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Electrodeposited manganese oxide films are promising catalysts for promoting the oxygen evolution reaction (OER), especially in acidic solutions. The activity of these catalysts is known to be enhanced by the introduction of Mn3+ We present in situ electrochemical and X-ray absorption spectroscopic studies, which reveal that Mn3+ may be introduced into MnO2 by an electrochemically induced comproportionation reaction with Mn2+ and that Mn3+ persists in OER active films. Extended X-ray absorption fine structure (EXAFS) spectra of the Mn3+-activated films indicate a decrease in the Mn-O coordination number, and Raman microspectroscopy reveals the presence of distorted Mn-O environments. Computational studies show that Mn3+ is kinetically trapped in tetrahedral sites and in a fully oxidized structure, consistent with the reduction of coordination number observed in EXAFS. Although in a reduced state, computation shows that Mn3+ states are stabilized relative to those of oxygen and that the highest occupied molecular orbital (HOMO) is thus dominated by oxygen states. Furthermore, the Mn3+(Td) induces local strain on the oxide sublattice as observed in Raman spectra and results in a reduced gap between the HOMO and the lowest unoccupied molecular orbital (LUMO). The confluence of a reduced HOMO-LUMO gap and oxygen-based HOMO results in the facilitation of OER on the application of anodic potentials to the δ-MnO2 polymorph incorporating Mn3+ ions.

We report on the theoretical prediction and experimental realization of new ternary zinc molybdenum nitride compounds. We used theory to identify previously unknown ternary compounds in the Zn-Mo-N systems, Zn3MoN4 and ZnMoN2, and to analyze their bonding environment. Experiments show that Zn-Mo-N alloys can form in broad composition range from Zn3MoN4 to ZnMoN2 in the wurtzite-derived structure, accommodating very large off-stoichiometry. Interestingly, the measured wurtzite-derived structure of the alloys is metastable for the ZnMoN2 stoichiometry, in contrast to the Zn3MoN4 stoichiometry, where ordered wurtzite is predicted to be the ground state. The formation of Zn3MoN4-ZnMoN2 alloy with wurtzite-derived crystal structure is enabled by the concomitant ability of Mo to change oxidation state from +VI in Zn3MoN4 to +IV in ZnMoN2, and the capability of Zn to contribute to the bonding states of both compounds, an effect that we define as "redox-mediated stabilization". The stabilization of Mo in both the +VI and +IV oxidation states is due to the intermediate electronegativity of Zn, which enables significant polar covalent bonding in both Zn3MoN4 and ZnMoN2 compounds. The smooth change in the Mo oxidation state between Zn3MoN4 and ZnMoN2 stoichiometries leads to a continuous change in optoelectronic properties-from resistive and semitransparent Zn3MoN4 to conductive and absorptive ZnMoN2. The reported redox-mediated stabilization in zinc molybdenum nitrides suggests there might be many undiscovered ternary compounds with one metal having an intermediate electronegativity, enabling significant covalent bonding, and another metal capable of accommodating multiple oxidation states, enabling stoichiometric flexibility.

The selective epoxidation of olefinic alcohols with t-BuOOH in the presence of vanadium catalysts proceeds in liquid carbon dioxide with high conversions and selectivities. Rates measured in liquid CO2 for the oxovanadium(V) triisopropoxide catalyzed epoxidation of allylic and homoallylic alcohols using tert-butyl hydroperoxide are comparable to those measured in methylene chloride, toluene, and n-hexane. The reactivity of the vanadium(IV) bis(acetylacetonato) oxide catalyst in liquid CO2 was found to be substantially lower than in organic solvents, presumably due to its low solubility in CO2. Highly fluorinated acac-type ligands increased the catalytic reactivity of VO(acac)2-catalyzed epoxidations by enhancing catalyst precursor solubility. Heterogeneous epoxidation reactions were also carried out in liquid CO2 using vanadium complexes supported on cation-exchange polymers.

We report results from a study of the reactivity of the halogenated porphyrins tetrakis(pentafluorophenyl)porphyrinato iron(III) chloride [Fe(TFPP)Cl] and β-octabromo-tetrakis(pentafluorophenyl)porphyrinato iron(III) chloride [Fe(TFPPBr8)Cl] with dioxygen and cyclohexene in supercritical carbon dioxide. A lower limit for the solubility of the iron porphyrins in sc CO2 was determined. Both halogenated metalloporphyrins were active catalysts for oxidation of cyclohexene to epoxide and allylic oxidation products in sc CO2. In 12 h at 80°C, up to 350 and 580 turnovers were observed for Fe(TFPP)Cl and Fe(TFPPBr8)Cl, respectively. We have also explored several organic solvent reactions at high temperature and pressure to benchmark relative activity and selectivity. Activity is higher in organic solvent, but accompanied by substantial oxidation of, or reaction with the solvent. Selectivity for epoxidation with Fe(TFPPBr8)Cl is higher in sc CO2 than in organic solvents, with up to 34% cyclohexene oxide produced.

The selective epoxidation of olefinic alcohols with t-BuOOH in the presence of vanadium catalysts proceeds in liquid carbon dioxide with high conversions and selectivities. Rates measured in liquid CO2 for the oxovanadium(V) triisopropoxide catalyzed epoxidation of allylic and homoallylic alcohols using tert-butyl hydroperoxide are comparable to those measured in methylene chloride, toluene, and n-hexane. The reactivity of the vanadium(IV) bis(acetylacetonato) oxide catalyst in liquid CO2 was found to be substantially lower than in organic solvents, presumably due to its low solubility in CO2. Highly fluorinated acac-type ligands increased the catalytic reactivity of VO(acac)2-catalyzed epoxidations by enhancing catalyst precursor solubility. Heterogeneous epoxidation reactions were also carried out in liquid CO2 using vanadium complexes supported on cation-exchange polymers.

Chemical manufacturing increasingly has to take environmental concerns into account, and chemical processes have to be designed accordingly. Baker and Tumas (page 1477) discuss recent developments in the area of homogeneous catalysis, where so-called biphasic catalysis is aiding separation of product and catalyst and facilitating catalyst recovery.

Benzimidazolines (dihydrobenzimidazoles) are shown for the first time to eliminate hydrogen (H2) by catalyzed reaction with protic compounds.

We have studied the influence of placing a soluble, homogeneous catalyst onto a solid support. We determined that such a 'heterogenized' homogeneous catalyst can have improved activity and selectivity for the asymmetric hydrogenation of enamides to amino acid derivatives. The route of heterogenization of RhDuPhos(COD){sup +} cations occurs via electrostatic interactions with anions that are capable of strong hydrogen bonding to silica surfaces. This is a novel approach to supported catalysis. Supported RhDuPhos(COD){sup +} is a recyclable, non-leaching catalyst in non-polar media. This is one of the few heterogenized catalysts that exhibits improved catalytic performance as compared to its homogeneous analog.

We report the activation of a 2,2′-bipyridine ligand within a class of (mono)cyclopentadienyl lanthanide complexes when reacted with carbon monoxide.

A new phase-separable catalysis concept is demonstrated using supercritical carbon dioxide and the room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate for hydrogenation of alkenes and carbon dioxide.

Non-covalent immobilization of [(R,R)-Me-(DuPHOS)Rh(COD)]OTf by interaction of the triflate counter ion with surface silanols of silica supports leads to an active, stable, enantioselective, asymmetric hydrogenation catalyst.

Catalysed alkene hydroboration proceeds in supercritical CO2 with several rhodium(I) complexes using tunable fluorinated ligands and shows higher regioselectivity relative to tetrahydrofuran or perfluoromethylcyclohexane.

Palladium-catalyzed carbon–carbon bond coupling reactions, the Heck and Stille reactions, proceed in supercritical carbon dioxide with a number of phosphine ligands including tris[3,5-bis(trifluoromethyl)phenyl]phosphine, which affords high conversions and selectivities.

Selective epoxidations with transition metal catalysts (V, Ti, Mo) and ButOOH proceed with high conversions and high selectivity in dense phase carbon dioxide.

We demonstrate that asymmetric catalytic hydrogenation reactions can be conducted in supercritical CO{sub 2} and that, in some cases, higher enantioselectivities can be achieved in this solvent relative to conventional solvents. These preliminary studies effectively demonstrate the feasibility of conducting highly enantioselective hydrogenation reactions in supercritical CO{sub 2}. Importantly, we have shown that higher enantioselectivities may be achieved in supercritical CO{sub 2} relative to conventional solvents. 16 refs., 2 tabs.

Les ions alcoolates subissent une elimination de molecules neutres pour donner des ions enolates

Infrared multiple photon (IRMP) photochemical activation of gas-phase ions trapped in an ion cyclotron resonance (ICR) spectrometer has been used to study the mechanism of a gas-phase negative ion unimolecular decomposition. Upon irradiation with a CO/sub 2/ laser (both high-power pulsed and low-power continuous wave (CW)), tert-butoxide anion, trapped in a pulsed ICR spectrometer, decomposes to yield acetone enolate anion and methane. The mechanism of this formal 1,2-elimination reaction was probed by measuring competitive hydrogen isotope effects (both primary and secondary) in the IR laser photolysis of 2-methyl-2-propoxide-1,1,1-d/sub 3/ (1) and 2-methyl-2-propoxide-1,1,1,3,3,3-d/sub 6/ (2) anions. Unusually large secondary isotope effects (pulsed laser, 1.9 for 1 and 1.7 for 2; cw laser, 8 for (1) and small primary isotope effects (pulsed laser, 1.6 for 1 and 2; cw laser, 2.0 for (1) were observed. These isotope effects, particularly the large difference in energy dependence of the primary and secondary effects, are consistent only with a stepwise mechanism involving initial bond cleavage to an intermediate ion-molecular complex followed by a hydrogen transfer within the intermediate complex. The observed secondary isotope effect have been modelled by using statistical reaction rate (RRKM) theory. The implications of this study for several previously reported unimolecular ionmore » decompositions are also discussed.« less

Metallacycles are key intermediates in organometallic chemistry and catalysis as well as useful reagents for organic synthesis. Despite numerous studies on the preparation and thermal chemistry of these complexes, few photochemical investigations have been reported. In this paper, they report photochemical studies of the bis(cyclopentadienyl)titanacyclobutanes 1. Photochemical activation of 1 results in clean reductive elimination to form cyclopropanes and titanocene which can be trapped by a number of added reagents. Stereochemical studies strongly suggest a stepwise mechanism in which the primary photochemical step involves metal-carbon bond homolysis to produce a metal-centered 1,4-biradical intermediate.

Metallacycles are key intermediates in organometallic chemistry and catalysis as well as useful reagents for organic synthesis. Despite numerous studies on the preparation and thermal chemistry of these complexes, few photochemical investigations have been reported. In this paper, they report photochemical studies of the bis(cyclopentadienyl)titanacyclobutanes 1. Photochemical activation of 1 results in clean reductive elimination to form cyclopropanes and titanocene which can be trapped by a number of added reagents. Stereochemical studies strongly suggest a stepwise mechanism in which the primary photochemical step involves metal-carbon bond homolysis to produce a metal-centered 1,4-biradical intermediate.

The k/sup 35//k/sup 37/ isotope effect for the ethoxide-promoted elimination of C/sub 6/H/sub 5/C/sup i/H(CH/sub 3/)CH/sub 2/Cl (I-Cl-h) coupled with that for I-Cl-d is evidence for the E2 pathway for the dehydrochlorination reaction. (DLC)