D0fN-METAL COMPLEXES SUPPORTED BY FERROCENE-BASED CHELATING LIGANDS

Abstract

Ferrocene is widely incorporated in pharmaceutical candidates, materials, and redox agents. In addition, ligand scaffolds make use of ferrocene groups because of their steric, electronic, and redox properties. In some cases, ferrocene is involved directly in the reactivity of a metal center even when it is part of the supporting ligand. While metal-ligand cooperation plays an important role in enzymatic catalysis, it is less developed in organometallic chemistry. Our group has focused on ferrocene-based chelating ligands because they possess unique electronic characteristics that make them especially versatile in supporting a wide range of reactivity behaviors for the resulting metal complexes. The present review discusses the chemistry of metal complexes with two types of ferrocene-based chelating ligands: (1) Schiff base; and (2) diamide. The first class of ligands supports yttrium and cerium alkoxides, while the second class is used for group 3 metal (scandium, yttrium, lutetium, and lanthanum) alkyls. Two series of Schiff base metal complexes are presented. The two ancillary ligands differ by the type of the N=X functionality that they incorporate: one ligand is based on an imine group, whereas the other is based on an iminophosphorane group. Cerium(IV) bis(alkoxide) complexes were targeted in order to determine whether the presence of a strongly oxidizing metal center would give rise to a non-innocent redox behavior in the supporting ligands. The experimental data indicated that iron remained in the +2 oxidation state and that cerium(IV) did not engage any part of the ancillary ligand in redox behavior. The reactivity of group 3 metal complexes supported by 1,1′-ferrocenylene diamide ligands toward aromatic N-heterocycles is also discussed. These reactions are compared to analogous reactions studied with group 3 metal complexes supported by pincer-type pyridine diamides. That comparison showed that similar reactions were observed with 1-methylimidazole, 2-picoline, and isoquinoline, although other types of reactions and a larger substrate scope were identified for the ferrocene- than for the pyridine-based complexes. Based on the reactions discussed herein and on isolated examples drawn from the literature, it is concluded that the ferrocene diamides represent a versatile and privileged ligand framework. It is proposed that the privileged status of these organometallic ancillary ligands is a consequence of iron's ability to accommodate changes in the electronic density at the metal center more readily than classical supporting ligands.