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 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.