Synthesis of chiral iron-based ionic liquids: modelling stable hybrid materials

1 FINAL VERSION ACCEPTED Synthesis of Chiral Iron-Based Ionic Liquids: Modelling Stable Hybrid Materials† Carmen Martin,a,b,* Israel Cano,c,* Fabio Scé,a Rubén Pérez-Aguirre,d Carolina Gimbert-Suriñach,e Pilar Lopez-Cornejob and Imanol de Pedroa ABSTRACT: Five chiral iron-containing ionic liquids were synthetized from several chiral ionic liquids (CILs) based on different imidazolium cations. These novel compounds were prepared through an easy synthetic method consisting in mixing 1 equivalent of iron salt FeX3 (X = Cl or Br) and 1 equivalent of chiral ionic liquid. The so-obtained chiral ironbased ionic liquids contain easily tunable imidazolium cations, which allow the preparation of a wide range of compounds within different functional groups and chiral moieties. These chiral iron-based ionic liquids have been fully characterized by a wide variety of techniques, including polarimetry, inductively coupled plasma optical emission spectrometry, elemental analysis, thermogravimetric analysis, differential scanning calorimetry, magnetic susceptibility measurements, and UV-vis, attenuated total reflection Fourier-transform infrared and Raman spectroscopies. Such experiments confirmed the structure of these newly materials, as well as their appealing properties. The combination of a chiral moiety with a haloferrate anion within the ILs structure allows the formation of very stable and multifunctional compounds with promising chiral, magnetic, optical and acidic properties. The resulting chiral iron-based ILs exhibit great potential for use in a range of applications such as enantioselective catalysis or chiral recognition, to name a few. Keywords: Acidic chiral paramagnetic ionic liquid • hybrid material • imidazolium-based halometallate complex • iron-containing ionic liquid a Universidad de Cantabria, CITIMAC, Facultad de Ciencias, Avda. de los Castros s/n, 39005 Santander, Spain. b Universidad de Sevilla, Department of Physical Chemistry, Faculty of Chemistry, c/Profesor García González s/n, 41012 Sevilla, Spain. c School of Chemistry, University of Nottingham, NG7 2RD, Nottingham, UK. d Universidad del País Vasco, Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Apartado 644, E-48080, Bilbao, Spain. e Institute of Chemical Research of Catalonia (ICIQ). Avinguda Països Catalans 16, 43007 Tarragona, Spain. † Electronic supplementary information (ESI) available: Crystallographic data, analytical and spectral data. CCDC: 1961691 and 1962239. 2 INTRODUCTION Multifunctional compounds based on ionic liquids (ILs) have gained considerable interest owing to the unique properties of ILs (e.g., good thermal stability, high ionic conductivity and wide electrochemical stability window).1,2,3,4 In particular, acidic ionic liquids (AILs) have been extensively investigated due to their capability to act as membranes, electrolytes for batteries and capacitors or catalysts (e.g., depolymerisation of cellulose, production of biodiesel, oxidation reactions, etc.).5,6 Among them, halometallate-based ILs7 are an important subgroup of AILs which contain a metal-based Lewis acid within their structure. This metal component enhances their features against purely organic ILs, such as lower viscosities and higher conductivities, and makes them very interesting compounds for a number of applications, including electrical-energy storage8 and catalysis (e.g. Friedel–Crafts reactions, Michael addition and polyethylene terephthalate (PET) glycolysis).9,10,11,12 Interestingly, their architecture can be easily modulated to create tailor-made ILs with different physical and chemical properties. In this regard, chiral centers or magnetic moieties within ILs structures can improve the AILs performance (e.g. in catalysis) and tune their properties. However, the development of new materials with simultaneous chiral, magnetic and acidic characteristic is still in its infancy and only two articles have been reported on the preparation of acidic chiral paramagnetic ionic liquids (ACMILs).13,14 Among them, it is worth mentioning the work of Warner and co-workers, in which the synthesis of chloroferrate(III) chiral ILs incorporating amino acid derived cations and their use as chiral recognition agents of fluorescent analytes is described.13 Recently, our group has developed new halometallate ILs based on iron.15,16,17 We have shown that these so-called halometallate-based ILs (containing a Lewis acid and a nucleophile within the structure) display a good catalytic activity in the glycolysis of PET, leading to better yields and selectivities than those reported with purely organic ILs.16,17 In a next step, we have decided to extend this work introducing chiral features into the AILs structures. In this context, the imidazolium cation is easily tunable and allows the introduction of functional groups and chiral moieties, opening the door to an enormous variety of compounds. Thus, herein we report the synthesis of five new and stable chiral imidazolium-based haloferrate(III) ILs with diverse architectures (Fig. 1). To investigate their stability and appealing chiral/magnetic properties, a detailed characterization of these multifunctional ILs was performed by the state-of-the-art techniques, such as polarimetry, inductively coupled plasma optical emission spectrometry (ICP-OES), elemental analysis (EA), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV-vis spectroscopy, magnetic susceptibility measurements, and 3 attenuated total reflection Fourier-transform infrared (ATR FT-IR) and Raman spectroscopies. These switchable ACMILs may be promising candidates as enantioselective catalysts (e.g. CO2 fixation into cyclic carbonates)18 or chiral recognition agents.13 Fig. 1 Overall scheme of the preparation of ACMLs, properties and applications. Therefore, the main objective of this work is the preparation of a series of new multifunctional ILs by the precise design of the imidazolium cation structure and the appropriate selection of the anion. EXPERIMENTAL General Procedures Solvents were purchased from Sigma-Aldrich as HPLC grade and dried by means of an Inert Puresolv MD purification system. All reagents were purchased from commercial suppliers (Sigma Aldrich and Across) and purified when required by literature procedures.19 The ion exchange resin Amberlite IRA-402(OH) was supplied by Alfa Aesar. Elemental analysis (EA). Elemental analyses were performed by the Elemental Analysis Service of the University of Nottingham. Nuclear magnetic resonance (NMR). 1H and 13C NMR spectra were recorded on a Bruker AV3400 400 MHz and a Bruker DPX500 500 MHz nuclear magnetic resonance spectrometers, and referenced to the residual NMR solvent signals. Signals are quoted as s (singlet), d (doublet), t (triplet), dd (double doublet), td (triplet doublet) and m (multiplet). 4 Electrospray ionization mass spectrometry (ESI-MS). ESI-MS analyses were carried out on a Bruker ESI-TOF MicroTOF II and Orbitrap Elite (Thermo Scientific) spectrometers. Fourier transform infrared spectroscopy (FT-IR). FT-IR measurements were performed on a Bruker Alpha Series FT-IR spectrometer equipped with an attenuated total reflectance (ATR) module. The ATR FT-IR spectra were recorded by collecting 24 scans of a compound in the ATR module. Non-polarized Raman spectra. The non-polarized Raman spectra were recorded in the backscattering geometry with a Horiba T64000 triple spectrometer with a confocal microscope in the subtractive mode which had a resolution of 0.6 cm-1, a 1800 grooves/mm grating and a 100-μm slit was equipped with a liquid N2-cooled CCD detector (Jobin-Yvon Symphony). A 647 nm line of a Coherent Innova Spectrum 70C Ar+-Kr+ laser was focused down with a 20x objective and kept the power on the sample below 5 mW to avoid laser-heating effects on the material being tested and the concomitant softening of the observed Raman peaks. UV-vis spectroscopy. UV−vis measurements were carried out on a Shimadzu UV- 2401PC spectrophotometer equipped with a photomultiplier detector, double beam optics, and D2 and W light sources. Samples were measured at 25°C and with dimethylformamide (DMF) as solvent. Differential scanning calorimetry (DSC). DSC measurements were carried out on a TA instruments Discovery from -90 °C to 150 °C at a rate of 10 °C/min under nitrogen atmosphere. Thermogravimetric analysis (TGA). Thermogravimetric measurements were carried out on a TA instruments Discovery at a heating rate of 10 °C/min and in a temperature range from 100 °C to 1000 °C under N2 in a platinum crucible. Note that the decomposition temperature (Td) was determined by TGA at 10% weight loss of the analyte. Inductively coupled plasma optical emission spectrometry (ICP-OES). The iron content determination was performed through analysis of the samples by ICP-OES on a Perkin Elmer Optima 2000 DV ICP-OES. The samples were prepared by diluting 25 mL of ionic liquid with approximated iron concentration of 150 mg/L (ppm) in HNO3 with a concentration of 2% (v/V). Calibration curves were prepared using standard solutions of concentrations between 0–250 ppm prepared by dilution of the standard solution. The iron Standard for ICP TraceCERT® (10′000 mg/L Fe in nitric acid) was supplied by Sigma Aldrich. Magnetization measurements. DC magnetic susceptibility measurements for ionic liquids 5-9 were carried out in a Quantum Design MPMS SQUID magnetometer using a 5 field of 1 kOe in the 2-300 K temperature range. Magnetization as a function of field (H) was measured using the same magnetometer in the -50 ≤ H/kOe ≤ 50 at 2 K after cooling the sample in zero field. Polarimetry. Optical rotations were measured in a 1.0 dm tube with a Perkin Elmer spectropolarimeter (model 341) equipped with a Na lamp. X-ray crystal structure determinations (CCDC: 1961691 for 1 and 1962239 for 4). X- ray crystal structure determinations were performed at the X-ray Service of the University of Nottingham. Crystals suitable for X-ray diffraction were selected and covered with “fomblin” (YR-1800 perfluoropolyether oil). The crystal was then mounted on a polymertipped MicroMountTM and cooled to 120 K. Single crystal X-ray diffraction data were collected using an Agilent SuperNova diffractometer, Atlas CCD area detector (mirrormonochromated Cu-Kα radiation source; λ = 1.54184 Å or graphite-monochromated Mo- Kα radiation source; λ = 0.7103 Å; ω scans) and an Agilent SuperNovaII diffractometer Titan S2 CCD area detector (mirror monochromated Cu-Kα radiation source; λ = 1.54184 Å; ω scans). Crystal structures were solved and refined by means of the Olex2 software package,20,21 program using Charge Flipping and refined with the ShelXL.22 Crystallographic data have been deposited in the Cambridge Crystallographic Data Center under reference numbers CCDC 1961691 and 1962239 for CILs 1 and 4, respectively. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_ request/cif. Gas Chromatography (GC). Analyses (determination of enantiomeric excess) of the reaction mixtures were performed on a Thermo Scientific Trace 1310 Gas Chromatograph with a Lipodex A (hexakis-(2,3,6-tri-O-pentyl)-α-cyclodextrin) 25m column. Synthetic Procedures Synthesis of chiral imidazolium-based ionic liquids. - (S)-1-(2-Methylbutyl)-2,3-dimethylimidazolium chloride ([S-methylBMMIm]Cl, 1). (S)- (+)-1-Chloro-2-methylbutane (12.0 g, 0.11 mol, 1.10 equivalents) and 1,2-dimethyl-1H- imidazole (9.81 g, 0.10 mol, 1.00 equivalent) were dissolved in 5.50 mL of CH3CN in a round bottom flask under inert atmosphere. The reaction mixture was heated for 5 days at 100 °C under reflux. Afterwards, the solution was cooled to room temperature (R.T.) and the volatile materials were evaporated under vacuum. The remaining orange oil was re-dissolved in the minimum amount of dry CH3CN and added dropwise via cannula to a well-stirred solution of ethyl acetate (CH3CN:ethyl acetate ratio ca. 1:5). Once the addition of the CH3CN solution was completed, the mixture was cooled at -30 °C in the freezer to crystallize. The final product was washed with ethyl acetate and dried under 6 vacuum, obtaining a pale yellow powder. Yield: 12.1 g (0.06 mol, 65 %). Crystals of 1 suitable for X-ray diffraction analysis were obtained at R.T. by slow diffusion of ethyl acetate vapour into an CH3CN solution of 1. 1H NMR (400 MHz, CDCl3): δ = 7.88 (d, 3JHH = 2.0 Hz, 1H, CHAr), 7.44 (d, 3JHH = 2.0 Hz, 1H, CHAr), 4.08 (dd, 2JHH = 15.0 Hz and 3JHH = 7.5 Hz, 1H, NCH2), 4.08 (s, 3H, NCH3), 3.94 (dd, 2JHH = 14.0 Hz and 3JHH = 9.0 Hz, 1H, NCH2), 2.77 (s, 3H, NCCH3), 1.90-1.77 (m, 1H,CHCH3), 1.44-1.32 (m, 1H, CH2CH3), 1.26-1.14 (m, 1H, CH2CH3), 0.90 (t, 3JHH = 7.0 Hz, 3H, CH2CH3), 0.89 (d, 3JHH = 6.0 Hz, 3H, CHCH3). 13C NMR (101 MHz, CDCl3): δ = 143.8 (1C, NCN), 123.4 (1C, CHAr), 121.7 (1C, CHAr), 54.6 (1C, NCH2), 36.1 (1C, NCH3), 35.7 (1C, NCCH3), 26.6 (1C, CHCH3), 16.62 (1C, CH2CH3), 11.1 (1C, CHCH3), 10.8 (1C, CH2CH3). ESI-MS (+ ve): m/z = 167.1553 [M-Cl]+ (calculated: m/z = 167.1548). Characteristic IR bands (cm-1): 3121- 3022 (C-H sp2), 2960-2849 (C-H sp3), 1587 (C=C/C=N), 1536 (C-C/C-N), 1120(C-N), 910 (C-H sp2), 820 (C-H sp2), 758 (C-H sp2). [α]  = -2.4 (c = 5 mg/1 mL MeOH). Td = 250 ºC. (Fig. S1, S2, S7, S18, S26 and S51 in Supporting Information). - (R)-3-(2,3-Dihydroxypropyl)-1-methylimidazolium chloride ([R-diOHPrMIm]Cl, 2) and (S)-3-(2,3-dihydroxypropyl)-1-methylimidazolium chloride ([S-diOHPrMIm]Cl, 3). 2 and 3 were prepared according to procedures described in the literature.23 the (R) or (S)-3- Chloro-1,2-propanediol (12.2 g, 0.11 mol, 1.10 equivalents) was placed in a round bottom flask with a stirring bar and dissolved in CH3CN (10.0 mL) under inert atmosphere. After addition of 1-methyl-1H-imidazole (9.55 g, 0.10 mol, 1.0 equivalent), the reaction mixture was heated at 90 °C for 48 h under reflux. Upon completion of the reaction, the solvent was removed in vacuo. Then, the reaction crude was re-dissolved in the minimum amount of dry CH3CN and added dropwise via cannula to a well-stirred solution of ethyl acetate (CH3CN:ethyl acetate ratio ca. 1:5). Finally, the mixture was cooled at -30 °C in the freezer to crystallize. The final product was washed with ethyl acetate and dried under vacuum, obtaining a pale orange powder. Yield: 15.4 g (0.08 mol, 80 %). 1H NMR (400 MHz, D2O): δ = 8.77 (s, 1H, N-CH-N), 7.52 (m, 1H, CHAr), 7.48 (m, 1H, CHAr), 4.40 (dd, 2JHH = 15.0 Hz and 3JHH = 3.0 Hz, 1H, NCH2), 4.22 (dd, 2JHH = 14.5 Hz and 3JHH = 3.5 Hz, 1H, NCH2), 4.11-4.04 (m, 1H, CHOH), 3.93 (s, 3H, NCH3), 3.63 (dd, 3JHH = 5.0 Hz and 3JHH = 1.5 Hz, 2H, CH2OH). 13C NMR (101 MHz, D2O): δ = 136.6 (1C, NCHN), 123.5 (1C, CHAr), 122.9 (1C, CHAr), 69.8 (1C, CHOH), 62.4 (1C, CH2OH), 51.7 (1C, NCH2), 35.7 (1C, NCH3). ESI-MS (+ ve): m/z = 157.0979 [M-Cl]+ (calculated: m/z = 157.0977). Characteristic IR bands (cm-1): 3325 (O-H), 3210 (O-H), 3165-3058 (C-H sp2), 2933-2830 (C-H sp3), 1632 (C=C/C=N), 1558 (C-C/C-N), 1164 (C- N), 1109 (C-O), 1059 (C-O), 941 (C-H sp2), 846 (C-H sp2), 773 (C-H sp2). (Fig. S3, S4, S8 and S19). 7 R-[diOHPrMIm]Cl, 2: [α]  = +15.6 (c = 5 mg/1 mL MeOH). Td (10%) = 300 ºC. (Fig. S27). S-[diOHPrMIm]Cl, 3: [α]  = -16.0 (c = 5 mg/1 mL MeOH). Td (10%) = 280 ºC. (Fig. S28). - 1-((1S,2R,4S)-(+)-Menthoxymethyl)-2,3-dimethylimidazolium chloride ([R-menthyl MMIm]Cl, 4). 4 was prepared according to procedures described in the literature.24 (1S,2R,4S)-(+)-Chloromethyl menthyl ether (5.00 g, 24.4 mmol, 1.05 equivalents) and 1,2-dimethyl-1H-imidazole (1.00 g, 23.2 mmol, 1.00 equivalents) were dissolved in CH3CN (75 mL) in a round bottom flask under inert atmosphere. The reaction mixture was heated for 24 h at 90 °C under reflux. Next, the solution was cooled to R.T. and the solvent was evaporated under vacuum. Finally, the product was washed three times with diethyl ether and dried under vacuum to give a white powder. Yield = 6.27 g (20.9 mmol, 90 %). Crystals of 4 suitable for X-ray diffraction analysis were obtained at 6 ºC in CH3CN. 1H NMR (400 MHz, CDCl3): δ = 7.88 (d, 3JHH = 2.0 Hz, 1H, CHAr), 7.77 (d, 3JHH = 2.0 Hz, 1H, CHAr), 5.71 (d, 2JHH = 11.0 Hz, 1H, NCH2O), 5.64 (d, 2JHH = 11.0 Hz, 1H, NCH2O), 4.02 (s, 3H, NCH3), 3.30 (td, 3JHH = 10.0 Hz and 3JHH = 4.0 Hz, 1H, OCH), 2.83 (s, 3H, NCCH3), 2.09-2.03 (m, 1H, CH2), 1.97-1.87 (m, 1H, CH2), 1.97-1.87 (m, 1H, CHCH(CH3), 1.69-1.58 (m, 2H, CH2), 1.45-1.32 (m, 1H,CHCH3), 1.26-1.18 (m, 1H, CHCH(CH3)2), 0.95-0.88 (m, 2H, CH2), 0.92 (d, 3JHH = 6.6 Hz, 3H, CHCH3), 0.88 (d, 3JHH = 7.5 Hz, 3H, CHCH(CH3)2), 0.51 (d, 3JHH = 7.0 Hz, 3H, CHCH(CH3)2). 13C NMR (101 MHz, CDCl3): δ = 145.1 (1C, NCN), 123.0 (1C, CHAr), 121.8 (1C, CAr), 79.6 (1C, OCH), 76.4 (1C, NCH2O), 47.9 (1C, CH2), 40.4 (1C, CH2),35.9 (1C, NCH3), 34.2 (1C, CHCH(CH3)2), 31.4 (1C, CHCH3), 25.6 (1C, CHCH(CH3)2), 22.9 (1C,CH2), 22.3 (1C, CHCH3), 21.0 (1C, CHCH(CH3)2), 15.7 (1C, CHCH(CH3)2), 10.8 (1C, NCCH3). ESI-MS (+ ve): m/z = 265.2276 [M-Cl]+ (calculated: m/z = 265.2280). Characteristic IR bands (cm- 1): 3159-3072 (C-H sp2), 2960-2855 (C-H sp3), 1634 (C=C/C=N), 1571 (C-C/C-N), 1166 (C-O-C), 1042 (C-O-C), 838 (C-H sp2), 747 (C-H sp2). [α]  = +112.0 (c = 5 mg/1 mL MeOH). Td = 212 ºC. (Fig. S5, S6, S9, S20, S29 and S52). 1H and 13C NMR, ESI-MS, FT-IR and [α]  data were consistent with those previously reported.24 Synthesis of iron-containing ionic liquids. 1.0 equivalent of FeCl3 was mixed with 1.0 equivalent of the corresponding imidazolium salt (1–4) in a Schlenk flask under Ar atmosphere (glovebox). After stirring for 12 h at 90 ºC, the acidic chiral paramagnetic ionic liquids 5–7 and 9 were obtained with quantitative conversion. Similarly, the acidic chiral paramagnetic ionic liquid 8 was prepared by mixing 1.0 equivalent of FeBr3 with 1.0 equivalent of (S)-3-(2,3-dihydroxypropyl)-1-methylimidazolium bromide (S- [diOHPrMIm]Br). The reaction was stirred under Ar atmosphere for 12 h at 90 ºC to obtain 8. 8 S-[diOHPrMIm]Br synthesis: A solution containing 950.0 mg of 3 (4.9 mmol) in 5 mL of H2O was passed through a column packed with anion-exchange Amberlite resin IRA- 402(OH) using H2O as eluent. Afterwards,4.9 mmol of HBr (aq.) 48 wt% was added to the aqueous solution of the hydroxide form of 3 collected. After addition of HBr, the reaction mixture was stirred overnight at R.T. Upon completion of the reaction the solvent was removed in vacuo (rotary evaporator), yielding S-[diOHPrMIm]Br. - (S)-1-(2-Methylbutyl)-2,3-dimethylimidazolium tetrachloroferrate(III) ([S-methylBMMIm] [FeCl4], 5). ESI-MS (+ ve): m/z = 167.15 [M-FeCl4]+ (calculated: m/z = 167.1548). ESIMS (- ve): m/z = 197.81 [FeCl4]- (calculated: m/z = 197.8074). Elemental Analysis: Obs. C, 32.07; H, 5.11; N, 7.82. Calc. C, 32.91; H, 5.25; N, 7.68. ICP-OES (% weight): Obs. Fe, 14.81. Calc. Fe, 15.30. Characteristic IR bands (cm-1): 3178-3086 (C-H sp2), 2966- 2854 (C-H sp3), 1586 (C=C/C=N), 1536 (C-C/C-N), 1165 (C-N), 837 (C-H sp2), 804 (C- H sp2), 739 (C-H sp2). Raman (cm-1): 334, 332, 141, 110 (Fe-Cl). [α]  = -2.6 (c = 5 mg/1 mL MeOH). Td (10%) = 334 ºC and Tg = -63 ºC. UV-Vis [λmax/nm (ε/M−1·cm−1)]: 315 (20000), 364 (18666). μeff = 5.88 μB/molecule. (Fig. S10, S11, S21, S30, S31, S40 and S45). - (R)-3-(2,3-Dihydroxypropyl)-1-methylimidazolium tetrachloroferrate(III) ([R-diOHPr MIm][FeCl4], 6). ESI-MS (+ ve): m/z = 157.0970 [M-FeCl4]+ (calculated: m/z = 157.0972). ESI-MS (- ve): m/z = 197.8073 [FeCl4]- (calculated: m/z = 197.8074). Elemental Analysis: Obs. C, 22.35; H, 3.72; N, 7.04. Calc. C, 23.69; H, 3.69; N, 7.89. ICP-OES (% weight): Obs. Fe, 14.75. Calc. Fe, 15.74. Characteristic IR bands (cm-1): 3513 (O-H), 3364 (O- H), 3149-3096 (C-H sp2), 2962-2881 (C-H sp3), 1690 (C=C/C=N), 1556 (C-C/C-N), 1164 (C-N), 1096 (C-O), 1037 (C-O), 926 (C-H sp2), 829 (C-H sp2), 740 (C-H sp2). Raman (cm-1): 330, 328, 138, 108 (Fe-Cl). [α]  = +12.2 (c = 5 mg/1 mL MeOH). Td (10%) = 259 ºC and Tg = -35 ºC. UV-Vis [λmax/nm (ε/M−1·cm−1)]: 316 (84000), 364 (85000). μeff = 5.89 μB/molecule. (Fig. S12, S13, S22, S32, S33, S41 and S46). - (S)-3-(2,3-Dihydroxypropyl)-1-methylimidazolium tetrachloroferrate(III) ([S-diOHPr MIm][FeCl4], 7). ESI-MS (+ ve): m/z =157.0970 [M-FeCl4]+ (calculated: m/z = 157.0972). ESI-MS (- ve): m/z = 197.8073 [FeCl4]- (calculated: m/z = 197.8074). Elemental Analysis: Obs. C, 22.54; H, 3.70; N, 7.02. Calc. C, 23.69; H, 3.69; N, 7.89. ICP-OES (% weight): Obs. Fe, 15.24. Calc. Fe, 15.74. Characteristic IR bands (cm-1): 3513 (O-H), 3364 (O- H), 3149-3096 (C-H sp2), 2962-2881 (C-H sp3), 1690 (C=C/C=N), 1556 (C-C/C-N), 1164 (C-N), 1096 (C-O), 1037 (C-O), 926 (C-H sp2), 829 (C-H sp2), 740 (C-H sp2). Raman (cm-1): 332, 330, 140, 111 (Fe-Cl). [α]  = -13.6 (c = 5 mg/1 mL MeOH). Td (10%) = 257 ºC and Tg = -34 ºC. UV-Vis [λmax/nm (ε/M−1·cm−1)]: 317 (12750), 363 (10500). μeff = 5.88 μB/molecule. (Fig. S23, S34, S35, S42 and S47). 9 - (S)-3-(2,3-Dihydroxypropyl)-1-methylimidazolium tetrabromoferrate(III) ([S-diOHPr MIm][FeBr4], 8). ESI-MS (+ ve): m/z = 157.0970 [M-FeBr4]+ (calculated: m/z = 157.0972). ESI-MS (- ve): m/z = 375.6045 [FeBr4]- (calculated: m/z = 375.6042). Elemental Analysis: Obs. C, 14.76; H, 2.19; N, 5.03. Calc. C, 15.74; H, 2.46; N, 5.26. ICP-OES (% weight): Obs. Fe, 10.83. Calc. Fe, 10.48. Characteristic IR bands (cm-1): 3529 (O-H), 3370 (O- H), 3155-3099 (C-H sp2), 2961-2875 (C-H sp3), 1690 (C=C/C=N), 1563 (C-C/C-N), 1168 (C-N), 1099 (C-O), 1039 (C-O), 925 (C-H sp2), 825 (C-H sp2), 741 (C-H sp2). Raman (cm-1): 295, 198 (Fe-Br). [α]  = -18.4 (c = 5 mg/1 mL MeOH). Td (10%) = 275 ºC and Tg = -31 ºC. UV-Vis [λmax/nm (ε/M−1·cm−1)]: 394(3333), 426 (2619), 473, (3048). μeff = 6.09 μB/molecule. (Fig. S14, S15, S24, S36, S37, S43 and S48). - (R)-1-((1S,2R,4S)-(+)-Menthoxymethyl)-2,3-dimethylimidazolium tetrachloro ferrate(III) ([R-menthylMMIm][FeCl4], 9). ESI-MS (+ ve): m/z = 265.2280 [M-FeCl4]+ (calculated: m/z = 265.2280). ESI-MS (- ve): m/z = 197.8073 [FeCl4]- (calculated: m/z = 197.8074). Elemental Analysis: Obs. C, 44.32; H, 6.99; N, 7.09. Calc. C, 41.50; H, 6.31; N, 6.05. ICP-OES (% weight): Obs. Fe, 11.54. Calc. Fe, 12.06. Characteristic IR bands (cm-1): 3154-3140 (C-H sp2), 2958-2841 (C-H sp3), 1684 (C=C/C=N), 1582 (C-C/C-N), 1168(C- O), 1091 (C-O), 838 (C-H sp2), 735 (C-H sp2). Raman (cm-1): 335, 332, 142, 112 (Fe-Cl). [α]  = +52.2 (c = 5 mg/1 mL MeOH). Td (10%) = 210 ºC and Tg = 8 ºC. UV-Vis [λmax/nm (ε/M−1·cm−1)]: 318 (20167), 364 (16333). μeff = 6.08 μB/molecule. (Fig. S16, S17, S25, S38, S39, S44 and S49). Catalytic experiments 1.665 mmol of epoxide was mixed with 0.083 mmol of the Fe-based ionic liquid in a glass vial with a magnetic stirrer. Then, the vial was placed in a small reactor with a volume of 2 mL and the reactor was flushed with carbon dioxide (CO2) three times before the pressure was kept constant (2 bar). After 24 h of reaction at the desired temperature, the reactor was slowly depressurized. The product was purified by extraction with diethyl ether (2×5 mL) and filtration with SiO2 by flash chromatography. Afterwards, the solvent was evaporated. The residue analyzed by 1H NMR to determine conversion and selectivity and by GC to determine the %ee. RESULTS AND DISCUSSION The Fe-containing CILs 5–9 were prepared in a two-step synthetic strategy (Scheme 1, for further details see sections Synthesis of chiral imidazolium-based ionic liquids and Synthesis of iron-containing ionic liquids). First, the imidazolium-based CILs 1–4 were prepared according to procedures described in the literature.23,24 Then, 1 equivalent of FeX3 (X = Cl or Br) was mixed with 1 equivalent of the corresponding imidazolium-based 16 Finally, we performed the kinetic resolution of a racemic mixture of styrene oxide through the asymmetric cycloaddition of CO2 to epoxides catalyzed by Fe-containing chiral ionic liquids 5 and 9 (Table 5). Although the enantiomeric excesses observed are very low, this preliminary experiment serves as a proof of concept for the potential application of ACMILs as enantioselective catalysts. Table 5. Kinetic resolution of racemic mixture of styrene oxide by cycloaddition of CO2 catalyzed by Fe-based chiral ionic liquids 2 and 3.a Entry Catalyst T (ºC) Time (h) Conv. (%)b Selectivity (%)c % ee (epoxide)d 1 5 R.T. 24 58 85 : 12 : 3 4 2 9 0 7 21 77 : 18 : 5 6 a Reagents and conditions: 5 mol % of ACMILs, styrene oxide (1.665 mmol), 24 h, neat and p(CO2) = 2.0 bar. b Conversions determined by 1H NMR spectroscopy and refer to the selective conversion of styrene oxide (average of two runs).c % selectivity referred to A : B : C. d %ee were determined by GC (average of two runs). CONCLUSIONS We present the synthesis and characterization of four based on different imidazolium cations. These compounds were used as enantioselective precursors to prepare five acidic chiral paramagnetic ionic liquids through an easy synthetic method. The obtained ACMILs were fully characterized using polarimetry, ICP-OES, EA, and UV-vis, ATR FT- IR and Raman spectroscopies. In addition, the magnetic properties of these chiral ironbased ionic liquids were studied, showing a Fe3+ oxidation state with a magnetic spin S = 5/2. UV-vis measurements corroborate the presence of the [FeX4]- anion in all the ACMILs, while Raman studies confirm a tetrahedral symmetry for this species. We have also performed the thermal characterization of these novel ACMILs by TGA and DSC, which display liquid physical state and high thermal stability (up to 200 ºC). These new ACMILs combine simultaneously the optical, magnetic and acidic properties of [FeX4]- anions with the chiral features of imidazolium cations. Such interesting properties envisage a new approach to develop ACMILs with applications in enantioselective catalysis (e.g. CO2 fixation into cyclic carbonates) or chiral recognition agents, among others. AUTHOR INFORMATION Corresponding Authors *E-mail: mm[email protected] (Carmen Martín). 17 *E-mail: [email protected] (Israel Cano) ORCID Carmen Martín: 0000-0001-8687-6887 Israel Cano: 0000-0003-3727-9327 CONFLICTS OF INTEREST There are no conflicts of interest to declare. ACKNOWLEDGMENTS Financial support from the Spanish Ministerio de Ciencia e Innovación (Projects MAT2014-55049-C2-R and MAT2016-75883-C2-1-P) and Universidad del País Vasco/Euskal Herriko Unibertsitatea (GIU17/50 and PPG17/37). Carmen Martín is grateful to the Proyecto Puente convocatoria 2018 from Universidad de Cantabria financed by SODERCAN and VI PPIT-2018 from Universidad de Sevilla. Israel Cano acknowledges financial support from the European Community through a Marie Skłodowska-Curie Individual Fellowships (IF-EF; Programme/Call: H2020-MSCA-IF- 2015; Proposal No: 704710–Sdchirnanocat). NOTES AND REFERENCES 1 K. Dong, X. Liu, H. Dong, X. Zhang, S. Zhang, Multiscale studies on ionic liquids, Chem. Rev., 2017, 117, 6636-6695, DOI: 10.1021/acs.chemrev.6b00776. 2 P. Wasserscheid, T. Welton, Ionic liquids in synthesis, John Wiley & Sons, 2008, DOI: 10.1002/9783527621194. 3 M. Galinski, A. Lewandowski, I. Stepniak, Ionic liquids as electrolytes, Electrochim. 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