Operando isotope selective ammonia quantification in nitrogen reduction studies via gas chromatography-mass spectrometry.

Rapid advances in electrocatalytic ammonia synthesis are impeded by laborious detection methods commonly used in the field and by constant risk of external contaminations, which generates misleading false positives. We developed a facile real-time GC-MS method for sensitive isotope NH3 quantification, requiring no external sample manipulations. This method ensures high detection reliability paramount to accelerate (electro-)catalyst screening.

Ammonia Synthesis at Ambient Conditions via Electrochemical Atomic Hydrogen Permeation.

Direct electrochemical nitrogen reduction holds the promise of enabling the production of carbon emission-free ammonia, which is an important intermediate in the fertilizer industry and a potential green energy carrier. Here we show a strategy for ambient condition ammonia synthesis using a hydrogen permeable nickel membrane/electrode that spatially separates the electrolyte and hydrogen reduction side from the dinitrogen activation and hydrogenation sites. Gaseous ammonia is produced catalytically in the absence of electrolyte via hydrogenation of adsorbed nitrogen by electrochemically permeating atomic hydrogen from water reduction. Dinitrogen activation at the polycrystalline nickel surface is confirmed with 15N2 isotope labeling experiments, and it is attributed to a Mars-van Krevelen mechanism enabled by the formation of N-vacancies upon hydrogenation of surface nitrides. We further show that gaseous hydrogen does not hydrogenate the adsorbed nitrogen, strengthening the benefit of having an atomic hydrogen permeable electrode. The proposed approach opens new directions toward green ammonia.

Hydrogenase Mimics in M12 L24 Nanospheres to Control Overpotential and Activity in Proton-Reduction Catalysis.

Hydrogenase enzymes are excellent proton reduction catalysts and therefore provide clear blueprints for the development of nature-inspired synthetic analogues. Mimicking their catalytic center is straightforward but mimicking the protein matrix around the active site and all its functions remains challenging. Synthetic models lack this precisely controlled second coordination sphere that provides substrate preorganization and catalyst stability and, as a result, their performances are far from those of the natural enzyme. In this contribution, we report a strategy to easily introduce a specific yet customizable second coordination sphere around synthetic hydrogenase models by encapsulation inside M12 L24 cages and, at the same time, create a proton-rich nano-environment by co-encapsulation of ammonium salts, effectively providing substrate preorganization and intermediates stabilization. We show that catalyst encapsulation in these nanocages reduces the catalytic overpotential for proton reduction by 250 mV as compared to the uncaged catalyst, while the proton-rich nano-environment created around the catalyst ensures that high catalytic rates are maintained.

How to Control the Rate of Heterogeneous Electron Transfer across the Rim of M6L12 and M12L24 Nanospheres.

Catalysis in confined spaces, such as those provided by supramolecular cages, is quickly gaining momentum. It allows for second coordination sphere strategies to control the selectivity and activity of transition metal catalysts, beyond the classical methods like fine-tuning the steric and electronic properties of the coordinating ligands. Only a few electrocatalytic reactions within cages have been reported, and there is no information regarding the electron transfer kinetics and thermodynamics of redox-active species encapsulated into supramolecular assemblies. This contribution revolves around the preparation of M6L12 and larger M12L24 (M = Pd or Pt) nanospheres functionalized with different numbers of redox-active probes encapsulated within their cavity, either in a covalent fashion via different types of linkers (flexible, rigid and conjugated or rigid and nonconjugated) or by supramolecular hydrogen bonding interactions. The redox probes can be addressed by electrochemical electron transfer across the rim of nanospheres, and the thermodynamics and kinetics of this process are described. Our study identifies that the linker type and the number of redox probes within the cage are useful handles to fine-tune the electron transfer rates, paving the way for the encapsulation of electroactive catalysts and electrocatalytic applications of such supramolecular assemblies.

Control of the overpotential of a [FeFe] hydrogenase mimic by a synthetic second coordination sphere.

Hydrogen as a renewable fuel is viable when produced sustainably via proton reduction catalysis (PRC). Many homogeneous electrocatalysts perform PRC with high rates, but they all require a large overpotential to drive the reaction. Natural hydrogenase enzymes achieve reversible PRC with potentials close to the thermodynamic equilibrium through confinement of the active site in a well-defined protein pocket. Inspired by nature, we report a strategy that relies on the selective encapsulation of a synthetic hydrogenase mimic in a novel supramolecular cage. Catalyst confinement decreases the PRC overpotential by 150 mV, and is proposed to originate from the cationic cage stabilizing anionic reaction intermediates within the catalytic cycle.

A Functional Hydrogenase Mimic Chemisorbed onto Fluorine-Doped Tin Oxide Electrodes: A Strategy towards Water Splitting Devices.

A diiron benzenedithiolate hydrogen-evolving catalyst immobilized onto fluorine-doped tin oxide (FTO) electrodes is prepared, characterized, and studied in the context of the development of water splitting devices based on molecular components. FTO was chosen as the preferred electrode material owing to its conductive properties and electrochemical stability. An FTO nanocrystalline layer is also used to greatly improve the surface area of commercially available FTO while preserving the properties of the material. Electrodes bearing a covalently anchored diiron catalyst are shown to be competent for electrocatalytic hydrogen evolution from acidic aqueous media at relatively low overpotential (500 mV) with a faradaic efficiency close to unity. Compared with bulk solution catalysts, the catalyst immobilized onto the electrode surface operates at roughly 160 mV lower overpotentials, yet with similar rates.

New tetracobalt cluster compounds for electrocatalytic proton reduction: syntheses, structures, and reactivity.

Reaction of Co2(CO)8 and 1,3-propanedithiol in a 1:1 molar ratio in toluene affords a novel tetracobalt complex, [(µ2-pdt)2(µ3-S)Co4(CO)6] (pdt = -SCH2CH2CH2S-, 1), which possesses some of the structural features of the active site of [FeFe]-hydrogenase. Carbonyl displacement reaction of complex 1 in the presence of mono- or diphosphine ligands leads to the formation of [(µ2-pdt)2(µ3-S)Co4(CO)5(PCy3)] (2) and [(µ2-pdt)2(µ3-S)Co4(CO)4(L)] [L = Ph2PCH=CHPPh2, 3; Ph2PCH2N(Ph)CH2PPh2, 4; Ph2PCH2N(iPr)CH2PPh2, 5]. Complexes 1-5 have been fully characterized by spectroscopy and single-crystal X-ray diffraction studies. Cyclic voltammetry has revealed that complexes 1-5 show a reversible first reduction wave and are active for electrocatalytic proton reduction in the presence of CF3COOH. Protonation reactions have been monitored by (31)P and (1)H NMR and infrared spectroscopies, which revealed the formation of different protonated species. The mono-reduced species of 1-5 have been spectroscopically characterized by EPR and spectro-electro-infrared techniques.

Terminal vs bridging hydrides of diiron dithiolates: protonation of Fe2(dithiolate)(CO)2(PMe3)4.

This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe(2)(xdt)(CO)(2)(PMe(3))(4), where xdt is edt (ethanedithiolate, 1), pdt (propanedithiolate, 2), and adt (2-aza-1,3-propanedithiolate, 3), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds 1-3 oxidize near -950 mV vs Fc(+/0). Crystallographic analyses confirm that 1 and 2 adopt C(2)-symmetric structures (Fe-Fe = 2.616 and 2.625 Å, respectively). Low-temperature protonation of 1 afforded exclusively [µ-H1](+), establishing the non-intermediacy of the terminal hydride ([t-H1](+)). At higher temperatures, protonation afforded mainly [t-H1](+). The temperature dependence of the ratio [t-H1](+)/[µ-H1](+) indicates that the barriers for the two protonation pathways differ by ∼4 kcal/mol. Low-temperature (31)P{(1)H} NMR measurements indicate that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate [Fe(2)(Hpdt)(CO)(2)(PMe(3))(4)](+) ([S-H2](+)). This intermediate converts to [t-H2](+) and [µ-H2](+) by first-order and second-order processes, respectively. DFT calculations support transient protonation at sulfur and the proposal that the S-protonated species (e.g., [S-H2](+)) rearranges to the terminal hydride intramolecularly via a low-energy pathway. Protonation of 3 affords exclusively terminal hydrides, regardless of the acid or conditions, to give [t-H3](+), which isomerizes to [t-H3'](+), wherein all PMe(3) ligands are basal.

Reaction of Aryl Diazonium Salts and Diiron(I) Dithiolato Carbonyls: Evidence for Radical Intermediates.

Treatment of Fe(2)(pdt)(CO)(4)(dppv) (1) with aryldiazonium salts affords the 34e(-) adducts [Fe(2)(pdt)(µ-N(2)Ar)(CO)(4)(dppv)](+) (pdt(2-) = 1,3-propanedithiolate, dppv = cis-C(2)H(2)(PPh(2))(2)). Under some conditions, the same reaction gave substantial amounts of [1](+), the product of electron-transfer. Consistent with the influence of electron transfer in the reactions of some electrophiles with Fe(I)Fe(I) dithiolates, the reaction of [Me(3)S(2)](+) and Fe(2)(pdt)(CO)(4)(dppbz) was found to give [Fe(2)(pdt)(CO)(4)(dppbz)](+) as well as Me(2)S and Me(2)S(2) (dppbz = 1,2-bis(diphenylphosphino)benzene).

Dichloromethane as solvent for the synthesis of polycationic clusters at room temperature--a link to standard organometallic chemistry.

Dichloromethane (DCM), CH(2)Cl(2) has been identified as an alternative reaction medium for room-temperature synthesis of subvalent homopolyatomic clusters of the group 15 element bismuth.