Application of Stimuli-Responsive and “Non-innocent” Ligands in Base Metal Catalysis

The development of efficient and selective catalysts is an important goal of modern research in chemistry – the science of matter and its transformations. Our society needs new catalysts to become more sustainable, and a desire for selectivity and efficiency in the preparation of medicines and materials has boosted our interest in developing new methods based on homogeneous catalysis, particularly on the development of new ligands that can be fine-tuned to specific needs. The properties of a metal complex as a whole are the result of the interaction between the metal center and its surrounding ligands. In traditional approaches, the steric and electronic properties of the spectator ligand are used to control the performance of the catalyst, but most of the reactivity takes place at the metal. Recent new approaches deviate from this concept and make use of ligands that play a more prominent role in the elementary bond activation steps in a catalytic cycle [1, 2]. The central idea is that the metal and the ligand can act in a synergistic manner to facilitate a chemical process. In this light, complexes based on the so-called “non-innocent” ligands offer interesting prospects and have attracted quite some attention. The term “non-innocent” is broadly used, and diverse authors give different interpretations to the term. It was originally introduced by Jørgensen [3] to indicate that assigning metal oxidation states can be ambiguous when complexes contain redox-active ligands. As such, ligands that get reduced or oxidized in a redox process of a transition metal complex are often referred to as “redox non-innocent.” [4, 5] With modern spectroscopic techniques, combined with computational studies, assigning metal and ligand oxidations states has become less ambiguous, and hence, many authors started to use the term “redox-active ligands” instead. Gradually, many authors also started to use the term “non-innocent” for ligands that are more than just an ancillary ligand, frequently involving ligands that have reactive moieties that can act in cooperative (catalytic) chemical transformations, act as temporary electron reservoirs, or respond to external triggers to modify the properties or reactivity of a complex.