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

Abstract

Five new tricompartmental ligands, each having three tridentate dibasic ONO sites, have been obtained from the condensation of benzene-1,3,5-tricarbohydrazide (bthz) and 4-formyl-5-pyrazolone H₆btzh(fp)₃ (I), 4-acetyl-5-pyrazolone H₆bthz(mp)₃ (II), 4-propionyl-5-pyrazolone H₆bthz(ep)₃ (III), 4-butyryl-5-pyrazolone H₆bthz(pp)₃ (IV) and 4-benzoyl-5-pyrazolone H₆bthz(php)₃ (V). The reaction of I–V with [Mo^VIO₂(acac)₂] (Hacac = acetylacetone) leads to the formation of tris{cis-[MoO₂]} complexes [{Mo^VIO₂(H₂O)}₃bthz(fp)₃] (1), [{Mo^VIO₂(H₂O)}₃bthz(mp)₃] (2), [{Mo^VIO₂(H₂O)}₃bthz(ep)₃] (3), [{Mo^VIO₂(H₂O)}₃bthz(pp)₃] (4) and [{Mo^VIO₂(H₂O)}₃bthz(php)₃] (5) having C₃ symmetry. All ligands and complexes are characterized by various techniques: FT-IR spectroscopy, UV/Visible, NMR (¹H and ¹³C), elemental analysis and thermal studies. X-Ray diffraction studies of complexes 2–5 confirm their distorted octahedral geometry, where each pocket of ligands coordinates through the enolate oxygen (of pyrazole), enolate oxygen (of hydrazide), and azomethine nitrogen atoms. Catalytic potentials of these complexes were achieved for the (i) oxidation of 2-methylnaphthalene to 2-methyl-1,4-naphthoquinone (vitamin K₃), (ii) oxidation of different olefins, and (iii) oxidative bromination of various phenol derivatives, including thymol. As high as 99% selectivity of 2-methyl-1,4-naphthoquinone has been obtained in high yield, while the selective olefin epoxidation resulted in the corresponding epoxide. The high yield of brominated products catalyzed by these complexes signifies them as useful functional models of vanadium-dependent haloperoxidases.