Unexpected reactivity of cyclometalated iridium(iii) dimers. Direct synthesis of a mononuclear luminescent complex

Abstract

A new synthetic method has been developed for the preparation of unexpected emissive iridium(III) complexes (A and B), directly obtained from the established [Ir(ppy)₂(μ-Cl)]₂ dimer, under reaction conditions in which such compounds are usually considered stable. Complex A ([Ir(ppy)₂(Oppy)], where Hppy = 2-phenylpyridine and HOppy = 2-(o-hydroxyphenyl)pyridine) was obtained from the dimer without the addition of further ancillary ligands in the reaction environment, but in the presence of a basic water environment in 2-ethoxyethanol as solvent at 165 °C. The complex evidences the unexpected insertion of an oxygen atom between the iridium(III) center and the carbon atom of one ppy moiety. Under specific reaction conditions, the mer-[Ir(ppy)₃] complex (B) was obtained. The presence of the right amount of water is important to maximize the formation of A relative to B. Both compounds were fully characterized by NMR spectroscopy and mass spectrometry (MS), and the X-ray structure of A was also determined. DFT calculations were used to shed light on the reaction mechanism leading to the unexpected formation of A, suggesting that the Oppy ligand is generated intramolecularly once the [Ir(ppy)₂(μ-OH)]₂ dimer is formed. The process is probably assisted by a redox reaction involving the second iridium(III) center in the dimer. The electrochemical and photophysical properties of complexes A and B were investigated in comparison with the well-known fac-[Ir(ppy)₃] analogue (C). Complex A displays a green emission (λ_max = 545 nm) with a photoluminescence quantum yield (PLQY) of nearly 40%, whereas the oxygen-free counterpart B is poorly emissive, exhibiting an orange emission (λ_max = 605 nm) with a PLQY below 10%. These findings may pave the way for the direct synthesis of neutral luminescent complexes with the general formula [Ir(C^N)₂(OC^N)], even using dimers with non-commercial or highly substituted C^N ligands, without the need for synthesizing the corresponding hydroxyl-substituted ancillary ligand, which may be hardly obtainable.