American Chemical Society
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Oxygen Atom Transfer to a Half-Sandwich Iridium Complex: Clean Oxidation Yielding a Molecular Product

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journal contribution
posted on 2014-03-12, 00:00 authored by Christopher R. Turlington, Peter S. White, Maurice Brookhart, Joseph L. Templeton
The oxidation of [Ir­(Cp*)­(phpy)­(NCArF)]­[B­(ArF)4] (1; Cp* = η5-pentamethylcyclopentadienyl, phpy = 2-phenylene-κC1′-pyridine-κN, NCArF = 3,5-bis­(trifluoromethyl)­benzonitrile, B­(ArF)4 = tetrakis­[3,5-bis­(trifluoromethyl)­phenyl]­borate) with the oxygen atom transfer (OAT) reagent 2-tert-butylsulfonyliodosobenzene (sPhIO) yielded a single, molecular product at −40 °C. New Ir­(Cp*) complexes with bidentate ligands derived by oxidation of phpy were synthesized to model possible products resulting from oxygen atom insertion into the iridium–carbon and/or iridium–nitrogen bonds of phpy. These new ligands were either cleaved from iridium by water or formed unreactive, phenoxide-bridged iridium dimers. The reactivity of these molecules suggested possible decomposition pathways of Ir­(Cp*)-based water oxidation catalysts with bidentate ligands that are susceptible to oxidation. Monitoring the [Ir­(Cp*)­(phpy)­(NCArF)]+ oxidation reaction by low-temperature NMR techniques revealed that the reaction involved two separate OAT events. An intermediate was detected, synthesized independently with trapping ligands, and characterized. The first oxidation step involves direct attack of the sPhIO oxidant on the carbon of the coordinated nitrile ligand. Oxygen atom transfer to carbon, followed by insertion into the iridium–carbon bond of phpy, formed a coordinated organic amide. A second oxygen atom transfer generated an unidentified iridium species (the “oxidized complex”). In the presence of triphenylphosphine, the “oxidized complex” proved capable of transferring one oxygen atom to phosphine, generating phosphine oxide and forming an Ir–PPh3 adduct in 92% yield. The final Ir–PPh3 product was fully characterized.