Linear Chain Compounds Research Articles

Palladium supported catalysts for the selective hydrogenation of sunflower oil

Selective hydrogenation of ethyl esters of traditional sunflower oil (SOEE) was carried out at low temperature (40 °C) in ethanol as solvent in the presence of supported palladium catalysts. In the range of studied dispersions (12–55%), the SOEE hydrogenation reaction is insensitive to the size of the palladium particles deposited on silica, but the largest metallic particles enhance the C18:1 cis– trans isomerization. The use of various oxide supports (α-Al 2O 3, γ c-Al 2O 3, TiO 2, MgO, ZnO, CeO 2, CeZrO 2) to deposit palladium does not allow to improve the selectivity of the reaction toward the cis C18:1 compared to Pd/SiO 2 catalysts. On the other hand, the modification of palladium by lead, introduced by surface redox reaction (catalytic reduction), promotes the selectivity in cis C18:1. In addition, this technique of preparation involves a moderate decrease of the palladium activity compared to a traditional method of successive impregnations. The introduction of amines in the reaction medium modifies the hydrogenating properties of the Pd/SiO 2 catalyst. According to the quantity and the nature of the added amine (aliphatic with linear or ramified chain and cyclic compounds), the catalytic activity can either be unchanged or inhibited. These evolutions result from a promoter electronic effect generated by the presence of the amine and from a poison geometric effect related to the adsorption of this nitrogen-containing compound on the palladium surface. Whatever the nature of the amine, it induces an increase of the selectivity toward the cis C18:1.

Incommensurate antiferromagnetic order in the [formula omitted] quantum chain compound LiCuVO 4

The linear spin chain (S= 1 2 ) compound LiCuVO 4 was investigated by elastic neutron diffraction, susceptibility and heat-capacity measurements on single crystals. Long-range incommensurate magnetic ordering with a propagation vector k=(0,0.532,0) is observed below 2.1(1) K by elastic neutron diffraction. The refinement of the magnetic structure indicates that the Cu 2+ moments lie within the ab-plane, to enclose an angle of ∼90° to each other along the chain, and have a magnitude of 0.31(1) μ B. These findings are attributed to the competition of nearest- and next-nearest-neighbour exchange coupling in the edge-sharing CuO 2 chains.

Low-Energy Charge Excitations in an Undoped Cuprate: Description Beyond the Standard pdσ-Model?

Within a joint experimental and theoretical study the density-density response of the prototypical linear chain compound Sr 2 CuO 3 has been investigated by means of polarization and momentum dependent electron energy loss spectroscopy (EELS) and exact diagonalizations including also the continued fraction technique for the determination of spectral densities such as the optical conductivity and the loss function, respectively. It has been shown that the lowest charge excitations of this 1D charge transfer insulator cannot be described properly within the usual pdσ-model for cuprates which is based on the Cu 3dx2-y2 and the corresponding O 2pσ states, only. The lowest energy charge excitations are dominated by O 2pπ final states within the CuO 4 plaquette plane. A detailed analysis of the EELS-data allows a classification into two types of charge transfer excitations with predominant π and σ character and various sizes of internal localization and dispersion.

Itinerant electrons and magnetism in the Co–C chain compound YCoC

Density-functional studies of the electronic structure of the Co--C linear-chain compound, YCoC are reported. The band structure near the Fermi energy shows a mixture of rather heavy Co-$d$-derived bands, and near-one-dimensional dispersive bands associated with the bonding of the Co--C chains. The resulting density of states is high, yielding an extremely weak ferromagnetic instability via the Stoner mechanism. However, the Fermi surfaces are strongly nested, suggesting competing antiferromagnetic spin fluctuations. An interesting possibility is that in clean samples a renormalized paramagnetic ground state may emerge, due to the combination of the weak competing ground states, the near one dimensionality of the bands, and quantum fluctuations.

Crystal Engineered Supramolecular Metal Phosphonates: Crown Ethers and Iminodiacetates

Three classes of metal phosphonates will be described in this brief review. First, we describe the zirconium derivatives of N-(phosphonomethyl)iminodiacetic acid, H2O3PCH2N(CH2COOH)2. The acid itself is a zwitterion formed by transfer of a phosphonic acid proton to the nitrogen. Consequently, the acid forms a phosphonic acid dimer (O···O, 2.61 A). Each carboxyl group then hydrogen bonds with phosphonate oxygens to form a three-dimensional network of short hydrogen bonds. A linear chain compound is formed when the phosphonic acid was combined with Zr(IV), but the addition of phosphoric acid as a spacer molecule results in the formation of layered compounds. When limited amounts of phosphoric acid were added, the layer that formed was of a new type embodying features of both the α- and γ-type zirconium phosphate layers. Increased addition of H3PO4 allowed the layers to revert to the α-type. A series of N-(phosphonomethyl) aza-crown ethers were prepared and reacted with M(IV) and M(II) ions. With zirconium, ...

ChemInform Abstract: Colopsinols D and E, New Polyhydroxyl Linear Carbon Chain Compounds from Marine Dinoflagellate Amphidinium sp.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Hydrated Cr(V) Peroxychromates M3CrO8·XH2O (M = Li, Na, Cs): Model 3d1Systems Exhibiting Linear Chain Behavior and Antiferromagnetic Interactions

A new class of Cr(V) peroxychromates, those containing waters of hydration (Li 3- CrO 8 .10H 2 O, Na 3 CrO 8 .14H 2 O and Cs 3 CrO 8 .3H 2 O), has been synthesized and characterized by single-crystal X-ray diffraction, magnetic susceptibility, specific heat, and EPR techniques. The Li salt crystallizes into the orthorhombic (Cmcm) group, whereas the Na and Cs salts exhibit the triclinic (P1) and monoclinic P2(1)/n space groups, respectively. Crystal structure analysis indicated the availability of low-dimensional spin-exchange pathways. The orientation dependence of the EPR line width exhibited the (3 cos 2 θ - 1) 4/3 behavior characteristic of linear chain compounds, with θ = 0 coinciding with the chain axis. Correlation of the EPR and X-ray data allowed for the explicit determination of the magnetic chain axes in the crystal and molecular frameworks. Magnetic susceptibility, X , measurements on oriented single crystals yielded negative values for the Curie-Weiss temperatures, indicating dominant antiferromagnetic interactions. The X data yielded the Curie constants that appear to be inversely related to the number of waters of hydration in the crystal structure; this observation is not yet understood. Correlation of the EPR line width, nearest neighbor distances, and Curie constants attested to the essential correctness of the linear chain spin exchange and dipolar fields in the Li and Cs salts, and to a lesser extent also for the Na salt. The specific heat data indicated that all these compounds have the potential to be useful for devices requiring strong heat sinks as well as magnetic refrigerants in the 250 mK temperature regime. They appear to be good as model systems for theoretical investigations on low-dimensional magnetic lattices and for understanding aspects of Cr(VI)-based catalysis and carcinogenesis.

Synthesis, crystal structure, EXAFS, and magnetic properties of catena [mu-tris(1,2-bis(tetrazol-1-yl)propane-N1,N1')iron(II)] bis(perchlorate). First crystal structure of an iron(II) spin-crossover chain compound.

[Fe(btzp)3](ClO4)2 (btzp = 1,2-bis(tetrazol-1-yl)propane) represents the first structurally characterized Fe(II) linear chain compound exhibiting thermal spin crossover. It shows a very gradual spin transition (T1/2 = 130 K) which has been followed by magnetic susceptibility measurements and 57Fe Mössbauer spectroscopy. The structure has been solved at 200 and 100 K by single-crystal X-ray analysis. It crystallizes in the trigonal space group P3c1 with Z = 2 Fe(II) units at both temperatures. The molecular structure consists of chains running along the c axis in which the Fe(II) ions are linked by three N4,N4' coordinating bis(tetrazole) ligands. The main difference between the two forms appears to be in the Fe-N bond lengths, which are 2.164(4) A at 200 K and 2.038(4) A at 100 K. The Fe-Fe separations are 7.422(1) A at 200 K and 7.273(1) A at 100 K. The EXAFS results are consistent with the crystal structure. In both spin states, the FeN6 octahedron is almost regular within the EXAFS resolution. The Fe-N distance is found as 2.16(2) A at 300 K and 2.00(2) A at 40 K. The absence of the "7 A peak" in the EXAFS spectra of [Fe(btzp)3](ClO4)2, in contrast with what has been observed for the [Fe(4-R-1,2,4-triazole)3]-(anion)2 chain compounds, confirms that this peak can be used as the signature of a metal alignment only when it involves a strongly enhanced multiple scattering M-M-M path, with M-M spacing less than 4 A. Irradiation with green light at 5 K has led to the population of the metastable high-spin state for the iron(II) ion. The nature of the spin-crossover behavior has been discussed on the basis of the structural features.

Colopsinols D and E, new polyhydroxyl linear carbon chain compounds from marine dinoflagellate Amphidinium sp.

Colopsinols D (1) and E (2), two new polyhydroxyl linear carbon chain compounds, have been isolated from the cultured marine dinoflagellate Amphidinium sp. The structures of 1 and 2 were elucidated on the basis of two-dimensional NMR and FAB-MS/MS data.

Simple linear-chain cobalt phosphates

Two simple one-dimensional cobalt phosphates containing linear chains of corner-shared 4-membered rings have been synthesized hydrothermally by the reaction between amine phosphates and cobalt(II) ions under non-aqueous conditions. The linear chains are held together by hydrogen bond interactions involving the hydrogens of the amine and the framework oxygens. The two cobalt phosphates reported are the first linear chain compounds discovered in the family of open-framework transition metal phosphates.

Preparation and Spectroscopic Characterization of [Pt(en)2I2][Pt(CN)4]: A New Quasi-One-Dimensional Mixed Valence Chain Material in a Completely Ordered Lattice

The unique linear chain compound [Pt(en){sub 2}I{sub 2}][Pt(CN){sub 4}] {l{underscore}brace}where en = ethylenediamine{r{underscore}brace} has been synthesized through a standard method. The results of a structural analysis by X-ray crystallography reveal this new MX compound to be quasi-one-dimensional with complete chain-to-chain registry to give a completely ordered structure. The results of resonance Raman, IR, diffuse reflectance, and X-ray crystallography reveal that the Pt centers are highly charge disproportionated with a large commensurate distortion of the iodide sublattice about the Pt center of higher charge. These results reveal that this material is the strongest CDW MX compound, containing iodide bridges, reported to date. Resonance Raman experiments have shown that five A{sub g} (C{sub 2h} site symmetry) Raman bands are strongly enhanced. These correspond to three in-chain Pt-I modes and two (chain perpendicular) CN stretches and are interpreted in terms of strong phonon coupling to the IVCT band.

Synthetic control of intrinsic ground-state defects in a mixed valence quasi-one-dimensional Pt halide chain

The new linear chain compound [Pt(en) 2Cl 2][Pt(en) 2][Pt(CN) 4] 2 has been synthesized through two independent methods to yield a quasi-one-dimensional chain with very strong hydrogen bonding interactions. The high charge density wave (CDW) character will be discussed in terms of an unusual hydrogen bonding network which bridges adjacent chains but does not bridge adjacent oxidized and reduced units within a chain. Such a network is unique and breaks from known CDW trends derived from the template effect. The results of the structural analysis by X-ray crystallography have shown that the chains grown from two independent methods are isostructural with one another. Spectroscopic results (resonance Raman and diffuse reflectance) clearly demonstrate the presence of ground-state polaronic defects in crystals grown from solution of stoichiometric amounts of subunits while crystals grown from Pt(IV) rich solutions display a complete absence of ground-state polaronic signatures. These results will be discussed in terms of how one can chemically control the presence and amount of ground-state reduced site defects in the general class of MX materials.

Anisotropic electrodynamics of low dimensional metals: Optical studies of (TMTSF)

Optical experiments are reported for the metallic state of the linear chain compound (TMTSF)2ClO4. For the electric field polarized both along the highly (a) and intermediately (b′) conducting directions, a zero energy (ZE) mode and a finite energy mode (FE) are observed. The large anisotropy in the spectral weight of the FE mode is consistent with the band structure, however the spectral weight of the ZE mode is surprisingly isotropic. In the least conducting (c*) direction, the low frequency optical conductivity along with the dc conductivity indicate the presence of a (small) Drude component only at temperatures below 10 K. These observations provide evidence for a correlation induced semimetallic state, with a 3D to 2D crossover with increasing temperature.

The hole distribution in cuprate chains

The electronic structure of the prototypical corner-sharing Sr{sub 2}CuO{sub 3} linear chain compound is investigated by combining several theoretical and experimental techniques. Band structure calculations within the local density approximation and using a local orbital basis provided the relevant orbitals and the transfer integrals for a four-band extended Hubbard pd-model, which was treated by means of exact diagonalization and of quantum Monte Carlo calculations for finite chain clusters. The band structure values of the transfer integrals t{sub pd}exceed the corresponding values for layered cuprates. Enhanced values of the intersite Coulomb interaction, V{sub pd} = 2 {hor{underscore}ellipsis} 2.5 eV, and a difference between the onsite energies of side and chain oxygen {Delta}{sub pp} = 0.5 {hor{underscore}ellipsis} 0.75 eV are deduced from the comparison of the model studies with the intensities of polarization dependent x-ray absorption spectra. The latter reflect the hold distribution over the oxygen sites.

Spin dynamics of organic linear chain compounds

We report about electron-spin-resonance measurements on single crystals of the organic spin-chain compounds (TMTTF) 2PF 6 and (TMTTF) 2ClO 4, where TMTTF stands for tetramethyltetrathiafulvalene, in the temperature range 2 K⩽ T⩽400 K. At high temperatures the spin susceptibility of both compounds can be described by a spin 1 2 antiferromagnetic Heisenberg chain with an exchange constant J≈420 K. In (TMTTF) 2ClO 4 the first-order structural phase transition at T AO=72.5 K is accompanied by a dimerization of the spin chain with a singlet–triplet gap Δ≈82 K. Below T AFM=12 K we find an additional transition to an antiferromagnetically ordered state. (TMTTF) 2PF 6 undergoes a spin–Peierls transition at T SP=19 K which can be well described using mean field theory with Δ(0)=34.5 K.

Formation of Metal-Oxygen Chain Structures by Bridging Dichromate and Coordination Compounds: [MLnCr2O7]∞ (MLn = Cu2+(bipy), Ni2+(en)2, and Ni2+(bipy)2)

Abstract Metal-oxygen linear chain compounds, [Cu(bipy)Cr2O7]∞ (1) (bipy = 2,2′-bipyridine), [Ni(en)2Cr2O7]∞ (2) (en = ethylenediamine), and [{Ni(bipy)2Cr2O7}(CH3CN)]∞ (3) have been prepared and characterized. X-ray structural analyses of 1 to 3 revealed three chain structures: parallel in 1, layers of chains which criss-cross in 2, and helical in 3.

LDA modeling of the electronic structure of the halogen-bridged transition-metal chain compounds

The electronic-structure for the halogen-bridged transition-metal (MX) linear chain compound Pt2X6(NH3)4 (X = Cl, Br, I) under uniaxial stress is studied using the full-potential linear muffin-tin method within the local density approximation. We have found that the ground state of the full structure (with the ammonia ligands) is insulating with a strong lattice distortion in good agreement with experiment. This highlights the importance of the ligand structure to the stability of MX systems and to its ground state nature. The amplitude of the charge density wave in these systems as a function of the metal-metal distance produced a linear shift of the core level energies of the two non-equivalent metal sites. This core level energy shift could be checked experimentally using X-ray absorption spectroscopy. The band structure along the chain is dominated by a strong coupling (pdσ) between Pt(d3z2−r2) and X(pz) orbitals, which causes the system to be of a quasi-one-dimensional character. The relevant band structure as a function of the metal-metal distance is nicely fitted to a two band tight-binding Su-Schrieffer-Heeger Hamiltonian with a single set of parameters. This model revealed that the mechanism of the dimerization is a consequence of a competition between a strong electron-phonon coupling along the chain and a repulsive anharmonic potential.

Reactivity of bimetallic dibridged complexes Cp 2Ta(H)(μ-H)(μ-PMe 2)M′(CO) 4 (M′ = Cr, Mo, W) toward two-electron donor ligands L (L = PR 3, Me 2P(CH 2) nPMe 2). Synthesis of linear trinuclear chain complexes Cp 2Ta(H) 2(μ-PMe 2)M′(CO) 4(Me 2P(CH 2) 2PMe 2)Cr(CO) 5 (M′ = Mo, W)

The reaction of the heterobimetallic phosphido- and hydrido-bridged complexes Cp 2TaH(μ-H)(μ-PMe 2)M′(CO) 4 (M′ = Cr, Mo, W) ( 1–3) with phosphines (L = PPh 2Me, PMe 2Ph) or diphosphines (L = dmpm, dmpe) leads to Cp 2Ta(H) 2(μ-PMe 2)M′(CO) 4(L) ( 1a, 1b, c, d-3b, c, d) with L regiospecifically coordinated to M′. Except for L = PPh 2Me, the reaction is stereospecific, since a cis arrangement (with respect to the PMe 2 bridge) on the M′ site is obtained. The new compounds Cp 2Ta(H) 2(μ-PMe 2)M′ (CO) 4(Me 2P(CH 2) 2PMe 2) (M′ = Mo, W) are able to bind [Cr(CO) 5] fragments affording the linear trinuclear chain compounds Cp 2Ta(H) 2(μ-PMe 2)M′(CO) 4(Me 2P(CH 5) ( 4,5). All new complexes have been fully characterized by NMR and IR spectroscopy, and by their elemental analyses.

The spin-density-wave gap in (TMTSF) 2ClO 4

We have measured the optical properties of the linear-chain compound (TMTSF) 2ClO 4 and AsF 6, in the spin-density-wave state which develops below T SDW. In the direction perpendicular to the chains, we observed clear signatures of the spin-density-wave gap, similar to our previous findings in the (TMTSF) 2PF 6 compound.

Cu-isotope effect on the spin-Peierls transition of CuGeO3

The Cu-isotope effect on the spin-Peierls transition temperature TSP in samples of the Cu2+ linear-chain compound CuGeO3 has been determined by dc-magneticsusceptibility and specific-heat measurements. A downshift of the spin-Peierls transition by about 0.1 K for the 65Cu-isotope enriched 65CuGeO3 as compared to natCuGeO3 containing the natural mixture of isotopes is found consistently in the magnetization and specific heat. This result suggests different isotope effects on the phonon characteristic frequency θ0 on the one hand and on the spin-phonon coupling constant g on the other hand. However, the isotope effect on TSP is smaller than estimated if all atoms were to contribute to ω0 and only the Cu atoms to g.