Synthesis and characterization of Mg-Al-layered double hydroxides intercalated with cubane-1,4-dicarboxylate anions

Article abstract of DOI:10.1039/c4dt03152k

In the present work, Mg₂Al-layered double hydroxide (LDH) intercalated with cubane-1,4-dicarboxylate anions was prepared from the reaction of solutions of Mg(ii) and Al(iii) nitrate salts with an alkaline solution of cubane-1,4-dicarboxylic acid by using the coprecipitation method. The successful preparation of a nanohybrid of cubane-1,4-dicarboxylate(cubane-dc) anions with LDH was confirmed by powder X-ray diffraction, FTIR spectroscopy and thermal gravimetric analysis (TGA). The increase in the basal spacing of LDHs from 8.67 ? to 13.40 ? shows that cubane-dc anions were successfully incorporated into the interlayer space. Thermogravimetric analyses confirm that the thermal stability of the intercalated cubane-dc anions is greater than that of the pure form before intercalation because of host-guest interactions involving hydrogen bonds. The interlayer structure, hydrogen bonding, and subsequent distension of LDH compounds containing cubane-dc anions were shown by molecular simulation. The RDF (radial distribution function), mean square displacement (MSD), and self-diffusion coefficient were calculated using the trajectory files on the basis of molecular dynamics (MD) simulations, and the results indicated that the cubane-dc anions were more stable when intercalated into the LDH layers. A good agreement was obtained between calculated and measured X-ray diffraction patterns and between experimental and calculated basal spacings.

Full text of DOI:10.1039/c4dt03152k

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Synthesis and characterization of Mg–Al-layered
double hydroxides intercalated with cubane-
Cite this: DOI: 10.1039/c4dt03152k
1,4-dicarboxylate anions
Zolfaghar Rezvani,* Farzad Arjomandi Rad and Fatemeh Khodam
2
In the present work, Mg Al-layered double hydroxide (LDH) intercalated with cubane-1,4-dicarboxylate
anions was prepared from the reaction of solutions of Mg(II) and Al(III) nitrate salts with an alkaline solution
of cubane-1,4-dicarboxylic acid by using the coprecipitation method. The successful preparation of a
nanohybrid of cubane-1,4-dicarboxylate(cubane-dc) anions with LDH was confirmed by powder X-ray
diffraction, FTIR spectroscopy and thermal gravimetric analysis (TGA). The increase in the basal spacing of
LDHs from 8.67 Å to 13.40 Å shows that cubane-dc anions were successfully incorporated into the inter-
layer space. Thermogravimetric analyses confirm that the thermal stability of the intercalated cubane-dc
anions is greater than that of the pure form before intercalation because of host–guest interactions invol-
ving hydrogen bonds. The interlayer structure, hydrogen bonding, and subsequent distension of LDH
compounds containing cubane-dc anions were shown by molecular simulation. The RDF (radial distri-
bution function), mean square displacement (MSD), and self-diffusion coefficient were calculated using
the trajectory files on the basis of molecular dynamics (MD) simulations, and the results indicated that the
cubane-dc anions were more stable when intercalated into the LDH layers. A good agreement was
obtained between calculated and measured X-ray diffraction patterns and between experimental and cal-
culated basal spacings.
Received 13th October 2014,
Accepted 30th October 2014
DOI: 10.1039/c4dt03152k
www.rsc.org/dalton
systems have been widely studied for the purpose of advancing
Introduction
their potential applications, and from a molecular simulations
5
Layered organic–inorganic hybrid compounds containing bio- point of view. Much attention has been paid to the many
logical and biomedical organic anions are a new class of organic anions incorporated in LDHs. These new materials
materials and represent a promising advance in the develop- show new photophysical behaviors and several useful advan-
1
ment of multifunctional materials. Recently, much work has tages for eco-friendly development. A homogeneous distri-
focused on the incorporation of various organic anions into bution of the organic anions at the molecular level can be
layered inorganic host structures for the purpose of improving facilitated by H-bond and van der Waals interactions and the
the thermal and physicochemical properties of these organic host–guest interaction based on electrostatic forces in the LDH
2
6
anions. The interlayer spacing of these inorganic host struc- galleries.
tures are easily expandable due to easy breaking of weak van
The anions in the interlayer gallery are generally exchange-
3
der Waals or hydrogen bonds. Among these layered inorganic able, and indeed anion exchange is the most widely used inter-
host structures, the layered double hydroxides (LDHs) have in calation method. Many different kinds of anions have been
particular attracted a lot of attention. Layered double hydrox- successfully intercalated into LDH, including almost all of the
ides consist of positively charged brucite-like layers and nega- common inorganic anions, many organic anions including
tively charged anions in the interlayer space. These structures carboxylates, sulfonates, benzoates, and biomolecular anions
7
can be represented by the general chemical formula such as amino acids, enzymes, DNA and biopolymers. Investi-
1−x x 2 2
M M (OH) O], where M and M are di- gation of the interlayer arrangement of these compounds is
] [(An−)x/n·mH
2
+
3+
x+
2+
3+
[
n−
and trivalent metal cations, respectively, and A represents an problematic as they usually exhibit a certain degree of dis-
exchangeable interlayer anion with a charge of n−. LDH order, which can prevent adequate structural analysis based
4
on diffraction data only. Combining molecular modeling with
5
experimental measurements, however, can solve this problem.
Department of Chemistry, Faculty of Basic Sciences, Azarbaijan Shahid Madani
Because of the considerable increase in computational
power over the past decades, molecular dynamics (MD) has
University, Tabriz, Iran. E-mail: zrezvani@azaruniv.ac.ir; Fax: +98 412 432 7541;
Tel: +98 412 432 7541
8
,9
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proved to be a useful tool to study molecular self-assembly and Syntheses of cubane-dc-LDH
this technique can yield detailed atomic-level insight into the
A solution containing cubane-1,4-dicarboxylic acid (0.172 g,
.001 mol) dissolved in NaOH (20 mL, 0.1 M) was slowly added
to an aqueous mixed solution (20 mL) of Mg(NO ) ·6H O
three-dimensional structure of the model system. The theore-
tical simulations provide a complementary and effective
method to study and predict the properties and structures of
new kinds of functional materials, and some impressive
0
3
2
2
(
3 3 2
0.5128 g,0.002 mol) and Al(NO ) ·9H O (0.375 g, 0.001 mol)
with vigorous stirring in a nitrogen atmosphere. The pH of the
solution was adjusted to 10.2 by addition of NaOH or HCl
dropwise, and the system was heated to reflux at 55 °C
for 72 h. The product (cubane-dc-LDH) was centrifuged at a
speed of 2000 rpm for 10 min. and the solid obtained
was washed thoroughly with deionized water and finally dried
at room temperature under vacuum. Anal. calculated for
6
results can be obtained.
1
0
Since Eaton and Cole synthesized cubane, there have
been numerous reports of syntheses and characterizations of
1
1–14
cubane derivatives.
Cubane derivatives are widely used as
novel organic anions for high-energy explosives, oligomeric
compound development and pharmaceutical materials. For
example, they are highly lipophilic molecules that bind avidly
to the envelope of the AIDS virus. The syntheses and characteri-
zations of nanohybrids of several organic compounds with
LDHs have been reported in last decade, but to the best of our
knowledge, synthesis, characterization and molecular model-
[
Mg Al (OH) ](C H O )·2.5H O: C, 20.33; H, 3.90; Mg, 16.47;
4 2 12 10 6 4 2
Al, 9.15. Found: C,19.9; H, 3.98; Mg, 16.0; Al, 8.7.
Molecular modeling
ing investigations of LDHs with cubane derivatives have not The entire simulation was conducted using the Discover and
1
6
been reported.
Forcite modules in the Materials Studio software package.
In the present work, the preparation of Mg–Al hydrotalcite These modules involve a range of well-validated force fields for
intercalated with cubane-1,4 dicarboxylate anions (hereafter dynamics simulations, minimization, and analysis searches
abbreviated as cubane-dc) is reported. The product was for periodic solids. It can be desirably employed in studying
obtained from the reaction of solutions of Mg(II) and Al(III) molecular systems and a variety of materials types. In this
salts with an alkaline solution of cubane-1,4-dicarboxylic acid article, a perfectly ordered LDH layer model with a hexagonal
using the coprecipitation method. Since the physicochemical supercell was made. At first, the DFT method was used to
properties of the intercalated material are influenced by the obtain the optimized geometrical configuration for the cubane-
structure of the interlayer space, molecular modeling was used dc. After determination of atomic charges, this information was
to propose the arrangement of cubane-dc anions in this space. used in the molecular dynamics simulations. The classical force
17–19
field CVFF was used for the cubane-dc-LDH system.
The
first step in the simulation was the preparation of the positively
charged Mg–Al-LDH layers as the host structure. The host
structure was constructed using atomic coordinates from the
Experimental
Materials
2
0
previously reported X-ray crystal structure of hydrotalcite
Mg Al (OH)12](CO )·3H O], which was refined in the rhombo-
hedral space group R3m with lattice parameters a = b =
.054 Å, c = 22.81 Å, α = β = 90°, and γ = 120°. In order to study
Cubane-1,4-dicarboxylic acid was prepared by the methods
described in the literature.
from the Merck chemical company and used without further
[
4
2
3
2
1
0,14,15
All reagents were purchased
ˉ
3
purification. NO –Mg–Al-LDH was prepared according to the
literature.
3
the arrangement of the guests in the interlayer space, a P1
superlattice with the dimensions 6a × 4a × 3dexp was developed
where dexp is d003 of cubane-dc-LDH, which was obtained from
the X-ray diffraction data. The interlamellar carbonate ions
1
Characterizations
Powder X-ray diffraction patterns (PXRD) of the samples were and water molecules were omitted; then a supercell in the
recorded with a Bruker AXS model D8 Advance diffractometer form 6a × 4b × 1.76c was developed with supercell parameters
α
using Cu-K radiation (λ = 1.54 Å) at 40 kV and 35 mA with 2θ 6a = 18.324 Å, 4b = 12.216 Å, 1.76c = 45.625 Å, α = β = 90° and
Bragg angles ranging from 2 to 70°. Fourier transform infrared γ = 120°. In fact, 3d003 or 3dexp(= 40.2 Å) of cubane-dc-LDH is
spectra (FT-IR) were recorded in the range of 4000–400 cm−
1
equal to the 1.76c of the hydrotalcite structure. The ratio of Mg
using the KBr pellet technique with a Perkin-Elmer spectro- and Al atoms in the framework was 2 : 1, and the Mg and Al
photometer. Thermogravimetric analysis (TGA) was carried out atoms were scattered in every layer arbitrarily. Every hydroxide
3
+
2+
with a Mettler-Toledo TGA 851e apparatus at a heating rate of layer had 8Al and 16Mg ions, with the latter arranged in
0 K min− in a nitrogen atmosphere. Scanning electron such a way that they were not placed in the adjacent octahedra.
1
1
microscopy (SEM) was used to study the morphology of The cubane-dc anions were randomly located in the inter-
samples using an ultrahigh resolution FESEM device (model lamellar area of the LDH to neutralize the positive charges for
ULTRA55, Carl Zeiss MST AG, Germany). Elemental (C, H, and N) the hybrid system, and then to form a tilted bilayer arrange-
analyses were carried out on a Perkin-Elmer 240B analyzer. ment. Ten molecules of water were also randomly inserted into
Mg and Al contents of the samples were determined by using the interlamellar area. The atomic charges can be determined
inductively coupled plasma spectroscopy (Jobin Yvon JY24) from the Mulliken charge, APT charge and ESP charge. In this
2
0,21
after dissolving the samples in nitric acid.
work, the Mulliken method
was used for calculation of
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were kept as rigid units during the energy minimization
period, and the cell parameters c, a, and β were allowed to
vary, which enabled the optimization of the arrangement of
the host layers; all the atomic positions in the interlayer region
6
were allowed to vary as well.
In this work, we concentrated on the aggregation structure
around the anions of cubane-dc intercalated into LDH. Simu-
lations were carried out on structures previously minimized
using the smart minimizer procedure of the materials studio
software package. The minimized structures were used to set
the equilibrium bond lengths and angles. The molecular
dynamics simulation was carried out in an NPT statistical
ensemble at room temperature (298 K). The time step was 0.5
fs and total time of simulation was 800 ps. Fig. 2 shows that
the potential energy reaches a stable equilibrium after 200 ps
of simulation time and the last 400 ps were used for further
analysis.
Fig. 1 Mulliken atomic charges of cubane-dc.
atomic charges. Fig. 1 shows the Mulliken charge of the
cubane-dc. The Dmol3 program at the GGA-PBE/DND level
optimized the cubane-dc structure. The atomic charges are
essential for explaining the cubane-dc structure and the MD
Results and discussion
XRD analyses
properties when cubane-dc anions are placed into the LDH The XRD patterns for the Mg–Al–NO LDH and the cubane-dc-
3
layer.
Layer charges were calculated using the charge equili- layered phase (Fig. 3).
bration (QEq) method. Since the CVFF force field is relatively the Mg–Al–NO LDHs shown in Fig. 3(a), there are sharp and
LDH showed characteristic reflections related to a crystalline
2
5,26
In the 2–70° range of 2θ values for
3
2
2
accurate for LDH materials, the use of this force field strong diffraction peaks at low 2θ values, which demonstrate
improved the LDH partial charges; the partial charges were set the good crystallinity of LDH nanoparticles.
to 1.231e or 1.278e for Al, to 0.615e or 0.638e for Mg, to
Mg–Al–NO LDH has a d
basal spacing of 8.67 Å. This
003
3
−
for H.
0.617e or −0.628e or −0.558e for O, and to 0.212e or 0.233e spacing, which represents the summation of the thickness of
2
7
the brucite-like layer (0.48 nm) and the gallery height, is a
The minimization was performed using the CVFF force function of the number, size and orientations of the interca-
2
8
field. The electrostatic energy was obtained using the Ewald lated anions.
2
3
summation method and the van der Waals energy was
The XRD pattern of the sample with the intercalated
2
4
shown using a Lennard-Jones potential. The total crystal cubane-dc anion is shown in Fig. 3(b). During the intercalation
energy minimization was conducted using a modified Newton of cubane-dc anions, the layers of LDH swell to host the
method with the following approach: all the layers of the host anions, and this expansion is reflected by the 13.40 Å value of
Fig. 2 Time profiles of the total energy (the last 200 ps).
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Fig. 3 PXRD patterns of the Mg–Al–NO3-LDH and Mg–Al-LDH intercalated with cubane-dc.
its d003. This swelling of the layers is due to the intercalation acid and the cubane-dc-LDH are shown in Fig. 4(b and c). The
of the cubane-dc anions, and the basal spacing corresponding FT-IR spectrum of the Mg–Al–NO
to diffraction by the (003) planes is much larger than that for comparison.
3
-LDH is shown in Fig. 4a for
the LDH. Taking into account that the thickness of the LDH
layer is 4.8 Å and the longitudinal van der Waals radius of the the literature.
The spectrum for NO -LDH is similar to that reported in
3
2
9,30
−1
Absorption at 1384 cm can be assigned to
cubane-dc anion is 7.20 Å (determined by the software Chem- the ν vibration of NO . A broad, strong absorption band cen-
3
3
Sketch), it was confirmed that cubane-dc anions were success- tered at 3451 cm− is ascribed to the stretching vibrations of
1
fully intercalated in their anionic form, and that they were hydroxyl groups and surface and interlayer water mole-
3
1,32
arranged as a monolayer within the interlayer.
cules,
pared to the O–H stretching in free water at 3600 cm
is attributed to the formation of hydrogen bonds between the
which are found at a lower frequency in LDHs com-
−
1 33
.
This
FTIR spectra
The structure of cubane-dc-LDH was also confirmed by FT-IR interlayer water and the different guest anions as well as with
spectroscopy. The FT-IR spectra of the cubane-1,4-dicarboxylic the hydroxide groups of the layers. A weaker band at
3
Fig. 4 FT-IR spectra of (a) cubane-1,4-dicarboxylic acid, (b) NO -LDH, and (c) cubane-dc-Mg–Al-LDH.
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Fig. 5 TGA thermograms of (a) cubane-1,4-dicarboxylic acid and (b) Mg–Al-LDH-cubane-dc.
1
1
638 cm− was due to the bending mode of water molecules. tion of the LDH layer. This temperature is higher than the
−
1
The bands centered at 446 and 672 cm are attributed to Al–O decomposition temperature of pure cubane-1,4-dicarboxylic
and Mg–O lattice vibrations.
2
9
acid, which occurs at 220 °C. It is confirmed that the thermal
In the case of pure cubane-1,4-dicarboxylic acid, strong stability of intercalated cubane-DC molecule was enhanced by
−1
absorption bands centered at 3416 and 3500 cm are attribu- 80 °C. The third mass loss in the temperature range of
ted to the stretching vibrations of OH groups. Furthermore, 400–600 °C is attributed to further dehydroxylation of LDH
−
1
absorption bands at 1692 and 1626 cm are related to the layers. The mass loss value of the cubane-dc-LDH at 850 °C is
CvO of carboxylate groups. In cubane-dc-LDH, the absence of 52.17%. Decomposition of cubane-dc-LDH leads to the for-
−
1
the NO3 bands at 1384 cm indicates that the preparation mation of MgO and Al O with an Mg/Al ratio of 2. On the
2
3
process was complete. The band spanning the wavelengths basis of the chemical formula of the cubane-dc-LDH,
−
1
4 2 6 4 2
from 1692 to 1626 cm due to the COOH group disappears, [Mg Al (OH)12](C10H O )·2.5H O, the calculated mass loss is
while the two bands at approximately 1537 and 1422 cm− due 55.61%. This difference (3.44%) between the calculated and
1
to the anti-symmetric and symmetric stretching vibrations of observed mass losses may be due to incomplete oxidation of
−
–
COO appear and shift to lower wavenumbers, compared to the carbon.
free –COOH in cubane-1,4-dicarboxylic acid, indicating that
To further confirm the improvement of thermal stability of
the intercalation in the interlayer space involves hydrogen cubane-DC anions intercalated into LDH layers, the virgin crys-
bonding, besides the obvious electrostatic attraction between tals of this sample were stored at 300 °C for 2 hours, and then
the electropositive cations in the layer and organic anions in an X-ray diffraction pattern and IR spectrum (Fig. 6) were
2
8,34,35
the interlayer.
recorded. As seen in Fig. 6(a), the layered structure of the
sample is preserved in the annealed sample, but the basal
spacing (d003) slightly decreased from 13.40 Å to 12.90 Å,
Thermal analyses
Thermogravimetric analyses (TGA) are suitable for determin- which confirms the loss of water molecules from the interlayer
ing the thermal stabilities of materials, and were thus used to spacing. The FTIR spectrum of the annealed sample (Fig. 6b)
investigate the thermal stabilities of the cubane and the shows all vibrations of cubane-dc-LDH, but as can be seen
cubane-dc-LDH. The TGA curves are shown in Fig. 5(a, b). In from the spectrum, the intensity of the water molecule
the case of cubane-1,4-dicarboxylic acid (Fig. 5a) there are two bending mode (1638 cm− ) decreased in comparison with the
distinct steps, one in the temperature range 220–300 °C and virgin sample (Fig. 5c), which confirms the loss of water mole-
the other between 300 and 500 °C, that are attributed to cules from the interlayer spacing.
1
decomposition of cubane-1,4-dicarboxylic acid. The mass loss
value of the cubane-1,4-dicarboxylic acid at 850 °C is 87.5%.
Cubane-dc-LDH shows major mass losses in three steps.
The first mass loss observed in the 50–200 °C range corres-
Results of molecular modeling
ponds to the loss of both adsorbed and interlayer water mole- The aggregation state of the structure at the microscopic level
cules. A following rapid mass loss between 300 and 400 °C is is a very important factor for determining the macroscopic be-
attributable to the cubane-dc decomposition and dehydroxyla- havior of the cubane-dc anions intercalated into the layers of
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Fig. 6 XRD pattern of cubane-DC -LDH (a), and FTIR spectrum of cubane-dc-LDH (b), annealed at 300 °C for 2 hours.
LDH. MD simulations can show the orientation of the interca-
lated molecules at the molecular level and lead to a determi-
nation of the arrangement of the guest cubane-dc anions in
the interlayer region. Fig. 7(a) and (b) show snapshots of equi-
librium structures of the cubane-dc-LDH system before and
after MD simulation. Note that the cubane-dc anions and
water molecules were, prior to the simulation, placed ran-
domly in the interlayer space of the LDH. Through analyzing
the trajectories, the equilibration of the simulation systems
reveals the arrangement of cubane-dc anions. Molecular
Fig. 8 RDFs between end-group and water.
dynamic calculations reveal that the long axis of the cubane-dc Fig. 8, the water molecules are dispersed in the interlayers of
anions in the interlayer space is tilted approximately 54° from LDH around the end-groups, and can be divided into two main
a perpendicular orientation to the LDH layer. These calcu- shells, which are respectively located about 0.97 Å and 1.57 Å
lations also showed that guest anions are not randomly distrib- from the cubane-dc anion end-groups. Most of the water mole-
−
uted in the interlayer space but they have a tendency to cules form close contacts with OH of the LDH layer and the
−
assemble in rows, albeit with a certain level of disorder.
–COO groups of the guest anions. Fig. 9 is a schematic of the
The end-group-water interaction can be described by partial H-bond structure between the end-groups and the water mole-
radial distribution functions (RDFs), which indicate how many cules, which shows that most of the hydrogen bonds in the
atoms of type A are on average a distance r from an atom of hybrid systems are made between O atoms of end-groups and
6
−
type B. Relevant RDFs of water molecules with cubane-dc OH in the LDH layers, and then with the water molecules.
anion end-groups were calculate over the last 800 ps of the
To study the effective delivery of cubane-dc anions, which
molecular dynamics calculation (Fig. 8). As can be seen from use LDH as carriers, we have obtained the mean square
Fig. 7 Snapshots of the equilibrium structures for cubane-dc-LDH (a) before and (b) after MD simulation.
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water molecules at the times t and 0, respectively. Once the
MSDs as a function of time are calculated, it is then possible
to calculate the self-diffusion coefficient. Eqn (1) implies that
MSD exhibits linear dependence on time, and the self-
diffusion coefficient is just the slope of the line. For short time
periods, the MSD dependence on time is clearly not linear, but
it does become linear for longer periods of time. Diffusion
coefficients should be determined only using data in the
region where MSD depends linearly on time, and by using
3
7
linear regression. Fig. 10 indicates the obtained MSD for the
dispersed water molecules in comparison with the obtained
MSD for the anions of cubane-dc. According to the MSD
curves, the self-diffusion coefficients of water and cubane-dc
−
7
2 −1
were calculated to be (1.647 ± 0.05) × 10 cm s and (6.543 ±
0
.05) × 10− cm s respectively. Based on this result, the self-
8
2 −1
Fig. 9 Hydrogen bonds in the hybrid system.
diffusion coefficient of cubane-dc is significantly lower than
that of the water molecule. From the above-mentioned analysis
we can conclude that the cubane-dc anions are restricted to
the layers of LDH and the cubane-dc anions are more stable
when intercalated into these layers. The stability is highly
important since it can overcome the thermal instability when
2
7
cubane-1,4-dicarboxylic acid is used alone.
Comparison of experimental and
calculated XRD
The orientation of cubane-dc anions within the interlayer
space depends on the space available to the guest anions in
the interlayer region. On the basis of the d003 value of cubane-
dc-LDH (measured by X-ray diffraction), the space available to
the cubane-dc anions in the interlayer space is 8.6 Å, which
can be approximately calculated by subtracting the 4.8 Å thick-
ness of the layer from the 13.4 Å interlayer spacing. According
to the longitudinal van der Waals radius of the cubane-dc
anion (7.20 Å), there is enough vacant space available for guest
anions. On the other hand, the orientation of guest anions
depends on the interactions of guest anions with LDH layers
and interlayer water molecules, as well as on interactions
among guest anions.
Fig. 10 Comparison of the calculated MSDs for cubane-dc and water
molecules.
displacements (MSDs) of the cubane-dc and water molecules
from the last 200 ps MD trajectory (time step = 0.5 fs and total
time of simulation = 400 ps). The analysis of the MSD yields
important information about the stability of the cubane-dc-
LDH system. The MSDs of the two different molecules in the
2
00 ps of the trajectory are shown in Fig. 10, in which the
Experimental XRD results combined with molecular simu-
lations can determine the potential orientations of the
cubane-dc guest anions in the interlayer space. The water
content of the interlayer space and the position of the inter-
layer water as well as the orientation of guest anions (cubane-
dc) strongly influence the X-ray diffraction patterns. By using
molecular modeling and molecular dynamic calculations, the
slopes of the MSD-versus-time curves are consistent with the
system achieving equilibrium. The MSD was obtained from
6
eqn (1),
*
+
n
X
1
N
2
MSDðtÞ ¼
½riðtÞ ꢀ rið0Þꢁ
ð1Þ
i¼1
i
where N is the number of target molecules and r (t) is the water molecules and guest anions were arranged within the
location of molecule i at time t. The diffusion coefficient (D) interlayer so that the calculated XRD pattern agrees with the
can then be obtained by using the famous Einstein relation measured one. Fig. 11 shows the calculated and experimental
3
6
(eqn (2))
XRD patterns of cubane-dc-LDH.
The experimental XRD pattern shows that the peak maxima
ð2Þ of d003, d006 and d009 were observed at 13.40, 6.92 and 4.52 Å,
respectively, while the calculated XRD pattern show them to be
i i
where d is the system dimensionality, and r (t) and r (0) are at 13.39, 6.69 and 4.46 Å, respectively. Note the particularly
1
d
2
Dα ¼ 2
lim h½riðtÞ ꢀ rið0Þꢁ i
dNα t!1 dt
the coordinates of the center of mass of the ith cubane-dc and good agreement between the calculated and experimental d003
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SEM
A scanning electron microscopy (SEM) image of a cubane-dc-
LDH nanohybrid is depicted in Fig. 12. The irregular sheet-like
character of cubane-dc-LDH nanohybrid can be clearly seen in
this figure. In fact, cubane-dc-LDH is composed of aggregates
of irregular sheet-like nanoparticles with sheet thicknesses in
the range of 40–60 nm. The broadening of the XRD pattern of
cubane-dc-LDH (Fig. 11) is due to the nanosize of the
particles.
Fig. 11 The calculated (solid) and experimental (dotted) x-ray diffrac-
tion patterns.
spacings. The slightly lower value for the calculated d006
spacing compared to that of the experimental d006 may be
related to the effect of crystal size on peak broadening as well
Conclusions
as to the high disorder of guests in the structural model, The synthesis and characterization of Mg–Al hydrotalcite inter-
5
which is not taken into account in the software.
calated with cubane-dc anions are reported. The successful
As can be seen from Fig. 11, the intensities of the experi- intercalation of cubane-1,4-dicarboxylate anions into the inter-
mentally determined (003) and (006) peaks are inverted com- layer space of LDH was confirmed by powder X-ray diffraction,
pared to those in the calculated XRD pattern. This observation FTIR spectroscopy and thermal gravimetric analysis (TGA)
.
is due to the roughness of the surface of the experimental During the intercalation of cubane-dc anions, the layers of
5
sample, which is not considered in the software. This effect is LDH swell to host the anions, and this expansion is reflected
common when the electron density of the interlayer space is by the value of d003. A molecular dynamics simulation was
comparable to or higher than that of the brucite sheet.
used to investigate the arrangement of the guest anions within
the interlayer space of LDH. The results show that the long
axis of cubane-dc anions is tilted approximately 54° from the
perpendicular orientation to the LDH layer. Based on RDF
results, the water molecules are dispersed in the interlayers as
two main shells, which are positioned about 0.97 Å and 1.57 Å
from the cubane-dc anion end-group. The dynamical pro-
perties such as mean-square displacements (MSD) and thus
self-diffusion coefficients (D) were also analyzed for interca-
lated cubane-dc anions and water molecules. The results of
the MD showed that the cubane-dc anions were more stable
when intercalated into layers of LDH.
Comparison with terephthalate-
intercalated Mg–Al-LDH
G. Lagaly et al. reported the syntheses, characterizations and
basal spacings of several dicarboxylate-intercalated-Mg–Al
3
8
layered double hydroxides. Among the reported cases, the
size of the terephthalate dianions is very close to that of the
cubane-dc anions. The distances between carboxylate end-
groups in the terephthalate and cubane-dc are 7.1 Å and 7.2 Å,
respectively. The basal spacing (d003) of terephthalate-Mg–Al-
LDH hybrid is 14.3 Å. This value is significantly higher (by
Acknowledgements
0
.9 Å) than the 13.40 Å basal spacing of the cubane-dc-Mg–Al-
LDH nanohybrid. This difference may be related to the tilted
arrangement of cubane-dc dianions in the interlayer region.
We are grateful for the support from Azarbaijan Shahid
Madani University. We also thank the research laboratory of
Dr M. Mahkam (Azarbaijan Shahid Madani University) for
preparation of cubane-1,4-dicarboxylic acid.
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