Chromium,molybdenum and ruthenium complexes of chloranilic acid

Chloranilic acid (H₂CA) reacts with Cr(CO)₆ and CrCl₃ to give the tris derivatives Cr(H₂CA)₃ and Cr(HCA)₃. Spectroscopic measurements on the two complexes reveal that the ligand–metal bond is of the semiquinone type. Susceptibility determinations showed effective magnetic moments of 1.82 and 1.07_B. Examination of Cr(H₂CA)₃ by cyclic voltammetry revealed two redox reactions due to tautomeric interconversion through electron transfer. Boiling Mo(CO)₆ with H₂CA in air gave MoO₃(HCA) in which the molybdenum atom is in the +5 oxidation state. The i.r. spectrum of the MoO₃(HCA) displayed bands due to terminal M=O bonds. The cluster compound Ru₃(CO)₁₂ reacted with H₂CA to give Ru₃(CO)₁₀(-H)(HCA), the i.r. spectrum of which revealed that the ligand is bound to the metal as a catechol moiety. The spectroscopic and magnetic studies of the molybdenum and ruthenium complexes confirmed the proposed structures.     

Adsorption of carbon monoxide on Lewis acid sites of alumina

It has been found that carbon monoxide is adsorbed on Lewis acid sites of a high acid strength. Potential applicability of carbon monoxide as a gaseous base to determine the strength and amount of acid sites is suggested.

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, . CO, , .

     

Ternary metal sulfides MzMoS2: synthesis using single-layer dispersions of molybdenum disulfide and study of the structure

The sulfur atoms of negatively charged (MoS₂)^x– layers in aqueous single-layer dispersions of molybdenum disulfide covalently bind the M²⁺ cations (M = Pb, Cd, Hg, Pd). This makes it possible to obtain ternary metal sulfides M_zMoS₂ containing guest M atoms in octahedral or tetrahedral vacancies between the S—Mo—S layers at room temperature. In the case of Pd and Hg, the formation of M_zMoS₂ is accompanied by the reduction of a fraction of Pd^II to Pd⁰ and Hg^II to Hg^I. During dispersion followed by the formation of ternary sulfides, the MoS₂ layers undergo structural transformations resulting in the formation of Mo—Mo bonds.     

Catalytic enantioselective oxidation of sulfides and disulfides by a chiral complex of bis-hydroxamic acid and molybdenum

A chiral bis-hydroxamic acid (BHA)-molybdenum complex was used for the catalytic asymmetric oxidation of sulfides and disulfides utilizing one equivalent of alkyl peroxide with yields up to 83% and ee up to 86%. An extension of our methodology combines the asymmetric oxidation with kinetic resolution providing excellent enantioselectivity (ee 92–99%).     

Thermal analysis and kinetics of oxidation of molybdenum sulfides

The thermal oxidation process of stoichiometric MoS₂ and nonstoichiometric “Mo₂S₃”, together with the kinetics of oxidation of MoS₂, were studied by using TG and DTA techniques in the Po₂ range 1-0.0890 atm. MoS₂ was oxidized completely to MoO₃ in one step: $$MoS_2 + 7/2O_2 \to MoO_3 + 2SO_2 $$ Irrespective of Po₂ and the heating rate, “Mo₂S₃” was oxidized finally to MoO₃, via the following four steps: $$\begin{gathered} ''Mo_2 S_3 ''\xrightarrow{I}\gamma - Mo_4 O_{11(sur)} + ''Mo_2 S_3 ''\xrightarrow{{II}} \hfill \\ MoO_{2(sur)} + ''Mo_2 S_3 ''\xrightarrow{{III}}MoO_2 \xrightarrow{{IV}}MoO_3 \hfill \\ \end{gathered} $$ where (sur) refers to the surface layer. The kinetic study revealed that the oxidation (α=0.01?0.90) of MoS₂ to MoO₂ was controlled by the kinetics $$1 - (1 - \alpha )^{1/3} = kt$$ and that the apparent activation energies determined with the isothermal and the nonisothermal (10 deg min^?1) method were 98.1±2.2 and 93.5±3.0 kJ mol^?1, respectively, over the temperature range 540–625? and the Po₂ range 0.612-0.129 atm. The relationship between the rate constantk and Po₂ was determined.     

Predicting the activity of single isolated Lewis acid sites in solid catalysts

An experimental study of the activity of Ti-, Zr- and Sn-beta catalysts in different types of oxidation reactions is combined with a quantum-chemical analysis of the electronic properties of the active sites and the adsorbed reactants. The differences observed in the catalytic behaviour of the three materials are explained in terms of the molecular orbital distribution of each system. The intrinsic Lewis acid strength of the isolated active site, the degree of back-donation from the catalyst to the empty orbitals of the organic reactant and the net atomic charges on selected atoms are proposed as predictors of reactivity.     

Nature, structure and strength of the acidic sites of amorphous silica alumina: an IR and NMR study

IR spectroscopy of probe molecules (pyridine, 2,6-dimethylpyridine, and CO) as well as high-resolution solid state NMR and especially double-resonance experiments give a new insight into the acidic sites of amorphous silica alumina (ASA). ASA samples are heterogeneous compounds that contain a silica alumina mixed phase as well as aluminum clusters and pure silica zones. The distribution of various forms depends both on the preparation method and on the Si/Al ratio. Formation of mixed phase leads to the creation of acidic hydroxyl groups of various strength, up to that present in dealuminated HY zeolite. Detailed spectroscopic analyses show that these acidic OH groups correspond to the silanol groups located in close vicinity to an Al atom in tetrahedral environment. The strength of the acidity of the OH species of ASA could be modified both by the location of the vicinal Al atom on the surface or in the bulk and by the number of aluminum atoms in the vicinity of silanol group. Cogelification of high silica-containing ASA appears as the best mean to prepare homogeneous amorphous aluminosilicate, which exhibits the strongest Br?nsted acidity.     

Polymer-micelle incarcerated ruthenium catalysts for oxidation of alcohols and sulfides

Highly active immobilized ruthenium catalysts, which can be used for oxidation of alcohols and sulfides, were developed on the basis of the polymer-micelle incarcerated (PMI) method. The catalysts could be recovered and reused several times without loss of activity and no metal leaching was observed. Selection of micelle-forming conditions and polymer structures were key in achieving high activities. TEM and SEM analyses were conducted to observe the structures of PMI-Ru.     

Adsorption of pyridine on the Lewis acid sites of microcrystalline γ-alumina: a quantum chemical cluster model study

Adsorption of pyridine on Lewis acid sites of microcrystalline γ-alumina was studied by quantum chemical cluster model approach at B3LYP and HF/6-31++G(d,p) levels of theory considering both the standard and the counterpoise-corrected potential energy surfaces (PESs). Harmonic vibrational frequency shifts of pyridine ν₈ and ν₁₉ internal mode components calculated at both levels of theory seem to excellently reproduce the experimental observations, the results for standard and counterpoise-corrected PESs being essentially identical. The interaction energies of pyridine with various clusters representing microcrystalline γ-Al₂O₃ were also calculated and the natural bond orbital and atoms in molecules analyses were performed.     

Non-empirical calculations of adsorbability of paired Lewis acid sites of alumina

The non-empirical LCAO SCF MO method on the STO-3G basis was applied to calculate adsorption of H₂O, NH₃ and CO molecules on paired Lewis acid sites (LAS) of alumina formed as a result of the removal of the OH groups which are common for two aluminium atoms.

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, STO-3G, H₂O, NH₃, CO () Al₂O₃, OH. , H₂O, NH₃, CO .

     

Iron and ruthenium Lewis acid catalyzed asymmetric 1,3-dipolar cycloaddition reactions between nitrones and enals

The single coordination-site transition metal Lewis acids [CpM(BIPHOP-F)][SbF6] (M = Fe, Ru) catalyze the [3+2] dipolar cycloaddition reaction between reactive nitrones and alpha,beta-unsaturated aldehydes to give chiral isoxazolidines with ee values of 75 to >96%. The stereochemistry of the major enantiomer is consistent with an endo approach of the nitrone to the Calpha-Si-face of the enal in the s-trans conformation in the (R,R)-catalyst site. The absolute configuration is based on an X-ray structure determination.     

Zero-flux surfaces of the electrostatic potential: the border of influence zones of nucleophilic and electrophilic sites in crystalline environment

The topology of the electrostatic potential varphi(r) has been studied for single molecules using geometries and electron distributions rho(r) determined from high-resolution single-crystal X-ray diffraction data. The electrostatic potential gradient nablavarphi(r), which is the negative of the electric field E = -nablavarphi, has been represented, revealing the position of zero-flux surfaces and critical points. Local maxima and minima of the electrostatic potential are interpreted in terms of electrophilic and nucleophilic sites, which present influence zones delimited by zero-flux surfaces containing saddle points. The influence zones of the nucleophilic and electrophilic sites define two alternative partitions of the space in disjoint volumes, the completeness of these partitions depending on either the neutral or ionic character of the molecule. The results obtained by using this methodology are useful for the interpretation of the saddle points of the electrostatic potential, which are related to the limits of the influence zones and reveal the path for preferred attack on reactive sites with finite influence zones.     

Lewis acid sites of bivalent silicon and quantum-chemical studies of interaction with CO

According to the results of quantum-chemical ab initio studies taking into account electron correlation, the structure of the adduct of the Lewis acid site R₂Si with CO where CO bond activation takes place, is suggested.

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R₂Si , CO.

     

Activation of Lewis acid catalysts in the presence of an organic salt containing a non-coordinating anion: its origin and application potential

In the presence of a soluble organic salt containing non-coordinating anion (e.g., [bmim][SbF(6)] or [NR(4)][SbF(6)]), the catalytic activity of Lewis acid (MX(n)) was dramatically enhanced due to the anion exchange between the Lewis acid and organic salt.     

富勒烯结构B_(80)分子静电势的理论研究

在混合密度泛函B3LYP理论下,用6-31G*基函数对富勒烯结构B80分子的3个异构体(1个具有Ih对称性,2个具有Th对称性)构型进行优化和分子静电势计算.结果表明:3个异构体球内全部为正电势,球外五元环中心所对应的区域都为负电势,B80Ih,Th(A)和Th(B)球外静电势的最大负值分别对应于20个六元环中心的B原子,五元环中心和12个六元环中心的B原子周围,它们组成了化学反应中最可能的活性点.     

Coordination of nitroxyl radical probes to the Lewis acid sites on the surface of gallium oxide: A quantum-chemical analysis

The results of ESR investigations and quantum-chemical studies on the coordination of 2,2,6,6-tetramethylpiperidine-N-oxyl to the Lewis acid sites (LAS) on the surface of gallium oxide are reported and analyzed. The structural, spin, electrostatic, and energy characteristics of the coordination of the simplest nitroxyl radical, H₂NO^., to the Ga(OH)₃ model cluster-type LAS were calculated by the unrestricted Hartree—Fock method in the 6–31G basis set and using the semiempirical PM3 approximation. The properties of gallium-containing nitroxyl surface complexes and their aluminium-containing analogs are compared. According to the results of cluster quantum-chemical calculations, the LAS on the surface of gallium oxide are stronger than those on the alumina surface. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 996–1001, June, 2000.     

Insight into the chemical bonding and electrostatic potential: A charge density study on a quinazoline derivative

2,3-Trimethylene-3,4-dihydroquinazoline shares the heterocyclic core with natural compounds and synthetic drugs. The hydrochloride of the compound forms excellent dihydrate crystals which have allowed us to collect high-resolution X-ray diffraction data and obtain the experimental charge density. The solid may be understood as built up from pairs of heterocyclic cations and chloride anions; a direct hydrogen bond links the halide to the formally cationic pyrimidine NH group. The hydrate water molecules interact with the anions, forming an infinite chain along the crystallographic a axis between the stacks of the heterocyclic cations. Based on the experimental charge density, a dipole moment of 16.1 Debye is calculated for a pair of the hydrogen-bonded quinazolinium cation and the chloride anion in the extended crystal structure.