A Biphenol-Based Chemosensor for Zn(II) and Cd(II) Metal Ions: Synthesis, Potentiometric Studies, and Crystal Structures

A new biphenol-dipicolylamine based ligand and its dinuclear Zn2+ complex as fluorescent sensors for ibuprofen and ketoprofen in aqueous solution

In this work, the study of the new ligand 3,3'-bis[N,N-bis(pyridine-2-ylmethyl)aminomethyl]-2,2'-dihydroxybiphenyl (L) is reported, where a central 2,2'-biphenol (BPH) fluorophore was functionalized at 3,3'-positions with two dipicolylamine (DPA) side arms as receptor units. Following the synthesis and full chemical-physical characterization, the acid-base and Zn2+-coordination abilities of L were investigated through a combination of potentiometric, UV-Vis, fluorescence, NMR, XRD and DFT measurements. The optical properties of the ligand turned out to be strongly dependent on the pH, being straightforwardly associated with the protonation state of the BPH moiety, whereas its peculiar design allowed to form stable mono and dinuclear Zn2+ complexes. In the latter species, the presence of two Zn2+ ions coordinatively unsaturated and placed at close distance to each other, prompted us to test their usefulness as metallo-receptors for two environmental pollutants of great relevance, ibuprofen and ketoprofen. Potentiometric and fluorescence investigations evidenced that these important non-steroidal anti-inflammatory drugs (NSAIDs) are effectively coordinated by the metallo-receptors and, of relevance, both the stability and the fluorescence properties of the resulting ternary adducts are markedly affected by the different chemical architectures of the two substrates. This study aims at highlighting the promising perspectives arising from the use of polyamino phenolic ligands as chemosensors for H+/Zn2+ and other additional anionic targets in their metal-complexed forms.

A New Family of Macrocyclic Polyamino Biphenolic Ligands: Acid-Base Study, Zn(II) Coordination and Glyphosate/AMPA Binding

In this study, the ligands 23,24-dihydroxy-3,6,9,12-tetraazatricyclo[17.3.1.1(14,18)]eicosatetra-1(23),14,16,18(24),19,21-hexaene, L1, and 26,27-dihidroxy-3,6,9,12,15-pentaazatricyclo[20.3.1.1(17,21)]eicosaepta-1(26),17,19,21(27),22,24-hexaene, L2, were synthesized: they represent a new class of molecules containing a biphenol unit inserted into a macrocyclic polyamine fragment. The previously synthesized L2 is obtained herein with a more advantageous procedure. The acid-base and Zn(II)-binding properties of L1 and L2 were investigated through potentiometric, UV-Vis, and fluorescence studies, revealing their possible use as chemosensors of H⁺ and Zn(II). The new peculiar design of L1 and L2 afforded the formation in an aqueous solution of stable Zn(II) mono (LogK 12.14 and 12.98 for L1 and L2, respectively) and dinuclear (LogK 10.16 for L2) complexes, which can be in turn exploited as metallo-receptors for the binding of external guests, such as the popular herbicide glyphosate (N-(phosphonomethyl)glycine, PMG) and its primary metabolite, the aminomethylphosphonic acid (AMPA). Potentiometric studies revealed that PMG forms more stable complexes than AMPA with both L1- and L2-Zn(II) complexes, moreover PMG showed higher affinity for L2 than for L1. Fluorescence studies showed instead that the L1-Zn(II) complex could signal the presence of AMPA through a partial quenching of the fluorescence emission. These studies unveiled therefore the utility of polyamino-phenolic ligands in the design of promising metallo-receptors for elusive environmental targets.

A New Benzoxazole-Based Fluorescent Macrocyclic Chemosensor for Optical Detection of Zn2+ and Cd2+

Background: Benzoxazole-containing ligands find many applications both in medicinal chemistry, catalysis and fluorescence chemosensing. Benzoxazole-containing macrocycles could be therefore a good strategy to achieve stable and selective fluorescent complexes with suitable metal ions. In this work, the synthesis, binding, and photochemical properties of a new fluorescent ligand (L) are reported. L is a cyclophane macrocycle containing the 1,3-bis(benzo[d]oxazol-2-yl)phenyl (BBzB) fluorophore and an aliphatic tetra-amine chain to form the macrocyclic skeleton. Methods: Spectrophotometric and spectrofluorimetric measurements, 1H NMR analysis, and DFT calculations were performed. Results: L behaves as a PET-mediated chemosensor, being emissive at 390 nm at acidic pH and non-emissive at basic pH. The chemosensor is able to detect Zn2+ and Cd2+ in an aqueous medium (acetonitrile–water, 4:1 v/v) at neutral pH through a CHEF effect upon metal ion coordination. Paramagnetic metal ions (Cu2+) and heavy atoms (Pb2+, Hg2+) resulted in a quenching of fluorescence or very low emission. Conclusions: The new cyclophane macrocycle L was revealed to be a selective PET-regulated chemosensor for Zn2+ and Cd2+ in an aqueous medium, being able to bind up to two and one metal cations, respectively. The molecule showed a shifted emission towards the visible region compared to similar systems, suggesting a co-planar conformation of the aromatic fragment upon metal coordination. All these data are supported by both experimental measurements and theoretical calculations.

Hetero-Tetranuclear Cu2+ /Ca2+ /Ca2+ /Cu2+ Architectures Based On Malten Ligand: Scaffold for Anion Binding

The hetero-tetranuclear Cu²⁺ /Ca²⁺ /Ca²⁺ /Cu²⁺ complex obtained with the N,N'-bis((3-hydroxy-4-pyron-2-yl)methyl)-N,N'-dimethylethylendiamine (Malten) ligand has been studied in solid and solution states as scaffold to bind anions. Three crystal structures showing the same metal ions sequence have been examined; they display a tetracharged complex cation neutralized by four monocharged anions. The anions play two different roles: as coordinated (two ClO₄ ^- , Cl^- or NO₃ ^- ) or ancillary (two ClO₄ ^- ) guests. The tetranuclear scaffold hosts two anions also in aqueous and ethanol solutions. Spectrophotometric studies in ethanol allowed to determine the addition constant values for Cl^- and Br^- (Log K_1-2 =4.43(4), 4.39(3) for Cl^- , 3.80(3), 3.54(2) for Br^- ) while the others, although bound, showed lower affinity for the scaffold. Both the crystals and the solutions change their color depending on the added anion, namely pink, dark green or blue in the presence of ClO₄ ^- , Cl^- or NO₃ ^- , respectively, thus the presence of the different anions is visible to the naked eye. The hetero-tetranuclear Cu²⁺ /Ca²⁺ /Ca²⁺ /Cu²⁺ complex is a versatile architecture to be used as scaffold for anion binding.

Playing with Structural Parameters: Synthesis and Characterization of Two New Maltol-Based Ligands with Binding and Antineoplastic Properties

Two maltol-based ligands, N,N′-bis((3-hydroxy-4-pyron-2-yl)methyl)-1,4-piperazine (L1) and N,N′,N′-tris((3-hydroxy-4-pyron-2-yl)methyl)-N-methylethylendiamine (L2), were synthesized and characterized. L1 and L2, containing, respectively, two and three maltol units spaced by a diamine fragment, were designed to evaluate how biological and binding features are affected by structural modifications of the parent compound malten. The acid-base behavior and the binding properties towards transition, alkaline-earth (AE) and rare-earth (RE) cations in aqueous solution, studied by potentiometric, UV-Vis and NMR analysis, are reported along with biological studies on DNA and leukemia cells. Both ligands form stable complexes with Cu(II), Zn(II) and Co(II) that were studied as metallo-receptors for AE and RE at neutral pH. L1 complexes are more affected than L2 ones by hard cations, the L1-Cu(II) system being deeply affected by RE. The structural modifications altered the mechanism of action: L1 partially maintains the ability to induce structural alterations of DNA, while L2 provokes single strand (nicks) and to a lesser extent double strand breaks of DNA.

Phenolic metabolism and related heavy metal tolerance mechanism in Kandelia Obovata under Cd and Zn stress

In the present study, a set of pot culture experiments was conducted to reveal how the metabolism process of phenolic compounds was affected by cadmium (Cd) and zinc (Zn) and to further uncover heavy metal tolerance mechanisms in Kandelia obovata. After 60d of treatment, the biomass and chlorophyll a content in the leaves were suppressed, but total phenolic compounds in roots and leaves were improved by the increasing gradient of Cd or Zn concentrations; Total phenolic compounds significantly increased by 3.6-44.6% in the roots, and by 0.4-126.6% in the leaves. At the meantime, the activity of Shikimate dehydrogenase (SKDH), cinnamyl alcohol dehydrogenase (CAD), and polyphenol oxidase (PPO) in the roots increased by 11.2-307.6%, 12.4-175.4% and - 2.7-392.8%, and the results were 3.4-69.5%, 1.7-40.0%, 16.0-99.7% in the leaves. Higher toxicity of Cd than Zn, as well as slight alleviating effect of 100 mg kg^-1 Zn on 2.5 mg kg^-1 Cd were found. Additionally, a significantly positive correlation coefficients for relationship between phenolic metabolism related enzyme activity and Cd/Zn contamination levels was found, and leaf SKDH, leaf CAD, and leaf PPO activities were moderately correlated with leaf Cd (r = 0.39, r = 0.43, and r = 0.57, respectively) and leaf Zn (r = 0.44, r = 0.41, r = 0.19, respectively) content, which indicate that Cd and Zn play a previously unrecognized but major role in phenolic compounds synthesis, transport, and metabolism in K. obovata. The results also provided evidence that the application of high levels of Cd and Zn was accompanied by three phenolic metabolism pathways participating in heavy metal tolerance process.

Improved Access to 1,8-Diformyl-carbazoles Leads to Metal-Free Carbazole-Based [2 + 2] Schiff Base Macrocycles with Strong Turn-On Fluorescence Sensing of Zinc(II) Ions

Development of a new and high yielding synthetic route to 1,8-diformyl-carbazoles 3 (3a 3,6-di- tert-butyl substituted; 3b 3,6-unsubstituted) is reported. Use of a Heck coupling reaction, followed by ozonolysis, has greatly facilitated the preparation of these interesting head units in useful quantities. An initial foray into the new generations of Schiff base macrocycles that ready access to these head units (3) opens up, has led to the direct (i.e., metal-free) synthesis of two [2 + 2] macrocycles from 3a or 3b with 1,2-diaminoethane, H₂L^tBu (4a) and H₂L^H (4b), respectively, obtained as yellow powders in high yields (87-88%). The dizinc complex [Zn₂L^H(OAc)₂] (5b) was isolated as a bright yellow solid in 83% yield, by 1:2:2 reaction of H₂L^H with zinc(II) acetate and triethylamine. Aldehydes 3a and 3b, macrocycle H₂L^H, and complex [Zn₂L^H(OAc)₂] (5b) have been structurally characterized. The carbazole NH makes bifurcated hydrogen bonds with the pair of flanking 1,8-diformyl-moieties in 3, or 1,8-diimine-moieties in H₂L^H, leading to a flat, all- cis conformation. The stepped conformation of the metal-free macrocycle H₂L^H is retained in [Zn₂L^H(OAc)₂], despite deprotonation and binding of two zinc(II) centers within the two tridentate pockets. The N₃O₂ coordination of the zinc ions is completed by one μ_1,1- and one μ_1,3- bridging acetate anion. Excitation of nanomolar [Zn₂L^H(OAc)₂] in DMF at 335 nm results in clearly visible blue fluorescence (λ_max = 460 nm). Further studies on the H₂L^H macrocycle revealed turn-on fluorescence, with selectivity (over Ca²⁺, Mg²⁺ and a range of 3d dications) and nanomolar sensitivity for zinc(II) ions, highlighting one of the many potential applications for these new carbazole-based Schiff base macrocycles.

Hexaphenylbenzene-based fluorescent aggregates for detection of zinc and pyrophosphate ions in aqueous media: tunable self-assembly behaviour and construction of a logic device

The aggregates of HPB derivative 7 exhibited “on–on” response towards Zn²⁺ ions and this in situ prepared 7-Zn2+ ensemble was utilized as a “not quenched” probe for detection of PPi ions in aqueous media.

Imidazole/benzimidazole-modified cyclotriphosphazenes as highly selective fluorescent probes for Cu2+: synthesis, configurational isomers, and crystal structures

We present a comprehensive work and discuss the fluoroprobe properties of synthesized compounds for copper ion detection with a stereochemical approach using X-ray crystallographic analysis results.