Structure–property relationships of unsupported diiridium(II) complexes with variation of axial ligand

Abstract

While iridium(III) complexes have been extensively studied, unsupported diiridium(II) complexes with Ir(II)–Ir(II) bonds remain underexplored. On the basis of our previous work on the related diiridium(II) complexes with variation of pincer ligand, herein we report the synthesis and characterization of two series of diiridium(II) complexes, anion series (axial X⁻ ligands) and P series (axial phosphine ligands), to investigate effect of axial ligands and charge on the modulation of molecular structure of Ir–Ir bonding, photophysics, and electrochemistry. Halide ions (I⁻, Br⁻, Cl⁻, SCN⁻) and phosphines with electron-donating/withdrawing substituents were introduced on the axial position and trans to the Ir–Ir bond. X-ray crystallography revealed shorter Ir–Ir bonds in the anion series (2.7567(3)–2.7807(5) Å) compared to the P series (2.9129(6)–2.9303(8) Å), attributed to the weaker trans influence. Photoluminescence studies showed tunable emission (581–676 nm) and photoluminescence quantum yields. Computational studies are also performed to corroborate the corresponding photophysical and electrochemical behaviours. Our findings reveal the significant impact of axial ligand characteristics on the bonding and photophysical properties of these less explored diiridium(II) complexes, contributing to a deeper understanding of their structure–property relationships.