Deaggregation properties and transmetalation studies of a zinc(II) salen-type Schiff-base complex.

This paper reports the synthesis and the deaggregation properties of a Lewis acidic Zn(II) salen-type Schiff-base complex derivative from diaminomaleonitrile and a systematic detailed study of its transmetalation with other metal ions in solution. In a solution of non-coordinating solvents, the complex is in a dimeric form, while in coordinating solvents or upon addition of a Lewis base it is stabilized as monomeric adducts. Experiments done in two solvents with different Lewis basicities indicate a major role of the stability of the starting adduct in transmetalation. Thus, using nitrate or perchlorate salts, acetonitrile solutions of the complex give an immediate and complete transmetalation with Cu2+, while with Co2+ and Ni2+ a much slower transmetalation rate is observed. Instead, using chloride salts a fast and complete transmetalation is observed for divalent ions of the first transition series (Mn2+, Fe2+, Co2+, Ni2+, Cu2+), indicating the role of the chloride in stabilizing the transition state of the transmetalation. On the other hand, DMF solutions of the complex are less prone to transmetalation, according with the greater basicity of the solvent and, hence, the greater stability of the related adducts with the complex. Therefore, the nature of the solvent and the counteranion allow controlling the transmetalation process of this Zn(II) Schiff-base complex.

Highly selective and sensitive colorimetric/fluorometric dual mode detection of relevant biogenic amines.

Biogenic amines are involved in physiological roles in living organisms, but their excessive production or intake can induce undesired toxicological effects. As biogenic amines can be found in the process of food spoilage, they are considered an indicator of food quality and freshness, and their detection is of crucial importance in food safety. In this contribution, we report the fast and direct colorimetric and fluorometric sensing of biogenic amines by means of a dinuclear Zn(ii) Schiff-base complex. The selective and sensitive detection involves the formation of stable adducts between the dinuclear complex, acting as the Lewis acidic molecular tweezer, and biogenic di- or polyamines. The selectivity towards biogenic amines, even in the presence of common aliphatic, primary, secondary, or tertiary monoamines, heterocyclic amines, and amino acids, is demonstrated by competitive experiments. The quantitation of histamine in a fish matrix is easily achieved using a standard extraction procedure followed by simple colorimetric or fluorometric measurements.

Dinuclear zinc(ii) salen-type Schiff-base complexes as molecular tweezers.

In this contribution, the synthesis and the unusual aggregation/deaggregation properties in solution of two dinuclear ZnII Schiff-base complexes of tetradentate Schiff-base units, having non-conjugated spacers between each molecular unit, are reported in comparison to the mononuclear model complex. Through detailed 1H NMR, DOSY NMR, optical absorption, fluorescence emission, and multivariate analysis of optical absorption data, emerge some interesting findings. In solution of non-coordinating solvents, these Lewis acidic species are characterized as monomers stabilized by formation of intramolecular aggregates, having distinct spectroscopic properties in comparison to intermolecular aggregates derived from the mononuclear model analogue. Instead, in coordinating solvents they exhibit a typical behaviour, with formation of stable adducts showing a strong fluorescence. Deaggregation studies using pyridine as reference Lewis base allowed establishing a larger thermodynamic stability of these intramolecular aggregates, in comparison to intermolecular aggregates, even larger than that of aggregates of conjugated multinuclear complexes. The combined analysis of spectroscopic data upon deaggregation with ditopic Lewis bases unambiguously demonstrated the formation of stable 1 : 1 adducts, with higher binding constants in comparison to those related to monotopic species. Therefore, the present Lewis acidic, dinuclear complexes behave as molecular tweezers of ditopic guests having a strong Lewis basicity.

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

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.

Aggregates of Defined Stereochemical Scaffolds: A Study in Solution of a Zinc(II) Schiff Base Complex Derived from the Enantiopure trans-1,2-Cyclopentanediamine.

Molecular aggregation of bis(salicylaldiminato) ZnII Schiff base, ZnL, complexes is a topic of current interest. In this paper, we report a novel complex derived from the enantiopure trans-1,2-cyclopentanediamine, (R)-1, in order to probe the effect of the defined stereochemistry on its coordination geometry and aggregation properties, through detailed 1H NMR, DOSY NMR, UV/vis, and circular dichroism spectroscopic studies and accurate DFT calculations. This complex shows several peculiarities in solution, behaving very differently from ZnL complexes and more importantly from the related trans-1,2-diaminocyclohexane derivative. Unexpectedly, experimental data suggest the existence in DMSO of two species in equilibrium, the classical monomeric adduct and a dimer, indicating that the DMSO is not sufficiently strong Lewis basic to completely deaggregate the complex. Also, in chloroform an unusual behavior is observed with the existence of a single defined dimeric species characterized as a dinuclear double helicate Zn2L2 structure which does not deaggregate even with the addition of pyridine in large stoichiometric excess. The formation of mononuclear adducts occurs only when dissolving the complex in the stronger Lewis base pyridine. All these data indicate the helicate (R)-1-h complex has a higher thermodynamic stability than that of the cyclohexane analogue, leading to its unique aggregation properties.

On the Lewis acidic character of bis(salicylaldiminato)zinc(ii) Schiff-base complexes: a computational and experimental investigation on a series of compounds varying the bridging diimine.

This contribution explores the effect of the 1,2-diimine bridge upon the Lewis acidic character of a series of bis(salicylaldiminato)zinc(ii), ZnL, Schiff-base complexes. The structure of the monomeric and dimeric ZnL complexes, and of the 1 : 1 adducts with pyridine, ZnL·py, is fully optimized by means of DFT calculations. The Gibbs free energy for the dimerization of ZnL complexes and for the formation of ZnL·py adducts is evaluated by accurate composite calculations. It accounts for their spontaneous dimerization and for the greater stability of the ZnL·py adducts with respect to the dimers. Calculated binding constants for the formation ZnL·py adducts are in excellent agreement with experimentally derived values, thus allowing establishing a relative Lewis acidity scale within this series. While the complex derived from the non-conjugated ethylenediamine reveals the lowest Lewis acidity, the complex derived from the diaminomaleonitrile represents the stronger Lewis acidic species. These findings are in good agreement with the greater catalytic activity observed for ZnL Schiff-base complexes derived from conjugated 1,2-diamines in comparison to the non-conjugated analogues. Both in ZnL dimers as well as in ZnL·py adducts the geometry of the coordination sphere seems to be a relevant feature to assess their relative stability. Thus, while the quasi-planarity of ZnL monomers of the conjugated diimines is an unfavourable feature in the dimerization process, it represents an important aspect in stabilizing ZnL·py adducts in a nearly perfect square-pyramidal coordination. These features are relevant for the sensing and catalytic properties of these complexes.

Supramolecular Aggregates of Defined Stereochemical Scaffolds: Aggregation/Deaggregation in Schiff-Base Zinc(II) Complexes Derived from Enantiopure trans-1,2-Diaminocyclohexane.

This contribution explores, through detailed 1H NMR, DOSY NMR, optical absorption, and circular dichroism spectroscopic studies, the aggregation properties in solution of noncoordinating solvents of some new ZnII Schiff-base complexes, (R)-1, (S)-1, and (R)-2, derived from the chiral trans-1,2-diaminocyclohexane. It is found that chloroform solutions of 1 are characterized by the presence of three species, the predominance of which consists of large oligomeric aggregates. For chloroform solutions of 1, upon heating or standing, all species are irreversibly converted into a dimer, 1C, which is very stable and hardly disaggregable. Analysis of 1H NMR, UV/vis, and CD spectroscopic data and chemical evidence allow proposing a double helicate Zn2L2 structure with a tetrahedral coordination around the ZnII ions for 1C, as a consequence of the defined stereochemistry of the trans-1,2-diaminocyclohexane chelate bridge. This represents a different, uncommon aggregation mode in ZnII complexes of tetradentate Schiff-bases.

Chirality of self-assembled achiral porphyrins induced by chiral Zn(ii) Schiff-base complexes and maintained after spontaneous dissociation of the templates: a new case of chiral memory.

Herein, we exploit the induction of chirality by chiral Zn(ii) Schiff-base complexes, followed by their spontaneous dissociation in aqueous solution, providing for the first time a possibility of overcoming the template removal steps, to demonstrate memorization of chirality in porphyrin hetero-aggregates.

Structure and aggregation properties of a Schiff-base zinc(II) complex derived from cis-1,2-diaminocyclohexane.

This contribution explores the effect of the bridging diamine upon the aggregation properties of a Zn(II) Schiff-base complex, , both in the solid state and in solution. The X-ray structure of , resulting from the harvest of good quality crystals using chloroform and diethyl ether as solvents, shows the presence of a densely packed dimer in the solid state which pentacoordinates two Zn atoms involved in a µ-phenoxo bridge. Detailed studies in solution, through (1)H NMR, DOSY NMR, and optical spectroscopic investigations, indicate the typical aggregation/deaggregation behaviour on switching from non-coordinating to coordinating solvents, in relation to the Lewis acidic character of such Zn(II) complexes. Thus, while in DMSO-d6 both (1)H NMR and DOSY studies suggest the existence of monomeric species, in chloroform solution experimental data support the existence of aggregates. However, unlike our previous studies, (1)H NMR data in chloroform solution indicate the existence of an asymmetric dimer, as observed in the X-ray crystal structure. This further evidences a very rigid backbone of the dimeric aggregate and can be related to the defined stereochemistry of the chelate cis-1,2-diaminocyclohexane bridge.

Synthesis, characterization, optical absorption/fluorescence spectroscopy, and second-order nonlinear optical properties of aggregate molecular architectures of unsymmetrical Schiff-base zinc(II) complexes.

This paper reports the synthesis and a detailed investigation in solution, through (1)H NMR, optical absorption, fluorescence spectroscopy and second-order nonlinear optical studies, on the structure and photophysical properties of a series of unsymmetrical alkoxy-derivatized Schiff-base Zn(II) species bearing on one side an alkyl ammonium bromide as a Lewis base. Overall results suggest that these complexes are present in solution of dichloromethane as dimeric aggregates in which each molecular unit mutually interacts with another unit through intermolecular Zn···Br(-) interactions. This represents an alternative approach to control the supramolecular architecture of Lewis acidic, Schiff-base Zn(II) complexes.

An unprecedented structural interconversion in solution of aggregate zinc(II) salen Schiff-base complexes.

This contribution explores the aggregation properties in solution of noncoordinating solvents of a series of amphiphilic Zn(salen) derivatives, through detailed (1)H NMR, DOSY NMR, and optical absorption spectroscopic studies. It is found that these aggregate species are involved in a unique structural interconversion between two defined dimers, A and B, driven by the concentration of water dissolved in chloroform. Dilute CHCl(3) solutions are characterized by the presence of dimeric species, A, in which both Zn(II) atoms of the Zn(salen) units mutually interact through a Zn···O axial coordination, likely adopting a square-base pyramidal structure. Investigations to higher concentrations indicate the existence of a new dimeric species, B, in equilibrium to that observed at lower concentrations, involving a coordination mode interconversion of an intermediate monomer presumably from a square-pyramidal to a trigonal bipyramidal structure. This behavior may be related to the nonconjugated, conformational flexible nature of the bridging diamine of the Schiff base, and is influenced by the solvent polarity. Variable-temperature (1)H NMR studies indicate the existence of a nonequivalent species B' in a fluxional equilibrium with species B.

Aggregation properties of bis(salicylaldiminato)zinc(II) Schiff-base complexes and their Lewis acidic character.

The synthesis, characterization, (1)H NMR, optical absorption and fluorescent properties of a series of amphiphilic Schiff-base bis(salicylaldiminato)zinc(II) complexes are reported. Detailed (1)H NMR, DOSY NMR, optical absorption and fluorescence spectroscopy studies indicate the existence of aggregate species in solutions of non-coordinating solvents. The degree of aggregation is related to the nature of the bridging diamine. Chloroform solutions of complexes where the bridging diamine contains a naphthalene or the pyridine nucleus are always characterized by the presence of defined dimer aggregates, whereas oligomeric aggregates are likely formed by complexes where the bridging diamine contains a benzene ring. In coordinating solvents or in the presence of coordinating species, a complete deaggregation of the complexes occurs, because of the axial coordination to the Zn(II) ion, accompanied by considerable changes in the (1)H NMR and optical absorption spectra. The effect of the alkyl chains length seems to play a minor role in the aggregation properties, as noticed by (1)H NMR data, optical absorption and fluorescence spectra, which remain almost unaltered on changing the chain length.

Design, synthesis and in vitro antitumour activity of new heteroaryl ethylenes.

Almond and VolSurf + modelling procedures allowed the structural design of new di- and mono-heteroaryl-ethylenes. The structural modifications suggested by the molecular modelling were verified by the synthesis of the designed molecules and by the evaluation of their in vitro activities against two lung tumour cell lines, A549 and H226. 2-{(E)-2-[5'-(Dibutylamino)-2,2'-bithien-5-yl]vinyl}-1-methylquinolinium iodide exhibited in vitro antiproliferative activity two orders of magnitude higher than that of the most active compound previously synthesized in our laboratory.

Supramolecular aggregation/deaggregation in amphiphilic dipolar Schiff-base zinc(II) complexes.

The synthesis, characterization, optical, and fluorescent properties of an amphiphilic Schiff-base bis(salicylaldiminato)zinc(II) complex are reported. Detailed (1)H nuclear magnetic resonance (NMR), diffusion-ordered spectroscopy (DOSY) NMR, optical absorption, and fluorescence spectroscopy studies indicate the existence of aggregate species in noncoordinating solvents. The degree and type of aggregation are related to the concentration and the polarity of the noncoordinating solvent. Dilute solutions are likely characterized by the presence of defined dimers, whereas larger oligomeric aggregates are conceivably formed at higher concentrations. The concentration needed to observe the formation of larger species depends upon solvent polarity. In coordinating solvents or in the presence of coordinating species, a complete deaggregation of the system occurs, because of the axial coordination to the Zn(II) ion, accompanied by considerable changes of (1)H NMR and optical absorption spectra. A dramatic enhancement of fluorescence emission is observed in dichloromethane solutions upon deaggregation with a coordinating agent. The formation of a defined 2:1 supramolecular structure is demonstrated in the case of a ditopic ligand as coordinating species. Overall, these complexes are promising systems for the development of new supramolecular architectures and supramolecular fluorescent probes.

Controlling the molecular aggregation. An amphiphilic Schiff-base zinc(II) complex as supramolecular fluorescent probe.

A designed amphiphilic Schiff-base bis(salicylaldiminato)zinc(II) complex, 1, forms defined aggregates in dilute solutions of non-coordinating solvents. The switching to the monomer can be driven by addition of a coordinating species and involves a dramatic enhancement of the fluorescence emission.

New cleft-like molecules and macrocycles from phosphonate substituted spirobisindanol.

We have synthetized medium-sized cyclophanes and macrocycles containing phosphonic groups, directly linked to the aromatic rings of the phanes or as pendant arms,for use as specific receptors for the selective complexation of neutral guests or for complexing lanthanides, as luminescent sensors and for diagnostic bioassays in medicine.Furthermore, because it would be of great interest for biochemistry as well as for pharmacological studies to dispose of preorganized rigid chiral hosts for biorelevant molecules we designed inter alia, some new chiral macrocycles capable of a triple binding mode and we used them for constructing macrocycles that could also be of interest for chiral recognition and chiral separations. Thus, in this paper we shall review the salient aspects of some macrocycles synthetized in our laboratory, all possessing the phosphonate moiety and a spirobisindanol scaffold and able to act as complexing agents for cations and organic substrates. In particular, we shall describe their NMR characterization, their stereochemistry in solution and in the solid state, and their use as chiral receptors for biorelevant molecules. Chiral HPLC resolution of some of them is also reported.