Number Of Coordination Sites Research Articles

Tricyanoborane-Functionalized Anionic N-Heterocyclic Carbenes: Adjustment of Charge and Stereo-Electronic Properties.

The 1‐methyl‐3‐(tricyanoborane)imidazolin‐2‐ylidenate anion (2) was obtained in high yield by deprotonation of the B(CN)3‐methylimidazole adduct 1. Regarding charge and stereo‐electronic properties, anion 2 closes the gap between well‐known neutral NHCs and the ditopic dianionic NHC, the 1,3‐bis(tricyanoborane)imidazolin‐2‐ylidenate dianion (IIb). The influence of the number of N‐bonded tricyanoborane moieties on the σ‐donating and π‐accepting properties of NHCs was assessed by quantum chemical calculations and verified by experimental data on 2, IIb, and 1,3‐dimethylimidazolin‐2‐ylidene (IMe, IIa). Therefore NHC 2, which acts as a ditopic ligand via the carbene center and the cyano groups, was reacted with alkyl iodides, selenium, and [Ni(CO)4] yielding alkylated imidazoles 3 and 4, the anionic selenium adduct 5, and the anionic nickel tricarbonyl complex 8, respectively. The results of this study prove that charge, number of coordination sites, buried volume (%V bur) and σ‐donor and π‐acceptor abilities of NHCs can be effectively fine‐tuned via the number of tricyanoborane substituents.

Atomic Scale Characterization of Fluxional Cation Behavior on Nanoparticle Surfaces: Probing Oxygen Vacancy Creation/Annihilation at Surface Sites.

Oxygen vacancy creation and annihilation are key processes in nonstoichiometric oxides such as CeO2. The oxygen vacancy creation and annihilation rates on an oxide's surface partly govern its ability to exchange oxygen with the ambient environment, which is critical for a number of applications including energy technologies, environmental pollutant remediation, and chemical synthesis. Experimental methods to probe and correlate local oxygen vacancy reaction rates with atomic-level structural heterogeneities would provide significant information for the rational design and control of surface functionality; however, such methods have been unavailable to date. Here, we characterize picoscale fluxional behavior in cations using time-resolved in situ aberration-corrected transmission electron microscopy to locate atomic-level variations in oxygen vacancy creation and annihilation rates on oxide nanoparticle surfaces. Low coordination number sites such as steps and edges, as well as locally strained sites, exhibited the greatest number of cation displacements, implying enhanced surface oxygen vacancy activity at these sites. The approach has potential applications to a much wider class of materials and catalysis problems involving surface and interfacial transport functionalities.

Coordination dependence of carbon deposition resistance in partial oxidation of methane on Rh catalysts

In this paper, we used the density functional theory (DFT) calculations for analyzing the property of carbon deposition resistance of size-dependence of Rh-clusters supported on anatase for partial oxidation of methane (POM). In addition, we used three-different coordination number of Rh-based models that were Rh1, Rh4 and Rh13. We considered two major routes for the carbon deposition processes which were CH4* dissociation and CHx* oxidation. The apparent energy barrier of CH4* dissociation on Rh1/Ana-OV (2.95 eV) was much higher than that on Rh4/Ana-OV (1.84 eV) and Rh13/Ana-OV (1.85 eV), whereas, the energy barrier of the reaction (CH2* + O* → CH2O*) enlarged with the growing coordination number: Rh1/Ana-OV (0.58 eV) < Rh4/Ana-OV (0.71 eV) < Rh13/Ana-OV (2.23 eV). Thus, the Rh-catalysts with low coordination number, Rh1/Ana-OV, could be the one with the strong resistance for the carbon deposition and high catalytic activity. In fact, the results of the energetic span model analysis indicated that the catalytic activity of Rh1/Ana-OV (3.32 × 106 s−1) was better than that of Rh4/Ana-OV (3.18 × 104 s−1) and Rh13/Ana-OV (1.65 × 10-2 s−1) at the temperature of 973 K. The possible reasons why the single Rh catalyst (Rh1/Ana-OV) has the strong carbon deposition resistance property can be explained as the dissociation of CHx* often requires more active sites because its products such as CH* or C*, which are much more active, and are usually adsorbed in threefold hollow site or more. However CHx* oxidation reaction needs relatively less active sites because the products like CH2O* is a more saturation species and often adsorbed on top or bridge sites. Moreover, the low coordination number sites can meet such kind of requirement. Thus, single-atom catalysts are the candidates to reduce the carbon deposition for POM processes: its low coordination number contributes to the CHx* oxidation process and hinder the CH4* dissociation except the first C–H bond scission step.

Coordination mode engineering in stacked-nanosheet metal\u2013organic frameworks to enhance catalytic reactivity and structural robustness

Optimising the supported modes of atom or ion dispersal onto substrates, to synchronously integrate high reactivity and robust stability in catalytic conversion, is an important yet challenging area of research. Here, theoretical calculations first show that three-coordinated copper (Cu) sites have higher activity than four-, two- and one-coordinated sites. A site-selective etching method is then introduced to prepare a stacked-nanosheet metal–organic framework (MOF, CASFZU-1)-based catalyst with precisely controlled coordination number sites on its surface. The turnover frequency value of CASFZU-1 with three-coordinated Cu sites, for cycloaddition reaction of CO2 with epoxides, greatly exceed those of other catalysts reported to date. Five successive catalytic cycles reveal the superior stability of CASFZU-1 in the stacked-nanosheet structure. This study could form a basis for the rational design and construction of highly efficient and robust catalysts in the field of single-atom or ion catalysis.

Explaining soil organic matter composition based on associations between OM and polyvalent cations

AbstractSite conditions and soil management determine the content and the composition of soil organic matter (SOM). Organic matter (OM) is characterized by functional groups, which preferentially interact with polyvalent cations and soil minerals. These interactions could perhaps explain the site‐specific composition of bulk SOM and a pyrophosphate‐soluble OM fraction (OM‐PY) using basic soil properties. The objective of this study was to test a simplified model for the interactions between OM and polyvalent cations (i.e., Ca, Mg, Al, Fe, and Mn) by using data from soils from long‐term field experiments. The model considered (1) OM–cation, (2) OM–cation‐mineral, and (3) OM–mineral associations and assumed that the availability of the cation's coordination sites for the interaction with OM depends on these three types of associations. The test was carried out using data (topsoil) from differently fertilized plots from three long‐term field experiments (Halle, Bad Lauchstädt, Rotthalmünster). The composition of SOM and OM‐PY was characterized by the relationship of the ratio of the C=O (i.e., here indicating both carbonylic and carboxylic groups) versus C–O–C absorption band intensities obtained from the Fourier transform infrared (FTIR) spectra with the content of exchangeable, oxalate‐, and dithionite‐extractable polyvalent cations. The assumed associations between the OM and cations and the availability of the coordination sites explained most of the variations in the C=O/C–O–C ratios of the SOM, and fewer variations in the OM‐PY, when using the site‐specific exchangeable and oxalate‐extractable cation contents. The C=O/C–O–C ratios of the OM‐PY were site‐independent for samples from plots that regularly received farmyard manure. The results suggested that a simplified model that considers the polyvalent cation content weighted by the number of coordination sites per cation according to the type of association could be used to improve the explanation of site‐specific differences in the OM composition of arable soils.

Density functional calculation of structural and electronic properties of Tin-xAlx (n =2–8, 13, x=0–n) clusters

Using density-functional theory, a systematic theoretical study on the structural and electronic properties of Tin-xAlx (n = 2–8, 13, x = 0-n) clusters has been performed. Tin-xAlx (n = 2–8, x = 1-n-1) clusters often employ the substitution patterns based on the structures of pure Tin cluster, while all Ti13-xAlx (x = 0–13) clusters adopt the icosahedral motif. The preference of Ti (Al) atom for high coordination number site experiences a transition with size increasing. In the range of n = 5, 6, the preference for high CN site is Ti atom, while for medium size, e.g., n = 13, Al atom. The clusters with around 7–8 atoms might represent a region of transition. The transition originates from the valent transition of Al atom from one to three. Ti2Al2, TiAl4, Ti4Al2, Ti4Al3, and Ti4Al4 clusters are the most stable ones for a given size when comparing with all possible compositions. TiAl4 cluster possesses the extraordinary stability originating from aromaticity.

Systematic syntheses and metalloligand doping of flexible porous coordination polymers composed of a Co(III)-metalloligand.

A series of flexible porous coordination polymers (PCPs) RE-Co, composed of a Co(III)-metalloligand [Co(dcbpy)3](3-) (Co; H2dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) and lanthanide cations (RE(3+) = La(3+), Ce(3+), Pr(3+), Nd(3+), Sm(3+), Eu(3+), Gd(3+), Tb(3+), Er(3+)), was systematically synthesized. X-ray crystallographic analysis revealed that the six carboxylates at the top of each coordination octahedron of Co(III)-metalloligand were commonly bound to RE(3+) cations to form a rock-salt-type porous coordination framework. When RE-Co contains a smaller and heavier RE(3+) cation than Nd(3+), the RE-Co crystallized in the cubic Fm-3m space group, whereas the other three RE-Co with larger RE(3+) crystallized in the lower symmetrical orthorhombic Fddd space group, owing to the asymmetric 10-coordinated bicapped square antiprism structure of the larger RE(3+) cation. Powder X-ray diffraction and vapor-adsorption isotherm measurements revealed that all synthesized RE-Co PCPs show reversible amorphous-crystalline transitions, triggered by water-vapor-adsorption/desorption. This transition behavior strongly depends on the kind of RE(3+); the transition of orthorhombic RE-Co was hardly observed under exposure to CH3OH vapor, but the RE-Co with smaller cations such as Gd(3+) showed the transition under exposure to CH3OH vapors. Further tuning of vapor-adsorption property was examined by doping of Ru(II)-metalloligands, [Ru(dcbpy)3](4-), [Ru(dcbpy)2Cl2](4-), [Ru(dcbpy)(tpy)Cl](-), and [Ru(dcbpy)(dctpy)](3-) (abbreviated as RuA, RuB, RuC, and RuD, respectively; tpy = 2,2':6',2″-terpyridine, H2dctpy = 4,4″-dicarboxy-2,2':6',2″-terpyridine), into the Co(III)-metalloligand site of Gd-Co to form the Ru(II)-doped PCP RuX@Gd-Co (X = A, B, C, or D). Three Ru(II)-metalloligands, RuA, RuB, and RuD dopants, were found to be uniformly incorporated into the Gd-Co framework by replacing the original Co(III)-metalloligand, whereas the doping of RuC failed probably because of the less number of coordination sites. In addition, we found that the RuA doping into the Gd-Co PCP had a large effect on vapor-adsorption due to the electrostatic interaction originating from the negatively charged RuA sites in the framework and the charge-compensating Li(+) cations in the porous channel.

ChemInform Abstract: Pyridine Coordination Chemistry for Molecular Assemblies on Surfaces

AbstractReview: 50 refs.

Pyridine coordination chemistry for molecular assemblies on surfaces.

CONSPECTUS: Since the first description of coordination complexes, many types of metal-ligand interactions have creatively been used in the chemical sciences. The rich coordination chemistry of pyridine-type ligands has contributed significantly to the incorporation of diverse metal ions into functional materials. Here we discuss molecular assemblies (MAs) formed with a variety of pyridine-type compounds and a metal containing cross-linker (e.g., PdCl2(PhCN2)). These MAs are formed using Layer-by-Layer (LbL) deposition from solution that allows for precise fitting of the assembly properties through molecular programming. The position of each component can be controlled by altering the assembly sequence, while the degree of intermolecular interactions can be varied by the level of π-conjugation and the availability of metal coordination sites. By setting the structural parameters (e.g., bond angles, number of coordination sites, geometry) of the ligand, control over MA structure was achieved, resulting in surface-confined metal-organic networks and oligomers. Unlike MAs that are constructed with organic ligands, MAs with polypyridyl complexes of ruthenium, osmium, and cobalt are active participants in their own formation and amplify the growth of the incoming molecular layer. Such a self-propagating behavior for molecular systems is rare, and the mechanism of their formation will be discussed. These exponentially growing MAs are capable of storing metal salts that can be used during the buildup of additional molecular layers. Various parameters influencing the film growth mechanism will be presented, including (i) the number of binding sites and geometry of the organic ligands, (ii) the metal and the structure of the polypyridyl complexes, (iii) the influence of the metal cross-linker (e.g., second or third row transition metals), and (iv) the deposition conditions. By systematic variation of these parameters, switching between linear and exponential growth could be demonstrated for MAs containing structurally well-defined polypyridyl complexes. The porosity of the MAs has been estimated by using electrochemically active probes. Incorporating multiple polypyridyl complexes of osmium and ruthenium into a single assembly give rise to composite materials that exhibit interesting electrochemical and electrochromic properties. These functional composites are especially attractive as they exhibit properties that neither of each metal complex possesses individually. Some of our MAs have very high coloration efficiencies, redox stability, fast responsive times and operate at voltages < 1.5 V. Moreover, their electrochemical properties are dependent on the deposition sequence of the polypyridyl complexes, resulting in MAs that possesses distinctive electron transfer pathways. Finally, some of these MAs are described in terms of their practical applications in electrochromic materials, storage-release chemistry, solar cells, and electron transport properties.

Crystal-bound vs surface-bound thiols on nanocrystals.

The use of thiol ligands as a sulfur source for nanocrystal synthesis has recently come en vogue, as the products are often high quality. A comparative study was performed of dodecanethiol-capped Cu2S prepared with elemental sulfur and thiol sulfur reagents. XPS and TGA-MS provide evidence for differing binding modes of the capping thiols. Under conditions where the thiol acts only as a ligand, the capping thiols are "surface-bound" and bond to surface cations in low coordination number sites. In contrast, when thiols are used as a sulfur source, "crystal-bound" thiols result that sit in high coordination sites and are the terminal S layer of the crystal. A (1)H NMR study shows suppressed surface reactivity and ligand exchange with crystal-bound thiols, which could limit further application of the particles. To address the challenge and opportunity of nonlabile ligands, dodecyl-3-mercaptopropanoate, a molecule possessing both a thiol and an ester, was used as the sulfur source for the synthesis of Cu2S and CuInS2. A postsynthetic base hydrolysis cleaves the ester, leaving a carboxylate corona around the nanocrystals and rendering the particles water-soluble.

Mean-field universality class induced by weak hyperbolic curvatures.

Order-disorder phase transition of the ferromagnetic Ising model is investigated on a series of two-dimensional lattices that have negative Gaussian curvatures. Exceptional lattice sites of coordination number seven are distributed on the triangular lattice, where the typical distance between the nearest exceptional sites is proportional to an integer parameter n. Thus, the corresponding curvature is asymptotically proportional to -n(-2). Spontaneous magnetization and specific heat are calculated by means of the corner transfer matrix renormalization group method. For all the finite n cases, we observe the mean-field-like phase transition. It is confirmed that the entanglement entropy at the transition temperature is linear in (c/6)ln n, where c = 1/2 is the central charge of the Ising model. The fact agrees with the presence of the typical length scale n being proportional to the curvature radius.

Bis[1,3-bis(2,4,6-trimethylphenyl)-2,3-dihydro-1H-imidazol-2-ylidene]dinitrosyl(tetrahydroborato-κ2H,H′)tungsten(0)

In the title paramagnetic 19-electron neutral complex, [W(BH4)(C21H24N2)2(NO)2], the W(0) atom is coordinated by two 1,3-bis­(2,4,6-trimethyl­phen­yl)imidazol-2-yl­idene (IMes) carbene ligands, two NO groups and two H atoms of an η2-tetra­hydro­borate ligand. Depending on the number of coordination sites (n) assigned to the BH4 − ligand, the coordination geometry of the W atom may either be described as approximately trigonal–bipyramidal (n = 1) or as very distorted octa­hedral with the bridging H atoms filling two coordination positions (n = 2). In the latter case, the coplanar NO groups and bridging H atoms (r.m.s. deviation = 0.032 Å) form one octa­hedral plane, with mutually trans-oriented carbene ligands. In the crystal, mol­ecules are connected via C—H⋯O inter­actions.

Ab initio ternary [formula omitted]-phase diagram: The Cr–Mo–Re system

For the first time, the formation enthalpies at 0 K of every ordered configuration of a ternary σ -phase, i.e. 5 3=243 configurations, have been calculated using the electronic density functional theory. The Cr–Mo–Re system has been chosen for the present investigation since the two binary Cr–Re and Mo–Re σ -phase are known to show opposite Re site preference: high coordination number sites in Cr–Re and low coordination number sites in Mo–Re. The ternary Cr–Mo–Re σ -phase diagram has been computed at 0 K. It presents at this temperature several distinct single-phase regions separated by a large number of miscibility gaps. Finite temperature properties have been calculated using only the configurational entropy, in the Bragg–Williams approximation. Only above 800 K, the Gibbs energy surface becomes convex. The occupancies of the inequivalent sites have been computed as a function of composition at several temperatures. Re site preference is shown to change progressively in the ternary field when passing from Cr–Re to Mo–Re binary borders.

First principles calculations of theσ andχ phases in the Mo–Re and W–Re systems

The total energies of all the ordered configurations of theσ andχ phases have been calculated by using first principles methodology in both Mo–Re andW–Re systems. These two complex structures possess 5 and 4 inequivalent sitesgenerating 32 and 16 different ordered configurations, respectively, for a binaryA–B system. The converged total energies of all the fully relaxed structures have been used tocompute the occupancy of the inequivalent sites as a function of compositionand temperature by using the Bragg–Williams approximation in the completecomposition range. It is shown that the configurational entropy stabilizes theσ andχ phases in the Mo–Re and W–Re systems. The results evidence the preference of Re forlower coordination number site occupancy and are in very good agreement with theexperimental measurements. Tentative ab initio phase diagrams have also been drawn.

Application of metal coordination chemistry to explore and manipulate cell biology.

Our desire to understand how the individual molecules that make up cells organize, interact, and communicate to form living systems has lead to the burgeoning field of chemical biology, an interfacial area of science that combines aspects of chemistry — the study of matter and its transformations, and biology — the study of living things and their interactions with the environment. The defining feature of chemical biology is the use of chemical approaches and small molecules to interrogate or manipulate biology.1,2 These small molecules are synthetic or naturally occurring ones that, for example, bind to DNA to affect protein expression levels, bind to proteins to inhibit their function, interact with lipids to alter membrane integrity, or become fluorescent in response to a metabolic event. Because small molecules can affect biochemical function, there is a clear link between chemical biology and pharmacology and medicine.3 While small molecules are usually implied as being organic compounds,4 inorganic small molecules also have a long history in both biology and medicine. Ancient civilizations used gold and copper for healing purposes, and the modern era of drug discovery was ushered in when arsenic-containing salvarsan was discovered as an anti-syphilis agent to become the world’s first blockbuster drug.5 Inorganic compounds should therefore not be overlooked in the realm of chemical biology, since their distinctive electronic, chemical, and photophysical properties render them particularly useful for a variety of applications.6-8 What are the properties of metal ions that impart utility to biology? Because inorganic elements comprise the bulk of the periodic table, the diversity of these properties is likewise broad and has been thoroughly covered by several books in the field of bioinorganic chemistry.9-11 A brief summary of the general chemical properties of metals is given below. Charge. Metal ions are positively charged in aqueous solution, but that charge can be manipulated depending on the coordination environment so that a metal complexed by ligands can be cationic, anionic, or neutral. Interactions with ligands. Metal ions bind to ligands (both organic and inorganic) via interactions that are often strong and selective. The ligands impart their own functionality and can tune properties of the overall complex that are unique from those of the individual ligand or metal. The thermodynamic and kinetic properties of metal—ligand interactions influence ligand exchange reactions. Structure and bonding. Metal—ligand complexes span a range of coordination geometries that give them unique shapes compared to organic molecules. The bond lengths, bond angles, and number of coordination sites can vary depending on the metal and its oxidation state. Lewis acid character. Metal ions with high electron affinity can significantly polarize groups that are coordinated to them, facilitating hydrolysis reactions. Partially filled d-shell. For the transition metals, the variable number of electrons in the d-shell orbitals (or f-shell for lanthanides) imparts interesting electronic and magnetic properties to transition metal complexes. Redox activity. Coupled with the variability of electrons in the d-shell is the ability for many transition metals to undergo 1-electron oxidation and reduction reactions. Biology has taken advantage of these chemical properties of metals to perform several functional roles, which are summarized in Table 1. This is by no means an exhaustive list, but rather a primer to highlight important themes. Some metal ions, particularly the alkali and alkaline earth metals, are stable in aqueous solution as cations, making Na+, K+ and Ca2+ ideal for maintaining charge balance and electrical conductivity.10 On the other hand, the distinct architectures accessible via metal—ligand bonding interactions impart important structural roles to metal ions that encompass both macroscopic structural stabilization, as in biomineralized tissues,12 as well as molecular structural stabilization, as in proteins and nucleic acids that are stabilized in a preferred fold by metal ions.13-16 Metal—ligand bonding is also significant in its reversibility. For example, Nature takes advantage of reversible binding of metal ions like Ca2+ and Zn2+ to proteins or other storage repositories in order to propagate various biochemical signals.13,17 Metal ions themselves can be their own signal to adjust DNA transcription, as in the case of metalloregulatory proteins.18,19 Reversible metal—ligand coordination is also exploited to bind and release molecules to and from a metal center, a prime example being O2 binding and release from hemoglobin. Table 1 Functional roles of inorganic elements found in biology, with selected representative examples.

Comparison of ionization behaviors of ring and linear carbohydrates in MALDI-TOFMS

Ionization efficiencies of cyclodextrins and their linear compounds in matrix-assisted laser desorption and ionisation (MALDI) analysis were compared, and differences in the ionization efficiencies of α- and β-cyclodextrins were also studied. The mass spectra showed a series of the [M+cation] + ions but not the [M+H] + ions. Alkali metal salts of Li +, Na +, K +, and Cs + were used as the cationizing agents to enhance the ionization efficiency. Relative ion intensities of the ring compounds (α- and β-cyclodextrins) were much larger than those of the linear ones (maltohexaose and maltoheptaose), and the difference showed an increasing trend with the size of the alkali metal cation. β-Cyclodextrin had higher ionization efficiency than α-cyclodextrin and the difference increased by increasing the size of the alkali metal cation. It was also found that the ionization efficiency was affected by the counter anion of the salt. The higher ionization efficiencies of cyclodextrins were explained with the number of coordination sites and the binding energies.

Ab Initio Molecular Dynamics Study of Methanol Adsorption on Copper Clusters

The preferential structures of small copper clusters Cun (n=2-9) and the adsorption of methanol molecules on these clusters are examined with first principles, molecular dynamics simulations. The results show that the copper clusters undergo systematic changes in bond length and bond order associated with altering their preferential structures from one-dimensional structures, to two-dimensional and three-dimensional structures. The results also indicate that low coordination number sites on the copper clusters are both the most favorable for methanol adsorption and have the greatest localization of electronic charge. The simulations predict that charge transfer between the neutral copper clusters and the incident methanol molecules is a key process by which adsorption is stabilized. Importantly, the changes in the dimensionality of the copper clusters do not significantly influence methanol adsorption.

Effects of ionic strength on the coordination of Eu(III) and Cm(III) to a Gram-negative bacterium,Paracoccus denitrificans

We studied the effect of ionic strength on the interactions of Europium(III) and Curium(III) with a Gram-negative bacteriumParacoccus denitrificans. Bacterial cells grown in 0.5-, 3.5-, and 5.0% NaCl were used in adsorption experiments and laser experiments that were performed at the same ionic strengths as those in the original growth media. The distribution ratio (logKd) for Eu(III) and Cm(III) was determined at pHs 3−5. To elucidate the coordination environment of Eu(III) adsorbed onP. denitrificans, we estimated the number of water molecules in the inner sphere and strength of the ligand field by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pHs 4−6. The logKdof Eu(III) and Cm(III) increased with an increase of pH at all ionic strengths because there was less competition for ligands in cells with H+at higher pHs, wherein less H+was present in solution: cation adsorption generally occurs through an exchange with H+on the functional groups of coordination sites. No significant differences were observed in the logKdof Eu(III) and Cm(III) at each pH in 0.5-, 3.5-, and 5.0% NaCl solutions, though competition for ligands with Na+would be expected to increase at higher NaCl concentrations. The logKdof Eu(III) was almost equivalent to that of Cm(III) under all the experimental conditions. TRLFS showed that the coordination environments of Eu(III) did not differ from each other at 0.5-, 3.5-, and 5.0% NaCl at pHs 4−6. TRLFS also showed that the characteristic of the coordination environment of Eu(III) onP. denitrificanswas similar to that on a halophile,Nesterenkonia halobia, while it significantly differed from that on a non-halophile,Pseudomonas putida. These findings indicate that the number of coordination sites for Eu(III) onP. denitrificans, whose cell surface may have similar structures to that of halophiles, increased with increasing ionic strength, though their structure remained unchanged.

Wide Frequency Range Dispersion and Relaxations in Ceramics of the K6Li4Ta10O30–Pb5Ta10O30 System

Dielectric and impedance measurements were performed on ceramics with composition Pb5xK6(1—x)Li4(1—x)Ta10O30, in wide frequency and temperature ranges, 101 to 109 Hz and 100 to 650 K, respectively. At low frequency and low temperature, a relaxor behavior is found for compositions corresponding to x > 0.70. Moreover, two other relaxations are observed: an ionic conduction relaxation at low frequency and high temperature and a network relaxation at high frequency and low temperature. The former is explained on the basis of the motion of Li+ cations in sites of coordination number 9 and the latter on the basis of correlation chains.

Diastereoselective epoxidation of allylic alcohols with hydrogen peroxide catalyzed by titanium-containing zeolites or methyltrioxorhenium versus stoichiometric oxidation with dimethyldioxirane: Clues on the active species in the zeolite lattice

Chiral, acyclic allylic alcohols 1 are epoxidized chemoselectively to the epoxy alcohols 2 by hydrogen peroxide, catalyzed by titanium-containing zeolites (TS-1, Ti-beta). For substrates with 1,3-allylic (A 1,3) strain, a high diastereoselectivity is observed with preference for the threo isomer, while derivatives with 1,2-allylic (A 1,2) strain or no allylic strain give a low threo diastereomeric excess. Comparison of the diastereomeric ratios of the titanium-containing zeolites with those for meta-chloroperbenzoic acid shows a good correspondence which suggests that the active species for the oxygen transfer in the epoxidations for zeolites is peracid- rather than peroxo-type. Comparison of the diastereomeric ratios achieved with the three-membered ring peroxide oxidants dimethyldioxirane and MTO/UHP (metal peroxo complex) disfavor the peroxo species since significantly lower thrio diastereoselectivities for substrates with 1,3-allylic strain were obtained. Direct coordination of the allylic alcohol through a metal alcoholate bond is unlikely because of the different diastereomeric ratios obtained for the heterogeneous and homogeneous titanium species with allylic alcohols that possess 1,2-allylic strain. Moreover, the number of coordination sites at the titanium atom in the zeolite framework is limited for steric reasons and the constrained space around the active center in the zeolite lattice presents severe geometrical problems for the stereoelectronically controlled linear S N2-type alignment of the oxygen donor (metal-activated peroxide bond) and the acceptor (metal-alcoholate-bonded substrate) during the epoxidation process.