Secondary Interactions or Ligand Scrambling? Subtle Steric Effects Govern the Iridium(I) Coordination Chemistry of Phosphoramidite Ligands

The like and unlike isomers of phosphoramidite (P*) ligands are found to react differently with iridium(I), which is a key to explaining the apparently inconsistent results obtained by us and other research groups in a variety of catalytic reactions. Thus, the unlike diastereoisomer (aR,S,S)‐[IrCl(cod)( 1 a )] ( 2 a ; cod=1,5‐cyclooctadiene, 1 a =(aR,S,S)‐(1,1′‐binaphthalene)‐2,2′‐diyl bis(1‐phenylethyl)phosphoramidite) forms, upon chloride abstraction, the monosubstituted complex (aR,S,S)‐[Ir(cod)(1,2‐η‐ 1 a ,κP)]⁺ ( 3 a ), which contains a chelating P* ligand that features an η² interaction between a dangling phenyl group and iridium. Under analogous conditions, the like analogue (aR,R,R)‐ 1 a′ gives the disubstituted species (aR,R,R)‐[Ir(cod)( 1 a′ ,κP)₂]⁺ ( 4 a′ ) with monodentate P* ligands. The structure of 3 a was assessed by a combination of X‐ray and NMR spectroscopic studies, which indicate that it is the configuration of the binaphthol moiety (and not that of the dangling benzyl N groups) that determines the configuration of the complex. The effect of the relative configuration of the P* ligand on its iridium(I) coordination chemistry is discussed in the context of our preliminary catalytic results and of apparently random results obtained by other groups in the iridium(I)‐catalyzed asymmetric allylic alkylation of allylic acetates and in rhodium(I)‐catalyzed asymmetric cycloaddition reactions. Further studies with the unlike ligand (aS,R,R)‐(1,1′‐binaphthalene)‐2,2′‐diyl bis{[1‐(1‐naphthalene‐1‐yl)ethyl]phosphoramidite} ( 1 b ) showed a yet different coordination mode, that is, the η⁴‐arene–metal interaction in (aS,R,R)‐[Ir(cod)(1,2,3,4‐η‐ 1 b ,κP)]⁺ ( 3 b ).     

Biofuel Combustion Chemistry: From Ethanol to Biodiesel

Biofuels, such as bio‐ethanol, bio‐butanol, and biodiesel, are of increasing interest as alternatives to petroleum‐based transportation fuels because they offer the long‐term promise of fuel‐source regenerability and reduced climatic impact. Current discussions emphasize the processes to make such alternative fuels and fuel additives, the compatibility of these substances with current fuel‐delivery infrastructure and engine performance, and the competition between biofuel and food production. However, the combustion chemistry of the compounds that constitute typical biofuels, including alcohols, ethers, and esters, has not received similar public attention. Herein we highlight some characteristic aspects of the chemical pathways in the combustion of prototypical representatives of potential biofuels. The discussion focuses on the decomposition and oxidation mechanisms and the formation of undesired, harmful, or toxic emissions, with an emphasis on transportation fuels. New insights into the vastly diverse and complex chemical reaction networks of biofuel combustion are enabled by recent experimental investigations and complementary combustion modeling. Understanding key elements of this chemistry is an important step towards the intelligent selection of next‐generation alternative fuels.     

Stereochemistry of lactide polymerization with chiral catalysts: new opportunities for stereocontrol using polymer exchange mechanisms

The synthesis of chiral aluminum and yttrium alkoxides and their application for lactide polymerization are reported. The complexes (SalBinap)MOR [4, M = Al, R = (i)Pr; 5, M = Y, R = (CH(2))(2)NMe(2)] are synthesized by reacting the ligand (SalBinap)H(2) [2,2'-[(1,1'-binaphthalene)-2,2'-diylbis(nitrilomethylidyne)]bisphenol] with the appropriate metal trisalkoxide. While enantiomerically pure yttrium complex 5 did not effect stereocontrol in the polymerization of either meso- or rac-lactide, homochiral 4 was found to exhibit excellent stereocontrol in a range of lactide polymerizations. Enantiomerically pure 4 polymerizes meso-lactide to syndiotactic poly(lactic acid) (PLA), while rac-4 polymerizes meso- and rac-lactide to heterotactic and isotactic stereoblock PLA, respectively. On the basis of the absolute stereochemistry of ring-opening of meso-lactide using (R)-4, a polymer exchange mechanism is proposed to account for the PLA microstructures resulting from rac-4.     

A new class of hybrid materials via salt inclusion: novel copper(II) arsenates Na(5)ACu(4)(AsO(4))(4)Cl(2) (A = Rb,Cs) composed of alternating covalent and ionic lattices

Two novel copper(II) arsenates Na5ACu4(AsO4)4Cl2 (A = Rb, Cs) were synthesized by conventional solid-state methods using reactive molten salt media. These compounds are isostructural and crystallize in an orthorhombic lattice (Fmmm, No. 69; Z = 8). The cell constants are a = 14.632(3) A, b = 18.872(2) A, c = 14.445(3) A, V = 3989(1) A3, for A = Rb; a = 14.638(3) A, b = 18.990(4) A, c = 14.418(3) A, V = 4008(1) A3, for A = Cs. Single-crystal structure studies reveal a new composite framework consisting of alternating covalent and ionic lattices. The covalent lattice contains highly oriented oligomeric mu-oxo [Cu4O12]16- tetrameric units with a cyclo-S8-like Cu4O4 magnetic core that resembles the building block of layered cuprates. The ionic slab consists of a novel framework of mixed alkali metal chloride lattice and rarely seen Na6O8 clusters. Similar to organic-inorganic hybrid materials, the title compounds present a new class of host-guest chemistry via salt inclusion reactions.     

Local dense structural heterogeneities in liquid water from ambient to 300 MPa pressure: evidence for multiple liquid-liquid transitions.

Difference and double-difference near-infrared DO-D and HO-H stretching overtone (2nuOD and 2nuOH) spectroscopy and a rigorous (physically substantiated) band deconvolution technique were applied to reveal three different kinds of inherent (interstitial) structures of liquid water, which determine its high density (compared to ice lh under ambient conditions), its compressibility (under hydrostatic pressure, up to 300MPa), and its high fragility (manifested under temperature variation). Our data processing allowed the rigorous discrimination of up to six vibrational components. On the basis of an extensive comparative analysis combined with available structural data (X-ray and neutron scattering) and molecular dynamics (MD) simulations for liquid water, as well as with experimental and computed data for small non-tetrahedrally arranged water clusters, the major four components could be ascribed to: i) The basic lh icelike substructure; ii) the temperature-dependent remote interstitial "defects" due to tetrahedral displacements (primarily responsible for transport properties); iii) the interstitial "defects" most probably arranged in quasiplanar noncyclic tetramers (totally absent in the ice structure); and iv) the interstitial "defects" formed with increasing pressure, probably arranged in cubic water octamers and composed of two pairs of noncyclic and cyclic tetramer fragments. The latter structures include, essentially, bent hydrogen bonds stabilized by resonance effects.     

Determination of the Molar-Mass Averages of Random Poly(aspartate-co-lactide) Copolymers by Tuning the Ionic Strength of the Solvent

Water-soluble sodium poly(aspartate-co-lactide) (PALNa) copolymers with a molar ratio of aspartate-to-lactide units equal to 1:0.6, 1:1.0 and 1:1.5 were studied using NMR spectroscopy to determine the composition as well as SEC-MALS and static light-scattering measurements to determine the molar-mass characteristics of the copolymers. In the copolymer aqueous solutions, high-molar-mass species were detected, most probably due to the incomplete dissolution of the samples. The molar-mass averages determined in water with added simple electrolyte, i.e., NaCl, were much lower than the values determined in pure water. The concentration of the salt, which allows dissolution on a molecular level, and the separation predominantly according to a size-exclusion mechanism depend on the chemical composition of the PALNa copolymers. The optimal mobile phase for the PALNa-1/0.6 and the PALNa-1/1.0 copolymers was 0.1 M NaCl at pH 9, and for the PALNa-1/1.5 copolymer with a higher content of lactide units it was 0.05 M NaCl at pH 9. The molar-mass averages of the PALNa-1/1.0 copolymer, determined by SEC-MALS and static light-scattering measurements, were comparable.

     

Two-electron-transfer redox systems,part 7: two-step electrochemical oxidation of the boron subhalide cluster dianions B6X 2- 6 (X = Cl,Br, I)

Boron subhalide cluster dianions B6X 2- 6 (X = Cl, Br, I) are electrochemically oxidized in two steps. According to cyclic voltammograms, the first step is chemically reversible and yields the corresponding radical anions B6X .- 6. The electron transfer is nearly diffusion controlled. The second, slower electron-transfer step leads to a species which we assume to be the hitherto not yet described neutral compounds B6X 2- 6. The voltammograms indicate a coupled fast catalytic reaction, producing the radical anions in a reduction by an electrolyte component. Computer simulations of the cyclic voltammograms reveal mechanistic details of the redox reactions, as well as quantitative values for formal potentials, rate constants, and diffusion coefficients. The results are compared to other BnXn redox systems.     

Selective Template Removal by Thermal Depolymerization to Obtain Mesostructured Molybdenum Oxycarbide

The carbon content of mesostructured organic‐inorganic hybrid material of a cylindrical block copolymer template of poly(2‐vinylpyridine)‐block‐poly(allyl methacrylate) (P2VP‐b‐PAMA) and ammonium paramolybdate (APM) could be reduced by thermal depolymerization. By calcination in vacuo at 320 °C the PAMA core can be completely removed while the remaining P2VP brush preserves the mesostructure. The P2VP‐APM composite can then be carburized in‐situ to MoO_xC_y in a second pyrolysis step without any additional carbon source but P2VP. The molybdenum oxycarbide nanotubes obtained, form hierarchically porous non‐woven structures, which were tested as catalyst in the decomposition of NH₃. They proved to be catalytically active at temperatures above 450 °C. The activation energy was estimated from an Arrhenius Plot to be 127 kJ · mol^–1.     

High-precision measurements of seawater Pb isotope compositions by double spike thermal ionization mass spectrometry

A new method for the determination of seawater Pb isotope compositions and concentrations was developed, which combines and optimizes previously published protocols for the separation and isotopic analysis of this element. For isotopic analysis, the procedure involves initial separation of Pb from 1 to 2 L of seawater by co-precipitation with Mg hydroxide and further purification by a two stage anion exchange procedure. The Pb isotope measurements are subsequently carried out by thermal ionization mass spectrometry using a ²⁰⁷Pb–²⁰⁴Pb double spike for correction of instrumental mass fractionation. These methods are associated with a total procedural Pb blank of 28 ± 21 pg (1sd) and typical Pb recoveries of 40–60%. The Pb concentrations are determined by isotope dilution (ID) on 50 mL of seawater, using a simplified version of above methods. Analyses of multiple aliquots of six seawater samples yield a reproducibility of about ±1 to ±10% (1sd) for Pb concentrations of between 7 and 50 pmol/kg, where precision was primarily limited by the uncertainty of the blank correction (12 ± 4 pg; 1sd). For the Pb isotope analyses, typical reproducibilities (±2sd) of 700–1500 ppm and 1000–2000 ppm were achieved for ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb, respectively. These results are superior to literature data that were obtained using plasma source mass spectrometry and they are at least a factor of five more precise for ratios involving the minor ²⁰⁴Pb isotope. Both Pb concentration and isotope data, furthermore, show good agreement with published results for two seawater intercomparison samples of the GEOTRACES program. Finally, the new methods were applied to a seawater depth profile from the eastern South Atlantic. Both Pb contents and isotope compositions display a smooth evolution with depth, and no obvious outliers. Compared to previous Pb isotope data for seawater, the ²⁰⁶Pb/²⁰⁴Pb ratios are well correlated with ²⁰⁷Pb/²⁰⁶Pb, underlining the significant improvement achieved in the measurement of the minor ²⁰⁴Pb isotope.