Oxidation of phenols by the excited state of an osmium(vi) nitrido complex

Abstract

The photoreaction of an osmium(VI) nitrido complex, [Os^VI(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, C–C bond cleavage occurred and [Os^IV(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 (R_x). A Hammett plot of log(R_x/R_H) versus σ_p is linear with a ρ value of −0.54. A linear correlation of log(R_x) 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(R_x) and O–H bond dissociation energy (BDE), as well as the pK_a of phenols. The oxidation of phenols 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.