New Functional Model Complexes of Intradiol-Cleaving Catechol Dioxygenases: Properties and Reactivity of Cu^II(L)(O₂Ncat)

Complexes Cu(O2Ncat)(tbeda) (1) and Cu(O2Ncat)(tmeda) (2) (tbeda = N,N,N‘,N‘-tetrabenzylethylenediamine, tmeda = N,N,N‘,N‘-tetramethylethylenenediamine, O2NcatH2 = 4-nitrocatechol) have been prepared by the reaction of copper(II) perchlorate with 4-nitrocatechol in the presence of triethylamine and the appropriate bidentate ligand. These compounds represent structural and functional model systems for the copper-containing catechol 1,2-dioxygenase. Both complexes have been structurally characterized by X-ray crystallography and by UV−vis, IR, and EPR spectroscopies. Upon protonation of 1 and 2 with perchloric acid, the bidentate coordination of O2Ncat could be reversible converted to the monodentate coordination of O2NcatH. The equilibrium constants were found to be 4200 and 3500, respectively, by measuring the UV−vis spectra in DMF. Back-titration with morpholine proved the reversibility of both reactions. Kinetic data on the oxygenation of 1 and 2 revealed overall second-order rate equations with kinetic parameters: ktbeda = (4.63 ± 0.23) × 10-2 mol-1 dm3 s-1, ΔH⧧ = 51 ± 6 kJ mol-1, ΔS⧧ = −137 ± 16 J mol-1 K-1; ktmeda = (0.89 ± 0.23) mol-1 dm3 s-1, ΔH⧧ = 85 ± 7 kJ mol-1, ΔS⧧ = −57 ± 19 J mol-1 K-1 at 365.16 K. Oxygenation of 1, 2, and [Cu(O2NcatH)(L)]ClO4 (L = tbeda, tmeda) in DMF solution at ambient conditions gives the corresponding intradiol ring-cleaved (2-nitro-muconato)copper(II) complexes. These data support the assumption that the reaction of the differently coordinated catecholate ligand with dioxygen shows only 1,2-dioxygenase activity.