Coordination Chemistry of Protic Pincer-Type Bis(pyrazolyl)pyridines and Related Compounds

Proton-responsive pincer-type ligands have been used as scaffolds to realize metal–ligand cooperative transformations. Unlike most of the ligands therein, 2,6-bis(pyrazol-3-yl)pyridines (3-bpp) ensure that two protic groups point to the remaining coordination site in the pincer plane. This article reviews the coordination chemistry of 3-bpp and related tridentate ligands with emphasis on the characteristics originating from the Brønsted acidity of the peripheral NH groups. Synthesis of 3-bpp complexes, which often associated with spontaneous deprotonation of the 3-bpp ligand and aggregation of the complex to give supramolecular structures, is first described. Deprotonation of the 3-bpp complexes along with the following reactivities is then discussed. Application of the 3-bpp complexes to catalytic reactions as well as materials exhibiting spin-crossover and fluorescence is also described. Finally, pincer-type complexes bearing protic N -heterocyclic carbene side arms are briefly mentioned as relevant molecules.