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

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.