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.

Lewis Acidic Zinc(II) Complexes of Tetradentate Ligands as Building Blocks for Responsive Assembled Supramolecular Structures

This review presents representative examples illustrating how the Lewis acidic character of the Zn(II) metal center in Zn(salen)-type complexes, as well as in complexes of other tetradentate ligands, and the nature of the medium govern their supramolecular aggregation, leading to the formation of a variety of supramolecular structures, either in solution or in the solid state. Stabilization of these Lewis acidic complexes is almost always reached through an axial coordination of a Lewis base, leading to a penta-coordinated square-pyramidal geometry around the metal center. The coverage is not exhaustive, mainly focused on their crystallographic structures, but also on their aggregation and sensing properties in solution, and on their self-assembled and responsive nanostructures, summarizing their salient aspects. The axial ligands can easily be displaced, either in solution or in the solid state, with suitable Lewis bases, thus being responsive supramolecular structures useful for sensing. This contribution represents the first attempt to relate some common features of the chemistry of different families of Zn(II) complexes of tetradentate ligands to their intrinsic Lewis acidic character.

New ternary Fe(III)-8-hydroxyquinoline-reduced Schiff base complexes as selective anticancer drug candidates

Due to the growing prevalence of cancer diseases, new therapeutic options are urgently needed, and drugs based on metal ions other than platinum are alternatives with exciting possibilities. We report the synthesis, characterization and biological effect of mixed-ligand Fe(III)-aminophenolate complexes derived from salicylaldehyde and L-tryptophan with quinoline derivatives as co-ligands, namely 8-hydroxyquinoline (8HQ), [Fe(L)(8HQ)(H₂O)] (1) and its 5-cloro derivative (Cl8HQ), [Fe(L)(Cl8HQ)(H₂O)] (2). The complex bearing the aminophenolate and lacking the quinoline co-ligand, [Fe(L)(Cl)(H₂O)₂] (3), was prepared for comparison. The analytical and spectroscopic characterization revealed that 1 and 2 are octahedral Fe(III) complexes with the aminophenolate acting as a dianionic tridentate ligand and 8HQ co-ligands as bidentate chelates. Spectroscopic techniques and molecular docking studies were used to evaluate the ability of these complexes to bind bovine serum albumin (BSA) and calf thymus DNA. Complex 2 [Fe(L)(Cl8HQ)(H₂O)] was the one showing higher affinity for both biomolecules. Cell viability was assessed in breast, colorectal and bone human cancer cell lines. 1 and 2 were found to be more active than cisplatin in all cell lines tested. A non-tumoral fibroblast line (L929, mouse non-tumoral fibroblasts) was used to evaluate selectivity. The results evidence that 2 shows much higher selectivity than 1 in all cell lines tested, but particularly in bone cancer cells in which selectivity index (SI) values are 8.0 and 18.8 for 1 and 2, respectively.

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.

Quenched Lewis Acidity: Studies on the Medium Dependent Fluorescence of Zinc(II) Complexes

Three new zinc(II) coordination units [Zn(1-3)] based on planar-directing tetradentate Schiff base-like ligands H2 (1-3) were synthesized. Their solid-state structures were investigated by single crystal X-ray diffraction, showing the tendency to overcome the square-planar coordination sphere by axial ligation. Affinity in solution towards axial ligation has been tested by extended spectroscopic studies, both in the absorption and emission mode. The electronic spectrum of the pyridine complex [Zn(1)(py)] has been characterized by MC-PDFT to validate the results of extended TD-DFT studies. Green emission of non-emissive solutions of [Zn(1-3)] in chloroform could be switched on in the presence of potent Lewis-bases. While interpretation in terms of an equilibrium of stacked/non-fluorescent and destacked/fluorescent species is in line with precedents from literature, the sensitivity of [Zn(1-3)] was greatly reduced. Results of a computation-based structure search allow to trace the hidden Lewis acidity of [Zn(1-3)] to a new stacking motif, resulting in a strongly enhanced stability of the dimers.

Structural Elucidation, Aggregation, and Dynamic Behaviour of N,N,N,N ‐Copper(I) Schiff Base Complexes in Solid and in Solution: A Combined NMR, X‐ray Spectroscopic and Crystallographic Investigation

A series of Cu(I) complexes of bidentate or tetradentate Schiff base ligands bearing either 1‐H‐imidazole or pyridine moieties were synthesized. The complexes were studied by a combination of NMR and X‐ray spectroscopic techniques. The differences between the imidazole‐ and pyridine‐based ligands were examined by ¹H, ¹³C and ¹⁵N NMR spectroscopy. The magnitude of the ¹⁵N_imine coordination shifts was found to be strongly affected by the nature of the heterocycle in the complexes. These trends showed good correlation with the obtained Cu−N_imine bond lengths from single‐crystal X‐ray diffraction measurements. Variable‐temperature NMR experiments, in combination with diffusion ordered spectroscopy (DOSY) revealed that one of the complexes underwent a temperature‐dependent interconversion between a monomer, a dimer and a higher aggregate. The complexes bearing tetradentate imidazole ligands were further studied using Cu K‐edge XAS and VtC XES, where DFT‐assisted assignment of spectral features suggested that these complexes may form polynuclear oligomers in solid state. Additionally, the Cu(II) analogue of one of the complexes was incorporated into a metal‐organic framework (MOF) as a way to obtain discrete, mononuclear complexes in the solid state.

Binding Properties of a Dinuclear Zinc(II) Salen-Type Molecular Tweezer with a Flexible Spacer in the Formation of Lewis Acid-Base Adducts with Diamines

In this paper we report the binding properties, by combined 1H NMR, optical absorption, and fluorescence studies, of a molecular tweezer composed of two Zn(salen)-type Schiff-base units connected by a flexible spacer, towards a series of ditopic diamines having a strong Lewis basicity, with different chain length and rigidity. Except for the 1,2-diaminoethane, in all other cases the formation of stable 1:1 Lewis acid-base adducts with large binding constants is demonstrated. For α,ω-aliphatic diamines, binding constants progressively increase with the increasing length of the alkyl chain, thanks to the flexible nature of the spacer and the parallel decreased conformational strain upon binding. Stable adducts are also found even for short diamines with rigid molecular structures. Given their preorganized structure, these latter species are not subjected to loss of degrees of freedom. The binding characteristics of the tweezer have been exploited for the colorimetric and fluorometric selective and sensitive detection of piperazine.

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.

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.

Substrate-dependent allosteric regulation by switchable catalytic molecular tweezers

Allosteric regulation is exploited by biological systems to regulate the activity and/or selectivity of enzymatic reactions but remains a challenge for artificial catalysts. Here we report switchable terpy(Zn-salphen)2 molecular tweezers and their metal-dependent allosteric regulation of the acetylation of pyridinemethanol isomers. Zinc-salphen moieties can both act as a Lewis acid to activate the anhydride reagents and provide a binding site for pyridinemethanol substrates. The tweezers’ conformation can be reversibly switched between an open and a closed form by a metal ion stimulus. Both states offer distinct catalytic profiles, with closed tweezers showing superior catalytic activity towards ortho substrates, while open tweezers presenting higher rate for the acetylation of meta and para substrates. This notable substrate dependent allosteric response is rationalized by a combination of experimental results and calculations supporting a bimetallic reaction in the closed form for ortho substrate and an inhibition of the cavity for meta and para substrates.

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.

Push–pull unsymmetrical substitution in nickel(ii) complexes with tetradentate N2O2 Schiff base ligands: synthesis, structures and linear–nonlinear optical studies

Push–pull unsymmetrical substitution efficiently modulates the electronic, linear and nonlinear optical properties of nickel(ii) complexes with salen-type ligands.

Zinc(II) as a Versatile Template for Efficient Dipolar and Octupolar Second-Order Nonlinear Optical Molecular Materials §

This short review outlines the main results obtained in the field of molecular materials based on zinc coordination compounds for second-order nonlinear optics. It presents an overview of the main classes of second-order nonlinear optical (NLO) active complexes bearing monodentate, bidentate, tridentate, or tetradentate π-delocalized ligands such as substituted stilbazoles, bipyridines, phenanthrolines, terpyridines, and Schiff bases. Macrocyclic ligands such as porphyrins and phthalocyanines are not covered. This paper shows how coordination to the Zn(II) center of π-delocalized nitrogen donor ligands produces a significant enhancement of their quadratic hyperpolarizability. Dipolar complexes are mainly presented, but octupolar zinc complexes are also presented. The coverage is mainly focused on NLO properties that are measured at the molecular level, working in solution, by means of the electric field-induced second harmonic generation (EFISH) or the hyper-Rayleigh scattering (HRS) techniques.

A luminescent benzothiadiazole-bridging bis(salicylaldiminato)zinc(ii) complex with mechanochromic and organogelation properties

Herein, a new bis(salicylaldiminato)Zn(ii) Schiff base complex, BTZn, derived from benzo[c][1,2,5]thiadiazole-5,6-diamine was designed and synthesized. It exhibited unique mechanical force-induced luminescence change characteristic. Upon mechanical grinding, the as-prepared BTZn solid crystalized from an ethanol/dichloromethane solution displays a high-contrast emission-colour variation from yellow (emission maximum λem = 545 nm) to red (λem = 645 nm), and this emission variation can be erased through solvent vapour treatment. The reversible emission colour alteration between yellow and red can be repeatedly performed. Thermal annealing of the as-prepared BTZn solid resulted in a more ordered orange phase with an emission maximum of 575 nm. The multi-stimuli-responsive luminescence mechanism has been investigated via SEM, powder X-ray diffraction (XRD), and thermal analyses. It is demonstrated that mechanical force can induce morphology transformation from the crystalline to the amorphous phase, which is accompanied by a change in the BTZn molecular packing modes. The BTZn-based solids have molecular packing-dependent emission characteristics. The XRD experimental results reveal that for the yellow emissive as-prepared BTZn solid, a columnar square molecular arrangement is adopted. On the other hand, the BTZn complex exhibits the ability to organize into organic luminescent gels constructed by one-dimensional BTZn molecular nanofibrils. The BTZn xerogel also displays mechanochromic properties. Accordingly, BTZn-based solids may be potential candidates for the development of new stimuli-responsive materials.

Photoluminescence properties of tetrahedral zinc(ii) complexes supported by calix[4]arene-based salicylaldiminato ligands

The synthesis and photophysical properties of four new hybrid salicylaldiminato-calix[4]arene ligands and their corresponding zinc(ii) complexes are described. The Schiff bases were obtained from condensation reactions between cone-25,27-di(aminoethoxy)-26,28-dihydroxy-calix[4]arene and salicylaldehyde (H2L1) or o-vanillin (H2L2) and 1,3-alt-25,27-di(aminoethoxy)-26,28-di(n-propyloxy)-calix[4]arene and 3,5-di-tert-butyl-salicylaldehyde (H2L3) or o-vanillin (H2L4). Complexation reactions were investigated by ESI-MS, IR, NMR, UV-vis absorption and steady-state and time resolved fluorescence spectroscopy, and X-ray crystallography. All ligands support 1 : 1 complexes (ZnL1-ZnL4), with equilibrium constants derived from absorption spectrophotometry in the range log K11 = 5.5-8.2 (MeCN or MeOH/CH2Cl2, I = 0.01 M). The zinc complexes show blue fluorescence, both in solution as well as in the solid state, with λem, Φf, and τ ranging from 472-504 nm, 0.11-0.60, and 2-9 ns, respectively. The nature of the substituents on the salicylaldiminato fragments was found to be the main parameter that influences the photophysical properties of the zinc complexes. Insights into the electronic nature of the UV-vis transitions were obtained with time dependent density functional theory (TD-DFT) calculations.

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

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 1H 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 Zn2L2 dimer.

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

A Zn^II Schiff-base complex shows vapochromic and chemiresistive behaviour when exposed to vapours of a Lewis base.

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

The Lewis basicity of relevant anions is reported for the first time and compared with that of neutral bases.