Ricardo F. Mendes

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.

Phosphonate Appended Porphyrins as Versatile Chemosensors for Selective Detection of Trinitrotoluene

Fluorescent molecular probes based on phosphonate-functionalized porphyrin derivatives have been designed for selective detection of nitroaromatics. It is shown that molecular recognition is based on cooperative hydrogen bonding and π-π stacking interactions with electron-deficient molecules (nitroaromatic compounds, NACs), displaying superior detection toward trinitrotoluene (TNT). The P═O functional groups decrease the lowest unoccupied molecular orbital (LUMO) energy level of the porphyrins and, consequently, facilitate the electron inoculation to TNT through a photoinduced electron transfer (PET) process. The hydroxyl groups of the phosphonates and pyrrole -NH protons are further engaged in donor-acceptor interactions with TNT by strong intermolecular hydrogen bonding interactions (as evidenced by single crystal X-ray, NMR, and density functional theory (DFT)) showing turn off fluorescence behavior. The nonplanarity of the porphyrins induced by protonation at the central core of the porphyrin H4TPPA(2+) undergoes additional interactions, furnishing an anomalous increase in the selectivity of TNT at nanomolar levels in solution (limit of detection, LOD ∼ 5 nM). Porphyrin-doped hybrid PMMA [poly(methyl methacrylate)] polymer films demonstrate the reversibility of the fluorescence behavior and exhibit high photostability. The formation of discrete molecular aggregates on the surface of hybrid films and efficient diffusion of TNT vapors (10 ppb) displayed high selectivity in the solid state. The hybrid films are further used to demonstrate the detection of NACs in the aqueous medium, ultimately providing a platform for a practical strategy and implementation for the detection of toxic NACs.

Transforming metal–organic frameworks into functional materials

This review focuses on a new alternative approach towards MOF preparation through the transformation of known MOF structures into other novel, more active MOFs that, in most cases, could not be obtained by conventional synthetic methods. These transformations are usually of the single-crystal-to-single-crystal type, usually accompanied by a modification of the dimensionality of the MOFs: e.g., 1D → 2D, 2D → 3D, 1D → 3D or even 3D → 3D′ (concerning modification of the connectivity of the primary building units). The literature contains several reports concerning MOF-to-MOF transformations but only in a handful of cases the authors aimed to design new functional compounds, pointing towards applications or the modification (or improvement) of the properties of the materials. This review aims to concisely describe the most significant reports concerning the transformation of MOFs into other more functional and active MOFs. Several types of transformations are possible including solvent removal or insertion, modification of the pH, metal exchange, release of active molecules, among others. These transformations can lead to significant improvements of the properties of MOFs, for example: increase of adsorption of different gases, such as nitrogen and carbon dioxide; creation of sensing centers for different chemical species; pH sensing with, in some cases, a concomitant change in crystal color; improvement of the luminescence properties by the removal of solvent molecules. Other transformations could lead instead to a complete modification of the properties of MOFs such as the appearance of magnetic properties, the creation of storage devices, the design of releasing materials by the incorporation of active molecules or water scavengers.

A lamellar coordination polymer with remarkable catalytic activity

A positively charged lamellar coordination polymer based on a flexible triphosphonic acid linker is reported. [Gd(H4 nmp)(H2 O)2 ]Cl⋅2 H2 O (1) [H6 nmp=nitrilotris(methylenephosphonic acid)] was obtained by a one-pot approach by using water as a green solvent and by forcing the inclusion of additional acid sites by employing HCl in the synthesis. Compound 1 acts as a versatile heterogeneous acid catalyst with outstanding activity in organic reactions such as alcoholysis of styrene oxide, acetalization of benzaldehyde and cyclohexanaldehyde and ketalization of cyclohexanone. For all reaction systems, very high conversions were reached (92-97 %) in only 15-30 min under mild conditions (35 °C, atmospheric pressure). The coordination polymer exhibits a protonic conductivity of 1.23×10(-5)  S cm(-1) at 98 % relative humidity and 40 °C.

Bifunctional Porphyrin-Based Nano-Metal–Organic Frameworks: Catalytic and Chemosensing Studies

The use of 5,10,15,20-tetrakis( p-phenylphosphonic acid)porphyrin (H10TPPA) as a linker in the preparation of porphyrin-based metal-organic frameworks (Por-MOFs) through coordination to lanthanides cations is reported. The resulting unprecedented materials, formulated as [M(H9TPPA)(H2O) x]Cl2· yH2O [ x + y = 7; M3+ = La3+ (1), Yb3+ (2), and Y3+ (3)], prepared using hydrothermal synthesis, were extensively characterized in the solid-state, for both their structure and thermal robustness, using a myriad of solid-state advanced techniques. Materials were evaluated as heterogeneous catalysts in the oxidation of thioanisole by H2O2 and as chemosensors for detection of nitroaromatic compounds (NACs). Nano-Por-MOFs 1-3 proved to be effective as heterogeneous catalysts in the sulfoxidation of thioanisole, with Por-MOF 1 exhibiting the best catalytic performance with a conversion of thioanisole of 89% in the first cycle and with a high selectivity for the sulfoxide derivative (90%). The catalyst maintained its activity roughly constant in three consecutive runs. Por-MOFs 1-3 can be employed as chemosensors because of a measured fluorescence quenching up to 70% for nitrobenzene, 1,4-dinitrobenzene, 4-nitrophenol, and phenol, with 2,4,6-trinitrophenol exhibiting a peculiar fluorescence profile.

Copper–Porphyrin–Metal–Organic Frameworks as Oxidative Heterogeneous Catalysts

The synthesis and structural characterization of novel metal–organic‐framework‐type materials obtained from the reaction of 5,10,15,20‐tetrakis[2,3,5,6‐tetrafluoro‐4‐(4‐pyridylsulfanyl)phenyl]porphyrin (H2P1) and 5,10,15,20‐tetrakis(4‐pyridyl)porphyrin (H2P2) with copper(II) acetate is reported. The new material, CuP1S, shows higher heterogeneous catalytic activity than the homogeneous porphyrin–copper(II) complex CuP1 in the oxidation of catechol into ortho‐benzoquinone in the presence of air (O2) or H2O2 (30 %).

Crystal structure of a compact three-dimensional metal-organic framework based on Cs+ and (4,5-dicyano-1,2-phenylene)bis(phosphonic acid)

The three-dimensional metal–organic framework compound prepared from Cs+ and the organic linker 4,5-dicyano-1,2-phenylene)bis(phosphonic acid is based on an irregular CsO8N2 coordination center comprising a single monodentate hydronium O-atom donor, together with multiple bridging links to the two phosphonate O-atom donors and to the two nitrile N-atom donors.