Oxidation of Phenols by the Excited State of an Osmium(VI) Nitrido Complex

Abstract

The photoreaction of an osmium(VI) nitrido complex, [OsVI(N)(L)(CN)3]‒ (OsN), with various phenols has been investigated. Upon irradiation of OsN with visible light, the excited state (OsN*) is generated which reacts readily with a variety of phenols. OsN* reacts with mono- and di-substituted phenols, including 2,6-dimethylphenol, 2,6-dichlorophenol and 4-methylphenol to afford the corresponding osmium(II) benzoquinone monoimine and osmium(IV) benzoquinone monoiminato complexes. On the other hand, in the reactions of OsN* with bulky tri-substituted phenol such as 2,4,6-tri-tert-butylphenol (tBu3PhOH), C-C bond cleavage occurred and [OsIV(L)(CN)3(N=tBu2Ph(-2H)O)]‒ was formed as the major product. The electronic effects of various para-substituents (X) on the oxidation of phenols were investigated by the method of initial rates (Rx). A Hammett plot of log(Rx/RH) versus σp is linear with a ρ value of ‒0.54. A linear correlation of log(Rx) with the oxidation potentials (E) of phenols was also found with a slope of ‒0.80. On the other hand, no correlations were found between log(Rx) and O-H bond dissociation energy (BDE), as well as the pKa of phenols. The oxidation phenol by OsN* exhibits a negligible kinetic isotope effect (KIE), k(C6H5OH)/k(C6D5OD) ~1. These results are consistent with a mechanism that involves an initial 1e‒ oxidation of the phenol followed by rapid proton transfer (ET-PT) to generate a phenoxy radical, this is followed by a N-rebound step to give the osmium products.