The role of a redox-active non-innocent ligand in additive-free C–C Glaser–Hay and Suzuki coupling reactions by an o-aminophenol palladium(ii) complex

Abstract

A novel mononuclear palladium complex with 2-(3,5-di-tert-butyl-2-hydroxyphenyl amino) benzonitrile as a non-innocent ligand (abbreviated as Pd^IIL₂^NIS) was synthesized, and characterized by IR, UV-Vis, ¹H and ¹³C NMR spectroscopies and elemental analysis. The crystal structure clearly showed that the metal center was in a square planar environment. The bond lengths obtained from X-ray structure analysis revealed that both ligands are in the o-iminobenzosemiquinone radical form. The neutral complex showed strong absorptions in the NIR region, corresponding to the ILCT (intra-ligand charge transfer). Catalytic tests performed for the coupling reaction of terminal alkynes showed that the palladium Pd^IIL₂^NIS complex acts as a highly effective catalyst for the base-free C–C coupling reactions, leading to diyne derivatives with excellent yields. The Pd^IIL₂^NIS complex in ethanol, as a green solvent, is demonstrated to be an exceptionally active phosphine-free catalyst for the Suzuki reaction of aryl iodides and bromides. The reaction can be carried out under mild conditions (room temperature) with high yields without using a microwave or phosphine ligands. This catalyst exhibits an interesting application of redox non-innocent ligands, the electron reservoir behavior, which makes it needless to use additional reagents. The theoretical calculation provides more details about the complex structure, molecular orbitals, and electronic state.