Unprecedented Iridium(VIII) Seen

Despite theoretical predictions that iridium is limited to a maximum oxidation state of 7+, a group led by Mingfei Zhou of Fudan University, in Shanghai, and Sebastian Riedel of Albert Ludwigs University, in Freiburg, Germany, has experimentally observed IrO₄ featuring an 8+ iridium center (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200902733). The researchers formed the compound by codepositing iridium with O₂ and a noble gas onto a CsI surface at 4 to 6 K. After annealing the sample to 30 K, they used infrared spectroscopy to detect (η¹-O₂)IrO₂, where (η¹-O₂) represents the end-on arrangement O–O–IrO₂. Irradiating the sample with infrared light produced (η²-O₂)IrO₂, where η²-O₂ indicates a side-on bound O₂ (forming a three-membered IrO₂ ring). Further irradiation with visible light produced the complex identified as IrO₄, which has four terminal Ir=O bonds. Computational analysis indicates that the latter compound is a d¹ species with the formal iridium oxidation state of 8+. That the molecule can exist, at least at low temperature, is likely due to the comparably low steric repulsions of the four divalent oxo ligands combined with the relatively large electronegativity of oxygen, the researchers say.