Generation and Reactivity of μ-1,2-Peroxo Cu<sup>II</sup>Cu<sup>II</sup> and Bis-μ-oxo Cu<sup>III</sup>Cu<sup>III</sup> Species and Catalytic Hydroxylation of Benzene to Phenol with Hydrogen Peroxide
Lena Siebe, Christoph Butenuth, Anja Stammler, Hartmut Bögge, Stephan Walleck, Thorsten Glaser
Abstract
Tetradentate-N 4 ligands stabilize dinuclear {Cu II (μ-1,2-peroxo)Cu II } and {Cu III (μ-O) 2 Cu III } species, and Cu II complexes of these ligands were reported to catalyze the oxidation of benzene with H 2 O 2 . Here, we report {Cu II (μ-1,2-peroxo)Cu II } and {Cu III (μ-O) 2 Cu III } intermediates of dinucleating bis(tetradentate-N 4 ) ligands depending on the absence or presence of 6-methyl substituents on the terminal pyridine donors, respectively, generated either from {Cu I Cu I } precursors with O 2 or from {Cu II Cu II } precursors with H 2 O 2 and NEt 3 . 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 H 2 O 2 between 30 and 50 °C indicate that both radical-based and {Cu 2 O n }-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 II (μ–OH)Cu II } reacts with excess H 2 O 2 at −40 °C to {Cu II (OOH)} 2 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.