Ionic liquids based on triethanolammonium salts of dicarboxylic acids (oxalic, malonic, succinic). Crystal structure and cation-anion interaction

Ionic liquids based on triethanolammonium salts of dicarboxylic acids (oxalic, malonic, succinic). Crystal structure and cation-anion interaction

    Ionic liquids based on Triethanolammonium Salts of Dicarboxylic Acids (oxalic, malonic, succinic). Crystal structure and cation-anion...

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    Ionic liquids based on Triethanolammonium Salts of Dicarboxylic Acids (oxalic, malonic, succinic). Crystal structure and cation-anion interaction V.S. Fundamensky, T.A. Kochina, Y.A. Kondratenko, A.A. Zolotarev, Yu.G. Vlasov, I.S. Ignatyev PII: DOI: Reference:

S0167-7322(16)32836-7 doi:10.1016/j.molliq.2016.12.111 MOLLIQ 6801

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

21 September 2016 29 November 2016 23 December 2016

Please cite this article as: V.S. Fundamensky, T.A. Kochina, Y.A. Kondratenko, A.A. Zolotarev, Yu.G. Vlasov, I.S. Ignatyev, Ionic liquids based on Triethanolammonium Salts of Dicarboxylic Acids (oxalic, malonic, succinic). Crystal structure and cation-anion interaction, Journal of Molecular Liquids (2017), doi:10.1016/j.molliq.2016.12.111

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ACCEPTED MANUSCRIPT Ionic liquids based on Triethanolammonium Salts of Dicarboxylic Acids (oxalic, malonic, succinic). Crystal structure and cation-anion interaction. V.S. Fundamenskya,b, T.A. Kochinab,c, Y.A. Kondratenkob, A.A. Zolotarevd, Yu.G. Vlasovc, I.S.

Grebenshchikov Institute of Silicate Chemistry RAS, 199034 St. Petersburg, Russia

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b

Saint Petersburg State Technology Institute (Technical University), 26 Moskovsky Pr., St. Petersburg, 190013 Russia

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Ignatyevc

c

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Department of radiochemistry, Institute of Chemistry, St. Petersburg State University, 199034 St. Petersburg, Russia d Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University, University Emb., 7/9, 199034 St. Petersburg, Russia Corresponding author: Y.A. Kondratenko, e-mail: [email protected], phone # +7 (812) 328 48 02, Grebenshchikov Institute of Silicate Chemistry RAS, 199034 St. Petersburg, Russia

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Keywords: triethanolammonium salts; triethanolamine; dicarboxylic acid; X-ray diffraction structure; IR spectra; protic ionic liquids. Abstract

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The series of ionic liquids based on triethanolammonium salts of dicarboxylic acids (oxalic, malonic and succinic) was synthesized, characterized by IR and H1, C13 NMR spectroscopy,

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thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Their structure was determined by single-crystal X-ray diffraction. It was found that triethanolammonium [(HOCH2CH2)3NH]+ cations in salts of oxalic (1), malonic (2) and succinic acids (3) containing

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monoanions [OOC(CH2)nCOOH]- (n=0-2) have the endo conformation and the ammonium proton (Ham) resides inside the “lampshade” formed by three CH2CH2OH branches connected with nitrogen atom. The asymmetric cell of the second modification of the succinic acid salt (4) includes

two

triethanolammonium

cations

and

a

succinate

dianion

[(HOCH2CH2)3NH]2+[OOC(CH2)2COO]2-. In this salt (4) one CH2CH2OH branch of the triethanolammonium cation is rotated around the N-C bond (endo-exo conformation) and forms infinite TEA chains. This pattern was firstly found in organic salts of TEA. The obtained results show that different structures of triethanolammonium cations in the salts of dicarboxylic acids have significant influence on the cation-anion interaction. Introduction Interaction of trithanolamine (TEA) with protic acid leads to the dissociation of the acid, the proton transfer to a nitrogen atom of TEA and the formation of triethanolammonium salts known as protaranes [1]. Protatranes, consisting of bulky organic cations [(HOCH2CH2)3NH]+ and 1

ACCEPTED MANUSCRIPT anions X- belong to the class of protic alkanolammonium ionic liquids [2-5]. Ionic liquids are large class of compounds which possess unique physicochemical properties (high thermal stability, negligible vapor pressure, good solubility, low melting point (below 100 °C), etc.) and have great potential applications in various fields (electrochemistry, organic synthesis (green

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solvents), biotechnology, chemical engineering, etc.) [6-9]. Single X-ray diffraction studies have

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allowed to determine about thirty structures of triethanolammonium salt [(HOCH2CH2)3NH]+X-

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with inorganic and organic anions, among which halogenides (F [10], Cl [11], Br [12], I [13]); indol-3-ylsulfanylacetate [14]; hexachloroplatinate (IV) [15]; cyclotetraphosphate [16]; 2formylbenzoate [17]; salicylate [18]; 1,3-benzothiazole-2-thiolate [19] and many others [20-23]. In most cases, three oxygen atoms of three hydroxyethyl group surround the hydrogen atom Ham

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of ammonium group and form weak (2.3-2.5 Å) trifurcated hydrogen bonds with him [10-14, 16, 18-23]. Lengths of N-Ham…O hydrogen bonds in these structures are substantially shorter than

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the sum of Van-der-Waals radii of oxygen and hydrogen atoms (2.72 Å). Location of the hydrogen atom Ham inside hydroxyethyl CH2CH2OH branches (structure «Chinese lantern») leads to formation «endo-conformation». In addition to this intramolecular hydrogen bonding

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structures of triethanolammonium salts are also stabilized by hydrogen bonds between oxygen

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atoms of cations and oxygen, halogen or other atoms of anions which contain these atoms [1023].

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The conversion of biologically active carboxylic acids into their triethanolammonium salts strongly increases their biological activity and expands the spectrum of their action [24-28]. On the basis of protatranes, drugs of the complex action (adaptogens, immunomodulators, etc.) –

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trecrezan or cresacin ([(HOCH2CH2)3NH]+·(2-СH3-C6H4OCH2COO-)) [24-26], chlorcresacin ([(HOCH2CH2)3NH]+·(4-Cl-C6H4OCH2COO-)) [26, 27], water-soluble analogues of aspirin without its side effects ([R3-nNH(CH2CH2OH)n]+·(2-CH3C(O)O-C6H4COO-), n=1-3; R=Me (n=1, 2)) [28], and others biologically active salts were developed. Antimicrobial [29] and enzyme activity [30] of protic alkanolammonium ionic liquids based on ethanolammonium salts were studied. IR and NMR spectra of organic salts [31] as well as equilibrium structures of complexes of TEA with carboxylic acids optimized at the B3LYP/6-311+C** level of theory [32] and anion– cation and ion–solvent interactions of ethanolammonium salts in solvents with different polarity [33] were also reported. In the present study, a series of four ionic liquids based on triethanolammonium salts, consisting of anions of dicarboxylic acid: hydrogenoxalate 1 (HOOC-COO-), hydrogenmalonate 2 (HOOCCH2COO-),

hydrogensuccinate

3

(HOOC(CH2)2COO-)

and

succinate

4

(-

OOC(CH2)2COO-), was synthesized, characterized by IR and H1, C13 NMR spectroscopy, TGA 2

ACCEPTED MANUSCRIPT and DSC. To explore the influence of the anion structure on the conformation of the triethanolammonium cation the structure of these salts 1-4 was determined by single-crystal Xray diffraction.

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Experimental Details

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Synthesis of ionic liquids

Triethanolammonium hydrogenoxalate (1). Equimolar amount of TEA and oxalic acid in

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methanol were refluxed with constant stirring for 1 h. After the completion of the reaction, methanol was evaporated under the reduced pressure. The resulting product was colorless viscous liquid. The crystals were grown from the liquid phase at low temperature (-10 °C) for

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three days. The final product was washed with diethyl ether and dried in vacuo. The yield of the reaction was 90 %. 1Н NMR spectrum (DMSO-d6, δ, ppm, J, Hz): 3.28 t (6H, 5.2, 5.2, HN+CH2-CH2-OH), 3.77 t (6H, 5.2, 5.2, HN+-CH2-CH2-OH), 6.08-7.40 m (5Н, HN+-CH2-CH2-OH, 13

С NMR spectrum (DMSO-d6, δ, ppm): 55.4 (HN+-CH2-CH2-OH), 55.6 (HN+-

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HOOC-COO-).

CH2-CH2-OH), 164.6-164.9 (HOOC-COO-). Elemental analysis calculated for C8H17NO7, %: С

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40.17; H 7.16; N 5.86, found, %: С 40.12; H 6.90; N 6.19. M.p. 70.01 °C. Triethanolammonium hydrogenmalonate (2) was prepared similarly to compound 1 from TEA and

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malonic acid (1:1). Yield 90%. 1Н NMR spectrum (DMSO-d6, δ, ppm, J, Hz): 2.84 s (2H, -OOCCH2-COOH), 3.14 t (6H, 6.3, 6.3, HN+-CH2-CH2-OH), 3.68 t (6H, 6.3, 6.3, HN+-CH2-CH2-OH),

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5.04-12.01 br. s (5Н, HN+-CH2-CH2-OH, HOOC-CH2-COO-). -

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С NMR (DMSO-d6, δ, ppm):

+

39.6 (HOOC-CH2-COO ), 55.5 (HN -CH2-CH2-OH), 56.0 (HN+-CH2-CH2-OH), 170.8-171.1 (HOOC-CH2-COO-). Elemental analysis calculated for C9H19NO7, %: C 42.68; H 7.56; N 5.53,

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found, %: C 42.68, H 7.84; N 5.47. M.p. 84.63 °C. Triethanolammonium hydrogensuccinate (3) was prepared similarly to compound 1 from TEA and succinic acid (1:1). Yield 93%. 1Н NMR spectrum (DMSO-d6, δ, ppm, J, Hz): 2.38 s (4H, HOOC-CH2-CH2-COO-), 2.70 t (6H, 5.9, 5.9, HN+-CH2-CH2-OH), 3.49 t (6H, 5.9, 5.9, HN+-CH2CH2-OH), 4.23-7.60 m (~7-8Н, HN+-CH2-CH2-OH; HOOC-(CH2)2-COO-).

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С NMR spectrum

(DMSO-d6, δ, ppm): 29.6 (НOOC-CH2-CH2-COO-), 56.7 (HN+-CH2-CH2-OH), 58.4 (HN+-CH2CH2-OH), 174.0 (НOOC-CH2-CH2-COO-). Elemental analysis calculated for С10H21NO7, %: C 44.94; H 7.92; N 5.24, found, %: C 44.30; H 8.44; N 4.93. M.p. 73.71 °C. Bis[triethanolammonium] succinate (4) was prepared similarly to compound 1 from TEA and succinic acid (2:1). Yield 95%. 1Н NMR spectrum (DMSO-d6, δ, ppm, J, Hz): 2.40 s (4H, -OOCCH2-CH2-COO-), 2.69 t (12H, 6.0, 6.0, HN+-CH2-CH2-OH), 3.49 t (12H, 6.0, 6.0, HN+-CH2-CH2OH), 5.20-6.50 m (8Н, HN+-CH2-CH2-OH). 13С NMR spectrum (DMSO-d6, δ, ppm): 29.9 (-OOCCH2-CH2-COO-), 56.9 (HN+-CH2-CH2-OH), 58.7 (HN+-CH2-CH2-OH), 174.2 (-OOC-CH2-CH23

ACCEPTED MANUSCRIPT COO-). Elemental analysis calculated for C16H36N2O10, %: C 46.15; H 8.71; N 6.73, found, %: C 46.12; H 9.18; N 6.84. M.p. 74.52 °C.

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FTIR spectroscopy. The infrared spectra of the samples 1-4 were obtained with a FTIR Nicolet 8700 (Thermo Scientific) spectrometer using the KBr disk technique in the region

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4000-400 cm-1.

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NMR spectroscopy. NMR spectra of compounds in DMSO-d6 2-5% solutions were registered on a Bruker Avance III spectrometer [400.13 (1Н), 100.613 MHz (13С)]. Chemical shifts presented below are relative to residual signals of dimethyl sulfoxide (2.50 ppm for 1Н and 39.52

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ppm for 13С [34].

Elemental analysis was performed on a C,H,N-analyzer Euro EA3028-НТ. DSC and TGA measurements were performed using a NETZSCH DSC 204 F1 Phoenix High

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Sensitivity with μ-Sensor under a nitrogen atmosphere in the temperature range from 0 to 145 °C and NETZSCH TG 209 F1 Libra Thermogravimetric Analyzer under argon atmosphere in the

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temperature range from 30 to 400 °C.

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X-ray Crystallography

The diffraction data for crystals 1-4 were collected at 100 K with an Oxford Diffraction SuperNova diffractometer using a Nova microsource and multi-layer monochromator (Cu Kα

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radiation, λ = 1.5418 Å) and processed using CrysAlisPro software [35]. Data were corrected for absorption effects using the multi-scan method and using spherical harmonics, implemented in SCALE3

ABSPACK

scaling

algorithm

[35]. All

structures

were

solved

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by Superflip Sp Software Package [36] and refined by full matrix least-squares on F2 by SHELXL 2014 [37]. Non-H atoms for 1-4 were refined anisotropically. H atoms were placed in an idealized position, with an N—H 0.88 Å and C—H distance of 0.97 Å Displacement parameters for all H atoms were assigned as Uiso(H) = 1.2Ueq(C,N). A summary of the structure determinations for 1-4 is given in Table 1. Bond lengths, bond angles, and selected torsion angles are given in Tables S1–S7 of the supporting information. Table 1. Main crystallographic data and refinement results for structures 1-4 1

2

3

4

Chemical formula

С8H17NO7

C9H19NO7

С10H21NO7

C16H35N2O10

Mr

239.23

253.25

267.28

415.46

Crystal system, space

Orthorhombic,

Monoclinic,

Monoclinic,

Monoclinic,

group

Pca21

P21/c

P21/c

P21/c 4

ACCEPTED MANUSCRIPT a, b, c (Å)

18.3406 (3), 10.90838(19), 11.2036 (2)

β (°)

10.1236 (4), 11.5577 (5), 10.0880 (4) 95.499 (4)

11.6402 (2), 11.2224 (2), 9.8894 (2)

16.7713 (3), 5.42820 (12), 22.6266 (4)

99.666 (2)

98.9585(18)

1273.51 (4)

2034.76 (7)

2241.46 (6)

1174.91 (9)

Z

8

4

μ (mm-1)

1.08

1.06

Crystal size (mm)

0.13×0.18×0.21

0.09×0.16×0.25

Temperature K

Oxford Diffraction Super-Nova 100

Oxford Diffraction Super-Nova 100

Oxford Diffraction Super-Nova 100

Oxford Diffraction Super-Nova 100

Tmin, Tmax

0.742, 1.000

0.480, 1.000

0.821, 1.000

0.877, 1.000

15742, 2652,

10838, 3791,

2486

3200

0.064

0.031

0.030

4.38, 75.84

3.85, 76.42

3.96, 70.00

θmin, θmax

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1.01 0.95 0.26×0.22×0.18 0.10×0.14×0.23

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0.033 4.05, 72.46

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Refinement

16516, 4372, 4372

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No. of measured, independent and observed [I >2α(I)] reflections Rint

4

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Data collection Diffractometer

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V (Å3)

7454, 2173, 1819

0.026, 0.067,

0.035, 0.095,

0.033, 0.053,

0.056, 0.117,

1.09

1.03

1.12

1.04

No. of reflections

4372

2173

2486

3200

No. of parameters

297

167

259

No. of restraints dif denmax, dif denmean, dif denav (e Å-3) Absolute structure Absolute structure parameter Identification code

1

158 0

0

0

0.18, -0.20,

0.379, -0.371,

0.33, −0.25,

0.79, -0.65,

0.006

0.031

0.029

0.092

1447679

1505413

1447670

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R[F2 > 2σ(F2)], wR(F2), S

Ref.38 -0.01 (7) 1447396

Results and discussion Interaction of TEA with dixarboxylic acids (oxalic, malonic and succinic) leads to the dissociation of acids, the proton transfer to nitrogen atom of TEA and the formation of triethanolammonium salts of oxalic (1), malonic (2) and succinic (3, 4) acids – protic 5

ACCEPTED MANUSCRIPT alkanolammonium ionic liquids. Ionic liquids based on triethanolammonium salts 1-4 have low melting point (70-89 °C, see experimental details) and possess good solubility in polar solvents (water, alcohols, DMSO, DMF, etc.). Salts 1-3, consisting of triethanolammonium cations [(HOCH2CH2)3NH]+ and monoanions -OOC(CH2)nCOOH (n=0-2) were prepared by dropping

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the equimolar amount (1:1) of the dicarboxylic acid to the methanol solution of TEA. Interaction

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of TEA with oxalic and malonic acids with 2:1 molar ratio led to the formation of viscous

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colorless liquids and only with succinic acid (2:1) led to formation solid product 4 – crystals suitable for X-ray diffraction studies.

DSC curves of salts 1-4 are characterized by a single intense endothermic peak in the region 70.01 – 84.63 °С (see experimental details and figure S5), corresponding to its melting points.

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Such values of melting points (below 100 °C) are typical for protic ionic liquids. TG studies showed that triethanolammonium salts of succinic acid, 3 and 4, with Td = 188.0 °С and 202.8

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°С, respectively, are the most stable among the investigated salts (figure S6). The thermal decomposition temperature (Td) was taken to be the value when a 5% mass loss was observed. Their destruction proceeds in several poorly expressed stages, and a high amount of residue

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(27.2 and 29.8 %) remains at 390.2 °С. The destruction process of triethanolammonium salts of

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oxalic (1) and malonic acid (2) starts at 174.1 and 142.3 °C, respectively. The destruction of 2 proceeds in three distinct stages (142.3-221.9 °C (-36.5%), 221.9-333.1 °C (-52.8 %) and 333.1-

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391.1 °C (-88.4 %)) in contrast to one-step decomposition of 1. Samples 1 and 2 showed approximately the same amount of residue (12.8 and 11.9%) at 390.2 °C (figure S6). The structures of ionic liquids 1-4 were studied using single crystal X-ray diffraction (XRD)

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analysis.

Triethanolammonium hydrogenoxalate (1) Triethanolammonium hydrogenoxalate (1), consisting of triethanolammonium cation and hydrogenoxalate [-OOCCOOH] anion, crystallizes in the orthorhombic system, space group Pca21. The asymmetric unit cell contains two cations and two hydrogenoxalate-anions (Fig. 1).

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Figure 1.

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Hydrogenoxalate anions form infinite chains running along the b-axis and linking head-to-tail through short alternating hydrogen bonds [1.73 Å (O2···H5) and 1.69 Å (O1···H8)]. Similar

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chains of oxalate anions were observed in potassium [39-42] and sodium [41] oxalates. All four oxygen atoms of COO and COOH groups of anion participate in H-bonding: one as a proton donor and other three as proton acceptors. The O2 atom forms a bifurcated H-bonds acting as a proton donor as well as an acceptor (Fig. 2). Triethanolammonium [(HOCH2CH2)3NH]+ cations form columns spreading along the b-axis formed from alternating non-equivalent cations. The NH bonds in one column are turned to one direction and in the column neighboring from the other side to the opposite one (Fig. 2).

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Figure 2.

This orientation of cations in columns and the fact that N-Ham bonds are effectively shielded by

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CH2CH2OH branches prevents the noticeable interaction between cations (Fig. 2). In the absence of prominent hydrogen bonds between [(HOCH2CH2)3NH]+ cations, the lattice is stabilized by hydrogen bonds between hydrogenoxalate chains and cationic columns. These are

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O10···H13 (1.92 Å), O2···H11 (1.97 Å), O6···H14 (1.99 Å), and O9···H12 (1.93 Å) (Table S4). In addition to the above-mentioned hydrogen bonds between hydrogenoxalate chains in the abplane shown in Fig. 2, there exist hydrogen bonds (not seen in the Fig.2) of the 1.95 Å length (O4···H3) in the bc-plane (Table S4). There exist short contacts between oxygen atoms of hydrogenoxalate anion and hydrogen atoms of CH2-groups of hydroxyethyl branches of cation: O9···H5B (2.39 Å), O5···H4A (2.56 Å), O10···H9B (2.59 Å), and between oxygen atoms of hydroxyethyl branches of cation and hydrogen atoms of CH2-group of neighboring symmetrically independent cation: O3···H7A (2.41 Å), which may be disputably assigned to weak hydrogen bonds [43, 44]. Several oxygen atoms of the hydrogenoxalate anion chain form bifurcated hydrogen bonds with hydrogen atoms of the free carboxyl group COOH of neighboring hydrogenoxalate anions as well as with OH groups of triethanolammonium cation (Fig. 2). The shortest hydrogen bond distances are those between hydrogenoxalate anions. 8

ACCEPTED MANUSCRIPT The difference in hydrogen bond strength reflects in the C-O bond lengths of the hydrogenoxalate anion chain (Fig.1 and 2, Table S4): to strongest hydrogen bonds between anions (which form chains) are 1.69 Å (H8···O1) and 1.73 Å (H5···O2). They correspond the longest C-O bonds in anions, i.e. C2-O8 and C18-O5 bonds with the 1.300 Å length (Table S1).

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To weaker H-bonds connecting cation and anion layers (1.93, 1.99 Å) correspond shorter CO

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bonds (1.225–1.214 Å). O2 and O10 atoms of anion interacting with two hydroxyls of one

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triethanolammonium cation form a cyclic 12-membered motif (C1-O2-H11-O11-C3-C10-N15C14-C13-O13-H13-O10-C2-C1). Note, that due to the involvement of all oxygen atoms in hydrogen bonding in the hydrogenoxalate anion chain this difference is less pronounced than the difference between C-O and C=O bond lengths observed in the gas phase of the oxalic acid

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molecule (1.339,1.208 Å) [44] and, thus cannot be classified either as a single or as a double bond.

Triethanolammonium

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Triethanolammonium hydrogenmalonate (2) hydrogenmalonate

(2),

[(HOCH2CH2)3NH]+[OOCCH2COOH]-,

crystallizes in the monoclinic system, space group P21/c. The asymmetric unit cell contains one

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triethanolammonium cation and one hydrogenmalonate anion (Fig. 3).

Figure 3. Similar to the case of triethanolammonium hydrogenoxalate 1, hydrogenmalonate anions form infinite chains which interlace cation columns. H-bond length of anionic chain in 2 (1.79 Å) are longer than those in 1 (1.73, 1.69 Å), but bond lengths between anions and cations in 2 (Table S4) are shorter (1.88 and 1.91 Å) than in 1 (1.97 and 1.95, as well as 1.93, and 1.92 Å). The structure and geometrical parameters of the triethanolammonium cation in the 2 practically do

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ACCEPTED MANUSCRIPT not differ from cations in the 1 and other protatranes (Table S1). Note, that in contrast to cations

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in 1 the direction of N-Ham bond in the cationic columns 2 alternates.

Figure 4.

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The infinite chains formed by hydrogen bonded hydrogenmalonate anions differ from the analogous chains of hydrogenoxalate anions: in contrast to 1 where all four oxygen atoms are involved in hydrogen bonding, in 2 the shortest C2-O7 bond (1.213 Å) do not form hydrogen bonds (Fig. 3). Two COOH and COO- groups in the hydrogenmalonate anion differ substantially. In COO- group at C1 carbon-oxygen bonds are nearly equal: 1.259 Å (С1-О1) and 1.256 Å (С1-О3). Both oxygen atoms form bifurcated H-bonds. Moreover, one of them (С1О1) accepts protons only from OH groups of cation (1.88 Å), while the other form hydrogen bonds with OH group of cation (1.91 Å) as well as with oxygen atom of anion (1.79 Å) (Table S4). In the carboxylic COOH group at C2 two CO bonds differ substantially. One of them (C2-O7) is a double bond (1.213 Å) and oxygen atom (O7) does not form hydrogen bonds, while the other bond (С2-О5) is a single one (1.315 Å) and donates protons to form hydrogenmalonate anion chains (Fig. 4, Table S4). Triethanolammonium hydrogensuccinate (3) 10

ACCEPTED MANUSCRIPT Triethanolammonium hydrogensuccinate 3, consisting of triethanolammonium cation and hydrogenoxalate [-OOC(CH2)2COOH] anion, crystallizes in the monoclinic system, space group P21/c. The asymmetric unit cell contains one triethanolammonium cation and one

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hydrogensuccinate anion (Fig. 5).

Figure 5.

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Each oxygen atom of hydroxyethyl branches of triethanolammonium cation are linked to the oxygen atoms of the carboxylate group of the corresponding hydrogensuccinate anion: H2···O1

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(1.88 Ǻ), H3···O1 (1.88 Ǻ) and H5···O4 (1.98 Ǻ). In contrast to triethanolammonium hydrogenmalonate 2, all four oxygen atoms of hydrogensuccinate anion are involved in

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hydrogen bonding. In the carboxylate group carbon-oxygen bond lengths are 1.271 Ǻ (C1-O1) and 1.245 Ǻ (C1-O7). O1 oxygen atom forms bifurcated H-bonds with OH-groups of two different [(HOCH2CH2)3NH]+ cations (1.88 Ǻ (O1···H3 and O1···H2), Table S4, fig. 6) similar

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to oxygen atom (O1) of COO- group in 2 (1.88 Ǻ (O1···H4 and O1···H2)). In contrast to 1 and 2 the second oxygen atom (O7) of COO- group form only one H-bond with hydrogen atom of carboxylic group of neighboring hydrogensuccinate anion (H6···O7, 1.73 Ǻ), acting as proton acceptor and forming infinite anionic chains (fig. 6). In the carboxylic group, consisting of double C2=O4 (1.219 Å) and single C2-O6 (1.314 Å) bonds, each oxygen atom forms one Hbond with OH-group of triethanolammonium cation (1.98 Å (O4···H5), acting as proton acceptor, and with oxygen atom of carboxylate group of neighboring hydrogensuccinate anion (H6···O7; 1.73 Å), acting as proton donor (Figure 6, Table S4). Similar H-bonding of carboxylic group of the anion was found in the salt 1. Thus, the triethanolammonium cations are linked to three different hydrogensuccinate anions, which, in turn, participate in four hydrogen bonds – three with cations and one with each other.

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Figure 6.

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Thus, the structure and geometry parameters of triethanolammonium cations in the salts 1-3 of general formula [(HOCH2CH2)3NH]+[OOC(CH2)nCOOH]- (n=0,1,2) similar to each other and

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practically coincide with those in the most famous triethanolammonium salts of inorganic and organic acids [10-14, 16, 18-23].

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Bis[triethanolammonium] succinate (4) Bis[triethanolammonium]

succinate,

consisting

of

two

triethanolammonium

+

[(HOCH2CH2)3NH] cations and one succinate dianion [OOC(CH2)2COO]2-, crystallizes in the monoclinic system, space group P21/c. The asymmetric unit contains two cations and one anion (two halves of two different anions are symmetrically independent, Fig. 7).

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Figure 7.

Succinate dianion in the salt 4 is doubly deprotonated in contrast to salts 1-3 in which only one

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proton is transferred to TEA. As a result of proton detachments the succinate dianion has nearly

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equal CO bond lengths. The symmetry of succinate [OOC(CH2)2COO]2- dianion was found to be C2 [45]. One of the reasons of the formation crystal phase in the 4 in contrast to salts of oxalic

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and malonic acids in the liquid state (the molar ratio of TEA and dicarboxylic acid – 2:1) may be the due to the higher hydrophobicity of the succinate dianion which affects the structural selforganization and dimensional isolation charges. CO bond lengths of carboxylate groups of

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succinate dianion nearly equal (1.251 and 1.266 Å; 1.268 and 1.245 Å, Fig. 7, Table S1). Oxygen atoms of succinate dianion forming longer CO bonds (C2-O7 and C5-O4) are connected with triethanolammonium cations and neighboring dianions by bifurcated H-bonds (1.81 and 1.86 for O4 and 1.80 and 1.85 Å for O7) (Fig. 8, Table S4). Since succinate dianion in 4 has no protons, it cannot form infinite anionic chains as anions in 13 salts do, but they form succinate columns connected with cationic columns by hydrogen bonds. Only in the salt 4, triethanolammonium cations form infinite chains. This peculiarity of hydrogen bonding in 4 leads to the necessity of rotation of one of three CH2CH2OH branches. In contrast to previous triethanolammonium salts 1-3 and numerous other protatrane structures [10-14, 16, 18-23], the triethanolammonium [(HOCH2CH2)3NH]+ cation in 4 do not have the ‘’lampshade” endo conformation. One of the CH2CH2OH branches of the [(HOCH2CH2)3NH]+ cation (exobranch) in 4 is rotated around the N-C bond and is linked to the succinate dianion by H-bonds of the 1.87 Å (H2···O5) and 1.79 Å (H6···O10) bond length. This leads to a considerable change in the structure of a cation (endo-exo conformation), although the geometry of separate 13

ACCEPTED MANUSCRIPT CH2CH2OH branches do not differ from those in “pure” endo conformation. The conformation of the rotated (exo) branch of the triethanolammonium cation in 4 is close to trans with HamNCC torsion angles (H11-N11-C6-C1 and H12-N12-C10-C9, Table S3) are - 165.7° and 171.0 °), while in two remaining branches they are gauche (H11-N11-C7-C11 = -50.3°, H11-

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N11-C4-C8 = 58.0°, H12-N12-C13-C3 = -36.3°, H12-N12-C14-C16 = -50.1°) and are close to

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similar values for all three CH2CH2OH branches in 1-3 (Table S3). Despite this substantial

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difference in the configuration of endo and exo branches in 4, the difference in the N···O interatomic distances is small and their values are close to similar distances in 1-3. Much greater is the difference in Ham···O and O···O interatomic distances in endo conformation of 1-3 and endo-exo conformation of triethanolammonium cations in 4. The Ham···O distances in 1-3 lie in

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the 2.21-2.39 Å range, while in 4 they are between 2.48 and 3.96 Ǻ for one cation and between 2.55 and 3.94 Ǻ for the second cation. For O···O interatomic distances corresponding values are

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3.508-3.914 Å in 1-3, and 4.290-5.900 for one cation and 4.853-5.442 Ǻ for the second cation in 4.

Similar structures with endo-exo conformation of cation were reported earlier in

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triethanolammonium 2-formylbenzoate [17] and bis[triethanolammonium] hexachloroplatinate

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(IV) [15]. Among structures retrieved from Cambridge Structural Database [46] we found only one salt with organic anion and similar conformation of the [(HOCH2CH2)3NH]+ cation, that is

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triethanolammonium 2-formylbenzoate [17], although no attention was paid to the unusual conformation of the cation in this work.

In this compound two neighboring cations form

[(HOCH2CH2)3NH]+ dimers by donating ammonium protons (Ham) to oxygen atoms of the

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CH2CH2OH endo branch. The exo branch as well as endo ones form hydrogen bonds with the 2formylbenzoate

anions.

In

contrast

to

triethanolammonium

2-formylbenzoate,

triethanolammonium cations in 4 stack along the b-axis linked by hydrogen bonds between ammonium protons (Ham) and oxygen atoms of the CH2CH2OH exo branch with distances (H11···O2) =2.08 Å in column 4a and (H12···O6) = 1.99 Å in column 4b (Fig. 8).

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Figure 8.

IR spectra

Differences in the anionic part of the discussed structures defined by X-ray analysis reflect also

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in IR spectra, primarily in characteristic bands in the CO stretching spectral region.

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Electron diffraction structure [47] and vibrational spectra of oxalic acid monomer [48] evidence that most stable conformers are those in which COOH groups are in trans- position.

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The analysis of IR spectra of the molecule isolated in Ne, Ar, and Xe matrices [49-51] shows that the most stable conformer is the cTc isomer (notations of ref. 50) belonging to the C2h symmetry point group and in which is stabilized by two intramolecular COOH…O=C(OH) hydrogen bonds. The characteristic features of the vibrational spectra of gaseous oxalic acid are

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bands at 3470 (νasOH), 1812 (νasC=O), 1800 (νsC=O), 1423 (νsC-O) 1329 (δ COH), and 1275 cm-1 (νasC-O) [48] . In the polycrystalline β-oxalic acid, structure of which is built from dimers, corresponding IR bands are 1714 (νasC=O), 1481 (δ COH), and 1219 (νasC-O) cm-1 [52]. IR and Raman spectra of potassium and sodium hydrogenoxalates were recorded and assigned by Shippey [41]. They are characterized by bands near 3400 (ν OH), 1720 (νasC=O ), 1665 (K) and 1572 (Na) (νsC=O), 1455 1427 (K) and 1415 (Na) (ô COH), 1289 (K) and 1214(Na) (νasCO) cm-1 [41]. Bands at 3360, 1720, 1630, 1410, and 1230 cm-1 observed in our IR spectrum of 1 (Fig. S1) fairly coincide with spectra of hydrogen oxalate chains in solid state spectra of potassium and sodium hydrogen oxalates [41]. In the spectrum of 1 bands in the 1740-1700 cm-1 region may be assigned to stretching vibrations of CO bonds C18-О6 (1.214 Å) and C2-O10 (1.215 Å) which form weak H-bonds with hydroxyl groups of cations. The strong 1630 cm-1 band possibly belongs to a slightly longer C1-O4 bond (1.232 Å) connected with OH group of cation by the hydrogen bond (1.952 Å) lying 15

ACCEPTED MANUSCRIPT in the bc-plane (O4···H3). Bands near 1320 cm-1 may be ascribed to the C1-O2 (1.266 Å) and C17-O1 (1.269 Å) stretchings, while those at 1280 and 1230 cm-1 to the longest bonds of the 1.300 Å length (C2-O8 and C18-O5) which link hydrogenoxalate anions in the chain by strongest O1···H8 (1.69 Å) and O2···H5 (1.73 Å) hydrogen bonds.

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The band at 3150 cm-1 may be assigned to NH bonds (N15-H15 and N16-H16) while bands in

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the 2800-3200 cm-1 and in the 1400-1500 cm-1 ranges to CH2 vibrations of CH2CH2OH branches

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of cation (Fig. S1).

In the IR spectra of 2 (Fig. S2) the band with the highest frequency in the CO stretching region, which, obviously, belongs to the shortest C2-O7 (1.213 Å) remains close (1715 cm-1) to corresponding bands in 1 (1740-1700 cm-1). The same is true for the 1650 cm-1 band (1630 cm-1

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in 1). The only difference between IR spectra of 1 and 2 is observed in the 1320-1230 cm-1 region: in 2 the intensity of the 1230 cm-1 band substantially decreases, while that of the 1320

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cm-1 band increases. This redistribution of intensities may be rationalized by the difference in the length between CO bonds donating protons to form anionic chains (1.300 Å in 1 vs. 1.315 Å in 2) and bonds at the carboxylate group of an anion (1.266/1.269 Å in 1 vs. 1.256 Å in 2).

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The IR spectrum of 3 in the 3400-1200 cm-1 spectral range contains all characteristic features of

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monoanions in compounds 1 and 2 with the exemption of bands in the 1750-1600 cm-1 region. Spectra of 1 and 2 contain strong bands near 1640 cm-1. This band is absent in 3. This feature

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may be assigned to a different pattern of hydrogen bonding between cations and anions: in 3 these bonds are shorter and corresponding CO bond lengths are longer. This leads to a retribution of band intensities in the 1750-1600 cm-1 region.

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The IR spectrum of 4 (Fig. S4) in the 1750-1200 cm-1 (CO stretching) region substantially differs from spectra of 1-3 (Fig. S1-S3): bands near 1700 cm-1 disappear and strong band at 1560 cm-1 may be observed in 4 (Fig. S4). This narrowing of CO stretching spectral range may be explained by smaller difference in CO bond lengths in the succinate dianion (1.245-1.268 Å) due to the absence of hydrogen atoms which form bonds between anions in 1-3 salts. This feature, i.e. the difference in the position of ν(CO) bands in the 1800-1200 cm-1 spectral range between mono- and dianions was also observed earlier. IR spectra of salts containing monoanions are characterized by the presence of bands in 1800-1700 cm-1spectral range [52-55]. On the contrary, bands in this region were not observed in completely deprotonated acids, i.e. carboxylate dianions [OOC(CH2)nCOO]2- (n=0-2) [56-60].

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ACCEPTED MANUSCRIPT Conclusions 1. Triethanolammonium salts of oxalic, malonic and succinic acids (1-4) belonging to the class protic alkanolammonium ionic liquids were synthesized and characterized by IR, H1, C13 NMR spectroscopy, elemental analysis, DSC and TGA. Ionic liquds 1-4 possess good solubility in

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polar solvents (water, alcohols DMSO, DMFA, etc.), have low melting points (70.01-84.63 °C)

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and are thermally stable up to 142.3-202.8 °C.

2. The crystal structure of triethanolammonium salts 1-4 was determined by single-crystal X-

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ray diffraction. 1-3 Salts, consisting of [(HOCH2CH2)3NH]+ cations and [OOC(CH2)nCOOH](n=0-2) anions, are characterized endo conformation of cation: three oxygen atoms (from CH2CH2OH branches) surround the hydrogen atom (Hamm) and form three intramolecular 3.

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trifurcate H-bonds.

In 1-3 salts, anions (hydrogenoxalate, hydrogenmalonate and hydrogensuccinate) form

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infinite chains linked head-to-tail by short (1.69-1.79 Å) hydrogen bonds. Triethanolammonium cations stack into columns held together by hydrogen bonds between their OH groups and oxygen atoms of anions.

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4. One of the CH2CH2OH branches of the triethanolammonium cation in 4 rotates around the C-

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N bond transforming the endo-conformation of 1-3 into the conformation which may be designated as endo-exo conformation. In compound 4, a continuous chain of TEA molecules was

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firstly found in the organic salts of TEA. This conformation allows cations to form hydrogen bonds (2.08 and 1.99 Å) inside cationic columns (H11···O2 and H12···O6). 5. In 4 due to the absence of protons succinate dianions (-OOC(CH2)2COO-), in contrast to 1-

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3, are not linked by hydrogen bonds to form infinite anionic chains, but rather form hydrogen bonds with hydroxyl groups of cations. 6. IR spectra in the CO stretching region (1750-1200 cm-1) do not differ significantly in 1-3, but in 4 this spectral range contracts in keeping with structural changes in anionic moieties on going from monoanions to dianions found by single-crystal X-ray diffraction.

Acknowledgement This work was performed using the equipment of the Resource Centers of St-Petersburg State University

«Centre for Optical and Laser Materials Research», «Chemical Analysis and

Materials Research Centre», «Centre for X-ray Diffraction Studies», «Magnetic Resonance Research Centre» and «Termogravimetric and Calorimetric Research Centre». The work was supported by the Grant of St. Petersburg State University (No 12.38.218.2015) and Grant of FASIE (No 10280GU (16.06.2016). 17

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Figure Captions

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Fig. 1. The structure of the asymmetric unit cell of 1 with atom-numbering scheme.

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Fig. 2. The fragment of the structure of 1 (O···H bond lengths, Å). View along the c-axis.

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Fig. 3. The structure of asymmetric the unit cell of 2 with atom-numbering scheme. Fig. 4. The fragment of the lattice structure of 2 (O···H and CO bond lengths, Å). Fig. 5. The structure of the asymmetric unit cell of 3 with atom-numbering scheme Fig. 6. The fragment of the structure of 3 (O···H bond lengths, Å).

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Fig. 7. The structure of the asymmetric unit of 4 with atom-numbering scheme. Halves of symmetry nonequivalent anions are expanded to full by corresponding symmetry operations.

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Fig. 8. The fragment of the structure of 4 (O···H bond lengths, Å). View along the b-axis.

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∙ Four protic ILs based on triethanolammonium salts of dicarboxylic acids were synthesized and characterized; ∙ Their structure was determined by single-crystal X-ray diffraction; ∙ Three salts containing monoanions [OOC(CH2)nCOOH]- (n=0-2) are characterized endo conformation of cation; ∙ Triethanolammonium cations in double deprotonated succinic acid salt have the endo-exo conformation.

22