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-R(1)-2-hydroxybenzyl)-2-R(2)-4,5-di-R(3)-imidazolium chlorides H(3)L(n)Cl (R(1) = (t)Bu, R(2) = R(3) = H, H(3)L(1)Cl, 1; R(1) = CH(3), R(2) = R(3) = H, H(3)L(2)Cl, 2; R(1) = (t)Bu, R(2) = H, R(3) = Cl, H(3)L(3)Cl, 3; R(1) = (t)Bu, R(2) = CH(3), R(3) = H, H(3)L(4)Cl, 4), were used to produce a novel series of ionic iron(III) complexes [H(3)L(n)][FeX(4)] (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.

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

“…Comparing entries 1, 7 and 8 showed the desired product is obtained in better yield when cyclohexyl bromide was used, and the general trend appears to be R-Br > R-I > R-Cl. This was the same pattern as that previously reported for [bmim][FeCl 4 ] [20] and related Fe(III)containing ILs [11]. Changing the cycloalkyl bromide to cyclopentyl bromide or cycloheptyl bromide yielded the desired product but in lower yields of 70% and 52%, respectively, with an extended reaction time ( Table 3, entries 9 and 10).…”

“…In a continuation of our work focused on the development of Fe(III)-containing imidazolium ILs for use in the cross-coupling of aryl Grignard reagents with alkyl halides [9][10][11][12], we report here on the use of ether-functionalized imidazolium salts in the facile synthesis of a series of Fe(III)-containing ILs 1-6, and their use as catalysts in cross-coupling the aryl Grignard with alkyl halides with -hydrogens. Ether-functionalized imidazolium salts have demonstrated attractive physicochemical properties for particular applications, e.g.…”

Combined Effects of Backbone and N-Substituents on Structure, Bonding, and Reactivity of Alkylated Iron(II)-NHCs

“…Along with their popularity in precious metal catalysis, NHCs have been used in a multitude of iron catalyzed reactions. , In cross-coupling, aryl–aryl, − aryl–alkenyl, alkyl–aryl, − aryl–alkyl, and alkyl–alkyl transformations have all been achieved using a variety of NHCs (Scheme ). In a popular example, Nakamura and co-workers reported in 2007 the Kumada coupling of aryl Grignard reagents with aryl halides using iron, nickel, and cobalt fluorides. , Here, SIPr is the most effective NHC for catalysis, outperforming IPr with 98% yield compared to only 25% cross-coupled product (Scheme ).…”

Charge-tagged ligands: useful tools for immobilising complexes and detecting reaction species during catalysis

“…In recycling experiments, 19 provided the coupling product with yields greater than 95% for six runs, however, in these experiments 5 mol% of iron (instead of 1 mol%) and a more active alkyl bromide were applied. 37 The authors also highlighted that 19 outperforms the catalytic activity of an imidazolium analogue, 40 especially in avoiding the formation of the β-elimination product.…”