Microwave-Assisted Synthesis of Amino Acid Schiff Base Complexes with Enhanced Nematicidal Properties against Meloidogyne hapla

Abstract

Microwave-assisted synthesis afforded benzylidene cysteine (BC), benzylidene glycine (BG), benzylidene methionine (BM), and benzylidene alanine (BA) Schiff bases in high yields (85–90%), followed by the preparation of their Cd(II), Co(II), Ni(II), and Zr(IV) complexes in moderate yields (18.5–68.3%). Spectroscopic analyses (FTIR and UV–vis) suggest metal coordination via azomethine nitrogen and oxygen donor sites, consistent with octahedral geometries except for Cd(II) complexes with a four-coordinate geometry.
Molar conductivity measurements suggested that the Schiff base ligands behave as non-electrolytes in solution, whereas the metal complexes exhibited electrolytic behaviour consistent with probable 1:2 electrolytes. Antinematicidal assays against Meloidogyne hapla demonstrated action of the ligands and metal complexes, particularly Cd(II) derivatives, against the nematodes. Free ligands exhibited moderate effects, with maximum mortality of 77.4 % (BA), 77.5 % (BC), and 77.8 % (BG) at 10.0 ppm after 15 hours, while BM showed lower activity (≤ 65.7 %). In contrast, metal complexes demonstrated markedly enhanced efficacy. Cd(II) complexes such as BA–Cd²⁺ and BC–Cd²⁺ achieved 99.2–100 % mortality across all concentrations (2.5 - 10.0 ppm) within 10–15 hours. Similarly, BG–Co²⁺ and BM–Zr⁴⁺ exhibited near-complete mortality (98.5–100 %) even at low concentrations. Other complexes, including BA–Ni²⁺ and BG–Ni²⁺, reached 98.5–100 % mortality at 10.0 ppm after 15 hours, which showed complete mortality at low concentrations. These findings highlight the role of metal coordination in enhancing the bioactivity of Schiff bases.