Separation of rhenium from molybdenum and vanadium by extraction with amyl alcohol

Summary A method is described for the separation of rhenium from molybdenum and vanadium by extraction with amyl alcohol. The sample solution containing Re^VII, Mo^VI, and V^V is boiled with hydrazine sulphate in order to obtain Mo^V and V^IV (Re remains in the heptavalent state). Rhenium is quantitatively extracted into amyl alcohol from this solution, along with very slight amounts of molybdenum and vanadium. After back-extraction, the residual molybdenum is completely removed by extraction of its thiocyanate with amyl acetate. The accompanying vanadium is usually too small to interfere in the thiocyanate method of rhenium estimation. The method is simple, rapid and applicable to all Mo:Re ratios generally met with.

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Zusammenfassung Ein Verfahren zur Abtrennung des Rheniums von Molybdän und Vanadium durch Extraktion mit Amylalkohol wird beschrieben. Rhenium soll als Re^VII, Molybdän als Mo^V und Vanadium als V^IV vorliegen (Mo^VI und V^V werden mit Hydrazinsulfat reduziert). Geringe Mengen Mo und V werden mitextrahiert. Nach Rückextraktion wird Mo vollständig durch Extraktion des Thiocyanats mit Amylacetat entfernt. Die vorhandenen Vanadiummengen sind zu gering, um die nachfolgende Rheniumbestimmung (nach der Thiocyanatmethode) zu stören. Das Verfahren ist einfach und rasch durchführbar und für alle üblichen Mo:Re-Verhältnisse anwendbar.

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Our sincere thanks are due to Prof. S. M. Mukherji, Head of the Chemistry Department, for facilities.     

Zur polarographischen Reduktion von Molybdän(VI) und Molybdän(V) in Gegenwart von Citronensäure

According to the ratio of citric acid to molybdenum different citratomolybdenum(VI) complexes exist at pH <2. Only one citratomolybdenum(VI) complex exists in solutions with a great excess of citric acid (20:1). In such solutions the polarographic reduction of Mo^VI proceeds in two waves: First Mo^VI → M^V, then Mo^V → Mo^IV, which disproportionates into Mo^V and Mo^III however. Furthermore, it is possible that the Mo^IV reacts with Mo^VI forming Mo^V, so that also a catalytic character can be attributed to the second wave besides a kinetic one.     

The effect of the sixth sulfur ligand in the catalytic mechanism of periplasmic nitrate reductase

The catalytic mechanism of nitrate reduction by periplasmic nitrate reductases has been investigated using theoretical and computational means. We have found that the nitrate molecule binds to the active site with the Mo ion in the +6 oxidation state. Electron transfer to the active site occurs only in the proton‐electron transfer stage, where the Mo^V species plays an important role in catalysis. The presence of the sulfur atom in the molybdenum coordination sphere creates a pseudo‐dithiolene ligand that protects it from any direct attack from the solvent. Upon the nitrate binding there is a conformational rearrangement of this ring that allows the direct contact of the nitrate with Mo^VI ion. This rearrangement is stabilized by the conserved methionines Met141 and Met308. The reduction of nitrate into nitrite occurs in the second step of the mechanism where the two dimethyl‐dithiolene ligands have a key role in spreading the excess of negative charge near the Mo atom to make it available for the chemical reaction. The reaction involves the oxidation of the sulfur atoms and not of the molybdenum as previously suggested. The mechanism involves a molybdenum and sulfur‐based redox chemistry instead of the currently accepted redox chemistry based only on the Mo ion. The second part of the mechanism involves two protonation steps that are promoted by the presence of Mo^V species. Mo^VI intermediates might also be present in this stage depending on the availability of protons and electrons. Once the water molecule is generated only the Mo^VI species allow water molecule dissociation, and, the concomitant enzymatic turnover. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Complexes of gold(iii) and titanium(iii) with 8-hydroxyquinoline

The preparation of an 8-hydroxyquinoline complex of trivalent gold by the action of oxine upon gold chloride and “monopyridine gold chloride” is described. The absorption spectrum of the complex, which approximated to the composition AuCl₂C₉H₆ON was plotted in the U.V. and visible regions of the spectrum.A partial separation between tri- and tetravalent titanium was obtained using paper chromatography. Absorption curves were obtained for oxine complexes of Ti(III) and Ti(IV) in the visible region of the spectrum.     

DFT Study of the Molybdenum‐Catalyzed Deoxydehydration of Vicinal Diols

The mechanism of the molybdenum‐catalyzed deoxydehydration (DODH) of vicinal diols has been investigated using density functional theory. The proposed catalytic cycle involves condensation of the diol with an Mo^VI oxo complex, oxidative cleavage of the diol resulting in an Mo^IV complex, and extrusion of the alkene. We have compared the proposed pathway with several alternatives, and the results have been corroborated by comparison with the molybdenum‐catalyzed sulfoxide reduction recently published by Sanz et al. and with experimental observations for the DODH itself. Improved understanding of the mechanism should expedite future optimization of molybdenum‐catalyzed biomass transformations.     

New Mixed-Valent Molybdenum Monophosphates with the KMo3P2O14 structure

New mixed valent molybdenum monophosphates AMo₃P₂O₁₄ have been synthesized for A = Ag, Rb, Na, Sr. The single crystal X-ray diffraction study of two of them (A = Ag, Sr) shows that they belong to the layer structure type KMo₃P₂O₁₄. Their structure consists of [Mo₃P₂O₁₄]_∞ layers involving MoO₆ octahedra and MoO₅ bipyramids, interleaved with A cations forming bicapped trigonal prisms AO₈. Bond valence calculations show a localisation of the Mo^V and Mo^VI species according to the formula A¹Mo^V_oct1Mo^VI_oct2Mo^VI_bipyP₂O₁₄ for A = Ag, Na and SrMo^V_oct1Mo^V_oct2Mo^VI_bipyP₂O₁₄. A comparison between the different Mo^V? Mo^VI phosphates is made.     

Structural Study and Solution Integrity of Dioxomolybdenum(VI) Complexes with Tridentate Schiff Base and Azole Ligands

Four new molybdenum complexes [Mo^VIO₂(L¹)(Him)] ( 1 ), [Mo^VIO₂(L¹)(3‐MepzH] ( 2 ), [Mo^VIO₂(L²)(3‐MepzH)] ( 3 ), and [(Mo^VIO₂)₂(μ‐L³)(MeOH)₂] ( 4 ) were synthesized and characterized by IR, NMR, ESI‐MS, and single‐crystal structure analysis [H₂L¹ = 2‐(salicylideneamino)‐2‐methyl‐1‐propanol, H₂L² = 2‐(3‐methoxysalicylideneamino)‐2‐methyl‐1‐propanol, H₄L³ = 1, 7‐bis(salicylidene)dihydrazide malonic acid, Him = imidazole and 3‐MepzH = 3‐methylpyrazole]. In all four structures the molybdenum atom has a distorted octahedral coordination with the three meridional donor atoms from the Schiff base di‐ or tetraanion (L^{1, 2})^2—/(L³)^4— and one oxo group occupying the sites of the equatorial plane. The other oxo group and the azole or methanol molecule occupy the apical sites. In 1—3 two centrosymmetrically related molecules form a hydrogen‐bonded pair through the (azole)N‐H···O(alkoxo) interaction. Additional crystal packing appears to be controlled mostly by π stacking between the aromatic rings of the salicyl moiety. ESI‐MS investigations reveal that the integrity of complexes 1—4 is largely retained in methanol solution. At the same time evidence is provided that di‐ to tetranuclear oligomers of formula [{Mo^VIO₂(L)}_x] and [{Mo^VIO₂(L)}_x(3‐MepzH)] with L = L¹, L², x = 2, 3, 4 are present simultaneously with 2 and 3 in methanol solution, respectively the tetranuclear species [{(Mo^VIO₂)₂(L³)}₂] with 4 .     

The First Anticancer Tris(pyrazolyl)borate Molybdenum(IV) Complexes: Tested in Vitro and in Vivo—A Comparison of O,O-, S,O-, and N,N-Chelate Effects

The synthesis, characterization and biological activity of molybdenum(IV) complexes containing Trofimenko's scorpionato ligand, hydrotris(3-isopropylpyrazolyl)borate (Tp^iPr), in addition to varying biologically active as well as other conventional ligands is described. Ligands employed include (O,O-) (S,O-) (N,N-) donors that have been successfully coordinated to the molybdenum center by means of oxygen-atom transfer (OAT) reactions from the known Mo^VI starting material, Tp^iPrMoO₂Cl. The synthesized complexes were characterized by standard analytical methods and where possible by X-ray diffraction analysis. The aqueous stability of the compounds was studied by means of UV/Vis spectroscopy and the impact of the attached ligand scaffolds on the oxidation potentials (Mo^IV to Mo^V) was studied by cyclic voltammetry. Utilizing polyvinylpyrrolidone (PVP) as a solubilizing agent, adequate aqueous solubility for biological tests was obtained. Anticancer activity tests and preliminary mode of action studies have been performed in vitro and in vivo.     

Modele de nitrogenase. activite de complexes molybdenecysteine dans la reduction de l'acetylene

The complex Mo₂O₄(cys)₂^2? disproportionates nito Mo^VI and Mo^III at a _pH higher than 9. Under our experimental conditions, the reduction of acetylene by molybdenum-cystéine complexes does only occur after addition of naBH₄·Mo^III; the molybdenum(III) species seems to be responsible for this activity.     

Synthesis,chemical and electrochemical studies of complexes of a tridentate ONS chelating ligand built around the elusive [MoVIOS]2+ core

Synthesis and characterization of four Mo(VI) complexes of a diprotic tridentate ONS chelating ligand (H₂L) containing the rather elusive [Mo^VIOS]²⁺ core is reported. These [Mo^VIOSL] complexes are obtained from their corresponding [Mo^VIO₂L] precursors using a combination of PPh₃ and PPh₃S. This process of oxo-abstraction and sulfido-inclusion affected by PPh₃–PPh₃S is reported for the first time and may be considered as a general method of converting [Mo^VIO₂L] complexes to the corresponding [Mo^VIOSL] complexes. Direct structural characterization of these complexes could not be done due to the ease of solvolysis of these oxosulfidomolybdenum(VI) complexes to the corresponding dioxomolybdenum(VI) analogues. However, the structure of these [Mo^VIOSL] complexes could be reasonably surmised from the corresponding structurally characterized [Mo^VIO₂L] complexes. Points of attachment of the potentially pentadentate but functionally tridentate ONS chelating ligands to [Mo^VIOS]²⁺ are located mainly through analysis of IR and UV-Vis spectral data and comparison with corresponding [Mo^VIO₂L] complexes of known structure. Conditions under which solvolysis of [Mo^VIOS]²⁺ to the [Mo^VIO₂]²⁺ core is significantly retarded have been identified and make us hopeful of obtaining single crystals of [Mo^VIOSL].     

Porous Capsules with a Large Number of Active Sites: Nucleation/Growth under Confined Conditions

This work deals with the generation of large numbers of active sites and with ensuing nucleation/ growth processes on the inside wall of the cavity of porous nanocapsules of the type (pentagon)₁₂(linker)₃₀≡{(Mo^VI)Mo^VI₅}₁₂{Mo^V₂(ligand)}₃₀. A first example refers to sulfur dioxide capture through displacement of acetate ligands, while the grafted sulfite ligands are able to trap {MoO₃H}⁺ units thereby forming unusual {(O₂SO)₃MoO₃H}^5? assemblies. A second example relates to the generation of open coordination sites through release of carbon dioxide upon mild acidification of a carbonate‐type capsule. When the reaction is performed in the presence of heptamolybdate ions, MoO₄^2? ions enter the cavity where they bind to the inside wall while forming new types of polyoxomolybdate architectures, thereby extending the molybdenum oxide skeleton of the capsule. Parallels can be drawn with Mo‐storage proteins and supported MoO₃ catalysts, making the results relevant to molybdenum biochemistry and to catalysis.     

Synthesis,structure, and properties of polynuclear molybdenum(v,vi) oxomethoxides with magnesium

General conditions for the formation of heterometallic clusters by the simultaneous methanolysis of MoCl₅ and MgCl₂ were determined. The resultant alkalinity of the reaction solution, the Mg/Mo molar ratio, and the presence of traces of water are key factors responsible for the composition and structure of the mixed magnesium molybdenum methoxides that formed. The new decanuclear mixed-valence Mo^V,VI Mg oxomethoxide [Mo^V ₄O₄(μ₃-O)₂(μ₂-O)₂Mo^VI ₂-O₄(OMe)₂Mg₄(MeOH)₆(μ₃-OMe)₆(μ₂-OMe)₈] (1) was synthesized by the reaction of lowernuclearity magnesium molybdenum oxoalkoxide complexes: NaMo^V of the complex Na(MeOH)Mo^V ₂O₂(μ₂-OMe)₃(OMe)₄ (2) and MgMo^VI of the complex [Mo^VIO₂Mg(MeOH)₂-(OMe)₄]₂ (3). The molecular structure of 1 was determined by X-ray diffraction.     

Mechanistic insight into the reactivity of oxotransferases by novel asymmetric dioxomolybdenum(VI) model complexes

The asymmetric molybdenum(VI) dioxo complexes of the bis(phenolate) ligands 1,4‐bis(2‐hydroxybenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐4‐methylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐3,5‐dimethylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐3,5‐di‐tert‐butylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐4‐flurobenzyl)‐1,4‐diazepane, and 1,4‐bis(2‐hydroxy‐4‐chlorobenzyl)‐1,4‐diazepane (H₂(L1)–H₂(L6), respectively) have been isolated and studied as functional models for molybdenum oxotransferase enzymes. These complexes have been characterized as asymmetric complexes of type [MoO₂(L)] 1–6 by using NMR spectroscopy, mass spectrometry, elemental analysis, and electrochemical methods. The molecular structures of [MoO₂(L)] 1–4 have been successfully determined by single‐crystal X‐ray diffraction analyses, which show them to exhibit a distorted octahedral coordination geometry around molybdenum(VI) in an asymmetrical cis‐β configuration. The Mo? O_oxo bond lengths differ only by ≈0.01 Å. Complexes 1 , 2 , 5 , and 6 exhibit two successive Mo^VI/Mo^V (E_1/2, ?1.141 to ?1.848 V) and Mo^V/Mo^IV (E_1/2, ?1.531 to ?2.114 V) redox processes. However, only the Mo^VI/Mo^V redox couple was observed for 3 and 4 , suggesting that the subsequent reduction of the molybdenum(V) species is difficult. Complexes 1 , 2 , 5 , and 6 elicit efficient catalytic oxygen‐atom transfer (OAT) from dimethylsulfoxide (DMSO) to PMe₃ at 65 °C at a significantly faster rate than the symmetric molybdenum(VI) complexes of the analogous linear bis(phenolate) ligands known so far to exhibit OAT reactions at a higher temperature (130 °C). However, complexes 3 and 4 fail to perform the OAT reaction from DMSO to PMe₃ at 65 °C. DFT/B3LYP calculations on the OAT mechanism reveal a strong trans effect.     

On the peroxidation reactions of monomolybdates

Summary The differences in kinetic behaviour of the reactions between Mo^VI and H₂O₂ in media at different pH are explained by the involvement of different Mo^VI species. In acidic medium a 6-coordinate Mo^VI undergoes an H⁺-independent substitution, while in neutral or alkaline solutions a proton-assisted addition takes place between the 4-coordinate Mo^VI and H₂O₂.     

Complex equilibria of molybdenum(V) and molybdenum(VI) witho-hydroxybenzylamine-N,N,O-triacetic acid (HBATA)

Summary Complex reactions between Mo^V.VI ando-hydroxybenzylamine-N,N,O-triacetic acid (HBATA) have been investigated in the 1–3 and 2.8–6.5 pH range by potentiometric titration at 30° C in 0.5 mol dm^–3 NaCl. The equilibrium data were analyzed with the SCOGS2 and MINIQUAD programs, taking into account side reactions of Mo^V.VI and HBATA with hydrogen ion. The favorable reaction model comprises two complexes, (1,1,1)⁺ and (1,2,2)^–, with formation constants log ₁₁₁ = 14.85 ± 0.11 and log ₁₂₂ = 28.51 ± 0.08 for the Mo^V-HBATA system and the two complexes (1,1,2)^3– and (1,1,3)^2– with formation constants log ₁₁₂ = 17.36 ± 0.01 and log ₁₁₃ = 20.60 ± 0.01 for the Mo^VI-HBATA system. The numbers in brackets refer to the chemical stoichiometric coefficients of molybdenum, HBATA and hydrogen ion in the complexes. The structure and coordinating behaviours of Mo^V and Mo^VI complexes are discussed. The equilibria studied for the polymerization of Mo^V indicates that dimeric, trimeric and tetrameric species are present at pH 1–3.     

Reactions of tungsten(VI) and molybdenum(V) chlorides,and tungsten(VI) and molybdenum(VI) oxychlorides,with azoxybenzene

Reactions of W^VI and Mo^V chlorides with azoxybenzene yield ionic species of W^VI and Mo^VI oxychlorides in which the cation is a protonated azobenzene. The reaction between MoCl₅ or MoOCl₄ and azoxybenzene gives, after extraction with methylene chloride—ethanol mixture, the complex [trans-MoOCl₄(OC₂H₅)]^? [C₁₂H₁₀N₂H]⁺. In contrast, WOCl₄ reacts with azoxybenzene to give a stable non-ionic adduct in which the organic moiety is coordinated through its oxygen atom trans to the WO bond. Several complexes of substituted azoxybenzene having similar structures are described.     

Two novel windmill-like tetrasupporting heteropolyoxometalates: [MoVI7MoVVIV8O40(PO4)][M(phen)2(OH)]2[M(phen)2(OEt)]2 (M=Co,Ni)

Two interesting neutral tetrasupporting heteropolyoxometalates: [Mo^VI₇Mo^VV^IV₈O₄₀(PO₄)][M(phen)₂(OH)]₂[M(phen)₂(OEt)]₂·xH₂O (phen=1,10-phenanthroline, EtOH=ethanol, M=Co, x=7, 1; M=Ni, x=6, 2) were hydrothermally prepared and structurally characterized. The mixed molybdenum–vanadium polyoxoanion [Mo^VI₇Mo^VV^IV₈O₄₀(PO₄)]⁴⁻ exist in both two complexes, which acts as a bridge to covalently link two pairs of transition metal complex fragments, generating neutral windmill-like trimetallic nanocluster polyoxometalates. Variable-temperature magnetic susceptibility measurements of complexes 1 and 2 reveal that antiferromagnetic exchange interaction exists in this type of trimetallic tetrasupporting heteropolyoxometalates.     

Struktur und katalytische Eigenschaften von Molybdänoxid-Trägerkatalysatoren bei einigen Oxydationsreaktionen

Structure and Catalytic Properties of Molybdenum Oxide Supported Catalysts in Some Oxidation Reactions Molybdenum supported catalysts were prepared by using different precursor compounds such as Mo(π-C₃H₅)₄, [Mo(OC₂H₅)₅]₂, MoCl₅, (NH₄)₆Mo₇O₂₄, and their catalytic behaviour in some oxidation reactions was studied. During the preparation process, as a result of interaction between the molybdenum compound used and the support, different surface compounds with strongly differing catalytic properties have been formed. MoO₃ and supported catalysts with MoO₃ crystallites on the surface, catalyse the H₂ oxidation at temperatures above 400°C and the CO oxidation at temperatures of about 500°C. The reaction proceeds according to a redox mechanism. On surface compounds of molybdenum which exist on the surface if organic complexes are used as precursors, the catalytic H₂ oxidation occurs even at 100°C with a high reaction rate. The catalytic CO oxidation on these catalysts occurs at temperatures of about 300°C. An associative mechanism on coordinative unsaturated Mo^VI sites is discussed.     

Polyoxomolybdate Bisphosphonate Heterometallic Complexes: Synthesis,Structure, and Activity on a Breast Cancer Cell Line

Six polyoxometalates containing Mn^II, Mn^III, or Fe^III as the heteroelement were synthesized in water by treating Mo^VI precursors with biologically active bisphosphonates (alendronate (Ale), zoledronate (Zol), an n‐alkyl bisphosphonate (BPC₉), an aminoalkyl bisphosphonate (BPC₈NH₂)) in the presence of additional metal ions. The Pt complex was synthesized from a polyoxomolybdate bisphosphonate precursor with Mo^VI ions linked by the 2‐pyridyl analogue of alendronate (AlePy). The complexes Mo₄Ale₂Mn, Mo₄Zol₂Mn, Mo₄Ale₂Fe, Mo₄Zol₂Fe, Mo₄(BPC₈NH₂)₂Fe, and Mo₄(BPC₉)₂Fe contain two dinuclear Mo^VI cores bound to a central heterometallic ion. The oxidation state of manganese was determined by magnetic measurements. Complexes Mo₁₂(AlePy)₄ and Mo₁₂(AlePy)₄Pt₄ were studied by solid‐state NMR spectroscopy and the photochromic properties were investigated in the solid state; both methods confirmed the complexation of Pt. Activity against the human breast adenocarcinoma cell line MCF‐7 was determined and the most potent compound was Mn^III‐containing Mo₄Zol₂Mn (IC₅₀≈1.3 μM ). Unlike results obtained with vanadium‐containing polyoxometalate bisphosphonates, cell growth inhibition was rescued by the addition of geranylgeraniol, which reverses the effects of bisphosphonates on isoprenoid biosynthesis/protein prenylation. The results indicate an important role for both the heterometallic element and the bisphosphonate ligand in the mechanism of action of the most active compounds.     

Cellular uptake of molybdenum and tungsten

Cells acquire molybdenum and tungsten as their highly soluble oxoanions, Mo^VIO₄^2? or W^VIO₄^2?, which they internalize by means of an active (i.e. energy requiring) transmembrane importer, for subsequent conversion into the metalloenzyme cofactors Moco or Wco (and FeMoco in nitrogen fixers). This import system has been studied as one of the models for the functioning of the protein complex superfamily of ABC (ATP binding cassette) transporters, but its mechanistic details are presently not clear. The complex exhibits interesting variants, known as the microbial Mod, Tup, and Wtp system, and the – less well defined – eukaryotic MOT1 system, which mutually differ in oxoanion coordination chemistry and in the control of intracellular Mo/W levels. This evolutionary diversity of Mo/W transporters has resulted in confusing nomenclature whose rectification is here proposed.