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

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. In the presence of suitable amounts of water and base, the archetypical [Ir(ppy) 2 (μ-Cl)] 2 dimer forms a complex with the formula [Ir(ppy) 2 (Oppy)].