Metal-ligand Coordination Bonds Research Articles

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.

H-bonds and metal-ligand coordination-enabled manufacture of palm oil-based thermoplastic elastomers by photocuring 3D printing

Thermoplastic elastomers exhibit high softness, stretchability, recoverability, and reprocessibility, but their manufacture requires high cost and long processing time. Photocuring 3D printing is a remedial processing method as it enables rapid fabrication at room temperature. However, it is difficult to print thermoplastic elastomers by photocuring 3D printing because the generated linear polymers easily dissolve in liquid precursors during printing. Herein, stretchable, self-healing, and renewable palm oil (PO)-based elastomers were prepared from UV-responsive vinyl PO monomers via photocuring 3D printing. Fast solid-liquid separation was achieved via hydrogen bonds and Zn 2+ -ligand coordination (non-covalent crosslinking) to slow down the dissolution of crosslinked polymers in the monomer liquid. The non-covalent bonds endowed the elastomers with high tensile strength (~4.2 MPa) and elongation at break (~851%). Additionally, the elastomers are mainly prepared from biobased, renewable, and low-cost PO feedstocks. Most importantly, the dual dynamic crosslinked network resulted in superior stress relaxation, shape programming, and self-healing behavior of the elastomers. The method described may enhance the application scope of thermoplastic elastomers and 3D printing. Highly stretchable, self-healable, and renewable palm oil (PO)-based thermoplastic elastomers were prepared from a UV curable vinyl PO monomer (POFA-EA) via photocuring 3D printing enabled by the construction of hydrogen bonds and Zn 2+ -ligand coordination. • Thermoplastic elastomers were prepared via simple and efficient photocuring 3D printing. • 3D-printing elastomers were preppared via fast solid-liquid separation by H- and metal-ligand coordination bonds. • The palm oil (PO)-based thermoplastic elastomers had excellent tensile stress (~4.2 MPa) and strain (~851%). • The PO-based thermoplastic elastomers had shape programming and self-healing behavior.

Quantum chemical approach (DFT) of the binary complexation of Hg(II) with L-canavanine and L-arginine.

The experimental study of the complexation of the two amino acids, L-canavanine, and L-arginine, with the mercuric ion Hg(II), was completed by the characterization by a quantum calculation based on the DFT method. This study covers electronic, energetic, and structural aspects in the neutral, deprotonated, and complexed states. The atomic net charges show that the active sites of the carboxyl, guanidyl, and amino groups are the oxygen and nitrogen atoms. In fact, the L-canavanine (Can) gave stable mercuric bidentate chelates via the amino and guanidyl groups. Hg(Can)(H 2 O) 2 , Hg(Can) 2 and Hg(OH)(H 2 O)(Can), while the L-arginine (Arg) resulted in engagement of carboxyl and amino groups to bidentate complexes: Hg(Arg)(H 2 O) 2 , Hg(Arg) 2 , Hg(Arg)(OH)(H 2 O). The metal-ligand coordination bond is more rigid with the guanidyl and carboxyl groups than with the amino group; and the bond formed with the amino group is more rigid in the L-canavanine than L-arginine.

Interfacial Adhesion of Mussel‐Inspired Hydrogels: Interfacial Adhesion of Fully Transient, Mussel‐Inspired Hydrogels with Different Network Crosslink Modalities (Adv. Mater. Interfaces 14/2021)

Fully transient, mussel-inspired hydrogels are supramolecular networks where polymers are crosslinked by reversible, metal-ligand coordination bonds. In article number 2100319, Niels Holten-Andersen, Jonathan T. Pham, and co-workers demonstrate an inverse correlation between tris- coordinate crosslinks and peak adhesive stress for hydrogels comprising either histidine-Ni2+ ion crosslinks or nitrodopamine-Fe3+ ion crosslinks. This exhibits tunability of supramolecular hydrogels for soft materials and interfacial applications.

Exploration of Metal-Ligand Coordination Bonds in Proteins by Single-molecule Force Spectroscopy

Thanks to the binding of various metal ions, metalloprotein plays an essential role in many different biological processes and represents an indispensable protein subgroup. Thus, the knowledge of the incorporated metal site and metal-ligand coordination bond in the protein is invaluable for understanding this critical bio-macromolecule and designing a new one. Over the last decade, atomic force microscopy-based single-molecule force spectroscopy (AFM-SMFS) has been used to explore the metalloprotein as an emerging methodology, focusing on measuring metal-ligand bond strength. By stretching the protein molecule along its peptide backbone, AFM-SMFS can unfold the protein secondary structure and break and quantify the metal-ligand bond/metal cluster. Moreover, the very recent development of enzymatic, site-specific protein conjugation and immobilization methods for the SMFS system enables the highly efficient and accurate measurement of the metal-ligand bond strength in proteins. As a result, comparing the strength among different types of metal-ligand bonds in proteins becomes possible, and the trend of their strength is gradually revealed, such as the Fe(III)-ligand bond in iron-binding proteins. Thus, we envision that in the future AFM-SMFS may provide a unique insight to uncover the general principle of how protein selectively binds to metal ion.

Swelling and Mechanical Properties of Polyacrylamide-Derivative Dual-Crosslink Hydrogels Having Metal-Ligand Coordination Bonds as Transient Crosslinks.

Hydrogels that have both permanent chemical crosslinks and transient physical crosslinks are good model systems to represent tough gels. Such “dual-crosslink” hydrogels can be prepared either by simultaneous polymerization and dual crosslinking (one-pot synthesis) or by diffusion/complexation of the physical crosslinks to the chemical network (diffusion method). To study the effects of the preparation methods and of the crosslinking ratio on the mechanical properties, the equilibrium swelling of the dual-crosslink gels need to be examined. Since most of these gels are polyelectrolytes, their swelling properties are complex, so no systematic study has been reported. In this work, we synthesized model dual-crosslink gels with metal–ligand coordination bonds as physical crosslinks by both methods, and we proposed a simple way of adding salt to control the swelling ratio prepared by ion diffusion. Tensile and linear rheological tests of the gels at the same swelling ratio showed that during the one-pot synthesis, free radical polymerization was affected by the transition metal ions used as physical crosslinkers, while the presence of electrostatic interactions did not affect the role of the metal complexes on the mechanical properties.

Review on the Conceptual Design of Self-Healable Nitrile Rubber Composites.

The search for suitable strategies to manufacture self-healable nitrile rubber (NBR) composites is the most promising part in the industrial field of polar rubber research. In recent years, some important strategies, specifically, metal–ligand coordination bond formation, ionic bond formation, and dynamic hydrogen bond formation, have been utilized to develop duly self-healable NBR composites. This paper reviews the continuous advancement in the research field related to self-healable NBR composites by considering healing strategies and healing conditions. Special attention is given to understand the healing mechanism in reversibly cross-linked NBR systems. The healing efficiency of a cross-linked NBR network is usually dependent on the definite interaction between functional groups of NBR and a cross-linking agent. Finally, the results obtained from successful studies suggest that self-healing technology has incredible potential to increase the sustainability and lifetime of NBR-based rubber products.

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.

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.

Coordination-promoted photoluminescence induced by configuration twisting regulation

In this work, we reported a coordination-promoted photoluminescence (CPPL) phenomenon of a new organometallic complex tBu(ONO)-W formed by ligand tBu(ONO) and metal tungsten for the first time. At the same time, the photoluminescence mechanisms of both the ligand tBu(ONO) and organometallic complex tBu(ONO)-W were investigated by using the steady-state and time-resolved spectroscopies as well as the time-dependent density functional theory (TDDFT) method. It was demonstrated that the weak photoluminescence of ligand tBu(ONO) could be ascribed as the destroyed planarity after significant configuration twisting in the excited state. However, the formation of the coordination bond between tBu(ONO) and the metal tungsten in the complex changes the two twisted parts of tBu(ONO) into a nearly planar configuration. The excited state configuration has a slight configuration change while the coplanarity of tBu(ONO)-W has not been destroyed. Therefore, the photoluminescence of tBu(ONO)-W is significantly enhanced compared to the ligand tBu(ONO). It can be proved that the coordination effect between the ligand and the metal inhibits the configuration twisting of tBu(ONO) and then promotes light emission. Moreover, it was also found that the emission peaks of tBu(ONO)-W are mainly originated from the relaxation of the ILCT and LMCT states. The experimental and theoretical results clearly show that the regulation of metal-ligand coordination bonds on the photoluminescence performance provides a new strategy for designing new high-performance luminescent materials.

Bioinspired design of elastomeric vitrimers with sacrificial metal-ligand interactions leading to supramechanical robustness and retentive malleability

Most elastomeric vitrimers suffer from mechanical weakness in practical applications. Inspired by the development of strong and tough biomaterials relying on sacrificial bond-detachment mechanisms, herein we describe the biomimetic design of elastomeric vitrimers with mechanical robustness, preservable malleability, and recyclability by engineering sacrificial metal-ligand coordination bonds into exchangeable networks. In particular, we use a commercially available metal complex, aluminum acetylacetonate (Al(acac)3), to catalyze cross-linking based on the silylation reaction between hydroxylated natural rubber and hydrosilanes, thus introducing dynamic silyl ether-based architectures into the rubber matrix. At the same time, the Al3+ ions can interact with the free oxygen-containing moieties on the rubber skeleton, enabling labile Al3+O coordination bonds in the covalent framework to substantially dissipate mechanical energy through reversible bond detachment/reattachment upon deformation. As the organic acetylacetonate ligands of Al(acac)3 can facilitate the dispersion of Al3+ ions in the matrix, incorporating a small amount of organometallic complex (0.68 wt% of elastomer matrix) achieves an unparalleled improvement of the strength, modulus, and toughness of the resulting vitrimers. Moreover, due to their temperature-dependent nature, the Al3+O coordination bonds will partially dissociate at elevated temperatures, which only slightly compromises the topological rearrangements of the silyl ether-based network, but barely affects the reprocessability.

Self-recovery, fatigue and anti-fatigue of supramolecular elastomers

Supramolecular elastomers (SMEs) with chains bridged by covalent and non-covalent bonds demonstrate self-healing and self-recovery at room temperature. A model is developed for the kinetics of self-recovery in SME subjected to cyclic deformation. Good agreement is shown between results of simulation and observations on SMEs with metal-ligand coordination bonds, hydrogen bonds, ionic hydrogen bonds, and temporary junctions formed via inter-polymer complexation. Analysis of multi-cycle tests on SMEs reveals their anti-fatigue property (the ability to eliminate cyclic softening by introducing short intervals of recovery between subsequent cycles of deformation). Two scenarios for anti-fatigue are discussed. For SMEs with the exponential kinetics of recovery, the optimal duration of rest between cycles is close to the time needed for total self-recovery. When the recovery process involves two stages, less than 1 min of rest between cycles suffices to eliminate cyclic softening.

Exploring various metal-ligand coordination bond formation in elastomers: Mechanical performance and self-healing behavior

Exploring various metal-ligand coordination bond formation in elastomers: Mechanical performance and self-healing behavior

Fixing Flexible Arms of Core-Shared Ligands to Enhance the Stability of Metal-Organic Frameworks.

In recent years, more and more research on metal-organic frameworks (MOFs) has focused on exploring their practical applications, where the stability is crucial. Besides the metal-ligand coordination bond, the configuration of the ligand also plays an important role in determining the stability of resulting MOFs. In this work, we demonstrate that fixing flexible arms of core-shared ligands can enhance the stability of their Zr(IV)-MOFs. Two groups, four core-shared tetracarboxylate ligands, 3,3',3″,3‴-(pyrene-1,3,6,8-tetrayltetrakis(benzene-4,1-diyl))tetraacrylate (PTSA4-) and 6,6',6″,6‴-(pyrene-1,3,6,8-tetrayl)tetrakis(2-naphthoate) (PTNA4-) with the pyrene core and 3,3',3″,3‴-((9H-carbazole-1,3,6,8-tetrayl)tetrakis(benzene-4,1-diyl))-tetraacrylate (CTSA4-) and 6,6',6″,6‴-(9H-carbazole-1,3,6,8-tetrayl)tetrakis-(2-naphthoate) (CTNA4-) with the carbazole core are rationally designed. Two ligands in each group have different flexibilities due to the distinct side arms: the styrene arm is flexible, whereas the naphthalene is rigid. Constructed with Zr6 clusters, four 4,8-connected Zr(IV)-MOFs, Zr6O4(OH)8(H2O)4(PTSA)2 (BUT-72) and Zr6O4(OH)8(H2O)4(PTNA)2 (BUT-73) with a sqc-a topologic framework structure and Zr6O4(OH)8(H2O)4(CTSA)2 (BUT-74) and Zr6O4(OH)8(H2O)4(CTNA)2 (BUT-63) with a scu-a structure are obtained, respectively. It is found that the stability of BUT-73 and -63 with the rigid naphthoate-based ligands is significantly enhanced compared with that of BUT-72 and -74 with the flexible phenyl acrylate-based ones. Moreover, stable BUT-63 represents outstanding performance in the molecular recognition of most solvents commonly used in organic synthesis and industrial manufacture.

Multistep Protein Unfolding Scenarios from the Rupture of a Complex Metal Cluster Cd3S9

Protein (un)folding is a complex and essential process. With the rapid development of single-molecule techniques, we can detect multiple and transient proteins (un)folding pathways/intermediates. However, the observation of multiple multistep (>2) unfolding scenarios for a single protein domain remains limited. Here, we chose metalloprotein with relatively stable and multiple metal-ligand coordination bonds as a system for such a purpose. Using AFM-based single-molecule force spectroscopy (SMFS), we successfully demonstrated the complex and multistep protein unfolding scenarios of the β-domain of a human protein metallothionein-3 (MT). MT is a protein of ~60 amino acids (aa) in length with 20 cysteines for various metal binding, and the β-domain (βMT) is of ~30 aa with an M3S9 metal cluster. We detected four different types of three-step protein unfolding scenarios from the Cd-βMT, which can be possibly explained by the rupture of Cd-S bonds in the complex Cd3S9 metal cluster. In addition, complex unfolding scenarios with four rupture peaks were observed. The Cd-S bonds ruptured in both single bond and multiple bonds modes. Our results provide not only evidence for multistep protein unfolding phenomena but also reveal unique properties of metalloprotein system using single-molecule AFM.

Hierarchically Porous and Water-Tolerant Metal–Organic Frameworks for Enzyme Encapsulation

Metal–organic frameworks (MOFs) for in situ enzyme encapsulation commonly possess weak metal–ligand coordination bonds and rather small pores, which are instable in aqueous solution and present rat...

Crosslinking behavior and enhanced mechanical properties of acrylonitrile‐butadiene rubber composites by incorporating aluminum ammonium sulfate particles

ABSTRACTThe crosslinked structure formed by the metal coordination bonding provides excellent and new properties for rubber materials. Herein, the crosslinking of acrylonitrile‐butadiene rubber (NBR) is induced by introducing aluminum ammonium sulfate (NH4Al(SO4)2·12H2O) particles. The crosslinking behavior, morphology, mechanical properties, and the Akron abrasion resistance of NBR/NH4Al(SO4)2·12H2O composites were fully explored. The results show that the three‐dimensional crosslinking structure is held together by metal–ligand coordination bonds between the nitrile group and AI(III). The coordination crosslink density exhibits a considerable increase with the addition of NH4Al(SO4)2·12H2O. Thus, the mechanical properties and abrasion resistance of the obtained composites are better than that of NBR/sulfur system. Interestingly, the elongation at break for NBR/NH4Al(SO4)2·12H2O composites is over 2000% due to the nature of coordination bonds. The abrasion volume loss decreases to 0.4 cm3 for NBR/NH4Al(SO4)2·12H2O composites with 20 phr NH4Al(SO4)2·12H2O particles as compared to 0.75 cm3 for NBR/sulfur system. The obtained NBR composites with facile preparation and excellent mechanical properties make the composites based on metal coordination bonding attractive for practical use. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 879–886

Self-Healable Dielectric Polydimethylsiloxane Composite Based on Zinc-Imidazole Coordination Bond

Self-healing material has been intensively studied in the last decade due to its capability of improving materials’ lifetime and safety. Among the numerous proposed self-healing mechanisms, reversible metal-ligand coordination bond is considered as a strong candidate for a healing moiety because of its high tunability of mechanical properties based on the thermodynamic characteristics and kinetic parameters of metal ions or ligands. Furthermore, the dipolar nature of coordination bond can enhance the dielectric property of the material. Herein, zinc-imidazole coordination bond, having a fast ligand exchange rate in ambient condition, was employed as a healing moiety to synthesize room temperature self-healable polydimethylsiloxane (PDMS). Imidazole modified PDMS cross-linked by ligand and zinc ratio 4.5 (IMZ-PDMS-1100 L/Z 4.5) exhibited ultimate tensile strength (UT) of 60.73 kPa with ultimate extensibility (UE) of 211%. These mechanical properties could be adjusted by varying L/Z ratio and IMZ content. In comparison with previously reported metal-ligand based self-healing PDMS, IMZ-PDMS-1100 L/Z 4.5 showed faster healing rate that recovered 98% of UT and UE after 31 h healing and even faster healing rate was achieved with higher IMZ content. Moreover, cross-linked IMZ-PDMS displayed high dielectric constant of 8.30±0.74 at 1 MHz, which was 2.7 times higher than conventional PDMS.

Baroplastics with Robust Mechanical Properties and Reserved Processability through Hydrogen-Bonded Interactions.

Conventional polymers are usually processed at a much higher temperature than room temperature, which inevitably leads to huge energy consumption and degradation of the polymers and thus a low recycling ability. Herein, we synthesized a poly( n-butyl acrylate)@polystyrene (PBA@PS) core-shell polymer to prepare a typical baroplastic (processible at room temperature). However, this type of baroplastics always has a low mechanical property. To solve this problem, in this work, we introduced hydrogen bonds into the matrix and successfully reinforced baroplastics for the first time. The hydrogen-bonded interaction was introduced by complexing PBA@PS with poly(acrylic acid) and poly(ethylene oxide). The results show that the reinforced baroplastics possessed notably enhanced mechanical properties and good processability. Their mechanical strength and modulus reached as high as 5.6 (by 73%) and 10 MPa (by 400%), respectively. Moreover, the baroplastics could be remolded many times at room temperature and, at the same time, still showed a higher tensile strength (10.5 MPa, 3.3 times that of the initial PBA@PS, which was never achieved in previous works), which resulted from the reversible hydrogen bonds and reserved orientation of molecular chains. Our work opened a new path to reinforce baroplastics and could widen their applications. Furthermore, not limited to the hydrogen bonds, more sacrificial bonds, such as ionic bonds, host-guest interactions, and metal-ligand coordination bonds, could be used to fabricate high-performance baroplastics.

8-Hydroxyquinolinate-Based Metal-Organic Frameworks: Synthesis, Tunable Luminescent Properties, and Highly Sensitive Detection of Small Molecules and Metal Ions.

Five new metal-organic frameworks, [Zn2L2]·2DMF·2MeOH (1), [Zn2L2(py)2] (2), [Cd2L2]·Diox·MeOH·6H2O (3), [Mn2L2]·2DMF·2MeOH (4), and [Co2L2]·2DMF·4H2O (5), were assembled by using a novel 8-hydroxyquinolinate derivative H2L with different metal ions. Complex 1 features a 3D porous network consisting of meso-helical chains ( P + M) built from metal-ligand coordination bonds. The adjacent dinuclear ZnII building blocks in 2 are connected together to generate a 2D grid network. In complex 3, each binuclear motif is bound to four ZnII ions to produce a 2D layer structure that stacks into a 3D porous structure. The framework of complex 4 is isostructural to 5, featuring a 21 helical chain built from [M2L2] units (M = Mn or Co). The adjacent meso-helices associated in parallel are interconnected by the phenolate μ2-O atoms of H2L to give rise to a 2D network. Distinct solid-state luminescence properties of 1-3 were observed, arising from their different metal nodes and frameworks. In particular, complex 1 exhibited excellent stability in both common organic solvents and H2O, thus facilitating its utility as a chemical sensor. Remarkably, luminescent 1 showed highly sensitive detection for nitroaromatic molecules in methanol and Fe3+ ion in H2O even in the presence of other interfering metal cations.