Copper(II) complexes of tripodal ligand scaffold (NO) as functional models for phenoxazinone synthase.

The copper(II) complexes [Cu(L)NO] (1-9) of newer NO 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-O 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 cm) that corresponds to square pyramidal geometry. The bonding parameters α, 0.760-0.825; β, 0.761-0.994; γ, 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 s in water. The reaction follows Michaelis-Menten enzymatic reaction kinetics with turnover numbers (k) 9.11 × 10 h for 1 and 4.66 × 10 h for 9 in water. The spectral, redox and kinetic studies were performed in water to mimic the enzymatic oxidation reaction conditions.