Effect of supramolecular polymeric aggregation in room temperature ionic liquids (RTILs) on catalytic activity in the synthesis of 4H-chromene derivatives and Knoevenagel condensation

Article abstract of DOI:10.1016/j.molliq.2020.114503

RTILs exhibit supramolecular self-assembled polymeric aggregation due to noncovalent interactions. The influence of the aggregation behaviour of RTILs on catalytic activity is evident but still poorly understood. The present work focuses on establishing a

Full text of DOI:10.1016/j.molliq.2020.114503

MOLLIQ-114503; No of Pages 12
Journal of Molecular Liquids xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Effect of supramolecular polymeric aggregation in room temperature
ionic liquids (RTILs) on catalytic activity in the synthesis of 4H-chromene
derivatives and Knoevenagel condensation
a
a
b
Shoaib Muhammad a, Firdous Imran Ali a, , Muhammad Naveed Javed , Agha Arsalan Wasim , Ahmed Bari ,
⁎
Faisal Rafique c, Muhammad Amjad Ilyas c, Kashif Riaz a,c, Syed Junaid Mahmood d,
Amir Ahmed d, Imran Ali Hashmi a,
⁎
a
Department of Chemistry, University of Karachi, Main University Road, Karachi 75270, Sindh, Pakistan
Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, P.O. Box 2457, Riyadh 11451, Saudi Arabia
Department of Applied Physics, University of Karachi, Main University Road, Karachi 75270, Sindh, Pakistan
PCSIR Labs Complex, Karachi 75280, Pakistan
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2020
Received in revised form 21 September 2020
Accepted 1 October 2020
Available online xxxx
RTILs exhibit supramolecular self-assembled polymeric aggregation due to noncovalent interactions. The influ-
ence of the aggregation behaviour of RTILs on catalytic activity is evident but still poorly understood. The present
work focuses on establishing a relationship between the role of supramolecular self-assembly of RTILs on catal-
ysis in organic reactions. Herein, we report four unreported, air and water stable, halide free, C2-symmetrical,
third generation, hydrophobic imidazolium-based room temperature ionic liquids (RTILs 1–4), their synthesis
and function as catalysts for the synthesis of 4H-chromene derivatives (terahydrobenzo[b]pyran) and
Knoevenagel condensation. The RTILs showed unprecedented solubility/miscibility behaviour. The RTILs showed
that 1,3-dihexylimidazolium 2-aminobenzoate [hhim][OAB] (RTIL-3) performed better in terms of yield and reac-
tion time. Supramolecular polymeric aggregation was explored by employing ESIMS, while the critical aggrega-
tion concentration (CAC) was calculated via recording electrical conductivity of RTILs in absolute ethanol. We
have also used rheometry experiments to explore forces governing the occurrence of aggregates in the liquid
phase of RTILs.
Keywords:
Supramolecular polymeric aggregation
Ionic liquids
4H-chromene
Knoevenagel condensation
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
[2,3]. The effect of noncovalent interactions (H-bonding, electrostatic
forces, π-π stacking, etc.) on the catalytic activity of various reactions in-
Self-assembly of molecules due to non-covalent (H-bonding, π-π
stacking ionic interactions, van der Waal's forces, etc.) generates supra-
molecular polymeric aggregates/clusters. The influence of supramolec-
ular polymeric self-assembly or material assembly (especially ionic
liquids) on catalysis is a neglected topic and is still in its infancy. Llansola
et al. observed remarkable differences in the catalytic activity of two dif-
ferent catalysts, out of which only one showed supramolecular poly-
meric aggregation [1]. Ionic liquids, especially imidazolium-based
ionic liquids, are considered self-assembled solvents that can accommo-
date guest organic molecules by creating cavities and hence provide an
excellent platform for catalysis. Not only the dramatic boost in the rate
of those reactions involving charge-separated intermediates or transi-
tion states observed due to imidazolium-based ionic liquids but also a
considerable influence on the reaction mechanism has been suggested
cluding, acetlylation reaction [4], esterification [5], Friedel-crafts reac-
tion, and other nucleophilic aromatic substitution reactions [6,7], the
multicomponent Biginelli reaction [8], the Baylis-Hillman reaction [9]
and N-formylation [10] etc. The role of supramolecular self-assembly
on the catalytic activity of RTILs in chemical reactions is not well
established or numeric. Hence, for precise applications of ILs as catalysts,
there is a need of a better understanding of the relationship between
this behaviour of RTILs and its effect on catalytic activity.
We previously reported the catalytic activity of hydrophilic ionic liq-
uids for C\\C bond forming reactions and explored the relationship of
supramolecular aggregation of imidazolium ionic liquids based on cata-
lytic activity [11,12]. In continuation of our studies, herein, we report
the catalytic activity of hydrophobic RTILs 1–4 on the synthesis of 4H-
chromene derivatives and Knoevenagel condensation. Despite a huge
number of published studies on chemical reactions in ILs, the use of hy-
drophobic ILs as catalysts, is rare. The catalytic activity of these new
RTILs for the synthesis of 4H-chromene derivatives (tetrahydrobenzo
⁎
Corresponding authors.
E-mail addresses: firdous@uok.edu.pk (F.I. Ali), iahashmi@uok.edu.pk (I.A. Hashmi).
https://doi.org/10.1016/j.molliq.2020.114503
0167-7322/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: S. Muhammad, F.I. Ali, M.N. Javed, et al., Effect of supramolecular polymeric aggregation in room temperature ionic
liquids (RTILs) on catalyti..., Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2020.114503

S. Muhammad, F.I. Ali, M.N. Javed et al.
Journal of Molecular Liquids xxx (xxxx) xxx
[b]pyran) and the C\\C bond forming reaction was investigated. The
4H-chromene was selected not only because it presents an important
structural motif and biologically significant class of compounds but
also because of the cascading C\\C bond forming reactions through
which it proceeds such as Knoevenagel condensation and Michael addi-
tion followed by heterocyclization [13]. Knoevenagel condensation is an
important C_C bond forming reaction that furnishes significant inter-
mediates for a number of value-added materials but needs an environ-
mentally benign protocol. The comparison of the present strategy is
compared with previously reported methods in Table 5. In addition,
the catalytic activity of RTILs 1–4 for the previously studied C\\C bond
forming reaction, Knoevenagel condensation, was also independently
demonstrated.
It has been reported that both ionic interactions and hydrogen bond-
ing can cause aggregations in ionic liquids [43]. Aggregates formed due
to hydrogen bonding tend to break easily and produce an effect that we
call shear thinning, a property possessed by non-Newtonian fluids [44].
Since our ILs are all Newtonian, we can easily exclude this hypothesis of
hydrogen bonding aggregation. Interionic interactions may also cause
aggregation via ion pairing, however, there are two possibilities. Either
the aggregate is overall neutral, or it is charged. It is reported that the
former would lead to non-Newtonian behaviour [45]. Aggregates
formed by ionic interactions possessing an overall charge are a prefera-
ble option that may be further supported by the evaluation of ionic
structural charges participating in ion pairing and the overall charge of
aggregates formed.
By definition, RTILs are assemblies of ions such as salts melting
below 100 °C [14]. These unique materials exhibit intrinsic physiochem-
ical properties, including thermal stability and low vapor pressure. Due
to these significant characteristics, these value-added materials have
been utilized in different applications, some of which are solvents and
catalysts in organic reactions [15–18], polymerization [19] and nano-
particle synthesis [20], separation methods for extraction of metal ions
[21–23], plant materials [24,25] etc. and as electrolytes in batteries,
fuel cells, dye sensitized solar cells [26,29,30], etc. and as thermal fluids
for heat storage devices [31]. Over the past decade, the use of ionic
liquids as catalysts and environmentally friendly solvent system as
conventional volatile organic solvents in organic reactions has been
well demonstrated [32,33]. The most commonly studied cations for
ionic liquids are immidazolium, ammonium, phosphonium, pyridinium,
piperidinium, pyrolidinium, etc. Due to the ease of synthesis, low viscos-
ity and chemical and thermal stability, imidazolium is the most studied
cation of ionic liquids [34–37].
Almost 40 years ago, John Wilkes et al. reported [38] chloride salts of
1,3-dialkyl imidazolium cations as first room temperature ionic liquids
(RTILs). The discovery was followed by second generation (air and
moisture stable RTILs) [39] and third generation (halide free) [40]
ionic liquids. The purpose of designing second-generation ILs was to
furnish an alternative medium for chemical reactions [41] considering
the non-volatile feature of ILs Another key fact is the hydrophobic
character, i.e., IL immiscibility with water can be attained by providing
suitable anions/cations. Hydrophobicity is introduced to avoid water
contamination.
The, relationship between supra-molecular aggregation and cata-
lytic performance in the chemical reactions of ionic liquids is hitherto
unknown. Herein, we have also demonstrated the effect of aggregation
behaviour of RTILs on the catalytic performance in the mentioned
chemical reactions of C\\C bond formation/heterocyclization.
2. Methods and materials
2.1. General
Oven dried glassware was utilized for all chemical reactions. All
chemicals were purchased from Sigma-Aldrich and verified via thin
layer chromatography (TLC) using precoated aluminium plates,
Kieselgel 60, F254, 0.25 mm, e. Merck, Darmstadt, Germany and FTIR.
To evaporate solvents, a rotary evaporator was used, maintaining
40 °C of the water bath. For visualization of chromatograms, iodine
spray or UV lamp (254 nm) was used. FTIR was recorded on a Shimadzu
spectrophotometer. Electron spray ionization mass Spectrometry (ESI-
MS) was performed on an AB SCIEX QSTA R XL LCMSMS Q-Tof
(pH 3.2, acetonitrile:water 1:1). 1H NMR and 13C NMR were obtained
from an AVANCE AV-500, and AV-125 BRUKER. Electrical conductivity
was measured on a Hanna (HI763100) conductivity meter. For melting
point measurement, Stuart Digital Melting Apparatus SMP 10 was used.
For rheology Brookfield DV-III rheometer with spindle 0 was used.
2.2. Synthesis of 1,3-dihexylimidazolium bromide ([hhim]+ [Br]−)
Therefore, our first objective was to regulate the thermal stability
and miscibility of RTILs by selecting appropriate ions. We selected imid-
azole to prepare 1,3-dihexyl imidazolium as a cation, while naturally oc-
curring substituted aromatic carboxylates were selected as anions for
two reasons: a) to avoid toxic halide and b) provide highly delocalized
bulky anions to introduce hydrophobic character and lower viscosity
(Fig. 1).
The behaviour and suitability of ILs for a number of applications de-
pends upon their molecular level structure. Electron spray ionization
mass spectrometry (ESI-MS) has been used to investigate supramolecu-
lar polymeric aggregation in newly prepared RTILs [41]. Furthermore,
measurement of electrical conductivities is used to calculate the critical
aggregation concentration (CAC) [42].
Then 29.4 mmol (1 equivalent) imidazole in 20 mL THF was added
dropwise in an ice cooled solution of 29.4 mmol (1 equivalent) of pow-
dered NaH in 20 mL THF. The imidazole solution was completely added,
followed by stirring of the mixture for two hours at room temperature.
First, 58.8 mmol (2 equivalents) 1-bromohexane was added dropwise,
followed by refluxing of the mixture for eight and half hours. The reac-
tion was monitored through TLC, and THF was evaporated under vac-
uum and [hhim]+ [Br]− is extracted using dichloromethane (DCM) as
a light brown waxy solid. Yield: 78% [46].
2.3. Synthesis of 1,3-dihexylimidazolium 2-hydroxybenzoate (RTIL-1)
A round bottomed flask containing 20 mmol o-hydroxybenzoic acid
(OHB) and 22 mmol Na2CO3 (sodium bicarbonate) in 12 mL water was
stirred at room temperature until neutralization was achieved (checked
via litmus paper). Then, 19 mmol of [hhim]+ [Br]− was added and stir-
ring and continued for up to 30 min [46]. RTIL-1 is extracted with CHCl3
(chloroform). The extracted RTIL-1 was dried and evaporated over a
week under vacuum in the presence of a cold trap to obtain pure RTIL-
1 as a waxy yellow and hydrophobic product. Yield: 90%.
IR νmax KBr cm−1: 3412.7 (phenolic OH), 3140.2, 3067.4 (aromatic
C\\H stretching), 2957.8, 2931.3, 2861.1 (aliphatic C\\H stretching due
to octyl chain hydrogen), 1629.8 (conjugated C_O), 1569.7 (C_N im-
idazole ring), 1464.2 (C_C imidazole ring), 1385.0 (C\\O), 1167.4
(C\\C).
Fig. 1. Charge distribution in ions.
2

S. Muhammad, F.I. Ali, M.N. Javed et al.
Journal of Molecular Liquids xxx (xxxx) xxx
1H NMR (DMSO-d6, 500 MHz) δ ppm: 9.31 (s, H-2, 1H), 7.82 (s, H-4
and H-5, 2H), 7.65 (dd, J = 8.0, 2.0 Hz, H-2’ 1H), 7.11 (dd, J = 8.0, 2.0 Hz,
H-4’ 1H), 6.59 (dd, J = 8.0, 2.0 Hz, H-3′, 1H), 6.57 (dd, J = 8.0, 2.0 Hz, H-
5′, 1H), 3.83 (d, J = 6.5 Hz, N-CH2, 4H), 1.79 (s, CH2- CH2N, 4H), 1.25
(broad singlet, CH2 x 3, 6H), 0.85 (s, terminal CH3 of alkyl chains at-
tached to imidazolium moiety, 6H).
chain), 30.9(CH2, C-2 of alkyl chain), 29.7 (CH2, C-3 of alkyl chain),
25.5 (CH2, C-4 of alkyl chain), 22.3 (CH2, C-5 of alkyl chain), 14.2 (termi-
nal CH3, C-6 of alkyl chain).
HR-ESI-MS + ve: m/z 237.2224 (C+, calcd. for C15H29N+2 m/z 237.
2325), HR-ESI-MS –ve: m/z 136.0374 (A− calcd. For C7H6NO2− m/z
136.0398); m/z 509.3118 [CA−2 , C29H41N4O4]−.
13C NMR (DMSO-d6, 125 MHz) δ ppm: 171.7 (-COO−), 163.5 (C-OH
aromatic), 136.5 (N-C=N imidazole ring), 131.7 (Anion C\\H, C-4′),
130.3 (Anion C\\H, 6′), 122.9 (C-1′), 120.9 (Imidazole C-4 and C-5),
116.2 and 116.1 (Anion C\\H, C-3′ and 5′), 49.3 (N-CH2, C-1 of alkyl
chain), 30.9 (CH2, C-2 of alkyl chain), 29.7 (CH2, C-3 of alkyl chain),
25.5 (CH2, C-4 of alkyl chain), 22.3 (CH2, C-5 of alkyl chain), 14.2 (Termi-
nal CH3, C-6 of alkyl chain).
2.6. Synthesis of 1,3-dihexylimidazolium 4-aminobenzoate (RTIL-4)
Sodium salt of p-aminobenzoic acid (PABA) was prepared, followed
by ion exchange with [hhim]+ [Br]− following the method described in
Section 2.3. A hydrophobic waxy dark brown product was obtained.
Yield: 90%.
HR-ESI-MS + ve: m/z 237.2224 (C+, calcd. for C15H29N+2 m/z
237.2325), m/z 321.3290 (C++Hexyl; C21H49N2+); m/z 153.1406
(C+ − Hexyl; C9H17N+2 ); HR-ESI-MS –ve: m/z 137.0167 (calcd. For
C7H5O−3 m/z 137.0238); m/z 93.0312 (C5HO2−, A− - CO2); m/z
511.2794 ([CA2]−, C29H39N2O−6 ), m/z 885.5353 ([C2A3]−, C51H73N4O−9 ).
IR νmax KBr cm−1: 3431.6, 3420.2 (Aromatic NH2), 3132.2, 3066.2
(aromatic C\\H stretching), 2957.5, 2930.6, 2858.3 (aliphatic C\\H
stretching due to octyl chain hydrogen), 1604.2 (Conjugated C_O),
1562.4 (C_N imidazole ring), 1463.1 (C_C imidazole ring), 1376.8
(C\\O), 1301.4 (C\\N), 1163.2 (C\\C).
1H NMR (DMSO-d6, 500 MHz) δ ppm:: 9.39 (s, N-CH-N, 1H), 7.64
(br. s, H-4 & H-5 imidazole ring, 2H), 7.19 (br. s, H-2′ & H-6′ anion,
4H), 6.91 (br. s, H-3′ & H-5′ anion, 4H), 4.17 (br. s, N-CH2, 4H), 1.69 (s,
CH2- CH2N, 4H), 11.26 (broad singlet, CH2 x 6, 12H), 0.85 (br. s, terminal
CH3, 6H).
2.4. Synthesis of 1,3-dihexylimidazolium 4-hydroxybenzoate (RTIL-2)
Sodium salt of p-hydroxybenzoic acid (PHB) was prepared, followed
by ion exchange with [hhim]+ [Br]− following the method described in
Section 2.3. A yellow-coloured waxy hydrophobic product was ob-
tained. Yield: 90%.
13C NMR (DMSO-d6, 125 MHz; δ in ppm): 122.92 (imidazole C-4 &
C-5), 49.29 (CH2, C-1 of alkyl chain), 46.44 (CH2, C-1 of alkyl chain),
31.13 (CH2, C-2 of alkyl chain), 30.99 (CH2, C-2 of alkyl chain), 29.71
(CH2, C-3 of alkyl chain), 26.04 (CH2, C-4 of alkyl chain), 25.58 (CH2,
C-4 of alkyl chain), 22.44 (CH2, C-5 of alkyl chain), 22.34 (CH2, C-5 of
alkyl chain), 14.30 (terminal CH3, C-6 of alkyl chain), 14.25 (terminal
CH3, C-6 of alkyl chain). Note: N-C=N imidazole ring and carbon-
based of anion could not be observed due to the exceptionally low sam-
ple quantity. Furthermore, the respective 1H NMR was conducted with
the same sample tube and observed according to the expected molecu-
lar structure.
IR νmax KBr cm−1: 3379.82 (phenolic OH), 3149.5, 3093.3 (aromatic
C\\H stretching), 2958.2, 2930.4, 2859.3 (aliphatic C\\H stretching due
to octyl chain hydrogen), 1611.6 (conjugated C_O), 1544.8 (C_N im-
idazole ring), 1425.9 (C_C imidazole ring), 1380.7 (C\\O), 1165.2
(C\\C).
1H NMR (DMSO-d6, 500 MHz) δ ppm: 16.5 (H-bonding), 9.29 (s, N-
CH-N, 1H), 7.82 & 7.62 (each s, H-4 & H-5 imidazole ring, 2H), 7.12 (s, H-
2′ & H-6′, 2H), 6.60 (s, H-3′ & H-5′, 2H), 4.17 (s, N-CH2, 4H), 1.79 (br. s,
CH2- CH2N, 4H), 1.68, 1.26 (broad singlet, CH2 x 6, 12H), 0.85 (s, termi-
nal CH3, 6H).
HR-ESI-MS + ve: m/z 237.2224 (C+, calcd. for C15H29N+2 m/z
237.2325), m/z 321.3255 (C++Hexyl; C21H49N2+); m/z 153.1406
(C+ − Hexyl; C9H17N+2 ); HR-ESI-MS –ve: m/z 136.0384 (A−calcd. For
C7H6NO−2 m/z 136.0398); m/z 509.3131 [C29H41N4O4]−.
13C NMR (DMSO-d6, 125 MHz) δ ppm: 163.4 (C-OH aromatic), 136.5
(N-C=N imidazole ring), 131.6 (Anion C\\H, C-2′ & C-6′), 130.3 (Anion
C\\H, C-1′), 122.9 (imidazole C-4 & C-5), 116.2 (Anion C\\H, C-3′ & C-
5′), 49.3 (N-CH2, C-1 of alkyl chain), 31.1 (CH2, C-2 of alkyl chain),
29.6 (CH2, C-3 of alkyl chain), 25.5(CH2, C-4 of alkyl chain), 22.3 (CH2,
C-5 of alkyl chain), 14.2 (tCH3, C-6 of alkyl chain).
2.7. Electrical measurement of RTILs
HR-ESI-MS + ve: m/z 237.2224 (C+, calcd. for C15H29N+2 m/z
237.2325), m/z 321.3255 (C++Hexyl; C21H49N2+); m/z 153.1261
(C+ − Hexyl; C9H17N+2 ); HR-ESI-MS –ve: m/z 137.0174 (calcd. For
C7H5O−3 m/z 137.0238); m/z 275.0570 (C14H11O6−, A−2 + H).
Electrical conductivities of the RTILs 1–4 were recoded on a Hanna
(probe model HI763100) conductivity meter at ambient temperature.
Each sample was prepared in ethanol with varying molar concentra-
tions. GLP Standard: 0 μS/cm, 12.88 mS/cm; Offset: 0.00 μS/cm, C.F.
(cm−1): 1.063; T. Coef (%/°C): 1.90; T. Ref (°C): 2; The results are listed
in Table S1 (Supporting information).
2.5. Synthesis of 1,3-dihexylimidazolium 2-aminobenzoate (RTIL-3)
Sodium salt of o-aminobenzoic acid (OAB) was prepared, followed
by ion exchange with [hhim]+ [Br]− following the method described
in Section 2.3. A hydrophobic waxy brown product (3) was obtained.
Yield: 90%.
2.8. Rheological studies of RTILs
The studies were performed using a Brookfield DV-III rheometer.
The spindle selected for measurements was Spindle 0. The selection of
spindles was based on the % torque limits of 10–100%. The prepared
ionic liquid was used as is, without any dilution, for the rheology
study. The volume of sample used was 16 mL as required by the UL
Adapter spindle. The sample was set on hold for 5 min to equilibrate be-
fore each run. For each run, the % torque, viscosity, shear stress, and
strain rate were recorded at different speeds (rpm). The data obtained
were used in rheological modeling.
Rheologicak data could be evaluated using several models to obtain
insight into the flow properties of the substance under study. Rheolog-
ical models can be categorized as empirical (e.g., power law model),
structural (e.g., Casson model), and theoretical (e.g., Newtonian)
models. These models can be divided into two further types viz. time-
independent and time-dependent models. In this study rheological
data were processed using the Newtonian, Bingham, Power law, and
IR νmax KBr cm−1: 3432.2, 3375.7 (NH2), 3138.1, 3073.6, (aromatic
C\\H stretching), 2957.9, 2927.5, 2857.4 (aliphatic C\\H stretching due
to octyl chain hydrogen), 1689.4 (conjugated C_O), 1566.6 (C_N im-
idazole ring), 1494.2 (C_C imidazole ring), 1297.3 (C\\N), 1161.1
(C\\C).
1H NMR (DMSO-d6, 500 MHz) δ ppm: 9.49 (s, N-CH-N, 1H), 7.83 (s,
H-4 & H-5, 2H), 7.71 (dd, J = 8.0, 1.5 Hz, H-2′, 1H), 6.90 (dd, J = 8.0,
1.5 Hz, H-4′, 1H), 6.49 (dd, J = 8.0, 1.5 Hz, H-3′, 1H), 6.34 (dd, J = 8.0,
1.5 Hz, H-5′, 1H), 4.18 (br. s, 4H), 1.78 (br. s, CH2- CH2N, 4H), 1.26
(broad singlet, CH2 x 4), 0.85 (br. s, 2 x terminal CH3, 6H).
13C NMR (DMSO-d6, 125 MHz) δ ppm: 172.2 (-COO−), 150.5 (C\\N
anion), 136.7 (N-C=N imidazole ring), 132.1 (Anion C\\H, C-4′), 129.7
(anion C\\H, 6′), 122.9 (imidazole C-4 and C-5), 121.8 (anion C\\H C-
1′), 115.4 & 114.0 (Anion C\\H, C-3′ and 5′), 49.2 (N-CH2, C-1 of alkyl
3

S. Muhammad, F.I. Ali, M.N. Javed et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Casson models, which are time-independent models. The equations
used for the models are as follows:
2.11. Recycling of RTILs after reaction
In Sections 2.9 and 2.10, after removal of the solid product, the water
and RTIL mixture was treated with ethyl acetate, resulting in three
layers, RTIL in the middle. After removal of both water and ethyl acetate,
the RTIL was again treated with ethyl acetate to remove unreacted or-
ganic reactants followed by drying.
Newtonian equation : τ ¼ ηD
´
Bingham equation : τ ¼ τ0 þ ηD
pffiffiffiffiffiffiffiffi
pffiffiffi pffiffiffiffiffi
Casson : τ ¼ τco þ ηcD
Power law : τ ¼ kDp
3. Results & discussion
where τ (Pa)is used to represent shear stress and D corresponds to
shear rate (s−1). For the Newtonian equation, the term η is the vis-
cosity (Pa.s). The terms τ0 and η in the Bingham equation are the
3.1. Synthesis and characterization of RTILs (1–4)
´
Using a two-step protocol by Dzyuba et al. [46], four C2-symmetrical
1,3-dihexylimidazolium-based RTILs 1–4 were prepared. Metathesis of
dihexylimidazolium with naturally occurring aromatic carboxylate an-
ions containing polar moieties such as hydroxyl or amino groups in
the C-2 or C-4 position makes them halide free, i.e., third generation
RTILs. Two equivalents of imidazole were quaternized using
hexylbromide in the presence of one equivalent of sodium hydride in
THF following the Dzyuba et al. [46] method. The next and crucial step
is the introduction of the desired anion. Sodium benzoates are obtained
through the reaction of substituted benzoic acids with aqueous sodium
bicarbonate solution. Upon mixing and stirring anion and cation solu-
tions at room temperature, ion exchange occurs within half an hour,
and sodium halide separates and RTILs result in excellent yields
(Scheme 1). The newly formed RTILs 1–4 showed very exceptional sol-
ubility behaviour. They are easily soluble, miscible with ethanol, meth-
anol, DMSO, chloroform and dichloromethane and insoluble in water,
ethyl acetate, n-hexane and diethyl ether. We utilized this property
for the separation of inorganic or organic impurities from mixtures
and to recycle RTILs.
yield stress and Bingham plastic viscosity, respectively. The term
τco is Casson yield stress and ηc is Casson plastic viscosity, where k
and p in the power law are the consistency index and flow index.
2.9. General method for 4H-chromene derivatives using Knoevenagel-
Michael-cyclocondensation
A mixture of dimedone (9 mmol), aromatic aldehyde (9 mmol),
malononitrile (9 mmol) and RTIL (2% mol, dried at 70 °C in vacuum
for 48 h before use) was refluxed in a suitable amount of ethanol. The
reaction was monitored through TLC hexane-ethyl acetate (8:2). Upon
completion, the reaction mixture was cooled to room temperature,
the solvent was evaporated under vacuum and the product precipitated
as a solid upon the addition of cold water. The precipitates were filtered
and washed with water. To obtain pure products (1a-1 h) recrystalliza-
tion was carried out using ethanol (95%). All compounds (1a-1 h) were
characterized through co-TLC with authentic sample and physical data
with those of previously documented literature [17,47,48]. To remove
unreacted organic material from ionic liquid, it was treated with ethyl
acetate.
We used 1H and 13C NMR spectroscopy, FTIR, and ESI-MS to charac-
terize of newly synthesized RTILs 1–4. There are distinct protons in the
1H NMR spectra of each RTIL. A singlet at δ 9.0–10.0 was attributed to H-
2 of imidazolium while two singlets (in sole cases both appear as one
singlet) in the aromatic region for H-4 and H-5. Likewise, anions exhibit
signals in aromatic regions. IL-2 shows a signal at δ 16.0 due to intramo-
lecular H-bonding later supported by ESI-MS displaying anion-anion ag-
gregation (A−2 ). In FTIR, a lower wavenumber medium intense signal at
approximately 1600–1690 cm−1 is suggestive of conjugated carbonyl
indicating the presence of carboxylate ions. Water is one of the impuri-
ties of even hydrophobic ionic liquids. The water was removed under
high vacuum for a period of one week. ESI-MS was used to determine
the mass of the ionic liquids, as described in Section 3.2.
2.10. General method for Knoevenagel cyclocondensation
Aromatic aldehyde (9 mmol) and malononitrile (9 mmol) were
taken in suitable amounts of ethanol and stirred at reflux. TLC after
5 min confirmed the completion of the reaction. Solvent evaporation
followed by precipitation with water yielded α-cyanoacrylates (2a-2j)
in excellent yields (Table S4, Supporting information). The isolated prod-
ucts were compared with authentic samples through TLC (hexane-ethyl
acetate) and comparison with physical data formerly reported in the
literature confirmed the product formation [11,12,47,48].
RTIL-1
RTIL-2
RTIL-3
RTIL-4
[hhim]+ [OHB]-
[hhim]+ [PHB]-
[hhim]+ [OAB]-
[hhim]+ [PAB]-
R1 = OH,
R1 = H,
R2 = H
R2 = OH
R2 = H
R1 = NH2,
R1 = H,
R2 = NH2
Scheme 1. Synthesis of RTILs 1–4
4

S. Muhammad, F.I. Ali, M.N. Javed et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 2. (a) ESIMS (negative) spectrum of RTIL-1. (b) ESIMS (negative) spectrum of RTIL-2. (c) ESIMS (negative) spectrum of RTIL-3. (d) ESIMS (negative) spectrum of RTIL-4.
Fig. 3. Conductivity at varying concentrations of RTILs 1–3.
5

S. Muhammad, F.I. Ali, M.N. Javed et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Table 1
anion O-H….O- interaction arrange ions in space to furnish supramolec-
ular anion-anion aggregation and reduce anion-anion electrostatic re-
pulsion [49]. Mata and Molins theoretically demonstrated the
formation of anionic dimers by oxoanions in the gaseous phase [50].
The anion-anion aggregation behaviour is observed only in p-
hydroxybenzoate. This change in behaviour also affects the catalytic ac-
tivity as described in the Section 3.6.
Conductivity, critical aggregation concentration (CAC) and degree of ionization (α) of
RTILs 1–3.
RTIL
Conductivity (μScm−1
)
CAC (mM)
α
1
2
3
6650
5400
4650
0.46
0.42
0.40
0.17
0.19
0.133
RTIL-3 and RTIL-4 display singly charged aggregates for [CA2]− at
m/z 509.3118 and m/z 509.3131, respectively. “A” corresponds to 2-
aminobenzoate (C7H6O2N−) and 4-aminobenzoate (C7H6O2N−),
while C = cation (Dihexylimidazolium; C15H29N+2 ).
3.2. ESI-MS and polymeric aggregation of RTILs (1–4)
On the other hand, ESI-MS positive not only exhibits a C+ peak in
each case with no cation-anion aggregation but also C-C6H13 at m/z
153 fragments (Figs. S1, S2, S3, vide Supporting information). The results
are suggestive of the effect of bulky symmetric cations on aggregate for-
mation. Formerly, we noted the anion-anion interaction of 1-hexyl-3-
methyl imidazolium [hmim]+ with the four benzoates (used in this
study as well) [11].
Four RTILs (1–4) were subjected to ESI-MS analysis to study their su-
pramolecular aggregation performance. RTIL-1, −3 and 4 exhibit
cation-anion supramolecular polymeric aggregates in ESIMS negative
mode (Fig. 2). On the other hand, the ESI-MS negative spectrum of
RTIL-2 displays anion-anion (Fig. 2b). The ESI-MS (negative) of RTIL-1
exhibits cation-anion aggregates with decreasing intensity at m/z
511.2794 corresponding to [CA2]-, where
C
=
cation
(Dihexylimidazolium; C15H29N2+) and A = anion (2-hydroxybenzoate;
C7H5O−3 ) and at m/z 885.5353 for [C2A3]−.
3.3. Conductivity measurments of RTILs 1–4
We did not observe cation-anion aggregation in RTIL-2 but a signif-
icant peak at m/z 275.0570 attributed to the [A2 + H]−, dianion of 4-
hydroxybenzoate, which is in agreement with previously reported liter-
ature on anion-anion interactions [49]. Braga, D., et al. suggest that inter
To further investigate aggregation behaviour and calculate the criti-
cal aggregation concentration (CAC) of RTILs, we recorded the electrical
conductivity of RTILs as a function of concentration [51]. Initially, we
Fig. 4. Model fitting of Rheological data for RTIL-1.
6

S. Muhammad, F.I. Ali, M.N. Javed et al.
Journal of Molecular Liquids xxx (xxxx) xxx
observed a linear increase in conductivity with increasing concentra-
tion. However, upon further increases concentration we witnessed de-
viations from linear behaviour due to increased viscosity that reducing
charge carrier mobility (Fig. 3). The reduction in mobility of charge car-
riers at higher concentrations again deviates and lowers conductivity,
supporting aggregate formation. The highest deviation exhibits the
“CAC” value demonstrated in Fig. 3 and Table 1. In addition, the degree
of ionization (α) of the aggregates was calculated according to method
reported by Wang et al. [51].
Fig. 3 demonstrates the non-linear variation of the conductivity ver-
sus concentration plot of RTILs 1–4. As cations are common among all
RTILs investigated in the present study, the variations are mainly attrib-
uted to the anions. RTIL-2 showed relatively lower conductivity values
and higher α, supporting anion-anion aggregation observed in negative
ESIMS (Fig. 2b).
temperatures. The obtained data for RTIL-1 were evaluated using rheo-
logical models and their plots are shown in Fig. 4. The plots for RTIL-3
and RTIL-2 are shown in Figs. 5 and 6 respectively. The model parame-
ters obtained for the synthesized ionic liquids are reported in Table S2
(Supporting information). It was observed that the correlation coeffi-
cients obtained for the selected models were quite high which inferred
that the models explained the rheological data quite well.
The shear stress varied proportionally with the shear rate. This be-
haviour is specific to Newtonian fluids. However, Newtonian plots
showed a nonzero intercept, indicating that the behaviour of the ionic
liquids can be explained using the Bingham model.
The Bingham model is used for fluids, which show high resistance to
initial flow. Once the flow starts, the sheer stress can be correlated to the
strain rate by either linear (Newtonian) or nolinear (non-Newtonian)
proportions. A measure of the resistance offered by a fluid to its initial
flow is termed the yield stress. The value is usually positive for most
of the substances, such as toothpastes, ketchups etc. Although high
values of the correlation coefficient were observed between shear stress
and the strain rate, the model produced negative values for yield stress,
τ0, as shown in Table S2 (Supporting information). The negative value of
yield stress is normally considered an unrealistic solution of a model in-
dicating ineptness of the model [53].
3.4. Rheology of RTILs
Ionic liquids have a tremendous scope in the field of catalysis. They
can also be used as solvents and are aimed at replacing traditional or-
ganic solvents in the field of separation sciences and synthesis [52].
The study of the rheological properties of ionic liquids is essential for
their effective utilization. The ionic liquids synthesized (RTIL 1–3) in
this study were subjected to rheological experiments at different
The Casson model showed high values for the correlation coefficient
indicating a good fit between the experimental data and model
Fig. 5. Model fitting for Rheological data for RTIL-2.
7

S. Muhammad, F.I. Ali, M.N. Javed et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 6. Model fitting for Rheological data for RTIL-3.
assumptions. The Casson yield stress, τco, values obtained were positive
and were negligible as compared to yield stress values for common
products shown in Table S3 (Supporting information). The Casson
plastic viscosity values obtained for ionic liquids were quite close to
the average viscosities determined experimentally.
The power law model is an empirical model with the capability to
differentiate Newtonian fluids from non-Newtonian fluids. The model
parameter p, the flow index, is a dimensionless parameter that classifies
a fluid as Newtonian if p = 1. A fluid is said to exhibit shear thinning be-
haviour if p < 1 and is classified as pseudoplastic, whereas, for p > 1 the
fluid is termed as dilatant, which shows shear thickening behaviour. The
Fig. 7. Effect of temperature on ionic liquid viscosity.
Fig. 8. Arrhenius plots of ionic liquids for activation energy of flow.
8

S. Muhammad, F.I. Ali, M.N. Javed et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 9. TGA data of RTILs 1–4.
Scheme 2. Synthesis of 4H-chromene-3‑carbonitrile using RTILs
values of p obtained for ionic liquids under study ranged from 0.98 to
1.20. The values were quite close to 1 and lead to the conclusion that
the ionic liquids possessed flow properties similar to those of the New-
tonian fluids.
Figs. 4, 5 and 6 show that the slope of the shear stress vs strain rate
plots decreased with an increasing temperature. Fig. 7 shows a plot of
Casson plastic viscosity obtained for RTIL-1, RTIL-3, and RTIL-2 against
temperature. The plot shows a nonlinear inverse relationship between
viscosity and temperature.
A plot of the logarithm of viscosity against the temperature inverse
can be used for the determination of activation energy for Newtonian
fluids with constant viscosity. The ionic liquids under study were
found to be Newtonian; hence, the Arrhenius equation was applied on
to the experimental data to obtain the activation energy for the flow.
The average experimental viscosity was used to construct the plots, as
shown in Fig. 8. The activation energy determined from the slope of
the Arrhenius plot was 43.5 kJ mol−1 for RTIL-1, 40.0 kJ mol−1 for
RTIL-3, and 38.5 kJ mol−1 for RTIL-2. The activation energy for water
flow within the range of working temperature range (303–353 K) is
14.9 kJ mol−1 [54], which is significantly lower than that of the ionic liq-
uids under study.
Ea 1
ln η ¼ B þ
R T
Table 2
Optimization of reaction conditions for the synthesis of 4H-chromene-3-carbonitrilea.
Entry
RTIL
Solvent
% RTIL
Yield%b
Time (min)
1
2
3
4
5
6
7
8
1
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
2
2
4
6
8
2
4
6
8
2
4
6
8
2
4
6
8
40.48
69.34
72.89
72.96
74.11
61.58
68.11
68.39
69.01
77.65
83.06
83.45
84.91
76.39
82.06
83.79
85.82
300
60
45
45
40
120
120
100
90
30
30
30
30
30
30
30
25
9
10
11
12
13
14
15
16
17
a
2-amino-7,7-dimethyl-4-(4-nitrophenyl)-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-
carbonitrile. Reaction Conditions: 4-nitrobenzaldehyde(9mmol), malononitrile(9mmol),
dimedone (9 mmol), RTILs in ethanol.
Fig. 10. Optimization of reaction conditions for the synthesis of 4H-chromene-
3‑carbonitrile.
b
Isolated yield.
9

S. Muhammad, F.I. Ali, M.N. Javed et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Scheme 3. Synthesis of α-cyanoacrylates using RTILs
3.5. Thermal stability of RTILs 1–4
type and orientation of substituents on the benzoate anion on the ther-
mal stability of RTILs.
The thermal stabilities of RTILs 1–4 were examined through thermo-
gravimetric analysis (TGA). For characterization of RTILs, Tonset was re-
corded. Tonset was set from 200 to 400 °C. The degradation
temperatures were found to be 295 °C, 230 °C, 260 °C and 290 °C for
RTIL1–4, respectively (Fig. 9). This shows the significant role of the
3.6. Catalytic activity of RTILs for synthesis of 4H-chromene carbonitrile
Initially, we carried out the reaction at room temperature using RTIL-
1 (2%) in water at room temperature without any base (Scheme 2).
Table 3
Substrate scope of Catalyst (RTIL-3) using optimal conditions.
and
10

S. Muhammad, F.I. Ali, M.N. Javed et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Table 4
4. Conclusion
Correlation of supramolecular polymeric aggregation in RTIL 1–4 and catalytic activity in
Knoevenagel condensation and 4H-chromene-3-carbonitrile synthesis.
Noncovalent interactions, among molecules or ions result in supra-
molecular self-assemblies. Hydrogen bonding aggregation and neutral
interionic aggregation are not compatible with our findings; we may
opt for charged ionic interactions for aggregation.
IL
% Abundance of
negatively charged
aggregates
Catalytic activity
Knoevenagel
condensation
4H-Chromene
synthesis
[A2 + H]− CA2− C2A−3 Time (min) % Yield Time (hr) % Yield
It was concluded from the results that the type of cation and anion
influences the extent of supramolecular polymeric aggregates, which
can be used to determine the catalytic performance of an ionic liquid
in organic reactions. Rheological experiments revealed that RTILs 1–4
are Newtonian and aggregates were formed due to electrostatic interac-
tions instead of H-bonding. In addition, the RTIL was recycled five times
without a significant drop in catalytic activity and change in chemical
structure of RTILs. Recyclability reduces cost for these industrially im-
portant reactions and makes it effective for large-scale industrial use.
Moreover, 1,3-dialkylimidazolium-based ionic liquids are already
being sold in 1 to 5 kg of packaging, while reagents to make the anions
are easily available. Ion pairing (self-assembled aggregates) has a direct
influence on the rate and mechanism of organic reactions. Previously,
the high reaction rate in C\\C bond formation reactions (Knoevenagel
condensation, Robbinson annulation) was attributed to imidazolidene
carbenes due to the use of inorganic bases as catalysts. We have ob-
served that no inorganic base is required, as RTILs can serve as base cat-
alysts. The use of ILs as reaction media or catalysts is now very common,
but the use of hydrophobic ILs as catalysts for organic reactions is rare.
RTIL-1
0
25
0
5
15
0
0
1
1
1
1
90.31
88.76
96.37
93.92
5
6
4
4
72.89
68.11
83.06
82.06
RTIL-2 10
RTIL-3
RTIL-4
0
0
12.5
0
Despite the hydrophobicity of ILs, water was used due to phase transfer
catalytic behaviour of RTILs. No reaction was detected after 24 h. There-
fore, we subjected the reaction in ethanol on reflux but only 40.48% of
the product was obtained after 5 h (Entry-1; Table 2). The refluxed reac-
tion mixture (in ethanol) yielded a better product within 30 min. Upon
increasing the % concentration of RTIL, an increase in the % yield was ob-
served. The performance of RTILs 2–4 was then explored (Fig. 10). RTIL-
2 took longer times, and reduced yields were obtained. RTIL-3,
([hhim]+ [OAB]−), appeared to be an excellent catalyst for the synthe-
sis of 4H-chromene-3‑carbonitrile among all tested RTILs (1–4).
To compare the effect of the catalytic activity of hydrophobic RTILs
with previous results [1], we probed Knoevenagel condensation
(Scheme 3). Similarly, instead of water, ethanol proved to be a better
solvent, and RTIL-3 was verified to be the best catalyst (Table S4,
Supporting Information). These results suggest the role of the anion
and supramolecular polymeric aggregation of RTIL in catalytic activity.
The scope of different aromatic aldehydes using optimal conditions
was investigated. Reactions with different substituted aromatic alde-
hydes resulted in moderate to high yields in both in 4H-chromene de-
rivatives and Knoevenagel condensation (Table 3).
CRediT authorship contribution statement
IA Hashmi, and FI Ali conceived the present idea, designed and di-
rected the project. S Muhammad, MN Javed, carried out synthesis and
purification. AA Waseem and M.Amir performed Rheological data. S
Muhammad, F Rafique, K. Riyaz and A Ilyas measured and analyzed con-
ductivity experiments. SG Musharraf and A. Bari performed spectros-
copy. S. Muhammad, MN Javed, SG Musharraf, A. Bari and FI ALI
performed characterization. SJ Mehmood and MN Javed recoded TGA
data. All Authors equally contributed to interpretation of the results, dis-
cussions and writing the manuscript.
3.7. Correlation of supramolecular polymeric aggregation and catalytic
activity
A remarkable relationship between supramolecular polymeric ag-
gregation of RTILs and catalytic activity was observed and found to be
in agreement with previous observations (Table 4) [1]. Earlier, two
types of ionic interactions were observed in all examined ILs,
i.e., cation-anion and anion-anion interactions and a decrease in cata-
lytic activity were observed with increased anion-anion interactions.
Table 1 clearly indicats that the only RTIL-2 exhibiting anion-anion in-
teractions shows comparatively poor activity. On the other hand,
RTIL-3 with the least supramolecular polymeric aggregation proves to
be an excellent catalyst for both reactions studied.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgment
The authors extend their appreciation to the Deanship of scientific
research at King Saud University for funding this work through research
group no RG-1440-009.
3.8. Catalyst recyclability
Despite the cost of RTILs, an enormous benefit of these unique mate-
rials is their recyclability, especially in the case of hydrophobic RTILs
that can easily be purified by treatment with water to remove inorganic
byproducts. After every run, RTIL-3 was recycled and reused in the sub-
sequent reactions four times without losing its efficiency and activity.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2020.114503.
Table 5
Catalytic evaluation of present study with some reported procedures for the reaction of dimedone, malononitrile and benzaldehyde.
Sr no.
Catalyst (mol/g)
Condition
Time (min)
Yield (%)
Ref
1
2
3
4
5
Ba(OTf)2
[cmmim]Br
Na2SeO4
N-Methylimidazole
RTIL-3
PEG-Water (50–50), rt
Solvent free, 115 °C
H2O/ EtOH (1:1), Reflux
H2O, rt
30
10
180
13
96
93
90
80
[13]
[17]
[17]
[17]
Ethanol, Reflux
30
86.93
Our work
11

S. Muhammad, F.I. Ali, M.N. Javed et al.
Journal of Molecular Liquids xxx (xxxx) xxx
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