Preparation of Tris-Heteroleptic Iridium(III) Complexes Containing a Cyclometalated Aryl-N-Heterocyclic Carbene Ligand (2018)

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)]<sub>2</sub> (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){κ<sup>2</sup>- C, C-(C<sub>6</sub>H<sub>4</sub>-ImMe)}{κ<sup>2</sup>- C, N-(C<sub>6</sub>H<sub>4</sub>-isoqui)}]<sub>2</sub> (3), [Ir(μ-Cl){κ<sup>2</sup>- C, C-(C<sub>6</sub>H<sub>4</sub>-ImMe)}{κ<sup>2</sup>- C, N-(C<sub>6</sub>H<sub>4</sub>-py)}]<sub>2</sub> (4), and [Ir(μ-Cl){κ<sup>2</sup>- C, C-(C<sub>6</sub>H<sub>4</sub>-ImMe)}{κ<sup>2</sup>- C, N-(C<sub>6</sub>F<sub>2</sub>H<sub>2</sub>-py)}]<sub>2</sub> (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{κ<sup>2</sup>- C, C-(C<sub>6</sub>H<sub>4</sub>-ImMe)}{κ<sup>2</sup>- C, N-(C<sub>6</sub>H<sub>4</sub>-isoqui)}(κ<sup>2</sup>- O, O-acac) (6a and 6b), Ir{κ<sup>2</sup>- C, C-(C<sub>6</sub>H<sub>4</sub>-ImMe)}{κ<sup>2</sup>- C, N-(C<sub>6</sub>H<sub>4</sub>-py)}(κ<sup>2</sup>- O, O-acac) (7a and 7b), and Ir{κ<sup>2</sup>- C, C-(C<sub>6</sub>H<sub>4</sub>-ImMe)}{κ<sup>2</sup>- C, N-(C<sub>6</sub>F<sub>2</sub>H<sub>4</sub>-py)}(κ<sup>2</sup>- 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{κ<sup>2</sup>- C, C-(C<sub>6</sub>H<sub>4</sub>-ImMe)}{κ<sup>2</sup>- C, N-(C<sub>6</sub>H<sub>4</sub>-isoqui)}(H<sub>2</sub>O)<sub>2</sub>]X [X = BF<sub>4</sub> (9a[BF<sub>4</sub>]), OTf (9a[OTf])]. The salt 9a[BF<sub>4</sub>] reacts with 2-(2-pinacolborylphenyl)-5-methylpyridine in the presence of 40 equiv of K<sub>3</sub>PO<sub>4</sub> to afford Ir{κ<sup>2</sup>- C, C-(C<sub>6</sub>H<sub>4</sub>-ImMe)}{κ<sup>2</sup>- C, N-(C<sub>6</sub>H<sub>4</sub>-isoqui)}{κ<sup>2</sup>- C, N-(C<sub>6</sub>H<sub>4</sub>-Mepy)} (10a). Complexes 6a, 6b, 7a, 7b, 8a, 8b, and 10a are phosphorescent emitters (λ<sub>em</sub> = 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/m<sup>2</sup> at a lower voltage and appears to give rise to higher luminous efficacy and power efficacy.