Synthesis, spectroscopic characterization, and catalytic activity of Mo(VI) complexes derived from 4-aminobenzohydrazones

Three Schiff base ligands containing ONO donor groups (LAC2, LAB2, and LAI2) were obtained by the condensation of 4-aminobenzohydrazide with 3,5-dichlorosalicylaldehyde, 3,5-dibromosalicylaldehyde, and 3,5-diiodosalicylaldehyde, respectively, using ethanol as a solvent. These ligands were subsequently treated with [MoO2(acac)2] (acac = acetylacetonato) in a 1:1 M ratio using methanol or DMF to give corresponding Mo(VI) complexes in good yield (58–64 %). FT-IR confirms the synthesis of complexes by showing an HC[dbnd]N band at 1604 cm−1, enolic C[sbnd]O at 1261–1265 cm−1, and cis-MoO22+ bands at 910–937 cm−1. NMR data reveal keto–imine tautomerism, evidenced by the disappearance of OH and NH signals, indicating deprotonation and chelation of the ligand to the molybdenum center. Results from the characterization showed that the ligands acted as tridentate, binding to the Mo(VI) center via O,N,O atoms, forming mononuclear homoleptic complexes (MAC2, MAB2, and MAI2). Furthermore, the synthesized complexes were evaluated for catalytic activity in the selective oxidation of sulfides to produce sulfoxides as the only products in high yields. Ethanol proved to be the optimal solvent, giving the highest conversions at its reflux temperature (78 °C), while H2O2 emerged as the superior oxidant, affording near-quantitative yields of sulfoxide.