Phenoxazinone Synthase-Like Catalytic Activity of Bi- and Trinuclear Copper(II) Complexes with 2-Benzylethanolamine: Experimental and Theoretical Investigations.

The self-assembly reaction of 2-benzylaminoethanol (Hbae) with CuCl2 or Cu(NO3)2 leads to the formation of binuclear [Cu2(bae)2(Cl)2] (1) and [Cu2(Hbae)2(bae)2](NO3)2 (2) complexes, while the trinuclear [Cu3(Hbae)2(bae)2(dmba)2](NO3)2 (3) compound was obtained using the auxiliar bulky substituted 2,2-dimethylbutyric acid (Hdmba). Crystallographic studies reveal the molecular structures of 1 and 2 based on the similar {Cu2(μ-O)2} core, while the structure of 3 features the {Cu3(μ-O)2} core with consecutive arranement of the metal centres, supported by the additional carboxylate bridges. The strong intermolecular hydrogen bonds join the molecular structures into 1D (for 1 and 3) or 2D (for 2) architectures. All three compounds act as catalysts for the aerobic oxidation of 2-aminophenol to the phenoxazinone chromophore (phenoxazinone synthase-like activity) with the maximum reaction rates up to 2.3×10-8 M s-1. The substrate scope involves methyl-, nitro- and chloro-substituted 2-aminophenols, disclosing the negligible activity of nitro-derivatives, while the 6-amino-m-cresol substrate shows the highest activity with the initial reaction rate of 5.8×10-8 M s-1. The mechanism of the rate-limiting reaction step (copper-catalysed formation of 2-aminophenoxyl radicals) was investigated at the DFT level. The combined DFT and CASSCF studies of the copper superoxo CuII-OO⋅ radical species as possible unconventional reaction intermediates resulted in a rational mechanism of H-atom abstraction, where the activation energies follow the experimental reactivity of substituted 2-aminophenols. The TDDFT and STEOM-DLPNO-CCSD theoretical calculations of the absorption spectra of substrates, phenoxazinone chromophores and putative polynuclear species containing 2-aminophenoxo ligand are reported.