S.F.G. Gildeh, H. Roohi, M. Mehrdad et al.
Journal of Molecular Structure 1245 (2021) 131123
changes in cation, anion or in the side chain (linker) resulted into
two head groups linked by a rigid or flexible spacer and two
monoanions [19].
during microwave irradiant was about 100 °C. After cooling at
room temperature, the resulting mixture was washed with di-
ethyl ether (3 × 5 mL) to remove the unreacted starting materi-
Compared to their monocationic counterparts, DILs usually dis-
play higher thermal stability: their thermal decomposition temper-
atures range from 330 °C up to 400 °C, while they can be as low
as 145–185 °C for monocationic ILs. Also, the viscosity of DILs is
more than that of the usual ionic liquid and the viscosity of DILs
can be tuned by varying the length of the chain linking and type
of anions [32–36]. DILs can be used in applications concerning lu-
brification at high temperatures, as an agent for the extraction of
phenolic compounds from oil mixtures, as a surfactant, or as cata-
lysts for the esterification reaction [32,36–38].
als and thereby the salt [C H CH DBU]Cl was obtained as a brown
6 5 2
solid in 96% yield. To derive the desirable MILs, [C H CH DBU]Cl
6
5
2
(1.12 g, 4 mmol) and a slightly excess amount (0.72 g, 4.6 mmol)
of [Li][OTF] or (1.32 g, 4.6 mmol) of [Li][NTf ] were added to ace-
2
tonitrile (15 mL) in a canonical flask equipped with a magnetic
stirrer. The flask was sealed and stirred at room temperature for
5 h. Next, the solid phase was removed by centrifugation and the
liquid phase was evaporated at 50 °C under reduced pressure to re-
move acetonitrile and any other volatile impurities. The viscous IL
remained after evaporation of the solvent was collected and dried
at 80 °C for 5 h. By this method, the MILs [C H CH DBU][OTF] and
Despite the wealth of papers on based-DBU monocationic ionic
liquids and their applications [8–17], there are a few experimen-
tal and computational studies on the corresponding DILs to un-
derstand the Gibbs free interaction energies, (structural, topolog-
ical and electronic) properties and vibrational frequency spectra.
Also, these relationships with physicochemical properties of the
DILs [39].
6
5
2
[C H CH DBU][NTf ] were obtained in 91% and 93% yields, respec-
6
5
2
2
tively, and have been sufficiently pure for synthetic applications, as
determined by their 1H NMR spectra (Scheme 1).
2.3. Synthesis of DILs [p-C H (CH DBU) ][OTF] and
6
4
2
2
2
[p-C H (CH DBU) ][NTf ] from the salt [p-C H (CH DBU) ][Br]
2
6
4
2
2
2 2
6
4
2
2
In previous work [40], we synthesized a series of DBU-based
−
−
−
MILs ([Bn-DBU] [Y1–6], (Y1–6 = [CH CO ] , [C H SO ] , [HCO ] ,
A mixture of p-xylylene dibromide (1.32 g, 5 mmol) and DBU
(1.52 g, 10 mmol) was heated under microwave (360 W) for 2 pe-
riods of 1 min irradiation with a break time (of ~25 s., for tempera-
ture measurement) between them. The average temperature of the
reaction mixture during microwave irradiant was about 100 °C. The
crude product was washed with diethyl ether (3 × 5 mL) to give
[p-C H (CH DBU) ]Br as a white hygroscopic solid in 97% yield.
3
2
6
5
2
3
− − −
CF CO ] , [BF ] and [SCN] )) and evaluated their electrochem-
3 2 4
[
ical stability by cyclic voltammetry (CV) and theoretical com-
putation. Also, in other research work [41], we introduced [Bn-
DBU][CF CO ] MIL as an efficient catalyst for the synthesis of 1H-
3
2
pyrazolo[1,2-b]phthalazine-5,10-diones via the three-component
reaction of phthalhydrazide, aromatic aldehydes, and active a-
methylene nitriles.
6
4
2
2
2
Next, to derive the desirable DILs, [p-C H (CH DBU) ]Br (2.27 g, 4
6
4
2
2
2
In this work, the four new ([p-C H (CH DBU) ][NTf ] and
mmol) and a slight excess amount of [Li][OTF] (1.44 g, 9.2 mmol)
were added to acetonitrile (15 mL) in a canonical flask contain-
ing a magnetic stirring bar. The reaction mixture was stirred for
5 h at 50–60 °C. After this time, the solid phase was removed
by centrifugation and the supernatant liquid phase was evaporated
at 80 °C under reduced pressure to remove acetonitrile and any
possible volatile impurities. This method gave the DIL [p-C H (CH
6
4
2
2
2 2
[
p-C H (CH DBU) ][OTF] ) DILs and [C H −CH DBU][NTf ])
6
4
2
2
2
6
5
2
2
and ([C H −CH DBU][OTF]) MILs based on para-xylyl linked
6
5
2
bis-1,8-diazobicyclo [5.4.0] undec-7-ene dication and its corre-
sponding monocation containing the bis(trifluoromethanesulfonyl)
−
−
imide [NTf2]
Fig. 1) were synthesized and characterized by 1HNMR and
13C-NMR.
The thermal stability of [p-C H (CH DBU) ][NTf ] ,
and trifluoromethanesulfonate [OTF]
anions
(
6
4
2
6
4
2
2
2
2
DBU) ][OTF]2 as a white amorphous solid in 95% yield, (Scheme 2).
2
[
[
p-C H (CH DBU) ][OTF] ,
[C H −CH DBU][NTf ]
and
The structure of this compound has been confirmed by IR, H NMR
and C NMR spectroscopy.
6
4
2
2
2
6
5
2
2
C H −CH DBU][OTF] ILs and their decomposition process were
6
5
2
investigated. In addition, the energy minimum structures of the
To achieve the synthesis of [p-C H (CH DBU) ][NTf ] , a mix-
6
4
2
2
2 2
ILs were calculated at M06-2X/6-311++G(d,p) level of theory.
ture of [p-C H (CH DBU) ]Br2 white powders (2.27 g, 4 mmol)
6 4 2 2
and a slight excess amount of LiNTf2 (2.64 g, 9.2 mmol) were
added to acetonitrile (15 mL) in a canonical flask containing a mag-
netic stirring bar. The suspension was stirred at room temperature
for 5 h. After this time, the suspension was filtered off. The fil-
trate was washed with deionized water, evaporated under a re-
duced pressure at 50 °C, and dried in an oven at 80 °C for 5 h
2. Experimental methods
2
.1. Material and methods
Chemicals were purchased from Fluka, Merck, and Aldrich
chemical companies and used without further purification. FT-IR
spectra of the products were obtained in KBr wafers on a VERTEX
to obtain the white powder of [p-C H4 (CH2 DBU) ][NTf ] (Mp.
6 2 2 2
98°C) in 95% yield (Scheme 3).
7
0 Brucker (Germany) instrument. 1H NMR and 13C NMR spec-
The structures of these DILs have been confirmed by IR, H
NMR and C NMR spectroscopy. Based on the NMR spectra, the
DILs are reasonably pure for synthetic applications. The ener-
tra were recorded on a Bruker (DRX-400 Avance) spectrometer
at 400.22 and 100.62 MHz, respectively. The chemical shifts were
measured in ppm relative to the resonance of the deuterated sol-
vent and TMS. Thermogravimetric analyses (TGA) were performed
on a DSC-TG analyzer model Q 600 TA (USA). The thermal behavior
of the samples was scanned from 25 to 800 °C at the rate of 20 °C
getically optimal structures of the MILs [C H CH DBU][OTF] and
6
5
2
[C H CH DBU][NTf ] as well as the DILs [p-C H (CH DBU) ][OTF]
2
6
5
2
2
6
4
2
2
and [p-C H (CH DBU) ][NTf ] have been investigated at M06–
6
4
2
2
2 2
2X/6-311++G(d,p) level of theory in the gas phase using DFT cal-
−
1
min under air atmosphere.
culations.
2
.2. Synthesis of [C H CH DBU][OTF] and [C H CH DBU][NTf ] MILs
3. Computational details
6
5
2
6
5
2
2
from [C H CH DBU][CL] salt
6
5
2
Density functional theory (DFT) was employed for predicting
the geometrical structure, energetic and electronic properties and
characterization of the nature of intermolecular interactions be-
tween the cation and anion of the ionic liquids. The M06–2X
exchange-correlation functional with the 6-311G++(d,p) basis set
was used to perform the conformational analysis and geometry
For the synthesis of [C H CH DBU]Cl, a mixture of DBU (0.76 g,
6
5
2
5
mmol) and benzyl chloride (1.01 g, 8 mmol) was heated un-
der the microwave (360 W) for 2 periods of 1 min irradiation
with a break time (of ~25 s, for temperature measurement) be-
tween them. The average temperature of the reaction mixture
2