Multi‐Tier Electronic Structure Analysis of Sita's Mo and W Complexes Capable of Thermal or Photochemical N₂ Splitting

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.

The nitrogen molecule cannot only be activated thermally and electrochemically, but also photochemically. A study of the electronic structure of four Mo and W complexes with similar ligand spheres that are either photochemically or thermally active shows that ligand-to-metal charge transfer excitations appear more often and with higher intensity in complexes capable of nitrogen photoactivation.