A Water-Stable Cationic SIFSIX MOF for Luminescent Probing of Cr2 O7 2- via Single-Crystal to Single-Crystal Transformation

The sensing and monitoring of toxic oxo-anion contaminants in water are of significant importance to biological and environmental systems. A rare hydro-stable SIFSIX metal-organic framework, SiF6 @MOF-1, {[Cu(L)2 (H2 O)2 ]·(SiF6 )(H2 O)}n , with exchangeable SiF6 2- anion in its pore is strategically designed and synthesized, exhibiting selective detection of toxic Cr2 O7 2- oxo-anion in an aqueous medium having high sensitivity, selectivity, and recyclability through fluorescence quenching phenomena. More importantly, the recognition and ion exchange mechanism is unveiled through the rarely explored single-crystal-to-single crystal (SC-SC) fashion with well-resolved structures. A thorough SC-SC study with interfering anions (Cl- , F- , I- , NO3 - , HCO3 - , SO4 2- , SCN- , IO3 - ) revealed no such transformations to take place, as per line with quenching studies. Density functional theory calculations revealed that despite a lesser binding affinity, Cr2 O7 2- shows strong orbital mixing and large driving forces for electron transfer than SiF6 2- , and thus enlightens the fluorescence quenching mechanism. This work inaugurates the usage of a SIFSIX MOF toward sensing application domain under aqueous medium where hydrolytic stability is a prime concern for their plausible implementation as sensor materials.

Highly Robust Metal-Organic Framework for Efficiently Catalyzing Knoevenagel Condensation and the Strecker Reaction under Solvent-Free Conditions

Metal-organic frameworks (MOFs) have been recognized as one of the most promising porous materials and offer great opportunities for the rational design of new catalytic solids having great structural diversity and functional tunability. Despite numerous inherent merits, their chemical environment instability limits their practical usage and demands further exploration. Herein, by employing the mixed-ligand approach, we have designed and developed a robust 3D Co-MOF, [Co2(μ2-O)(TDC)2(L)(H2O)2]·2DMF (H2TDC = 2,5-thiophenedicarboxylic acid, L = 3,3'-azobispyridine), IITKGP-50 (IITKGP stands for the Indian Institute of Technology Kharagpur), which exhibited excellent framework robustness not only in water but also in a wide range of aqueous pH solutions (pH = 2-12). Taking advantage of superior framework robustness and the presence of high-density open metal sites, IITKGP-50 was further explored in catalyzing the two-component Knoevenagel condensation reaction and three-component Strecker reactions. Moreover, to verify the size selectivity of IITKGP-50, smaller to bulkier substrates in comparison with the MOF's pore cavity (8.1 × 5.6 Å2) were employed, in which relatively lesser conversions for the sterically bulkier aldehyde derivatives confirmed that the catalytic cycle occurs inside the pore cavity. The easy scalability, lower catalyst loading compared to that of benchmark MOFs, magnificent conversion rate over a wide range of substrates, and excellent recyclability without significant performance loss made IITKGP-50 a promising heterogeneous catalyst candidate.

Highly Luminescent Crystalline Sponge: Sensing Properties and Direct X-ray Visualization of the Substrates

A phenomenon of crystalline sponge is represented by guest-dependent structural fluidity of the host polymeric lattice in highly crystalline sorbents, such as metal-organic frameworks, driven by multiple weak intermolecular interactions. Such induced fitting in MOFs is a valuable property in selective adsorption, guest determination by single-crystal XRD and in-situ structural analysis under external stimuli. In this work, a porous three-dimensional metal-organic framework [Eu₂(DMF)₄(ttdc)₃]·4.45DMF (1_DMF; DMF = N,N-dimethylformamide, ttdc^2- = trans-thienothiophenedicarboxylate anion) was applied as a crystalline sponge bearing luminescent functionality to couple its sensing properties with direct structural determination of the adsorbed molecules. As a result, the paper discusses crystal structures and luminescent properties for the successfully obtained new adducts with the crystallographic formulae [Eu₂(DMSO)₄(ttdc)₃]·2.5DMSO·2.2H₂O (1_DMSO; DMSO = dimethylsulfoxide), [Eu₂(DMF)₄(ttdc)₃]·3phet (1_phet; phet = phenylethanal) and [Eu₂(DMF)_3.5(cin)_0.5(ttdc)₃]·1.64cin (1_cin; cin = trans-cinnamaldehyde). As a result of inclusion of DMSO into 1, a slight increase in the quantum yield and excited state phosphorescence lifetime was observed, while the adsorption of phet leads to a considerable (up to three times) decrease in the corresponding values. The incorporation of cinnamal results in a full quenching of QY, from 20% down to zero, and a more than order of magnitude diminishing of the excited state lifetime compared to the initial 1_DMF. The effective sensing of cinnamal was explained from the structural point of view by its direct coordination to the Eu³⁺ emitter, as well as by multiple weak intermolecular interactions with ttdc antenna ligand, both capable of enhancing the non-radiative energy dissipation.

Variable Dimensionality of Europium(III) and Terbium(III) Coordination Compounds with a Flexible Hexacarboxylate Ligand

A reaction between 4,4',4″-(benzene-1,3,5-triyltris(oxy))triphthalic acid (H₆L) and lanthanide(III) nitrates (Ln = Eu³⁺, Tb³⁺) in water under the same conditions gave a molecular coordination compound [Tb(H_4.5L)₂(H₂O)₅]∙6H₂O in the case of terbium(III) and a one-dimensional linear coordination polymer {[Eu₂(H₃L)₂(H₂O)₆]∙8H₂O}_n in the case of europium(III). The crystal structures of both compounds were established by single-crystal X-ray diffraction, and they were further characterized by powder X-ray diffraction, thermogravimetric analysis and infrared spectroscopy. The compounds demonstrated characteristic lanthanide-centered photoluminescence. The lanthanide-dependent dimensionality of the synthesized compounds, which are the first examples of the coordination compounds of hexacarboxylic acid H₆L demonstrates its potential as a linker for new coordination polymers.