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

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.