Elsa E. Sileo

Structural studies on the manganese for iron substitution in the synthetic goethite-jacobsite system

Abstract A set of synthetic oxides were prepared from Fe(III) and Mn(II) nitrate solutions in alkaline media with a Mn/(Mn+Fe) ratio up to 0.5. The structural characterization of the solid phases obtained was carried out by X-ray diffraction (XRD). XRD analyses showed that in preparations with a Mn mol fraction (xMn) lower than 0.14 (samples G1–G4) a Mn-substituted goethite was the only crystalline phase present. A second phase, jacobsite appeared together with goethite in the range 0.45≥xMn≥0.14 (samples G5–G8). For xMn=0.55 the majority and minority phases detected were jacobsite and hausmannite. Atomic and cell parameters for the nine samples synthesized were obtained by the Rietveld refinement of the XRD data. The relative proportion of the different phases in the system was also estimated by the refinement program and indicates that at xMn=0.28, jacobsite and goethite-like phases are in the same proportion. Refined cell parameters show that the unit cell in the goethite-like phase is distorted as a function of xMn. Deviations from the Vegard are observed and suggest a non-homogeneous substitution of Mn in the goethite phase.

The crystal chemistry of copper(II) dipicolinates

Abstract The molecular structure of a new copper dipicolinate, Cu(dipic)(H2dipic)·H2O, and a refinement of the previously reported Cu(dipic)(H2dipic)·3H2O are presented. The Jahn-Teller effect, as operative in six-coordinated CuII surrounded by two planar tridentate ligands, is responsible for the presence of a dianion and a neutral acid molecule in the structure. As opposed to previously reported copper dipicolinates, water molecules do not coordinate and in the trihydrate dehydration is facile because interstital water finds easy dehydration pathways; the mechanism involves nucleation and fast bidimensional growth. The monohydrate, however, presents tightly bound (through hydrogen bonds) water and no easy dehydration pathways are available; the dehydration mechanism involves fluid-flux nucleation, mediated by fusion. Five different copper(II) dipicolinates are known; because of the strong tendency of this ligand to form tridentate mononuclear complexes, all of them, except monoclinic Cu(dipic)·2H2O, belong to the class of molecular solids. In the monoclinic dihydrate, carbonyl O atoms are involved in defining chains that run parallel to [001] planes.

Structure and reactivity of synthetic Co-substituted goethites

Abstract A set of synthetic goethites were prepared from Fe3+- and Co2+-nitrate solutions in alkaline media with a Co/(Co + Fe) ratio (xCo) up to 10 mol%. The structural characterization of the resultant solid phases was carried out by X-ray diffraction (XRD). XRD analyses showed that in preparations with xCo <10 mol%, Co-substituted goethite was the only crystalline phase present. Atomic and cell parameters for the samples synthesized were obtained by the Rietveld refinement of the XRD data, and showed that the unit cell in the goethite-like phase is contracted as a function of xCo. Little deviation from the Vegard rule was observed for all unit-cell parameters. Cobalt substitution produces an increase in the surface area of the goethite, as well as an increment in the dehydroxylation temperature. The acid dissolution of all Co-goethites showed an increase in dissolution rate with the Co content, and a congruent behavior was observed. The activation energy for dissolution was obtained two samples. A modified first-order Kabai equation best describes the dissolution data.

The thermal decomposition of copper(II) nicotinate and isonicotinate

Abstract The thermal decomposition of copper(II) nicotinate and isonicotinate yields, in a sharp transition, metallic copper as the final solid residue along with the release of carbon dioxide and pyridine. In the case of the nicotinate, small amounts of nicotinic acid are also detected. The decomposition is started by homolytic RC(O)O-Cu bond scission, which is equivalent to electron transfer from RC(O)O− to Cu(II).

Structure and reactivity of copper (II) carboxylates I. Copper (II) dipicolinates

Abstract The crystal and molecular structures of a series of salts formed by copper (II) with dipicolinic acid were used to interpret the dehydration behavior. The morphology of single crystals dehydration of Cu(dipic)·2H 2 O (monoclinic and triclinic symmetry), Cu(dipic)·3H 2 O and Cu(dipic) (H 2 dipic)· x H 2 O was shown to be governed by the arrangement of water molecules along specific orientations.

Thermal decomposition of mixed-valent diruthenium pentakis(alkanoates) enhanced by their liquid crystalline state

Abstract The thermal decomposition of mixed-valent diruthenium pentakis(alkanoates) of general formula Ru2(II,III)(O2C(CH2)n−2CH3)5 (for n=3, 4, 5, 6, 8, 10, 12, 16), studied by a combination of several complementary physicochemical techniques, has been found to take place in two differentiated steps. The first one gives the divalent analogues Ru2(II,II)(O2C(CH2)n−2CH3)4 as the main product, in a one-electron reduction of the bimetallic core likely provided by oxidative decarboxylation of the axial anion. The second one involves the reduction of the binuclear core to metallic ruthenium, and the oxidation of the equatorial carboxylates. The first step is very much influenced by the aggregation state of the system, the occurrence of a liquid–crystalline phase in compounds with n>6 facilitates the thermolysis: decomposition temperatures fall from ≈300°C for compounds with n=3 to ≈175°C for compounds with n=10–16.

Comparison of the thermal behaviour of the metal salts of simple and polymeric carboxylates

Abstract The thermal decomposition of several metal polyacrylates and poly(meta)acrylates is compared with the TG of simple carboxylates. The applicability of a general scheme of decomposition is discussed.

The different pathways of the thermal decomposition of metal nicotinates and isonicotinates

Abstract The thermal decomposition (TD) of basic iron nicotinate and isonicotinate proceeds in several stages: (a) dehydration to yield a hydroxocompound; (b) dehydration of hydroxide bridges, overlapping with water attack on the anion and release of H(nic) and H(isonic); (c) homolytic RC(O)O-Fe(III) bond breaking; (d) formation of metallic iron. Sodium salts decompose yielding Na 2 CO 3 + C, that later evolves CO. The different types of chemical reactions involved in the TD of metal nicotinates and isonicotinates are compared.

Synthesis and thermal behaviour of a series of pentacyanoligandferrate(II) complexes

Abstract [M m Fe(CN) 5 L·xH 2 O] (M = Mn, Fe, Co, Ni, Cu; L = pyridine or pyrazine derivative) have been synthesized, and their UV-vis and IR spectral characteristics are reported. Thermolysis of the solids leads to water release, ligand release and/or decomposition, redox interaction between Fe 3+ (or Cu 2+ ) with CN − or L, and redox interaction between M 2+ (or Cu + ) and CN − , yielding elemental metal and metal carbides.

The Influence of Mn(II) and Aging Time in the Ferrihydrite to Goethite Transformation

The ferrihydrite-goethite transformation in samples of pure- and Mn-ferrihydrite was investigated in order to elucidate the effect of Mn(II) in the aging process. Samples of pure- and Mn-ferrihydrites were aged for different times, and their structural changes were described on the basis of Rietveld structure refinements. A comparison of two series of samples with (A-series) and without Mn (F-series) shows b- enlargement and a- and c- shortening due to the presence of Mn during the complete aging process. However, with aging different trends are observed in both the series. In the sam- ples synthesized without Mn(II) the variations in all the unit-cell constants are virtually negligible, whereas in the series with Mn(II) more significant changes are reflected by the enlargement of a- and c- and the shortening of the b. These re- sults confirm that the dimensions of the lattice parameters depend not only on the substitution ratio but also on the aging history of the sample.