Metal-ligand Coordination Research Articles

Design and fabrication of mechanically strong and self-healing rubbers via metal-ligand coordination bonds as dynamic crosslinks

Developing mechanically strong and efficient self-healing rubbers is booming and shows great potential in a wide range of applications. Herein, by introducing pyridine ligands onto rubber chains via ring-opening reaction between epoxy groups and aminopyridine, we constructed dynamic Fe3+-pyridine coordination bonds in commercially available epoxidized natural rubber (ENR). The reversible nature of the coordination bonds endows the rubbers with efficient self-healing behavior under moderate conditions. Meanwhile, the coordination bonds can also serve as crosslinking points, which enhanced mechanical properties of the fabricated rubbers. When the molar ratio of Fe3+ to pyridine was 1 : 4, the tensile strength and 100% modulus reached 2.23 and 0.78 MPa, which are 18-times and 8-times that of the neat sample. Noteworthy, the sample also shows excellent self-healing capability with healing efficiency of 87% (tensile strength). Furthermore, by embedding conductive materials, we designed a conductive rubber with sandwich structure, which exhibits sensitive strain sensing behavior in detecting small strains.

Photoresponsive supramolecular coordination polyelectrolyte as smart anticounterfeiting inks

While photoluminescence printing is a widely applied anticounterfeiting technique, there are still challenges in developing new generation anticounterfeiting materials with high security. Here we report the construction of a photoresponsive supramolecular coordination polyelectrolyte (SCP) through hierarchical self-assembly of lanthanide ion, bis-ligand and diarylethene unit, driven by metal-ligand coordination and ionic interaction. Owing to the conformation-dependent photochromic fluorescence resonance energy transfer between the lanthanide donor and diarylethene acceptor, the ring-closure/ring-opening isomerization of the diarylethene unit leads to a photoreversible luminescence on/off switch in the SCP. The SCP is then utilized as security ink to print various patterns, through which photoreversible multiple information patterns with visible/invisible transformations are realized by simply alternating the irradiation with UV and visible light. This work demonstrates the possibility of developing a new class of smart anticounterfeiting materials, which could be operated in a noninvasive manner with a higher level of security.

MONO-NUCLEATED TRANSITION METAL (M2+) COMPLEXES OF A NEW SCHIFF-BASE AS LIGAND: SYNTHESIS, STRUCTURAL CHARACTERIZATION AND ANTI-MICROBIAL ACTIVITY

We have synthesized a new class of Schiff-base Ligands with m-pthaladehyde and 3-amino benzoic acid. This adduct act as bi-dentate (N8#x38;O) ligand in the formation of mononuclear transition metal complexes of nickel (II), zinc (II), copper (II), cobalt (II), and cadmium (II). Ligand and complexes are characterized and their formation was confirmed by spectroscopic techniques i.e. 1HNMR, FTIR, and UV-Vis spectroscopy. We have synthesized Ligand-metal coordination sphere [M2+(L)2] (where M = Cu(II), Co(II), Cd(II),Ni(II)and Zn(II)) and L=3,3'-((1E,1'E)-(1,3 phenylenebis (methanylylidene)) bis (azanylylidene)) dibenzoic acid and (E)-N-((1H-indol-3-yl) methylene)-4H-1,2,4-triazole-4- amine. Both Ligand and their metal complexes showed magnificent antimicrobial property against Pseudomonas aeruginosa (MTCC 741) which causes nosocomial infections and E. coli (MTCC 1610) which causes intestinal infection.

Iron (III) cross-linked thermoplastic nitrile butadiene elastomer with temperature-adaptable self-healing property

Fe cross-linked thermoplastic carboxylated nitrile butadiene rubber (XNBR) materials using Fe-COOH coordination as dynamic cross-links are reported. The transparent and brown Fe cross-linked XNBR (Fe/XNBR) elastomer was prepared by drying of Fe/XNBR organogels, which were fabricated by simple mixing of Fe3+ ethanol solution and XNBR dioxane solution. Tensile tests showed that only a small amount of Fe3+ ions (e.g., COOH/Fe=1/0.086, mol/mol) made the Fe/XNBR had even better tensile strength and stretchability than 2 phr sulphur vulcanized XNBR. Dynamic mechanical analysis demonstrated that the Fe-COOH interaction not only increased the moduli at higher temperatures, but also made the glass transition temperature high-temperature-shifted. In-situ infrared spectra measurements suggested that higher temperatures (e.g, 100°C) generated more trifunctional Fe-COOH cross-links and made the Fe-COOH interaction highly dynamic. The dynamic nature of the Fe-COOH interaction and generation of more trifunctional cross-links at high temperatures endowed fast self-healing and re-molding properties to the Fe/XNBR elastomers. Our result as a proof-of-concept illustrated a simple and cost-efficient way to fabricate self-healable thermoplastic elastomers using metal-ligand coordination in commercial rubbers.

Network-Forming Liquids from Metal-Bis(acetamide) Frameworks with Low Melting Temperatures.

Molten phases of metal-organic networks offer exciting opportunities for using coordination chemistry principles to access liquids and glasses with unique and tunable structures and properties. Here, we discuss general thermodynamic strategies to provide an increased enthalpic and entropic driving force for reversible, low-temperature melting transitions in extended coordination solids and illustrate this approach through a systematic study of a series of bis(acetamide)-based networks with record-low melting temperatures. The low melting temperatures of these compounds are the result of weak coordination bonds, conformationally flexible bridging ligands, and weak electrostatic interactions between spatially separated cations and anions, which collectively reduce the enthalpy and increase the entropy of fusion. Through a combination of crystallography, spectroscopy, and calorimetry, enthalpic trends are found to be dictated by the strength of coordination bonds and hydrogen bonds within each compound, while entropic trends are strongly influenced by the degree to which residual motion and positional disorder are restricted in the crystalline state. Extended X-ray absorption fine structure (EXAFS) and pair distribution function (PDF) analysis of Co(bba)3[CoCl4] [bba = N,N'-1,4-butylenebis(acetamide)], which features a record-low melting temperature for a three-dimensional metal-organic network of 124 °C, provide direct evidence of metal-ligand coordination in the liquid phase, as well as intermediate- and extended-range order that support its network-forming nature. In addition, rheological measurements are used to rationalize differences in glass-forming ability and relaxation dynamics. These results provide new insights into the structural and chemical factors that influence the thermodynamics of melting transitions of extended coordination solids, as well as the structure and properties of coordination network-forming liquids.

Conjugated Polymer Nanoparticles Based on Copper Coordination for Real-Time Monitoring of pH-Responsive Drug Delivery.

Metal coordination-driven composite systems have excellent stability and pH-responsive ability, making them suitable for specific drug delivery in physiological conditions. In this study, an anionic conjugated polymer PPEIDA with a poly(p-phenylene ethynylene) backbone and iminodiacetic acid (IDA) side chains is used as a drug carrier to construct a class of pH-responsive nanoparticles, PPEIDA-Cu-DOX conjugated polymer nanoparticles (CPNs), by taking advantage of the metal coordination interaction of Cu2+ with PPEIDA and the drug doxorubicin (DOX). PPEIDA-Cu-DOX CPNs have high drug loading and encapsulation efficiency (EE), calculated to be 54.30 ± 1.10 and 95.80 ± 0.84%, respectively. Due to the good spectral overlap, Förster resonance energy transfer (FRET) takes place between PPEIDA and the drug DOX, which enables the observation of the loading and the release of DOX from CPNs in an acidic environment by monitoring fluorescence emission changes. Therefore, PPEIDA-Cu-DOX CPNs can also be used in real-time cell imaging to monitor drug release in addition to delivering DOX targeting tumor cells. Compared with free DOX, PPEIDA-Cu-DOX CPNs show a similar inhibition to tumor cells and lower toxicity to normal cells. Our results demonstrate the feasibility and potential of constructing pH-responsive CPNs via metal-ligand coordination interactions for cancer treatment.

In situ mechanical reinforcement of polymer hydrogels via metal-coordinated crosslink mineralization

Biological organic-inorganic materials remain a popular source of inspiration for bioinspired materials design and engineering. Inspired by the self-assembling metal-reinforced mussel holdfast threads, we tested if metal-coordinate polymer networks can be utilized as simple composite scaffolds for direct in situ crosslink mineralization. Starting with aqueous solutions of polymers end-functionalized with metal-coordinating ligands of catechol or histidine, here we show that inter-molecular metal-ion coordination complexes can serve as mineral nucleation sites, whereby significant mechanical reinforcement is achieved upon nanoscale particle growth directly at the metal-coordinate network crosslink sites.

N-Heterocyclic Carbene-Stabilized Gold Nanoparticles: Mono- Versus Multidentate Ligands

Over the past decade, N-heterocyclic carbenes (NHCs) have attracted remarkable attention as metal-coordinating ligands because of their ability to strongly interact with transition metal complexes ...

Flexible, stretchable and magnetic Fe3O4@Ti3C2Tx/elastomer with supramolecular interfacial crosslinking for enhancing mechanical and electromagnetic interference shielding performance

Electromagnetic interference (EMI) and radiation of electronic devices are ubiquitous, which are potentially hazardous to the normal operation of electronic equipment and human health. MXenes are extremely attractive in the preparation of EMI shielding materials due to their excellent metallic conductivity and tunable surface chemistry. Herein, by virtue of the designed nanostructure and regulation of interface interactions, we fabricated flexible Fe3O4@Ti3C2Tx MXene/3,4-dihydroxyphenylacetic acid (DOPAC)-epoxidized natural rubber (ENR) elastomers (FMDE) with 3D segregated interconnected structures. The elaborately designed metal-ligand coordination crosslinking between Fe3O4 nanoparticles and DOPAC ligand molecules provides strong interfacial interactions, resulting in significantly reinforced mechanical properties. Compared with Ti3C2Tx/ENR elastomers, the maximum tensile strength and toughness of FMDE are elevadted by ~306% and 475%, respectively. Moreover, the 3D segregated conductive network constructed by Fe3O4@Ti3C2Tx nanoflakes resulted from volume exclusion effect of ENR latex and the introduction of magnetic Fe3O4 nanoparticles with enhanced electromagnetic wave absorption greatly improved the EMI shielding performance of FMDE, exhibiting an excellent EMI shielding effectiveness of up to 58 dB in the X band (8.2–12.4 GHz) and stable EMI shielding capability during repeated deformations. This work provides a promising strategy for the design and manufacture of novel flexible EMI shielding materials.

Controllable DNA strand displacement by independent metal-ligand complexation.

Introduction of artificial metal–ligand base pairs can enrich the structural diversity and functional controllability of nucleic acids. In this work, we revealed a novel approach by placing a ligand-type nucleoside as an independent toehold to control DNA strand-displacement reactions based on metal–ligand complexation. This metal-mediated artificial base pair could initiate strand invasion similar to the natural toehold DNA, but exhibited flexible controllability to manipulate the dynamics of strand displacement that was only governed by its intrinsic coordination properties. External factors that influence the intrinsic properties of metal–ligand complexation, including metal species, metal concentrations and pH conditions, could be utilized to regulate the strand dynamics. Reversible control of DNA strand-displacement reactions was also achieved through combination of the metal-mediated artificial base pair with the conventional toehold-mediated strand exchange by cyclical treatments of the metal ion and the chelating reagent. Unlike previous studies of embedded metal-mediated base pairs within natural base pairs, this metal–ligand complexation is not integrated into the nucleic acid structure, but functions as an independent toehold to regulate strand displacement, which would open a new door for the development of versatile dynamic DNA nanotechnologies.

Anion assisted extraction of U(VI) in alkylammonium ionic liquid: Experimental and DFT studies

The extraordinary extraction of metal ions in imidazolium ionic liquid phase containing neutral extractants was most often attributed to the cation exchange mechanism in addition to the usual solvation-type mechanism. The cation exchange mechanism results in an irreversible loss of the ionic liquid cation, which eventually leads to the pollution of aqueous phase also. To minimize the limitations of imidazolium based ionic liquid, strongly hydrophobic ionic liquids containing tetra-alkyl ammonium ion have been introduced and studied for actinide separation. Even though these tetra-alkyl ammonium ionic liquids did not undergo cation exchange, but showed exceptional extraction of actinides for the reason, which was not established so far. To unravel the unique role of these hydrophobic ionic liquids, the extraction behavior of U(VI) was studied in a solution of tri-n-octylmethylammonium nitrate ([N1888 ][NO3]) in conjunction with a neutral ligand, 2-hydroxy-N,N-dioctyl acetamide (DOHyA). The extraction of U(VI) was studied as a function of various extraction parameters and the organic phase obtained after extraction was probed by FTIR and Raman spectroscopy to elucidate the mechanism of U(VI) extraction in ionic liquid phase. The result revealed that U(VI) was extracted into ionic liquid phase by a couple of extraction modes, namely through metal–ligand coordination and by loss-less anion exchange mode and these pathways did not cause any undue burden to the organic and aqueous phases, unlike the traditional imidazolium ionic liquids. Further, DFT studies were performed to elucidate the underlying mechanism of U(VI) complexation in ionic liquid phase. The calculated values of binding energy (ΔE) and Gibbs free energy (ΔG) for the complexation of U(VI) with DOHyA in dodecane and tetra-alkyl ammonium ionic liquids medium were found in good agreement with the experimentally determined complexation trends of U(VI) ion with DoHyA. The DFT calculations further established that the cation exchange mechanism by both the ionic liquids, [N1888][NO3] and [N1888][NTf2] was not feasible, but anion exchange mechanism could be possible as observed in the experimental studies.

Di(2-picolyl)amine-functionalized poly(ethylene glycol) hydrogels with tailorable metal–ligand coordination crosslinking

A new class of metallo-hydrogels has been developed using di(2-picolyl)amine (DPA)-functionalized 4-arm polyethylene glycol (4A-PEG-DPAn) polymers crosslinked by metal–ligand coordination.

Adhesion enhancementviathe synergistic effect of metal–ligand coordination and supramolecular host–guest interactions in luminescent hydrogels

We report an approach to achieve adhesion enhancementviathe synergistic effect of metal–ligand coordination and supramolecular host–guest interactions in luminescent hydrogels without affecting their luminescence behavior.

Dynamic tuning of metal–ligand coordination through water molecules to induce multicolor fluorescence variations for humidity monitoring and anti-counterfeiting applications

We proposed a simple and effective strategy to construct a luminescent ratiometric RH sensor that can exhibit fluorescence color changes from blue to yellow upon increasing the RH.

Metal-ligand coordination nanomaterials for radiotherapy: emerging synergistic cancer therapy.

Radiotherapy (RT) plays a central role in curing malignant tumors. However, the treatment outcome is often impeded by low radiation absorption coefficients and radiation resistance of tumors along with normal tissue radio-toxicity. With the development of nanotechnology, nanomaterials in combination with RT offer the possibility to improve the therapeutic efficacy yet reduce side-effects. Metal-ligand coordination nanomaterials, including nanoscale metal-organic frameworks (NMOFs) and nanoscale coordination polymers (NCPs), formed by coordination interactions between inorganic metal ions/clusters with organic bridging ligands, have shown great potential in the field of radiation oncology in recent years in view of their unique advantages including the porous structure, high surface area, periodic frameworks, and diverse selections of both metal ions/clusters and organic ligands. In this review, we summarize the recent advances in NMOF/NCP-mediated synergistic RT in combination with hypoxia relief, chemotherapy, photodynamic therapy, photothermal therapy, chemodynamic therapy or immunotherapy, which emerged in the last 3 years, and describe cooperative enhancement interactions among these synergistic combinations. Moreover, the potential challenges and future prospects of this rapidly growing direction were also addressed.

Orthogonal Self-Assembly of a Two-Step Fluorescence-Resonance Energy Transfer System with Improved Photosensitization Efficiency and Photooxidation Activity.

During the past few decades, fabrication of multistep fluorescence-resonance energy transfer (FRET) systems has become one of the most attractive topics within supramolecular chemistry, chemical biology, and materials science. However, it is challenging to efficiently prepare multistep FRET systems with precise control of the distances between locations and the numbers of fluorophores. Herein we present the successful fabrication of a two-step FRET system bearing specific numbers of anthracene, coumarin, and BODIPY moieties at precise distances and locations through an efficient and controllable orthogonal self-assembly approach based on metal-ligand coordination and host-guest interactions. Notably, the photosensitization efficiency and photooxidation activity of the two-step FRET system gradually increased with the number of energy transfer steps. For example, the two-step FRET system exhibited 1.5-fold higher 1O2 generation efficiency and 1.2-fold higher photooxidation activity than that of its corresponding one-step FRET system. This research not only provides the first successful example of the efficient preparation of multistep FRET systems through orthogonal self-assembly involving coordination and host-guest interactions but also pushes multistep FRET systems toward the application of photosensitized oxidation of a sulfur mustard simulant.

Chemically Modified Biopolymers for the Formation of Biomedical Hydrogels.

Biopolymers are natural polymers sourced from plants and animals, which include a variety of polysaccharides and polypeptides. The inclusion of biopolymers into biomedical hydrogels is of great interest because of their inherent biochemical and biophysical properties, such as cellular adhesion, degradation, and viscoelasticity. The objective of this Review is to provide a detailed overview of the design and development of biopolymer hydrogels for biomedical applications, with an emphasis on biopolymer chemical modifications and cross-linking methods. First, the fundamentals of biopolymers and chemical conjugation methods to introduce cross-linking groups are described. Cross-linking methods to form biopolymer networks are then discussed in detail, including (i) covalent cross-linking (e.g., free radical chain polymerization, click cross-linking, cross-linking due to oxidation of phenolic groups), (ii) dynamic covalent cross-linking (e.g., Schiff base formation, disulfide formation, reversible Diels-Alder reactions), and (iii) physical cross-linking (e.g., guest-host interactions, hydrogen bonding, metal-ligand coordination, grafted biopolymers). Finally, recent advances in the use of chemically modified biopolymer hydrogels for the biofabrication of tissue scaffolds, therapeutic delivery, tissue adhesives and sealants, as well as the formation of interpenetrating network biopolymer hydrogels, are highlighted.

Coordination Polymers Integrating Metalloimmunology with Immune Modulation to Elicit Robust Cancer Chemoimmunotherapy

Nanoscale coordination polymers (NCPs) constructed by metal ions and organic ligands via metalligand bonds have attracted great attention for their biomedical application. Herein, a new type of NCP...

Hammett Relationship in Oxidase-Mimicking Metal-Organic Frameworks Revealed through a Protein-Engineering-Inspired Strategy.

While the unique physicochemical properties of nanomaterials that enable regulation of nanozyme activities are demonstrated in many systems, quantitative relationships between the nanomaterials structure and their enzymatic activities remain poorly understood, due to the heterogeneity of compositions and active sites in these nanomaterials. Here, inspired by metalloenzymes with well-defined metal-ligand coordination, a set of substituted metal-organic frameworks (MOFs) with similar coordination is employed to investigate the relationship between structure and oxidase-mimicking activity. Both experimental results and density functional theory calculations reveal a Hammett-type structure-activity linear free energy relationship (H-SALR) of MIL-53(Fe) (MIL = Materials of Institute Lavoisier) nanozymes, in which increasing the Hammett σm value with electron-withdrawing ligands increases the oxidase-mimicking activity. As a result, MIL-53(Fe) NO2 with the strongest electron-withdrawing NO2 substituent shows a tenfold higher activity than the unsubstituted MIL-53(Fe). Furthermore, the generality of H-SALR is demonstrated for a range of substrates, one other metal (Cr), and even one other MOF type (MIL-101). Such biologically inspired quantitative studies demonstrate that it is possible to identify quantitative structure-activity relationships of nanozymes, and to provide detailed insight into the catalytic mechanisms as those in native enzymes, making it possible to use these relationships to develop high-performance nanomaterials.

Coordination-Assembled Molecular Cages with Metal Cluster Nodes.

Molecular cages have attracted great attention because of their fascinating topological structures and well-defined functional cavities. These discrete cages were usually fabricated by coordination assembly approach, a process employing directional metal-ligand coordination bonds due to the nature of the divinable coordination geometry and the required lability to encode dynamic equilibrium/error-correction. Compared to these coordination molecular cages with mononulcear metal-nodes, an increasing number of molecular cages featuring dinuclear and then polynuclear metal-cluster nodes have been synthesized. These metal-cluster-based coordination cages (MCCCs) combine the merits of both metal clusters and the cage structure, and exhibit excellent performances in catalysis, separation, host-guest chemistry and so on. In this review, we highlight the syntheses of MCCCs and their potential functions that is donated by the metal-cluster nodes.