Direct conversion of N2 and O2: status, challenge and perspective

Eliminating Concentration Polarization with Cationic Covalent Organic Polymer to Promote Effective Overpotential of Nitrogen Fixation

Electrocatalytic nitrogen reduction reaction offers a sustainable alternative to the conventional Haber-Bosch process. However, it is currently restricted by low effective overpotential due to the concentration polarization, which arises from accumulated products, ammonium, at the reaction interface. Here, a novel covalent organic polymer with ordered periodic cationic sites is proposed to tackle this challenge. The whole network exhibits strong positive charge and effectively repels the positively charged ammonium, enabling an ultra-low interfacial product concentration, and successfully driving the reaction equilibrium to the forward direction. With the given potential unchanged, the suppressed overpotential can be much liberated, ultimately leading to a continuous high-level reaction rate. As expected, when this tailored microenvironment is coupled with a transition metal-based catalyst, a 24-fold improvement is generated in the Faradaic efficiency (73.74 %) as compared with the bare one. The proposed strategy underscores the importance of optimizing dynamic processes as a means of improving overall performance in electrochemical syntheses.

Coupling of N2 and O2 in the Gas Phase to Synthesize Nitric Oxide at Room Temperature: A Zeldovich-Like Strategy

Dinitrogen (N2) activation and its chemical transformations are some of the most challenging topics in chemistry. Herein, we report that heteronuclear metal anions AuNbBO- can mediate the direct coupling of N2 and O2 to generate NO molecules. N2 first forms the nondissociative adsorption product AuNbBON2- on AuNbBO-. In the following reactions with two O2 molecules, two NO molecules are gradually released, with the formation of AuNbBO2N- and AuNbBO3-. In the reaction with the first O2, the generated nitrene radical (N••-) originating from the dissociated N2, induces the activation of O2. Subsequently, the second O2 is anchored and forms a superoxide radical (O2•-); this radical attacks the other N atom to form an N-O bond, releasing the second NO. The N••- and O2•- radicals play key roles in the reactions. The mechanism adopted in this direct oxidation of N2 by O2 to NO can be labeled as a Zeldovich-like mechanism.

Pulsed Electrocatalysis Enabling High Overall Nitrogen Fixation Performance for Atomically Dispersed Fe on TiO2

Atomically dispersed Fe was designed on TiO₂ and explored as a Janus electrocatalyst for both nitrogen oxidation reaction (NOR) and nitrogen reduction reaction (NRR) in a two-electrode system. Pulsed electrochemical catalysis (PE) was firstly involved to inhibit the competitive hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Excitingly, an unanticipated yield of 7055.81 μmol h^-1  g^-1 _cat. and 12 868.33 μmol h^-1  g^-1 _cat. were obtained for NOR and NRR at 3.5 V, respectively, 44.94 times and 7.8 times increase in FE than the conventional constant voltage electrocatalytic method. Experiments and density functional theory (DFT) calculations revealed that the single-atom Fe could stabilize the oxygen vacancy, lower the energy barrier for the vital rupture of N≡N, and result in enhanced N₂ fixation performance. More importantly, PE could effectively enhance the N₂ supply by reducing competitive O₂ and H₂ agglomeration, inhibit the electrocatalytic by-product formation for longstanding *OOH and *H intermediates, and promote the non-electrocatalytic process of N₂ activation.

Dinitrogen Functionalization Affording Chromium Diazenido and Side-on η2-Hydrazido Complexes

Isolation of key intermediate complexes in dinitrogen functionalization is crucial for elucidating the mechanistic details and further investigation. Herein, the synthesis and characterization of (μ-η¹:η¹-N₂)(η¹-N₂)-Cr(I) 3 and (η¹-N₂)₂-Cr(0) complexes 4 supported by Cp* (Cp* = C₅Me₅) and NHC ligands were reported. Further functionalization of Cr(0)-N₂ complex 4 with silyl halides delivered the key intermediates in the alternating pathway, the chromium diazenido complex 5 and the chromium side-on η²-hydrazido complex 6. Protonation of 6 led to the quantitative formation of N₂H₄. Moreover, the [η²-Me₃SiNNSiMe₃]^2- unit in 6 enabled N-C bond formation reactions with CO₂ and ^tBuNCO, giving the corresponding N,O-chelating hydrazidochromium complexes 7 and 8, respectively.

Direct Conversion of N2 and O2 to Nitric Oxide at Room Temperature Initiated by Double Aromaticity in the Y2BO+ Cation

The conversion of dinitrogen to more useful and reactive molecules has been the focus of intense research by chemists. In contrast to reductive N₂ fixation, direct oxidation of N₂ by O₂ to nitric oxide under mild conditions via a thermochemical process is extremely challenging. Herein, we report the first example of N₂ and O₂ activation and coupling under thermochemical conditions through the remarkable ability of Y₂BO⁺ to react with one N₂ and two O₂ molecules. Detailed mechanistic studies using mass spectrometry and quantum chemical calculations revealed that the N₂ activation by Y₂BO⁺ is facilitated by the double aromatic character of the Y₂BON₂⁺ intermediate. Subsequent oxidation with O₂ releases NO in a dearomatization process driven by the formation of stronger Y-O bonds over the Y-N bonds. Our findings represent the first example of N₂ and O₂ activation and coupling under thermochemical conditions at room temperature, providing a novel strategy for small-molecule activation.

Synthesis of pyrimidines from dinitrogen and carbon

The element nitrogen and nitrogenous compounds are vital to life. The synthesis of nitrogen-containing compounds using dinitrogen as the nitrogen source, not through ammonia, is of great interest and great value but remains a grand challenge. Herein, we describe a strategy to realize this transformation by combining the heterogeneous approach with the homogeneous methodology. The N₂ molecule was first fixed with carbon and LiH through a one-pot heterogeneous process, forming Li₂CN₂ as an 'activated' nitrogen source with high efficiency. Then subsequent homogeneous treatments of Li₂CN₂ to construct the organic synthon carbodiimide and the RNA/DNA building block pyrimidines were fulfilled. By using ¹⁵N₂ as the feedstock, their corresponding ¹⁵N-labeled carbodiimide and pyrimidines were readily obtained. This homogeneous-heterogeneous synergy strategy will open a new chapter for N₂ transformation.

Dinitrogen activation by a phosphido-bridged binuclear cobalt complex

The reduction of PNPCoBr under a N2 atmosphere yielded a binuclear cobalt dinitrogen anion complex via the C–P bond cleavage of the PNP ligand.