O 2 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 (H 3 L) coordinated to octahedral ruthenium(II) and osmium(II) ions activates an 3 O 2 molecule spontaneously. Hydrogen atom transfer (HAT) from the -NH- function of H 3 L to 3 O 2 and subsequent (2e + 2H + ) oxidation forming (1,3,4-trioxonaphthalen)=N-N=C(OH)Ph (HL OX ) have been established. The H 3 L → 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 H 2 L •- and hydroperoxide (OOH • ) radicals. H 2 L •- 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 (H 2 L - )(PPh 3 ) 2 X], [M II (HL OOH - )(PPh 3 ) 2 X], and trans -[M II (L OX - )(PPh 3 ) 2 X] 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 (H 2 L - )(PPh 3 ) 2 X] → [M II (HL OOH- )(PPh 3 ) 2 X] conversions determined by time-driven UV-vis spectroscopy in dry CH 2 Cl 2 , wet CH 2 Cl 2 , and D 2 O wet CH 2 Cl 2 in air at 298 K follow the order k CH 2 Cl 2 -H 2 O > k CH 2 Cl 2 -D 2 O > k CH 2 Cl 2 . It is established that the rate constants are dependent on the 3 O 2 content of the solution but not on the concentration of the complex.