Reversible dioxygen binding and phenol oxygenation in a tyrosinase model system.

The complex [Cu2(L-66)]2‡ (L-66 ˆ a,a'-bis{bis[2-(1'-methyl-2'- benzimidazolyl)ethyl]amino}-m-xylene) undergoes fully reversible oxygenation at low temperature in acetone. The optical [lmaxˆ362 (e 15 000), 455 (e 2000), and 550 nm (e 900mÿ1cmÿ1)] and resonance Raman features (760 cmÿ1, shifted to 719 cmÿ1 with 18O2) of the dioxygen adduct [Cu2(L-66)(O2)]2‡ indicate that it is a m-h2 :h2-peroxodicopper(ii) complex. The kinetics of dioxygen binding, studied at ÿ78 8C, gave the rate constant k1ˆ1.1mÿ1 sÿ1, for adduct formation, and kÿ1ˆ7.8 10ÿ5 sÿ1, for dioxygen release from the Cu2O2 complex. From these values, the O2 binding constant Kˆ 1.4104mÿ1 at ÿ78 8C could be determined. The [Cu2(L-66)(O2)]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 ÿ608C. Therefore, [Cu2(L-66)]2‡ is the first synthetic complex to form a stable dioxygen adduct and exhibit true tyrosinase- like activity on exogenous phenolic compounds.