Multi‐Tier Electronic Structure Analysis of Sita's Mo and W Complexes Capable of Thermal or Photochemical N₂Splitting

Abstract

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.

Understanding Terminal versus Bridging End-on N2 Coordination in Transition Metal Complexes

“…To elucidate the origin of this effect, we inspect in Figure the π-MOs in the octahedral terminal and bridging N 2 complexes of the symmetrical d 6 fragment [HRe­(NH 3 ) 4 ]. The ammonia and hydride ligands in [HRe­(NH 3 ) 4 ] prevent π-MO delocalization beyond the M–N 2 core of interest, which was observed for the complexes in Scheme (Figure S1–S4) and which is known to plague MO analyses of other studies of related complexes. , To aid the discussion we also utilize general qualitative π-MO diagrams for free N 2 and the MNN and MNNM moieties.…”

The role of excited-state character, structural relaxation, and symmetry breaking in enabling delayed fluorescence activity in push–pull chromophores

“…The CT numbers constitute a versatile tool and have been applied successfully for, e.g., interacting chromophores, 60 pushpull systems 31 and transition metal complexes. 61,62 However, a downside of this analysis is that it depends on the a priori definition of a fragmentation scheme. To overcome this problem and obtain a more fundamental measure of charge transfer, we compute an exciton size defined as the root-mean-square separation of electron and hole…”

Stickstoff aktivieren mit Licht

“…Dazu dienen neben der Analyse der berechneten MOs Techniken wie Differenzdichten. 10) In einem der bisher bekannten Komplexe, [(µ-N 2 ){ReCl-(PNP iPr )} 2 ], weist die photoaktive Region Übergänge des Typs δ/δ*→π*π*π* auf. 11) In den relevanten angeregten Zuständen wird also die π-Bindung zwischen den Stickstoffatomen und zwischen den Stickstoff-und Metall ato men geschwächt, was etwa zu einer Abwinklung der linearen Re-N-N-Re-Einheit führen könnte.…”