Synthetic control to achieve lanthanide(III)/pyrimidine-4,6-dicarboxylate compounds by preventing oxalate formation: structural, magnetic, and luminescent properties

Control over the synthetic conditions in many metal/diazinedicarboxylato systems is crucial to prevent oxalate formation, since dicarboxylato ligands easily undergo degradation in the presence of metal salts. We report here an efficient route to obtain oxalato-free compounds for the lanthanide/pyrimidine-4,6-dicarboxylato (pmdc) system on the basis of the reaction temperature and nonacidic pH or oxygen free atmosphere. Two different crystal architectures have been obtained: {[Ln(μ-pmdc)(1.5)(H(2)O)(3)]·xH(2)O}(n) (1-Ln) and {[Ln(2)(μ(4)-pmdc)(2)(μ-pmdc)(H(2)O)(2)]·H(2)O}(n) (2-Ln) with Ln(III) = La-Yb, except Pm. Both crystal structures are built from distorted two-dimensional honeycomb networks based on the recurrent double chelating mode established by the pmdc. In compounds 1-Ln, the tricapped trigonal prismatic coordination environment of the lanthanides is completed by three water molecules, precluding a further increase in the dimensionality. Crystallization water molecules are arranged in the interlamellar space, giving rise to highly flexible supramolecular clusters that are responsible for the modulation found in compound 1-Gd. Two of the coordinated water molecules are replaced by nonchelating carboxylate oxygen atoms of pmdc ligands in compounds 2-Ln, joining the metal-organic layers together and thus providing a compact three-dimensional network. The crystal structure of the compounds is governed by the competition between two opposing factors: the ionic size and the reaction temperature. The lanthanide contraction rejects the sterically hindered coordination geometries whereas high-temperature entropy driven desolvation pathway favors the release of solvent molecules leading to more compact frameworks. The characteristic luminescence of the Nd, Eu, and Tb centers is improved when moving from 1-Ln to 2-Ln compounds as a consequence of the decrease of the O-H oscillators. The magnetic properties of the compounds are dominated by the spin-orbit coupling and the ligand field perturbation, the exchange coupling being almost negligible.     

Transition metal modified and partially calcined inorganic-organic pillared clays for the adsorption of salicylic acid, clofibric acid, carbamazepine, and caffeine from water

Pharmaceutical and Personal Care Products (PPCPs) are considered emerging contaminants, and their efficient removal from water is going to be a challenging endeavor. Microporous adsorbent materials, including pillared clays, could offer a potential solution if tailored properly. Although pillared clays have been employed previously for the removal of organics, the effective removal of PPCPs will only be possible if their surface and textural properties are manipulated from the bottom-up. This work presents the use of modified inorganic-organic pillared clays (IOCs) for the adsorption of salicylic acid, clofibric acid, carbamazepine, and caffeine. The IOCs have been modified with Co(2+), Cu(2+), or Ni(2+) to induce complexation-like adsorbate-adsorbent interactions at ambient conditions, in an attempt to provide an efficient and yet reversible driving force in the sub-ppm concentration range. Furthermore, the IOCs were partially calcined to increase effective surface area by an order of magnitude while preserving some hydrophobicity. In general, the Ni(2+) IOCs exhibited the greatest interaction with salicylic and clofibric acids, respectively, while the Co(2+) adsorbents excelled at adsorbing caffeine at low concentrations. All of the metal-modified IOCs showed comparable adsorption capacities for the case of carbamazepine, probably due to the lack of availability of particular functional groups in this adsorbate.     

Azo Monomers Exhibiting Low Layer Shrinkage at the SmA–SmC Transition and trans–cis Light‐Induced Isomerization

Methacrylic monomers containing a (phenylene)azobenzene unit substituted with a lateral cyano group and alkyl chains of different length are synthesized and characterized by NMR techniques. Their liquid‐crystalline properties are studied by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction. All monomers exhibit a mesomorphic behavior that extends over wide temperature ranges with nematic and orthogonal or tilted smectic‐type mesophases, depending on the length of the terminal chain. The smectic structures are determined to be single‐layered with a low layer shrinkage (<5 %) at the SmA–SmC transition. This atypical behavior is attributed to the combination of a high smectic order promoted by both π–π and bond dipole–bond dipole interactions between cyano‐substituted central cores, and a low correlation between neighboring layers arising from dispersive forces between the end groups (methacrylic group and alkyl chain) of the monomer. On the other hand, the trans–cis isomerization of monomers is induced in solution by irradiating with a UV lamp. High cis‐isomer contents (≥96 %) are obtained at the photostationary state, which is reached in a relatively short time (40 s).     

Molybdenum(VI) catalysts obtained from η3-allyl dicarbonyl precursors: synthesis, characterization and catalytic performance in cyclooctene epoxidation

The oxidative decarbonylation of the η(3)-allyl dicarbonyl complexes [Mo(η(3)-C(3)H(5))Cl(CO)(2)(L)] (L = 2,2'-bipyridine (bipy) (1), 4,4'-di-tert-butyl-2,2'-bipyridine (di-tBu-bipy) (2)) by reaction with aqueous tert-butylhydroperoxide (TBHP) or H(2)O(2) gave the following compounds in good to excellent yields: the oxo-bridged dimers [MoO(2)Cl(L)](2)O (L = bipy (3), di-tBu-bipy (6)) using TBHP(10 equiv.)/CH(3)CN/r.t.; the molybdenum oxide/bipyridine hybrid material {[MoO(3)(bipy)][MoO(3)(H(2)O)]}(n) (4) and the octanuclear complex [Mo(8)O(24)(di-tBu-bipy)(4)] (7) using TBHP(50 equiv.)/H(2)O/70 °C; the oxodiperoxo complexes MoO(O(2))(2)(L) (L = bipy (5), di-tBu-bipy (8)) using H(2)O(2)(10 equiv.)/CH(3)CN/r.t. The structure of 7·x(solvent) (where solvent = CH(2)Cl(2) and/or diethyl ether) was determined by single crystal X-ray diffraction. Despite possessing the same windmill-type complex as that described previously for 7·10CH(2)Cl(2), the crystal structure of 7·x(solvent) is unique due to differences in the crystal packing. Compounds 1-8 were examined as catalysts or catalyst precursors for the epoxidation of cyclooctene using aqueous TBHP or H(2)O(2) as oxidant at 55 or 70 °C. Reactions were performed without co-solvent or with the addition of water, ethanol or acetonitrile. Cyclooctene oxide was always the only reaction product. Solids recovered after 24 h reaction at 70 °C were identified by FT-IR spectroscopy as the hybrid 4 from (1,3-5)/TBHP, complex 5 from (1,3-5)/H(2)O(2), and complex 8 from (2,6-8)/H(2)O(2). With TBHP as oxidant, the highest epoxide yields (for 24 h reaction at 70 °C) were obtained using excess H(2)O as solvent (28-38% for 1,3-5; 87-98% for 2,6-8), while with H(2)O(2) as oxidant, the highest epoxide yields were obtained using CH(3)CN as solvent (54-81% for 3-8).     

On the curvature of vortex moduli spaces

We use algebraic topology to investigate local curvature properties of the moduli spaces of gauged vortices on a closed Riemann surface. After computing the homotopy type of the universal cover of the moduli spaces (which are symmetric products of the surface), we prove that, for genus $g>1$ , the holomorphic bisectional curvature of the vortex metrics cannot always be nonnegative in the multivortex case, and this property extends to all Kähler metrics on certain symmetric products. Our result rules out an established and natural conjecture on the geometry of the moduli spaces.     

A solution to a problem of Grünbaum and Motzkin and of Erdős and Purdy about bichromatic configurations of points in the plane

Let P be a set of n blue points in the plane, not all on a line. Let R be a set of m red points such that P ∩ R = ? and every line determined by P contains a point from R. We provide an answer to an old problem by Grünbaum and Motzkin [9] and independently by Erd?s and Purdy [6] who asked how large must m be in terms of n in such a case? More specifically, both [9] and [6] were looking for the best absolute constant c such that m ≥ cn. We provide an answer to this problem and show that m ≥ (n?1)/3.     

The effect of temperature on the free radical copolymerization of methyl methacrylate with ethyl acrylate

Methyl methacrylate has been copolymerized with ethyl acrylate at temperatures between 35 and 65′ using 2,2′-azobisisobutyronitrile as initiator. For both polymer radicals, crosspropagation is favoured energetically whereas self-propagation is favoured entropically. Values of Arrhenius parameters indicate that cross-propagation predominates for ethyl acrylate and self-propagation for methyl methacrylate.     

Spectrofluorimetric determination of boron in soils, plants and natural waters with Alizarin Red S.

A simple and sensitive spectrofluorimetric method for the determination of boron with Alizarin Red S is described. The fluorescence intensity of the complex formed in aqueous solution develops immediately and heating is not required. The excitation and emission wavelengths are 465 and 615 nm, respectively. The detection limit is 7.2 ng cm-3. A study was made of the influence of several ions as potential interferents. The method was applied satisfactorily to the determination of boron in soils, plants and natural waters.     

Photobiological properties of hydroxy-substituted flavothiones

Flavothione (FT) and a series of 18 hydroxy- and methoxy-substituted flavothiones were screened for photobiological activity. The 5-hydroxy-substituted compounds (group 3) and the methoxy-substituted flavothiones were inactive. FT and the remaining hydroxy-substituted compounds, all displayed photobiological activity. Among these, the 3-hydroxy-substituted compounds (group 2) were the most efficient photosensitizers overall in spite of their concurrent fast photodegradation. FT and all other hydroxyflavothiones, not substituted in the 3- or 5-positions (group 1), were inefficient compared with group 2. Detailed photobiological tests were carried out for four flavothiones of groups 1 and 2. The biological tests included fungi, several strains of Escherichia coli, Salmonella typhimurium and mammalian cells. In addition, the ability of these flavothiones to perform lipid peroxidation was evaluated. FT and 6-hydroxyflavothione (group 1) induce DNA damage via H-atom abstraction from the lowest n, pi* triplet state of the thione (oxygen independent). For 3-hydroxy and 3,6-dihydroxyflavothione (group 2), both DNA and the membrane are targets. The mechanism likely involves both energy transfer and electron transfer from the lowest pi, pi* triplet state to oxygen, to form singlet oxygen and the superoxide anion. Some of these compounds could be considered as models for environmentally safe photopesticides.     

Time-Dependent Lagrangians Invariant by a Vector Field

We study the reduction of nonautonomous regular Lagrangian systems by symmetries, which are generated by vector fields associated with connections in the configuration bundle of the system Q × . These kind of symmetries generalize the idea of time-invariance (which corresponds to taking the trivial connection in the above trivial bundle).