A review on dual role of embelin in metal complexes: structural ligand and bioactive agent

Embelin, a naturally occurring hydroxyl benzoquinone isolated from Embelia ribes, has been extensively investigated for its diverse pharmacological properties, including antitumor, antidiabetic, antioxidant, and antimicrobial activities. In recent years, the coordination chemistry of embelin has emerged as an important area of research, as metal complexation can notably influence its physicochemical and biological behaviour. This review critically compiles and analyzes the reported literature on metal complexes of embelin and its derivatives synthesized through various chemical and physical approaches, focusing on their structural features, spectroscopic characterization, thermal behavior, and biological activities. The coordination mode of embelin is discussed based on ultraviolet–visible (UV), Fourier transform infrared (FT-IR), nuclear magnetic resonance spectroscopy (NMR), thermogravimetric analysis, and elemental (CHNS) studies, which collectively confirm bidentate chelation through enolic and quinonic oxygen atoms, leading predominantly to octahedral or distorted octahedral geometries. Biological evaluations indicate that complexation of embelin with metal ions results in a notable reduction in antioxidant activity compared to free embelin, while leading to enhanced antimicrobial efficacy upon complexation with nickel, cobalt, and iron ions. The observed modulation of biological activity upon metal coordination is discussed in terms of structural factors. Overall, this review provides a comprehensive and updated account of embelin-based metal complexes, highlighting their potential relevance in medicinal and coordination chemistry.