Design of multifaceted acidic 1,3-disulfoimidazolium chlorometallate ionic systems as heterogeneous catalysts for the preparation of β-amino carbonyl compounds

Article abstract of DOI:10.1016/j.molcata.2016.02.007

A new series of 1,3-disulfoimidazolium transition metal chloride systems: [Dsim]₂[ZnCl₄], [Dsim][FeCl₄] and [Dsim]₂[NiCl₄] were synthesized as Br?nsted-Lewis acidic solid material and characterized via NMR, FT-IR, Raman, TGA, SEM, EDX, Powder XRD, UV-visible, ICP-OES and elemental analysis. Semiconductor properties of these materials were studied through electronic spectroscopy. The Br?nsted acidity of these systems was determined from Hammett plot. All these three materials were successfully tested as reusable catalyst for the one pot three component Mannich-type syntheses of β-amino carbonyl compounds within considerable time frame.

Full text of DOI:10.1016/j.molcata.2016.02.007

Accepted Manuscript
Title: Design of multifaceted acidic 1,3-disulfoimidazolium
chlorometallate ionic systems as heterogeneous catalysts for
the preparation of ␤-amino carbonyl compounds
Author: Susmita Saikia Pinky Gogoi Arup Kumar Dutta
Parishmita Sarmah Ruli Borah
PII:
S1381-1169(16)30041-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.02.007
MOLCAA 9772
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
15-12-2015
3-2-2016
4-2-2016
Please cite this article as: Susmita Saikia, Pinky Gogoi, Arup Kumar Dutta,
Parishmita Sarmah, Ruli Borah, Design of multifaceted acidic 1,3-disulfoimidazolium
chlorometallate ionic systems as heterogeneous catalysts for the preparation of
rmbeta-amino carbonyl compounds, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.02.007
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Design of multifaceted acidic 1, 3-disulfoimidazolium
chlorometallate ionic systems as heterogeneous catalysts for the
preparation of -amino carbonyl compounds
Susmita Saikia, Pinky Gogoi, Arup Kumar Dutta, Parishmita Sarmah, Ruli Borah*
ruli@tezu.ernet.in
Department of Chemical Sciences, Tezpur University, Napaam-784028, Tezpur, India
*Corresponding author.
1

Graphical abstract
2

Highlights
 A new class of acidic 1,3-disulfoimidazoliumchlorometallate ionic solid system is
prepared
 All three materials are characterized by various analytical techniques such as NMR,
elemental analysis, UV-vis, FT-IR, Raman, TGA, SEM-EDX etc.
 Metal anion speciation regarding the structure is further confirmed by electronic spectra
 Calculated band gap energies prove these materials as better semiconductors
 Suitable to catalyze as heterogeneous acid catalyst for the synthesis of β–amino carbonyl
compounds
3

Abstract
A new series of 1,3-disulfoimidazolium transition metal chloride systems: [Dsim]2[ZnCl4],
[Dsim][FeCl4] and [Dsim]2[NiCl4] were synthesized as Brönsted-Lewis acidic solid material and
characterized via NMR, FT-IR, Raman, TGA, SEM, EDX, Powder XRD, UV-visible, ICP-OES
and elemental analysis. Semiconductor properties of these materials were studied through
electronic spectroscopy. The Brönsted acidity of these systems was determined from Hammett
plot. All these three materials were successfully tested as reusable catalyst for the one pot three
component Mannich-type syntheses of -amino carbonyl compounds within considerable time
frame.
Keywords: Multifaceted acidic solid; 1, 3-disulfoimidazoliumchlorometallates; reusable
catalyst; Mannich-type; -amino carbonyl compound
4

1. Introduction
Chlorometallate ionic liquids (IL) are considered as functional materials for their distinct and
potentially useful properties [1, 2]. The specific characters of such ionic liquid depend on both
the nature of metal present and the ratio of metal chloride to organic chloride salt used for this
synthesis. The anionic component [MxCly]n- of chlorometallate IL can be varied by considering
these two factors and thereby, control its stability and catalytic activity using Lewis acidity or
basicity for a given composition. The characteristics and individual properties of these ILs
provide wide uses in catalysis, electrochemistry, separation/extraction processes, and synthesis
of inorganic semiconductors and conversion of biomass to energy products [3-6].
Chlorometallate systems based on Al were extensively studied for catalysis and electrochemical
uses [3, 7-8]. Unfortunately, their extreme sensitivity towards moisture created a major barrier
for various applications. Till date several transition metal chlorides have been investigated to
design different compositions of chlorometallate anionic ILs [1]. It was seen that most of these
ILs were prepared with alkyl group containing imidazolium or pyridinium cations and little work
had been done with functionalized cations in halometallate systems [9-12]. Addition of acidic or
basic functional groups into the cation may further modify the characteristic properties of each
IL. In this context, we tried to develop new bifunctional Brönsted-Lewis acidic chlorometallate
ionic salt systems using two –SO3H group into the cation as an extension of the previous work
[13]. Herein, we synthesized and characterized three multifaceted acidic 1, 3-disulfoimidazolium
chlorometallate systems: [Dsim]2[ZnCl4], [Dsim][FeCl4] and [Dsim]2[NiCl4] in solid state from
the reaction of 1,3-disulfoimidazolium chlorides ([Dsim]Cl) with transition metal chlorides
(FeCl3, NiCl2, ZnCl2 ) in different molar ratios (1:1 and 2: 1) under neat condition at 60°C for 2 h
(Scheme-1). The bifunctional acidic sites make them as strong acid catalysts for potential
organic reactions with various advantages of heterogeneous catalysis [14, 15].
5

The acid catalyzed multicomponent Mannich-type reaction is an important procedure for the
preparation of β-amino carbonyl compounds (Scheme-2) which act as precursors of various
natural products and synthetic bioactive compounds, pharmaceuticals and functional materials
[16, 17]. The search of literature provided several modified synthetic methods for its preparation
using heterogeneous acid catalyst, ionic liquids, solvent-free condition, microwave energy and
aqueous media [18-21]. Various limitations of such procedures also require mitigation of
methodology in terms of enhancing the catalytic activities. Therefore, we checked the efficiency
of these chlorometallate ionic salts as heterogeneous catalysts for the Mannich-type reaction of
acetophenone, aromatic aldehydes and substituted aniline in ethanol at room temperature
according to the Scheme-2.
2. Experimental
2.1. General techniques
All chemicals were purchased from Merck and used without any further treatment. For catalytic
activity comparisons, the known 3-methyl-1-sulfonic acid imidazolium transition metal chlorides
[Msim][X] (where X= [ZnCl3]- , [FeCl4]- ), 1-butyl-3-methylimidazolium chlorometallates
[BMIm]n[X] (7a-7c) (m=2 and 1; [X] = [ZnCl4]2-,[FeCl4]-,[NiCl4]2-) and 1, 3-disulfonic acid
imidazolium chloride or carboxylate without metal center [Dsim][X](8a-8c) ([X]= Cl-, CF3COO-
, CCl3COO-) were synthesized using the standard procedure available in literature [13,22-24, 25].
1
13
The IR spectra were run on a Nicolet Impact- 410 spectrophotometer. The H NMR and C
NMR were recorded on a JEOL 400 MHz spectrophotometer (δ in ppm) in DMSO-d6 and CDCl3
as solvents. The Hammett plots of the solid acids were obtained on a UV 2550
spectrophotometer using 4- nitroaniline as basic indicator. The TGA analysis was done on
6

Shimadzu TGA-50. Scanning electron microscopy was performed on a JEOL JSM – 6390LV
SEM, equipped with energy dispersive X–ray analyzer (EDX). The Raman spectroscopy
analyses were done on Horiba LabRAM HR spectrophotometer equipped with a He-Ne laser of
excitation wavelength of 514.5 nm. Powder X – Ray diffraction spectra were recorded on Rigaku
Multiflex instruments using a nickel filtered CuKα (0.15418 nm) radiation source and
scintillation counter detector. Electronic spectra were recorded on a UV 2450, Shimadzu
spectrophotometer. ICP-OES analysis was done by ICP-OES Perkin Elmer Optima 2100DV
instrument. All the melting points were recorded on Büchi– 560 apparatus.
2.2. Preparation of 1, 3-disulfoimidazolium chlorometallates [Dsim][FeCl4] and [Dsim]2[X]
(2a-c), where X- = [ZnCl4]2-, [NiCl4]2- :
The three 1, 3-disulfoimidazolium chlorometallates were prepared by two step reactions
(Scheme 1). The first step involved with the dropwise addition of chlorosulfonic acid (20 mmol)
to a stirred solution of imidazole (10 mmol) in dry CH2Cl2 (30 mL) at 0°C within a period of 5
min under nitrogen atmosphere in a 100 mL round bottom flask. Then the mixture was stirred for
one hour to complete the synthesis of 1, 3-disulfoimidazolium chloride [Dsim][Cl].The viscous
ionic liquid layer was washed three times with fresh CH2Cl2 (3× 20 mL) by decantation of
dichloromethane solution. The residue was dried under vacuum which produced 98% pale
yellow viscous oil of [Dsim][Cl]. The 2nd step was performed by mixing of respective metal
chlorides with the [Dsim][Cl]. Mole fractions of ZnCl2, FeCl3, and NiCl2 in their respective
reaction mixture with [Dsim]Cl were calculated as 0.33 for ZnCl2 (2a), 0.5 for FeCl3 (2b) and
0.33 for NiCl2 (2c) [26]. As an example, synthesis of 2b with 0.5 mol fraction of FeCl3 (1.62 gm,
10 mmol) was added into [Dsim][Cl] (2.67gm, 10 mmol) and stirred at 60°C in an oil bath for 2
hour. The solid product was washed with dry CH2Cl2 (2 x 5 mL) and then dried in vacuum oven
7

at 80 °C to get 96-97 % yields of 2a-c. Finally they were kept in vacuum desiccator. Melting
points of the three ionic salts were observed up to 400°C in Büchi-560 melting point apparatus
but their initial colours changed into black or brown without going to liquid state. They may lose
their composition at high temperature as shown by TGA analysis (Fig-5).
2.3. Characterization of 1, 3-disulfoimidazoliumtransition metal chlorides [Dsim]2[X] and
[Dsim][X]
[Dsim]2[ZnCl4] 2a : Off white solid, 96 % yield; FT-IR (KBr): 3438, 1635, 1588, 1435, 1202,
1054, 884, 761 and 579 cm-1; 1H NMR (DMSO- d6, 400 MHz): 14.3 (s, 4H), 9.03(s, 2H), 7.64(s,
4H); 13C NMR (DMSO-d6, 100 MHz): 134.8, 119.8; CHN analysis (Mol .formula
C6H16Cl4N4O12S4Zn): calculated (%) C 10.73, H 2.40, N 8.34; Found C 10.66, H 2.36, N 8.22.
[Dsim][FeCl4] 2b: Yellow solid; 97 % yield; FT-IR (KBr): 3405, 1635, 1588, 1444, 1194, 1054,
982, 766 and 604 cm-1; CHN analysis (Mol .formula C3H8Cl4N2O6S2Fe) calculated (%) C 8.38,
H 1.88, N 6.52; Found C 8.31, H 1.80, N 6.48.
[Dsim]2[NiCl4] 2c: Yellow solid ; 96% yield; FT-IR (KBr): 3386, 1603, 1584,1448, 1177, 1050,
884, 762 and 584 cm-1; 1H NMR (DMSO-d6):14.22 (s, 4H), 9.00 (s, 2H), 7.61 (s, 4H); 13C NMR
(DMSO-d6, 100 MHz):134.7, 119.8, 40.02, 39.8, and 39.6 ; CHN analysis ( Mol .formula
C6H16Cl4N4O12S4Ni) : calculated (%) C 10.84, H 2.43, N 8.43; Found C 10.81, H 2.40, N 8.35.
2.4. Typical procedure for the synthesis of -amino carbonyl compounds (6)
A three component reaction of acetophenone (1 mmol), aromatic aldehyde (1 mmol) and
aromatic amine (1 mmol) in absolute ethanol ( 5 mL) was conducted in a 50 mL round bottom
flask at ambient temperature with continuous stirring in presence of 5 mol % of 1,3-
8

disulfoimidazolium chlorometallates [Dsim]2[X] or [Dsim][FeCl4] where X=[ZnCl4]2-, NiCl4]2-
respectively. The progress of the reaction was monitored on thin layer chromatographic plate
using a mixture of EtOAc and petroleum ether (1: 5) as solvent system. The solvent ethanol was
eliminated under reduced pressure in rotary evaporator after completion of the reaction. The
solid acidic catalyst was recovered from the dichloromethane solution (5 mL) by filtration which
was remaining as insoluble residue. The catalyst was reactivated after washing with CH2Cl2
solvent (5 mL) and then dried in vacuum oven at 80 °C for 2 hour for next run. The product was
precipitated out from the saturated solution of CH2Cl2 by dropwise addition of hexane with
continuous stirring at room temperature. The solid product was filtered and again recrystallized
from ethanol solution to generate analytically pure product.
3. Results and discussions
3.1. Characterization of the catalyst:
3.1.1. IR spectroscopy analyses:
FT-IR spectroscopic analyses of the three solid acids are represented in Fig. 1. Strong absorption
at 1177-1202, 1050-1054 and 579-604 cm-1 were assigned respectively for S-O symmetric and S-
O antisymmetric stretching and bending vibration of –SO3H groups. The presence of –C=N- and
-C=C- bonds of basic imidazole unit was confirmed from the stretching frequencies at 1584-
1588 cm-1 and 1603-1635 cm-1 respectively. The peak around 884-982 cm-1 expressed both N-S
stretching and in plane imidazole ring bending vibrations. The band at 761-766 cm-1 appeared
because of out of plane ring bending of C-H bond. The broad –OH peak of the three solids at
3386-3438 cm-1 expressed the involvement of two –SO3H groups in H-bonding [27-28]. The IR
spectra of 2b and 2c in the region 400-200 cm-1 were characterized by the distinct metal-chloride
9

vibration of [FeCl4]- at 380 cm-1 and around 285 cm-1 for [NiCl4]2- according to the reported
literature [29-30]. No characteristic absorptions were observed for 2a in this region.
3.1.2. Electronic spectra:
Electronic spectra in Fig.2 (a) shows different absorbance peaks for different halometallate
anions in disulfoimidazolium ionic liquids. Absorbance at 222 nm and 340 nm for 2a can be
assigned as intra-ligand charge transfer in 2a[31-32]. For 2b, peaks at 270 nm and 385 nm
originate from ligand to metal charge (L Fe (+3)) transfer transitions and the other absorption
-
band at 572 nm can be attributed for d-d characteristic transition of FeCl4 [33-35]. The
absorbance spectrum of 2c exhibits two bands of almost same intensity at 680 nm and 744 nm
for the tetrahedral moiety of NiCl42- as supported by the literature data [24]. Band at 261 nm can
also be assigned as L M charge transfer transition for tetrahedral complexes on the basis of
ligand field theory. Also the peak at 405 nm can be reasoned for the equilibrium mixture of a
2-
little mono and dichloro nickel species present along with the NiCl4 complex [36].
The confirmation of anionic species present in the three materials from the electronic spectra
also provided a chance to focus on the semiconducting properties of these materials by
calculation of optical band gaps using Tauc equation-1. With the symbols having their usual
meaning, the Tauc relation (equation-1) can estimate optical band gap of systems under
investigation [37].
αhʋ ∝ (hʋ- Eg)n
(equation-1)
Where, α is the absorption coefficient, hʋ is the photon energy and Eg is the optical band gap for
indirect transitions (n=2) or direct transitions (n=1/2). Assuming all transitions are direct, and
allowed type (n=1/2), the estimated Eg values from the intercepts of plot Fig.2(b) hʋ vs (αhʋ)2
10

are found to be 2.55, 2.41 and 2.30 eV for 2a, 2b and 2c respectively. Thus the three solid
compounds with significant band gaps similar to that of semiconductors firmly indicate their
importance.
1
3.1.3 H and 13C NMR analyses:
1
The four proton singlets at 14.0-14.3 ppm in H NMR spectra of 1, 3-disulfoimidazolium
chlorometallates 2a and 2c supported the presence of two –SO3H groups in imidazole unit. The
1
lower solubility of 2b in DMSO-d6 restricted to take the H and 13C NMR spectra of
13
[Dsim][FeCl4]. The characteristic signals of imidazole ring carbons were observed in the C
NMR spectra of 2a and 2c. The CHN elemental analyses also confirmed the formation of
disulfoimidazolium chlorometallates.
3.1.4 Elemental analyses:
The CHN analyses of these solids showed the percentage of C, H and N in the
disulfoimidazolium chlorometallates as included in the experimental section after treatment at
90°C in vacuum oven to remove the physisorbed water for 3 hour. The amount of metal content
in each of the solid was estimated from a 30 ppm solution in aqua regia using Inductive Coupled
Plasma (ICP) analysis. The measured values were comparable with the original metal content in
the respective ionic solid. (Table 1)
3.1.5. Surface morphology:
The SEM images of the three organochlorometallates in Fig.3 displayed some cracks and
deformities on the surfaces of the catalyst particles. Variations of structural arrangement were
observed in the solid state of 2a-c based on the nature of transition metal chlorides. The SEM
11

image (a) portrays a gloomy surface of 2a where particle visibility is not satisfactory. Image (b)
displays the existence of some solid particles even though the surface image is found to be
contorted. SEM image (c) demonstrates a surface looked like fused particles. These
discontinuities may be accounted to the different moisture sensitivity of the catalysts.
3.1.6. EDX analysis:
Energy dispersion X-ray (EDX) analysis was performed for the three solid acids which
confirmed the presence of respective metal chloride along with other constituent’s elements of
disulfonic imidazolium metal chlorides as shown in Fig.4.
3.1.7. TGA analysis:
The thermal stabilities of the three catalysts were investigated by thermo gravimetric analysis
(Fig.5).The TGA curve for [Dsim]2[ZnCl4] showed initial 12 % weight loss below 100°C which
can be assigned for physisorbed water followed by another decompositions at 250°C. The TGA
analysis of [Dsim][FeCl4] salt expressed total 25% weight loss up to 310°C including 14%
weight loss of absorbed moisture at 90°C along with two other breaks around 125°C and 310°C
respectively. For [Dsim]2[NiCl4], we observed three steps decomposition around 100 ,220°C and
400 °C with total weight loss of 45% which includes 25 % of moisture from the ionic salt.
Thermal decomposition of each material above 100 °C can be attributed for stepwise losing of
two –SO3H groups present in the imidazolium cation by interaction with chlorometallate anion.
Analysis of the curves also indicates the presence of certain amount of non-volatile residue
which may be accounted for metallic chlorides of the respective metals. Among the three metal
salts, this study displayed higher thermal stability of [Dsim]2[ZnCl4] up to 250°C.
3.1.8. Powder X-ray diffraction analysis:
12

Powder XRD pattern of 2a was displayed in Fig. 6. The characteristic XRD diffraction peaks of
ZnCl2 in [Dsim]2[ZnCl4] were observed at 2Ɵ = 16.6, 25.5, 29.4 and 34.9° and matched
satisfactorily with the XRD database (JCPDS card no. 74-0517) corresponding to (002)
,(101),(102) and (103) reflection planes respectively. The powder XRD analyses of
[Dsim][FeCl4] and [Dsim]2[NiCl4] didn’t produce the actual pattern which can be attributed for
the strong hygroscopic character of these two solids as observed from the TGA graph(Fig. 5 ).
3.1.9. Raman analysis:
In Fig.7, Raman spectra of 2a showed one strong peak at 283 cm-1 and one medium peak at 306
cm-1. These Raman shift values can be assigned to the mixture of major amount of [ZnCl4]2- and
minor amount of dimeric [Zn3Cl6]2- unit [38-39]. Till date there is no experimental evidence for
-1
-
the support of [ZnCl3] in either the solid or liquid state. A strong peak at 336 cm referred to the
literature value for tetrahedral [FeCl4]- species in 2b confirmed the presence of this
chlorometallate anion [40]. The Raman spectrum of 2c satisfactorily produced the characteristic
2-
peak at 261 cm-1 for NiCl4 [41].
3.2. Hammett acidity measurement:
The acidity of the acidic ionic salts was evaluated as Hammett acidity function Ho (Equation-2)
obtained on UV-visible spectrophotometer using 4-nitroaniline as basic indicator by Hammett
plots (Fig.8) [42-43]. The indicator [I] showed maximum absorbance at 378 nm without
protonation. As the absorbance of 4-nitroaniline decreases, the acidity of solid acid increases
because of higher concentration of low molar absorptivity protonated indicator [HI] in ethanol
solution. The ratio of [I]/[IH+] can be measured from the absorption differences and pK(I)aq
represents the pKa value of the indicator referred to an aqueous solution.
13

Ho= pK (I) aq + log [I]/ [IH+]
(Equation-2)
The procedure involved with the mixing of equal concentration of 4-nitroaniline (5mg/L, pKa =
0.99) and the solid acid (5mmol/L) in ethanol solution. The decreasing Brönsted acidity order of
the three solid acids against their observed Ho (table-2) values was as: [Dsim][FeCl4]
>[Dsim]2[NiCl4] > [Dsim]2[ZnCl4] and was further confirmed by the catalytic study for the one-
pot Mannich-type synthesis of -amino carbonyl compounds.
3.3. Leaching test:
The leaching test of the three salts was performed in absolute EtOH, H2O, CH2Cl2 and EtOAc
etc. 20 mg of each 2a, 2b and 2c was stirred separately in 5 mL solvent for 2.5 hour at room
temperature and then filtered to measure the pH of the filtrate. This study expressed neutral
character of EtOH, CH2Cl2 and EtOAc solution except in case of H2O (pH= 1-2).
3.4. Catalytic test:
3.4.1. Reaction condition optimization:
The catalytic efficiency of these three multi-functionalized acidic salts were investigated
for the three component Mannich-type synthesis of -amino ketones utilizing acetophenone (1
mmol), benzaldehyde (1 mmol) and aniline (1 mmol) in ethanol using 5, 10 and 20 mol % of
each catalyst at room temperature (table-3, entries 1-3 ). It was perceived that 5 mol % of the
each solid catalyst could be preferable to produce 85-94% yield of 6a within 45-80 min in EtOH
under mild condition. Thus, the acidity order of three catalysts as observed from the Hammett
study strictly followed the same catalytic activity order for the preparation of 6a in terms of
reaction time (table-3, entries 1-3).
The three catalysts expressed similar type of catalytic activity in dichloromethane and
ethyl acetate with 5 mol% at room temperature stirring (table-3, entries 4-5). From the solvent
study results, we selected ethanol as the suitable safer solvent to extend the synthesis of  -
14

amino carbonyl compound with variety of substituted aromatic aldehydes and substituted
anilines.
It was specially mentioned in literature that this one-pot reaction is sensitive to
temperature for further decomposition of -amino ketones to side products in presence of acidic
catalyst [44-46]. To avoid these problems, low temperature reactions are highly desirable for this
reaction and therefore we optimized the reaction condition at ambient temperature.
3.4.2. Substrate scope study and plausible mechanism of the Mannich-type reaction:
From the optimization study we found that the hygroscopic nature of 2b and 2c didn’t affect the
overall efficacy of the catalysts. As results, the three catalysts worked successfully under the
optimized conditions to generate library of -amino carbonyl derivatives from the three
component reaction of acetophenone with substituted aromatic aldehyde and different group
bearing aniline in ethanol (table-4). Most of the reactions were completed within the specific
reaction time for each catalyst irrespective of the nature of substituent present in both aromatic
aldehyde and aniline derivatives (table-4).
The possible mechanism of [Dsim][FeCl4] catalyzed three component synthesis of -amino
ketone derivative can be proposed through interaction of aldehyde functionality by –SO3H group
¯
of IL catalyst for nucleophilic attack with FeCl4 activated amino group of aniline according to
Scheme-3.
3.4.3. Comparison of catalytic efficiency with other ionic liquid systems:
The model reaction was extended for 3-methyl-1-sulfonic acid imidazolium transition metal
chlorides [Msim][X] (where X= [ZnCl3]- , [FeCl4]- ) using 5 mol % of the solid acid in ethanol at
ambient condition (table-5, entry-1)[13]. Disulfonic analogues 2a and 2b expressed better
catalytic efficiency than monosulfonic analogues (table-5, entry 4). The role of two –SO3H
groups attached to imidazolium cation in enhancement of the activity of three catalytic systems
(2a, 2b and 2c) was also clearly understood by performing the above synthesis with 5 mol % of
1-butyl-3-methylimidazolium chlorometallates [BMIm]n[X] (7a-7c) (m=2 and 1; [X] = [ZnCl4]2-
,[FeCl4]-,[NiCl4]2- ) at room temperature stirring to afford 65-75 % yields of 6a (table-5, entry 2)
15

[22-24]. Also the activity of 1, 3-disulfoimidazolium chloride and carboxylate ILs without metal
center [Dsim][X](8a-8c) ([X]= Cl-, CF3COO-, CCl3COO-) were observed for comparison with
the disulfonic chlorometallates 2a, 2b and 2c (table-5, entry 3) [25]. This table clearly
demonstrated the higher catalytic activity of the three chlorometallates under the optimized
condition for the preparation of model compound 6a (table-5, entry 4).
3.5. Reusability test of catalysts:
To determine the reusability of the three catalysts we performed the model reaction of
optimization study in 5 mmol scale in ethanol. The catalysts can be recovered as solid residue
after filtration of the product solution in CH2Cl2 which was suitable for further use. Fig. 9 depicts
the clear picture of reusability of each catalyst for the preparation of 6a in absolute ethanol for
three runs. An appreciable catalytic activity is noted for all the three catalysts. The spent
catalysts were again characterized by ICP-OES analysis (table-1) which presented the similar
amount of metal content to describe the catalytic activity up to three runs.
4. Conclusion
In our work, we have developed a new series of –SO3H functionalized imidazolium
2-
chlorometallate ionic salt systems: [Dsim][FeCl4] and [Dsim]2[X], where X= ZnCl4 and
2-
NiCl4 . The systems were fully characterized by various analytical techniques such as NMR,
FT-IR, Raman, UV-visible, PXRD, SEM-EDX, ICP-OES, TGA, CHN etc. Anions speciations of
the solid ionic systems were established via electronic spectroscopy and Raman analysis. All the
three systems were demonstrated as semiconducting material with required band gap energies.
Considering the enhanced acidity and the thermal stability of these systems, they were
satisfactorily employed as catalysts for the synthesis of - amino carbonyl compounds. It was
16

also relevant that we have recovered the catalysts and utilized them up to three consecutive
cycles. In these imidazolium chlorometallate systems, the presence of functionalized
imidazolium cation played a major role in determining the behavior of these solids in
conjunction to the chlorometallate anions. Both the cation and the anionic part of the systems
open the possibility of an easy and considerable tunability of these materials for future prospects.
Acknowledgement
The authors are thankful to Sophisticated Analytical Instrumentation Centre, Tezpur University
and Central Instrument Facility (CIF), IIT-Guwahati for analyses of various samples for this
work.
17

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Figure Captions
Fig. 1: FT-IR spectra of 1, 3- disulfoimidazolium chlorometallates
22

(a)
(b)
Fig. 2: (a) UV-visible absorbance spectra of 2a, 2b and 2c and (b) TAUC plot for the
determination of band gap of prepared chlorometallates
23

(a)
(b)
(c)
Fig. 3: SEM images of (a) [Dsim]2[ZnCl4], (b) [Dsim][FeCl4] and (c) [Dsim]2[NiCl4]
24

(a)
(b)
(c)
Fig. 4 EDX images of (a) [Dsim]2[ZnCl4], (b) [Dsim][FeCl4] and (c) [Dsim]2[NiCl4]
25

Fig. 5: TGA curves of the three ionic solid acids
26

Fig. 6: Powder XRD pattern of 2a
27

Fig. 7 : Raman spectra of 2a , 2b and 2c
28

Fig. 8: Absorption spectra of p-nitroaniline in the presence of ionic salts in ethanol
29