Reversible dioxygen binding and phenol oxygenation in a tyrosinase model system

The complex [Cu-2(L-66)](2+) (L-66 = alpha,alpha'-bis{bis[2-(1'-methyl-2'-benzimidazolyl)ethyl] amino}-m-xylene) undergoes fully reversible oxygenation at low temperature in acetone. The optical [lambda(max) = 362 (epsilon 15 000), 455 (epsilon 2000), and 550 nm (epsilon 900 M-1 cm(-1))] and resonance Raman features (760 cm(-1), shifted to 719 cm(-1) with O-18(2)) of the dioxygen adduct [Cu-2(L-66)(O-2)](2+) indicate that it is a mu-eta(2):eta(2)-peroxodicopper(II) complex. The kinetics of dioxygen binding, studied at - 78 degrees C, gave the rate constant k(1) = 1.1 M-1 s(-1), for adduct formation, and k(-1) = 7.8 x 10(-5) s(-1), for dioxygen release from the Cu2O2 complex. From these values, the O-2 binding constant K = 1.4 x 10(4) M-1 at -78 degrees C could be determined. The [Cu-2(L-66)(O-2)](2+) complex performs the regiospecific ortho-hydroxylation of 4-carbomethoxyphenolate to the corresponding catecholate and the oxidation of 3,5-di-tert-butylcatechol to the quinone at -60 degrees C. Therefore, [Cu-2(L-66)](2+) is the first synthetic complex to form a stable dioxygen adduct and exhibit true tyrosinase-like activity on exogenous phenolic compounds.