Ionic iron(iii) complexes of bis(phenol)-functionalized imidazolium cations: synthesis, structures and catalysis for aryl Grignard cross-coupling of alkyl halides

Abstract

A series of bis(phenol)-functionalized imidazolium salts, 1,3-bis(4,6-di-R¹-2-hydroxybenzyl)-2-R²-4,5-di-R³-imidazolium chlorides H₃LⁿCl (R¹ = ^tBu, R² = R³ = H, H₃L¹Cl, 1; R¹ = CH₃, R² = R³ = H, H₃L²Cl, 2; R¹ = ^tBu, R² = H, R³ = Cl, H₃L³Cl, 3; R¹ = ^tBu, R² = CH₃, R³ = H, H₃L⁴Cl, 4), were used to produce a novel series of ionic iron(III) complexes [H₃Lⁿ][FeX₄] (n = 1, X = Cl, 5; n = 2, X = Cl, 6; n = 3, X = Cl, 7; n = 4, X = Cl, 8; n = 1, X = Br, 9; n = 3, X = Br, 10). All of the complexes were characterized by Raman spectroscopy and electrospray ionization mass spectrometry. Elemental analysis and X-ray crystallography were also used. All of the complexes were non-hygroscopic and air-stable, with five of them existing as solids (5, 7–10) and one as an oil (6) at room temperature. A preliminary catalytic study on the cross-coupling reactions of aryl Grignard reagents with primary and secondary alkyl halides bearing β-hydrogens, revealed that all of the ionic iron(III) complexes exhibited good to excellent catalytic activity. Complexes 5, 6 and 8 exhibited optimal activity, whereas 7, 9 and 10 showed only moderate activity. Furthermore, by simply decanting the cross-coupling product in the ether layer, complexes 5 and 6 could be reused in at least seven successive runs without significant loss in catalytic activity.