Synthesis, Structural Versatility, Magnetic Properties, and I- Adsorption in a Series of Cobalt(II) Metal-Organic Frameworks with a Charge-Neutral Aliphatic (O,O)-Donor Bridge.

Four new metal-organic frameworks based on cobalt(II) salts and 1,4-diazabicyclo[2.2.2]octane N,N'-dioxide (odabco) were obtained. Their crystallographic formulae are [Co3(odabco)2(OAc)6] (1, OAc- = acetate), [Co(H2O)2(HCOO)2]·odabco (2), [Co2(H2O)(NO3)(odabco)5](NO3)3·3.65H2O (3), and [Co2(DMF)2(odabco)4](NO3)4·3H2O (4; DMF = N,N-dimethylformamide). Crystal structures of 1-4 were determined by single-crystal X-ray crystallography. Coordination polymer 1 comprises binuclear and mononuclear metal-acetate blocks alternating within uncharged one-dimensional chains, in which odabco acts as a bridging ligand. A layered Co(II) formate 2 contains odabco only as guest molecules located in the interlayer space. Layered compound 3 and three-dimensional 4 have cationic coordination frameworks with 26% and 34% specific void volumes, respectively, unveiling high structural diversity of Co(II)-odabco MOFs based on quite a rare aliphatic moiety. Magnetization measurements were performed for 1, 3, and 4 and the obtained data were interpreted on the basis of their crystal structures. A strong (J/kB~100 K) antiferromagnetic coupling was found within binuclear metal blocks in 1. Ion exchange experiments revealed a considerable iodide uptake by 3 resulting in an up to 75% guest nitrate substitution within the voids of a coordination framework, found by capillary zone electrophoresis data and confirmed by single-crystal XRD. A preservation of 3 crystallinity during the exchange allowed for the guest I- positions within a new adduct with the formula [Co2(H2O)(NO3)(odabco)5]I2(NO3)·1.85H2O (3-I) to be successfully determined and the odabco aliphatic core to be revealed as a main adsorption center for quite large and easily polarizable iodide anions. In summary, this work presents a comprehensive study for a series of 1,4-diazabicyclo[2.2.2]octane N,N'-dioxide-based MOFs of cobalt(II) within the framework of magnetic properties and reports the first example of anion exchange in odabco-based coordination networks, supported by direct X-ray structural data. The reported results unveil promising applications of such frameworks bearing ligands with an aliphatic core in the diverse structural design of selective adsorbents and other types of functional materials.

Diastereoselective guest-shape dependent [2+2]-photodimerization of 2-cyclopenten-1-one trapped within a metal-organic framework.

UV-induced [2+2] dimerization of 2-cyclopenten-1-one and 2-methyl-2-cyclopenten-1-one was successfully carried out in a single-crystal-to-single-crystal manner within a porous metal-organic framework. Intermolecular contacts direct the orientation of the α,ß-enone molecules within the host channels, which drives the subsequent photoaddition reaction in a facile and diastereoselective fashion, yielding head-to-tail anti dimers only.

Properties of Aliphatic Ligand-Based Metal-Organic Frameworks.

Ligands with a purely aliphatic backbone are receiving rising attention in the chemistry of coordination polymers and metal-organic frameworks. Such unique features inherent to the aliphatic bridges as increased conformational freedom, non-polarizable core, and low light absorption provide rare and valuable properties for their derived MOFs. Applications of such compounds in stimuli-responsive materials, gas, and vapor adsorbents with high and unusual selectivity, light-emitting, and optical materials have extensively emerged in recent years. These properties, as well as other specific features of aliphatic-based metal-organic frameworks are summarized and analyzed in this short critical review. Advanced characterization techniques, which have been applied in the reported works to obtain important data on the crystal and molecular structures, dynamics, and functionalities, are also reviewed within a general discussion. In total, 132 references are included.

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 (1DMF; DMF = N,N-dimethylformamide, ttdc2- = 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 (1DMSO; DMSO = dimethylsulfoxide), [Eu2(DMF)4(ttdc)3]·3phet (1phet; phet = phenylethanal) and [Eu2(DMF)3.5(cin)0.5(ttdc)3]·1.64cin (1cin; 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 1DMF. 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.

Cinnamal Sensing and Luminescence Color Tuning in a Series of Rare-Earth Metal-Organic Frameworks with Trans-1,4-cyclohexanedicarboxylate.

Three isostructural metal-organic frameworks ([Ln2(phen)2(NO3)2(chdc)2]·2DMF (Ln3+ = Y3+ for 1, Eu3+ for 2 or Tb3+ for 3; phen = 1,10-phenanthroline; H2chdc = trans-1,4-cyclohexanedicarboxylic acid) were synthesized and characterized. The compounds are based on a binuclear block {M2(phen)2(NO3)2(OOCR)4} assembled into a two-dime nsional square-grid network containing tetragonal channels with 26% total solvent-accessible volume. Yttrium (1)-, europium (2)- and terbium (3)-based structures emit in the blue, red and green regions, respectively, representing the basic colors of the standard RGB matrix. A doping of Eu3+ and/or Tb3+ centers into the Y3+-based phase led to mixed-metal compositions with tunable emission color and high quantum yields (QY) up to 84%. The bright luminescence of a suspension of microcrystalline 3 in DMF (QY = 78%) is effectively quenched by diluted cinnamaldehyde (cinnamal) solutions at millimolar concentrations, suggesting a convenient and analytically viable sensing method for this important chemical.

Metal-organic framework application for mercury speciation using solid phase extraction followed by direct thermal release-electrothermal atomization atomic absorption spectrophotometric detection (ETA AAS).

Metal-organic frameworks (MOFs) are increasingly used in analytical chemistry for pre-concentration of trace elements followed by their determination using modern analytical techniques. However, there are a limited number of publications concerning the use of MOFs for speciation purposes, while their structural and functional features are perspective for the element species selective extraction and pre-concentration. It is known that mercury refers to the most hazardous elements which species demonstrate different toxicity, migration routes and bioavailability as well. Consequently the development of new approaches for mercury speciation in environments remains an actual objective of analytical chemistry. In present work a new methodology for inorganic and organic mercury speciation in water was proposed. This approach is based on pre-concentration using solid phase extraction (SPE) followed by their determination directly from the solid phase with the application of the thermal release - electrothermal-atomic-absorption technique (TR-ETA-AAS). An original SPE-procedure based on the use of UIO-66 [Zr6O4(OH)4(bdc)6] in two different modes (non-modified and modified with cysteine) as a sorbent was designed. As a result of SPE as well as TR-ETA-AAS optimization the detection limits (LOD) for all listed species at the level of 0.06 µg L-1 have been achieved. It was also shown that the presence of the other elements (K, Na, Ca, Mg at the level of 100 µg L-1, and Mn, Fe, Cr, Al, Zn, Cd, Pb - of 25 µg L-1) does not affect the results obtained. The developed assay demonstrates a high efficiency, low LODs, wide linear range and admissible analysis duration. The reliability of the data obtained was confirmed by the standard addition approach and by a comparison with the results of independent analytical methods.

Intense multi-colored luminescence in a series of rare-earth metal-organic frameworks with aliphatic linkers.

Two series of highly luminescent yttrium(iii), europium(iii) and terbium(iii) metal-organic frameworks containing diimine aromatic ligands and the dicarboxylate linker trans-1,4-cyclohexanedicarboxylate (chdc2-) which can be described by the general formulas [M2(bpy)2(chdc)3], where M = Y3+ (1), Eu3+ (2), and Tb3+ (3) and bpy = 2,2'-bipyridyl, and [M2(phen)2(chdc)3], where M = Y3+ (4), Eu3+ (5), and Tb3+ (6) and phen = 1,10-phenanthroline, were synthesized and characterized. All compounds are based on the same dinuclear {M2(L)2(OOCR)6} building blocks and possess a similar topology of the 3D framework with narrow pores. The chelate aromatic ligands act as efficient light-harvesting antennas for subsequent energy transfer to the emitting metal center (M = Eu3+, Tb3+) or intraligand photoemission (M = Y3+). As a result, the reported compounds display intense emission in the red (Eu3+), green (Tb3+) or blue (Y3+) regions representing three basic colors (RGB) of visible light. The measured quantum yields (QYs) of the solid-state luminescence for individual compounds were found to be 63% (1), 46% (2), 59% (3), 2.3% (4), 55% (5) and 49% (6). The drastic reduction of the luminescence efficiency for 4 is explained by the strong disorder of phen ligands. The high thermal stability (up to 300 °C) and exceptional moisture resistance of the bpy-based frameworks 1-3 were confirmed by TG and PXRD measurements. Various bimetal or trimetal compositions were also prepared for the bpy-series. The luminescence properties of these mixed-metal compounds depend on both the chemical composition and excitation wavelength (λex). Remarkably, pure white emission with color temperature = 6126 K was achieved for [Y1.68Eu0.08Tb0.24(bpy)2(chdc)3] at λex = 360 nm with QY = 20%. The reported results suggest that the obtained coordination framework series is a convenient platform for the design of highly efficient light emitting materials with tunable properties.

Structural Dynamics and Adsorption Properties of the Breathing Microporous Aliphatic Metal-Organic Framework.

A mixed-ligand metal-organic framework [Zn2(chdc)2(dabco)]·2NMP (chdc2- = trans-1,4-cyclohexanedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane; NMP = N-methylpyrrolidone) was synthesized under solvothermal conditions. This coordination compound demonstrates a guest-driven framework breathing due to a conformational change between e,e-chdc and a,a-chdc forms of the linkers with a reversible restoration of crystallinity. Both the local and longer-range coordination environment of the metal centers were extensively studied by electron paramagnetic resonance on a Cu(II)-doped compound. This approach allowed the detailed investigation of the ligand structural conformations and the framework structural dynamics, supported by an X-ray diffraction method. Carbon dioxide and methane adsorption measurements as well as vapor sorption of benzene and cyclohexane at 298 K of the activated compound were studied. While adsorption of small gas molecules, such as CO2, CH4, and N2, is moderate and does not induce the phase transition, the multistepped character of C6H6 and C6H12 adsorption isotherms characterize the breathing nature of [Zn2(chdc)2(dabco)]. The uptake of benzene from the vapor phase reaches 125 mL·g-1 at 298 K, which surpasses most of benzene uptake values reported for microporous metal-organic frameworks.

A Selenophene-Incorporated Metal-Organic Framework for Enhanced CO2 Uptake and Adsorption Selectivity.

A new metal-organic coordination polymer [Zn2(sedc)2(dabco)] (1se; sedc2- = selepophene-2,5-dicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane) was synthesized and characterized by single-crystal X-ray diffraction analysis. This MOF is based on {Zn2(OOCR)4N2} paddle wheels and is isoreticular to the family of [Zn2(bdc)2(dabco)] derivatives (1b; bdc2- = 1,4-benzenedicarboxylate) with pcu topology. The gas adsorption measurements revealed that 1se shows a 15% higher CO2 volumetric uptake at 273 K and 28% higher CO2 uptake at 298 K (both at 1 bar) compared to the prototypic framework 1b. Methane and nitrogen adsorption at 273 K was also investigated, and IAST calculations demonstrated a pronounced increase in CO2/CH4 and CO2/N2 selectivity for 1se, compared with 1b. For example, the selectivity factor for the equimolar CO2/CH4 gas mixture at 1 bar = 15.1 for 1se and 11.9 for 1b. The obtained results show a remarkable effect of the presence of selenium atom on the carbon dioxide affinity in the isoreticular metal-organic frameworks with very similar geometry and porosity.

Transition Metal Coordination Polymers with Trans-1,4-Cyclohexanedicarboxylate: Acidity-Controlled Synthesis, Structures and Properties.

Five trans-1,4-cyclohexanedicarboxylate (chdc2-) metal-organic frameworks of transition metals were synthesized in aqueous systems. A careful control of pH, reaction temperature and solvent composition were shown to direct the crystallization of a particular compound. Isostructural [Co(H2O)4(chdc)]n (1) and [Fe(H2O)4(chdc)]n (2) consist of one-dimensional hydrogen-bonded chains. Compounds [Cd(H2O)(chdc)]n∙0.5nCH3CN (3), [Mn4(H2O)3(chdc)4]n (4) and [Mn2(Hchdc)2(chdc)]n (5) possess three-dimensional framework structures. The compounds 1, 4 and 5 were further characterized by magnetochemical analysis, which reveals paramagnetic nature of these compounds. A presence of antiferromagnetic exchange at low temperatures is observed for 5 while the antiferromagnetic coupling in 4 is rather strong, even at ambient conditions. The thermal decompositions of 1, 4 and 5 were investigated and the obtained metal oxide (cubic Co3O4 and MnO) samples were analyzed by X-ray diffraction and scanning electron microscopy.

A Cryptand Metal-Organic Framework as a Platform for the Selective Uptake and Detection of Group I Metal Cations.

The metal-organic framework (MOF) complex (H3 O)2 [Zn4 (ur)(Hfdc)2 (fdc)4 ] (1, ur=urotropine, H2 fdc=furan-2,5-dicarboxylic acid) incorporates cryptand-like cavities, which can be used to separate and detect Rb+ and Cs+ optically. This is the first example of the effective employment of a MOF material for optical detection of these cations.