Generation and Reactivity of μ-1,2-Peroxo CuCu and Bis-μ-oxo CuCu Species and Catalytic Hydroxylation of Benzene to Phenol with Hydrogen Peroxide.

Tetradentate-N ligands stabilize dinuclear {Cu(μ-1,2-peroxo)Cu} and {Cu(μ-O)Cu} species, and Cu complexes of these ligands were reported to catalyze the oxidation of benzene with HO. Here, we report {Cu(μ-1,2-peroxo)Cu} and {Cu(μ-O)Cu} intermediates of dinucleating bis(tetradentate-N) ligands depending on the absence or presence of 6-methyl substituents on the terminal pyridine donors, respectively, generated either from {CuCu} precursors with O or from {CuCu} precursors with HO and NEt. Both intermediates are not stable even at low temperatures, but they show no electrophilic HAT reactivity with DHA. Catalytic investigations on the hydroxylation of benzene with excess HO between 30 and 50 °C indicate that both radical-based and {CuO}-based mechanisms depend strongly on the catalytic conditions. In the presence of a radical scavenger, TONs of ∼920/∼720 have been achieved without/with the 6-methyl group of the ligand. Although {Cu(μ-OH)Cu} reacts with excess HO at -40 °C to {Cu(OOH)} species, these are only stable for seconds at 20 °C and cannot account for catalytic oxidations over a period of 24 h at 30-50 °C.