Multifrequency EPR and redox reactivity investigations of a bis(mu-thiolato)-dicopper(II,II) complex

From a new tripodal ligand [N2SS'H] with mixed N, S(thioether), and S(thiolate) donor set, the corresponding bis(mu-thiolato)dicopper(II) complex has been prepared and characterized. X-ray crystallographic analysis of the complex [Cu2(N2SS')2](ClO4)2.C4H10O (1) demonstrates that the two five-coordinated Cu atoms are bridged by two thiolates leading to a nearly planar Cu2S2 core with a Cu1...Cu1* distance of 3.418(8) A and a large bridging angle Cu1S1Cu1* of 94.92 degrees. X-band (10 GHz), Q-band (34 GHz), and F-Band (115 GHz) EPR spectra of 1 are consistent with a weakly coupled dicopper(II,II) center attributed to an S = 1 state. Simulations for the three frequencies are obtained with a unique set of electronic parameters. The mean values of the spin Hamiltonian parameters for 1 are D = 0.210(3) cm(-1), E = 0.0295(5) cm(-1), |E/D| = 0.140, gx = 2.030(2), gy = 2.032(2), gz = 2.128(2). The electrochemical one-electron reduction of 1 generates the mixed-valent CuIICuI species. EPR and UV-vis spectra are consistent with a type I localized mixed-valent species, while dinuclear CuA centers of native cytochrome c oxidase (CcO)1-3 or nitrous oxide reductase (N2OR)4 have a delocalized CuIICuI mixed-valent state. After reoxidation of the CuIICuI species, the initial complex 1 is regenerated through a reversible interconversion process.     

Application of heat conduction calorimetry to high explosives

This paper describes some thermal analysis experiments conducted on high explosive samples. These employ differential scanning calorimetry to monitor thermal effects at elevated temperatures (around 200 °C) and heat conduction calorimetry to record thermal effects at much lower temperatures (below 100 °C).The work shows that, due to the generally high thermal stability of many high explosive compositions, heat generation rates are very low, if detectable at all, at normal storage temperatures, even when using a very sensitive instrument. The sensitivity and reproducibility of this technique has been investigated in detail by Wilker et al. [S. Wilker, U. Ticmanis, G. Pantel, Detailed investigation of sensitivity and reproducibility of heat flow calorimetry, in: Proceedings of the 11th Symposium on Chemical Problems Connected with the Stability of Explosives, Sweden, 1998] and shown to be capable of recording heat generation rates of less than a microwatt. This allows continuous measurement of decomposition processes in nitrate ester based propellants at temperatures as low as 40 °C. However, the measurement of very low levels of heat generation is difficult, time consuming and therefore expensive. If the assumption is made that the life limiting process is invariably the slow decomposition of the energetic component, this will frequently lead to very long service lifetime predictions.A number of possible complications are identified. Firstly, due to its low detection threshold, a heat conduction calorimeter may detect other reactions which will not lead to failure, but which may still dominate the heat flow signal. Secondly, the true failure process may generate little energy and be overlooked. In view of these considerations, at present it seems unwise to rely on heat conduction microcalorimetry as the only tool for the assessment of the life of high explosive energetic systems.Based on examples of life terminating processes in high explosives during storage and use, it is clear that decomposition of the energetic material is not invariably the cause of system failure. It is also by no means the only reaction that may take place in, and be observed by, a heat conduction calorimeter.     

Mesoporous‐Silica‐Functionalized Nanoparticles for Drug Delivery

The ever‐growing interest for finding efficient and reliable methods for treatment of diseases has set a precedent for the design and synthesis of new functional hybrid materials, namely porous nanoparticles, for controlled drug delivery. Mesoporous silica nanoparticles (MSNPs) represent one of the most promising nanocarriers for drug delivery as they possess interesting chemical and physical properties, thermal and mechanical stabilities, and are biocompatibile. In particular, their easily functionalizable surface allows a large number of property modifications further improving their efficiency in this field. This Concept article deals with the advances on the novel methods of functionalizing MSNPs, inside or outside the pores, as well as within the walls, to produce efficient and smart drug carriers for therapy.     

Optical characterization of photochromic effect

Photochromic powders are powders whose color is sensitive to UV irradiation. When such a powder is exposed to the sunlight or any other UV rays, it becomes darker and then looks back progressively to its initial color. An optical method has been set up in order to quantify these colors variations. We can then quantify the variations in the colorimetric space, and the time response of the coming back phenomenon.     

Rheology and gelation kinetics of PVC plastisols

Oscillatory rheological experiments at different temperatures and over a wide range of frequencies have been used to investigate the gelation process and, more particularly, the sol–gel transition of various poly(vinyl chloride) (PVC) plastisols. The sol–gel transition process was found to be universal with respect to the temperature and solid volume fraction according to the similarity of the fractal structure in PVC plastisols. The variation of the gel time (t _gel) with temperature for any composition of PVC plastisols was predicted from the Dickinson’s model (E. Dickinson, J Chem Soc Faraday Trans, 93:111–114, 1997). Dynamic viscoelastic properties of PVC plastisols have also been studied as a function of temperature that allowed us to follow the gelation process of various plastisols. Thus, the influence of the type and concentration of PVC resins in gelation process was investigated. The variation of the complex shear modulus at a constant frequency was depicted by a master curve regarding the dependence of the moduli on PVC concentrations.     

Influence of mixed thiolate/thioether versus dithiolate coordination on the accessibility of the uncommon +I and +III oxidation states for the nickel ion: an experimental and computational study

Sulfur-rich nickel metalloenzymes are capable of stabilizing Ni(I) and Ni(III) oxidation states in catalytically relevant species. In an effort to better understand the structural and electronic features that allow the stabilization of such species, we have investigated the electrochemical properties of two mononuclear N(2)S(2) Ni(II) complexes that differ in their sulfur environment. Complex 1 features aliphatic dithiolate coordination ([NiL], 1), and complex 2I is characterized by mixed thiolate/thioether coordination ([NiL(Me)]I, 2I). The latter results from the methylation of a single sulfur of 1. The X-ray structure of 2I reveals a distorted square planar geometry around the Ni(II) ion, similar to what was previously reported by us for 1. The electrochemical investigation of 1 and 2(+) shows that the addition of a methyl group shifts the potentials of both redox Ni(II)/Ni(I) and Ni(III)/Ni(II) redox couples by about 0.7 and 0.6 V to more positive values. Through bulk electrolyses, only the mononuclear dithiolate [Ni(I)L](-) (1(-)) and the mixed thiolate/thioether [Ni(III)L(Me)](2+) (2(2+)) complexes were generated, and their electronic properties were investigated by UV-vis and EPR spectroscopy. For 1(-) (Ni(I), d(9) configuration) the EPR data are consistent with a d(x(2))(-)(y(2)) based singly occupied molecular orbitals (SOMOs). However, DFT calculations suggest that there is also pronounced radical character. This is consistent with the small g-anisotropy observed in the EPR experiments. The spin population (Mulliken analysis) analysis of 1(-) reveals that the main contribution to the SOMO (64%) is due to the bipyridine unit. Time dependent density functional theory (TD-DFT) calculations attribute the most prominent features observed in the electronic absorption spectrum of 1(-) to metal to ligand charge transfer (MLCT) transitions. Concerning 2(2+), the EPR spectrum displays a rhombic signal with g(x) = 2.236, g(y) = 2.180, and g(z) = 2.039 in CH(3)CN. The g(iso) value is larger than 2.0, which is consistent with metal based oxidation. The unpaired electron (Ni(III), d(7) configuration) occupies a Ni-d(z(2)) based molecular orbital, consistent with DFT calculations. Nitrogen hyperfine structure is observed as a triplet in the g(z) component of the EPR spectrum with A(N) = 51 MHz. This result indicates the coordination of a CH(3)CN molecule in the axial position. DFT calculations confirm that the presence of a fifth ligand in the coordination sphere of the Ni ion is required for the metal-based oxidation process. Finally, we have shown that 1 exhibits catalytic reductive dehalogenation activity below potentials of -2.00 V versus Fc/Fc(+) in CH(2)Cl(2).     

Selective conversion of {Mo132} Keplerate ion into 4-electron reduced crown-capped Keggin derivative [Te5Mo15O57](8-). A key intermediate to single-phase M1 multielement MoVTeO light-alkanes oxidation catalyst

{Mo(132)} Keplerate anion reacts with tellurites to give a soluble precursor to produce in hydrothermal conditions single-phase M1 MoVTeO light-alkanes oxidation catalyst. Characterization of this Te-containing intermediate by single-crystal X-ray diffraction, (125)Te NMR, UV-visible and redox titration reveals a molybdotellurite anion as a crown-capped Keggin derivative.