Exploring Oxidation of Half-Sandwich Rhodium Complexes: Oxygen Atom Insertion into the Rhodium-Carbon Bond of κ2-Coordinated 2-Phenylpyridine

The reactions of oxygen atom transfer reagents with Rh(Cp*) complexes, each with a bidentate ligand and an accessible coordination site, are described (Cp* = eta(5)-pentamethylcyclopentadienyl). When [Rh(Cp*)(phpy)(NCArF)][B(Ar-F)(4)] (1, phpy = 2-phenylene-kappa C-l'-pyridine-kappa N, NCArF = 3,5-bis(trifluoromethyl)benzonitrile, B(Ar-F)(4) = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) was treated with the soluble oxygen atom transfer reagent 2-tert-butylsulfonyliodosylbenzene (sPhIO), oxygen atom insertion into the rhodium-carbon bond of coordinated phpy was observed. This resulted in the formation of a kappa(2) 2-(2-pyridyl)phenoxide ligand. Following insertion to form a new bidentate ligand, a second equivalent of sPhIO, acting as a neutral, two-electron donor ligand, coordinated to the rhodium center through the iodosyl oxygen. Over time, the sPhIO ligand dissociates and dimerization occurs to generate a phenoxide-bridged dinuclear species. The 2-(2-pyridyl)phenoxide ligand could be protonated and cleaved from the mononuclear rhodium-(sPhIO) adduct by treating with a carboxylic acid (pivalic acid) at room temperature. In addition, when rhodium complex 1 was treated with excess phpy (14 equiv), hydrogen peroxide, and acetic acid, 5 equiv of 2-(2-pyridyl)phenol formed. Deactivation of the organometallic species, probably due to oxidative degradation of Cp*, severely limited this catalysis.