Chemische und biologische Aspekte der Fixierung und Reduktion molekularen Stickstoffs

Dinitrogen (N2) is the most abundant gas in Earth's atmosphere, but its inertness hinders its use as a nitrogen source in the biosphere and in industry. Efficient catalysts are hence required to ov. ercome the high kinetic barriers associated to N2 transformation. In that respect, molecular complexes have demonstrated strong potential to mediate N2 functionalization reactions under mild conditions while providing a straightforward understanding of the reaction mechanisms. This Review emphasizes the strategies for N2 reduction and functionalization using molecular transition metal and actinide complexes according to their proposed reaction mechanisms, distinguishing complexes inducing cleavage of the N≡N bond before (dissociative mechanism) or concomitantly with functionalization (associative mechanism). We present here the main examples of stoichiometric and catalytic N2 functionalization reactions following these strategies.

Section: Dissociative Mechanismmentioning

confidence: 99%

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

Masero

Perrin

Dey

et al. 2020

“…Instead, the particular excited state must enable a well-directed activation of N 2 through excitation in an antibonding molecular orbital. This idea of utilizing p* orbitals of the N 2 moiety is straightforward and has therefore been suggested earlier in the literature; [31] certain bands in the electronic spectra of various dinitrogen complexes could indeed be assigned to transitions into p* orbitals (in an idealized, qualitative quantum chemical one-particle picture). [32][33][34] However, it is by far less clear whether the occupation of a p*-antibonding molecular orbital can then produce a diazene-like conformation with N 2 possessing a double bond and two lone pairs at the nitrogen atoms as depicted in Figure 1.…”

Section: Introductionmentioning

confidence: 97%

A Photochemical Activation Scheme of Inert Dinitrogen by Dinuclear Ru^II and Fe^II Complexes

Reiher

Kirchner

Hutter

et al. 2004

A general photochemical activation process of inert dinitrogen coordinated to two metal centers is presented on the basis of high-level DFT and ab initio calculations. The central feature of this activation process is the occupation of an antibonding pi* orbital upon electronic excitation from the singlet ground state S0 to the first excited singlet state S1. Populating the antibonding LUMO weakens the triple bond of dinitrogen. After a vertical excitation, the excited complex may structurally relax in the S1 state and approaches its minimum structure in the S1 state. This excited-state minimum structure features the dinitrogen bound in a diazenoid form, which exhibits a double bond and two lone pairs localized at the two nitrogen atoms, ready to be protonated. Reduction and de-excitation then yield the corresponding diazene complex; its generation represents the essential step in a nitrogen fixation and reduction protocol. The consecutive process of excitation, protonation, and reduction may be rearranged in any experimentally appropriate order. The protons needed for the reaction from dinitrogen to diazene can be provided by the ligand sphere of the complexes, which contains sulfur atoms acting as proton acceptors. These protonated thiolate functionalities bring protons close to the dinitrogen moiety. Because protonation does not change the pi*-antibonding character of the LUMO, the universal and well-directed character of the photochemical activation process makes it possible to protonate the dinitrogen complex before it is irradiated. The pi*-antibonding LUMO plays the central role in the activation process, since the diazenoid structure was obtained by excitation from various occupied orbitals as well as by a direct two-electron reduction (without photochemical activation) of the complex; that is, the important bending of N2 towards a diazenoid conformation can be achieved by populating the pi*-antibonding LUMO.

“…The idea of using electronic excitations to populate a N‐N antibonding orbital to induce N 2 splitting has been discussed before. In a thought experiment by Fischler and von Gustorf, they noted that in an end‐on bound metal‐N 2 complex an excitation from a π‐ to a σ*‐orbital would weaken the M−N bond in a twofold way and thus likely induce metal‐nitrogen bond cleavage, while a MLCT transition into a π*‐orbital would increase the nitrogen basicity and thus facilitate protonation . The previous computational study by Reiher et al.…”

Section: Resultsmentioning

confidence: 96%

A Valence‐Delocalised Osmium Dimer capable of Dinitrogen Photocleavage: Ab Initio Insights into Its Electronic Structure

Krewald

González

2018

The search for molecular catalysts that efficiently activate or cleave the dinitrogen molecule is an active field of research. While many thermal dinitrogen cleavage catalysts are known, the photochemical activation of N has received considerably less attention. In this paper, the first computational study of the osmium dimer [Os (μ-N )(NH ) ] , which was shown to be capable of dinitrogen photocleavage, is presented. Despite its deceptively simple geometry, it has a complex electronic structure with a valence-delocalized and electronically degenerate ground state. Using multiconfigurational methods, the electronic structure at the ground state geometry and along the dinitrogen cleavage coordinate was investigated. The results indicate that an unoccupied molecular orbital with σ-bonding character between osmium and μ-N atoms and σ-antibonding dinitrogen character is most affected by N-N distance elongation. This implies that a lower barrier for thermal or photochemical N activation in linear M-N-N-M complexes can be achieved by lowering the energetic separation between this unoccupied orbital and the HOMO, representing a specific target for future catalyst design.