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

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.