Encapsulation of metal cations and anions within the cavity of bis(1,4,7-triazacyclononane) receptors

Neutral Ligands as Potential 64Cu Chelators for Positron Emission Tomography Imaging Applications in Alzheimer's Disease

Positron emission tomography (PET), which uses positron-emitting radionuclides to visualize and measure processes in the human body, is a useful noninvasive diagnostic tool for Alzheimer's disease (AD). The development of longer-lived radiolabeled compounds is essential for further expansion of the use of PET imaging in healthcare, and diagnostic agents employing longer-lived radionuclides such as ⁶⁴Cu (t_1/2 = 12.7 h, β⁺ = 17%, β^- = 39%, electron capture EC = 43%, and E_max = 0.656 MeV) can accomplish this task. One limitation of ⁶⁴Cu PET agents for neuroimaging applications is their limited lipophilicity due to the presence of several anionic groups needed to ensure strong Cu chelation. Herein, we evaluate a series of neutral chelators containing the 1,4,7-triazacyclononane or 2,11-diaza[3.3](2,6)pyridinophane macrocycles that have pyridyl-containing arms incorporating Aβ-peptide-interacting fragments. The crystal structures of the corresponding Cu complexes confirm that the pyridyl N atoms are involved in binding to Cu. Radiolabeling and autoradiography studies show that the compounds efficiently chelate ⁶⁴Cu, and the resulting complexes exhibit specific binding to the amyloid plaques in the AD mouse brain sections versus wild-type controls.

Fluorescence modulation of naphthalene containing salicyl hydrazide-based receptor through aggregation-induced emission enhancement approach: Dual detection of lanthanum and cyanide ions in semi-aqueous medium

The sensing activity of naphthalene containing salicyl hydrazide-based fluorescence receptor has been improved through aggregation-induced enhanced emission mechanism approach in semi-aqueous medium. The receptor has been found to be selective toward La³⁺ with approximately 70-fold fluorescence enhancement due to a combined effect of keto-enol tautomerism inhibition and chelation enhanced fluorescence with a detection limit of 3.91 × 10^-6 M. In addition, the receptor is also able to sense CN^- with a detection limit of 3.55 × 10^-6 M via deprotonation effect, justifying its multiple analyte sensing behaviour. Hence, the current analytical methodology improves the sensing activity of the probe and also provides a greener alternative for La³⁺ and CN^- detection.

Protonation of cyclen-based chelating agents containing fluorescent moieties

The fluorescence emission properties of 1,4,7,10-tetraazacyclododecane-based receptors with appended heteroaromatic fluorophores are tuned by photoinduced electron and proton transfer processes.

Aza- and Mixed Thia/Aza-Macrocyclic Receptors with Quinoline-Bearing Pendant Arms for Optical Discrimination of Zinc(II) or Cadmium(II) Ions

The synthesis and coordination properties of two fluorescent chemosensors, featuring [9]aneN₃ (1,4,7-triazacyclononane; L1) and [12]aneNS₃ (1-aza-4,7,10-trithiacyclododecane; L2) as receptor units, and a quinoline pendant arm with an amide group as a functional group spacer are described. The optical responses of L1 and L2 in the presence of several metal ions were analysed in MeCN/H₂ O (1 : 4 v/v) solutions. A selective chelation enhancement of fluorescence (CHEF) effect was observed in the presence of Zn²⁺ in the case of L1, and in the presence of Cd²⁺ in the case of L2, following the formation of a 1 : 1 and a 1 : 2 metal/ligand complex, respectively, which was also confirmed by potentiometric measurements. ¹ H and ¹³ C NMR measurements in CD₃ CN/CDCl₃ in combination with molecular mechanics calculations show that for both complexes of L1 and L2 with Zn²⁺ and Cd²⁺ , respectively, the coordination of the carbonyl group from the pendant arm could be the origin of the observed optical selectivity.

[9]aneN3-based fluorescent receptors for metal ion sensing, featuring urea and amide functional groups

The sensing and recognition properties of three new [9]aneN₃-based chemosensors have been studied both in solution and in the solid state.

A Bis-Triazacyclononane Tris-Pyridyl N9 -Azacryptand "Beer Can" Receptor for Complexation of Alkali Metal and Lead(II) Cations

A new bis-triazacyclononane tris-pyridyl N₉ -azacryptand ligand is prepared via a convenient one-pot [2+3] condensation reaction between triazacyclononane and 2,6-bis(bromomethyl) pyridine in the presence of M₂ CO₃ (M=Li, Na, K). The proton, lithium, sodium, potassium and lead(II) complexes of the ligand are characterised in the solid state. Preliminary solution-phase competition experiments indicate that the cryptand ligand preferentially binds lead(II) in the presence of sodium, calcium, potassium and zinc cations in methanol solution.

Di- and Triphosphate Recognition and Sensing with Mono- and Dinuclear Fluorescent Zinc(II) Complexes: Clues for the Design of Selective Chemosensors for Anions in Aqueous Media

The synthesis of a new ligand (L1) containing two 1,4,7-triazacyclononane ([9]aneN₃ ) moieties linked by a 4,5-dimethylenacridine unit is reported. The binding and fluorescence sensing properties toward Cu²⁺ , Zn²⁺ , Cd²⁺ , and Pb²⁺ of L1 and receptor L2, composed of two [9]aneN₃ macrocycles bridged by a 6,6''-dimethylen-2,2':6',2''-terpyridine unit, have been studied by coupling potentiometric, UV/Vis absorption, and emission measurements in aqueous media. Both receptors can selectively detect Zn²⁺ thanks to fluorescence emission enhancement upon metal binding. The analysis of the binding and sensing properties of the Zn²⁺ complexes toward inorganic anions revealed that the dinuclear Zn²⁺ complex of L1 selectively binds and senses the triphosphate anion (TP), whereas the mononuclear Zn²⁺ complex of L2 displays selective recognition of diphosphate (DP). Binding of TP or DP induces emission quenching of the Zn²⁺ complexes with L1 and L2, respectively. These results are exploited to discuss the role played by pH, number of coordinated metal cations, and binding ability of the bridging units in metal and/or anion coordination and sensing.

Bi-Nuclear Metal Complexes of 2,6-Bis(1,4,7-triazacyclonon-1-yl-methylene)pyridine with Zinc(II), Copper(II), and Nickel(II)

The coordination chemistry of 2,6-bis(1,4,7-triazacyclonon-1-ylmethyl)pyridine (Lpyx, 1) has been investigated and shown to yield bi-nuclear metal complexes in the presence of zinc(ii), copper(ii), or nickel(ii) ions. The reaction of (Lpyx)·7HCl (2) with Zn(NO3)2 gave the monomeric [Zn2(μ-Cl)2(Lpyx)(H2O)](ClO4)2 (3) in which the ligand encapsulates a Zn2(μ-Cl)2 moiety. Similar treatment of 2 with Cu(NO3)2 gave a bi-nuclear complex cation which exists as a 1 : 1 co-crystal {[Cu(Cl)(Lpyx)Cu(μ-Cl)(Cl)](BF4)(H2O)}2 and {[Cu(Cl)(Lpyx)Cu(μ-Cl)(H2O)](BF4)2(H2O)}2 (4) with two discrete CuII centres bridged by the Lpyx ligand and dimerised through an unsymmetrical Cu2(μ-Cl)2 interaction. Similarly, reaction of 2 with Ni(NO3)2 also gave a dimeric complex {[Ni(Cl)(μ-Cl)(Lpyx)Ni(NO3)(H2O)](PF6)}2 (5) containing two discrete NiII centres with dimerisation occurring through a symmetrical Ni2(μ-Cl)2 interaction. In all cases, the Lpyx ligand binding is unsymmetrical between the two metal centres being tridentate to one and tetradentate to the other through an additional coordination of the pyridyl linker.

Selective chelation of Cd(II) and Pb(II) versus Ca(II) and Zn(II) by using octadentate ligands containing pyridinecarboxylate and pyridyl pendants

Herein we report the coordination properties toward Cd(II), Pb(II), Ca(II), and Zn(II) of a new octadentate ligand (py-H(2)bcpe) based on a ethane-1,2-diamine unit containing two picolinate and two pyridyl pendants, which is structurally derived from the previous reported ligand bcpe. Potentiometric studies have been carried out to determine the protonation constants of the ligand and the stability constants of the complexes with these cations. The introduction of the pyridyl pendants in bcpe provokes a very important increase of the logK(ML) values obtained for the Pb(II) and Cd(II) complexes, while this effect is less important in the case of the Zn(II) analogue. As a result, py-bcpe shows a certain selectivity for Cd(II) and Pb(II) over Zn(II) while keeping good Pb(II)/Ca(II) and Cd(II)/Ca(II) selectivities, and the new receptor py-bcpe can be considered as a new structural framework for the design of novel Cd(II) and Pb(II) extracting agents. Likewise, the stabilities of the Cd(II) and Pb(II) complexes are higher than those of the corresponding EDTA analogues. The X-ray crystal structure of [Zn(py-bcpe)] shows hexadentate binding of the ligand to the metal ion, the coordination polyhedron being best described as a severely distorted octahedron. However, the X-ray crystal structure of the Pb(II) analogue shows octadentate binding of the ligand to the metal ion. A detailed investigation of the structure in aqueous solution of the complexes by using nuclear magnetic resonance (NMR) techniques and density functional theory (DFT) calculations (B3LYP) shows that while in the Zn(II) complex the metal ion is six-coordinated, in the Pb(II) and Ca(II) analogues the metal ions are eight-coordinated. For the Cd(II) complex, our results suggest that in solution the complex exists as a mixture of seven- and eight-coordinated species. DFT calculations performed both in the gas phase and in aqueous solution have been also used to investigate the role of the Pb(II) lone pair in the structure of the [Pb(py-bcpe)] complex.