Ni ion-containing ionic liquid salt and Ni ion-containing immobilized ionic liquid on silica: Application to Suzuki cross-coupling reactions between chloroarenes and arylboronic acids

Bis(1-n-butyl-3-methyl-imidazolium)tetrachloronickelate ([Bmim]2[NiCl4]), which is a nickel ion-containing ionic liquid, and immobilized nickel ion-containing ionic liquid (ImmNi2+_IL) on silica surface were prepared and applied as new catalysts for Suzuki cross-coupling reactions between aryl chlorides and arylboronic acids. It was found that pretreatment of the catalysts and the addition of triphenylphosphine to the reaction system greatly promoted the reactions. Before the addition of substrates, [Bmim]2[NiCl4] should be treated with K3PO4 in dioxane at a refluxing temperature for 1 h, and ImmNi2+_IL should be treated with NaOtBu in dioxane at room temperature for 30 min. An active precursor for the catalytic reactions was found to be a Ni carbene species formed by pretreatment with the bases as characterized by NMR and EXAFS. Reusability was confirmed for ImmNi2+_IL.