Metal-Ligand Cooperation at Phosphine-Based Acceptor Pincer Ligands

Acceptor ligands, which predominantly withdraw electron density from a transition metal center, often engage in weak metal-ligand interactions. These can be stabilized by flanking the acceptor moiety with strongly binding phosphines in a pincer motif, affording more robust complexes in which bond activation and/or bond-forming events can take place while preserving the integrity of the molecule as a whole. This contribution highlights recent developments in this area. Compounds incorporating a borane at the central position are discussed first, followed by compounds incorporating an electrophilic C = E (E = C, O, N) p-bond. In both cases, recent examples highlight the ability of these ligands to (1) respond to electronic changes at the metal by modifying their binding mode and (2) accept a nucleophilic fragment (e.g., hydride) from substrate molecules. Applications of acceptor pincer ligands as cooperative catalysts are discussed.