Syntheses, Structures, and Properties of Phosphorescent Iridium(III) Complexes Containing N-Heterocyclic Carbene Ligands

The organic salts 1-(2-pyridylmethyl)-3-alkylbenzimidazolium halide (pm-RbH⁺X⁻) and 1-(2-pyridylmethyl)-3-alkylimidazolium halide (pm-R′iH⁺X′⁻) were prepared (where R = 4-, 3-, 2-fluorobenzyl (4f, 3f, and 2f, respectively), 4-, 3-, 2-chlorobenzyl (4c, 3c, and 2c, respectively); 4-methoxybenzyl (4mo); 2,3,4,5,6-pentafluorobenzyl (f₅); benzyl (b); and methyl (m)); X = Cl and Br; R′ = benzyl (b) and methyl (m); and X′ = Cl and I. From these salts, heteroleptic Ir(III) complexes containing one N-heterocyclic carbene (NHC) ligand [Ir(κ²-ppy)₂(κ²-(pm-Rb))]PF₆ (R = 4f, 1(PF₆); 3f, 2(PF₆); 2f, 3(PF₆); f₅b, 4(PF₆); 4c, 5(PF₆); 3c, 6(PF₆); 2c, 7(PF₆); 4mo, 8(PF₆); b, 9(PF₆); m, 10(PF₆)) and [Ir(κ²-ppy)₂(κ²-(pm-R′i))]PF₆ (R = b, 11(PF₆); m, 12(PF₆)), were synthesized, and the crystal structures of 1(PF₆), 2(PF₆), 3(PF₆), 5(PF₆), 6(PF₆), 7(PF₆), 9(PF₆), 10(PF₆), and 12(PF₆) were determined by X-ray diffraction. The neutral NHC ligands 1-(2-pyridylmethyl)-3-alkylbenzimidazolin-2-ylidene (pm-Rb) and 1-(2-pyridylmethyl)-3-alkylimidazolin-2-ylidene (pm-R′i) of all cations were found to be involved in the intermolecular π−π stacking interactions with the surrounding cations in the solid state, thereby probably influencing the photophysical behavior in the solid state and in solution. The absorption and emission properties of all the complexes show only small variations.