Tetracarboxylate Ligand Research Articles

Ligand controlled structure of cadmium(II) metal-organic frameworks for fluorescence sensing of Fe3+ ion and nitroaromatic compounds

Three cadmium(II) metal-organic frameworks (MOFs) based on tetracarboxylate ligands, namely [Cd2(TTTA)(DMF)3]·2DMF (1), [Cd2(TB)(H2O)4]·3DMF·H2O (2) and [Cd(TEB)0.5]·2DMF·4H2O (3) have been designed and synthesized. Complex 1 is a 2-dimensional (2D) 3,4-connected network with 3,4L13 topology, complex 2 features a 3-dimensional (3D) 3,4-connected tfa topology with a 2-fold interpenetrating structure and complex 3 has a 3D 4-connected dia topology with a 4-fold interpenetrating structure. Interestingly, 2 exhibits permanent pores and selective adsorption of CO2 over CH4. In addition, 2 shows fluorescence sensing of Fe3+ ion and rapid detection of nitroaromatic compounds (NACs) through fluorescence quenching.

A water stable metal–organic framework with 1D nanotube pores for 5-fluorouracil delivery and anti-brain tumor activity

The employment of porous carriers for anticancer drug delivery has gained much attention in recent years for their excellent drug-loading capacity and controlled release performance. In this work, by utilizing a rigid organic aromatic carboxylic acid ligand, a new Zn(II)–organic framework {[Zn2(bptc)(H2O)]·(DMAc)3(H2O)4}n (1, DMAc is in short of N,N-dimethylacetamide, H4bptc is in short of biphenyl-3,3′,5,5′-tetracarboxylic acid ligand), with considerable water stability has been prepared using the solvothermal technology. The structural analysis result reveals that complex 1 is constructed from fourfold helical metal chains whose net could be simplified into a (6,8)-connected topology. Moreover, complex 1 shows 1D nanotube channels with a window size of 10.7 × 10.7 A2 along the [001] direction and reveals a large accessible voids of 51.7%. The desolvated 1 (1a) was used to study the loading and releasing of the 5-fluorouracil (5-Fu), which reveals that 1a could uptake around 51.2 wt% of 5-Fu and nearly 85.6% of the absorbed 5-Fu could be released in PBS (7.4 for the pH value) in 100 h at 37 °C. In connection to these, in vitro cytotoxicity of compound 1 together with the anticancer performance of 5-Fu-loaded 1a was evaluated against human oral epidermal cells and human brain tumor cells SF17 via MTT assays.

Metal–Organic Frameworks with Extended Viologen Units: Metal-Dependent Photochromism, Photomodulable Fluorescence, and Sensing Properties

The photoresponsive viologen unit has been widely used to endow metal–organic materials with photochromic and other photomodulable properties. Herein we report the first examples of the metal–organic materials functionalized by extended viologens (ExVs), of general formula [ML]·2H2O (M = Zn for 1, M = Mn for 2, M = Co for 3, and L is a tetracarboxylate ligand with the p-phenylene-extended viologen spacer). Of the three isomorphic metal–organic frameworks, only 1 is photochromic owing to formation of extended viologen radicals through photoinduced electron transfer (PET). The incapability of 2 and 3 to undergo photochromism can be ascribed to longer intermolecular donor–acceptor contacts, emphasizing the sensitivity of solid-state PET to structural changes. 1 also shows strong fluorescence owing to interligand charge transfer, and the fluorescence can be reversibly modulated and switched on/off in the photochromic process. Furthermore, 1 shows excellent hydrolytic stability and can be used as a sensitive, ...

Cryogenic Luminescent Tb/Eu-MOF Thermometer Based on a Fluorine-Modified Tetracarboxylate Ligand.

A fluorine-modified tetracarboxylic acid ligand, namely, 2'-fluoro-[1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylic acid (H4FTPTC), with suitable triplet energy excited state, was designed and applied to construct the luminescent lanthanide metal-organic frameworks (LnMOFs) for cryogenic temperature sensing. With the lanthanides codoping strategy, we developed a new Tb3+/Eu3+ mixed LnMOF system Tb1- xEu xFTPTC ( x = 0.05, 0.1, 0.2), which feature excellent linear responses to temperature with high relative sensitivity in the cryogenic range of 25-125 K. It was found that the relative sensitivity of such mixed LnMOF could readily be tuned by adjusting the incorporation amount of Eu3+ ions in the host framework. In addition, the energy transfer efficiency between the Tb3+ and Eu3+ ions in the framework with different Tb3+/Eu3+ ratios are also investigated and discussed.

Biporous Metal-Organic Framework with Tunable CO2/CH4 Separation Performance Facilitated by Intrinsic Flexibility.

In this work, we report the synthesis of SION-8, a novel metal–organic framework (MOF) based on Ca(II) and a tetracarboxylate ligand TBAPy4– endowed with two chemically distinct types of pores characterized by their hydrophobic and hydrophilic properties. By altering the activation conditions, we gained access to two bulk materials: the fully activated SION-8F and the partially activated SION-8P with exclusively the hydrophobic pores activated. SION-8P shows high affinity for both CO2 (Qst = 28.4 kJ/mol) and CH4 (Qst = 21.4 kJ/mol), while upon full activation, the difference in affinity for CO2 (Qst = 23.4 kJ/mol) and CH4 (Qst = 16.0 kJ/mol) is more pronounced. The intrinsic flexibility of both materials results in complex adsorption behavior and greater adsorption of gas molecules than if the materials were rigid. Their CO2/CH4 separation performance was tested in fixed-bed breakthrough experiments using binary gas mixtures of different compositions and rationalized in terms of molecular interactions. SION-8F showed a 40–160% increase (depending on the temperature and the gas mixture composition probed) of the CO2/CH4 dynamic breakthrough selectivity compared to SION-8P, demonstrating the possibility to rationally tune the separation performance of a single MOF by manipulating the stepwise activation made possible by the MOF’s biporous nature.

From Zn(II) to Cu(II) framework via single-crystal to single-crystal metathesis with superior gas uptake and heterogeneous catalytic properties

In this work, a Zn(II) framework, {[Zn2(L)4(H2O)4]·(7DMF)(7H2O)}n (1Zn), has been synthesized using a bent tetracarboxylic acid ligand (H4L). The structure of 1Zn contains [Zn2(COO)4] paddle-wheel secondary building units with axial sites occupied by water molecules. Interestingly, the isostructural Cu(II) framework {[Cu2(L)4(H2O)4]·(7DMF)(7H2O)}n (1Cu) has been obtained via single-crystal to single-crystal metathesis reaction. The activated 1Cu (denoted as a1Cu; a stands for activated) has open coordination sites. This species showed enhanced CO2 adsorption and heterogeneous catalytic properties for the Hantzsch coupling reaction involving condensation of an aldehyde with ethyl acetoacetate and ammonium acetate to produce 1,4-dihydropyridines, and three-component coupling of amines, aldehydes and alkynes to generate propargylic amines with high efficiency.

Microporous Cu metal-organic framework constructed from V-shaped tetracarboxylic ligand for selective separation of C2H2/CH4 and C2H2/N2 at room temperature

A novel microporous Cu-MOF [Cu3(L)2(DMF)]·(H3O)2·3DMF, 1, with dual functionalities of open metal sites (OMSs) and Lewis basic sites (LBSs) based on a V-shaped organic ligand 5,5'-azanediyldiisophthalate was synthesized by hydrothermal method. The framework contains two types of 1D channels with BET surface areas 419 m2 g−1 and pore volume 0.23 cm3 g−1. It shows moderately high adsorption selectivity for C2H2/CH4 and C2H2/N2 at room temperature, the selectivity is 122 and 7.8 (50% and 50%), 181 and 165.7 (5% and 95%).

Tetracarboxylate Bis-Bipyridine Ruthenium Dyes: Synthesis, Structural and Electronic Characterisation.

The preparation of ruthenium complexes with novel 2,2'-bipyridine (bpy) ligands bearing four carboxylic acid groups was investigated with a view to creating dyes containing more than two potential anchoring groups per bpy unit for attachment to a titania surface. Synthetic challenges are encountered upon using the 2,2'-bipyridine-3,3',4,4'-tetracarboxylic acid ligand because it readily decarboxylates. The use of the methyl esterified derivative (3) proved to be more successful for complex preparation, with a robust preparation of the [Ru(3)2 Cl2 ] complex identified with diglyme as the solvent. This complex was further converted into the thiocyanato complex, [Ru(3)2 (NCS)2 ], which could not be completely de-esterified. X-ray analysis of crystals obtained from a mixture of isomers for this complex provided data for the S,S- and N,S-coordinated isomers; both showed a twisted arrangement of the pyridine rings in the 2,2'-bipyridine-3,3',4,4'-tetracarboxylic acid ligand, owing to steric hinderance. Conversely, the isosteric 2,2'-bipyridine-4,4',5,5'-tetracarboxylic acid ligand was easily converted into the desired [Ru(2)2 (NCS)2 ] complex through a standard one-pot procedure in N,N-dimethylformamide solvent. All of the complexes presented herein exhibit a significant redshift for the metal to ligand charge-transfer bands, relative to the benchmark ruthenium dye N719 and derivatives thereof. All complexes exhibit a quasi-reversible process for the ruthenium(II/III) couple at approximately 0.4 V versus the ferrocene couple, comparable to analogous ruthenium dyes.

Switchable Ferro-, Ferri-, and Antiferromagnetic States in a Piezo- and Hydrochromic Metal-Organic Framework.

The Mn(II) metal-organic framework with a viologen-based tetracarboxylate ligand exhibits reversible optical (color) and magnetic changes concomitant with stimuli-induced electron transfer from carboxylate to viologen. Compression causes a magnetic transformation from ferro- to ferrimagnetic, while water release/reuptake switches the magnetic behavior between ferro- and antiferromagnetic.

Synthesis, structure and characterization of a new highly porous zirconium-based metal-organic frameworks

A new highly porous zirconium-based metal-organic frameworks MOF-1 was constructed from a vinyl tetracarboxylic ligand and inorganic building blocks [Zr6O4(OH)4(CO2)12] under solvothermal conditions. MOF-1 crystallize in the cubic Pm-3m space group with a unique ftw topology. Additionally, MOF-1 performs a prominently chemical, thermal stability and high nitrogen absorption.

A phase transformable ultrastable titanium-carboxylate framework for photoconduction

Porous titanium oxide materials are attractive for energy-related applications. However, many suffer from poor stability and crystallinity. Here we present a robust nanoporous metal–organic framework (MOF), comprising a Ti12O15 oxocluster and a tetracarboxylate ligand, achieved through a scalable synthesis. This material undergoes an unusual irreversible thermally induced phase transformation that generates a highly crystalline porous product with an infinite inorganic moiety of a very high condensation degree. Preliminary photophysical experiments indicate that the product after phase transformation exhibits photoconductive behavior, highlighting the impact of inorganic unit dimensionality on the alteration of physical properties. Introduction of a conductive polymer into its pores leads to a significant increase of the charge separation lifetime under irradiation. Additionally, the inorganic unit of this Ti-MOF can be easily modified via doping with other metal elements. The combined advantages of this compound make it a promising functional scaffold for practical applications.

Mg(II) Coordination Polymers Based on Flexible Isomeric Tetracarboxylate Ligands: Syntheses, Structures, Structural Transformation and Luminescent Properties.

By using a new flexible tetracarboxylic acid, bis(3,5-dicarboxyphenyl) adipoamide, H4L1, and its isomer, bis(2,5-dicarboxyphenyl)adipoamide, H4L2, three Mg(II) coordination polymers, {[Mg2(L1)(H2O)2]·2EtOH·3H2O}n, 1, [Mg2(L1)(H2O)8]n, 2, and {[Mg2(L2)(H2O)6]·H2O}n, 3, have been hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction. Complexes 1 and 2 are the solvent ratio-dependent hydrothermally stable products. The tetracarboxylate ligand of complex 1 connects eight Mg(II) ions through nine oxygen atoms, resulting in a three-dimensional (3D) 5-connected uninodal net with a rare non-interpenetrating (44.66)-pcu-5-Pmna topology, whereas those of 2 and 3 link four Mg(II) ions through four oxygen atoms and six Mg(II) ions through six oxygen atoms, forming a 1D linear chain and a 3,6-connected 2-nodal 3D net having {4.62}2{42.610.83}-rtl topology, respectively. Complex 1 shows a series of structural transformations on heating to 200 °C and almost reversible structural transformation when the activated products were immersed in a mixture of ethanol and water or on hydrothermal. Likewise, complex 2 exhibits a reversible structural transformation on heating/hydrothermal, while 3 exhibits irreversible structural transformations. All three complexes exhibit blue light emissions, with that of complex 3 being much more intense.

Electron Injection from Photoexcited Metal-Organic Framework Ligands to Ru2 Secondary Building Units for Visible-Light-Driven Hydrogen Evolution.

We report the design of two new metal-organic frameworks (MOFs), Ru-TBP and Ru-TBP-Zn, based on Ru2 secondary building units (SBUs) and porphyrin-derived tetracarboxylate ligands. The proximity of Ru2 SBUs to porphyrin ligands (∼1.1 nm) facilitates multielectron transfer from excited porphyrins to Ru2 SBUs to enable efficient visible-light-driven hydrogen evolution reaction (HER) in neutral water. Photophysical and electrochemical studies revealed oxidative quenching of excited porphyrin by Ru2 SBUs as the initial step of the HER process and the energetics of key intermediates in the catalytic cycle. Our work provides a new strategy to building multifunctional MOFs with synergistic ligands and SBUs for efficient photocatalysis.

A bifunctional luminescent Tb(III)-metal-organic framework by a tetracarboxylate ligand for highly selective detection of Fe3+ cation and Cr2O72- anion

Reaction of Tb3+ ions with p-terphenyl-3,3″,5,5″-tetracarboxylic acid (H4ptptc) in a mixed solvent system has afforded a new metal-organic framework formulated as [Tb2(ptptc)1.5(H2O)2]n (1). Compound 1 displays a 3D (5,6,8)-connected framework with fascinating one-dimensional triangle open channels. The luminescence explorations demonstrated that 1 exhibits highly selective and sensitive response to Fe3+ in DMF solution and biological system through luminescence quenching effects. In addition, 1 also shows high detection for the Cr2O72-, making it a promising dual functional materials for detecting Fe3+ cation and Cr2O72- anion with high sensitivity and selectivity.

Two Chemically Stable Cd(II) Polymers as Fluorescent Sensor and Photocatalyst for Aromatic Dyes.

Two new 2D Cd(II)-based coordination polymers (CPs), viz. [Cd2(H2L)2(2,2’-bipy)2] (1) and [Cd(L)0.5(phen)·0.5H2O] (2), have been constructed using ethylene glycol ether bridging tetracarboxylate ligand 5,5′(4,4′-phenylenebis(methyleneoxy)) diisophthalic acid (H4L). Both CPs behaved as profound fluorescent sensor for Fe3+ ion and nitro-aromatics (NACs), specifically 2,4,6-trinitrophenol (TNP). The stability at elevated temperature and photocatalytic behaviors of both 1 and 2 for photo-decomposition of aromatic dyes have also been explored. An attempt has been made to explore the plausible mechanism related with the decrease in fluorescence intensity of 1 and 2 in presence of NACs using theoretical calculations. Additionally, the probable mechanism of photo catalysis by 1 and 2 to photo-degrade aromatic dyes has been explained using density of states (DOS) calculations.

A porous rhodium(III)-porphyrin metal-organic framework as an efficient and selective photocatalyst for CO2 reduction

A rhodium(III)-porphyrin zirconium metal-organic framework (Rh-PMOF-1(Zr)) has been prepared from the self-assembly of a Rh-based metalloporphyrin tetracarboxylic ligand Rh(TCPP)Cl (TCPP = tetrakis(4-carboxyphenyl)porphyrin) with ZrCl4. The framework of Rh-PMOF-1 is stable up to 270 °C as disclosed by the variable-temperature powder X-ray diffraction (VT-PXRD) measurements, and possesses good chemical stability over a wide range of solvents including water. The single-crystal structural analysis reveals that Rh-PMOF-1 contains 3-D channels (1.9 × 1.9 nm2), and the Rh-porphyrin units are exposed to the cavities. The calculation based on the N2 adsorption at 77 K shows Rh-PMOF-1(Zr) has a high BET surface area (3015 m2g−1). The luminescence decay of Rh-PMOF-1 is well fitted to a tri-exponential curve featuring a long average lifetime of 207 μs at 298 K under vacuum, which represents a rare example of room-temperature phosphorescence of Rh-porphyrin complexes. Under 1 atm, it displays CO2 uptake up to 42, 53 and 98 cm3g−1 at 308, 298 and 273 K, respectively. Catalytic results show that, under the visible light (≥400 nm) irradiation without any additional photosensitizer, Rh-PMOF-1 is powerful to catalyze CO2 reduction to the formate ion with up to 99% selectivity, and can be recycled and reused for 3 runs. Theoretical study was further carried out to reveal the energy levels of the frontier orbitals of Rh-PMOF-1 and the preferred binding sites of CO2 in the framework.

Two 3D Coordination Polymers with New Topology Based on Mixed Ligands: Synthesis, Structure, and Photoluminescence Properties

Employing tetracarboxylate and imidazole mixed ligands to react with different transition meat salts afford two new 3D coordination polymers, {[Zn2(BPTC)(BBI)2]·(H2O)3·DMSO}n (1) and {[Cd2(BPTC)(BBI)2]·H2O}n (2) (H4BPTC = biphenyl-3,3′,4,4′-tetracarboxylic acid, BBI = 1,1′-(1,4-butanediyl)bis(imidazole)). Both these two coordination polymers are 4-connected network topologies. Compound 1 features a unprecedented 3-nodal (4,4,4)-connected network topology with the point symbol {64·82}, and 2 displays a (4,4)-connected binodal network bearing new topology with the point symbol of {72·84}2{73·83}. Additionally, thermal stability and photoluminescence properties of 1 and 2 were investigated.

Unusual and Tunable Negative Linear Compressibility in the Metal-Organic Framework MFM-133(M) (M = Zr, Hf).

High-pressure single-crystal X-ray structural analyses of isostructural MFM-133(M) (M = Zr, Hf) of flu topology and incorporating the tetracarboxylate ligand TCHB4- [H4TCHB = 3,3',5,5'-tetrakis(4-carboxyphenyl)-2,2',4,4',6,6'-hexamethyl-1,1'-biphenyl] and {M6(μ3-OH)8(OH)8(COO)8} clusters confirm negative linear compressibility (NLC) behavior along the c axis. This occurs via a three-dimensional wine-rack NLC mechanism leading to distortion of the octahedral cage toward a more elongated polyhedron under static compression. Despite the isomorphous nature of these two structures, MFM-133(Hf) shows a higher degree of NLC than the Zr(IV) analogue. Thus, for the first time, we demonstrate here that the NLC property can be effectively tuned in a framework material by simply varying the inorganic component of the frameworks without changing the network topology and structure.

Shedding Light on the Protonation States and Location of Protonated N Atoms of Adenine in Metal–Organic Frameworks

We report the syntheses and structures of five metal–organic frameworks (MOFs) based on transition metals (NiII, CuII, and ZnII), adenine, and di-, tri-, and tetra-carboxylate ligands. Adenine, with multiple N donor sites, was found to coordinate to the metal centers in different binding modes including bidentate (through N7 and N9, or N3 and N9) and tridentate (through N3, N7, and N9). Systematic investigations of the protonation states of adenine in each MOF structure via X-ray photoelectron spectroscopy revealed that adenine can be selectively protonated through N1, N3, or N7. The positions of H atoms connected to the N atoms were found from the electron density maps, and further supported by the study of C–N–C bond angles compared to the literature reports. DFT calculations were performed to geometrically optimize and energetically assess the structures simulated with different protonation modes. The present study highlights the rich coordination chemistry of adenine and provides a method for the determination of its protonation states and the location of protonated N atoms of adenine within MOFs, a task that would be challenging in complicated adenine-based MOF structures.

Syntheses, crystal structures, and properties of two dinuclear iron(III) complexes constructed with 1,2,3,5-benzenetetracarboxylic acid and chelating N-donor auxiliary coligands

Two dinuclear Fe(III) metal–organic complexes with tetracarboxylate and chelating N-donor ligands, [Fe(Hbtec)(phen)(H2O)]2·2H2O (1) and [Fe(Hbtec)(bpy)(H2O)]2·2H2O (2) (H4btec = 1,2,3,5-benzenetetracarboxylic acid, phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine) have been prepared and characterized by elemental analysis, IR spectroscopic, and X-ray diffraction methods. Both complexes crystallize in the monoclinic space group P21/c with two Fe(III) ions bridged by two Hbtec3− ligands into a dinuclear unit. Hydrogen bonding connects the dinuclear units into a 3-D framework. The dinuclear units are 10-connected nodes that produce a 3-D framework with topology Schläfli symbol as (312·428·55). Thermal stabilities and luminescent properties of the two complexes have also been investigated.