Abstract
The zinc(II) ion forms stable complexes with a wide variety of ligands, but those related to Schiff-bases are among the most largely investigated. This review deals with the peculiar aggregation characteristics of Zn(II) Schiff-base complexes from tetradentate N2O2 salen-type ligands, L, derivatives from salicylaldehydes and 1,2-diamines, and is mostly focused on their spectroscopic properties in solution. Thanks to their Lewis acidic character, ZnL complexes show interesting structural, nanostructural, and aggregation/deaggregation properties in relation to the absence/presence of a Lewis base. Deaggregation of these complexes is accompanied by relevant changes of their spectroscopic properties that can appropriately be exploited for sensing Lewis bases. Thus, ZnL complexes have been investigated as chromogenic and fluorogenic chemosensors of charged and neutral Lewis bases, including cell imaging, and have shown to be selective and sensitive to the Lewis basicity of the involved species. From these studies emerges that these popular, Lewis acidic bis(salicylaldiminato)Zn(II) Schiff-base complexes represent classical coordination compounds for modern applications.
Highlights
Molecular aggregation is a topic of current interest [1] playing a crucial role in the development of new materials [2,3,4,5] including mimicking biological systems [6,7], with different spectroscopic, electrical, or magnetic properties with respect to those of component molecules.Schiff-base derivatives from substituted salicylaldehydes and amines are suitable template ligands for many metal ions
Lewis basicity of the involved species. From these studies emerges that these popular, Lewi acidic bis(salicylaldiminato)Zn(II) Schiff-base complexes represent classical coordination compounds for modern applications
Because of the d10 electron configuration of the Zn(II) ion, it is not subjected to ligand field stabilization, and forms stable complexes with various Schiff-base ligands
Summary
Molecular aggregation is a topic of current interest [1] playing a crucial role in the development of new materials [2,3,4,5] including mimicking biological systems [6,7], with different spectroscopic, electrical, or magnetic properties with respect to those of component molecules. ZnL complexes are versatile synthons to supramolecular architectures [14,15,16] This is because these complexes are Lewis acidic species which are stabilized through the axial coordination of a donor, saturating their coordination sphere, with formation of adducts or aggregate species having penta-coordinated, distorted square-pyramidal structures [15]. A variety of molecular aggregates and supramolecular assemblies were achieved [10,11,12,13,14,15,16] Another interesting aspect related to the Lewis acidic character of these ZnL complexes is their potential application as sensors of Lewis bases, because of their spectroscopic changes owing to the formation of monomeric adducts [11,17,18]. Zn· · · Br Zn⋯Br interactions (Figure 3), while 3), related symmetrical species lead to thelead formation formation of branched nanostructures [59]
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