Immobilization and DFT studies of Tin chloride on UiO-66 metal–organic frameworks as active catalyst for enamination of acetylacetone

Article abstract of DOI:10.1007/s13738-019-01693-4

An efficient synthetic method for the preparation of β-enaminones catalyzed by immobilization of SnCl₂ on Zr-metal–organic frameworks (UiO-66 MOF) and designated as SnUiO66 is described. Also the prepared catalyst was characterized by Fourier t

Full text of DOI:10.1007/s13738-019-01693-4

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-019-01693-4
ORIGINAL PAPER
Immobilization and DFT studies of Tin chloride on UiO‑66
metal–organic frameworks as active catalyst for enamination
of acetylacetone
Faezeh Farzaneh1 · Nasim Kabir1 · Elham Geravand1 · Mina Ghiasi1 · Mehdi Ghandi2
Received: 10 November 2018 / Accepted: 11 May 2019
© Iranian Chemical Society 2019
Abstract
An efcient synthetic method for the preparation of β-enaminones catalyzed by immobilization of SnCl2 on Zr-metal–organic
frameworks (UiO-66 MOF) and designated as SnUiO66 is described. Also the prepared catalyst was characterized by Fou-
rier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy and
N2 adsorption and desorption techniques. XPS and DFT studies on Sn UiO66 were consistent with the immobilization of
Sn chloride as SnIV on MOF. A variety of β-enaminones were synthesized via reaction of primary aromatic amines with
acetylacetone. It was found that SnUiO66 shows moderate-to-high catalytic activity for enamination reactions. The catalyst
was found to be reusable up to four catalytic cycles without any appreciable loss in activity.
Keywords Modifcation · Metal–organic frameworks · Enamination · SnUiO-66
Introduction
[5], BF3∙OEt2 [6], Zn(ClO4)∙6H2O [7], ceric ammonium
nitrate [8] and Bi(OTF)3 [9]. Utilization of homogeneous
catalysts is associated with certain drawbacks like use of
moisture-sensitive metal trifates and tedious workup, use of
harmful reagents and nonrecyclable catalysts. On the other
hand, heterogeneous catalysts such as clays [10], supported
tungstophosphoric acid [11], silica-supported perchloric
acid [12] and silica chloride [13] have been used for enami-
nation reactions. Due to the signifcantly more expensive
non-recoverable catalysts, use of the more attracting, easily
separable and recyclable heterogeneous systems has been
developed [14–18].
An increasing interest has been directed toward the enamina-
tion of 1,3-dicarbonyl compounds to β-enamino ketones and
esters due to their importance in organic synthesis [1]. These
compounds are signifcant intermediates in the synthesis of
heterocyclic compounds and also in medicinal applications,
and they are also known as anticonvulsant, anti-infamma-
tory, antitumor and analgesic agents [2]. There are a few
reported methods for the synthesis of β-enaminones through
reaction of dicarbonyl compounds with amines using cata-
lysts such as metal oxides under microwave or ultrasound
irradiation [3, 4]. Several homogeneous and heterogeneous
catalysts have also been used for enamination reactions.
Examples of homogeneous catalysts include Ca(CF3COO)2
A new class of microporous materials known as
metal–organic frameworks (MOFs) has emerged in recent
years which are comprised of metal “nodes” linked with
“rod”-shaped organic linkers. These materials have attracted
considerable attention due to their profound range of appli-
cations including separation, gas storage and catalysis [19].
The use of MOFs as heterogeneous catalysts is particularly
interesting since the pore size and functionality of the frame-
work can be adjusted over a wide range for a variety of cata-
lytic reactions [20].
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-019-01693-4) contains
supplementary material, which is available to authorized users.
* Faezeh Farzaneh
faezeh_farzaneh@yahoo.com; farzaneh@alzahra.ac.ir
1
2
Modifcation of some MOFs to increase their acidity
and use as catalyst in the synthesis of β-enaminones via
reaction of primary aromatic amines with acetylacetone
is described in this presentation. The DFT calculation on
Department of Chemistry, Faculty of Physics and Chemistry,
Alzahra University, Vanak, Tehran, Iran
School of Chemistry, College of Science, University
of Tehran, P.O. Box 14155 6455, Tehran, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
all studied complexes has been carried out for comparison
of the experimental and theoretical results. The DFT as a
suitable and popular method can be used for calculation of
compounds including metals. The most important advantage
of using the DFT methods is obtaining a signifcant increase
in computational accuracy without the additional increase
in computing time.
subsequently fltered, washed with MeOH for several times
and dried at 110 °C within 4 h.
Typical method for the synthesis of β‑enaminones
and β‑enamino esters
To a 25-ml round-bottom fask containing MeOH (5 mL)
were added dicarbonyl compound (1 mmol), amine
(1 mmol, 1.1 mmol) and catalyst (20 mg) and the mixture
heated at refux for 2 h. After separation of the catalyst by
fltration, the fltrate was subjected to GC and GC–Mass
analyses. The GC–Mass of products was reported in
supplementary.
Experimental section
Materials and characterization
All materials were of commercial reagent grade and used
without further purifcation. FT-IR spectra were recorded on
a Bruker Tensor 27 FT-IR spectrometer using KBr pellets
over the range of 4000–400 cm−1. The catalysts were ana-
lyzed by powder X-ray difractometer (XRD) PHILIPS PW
1800, using Cu Kα radiation (λ=0.15406 nm) and in the 2θ
range of 0°–60°. The percentages of immobilized metal ions
were determined by atomic absorption double-beam instru-
ment. Energy-dispersive X-ray spectroscopy (EDX) VEGA3
LMU analysis was performed by TESCAN Company. The
X-ray photoelectron spectroscopy (XPS) measurements were
performed using a Thermo Scientifc ESCALAB 250Xi ana-
lyzer equipped with Mg X-ray source. Products were ana-
lyzed by GC and GC–Mass using Agilent 6890 series with a
FID detector, HP-5,5% phenylmethylsiloxane capillary and
Agilent 5973 network, mass selective detector, HP-5 MS
6989 network GC.
Computational details
The structures and energy of all studied compounds includ-
ing [Zr6O4(OH)4] clusters and the complexes between Tin
and [Zr6O4(OH)4] from diferent positions with diferent
oxidation numbers were calculated by using the density
functional theory (DFT) [22] without any symmetry con-
straints. The hybrid of Beck’s nonlocal three-parameter
exchange and corrected functional with Lee–Yang–Parr
correlation functional (B3LYP) [23] for the closed shell
system has been used. The 6-311++G** basis set was used
for H and O atoms. Moreover, the efective core potentials
(ECPs) of Hay and Wadt with double-ζ valance basis set
(Lan12DZ) [24–26] were chosen to describe the Zr and Tin
atoms. The core behavior parameter controls the electrons
treatment modes in the lowest lying atomic orbitals. In the
case of heavier elements especially in MOF, it is important
to take into account the relativistic efects of the central elec-
trons. One prevalent method to incorporate these efects is
replacing the complex efects of the core electrons with an
efective potential or pseudo-potential [27, 28]. This method
which is named efective core potential (ECP) approximates
the Schrodinger equation in heavy systems [29].
Synthesis of UiO‑66
The UiO-66 was prepared via modifcation of the previ-
ously reported procedure [21]. To a suspension of ZrCl4
(0.63 g, 2.7 mmol) in DMF (25 mL) was added concen-
trated HCl (5 mL) and the mixture sonicated with ultrasound
for several minutes until it becomes clear. Terephthalic acid
(0.62 g; 3.75 mmol) dissolved in DMF (50 mL) was then
added into the ZrCl4 and the mixture stirred at 80 °C for
24 h. After cooling to RT, the white solid was separated
and resuspended in DMF. After stirring for 24 h in RT, the
precipitate was separated by fltration and washed with DMF
(4×10 mL). It was then resuspended in MeOH and stirred
for 24 h at RT followed by fltration and washing the solid
with MeOH. This procedure was repeated twice in order to
remove all impurities.
The harmonic vibrational frequencies have been com-
puted to confrm that an optimized geometry correctly cor-
responds to a local minimum that has only real frequen-
cies. In addition, the results of frequency calculations have
been employed to calculate thermodynamic functions such
as enthalpies and Gibbs free energies at 298.15 K and 1.0
atmosphere pressure. All reported enthalpies were zero-point
(ZPE) corrected with unscaled frequencies. The solvent
efects on the conformational equilibrium and contribution
to the total enthalpies were investigated by using polarized
continuum (overlapping spheres) model (PCM) of Tomasi
and coworkers [30] at the same level for the methanol sol-
vent. All calculations were performed using the Gaussian
2003 [31] software.
Preparation of Sn UiO‑66
To a dispersed UiO-66 (0.1 g) in MeOH (1 mL) was added
SnCl2·H2O (0.01 g; 10 wt%) dissolved in MeOH (1 mL)
and the mixture stirred for 4 h at RT. The white solid was
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Journal of the Iranian Chemical Society
Results and discussion
In this study, the UiO-66 was prepared with ZrCl4 and tere-
phthalic acid based on the report of [21], followed by immo-
bilization SnCl2 on it.
Characterization of prepared catalyst
The FTIR spectra of ZrCl4, terephthalic acid and UiO-66 are
shown in Fig. S1. The spectrum of acid shows two peaks at
3742 and 3389 cm−1 related to the O–H stretching vibra-
tions of COOH. Whereas two peaks appeared at 1424 cm−1
and 1568 cm−1 are due to the symmetric and asymmetric
vibrations of carboxylate anions, the band observed around
1682 cm−1 can be signifcantly assigned to acidic C=O
stretching vibration (Fig. S1b). After formation of MOF
(UiO-66), the C=O vibration is shifted to 1581 cm−1 (Fig.
S1c).. Zr–O vibrations are observed at 482, 545, 661 and
745 cm−1. The strong and weak peaks located at 1399 and
1508 cm−1 are attributed to C–O and aromatic organic linker
C=C stretchings, respectively [32–34].
Fig. 2 XRD patterns for UiO-66, SnUiO-66 before and (c) after being
used as catalyst
The energy-dispersive X-ray (EDX) for SnUiO-66 is
shown in Fig. 3. The presence of C, O, Cl, Zr and Sn is
clearly observed in EDX map. Signifcantly, the immobilized
amount for Sn before and after reaction was determined by
ICP techniques and found to be 0.2697 and 0.2511 weight
percentage, respectively.
The FTIR spectra of UiO-66, Sn UiO-66 before and after
being used as a catalyst are shown in Fig. 1a–c, respectively.
No signifcant change in catalyst is observed on the basis of
these spectra.
The XRD patterns of UiO-66, SnUiO-66 before and
after being used as a catalyst exhibit a typical difraction at
2θ = 7.45°, 8.58°, similar to those reported previously for
UiO-66 [28] (Fig. 2a–c). Observation of no change in the
XRD patterns after immobilization of SnCl2 signifcantly
confrms the stability of the prepared MOF crystal structure.
In order to obtain support for immobilization of metal
ions on UiO-66, the SnUiO-66 was used as a model for this
study. The N2 adsorption/desorption isotherms of UiO-66
and Sn UiO-66 are exhibited in Fig. 4a, b. It seems that
the isotherms of two samples are type I according to the
IUPAC classifcation. This type of isotherm is observed for
solids with micropores such as activated carbons or zeolites
[35]. As seen in Fig. 4, the amount of nitrogen adsorption
Fig. 1 FTIR spectra of (a) UiO-66, (b) SnUiO-66 before and (c) after
being used as catalyst
Fig. 3 EDX image of SnUiO-66
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Table 1 Texture parameters of UiO-66 and Sn2+/UiO-66
MOF
Vm
as, BET
Total pore Mean pore
[m3(STP) g−1
]
(m2 g−1
)
volume
diameter
(nm)
(cm3 g−1
)
UiO-66
316.39
1377.1
1205
0.851
0.7262
2.47
2.41
Sn2+/UiO- 276.92
66
to 1205 [m2 g−1], 0.7262 [cm3 g−1], 2.41 nm and 276.92
[m3(STP) g−1] after immobilization of Sn within UiO-66.
These results indicate that whereas the most Sn atoms of
complex are located on the surface of MOF, some are located
at internal surface based on the obtained results. Although
such a decrease in the textural parameters is observed, the
structure of UiO-66 remained unchanged after immobiliza-
tion of SnCl2, consistent with XRD and FTIR characteriza-
tion results.
XPS spectra were used for analyses of the SnUiO-66
chemical status and surface chemical composition. Figure 5a
shows the survey scan of sample, which indicates that Sn,
Cl, Zr, O and C elements exist in the SnUiO-66. Figure 5b
indicates the binding energies of Zr 3d5/2 and Zr 3d3/2, which
are observed at approximately 182.96 eV and 185.33 eV.
This binding energy confrmed the existence of ZrIV for the
zirconium-oxo cluster [36, 37].
The Gaussian fit of the C1s spectrum of SnUiO-66
(Fig. 5c) exhibited two peaks as follows: non-oxygenated
C–C (284.94 eV) and O–C=O (288.90 eV) components that
both of them were attributed species [38, 39]. Figure 5d pre-
sents Sn3d5/2 and Sn3d3/2 binding energies of 487.28 and
495.69 eV, respectively. The peak positions agree well with
the presence of SnIV species as SnUiO-66 [40, 41]. The
presence of chloride and oxygen is confrmed by the energy
binding peak of Cl2p and O1s at 198.86 eV and 531.89 eV
in Fig. 5e, d [36, 42].
Optimization of enamination reaction conditions
Synthesis of β-enaminone via reaction of aniline with acety-
lacetone was selected as model to study the catalytic activity
of SnUiO-66 at RT. The efects of various reaction parame-
ters including time, catalyst amount and solvent were studied
(Figs. 6, 7 and Table 2). High reaction yield was obtained
using 20 mg of catalyst in refuxing MeOH within 2 h. As
seen in Fig. 7, increasing the amount of catalyst from 5 to
20 mg enhances the conversion from 64 to 93% with 100%
selectivity (Fig. 7).
Fig. 4 N2 adsorption/desorption of UiO-66 and SnUiO-66
for the immobilized SnUiO-66 is less than that of UiO-66.
The specifc surface area and pore size distribution were
determined using Brunauer–Emmett–Teller (BET) and Bar-
rett–Joyner–Halenda (BJH) methods. The measured BET
surface area, pore volume, mean pore diameter and the vol-
ume of a monolayer were determined as 1377.1 [m2 g−1],
0.851 [cm3 g−1], 2.47 nm and 316.39 [m3 (STP) g−1], respec-
tively (Table 1). The textural parameters calculated by N2
adsorption–desorption isotherms were found to be reduced
Table 2 presents the infuence of solvent on the conden-
sation reaction condition for the synthesis of β-enaminone
using SnUiO-66 as catalyst. Based on the obtained results,
better yield was obtained in ethanol and methanol if an
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Journal of the Iranian Chemical Society
Fig. 5 XPS spectra of SnUiO-66 a survey scan, b Zr3d, c C1s, d Sn3d, e O1s and f Cl2p
excess amount of acetylacetone with respect to aniline
(1.2/1) was used (entries 4–5, Table 2).
seen in Table 3, utilization of aniline (entry 1, Table 3) and
substituted aniline either with strong electron-donating
(entries 2–3, Table 3) or modest electron-withdrawing sub-
stituents (entries 4–6, Table 3) afords the corresponding
Enamination of acetylacetone with various amines was
then investigated under optimized reaction conditions. As
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Table 2 Efect of solvent on enaminone 2a obtained via reaction of
acetylacetone with amine 1a catalyzed by SnUiO-66
Fig. 6 Conversion and selectivity of β-enaminones at diferent times
in the presence of the SnUiO-66 as catalyst. Reaction conditions:
acetylacetone (1.1 mmol), aniline (1 mmol), catalyst amount (20 mg),
methanol (5 mL) and temperature (60 °C)
Acetylacetone (1.1 mmol), aniline (1 mmol), catalyst amount
(20 mg), solvent (5 mL), temperature (60 °C) and time (2 h)
Based on the obtained results, SnUiO-66 has been used in
further theoretical studies to fnd the position of immobilized
tin chloride on UiO-66.
UiO-66 is known to have missing some linkers due to
coordinating water and solvent molecules as reported before
[43, 44]. Huong Giang indicated that vanadium (V) can be
directly incorporated onto the [Zr6O4(OH)4] clusters of UiO-
66 by reacting with exposed OH groups, especially the OH
groups arisen from missing linkers. According to this article,
fve complexes are formed from three proposed structures
A, B and C (Fig. 9) [43]. Based on the above-mentioned
article, in the present study, ten diferent complexes that
arise from the connection between two ions (SnII and SnIV)
onto [Zr6O4(OH)4] clusters of UiO-66 from diferent posi-
tions have been investigated using density functional theory
(Fig. 10). Our goal is to determine the geometrical details
and energetic state of the most stable complex based on ther-
modynamic point of view.
Fig. 7 Conversion and selectivity of β-enaminones at diferent
amounts of SnUiO-66 as catalyst. Reaction conditions: acetylacetone
(1.1 mmol), aniline (1 mmol), methanol (5 mL) and temperature
(60 °C), time 2 h
β-enaminones in excellent yields. On the other hand, alde-
hydes containing strong electron-withdrawing substituents
are totally inefcient toward the formation of β-enaminone
(entries 7–8, Table 3).
Particularly signifcant are the stability and reusability of
the prepared catalyst since the obtained results indicate that
not only the activity in methanol solvent is remarkable, but
also it is recycled up to four consecutive cycles without any
notable loss in catalytic activity (Fig. 8).
Geometry optimization of [Zr6O4(OH)4] cluster
The [Zr6O4(OH)4] cluster from UiO-66 (Zr) was assembled
by using the crystallographic structure available in litera-
ture [44] and has been selected as a model system. Com-
parison of the average bond distances obtained from X-ray
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Table 3 Enaminones 2a-
h obtained via reaction of
acetylacetone with amines 1a-h
catalyzed by Sn UiO-66.a
Entry
Amine
Product
Conversion (%)
NH2
1
93
O
HN
2a
1a
CH3
H3C
NH2
NH2
89
O
HN
2
3
1b
1c
2b
2c
OCH3
H3CO
O
HN
97
96
Br
Br
NH2
O
HN
4
5
1d
1e
1f
2d
Cl
Cl
NH2
95
O
HN
2e
F
F
NH2
6
91
O
HN
2f
NH2
O2N
7
-
-
1g
2g
O2N
NH2
NO2
O
HN
2h
8
1h
Acetylacetone (1.1 mmol), aniline (1 mmol), catalyst amount (20 mg), methanol
(5 mL), temperature (60 °C) and time (2 h)
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Journal of the Iranian Chemical Society
theoretical prediction which is about 0.0099 standard devia-
tion of bond distances at the B3LYP/6311+G*U Lan12DZ,
Table 4.
Interaction between Tin and [Zr6O4(OH)4] clusters
According to the experimental data, diferent complexes
from diferent positions of [Zr6O4(OH)4] cluster and tin
(state of SnII and SnIV) have been constructed and were
named as Com I, Com II, Com III, Com IV, Com V (Fig. 10a
and b). To analyze the interaction between [Zr6O4(OH)4]
cluster and tin (state of SnII and SnIV) in the optimized
geometries, the complexation energy (∆Ecom) of ten pos-
sible complexes that is presented in Table 5 is calculated
by using Eq. 1:
(
)
Fig. 8 Efect of recycling of Sn UiO-66 as catalyst in enamination of
ΔEcom = Eopt[Zr6O4(OH4)]∕Sn − Eopt[Zr6O4(OH4)] + Eopt sn
acetylacetone with aniline
(1)
The complexation energy has been calculated as the dif-
ference in energies of isolated [Zr6O4(OH)4] cluster and Tin
(state of SnII and SnIV) at their optimized geometry and the
crystallography and optimized structure is shown in Table 4.
The structure of [Zr6O4(OH)4] cluster was optimized at
B3LYP/6311+G*U Lan12DZ level with no initial symme-
try restrictions. The average bond distance between Zr and O
atoms at the optimized structure and X-ray crystallography
data was found to be 2.28Å and 2.26 Å, respectively. The
result indicates a good accuracy between experimental and
optimized complex, [Zr6O4(OH)4]/Sn (Eq. 1).
◦
ΔEcom and all thermodynamic data of the complexation
◦
◦
including enthalpies (ΔHcom), Gibbs free energies (ΔGcom
)
◦
and entropies (ΔScom) are calculated in the solvents for ten
formed complexes with SnII and SnIV (Table 5). As seen
Fig. 9 Left: structure of UiO-66 and its (A) proposed idealized and (B, C) defective linker presenting nodes. Right: proposed possible products
(I, II, III, IV, V)
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Fig. 10 a, b Comparison of the stability order of complexes [Zr6O4(OH)4]-SnIV and [Zr6O4(OH)4]-SnII based on the thermodynamic data ΔEcom
°
in Table 5, the calculated results show the negative ∆Ecom
value for all possible complexes and also enthalpy result
demonstrated the exothermic interaction between tin and
[Zr6O4(OH)4] cluster. According to thermodynamic data
in Table 5, the stability order of diferent complexes of
[Zr6O4(OH)4]/Sn is Com(III) > Com(I) > Com(II) > Com
(IV) > Com(V) for both states of SnII and SnIV (Fig. 10a,
b). So, the Com (III) complex was selected as the most
stable complex for both states of SnII and SnIV. Compari-
son result between ∆Ecom of two Com (III)-SnII and Com
(III)-SnIV complexes indicates that SnIV is more stable
than SnII by about 47.11 kcal/mol. On the other hand, the
Com (III)-SnIV complex was determined as the most sta-
ble complex between ten possible complexes (Table 5).
Therefore, the Com (III)-SnIV complex is formed via con-
nection of SnCl2, to the O atom of [Zr6O4(OH)4] cluster
and formation of one coordination bond. This observa-
tion is consistent with the result of Sn ion obtaind using
XPS analysis that shows the existence of Sn with oxidation
numer of four. The good agreement between theoretical
and experimental results confrms the accuracy of selected
method for calculations of these kinds of compounds.
Conclusion
In conclusion, a high-performance catalyst for the synthe-
sis of β-enaminones from acetylacetone and primary aro-
matic amines with SnUiO-66 was prepared. Interestingly,
the prepared SnUiO66 as catalyst revealed a remarkable
activity in methanol solvent and was recycled up to four
consecutive cycles without any notable loss in catalytic
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Table 4 Comparison between some selected experimental (Exp. and
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◦
◦
◦
◦
aΔE
cΔH
dΔG
ΔS
com
com
com
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Complex I (SnII)
Complex II (SnII)
Complex III (SnII)
Complex IV(SnII)
Complex V (SnII)
Complex I (SnIV)
Complex II (SnIV)
Complex III (SnIV)
Complex IV(SnIV)
Complex V (SnIV)
−79.88
−74.24
−88.21
−61.32
−24.82
−122.54
−107.32
−135.32
−73.82
−23.15
−80.47
−74.83
−88.79
−61.91
−25.41
−123.13
−107.91
−135.91
−74.41
−23.74
−74.76
−69.37
−82.60
−58.44
−20.45
−117.67
−103.69
−129.95
−70.93
−18.53
−0.023
−0.022
−0.025
−0.014
−0.020
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−0.017
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