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With ammonium tetrafluoroborate; In acetonitrile; for 24h;Reflux;
General procedure: The second step of synthesis of [C4mim][BF4] is the substitution of the bromide ion with the BF4- ion. Tetrafluoroborate salt was synthesized by metathesis reactions from the corresponding bromide. [C4mim][Br] (0.1 mol) was dissolved in acetonitrile (50 mL), and ammonium tertrafluoroborate (0.11 mol) was added. The mixture was refluxed for at least 24 h. When it was cooled to room temperature, NH4Br precipitate was removed by filtration. Any remaining precipitate was removed by further filtration at this step. The remaining acetonitrile was removed by rotary evaporation to get crude 1-butyl-3-methylimidazolium tetrafluorobarate. Crude [C4mim][BF4] was dissolved in dichloromethane (50 mL) and cooled below 278 K. Deionized water and a separation funnel were also cooled to below 278 K. The dichloromethane solution was washed with cooled deionized water (30 mL) five times until the aqueous solution did not form any precipitate with 0.1 mol L-1 AgNO3 solution. The solvent dichloromethane was removed by rotary evaporation, and the [C4mim][BF4] was dried under high vacuum at (323-333)K for at least 6 h (Scheme 2).
General procedure: Another anion exchange reaction was carried out to achieve IL with boron tetrafluorides as a counter anion (IL-BF4) by metathesis reaction as per reported literature with minor modifications (Scheme 2). A demonstrative example for the synthesis of [RMIM]BF4 (i.e., [EMIM]BF4, [BMIM]BF4, [HMIM]BF4, [OMIM]BF4, [DMIM]BF4) and [RBZMIM]BF4 (i.e., [EBZMIM]BF4, [BBZMIM]BF4, [HBZMIM]BF4, [OBZMIM]BF4, [DBZMIM]BF4) in Scheme 2 (step-3) derivatives are as follows: 1:2 ratio of the 1-methyl-3-alkylimidazolium bromide or 1-methyl-3-alkylbenzimidazolium bromide ([RBZMIM]Br) and NaBF4 were added distinctly in to the round bottom flask containing 25mL of methanol and stirred at room temperature for 24h. Afterward, the salt was removed by filtering the reaction mixture using Whatman filter paper. Further, the filtrate was centrifuged for 10min at 3000rpm to isolate the residual salts. Solvent was evaporated under reduced pressure, vacuum dried and characterized (using 1H and 13C NMR and mass spectroscopy). The spectral details of the synthesized ILs are provided in the supplementary information Figs. S1 to S30.