Multidentate unsymmetrically-substituted Schiff bases and their metal complexes: Synthesis, functional materials properties, and applications to catalysis

•Syntheses, structures and properties of tridentate Schiff bases and metal complexes.•Syntheses of unsymmetrically-substituted tetradentate Schiff base and metal complexes.•Discussion of structures and physico-chemical properties of SB metal complexes.•A panorama of 13 organic transformations catalyzed by SB metal complexes is presented. This review focuses on the recent developments of unsymmetrically-substituted multidentate Schiff bases whose steric and electronic characteristics are easily manipulated by selecting suitable condensing aldehydes or ketones and primary amines, and on their metal complexes. After a brief historical introduction, this manuscript is divided in three main sections. In the two first parts, the synthesis, reactivity, functions, and properties of tridentate Schiff base precursors and of quadridentate Schiff base metal complexes, respectively, are discussed through a literature survey including examples of research from the authors’ groups. More specifically, the second section is formed of seven subsections with the synthesis of unsymmetrically-substituted tetradentate Schiff bases and their transition metal (V, Mn, Fe, Co, Ni, Cu, Zn, Ru, Pd, Pt) and uranyl complexes. Emphasis is given to our research work based on ferrocenyl-containing tri- and tetradentate unsymmetrically-substituted Schiff base complexes of Ni(II) and Cu(II) starting from variously substituted ferrocenyl-β-diketones. The unsymmetrically-substituted Schiff base complexes present a wide range of remarkable properties that are also summarized in this section, including structural, biocidal, magnetic, and second-order nonlinear optical properties. The third section is devoted to the catalytic activity of Schiff base metal complexes that is discussed through thirteen major organic reactions, including copper-catalyzed azide–alkyne cycloaddition (CuAAC), Henry and nitro-Mannich reactions, hydrosilylation of ketones, aldol, cyclopropanation and epoxidation reactions, among others.