Synthesis of Novel Azo-Azomethine-Based Pd(II) Complexes and Catalytic Activities in the Synthesis of Vitamin K₃ (2-Methyl-1,4-Naphthoquinone)

Abstract

Five types of azo-azomethine ligands (6–10) have been synthesized from the reaction of azo aldehyde derivatives with aniline. Structures of the synthesized ligand were characterized with ¹³C-NMR, ¹H-NMR, UV–Visible spectroscopy, elemental analysis, and FT-IR. The Pd(II) complexes were synthesized from the reaction of free ligands with Na₂PdCl₄. From FTIR, UV–visible, ¹³C-NMR, ¹H-NMR, elemental analysis, and HRMS analysis results, it was understood that palladium metal is bound by N and O atoms in the ligands. The catalytic activities of complexes were investigated in the synthesis of 2-methyl-1,4-naphthoquinone (vitamin K₃, 2MNQ, and menadione) with the oxidation of 2-methyl naphthalene (2MN) substrate. Acidic media (sulfuric acid, acetic acid, and hydrogen peroxide) was used as a strongly oxidizing agent in the reaction. Selectivities were reached over 75% with conversions over 89% using the Pd(II) complexes as catalysts.

Highlights

This paper reports the preparation, characterization and catalytic activities for some palladium(II) complexes of Schiff base ligands that include azo group. The products have been characterized using elemental analysis, HRMS, FTIR, UV–Vis, and NMR (¹H, ¹³C) spectroscopy. All the novel complexes were used as catalysts in the oxidation of 2-methyl naphthalene (2MN) to 2-methyl-1, 4-naphthoquinone (vitamin K₃, menadione, and 2MNQ) in the presence of hydrogen peroxide and acetic acid. Selectivities reached over 75% with the conversions over 89% using the Pd(II) complexes as catalysts.

Keywords:

• Azo-azomethine
• metal complexes
• catalysis
• vitamin K₃

Disclosure statement

No potential conflict of interest was reported by the authors.