M.S. Mirak-Mahaleh, K. Rad-Moghadam / Journal of Molecular Liquids 307 (2020) 112989
3
DMSO-d6): δH 4.52 (1H, s, 4-H), 7.24 (2H, d, J 8.4 Hz 2′- and 6′-H), 7.30
(1H, dt, J 8.0 and 1.2 Hz, 9-H), 7.32 (2H, s, NH2), 7.34 (1H, d, J 8.4 Hz, 7-
H), 7.35 (2H, d, J 8.4 Hz, 3′-H and 5′-H), 7.59 (1H, dt, J 7.8 and 1.2 Hz, 8-
H), 7.91 (1H, dd, J 8.0 and 1.2 Hz, 10-H), 11.80 (1H, s, N\\H).
2-Amino-4-(4-nitrophenyl)-5-oxo-5,6-dihydro-4H-pyrano[3,2-c]
quinoline-3-carbonitrile 8e:
of TMG.HCl and Et3N.HCl, i.e. the salts with the moderate melting
point temperatures of 210 °C and 255 °C, respectively. Certainly, the
melting point depression of these simple salts is related to the stabiliza-
tion of Cl− by extensive H-bonding with bisulfate ions.
The FT-IR spectrum of the pure CS indicates a strong broad band
peaking at 3377 cm−1 and a subsidiary broad shoulder at around
3190 cm−1 corresponding to O\\H and N\\H stretching vibrations.
The split of these vibrations into two distinguishable bands suggests
that certain, but different, H-bonds are involved in defining the struc-
ture of the CS. Interesting to notice are the bands (1191, 1058, and
879 cm−1) appeared in the S_O stretching vibrations region [36]. The
presence of these three bands indicates that the sulfate ions in the CS
are no longer isolated ions but exist with less symmetry due to strong
H-bonding with other species in the salt. In addition, the isolated
SO24− belongs to the higher symmetry point group (Td), and only one vi-
brational band is expected for this symmetric ion in the S_O stretching
region [37,38]. The deformation vibrations of SO24− ions resulted in a
IR (KBr) υmax (cm−1): 1348 and 1514 (NO2), 1387, 1632, 1674, 2202
(C`N), 3335 and 3406 (N\\H). 1H NMR (400.13 MHz, DMSO-d6): δH
4.69 (1H, s, 4-H), 7.31 (1H, t, J 7.6 Hz, 9-H), 7.34 (1H, d, J 9.2 Hz, 7-H),
7.42 (2H, s, NH2), 7.51 (2H, d, J 8.8 Hz, 2′- and 6′-H), 7.60 (1H, dt, J 7.8
and 1.2 Hz, 8-H), 7.93 (1H, d, J 8.0 Hz, 10-H), 8.17 (1H, d, J 8.8 Hz, 3′-H
and 5′-H), 11.84 (1H, s, N H).
2-Amino-4-(4-methoxyphenyl)-5-oxo-5,6-dihydro-4H-pyrano[3,2-
c]quinoline-3-carbonitrile 8f: IR (KBr) υmax (cm−1): 1379 (C\\O pyran),
1632 (N\\H bend), 1676 (C_O), 2183 (C`N), 3163, 3246, 3283, 3337
(N\\H). 1H NMR (400.13 MHz, DMSO-d6): δH 3.71 (3H, s, OCH3), 4.44
(1H, s, 4-H), 6.84 (2H, d, J 8.8 Hz, 3′-H and 5′-H), 7.12 (2H, d, J 8.8 Hz,
2′-H and 6′-H), 7.23 (2H, s, NH2), 7.29 (1H, dt, J 7.6 and 1.2 Hz, 9-H),
7.33 (1H, d, J 8.0 Hz, 7-H), 7.57 (1H, dt, J 7.8 and 1.6 Hz, 8-H), 7.90
(1H, dd, J 8.0 and 0.8 Hz, 10-H), 11.75 (1H, s, N\\H).
moderate band at 595 cm−1 flanked by a small shoulder at 612 cm−1
.
Other characteristic bands in the spectrum can be attributed to the
C\\N stretching (1120 cm−1), CH3 deformation (1470 cm−1), N\\H
bending (1613 cm−1), C_N stretching (1648 cm−1), and the aliphatic
C\\H stretching (2940 and 2973 cm−1) vibrations. The 1H NMR spec-
trum of the CS in DMSO-d6 exhibited a broad singlet peak in the down-
field end at δ 9.82 due to the proton attached to triethylamine along
with a singlet peak at δ 7.88, which is readily conceived as arising
from the two equivalent protons of C_NH2 group in the protonated
TMG. Due to similar couplings with the adjacent N\\H and CH3 protons
(J 6.8 Hz), the enantiotopic CH2 protons in Et3N-H appeared as a pentet
peak at δ 3.08. It is worth noting that the four methyl groups of the TMG-
H cation in the CS resonated as a singlet peak with the chemical shift (δH
2.91) close to that of pristine TMG (δH 2.73), indicating that neither of
the two methylated nitrogen atoms are protonated. The preferred pro-
tonation of TMG at its sp2-hybridized nitrogen atom allows the π-
electrons and consequently the generated positive charge to be
delocalized over the entire molecule. On the other hand, the proton-
ation of TMG at either of the methylated nitrogen atoms would inter-
rupt the resonance of its π-system. Therefore, TMG-H is too weak to
neutralize the remaining acidic species of the CS. In other words, the
HSO−4 ions, instead of participating in the neutralization reactions,
take part as structural units in a network of H-bonds by trapping the
HCl molecule of the CS within their acidic H-bonds. These acids resulted
in a broad peak with the integral of 3H at δH 5.94 ppm, which is a con-
vincing evidence to confirm the structure of the CS. Because of its struc-
tural features, the CS is a solid acid as its 0.01 M solution in water has the
pH = 2.
3. Results and discussion
Although room-temperature ILs are ideal for most applications, han-
dling and storage of these compounds, particularly their acidic variants,
are tedious. Thus, developing solid salts with amphipathic character
should be emphasized. These salts, despite their solid state above
100 °C, melt easily at lower temperatures when are used in organic re-
actions, since a considerable measure of their fusion enthalpy is pro-
vided by solvating the organic substrates of reaction mixtures. Our
approach to prepare a new amphipathic complex salt (CS) is outlined
in Scheme 1. The acid-base reaction between triethylamine and two
equivalents of chlorosulfonic acid gave the IL triethylammonium
chlorosulfonate 1. Addition of this water-sensitive IL to an equimolar
amount of 1,1,3,3-tetramethylguanidine in CH2Cl2 under moist air led
to the formation of the complex salt [TMG-H][Et3N-H][(HCl)(HSO4)2],
which melts at 118 °C. The stoichiometric contents of one chloride ion
and two bisulfate ions in the empirical formula of this salt were con-
firmed through separate titration measurements of the ions with Ag+
and Ba2+ [35]. Dissolving a certain amount of the salt into aqueous
BaCl2 solution results in precipitating two equivalents of BaSO4, imply-
ing that TMG has not been sulfonated by 1 and the sulfur content of
the salt exists as bisulfate anion. Interestingly, the CS contains the ions
The thermal behavior of the CS was studied by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) in the tem-
peratures between 25 and 600 °C (Fig. 2). The TG curve of the CS exhib-
ited an initial slight mass loss of ~ 6% up to 78 °C due to the evaporation
of the moisture adsorbed by the salt. After this preliminary stage, the
curve becomes almost a plateau up to 187 °C and then smoothly
Scheme 1. The reaction steps for synthesis of the CS, [TMG-H][TEA-H][(HCl)(HSO4)2]
Fig. 2. The TGA and the DSC thermograms of the CS.