Electronic Properties of Oxidized Cyclometalated Diiridium Complexes: Spin Delocalization Controlled by the Mutual Position of the Iridium Centers

Article abstract of DOI:10.1002/chem.201904894

Four cyclometalated diiridium complexes, with IrCp*Cl (Cp=η⁵-C₅Me₅ ^?) termini bridged by 1,4- and 1,3-bis(p-tolyliminoethyl)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 centers in complexes 1 and 3 are thus bound at the central benzene ring in the para-position (trans-Ir2), whereas those in complexes 2 and 4 are 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 than that for 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⁺ whereas 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 the highest occupied spin-orbitals (HOSO) and the lowest unoccupied spin-orbital (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 intervalence charge-transfer (IVCT) absorption in the SWIR region.