Polymerization of Ethylene by Silica‐Supported Dinuclear CrIII Sites through an Initiation Step Involving CH Bond Activation

The insertion of an olefin into a preformed metal–carbon bond is a common mechanism for transition‐metal‐catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal–carbon bond is not present in the precatalyst. The Phillips catalyst, CrO₃ dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first Cr? C bond is formed remain unknown. We synthesized well‐defined dinuclear Cr^II and Cr^III sites on silica. Whereas the Cr^II material was a poor polymerization catalyst, the Cr^III material was active. Poisoning studies showed that about 65 % of the Cr^III sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si–(μ‐OH)–Cr^III species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr^III? O bonds.     

Polymerization of Ethylene by Silica‐Supported Dinuclear CrIII Sites through an Initiation Step Involving C?H Bond Activation

The insertion of an olefin into a preformed metal–carbon bond is a common mechanism for transition‐metal‐catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal–carbon bond is not present in the precatalyst. The Phillips catalyst, CrO₃ dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first Cr C bond is formed remain unknown. We synthesized well‐defined dinuclear Cr^II and Cr^III sites on silica. Whereas the Cr^II material was a poor polymerization catalyst, the Cr^III material was active. Poisoning studies showed that about 65 % of the Cr^III sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si–(μ‐OH)–Cr^III species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr^III O bonds.     

Specific and random fragmentations in the mass spectra of some neopentyl compounds

The mass spectra of neopentyl alcohol, bromide and chloride and some ¹³C and ²H labelled analogues have been studied. Most fragmentations of the molecular ions of these compounds occur by simple bond cleavages and do not involve rearrangement before fragmentation. We propose that in the [M ? CH₃]⁺ fragment ions, seven of the eight hydrogen atoms and all four carbon atoms are involved in randomisation when an ethylene molecule is ejected. The eighth hydrogen atom (which comes from a methyl group) is probably associated with the heteroatom. The neopentylcation, observed only in the mass spectrum of the bromide, fragments mainly by loss of an ethylene molecule, also containing randomly selected hydrogen and carbon atoms. The [C₄H₇]⁺ ion also was observed to undergo complete atom scrambling.     

Catalytic activity and physicochemical properties of Ni-Au/Al<Subscript>3</Subscript>CrO<Subscript>6</Subscript> system for partial oxidation of methane to synthesis gas

This work is focused on the role of gold and Al₃CrO₆ support for physicochemical properties, and catalytic activity of supported nickel catalysts in partial oxidation of methane (POM). Catalysts, containing 5% Ni and 5% Ni-2% Au active phases dispersed on mono- (Al₂O₃, Cr₂O₃) and bi-oxide Al₃CrO₆ support, were investigated by TPR, BET and XRD methods, and the activity tests in POM reaction were carried out. Bimetallic Ni-Au catalysts dispersed on Al₃CrO₆ support remained highly stable and active. The amorphous binary oxide Al₃CrO₆ can stabilize considerable amount of Cr⁴⁺, Cr⁵⁺, and Cr⁶⁺ species in Ni-Au/Al₃CrO₆ catalyst network during its calcination in the air. Nickel supported on binary oxide Ni/Al₃CrO₆ can form Ni(III)CrO₃ bi-oxide phase in reductive conditions. During TPR H2 reduction of Ni-Au/Al₃CrO₆ catalyst chromium(II) oxide Cr(II)O phase is observed. After POM reaction the existence of bimetallic Au-Ni alloy was experimentally confirmed on mono-oxide Al₂O₃ support surface, but its formation was not identified on bioxide Al₃CrO₆ support. Published in Russian in Kinetika i Kataliz, 2009, Vol. 50, No. 1, pp. 149–156. The article is published in the original. Based on a report at the VII Russ. Conf. on Mechanisms of Catalytic Reactions (with international participation), St. Petersburg, July 2–8, 2006.     

Heteroelemental diatomic secondary ions as a probe for molecular states

Summary The complete chemical characterization of environmental particles requires a depth resolved analysis of the chemical state of the elements. Using dynamic SIMS, a depth-resolved determination of the chemical state of chromium was derived from measurements of ten different chromium salts and oxides. The evaluation is based on the signals of Cr ₂ ⁺ , CrO⁺ and Cr₂O⁺ which are the most intense molecular ions. The valence state of chromium can be determined for concentrations far less than 1 atom %. The transferability of this kind of analysis is shown for manganese compounds. Additionally binary mixed metal clusters are shown to indicate binding partners of compounds in mixtures.     

Cs2Cr4O13: a new structure type among alkali tetrachromates

Dicaesium tetrachromium(VI) tridecaoxide, Cs₂Cr₄O₁₃, contains finite [Cr₄O₁₃]²⁻ anions composed of four corner‐linked CrO₄ tetrahedra. These anions are linked by Cs⁺ cations whose Cs—O bond lengths range between 3.015 (2) and ∼3.7 Å. Although Cs₂Cr₄O₁₃ is not isotypic with its NH₄, K or Rb analogs, the [Cr₄O₁₃]²⁻ anions in all these compounds exhibit a similar zigzag‐like geometry.     

Applications involving oxidation with KBrO3: Potentiometric determination of some inorganic white and yellow pigments

Rapid potentiometric methods were given for determination of some sulfide and chromate pigments in pure as well as adulterated samples. The methods are based on oxidation of S^2? or Cr³⁺ with known excess of KBrOs³ to the corresponding (S ↓) or Cr^6?. Oxidation of trivalent chromium proceeds only toward completion in presence of a catalyst such as Co²⁺. Sulfide oxidation needs gradual liberation of H₂S—which can be achieved by dropwise additions of the acid—together with the presence of excessive amount of water, in order to obtain accurate results excess oxidant was subsequently reduced with SO₂ to produce Br^? that can be titrated against Hg(I). The potential breaks are quite sharp for accurate determination of the equivalent points.     

Optimized structures of Cr2(CO)6+ with three different symmetries by density functional calculation

Density functional calculations have been made on a binuclear metal carbonyl ion Cr₂(CO)₆⁺ found in our laser ablation–molecular beam (LAMB) experiment. Optimized structures are calculated for three different conformations: T33 of D_3d symmetry with three terminal carbonyl groups on each chromium atom, B2T22 of D_2h symmetry with two bridging carbonyl groups and two terminal carbonyl groups on each chromium atom, and B4T11 of D_4h symmetry with four bridging carbonyl groups and one terminal carbonyl group on each chromium atom. The most stable conformation is T33 which is 36.76 and 286.44 kJ mol⁻¹ lower in energy than B2T22 and B4T11, respectively. The difference of conformation exerts a significant influence on the internuclear distance between chromium and the carbon of terminal CO, but hardly on the Cr–Cr bond length. For B2T22 and B4T11, longer C–O distances for bridging carbonyls compared with those for terminal ones indicate effective π*-back donation from the chromium atom to the bridging carbonyl groups. Furthermore, the relative abundance of Cr₂(CO)_n⁺ (n = 0–6) observed in our previous experimental study can be explained qualitatively by comparison of the excess energy produced in the formation of a Cr⁺–Cr bond with the CO dissociation energy of Cr₂(CO)₆⁺. © 1998 John Wiley & Sons, Ltd.     

Synthesis,Properties, and Structures of Chromium(VI) and Chromium(V) Complexes with Heterocyclic Nitrogen Ligands

The reaction of CrO₂Cl₂ with 2, 2′‐bipyridyl or 1, 10‐phenanthroline (diimine) in CCl₄ or anhydrous CH₃CO₂H solution, produces orange‐brown diamagnetic [CrO₂Cl₂(diimine)]. The X‐ray structure of [CrO₂Cl₂(2, 2′‐bipy)] shows a six‐coordinate central chromium(VI) atom with cis‐dioxo groups trans to the diimine. In contrast, the diimines react with CrO₃ in CH₃CO₂H / conc. aqueous HCl to form bright red paramagnetic Cr^V complexes, [CrOCl₃(diimine)]. The X‐ray structure of [CrOCl₃(2, 2′‐bipy)] shows a six‐coordinate central chromium atom with mer‐chlorines and the diimine trans to O/Cl. The addition of [2, 2‐bipyH₂]Cl₂ to a solution of CrO₃ in CH₃CO₂H saturated with HCl gas, produces the Cr^V anion [2, 2′‐bipyH₂][CrOCl₄]Cl, which loses HCl on heating in vacuo to form [CrOCl₃(2, 2′‐bipy)]. IR, UV/Vis, and ¹H NMR spectra (Cr^VI only) are reported for the new complexes. Attempts to extend these routes to oxygen donor ligands, including ethers and phosphine oxides, were unsuccessful. The diimine complexes are the first structurally autheticated adducts of chromium(VI) and (V) oxide‐chlorides with neutral ligands.     

Ca10(CrVO4)6(CrVIO4), a disordered mixed‐valence chromium compound exhibiting inversion twinning

The structure analysis of so‐called 9CaO·4CrO₃·Cr₂O₃ proved it to be the title compound, decacalcium hexakis[chromate(V)] chromate(VI), with the simultaneous presence of unusual chromium oxidation states. The structure determination was carried out on a crystal that had inversion twinning. The Cr^VIO₄ tetrahedron is situated on a threefold axis and is disordered over two possible orientations that share three O atoms, while the Cr^VO₄ tetrahedra are in general positions and are ordered. The charge is balanced by Ca²⁺ cations, one of which is located on a threefold axis. The Ca²⁺ ions are coordinated by six, seven or eight O atoms. The compound is a significant phase in the CaO–CrO_x system and its formation reduces the refractoriness of calcium‐rich compositions in an oxidizing atmosphere.     

Reactions anodiques et cathodiques des composes de Cr(VI) en milieu de nitrates alcalins fondus

The electrochemical behavior of chromate and dichromate, ions in molten NaNO₃-KNO₃ at 250°C has been studied by single-sweep voltammetry and cyclic voltammetry. The anodic dimerization of CrO₄^2- ions has been shown to proceed in three steps, the reaction: CrO₄^2-→CrO_4ad^?+e^? being rate-determining. The cathodic reduction of Cr₂O₇^2- ions includes the coupled acid-base reactions; it forms solid chromium oxide Cr₂O₃ and the soluble chromate and chromite ions.     

A Heterobimetallic Superoxide Complex formed through O2 Activation between Chromium(II) and a Lithium Cation

The reaction of 1,1,3,3‐tetraphenyl‐1,3‐disiloxandiol (LH₂) with n‐butyllithium and CrCl₂ results in a mononuclear chromium(II) complex ( 1 ) that further reacts with O₂ at low temperatures to yield a mononuclear chromium(III) superoxide complex [L₂CrO₂(THF)][Li₂(THF)₃] ( 2 ). The crystal structure revealed that the chromium superoxido entity is stabilized by the coordination to an adjacent lithium cation. Complex 2 thus contains an unprecedented heterobimetallic [Cr^III(μ‐O₂)Li⁺] core; beyond this it is the first chromium superoxide for which a temperature‐dependent magnetic characterization could be achieved, and the first structurally characterized representative with chromium in an exclusive O‐donor environment.     

Infrared absorption of chromium trioxide and its suboxides

The chromium oxides, obtained by the thermal decomposition of chromium trioxide, presented a bewildering array of possible mixed valency compounds. The IR absorption measurements in the range 400–5000 cm^?1 revealed the presence of the valencies +6, +5, +4, +3 in the samples between CrO₃ and Cr₂O₃.     

Dinuclear cage-like metalloorganosiloxane containing CrIII ions

Cage-like chromium sodium phenylsiloxane (PhSiO_1.5)₁₂(CrO_1.5)₂(NaO_0.5)₆·3BuOH·9EtOH·2H₂O containing two Cr^III ions was prepared for the first time by the exchange reaction of sodium phenylsilanolate with CrCl₃. The coordination sphere of the metal atom is filled by silanolate anions and water molecules located in the inner cavity of the cage. The structure of this compound was established by X-ray diffraction.     

Cu(Cu0.44Cr4.56)Ge2O12: a close‐packed oxide with CuO4 tetra­hedra

The structure of copper(I,II) penta­chromium(III) germanate, Cu(Cu_0.44Cr_4.56)Ge₂O₁₂, contains one Cu position (m2m), one Ge position (m) and three Cr positions (2/m, m and 2). The close‐packed structure is described in terms of slabs of edge‐sharing Cr³⁺O₆ octa­hedra and isolated CuO₄ and GeO₄ tetra­hedra. These slabs are aligned parallel to the bc plane and are separated from each other by GeO₄ tetra­hedra along a. The tetra­hedral coordination observed for the Cu⁺/Cu²⁺ ions represents an unusual feature of the structure. The Cr—O and Cu—O bond lengths are compared with literature data.     

Synthesis of LPPE by gas-phase copolymerization of ethylene with 1-butene: A highly efficient chromium oxide supported catalyst

A modified chromium oxide supported catalyst has been developed and applied in industry for the manufacture of LPPE via the gas-phase (co)polymerization of ethylene. The catalyst contains surface chromium oxide in the oxidation number Cr²⁺, two modifiers (aluminum oxide and fluorine surface compounds), and silicon dioxide as a support. The activity of the new chromium oxide catalyst in the gas-phase copolymerization of ethylene with 1-butene is higher by a factor of 4–5 than that of the traditional commercial catalytic system based on the supported bis(triphenylsilylchromate) catalyst. An increased reactivity of 1-butene in its copolymerization with ethylene in the presence of the chromium oxide catalyst makes it possible to reduce the consumption of 1-butene in the synthesis of a linear medium-density PE (0.937–0.938 g/cm³). Gas pipes made of PE prepared with the new catalyst are characterized by improved resistance to crack propagation.     

Tetrapotassium dicarbonato­dioxo­peroxo­uranium(VI) 2.5‐hydrate,K4[U(CO3)2O2(O2)]·2.5H2O

The title compound was obtained by reacting UO₂ powder in 2 M K₂CO₃ with hydrogen peroxide. The compound contains individual [U(CO₃)₂O₂(O₂)]⁴⁻ ions, which are linked via an extended network of K atoms and hydrogen bonding. The U atom is coordinated to two trans‐axial O atoms and six O atoms in the equatorial plane, forming distorted hexagonal bipyramids. The carbonate ligands are bound to the U center in a bidentate manner, with U—O bond distances ranging from 2.438 (5) to 2.488 (5) Å. The peroxo group forms a three‐membered ring with the U atom, with U—O bond distances of 2.256 (6) and 2.240 (6) Å. The U=O bond distances of 1.806 (5) and 1.817 (5) Å, and an O—U—O angle of 175.3 (3)° are characteristic of the linear uranyl(VI) unit.     

A tandem mass spectrometry study of the gas phase RSC6H5Cr+L (R = H,C6H5; L = arene) and C6H5SSC6H5Cr+ ions

Ion-molecule reactions of chromium containing ions with arylsulfides have been studied in the gas phase and their products have been characterized by tandem mass spectrometry. C₆H₅SH and (C₆H₅)₂S react as typical aromatic compounds and give rise to Cr⁺C₆H₅SR] and RC₆H₅Cr⁺QH₅SR′ [R = H, CH₃, CH(CH₃)₂; R′ = H, C₆H₅] ions. Metastable ion mass spectra of the latter species show that the metal is more strongly bound to diphenylsulfide than to alkylbenzenes. C₆H₅SSC₆H₅ reacts with chromium-containing ions to form only Cr⁺(C₆H₅SSC₆H₅). The decomposition characteristics of this ion and, in particular, the presence of a recovery signal in the neutralization-reionization mass spectrum are in keeping with the formation of a 1,2-dithia[2]cyclophane complex ion, which rearranges into a structurel(s) that contains Cr?S bond(s). No evidence was found for metal atom insertion into S?S, C?S, or S?H bonds.     

X-Ray CrKβ emission and K absorption spectra in mixed copper chromium oxides

The Kβ emission spectra of chromium in oxidic compounds were investigated by means of the fluorescent method. The origin of the main Kβ lines is discussed using MO theory and the dependence of the energy positions on the chemical bond is reported. The compounds studied include the oxides Cr₂O₃ CrO₂ and CrO₃, and the mixed copper chromium oxides CuCrO₂, CuCr₂O₄ and CuCrO₄. High-resolution chromium K-edge absorption spectra have been recorded using the synchroton radiation available at LURE (Orsay, France). The object of this investigation is to obtain further information about the chemical state analysis of chromium in catalysts: an evaluation of the atomic Cr(VI):Cr_total ratio in Cu_xCr_yO₄ catalysts is reported.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. I. Fixation of chromium on wood

The kinetic behavior of CrO₃ in its reaction with wood has been elucidated. Various reactions take place between CrO₃ and the lignin and cellulose in wood. CrO₃ reacts with cellulose in a two-step reaction: the first step is an adsorption of Cr^VI onto the cellulose to form Cr^VI/cellulose activated complexes. The second step is a Cr^VI → Cr^III reduction taking place on the cellulose surface. The Cr^III formed is only physically adsorbed to the cellulose or very weakly bound as small amounts of Cr^III can be released into the reaction medium. The Cr^VI adsorbed by cellulose appears mainly to be reduced to Cr^III. The reaction of Cr^VI with lignin has been shown to be the composition of the three successive reaction of Cr₂O₇^2?, HCrO₄^?, and CrO₄^2? with the guaiacyl units of lignin. Insoluble and stable Cr^VI/lignin complexes in which chromium maintains its hexavalent oxidation state are formed. Rate constants and energies of activation for all the reactions have been determined. The fixation of CrO₃-derived compounds on wood has been explained as the combination of the various reactions investigated. The results indicate that 60% of Cr is fixed irreversibly to the lignin of wood as Cr^VI and 40% is weakly bound, probably just precipitated, on the cellulose surface as Cr^III of which small amounts can be released in a water medium. The complex Cr^VI and Cr^III species forming complexes with the guaiacyl units have been identified.