Ionic Liquid or Zwitterionic - ACS Publications - American Chemical

Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Structural Identification for the Reaction of Chlorosulfonic Acid with Tertiary N‑Donor Ligand − Ionic Liquid or Zwitterionic Compound? Rajkumar Kore,†,‡ Victor Day,§ and Mark B. Shiflett*,†,‡ †

Department of Chemical and Petroleum Engineering, University of Kansas, 1530 West 15th, Lawrence, Kansas 66045, United States ‡ Center for Environmentally Beneficial Catalysis, University of Kansas, 1501 Wakarusa Drive, Lawrence, Kansas 66047, United States § X-ray Crystallography Laboratory, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045, United States ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by TULANE UNIV on 02/09/19. For personal use only.

S Supporting Information *

ABSTRACT: Acid catalysts derived from a well-known reaction of 1-methylimidazole and chlorosulfonic acid system are considered very important, but sufficient details are lacking on their structure information. We studied and report the structural characterization of products from a reaction of 1-methylimidazole and chlorosulfonic acid. Our results from crystallography, nuclear magnetic resonance spectroscopy (NMR), elemental analysis, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) indicate the formation of a unique reaction mixture of zwitterion-type salt [C1im-SO3] and ionic liquid [C1im-SO3H]Cl at a 7:3 ratio instead of only ionic liquids as proposed by others. KEYWORDS: Zwitterion, Ionic liquid, Brönsted acidic ionic liquid, Sulfonic acid functionalized ionic liquid, Acid catalyst

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INTRODUCTION

in the motional freedom of each ion and strong intermolecular interactions.5 These ZI-type ILs have been studied as electrolyte materials for lithium batteries and fuel cells,16,17 acid catalysts,18 desulfurization of fuels,19 solubilization of metal oxides,20 and liquid crystals.21,22 Another interesting attribute of ZIs is their ability to act as precursors for Brönsted acidic IL preparations. These ILs can be used as solvents as well as catalysts in various acid-catalyzed organic transformations.4,23−27 Brönsted acidic ILs have been reported as novel eco-friendly catalysts for acid catalyzed reactions.2,4 The reactions can be carried out using solvent-free conditions and products can be separated by easy decantation which allows the ILs to be recycled. These characteristics make Brönsted acidic ILs more popular versus conventional homogeneous and heterogeneous acid catalysts. Recently, many research groups have reported making super acidic Brönsted ILs by reacting chlorosulfonic acid (ClSO3H) with tertiary N-donor molecules (e.g., 1methylimidazole (C1im) and triethylamine (N222)).28−34 The researchers proposed [L-SO3H]Cl (L = tertiary N-donor ligand) IL structures (e.g., [C1im-SO3H]Cl, [N222−SO3H]Cl), as a result of these studies. To the best of our knowledge, there is no structural evidence about ILs prepared from the reaction of ClSO3H and tertiary N-donor ligand in the literature. On the basis of our studies of sulfamation chemistry, we followed

Ionic liquids (ILs) are molten salts defined with a melting temperature below 100 °C and have attracted significant attention from researchers due to their favorable physicochemical properties. Ionic liquids are being used for a variety of industrial applications.1−4 There is increasing demand and considerable research taking place to discover and synthesize new ILs for a diverse variety of applications.5 Some of this research has led to the science of functional zwitterions (ZIs) or zwitterionic salts, which have found applications in multidisciplinary research areas.5−15 Zwitterionic salts are organic salts consisting of both a cation and an anion tethered covalently; whereas, in the ILs, ions are held together by electrostatic interactions (Figure 1). The most common ZItype ILs are nitrogen/phosphorus-based heterocycles with sulfonate/carboxylate functions. Examples of representative structures are given in Scheme 1. The typical melting point of ZIs is generally higher than classical ILs because of a decrease

Received: December 15, 2018 Revised: January 11, 2019 Published: February 5, 2019

Figure 1. Ion pairs in ionic liquid and zwitterion-type salt. © XXXX American Chemical Society

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DOI: 10.1021/acssuschemeng.8b06573 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering Scheme 1. Common Zwitterion-Types of Salt Structures

Scheme 2. Schematic Representation for the Reaction of C1im and ClSO3H

Figure 2. The 50% probability ellipsoid plot of [C1im-SO3] zwitterion salt (all bond lengths in the figure have an esd of 0.003 Å) (left) and PXRD of spectrum of C1im/ClSO3H reaction system compared with the corresponding SCXRD (right). methane solvent (400 mL) at room temperature. After the addition was completed, the reaction mixture was stirred for 12 h and left to stand for 24 h. After reaction, a solid precipitate was obtained. While the reaction mixture was kept in dichloromethane at room temperature for 24 h, a bundle of crystals was obtained. The dichloromethane solvent was later decanted. The residue was washed twice with dry dichloromethane solvent. The remaining dichloromethane solvent from the residue was removed using a rotaevaporator under high vacuum, and a crystalline solid compound was obtained. The solid compound was characterized by NMR, XRD, elemental analysis, and TGA-DSC techniques. Spectral data for 1H NMR (DMSO-d6) δ (ppm): mixture of [C1im-SO3] and [C1imSO3H]Cl set of peaks at 7:3 ratios; 1st set major compound [C1imSO3] at 3.87 (s, 2.1H, CH3), 7.64 (m, 0.7H, ImH), 7.68 (m, 0.7H, ImH), 9.07 (s, 0.7H, ImH); 2nd set minor compound [C1im-

our intuition that tertiary N-donor ligand will attack sulfur resulting in the evolution of HCl and form mostly [L-SO3] ZI salt, instead of [L-SO3H]Cl IL. In this communication, the unambiguous structural evidence by crystallography, NMR, and elemental analysis of a zwitterionic salt is found to be the major product derived from the well-known reaction of ClSO3H with the tertiary N-donor ligand molecule like C1im.

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EXPERIMENTAL SECTION

Synthetic Procedure for the Reaction of Chlorosulfonic Acid/1-Methylimidazole. The reaction of 1-methylimidazole and chlorosulfonic acid was carried out by following the reported procedure.28,29 In a typical synthesis, 1-methylimidazole (100 mmol) was reacted with ClSO3H (104 mmol) in dry dichloroB


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ACS Sustainable Chemistry & Engineering

Figure 3. NMR spectra of C1im/ClSO3H reaction system for 1H (top left), 13C (top right), COSY (bottom left), and HSQC (bottom right). SO3H][Cl] at 3.85 (s, 0.9H, CH3), 7.67 (m, 0.3H, ImH), 7.76 (m, 0.3H, ImH), 9.28 (s, 0.3H, ImH), 11.80 (s, 0.2H, −SO3H), 14.37 (s, 0.2H, −SO3H). Spectral data for 13C NMR (DMSO-d6) δ (ppm): mixture of [C1im-SO3] and [C1im-SO3H]Cl set of peaks; 1st set major compound [C1im-SO3] at 35.85 (CH3), 120.08 (ImH), 123.59 (ImH), 136.22 (ImH); 2nd set minor compound [C1im-SO3H][Cl] at 36.16 (CH3), 120.37 (ImH), 123.98 (ImH), 135.83 (ImH). Elemental analysis details: anal. calcd. for [C1im-SO3] at C 29.63, H 3.73, N 17.28, Cl 0; anal. calcd. for [C1im-SO3H]Cl at C 24.19, H 3.55, N 14.10, Cl 17.85; found for C1 im/ClSO3H reaction system at C 26.46, H 3.74, 15.37, Cl 4.78. SCXRD: The crystal from the C1 im/ ClSO3H reaction system was analyzed by SCXRD and confirmed a zwitterionic salt structure ([C1im-SO3]. PXRD of the bulk sample [C1im-SO3] was compared with the SCXRD data and confirmed that all the bulk material is the same as the zwitterionic salt. Chemicals and Instruments. Details are provided in the Supporting Information.

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RESULTS AND DISCUSSION To confirm our hypothesis about the structure of the product from the well-known reaction of ClSO3H with one of the tertiary N-donor ligands C1im, ClSO3H was reacted with C1im at 1.05:1 molar ratios in excess dichloromethane solvent following the procedure from the literature28,29 (Scheme 2). After ClSO3H and C1im were reacted, an exothermic reaction was observed, and a solid precipitate was obtained in the reaction mixture. While the reaction mixture was kept in dichloromethane at room temperature for 24 h, a bundle of crystals was observed. The dichloromethane solvent was evaporated using a rota-evaporator, and a colorless solid

Figure 4. TGA-DSC results (5 °C min−1) in nitrogen for the [C1imSO3] ZI salt (TGA spectra in the black and DSC spectra in the red).

compound was obtained. The crystals from the reaction system were analyzed by single crystal X-ray diffraction (SCXRD), and a zwitterionic salt structure [C1im-SO3] (Figure 2, left) was identified. The SCXRD data suggest that tertiary N-donor ligand molecules like C1im react with Cl−SO3H and produce a [C1im-SO3] zwitterionic salt; however, no [C1im-SO3H]Cl IL C


Letter

ACS Sustainable Chemistry & Engineering Scheme 3. Plausible Reaction Mechanism of 1-Methylimidazole/ClSO3H System

SO3H]Cl confirms the mixture of these two compounds, see the Experimental Section. The thermal stability of the new ZI salt was investigated using thermogravimetric analysis (TGA) (Figure 4). The TGA results confirm that the [C1im-SO3] ZI salt does not decompose below 325 °C. Differential scanning calorimetry (DSC) confirms a high melting point (Tm= 325.8 °C) for the ZI salt. The onset of melting temperature is 325 °C, and the heat absorbed during salt decomposition is 525.1 J/g. While C1im reacts with ClSO3H, the reaction can follow two pathways (Scheme 3). In path A, C1 im reacts with ClSO3H and results in [C1im-SO3H]Cl IL.28,29 In path B, the C1im reacts with ClSO3H and results in [C1im-SO3] zwitterion as a major product by releasing HCl (gas/liquid) as a side product. The released HCl can again react with zwitterion and generates [C1im-SO3H]Cl IL.

was detected due to the amorphous nature of the compound. The single crystal structure determination was unambiguous (see Table S1 and the Supporting Information). All hydrogen atoms were located using a single difference Fourier analysis after anisotropically refining all non-hydrogen atoms. The hydrogen atoms were included in the structural model as isotropic atoms. The bond lengths (Figure 2, left) clearly indicate a positive charge delocalized over the five-membered ring. The PXRD of the bulk sample [C1im-SO3] was also compared with the SCXRD (Figure 2, right). The PXRD and SCXRD data match well, indicating that all the bulk crystalline material is the same as the ZI single crystal salt structure. 1 H and 13C NMR spectra of the C1im/ClSO3H reaction system were investigated to understand the structure of the product, and the spectral data have been provided in the Experimental Section. The graphical 1H and 13C NMR spectra of the reaction system are presented in Figure 3. Both the 1H and 13C NMR data for the reaction system showed two set of peaks, indicating a mixture of compounds ([C1im-SO3] and [C1im-SO3H]Cl at a 7:3 ratio). The COSY and HSQC of the C1im/ClSO3H reaction system were investigated to characterize the compounds and supported the identification (Figure 3). The two peaks at 14.4 and 11.8 ppm are assigned to the acidic proton of the −SO3H group in [C1im-SO3H]Cl IL due to the mixture of ZI and IL. The reason for getting two acidic proton peaks for the C1im/ClSO3H reaction system was a function of the exchangeable acidic proton. To confirm the exchangeable protons, a control experiment was carried out to prepare pure IL [C1im-SO3H]Cl from a mixture of [C1imSO3] and [C1im-SO3H]Cl. In the control experiment, pure IL [C1im-SO3H]Cl was prepared by reacting the products of the C1im/ClSO3H system with HCl at a 1:1 molar ratio. NMR confirmed for the prepared pure IL [C1im-SO3H]Cl that only one set of peaks is present, which is similar to the [C1imSO3H]Cl (the [C1im-SO3] set of peaks is no longer present in both 1H and 13C NMR spectra). The acidic peak at 11.8 ppm disappears, and only the peak at 14.4 ppm which is assigned to the acidic proton for the −SO3H group in [C1im-SO3H]Cl IL remains. The control experiment results strongly support that the two acidic protons obtained for the mixture of ZI and IL are due to the exchangeable nature of the acidic protons. The elemental analysis for the C1im/ClSO3H reaction system compared with the pure compounds [C1im-SO3] and [C1im-

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CONCLUSIONS In summary, the well-known reaction of ClSO3H with a tertiary N-donor ligand such as 1-methylimidazole (C1im) forms a mixture of zwitterion-type salt ([C1 im-SO3] and IL [C1im-SO3H]Cl at a 7:3 ratio) rather than only IL as proposed by other groups. The SCXRD and PXRD results confirm that the single crystals analyzed are a representation of the bulk crystalline material. TGA-DSC results confirm that these ZI salts are stable with a high melting temperature around 325 °C. The ZI salts can be used as precursors for the preparation of many new ILs with tunable acidity. This study provides new possibilities for the preparation as well as the application of new ILs in multidisciplinary research areas. Our investigation of new applications for ZIs in catalysis is in progress.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b06573. Additional experimental details and the crystal data and structure refinement for C1im-SO3 ZI salt (Table S1) (PDF) D


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ACS Sustainable Chemistry & Engineering Accession Codes

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CCDC 1873144 contains the supplementary crystallographic data for this paper. This data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: Mark.B.Shifl[email protected]. ORCID

Rajkumar Kore: 0000-0002-3361-1188 Mark B. Shiflett: 0000-0002-8934-6192 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank The University of Kansas for supporting this exploratory research. The authors also thank the National Science Foundation−Major Research Instrumentation Program (NSF-MRI CHE-0923449, for X-ray diffractometer; NSF 0320648, for NMR) and the National Institutes of Health (NIH S10RR024664, for NMR) for funding that was used to purchase instrumentation used in this study.

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REFERENCES

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Ionic Liquid or Zwitterionic - ACS Publications - American Chemical

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