727 × 0.7990 nm2 along [010] direction (Fig. 1B). The nearest
intrachain Cd…Cd distance separated by the IIP2- bridge is 10.461 Å and the
interstitial water molecules are situated close to the wall of the tube.
If water molecules are ignored, calculations with the SQUEEZE program from the
PWT19 show that, in the evacuated regions of SWCOT 1, the available volume for the
inclusion is 299.3 Å3 per cell unit, i.e. 7.7 % of the crystal volume of 3865.0 Å3.
Both the coordinated and lattice water molecules form a water dimer. The
intertubular hydrogen bonds further extend the discrete tube into 2D supramolecular
layer [O1W-H1WA…O4 2.790(9), O1W-H1WB…N2 3.010(11), O2W-H2WA…O2
2.891(7) and O2W-H2WB…O3 3.007(8) Å] (Fig. 1C, Table 2). The lattice water
O2W acts as hydrogen bond donor and acceptor, while the coordinated water
molecule O1W serves as double hydrogen bond donor. Interestingly, 2D
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supramolecular layer interpenetrates each other through the halogen bond C-I…I
(3.791 Å), resulting in the formation of a 3D supramolecular microporous
interdigitated columnar network (Fig. 1D).
A
B
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C
D
Fig. 1 A The coordination environment of Cd(II) and the conformation of bte in 1. (Symmetry
codes: #1 -x+2,-y,-z+1 #2 x,y-1,z).
B Ball and stick representation of 1D tube along [010] direction with internal dimensions.
C 2D supramolecular layer, showing the coordination mode of IIP2- and the hydrogen
bonding pattern (red dotted lines).
D 2D supramolecular layer interpenetrates each other through the halogen bond C-I…I
(pink dotted lines)), resulting in a 3D supramolecular microporous framework.
Structural description of crystalline polymer [Cd(IIP)(bpp)(H2O)]n (2).
Single crystal X-ray structural analysis shows that 2 crystallizes in the monoclinic
system with space group C2/c and exhibits a two-fold interpenetrated 3D diamond
network architecture. The asymmetric unit consists of one Cd(II) ion, one IIP2-, one
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bpp ligand and one coordinated water. Each six-coordinated Cd(II) ion is bound by
two N atoms from two bpp ligands, four oxygen atoms from two IIP2- ligands and
coordinated water, which exhibits a distorted octahedral coordination geometry with
the subtended angles ranging from 52.65(12) to 175.77(13)° (Fig. 2A, Table 2). One
of the carboxylate groups in the IIP2- ligand exhibits the monodentate bridging
conformation, while the other one exhibits the chelating coordination mode, So the
IIP2- ligand adopts the (k1)-(k2)-ꢀ2 coordination mode to connect the adjacent Cd(II)
ions into a zigzag-like double chain along [100] direction (Fig. 2B). These chains are
further connected by bpp ligands in TG(trans-gauche) conformation (Scheme 1),
leading to the formation of 3D architecture.
If the Cd(II) ions are treated as nodes, both the IIP2- and bpp ligands as linkers, the
framework of 2 can be viewed as a two-fold interpenetrated 3D diamond network,
which are further connected with each other through π…π [(3.711(2) Å] stacking and
C-H…π interactions [H…pyridyl centroid: 2.954(2) Å] (Fig. 2 C-D).
A
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B
C
D
Fig. 2 A View of the coordination environment of Cd(II) in 2 ( #5 x,1-y,1/2+z #6 -1/2+x , 1/2-y,
-1/2+z).
B View of the 1D carboxylate-bridged zigzag chain in 2 along c axis.
C View of the two-fold interpenetrated 3D nets connected with each other through π…π
stacking and C-H…π interactions (blue dashed lines).
D Schematic drawing of the two-fold interpenetrated diamond network.
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Structural description of the nanotube {[Cd(IIP)(bbtz)(H2O)]·H2O}n (3)
Replacement of the auxiliary ligands bte and bpp by a more flexible ligand with
longer spacer length, bbtz, yielded the novel discrete Cd(II)-organic nanotube 3 which
was obtained at room temperature. Different from 1, compound 3 crystallizes in the
triclinic system with space group P-1 and has a novel discrete nanotubular chain
structure. The central Cd(II) atom adopts the pentagonal bipyramidal coordination
geometry through bonding to four O-donors from two IIP2- ligands, two triazole
N-donors from two bbtz ligands and one O-donor from the coordinated water
molecule (Fig. 3A,Table 2).
As expected, two bbtz ligands coordinate to two Cd(II) atoms to form an nanosized
elliptical dimeric metallamacrocycle ring [Cd(bbtz)2Cd] in which the auxiliary
flexible ligand bbtz adopts GG(gauce-gauce) conformation (Scheme 1). Compared
with bte in SWCOT 1, the auxiliary ligand bbtz in SWCONT 3 has a bigger bent
angle between two triazole rings (67.47°) and a longer intramolecular N1…N6
distance [9.2159(12) Å], which are mainly responsible for the formation of nanosized
elliptical dimeric subunit. The elliptical dimeric rings thus created are then further
linked with each other through the rigid IIP2- ligand in (k2)-(k2)-ꢀ2 fashion to form a
novel discrete single-walled elliptical nanotube (Fig. 3B). The nanotube lies parallel
to the crystallographic a-axis and has an internal dimension of 1.22950 × 1.13206
nm2.
Quite different from SWCOT 1, there are large solvent-accessible voids in
SWCONT 3. Calculations with the SQUEEZE program from PWT19 show that the
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total solvent-accessible volume of 3 is 339.0 Å3 per unit cell, even including both the
lattice water and coordinated water molecules, comprising 23.7 % of the crystal
volume (1432.64 Å3). While in the structure of SWCOT 1, there are no
solvent-accessible voids if both the coordinated and lattice water molecules are
included. If both the lattice water and coordinated water molecules are excluded, the
total solvent-accessible volume in SWCONT 3 reaches 410.6 Å3 per unit cell,
comprising 28.7 % of the crystal volume, which is quite larger than that in SWCOT 1.
This may result in its distinct adsorption/desorption properties to the guest molecules
as described below.
Similar to SWCOT 1, in the proximity of the wall of SWCONT 3 is situated one
ordered interstitial H2O molecule. Both the interstitial water molecule and the
coordinated one are organized into a discrete water dimer. The Ow…Ow distance
[2.709(14) Å] is comparable to that for the ice II phase (2.77-2.84 Å).20 The O1W and
O2W are also hydrogen-bonded to both the coordinated and uncoordinated
carboxylate oxygen atoms of the neighboring nanotubes by O1W-H1WA…O2
[2.714(17) Å)] and O2W-H2WA…O4 [2.682(11) Å)] (Fig. 3C). Thus, the nanotubes
are aligned into a 2D columnar sheet driven by such O-H…O hydrogen bonds and
face to face π…π stacking interactions between the benzene and triazole rings of the
adjacent tubes [the centroid…centroid distance: 3.7216(4) Å]. The 2D supramolecular
layer is further linked through face-to-face π…π stacking interactions between two
benzene rings of the adjacent nanotubes with the centroid…centroid distance of
3.8970(4) Å, ultimately resulting in a 3D supramolecular microporous assembly
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without interpenetration (Fig. 3D), which is quite different from the packing of
SWCOT 1.
A
B
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C
(Top)
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(Bottom)
D
Fig. 3 A The coordination environment of Cd(II) and the elliptical dimeric metallamacrocycle ring
in 3. (Symmetry codes: #1 1-x, -y, 1-z #2 x-1, y, z).
B 1D discrete single-walled nanotube, showing the lattice water molecules reside just in the
wall of the tube.
C Supramolecular columnar layer formed through O-H…O hydrogen bonds and face to
face π…π stacking interactions.
D 3D supramolecular microporous framework without interpenetration, including both the
coordinated and lattice water molecules.
Top: Stick representation;
Bottom: Space filling diagram, showing the larger cavities within the nanotube.
Structural description of the framework [Cd(IIP)(bbtz)(H2O)]n (4)
When the reaction used to prepare compound 3 was performed under solvothermal
condition at 120 °C, an interesting 2D framework 4 was obtained. Compound 4
crystallizes in the monoclinic system with space group P21/c. The asymmetric unit of
4 contains one Cd(II) ion, one IIP2- ligand, one bbtz ligand and one coordinated water
molecule. The Cd(II) ion is heptacoordinated by four carboxylate oxygen atoms from
two IIP2- ligands, two nitrogen atoms from two bbtz ligands and one water molecule,
resulting in a pentabipyramidal coordination geometry (Fig. 4A). The bbtz ligand
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links the adjacent Cd(II) nodes to yield a 1D zigzag chain, which is further linked by
the IIP2- ligands to form a 2D (4, 4) topological net along [100] direction (Fig. 4B).
The two carboxylate groups of IIP2- ligand adopt the same chelating bidentate
coordination mode, while the bbtz ligand adopts TG(trans-gauce) conformation
(Scheme 1).18 Thus, 4-membered macrometalcyclic rings with dimensions of 12.820
× 10.253 Å2 are formed.
The 2D (4, 4) topological net is further connected through the hydrogen bonds
O1W - H1WA … O3 2.746(4) Å and O1W - H1WB … N5 2.848(6) Å, leading to the
formation of a 2D supramolecular “embracing” double-layer (Fig. 4C). These double
arrays are further extended into a two fold 3D “embracing” supramolecular
architecture via O-H…O hydrogen bonds (see Table 3) (Fig. 4D).
A
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B
Top
Bottom
C
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D
Fig. 4 A View of the Cd(II) coordination environment and the conformation of the bbtz ligand in 4.
Symmetry code: #1 x-1,y,z-1 #2 -x+2,y+1/2,-z+1/2.
B 2D (4, 4) topological net.
C The adjacent 2D (4, 4) nets embrace with each other through hydrogen bonds, leading to
the formation of 2D supramolecular double layer.
Top: View along [-8.90, 2.65, 6.20]; Bottom: Side view along [-3.41, 0.41, -6.85]
direction.
D Two-fold 3D “embraced” supramolecular framework through hydrogen bonds.
FT-IR spectra and thermal Stability
The FT-IR spectral data show features attributable to the carboxylate stretching
vibrations of 1-3 and 4A. The absence of bands in the range of 1680-1760 cm-1
indicates the complete deprotonation of H2IIP in these four complexes. The
characteristic bands of the carboxylate groups appear in the range 1552-1598 cm-1 for
the asymmetric stretching and 1384-1490 cm-1 for the symmetric stretching,
respectively. The broad band at 3120 cm-1 corresponds to the vibration of the water
molecules in 1-3 and 4A.21
To study the thermal stability of the compounds, thermogravimetric analysis (TGA)
was performed on all samples under a nitrogen atmosphere in flowing N2 with a
heating rate of 10 °C·min-1 (Fig. 4). TGA of 1 suggests that the release of the water
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molecules begins at about 67 °C. The total weight loss of 5.90 % up to 153 °C
corresponds to the loss of both the coordinated water and lattice water molecules
(calcd. 5.98 %).
For 2, the first step weight loss of 2.56 % (calcd. 2.91 %) from 111 to 170 °C
corresponds to the removal of one coordinated water molecule.
For 3, the first step weight loss of 4.76 % (calcd. 5.30 %) in the temperature range
20 to 85 °C corresponds to the removal of two water molecules.
The weight loss of 0.84 % from 53 to 80 ˚C corresponds to the release of water
molecule, which further reveals that the formula of the partly dehydrated phase of
[Cd(IIP)(bbtz)(H2O)]n (4) is [Cd(IIP)(bbtz)]·0.5H2O (4A) (calcd. 1.38 %). The second
step begins from 240 ˚C and the whole decomposition process is complete at 800 ˚C.
X-ray powder diffraction measurement further shows that the structure of 4A is
different from that of 4 (Fig. 5).
100
80
60
40
20
0
0
100 200 300 400 500 600 700 800 900
0
Temperature / C
Fig. 4 TG curves of 1 (green), 2 (blue), 3 (black) and 4A (red).
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10
20
30
40
50
2θ
/degree
Fig. 5 PXRD patterns: (black) simulated from the single-crystal data of 4, (red) for the powder
sample 4A.
Dehydration-rehydration properties of 1-3
Thermogravimetric analyses (TGA) show clearly that both the interstitial and
coordinated water molecules in 1-3 could be excluded upon heating. To study the
dehydration-rehydration properties of 1–3, the in situ variable temperature PXRD was
performed on 1-3 over the temperature range 30-155 °C for 1, 30-170 °C for 2 and
30-85 °C for 3, respectively, in static air (Fig. 6-8). Remarkably, the PXRD patterns
clearly show that 1-3 undergo reversible dehydration and rehydration; upon exposure
of the dehydrated material in the static air, the dehydrated phase rehydrates rapidly,
resulting in the re-establishment of the original crystal structure. This is further
verified by the TG analysis of the rehydrated material of 1-3 [the first step mass loss
corresponding to water molecules in TGA: 5.81 % (calcd. 5.98 %) for 1, 2.88 %
(calcd. 2.91 %) for 2 and 5.34 % (calcd. 5.30 %) for 3 (see Fig. S1)]. It should be
pointed out that the dehydrated phases of 1-3 still remain integrity after the water
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molecules are lost, but the structure of the desolvated materials are changed, as
supported by the in-situ variable temperature powder X-ray diffraction (PXRD)
patterns. This observation clearly indicates that during the process of dehydration and
rehydration in 1-3 the frameworks exhibit a “breathing” property which may mainly
be attributed to the flexibility of the auxiliary ligands bte, bpp and bbtz in the
corresponding frameworks 1-3 since the main ligand IIP2- is rigid.
The framework integrity of 1-3 can be maintained after a number of
desorption/adsorption cycles, which indicate that 1-3 may be used as potential novel
adsorbent as well as sensing materials to water molecules.
40000
30000
20000
10000
0
30℃
155℃
30℃
10
15
20
2θ /degr ee
25
Complex 1
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25000
20000
15000
10000
5000
0
30℃
170℃
30℃
10
15
20
2θ /degr ee
Complex 2
20000
15000
10000
5000
0
30℃
85℃
30℃
10
20
30
40
50
2θ /degr ee
Complex 3
Fig. 6 VT-XRPD patterns recorded during the heating of 1-3 showing the complete departure of
the water molecules at 155 °C (green line) for 1, 170 °C (red line) for 2, and 85 °C (red
line) for 3, leading to a new dehydrated phase which further shows rapid in situ reversible
rehydration just in the static air. [(Black) the simulated XRPD patterns calculated from
single-crystal data of 1-3 with Mercury 1.4.2.]
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Desorption/adsorption to gas/vapors.
In order to test the ability of both the dehydrated SWCOT 1 and SWCONT 3 to host
molecules other than water, several sorption experiments were carried out. For such a
purpose, we considered molecules that exhibit current interest for storage, for
example, carbon dioxide (CO2) and nitrogen (N2). Further, gas sorptions were
performed at 278 K, to give more reliable results related to crystal structures
determined at room temperature. The N2 adsorption isotherm of the dehydrated 1 and
3 showed almost no uptake capacity. The gas sorption behavior of the dehydrated 1
and 3 showed distinct behavior in CO2 uptake when compared to other MOFs
reported in the literature (Fig. 8). Furthermore, the CO2 adsorption isotherm of the
dehydrated 1 and 2 showed quite different behavior with each other. For the
dehydrated 1, a significant hysteresis loop was observed. Above 20 atm, a large
increase in gas sorption is observed, resulting in a final uptake of 65 mg.g-1 (6.5 wt %)
at 38 atm. In addition, the desorption isotherm also displayed one phase transition at
10 bar. The dramatic change in gas sorption behavior indicates the framework
undergoes a phase change.22 But, the dehydrated 3 uptakes a relative lower amount of
CO2 (21 mg.g-1; 2.1 wt %) at 38 bar, which may be due to the size of the resulting
channel of 1 is much closer to the kinetic diameter of carbon dioxide (3.3 Å) than that
of 3. Quite different from that of the dehydrated 1, no dramatic change in gas sorption
was observed during both desorption and adsorption process, indicative of no phase
transition occurring in the process. Overall, both the dehydrated 1 and 3 showed
selective uptake of CO2 over N2 at 278 K under high pressure. The selective sorption
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of CO2 over N2 gases may be attributed to both the smaller kinetic diameters of CO2
compared to that of N2 (CO2, 3.3; N2, 3.6 Å) and the significant quadrupole moment
of CO2 (-1.4 × 10-39 cm2), which generates specific interactions with the host
framework.23
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
35
40
P / bar
Fig. 8 Sorption isotherms of CO2 measured at 278 K (1: pink; 3: red).
In addition, the dehydrated SWCOT 1 and SWCONT 3 were also subjected to
some protic and aprotic solvent vapor adsorption, such as methanol, ethanol and
acetone vapors. The adsorption to vapors was measured at 298 K. For the dehydrated
1, the amounts adsorbed for these three corresponding vapors are 16.5, 7.8 and 3.6
mg·g-1, respectively, while the dehydrated 3 showed much stronger adsorptions to
these three corresponding vapors, which are 61.1, 93.2 and 16.8 mg·g-1 (P/P0: 0.73 for
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methanol; 0.62 for ethanol and 0.72 for acetone), respectively. That the dehydrated 1
and 3 can adsorb methanol and ethanol vapor more strongly than acetone may be
attributed to the fact that the protic polar solvents can form hydrogen bonds with the
host tube. Furthermore, the strong adsorption ability to methanol and ethanol vapors
of the dehydrated 3 reveals that the dehydrated 3 may be used as effective adsorbent
material to some protic polar solvents.
Fluorescent properties
To well compare the relative fluorescent intensities of 1-3, 4A and free acid H2IIP, we
determined all the emission spectra with the same excitation wavelength. Excitation
of the samples 1-3 and 4A at 290 nm leads to the generation of similar blue
fluorescent emissions with the maximum emission centered on 472 nm and a shoulder
at about 412 nm (Fig. 9). To further understand the origin of these emission bands, the
fluorescent spectrum of free H2IIP acid has also been measured. The free H2IIP acid
exhibits the same maximum fluorescent emission and shoulder (λex = 290 nm) as
those in 1-3 and 4A. These observations suggest that the coordination of the IIP2-
ligand with Cd(II) ion in the presence of bte, bpp or bbtz as auxiliary ligands has no
influence on the emission mechanism of the Cd(II)–organic frameworks.24 So, the
emissions of these three complexes may be assigned to an intraligand π…π*
transition.25 The strongest fluorescent intensity of 4A compared with those of 1-3 may
be attributed that 4A has the least lattice water molecules among the three complexes.
The enhancement of fluorescence in the complexes 1-3 and 4A may be attributed to
the ligation of the ligand to the metal center, which further enhances the rigidity of the
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ligand IIP2- and reduces the loss of energy through a radiationless pathway.28
300
350
400
450
500
550
600
Wavelength(nm
)
Fig. 9 Solid-state fluorescence emissions recorded at room temperature for H2IIP (pink), 1 (red),
2 (blue), 3(black), and 4A (green) (λex = 290 nm).
Conclusions
In summary, a novel temperature-independent discrete single-walled coordination
tube 1 and a two-fold interpenetrated 3D diamond network 2 were successfully
obtained in the presence of bte and bpp as auxiliary ligand, respectively. In the case of
using a longer spacer auxiliary ligand bbtz with more flexibility, a pair of
temperature-dependent frameworks, 3 and 4, has been synthesized. Among them, the
novel SWCONT 3 was obtained at ambient temperature, while 4 was prepared under
solvothermal condition at 120 °C and features an interesting two-fold 2D “embracing”
(4, 4) topological network architecture. The different conformations of the auxiliary
flexible ligand bbtz in 3 and 4 must be induced by different temperatures and should
be responsible for the different architectures of these two frameworks. This reveals
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that the more flexible is the auxiliary ligand, the assembly will be more dependent on
the temperature used in synthesis. The present study also reveals that an effective
strategy for the formation of novel discrete SWCONT is the use of connecting 0D
elliptical dimeric nanometer-sized subunits through the rigid ligand IIP2-. To our best
knowledge, 1 and 3 are the first discrete single-walled Cd(II)-organic tubes.
Furthermore, 1-3 show in-situ rapid reversible dehydration-rehydration behavior just
in the static air, revealing that they may be useful as potential water adsorbent or
sensing materials. 1-3 and 4A have been found to exhibit blue fluorescence in the
solid state. The dehydrated 1 and 3 show selective gas adsorption to CO2 over N2
under high pressure. Furthermore, the dehydrated 3 may be used as adsorbent material
to some protic organic solvents. Further work is in progress to prepare other novel
MOFs based on H2IIP in our laboratory.
Experimental
Materials and characterization
The starting materials, H2IIP, bte, bpp and bbtz, were prepared according to the
previously reported procedure. 15, 27 The other reagents were purchased commercially
and used without further purification. Elemental analyses (C, H and N) were carried
out on a 240 C Elemental analyzer. FT-IR spectra (4000 - 400 cm-1) were recorded
from KBr pellets in Magna 750 FT-IR spectrophotometer. The solid state fluorescence
emission spectra were recorded using an F-4500 Fluorescence spectrophotometer
(Hitachi). Both the excitation and emission pass width are 5.0 nm. X-ray powder
diffraction data were collected on a computer controlled Bruker D8 Advanced XRD
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diffractometer operating with Cu-Kα1/2 (λ = 1.5418 Å) at a scanning rate 0.024 °/s
from 5° to 50° using a VÅNTEC solid-state detector. The diffractometer was fitted
with an Anton-Parr XRK900 heating stage for variable temperature studies.
Thermogravimetric analysis (TG) was taken on NETZSCH STA 409 PG/PC
instrument under a stream of N2 (10 mL/min). The sorption experiments for gases and
vapors were measured with an automatic gravimetric adsorption apparatus (Magnetic
Suspension Balances, Rubotherm Ltd. Germany) at 298 K. The initial outgassing
process for the samples 1 and 3 was carried out under a high vacuum at 155 and 85 °C
for ca. 4 h, respectively.
Synthesis of [Cd(IIP)(bte)(H2O)]·H2O (1)
A solution of Cd(NO3)2·4H2O (15.4 mg, 0.05 mmol) was dissolved in water (4 ml)
and then a solution of H2IIP (14.6 mg, 0.05 mmol) in DMF/H2O (10 ml, V/V = 1:1)
(DMF = N,N’-Dimethylformamide) was added whilst stirring. To this solution bte
(8.2 mg, 0.05 mmol) in water (4 ml) was added and then filtered. The filtrate was kept
at ambient temperature for several days and pale yellow block crystals were formed.
Complex 1 can also be obtained using the following hydrothermal method. A
mixture containing CdCl2·2.5H2O (11.4 mg, 0.05 mmol), NaOH (3.0 mg, 0.075
mmol), H2IIP (14.6 mg, 0.05 mmol), bte (8.0 mg, 0.05 mmol) and water (6 ml) was
sealed in a Teflon reactor, which was heated at 120 °C for 3 days and then it was
cooled to room temperature at 5 °C·h-1. Yellow block crystals were collected in a
69 % yield (based on H2IIP). Anal. Calc. for: C14H15CdIN6O6 C, 27.90; H, 2.51; N,
13.95. Found: C, 27.99; H, 2.60; N, 13.86 %. (yield: 67 % based on H2IIP). IR/cm-1
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(KBr): 3329 (m), 3162(w), 1595 (vs), 1544(vs), 1424(s), 1370(s), 1281(m), 1012(m),
926(m), 729(s), 670(s).
Synthesis of [Cd(IIP)(bpp)(H2O)] (2)
A mixture containing CdCl2·2.5H2O (22.8 mg, 0.1 mmol), H2IIP (29.2 mg, 0.1 mmol),
bpp (19.8 mg, 0.1 mmol), NaOH (8.0 mg, 0.2 mmol) and water (6 ml) was sealed in a
Teflon reactor, which was heated at 160 °C for 3 days, and then it was cooled to room
temperature at 10 °C·h-1. Colorless block crystals of 24 were collected in a 85 % yield
(based on H2IIP).
Complex 2 can also be obtained using the following typical solution method. A
mixture of H2IIP (29.0 mg, 0.1 mmol) and NaOH (4.0 mg, 0.1 mmol) was dissolved
in water (10 ml) and then an aqueous solution of CdCl2·2.5H2O (22.0 mg, 0.1 mmol)
in water (5 ml) was added whilst stirring. To this solution bpp (20.0 mg, 0.1 mmol) in
N, N’-dimethylformamide (DMF) (5 ml) was added and then filtered. The filtrate was
kept at ambient temperature for several days and yellow block crystals were formed.
(yield: 70 % based on H2IIP). Anal. Calc. for: C21H19CdIN2O5: C, 40.76; H, 3.09; N,
4.53. Found: C, 40.67; H, 3.12.; N, 4.71 %.
Synthesis of [Cd(IIP)(bbtz)(H2O)]·H2O (3)
A mixture of H2IIP (29.0 mg, 0.1 mmol) and NaOH (4.0 mg, 0.1 mmol) was dissolved
in DMF (5 ml) and then an aqueous solution of CdCl2·2.5H2O (22.8 mg, 0.1 mmol) in
water (5 ml) was added whilst stirring. To this solution bbtz (24.0 mg, 0.1 mmol) in
water (5 ml) was added and then filtered. The filtrate was kept at ambient temperature
for several days and yellow block crystals were formed (yield: 59 % based on H2IIP).
29