Preparation of tris-heteroleptic iridium(III) complexes containing a cyclometalated aryl-N-heterocyclic carbene ligand

Abstract

A new class of phosphorescent tris-heteroleptic iridium(III) complexes has been discovered. The addition of PhMeImAgI (PhMeIm = 1-phenyl-3-methylimidazolylidene) to the dimer [Ir(μ-Cl)(COD)]2 (1; COD = 1,5-cyclooctadiene) affords IrCl(COD)(PhMeIm) (2), which reacts with 1-phenylisoquinoline, 2-phenylpyridine, and 2-(2,4-difluorophenyl)pyridine to give the respective dimers [Ir(μ-Cl){κ2-C,C-(C6H4-ImMe)}{κ2-C,N-(C6H4-isoqui)}]2 (3), [Ir(μ-Cl){κ2-C,C-(C6H4-ImMe)}{κ2-C,N-(C6H4-py)}]2 (4), and [Ir(μ-Cl){κ2-C,C-(C6H4-ImMe)}{κ2-C,N-(C6F2H2-py)}]2 (5), as a result of the N-heterocyclic carbene (NHC)- and N-heterocycle-supported o-CH bond activation of the aryl substituents and the hydrogenation of a C–C double bond of the coordinated diene. In solution, these dimers exist as a mixture of isomers a (Im trans to N) and b (Im trans to Cl), which lie in a dynamic equilibrium. The treatment of 3–5 with Kacac (acac = acetylacetonate) yields isomers a (Im trans to N) and b (Im trans to O) of Ir{κ2-C,C-(C6H4-ImMe)}{κ2-C,N-(C6H4-isoqui)}(κ2-O,O-acac) (6a and 6b), Ir{κ2-C,C-(C6H4-ImMe)}{κ2-C,N-(C6H4-py)}(κ2-O,O-acac) (7a and 7b), and Ir{κ2-C,C-(C6H4-ImMe)}{κ2-C,N-(C6F2H4-py)}(κ2-O,O-acac) (8a and 8b), which were separated by column chromatography. The treatment of 6a with HX in acetone–water produces the protonation of the acac ligand and the formation of the bis(aquo) complex [Ir{κ2-C,C-(C6H4-ImMe)}{κ2-C,N-(C6H4-isoqui)}(H2O)2]X [X = BF4 (9a[BF4]), OTf (9a[OTf])]. The salt 9a[BF4] reacts with 2-(2-pinacolborylphenyl)-5-methylpyridine in the presence of 40 equiv of K3PO4 to afford Ir{κ2-C,C-(C6H4-ImMe)}{κ2-C,N-(C6H4-isoqui)}{κ2-C,N-(C6H4-Mepy)} (10a). Complexes 6a, 6b, 7a, 7b, 8a, 8b, and 10a are phosphorescent emitters (λem = 465–655 nm), which display short lifetimes in the range of 0.2–5.6 μs. They show high quantum yields both in doped poly(methyl methacrylate) films (0.34–0.87) and in 2-methyltetrahydrofuran at room temperature (0.40–0.93). From the point of view of their applicability to the fabrication of organic-light-emitting-diode devices, a notable improvement with regard to those containing two cyclometalated C,N ligands is achieved. The introduction of the cyclometalated aryl-NHC group allows one to reach a brightness of 1000 cd/m2 at a lower voltage and appears to give rise to higher luminous efficacy and power efficacy.