Phenoxazinone synthase-like catalytic activity of novel mono- and tetranuclear copper(ii) complexes with 2-benzylaminoethanol

Abstract

Three novel coordination compounds, [Cu(ca)₂(Hbae)₂] (1), [Cu(va)₂(Hbae)₂] (2) and [Cu₄(va)₄(bae)₄]·H₂O (3), have been prepared by self-assembly reactions of copper(II) chloride (1 and 2) or tetrafluoroborate (3) and CH₃OH (1 and 3) or CH₃CN (2) solution of 2-benzylaminoethanol (Hbae) and cinnamic (Hca, 1) or valeric (Hva, 2 and 3) acid. Crystallographic analysis revealed that both 1 and 2 have mononuclear crystal structures, wherein the complex molecules are H-bonded forming extended supramolecular chains. The tetranuclear structure of 3 is based on the {Cu₄(μ₃-O)₄} core, wherein the metal atoms are bound together by μ₃ oxygen bridges from 2-benzylaminoethanol forming an overall cubane-like configuration. The strong hydrogen bonding in 1–3 leads to the joining of the neighbouring molecules into 1D chains. Concentration-dependent ESI-MS studies disclosed the equilibria between di-, tri- and tetranuclear species in solutions of 1–3. All three compounds act as catalysts for the aerobic oxidation of o-aminophenol to the phenoxazinone chromophore (phenoxazinone synthase-like activity), with the maximum reaction rates of 4.0 × 10⁻⁷, 2.5 × 10⁻⁷ and 2.1 × 10⁻⁷ M s⁻¹ for 1, 2 and 3, respectively, supported by the quantitative yield of the product after 24 h. The dependence of the reaction rates on catalyst concentrations is evidence of reaction orders higher than one relative to the catalyst. Kinetic and ESI-MS data allowed us to assume that the tetranuclear species, originating from 1, 2 and 3 in solution, possess considerably higher activity than the species of lower nuclearity. Mechanistic and isotopic ¹⁸O-labelling experiments suggested that o-aminophenol coordinates to Cu^II species with the formation of reactive intermediates, while the oxygen from ¹⁸O₂ is not incorporated into the phenoxazinone chromophore.