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

Abstract

Two mononuclear cis-[Mo VI O 2 ] complexes with Schiff base ligands, H 4 L 1 (I) and H 4 L 2 (II) (derived from the condensation of 3,5-diformyl-4-hydroxybenzoic acid with isonicotinoylhydrazide and nicotinoylhydrazide) were synthesized using [Mo VI O 2 (acac) 2 ] (Hacac = acetylacetone) and ligand in 1 : 1 molar ratio in methanol. The obtained homogeneous complexes [Mo VI O 2 (H 2 L 1 )] n (1) and [Mo VI O 2 (H 2 L 2 )(MeOH)] (2) were supported on titania functionalized with amine group (APTMS-TiO 2 ) to obtain heterogeneous complexes [Mo VI O 2 (HL 1 )(MeOH)]-@APTMS-TiO 2 (3) and [Mo VI O 2 (HL 2 )(MeOH)]@APTMS-TiO 2 (4), respectively. All the isolated complexes were characterized by various spectral techniques (FT-IR, UV-visible, 1 H and 13 C 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 biomimicking 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. H 2 O 2 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 À 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.

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

“…2-aminophenoxazin-3-one (APX, m.p. = 254–256 °C) 36 are acquired after recrystallizing it from methanol. The 1 H and 13 C NMR spectra of the APX are displayed in Fig.…”

“…[ [ 36 After achieving the optimized reaction conditions, immobilized ligand II, APTMS-TiO 2 , H 2 O 2 treated TiO 2 and homogeneous complex 1 were also tested as catalysts for the oxidative coupled product of o-AP. As expected, the yield of APX obtained for immobilized ligand II, and APTMS-TiO 2 is very less due to the unavailability of an active metal centre.…”

“…Though SiO 2 , Fe 3 O 4 and other inorganic materials are available for the immobilization of metal complexes, [32][33][34][35] we recently observed that TiO 2 has worked as an excellent support in immobilizing molybdenum complexes. 26,36 Further, such TiO 2supported metal complexes based catalysts are recyclable and reusable. [37][38][39][40][41][42] Additionally, the kinetic potential of the homogeneous dioxidotungsten(VI) complex has been explored for the coupling of o-aminophenol under oxidizing conditions.…”

Trinuclear dioxidomolybdenum(vi) complexes derived from benzene-1,3,5-tricarbohydrazide and study of the catalytic activity

“…1 may also be considered, wherein an additional coordinating group like pyridine that is present within the ligand interacts with the Mo centre of the dioxidomolybdenum( vi ) complex and facilitates an oligomeric structure. 22 However, the better catalytic activity for any one of the above categories can be co-related with the one that will have a high tendency to interact with the oxidant and form a catalytically active intermediate. 23 Therefore, the design of dioxidomolybdenum( vi ) complexes of type (a) is highly desirable, where the weakly coordinated solvent molecule may facilitate better interaction with the oxidant and/or substrate.…”