Synthesis, Characterization and Biomimetic Activity of Heterogenized Dioxidomolybdenum(VI) and Analogous Homogeneous Complexes

Two mononuclear cis-[(MoO2)-O-VI] complexes with Schiff base ligands, H4L1 (I) and H4L2 (II) (derived from the condensation of 3,5-diformyl-4-hydroxybenzoic acid with isonicotinoylhydrazide and nicotinoylhydrazide) were synthesized using [(MoO2)-O-VI(acac)(2)] (Hacac=acetylacetone) and ligand in 1 : 1 molar ratio in methanol. The obtained homogeneous complexes [(MoO2)-O-VI(H2L1)](n) (1) and [(MoO2)-O-VI(H2L2)(MeOH)] (2) were supported on titania functionalized with amine group (APTMS-TiO2) to obtain heterogeneous complexes [(MoO2)-O-VI(HL1)(MeOH)]@APTMS-TiO2 (3) and [(MoO2)-O-VI(HL2)(MeOH)]@APTMS-TiO2 (4), respectively. All the isolated complexes were characterized by various spectral techniques (FT-IR, UV-visible, H-1 and C-13 NMR), elemental (CHN) and thermogravimetric analyses; heterogeneous complexes were additionally characterized by transmission electron microscopy, diffuse reflectance, X-ray photoelectron spectroscopy and Powder-X-ray diffraction studies. The single-crystal X-ray diffraction analysis of complexes 1 and 2 confirms the distorted octahedral geometry for both complexes, where ligands coordinate to the metal centre through O-phenolate, N-azomethine, and O-enolate. Complex 1 has a 3D polymeric structure, where the sixth site of one molecule of Mo is coordinated to the pyridinic nitrogen of the other molecule while 2 is monomeric. These complexes are explored as bio-mimicking model catalysts for the type II copper site in phenoxazinone synthase. These complexes catalyze the oxidative condensation of o-aminophenol (OAP) into 2-aminophenoxazine-3-one (APX) utilizing aq. H2O2 in the MeCN-MeOH mixture. For complexes 1 and 2, the value of turnover number (k(cat)) obtained for phenoxazinone synthase-like activity is 2.14 and 1.76 h(-1), respectively. Under the optimized reaction parameters, the yield of 2-aminophenoxazine-3-one (APX) obtained is 97 and 92 % for complexes 3 and 4, respectively.

Automated summary

In this study, two mononuclear cis-[(MoO2)-O-VI] complexes were synthesized and then supported on amine-functionalized titanium dioxide to obtain heterogeneous complexes. Various characterization techniques were used to analyze the isolated complexes, and the geometric structure of the complexes was determined by single-crystal X-ray diffraction analysis. The study found that these complexes can serve as biomimetic catalysts for phenoxazinone synthase, effectively catalyzing the reaction and achieving high yields.