Syntheses, structures and properties of cyclometalated iridium(III) complexes based on 2-phenyl-5-nitropyridyl and different strong-field ancillary ligands

The reactions of [Ir(5-NO₂-ppy)₂(PPh₃)Cl] (5-NO₂-ppy = 2-phenyl-5-nitropyridyl, PPh₃ = triphenylphosphine, 1) with pseudohalides, replacing the weak π donor Cl ^- with strong σ donors and π acceptors, lead to three new iridium(III) cyclometalated complexes [Ir(5-NO₂-ppy) ₂(PPh₃)(L)] (L = NCO^-, 2; NCS^-, 3; N₃^-, 4) based on 2-phenylpyridyl (ppy) derivative, PPh₃ and different pseudohalogen strong-field ancillary ligands. 2-4 are unambiguously characterized and their crystal structures are also determined by X-ray crystallography. The HOMO-LUMO electronic energy gaps of 1-4 estimated from the cyclic voltammetry measurements are comparable with those calculated from the UV-Vis absorption spectra, which are both in the order of 3 > 1 > 2 > 4. Furthermore, the electrochemical and photophysical properties of 1-4 show that the introduction of the electron-withdrawing nitro group to the pyridyl ring of the ppy backbone narrows the HOMO-LUMO electronic energy gaps, compared with those of corresponding ppy-containing iridium(III) complexes.