Carboxylate Ligands Research Articles

Mixed Carboxylate Ligands Bridging Tetra-Pr3+-Encapsulated Antimonotungstate: Syntheses, Structure, and Catalytic Activity for Imidazoles Synthesis.

Multinuclear Pr-containing antimonotungstate [Pr4(H2O)10W6O13(mal)2(OAc)(B-α-SbW9O33)4]21- (Pr-1, mal = malate anion, OAc = acetate anion), bridged by organic carboxylic acid, was synthesized through a one-pot assembly reaction and structurally characterized. Pr-1 is composed of four [B-α-SbW9O33]9- fragments fused together by an organic-inorganic hybrid central [Pr4(H2O)10W6O13(mal)2(OAc)]15+ cluster core through 24 μ2-O atoms. Notably, the central cluster comprises unprecedented decanuclear Pr4(H2O)10W6O13 jointly decorated by two types of carboxylic acid ligands. This integration of rare earth-containing antimonotungstate with mixed organic carboxylate ligands is very rare in POMs chemistry. Pr-1 exhibits excellent catalytic activity in the cyclo-condensation reaction involving benzil, aldehyde, and NH4OAc. A series of 2,4,5-trisubstituted imidazoles were synthesized in remarkable yields using iPrOH as a green solvent.

Leveraging Ligand Desymmetrization to Enrich Structural Diversity of Zirconium Metal-Organic Frameworks for Toxic Chemical Adsorption.

The discovery of metal-organic frameworks (MOFs) with novel structures provides significant opportunities for developing porous solids with new properties and enriching the structural diversity of functional materials for various applications. The rational design of building units with specific geometric conformations is essential to direct the construction of MOFs with unique properties. Herein, we leverage a ligand desymmetrization approach to construct a series of new MOFs. A flexible tetratopic carboxylate ligand with a tetrahedral geometry was designed and assembled with a Zr6 cluster, generating four Zr-based MOF structures: NU-2600, NU-2700, NU-2800, and NU-1802, in which the ligand configurations and Zr6 cluster connectivities can be controlled by varying solvents and modulators during synthesis. Except for NU-1802, these represent entirely new topologies. Notably, NU-1802 exhibits structural flexibility, with up to a 74% reduction in the unit cell volume as confirmed by single-crystal X-ray diffraction studies. Given their microporous structures, we studied the adsorption behaviors of n-hexane and 2-chloroethyl ethyl sulfide to explore the structure-property relationships of these MOFs. Overall, this work highlights ligand desymmetrization as a powerful method to enrich MOF structural diversity and access complex MOFs with non-default topologies suitable for applications such as toxic gas capture.

Leveraging Ligand Desymmetrization to Enrich Structural Diversity of Zirconium Metal–Organic Frameworks for Toxic Chemical Adsorption

The discovery of metal–organic frameworks (MOFs) with novel structures provides significant opportunities for developing porous solids with new properties and enriching the structural diversity of functional materials for various applications. The rational design of building units with specific geometric conformations is essential to direct the construction of MOFs with unique properties. Herein, we leverage a ligand desymmetrization approach to construct a series of new MOFs. A flexible tetratopic carboxylate ligand with a tetrahedral geometry was designed and assembled with a Zr6 cluster, generating four Zr‐based MOF structures: NU‐2600, NU‐2700, NU‐2800, and NU‐1802, in which the ligand configurations and Zr6 cluster connectivities can be controlled by varying solvents and modulators during synthesis. Except for NU‐1802, these represent entirely new topologies. Notably, NU‐1802 exhibits structural flexibility, with up to a 74% reduction in the unit cell volume as confirmed by single‐crystal X‐ray diffraction studies. Given their microporous structures, we studied the adsorption behaviors of n‐hexane and 2‐chloroethyl ethyl sulfide to explore the structure‐property relationships of these MOFs. Overall, this work highlights ligand desymmetrization as a powerful method to enrich MOF structural diversity and access complex MOFs with non‐default topologies suitable for applications such as toxic gas capture.

Dirhodium-Catalyzed [2 + 2 + 2] Cycloaddition of 1,6-Diynes and Alkynes.

A facile method for the construction of fused arenes has been developed through dirhodium-catalyzed [2 + 2 + 2] cycloaddition, which represents a new application of dirhodium complexes. This protocol is convenient to handle without the addition of extra ligands and reductants and tolerates a broad range of functional groups. Mechanistic studies revealed that the two-electron oxidation process, carboxylate ligand departure, and heteroatom coordination-promoted [2 + 2 + 2] cycloaddition were possibly involved.

Efficient paddle-wheel copper cluster-based metal-organic framework for catalytic conversion of CO2 to oxazolidinones under mild conditions

Efficient utilization of CO2 and its conversion into high-value-added products contribute to mitigating environmental issues such as the greenhouse effect. Among these methods, the carboxylation cyclization of CO2 with propargylic amine represents a green and atom-economical approach. Developing cost-effective catalyst that can promote this reaction under mild condition is desirable but challenging. Herein, a unique two-dimensional (2D) metal–organic framework (MOF) {[Cu(QCA)2]∙0.5DMF∙0.5H2O}n (1) (QCA =6-quinolinecarboxylic acid, DMF = N,N-dimethylformamide) have been synthesized and thoroughly characterized. Compound 1 was constructed from paddle-wheel dinuclear copper clusters [Cu2(COO)4] and carboxylate ligands, presenting a two-dimensional framework with new topology. Stability tests indicate that compound 1 undergoes reversible structural changes in different solvents and exhibits good thermal stability. 1 as heterogeneous catalyst enabled the transformation of CO2 and propargylic amine into oxazolidinones under mild conditions with broad substrate generality. In addition, 1 could maintain its catalytic performance with five continuous reactions. This work presents an infrequent example of a 2D noble metal free MOF-based catalyst that shows high catalytic efficiency under mild conditions.

Five novel Pb/Zn/Co metal-organic frameworks using 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene and carboxylate ligands: Synthesis, characterization, DFT calculation and catalytic PMS degradation of RhB

Five new metal-organic frameworks (MOFs), namely {Pb(atpt)(3-bpd)0.5}n (1); {Co(cbz)2(3-bpd)(H2O)2·2H2O}n (2); {Co(imdc)(3-bpd)(H2O)·H2O}n (3); {Co(tdca)(3-bpd)(H2O)2·DMF}n (4); {Zn(tdca)(3-bpd)(H2O)2·DMF}n (5), were assembled with metal ions Pb²⁺, Zn²⁺, and Co²⁺ using 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (3-bpd) as the main ligand, combined with 2-aminoterephthalic acid (H2atpt), 4-cyanobenzoic acid (Hcbz), 4,5-imidazoledicarboxylic acid (H2imdc), and thiophene-2,5-dicarboxylic acid (H2tdca) as auxiliary ligands via hydro/solvothermal methods. Their molecular structures were analyzed by single-crystal X-ray diffraction and characterized by FT-IR and PXRD. Complex 1 has a three-dimensional structure with a hepta-coordinated, semi-directed {PbNO6} metal center. The central metal ions in complexes 2–5 exhibit a six-coordinated {MN2O4} (M = Co or Zn) octahedral geometry. In complex 2, the carboxylate ligand does not function as a bridging ligand, resulting in a 1D chain structure, while complexes 3- 5 extend into 2D layered structures. Thermogravimetric analysis indicates good thermal stability for complexes 1–5. Then, the prepared complexes were used as catalysts to degrade Rhodamine B (RhB) during the monopersulfate (PMS) activation. Complexes 3 and 4 demonstrated significant efficiency, achieving degradation rates of 93.61 % and 98.56 % within 10 min. Radical scavenging experiments and EPR analysis identified that the activation of PMS for dye degradation by the two catalysts occurs through both radical and non-radical pathways, with SO4•− and •OH radicals being the dominant species. Density functional theory (DFT) calculations explained the superior catalytic performance of complex 4 compared to complex 3.

Structural and Biological Studies of Bioactive Silver(I) Complexes with Coumarin Acid Derivatives.

Two new Ag(I) complexes with coumaric carboxylic acid derivatives have been synthesized. Structural studies of these noncrystalline complexes have been performed using a methodology that combines laboratory and synchrotron techniques, supported by density functional theory calculations. The arrangement of ligands around the Ag(I) cation has been refined using infrared, extended X-ray absorption fine structure, and X-ray absorption near edge structure spectroscopies. Different coordination modes of carboxylate ligands are observed for the studied compounds. Carboxylate bridges are characteristic for the Ag(I) complex with 4-oxo-4H-1-benzopyran-2-carboxylic acid (1), while a bidentate chelating motif was found for the complex with 2-oxo-2H-1-benzopyran-3-carboxylic acid (2). Additionally, the carbonyl oxygen atom of the coumarin ring coordinates to the silver cation in complex 2, while it is inactive in complex 1. Antimicrobial evaluation has been performed for both compounds. The complexes show activity against selected bacteria as well as Candida yeast. This activity is slightly lower for bacteria and the same or higher for Candida in relation to the reference substances: ciprofloxacin or fluconazole.

Uranium Repartitioning during Microbial Driven Reductive Transformation of U(VI)-Sorbed Schwertmannite and Jarosite.

This study exposes U(VI)-sorbed schwertmannite and jarosite to biotic reductive incubations under field-relevant conditions and examines the changes in aqueous and solid-phase speciation of U, Fe, and S as well as associated microbial communities over 180 days. The chemical, X-ray absorption spectroscopy, X-ray diffraction, and microscopic data demonstrated that the U(VI)-sorbed schwertmannite underwent a rapid reductive dissolution and solid-phase transformation to goethite, during which the surface-sorbed U(VI) was partly reduced and mostly repartitioned to monomeric U(VI)/U(IV) complexes by carboxyl and phosphoryl ligands on biomass or organic substances. Furthermore, the microbial data suggest that these processes were likely driven by the consecutive developments of fermentative and sulfate- and iron- reducing microbial communities. In contrast, the U(VI)-sorbed jarosite only stimulated the growth of some fermentative communities and underwent very limited reductive dissolution and thus, remaining in its initial state with no detectable mineralogical transformation and solid-phase U reduction/repartitioning. Accordingly, these two biotic incubations did not induce increased risk of U reliberation to the aqueous phase. These findings have important implications for understanding the interactions of schwertmannite/jarosite with microbial communities and colinked behavior and fate of U following the establishment of reducing conditions in various acidic and U-rich settings.

The structure of tetranuclear zirconium pivalate ZR<sub>4</sub>O<sub>2</sub> [(CH<sub>3</sub>)<sub>3</sub>CCO<sub>2</sub>]<sub>12</sub> according to X-ray diffraction analysis and quantum chemical calculations

The crystal and molecular structure of a polynuclear pivalate complex obtained by the interaction of ZrCl4 with pivalic acid was determined by X-ray diffraction analysis. The compound C71H124O28Zr4 (compound 1) crystallizes in the monoclinic crystal system. The crystal structure was refined in the nonstandard space group I2. The asymmetric part of the structure includes three Zr atoms, six pivalate ligands, a bridging µ3-O oxygen atom, as well as disordered crystallization molecules of pivalic acid with an occupancy of 50% and benzene with an occupancy of 50%. The zirconium complex molecule is a tetranuclear cluster that contains three types of Zr atoms that differ in ligand environment. Comparison of the results of quantum chemical calculations of the model reaction ZrCl4 with acetic acid with the literature data on reactions of ZrCl4 with aliphatic acids have shown the possibility of the formation of both mononuclear Zr(RCO2)4 and polynuclear clusters in this reaction, which is a new route for obtaining polynuclear zirconium clusters. The structure of the clusters formed depends on the steric properties of carboxylate ligands.

Temperature-Dependent Supramolecular Isomeric Co-CPs for Luminescence Recognition and Catalytic Oxidation.

Two Co-based supramolecular isomers were synthesized from a fluorinated carboxylic acid ligand under hydrothermal conditions at varying temperatures. Both exhibited similar one-dimensional chain structures while different bending connections of the aromatic rings led to different supramolecular structures, namely CoCP-1 and CoCP-2, respectively. The structural differences of two isomers resulted in discrepant performance with regards to luminescence sensing and catalysis. CoCP-1 demonstrated more significant luminescence quenching activity toward biomarkers 2,6-dipyridinoic acid (DPA) and high vanilloid acid (HVA), which could be distinguished in the presence of Eu3+. The limit of detection (LOD) was found to be as low as 3.4 and 1.3 μM, respectively. The recovery rate of for HVA and DPA was within the range of 89.6-101.2% and 99.7-117.9% in simulated urine and serum, respectively, indicating potential reliability in monitoring these two analytes in real samples. Notably, CoCP-2 exhibited catalytic activity for the oxidation of thioethers to sulfoxides. Our finding here suggests that the coordination conformation of the ligands within supramolecular isomers plays a pivotal role in determining the structure and luminescence sensing/catalysis performance.

Charged organic ligands inserting/supporting the nanolayer spacing of vanadium oxides for high-stability/efficiency zinc-ion batteries.

Given their high safety, environmental friendliness and low cost, aqueous zinc-ion batteries (AZIBs) have the potential for high-performance energy storage. However, issues with the structural stability and electrochemical kinetics during discharge/charge limit the development of AZIBs. In this study, vanadium oxide electrodes with organic molecular intercalation were designed based on intercalating 11 kinds of charged organic carboxylic acid ligands between 2D layers to regulate the interlayer spacing. The negatively charged carboxylic acid group can neutralize Zn2+, reduce electrostatic repulsion and enhance electrochemical kinetics. The intercalated organic molecules increased the interlayer spacing. Among them, the 0.028EDTA · 0.28NH4 + · V2O5 · 0.069H2O was employed as the cathode with a high specific capacity (464.6mAhg-1 at 0.5Ag-1) and excellent rate performance (324.4mAhg-1 at 10Ag-1). Even at a current density of 20Ag-1, the specific capacity after 2000 charge/discharge cycles was 215.2mAhg-1 (capacity retention of 78%). The results of this study demonstrate that modulation of the electrostatic repulsion and interlayer spacing through the intercalation of organic ligands can enhance the properties of vanadium-based materials.

Synthesis and electrochemical properties of two cobalt(II) and nickel(II) metal-organic frameworks for excellent performance supercapacitors

Two novel metal-organic frameworks, [Co(HNTB)(bif)]·3H2O·DMA (1) and [(Ni2(μ2O)(HNTB)2(bif)2]·2H2O·2H3O·2DMA (2), were obtained with a tripod-type carboxylic ligand (4,4′,4″-Nitrilotrisbenzoic acid, H3NTB) and a rigid N-donor ligand (2,7-bis(1-imidazoly)fluorene, bif) by solvothermal method. Complex 1 exhibits a 2D layered structure and 2 is a 3D microporous structure with an anionic skeleton. Significantly, their electrochemical tests show that both of them own excellent capacitive performance. Complex 1 has a relatively high specific capacitance (301.2 F g−1 at 1 A g−1) with favorable cycling durability (retained 73.2 % over 3000 cycles). Complex 2 presents a high specific capacitance (329.6 F g−1 at 1 A g−1) with a relatively poor cycling durability (retained 58.9 % over 3000 cycles). Additionally, complex 1 // activates carbon (AC) as an asymmetrical ultra-capacitor exhibits a maximum energy density of 20.9 Wh kg−1 at 800.0 W kg−1 and a high cycle stability (retained 89.2 % over 8000 cycles). Complex 2 // AC owns an energy density of 19.3 Wh kg−1 at 750.0 W kg−1 and a satisfactory cycle stability (retained 88.6 % over 8000 cycles).

Machine learning force field study of carboxylate ligands on the surface of zinc-blende CdSe quantum dots

Machine learning force field study of carboxylate ligands on the surface of zinc-blende CdSe quantum dots

Synthesis, crystal structures, characterization and luminescent thermal stability of Eu(III) and Gd(III) complex

Two new dinuclear lanthanide complexes [Ln2(Hsal)3(NO3)6]·(C2H5N)3·nH2O (Ln = Gd, Eu) were synthesized with an aromatic carboxylic acid ligand (salicylate). Both complexes were extensively characterized by various techniques, including elemental analysis, infrared spectroscopy, powder X-ray diffraction, and X-ray single-crystal diffraction. The thermal behavior and photoluminescence properties of lanthanide complexes were investigated. It was found that the two complexes had good thermal stability even after heating at 180 °C for 1 h. Furthermore, the two complexes exhibit a unique luminescent thermal stability.

The fate of cadmium during ferrihydrite phase transformation affected by dissolved organic matter: Insights from organic-mineral interaction

Ferrihydrite (Fh) is an important scavenger for sequestering cadmium (Cd). However, Fh is poorly crystallized and tends to undergo phase transformation under the impact of dissolved organic matter (DOM). The roles of DOM in Fh transformation pathway and the consequent Cd2+ remobilization behavior remain debate due to its structural heterogeneity and diversity. Herein, formic acid (FA), oxalic acid (Ox), and citric acid (CA) were selected as model DOM, which have one, two, and three carboxyl ligands, respectively. Fe K-edge extended X-ray absorption fine structure (EXAFS) spectra revealed an organic-specific correlation between Fh transformation and Cd2+ transport. FA with monodentate carboxyl ligand, lacking binding with Fh particles, had negligible influence on transformation dynamics and Cd2+ transport. The complexation of bidentate Ox with Fh particles impaired the stability of Fe(O,OH)6 octahedra, accelerating Fh phase transformation, thereby increasing Cd2+ release. During the subsequent recrystallization, Ox guided crystal growth via oriented attachment, which possibly occluded the remaining Cd2+ within structural defects of the secondary minerals. Conversely, tridentate CA exhibited a strong affinity with Fh, resulting in the formation of ferric citrate surface complexes that stabilized Fh against transformation and thus enhanced the long-term immobilization of Cd2+. These findings highlight the significant role of organic-ion/mineral interactions in Fh transformation, which are fundamentally linked to predicting Cd2+ transport behavior in environmental systems.

Coordination Stabilization of Fe by Porphyrin‐Intercalated NiFe‐LDH Under Industrial‐Level Alkaline Conditions for Long‐Term Electrocatalytic Water Oxidation

AbstractThe durable and economic electrocatalysts with high current density under industrial alkaline conditions are critical for advancing the industrial production of hydrogen energy by water electrolysis. The industrial highly alkaline electrolyte exacerbates the Fe dissolution of NiFe layered double hydroxide (NiFe‐LDH), leading to the dramatic degradation of stability and activity. The NiFe‐LDH intercalated Tetrakis(4‐carboxyphenyl)porphyrin (TCPP) (NiFe‐TCPP), 1,4,7,10‐Tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA) (NiFe‐DOTA) and CO32− (NiFe‐CO3) are fabricated respectively for electrocatalytic water oxidation under industrial alkaline conditions. In 10 m KOH, compared to NiFe‐DOTA (335.0 mV) and NiFe‐CO3 (499.2 mV), the resultant NiFe‐TCPP exhibits the lowest overpotentials of 290.2 mV at 1000 mA cm−2. The NiFe‐TCPP also operates continuously for 1000 h at 500 mA cm−2 with near‐zero attenuation. The theoretical and experimental studies reveal that the strong coordination between conjugated carboxylate ligand TCPP and Fe of the laminate inhibits the Fe leaching by increasing the dissolution energy barriers to 4.29 eV and improving the self‐healing ability, thus enhancing the stability. Furthermore, the charge redistribution induced by the strong coordination optimizes the d‐band centers (‐2.81 eV) and decreases the reaction energy barriers (1.47 eV), thereby increasing the catalytic activity.

Intramolecular C-H Oxidation in Iron(V)-oxo-carboxylato Species Relevant in the γ-Lactonization of Alkyl Carboxylic Acids.

High-valent oxoiron species have been invoked as oxidizing agents in a variety of iron-dependent oxygenases. Taking inspiration from nature, selected nonheme iron complexes have been developed as catalysts to elicit C-H oxidation through the mediation of putative oxoiron(V) species, akin to those proposed for Rieske oxygenases. The addition of carboxylic acids in these iron-catalyzed C-H oxidations has proved highly beneficial in terms of product yields and selectivities, suggesting the direct involvement of iron(V)-oxo-carboxylato species. When the carboxylic acid functionality is present in the alkane substrate, it acts as a directing group, enabling the selective intramolecular γ-C-H hydroxylation that eventually affords γ-lactones. While this mechanistic frame is solidly supported by previous mechanistic studies, direct spectroscopic detection of the key iron(V)-oxo-carboxylato intermediate and its competence for engaging in the selective γ-C-H oxidation leading to lactonization have not been accomplished. In this work, we generate a series of well-defined iron(V)-oxo-carboxylato species (2c-2f) differing in the nature of the bound carboxylate ligand. Species 2c-2f are characterized by a set of spectroscopic techniques, including UV-vis spectroscopy, cold-spray ionization mass spectrometry (CSI-MS), and, in selected cases, EPR and Mössbauer spectroscopies. We demonstrate that 2c-2f undergo site-selective γ-lactonization of the carboxylate ligand in a stereoretentive manner, thus unequivocally identifying metal-oxo-carboxylato species as the powerful yet selective C-H cleaving species in catalytic γ-lactonization reactions of carboxylic acids. Reactivity experiments confirm that the intramolecular formation of γ-lactones is in competition with the intermolecular oxidation of external alkanes and olefins. Finally, mechanistic studies, together with DFT calculations, support a mechanism involving a site-selective C-H cleavage in the γ-position of the carboxylate ligand by the oxo moiety, followed by a fast carboxylate rebound, eventually leading to the selective formation of γ-lactones.

Unactivated Pd-catalyzed oxidative cross-coupling reactions of arenes with benzene: a theoretical and experimental study

Oxidative C–H bond activated palladium-catalyzed aromatic cross-coupling reactions offer advantages of fewer required synthetic steps but disadvantages of loss of inherent regioselectivity and selectivity. In one proposed catalytic cycle, each arene adds to a palladium carboxylate catalyst according to a concerted metalation deprotonation mechanism, followed by reductive elimination to couple the arenes and the replenishment of the catalyst by an oxidant. Using this proposed mechanism, molecular structures and free energy profiles were calculated for coupling of benzene with a range of heteroarenes containing various substituents and with several Pd-carboxylate catalysts. Reaction kinetics were analysed using the energetic span model, predicting turnover frequencies (TOFs) of heterocoupling (HB and BH) and homocoupling (HH and BB) pathways and assessing degrees of turnover frequency control (XTOFs). More electron-withdrawing carboxylate ligands and more electron-donating heteroarene substituents decrease activation and reaction free energies of addition steps, with C-2 more favourable than C-3. When heteroarene and catalyst choice are particularly favourable, the first association can become exergonic, causing a significant change in catalytic kinetics. When not exergonic, free energy for addition of benzene is almost always higher, leading to large energetic spans and slower TOFs (BB < HB < BH < HH). Non-exergonic pathways are also faster with more electron-withdrawing ligands and more electron-donating heteroarene substituents because of lower energy addition steps and a smaller energetic span. For exergonic associations, the low-energy arene–catalyst intermediate creates a larger energetic span, slowing down the HH and HB pathways. More electron-withdrawing ligands and more electron-donating substituents increase the relative TOF for BH, and the reaction requires only small excesses of benzene to induce heterocoupling.

Three Metal Organic Frameworks of Cd(II) with Tri (4‐Carboxybenzyl) Amine and their Photoluminescence Properties

AbstractUtilizing the flexible carboxylic acid ligand, tri (4‐carboxybenzyl) amine (H3L), three new metal organic frameworks Na[Cd(L)]n ⋅ 3nH2O (1), [Cd(HL)(4,4’‐bipy)0.5(H2O)]n (2) (4,4’‐bipy=4,4’‐bipyridine) and [Cd(HL)(2,2’‐bipy)]n (3) (2,2’‐bipy=2,2’‐bipyridine) have been successfully synthesized. Compounds 1 and 2 are 2D structures with (3,6) 2‐nodal as kgd topology, while compound 3 is a 1D chain structure. The powder X‐ray diffraction and thermal stability of 1–3 were studied. In addition, the solid‐state luminescence properties of compounds 1–3 at room temperature were also tested, indicating their application prospects in fluorescent materials.

Cathodoluminescence of terbium coordination compounds

The cathodoluminescence (CL) of terbium coordination compounds is studied in order to establish its correlation to photoluminescence (PL), as well as to increase its intensity and stability over time. A special setup was designed for this purpose. It is found that terbium complexes anionic carboxylate ligands, as well as NO3, and neutral ligands of different nature display luminescence of various intensities under excitation by fast electrons. Their spectra contain only characteristic narrow bands of terbium f–f transitions. Terbium terephthalate shows the highest CL intensity, as well as high stability over time.