O Activation by a Coordinated -NH- Function: Hydrogen Atom Transfer and Aromatic Ring Oxidation.

Herein, we disclose a unique method of oxidation of a 1,4-naphthoquinone ring in air. We report that (1,4-naphthoquinone)-NH-N=C(OH)Ph (HL) coordinated to octahedral ruthenium(II) and osmium(II) ions activates an O molecule spontaneously. Hydrogen atom transfer (HAT) from the -NH- function of HL to O and subsequent (2e + 2H) oxidation forming (1,3,4-trioxonaphthalen)=N-N=C(OH)Ph (HL) have been established. The HL → HL transformation occurs via (3-hydroperoxy-1,4-naphthoquinone)=N-N=C(O)Ph (HL) as an intermediate. The primary step is HAT generating HL and hydroperoxide (OOH) radicals. HL is delocalized over the aromatic ring and incites coupling reactions via ortho carbon and produces coordinated HL. In solution, the homolytic cleavage of the peroxo bond leads to aromatic ring oxidation, affording L. Ruthenium(II) and osmium(II) complexes of the types [M(HL)(PPh)X], [M(HL)(PPh)X], and -[M(L)(PPh)X] were successfully isolated in good yields. Notably, the cyclic voltammograms of all of the complexes exhibit reversible anodic waves due to M/M redox couples. The rate constants of the [M(HL)(PPh)X] → [M(HL)(PPh)X] conversions determined by time-driven UV-vis spectroscopy in dry CHCl, wet CHCl, and DO wet CHCl in air at 298 K follow the order > > . It is established that the rate constants are dependent on the O content of the solution but not on the concentration of the complex.