Stimuli-Responsive Properties of a Zinc(II) Salen-Type Schiff-Base Complex and Vapochromic Detection of Volatile Organic Compounds.

This contribution reports, through a combined thermogravimetric analysis, differential scanning calorimetry, UV-vis, powder X-ray diffraction, and Rietveld refinement analysis, on the stimuli-responsive chromic properties of a substituted Zn(salmal) Schiff-base Lewis acidic complex with unique and distinct thermo- and vapochromic characteristics. The solid complex obtained in air or by evaporation of the solvent from their THF solutions shows a marked thermochromism associated with a phase transition, unusually triggered by the reversible desorption/adsorption of one lattice water molecule. In contrast, the anhydrous solid, achieved from THF solutions of the complex by evaporation of the solvent under anhydrous conditions, behaves very differently as it does not show any absorption of water or thermochromism and exhibits varied vapochromic properties. Detection of volatile organic compounds having Lewis basicity is demonstrated by using the anhydrous solid or the related cast films on glass or paper substrates. In both cases, a marked vapochromism is observed upon exposure to vapors of various volatile species and involves well-defined optical absorptions and naked-eye color changes, also allowing the discrimination of primary aliphatic amines. Vapochromic behavior with the formation of stable, stoichiometric adducts is also demonstrated for both the solid obtained in air and the anhydrous solid or for the related cast films after exposure to vapors of pyridine.

A Zinc(II) Schiff Base Complex as Fluorescent Chemosensor for the Selective and Sensitive Detection of Copper(II) in Aqueous Solution.

The development of chemosensors able to detect analytes in a variety of sample matrices through a low-cost, fast, and direct approach is of current interest in food, health, industrial, and environmental fields. This contribution presents a simple approach for the selective and sensitive detection of Cu2+ ions in aqueous solution based on a transmetalation process of a fluorescent substituted Zn(salmal) complex. Transmetalation is accompanied by relevant optical absorption changes and quenching of the fluorescence emission, leading to high selectivity and sensitivity of the chemosensor, with the advantage of not requiring any sample pretreatment or pH adjustment. Competitive experiments demonstrate a high selectivity of the chemosensor towards Cu2+ with respect to the most common metal cations as potential interferents. A limit of detection down to 0.20 µM and a dynamic linear range up to 40 µM are achieved from fluorometric data. By exploiting the fluorescence quenching upon formation of the copper(II) complex, simple paper-based sensor strips, visible to naked eyes under UV light, are used for the rapid, qualitative, and quantitative in situ detection of Cu2+ ions in aqueous solution over a wide concentration range, up to 10.0 mM, in specific environments, such as in industrial wastewater, where higher concentrations of Cu2+ ions can occur.

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.

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.

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.

Phase transition and vapochromism in molecular assemblies of a polymorphic zinc(II) Schiff-base complex.

This paper reports for the first time the irreversible thermally induced phase transition, accompanied by color change, and the vapochromic behavior of an amphiphilic, Lewis acidic Zn(II) Schiff-base complex, through detailed X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry, and optical absorption studies. The unprecedented irreversible phase transition for such kind of complexes is associated with a thermal, lamellar-to-hexagonal columnar structural transition, which involves a different arrangement of each molecular unit within the assembled structure, H- and J-type aggregates, respectively, responsible for the thermochromic behavior. The vapochromism, investigated either in powder samples or in thermally annealed cast films, is related to the formation of 1:1 adducts upon exposure to vapors of strong Lewis bases and implies dramatic optical absorption variations and naked-eye observation of the change in color from red-brown to red. The chemisorption process is fast, completely reversible, reproducible, and selective for amines. The reversible switching of the chemisorption-desorption process in cast films is demonstrated by successive cycles, amine exposure and subsequent heating, by monitoring the substantial optical absorption changes in the visible region. Vapochromism of this material can potentially be used to detect vapors of volatile amines.

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.

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.

A Lewis basicity scale in dichloromethane for amines and common nonprotogenic solvents using a zinc(II) Schiff-base complex as reference Lewis acid.

A consistent, reliable scale of Lewis basicity in dichloromethane for 26 bases, involving amines and nonprotogenic solvents, is presented. A Lewis acidic Zn(II) Schiff-base complex, involving formation of stable 1:1 adducts is used as reference acid. Evaluation of binding constants is achieved from spectrophotometric titrations, by the least-squares nonlinear regression of multiwavelength spectrophotometric data. This Lewis basicity scale represents a unique set of data reflecting the actual Lewis basicity with respect this "real world" Lewis acidic species. The comparison of present Lewis basicity scale with data reported in the literature indicates that while for the involved solvents their relative basicity is scarcely affected by the reference Lewis acid, in contrast for sterically encumbered amines the Lewis basicity seems to be dependent from the reference species. Thus, Lewis basicity is governed by the steric hindrance at the donor atom and involves very different relative basicities than those predicted considering typical reference Lewis acids. This is expected to have a major involvement in the organic synthesis and catalysis, given the sterically encumbered nature of commonly involved Lewis acidic organometallic complexes.

Sensitive fluorescent detection and Lewis basicity of aliphatic amines.

In this contribution is reported the sensitive properties of the Zn(II) Schiff base complex, 1, in dichloromethane with respect a series of primary, secondary, and tertiary aliphatic amines through the study of fluorescence enhancement upon amine coordination to the Lewis acidic Zn(II) metal center with formation of 1:1 adducts. It is found that complex 1 exhibits selectivity and nanomolar sensitivity for primary and alicyclic amines. A distinct selectivity is also observed along the series of secondary or tertiary amines, paralleling the increasing steric hindrance at the nitrogen atom. The binding interaction can be related to the Lewis basicity of the coordinating amine; thus, complex 1 represents a suitable reference Lewis acid, and estimated binding constants within the investigated amine series can be related to their relative Lewis basicity. A relative order of the Lewis basicity can be established for acyclic amines, primary > secondary > tertiary, while an inverted order, tertiary > secondary ≈ primary (acyclic), is found in the case of alicyclic amines. The present approach represents a simple, suitable method to ranking the relative Lewis basicity of aliphatic amines in low-polarity, nonprotogenic solvents.

Lewis acidic zinc(II) salen-type Schiff-base complexes: sensing properties and responsive nanostructures.

In this frontier article some peculiar characteristics of Zn(salen)-type Schiff-base complexes are reviewed. The paper is mainly focused on the most recent and relevant achievements on responsive supramolecular nanostructures and sensing properties, both of them related to the Lewis acidic character of the ZnII centre in these molecular species, providing an interpretation of these features. The sensing properties of Zn(salen)-type complexes mainly originate from optical spectroscopic changes associated with the formation of the adducts upon addition of a Lewis base (analyte), either by deaggregation of dimeric species or displacement of the solvent coordinated to the metal centre. In both cases the direct sensing is related either to the Lewis acidic character of the complex as well as to the Lewis basicity of the analyte. The formation of responsive nanostructures with fluorescent, and/or vapochromic, mechanochromic, and thermochromic characteristics is driven by non-mutual intermolecular ZnO interactions, further stabilized by π-π stacking interactions and/or interdigitation of the alkyl side groups. The Lewis acidic character is not a prerogative of Zn(salen)-type complexes of tetradentate Schiff-bases. Many other classes of ZnII complexes can possess this property. A correct interpretation of their chemistry is certainly useful for further development of these classical coordination compounds as new molecular materials.

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.

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.

Nanoscale uniform self-assembled monolayers of fluorescent zinc(II) complexes on the Si(100) surface.

The synthesis of self-assembled monolayers on Si(100) substrates of a new fluorescent Zn(II) Schiff-base complex is reported. Chemisorbed species are characterized by the combination of fluorescence scanning near-field optical/atomic force microscopy (SNOM/AFM), and by fluorescence spectroscopy. Both SNOM/AFM results indicate the existence of a monolayer on the surface, while optical SNOM images highlight the contribution of the monolayer to the local fluorescence. While chemisorbed molecular monolayers exhibit a distinct fluorescence, analogous to that observed in solution, cast thin films do not show any emission. Photoluminescent properties of the monolayer can be related to its nanoscale uniform, ordered structure.

Vapochromic and chemiresistive characteristics of a nanostructured molecular material composed of a zinc(ii)-salophen complex.

The vapochromic and chemiresistive properties of a nanofibrillar molecular material composed of an amphiphilic ZnII Schiff-base complex, 1, are explored with the aim to obtain new materials with multifunctional properties. The solid 1 is characterized by the existence of two stable dimorphs, both exhibiting a nanofibrillar morphology. The thermal annealing of the ground solid 1 induces an irreversible lamellar-to-square columnar structural phase transition, as assessed by differential scanning calorimetry and X-ray diffraction analysis. When exposed to vapours of a Lewis base, spin coated films of the square columnar dimorph exhibit a marked change of both the optical absorption spectrum and the resistance, because of chemisorption of the Lewis base with the formation of 1·Lewis base adducts. The chemisorbed material is easily restored by thermal heating (150 °C), inducing desorption of the Lewis base. The changes of the properties are reproducible in successive cycles of exposure/restoring. Therefore, the present contribution provides new insights into the properties of this molecular material, potentially useful as a sensor for volatile organic Lewis bases.

Lewis basicity of relevant monoanions in a non-protogenic organic solvent using a zinc(ii) Schiff-base complex as a reference Lewis acid.

Anions are ubiquitous species playing a primary role in chemistry, whose reactivity is essentially dominated by their Lewis basicity. However, no Lewis basicity data, in terms of Gibbs energy, are reported in the literature. We report here the first Lewis basicity of relevant monoanions through the determination of binding constants for the formation of stable 1 : 1 adducts, using a ZnII Schiff-base complex, 1, as a reference Lewis acid. Binding constants for equilibrium reactions were achieved through a nonlinear regression analysis of the binding isotherms from spectrophotometric titration data. The Lewis acidic complex 1 is a proper reference species because it forms stable adducts with both neutral and charged Lewis bases, thus allowing ranking their Lewis basicity. Binding constants indicate generally a strong Lewis basicity for all involved anions, rivalling or exceeding that of the stronger neutral bases, such as primary amines or pyridine. The cyanide anion results to be the strongest Lewis base, while the nitrate is the weaker base within the present anion series. Moreover, even the weaker base anions behave as stronger bases than the most common non-protogenic coordinating solvents.

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.

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.