Because of the continually rising levels of CO2 in the atmosphere, research for the conversion of CO2 into fuels using carbon-neutral energy is an important and current topic in catalysis. Recent ...research on molecular catalysts has led to improved rates for conversion of CO2 to formate, but the catalysts are based on precious metals such as iridium, ruthenium and rhodium and require high temperatures and high pressures. Using established thermodynamic properties of hydricity (ΔG H– ) and acidity (pK a), we designed a cobalt-based catalyst system for the production of formate from CO2 and H2. The complex Co(dmpe)2H (dmpe is 1,2-bis(dimethylphosphino)ethane) catalyzes the hydrogenation of CO2, with a turnover frequency of 3400 h–1 at room temperature and 1 atm of 1:1 CO2:H2 (74 000 h–1 at 20 atm) in tetrahydrofuran. These results highlight the value of fundamental thermodynamic properties in the rational design of catalysts.
Several models exist to describe the growth and evolution of Earth; however, variables such as the type of precursor materials, extent of mixing, and material loss during accretion are poorly ...constrained. High-precision palladium-silver isotope data show that Earth's mantle is similar in ¹⁰⁷Ag/¹⁰⁹Ag to primitive, volatile-rich chondrites, suggesting that Earth accreted a considerable amount of material with high contents of moderately volatile elements. Contradictory evidence from terrestrial chromium and strontium isotope data are reconciled by heterogeneous accretion, which includes a transition from dominantly volatile-depleted to volatile-rich materials with possibly high water contents. The Moon-forming giant impact probably involved the collision with a Mars-like protoplanet that had an oxidized mantle, enriched in moderately volatile elements.
We report a nickel complex for catalytic oxidation of ammonia to dinitrogen under ambient conditions. Using the aryloxyl radical 2,4,6‐tri‐tert‐butylphenoxyl (tBu3ArO⋅) as a H atom acceptor to cleave ...the N−H bond of a coordinated NH3 ligand up to 56 equiv of N2 per Ni center can be generated. Employing the N‐oxyl radical 2,2,6,6‐(tetramethylpiperidin‐1‐yl)oxyl (TEMPO⋅) as the H‐atom acceptor, up to 15 equiv of N2 per Ni center are formed. A bridging Ni‐hydrazine product identified by isotopic nitrogen (15N) studies and supported by computational models indicates the N−N bond forming step occurs by bimetallic homocoupling of two paramagnetic Ni−NH2 fragments. Ni‐mediated hydrazine disproportionation to N2 and NH3 completes the catalytic cycle.
A nickel complex for catalytic oxidation of NH3 to N2 is demonstrated using the radicals 2,4,6‐tri‐tert‐butylphenoxyl (tBu3ArO⋅) or 2,2,6,6‐(tetramethylpiperidine‐1‐yl)oxyl (TEMPO) as the H‐atom acceptor to cleave the N−H bond of a NH3 ligand. Two Ni−NH2 fragments form an N−N bond in a bridging Ni‐hydrazine product. Ni‐mediated hydrazine disproportionation affords N2 and NH3 in the proposed catalytic cycle. (BDFE=Bond Dissociation Free Energy)
Catalysts for the oxidation of NH3 are critical for the utilization of NH3 as a large‐scale energy carrier. Molecular catalysts capable of oxidizing NH3 to N2 are rare. This report describes the use ...of Cp*Ru(PtBu2NPh2)(15NH3)BArF4, (PtBu2NPh2=1,5‐di(phenylaza)‐3,7‐di(tert‐butylphospha)cyclooctane; ArF=3,5‐(CF3)2C6H3), to catalytically oxidize NH3 to dinitrogen under ambient conditions. The cleavage of six N−H bonds and the formation of an N≡N bond was achieved by coupling H+ and e− transfers as net hydrogen atom ion (HAA) steps using the 2,4,6‐tri‐tert‐butylphenoxyl radical (tBu3ArO.) as the H atom acceptor. Employing an excess of tBu3ArO. under 1 atm of NH3 gas at 23 °C resulted in up to ten turnovers. Nitrogen isotopic (15N) labeling studies provide initial mechanistic information suggesting a monometallic pathway during the N⋅⋅⋅N bond‐forming step in the catalytic cycle.
Totally radical: A molecular Ru complex catalytically oxidizes NH3 to dinitrogen under ambient conditions. The cleavage of six N−H bonds and the formation of an N≡N bond was achieved by coupling H+ and e− transfers as net hydrogen atom ion (HAA) steps using the 2,4,6‐tri‐tert‐butylphenoxyl radical (tBu3ArO.) as the H atom acceptor, resulting in up to 10 turnovers.
We report the first discrete molecular Cr-based catalysts for the reduction of N2. This study is focused on the reactivity of the Cr-N2 complex, trans-Cr(N2)2(PPh 4NBn 4) (P 4 Cr(N 2 ) 2 ), ...bearing a 16-membered tetraphosphine macrocycle. The architecture of the 16-PPh 4NBn 4 ligand is critical to preserve the structural integrity of the catalyst. P 4 Cr(N 2 ) 2 was found to mediate the reduction of N2 at room temperature and 1 atm pressure by three complementary reaction pathways: (1) Cr-catalyzed reduction of N2 to N(SiMe3)3 by Na and Me3SiCl, affording up to 34 equiv N(SiMe3)3; (2) stoichiometric reduction of N2 by protons and electrons (for example, the reaction of cobaltocene and collidinium triflate at room temperature afforded 1.9 equiv of NH3, or at −78 °C afforded a mixture of NH3 and N2H4); and (3) the first example of NH3 formation from the reaction of a terminally bound N2 ligand with a traditional H atom source, TEMPOH (2,2,6,6-tetramethylpiperidine-1-ol). We found that trans-Cr(15N2)2(PPh 4NBn 4) reacts with excess TEMPOH to afford 1.4 equiv of 15NH3. Isotopic labeling studies using TEMPOD afforded ND3 as the product of N2 reduction, confirming that the H atoms are provided by TEMPOH.
Phytoplankton forms the basis of primary production in mangrove environments. The phylogeny and diversity based on the amplification and sequencing of rbcL, the large subunit encoding the key enzyme ...ribulose‐1, 5‐bisphosphate carboxylase/oxygenase was investigated for improved understanding of the community structure and temporal trends of chromophytic eukaryotic phytoplankton assemblages in Sundarbans, the world's largest continuous mangrove. Diatoms (Bacillariophyceae) were by far the most frequently detected group in clone libraries (485 out of 525 clones), consistent with their importance as a major bloom‐forming group. Other major chromophytic algal groups including Cryptophyceae, Haptophyceae, Pelagophyceae, Eustigmatophyceae, and Raphidophyceae which are important component of the assemblages were detected for the first time from Sundarbans based on rbcL approach. Many of the sequences from Sundarbans rbcL clone libraries showed identity with key bloom forming diatom genera namely Thalassiosira, Skeletonema and Nitzschia. Similarly, several rbcL sequences which were diatom‐like were also detected highlighting the need to explore diatom communities from the study area. Some of the rbcL sequences detected from Sundarbans were ubiquitous in distribution showing 100% identities with uncultured rbcL sequences targeted previously from the Gulf of Mexico and California upwelling system that are geographically separated from study area. Novel rbcL lineages were also detected highlighting the need to culture and sequence phytoplankton from the ecoregion. Principal component analysis revealed that nitrate is an important variable that is associated with observed variation in phytoplankton assemblages (operational taxonomic units). This study applied molecular tools to highlight the ecological significance of diatoms, in addition to other chromophytic algal groups in Sundarbans.
We report ammonia oxidation by homolytic cleavage of all three H atoms from a MoNH3+ complex using the 2,4,6-tri-tert-butylphenoxyl radical to yield a Mo-alkylimido (MoNR+) complex (R = ...2,4,6-tri-tert-butylcyclohexa-2,5-dien-1-one). Chemical reduction of MoNR+ generates a terminal MoN nitride complex upon NC bond cleavage, and a MoNH+ complex is formed by protonation of the nitride. Computational analysis describes the energetic profile for the stepwise removal of three H atoms from MoNH3+ and formation of MoNR+.
The geometric constraints imposed by a tetradentate P4N2 ligand play an essential role in stabilizing square planar Fe complexes with changes in metal oxidation state. The square pyramidal ...Fe0(N2)(P4N2) complex catalyzes the conversion of N2 to N(SiR3)3 (R = Me, Et) at room temperature, representing the highest turnover number of any Fe-based N2 silylation catalyst to date (up to 65 equiv N(SiMe3)3 per Fe center). Elevated N2 pressures (>1 atm) have a dramatic effect on catalysis, increasing N2 solubility and the thermodynamic N2 binding affinity at Fe0(N2)(P4N2). A combination of high-pressure electrochemistry and variable-temperature UV–vis spectroscopy were used to obtain thermodynamic measurements of N2 binding. In addition, X-ray crystallography, 57Fe Mössbauer spectroscopy, and EPR spectroscopy were used to fully characterize these new compounds. Analysis of Fe0, FeI, and FeII complexes reveals that the free energy of N2 binding across three oxidation states spans more than 37 kcal mol–1.
Catalysts that are able to reduce carbon dioxide under mild conditions are highly sought after for storage of renewable energy in the form of a chemical fuel. This study describes a systematic ...kinetic and thermodynamic study of a series of cobalt and rhodium bis(diphosphine) complexes that are capable of hydrogenating carbon dioxide to formate under ambient temperature and pressure. Catalytic CO2 hydrogenation was studied under 1.8 and 20 atm of pressure (1:1 CO2/H2) at room temperature in tetrahydrofuran with turnover frequencies (TOF) ranging from 20 to 74 000 h–1. The catalysis was followed by 1H and 31P NMR spectroscopy in real time under all conditions to yield information about the rate-determining step. The cobalt catalysts displayed a rate-determining step of hydride transfer to CO2, while the hydrogen addition and/or deprotonation steps were rate limiting for the rhodium catalysts. Thermodynamic analysis of the complexes provided information on the driving force for each step of catalysis in terms of the catalyst hydricity (ΔG°H– ), acidity (pK a), and free energy for H2 addition (ΔG°H2 ). Linear free-energy relationships were identified that link the kinetic activity for catalytic hydrogenation of CO2 to formate with the thermodynamic driving force for the rate-limiting steps of catalysis. The catalyst exhibiting the highest activity, Co(dmpe)2H, was found to have hydride transfer and hydrogen addition steps that were each downhill by approximately 6 to 7 kcal mol–1, and the deprotonation step was thermoneutral. This indicates the fastest catalysts are the ones that most efficiently balance the free energy relationships of every step in the catalytic cycle.
We report a rare example of a Cr–N2 complex supported by a 16-membered phosphorus macrocycle containing pendant amine bases. Reactivity with acid afforded hydrazinium and ammonium, representing the ...first example of N2 reduction by a Cr–N2 complex. Computational analysis examined the thermodynamically favored protonation steps of N2 reduction with Cr leading to the formation of hydrazine.
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