Abstract
The reaction of organic molecules mediated by a metal center (template synthesis) 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 template effect 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.
Highlights
The term template synthesis can be referred to ligand-based reactions that occur between two or more organic molecules coordinated to a metal center
Due to the vastness of studies on Schiff bases, which have been known and applied by chemists since the early stages of coordination chemistry, during the last century and far from being exhaustive, this review focuses on the essential template effect given by transition metal ions, with particular attention to copper(II), in obtaining tridentate N2 O half units and tetradentate salen-type Schiff base ligands derived from G-salH and primary aliphatic diamines, not accessible without the presence of the metal center, and their complexes
Tetradentate salen-type Schiff base ligands derived from G-salH and primary diamines have clearly highlighted several critical but fundamental points, which can be summarized as follow: (i) Employment of aromatic vs. aliphatic primary diamines: the first ones, due to their lower nucleofilicity, permit mono-condensation with G-salH yielding tridentate
Summary
The term template synthesis can be referred to ligand-based reactions that occur between two or more organic molecules coordinated to a metal center. The term Schiff base is commonly applied to these compounds since the first report by Hugo Schiff more than one and a half century ago [6], and the lone pair of the iminic nitrogen atom makes Schiff bases popular as ligands for metal centers [7]. The term Schiff base is commonly applied to these compounds polydentate ones, in which coordinative sites more are combined with the iminic nitrogen since theother first report by Hugo Schiff than one and a half century ago [6], and the atom within the organic skeleton. [33,34,35]
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