Metal-ligand Bond Research Articles

Nanoscale Metal-Organic Frameworks and Their Nanomedicine Applications.

Abundant connectivity among organic ligands and inorganic metal ions makes the physical and chemical characters of metal-organic frameworks (MOFs) could be precisely devised and modulated for specific applications. Especially nanoscale MOFs (NMOFs), a unique family of hybrid nanomaterials, with merits of holding the nature as the mainstay MOFs and demonstrating particle size in nanoscale range which enable them prospect platform in clinic. Adjustability of composition and structure allows NMOFs with different constituents, shapes, and characteristics. Oriented frameworks and highly porous provide enough space for packing therapeutic cargoes and various imaging agents efficiently. Moreover, the relatively labile metal-ligand bonds make NMOFs biodegradable in nature. So far, as a significant class of biomedically relevant nanomaterials, NMOFs have been explored as drug carriers, therapeutic preparation, and biosensing and imaging preparation owing to their high porosity, multifunctionality, and biocompatibility. This review provides up-to-date developments of NMOFs in biomedical applications with emphasis on size control, synthetic approaches, and surfaces functionalization as well as stability, degradation, and toxicity. The outlooks and several crucial issues of this area are also discussed, with the expectation that it may help arouse widespread attention on exploring NMOFs in potential clinical applications.

Remotely controllable supramolecular rotor mounted inside a porphyrinic cage

Remotely controllable supramolecular rotor mounted inside a porphyrinic cage

Quantifying the Steric Effect on Metal-Ligand Bonding in Fe Carbene Photosensitizers with Fe 2p3d Resonant Inelastic X-ray Scattering.

Understanding the electronic structure and chemical bonding of transition metal complexes is important for improving the function of molecular photosensitizers and catalysts. We have utilized X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L3 edge to investigate the electronic structure of two Fe N-heterocyclic carbene complexes with similar chemical structures but different steric effects and contrasting excited-state dynamics: [Fe(bmip)2]2+ and [Fe(btbip)2]2+, bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)pyridine and btbip = 2,6-bis(3-tert-butyl-imidazole-1-ylidene)pyridine. In combination with charge transfer multiplet and ab initio calculations, we quantified how changes in Fe-carbene bond length due to steric effects modify the metal-ligand bonding, including σ/π donation and π back-donation. We find that σ donation is significantly stronger in [Fe(bmip)2]2+, whereas the π back-donation is similar in both complexes. The resulting stronger ligand field and nephelauxetic effect in [Fe(bmip)2]2+ lead to approximately 1 eV destabilization of the quintet metal-centered 5T2g excited state compared to [Fe(btbip)2]2+, providing an explanation for the absence of a photoinduced 5T2g population and a longer metal-to-ligand charge-transfer excited-state lifetime in [Fe(bmip)2]2+. This work demonstrates how combined modeling of XAS and RIXS spectra can be utilized to understand the electronic structure of transition metal complexes governed by correlated electrons and donation/back-donation interactions.

Group 13 monohalides [AX (A = B, Al, Ga and In; X = Halogens)] as alternative ligands for carbonyl in organometallics: Electronic structure and bonding analysis

Iron pentacarbonyl complexes have received a lot of interest because of their unique bonding nature and trigonal bipyramidal structure. When one carbonyl in each of the axial and equatorial positions is replaced with a group 13 monohalide, the molecule exhibits unusual bonding properties. A theoretical investigation of 32 Fe(CO)4(A-X) complexes are presented in this study. A variety of methods were used to investigate the nature of the ligand–metal bond, including atomic partial charge, FMO, WBI, and bond order. The bond index of the Fe-AX bond was determined using WBI analysis. The HOMO-LUMO energy gap is 3.71 to 5.52 eV, according to the FMO analysis. The Fe atom contributes less to the bond than the ligands (AX) atom, which is equal to the carbonyl, and the C atom contributes more than the A in the complexes, according to NBO analysis.

A computational study on the nature, strength and cooperativity of bonds in [M(η5–C60Me5)(CO)n] and [M(η5–Cp)(CO)n] (n = 3, M = Mn(i), Tc(i), Re(i); n = 2, M = Co(i), Rh(i), Ir(i)) complexes

It is shown that, due to cooperativity versus anticooperativity of bonds, the total interaction energy of a complex, having weaker metal–ligand bonds, can be comparable to or even larger than that of a similar complex having stronger bonds.

Transformation networks of metal-organic cages controlled by chemical stimuli.

The flexibility of biomolecules enables them to adapt and transform as a result of signals received from the external environment, expressing different functions in different contexts. In similar fashion, coordination cages can undergo stimuli-triggered transformations owing to the dynamic nature of the metal–ligand bonds that hold them together. Different types of stimuli can trigger dynamic reconfiguration of these metal–organic assemblies, to switch on or off desired functionalities. Such adaptable systems are of interest for applications in switchable catalysis, selective molecular recognition or as transformable materials. This review highlights recent advances in the transformation of cages using chemical stimuli, providing a catalogue of reported strategies to transform cages and thus allow the creation of new architectures. Firstly we focus on strategies for transformation through the introduction of new cage components, which trigger reconstitution of the initial set of components. Secondly we summarize conversions triggered by external stimuli such as guests, concentration, solvent or pH, highlighting the adaptation processes that coordination cages can undergo. Finally, systems capable of responding to multiple stimuli are described. Such systems constitute composite chemical networks with the potential for more complex behaviour. We aim to offer new perspectives on how to design transformation networks, in order to shed light on signal-driven transformation processes that lead to the preparation of new functional metal–organic architectures.

Radiative properties of ‘Eu’in Li–Al–Si–O ceramics: Effect of ‘Si’ to ‘Li’ ratio

Four Li–Al–Si– O -based ceramic compounds with varying Si to Li ratios were synthesized via a high-temperature reaction route and characterized by X-Ray diffraction, scanning electron microscopy, and photoluminescence techniques. Eu was used as an activator in these four compositions namely, LiAlSiO 4 , LiAlSi 2 O 6 , LiAlSi 3 O 8 , and LiAlSi 4 O 10 , and the radiative properties were evaluated. The activator concentration was optimized in all these samples to 2 mol%. The photoluminescence emission intensity and decay time values increased with the Si to Li ratio. The relative intensities of the emission spectra and the lifetime values indicated that the rare earth ion stabilizes itself at interstitial positions in the lattice having approximately C 2v symmetry. The chromaticity indices for the four ceramic samples doped with Eu were calculated that showed near red emission for all. To get an idea about their commercial utility, color purity, quantum efficiencies for the phosphors were evaluated. The emission profile of the prepared samples was compared with a commercial sample. The Judd-Ofelt parameters, transition probabilities, branching ratios, and quantum efficiencies were also calculated for these systems. It was observed that the metal-ligand bond covalency decreased with an increase in the Si to Li ratio which was responsible for the increase in the emission intensities and luminescence decay time values.

A Discrete Platinum(II) Metallacycle Harvesting Triplet Excitons for Solution‐Processed Deep‐Red Organic Light‐Emitting Diodes

AbstractPlatinum(II) coordination‐driven architectures have exhibited unique features in fabricating functional supramolecular materials. By introducing luminescent moieties into the ligand structure, various light‐emitting metallacycles and metallacages have been facilely prepared, presenting specific applications in chemical sensing, light‐harvesting, and bio‐imaging. Except for building up the metal–ligand bonds, the platinum(II) center should also benefit the ultimate luminescence due to its unique photophysical traits. Here, a platinum(II) metallacycle with deep‐red emission for solution‐processed organic light‐emitting diodes is reported. This metallacycle is assembled by mixing a 180° di‐Pt(II) acceptor with a pyridyl‐decorated ligand functionalized by a deep‐red fluorescent emitter. Notably, the platinum(II) acceptor permits the efficient intramolecular transfer of all electrically generated singlet and triplet excitons from itself to the fluorescent moiety, which dramatically enhances the external quantum efficiency of the device compared with the one consisting of the sole ligand. The present results reveal the function of platinum(II) metallacycles in light‐emitting devices, a finding which should apply to other coordination‐driven architectures with versatile properties.

N-Heterocyclic Carbene Ligand Stability on Gold Nanoparticles in Biological Media.

The ability to functionalize gold nanoparticle surfaces with target ligands is integral to developing effective nanosystems for biomedical applications, ranging from point-of-care diagnostic devices to site-specific cancer therapies. By forming strong covalent bonds with gold, thiol functionalities can easily link molecules of interest to nanoparticle surfaces. Unfortunately, thiols are inherently prone to oxidative degradation in many biologically relevant conditions, which limits their broader use as surface ligands in commercial assays. Recently, N-heterocyclic carbene (NHC) ligands emerged as a promising alternative to thiols since initial reports demonstrated their remarkable stability against ligand displacement and stronger metal–ligand bonds. This work explores the long-term stability of NHC-functionalized gold nanoparticles suspended in five common biological media: phosphate-buffered saline, tris-glycine potassium buffer, tris-glycine potassium magnesium buffer, cell culture media, and human serum. The NHCs on gold nanoparticles were probed with surface-enhanced Raman spectroscopy (SERS) and X-ray photoelectron spectroscopy (XPS). SERS is useful for monitoring the degradation of surface-bound species because the resulting vibrational modes are highly sensitive to changes in ligand adsorption. Our measurements indicate that imidazole-based NHCs remain stable on gold nanoparticles over the 21 days of examination in all tested environments, with no observed change in the molecule’s SERS signature, XPS response, or UV–vis plasmon band.

Modified MXene-doped conductive organosilicon elastomer with high-stretchable, toughness, and self-healable for strain sensors

Polymers/nanomaterials conductive elastomers based on reversible interactions are of great significance in the application of resistive sensors because they repair themselves after mechanical injury, just like living things. The electronic devices developed with such materials will be more stable and reliable. Herein, we innovatively used ring opening polymerization (ROP) of sulfur-containing five membered rings to endow organosilicon supermolecule and MXene with hyperbranched structure which had terminal carboxyl groups. Subsequently, organosilicon conductive elastomer (PDMS@MXene) crosslinked by hydrogen bonds and metal–ligand bonds was prepared. The fully disconnected PDMS@MXene could recover 91.7% of its original mechanical properties after spontaneous repair. The well-organized composite had high stretchability and excellent tensile strength, which enhanced its application prospects in soft robots, motion monitoring and other fields. This novel design idea and simple preparation process will provide a reference for the development of self-healable conductive elastomer in the future.

Reduction of Electron Repulsion in Highly Covalent Fe-Amido Complexes Counteracts the Impact of a Weak Ligand Field on Excited-State Ordering.

The ability to access panchromatic absorption and long-lived charge-transfer (CT) excited states is critical to the pursuit of abundant-metal molecular photosensitizers. Fe(II) complexes supported by benzannulated diarylamido ligands have been reported to broadly absorb visible light with nanosecond CT excited state lifetimes, but as amido donors exert a weak ligand field, this defies conventional photosensitizer design principles. Here, we report an aerobically stable Fe(II) complex of a phenanthridine/quinoline diarylamido ligand, Fe(ClL)2, with panchromatic absorption and a 3 ns excited-state lifetime. Using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L-edge and N K-edge, we experimentally validate the strong Fe-Namido orbital mixing in Fe(ClL)2 responsible for the panchromatic absorption and demonstrate a previously unreported competition between ligand-field strength and metal-ligand (Fe-Namido) covalency that stabilizes the 3CT state over the lowest energy triplet metal-centered (3MC) state in the ground-state geometry. Single-crystal X-ray diffraction (XRD) and density functional theory (DFT) suggest that formation of this CT state depopulates an orbital with Fe-Namido antibonding character, causing metal-ligand bonds to contract and accentuating the geometric differences between CT and MC excited states. These effects diminish the driving force for electron transfer to metal-centered excited states and increase the intramolecular reorganization energy, critical properties for extending the lifetime of CT excited states. These findings highlight metal-ligand covalency as a novel design principle for elongating excited state lifetimes in abundant metal photosensitizers.

Mechanisms of the Cu(I)-Catalyzed Intermolecular Photocycloaddition Reaction Revealed by Optical and X-ray Transient Absorption Spectroscopies.

The [2 + 2] photocycloaddition provides a simple, single-step route to cyclobutane moieties that would otherwise be disfavored or impossible due to ring strain and/or steric interactions. We have used a combination of optical and X-ray transient absorption spectroscopies to elucidate the mechanism of the Cu(I)-catalyzed intermolecular photocycloaddition reaction using norbornene and cyclohexene as model substrates. We find that for norbornene the reaction proceeds through an initial metal-to-ligand charge transfer (MLCT) state that persists for 18 ns before the metal returns to the monovalent oxidation state. The Cu K-edge spectrum continues to evolve until ∼5 μs and then remains unchanged for the 50 μs duration of the measurement, reflecting product formation and ligand dissociation. We hypothesize that the MLCT transition and reverse electron transfer serve to sensitize the triplet excited state of one of the norbornene ligands, which then dimerizes with the other to give the product. For the case of cyclohexene, however, we do not observe a charge transfer state following photoexcitation and instead find evidence for an increase in the metal-ligand bond strength that persists for several ns before product formation occurs. This is consistent with a mechanism in which ligand photoisomerization is the initial step, which was first proposed by Salomon and Kochi in 1974 to explain the stereoselectivity of the reaction. Our investigation reveals how this photocatalytic reaction may be directed along strikingly disparate trajectories by only very minor changes to the structure of the substrate.

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.

SYNTHESIS, CHARACTERIZATION AND ANTIMICROBIAL ACTIVITY OF MIXED LIGAND COMPLEXES OF INNER TRANSITION METAL WITH L-ASCORBIC ACID, ITS DERIVATIVE AND ADENINE BASE

Complexes of La(III), Ce(III), Nd(III) and Gd(III) with mixed Ligand of 2-aminopurene and L-ascorbic acid derivative were prepared in aqueous ethanol. The two types of mixed ligand complexes [M(AA)(ADE)(H2O)2]+and [M(IPAA) (ADE)(H2O)2]+ have been synthesized. All complexes were characterized on the basis of elemental analysis, molar conductance, magnetic susceptibility measurements, IR, UV-VISSIBLE, NMR, ESR and Mass spectral studies. IR spectra of the complexes reveal that the complex formation occurred through both nitrogen and oxygen atoms. On the basis of electronic spectral data and magnetic susceptibility measurement octahedral geometry has been proposed for the complexes. The ESR spectral data of the M (III) complexes showed that the metal-ligand bonds have considerable covalent character. The electrochemical behaviour of mixed Ligand M(III) complexes was studied which showed that complexes of IPAA appear at more positive potential as compared to those for corresponding AA complexes. In addition, biological activity of the synthesised metal complexes against Salmonella typhi, Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae bacteria and Aspergillus niger, Aspergillus flavus, Fusarium oxisporium and Trichoderma viride fungi respectively were examined in-vitro. Some of the metal complex displayed pounced biological activity.

Cooperativity and Metal–Linker Dynamics in Spin Crossover Framework Fe(1,2,3-triazolate)2

Cooperative interactions are responsible for the useful properties of spin crossover (SCO) materials─large hysteresis windows, critical temperatures near room temperature, and abrupt transitions─with hybrid framework materials exhibiting the greatest cooperativity and hysteresis of all SCO systems. However, little is known about the chemical origin of cooperativity in frameworks. Here, we present a combined experimental–computational approach for identifying the origin of cooperativity in the metal–organic framework (MOF) Fe(1,2,3-triazolate)2 (Fe(TA)2), which exhibits the largest known hysteresis window of all SCO materials and unusually high transition temperatures, as a roadmap for understanding the manipulation of SCO behavior in general. Variable-temperature vibrational spectroscopy provides evidence that “soft modes” associated with dynamic metal–linker bonding trigger the cooperative SCO transition. Thermodynamic analysis also confirms a cooperativity magnitude much larger than those of other SCO systems, while electron density calculations of Fe(TA)2 support previous theoretical predictions that large cooperativity arises in materials where SCO produces considerable differences in metal–ligand bond polarities between different spin states. Taken together, this combined experimental–computational study provides a microscopic basis for understanding cooperative magnetism and highlights the important role of dynamic bonding in the functional behavior of framework materials.

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.

Structures, Bonding and Sensor Properties of Some Alkaline o-Phthalatocuprates.

Comprehensive study of the structure and bonding of disodium, dipotassium and diammonium di-o-phthalatocuprates(II) dihydrates has been undertaken. The crystal structure of ammonium o-phthalatocuprate has been determined. The identity of structures of phthalatocuprate chains in potassium and ammonium salts has been revealed. Vibrational spectra of all three compounds have been recorded, and the assignment of vibrational bands has been made. Force field calculations have shown a minor effect of outer-sphere cations (Na+, K+, NH4+) on both intraligand (C-O) and metal–ligand bond strengths. Synthesized compounds have been tested as electrochemical sensors on D-glucose, dopamine and paracetamol. Their sensitivity to analytes varied in the order of Na+ > K+ > NH4+. This effect has been explained by the more pronounced steric hindrance of copper ions in potassium and ammonium salts.

Isomerism, molecular structure, and vibrational assignment of tris(triflouroacetylacetonato)iron(III): An experimental and theoretical study

The isomerism, optimized molecular structure, UV spectrum, metal-ligand bond strength, and vibrational assignment of tris(triflouroacetylacetonato)iron(III), Fe(TFAA)3, were investigated by the aid of theoretical calculations (using DFT and Atoms-in-Molecules (AIM) at the B3LYP/6-311++G(d,p) level) and experimental methods (vibrational and UV spectroscopy). To explore the effect of the CF3 substituent in the β-position on the properties of complex, the above theoretical and experimental results of the titled complex compared with the corresponding data for tris(acetylacetonato) iron(III), Fe(AA)3. Both theoretical and experimental results confirmed that there is no significant difference in the strength of the Fe−O bond in these complexes. The effect of the CF3 group on the experimental vibrational bands of the chelated ring agrees with the calculated results. Comparing the observed and calculated vibrational spectra suggests that vibrational spectroscopy cannot be used to determine the type of isomer in the sample. However, due to the small difference of energy between the fac and mer isomers in the Fe(TFAA)3, the presence of both isomers in the sample is possible. The computed quantum chemical descriptors of Fe(TFAA)3 and Fe(AA)3 were also compared.

Tuning the metal-ligand bond in the σ-complexes of stannylenes and azabenzenes.

The metal-ligand bond in a set of 60 σ-complexes has been investigated by electronic structure computations. These σ-complexes originate from the unique combination of 12 stannylenes (SnX2 ) with five azabenzene ligands (pyridine, pyrazine, pyrimidine, pyridazine, and s-triazine), where the nitrogen center of the ligand acts as σ-donor and the tin(II) center as σ-acceptor in a 1:1 fashion. The Sn ← N bond and the total interaction between the stannylene and azabenzene moieties of the σ-complexes are characterized in depth to relate the Sn ← N strength to the substitution pattern at SnX2 and to the number and the positioning of N atoms in the azabenzenes. Such X substituents as (iso)cyano and trifluoromethyl groups enhance the interaction strength, while the presence of alkyl, phenyl, and silyl substituents in SnX2 diminishes the stability of σ-complexes. A gradual weakening of the total interaction is associated with the growing number of N atoms in the azabenzenes, while the N-atom positioning in pyridazine is particularly effective in strengthening the interaction with stannylenes. Variations in the Sn ← N bond strength usually follow those in the total interaction between the moieties but the interacting quantum atoms picture of Sn ← N reveals certain intriguing exceptions.

Neutral Nitrogen Donor Ligand-based MOFs for Sensing Applications.

Neutral nitrogen donor (N-donor) ligand-based MOFs, with their enticing features inclusive of facile synthesis, labile metal-ligand bond, framework flexibility, atomic level tunability renders them appealing in molecular recognition-based studies. Intriguingly, the flexibility in such systems (owing to weaker metal-nitrogen bonds) promote maximization of host-analyte interactions, which is critical for the manifestation of a signaling response. Such host-analyte interactions can be tapped by discerning any change in the physical properties associated with the system, such as optical, fluorometric, chemiresistive, magnetic, dielectric constant, mass. This minireview presents a brief discussion on the various types of signal transduction pathways unveiled hitherto using neutral N-donor ligand-based MOFs and the fundamental insight into the signal's origin. Moreover, an elaborate compilation of the recent examples in this field has been presented. Also, the untapped prospects have been highlighted, which may serve as a beacon to drive future research.