Copper(II) complexes of tripodal ligand scaffold (N3O) as functional models for phenoxazinone synthase

Abstract

The copper(II) complexes [Cu(L)NO3] (1–9) of newer N3O ligands (L1-L9) have been synthesized and characterized. The molecular structure of 1, 4, and 7 exhibited nearly a perfect square pyramidal geometry (τ, 0.04–0.11). The Cu-OPhenolate bonds (~ 1.91 Å) are shorter than the Cu-N bonds (~ 2.06 Å) due to the stronger coordination of anionic phenolate oxygen. The Cu(II)/Cu(I) redox potentials of 1–9 appeared around −0.102 to −0.428 V versus Ag/Ag+ in water. The electronic spectra of the complexes showed the d-d transitions around 643–735 nm and axial EPR parameter (g||, 2.243–2.270; A||, 164–179 × 10−4 cm−1) that corresponds to square pyramidal geometry. The bonding parameters α2, 0.760–0.825; β2, 0.761–0.994; γ2, 0.504–0.856 and K||, 0.698–0.954 and K⊥, 0.383–0.820 calculated from EPR spectra and energies of d-d transitions. The complexes catalyzed the conversion of substrate 2-aminophenol into 2-aminophenoxazine-3-one using molecular oxygen in the water and exhibited the yields of 41–61%. The formation of the product is accomplished by the appearance of a new absorption band at 430 nm and the rates of formation were calculated as 6.98–15.65 × 10−3 s−1 in water. The reaction follows Michaelis-Menten enzymatic reaction kinetics with turnover numbers (kcat) 9.11 × 105 h−1 for 1 and 4.66 × 105 h−1 for 9 in water. The spectral, redox and kinetic studies were performed in water to mimic the enzymatic oxidation reaction conditions.