Biomimetic modelling of copper enzymes: Synthesis, characterization, EPR analysis and enantioselective catalytic oxidations by a new chiral trinuclear copper(II) complex

The new octadentate ligand (R)-(+)-N,N′-dimethyl-N,N′-bis{-2-[bis(1-methyl-2-benzimidazolylmethyl)]-2-methylaminoethyl}-1,1′-binaphthyl-2,2′diamine [(R)-(+)-DABN-L-Ala-Bz4; L] was employed for the synthesis of dinuclear and trinuclear copper(II) complexes. The ligand design is based on the insertion of chiral residues derived from L-alanine between the diaminobinaphthyl (R)-(+)-DABN spacer and the aminobis(benzimidazole) metal binding units. The chiroptical properties of the ligand and the complexes are described. EPR experiments were performed on [Cu2L]4+ and [Cu3L]6+ at low temperatures. In the case of [Cu2L]4+, a weak dipolar interaction between the two spin centres was found. A similar weak spin interaction occurred in the trinuclear copper cluster, which could be treated likewise as a weakly coupled three-spin system. The analysis was substantiated by studying the complex EPR temperature behaviour, where population of the quartet state occurred only at high temperature (77 K), whereas at cryogenic temperatures (4 K) the system adopted a doublet state as a ground-state spin configuration. Titration with sodium azide of [Cu2L]4+ was consistent with terminal binding of one N3– molecule to each copper ion. Furthermore, dipolar interactions between spin centres were strongly suppressed in this case. For [Cu3L]6+, the adduct formed by the interaction with two azido molecules induced formation of a μ-azido bridges among the three Cu2+ ions and led to population of the quartet state even at cryogenic temperatures. This bridged configuration was, however, lost upon further addition of azido molecules. The copper(II) complexes were tested as catalysts in the oxidation of biogenic catechols and flavonoids by dioxygen to give the corresponding quinones, which were trapped as adducts with MBTH. The dinuclear complex [Cu2L]4+ displays poor substrate enantiodifferentiating ability, even though it exhibits catalytic activity comparable to that of [Cu3L]6+. The trinuclear complex [Cu3L]6+ exhibits significant enantioselectivity in the oxidations of the catecholamines L-/D-dopa methyl ester and L-/D-norepinephrine. The origin of this enantioselectivity must be associated with the mode of substrate binding, as it depends almost entirely on KM.