Titania supported dioxidotungsten(vi) complex as bio-mimic for the type II copper-containing oxidase enzyme phenoxazinone synthase

Abstract

Mononuclear dioxidotungsten(VI) complex [W^VIO₂(CH₃-hptb)(H₂O)] (1) of deferasirox {4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzoic acid, H₃hptb, I} has been synthesized by the reaction of [W^VIO₂(acac)₂] with I in MeOH and successfully characterized by various spectral techniques (FT-IR, UV-visible, ¹H and ¹³C NMR), elemental (CHN) and thermogravimetric analyses. During the metal complex formation, the esterification of the carboxylic group of ligand occurs. A similar reaction of [W^VIO₂(acac)₂] with heterogeneous ligand, [H₂hptb]@APTMS-TiO₂ (II) (i.e. ligand I immobilized on amine-functionalized titania) in MeOH produced the heterogeneous complex [W^VIO₂(hptb)(H₂O)]@APTMS-TiO₂ (2) which was characterized by additional techniques such as diffuse reflectance spectroscopy (DRS), powder-X-ray diffraction (P-XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The thermal stability and metal content of complex 2 have been achieved through thermogravimetric analysis and microwave plasma atomic emission spectroscopy (MP-AES), respectively. Both complexes have been utilized in bio-mimicking the type II copper-containing enzyme phenoxazinone synthase (PHS). In the presence of a green oxidant (aq. H₂O₂), both complexes produce 2-aminophenoxazine-3-one through the catalytic oxidative coupling of o-aminophenol in MeOH. Kinetic analysis of PHS-like activity has been carried out on complex 1, and the value of turnover number (k_cat) obtained was found to be 1.18 h⁻¹. Under the well-optimized reaction parameters, 80 and 93% yield, respectively, of APX by complexes 1 and 2 has been achieved.