Addition of Amine-Substituted Heteroaromatic Rings to Alkynyl Bridging Ligands to Generate Phosphorescent Emitters Incorporating Iridaimidazoles with Fused Heterocyclic Rings

Alkynyl ligands are versatile building blocks in the design of luminescent iridium(III) complexes due to their ability to support postcoordination functionalization that gives rise to ligands non available through conventional coordination chemistry. Here, we report the synthesis and characterization of a new family of heteroleptic iridium(III) green emitters based on cyclometalated 2-p-tolylpyridine as main ligands and new C,N-chelating units as the auxiliary ligand that yield iridaimidazole structures. The alkynyl bridging dimer cis-[Ir(μ-CCtBu){κ2-C,N-(MeC6H3-py)}2]2 (1) reacts with amine-substituted five-membered heterocycles bearing two heteroatoms, such as 1-methyl-1H-imidazol-2-amine, 1-methyl-1H-benzo[d]imidazol-2-amine, 4-methyloxazol-2-amine, benzo[d]oxazol-2-amine, 4-methylthiazol-2-amine, and benzo[d]thiazol-2-amine, giving the iridaimidazole derivatives Ir{κ2-C,N-(MeC6H3-py)}2{κ2-C,N-[C(CH2tBu)N-im]} (im = imidazole, 2), Ir{κ2-C,N-(MeC6H3-py)}2{κ2-C,N-[C(CH2tBu)N-bzim]} (bzim = benzimidazole, 3), Ir{κ2-C,N-(MeC6H3-py)}2{κ2-C,N-[C(CH2tBu)N-oxazol]} (4), Ir{κ2-C,N-(MeC6H3-py)}2{κ2-C,N-[C(CH2tBu)N-bzoxazol]} (5), Ir{κ2-C,N-(MeC6H3-py)}2{κ2-C,N-[C(CH2tBu)N-thiazol]} (6), and Ir{κ2-C,N-(MeC6H3-py)}2{κ2-C,N-[C(CH2tBu)N-bzthiazol]} (7), respectively. The iridium center in these complexes is in an octahedral environment with nitrogen and carbon atoms in a facial disposition. Complexes 2, 4, and 6 feature one five-membered heteroaromatic ring fused to the iridaimidazole cycle whereas 3, 5, and 7 additionally contain a benzo group fused to the organic heterocycle. All complexes are green phosphorescent emitters (474–558 nm) upon photoexcitation, with high quantum yields (0.55–0.92) in poly(methyl methacrylate) films and 2-MeTHF at 298 K.