Sample preparation and characterization

Hydrogenated C4mimFeCl₄ (h-C4mimFeCl₄) with purity specified as > 98% was purchased from Tokyo Chemical Industry and used for magnetization measurements and a neutron scattering experiment to investigate vibrational excitations. The deuterated analogue (d-C4mimFeCl₄) was used to probe for magnetic scattering, to avoid strong incoherent scattering contribution from H atoms. d-C4mimFeCl₄ was prepared via the procedure as follows. 1-Butyl-3-methylimidazolium chloride (C4mimCl) was first synthesized using a literature method with modifications⁴². Reagents: 1-methylimidazole-d₆ (98% atom D, CDN isotopes), 1-chlorobutane-d₉ (98% atom D, CDN isotopes), iron(III) chloride (98%, Alfa Aesar), super dehydrated toluene (99.5%, FUJIFILM Wako Pure Chemical Co., Ltd.), dichloromethane (99.5%, FUJIFILM Wako Pure Chemical Co., Ltd.), and deuterium oxide (99.9%-d, Sigma-Aldrich).

In an argon-filled glove bag, 1-methylimidazole-d₆ (2.0 g, 22.7 mmol) was dissolved in dehydrated toluene (10 mL). 1-Chlorobutane-d₉ (2.0 g, 19.7 mmol) was then added and the mixture was stirred at 20 °C with argon bubbling for 10 min. After, the resulting mixture was refluxed in an oil bath at 120 °C for 7 days. When the reaction was complete, the solvent was cooled to 20 °C and evaporated to dryness under reduced pressure. The crude product was washed a third time with dichloromethane (30 mL), and the remaining solution was evaporated to dryness. The yield of deuterated C4mimCl was 38.2% (1.53 g). The deuteration level was estimated to be greater than 99% by nuclear magnetic resonance and electrospray ionization mass spectrometry (Supplementary SI-¹).

C4mimCl-d₁₅ (0.97 g, 5.1 mmol) and FeCl₃ (0.865 g, 5.3 mmol) were dissolved in 10 mL of D₂O, and the mixture was stirred at 20 °C for 10 min. Then, the mixture was filtrated through a polypropylene filter (0.45 μm pore size) to remove the small amount of insoluble material. The filtered solution was evacuated at 70 °C for 24 h to dryness under reduced pressure. The yield of d-C4mimFeCl₄ was 94.3% (1.73 g).

The thermodynamic and paramagnetic properties of both h- and d-C4mimFeCl₄ were examined by differential scanning calorimetry (DSC) and magnetic susceptibility measurements. Deuterium isotope effects on these properties were not confirmed (Supplementary SI-²).