A Multivariate 2D Metal-Organic Framework with Open Metal Sites for Catalytic CO2 Cycloaddition and Cyanosilylation Reactions

This work reports the synthesis of a dual functional 2D framework, {[Zn2(4-tpom)2(oxdz)2]·4H2O}n (1), at room temperature, where a bent dicarboxylate, oxdz2- (4,4'-(1,3,4-oxadiazole-2,5-diyl)dibenzoate), and a neutral flexible N-donor linker, 4-tpom (tetrakis(4-pyridyloxymethylene)methane), are utilized. Its single-crystal X-ray analysis indicated a 2-fold interpenetrated 2D framework having tetracoordinated Zn(II) centers and dangling pyridyl groups. Its further characterization was carried out with elemental microanalysis, FTIR spectroscopy, TG analysis, and powder X-ray diffraction. The tetracoordinated Zn(II) centers as active Lewis acidic sites and the N atoms of 4-tpom as Lewis basic sites in 1 are explored for its functioning as a heterogeneous catalyst in two important reactions, (i) cycloaddition of CO2 with various epoxides and (ii) cyanosilylation reaction under solvent-free conditions. We could successfully show the cycloaddition of styrene oxide with CO2 (99% conversion) under balloon pressure with low catalyst (0.2-0.3 mol %) and cocatalyst (0.5-0.75 mol %) loadings, which is otherwise difficult to achieve. It was observed that all the substrates (aromatic and aliphatic), irrespective of their sizes, showed conversion percentage >99%. In the cyanosilylation reaction, a conversion of 96% was obtained with 1.5 mol % of 1 at room temperature for 12 h. This framework emerged as an excellent recyclable catalyst for both the reactions.

Effect of Unsaturated Metal Site Modulation in Highly Stable Microporous Materials on CO2 Capture and Fixation

We have designed and synthesized two unprecedented microporous three-dimensional metal-organic frameworks, {[Cd₆(TPOM)₃(L)₆]·12DMF·3H₂O}_n (1) and {[Zn₂(TPOM)(L)₂]·2DMF·H₂O}_n (2), based on a flexible quadritopic ligand, tetrakis(4-pyridyloxymethylene)methane (TPOM), and a bent dicarboxylic acid, 4,4'-(dimethylsilanediyl)bis-benzoic acid (H₂L). The networks of 1 and 2 share a 4-c uninodal net NbO topology but exhibit different metal environments due to coordination preferences of Cd(II) and Zn(II). The Cd(II) center in 1 is six-coordinated, whereas the Zn(II) center in 2 is only four-coordinated, making the latter an unsaturated metal center. Such modulation of coordination atmosphere of metal centers in MOFs with the same topology is possible due to diverse binding of the carboxylate groups of L^2-. Both 1 and 2 have relatively high thermal stability and exhibit permanent porosity after the removal of guest solvent molecules based on variable temperature powder X-ray diffraction and gas adsorption analysis. These materials exhibit similar gas adsorption properties, especially highly selective CO₂ uptake/capture over other gases (N₂ and CH₄). However, because of the presence of an unsaturated Lewis acidic metal site, 2 acts as a very efficient heterogeneous catalyst toward the chemical conversion of CO₂ to cyclic carbonates under mild conditions, whereas 1 shows very less activity. This work provides experimental evidence for the postulate that an unsaturated metal site in MOFs enhances adsoprtion of CO₂ and promotes its conversion via the Lewis-acid catalysis.

Strategic Construction of Highly Stable Metal-Organic Frameworks Combining Both Semi-Rigid Tetrapodal and Rigid Ditopic Linkers: Selective and Ultrafast Sensing of 4-Nitroaniline in Water

In this work, we have designed two new 3D metal-organic frameworks (MOFs), {Zn₄(TPOM)(1,4-NDC)₄} _n (1) and {Ni₂(TPOM)(1,4-NDC)₂(H₂O)₂} _n (2), utilizing both semi-rigid tetrapodal neutral linker, tetrakis(4-pyridyloxymethylene)methane (TPOM) and rigid ditopic anionic linker, 1,4-naphthalene dicarboxylic acid (H₂(1,4-NDC)). On the basis of the single-crystal X-ray diffraction, 1 has a 3D structure with star-shaped pores arising from four-fold symmetry due to the presence of a paddle-wheel core [Zn₂(O₂CC₁₂H₆)₄(C₆H₄N)₂] as a subunit, whereas 2 consists of a zig-zag 3D framework with strong hydrogen bonding between the coordinated water molecules and coordinated carboxylate groups. Their thermogravimetric analysis indicates an extraordinary thermal stability: 1 up to 400 °C and 2 up to 350 °C. In addition to elemental microanalysis and spectroscopic characterization (UV-vis and infra-red spectroscopy), the bulk phase purity of 1 and 2 as well as hydrolytic stability of 1 are established by powder X-ray diffraction. Exploiting the luminescence nature of 1, both solvent-dependent fluorescence properties and sensing of various amines in aqueous medium are demonstrated. It exhibits good sensing ability toward 4-nitroaniline (4-NA) and 2,6-dichloro-4-nitroaniline (2,6-DCNA; a broad spectrum pesticide belonging to toxicity class III) with the lowest detection limit of 88 ppb and 0.28 ppm, respectively. The mechanism of action has been established through Stern-Volmer plots, time-resolved fluorescence studies, spectral overlap, and density functional theory calculations. The recyclability and stability of 1 after sensing experiments also reveal no change in its crystallinity. Furthermore, selectivity test and time-dependent detection for 4-NA have been successfully demonstrated. For practical applications, naked eye detection of 4-NA using test paper strips is also displayed.

Linking cobalt–water chains with cis-1,4-cyclohexanedicarboxylate bridges to form a 2D network exhibiting spin-canted antiferromagnetism

A 2D magnetic network consisting of water-bridged 1D Co²⁺ chains and cis-CHDA bridges displays spin-canted antiferromagnetism with spin-glass behavior.

Controllable assembly of Cu(ii) coordination compounds based on a flexible zwitterionic benzimidazole–dicarboxylate ligand: synthesis, structural diversity, reversible SCSC transformation and magnetic properties

The reversible SCSC transformation of a paddle-wheel cluster to a 1D linear polymer could be accomplished.

A porous framework based on tetrakis(4-pyridyloxymethyl)methane fine-tuned by metal ions: synthesis, crystal structures and adsorption properties

Solvothermal reactions of tetrakis(4-pyridyloxymethyl)methane and 4,4′-oxybisbenzoic acid with different metal ions produced three new coordination polymers.

A new family of 1D, 2D and 3D frameworks aggregated from Ni5, Ni4 and Ni7 building units: synthesis, structure, and magnetism

Three polynuclear nickel(ii) complexes with 1D, 2D and 3D structures are controlled by carboxylate bridges of biphenyl-3,4′,5-tricarboxylic acid. Magnetic studies reveal that the polymers have ferromagnetic coupling features for 1 and 2 and an alternating magnetic chain behavior for 3.

Structural diversities and magnetic properties of azide-containing coordination polymers based on flexible tetra-pyridinate ligands

By utilizing two quadritopic ligands, four new azido-containing coordination polymers have been synthesized and structurally characterized.

A metal–organic framework constructed using a flexible tripodal ligand and tetranuclear copper cluster for sensing small molecules

A porous MOF based on a tetranuclear copper cluster with intracluster antiferromagnetic interactions was synthesized, which reveals highly sensitive and selective sensing of small molecules.

A series of coordination polymers based on varied polycarboxylates and different imidazole-containing ligands: syntheses, crystal structures and physical properties

To investigate the effect of organic ligands on the coordination frameworks, seven nickel(ii) coordination polymers, based on varied polycarboxylates and different imidazole-containing ligands, are synthesized under hydrothermal method.

In situ generation of functionality in a reactive binicotinic-acid-based ligand for the design of multi-functional copper(ii) complexes: syntheses, structures and properties

Five copper binicotinic complexes have been synthesized depending on the reaction conditions. Complexes 1 and 2 display excellent catalytic activity for Strecker reaction of imines.

Temperature-induced one-dimensional chiral Ag(i) linear chains and left-handed 21 helices: DFT studies, luminescence and SHG response

Two pairs of chiral coordination polymers have been synthesized with the same materials at 25 and 90 °C, respectively.