Schiff Base Metal Complexes of 4-Hydroxyquinolin-2-one: A Short Review

Schiff base metal complexes derived from 4-hydroxyquinolin-2-one has drawn significant interest owing to its structural broad spectrum, coordination adaptability, and potential biological applications. This review provides a comprehensive overview of the synthesis, characterization, coordination behaviour, and functional properties of Schiff base ligands based on the 4-hydroxyquinolin-2-one scaffold and their corresponding metal complexes. The presence of multiple donor sites, including azomethine nitrogen, phenolic oxygen, and carbonyl functionalities, enables these ligands to form stable complexes with a wide range of transition metal ions such as Cu(II), Ni(II), Co(II), Zn(II), and Fe(III). Various synthetic strategies, including conventional reflux and green chemistry approaches, are discussed in detail, highlighting recent advancements in ligand design and complex formation. Spectroscopic, analytical, and crystallographic techniques employed for structural elucidation, such as FT-IR, NMR, UV–Vis, mass spectrometry, and X-ray diffraction, are critically examined to establish coordination modes and geometrical configurations. The review further explores the electronic, magnetic, and photophysical properties of these complexes, emphasizing their structure–property relationships. Overall, this review aims to consolidate current knowledge and identify emerging trends in the development of 4-hydroxyquinolin-2-one-based Schiff base metal complexes. It underscores their potential as multifunctional agents in medicinal chemistry and materials science, while also outlining future research directions to address existing challenges related to selectivity, toxicity, and mechanistic understanding.