Pyridylidene Amines and Amides: Donor-Flexible Ligands for Catalysis

Ligands with flexible donor properties exhibit the unique ability to transition within the continuum of neutral (L-type) and anionic (X-type) metal bonding in response to their local electronic environment. This flexibility stabilizes transition-metal complexes in unparalleled ways and facilitates catalytic turnovers, especially when the catalytic cycle features the active metal center in different oxidation states. Pyridylidene amines (PYEs) and pyridylidene amides (PYAs) are examples of nitrogen-based donor-flexible ligands. In the past decade, the application of PYE and PYA ligands in transition-metal catalysis has been growing, to now include reactions such as hydrogen transfer reactions, water oxidation, and CO2 utilization, among many others. Here we provide an overview of these powerful ligands, including their synthesis, their donor properties, and in particular their catalytic application─highlighting the key ligand features that impart high activity, as well as the wide potential of this rather underexplored ligand system for diverse catalytic applications.