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

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.

Substituent-Guided Cluster Nuclearity for Tetranuclear Iron(III) Compounds with Flat {Fe4(μ3-O)2} Butterfly Core

“…Schiff bases are well-known ligands that have been used for decades in the synthesis of metal complexes for applications in different fields, such as catalysis [ 14 ], new materials and optics [ 15 , 16 , 17 ]. From the most famous N 2 O 2 tetradentate Schiff base H 2 salen, formed by the condensation of salicylaldehyde (salH) with ethylenediamine (en) [ 18 , 19 , 20 ], new functionalized families of structurally modified molecules have been designed for the purpose of obtaining new polynuclear complexes with desired structural features and functionalities [ 21 , 22 , 23 ]. In particular, the substitution of en with the shorter methylenediamine in condensation with salH yields ligands (called H 2 salben’s when the methylene bridge between the two nitrogen atoms carries a phenyl ring) that preferentially produce oligonuclear compounds by complexation [ 24 , 25 , 26 , 27 ].…”

Stereochemical Properties of Two Schiff-Base Transition Metal Complexes and Their Ligand by Using Multiple Chiroptical Spectroscopic Tools and DFT Calculations

“…Detailed stereochemical properties, such as conformational distribution, ligand chirality and absolute configuration of the metal centers, are of significant importance for further exploration of this fascinating class of chiral molecular systems. Historically, such chirality-related information has been typically obtained using X-ray crystallography, where a high-quality single crystal is needed [ 10 , 11 ]. However, to extract the stereochemical properties of these systems directly in solution, chiral spectroscopic methods, such as electronic circular dichroism (ECD) [ 12 ], vibrational circular dichroism (VCD) [ 13 , 14 , 15 , 16 ], and vibrational Raman optical activity (ROA) [ 17 ] have been utilized.…”

Cation-/Ligand-Induced Solvent-Assisted Transformations of Zn(II) and Cu(II) Complexes Featuring Single-Pocket Multidentate Chelating Members

“…Over time, the inorganic chemistry community has progressively developed many unconventional routes to devise and prepare unique inorganic systems. The major significant synthetic impediment for inorganic chemists is frequently acquiring pure chelates and bringing forth their coordination to suitable metal centers or reactive metal precursors that allow for mild reaction conditions, short reaction times, and, most importantly, high yields and product purity. − This can be accomplished, for example, by employing renowned synthetic pathways such as cation-induced solvent-assisted (CISA) and ligand-induced solvent-assisted (LISA) transformation reactions. − Solvent-assisted transformations render one of the most indispensable strategies for preparing novel complexes which otherwise cannot be obtained via conventional routes. − Generally, solvent-assisted transformations admit to bond breaking/formation. − As such, self-assembly transitions can be reversible , or irreversible − depending either on thermodynamic or kinetic factors governing the metal’s reactivity (i.e., preferable coordination number and geometry, availability of d-orbitals, change in ionic radii, and oxidation state of the metal). − Furthermore, crystallographic transformations are highly desirable for comprehending structural changes. However, one major challenge is obtaining the crystalline phase of both the reactant and the product without crystal degradation, in which case, these solution-mediated transformations, more often than not, are accompanied by single-crystal to amorphous-phase transitions. , …”