O2 Activation by a Coordinated -NH- Function: Hydrogen Atom Transfer and Aromatic Ring Oxidation

Abstract

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 (H₃L) coordinated to octahedral ruthenium(II) and osmium(II) ions activates an ³O₂ molecule spontaneously. Hydrogen atom transfer (HAT) from the -NH- function of H₃L to ³O₂ and subsequent (2e + 2H⁺) oxidation forming (1,3,4-trioxonaphthalen)=N-N=C(OH)Ph (HL^OX) have been established. The H₃L → HL^OX transformation occurs via (3-hydroperoxy-1,4-naphthoquinone)=N-N=C(O^-)Ph (HL^OOH-) as an intermediate. The primary step is HAT generating H₂L^•- and hydroperoxide (OOH^•) radicals. H₂L^•- is delocalized over the aromatic ring and incites coupling reactions via ortho carbon and produces coordinated HL^OOH-. In solution, the homolytic cleavage of the peroxo bond leads to aromatic ring oxidation, affording L^OX-. Ruthenium(II) and osmium(II) complexes of the types [M^II(H₂L^-)(PPh₃)₂X], [M^II(HL^OOH-)(PPh₃)₂X], and trans-[M^II(L^OX-)(PPh₃)₂X] were successfully isolated in good yields. Notably, the cyclic voltammograms of all of the complexes exhibit reversible anodic waves due to M^III/M^II redox couples. The rate constants of the [M^II(H₂L^-)(PPh₃)₂X] → [M^II(HL^OOH-)(PPh₃)₂X] conversions determined by time-driven UV-vis spectroscopy in dry CH₂Cl₂, wet CH₂Cl₂, and D₂O wet CH₂Cl₂ in air at 298 K follow the order $k_{{\mathrm{CH}}_2{\mathrm{Cl}}_2{\text{-H}}_{2}O}$> $k_{{\mathrm{CH}}_2{\mathrm{Cl}}_2{\text{-D}}_{2}O}$> $k_{{\mathrm{CH}}_2{\mathrm{Cl}}_2}$. It is established that the rate constants are dependent on the ³O₂ content of the solution but not on the concentration of the complex.