Synthesis and Properties of Magnetic Aryl-Imidazolium Ionic Liquids with Dual Brønsted/Lewis Acidity

Abstract

A series of unique tunable aryl-imidazolium magnetic ionic liquids (MILs) with dual acidity that contain both Brønsted and Lewis acidic sites (abbreviated as B-L MILs) were synthesized and characterized using nuclear magnetic resonance and mass spectrometry. Physical properties, such as thermal properties, magnetic susceptibility, and Brønsted and Lewis acidity, were measured. These properties were found to depend on the cation structure. These B-L MILs had good solubility in many organic solvents, good thermal stability, and low melting points, and exhibited magnet-like behavior. For these B-L MILs, the Brønsted acidity was measured using ultraviolet-visible (UV-Vis), and the Lewis acidity was measured using Fourier transform infrared spectroscopy (FTIR). The results showed that B-L MILs with an electron-withdrawing group in the aryl-imidazolium moiety had higher Brønsted acidity, whereas those with an electron-donating group had higher Lewis acidity. This type of ionic liquid, with both Brønsted and Lewis acidic sites, is expected to be a useful solvent and catalyst for organic reactions.

Novel Aryl-Imidazolium Ionic Liquids with Dual Brønsted/Lewis Acidity as Both Solvents and Catalysts for Friedel–Crafts Alkylation

“…The B-L MILs were synthesized following the procedure (shown in Scheme 1) discussed in our previous work [51]. p-Xylene was purchased from Sigma-Aldrich (St. Louis, MO, USA), and benzyl chloride (BnCl) was purchased from Acros (Morris Plains, NJ, USA).…”

“…Since the details of the structure (shown in [51]) and property characterization of the B-L MILs 5a-d with dual Brønsted-Lewis properties were published in our previous paper [51], only a brief description is provided here. The B-L MILs 5a-d with close to a 100% atom economy were synthesized using three steps: Ullmann-type coupling, treatment with HCl, and coordination with FeCl 3 (seen in [51]).…”

“…Although studies using ILs as catalysts have been rapidly increasing in number, the cationic structures described in these studies mainly focus on carrying sp3-hybridized carbon atoms as substituents at both nitrogen heterocyclic atoms [13,49,50]. Very recently, a unique series of tunable aryl-imidazolium iron-containing ILs with dual Brønsted-Lewis acidity (abbreviated as B-L MILs) were reported by our group [51]. These B-L MILs have a combination of one proton and sp2 aryl substituents at the nitrogen atoms of the imidazolium core.…”

“…These B-L MILs have a combination of one proton and sp2 aryl substituents at the nitrogen atoms of the imidazolium core. The physical-chemical properties of B-L MILs are tuned by specific electronic effects (the σand π-system) and even by the substituent effect in the aryl moiety [51]. Even though the molar ratio of FeCl 3 to [HPhImX][Cl] (X = H, CH 3 , OCH 3 , and NO 2 ) is only 0.5 in these B-L MILs systems, they have been shown to have good Lewis acidity [51].…”

“…The physical-chemical properties of B-L MILs are tuned by specific electronic effects (the σand π-system) and even by the substituent effect in the aryl moiety [51]. Even though the molar ratio of FeCl 3 to [HPhImX][Cl] (X = H, CH 3 , OCH 3 , and NO 2 ) is only 0.5 in these B-L MILs systems, they have been shown to have good Lewis acidity [51]. In this study, we demonstrate the utility of the B-L MILs as both solvents and catalysts for the Friedel-Crafts alkylation reaction of p-xylene with benzyl chloride.…”