When the Metal Makes the Difference: Template Syntheses of Tridentate and Tetradentate Salen-Type Schiff Base Ligands and Related Complexes

• Main Authors: Rita Mazzoni, Fabrizio Roncaglia, Luca Rigamonti
• Format: Article
• Language: English
• Published: MDPI AG 2021-04-01
• Series: Crystals
• Subjects: template effect; Schiff base ligands; metal coordination; imine formation; oligonuclear complexes; aliphatic diamine
• Online Access: https://www.mdpi.com/2073-4352/11/5/483

The reaction of organic molecules mediated by a metal center (<i>template synthesis</i>) can result in a final connectivity that may differ from the one obtained in the absence of the metal. The condensation of carbonyl fragments with primary amines form C=N iminic bonds, the so-called Schiff bases, which can act as ligands for the templating metal center by means of the lone pair on the nitrogen atom. This review focuses on the template methods for the reaction between a carbonyl compound (mainly salicylaldehyde) and a primary aliphatic diamine able to prevent the double condensation on both amine groups and obtain tridentate N₂O ligands. These adducts, still having one free amino group, can further react, yielding tetradentate salen-type Schiff base ligands. A screening over the transition metals able to show such a <i>template effect</i> will be presented, with particular attention to copper(II), together with their peculiar reactivity and the available crystal structure of the metal complexes and related coordination geometries.