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The sustainable fixation of atmospheric N2 and its conversion into industrially relevant molecules is one of the major current challenges in chemistry. Besides nitrogen activation with transition metal complexes, a "push‐pull" approach that fine‐tunes electron density along the N−N bond has shown success recently. The "pushing" is performed by an electron rich entity such as a transition metal complex, and the "pulling" is achieved with an electron acceptor such as a Lewis acid. In this contribution, we explore the electronic structure implications of this approach using the complex trans‐[ReICl(N2)(PMe2Ph)4] as a starting point. We show that borane Lewis acids exert a pull‐effect of increasing strength with increased Lewis acidity via a π‐pathway. Furthermore, the ligand trans to dinitrogen can weaken the dinitrogen bond via a σ‐pathway. Binding a strong Lewis acid is found to have electronic structure effects potentially relevant for electrochemistry: dinitrogen‐dominated molecular orbitals are shifted into advantageous energetic positions for redox activation of the dinitrogen bond. We show how these electronic structure design principles are rooted in cooperative effects of a transition metal complex and a Lewis acid, and that they can be exploited to tailor a complex towards the desired thermal, electrochemical or photochemical reactivity. [ABSTRACT FROM AUTHOR]

An emerging approach for the activation of the nitrogen molecule is the light‐driven splitting of the N–N bond. Less than ten examples for complexes capable of N2 photoactivation are currently known, and the underlying photophysical and photochemical processes after light absorption are largely unresolved. All complexes have a central [M(µ‐η1:η1‐N2)M] unit with equivalent ligand spheres around each metal. For several of these complexes, small modifications of the ligand sphere result in thermal rather than photochemical activity. Herein, we analyse the electronic structures and computed UV/Vis spectra of four complexes: two thermally and two photochemically active complexes, each either involving molybdenum or tungsten. The analysis of electronic structures and spectra is based on the molecular orbitals, difference densities and the charge‐transfer numbers provided by TheoDORE. We find that the spectra of the photochemically active complexes contain excitations with more ligand‐to‐metal charge‐transfer character and higher intensity, providing a plausible explanation for light‐induced nitrogen splitting. [ABSTRACT FROM AUTHOR]

Abstract: 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 N2 has received considerably less attention. In this paper, the first computational study of the osmium dimer [Os(II,III)2(μ‐N2)(NH3)10]5+, 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 N2 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. [ABSTRACT FROM AUTHOR]

Heterogeneity in intermediate catalytic states of the oxygen-evolving complex (OEC) of Photosystem II is known from a wide range of experimental and theoretical data, but its potential implications for the mechanism of water oxidation remain unexplored. We delineate the consequences of structural heterogeneity for the final step of the catalytic cycle by tracing the evolution of three spectroscopically relevant and structurally distinct components of the last metastable S 3 state to the transient O 2 -evolving S 4 state of the OEC. Using quantum chemical calculations, we show that each S 3 isomer leads to a different electronic structure formulation for the active S 4 state. Crucially, in addition to previously hypothesized Mn(IV)-oxyl species, we establish for the first time, how a genuine Mn(V)-oxo can be obtained in the catalytically active S 4 state: this takes the form of a five-coordinate and locally high-spin (S Mn = 1) Mn(V) site. This formulation for the S 4 state evolves naturally from a preceding S 3 -state structural intermediate that contains a quasi-trigonal-bipyramidal Mn(IV) ion. The results strongly suggest that water binding in the S 3 state is not prerequisite for reaching the oxygen-evolving S 4 state of the complex, supporting the notion that both substrates are preloaded at the beginning of the catalytic cycle. This scenario allows true four-electron metal-centered hole accumulation to precede O O bond formation and hence the latter can proceed via a genuine even-electron mechanism. This can occur as intramolecular nucleophilic coupling of two oxo units synchronously with the binding of a water substrate for the next catalytic cycle. A new possible formulation is suggested for the catalytically active S 4 state of the oxygen evolving complex in Photosystem II: it contains a high-spin five-coordinate Mn(V)-oxo entity as a result of decoupling water binding from cofactor oxidation and can support a genuine even-electron mechanism for water oxidation. Unlabelled Image • The oxygen-evolving cluster of photosystem-II exists in different structural forms. • Each S 3 isomer progresses into a distinct O 2 -evolving S 4 state. • S 3 structures with six-coordinate Mn ions form Mn(IV)-oxyl S 4 species. • Delayed water binding allows Mn-centered oxidation, forming a genuine Mn(V)-oxo in S 4. • This enables genuine four-electron nucleophilic coupling in dioxygen evolution. [ABSTRACT FROM AUTHOR]