Luís Cunha-Silva

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.

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.

Synthesis, characterization and antibacterial studies of a copper(II) levofloxacin ternary complex.

Solution behavior of levofloxacin (lvx) complexes with copper(II) in the presence and absence of phen was studied in aqueous solution, by potentiometry. The results obtained show that under physiological conditions (micromolar concentration range and pH 7.4) only copper(II):lvx:phen ternary complexes are stable. Hence, a novel copper(II) ternary complex of fluoroquinolone levofloxacin with nitrogen donor heterocyclic ligand phen was synthesized and characterized by means of UV-Visible and IR spectroscopy, elemental analysis and X-Ray crystallography. In the synthesized complex (1), [Cu(lvx)(phen)(H(2)O)](NO(3)).2H(2)O, levofloxacin acts as a bidentate ligand coordinating to the metal, in its anionic form, through the carbonyl and carboxyl oxygens and phen coordinates through two N-atoms forming the equatorial plane of a distorted square-pyramidal geometry. The fifth ligand of the penta-coordinated Cu(II) centre is occupied axially by an oxygen atom from a water molecule. Minimum inhibitory concentration (MIC) determinations of the complex and comparison with free levofloxacin in various E. coli strains indicated that the Cu-complex is as efficient an antimicrobial as the free antibiotic (both in the case of the dissolved synthesized complex and the complex formed following stoichiometric mixture of the individual components in solution). Moreover, results strongly suggest that the cell intake route of both species is different supporting, therefore, the complexs suitability as a candidate for further biological testing in fluoroquinolone-resistant microorganisms.

Coordination Networks with Carborane Anions: Ag(i) and Nitrogen Bridging Ligands

New crystalline coordination networks based on Ag(i) and N-bridging ligands — pyrazine (pyz), 4,4′-bipyridine (bpy), and 2,3-bis-(2-pyridyl)pyrazine (bppz) — incorporating carborane monoanions (CB11H12)− or [Co(C2B9H11)2]− were isolated and characterized using single crystal X-ray diffraction, microanalysis, and infrared spectroscopy. All the complexes with the bpy and bppz ligands reveal 1D coordination infinite chains involving the Ag(i) and the ligands, and layered extended structures with rows of carborane anions between the layers of chains. Interestingly, two polymorphs of the complex [Ag(bpy)(CH3CN)][Co(C2B9H11)2] were observed. The complex [Ag(pyz)(CH3CN)2][Co(C2B9H11)2] shows a 1D coordination polymer, while the pyz complex with the smaller carborane anion {[Ag(pyz)](CB11H12)} exhibits a 3D coordination network structure with four Ag···H−B interactions between the silver centre and carborane anions.

Novel Composite Material Polyoxovanadate@MIL-101(Cr): A Highly Efficient Electrocatalyst for Ascorbic Acid Oxidation

A novel hybrid composite material, PMo10V2@MIL-101 was prepared by the encapsulation of the tetra-butylammonium (TBA) salt of the vanadium-substituted phosphomolybdate [PMo10V2O40](5-) (PMo10V2) into the porous metal-organic framework (MOF) MIL-101(Cr). The materials characterization by powder X-ray diffraction, Fourier transform infrared spectra and scanning electron microscopy confirmed the preparation of the composite material without disruption of the MOF porous structure. Pyrolytic graphite electrodes modified with the original components (MIL-101(Cr), PMo10V2), and the composite material PMo10V2@MIL-101 were prepared and their electrochemical responses were studied by cyclic voltammetry. Surface confined redox processes were observed for all the immobilized materials. MIL-101(Cr) showed one-electron reduction process due to chromium centers (Cr(III) → Cr(II)), while PMo10V2 presented five reduction processes: the peak at more positive potentials is attributed to two superimposed 1-electron vanadium reduction processes (V(V) → V(IV)) and the other four peaks to Mo-centred two-electron reduction processes (Mo(VI) → Mo(V)). The electrochemical behavior of the composite material PMo10V2@MIL-101 showed both MIL-101(Cr) and PMo10V2 redox features, although with the splitting of the two vanadium processes and the shift of the Mo- and Cr- centered processes to more negative potentials. Finally, PMo10V2@MIL-101 modified electrode showed outstanding enhanced vanadium-based electrocatalytic properties towards ascorbic acid oxidation, in comparison with the free PMo10V2, as a result of its immobilization into the porous structure of the MOF. Furthermore, PMo10V2@MIL-101 modified electrode showed successful simultaneous detection of ascorbic acid and dopamine.

Synthesis and catalytic properties of molybdenum(VI) complexes with tris(3,5-dimethyl-1-pyrazolyl)methane.

The complex [MoO(2)Cl{HC(3,5-Me(2)pz)(3)}]BF(4) (1) (HC(3,5-Me(2)pz)(3) = tris(3,5-dimethyl-1-pyrazolyl)methane) has been prepared and examined as a catalyst for epoxidation of olefins at 55 °C using tert-butyl hydroperoxide (TBHP) as the oxidant. For reaction of cis-cyclooctene, epoxycyclooctane is obtained quantitatively within 5 h when water is rigorously excluded from the reaction mixture. Increasing amounts of water in the reaction mixture lead to lower activities (without affecting product selectivity) and transformation of 1 into the trioxidomolybdenum(VI) complex [{HC(3,5-Me(2)pz)(3)}MoO(3)] (4). Complex 4 was isolated as a microcrystalline solid by refluxing a suspension of 1 in water. The powder X-ray diffraction pattern of 4 can be indexed in the orthorhombic Pnma system, with a = 16.7349(5) Å, b = 13.6380(4) Å, and c = 7.8513(3) Å. Treatment of 1 in dichloromethane with excess TBHP led to isolation of the symmetrical [Mo(2)O(4)(μ(2)-O){HC(3,5-Me(2)pz)(3)}(2)](BF(4))(2) (2) and unsymmetrical [Mo(2)O(3)(O(2))(2)(μ(2)-O)(H(2)O){HC(3,5-Me(2)pz)(3)}] (3) oxido-bridged dimers, which were characterized by single-crystal X-ray diffraction. Complex 2 displays the well-known (Mo(2)O(5))(2+) bridging structure where each dioxidomolybdenum(VI) center is coordinated to three N atoms of the organic ligand and one μ(2)-bridging O atom. The unusual complex 3 comprises dioxido and oxidodiperoxo molybdenum(VI) centers linked by a μ(2)-bridging O atom, with the former center being coordinated to the tridentate N-ligand. The dinuclear complexes exhibit a similar catalytic performance to that found for mononuclear 1. For complexes 1 and 2 use of the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate and N-butyl-3-methylpyridinium tetrafluoroborate as solvents allowed the complexes to be completely dissolved, and in each case the catalyst and IL could be recycled and reused without loss of activity.

Using pyridine amidoximes in 3d-metal cluster chemistry: a novel ferromagnetic Ni12 complex from the use of pyridine-2-amidoxime

The use of pyridine-2-amidoxime in reaction with NiCl2.6H2O in the presence of NaOMe has led to a cationic NiII 12 cluster with novel structural features; each cluster molecule is a ferromagnet with a spin ground state of S = 6 arising from ferromagnetic exchange between the six octahedral NiII ions, but with intercationic interactions also affecting the magnetic properties.

Slow magnetization relaxation in unprecedented Mn(III)4Dy(III)3 and Mn(III)4Dy(III)5 clusters from the use of N-salicylidene-o-aminophenol.

The first use of N-salicylidene-o-aminophenol in 3d/4f chemistry has led to Mn(III)(4)Dy(III)(5) and Mn(III)(4)Dy(III)(3) clusters with unprecedented metal topologies and stoichiometries; both compounds exhibit out-of-phase signals indicative of the slow magnetization relaxation of a single-molecule magnet.

Oxidative catalytic versatility of a trivacant polyoxotungstate incorporated into MIL-101(Cr)

The first immobilization of the trivacant Keggin-type polyoxometalate ([A-PW9O34]9−, PW9) to prepare a novel heterogeneous oxidative catalyst is here reported. PW9 was incorporated into the cavities of the chromium terephthalate metal–organic framework MIL-101(Cr). Characterization of the composite PW9@MIL-101 by powder X-ray diffraction, SEM-EDX, FT-IR, FT-Raman spectroscopy, N2 adsorption–desorption isotherms and 31P solid-state NMR confirmed that the structures of MIL-101 and the polyoxometalate anion were retained after immobilization. The composite PW9@MIL-101 revealed versatility as a heterogeneous catalyst to oxidize efficiently monoterpenes as well as to reach a complete desulfurization of a model oil containing the most refractory sulfur compounds in fuel, using in both systems acetonitrile as the solvent and H2O2 as the oxidant. Complete conversion of geraniol to 2,3-epoxygeraniol was achieved after the first 30 min at room temperature, while the total desulfurization of the model oil containing 1707 ppm of sulfur was attained after 2 h. In both systems the catalyst was recyclable for various cycles without a significant loss of activity. The stability and heterogeneity of the catalyst were confirmed by several techniques and by leaching tests.