Efficient Synthesis of Tris-Heteroleptic Iridium(III) Complexes Based on the Zn²⁺-Promoted Degradation of Tris-Cyclometalated Iridium(III) Complexes and Their Photophysical Properties

Abstract

We report on the efficient synthesis of tris-heteroleptic iridium (Ir) complexes based on the degradation of tris-cyclometalated Ir complexes (IrL₃, L: cyclometalating ligand) in the presence of Brønsted and Lewis acids such as HCl (in 1,4-dioxane), AlCl₃, TMSCl, and ZnX₂ (X = Br or Cl), which affords the corresponding halogen-bridged Ir dimers (μ-complexes). Tris-cyclometalated Ir complexes containing electron-withdrawing groups such as fluorine, nitro, or CF₃ moieties on the ligands were less reactive. This different reactivity was applied to the selective degradation of heteroleptic Ir complexes such as fac-Ir(tpy)₂(F₂ppy) (fac-12) (tpy: 2-(4'-tolyl)pyridine and F₂ppy: 2-(4',6'-difluorophenyl)pyridine), mer-Ir(tpy)₂(F₂ppy) (mer-12), and mer-Ir(mpiq)₂(F₂ppy) (mer-15) (mpiq: 1-(4'-methylphenyl)isoquinoline). For example, the reaction of mer-12 with ZnBr₂ gave the heteroleptic μ-complex [{Ir(tpy)(F₂ppy)(μ-Br)}₂] 27b as a major product, resulting from the selective elimination of the tpy ligand of mer-12, and treatment of 27b with acetylacetone (acacH) afforded the corresponding tris-heteroleptic Ir complex Ir(tpy)(F₂ppy)(acac)18. In addition, another tris-heteroleptic Ir complex 35a having 8-benzenesulfonylamidoquinoline (8BSQ) ligand was synthesized. Mechanistic studies of this degradation reaction and the photochemical properties, especially a dual emission, of these newly synthesized tris-heteroleptic Ir complexes are also reported.