Anion exchange separation of chromium and molybdenum from iron,vanadium, uranium and other elements in malonic acid solution

Summary Anion-exchange behaviour of chromium (III) and molybdenum (VI) was studied in malonate media. They form anionic complexes with malonic acid at pH 5.6. Various eluants, such as mineral acids and their salts were tested and a selectivity scale evolved. Cr and Mo were separated from Tl(I), alkali and alkaline earth elements by selective sorption and from Co(II), Ni(II), Mn(II), Zn(II) and Cd(II) by selective washing with water. They were separated from many other elements by selective elution. The sequential separation of Fe(III) V(IV), Cr(III), Mo(VI) and U(VI) was significant.     

Chemistry of tricarbonylrhenium(I) chelates of azopyrimidines, azoimidazoles and anion radicals thereof

The azopyrimidine and azoimidazole ligands (general abbreviations, RL) used in the present work are 2-(p-R-C₆H₄NN)C₄H₃N₂, RL_pm (R=H, Cl) and 2-(p-R-C₆H₄NN)-1-(Me)C₃H₂N₂, RL_im (R=Me, Cl), respectively. The reaction of Re(CO)₅Cl with a slight excess of RL in boiling benzene has furnished blue-violet complexes of type Re(CO)₃Cl(RL) which have been spectrally characterized. In Re(CO)₃Cl(HL_pm) and Re(CO)₃Cl(ClL_im) the Re-N^h, Re-N^a distances are 2.173(6), 2.136(6) Å and 2.150(5), 2.166(5) Å, respectively (N^h and N^a are heterocyclic and azo N atoms, respectively). Their N-N lengths (1.271(8), 1.281(7) Å) implicate relatively weak Re-azo(π^*) back-bonding. In the lattice of Re(CO)₃Cl(HL_pm), pair-wise C-H?O hydrogen bonding between symmetry related molecules is present (C?O; 3.264(9) Å, H?O; 2.460(10) Å; C-H?O; 130.6(5)°). The lattice of Re(CO)₃Cl(ClL_im) also consists of centrosymmetric dimers held by aromatic π-π stacking between parallely placed pendant aryl rings (centroid?centroid distance, 3.781(9) Å). Extended Hückel calculations reveal that the LUMO of Re(CO)₃Cl(RL) is ∼60% azo in character. One-electron quasireversible electrochemical reduction occurs near −0.1 and −0.4 V vs. SCE in the cases of Re(CO)₃Cl(RL_pm) and Re(CO)₃Cl(RL_im), respectively. The redox orbital is believed to be to the above noted LUMO. Electrogenerated Re(CO)₃Cl(RL⁻) underwent spontaneous solvolytic chloride displacement in MeCN furnishing Re(CO)₃(MeCN)(RL⁻) which has been isolated. The latter in turn reacted with imidazole and triphenyl phosphine furnishing Re(CO)₃(C₃H₄N₂)(RL⁻) and Re(CO)₃(PPh₃)(RL⁻), respectively. The pattern of carbonyl stretching frequencies of these radical anion complexes is similar to that of Re(CO)₃Cl(RL) but for shifts to lower frequencies by 10-40 cm⁻¹. All the three radical anion systems are one-electron paramagnets (1.7-1.8 μ_B). The unpaired electron is primarily localized in a predominantly azo-π^* orbital. A small metal contribution (^{185, 187}Re, I=5/2) is present and both Re(CO)₃(MeCN)(RL⁻) and Re(CO)₃(C₃H₄N₂)(RL⁻) display six-line EPR spectra (A∼28 G). The line shapes and intensities are characteristic of the presence of g-strain. In the case of Re(CO)₃(PPh₃)(RL⁻) seven nearly equispaced lines are observed due to virtually equal coupling with metal and ³¹P (I=1/2) nuclei. The g values of the radical species span the range 2.0033-2.0066.     

Chemistry of the rhenium-azopyridine family: an oxo parent and derivatives thereof including a novel oxo-imido dimer

The concerned azo ligands are 2-(phenylazo)pyridine (HL) and 2-((p-chlorophenyl)azo)pyridine (ClL). The reaction of KReO4 with HL in hot concentrated HCl is attended with metal reduction and ligand chlorination affording the oxo complex ReVOCl3(ClL), 2, which furnishes ReIII(OPPh3)Cl3(ClL), 3, upon treatment with PPh3. Aromatic amines, ArNH2, convert 2 to the imido complex ReV(NAr)Cl3(ClL), 5, and the unusual oxo-imido dimer (ClL)-Cl2(O)ReVOReV(NAr)Cl2(ClL), 7. The complex ReIII(OPPh3)Cl3(HL), 4, has been generated from ReVOCl3(PPh3)2 and HL. Reaction of 4 with HL has yielded ReV(NPh)Cl3(HL), 6, via azo splitting. The complexes have been characterized with the help spectral, magnetic, and X-ray structural data (2, 3, 5c (Ar = pClC6H4) and 7.CH2Cl2 (Ar = pMeC6H4)). In 2, 3, and 5c the ReCl3 fragment is meridionally disposed, and in 7 the ReCl2 fragments have a trans configuration. The Re-O(oxo) bond, 1.663(6) A, in 2 and Re-N(imido) bond, 1.719(5) A, in 5c are triple bonds. The corresponding bonds are slightly longer in 7 wherein the (O)Re(1)-O(2)-Re(2)(NAr) bridge is angular (151.0(5) degrees) and unsymmetrical, the Re(1)-O(2) bond, 1.849(7) A, having a large double-bond character (Re(2)-O(2), 1.954(7) A). In effect, cis-ReVO2 acts as a monodentate oxygen ligand toward ReVNAr in 7. In all cases the pyridine nitrogen binds trans to the oxo, OPPh3, or NAr donor. Bond length data are consistent with the presence of substantial d(Re)-pi*(azo) back-bonding. In acetonitrile solution the complexes display electrochemical one-electron metal (ReVI/ReV or ReIV/ReIII) and azo redox. The imido ligand in 5 stabilizes the ReVI state (E1/2 approximately 1.4 V) better than the oxo ligand in 2 (approximately 1.9 V). Parallely it is more difficult to reduce the azo group in 5 (approximately -0.4 V) than in 2 (approximately 0.0 V). In 7 the metal (approximately 1.0 V) and azo (approximately -0.4 V) couples correspond to the imido and oxo halves, respectively. The significantly higher (by 0.2-0.6 V) metal reduction potentials of the azopyridine compared to pyridine-2-aldimine complexes is ascribed to the superior pi-acidity and electron-withdrawing character of the azo function relative to the aldimine function. This also makes the transfer of the ReVO oxygen function much more facile under azopyridine chelation as in 2. For the same reason, ReOCl3(PPh3)2 reacts with HL affording only 4 while it reacts with pyridine-2-aldimines furnishing oxo species. Crystal data for the complexes are as follows: 2, empirical formula C11H8Cl4N3ORe, crystal system triclinic, space group P1, a = 7.118(4) A, b = 8.537(4) A, c = 13.231(9) A, alpha = 79.16(5) degrees, beta = 78.03(5) degrees, gamma = 70.96(4) degrees, V = 737.2(7) A3, Z = 2; 3, empirical formula C29H23Cl4N3OPRe, crystal system monoclinic, space group P2(1)/n, a = 11.264(2) A, b = 15.221(3) A, c = 17.628(4) A, beta = 94.21(3) degrees, V = 3014(1) A3, Z = 4; 5c, empirical formula C17H12Cl5N4Re, crystal system triclinic, space group P1, a = 9.683(3) A, b = 10.898(3) A, c = 11.522(3) A, alpha = 63.67(2) degrees, beta = 71.24(2) degrees, gamma = 86.79(2) degrees, V = 1026(1) A3, Z = 2; 7.CH2Cl2, empirical formula C30H25Cl8N7O2Re2, crystal system triclinic, space group P1, a = 12.522(6) A, b = 12.857(8) A, c = 13.182(7) A, alpha = 67.75(4) degrees, beta = 88.30(4) degrees, gamma = 82.09(4) degrees, V = 1945(2) A3, Z = 2.     

Surface optical Raman modes in GaN nanoribbons

Raman scattering studies were performed in GaN nanoribbons grown along [1 0 0]. These samples were prepared inside Na‐4 mica nanochannels by the ion‐exchange technique and subsequent annealing in NH₃ ambient. Detailed morphological and structural studies including the crystalline orientation were performed by analyzing the vibrational properties in these GaN nanoribbons. Pressure in the embedded structure was calculated from the blue shift of the E₂(high) phonon mode of GaN. Possible red shift of optical phonon modes due to the quantum confinement is also discussed. In addition to the optical phonons allowed by symmetry, two additional Raman peaks were also observed at ∼633 and 678 cm⁻¹ for these nanoribbons. Calculations for the wavenumbers of the surface optical (SO) phonon modes in GaN in Na‐4 mica yielded values close to those of the new Raman modes. The SO phonon modes were calculated in the slab (applicable to belt‐like nanoribbon) mode, as the wavenumber and intensity of these modes depend on the size and the shape of the nanostructures. The effect of surface‐modulation‐assisted electron–SO phonon scattering is suggested to be responsible for the pronounced appearance of SO phonon modes. A scaling factor is also estimated for the interacting surface potential influencing the observed SO Raman scattering intensities. Copyright © 2010 John Wiley & Sons, Ltd.     

Neutron diffraction studies on La2−x Dy x Ca2xBa2Cu4+2x Oz superconductors

Structural studies on Dy-substituted La-2125 type superconductors have been carried out by neutron diffraction experiments at room temperature using a monochromatic neutron beam of wavelength (λ) = 1.249 Å. A series of samples with La_2-xDy_xCa_2xBa₂Cu_4+2xO_z stoichiometric composition, forx = 0.1–0.5, have been studied for their structural properties. A tetragonal Y-123 unit cell was taken as the starting model for the Rietveld analysis. All the samples fit into the starting model, with no structural transition taking place with increasing dopant concentration. The results of Rietveld analysis and structural properties will be discussed in detail.

     

New Efficient RCM-Mediated Synthesis of Pyrrolidine Derivatives

Fast and effective ring-closing olefin metathesis (RCM) of diallyamine derivatives of coumarin, quinolone, pyridine, and substituted benzene, using first-generation RCM ruthenium-based catalyst, leads to corresponding pyrrolidine derivatives in 70–95% yields under very mild conditions.     

Do the recent heavy-ion collisions indicate a baryonic dictatorship in the cosmic world?

Summary The recent AGS experiments at BNL involving the heavy ions have reported some experimental measurements on production cross-sections of antiprotons. The brief communication presented here attempts at dealing with the relevance and reflections of these findings on the cosmic measurements of antiproton flux, and their final cosmological implications.