Supramolecular Aggregation of a New Substituted Bis(salicylaldiminato)zinc(II) Schiff-Base Complex Derived from trans-1,2-Diaminocyclohexane

Abstract

Abstract:In this contribution is reported the synthesis, characterization, and aggregation properties in solution of a novel Zn(II) complex, (R)-2, derived from the enantiopure chiral trans-1,2-diaminocyclohexane and a substituted salicylaldehyde. Detailed 1 H NMR, DOSY NMR, optical absorption, and circular dichroism spectroscopic studies and chemical evidence allowed to investigate the nature of aggregate species in solution. The high solubility of (R)-2 in solution of the non-coordinating chloroform solvent leads to formation of various aggregates, some of them consisting of large oligomers estimated to contain up to 27 monomeric units. The chiral trans-stereochemistry of the bridging diamine favors a different aggregation mode in these complexes, both in the oligomers and dimers, involving a tetrahedral coordination geometry around the metal center. Overall data suggest the formation of helical oligomers, (ZnL) n , in freshly prepared chloroform solutions which, by standing or heating, evolve towards a more thermodynamically stable, dinuclear double-helicate Zn 2 L 2 dimer.

On the Aggregation and Sensing Properties of Zinc(II) Schiff-Base Complexes of Salen-Type Ligands

“…Some chiral ZnL complexes ( 9 – 13 ) were also investigated (Figure 7). In particular, complexes derived from the enantiopure chiral (1 S ,2 S )-(+)- or (1 R ,2 R )-(‒)- trans -1,2-diaminocyclohexane and (1 R ,2 R )- trans -1,2-cyclopentanediamine were synthesized and studied in solution [66,67,68], in comparison to previous achiral complexes [27,60,61,62,63].…”

“…In particular, the greater solubility of the complex enables a higher degree of aggregation in solution, up to ca. 27 monomeric units [67]. For these aggregates, obtained in freshly-prepared solutions of non-coordinating solvents, a helical oligomeric structure was proposed, (ZnL) n , with a tetrahedral coordination geometry around the metal center.…”

“…Therefore, the formation of the monomeric adducts involves relevant changes of optical spectroscopic properties. In contrast, for the non-conjugated bridging diimine derivatives 1 , 2 , and the chiral 9 – 11 , and 13 complexes, are observed minor variations of the UV/Vis absorption spectra on changing the coordinating nature of the solvent, indicative of weak interligand interactions between each unit in aggregate species of these complexes [27,60,66,67,68].…”

“…Conversely, the chiral trans - 9 and 11 derivatives exhibit a further interesting behavior. While UV/Vis spectra of fresh prepared solutions in both coordinating/non-coordinating solvents are very similar one each other, spectra of 9C and 11-h , achieved upon heating/standing chloroform solutions of 9 and 11 , are characterized by the appearance of a new, intense absorption band at longer wavelengths [66,67]. This is consistent with a change of their structure, from the oligomeric (ZnL) n to the double-helicate Zn 2 L 2 dimer, the latter being characterized by stronger interligand interactions.…”

A Chiral Bis(salicylaldiminato)zinc(II) Complex with Second-Order Nonlinear Optical and Luminescent Properties in Solution

“…Remarkably, some of these tetradentate [N 2 O 2 ] Schiff-base zinc complexes also show interesting luminescent properties [70]. Thanks to their Lewis acidic character, this kind of complexes are characterized by fascinating aggregation/deaggregation properties in relation to the absence or presence of a Lewis base, accompanied by significant changes of their spectroscopic properties that can find a lot of applications [71][72][73][74][75][76][77].…”