Filipe A. Almeida Paz

Luminescent and Magnetic Cyano-Bridged Coordination Polymers Containing 4d-4f Ions: Toward Multifunctional Materials

A new family of cyano-bridged coordination polymers Ln(H(2)O)(5)[M(CN)(8)] (Ln = Eu, Tb, Sm, Gd; M = Mo, W) were obtained and characterized by X-ray diffraction, photoluminescence spectroscopy, and magnetic analyses. These compounds are isomorphous and crystallize in the tetragonal system P4/nmm, forming two-dimensional gridlike networks. The Eu- and Tb-containing coordination polymers are room-temperature optically active emitters displaying the characteristic (5)D(0) --> (7)F(0-4) (Eu(3+)) and (5)D(4) --> (7)F(6-2) (Tb(3+)) transitions. All of the coordination polymers except Eu(H(2)O)(5)[M(CN)(8)] present long-range magnetic ordering at low temperatures. The coexistence of luminescence with ferromagnetic ordering for Tb(H(2)O)(5)[M(CN)(8)] (M = Mo, W) suggests that these compounds may be considered as bifunctional magneto-luminescent coordination polymers exhibiting diverse physical responses when subjected to various external stimuli.

A Luminescent and magnetic cyano-bridged Tb3+-Mo5+ coordination polymer: toward multifunctional materials.

A new cyano-bridged coordination polymer network Tb(H2O)5-[Mo(CN)8] was obtained and characterized. This compound has a two-dimensional layered structure and presents luminescence along with a magnetic transition at low temperature.

Packing Interactions in Hydrated and Anhydrous Forms of the Antibiotic Ciprofloxacin: a Solid-State NMR, X-ray Diffraction, and Computer Simulation Study

We present an experimental NMR, X-ray diffraction (XRD), and computational study of the supramolecular assemblies of two crystalline forms of Ciprofloxacin: one anhydrate and one hydrate forming water wormholes. The resonance assignment of up to 51 and 54 distinct (13)C and (1)H resonances for the hydrate is reported. The effect of crystal packing, identified by XRD, on the (1)H and (13)C chemical shifts including weak interionic H-bonds, is quantified; (1)H chemical shift changes up to ∼-3.5 ppm for CH···π contacts and ∼+2 ppm (CH···O((-))); ∼+4.7 ppm (((+))NH···O((-))) for H-bonds. Water intake induces chemical shift changes up to 2 and 5 ppm for (1)H and (13)C nuclei, respectively. Such chemical shifts are found to be sensitive detectors of hydration/dehydration in highly insoluble hydrates.

A high-nuclearity 3d/4f metal oxime cluster: an unusual Ni(8)Dy(8) "core-shell" complex from the use of 2-pyridinealdoxime.

The initial employment of 2-pyridinealdoxime in 3d/4f chemistry has led to a Ni(II)(8)Dy(III)(8) cluster with an unprecedented metal topology; the compound has an unusual structure, is the highest-nuclearity metal oxime cluster to date, and exhibits slow magnetization relaxation.

Multi-functional rare-earth hybrid layered networks: photoluminescence and catalysis studies

Hydrothermal reactions between rare-earth (RE) chloride salts and N-(carboxymethyl)iminodi(methylphosphonic acid) (H5cmp) led to the isolation of a series of layered networks formulated as [RE(H2cmp)(H2O)] [RE3+ = Y3+, La3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+ and Er3+]. All compounds were isolated as micro-crystalline powders (many of which were nano-sized in thickness), with the plate-like crystallites found to exhibit preferential growth perpendicular to the [002] vector, a feature which seems to favour catalytic performance. Full structural elucidation was attained by the combination of synchrotron radiation (micro-crystal and powder) diffraction data, solid-state NMR studies (1H, 13C and 31P) and photophysical measurements. Materials consist of ∞2[RE(H2cmp)(H2O)] layers in the ab plane of the unit cell, constructed from a single RE3+ centre (in a highly distorted dodecahedral coordination environment with one water molecule in the first coordination sphere) and one H2cmp3− ligand present in a zwitterionic form. Connections between layers along the c-axis are assured by strong and highly directional O–H⋯O hydrogen bonds involving the protonated phosphonate group (donor) of one layer and one oxygen atom (acceptor) of the carboxylate group in the adjacent layer. The network is an unprecedented 12-connected uninodal plane net with total Schafli symbol 330.434.52. The Eu3+ material is photoluminescent at room temperature and 12 K with 5D0 lifetimes of 0.86 ± 0.01 ms and 0.89 ± 0.01 ms, respectively. Studies of the mixed-lanthanide diluted [(Gd0.95Eu0.05)(H2cmp)(H2O)] material showed that Gd3+-to-Eu3+ energy transfer occurs within the layers. The coordinated water molecule plays a decisive role in the non-radiative relaxation process of the Eu3+ emission. All synthesised materials were further tested in the cyclodehydration of xylose to furfural, with the observed results comparing quite favourably with those from other solid acid catalysts used in the same reaction under similar conditions. A detailed catalytic study was performed for [Y(H2cmp)(H2O)]: selectivity increased to 84% as the conversion reached 83%; this solid was also re-used successfully in three consecutive 4 h runs after separation from the liquid phase by centrifugation and regeneration using either thermal treatment at 280 °C or repeated washing with solvents. All materials have been routinely characterized using vibrational spectroscopy (ATR-FT-IR and FT-Raman), thermogravimetric analyses, SEM investigations and CHN elemental composition.

Evaluation of [Ln(H2cmp)(H2O)] Metal Organic Framework Materials for Potential Application as Magnetic Resonance Imaging Contrast Agents

Aqueous suspensions of metal organic frameworks (MOF) containing different Ln(3+) ions, consisting of a series of layered Ln(3+) networks formulated as [Ln(H(2)cmp)(H(2)O)] (where H(5)cmp is (carboxymethyl)iminodi(methylphosphonic acid), with a relatively wide size distribution (400 nm to 1 microm) were studied by relaxometry. The water (1)H longitudinal (r(1)) and transverse (r(2)) relaxivities were obtained for aqueous suspensions of these materials with different lanthanide ions. The values of r(1) are very small and varied only slightly with the effective magnetic moment (mu(eff)) of the lanthanide ions, while r(2) values are larger and proportional to the value of mu(eff)(2). The dependence of R(2) on tau(CP) (the time interval between two consecutive refocusing pulses in the train of 180 degrees pulses applied in a CPMG pulse sequence) was evaluated. The value of R(2) initially increases with tau(CP) and then saturates at higher tau(CP) at a value that is about 3 to 5 times lower than R(2p)*. This can be explained by the static dephasing regime (SDR) theory, in which the diffusion effect is taken into account and where the condition tau(D) > Delta omega(r(p))(-1) holds (tau(D) = r(p)(2)/D, where D is the diffusion coefficient, r(p) is the radius of the particle, and Delta omega(r(p)) is the Larmor frequency shift at the particles surface). Separation of the particles into two fractions with different particle sizes led to a significant enhancement of the r(2) relaxivity of the smaller particles with a narrow size distribution. Magnetometric measurements performed with the particles containing Dy(III), Ho(III), and Gd(III) showed a typical paramagnetic behavior from 4 to 100 K, used to determine the Curie constants.

Theoretical and Experimental Studies of the Photoluminescent Properties of the Coordination Polymer [Eu(DPA)(HDPA)(H2O)2]·4H2O

We report on the hydrothermal synthesis of the [Eu(DPA)(HDPA)(H(2)O)(2)].4H(2)O lanthanide-organic framework (where H2DPA stands for pyridine-2,6-dicarboxylic acid), its full structural characterization including single-crystal X-ray diffraction and vibrational spectroscopy studies, plus detailed investigations on the experimental and predicted (using the Sparkle/PM3 model) photophysical luminescent properties. We demonstrate that the Sparkle/PM3 model arises as a valid and efficient alternative to the simulation and prediction of the photoluminescent properties of lanthanide-organic frameworks when compared with methods traditionally used. Crystallographic investigations showed that the material is composed of neutral one-dimensional coordination polymers infinity(1)[Eu(DPA)(HDPA)(H(2)O)(2)] which are interconnected via a series of hydrogen bonding interactions involving the water molecules (both coordinated and located in the interstitial spaces of the structure). In particular, connections between bilayer arrangements of infinity(1)[Eu(DPA)(HDPA)(H(2)O)(2)] are assured by a centrosymmetric hexameric water cluster. The presence of this large number of O-H oscillators intensifies the vibronic coupling with water molecules and, as a consequence, increases the number of nonradiative decay pathways controlling the relaxation process, ultimately considerably reducing the quantum efficiency (eta = 12.7%). The intensity parameters (Omega(2), Omega(4), and Omega(6)) were first calculated by using both the X-ray and the Sparkle/PM3 structures and were then used to calculate the rates of energy transfer (W(ET)) and back-transfer (W(BT)). Intensity parameters were used to predict the radiative decay rate. The calculated quantum yield obtained from the X-ray and Sparkle/PM3 structures (both of about 12.5%) are in good agreement with the experimental value (12.0 +/- 5%). These results clearly attest for the efficacy of the theoretical models employed in all calculations and create open new interesting possibilities for the design in silico of novel and highly efficient lanthanide-organic frameworks.

An efficient oxidative desulfurization process using terbium-polyoxometalate@MIL-101(Cr)

An efficient and recyclable oxidative desulfurization process (ODS) to remove the most refractory sulfur-compounds (dibenzothiophene, 1-benzothiophene and 4,6-dimethyldibenzothiophene) from fuel is reported. The ODS process was catalyzed by terbium-polyoxometalate [Tb(PW11O39)2]11− (Tb(PW11)2) and its composite Tb(PW11)2@MIL-101. The tetrabutylammonium (TBA) salt of Tb(PW11)2 was prepared and further incorporated in the porous metal–organic framework MIL-101(Cr). The TBA compound and its composite were characterized by various techniques (powder X-ray diffraction, FT-IR, FT-Raman, SEM and elemental analysis), and their electrochemical behavior was investigated, indicating that the structure of the polyoxometalate anion must be retained after immobilization. The studied ODS process was based on a biphasic system formed by a model oil with various refractor sulfur-compounds and an extracting solvent using H2O2 as the oxidant. Two main steps in the process were carefully investigated: the initial extraction and the oxidative catalytic stage. The optimization of the ODS process was performed by the analysis of the most suitable extracting solvent and also comparing the desulfurization performance of the homogeneous Tb(PW11)2 and the heterogeneous Tb(PW11)2@MIL-101 catalysts. Acetonitrile was selected as the best solvent because it allowed the highest desulfurization rate, conciliating good initial extraction and high catalytic performance. The presence of the porous catalyst Tb(PW11)2@MIL-101 seemed not to influence the initial extraction step; however, with this porous hybrid catalyst were obtained higher desulfurization rates during the catalytic stage. Remarkably, using Tb(PW11)2@MIL-101 and the oil–acetonitrile system complete desulfurization of oil was achieved only after 5 h. The recyclability of the solid catalyst was investigated for three consecutive ODS cycles and its stability was confirmed by several techniques.

Structural and Photoluminescence Studies of a Europium(III) Tetrakis(β-diketonate) Complex with Tetrabutylammonium, Imidazolium, Pyridinium and Silica-Supported Imidazolium Counterions

Tetrakis(naphthoyltrifluoroacetonato)lanthanate(III) complexes (Ln = Eu, Gd) containing the cations tetrabutylammonium, [NBu(4)](+); 1-butyl-3-methylimidazolium, [C(4)mim](+); and 1-butyl-3-methylpyridinium, [C(4)mpyr](+), have been prepared and structurally characterized by single-crystal X-ray diffraction. The {EuO(8)} coordination sphere in [NBu(4)][Eu(NTA)(4)] is best described as a distorted dodecahedron, where the metal ion is located at the 4-fold inversion axis with only one crystallographically independent NTA residue. In [C(4)mim][Eu(NTA)(4)] and [C(4)mpyr][Gd(NTA)(4)], the central Ln(3+) ions are coordinated by eight oxygen atoms from four distinct beta-diketonate ligands, in an overall distorted square-antiprismatic geometry. Besides electrostatic interactions, the crystal packing in all three structures is stabilized by offset pi-pi interactions involving the naphthyl rings of neighboring complexes (and, for [C(4)mim][Eu(NTA)(4)] and [C(4)mpyr][Gd(NTA)(4)], neighboring naphthyl/imidazolium and naphthyl/pyridinium rings) and C-H...pi contacts. The photoluminescence properties of the three Eu(III) complexes were studied at room temperature and -259 degrees C by measuring emission and excitation spectra, (5)D(0) emission decay curves, and absolute emission quantum yields. Under ligand excitation (lambda(ex) = 290-395 nm), the quantum yields (room temperature) were in the range 0.72-0.77 for the 1-butyl-3-methylimidazolium salt. An immobilized analogue of this complex was prepared by supporting [Eu(NTA)(4)](-) on an ordered mesoporous silica derivatized with 1-propyl-3-methylimidazolium groups. The disappearance of the intra-4f(6) lines in the excitation spectrum of the supported material indicated an increase in the ligands sensitization process of the Eu(3+) ions, relative to direct intra-4f(6) excitation. The emission quantum yield measured for the supported material (0.32-0.40, for excitations between 265 and 360 nm) is the highest so far reported for lanthanide-containing ordered mesoporous silicas.

Photoluminescent Thermometer Based on a Phase-Transition Lanthanide Silicate with Unusual Structural Disorder

The hydrothermal synthesis of the novel Na[LnSiO4] (Ln = Gd, Eu, Tb) disordered orthorhombic system is reported. At 100 K, and above, these materials are best described in the centrosymmetric orthorhombic Pnma space group. At lower temperatures (structure solved at 30 K) the unit cell changes to body-centered with Imma symmetry. The materials exhibit unique photophysical properties, arising from both, this phase transformation, and the disorder of the Ln(3+) ions, located at a site with D2d point symmetry. Na[(Gd0.8Eu0.1Tb0.1)SiO4] is an unprecedented case of a luminescent ratiometric thermometer based on a very stable silicate matrix. Moreover, it is the first example of an optical thermometer whose performance (viz., excellent sensitivity at cryogenic temperatures <100 K) is determined mainly by a structural transition, opening up new opportunities for designing such devices.