Macrocyclic Receptor Showing Extremely High Sr(II)/Ca(II) and Pb(II)/Ca(II) Selectivities with Potential Application in Chelation Treatment of Metal Intoxication

Double-Armed 18- and 21-Membered Macrocycles as Potential Chelators for Lead and Bismuth Radiopharmaceuticals

With increasing clinical applications and interest in targeted alpha therapy, there is growing interest in developing alternative chelating agents for [212Pb]Pb2+ and [212/213Bi]Bi3+ that exhibit rapid radiolabeling kinetics and kinetic inertness. Herein we report the synthesis and detailed investigation of diacetate and dipicolinate 18- and 21-membered macrocyclic chelators BADA-18, BADA-21, BADPA-18, and BADPA-21 for the complexation of Pb2+ and Bi3+ ions with potential use in the preparation of radiopharmaceuticals. The formation of mononuclear complexes was established by using ESI-mass spectrometry, and their stability constants were determined by potentiometric titration. A thorough study of the structure of the metal complexes was carried out by using X-ray diffraction and NMR spectroscopy. It was shown how the stability of the complex is influenced by an increase in the size of the macrocycle, the replacement of acetate arms with picolinate ones, the rigidity of the ligand, as well as the type of conformation (syn- or anti-) of the metal complex. The new ligands were radiolabeled with [210Pb]Pb2+ and [207Bi]Bi3+, and the in vitro stability of the resulting complexes in a competitive environment of serum and biologically significant metal ions was assessed. Rapid complex formation in 1-2 min at room temperature, as well as the high kinetic inertness of the complexes Pb(BADPA-18) and Bi(BADPA-18) in biological media, demonstrate its potential for use in targeted radionuclide therapy.

Incorporation of Carboxylate Pendant Arms into 18-Membered Macrocycles: Effects on [nat/203Pb]Pb(II) Complexation

We present a detailed investigation on the coordination chemistry of [nat/203Pb]Pb(II) with chelators H4PYTA and H4CHX-PYTA. These chelators belong to the family of ligands derived from the 18-membered macrocyclic backbone PYAN and present varying degrees of rigidity due to the presence of either ethyl or cyclohexyl spacers. A complete study of the stable Pb(II) complexes is carried out via NMR, X-Ray crystallography, stability constant determination and computational studies. While these studies indicated that Pb(II) complexation is achieved, and the thermodynamic stability of the resulting complexes is very high, a certain degree of fluxionality does exist in both cases. Nevertheless, radiolabeling studies were carried out using SPECT (single photon emission computed tomography) compatible isotope lead-203 (203Pb, t1/2=51.9 h), and while both chelators complex the radioisotope, the incorporation of carboxylate pendant arms appears to be detrimental towards the stability of the complexes when compared to the previously described amide analogues. Additionally, incorporation of a cyclohexyl spacer does not improve the kinetic inertness of the system.

Reining in Radium for Nuclear Medicine: Extra-Large Chelator Development for an Extra-Large Ion

Targeted α therapy (TAT) of soft-tissue cancers using the α particle-emitting radionuclide 223Ra holds great potential because of its favorable nuclear properties, adequate availability, and established clinical use for treating metastatic prostate cancer of the bone. Despite these advantages, the use of 223Ra has been largely overshadowed by other α emitters due to its challenging chelation chemistry. A key criterion that needs to be met for a radionuclide to be used in TAT is its stable attachment to a targeting vector via a bifunctional chelator. The low charge density of Ra2+ arising from its large ionic radius weakens its electrostatic binding interactions with chelators, leading to insufficient complex stability in vivo. In this study, we synthesized and evaluated macropa-XL as a novel chelator for 223Ra. It bears a large 21-crown-7 macrocyclic core and two picolinate pendent groups, which we hypothesized would effectively saturate the large coordination sphere of the Ra2+ ion. The structural chemistry of macropa-XL was first established with the nonradioactive Ba2+ ion using X-ray diffraction and X-ray absorption spectroscopy, which revealed the formation of an 11-coordinate complex in a rare anti pendent-arm configuration. Subsequently, the stability constant of the [Ra(macropa-XL)] complex was determined via competitive cation exchange with 223Ra and 224Ra radiotracers and compared with that of macropa, the current state-of-the-art chelator for Ra2+. A moderate log KML value of 8.12 was measured for [Ra(macropa-XL)], which is approximately 1.5 log K units lower than the stability constant of [Ra(macropa)]. This relative decrease in Ra2+ complex stability for macropa-XL versus macropa was further probed using density functional theory calculations. Additionally, macropa-XL was radiolabeled with 223Ra, and the kinetic stability of the resulting complex was evaluated in human serum. Although macropa-XL could effectively bind 223Ra under mild conditions, the complex appeared to be unstable to transchelation. Collectively, this study sheds additional light on the chelation chemistry of the exotic Ra2+ ion and contributes to the small, but growing, number of chelator development efforts for 223Ra-based TAT.

Bispidine Chelators for Radiopharmaceutical Applications with Lanthanide, Actinide, and Main Group Metal Ions

Octadentate and specifically nonadentate ligands with a bispidine scaffold (3,7-diazabicyclo[3.3.1]nonane) are known to be efficiently coordinated to a range of metal ions of interest in radiopharmaceutical chemistry and lead to exceedingly stable and inert complexes. Nonadentate bispidine L2 (with a tridentate bipyridine acetate appended to N3 and a picolinate at N7) has been shown before to be an ideal chelator for 111In3+, 177Lu3+, and 225Ac3+, nuclides of interest for diagnosis and therapy, and a proof-of-principle study with an SSTR2-specific octreotate has shown potential for theranostic applications. We now have extended these studies in two directions. First, we present ligand derivative L3, in which the bipyridine acetate is substituted with terpyridine, a softer donor for metal ions with a preference for more covalency. L3 did not fulfill the hopes because complexation is much less efficient. While for Bi3+ and Pb2+ the ligand is an excellent chelator with properties similar to those of L2, Lu3+ and La3+ show very slow and inefficient complexation with L3 in contrast to L2, and 225Ac3+ is not fully coordinated, even at an increased temperature (92% radiochemical yield at 80 °C, 60 min, [L3] = 10-4 M). These observations have led to a hypothesis for the complexation pathway that is in line with all of the experimental data and supported by a preliminary density functional theory analysis, which is important for the design of further optimized bispidine chelators. Second, the coordination chemistry of L2 has been extended to Bi3+, La3+, and Pb2+, including solid state and solution structural work, complex stabilities, radiolabeling, and radiostability studies. All complexes of this ligand (La3+, Ac3+, Lu3+, Bi3+, In3+, and Pb2+), including nuclides for targeted α therapy (TAT), single-photon emission computed tomography, and positron emission tomography, are formed efficiently under physiological conditions, i.e., suitable for the labeling of delicate biological vectors such as antibodies, and the complexes are very stable and inert. Importantly, for TAT with 225Ac, the daughter nuclides 213Bi and 209Pb also form stable complexes, and this is important for reducing damage to healthy tissue.

Equilibrium Thermodynamics of Macropa Complexes with Selected Metal Isotopes of Radiopharmaceutical Interest

To pursue the design of in vivo stable chelating systems for radiometals, a concise and straightforward method toolbox was developed combining NMR, isothermal titration calorimetry (ITC), and europium time-resolved laser-induced fluorescence spectroscopy (Eu-TRLFS). For this purpose, the macropa chelator was chosen, and Lu3+, La3+, Pb2+, Ra2+, and Ba2+ were chosen as radiopharmaceutically relevant metal ions. They differ in charge (2+ and 3+) and coordination properties (main group vs lanthanides). 1H NMR was used to determine four pKa values (±0.15; carboxylate functions, 2.40 and 3.13; amino functions, 6.80 and 7.73). Eu-TRLFS was used to validate the exclusive existence of the 1:1 Mn+/ligand complex in the chosen pH range at tracer level concentrations. ITC measurements were accomplished to determine the resulting stability constants of the desired complexes, with log K values ranging from 18.5 for the Pb-mcp complex to 7.3 for the Lu-mcp complex. Density-functional-theory-calculated structures nicely mirror the complexes' order of stabilities by bonding features. Radiolabeling with macropa using ligand concentrations from 10-3 to 10-6 M was accomplished by pointing out the complex formation and stability (212Pb > 133La > 131Ba ≈ 224Ra > 177Lu) by means of normal-phase thin-layer chromatography analyses.

Elucidating the coordination chemistry of the radium ion for targeted alpha therapy

The coordination chemistry of Ra²⁺ is poorly defined, hampering efforts to design effective chelators for ²²³Ra-based targeted alpha therapy. Here, we report the complexation thermodynamics of Ra²⁺ with the biomedically-relevant chelators DOTA and macropa. Our work reveals the highest affinity chelator to date for Ra²⁺ and advances our understanding of key factors underlying complex stability and selectivity for this underexplored ion.

Evaluation of the Effect of Macrocyclic Ring Size on [203Pb]Pb(II) Complex Stability in Pyridyl-Containing Chelators

As an element-equivalent theranostic pair, lead-203 (²⁰³Pb, 100% EC, half-life = 51.92 h) and lead-212 (²¹²Pb, 100% β^-, half-life = 10.64 h), through the emission of γ rays and an α particle in its decay chain, respectively, can aid in the development of personalized targeted radionuclide treatment for advanced and currently untreatable cancers. With these isotopes currently being used in clinical trials, an understanding of the relationship between the chelator structure, ability to incorporate the radiometal, and metal-complex stability is needed to help design appropriate chelators for clinical use. Herein, we report an investigation into the effect of ring size in macrocyclic chelators where pyridine, an intermediate Lewis base, acts as an electron donor toward lead. Crown-4Py (4,7,13,16-tetrakis(pyridin-2-ylmethyl)-1,10-dioxa-4,7,13,16-tetraazacyclooctadecane), cyclen-4Py (1,4,7,10-tetrakis(pyridin-2-ylmethyl)-1,4,7,10-tetraazacyclododecane), and NOON-2Py (7,16-bis(pyridin-2-ylmethyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) were synthesized and analyzed for their ability to coordinate Pb²⁺. Metal complex stability was investigated via [²⁰³Pb]Pb²⁺ radiolabeling studies, ¹H NMR spectroscopy, X-ray crystallography, and potentiometry. With the smallest macrocyclic backbone, cyclen-4Py had the highest radiochemical yield, while, in descending order, crown-4Py and NOON-2Py had the lowest. Thermodynamic stability constants (log K_ML) of 19.95(3), 13.29(5), and 11.67 for [Pb(Cyclen-4Py)]²⁺, [Pb(Crown-4Py)]²⁺, and [Pb(NOON-2Py)]²⁺, respectively, correlated with their radiochemical yields. The X-ray crystal structure of the least stable complexes [Pb(NOON-2Py)]²⁺ revealed a hemidirected Pb²⁺ center, as reflected by a void within the coordination sphere, and [Pb(Crown-4Py)]²⁺ showed an average Pb-N pyridine interatomic distance of >3 Å. By contrast, the crystal structure of [Pb(Cyclen-4Py)]²⁺ showed shorter Pb-N pyridine interactions, and in solution, only one highly symmetric isomer existed for this complex, whereas conformational flexibility was observed for both [Pb(Crown-4Py)]²⁺ and [Pb(NOON-2Py)]²⁺ at the NMR timescale. This study illustrates the importance of the macrocyclic backbone size when incorporating bulky electron-donor groups into the design of a macrocyclic chelator as it affects the accessibility of lead to the donor arms. Our results show that cyclen-4Py is a promising chelator for future studies with this theranostic pair.

Advancing Chelation Strategies for Large Metal Ions for Nuclear Medicine Applications

Nuclear medicine leverages radioisotopes of a wide range of elements, a significant portion of which are metals, for the diagnosis and treatment of disease. To optimally use radioisotopes of the metal ions, or radiometals, for these applications, a chelator that efficiently forms thermodynamically and kinetically stable complexes with them is required. The chelator also needs to attach to a biological targeting vector that locates pathological tissues. Numerous chelators suitable for small radiometals have been established to date, but chelators that work well for large radiometals are significantly less common. In this Account, we describe recent progress by us and others in the advancement of ligands for large radiometal chelation with emerging applications in nuclear medicine.First, we discuss and analyze the coordination chemistry of the chelator macropa, a macrocyclic ligand that contains the 18-crown-6 backbone and two picolinate pendent arms, with large metal ions in the context of nuclear medicine. This ligand is known for its unusual reverse size selectivity, the preference for binding large over small metal ions. The radiolabeling properties of macropa with large radiometals ²²⁵Ac³⁺, ^132/135La³⁺, ¹³¹Ba²⁺, ²²³Ra²⁺, ²¹³Bi³⁺, and related in vivo investigations are described. The development of macropa derivatives containing different pendent donors or rigidifying groups in the macrocyclic core is also briefly reviewed.Next, efforts to transform macropa into a radiopharmaceutical agent via covalent conjugation to biological targeting vectors are summarized. In this discussion, two types of bifunctional analogues of macropa reported in the literature, macropa-NCS and mcp-click, are presented. Their implementation in different radiopharmaceutical agents is discussed. Bioconjugates containing macropa attached to small-molecule targeting vectors or macromolecular antibodies are presented. The in vitro and in vivo evaluations of these constructs are also discussed.Lastly, chelators with dual size selectivity are described. This class of ligands exhibits good affinities for both large and small metal ions. This property is valuable for nuclear medicine applications that require the simultaneous chelation of both large and small radiometals with complementary therapeutic and diagnostic properties. Recently, we reported an 18-membered macrocyclic ligand called macrodipa that attains this selectivity pattern. This chelator, its second-generation analogue py-macrodipa, and their applications for chelating the medicinally relevant large ¹³⁵La³⁺, ²²⁵Ac³⁺, ²¹³Bi³⁺, and small ⁴⁴Sc³⁺ ions are also presented. Studies with these radiometals show that py-macrodipa can effectively radiolabel and stably retain both large and small radiometals. Overall, this Account makes the case for innovative ligand design approaches for novel emerging radiometal ions with unusual coordination chemistry properties.

Tuning the Kinetic Inertness of Bi3+ Complexes: The Impact of Donor Atoms on Diaza-18-Crown-6 Ligands as Chelators for 213Bi Targeted Alpha Therapy

The radionuclide ²¹³Bi can be applied for targeted α therapy (TAT): a type of nuclear medicine that harnesses α particles to eradicate cancer cells. To use this radionuclide for this application, a bifunctional chelator (BFC) is needed to attach it to a biological targeting vector that can deliver it selectively to cancer cells. Here, we investigated six macrocyclic ligands as potential BFCs, fully characterizing the Bi³⁺ complexes by NMR spectroscopy, mass spectrometry, and elemental analysis. Solid-state structures of three complexes revealed distorted coordination geometries about the Bi³⁺ center arising from the stereochemically active 6s² lone pair. The kinetic properties of the Bi³⁺ complexes were assessed by challenging them with a 1000-fold excess of the chelating agent diethylenetriaminepentaacetic acid (DTPA). The most kinetically inert complexes contained the most basic pendent donors. Density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) calculations were employed to investigate this trend, suggesting that the kinetic inertness is not correlated with the extent of the 6s² lone pair stereochemical activity, but with the extent of covalency between pendent donors. Lastly, radiolabeling studies of ²¹³Bi (30-210 kBq) with three of the most promising ligands showed rapid formation of the radiolabeled complexes at room temperature within 8 min for ligand concentrations as low as 10^-7 M, corresponding to radiochemical yields of >80%, thereby demonstrating the promise of this ligand class for use in ²¹³Bi TAT.

H2O coordination in macropa complexes of f elements (Ac, La, Lu): feasibility of the 11th coordination site

The feasibility of an additional ligand coordination at the 11th coordination site of actinium, lanthanum, and lutetium ions in 10-fold coordinated macropa complexes has been studied by means of density functional theory calculations. The study covered the two main macropa conformers, Δ(δλδ)(δλδ) and Δ(λδλ)(λδλ), favoured by larger (Ac3+, La3+) and smaller (Lu3+) ions, respectively. At the molecular level, the coordination of H2O is the most favourable to the largest Ac3+ while only slightly less to La3+. Protonation of the picoline arms enhances the coordination by shifting the metal ion closer to the open site of the ligand. The choice of macropa conformer has only a slight influence on the strength and bonding properties of the H2O coordination. Aqueous solution environment decreases considerably the energy gain of H2O coordination at the 11th coordination site.

Towards the stable chelation of radium for biomedical applications with an 18-membered macrocyclic ligand

Targeted alpha therapy is an emerging strategy for the treatment of disseminated cancer. [223Ra]RaCl2 is the only clinically approved alpha particle-emitting drug, and it is used to treat castrate-resistant prostate cancer bone metastases, to which [223Ra]Ra2+ localizes. To specifically direct [223Ra]Ra2+ to non-osseous disease sites, chelation and conjugation to a cancer-targeting moiety is necessary. Although previous efforts to stably chelate [223Ra]Ra2+ for this purpose have had limited success, here we report a biologically stable radiocomplex with the 18-membered macrocyclic chelator macropa. Quantitative labeling of macropa with [223Ra]Ra2+ was accomplished within 5 min at room temperature with a radiolabeling efficiency of >95%, representing a significant advancement over conventional chelators such as DOTA and EDTA, which were unable to completely complex [223Ra]Ra2+ under these conditions. [223Ra][Ra(macropa)] was highly stable in human serum and exhibited dramatically reduced bone and spleen uptake in mice in comparison to bone-targeted [223Ra]RaCl2, signifying that [223Ra][Ra(macropa)] remains intact in vivo. Upon conjugation of macropa to a single amino acid β-alanine as well as to the prostate-specific membrane antigen-targeting peptide DUPA, both constructs retained high affinity for 223Ra, complexing >95% of Ra2+ in solution. Furthermore, [223Ra][Ra(macropa-β-alanine)] was rapidly cleared from mice and showed low 223Ra bone absorption, indicating that this conjugate is stable under biological conditions. Unexpectedly, this stability was lost upon conjugation of macropa to DUPA, which suggests a role of targeting vectors in complex stability in vivo for this system. Nonetheless, our successful demonstration of efficient radiolabeling of the β-alanine conjugate with 223Ra and its subsequent stability in vivo establishes for the first time the possibility of delivering [223Ra]Ra2+ to metastases outside of the bone using functionalized chelators, marking a significant expansion of the therapeutic utility of this radiometal in the clinic.

Theoretical Study of Actinide Complexes with Macropa

The complex formation of actinium (Ac³⁺) and californium (Cf³⁺) ions with macropa (a promising ligand for medical applications, e.g., in targeted α therapy) has been studied by means of density functional theory (DFT) calculations. This work is focused on the structural and bonding properties, the latter on the basis of charge transfer data and topological properties of the electron density distribution. The effect of water solvent on the energetics has been investigated using the SMD model. A comparative analysis with the related properties of two representative lanthanide (La, Lu) complexes has been performed.

Toxicity, migration and transformation characteristics of lead in soil-plant system: Effect of lead species

Lead (Pb) is a typical hazardous element of high concern in species characteristics involving toxicity, migration and transformation. A greenhouse experiment was conducted using Solanum nigrum L. grown in soils treated by divalent (Pb²⁺), tetravalent (Pb⁴⁺), trimethyl (TML) and triethyl (TEL) lead for 60 days. Results of physio-biochemical parameters indicated Pb toxicity was ranked as TEL > TML > Pb²⁺ > Pb⁴⁺ in a dose-dependent manner, and the correlation levels of organic species were higher than inorganic species. S. nigrum L. adopted phytostabilization strategy through fixing Pb in roots and restricting its transfer to shoots. More phytotoxic Pb was absorbed from soils treated by Pb²⁺ than Pb⁴⁺ as well as TEL than TML. In soils, inorganic Pb species were mainly present in residues while organic Pb species in Fe/Mn oxide and exchangeable fractions. Although most of Pb species in plant existed in the low-bioavailable extractions of 1 M NaCl and 2% HAC, the water-soluble Pb extracted by d-H₂O and 80 % ethanol were increased to a large extent under high-level exposure. The occurrence of reduction and (de)alkylation were considered as the major pathways in the biotransformation of Pb species. This study will conduce to the ecological risk management for Pb-contaminated soils.

225Ac-H4py4pa for Targeted Alpha Therapy

Herein, we present the syntheses and characterization of a new undecadendate chelator, H₄py4pa, and its bifunctional analog H₄py4pa-phenyl-NCS, conjugated to the monoclonal antibody, Trastuzumab, which targets the HER2+ cancer. H₄py4pa possesses excellent affinity for ²²⁵Ac (α, t_1/2 = 9.92 d) for targeted alpha therapy (TAT), where quantitative radiolabeling yield was achieved at ambient temperature, pH = 7, in 30 min at 10^-6 M chelator concentration, leading to a complex highly stable in mouse serum for at least 9 d. To investigate the chelation of H₄py4pa with large metal ions, lanthanum (La³⁺), which is the largest nonradioactive metal of the lanthanide series, was adopted as a surrogate for ²²⁵Ac to enable a series of nonradioactive chemical studies. In line with the ¹H NMR spectrum, the DFT (density functional theory)-calculated structure of the [La(py4pa)]^- anion possessed a high degree of symmetry, and the La³⁺ ion was secured by two distinct pairs of picolinate arms. Furthermore, the [La(py4pa)]^- complex also demonstrated a superb thermodynamic stability (log K_[La(py4pa)]^- ∼ 20.33, pLa = 21.0) compared to those of DOTA (log K_[La(DOTA)]^- ∼ 24.25, pLa = 19.2) or H₂macropa (log K_{[La(macropa)]}^- = 14.99, pLa ∼ 8.5). Moreover, the functional versatility offered by the bifunctional py4pa precursor permits facile incorporation of various linkers for bioconjugation through direct nucleophilic substitution. In this work, a short phenyl-NCS linker was incorporated to tether H₄py4pa to Trastuzumab. Radiolabeling studies, in vitro serum stability, and animal studies were performed in parallel with the DOTA-benzyl-Trastuzumab. Both displayed excellent in vivo stability and tumor specificity.

An insight into the non-covalent Pb⋯S and S⋯S interactions in the solid-state structure of a hemidirected lead(ii) complex

A hemi-directed copper(ii)/lead(ii) complex has been synthesized and characterized. The energy of chalcogen–chalcogen and tetrel bonding interactions in this complex was analyzed by DFT calculations.

Supramolecular Assemblies in Pb(II) Complexes with Hydrazido-Based Ligands

Herein, we describe the synthesis and single crystal X-ray diffraction characterization of several Pb(II) complexes using Schiff base hydrazido-based ligands and different counterions (NO3−, I– and ClO4). In the three complexes reported in this work, the lead(II) metal exhibits a high coordination number (n > 8) and thus it is apparently not involved in tetrel bonding interactions. Moreover, the aromatic ligands participate in noncovalent interactions that play an important role in the formation of several supramolecular assemblies in the solid state of the three Pb(II) complexes. These assemblies have been analyzed by means of Hirshfeld surface analysis and DFT calculations.

Synthesis, structure and metal ion coordination of novel benzodiazamacrocyclic ligands bearing pyridyl and picolinate pendant side-arms

Complex formation of benzodiazacrown ethers with heavy and transition metal ions was studied using NMR spectroscopy, potentiometry and X-ray crystallography.

Rapid Dissolution of BaSO4 by Macropa, an 18-Membered Macrocycle with High Affinity for Ba2

Insoluble BaSO4 scale is a costly and time-consuming problem in the petroleum industry. Clearance of BaSO4-impeded pipelines requires chelating agents that can efficiently bind Ba2+, the largest nonradioactive +2 metal ion. Due to the poor affinity of currently available chelating agents for Ba2+, however, the dissolution of BaSO4 remains inefficient, requiring very basic solutions of ligands. In this study, we investigated three diaza-18-crown-6 macrocycles bearing different pendent arms for the chelation of Ba2+ and assessed their potential for dissolving BaSO4 scale. Remarkably, the bis-picolinate ligand macropa exhibits the highest affinity reported to date for Ba2+ at pH 7.4 (log K' = 10.74), forming a complex of significant kinetic stability with this large metal ion. Furthermore, the BaSO4 dissolution properties of macropa dramatically surpass those of the state-of-the-art ligands DTPA and DOTA. Using macropa, complete dissolution of a molar equivalent of BaSO4 is reached within 30 min at room temperature in pH 8 buffer, conditions under which DTPA and DOTA only achieve 40% dissolution of BaSO4. When further applied for the dissolution of natural barite, macropa also outperforms DTPA, showing that this ligand is potentially valuable for industrial processes. Collectively, this work demonstrates that macropa is a highly effective chelator for Ba2+ that can be applied for the remediation of BaSO4 scale.

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.

Non-covalent tetrel bonding interactions in hemidirectional lead(ii) complexes with nickel(ii)-salen type metalloligands

Tetrel bonding interactions have been investigated in hetero-dinuclear nickel(ii)/lead(ii) complexes using MEP and DFT calculations.

On the importance of Pb⋯X (X = O, N, S, Br) tetrel bonding interactions in a series of tetra- and hexa-coordinated Pb(ii) compounds

We study Pb⋯X (X = N, S, Br) tetrel bonding interactions in a series of tetra- and hexa-coordinated Pb(ii) compounds that exhibit hemidirectional coordination.

Bis(picolinato) complexes of vanadium and zinc as potential antidiabetic agents: synthesis, structural elucidation and in vitro insulin-mimetic activity study

The studied vanadium(iv), vanadium(v) and zinc(ii) complexes show inhibition of the free fatty acid release from rat adipocytes.

Vanadium and zinc complexes of 5-cyanopicolinate and pyrazine derivatives: synthesis, structural elucidation and in vitro insulino-mimetic activity study

Inhibition of free fatty acid release from rat adipocytes was observed for vanadium(iv), vanadium(v) and zinc(ii) complexes.

[Cd(H2O)6]@{Cd6Cl4(nico)12[Hg(Tab)2(μ-Cl)]2}: a heterometallic host–guest icosidodecahedron cage via hierarchical assembly

A reaction of [Hg(Tab)₂(nico)](PF₆) (Tab = 4-(trimethylammonio)benzenethiolate, nico = nicotinate) with equimolar CdCl₂·2.5H₂O afforded a unique heterometallic cage complex [Cd(H₂O)₆]@{Cd₆Cl₄(nico)₁₂[Hg(Tab)₂(μ-Cl)]₂}.

The role of the capping bond effect on pyclen natY3+/90Y3+ chelates: full control of the regiospecific N-functionalization makes the difference

A dissymmetric pyclen based ligand shows astonishing ^natY³⁺ and ⁹⁰Y³⁺ complexation properties.

Nano drug delivery systems: a new paradigm for treating metal toxicity

INTRODUCTION

The standard medical treatment for metal toxicity is chelation therapy. Chelating agents work by forming less toxic complexes with the toxic metal ions which are readily excreted from the body. These compounds, based on their hydrophilic/lipophilic property, can either remove toxic metal ions from extracellular sites or can penetrate the intracellular compartments to facilitate the removal of toxic metal ions. However, there are various disadvantages associated with this kind of therapy, notably, selectivity. Other problems and challenges are that the therapy regime is expensive, time consuming and has poor patient compliance. Two chelating agents, dimercaptosuccinic acid (DMSA) and dimercaptopropionicsulfonate (DMPS) have gained increased acceptance among clinicians, undoubtedly improving the management of metal intoxications.

AREAS COVERED

The present review provides an insight into the conventional chelating agents, new chelators under development, and the new opportunities presented by the use of nanotherapy for the treatment of metal poisoning cases.

EXPERT OPINION

Today's research should not only focus towards development of alternate chelators but also targeted therapy such as the nanotherapy.

Polyether complexes of groups 13 and 14

Notable aspects of the chemistry of polyether complexes of group 13 and 14 elements are reviewed.

Coordination of lead(II) in the supramolecular environment provided by a "two-story" calix[6]arene-based N6 ligand

First insights into the coordination properties and host-guest behavior of a "two story" calix[6]aza-cryptand (1) are described. The ligand is constituted of a triazacyclononane (TACN) cap and three pyridine (PY) spacers connected to the calix[6]arene small rim. The resulting N6 donor site coordinates Pb(II) ions to give complexes that are highly stable. X-ray diffraction structures reveal a hemidirected environment for Pb(II) with strong coordination to the TACN cap and weaker bonds with the three PY residues. A guest molecule, either water or EtOH, sitting in the calixarene macrocycle and hydrogen-bonded to the phenoxyl units at the level of the small rim further stabilizes the complexes through electrostatic interactions with the metal center and the calixarene core. In-depth (1)H NMR studies confirm the host-guest behavior of the complexes in solution, with evidence of embedment of neutral guest molecules such as EtOH, BuOH, and N-Me-formamide. Hence, in spite of the presence of a N6 donor, the calixarene macrocycle can be open to guest interaction, giving rise to seven-coordinate dicationic complexes. Noteworthy also is the flexibility of the macrocycle that allows Pb(II) to adopt its preferred hemidirected environment in spite of the three covalent links connecting the calixarene core to the three PY groups. The flexibility of the system is further illustrated by the possible coordination of an exogenous anionic ligand in the exo position. Hence, compared to the previously described "one story" calix[6]aza-cryptands, ligand 1 displays several similar but also new features that are discussed.