Basic ionic liquid anchored on UiO-66-NH2 metal–organic framework: a stable and efficient heterogeneous catalyst for synthesis of xanthenes

Article abstract of DOI:10.1007/s11164-021-04439-1

In this study, the Zr-based metal–organic framework, UiO-66-NH₂, was modified with ionic liquid (dibutylimidazolium bromide) to generate a platform for supporting an organocatalyst of guanidine. After an anion exchange, the prepared hybrid syst

Full text of DOI:10.1007/s11164-021-04439-1

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-021-04439-1
Basic ionic liquid anchored on UiO‑66‑NH2 metal–organic
framework: a stable and efcient heterogeneous catalyst
for synthesis of xanthenes
Saeed Askari1 · Mohammad Mehdi Khodaei1,2 · Mohammad Jafarzadeh1,2
Received: 11 December 2020 / Accepted: 25 March 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract
In this study, the Zr-based metal–organic framework, UiO-66-NH2, was modifed
with ionic liquid (dibutylimidazolium bromide) to generate a platform for support-
ing an organocatalyst of guanidine. After an anion exchange, the prepared hybrid
–
system was denoted as UiO-66-NH2-ILPF6 -guanidine. The catalyst was charac-
terized by diferent techniques such as PXRD, FTIR, TEM, SEM–EDS, TGA, and
BET. The basic heterogeneous catalyst was then utilized for the synthesis of xan-
thene derivatives via a reaction of aldehydes with dimedone. Various types of aro-
matic aldehydes were employed for the synthesis with medium to high yield. The
study of catalytic activities showed that the heterogenization of guanidine signif-
cantly promotes catalytic performance in comparison with guanidine in a homoge-
neous form. The heterogeneous catalyst is stable in the reaction medium and can be
recycled for at least four times without signifcant reduction in activity. The stability
of the catalyst was also investigated by XRD, FTIR, SEM/elemental mapping, and
leaching test.
Keywords Heterogeneous catalyst. · Metal–organic framework. · Ionic-liquid.
Guanidine. · Xanthene
* Mohammad Mehdi Khodaei
mmkhoda@razi.ac.ir
* Mohammad Jafarzadeh
1
Department of Organic Chemistry, Razi University, 67149-67346 Kermanshah, Iran
2
Nanoscience & Nanotechnology Research Center (NNRC), Razi University,
67149-67346 Kermanshah, Iran
Vol.:(0123456789)
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S. Askari et al.
Introduction
A vast application of ionic liquids (ILs) in science and technology is related to
their interesting behaviors of high polarity, non-volatility, high thermal stability,
high conductivity, and a large variety of components in their structures [1]. Their
applications in catalysis have boomed with the introduction and development of
Task-Specifc ILs (TSILs) [2]. Numerous organic reactions were carried out by
applying TSILs. Recyclability, reusability, and leaching of TSILs are major con-
cerns in the development of these materials for industrial applications. By het-
erogenization of ILs to porous solid supports (e.g., polymer, silica, MCM-41,
SBA-15, metal oxides, MOFs), generation of supported ILs (SILs), the physico-
chemical properties of ILs transfer to the support [3]. Besides a synergic efect
between IL and support, the recyclability can be promoted and the leaching mini-
mized [4, 5].
Metal–organic frameworks (MOFs) are composed of metallic oxides (nodes)
and bi-/multi-functional ligands. Their reticular structures provide high crystal-
linity, high porosity (large surface area, micro/mesopores, large pore volume),
tunable synthetic methods, reasonable chemical/thermal stabilities with a broad
range of applications, mainly gas adsorption [6], catalysis [7–10], drug deliv-
ery [11–13], etc. MOFs could use as catalysts individually (metal node as a
Lewis acid site) [14–17] and being a support for metallic [18, 19], organometal-
lic [20, 21], and organic [22, 23] catalytic systems. MOFs have been also used
as support for ILs (i.e., SILs), particularly TSILs [24]. ILs could either confne
in MOF pores [25] or covalently attached to the MOF nodes/ligands [4]. MOF-
supported ILs have been found in many applications in the synthesis of a large
variety of organic compounds [26–32]. As follow, we describe the application
of the selected catalytic systems reported recently. Chong et al. used 1-butyl-
3-methylimidazolium bromide (BmimBr)/NH2-MIL-101(Cr) for the synthesis of
N-aryl oxazolidin-2-ones from carbon dioxide and epoxides, via confnement of
the IL in the MOF pores [33]. The catalyst was recycled and reused and showed
a synergic efect of the IL and the MOF. Jin et al. reported a catalytic system
of IL/polyoxometalate@MOF for epoxidation of cycloolefns with H2O2 [34]. A
synergistic efect between 1-octyl-3-methylimidazolium bromide, PMo10V2, and
MIL-100(Fe) was observed by activation of POM with imidazolium cations of
the IL. Chen et al. reported [HVIm-(CH2)3SO3H]HSO4@HKUST-1(Cu) catalyst,
with available Lewis and Brønsted acid sites, for esterifcation of oleic acid [35].
Xanthenes, with an oxygen-containing ring in the core, can be found in com-
pounds with biological activities (e.g., antiviral, antibacterial), fuorescent dyes
for applications in laser technology and sensing [36], and used as a corrosion
inhibitor [37]. They can be prepared by reaction of aryl aldehydes or aromatic
hydroxyls (e.g., â-naphthol) with dimedone (i.e., 5,5-dimethyl-cycloheaxan-
1,3-dione) in the presence of acidic [36, 38], basic, and metallic [39] catalysts
under conventional, solvent-free [40], microwave [41] and ultrasonic [42] con-
ditions. Various catalytic systems were introduced in the literature such as
CoFe2O4/OCMC/Cu(BDC) [43], L-Proline [44], Cu(OAc)2 [45], cyclodextrin/
1 3

Basic ionic liquid anchored on UiO-66-NH2 metal–organic…
melamine/Ag [46], Fe-Cu/ZSM-5 [47], MWCNs/Ru(CO)4 [48], NiFe2O4@Cu/
montmorillonite [49], ZnCl2/urea as a deep eutectic solvent (DES) [50], cerium
(IV) ammonium nitrate/ultrasonic [51], 1,1´-butylenebis(3-sulfo-3H-imidazol-
–
1-ium) chloride as a binuclear TSIL [52], [(n-propyl)2NH2][HSO4 ] [53] and
perlite nanoparticles@IL/ZrCl4 [54]. Most recently, Lewis acid sites in MOFs
(i.e., Cu/Cr-MIL-101) catalyzed the reaction to xanthenes [55]. The bimetallic
MOFs showed a synergic efect with higher catalytic activity compared to the
corresponding MOFs, Cu-BDC and Cr-MIL-101, individually. Abo El-Yazeed
and co-workers employed a bimetallic Fe–Mg MOF for the catalytic synthesis of
xanthenes [56]. Core–shell nanoparticle-supported imidazolium IL (i.e., Fe3O4@
–
propylsilane-histidine[HSO4 ]) was introduced for the synthesis of xanthene
derivatives [57]. Zwitterionic and ionic structures, 4-[3-(4-sulfo-butyl)-imidazol-
1-yl]-butane-1-sulfonate and 4-imidazol-1-yl-butane-1-sulfonate imidazolium,
were also used for the synthesis of xanthenes [58].
Recently, we have immobilized butylimidazolium IL to a zirconium-based MOF,
UiO-66-NH2, with a subsequent functionalization of the IL using urea [59]. The
basic IL was used for the synthesis of oximes. Since guanidine is recognized as a
base catalyst, which has been used recently for transesterifcation to biodiesel [60],
it was used for the functionalization of UiO-66-NH2-supported butylimidazolium in
–
this study. The UiO-66-NH2-ILPF6 -guanidine as a heterogeneous catalyst was then
used for the base-catalyzed synthesis of xanthenes. The efect of solvent type, tem-
perature, catalyst amount, and catalyst recoverability, and reusability were studied.
Experimental
Material and physical measurements
The materials were purchased from Merck and used without any additional purif-
cation. Powder X-ray difraction (XRD) was conducted by a Philips PW1730 dif-
fractometers (Cu Kα, λ=1.5418 Å). Infrared spectra were recorded with FT-IR
spectrometer (WGH-30/6300), in the range of 400–4000 cm–1. Transmission elec-
tron microscopy (TEM) images were obtained using a Philips EM 208S instrument
at an accelerating voltage of 100 kV. Field emission scanning electron microscopy
(FESEM, Model: TESCAN MIRA3) and energy-dispersive spectroscopy (EDS)
were used for morphological and compositional studies, respectively. Thermo-
gravimetric analysis (TGA, model: Q600) was carried out at the heating rate of
7 °C min–1 under nitrogen fow. The BET surface area and pore volume were meas-
ured by a Belsorp mini II porosimeter. The metal content was determined by induc-
tively coupled plasma-optical emission spectroscopy (ICP-OES, model: Perkin-
1
Elmer DV5300). The organic products were characterized by H and 13C nuclear
magnetic resonance (NMR, Model: Bruker AVANCE 300 MHz) spectroscopy.
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S. Askari et al.
Preparation of UiO‑66‑NH2‑ILBr–
1,3-Bis-(4-bromobutyl)-3H-imidazol-1-ium bromide (ILBr–) was initially prepared
according to our reported method [59]. The preparation of UiO-66-NH2 was car-
ried out by adopting a method reported in the literature [61]. For the preparation of
immobilized IL on the MOF, UiO-66-NH2 (0.1 g) was dispersed in 10 mL DMF
under ultrasonic irradiation and ILBr– (0.03 mmol, 0.01 g) was then added, which
was stirred at room temperature for 24 h.
Preparation of UiO‑66‑NH2‑ILBr–‑guanidine
Guanidine hydrochloride (0.06 mmol, 0.005 g) was added to UiO-66-NH2-
ILBr– (0.1 g) dispersed in 10 mL DMF and stirred at room temperature for 24 h.
The product (UiO-66-NH2-ILBr–-guanidine) was isolated by centrifugation, washed
with DMF and ethanol several times, and dried at 80 °C in an oven for 24 h.
Preparation of UiO‑66‑NH2‑ILPF6–‑guanidine
–
The Br– counter anion was exchanged with hexafuorophosphate (PF6 ) using potas-
sium hexafuorophosphate (0.75 mmol, 0.138 g) in acetone (2 mL) under stirring
–
at room temperature for 24 h. The fnal product (UiO-66-NH2-ILPF6 -guanidine)
was fltered, washed with H2O (3×10 mL), EtOH (3 mL), Et2O (2.5 mL), acetone
(2.5 mL), and dried at 50 °C in an oven overnight.
Typical procedure for the synthesis of 3,3,6,6‑tetramethyl‑9‑phenyl‑3,4,5,6‑tet‑
rahydro‑9H‑xanthene
–
UiO-66-NH2-ILPF6 -guanidine (0.03 g) dispersed in 1 mL ethanol was added to a
mixture of benzaldehyde (1 mmol, 0.11 mL) and dimedone (2 mmol, 0.280 g). The
mixture was stirred at room temperature at an appropriate time to complete the reac-
tion. The reaction progress was monitored by TLC. After completion of the reaction,
the catalyst was separated from the reaction medium by centrifugation, washed with
ethanol, and dried in an oven (70 °C) overnight. The organic products were isolated
from the reaction mixture with the evaporation of the solvent and then purifed by
recrystallization. The xanthene derivatives were identifed by 1H and 13C NMR (300
and 75 MHz, respectively). The catalyst was also reused several times.
The spectral data for the selected products
3,4,6,7‑Tetrahydro‑9‑(4‑methoxyphenyl)‑3,3,6,6‑tetramethyl‑2H‑xanthe
ne‑1,8‑(5H,9H)‑dione (Fig. S1)
1H NMR (300 MHz, CDCl3); δ (ppm)=1.02 (s, 6H, 2×CH3), 1.11 (s, 6H, 2×CH3),
2.23 (q, 4H, 2×CH2), 2.49 (s, 4H, 2×CH2), 3.74 (s, 3H, OCH3), 4.70 (s, 1H,
CH), 6.73 (d, 2H, J=6.75 Hz, Ar–H), 7.28 (d, 2H, J=7.25 Hz, Ar–H); 13C NMR
1 3

Basic ionic liquid anchored on UiO-66-NH2 metal–organic…
(75 MHz, CDCl3): δ (ppm)=27.4, 29.8, 30.9, 32.3, 40.6, 50.8, 115.2, 116.0, 129.3,
135.3, 154.6, 162.4, 197.5.
3,4,6,7‑Tetrahydro‑9‑(4‑hydroxyphenyl)‑3,3,6,6‑tetramethyl‑2H‑xan‑
thene‑1,8‑(5H,9H)‑dione (Fig. S2)
1H NMR (300 MHz, CDCl3); δ (ppm)=1.02 (s, 6H, 2×CH3), 1.13 (s, 6H, 2×CH3),
2.22 (q, 4H, 2×CH2), 2.48 (s, 4H, 2×CH2), 3.76 (s, 1H, OH), 4.73 (s, 1H, CH),
6.5 (d, 2H, J=6.58 Hz, Ar–H), 7.0 (d, 2H, J=7.09 Hz, Ar–H); 13C NMR (75 MHz,
CDCl3): δ (ppm)=27.10, 29.43, 30.57, 32.01, 40.61, 51.01, 55.04, 113.16, 116.03,
136.50, 158.06, 162.08, 196.64.
3,4,6,6‑Tetramethyl‑9‑phenyl‑3,4,5,6,7,9‑hexahydro‑1H‑xanthene‑1,8(2H)‑dione
(Fig. S3)
1H NMR (300 MHz, CDCl3); δ (ppm)=1.13 (s, 6H, 2×CH3), 1.27 (s, 6H, 2×CH3),
2.20 (s, 4H, 2×CH2), 2.42 (q, 4H, 2×CH2), 5.58 (s, 1H, CH), 7.11–7.22 (m, 5H);
13C NMR (75 MHz, CDCl3): δ (ppm)=27.1, 29.6, 31.4, 32.6, 46.1, 47.5, 115.5,
125.6, 126.9, 128.4, 137.5, 159.0, 189.2.
3,3,6,6‑Tetramethyl‑9‑(3‑nitrophenyl)‑3,4,5,6,7,9‑hexahydro‑1H‑xanthene‑1,8(2H)‑d
ione (Fig. S4)
1H NMR (300 MHz, CDCl3); δ (ppm)=1.15 (s, 6H, 2×CH3), 1.30 (s, 6H, 2×CH3),
2.40–2.48 (m, 4H, 2×CH2), 2.48–2.56 (m, 4H, 2×CH2), 5.61 (s, 1H, CH),
7.21–8.06 (m, 4H); 13C NMR (75 MHz, CDCl3): δ (ppm)=27.4, 29.5, 31.2, 32.9,
46.6, 47.0, 114.9, 121.2, 122.1, 129.1, 133.3, 140.8, 148.5, 189.7.
Results and discussion
Preparation and characterization of UiO‑66‑NH2‑ILPF6–‑guanidine
–
The preparation steps of UiO-66-NH2-ILPF6 -guanidine are summarized in
Scheme 1. Initially, the ILBr– was prepared according to our previously reported
method [59] and then immobilized on the surface of UiO-66-NH2 by post-synthesis
modifcation (PSM) method. Guanidine hydrochloride was attached to the supported
ionic liquid (UiO-66-NH2-ILBr–) and the Br– was then exchanged by PF6– to fnally
generate supported ionic liquid catalyst (SILC). The prepared catalyst, UiO-66-NH2-
–
ILPF6 -guanidine, was characterized by diferent techniques including XRD, FT-IR,
TEM, SEM–EDS, TGA, and BET.
To evaluate the crystal stability of UiO-66-NH2 after the introduction of IL,
powder XRD was used over the difraction angle (2θ) of 5–80° (Fig. 1). The char-
acteristic difractions of the UiO-66-NH2 were found to be almost similar to the
pattern of that framework in the literature [62]. The difractions at angles of 7.6°,
7.9°, 14°, 16.4°, 21.4°, 24.8°, 29.8°, 32.2°, 34.9°, 36.8°, 40.1°, 42.7°, 49.3°, and
1 3

S. Askari et al.
–
Scheme 1 Preparation of ILBr– and UiO-66-NH2-ILPF6 -guanidine
UiO-66-NH2
UiO-66-NH2-ILBr–
UiO-66-NH2-ILPF6–-guanidine-Fresh
UiO-66-NH2-ILPF6–-guanidine-Reused
0
10
20
30
40
50
60
70
80
2θ (angle)
–
Fig. 1 XRD patterns of UiO-66-NH2, UiO-66-NH2-ILBr–, and UiO-66-NH2-ILPF6 -guanidine (fresh and
reused)
55.9° were observed corresponding to the (111), (200), (222), (400), (511), (600),
(711), (731), (820), (751), (664), (933), (955), and (12 42) planes, respectively.
–
For UiO-66-NH2-ILBr– and UiO-66-NH2-ILPF6 -guanidine, similar patterns to
the pattern of UiO-66-NH2 were observed indicating the structural preservation
of the MOF framework during PSM with IL and the attachment of guanidine. A
slight refection shift, corresponding to the angles of 7.6°, 7.9°, 24.8°, and 43.7°,
toward higher angles was observed for the functionalized MOF with IL (Fig. S5).
1 3

Basic ionic liquid anchored on UiO-66-NH2 metal–organic…
It indicates that the ILs on the surface compress the fexible framework of the
MOF to form a smaller unit cell.
Post-synthetic modifcation of the UiO-66-NH2 with ILBr− was further con-
frmed by FT-IR spectroscopy.−The FT-IR spectra of UiO-66-NH2, UiO-66-NH2-
ILBr− and UiO-66-NH2-ILPF6 -guanidine are presented in Fig. 2. The stretching
peaks at 3460 and 3336 cm−1 are attributed to the NH2 band of primary amine,
which indicates the presence of the amine group within the framework of UiO-
66-NH2 [61]. The stretching peaks at 1573, 1496, 1253, and 600–750 cm−1 are
assigned to C = C (aromatic ring), C-N (C-NH2), and Zr-O bonds of UiO-66-NH2,
respectively [62]. After modifcation, the C-NH2 band of a primary amine was
converted to C-NH of a secondary amine and the stretching vibration of the
NH bond appeared at 3452 cm−1. The observed band at 1654 cm−1 is related
to stretching vibration of C = N of imidazolium ring, and the stretching band at
663 cm−1 is corresponded to CH2-Br which overlapped with the UiO-66-NH2
peaks. These results confrmed the incorporation of ILBr− to UiO-66-NH2.
Finally, the binding of guanidine to the ionic liquid results in the formation of
two bands at 3452 and 3334 cm−1, which are corresponded to NH2 of guanidine.
–
The morphology of UiO-66-NH2 and UiO-66-NH2-ILPF6 -guanidine was
investigated by TEM and SEM (Fig. 3). The TEM images of UiO-66-NH2 and
–
UiO-66-NH2-ILPF6 -guanidine show that the MOF particles were aggregated,
and the size of aggregated particles was found to be in the range of~ 50–100 nm.
The SEM images show irregular and spherical shapes for UiO-66-NH2 and
–
UiO-66-NH2-ILPF6 -guanidine, respectively. The EDS spectrum of UiO-66-
–
NH2-ILPF6 -guanidine confrmed the presence of corresponding elements of the
UiO-66-NH2
UiO-66-NH2-IL-PF6--Guanidine-Fresh
UiO-66-NH2-ILBr-
UiO-66-NH2-ILPF6--Guanidine-Reused
4000
3600
3200
2800
2400
2000
1600
1200
800
400
Wavenumber (cm-1)
–
Fig. 2 FT-IR spectra of UiO-66-NH2, UiO-66-NH2-ILBr−,UiO-66-NH2-IL ILPF6 -guanidine (fresh and
reused)
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S. Askari et al.
–
Fig. 3 TEM and SEM images of (a,b) UiO-66-NH2 and (c,d) UiO-66-NH2-ILPF6 -guanidine
framework, IL, and guanidine (Fig. 4). The elemental mapping demonstrated that
the elements were distributed homogeneously in the structure.
–
The thermal behavior of UiO-66-NH2-ILPF6 -guanidine was investigated
by thermogravimetric analysis and three steps of weight loss observed (Fig. 5).
In the frst step, a weight loss of ~ 20% was found in the temperature range of
25–140 °C, attributed to physically adsorbed solvents (i.e., DMF and ethanol) and
moisture in the pores. The second step in the temperature range of 140–470 °C
was observed with ~ 14.7% weight loss, related to the removal of coordinated
DMF from the framework, degradation of IL and guanidine, and the partial
degradation of the MOF frameworks [63]. The content of ILBr– was estimated
at ~ 6.3% by comparison of the second step weight losses of UiO-66-NH2 and
UiO-66-NH2-ILBr– according to our previous report [59]. We are expecting that a
part of IL has been functionalized by guanidine. The decomposition of the entire
1 3

Basic ionic liquid anchored on UiO-66-NH2 metal–organic…
Element wt. %
at. %
50.31
5.36
C
F
19.06
3.22
Zr
O
P
16.56
10.46
5.82
4.42
20.73
5.95
8.35
N
3.69
–
Fig. 4 EDS spectrum and elemental mapping of the UiO-66-NH2-ILPF6 -guanidine (fresh)
framework occurred in the third step at a temperature above 470 °C with a weight
loss of ~ 9.9%.
Figure 6 shows the N2 adsorption/desorption isotherms and the corresponding
–
pore size distributions of UiO-66-NH2 and UiO-66-NH2-ILPF6 -guanidine. The
pattern of isotherms is consistent with the gas sorption behavior of microporous
materials (type I) [64]. The data of the surface area and the porosity are given in
Table 1. As expected, the BET specifc surface area and total pore volume of the
1 3

S. Askari et al.
100
90
80
70
60
50
40
30
0
100
200
300
400
500
600
700
Temperature (°C)
–
Fig. 5 TGA thermogram of UiO-66-NH2-ILPF6 -guanidine
UiO-66-NH2 decreased after functionalization with IL and guanidine. The pore size
–
of UiO-66-NH2-ILPF6 -guanidine was found to be the same as that in UiO-66-NH2,
which is consistent with the pore size distribution curves. The high surface area of
the catalyst provides a high number of the available active sites on the surface to
interact with reactants. Moreover, the large pore volume facilitates easy access to the
pore interior for efcient contact between the reactants and the catalyst active sites.
The catalytic activity of the catalyst
–
The catalytic performance of the UiO-66-NH2-ILPF6 -guanidine was evaluated
in the synthesis of xanthenes. A model reaction, using benzaldehyde (1 mmol)
and dimedone (2 mmol) as precursors, was chosen to fnd the optimum condi-
tions by considering solvent type, temperature, and catalyst amount. The results
of optimal conditions are presented in Table 2. Various solvents such as CH2Cl2,
CH3CN, H2O, EtOH, and EtOH:H2O (1:1) were used for the reaction. Solvents
with medium polarity (Table 2, entries 3, 5, 6) exhibited better performance for
the synthesis compared to a non-polar solvent (CH2Cl2) and a solvent with high
polarity (H2O). The highest reaction efciency was obtained by utilizing EtOH;
a suitable medium for the reaction components. In contrast, the lowest yield
of 33% was observed in the absence of solvent. In other words, the solvent is
vital for the efcient contacts of the precursors. The efect of elevated tempera-
ture on the reaction efciency was examined (Table 2, entries 6–8). It was found
that the reaction efciency decreased by increasing the reaction temperature to
60 and 80 °C. The same phenomenon has been observed in our previous work
1 3

Basic ionic liquid anchored on UiO-66-NH2 metal–organic…
400
ADS-UiO-66-NH2
DES-UiO-66-NH2
ADS-UiO-66-NH2-ILPF6--Guanidine
DES-UiO-66-NH2-ILPF6--Guanidine
300
200
100
0
0.04
0.03
0.02
0.01
0
0
0.2
0.4
0.6
0.8
1
Relative pressure (P/P°)
UiO-66-NH2
UiO-66-NH2-ILPF6--Guanidine
1
10
Pore width (nm)
100
Fig. 6 N2 adsorption–desorption isotherms and the corresponding pore size distribution curves of UiO-
–
66-NH2 and UiO-66-NH2-ILPF6 -guanidine
–
using MOF/ILBF4 /Urea catalyst in the synthesis of oximes [59]. However, the
room temperature was chosen for further study (Table 2, entry 6). The quantity
of the catalyst was studied, and the results showed that by increasing the catalyst
1 3

S. Askari et al.
Table 1 The surface area and
the porosity data for UiO-
66-NH2 and UiO-66-NH2-
Materials
Specifc surface Total pore vol-
Mean pore
size (nm)c
area (m2g–1)a
ume (cm3g–1)b
–
ILPF6 -guanidine
UiO-66-NH2
UiO-66-NH2-
1072
648
0.56
0.34
2.09
2.10
–
ILPF6 -guan-
idine
aSBET
bTotal pore volume determined at P/P0 =0.99
cMean pore diameter determined using the BJH method
Table 2 Optimization of the reaction conditionsa
Entry
Solvent
Catalyst
amount (g) ture (°C)
Tempera- Yield (%)b
1
2
3
4
5
6
7
8
9
10
Solvent-Free
CH2Cl2
CH3CN
H2O
EtOH: H2O (1:1)
EtOH
EtOH
EtOH
EtOH
EtOH
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.05
0.1
25
25
25
25
25
25
60
80
25
25
33
45
70
55
65
95
95
95
95
95
aReaction conditions: benzaldehyde (1 mmol), dimedone (2 mmol),
–
solvent (1 mL), UiO-66-NH2-ILPF6 -guanidine, 45 min
bIsolated yield
1 3

Basic ionic liquid anchored on UiO-66-NH2 metal–organic…
Table 3 Synthesis of xanthenes via reactions of aldehydes with dimedone
Entry
Aldehydes
Yield (%)a
Time (min)
m.p. (°C) found
m.p. (°C) reported
1
2
3
4
5
6
7
8
C6H5CHO
95
93
92
94
82
85
95
90
45
50
60
55
50
55
45
45
194–198
136–140
128–132
190–192
228–232
196–200
188–192
166–170
202–204 [65]
138 [66]
4-OMe-C6H4CHO
4-Me-C6H4CHO
4-OH-C6H4CHO
4-Cl-C6H4CHO
2,4-Cl2-C6H3CHO
4-NO2-C6H4CHO
3-NO2-C6H4CHO
132–133 [65]
194–196 [67]
230–232 [65]
197–199 [66]
192–194 [66]
166–168 [68]
–
Reaction conditions: aldehyde (1 mmol), dimedone (2 mmol), ethanol (1 mL), UiO-66-NH2-ILPF6 -
guanidine (0.03 g), rt
aIsolated yield
content from 0.03 to 0.05 and 0.1, the efciency of the reaction did not change
(Table 2, entries 6, 9, 10). Therefore, the best conditions for the reaction were
found by employing ethanol, room temperature, and 0.03 g of the catalyst.
To examine the capability of various aldehydes in the synthesis of xanthenes,
aromatic aldehydes were used to react with dimedone (Table 3). Aryl aldehydes
with electron-donating and electron‐withdrawing groups, including –OMe, –Me,
–OH, –Cl, and –NO2, exhibited high yields in the optimized reaction conditions.
In other words, the nature of the substituent did not afect signifcantly the ef-
ciency of the reaction.
The type of catalyst was another efective parameter on reaction efciency
(Table 4). The support (i.e., UiO-66-NH2) and the IL (in the homogeneous form)
–
were less reactive in the synthesis, while guanidine and ILPF6 /guanidine, both
–
in homogeneous form, exhibited relatively good yields. Although ILPF6 /guani-
dine showed a slightly higher yield than guanidine hydrochloride, the synergistic
efect between IL and guanidine was not signifcant. Since the high surface area and
Table 4 The catalytic activity of
Entry Catalyst
Time (min) Yield (%)a
the catalyst, the support, and the
ILs (in homogeneous phase)
–
1
2
3
4
5
6
7
UiO-66-NH2-ILPF6 -guanidine 45
95
UiO-66-NH2-ILBr–-guanidine
UiO-66-NH2
60
80
90
35
ILBr–
ILPF6
ILPF6 -guanidine
120
120
60
Trace
25
73
–
–
Guanidine hydrochloride
60
70
Reaction conditions: benzaldehyde (1 mmol), dimedone (2 mmol),
ethanol (1 mL), catalyst (0.03 g), rt
aIsolated yield
1 3

S. Askari et al.
–
Fig. 7 The proposed mechanism for the synthesis of xanthenes using UiO-66-NH2-ILPF6 -guanidine
porosity of the support provide a high number of active sites for immobilizing IL
and then guanidine, therefore higher performance was observed for the supported
catalyst (Table 4, entries 1 and 2). In other words, a synergistic efect was observed
between the support and the IL/guanidine. Furthermore, the type of counter anion of
–
the IL was found to be efective on the reaction yield, as ILPF6 (Table 4, entries 1
and 5) showed higher activity than ILBr– (Table 4, entries 2 and 4).
1 3

Basic ionic liquid anchored on UiO-66-NH2 metal–organic…
The mechanism of the synthesis of xanthenes involves a tandem reaction
of Knoevenagel, Michael, and intramolecular cyclization catalyzed by a base
(Fig. 7) [39]. The supported guanidine plays the role of the base in deprotonation
of the acidic proton of the dimedone, while the metallic nodes of the MOF acti-
vate the carbonyl of aldehydes. In addition, the supported IL can polarize the area
near the surface of the catalyst and facilitate the contact of the starting materials
and the catalyst surface.
Reusability of the catalyst
After completion of the reaction under optimized reaction conditions, the catalyst
was recycled by fltration, washed with ethanol, and dried at 70 °C in an oven over-
–
night. The UiO-66-NH2-ILPF6 -guanidine was reused four times without a signif-
cant loss in its catalytic activity (Fig. 8). The slight reduction in the catalyst per-
formance could be related to the loss of the catalyst within the recycling process,
proved by ICP-OES. However, the remained catalyst activity indicates the high
–
structural stability of the catalyst. The stability of UiO-66-NH2-ILPF6 -guanidine
heterogeneous catalyst was evaluated by XRD, FTIR, and SEM/elemental mapping.
The XRD pattern of the recovered catalyst (after four times reused) indicates that
the crystal structure of the framework has been preserved (Fig. 1). The FTIR spec-
trum showed that the catalyst retained its structure after four times reusing (Fig. 2).
The SEM image and elemental mapping exhibited that the morphology of the par-
ticles and the elemental distribution in the structure of the catalyst was remained
unchanged after four times reusing the catalyst (Fig. S6).
–
Moreover, the stability of the catalyst, UiO-66-NH2-ILPF6 -guanidine, was
examined by the leaching test. The test was performed for the model reaction in two
steps (Fig. S7) In the frst step, the catalyst was separated from the reaction medium
by centrifugation after 10 min and the reaction let be continued until 45 min in
the absence of the catalyst. A reaction yield of 20% was obtained. The content of
100
95
95
93
92
90
89
90
85
80
75
1
2
3
4
5
Number of runs
Fig. 8 Reusability of the catalyst
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S. Askari et al.
1 3

Basic ionic liquid anchored on UiO-66-NH2 metal–organic…
zirconium in the recovered catalyst was found to be 5.70 mol%, determined accord-
ing to the method described in the literature [69]. A slight decrease in the zirconium
content, compared to the content in the fresh catalyst (6 mmol%), might be related
to the inevitable detriment in catalyst recycling and regenerating (i.e., centrifugation
and rinsing with solvents). In the second step, the catalyst was removed after 25 min
and the reaction continued till 45 min, and a yield of~50% was found. The zirco-
nium content was also determined as 5.11 mol% in the recovered catalyst. A com-
parison between these results and the results in the optimized conditions (95% yield
within 45 min) indicates that the leaching of the guanidine did not happen, which
could catalyze the reaction after removal of the catalyst.
The performance of our catalyst was compared with other catalytic systems
such as MOFs, ILs, and supported ILs, reported in the literature recently (Table 5)
[51–58]. The MOFs (entries 1 and 2) as heterogeneous catalysts exhibited great cat-
alytic activity in high temperatures, solvent-free conditions, and long reaction times.
The ILs (entries 3–6) as homogeneous catalysts/media showed good yields, without
the utilization of additional solvent, in temperatures higher than room temperature,
in short reaction times. The nanoparticle-supported ILs (heterogeneous catalysts)
exhibited good performance in elevated temperatures (entries 7 and 8). In the pre-
sent work, the synthesis was carried out in non-toxic/non-volatile solvent and room
temperature with high yield. Our catalytic system showed comparable efciency
with reported results and proving milder conditions in temperature and reaction
medium.
Conclusion
A supported ionic liquid catalyst was presented for the synthesis of xanthenes. The
basic heterogeneous catalyst was prepared by the functionalization of Zr-based
MOF with an ionic liquid. Guanidine was then attached to the ionic liquid, along
–
with an anion exchange by replacing Br– with PF6 . Various types of aldehydes were
successfully reacted with dimedone to produce xanthenes in good to high yield in
the presence of the catalyst. The type of solvent, temperature, and catalyst amount
were efective parameters in the reaction. Furthermore, a synergistic efect between
the MOF and the IL/guanidine was observed. The catalyst was capable of recycling
and reuse several times. The stability of the catalyst was confrmed by XRD, FTIR,
SEM/elemental mapping, and also leaching test. It is expected that such a hybrid
system, MOF/IL/organocatalyst, fnds further development and deep understanding
to know the role of each catalyst component in the organic synthesis. Moreover, the
high surface area and porosity of MOFs provide a great opportunity for organocata-
lysts to stabilize on the surface of porous materials and generate a unique heteroge-
neous catalytic system.
Supplementary Information The online version contains supplementary material available at https://doi.
org/10.1007/s11164-021-04439-1.
Acknowledgements The authors gratefully acknowledge Razi University for fnancial support.
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S. Askari et al.
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