Design and Synthesis of Transition Metal Complexes for Catalytic Applications in Green Chemistry

Abstract

The mounting pressure of environmental degradation and climate change has intensified the search for sustainable chemical processes. Transition metal complexes (TMCs) occupy a central position in green chemistry owing to their tuneable electronic structures, diverse coordination geometries, and remarkable catalytic versatility. These coordination compounds can activate a wide range of substrates under mild conditions, reducing energy consumption and hazardous by-product generation. Despite their potential, the widespread adoption of TMC-based catalysts faces several formidable challenges: achieving high selectivity in complex substrate environments, preventing metal leaching into products, designing robust recyclable systems, and developing atom-economical synthetic pathways for the complexes themselves. Balancing reactivity with stability under diverse reaction conditions remains a persistent bottleneck. This study presents the rational design, synthesis, and characterisation of a series of novel TMCs incorporating multidentate N,N-, N,O-, and P,P-donor ligands coordinated to Cu(II), Pd(II), Ru(II), Fe(III), Ni(II), and Mn(III) centres. Complexes were fully characterised using IR, UV-Vis, NMR, ESI-MS, XRD, and magnetic susceptibility measurements. Catalytic performance was evaluated in aerobic oxidations, C–C coupling, photocatalytic water splitting, epoxidation, and hydrogenation reactions under green solvent conditions. The synthesised complexes demonstrated excellent catalytic activity with yields ranging from 85–97% and turnover numbers (TON) up to 30 × 10³. The [Pd(PPh₃)₄] and [Cu(bpy)₂]²⁺ systems exhibited outstanding recyclability over ten reaction cycles. Photocatalytic hydrogen evolution with [Ru(bpy)₃]²⁺ achieved a quantum efficiency of 12.4%, while [Fe(salen)Cl] gave 91% enantioselectivity in asymmetric epoxidation. This work demonstrates that judicious ligand engineering and metal selection can produce highly active, selective, and recyclable catalytic systems fully aligned with the principles of green chemistry, providing a robust platform for next-generation sustainable catalytic processes.