Coordination polymers with cyanoaurate building blocks: Potential new industrial applications for gold

This report illustrates the concept that aurophilic interactions of gold-containing building blocks, particularly cyanoaurates, could be used as a tool to increase structural dimensionality in systems containingother metals in addition to gold(I). Such high-dimensionality systems may have useful optical, magnetic, conducting or porous materials properties. Recent successes from our group and others in using the neglected, luminescent [Au(CN)₂]⁻ building block to synthesize supramolecular coordination polymers with interesting and potentially commercially applicable physical properties will be surveyed. In most heterometallic [Au(CN)₂]-based polymers, aurophilic interactions increase the structural dimensionality of the system and can impart increased thermal stability. The gold(I) ion can mediate significant magnetic interactions between transition-metal centres or influence iron(II) spin-transition behaviour in the polymers. The Cu[Au(CN)₂]₂(solvent)_x polymer system is dynamically vapochromic, i.e., it shows large, reversible colour changes upon exposure to solvent vapours, thereby illustrating a sensor-type application. The related d⁸, square-planer [Au(CN)₄]⁻ building block, which has only recently been incorporated into coordination polymers, does not form any aurophilic interactions; weak Au-N(cyano) interactions control the intermolecular packing. Several structural examples of cyanoaurate-based coordination polymers are presented, including 2-D and 3-D arrays. The incorporation of cyanoaurates as components of advanced materials would provide a new utility and market for these key compounds of the gold mining and refining industry.     

Gas adsorption at the components of the GaAs-CdS system

Adsorption properties of the GaAs-CdS-system solid solutions and binary compounds are studied with respect to CO and NH₃ by the piezoelectric quartz microweighing, programmed thermal desorption, and IR-spectroscopy. Based on the experimental dependences (α_p = f(T), αT = f(P), αT = f(t)), thermodynamic and kinetic characteristics of adsorption, analysis of the surface chemical composition, the adsorbents’ acid-base and other physicochemical characteristics. With due allowance for electron structure of the adsorbate’s molecules, the mechanism and specifity of adsorption processes are determined as functions of the operation conditions and system composition. By comparing the adsorption properties of the binary compounds (GaAs and CdS) with those of the solid solutions (GaAs) _x (CdS)_1−x , their similarities, as well as the specificity of the solid solutions as multicomponent systems, were revealed. Optimal adsorbent compositions to be used in primary transducers in the medicine-and environment-orientated sensors are found. Original Russian Text ? I.A. Kirovskaya, 2008, published in Zashchita Metallov, 2008, Vol. 44, No. 2, pp. 197–203.     

Gold complexes with

The reactions of methane with the gold complexes [Au(OH)]⁻, [Au(OCH₃)₄]⁻, [Au(O(CO)₂O₂]⁻ and [Au(O₂CH)₂]⁺, [Au^I(acac)], [Au^III(acac)₂]⁺ (acac-acetylacetonato) were studied using the DFT/PBE method with the SBK basis set. High activation barriers were obtained for the electrophilic substitution in [Au(OH)]⁻, [Au(OCH₃)⁴]⁻, [Au(O(CO)₂O)₂]-and [Au^III(acac)₂]⁺ complexes, which excludes the possibility that these reactions might proceed under mild conditions. The reactions of the [Au(HCO₂)₂]⁺ and [AuI(acac)] complexes with methane have rather low energy barriers and proceed through the formation of an intermediate complex. The alternative mechanism of methane oxidation with a gold complex in the presence of oxygen is simulated.     

Adsorption properties of immobilized metal nanoparticles

The adsorption properties of the original TiO₂, MgO, and SiO₂ surfaces and those modified with nanoscale gold and nickel particles are studied using the dynamic sorption in a range of low coverages. N-alkanes (C₆–C₉) and adsorbate molecules with different donor and acceptor properties were taken as the reference compounds. Differential molar heats of adsorption of the adsorbates (Q _ν) are determined from the experimental retention data and, in the case of polar adsorbates, contributions from the dispersion and specific interactions to Q _ν are found. Donor and acceptor characteristics of the original and modified oxide surfaces are estimated. The immobilization of gold and nickel nanoparticles is shown to result in the formation of less active donor (K _D ) and acceptor (KA) sites compared to those on the original substrate surfaces. Changes in the surface chemistry and adsorption properties of Au/SiO₂ composites compared to the original substrate are less pronounced than in the case of Au,Ni/TiO₂ and Au,Ni/MgO composites.     

Synthesis and antimicrobial activities of gold(I) sulfanylcarboxylates

Reaction of NaAuCl₄·H₂O and thiodiglycol (1:3 molar ratio) with 3-(aryl)-2-sulfanylpropenoic acids, H₂ xspa = [x:p = 3-phenyl-, f = 3-(2-furyl)-, t = 3-(2-thienyl)-, o-py = 3-(2-pyridyl)-, Clp = 3-(2-chlorophenyl)-, -o-mp = 3-(2-methoxyphenyl)-, -p-mp = 3-(4-methoxyphenyl)-, -o-hp = 3-(2-hydroxyphenyl)-, -p-hp = 3-(4-hydroxyphenyl)-, diBr-o-hp = 3-(3,5-dibromo-2-hydroxyphenyl)] and 2-cyclopentylidene-2-sulfanylacetic acid (H₂cpa) in a 1:1 metal/ligand molar ratio gave compounds of the type [Au(Hxspa)] or [Au(Hcpa)]. These compounds were reacted with diisopropylamine to afford [HQ][Au(xspa)] or [HQ][Au(cpa)] (HQ = diisopropylammonium) and with NaOH to afford Na[Au(xspa)]·H₂O and Na[Au(cpa)]·H₂O. All of the new compounds were isolated and characterised by IR and ¹H and ¹³C NMR spectroscopy. The antimicrobial activities of the complexes against Escherichia coli, Staphylococcus aureus, Bacillus subtilis, Candida albicans, Pseudomonas aeruginosa and carbapenem-resistant P. aeruginosa were evaluated and compared to those of the equivalent silver(I) complexes. The comparison shows that the gold compounds generally show better activity than the silver analogues against S. aureus and B. subtilis, but low sensitivity against E. coli, P. aeruginosa and C. albicans, suggesting a different mode of antimicrobial action for equivalent silver and gold compounds.     

Design of luminescent sulfur-containing polynuclear gold(I) complexes for advanced nanomaterials and chemosensors

A series of high-nuclearity gold(I) sulfido complexes with bridging diphosphine ligands have been synthesized and isolated, with the general formulae of [Au₁₀(μ-P^P)₄(μ₃-S)₄]X₂ and [Au₁₂(μ-P^P)₄(μ₃-S)₄]X₄ (P^P = diphosphine; X = PF₆ or ClO₄). Intense green and orange emissions were observed upon excitation at λ > 350 nm both in the solid state and in solutions. The green emission has been attributed to originate from excited states derived from the metal-perturbed intraligand (IL) transition while the orange emission arises from the ligand-to-metal-metal bond charge-transfer (LMMCT; S(Au...Au) states. The switching on and off of the LMMCT emission in a series of dinuclear gold(I) thiolate complexes with crown ether pendants induced by ion-binding has been demonstrated, and this provides a new strategy for the design of luminescence signalling and chemosensing devices in optoelectronics and sensor technology.     

Magnetic anisotropy of Ho–Fe–Co–Cr intermetallic compounds

Starting with a Ho₃(Fe_1−xCo_x)_29−yCr_y, (x,y) = (0.6,4.5) and (0.8,5.5) nominal stoichiometry, a disordered variant of the hexagonal 2:17 phase (Th₂Ni₁₇-type, S.G. P6₃/mmc) occurs, since both the monoclinic 3:29 and the transition-metal-rich disordered Th₂Ni₁₇-type hexagonal compounds have the same rare earth to transition metal ratio, 1:9.7. The magnetic properties and the magnetocrystalline anisotropy of these compounds have been investigated. The anisotropy constant, K's, and the anisotropy field, μ₀H_A, values have been deduced from the magnetization curves measured on powder samples magnetically aligned in a rotating magnetic field. The compound with (x,y) = (0.8,5.5) shows a compensation point at about 55 K. The magnetic anisotropy of both compounds is that of easy-plane from room temperature to low temperatures down to 5 K.     

Support effects in the selective gas phase hydrogenation ofp-chloronitrobenzene over gold

The catalytic continuous gas phase hydrogenation of p-chloronitrobenzene (P=1 atm;T=423 K) has been investigated over a series of oxide (Al₂O₃, TiO₂, Fe₂O₃ and CeO₂) supported Au (1 mol %) catalysts. The application of two catalyst synthesis routes,i.e. impregnation (IMP) and deposition-precipitation (DP), has been considered where the DP route generated smaller mean Au particle sizes (1.5-2.8 nm) compared with the IMP preparation (3.5-9.0 nm). The catalysts have been characterised in terms H2 chemisorption and BET area measurements where the formation of metallic Au post-activation has been verified by diffuse reflectance UV-Vis, XRD and HRTEM analyses.p-Chloroaniline was generated as the sole reaction product over all the Au catalysts with no evidence of C-Cl and/or C-NO₂ bond scission and/or aromatic ring reduction. The specific hydrogenation rate increased with decreasing Au particle size (from 9 to 3 nm), regardless of the nature of the support. This response extends to a reference Au/TiO₂ catalyst provided by the World Gold Council. A decrease in specific rate is in evidence for smaller particles (< 2 nm) and can be attributed to a quantum size effect. The results presented establish the basis for the design and development of a versatile catalytic system for the clean continuous production of high value amino compounds under mild reaction conditions.     

Anodic Oxidation of Silver and Ag–Au Alloys in Alkaline Medium

Anodic oxidation of Ag and Ag–Au alloys containing from 0.1 to 30 at. % Au under the conditions of Ag(I) oxide formation at t 0.5 s is determined by the peculiarities of migration transfer of charge carriers through phase oxide layer. The Ag₂O film is two-layer; this is reflected in the rise of additional current peaks in cyclic ivs. E(t) curves and also in the variation in the slopes of linear segments of chronoammograms presented in the i vs. t ^–1/2 coordinates. The conductivity of Ag₂O film slightly varies with its thickness; it is virtually independent of small gold additions to silver, but steeply decreases at 15 and 30 at. % Au. The formation and reduction of AgO occur via single-electron conversions of Ag₂O proceeding in the oxide phase. At all stages of oxidation of Ag and Ag–Au alloys in alkaline medium, the charge-transition stage remains reversible.     

Stability and electronic structure of the Co-P compounds from first-principle calculations

First-principle calculations based on density functional theory (DFT) were performed to investigate the structural stability and electronic properties of Co-P compounds such as Co₂P(I) (orthorhombic), Co₂P(II) (hexagonal), CoP, CoP₂ and CoP₃. The cohesive energies of Co-P compounds are all negative, which indicates that they are thermodynamically stable. Furthermore, the stability of Co-P compounds decreases with the increase of P element. By analyzing the electronic structures of Co-P compounds, we have found that Co₂P(I) (orthorhombic), Co₂P(II) (hexagonal) and CoP show metallic character, while CoP₂ and CoP₃ show semiconductor character. The bonding behaviour between Co atom and P atom in Co-P compounds is a combination of covalent and ionic nature.     

On the crystal structure and magnetic properties of the ternary rare earth compounds RETSb2 with RE rare earth and T Ni, Pd, Cu and Au

Novel compounds RETSb₂ have been prepared and characterized for T Cu (RE rare earth from La to Lu), Ni (RE La to Ho), Pd (RE La to Tb) and Au (RE La to Sm). From X-ray powder diffraction analyses all compounds were found to crystallize as the ZrCuSi₂ type. Magnetic susceptibilities were generally measured in the temperature range from 4 to 100 K. YCuSb₂ and LaTSb_z are temperature-independent paramagnets. RETSb₂ compounds are found to order antiferromagnetically below T = 20 K. PrPdSb₂ and TbPdSb₂ undergo metamagnetic transitions, whereas PrCuSb₂ and ErCuSb₂ are simple ferromagnets. The Sm-containing compounds are typical Van Vleck paramagnets owing to the closely spaced multiplets.     

Positions and mobility of hydrogen atoms in Hf2CoH x (D x ) with a structure of the Ti2Ni type: Study by the NMR and neutron diffraction methods

Spin-lattice relaxation rates of ¹H and ²D nuclei in Hf₂CoH_1.2 and Hf₂CoD_1.3 compounds with a structure of the Ti₂Ni type have been measured in the temperature range of 80–424 K. The experimental data indicate the coexistence of at least two types of jumplike motion of H (D) atoms with different characteristic frequencies. A neutron-diffraction study of the structure of the Hf₂CoD_1.3 compound showed that the deuterium atoms in this compound occupy octahedral positions 16c and 8a, as well as tetrahedral positions 32e ₁ and 96g ₂. An analysis of the sublattice of positions partially filled with H (D) atoms makes it possible to qualitatively explain the difference in the behavior of the jumplike motion of hydrogen in Hf₂CoH _x (D _x ) from the corresponding behavior in the related system Ti₂CoH _x (D _x ). Original Russian Text ? A.L. Buzlukov, A.V. Soloninin, A.V. Skripov, E.Yu. Medvedev, V.I. Voronin, I.F. Berger, 2009, published in Fizika Metallov i Metallovedenie, 2009, Vol. 107, No. 1, pp. 78–84.     

Sensing behavior of silica-coated Au nanoparticles towards nitrobenzene

In the present work, we report silica-stabilized gold nanoparticles (SiO₂/Au NPs) as a wide-range sensitive sensing material towards nitrobenzene (NB). Surface hydroxyl groups of silica selectively form Meisenheimer complex with electron-deficient aromatic ring of NB and facilitate its immobilization and subsequent catalytic reduction by Au cores. Silica-coated Au NPs were synthesized and characterized for their chemical, morphological, structural, and optical properties. SiO₂/Au NPs-modified electrodes were characterized with impedometric and cyclic voltammetric electrochemical techniques. SiO₂/Au NPs are found to have a higher optical detection window of range, 0.1 M to 1 μM and a lower electrochemical detection window of range, 10⁻⁴ to 2.5 × 10⁻² mM with a detection limit of 12.3 ppb. A significant enhancement in cathodic peak current, C ₁, and sensitivity (102 μA/mM) was observed with modified electrode relative to bare and silica-modified electrodes. The I _P was found to be linearly co-related to NB concentration (R ² = 0.985). The interference of cationic and anionic species on sensor sensitivity was also studied. Selectivity in the present sensing system may be further improved by modifying silica with specific functional moieties.     

Thermodynamic Description of the Al-Mo and Al-Fe-Mo Systems

The Al-Mo and Al-Fe-Mo systems were critically assessed using the CALPHAD technique. The solution phases (liquid, fcc and bcc) were described by a substitutional solution model. The non-stoichiometric compound AlMo₃ was described by a two-sublattice model (Al,Mo)(Al,Mo)₃ in the Al-Mo binary system and (Al,Fe,Mo)(Al,Fe,Mo)₃ in the Al-Fe-Mo ternary system. Other compounds Al₆₃Mo₃₇, Al₈Mo₃, Al₃Mo, Al₄Mo, Al₁₇Mo₄, Al₂₂Mo₅, Al₁₂Mo and Al₅Mo in the Al-Mo system were treated as stoichiometric compounds in the binary system and as line compounds Al _m (Fe,Mo) _n in the Al-Fe-Mo ternary system. The compounds μ and Fe₂Mo in the Fe-Mo system were treated as (Al,Fe)₇Fe₂(Fe,Mo)₄ and (Fe,Mo)₂(Al,Mo) in the Al-Fe-Mo system, respectively. Compounds Al₅Fe₄, Al₂Fe, Al₅Fe₂ and Al₁₃Fe₄ in the Al-Fe system were treated as (Al,Fe,Mo), Al₂(Fe,Mo), (Al,Fe)₅(Al,Fe,Mo)₂ and (Fe,Mo)_0.235Al_0.6275(Al,Va)_0.1375 in the Al-Fe-Mo system, respectively. Ternary compounds τ₁ and τ₂ were treated as Al₈(Al,Fe)Mo₃ and (Al,Fe,Mo)(Va)₃, respectively. A set of self-consistent thermodynamic parameters of the Al-Fe-Mo system was obtained.     

Structure and magnetic properties of Gd3(Fe1−xTix)29 (x=0.011–0.034)

Single-phase compounds Gd₃(Fe_1−xTi_x)₂₉ (x=0.0110.034) have been synthesized. Gd₃(Fe_1−xTi_x)₂₉ crystallises in a monoclinic lattice with space group P2₁/c, and the crystal structure is refined by the Rietveld technique based on X-ray powder diffraction data. Thermomagnetic analysis indicates that the Curie temperature of the compounds ranges from 517 K to 538 K. The saturation magnetizations of the Gd₃(Fe_1−xTi_x)₂₉ (x=0.011, 0.022, 0.034) at 1.5 K are 103.6, 102.0 and 94.3 Am²/kg, and the anisotropy fields at 1.5 K are 6.0, 6.2 and 6.4T, respectively.     

On Synthesis and Magnetic Properties of Nd(Fe, Mo, Ti)12Zx (Z=N, H)

NdFe_10+xMo_2−2xTi_x compounds with 0≤x≤1.0 have been prepared by using the reduction–diffusion process with superfine precursors as starting materials. With increasing Ti-content, the intrinsic magnetic properties, such as the Curie temperature, saturation magnetization and magnetocrystalline field, are improved. The interstitial compounds NdFe_10+xMo_2−2xTi_xZ_y (Z=N, H) were obtained and exhibit significant enhancement of the intrinsic magnetic properties upon nitrogenation.     

Liquid-liquid extraction of Au(I) by Amberlite LA2 and its application to a polymer-immobilized liquid membrane system

The liquid-liquid extraction of Au(I) by Amberlite LA2 (secondary amine) in kerosene from cyanide media has been investigated. The analysis of metal distribution data suggested the formation of R₂NH₂ ⁺Au(CN)₂-species in the organic phase with formation constant log K_ext=6.5 ± 0.03, whereas the use of mixtures of the amine with a solvation extractant (e.g. a phosphine oxide) shifted the gold-pH_1/2 extraction towards more alkaline values. The results obtained on Au(I) distribution have been implemented in a polymerimmobilized liquid membrane system, using NaOH solutions (e.g. 0.01 mol/L) to strip the metal from the organic phase.     

Perfluorinated oxygen- and sulfur-containing compounds as extractants for gold(III)

A series of highly fluorinated compounds of the type {CF₃(CF₂)₅CH₂CH₂}₃P=O, [{CF₃(CF₂)₅CH₂CH₂}₂P(E)CH₂CH₂P(E){CH₂CH₂(CF₂)₅CF₃}] and {CF₃(CF₂)₅CH₂CH₂}₂C=E (E = O or S) have been examined for their ability to extract gold(III) from aqueous solutions. The phosphine oxides have been studied under liquid–liquid extraction conditions from water into perfluorohexane and found to give poor distribution ratios. The bidentate phosphine oxide, ketone and thioketone were studied under solid–liquid extraction conditions and were substantially better with extraction of up to 80% of Au(III). In addition, the crystal structure of {CF₃(CF₂)₅CH₂CH₂}₃P=O has been determined.     

Thermodynamic properties and phase equilibria in the Cr-P system

A Knudsen effusion method with mass-spectrometric analysis of gaseous phase has been applied to investigate the thermodynamic properties of the chromium phosphides (1341 to 1704 K) and Cr-P liquid alloys (1664 to 1819 K). Simultaneously, DSC has been used to measure heat capacities of chromium phosphides Cr₃P and Cr₁₂P₇ in the temperature range of113 to 873 K. The entropies of formation of chromium phosphides calculated according to the second and third laws of thermodynamics agree within the limits of experimental error. The Gibbs energies of formation of the phosphides from solid Cr and P₂ gas have been approximated with the following equations (in J/mol): A_fG⁰(Cr₃P) = −(244 112 ±2800) + (70.95 ±1.80)T ΔG⁰(Cr₁₂2P₇) = −(1563 678 ±15 350) + (440.6 ±9.90)T Thermodynamic properties of liquid solutions have been described with the ideal associated-solution model assuming that CrP, Cr₂P, Cr₃P, and Cr₃P₂ complexes exist in the melt. The phase diagram computed with the help of the thermodynamic data agrees with the published information.     

Kinetic Parameters and Mechanism of Gold Electrooxidation in Thiocarbamide Solutions

Kinetics and mechanism of gold electrooxidation in sulfuric acid solutions of thiocarbamide (TCA) are studied by measuring voltammograms (VA) of gold and glassy carbon (GC) electrodes. In a potential (E) range from –0.2 to 1.2 V (SCE), VA of a gold electrode demonstrate three peaks. The first and the second peaks correspond to gold electrooxidation to [AuTCA_ads] + and [Au(TCA_ads)₃]³⁺ ions at E of 0.152 and 0.554 V, while the third one corresponds to TCA electrooxidation. The electrooxidation of TCA on GC electrodes is characterized by two VA peaks at 0.983 and 1.437 V. The first peak corresponds to the formation of formamidine disulfide (FADS) (k _a1 = 6.40 × 10^–4 cm/s), while the second peak corresponds to TCA oxidation to sulfides and hydrosulfides (k _a2 = 7.78 × 10^–5 cm/s). The composition of adsorption layers formed at the Au electrode is analyzed by Auger spectroscopy. The introduction of sodium sulfide into TCA solutions eliminates the formation of adsorption layers and accelerates Au oxidation in such solutions. The rate constants of gold electrooxidation k _a = 6.31 × 10^–3 cm/s and Au(I) electroreduction k _c = 5.46 × 10^–4 cm/s in TCA solutions are estimated. Kinetic parameters (charge transfer coefficients, reactions orders in carbamide) are determined and the mechanisms of Au and TCA oxidation in sulfuric acid solutions are proposed.__________Translated from Zashchita Metallov, Vol. 41, No. 3, 2005, pp. 316–325.Original Russian Text Copyright © 2005 by Kozin, Kozina, Bogdanova.