Complex Formation of 2, 6-Bis-(2?-hydroxyphenyl)pyridine with AlIII, FeIII and CuII

An Iron Pincer Complex in Four Oxidation States

The synthesis and characterization of a series of homoleptic iron complexes [Fe(^benzNHCOCO)₂]^2-/1-/0/1+ supported by the tridentate bis-aryloxide benzimidazolin-2-ylidene pincer ligand ^benzNHCOCO^2- (II) is presented. While the reaction of 2 equiv of free ligand II with a ferrous iron precursor leads to the isolation of the coordination polymer [Fe(^benzNHCOCOK)₂]_n (1), treatment of II with ferric iron salts allows for the synthesis and isolation of the mononuclear, octahedral bis-pincer compound K[Fe(^benzNHCOCO)₂] (2) and its crown-ether derivative [K(18c6)(THF)₂][Fe(^benzNHCOCO)₂] (3). Electrochemical studies of 2 suggested stable products upon further one- and two-electron oxidation. Hence, treatment of 2 with 1 equiv of AgPF₆ yields the charge-neutral species [Fe(^benzNHCOCO)₂] (4). Similarly, the cationic complex [Fe(^benzNHCOCO)₂]PF₆ (5) is obtained by addition of 2 equiv of AgPF₆. The characterization of complexes 1, 3, and 4 reveals iron-centered reduction and oxidation processes; thus, preserving the dianionic, closed-shell structure of both coordinated ^benzNHCOCO pincer chelates, II. This implies a stabilization of a highly Lewis acidic iron(IV) center by four phenolate anions rather than charge distribution across the ligand framework with a lower formal oxidation state at iron. Notably, the overall charge-neutral iron(IV) complex undergoes reductive elimination of the pincer ligand, providing a metal-free compound that can be described as a spirocyclic imidazolone ketal (6). In contrast, the ligand-metal bonds in 5, formally an iron(V) complex, are considerably covalent, rendering the assignment of its oxidation state challenging, if not impossible. All compounds are fully characterized, and the complexes' electronic structures were studied with a variety of spectroscopic and computational methods, including single-crystal X-ray diffraction (SC-XRD), X-band electron paramagnetic resonance (EPR), and zero-field ⁵⁷Fe Mössbauer spectroscopy, variable-field and variable-temperature superconducting quantum interference device (SQUID) magnetization measurements, and multi-reference ab initio (NEVPT2/CASSCF) as well as density functional theory (DFT) studies. Taken altogether, the electronic structure of 5 is best described as an iron(IV) center antiferromagnetically coupled to a ligand-centered radical.

Synthesis and Study of Multifunctional Cyclodextrin-Deferasirox Hybrids

Metal dyshomeostasis is central to a number of disorders that result from, inter alia, oxidative stress, protein misfolding, and cholesterol dyshomeostasis. In this respect, metal deficiencies are usually readily corrected by treatment with supplements, whereas metal overload can be overcome by the use of metal-selective chelation therapy. Deferasirox, 4-[(3Z,5E)-3,5-bis(6-oxo-1-cyclohexa-2,4-dienylidene)-1,2,4-triazolidin-1-yl]benzoic acid, Exjade, or ICL670, is used clinically to treat hemosiderosis (iron overload), which often results from multiple blood transfusions. Cyclodextrins are cyclic glucose units that are extensively used in the pharmaceutical industry as formulating agents as well as for encapsulating hydrophobic molecules such as in the treatment of Niemann-Pick type C or for hypervitaminosis. We conjugated deferasirox, via an amide coupling reaction, to both 6A -amino-6A -deoxy-β-cyclodextrin and 3A -amino-3A -deoxy-2A (S),3A (S)-β-cyclodextrin, at the upper and lower rim, respectively, creating hybrid molecules with dual properties, capable of both metal chelation and cholesterol encapsulation. Our findings emphasize the importance of the conjugation of β-cyclodextrin with deferasirox to significantly improve the biological properties and to decrease the cytotoxicity of this drug.

Design, synthesis and evaluation of a novel metal chelator as multifunctional agents for the treatment of Alzheimer's disease

A series of compounds following the lead compounds including deferasirox and tacrine were designed, synthesized and evaluated as multifunctional agents against Alzheimer's disease (AD). In vitro studies showed that most synthesized compounds exhibited good multifunctional activities in inhibiting acetylcholinesterase (bAChE), and chelating metal ions. Especially, compound TD_e demonstrated significant metal chelating property, a moderate acetylcholinesterase (AChE) inhibitory activity and an antioxidant activity. Results from the molecular modeling indicated that TD compounds were mixed-type inhibitor, binding simultaneously to the catalytic anionic site (CAS) and the peripheral anionic site (PAS) of TcAChE. Moreover, TD_e showed a low cytotoxicity but a good protective activity against the injury caused by H₂O₂. These results suggest that TD compounds might be considered as attractive multi-target cholinesterase inhibitor and will play important roles in the treatment of AD.

Crystal structure of 2,6-bis(2-hydroxy-5-methylphenyl)-4-phenylpyridinium bromide dichloromethane hemisolvate hemihydrate

The asymmetric unit in the structure of the title compound, C₂₅H₂₂NO₂⁺·Br ⁻·0.5CH₂Cl₂·0.5H₂O, comprises two pseudosymmetry-related cations, two bromide anions, a di­chloro­methane molecule and a water mol­ecule of solvation. The two independent cations are conformationally similar with the comparative dihedral angles between the central pyridine ring and the three benzene substituent rings being 3.0 (2), 36.4 (1) and 24.2 (1)°, and 3.7 (2), 36.5 (1) and 24.8 (1)°, respectively. In the crystal, the cations, anions and water mol­ecules are linked through O—H⋯O and O—H⋯Br hydrogen bonds, forming an insular unit. Within the cations there are also intra­molecular N—H⋯O hydrogen bonds. Adjacent centrosymmetrically related aggregates are linked by π–π stacking inter­actions between the pyridine ring and a benzene ring in both cations [ring-centroid separations = 3.525 (3) and 3.668 (3) Å], forming chains extending across the ac diagonal. Voids between these chains are filled by dichloromethane molecules.

A boronate prochelator built on a triazole framework for peroxide-triggered tridentate metal binding

Iron chelating agents have the potential to minimize damage associated with oxidative stress in a range of diseases; however, this potential is countered by risks of indiscriminant metal binding or iron depletion in conditions not associated with systemic iron overload. Deferasirox is a chelator used clinically for iron overload, but also is cytotoxic to cells in culture. In order to test whether a prodrug version of deferasirox could minimize its cytotoxicity but retain its protective properties against iron-induced oxidative damage, we synthesized a prochelator that contains a self-immolative boronic ester masking group that is removed upon exposure to hydrogen peroxide to release the bis-hydroxyphenyltriazole ligand deferasirox. We present here the synthesis and characterization of this triazole-based, self-immolative prochelator: TIP (4-(5-(2-((4-boronobenzyl)oxy)phenyl)-3-(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl)benzoic acid). TIP does not coordinate to Fe(3+) and shows only weak affinity for Cu(2+) or Zn(2+), in stark contrast to deferasirox, which avidly binds all three metal ions. TIP converts efficiently in vitro upon reaction with hydrogen peroxide to deferasirox. In cell culture, TIP protects retinal pigment epithelial cells from death induced by hydrogen peroxide; however, TIP itself is more cytotoxic than deferasirox in unstressed cells. These results imply that the cytotoxicity of deferasirox may not derive exclusively from its iron withholding properties.

Tris[3,6-di-tert-butyl-1-(isoquinolin-1-yl)naphthalen-2-olato-κ2N,O]aluminium(III) toluene sesquisolvate

The central Al(III) atom of the title compound, [Al(C(27)H(28)NO)(3)]·1.5C(7)H(8), has octahedral geometry in which the three N atoms are arranged in a meridional fashion. One of the toluene solvent molecules is located on a general position, while the second is disordered around a centre of inversion. The ligand molecule has axial chirality, and two of the three ligands in the complex exhibit the same stereochemistry. The three independent chelate rings exhibit significantly different bite angles at the metal atom, with one [83.72 (8)°] notably smaller than the other two [87.22 (8) and 87.13 (8)°]. Calculation of the solid angle covered by the ligands at the metal atom reveals that coverage is greatest for the ligand group with the shortest Al-O bond distance.

Bis[2,6-bis-(2-meth-oxy-phen-yl)pyridinium] di-μ-bromido-bis-[dibromidocuprate(II)]

The title salt, (C₁₉H₁₈NO₂)₂[Cu₂Br₆], was obtained from an attempt to synthesize the copper(II) complex of 2,6-bis­(2-meth­oxy­phen­yl)pyridine (L) from a reaction between CuBr₂ and one equivalent of L in CH₂Cl₂ at room temperature. The resulting compound is the salt of the 2,6-bis­(2-meth­oxy­phen­yl)pyridinium cation and 0.5 equivalents of a hexa­bromido­dicuprate(II) dianion. Both meth­oxy groups of the cationic pyridinium moiety are directed towards the N atom of the pyridine ring as a result of intra­molecular N—H⋯O hydrogen bonds. The centrosymmetric hexabromidodicuprate dianion possesses a distorted tetra­hedral geometry at the copper ion. The Cu—Br bond lengths are 2.3385 (7) and 2.3304 (7) Å for the terminal bromides, whereas the bond length between the Cu atom and two bridging bromides is slightly longer [2.4451 (6) Å].