Ligand relay catalysis: concepts, principles, and applications in transition-metal catalysis

In transition-metal catalysis, traditional single metal-ligand systems (M/L) often struggle when different steps of the catalytic cycle impose conflicting demands on the catalyst. Emerging as a powerful alternative, ligand relay catalysis (LRC) employs one metal with two distinct ligands that dynamically switch between two activated catalyst states (ML_A and ML_B) during the catalytic cycle. This strategy enhances reaction efficiency, improves selectivity, and unlocks unprecedented reactivities or even entirely new reactions. This tutorial review offers a comprehensive overview of recent advances in this rapidly growing area. Specifically, we focus on multiligand catalytic systems involving two distinct activated catalysts, excluding those relying on only a single active species. Mechanistically, ligand relay catalysis (LRC) can be broadly classified into three categories: (1) dynamic ligand relay catalysis, (2) sequential ligand relay catalysis, and (3) synergistic ligand relay catalysis. We hope this review not only summarizes the state-of-the-art in ligand relay catalysis (LRC) but also inspires the development of more efficient and innovative catalytic systems in the near future.