Coordination Chemistry of Bulky Aminopryridinates with Main Group and Transition Metals

The coordination chemistry of bidentate aminopyridinato ligands (ApH), in particular 2-aminopyridines, is a highly popular area of research. Due to easy accessibility and versatility, 2-aminopyridines have played a prominent role as alternatives to cyclopentadienyl ligands in coordination chemistry. Easily modifiable steric bulks and the ability for fine-tuning of electronic effects have allowed researchers to control not only the metal-to-ligand stoichiometry but also the properties of their metal complexes. Previously, ligand redistribution was frequently observed for ligands of small steric demands. Bulky aminopyridinato ligands refer to ligands that incorporate alkyl-substituted phenyl groups at the amine/amido nitrogen and at the sixth position of the pyridine ring. The steric crowding allowed the stabilization of transition metals in unusually low oxidation conditions. One of the remarkable developments, for example, is the stabilization of metal-metal quintuple bonds by these ligands, thus providing a diamagnetic platform to study such systems chemically. Application of metal aminopyridinates in homogeneous catalysis has also broadened considerably in recent years. This review provides a comprehensive account of advances made with such ligands since their development for main group and transition elements.