Current Inorganic Chemistry (Discontinued) - Volume 1, Issue 2, 2011

The vast field of oxidation catalysis is explored by many groups worldwide. Most of these transformations rely on the use of metal centered catalysts and take advantage of the plethora of oxidation states that metal centers may assume, both in homogeneous and in heterogeneous phases. Biological systems provide a major source of inspiration for these systems, since metalloenzymes conduct such reactions across very elegant and efficient pathways, which are still far from being unravelled. The aim of this special issue in the inaugural volume of Current Inorganic Chemistry is to provide an overview, as broad as possible, dealing with recent developments in oxidation catalytic systems. Therefore, Arzoumanian wrote a critical mini-review on the use of molybdenum oxo and peroxo complexes to accomplish oxygen atom transfer reactions using dioxygen molecules, reflecting the crescent environmental concern on the use of oxidants in these reactions. The second paper, Royo describes the use of MoO2 complexes with chiral N-heterocyclic ligands for olefin epoxidation, addressing also the role of different oxidants. The next two papers take us into the different field of heterogeneous catalysis. The paper by Calhorda describes the application of layered double hydroxides as useful inorganic supports for active catalysts in olefin epoxidation, while the following work by Nunes addresses the influence of surface treatment in MCM-41 derived materials in product selectivity when using them as olefin epoxidation catalysts. Last but not least, the paper by Kühn reviews some important advances on the use of environmentally friendly solutions for oxidation catalysis - ionic liquids. While not covering every aspect of oxidation catalysis, these works provide a wide-ranging illustration of several areas, making this issue attractive for all those interested in the development and understanding of olefin oxidation reactions. Finally, as a guest editor for this special issue, I am very grateful for the valuable and excellent contributions from my colleagues, and I am also highly indebted to all the other colleagues who acted as referees for their expert comments. A final word of acknowledgment is also due to Ms. Humaira Bilal and Ms. Anila Mufti (Managers of the Publication Department, Current Inorganic Chemistry) for their enthusiasm, dedication and organization of this issue. Thank you all!

Novel cis-dioxomolybdenum(VI) complexes containing chiral ligands have been prepared and fully characterized, including structural determinations by X-ray diffraction. The first monometallic dioxomolybdenum(VI) complex containing a chiral N,N',O-tridentated ligand is reported. The new complexes were evaluated as catalysts for epoxidation of olefins using tert-butyl hydroperoxide and H2O2 as oxidants. They were found to be efficient catalysts affording good chemoselectivity but low enantioselectivity.

The chemistry of Oxygen Atom Transfer catalyzed by metal-dioxo and metal-oxo-peroxo complexes, described in this mini review is limited to processes in which the primary oxidant is dioxygen. Contrary to the vast majority of OAT catalytic systems reported in the literature and involving oxidant such as hydroperoxide, peracid, hydrogen peroxide, DMSO or N2O, the use of dioxygen is certainly more arduous since it brings up more constraints. First, experimental conditions must minimize free radical chain pathways in order to avoid unselective product formation. Second, it is essential that the catalytic system be able , not only, to activate O2 but also transfer both oxygen atoms to a substrate. Examples from the literature and from our laboratory are given for systems under homogeneous conditions and with recently reported more performant solid matrix anchored catalysts.

[MoBr2(CO)3(L)2] complexes were synthesized with L = nicH (pyridine-3-carboxylate), picH (pyridine-2- carboxylate) and pydcH2 (pyridine-2,6-dicarboxylic acid). New lamellar materials intercalated with molybdenum(II) complexes were prepared by first calcinating the starting lamellar material at 823 K for four hours, to eliminate all the carbonate ions; the layered structure was reconstructed after treatment with a basic solution of either nicH, picH, or pydcH2 in a dmf solution of NaOH at 343 K. Impregnation with a solution of the organometallic precursor [MoX2(CO)3(NCCH3)2] (X= Br, X= I) led to substitution of the nitriles by two L ligands. All the Mo(II) complexes were characterized by FTIR, elemental analysis, and 1H and 13C solution NMR, and the materials by powder X-ray diffraction, FTIR and 13C CP MAS-DD solid state NMR. Both the complexes, their iodide analogues, these new materials (3.80 wt% and 1.25 wt% Mo for pic or pydc) and their iodide analogues (4.54 wt% and 6.33 wt% Mo for pic or pydc) catalyze the epoxidation of ciclooctene and styrene and the chiral olefins R(+)- and S(-)-limonene at 328 K. Higher activity is associated with presence of iodide and nicH.

The mesoporous silica MCM-41 was first treated with an acetonitrile solution of the ligand pyCOCl (2) under reflux. Before isolating the grafted materials, the silylating agents R3SiCl (R = Ph, Me) were added to remove the residual Si-OH groups on the surface of the mesoporous materials. Mo(II) precursor complex [MoI2(CO)3(CH3CN)2], was subsequently introduced into the ligand-silicas by pore volume impregnation of a solution of the complex in CH2Cl2 at room temperature. The modified materials have been characterized by powder X-ray diffraction, FTIR, N2 adsorption, and solid-state CP-MAS NMR (13C, 29Si). The grafted MCM materials (2-3 wt.-% Mo) were tested as catalysts for the epoxidation of cyclooctene, styrene and geraniol with tert-butylhydroperoxide (TBHP) at 328 K. Selectivity to the epoxide in cyclooctene was complete. On recycling one time, some activity was lost from the first to second run, but not significantly. In the case of styrene and geraniol the systems were not as selective yielding benzaldehyde and geranial, respectively. In fact, surface tuning towards hydrophobicity proved to be adequate over product formation control to yield either the epoxide or aldehyde products.

This review deals with the catalytic application of ionic transition-metal complexes using different ionic liquids as solvents. Additionally, so-called organometallic ionic liquids have been described with regard to their activity in several types of reaction. In most cases the catalysis was additionally performed under homogeneous conditions allowing a comparison between the use of ILs and conventional organic solvents. In all described cases the biphasic performance led to a higher recyclability of the catalytic system.

We prepared Ce-doped (Ba0.87Sr0.04Ca0.09)(Ti0.90Zr0.04Sn0.06)O3 (BSCTZS) powders and ceramics using the solgel process. The samples were characterized by means of thermogravimetric, Fourier-transform infrared, X-ray diffraction, and transmission and scanning electron microscope analysis. We also determined the dielectric properties of the ceramics. The results indicated that the SrZrO3 cubic perovskite phase first appeared at a calcining temperature of 600 °C, and facilitated formation of the BaTiO3-based solid solution. The as-synthesized powders calcined at 900 °C for 2 h had a nanometer-scale grain size, with the grains mainly composed of the cubic BaTiO3 phase. After sintering, the ceramics consisted entirely of the cubic BaTiO3 phase. The average grain size and the relative density decreased as the Ce concentration increased. Moreover, the maximum dielectric constant (εmax) decreased and the Curie temperature (Tm) shifted to a lower temperature. Furthermore, the Ce-doped BSCTZS ceramics had a relatively high dielectric constant over a wide temperature range. When the Ce concentration was 0.15 mol%, the BSCTZS ceramics met the Electronic Industries Alliance (EIA) Y5V specifications and the εr (25 °C) was higher than 7500. These merits of the BSCTZS nanopowders and ceramics prepared using the sol-gel method indicate that they will permit a significant reduction in the thickness of multilayer ceramic capacitors and will help meet the current demand for device miniaturization in the electronics industry.

The study of thiosemicarbazones and their metal complexes has become a fruitful area of the coordination chemistry due to the interesting properties exhibited by these compounds, in particular those dealing with the biological activity. Our efforts have focused on the structure-properties relationship in copper(II) derivatives of pyridine-2- carbaldehyde thiosemicarbazone. Usually, this molecule acts as a NNS terdentate ligand and gives rise to [Cu(HL)]2+ / [CuL]+ rigid planar blocks, which can form dimers by bonds through the thiosemicarbazone S atom (S-bridged systems) or the X atom belonging to different coligands (X-bridged systems). Such structural features, together with the high stability of these compounds, allow the synthesis of complexes with small structural modifications. This fact has prompted us to carry out systematic studies in which variations of physic and chemical parameters (as pH, temperature or reactants present in the reaction medium) induce changes in the ligand or modify the structural, spectroscopic and magnetic properties of the complexes. In this sense, an appropriate choice of ancillary ligands (halides, oxalate, carboxylates, polyoxometalate or biomolecules, among others) is essential to attain different behaviors. Oxalato derivatives with relatively high ferromagnetism and unusually twisted carboxylato groups, structures of thiosemicarbazone-polyoxometalate hybrid systems and nucleic acid base-thiosemicarbazone complexes, reactivity with glutathione, interaction and cleavage of DNA and biological activity against tumoral cell lines are some of the aspects discussed here. Some conclusions of our research can be extrapolated to a broad range of compounds.