Physical properties and mechanisms of formation of nitrous oxide

Changes in agricultural nitrogen (N) balance of OECD countries and its causes and impacts

The Organization for Economic Co-operation and Development (OECD) developed soil surface nutrient balance and made it mandatory for member countries to report annual nutrient budgets since 1990. This study aimed to evaluate the status of nitrogen (N) management in member countries and to figure out why N surplus levels differ across countries and how they relate to other agri-environmental indicators, by analyzing the N budgets from 35 OECD countries over the last 30 years. Of the three factors determining N balance (agricultural land area, N input, and N output), agricultural land area decreased in most OECD countries, negatively affecting N balance reduction. However, OECD's average N balance highly decreased from 91 to 46 kg ha-1 over the last 30 years due to the decrease in N input through inorganic fertilizers and manure, especially in EU countries with high N input levels, while N output did not meaningfully change. In comparison, in Japan and Korea, the N balance slightly increased and they became the highest N balance country recently. A higher N balance led to lower N use efficiency and higher ammonia (NH3) and nitrous oxide (N2O) emission intensities. More densely populated countries with smaller agricultural land per capita (ranging from 0.03 to 0.47 ha capita-1) showed a higher N balance (228-80 kg ha-1), presumably due to higher N input for more agricultural production on limited land. The most densely populated countries among OECD members (Belgium, the Netherlands, Korea, and Japan) had similar N input levels. However, two EU countries had much higher N output than two Asian countries due to higher pasture production, which led to a lower N balance and higher N use efficiency. Therefore, highly populated countries with small arable land areas per capita might need multilateral efforts to alleviate agricultural N balance.

Triazenide-supported [Cu4S] structural mimics of CuZ that mediate N2O disproportionation rather than reduction

As part of the nitrogen cycle, environmental nitrous oxide (N2O) undergoes the N2O reduction reaction (N2ORR) catalyzed by nitrous oxide reductase, a metalloenzyme whose catalytic active site is a tetranuclear copper-sulfide cluster (CuZ). On the other hand, heterogeneous Cu catalysts on oxide supports are known to mediate decomposition of N2O (deN2O) by disproportionation. In this study, a CuZ model system supported by triazenide ligands is characterized by X-ray crystallography, NMR and EPR spectroscopies, and electronic structure calculations. Although the triazenide-ligated Cu4(μ4-S) clusters are closely related to previous formamidinate derivatives, which differ only in replacement of a remote N atom for a CH group, divergent reactivity with N2O is observed. Whereas the formamidinate-ligated clusters were previously shown to mediate single-turnover N2ORR, the triazenide-ligated clusters are found to mediate deN2O, behavior that was previously unknown to natural or synthetic copper-sulfide clusters. The reaction pathway for deN2O by this model system, including previously unidentified transition state models for N2O activation in N-O cleavage and O-O coupling steps, are included. The divergent reactivity of these two related but subtly different systems point to key factors influencing behavior of Cu-based catalysts for N2ORR (i.e., CuZ) and deN2O (e.g., CuO/CeO2).

Mechanisms in the Catalytic Reduction of N2O by CO over the M13@Cu42 Clusters of Aromatic-like Inorganic and Metal Compounds

Metal aromatic substances play a unique and important role in both experimental and theoretical aspects, and they have made tremendous progress in the past few decades. The new aromaticity system has posed a significant challenge and expansion to the concept of aromaticity. From this perspective, based on spin-polarized density functional theory (DFT) calculations, we systematically investigated the doping effects on the reduction reactions of N₂O catalyzed by CO for M₁₃@Cu₄₂ (M = Cu, Co, Ni, Zn, Ru, Rh, Pd, Pt) core-shell clusters from aromatic-like inorganic and metal compounds. It was found that compared with the pure Cu₅₅ cluster, the strong M-Cu bonds provide more structural stability for M₁₃@Cu₄₂ clusters. Electrons that transferred from the M₁₃@Cu₄₂ to N₂O promoted the activation and dissociation of the N-O bond. Two possible reaction modes of co-adsorption (L-H) and stepwise adsorption (E-R) mechanisms over M₁₃@Cu₄₂ clusters were thoroughly discovered. The results showed that the exothermic phenomenon was accompanied with the decomposition process of N₂O via L-H mechanisms for all of the considered M₁₃@Cu₄₂ clusters and via E-R mechanisms for most of the M₁₃@Cu₄₂ clusters. Furthermore, the rate-limiting step of the whole reactions for the M₁₃@Cu₄₂ clusters were examined as the CO oxidation process. Our numerical calculations suggested that the Ni₁₃@Cu₄₂ cluster and Co₁₃@Cu₄₂ clusters exhibited superior potential in the reduction reactions of N₂O by CO; especially, Ni₁₃@Cu₄₂ clusters are highly active, with very low free energy barriers of 9.68 kcal/mol under the L-H mechanism. This work demonstrates that the transition metal core encapsulated M₁₃@Cu₄₂ clusters can present superior catalytic activities towards N₂O reduction by CO.

Beyond Purification: Highly Efficient and Selective Conversion of NO into Ammonia by Coupling Continuous Absorption and Photoreduction under Ambient Conditions

Although the selective catalytic reduction technology has been confirmed to be effective for nitrogen oxide (NO_x) removal, green and sustainable NO_x re-utilization under ambient conditions is still a great challenge. Herein, we develop an on-site system by coupling the continuous chemical absorption and photocatalytic reduction of NO in simulated flue gas (C_NO = 500 ppm, GHSV = 18,000 h^-1), which accomplishes an exceptional NO conversion into value-added ammonia with competitive conversion efficiency (89.05 ± 0.71%), ammonia production selectivity (95.58 ± 0.95%), and ammonia recovery efficiency (>90%) under ambient conditions. The anti-poisoning capacities, including the resistance against factors of H₂O, SO₂, and alkali/alkaline/heavy metals, are also achieved, which presents strong environmental practicability for treating NO_x in flue gas. In addition, the critical roles of corresponding chemical absorption and catalytic reduction components are also revealed by in situ characterizations. The emerging strategy herein not only achieves a milestone efficiency for sustainable NO purification but also opens a new route for contaminant resourcing in the near future of carbon neutrality.

The role of coupled DNRA-Anammox during nitrate removal in a highly saline lake

Nitrate (NO₃^-) removal from aquatic ecosystems involves several microbially mediated processes, including denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and anaerobic ammonium oxidation (anammox), controlled by slight changes in environmental gradients. In addition, some of these processes (i.e. denitrification) may involve the production of undesirable compounds such as nitrous oxide (N₂O), an important greenhouse gas. Saline lakes are prone to the accumulation of anthropogenic contaminants, making them highly vulnerable environments to NO₃^- pollution. The aim of this paper was to investigate the effect of light and oxygen on the different NO₃^- removal pathways under highly saline conditions. For this purpose, mesocosm experiments were performed using lacustrine, undisturbed, organic-rich sediments from the Pétrola Lake (Spain), a highly saline waterbody subject to anthropogenic NO₃^- pollution. The revised ¹⁵N-isotope pairing technique (¹⁵N-IPT) was used to determine NO₃^- sink processes. Our results demonstrate for the first time the coexistence of denitrification, DNRA, and anammox processes in a highly saline lake, and how their contribution was determined by environmental conditions (oxygen and light). DNRA, and especially denitrification to N₂O, were the dominant nitrogen (N) removal pathways when oxygen and/or light were present (up to 82%). In contrast, anoxia and darkness promoted NO₃^- reduction by DNRA (52%), combined with N loss by anammox (28%). Our results highlight the role of coupled DNRA-anammox, which has not yet been investigated in lacustrine sediments. We conclude that anoxia and darkness favored DNRA and anammox processes over denitrification and therefore to restrict N₂O emissions to the atmosphere.

K T, Ismail TM, Sajith PK. Theoretical investigation into the effect of water on the N2O decomposition reaction over Cu-ZSM-5 catalyst

Copper exchanged zeolites are an admirable catalyst for the direct decomposition reaction of harmful N2O gas. However, the inhibition of the decomposition reaction in the presence of water vapor greatly...

Decomposition of nitrous oxide in hydrated cobalt(I) clusters: a theoretical insight into the mechanistic roles of ligand-binding modes

Hydrated cobalt(i) cluster ions, [Co(H2O)n]+, can decompose the inert nitrous oxide molecule, N2O. Density functional theory suggests that N2O can anchor to Co+ of [Co(N2O)(H2O)n]+ through either O end-on (η1-OL) or N end-on (η1-NL) coordinate mode. The latter is thermodynamically more favorable resulting from a subtle π backdonation from Co+ to N2O. N2O decomposition involves two major processes: (1) redox reaction and (2) N-O bond dissociation. The initial activation of N2O through an electron transfer from Co+ to N2O yields anionic N2O-, which binds to the metal center of [Co2+(N2O-)(H2O)n] also through either O end-on (η1-O) or N end-on (η1-N) mode and is stabilized by water molecules through hydrogen bonding. From η1-O, subsequent N-O bond dissociation to liberate N2, producing [CoO(H2O)n]+, is straightforward via a mechanism that is commonplace for typical metal-catalyzed N2O decompositions. Unexpectedly, the N-O bond dissociation directly from η1-N is also possible and eliminates both N2 and OH, explaining the formation of [CoOH(H2O)n]+ as observed in a previous experimental study. Interestingly, formation of [CoO(H2O)n]+ is kinetically controlled by the initial redox process between Co+ and the O-bound N2O, the activation barriers of which in large water clusters (n ≥ 14) are higher than that of the unexpected N-O bond dissociation from the N-bound structure forming [CoOH(H2O)n]+. This theoretical discovery implies that in the present of water molecules, the metal-catalyzed N2O decomposition starting from an O-bound metal complex is not mandatory.

Catalytic Oxygenation of Hydrocarbons by Mono-μ-oxo Dicopper(II) Species Resulting from O-O Cleavage of Tetranuclear CuI /CuII Peroxo Complexes

One of the challenges of catalysis is the transformation of inert C-H bonds to useful products. Copper-containing monooxygenases play an important role in this regard. Here we show that low-temperature oxygenation of dinuclear copper(I) complexes leads to unusual tetranuclear, mixed-valent μ₄ -peroxo [Cu^I /Cu^II ]₂ complexes. These Cu₄ O₂ intermediates promote irreversible and thermally activated O-O bond homolysis, generating Cu₂ O complexes that catalyze strongly exergonic H-atom abstraction from hydrocarbons, coupled to O-transfer. The Cu₂ O species can also be produced with N₂ O, demonstrating their capability for small-molecule activation. The binding and cleavage of O₂ leading to the primary Cu₄ O₂ intermediate and the Cu₂ O complexes, respectively, is elucidated with a range of solution spectroscopic methods and mass spectrometry. The unique reactivities of these species establish an unprecedented, 100 % atom-economic scenario for the catalytic, copper-mediated monooxygenation of organic substrates, employing both O-atoms of O₂ .

Side-on Coordination in Isostructural Nitrous Oxide and Carbon Dioxide Complexes of Nickel

A nickel complex incorporating an N₂ O ligand with a rare η² -N,N'-coordination mode was isolated and characterized by X-ray crystallography, as well as by IR and solid-state NMR spectroscopy augmented by ¹⁵ N-labeling experiments. The isoelectronic nickel CO₂ complex reported for comparison features a very similar solid-state structure. Computational studies revealed that η² -N₂ O binds to nickel slightly stronger than η² -CO₂ in this case, and comparably to or slightly stronger than η² -CO₂ to transition metals in general. Comparable transition-state energies for the formation of isomeric η² -N,N'- and η² -N,O-complexes, and a negligible activation barrier for the decomposition of the latter likely account for the limited stability of the N₂ O complex.

Probing the electronic and mechanistic roles of the μ4-sulfur atom in a synthetic CuZ model system

Nitrous oxide (N2O) contributes significantly to ozone layer depletion and is a potent greenhouse agent, motivating interest in the chemical details of biological N2O fixation by nitrous oxide reductase (N2OR) during bacterial denitrification. In this study, we report a combined experimental/computational study of a synthetic [4Cu:1S] cluster supported by N-donor ligands that can be considered the closest structural and functional mimic of the CuZ catalytic site in N2OR reported to date. Quantitative N2 measurements during synthetic N2O reduction were used to determine reaction stoichiometry, which in turn was used as the basis for density functional theory (DFT) modeling of hypothetical reaction intermediates. The mechanism for N2O reduction emerging from this computational modeling involves cooperative activation of N2O across a Cu/S cluster edge. Direct interaction of the μ4-S ligand with the N2O substrate during coordination and N-O bond cleavage represents an unconventional mechanistic paradigm to be considered for the chemistry of CuZ and related metal-sulfur clusters. Consistent with hypothetical participation of the μ4-S unit in two-electron reduction of N2O, Cu K-edge and S K-edge X-ray absorption spectroscopy (XAS) reveal a high degree of participation by the μ4-S in redox changes, with approximately 21% S 3p contribution to the redox-active molecular orbital in the highly covalent [4Cu:1S] core, compared to approximately 14% Cu 3d contribution per copper. The XAS data included in this study represent the first spectroscopic interrogation of multiple redox levels of a [4Cu:1S] cluster and show high fidelity to the biological CuZ site.

N2 O Reductase Activity of a [Cu4 S] Cluster in the 4CuI Redox State Modulated by Hydrogen Bond Donors and Proton Relays in the Secondary Coordination Sphere

The model complex [Cu₄ (μ₄ -S)(dppa)₄ ]²⁺ (1, dppa=μ₂ -(Ph₂ P)₂ NH) has N₂ O reductase activity in methanol solvent, mediating 2 H⁺ /2 e^- reduction of N₂ O to N₂ +H₂ O in the presence of an exogenous electron donor (CoCp₂ ). A stoichiometric product with two deprotonated dppa ligands was characterized, indicating a key role of second-sphere N-H residues as proton donors during N₂ O reduction. The activity of 1 towards N₂ O was suppressed in solvents that are unable to provide hydrogen bonding to the second-sphere N-H groups. Structural and computational data indicate that second-sphere hydrogen bonding induces structural distortion of the [Cu₄ S] active site, accessing a strained geometry with enhanced reactivity due to localization of electron density along a dicopper edge site. The behavior of 1 mimics aspects of the Cu_Z catalytic site of nitrous oxide reductase: activity in the 4Cu^I :1S redox state, use of a second-sphere proton donor, and reactivity dependence on both primary and secondary sphere effects.

Rhodium(I) Pincer Complexes of Nitrous Oxide

The synthesis of two well-defined rhodium(I) complexes of nitrous oxide (N2 O) is reported. These normally elusive adducts are stable in the solid state and persist in solution at ambient temperature, enabling comprehensive structural interrogation by 15 N NMR and IR spectroscopy, and single-crystal X-ray diffraction. These methods evidence coordination of N2 O through the terminal nitrogen atom in a linear fashion and are supplemented by a computational energy decomposition analysis, which provides further insights into the nature of the Rh-N2 O interaction.

Non-innocent PNN ligand is important for CO oxidation by N2O catalyzed by a (PNN)Ru-H pincer complex: insights from DFT calculations

Milstein et al. developed an efficient and mild method for CO oxidation by N₂O to give CO₂ and N₂ catalyzed by a (PNN)Ru-H pincer complex. To gain mechanistic information on this catalytic transformation, the reaction mechanism has been studied using density functional theory (DFT) calculations. It was found that the catalytic cycle for CO oxidation by N₂O proceeds in three stages: N₂O activation to form a (PNN)Ru-OH intermediate, CO insertion into the Ru-OH bond to form a (PNN)Ru-COOH intermediate and CO₂ release from (PNN)Ru-COOH. In the CO₂ release stage, CO₂ is not released via a β-H elimination mechanism as proposed in experiments, instead it is released via a deprotonation mechanism. The calculations demonstrated that the Ru-H bond of the catalyst plays an important role in facilitating the activation of N₂O, which is the rate-determining step for the whole catalytic cycle, and the non-innocent PNN ligand is very important for CO oxidation by N₂O. Our theoretical results are consistent with the experimental observations and could help design highly efficient catalysts for N₂O activation.

Synthesis and reactivity of a nickel(ii) thioperoxide complex: demonstration of sulfide-mediated N2O reduction

The “masked” terminal nickel sulfide [K(18-crown-6)][L^tBuNi^II(S)] mediates the reduction of N₂O by CO, via the thioperoxide complex [K(18-crown-6)][L^tBuNi^II(η²-SO)].

The thiohyponitrite ([SNNO]^2–) complex, [K(18-crown-6)][L^tBuNi^II(κ²-SNNO)] (L^tBu = {(2,6-ⁱPr₂C₆H₃)NC(^tBu)}₂CH), extrudes N₂ under mild heating to yield [K(18-crown-6)][L^tBuNi^II(η²-SO)] (1), along with minor products [K(18-crown-6)][L^tBuNi^II(η²-OSSO)] (2) and [K(18-crown-6)][L^tBuNi^II(η²-S₂)] (3). Subsequent reaction of 1 with carbon monoxide (CO) results in the formation of [K(18-crown-6)][L^tBuNi^II(η²-SCO)] (4), [K(18-crown-6)][L^tBuNi^II(S,O:κ²-SCO₂)] (5), [K(18-crown-6)][L^tBuNi^II(κ²-CO₃)] (6), carbonyl sulfide (COS) (7), and [K(18-crown-6)][L^tBuNi^II(S₂CO)] (8). To rationalize the formation of these products we propose that 1 first reacts with CO to form [K(18-crown-6)][L^tBuNi^II(S)] (I) and CO₂, via O-atom abstraction. Subsequently, complex I reacts with CO or CO₂ to form 4 and 5, respectively. Similarly, the formation of complex 6 and COS can be rationalized by the reaction of 1 with CO₂ to form a putative Ni(ii) monothiopercarbonate, [K(18-crown-6)][L^tBuNi^II(κ²-SOCO₂)] (11). The Ni(ii) monothiopercarbonate subsequently transfers a S-atom to CO to form COS and [K(18-crown-6)][L^tBuNi^II(κ²-CO₃)] (6). Finally, the formation of 8 can be rationalized by the reaction of COS with I. Critically, the observation of complexes 4 and 5 in the reaction mixture reveals the stepwise conversion of [K(18-crown-6)][L^tBuNi^II(κ²-SNNO)] to 1 and then I, which represents the formal reduction of N₂O by CO.

Photo release of nitrous oxide from the hyponitrite ion studied by infrared spectroscopy. Evidence for the generation of a cobalt-N2O complex. Experimental and DFT calculations

The solid state photolysis of sodium, silver and thallium hyponitrite (M₂N₂O₂, M=Na, Ag, Tl) salts and a binuclear complex of cobalt bridged by hyponitrite ([Co(NH₃)₅-N(O)-NO-Co(NH₃)₅]⁴⁺) were studied by irradiation with visible and UV light in the electronic absorption region. The UV-visible spectra for free hyponitrite ion and binuclear complex of cobalt were interpreted in terms of Density Functional Theory calculations in order to explain photolysis behavior. The photolysis of each compound depends selectively on the irradiation wavelength. Irradiation with 340-460nm light and with the 488nm laser line generates photolysis only in silver and thallium hyponitrite salts, while 253.7nm light photolyzed all the studied compounds. Infrared spectroscopy was used to follow the photolysis process and to identify the generated products. Remarkably, gaseous N₂O was detected after photolysis in the infrared spectra of sodium, silver, and thallium hyponitrite KBr pellets. The spectra for [Co(NH₃)₅-N(O)-NO-Co(NH₃)₅]⁴⁺ suggest that one cobalt ion remains bonded to N₂O from which the generation of a [(NH₃)₅CoNNO]⁺³ complex is inferred. Density Functional Theory (DFT) based calculations confirm the stability of this last complex and provide the theoretical data which are used in the interpretation of the electronic spectra of the hyponitrite ion and the cobalt binuclear complex and thus in the elucidation of their photolysis behavior. Carbonate ion is also detected after photolysis in all studied compounds, presumably due to the reaction of atmospheric CO₂ with the microcrystal surface reaction products. Kinetic measurements for the photolysis of the binuclear complex suggest a first order law for the intensity decay of the hyponitrite IR bands and for the intensity increase in the N₂O generation. Predicted and experimental data are in very good agreement.

Dehydrogenation of benzyl alcohol with N2O as the hydrogen acceptor catalyzed by the rhodium(i) carbene complex: insights from quantum chemistry calculations

The detailed mechanisms of the dehydrogenation of benzyl alcohol with N₂O as the hydrogen acceptor catalyzed by the rhodium(i) carbene complex for the formation of the corresponding carboxylic acid or ester have been investigated via density functional theory (DFT) calculations at the M06 level of theory. Three cycles were considered for the formation of benzaldehyde, benzyl benzoate and benzoic acid. On the basis of the calculations, the rate-determining step for these three cycles is involved in N₂O activation by the rhodium ammine hydride complex with an activation barrier of only 22.6 kcal mol^-1, which is different from the previous mechanism proposed by Gianetti and co-workers, where the hydride is transferred from the Rh atom to the oxygen atom of N₂O with a barrier of 30.5 kcal mol^-1. In addition, the calculations also demonstrated that one more N₂O is necessary to give benzoic acid, and the reaction can only take place under anhydrous conditions. Present calculations are in good agreement with the experimental observations and provide new insights into the dehydrogenation of benzyl alcohol with N₂O as the hydrogen acceptor.

Oxidation of phenyl and hydride ligands of bis(pentamethylcyclopentadienyl)hafnium derivatives by nitrous oxide via selective oxygen atom transfer reactions: insights from quantum chemistry calculations

The mechanisms for the oxidation of phenyl and hydride ligands of bis(pentamethylcyclopentadienyl)hafnium derivatives (Cp* = η(5)-C5Me5) by nitrous oxide via selective oxygen atom transfer reactions have been systematically studied by means of density functional theory (DFT) calculations. On the basis of the calculations, we investigated the original mechanism proposed by Hillhouse and co-workers for the activation of N2O. The calculations showed that the complex with an initial O-coordination of N2O to the coordinatively unsaturated Hf center is not a local minimum. Then we proposed a new reaction mechanism to investigate how N2O is activated and why N2O selectively oxidize phenyl and hydride ligands of . Frontier molecular orbital theory analysis indicates that N2O is activated by nucleophilic attack by the phenyl or hydride ligand. Present calculations provide new insights into the activation of N2O involving the direct oxygen atom transfer from nitrous oxide to metal-ligand bonds instead of the generally observed oxygen abstraction reaction to generate metal-oxo species.

It is no laughing matter: nitrous oxide formation in diesel engines and advances in its abatement over rhodium-based catalysts

N₂O appears as one of the undesired by-products in exhaust gases emitted from diesel engine aftertreatment systems, such as diesel oxidation catalysts (DOC), lean NO_x trap (LNT, also known as NO_x storage and reduction (NSR)) or selective catalytic reduction (NH₃-SCR and HC-SCR) and ammonia slip catalysts (ASC, AMOX, guard catalyst).

Nitrous oxide activation by a cobalt(ii) complex for aldehyde oxidation under mild conditions

A new cobalt(ii) complex reacts with N₂O under mild conditions and it catalytically performs the oxidation of aldehydes.

Functionalization of Complexed N2O in Bis(pentamethylcyclopentadienyl) Systems of Zirconium and Titanium

Methyl triflate reacts with the metastable azoxymetallacyclopentene complex Cp*2Zr(N(O)NCPhCPh), generated in situ from nitrous oxide insertion into the Zr-C bond of Cp*2Zr(η2-PhCCPh) at -78 °C, to afford the salt [Cp*2Zr(N(O)N(Me)CPhCPh)][O3SCF3] (1) in 48% isolated yield. A single-crystal X-ray structure of 1 features a planar azoxymetallacycle with methyl alkylation taking place only at the β-nitrogen position of the former Zr(N(O)NCPhCPh) scaffold. In addition to 1, the methoxy-triflato complex Cp*2Zr(OMe)(O3SCF3) (2) was also isolated from the reaction mixture in 26% yield and fully characterized, including its independent synthesis from the alkylation of Cp*2Zr=O(NC5H5) with MeO3SCF3. Complex 2 could also be observed, spectroscopically, from the thermolysis of 1 (80 °C, 2 days). In contrast to Cp*2Zr(N(O)NPhCCPh), the more stable titanium N2O-inserted analogue, Cp*2Ti(N(O)NCPhCPh), reacts with MeO3SCF3 to afford a 1:1 mixture of regioisomeric salts, [Cp*2Ti(N(O)N(Me)CPhCPh)][O3SCF3] (3) and [Cp*2Ti(N(OMe)NCPhCPh)][O3SCF3] (4), in a combined 65% isolated yield. Single-crystal X-ray diffraction studies of a cocrystal of 3 and 4 show a 1:1 mixture of azoxymetallacyle salts resulting from methyl alkylation at both the β-nitrogen and the β-oxygen of the former Ti(N(O)NCPhCPh ring. As opposed to alkylation reactions, the one-electron reduction of Cp*2Ti(N(O)NCPhCPh) with KC8, followed by encapsulation with the cryptand 2,2,2-Kryptofix, resulted in the isolation of the discrete radical anion [K(2,2,2-Kryptofix)][Cp*2Ti(N(O)NCPhCPh)] (5) in 68% yield. Complex 5 was studied by single-crystal X-ray diffraction, and its solution X-band EPR spectrum suggested a nonbonding σ-type wedge hybrid orbital on titanium, d(z2)/d(x2-y2), houses the unpaired electron, without perturbing the azoxymetallacycle core in Cp*2Ti(N(O)NCPhCPh). Theoretical studies of Ti and the Zr analogue are also presented and discussed.

No laughing matter: the unmaking of the greenhouse gas dinitrogen monoxide by nitrous oxide reductase

The gas nitrous oxide (N₂O) is generated in a variety of abiotic, biotic, and anthropogenic processes and it has recently been under scrutiny for its role as a greenhouse gas. A single enzyme, nitrous oxide reductase, is known to reduce N₂O to uncritical N₂, in a two-electron reduction process that is catalyzed at two unusual metal centers containing copper. Nitrous oxide reductase is a bacterial metalloprotein from the metabolic pathway of denitrification, and it forms a 130 kDa homodimer in which the two metal sites CuA and CuZ from opposing monomers are brought into close contact to form the active site of the enzyme. CuA is a binuclear, valence-delocalized cluster that accepts and transfers a single electron. The CuA site of nitrous oxide reductase is highly similar to that of respiratory heme-copper oxidases, but in the denitrification enzyme the site additionally undergoes a conformational change on a ligand that is suggested to function as a gate for electron transfer from an external donor protein. CuZ, the tetranuclear active center of nitrous oxide reductase, is isolated under mild and anoxic conditions as a unique [4Cu:2S] cluster. It is easily desulfurylated to yield a [4Cu:S] state termed CuZ (*) that is functionally distinct. The CuZ form of the cluster is catalytically active, while CuZ (*) is inactive as isolated in the [3Cu(1+):1Cu(2+)] state. However, only CuZ (*) can be reduced to an all-cuprous state by sodium dithionite, yielding a form that shows higher activities than CuZ. As the possibility of a similar reductive activation in the periplasm is unconfirmed, the mechanism and the actual functional state of the enzyme remain under debate. Using enzyme from anoxic preparations with CuZ in the [4Cu:2S] state, N2O was shown to bind between the CuA and CuZ sites, suggesting direct electron transfer from CuA to the substrate after its activation by CuZ.