NiBr2 complexes with triphenylarsine oxide (Ph3AsO). Two new interesting structures

The synthesis and crystal structure of two complexes resulting from interaction between NiBr2 and triphenylarsine oxide (Ph3AsO) is described. Green and orange complexes can be obtained from the blue, probably tetrahedral complex [NiBr2(Ph3AsO)2], depending on the solvents used for recrystallization. NiBr2·4[(C6H5)3AsO]·8H2O (green): M = 1650.2, P21/c, a = 13.731(2), b = 16.267(3), c = 17.647(2) Å, β = 112.04(1)°, V = 3651.4 Å3, Z = 2, Dx = 1.501 g cm-3, CuKα, λ = 1.54184 Å, μ = 38.67 cm-1, R = 0.039, 3741 unique reflections, 3203 with I > 3σ(I). NiBr2·4[(C6H5)3AsO]·3|2(C6H5CH3)·H2O (orange): M = 1663.7, P1, a = 12.647(8), b = 13.953(5), c = 22.853(6) Å, α = 90.91(3), β = 96.70(4), γ = 111.16(4)°, V = 3727.4 Å3, Z = 2, Dx = 1.482 g cm-3, MoKα, λ = 0.71073 Å, μ = 30.48 cm-1, R = 0.087, 8600 unique reflections, 4293 with I > 3σ(I). In the green complex the Ni(II) ion is sited on a center of symmetry and is octahedrally coordinated to six water molecules, hydrogen bonded to the Ph3AsO molecules and to the bromide anions forming a second coordination sphere in a nearly octahedral arrangement. In the orange complex the cation is pentacoordinated with the four oxygen atoms of the Ph3AsO ligands forming the basis of a tetragonal pyramid and with one Br- anion in the apical position. The absorption spectrum of the orange complex is compared with the spectra of other Ni(II) square pyramidal complexes described in the literature. © 1984.

Synthesis and solid-state structural characterization of bis (thiocyanatemercury)tetracarbonyliron

The bis (thiocyanatemercury)tetracarbonyliron, [Fe(CO)4(HgSCN)2], was prepared from [Fe(CO) 5] and Hg(SCN)2, and studied by IR spectroscopy and X-ray diffraction. The compound crystallizes in the tetragonal space group I4,1/a. The unit cell, with dimensions of a = 13.778(3), c = 13.234(3) Å, V = 2512.3(9) Å3, contains four molecules. The iron atom is octahedrally coordinated by four carbonyl groups and two mercury atoms in cis positions. The coordination of the mercury atoms is distorted square-planar, since, besides mercury-iron and mercury-sulphur bonds, there are also mercury-mercury and mercury-nitrogen interactions. The FeHg distance is 2.506(5)Å and the HgFeHg angle is 78.0(1)°. © 1987.

Mechanism of phase formation in Pb(ZrxTi1-x)O3 synthesized by a partial oxalate method

The phase formation mechanism, as well as the morphotropic phase boundary, of lead zirconate titanate (PZT) processed by a partial oxalate method was investigated by simultaneous thermal analysis (TG-DTA) and by qualitative and quantitative X-ray diffraction (XRD). The results show that the ZrxTi1-xO2 (ZT) phase reacts with PbO forming the PZT phase without intermediate phases. XRD analysis showed the coexistence of rhombohedral and tetragonal phases for 0.47 ≤ x ≤ 0.55 with the phase boundary composition for x = 0.51. For low calcination temperatures, preferential formation of the PZT rhombohedral phase was observed. A model for phase formation of PZT by the partial oxalate method is proposed based on the existence of two interfaces of reaction (PbO-PZT and PZT-ZT) and diffusion of cations.

Scandium fluorides

A new modification of scandium fluoride has been synthesised. The compound is deficient in fluorine, with the composition ScF2.76. It belongs to the tetragonal system, lattice parameters being a = 3.792 and c = 6.740 Å and may be obtained at low temperatures by the decomposition of the precursor NH4ScF4. The reaction is topotactic, tetragonal parameters of the precursor are a = 4.021 and c = 6.744 Å. Structural relationships with various fluorides and ammonium aminofluorides are discussed. This synthesis route with IR-assisted decomposition should be considered as a soft-chemistry approach. © 1997 Elsevier Science S.A.

Structure of cobalt (II) perchlorate adsorbed on silica gel surface chemically modified with benzimidazole molecule

Covalently attached benzimidazole molecules on silica gel surface, ≡SiL (where L = N-propyl-benzimidazole), adsorbs Co(ClO4)2 from non-aqueous solvent by forming a surface complex according to the reaction: m ≡SiL + Co(ClO4)2 → (≡SiL)mCo(ClO4)2. The equilibrium constant and the adsorption capacity, determined by applying the Langmuir equation were b = 3.0 × 103 L mol-1 and Ns= 0.098 × 10-3 mol g-1, respectively. The metal is bonded through the nitrogen atom and the perchlorate ion is not coordinated. The ESR study indicated that the complex has essentially an octahedral geometry with tetragonal distortion, with the electrons of the four nitrogen atoms interacting with the cobalt central metal ion in the equatorial plane. Only one complex species was detected on the surface.

Thermally activated dynamics of the tilts of the CuO6 octahedra, hopping of interstitial O, and possible instability towards the LTT phase in La2CuO4+δ

The anelastic spectrum (dynamic Young's modulus and elastic energy absorption) of La2CuO4+δ has been measured between 1 and 700 K with 0<δ<0.02. The spectrum of stoichiometric La2CuO4 in the low-temperature orthorhombic (LTO) phase is dominated by two intense relaxation processes which cause softenings of 16% around 150 K and 9% below 30 K at f∼1 kHz. The relaxation at 150 K is attributed to the presence of a fraction of the CuO6 octahedra which are able to change their tilted configuration by thermal activation between orientations which are nearly energetically equivalent, possibly within the twin boundaries. The relaxation below 30 K is governed by tunneling, and involves a considerable fraction of the lattice atoms. It is proposed that the double-well potentials for the low-temperature relaxation are created by the tendency of the LTO phase to form low-temperature tetragonal (LTT) domains, which however are not stabilized like when La is partially substituted with Ba. On doping with excess O, the relaxation rates of these processes are initially enhanced by hole doping, while their intensities are depressed by lattice disorder; an explanation of this behavior is provided. Excess O also causes two additional relaxation processes. The one appearing at lower values of δ is attributed to the hopping of single interstitial O2- ions, with a hopping rate equal to τ-1=2×10-14exp(-5600/T) s. The second process is slower and can be due to O pairs or other complexes containing excess O.

Adsorption and structure of copper(II) complexes on a silica gel surface chemically modified with 2-aminothiazole

The isotherms of adsorption of CuX2 (X=Cl-, Br-, ClO- 4) by silica gel chemically modified with 2-aminothiazole were studied in acetone and EtOH solutions, at 25°C. The 2-aminothiazole molecule, covalently bond to the silica gel surface, adsorbs CuX2 from solvent by forming a surface complex. At low loading, the electronic and E.S.R. spectral parameters indicate that the Cu2+ complexes have a distorted tetragonal symmetry. The d-d eletronic transition spectra show that for ClO- 4 complex, the peak of absorption do not change for any degree of metal loading whilst for Cl- and Br- complexes, the peak maxima shift to higher energy with lower metal loading. © Elsevier Science Ltd.

Direct amorphous-to-cubic perovskite phase transformation for lead titanate

The phase evolution of lead titanate processed by the polymeric precursor method was investigated by thermal analysis, X-ray diffraction, and high-resolution transmission electron microscopy. The results showed that the cubic perovskite PbTiO3 (PT) phase is formed from an inorganic amorphous precursor at a temperature of 444 °C. A gradual transition from cubic to tetragonal perovskite PT was observed with the increase of calcination time at this temperature. HRTEM results showed that the cubic PT particles have a size of around 5 nm. The identification of cubic PT as an intermediate phase supports the hypothesis that the chemical homogeneity was kept at the molecular level during the synthesis process, with no cation segregation.

Structure of tick anticoagulant peptide at 1.6 Å resolution complexed with bovine pancreatic trypsin inhibitor

The structure of tick anticoagulant peptide (TAP) has been determined by X-ray crystallography at t.6 Å resolution complexed with bovine pancreatic trypsin inhibitor (BPTI). The TAP-BPTI crystals are tetragonal, a = b = 46.87, c = 50.35 Å, space group P41, four complexes per unit cell. The TAP molecules are highly dipolar and form an intermolecular helical array along the c-axis with a diameter of about 45 Å. Individual TAP units interact in a head-to-tail fashion, the positive end of one molecule associating with the distal negative end of another, and vice versa. The BPTI molecules have a uniformly distributed positively charged surface that interacts extensively through 14 hydrogen bonds and two hydrogen bonded salt bridges with the helical groove around the helical TAP chains. Comparing the structure of TAP in TAP-BPTI with TAP bound to factor Xa(Xa) suggests a massive reorganization in the N-terminal tetrapeptide and the first disulfide loop of TAP (CyS5(T)- Cys 15(T)) upon binding to Xa. The Tyr1(T)OH atom of TAP moves 14.2 Å to interact with Asp189 of the S1 specificity site, Arg3(T)CZ moves 5.0 Å with the guanidinium group forming a cation-π-electron complex in the S4 subsite of Xa, while Lys7(T)NZ differs in position by 10.6 Å in TAP-BPTI and TAP-Xa, all of which indicates a different pre-Xa-bound conformation for the N- terminal of TAP in its native state. In contrast to TAP, the BPTI structure of TAP-BPTI is practically the same as all those of previously determined structures of BPTI, only arginine and lysine side-chain conformations showing significant differences.

Crystal structure of BaBi2Ta2O9

The crystal structure of the Aurivillius compound Bi2BaTa2O9 prepared via the chemical route was determined by direct methods using EXPO97, and refined using the Rietveld method with conventional X-ray diffraction data. The structure was found to be tetragonal (space group I4/mmm, number 139) and Z = 2, isomorphic of the Bi2BaNb2O9 reported by Blake and co-workers in the literature (1997). Two refinements were performed using the two asymmetry functions of DBWS-9807 (release 20/May/99). The unit cell for each case are: a = 3.932 22(6) Å, c = 25.5053(6) Å (RA) and a = 3.93250(7) Å, c = 25.5069(6) Å (RCF). The differences for atom positions, interatomic distances and angles are in the range of one standard deviation. Final agreements factors are: Rwp = 7.97%, S = 1.84, RBragg = 4.28%(RA), Rwp = 7.98%, S = 1.84, RBragg = 4.30% (RCF). The occupancies of Ba and Bi in site 2b were refined but constrained to have their summation equal to 1.00. The same constraints were applied to the Ba and Bi of the 4e site. The results show that on site 2b there are 70% of Ba and 30% of Bi and on the site 4e there are 82% of Bi and 18% of Ba. The charge equilibrium is maintained for one standard deviation of the site occupancies. © 2000 International Centre for Diffraction Data.