Coordination Polymers Based on a Biphenyl Tetraphosphonate Linker: Synthesis Control and Photoluminescence.

In this work, we used the rigid tetrapodal organic linker, [1,1'-biphenyl]-3,3',5,5'-tetrayltetrakis(phosphonic acid) (H8btp), for the preparation of two lanthanide-organic framework families of compounds: layered [Ln7(H5btp)4(H5.5btp)2(H6btp)2(H2O)12]∙23.5H2O∙MeOH [where Ln3+ = Eu3+ (1Eu) and Gd3+ (1Gd)], prepared using microwave-irradiation followed by slow evaporation; 3D [Ln4(H3btp)(H4btp)(H5btp)(H2O)8]∙3H2O [where Ln3+ = Ce3+ (2Ce), Pr3+ (2Pr), and Nd3+ (2Nd)], obtained from conventional hydro(solvo)thermal synthesis. It is shown that in this system, by carefully selecting the synthetic method and the metal centers, one can increase the dimensionality of the materials, also increasing structural robustness (particularly to the release of the various solvent molecules). Compound 1 is composed of 2D layers stacked on top of each other and maintained by weak π-π interactions, with each layer formed by discrete 1D organic cylinders stacked in a typical brick-wall-like fashion, with water molecules occupying the free space in-between cylinders. Compound 2, on the other hand, is a 3D structure with small channels filled with crystallization water molecules. A full solid-state characterization of 1 and 2 is presented (FT-IR spectroscopy, SEM microscopy, thermogravimetric studies, powder X-ray diffraction and thermodiffractometry). The photoluminescence of 1Eu was investigated.

Metal-organic framework assembled from erbium and a tetrapodal polyphosphonic acid organic linker.

A three-dimensional metal-organic framework (MOF), poly[[µ6-5'-pentahydrogen [1,1'-biphenyl]-3,3',5,5'-tetrayltetrakis(phosphonato)]erbium(III)] 2.5-hydrate], formulated as [Er(C12H11O12P4)]·2.5H2O or [Er(H5btp)]·2.5H2O (I) and isotypical with a Y3+-based MOF reported previously by our research group [Firmino et al. (2017b). Inorg. Chem. 56, 1193-1208], was constructed based solely on Er3+ and on the polyphosphonic organic linker [1,1'-biphenyl]-3,3',5,5'-tetrakis(phosphonic acid) (H8btp). The present work describes our efforts to introduce lanthanide cations into the flexible network, demonstrating that, on the one hand, the compound can be obtained using three distinct experimental methods, i.e. hydro(solvo)thermal (Hy), microwave-assisted (MW) and one-pot (Op), and, on the other hand, that crystallite size can be approximately fine-tuned according to the method employed. MOF I contains hexacoordinated Er3+ cations which are distributed in a zigzag inorganic chain running parallel to the [100] direction of the unit cell. The chains are, in turn, bridged by the anionic organic linker to form a three-dimensional 6,6-connected binodal network. This connectivity leads to the existence of one-dimensional channels (also running parallel to the [100] direction) filled with disordered and partially occupied water molecules of crystalization which are engaged in O-H...O hydrogen-bonding interactions with the [Er(H5btp)] framework. Additional weak π-π interactions [intercentroid distance = 3.957 (7) Å] exist between aromatic rings, which help to maintain the structural integrity of the network.

Robust Multifunctional Yttrium-Based Metal-Organic Frameworks with Breathing Effect.

Phosphonate- and yttrium-based metal-organic frameworks (MOFs), formulated as [Y(H5btp)]·5.5H2O (1), [Y(H5btp)]·2.5H2O (2), (H3O)[Y2(H5btp)(H4btp)]·H2O (3), and [Y(H5btp)]·H2O·0.5(MeOH) (4), were prepared using a "green" microwave-assisted synthesis methodology which promoted the self-assembly of the tetraphosphonic organic linker [1,1'-biphenyl]-3,3',5,5'-tetrayltetrakis(phosphonic acid) (H8btp) with Y3+ cations. This new family of functional materials, isolated in bulk quantities, exhibits a remarkable breathing effect. Structural flexibility was thoroughly studied by means of X-ray crystallography, thermogravimetry, variable-temperature X-ray diffraction, and dehydration and rehydration processes, ultimately evidencing a remarkable reversible single-crystal to single-crystal (SC-SC) transformation solely through the loss and gain of crystallization solvent molecules. Topologically, frameworks remained unaltered throughout this interconversion mechanism, with all compounds being binodal 6,6-connected network with a Schäfli symbol of {413.62}{48.66.8}. Results show that this is one of the most stable and thermally robust families of tetraphosphonate-based MOFs synthesized reported to date. Porous materials 2 and 3 were further studied to ascertain their performance as heterogeneous catalysts and proton conductors, respectively, with outstanding results being registered for both materials. Compound 2 showed a 94% conversion of benzaldehyde into (dimethoxymethyl)benzene after just 1 h of reaction, among the best results registered to date for MOF materials. On the other hand, the protonic conductivity of compound 3 at 98% of relative humidity (2.58 × 10-2 S cm-1) was among the highest registered among MOFs, with the great advantage of the material to be prepared using a simpler and sustainable synthesis methodology, as well as exhibiting a good stability at ambient conditions (temperature and humidity) over time when compared to others.

Fast detection of nitroaromatics using phosphonate pyrene motifs as dual chemosensors.

A new class of dual fluorescent chemosensors for nitroaromatic compounds (NACs) based on phosphonated pyrene derivatives is reported, showing high selectivity towards trinitrotoluene (TNT). The strong intermolecular interactions (π-π stacking and hydrogen bonding) allow high fluorescence quenching with visual detection in short response times.

Lanthanide-polyphosphonate coordination polymers combining catalytic and photoluminescence properties.

A rapid, mild and high-yield microwave synthesis of 1D isotypical [Ln(H4bmt)(H5bmt)(H2O)2]·3H2O coordination polymers is presented. The La(3+)-based material is highly active as a heterogeneous catalyst in the methanolysis of styrene oxide at nearly room temperature. Eu(3+)- and Tb(3+)-doped materials are very effective UV-to-visible light converters.