Concluding remarks: Sustainable nitrogen activation - are we there yet?

The activation of dinitrogen as a fundamental step in reactions to produce nitrogen compounds, including ammonia and nitrates, has a cornerstone role in chemistry. Bringing together research from disparate fields where this can be achieved sustainably, this Faraday Discussion seeks to build connections between approaches that can stimulate further advances. In this paper we set out to provide an overview of these different approaches and their commonalities. We explore experimental aspects including the positive role of increasing nitrogen pressure in some fields, as well as offering perspectives on when ¹⁵N₂ experiments might, and might not, be necessary. Deconstructing the nitrogen reduction reaction, we attempt to provide a common framework of energetic scales within which all of the different approaches and their components can be understood. On sustainability, we argue that although green ammonia produced from a green-H₂-fed Haber-Bosch process seems to fit the bill, there remain many real-world contexts in which other, sustainable, approaches to this vital reaction are urgently needed.

Photoexcited cobalt catalysed endo-selective alkyl Heck reaction

Herein, we report an intramolecular endo-selective Heck reaction of iodomethylsilyl ethers of phenols and alkenols. The reaction leads to the formation of seven- and eight-membered siloxycycles in excellent yields, which could be further converted into the corresponding allylic alcohols upon oxidation. Thus, this method could be used for the selective (Z)-hydroxymethylation of o-hydroxystyrenes and alkenols. Rapid scan EPR experiments and DFT calculations suggest a concerted β-hydrogen elimination event to take place in the triplet state.

Effect of Ligands on the Stability of Gold Nanoclusters

Gold nanoclusters (AuNCs) are atomic architectures that can be precisely tailored for catalytic applications. In this work, we studied two benchmark AuNCs, Au₂₅(SR)₁₈ and Au₁₄₄(SR)₆₀, covered by aromatic and aliphatic ligands to envision how the 3D structure of the ligand impacts the stability of the nanomaterial. Surprisingly, we found that increasing the alkanethiol length has a poor or null effect on the stability of the AuNCs, a trend opposite to that on Au(111) surfaces. When considering the aromatic or aliphatic nature, the AuNC stability follows the same trend as on Au(111): the thermodynamical stability is dictated by the ligand density rather than its chemical nature, where the aliphatic ligand imparts more stability than the aromatic one. Our findings provide a tool to predict how an ultrasmall gold core can interact with the environment, substrate, and themselves according to the stability of its protecting ligand shell.

Competition between metal-catalysed electroreduction of dinitrogen, protons, and nitrogen oxides: a DFT perspective

Metals are common electrocatalysts for N2-into-NH3 reduction. In protic media, H+ competes with N2 to be reduced into H2. NOx, common air pollutants, are predicted to be more selectively converted into NH3 than N2, and even more than H+ into H2.

Stabilisation of dianion dimers trapped inside cyanostar macrocycles

Interanionic H-bonds (IAHBs) are unfavourable interactions in the gas phase becoming favoured when anions are in solution. Dianion dimers are also susceptible to being trapped inside the cavities of cyanostar (CS) macrocycles, and thus, the formation of 2 : 2 anion : cyanostar aggregates is mainly supported by three kinds of interactions: IAHBs between the dianions, π-π stacking between the confronted cyanostars, and the presence of an intricate network of multiple C(sp²)HO H-bonds between cyanostar ligands and the anionic moieties. An analysis of the interaction energies supported by NBO reveals a slight cooperative effect of the CSs on the IAHB stabilisation.

Liquefied Sunshine: Transforming Renewables into Fertilizers and Energy Carriers with Electromaterials

It has become apparent that renewable energy sources are plentiful in many, often remote, parts of the world, such that storing and transporting that energy has become the key challenge. For long-distance transportation by pipeline and bulk tanker, a liquid form of energy carrier is ideal, focusing attention on liquid hydrogen and ammonia. Development of high-activity and selectivity electrocatalyst materials to produce these energy carriers by reductive electrochemistry has therefore become an important area of research. Here, recent developments and challenges in the field of electrocatalytic materials for these processes are discussed, including the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the nitrogen reduction reaction (NRR). Some of the mis-steps currently plaguing the nitrogen reduction to ammonia field are highlighted. The rapidly growing roles that in situ/operando and quantum chemical studies can play in new electromaterials discovery are also surveyed.

Conversion of racemic alcohols to optically pure amine precursors enabled by catalyst dynamic kinetic resolution: experiment and computation

An unprecedented base metal catalysed asymmetric synthesis of α-chiral amine precursors from racemic alcohols is reported.

In silico design of novel NRR electrocatalysts: cobalt–molybdenum alloys

Cobalt-molybdenum alloys can stabilise the elusive N₂H as the first reduced intermediate species in nitrogen reduction reaction (NRR).

Manganese-Catalyzed Multicomponent Synthesis of Pyrroles through Acceptorless Dehydrogenation Hydrogen Autotransfer Catalysis: Experiment and Computation

A new base metal catalyzed sustainable multicomponent synthesis of pyrroles from readily available substrates is reported. The developed protocol utilizes an air- and moisture-stable catalyst system and enables the replacement of themutagenic α-haloketones with readily abundant 1,2-diols. Moreover, the presented method is catalytic in base and the sole byproducts of this transformation are water and hydrogen gas. Experimental and computational mechanistic studies indicate that the reaction takes place through a combined acceptorless dehydrogenation hydrogen autotransfer methodology.

Diastereoselective diazenyl formation: the key for manganese-catalysed alcohol conversion into (E)-alkenes

The proposed reaction mechanism for the unprecedented direct transformation of primary alcohols into alkenes catalysed by Mn(i)-PNP complexes consists of two cycles.

Regression analysis of properties of [Au(IPr)(CHR2)] complexes

New [Au(IPr)(CHR₂)] complexes have been synthesised through protonolysis reactions of [Au(IPr)(OH)] with moderately acidic substrates, CH₂R₂.

Quantifying electronic similarities between NHC–gold(i) complexes and their isolobal imidazolium precursors

The σ and π donor/acceptor properties of the carbon–gold bond are manifested by changes in ¹⁹⁷Au spectroscopic data.

Energy-Efficient Nitrogen Reduction to Ammonia at Low Overpotential in Aqueous Electrolyte under Ambient Conditions

The electrochemical nitrogen reduction reaction (NRR) under ambient conditions is a promising alternative to the traditional energy-intensive Haber-Bosch process to produce NH₃ . The challenge is to achieve a sufficient energy efficiency, yield rate, and selectivity to make the process practical. Here, we demonstrate that Ru nanoparticles (NPs) enable NRR in 0.01 m HCl aqueous solution at very high energy efficiency, that is, very low overpotentials. Remarkably, the NRR occurs at a potential close to or even above the H⁺ /H₂ reversible potential, significantly enhancing the NRR selectivity versus the production of H₂ . NH₃ yield rates as high as ≈5.5 mg h^-1  m^-2 at 20 °C and 21.4 mg h^-1  m^-2 at 60 °C were achieved at a redox potential (E) of -100 mV versus the reversible hydrogen electrode (RHE), whereas a highest Faradaic efficiency (FE) of ≈5.4 % is achievable at E=+10 mV vs. RHE. This work demonstrates the potential use of Ru NPs as an efficient catalyst for NRR at ambient conditions. This ability to catalyze NRR at potentials near or above RHE is imperative in improving the NRR selectivity towards a practical process as well as rendering the H₂ viable as byproduct. Density functional theory calculations of the mechanism suggest that the efficient NRR process occurring on these predominantly Ru (0 0 1) surfaces is catalyzed by a dissociative mechanism.

Hydrogenation of CO2 -Derived Carbonates and Polycarbonates to Methanol and Diols by Metal-Ligand Cooperative Manganese Catalysis

The first base-metal-catalysed hydrogenation of CO₂ -derived carbonates to alcohols is presented. The reaction proceeds under mild conditions in the presence of a well-defined manganese complex with a loading as low as 0.25 mol %. The non-precious-metal homogenous catalytic system provides an indirect route for the conversion of CO₂ into methanol with the co-production of value-added (vicinal) diols in yields of up to 99 %. Experimental and computational studies indicate a metal-ligand cooperative catalysis mechanism.

Hydrogen bonding effect between active site and protein environment on catalysis performance in H2-producing [NiFe] hydrogenases

The interaction between the active site and the surrounding protein environment plays a fundamental role in the hydrogen evolution reaction (HER) in [NiFe] hydrogenases. Our density functional theory (DFT) findings demonstrate that the reaction Gibbs free energy required for the rate determining step decreases by 7.1 kcal mol^-1 when the surrounding protein environment is taken into account, which is chiefly due to free energy decreases for the two H⁺/e^- addition steps (the so-called Ni-SIa to I1, and Ni-C to Ni-R), being the largest thermodynamic impediments of the whole reaction. The variety of hydrogen bonds (H-bonds) between the amino acids and the active site is hypothesised to be the main reason for such stability: H-bonds not only work as electrostatic attractive forces that influence the charge redistribution, but more importantly, they act as an electron 'pull' taking electrons from the active site towards the amino acids. Moreover, the electron 'pull' effect through H-bonds via the S^- in cysteine residues shows a larger influence on the energy profile than that via the CN^- ligands on Fe.

An intensified π-hole in beryllium-doped boron nitride meshes: its determinant role in CO2 conversion into hydrocarbon fuels

DFT investigations on beryllium-doped boron nitride meshes or sheets (BNs) predict the existence of a very reactive kind of novel material capable of spontaneously reducing the first hydrogenation step in the CO₂ conversion mechanism.

A DFT study of planar vs. corrugated graphene-like carbon nitride (g-C3N4) and its role in the catalytic performance of CO2 conversion

Graphene-like carbon nitride (g-C₃N₄), a metal-free 2D material that is of interest as a CO₂ reduction catalyst, is stabilised by corrugation in order to minimise the electronic repulsions experienced by the N lone pairs located in their structural holes.

Ionic liquid electrolytes for reversible magnesium electrochemistry

Mg has great potential as the basis for a safe, low cost energy storage technology, however, cycling of magnesium is difficult to achieve in most electrolytes.

Why is a proton transformed into a hydride by [NiFe] hydrogenases? An intrinsic reactivity analysis based on conceptual DFT

The hydrogen evolution reaction (HER) catalysed by [NiFe] hydrogenases entails a series of chemical events involving great mechanistic interest.

Chalcogen bonds in complexes of SOXY (X, Y = F, Cl) with nitrogen bases

SOF2, SOFCl, and SOCl2 were each paired with a series of N bases. The potential energy surface of the binary complexes were characterized by MP2 calculations with double and triple-ξ basis sets, extrapolated to complete sets. The most stable configurations contained a S···N chalcogen bond with interaction energies as high as 6.8 kcal/mol. These structures are stabilized by a Nlp → σ*(S-Z) electron transfer (Z = O, F, Cl), complemented by Coulombic attraction of N to the σ-hole opposite the Z atom. N···S-F and N···S-Cl chalcogen bonds are stronger than N···S═O interactions. Formation of each chalcogen bond elongates all of the internal covalent bonds within SOXY, especially the S-Cl bond. Halogen-bonded (N···Cl-S) complexes were also observed, but these are more weakly bound, by less than 3 kcal/mol.

Strongly bound noncovalent (SO3)n:H2CO complexes (n = 1, 2)

SO₃and H₂CO dimers and trimers are held together by S⋯O chalcogen bonds, supplemented by weaker CH⋯O and/or O⋯C bonds.

Complexes containing CO2and SO2. Mixed dimers, trimers and tetramers

Two stable minima for the 1 : 1 heterodimer of CO₂ : SO₂, both bound by about 2 kcal mol⁻¹. Binding is dominated by charge transfer from O lone pairs of SO₂to CO π* antibonding orbitals.

Conformational preference and chiroptical response of carbohydrates D-ribose and 2-deoxy-D-ribose in aqueous and solid phases

This work targets the structural preferences of D-ribose and 2-deoxy-D-ribose in water solution and solid phase. A theoretical DFT (B3LYP and M06-2X) and MP2 study has been undertaken considering the five possible configurations (open-chain, α-furanose, β-furanose, α-pyranose, and β-pyranose) of these two carbohydrates with a comparison of the solvent treatment using only a continuum solvation model (PCM) and the PCM plus one explicit water molecule. In addition, experimental vibrational studies using both nonchiroptical (IR-Raman) and chiroptical (VCD) techniques have been carried out. The theoretical and experimental results show that α- and β-pyranose forms are the dominant configurations for both compounds. Moreover, it has been found that 2-deoxy-D-ribose presents a non-negligible percentage of open-chain forms in aqueous solution, while in solid phase this configuration is absent.