Vanadium(iv and v) complexes of pyrazolone based ligands: Synthesis, structural characterization and catalytic applications

Abstract

The ONO donor ligands obtained from the condensation of 4-benzoyl-3-methyl-1-phenyl-2-pyrazoline-5-one (Hbp) with benzoylhydrazide (H₂bp-bhz I), furoylhydrazide (H₂bp-fah II), nicotinoylhydrazide (H₂bp-nah III) and isonicotinoylhydrazide (H₂bp-inh IV), upon treatment with [V^IVO(acac)₂], lead to the formation of [V^IVO(bp-bhz)(H₂O)] 1, [V^IVO(bp-fah)(H₂O)] 2, [V^IVO(bp-nah)(H₂O)] 3 and [V^IVO(bp-inh)(H₂O)] 4, respectively. At neutral pH the in situ generated aqueous K[H₂V^VO₄] reacts with ligands I and II, forming potassium salts, K(H₂O)₂[V^VO₂(bp-bhz)] 5 and K(H₂O)₂[V^VO₂(bp-fah)] 6, while ligands III and IV give neutral complexes, [V^VO₂(Hbp-nah)] 9 and [V^VO₂(Hbp-inh)] 10, respectively. Acidification of aqueous solutions of 5 and 6 with HCl also gives neutral complexes [V^VO₂(Hbp-bhz)] 7 and [V^VO₂(Hbp-fah)] 8, respectively. Complexes 1–4, upon slow aerial oxidation in methanol, convert into monooxidovanadium(V) complexes, [V^VO(bp-bhz)(OMe)] 11, [V^VO(bp-fah)(OMe)] 12, [V^VO(bp-nah)(OMe)] 13 and [V^VO(bp-inh)(OMe)] 14, respectively. All complexes were characterized by various spectroscopic techniques like FT-IR, UV-visible, EPR (for complexes 1–4) and NMR (¹H, ¹³C and ⁵¹V), elemental analysis, thermogravimetry and single crystal X-ray diffraction (for complexes 5–10 and 12). In the solid state, all complexes characterized by X-ray diffraction show the metal ion 5-coordinated in a distorted square pyramidal geometry. Complexes 11–14 were tested as catalysts for the one-pot three-component (ethylacetoacetate, benzaldehyde and ammonium acetate) dynamic covalent assembly, via Hantzsch reaction, using hydrogen peroxide as oxidant in solution and under solvent-free conditions. The complexes are also active catalysts for the oxidation of tetralin to tetralone with H₂O₂ as oxidant. The influence of the amounts of catalyst and oxidant, and solvent, temperature and time on the catalyzed reactions was investigated.