Soluble Metal Complexes Research Articles

The role of electrophilic species in the Fischer–Tropsch reaction

The heterogeneously catalysed Fischer-Tropsch (FT) synthesis converts syngas (CO+H2) into long chain hydrocarbons and is a key step in the economically important transformation of natural gas, coal, or biomass into liquid fuels, such as diesel. Catalyst surface studies indicate that the FT reaction starts when CO is activated at imperfections on the surfaces of late transition metals (Fe, Ru, Co, or Rh) and at interfaces with "islands" of promoters (Lewis acid oxides such as alumina or titania). Activation involves CO cleavage to generate a surface carbide, C(ad), which is sequentially hydrogenated to CHx(ad) species (x=1-4). An overview of practical aspects of the FT synthesis is followed by a discussion of the chief mechanisms that have been proposed for the formation of 1-alkenes by polymerisation of surface C1 species. These mechanisms have traditionally postulated rather non-polar intermediates, such as CH2(ad) and CH3(ad). However, electrophiles and nucleophiles are well-known to play key roles in the reactions of organic and organometallic compounds, and also in many reactions homogeneously catalysed by soluble metal complexes, including olefin polymerisation. We have now extended these concepts to the Fischer-Tropsch reaction, and show that the polymerisation reactions at polarising surfaces, such as oxide-metal interfaces, can be understood if the reactive chain carrier is an electrophilic species, such as the cationic methylidyne, CH(delta+)(ad). It is proposed that the key coupling step in C-C bond formation involves the interaction of the electrophilic methylidyne with an alkylidene (RCH(ad), R=H, alkyl), followed by an H-transfer to generate the homologous alkylidene: CHdelta+(ad)+RCH(ad)-->RCHCH(ad) and RCHCH(ad)+H(ad)-->RCH2CH(ad). If the reactions occur on non-polarising surfaces, an alternative C-C bond forming reaction such as the alkenyl+methylene, RCH=CH(ad)+CH2(ad)-->RCH=CHCH2(ad), can take place. This approach explains important aspects of the enigmatic Fischer-Tropsch reaction, and allows new predictions.

Selective epoxidation of olefins catalyzed by oxodiperoxomolybdenum(VI) complexes immobilized over highly ordered 2D-hexagonal mesoporous silica

Two oxodiperoxomolybdenum(VI) complexes of 8-quinolinol have been immobilized over a highly ordered 2D-hexagonal mesoporous silica. The latter has been synthesized hydrothermally by using cationic–non-ionic mixed surfactant templating route. The mesoporous silica and the immobilized materials were thoroughly characterized by powder X-ray diffraction, N 2 sorption, transmission electron microscopy (TEM), UV–vis and FT-IR spectroscopic tools and elemental analyses. Experimental results revealed 6.7–7.1 wt% loading of metal complexes on the mesoporous silica host. The immobilized nanostructured materials showed very good catalytic activity and excellent recycling efficiencies over their respective homogeneous catalysts for the epoxidation of different olefins in the presence of H 2O 2/NaHCO 3 system. High catalytic efficiency could be attributed to the heterogenization of soluble metal complexes on the high surface area mesoporous host.

How Molecules Turn into Solids: the Case of Self-Assembled Metal−Organic Frameworks

This perspective proposes a rational and intuitive mechanism for the formation of metal−organic framework compounds (MOF). As soon as a metal salt is dissolved in water or a nonaqueous solvent, a soluble metal complex is formed. Knowing how this complex organizes with the organic groups in the medium of the solvent is crucial to understanding the transformation of these molecules into the final solid through hydrolysis and condensation. Condensation is strongly believed to occur when point zero charge molecules (pzc) are formed at the isoelectric point. Several examples from the literature are readily explained using this mechanism.

Hydrogenation of N‐Heteroaromatic Compounds in the Presence of Soluble Metal Complexes

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Atom-economical reduction of carbon monoxide to methanol catalyzed by soluble transition metal complexes at low temperatures

Hydrocarbons and methanol are considered the preferred products of catalytic reduction of carbon monoxide derived from clean natural gas. In this paper, we focus on atom-economical synthesis of methanol catalyzed by soluble transition metal complexes as single-site catalyst precursors under mild reaction conditions. Of the metal systems reported in the literature, Ni complexes activated by alkoxide bases affected homogeneous CO reduction to methanol at low temperatures (80°–130 °C) and low pressures (2000–5000 kPa) to achieve CO conversion and methanol selectivity of >90 and >95, respectively. The involvement of mono- and multi- nuclear Ni species that readily interconvert under methanol producing conditions is invoked. A process based on such an active catalyst system would position methanol to be considered an inexpensive feedstock for the synthesis of other derivatives.

Mass, IR, electronic and EPR spectral studies on transition metal complexes with a new tetradentate 12-membered new macrocyclic ligand

Complexes of Cr(III), Co(II), Ni(II) and Cu(II) containing a novel macrocyclic tetradentate nitrogen donor (N 4) ligand prepared via reaction of 2,3-hexanedione and ethylenediamine has been prepared and characterized. The newly synthesized ligand (L) and its complexes have been characterized on the basis of elemental analysis, molar conductance, magnetic moment susceptibility, EI-Mass, IR, Electronic and EPR spectral studies. The complexes are of high-spin type and four coordinated tetrahedral, five coordinated square pyramidal and six coordinated octahedral/tetragonal geometries. The ligand (L) and its soluble transition metal complexes have also been screened against different bacteria and plant pathogenic fungi in vitro.

Bioavailability of cadmium-organic complexes to soil alga--an exception to the free ion model.

It is generally considered that cadmium bioavailability shows a considerable dependence on chemical speciation of Cd in solution, correlates best with the activity of free metal ion (Cd2+) in solution, and is largely indifferent to soluble metal complexes. The role of soluble organic matter (DOM) and soluble metal-organic complexes in metal bioavailability and toxicity, however, is not clear. Growth studies with a soil alga (Chlorococcum sp.) were conducted on a growth medium and pore water of Cookes Plain soil (Paleuxeralf), spiked with Cd as Cd(NO3)2. Speciation of the Cd in pore water, and in growth medium with and without citrate, was performed using the MINTEQA2 computer model incorporating updated values of the stability constants of Cd-DOM complexes, as well as using anode stripping voltammetry. Analysis of the toxicity data showed that Cd-citrate, as well as the Cd-DOM complexes, is bioavailable and contributes toward the toxicity to alga. These data contradict the long-held notion that Cd-DOM complexes are not bioavailable to soil biota although they may increase the mobility of Cd.

Selective Oxidation of Organic Sulfides to Sulfoxides by Hydrogen Peroxide with Trirutile-Type Solid Oxide Catalyst HNbMoO6

Selective oxidation of organic sulfides to sulfoxides has been the subject of extensive research because of their importance of sulfoxides as intermediates in various organic reactions. One of the simplest methods for oxidation of sulfides to sulfoxides is the use of hydrogen peroxide as an oxidant in the presence of a catalyst. Though a number of catalysts including soluble metal complexes have been used for the reaction, there are a few reports employing a heterogeneous catalyst despite the advantage in separation and reuse of the catalyst. In this paper, we introduce a new heterogeneous catalyst, HNbMoO6, which shows an excellent catalytic activity in the oxidation of sulfides. Trirutile-type solid oxide, AMM’O6, was first synthesized in 1970. Since then, many applications of the solid oxide in various fields such as non-linear optics and composites have been reported. However, the catalytic activity of the oxide in organic reactions has not been well studied in spite of the potentials expected from the two different transition metals contained in the solid oxide. Recently, we reported a selective sulfoxidation of allylic sulfides catalyzed by the trirutile-type solid oxide, LiNbMoO6 and successfully applied the oxidation method to the synthesis of Lansoprazole. Because of the good catalytic activity shown by the LiNbMoO6 in the sulfoxidation of sulfides, we decided to test other trirutile-type solid oxides, AMM’O6 (A=H, Li, M=Nb, Ta, M’=Mo, W), as a catalyst in the hydrogen peroxide oxidation of sulfides to sulfoxides (Eq. 1).

Enhanced Electrokinetic Remediation of Heavy Metals in Glacial Till Soils Using Different Electrolyte Solutions

Previous electrokinetic remediation studies involving the geochemical characterization of heavy metals in high acid buffering soils, such as glacial till soil, revealed significant hexavalent chromium migration towards the anode. The migration of cationic contami- nants, such as nickel and cadmium, towards the cathode was insignificant due to their precipitation under the high pH conditions that result when the soil has a high acid buffering capacity. Therefore the present laboratory study was undertaken to investigate the performance of different electrolyte ~or purging! solutions, which were introduced to either dissolve the metal precipitates and/or form soluble metal complexes. Tests were conducted on a glacial till soil that was spiked with Cr~VI! ,N i~II!, and Cd~II! in concentrations of 1,000, 500, and 250 mg/kg, respectively, under the application of a 1.0 VDC/cm voltage gradient. The electrolyte solutions tested were 0.1M EDTA ~ethylenediaminetetraacetic acid!, 1.0M acetic acid, 1.0M citric acid, 0.1M NaCl/0.1M EDTA, and 0.05M sulfuric acid/0.5 M sulfuric acid. The results showed that 46 - 82% of the Cr~VI! was removed from the soil, depending on the purging solution used. The highest removal of Ni~II! and Cd~II! was 48 and 26%, respectively, and this removal was achieved using 1.0M acetic acid. Although cationic contaminant removal was low, the use of 0.1M NaCl as an anode purging solution and 0.1M EDTA as a cathode purging solution resulted in significant contaminant migration towards the soil regions adjacent to the electrodes. Compared to low buffering capacity soils, such as kaolin, the removal of heavy metals from the glacial till soil was low, and this was attributed to the more complex composition of glacial till. Overall, this study showed that the selection of the purging solutions for the enhanced removal of heavy metals from soils should be primarily based upon the contaminant characteristics and the soil composition.

Solubility and freezing effects of Fe2+ and Mg2+ in H2SO4 solutions representative of upper tropospheric and lower stratospheric sulfate particles

Chemical elements characteristic of earth minerals and meteorites are present within background tropospheric and stratospheric sulfate aerosol particles. However, it is unknown if these elements are present predominantly as solids, including possible sulfates, carbonates, and oxides, or rather as soluble aqueous metal ions or complexes. Further, it is unclear how these impurities could affect particle freezing. To address these questions, we have determined the total equilibrium metal solubility ([Fe2+]T and [Mg2+]T) in 20–90 wt % sulfuric acid solutions over the temperature range 200–300 K. We have measured solubilities using samples of MgSO4, FeSO4·7H2O, and a mixture of metal sulfates representative of some meteorite samples. Because Fe2+ oxidation by O2 was found to be negligible over a 28 day period when air was bubbled through the solutions, we conclude that aqueous Fe2+ may be present at high concentrations in atmospheric sulfuric acid particles. We estimate that soluble Fe2+ and Mg2+ compose ∼0.01–0.28 wt % of the H2SO4/H2O/{Fe2+, Mg2+} solutions at temperatures and acid compositions representative of the atmosphere. Compared to composition measurements done by mass spectrometry of atmospheric particles showing ∼0.75 wt % Fe and 0.20 wt % Mg, we conclude it is plausible that solid Fe2+ and Mg2+ minerals often occur inside atmospheric sulfuric acid particles. Bulk freezing experiments were also carried out on H2SO4 solutions containing amounts of dissolved metal determined by the low‐temperature solubility studies. It was found that 57.6 wt % H2SO4 containing soluble meteoritic metal and 60 wt % H2SO4 containing either soluble Fe or soluble meteoritic metal froze ∼12–20 K higher than solutions containing no soluble metal. Soluble metal ions in sulfuric acid solution may therefore provide a mechanism for sulfuric acid tetrahydrate formation in the lower stratosphere.

Catalysis with Soluble Hybrids of Highly Branched Macromolecules with Palladium Nanoparticles in a Continuously Operated Membrane Reactor

AbstractThe continuous recovery and recycling of soluble metal nanoparticles by means of ultrafiltration is described, employing hybrids of palladium nanoparticles with highly branched amphiphilic polyglycerol as a catalyst for cyclohexene hydrogenation as a model reaction. In a continuously operated membrane reactor a productivity of 29000 TO over 30 exchanged reactor volumes was observed for nanoparticles of 2.2 nm size, with a maximum rate of 1200 TO h−1. Catalysis by soluble metal complexes can be excluded. After 30 hours of operation, some decrease in activity is observed which is due to deposition of palladium on the ultrafiltration membrane, however this material does not contribute to catalytic activity.

Synthesis and photophysical characterisation of soluble photoluminescent metal complexes with substituted 8-hydroxyquinolines

The spectroscopic characterization of the metal complexes the 5-amidopyridine-8-hydroxyquinoline (HQ′NHCOC 5H 4N) form with M = Be, Zn, Cd, Al, Ga and In is reported. These compounds, M(Q′NHCOC 5H 4N) 2 (M = Be, Zn, Cd) and M(Q′NHCOC 5H 4N) 3 (M = Al, Ga, In), are highly soluble in DMSO and, in addition, the gallium and indium products dissolve in methyl or ethyl alcohol. These species are very stable in solution and emit in the range 520 (Be)–590 (Ga and In) nm with photoluminescent quantum yields ranging from 0.010 (Al) to 0.001 (In). Processability and color emission suggest that some of these new materials can be of interest for practical applications in optoelectronic devices.

Synthesis of Primary Amines: First Homogeneously Catalyzed Reductive Amination with Ammonia.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Speciality zeolites in catalysis 1992-2001

Catalytic oxidations in the liquid phase generally employ soluble metal salts or complexes as the catalyst. So-called redox molecular sieves, unlike conventional supported catalysts, possess a regular microenvironment with homogeneous internal structures, consisting of uniform, well-defined cavities, and channels. Confinement of the redox active site in these channels and/or cavities can endow the catalyst with unique activity, as a result of strong electrostatic interactions between acidic and basic sites on the internal surface, and the substrate or reaction intermediate analogous to interactions with acidic carboxyl and basic amino groups of amino acid residues in the active site of (redox) enzymes. Zeolitcs and zeotypes are crystalline oxides, comprising corner sharing TO4 tetrahedra (T = Si, AI, P, etc.), and consisting of a regular pore system with diameters of molecular dimensions; hence, the term molecular sieve. The framework of molecular sieves is not completely rigid and the incoming molecules are able to induce slight structural changes. Three types of redox molecular sieve can be distinguished on the basis of the method of synthesis: ion exchange, framework substitution, and encapsulation. Redox molecular sieves have one important advantage over other heterogeneous catalysts; it is possible to influence that substrate molecules approach the active site by a suitable choice of molecular sieve and solvent. Hence, by using a redox molecular sieve, it is possible to obtain much higher selectivities to primary oxidation products than with analogous homogeneous catalysts. Titanium(IV) silicalite (TS-1), the first example of a framework-substituted redox molecular sieve, catalyzes a variety of synthetically useful oxidations with 30% aqueous hydrogen peroxide under mild conditions. Similar heterolytic mechanisms can be envisaged for other nucleophilic substrates like ammonia, amines, sulfides, phenols, and alcohols. Based on these recent developments, ship-in-a-bottle catalysts appear to hold much promise, extending even to the design of chiral “ships” for enantioselective oxidation.

Synthesis of primary amines: first homogeneously catalyzed reductive amination with ammonia.

[reaction: see text] The synthesis of primary amines via reductive amination of the corresponding carbonyl compounds with aqueous ammonia is achieved for the first time with soluble transition metal complexes. Up to an 86% yield and a 97% selectivity for benzylamines were obtained in the case of various benzaldehydes by using a Rh-catalyst together with water-soluble phosphine and ammonium acetate. In the case of aliphatic aldehydes, a bimetallic catalyst based on Rh/Ir gave improved results.

A novel solution-based method for the preparation of nanocrystalline single-phase 0.9Pb(Mg 1/3Nb 2/3)O 3–0.1PbTiO 3 powders

Nanocrystalline powders of solid solution of lead magnesium niobate–lead titanate with the ratio 9:1{0.9Pb(Mg 1/3Nb 2/3)O 3–0.1PbTiO 3}, have been prepared using a single-step chemical synthetic process starting from a precursor solution containing niobium/titanium tartarate and lead/magnesium triethanolamine (TEA) complexes. The soluble metal complex solution on complete dehydration followed by decomposition forms a black fluffy carbonaceous precursor material, which on calcination at 750 °C (2 h) results in single-phase pyrochlore-free pure 0.9Pb(Mg 1/3Nb 2/3)O 3–0.1PbTiO 3(0.9PMN–0.1PT) powders. The precursors and heat-treated powders have been characterized by infrared (ir) spectroscopy, thermal analysis, X-ray diffraction studies (XRD) and transmission electron micrograph (TEM). The crystallite size and average particle size as measured from XRD line broadening and TEM has been found to be 38 and 40 nm, respectively.

REMOVAL OF HEAVY METALS FROM SOIL COMPONENTS AND SOIL BY NATURAL CHELATING AGENTS Part I: Displacement from Clay Minerals and Peat by L-Cysteine and L-Penicillamine

The β-thiol group containing amino acids L-cysteineand L-penicillamine were tested for their ability to releaseheavy metals (Cd, Cr, Cu, Hg, Ni, Pb, Zn) from bondings ontotypical soil components (peat, bentonite, illite) at neutral pHformed by batch adsorption procedure. Following equilibrationthe metal loaded sorbents were extracted by aqueous amino acidsolutions under various physico-chemical conditions.The extractability of metals from peat (metal contents between62.0 and 6170 mg kg-1) increased in thepresence of L-penicillamine following the order Cr < Hg < Pb <Ni ≈ Zn ≈ Cd, ranging from 9% (Cr) to 97% (Cd).The extractability by L-cysteine was slightly lower, followingthe sequence Cr < Pb < Cd < Ni < Zn. In both cases the sequencesdid not correlate with the order of the stability constants ofthe corresponding amino acid complexes. The recovery of metalsfrom bentonite was higher generally. The extent of removal increased with increasing molar excess of the amino acids and,to a lower extent, with increasing extraction volume.Presumably initiated by oxidation of amino acids or by theformation of sparingly soluble polymeric metal complexes, theconcentrations of dissolved metals dropped after an agitationperiod of 2 to 24 hr in most of the extracts. Aspects of thisprocess were discussed in detail.

Dissolved Pb(II) Speciation in a Polluted River

Soluble Pb(II) speciation in a polluted river (Este River, Northern Portugal) was studied using differential pulse anodic stripping voltammetry (DPASV). Three sampling sites were selected: Braga (downstream the pollution sources), Foz (near the river mouth) and an intermediate point called Louro. Studies have been done with samples collected in different times of the year, namely spring, summer, autumn and winter. Centrifuged and filtered samples were titrated with Pb(II) and the labile metal concentration measured by DPASV. Results show that soluble metal complexes are labile, in the time scale of the analytical technique, with DML<DM. Pb(II) complexes have been studied in terms of discontinuous and continuous heterogeneity models. Values of affinity parameters and concentration of sites have been estimated. From complexometric results it was shown that, in the river mouth the effects of organic pollution are less preceptible, specially in dry weather conditions, since ligands are more similar to humic substances.

Comparison of diffusive gradients in thin films and equilibrium dialysis for the determination of Al, Fe(III) and Zn complexed with dissolved organic matter

The distinction between ‘free’ metals and organically complexed metals in aqueous solutions is important for research involving the mobility or bioavailability of metals in the environment. In this study, the applicability of equilibrium dialysis (molecular weight cut-off=1000 Da) and diffusive gradients in thin films (DGT) to determine ‘free’ Al, Fe(III) and Zn in four forest soil solutions was compared. The ‘free’ metals as measured by both methods, consist of hydrated metal cations and soluble inorganic metal complexes. In addition, dialysis measures any organic complexes<1000 Da and DGT measures a portion of smaller labile organic complexes. The four soil solutions were prepared by water extraction of an organic soil horizon (H) from a Fimic Anthrosol, and contained either 20 or 75 mg C/l dissolved organic matter at pH 4.0 and pH 7.0. To test the performance of both methods and optimize experimental volume and time, experiments using metal nitrate solutions were carried out. In the solutions at pH 4.0, no significant differences in average ‘free’ metal contents were found for Al, Fe(III) or Zn. This makes DGT a viable alternative for equilibrium dialysis for the study of the complexation of all three metals in acidic soils. At pH 7.0, again no significant difference between both methods were found for Al and Fe(III), but the DGT results for Zn were significantly higher. This was likely caused by labile organic complexes that were only detected by DGT.

Simultaneous spectrophotometric determination of chromium(VI) and iron(III) with chromogenic mixed reagents by H-point standard addition method and partial least squares regression

Simultaneous determination of Cr(VI) and Fe(III) by partial least squares (PLS) calibration and H-point standard addition method (HPSAM) is described. Mixed reagents of diphenylcarbazide and 1,10-phenanthroline in a non-ionic micellar solution of Triton X-100 was used as a selective chromogenic system for determination of Cr(VI) and Fe(III). The presence of a micellar system allows one to eliminate the previous solvent extraction step that is necessary for the determination of slightly soluble metal complexes in the absence of micelles. This reduces the cost and toxicity of the method. The total relative standard error for applying the PLS method to 15 synthetic samples in the ranges 0.20–6.00 μg ml −1 Cr(VI) and 0.20–8.00 μg ml −1 was 1.5%. The results of applying the H-point standard addition method showed that Cr(VI) and Fe(III) can be determined simultaneously with weight concentration ratios of Cr(VI) to Fe(III) varying from 15:1 to 1:30 in the mixed sample. Both proposed methods (PLS and HPSAM) were also successfully applied to the determination of Cr and Fe in several synthetic alloy solutions.