Molecular redox: revisiting the electronic structures of the group 9 metallocorroles

The electronic structures of monocationic tris[(5,10,15-pentafluorophenyl)-corrolato]iridium compounds, [Ir(tpfc)L2](+), where L = 4-cyanopyridine [1](+), pyridine [2](+), 4-methoxypyridine [3](+), or 4-(N,N'-dimethylamino)pyridine [4](+), have been probed by electron paramagnetic resonance (EPR) spectroscopy, Ir L3,2-edge X-ray absorption spectroscopy (XAS), UV/visible (UV-vis) spectroelectrochemistry, and density functional theoretical (DFT) calculations. The data demonstrate that these complexes, which have been previously formulated as either of the limiting cases [Ir(III)(tpfc(•))L2](+) or [Ir(IV)(tpfc)L2](+), are best described as possessing a singly occupied molecular orbital (SOMO) dominated by tpfc with small but significant Ir admixture. EPR g-values and electronic absorption spectra are reproduced well using a simple DFT approach. These quantities depend profoundly upon Ir orbital contribution to the SOMO. To wit, the calculated Ir spin population ranges from 10.6% for [1](+) to 16.3% for [4](+), reflecting increased Ir d mixing into the SOMO with increasingly electron-rich axial ligation. This gives rise to experimentally measured gz values ranging from 2.335 to 2.533, metal-to-ligand charge transfer (MLCT) bands ranging from 14730 and 14330 cm(-1), and [Ir(tpfc)L2](+/0) reduction potentials ranging from 0.305 to 0.035 V vs Fc(+/0). In addition, the calculated Ir character in the SOMO tracks with estimated Ir L3,2 XAS branching ratios (EBR), reflecting the increasing degree of Ir d orbital character upon proceeding from [1](+) to [4](+).