Metal-ligand Coordination Bonds Research Articles

Metal Complexes of Cobalt Chloride with Vinyl‐, Allyl‐ and Styrylbenzimidazole Ligands: Synthesis, Structure, and Properties

AbstractThree N‐substituted benzimidazole cobalt(II) complexes have been synthesized and fully characterized by elemental analysis, FT‐IR spectroscopy, and X‐ray crystallography. The crystal packing formation features were studied by AIM analysis, and the energy and nature of both weak noncovalent interactions in the crystal and metal–ligand coordination bonds were assessed. These N‐coordinated cobalt(II) complexes were screened for antimicrobial activities against Gram‐negative (Escherichia coli, Bacillus subtilis) and Gram‐positive (Enterococcus durans) bacteria. Minimal inhibitory concentrations (MIC = 6.2 mg/mL) were found for complex cobalt chloride with allylbenzimidazole ligands against E. durans and E. coli. Its antimicrobial activity is several times higher compared to the reference gentamicin. Molecular docking made it possible to consider the nature of binding of cobalt metal complexes with vinyl‐, allyl‐ and styrylbenzimidazole ligands to biological targets and estimate their energy. The binding energy meanings depend on protein and ligand type, and reaches 8 kcal/mol.

Improving the hydrostability of zeolitic imidazolate framework coatings using a facile silk fibroin protein modification method

Zeolitic imidazolate frameworks (ZIFs) are an important subclass of metal-organic frameworks (MOFs) with zeolite-type topology, which can be fabricated under ambient synthesis conditions. However, the applications of ZIFs are commonly limited due to the weak hydrostability of their metal–ligand coordination bonds, particularly under humid and aqueous conditions. In this work, as an example, the hydrolysis behaviours of ZIF-L with a special focus on ZIF-L coatings were tested at aqueous conditions with a wide range of pHs to systematically study and fundamentally understand their structural stability and degradation mechanism. Pristine ZIF-L powder and ZIF-L coatings were severely damaged after only 24 h in aqueous media. Interestingly, the ZIF-L coatings showed two distinct hydrolyzation pathways regardless of pH conditions, exhibiting either a ring-shaped etching or unfolding behaviours. While the ZIF-L powders were hydrolyzed almost identically across all pH conditions. With this new understanding, a facile silk fibroin (SF) protein modification method was developed to enhance the hydrostability of ZIF-L coatings in aqueous media. The effect of protein concentration on surface coating was systemically studied. ZIF-L coating retained its surface morphology after soaking in water and demonstrated switchable super wetting properties and superior separation performance for oil/water mixture. As a result, the quick SF protein modification significantly enhanced the stability of ZIF-L coatings under various pHs, while retaining their switchable wetting property and excellent separation performance.

Strong Second-Harmonic Generation Induced by a Triphenylamine-Based Bismuth-Organic Framework for Photocatalytic Activity Enhancement.

Taking advantage of the well-defined geometry of metal centers and highly directional metal-ligand coordination bonds, metal-organic frameworks (MOFs) have emerged as promising candidates for nonlinear optical (NLO) materials. In this work, taking a photoresponsive carboxylate triphenylamine derivative as an organic ligand, a bismuth-based MOF, Bi-NBC, NBC = 4',4‴,4‴″-nitrilotris(([1,1'-biphenyl]-4-carboxylic acid)) is obtained. Structure determination reveals that it is a potential NLO material derived from its noncentrosymmetric structure, which is finally confirmed by its rarely strong second harmonic generation (SHG) effect. Theoretical calculations reveal that the potential difference around Bi atoms is large; therefore, it leads to a strong local built-in electric field, which greatly facilitates the charge separation and transfer and finally improves the photocatalytic performance. Our results provide a reference for the exploration of MOFs with NLO properties.

Hierarchically Micro- and Mesoporous Zeolitic Imidazolate Frameworks Through Selective Ligand Removal.

A new method to engineer hierarchically porous zeolitic imidazolate frameworks (ZIFs) through selective ligand removal (SeLiRe) is presented. This innovative approach involves crafting mixed-ligand ZIFs (ML-ZIFs) with varying proportions of 2-aminobenzimidazole (NH2-bIm) and 2-methylimidazole (2-mIm), followed by controlled thermal treatments. This process creates a dual-pore system, incorporating both micropores and additional mesopores, suggesting selective cleavage of metal-ligand coordination bonds. Achieving this delicate balance requires adjustment of heating conditions for each mixed-ligand ratio, enabling the targeted removal of NH2-bIm from a variety of ML-ZIFs while preserving their inherent microporous framework. Furthermore, the distribution of the initial thermolabile ligand plays a pivotal role in determining the resulting mesopore architecture. The efficacy of this methodology is aptly demonstrated through the assessment of hierarchically porous ZIFs for their potential in adsorbing diverse organic dyes in aqueous environments. Particularly striking is the performance of the 10%NH2-ZIF-2h, which showcases an astonishing 40-fold increase in methylene blue adsorption capacity compared to ZIF-8, attributed to larger pore volumes that accelerate the diffusion of dye molecules to adsorption sites. This versatile technique opens new avenues for designing micro/mesoporous ZIFs, particularly suited for liquid media scenarios necessitating efficient active site access and optimal diffusion kinetics, such as purification, catalysis, and sensing.

Insight into the enhanced interfacial adhesion of carbon fiber reinforced composites: A facile ferric ion and tannic acid self-assembly strategy

Applications of carbon fiber reinforced polymer composites are subjected the poor interfacial adhesion due to the smooth and chemically inert of CF surface. In this work, we proposed an effective and simple approach to improve the interfacial properties between CF and matrix by self-assembling the ferric ion (Fe3+) and tannic acid (TA) on the carbon fiber surface within minutes. TA can rapidly coordinate with Fe3+ ions into three-dimensional Fe3+-TA complex networks, the morphologies of which were controlled by Fe3+/TA molar ratio and deposition cycles, which played a key role in the interfacial performance. As Fe3+/TA molar ratio is optimized to 4, Fe3+-TA complex aggregated to nano-sized bulges on the fiber surface, affording numerous mechanical interlocking points which can promote the stress transfer from matrix to fiber and induce the deflection of cracks. Moreover, breaking metal–ligand coordination bonds of Fe3+-TA complex as cracks propagate can dissipate lots of fracture energy, leading to the interfacial shear strength of Fe3+-TA complex modified CF/epoxy vinyl resin composites being improved by ∼ 80 % as compared with the pristine sample. This strategy is facile, mild, and eco-friendly, which opens a feasible avenue for enhancing the interfacial adhesion of CFRPs.

Exploiting Chirality in Confined Nanospaces.

Spatial organization using confinement has been of great interest since the early ages of supramolecular chemistry. Application such as sensing, catalysis and delivery are continuously emerging. This minireview highlights the evolution of chiral supramolecular cages (CSC) applications in the fields of catalysis, sensing and chiroptical properties. More in detail, beside the description of the strategies adopted for the preparation of chiral supramolecular cages, either of purely organic supramolecular architectures or prepared using metal-ligand coordination bonds, recent findings on their applications, with particular attention to stereodynamic systems, are presented to highlight the recent scientific interests and the future opportunities.

Generating crosslinking network in XNBR based on copper (I)–carboxylate interaction

AbstractCarboxylated nitrile rubber (XNBR) is crosslinked via metal–ligand coordination bond by simple mixing and compounding as an alternate to chemically rich traditional vulcanization route. Basis of the generation of reversible non‐covalent crosslinks, in the rubber matrix, is copper (I)–carboxylate metal–ligand interaction that is evidenced by XPS, FTIR, and rheological studies. At low copper content (5 phr), self‐healing property is exhibited by the composite while adequate mechanical strength is obtained at higher copper content (20 phr). For 20 phr filled composite (XNBR–Cu20), the tensile strength reaches up to 5.41 ± 0.28 MPa, which is almost 19 times higher than that of pure XNBR (0.29 ± 0.02 MPa). On the other hand, tensile strength is not so high (1.95 ± 0.20 MPa) for composite XNBR–Cu5, however, this one shows the self‐healing efficiency of around 75% for the first cycle, 71% for second cycle, and 56% for the third cycle. The positive shift of glass transition temperature (Tg) takes place with the increasing content of copper (I)–carboxylate crosslinking that is caused by the lowering in segmental mobility of the rubber chains. Reversible nature of the metal–ligand coordination, recoverability, and hysteresis in the XNBR–Cu (I) composite are studied by cyclic stress–strain loading. The CuCl content greatly influences the inherent crosslinking nature of the elastomeric network and the ultimate properties of the composites.

Metal–organic frameworks doped with metal ions for efficient sterilization: Enhanced photocatalytic activity and photothermal effect

Photocatalytic and photothermal disinfection is a promising strategy for addressing the challenges of environmental microbial contamination. In this work, we choose a metal–organic framework (MOF), ZIF-8, as an inexpensive and ideal model for metal ion doping, and manipulate the band structure, thermal vibration in molecules, charge distribution, and robustness of the metal–ligand coordination bond of the metal-ion-doped ZIFs for their use in photo-disinfection. The effects of their absorption edge, rate of the photo-induced temperature rise, transient photocurrent response, photo-generated reactive oxygen species (ROS) type, and crystal stability on the photo-disinfection performance are systematically studied by varying the metal ion type (Co2+, Ni2+, or Cu2+) and doping concentration (1–100%). The results show that the efficiency of light harvesting and photogenerated carrier separation is facilitated in all doped ZIFs. The photothermal conversion gradually improves with the increasing concentration of doped Co2+/Cu2+. Remarkably, the photo-generated ROS type changes from the original singlet oxygen (1O2) to multiple ROS (1O2 and •O2−) due to the introduction of Co(II) sites. Consequently, compared with pristine ZIF-8 and other doped ZIFs, Co2+-doped ZIF-8 with a 5% doping concentration shows the highest sterilization efficiency (6.6 log10 CFU mL−1) against Escherichia coli (E. coli) under simulated sunlight within one hour by virtue of the enhanced photothermal effect and the generation of multiple ROS. This work provides insights into the application of metal-ion-doped MOF photocatalysts to the disinfection of environments with pathogenic microorganisms.

Dramatically Enhanced Reactivity of Fullerenes and Tetrazine towards the Inverse-Electron-Demand Diels-Alder Reaction inside a Porous Porphyrinic Cage.

Inverse-electron-demand Diels-Alder reaction (IEDDA) between fullerenes and 1,2,4,5-tetrazine generally requires harsh conditions and long reaction times due to their strong electron-accepting nature. Herein, we report a dramatic enhancement in the reactivity of the fullerenes (C60 /C70 )-tetrazine reaction inside a porous Zn-porphyrinic cage (Zn-PB) under sustainable conditions by installing a tetrazine-based axle (LA) via metal-ligand coordination bond, which modulates the cavity size to facilitate the encapsulation of fullerenes. Upon encapsulation, the close proximity of fullerenes and the tetrazine group of LA dramatically increase their reactivity towards the IEDDA reaction to form fullerene-tetrazine adducts. Furthermore, the C60 -tetrazine adduct is rearranged upon hydration to a bent-shaped C60 -pyrazoline adduct that can be released from the Zn-PB cavity in the presence of excess LA, thus catalyzing the formation of C60 -pyrazoline adduct inside Zn-PB without product inhibition.

Self-healing and anticorrosion coatings based on responsive polymers with metal coordination bonds

Coatings containing corrosion inhibitors are effective materials for hindering corrosion of metals and alloys structures. However, shortcomings of these coatings include an uncontrolled leaching of inhibitors and susceptibility to mechanical damages, which lead to a shortened anticorrosion property. To overcome these issues, we prepare a new smart coating with the abilities of releasing selectively corrosion inhibitors and repairing autonomously mechanical damages. This dual-function is imparted by a pH-responsive polymer containing a corrosion inhibitor which coordinates cerium ions. Upon corrosion, the polymer releases the corrosion inhibitor and cerium which then forms insoluble cerium oxides or cerium hydroxides on cathodic sites. The corrosion rate of aluminum alloys coated with the polymer conjugate complex is ∼ 31 times lower than for the coating of the polymer conjugate that is not complexed. Self-healing after mechanical damage is enabled by the metal–ligand coordination bond between cerium ions and conjugated 8-hydroxyquinoline (8HQ), leading to a restauration of ∼ 98 % of the anticorrosion properties.

Mechano-Regulable and Healable Silk-Based Materials for Adaptive Applications.

Mechanically adaptive materials responsive to environmental stimuli through changing mechanical properties are highly attractive in intelligent devices. However, it is hard to regulate the mechanical properties of most mechanically adaptive materials in a facile way. Moreover, it remains a challenge to achieve mechano-regulable materials with mechanical properties ranging from high strength to extreme toughness. Here, inspired by the reversible nanofibril network structure of skeletal muscle to achieve muscle strength regulation, we present a mechano-regulable biopolymeric silk fibroin (SF) composite through regulating dynamic metal-ligand coordination bonds by using water molecules as competitive regulators. Efficient interfacial hydrogen bonds between tannic acid-tungsten disulfide nanohybrids and the SF matrix endow the composite with high mechanical strength and self-healing ability. The resulting composite exhibits 837-fold change in Young's modulus (5.77 ± 0.61 GPa to 6.89 ± 0.64 MPa) after water vapor triggering, high mechanical properties (72.5 ± 6.3 MPa), and excellent self-healing efficiency (nearly 100%). The proof-of-concept ultraconformable iontronic skin and smart actuators are demonstrated, thereby providing a direction for future self-adaptive smart device applications.

Room-temperature self-healing supramolecular polyurethanes based on the synergistic strengthening of biomimetic hierarchical hydrogen-bonding interactions and coordination bonds

Developing a new generation of polymeric materials that integrate robust mechanical properties with ultra-high self-healing efficiency at ambient temperature remains a formidable challenge due to the proverbial trade-offs among strength, toughness, and self-healing ability. Herein, inspired by biological systems (titin protein molecular structure and Nereis jaws), a series of updated supramolecular polyurethanes have been successfully synthesized by incorporating hierarchical hydrogen-bonding motifs into the polymer matrix and subsequently coordinated with Zn2+ ions. Relying on collaborative reinforcement of optimized hierarchical hydrogen bonds and metal-ligand coordination bonds, the resulted supramolecular elastomers exhibited a robust strength of ∼14.15 MPa, an excellent toughness of ∼47.57 MJ m−3, and Young’s modulus of ∼146.92 MPa. Benefiting from the rational design of hard domains, high mobility of chains, and the synergistic effect of multiple non-covalent interactions, the mechanical properties are far superior to the previously reported room‐temperature self‐healing materials. In addition, with the structural design of “inner soft and external hard” model, we also fabricated a bilayer polymer film with a gradient distribution that can achieve rapid self-healing, with an unexpectedly high self-healing efficiency of 95% at ambient temperature in 24 h. This work opens up a new avenue for fabricating room-temperature self-healing materials with robust mechanical strength and toughness at the same time.

Switchable Nonlinear Optical Absorption of Metal–Organic Frameworks

AbstractMetal–organic frameworks (MOFs) are widely explored owing to their excellent nonlinear optical (NLO) properties. However, researchers have mainly focused on designing new structures of MOF crystals to adjust the NLO response while ignoring the influence of the number of metal–ligand coordination bonds on the NLO properties of MOFs. In this study, the influence of the coordination numbers of MOFs on their NLO properties is studied for the first time. Herein, MOFs with different coordination numbers, using trifluoroacetic acid as an auxiliary agent, are synthesized and their NLO properties are tested using the Z‐scan technique. The results reveal that the NLO absorption properties of MOFs with high coordination numbers can be effectively modulated from saturable absorption to reverse saturable absorption by increasing the excitation intensity. First‐principles calculations show that a change in the coordination numbers leads to a change in the charge transfer from metal to ligand, thereby resulting in different NLO responses. The MOFs with a high coordination number have potential applications in novel optical switches or logic gates. These results provide a reference for the precise adjustment of the NLO properties of MOFs.

Room temperature Self-healable and extremely stretchable elastomer with improved mechanical Properties: Exploring a simplistic Metal-Ligand interaction

The dynamic supramolecular crosslinking reactions like imine bond formation, disulfide metathesis reactions, Diels Alder reactions, metal–ligand interactions, H-bonding, and ionic interactions are broadly used for the self-healing application of elastomers. Herein, we have developed a new self-healable and extremely stretchable elastomer based on robustness and dynamic metal–ligand coordination bonds between the API ligand and the Zn2+ metal ion moieties incorporated onto the XNBR rubber backbone. The FT-IR analysis confirmed the formation of amide linkage and metal–ligand coordination bonds into the XNBR rubber backbone. The healing performance and mechanical properties of various rubber compounds depend on the dosages of metal ions and the type of crosslinking systems. The metal–ligand crosslink compounds exhibit excellent healing performance over the sulfur crosslinking system. Differential scanning calorimetry, rubber process analyzer, crosslinking density, and morphology studies are indicated to characterize the metal–ligand interactions in the XNBR rubber matrix. The metal–ligand cross-linked XNBR rubber with 3 parts of Zn2+ metal ion demonstrates an excellent healing performance of 91.2%, with the tensile strength of 5.7 ± 0.8 MPa at room temperature for 24 h is much higher than the covalently cross-linked elastomers. The metal–ligand coordination bonds are entirely dynamic and thermoreversible during the rebuilding process and obtain an excellent self-healing property than the sulfur curing system.

One-Pot Synthesis and Characterization of Three Chromotropic Supramolecular Isomeric Cu(II) Coordination Polymers as 1-D Zigzag or Helical Chains with Homochiral or Heterochiral Cu(II) Coordination Centres

The reaction of 1,10-phenanthroline (Phen) and 4′,4′''-(perfluoropropane-2,2-diyl) bis(([1,1′-biphenyl]-4-carboxylic acid)) (H2L2) with Cu(CH3COO)2⋅H2O in DMF/H2O at 50 ∼ 90°C in capped glass vials afforded three products with distinctively different colours and shapes. Single crystal X-ray diffraction analysis revealed that the three products, namely {[Cu(phen)(L2)] ⋅ 1.25H2O} (1) (teal acicula, KPI = 60.6%), {[Cu(phen)(L2)]⋅ H2O⋅DMF}n(2) (sapphire blue plate, KPI = 65.6%) and {[Cu(phen)(L2)]⋅0.31H2O}n (3) (cool grey plate, KPI = 66.8%) are 1-D supramolecular Cu(II) coordination polymers. Both 1 and 2 are composed of 1-D zigzag chains with 1 contains heterochiral Cu(II) (Λ- and Δ- alternatively) centres in the same chain and 2 contains homochiral Cu(II) (Λ- or Δ-) centres in the same chain, while 3, containing homochiral Cu(II) (Λ- or Δ-) centres, has a helical structure along a axis, with a rectangular channel of a size 19.61 × 16.61 Å2. Each helical turn spans 10.74 Å, composed of two [Cu(L2)(Phen)] units. With the largest KPI (∼66.3%), the highest thermal stability, highest formation energy from DFT calculations and lowest hygroscopicity, 3 can be obtained as the only product at higher temperature (95°C) and/or higher ratio of water (DMF/water (v/v) = 20:6), which implies that it is the thermodynamic product. 2 can be obtained at mediate temperature (60 ∼ 80°C) as the only product or co-exist with other products, while 1 only appeared at low temperature (60 and 70 °C) in the solvents of DMF/water (v/v) = 20:2 or 20:3 at a very small amount. The chromotropism of the three isomers was examined using UV-Vis spectroscopic technique, and the coordination geometry of Cu(II) was examined with Hathaway's tetragonality analysis and Continuous Shape Measures Calculation using SHAPE 2.1 programme. Linear correlations between the solid-state UV-Vis absorption maxima of the three isomers against the tetragonality (T) or the distortion percentage of the coordination geometry of Cu(II) from a perfect octahedral (Doct) were observed. The three complexes were also analyzed by FT IR, TGA and DFT formation energy calculations. The thermodynamic stability and the thermal stability of the three isomer increases in order of 1 < 2 < 3, supported by the analysis of the metal-ligand coordination bonds and the intermolecular interactions, the study of chromotropism and crystal fields, the effects of solvent polarity and temperature to the formation of isomers, the close-packing theory (KPI calculations), the DFT calculations and the TGA analysis.

Thermo-Induced Fluorochromism in Two AIE Zinc Complexes: A Deep Insight into the Structure-Property Relationship.

Solid-state emitters exhibiting mechano-fluorochromic or thermo-fluorochromic responses represent the foundation of smart tools for novel technological applications. Among fluorochromic (FC) materials, solid-state emissive coordination complexes offer a variety of fluorescence responses related to the dynamic of noncovalent metal-ligand coordination bonds. Relevant FC behaviour can result from the targeted choice of metal cation and ligands. Herein, we report the synthesis and characterization of two different colour emitters consisting of zinc complexes obtained from N,O bidentate ligands with different electron-withdrawing substituents. The two complexes are blue and orange solid-state fluorophores, respectively, highly responsive to thermal and mechanical stress. These emitters show a very different photoluminescent (PL) pattern as recorded before and after the annealing treatment. Through X-ray structural analysis combined with thermal analysis, infrared (IR) spectroscopy, PL, and DFT simulation we provide a comprehensive analysis of the structural feature involved in the fluorochromic response. Notably, we were able to correlate the on-off thermo-fluorochromism of the complexes with the structural rearrangement at the zinc coordination core.

Chiral Metal-Organic Frameworks.

In the past two decades, metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) assembled from metal ions or clusters and organic linkers via metal-ligand coordination bonds have captivated significant scientific interest on account of their high crystallinity, exceptional porosity, and tunable pore size, high modularity, and diverse functionality. The opportunity to achieve functional porous materials by design with promising properties, unattainable for solid-state materials in general, distinguishes MOFs from other classes of materials, in particular, traditional porous materials such as activated carbon, silica, and zeolites, thereby leading to complementary properties. Scientists have conducted intense research in the production of chiral MOF (CMOF) materials for specific applications including but not limited to chiral recognition, separation, and catalysis since the discovery of the first functional CMOF (i.e., d- or l-POST-1). At present, CMOFs have become interdisciplinary between chirality chemistry, coordination chemistry, and material chemistry, which involve in many subjects including chemistry, physics, optics, medicine, pharmacology, biology, crystal engineering, environmental science, etc. In this review, we will systematically summarize the recent progress of CMOFs regarding design strategies, synthetic approaches, and cutting-edge applications. In particular, we will highlight the successful implementation of CMOFs in asymmetric catalysis, enantioselective separation, enantioselective recognition, and sensing. We envision that this review will provide readers a good understanding of CMOF chemistry and, more importantly, facilitate research endeavors for the rational design of multifunctional CMOFs and their industrial implementation.

A Strong and Rigid Coordination Adaptable Network that Can Be Reprocessed and Recycled at Mild Conditions

A Strong and Rigid Coordination Adaptable Network that Can Be Reprocessed and Recycled at Mild Conditions

Remotely controllable supramolecular rotor mounted inside a porphyrinic cage

Remotely controllable supramolecular rotor mounted inside a porphyrinic cage

Transformation of the coordination nanostructures of 4,4',4''-(1,3,5-triazine-2,4,6-triyl) tribenzoic acid molecules on HOPG triggered by the change in the concentration of metal ions.

The modulation effects of Cu2+/Fe3+ ions on the hydrogen-bonded structure of 4,4′,4′′-(1,3,5-triazine-2,4,6-triyl) tribenzoic acid (TATB) on a HOPG surface have been investigated at the liquid–solid interface by scanning tunneling microscopy (STM). STM observations directly demonstrated that the self-assembled honeycomb network of TATB has been dramatically transformed after introducing CuCl2/FeCl3 with different concentrations. The metal–organic coordination structures are formed due to the incorporation of the Cu2+/Fe3+ ions. Interestingly, a Cu2+ ion remains coordinated to two COOH groups and only the number of COOH groups involved in coordination doubles when the concentration of Cu2+ ions doubled. A Fe3+ ion changes from coordination to three COOH groups to two COOH groups after increasing the concentration of Fe3+ ions in a mixed solution. Such results suggest that the self-assembled structures of TATB molecules formed by metal–ligand coordination bonds can be effectively adjusted by regulating the concentration of metal ions in a mixed solution, which has rarely been reported before. It explains that the regulatory effect of concentration leads to the diversity of molecular architectures dominated by coordination bonds.