Unusual Dinuclear and Mononuclear Cyclometalated Iridium Complexes of 2,5-Diaryl-1,3,4-oxadiazole Derivatives

A family of new 2,5-diphenyl-1,3,4-oxadiazole (OXD) derivatives 8−11 bearing ortho-alkyl substituents on one of the phenyl rings is reported. The reactions of these OXDs with IrCl3 under standard cyclometalating conditions did not give the usual μ-dichloro bridged diiridium OXD complexes. Instead, the novel diiridium complexes 12−14 and the monoiridium complex 15 were isolated and characterized by X-ray crystallography. It is proposed that the unusual structures arise because of the ortho-alkyl substituents leading to a substantial twisting of part of the OXD system which, for steric reasons, changes the normal course of the metal−ligand coordination reactions. Subsequent reactions of 13 and 15 gave the mononuclear complexes 16−18 with acac and picolinate anciliary ligands. The crystal structures of 16 and 18 are reported. Photoluminescence is observed in the green (16) and blue-green regions (17 and 18) at room temperature. Complexes 16−18 are phosphorescent at low temperature, with triplet lifetimes of 4.2−5.7 μs at 77 K.