Electronic properties of oxidized cyclometalated diiridium complexes: Spin delocalization controlled by the mutual position of the iridium centres

Four cyclometalated diiridium complexes, with IrCp*Cl (Cp* = η5-C5Me5‒) termini bridged by 1,4- and 1,3-bis(p-tolylimino-ethyl)benzene (1, 2), or 1,4- and 1,3-bis(2-pyridyl)benzene (3, 4), were prepared and characterized by nuclear magnetic resonance (NMR) spectroscopy and single-crystal X-ray diffraction (complexes 1, 2 and 4). The two iridium centres in complexes 1 and 3 are thus bound at the central benzene ring in the para-position (trans-Ir2), while those in complexes 2 and 4 in the meta-position (cis-Ir2). Cyclic voltammograms of all four complexes show two consecutive one-electron oxidations. The potential difference between the two anodic steps in 1 and 3 is distinctly larger compared to 2 and 4. The visible - near-infrared (NIR) - short-wave infrared (SWIR) absorption spectra of trans-Ir2 monocations 1+ and 3+ are markedly different from those of cis-Ir2 monocations 2+ and 4+. Notably, strong near-infrared electronic absorption appears only in the spectra of 1+ and 3+ while 2+ and 4+ absorb only weakly in the NIR-SWIR region. Combined DFT and TD-DFT calculations have revealed that (a) 1+ and 3+ (the diiridium-benzene trans-isomers) display HOSO and LUSO evenly delocalized over both molecule halves, and (b) their electronic absorptions in the NIR-SWIR region are attributed to mixed metal-to-ligand and ligand-to-ligand charge transfers (MLCT and LLCT). In contrast, cis-isomers 2+ and 4+ do not feature this stabilizing π-delocalization but a localized mixed-valence state showing a weak IVCT absorption in the SWIR region.