Friday, May 14, 2021

Thermodynamic Trends for Reduction of CO by Molecular Complexes | Organometallics

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Selective reduction of CO2 into fuels and chemical feedstocks is highly desirable to reduce our dependence on fossil fuels. Most molecular catalysts afford 2e reduction products, such as CO or HCO2, as opposed to more reduced products. Here we present an analysis of the thermodynamic limitations for reduction of the CO ligand in the form of a series of isostructural group 6 carbonyl complexes, Cp*M(CO)3(P(OMe)3)+ (M = Cr, Mo, and W). The free energy for stepwise transfer of a hydride (H) and a proton (H+) to the CO ligand, resulting in a hydroxycarbene (CHOH) complex, was measured by equilibration with H/H+ donors and acceptors with known hydricity or acidity. Together, these two reaction steps are equivalent to a net addition of H2 across the CO ligand. A large and unfavorable free energy for H2 addition (ΔG°H2) was measured for all three complexes and decreases in the order Cr > Mo > W. The trend for these complexes is opposite to the trend previously reported for group 7 carbonyl complexes, for which ΔG°H2 decreases moving up the group, Re > Mn. Computational analysis indicates the trends can be described in terms of electrostatic effects, where a low ΔG°H2 is obtained in complexes that balance the atomic charges of the M–C≡O fragment of the complex. These findings can be used to design metal carbonyl complexes with a more energetically accessible H2 addition, which will facilitate the development of molecular catalysts for reduction of CO.

Convenient Syntheses of Trivalent Uranium Halide Starting Materials without Uranium Metal | Inorganic Chemistry

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Low-valent uranium coordination chemistry continues to rely heavily on access to trivalent starting materials, but these reagents are typically prepared from uranium turnings, which are becoming increasingly difficult to acquire. Here we report convenient syntheses of UI3(THF)4 (THF = tetrahydrofuran) and UBr3(THF)4 from UCl4, a more accessible uranium starting material that can be prepared from commercially available uranium oxides. UCl3(THF)2 (1), UBr3(THF)4 (2), and UI3(THF)4 (3) were prepared by single-pot reductions from UCl4 using KH and KC8 and converted to 2 or 3 by halide exchange with the corresponding Me3SiX (where X = Br or I). Reduction of UI4(Et2O)2 (4; Et2O = diethyl ether) and UI4(1,4-dioxane)2 (5) was also shown to cleanly yield 3. Complex 1 was also synthesized separately by the addition of anhydrous HCl to U(BH4)3(THF)2, which was prepared by thermal reduction of U(BH4)4. All three trivalent uranium halide complexes were isolated in high crystalline yields (typically 85–99%) and their formulations were confirmed by single-crystal X-ray diffraction, elemental analysis, and 1H NMR and IR spectroscopy. Elemental analysis conducted on triplicate samples of 13 exposed to vacuum for different time intervals revealed significant THF loss for all three complexes in as little as 15 min. Overall, these results offer expedient entry into low-valent uranium chemistry for researchers lacking access to uranium turnings.

Thursday, May 13, 2021

Natural plutonium from supernovae | Science

The rapid neutron capture process (r-process) produces many of the heavy chemical elements, but the astrophysical settings where it occurs remain unclear. Leading candidates are neutron star mergers and some types of supernovae. Wallner et al. analyzed the plutonium content of a deep-sea crust sample, identifying a few dozen atoms of the r-process isotope plutonium- 244 that were delivered to Earth within the past few million years. There was a simultaneous signal of iron-60, which is known to be produced in supernovae. Comparing the ratios of these isotopes constrains the relative contributions of supernovae and neutron star mergers to r-process nucleosynthesis.

Chemical Characterization of a Volatile Dubnium Compound, DbOCl3 - Chiera - - Angewandte Chemie International Edition - Wiley Online Library

The formation and the chemical characterization of single atoms of dubnium (Db, element 105), in the form of its volatile oxychloride, was investigated using the on‐line gas phase chromatography technique, in the temperature range 350‐600 °C. Under the exactly same chemical conditions, comparative studies with the lighter homologs of group‐5 in the Periodic Table clearly indicate the volatility sequence being NbOCl3 > TaOCl3 ≥ DbOCl3. From the obtained experimental results, thermochemical data for DbOCl3 were derived. The present study delivers reliable experimental information for theoretical calculations on chemical properties of transactinides.

Cationic Heterobimetallic Mg(Zn)/Al(Ga) Combinations for Cooperative C‐F Bond Cleavage - Harder - - Angewandte Chemie International Edition - Wiley Online Library

Low‐valent ( Me BDI)Al and ( Me BDI)Ga act as Lewis bases for complexation of highly Lewis acidic cations in [( tBu BDI)M + ·C 6 H 6 ][(B(C 6 F 5 ) 4 ˉ] (M = Mg or Zn, Me BDI = HC[C(Me)N‐DIPP], tBu BDI = HC[C( t Bu)N‐DIPP], DIPP = 2,6‐diisopropylphenyl) yielding heterobimetallic complexes containing the cations [( tBu BDI)Mg‐Al( Me BDI) + ], [( tBu BDI)Mg‐Ga( Me BDI) + ] and [( tBu BDI)Zn‐Ga( Me BDI) + ]. These cations do not show interaction with B(C 6 F 5 ) 4 ˉ and feature long Mg‐Al(or Ga) distances or a short Zn‐Ga bond. The complex with the [( tBu BDI)Zn‐Al( Me BDI) + ] cation could not be obtained but should, according to DFT calculations, be stable. Combined AIM and charge calculations suggest that the metal‐metal bonds to Zn are considerably more covalent whereas those to Mg should be described as weak Al I (or Ga I )→Mg 2+ donor bonds. Failure to isolate the Zn‐Al combination originates from cleavage of the C‐F bond in the solvent fluorobenzene to give ( tBu BDI)ZnPh. The other product cation ( Me BDI)AlF + is expected to be extremely Lewis acidic and was not observed. Howver, a complex containing the cation ( Me BDI)Al(F)‐(m‐F)‐(F)Al( Me BDI) + was verified by X‐ray diffraction. DFT calculations show that the remarkably facile C‐F bond cleavage is a low‐energy route in which p‐bound PhF in ( tBu BDI)Zn + ·(p‐PhF) is attacked from the rear‐side by ( Me BDI)Al. Instead of a S N Ar type mechanism in which F is directly replaced by a nucleophile, a low‐energy transition state for addition to an aromatic C=C bond was found. This  leads to dearomatizion of the Ph ring to give a non‐planar cyclohexadiene intermediate which eliminates ( Me BDI)AlF + to give ( tBu BDI)ZnPh.

Wednesday, May 12, 2021

Silylated Sulfuric Acid: Preparation of a Tris(trimethylsilyl)oxosulfonium [(Me3Si−O)3SO]+ Salt - Bläsing - - Angewandte Chemie International Edition - Wiley Online Library

Silylated Sulfuric Acid: Preparation of a Tris(trimethylsilyl)oxosulfonium [(Me3Si−O)3SO]+ Salt

The “larger” analog of protonated sulfuric acid, [H3SO4]+, the silylated oxosulfonium ion, [(Me3Si)3SO4]+, was obtained in the reaction of silylated sulfuric acid with a [Me3Si]+ transfer reagent such as [Me3Si−H−SiMe3]+[wca], when a weakly coordinating anion ([wca]) is used as counterion.

Tuesday, May 11, 2021

Nitrogen Fixation via Splitting into Nitrido Complexes | Chemical Reviews

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The large carbon footprint of the Haber–Bosch process, which provides ammonia for fertilizers but also the feedstock for all nitrogenous commercial products, has fueled the quest for alternative synthetic strategies to nitrogen fixation. Owing to the extraordinarily strong N≡N triple bond, the key step of the Haber–Bosch reaction, i.e., the dissociative adsorption of N2, requires high temperatures. Since the first report in 1995, a wide variety of molecular transition metal and f-block compounds have been reported that can fully cleave N2 at ambient conditions and form well-defined nitrido complexes. We here provide a comprehensive survey of the current state of N2 splitting reactions in solution and follow-up nitrogen transfer reactivity. Particular emphasis is put on electronic structure requirements for the formation of suitable molecular precursors and their N–N scission reactivity. The prospects of N2 splitting for the synthesis of nitrogen containing products will be discussed, ranging from ammonia and heterocumulenes to organic amines, amides or nitriles via proton coupled electron transfer, carbonylation, or electrophilic functionalization of N2 derived nitrido complexes. Accomplishments and challenges for nitrogen fixation via N2 splitting are presented to offer guidelines for the development of catalytic platforms.

Isolation of a [Fe(CO)4]2–-Bridged Diuranium Complex Obtained via Reduction of Fe(CO)5 with Uranium(III) | Organometallics

  • Robert J. Ward
  • Daniel Pividori
  • Ambre Carpentier
  • Michael L. Tarlton
  • Steven P. Kelley
  • Laurent Maron*
  • Karsten Meyer*
  • , and 
  • Justin R. Walensky*

Abstract

Treatment of the U(III) complex, [(C5Me5)2U(OMes)(THF)], Mes = 2,4,6-Me3C6H2, with Fe(CO)5 forms [{(C5Me5)2(MesO)U}22-(OC)2Fe(CO)2)] with the bridging, tetrahedral Fe(CO)4 moiety. This complex has been studied using 1H NMR, IR vibrational, UV–vis electronic absorption, and zero-field 57Fe Mössbauer spectroscopy as well as single-crystal X-ray diffraction analysis, magnetic measurements, and DFT calculations.

UC Berkeley gets new start-up hub

Astart-up incubator focused on life science, engineering, and data science is coming to the University of California, Berkeley, this fall. The non-profit Bakar BioEnginuity Hub will stand out in the entrepreneurial Bay area, organizers say, for its size, affiliation with the university, and focus on innovations that benefit society.

The hub will be located in Berkeley’s former art museum. When renovations are complete, Bakar Labs will have 3,700 m2 of space for as many as 80 start-ups. Other programs will include fellowships and programming for Berkeley students and researchers, paid access to advanced facilities on campus, and connections to potential investors. Founders can get help from experts at the university’s business and law schools.

Monday, May 10, 2021

Titanocene Pnictinidene Complexes - Chemical Communications (RSC Publishing)

The phospha-Wittig reagent MesTerPPMe3 (MesTer = 2,6-{2,4,6-Me3-C6H2}-C6H3) and arsa-Wittig reagent DipTerAsPMe3 (DipTer = 2,6-{2,6-iPr2-C6H3}-C6H3) have been employed to synthesize the titanocene complexes Cp2Ti(PMe3)PnAr (Pn = P, As) with terminal phosphinidene or arsinidene ligands, respectively. Ab initio studies show that the description as singlet biradicaloids in their ground state is warranted.

Relevance of Single-Transmetalated Resting States in Iron-Mediated Cross-Couplings: Unexpected Role of σ-Donating Additives | Inorganic Chemistry

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Control of the transmetalation degree of organoiron(II) species is a critical parameter in numerous Fe-catalyzed cross-couplings to ensure the success of the process. In this report, we however demonstrate that the selective formation of a monotransmetalated FeII species during the catalytic regime counterintuitively does not alone ensure an efficient suppression of the nucleophile homocoupling side reaction. It is conversely shown that a fine control of the transmetalation degree of the transient FeIII intermediates obtained after the activation of alkyl electrophiles by a single-electron transfer (SET), achievable using σ-donating additives, accounts for the selectivity of the cross-coupling pathway. This report shows for the first time that both coordination spheres of FeII resting states and FeIII short-lived intermediates must be efficiently tuned during the catalytic regime to ensure high coupling selectivities.

Thermodynamic Trends for Reduction of CO by Molecular Complexes | Organometallics

Selective reduction of CO 2  into fuels and chemical feedstocks is highly desirable to reduce our dependence on fossil fuels. Most molecular...