Possibility of Protic Ionic Liquids Formation From Triethanolamine with

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Possibility of Protic Ionic Liquids Formation from Triethanolamine with Sulfonamides Matvey Sergeevich Gruzdev, Michael A. Krestyaninov, Evgeniy N. Krylov, Liudmila E. Shmukler, and Lyubov P. Safonova J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b02981 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Journal of Physical Chemistry

Possibility of Protic Ionic Liquids Formation From Triethanolamine with Sulfonamides

Gruzdev M.S.a*, Кrestyaninov М.А.a, Krylov Е.N.b, Shmukler L.E.a, Safonova L.P.a

a

G.A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences

Akademicheskaya St.1, Ivanovo, 153045 Russia b

Ivanovo State University, Yermak St. 39, Ivanovo, 153025 Russia

*[email protected]

*Corresponding Author Address: G.A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences Akademicheskaya St. 1, Ivanovo, 153045 Russia E-mail: [email protected]

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ABSTRACT. In this work, we have studied the products of interaction of triethanolamine with sulfonamides (4-chloro- and 4-nitrobenzenesulfonamide) and with bis(trifluoromethanesulfonyl)amide to show that it is possible to form protic ionic liquids. By using the hybrid functional B3LYP, we have made quantum-chemical calculations of the structure and energy of the formed compounds, and as part of the Natural bond orbital analysis, we have calculated the hydrogen bonds parameters. The structures of the obtained compounds have been confirmed by IR and NMR spectroscopy. Based on the obtained data, we have made a conclusion that triethanolamine with 4-chloro- and 4-nitrobenzenesulfonamides forms hydrogen-bonded complexes, whereas with bis(trifluoromethanesulfonyl)amide – a salt. We have determined the thermal characteristics of all of the obtained compounds, and for bis(trifluoromethanesulfonyl)imide tris(2-hydroxyethyl)ammonium salt – the electric conductivity as well.


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Introduction Protic ionic liquids (PILs) formed via a proton transfer from Bronsted acid to Bronsted base are a subclass of ionic liquids. The feature distinguishing the PILs from aprotic ionic liquids is the presence of an “active” proton. The protonic feature of PILs is a fundamental characteristic that has been explored in a number of areas such as biological applications1, organic synthesis2 and chromatography.3 In addition, protic ionic liquids must be considered of interest in electrochemical studies and in electrochemical devices.4 Protic ionic liquids are also used for designing drugs and optimizing pharmaceutical protocols.5 Protic ionic liquids are normally synthesized through a stoichiometric neutralization reaction of Bronsted acid with a Bronsted base. In this process the acid donates the proton to the base to form a salt, whereas the possibility of a reverse proton transfer is negligibly small. However, the degree of proton transfer can vary depending on the acid and base strength, and the proton can participate in the formation of a hydrogen bond between the cation and the anion. This will produce either a molecular hydrogen-bonded complex or an ion pair. It should be mentioned that the distinguishing feature of protic ionic liquids is their ability to form strong hydrogen bonds between the cation and anion, which leads to the formation of hydrogen-bonded ion pairs (Z-bonds).6 A wellknown criterion of PILs formation is the ∆pKa value (the difference between the pKa values of the base and the acid in water) that, according to,7 must be higher than 10. At the same time, it has been shown in8 that a complete proton transfer occurs at ∆pKa = 4. If ∆pKa is about 1, ionic particles are not formed. The properties and applications of PILs have been comprehensively overviewed in a number of publications.9,10 The most frequently used cations for obtaining PILs are ammonium and imidazole ions. As described in several papers11-13 it is possible to determine PILs ionicity by spectral methods such as FT-IR and NMR instead of using ∆pKa. The degree of proton transfer at PILs formation was determined in works14-17 by the quantumchemical calculations. Many of the PILs had a nanostructure resulting from the segregation of the polar and nonpolar components of the ionic liquid. It was found that this segregation was enhanced for longer alkyl chains, with a corresponding increase in the length scale, whereas the presence of hydroxyl groups on the alkyl chains produced much less ordered liquids.18 The structural properties such as density, radial distribution functions, spatial distribution functions and structure factors and hydrogen bond dynamics of protic ionic liquids, 2-hydroxyethylammonium acetate, ethylammonium hydroxyacetate and 2hydroxyethylammonium hydroxyacetate at ambient conditions have been calculated using classical molecular dynamics simulations.16 Information about the physicochemical properties of PILs based on ammonium salts can be also found in works.19-23 The works of Verkade et al.24 in which they attempted to obtain salts with a tricyclic atrane structure became the basis for creating salts with a closed intramolecular hydrogen bond. Ionic compounds with a common formula [N+(CH2CH2OH)3]A- formed by quaternization of triethanolamine (TEOA) with inorganic or

carboxylic

acids

were

named

protatranes.

It

was

shown25

that

the

interaction

of

(tris(2)hydroxyethyl)amine) triethanolamine with carboxylic biologically active acids gives salts with different melting points but capable of remaining in a molten metastable state for a long time. To obtain such salts, Voronkov et al.25,26 applied a method based on TEOA interaction with the corresponding ammonium salts NH4X (Х = Cl-, Br-, I-, NO3-, ClO4-) in aqueous and nonaqueous solvents. This method ACS Paragon Plus Environment

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excludes the use of volatile corrosive acids and direct protonation. The synthesis of aroxyprotatranes (salts of triethanolamine with phenol, 2-, 2,4-di-, 2,4,6- trinitrophenols) for obtaining new potentially biologically active compounds was described.27 All the obtained compounds27 are well soluble in water and their 0.1 N aqueous solutions have a high electric conductivity. Studies of the crystal structure of triethanolammonium salts with different acids28,29 has shown that in most cases, three hydrogen atoms of three hydroxyl groups of the cation surround the hydrogen atom of the ammonium group forming weak hydrogen bonds with it. Physicochemical properties such as thermal behavior, ionic conductivity, viscosity and density, and acidbase properties of 2-hydroxyethylammonium-based protic ionic liquids (PILs) were investigated.30 With regard to [(EtOH)nEt(3–n)NH+][TFS–] (n = 0 – 3, TFS, trifluoromethanesulfonate), the ion-ion interactions between cation-anion and cation-cation were enhanced with increasing of the 2-hydroxyethyl groups. The structure–property relationship of 15 PILs that are primarily composed of alkanolammonium cations and organic acid anions was studied.31 The inﬂuence of both the nature and number of alkanol substituents on the cation and the anion nature on the densities, viscosities and electrical conductivities at ambient and elevated temperatures were discussed. The physicochemical properties of 22 protic ionic liquids (PILs) were reported.32 The structure-property relationships have been explored for the PILs, including the effect of increasing the substitution of ammonium cations and the presence of methoxy and hydroxyl moieties in the cation. Recently the results of obtaining TEOA salts with a number of sulfonic acids as protic ionic liquids have been presented.20 However, the interaction of triethanolamine with sulphonamides (sulfonic acid amides) has been largely disregarded. The latter have high pKa values (Bronsted acids) and represent a class of synthetic drug compounds producing a bacteriostatic effect on bacteria, viruses, fungi and protozoa.33-35

Experimental and Theoretical Methods Triethanolamine and bis(trifluoromethanesulfonyl)amide (from “Acros Organics”, (99+%) used in this work were not further purified. 4-chloro- and 4-nitrobenzenesulfonamide had been obtained earlier according to the technique described.36 The chemical shifts of 1Н,

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С and 1H-15N-HMBC nuclei were measured in

solutions of deuterated solvent (DMSO-d6) on an NMR spectrometer Bruker AVANCE-500. The signal of a tetramethylsilane (TMS) molecule proton was used as the external standard. The IR-spectra were registered on a FT-IR-spectrometer VERTEX 80V (Bruker) at room temperature in the range of 4000-400 cm-1 by averaging 32 scans with a 1 cm-1 resolution. The thermal analysis was done by the method of differential scanning calorimetry on a NETZCH DSC 204 F1 apparatus. The crucible was made from Al, and the mass of the samples equaled 10 – 20 mg. The experiment was made in an argon atmosphere (the cooling was made with gaseous N2) within a temperature range from –80 to 200ºС at the heating and cooling rate of 10ºС/min. The thermogravimetric analysis of ionic liquids was done on a NETZCH TG 209 F1 analyzer in an argon stream of 20ml/min. at the heating rate of 10 оС/min. The temperature measurement accuracy was ± 0.1oC. The conductivity of ionic liquids was determined by the electrochemical impedance method with an impedance/gain-phase analyzer Solartron 1260A complete with с electrochemical interface (Solartron SI1287A) over the frequency range of 0.1 Hz – 1 MHz with the signal amplitude of 10 mV and with the ACS Paragon Plus Environment

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accuracy higher than 0.2%. The temperature dependence of the cell constants was described by the equations from.37 The temperature was maintained with a HAAKE DC50-K35 thermostat within ± 0.01 K. The calculations were carried out in the software package Gaussian 09.38 The optimization of the geometric parameters was made by the hybrid functional B3LYP39 with basis sets 6-31++g(d,p)40 and 6311++g(2d,2p),41 and the paper presents the data calculated with the basis set 6-311++g(2d,2p). The initial molecule configurations were created in the GaussView program, and the initial configurations of the complexes were prepared by using optimized molecule structures. Each of the obtained structures corresponded to the minimum of energy, which was confirmed by frequency calculations. The changes in the total electronic energy in the reaction of complex formation from triethanolamine and amide in gas phase ∆E, changes in the total electronic energy, accounting for zero-point vibrations, ∆E0, and changes in the free energy ∆G of the reaction were calculated as a difference between the corresponding energies of the complex and the initial molecules. The free energy of the molecule is understood to be equal to the sum of total electronic energy and the corresponding thermal correction. The energy of intermolecular interaction in the studied complexes ∆Еint taking into account the superposition error BSSE was calculated by the following relations:42 ∆Eint = E(AB, aUb, R) – [E(A, aUb, R) + E(B, aUb, R)]. The basis set superposition error (BSSE) was calculated by the equation: BSSE = [E(A, aUb, R) – E(A, a, R)] + [E(B, aUb, R) – E(B, b, R)], where E(AB; aUb; R), E(A, aUb, R), E(B, aUb, R), E(A; a; R), and E(B; b; R) are the energies of the complex and the particles constituting it. Particles A and B are separated by the distance R in the AB complex; a and b are the basis set of the isolated molecules, and aUb is the basis set of the AB complex. As part of the Natural Bond Orbital Analysis (NBO analysis), we calculated the stabilization energies of the hydrogen bond (Е2) and the value of the transferred charge (q):43,44 E2 = – FIJ2/∆E q = 2(FIJ/∆E)2, where FIJ = 〈n|F|σ*〉 ∆E = 〈σ*|F|σ*〉 − 〈n|F|n〉 , n, σ* is the lone electron pair orbital and the antibonding orbital of the X-H bond of the molecule donating the proton, correspondingly, FIJ is the nondiagonal element of the effective orbital Hamiltonian (F) characterizing the interaction (overlap) of these orbitals; ∆Е is the difference of the orbital energies.

Synthesis of compounds. In this work, we have synthesized and characterized compounds obtained via interaction of triethanolamine

with

p-chlorobenzenesulfonamide

(I),

p-nitrobenzenesulfonamide

(II)

and

bistrifluoromethanesulfonimide (III). To synthesize all these compounds, we used the interaction reaction of equimolar amounts of amine and acid (sulfonamide) in an atmosphere of inert argon gas. Argon, like cooling, is used to prevent partial oxidation of the amine during its interaction with the amide and to remove ACS Paragon Plus Environment

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the moisture from the reaction mixture. The obtained viscous syrup-like substances were dried in vacuum and transferred into a hermetically sealed vial. Tris(2-hydroxyethyl)amine p-chloro-benzenesulfonamide TEOA/ClBSAd (I). 12.84 g (0.067 mole) of p-chloro-benzenesulfonamide was added to 10 g (0.067 mole) of tris(2-hydroxyethyl)amine. After adding all the calculated amount of amide, the reaction mass was heated on an oil bath to 140 ºС under constant mixing for 8 hours in an argon atmosphere. The obtained product was dried in vacuum for about 8 hours yielding I as a light-yellow gel (21 g, 91.9%). IR spectrum, ν, cm-1: 3337 s (ОН), 3240 s (NH2), 3121-3088 s (Ar-H), 2955-2827 s (СН2), 2739-2467 w (R3NH+), 1653 s (primary amine NH), 1474-1401 s (Ar-H), 1327 s (νas(S=O)), 1152 s (νs(S=O)), 826 s (vibrations CH of 1,4-substituted aromatic ring), 752 s (CH), 628 s (primary amine NH), 720-638 s (CH2). 1H NMR spectrum δ, ppm: 7.83-7.82 (d, 2H, J = 7.94 Hz, Ph-H,), 7.66-7.64 (d, 2H, J = 7.94 Hz, Ph-H), 7.46 (s, 1H, -NH); 4.34 (s, 3H, OH), 3.4 (s, 6H, CH2); 2.55-2.53 (t, 6H, J=5.49 Hz, CH2,). 13C NMR spectrum, δ, ppm: 143.65 (Ph-H), 137.27 (Ph-H); 129.75 (Ph-H); 128.29 (PhH); 59.4 (CH2); 57.3 (CH2). 1H-15N-HMBC NMR spectrum: δN -355.36 ppm. Tris(2-hydroxyethyl)amine p-nitro-benzenesulfonamide TEOA/p-NO2BSAd (II) was obtained in the same procedure as compound (I). The crystals were dried under vacuum, yielding II as yellow crystals (21.0 g, 74.1%). IR spectrum, ν, cm-1: 3344 s (H2O), 3253 s (NH2), 3155-3127 s (Ar-H), 2952-2836 s (СН2), 2735-2465 w (R3NH+), 1647 w (primary amine NH), 1613 s (Ar-H), 1570 s (νas (Ar-NO2)), 1356 s (νs (ArNO2)), 1521 s (νas (SO2)), 1163 s (νs (SO2)), 858 s (vibrations CH of 1,4-substituted aromatic ring), 752 s (CH), 684 s (primary amine NH), 539 s (CH2). 1H NMR spectrum δ, ppm: 8.42-8.43 (d, 2H, J=8.66 Hz, PhH); 8.07-8.09 (d, 2H, J=8.66 Hz, Ph-H); 7.76 (s, 1Н, NH+); 4.37 (s, 3H, OH); 3.40 (t, 6H, J=6.04 Hz, СH2,); 2.56 (t, 6H, J=6.05 Hz, СH2,). 13C NMR spectrum δ, ppm: 148.94 (Ph-H); 126.79 (Ph-H); 124.00 (Ph-H); 59.19 (СH2); 57.16 (СH2). 1H-15N-HMBC NMR spectrum δN-354.78 ppm. Bis(trifluoromethanesulfonyl)imide tris(2-hydroxyethyl)ammonium TEOA/BisFImine (III) was obtained in the same procedure as compound (I). After adding the amide, the reaction mass was heated on an oil bath to 80 ºС under constant mixing for 8 hours in an argon atmosphere. The obtained product was dried in vacuum yielding III as a yellow liquid (5.8 g, 95.7%). IR spectrum, ν, cm-1: 3360 s (ОН), 3151 s (CH2OH, intramolecular hydrogen bond), 2969-2816 s (СН2), 2766-2436 w (R3NH+), 1486-1406 s (C-F), 1352 s (νas(S=O)), 1198 s (νs(S=O)), 752 s (CH), 720-638 s (CH2). 1H NMR spectrum, δ, ppm: 5.1 (s, 4H, OH, NH+), 3.6-3.57 (t, 6H, J=5.49 Hz, CH2); 2.94 (s, 6H, -CH2-). 1

13

C NMR spectrum δ, ppm: 123.4 - 115.58

15

(CF3); 57.1 (CH2); 56.13 (CH2). H- N-HMBC NMR spectrum: δN -344.62 ppm. Results and Discussion Theoretically, triethanolamine can react with sulfonamides forming both protic ionic liquids (PILs) and hydrogen-bonded complexes according to scheme 1 shown below.


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OH "А" o 0 C - 140oC Ar, 8 hours

OH

HO

O S NH2

R

HAmide

N

O

OH

R= -Cl (I), -NO2 (II) Amide- =

HO

N

+ HAmide OH

"B" - 80oC Ar, 8 hours

HO

0oC

+

OH H OH N

F F F

N

F F F

S O O O O (III) S

Scheme 1. Reaction of triethanolamine interaction with amides.

To determine the characteristics of the formed compounds, we used the hybrid functional B3LYP to optimize the molecular hydrogen-bonded complexes of TEOA with the studied sulfonamides of different conformations (4 complexes for compounds (I) and (III), 5 – for compound (II)) and ion pairs of different conformations (4 for compound (I), 5 for compound (II) and 8 for compound (III)). The structure of the ion pairs and molecular complexes with lower formation energy is shown in Figure 1. Table 1 contains the values of total electronic energy changes in the reaction of complex formation from triethanolamine and amide in the gas phase ∆E, changes in the total electronic energy, accounting for the zero-point vibrations, ∆E0, and changes in the reaction free energy ∆G, scheme 1. The hydrogen-bonded molecular complexes (Fig. 1 a, c) of sulfonamides (1) and (II) are more stable than the ion pairs (Fig. 1 b, d). The energy parameters of the molecular complexes for compounds (I) and (II) are similar to each other, just as these values for the ion pairs. The calculation of the complexes taking into the medium by the CPCM method45 with a dielectric constant of 20 and 40 has not significantly changed the complexes structure; the molecular complexes have also remained the most stable ones. The interaction energy in the ion pairs is much higher than in the hydrogen-bonded molecular complexes due to the electrostatic interaction of the charged particles (Table 1). The positive values of the formation free energy change ∆G show that complex formation is impossible in the gas phase but solvation stabilizes these systems reducing the energy values. We suppose that the trend that we observed in the gas phase will be also observed in the condensed phase. In contrast to complexes (I) and (II) that have been discussed above, the ion pairs in the compounds of triethanolamine with (CF3SO2)2NH (Fig. 1f) are the most stable ones compared to the molecular complexes (Fig. 1e). The interaction energy in ion pair (III) is lower than that of the ion pairs with para-chloro- and para-nitrosulfonamides (Table 1), which is caused by the charge delocalization in the anion of bistrifluoromethanesulfonimide and, consequently, by the reduction of the electrostatic interaction. The calculation taking into account the medium has also shown that in this case the ion pairs have a higher stability as well. ACS Paragon Plus Environment

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a

b

c

d

e

f

Figure 1. Structures of the most stable molecular (a, c, e) complexes and ion pairs (b, d, f) of TEOA/pClBSAd (I) (a, b), TEOA/p-NO2BSAd (II) (c, d) and TEOA/BisFImine (III) (e, f). Table 1. Changes in the total electronic energy ∆E taking into account zero-point vibrations ∆E0 and Gibbs free energy ∆G in the reaction of formation of molecular complexes and ion pairs from amide (ClBSAd, pNO2BSAd, BisFImine) and triethanolamine in the gas phase and interaction energy in the most stable complexes. Complex type

∆E,

∆E0,

∆G,

∆Eint (BSSE),

kJ/mol

kJ/mol

kJ/mol

kJ/mol

TEOA/p-ClBSAd (I)


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I

Mol. (Fig. 1a)

-42.04

-39.17

4.14

-59.75 (4.54)

Ion. (Fig. 1b)

-2.23

4.52

52.53

-520.95 (7.34)

TEOA/p-NO2BSAd (II) II

Mol. (Fig. 1c)

-40.44

-38.27

4.46

-58.24 (4.53)

Ion. (Fig. 1d)

-4.25

3.64

52.95

-495.43 (7.06)

TEOA/BisFImine (III) III

Mol. (Fig. 1e)

-60.96

-59.23

-13.66

-69.83 (6.68)

Ion. (Fig. 1f)

-91.90

-80.65

-32.86

-353.55 (8.21)

To estimate the possibility of hydrogen bond formation in both molecular complexes and ion pairs, we carried out NBO analysis. The geometric parameters of the hydrogen bonds and the NBO analysis data, the stabilization energy of the hydrogen bond E2 and the value of the transferred charge q are given in Table 2. Four hydrogen bonds are formed in the molecular complexes of triethanolamine both with chloro- and nitro-benzenesulfonamide: 3 intermolecular (between two protons of the two OH amine groups and sulfogroup oxygens, between the proton of the acid NH2 group and the oxygen of the OH group of triethanolamine) and one intramolecular bond between the OH amine groups (Fig. 1 a, c). The values of the hydrogen bond parameters (Table 2) show that the strongest hydrogen bond in these complexes is the intramolecular H-bond. A weaker bond, in comparison with the first one, is formed between the protons of the amide NH2 group and the oxygen of the triethanolamine OH group. And the weakest of the considered bonds is the one between the proton of the OH group of triethanolamine and the oxygen of the acid S=O group, and it is close to the threshold value of the NBO H-bond criterion (the charge transfer value is lower than 0.01). The molecular complex of triethanolamine with bistrifluoromethanesulfonamide (Fig. 1e) also has 4 hydrogen bonds: two intermolecular ones (between the proton of the acid NH group and the oxygen of the acid S=O group) and two intramolecular ones (between the amine OH groups). The strongest hydrogen bond (Table 2) is formed between the proton of the bistrifluoromethanesulfonimide NH group and the oxygen atom of the triethanolamine OH group. The next two intramolecular bonds between the triethanolamine OH groups are similar to each other in strength but are weaker than the first one. The weakest of the considered bonds is the one between the proton of the triethanolamine OH group and the acid S=O group oxygen that, according to the geometric parameters (Table 2) is a weak hydrogen bond but is beyond the NBO bonding criterion. The ion pairs formed via the interaction of triethanolamine with p-chloro- and pnitrobenzenesulfonamide also have 4 hydrogen bonds. All of them are formed between the triethanolammonium cation and the acid anion but these complexes differ a lot from each other. In case of the amide nitro-derivative (Fig. 1 d), the proton of the triethanolammonium cation NH+ group forms a hydrogen bond with the nitrogen of the acid anion NH–group, whereas in case of the chloro-derivative (Fig. 1 b), the proton of the NH+ group forms a hydrogen bond with the oxygen of the S=O group of the acid anion. The other three hydrogen bonds in both ion pairs are formed between the protons of the three OH ACS Paragon Plus Environment

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groups of the cation and the nitrogen of the NH–group and the oxygens of the two anion S=O groups. No intramolecular hydrogen bonds are formed, in contrast to the molecular complexes. The OH bonds of the cation protons with the oxygen of the S=O group are stronger than in the molecular complexes. In case of the chloro-derivative, the strongest hydrogen bond (Table 2) is formed between the proton of the OH group of the triethanolammonium cation and the nitrogen atom of the amide NH– group of the anion, whereas in the nitro-derivative the strongest hydrogen bond is formed between the proton of the triethanolammonium NH+ group and the nitrogen atom of the amide NH–group of the anion. There are three hydrogen bonds in the ion complex of triethanolammonium with bistrifluoromethanesulfonimide (Fig. 1 f): two of them between the protons of the cation OH groups and the oxygens of two anion S=O groups and one intramolecular bond – between the OH groups of triethanolammonium. The inter-ion OH… S=O hydrogen bonds are not equally strong: one is stronger than the other. The weakest H-bond in this complex is the intramolecular bond between the OH groups of the triethanolammonium ion. It should be said that none of the ion pair structures that we optimized had a tricyclic atrane structure in the triethanolammonium ion characteristic of the solid phase.

Table 2. Geometric parameters of the hydrogen X-H…Y bonds, stabilization energy of the hydrogen bond E2 and the value of charge transfer q for the most stable calculated complexes. Complex type

Bond type X-H...Y*

R(X…Y), A

R(Y...H),

α(X-H...Y),

E2,

q,

A

0

kJ/mol

a.u. (atomic units)

TEOA/p-ClBSAd (I) N1-H1...O2

2.876

1.913

105.1

42.97

0.020

Mol.

O2-H2...O2

2.810

1.831

179.7

63.18

0.029

(Fig. 1a)

O2-H2...O1

2.931

2.017

156.8

21.17

0.009

O2-H2...O1

2.929

1.975

169.3

24.64

0.010

N2-H2...O1

2.761

1.741

163.4

115.65

0.057

Ion.

O2-H2...O1

2.746

1.834

154.9

40.25

0.016

(Fig. 1b)

O2-H2...O1

2.772

1.799

174.0

61.42

0.030

O2-H2...N1

2.743

1.755

172.6

126.86

0.070

I

TEOA/p-NO2BSAd (II) N1-H1...O2

2.857

1.897

153.6

45.90

0.021

Mol.

O2-H2...O2

2.814

1.835

179.7

62.51

0.029

(Fig. 1c)

O2-H2...O1

2.961

2.058

154.9

18.37

0.008

O2-H2...O1

2.946

1.994

168.7

22.26

0.009

N2-H2…N1

2.871

1.825

167.5

258.78

0.304

Ion.

O2-H2…N1

2.835

1.905

157.7

53.97

0.024

(Fig. 1d)

O2-H2…O1

2.747

1.812

159.6

52.59

0.026

O2-H2…O1

2.821

1.876

162.5

47.57

0.023

II


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TEOA/BisFImine (III) N2-H2…O1

2.678

1.614

177.8

144.52

0.063

Mol.

O2-H2…O2

2.888

1.909

176.9

48.83

0.022

(Fig. 1e)

O2-H2…O2

2.860

1.905

165.1

45.15

0.021

O2-H2…O1

2.952

2.084

149.0

9.96

0.004

O2-H2…O1

2.775

1.853

156.6

49.03

0.023

O2-H2…O2

2.892

1.983

155.1

27.36

0.012

O2-H2…O1

2.711

1.740

169.4

78.45

0.036

III Ion. (Fig. 1e)

*1 – amide or its anion, 2 – amide or its cation.

Thus, the calculation results show that when TEOA interacts with p-chlorobenzenesulfonamide or pnitrobenzenesulfonamide, the reaction (scheme 1) follows path “A” and produces a hydrogen-bonded molecular complex, while in case of triethanolamine interaction with bistrifluoromethanesulfonimide, the reaction follows path “B” to produce an ion pair. All the obtained compounds were also characterized by NMR-, IR spectroscopy and differential scanning calorimetry. Detailed spectral data is given in the experimental part of the text. Comparison of 1H and 13C chemical shifts of the protons and carbon in the obtained compounds with the corresponding values in pure amine shows that the chemical shifts of the protons in the NCH2 and OCH2 groups in compounds (I) and (II) are displaced in the direction of the stronger field (0.1-0.2 ppm), while in compound (III), the displacement is in the direction of the weaker field (0.4 ppm). Similarly, the position of the 13С chemical shifts in compounds (I) and (II) changes insignificantly (0.4-0.1 ppm) in comparison with compound (III) in which the displacement is 1-3 ppm in the direction of the strong field. The chemical shifts of the onium cation nitrogen in the correlation 1Н-15N-НМВС spectra of the amine salts, as a rule, are displaced by 20-10 ppm to the weaker field compared to the TEOA signal, which indicates the presence of the positively charged N+H.13,20 In salt (III), the value δN =-344.62 ppm (∆δN = 10 ppm), which clearly indicates amine protonation during salt formation. In compounds (I) and (II), the similar nitrogen signal is equal to -354 – -355 ppm, which corresponds to the nitrogen signal position in the initial triethanolamine. It can be explained due to the formation of a compound similar to a hydrogen-bonded complex.13 Such hypothesis is also confirmed by the FT-IR spectra. The compound is characterized by a low-intensity band in the region of 3140 - 3155 cm-1, which indicates a weak intermolecular interaction (N…H-О). Similar spectral characteristics of the hydrogen-bonded complex of triethanolamine with propionic acid are given in work.21 The IR-spectra of the samples have a number of vibration bands in the region of 2800-3000 cm-1 indicating that sulfonamides interact with amine. The vibration band ν(ОН) observed in the region of 33603060 cm-1 is produced by cation hydroxyl groups vibrations. The oxygen atoms of the alcohol groups participate in the formation of hydrogen bonds with the proton of the (N+H) and (N…H-О) groups. This interaction is typical of all the samples, which is confirmed by the formation of a new band at 3167 cm-1 (I), 3152 cm-1 (II), 3151 cm-1 (III). The most intensive vibration is observed in compound (III). In compounds ACS Paragon Plus Environment

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(I) and (II), we associate such vibrations with the formations of a strong hydrogen bond between the amide and amine. The characteristics of such formation have been discussed in the calculation part of the paper. The main characteristic bands of the sulfonamide functional group vibrations are given in Table 3.

Table 3. Characteristic absorption bands of sulfonamide fragment in the synthesized compounds ν, O=S=O

ν, C–HAr

ν, C=CAr

ν, Ar-NO2

TEOA/p-ClBSAd

1331, 1154

3065-3128

1573

–

TEOA/p-NO2BSAd

1354, 1164

3074-3129

1571

1521, 1351

TEOA/BisFImine

1352, 1195

–

–

–

Table 4 shows the values of the thermodynamic parameters obtained by differential scanning calorimetry (DSC) and thermogravimetry (TG) in the cycles of heating of the prepared compounds.

Table 4. Phase characteristics of compounds I – III. PILS

Tcr o

∆H -1

Tm o

∆H -1

Tg o

∆cp -1

Tdec -1

o

Н2О

( C)

(J·g )

( C)

(J·g )

( C)

(J·g K )

( C)

(%)

-

-

--

-

-25.7

0.718

238.7

0.2

TEOA/p-NO2BSAd (II)

128.2

-48.8

129.5

43.38

-32.4

0.354

-2.7

0.125

247,6

0.3

TEOA/BisFImine (III)

-

-

-

-

-54.2

0.723

223.8

0.5

TEOA/p-ClBSAd (I)

Hydrogen-bonded complexes (I) and (II) that we have obtained in this work are in different aggregative states. Compound (II) based on para-nitrobenzenesulfonamide can be classified as a compound that has a high melting point, is quickly crystallized from the melt and, evidently, forms a molecular crystal. Compound (I) can be called an amorphous substance that is only characterized by glass transition and the ability to remain for a long time in a glassy state (Figure 2a). In its turn, compound (III) belongs to the group of low-temperature ionic liquids. The high viscosity of this compound makes it only capable of glasstransition, and the sample itself can remain in a metastable glassy state for a long time. Figure 2b shows the DSC-curves of compound (III) and the initial substances. It is evident that the glass-transition temperature of the salt is 13 оС higher than the triethanolamine Tg without melting or crystallization. It should be noted that all the synthesized substances are thermally stable up to the temperature of 220 оС.


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a

b

Figure 2. DSC curves of triethanolamine molecular complex with p-chloro-benzenesulfonamide (TEOA/ClBSAd) (a) and bis(trifluoromethanesulfonyl)imide triethanolammonium salt (b). The figures also show the values of the initial substances: triethanolamine (a, b), p-chloro-benzenesulfonamide (a) and bis(trifluoromethanesulfonyl)amide (b).

To confirm the type (ionic/molecular) of the obtained compounds, we have carried out an experiment to determine their electric conductivity and found that only TEOA/BisFImine is electroconductive. The impedance spectra of TEOA/BisFImine at several selected temperatures are presented in Fig 3. Such type of Nyquist plots is typical of ionic liquids based on TEOA with a high electric conductivity.20

Figure 3. Impedance spectra of TEOA/BisFImine at several selected temperatures.

The impedance hodographs for this ionic liquid represent a straight line with an insignificant deflection. The spectra correspond to the equivalent circuit consisting of volume resistance of the electrolyte (R) connected in series with double layer capacity (Сdl). Fig. 4 shows a temperature dependence of specific conductivity of the TEOA/BisFImine salt.


13


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Fig 4. Arrhenius plot for TEOA/BisFImine specific conductivity.

Comparing the values of the TEOA/BisFImine electric conductivity with the literature data for triethanolamine-based salts with anions of different acids, we can conclude that this salt has a rather high conductivity.20,21, 46-48 The electric conductivity of TEOA/BisFImine is close to that of triethanolamine triflate and trifluoroacetate.21,30 Besides, the electric conductivity value increases by two orders of magnitude with the temperature growth (from 10-4 to 10-2). The Arrhenius plots (Fig.4) show that the temperature dependences of the ionic conductivity are not strictly linear. Therefore, these dependences can be fitted with the nonlinear Vogel–Tamman–Fulcher (VTF) equation:49-51  −B    T − To 

κ = κ o ⋅ exp 

Here, кo is the conductivity at an infinitely high temperature, B is a coefficient related to the activation energy Ea, and To is the ideal glass transition temperature. The VFT parameters calculated from the ionic conductivities of the TEOA/BisFImine are: кo = 0.93±0.06 S·cm-1; B = 864±23 K; To = 194±2 K; R2 = 0.999. The calculated values of To are by ~ 25 K lower than the glass transition temperatures determined experimentally, Table 4.

Conclusion Thus, the results of both the quantum-chemical calculations and the spectral characteristics of the obtained compounds and conductometric experiment show that TEOA interaction with aromatic sulfonamides leads to the formation of hydrogen-bonded molecular complexes, while its interaction with aliphatic fluorinated sulfonamide produces a salt that can be classified as a protic ionic liquid. By using the hybrid functional B3LYP, we have made quantum-chemical calculations of the structure and energy of the formed compounds, and as part of the NBO analysis, we have calculated the hydrogen bonds parameters. The structures of the obtained compounds have been confirmed by IR and NMR spectroscopy. Based on the


14

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obtained

data,

we

have

nitrobenzenesulfonamides

made

a

conclusion

forms

that

triethanolamine

hydrogen-bonded

with

complexes,

4-chlorowhereas

and

4with

bis(trifluoromethanesulfonyl)amide – a salt. We have determined the thermal characteristics of all of the obtained compounds, and for bis(trifluoromethanesulfonyl)imide tris(2-hydroxyethyl)ammonium salt – the electric conductivity as well.

Acknowledgment The investigations were carried out on the equipment of the Interlaboratory scientific center “The Upper Volga Region Center of Physicochemical Research”. The synthesis of the compounds and the conductometric experiment were carried out as part of the Russian Science Fund grant No.16-13-10371.

AUTHOR INFORMATION Corresponding Author Dr. Gruzdev M.S. *E-mail: [email protected].

ORCID Gruzdev Matvey: 0000-0001-8408-0092

NOTES The authors declare no competing financial interest.


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Possibility of Protic Ionic Liquids Formation From Triethanolamine with

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