NN Bond Research Articles

Boosting electrochemical ammonia synthesis via dynamic nitrogen carriers coupled with electron-rich Lewis acidic sites on TiO2−x nanofiber

In light of the high energy consumption and substantial carbon emissions associated with traditional NH3 production based on the Haber–Bosch process, the aqueous electrochemical nitrogen reduction reaction (NRR) offers a clean and sustainable alternative production route. Nevertheless, activating the NN bonds at room temperature is challenging due to the high bond energy, severely hindering the development and commercialization of the electrochemical NRR. Herein, we report a synergistic strategy for achieving efficient N2 activation at ambient conditions that combines electrolyte engineering with catalytic site-modulated TiO2−x nanofiber electrocatalysts. The synthesized TiO2−x nanofiber electrocatalysts contained abundant intrinsic oxygen vacancies and were further modified with hydroxyl groups to create electron-rich Lewis acidic Ti sites. Additionally, BF3 was engineered into the electrolyte microenvironment, and it could form adducts with N2, serving as a dynamic carrier for N2 transport. The electron-rich Lewis acidic sites and the dynamic carriers exerted a ‘pull–pull’ effect on N2, thereby weakening the NN bonds. Through electrochemical performance evaluation, the designed electrocatalytic scheme achieved an NH3 yield of ∼57.15 μg h−1 mg−1 and a Faradaic efficiency of ∼15.14 %. We anticipate that this methodology will provide new insights into the development of electrochemical ammonia synthesis, particularly in relation to multifaceted design.

The active site of Mδ+ tailed by B (or N)-doped graphyne for nitrogen reduction reaction through DFT study

NN bond activation is one of the technical problems in the conversion of N2-to-NH3. In this work, we researched Cr, Mn, Co, Ni, Cu and Pt metal single atoms anchored on B/N-doped graphyne (M/X-GY) catalysts systematically by means of DFT. The results of Bader charge and charge density verified the charge transfers between single metal and the doped graphyne in M/X-GY catalysts. Cr (+1.001 e) and Mn (+0.800 e) exhibit higher positive valence, which is favorable for N2 adsorption. Based on the "donation and back-donation" mechanism, the energies gaps of 5σ-N2 and d-M for M=Cr, Mn and Co have the lower values of 3.01, 3.18, 3.18 eV, respectively, and the N2 adsorption energies on Cr/GY (−2.046 eV) and Mn/GY (−1.622 eV) have the more negative values. As a result, the catalyst of Cr metal atom anchored on GY is promising candidate. The substitution of B can effectively promote the activation of N2 and suppress the hydrogen evolution, while the substitution of N has the minimum ΔG*N-NH of 0.26 eV and ΔGNH3 of 0.08 eV. Compared with two different NRR pathways, the distal mechanism process is more preferential than the alternating pathway. The catalyst of Cr/N-GY has higher stability and can effectively activate N2 to generate *N2H, which can serves as promising catalysts throughout the NRR process.

Constructing S-scheme heterojunction Cs3Bi2Br9/BiOBr via in-situ partial conversion to boost photocatalytic N2 fixation

The judicious construction of interfaces with swift charge communication to enhance the utilization efficiency of photogenerated carriers is a viable strategy for boosting the photocatalytic performance of heterojunctions. Herein, an in-situ partial conversion strategy is reported for decorating lead-free halide perovskite Cs3Bi2Br9 nanocrystals onto BiOBr hollow nanotube, resulting in the formation of an S-scheme heterojunction Cs3Bi2Br9/BiOBr. This unique in-situ growth approach imparts a closely contacted interface to the Cs3Bi2Br9/BiOBr heterojunction, facilitating interfacial electron transfer and spatial charge separation compared to a counterpart (Cs3Bi2Br9:BiOBr) fabricated via traditional electrostatic self-assembly. Additionally, the establishment of an S-scheme charge transfer pathway preserves the robust redox capability of photogenerated carriers. Furthermore, the free electron transfer from Cs3Bi2Br9 to BiOBr promotes the activation of the NN bond and diminishes the energy barrier associated with the rate-determining step in the N2 reduction process. Consequently, the Cs3Bi2Br9/BiOBr heterojunction exhibits highly selective photocatalytic N2 reduction to NH3 (nearly 100 %) at a rate of 130 μmol g−1 h−1 under simulated sunlight (100 mW cm−2), surpassing BiOBr, Cs3Bi2Br9, and Cs3Bi2Br9:BiOBr by factors of 6, 4, and 2, respectively.

Ti3+-mediated MIL-125(Ti) by metal substitution for boosting photocatalytic N2 fixation

Photocatalysis, which uses sunlight, N2 and H2O to produce NH3, is a more sustainable approach to N2 fixation than the Haber-Bosch process. However, its efficiency is severely limited by the difficulty of activating NN bonds. This work presents metal (M = Cu, Fe, V)-substituted MIL-125(Ti) (MIL-(MTi)) for photocatalytic N2 fixation without using any sacrificial agents. Structural characterizations reveal that the active sites including oxygen vacancies (OV) and Ti3+ species are formed by the resulting crystal distortion due to the partial substitution of Ti4+ by other metal ions (Cu+, Fe2+, V3+) in MIL-125(Ti). MIL-(CuTi) possesses a larger number of OV and Ti3+ compared to MIL-(FeTi) and MIL-(VTi) due to the larger valence difference between Cu+ and Ti4+. These active sites not only promote the adsorption and activation of N2 and H2O, but also facilitate the photogenerated charge mobility. Photogenerated holes oxidize H2O to produce O2 and H+. Photogenerated electrons reduce N2 activated on Ti3+ sites by combining with H+ to form NH4+. Therefore, MIL-(CuTi) shows the highest NH4+ production rate 46.5 µmol·h−1·g−1, which is much higher than that (1.2 µmol·h−1·g−1) of the pristine MIL-125(Ti). This work provides a new insight into rational design for artificial N2 fixation systems by the construction of the active site.

Sulfur defect engineering boosted nitrogen activation over FeS2 for efficient electrosynthesis of ammonia

Electrocatalytic nitrogen (N2) reduction to ammonia (NH3) reaction (eNRR) supplies a promising alternative to the Haber-Bosch technology. However, the dissociation of NN bond hinders its development. Herein, sulfur vacancies are introduced into FeS2 for promoting N2 activation and thus stimulating the eNRR progress. Experimental investigations and density functional theory (DFT) calculations reveal that the electrons could transfer from Fe 3d orbits to N2 2π* orbital, thus facilitating the cracking of inert N2 molecules. And the electron transfer is easier for those Fe atoms with S vacancies in adjacent positions. Furthermore, we find that eNRR process on the FeS2 surface follows the distal and alternating hybrid pathway. Also, the water molecules in the electrolyte facilitate the first hydrogenation of N2 (*N2 → *NNH). Notably, FeS2 with rich sulfur vacancies exhibits an excellent NH3 yield rate of 67.5 μg h−1 mgcat.−1, which outperforms most of the reported eNRR activities of Fe-based catalysts.

Mechanism and toxicity assessment of Cu0/Fe3O4 activated persulfate based on the acceleration cycle of Fe3+/Fe2+ for the degradation of methyl orange

Accelerating the circulation of Fe3+/Fe2+ contributed to the production of SO4·- during persulfate activation in the treatment of refractory wastewater. In this study, magnetic Cu0/Fe3O4 was prepared to enhance the Fe3+/Fe2+ circulation and the separation of Cu0/Fe3O4. The composition, structure, catalytic activity and toxicity of Cu0/Fe3O4 were investigated. The degradation mechanisms of MO were investigated, as well as the toxicity of MO and the breakdown products were evaluated. The results showed that Cu0/Fe3O4(1:3) was more efficient for PS activation as compared with single Cu0 or single Fe3O4. Removal efficiency of MO was up to 94.2% and ·OH and 1O2 played the dominant role. Cu0 and in-situ generated Cu+ accelerated the circulation of Fe3+/Fe2+ and electron transfer efficiency from Cu0/Fe3O4(1:3) to PS. MO molecule was effectively decomposed through hydroxylation and the cleavage of NC and NN bonds. The leachate toxicity of Cu0/Fe3O4(1:3) was low, and the toxicity of intermediate products produced during the reaction was significantly less than MO. In conclusion, this study proposed a feasible pathway for rapid cycle of Fe3+/Fe2+ on the Fe3O4 surface and separation of Cu0 by using Cu0/Fe3O4 as a promising activator for PS, effectively improving the degradation efficiency of MO.

Unusual C-N coupling mechanism via NO dimerization for the electrochemical synthesis of urea on transition metal dimers supported by two-dimensional expanded phthalocyanine

Electro-synthesis of urea is a powerful alternative to traditional demanding industrial engineering by selective reduction of N2 and CO2. Due to the existence of inert NN bond, usual C-N coupling has always hindered the design and performance of catalysts. Herein, we present an unusual C-N coupling mechanism for highly electrochemical catalytic synthesis of urea via NO dimerization on transition metal dimers supported by two-dimensional expanded phthalocyanine (TMTM-Pc) as a prototype through first principles calculations. The key is that the adsorbed *NO dimer can directly jump over challenging step of N-N bond breaking and associate with CO. Particularly, FeFe-Pc is singled out in terms of activity, selectivity, and stability via proposed reaction pathways. What's more, we discovered an easy descriptor, namely, the difference of the adsorption energies of *NO-CO-NO* and *NO-NO*, ∆E(*NO-CO-NO* - *NO-NO*), to consummately depict the thermal-kinetic relationships for the C-N coupling. This work may broaden the understanding of urea formation mechanism and provide more options for future research.

Impact of regioisomerism on unimolecular decomposition reactions of energetic materials: Quantum chemistry modeling of ICM-103 and NAPTO

Energetic regioisomers usually behave different physicochemical properties, and the thermal decomposition mechanism might also be different. In this work, the unimolecular decomposition reactions of two novel energetic isomers, ICM-103 and NAPTO, were investigated through quantum chemistry calculations to reveal the decay mechanism of energetic regioisomers. The molecular properties were first explored to reveal their differences in the molecular properties of ICM-103 and NAPTO. Then, six initial decomposition channels were obtained, including N2 release of azide group, H transfer, O transfer, nitro dissociation, nitro-nitrite isomerization, and ring-opening reaction. Thermochemistry results show that the most possible decay reaction is the cleavage of NN bonds of azide group to generate N2, and the energy barrier of ICM-103 is lower than that of NAPTO, which is consistent with the conclusion that NAPTO is more stable form the experiment. Specially, two molecules react differently, and a sequential N2 loss is expected in one, and N2O loss in the other is almost lost in the clutter. Besides, the nitro dissociation, nitro-nitrite isomerization, and ring-opening reaction energy of ICM-103 are also lower than that of NAPTO. However, for the other same reaction, the energy barrier of ICM-103 is higher than that of NAPTO. These findings in the initial decomposition pathways of ICM-103 and NAPTO could gain more understanding of the effect of regiochemical control on the thermal stability of energetic materials.

Electrochemical Reduction of N2 to Ammonia Promoted by Hydrated Cation Ions: Mechanistic Insights from a Combined Computational and Experimental Study.

Alkali ions, major components at the electrode-electrolyte interface, are crucial to modulating reaction activity and selectivity of catalyst materials. However, the underlying mechanism of how the alkali ions catalyze the N2 reduction reaction (NRR) into ammonia remains elusive, posing challenges for experimentalists to select appropriate electrolyte solutions. In this work, by employing a combined experimental and computational approach, we proposed four essential roles of cation ions at Fe electrodes for N2 fixation: (i) promoting NN bond cleavage; (ii) stabilizing NRR intermediates; (iii) suppressing the competing hydrogen evolution reaction (HER); and (iv) modulating the interfacial charge distribution at the electrode-electrolyte interface. For N2 adsorption on an Fe electrode with cation ions, our constrained ab initio molecular dynamic (c-AIMD) results demonstrate a barrierless process, while an extra 0.52 eV barrier requires to be overcome to adsorb N2 for the pure Fe-water interface. For the formation of *NNH species within the N2 reduction process, the calculated free energy barrier is 0.50 eV at the Li+-Fe-water interface. However, the calculated barrier reaches 0.81 eV in pure Fe-water interface. Furthermore, experiments demonstrate a high Faradaic efficiency for ammonia synthesis on a Li+-Fe-water interface, reaching 27.93% at a working potential of -0.3 V vs RHE and pH = 6.8. These results emphasize how alkali metal cations and local reaction environments on the electrode surface play crucial roles in influencing the kinetics of interfacial reactions.

Mussel-inspired poly-norepinephrine coatings as “slippy-sticky interlayer” for fabricating high-energy insensitive energetic composites

Inspired by the super-adhesive properties in marine mussel, a desensitized energetic powder constituted of a slippy-sticky polynorepinephrine interlayer and CL-20 microcore is reported. The slow polymerization process enables the extremely smooth and multifunctional thin-coating formed onto the CL-20 surface. Based on the designed composites, the calculated electronic energy values of all 15 optimized structures reveals a spontaneous thermodynamic process and a stronger bonding strength with the increase of polymerization degree. The H50values of NE/CL-20 structure are calculated and show a significant increase for new composite, produced by the bonding of norepinephrine with CL-20, versus pure CL-20. The subsequent experimental results demonstrate that the slippy CL-20@polynorepinephrine structure exhibits a high drop height (H50, 35.5 cm) and a low explosion probability (60 %) than that of pure CL-20, even exhibits a considerable increase and decreases by 13.3 cm and 12 % in turn versus CL-20@polydopamine under the identical experimental conditions. The sensitive simulation results indicate that the shorter NN bond length and lower area percent in electrostatic potential of samples are positively correlated with the decreasing of impact sensitivity. Note that the ultrasmooth bionic interlayer gives the super-adhesive properties of energetic powder, greatly improves the coating degree of 2D material, thereby shows a lower sensitivity and a high detonation heat for the constructed double-coating composites (CL-20@PNE@GO). To sum up, this study will provide an innovative route to construct a high-safety powder based on a bionic sticky interlayer strategy.

Multifold nanotwin-enhanced catalytic activity of Ni-Zn-Cu for hydrazine oxidation

The hydrazine oxidation reaction (HzOR) plays a critical role in direct hydrazine fuel cells and presents a promising alternative to the kinetics-sluggish oxygen evolution reaction in water reduction. Engineering catalysts with high-density multifold nanotwin structures has emerged as a compelling strategy to enhance catalytic activity. However, the exploration of multifold nanotwins for HzOR applications remains absent. Herein, we report the successful synthesis of a porous Ni-Zn-Cu catalyst featuring high-density twofold and fivefold nanotwins. The Ni-Zn-Cu catalyst supported on Ni foam (Ni-Zn-Cu/Ni foam) exhibits exceptionally high catalytic activity and stability toward the HzOR, achieving a current density of 204.3 mA cm−2 at 0.0 V vs. RHE and retaining up to 89.1 % after 20 h of continuous electrolysis. Online mass spectrometer (MS) analysis indicates that the Ni-Zn-Cu coating facilitates NN bond cleavage of N2H4. This study offers valuable insights into the development of multifold nanotwin catalysts for advancing HzOR applications.

Study of mesomorphism behavior and protonation-induced emission of symmetrical benzalazines

Rod-shaped and polycatenar symmetric benzalazines with protonation-induced emission were synthesized, characterized, and studied for their thermal and optical behavior. Benzalazines lacking hydroxy groups at the ortho position did not show luminescence in the solid state or solution. In the presence of HCl (either in solution or vapor form) these compounds became good emitters. The protonation of azine nitrogen that may hinder the NN bond rotation by intermolecular interactions toward aggregates provided an explanation for the emission characteristics observed. The protonation-induced emission was thermally reversible. A benzalazine derived from 4-alkoxysalicylaldehyde with AIE property was prepared. For this compound with ortho-OH groups, the optical properties were not affected by the presence of acidic media. It exhibited a stable SmC phase characterized by POM, DSC, and XRD. The luminescence capability of the molecular arrangement within the smectic phase was investigated. The intense yellow-green emission in the crystals decreased significantly at the phase transition due to the non-radiative decay that occurs with increasing temperature, and loss of intermolecular H-bonding which prevents molecular rotations in the crystal.

Constructing a interfacial electric field for efficient reduction of nitrogen to ammonia

Electrochemical nitrogen reduction (eNRR) is a cost-effective and environmentally sustainable approach for ammonia production. MoS2, as a typical layered transition metal compound, holds significant potential as an electrocatalyst for the eNRR. Nevertheless, it suffers from a limited number of active sites and low electron transfer efficiency. In this study, we constructed a heterostructure by depositing SnO2 (an n-type semiconductor) nanoparticles on MoS2 (a p-type semiconductor). This unique interfacial structure not only generates abundant interfacial contacts but also facilitates the transfer of electrons from SnO2 to MoS2, leading to the formation of an interfacial electric field. Theoretical calculations demonstrate that this electric field increases the number of active electrons, facilitating N2 adsorption and NN bond activation. Moreover, it increases the degree of orbital overlap between N2 and SnO2/MoS2, effectively reducing the energy barrier of the rate-determining step. Benefiting from the interfacial electric field effect, the SnO2/MoS2 catalyst exhibits significant catalytic activity and selectivity towards eNRR, with an ammonia yield of 47.1 µg h−1 mg−1 and a Faraday efficiency of 19.3 %, surpassing those reported for the majority of MoS2- and SnO2-based catalysts.

Reduction of molecular oxygen in flavodiiron proteins - Catalytic mechanism and comparison to heme-copper oxidases

The family of flavodiiron proteins (FDPs) plays an important role in the scavenging and detoxification of both molecular oxygen and nitric oxide. Using electrons from a flavin mononucleotide cofactor molecular oxygen is reduced to water and nitric oxide is reduced to nitrous oxide and water. While the mechanism for NO reduction in FDPs has been studied extensively, there is very little information available about O2 reduction. Here we use hybrid density functional theory (DFT) to study the mechanism for O2 reduction in FDPs. An important finding is that a proton coupled reduction is needed after the O2 molecule has bound to the diferrous diiron active site and before the OO bond can be cleaved. This is in contrast to the mechanism for NO reduction, where both NN bond formation and NO bond cleavage occurs from the same starting structure without any further reduction, according to both experimental and computational results. This computational result for the O2 reduction mechanism should be possible to evaluate experimentally. Another difference between the two substrates is that the actual OO bond cleavage barrier is low, and not involved in rate-limiting the reduction process, while the barrier connected with bond cleavage/formation in the NO reduction process is of similar height as the rate-limiting steps. We suggest that these results may be part of the explanation for the generally higher activity for O2 reduction as compared to NO reduction in most FDPs. Comparisons are also made to the O2 reduction reaction in the family of heme‑copper oxidases.

Phosphomolybdic acid regulated the defective metal-organic framework UiO-66 for electrochemical nitrogen reduction reaction

In recent years, electrochemical ammonia synthesis has been considered a green process with excellent application prospects. The nitrogen reduction process (NRR) needs more activation energy when NN bonds are broken, which decreases the ammonia yield rate and Faraday efficiency (FE) for ammonia synthesis. Herein, UiO-66-xHPMo electrocatalysts are effectively synthesized by regulating the defects of the metal-organic framework (UiO-66) using phosphomolybdic acid (HPMo) as a defect regulator. XRD, FT-IR, TEM, XPS, TPD, etc., characterized the catalysts. As a result, the reasons for the increase in NH3 yield rate (RNH3) are catalysts losing the ligand of terephthalic acid, exposing more Zr6 nodes as active sites, and providing numerous active sites for NN cleavage. However, introducing HPMo makes catalysts more alkaline and have lower charge transfer resistance, which are more inclined to adsorb H+ and enhance hydrogen evolution reaction (HER), reducing FEs. Thus, with a voltage of −0.3 V (vs. RHE) and a neutral electrolyte (0.1 mol L−1 Na2SO4), UiO-66–2HPMo has excellent NRR performance (RNH3: 36.61 μg h−1 mg−1, FE: 31.09%).

Synergistic polarity interaction and structural reconstruction in Bi2MoO6/C3N4 heterojunction for enhancing piezo-photocatalytic nitrogen oxidation to nitric acid

Constructing heterojunctions has emerged as a widely embraced strategy for augmenting piezo-photocatalytic activities. However, the synergistic pressure response, the construction of charge transfer, polar direction sites and active site are often left in the basket. Here, the carboxylated C3N4 and Bi2MoO6 S-scheme heterostructure was elaborately designed for piezo-photocatalytic nitrogen oxidation towards nitric acid. Extensive research evidence proves that Bi-COOH interaction between Bi2MoO6 and g-C3N4 leads to the occurrence of polarity interaction and structural reconstruction. Those initiatives facilitate the efficient distribution of charges and acts as a pathway for carrier migration, thereby promoting charge transfer and the large intrinsic dipole moment. Furthermore, the combination of polarity interaction and structural reconstruction strengthens N2 polarization and electron transfer, facilitating the breaking of NN bonds and reducing activation energy. Consequently, the optimal BCO-3 catalyst revealed outstanding nitric acid production rates of 5930 μg g−1 h−1 under the stimulation of ultrasonic and light illumination, which is 3.66 times higher than that of C3N4-Bi2MoO6 heterostructure and superiors to various piezo-photocatalysts. Our research endeavors present promising prospects for the design of advanced catalysts and contribute to a profound comprehension of molecular activation processes in heterojunction.

Theoretical study of N2 adsorption and dissociation on Ir/Cu loaded Ir(100) catalyst

Electrocatalytic nitrogen reduction (NRR) is a promising method for NH3 synthesis. However, the design of catalysts with high activity for N2 dissociation remains a key challenge. Herein, we have designed several catalysts based on Ir, including pure Ir(100), and Ir(100) with Ir (Cu) atom loaded on it (denoted as Ir(a)@Ir(100) and Cu(a)@Ir(100)), to study the reactivity of N2 dissociation. The results showed that Ir(a)@Ir(100) and Cu(a)@Ir(100) can effectively activate NN bond with ultralow dissociation barriers of 0.31 eV and 0.61 eV. However, the adsorption strength of N2 is significantly poor on Ir(a)@Ir(100) (−0.24 eV) compared to that on Cu(a)@Ir(100) (−0.62 eV). This can be interpreted from the electronic properties: The Ir-5d states can hybridize with N-2π* states significantly near the Fermi level, which is absence for Cu-3d states. Therefore, the loaded Cu atom on Cu@Ir(100) can effectively decrease the occupation of N2 antibonding orbitals (ICOHP = −7.68) compared to the situation on Ir@Ir(100) (ICOHP = −7.35). Therefore, Cu(a)@Ir(100) can be screened as the favorable candidate although a little higher dissociation barrier of N2 (0.61 eV), compared to the situation on Ir(a)@Ir(100) (0.31 eV). However, a barrier of 0.61 eV can also be easily overcome at room temperature as 0.31 eV on Ir(a)@Ir(100). We firmly believe that this work can not only open a novel way for the design of Ir-based catalysts, but also provide a promising strategy of N2 dissociation for experimental works.

Boosting N[tbnd]N tri-bond activation and NH3 generation rate through simple surface Ag modification

NH3 photosynthesis from nitrogen remains a big challenge due to the extreme difficulty for the activation of NN bond. In this work, Ag-modified MoO3·0.55 H2O nanorods (named as Ag@Mo) are prepared by a simple photoreduction method. Under visible light (λ ≥ 420 nm) for 60 min, the NH3 yield over 2%Ag@Mo (molar ratio of Ag to MoO3·0.55 H2O) reaches 242.0 μmol·gcat−1, which is about 2.8 times higher than MoO3·0.55 H2O under the same conditions. The effect of Ag modification on photocatalytic ammonia synthesis is explored through experiments and theoretical calculations. The excellent photocatalytic performance is mainly attributed to the localized surface plasmon resonance effect of Ag and the successful construction of Mott-Schottky junction, which improves light utilization efficiency and reduces carrier recombination efficiency, respectively. Moreover, density functional theory calculations theoretically prove that over Ag@Mo, Ag provides new adsorption sites, and the NN tri-bond length is elongated by about 16.2%, confirming that Ag modification obviously promotes the activation of NN tri-bond. The ammonia generation rate is obviously higher than the previously reported results.

Insertion reactions and structural studies of [(NHC)CuH]2 with nitrogen-based substrates

The stoichiometric and catalytic reactions of Cu-H dimers supported by N-heterocyclic carbenes (NHCs) have mainly focused on the insertions of aldehydes, ketones, CO2, and unsaturated hydrocarbons. In this report, we investigated the stoichiometric reactions of dimeric [(NHC)CuH]2 (NHC = IPr*, 6Dipp) with unsaturated nitrogen-based substrates of PhN=NPh, N3Ad, pyrazine, and N2O. The spectroscopic and structural chacterizations of the resulting monomeric, two-coordinate Cu(I) complexes containing diphenyl hydrazido, triazenido, pyrazinyl, and hydroxide ligands are discussed. The hydrazido complex of (IPr*)Cu(NPh-NHPh) and triazenido complex of (IPr*)Cu(HNNNAd) are resistant to NN bond cleavage and loss of N2, respectively, to form the corresponding amido complexes even when heated at 80 °C over several hours.

Enhancing biocatalytical N[sbnd]N bond formation with the actinobacterial piperazate synthase KtzT

Natural compounds with nitrogen-nitrogen bonds are diverse and have applications in medicine and agriculture. l-Piperazic acid (Piz), an α-hydrazino acid, is one of few naturally occurring compounds of its kind. Yet, Piz and its derivatives are valuable building blocks for bioactive compounds. Few NNzymes, enzymes capable of forming NN bonds, have been identified thus far. The hemoenzyme KtzT from Kutzneria sp. 744 catalyzes the intramolecular NN bond formation of N5‑hydroxy-l-ornithine (OH-Orn) to form Piz, a natural building block of kutznerides. The latter has antifungal and antibiotic properties. In our study, we established an improved expression method, with significantly improved yields (ca. 35-fold) of heme-loaded enzyme, making the enzyme much more accessible for laboratory studies. In vitro biochemical characterization under conditions for NN bond formation indicated a considerable thermo- and pH-flexibility, with optimal reaction conditions at 30 °C and 10 mM Tris buffer at pH 9 together with low salinity, paving the way for more complex applications involving KtzT. We have also identified two homologous enzymes from extremophilic organisms to exhibit piperazate-forming activity. In silico structural studies, combined with phylogenetic analysis, resulted in a heme- and substrate-binding model, suggesting target enzyme residues that we propose are critical for the structural integrity and catalytic activity of KtzT. Following this approach, we investigated the potential role of a cysteine residue in a dimer-stabilizing disulfide bridge. The interplay of in vitro and in silico data therefore provides crucial functional information on this enzyme class.