Synthesis,Structure, and Reactivity of Lewis Base Stabilized Plumbacyclopentadienylidenes

Plumbacyclopentadienylidenes, in which the lead atoms have divalent states and are coordinated by THF, pyridine and N‐heterocyclic carbene, were synthesized and characterized. The THF‐ and pyridine‐stabilized compounds can be regarded as rare examples of hypervalent 10‐X‐4 species. The equilibrium between the THF adduct and the free plumbacyclopentadienylidene was evidenced by spectroscopic analysis and theoretical calculations. The THF adduct in benzene converted into a plumbylene dimer, where one of the lead centers is coordinated by THF and the other lead atom is coordinated by a divalent lead atom, the dimer gradually decomposing into spiroplumbole. The THF adduct unexpectedly reacted with trifluoroborane and trichlorogallane to afford fluoroborole and chlorogallole, which are the first examples of non‐annulated fluoroborole and gallole, respectively.     

Hypercoordinated carbon in C-doped boron fullerenes: a quantum chemical study

The structures and stability of C-doped boron fullerenes with the three-dimensional arrangement of non-classical pentacoordinated quasi-flat carbon centers were studied using the density functional theory (DFT) B3LYP/6-311+G(d,p) method. The doping with carbon atoms in apical positions above the five-membered rings stabilizes the spherical boron fullerene forms due to multicenter interactions of p_z-orbitals of the carbons and adjacent boron atoms. Increasing in the size of the fullerene cluster is accompanied by change in the bonding pattern and by flattening of the hypercoordinated carbon centers. Endohedral metal atoms significantly affect on the structure and stability of the fullerene systems with hypercoordinated carbon centers.     

Stereoselective Syntheses of Building Blocks with Three Consecutive Stereogenic Centers: Important Precursors of Polyketide Natural Products [New Synthetic Methods (68)]

Sequences of directly adjacent stereogenic centers were first discovered for the monosaccharides, which therefore constituted the first focus of interest in Stereoselective synthesis. The structures of many polyketide-derived natural products that have been elucidated in recent years often exhibit longer sequences of stereogenic centers in which hydroxy-substituted secondary carbon atoms alternate with tertiary carbon atoms. This finding inspired the development of methods allowing the specific construction of first two and then—more importantly–three neighboring stereogenic centers. The diverse methodologies that have thereby emerged reveal common principles and stress the variety of approaches possible in stereoselective synthesis.     

Properties of atoms in molecules

The electronic charges and the positions of the centers of these charges have been calculated for the atoms of a number of second- and third-row heteronuclear diatomic molecules. For both the oxygen and the fluorine atoms, the charge associated with one of these atoms can be correlated, within a series of molecules containing that atom, with both the orbital energy of the atom's 1s electrons and also with the difference in electronegativities of the atoms that comprise the molecule. The centers of electronic charge are outside of the internuclear regions, except for the positive atoms in the more ionic molecules and in HF.     

Quantum-chemical modeling of defects in PbTe crystals doped with VTe<Subscript>2</Subscript> and V<Subscript>3</Subscript>Te<Subscript>4</Subscript>

Quantum-chemical modeling of defects in PbTe crystals doped with VTe₂ and V₃Te₄ (PbTe(V)) has been performed. It has been shown that, at the B3LYP/LanL2DZP level, the effect of single defects (like vanadium atoms or lead vacancies) in the charges and electrostatic potentials is small and does not exceed 0.1e and 0.3 eV, respectively. Calculations predict that the location of a lead vacancy in the vicinity of a vanadium atom is energetically favorable. Fragments of chains of tellurium atoms formed upon association of vacancies can serve as active annihilation centers (AACs). In PbTe clusters, Te-Te chains are most abundant, Te-Te-Te-Te chains are less abundant, and Te-Te-Te chains are even less frequently encountered. AACs form beginning with association of three vacancies in PbTe clusters and beginning with association of two vacancies in PbTe(V) clusters. This fact that a smaller number of vacancies are required for formation of AACs in the PbTe(V) cluster can be responsible for that the number of annihilation centers experimentally found by the positron annihilation method in PbTe(V) is ∼1.2–1.4 times as large as the number of AACs in undoped PbTe.     

Electronic structure of calix[4]arene thioethers and their complexes with palladium,as studied by X-ray emission spectroscopy,X-ray photoelectron spectroscopy,and quantum chemistry

The charge state of calix[4]arene thioethers and their complexes with palladium was studied by X-ray emission spectroscopy and X-ray photoelectron spectroscopy. Experimental data were compared with the results of quantumchemical calculations. The involvement of the sulfur lone electron pairs in the coordination to Pd atoms causes a decrease in the energies of the corresponding MOs having contributions from the sulfur 3p AOs and, consequently, a decrease in the intensity of the maximum A on going from the S(Kβ) spectra of free ligands to the spectra of complexes. The binuclear nature of the complexes does not lead to the onset of significant additional interactions between two metal centers.     

Theoretical and experimental study of the reactivity of PbTe in interaction with oxygen: Effect of germanium impurity atoms

High-resolution X-ray photoelectron spectroscopy (BESSY II, Berlin) has shown that germanium atoms doped into lead telluride are the centers of the oxidation reaction with dioxygen. The structure, stability, and core-level binding energies in the reaction centers have been calculated in the framework of the cluster approach with the use of the hybrid density functional (B3LYP); i.e., theoretical X-ray photoelectron spectra have been obtained. Different variants of the attachment of one to three O₂ molecules to the Ge atom located at the surface of a crystal or in the subsurface layer have been considered. It has been shown that chemisorption leads to structures with close energies, which give rise to a set of components in the spectra with unresolvable binding energies. The calculation and experimental results are in good agreement.     

Molecular orbital description of the metal-semiconductor interface of AgAgBr

Extended Hückel (EH) and CNDO methods are applied to a cluster (up to 35 ions) of an ionic semiconductor (AgBr) and its interface with a metal (Ag). It is shown that EH suitably modified to include charge and Madelung effects and CNDO provide a suitable treatment for these systems. Comparison of calculated AgBr data with experimental data shows 10–20% agreement. Significant electron transfer from silver atoms to silver bromide takes place, but as the silver cluster is made larger the average charge transferred decreases. Calculated data for this system lead to a thermodynamic pathway for the photochemical decomposition of AgBr involving electron capture by odd-size silver centers and silver ion capture by even-size silver centers.     

Enthalpies of specific interaction of heteroatom derivatives of three-coordinate phosphorus with chloroform

The nature of hydrogen bonds formed by chloroform with three-coordinate phosphorus derivatives containing sulfur, silicon, and chlorine atoms and their carbon analogs was revealed by analysis of the experimental enthalpies of specific interactions. The main donor centers in the examined compounds were found to be oxygen atoms and π-electron systems of aromatic rings. Sulfur atoms give rise to weaker complexes as compared to oxygen. Phosphorus, chlorine, and silicon atoms are not involved directly in specific interactions with the solvent.     

Formation of Pd dimers at regular and defect sites of the MgO(1 0 0) surface: cluster model calculations

The formation of Pd dimers on the surface of MgO has been studied by means of density functional theory (DFT) cluster model calculations. The following surface sites have been considered: regular five-coordinated anions at the (1 0 0) terraces, monoatomic steps, OH groups, and neutral vacancies (F centers). We discuss the energy balance of forming a dimer at a given site with respect to two isolated Pd atoms, one adsorbed at the defect and one on a regular terrace site. We found that all the defects considered lead to an energy gain when the dimer is formed, suggesting that they can be involved in nucleation and growth processes of metal clusters on the MgO surface. The dimerization energy is moderate for steps (≈0.8 eV), large for OH groups (≈1.3 eV) and rather small (<0.5 eV) for F centers.     

Mn(II) ion centers in novel coordination environment: a new two-dimensional coordination polymer [Mn(3,5-pdc)·2H2O]

The hydrothermal synthesis and structure of the coordination polymer [Mn(3,5-pdc)·2H₂O] (3,5-pdc=3,5-pyridinedicarboxylic acid) with a novel seven-coordination mode of Mn(II) ion is reported. The metal ion center is in the pentagonal bipyramid coordination environment. Oxygen atoms from two waters hold the axial sites, and four oxygen atoms from two chelated carboxylic groups and one nitrogen atom from one pyridine ring occupy the five planar sites. This novel coordination environment of Mn(II) ion may be due to the smaller steric effect of chelated carboxylic groups. Each 3,5-pdc ligand is in the same coordination mode to bridge three Mn(II) ion centers and lead to two-dimensional layers with water molecules between the layers. Hydrogen bonds, which are generated by these water molecules and carboxylic groups, connect the layers to form a three-dimensional structure.     

Syntheses, Characterization and Crystal Structures of Two-dimensional 4,4''-Bipyridyl Lead Halides,PbI2 (4,4'' -bpy) and PbBr2 (4,4'' -bpy)

Two-dimensional 4,4-bipyridyllead halides, PbI2 (4,4'-bpy) (1) and PbBr2 (4,4'-bpy) (2), were synthesized. The structures were determined by means of X-ray single crystal diffraction.The structure shows a distorted octahedral configuration with six-coordinated central lead atoms. In crystals 1 and 2, the molecules are packed in a two-dimensional network structure through bridging halide atoms and 4,4'-bipyridine ligands between the adjacent lead atoms.     

Syntheses, Characterization and Crystal Structures of Two-dimensional 4,4＇-Bipyridyl Lead Halides, PbI2 （4,4＇-bpy） and PbBr2 （4,4＇-bpy）

Two-dimensional 4,4-bipyridyllead halides, Pbl2 (4,4‘-bpy) (1) and PbBr2 ( 4,4‘-bpy ) (2), were synthesized. The structures were determined by means of X-ray single crystal diffraction. The structure shows a distorted octahedral configuration with six-coordinated central lead atoms. In crystals 1 and 2, the molecules are packed in a two-dimensional network structure through bridging halide atoms and 4,4‘-bipyridine ligands between the adjacent lead atoms.     

Structural Tunability and Diversity of Two-Dimensional Lead Halide Benzenethiolate

Two-dimensional (2D) organic-inorganic hybrid materials are currently of great interest for applications in electronics and optoelectronics. Here, the synthesis and optical properties of a new type of halide-organothiolate-mixed 2D hybrid material, Pb₂X(S-C₆H₅)₃, are reported, in which X is a halide (I, Br, or Cl). Different from conventional lead-based 2D layered materials, these compounds feature unusual five-coordinated lead centers with a stereochemically active electron lone pair on the lead atoms and four-coordinated iodine atoms. The Pb₂X(S-C₆H₅)₃ materials feature an indirect bandgap, strongly emissive long-lived self-trap states, and an extremely large Stokes shift. Interestingly, the optical bandgap of the materials can be tuned through variation of the halides; however, the photoluminescence is less sensitive to the composition and is more likely dominated by lead-sulfur lattice interactions or the lead lone-pair electrons. Our results support that a halide–organothiolate mixed anion hybrid structure offers a unique platform for discovering new exciting 2D electronic materials.     

Chemistry at corners and edges: generation and adsorption of H atoms on the surface of MgO nanocubes

We used UV light to generate site-selective O- hole centers at three-coordinated corner oxygen sites on MgO nanocubes. These highly reactive O- radicals split H2 homolytically and, in the course of this reaction, become hydroxylated and produce hydrogen atoms. The hydrogen atoms adsorb predominantly at cube edges and dissociate into surface-trapped electrons and protons. We propose that the experimentally observed (H+)(e-) centers are formed adjacent to the hydroxyl groups generated in the homolytic splitting process and can be defined as (H+)3C...(e-)(H+)NC centers where 3C and NC refer to the coordination numbers of the corresponding hydroxylated oxygen sites. Our ab initio embedded cluster calculations reveal that the electronic properties of (H+)3C...(e-)(H+)4C centers situated along MgO nanocube edges are consistent with both the electron-paramagnetic-resonance signal parameters and the reported optical-absorption properties. The transformation of corner O- centers into the (H+)3C...(e-)(H+)NC-type centers prevents their recombination with electronic surface centers and, hence, significantly alters the electronic structure of MgO nanocubes by introducing shallow electron traps.     

Stereoelectronic structure of 3-(trichlorogermyl)propionic acid by the results of <Emphasis Type="Italic">ab initio</Emphasis> calculations

Two structures of the 3-(trichlorogermyl)propionic acid molecule and its dimer were calculated by the RHF/6-31G(d) method with the full geometry optimization. The structure with pentacoordinated germanium atom is by 4.71 kcal mol^?1 more favorable than that with tetracoordinated germanium. The strength of coordination bond in the first structure increases with the absolute values of charge on the Ge and O coordination centers. The relatively small values of these charges result in a weak coordination bond. In the first structure, this bond is weaker than in the dimer, since the Ge…O distance in it (3.016 Å) is larger than in the latter (2.898 Å). This bond is formed due to the rapproachment of the Ge and O coordination centers at their electrostatic interaction. This provides the transfer of electron density from the atoms of the donor fragment of the molecule to the atoms of the germanium coordination polyhedron. The coordination centers serve as the conductors for the electron density transfer.     

Electronic properties of Strandberg anions: A DFT study of [X2Mo5O23]n−, (X = PV,SVI, AsV,SeVI), and [(RP)2Mo5O21]4− (R = H,CH3, C2H5)

The electronic properties of the anions mentioned in the title polyanions were calculated by means of Density Functional Theory (DFT). The redox properties and the basicity of the external oxygen sites of those polyanions were analyzed. The results show that the redox properties of Strandberg anions depend on the nature of heteroatom X. The organic group bonded to the heteroatom modifies the redox property of the cluster. The oxygen basicities of the polyanions were analyzed by virtue of molecular electrostatic potential (MEP). The MEP distribution suggests that the most basic centers are triple‐bridging oxygen atoms, one of which is shared with two metal atoms and one heteroatom X in [P₂Mo₅O₂₃]^6? and [As₂Mo₅O₂₃]^6?. In [(RP)₂Mo₅O₂₁]^4?, the triple‐bridging oxygen atoms and the double‐bridging oxygen atoms bonded to two Mo atoms identified as the most basic centers. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Studies on the Electron Transfer Centers of Amino Oxidase Immobilized on Self-Asembly Monolayer Using Electrochemical Methods

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The effect of the disperse composition of a titanium catalyst on the kinetic heterogeneity of isoprene polymerization centers

The kinetic heterogeneity of centers of isoprene polymerization on fractions of titanium catalyst particles is studied. It is found that the isoprene polymerization with a catalyst consisted of particles 0.03–0.14 μm in diameter involves centers of one type with low reactivity. On catalyst particles 0.15–4.50 μm in diameter, the active centers of polymerization of two types with high reactivity may be formed. The addition of modifiers, a reduced temperature of catalyst formation, and the hydrodynamic effect result in the appearance of a narrow fraction of particles 0.15–0.18 μm in diameter with one type of surface active center that generates high-molecular-mass cis-1,4-polyisoprene. The obtained results are in accordance with the concept of particles 0.15–4.50 μm in diameter as aggregates of the elementary crystallites of β-TiCl₃ connected via additional Al-Cl bonds to surface titanium atoms. At the same time, catalyst particles 0.03–0.14 μm in diameter are formed by the minimum number of elementary crystallites, where titanium atoms are bound to a smaller number of chlorine atoms.     

Magnetic properties of copper(II) complexes with some hydrazones of substituted salicylaldehydes

A series of novel mono-and binuclear copper(II) complexes with substituted salicylaldehyde acylhydrazones H₂L of the formula CuL · xH2O (x = 0 and 1) and [Cu(HL)](ClO₄)(CH₃OH) were synthesized. The isolated dimeric complexes of copper acetate were found to exist as isomers with different bridging atoms. In dimers showing a superexchange between the paramagnetic centers through bridging phenoxide O atoms, the antiferromagnetic exchange couplings were much stronger than those in complexes with bridging O atoms of the a-oxyazine fragment.