One-electron redox reactions involving chromium(0) and molybdenum(0) bis-1,3,5-trimethylbenzene derivatives

The reaction of M(η⁶-1,3,5-Me₃C₆H₃)₂, M = Cr, Mo, with the tetrahalides of Groups 4 and 5 elements proceeds with the monoelectronic oxidation of the metal bis-arene to the [M(η⁶-Me₃C₆H₃)₂]⁺ cation. In the case of MX₄, M = Ti, X = Cl, Br, M = V, X = Cl, and of Nb₂Cl₁₀ the reduction products are the titanium(III), vanadium(III) halides and the niobium(IV) chloride, isolated as the solvate anions [MCl₄(THF)₂]⁻ and [NbCl₄(CH₃CN)]⁻. The reaction of the tetrachloro complexes MCl₄(THF)₂, M = Zr, Hf, with Cr(η⁶-1,3,5-Me₃C₆H₃)₂ in THF produces the ionic [Cr(η⁶-1,3,5-Me₃C₆H₃)₂][MCl₅(THF)], which has been characterized by single-crystal X-ray diffraction in the case of hafnium.     

Unsymmetrically substituted triazacyclohexanes and their chromium complexes

The unsymmetrically N-substituted N,N′-Ar₂-N″-R-1,3,5-triazacyclohexanes 1–4 (Ar = ortho- or para-fluorophenyl, R = n- or iso-propyl) can be obtained in good yields from a one-step condensation reaction with excess amine. Solid state structures of 1–4 resemble closely those of their triaryl-substituted analogues. The condensation reaction to 4 was looked at by detailed NMR investigations and revealed that amine/aniline exchange is occurring in solutions containing free aniline even at ambient conditions setting up an equilibrium between all possible symmetrical and unsymmetrical triazacylcohexanes. Selective crystallisation of 4 from the solution drives the reaction to high yields of 4. Complexes 1–4 react readily with CrCl₃ or CrCl₃(THF)₃ to form the corresponding CrCl₃ complexes. The complexes are insoluble in non-polar solvents and decompose under decomplexation in coordinating solvents.     

Perhydroxylated 1,7-Dioxaspiro[5.5]undecanes (“Spiro Sugars”): Synthesis,Stereochemistry, and Structure

As spiro sugars is an apt way of considering perhydroxylated 1,7-dioxaspiro[5.5]undecanes–a class of compounds which has not been found in nature up to now. The crystal structure of such a spiroacetal, in which the two pyran rings show the β-D -manno configuration, is depicted. Note that the all-trans arrangement of C-6, C,-5, O_pyr, C_spiro, O_pyr^′, C-5′, and C-6′ does not allow any of the stereoelectronic effects that are typical of carbohydrates.     

Inclusion of methano[60]fullerene derivatives in cavitand-based coordination cages

The X-ray study of self-assembled coordination cage 1, constituted of two tetrapyridyl-substituted resorcin[4]arene cavitands coupled through four square-planar palladium complexes is reported. The coordination cage, embracing an internal cavity of ca. 840 Å³, reveals to have the right size for the inclusion of large molecules such as fullerenes. Cage 1 forms 1:1 complexes with methano[60]fullerene derivatives 3 and 4 bearing a dimethyl and a diethyl malonate addend, respectively. Evidence for inclusion complexation was provided by ¹H NMR spectroscopic studies and ESI-MS investigations, which unambiguously showed the formation of 1:1 fullerene-cage complexes. The association constants (K_a) were experimentally determined to be ca. 150 M⁻¹ at 298 K in CD₂Cl₂. In both complexes 1·3 and 1·4, the malonate residue is threaded through one of the four lateral portals, as clearly shown by docking simulations.     

Rapid ion-exchange technique for the separation and preconcentration of chromium(VI) and chromium(III) in fresh waters

A technique for the separation and preconcentration of Cr(VI) and Cr(III) in fresh waters is presented. The analytical procedure involves the use of anion- and cation-exchange columns. The columns are eluted and the eluate is analysed for chromium using a graphite furnace atomic absorption spectrometer. The recovery of Cr(VI) and Cr(III) is 97.86 ± 1.31% and 102.36 ± 1.25% (95% confidence), respectively. The detection limits are 0.019 and 0.020 μg 1^?1 for 200 ml of sample (twice the standard deviation of eleven replicate blanks). The method is rapid and the need for minimum sample handling avoids contamination problems.     

Optimization of species stability and interconversion during the complexing reaction for chromium speciation by high‐performance liquid chromatography with inductively coupled plasma mass spectrometry

High‐performance liquid chromatography coupled with inductively coupled plasma mass spectrometry was employed for the determination of chromium species. For simultaneous separation of both chromium species by an anion‐exchange column, ethylenediaminetetraacetic acid was induced to form negatively charged complex with Cr(III) normally. Cr(III) chelating reactions are known to be slow, so a high temperature and long reaction time are needed to ensure the completion of the complexing reaction. However, the stability and interconversion of chromium species during the complexing reaction have not been studied earlier. The main aim of this work was to optimize and investigate complexing reaction conditions between ethylenediaminetetraacetic acid and Cr(III). Through optimizing conditions, the reaction will be finished completely in 15 min at pH 7 and 70°C without any obvious interconversion between Cr(VI) and Cr(III). By compromising analysis time, chromatographic resolution, and sensitivity, 60 mM NH₄NO₃ as competing ion concentration and 1.2 mL/min as flow rate have been selected for real‐sample application. Detection limits for Cr(VI) and Cr(III) were 0.051 and 0.078 μg/L, respectively. The proposed method was used for the determination of chromium species in tap and surface water samples with an acceptable range of spiked recoveries of 95–109%.     

Accurate and precise reference method for the determination of chromium in high-alloy steel

Methods for determining chromium in high-alloy steels based on potentiometric titration after oxidation of chromium(III) to chromium(VI) with peroxodisulphate were studied using different dissolution procedures, viz., dissolution in HClHNO₃ and fuming with H₂SO₄H₃PO₄, dissolution in HClHNO₃ and fuming with HClO₄, dissolution in HClHNO₃HF in a microwave oven, fusion in sodium peroxide in a zirconium crucible and dissolution in dilute H₂SO₄ and oxidation with H₂O₂. A back-titration was used with dichromate after addition of solid ammonium iron (II) sulphate.The dissolution procedures were tested on 24 certified reference materials (0.01–3.3% C, 10–325% Cr). All procedures except the second gave good results for samples with ? 0.8% C. For samples with ? 0.8% C, the third and fourth procedures gave significantly higher values and better precisions, and gave the best results for all samples. The relative standard deviations were, with few exceptions, below 0.2%.     

Synthesis,structural characterization,and properties of an entangled metal–organic framework based on a flexible dicarboxylate and a rigid N-donor

Self-assembly of manganese acetate with 1,3-bis(4-carboxy-phenoxy)propane (H₂bcp) and 1,2-bis(4-pyridyl)ethene (bpe) under solvothermal conditions yielded a polymer {[Mn₂(bcp)₂(bpe)(DMF)]} _n (1), which shows 2-D?→?3-D inclined interpenetration with polyrotaxane character. The magnetic behavior of 1 shows antiferromagnetic exchange between Mn magnetic centers.     

Self-assembly of nanosize coordination cages on si(100) surfaces

Bottom-up fabrication of 3D organic nanostructures on Si(100) surfaces has been achieved by a two-step procedure. Tetradentate cavitand 1 was grafted on the Si surface together with 1-octene (Oct) as a spatial spectator by photochemical hydrosilylation. Ligand exchange between grafted cavitand 1 and self-assembled homocage 2, derived from cavitand 5 bearing a fluorescence marker, led to the formation of coordination cages on Si(100). Formation, quantification, and distribution of the nanoscale molecular containers on a silicon surface was assessed by using three complementary analytical techniques (AFM, XPS, and fluorescence) and validated by control experiments on cavitand-free silicon surfaces. Interestingly, the fluorescence of pyrene at approximately 4 nm above the Si(100) surface can be clearly observed.     

Topological aspects of metals in carbon cages: Analogies with organometallic chemistry

Metals can interact with carbon cages in the following ways: (1) stable carbon cages (i.e., fullerenes) function as electronegative olefins in their exohedral η₂ bonding to transition metals; (2) endohedral metallofullerenes with a highly electropositive lanthanide (Ln) inside the carbon cage can be considered to be ionic with lanthanide cations, Ln³⁺, and fullerene anions; (3) fullerenes too small for independent existence can be stabilized by internal covalent bonding to an endohedral metal atom using the central carbon atoms of pentagon triplets,i.e triquinacene, units, in complexes such as M@C₂₈ (M=Ti, Zr, Hf, and U), derived from the tetrahedral fullerene C₂₈; (4) metal atoms can occur as vertices of binary mixed metal-carbon cages in both early transition metal complexes of the types M₁₄C₁₃, M₈C₁₂, and M₁₃C₂₂ (e.g., M=Ti) and copper-carbon cages of the types Cu_2n +1C_2n ⁺ (n≤10), Cu₇C₈ ⁺, Cu₉C₁₀ ⁺ and Cu₁₂C₁₂ ⁺. The presence of metal atoms as vertices of carbon cages changes radically their stoichiometries and thus their structures. Thus, early transition metals form cages such as Ti₁₄C₁₃ assumed to have titanium atoms at the vertices and face midpoints of a 3×3×3 cube and carbon atoms at the edge midpoints and center of the cube and Ti₁₃C₂₂ assumed to have titanium atoms at the edge midpoints and center of a 3×3×3 cube as well as C₂ units and carbon atoms at the vertices and face midpoints, respectively, of the cube. Elimination of the face metal atoms from the Ti₁₄C₁₃ structure as well as the center carbon atom, which has been achieved experimentally by photofragmentation, leads to the Ti₈C₁₂ cluster. The structure of this cluster is based on a tetracapped tetrahedron withT _d symmetry with two distinct quartets of titanium atoms, six distinct C₂ pairs, and 36 direct Ti−C interactions. The copper-carbon cages of various stoichiometries are suggested to have prismatic, antiprismatic, or cuboctahedral structures in which the electronic configurations of the copper atoms approach the favored 18-electron rare gas configuration. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 862–869, May, 1998.     

Noble metal nanoparticles meet molecular cages: A tale of integration and synergy

Noble metal nanoparticles attract growing interest owing to their high surface-to-volume ratio and unique optical, electric and catalytic properties. Fine-tuning these properties and broadening potential applications can be envisaged if nanoparticles are coupled to supramolecular cages that afford a highly tailorable inner environment as well as rich endo-/exo-functionalization. Due to rich chemical/physical properties of cages, integration of multiple host-guest interactions in confined cavities through endo-molecular design has been achieved. Such cages provide ideal confined templates for size-controlled synthesis of ultrafine nanoparticles with superior catalytic activities. Moreover, exo-functionalization of cages offers huge opportunities to couple with nanoparticles, generating cage-nanoparticle hybrids or hierarchical assemblies that combine merits of both. The present review provides recent advances in cage-mediated nanoparticle systems with synergistic effects and integrated functions, and demonstrates their applications in catalysis, sensing, chiral amplification, plasmonic switches, imaging and cell therapy. Finally, we highlight key challenges and identify emerging directions in the coming years.     

Tricarbonyl-6,6′-dimethylfulvene chromium(0) - Structural properties

The crystal structure of tricarbonyl-6,6′-dimethylfulvene chromium(0) was determined using X-ray diffraction at room temperature and at 120 K. The title compound crystallizes in the centrosymmetric, monoclinic space group P2₁/n (no. 14) with one complete molecule in the asymmetric unit. The deviation of the chromium(0) complex from C_s(m) symmetry is negligible in agreement with results derived from density functional calculations using three different methods as well as ab initio calculations (MP2). The 6,6′-dimethylfulvene ligand is bent by 30.9° as a result of the π-η²:π-η²:π-η² coordination to the Cr(0) metal. In addition, the chromium(0) complex was studied by IR and Raman spectroscopy and selected vibrational data are compared to values derived from DFT calculations.     

Eu4Bi3: Crystal Structure,Magnetic and Electronic Properties

The Eu? Bi system contains the phases Eu₅Bi₃, Eu₄Bi₃ and Eu₁₁Bi₁₀. The structure types of these phases have been determined by powder X-ray diffraction. Crystals of Eu₄Bi₃ (cubic, space group I4 3d; a = 9.920 Å, Z = 4, T = 130 K, R1/wR2 = 4.86/10.84%) were obtained in low yield by reaction of Eu, Mn, and Bi in the ratio 14:1:11 in a closed niobium tube (heating rate 30°C/h; reaction at 1050°C for 300 h, cooling rate 100°C/h). The crystal structure consists of distorted octahedra made up of six Bi coordinated to a central Eu atom. Eu is also coordinated to a three other Eu atoms and forms a three-dimensional network composed of interconnected rings. The Bi atoms are coordinated to eight Eu atoms. High yields of Eu₄Bi₃ can be prepared by reacting stoichiometric amount of the elements in a sealed tantalum tube at 1100°C for 24 h. Temperature dependent magnetic susceptibility is consistent with antiferromagnetic behavior with an ordering temperature of 18 K. The data could be fit with the Curie-Weiss law and a moment of 7.38 μ_B/Eu is obtained, consistent with all Eu atoms being Eu¹¹. Temperature dependent resistivity indicates that Eu₄Bi₃ is a metal with a room temperature resistance of 1.3 Ωcm.     

Synthesis,crystal structure,and properties of copper(II) and manganese(II) complexes with azide and 3,4-di(2′-pyridyl)-1,2,5-oxadiazole

Two transition metal complexes with azide and 3,4-di(2′-pyridyl)-1,2,5-oxadiazole (dpo), [Cu₂(dpo)₂(N₃)₄] (1), and [Mn(dpo)₂(N₃)₂] (2), have been synthesized and characterized by single-crystal X-ray diffraction. The Cu(II) complex is binuclear with double end-on (EO) azido bridges, in which each Cu(II) ion assumes a distorted square pyramidal geometry, and each EO azido bridge adopts a quasi-symmetric fashion. In contrast, the Mn(II) complex is mononuclear, in which the Mn(II) ion is ligated by two dpo ligands and two terminal azide ions, with a distorted octahedron geometry. Magnetic studies on the Cu(II) complex revealed that the double EO azido bridge mediates ferromagnetic coupling with J=12.8 cm⁻¹.     

Speciation of hexavalent chromium in waters by liquid-liquid extraction and GFAAS determination

Diperoxo chromium oxide is produced by reaction of hydrogen peroxide on chromium(VI). Diperoxo chromium creates a complex with ethyl acetate, while chromium(III) remains in an unchanged form in the aqueous phase. By this means chromium(VI) can be extracted into ethyl acetate from the aqueous phase. The optimal conditions of Cr(III)-Cr(VI) separation, as well as the chromium content of the ethyl acetate phase were determined with graphite furnace atomic absorption spectrometry. In the second extraction of Cr(VI) from ethyl acetate back into water phase an additional preconcentration of chromium(VI) can be carried out. The detection limit (3σ) of the developed method found to be 200 ng dm^− 3 for the first extraction and 50 ng dm^− 3 after using the twofold extraction. In consequence of the matrix free ethyl acetate phase after the first extraction, with this separation a really extensive preconcentration of chromium(VI) can be realized.     

Determination of total chromium in phosphate rocks by ion chromatography

Chromium(VI) is one of seven elements which is classified in the fertilizer industry as being harmful to plants and biological systems. Phosphate rocks represent the raw material for complex fertilizer production in the world. This paper investigates for the first time the determination of total chromium in phosphate rocks by ion chromatography. The developed analytical method involves the digestion of phosphate rocks with nitric acid followed by sample treatment of the resulting solution. The digestion solution obtained was treated with an oxidising agent (potassium peroxosulphate) to convert all chromium to the hexavalent state. The analytical method developed utilizes anion-exchange ion chromatography to achieve the separation and spectrophotometric post-column reaction for detection with diphenylcarbazide. The relative standard of deviation from analytical data comparison of six different phosphate rocks with atomic absorption spectrometry and inductively coupled plasma atomic emission spectrometry techniques, and cross-analysis data against an internationally certified phosphate rock standard were between 0.58 and 1.45%. Calibration curve between 0.2 and 0.9 μg/ml was excellent, and the method has a detection limit for Cr(VI) of 0.05 ng. The developed method offers a fast, a reliable and an alternative procedure for the determination of total chromium in phosphate rock deposits by ion chromatography.     

Determination of chromium(III) and total chromium using dual channels on glass chip with chemiluminescence detection

A simple, rapid and sensitive method for the determination of chromium(III) and total chromium using the simple dual T channels on glass chip with negative pressure pumping system and chemiluminescence (CL) detection is presented. The CL reaction was based on luminol oxidation by hydrogen peroxide in basic aqueous solution catalyzed by chromium(III). Total chromium in form of chromium(III) was achieved after chromium(VI) was completely reduced by acidic sodium hydrogen sulfite. Total chromium could then be determined with the same strategy as the chromium(III). The CL reagent was composed of 1.0 × 10⁻⁴ mol/L luminol, 1.0 × 10⁻² mol/L hydrogen peroxide and 0.10 mol/L sodium bromide in 0.050 mol/L carbonate buffer (pH 11.00). The 1.0 × 10⁻² mol/L ethylenediaminetetraacetic acid was added into the sample solution in order to improve the selectivity. Chromium(III) could be detected at a notably concentration of 1.6 × 10⁻¹⁶ mol/L and a linear calibration curve was obtained from 1.0 × 10⁻¹⁵ to 1.0 × 10⁻¹³ mol/L. The sample and CL reagent consumption were only 15 and 20 μL, respectively. The analysis time was less than 1 min per sample with the precision (%R.S.D.) was 4.7%. The proposed method has been applied successfully to the analysis of river water, mineral waters, drinking waters and tap water. Its performance was verified by the analysis of certified total chromium-reference materials and by recovery measurement on spiked synthetic seawater sample.