How a Redox‐Innocent Metal Promotes the Formal Reductive Elimination of Biphenyl Using Redox‐Active Ligands

One of the most compelling strategies for utilizing redox‐active ligands is to perform redox events at the ligands to avoid accessing prohibitively high energy oxidation states at the metal center. This has been demonstrated experimentally in many systems, yet there is little understanding of the fundamental electronic structures involved with these transformations or how to control them. Here, the reductive elimination of biphenyl from [M(isq)₂Ph₂] (M=Ti, Zr, and Hf and isq=2,4‐di‐tert‐butyl‐6‐tert‐butyliminosemiquinone) was studied computationally. It was found that the metal remains in the +IV oxidation state and all redox chemistry was mediated by the redox‐active ligands. Two types of electron‐transfer mechanisms were identified, an asymmetric unpaired electron transfer (UET) and a symmetric pairwise electron transfer (PET), the former always being lower in energy. The energetic differences between these two mechanisms were explained through simple molecular orbital theory arguments. Despite the metal’s redox‐inactivity, it still has a marked influence on the calculated energetics of the reaction, with the Ti systems being much more reactive than the Zr/Hf systems. This primarily originates from the shorter Ti?Ph bond, which leads to a stronger filled‐filled interaction between these ligands at the reactant state. This greater reactant destabilization leads to the lower activation energies.     

Abiotic reduction of uranium by mackinawite (FeS) biogenerated under sulfate-reducing condition

Sulfate-reducing bacteria and their by-products, such as iron sulfides, are widely distributed in groundwater and sediments, and can affect subsurface aqueous chemistry. Here we show the catalytic reduction of hexavalent uranium by FeS particles, which were largely generated by the activities of Desulfovibrio desulfuricans and D. vulgaris in anaerobic condition. Characterization of FeS particles by X-ray diffraction and high-resolution transmission electron microscopy revealed the presence of mackinawite having thin and flexible platy sheets with 0.5-nm lamellar spacing. This biogenic phase mediated abiotic reduction of U(VI) to U(IV) which was confirmed by UV–Vis absorption spectroscopy. The U conversion occurred through surface catalysis that involved adsorption of aqueous U(VI)–carbonate complexes (predominantly UO₂(CO₃) ₃ ^4? ) onto the mackinawite, but the transformed uranium was then released and remained in suspended form in the solution phase. This surface catalysis and subsequent U(IV) remobilization has not been reported as a pathway to occur under sulfate-reducing conditions. Our results suggest that the iron sulfide solid, which is characteristic of conductive property, is very sensitive and variable depending on the electron supplying and transferring environment, negatively affecting the surface uranium to be strongly stabilized and fixed on the FeS surface.     

Sorption of selenite onto chlorite considering mineral dissolution

In this study we investigated the sorption of selenite (SeO₃ ^2?) onto chlorite as a function of Se(IV) concentration, pH, and ionic strength. The sorption isotherm of Se(IV) onto chlorite was successfully presented by both the Langmuir isotherm and Tempkin equation although the Langmuir isotherm is somewhat better than the Tempkin equation. The sorption of Se(IV) onto chlorite was maintained to be constant at an acidic pH region, while the sorption decreased with an increasing pH at neutral and alkaline pH regions. However, the Se(IV) sorption onto chlorite was independent of the ionic strength of NaClO₄ solution. The amount of Se(IV) sorbed onto chlorite was significantly low compared to those of iron oxides such as apatite, goethite, hematite, and magnetite because of the lower content of Fe. We also investigated the effect of Fe(II) ions dissolved from chlorite on the Se(IV) sorption as a function of contact time. The chemical oxidation states of selenium sorbed onto chlorite surface were identified using X-ray absorption near edge structure (XANES) at the Pohang synchrotron light source. The amount of Fe(II) dissolved was increased by the contact time of 28 days but decreased after 28–56 days although the amount of dissolved Fe(II) ions was significantly small. This decrease of the dissolved Fe(II) may be due to the formation of Fe-oxyhydroxides such as ferrihydrite. The results of XANES measurements also showed that the Se(IV) sorbed onto chlorite was not reduced into Se(0) or Se(-II) even in the presence of Fe(II) ions in the solution because of the low Fe content of the chlorite although the mechanism was not clearly understood.     

Synthesis and Structural Characterization of Hexacoordinate Silicon,Germanium, and Titanium Complexes of the E. coli Siderophore Enterobactin

The E. coli siderophore enterobactin, one of the strongest Fe^III chelators known to date, is also capable of binding Si^IV under physiological conditions. We report on the synthesis and structural characterization of the tris(catecholate) Si^IV–enterobactin complex and its Ge^IV and Ti^IV analogues. Comparative structural analysis, supported by quantum‐chemical calculations, reveals the correlation between the ionic radius and the structural changes in enterobactin upon complexation.     

Acoustic attenuation, phase and group velocities in liquid-filled pipes II: simulation for Spallation Neutron Sources and planetary exploration

This paper uses a finite element method (FEM) to compare predictions of the attenuation and sound speeds of acoustic modes in a fluid-filled pipe with those of the analytical model presented in the first paper in this series. It explains why, when the predictions of the earlier paper were compared with experimental data from a water-filled PMMA pipe, the uncertainties and agreement for attenuation data were worse than those for sound speed data. Having validated the FEM approach in this way, the versatility of FEM is thereafter demonstrated by modeling two practical applications which are beyond the analysis of the earlier paper. These applications model propagation in the mercury-filled steel pipework of the Spallation Neutron Source at the Oak Ridge National Laboratory (Tennessee), and in a long-standing design for acoustic sensors for use on planetary probes. The results show that strong coupling between the fluid and the solid walls means that erroneous interpretations are made of the data if they assume that the sound speed and attenuation in the fluid in the pipe are the same as those that would be measured in an infinite volume of identical fluid, assumptions which are common when such data have previously been interpreted.     

Ab initio SCF calculations of the linear infinite chain of LiH according to the pseudo-lattice method

The pseudo-lattice (PL) method has been reformulated for ab initio self-consistent-field (SCF) calculations. The translational symmetries of infinite systems have been applied to the finite model chain by manipulating all the intramolecular and intermolecular Fock matrices. The nuclear repulsion energy has been corrected accordingly. The method has been tested for the linear chain of lithium hydride under the constraint of equidistance between all neighboring lithium and hydrogen atoms. The calculated results of the infinite chain have been compared with those of finite chains of lithium hydride under the same geometric constraint. The equilibrium geometries, band structures, intermolecular stabilization energies and potential curves have been studied. It is found that the infinite systems cannot be described by considering only first nearest neighbor interactions, and the intermolecular interactions must be considered at least up to third nearest neighbors in order to obtain accurate value of force constant of infinite systems. We can conclude from band structures of infinite chains that the boundary effect of the finite model chain is effectively removed by the PL method.     

Structure-property relationships of copolyamides. I. Thermal properties and crystallization

Six nylon salts [hexamethylenediammonium adipate (6.6), hexamethylenediammonium terephthalate (6.T), hexamethylenediammonium isophthalate (6.I), p-xylylenediammonium adipate (PXD.6), m-xylylenediammonium adipate (MXD.6), and m-xylylenediammonium isophthalate (MXD.I)] were copolymerized with ε-caprolactam. The resulting random copolyamides showed different modes of crystallization as confirmed by the melting temperature depression and the decrease in the isothermal crystallization rate. By selective hydrolysis it was found that the differences in T_m depression and isothermal crystallization rate were due to partial inclusion of comonomers in the crystal lattice. The effect of comonomer structure on the crystallization rate is also discussed.     

Proteomic analysis of Helicobacter pylori cellular proteins fractionated by ammonium sulfate precipitation

Among 1590 ORFs in the Helicobacter pylori genome, >250 have been identified as authentic genes by proteomic analysis. Low-abundance proteins need to be enriched to a minimal amount for MALDI-TOF analysis and salt precipitation has generally been used for protein enrichment. Here, a whole-cell extract of H. pylori strain 26695 was subjected to protein fractionation with stepwise concentrations of ammonium sulfate and the proteins were displayed by 2-DE. The protein spots were quantified using PDQUEST software and identified by peptide fingerprinting. The 2-DE profiles and intensities of individual protein spots differed among the protein fractions. Out of the 98 identified proteins, 61 were found in the stepwise ammonium sulfate fractions but not in the whole-cell extract. Out of these, 37 proteins, including KdsA, were found exclusively in a single fraction. In contrast, GroEL, UreA, UreB, TrxA, NapA, and FldA were ubiquitously present in all fractions. Iron-containing proteins such as NapA, SodB, CeuE, and Pfr were found predominantly in the 100% saturated ammonium sulfate precipitate. Additionally, 29 proteins were newly identified in this study. These data will facilitate the preparation of significant H. pylori proteins, as well as provide information about low-abundance proteins.     

Intermolecular C-H bond activation reactions promoted by transient titanium alkylidynes. Synthesis, reactivity, kinetic, and theoretical studies of the Ti[triple bond]C linkage

The neopentylidene-neopentyl complex (PNP)Ti=CH(t)Bu(CH2(t)Bu) (2; PNP(-) = N[2-P(CHMe2)(2-)4-methylphenyl]2), prepared from the precursor (PNP)Ti[triple bond]CH(t)Bu(OTf) (1) and LiCH2(t)Bu, extrudes neopentane in neat benzene under mild conditions (25 degrees C) to generate the transient titanium alkylidyne, (PNP)Ti[triple bond]C(t)Bu (A), which subsequently undergoes 1,2-CH bond addition of benzene across the Ti[triple bond]C linkage to generate (PNP)Ti=CH(t)Bu(C6H5) (3). Kinetic, mechanistic, and theoretical studies suggest the C-H activation process to obey pseudo-first-order in titanium, the alpha-hydrogen abstraction to be the rate-determining step (KIE for 2/2-d(3) conversion to 3/3-d(3) = 3.9(5) at 40 degrees C) with activation parameters DeltaH = 24(7) kcal/mol and DeltaS = -2(3) cal/mol.K, and the post-rate-determining step to be C-H bond activation of benzene (primary KIE = 1.03(7) at 25 degrees C for the intermolecular C-H activation reaction in C6H6 vs C6D6). A KIE of 1.33(3) at 25 degrees C arose when the intramolecular C-H activation reaction was monitored with 1,3,5-C6H3D3. For the activation of aromatic C-H bonds, however, the formation of the sigma-complex becomes rate-determining via a hypothetical intermediate (PNP)Ti[triple bond]C(t)Bu(C6H5), and C-H bond rupture is promoted in a heterolytic fashion by applying standard Lewis acid/base chemistry. Thermolysis of 3 in C6D6 at 95 degrees C over 48 h generates 3-d(6), thereby implying that 3 can slowly equilibrate with A under elevated temperatures with k = 1.2(2) x 10-5 s(-1), and with activation parameters DeltaH = 31(16) kcal/mol and DeltaS = 3(9) cal/mol x K. At 95 degrees C for one week, the EIE for the 2 --> 3 reaction in 1,3,5-C6H3D3 was found to be 1.36(7). When 1 is alkylated with LiCH2SiMe3 and KCH2Ph, the complexes (PNP)Ti=CHtBu(CH2SiMe3) (4) and (PNP)Ti=CHtBu(CH2Ph) (6) are formed, respectively, along with their corresponding tautomers (PNP)Ti=CHSiMe3(CH2tBu) (5) and (PNP)Ti=CHPh(CH2tBu) (7). By means of similar alkylations of (PNP)Ti=CHSiMe3(OTf) (8), the degenerate complex (PNP)Ti=CHSiMe3(CH2SiMe3) (9) or the non-degenerate alkylidene-alkyl complex (PNP)Ti=CHPh(CH2SiMe3) (11) can also be obtained, the latter of which results from a tautomerization process. Compounds 4/5 and 9, or 6/7 and 11, also activate benzene to afford (PNP)Ti=CHR(C6H5) (R = SiMe3 (10), Ph (12)). Substrates such as FC6H5, 1,2-F2C6H4, and 1,4-F2C6H4 react at the aryl C-H bond with intermediate A, in some cases regioselectively, to form the neopentylidene-aryl derivatives (PNP)Ti=CHtBu(aryl). Intermediate A can also perform stepwise alkylidene-alkyl metatheses with 1,3,5-Me3C6H3, SiMe4, 1,2-bis(trimethylsilyl)alkyne, and bis(trimethylsilyl)ether to afford the titanium alkylidene-alkyls (PNP)Ti=CHR(R') (R = 3,5-Me2C6H2, R' = CH2-3,5-Me2C6H2; R = SiMe3, R' = CH2SiMe3; R = SiMe2CCSiMe3, R' = CH2SiMe2CCSiMe3; R = SiMe2OSiMe3, R' = CH2SiMe2OSiMe3).     

Facile one-step synthesis of Ru doped NiCoP nanoparticles as highly efficient electrocatalysts for oxygen evolution reaction

Transition metal phosphides (TMPs) as ever-evolving electrocatalytic materials have attracted increasing attention in water splitting reactions owing to their cost-effective, highly active and stable catalytic properties. This work presents a facile synthetic route to NiCoP nanoparticles with Ru dopants which function as highly efficient electrocatalysts for oxygen evolution reaction (OER) in alkaline media. The Ru dopants induced a high content of Ni and Co vacancies in NiCoP nanoparticles, and the more defective Ru doped NiCoP phase than undoped NiCoP ones led to a greater number of catalytically active sites and improved electrical conductivity after undergoing electrochemical activation. The Ru doped NiCoP catalyst exhibited high OER catalytic performance in alkaline media with a low overpotential of 281 mV at 10 mA cm⁻² and a Tafel slope of 42.7 mV dec⁻¹.