Development of Fe(III)-containing ether-functionalized imidazolium ionic liquids for aryl Grignard cross-coupling of alkyl halides

A series of Fe(III)-containing imidazolium-based ionic liquids containing ether substituents, including [C₃OMim][FeCl₄] (1, [C₃OMim] = 1-(2-methoxyethyl)-3-methylimidazolium), [C₃O ⁱ Pim][FeCl₄] (2, [C₃O ⁱ Pim] = 1-isopropyl-3-(2-methoxyethyl) imidazolium), [C₃OBim][FeCl₄] (3, [C₃OBim] = 1-butyl-3-(2-methoxyethyl)imidazolium), [(C₃O)₂im][FeCl₄] (4, [(C₃O)₂im] = 1,3-bis(2-methoxyethyl)imidazolium), [C₃OMim][FeBr₄] (5) and [(C₃O)₂im][FeBr₄] (6), were prepared and characterized by elemental analysis, Raman spectroscopy and electrospray ionization mass spectrometry. The catalytic performances of 1–6 and related Fe(III)-based catalysts in the cross-coupling of aryl Grignard reagents with alkyl halides bearing β-hydrogens were studied, revealing that mono(ether) functionality improves the catalytic activity and that bis(ether) functionality improves the reusability. After simply decanting the product contained in the ethereal layer, complex 4, which containing bis(ether)-functionalized imidazolium cation, could be successfully recycled seven times.