Molecular Redox: Revisiting the Electronic Structures of the Group 9 Metallocorroles

Abstract

The electronic structures of monocationic tris[(5,10,15-pentafluorophenyl)-corrolato]iridium compounds, [Ir(tpfc)L_2]^+, 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 L_(3,2)-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^•)L_2]^+ or [Ir^(IV)(tpfc)L_2]^+, 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 g_z 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)L_2]^(+/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 L_(3,2) XAS branching ratios (EBR), reflecting the increasing degree of Ir d orbital character upon proceeding from [1]^+ to [4]^+.

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