Ultrasound-assisted synthesis and characterization of a new metal-organic framework based on azobenzene-4,4-dicarboxylic acid: Precursor for the fabrication of Co3O4 nano-particles

Article abstract of DOI:10.1016/j.ultsonch.2018.03.014

Azobenzene groups are widely known as photochromic ligands thus are particularly interesting building blocks for designing receptors for neutral or charged guests. A new metal-organic framework, [Co₃(adc)₃(DMF)₄].2DMF (compound 1) (adc = azobenzene-4,4-dicarboxylic acid, DMF = N,N-dimethylformamide), was synthesized by solvothermal methods and structurally characterized using X-ray crystallography and a range of spectroscopic techniques. Also, nanorods of compound 1 have been synthesized by a sonochemical process and characterized by field emission scanning electron microscopy (FE-SEM) and powder X-ray diffraction (PXRD). The effect of sonication time and concentration of the initial reagents on the size and morphology of the MOF have been optimized. Results indicate that decreasing of initial concentration and increasing ultrasound radiation time lead to small size nanorods of compound 1. Thus, ultrasound radiation affects the size of nanorods. After heat treatment, the cobalt ion-based metal organic framework nanorods can be converted into porous Co₃O₄ nanoparticles.

Full text of DOI:10.1016/j.ultsonch.2018.03.014

Accepted Manuscript
Ultrasound-assisted synthesis and characterization of new metal-organic frame-
work based on azobenzene-4,4-dicarboxylic acid: Precursor for the fabrication
of Co O nano-particles
3
4
Fatemeh Parsa, Massomeh Ghorbanloo, Mohammad Yaser Masoomi, Ali
Morsali, Peter C. Junk, Jun Wang
PII:
S1350-4177(18)30476-0
DOI:
https://doi.org/10.1016/j.ultsonch.2018.03.014
ULTSON 4127
Reference:
To appear in:
Ultrasonics Sonochemistry
Received Date:
Revised Date:
Accepted Date:
2 December 2017
4 March 2018
24 March 2018
Please cite this article as: F. Parsa, M. Ghorbanloo, M.Y. Masoomi, A. Morsali, P.C. Junk, J. Wang, Ultrasound-
assisted synthesis and characterization of new metal-organic framework based on azobenzene-4,4-dicarboxylic acid:
Precursor for the fabrication of Co O nano-particles, Ultrasonics Sonochemistry (2018), doi: https://doi.org/
3
4
1
0.1016/j.ultsonch.2018.03.014
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Ultrasound-assisted synthesis and characterization of new metal-
organic framework based on azobenzene-4,4-dicarboxylic acid:
Precursor for the fabrication of Co O nano-particles
3
4
a
a
b
b
Fatemeh Parsa, Massomeh Ghorbanloo, Mohammad Yaser Masoomi, Ali Morsali,* Peter C.
c
c
Junk , Jun Wang
a
Department of Chemistry, Faculty of Science, University of Zanjan, Zanjan, Iran
b
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-
1
75, Tehran, Iran. E-mail: Morsali_a@modares.ac.ir
c
College of Science & Engineering, James Cook University, Townsville Queensland, 4811,
Australia
ABSTRACT
Azobenzene groups are widely known as photochromic ligands thus particularly interesting
building block for designing receptors for neutral or charged guests. A new metal-organic
framework, [Co (adc) (DMF) ].2DMF (compound 1) (adc = azobenzene-4,4-dicarboxylic acid,
3
3
4
DMF = N,N-dimethylformamide), was synthesized by solvothermal methods and structurally
characterized using X-ray crystallography and a range of spectroscopic techniques. Also,
nanorods of compound 1 have been synthesized by a sonochemical process and characterized by
field emission scanning electron microscopy (FE-SEM) and powder X-ray diffraction (PXRD).
The effect of sonication time and concentration of the initial reagents on the size and
morphology of the MOF have been optimized. Results indicate that decreasing of initial
concentration and increasing ultrasound radiation time lead to small size nanorods of compound
1
. Thus, ultrasound radiation affects the size of nanorods. After heat treatment, the cobalt ion-
based metal organic framework nanorods can be converted into porous Co3O4 nanoparticle.

Keywords: Metal-organic framework; Sonochemical; Ultrasound; Nano-structure; Single crystal
X-ray diffraction.
1
. INTRODUCTION
Metal–organic frameworks (MOFs) are known as porous materials which have attracted great
attention due to their potential applications in gas storage [1, 2], separation [3, 4], ion exchange
[
5], sensing [6, 7], catalysis [8, 9], and drug delivery [10]. An interesting feature of MOFs over
other porous materials is the ability to tune their channel size, topology and functionality by
deliberate design [11]. Flexible design is one of the essential properties of these materials that
can be employed by choosing suitable organic ligands and inorganic secondary building units
(SBUs) [12, 13].
Material growth control at the scale of sub-micrometer is of great importance in the field of
nanotechnology. Additionally, reducing the size of MOFs to the micro- and nanoscale has been
quite attractive [14-17]. Various parameters such as molar ratio of reactants, solvent, reaction
time, pH, pressure and etc affect the size and morphology of pre-designed supramolecular
assemblies. Nowadays a variety of synthetic strategies have been employed by various research
groups in supramolecular synthesis including microwave-assisted heating, electrochemical
synthesis, mechanochemistry and ultrasound irradiation that result in different yields, reaction
time, particle size, morphology and final products [18, 19]. Among these synthetic strategies,
ultrasound synthesis is one of the nontraditional techniques, facile, fast, and environmentally friendly
and energy efficient method with advantages in operation under ambient conditions and lowered
reaction times [20].
In recent years there have been many efforts to design functionalized MOFs with photochromic
sites. The introduction of H2azdc in the formation of CPs and MOFs has resulted in limited

success. Azobenzene and its derivatives are widely known as photochromic ligands which could
sustain clean and efficient invertible photoisomerization for the azo bond between cis and trans
states under irradiation or heating, as well as other conditions [21].
MOFs are useful precursors for the preparation of inorganic nano-materials such as metal oxides
via calcination at high temperature in the air [22, 23]. The chemical and physical properties of
solid materials mostly depend on the size and shape of the materials which the compounds are
made especially in the case of nano-materials [24-26]. In recent years, Co O as a magnetic p-
3
4
type semi-conductor has attracted great attention due to interesting properties such as excellent
cycle reversibility and high specific capacity [27,28].
Herein, we report the simple preparation of micro and nano structures of an azine-functionalized
MOF by two solvothermal and sonochemical methods. The effects of sonication time and initial
concentration of reagents on shape and size of the nanostructures have been investigated.
Moreover, similar to other solid-phase fabrication approaches, metal–organic frameworks can be
easily transformed into metal oxides via simple calcination at high temperature in the air. To
proceed, we report Co O nano-structures synthesized by direct calcination of the single crystals
3
4
°
and nano-sized compound 1 at 550 C.
2
. EXPERIMENTAL
Materials and Physical Techniques
All reagents for the syntheses and analysis were commercially available and used as received.
The infrared spectra were recorded on a Nicolet Fourier Transform IR, Nicolet 100 spectrometer
-1
in the range 500-4000 cm using the KBr disk technique. A single crystal of compound 1 was
selected and mounted on a loop in inert oil and transferred to the cold gas stream of the
diffractometer on the MX1 Beamline at the Australian Synchrotron. The data were corrected for

absorption and beam corrections based on the multiscan technique as implemented in SADABS.
The structures were solved by conventional methods and refined by full-matrix least-squares on
2
all F data using SHELX-2014 in conjunction with the Olex2 graphical user interface. Melting
points were measured on an Electrothermal 9100 apparatus. X-ray powder diffraction (XRD)
measurements were performed using a Philips X’pert diffractometer with mono chromated Cu-
Kα radiation. The molecular structure scheme and simulated XRD powder pattern based on
single crystal data were prepared using Mercury software. The sonicator used in this study was a
Misonix Sonicator 3000 with an adjustable power output (maximum 600 W at 50/60 kHz). A
horn type tube Pyrex reactor was custom-made and fitted to the sonicator bar. The samples were
also characterized by field emission scanning electron microscope (FE-SEM) TESCAN MIRA
with a gold coating.
Synthesis of azobenzene-4,4-dicarboxylic acid (adcH ) ligand
2
4
-nitrobenzoic acid (2.0 g, 12 mmol) and NaOH (5.75 g, 144 mmol) were dissolved in 30 mL of
0
water. As 20 mL of glucose (13.0 g, 72 mmol) aqueous solution was added at 70 C, a yellow
precipitate formed which was vigorously stirred and upon further addition of glucose a brown
solution resulted. The mixture was heated to reflux under a stream of air overnight. The light
brown precipitate was dissolved in 10 mL of water, and acetic acid was used to adjust the pH of
the solution in the range of 5 to 6. The light pink precipitate was filtered and washed several
times with distilled water and dried in vacuum to give azobenzene-4,4-dicarboxylic acid
(
Scheme 1). Yield: 60%. 1H NMR (500 MHz, DMSO-d ) d (ppm): 8.3 (d, 4H, J = 8.4 Hz), 8.15
6
(d, 4H, J = 8.4 Hz).

COOH
NO2
NaOH
N
N
C H O , air
6
12
6
COOH
COOH
Scheme 1. Schematic representation of the production of adcH ligand.
2
Synthesis of single crystals of [Co (adc) (DMF) ].2DMF
3
3
4
To isolate single crystals of [Co (adc) (DMF) ].2DMF, azobenzene-4,4-dicarboxylic acid (0.27
3
3
4
g, 1mmol), Co(NO ) .6H O (0.3g, 1 mmol) and DMF (15 ml) were placed in a Teflon-lined
3
2
2
stainless steel autoclave under autogenous pressure and heated at 100 °C for 3 days. The mixture
was then gradually cooled to room temperature over 48 hours. Red crystals formed on the walls
-1
of the container with a 61% yield m.p. > 300 °C. FT-IR data (KBr pellet, cm ): selected bands
are: 682(m), 786(m), 1112(m), 1388(m), 1615(s), 1658(vs), 2923(br) and 3409(br).
Synthesis of [Co (adc) (DMF) ].2DMF nano-structure by a sonochemical process
3
3
4
To prepare nano-sized compound 1, 15 ml solution of Cobalt(II) acetate tetrahydrate and
azobenzene-4,4-dicarboxylic acid (1:1) in DMF (Table 1) were positioned in a high-density
ultrasonic probe, operating with a maximum power output of 12 W. The obtained precipitate was
filtered off, washed with DMF and then dried in air, m.p. > 300 °C. IR bands are 682(m),
7
86(m), 1112(m), 1388(m), 1615(s), 1658(vs), 2923(br) and 3409(br).

The morphology and size of nanostructures mostly depend on various parameters such as the
concentration of starting reagents, ultrasonic irradiation time and power. For investigation, the
effect of starting material concentration on size and morphology of compound 1, the synthesis
processes were carried out with different concentrations (0.1M, 0.05M, 0.02M, and 0.0125M).
Moreover, for studying the role of ultrasonic irradiation time on size and morphology of
compound 1 the process with the initial reagents concentration of 0.1 M has been done in two
different ultrasonic times (table 1).
Synthesis of Co O nanostructures
3
4
0
To prepare Co O nano-particles, samples of compound 1 were calcined at 550 C for 5 h. After
3
4
cooling, a brown precipitate was obtained. SEM images of these materials show the formation of
Co O nanoparticle in the range of 70–120 nm.
3
4
Table 1: Experimental details for synthesis of nano compound 1.
Sample name Concentration (M)
adc]/[Co(OAc)2]
Time
power
Morphology
[
Sample A
Sample B
Sample C
Sample D
Sample E
[0.1]/[0.1]
60min
3:30h
3:30h
3:30h
3:30h
12W
12W
12W
12W
12W
non-uniform rod
non-uniform rod
uniform rod
[0.1]/[0.1]
[0.05]/[0.05]
[0.02]/[0.02]
[0.0125]/[0.0125]
uniform rod
uniform rod
3
. RESULTS AND DISCUSSION

Single crystals of compound 1 were obtained by mixing azobenzene-4,4-dicarboxylic acid and
Co(NO ) .6H O, in DMF at 100 °C. The FT-IR spectrum of compound 1 shows the symmetric
3
2
2
-1
ν (COO) and asymmetric ν (COO) vibrations of the carboxylate groups at 1409 cm and 1604
sym
as
-1
cm , respectively for both crystals and ultrasonic synthesized compound 1. Also the
-1
characteristic absorption peak (νC=O = 1659 cm ) of DMF is observed in the IR spectrum (Fig.
).
1
Ultrasound
compound 1-crystal
3900
3400
2900
2400
1900
1400
900
400
Wevelength(nm)
Fig. 1. FT-IR spectra of simulated and ultrasound compound 1
X-ray crystallography reveals that [Co (adc) (DMF) ].2DMF (compound 1) crystallizes in the
3
3
4
monoclinic space group P2 /n (Table 2). There are two different Co(II) centers in the asymmetric
1
unit of 1 in which Co1 is octahedrally coordinated by six oxygen atoms (CoO ) of six different
6
adc ligands, while Co2 has distorted octahedral geometry interacting with four oxygen atoms of
three adc ligands and two oxygen atoms from coordinated DMF molecules. Two of the adc

2
ligands adopt µ ligation and the other coordinates as a chelating κ O ligand and extends the
2
structure in three dimensions (Fig. 2). The supramolecular features in this complex are guided
and controlled by weak directional intermolecular interactions. The chains interact with each
other through p–p stacking interactions creating a 3D supramolecular framework. Compound 1
shows limited porosity with the calculated void space per unit cell for guest-free framework of
3
3
1.6% (1205.8 Å ). The topological analysis of Compound 1 exhibits a uninodal 6-connected
6
6
3
hxl/Shubnikov plane net (3,6) with {3 .4 .5 } topology. Also compound 1 is nonporous toward
N2 at 77 K and 1 bar.

Fig. 2 (a) SBU of compound 1 showing coordination environment about Co1 and Co2. (b)
Representation of the three-dimensional structure of compound 1. The hydrogen atoms were
omitted for clarity.
Thermogravimetric analysis of compound 1 synthesized by the ultrasound technique shows that
the trapped guest DMF molecules are released from the network upon heating to about 200˚C,
which causes 16% weight loss of the material corresponding to two DMF molecules (calculated
1
6.5%). The second gradual weight loss appears between 200-380 ºC (~5%), the third sharp

weight loss appears between 380-500 ºC (~40%) and corresponds to the decomposition of the
framework (Fig. 3).
Fig. 3. (a) Thermogravimetric analysis and (b) Derivative thermogravimetry of compound 1
For studying the effect of sonication time as a parameter on size and morphology of nano-
structured compound 1, the synthesis process was performed at a constant concentration of initial

reagents of 0.1 M but at two different sonication times. At 60 min and 3:30 h, aggregated
nanoparticles were obtained, and at the longer reaction time, the particles were smaller than the
shorter reaction time (Fig. 4). For monitoring the role of the concentration of initial reagents on
size and morphology of compound 1 the process at the optimized time of 3:30 h has been done in
three different concentration of starting material (0.05M, 0.02M, 0.0125M). The results indicate
that the higher concentration of reagents causes increasing particle size. Lower concentrations of
initial reagents reduce the size of particles and lead to a uniform distribution of particle size (Fig.
5
).
Fig. 4: SEM images of compound 1 synthesized by the sonochemical method at two different
reaction times, (a) sample A and (b) sample B. In both cases the initial concentration of reagents
were [0.1] M.

Fig. 5. SEM images of compound 1 synthesized by the sonochemical method at different
concentration of starting material (a) sample C, (b) sample D and (c) sample E. In all three cases
the reaction time was 3:30 h.
A comparison between powder X-ray diffraction (PXRD) patterns of the simulated (derived from
the single crystal structure of 1 and experimental (resulting from the sonochemical process)
shows a good match between these two PXRD patterns indicating that the sonochemically
synthesized 1 is structurally identical to that prepared through solvothermal reaction
Fig. 6: PXRD patterns of simulated and ultrasound of compound 1

Furthermore, the sonochemically synthesized nanorods of compound 1 were used as a precursor
for the preparation of MOF-derived Co O nano-structures. Calcination of the nano-sized
3
4
compound 1 has been done at 550 ºC in a static atmosphere of air for 5 h. Fig. 7 shows the XRD
pattern of the Co O nanoparticles. These nanoparticles have shown the good crystallinity
3
4
because of the existence of sharp peaks in the XRD pattern. The Bragg diffraction peaks in the
range of 2 = 20–80 indicate the typical cubic spinel structure with the lattice parameters of a =
8
.0837 Å and Z = 8 and S.G = Fd3m which are in JCPDS card file No. 42-1467. An SEM image
of the residue obtained from the direct calcination of compound 1 shows the formation of Co O
3
4
nano particles in the range of 70–120 nm (Fig. 9).
Fig. 7. PXRD patterns of Co O form of compound 1
3
4

Fig.8. SEM images of compound 1 synthesized by the solvothermal
Fig.9. SEM images of Co O nanoparticles
3
4

4
. Conclusion
A new Co(II) based metal–organic framework, [Co (adc) (DMF) ].2DMF has been readily and
3
3
4
quickly synthesized by the solvothermal and sonochemical process. Compound 1 was
characterized using different spectroscopic techniques and X-ray crystallography. Ultrasonic
method for preparation of the MOFs has some advantages such as: needing shorter reaction
times, production of MOFs with better yields and also at nano-sizes. Morphology and sizes of
the nano-structures depend on the concentrations of initial reagents and the reaction time. The
results show that a lower concentration of reagents leads to decrease in particle size. Also,
increasing of reaction time leads to nano-structures with rod-like morphology and decreases the
particle size of the nano-structures. We believe that it would be of interest to pursue such studies
to clarify the design of MOFs with favorite morphology, size distribution and shape. Nano-
0
structures of Co O were produced by calcination of compound 1 at 550 C for 5 h.
3
4
Acknowledgment
Support of this investigation by Tarbiat Modares University is gratefully acknowledged.
Appendix A. Supplementary data
The supplementary crystallographic data was deposited with the Cambridge Crystallographic
Data Centre (CCDC) as entry CCDC 1588088.

Table 2. Crystal data and structure refinement of [Co (adc) (DMF) ]·2DMF (1).
3
3
4
CCDC Identification code
Empirical formula
1588088
C H Co N O
14 20
66
74
3
Formula weight
Temperature
1560.18
100.15 K
Wavelength
0.71073 Å
monoclinic
Crystal system
Space group
P2 /n
1
Unit cell dimensions
a = 14.765(3) Å
b = 17.325(4) Å
c = 16.395(3) Å
α = 90.00°
β = 114.59(3)°
γ = 90.00°
3
3813.6(16)
Cell volume, Å
Z value
2
-
3
Density (calc.) (g.cm )
1.359
-1
Absorption coefficient
0
.722 mm
F(000)
1618
.11  0.05  0.02 mm
Crystal size
3
0
θ range for data collection
1.95 to 24.99°
Index ranges
-17h17
-
20k20
19l19
-
Reflections collected/unique
Data/restraints/parameters
34707/6683 [R(int) = 0.0457]
6683/106/419
2
Goodness-of-fit on F
1.075
Final R indices (I>2.00σ(I))
R indices (all data)
R1 = 0.0955, wR2 = 0.2462
R1 = 0.0997, wR2 = 0.2501
-3
Largest diff. Peak, hole
2
.180 and -1.071 e.Å

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Highlights


Synthesis of a new Co(II) based metal-organic framework.
Effects of sonication time and initial concentration of reagents on shape and size of
nanostructures have been investigated.



High concentrations of initial reagents increased particles size of compound 1.
Low concentrations of initial reagents leaded to nanostructures.
Preparation of Co3O4 nanoparticles by thermolysis of metal-organic framework.