Amino acid-containing reduced Schiff bases as the building blocks for metallasupramolecular structures

Water-soluble chiral coordination polymers of Li+, Na+, K+, and Ba2+ with an anionic iron(III) complex of a L-threonine derivative and a significant red shift of visible spectra with Al3+ salt

Four salts of an anionic iron(III) bis-complex, [Fe(LL-thr)2]1-, were synthesized from water or methanol. H2LL-thr is a tridentate ligand derived from the L-threonine amino acid, and the cations used are Li+ (1), Na+ (2), K+ (3), and Ba2+ (4). Single-crystal X-ray diffraction showed that all the complexes are coordination polymers of different dimensionalities. The iron(III) complex binds to cations through its coordinated phenolate and non-coordinated carboxylate oxygen atoms. While Li+ forms a linear chain, all others have a pair of bridged cations intervening the iron(III) complexes. The 3D network of Ba2+ salt has a sizeable solvent-accessible space occupied by aquated chloride ions. The differences in circular dichroism (CD) spectra and significantly lower conductance values in water and methanol support partial retention of the polymeric nature in methanol. The visible spectra of 4 in methanol or water showed an ∼10 nm shift of the charge transfer bands from 3. However, the addition of Al3+ salt to 2 showed a significant colour shift. Further investigation confirmed that the colour shift is due to partial protonation of the complex with protons generated from salt hydrolysis. Most reports on visual aluminium detection consider aluminium's binding as the shift's source. The present results show that protonation due to hydrolysis of aluminium salt can skew the observation.

Synthesis and Characterization of Schiff Base Polymers via Metal Coordination and Its Application in Infrared Stealth Coating

In order to reduce the infrared emissivity to meet the requirements of modern warfare for infrared stealth materials, we prepared the polymers containing Schiff base moieties using polyetheramine and 2,6-pyridinedicarboxaldehyde by solution polycondensation and coordinated with Ni2+, Cu2+, and Sm3+ ions to prepare organic coatings. The structure and the thermal and mechanical properties of the coatings were studied in detail. Meanwhile, the effect of the conductivity change of coordination polymers on infrared emissivity was studied systematically. The results showed the polymer coordinated with Sm3+ ions had the lowest energy band gap, which was 2.99 eV, and the best electrical conductivity of 3.54 × 10−4 S/cm compared with Ni2+ and Cu2+ coordination polymers. The infrared emissivity was the lowest in the 2–22 μm infrared waveband range, which reached 0.58, suggesting the polymers containing Schiff base moieties and their coordination polymers may have a great potential to be applied as infrared stealth materials in military applications.

Iminium-Bridged Resorcinol-Silane Networks and Their Pyrolyzed Derivatives as Electrode Materials for the Electrocatalytic Oxygen Reduction Reaction and Supercapacitors

We report the synthesis of iminium-bridged resorcinol-silane (I-R-S) gel networks through the reaction of resorcinol with 3-aminopropyl triethoxysilane (APTES) in the presence of acetone under ambient conditions. Ambient pressure drying leads to monolithic gels with minimal shrinkage and cracks, eliminating the use of conventional supercritical drying. The gels were found to have bulk and skeletal densities of 0.90 and 1.23 g/cm³, respectively. Interestingly, it was found that acetone plays the dual role of solvent (solubilizing the precursors, APTES, and resorcinol) and reactant by reacting with amine present in APTES to form imine functionalities. Furthermore, the imine was found to react with resorcinol to form I-R-S gel networks. The I-R-S gel was found to be hydrophobic with a contact angle of between 99 and 103°. Upon pyrolysis at higher temperatures (600, 800, and 1000 °C), the I-R-S gels transformed to silica containing N-doped carbon as confirmed by X-ray photoelectron spectroscopy (XPS). Further removal of silica by hydrofluoric acid (HF) treatment lead to porous N-doped carbon which was found to effectively electrocatalyze the oxygen reduction reaction (ORR) by mixed 3e^- pathway. Note that the sample pyrolyzed at 600 °C followed by treatment with HF displayed an ∼2e^- pathway with a higher peroxide yield of 91%. Due to the high specific surface area and increased microporosity, the pyrolyzed samples treated with HF were found to exhibit a specific capacitance of 213 F/g with capacity retention of 90% for up to 2100 cycles.

Synthesis, structural and electrochemical properties of a new family of amino-acid-based coordination complexes

Metalloproteins involved in oxidation-reduction processes in metabolism are fundamental for the wellbeing of every organism. The use of amino-acid-based compounds as ligands for the construction of biomimetic coordination systems represents a promising alternative for the development of new catalysts. Herein is presented a new family of copper, zinc and nickel coordination compounds, which show four-, five- and six- coordination geometries, synthesized using Schiff base ligands obtained from the amino acids L-alanine and L-phenylalanine. Structural analysis and property studies were performed using single-crystal X-ray diffraction data, spectroscopic and electrochemical experiments and DFT calculations. The analysis of the molecular and supramolecular architectures showed that the non-covalent interactions developed in the systems, together with the identity of the metal and the amino acid backbone, are determinants for the formation of the complexes and the stabilization of the resultant geometries. The Cu^II complexes were tested as candidates for the electrochemical conversion reduction of nitrite to NO, finding that the five-coordinate L-phenylalanine complex is the most suitable. Finally, some insights into the rational design of ligands for the construction of biomimetic complexes are suggested.

Supramolecular Helical Assemblies of Dirhodium(II) Paddlewheels with 1,4-Diazabicyclo[2.2.2]octane: A Remarkable Substituent Effect on the Helical Sense Preference and Amplification of the Helical Handedness Excess of Metallo-Supramolecular Helical Polymers

We report unique coordination-driven supramolecular helical assemblies of a series of dirhodium(II) tetracarboxylate paddlewheels bearing chiral phenyl- or methyl-substituted amide-bound m-terphenyl residues with triethylene glycol monomethyl ether (TEG) or n-dodecyl tails through a 1:1 complexation with 1,4-diazabicyclo[2.2.2]octane (DABCO). The chiral dirhodium complexes with DABCO in CHCl₃/n-hexane (1:1) form one-handed helical coordination polymers with a controlled propeller chirality at the m-terphenyl groups, which are stabilized by intermolecular hydrogen-bonding networks between the adjacent amide groups at the periphery mainly via a cooperative nucleation-elongation mechanism as supported by circular dichroism (CD), vibrational CD, and variable-temperature (VT) absorption and CD analyses. The VT visible-absorption titrations revealed the temperature-dependent changes in the degree of polymerization. The columnar supramolecular helical structures were elucidated by X-ray diffraction and atomic force microscopy. The helix sense of the homopolymer carrying the bulky phenyl and n-dodecyl substituents is opposite those of other chiral homopolymers despite having the same absolute configuration at the pendants. A remarkably strong "sergeants and soldiers" (S&S) effect was observed in most of the chiral/achiral copolymers, while the copolymers of the bulky chiral phenyl-substituted dirhodium complexes with n-dodecyl chains displayed an "abnormal" S&S effect accompanied by an inversion of the helix sense, which could be switched to a "normal" S&S effect by changing the solvent composition. A nonracemic dirhodium complex of 20% enantiomeric excess bearing the less bulky chiral methyl substituents with n-dodecyl chains assembled with DABCO to form an almost one-handed helix (the "majority rule" (MR) effect), whereas the three other nonracemic copolymers showed a weak MR effect.

Large interaction energy for the homodimer and the heterodimer extracted from the supramolecular chain of a bent trinuclear zinc(ii) complex with a reduced Schiff base ligand

The coordinated amino groups in a trinuclear zinc complex participate in strong H-bonding interactions that have been analysed. The dimerization energy is very large for both the homodimer and the heterodimer in the 1D supramolecular chain.

Catechol oxidation promoted by bridging phenoxo moieties in a bis(μ-phenoxo)-bridged dicopper(ii) complex

A dinuclear copper(ii) complex [Cu₂(papy)₂(CH₃OH)₂] has been synthesized by reaction of one equiv. of Cu(OAc)₂·2H₂O with one equiv. of the tetradentate tripodal ligand H₂papy [N-(2-hydroxybenzyl)-N-(2-picolyl)glycine] and has been characterized by various spectroscopic techniques and its solid state structure has been confirmed by X-ray crystal structure analysis. The single-crystal structure of the complex reveals that the two copper centers are hexa-coordinated and bridged by two O-atoms of the phenoxo moieties. The variable temperature magnetic susceptibility measurement of the complex reveals weak ferromagnetic interactions among the Cu(ii) ions with a J value of 1.1 cm⁻¹. The catecholase activity of the complex has been investigated spectrophotometrically using 3,5-di-tert-butyl catechol as a model substrate in methanol solvent under aerobic conditions. The Michaelis–Menten kinetic treatment has been applied using different excess substrate concentrations. The parameters obtained from the catecholase activity by the complex are K_M 2.97 × 10⁻⁴ M, V_max 2 × 10⁻⁴ M s⁻¹, and k_cat 7.2 × 10³ h⁻¹. A reaction mechanism has been proposed based on experimental findings and theoretical calculations. The catechol substrate binds to dicopper(ii) centers and subsequently two electrons are transferred to the metal centers from the substrate. The bridging phenoxo moieties participate as a Brønsted base by accepting protons from catechol during the catalytic cycle and thereby facilitating the catechol oxidation process.

A bis(μ-phenoxo)-bridged dicopper(ii) complex capable of oxidizing catechol with the highest efficiency amongst any bis(μ-phenoxo)-bridged dicopper(ii) complexes is reported.

Cu(ii) and V(iv)O complexes with tri- or tetradentate ligands based on (2-hydroxybenzyl)-l-alanines reveal promising anticancer therapeutic potential

New Cu^II- and V^IVO amino acid complexes show antiproliferative activity mediated by apoptosis and genomic damage.

Equilibria in Aqueous Cobalt(II)-Reduced Schiff Base N-(2-hydroxybenzyl)alanine System: Chemical Characterization, Kinetic Analysis, Antimicrobial and Cytotoxic Properties

The present study describes the coordination properties of a reduced Schiff base, N-(2-hydroxybenzyl)alanine, towards cobalt(II) using potentiometric as well as spectroscopic (UV-Vis and ESI-MS) methods. The results indicate the formation of six mononuclear complexes showing high stability in aqueous solution. Coordination occurs in the {O⁻_phenolic,N,O⁻_carboxyl} and {N,O⁻_carboxyl} chelation modes, depending on the degree of ligand deprotonation. Examination of the complexation equilibria at pH ca 7, which is important from a biological point of view, allowed to identify two species: [CoL] and [CoL₂H]⁻. The kinetic analysis showed a structural change of those cobalt(II) complexes from octahedral to tetrahedral in accordance with a first-order time relationship. The antimicrobial properties of N-(2-hydroxybenzyl)alanine, cobalt(II) nitrate and of the Co(II) – ligand complexes were determined against Gram-positive bacteria (Enterococcus faecalis, Staphylococcus aureus, Staphylococcus epidermidis), Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli, Helicobacter pylori) and a fungal strain (Candida). The results indicate that the complexes are more active for more strains than the ligand alone. Nevertheless, the complexes induce a higher decrease in the metabolic activity of cells but without damage to nuclei. Tetrahedral structures show stronger anti-cellular toxicity than octahedral complexes, which is most likely due to the higher accessibility of the cobalt(II) center.

Representation of a photosensitive Schottky barrier diode made with hetero-dinuclear cobalt(iii)/sodium building blocks

A hetero-dinuclear cobalt(iii)/sodium complex has been synthesized and characterized. The complex which has been used to construct photosensitive Schottky devices, shows interesting C–H⋯π(N₃) interactions in the solid state.

A theoretical insight on the rigid hydrogen-bonded network in the solid state structure of two zinc(ii) complexes and their strong fluorescence behaviors

A reduced Schiff base used as a fluorescence chemo-sensor for selective detection of zinc(ii). Strong fluorescence behaviors of two zinc(ii) complexes are correlated with the presence of a rigid hydrogen-bonded network in their solid state structure.

Mechanosynthesis of N-Methyl Imines Using Recyclable Imidazole-Based Acid-Scavenger: In Situ Formed Ionic Liquid as Catalyst and Dehydrating Agent

1,1′-(1,4-Butanediyl)bis(imidazole) was prepared by a modified method and its application as an efficient promoter was demonstrated for the mechanosynthesis of N-methyl imines using ball milling as a non-conventional process under solvent-free conditions. In this new protocol design, the bis-imidazole acted as a recyclable acid-scavenging agent. This efficient approach to the N-methyl imines displays a combination of the synthetic virtues of a non-conventional condensation reaction with ecological benefits and convenience of a facile mechanosynthetic process. The current method has advantages such as reduced waste by avoiding solvent, exclusion of hazardous materials during the reaction, elimination of handling an anhydrous gas in an evacuated container or a solution of methylamine in ethanol, good yields for relatively unreactive benzaldehydes containing electron-donating substituents, short reaction times, and metal- and acid-free conditions. Furthermore, the promoter was easily regenerated and reused several times with no significant loss of activity.

A series of trinuclear zinc(ii) complexes with reduced Schiff base ligands: turn-off fluorescent chemosensors with high selectivity for nitroaromatics

A series of trinuclear zinc(ii) complexes have been synthesized and characterized by X-ray crystallography. The ability of the complexes to act as sensors for the detection of nitroaromatics in DMF has been assessed.

Encapsulation of a Water Octamer Chain in a Chiral 2D Sheetlike Supramolecular Coordination Network Composed of Dinickel-Dicarboxylate Subunits

Four new chiral supramolecular coordination networks of Ni(II) of general formula [Ni₂(Hhissal)₂(dicarboxylate)(H₂O)₂]·nH₂O (where Hhissal = histidinesalicylate; dicarboxylate = adipate; n = 8 for 1, succinate; n = 4 for 2, maleate; n = 4 for 3, fumarate; and n = 6 for 4) are reported. On the basis of the single-crystal X-ray study, an unprecedented zig-zag chain structure of water octamer encapsulated in 1 has been identified. The supramolecular network of the dimetal subunits is formed through hydrogen bonding interactions between the amine N-H of Hhissal and the oxygen atom of the coordinated water molecule of one subunit with the uncoordinated oxygen atom and the coordinated oxygen atom of the carboxylate group of Hhissal of the next subunit, respectively. The strength of hydrogen bonding within this water cluster (the range of O···O distances is 2.702-2.760 Å) is similar to that found in ice. These networks are further characterized by elemental analysis, IR spectroscopy, powder X-ray diffraction, polarimetry, UV-vis/diffuse reflectance and circular dichroism spectroscopy, and thermogravimetric analysis. A comparison of their properties indicates that these are isostructural with a variation of encapsulated water clusters.

Supramolecular Assemblies, Luminescence and Anion Exchange of Helical Pb(II) Complexes Based on Enantiomeric N-(2-pyridylmethyl) Alanine Ligands

Two pairs of PbII coordination polymers have been synthesised using pyridine-functionalised amino acid derivatives N-(2-pyridylmethyl)-L(D)-alanine hydrochlorides [(L-Hpala)Cl or (D-Hpala)Cl] and PbII salts. Their X-ray structures confirm their formulations as [Pb(L-pala) (NO3)]n, [Pb(D-pala)(NO3)]n, [Pb(L-pala)(Cl)]n and [Pb(D-pala)(Cl)]n. They show similar coordination environments of PbII cations with two kinds of five-membered chelating rings, which are further assembled into 1D helical chains through the bridging carboxylates from amino acids. Thermal and fluorescent properties of these complexes have also been investigated.

Studies on some metal complexes of quinoxaline based unsymmetric ligand: Synthesis, spectral characterization, in vitro biological and molecular modeling studies

Mononuclear Co(II), Ni(II), Cu(II) and Zn(II) complexes of an unsymmetric Schiff base ligand, 3-(-(3-(-3,5-dichloro-2-hydroxybenzylideneamino)propylimino)methyl)quinoxalin-2(1H) -one (L) were synthesized and characterized by various analytical and spectral techniques. The molar conductance values of metal complexes indicate non-electrolytic behavior of the metal complexes. The Schiff base act as tetra dentate ONNO donor ligand in Co(II), Ni(II), Zn(II) complexes and tridentate NNO donor in Cu(II) complex. Thermal stabilities of the newly synthesized compounds were determined by thermal analysis. Crystallinity, average grain size and unit cell parameters were determined from powder X-ray diffraction study. Electrochemical behaviors of the compounds were examined by cyclic voltammetry technique. The Schiff base and its complexes have been screened for their in vitro antimicrobial activities against some bacterial and fungal strains by disc diffusion method. The interaction of the compounds with calf thymus DNA (CT DNA) has been investigated by electronic absorption spectral titration and viscosity measurement (hydrodynamic) methods. Furthermore, the pUC18 DNA cleavage activities of the complexes have been explored. The compounds were also subjected to in vitro antioxidant, anticancer activity screening, druglikeness and bioactivity predictions using Molinspiration software. Molecular docking studies of the present compounds were carried out against B-DNA dodecamer d(CGCGAATTCGCG)2 and vascular endothelial growth factor receptor (VEGFR-2) kinase. Quantum chemical calculations were done with DFT method to determine the optimum geometry of the ligand and its metal complexes. From the quantum chemical parameters, the reactivity parameters of the compounds were established.

Emerging enantiomeric resolution materials with homochiral nano-fabrications

The major scientific challenge of enantiomeric separation is to develop simple, rapid, and sensitive routine analytical methods. Generally, enantio-resolution is still based on "three-point interaction" theory, which indicates that homochiral sites are needed for enantio-selective interaction. However, in recent years, advanced materials with precise homochiral fabrication at the nanoscale have been synthesized, and have shown great potential in development of high-throughput enantio-resolution methods. This tutorial review summarizes fabrication and applications of homochiral materials for enantio-selective recognition and separation. These materials, which include intrinsic and restructured chiral metal surfaces, plasmonic nanostructures, coordination polymers, organic polymer sensors, and molecularly imprinted polymers, have been applied as sensors or chiral stationary phases (CSPs) for efficient enantio-resolution.