Insights into the Electrochemical CO2RR Performance and Binding of Small Molecules on Quaternary Thiospinels Ag2FeSn3S8 and Cu2FeSn3S8.

Using a mechanical synthesis method in the form of ball milling and an additional annealing step, a novel and accelerated route for the synthesis of the thiospinels toyohaite (Ag2FeSn3S8) and rhodostannite (Cu2FeSn3S8) was discovered. Both thiospinels display faradaic efficiencies of up to 73% for CO2 reduction to CO using an organic electrolyte in an H-type cell. The materials were furthermore implemented in a zero-gap electrolyzer, with toyohaite producing 22% CO and 52% H2 at 100 mA cm-2 and rhodostannite 28% CO and 37% H2. The catalytically active sites are studied using density functional theory, revealing strong CO binding interactions on both Ag and Cu, whereas Sn is found to contribute to the decomposition of Ag2FeSn3S8 and Cu2FeSn3S8 by coordination with oxygen. Postmortem analysis of the thiospinel-based electrodes by means of SEM-EDX, XRD, XPS, and Mössbauer spectroscopy showed sulfur leaching from the catalysts after applying 100 mA cm-2. These spectroscopic results-in conjunction with DFT calculations of the oxidized surfaces-suggest that the catalytically active species consists of metal oxides. As a conversion of the metal sulfides into the corresponding metallic species was observed via XRD, the decomposition pathways of both catalysts were also computed using DFT; thus, elucidating the energetically most favorable decomposition products and expanding the possible composition of the catalysts postelectrolysis.

Molecularly Defined Manganese Catalyst for Low-Temperature Hydrogenation of Carbon Monoxide to Methanol.

Methanol synthesis from syngas (CO/H2 mixtures) is one of the largest manmade chemical processes with annual production reaching 100 million tons. The current industrial method proceeds at high temperatures (200-300 °C) and pressures (50-100 atm) using a copper-zinc-based heterogeneous catalyst. In contrast, here, we report a molecularly defined manganese catalyst that allows for low-temperature/low-pressure (120-150 °C, 50 bar) carbon monoxide hydrogenation to methanol. This new approach was evaluated and optimized by quantum mechanical simulations virtual high-throughput screenings. Crucial for this achievement is the use of amine-based promoters, which capture carbon monoxide to give formamide intermediates, which then undergo manganese-catalyzed hydrogenolysis, regenerating the promoter. Following this conceptually new approach, high selectivity toward methanol and catalyst turnover numbers (up to 3170) was achieved. The proposed general catalytic cycle for methanol synthesis is supported by model studies and detailed spectroscopic investigations.

Matched/Mismatched Cases in Proline-Catalyzed Cascade Reactions with Carbohydrates: A Computational Insight into the Role of d- and l-Proline.

The cascade reactions of carbohydrates with methyl ketones in the presence of proline feature complex running reaction steps. By extensive quantum mechanical simulation, a coherent reaction mechanism was identified matching the experimental data. The present calculations indicate a Mannich reaction/proline hydrolysis/retro aza-Michael cascade to form an intermediate α,ß-unsaturated ethyl ketone. This key precursor yields C-glycosides by a final intramolecular amine-catalyzed oxa-Michael addition. Additionally, the formation of this intermediate determines the rate and selectivity of the overall cascade reaction. Strongly matched and mismatched cases were observed when used with d- or l-proline. They are consistent with the calculated energy barriers of the corresponding transition states.

Isolation and structure elucidation of natural products of three soft corals and a sponge from the coast of Madagascar.

We investigated the three soft corals Sarcophyton stellatum, Capnella fungiformis and Lobophytum crassum and the sponge Pseudoceratina arabica, which have been collected at the coast of Madagascar. In addition to previously known marine natural products, S. stellatum provided the new (+)-enantiomer of the cembranoid (1E,3E,11E)-7,8-epoxycembra-1,3,11,15-tetraene (2). Capnella fungiformis afforded three new natural products, ethyl 5-[(1E,5Z)-2,6-dimethylocta-1,5,7-trienyl]furan-3-carboxylate (6), ethyl 5-[(1E,5E)-2,6-dimethylocta-1,5,7-trienyl]furan-3-carboxylate (7) and the diepoxyguaiane sesquiterpene oxyfungiformin (9a). The extracts of all three soft corals exhibited moderate activities against the malarial parasite Plasmodium falciparum. Extracts of the sponge Pseudoceratina arabica proved to be very active against a series of Gram-positive and Gram-negative bacteria.

Unravelling the Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed by Ru-PNP Pincer Complexes.

Ruthenium PNP complex 1a (RuH(CO)Cl(HN(C2H4Pi-Pr2)2)) represents a state-of-the-art catalyst for low-temperature (<100 °C) aqueous methanol dehydrogenation to H2 and CO2. Herein, we describe an investigation that combines experiment, spectroscopy, and theory to provide a mechanistic rationale for this process. During catalysis, the presence of two anionic resting states was revealed, Ru-dihydride (3-) and Ru-monohydride (4-) that are deprotonated at nitrogen in the pincer ligand backbone. DFT calculations showed that O- and CH- coordination modes of methoxide to ruthenium compete, and form complexes 4- and 3-, respectively. Not only does the reaction rate increase with increasing KOH, but the ratio of 3-/4- increases, demonstrating that the "inner-sphere" C-H cleavage, via C-H coordination of methoxide to Ru, is promoted by base. Protonation of 3- liberates H2 gas and formaldehyde, the latter of which is rapidly consumed by KOH to give the corresponding gem-diolate and provides the overall driving force for the reaction. Full MeOH reforming is achieved through the corresponding steps that start from the gem-diolate and formate. Theoretical studies into the mechanism of the catalyst Me-1a (N-methylated 1a) revealed that C-H coordination to Ru sets-up C-H cleavage and hydride delivery; a process that is also promoted by base, as observed experimentally. However, in this case, Ru-dihydride Me-3 is much more stable to protonation and can even be observed under neutral conditions. The greater stability of Me-3 rationalizes the lower rates of Me-1a compared to 1a, and also explains why the reaction rate then drops with increasing KOH concentration.

Trimetallic Pentlandites (Fe,Co,Ni)9S8 for the Electrocatalytical HER in Acidic Media.

Recently, pentlandite materials have been shown to exhibit promising properties with respect to the hydrogen evolution reaction (HER). A whole series of trimetallic FeCoNi-pentlandite materials and composites have been synthesized from the elements using high-temperature synthesis and categorized in terms of purity. Furthermore, the electrocatalytic properties regarding the HER were determined and correlated to hydrogen adsorption energies, which were determined by means of density functional theory (DFT) calculations. The relationships between activity and its origin generated in this way help to better understand the pentlandite system and provide meaningful approaches for catalyst synthesis.

Comprehensive Thermochemical Benchmark Set of Realistic Closed-Shell Metal Organic Reactions.

We introduce the new MOR41 benchmark set consisting of 41 closed-shell organometallic reactions resembling many important chemical transformations commonly used in transition metal chemistry and catalysis. It includes significantly larger molecules than presented in other transition metal test sets and covers a broad range of bonding motifs. Recent progress in linear-scaling coupled cluster theory allowed for the calculation of accurate DLPNO-CCSD(T)/CBS(def2-TZVPP/def2-QZVPP) reference energies for 3d,4d,5d-transition metal compounds with up to 120 atoms. Furthermore, 41 density functionals, including seven GGAs, three meta-GGAs, 14 hybrid functionals, and 17 double-hybrid functionals combined with two different London dispersion corrections, are benchmarked with respect to their performance for the newly compiled MOR41 reaction energies. A few wave function-based post-HF methods as, e.g., MP2 or RPA with similar computational demands are also tested and in total, 90 methods were considered. The double-hybrid functional PWPB95-D3(BJ) outperformed all other assessed methods with an MAD of 1.9 kcal/mol, followed by the hybrids ωB97X-V (2.2 kcal/mol) and mPW1B95-D3(BJ) (2.4 kcal/mol). The popular PBE0-D3(BJ) hybrid also performs well (2.8 kcal/mol). Within the meta-GGA class, the recently published SCAN-D3(BJ) functional as well as TPSS-D3(BJ) perform best (MAD of 3.2 and 3.3 kcal/mol, respectively). Many popular methods like BP86-D3(BJ) (4.9 kcal/mol) or B3LYP-D3(BJ) (4.9 kcal/mol) provide significantly worse reaction energies and are not recommended for organometallic thermochemistry considering the availability of better methods with the same computational cost. The results regarding the performance of different functional approximations are consistent with conclusions from previous main-group thermochemistry benchmark studies.