This is the accepted version of the journal article: Mena Fernández, Silvia; Guirado López, Gonzalo. «One-pot sustainable syn- thesis of tetrabutylammonium bis(trifluoromethanesulfonyl)imide ionic liquid». Journal of Molecular Liquids, Vol. 312 (August 2020), art. 113393. DOI 10.1016/j.molliq.2020.113393 This version is available at https://ddd.uab.cat/record/274574 under the terms of the license 1 One-pot sustainable synthesis of tetrabutylammonium 2 bis(trifluoromethanesulfonyl)imide ionic liquid 3 Silvia Mena and Gonzalo Guirado* 4 Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, 5 Barcelona, Spain 6 7 Abstract 8 Tetrabutylammonium bis(trifluoromethanesulfonyl)imide (TBA TFSI) and 9 tetrafluororate-based anion ionic liquids (ILs) were obtained simultaneously by using a 10 fast, versatile, cheap, low cost, and one-pot process. The process relies on dissolving 11 tetrabutylammonium tetrafluoroborate salt, an organic-based 12 bis(trifluoromethanesulfonyl)imide, ILs, such as 1-ethyl-3-methylimidazolium, N- 13 trimethyl-N-butylammonium, 1-butyl-1-methylpyrrolidinium, 1-methyl-1- 14 propylpiperidinium derivatives in dimethylformamide. A further extraction process using 15 water and ether allows the selective recovery of the TBA TFSI in the organic phase, and 16 the organic-based tetrafluoroborate IL in the aqueous phase. All the ionic liquids 17 synthetized were characterized using Infrared Spectroscopy and Nuclear Magnetic 18 Resonance, and compared with commercially available samples. 19 20 21 Graphical Abstract 22 23 DMF Aqueous phase 24 25 Ether 26 H2O Organic phase 27 28 29 30 31 1/14 32 1. Introduction 33 Solvents define a major part of the performance of processes in the chemical 34 industry, and have an impact on cost, safety, and health issues. The use of alternative 35 solvents represents a major entry in the general green chemistry toolkit, and is the subject 36 of an enormous research effort[1–8]. In this sense, an attractive alternative to conventional 37 organic solvents is the use of ionic liquids, ILs, due to both their benign chemical features 38 (non-flammable, non-volatile, and thermally stable over a wide range of temperatures)[9– 15] 39 and their electrochemical characteristics. ILs have from moderate to high 40 conductivities because they are liquids consisting of only ions and have good 41 electrochemical stability (stable from -3.00 V to +1.7 V vs. Saturated Calomel Electrode 42 (SCE) for reduction-oxidation processes)[16–20]. However, the processes and the reactant 43 used for synthetizing ILs require a critical analysis from an environmental point of view. 44 In most of the cases, it is not possible to obtain the IL with the desired anion in one single 45 step, so two main different reaction pathways are used: 1) Lewis-acid-base reaction, or 2) 46 anion metathesis[21–25]. Both types of reactions are performed from the halide salts of ionic 47 liquids, producing a considerable amount of non-desired byproducts. In this regard, one 48 important drawback associated with synthetic routes is to remove the impurities or 49 byproducts of the ILs, which is a hard and tedious task (Scheme 1). Note that pure ILs 50 are specially required for homogeneous catalysis, supporting electrolyte and electronic 51 applications, such as gating in transistors[26,27]. 52 53 Scheme 1. Reaction pathways 54 In this sense the tetrabutylammonium bis(trifluoromethylsulfonyl)imide (TBA TFSI) is 55 one of the most promising IL for electronic applications, more exactly, for transistor 56 design. However, up until now, there are only three synthetic routes for obtaining TBA 57 TFSI IL, and all of them generate a considerably amount of lithium salts or other 58 byproducts [9,10,21,28] . The aim of this work is to design a cleaner strategy for obtaining 59 TBA TFSI by mixing an organic TFSI ionic liquid and a tetrabutylammonium 60 tetrafluoroborate salt (TBA BF4) in dimethylformamide (DMF). This new synthetic route 2/14 61 will potentially allow two different ionic liquids, TBA TFSI and organic cation, C+ BF4- 62 to be produced after an ion exchange process. It is important to highlight that following 63 this new approach, then main product (TBA TFSI) and the byproducts formed in the ion 64 exchange process are also ILs, which will potentially have their own market. Finally, it is 65 remarkable there is the higher recyclability of the solvents involved in the chemical 66 process. 67 68 Scheme 2. Chart of structures 69 70 2. Experimental 71 2.1 Chemicals 72 Ionic liquids (1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM 73 TFSI), N-Trimethyl-N-butylammonium bis(trifluoromethanesulfonyl)imide (N1114 74 TFSI), 1-Butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14 TFSI), 75 1-Methyl-1-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13 TFSI)) were 76 purchased from Solvionic and used without further purification. Tetrabutylammonium 77 tetrafluoroborate (TBA BF4), Tetraethylammonium tetrafluoroborate (TEA BF4), 78 Tetramethylammonium hexafluorophosphate (TMA PF6), Tetrabutylammonium bis 79 (trifluoromethanesulfonyl)imide (TBA TFSI)) and N,N-dimethylformamide (DMF), 80 99.8%, were supplied by Sigma-Aldrich and used as received (Scheme 2). 81 82 2.2 Analysis and Product Characterization 83 Products obtained were characterized by Nuclear Magnetic Resonance and Near Infrared 84 Spectroscopy and compared with commercially available samples. 3/14 85 86 2.2.1 Nuclear Magnetic Resonance (NMR) 87 Synthetized ionic liquids and commercially available samples of the synthesized ionic 88 liquids were characterized by 1H NMR, 13 C NMR and 19 F NMR. Measurements were 89 performed in a Bruker DPX360 (360 MHz) (Billerica, MA, USA) spectrometer, and 90 Bruker DPX250 (250 MHz) with a Quattro Nucleus Probe (QNP). Proton chemical shifts 91 were reported in ppm (δ) (CDCl3, δ = 7.26 or CD3CN, δ = 1.94). Carbon chemical shifts 92 are reported in ppm (δ) (CDCl3, δ = 77.2 or CD3CN, δ = 1.32). The J values are reported 93 in Hz. Fluor chemical shifts are reported in ppm (δ) (CDCl3). 94 95 2.2.2 Infrared Spectroscopy (IR) 96 Near infrared spectra were recorded in attenuated total reflectance (ATR) mode on a 97 Model Tensor 27 spectrophotometer from Bruker that was governed via the software 98 OPUS 5.5, also from Bruker. 99 100 101 3. Results and discussion 102 103 Scheme 3 describes the simple ionic exchange process between TBA BF4 (organic salt) 104 and EMIM TFSI (ionic liquid). An equimolar amount of the above-mentioned salt and 105 IL are dissolved in dimethylformamide (DMF) by stirring the mixture for five minutes at 106 room temperature. After that the DMF solution was poured into a separator funnel, which 107 contained the same amount of water and ether. Due to the fact that neither EMIM TFSI 108 nor TBA BF4 is soluble in water, an ion exchange process takes place. Hence, in the 109 organic phase, after washing and drying this phase with water and anhydrous sodium 110 sulphate, the desired IL (tetrabutylammonium bis(trifluoromethanesulfonyl)imide) is 111 obtained with an 89% yield. Moreover, the byproduct of the ion exchange between 112 tetrabutylammonium tetrafluoroborate and 1-ethyl-3-methylimidazolium 113 bis(trifluoromethanesulfonyl)imide, EMIM BF4, is easily recovered from the aqueous 114 phase after several washings with dichloromethane. Distillation of dichloromethane 115 enables EMIM BF4 to be obtained in a 66.7% yield. It is important to highlight that the 116 one-pot synthetic process above described enables two highly pure ionic liquids to be 117 obtained following several principles of Green Chemistry, such as high atom economy, 4/14 118 the utilization of by-products, recovery of the solvents, as well as recyclability of the 119 reactants. 120 121 122 Distillation 123 124 125 H2O + ether (1:1) + ether 126 CH2Cl2 127 Δ 128 + H2O + reagents reagents + CH2Cl2 + H2O 129 130 Scheme 3. Ionic exchange for EMIM TFSI and TBA BF4 131 132 Standard spectroscopy techniques were used as useful tools to identify and characterize 133 the products formed. Quick comparisons between the spectroscopic features of pure 134 reactants, compounds obtained, and pure commercial available samples of the compounds 135 obtained verified the exchange process. [28–30] Figure 1 (a-b) shows the different IR spectra 136 for the TBA BF4 and EMIM TFSI reagents, and the synthesized ionic liquids, TBA TFSI 137 and EMIM BF4. All frequencies described in IR for the EMIM+, TBA+cations, and TFSI-, 138 BF4- anions are summarized in Table S1. The stretching/bending vibrations for the cation 139 part could be distinguished, focusing between 2880 - 2970 cm-1 for TBA+ cation and 140 between 2950 - 3165 cm-1 for EMIM+. Furthermore, the imidazolium ring of EMIM+ 141 shows characteristic vibrations between 1300 – 1600 cm-1. In the anion part, the spectra 142 for BF4- is easily described as it shows a huge peak at 1050 cm-1; however, in the case of 143 the TFSI- anion the spectra show four different peaks that describe the stretching and 144 bending for CF3, SO2, and S-N-S bonds. 145 146 5/14 a) 1,0 0,8 Trasmittance (%) 0,6 0,4 0,2 TBA BF4 EMIM TFSI 0,0 3200 3000 2800 1600 1400 1200 1000 800 600 -1 Wavenumber (cm ) 147 b) 1,0 0,8 Transmittance (%) 0,6 0,4 0,2 EMIM BF4 TBA TFSI 0,0 3200 3000 2800 1600 1400 1200 1000 800 600 -1 Wavenumber (cm ) 148 149 Figure 1. IR spectra of ionic exchange process between TBA BF4 and EMIM TFSI a) before mixing and b) after a 150 mixing and a purification process 151 152 On the other hand, figure 2 and 3 show the 1H-NMR spectrum for EMIM BF4, EMIM 153 TFSI, TBA BF4 and TBA TFSI. It is easy to distinguish between cation EMIM+ and TBA+ 154 because they have very different kind of protons in their structure; TBA+ presents 4 155 different signals of its alkyl protons, whereas EMIM+ has 6 different proton types, which 156 appear at a higher chemical shift because of the resonance of the imidazolium ring. 157 Moreover, the EMIM+ cation described an ion-pair formation equilibrium with BF4- and 158 TFSI- anions in dilute low dielectric solutions, such as chloroform[31]. Hence, 1H-NMR 6/14 159 describes two concurrent sets of signals due to the presence in the sample of freely 160 dissolved ions and ion-pair aggregates, these signals are different depending on anion. 161 Reagent EMIM TFSI 162 c b 163 e f a b 164 d 165 a c e d 166 f 167 168 Product EMIM BF4 169 b CDCl3 c 170 e a f b d 171 172 a d e f c 173 174 175 176 177 Figure 2. 1H NMR of EMIM BF4 and EMIM TFSI ILs 178 179 7/14 Reagent TBA BF4 c a a d b d c b a Product TBA TFSI c b a d c d b 180 181 Figure 3. 1H NMR of TBA BF4 salt and TBA TFSI IL 182 183 Furthermore, the combined use of 13C NMR and 19F NMR allow the nature of the counter 13 184 anion present to be identified in the ILs formed. Figure 4 shows the C NMR for the 185 TBA+ cation paired with TFSI- and BF4-. An extra signal (C5) TBA TFSI, which is related 186 to CF3 group. Table S2 summarizes the 1H-NMR and 13C-NMR signals for TBA TFSI. 19 187 F NMR data are reported in Figure S1, according to previously published data-) [32–34] 188 two different signals are observed for BF4- and TFSI- anions at 152.4 and 79.9 ppm, 189 respectively. 190 191 192 193 194 8/14 C1 C2 C3 C4 C4 C2 C3 C1 C3 C4 C1 C2 C4 C2 C3 C5 C1 C5 195 196 Figure 4. 13C NMR for TBA BF4 and TBA TFSI 197 198 The current methodology can be extended to other similar 199 bis(trifluoromethanesulfonyl)imide based ionic liquids by a simple mixture with TBA 200 BF4. Table 1 summarizes the yields obtained after the ionic exchange and phase 201 separation process described in Scheme 3, when N-Trimethyl-N-butylammonium 202 bis(trifluoromethanesulfonyl)imide (N1114 TFSI), 1-Butyl-1-methylpyrrolidinium 203 bis(trifluoromethanesulfonyl)imide (Pyr14 TFSI), 1-Methyl-1-propylpiperidinium 204 bis(trifluoromethanesulfonyl)imide (PP13 TFSI) are used as reagents. Notice that in all 205 cases are obtained TBA TFSI is obtained a good yield, whereas the by-product (BF4- 206 based ionic liquid) is obtained in moderate yields. However, the ionic exchange process 207 is strongly dependent on the length of the alkyl chain present in the tetraalkylammonium 208 salt, as it was previously described in the literature [9] Hence, no ionic exchange products 209 were obtained when Tetraethylammonium tetrafluoroborate (TEA BF4) or salt is used as 210 starting material. This fact can be rationalized by taking into account the different 211 hydrophobicity of both cations, TEA+ and TBA+; therefore, it is soluble in aqueous phase. 212 213 214 9/14 Reactive + TBA BF4 Products + TBA TFSI TBA TFSI Yield (%) C+ BF4- Yield (%) 73.4 31.2 67.3 35.4 71.2 41.4 215 216 Table 1. Ionic liquid yields obtained after ionic exchange and purification processes. 217 218 The purity of the ionic liquids synthetized following this methodology are determined 219 employing the 1H NMR data using of 13C satellites of imidazolium N-methyl group as 220 internal standard[35] as well as the integration of similar protons in non-imidazolium ILs. 221 In the case of TBA TFSI the purity of IL obtained was from 96.2% to 99.3 % depending 222 on the type of C+TFSI used as reagent. For the rest of the ILs obtained as a byproducts 223 purities obtained were the following: 99.2% for Pyr14 TFSI, 93.5% for PP13 TFSI, 98.0% 224 for N1114 TFSI and 99.1% for EMIM TFSI. Finally, it is remarkable that a comparison in 225 terms of atom economy between our methodology and a classical one starting from 226 similar products reveals that are very similar in both cases (c.a. 80 %). However, the 227 byproducts obtained using our methodology are also ILs and the solvent used are 228 recyclable, whereas in the case the classical route the byproduct obtained is a salt (lithium 229 bromide). 230 231 4. Conclusions 232 233 In the current manuscript we have been able to design a new synthetic route for 234 synthetizing the tetrabutylammonium bis (trifluoromethanesulfonyl) imide ionic liquid 235 (TBA TFSI) following a sustainable one-pot methodology based on an ion exchange 236 process with purities from 96 to 99 %. It is important to highlight tetrafluoroborate based 237 ionic liquids are also obtained as a secondary product with purities from 98 to 99 %. In 238 terms of cost-effectiveness analysis, our exchange ionic approach is a low-cost alternative 239 since the products obtained at the end (ILs) have increased their value by ca 40% in 240 comparison with the starting IL and salt used. 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