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

Abstract

The oxidation of [Ir(Cp*)(phpy)(NCAr(F))][B(Ar(F))4] (1; Cp* = η(5)-pentamethylcyclopentadienyl, phpy = 2-phenylene-κC(1')-pyridine-κN, NCAr(F) = 3,5-bis(trifluoromethyl)benzonitrile, B(Ar(F))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)(NCAr(F))](+) 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.

Iridium-based complexes for water oxidation

“…44,48,[76][77][78] These reactions use Cp*Ir precursors with water as a solvent or co-solvent when oxidants like NaIO 4 or CAN are used; other studies use OAT reagents, but these will not be discussed here as they require anhydrous conditions. 64 For aqueous C-H oxidation reactions, WO occurs in tandem with C-H oxidation, but the latter is greatly favored, indicating that alkyl C-H bonds are much more reactive than water. Thus, it remains possible that the Cp*Ir precursors can oxidize reactive C-H substrates, such as tetrahydrofuran, in preference to attacking a Cp* ligand-particularly if the oxidative degradation of the Cp* is bimolecular-and thus carry out oxidation catalysis as a high-valent monomeric species with the Cp* still intact.…”

Half-sandwich Rh(Cp*) and Ir(Cp*) complexes with oxygen atom transfer reagents as ligands

“…Compound 1, Rh(Cp*)[(3,5-bis(trifluoromethyl)benzoyl)(2-(2-pyridyl)phenyl)amide]Cl, was synthesized from [Rh(Cp*)Cl 2 ] 2 and the proligand 3,5-bis(trifluoromethyl)-N-(2-(2-pyridyl)phenyl)benzamide by following a procedure described for metalation of the same proligand with [Ir(Cp*)Cl 2 ] 2 . 10 Deprotonation of the proligand by the insoluble NaH base, followed by filtration of the THF solution onto solid [Rh(Cp*)Cl 2 ] 2 , yielded complex 1 after stirring overnight (Scheme 2). The product was stable to both air and water.…”

“…4 While metal-oxo complexes have been proposed as reactive intermediates in biological applications, 5 industrial applications, 6 and water oxidation catalysis, 7 they are often difficult to detect or study. Metal-oxo compounds of the late transition metals (groups [9][10][11] are especially rare, 8 as the metal-oxo bond is destabilized by electronic repulsion between filled dπ-orbitals on the metal and filled p-orbitals on the oxo ligand. 9 By combining oxygen atom transfer reagents with labile Cp* derivatives of rhodium and iridium we have been able to probe the scope of reactions available to these strong oxidants.…”

“…Recent efforts utilizing labile late transition metal complexes revealed unexpected OAT to the labile ligand. 10 While coordination of nucleophilic OAT reagents typically forms transient intermediates, some stable metal complexes with coordinated OAT reagents have been isolated. Such compounds serve as welldefined starting points for OAT reactions because the initial metal-oxygen bond is already established.…”

“…[14][15] A Rh(Cp*) complex with a coordinated iodosylbenzene derivative and an Ir(Cp*) compound with a coordinated pyridine-N-oxide are also known, but oxidation to form terminal metal-oxo complexes was not described. 10,16 Compounds with coordinated OAT reagents are relatively easy to handle and serve as well-defined precursors for oxygen atom transfer reactions. Activation of coordinated OAT reagents with heat or light, therefore, is an attractive method to probe either oxidation to form an oxo complex or alky/aryl migration to the oxygen atom of the OAT reagent.…”